12th International Congress on Catalysis (Studies in Surface Science and Catalysis)

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Studies in Surface Science and Catalysis 130 Part A

12th INTERNATIONAL CONGRESS ON CATALYSIS

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Studies in Surface Science and Catalysis Advisory Editors: B. Delmon and J.T. Yates

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12th INTERNATIONAL CONGRESS ON CATALYSIS PART A Proceedings of the 12th ICC, Granada, Spain, July 9-14,2000 Editors

Avelino Corma Francisco V. Melo lnstituto de Tecnologia Quimica, UPV-CSIC,Avda. de 10s Naranjos s/n, 46022 - Valencia, Spain

Sagrario Mendioroz Jose Luis G. Fierro lnstituto de CatAlisis y Petroleoquimica, CSlC, Campus UAM Cantoblanco, 28049- Madrid, Spain

2000 ELSEVIER

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Amsterdam Lausanne New York Oxford Shannon Singapore -Tokyo

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ELSEVIER SCIENCE B.V. Sara Burgerhartstraat 25 P.O. Box 2 1 1, I000 A E Amsterdam, The Netherlands

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2000 Elsevier Science B.V. All rights resewed.

This work is protected under copyright by Elsevier Science, and the following terms and conditions apply to its use: Photocopying Single photocopies of single chapters may be made for personal use as allowed by national copyright laws. Permission of the Publisher and payment of a fee is required for all other photocopying, including multiple or systematic copying, copying for advertising or promotional purposes, resale, and all forms of document delivety. Special rates are available for educational institutions that wish to make photocopies for non-profit educational classroom use. Permissions may be sought directly from Elsevier Science Global Rights Department, PO Box 800, Oxford OX5 IDX, UK; phone: (+44) 1865 843830, fax: (+44) 1865 853333, e-mail: [email protected]. You may also contact Global Rights directly through Elsevier's home page (http://www.elsevier.nl), by selecting 'Obtaining Permissions'. In the USA, users may clear permissions and make payments through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA; phone: (978) 7508400, fax: (978) 7504744, and in the UK through the Copyright Licensing Agency Rapid Clearance Service (CLARCS), 90 Tottenham Court Road, London W IP OLP, UK; phone: (+44) 171 63 1 5555; fax: (+44) 171 63 1 5500. Other countries may have a local reprographic rights agency for payments. Derivative Works Tables of contents may be reproduced for internal circulation, but permission of Elsevier Science is required for external resale or distribution of such material. Permission of the Publisher is required for all other derivative works, including compilations and translations. Electronic Storage or Usage Permission of the Publisher is required to store or use electronicallyany material contained in this work, including any chapter or part of a chapter. Except as outlined above, no part of this work may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without prior written permission of the Publisher. Address permissions requests to: Elsevier Science Global Rights Department, at the mail, fax and e-mail addresses noted above. Notice No responsibility is assumed by the Publisher for any injury andlor damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Because of rapid advances in the medical sciences, in particular, independent verification of diagnoses and drug dosages should be made.

First edition 2000

Library of Congress Cataloging-in-PublicationData International Congress on Catalysis (12th :2000 : Granada,Spain) 12th International Congress on Catalysis : proceedings of the 12th ICC, Granada, Spain, July 9-14,2000 1editors Avelino Corma ... [et al.1.-- 1st ed. p. m. Includes index. ISBN 0444-50480-X (pt. A) 1. Catalysis--Congresses. I. Corma, Avelino, 1951- 11. Title. QDSOS .I57 2000 541.3'95--dc21

ISBN:

0 444 50480 X

@ The paper used in this publication meets the requirements of ANSIiNISO 239.48-1992 (Permanence of Paper). Printed in The Netherlands.

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PREFACE The twelfth Congress on Catalysis was held in Granada (Spain) under the auspices of the International Association of Catalysis Societies and the Spanish Society of Catalysis. Those Proceedings are the expression of the Scientific Sessions which constituted the main body of the Congress.

They include 5 plenary lectures, 1 award lecture, 8 keynote lectures, 124 oral presentations and 495 posters. The oral and poster contributions have been selected on the basis of the reports of at least two international reviewers, according to standards comparable to those used for specialised journals, among 1045 two-page abstracts received from 53 countries. The submitted camera-ready manuscripts were then evaluated by the International Scientific Board. Fortunately, most of the corrected manuscripts were received in due course and have been included as such in the Proceedings; however, in a few exceptions, no answer was obtained from the authors; in those cases, a first version of the manuscript appears in the Proceedings. In order to accommodate all these contributions, the Congress was divided in four parallel sessions and three additional sessions in which all the posters were displayed. The management of this fantastic volume of work forced us to take several decisions. As the contributions are published prior to the meeting for distribution to all delegates who attend the Congress in Granada, no discussions at the meeting have been included. Besides this, for space reasons we were restricted to expand the works to only six-page text.

Financial contribution from the Ministry of Education and Culture, the National Council of Scientific Research, other local and national institutions or corporations, chemical, refining and petrochemical companies made it possible to balance the budget of the Congress. Allowance for young students to pay a reduced registration fee was also possible from this income.

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We are grateful to the outstanding scientists, expert in different fields of catalysis, who

accepted our invitation to overview vital research areas in plenary lectures, the 1998 awardee by his illustrative conference and the keynote lecturers that introduce the various topics of the sessions covered by the Congress. The Organisers are indebted to all the scientists who accepted our invitation to come to Spain and made this meeting another outstanding success in the 44 year tradition of this event. We hope everybody will enjoy the meeting and will find these Proceedings a useful book to be added to the catalysis library. We are also grateful to Drs. A. Jongejan, managing director, Dr. P.S. Jackson, publishing director, and specially to Drs. Huub Manten of Elsevier Science Publishers for the guidance and co-operation provided in getting these four volumes printed before the Congress.

Granada, July 2000 The Editors

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LIST OF SPONSORS (by May 17, 2000)

PA TR ONS 1- Ministerio de Ciencia y Tecnologia (Spain) 2- Junta de Andalucia (Spain) 3- Ayuntamiento de Granada (Spain) 4- Consejo Superior de Investigaciones Cientificas (Spain) 5- Universidad de Granada (Spain)

DONORS

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CEPSA (Spain)

7- REPSOL- YPF (Spain) 8- UOP (USA) 9- DEGUSSA HILLS AG (Germany) 10- PROCATALYSE (France) 11- INSTITUT FRAN(~AIS DU PETROLE (France) 12- ENGELHARD (The Netherlands) 13- NOVARTIS AG (Switzerland) 14-SHELL International Chemicals B.V.A. (The Netherlands) 15-BP AMOCO Chemicals Co. (UK) 16- CHEVRON (USA) 17- EXXON-MOBIL Research and Engineering (USA) 18- GRACE Co. (USA) 19- DSM Research (The Netherlands) 20- SI]D-CHEMIE Inc. (USA)

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EXHIBITORS 21-HALDOR TOPSOE (Denmark) 22-SE Reactor, Inc. (USA) 23- ENGELHARD (Italy) 24-ACADEMIC PRESS, Inc. (USA) 25-VINDUM ENGINEERING, Inc. (USA) 2 6 - P S R - SOTELEM (The Netherlands) 27-SPRINGER-VERLAG Ib&ica S.A. (Spain) 28-CHAMBERS HISPANIA S.L. (Spain) 29-JOHNSON MATTHEY (USA) 30-GENERAL ELECTRIC Plastics S.A. (Spain) 31- HIDEN ANALYTICAL (UK) 32-KAISER Optical Instruments Industries. (France) 33-THERMO QUEST- CE Instruments (Italy) 34-AIR LIQUIDE S.A. (Spain) 35-IBERFLUID Instruments S.A. (Spain) 36-ARGONAUT Technologies AG (Switzerland) 37- IN-SITU Research Instruments (USA) 38-VINCI Technologies (France) 39-MEL Chemicals (UK) 40- ISCOA Inc. (USA) 41- CRI Katalema (UK) 42- PARR Instrument (Germany) 43-ELSEVIER SCIENCE (The Netherlands)

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TABLE OF CONTENTS

PLENARY LECTURES

PL-1. In Situ Characterization of Catalysts H. Topsere .

1

PL-2. Pollution Abatement through Heterogeneous Catalysisfor Preserving Clean Air M.Iwamoto .

23

PL-3. Molecular Design of Heterogeneous Catalysts M.E. Davis .

49

PL-4. Millisecond Chemical Reactions and Reactors L.D. Schmidt .

61

PL-5. Catalysisfor Oil Refining and Petrochemistry, Recent Developments and Future Trends G. Martino .

83

AWARD CONFERENCE New Catalysisfiom Metal Oxide Surface Science M. A. Barteau .

105

KEYNOTE LECTURES KN- 1 From Unit Operations to Elementary Processes: A Molecular and Multidisciplinary Approach to Catalyst Preparation M. Che

115

KN-2 Acidity in Zeolite Catalysis R.A. van Santen and F.J.M.M. de Gauw.

127

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KN-3 Engineered Solid Catalysisfor Synthesis of Fine Chemicals in Multiphasic Reactant System D.E. De Vos, B.F. Sels, W.M. Van Rhijn and P.A. Jacobs .

137

KN-4 New Solid Acid Based Breakthrough Technologies S.A. Gembicki

147

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KN-5 Application of Titanium Oxide Photocatalysts and Unique Second-Generation Ti02 Photocatalysts able to Operate under Visible Light Irradiationfor the Reduction of Environmental Toxins on a Global Scale M.Anpo .

157

KN-6 Catalysis for Fine Chemicals: an Industrial Perspective P. Mktivier .

167

KN-7 Oxidative Methanol Reforming Reactionsfor the Production of Hydrogen J.L.G. Fierro .

177

KN-8 Supported Metallocenes: Monosite and Multisite Catalystsfor OIefn Polimerization . F. Ciardelli, A. Altomare and S. Bronco

187

ORAL COMMUNICATIONS Session A Preparation of Catalysts

A0 1

A02

A03 A04

A05

A New Method for Preparing Nanometer-size Perovskitic Catalystsfor CH, Flameless Combustion R.A.M. Giacomuzzi, M . Portinari, I. Rossetti and L. Forni .

197

Experimental Investigation and Modelling of Platinum Adsorption onto Ion-modified Silica and Alumina . W. Spieker, J. Regalbuto, D. Rende, M. Bricker and Q. Chen

203

Nanocrystalline Thin Films as a Model System for Sulfated Zirconia . F.C. Jentoff, A. Fischer, G. Weinberg, U. Wild and R. Schlogl

209

Nanoparticle Arrays as Model Catalysts: Microstructure, Thermal Stability and Reactivity of Pr/SiO2 Fabricated by Electron Beam Lithography . G. Rupprechter, A.S. Eppler, A. Avoyan and G.A. Somorjai

215

Epoxidation of Ole$ns on M-SiO2 (M=Ti, Fe, Catalysts with Highly Isolated Transition Metal Ions Prepared by Ion Beam Implantation Q. Yang, C. Li, S. Wang, J. Lu, P. Ying, Q. Xin, W. Shi .

22 1

Catalysis on Metals and Metal Oxides

A06

Cyclohexane Ring Opening on Metal-Oxide Catalysts L.M. Kustov, T.V. Vasina, O.V. Masloboishchikova, . E.G. Khelkovskaya-Sergeeva and P. Zeuthen

227

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A07

A08

A09

xi OleJins*om Chlorocarbons: Reactions of I , 2-Dichloroethane Catalyzed by Pt-Cu L. Vadlamannati, D. Luebke, V. Kovalchuk and J.L. d’Itri .

233

Artijkial Control of Selectivity by Dynamic Lattice Displacement of Acoustic Wave Effects: Decomposition and Oxidation of Alcohol on Ag and Pd catalysts . N. Saito, H . Nishiyama, K. Sat0 and Y. Inoue

239

The Effect of Hydrogen Concentration on Propyne Hydrogenation over a Carbon Supported Palladium Catalyst Studied under Continuous Flow Conditions D. Lennon, R. Marshall, G. Webb and S.D. Jackson

245

Refining and Petrochemistry

A10

A1 1 A12 A13

A14 A15

A16 A17

Suggestion of a New Alternative Mechanism of the Sulfuric Acid Catalyzed Isoparafln-OleJn Alkylation V.B. Kazansky and T.V. Vasina .

25 1

Hydroisomerization of n-Decane in the Presence of Sulfur and Nitrogen L.B. Galperin .

257

State of Metals in the Supported Bimetallic Pt-Pd/SOt--ZrOz System . A.V. Ivanov, A.Yu. Stakheev and L.M. Kustov

263

Dehydroisomerization of n-Butane over Pt Promoted Ga-Substituted Silico-aluminophosphates A. Vieira, M.A. Tovar, C. Pfaff, P. Betancourt, B. Mtndez, C.M. Lbpez, . F.J. Machado, J. Goldwasser and M.M. Ramirez de Agudelo

269

Pore Mouth Catalysis over Acidic Zeolites. Nature of Active Species P. Magnoux, M. Guisnet and I. Ferino .

275

Rediscovery of the Paring Reaction: The Conversion of l,Z,CTrimethylBenzene over HZSMS at Elevated Temperature H.P. Roger, M. Bohringer, K.P. Moller and C.T. O’Connor

28 1

SAPO-34 Catalystfor Dimethylether Production Gr. Pop and C. Theodorescu .

287

Catalytic Performances of Pillared Beidellites Compared to Ultrastable Y Zeolites in Hydrocracking and Hydroisornerization Reactions J.A. Martens, E. Benazzi, J. Brendlt, S. Lacombe and R. le Dred .

293

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A18

A19

A20

A2 1

A22

A23

A24

A25

A26

A27 A28

Catalyst Characterisation Effect of Ni on K-Doped Molybdenum-on-Carbon Catalysts: TemperatureProgrammed Reduction and Reactivity to Higher-Alcohol Formation E.L. Kugler, L. Feng, X. Li and D.B. Dadybujor .

299

Quantitative Determination of the Number of Active Surface Sites and the Turnover Frequenciesfor Methanol Oxidation over Metal Oxide Catalysts L.E. Briand and I.E. Wachs .

305

Metal Particles on Oxide Surfaces: Structure and Adsorption Behaviour M. Baumer, M. Frank, P. Kiihnemuth, M. Heemeier, S. Stempel and H.-J. Freund .

31 1

An Atomic XIFS Study of the Metal-Support Interaction in Pt/Si02-A1203 and Pt/MgO-A1203 Catalysts: An Increase in Ionization Potential of Platinum with Increasing Electronegativity of the Support Oxygen Ions D.C. Konigsberger, M.K. Oudenhuijzen, D.E. Ramaker and J.T. Miller .

317

Transition State and Difision Controlled Selectivity in Skeletal Isomerization of Olefns L. Domokos, M.C. Paganini, F. Meunier, K. Seshan and J.A. Lercher

323

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Development and Application of 3-Dimensional Transmission Electron Microscopy (3D-TEM)for the Characterisation of Metal-Zeolite Catalyst Systems A.J. Koster, U . Ziese, A.J. Verkleij, A.H. Janssen, J. de Graaf, J. W. Geus . and K.P. de Jong

329

Interrogative Kinetic Characterisation of Active Catalyst Sites Using TAP Pulse Experiment J.T. Gleaves, G.S. Yablonsky, S.O. Shekhtman and P. Phanawadee .

335

W Resonance Raman Spectroscopic Identijcation of Transition Metal Ions Incorporated in the Framework of Molecular Sieves G. Xiong, C. Li, Z. Feng, J. Li, P. Ying, H. Li and Q. Xin .

34 1

Surface Mobility of Oxygen Species on Mixed-Oxides Supported Metals . C. Descorme, Y. Madier, D. Duprez and T. Birchem

347

SO*-Promoted Propane Oxidation over Pt/A1203 Catalysts A.F. Lee, K. Wilson and R.M. Lambert .

353

Selective Oxidation of Toluene to Benzaldehyde: Investigation of StructureReactivity Relationships by in situ-Methods A. Brtickner, U . Bentrup, A. Martin, J. Radnik, L. Wilde and G.-U. Wolf

359

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A29

A30

A3 1

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Observation of Unstable Reaction Intermediate by Picosecond Tunable Inpared Laser Pulses K. Domen, K. Kusafika, A. Bandara, M. Hara, J.N. Kondo, J. Kubota, K. Onda, A. Wada and C. Hirose .

365

Changes in Morphology of y-AI203-SupportedPt Clusters under Reaction Conditions: Evidence@om In-Situ EXAFS Spectroscopy 0.Alexeev and B.C. Gates .

371

In Situ FT-IR Investigation of Hydrocarbon Reactions over Zeolite Based Bifunctional Catalysts M. Hochtl, Ch. Kleber, A. Jentys and H. Vinek .

377

Session B Fischer-Tropsch Synthesis

B01

B02

B03

A Steady State Isotopic Transient Kinetic Analysis of the Fischer-Tropsch Synthesis Reaction over a Cobalt-Based Catalyst . H.A.J. van Dikj, J.H.B.J. Hoebink and J.C. Schouten

383

Use of Membranes in Fischer-Tropsch Reactors R.L. Espinoza, E. du Toit, J. Santamaria, M. MenCndez, J. Coronas and S. Irusta .

389

Egg-Shell Catalystfor the Synthesis of Middle Distillates C . Galarraga, E. Peluso and H. de Lasa .

395

Application of Theoretical Methods to Catalysts

B04

B05

Computer Aided Design of Novel Heterogeneous Catalysts A Combinatorial Computational Chemistry Approach K. Yajima, Y. Ueda, H. Tsuruya, T. Kanougi, Y. Oumi, S.S.C. Ammal, S. Takami, M. Kubo and A. Miyamoto .

40 1

Applications of Densily Functional Theory to IdentrfL Reaction Pathways for Processes Occuring in Zeolites and on Dispersed Metal Oxides . J.A. Ryder, M.J. Rice, F. Gilardoni, A.K. Chakraborty and A.T. Bell

407

Fuel Cells and Electrocatalysis

B06 B07

Electrocatalytic Synthesis of Ammonia at Atmospheric Pressure . G. Marnellos, G. Karagiannakis, S. Zisekas and M. Stoukides

413

Electrocatalysis at a Pt Electrode Surface in a Fuel Cell as Observed In Situpom Pt-H and Pt-0 Shape Resonances and E U F S Scattering in Pt Lz,3 XANES D.E. M a k e r and W.E. O’Grady .

419

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B08

In situ Catalytic and Electrocatalytic Studies of Internal Reforming in Solid Oxide Fuel Cells Running on Natural Gas C.M. Finnerty, R.H. Cunningham and R.M. Ormerod

B09

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Efectof Gd203Doping and Steadcarbon Ratio on the Activity of Catalystfor Internal Steam Reforming in Molten Carbonate Fuel Cell Y.J. Shin, H.-D. Moon, T.-H. Lim and H.-I. Lee .

425

43 1

Reactor Engineering and Catalytic Processes

B10

B11 B12

B13 B14

A Microstructured Catalytic ReactodHeat Exchangerfor the Controlled Catalytic Reaction between H2 and 0 2 M. Janicke, A. Holzwarth, M. Fichtner, K. Schubert and F. Schuth

437

Catalytic Water Denitrification in Membrane Reactor O.M. Ilinitch, F.P. Cuperus, L.V. Nosova and E.N. Gribov .

443

Reactivity and Thermal ProJle of Metahne Partial Oxidation ar Very Short Residence Time F . Basile, G. Fomasari, F. TrifirC, and A. Vaccari .

449

Supercritical Synthesis of Dimethyl Carbonate@om CO2 and Methanol T. Zhao, Y. Han and Y. Sun .

455

A New Catalystfor an Old Process Driven by Environmental Issues L. Abrams, W.V. Cicha, L.E. Manzer and S. Subramoney .

46 1

Catalysis on Sulfides, Nitrides and Carbides

B15 B16

B17

B18

Mechanism of HDN over Mo and Nb-Mo Carbide Catalysts V. Schwartz, V.L. Teixeira da Silva, J.G. Chen and S.T. Oyama .

467

In Situ Characterisation of Transition Metal Surfde Catalysts by IR Probe Molecules Adsorption and Model Reactions G. Berhault, M. Lacroix, M. Breysse, F. MaugC and J.-C. Lavalley

473

Comprehension of the Promoting EfSect in the MCrzSd Mixed Surfde Catalysts . P. Afanasiev, A. Thiollier, P. Delichere and M. Vrinat

479

Potassium at Catalytic Surfaces - Stability, Electronic Promotion and Excitation A. Kotarba, G. Adamski, Z. Sojka, S. Witowski and G. Djega-Mariadassou

485

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B19

B20

B2 1

B22

B23

B24

B25

B26

B27

B28

B29

xv

Catalysis for Fine Chemical Synthesis Heterogeneous Catalysis of Aldolisations on Activated Hydrotalcites J. Lhpez, R. Jacquot and F. Figueras .

49 1

The Influence of Metal-Support Interactions During Liquid-Phase Hydrogenation of an GPUnsaturated Aldehyde over Pt U.K. Singh and M.A. Vannice .

497

Tailoring of Acido-basic Properties and Metallic Function in Catalysts Obtainedfrom LDHs for the Hydrogenation of Nitriles and of GpUnsaturated Aldehydes D. Tichit, B. Coq, S. Ribet, R. Durand and F. Medina .

503

Chemical versus Enzymatic Catalysisfor the Regioselective Synthesis of Sucrose Esters of Fatty Acids M. Ferrer, M.A. Cruces, F.J. Plou, E. Pastor, G. Fuentes, M. Bernabe, J.L. Parra and A. Ballesteros .

509

The Higly Selective Conversion of Toluene into 4-Nitrotoluene and 2,4Dinitrotoluene Using Zeolite H-Beta D. Vassena, A. Kogelbauer and R. Prins .

515

Catalytic Asymmetric Heterogeneous Aziridination and Epoxidation of Alkenes using ModiJied Microporous and Mesoporous Materials G.J. Hutchings, C. Langham, P. Piaggio, S. Taylor, P. McMorn, D.J. Willock, D. Bethell, P.C. Bulman Page, C . Sly, F. Hancock and F. King

521

Catalytic Hydrogenation of Nitriles to prim., sec. and tert. Amines over Supported Mono- and Bimetallic Catalysts Y.-Y. Huang and W.M.H. Sachtler .

527

Alkali Promoted Regio-Selective Hydrogenation of Styrene Oxide to Phenethyl Alcohol C.V. Rode. M.M. Telkar and R.V. Chaudhari

533

Production of Fatty Alcohols by Heterogeneous Catalysis at Supercritical Single-Phase Conditions S . van den Hark, M. Harrod and P. M ~ l l e r .

539

Regioselective Oxidation of Primary Hydroxyl Groups of Sugar and its Derivatives Using a New Catalytic System Mediated by TEMPO H. Kochkar, M. Morawietz and W.F. Holderich .

545

Aspects of Regioselective Control in the Hydroformylation of Methyl Methacrylate with the in situ Formed (0-Thiomethylphenyu diphenylphosphine Rhodium Complex H.K. Reinius, R.H. Laitinen, A.O.I. Krause and J.T. Pursiainen .

55 1

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B30

B3 1

Highly Eficient Synthesis of N,N-Dialky~ormamides@omCarbon Dioxide and DialRylamines over Ruthenium-Silica Hybrid Gels, L. Schmid and A. Baiker .

557

Metal Complex Catalyzed Functionalization of Naturally Occurring Monoterpenes: Oxidation, Hydroformylation, Alkoxicarbonylation E.V. Gusevskaya, E.N. dos Santos, R. Augusti, A.O. Dias, P.A. Robles-Dutenheher, C.M. Foca and H.J.V. Barros .

563

Session C Environmental Catalysis. Combustion VOCs

co1

Ahocat: AhorptiodCatalytic Combustionfor VOC and Odour Control . E. Kullavanijaya, D.L. T r i m and N. W. Cant

569

c02

Structure Sensitivity of the Hydrocarbon Combustion Reaction over Alumina-Supported Platinum Catalysts T.F. Garetto and C.R. Apesteguia .

575

Ceria-Zirconia-SupportedPlatinum Catalystfor Hydrocarbons Combustion: Low-Temperature Activity, Deactivation and Regeneration . C. Bozo, E. Garbowski, N. Guilhaume and M. Primet

581

Characterisation of a y-MnO2 Catalyst Used in VOC Abatement C. Lahousse, C. Cellier, B. Delmon and P. Grange .

587

New AluminaiAluminium Monolithsfor the Catalytic Elimination of vocs N . Burgos, M. Paulis, A. Gil, L.M. Gandia and M. Montes.

593

Hydrotalcite-Derived Catalystsfor Removal of Nitrogen-Containing Volatil Organic Compounds J. Haber, K. Bahranowski, J. Janas, R. Janik, T. Machej, L. Matachowski, . A. Michalik, H. Sadowska and E.M. Senvicka

599

C03

C04 C05

C06

Environmental Catalysis. NO,

C07

C08

Surface Catalytic Reactions Assisted by Gas Phase Molecules on Supported Co-ensemble Catalysts A. Yamaguchi, T . Shido, K. Asakura and Y. Iwasawa .

605

Fresh and Used V205-W O g i 0 2 SCR EUROCAT Standard Catalyst: An European Collaborative Characterization J.C.Vedrine .

61 1

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C09 c10

c11

c12 C13

C14

C15

xvii

Lean NO, Reduction over Snl,Zr,O2 Solid Solutions J. Ma, Y. Zhu, J. Wei, X. Cai and Y. Xie .

617

Concentration Programmed Aakorption-Desorption / Surface Reaction Study of the SCR-DeNOx Reaction I . Nova, L. Lietti, E. Tronconi and P. Forzatti .

623

Bifunctional Nature of Sn02/y-A1203 Catalysts in the Selective Reduction of NO, . A. Yezerets, Y. Zheng, P.W. Park, M.C. Kung and H.H. Kung

629

SO2 Resistant Fe/ZSM-.5 Catalystfor the Conversion of Nitrogen Oxides G. Centi, G. Grasso, F. Vanzzana and F. Arena .

635

Mechanistic Studies of the NO, Reduction by Hydrocarbons in Oxidative Atmosphere S. Schneider, S. angler, P. Girard, G. Maire, F. Garin and D. Bazin .

64 1

NO Reduction in Presence of Methane and Ethanol on Pd-Mo/A1203 Catalysts L.F. De Mello, M.A.S. Baldanza, F.B. Noronha and M. Schmal .

647

Fe- Vanadyl Phosphates/TiO2 as SCR Catalysts G. Bagnasco, P. Gilli, M.A. Larmbia, M.A. Massucci, P. Patrono, G. Ramis and M. Turco .

653

Photochemistry

C16 C17

C18

Photoinduced Non-Oxidative Methane Coupling over Silica-Alumina H. Yoshida, Y . Kato and T. Hattori .

659

Photocatalytic Oxidation of Gaseous Toluene on Polycrystalline Ti02: FT-IR Investigation of Surface Reactivity of Diferent Types of Catalysts G. Martra, V. Augugliaro, S. Coluccia, E. Garcia-Lbpez, V. Loddo, . L. Marchese, L. Palmisano and M. Schiavello

665

Investigation of Environmental Photocatalysis by Solid-state NMR Spectroscopy D. Raflery, S . Pilkenton, C.V. Rice, A. Pradhan, M. Macnaughtan, S . Klosek and T. Hou .

67 1

C1 Chemistry

C19

A Study of CHd Reforming by C02 and H20 on Ceria-Supported Pd S . Sharma, S . Hilaire and R.J. Gorte .

677

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xviii

c20

c 21

c22

C23

C24

C25

C26

Catalytic Behaviour on Ni Containing Catalysts in Vaporeforming of Methane with Low HzO/CHd Ratio and Free Carbon Deposition H . Provendier, C. Petit and A. Kiennemann .

683

Carbon Deposition and Reaction Steps in CO2/CH4 Reforming over Ni-LazOd5A Catalyst J.Z. Luo, L.Z. Gao, Z.L. Yu and C.T. Au .

689

The Autothermal Partial Oxidation Kinetics of Methanol to Produce Hydrogen E. Newson, P. Mizsey, T. Truong and P. Hottinger .

695

New Reaction Mechanismfor Methane Formation in CO Hydrogenation over PdCeOr S. Naito, S . Aida and T. Miyao .

70 1

Methane Coupling over SrCo03-Based Perovskites in the Absence of Gas-Phase Oxygen Yu.1. Pyatnitsky, N.I. Ilchenko, L.Yu. Dolgikh and N.V. Pavlenko

707

Novel Conception ofthe Methanol Synthesis Mechanism on Conventional Cu-Containing Catalysts G.I. Lin, K.P. Kotyaev and A.Ya. Rozovskii

713

“Real and “Inverse’’ Model Catalystsfor Studies of Metal-Support Interactions: CO Hydrogenation on Titania and Vanadia Supported Rh W . Reichl and K. Hayek .

719

”

Environmental Catalysis. Catalysis for Clean Processes and Fuels

C27

C28

C29 C30 C3 1

Agglomeration of Pt Particles and Potential Commercial Application for Selective Hydrodechlorination of CCId Z.C. Zhang and B.C. Beard . Catalytic Diesel Soot Elimination on Co-WLa203 Catalysts: Reaction Mechanism and the Effect of NO Addition E.E. Miro, F . Ravelli, M.A. Ulla, L.M. Cornaglia and C.A. Querini

725

.

73 1

Environmentally Benign Carbonylation of Nitrobenzene and Aniline S.S.C. Chuang, M.V. Konduru, Y. Chi and P. Toochinda .

737

Nitrous Oxide - Waste to Value A.K.Uria.rte .

743

Catalytic Wet Peroxide Oxidation over Mixed (AI-Fe) Pillared Clays J. Barrault, C. Bouchoule, J.-M. Tatibouet, M. Abdellaoui, A. MajestC, I. Louloudi, N. Papayannakos and N.H. Gangas .

749

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Session D Selective Oxidation (ODH)

DO 1

DO2

DO3

DO4

DO5

DO6

A new Iron Phosphate Crystalline Phase and its Catalytic Activity in Oxidative Dehydrogenation of Lactic Acid and Glycolic Acid M . Ai and K. Ohdan .

755

Dynamic of the Oxidative Dehydrogenation of Propane over VMgO Catalysts Studied by In Situ Electrical Conductivity and Step Transients H.W. Zanthoff, J.C. Jalibert, Y. Schuurman, P. Slama, J.M. Herrmann . and C. Mirodatos

76 1

EfSect of Potassium on the Structure and Reactivity of Vanadium Species in VO/Al,O3 Catalysts P. Concepcion, S . Kuba, H. Knozinger, B. Solsona and J.M. Lopez Nieto .

767

Strategies for Combining Light Parafin Dehydrogenation (OH) with Selective Hydrogen Combustion (SHC) R.K. Grasselli, D.L. Stem and J.G. Tsikoyiannis .

773

Oxidative Dehydrogenation over Promoted Chromia Catalysts at Short Contact Times D.W. Flick and M.C. Huff .

779

Propane Oxidative Dehydrogenation by Continuous and Periodic Operating Flow Reactor with a Nickel Molybdate Catalyst C . Mazzocchia, A. Kaddouri, E. Tempesti and R. del Rosso

785

Selective Oxidation (General Papers) DO7

DO8

DO9

D10

A Novel Catalyst of Copper Hydroxyphosphate (Cu2(OH)PO,) with High Activity in Hydroxylation of Phenol by Hydrogen Peroxide F . 4 . Xiao, J. Sun, R. Yu, X. Meng, H. Yuan, D. Jiang and R. Xu .

79 1

Oxidation of Alkanes with Hydrogen Peroxide Catalyzed by di-lronSubstituted Inorganic Synzyme N. M i m o , Y. Nishiyama, I. Kiyoto and M. Misono

797

Vanadyl-Aluminum Binary Phosphate: Eflect of Thermal Treatment over its Structure and Catalytic Properties in Selective Oxidation of Aromatic Hydrocarbons F.M. Bautista, J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas, A.A. Romero and M.T. Siles .

803

Direct Synthesis of Phenol ffom Benzene with 0 2 over VMo-Oxide/SiO2 Catalyst I . Yamanaka, M. Katagiri, S. Takenaka and K. Otsuda .

809

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D11

D12

D13 D14

Eu-Ti-Pt-Catalytic System for Direct Hydroxylation of Benzene by 0 2 and H2 under Mild Conditions . I . Yamanaka, T. Nabeta, S. Takenaka and K. Otsuka

815

High Yield Butane to Maleic Anhydride Direct Oxidation on New Supported Catalysts M.J. Ledoux, V. Turines, C. Crouzet, K. Kourtakis, P.L. Mills and J.J. Lerou

82 1

Propylene Epoxidation over Gold-Titania Catalysts E.E. Stangland, K.B. Stavens, R.P. Andres and W.N. Delgass

.

827

Vapor-Phase Epoxidation of Propene using HZand 0 2 over AdTiMCM-41 and Au/Ti-MCM-48 B.S. Uphade, M. Okumura, N. Yamada, S. Tsubota and M. Haruta

833

Molecular Sieves

D15

D16

D17

D18

D19 D20

D2 1 D22

Ring-Opening Reactions of Ethyl- and Vinyloxirane on HZSM-5 and Cu-ZSM-5 Catalysts A. Fasi, I . Palink6 and I. Kiricsi .

839

Controlling the Distribution of Framework Aluminum in High-Silica Zeolites . D.F. Shank, R.F. Lobo, C. Fild and H. Koller

845

Toluene Disproportionation Catalysis Using EUO Type Zeolites: Initial Optimisation and Development J.L. Casci and A. Stewart .

85 1

Influence of the Aluminium Content on the Acidity and Catalytic Activity of MTW-Type Zeolites A. Katovic, B.H. Chiche, F. di Renzo, G. Giordano and F. Fajula .

857

Synthesis of Ethylbenzenefiom 1,3-Butadiene Using Basic Zeolite Catalysts J. Ackermann, E. Klemm and G. Emig .

863

Activity/Selectivity and Ligation of the Co Ions in Zeolites in Ammoxidation of Ethane to Acetonitrile B. Wichterlova, Z. Sobalik, Y. Li and J.N. Armor .

869

Room-Temperature Oxidation of Hydrocarbons over FeZSM-5 Zeolite M.A. Rodkin, V.I. Sobolev, K.A. Dubkov, N.H. Watkins and G.I. Panov Selective Oxidation of Hydrocarbons by Active Oxygen Formedfiom NzO on ZSM-5 at Moderate Temperatures S.N. Vereshchagin, N.P. Kirik, N.N. Shishkina, S.V. Morozov, . A.I. Vyalkov and A.G. Anshits

.

875

88 1

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

D23

D24

D25

D26

D27

Redox Molecular Sieve Catalystsfor the Aerobic Selective Oxidation of Hydrocarbons . J.M. Thomas, R. Raja, G. Sankar and R.G. Bell

887

Catalytic Behaviour of H-Y Type Zeolites in the Decomposition of Chlorinated VOCs R. Ldpez Fonseca, P. Steltenpohl, J.R. Gonzilez Velasco, A. Aranzabal, and J.I. Gutitrrez Ortiz

896

Coupling Alkane Dehydrogenation with Hydrogenation Reactions on Cation-Exchanged Zeolites W. Li, S.Y. Yu and E. Iglesia

899

Water Vapor Tolerance of PdHZSM-5 in the NO-Methane-02 Reaction: Its Relation with Very Strong Solid Acidity of Zeolite Support J. Amano, 0. Shoji, K. Okumura and M. Niwa .

905

Simultaneous NO and N20 Decomposition on Cu-ZSM5 R. Pirone, P. Ciambelli, B. Palella and G . Russo .

91 1

Polymerization and Organometallics

D28

D29

D30

D3 1

Catalytic Properties of Silica-Supported Tantalum and Tungsten Hydrides in the Cleavage and Formation of C-C Bonds of Alkanes 0.Maury, J. Lefort, G . Saggio, C. Coptret, M. Taoufick, M. Chabanas, J. Thivolle-Cazat and J.M. Basset .

917

Characteristics of Ethylene Polymerization Catalyzed over Ziegler-Natta / Metallocene Hybrid Catalysts: Comparison between Silica-Based and Magnesium-Based Supports H.S. Cho, D.J. Choi, I.K. Song and W.Y. Lee .

923

Ethylene Polymerization with Zirconocene-MA0 Supported on Molecular Sieves I.S. Paulino, A.P. de Oliveira Filho, J.L. de S o u and U. Schuchardt

929

In Situ and Ex Situ Study of Propylene Polymerization with a MgC12Supported Ziegler-Natta Catalyst . V. Oleshko, P. Crozier, R. Cantrell and A. Westwood

935

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POSTER SESSIONS Session P1 Preparation of Catalysts Nanoheterogeneous Metal-Polymer Composites as the New Type of Eflective Selective Catalysts L.I. Trakhtenberg, G.N. Gerasimov, E.I. Grigoriev, S.A. Zavjalov, O.V. Zagorskaja, V.Yu. Zufman and V.V. Smimov. Design of Heteropolyacid-Polymer Composite Films as Catalytic Materials for Heterogeneous Reactions G.I. Park, S.S. Lim, Y.H. Kim, I. K. Son and W.Y. Lee . Catalyts Based on Heteropolyacids Supported on Zirconia L. Pizzio, P. Vhzquez, C. Caceres and M. Blanco . Dependence of structure andphysico-chemical properties of the catalysts containing heteropolyacids on the conjugatedpolymer matrix E. Stochmal-Pomarzanska and W. Turek . Mesoporous Silica Supported Solid Acid Catalysts S. Choi, Y. Wang, Z. Nie, D. Kambapati, J. Liu and C.H.F. Peden. Study of Acidic and Catalytic Properties of Sulfated Zirconia Prepared by Sol-Gel Process: Influence of Preparation Conditions L.B. Hamouda, A. Ghorbel and F. Figueras . Synthesis and Characterization of Ta-Pillared Ilerite Y.Ko,M.H.Kim,S.J.KimandY.S.Uh . Al-Pillared Hectorite and Montmorillonite Preparedfiom Concentrated Suspensions: Structural,Textural and Catalyic Properties R. Molina, S. Moreno and G. Poncelet . Synthesis of High Surface Area Transition Metal Carbide Catalysts A.P.E. York, J.B. Claridge, V.C. Williams, A.J. Brungs, J. Sloan, A. Hanif, H. Al-Megren and M.L.H. Green . Use of Carbon Fabrics as Supportfor Hydrogenation Catalysts Usable in Polyphasic Reactors J.P. Reymond and P. Fouilloux . Preparation, Characterization and Reactivity of Activated Carbon Supported Platinum Catalysts by Fluidized Bed Organometallic Chemical Vapor Deposition (FBMOCVD) Ph. Serp, J-C. Hierso, R.Feurer, R. CorratgC, Y. Kihn and Ph. Kalck

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

PO 12 Metal-Carbon Aerogels as Catalysts and Catalyst Supports F.J. Maldonado Hbdar, C. Moreno Castilla, J. Rivera Utrilla and M.A. Ferro Garcia .

1007

PO13 Preparation and Characterisation of Carbon Supported Pt-Ce02 Catalysts A. Sepclveda Escribano, J. Silvestre Alvero, F. Coloma and F. Rodriguez-Reinoso .

1013

PO14 Highly Dispersed Pd Based Catalystsfor Selective Hydrogenation Reactions M. Delage, B. Didillon, Y. Huiban, J. Lynch and D. Uzio .

1019

PO 15 Preparation of New Type of Supported Sn-Pt Bimetallic Catalysts Containig Lewis Acid Sites Anchored to the Platinun J.L. Margitfalvi, I. Borbith, M. Hegediis, S. GBbolos, A. Tompos, andF.Lonyi .

1025

PO16 Influence of Small Amounts of Rhodium on the Structure and the Reducibility of CdA1203 and Cu/Ce02-A1203 Catalysts X . Courtois, V . Perrichon, M. Primet and G. Bergeret .

1031

PO17 Synthesis, Characterization and Catalytic Properties of Chromium Silicate Xerogel R. Serpa da Cruz, M. de Magalhaes Dauch, U. Schuchardt and R. Kumar .

1037

PO18 Thermal Chemistry of Oxide-Supported Platinum Catalysts: A Comparative Study J.F. Lambert, E. Marceau, B. Shelimov, J. Lehman, V. Le Be1 de Penguilly, X. Carrier, S. Boujday, H. Pernot and M. Che

1043

.

PO19 Geochemistry and Catalysts Preparation: Alumina as a Chemical Reactant in the Synthesis of M0OdAl203 and WOJAl203 Systems X . Carrier, J.F. Lambert and M. Che .

1049

PO20 Chemistry of the Preparation of Silica-Supported Cobalt Catalysts@om Co(I4 and Co(II4 Complexes: Grafting versus Phyllosilicate Formation . R. Trujillano, J. Grimoult, C. Louis and J.-F. Lambert

1055

PO2 1 New Method of the Preparation of Heterogeneous Ni(0) Complexesfor Ethylene Oligomerization M.K. Munshieva .

1061

PO22 Synthesis, Characterization and Catalytic Activity of [ c u ( N H ~ ) ~ ] ~ + Supported on Hydrous Oxides D. Pate1 and A. Pate1 .

1067

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Preparation and Characterization of CuQnO and P m n O Catalysts for Partial Oxidation of Methanol. Control of Catalyst Surface Area and Particle Size Using Microemulsion Technique J. Agrell, K. Hasselbo, S. Jaras and M. Boutonnet . Mechanism of Silation of Alumina and Silica with Hexamethyldisilazane S.V. Slavov, A.R. Sanger and K.T. Chuang . Solvent Influence in Silica-Alumina Sol-Gel Synthesis A. Carati, C. Rizzo, M. Tabliabue and C. Perego . Ru, Co and RuCo/SiOz Catalyst Prepared by Sol-Gel Methods B. Botti, D. Cauzzi, P. Moggi, G. Predieri and R. Zanoni . Structure and Catalytic Properties of Co-Re Bimetallic Catalysts Prepared by SOWGEL Method and by Ion Exchange in Nay Zeolite L. Guczi, G. Stefler Z. Schay, I. Kiricsi, F. Mizukami, M. Toba and S. Niwa . On the Effect ofthe Preparation Conditions on the Localization and the Size ofPlatinum Particles on Zeolite NaX L.V. Mattos, M.C.M. Alves, F.B. Noronha, B. Moraweck and J.L.F. Monteiro Hollow Metallic Particles Obtained by OxidatiordReductionTreatment of Organometallic Precursors . F.J. Cadete Santos, R. Darji, J.F. Trillat, A. Howie and A. Renouprez Synthesis, Characterization and Catalytic Application of Inorganic Nanotubes . I. Kiricsi, A. Kukovecz, A. Fudala, Z. Konyia and J.B. Nagy Nanostructuring Surfaces by Lithography: The Production and use of Model Catalysts M. Schildenberger, Y. Bonetti and R. Prins . New Catalyst Supports and Catalytic Systems on the Basis of Metallic Gauzes Coated with 11-IV Group Element Oxides M.P. Vorob'eva, A.A. Greish, A.Yu. Stakheev, N.S. Telegina, A.A. Tyrlov, E.S. Obolonkova and L.M. Kustov .

Fischer-Tropsch Synthesis Precipitated Iron Fischer-Tropsch Catalysts: The Effect of SuFde Ions . T.C. Bromfield, T.H. Dlamini, F. Marsicano and N.J. Coville The Nature of the Active Phase in Iron Fischer-Tropsch Catalysts A.K. Datye, Y. Jin, L. Mansker, R.T. Motjope, T.H. Dlamini and N.J. Coville

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

PO35 Selective Inhibition: The Intrinsic Feature of Fisher-Tropsch Synthesis: Transient Initial Episodes in FT Synthesis on Cobalt Catalysts H. Schulz and Z. Nie .

1145

PO36 Eflects of Support on Methane Selectivity in Cobalt-Catalyzed FischerTropsch Synthesis: A Kinetic Model C.H. Bartholomew and W.-H. Lee .

1151

PO37 Does Mono-Atomic Ru Catalyse the Fischer-Trosph Synthesis? . M. Claeys. M. Hearshaw, J.R. Moss and E. van Steen

1157

PO38 Fischer-Tropsch Synthesis Using Monolithic Catalysts A.-M. Hilmen, E. Bergene, O.A. Lindvag, D. Schanke, S. Eri and A. Holmen

1163

Application of Theoretical Methods to Catalysis PO39 The Design ifCatalysts SurJaces by the Use of Stochastic Optimisation Algorithms A.S. McLeod and L.F. Gladden .

1169

PO40 Modeling of Structure and Properties of Active Centers of Catalysts on the Base of Metalorganosiloxanes A.V. Nernukhin, I.M. Kolesnikov and V.A. Vinokurov .

1175

PO41 Theoretical Study on Active sites of Molybdena-Alumina Catalystfor Ole$n Metathesis J. Handzlik and J. Ogonowski

1181

PO42 Modeling the Oxygen Activation on Dinuclear Iron MMO Mimics, a Quantum Mechanic Study P.-P. Knops-Gerrits, P.A. Jacobs and W.A. Goddard I11 .

1187

PO43 Quantum-Chemical and Experimental Study of an Interaction between C 0 2and Propylene over Rh-Co/Al~ojCatalysts L.B. Shapovalova, G.D. Zakumbaeva, A.V. Gabdrakipov, I.A. Shlygina and A.A. Zhurtbaeva .

1193

PO44 Ab-Initio Study of the Vacancy Formation in Keggin and Linqvisr Heteropolyanion J.F. Paul and M. Founier .

1199

PO45 Theoretical Studies o f the Interaction of Butane and Butene Isomers in H-Ferrierite and H-Mordenite D.E. Galindo, M.J. Goncalves and M.M. Ramirez Corredores. .

1205

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Theoretical Investigation of the Intrinsic Reaction Path (IRC) of the Methylation of mono-Substituted Aromatic Molecules over Faujasite Catalysts K.H.L. Nulens, A.M. Vos, G.O.A. Janssens and R.A. Schoonheydt Environmental Catalysis. Combustion VOCs

Catalysts or Combustion of Halogenated Volatile Organis Compounds . J. Trawczynski, J. Walendziewski and M. Kulazynski Non-Stoichiometry and Catalytic Activity in AB03 Perovskites: LaMn03 and LaFe03 A.A. Barresi, D. Mazza, S. Ronchetti, R. Spinicci and M. Vallino . Deep Catalytic Oxidation of Chlorinated VOC Mixturesfiom Groundwater Stripping Emissions A. A r d b a l , J.A. G o n d e z Marcos, R. Lopez Fonseca, M. A. GutiCrrez Ortiz and J.R. G o d l e z Velasco . Transformation of Chlorinated Compounds on Dzflerent Zeolites under Oxidative and Reductive Conditions I. Hannus, A. Tamhi, Z. Konya, S.-I. Niwa, F. Mizukami, J.B. Nagy and I. Kiricsi . Biofiltration of Gasoline VOCs with Dzflerent Support Media I. Ortiz, M. Morales, C. GobbCe, S. Revah, V.M. Guerrero, I. Shifter, R. Auria, G.A. PCrez and L.A. Garcia Environmental Catalysis. NOx

An Unified View on Nitrogen Oxide Abatement J. Paul . Measuring the Adsorption of Reactive Probes as a Toolfor Understanding the Catalytic Properties of De-NO, Catalysts A. Gervasini and A. Auroux . Reactivity of the NO, Sur$ace Species Formed Afrer Co-adsorption of NO + 0 2 on a WO&'rOz Catalyst:An FTIR Spectroscopic Study S. Kuba, K. Hadjiivanov and H. Knozinger . The Mechanism of the Selective NO, Adsorption on 12- Tungstophoshoric Acid hexa-Hydrate (HPW) S. Hodjati, C. Petit, V. Pitchon and A. Kiennemann. Activity and DRIFT Spectroscopy Studies of NO Oxidation and Reduction by C3Ht in Excess 0 2 over A1203and AdAl203 G.R. Barnwenda, A. Obuchi, S. Kushiyama and K. Mizuno

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

PO57 Transient Study of NO, N20, C3H6 Interactions over a Silica-Supported Pt Catalyst P. Denton, Y . Schuurman, A. Giroir-Fendler, H. Praliaud, M. Primet and C. Mirodatos . PO58 Electrochemical Promotion of NO Reduction by C3H6 on RWYSZ Catalyst-Electrodes and Investigation of the Origin of the Promoting Action Using TPD and WF Measurements C. Raptis, Th. Badas, D. Tsiplakides, C. Pliangos and C.G. Vayenas

1277

.

1283

PO59 The Reaction of Propane with Mixtures of NO, N2O and 0 2 Over Platinum, Palladium and Rhodium Catalysts D.C. Chambers, D.E. Angove and N. W. Cant .

1289

PO60 Kinetics of NO Reduction by CO on Rh(l1 I): A Molecular Beam Study F. Zaera and C.S. Gopinah .

1295

PO61 The Surface Migration of NO, Adspecies as a Factor Determininig the Reactivity of Supported Pt Catalystsfor the Treatment of Lean Burn Engine Emissions G.E. Arena, A. Bianchini, G. Centi and F. Vazzana .

1301

PO62 Pd/Sn02 as deNO, Catalysts: Preparation, Characterization and Activity D. Amalric-Popescu, J.-M. Herrmann and F. Bozon-Verduraz .

1307

PO63 Decomposition of NO Over Copper-Manganese Oxide Catalysts at Room Temperature I. Spassova, M . Khrishtova, N. Nyagolova and D. Mehandjiev .

1313

PO64 Catalytic Performance of SrTi03-Based Catalystsfor NO Direct Decomposition Y. Yokoi, I. Yasuda, H. Uchida, 0 . Okada, Y. Nakamura, H. Ishikawa and H. Kimura 1319 PO65 CO and NO Elimination over Pd-Cu Catalysts M. Fernhdez Garcia, A. Martinez Arias, J.A. Anderson, J.C. Conesa and J. Soria

1325

PO66 NO, Storage and Reduction over Pt/Ba/A1103 J.A. Anderson, A.J. Paterson and M. Fernhdez Garcia

1331

.

PO67 Nitrogen Emission Process in the Decomposition of Nitrogen Oxides on Pd(1 lo): An Angular and Velocity Distribution Stua'y I . Kobal, K. Kimura, Y. Ohno, H. Horino, I. Rzeznicka and T. Matsushima

1337

PO68 Reduction of NO by Propylene over ModiJied RhITi02 Catalysts in the Presence of Oxygen. T.I. Halkides, D.I. Kondarides and X.E. Verykios .

1343

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

E'ect of sulfirr on the oxygen storage/release capacity of RWCe02 and RWCeO2-Zr02 model TWCs M. Boaro, C. de Leitenburg, G. Dolcetti and A. Trovarelli . Thermal Stability and Oxygen Storage Capacity of Noble Metal/CeriaZirconia Catalystsfor the Automotive Converters with the On-Board Diagnostics (OBD) P. Fornasiero, R. Di Monte, T. Montini, J. Kaspar and M. Graziani

.

N 2 0 Formation over Ce02-Zr02Mixed Oxides Supported Metal (Pt, Pd Rh), Catalystsfor Automotive Exhaust Control J.L Marc, J.R. G o d e z Velasco, M. A. Gutierrez Ortiz, J.A. Botas, M.P. G o d e z Marcos and G. Blanchard . Model Studies of Automobile Exhaust Catalysis Using Single Crystals of Rhodium and CeriaRirconia C.H.F. Peden, G.S. Herman, S.A. Chambers, Y. Gao, Y.-J. Kim and D.N. Belton . Thermally Stable, High-Surface-Area, Pro,-CeOz-Based Mixed Oxides for Use in Automotive-Exhaust Catalysts A.N. Shigapov, H.-W. Jen, G.W. Graham, W. Chun and R.W. McCabe . Improvement of Three Way Catalytic Perfomance by Optimizing Ceria and Promoters in Pd only Catalyst Prepared bu Sol-Gel Method H.-S. So, 0.-B. Yang, D.H. Kim and S.I. Woo . Flue Gas NO, Removal by SCR with NH3 on CuO/AC at Low Temperatures Z.P. Zhu, Z.Y. Liu, S.J. Liu, H.X. Niu, T.D. Hu, T. Liu and Y.N. Xian . Low temperature Monolithic SCR Catalystsfor Tail Gas Treatment in Nitric Acid Plants J. Blanco, P. Avila, L. Marzo, S. Suarez and C. Knapp . Effect of La203 Concentration in La203-A1203Supports and Pd/La203-AG03 Catalysts in Reduction of NO by H2 N.E. Bogdanchikova, A. Barrera, S. Fuentes, G. Diaz, A. Gomez-CortCs, A. Boronin, M.H. Farias and M. Viniegra R.. Molecular Basis for the Design of Transition-Metal Oxide Catalystsfor Selective Catalytic Reduction of NO by Hydrocarbons K. Shimizu, H. Maeshima, H. Kawabata, H. Yoshida, A. Satsurna and T. Hattori . Silver as Promoter for the Catalytic Decomposition of NO, under Oxygen-Excess Condition: Evidence for Oxygen Spilloverfrom Noble Metals to Silver W.X. Huang, J. W. Teng, T.X. Cai and X.H. Bao .

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

PO80 Reactiviry of Surface Species in the Reduction of NO, by Ethanol on Ag/AI203 Catalyst T . Chafik, S. Kameoka, Y. Ukisu and T. Miyadera.. PO8 1 Modelling of Uncatalysed and Barium Catalysed NO Reduction by Activated Carbon S.A. Carabineiro, F.B. Fernandes, J.S. Vital, A.M. Ramos and I.F. Silva PO82 Selective Catalytic Reduction of NO, with C3H,j under Lean-Burn Conditions on Activated Carbon-Supported Metals J.M. Garcia Cortts, M.J. Illin Gbrnez, A. Linares Solano and C. Salinas Martinez de Lecea. PO83 Role of Acidic Sites in the DENO, Process on Supported Tin Oxides A. Auroux, D. Sprinceana and A. Gervasini . PO84 Sulphated-Zr02 Prepared by Impregnation with Ammonium, Sodium or Copper Sulphate: Catalytic Activity for NO Abatement with Propene in the Presence of Oxygen V. Indovina, M.C. Campa and D. Pietrogiacomi . PO85 Co.Based exTHlcfor the Decomposition of N20: Tailoring Catalystsfor Active and Stable Operation J. Perez Ramirez, G. Mul, X. Xu, F. Kapteijn and J.A. Moulijn .

1445

PO86 Cold Start Vehicle Emission Control Using Trapping and Catalyst Technology N.R. Burke, D.L. Trirnm and R. Howe .

1451

PO87 Oscillation of the NO, Concentration in its Selective Catalytic Reduction on Platinum Containing Zeolite Catalysts Y . Traa, B. Burger and J. Weitkamp .

1457

PO88 Experimental and Theoretical Description of Transition Metal Ion Structures in Zeolites Relevant to DENO, Catalysis Z . Sobalik, J.E. Sponer and B. Wichterlova .

1463

PO89 Cooperation of Pt and Pd over H-Mordenite for the Lean SCR of NO, by Methane C. Montes de Correa, F. Cordoba C. and F. Bustamante L. .

1469

PO90 The Selective Catalytic Reduction of NzO by NH3 on a Fe-BEA Catalysts M. Mauvezin, G. Delahay, B. Coq and S. Kieger .

1475

PO91 A New Mechanismfor the Selective Catalytic Reduction of NO, by NH3 on Cu-zeolite Catalysts B. Coq, S. Kieger, G. Delahay, D. Berthomieu and A. Goursot .

1481

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PO92 In Situ FT-I. Shrdy of the Selective Catalytic Reduction of NO by Propane on Cu-ZSM-5: Evidence of a Reaction Pathway by Oxygen Pulses F. Poignat, J.L. Freysz, M. Daturi, J. Saussey and J.C. Lavalley .

1487

PO93 Catalytic Removal of NO: the Eflect of Cu Loading and Type of Binder on Catalytic Properties of Cu/ZSM-5 Catalyst V. Tomasic, A. Gerzina, Z. Gornzi and S. Zrncevic .

1493

PO94 Changes in Cation Coordination during Deactivation of Cu-ZSM-5 deNOx Catalysts . S.A. Gbmez, A. Campero, A. Martinez Hernindez and G.A. Fuentes

1499

PO95 On the Role of Oxygen in the Selective Reaction ofNO Reduction with Methane over Co/ZSM-5 Catalyst E.M. Sadovskaya, A.P. Suknev, L.G. Pinaeva, V.B. Goncharov, C. Mirodatos and B.S. Bal'zhinimaev

1505

PO96 Eflect of SiLAl Ratio of Mordenite and ZSM-5 Type Zeolite Catalysts on Hydrothermal Stabilityfor NO Reduction by Hydrocarbons . S.Y. Chung, B.S. Kim, S.B. Hong, I.-S. Nam and Y.G. Kim

1511

PO97 Precipitation of Mn02 onto the External Surface of Zeolite Microcrystals. Structure of the Manganese Oxide and its Role in the Removal of NO, by NH3 M. Richter, H. Kosslick and R. Fricke .

1517

PO98 Catalflic Properties of Mesoporous Molecular Sievesfor the Reduction of NO, 1523 A. Jentys, W . Schieber and H. Vinek PO99 Catalytic Activity of High Surface Area Mesoporous Mn-Based Mixed Oxidesfor the Deep Oxidation of Methane and Lean-NO, Reduction V.N. Stathopoulos, V.C. Belessi, C.N. Costa, S. Neophytides, P. Falaras, A.M. Efstathiou and P.J. Pomonis .

1529

PlOO Characterization of the Role of Pt on Copt and InPt Ferrierite Activity and Stability upon the SCR of NO with CH4 L. Gutierrez, L. Cornaglia, E. Mir6 and J. Petunchi .

1535

Environmental Catalysis. Catalysis for Clean Processes and Fuels P 101 Cu-Cr Oxide Catalystsfor Complete Oxidation of Aromatic Hydrocarbons V. Georgescu . PI02 Ag/Lao.&r,dMnO/y-A1203 Catalystfor Complete Oxidation of Methanol at Low Concentration H.-B. Zhang, W . Wang, Z.-T. Xiong and G.-D. Lin .

1541

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P103 Treatment of Aqueous Solutions of Organic Pollutants by Heterogeneous Catalytic Wet Air Oxidation (CWAO) . M. Besson, J.-C. Beziat, B. Blanc, S. Durecu and P. Gallezot P104 Efect of the Supportfor Pt Catalysts on Soot Oxidation J. Obuchi, J. Oi-Uchisawa, R. Enomoto, S. Liu, T. Nanba and S. Kushiyama P105 Dimethyl Carbonate Synthesis From Carbon Dioxide and Methanol over Ni-Cw'MoSiO (VSiO) Catalysts . S.H. Zhong, J.W. Wang, X.F. Xiao and H.S. Li P106 Promotion of Support in Synthesis of Propylene Carbonatefrom COr and Propylene Oxide T. Zhao, Y . Han and Y. Sun . P107 A Novel Process for the Removal of Nitrates from Drinking Water A. Pintar, J. Batista and G. Bercic . PI08 Greenhouse Gases and Emissions Control by New Catalysts Free of Precious Metals V. Gryaznov and Y. Serov . P 109 Study on the Initial Steps of the Polyethylene Cracking over DifSerent acid Catalysts D.P. Serrano, R. Van Grieken, J. Aguado, R.A. Garcia, C. Rojo and F. Temprano . P 110 Catalytic Conversion of Traces in Biomass Gasijkation Fuel Gases with Nickel Activated Ceramic Filters D.J. Draelants. H.-B. Zhao and G.V. Baron . Pl 1 1 An Environmental Friendly Catalystfor the High Temperature Shift Reaction G.C. Araujo and M.C. Range1 P112 Direct Catalytic Conversion of Chloromethane to Higher Hydrocarbons over Various Protonic and Cationic Zeolite Catalysts as Studied by In-Situ FTIR and Catalytic Testing D. Jaumain and B.-L. Su . P 113 Catalytic Conversion of 2-Chloropropane in Oxidizing Conditions: A FT-IR and Flow Reaction Study C. Pistarino, F. Brichese, E. Finocchio, G. Romezzano, R. di Felice, M. Baldi and G. Busca P114 Catalyst Degradation of Polychlorinated Biphenyls at Low Temperature D.-K. Lee, E.-S. Byun, I.-C. Cho and S.-W. Kim .

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P 1 1 5 Microwave Eflects in Exhaust Catalysis M. Turner, R.L. Laurence, K.S. Yngvesson and W.C. Conner

.

1625

Selective Oxidation (General Papers)

P 1 16 A New Way to Ti-containingZeolite Beta Catalystsfor Selective Oxidation F. Di Renzo, S. Gomez, R. Teissier and F. Fajula .

1631

P117 Synthesis of Organically ModJied Titania Silica Aerogels: Application for Epoxidation of Cyclohexenol A. Gisler, C.A. Miiller, M. Schneider, T. Mallat and A. Baiker .

1637

Pll8 Characterization and Catalytic Properties of Titanium Pillared Clays in the Epoxidation of Allylic Alcohols . L. Khalfallah-Boudalli, A. Ghorbel, F. Figueras and C. Pine1

1643

PI 19 Epoxidation of Allylic Alcohol by Vanadium-Montmorillonite Catalyst I. Kheder and A. Ghorbel .

1649

P120 Activity and Stability of Thermally Stable Polyimide Supported Mo(V.. Complexesfor Epoxidation of Olefins J.H. Ahn, C.G. Oh, S.K. Ihm and D.C. Sherrington .

1655

P121 Epoxidation of Soybean Oil Catalysed by CHjReOdH202 H. Sales, R. Cesquini, S. Sato, D. Mandelli and U. Schuchardt

1661

.

P122 Epoxidation of Limonene with Layered Double Hydroxides as Catalysts M.A. Aramendia, V . Borau, C. JimCnez, J.M. Luque, J.M. Marinas, F.J. Romero, J.R. Ruiz and F.J. Urbano .

1667

P 123 Epoxidation of Electron-Deficient Alkenes Using Heterogeneous Basic Catalysts J.M. Fraile, J.I. Garcia, D. Marco, J.A. Mayoral, E. Shnchez, A. Monzon and E. Romeo.

1673

P124 Hydroxylation of Benzene to Phenol with Nitrous Oxide on Fe-Silicalites R. Monaci, E. Rombi, M.G. Cutrufello, V. Solinas, G. Berlier and G. Spoto

1679

P125 Ammoxidation of Propane to Acrylonitrile over V-Sb-Al-Oxide Catalysts - Influence of Catalyst Preparation on Reaction Kinetics 0.Ratajczak, H.W. Zanthoff and S. Geisler .

1685

P126 Ammoxidation of Propylene to Acrylonitrile Catalyzed by Multimetal Molybdateand Iron Antimoniate-BasedActive Compounds Dispersed in Oxidic Matrixes: The Efect of the Dispersing Oxide on the Catalytic Performance R. Catani, F. Cavani, U. Cornaro, A. Del Bianco, E. Frontani, D. Ghisletti, 1691 A. Tasso and F. Trifiri,

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P127 Mastering of Chemical State of Mo at the Surface of Oxide Catalysts in the Selective Oxidation of Hydrocarbons: Towards a Fine Optimization of Catalytic Performances E.M. Gaigneaux, M-L. Naeye, E. Godard, H.M. Abdel Dayem, P. Ruiz and B. Delmon

1697

P128 Oligomerization of Higher OleJins and Oxidation in a Series of Isobutanol - Isobutyric Aldehyde - Isobutyric Acid in the Presence of Some Heteropoly Compounds S.M. Zulfugarova, M.K. Munshieva, D.B. Tagiev and P.S. Mamedova .

1703

PI29 ModiJications of Vanadium Phosporous Oxides by Aluminium Phosphate for n-Butane Oxidation to Maleic Anhydride S. Holmes, L. Sartoni, A. Burrows, V. Martin, G.J. Hutchings, C. Kiely and J.-C. Volta

1709

P130 Selective Oxidation of n-Butane over Novel V-P-O/Silica-Composites Prepared Through Intercalation and Exfoliation of Layered Precursor N. Hiyoshi, N. Yamamoto, N. Terao, T. Okuhara and T. Nakato .

1715

P 131 Molecular Structure-Reactivity Relationships in n-Butane Oxidation over Bulk VPO and Supported Vanadia Catalysts: Lessons for Molecular Engineering of New Selective Catalystsfor Alkane Oxidation . V.V. Giulants, J.B. Benziger, S. Sundaresan and I.E. Wachs

1721

P132 The Nature of the Cobalt Salt Affects the Catalytic Properties of Promoted VPO 1727 . L. Cornaglia, C. Carrara, J. Petunchi and E. Lombardo P133 Oxidation of Toluene to Benzaldehyde over VSb,,Ti,Od Kinetic Studies A. Barbaro, S. Larrondo and N. Amadeo .

Catayst.

P134 Interaction of Surface- and Bulk-Oxygen in Mo/V-Mixed Oxides during the Partial Oxidation of an UnsaturatedAldehyde A Concentration-Programmed-ReductionStudy R. Bohling, A. Drochner, M. Fehlings, K. Kraub and H. Vogel .

1733

1739

P135 Steady State and Transient Kinetic Experimentsfor Rational Catalyst Design: Dzfferences of Acrolein and Methacrolein Oxidation on Mixed Oxide Catalysts 1745 H. Bohnke, J.C. Petzoldt, B. Stein and J.W. Gaube . P136 The Effects ofParticle Size on the Performance of Er203and 30 mol% BaC12/Er203 Catalysts in the Selective Oxidation of Ethane to Ethene W. Zhong, H.X. Dai, C.F. Ng, and C.T. Au .

1751

P137 Perovskite Type Chloro Oxide S ~ C O O ~ - ~ACNovel I ; and Durable Catalystfor the Selective Oxidation of Ethane to Ethene H. X . Dai, C.F. Ng, and C.T. Au .

1757

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P138 Preparation and Activity of Copper, Nickel and Copper-Nickel-A1Mixed Oxides via Hydrotalcite-Like Precursorsfor the Oxidation of Phenol Aqueous Solutions A. Alejandre, F. Medina, X. Rodriguez, P. Salagre and J.E. Sueiras

.

1763

P139 Influence of Temperature and Catalyst Loading on the Aqueous-Phase Catalytic Oxidation of Phenol . A. Santos, P. Yustos, B. Durbhn and F. Garcia Ochoa

1769

PI40 Glyoxal Synthesis by Vapour - Phase Ethylene Glycol Oxidation on a Silver and Copper Catalysts O.V. Vodyankina, L.N. Kurina, A.I. Boronin and A.N. Salanov .

1775

P 141 Production of Alkenes through Oxidative Cracking of n-Butane over OCM Catalysts M. Nakamura, S. Takenaka, I. Yamanaka and K. Otsuka .

1781

P142 Selective Oxidation of Monosaccharides using Pt and Pd-Containing Cctalysts E. Sulman, N. Lakina, M. Sulman, T. Ankudinova, V. Matveeva, A. Sidorov and S. Sidorov 1787 PI43 Synthesis of New Neocarboxylic Acids via Rearragement and Oxidation of I , 3-Dioxanes J . Fischer and W F. Holderich

1793

P 144 Low Temperature Radical Oxidation of Butane over In-Situ Prepared Silica Species A. Satsuma, N. Sugiyama. Y. Kamiya, T. Kamatani and T. Hattori

1799

P145 Mechanochemical Preparation of V-Ti-0 Catalystsfor o-Xylene Low Temperature Oxidation V.A. Zazhigalov, J. Haber, J. Stoch, A.I. Kharlamov, A. Marino, L. Depero and I.V. Bacherikova .

1805

P146 A Common Concept Accountingfor Selectivity in Mild and Total Oxidation Reactions: The Optical Basicity of Catalysts P. Moriceau, B. Taouk, E. Bordes and P. Courtine .

1811

P 147 Copper Species in CuC12/y-Alr03Catalystfor Ethylene Oxychlorination M . Garilli, D. Carmello, B. Cremaschi, G. Leofanti, M. Padovan, A. Zecchina, G. Spoto, S. Bordiga and C. Lamberti .

1817

P 148 A Study of the Main Path and of Side-Reactions upon Ethylene Oxychlorination over CuCI2-Al203 Based Catalysts A. Marsella, D. Carmello, E. Finocchio, B. Cremaschi, G. Leofanti, M. Padovan and G. Busca .

1823

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Selective Oxidation (ODH)

P149 Oxidative Dehydrogenation of Ethane on Lithium Promoted Oxide Catalysis S. Wang, K. Murata, T. Hayakawa, S. Hamakawa and K. Suzuki . PI50 Oxidative Dehydrogenation of Ethane to Ethylene over NiO/AI203 Catalysts X. Zhang, G. Yu, Y. Gong, D. Jiang and Y. Xie . PI5 1 Oxidative Dehydrogenation of Ethane over Catalysts Prepared Via Heteropolycompounds N . Haddad, C. Rabia, M.M. Bettahar and A. Bararna

P152 The Importance of Nonstoichiometric Oxygen in NiO for the Catalytic Oxidative Dehydrogenation of Ethane T. Chen, W. Li, C. Yu, R. Jin and H. Xu

.

P153 Oxidative Dehydrogenation of Ethane on Vanadium-PhosphorusOxide Catalysts J.M. L6pez Nieto, V.A. Zazhigalov, B. Solsona and I.V. Bacherikova

.

P154 High Throughput Synthesis and Screening of Nb and Ta Containing

Mixed Metal Oxide Librariesfor Ethane Oxidative Dehydrogenation Y. Liu, P.Cong, R.D. Doolen, H.W. Turner and W.H. Weinberg .

P 1 55 Chromium-ImpregnatedMesoporous Silica as Catalystsfor the Oxidative Dehydrogenation of Propane J. MCrida Robles, M. Alciintara Rodriguez, E. Rodriguez Castellbn, J. Santaman'a Gonzdlez, P. Maireles Torres and A. Jimenez L6pez.

P156 Transient Kinetic Studies of the Propane Oxidehydrogenationon V~OdTi02( S. Pietrzyk and F. Genser

.

PI 57 Vanadium-Doped Titania-Pillared Montmorillonites as Catalystsfor

the Oxidative Dehydrogenation of Propane K. Bahranowski, R. Dula, R. Grabowski, E.M. Senvicka and K. Wcislo .

P158 Oxidative Dehydrogenation of Propane over Alkali-Mo Catalysts Supported on Sol-Gel Silica-Titania Mixed Oxides R.B. Watson and U.S. Ozkan.

P 159 Vanadium Oxide Supported on Gallium and Indium Oxides: Synthesis, Physicochemical and Catalytic Properties B. Sulikowski, A. Kubacka, E. Wloch, Z. Schay, V. CortCs Corberiin and R.X. Valenzuela .

P160 Oxidative Dehydrogenation of Cyclohexane over Heteropolymolybdates . S. Hocine, C. Rabia, M.M. Bettahar and M. Fournier

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P161 Transient Response Studies of lsobutane Oxidative Dehydrogenation over Molybdenum Catalysts N.V. Nekrasov, N.A. Gaidai, Yu.A. Agafonov, S.L. Kiperman, V. Cortes Corberhn and M.F. Portela

P 162 Olefins Formation by Oxidative Dehydrogenation of Propane over Monoliths at Short Contact Times V.A. Sadykov, S.N. Pavlova, N.F. Saputina, I.A. Zolotarski, N.A. Pakhomov, E.M. Morov, V.A. Kumin, A.V. Kalinkin, A.N. Salanov, I.G. Dalinova and E.A. Paukshtis .

1907

P 163 Experimental and Theoretical Investigation on the Roles of Heterogeneous and Homogeneous Phases in the Oxidative Dehydrogenation of Light Parafins in Novel Short Contact Time Reactors A. Beretta, E. Ranzi and P. Forzatti .

1913

Fuel Cells and Electrocatalysis

P 164 Composite Nickel-Oxide Surfaces, Modified by Palladium R.G. Baisheva, Z.K. Kanaeva, K.A. Zhubanov and Zh.K. Kairbekov

.

1919

Photochemistry P 165 Photocatalyst-CoatedAcrylic Waveguidesfor Oxidation of Organic Compounds L.W. Miller, M.I. Tejedor Tejedor, M. PCrez Moya, R. Johnson and M.A. Anderson . 1925

P166 Preparation of Eflcient Titanium Oxide Photocatalysts by an Ionizer Cluster Beam Method and Their Application for the Degradation of Propanol Diluted in Water H . Yarnashita, M. Harada, A. Tanii, M. Honda, M. Takeuchi, Y. Ichihashi and M. Anpo .

1931

P167 Thermal Treatment of Titanium Alkoxides in Organic Media: Novel Synthesis Methods for Titanium (IV) Oxide Phot~catalystof Ultra-HighActivity H. Kominami, J. Kato, S. Murakami, M. Kohno, Y. Kera, S. Nishimoto, . M. Inoue, T. Inui and B. Ohtani

1937

P 168 Photocataliytic Water Decomposition by Layered Perovskites T. Takata, A. Tanaka, M. Hara, J.N. Kondo and K. Domen.

1943

P 169 Catalyst Preparation and Reactor Designfor Gas-Phase Photocatalytic Oxidation of Trichloroethylene (TCE) Pollutant A.J. Maira, K.L. Yeung, C.K. Chan, J.F. Porter and P.L. Yue

.

P170 Formation of Ethylene Oxide by Photooxidation of Ethylene over Silica ModiJied with Copper Y. Ichihashi and Y. Matsumura .

1949

1955

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P171 Photocatalytic Oxidation of n-Butane at a Steady Rate over Rb-Mod$ed Vanadium Oxide Supported on Silica T. Tanaka, T. Ito, T. Funabiki and S. Yoshida .

1961

P 172 Photocatalytic Oxidation of Toluene on PlatinudTitania Catalysts M.C. Blount and J.L. Falconer .

1967

PI73 Photocatalytic Degradation of Toluene in Aqueous Suspensions of Polycrystalline Ti02 in the Presence of the Surfactant Tetradecyldimethylamino-Oxide V . Augugliaro, V . Loddo, G. Marci, L. Palmisano, C. Sbriziolo, M. Schiavello and M.L. Turco Liveri

1973

P174 Photooxidation of Benzene to Phenol by Ru Complex Occluded in Mesoporous FSM-16 K. Fujishima, A. Fukuoka and M. Ichikawa .

1979

Session P2 Catalysis on Metal and Metal Oxides

P 175 Hydrodechlorination of CC12F-CF3 (CFC-114a) over Silica-Supported Noble Metal Catalysts T. Mori and Y. Morikawa .

1985

P176 Hydrodechlorination of CC12F2(CFC-12) by Carbon- and MgF2Supported Palladium and Palladium-Gold Catalysts A. Malinowski, W. Juszczyk, J. Pielaszek, M. Bonarowska, M. Wojciechowska and Z. Karpinski. .

1991

P177 C-C Bond formation during Hydrodechlorinarion of CCI4 on PdContaining Catalysts E.S. Lokteva, V.V. Lunin, E.V. Golubina, V. I. Simagina, M. Egorova and I.V. Stoyanova .

1997

P178 Hydrodehalogenation of Aryl Halides by Hydrogen Gas and Hydrogen Transfer in the Presence of Palladium Catalysts M.A. Ararnendia, V . Borau, I.M. Garcia, C. JimCnez, J.M. Marinas, A. Marinas and F.J. Urbano .

2003

PI79 Carbon-Supported Palladium Catalystsfor Liquid-Phase Hydrodechlorination of Carbon Tetrachloride to Chloroform L.M. G6mez Sainero, J.M. Grau, A. Arcoya and X.L. Seoane

2009

P180 Dispersed Pd-Ag Alloys for Selective Production of 0leJinsJi.om Chlorinated Alkanes B. Heinrichs, J.-P. Schoebrechts and J.-P. Pirard .

.

2015

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P18 1 Surface Structure and Methylcyclobutane Hydrogenolysis Activity of Pt/AlzO3 and Pt/CeOz a+ Reduction at Increasing Temperature (3 73 to 973 K) G. Rupprechter, J.J. Calvino, C. Lopez-Cartes, M. Fuchs, J.M. Gatica, J.A. PCrez Omil, K. Hayek and S. Bernal .

PI 82 Hydrogenation ofXylose to Xylitol :Three Phase Catalysis by Promoted Raney Nickel, Catalyst Deactivation and In-situ Sonochemical Catalyst Rejuvenation J.-P. Mikkola, T. Salmi, R. Sjoholm, P. M&i-Arvela and H. Vainio PI 83 Immobile Silica Fibre Catalyst in Liquid Phase Hydrogenation T . Salmi, P. M&i-Arvela, E. Toukoniitty, A.Kalantar Neyestanaki, . L.E Lindfors, R. Sjoholm and E. Laine PI 84 Lowering the Trans-Isomer Content in Hydrogenation of Triglycerides of Unsaturated Fatty Acids at Ambient Temperatures I.A. Makaryan, O.V. Matveeva, G.I. Davydova and V.I. Savchenko PI 85 Size Particle Effect and Copper or Silver Addition Effect on Catalytic Properties of Rhodium Supported onto Amorphous Silica Z. Ksibi, A. Ghorbel, B. Bellamy and A. Masson . P186 Kinetic Studies on the Hydrogenation of 1,3- Butadiene, I-Butyne and their Mixtures P. Shafer, N. Wuchter and J. Gaube . P187 Selective Hydrogenation of Nitrobenzene in Phenylhydroxylamine on Silica Supported Platinum Catalysts L. Pernoud, J.P. Candy, B. Didillon, R. Jacquot and J.M. Basset . PI88 Hydrogenation of Carbonylic Compounds on Pt/Si02 Catalysts Modified with SnBu4 G.F. Santori, M.L. Casella, G.J. Siri, O.A. Ferretti, A. Moglioni and G. Moltrasio Iglesias . PI 89 Catalytic Behaviour of Several Ni/NiAl20 Catalysts in the Hydrogenation of 1,2,4-Trichlorobenzeneand Benzene to Cyclohexane Y . Ceseros, P. Salagre, F. Medina and J.E. Sueiras . P 190 Catalytic Lifetime of Amine Metal Complexes Supported on Carbons in Ciclohexene Hydrogenation J.A. Diaz Auiion, M.C. R o m b Martinez, P.C. l'hgentiere and C. Salinas Martinez de Lecea. P 191 Promoting Effect of Ni in Semi-Hydrogenation of 1,3-Butadieneover Ni-Pd Catalysts A. Sarkany .

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P192 Influence of the Surface Structure of Co on the Selective Hydrogenation of Crotonaldehyde E.L. Rodrigues, C.E.C. Rodrigues, A.J. Marchi, C.R. Apesteguia and J.M.C. Bueno . P193 Hydrogenation of Olejns in Aqueous Phase, Catalized by LigandProtected and Polymer-Protected Rhodium Colloids A. Borsla, A.M. Wilhelm, J.P. Canselier and H. Delmas . P194 Preparation and Characterisation of Ni-Mg-A1 Hydrotalcites as Hydrogenation Catalysts E. Romeo, C. Royo, A. Monzbn, R. Trujillano, F.M. Labajos and V. Rives P 195 Hydrogenation of Acrylonitrile-Butadiene Rubbers with Palladium Loaded Mesopore-Size Controlled Clay Minerals M. Shirai, K. Torii and M. Arai . P196 Single-Stage Liquid Phase Hydrogenation of Maleic Anhydride to y-Butyrolactone, 1,4-Butanediol and Tetrahydrofurane on Cu/ZnO/Al203catalysts A. Kuksal, E. Klemm and G. Emig . PI97 Methanol Decomposition to Synthesis Gas Over Supported Pd Catalysts Preparedfiom Synthetic Anionic Clays T. Shishido, H . Sameshima, T. Hayakawa, S. Hamakawa, E. Tanabe, K. Ito and K. Takehira P198 Methanol Decomposition on Unpromoted and Zn Promoted Cu/Si02 Catalysts M . Clement, Y . Zhang, D.S. Brands, E.K. Poels and A. Bliek . P199 Comparative Study of P and Mn Mod$ed CuO/Si02Catalystfor Methanol Dehydrogenation R. Zhang, Y . Sun and S. Peng P200 Decomposition of Ethanol on Rh(100) and Rh(100)c(2x2)-Mn Surfaces . R. Zhai, Z. Tian, H. Luo, D. Liang and L. Lin P201 Use of New Tin orthophosphates as Catalystsfor the Gas Phase Dehydrogenation-Dehydrationof Alcohols M.A. Aramendia, V . Borau, C. JimCnez, J.M. Marinas, M.L. Ortega, F.J. Romero, J.R. Ruiz and F.J. Urbano . P202 Steam Reforming of Ethanol Using Cu-Ni Supported Catalysts . F. Man'fio, M. Jobbagy and G. Baronetti, M. Laborde P203 Activity and Stability of Single and Perovskite-Type Manganese and Cobalt Oxides in the Catalytic Combustion of Acetone A. Gil, N . Burgos, M. Paulis, M. Montes and L.M. Gandia.

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P204 Efect of Copper Addition on the Activity and Selectivity of Oxide Catalysts in the Combustion of Carbon Particulate C. Pruvost, J.F. Lamonier, D. Courcot, E. Abi Aad and A. Aboukai's

.

2159

P205 The Efect of Mn Substitution on the Catalytic Properties of Ferrites L.C.A. Oliveira, R.M. Lago, R.V.R.A. Rios, P.P. Sousa, W.N. Mussel and J.D. Fabris

2165

P206 Oxygen Nonstoichiometry and Linear Deformation of La-Sr-Fe-Co-0 Perovskites within the Redox Cycle L.A. Rudnitsky, V.V. Aleksandrovskii and S.Y. Stefanovich .

2171

P207 Molecular Mechanism of Surface Recognition during the Adsorption/ Degradation of Organic Compounds on Iron Oxides J. Bandara, J. Mielzcarski and J. Kiwi .

2177

P208 Characterization and Catalytic Activity of Mixed Cu-Cr/Ce02 Catalyst W. Daniell, P. Grotz, H. Knozinger, N.C. Loyd, C. Bailey and P.G. Harrison .

2183

P209 Acid Strength of Support Materials as a Factor Controlling Catalytic Activity of Noble Metal Catalystsfor Catalytic Combustion Y. Yazawa, H. Yoshida, N. Takagi, N. Kagi, S. Komai, A. Satsurna, Y. Murakami and T. Hattori .

2189

P210 Structural Features and Activity for CO Oxidation of LaFeXNil,03+6 Catalysts 2195 H. Falcbn, J. Baranda, J.M. Campos Martin, M.A. Peiia and J.L.G. Fierro . P2 11 Amorphous Alloys as Catalystsfor Hydrogen Oxidation M. Stancheva, St. Manev, D. Lazarov and E. Stancheva

.

220 1

P2 12 Precursor of Iron Catalystfor Ammonia Synthesis: Fe3O4, Fel,O, Fe203 or their Mixture? L. Huazhang and L. Xiaonian

2207

P213 Production ofAlkenes over Chromia Catalysts: Effect of Potassium on Reaction Sites and Mechanism S.D. Jackson, I.M. Matheson, M.-L. Naeye, P.C. Stair, V.S. Sullivan, S.R. Watson and G. Webb .

2213

P2 14 Acid-base Site Requerimentsfor Elimination Reactions on Alkali Promoted MgO V.K. Diez, C.R. Apesteguia and J.I. Di Cosimo .

2219

P2 15 Structure-Reactivity Correlations Across the Pressure-Gap Studied on Epitaxial Iron Oxide Model Catalyst Films C. Kuhrs and W. Weiss .

2225

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P216 Catalytically-Mediated Generation of HNO2 in Highly HN03 Concentrated Media S. Guenais-Langlois, C. Bouyer, J.-C. Broudic, A. Ananiev and B. Coq . P2 17 Catalytic Nitrate Reduction: Kinetic Investigations . U . Priisse, J. Daum, C. Bock and K.-D. Vorlop P218 Liquid Phase Isomerization of Propadiene to Methyl Acetylene on Modi3ed Alumina Catalysts R. Dotzel, M. Reif and E. Klernm . P2 19 The Eflects of Raney Alloy Structure on the Activation Process and the Properties of the Resulting Catalyst S. Knies, M. Berweiler, P. Panster, H.E. Exner and D.J. Ostgard . P220 Raney-Nickel-Iron Catalysts Obtained by Mechanical Alloying: Characterization and Hydrogenation Activity J. Salmones, B.H. Zeifert, J.A. Hernandez, R. Reynoso, N. Nava, J.G. Cabafias Moreno and G. Aguilar Rios . P22 1 Reduction of Benzaldehyde on Copper Supported on SiO2. Effect of Method of Preparation . A. Saadi, M.M. Bettahar and Z. Rassoul P222 Some Phenomenological and Mechanistic Aspects of the Use of Copper as Catalyst in Trichlorosilane Synthesis H. Ehrich, T . Lobreyer, K. Hesse and H. Lieske . P223 The Role of Oxygen Vacancies in Zirconia on the Dispersion, Stabilisation and Reactivity in the Presence of 0 2 of Supported Rh Particles G. Centi, B. Panzacchi, S. Perathoner and F. Pinna . P224 Interactions among Rh, Mn and Li Components in the Rh-Based Catalysts Y . Wang, Z . Song, D. Ma, H. Luo, D. Liang and X. Bao . P225 Direct Synthesis of Propionamide and Propiononitrilefrom Ethylene Carbon Monoxide, and Ammonia Using Supported Ru Catalyst S.-P. Zhao, S.-I. Sassa, H. Inoue, M. Yamakazi, H. Watanabe, T. Mori . and Y. Morikawa P226 Structure, Surface State and Catalytic Properties of a Model Platinum Catalyst J. Find, Z. Pail, H. Sauer, R. Schlogl, U. Wild and A. Wootsch . P227 Structure of Ultradisperse Pt Powders and their Performance in the Partial Oxidation of C-H Bonds by Molecular Oxygen A.P. Suknev, V.B. Goncharov, V.I. Zaikovskii, A.S. Belyi, N.I. Kuznetsova, V.A. Likholobov and B.S. Bal'zhinimaev .

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P228 Eflect of Water Vapour on the Catalytic Activity of Supported Platinum Catalysts S. Lunzer and R. Krarner .

2303

P229 Stabilization of Molecular Pd Species in a Heterogenized Wacker-Type Catalystfor Low Temperature CO Oxidation E.D. Park and J.S. Lee

2309

P230 Surface Properties of Palladium Supported on Cerium Oxide and its Catalytic Activity for Methanol Decomposition Y . Matsurnura, Y . Ichihashi, Y. Morisawa, M. Okurnura and M. Haruta .

2315

P23 1 Deactivation of Palladium Catalysts Supported on Functionalized Resins in the Reduction ofAromatic Nitrocompounds M. Krhlik, V . Kratky, M. Hronec, M. Zecca and B. Corain .

2321

P232 Metal Palladium Dispersed Inside Macroporous Ion-Exchange Resins: Textural Characterization and Accessibility to Gaseous Reactants A. Biffis, K. Jerabek, A,A, D'Archivio, L. Galantini and B. Corain

2327

P233 Novel Supported Heterogeneous Palladium Catalystsfor Carbon-Carbon Forming Reactions E.B. Mubofu, J.H. Clark and D.J. Macquarrie .

2333

P234 Passivation of Nickel Activity in the Nickel-Nb205-Si02System at Low Metal Contents M.M. Pereira, E.B. Pereira, Y.L. Lam, L.T. dos Santos and M. Schmal

2339

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P235 Gold Based Environmental Catalyst L.A. Petrov .

2345

P236 Evolution of a New Reaction Pathway in Propene-Deuterium Exchange Reaction by the Co-adsorption of CO over Rh, Ir, and 0 s Carbonyl Cluster Derived Catalysts Supported on Alumina S. Naito, K. Oguni, T. Naito and T. Miyao .

235 1

P237 Studies on a ChromidLanthanalZirconiaAromatization Catalyst D.L. Hoang, A. Trunschke, A. Briickner, J. Radnik and H. Lieske .

2357

P238 The Reforming Catalytic Properties of Bulk and Supported Molybdenum and Tungsten Dioxides, X02 (X = Mo, W). A. Katrib, L. Urfels and G. Maire .

2363

Refining and Petrochemistry

P239 Active Sites of the Isomerisation of n-Butane over Oxygen-Modijed Molybdenum Carbide and Molybdenum Oxides S . Liebig, T. Gerlach, Kh. Doukkali and W. Griinert

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AlzOj-Promoted Sulfared Zirconia Catalystsfor the Isomerization of n-Butane R. Olindo, F. Pima, G. Strukul, P. Canton, P. Riello, G. Cerrato, . G. Meligrana and C. Morterra n-Pentane Isomerization over Mesoporous Sulfated Zirconia Catalysts M. Risch and E.E. Wolf . Bzfunctional Ni, Pt Zeolite Catalystsfor the Isomerization of n-Hexane M.H. Jordao, V. Simoes, A. Montes and D. Cardoso About the Importance of Surface Hydrogen Availability during n-Hexane Isomerization over Platinum on Tungsten Oxide Promoted Zirconia M.G. Falco, S.A. Canavese, R.A. Comelli and N.S. Figoli . Highly Dispersed Framework Zirconium Phosphates-Acid Catalysts of Hexane Isomerization S.N. Pavlova, V.A. Sadykov, G.V. Zabolotnaya, D.I. Kochubey, R.I. Maximouskaya, V.I. Zaikovskii, V.V. Kriventsov, S.V. Tsybulya, E.B. Burgina, A.M. Volodin, N.M. Ostrovskii, A.M. Simakov, T.A. Nikoro, V.B. Fenelonov, M.V. Chaikina, N.N. Kmetsova, V.V. Lunin, D. Agrawal and R.Roy . Zeolites and Natural Materials as Catalystsfor n-Heptane Hydroisomerization Reaction A. Brito, M.C. Alvarez, F.J. Garcia, M.E. Borges and M. Torres . Influence of the Zeolite Structure on the Hydroisomerization of n-Heptane A. Patrigeon, E. Benazzi, Ch. Travers and J.Y. Bernhard . Hydroisomerization of Octane on Pt/AI-Pillared Vermiculite and Phlogopite and Comparison with Zeolites F.J. del Rey P.C., M.L. Shchez Henao and G. Poncelet . Slurry Isomerization of n-Hexadecane over Moo3-Carbon-Mod$ed, Pt/PZeolite, Pt/ZSM 22 and Pt/SAPO 1 I Catalysts at Medium Pressure S. Roy, C. Bouchy, C. Pham-Huu, C. Crouzet and M.J. Ledoux . Zinc Oxide Modified by Alkylsilylation as an Efficient Catalystfor Isomerization of Hydrocarbon Y . Imizu, T. Narita, Y. Fujito and H. Yamada . Eflect of Redox Treatments on the Surface State of Pd- W-Based Catalysts and on the Skeletal Rearrangement of Hydrocarbons. C. Bigey, F. Garin, L. Hilaire and G. Maire .

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P25 1 Acid or Bzfunctional Mechanism in ParafJin Isomerization Reaction and Pt/W03-Zr02 Catalysts on P~/SO;--Z~O~ J.C. Yori, C.L. Pieck and J.M. Parera

P252 Catalytic Cracking of Heavy Oil Fractions over Natural Zeolite Contained Composites R.H. Ibrasheva and K.A. Zhubanov . P253 Evidence for DifSerent Reaction Mechanisms as the Cause of the Enhanced Hydrocarbon Cracking Activity of Steamed Y Zeolites . B.A. Williams, W. Ji, J.T. Miller, R.Q. Snurr and H.H. Kung P254 TPR and C02-TPDof Composite Titania (Anatase)-Alumina Systems as Potential Vanadium Trapsfor Fluid Catalytic Cracking (FCC) F. Hernimdez Beltrim, E. Mogica Martinez, E. L6pez Salinas and M.E. Llanos Serrano . P255 Formation of SulJitr-Containing Compounds Under Fluid Catalytic Cracking Reactions P. Leflaive, J.L. Lemberton, G. Perot, C. Mirgain, J.Y. Carriat and J.M. Colin P256 The Use of Catalytic Cracking Technologyfor the Creation of Novel Processes for Oil Refinery and Petrochemistry M.I. Levinbuk and N.Y. Usachev . P257 Transformation of Methylcyclohexane over Fresh HFA U and HMFI Zeolites: Reaction Scheme and Kinetic Modeling H.S. Cerqueira, M.G.F. Rodrigues, P. Magnoux, D. Martin and M. Guisnet P258 Mechanism of n-Hexane Cracking on MFI (Si/Al= 10) L. Isernia, A. Quesada, J. Lujano and F.E. Imbert . P259 Hydrocracking of Heavy Straight Run Naphtha with Pt Supported on Zeolites Y, USY, ZSM5 and Beta H. Ortiz Soto and L.J. Hoyos Marin . P260 Hydrocracking of Phenanthrene over Pt/Si02-A1203,Pt/H-Y, Pt/H-Beta and Pt/H-ZSM-5 Catalysts: Reaction Patway and Products Distribution L. Leite, E. Benazzi, N. Marchal-George and H. Toulhoat . P261 Hydrogenation and Dehydrogenation of Hydrocarbons over Ni Supported on Alumina- and Silica-Promoted Titania G. Ptrez and T. Viveros . P262 Dehydroisomerization of N-Butane on PT-ZN/H-Mordenite M.M. Ramirez-Coreedores, T . Romero and M. Gordlez .

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P263 Bimetallic Pt-M(M=Ga,ln,T1) Silica Supported Catalystsfor lsobutane Dehydrogenation J. Llorca, P. Ramirez de la Piscina, J. Lebn, J. Sales, J.L.G. Fierro and N. Homs .

2513

P264 Aromatization of Ethane on Modijied Zeolites in the Presence of co-reactans F. Roessner, A. Hagen and J. Heemsoth .

2519

P265 Effect of Barium and Lanthanum Oxides on the Properties of Pt/KL Catalysts in the n-Heptane Dehydrocyclization J.M. Grau, L.M. Gomez Sainero, L. Daza, X.L. Seoane and A. Arcoya

2525

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P266 Effect of Lanthanum Doped y-A1203 in the Selectivity of Reforming Pt-Pb Supported Catalysts. G. del Angel, G. Torres and V. Bertin .

2531

P267 Regeneration and Oxidation-Reduction Cycles of Vapor Phase and Incipient Wetness Impregnation Pt/KL Catalysts F. Ghadiali, G. Jacobs, A. Pisanu, A. Borgna, W.E. Alvarez and D.E. Resasco

2537

P268 Mechanism of Iso-butane Alkylation by Butenes over H3PW12040, HflTiW11039and Zr Supported Catalysts E.A. Paukshtis, V.K. Duplyakin, V.P. Finevich, A.V. Lavrenov, V.L. Kirillov, L.I. Kuznetsova, V.A. Likholobov and B.S. Bal'zhinimaev.

2543

P269 Porous 12-TungstophosphoricAlkaline Salts for Isobutane/Butene Alkylation. Influence of Protonic Density and Surface Polarity of the HPA. Effects of the Supercritical Phase P.Y. Gayraud, N . Essayem and J.C. Vedrine

2549

P270 Influence of the Metal Content on the Amount and the Nature of the Coke Formed during Isobutane/2-Butene Alkylation over Ni-Y Zeolite P.A. Arroyo, C.A. Henriques, E.F. Sousa Aguiar, A. Martinez and J.L.F. Monteiro .

2556

P271 The Role of Hydride Transfer in Zeolite Catalyzed Isobutane/Butene Alkylation 2561 . G.S. Nivarthy, A. Feller, K. Seshan and J.A. Lercher P272 Mechanistic Routes of Low Temperature Alkane Activation over Zeolites J.A.Z. Pieterse, K. Seshan and J.A. Lercher .

2567

P273 Catalytic Conversion of n-Parafins in Supercritical Phase W. Wei, F. Li, J. Ren, Y. Sun and B. Zhong.

2573

P274 Sulphur Resistant Palladium-Platinum Catalysts Preparedfiom Mixed Acetylacetonates A.J. Renouprez, A. Malhomme, J. Massardier, M. Cattenot and G. Bergeret .

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P275 Production of High Cetane Diesel Fuels by Simultaneous Hydrogenation of Aromatics and Ring Opening of Naphthenes over Bzfinctional Molecular Sieves Based Catalysts M.A. Arribas and A. Martinez

P276 Formation of Diisopropyl EtherJLoin 2-Propanol Using Keggin-Type H3[W12P040]and H4[W12Si040]Heteropolyacids Supported on Zirconia E. Lopez Salinas, I.G. Hemiindez Cortez, J. Navarrete, M. Salmon and I. Shifter . P277 The Mechanism of Diisopropyl Ether Synthesispom a Feed of Propylene and Isopropanol over Ion Exchange Resin F.P. Heese, M.E. Dry and K.P. Moller . P278 The Conversion of 2,3-Butanediol to Methyl Ethyl Ketone over Zeolites J. Lee, J.B. Grutzner, W.E. Walters and W.N. Delgass . P279 Effect of Internal Dzffusion on Liquid-Phase Synthesis of MTBE R. Pla, J. Tejero, F. Cunill, J. Felipe Izquierdo, M. Iborra and C. Fit6 P280 Acid and Catalytic Properties of Supported Sulfpolyphenylketones in the Formation of DIPE, MTBE and TAME Ethers T. Jarecka, St. Miescheriakow and J. Datka . P28 1 Effect of the Addition ofAcidic and Basic Compounds on the Catalytic Behavior of Exchanged Zeolites in the Alkylation of Toluene with Methanol . A. Borgna, J. Sepulveda, S. Magni and C. Apesteguia P282 Zeolite Beta: Selective Molecular Sievefor Synthesis ofXylenesfiom Trimethylbenzenes J. Cejka and A. Krejci P283 Characterisation and Reactivity of Supported and Unsupported B/P/O Catalysts in the o-Alkylation of Diphenols with methanol F. Cavani, T . Monti and D. Paoli . P284 Oxidative Alkylation of Isobutene, Propene and Toluene with Methanol . I.P. Belomestnykh and G.V. Isaguliants P285 Side Chain Methylation of Toluene and Ethylbenzene with Dimethylcarbonate over Alkaline X-Zeolite R. Bal and S. Sivasanker . P286 The effect of the particle size on methanol conversion to ligh olefins De Chen, K. Moljord and A. Holmen

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P287 Synthesis of N-Isopropylacrylamide Synthesis Acrylonitrile and Isopropyl Alcohol over Solid Acids X . Chen, H. Matsuda and T. Okuhara

2657

P288 Pumice as Catalyst in the Claus Reaction A. Brito, R. Larraz, R. Arvelo, M.T. Garcia and M.E. Borges

2663

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P289 Supported Alkali Salt Catalysts Active for the Guerbet Reaction between Methanol and Ethanol K. Gotoh, S . Nakamura, T. Mori and Y. Morikawa .

2669

P290 Catalyst Porous Structure and Acidity Effects on Alkylphenol Transformations 2675 . J.C. Hernhdez, F.E. Imbert, P. Ayrault, M. Guisnet and N.S. Gnep Reactor Engineering and Catalytic Processes

P291 Natural Gas Utilization through CO2 Reforming in a Membrane Reactor T.M. Raybold and M.C. Huff.

268 1

P292 Activity and Selectivity of a Pd/y-A1203 Catalytic Membrane in the Partial Hydrogenation of Acetylene C. Lambert, M. Vincent, J. Hinestroza, N. Sun and R. Gonzalez .

2687

P293 Catalysis of Palladium Salt Reduction in a Gas-Liquid Membrane Reactor S. Miachon, A. Mazuy, J.-A. Dalmon .

2693

P294 Propane Aromatization in a Silicalite-1 Membrane Reactor W . Yang, P. Yang, X. Xu and L. Lin. P295 Effects of Operation Modes on the Oxidation of Propane to Acrolein in a Membrane Reactor W . Yang, P. Yang, W. Fang and L. Lin .

2705

P296 The Conversion of Methanol to Hydrocarbons over ZSMS: Fixed Bed vs. Recycle Reactor K.P. Moller, W . Bohringer, A.E. Schnitzler and C.T. O'Connor

.

271 1

P297 Catalytic Dehydrogenation of n-Butane in a Fluidized Bed Reactor with Separated Coking and Regeneration Zones C. Callejas, J. Soler, J. Herguido, M. Menendez and J. Santarnaria.

2717

P298 Study in a Riser Simulator Reactor of the Role of a HZSM-5 Zeolite as FCC Catalyst Additive J.M. Arandes, I . Abajo, I. Fernhdez, J. Bilbao, H.I. de Lasa .

2723

P299 Difision Analysis of Cumene Cracking over ZSMS Using a Jetloop Reactor P. Schwan, K.P. Moller and J.P. Henry .

2729

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Monolithic Catalysts as more Eficient Reactors in Three-PhaseApplications T.A. Nijhuis, F. Kaptjein and J.A. Moulijn . Synthesis of Ethylene Oxide in a Catalytic Microreactor System H. Kestenbaum, A. Lange de Oliveira, W. Schmidt, F. Schiith, W. Ehrfeld, K. Gebauer, H. Lowe and Th. Richter Develoment of a Novel Structured CataIytic Reactorsfor Highly Exothermic Reactions . G. Groppi, C. Cristiani, M. Valentini and E. Tronconi Simulation of Alternative Catalyst Bed Configurations in Autothermal Hydrogen Production A.K. Avci, D.L. Trimm and Z.I. 0nsan . Development and Study of Metal Foam Heat-Exchanging Tubular Reactor: Catalytic Combustion of Methane Combined with Methane Steam Reforming Z.R. Ismagilov, O.Yu. Podyacheva, V.V. Pushkarev, N.A. Koryabkina, V.N. Antsiferov, Yu.V. Danchenko, O.P. Solonenko and H. Veringa . Optimization in Design, Testing and Reactor Use by Statistical Modeling of the Reaction Variables, Energy Input and Reaction Time E. Balanosky, F. Herrera and J. Kiwi Catalysis on Sulfides, Nitrides and Carbides

Synthesis of Highly Dispersed Molybdenum and Ruthenium Sulfides Using Tetraalkylammonium Surfactant P. Afanasiev, G.-F. Xia, B. Jouguet and M. Lacroix. Properties of Sonochemically Synthesized, Highly Dispersed MoS2/AI203 Catalysrsfor the Hydrodedulfurization of Dibenzothiophene and 4,6Dimethyldibenzothiophene J.J. Lee, C. Kwak, Y.J. Yoon, T. Hyeon and S.H. Moon . Structure and Catalysis of Intrazeolite Co-Mo Binary Sulfide Model Clustersfor Hydrodesuljkrization Y. Okamoto, H. Okamoto and T. Kubota . An Inelastic Neutron Scattering Study of the Interaction of Thiophene with a Hydrodesulfurisation Catalyst P.C.H. Mitchell, D.A. Green, E. Payen, J. Tomkinson and S.F. Parker Characterization of Oxide Catalysts Using Time-Resolved XRD and XANES: Properties of Pure and Sulfided CoMo04 and NiMoO4 J.A. Rodriguez, J.C. Hanson, S. Chaturvedi and J.L. Brito .

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E$ect of the Support Porosity on the Thiophene and Dibenzothiophene Hydrodesulfirization Reactions. A1203-Ti02Mixed Oxide Support T . Klimova, J. Ramirez, R. Cuevas and H. GonAlez . TPR-S and TPS Studies of CoMo and NiMo Catalysts Supported on Al203-Ti02 Mixed Oxides L. Cedefio, R. Zanella, J. Ramirez and A. L6pez Agudo . Characterization of Reduced and Sulphided Ru-V/A1203Catalysts C.E. Scott, J. Guevara, A. Scaffidi, E. Escalona, C. Bolivar C., M.J. PCrez Zurita and J. Goldwasser . Sulfir Effect on Mo2N/y-ALO3Catalyst Studied by In Situ FT-IR Spectroscopy Z. Wu, Y. Chu, S. Yang, Z. Wei, C. Li and Q. Xin . Concerted Mechanisms in the Heterogeneous Catalysis by SulJides A.N. Starsev . Selective Promoting Effect on Alkylbenzothiophenes HydrodesulJitrization Pathways F. Bataille, J. Mijoin, J.L. Lemberton, G. Perot, C. Berhault, M. Lacroix, . F. Mauge, S. Kasztelan and M. Breysse Deep Hydrodenitrogenation on Pt Supported Catalysts in the Presence of Hfi, Comparison with NiMo Sulfide Catalyst E. Peeters, C. Geantet, J.L. Zotin, M. Breysse and M. Vrinat . Hydrotreating of Heavy Gas Oil on Unsupportesd and Supported Mo-, W-, Nb-Nitrides J.A. Melo Banda, J.M. Dominguez and G. Sandoval Robles . Evidence for the Effect of Phosphorus on Molybdenum Oxynitrides and Oxycarbides, Activity and Selectivity in Propene Hydrogenation and n-Heptane Isomerization P. PCrez Romo, C. Potvin, J.-M. Manoli and G. Djeda-Mariadassou Degradation of a Spent Hydrotreating Cata1yst:interaction with Enviroment J.C. Afonso, T . Siqueira de Lima and P.C. Campos . Influence of Sulfur Containing Compounds on High Temperature Coke Formation on Hydrotreating Catalyst R. Lebreton, S. Brunet, G. Ptrot, V. Harle and S. Kasztelan Deactivation of MoS2 Cataysts during the HDS of Thiophene. Effect of Nickel Promoter F. Pedraza, S. Fuentes, M. Vrinat and M. Lacroix .

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P323 Deactivation and Testing of Molybdenum Nitride Catalysts in Hydrodesulfirization of Dibenzothiophene M. Nagai, M. Kiyoshi and S. Omi .

P324 Development of a Method for the S t u 4 of the Deactivation of HDS Catalysts J.M. Trejo and F. Albertos . P325 Deactivation by Oxygen and Subsequent Activation of Bulk Mo2C for Benzene Hydrogenation at 298 K J.-S. Choi, G. Bugli and G. Djega-Mariadassou . P326 Silicon Carbide Supported NiS2 Catalystfor the Selective Oxidation of H2S in Claus Tail Gas M.J. Ledoux, C. Pham-Huu, N. Keller, J.B. Nougayrede, S. Savin-Poncet and J. Bousquet . Molecular Sieves

P327 Novel Method of Preparing Zeolite Filmsfrom Colloidal Zeolite Nay H-Q. Lin, Z-S. Chao, T-H. Wu, G.-Z. Chen, F-X. Zhang, H-L. Wan And K.-R. Tsai P328 Evidence of the Supermicropores Creation in Zeolites. Dealumination of Narrow-Pore Zeolites P. Hudec, A. Smieskova, Z. Zidek and E. Rojasova . P329 Transition through Various Regimes of Reaction Kinetics and Selectivity Driven by Progressing Modification of the External Surface of Zeolites H.P. Roger, K.P. Moller, W. Bohringer and C.T. O'Connor P330 Polycarbonylic and Polynitrsylic Species in ~ u ' - ~ x c h a n ~ZSM-5, e d Beta, Mordenite and Y Zeolites: Comparison with Homogeneous Complexes G. Turnes Palomino, A. Zecchina, E. Giamello, P. Fisicaro, G. Berlier and C. Lamberti . P33 1 The Characterization of Zeolite Acidity by Dinamic Methods Gy. Onyesty&, J.Valyon and L.V.C.Rees . P332 Textural Characterization of Zeolites by Adsorption of Molecules of Different Size Followed by Adsorption Microcalorimetry J.M. Guil, R. Guil Lopez and J.A. Perdigon Mel6n . P333 Unusual bomerization Routes of n-Butenes on the Acidic OH(0D) Groups on Ferrierite Zeolite Studied by FT-IR J.N. Kondo, E. Yoda, M. Hara, F. Wakabayashi and K. Domen .

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P334 Use o f ' 2 9 ~N eMR Spectroscopyfor the Study of Gaseous Reactant Dzfision in a Fixed Bed of Zeolite; Application to Benzene Difision in HZSM-5 P. N'Gokoli-Kekele, M.-A. Springuel-Huet, J.-L. Bonardet and J. Fraissard

P335 Ti-0-Si and Fe-0-Si Valence Angles of Tetrahedral Ti and Fe Sites in Porous Silicas of Diflerent Structures F. BCland and B. Echchahed and L. Bonneviot . P336 FTIR Study of the Proton Transfer in Al-MCM-22 Zeolite to Insaturated Hydrocarbons B. Onida, F. Geobaldo, F. Testa, F. Crea and E. Garrone . P337 Sorption of Methanol in Alkali Exchanged Zeolites M. Rep, A.E. Palomares, J.G. van Ohrnmen, L. Lefferts and J.A. Lercher . P338 Structural Lewis Sites in Zeolite Beta - Role on Coking of the Catalyst A. Vimont, F. Thibault-Startzyk and J.C. Lavalley . P339 Mechanism of Coking and Regeneration of Catalysts Containing ZSM-5 Zeolite V.G. Stepanov, E.A. Paukshtis, V.V. Chesnokov and K.G. Ione . P340 Effect of Dealumination of Mordenite Catalystfor Amination Reaction of Ethanolamine K. Segawa and T. Shimura . P34 1 Synthesis and Characterization of Pd-Zr- and Pd-Rh-Beta Zeolite Catalystsfor Removal of EmissionsJi.om Natural Gas Driven Vehicles N. Kurnar, F. Klingstedt and L.-E. Lindfors . P342 Catalytic Behavior of the Microporous Hexagonal Zincophosphate CZP in Base-Catalyzed Reactions L.A. Garcia Serrano, J. PCrez Pariente, F. Shchez and E. Sastre . P343 Basic CsBeta - A New Support for Pt Nanoparticles Active in Aromatization of Parafins C. Jia, A.P. Antunes, J.M. Silva, M.F. Ribeiro, M. Lavergne, M. Kermarec and P. Massiani P344 Hydroformylation of Olefins Using Encapsulated HRh(CO)(PPhj) 3 in Nay Zeolite as a Catalyst K. Mukhopadhyay, V.S. Nair and R.V. Chaudhary . P345 Spectroscopic and Catalytic Investigation ofthe NO Reactivity on CoAPOs with Chabasite-Like Structure L. Marchese, E. Gianotti, B. Palella, R. Pirone, G. Martra, S. Coluccia . and P. Ciambelli

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P346 Bifinctional Zeolite Catalystsfor the Selective Synthesis in One Step of Various Ketones P. Magnoux, N. Lavaud, L. Melo, G. Gianetto, A.I. Silva, F. Alvarez and M. Guisnet

301 1

P347 Considerations about the Bzfunctionallity and the Gallium Species Role in the Propane Aromatization Reaction over [Gal- and [Ga,AI]-ZSM-5 Catalysts A. Montes and G. Gianetto .

3017

P348 Novel Shape-Selective Catalystsfor Synthesis of 4, 4'-Dimethylbiphenyl J.-P. Shen, L. Sun and C. Song .

3023

P349 Investigation of n-Pentane and Cyclohexane Total Oxidation on the Cu-Containing ZSM-5 Zeolites M.A. Botavina, M.-V. Nekrasov and S.L. Kiperman

3029

P350 Textural Properties, Stability and Catalytic Activity of Acidic Mesoporous Materials Obtained By Direct Incorporation of Aluminum Y.-H. Yue, A. GCdCon, J.-L. Bonardet, J.B. D'Espinose and J. Fraissard .

3035

P35 1 Templating Fabrication and Catalysis of Platinum Nanowires in Mesoporus Channels of FSM-I6 A. Fukuoka, N . Higashimoto, M. Sasaki, M. Harada, S. Inagaki, Y. Fukushima and M. Ichikawa .

3041

P352 Physico-Chemical and Catalytic Properties of Ni-Containing Mesoporous Molecular Sieves of MCM-41 Type M. Ziolek, I. Nowak, I. Sobczak and H. Poltorak . P353 Preparation of Highly Structured V-MCM-41 and Determination of its Acidic Properties S. Lim and G.L. Haller

3053

P354 Post-Synthetic Preparation of Ti-MCM-41 Oxidation Catalysts with Titanium Alkoxides A. Hagen, K. Schueler and M. Voskamp .

3059

P355 Fe-MCM-41 as a Novel Sulfuric Acid Catalystfor SO2 Rich Feeds A. Wingen, N . Anastasieviec, L. Hollnagel, D. Werner and F. Schiith

3065

.

Session P3 Catalyst Characterisation

P356 In situ Inpared Spectroscopic Study of the Reaction between CO and Hz over an Sm203Catalyst . Y. Sakata, T . Arimoto, H. Imamura and S. Tsuchiya

3071

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liii

P357 In Situ Inpared Spectroscopic Investigation of 12-Tungstphosphoric Acid and its Cesium Hydrogen Salts in Relation to Catalytic Activiry G. Koyano, T . Saito, M. Hashimoto and M. Misono

3077

P358 The Oxidation State of Ru Catalysts under Conditions of Partial Oxidation of Methane Studied by XPS and FTIR Spectroscopy C. Elmasides, D.I. Kondarides, S. Neophytides and X.E.Verykios .

3083

P359 Correlation between "In Situ " XPS Characterization and Catalytic Activity of Mo03/cr-A1203for Hexenes and Hexanes Isomerization. Mechanistic Approach Using Probe Molecules V. Keller, F. Barath, F. Garin and G. Maire .

3089

P360 In Situ Observation of a Surface Catalysed Chemical Reaction by Fast X-Ray Photoelectron Spectroscopy A.F. Lee, K. Wilson and R.M. Lambert .

3095

P361 The Structure of Vanadium Oxide on y-Alz03: An In Situ X-Ray Absorption Spectroscopy Study during Catalytic Oxidation M . Ruitenbeek, F.M.F.de Groot, A.J. Van Dillen and D.C. Koningsberger

3101

P362 Flux Response Analysis - A New In Situ Techniquefor Catalyst Characterization K. Hellgardt, B.A. Buffham, M.J. Heslop, G. Mason and D.J. Richardson

3107

P363 New Method of the Surface Characterisation of a Metal Catalyst under Real Reactor Conditions using Electron Microscopy W. Arabczyk, K. Kalucki, U. Narkiewicz, D. Moszynski and A.W. Morawski

31 13

P364 Direct Observation of Reduction of PdO to Pd Metal by In Situ Electron Microscopy P.A. Crozier and A.K. Datye .

31 19

P365 Identification and Roles of the Different Active Sites in Supported Vanadia Catalysts by In Situ Techniques M.A. Baiiares, M. Martinez-Huerta, X. Gao, I.E. Wachs and J.L.G. Fierro

3125

P366 Design of an SFG-Compatible Uhv-High Pressure Reaction Cell: Studies of CO and NO Adsorption on Ni and NiO(l00) by IR-Vis Sum Frequency Generation Vibrational Spectroscopy G. Rupprechter, T . Dellwig, H. Unterhalt and H-J. Freund .

3131

P367 The Combined Use of Acetonitrile and Adamantane-Carbonitrilefor FTIR Spectroscopic Characterization of Acidity in Zeolites C. Otero Areh, M. Rodriguez Delgado, M. Peiiarroya Mentruit, . F.X. Llabres i Xamena and C. Morterra

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P368 Infiared Spectroscopic Evidence for Two Ways of Adsorbed CO Coordination A.A. Tsyganenko, C. Otero Arein and E. Escalona Platero .

3143

P369 Study of Layer-by-Layer Growth of Silica on Alumina and Alumina on Silica Using IR Spectroscopy of Adsorbed CO A.A. Tsyganenko, O.V. Manoilova, K.M. Bulanin, P.Yu. Storozhev, S. Haukka, A. Palukka and M. Lindblad .

3 149

P370 Applications of Inelastic Incoherent Neutron Scattering in Technical Catalysis P. Albers, G. Prescher, K. Seibold and S.F.Parker .

3155

P371 Hydrogen Species in Cerium - Nickel Oxide Catalysts: Inelastic and Compton Neutron Scattering Studies C. Lamonier, E.Payen, P.C.H.Mitchel1, S.F. Parker, J. Mayers and J.Tornkinson .

3161

P372 Using Atomic Force Microscopy (AFW to Study the Surface Structure of Oxide and Metal-Decorated Oxide Particules E. Dokou, W.E. Farneth and M.A. Barteau .

3167

P373 Study of Catalyst Preparation Processes by Atomic Force Microscopy (AFM: Adsorption of a Pt Complex on a Zeolite Surface M.KomiyamaandN.Gu .

3173

P374 Chemistry of SO2 on Model Metal and Oxide Cata1ysts:Photoemission andXANES Studies J.A. Rodriguez, T. Jirsak, S. Chaturvedi, J. Hrbek, A. Freitag and J.Z. Larese

3 177

P375 Changes in the Electronic Structure of Pt/CeOz Based DENOXCatalysts as a Function of the Reduction Treatment. Detection of a Pt-H Anti-Bonding Shape Resonance and Pt-H EXAFS J.H. Bitter, M.A. Cauqui, J.M. Gatica, S. Bernal, D.E. Ramaker and D.C. Koninsberger .

3183

P376 Bimetallic Nano-Particle Formation in the Pt-Re Reforming Catalysts Revealed by STEWEDX XANES/EAYFS and Chemical Characterization Techniques. EfSects of Water and Chlorine T . Gjeman, M. bnning, R. Prestvik, B. Tstdal, C.E. Lyman and A. Holmen

3189

P377 Hyperfine Interactions on Pt-In Catalysts F.B. Passos, P.R.J. Silva and H. Saitovich .

3195

P378 Oxygen Atom Radical Formation on the Sol-Gel Molybdenum-Silica Catalysts Characterized by X-Ray Absorption Fine Structure Spectroscopy Y . Izumi .

3201

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P379 Structural Characterization of Pd-Ag and Pd-Cu Bimetallic Catalysts by means of EX4FS WAXS and XPS A. Longo, A. Balema, F. Deganello, L.F. Liotta, C. Meneghini, . A. Martorana and A.M. Venezia P380 Super Acidity Confirmed on a Monolayer of Sulfare Species Loaded on Zirconia N. Kadata, J-I. Endo, K-I. Notsu, N. Yasunobu, N. Naito and M. Niwa

.

P38 1 Characterization and Catalytic Properties of Aerogels Suyated Zirconia . M.K.Younes, A. Ghorbel, A. Rives, R. Hubaut P382 Structure and Surface Properties of Zr02-Supported W03Nanostructures C.D. Baertsch, R.D. Wilson, D.G. Barton, S.L. Soled and E. Iglesia . P383 Proton Afinity in Heterogeneous Acid Base Catalysis. Measurements and Usefor Analysis of Catalytic Reaction Mechanism E.A. Paukshtis P384 Basic Sites on Mixed Nitrited Galloaluminiophosphates "AIGaPON" Confrontation of Spectroscopic and Catalytic Data S.Delsarte and P. Grange . P385 Study of Relevant properties injluencing the catalytic activity of layered double hydroxides in the Meixnerite-likeform F. Prinetto, G. Ghiotti and D. Tichit . P386 Copper Redox Chemistry in CuZSM-5 Zeolites: EPR, IR and DFT Investigations B. Gil, J. Datka, S. Witkowski, Z. Sojka and E. Broclawik . P387 Dynamics of Adsorbed Benzene on Ag-Y Zeolite Studied by Solid-State NMR A. GedCon, D.E. Favre and B.F. Chmelka . P388 Interaction of CO and NH3 with Noble Metal Cations Dispersed in ZSM-5 Zeolites. Spectroscopic and Microcalorimetric Investigation V . Bolis, S. Bordiga, V. Graneris, C. Lamberti, G. Turnes Palomino and A. Zecchina . P389 The Use of NMR Imaging and Mercury Porosimetry in the Modeling and Measurement of Coke Profiles in Deactivated Catalyst Pellets S.P. Rigby . P390 Low-loaded Metal Pd-Au Supported Catalysts on Active Carbon. Recent Developments of the X-Ray Dzji-action Analysis to Detect Simultaneously Nanoclusters and Larger Particles . P.Riello, P. Canton, A. Minesso, F. Pinna and A. Bennedetti.

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P39 1 Characterisation of Aukorbates and Surface Functional Groups on Polycrystalline Oxides by UltravioletPhotoelectron Spectroscopy (UPS) M. Heber and W.Griinert .

P392 Evidence of an Alloying Eflect in Zeolite Supported Pt-Pd Systems as Seen by Hidrogen Chemisorptionand Proton NMR T . Rades, M. Polisst-Thfoin and J. Fraissard. P393 Catalytic Probe ofAg Venters on Silver Halide and Supported Silver 1.1. Mikhalenko and V.D. Yagodovskii . P394 Change in the Redox Properties of PdRWCe02-ZrO2 Catalysts afier Ageing at 1323 and 1423 K S. Salasc, V. Perrichon, M. Primet, M. Chewier and N. Mouaddib-Moral . P395 XPS Study of AuITiO2 Catalytic Systems J.W. Sobczak and D. Andreeva . P396 Procedure-Sensitive HTR for Au/TiOz M.-Y. Lee, Y.-J. Lee and S.D. Lin . P397 Characterization of Amino-ModiJied Silicate Xerogels Complexed with Copper (IQ, Cobalt (III) and Chromnium (IIQ W.K. Jozwiak, E. Szubiakiewicz, A.M. Klonkowski, T. Widemik, W. Ignaczak and T. Paryjczak . P398 Active Sites on Well-CharacterizedPd/Si02 Determined by (-)-Apopinene Deuteriumation, Cyclohexene Hydrogenation and CS2 Titration G.V. Smith and D.J. Ostgard . P399 Structural and Catalytic Properties of Zr-Ce-0 Mixed Oxides. Role of the Anionic Vacancies S. Rossignol, C. Micheaud Especel and D. Duprez . P400 An X-Ray Photoelectron Spectroscopy Investigation of a-Alumina-Supported Nickel Catalysts Preparaedfiom Nickel(I~Acetylacetonate R. Molina, M. Genet and G. Poncelet P401 The Role of Oxide Promoters in the Dissociation of CO and its Reaction with Hydrogen on Pd (111) and Rh (I 1I): A Molecular Beam Study B. Klozter and K. Hayek . P402 Characterization of the Active Sites of Ni-Si-A1 Sol-Gel Hydrogenation Catalysts C. Guimon, N. El Horr, E. Romero and A. Monzon.

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Catalysis for Fine Chemical Synthesis

P403 A New Type of Anchored Homogeneous Catalyst R.L. Augustine, S.K. Tanielyan, S. Anderson, H. Yang and Y. Gao

.

P404 Regioselective Homogeneous Hydrogenation of Heteroaromatic Nitrogen as the Precatalyst Compounds by Use of [OSH(CO)(NCM~)~(PP~~~)Z]BF~ M. Rosales, F. Arrieta, J. Castillo, A. Gonzalez, J. Navarro and R. Vallejo P405 Catalyst Selection and Solvent EfSects in the Enantioselective Hydrogenation of 1-Phenyl-l,2-propanedione E. Toukoniitty, P. M&i-Arvela, A.Kalantar Neyestanaki, T. Salmi, A. Villela, R. Leino, R. Sjoholm, E. Laine, J. Vayrynen and T. Ollonqvist P406 Catalytic Asymmetric Hydrogenation and Hydroformylation Reactions with Chiral N or N,S Ligands M.L. Tomrnasino, M. Casalta, J.A.J. Breuzard, M.C. Bonnet and M. Lemaire P407 Heterogeneous Asymmetric Hydrogenation of Ethyl Pyruvate on Chirally modified Pt/A1203Catalyst with Fixed-Bed Reactor X. You, Xi. Li, S. Xiang, S. Zhang, Q. Xin, Xu. Li and C. Li . P408 EfSect of Coacid Acidity on the Cinchona-Mod$ed Pt-Catalized Enantioselective Hydrogenations B. Torok, K. Balkzsik, K. Felfdldi and M. Bartok . P409 Lactones 8. [I] Enantioselective Hydrolysis of y-Acetoxy-&lactones T. Olejniczak and C. Wawrzenczyk . P410 New Environmentally Friendly Base Catalystfor Enantioselective Reactions. Heterogenisation of Chiral Amines to USY and MCM-41 Zeolites M . Iglesias Hemhndez and F. Shnchez Alonso . P411 Selective Enzymatic Production ofAmide EmulsifiersJi.om Ethanolamine and Fatty Acids M . Femhndez Ptrez and C. Otero . P412 Rapid Solvent-Free Esterification of Conjugated Linoleic Acid and Glycerol in a Packed Bed Reactor Containing an Immobilized Lipase J.A. Arcos and C.G. Hill Jr. . P4 13 EtheriJication over a Novel Acid Catalyst R.S. Karinen, A.O.I. Krause, K. Ekman, M. Sundell and R. Peltonen P4 14 Esterification of Lauric Acid with Glycerol Using Modified Zeolite Beta as Catalyst M. da Silva Machado, D. Cardoso, J. Perez Pariente and E. Sastre .

.

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P415 Preparation of Alkylglucosides using Al-MCM-41 Molecular Sieves as Catalysts H.M.C. Andrade, A.R.E. Gonzaga and L.G. Aguiar .

3423

P4 16 Novel Supported Solid Acid Catalystsfor Environmentally Friendly Organic Synthesis K. Wilson, J.K. Shorrock, A. Renson, W. Hoyer, B. Gosselin, D.J. Macquarrie and J. H. Clark .

3429

P417 Catalytic Activity of Polymer-Anchored Pd(I4 SchlflBase Complexes: Hydrogenation of Styrene and Oxidation of Benzyl Alcohol M.K. Dalal and R.N. Ram .

3435

P418 Highly Promising Activity of Na$on/Silica Composite Catalyst in Acylation Reactions A. Heidekum, M.A. Harmer and W.F. Hoelderich .

344 1

P419 Alcohol Coupling to Unsymmetrical Ethers over Solid Acid Catalysts K. Klier, H.-H. Kwon, R.G. Herman, R.A. Hunsicker, Q. Ma and S.J. Bollinger 3447 P420 Gas versus Liquid Phase a-Pinene Transformation over Acid Molecular Sieves of Different Topolog~and Composition C.M. Lbpez, F.J. Machado, K. Rodriguez, D. Arias and M. Hasegawa .

3453

P42 1 Ruthenium-Catalyzed [2+2] Cross-Addition of Norbornene Derivatives and Dialkyl Acetylenedicarboxilates H. Suzuki, I. Hashiba, T.-A. Mitsudo and T. Kondo.

3459

P422 Shape-Selective N-Alkylation of Melamine Using Alcohol as a Alkylating Agent with RuIMordenite Catalyst in the Liquid Phase . S. Shinoda, K.-I. Inage, T. Ohnishi and T. Yamakawa

3465

P423 Diastereoselective Hydrogenation of a Prostaglandin Intermediate over Ru Supported on MCM-41 and MCM-48 S. Coman, R. Caraba, F. Cocu, V.I. Parvulescu, H. Bonnemann, B. Tesche, C. Danumah and S. Kaliaguine .

3471

P424 Generation of Uniformly Sized and Dispersed Copper Particles in New Cu-A1 Based Mesoporous Catalysts and their Role in Selective Hydrogenation of Conjugated Unsaturated Carbonyl Groups . S. Valange, Z. Gabelica, A. Derouault and J. Barrault P425 Selective Hydrogenation of Furfural on Ni-P-B Nanometals S.-P. Lee and Y.-W. Chen . P426 A Sulfur Based Catalytic Systemfor the Carbonylative Reduction and the Reductive Carbonylation . V . Macho and M. Kralik

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lix

P427 Rapid Synthesis of Ketenes From Carboxylic Acids on Functionalized Silica Monoliths R. Martinez, M.C. Huff and M.A. Barteau .

3495

P428 Nitroaldol Condensation Promoted by Organic Bases Tethered to Amorphous Silica and MCM-41-Type Materials F. Bigi, S. Carloni, R. Maggi, A. Mazzacani and G. Sartori.

3501

P429 Alcoholysis of Ester and Epoxide Catalyzed by Solid Bases H. Hattori, M. Shima and H. Kabashima . P430 Beckmann Rearrangement over Anionic Clays L. Forni, G. Fornasari, R. Trabace, F. Trifiro, A. Vaccari and L. Dalloro .

3513

C1 Chemistry P43 1 Partial Oxidation of Methane over Supported Nickel Catalysts A.M. Diskin and R.M. Ormerod .

3519

P432 Partial Oxidation of CH4 into Synthesis Gas on Ni/Perovskite Catalysts Prepared by SPC Method K. Takehira, T. Shishido. M. Kondo, R. Furukawa, E. Tanabe, K. Ito, S. Harnakawa and T. Hayakawa .

3525

P433 Methane Partial Oxidation in New Iron Zeolite Topologies P.P. Knops-Gerrits and W.J. Smith .

353 1

P434 Investigations of the Selective Partial Oxidation of Methanol and the Oxidative Coupling of Methane on Copper Catalysts H.-J. Wolk, A. Scheffler, G. Mestl and R. Schlogl .

3537

P435 Partial Oxidation and Chemisorption of Methane over Ni/A1203 Catalysts Ya. Chen, C. Hu, M. Gong, Yu. Chen and A. Tian .

3543

P436 Carbon Deposition on Ni/A1203 Catalyst during Partial Oxidation of Methane to Syngas Q.-G. Yan, Z.-S. Chao, T.-H. Wu, W.-Z. Weng, M.-S. Chen and H.-L. Wan

3549

P437 Mechanistic Studies of Methane Partial Oxidation to Synthesis Gas over Si02-Supported Rhodium Catalyst Z.-S. Chao, Q.-G. Yan, T.-H. Wu, W.-Z. Weng, H.-Q. Lin, L. Yang, J.-L. Ye, M.-S. Chen, H.-L. Wan and K.-R. Tsai .

3555

P438 Reaction Kinetics and Product Selectivity in the Oxidation of Methane over Pd/Si3N4 D. Whang, F. Monnet and C. Mirodatos .

3561

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P439 Sustainable Ni Catalystfor Partial Oxidation of CH4 to Syngas at High Temperature S. Liu, G. Xiong, H. Dong, W. Yang, S. Sheng, W. Chu and Z. Yu

3567

P440 Improvements of Ceria Promoted Nickel Catalystsfor Natural Gas Oxidation to Syngas W. Chu, Q. Yan, S. Liu and G . Xiong .

3573

P441 Partial Oxidation of Methane Catalyzed by Mo-Incorporated SBA-I T. Tatsumi, N . Hamakawa and L.X. Dai .

3579

P442 Kinetic and Mechanistic Study of the Partial Oxidation of Methane to Formaldehyde on Silica Catalysts . F. Arena, F. Frusteri, A. Mezzapica and A. Parmaliana

3585

P443 Two Step Synthesis of Methyl Formatefi.om CH4and Air via Formaldehyde: Surface Reactivity of Oxide Catalysts towards HCHO G. Martra, F. Arena, S. Coluccia, F. Frusteri, A. Mezzapica and A. Parmaliana 3591 P444 Oxidation of Methanol on Sodium Mod$ed Mo-Based Catalysts . K. Ivanov, S. Krustev, P. Litcheva and I. Mitov

3597

P445 Novel Rhenium Based Catalystsfor Direct Dehydroaromatization of Methane with CO/C02 towards Ethylene and Benzene R. Ohnishi, K. Issoh, L. Wang and M. Ichikawa .

3603

P446 H-ZMS-8 Supported Mo-Based Catalystsfor Methane Conversion under Non-oxidative Condition S . Li, C.-L. Zhang, Q.-B. Kan, D.-Y. Wang, Y. Yuan and T.-H. Wu

.

3609

P447 Methane Dehydro-aromatization and NO Adsorption on Mo/HZSM-5 W. Liu and Y.-D. Xu .

3615

P448 The Location, Structure and Role of MOO,and MoC, Species in Mo/H-ZSM5 Catalystsfor Methane Aromatization . W. Li, G.D. Meitzner, Y.-H. Kim, R.W. Borry and E. Iglesia

3621

P449 Oxygen-Free Conversion of Methane in the Presence of Intermetallic Hydrogen ~ b c e ~ t o r A. Sauvage, M. Mercy, H. Amariglio, J.-C. Gachon and P. Pareja .

3627

P450 Conversions of Substituted Methanes Over ZSM-Catalysts C.E. Taylor .

3633

P45 1 A Study of Coke Formation Kinetics by a Conventional and an Oscillating Microbalance on Steam Reforming Catalysts R. Lerdeng, D. Chen, C.K. Jakobsen and A. Holmen

3639

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P452 Methanol Reforming over CuO/ZnO under Oxidizing Conditions T.L. Reitz, S. Ahrned, M. Krumplet, R. Kumar and H.H. Kung .

P453 Methane COz Reforming over a Stable Ni/Al203 Catalyst A. Castro Luna, A. Becerra, M. Dimitrijewits and C. Arciprete

.

P454 Optimization of Ni and Ru Catalysts Supported on LaMn03for the Carbon Dioxide Reforming of Methane E. Pietri, A. Barrios, M.R. Goldwasser, M.J. Perez Zurita, M.L. Cubeiro, J. Goldwasser, L. Leclercq, G. Leclercq and L. Gingembre. P455 Metal Support Interaction on Pt/ZrOz Catalystsfor the C 0 2 Reforming of CH4 S.M. Stagg-Williams, R. Soares, E. Romero, W.E. Alvarez and D.E. Resasco P456 Methane Reforming with C02 over Ni/ZrO2-CeO2 and Ni/ZrO2-MgO Catalysts Synthesized by Sol-Gel Method J.A. Montoya, E. Romero, A. Monzon and C. Guimon . P457 Support Effect over Rhodium Catalysts during the Reforming of Methane by Carbon Dioxide P. Ferreira Aparicio, M. Femindez Garcia, I. Rodriguez Ramos and A. Guerrero Ruiz . P458 C 0 2Reforming of CH4over Mo Promoted Nickel-Based Catalysts C.E. Quincoces, S. Ptrez de Vargas and M.G. Gonzalez . P459 Stable Ni/Zr02 Catalystfor Carbon Dioxide Reforming of Methane J.-M. Wei, B.-Q. Xu, Z.-X. Cheng, J.-L. Li and Q.-M. Zhu. P460 Dependence ofActivity of ZrO2 Catalystsfor Isobutene Formation in CO Hydrogenation on the Phase Structure K-I. Maruya, T. Komiya and M. Yashima . P461 Long-chain Alcohols fiom Syngas A. Frennet, C. Hubert, E. Ghenne, V. Chitry and N. Kruse . P462 Preparation and Catalytic Properties of Copper-Ytterbium Oxide System for CO Hydrogenation Y. Sakata, S. Tsuchiya, N. Kouda, F. Takahashi and H. Imamura . P463 Mechanistic Studies of CO and C02 Hydrogenation to Methanol over a 50Cd45Zd5Al Catalyst by In Situ FT-IR, Chemical Trapping and Isotope Labelling Methods L.Z. Gao, J.T. Li and C.T. Au P464 Eflect of Nb2Os Addition to Co/A1203Catalyst on CO Hydrogenation Reaction 3717 F.T. Mendes, F.B. Noronha and M. Schmal..

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P465 Investigation of Alternative Support Materials for Monometallic and Bimetallic Catalystsfor CO Hydrogenation T. Biilbiil, A.I. Isli, A.E. Alsoylu and Z.I. dnsan .

3723

P466 Isotopic Transient Inffared Study of Reaction Pathway and Intermediates for CO Insertion and Hydrogenation S.A. Hedrick and S.S.C. Chuang .

3 729

P467 Relationship between Formation of DME, CH30Hand i-C4Hs in Isosynthesis . C.-L. Su, D.-H. He, J.-R. Li, B.-Q. Xu, Z.-X. Cheng and Q.-M. Zhu

3735

P468 CO Hydrogenation on RWAI-PILC: The Effect of the Metallic Precursor S. Mendioroz, B. Asenjo, P. Terreros, P. Salerno and V. Muiioz .

3741

P469 Enhancement of the Catalytic Performance to Methanol Synthesis @om C02/H2 by Gallium Addition to Palladium/Silica Catalysts A.L. Bonivardi, D.L. Chiavassa, C.A. Querini and M.A. Baltanas .

3747

P470 A Comparison of the Catalytic Performancefor Low-Temperature Methanol Synthesis in a Liquid Medium S. Ohyama .

3753

P471 A Novel Effect of Li Additive: Dynamic Control of Rh Mobility during C 0 2Hydrogenation Reaction . K.K. Bando, H. Arakawa, N. Ichikuni and K. Asakura

3759

P472 Structure and Characteristics of Supported Metal SPR Catalystsfor C02 Hydrogenation T. Uematsu, D. Li, N. Ichikuni and S. Shimazu .

3765

P473 The Nature of the Active Species on the Fe/MgO Used in the Combustion of Methane R. Spretz, S.G. Marchetti, M.A. Ulla and E.A. Lombardo .

3771

P474 Oxidation of Methane over Pt-Cr(Mo, W)/A1203Catalysts Z . Sarbak and S.L.T. Andersson . P475 Monolith Honeycomb Mixed Oxide Catalystsfor Methane Oxidation L.A. Isupova, V.A. Sadykov, G.M. Alikina, 0.1. Snegurenko, . S.V. Tsybulya, A.N. Salanov and V.A. Rogov P476 Catalytic Properties ofActive Phase of Glass Crystal Microespheres in the Reaction of Methane Oxidation A.G. Anshits, E.V. Kondratenko, E.V. Fomenko, A.M. Kovalev, O.A. Bajukov and A.N. Salanov .

3783

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Catalytic Methane Combustion over Alumina Supported Palladium Catalysts Prepared by Sol-Gel Method: Investigation of the Activity Evolution S . Fessi, A. Ghorbel, A. Rives and R. Hubaut . Study of PdO/Pd Transformation over Alumina Supported Catalystsfor Natural Gas Combustion G.Groppi, C. Cristiani, L. Lietti and P. Forzatti . Catalytic Activity of Tungsten Phosphate ( I v , (V), (VI) at Carbon Monoxide Oxidation V.V. Lesnyak, N.S. Slobodyanik, V.K. Yatsimirsky and N.A. Boldyreva . A Study of Pt/ZrOz Catalystsfor Water-Gas Shift Reaction in Presence of H2S E. Xue, M. O'Keeffe and J.R.H. Ross Reduction Requirementsfor Ru(Na)/Fe203 Catalytic Activity in Water Gas Shift Reaction W.K. Jozwiak, A. Basinska, J. Goralski, T.P. Maniecki, D. Kincel and F. Domka . Probing the Elementary Steps of the Water-Gas Shift Reaction over Cu/ZnO/AI203 with Transient Experiments . 0 . Hinrichen, T. Genger and M. Muhler Highly Selective and Highly Efficient Catalytic Conversion of Methane into Ethylene by the Oxidative Coupling . A. Machocki, A. Denis, J. Gryglicki and H. Mlynarska Oxidation of Methane to Methanol by Hydrogen Peroxide on a Supported Hematin Catalyst T.M. Nagiev and M.T. Abbasova . Polymerization and Organometallics

Investigation of a Cr/Silica Polyethylene Polymerisation Catalyst by In-situ Inpared Spectroscopy S.F. Parker, C.C.A. Riley and J.E. Baker . Supported (Cp)2 ZrCI2 as a Catalystfor Ethylene Polymerization S. Teixeira Brandao, M.L. dos Santns Correa, J.S. Boaventura, . and S. Carneiro Vianna On the EfSect ofAdding a Lewis Acid as a ModiJier of Metallocene Based Polymerization Catalysts P.G. Belelli, M.L. Ferreira and D.E. Darniani Surface Science Studies of Model Ziegler-Natta Polymerization Catalysts S.H. Kim, E. Magni and G.A. Somorjai .

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

P489 Ethylene Polymerization Catalyzed over Single Site Ni Complex Catalysts Supported on Silica D.S. Hong, T.S. Seo and S.I. Woo .

P490 DRIFTS, TPRS and NMR Investigations of Unsupported and Supported Pentamethylcyclopentadienyldiallyl-Complexes-Catalystsfor the Polymerization of Butadiene . H. Landmesser, H. Berndt, D. Miiller and D. Kunath P491 SchrfJBases Containing Metal ComplexesAnchored on Aerosil as Catalysts of Low-Temperature Ozone Descomposition T.L. Rakitskaya, A.A. Golub, A.A. Ennan, L.A. Raskola, V.Ya. Paina, A.Yu. Bandurko and L.L. Ped P492 Carbon Nanotubes-Supported Rh-Phosphine Complex Catalystsfor Propene Hydroformylation . H.-B. Zhang, Y . Zhang, G.-D. Lin, Y.-2. Yuan and K.R. Tsai P493 Model Rhodium Organometallic Complexes as Catalyst Precursors for CO Hydrogenation P. Terreros, R. Fandos, M. L6pez Granados, A. Otero, S. Rojas and M.A. Vivar Cerrato . P494 An OrganometallicApproachfor Tin Promotion Enhancement over Pt/y-A1203 Alkane Dehydrogenation Catalysts G.F. Santori, M.L. Casella, G.J. Siri, O.A. Ferreti and J.L.G. Fierro P495 Il9sn Mossbauer Study and Catalytic Properties of Magnesia-Supported Platinum-Tin Catalysts Prepared by Surface Organometallic Chemistry L. Stievano, F.E. Wagner, S. Calogero, S. Recchia, C. Dossi and R. Psaro. AUTHORINDEX

.

SUBJECT INDEX

.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

In Situ Characterization of Catalysts Henrik Topsoe Haldor Topsoe Research Laboratories, DK-2800 Lyngby, Denmark

Abstract The state of a catalyst depends intimately on the conditions under which the catalyst operates. Such dependencies are often related to adsorbate induced relaxations or reconstructions, and small variations in conditions may result in dramatic changes in the catalyst structure. As a consequence, characterization studies should ideally be carried out in situ during catalysis and studies performed in the absence of the reactants and products may yield limited insight. This present paper will discuss some recent developments of in situ methods. Some examples will be given to illustrate how the access to in situ information may greatly facilitate the understanding of the catalysis and contribute to more rational design of catalysts. 1. INTRODUCTION In order to perform rational catalyst research, it is highly desirable to have atomic-scale information on the state of the catalyst inside the catalytic reactor. With such information, one has the possibility to establish structure-activity relationships, which can provide useful guidelines for rational catalyst research and developments. Without such insight one is very much left with treating the reactor as a black box. Heterogeneous catalysts are typically very complex solids and a multitude of structural features may coexist at the atomic-scale. In order to characterize catalysts, it is therefore a goal to find suitable techniques, which can provide detailed chemical and structural information for such systems. Besides the intrinsic complexities discussed above, one also has the added complexity that the catalyst structures may change dynamically depending on the reaction conditions. This is typically related to adsorbate induced restructuring of the catalysts, and it has been observed that even small changes in the environment may result in dramatic changes in the structures. Clearly, the state of the catalyst inside the reactor will generally be quite different from that after removal from the reactor. It is therefore important that the techniques employed in catalyst studies can both deal with the intrinsic complexities and also provide insight under in situ conditions. Over the years, great efforts have been devoted to develop methods, which allow in situ studies (see e.g. the reviews and conference proceedings (1-14)). In spite of this, many important techniques used in catalyst research are not suited for studies at the high pressures and temperatures encountered in many catalytic reactions. In view of these difficulties, simplifications have often been adopted. Thus, the investigations termed in situ in the literature often refer to studies carried out under conditions, which deviate from those encountered during catalysis. Frequently, the characterization is performed on the catalyst

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

atter its transfer from the reactor to an analysis station. For example, in order to utilize many of the surface sensitive spectroscopies, the investigations are typically carried out after transfer of the catalyst to UHV conditions. Although, important insight may be obtained from such studies (see sec. 3), the state of the catalyst after transfer may, as discussed above, be quite different from that existing during catalysis. The differences may not only be limited to simple differences in the coverage of adsorbates, but may involve complete surface and bulk structural changes. The following sections will give several examples, which illustrate the dynamic nature of catalysts and the need to carry out in situ studies. Some experimental requirements and new possibilities for performing combined catalysis and in situ on-line characterization studies will also be discussed. The range of applications is illustrated using a few selected studies of catalysts for ammonia synthesis, methane conversions, hydrotreating, DeNOx and methanol synthesis. The present paper will not attempt to give a complete overview of all in situ techniques. In fact, many important techniques, like Raman spectroscopy, isotopic tracing, microscopy, NMR, SAXS/WAXS, ASAXS, gravimetry and SFG, will not be discussed here. For these topics the reader is referred to other recent papers and reviews (11-25). 2. ADSORBATE INDUCED RECONSTRUCTION Although reconstruction of surfaces by adsorbates has been known for a long time, in situ insight for real catalyst systems has been limited. One of the first techniques, which provided in situ information about such phenomena, was M6ssbauer spectroscopy (1,3). Studies using this technique revealed, for example, that the surface composition of alloy catalysts (26,27), the surface structure of ammonia synthesis catalysts (28-30) and the magnetic properties of supported catalysts (31,32) depend critically on the reaction environment. Scanning Tunneling Microscopy (STM) is one of the newer techniques, which has provided detailed atomic scale insight into adsorbate induced restructuring. The studies show that structural relaxations and reconstructions are far more common and complex than often assumed in the past. Figure 1, taken from the work of Ruan et al. (33), illustrates some of the complexities, one encounters during the reaction of H2S with preadsorbed oxygen on a Ni(110) surface. It is observed that the starting Ni surface has reconstructed upon the exposure to oxygen (Fig. 1a) and as the reaction proceeds, a significant surface roughening takes place and islands are also formed. The final surface (Fig. l f), which only contains sulfur, is reconstructed again. It is interesting that the structure of this surface is different from that observed upon direct exposure of the Ni (110) surface to H2S under the same conditions. Thus, the initial reconstruction caused by oxygen activates the Ni surface for new reaction pathways, and this leads to the production of structures which otherwise would not have formed. The above results also suggest that kinetic treatments using for example Langmuir-Hinshelwood mechanisms may provide too simplified descriptions of complex catalytic reactions. Ertl has shown that due to time and spatial dependencies of the surface reconstructions, unusual reaction behaviors (oscillations and chaos) may be observed (34). The interesting effect of oxygen reconstruction on the final structure of the S on Ni (110) may have analogies to effects observed on real catalysts. For example, it was observed (28) that an ammonia treatment of a reduced Fe/MgO catalyst prior to exposure to a H2:N2 synthesis gas gave rise to special highly active surfaces, which could not be produced by direct exposure of the reduced catalyst to synthesis gas. In a recent study, Nerlov and Chorkendorff (35) presented a nice example, which demonstrated that the presence of molecules, which do not participate in the catalytic

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i Fig. 1. STM images of the reaction of H2S with preadsorbed oxygen on Ni(110). The images are taken after progressively higher exposures to H2S. (a) O L, (b) 3 L, (c) 8 L, (d) 20 L, (e) 25 L, (f) 35 L. (a)-(e) recorded on a 85x91A2 area. (f) shows an area of 59x63A2. According to Ref. (33). cycle, may still alter the catalytic properties dramatically through adsorbate induced reconstructions/segregations. Multicomponent catalyst systems (supported catalysts, alloys, mixed oxides, promoted catalysts, etc.) are expected to exhibit even more complex behaviors, since an even larger variety of adsorbate interactions may exist. The Cu catalyst system discussed in sec. 7 illustrates that dynamic changes may involve structural changes in the support as well as changes in the surface and bulk structure of the metal. All the above-mentioned studies clearly demonstrate the necessity to perform in situ studies on actual operating catalysts and it is suggested that many of the controversies existing in the literature regarding the structure and performance of catalysts are related to the lack of in situ insight. 0

COMBINED SURFACE SCIENCE AND CATALYSIS STUDIES. AMMONIA SYNTHESIS OVER Ru

In order to elucidate the role of different surface structures in catalysis, it may be attractive to place a high-pressure reactor inside a typical surface science UHV chamber (Fig. 2). In this way one can perform catalysis experiments using well-defined starting surfaces like single crystals. By transferring the catalyst from the reactor to the UHV system, one can directly perform post analysis. Furthermore, one may also modify the surface in the UHV system before doing new catalysis experiments. There has already been

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Fig. 3. Arrhenius plot of the measured thermal sticking coefficients of N2 on a clean Ru(0001) surface and after depositing 0.01-0.02 ML of gold on the same surface. According to Ref. (36).

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DFT calculations

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Experimental output (% NH 3) Fig. 4. Top panel: Energy barriers for N2 dissociation on terrace sites and step sites on the Ru(0001) surface obtained by DFT calculations. According to Ref. (36). Bottom panel: The ability of a microkinetic model to predict the NH3 production over a supported Ru catalyst and a Ru single crystal surface for a wide range of conditions. According to Ref. (37).

many successful applications of this approach. However, such studies - and others requiring sample transfer - are of course still not proper in situ studies, and it is not certain to what extent, the surfaces characterized before and after transfer resemble those present during the catalysis. Nevertheless, many useful features may be addressed and the recent studies by Dahl et al. (36,37) demonstrate nicely the special advantages of this approach. They used the above type set-up to study the N2 dissociation on Ru(0001) and examined to what extent, the presence of small amount of steps may play a role. The results shown in the top half of Fig. 3 are data for the clean Ru(0001) surface. The observed activation energy is much lower than that predicted from DFT calculations on this surface

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

(Fig. 4). On a clean Ru(0001) surface one may always have up to one percent of steps and in order to estimate the possible role of such step sites, the N2 dissociation experiment was repeated after such sites were "blocked". This could be done by depositing a few percent of a monolayer of gold, which is known to preferentially decorate the steps. This resulted (Fig. (3)) in a many order decrease in the activity. The large difference in activation energy for N2 dissociation (the rate-limiting step in NH3-synthesis) is in-line with that predicted by DFT calculations (Fig. 4). It can be estimated that the rate on the (0001) surface is about 9 orders of magnitude smaller than that on the step sites. This is probably the largest measured difference in structure sensitivity, and it is clear that for Ru, the catalysis will be dominated by the presence of a few sites resembling the step sites. The insight was used to construct a microkinetic model which can account for the reactions both over single crystals and supported Ru catalysts (Fig. 4). It is likely that other reactions reported in the literature may also have been influenced by small concentration of step, edge or defect sites, and this old topic needs to be reexplored further. It is indeed the goal of many in situ studies to obtain information about the fraction of the total amount of surface sites, which are responsible for the catalysis. Studies discussed in subsequent sections also show that for many systems the active sites may only represent a small fraction of the total concentration of surface sites.

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家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

4. EXPERIMENTAL CONSIDERATIONS

In view of the strong dependence of the catalyst structure on the environment, catalyst characterizations should ideally be done under the exact conditions of the catalysis, and in order to relate the obtained information to the catalytic activities, it is furthermore desirable to perform simultaneous activity and characterization studies. However, many sample cells used in catalyst studies have geometry, which is not ideal for in situ studies (9). Examples of typical XRD cells are shown in Figure 5. Such cells may provide good quality diffraction data, but one will typically have large temperature and concentration gradients and poorly defined conditions at the position of the catalyst. Many designs for in situ cells for other techniques also suffer from such problems. In order to circumvent these difficulties, we have aimed at adapting the same geometry in the in situ cell, as that encountered in a plug flow reactor (38,39). An attractive solution based on miniaturization is shown in Figure 6 (39). This simple design facilitates simultaneous spectroscopy and on-line activity measurements. This approach has been used for in situ XRD (39) and combined XRD/QEXAFS experiments (40) but can also be adapted to other techniques. In order to obtain sufficient penetration of the radiation, thin capillary reactors were utilized. These also allow studies to be carried out under high pressures. The small mass of the reactor makes it ideal for transient studies and such capillary reactors have, for example, been used in TPR, TPS or transient catalysis studies. In Ref. (39), it was shown that good catalytic activity measurements can be obtained in the in situ cell. X-ray diffraction is probably the most widely used technique in catalysis R&D for obtaining structural information. Beside the normal angle-dispersion mode, the energydispersive mode can also be used and it has certain advantages for small particle and in situ studies (41). Nevertheless, it is important to note that XRD is not sensitive to structures, where the dimension of order is less than about 2 nm. This is a serious limitation since in catalysis research, it is often the goal to prepare highly dispersed catalysts with dimensions less than this value. For such systems, EXAFS has become an indispensable tool (4,5,9,10) but this technique also has several limitations since it is a local environment technique. It is clear that XRD and EXAFS in many respects provide complimentary information, and it is therefore desirable to combine these two methods in order to obtain a more complete structural description of catalysts. An attractive solution is the combined XRD/QEXAFS technique (Fig. 7)

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developed by Clausen et al. (40). By use of a position sensitive detector, the complete XRD pattern and fast EXAFS data are recorded simultaneously. Using the Quick EXAFS mode, the time resolution is about 0,01 to 2 s on a routine basis. Recently, time resolutions of about lms have been demonstrated using a so-called Piezzo EXAFS mode (42). The combined XRD/QEXAFS set-up (Fig. 7) also uses the in situ cell described above for simultaneous reaction measurements. e-

Synchrotron radiation source

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DEACTIVATION Heterogeneous catalysts are non-equilibrium solids and their structures depend critically on the choice of preparation and activation parameters. In situ studies can also be used to obtain information about such parameters. For example, a small angle x-ray scattering (SAXS) has been used to obtain information about nucleation and crystallization processes (25). Figure 8 illustrates how the combined XRD/QEXAFS technique can be used to elucidate the processes occurring during the reduction of the precursors to a Cu-based methanol synthesis catalyst (40). Product analysis was monitored by simultaneous gas analysis. In situ studies may also be used to examine whether certain catalyst design strategies have resulted in the desired structures in the final catalyst. For example, in situ EXAFS was recently used to test whether one could succeed in preparing small Ni-Au catalysts particles, having the Au atoms present as a surface Ni-Au alloy (43). The starting point in this project was the observation by STM of the existence of a Ni-Au surface alloy in single crystal model

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

systems (44). The STM picture indicates Figure 9 that the Ni atoms neighboring the Au atoms have altered electronic properties. Subsequent DFT calculations and molecular beam experiments (43) predicted, that these atoms should exhibit different properties for activating the methane molecule compared to pure Ni. In view of these results, preparation of porous, high surface area catalysts containing small Au-Ni nano-crystals with the same surface structures was initiated. In situ EXAFS studies played an essential role in examining the results of different preparation strategies (43,45). This was important, since many procedures failed due to, for example, segregation of separate Au particles. Figure 9 shows the EXAFS spectrum of a preparation yielding high amounts of the desired small Ni-Au nano-particles. In situ studies may also be used to elucidate processes occurring during catalyst deactivation. Sintering is one of the important processes leading to catalyst deactivation. Figure 10 shows the result of in situ XRD studies of the sintering behavior of two Cu-based catalysts (46). The insight gained from such studies can be used to elucidate the sintering mechanisms and aid in the development of catalysts in which sintering is minimized. It is

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6. I n situ FTIR STUDIES OF V/TiO2 DeNO,, CATALYSTS. REACTION INTER-

MEDIATES AND SPECTATORS Infrared spectroscopy is one of the most important techniques in catalysis research, and it has been used extensively to provide information about surface functional groups and adsorbed species. Recently, combined in situ FTIR and on-line reaction studies were performed to elucidate the SCR deNOx reaction over V/TiOz catalysts (48-51). It was found very useful to perform both steady state and transient studies. By comparing the mass spectrometric analysis of the reactants and products with the FTIR information (one example is shown in Figure 11, the changes in reaction rates could be related to the changes in the concentration of different surface sites and adsorbed species. Thus, combining the information from several such in situ on-line experiments, one could distinguish between the active reaction intermediates and spectator species, which were present in quite large amounts, but did not contribute significantly to the catalysis. It is a common problem in catalysis research to distinguish between reaction intermediates and spectator species, and simple adsorption experiments do not allow this discrimination. Some of the FTIR studies mentioned above also showed that Temperature Programmed Surface Reaction (TPSR) studies may also have difficulties in distinguishing between reaction intermediates and spectator species. Specifically, it was observed in some TPSR experiments that NO could react with certain adsorbed ammonia species to yield the deNOx reaction products (49,50). Nevertheless, these species should still be regarded as spectator species since the FTIR measurements showed that the species are not involved in the catalytic cycle under steady-state catalysis. The above studies demonstrate the variety of insight, which can be gained from in situ on-line studies. The studies formed the basis for the

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Fig. 11. In situ FTIR (a) and on-line mass spectrometry (b) results obtained during TPSR deNOx experiments. (c) Catalytic cycle for the SCR deNOx reaction over V/TiO2 catalysts. According to Refs. (48, 49) development of a catalytic cycle for the deNOx reaction (Fig. 11) (48) and for the formulation of a microkinetic model which can describe the industrial operation (50).

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situ STUDIES OF THE DYNAMICAL BEHAVIOR OF Cu/ZnO BASED M E T H A N O L SYNTHESIS CATALYSTS

7. In

Recent in situ studies of Cu/ZnO based methanol synthesis catalysts have revealed that structural changes may occur as a result of small changes in reaction conditions (52-55). It has furthermore been shown that the structural changes are not restricted to the surfaces, but involve complete changes in the morphology of the Cu particles. The insight gained from the in situ studies has allowed the formulation of dynamic microkinetic models for the industrial performance. Some of the recent studies will be discussed below. Previous in situ EXAFS studies of Cu/ZnO based catalysts showed that Cu is present as metallic Cu particles (56). In recent years, there has therefore been an interest in exploring to what extent, surface science information obtained on single crystal Cu surfaces may be used as a basis for formulating a microkinetic model capable of explaining the industrial macrokinetics. Using results from Maddix's and Campbell's groups and their own results, Chorkendorff, Stoltze, Norskov and co-workers formulated a microkinetic model including both the water gas shift reaction and methanol synthesis (57,58). It was shown that such a model could account for many of the observed laboratory pilot plant results, and the model also gave an adequate quantitative description of the observed rates. Nevertheless, the model did not account for all the apparent reaction orders and the presence of special transient phenomena for the ZnO containing catalysts (52, 54). Furthermore, when analyzing the results of the microkinetic modeling of the industrial rates on Cu/ZnO/A1203 catalysts, it was noticed (see Fig. 12) that in the most reducing synthesis gas mixtures, the measured rates were larger than those predicted by the microkinetic model (53). The opposite was the case for the experiments using more oxidizing synthesis gases. The different observations suggest that the catalysis cannot be explained completely by only considering a stationary copper metal component in the catalyst. Specifically, it appeared that ZnO also plays a key role. The results in Figure 12 indicate that the state of the catalysts may depend quite sensitively on the reduction potential of the synthesis gas. Detailed insight into this feature was obtained by the

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interfase, and this results in increased interaction with the copper particles and a tendency to form non-spherical Cu particles with lower average coordination numbers and higher surface area. The insight gained from the in situ studies led to the development of a semi-quantitative dynamic microkinetic model (53), and it provided a much better basis for explaining the steady state and transient kinetic data (53,54). Recent infrared studies (59) and experiments performed on different systems (60-62) indicate, that the Cu/ZnO may be even more complex and several questions need to be addressed (e.g. the role of surface alloys). Nevertheless, it is likely that further progress will continue to be dependent on the development and application of in situ methods. 12 10 = l

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Number of Co atoms present as Co-Mo-S (x1020/g catalyst) Fig. 15. Top panel" DFT calculations of the active Co-Mo-S structure ((a) top view, (b) side view). According to Ref. (67). Bottom panel: Correlation between the HDS activity and the number of Co atoms present in Co-Mo-S as determined by in situ MES. According to Refs. (63, 65). of the promoter atoms present in the so-called Co-Mo-S structure (Fig. 15)(63,65). This structure can be considered as Co located at the edges of small nano-clusters of MoS2 (63,66,67). Important insight into these structures has subsequently been obtained by combining the MES studies with in situ studies using other techniques such as XAFS, FTIR, EPR etc. (63). It is evident from all these studies that the catalytic activity is related to the edge CoMo-S structures in promoted catalysts and to the edge MoS2 structures in unpromoted catalysts. This in situ insight has provided important guidelines for many subsequent studies, and it is clear that a more detailed understanding of hydrodesulfurization requires atomic-scale

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insight into the structure of the edges and the catalytic processes occurring there. Recent DFT calculations are interesting in this respect, since these are now capable of treating a variety of realistic edge structures (66-68)(see also Fig. 15). Recently, it has also been possible for the first time to obtain direct atom-resolved STM images of the MoS2 edges. By choosing the herringbone reconstructed Au (111) surface as a template, Helveg et al. (69) succeeded in preparing small single layer nano-crystals of MoS2 (Fig. 16). The nano-crystals are single layer thick and this allowed the recording of atom-resolved STM images of the active edge structures of MoS2. It is apparent from the images that the structure of the edges is reconstructed with respect to that of the bulk. This new insight is interesting since it implies that the edge structures may be quite different from those typically assumed in most studies. In order to get further insight into the nature of the active sites, the sulfided nanocrystals shown in Figure 16 were also treated with atomic hydrogen and this resulted in the creation of vacancies at the MoS2 edges. The observation of such sites is interesting, since in the literature (63) it is commonly assumed that vacancies are the active sites for hydrodesulfurization, but direct evidence of the nature of such sites has been lacking. At present, the conditions during the STM experiments are quite different from those during catalysis, and it is therefore uncertain if the structures observed under the present conditions are typical for an active catalyst. Nevertheless, developments in STM are occurring rapidly

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and measurements under higher pressures and temperatures have become feasible. It should also be possible to carry out studies with more realistic supports. In this connection it is interesting that Hojrup Hansen et al. (70) recently have been able to record atomic resolved images for Pd clusters deposited on thin A1203 films (Fig. 17).

Fig. 17. An atom-resolved STM image of a small Pd cluster on a thin aluminum oxide film. According to Ref. (70). 9. CONCLUSION The possibilities of obtaining detailed in situ information about the state of a catalyst and the processes occurring inside a reactor have had an important impact on catalyst research and developments. Thus, today the catalytic reactor no longer has to be regarded as a black box and this opens new opportunities for moving away from the trial-and-error era. Due to dynamic reconstructions, the state of catalyst during reaction may be quite different from that present without exposure to reactants and products. Also, small changes in reaction conditions may result in quite dramatic structural changes, and the structure of a catalyst'may change with the position in the reactor. All these effects further emphasize the need to carry out in situ studies. The present paper discussed some of the novel in situ on-line techniques, which can address the complex problems encountered in catalyst research. The paper also demonstrated that it can be very beneficial to employ multidisciplinary approaches and integrate in situ studies into research efforts that also involve theoretical calculations and surface science experiments. ACKNOWLEDGEMENTS The author would like to acknowledge the many colleagues and collaborators involved in the research discussed in this paper. Particular thanks go to Nan-Yu Topsoe, Bjeme Clausen, Soren Dahl, Jan-Dierk Griinwaldt, Charlotte Ovesen, Ib Chorkendorff, Stig Helveg

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and Flemming Besenbacher for comments and assistance during the preparation of the manuscript. REFERENCES

1. J.A. Dumesic and H. Topsoe, Advances in Catalysis 26 (1977) 121. 2. K. Tamaru, Dynamic Heterogeneous Catalysis, Academic Press, 1978. 3. H. Topsoe, J.A. Dumesic, and S. M~rup: "Applications of Mrssbauer Spectroscopy", (R.L. Cohen, Ed.), Vol. II, 55 (1980). 4. F.W. Lytle, H. Via and J.H. Sinfelt, in "Synchrotron Radiation Research" (H. Winick and S. Doniach, eds.), Plenum, New York (1980) chapter 12 5. P. Gallezot in "Catalysis Science and Technology" (J.R. Anderson and M. Boudart, eds.), Springer-Verlag, Berlin, vol. 5 (1984). 6. In situ Methods in Catalysis, Catal. Today 9, No. 1-2 (1991) 1-236. 7. J.W. Niemantsverdriet, Spectroscopy in Catalysis, VCH 1993. 8. Frontiers in Catalysis: Ammonia Synthesis and Beyond (H. Topsoe, M. Boudart, and J.K. Norskov, eds), Topics in Catalysis, Vol. 1, No. 3,4, (1994). 9. B.S. Clausen, H. Topsoe, R. Frahm, Advances in Catalysis 42 (1998) 315. 9. R. Prins and D.C. Koningsberger, in X-ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS, and XANES, (D.C. Koningsberger, and R. Prins, Eds.) Wiley, New York, (1998), 321. 10. E.G. Derouane, S.B. Derouane-abd Hamid, I.I. Ivanova, H. He, and J.C. Vedrine, Combinatorial Catalysis and High Throughput Catalyst Design and Testing (E.G. Derouane, C. Adams, F. Ramoa Ribeiro, A, Corma, Eds). (1999), 37. 11. J.C. Vedrine, and E.G. Derouane, Combinatorial Catalysis and High Throughput Catalyst Design and Testing (E.G. Derouane, C. Adams, F. Ramoa Ribeiro, A, Corma, Eds). (1999), 63. 13 Catalyst Characterization under Reaction Conditions, Topics in Catalysis, Vol. 8 (1999) No. 1,2. 14. G.A. Somorjai, CaTTech, Vol. 3, No. 1 (1999) 84. 15. R.T.K. Baker, Catal. Rev. Sci. Eng. 19 (1979) 161. 16. F. Herschkowitz and P.D. Madiara, Ind. Eng. Chem. Res. 32 (1993) 2969-2974. 17. S.C. Fung, C.A. Querini, K. Liu, D.S. Rumschitzki, T.C. Ho, Stud. Surf. Sci. and Catal. Vol. 88 (1994) 18. D. Chen, H.P. Rebo, K. Moljord, and A. Holmen, Chem. Eng. Sci. Vol. 5, No. 11 (1996) 2687. 19. S. Xie, G. Mestl, M.P. Rosynek, J.H. Lunsford, J. Am. Chem. Soc. 119, No. 42 (1997) 10186. 20. F. Eisert and A. Rosrn, Surface Science 377-379 (1997) 759. 21. W. Zhu, J.M. van de Graaf, L.J.P. van den Broecke, F. Kapteijn, and J.A. Moulijn, Indu. Chem. Res. Vol. 37 (1998) 1934. 22. X. Gao, M.A. Banares, I.E. Wachs, J. Catal. 188 (1999) 325. 23. P.L. Gai, Topics in Catal. 8 (1999) 97. 24. F. Berg Rasmussen, A.M. Molenbroek, B.S. Clausen and R. Feidenhans'l, J. Catal. (in press). 25. P.-P.E.A. de Moor, T.P.M. Beelen. B.U. Komanschek, O. Diat, R.A. van Santen, J. Phys. Chem. B 101 (1997) 11077. 26. C.H. Bartholomew and M. Boudart, J. Catal. 29 (1973) 278.

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27. P.H. Christensen, S. Morup, B.S. Clausen, and H. Topsoe, in "Proc. 8th Int. Congr. Catal.", Verlag Chemie: Weinheim, 1984, Vol. II, 545. 28. J.A. Dumesie, H. Topsoe, S. Khammotana and M. Boudart, J. Catal. 37 (1975) 503. 29. J.A. Dumesie, H. Topsoe and M. Boudart, J. Catal. 37 (1975) 513. 30. M. Boudart, H. Topsoe, and J.A. Dumesie, in "The Physical Basis for Heterogeneous Catalysis" (E. Draughlish & R.I. Jaffee, Eds.) pp. 337, Plenum Press, New York (1975). 31. B.S. Clausen, S.Morup and H. Topsoe. Surface Science 82 (1979) L589 32. S. Morup, B.S. Clausen and H. Topsoe. J. de Physique, 41 (1980) C1-331 33. L. Ruan, F. Besenbaeher, I. Stensgaard and E. L~egsgaard, Phys. Rev. Lett. 69 (1992) 3523. 34. G. Ertl, Topics in Catalysis 1 (1994) 305. 35. J. Nerlov and I. Chorkendorff, J. Catal. 181 (1999) 271 36. S. Dahl, A. Logadottir, R.C. Egeberg, J.H. Larsen, I. Chorkendorff, E. T6mqvist and J.K. Norskov, Phys. Rev. Lett. 83 (1999) 1814. 37. S. Dahl, J. Sehested, C.H.J. Jacobsen, E. T6mqvist and I. Chorkendorff, Submitted to J. Catal. 38. B.S. Clausen and H. Topsoe, Catalysis Today 9 (1991) 189 39. B.S. Clausen, G. Steffensen, B. Fabius, J. Villadsen, R. Feidenhans'l, and H. Topsoe, J. Catal. 132 (1991) 524. 40. B.S. Clausen, L. GrAb~ek, G. Steffenen, P.L. Hansen, and H. Topsoe, Catal. Let., 20 (1993) 23. 41. L. Gerward, S. Morup, and H. Topsoe, J. Appl. Phys. 47 (1976) 822. 42. H. Bomebusch, B.S. Clausen, G. Steffensen, D. Liitzenkirchen-Hecht and R. Frahm, J. Synchrotron Rad. (1999). 6, 209 43. F. Besenbacher, I. Chorkendorff, B.S. Clausen, B. Hammer, A.M. Molenbroek, J.K. Norskov and I. Steensgaard, Science 279 (1998) 1913. 44. L.P. Nielsen, F. Besenbacher, I. Stensgaard, E. La~gsgaard, C. Engdahl, P. Stoltze, K.W. Jacobsen and J. K. Norskov, Phys. Rev. Lett. 71 (1993) 754 45. A.M. Molenbroek and B.S. Clausen, unpublished results 46. L .GrAbaek, B.S. Clausen, G. Steffensen, H.Topsoe, Mat. Sci. Forum 133-136 (1993) 255 47. H. Kn6zinger and E. Taglauer, A Specialist Periodical Report, Catalysis, 10, (1993), 1 48. N.-Y. Topsoe, Science, Vol. 265 (1994) 1217 49. N.-Y. Topsoe, H. Topsoe and J.A. Dumesic, J. Catal 151 (1995) 226. 50. N.-Y. Topsoe, J.A. Dumesie and H. Topsoe, J. Catal 151 (1995) 241. 51. N.-Y.Topsoe, CaTTech, Vol. 1 (1997), No. 2, 125. 52. B.S. Clausen, J. Schie~z, L. GrAbsek, C.V. Ovesen, K.W. Jacobsen, J.K. Norskov and H. Yopsoe. Topics Catal. 1 (1994) 367 53. C.V. Ovesen, B.S. Clausen, J. Schi~z, P. Stolze, H. Topsoe and J.K. Norskov. J. Catal. 168 (1997) 133 54. H. Topsoe, C.V. Ovesen, B.S. Clausen, N.-Y. Topsoe, P.E. Hojlund Nielsen, E. T6rnqvist, J.K., Stud. Surf. Sci. Vol. 109, (1997), 121. 55. J.D. Grunwaldt, A.M. Molenbroek, B.S. Clausen, C.V. Ovesen, N.-Y. Topsoe and H. Topsoe. To be submitted. 56. B.S. Clausen, B. Lengeler, B.S. Rasmussen, W. Niemann and H. Topsoe, J. Phys. (Paris) C8 (1986) 237 57. P.B. Rasmussen, P.M. Holmblad, T.S. Askgaard, C.V. Ovesen, P. Stoltze, J.K. Norskov and I. Chorkendorff. Catal. Lett. 26 (1994) 373 58. T.S. Askgaard, J.K. Norskov, C.V. Ovesen and P. Stoltze. J. Catal. 156 (1995) 229

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59. N.-Y. Topsoe and H. Topsoe, J. Mol. Catal. A 141 (1999) 95 60. T. Fujitani and J. Nakamura. Catal. Lett. 56 (1998) 119 61. M.M. Viitanen, W.P.A. Jansen, R.G. van Welzenis, H.H. Brongersma, D.S. Brands, E.K. Poels and A.Bliek. Phys. Chem. B 103 (1999) 6025 62. E.K. Poels and D.S. Brands, Appl. Catal. A 4816 (1999) 1 63. H. Topsoe, B.S. Clausen, and F.E. Massoth, "Hydrotreating Catalysis: Science and Technology", in Catalysis, Science and Technology (eds. J.R. Anderson and M. Boudart), Springer Verlag, Vol. 11 (1996). 64. H. Topsoe, B.S. Clausen, R. Candia, C. Wivel and S. Momp, J. Cat. 68 (1981) 435. 65. C. Wivel, R. Candia, B.S. Clausen, S. Momp and H. Topsoe, J. Catal. 68 (1981) 453. 66. L.S. Byskov, B. Hammer, J.K. Norskov, B.S. Clausen, H. Topsoe, Catal. Lett.,47 (1997) 177. 67. L.S. Byskov, J.K. Norskov, B.S. Clausen, and H. Topsoe, J. Catal. 187 (1999) 109. 68. P. Raybaud, J. Hafner, G. Kresse and H. Toulhoat, Phys. Rev. Lett. 80 (1998) 1481 69. S. Helveg, J.V. Lauritsen, E. L~egsgaard, I. Steensgaard, J.K. Norskov, B.S. Clausen, H. Topsoe, and F. Besenbacher, Phys. Rev. Lett. 84(2000)951. 70. K. Hojrup Hansen, T. Worren, S. Stempel, E. L~egsgaard, M. B/iumer, H.-J. Freund, F. Besenbacher, I. Stensgaard, Phys. Rev. Lett. 83, No. 20 (1999) 4120.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Air Pollution Abatement through Heterogeneous Catalysis Masakazu Iwamoto*

Research Laboratory of Resources Utilization, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama 226-8503, Japan Recem progresses in catalytic technologies for preserving clean air have been reviewed. The major part of this paper was devoted to the novel catalytic removal of nitrogen monoxide from exhausts, especially the selective catalytic reduction of NO with hydrocarbons in the presence of excess oxygen. At present there are two suggestions for the NO reduction. First is the development of a catalyst which can yield N2 selectively in the continuous flow of mixture of NO, oxygen, and hydrocarbons. Second is the separation of oxidation process of NO to NO2 with oxygen and subsequent reduction process of NO2 to N2 with hydrocarbons. In the first strategy the catalytic activities, durability, and characterization of Cu- and Fe-zeolites were summarized. In the second one the storage-reduction method and the intermediate addition of reductant have been introduced.

I. Introduction The use of catalytic processes in pollution abatemem and resource recovery is widespread and of significant economic importance for the realization of sustainable chemistry/industry [1]. As has widely been recognized, there are following five classes as environmemally benign catalyses. (1) Control of emissions of environmentally unacceptable compounds, especially in flue gases and car exhaust gases. (2) Conversion of solid or liquid waste into environmentally acceptable products. (3) Selective manufacture of alternative products that can replace environmemally harmful compounds, such as some chlorofluorocarbons (CFCs). (4) Replacement of environmentally hazardous catalysts in existing processes. (5) Developmem of catalysts that enable new technological routes to valuable chemical products without the formation of polluting by-products. The targets of environmentally benign catalyses are lying in air, water, and soil. In this paper the first topic, the heterogeneous catalysis for materials emitted into air, will mainly be taken up because the compositions and quality of fuels and emission control * Previous address: CatalysisResearch Center, HokkaidoUniversity, Sapporo 060-0811, Japan.

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during fuel utilization are strongly dependent upon the application of heterogeneous catalysis. Problems and opportunities in water, however, are also of increasing importance [2-4]. The amount of water consumption in industrialized countries is continuously increasing and in several countries the depletion of underground sources and/or their increasing level of contamination has become a central question [2]. Rational use of water resources is one of key issues for sustainable growth. Although technologies for treating recycled rinse water are available commercially, there are limitations in terms of cost of chemicals/technology, efficiency of removal of pollutants, production of side streams, severity of operation, range of conditions for operation, etc., for which innovative solutions are required. The use of solid catalysts would overcome or reduce the limitations. From these viewpoints there are two classes of studies; oxidation processes [3] and heterogeneous photocatalysis [4]. The progresses in these studies are highly expected.

2. Present Positions of Catalyses for Sulfur, Soot, and Organic Compounds Air pollution and acid rain seriously affect the terrestrial and aquatic ecosystems and therefore are very important social problems that must be solved as soon as possible. The exhaust gases from engines of vehicles and industrial boilers contain mainly carbon oxides, nitrogen oxides (NOx), hydrocarbons, sulfur dioxide, particles, and soot. In this section the studies on sulfur, soot, and organic compounds would be reviewed. 2.1. Sulfur Sulfur compounds produce SOx during combustion in engines and during catalytic regeneration in catalytic cracking units, leading to local contamination and to the poisoning of automotive exhaust catalysts [1,5]. Recent research has been conducted from two viewpoints, developments of new active catalysts for desulfurization of some organic sulfur compounds and of reduction catalysts of SO2 to elemental sulfur by CO or hydrocarbons. Very recently desulfurization of thiophene via hydrogen transfer from alkanes was reported [6]. The direct use of hydrocarbons in desulfurization reaction should be considered because of the ubiquitous presence of light alkanes in refinery and petrochemical streams and their frequent use for H2 generation. Stoichiometric hydrogen scavengers such as oxygen, CO, and CO2 can remove hydrogen formed in C-H activation steps and increase alkane dehydrogenation selectivity on H-MFI and Zn-/H-MFI. Propane coreactants (20kPa) led to desulfurization rates and H2S selectivities much higher than expected at the H2 pressures prevalent during propane-thiophene reactions (1-3kPa) and similar to those obtained at 50-300kPa HE. Catalytic desulfurization occurred without significant formation of benzothiophene or of unreactive sulfur-catalyst adducts and without requiring gas phase H2. These results show that dehydrogenation reactions can be coupled kinetically with hydrogenation reactions of thiophene or its fragments.

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Many kinds of materials have been examined as soot oxidation catalysts: single or mixed oxides [16,17], perovskite-type oxides [18], various vanadates and molybdates [19,20], and Pt-loading materials [ 15,21]. The effect of support was also tested [19,22]. The experimental conditions such as the intensity of contact between soot and catalysts may affect the activities these solids. For example, it was reported that the catalytic activity of cobalt oxide or iron oxide was dependent on the degree of contact; tight contact is better for the catalysis [23]. From this viewpoint it is interesting that Cu/K/Mo/C1 shows high soot oxidation activity under loose contact conditions [24]. Three reaction mechanisms have been suggested for the soot combustion on solid catalysts. First, the reaction is catalyzed by redox behavior of the catalytic materials [16,25]. In a recent kinetic study [26], a half-order kinetics in the partial pressure of oxygen was obtained on CuFe204 and the formation of reactive oxygenated intermediates on the soot surface is suggested to be the rate-determining step. In this mechanism the tight contact is essential for the promotion of the catalytic oxidation [16]. Secondly the formation of liquid eutectic phases is reported to be a key in determining the activity on Cu-K-V oxide [27] and Cs-V-Mo oxides [20]. The catalytic activity increased dramatically upon the improvement of the catalyst-carbon contact at or above the temperature at which liquid phases are formed. Thirdly the activity of MoO3 as a soot oxidation catalyst is claimed to be due to gas-phase contact [20]. Significant loss of catalyst through vapor phase was recognized in the experiments, which can result in catalyst emission and loss of catalytic activity in time. The catalytic activity of MoO3 is very good but it is probably not applicable as soot oxidation catalyst. In the cases of second and third reaction mechanism the tight contact is not essential. It should be noteworthy to add that NO increased the soot combustion rate on several catalysts [14,15,26] and this would be the formation of NO2 on the catalysts and the subsequent oxidation of carbon with NO2.

2.3. Organic Compounds Fully halogenated chlorofluorocarbons (CFCs) are responsible for the depletion of the ozone layer. The Program for Alternative Fluorocarbon Toxicity Testing has recommended a guide for transforming CFCs into hydrofluorocarbon compounds (HFCs). HFCs show no effect for the ozone-depletion. To recover CFCs and destroy them is a logical step forward. Many destruction techniques have been proposed [28]. Very recently, however, converting CFCs into valuable chemical compounds have been studied as a better choice. This technique involves the selective hydrodechlorination of CFCs to HFCs on supported palladium [29] or non-noble metal such as nickel [30]. As a consequence of its refractory nature and large-scale production tetrachloromethane (TCM) is environmentally ubiquitous. Since TCM is not easily decomposed under ambient conditions, and is a suspected human carcinogen, a wide variety of studies have been carried out on the decomposition of TCM over heterogeneous catalysts [31] and on selective catalytic hydrogenation of TCM to CHC13 [32]. In the

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Modem hydrocracking catalysts often consist of a combination of a sulfidic Ni (Co)-Mo or Ni-W phase [5] and an acidic zeolite. Although the reaction mechanism on the former catalysts is widely studied, the acid catalyzed reactions are not well understood. A basis to a better understanding of the reaction was provided from the theoretical study of reaction paths of sulfur comaining molecules upon contact with Bronsted acid sites [7]. It was clarified that the presence of hydrogen does not affect significantly activation barriers but dramatically changes the overall enthalpy of reaction. Currently operating desulfurization, based on the SO2 scribing with lime or limestone, is a costly process requiring a large space with complicated facilities and disposal of the used sorbents. Direct catalytic reduction of SO2 to elemental sulfur has received much attemion because it is easier to design and operate. The process can be applied directly to flue gases containing a small amount of oxygen or to the case where SO2 in the flue gas is isolated or concentrated using a proper adsorption/regeneration system. SO2 reduction with CO has been studied on several types of catalysts. In the early developmem of catalyst, a substantial amount of COS, which is much more harmful than SO2, was formed as byproduct. Recently Co304-TiO2 has been reported to show the highest catalytic activity for the reduction of SO2 by CO among catalysts reported so far [8]. There existed a strong synergistic promotional effect in the conversion of SO2 when cobalt was mixed with TiO2. It is claimed that the COS intermediate can react with SO2 to produce an additional sulfur and also behaves as a strong reductant to keep oxygen vacancies on the TiO2. On the other hand sulfided CoMo/A1203 has been found to exhibit outstanding activity for the reduction of SO2 with CO [9], though a certain amount of COS was produced. On the catalyst CO adsorbs exclusively on CoMo phase and SO2 predominantly on 7-alumina. Ceria-based catalysts have also been reported [ 10]. Methane is an attractive reductant, due to abundant and cheap natural gas. The reaction between SO2 and CH4 has been studied for a long time but the catalytic activity was insufficient. Cobalt oxide was recently reported as an active catalyst over alumina [ 11], but only at high temperatures (>973K). In contrast, La-doped and undoped ceria were found to catalyze the SO2 reduction by CH4 at 823-1023K at atmospheric pressure [12]. The addition of copper or nickel into La-doped ceria has given the improved selectivity to elemental sulfur or H2S, respectively. With proper further development, this class of catalysts offers promise for practical application to sulfur recovery from various SO2-1aden gas streams.

2.2. Diesel Soot Particulate matter as well as NOx is one of the main pollutants in diesel engine emissions. The combination of traps and oxidation catalysts appears to be the most plausible after-treatment technique to eliminate soot particles [ 13]. The possibility of promoting both oxidation and NOx reduction in a single catalyst has also been investigated [14,15]. The present position of soot combustion catalysts was summarized by Querini [ 16].

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former reaction the requirement of the input of substantial quantities of energy is the disadvantage, while in the latter catalyst deactivation is a major point to be solved. The stringent limiting value for emissions of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs), 0.1 ng/Nm 3 for municipal and hazardous waste incinerators, has been in effect in several European countries since the early 1990s and in Japan since January 1997. It has been shown that the TiO2-based V205-WO3 catalysts originally designed for the removal of nitrogen oxides (NOx) through selective catalytic reduction (SCR) are very effective in the decomposition of PCDD/PCDF at the same temperatures as are used for the deNOx reaction. In the last few years, the commercial SCR catalysts have been optimized for the combined dioxin~Ox destruction. This was achieved mainly by increasing the oxidation potential of the catalysts [33].

3. Catalytic Decomposition and Adsorption of Nitrogen Monoxide 3.1. Necessity of New DeNOx Technologies At present, one of the significant problems in air pollution is removal of NOx, which is produced during high temperature combustion. In particular, the removal of nitrogen monoxide (NO) is a dominant target to be achieved because NO is an inert and major component of NOx in the exhaust gases [34]. It is well known that NO is thermodynamically unstable relative to N2 and 02 at temperatures below 1200K, and its catalytic decomposition is the simplest and most desirable method for its removal. To date, however, no suitable catalyst with sustainable high activity has been found. This is due to the fact that oxygen contained in the feed or produced in the decomposition of NO, competes with NO for adsorption sites. As a result, high reaction temperature and/or gaseous reluctant is required to remove surface oxygen and regenerate catalytic activity. The catalytic reduction processes employing NH3, CO, or hydrocarbons as reductant on TiO2(-V205)-WO3 or Pt-Pd(.Rh) catalysts have been put to practical use. Although many efforts have been devoted to improve the processes, the disadvantages or problems which each of the present reduction processes suffers are summarized as follows. (a) In the selective catalytic reduction system with ammonia (NH3-SCR) there are several disadvantages such as high costs of facilities and running and leakage of unreacted dangerous ammonia. (b) The automobile catalytic converter is the only technology available for the most stringent emission standards. In this technology so called three-way catalysts are preferentially used with several limitations such as using unleaded gasoline and maintaining a specified air/fuel ratio. However, this system cannot meet the requirements of newly developed engines in which the air/fuel ratio has been made lean to an air-rich region, because the exhaust contains a considerable amount of oxygen and the present catalysts do not work under such conditions. (c) The greater use of diesel-engine vehicles is a major trend observed worldwide over the last decade. Co-generation systems using diesel engines have also been under development. Although inherently cleaner than gasoline engines from the viewpoint

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of CO and hydrocarbons, diesels produce more aldehydes, SOx, NOx, smoke, and odor. In this instance the problem similar to the above, i.e., removal of NO in the presence of oxygen and SOx remains unsolved. Much effort has recently been devoted to develop alternative methods for the removal of NO, that is, decomposition, reduction with hydrocarbons, or adsorption. Future opportunities of catalytic removal of NO can be classified as follows. This review will be described in this order. Removal of NO without Reductant Catalytic decomposition to nitrogen and oxygen molecules Adsorption-Enrichment-Aftertreatment Removal of NO with Reductant Selective catalytic reduction with hydrocarbons (HC-SCR) in the presence of excess oxygen One-stage treatment of continuous flow of NO, oxygen, and hydrocarbons Two-stage treatment separating oxidation of NO and reduction of NOx

3.2. Catalytic Decomposition

Catalysts. NO decomposition to molecular nitrogen and oxygen is the simplest, most attractive, and most challenging approach to NOx abatement. We have first reported that Cu ions exchanged in the MFI matrix exhibit unique and stable activity among Cu-zeolites [35]. In particular, the over-exchanged Cu-MFI (Cu2+/AI > 0.5) reaches very high decomposition activity [36]. These results were confirmed by Li and Hall [37]. The effects of the Si/A1 atomic ratio of the parent zeolites used to prepare the catalysts were also investigated. Since the discovery of the remarkable NO decomposition activity of Cu-MFI catalysts in 1986 [34, 35], a lot of effort has been devoted to develop active catalysts. In the case of metal oxides, CO304 is one of the most active, single component, metal oxides for NO decomposition [38]. Its activity can be enhanced by addition of Ag, presumably by modifying the extent of oxygen suppression [39]. It has also been shown that the modification of Co304 by alkali metal ions, in particular Na, is very effective for the enhancement of decomposition activity [40]. YBa2Cu3Oy[41a] and Sr2+-substituted perovskite oxides [41b] have been reported as candidates for the catalyst. Lunsford et al. have reported high activities of Ba/MgO [42]. In the case of zeolites or porous materials, it has been claimed that Co-MFI zeolite that contain Co in the framework has considerably larger maximum activity for NO decomposition than does Cu-MFI [43] though no data has been reported in a continuous flow system. Wichterlova et al. have found that Cu-MeA1PO-1 ls (Me - Mg or Zn) exhibit constant conversion in NO decomposition and the turnover frequency values at 770K are comparable to those of Cu-MFI with high silica matrix [44]. On the other hand, the decomposition activity of Pt metal has been established for a long time. Recently, the formation of Tb-nitrate intermediate was observed to be important in NO decomposition over Tb-promoted Pt catalyst [45]. The relative catalytic activities of these catalysts are roughly compared in Figure 1

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化

1.25 I

r 1.00

Cu-ZSM-5

r

r

9Lao.8Sro.2Co03

0

:.E'_ 0.75 r~

0

Na/C0304

E 0

o 0.50

9YBa2Cu3OT.x/MgO 9 ka~.sSro.sCuO4 9

"0

>

0.25 n"

0.00

Cu-MgAIPO

BaFeO3.x i C0304Pt/AI203 Ag-C0304 9 I~a/Mgu 1100

1000

900

800

700

Temperature giving maximum decomposition activity / K Figure 1.

Decomposition activity of various catalysts reported so far.

where each decomposition activity is plotted on the basis of very vague calculation since the experimental conditions are dependent on each research group. The figure indicates that the key components for direct decomposition of NO are Cu and Co and that their catalytic activities can be improved by addition of precious metal and so on. It has been reported that the increase of 1 order of magnitude in the turnover frequency could lead to a practical catalyst [46,47]. NO decomposition still offers a very attractive approach to NOx removal. However, since any combustion process is going to produce 10-20% water vapor, one must focus on a catalyst that is stable for long times in such wet environments.

Characterization o f and Decomposition Mechanism on Cu-MFI. It is clear that demonstrating the structure of the active sites, and learning the limitations of the Cu-MFI, could lead to develop new more active and stable catalysts that could find practical application. Several excellent reviews have been published and it is apparent that no general consensus of opinion exists with respect to the nature of the active site involved or indeed the reaction mechanism occurring, as pointed out by Mackinnon and coworkers [48]. The main points of dispute can be summarized as follows. (1) Considerable evidence has been provided to indicate that Cu + species participate in the reaction [49,50]. On the other hand, the reaction on Cu 2+ ion with no contribution of Cu + has also been postulated [46]. In my opinion, however, there is no doubt that the NO decomposition is a redox process. (2) The NO decomposition reaction is promoted on over-exchanged Cu-MFI catalysts and this behavior may correlate with the availability of extra-lattice oxygen

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(ELO) species [49]. The identity of the ELO is not clear. We [49] and Sachtler and coworkers [51] have proposed that it is of the form Cu2+-O2"-Cu2+, whereas Bell and coworkers have suggested that the ELO is associated with isolated Cu 2§ sites and is of the structure Cu2§ or Cu2+O2 [52]. Very Recent investigations [48,53] have supported the presence of Cu2+O" or Cu2+O2 species. (3) The mechanism for coupling of nitrogen species to form product N2 is a topic of controversy. There are two kinds of problems to be solved. First significant problem is that the number of copper ions working as active site is one or two. Second argument is the form of intermediates to produce N2; a nitrosyl species, a nitro species, a nitrate species, and dissociatively chemisorbed NO, etc. have been suggested. The first point of the third item will be discussed here in more detail. It was demonstrated that the most active catalysts are those with the low Si/AI atomic ratios and Cu exchange levels in the range of 90-150%. These results have leaded to two kinds of possibilities for copper active sites in Cu-MFI catalysts. It has been suggested the active site responsible for the high catalytic activity is a unique dimeric Cu species which is stabilized by zeolite framework. Adsorption of NO on this dimeric species to form a cuprous hyponitrite that decomposes to form N20 and then N2 is proposed to be a possible reaction mechanism [49,51,53,54]. The species Cu2+-O2-Cu 2+, Cu+-O2"-Cu2+, and Cu+...Cu2+O [53] are suggested. In contrast, the monomeric Cu site was suggested as an active site by several researchers [52,55,56]. Although Cu+(NO)2 has been proposed as precursor for N20 formation in previous studies, the lack of correlation between Cu+(NO)2 and N2 formation [57] and first principles quantum mechanical calculations [58] suggest that Cu+(NO)2 is not formed under reaction conditions. Thus, Cu+(NO)2 as precursor is ruled out. The Cu2+O or Cu2+O2 species may form on the over-exchanged Cu-MFI and act as the active sites

[56].

Detailed characterization of Cu-zeolites has now been carried out by Wichterlova and coworkers [59,60], Kuroda and coworkers [61,62] and other researchers [63,64] eagerly, being expected to solve the above controversial reaction mechanism. For example, very recently the locations of Cu § ions are proposed on the basis of experimental [60] and theoretical [63] studies and their conclusions are in good agreement with each other. In addition Kuroda et al have claimed that zeolite having an appropriate Si/AI ratio, in which it is possible for the copper ions to exist as dimer species, may provide the key to the redox cycle of copper ion as well as catalysis in NO decomposition [62]. This conclusion coincides with the results of theoretical calculation [64] in which bent Cu-Ox-Cu structures are found in Cu-MFI and these are suggested to be the part of a catalytic cycle.

3.3. Adsorptive Removal of NO It is widely accepted that selective adsorption is one of the most suitable techniques for removal and/or enrichment of low concentration pollutants. In particular, pressure swing adsorption (PSA) has widely been applied to various processes;

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Table I. NO Adsorption Properties of Various Cation-Exchanged MFI Zeolites j amount of NO adsorbed/(cm3.g -1) adsorbent

content of cation/(wt %)

reversible

irreversible

Na-MFI(23.3)-100 b Ca-MFI(23.3)-54 Sr-MFI(23.3)-105 Ba-MFI(23.3)-80 Mg-MFI(23.3)-46 Cu-MFI(23.3)-157 Ag-MFI(23.3)-90 Co-MFI(23.3)-90 Mn-MFI(23.3)-127 Ni-MFI(23.3)-68 Zn-MFI(23.3)-96 Fe-MFI(23.3)-62 Cr-MFI(23.3)-41 Ce-MFI(23.3)-8 La-MFI(23.3)-7 H-MFI(23.3)-100

2.81 1.32 5.45 6.44 0.69 5.90 10.85 3.06 4.20 2.41 3.79 2.12 0.87 0.43 0.40 0.13

0.16(0.006) c 1.81 (0.246) 2.71 (0.195) 1.50(0.143) 0.69(0.109) 4.28(0.206) 3.38(0.150) 1.52(0.131) 1.19(0.069) 1.03(0.112) 1.01 (0.078) 0.52(0.061) 0.38(0.101) 0.34(0.496) 0.25(0.388) 0.12(0.004)

0.00(0.000) c 1.56(0.212) 0.20(0.014) 1.44(0.137) 0.22(0.035) 14.90(0.716) 0.54(0.024) 19.69(1.693) 5.81 (0.339) 6.64(0.727) 0.50(0.039) 3.08(0.362) 1.16(0.308) 0.34(0.496) 0.24(0.372) 0.32(0.011)

a Adsorption time, 45 min; desorption time, 60 min; concentration of NO, 997 ppm; adsorption temperature, 273 K; adsorbent weight, 0.5 g; flow rate, 100 cm3.min -1. b Concentration of NO, 1910 ppm. c Unit, (NO molecules)-(cation) -1. therefore, the PSA is expected to be an effective method to remove or enrich NOx diluted in air. Although active carbon, carbon fiber, silica, zeolite, and chelate resin have been reported as the candidates so far, little is known of the respective amounts of reversible and irreversible adsorption of NO on metal ion-exchanged zeolites. We measured them by a fixed bed flow adsorption apparatus [65-67]. The amount of reversible and irreversible adsorption of NO per weight of adsorbent (denoted as qrev and qirr, respectively) measured at 273 K on various cation exchanged MFI zeolites are summarized in Table 1. The values in parentheses are the amounts of reversible and irreversible adsorption of NO per cation (q*rev and q*irr)- The qrev and qirr greatly changed with the metal ion. With MFI zeolites, the order of qrev was transition metal ion = alkaline earth metal ion > rare earth metal ion ~ alkali metal ion proton. Among the adsorbents listed in the table, Cu-MFI-157 and Co-MFI-90 showed the largest qrev and qirr, respectively. The dependency of the NO adsorption upon the exchange level of copper ion was studied on MFI zeolite at 273 K. qrevand qirr were linearly proportional to the exchange level of copper ion, showing that q*rev

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and q*~ are constant, approximately 0.23 and 0.64 NO molecules-Cul, respectively. It follows that the effectiveness of each copper ion in MFI zeolite for NO adsorption is independent of its loading level. The amounts of reversible and irreversible adsorption of NO were dependent not only on metal ion but also on silica/alumina ratio. Both q*rCvand q*~rrdecreased with increment of the aluminum content in zeolites. It indicates that the adsorbability of NO is mainly controlled by the aluminum content and not by the zeolite structure. The dependencies of qr~v and qirr upon the adsorption temperature are also studied. With increasing adsorption temperature q~r on Cu-MFI-157 significantly decreased. On the other hand, qr~v gradually increased with temperature, reached the maximum (4.35cm3gl ) at 243 K, and then decreased. The maximum qrev on Co-MOR-65 was 5.42cm3gl at 373 K. A high capacity for reversible adsorption of NO is required for PSA. At present Co-MOR [66] or Ag-MOR [67] are strong candidates for NO adsorbents in high or low temperature PSA. In real exhaust gases, there coexist various gases and therefore it is important from a practical point of view to clarify their influence on adsorption properties. The preadsorption of NO2 on Cu-MFI-147 resulted in the enhancement of qrev (from 4.35 9 cm 3 g-1 without preadsorbed NO2 to 7.14 after the preadsorption of NO2). At low temperature N203 is known to be in equilibrium with NO and NO2. This suggests that NO2 irreversibly adsorbed can work as new active sites for the reversible adsorption of NO. When 02, CO2, or SO2 was preadsorbed, qrCvwas little reduced (4.26, 4.25, or 3.92cm3gl, respectively). CO or H20 poisoned the adsorbability (1.39 or 0.22cm3g1, respectively). On the other hand, qirr is always decreased by the preadsorption of these gases.

4. Continuous Reduction of Mixture containing NO, Oxygen, and Hydrocarbons Cu-MFI is the most active catalyst for the decomposition of NO. However, the activity greatly decreases in the presence of excess oxygen, water vapor, and SO2, as mentioned in the previous section. Selective reduction of NO with hydrocarbons in an oxidizing atmosphere over Cu-MFI has first been reported by the present group [34]. At the same time, Held and coworkers have reported similar findings independently and Toyota Motor Co. also applied for the patents. The distinguishing characteristic of this new technology is that the presence of oxygen is indispensable for the progress of the reduction of NO. This new selective reduction of NO proceeds even in the presence of excess 02, and has the possibility to overcome the disadvantages of the present reduction systems, NH3-SCR and three-way catalytic system. Several reviews [34,68] have already summarized the progress of HC-SCR in 1996 or before. The catalysts developed before 1996 therefore will briefly be introduced here and then recent progress will be reviewed in the latter sections. Since the finding of the HC-SCR technology, a lot of catalysts have been reported to be active. Some zeolite-based catalysts including Cu, Co, Fe, Ag, or Pt show high initial activities with hydrocarbons as reducing agents as well as ammonia. So far,

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化

100

In-Fe203/H-MFI 9

80o

=

oo

60-

9 Z E 40E

o~

20 -

Au/AI203+Mn203 O

A Sn/Ga203.AI203 A Ga203-AI203

Fe2+.MFI ~ Au/AI203 Co-BEA O O

A In/Co.Al203

A Ag/Al203 Ag/Co-MFI C~3(PO4)2 ~ OCo-MFIA ln/Ga203-A1203 ~Ag/TiO2"ZrO2 ~' ln/Al203 Cu-P-MFI~ Pd-In/TiO2-ZrO2 Cu-SAPO34 9 Co-MOR in/C~lOx A AI203 Pt/MOR(~ Pt+Zn-MFI(IAR) Pd/H-MOR Ni~1203a Co/AI203 O Pt/WO3 O Cu-MFI in//~a203.A1203 A Ag/AI203 Pt-silicate O O Pt/AI203 O Ir/A1203 OCo-silicate Pt-B/YPO4 0 ~ ~ Ga/H-MFI A CoAIOx Pt-B/LaPO4 0 Fe3+-MFI ~ In/TiO2-ZrO2 Sn/Al203 A Al203 ad/Al203 O O Pt/SiO2 O Mn203+Sn-MFI ~ Pt/MFI ~ Pt/SiO2

O Pt/Co-silicate

at/gl203

A Co/A1203

A AIPO4

0 Rh/Al203 0 373

I

I

!

I

473 573 673 773 Temperature giving maximum NOx conversion / K

I

873

Figure 2. Reduction activity of various catalysts reported so far. Open and closed circles and triangles roughly correspond precious metals, microporous materials, and metal oxides. The change in catalytic activity resulting from the difference of experimental conditions has not been taken into account at all. however, the hydrothermal stability of zeolite catalysts appears to be limited. Hydrothermal deactivation can have several causes, which are structural collapse, dealumination, agglomeration of active cations to small oxide islands, and migration of the cations to inaccessible sites. As the stability is of major importance from application, improvement of zeolite catalysts should be aimed at stability too rather than initial activity only. On the other hand alumina and composite metal oxides are reported as active catalysts. Some solid acids also show catalytic activity. In the case of metal oxide catalysts the reaction rates are not sufficient, which means that a big reactor or low gas hourly space velocity is needed for the practical application. All of the catalytic activities have been measured under the experimental conditions of the respective researchers. The kinds of hydrocarbons used, the concentrations of the respective reactants, the space velocity, the shape of the reactor, and the pretreatment of the catalyst can all influence the reaction results, that is, the apparent catalytic activities. For example, we can employ ethene as reductant and probably obtain good results when we use a catalyst with high performance for hydrocarbon

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oxidation, while the use of propene could be recommended for the catalysts with low oxidation power. With the catalysts not so active for the hydrocarbon oxidation, the low space velocity can be set to get high conversion of hydrocarbons and NOx. The molar ratio of NOx and hydrocarbons also determines the apparent catalytic activity for deNOx reaction. On understanding of these situations, many results reported have been plotted in one figure to reveal general features of HC-SCR [34]. In Figure 2 the difference of experimental conditions in the respective reports has not been taken into account at all. Open and closed circles and triangles roughly correspond precious metals, microporous materials, and metal oxides, though there are many combined catalysts. The active temperature regions of catalysts are clearly depending on the type of active centers. Precious metal catalysts are active at the lowest temperature, transition metal-ion exchanged zeolites work at the middle temperature region, and the active temperatures of metal oxide catalysts are the highest. Figure 2 also indicates that the active components are Pt, Cu, Co, Fe, Ag, In, Ga, Sn and so on and that the supports frequently used are alumina and zeolites. I am guessing that the practical application might be achieved on precious metal-, Cu-, Co-, or Fe-containing catalysts. For lack of space, Cu and Fe would be reviewed here in more detail and the investigations on Pt [69], Pd [70], Rh [71], Ag [72], and Co [73] were omitted. When we consider the practical application of the present HC-SCR method, the best way is the simultaneous abatement of NOx and hydrocarbons on one catalyst bed in a continuous flow. The second best is the separation of oxidation of NO to NO2 and reduction of NO2 with hydrocarbons. In this section active catalysts for the first method will be introduced. The latter way will be described in the next section.

4.1. Copper Ion-exchanged MFI Zeolites Much effort has been devoted to the study on Cu-MFI. The major research targets are characterization of copper ions in zeolite frameworks, clarification of deactivation mechanism and development of procedures to preserve the activity, and elucidation of reduction mechanism of NO. The first target highly overlaps with that in the studies on catalytic decomposition. The latter would be discussed here. The most significant problem of Cu-MFI is deactivation during the catalytic run at high temperatures in the presence of water vapor [68]. Too much loading of copper or severe treatment of Cu-MFI has widely been reported to result in the formation of CuO particles or the destruction of lattice, and therefore one could avoid rapid deactivation if the catalyst was used under the proper conditions. The preparation of heat-resistant zeolite is a future problem. The mechanism of gradual deactivation under relatively mild conditions has not been identified. Formation of CuO particles [74] or clusters [75,76] and migration of Cu a§ ion into inert sites [77,78] have been suggested as the causes. The fresh Cu-MFI samples pretreated at 673-773 K usually show two kinds of ESR spectra with g//=2.31-2.33 and A//=140-155 G (CuA) and g//=2.27-2.29 and A//=155-175 G (CUB). The spectra have been assigned to the Cu 2§ species in square-pyramidal and

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square-planar coordinations, respectively. A few research groups [77-79] have independently reported that the treatment of Cu-MFI at 1073 K causes the elimination of the CuA and CuB species, the formation of new CuC species with gH=2.30-2.32 and Aa=155 - 160 G, and the simultaneous dealumination of the zeolite lattice. It has been suggested that the dealumination brought about the change in the location of Cu ions and the resulting migration of Cu ions to inert sites is the origin of the deactivation under the mild conditions [78,79]. On the other hand, Tabata et al. [75] have not found any dealumination under the similar conditions but observed the formation of Cu---Cu bonds by EXAFS. The formation of CuO clusters has been suggested for the deactivation. There is another report [76] in which the CuAI204 formation is associated with the deactivation. We have independently compared the ESR, IR, XRD, and 27A1 MASNMR spectra and the surface area of the hydrothermally treated Cu-MFI with those of the fresh one [80]. The results have indicated that the migration of Cu ions to inert sites without dealumination resulted in the deactivation and the change in zeolite lattice occurred under more severe reaction conditions. There are many reports for improvement of the stability of Cu-MFI. Cucontaining silicate has been reported to show better stability than Cu-MFI [81]. The coloading of La or Ce [82], Cr [83], or P [84] stabilized the catalytic activity of Cu-MFI. In particular, the addition of P is very effective. The catalyst treated at 923 K for 50 h in water vapor has still possessed the reduction activity though the active temperature region became higher. The addition of Ca onto the Cu-P zeolite is reported to be effective for the further improvement of durability. At present two kinds of reaction mechanisms have been suggested for the role of hydrocarbons. Some research groups have proposed that no direct interaction between hydrocarbons and NO is required [85]. In the mechanism, decomposition of NO proceeds first to yield N2 and surface oxygen species, and then the hydrocarbons clean up the surface oxygen adsorbates, or the hydrocarbons-O2 mixture reduce the active sites for the NO decomposition reaction which occurs by a redox mechanism. The other researchers have claimed the direct interaction between hydrocarbons and NO on the catalysts [86, 87]. In this view, carbonaceous deposits, partially oxidized hydrocarbons, hydrocarbons themselves, or ammonia are postulated as the active species, and NO, NO2, N203, and NO3" are proposed as the reactive nitrogen oxides. The latter mechanism is promising on Cu-MFI. Many types of reaction mechanisms have been suggested on Cu-zeolites, the majority of which are still controversial. It should be careful in the research on reaction mechanism that the data were obtained on over-exchanged or low-exchanged Cu-MFI [86]. For example, some types of adsorbed NO were observed on over-exchanged ones, while nitrosyl and nitrite-nitrate adsorbates were found on low-exchanged ones. The behavior of some surface N-containing intermediates such as nitrosopropane [88] was greatly dependent on the exchange level of copper and the atmosphere of the catalysts. The role of N-containing surface species in the HC-SCR has very recently been summarized by Sachtler and coworkers [89].

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4.2. Iron Ion-exchanged MFI Zeolites Numerous zeolite-based catalysts show promising activities for the reduction of nitrogen oxides with hydrocarbons, but have not yet been commercialized for this purpose, except for Co-[~ zeolite. This is due to a lack of long-term stability, especially in the presence of sulfur dioxide and water vapor [90]. Recent results indicate that iron ion-exchanged MFI zeolites exhibit remarkable stability under realistic off-gas conditions. Feng and Hall [91] reported a very high and stable catalytic activity for the reduction of NO with iso-butane at 723K in the presence of 20% H20 and 150ppm SO2. Although the very high catalytic activities could not be reproduced by other groups [92-94] and by themselves [95], Chen and Sachtler clearly demonstrated that the high activity under wet conditions continues for at least 100h at 623K [93]. The problem on reproducibility of active catalysts is attributable to the difficulty of preparation of zeolites containing unstable Fe 2+ ions as follows. Active Fe-MFI catalysts described so far have been obtained under anaerobic conditions. This is due to that Fe 2+ ions are easily oxidized in aqueous medium giving rise to the formation of iron hydroxide species [96]. In the first report, iron oxalate was used in a glass apparatus with separate supply of zeolite and iron salt under nitrogen atmosphere and the F e/AI atomic ratio reached 1.0. Chen and Sachtler [92], however, could not achieve such high degree of ion exchange in their attempt to reproduce the results. A better way for introducing iron was found to be the sublimation of a volatile iron salt, FeC13, into the hydrogen form of the parent zeolite under inert atmosphere [92,93]. Pophal et al. [97] employed iron sulphate during aqueous ion exchange at 323K under N2. On the other hand, K6gel et al. have used the solid-state ion exchange procedure [98,99] to prepare iron exchanged MFI zeolites in air [94,100]. This method using FeCI24H20 in air would be useful for the preparation of practical Fe-zeolites catalysts. The activity of Fe-MFI could be improved by the addition of La [93]. In particular the activity at higher temperature region could be much increased and the temperature window of Fe-MFI became wider. Very recently 10h exposure of Fe-MFI, prepared by sublimation of iron chloride, to wet exhaust gas at 873K was reported to cause severe deactivation of the catalyst [ 101 ]. The temperature would be too high for maintaining the zeolite structure, as has been discussed in the section 4.1. It was suggested that the second sublimation brings about an improvement in the stability of the Fe-MFI catalyst though its deNOx activity is decreased. The state of Fe dispersion in Fe-MFI was investigated by means of IR, TPD, and TPR. For samples with an Fe/AI ration <0.56, Fe exchanges with Broensted acid protons on a one-to-one basis, while higher weight loading of Fe result in the formation of small particles of FeOx [102]. For Fe/AI<0.19 most of the Fe is present as Fe 3+, and at higher values of Fe/AI as a mixture of Fe 3+ and Fe 2+. Meanwhile it was clarified that the population of oxo-complexs that lose oxygen by heating depends on the Si/AI ratio. This dependence is in qualitative agreement with the model of (2+) charged binuclear ions [HO-Fe-O-Fe-OH] 2+. Upon reacting with NO, the bridging O atom would yield NO2 [ 103], which is a key compound in HC-SCR.

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The study on reaction mechanism on Fe-MFI has just been started by several groups [104]. All researchers suggest the formation of nitrogen containing deposit/ adsorbate on the catalyst and the subsequent reaction with gas phase NO or NO2. It should be noteworthy that recent FTIR analysis of the gaseous products on Fe-MFI clarified the substantial production of HCN [105]. The formation was strongly dependent on the dry/wet conditions. The results would be useful to elucidate the reaction mechanism. An interesting feature of Fe-MFI is that not only NO but also N20 can be reduced to nitrogen by using hydrocarbons. Concern about nitrous oxide emissions has recently increased due to the reported potential for global warming and stratospheric ozone depletion [106]. Segawa and co-workers [97] used propene as reductant and achieved high nitrous oxide conversion, also in the presence of oxygen and up to 15% water. Subsequently it has shown that substantial conversions of NO and N20 can be achieved simultaneously, using propane as reducing agent [94,100].

5. Two Stage Treatments Consisting of Oxidation of NO to NO2 and Subsequent Reduction of NO2 with hydrocarbons Various metal oxides, precious metals, and metal ion-exchanged zeolites have been reported to be active for the HC-SCR and there are many discussions on the reaction mechanism. The attention of many investigators has been given to the concept that NO is first oxidized to NO2 with oxygen and then the resulting NO2 reacts with hydrocarbons to form an active intermediate for the formation of N2. Much effort has so far been devoted to develop bi-functional catalysts with an oxidation and a reduction site. For example, in the reaction systems of NO+O2+CH4 on Pt/In/MFI [107] and of NO+O2+C3H6 on the physical mixture of Mn203 and Ce-MFI [108], platinum and Mn203 are reported to be active for the oxidation of NO to NO2, respectively, and the other components are suggested to be the sites for reaction of resulting NO2 with hydrocarbons. One of the most significant problems in these systems is that the oxidation catalyst or site for NO is often active for oxidation of the hydrocarbons, which would result in decrease in efficiency of the reductant. In case of CH4 as the reductant, Pt can be used for the NO oxidation because of the low oxidation activity of Pt to CH4 [107]. For C3H6, Mn203 or CeO2 should be employed because Pt or other oxide catalysts are too active for the C3H6 oxidation [ 108]. A different concept to solve this problem is dividing of the oxidation and the reduction processes. There are two kinds of suggestions. Time-resolved HC-SCR is the NOx storage-reduction (NSR) system, while site-resolved HC-SCR is the intermediate addition of reductant (IAR) system. The former was developed by Toyota Motor Co. and practically used for the lean-bum gasoline engines. The latter was suggested by the present author's group.

5.1. NOx Storage-Reduction NOx storage catalyst is used in an engine that operates alternatively under lean or

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Lean A/F ]

l Rich A/F

I

(NO +02) NO2

[Stored as nitrate

]

[Reduced to nitrogen

Figure 3. Concept of the NOx storage-reduction method. ratio of a gasoline engine.

]

A/F means the Air/Fuel

rich conditions [ 109,110]. During lean operation, the nitrogen oxides in the exhaust are stored in the catalyst. Upon turning the engine to rich conditions for a short period, the stored NOx is released and reduced with hydrocarbons and CO over the catalysts. The NOx storage catalyst reported by Takahashi et al. [109] consists of noble metals (mainly Pt), alkaline earth metals (mainly Ba), and alumina. Noble metals catalyze oxidation and reduction reactions of NOx, alkaline earth metals are NOx storage components, and alumina is a high surface area support. The concept of NSR reaction is summarized in Figure 3. Several metal oxides, mainly alkaline earth metal oxides, are effective in forming nitrates with NO2, which decompose at high temperatures [110]. The metal oxides could be used for reversible storage of NOx. Typical examples of such metals are barium and strontium. The reaction sequence in the NOx storage/release cycle has been discussed [111]. Before NOx storage reaction the oxidation of NO to NO2 is assumed to be a necessary initial step during lean conditions. With BaO as the storing component, the storage would proceed through the following reaction. 2NO2 + O + BaO ~

Ba(NO3)2

This simple step, however, already includes several uncertainties. The atomic oxygen included in the left side of reaction is needed for mass balance but has not been proven experimentally. The types of barium compounds are also uncertain. The decomposition of stored NOx during the rich period would be reverse of the above reaction. Several confusions are as follows. Takahashi et al. reported that the amount of NOx stored increased with increasing oxygen content in the exhaust gas [109]. In contrast it has been reported that the capacity increased when the oxygen

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content increased from 0% to 3% but above 3% no effect was exhibited [112]. B6gner et al. [ 113] concluded that their catalyst stores NOx as a surface metal nitrate. Very recently it has been revealed [114] that stable carbonates are formed on BaO whereas no carbonate is formed on barium aluminate and that the structure of the nitrates formed on these two compounds is very different. An N-bounded nitrate is formed on barium aluminate and not on bulk BaO. Some problems still remain, mainly of deactivation of the catalysts. The deactivation of would be due to reaction of the adsorbents with gas-phase compounds and to particle growth of both the precious metals and the adsorbents [115]. In particular, the adsorbents have to be improved in their resistance to sulfur poisoning due to the formation of very stable sulfates [ 116]. SO2 was found to be accumulated in the catalysts as sulphate [117]. 5.2. Intermediate Addition of Reductant between an Oxidation and a Reduction Catalysts

It is well known that the diesel exhausts contain very small amounts of unburned hydrocarbons though the concentration of NO is rather high [118]. The addition of fuels or hydrocarbons into the diesel exhausts is required to reduce NO through the HC-SCR method. On the basis of the reaction mechanism described in the section 4.1 and the necessity of hydrocarbons addition, we proposed a novel strategy to reduce NO selectively; intermediate addition of a reductant into the NO-containing exhaust between an oxidation catalyst of NO to NO2 and a reduction catalyst to N2 (IAR method) [ 119]. The concept of the IAR method can be described as follows. Conventional HC-SCR NO + 02 + HC I~ N2 + CO2 + H20 Catalyst IAR method HC NO + 02

~ NO2 + 02 ~ N2 + CO2 + H20 Ox. Cat. Red. Cat. The IAR method is expected to prevem the combustion or wasting of hydrocarbons with 02 on oxidation sites. The experimental results on the IAR method would also be useful to reveal the reaction mechanism of the HC-SCR. In our research Pt-MFI has been used as the oxidation catalyst on the basis of previous results [120]. Many kinds of metal ion-exchanged MFI zeolites have been examined as the reduction catalysts to investigate the validity of the IAR method. When H, Zn, Ag, In, Ga, Mg, Ba, Ca, and Na-MFI were used as the latter catalysts, the conversion levels of NO to N2 greatly increased from those on the respective systems without the oxidation catalyst. On the other hand the conversions on Co, Cu, Mn, Ni, Fe, and Pt-MFI little changed with or without the preoxidation of NO. The combination of Pt-MFI and Zn-MFI has been found to be the most effective for the IAR method under the present conditions. For example, the degree of conversion of NO to N2 at 573 K was increased to 54% on the Pt- and Zn-MFI combination from 5%

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o~ z

o 0

100 Co

80

Cu~ Mn-.-'~

z ~

0 tO r~ r

~ Ni

60

!._

tO 0

E E

40

20

x

Na/ .0

,,, 0.5

- , Mg

Ba

1.0

1.5

EHC Figure 4. Change in catalytic activity of metal ion-exchanged MFI zeolites upon the IAR combination with Pt-MFI. The ending point of each arrow indicates the result of IAR method and the starting point is that of the zeolite alone. on Zn-MFI alone or 6% on Pt-MFI alone. The efficiency of ethene (EHc), namely the molar ratio of the amount of NO reduced to that of ethene consumed, reached about 2 and was much higher than 1.4 of conventional reduction on Cu-MFI. The correlation of EHC and the N2 yield are plotted in Figure 4. The present results conclude the great effectiveness of the IAR method to reduce NO selectively with hydrocarbons in the presence of excess oxygen. Although the effect of water addition to the system and the dependence of the IAR reaction on the partial pressures of the reactants should be clarified in the future, it is clear that we don't need to develop or look for the catalyst which does oxidize NO to NO2 but does not oxidize the reductant, if we employ this method. There lie many possibilities for the proper combination of the oxidation and the reduction catalyst, depending on the conditions of exhausts and the kinds of reductants. In addition, the results would be useful to understand the reaction mechanism of the SCR-HC on each catalyst. Finally this IAR method will be compared with the NOx storage-reduction method summarized in the section 5.1. In NSR system NO is oxidized to NO2 under the lean-burn conditions and the resulting NO2 can be sorbed as NO3 on or in the support of the three way catalyst. The sorbed NO3 species are periodically reduced with hydrocarbons which are supplied on the catalyst by applying the rich-bum conditions. In the present IAR method we need to supply hydrocarbons onto the second catalyst bed to reduce NO2 produced on the first bed. One can recognize that both systems

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divide the SCR-HC to the oxidation and the reduction steps. Namely, the storage-reduction system is the time-resolved SCR-HC and the IAR system is the site-resolved one. Each system has advantages and disadvantages. 6. Conclusions

In this paper, the removal of nitrogen oxides has mainly reviewed. Clearly, the concepts or strategies for developing catalysts for this target are quite different from those for conventional petrochemical catalysts. In the latter case one can use purified raw materials and select the best reaction conditions for the production of objective materials. In contrast, environmental catalysts have to work under very severe conditions; very wide temperature ranges, high space velocities, low concentrations of target materials, very high concentrations of co-existing gases and poisons, and considerable changes in the reaction conditions. Namely, environmental catalysts must have extremely high activities, selectivities, and durabilities. We expect much progress in the near future, both with respect to the development of environmentally benign technology and in the scientific understanding of the catalytic action of deNOx. REFERENCES [1] R. J. Farrauto and C. H. Bartholomew, "Fundamentals of Industrial Catalytic Processes", Blackie Academic & Professional, London, 1997, pp.580-666. [2] G. Centi and S. Perathoner, Catal. Today, 53(1999), 11. [3] F. E. Hancock, Catal. Today, 53(1999), 3. F. Luck, Catal. Today, 53(1999), 81. A. Foruny, J. Font, and A. Fabregat, Appl. Catal. B, 19(1998), 165. R. Ukropec, B. F. M. Kuster, J. C. Schoulten, R. A. van Santen, Appl. Catal. B., 23(1999), 45. [4] J. -M. Herrmann, Catal. Today, 53(1999), 115. B. Bems, F. C. Jentoft, and R. SchlSgl, Appl. Catal. B, 20(1999), 155. T. Mori, J. Suzuki, K. Fujimoto, M. Watanabe, and Y. Hasegawa, Appl. Catal. B, 23(1999), 283. D. Chen and A. K. Ray, Appl. Catal. B, 23(1999), 143. K. -H. Wang, Y. Hsieh, M. -Y. Chou, and C. -Yu Chang, Appl. Catal. B, 21 (1999), 1. [5] N. -Y Topsoe, "Infrared Spectroscopic Investigation on Environmental DeNOx and Hydrotreating Catalysts", Haldor Topsoe Res. Labo., Lyngby (Denmark), 1998, pp. 121-210. [6] E. Iglesia, J. E. Baumgartner, and G. L. Price, J. Catal., 134(1992), 549. E. Iglesia and J. E. Baumgartner, Catal. Lett., 21(1993), 55. J.A. Biscardi and E. Iglesia, J. Phys. Chem. B, 102(1998), 9284. J.A. Biscardi and E. Iglesia, J. Catal., 182(1999), 117. S.Y. Yu, W. Li, and E. Iglesia, J. Catal., 187(1999), 257. [7] X. Saintigny, R. A. van Santen, S. C16mendot, and F. Hutschka, J. Catal., 183(1999), 107. [8] H. Kim, D. W. Park, H. C. Woo, and J. S. Chung, Appl. Catal. B, 19(1998), 233. [9] S. -X. Zhuang, H. Magara, M. Yamazaki, Y. Takahashi, and M. Yamada, Appl. Catal. B, 24(2000), 89. [10] W. Liu, A.F. Sarofim, and M. F. -Stephanopoulos, Appl. Catal. B, 4(1994), 167. W. Liu, Sc. D. Thesis, Massachusetts Institute of Technology, 1995. W. Liu, C. Wadia, M. F.-Stephanopoulos, Catal. Today, 28(1996), 391. [ 11] J.J. Yu, Q. Yu, Y. Jin, S. G. Chang, Ind. Eng. Chem. Res., 36(1997), 2128. [12] T. Zhu, A. Dreher, and M. F. -Stephanopoulos, Appl. Catal. B, 21(1999), 103. [13] J. Widdershoven, F. Pischinger, and G. Lepperhof, SAE paper 860013, Detroit, 1996.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Molecular design of heterogeneous catalysts M. E. Davis Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA A discussion on the molecular design of solid catalysts is presented. Rather than reviewing the many definitions of design principles and methodologies for synthesis of classes of catalysts, a specific case study is provided. Salient features of protein-based (enzyme, antibody) catalysts are enumerated and used to guide the creation of two new types of solid catalytic materials, namely, organic functionalized molecular sieves and amorphous imprinted organosilicates. The solid catalysts are compared to an antibody catalyst and their behaviors contrasted. 1.

INTRODUCTION

The molecular design of heterogeneous catalysts encompasses many different materials, e.g., zeolites, immobilized organometallics, etc., and methodologies for their preparation. Rather than enumerate these reports, I choose here to illustrate and discuss one line of thought on the molecular design of catalytic solids. One conceptual approach to the rational design of nonbiologically-derived, catalytic entities is the attempt to translate the principles of enzyme catalysis to abiological, catalytic materials. The key issue is not to mimic the biological catalyst but rather to learn from protein-based systems and to translate the essential features to nonbiologically-derived materials. It is this conceptual pathway that I will illustrate here using examples primarily from my own work. I do not wish to give the impression that I am ignoring the work of others in the area of molecularly designed catalysts, but rather wish to present a concise story to illustrate an example of how I view molecular design of heterogeneous catalysts at this stage of its conceptual and practical development.

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ENZYME/ANTIBODY CATALYSIS: A MODEL

The elegance of enzyme reactivity is unparalleled in solid catalytic materials. Typically, it is thought that: (i) there is a precise three-dimensional configuration of amino acid functional groups that provide multipositional interactions with an appropriate reactant in order for catalysis to occur, (ii) the binding of the reactant produces an appreciable change in the three-dimensional conformation of the amino acids in the active site region and (iii) the changes in the protein structure produced by reactant binding bring about proper alignment between protein functional groups and the reactant to allow catalysis. A nonreactant may be able to bind but not react due to the misalignment of the appropriate interactions. As was advanced by Pauling, enzymes accelerate reactions by lowering the activation barrier to the transition state [1], and it is likely that evolution has selected for proteins that have optimized both the bound and unbound states [2] in order to do so. Pauling first proposed over a half a century ago that enzymes selectively bind transition states of reactions while antibodies bind molecules in their ground state [3]. This paradigm led to the idea of raising antibodies to ground state molecules that approximated transition states, i.e., stable transition-state analogue (TSA). By raising an antibody to the TSA, the antibody may have a binding region complementary to the transition state for which the TSA is a ground state approximation. By creating an environment that provides stabilization of the transition state, the activation energy is reduced and rate accelerations accomplished. This concept is illustrated in Fig. 1 (TSA denoted as imprint in this figure). Lerner and co-workers [4] and Schultz and colleagues [5] independently demonstrated that catalytic antibodies could be prepared following this paradigm. Although catalytic antibodies that function in this manner cannot always be obtained, they are wonderful examples of catalytic materials by design. While catalytic antibodies rarely yield rate accelerations comparable to enzymes, the rates can be quite fast when compared to nonbiologically-derived catalysts. Additionally, it is clear that the structural dynamics of antibodies are not optimized for catalysis as are those of enzymes and this point is discussed further below.

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A

C+R ~

direct reaction

catalyst (C) + reactant

CR

C + product (P)

Reaction Coordinate

B imprint (I) z

CR T

C I

D

synthesis machinery

removal of imprint ~- catalytic material (C) ~ catalytic material 9 I

kcat

p

R ~....~ kdirect

k c a t >> k d i r e c t

P

Figure 1. Conceptual steps for an imprinted catalyst (C). In the energy diagram A), CR* is the transition-state for the catalytic path. Schematic B) illustrates the fact that the imprint (I) should somehow approximate the transition-state. Schematic C) shows the steps in preparing the imprinted material after which the imprint is removed to create the catalytic material. The goal of creating the imprinted material to accelerate the rate over that of the direct reaction is illustrated in schematic D.

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Although catalytic antibodies show higher binding affinities for their TSAs than reactants (implying that they can function by stabilizing the reaction transition state), the question arises as to whether they can catalyze reactions in pathways similar to enzymes. There are clearly cases where the catalysis does not conform to the TSA concept, as well as numerous examples where they do. Figure 2 illustrates the structure of a bound TSA in the enzyme chorismate mutase [6] and a catalytic antibody [7]. The structural data reveal that both the enzyme and the antibody provide environments complementary to a conformationally restricted TSA and strongly suggest that they both catalyze the isomerization of chorismate (shown in Fig. 2) by stabilizing the same transition state that occurs in the direct reaction [7]. Although the differences in the specific interactions may

CO~-

L

OH

i

3 OH

OH

\NH

H "'" O~--~All"m7 O~H ",

AIg7 HNJ

0

+~H~-~).~ H-aN~ NH O . ]

.

O"~

A,m-mo

--"'J ~ 0 0 - H2AI HO...~ .

~o~ ~ , , . ~

H2N~+ N ~

-., o A~O. NH ~.O

(~

Tyrl0B

. HO

ta'lN~+NH~ a' :

~,,.,,

o---- HO O~TS.j sH

Antibody

Enzyme

Rate (enzyme)> 102-4Rate (Antibody)> 106Rate (direct) Figure 2.

attribute to the lower reactivity of the antibody, it is important to point out that both proteins utilize multipositional interactions. Additionally, there is most likely a relationship between binding and conformational flexibility. For example, Schultz and co-workers have shown that the initial antibodies produced (denoted germline) have greater conformational changes between the unbound and TSA bound states than the mature antibodies, and the mature antibodies bind the TSA with a 13,500 fold increase in affinity [8, 9]. Additionally, the mature antibody catalyzes the reaction 82 times faster than the germline antibody. Thus, while the germline antibody possesses conformational flexibility more in line with

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enzymes, the increase in binding affinity of the mature antibody compensates for the increase in rigidity. Schultz and co-workers provide another example where the germline and mature antibody both have the same affinity for the TSA and the germline antibody has a 35 fold greater rate [ 10]. Although it is not clear as to the origin of the declining rate with maturation, it is consistent with the concept of mature antibodies being more rigid and this loss of conformational flexibility leads to a lowering of the reaction rate. Some of the essential features of the protein-based catalysts that appear to be conserved are the multipositional binding of reaction species to amino acid functional groups via non-covalent interactions, the local (around the active site) conformational flexibility and the interplay of these two features. Additionally, these active site regions can provide environments that are sufficiently different from the bulk aqueous solution to allow for partitioning of reactants and products between the active site region and the solvent (the active site can provide a "hydrophobic solution environment" [ 11 ]). 3.

M O L E C U L A R DESIGN OF SOLID, CATALYTIC MATERIALS

Based on analyses like that presented in Section 2 of this paper, we explored the possibility of preparing solid, catalytic materials endowed with at least a portion of the features deemed important to the success of protein-based catalysts. In order to bring about the ability to partition reactants/products between solvents and the catalytic active sites, we focused on silica-based, microporous solids (microporous space in silica can be hydrohobic). Earlier, we showed that the hydrophobicity of the titanosilicate TS-1 was critical to the use of aqueous H202 for the oxidation of alkenes and alkanes [12]. Recent quantitative measurements of partition coefficients for alkanes, alkenes, polar products and water with TS-1 [13] support our previous conclusions. The concept of partitioning between hydrophilic bulk solvents and active site in hydrophobic regions of titanosilicates has been extended from TS-1 to amorphous, microporous titanosilicates [14]. Thus, we chose to prepare microporous materials to provide for the possibility of partitioning of reactants/products from bulk solvent to catalytic active sites. We explored the feasibility of having organic functional groups comprise the catalytic active centers that are located within the micropores of the silica. Silica was used as the solid support to provide the microporous network of voids. In general organic polymer supports tend to have dynamic structural properties under liquid-phase reaction conditions (unless highly crosslinked) and therefore do not provide consistent porosity characteristics that may be useful for creating shape-selective reactions. Since the silica is rigid, we speculated that short organic "spacers" needed to be placed between the silica and the organic

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functional groups in order to allow for some conformational flexibility of the active site. Starting with organosilanes and silica precursors, we prepared microporous, crystalline molecular sieves containing intracrystalline organic functionalities, e.g., phenethyl, aminopropyl, etc., covalently attached to framework silicon atoms [15-17], and amorphous, microporous organosilicates with multipositioned functional groups [18]. Figure 3 summarizes our synthetic strategies while Fig. 4 provides a schematic illustration of the preparation of an imprinted silica. Both classes of materials are able to perform shape-selective catalysis [15, 18] and are the outcome of rationally extending concepts from protein-based catalysts to the synthesis of solid catalytic materials.

low temperature ~40~ S

[ organosilanes+ silica ]

limprinted silica ] imprintremoval

amorphous, microporous organosilicates withmultipositional functionalgroups

Figure 3.

~

high temperatures, 75~

organic-functionalized, as-synthesized,crystalline molecular sieve structure-directing agent removal organic-functionalized crystallinemolecularsieve

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55

m

TEOS

three point or~anosilane imprint OCHICH~c CHt-O-.C --N .--(CH1).~-,~d--OCH1 H~ e~ ~l 1~. . . . .

c a , c a , o - ~ - 4 c a ps--N--c-o-aa t~CIIICH, ~

....~ ~ ~, .

. . . mree pom[ 9 .._:_,^.i'~,^_:~ lm~lllgltCU ili~IIII,CII~III

(lirotected form)

, .o---o-~-o ~ ~,si o', o,

~-.o-~-

ca~. . . .

-,ca,)-,~-o-.-o~

"s4 ~ ' o-si---o ~ ..s ca ~ XO o r o o " No ~.NO,. s o' 9 t tt JJ . a- o a--OH "si~o.~q-o-~--~ca ) - ~ - c - o - - a c ~ , ~ C a r O . ~ ~ C H ) ~ 0_" - " ~a ' / ~' L ~ -1 ~V I - " ~ _ s i . ,

".o

7s~'--o'~-.o..'~-.o "

~/s~.-o"si\~

s~

0r siXO "~--6"

cn, ~ o, 01 CIt I o -.o.'~N,. ~=" ~---o-Si,,o~,, o "si ~o o "si d ~.-s~~ o/ si.~ " s i " o ' ~ ' " xo ",i-o-,

. c' ~ l " o

three

point

(deprotected

form)

TMS-capping of snaaols

,o ,.o-~-o %,-0-'-0-,-0---0- .... g":~. o,~i' ~ -o ' .

o

o'

. ."0 . .t

si

"si

o'

~Si...O...Si"

~,~ , o

o~ "

0 2S M T M S i ill atttonitrile

" . . matenal washing

ca,

~si...~..~i --ot s~r t ' O/Si .,.OISi --O" " CH,

sf"

n , c - ,~#--O''Si &,

si

-\

o

o si_ si si sisr sa" b "o H N - ( C H . ) T r ~ - o - S ~ - o -'m" 2 * ~t , o-si~ o - si .,o...si -.o CHs

~~

H q'~" 3" H ;~i-k. a CH3

~ o .~" " ~ - - c . o- ~. ' H2N-(CHz)~ "sd " ~ o " ~ " o s

~si-(CH2)~ NHz S,.o.~

-o~ ~,si o'

o, ~ %

"si'" "~sir ~ " ~ ot ca, ,.~ "SI......~....~ ---CH ~" ~ " J,u s ,,/si.,.,~,,.~....o/

imprinted material

acid-catalyzed hydrolysis

oca~ca, 150 CII,CHIO- ~ -'OCH:CH~ ~,ca,

CHrO--~--N--(CH )- ~i--OCHtCH, ~ l'l &IHIIClt,

. o

o~-o/~\ s~" c.,

"si-.o.A. . . . . S~ ~ 1 1 ,

o

o.~.o..Si_o, o "s~7 ~ ..

~

o/ ca, "s~_...~---ca, s~ ~n,

o--~-o.-~,-o"

O/si-O/Si'''O/ S(" %0 s( cHj ~o s, "o ,, c A , - o .'s," o sr" ' ~H, "Si ..~

Figure 4. Schematic illustration of preparing an imprinted silica. The first step provides the imprint-material composite, while the second step yields the final catalytic material (refer to Figure 2). This type of material reveals microporosity site pores in the 10/~ range and is able to perform base catalysis on liquid-phase reactants. 4.

COMPARISON OF CATALYTIC ANTIBODIES ORGANOSILICATE, CATALYTIC MATERIALS

TO

Lerner and co-workers have generated catalytic antibodies that mimic class I aldolase e n z y m e s [19] and can catalyze a broad spectrum of aldol condensations and retroaldol reactions [20]. The active site in the catalytic antibody 38C2 was shown to contain a lysine residue that provides a primary amine in a hydrophobic cavity of approximately 1 nm in size. This primary amine could be reacted with acetylacetone to give an enamine with a U V absorption at 316 nm. With the catalytic antibody 38C2 (initially provided by Dr. Lerner and subsequently purchased from Aldrich), we reproduced the results of

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Lerner et al. for the enamine formation with 38C2 and acetylacetone and the kinetics of the following retroaldol reaction in phosphate buffered saline (PBS) [21]: H

I

6-(4'-dimethylaminophenyl)-4-hydroxy-5-hexen-2-one

acetone

dimethylaminophenylcinnamaldehyde

Subsequently, we prepared both an organic functionalized molecular sieve (OFMS) [16] and an imprinted, amorphous silica [18] containing aminopropyl groups. These solids possess microporous voids of size close to 1 nm and aminopropyl groups in hydrophobic regions. Thus, they have numerous properties similar to the active site region of antibody 38C2. The organosilicates are able to form an enamine like antibody 38C2 (see Fig. 5 for results with the imprinted silica). When performing the retroaldol reaction listed above with the

+ H 2 ~ acetylacetone

pH 7.4

imprintedsilica

316 nm, enamine

:3 >,

I,, .Q l... , . .

316 nm, enamine

=:

_= . . . . . . . 200.00

250.00

. , .-.---~.. 300.00

,-..L...,, 350.00

400.00

wavelength ( n m )

Figure 5. Enamine formation from an imprinted silica containing aminopropyl groups and acetylacetone

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OFMS in PBS, the reactant was absorbed so strongly that no products were observed. In order to change the partitioning/adsorption characteristics of the reactant, numerous solvents were explored. However, we were unable to find a single or mixed solvent system that would allow direct comparison between antibody 38C2 (which does not catalyze the reaction in pure organic solvents) and the OFMS [21]. The aminopropyl-containing OFMS was able to catalyze the retroaldol reaction in hexane solvent. If the moles of product per second per site is compared for antibody 38C2 in water and for OFMS in hexane both at 296 K, the values are approximately 5 x 1 0 "3 and 5 x 106, respectively [21]. Clearly, all antibody sites are accessible while such is not likely the case with the OFMS. The OFMS crystals are approximately 4 microns in size and it is highly unlikely that aminopropyl groups in the center of the crystal are utilized. The value of 5 x 10 .6 is therefore a lower bound based on the assumption that all aminopropyl groups are active. An upper bound using an estimate for only the number density of cages with direct communication to the bulk solvent (in analogy to the antibody) gives a value near 5 x 10z. The true value is somewhere between these bounds and remains unknown until a method of counting the active aminopropyl groups can be developed. The above comparative example does not address the issues of multipositional binding of reactant and conformational flexibility at the active site. However, it does reveal the importance of partitioning and the problem of active site accessibility. Clearly, the OFMS material is too hydrophobic since it will not desorb the reactant/product in PBS. If an OFMS could be produced with more hydrophilicity, then it may be possible to make the direct comparison between it and the antibody in PBS. Additionally, it is clear that the solid materials have the disadvantage of long pathlengths from bulk solution to active sites within the centers of the solid particles. This effect can be reduced by creating small particles but cannot approach the single molecule (antibody) in a practical sense. This limitation is counterbalanced by the ability of the solids to operate in more diverse reaction conditions, e.g., higher temperatures, higher concentrations, in organic solvents, etc. The issues of multipoint binding during catalysis and conformational flexibility of the active site are currently under investigation. We have shown that multipositional binding can be accomplished with the imprinted organosilicates [ 18] and have prepared solids that can test for conformational flexibility effects on catalysis, e.g., we have prepared OFMS from the following organosilanes (phenyl group sulfonated to create acid site):

)

$i

/1\

/

\

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Additionally, the protein-based catalysts can perform chiral reactions. In order to obtain chiral induction, there must be at least three positions of interaction between the reactant (or more specifically the transition state) and the catalyst. We have successfully positioned three functional groups in an amorphous, imprinted organosilicate (see Fig. 4 for schematic [18]) in anticipation of extensions to chiral arrangements. 5.

SUMMARY

The idea of molecularly designed heterogeneous catalysts has been in the literature for a long time. However, as new information arises, e.g., the interplay of binding affinities and conformational flexibility on catalysis with the antibody, it can be exploited to develop more effective solid catalytic materials. The case study provided here serves only to illustrate how a translational strategy can be developed and exploited. 6.

ACKNOWLEDGEMENTS

I thank my co-authors Chris Jones, Katsuyuki Tsuji and Alex Katz (refs. 15-18) and Ken Carlgren (ref. 21) for their patience with me over the past few years. We thank Dr. Richard Lerner for initially providing us with antibody 38C2. REFERENCES .

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

L. Pauling, Chem. Eng. News, 24 (1946) 1375. D.E. Koshland Jr., Angew. Chem., Int. Ed. Engl., 33 (1994) 2375. L. Pauling, Am. Sci., 36 (1948) 51. A. Tramontano et al., Science, 234 (1986) 1566. S.J. Pollack et al., Science, 234 (1986) 1570. Y.M. Chook et al., Proc. Nat. Acad. Sci. U.S.A., 90 (1993) 8600. M.R. Haynes et al., Science, 263 (1994) 646. P.A. Patten et al., Science, 271 (1996) 1086. G.J. Wedemayer et al., Science, 276 (1997) 1665. H.D. Ulrich et al., Nature, 389 (1997) 271. S.N. Thorn et al., Nature, 373 (1995) 228. C.B. Khouw et al., J. Catal., 149 (1994) 195. G. Langhendries et al., J. Catal., 187 (1999) 453. S. Klein and W.F. Maier, Angew. Chem., Int. Ed. Engl., 35 (1996) 2230. C.W. Jones et al., Nature, 393 (1998) 52. K. Tsuji et al., Microporous and Mesoporous Mater., 29 (1999) 339.

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17. 18. 19. 20. 21.

59 C.W. Jones et al., Microporous and Mesoporous Mater., 33 (1999) 223. A. Katz and M.E. Davis, Nature, 403 (2000) 286. J. Wagner et al., Science, 270 (1995) 1797. T. Hoffman et al., J. Am. Chem. Soc., 120 (1998) 2768. K. Carlgren, M.S. Thesis, California Institute of Technology (1998).

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Millisecond Chemical Reactions and Reactors*

Lanny D. Schmidt Department of Chemical Engineering and Materials Science University of Minnesota Minneapolis MN 55455

Abstract Short contact time chemical reactors have many features which are very different from conventional packed bed reactors in that temperatures are determined by inlet parameters only, performance is nearly unchanged over wide variations in flow rate, and highly nonequilibrium products can be obtained at high conversions. Chemical reactions occur in regions of large gradients in composition and temperature, so the kinetics of the processes cannot be simply extracted from experiments. We illustrate these systems with recent examples of oxidative dehydrogenation of ethane at 85% ethylene selectivity and cyclohexane partial oxidation to oxygenates and olefins at 80% selectivity. A major feature of both reaction systems is the probable existence of two zones in the monolith where oxidation reactions occur close to the entrance and successive reactions such as homogeneous pyrolysis occur after all 02 is consumed and the surface is passivated with carbon. For the single gauze reactor only homogeneous oxidation occurs after the gauze where the temperature decreases strongly and quenches further decomposition of olefins and oxygenates. Considerations of scaleup of these reactors for large-scale chemicals production are discussed.

* This research partially sponsored by grants from NSF and DOE.

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Introduction

In order to compete with an existing technology, a new chemical process must have lower capital and operating costs and produce higher selectivities and conversions or some combination of these advantages to give a significant economic incentive for its implementation. Partial oxidation processes are among the most promising alternatives, and partial oxidation catalysis is among the most practiced research and development areas 1-7. Partial oxidation processes are mildly exothermic, so process heat requirements are small or negligible, in contrast to endothermic reactions where the reaction heat must be supplied externally. The downside to partial oxidation is that it competes with total oxidation to CO2 and H20, which is much more exothermic. Another significant problem with catalytic partial oxidation is that total oxidation can also react homogeneously in free radical chain reactions which can cause flames and explosions. Another advantage in partial oxidation processes is that reactions can be very fast, thus requiring smaller equipment. Beginning with the Ostwald process for ammonia oxidation to NO (actually a superoxidation), short contact time reaction processes have been practiced commercially for nearly a century ~. As listed in Table I, several short contact time partial oxidation processes are commercially practiced or appear attractive for commercialization. Millisecond reactors usually operate close to adiabatic because it is usually impossible to add or remove heat in millisecond times even in small lab reactors, and therefore the temperature can be adjusted only by varying the preheat and composition of the feed into the reactor. The other important factor controlling the reactor temperature is of course the conversion and selectivity. If the conversion is zero, obviously no heat is generated in the reactor. The selectivity is also crucial in determining the reactor temperature, with the temperature being close to the adiabatic calculation. As the selectivities go from partial to total oxidation, the temperature should rise significantly (400 ~ for

C H 4 to

syngas and 2000 ~ for total combustion). Therefore, the only way to have

stable operation in a millisecond partial oxidation process is for the selectivity to partial

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oxidation processes to increase with increasing temperature. Otherwise a positive feedback situation arises where the temperature will run away to total combustion.

Fortunately, most reactions favor partial oxidation as temperature is increased as long as the reaction is run in excess fuel. Millisecond partial oxidation processes usually go to completion in 02 as long as they are operated in excess fuel 917. The 02 conversion usually cannot be controlled except by adjusting input variables. This is usually desired because it can be quite hazardous to have any 02 in products. This also requires that the fuel conversion be fixed by the selectivities.

In this paper we will briefly summarize recent results with partial oxidation in millisecond reactors in order to consider some of the dominant issues in understanding the mechanisms and in designing optimal reactor configurations. We will show that these apparently simple processes are in fact quite complex, and many issues must be included in their interpretation. We will focus on two recent examples, the oxidative dehydrogenation of ethane to ethylene and the partial oxidation of cyclohexane to oxygenates. In ethane to ethylene we have recently found that, by using Pt-Sn instead of Pt as catalyst and adding large amounts of H2, the selectivity to C2H4 rises from 65% to 85%. The only way we have found to explain these results is that addition of H2 causes catalytic H20 formation in the first zone which consumes all 02, suppresses COx formation, and allows dehydrogenation to occur selectively in the second zone. In the second example, we have recently found that cyclohexane can be partially oxidized using a single layer of Pt gauze as catalyst. This process produces -40% oxygenated products and -40% olefins with little dissociation of the parent alkane, and this also must occur in two reaction zones with homogeneous reaction producing most oxygenates.

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Millisecond Reactors and Reactions The first observation we should make is that millisecond reactors are not at all

new. In the nineteenth century Sir Humphrey Davy and Michael Faraday used Pt wires to catalyze the oxidation of methane, ammonia, and other fuels in air with a millisecond contact time on metal wires. Several millisecond reaction systems have been practiced commercially and several are attractive. These reactions are summarized in Table I. Early in the twentieth century Wilhelm Ostwald repeated the Davy Faraday experiment with woven wires of Pt and showed that large amounts of NO could be produced by oxidizing NH3 in air in an excess air environment 8, NH3 + 5/402 --> NO + 3/2H20. This process has evolved to be the dominant process to produce ntric acid, now typically using -5% NH3 at - 10 atmospheres. In the 1950s Andrussow showed that by adding C H 4 to a NH3+O2 mixture in a 1/1/1 ratio, it was possible to obtain -70% selectivity to HCN in the reaction CH 4 +

NH3 + 3/202 --> HCN + 3H20

based on both CH4 and NH3, and this process is widely used to produce HCN in Nylon and MMA synthesis 8"~8. A third industrial process using millisecond reactors is the oxidation of methanol to formaldehyde 8, CH3OH + 1/202 --->HCHO + H20, which takes place on silver needles in a thin layer -5 mm thick with high gas velocities to achieve a residence time of several milliseconds. The gas composition and diluents are adjusted to that the adiabatic temperature is -600~ We and others have been exploring other millisecond reaction systems. One is the partial oxidation of CH4 to syngas 459~3 CH4 + 1/202 --->CO + H2, which competes with steam reforming (an endothermic catalytic reaction) and autothermal oxidation (a homogeneous process). The oxidative dehydrogenation of light alkanes to olefins 3 14 9 "17 C2H6 + 1/202 ---> C2H 4 + H 2 0

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is also an attractive alternative to steam cracking s C2H 6 ---)C2H 4 + H 2,

because the oxidation processes are exothermic and do not require furnaces to heat the reactants and supply the heat of endothermic reactions. Another type of millisecond reactor is the single gauze reactor where a single layer of Pt or Ptl0%Rh gauze is used as the catalyst ~922. This configuration uses the catalyst to ignite the reaction. Homogeneous reaction then occurs downstream of the gauze, and this produces both olefins and oxygen containing products with only 20% of the alkanes being converted to the total oxidation products CO and CO_,. An example we have recently investigated is partial oxidation of cyclohexane, cyclo-C6Hl2 + 1/202 ~ olefins + oxygenates + H20. In this reaction system -40% selectivity to olefins and 40% selectivity to ozygenates is observed with the dominant products being cyclohexene and 5-hexanal respectively. Figure 1 summarizes these results for C H 4 to syngas (Rh is the preferred catalyst), C2H 6 to C2H 4 (Pt

or PtSn are the preferred catalysts), and cyclohexane to olefins and

oxygenates on a single gauze. The optimal yield (selectivity multiplied by conversion) occurs near the fuel/O2 ratio predicted by the reaction stoichiometry, and the horizontal . axis of figure 1 is alkane/O2, which are optimized at 2/1 for syngas and for C2H4 and at -3 for cyclohexane/O2. The table and graph show typical results, and slightly better performance than those shown can usually be obtained using preheat or by optimizing other variables. Some reactions are clearly not suitable for millisecond reactor operation. Millisecond reaction systems must be fast and exothermic overall. High temperatures are obviously necessary for significant reaction in 103 sec. For a first order reaction we require that kz-1 or ko exp(-ER/RT)= 1/z = 103 sec ~. This can only be achieved if the rate is sufficiently high, and this requires that T be large for normal values of preexponential ko and the reaction activation energy ER, typically above 900K.

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For the systems described above, the kinetics are fast enough that reaction is

essentially complete in times less than 1 millisecond, and this requires that reaction times on surfaces are much shorter than this. The adsorption lifetimes of molecules on a surface at 1300K are extremely short. Assuming 1: = 1/k = 10~3 exp(+EJRT) with Ed the desorption activation energy, one predicts that for bonds of 25 kcal/mole (chemisorption), 10 kcal/mole (weak chemisorption), and 3 kcal/mole (van der Waals bonds) the adsorption lifetimes are 109, 10~'-, and 10~5 sec. These are extremely short adsorption lifetimes, and it is reasonable to expect that rate parameters measured at low temperature might not be accurate at high temperatures because of lack of energy equipartition at these temperatures.

Reactor Geometry The performance of millisecond reactors depends crucially on the geometry of the catalyst, the gas flow pattern, and the temperature and composition profiles created. The flow configuration and the temperatures of gases and catalysts are sensitive parameters in controlling selectivity and conversion 9~7. Foam monoliths.

Sketched in figure 2 are the temperature and concentration

profiles expected in millisecond reactors using ceramic supported metals (left) and for the single woven gauze (right). Monolith ceramics (typically low area c~-A1203 in forms such as a foam, extruded, fiber, or sphere bed with 0.1 to 1.0 mm channels) are used as supports, and metal is deposited on their surfaces from salt solutions. Metal weight loadings are 0.1 to 20%, and performance is fairly insensitive to loading because metals form micron size particles which coat the walls of the ceramic monolith. Therefore, to a good approximation the metal on a porous ceramic catalyst is a continuous film, not dissimilar to a metal gauze in size of structures and flow patterns. However, partial oxidation catalysts are quite different from conventional metals supported on high area porous oxides where surface areas determine performance and loss of surface area by sintering or poisoning is a major problem. Surface area is to a good approximation not an issue in short contact time reactors. Poisons such as sulfur are not a problem on metals

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such as Pt and Rh because temperatures are too high to form significant coverages of

most poisons. The monolith temperature is measured to be uniform to within ~ 100~ as sketched in figure 2. The gas temperature rises from -25~ (or higher with preheat) to the monolith temperature within ~ 1 mm, and to a good approximation the gas and surface temperatures are equal within 1 mm of the entrance. After the gases leave the catalyst. they cool slowly by conduction from the tube walls such that the temperature is -100~ within 20 cm after leaving the catalyst. Most reaction occurs within 1 mm of the entrance to the catalyst as predicted by detailed 2D modeling and observed by using a monolith only 1 mm thick which shows that all O~. is consumed in this distance. All reactions after the 02 is consumed must be decomposition of the remaining alkane or reaction of products with fuel molecules or products or with H:O. Full 2D modeling also confirms that the temperature reaches the surface temperature in less than 1 mm and that near the entrance a boundary layer of <0.1 mm thickness is set up. This predicts temperature gradients of up to 106 K/sec and 105 K/cm. Single gauze reactor. The temperature and composition profiles are quite different for the single gauze reactor as sketched in the right panel of figure 2. Approximately 20% of the reactants contact the gauze surface, heating some of the gases to the surface temperature of--800~ and producing mostly CO and CO2 by surface reactions. For CH 4 through C3H8 as fuels, surface reactions dominate, producing mostly CO and olefins, with little oxygenates and considerable 02 breakthrough. For CaHl0 most 02 is consumed, and multiple steady states 2~can be observed in which homogeneous reaction could be ignited or not ignited, depending on whether a composition was attained starting from richer or leaner composition. With higher alkanes in the single gauze reactor, the 02 conversion is >90% and considerable oxygenates are formed. This must arise through homogeneous reactions which occur downstream of the gauze, and the unique temperature profile of the single gauze reactor allows homogeneous reactions which form oxygenates without subsequent reactions which would decompose them.

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The 20% of the reactants which pass near the surface are heated to ~800~

the gases that bypass the gauze wires remain at 25~

while

These gases then mix rapidly

downstream of the gauze to give a temperature profile as sketched in figure 2. The temperature measured a few millimeters downstream of the gauze is 400~

and this is

close to the calculated adiabatic temperature for the conversions and products observed, so the overall process is nearly adiabatic. The single gauze reactor is thus a rapid heat, rapid quench reactor. Rapid heating prevents reaction before the catalyst because the gases remain cold. Rapid quenching downstream of the gauze caused by mixing of cold and hot gases cools the products to a sufficiently low temperature that subsequent decomposition reactions (dehydration of alcohols, decarbonylation of aldehydes, and decarboxylation of carboxylic acids) is suppressed. In all experiments with conventional monoliths the oxygenates observed are always less than 0.1% for any conditions. We attribute this to the fact that any of these products which form would react quickly in the presence of surfaces to that none of them survive. The exact temperature axially and radially around the gauze wires is not known because it depends on the details of mixing and the reactions that occur in this region. Most of the homogeneous reactions are exothermic, and the temperature could in fact rise above the surface temperature just downstream of a wire. Detailed 2D calculations will be necessary to determine this temperature profile. While the Reynolds number is low (~ 1) so the flow is laminar and detailed flow calculations should be possible, the properties vary strongly with temperature, and the temperature profiles cannot be predicted intuitively. Further, the reactions and their rates are uncertain for these low temperatures and in excess fuels, and these must be known before detailed simulations are possible. The most important species in homogeneous reactions at low temperatures are probably the alkyl peroxy radicals and alkyl hydroperoxides which propagate chains, forming oxygenates without chain branching and further oxidation. Kinetics of reactions of these species have not been characterized in detail.

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Two-Zone Model of Millisecond Reactors Millisecond partial oxidation reactors have an unusual feature that is not seen in

most chemical reactors in that reactions always go to completion because >99% of 02 is consumed in essentially all experiments in excess 0:. It is virtually impossible to have 02 breakthrough except by having bypass because of poor sealing of the catalyst in the reactor tube or having dead zones in the catalyst. Even in the single gauze reactor, more than 90% 02 conversion is typical. Further, partial oxidation processes can be run with typically a factor of at least 10 variation in flow rate with almost no change in conversion or selectivities ~'23. Both of these limits are associated with cooling of the catalyst significantly below the adiabatic temperature. The lower limit to flow rate occurs when heat removal through the walls or by radiation is sufficient to cool the catalyst to a temperature where coke or

CO,

formation occurs. The upper limit to flow rate is where blowout begins by having the front face of the catalyst cool off sufficiently that reaction slows down so that heat generation slows. In industrial HCN synthesis no 02 is ever detected in the product stream, and in nitric acid synthesis there is no NHs in the product stream (the Ostwald process is run in excess 02) unless the gauze catalyst has holes in it. This is true for a wide range of process conditions, including large variations in flow rates, which are basically limited in commercial processes by pumps or heating or cooling limitations. It is essentially impossible to flow too fast in these commercial processes s. This is basically good in that one wants no unreacted 02 in the product because of separation problems and possible unwanted downstream reactions. It also allows the process designer great flexibility in sizing equipment. This occurs because all 02 is consumed in the entrance region of the monolith (within the first millimeter in most experiments), as sketched in figure 3. Higher flow rates simply extend the O2-containing zone farther downstream into the monolith. The limit is of course when the flow rate is so high that 02 escapes, but this is frequently associated with blowout when the front face cools.

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Millisecond reactors therefore have a short oxidation zone where the surfaces are

covered with oxygen, followed by an O2-free zone. This zone is in highly reducing conditions, and models of syngas and olefins predict that these surfaces are covered with nearly a monolayer of carbon, which may passivate the reactor against further reaction. However, additional reaction of the alkane may occur in the second zone as sketched in figure 3, and the control of these reactions is an important factor in designing partial oxidation processes. Syngas. For CH4 oxidation to syngas, CO and H2 are equilibrium products, so no further reaction of these species can occur. However, unreacted C H 4 can still react in steam reforming and CO2 reforming CH4 + H20 ~ CO +3H2, and c n 4 + C O 2 --4 2 C O +2H2,

and it has been frequently suggested 46 that syngas from methane is produced through a two-stage process where the first reaction is total combustion CH4 + 202 ---) CO2 +2H20,

followed by steam and CO2 reforming. We believe that under most situations, particularly where the temperature is >800~

most syngas is produced in a direct

reaction rather than a two-stage process, in contrast to the two-zone picture of figure 3. Addition of excess H20 and CO2 suggest that these reactions are too slow to be significant in producing syngas on Rh except at even higher temperatures. Ethylene. For ethane to ethylene C2H 6 + 11202 --~ CzH 4 + H20,

we believe that the process in fact occurs in two stages. In the absence of added Hz, the two reactions in the oxidation zone are C2H 6 + 02 ~

2 C O + 3H2,

and C2H 6 + 1/202 ----) C2H 4 +

H20

along with reaction of the H 2 produced by the first reaction to form water H 2 + 1/202 ~

H20.

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Then, when all 02 is consumed, the remaining C2H6 dehydrogenates to form more ethylene, C2H 6 ---> C2H 4 + H 2.

Since on Pt without H E added, -20% of the carbon in C2H 6 is converted to CO and CO:, this fraction of C2H 6 converted to COx in the oxidation zone. In experiments where H2 is added to the reactants, the dominant reaction in the oxidation zone is n 2 + 1/202 --> H20,

while in the second zone the major reaction is direct hydrogenation C2H 6 ---> C2H 4 + H 2.

Since with H2 added (with a Pt-Sn catalyst rather than Pt alone) the selectivity to CO falls to 5%, the direct oxidation of C2H 6 to C O must consume only this fraction of the C2H6 because most 02 reacts with H2 rather than attacking C2H 6.

In both of these situations the reactions are strongly exothermic in the first zone and strongly endothermic in the second zone, as sketched in figure 3. Thus, reaction should pull the temperature up in the first zone and down in the second, although radiation and solid conduction should make the monolith temperature fairly uniform. In the single gauze reactor there are obviously two zones because only homogeneous reaction can occur downstream of the catalyst. In this situation both sets of reactions are exothermic, and the temperature profiles are not known.

Scaleup and Scaledown One of the virtues of millisecond reactors is that they can be scaled up and scaled down very easily. In contrast to conventional packed bed reactors, millisecond reactors process large amounts of reactants, typically 1 kg/day of products from a 1 cm 3 catalyst support and <0.1 gram of catalyst metal. Scaling this throughput (space velocities of 105 to 106 hr~), one predicts several tons of product/day from a 1 foot diameter reactor, and perhaps 1000 tons/day from a 6 foot diameter reactor. Laboratory millisecond reactors operate nearly adiabatically for reactor volumes as small as 1 c m 3 in that temperatures are

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within 100~ of predicted adiabatic temperatures. Consequently, a larger version of a lab

reactor should give nearly identical performance with scaleup by simply increasing the monolith catalyst diameter while keeping its length (bed depth) the same. A large-scale reactor might be constructed as sketched in figure 4. We envision a reactor configuration similar to those used for ammonia oxidation in the Haber process or HCN synthesis in the Andrussow process (see Table I). These are both millisecond reactors operating at superficial velocities of 1 to 20 m/sec at pressures of 1 to 10 atm. The catalyst is a gauze pack consisting of--20 layers of woven gauze for a monolith thickness of less than 1 cm. The monolith catalyst is mounted on a ceramic support which acts as a radiation shield, and a top radiation shield is sometimes added to minimize backflow of heat by radiation in the mixing chamber above the catalyst. This chamber is usually cone-shaped to minimize the premixed volume, and a static mixer above this cone combines the reactants rapidly. These reactors are constructed of stainless steel with hot regions lined with ceramics to protect against extremely high temperatures. Mixing is one of the crucial engineering features of these reactors because a major concern is flames and explosions in this zone if gases are improperly or incorrectly mixed. The flow velocity should be a large as possible so that the flow velocity in this zone exceeds the flame velocity, although gases are usually nonflammable for designed conditions. Preheat is usually accomplished by heating gases individually before mixing. Below the catalyst layer is a heat exchanger which cools the products quickly to avoid reactions subsequent to the catalyst from the reaction temperature of 800~ (ammonia oxidation) or 1100~ (HCN synthesis) to the temperature needed for subsequent processing. These millisecond reactors have been operated for decades at large scale for nitric acid, Nylon, and MMA synthesis, and they have been optimized for safe, efficient, and reliable operation. Catalyst stability is an important issue in these high temperature reactors, and the industrial catalysts are quite stable for operating times of many months. Metal loss is a problem for NH 3 oxidation but not for HCN synthesis where 1 year of continuous operation without shutdown is common even though the catalyst is at 1100~

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more than 100 ~ hotter than used for syngas or olefins reactors. The metal losses in NH 3 oxidation even at temperatures as low as 800~ occur because the reactor operates in

excess O_, where volatile Pt oxides evaporate. Possible catalyst supports for a large-scale syngas reactor (Rh) or an olefins reactor PtSn) might be ceramics in foam, extruded, fiber, or sphere forms. All of these supports give comparable performance within a few percent ~~ Obviously, catalyst and support stability is a major concern in scaling up these processes, and the thermal stability and especially thermal shock resistance must be designed appropriately. Scaledown. Another virtue of millisecond reactors is that they can be scaled down easily for small applications where one needs small amounts of product such as for on-site generation of hazardous chemicals or for distributed power applications such as the automotive fuel cell. The lab scale reactor should be directly transferable into these applications if gas flows and heat removal can be designed correctly.

Summary Millisecond reactors introduce fascinating complexities to partial oxidation compared to conventional reactors. Both surface and homogeneous reactions occur simultaneously, and a major consideration is the design of reactor geometry to optimize the contributions of surface and homogeneous reactions by choosing temperatures and times at which they occur to maximize desired selectivities and conversions. We have considered several aspects such as the two-zone model which seems to fit observed behavior fairly well. These reactors are easily scaled up and down by simply changing the diameter of the catalytic monolith without changing other operating parameters because reactors operate nearly adiabatically at all sizes. We suggest that there are probably many other situations where oxidation reactions can be run at very short contact times to produce high yields of nonequilibrium products.

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References

Third World Congress on Oxidation Cata!ysjs, R. K. Grasselli et al, editors, Elsevier, 1997. S., Cavani F., Trifiro F., "Key Aspects of Catalyst Design for the Selective Oxidation of Paraffins", Catalysis Reviews-Science & Engineering 38(4) (1996) 413-438. H. H. Kung, "Oxidative Dehydrogenation of Light Alkanes", Advances in Catalysis 40, 1, (1994). Choudhary V. R., Uphade B. S., Belhekar A. H. "Oxidative Conversion of Methane to Syngas over LaNiO3 Perovskite with or without Simultaneous Steam and CO2 Reforming Reactions - Influence of Partial Substitution of La and Ni", J Catal 163(2) (1996) 312-318; "Beneficial Effects of Noble Metal Addition to Ni/A1203 Catalyst for Oxidative Methane-to-Syngas Conversion" J Catal 157(2)(1995) 752-754.

.

Tsang S. C., Claridge J. B., Green M. LI H., "Recent Advances in the Conversion of Methane to Synthesis Gas", Cat. Today 23(1) (1995) 3-15. .

.

.

.

Claridge J. B., Green M. L. H., Tsang S. C., 'Methane Conversion to Synthesis Gas by Partial Oxidation and Dry Reforming over Rhenium Catalysts' Cat. Today 21 (23) (1994) 455-460. A. Bielanaki and J. Haber, Oxygen in Catalysis (Marcel Dekker, 1991. Charles N. Satterfield, Heterogeneous Catalysis in Practice, McGraw Hill, 1980. D. A. Hickman and L. D. Schmidt, "Syngas Formation by Direct Catalytic Oxidation of Methane", Science 259, 343-346 (1993): "Syngas Production by Direct Oxidation of Methane on Monoliths", J. Catalysis 138, 267-282 (1992).: "Synthesis Gas Formation by Direct Oxidation of Methane over Monoliths", Catalytic Selective Oxidation, ACS Proceedings 32, 416-426 (1993).: "Elementary Steps in Methane Oxidation on Pt and Rh: Reactor Modeling at High Temperature", A.I.Ch.E Journal 39, 1164-1177 (1993).:, "Syngas Formation by Direct Oxidation of Methane over Rh Monoliths", Catalysis Letters 17, 223 (1993).

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10.

P. Torniainen and L. D. Schmidt, "Comparison of Monolith Supported Metals for the Direct Oxidation of Methane to Syngas", J. Catalysis 146, 1-10 (1994)

11.

K. Hohn, P. Witt, and L. D. Schmidt, "Oxidative Coupling of Methane to Acetylene on Monolith Catalysts at Millisecond Contact Times", Catalysis Letters 54, 105-112 (1998).

12.

O. Deutschman and L. D. Schmidt, "Partial Oxidation of Methane in a Short Contact Time .2465-2477 (1998).

13.

O. Deutschmann and L. D. Schmidt, "Two-Dimensional Modeling of Partial Oxidation of Methane in a Short Contact Time Reactor", Twenty-Seventh Symposium (International) on Combustion, The Combustion Institute, 2283-2291 (1998).

14.

M. Huff and L. D. Schmidt, "Olefin and Syngas Formation by Direct Catalytic Oxidation of Ethane at Short Contact Times", J. Phys. Chem. 97, 11815-11822 (1993).

15.

A. Dietz III and L. D. Schmidt, " Effect of Pressure on Three Catalytic Oxidation Reactions", Catalysis Letters 33, 15-29 (1995).

16.

S. Bharadwaj and L. D. Schmidt, "HCN Synthesis by Ammoxidation of Methane and Ethane on Pt Monoliths", J. Mol. Catalysis A: Chem, ~105, 145-148 (1996)

17.

A. Bodke, D. Olschke, L. D. Schmidt, and E. Ranzi, "High Selectivities to Ethylene by Partial Oxidation of Ethane", Science 285, 712, (1999).

18.

L. D. Schmidt and D. A. Hickman, "Surface Chemistry and Engineering of HCN Synthesis", in Catalysis of Organic Reactions, Kosak and Johnson eds., pages 195212 (1993).

19.

D. Goetsch and L. D. Schmidt, "Microsecond Catalytic Partial Oxidation of Alkanes", Science 271, 1560-1562, March 15 (1996).

20.

D. Iordanoglou and L. D. Schmidt, "Oxygenate Formation from n-Butane Oxidation at Short Contact Times: Different Gauze Sizes and Multiple Steady States", J. Catalysis 176, 503-512 (1998).

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21.

D. Iordanoglou and L. D. Schmidt, "Oxygenates from Alkanes in Single Gauze Reactors at Short Contact Times", J. Catalysis 87, 400-409 (1999).

22.

R. P. O'Conner and L. D. Schmidt, " Partial Oxidation of Cyclohexane in a Single Gauze Reactor", J. Catalysis, to be published.

23.

P. Witt and L. D. Schmidt, "Effect of Flow Rate on the Partial Oxidation of Methane and Ethane", J. Catalysis 163, 465-475, (1996).

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>.., ~ ~

77

100 ~- CO from CH4 on Rh

> r,.)

C H4 from C . H { on PI-Sn with H.

a,~

80-

=

--

-

Oxygenates§ C)leflrtS fr,~m c-C Hi: on PI-10%Rt~

-

12"

m

03

E o < I

o

o

s

C=H= from C2H6 Oll PI-Sn

60Oxygenates from c-C6H12 on PI-10%Rh "

40

-

1 7 6 L.~.a,..~

I~ " = ~ ~ ~

"

E~,

-------- . . . .

C

....... ~ "- 9 Olefins from c-CsH12 on PI-10%Rh dlA

o,...,..=

~ o , o . 00 " d r " " " " " ~ " 0 " " " ' ; "

,

@

............

~

c

-

,4P

20-

0

1

1.5

2

2.5

C/0 n

100

3 2

3.5

4

4.5

in Feed

on R-Sn with H

.,.._.

< o ~

0

80

60

L_

> r-.o

o

40 9~ C y e l o h o e x a n e

20

0

1

i

I

1.5

2

on Pt-lO%Rh

!

2.5

C/0 n

2

I

I

3

3.5

~

o

4

4.5

in Feed

Figure I. Selectivities and alkane conversion for three partial oxidation processes versus alkandO: ratio for room temperature feeds with catalysts indicated. For CH4 to syngas -90% selectivity at 90% CI'14conversion is obtained using Rh on c~A1_,O3foam. For ethane to ethylene -70% selectivity is obtained using PtSn an a-Al:O3 foam and -85% selectivity using this catalyst with H: added (single point on 6aph) with a feed of C2I-~0.JH2=2/1/2. Using cyclohexane on a single Pt gauze -80% olefins+oxygeaates are obtained, although this occurs at cyclohexane/O2--3 where the cyclohexane conversion is -20%.

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Single Gauze Reactor

化

Monolith Reactor

o

~

I

o o

~ \1 o

1300K

%

-

T 300KI

9

fuel

Cj

\

~oxidation I

02

I I I I I I

1000K

T

I

I

I

Y

300K fuel

Cj.

r

!

I

i desired I products I

homogenous zone

02

I

I

II

II

z-~

desired products

I

Figure 2. Sketches of temperature and concentration profiles expected in millisecond reactors for ceramic foam (left) and single gauze (fight) reactors. Temperatures are fairly uniform in the foam, and gases and surface attain the same temperature within -1 mm of the entrance. For the single gauze reactor part of the reactants are heated rapidly in -100 ktsec, but most gases bypass the catalyst (lower T curve) and are heated by mixing with hot gases downstream of the catalyst where homogeneous reaction occurs and the gases are quenched in ---10.3 sec to preserve oxygenates.

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化

Two Z o n e Model olefins:

1

C2H 6 ~ C2H 4 + H 2

H2 + ~ 02 ~ H20

syngas"

CH 4 + H20 --* CO + 3H 2

CH 4 + 202 .-. 0 0 2 + 2 H 2 0

CH 4 + CO 2 ~ 2CO + 2H 2

fuel 02

Cj

i

I 1

I

i

i

I

I

I

,,product pfuuu~

L

o

T '

'

'

1

'

Figure 3. The two-zone model of millisecond partial oxidation, indicating that 02 is consumed within a short distance in the reactor. After this zone, the surface becomes carbon covered and homogeneous reactions can be important, such as for formation of ethylene by pyrolysis. A similar situation exists with the single gauze reactor although the second zone occurs downstream of the gauze.

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mixer

""~

heat

Figure 4. Illustration of scaleup of a millisecond partial oxidation process from a single tube -2 cm diameter to a large reactor such as used in NH3 oxidation and HCN synthesis. Conditions are similar in both situations, and the major problems are mixing in the larger reactor and thermal stability of the large diameter monolith.

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Table I Some Millisecond Reaction Processes

temperature

catalyst

selectivity"

Name conversion

major reaction

Ostwald

NH3+O ~ NO+H20

800~

Pt 10%Rh gauze 90%

90%

Andrussow

CH4+NH3+O 2 --> HCN+H20

IO0~

Ptl0%Rh gauze 70

70

Formaldehyde CH3OH+O 2 --->HCHO+HEO

600~

Ag needles

80

90

Syngas

C H 4 + O 2 ---)

1000~

Rh on monolith

95

95

Olefins

C2H6+O2 ~ C_,H4+H2

1000~ 950~

Pt on monolith Pt-Sn +H,

65 85

80 70

800~

Pt single gauze

40 40

20

CO+2H2

Single Gauze C.H2.+2+O_, ---) olefins ---) oxygenates

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This Page Intentionally Left Blank

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

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C a t a l y s i s for oil r e f i n i n g a n d and future trends

petrochemistry,

recent

developments

G. Martino Institut Fran~ais du Petrole l&4 avenue de Bois Preau, 92582 Rueil-Malmaison Cedex 1. I N T R O D U C T I O N Catalysis a multidisciplinary science is a very important tool for most of the industries. Industrial catalysis contributes to about 25% of the whole Gross Domestic Products (GDP) [1] and in the most developped countries the generation of a million s added GDP is linked to the spending of about 300~ of catalysts [2]. At present, worlwide catalysts demand is at about 10 Billion C. Oil refining, chemical and petrochemical industries and environmental protection are the main users of catalysts as presented in table 1. Table 1 2000 Worlwide catalysts demand (- 10 G s Refining 25% FCC HDT/HYC Others 42% Chemical processing Chemicals Polymers Emission control 33% Motor vehicules Others

10 10 5 28 14 30 3

Future projections of this demand is difficult to make but most manufacturers [3] are looking for growth in the next five years and even beyond. Heterogeneous catalysis remains the most important one, most of the developments in the fields of environnental protection are based on solids, mainly metals and oxides supported on monoliths. A lot of new polymerisation catalysts are solids and their preparation is based on several organometallic chemistry steps.

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The worldwide foreseen growing rates for the different domains are, rather low for oil refining (2 to 3% per annum), reasonable for environmental protection (4 to 5%) and rather high for chemicals (more than 6%). Regional growths may be significantly different, due to the contrasted starting points as presented in Table 2. The biggest growth rates are expected to take place in the Asia Pacific area. Table 2 2000 Estimates for specific countries (G (~) USA Refining 1.1 Chemical processing 1.1 Emission control 0.7 Total 2.9 * AP/J Asia Pacific+Japan

EUROPE 0.5 1.9 1.0 3.4

AP/J* 0.4 0.9 0.5 1.8

These trends seem reasonable for the oil refining and petrochemical industries which the present paper will be devoted to. It will be limited to the manufacturing of hydrocarbons, cuts or pure compounds like motor fuels, olefins and basic aromatics. 2. O I L R E F I N I N G 2.1. P r e s e n t s i t u a t i o n

At present, there are about 600 refineries running worldwide. Catalytic reforming and hydrotreating capacities as well as the necessary flue gas treatment are present in most of them. Thermal units like visbreaking or coking are also used. But only about one half of the refineries have a catalytic cracking unit and one on four only have implemented an hydrocracker. As a consequence, only a few families of the different catalysts are largely present in all the refineries (table 3). For each family several catalysts manufacturers offer proprietary products [4]. For most on these catalysts, important improvements have been introduced during the recent years but only a few catalysts, clearly different in concept of the existing ones, have been brought up and introduced into a refinery since 1990. These improvements have largely benefited of the tremendeous efforts made by the catalyst community in the last 20 y e a r s ; most of it has been brought together in the handbook edited by E. Ertl, H. Knozinger and J. Weitkamp [5]. Sophisticated physical technics, surface and material sciences as well a molecular modeling have allowed heterogeneous catalysis to move from , , black art to atomic understanding ~,.

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Table 3 Main basic refining catalysts Catalysts Ni, Pd, Pt/carriers Metals Pt + M/alumina Pt/chlorided alumina or mordenite Group VI + Group VIII Sulfides on carriers USY/carriers Solid H3P04/silica Acids Resins HF, H~SO 4 Modified aluminas Others Co Phtalocyanine/carbon

Processes Hydrogenations Catalytic reforming Paraffin isomerization Hydrotreating Hydrocracking FCC Polymerization MTBE Alkylation CLAUS Sweetening

Chemical engineering has made a lot of progresses in kinetic modeling and transport phenomena understanding [6] and has allowed an optimized use of industrial catalysts.

2.2. D r i v i n g f o r c e s The evolution of the refining industry is m a r k e t driven but the market rules are heavily puzzled by environmental concerns and the interference of political decisions well ahead of scientific consensus. For instance MTBE production peaked in 1998 [7a] and was hailed as the environmentaly friendly component for gasoline but it's future now looks in doubt, at least in California and other states in the USA [7b,c] and perhaps elsewhere. That means t h a t any forecast has to be done at our best knowledge on a ,, as today business ~ basis. 2.2.1. P r o d u c t d e m a n d e v o l u t i o n Worldwide oil consumption is foreseen to increase slightly beetwen now and 2010 as indicated in Table 4. For 2020, the increase may continue but it will be linked to the introduction of new kinds of transportations vehicules. Mainly the ratio of heavy ends to transportation fuels will continue to decrease ; n a t u r a l gas and renewable energies taking over as fuels for fixed power stations. Table 4 World oil consumption (GT) Transportation fuels Petrochemicals Other now energy uses Heating & ind. fuels Total

1995 1.600 0.192 0.192 1.216 3.200

2000 1.870 0.250 0.215 1.265 3.600

2010 2.320 0.300 0.250 1.430 4.300

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An other i m p o r t a n t point for the refiners is the continuous evolution of the ratio beetween gasoline and middle distillates as indicated in table 5. This raises the question of the hydrogen to carbon ratio in the refinery finished p r o d u c t s ; the hydrogen richest product is diesel oil and in the future the hydrogen content m a y even increase with the decrease of their aromatic content. The same trend is expected for gasoline with the projected aromatics and olefins content reduction.

Table 5 World distillates d e m a n d (MT) 1995 Gasoline (~) 860 Middle distillates (2) 1085 (1) including naphtha- (2) including heating oil

2000 950 1250

2010 1150 1550

Even w i t h o u t taking into account the most controversial concerns on climate change arising from Greenhouse Gases (GHG) effects, motor fuels are u n d e r p r e s s u r e in order to contribute with the engine's modification to meet the future tighter tailpipe emissions. This p r e s s u r e has led to the i m p l e m e n t a t i o n of new specifications all around the world and by j a n u a r y I st 2000 in Europe where more severe ones are expected to become m a n d a t o r y by 2005. Table 6 shows the existing specifications for gasoline. The most stringent ones concern benzene, reduced to 1% volume, sulphur, brought down to 150 ppm. The b a n of lead in most european countries gasoline has started by the i st of j a n u a r y 2000. By 2005, vapor pressure reduction will be mandatory, aromatics content will be limited to 35% volume. For s u l p h u r a m a x i m u m of 50 ppm has been decided but the possibility to move down to 30 or even to 10 ppm are u n d e r consideration. For C a n a d a and USA, 30 ppm average have been adopted. The possible lowering of the specification on olefins would severely h u r t all refiners using FCC as a work horse for conversion. Table 6 Evolution of gasoline specifications in E U Property 2000 TVR s u m m e r (Kpa) 70 Benzene (% vol) 1 Aromatics (% vol) 42 Olefins (% vol) 18 02 m a x (%wt) 2.3 S max ppm 150 Pb (max) g/1 0.005

2005 60 1 35 (i0) 50 (30) -

The situation for diesel oil (table 7) indicates the clear bend to reduce s u l p h u r in order to contribute to a reasonnable solution to NO x emissions. Europe h a s

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moved from 500 ppm to 350 by j a n u a r y this year. C e t a n e n u m b e r h a s been increased, polyaromatics are limited to 11% wt, 95% volume distillation point h a s been brought down from 370~ to 360~ Table 7 Evolution of diesel specifications in E U Property 2000 S u l p h u r m a x (ppm) 350 Cetane n u m b e r min 51 Polyaromatics m a x (%w) 11 Density (Kg/l) 0.845 T95 m a x (~ 360

2005 50 (30) (55) - (1) - (0.84) (340)

There is a definitive obligation to reach the 50 ppm level of s u l p h u r in diesel by 2005 in Europe. O t h e r countries have not yet m a d e their decision. Figures as low as 30 or 10 ppm are, even today, requested by refiners for the design of their new h y d r o t r e a t i n g units. O t h e r properties m a y give rise to new specifications as indicated in column 2 u n d e r brackets. Cetane n u m b e r of 55 and polyaromatics at 1% wt would need expensive investments. Reduction of T95 and density would reduce the a m o u n t of diesel oil available and require the implementation of several hydrocrackers. 2.2.2. O t h e r c o n s t r a i n t s As any industrial operation, oil refining generates different waste s t r e a m s which have to be h a n d l e d and induce operating costs. W a t e r -up to a few cubic m e t e r are used per ton of crude- is more and more u n d e r scrutinizing [8]. Solids like catalysts are more and more subject to r e t r e a t m e n t s . S u l p h u r which h a d a sales price in the past m a y become an issue as h y d r o t r e a t i n g is growing. Gaseous s t r e a m s are the m a i n concerns today [9]. For nitrogen oxides, sulphur dioxide, particulates and VOC's emissions new limits have been adopted in Europe for 2007 (table 8). Table 8 Limits of emissions in E U (after 2007) Pollutants Power >500 MW SO 2 mg/Nm 3 400 NO x mg/Nm 3 450 Particulates mg/Nm 3 50

Power <300 MW 1700 450 50

Other products emissions m a y also in the future be more strictly limited, even though their concentrations are very low at the outlet of incinerators [10].

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The issues concerning GHG is not yet clear; carbon dioxide is the main involved gas but not the only one. CO~ is produced by any energy production from hydrocarbons ; taxation will have important impacts, directly or indirectly on the refining industry. Inside the refinery, where 5 to more than 10% of the crude is used to make energy or hydrogen, there will be a renewed fight for energy saving [11]. A new insight in hydrogen production as well as its utilization [12] will come up with technologies which could introduce new hydrogen producing routes. Any ton of hydrogen produced starting from resids leads to 15 tons of CO~ and even with natural gas about 10 tons of CO~ are generated. This could even modify tremendously some, today, classical refining schemes and push to non hydrogen using processes. It could also severely impact the price of the new high hydrogen containing fuels and fasten the implementation of a new generation of cars with high efficiency propulsion systems like hybrid vehicules or fuel cells engined cars. A fast evolution in that way, seems not to be the most likely ; it would bring us out of a forecast of , , business as usual -. The heavy investments required in such a case would be insustainable for refiners faced with low margins of less than 20 Eurosfron of crude in most cases. 2.2.3. C o n c l u s i o n s Without taking into account this extreme issue of severe CO 2 taxation, refiners will have to : 1. make more and cleaner transportation fuels while reducing production costs and coping with waste management. 2. handle the incertainties concerning the agenda of the implementation of the new specifications not only for the motor fuels but also for the other products like resids. 3. face competition for their resids in fixed power stations ; natural gas gives less CO 2 and contains less sulphur. 4. find ways to increase profitability through specialties productions as well as petrochemical intermediates.

2.3. R e c e n t c h a n g e s a n d f u t u r e t r e n d s Refiners have been able to satisfy the demand of products by continuously improving their units. Their refining schemes have been modified to cope with the crude oils they could buy. Until 2010 it seems that rather conventional crude oils could be available ; this means that only a few heavy crude upgrading refineries will be erected in the next years. As the use of heavy ends will continue to decline, new resid conversion units will be implemented and on going R&D will be necessary in order to reduce operating costs and investments. Resid desulphurization will be needed before combustion ;if not, flue gas treatment will be required.

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As indicated before, this paper will now focus on motor fuels production in the next decade.

2.3.1. Gasoline m a n u f a c t u r i n g The main blends contributing to the gasoline pool are reformate, isomerate, catalytic cracking, coking and visbreacking gasolines, polymerization and alkylation gasolines as well as ethers. The ban of ethers (~ 30 MT/year), will be difficult to handle if other limitations, mainly on aromatics, are introduced at the same time. Catalytic reforming Catalytic reforming has been questionned several years ago when limitations on benzene were introduced. This issue has got several solutions. It is admitted t h a t catalytic reforming is the only cheap, almost without CO2, hydrogen producer in a refinery. Most of the recent improvements have been dealing with how to get a high continuous production of hydrogen in order to maintain the whole refinery on stream. More and more refineries have implemented continuous catalysts regeneration and catalyst have been improved as indicated in table 9 [13, 14]. The same trends will remain for the future. Table 9 Low pressure catalytic reforming with continuous regeneration Catalyst changes Impact Activity yl Throughput yl Selectivity yl H 2 + aromatics yl Recently Coke formation Benzene Thermal stability of carrier yl Throughput ;~ Time on stream yl Tomorrow Same trends

Paraffin i s o m e r i z a t i o n (table 10) The use of light paraffins isomerization has grown during the last decade. Mordenite based catalysts have been less successful t h a n platinum on chlorinated aluminas. For the latest, some major changes have been introduced : improved catalysts which allow operations with very low hydrogen to hydrocarbon ratios and which have lower sensitivity to the higher cyclo alkane concentration linked with the benzene management [15]. The off site regeneration of some of these catalysts has also been demonstrated.

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Table 10 Paraffins isomerization Catalyst changes Stability of Pt/alumina Activity Pt/alumina Recently Off-site regeneration Pt/alumina New sulfated oxides

Activity ~ Tomorrow Cracking ~

Impact H 2 recycling Cyclo alkanes in feed ;~ Life Stability/impurities No chlorides But activity Octane :~ or Throughput ~t T r e a t m e n t of C7/C8's

A new generation of catalysts based on sulfated zirconia [16] has shown interesting features: higher activity than mordenite and lower sensitivity to poisons but, until now, lower activity than chlorinated aluminas. Nevertheless, their operation without producing waste chlorides might save money and certainly gives them a , , green ~ image. There is a lot of progress possible in that line and work is going on in numerous laboratories for C4,s [17] but also for C5, C 6 and other paraffins. Zeolites may not be forgotten on the way of treating even higher paraffins [18]. The coupling with separations processes able to screen dibranched paraffins from monomethyl and normal paraffins will promote other catalysts.

Fluid catalytic cracking Fluid catalytic cracking remains the most important process to convert vacuum distillates, deasphalted oil and partly pretreated resids into lighter ends. It has benefited of continuous improvements as indicated in table 11. The big recently rised issue is the sulphur content of the FCC gasoline - Several solutions have been developped to reduce it, although limiting the octane loss. Table 11 Fluid catalytic cracking Catalyst changes New matrix Recently

Tomorrow

Zeolitic additives De S O additives New zeolites Coke formation

Impact Bottom conversion Coke formation Olefins production S O formation Olefins ;~ Heavier feeds ~I

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Mobil's Octgain as well as Intevep's Isal couple hydrodesulphurization with reactions (isomerization, alkylation) which allow to recover the octane loss due to olefins saturation. Exxon/Akzo [19] and IFP/Procatalyse have choosen the way of selective hydrotreating catalyst developments and their optimized use. CDTech [20] has taken advantage of the catalytic distillation process to reach the goal. Adsorption has been priviledged by Phillips [21] and others [22] to eliminate sulphur. Processes will be demonstrated soon. Even if catalytic cracking produces a very bad diesel oil (LCO), it seems that it will continue to be used in the future. Feed pretreatment with more or less conversion may help to get more and better diesel. FCC, as indicated in tableau 11, will remain as a conversion work horse oriented to convert heavier feeds and produce more light olefins. Cost reduction of gasoline desulphurization will remain an important issue mainly if hydrogen becomes more expensive.

Alkylation of light olefins (table 12) Alkylation of light olefins with isobutane leads to good gasoline blends. The environmental concerns about liquids acids (HF, H2SO 4) have at least been partially solved. On the other side, solid catalyst alkylation does not show, beside of being more environmental friendly, any significant economical advantage.There have been progresses, concerning mainly regeneration [23, 24] and new solid acids [25] have been claimed. Table 12 Light olefins alkylation Catalyst changes HF complexation H~SO 4 on site regeneration Recently H 2 regeneration of solid acid New Solids Tomorrow Other solids (low cracking)

Impact Vapor pressure Transport Costs :~ Investments Quality of product 71 Convert higher olefins

If ethers are banned, a strong need for more alkylation and more C~ olefins alkylation too will appear.

Polymerization of C ] C 4 FCC cuts (table 13) Polymerization, in fact, oligomerization, uses at present mainly two kind of catalysts : supported phosphoric acid and homogeneous Ziegler type nickel. Both have environmental drawbacks and are best suited for C 3 cuts or C t cuts without isobutene.

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Table 13 Polymerization of C]C~ FCC cuts Catalyst change Solid regenerable catalysts Recently Solid for iso C: -~ i C~

Tomorrow

Selectivity yl -~ branched C 6 - C~ Selectivity yl linear > C~ Selective solids

Impact Waste Replacement of MTBE Gasoline 71 Diesel yl C~-C7 olefins conversion

Recently the possible use of other solids have been demonstrated with long cycles and several regenerations. The ether ban could bring up a new d e m a n d for the use of isobutylene. Revamping of MTBE units has been announced by Snamprogetti [26] and indirect alkylation has been claimed [27]. Iso octenes are produced ; they can further be hydrogenated into iso octanes. Tomorrow, more olefins will be produced in FCC and more propylene will go to the petrochemistry. There is a clear need to convert not only C4.s but also higher olefins into valuable products, gasoline or diesel oil.

2.3.2. Diesel oil production The trends for diesel oil future d e m a n d as well as quantities and qualities are concerned are clear : more and better products are needed. Although the final specifications are not yet defined, m a n y refiners prepare themselves to the most stringent situation.

Deep desulphurization A lot of efforts have been made by the refining industry with the help of technology and catalyst suppliers to reach the 500 ppm specification as indicated in table 14. Each catalyst m a n u f a c t u r e r has brought up more active catalysts in order to avoid investments [28, 29, 30] ; activity increases by factors up to 2 are claimed. Intrinsic activities have been enhanced through better carriers, higher metal loadings. A lot has been added by chemical engineering contributions : control of fluid distribution in reactors [31], use of series of different catalysts, best adapted to the chemical t r a n s f o r m a t i o n requested, hydrogen effects on the different reactions have been reanalyzed and mastered.

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Table 14 Deep desulphurization of diesel oil Catalyst change Recently

Tomorrow

Activity ;~

Different ones Activity yl Stability yl Regenerability yl

Impact Throughput yl or Cycle yl or HDS ;n HDS yl HDS Y, Cycle Life :~

In order to keep most investments as low as possible further improvements of catalysts activities are needed. These improvements may arise from changes in the active phases, nature and porosities of the carriers but also in changes in startup procedures for instance. There may be on important competition between catalysis and other technologies like adsorption, oxidation followed by washing or biodesulphurization [32].

Deep hydrogenation (table 15) A few deep hydrogenation units, have been implemented in order to provide city diesel. These processes are based on sulphur resistant noble metal catalysts [33, 34] ; their main drawbacks are their cost and the need to work at rather low sulphur levels (< 50 ppm). This has been overcome with process modification by Lummus/Criterion with the Synsat process [35] where hydrogen flows at countercurrent to the feed and allows for the highest hydrogen pressure at the outlet. Implementation of many deep hydrogenations will require a lot of hydrogen units and, alternatives to reduce the large amount of noble metal catalysts. There is a need for a better sulphur resistance or for other more active catalysts. There may be a place for alternative ; non hydrogen based processes. Table 15 Deep hydrogenation of diesel oil Catalyst change Zeolite supported metals Recently Bimetallic noble metals New carrier Tomorrow New bimetallics Non noble metals

Impact Activity yl Activityyl Activity yl Activity yl Cost

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H y d r o c r a c k i n g (table 16) The existing move to the erection of maxi distillates hydrocrackers, with selective amorphous catalysts operated at high pressure in 2 stages units, fills the gap for the increased middle distillate demand. Products are of good quality but investments as well as hydrogen consumption are high (> 2% wt/feed) [36, 37].

Table 16 Hydrocracking atalyst change lectivity ;n

Impact Middle distillate yields

Recently

Tomorrow

Stability 71 Selectivity 71 Stability Selective ring opening

Pressure ~ or cycle length 71 Yields 71 H 2 consumption Pressure Cetane 71

It is important to reduce the cost of investment by lowering operating pressures and by reducing the size of units with new more active catalysts. The processing of LCO or naphthenes rich crudes requires not only hydrogenation but also ring opening activity [38] while minimizing hydrogen consumption. Competition may arise in the future from Fischer Tropsch products. 2.3.3. C o n c l u s i o n s The refining industry is a mature industry where costs reduction is the day to day issue. Important changes in quantity but mainly in quality of the demand are foreseen. The possible GHG taxes implementation add new incertainties. Newertheless refiners will adapt themselves, while catalysts manufacturers and research and development will have to play their respective roles. Catalyst manufacturers will have to improve continuously the existing products and find, in some cases, new solutions. Paraffins isomerization, olefins alkylation, deep hydrogenation of diesel oil and ring opening may be the domains where some breakthroughs can be foreseen. 3. P E T R O C H E M I S T R Y

As presented before this topic will be limited to the routes for the production of the main basic olefins and aromatics.

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3.1. P r e s e n t s i t u a t i o n At present, olefins are mainly manufactured by steam cracking of ethane, naphtha and heavier cuts. Aromatics, benzene, toluene and xylenes are coproduced in steam crackers when naphtha is used as a feed, but the main source remains catalytic reforming of n a p h t h a . The industry uses a limited number of kind of catalysts (table 17) ;noble metals on different carriers, solids acids like zeolites which tend to progressively replace liquid acids and some mixed oxides, for dehydrogenation mainly. Table 17 Main basic petrochemical catalysts Catalysts Metals Pt+(M1)/alumina on

oxides Metals on zeolites Zeolites + other acids Oxides

Pd+(M2)/alumina or silica Pt]MFI P t ~ mordenite Ni/H mordenite MFI HM, H beta HY or A1Cl~ Iron+ alkaline oxides Chromium oxide + alkaline oxides

Processes Aromatization Dehydrogenation of paraffins Selective hydrogenation Xylenes isomerization Xylenes isomerization Transalkylation of aromatics Xylenes isomerization Benzene alkylation with light olefins Dehydrogenation of ethylbenzene Dehydrogenation of light olefins

Unlike oil refining, catalysts are tuned to each operation, this opens the need to more specific research and developments.

3.2. D r i v i n g forces Costs reduction of the whole production chain from raw materials to the intermediate olefins or aromatics is the main driving force. The high required purity is increasing in connection with the quality improvements of the final products : polyethylenes, polypropylenes or polyterephtalates for instance. Separation and purification operations have a very important role. The intrinsic selectivity of the catalyst used not only saves raw materials but also decreases the cost of purification. The evolution of the global demand is another important driving force, olefins demand (table 18) will grow at a rather high rate [39]. The differences of growing rates for the different olefins, 3,5% for ethylene, more than 5% for propylene and more than 8% for some of the higher olefins, introduce a need of different production routes.

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Table 18 Worldwide main olefins demand (MT)

Ethylene Propylene 1 Butene Higher a olefins

1998 80 45 0.8 1.0

2010 120 82 1.4 2.2

Aromatics demand (table 19) is growing at a reasonnable rate of about 4%, paraxylene much faster (6%) and, even m xylene or dimethylnaphtalene are supposed to grow at much higher rates. Table 19 Worldwide main aromatics demand (MT) 1998 Benzene 27 Toluene 13 p Xylene 14

2010 40 21 30

There are big differences in regional growing rates ; South East Asia is expected to have the highest ones. Increased demand and better quality at lower cost push the industry to maintain an important research and development effort.

3.3. R e c e n t c h a n g e s and future t r e n d s 3.3.1. Olefins Ethylene Ethylene remains the most important olefin ; most of it is obtained from steam cracking of ethane (40%) or of heavier hydrocarbons. Recent evolutions in steam crackers have been devoted to cost reduction through several modifications (table 20). Table 20 Ethylene Recently

Tomorrow

Millisecond furnace 1" conversion Better selective hydrogenation catalysts Use of condensates Increase conversion to more than 85% New adapted hydrogenation catalysts Use of selected feeds New routes

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The use of cheaper feeds like gas condensates have brought up some concerns on arsenic and mercury removal. Another move to get higher per pass conversion is likely. This will lead to the formation of more acetylenics and will requires renewed downstream hydrogenation processes. New routes are of interest. Oxidative dehydrogenation of ethane which is a big challenge could have a better chance to succeed compared to the oxidative coupling of methane to which a tremendous amount of work has been devoted during the 80's. Methanol to olefins (MTO), demonstrated by Mobil [40] in the 80's, could take advantage of the excess capacity of methanol which could result of an MTBE ban. UOP and Norsk Hydro have brought up a new process [41] which gives propylene to ethylene ratios of up to 0,7 which is higher t h a n those obtained by n a p h t h a steam cracking.

Propylene More t h a n 60% of the needed propylene is produced by steam cracking of naphtha. In the recent years several propane dehydrogenation units have been erected but the growing source of propylene has been FCC units. The implementation of FCC splitters, the design of dedicated olefins producing units like the DCC of RIPP/Stone & Webster [42] and the use of additives, MFI type zeolites, have helped to provide the required amount of propylene to the market. Table 21 Propylene Recently

Tomorrow

Propane dehydrogenation processes & catalysts More propylene ex FCC (new catalyst + processes) More propylene ex FCC (new catalyst + processes) FCC more oriented to olefins/new additives MTO propylene oriented Metathesis implementation

These trends will be maintained and there is place for new additives and new modifications of the FCC units. Dehydrogenation may be favored in places where FCC units are too far away [43]. Considered today as rather expensive, dehydrogenation can be expected to be improved on both catalysts and process sides. The use of m e m b r a n e reactors as well as the introduction of oxidative dehydrogenation are considered as possible future solutions [44]. Metathesis of ethylene and 2 butene is another alternative which helps to get more propylene while using the low value C 4 cut [45]. The IFP/CPC's low temperature rhenium based process and the Phillips ABB Lummus, high temperature process, have to cope with catalyst deactivation and the need of frequent regenerations. Progresses can be expected for both processes.

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MTO as presented previously may lead to interesting ratios of propylene to ethylene. Some older datas obtained on mordenite at IFP have shown that yields of up to 60% wt of C 2 to C 4 olefins can be obtained with a ratio of propylene to ethylene of more than 5. This indicates that it may be possible to get out of MTO, the desired slate of olefins. Further studies will allow to move in that direction.

Higher a olefins Butene 1 can be extracted from C 4 cuts. It is also obtained by selective dimerization of ethylene with homogeneous catalysts [39] ; higher a olefins are mainly obtained via oligomerization of ethylene. As a consequence only higher a olefins with even numbers of carbon atoms are produced in most cases with a Schulz Flory like distribution. Recently pure hexene-1 has been obtained with the new Phillips catalyst. Fischer Tropsch oriented toward olefins production gives a olefins with any carbon atom number but their purification is difficult. Sasol has implemented a separation for hexene 1 and has brought hexene 1 (ex FT) on the market. Improvements in both catalysts and separations may make that route more competitive. 3.3.2. A r o m a t i c s BTX (table 22) The starting point of the production of aromatics is the mixture of benzene, toluene, xylenes (BTX) which is mainly obtained, at present, by catalytic reforming of naphtha (60%). The other way is steam cracking of naphtha (25%). Catalytic reforming has benefited of the improvements described in the gasoline section (2.3.1.).

Table 22 BTX production

Recently

Tomorrow

Improved catalytic reforming and solvent extractions Distillations and other separation processes Specific feeds ; dedicated catalysts - Ba, K/L C 6,C 7 feeds - Ga/MFI C~, C 4 feeds Further improvements of catalysts Increased use of shape selectivity

Some dedicated processes based on new catalysts have been proven at large scale and several industrial units are scheduled to start in the near future. Platinum on baryum/potassium exchanged zeolite L initially patented by ELF [46] and further developed by Chevron is well adapted to get more aromatics out of C 6 and C 7 paraffins. The advantage on platinum on alumina catalysts seems not to be as high for C s hydrocarbons.

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The BP/UOP cyclar process [47] based on Ga/MFI, converts LPG's, a rather cheap feed in some places, and produces an important amount of hydrogen as well as fuel gas. Both kind of catalysts could be improved. There might be place for specific catalysts for other niche applications.

Xylenes production (table 23) The full range BTX feed is handled in different units where benzene, toluene, C s mixtures and heavier aromatics are separated. Only benzene and p xylene are largely used ; the excess of the others is converted as much as possible into benzene and p xylene through transalkylation, isomerization or dealkylation. Several new catalysts have been introduced into the market place for xylenes isomerization. Mobil's MFI based processes remain at the top for isomerization of xylenes with dealkylation of ethylbenzene. The mordenite based catalyst, which isomerizes ethylbenzene into xylenes is being challenged by other zeolites based catalysts [48, 49] which lead mainly to better selectivities toward p xylene. Table 23 Xylenes production Recently

Tomorrow

Better catalysts for xylenes isomerization Improved catalysts for transalkylation Use of shape selective catalysts to convert toluene -~ p xylene + benzene More use of shape selectivity Integration of reaction/separation

Transalkylation of toluene with C 9 aromatics has been developed on Ni/mordenite based catalysts. Their main limitation was the quantity of Cgs which could be fed to the reactor. The most recently developed catalysts seem to overcome that difficulty ; the TAC9 process claims to be able to handle up to 70% of C9 aromatics. The better understanding of coking of this kind of catalyst can help to increase their performances. Toluene disproportionation is another way to get more benzene and more xylenes. The Mobil MSTDP has been demonstrated several years ago [50]. Based on ZSM 5, it has been continuously improved. The two main features of xylenes production are : - the involved reactions are equilibrated and mainly catalyzed by zeolites. - products with close ebullating temperatures are obtained which makes separation by distillation difficult.

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The future trends take into account both these facts. Catalysts selectivity mainly shape selectivity and integration of reactions and separations will be main lines of improvements.

Styrene (table 24) Styrene is produced via a two step process ; benzene alkylation with ethylene and further dehydrogenation. Table 24 Styrene Recently Tomorrow

New alkylation catalysts/processes Improved DH with higher conversion Introduction of post-DH, selective hydrogenation Higher conversions/selectivities in dehydrogenation New routes

For the first step zeolitic catalysts are, since the 70's, progressively replacing the other catalysts. The Mobil Badger process operates in vapor phase with MFI type catalysts. Recent processes use liquid phase with other zeolitic catalysts. CD Tech has introduced the use of catalytic distillation ; this allows better heat integration and avoids polyalkylation. There is place for further improvements. The dehydrogenation step, limited by equilibrium has been improved by subatmospheric pressure operation and selective hydrogen removal (Styro Plus process of UOP). These higher conversions lead to acetylenics formation and require the addition of a specific purification. The main expected improvements concerning the dehydrogenation step are higher conversion and better selectivity which would help to reduce costs. Membrane technologies have also been considered [51]. New routes have to be considered ; the most advanced one is based on butadiene dimerization into vinylcyclohexene and further dehydrogenation into styrene ; DSM and DOW have been the most active companies in this field ; it is a way to use excess butadiene. These two steps processes have to be improved in order to become competitive.

3.4. Conclusions In the future the petrochemical industry has to face an increase in the demand of existing products and a requirement for higher purities and for lower costs. A rise-up of niche products can also be foreseen. In order to tackle both these challenges, innovative solutions have to be brought up. More extensive use of condensates and better integration with the refining industry will give access to lower priced feeds.

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More selective even integration expenditures and New catalytic products.

101 catalysts, more efficient catalytic and separation processes and of both kinds of processes will help in lowering capital operating costs. routes may open the road to real breackthroughs and new

4. G E N E R A L C O N C L U S I O N S Without a severe taxation of GHG, mainly carbon dioxide, it appears that both the oil refining and the petrochemical industries will see an increasing demand for their products ; nevertheless the average growing rate is much higher for petrochemicals. The production of environmental more friendly fuels and higher purity petrochemicals creates a need for improvements in catalysts and processes. There is a strong need for new catalytic materials with well defined sites [52] but also a deeper integration of catalysis and reaction engineering [53] taking into account the overall process, catalytic and separation sections. Modeling and simulation will used more and more and be of great help. Combinatorial methods [54] could become a must for future new solids screening. REFERENCES

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44. T.G Tsikocannis - D.L. Stern - R.K. Grasseli, Journal of Catalysis 184 (1999), pp. 77-86 45. J.A. C h o d o r g e - D. Commereuc, PTQ (spring 1997), p. 109 46. J.R. Bernard, Proceeding of the fifth Int. Zeolite Conference, Ed. L.V. Rees H e y d e n - London (1980), p. 686 47. P.C. D o o l a n - P.R. Pujado, Hydrocarbon Processing (1989), 68 (72) 48. H.H. J o h n - H.D. N e u b a u e r - P. Birke, Catalysis today 49 (1999), pp. 211-220 49. J. R a u l t - P. R e n a r d - F. Alario, DGMK Erlangen, (Oct. 1999) proceeding pp.131-138 50. F. G o r i a - L.L. Breckenridge - W. G u y - R. Sailor, Oil & Gas, Oct. 12, (1992), p. 6O 51. F. Tiscarenp-Lachuga- C.G. Hill - M. Anderson, Applied Catalysis (1993), 96 pp. 33-35 52. J.M. Thomas, J. Molecular Catalysis A Chemical 146 (1999) pp.77-85 53. F. D a u t z e n b e r g - H.E. B a r n e r - M. Maraschino, Cat Con 98 Houston (June 1999) 54a L.L. Bellavance, Chemical Engineering (Sept. 1999), pp. 76-80 54b B. Jandeleit - H.W. Turner - T. Uno - J.A.M. Van Beek - W.H. Weinberg Cattech V2(2), Balzer Science Publication (Dec. 1999) ACKNOWLEDGEMENTS

I wish to thank J.F. Gruson from IFP's strategy and economics division and Dr. Ph. Courty for their help in preparing this paper.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

New Catalysis from Metal Oxide Surface Science Mark A. Barteau Center for Catalytic Science and Technology, Department of Chemical Engineering University of Delaware, Newark, DE 19716 I. Introduction Surface Science---particularly studies of the chemistry and physics of solid surfaces under ultrahigh vacuum (UHV) conditions, to which this label is oRen applied--has long been recognized for its potential impact on diverse technologies. These range from corrosion, to adhesion, to tribology, to semiconductor fabrication, but perhaps the most important technological motivation is, as suggested by Woodruff and Delchar [1], "the goal of understanding heterogeneous catalysis." Beyond (or perhaps as a result of) achieving that understanding, one can look toward the invention of new catalysts from surface science discoveries. The power of surface science is clearmto define the site requirements for surface reactions by application of chemical and spectroscopic probes on surfaces with well-defined structures. Research on macroscopic single crystals has provided clear connections between reactivity and surface structure for a number of reactions, both on metals and metal oxides. The question of whether stable adsorbates examined in surface spectroscopic studies are relevant to catalytic processes or whether they represent unreactive dead-ends has also been debated, but in a number of cases, especially oxidation reactions, the connection between surface spectroscopy and catalytic phenomena is simply inescapable. One question that arises about surface science on both metals and non-metals is its impact on the development of n e w catalysis. While fundamental studies have made clear contributions to the understanding and improvement of existing catalytic processes, it is more difficult to identify new catalysts or processes that have originated from surface science discoveries. While one can debate whether or not a "first principles" approach is necessarily the most direct or efficient route to new technology, the potential for application is often proffered as the justification of much fundamental research in this and other fields. There are at least two other reasons to pursue this strategy. First, the chemistry investigated on solid surfaces in UHV is usually stoichiometric, not catalytic. The establishment of new paradigms for turning stoichiometric reactions into catalytic ones, whether on solid surfaces or in homogeneous gas or liquid phases, will have ramifications far beyond applications of surface science. Indeed, such issues are at the heart of the pursuit of "atom economy" in organic syntheses [2]. A related motivation comes from the impetus to develop ever more environmentally benign processes. One of the challenges facing catalysis research is to develop efficient, selective, and economical replacements for multistep stoichiometric organic syntheses. To the extent that new catalytic transformations are needed to meet such challenges, new sources of potential catalytic chemistry, including surface science, have an important contribution to make. Considered below are two examples from our laboratories of the translation of principles and chemical discoveries from UHV surface science on single crystal oxides into new catalysis. We have previously reported the first of these, dehydration of carboxylic acids to produce ketenes [3,4], and development cominues [5]. The second example, the reductive coupling of carbonyl compounds, represents a more difficult challenge, but it can be performed catalytically [6], as demonstrated below. Our hope is that these examples are but early members of a series of surfacescience-driven creation of new catalysts and processes.

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II. Catalyst Design for Ketene Synthesis

Ketenes, highly reactive species of the general form RR'C=C=O, are much more common as reaction products in UHV surface science experiments than in catalysis. Since the leap from 1 cm2 single crystals at 10-10 torr to ca. 100 mg samples of catalyst at 1 atm in a microreactor is one of 13 orders of magnitude in pressure and typically at least 4 orders of magnitude in surface area, the potential obstacles to such scale-up are many. In the case of selective ketene production, higher solid surface area and/or higher pressure may contribute to both parallel (non-selective) reactant consumption channels and series reactions consuming the desired product. The challenge of creating a catalyst for ketene synthesis is one of overcoming these obstacles. The motivation is that catalytic processes have not found application to production of these versatile acylating agents. Ketene and its low carbon number homologs are typically produced by pyrolysis of carboxylic acids or ketones at 600-800~ [7]. Larger aliphatic and aromatic ketenes are produced via a multistep sequence involving synthesis of the acyl halide from the carboxylic acid or its ester, followed by dehydrohalogenation of the acyl halide with tertiary amines [7]. A catalytic alternative to either or both of these routes would be desirable from the perspective of waste minimization. A number of examples exist in the surface science literature of ketene production by temperature programmed reaction of carboxylate ligands adsorbed on metal and metal oxide surfaces. Such ligands are formed in most cases either by dissociative adsorption of carboxylic acids or by oxidation the surface of the corresponding aldehydes or alcohols. Among metal surfaces, both Cu [8] and Ag [9] produce ketene (H2C=C=O) by decomposition of adsorbed acetates in temperature programmed reaction/desorption experiments. Group VIII metals tend to produce unselective decomposition of adsorbed acetates, with C-C bond scission leading ultimately to carbon oxides and C~ hydrocarbon moieties. In contrast, a variety of metal oxides from across the periodic table have been shown to produce ketene from adsorbed acetates in UHV studies on single crystals; examples include ZnO [10] TiO2 [11], MgO [12], and SnO2 [13]. On some of these oxides the ketene selectivities in adsorption and temperature programmed reaction/desorption experiments are quite high: 70% on the {011} faceted structure of the TiO2(001) surface, 100% on the MgO(100) surface [11,12,14]. Since the dissociation of carboxylic acids on most metal oxide surfaces is facile, these surface science results suggest the feasibility of catalytic production of ketenes from carboxylic acids via dissociative adsorption and carboxylate decomposition on metal oxides, provided of course that the surface area and pressure gaps noted above can be surmounted. One of the key issues to emerge from surface science studies of oxide single crystals is the recognition of the controlling influence of surface cation coordinative unsaturation--not only on activity, but on the selectivity of surface reactions. The surfaces noted above that are both active and selective for ketene synthesis from carboxylic acids in UHV are those on which each surface cation possesses a single coordination vacancy (at least in the ideal, stoichiometric surface structure). Surface structures that give rise to cations with multiple coordination vacancies, e.g., the TiO2 (001)-{ 114} faceted surfaces, exhibit lower selectivities for ketene formation and higher selectivities for the bimolecular coupling of carboxylates to produce ketones [14-16]. Sites that afford opportunities for coordination of multiple carboxylate ligands at individual cations are undesirable from the point of view of ketene synthesis~what is wanted are sites on which isolated carboxylate ligands can be formed and converted by unimolecular dehydration to ketenes. Beyond the specification of singly coordinatively unsaturated surface cations, an additional design criterion for a ketene synthesis catalyst can also be inferred ~om the single crystal results. The ZnO(0001), MgO(100), and TiO2(001)-{011 }-faceted surfaces all possess one coordination vacancy per surface cation and thus yield no bimolecular coupling products. However, their selectivities for selective unimolecular reaction of surface acetates to form ketene (vs. decarboxylation to yield carbon oxides and carbonaceous deposits) vary widely. The decarboxylation/dehydration selectivity ratio appears to decrease with increasing difficulty of

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oxide reduction [14-17], thus in UHV experiments MgO(100) is the most selective surface for ketene formation. This catalyst specification: a non-reducible oxide with predominantly singly coordinatively unsaturated cations exposed, especially in high surface area, polycrystalline powder samples, can now be applied in search for a ketene catalyst. It should be recognized that the design criteria developed from surface science studies of reactions on oxide single crystals are at best necessary, but not sufficient conditions to identify a selective ketene synthesis catalyst. This becomes apparent if one examines the catalytic properties of the optimum ketene-producing oxide from UHV single crystal studies--MgO. Not only does the MgO(100) surface produce the highest selectivity observed for ketene formation ~om acetic acid in UHV, it fits well with the design criteria for a ketene catalyst formulated fi-om surface science results. It exposes cations, all of which possess the desired single degree of coordinative unsaturation, and it is essentially nonreducible. Moreover, high surface area MgO powders which consist of cubic particles terminated by (100) faces can easily be synthesized, and the single crystal reaction data available on the (100) plane thus represent studies of the dominant surface of typical MgO powders. In short, the surface science results and the principles deduced from them point decisively to MgO as the material of choice for selective ketene synthesis from carboxylic acids. Unfortunately, MgO is not a selective catalyst for ketene synthesis at more realistic temperatures. As demonstrated by Sugiyama et al. [18], MgO is a selective catalyst for acetone, rather than ketene, formation from acetic acid at atmospheric pressure. This observation is initially surprising if viewed solely from the surface science perspective, and may lead one to question whether the single crystal surface represents an adequate or appropriate model for that of the high surface area catalyst. The explanation is, in fact, simpler and more reassuring regarding the extrapolation of structure-reactivity principles from oxide single crystals. MgO is slightly soluble in acetic acid. The consequence is that acetic acid uptake does not stop at one monolayer (bound at the surface), in effect exposure of MgO to acetic acid vapor at quite modest pressures (10 tort and up) converts the bulk oxide to the bulk acetate. This can be demonstrated by measuring uptake or breakthrough of acetic acid vapor flowing through a packed bed of MgO powdermthe amounts taken up are far in excess of those required to decorate a monolayer of surface cations with adsorbed acetate groups [14]. Each cation in bulk magnesium acetate is bonded to a pair of acetate ligands, these couple upon pyrolysis of the salt, releasing acetone. While this phenomenon is pleasingly consistent with the site requirement deduced for ketone formation at surfaces by coupling of pairs of carboxylates bound to a common cation, its contribution to the problem of designing a ketene synthesis catalyst is a further restriction: the catalyst must be insoluble in the reactants. This is, no doubt, a general principle of heterogeneous catalysis that is intuitively obvious without surface science. It provides a useful reminder, however, of the mundane but unavoidable pitfalls in "scaling-up" surface science results across the enormous ranges of surface area and pressure noted above. Simple traps permit simple escapes, and the material we have identified as an effective ketene catalyst at higher pressures is simple yet surprising because of the lack of catalytic chemistry usually ascribed to it. That material is silica [3,4]. Carboxylic acids can be adsorbed dissociatively from the vapor phase onto the surface of silica powders. The adsorbed carboxylate intermediates decompose at ca. 700 to 800 K, liberating the corresponding ketenes with selectivities from ca. 40 to 85% for linear and branched C2 to C5 carboxylic acids [3,4]. Typical side products are carbon oxides and the synmletric ketones ((CnH2n+1)2CO) formed by coupling of CnH2,+ICOOH reactants. When operated in a steady state mode, high conversions of acetic acid on silica catalysts are attained, and we have observed selective production of ketene without catalyst deactivation for up to 20 hours. One can indeed produce ketenes by catalytic dehydration of carboxylic acids. As reported elsewhere in these Proceedings, we have achieved per-pass yields of 80% for ketene production from acetic acid, and higher yields of dimethyl ketene from isobutyric acid [5]. One question concerns the nature of the active site on silica and its relationship to the principles deduced from surface science. The single crystal experiments are carried out in UHV in the absence of co-adsorbates that might influence the behavior of adsorbed carboxylates, e.g., surface hydroxyl groups. The active sites for carboxylic acid dissociation are the cation-anion, acid-base site pairs at the surface, with the carboxylate ligands formed by dissociative adsorption

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occupying coordination sites at the surface cation. In contrast, if one produces highly dehydroxylated silicas by high temperature treatments [19], they show little activity for carboxylic acid dissociation and reaction. "Bare" silicas produced at high temperature show little activity for this chemistry. Upon heating, the silanol groups (Si-OH) on silica surfaces eliminate water and form strained siloxane (Si-O-Si) groups. At high temperatures (>500~ the strained siloxanes convert to a more stable form [19]. The silicon atoms in these siloxane linkages are coordinatively saturated, and the stable siloxanes are not easily hydrolyzed, i.e., the Si-O bonds are difficult to break. The relatively inert character is easily reconciled with the need for sites of coordinative unsaturation in order to perform surface acid-base chemistry, illustrated previously by surface science experiments on the polar planes of zinc oxide [10]. An alternative to maintaining coordination vacancies at the surface is to fill these with ligands which are readily displaced by the reactant. This is standard practice in homogeneous catalysis by transition metals, where the soluble metal complex that one introduces into the reaction mixture is often not a direct participant in the catalytic cycle, but serves to introduce active centers into the cycle, often by exchanging ligands which stabilize it in the absence of the reaction mixture [20]. It is perhaps less common to see this strategy applied or identified in heterogeneous catalysis, although one could argue that common practices such as passivation of finely divided metal particles by controlled oxidation is a variation on the same theme. In any case, the maintenance of acid-base sites on silica surfaces by hydroxyl groups is essentially analogous to the example of homogeneous catalysis. Surface silanol groups can be displaced from the surface with stronger acids such as carboxylic acids:

R~

H

I

RCOOH +

O

i

>

I

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I

i

~O + H20

~ 1 si ~

In effect, their role here is to serve as "place holders," to maintain accessible coordination sites for conjugate base ligands at surface silicons, sites which would otherwise be lost by thermally driven dehydration of the surface to form stable coordinatively saturated siloxane linkages. The role of surface OH groups in providing active sites for carboxylate formation from the vapor phase on silica is illustrated in Fig. 1. This figure depicts correlations, obtained both by infrared spectroscopy and by gravimetry, between the initial hydroxyl population on silica, and the capacity for formation of surface acetates. Gravimetric measurements, in particular permit one to determine the surface hydroxyl coverage produced by various catalyst pretreatment procedures, and thus permit the catalyst activity to be described in terms of the turnover frequency of the catalyst sites. Typical values for the turnover frequency of ketene synthesis from acetic acid with silica catalysts at 750 K are 10.3 s1. In order for the reaction to be catalytic, the active site must be regenerated in the course of the reaction sequence. The reaction of surface carboxylates to produce ketene involves the net loss of OH from the carboxylate: RR'CHOO(ad)

>

R

i C=C=O + OH (ad)

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Transmission Infrared Spectroscopy Hydroxyl Peak Area (arbitraryunits) 5

0.3

10

15

,-

0.25 r

-,m

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9

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Hydroxyl Population(OH/nm2) from Thermogravimetry Figure 1. Correlation of acetate coverages following acetic acid adsorption on silica catalysts, with initial hydroxyl population.

This hydroxyl group can then serve as the site for reaction of another molecule of acetic acid, starting the cycle over again. Although the catalyst ultimately identified for the one-step synthesis of ketenes was not one of the materials investigated in single crystal surface science experiments, its origin is directly traceable to the design principles identified by such experiments. This is one of the first examples of the invention of a new catalytic process driven by surface science studies of metal oxides.

IlL

Catalytic Reductive Carbonyi Coupling

We reported previously the first example of reductive coupling of carbonyl compounds, the McMurry reaction [21], as a gas-solid reaction [22]. This reaction, depicted below, is a net 4electron reduction of the carbonyl reactants, and therefore requires reduced surface sites. 2 RR'C = O + nTi x+ ~ RR'C = CRR' + 2[0]+ nTi (X+4/n)+) In this scheme n represents the number of reduced Ti cations involved in this 4-electron reduction and [O] represents the oxygen atoms deposited on the surface by this reaction.

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Although the stoichiometric, liquid-solid version of the McMurry reaction had been known for several decades [21], the work of Idriss et al. [22-25] was the first to demonstrate that this chemistry could be carried out as a gas-solid reaction. Remarkably, this discovery was made in reaction studies on reduced (by ion bombardment) single crystals of TiO2 carded out in UHV. In other words, it was found first on single crystals [22,23] and only later demonstrated on higher surface area powder samples [24]. Our single crystal studies have subsequently shown that this gas-solid reaction can be applied to couple a wide variety of carbonyl compounds, with yields approaching 60% in TPD experiments [23,26]. These studies have demonstrated that the activity of reduced titania surfaces for reductive coupling tracks the extent of reduction of surface cations below the (+4) oxidation state, but that zero valent Ti is not required [23], in contrast to earlier suggestions by McMurry [21 ]. A variety of circumstantial evidence points to the intermediacy of a pinacolate intermediate

2 RCH = O ~

H

H

I

I

R---C--~mR ~

RCH = CHR

I I

O O in this reaction, as suggested by the literature of its liquid solid counterpart. However, spectroscopic results, while consistent with that conclusion, have not permitted definitive identification of the pinacolate. We have also shown that the McMurry reaction can be carried out as a stoichiometric reaction on TiO2 powders, the surfaces of which had been previously reduced by treatment in H2 at elevated temperature [24]. This observation led us, both to quantitative determination of the kinetics of reduction of anatase and rutile TiO2 powders [27], and to the proposal of the following hypothetical cycle for reductive carbonyl coupling [24].

RCH=RCH

Ti 4+

2 RCHO TiX<4*

H2

H20 Figure 2: Proposed catalytic cycle for reductive carbonyl coupling [24].

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Comparison of the single crystal and powder results for reductive carbonyl coupling by reduced TiO2 and for reduction of TiO2 with H2 makes it readily apparent that the ratedetermining step in the above catalytic cycle is likely to be the surface reduction step [27]. Carbonyl coupling occurs with high yield at temperatures of 300-400 K on reduced single crystals in UHV; in contrast, reduction of TiO2 in flowing H2 at atmospheric pressure occurs at a rate of ca. 10.3 oxygen atoms/surface titanium atom see at 773 K. Can we approach this theoretical limit, indeed can reductive coupling be carded out catalytically by reaction of a gaseous stream of 1-12and aldehydes or ketones over a reduced TiO2 catalyst? If catalytic, how selective is the reaction? We have carded out catalytic reductive coupling of four different carbonyl compounds: acetaldehyde, acetone, benzaldehyde and pivalaldehyde in steady state reactions on reduced titania to address these questions [6]. In each case the feed consisted of the carbonyl compound at partial pressures of 5-150 torr in 1 atm H2 flowing over the oxide catalyst at 773 K. Table 1 reports results for the reactions of acetaldehyde and acetone with hydrogen over reduced TiO2 rutile and anatase catalysts at 773 K. The expected reductive coupling products are 2-butene from acetaldehyde coupling and 2,3-dimethyl-2-butene from acetone. These products were indeed observed throughout experimental runs of more than ten hours. However the selectivities to the reductive coupling products for steady-state operation of these catalysts (reached after about two hours on stream) were only about 10% of the carbonyl reactants converted. In each case the principal products were those produced by aldol condensation: crotonaldehyde and crotyl alcohol from acetaldehyde, and mesityl oxide, isomesityl oxide, and isophorone from acetone. Small amounts of hydrocarbons with carbon numbers both lower and higher than those of the expected reductive coupling products were also observed from both acetaldehyde and acetone. Table 1.

Carbonyl coupling yields for acetaldehyde and acetone conversion on reduced titania (10 hours on stream) [6].

Reactant CH3CHO

Catalyst Conversion Coupling Yield TOF (s -~) Anatase 62% 5.4% 0.13 Rutile 74% 6.2% 0.27 CH3COCH3 Anatase 51% 5.8% 0.12 Rutile 63% 7.1% 0.28 Initial Conditions: 100 mg of each catalyst; catalyst reduced 24 hours at 773 K in 1 atm 1-I2 Reaction Conditions: Temperature = 773 K; 150 Torr acetaldehyde or 25 Torr acetone; balance I-I2to 760 Torr total pressure; total gas flow = 30 cm3/min Two important points may be drawn from these results. First, reductive coupling of acetone and acetaldehyde is a catalytic gas-solid reaction on TiO2. The turnover frequencies in Table 1 were calculated on the basis of the adsorption capacities measured at 300 K for acetaldehyde on rutile (3.4 molecules/nm 2) and anatase (2.2 molecules/mm 2) and for acetone (2.9 and 1.8 molecules/nm L on rutile and anatase, respectivel~r [6]. Reductive coupling products were generated with turnover frequencies of 0.01-0.03 s- for more than ten hours of operation, clearly demonstrating catalysis of this reaction. Second, it is not surprising that the dominant products of the reactions of acetaldehyde and acetone were not reductive coupling products. We have previously shown that the principal reaction of these species on oxidized TiO2 surfaces is aldol condensation [28,29]. Since each reductive coupling event reoxidizes the surface cations at which it occurs, and since the rate of surface reduction limits the reductive coupling rate, the steady state extent of reduction of the surface under the conditions utilized for reductive coupling will be relatively small. Thus most adsorbing carbonyl reactants will see oxidized surface sites, and it is

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not surprising that the chemistry of their conversion is that of the stoichiometric TiO2 surface--aldol condensation. One way to preclude aldol condensation is to choose carbonyl reagents lacking the a-H required for this reaction. Benzaldehyde (Ph-CHO) and pivalaldehyde ((CH3)3CCHO) are two such molecules. The expected reduetive coupling products from these two reagents are stilbene (Ph-CH=CH-Ph) and di-t-butyl ethylene ((CH3)3 CCH = CHC(CH3)3), respectively. Tables 2 and 3 report results for reductive coupling of benzaldehyde and pivalaldehyde by reduced anatase and rutile [6]. As shown by the product distributions, these reactants do indeed yield more of the reductive coupling products, and there is little formation of other oligomers, since aldol condensation is not possible. However for these aldehydes the dominant products are those of hydrogenation and/or crackingmtoluene is the most abundant product from benzaldehyde, isobutane is the main product from pivalaldehyde. These results are again consistent with catalyst reduction being the rate determining step in reductive coupling. In the case of benzaldehyde and pivalaldehyde, the reaction of adsorbed hydrogen with adsorbed organics appears to be faster than its reaction with surface lattice oxygen to produce reduced sites for reductive coupling; reductive coupling loses to hydrogenation. Nevertheless, the reductive coupling reaction is catalytic; production per site for the 8 to 40 hour runs conducted corresponds to 300 to 1500 reductive coupling product molecules at an turnover frequency for carbonyl consumption of 0.12 s-1 and a reductive coupling selectivity of 10%. While these rates and selectivities are not at the level expected for a practical process, they do represent the first example of heterogeneously catalyzed reductive carbonyl coupling. Perhaps equally important, this new demonstration of catalysis finds its origin in gas-solid reactions first observed in single crystal surface science experiments in ultrahigh vacuum. Table 2. Summary ofbenzaldehyde coupling on reduced TiO2 (8 hours on stream) [6]. Catalyst Rate (x 10 3, s "l) COUPLING YIELD (%)

TiO2 (Anatase)) 135 8.4

TiO2 (Rutile) 271 10.2

CARBON BASIS SELECTIVITIES (%) Reductive Coupling Product Stilbenes Hydrogenation Products Toluene Methylcyclohexane C7 Paraffins Others Benzene Light HCs Heavies

13.5

13.8

56.9 6.2 12.7

60.5 6.5 9.5

9.1 0.4 1.2

5 0.8 3.9

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Table 3.

113 Sunmaary of pivalaldehyde carbonyl coupling on reduced TiO2 (8 hours on stream) [6]. Catalyst

Rate (x 103, sl) COUPLINGYIELD (%)

TiO2 (Anatase) 118 6.2

TiO2 (Rutile) 247 6.8

CARBON BASIS SELECTIVITIES (%) Reductive Coupling Product DTBE* Hydrogenation Products Isobutane Branched C5 Paraffins Others Methane Light HCs (C2-C3) C6 HCs

12.2

10.8

65.3 12.7

68.3 15.1

4.8 4.4 0.6

4.2 0.7 0.9

* DTBE: di-t-butylethylene, or (cis-/trans-) 2,2,5,5-tetramethyl-3-hexene.

IV. Summary The examples above represent very different chemistries brought forth from surface science studies on metal oxides and made catalytic on high surface area materials. There is every reason to believe that these represent but the first of many other examples that will emerge in the future. The crucial hurdle to scaling up single crystal results to higher surface area materials operating at higher pressures is in making and maintaining comparable active sites under the very different conditions of catalysis vs. surface science experiments. The advantage of the surface science approach is that by defining active site requirements in terms of surface structure, coordination environment, oxidation state, etc., one may recognize ways to create sites on working catalysts that fulfill these requirements, even when the materials studied in UHV are not amenable to catalytic operation.

Acknowledgment I am grateful to the National Science Foundation and the Department of Energy, as well as to the Sponsors of the Center for Catalytic Science and Technology, for their steadfast support of my research. All of the students, postdocs, and collaborators with whom I have had the pleasure to work have had an influence on the ideas expressed in this paper. Particular credit is due to Dr. Paul A. Watson and Dr. James E. Rekoske, some of whose previously unpublished results appear in Sections II and III, respectively. I would especially like to acknowledge the contributions of Dr. Hicham Idriss, now at the University of Auckland, who first challenged me to turn surface science discoveries into catalysis, and who has help us to take up that challenge ever since.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

From unit operations to elementary processes: a molecular and multidisciplinary approach to catalyst preparation M. Che Institut Universitaire de France and Laboratoire de R6activit6 de Surface, UMR 7609-CNRS, Universit6 Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France Using a molecular approach based on transition metal ions, the combined deployment of physical techniques and stable isotopes, it is possible to identify the elementary processes occurring during the unit operations (preparation steps) leading to oxide-supported catalysts at the industrial (laboratory) scale. These processes correspond to various disciplines of chemistry, the concepts of which can be applied to catalyst preparation, thus giving a stronger conceptual framework to the field. 1. INTRODUCTION In all textbooks on catalysis, the definition of catalysis is based on that of the catalyst. Many definitions have been given but the most informative and molecular one appears to be that proposed by Boudart: a catalyst is a substance that transforms reactants into products, through an uninterrupted and repeated cycle of elementary steps in which the catalyst participates while being regenerated in its original form at the end of each cycle during the life of the catalyst. The catalytic cycle is the principle of catalytic action [1]. Despite the importance given to the catalyst by this definition, textbooks on catalysis pay very little attention to the preparation of catalysts, in contrast to the large number of documents on this subject. This mismatch lies in the fact that most of them, if not all, give preparation recipes, making any synthesis, usually required for textbooks, rather difficult. As suggested earlier [2], this situation is probably due to the fact that the number of papers on catalyst characterization, while steadily increasing, remains almost one order of magnitude smaller than for catalyst preparation. In recent years, major advances have been made in physical techniques [3] particularly for the in s{tu characterization of catalysts [4]. A molecular approach based on the use of such techniques, stable isotopes, and transition metal elements was developed and applied to the study of catalysts prepared from oxide supports and transition metal complexes in aqueous media [5].

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At the 10th ICC [5], the oxide support was shown to act as a ligand, thus bridging the gap between molecular chemistry and solid state chemistry and leading to the field of interracial coordination chemistry [5-7]. More recently, at EuropaCat-IV [8], the emphasis was laid on the role of the oxide support as reactant and on the corresponding concepts of geochemistry which can be applied in conditions where the oxide support can lead to dissolution-reprecipitation phenomena [5,9,10] with subsequent formation of mixed molecular species [11] or mixed solid phases [9,10]. The present contribution concerns the identification of the elementary processes occurring during the successive steps of catalyst preparation. It also reviews the various disciplines of chemistry, including interfacial coordination chemistry and geochemistry, related to these processes, thus giving a more firm conceptual basis to the field. 2. THE CONTEXT and THE MOLECULAR APPROACH

Several strategies exist to design and synthesize solid catalysts and Thomas has recently reviewed one of them, the integrated strategy, for active site engineering in micro- and mesoporous solids [12]. When large quantities of catalysts are required, some strategies however become difficult or impossible to apply. The present review concerns such types of catalysts, particularly oxide-supported ones, produced at the industrial scale from conventional oxides (alumina, zeolites, silica) using water-soluble molecular complexes as precursors of the catalytically active phase. In oxide-supported catalysts, active sites are difficult to characterize because they are located at the frontiers of two or three phases (solid catalyst, on one hand, and liquid and/or gas, on the other hand), where the structure is ill-defined and where techniques and theories do not always apply easily. In the molecular approach we developed, the role of transition metal complexes is threefold [5]: i) transition elements generally have remarkable catalytic properties, ii) their partly filled d orbitals provide them also with unique optical and magnetic properties, iii) such properties followed by suitable spectroscopies strongly depend upon the immediate environment of the transition metal elements which can thus function as probes of their own interactions with the oxide support in the course of preparation, thus providing very valuable information on the role played by the support. The preparation of catalysts involves several stages called unit operations in the industry [13] or preparation steps in the laboratory, such as impregnation, elimination of the solvent, drying, calcination, sulfidation and reduction. Industry and laboratory procedures are the same and only differ by the quantities of catalysts prepared, typically of the order of the ton and gram respectively. Therefore, we shall use either unit operation or preparation step in what follows.

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A given sequence of preparation steps together with the catalytic reaction constitute the catalyst history, which we propose to conveniently summarize by a temperature vs time, T(t), profile as shown in Fig. 1. The T(t) profiles can be divided essentially in three parts, the first involving the liquid-solid or wet interface with deposition of the active component precursor as a divided form on the support, and the second involving the gassolid or dry interface with transformation of the precursor into the required catalytically active phase. 800 -

02 H2 02

vacuo

800 "

H2

700

J

700 -

600

; reaction

~' ~, 500

H20 /

02

~ 600 H20

500 drying

400

~

i

400

300

300

0

;

~0

~

t (h)

a)

~0

25

~0

o

1

dry'g

!

i! J

H2 i

"reaction

;o ~o

t (h)

go io 8o ~o

b)

Fig. 1. The temperature vs time, T(t), profile for a) a Cu/SiO2 catalyst prepared by deposition-precipitation and used in the hydrogenolysis of methyl acetate [14]; b) a Ni/SiO2 catalyst prepared by competitive ion exchange and used in the dimerization of olefins [15]. Compared to the dry interface, the wet interface has been rather neglected until recently, probably for three main reasons: i) the presence of the liquid phase makes it more difficult to use in situ spectroscopies, ii) it also leads to two types of speciation, one concerning the liquid phase and the other the solid phase: by speciation we mean the distribution of an element in its various chemical forms, e.g. Pt in the form of [PtC16] 2-, [PtC15(H20)]-, [PtC14(H20)], etc.. iii) it is often believed that phenomena occurring at the wet interface, i.e., at an early stage of preparation lose their importance during the remainder of catalyst history, particularly during thermal treatment. This however is in sharp contrast to practice in industry. There, for a long time, different methods of deposition have been employed to prepare a catalyst depending upon the reaction it has to catalyze. This constitutes a strong de facto support to the idea that catalysts have a

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memory effect [5] and that the phenomena initially occurring at the wet interface do contribute to the final properties of the catalyst [16]. The molecular approach described has been applied to the deposition methods most frequently used in the industry: impregnation, equilibrium adsorption, and deposition-precipitation, and the catalysts obtained have been characterized both at the molecular level with spectroscopies such as diffuse reflectance UV-Visible, NMR, and EXAFS, and at the macroscopic level with techniques such as TEM, TPR and XRD. 3. THE MULTIDISCIPLINARY APPROACH Although preparation procedures involve a lot of experimental parameters, the search for elementary processes can be simplified by considering the most important parameters, i.e., the temperature/time history typically chosen for the different preparation steps: deposition of the precursor following one of the selected methods, drying, freeze-drying and aging (Fig. 2). As will be seen, the use of this T(t) plot emphasizes the role of kinetics in favouring/inhibiting some of the elementary processes.

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I /~!!ii!iiii}iiii!iliiiii!i!i!iii!iiiiiiii! (10g t) Fig. 2. A T(t) diagramme showing the temperature and time duration typical of different preparations steps (FD: freeze-drying, Imp: impregnation, EA: equilibrium adsorption, Aging often referred to also as maturation, D P: deposition-precipitation, Dr: conventional drying) [19]. The multidisciplinary approach consists in the three following aspects: i) the identification, in each preparation step, of the elementary processes involved and of their corresponding experimental key parameters which constitute tools to modify the state of the catalyst in the course of preparation.

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ii) the determination of the state and properties (mobility, accessibility, reactivity) of the molecular complex in various types of interaction with the oxide surface which are characteristic of certain disciplines of chemistry with their associated concepts. This multidisciplinary approach considerably broadens our vision of catalyst preparation. iii) the modification, based on this knowledge, of the unit operations along an established sequence or the proposition of new sequences, in order to confer to the catalytic system some specific properties. This is known as "catalysts design". Even though this catalyst design has not yet reached the sophistication attained in the field of homogeneous catalysis, this is not an impossible goal and some recent reviews have contributed to build bridges between these two fields [5,12,16-18]. 3.1. Colloid chemistry: formation of a surface charge [20] Zeolites are known to have a permanent and pH-independent charge. Conventional oxides in aqueous solution can be electrically charged also. Colloid chemistry explains that, due to their amphoteric character, surface hydroxyl groups may be protonated (reverse of reaction(I)) or deprotonated (reaction (2)), according to the solution pH:

SOH2 +

~=~

SOH

+

H§

K1

(1)

SOH

~=~

SO-

+

H§

K2

(2)

Therefore, a pH-dependent surface charge develops. There is a particular value of pH for which the total electric charge of the surface is zero: the point of zero charge, PZC. Typical PZCs are 2-2.5 for silica, 8-8.5 for alumina and 6.5 for titania. The role of the oxide support is thus to constitute a reservoir of OH groups and the basic phenomenon is the formation of a surface charge, using the pH as a molecular switch [5]. 3.2. Interfacial electrochemistry: formation of an electrostatically held complex [16] At pH < PZC, the surface bears a global positive charge and anions can be electrostatically adsorbed, while at pH > PZC, the surface bears a global negative charge, allowing the cations to be adsorbed. As one moves away from the surface, the concentration of the counter ions of charge opposite to that of the surface quickly decreases to the value found in the bulk solution. The thickness of this diffuse layer is a few tens of A in realistic situations. The resulting "double layer" (surface + diffuse layer) can be regarded at the macroscopic level as a condenser, and at the molecular level as an ion pair which is stable because of the low dielectric constant of water in the vicinity of the charged surface [5]. Triple layers may be formed such as in the case of the (NH4)6Mo7024/alumina system prepared by equilibrium adsorption(see Fig. 2 for T / t values). At pH < PZC, the charge of the alumina surface is positive but the

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Mo70246- surface concentration is such that it is overcompensated, leading to a coadsorption of NH4 § ions, thus forming the third layer [21]. The electrostatic interactions do not prevent the adsorbed precursor complex to have some rotational freedom, although it is decreased with respect to that in bulk water because of the higher viscosity close to the charged surface [22], a phenomenon called electrostriction. Here the oxide support constitutes a charged surface and the basic phenomenon is the formation of the double (triple) layer based on non specific electrostatic interactions. 3.3. Heterosupramolecular chemistry: formation of an outer sphere complex [16] An intermediate situation between the non specific electrostatic adsorption and the highly specific inner sphere complex formation (vide supra, w is the formation of an outer sphere complex where the original ligands remain coordinated to the metal ion but form weak specific bonds with the surface, for instance hydrogen bonds. This is closely related to the field of supramolecular chemistry [23]. Unless the precursor complex bears no charge (molecular complex) and/or the surface is neutral (pH = PZC), this supramolecular interaction is accompanied by an electrostatic interaction as described earlier (see w 3.2). Outer sphere complex formation can be followed in the [PtC16]2-/silica system [24]. At the liquid-solid interface, 195pt NMR spectra indicate that the main species are [PtC15(H20)]-and [PtC14(H20)2]. By comparison with the [PtC15(H20)]-/crown ether 18C-6 model system, which has been studied by several techniques (particularly X-Ray diffraction) and shows a very similar 195pt NMR shift [25], it has been proposed that the [PtC15(H20)]- complex is bonded to the surface of silica by means of a solvent molecule and hydrogen bonds indicated by dotted lines in Fig. 3. The three oxygens represented belong either to three of the 6 oxgens of the crown ether in the model system or to three oxygens of (Si-O)6 cycles at the silica surface. It seems that there is a molecular recognition phenomenon between the outer sphere Pt complex and the oxide support, and that the solvent has an important role in this three-partner (precursor complex-solvent-oxide support) system [24]. While the complex is the guest, the oxide support acts as the receptor, the basic phenomenon being here self-assembly, based on hydrogen bonding and van der Waals interactions. This type of specific interaction falls into the so-called heterosupramolecular chemistry [26]; in this context, the [PtC15(H20)]-/silica system can be seen as a non-covalent heterosupramolecular assembly.

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~ : Pt ( ~ " CI

| 0..., " H20 \ Fig. 3. A possible model for the [PtC15(H20)]-/silica molecular recognition [24].

3.4 Interfacial coordination chemistry: formation of an inner sphere complex [5,6] This type of chemistry has been described earlier and its concepts established [57]. It concerns the so-called grafting, i.e., the formation of an inner sphere complex: the surface groups of the support enter the first coordination sphere and form direct bonds with the transition metal ion of the precursor complex. The [Ni(NH3)6]2+/silica system was studied, after equilibrium adsorption (see Fig. 2 for T / t values), by diffuse reflectance UV-Visible spectroscopy at pH > PZC for which the surface presents SiO- groups [27]. The change of the crystal field parameter A was measured upon NH3/SiO- ligand substitution leading to [Ni(NH3)4(SiO)2]. Silica via its chelating group (SiO)2 is found to act as a weak c~ donor-x donor ligand and the solid SiO-group was introduced in the ligand spectrochemical series [27]" ASiO- < AH20 < ANH3 Pure cases of inner sphere complex formation are scarce since other phenomena such as dissolution-reprecipitation (see w 3.5) may occur in parallel. The clearest instances are observed when "spectator" ligands are inert to substitution, either because of chelate effect (cis-[Ni(en)2(H20)2] 2§ (en = NH2-CH 2CH2-NH2) on various supports [28])or because of high crystal field activation energy ([Co(NH3)5(RO)] x§ on TI-A1203 [29], RO = OH, H 2 0 or alcohol). After deposition from the liquid phase and drying at room temperature, the cis-[Ni(en)2(H20)2] 2§ complex with its two labile H 2 0 ligands binds to SiO2, yA1203 or Y zeolite to lead to cis-[Ni(en)2(SO)2] x§ Using the crystal field parameter A, a spectrochemical series of oxide supports (in bold) can be proposed and introduced in the ligand spectrochemical series [6,28]: ACl-< AA10- < AZO-< ASiO- < AH20 < ANH3 < Aen

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The results presented here show that the oxide support behaves as a weak polydentate ligand, as expected from the theory because of its (~ donor-~ donor character [5,6]. The basic phenomenon leading to grafting is ligand substitution and involves highly specific and iono-covalent interactions. 3.5. Geochemistry: formation of mixed phases or molecular species [5,8] There has been many reports on the formation, during the deposition step itself, of mixed phases or molecular species, containing both the transition metal and an element from the support. Typical examples are the formation of the mixed phase nepouite, Si2Ni305(OH)4, for the Ni(II)/SiO 2 system [30] or the molecular heteropolyanion, Al(OH)6Mo60183-, for the Mo70246-/~,-A1203 system [11]. The phenomenon is quite general, since it occurs upon contact of conventional oxides (silica, alumina) with aqueous solutions of precursor complexes, be they cationic, e.g. [Ni(NH3)6] 2§ or anionic, e.g. Mo70246-, and for different methods of deposition (impregnation, equilibrium adsorption, deposition-precipitation). A parallel can be made with geochemistry phenomena, where the alteration of primary minerals on weathering, i.e., produced by climatic factors, including exposure to the soil solution, leads to dissolution followed by neoformation of secondary phases. The primary mineral and secondary phase in geochemistry are the equivalent of the silica support and mixed phase in catalyst preparation respectively. One often uses the terms "dissolution-reprecipitation" to mean dissolution of the oxide support followed by reprecipitation of the mixed phase or molecular species. The dissolution of the oxide support leads to support monomers which can react with the precursor complex to form mixed entities, following an oxolation or olation reaction [5], written for the Ni(II)/SiO 2 system as: Si-OH Si-OH

+ +

H20-Ni HO-Ni

~=~ ~=~

Si-OH-Ni Si-O-Ni

+ H20 + H20

(3) (4)

where no attempt is made to indicate the complete coordination sphere; only the pertinent ligands around Si(IV) and Ni(II) are given. Olation (reaction 3) and oxolation (reaction 4) are heterocondensation processes in which a hydroxy or an oxo bridge between Si and Ni are formed respectively. Because they have at least two lone pairs, H20, OH or O are bridging ligands can form bonds with Si and Ni. If they now are substituted by non bridging ones, such as NH3 or en with only one lone pair per N, the heterocondensation will be inhibited [5,31]. Several parameters may influence the dissolution of the oxide support and its kinetics, either separately or in synergy. For instance, it has long been known that

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the pH of the solution, in absence of any precursor complex, favours quantitatively the dissolution of the oxide: the more distant the pH of the solution from the PZC, the more important the dissolution of the oxide [20]. It has been also noted that the dissolution increases with the time of contact between the oxide (e.g. silica and alumina) and the aqueous solution of the precursor and this is most probably the main phenomenon occurring during aging or maturation (see Fig. 2 for T/t values) [5,8]. Recently, a systematic study was performed on Ni(II)/SiO 2 catalysts prepared by the deposition-precipitation (DP) (see Fig. 2 for T/t values) [30]. Because it takes place at a temperature of about 90~ this method considerably increases the kinetics of dissolution and therefore of the appearance of mixed phases. For short DP time and silica of low surface area, the supported Ni(II) phase is mainly a turbostratic Ni hydroxide, while for silica of high surface area, it is mainly an illcrystallized 1:1 Ni phyllosilicate. For long DP time and silica of both types, the Ni(II) phase is the 1:1 Ni phyllosilicate [30]. In order to explain these results, a molecular mechanism was proposed based on the kinetic competition between two types of reaction: i) a Ni-O-Si oxolation/heterocondensation leading to the formation and growth of 1:1 Ni phyllosilicate prevailing for long DP time and silica of both types and ii) an olation/homocondensation leading to the formation of nickel hydroxide for short DP time and silica of low surface area [32]. Another parameter is the precursor complex. It has long been known in geochemistry that complex anions in solution can modify the solubility of oxides, as shown for the oxalate/halloysite (an aluminosilicate clay mineral) system [33], and this is known as the ligand-promoted dissolution of oxides [34,35]. We have evidenced the same phenomenon for the Mo70246-/~-A1203 system prepared by equilibrium adsorption (see Fig. 2 for T/t values) [11] and this can be explained using Fig. 3. The specific (probably inner sphere) adsorption of heptamolydate ligand ions on alumina labilizes the lattice A1-O bonds, thus facilitating the detachment of surface A1 from the oxide. Because this detachment is rate-determining, it is clear that specific adsorption has a kinetic effect on the dissolution of alumina. The oxide dissolution can be thermodynamically modified also. The formation of ligand-aluminum complexes in solution will enhance the solubility of alumina by consuming the aluminum ions in solution. The dissolution equilibrium is thus shifted to the right with an increased aluminum concentration in solution. By reaction of the ligand isopolymolybdate M o 7 0 2 4 6 - w i t h dissolved aluminum (Fig. 3), a previously unrecognized species, i.e., the Anderson-type heteropolymolybdate Al(OH)6Mo60183- is formed and observed by 27A1 CP-MAS NMR, both in solution and in the adsorbed state [11]. Similar results have been found for the polytungstate/alumina system [36].

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0

i l l

l+

Electrostatic

Fig. 4. The concept of ligand-promoted dissolution of alumina evidenced for the MoOx/~/-A1203 system prepared by equilibrium adsorption using Mo70246- as precursor [11]. It is also possible to observe mixed phases with the same (NH4)6Mo7024/TA1203 system, but prepared by incipient wetness impregnation (see Fig. 2 for T/t values) [37]. The formation of bulk (NH4)3[Al(OH)6Mo6018 ] can be identified by XRD one hour after the start of impregnation, showing an extensive reaction of molybdate ions with dissolved aluminum ions during the preparation procedure. The aluminomolybdic species are not molecularly dispersed but precipitate as a mixed bulk phase because of the presence of nearby and available ammonium ions. The formation of the bulk phase can be avoided by using a very short impregnation step followed by freezing the system in liquid nitrogen and further drying under vacuum (freeze-drying, see Fig. 2 for T/t values). The impregnation and drying steps have been modified so as to quench the dissolution of alumina. After such a procedure, the bulk phase is no longer observed by XRD [37]. The catalyst obtained after calcination at 400~ strongly depends on the nature of the molybdic species present after the initial impregnation step. For catalysts prepared by incipient wetness impregnation, bulk MoO3 is identified by XRD for loadings as low as 7 Mo wt%, while for catalysts obtained by freeze-drying, the formation of bulk MoO3 can be avoided for loadings as high as 12 Mo wt% [37]. In the conventional drying step at temperatures between 90 and 120~ (see Fig. 2 for T/t values), some water solution may remain in the porosity for a significant lapse of time before drying is completed, and of course dissolution reactions will be speeded up with respect to room temperature [38]. From the preceding results, the following elementary steps have emerged: the bonding of the ligand precursor to the oxide surface, followed by the ligandpromoted dissolution of the oxide, and then precipitation of the mixed phase or deposition of the mixed molecular species on the oxide support. The mechanism

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of formation

125 of mixed

molecular

species is largely unknown,

e.g., the

transformation of the non planar isopolymolybdate Mo70246- into the Andersontype planar heteropolymolybdate Al(OH)6Mo60183- (Fig. 3) appears to involve an important change of structure. The previous results show that the oxide support may act as a real reactant. The basic and main phenomena are the ligand-promoted dissolution of the oxide followed by (ox)olation reactions and essentially involve chemical interactions, but the subject is far from being exhausted. It is interesting to note that Lavoisier was the first to demonstrate the phenomenon of the dissolution of glass by water upon prolonged distillation, thereby closing the debate on the possibility of conversion of water into earth [39]. 4. CONCLUSIONS Using a molecular approach to catalyst preparation, it is possible to evidence the elementary processes which develop, particularly at the liquid-solid interface. These processes find their origin in various disciplines of chemistry, which have been identified. An attempt has been made to give in each case, the basic phenomenon involved, the role plaid by the support, and the interactions taking place between the precursor complex and the support. Those are first physical and then chemical, as the precursor complex interacts more strongly with the oxide support. We earlier gave an estimate of the time scales attached to the various processes [16], which are likely to develop during the unit operations and showed that depending upon the experimental conditions, some become predominant. Even though all the experimental parameters have not been identified and their influence fully estimated, the conceptual framework appears now reasonably well established, if we include solid state chemistry, which is likely to be important for the gas-solid interface subjected to thermal treatments taking place at temperatures of several hundreds of ~ This conceptual framework opens up the possibility to modify an established sequence of preparation steps or to propose new sequences [40], in order to confer to the catalytic system specific properties.

5. Acknowledgments

The author would like to gratefully acknowledge all the persons who have contributed to the work reported here, particularly those cited in the references and Dr J. Blanchard for kindly preparing Figure 1.

REFERENCES M. Boudart, in Handbook of Heterogeneous Catalysis, G. Ertl, H. Kn6zinger and J. Weitkamp, eds, Wiley-VCH, Weinheim, 1997, p. 1.

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2.

3.

4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40.

M. Che, O. Clause and C. Marcilly, in Preparation of Solid Catalysts, G. Ertl, H. Kn6zinger and J. Weitkamp, eds, Wiley-VCH, Weinheim, 1999, p. 315. Catalyst Characterization: Physical Techniques for Solid Materials; B. Imelik and J.C. V6drine, eds, Plenum Press, New York, 1994. Catalyst Characterization under Reaction Conditions, Topics Catal., 8 (1999) 1-140. M. Che, Proc. 10th Int. Cong. Catal., Budapest, Stud. Surf. Sci. Catal., 75A (1993) 31. C. Lepetit and M. Che, J. Mol. Catal., 100 (1995) 147. K. Dyrek and M. Che, Chem. Rev., 97 (1997) 305. M. Che, Book of Abstracts, EuropaCat-IV, Rimini, 1999, p. KN8. O. Clause, M. Kermarec, L. Bonneviot, F. Villain and M. Che, J. Am. Chem. Soc., 114 (1992) 4079 J.B. d'Espinose de la Caillerie, M. Kermarec and O. Clause, J. Am. Chem. Soc., 117 (1995) 11471. X. Carrier, J.F. Lambert and M. Che, J. Am. Chem. Soc., 119 (1997) 10137. J.M. Thomas, Angew. Chem. Int. Ed., 38 (1999) 3588. J.F. Lepage et al., Catalyse de Contact, Technip, Paris, 1978. C.J.G. van der Grift, A.F.H. Wielers, B.P.J. Joghi, M. de Boer, M. Versluijs-Helder and J.W. Geus, J. Ca tal., 131 (1991) 178. L. Bonneviot, D. Olivier and M. Che, J. Mol. Catal., 21 (1983) 415. J.-F. Lambert and M. Che, Stud. Surf. Sci. Catal., 109 (1997) 91. J.A. Schwarz, C. Contescu and A. Contescu, Chem. Rev., 95 (1995) 477. K.I. Zamaraev, Topics Catal., 3 (1996) 1. Fig. 2 is reproduced in X. Carrier, J.-F. Lambert and M. Che, this Congress. J.P. Brunelle, Pure Appl. Chem., 50 (1978) 1211. N. Spanos, L. Vordonis, C. Kordulis and A. Lycourghiotis, J. Catal., 124 (1990) 301. M. Che and L. Bonneviot, Pure Appl. Chem., 60 (1988) 1369. J.M. Lehn, Supramolecular Chemistry, VCH, Weinheim, 1995. S. Boujday, J. Lehman, J.-F. Lambert and M. Che, Book of Abstracts, EuropaCat-IV, Rimini, 1999, p. 132; to be published D. Steinborn, O. Gravenhorst, H. Hartung and U. Baumeister, Inorg. Chem., 36 (1997) 2195. X. Marguerettaz, A. Merrins and D. Fitzmaurice, J. Mater. Chem., 8 (1998) 2157. L. Bonneviot, O. Legendre, M. Kermarec, D. Olivier and M. Che, J. Colloid Interf. Sci., 134 (1990) 534. J.-F. Lambert, M. Hoogland and M. Che, J. Phys. Chem., 101 (1997) 10347. Y. Chen, J. Hyldtoft, C.J.H. Jacobsen, D.H. Christensen and O.F. Nielsen, Spectrochim. Acta 50A (1994) 1879. P. Burattin, M. Che and C. Louis, J. Phys. Chem. B, 101 (1997) 7060; J.Y. Carriat, M. Che, M. Kermarec, M. Verdaguer and A. Michalowicz, J. Am. Chem. Soc., 120 (1998) 2059 P. Burattin, M. Che and C. Louis, J. Phys. Chem. B, 102 (1998) 2722. R.W. Smith, Coord. Chem. Rev., 149 (1996) 86. W. Stumm, Chemistry of the Solid-Water Interface, J. Wiley & Sons, New York, 1992. G.E. Brown et al., Chem. Rev., 99 (1999) 77. X. Carrier, J.B. d'Espinose de la Caillerie, J.F. Lambert and M. Che, J. Am. Chem. Soc., 121 (1999) 3377. X. Carrier, J.F. Lambert and M. Che,, Stud. Surf. Sci. Catal., 121 (1999) 311. C. Louis, Z.X. Cheng and M. Che, J. Phys. Chem., 97 (1993) 5703. A.L. Lavoisier, M6m. Acad. R. Sci., 1770 (1773) 73. M. Che, Z.X. Cheng and C. Louis, J. Am. Chem. Soc., 117 (1995) 2008.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

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Acidity in zeolite catalysis R.A. van Santen and F.J.M.M. de Gauw Schuit Institute of Catalysis, Eindhoven University of Technology P.O. Box 513, 5600 MB Eindhoven, The Netherlands A perspective is provided on our current understanding of the activation of hydrocarbons by protonic zeolites. One has to distinguish the proton affinity of a zeolite, measured in an equilibrium experiment, from proton activation that determines a kinetic catalytic result. The proton affinity depends on the heat of adsorption and the intrinsic protonation energy of a substrate. The heat of adsorption is strongly micropore size and shape dependent. Kinetic proton activation depends on the activation energy of the proton activated rate limiting step and also on the heat of reactant adsorption. Differences in catalytic performance between zeolites are often rather due to differences in heat of adsorption than to intrinsic proton property differences. 1. INTRODUCTION The past decade has shown a significant increase in our understanding of the relation between the chemical reactivity of protons of solid acid materials as zeolites and catalytic performance. A major hurdle to overcome was the insight that the chemical Bronsted definition of acidity in terms of a pH has no analogue in solid acid catalysis. The pH is defined as the proton concentration of an acid dissolved in water, which is determined by the equilibrium: HX +(H20).

H30;e r +

(1)

X;d ~

The pH relates to the concentration o fhydronium ions. In equilibrium 1 the hydration energies of the different intermediates play a crucial role, as well as the deprotonation energy of the HX molecule. For acidic water solutions this value influences the proton concentration, but not the proton activation of a reactant molecule, that is determined by the reaction with H30 + in the water phase. In superacids similar considerations play a role, but now acidity and pH is determined by self-dissociation equilibria as: 2HF

(HFH +)solv + (F-)sotv

(2)

The acidity is now determined by the (HFH+)solvconcentration, which will be low, and its deprotonation energy, which is significantly decreased compared to that of H30 § Olah [1] discovered that the equilibrium shifts to the fight when Lewis acids are used to stabilize

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the anions. The major difference with the zeolite case is that usually no initial equilibrium with a solvent is established (except when zeolites in solution are used), since reactions ot~en occur at the gas-solid surface interphase and with a-polar molecules. Remember that the dielectric constant of a zeolite is only four times the vacuum value. In zeolites an important question is whether the equilibrium: ZH

+R

Z-(RH) +

(3)

(R is a reactant) exists and also whether the equilibrium constant of equation 3 and the zeolite proton concentration have any meaning as parameters controlling activity in zeolite catalysis. Since the breakthrough papers ofHaag et al. [2], it can be considered as established that zeolite activity and zeolite-proton concentration have a direct relationship. This statement can be refined by limiting the number of reactive protons to those accessible to the reacting molecule. The next issue is the question of the parameters that control the free energies in equilibrium equation 3 and the issue whether the equilibrium equation 3, or the rate constants involved in the protonation reaction are really the relevant parameters. Also of interest is the question whether there is a useful definition of the intrinsic acidity per proton, independent of reactant. This paper aims to describe our current understanding of the relative importance of the three questions as formulated. The question of equilibrium versus kinetic definition will be dealt with after a discussion of the intrinsic acidity of the zeolitic proton. We will answer these questions by first summarizing our current understanding of the elementary steps the protonated molecule undergoes when reacting with the zeolite from the gasphase. In this discussion we will summarize results of quantum-chemical and Monte Carlo simulations as well as supporting NMR, IR and adsorption experiments. In a second section we will discuss the implications of the model for the rate of a zeoliteactivated reaction. As an example we have chosen a comparison of the hydroisomerization rate ofhexane catalysed by different zeolites. 2. THE PROTON AFFINITY The Hammett indicator method has been extensively studied to characterize the acid strength of a solid acid. Color changes of adsorbed indicators have been used as measures of proton donation power. From measurements of the Hammett's acidity function Ho [3]: H o = pK a -

log C B n .

/ CB

(4)

with different bases B for a particular solvent (usually H20). Ho can be defined for a particular acid concentration, pKa is the equilibrium constant for protonation of base B. CBH§ and CB are the respective concentrations of protonated and free base. In this way, using the indicator method, Hall e.a. [4] deduced a Ho value for Mordenite between-13.7 and-12.4. This is within the range of superacids as neat H2804 or HF/SbFs.

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If one accepts these results, one has the difficulty to understand the widely different conditions for acid catalysis by solid acids compared to reactions in superacids. For molecules as the relatively inert alkanes typically a difference in temperature of 200~ is found. Another indication that the indicator method may strongly overestimate apparent solid acid strength is the now widely accepted observation that adsorption of a single H20 or CH3 OH molecule to a zeolitic proton will lead to strong hydrogen bonding and the water molecule is adsorbed close to the hydronium ion stage [5]. For a singly adsorbed water molecule the hydronium ion state is part of a transition state. The hydronium ion is formed, when two water molecules are adsorbed to a single proton, which implies a pH ~ -0.7. In this context it is also relevant that the gas phase deprotonation energy of H30 § is only 648 kJ/mol [6], whereas that of the zeolitic proton is 1329 kJ/mol [7], indicating that in the gas phase hydronium ions are more acidic than zeolitic protons. Theory has shown that coadsorbed H20 may dramatically increase the rate of acid catalyzed reactions [8]. The weakness of the indicator method to provide an acceptable measure for intrinsic solid acidity probably relates to the unique stabilizing interaction of polarizable solid and protonated molecule. We will return to this point below. Figure 1 illustrates the quantum-chemical model of protonation of a molecule (ethene) that reacts with a proton in a zeolite. Essential is the decoupling of adsorption of the molecule in the micropore and protonation. Experiment and theory has provided a very clear insight into the physical chemistry of adsorption. This is best understood for adsorption in the siliceous zeolites. For an example, table 1 and figure 2 illustrate key results for the adsorption of linear hydrocarbons. Basic to these results is the understanding that the interaction between hydrocarbon and zeolite lattice is determined by the van der Waals dispersive interactions between the large highly polarizable oxygen atoms and polarizable CH2 and CH3 alkane sub units.

j i m B i B e

9

3oIi

..........

~

"-..:"""]cOte...... !. . . . . . . . . . . .

-

' I -

70

Figure 1. Thermodynamics of proton activation according to quantum-chemical theory.

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Using as analytical expressions additive Buckingham or Lennard-Jones potentials for the van der Waals interaction Configurationally Biased Monte Carlo simulations enable prediction of adsorption isotherms as well as siting of the molecules in the zeolitic micropore [9]. A most remarkable result is the strong relation between hydrocarbon size and shape and zeolite micropore pore geometry. As figure 2 shows, the heat of adsorption increases with micropore size, until pores are too small to allow access. The values for average micropore sizes less than 4.8A, refer to adsorption in larger micropores in the bimodal zeolites concerned. One notes also the incremental increase in the heat of adsorption with hydrocarbon length. Table 1 provides a comparison of some Henry constants, e.g. compare H-Mor with onedimensional 12 ring channels with ZSM-22 with one-dimensional 10 ring channels. One notes an optimum in the Henry coefficient when the micropore size decreases. When the micropore size is large, adsorption is limited by the relatively low heat of adsorption. However when the micropore size is small, adsorption is limited because of the large loss in entropy. In a small micropore mobility is low compared to this in a large micropore. The medium sizel2 ring channel of Mordenite provides an optimum for the Henry coefficient. As we will discuss later careful measurements of the hydroisomerization ofhexane in different zeolites show an optimum TOF for Mordenite [ 10]. Within the model ofprotonation shown in figure 1, it implies that the protonation energy will depend strongly on micropore size, because of differences in adsorption energy. Quantum-chemistry in combination with spectroscopy has considerately elucidated the reactivity of zeolitic protons [11 ]. This provides a measure of its intrinsic acidity. A similar situation holds for the IR spectra. Interaction with weakly interacting molecules as CH4, N2 or benzene increases the proton chemical shifts [ 12] and decreases and broadens the zeolite OH frequencies and adsorption bonds [ 12]. Proton NMR chemical shifts of free zeolitic protons are relatively independent of zeolite [12], as long as the A1/Si ratio is less than 0.1. Interaction induces a positive charge on the proton. Again when molecules are small and zeolite composition such that flame work position does not affect the proton bond, the response of the OH groups turns out to be very similar for different zeolites. Table 1. Henry's constants (KH) for adsorption of n-hexane on various zeolites at 50 and 240 ~ The values between brackets represent the pore dimensions in/~. KH(mol/kg/Pa) H-Beta a (6.4x7.6)

(5.5x5.5)

50 ~ 240 ~

1 1.3"10 -4

H-Mordenite b (6.5x7.0)

H-ZSM-5 b (5.3x5.6)

H-ZSM-22 b (4.4x5.5)

2.6 1.9"10 -4

3.5 4.2"10 -5

0.64 7.9"10 -6

(5.1x5.5)

aCalculated from pulse chromatographic measurements [ 10] bCalculated from data published in reference [22]

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140 r 120 r I

'r

m

0

E imll.4

0

|

100 ,80 ~011 60 L-C7 40

ic4

20

~ ,,

': . . . . . . . . . . . .

L

3

. . . . . . . .

4

i. . . . . . . . . . . .

5

,~ . . . . . . . . . . .

6

i . . . . .

7

8

average pore diameter [A] Figure 2. Heat of adsorption (Q) of n-alkanes in zeolites as a function of mean pore diameter [9a]. This suggests a localized behavior of the intrinsic acid strength of a zeolitic proton. Calculations on the protonation of olefins tend to confirm this, however local steric constraints of the topology of the zeolite micropore may result in differences [ 14]. According to the view presented here the proton affinity energy of a solid acid with respect to adsorption from the gas phase can be written as" E prot :

E aas + l ~ E protonation

(5)

Eads is the energy of adsorption of a molecule in a micropore, without contact to a proton. AEprotonationis the change in interaction energy of the molecule adsorbed in a micropore upon contact with a proton. This may have the form of a strong hydrogen bond or actual proton transfer may occur. Adsorption measurements of heats of adsorption to protons may show significant differences, when different zeolites are compared [16]. A significant part of this difference however relates to the strong micropore size dependence of Eads and to a lesser extent to AEprotonation. Of course when zeolite framework composition changes, this will affect the intrinsic acid strength, as for instance observed by an altered vibrational shift [ 12]. 3. AEprotonatio n and E protonation ~ct

DFT cluster calculations have provided substantial insights into the chemical bonding characteristics and energetics of the proton transfer reaction [ 17]. Of course their main drawback is the absence of any information on the micropore. Recently using periodical structure codes calculations on proton transfer in zeolites have become possible. Whereas the initial n bonded and final alkoxy-states energy differences are

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quite comparable for chabasite, a decrease of the order of 50 kJ/mol in the periodical structure activation energies is found compared to the cluster calculation values [ 14]. The periodical structure calculation allows for dielectric screening of the transitionstate dipole. When the size of the protonated molecule is such that the micropore gives a steric constraint, alkoxy-formation may become suppressed. This has been demonstrated using a QM-MM calculation for the protonation of isobutene in chabasite [ 15]. The protonated state is less favorable than the n-bonded state, but is a stable carbocation intermediate. The drawback of DFT calculations is that no adequate estimate can be given of the van der Waals interactions responsible for the contribution to the bond energy of physical adsorption. Hence other techniques, as empirical force field methods have to be used to estimate the changes in van der Waals interaction, when geometrics change by protonation [ 14]. One finds usually that these changes are small compared to Eads. 4. THE H Y D R O I S O M E R I Z A T I O N OF n - H E X A N E to i-HEXANE

Haag [18a] and Lercher [18b] demonstrated for cracking of linear alkanes a linear relation between the apparent activation energy of the overall cracking reactions and the heats of adsorption of the molecules, by varying the length of the hydrocarbons. This is the expected relationship when the surface coverage of adsorbed molecules is low [ 19]. Because of deactivation effects it is much more difficult to measure reliably differences in Turnover Frequencies (TOF) and apparent activation energies between different zeolites. We have chosen the hydroisomerization reaction of n-hexane to determine the difference in catalytic acidity between different zeolites. Details of the deactivation studies and kinetic analysis will be reported elsewhere [ 10]. Mordenite, ZSM-5, zeolite 13and ZSM-22 were compared. The zeolites were activated with Pt, such that at reaction conditions (- 240~ PcoH,, / Pn~ = 1 O, etot 1 atm) the hexanehexene equilibrium is established. Measured TOF's normalized per proton are reported in table 2. Data were analyzed using the schematic reaction energy diagram shown in figure 3. =

Table 2. Measured Turn over frequencies per proton for different zeolites at 240 ~ and PcrM,, = 779 Pa

TOF (sec -l)

H-Beta

H-Mordenite

H-ZSM-5

H-ZSM-22

5.0"10 -3

1.1"10 .2

4.1"10 -3

1.6"10 .3

As rate limiting step the isomerization of adsorbed linear alkoxy to branched alkoxy was taken. From the literature ref-21 and pulse chromatographic measurements [ 10] the Henry coefficients for adsorption ofhexane in the four different zeolites was taken and for the rtinteraction between proton and olefins quantum-chemical data [ 14]. A kinetic scheme based on the bifunctional reaction mechanism [20] then enables to obtain the activation energy of proton activation by fitting with initial kinetic data. Table 3 shows obtained values for AHprot and

EiaCo t .

One notes the relatively small variation in activation

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Gas phase alkene I

dk

.

i

ik

Eads

n n i m g m

r .....

~A d s o r b e d alk 9 ene

."

".:

" "

30 1 . . . . . . . . .

~~ - c o m p l e x -

~.

'AHrotI' i

ann l

l

n i

i

I

l

i

u am l

m n i

I

m m l

i

I

E act iso

i n

"

r

l a m l i l l l i l l m l l n l l i l l m m m

~-complex Figure 3. Reaction energy diagram for olefin isomerization (schematic). Table 3. Measured values for Anprot , Ea~o' (kJ/mol) and kao (sec -1) [10].

H-Beta H-Mordenite H-ZSM-5 H-ZSM-22

Anprot

Ei",~ot

-46 -19 -23 -35

145 120 105 106

ki~ o

1.7" 10 -2 2.7"10 .2 2.8* 10 -2 1.7* 10 -2

*measured at 240 ~ energies of the rate limiting isomerization step compared to the large variation in Turnover Frequencies shown in table 2. The results in table 3 show that the differences in Turnover Frequencies can be completely assigned differences in the Henry coefficients (see also table 1). Whereas Turn Over Frequencies can vary by a factor of ten, one notes that the elementary rate constants for isomerization kiso for the different zeolites are quite similar. All of the zeolites investigated have A1/Si rates less than 10. Conclusion: Similarly to the energy of protonation also the apparent activation energy is seen to depend on the adsorption energy. For a monomolecular reaction the apparent activation energy is [19]: Ea,t, act = Eact + (1_ 0)Eaes

(6)

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Equilibrium proton affinities of solid acidic zeolites are also controlled to a significant extent by differences in energies of adsorption as is indicated in equation 5. Catalytic proton activation by zeolites depends to a significant extent on the free energies of adsorption that show an optimum for medium size zeolites. REFERENCES

1. G.A. Olah, S. Prakash, J. Sommer, Superacids, Wiley (1985) 2. a. Haag, W.O., Lago, R.M., Weisz, P.B., Nature 309, 589 (1984) b. Olson, D.H., Haag, W.O., Lago, R.M., J. Catal. 61,390 (1980) 3. L.P. Hammett, A.J. Deyrup, J. Am. Chem. Soc. 59, 2721 (1932) 4. B.S. Umansky, W.K. Hall, J. Catal. 124, 97 (1990) 5. A.G. Pelmenschikov, R.A. van Santen, J. Phys. Chem. 97, 10 678 (1993) 6. K.N. Honk, in "Acidity and Basicity of Socials" J. Fraissard, L. Petrakis (eds.), NATO ASI series C, 444, p. 33-51 7. E.A. Pankshtis, E.N. Yurchenko, Uspekki Khitin 52, 426 (1983) 8. a.S.R. Blaskowski, R.A. van Santen, J. Am. Chem. Soc. 119, 5020 (1997) b. S.R. Blaskowski, R.A. van Santen, in "Transition State Modelling for Catalysis", D.G. Trehlav, K. Morokuma (eds.), ACS symposium series 721, (1999) p. 307 9. a.S.P. Bates, W.J.M. van Well, R.A. van Santen, B. Smit, J. Am. Chem. Soc. 118, 6753 (1996) b. S.P. Bates, W.J.M. van Well, R.A. van Santen, B. Smit, J. Phys. Chem. 100, 17573 (1996) 10. F.J.M.M. de Gauw, J. van Grondelle, R.A. van Santen, in preparation 11. a. R.A. van Santen, G.J. Kramer, Chem. Rev. 95,637 (1995) b. H. Koller, G. Engelhardt, R.A. van Santen, Top. Catal. 9, 163 (1999) 12. a. H. Knrzinger, S.J. Huber, Chem. Sot., Faraday Trans. 94, 2047 (1998) b. F. Wakabayashi, J. Kondo, K. Domen, C. Hirose, Stud. Surf. Sci. and Catal. 90, 157 (1994) c. S.G. Hedge, P. Ratnasamy, L.M. Kustov, V.B. Kazansky, Zeolites 8, 137 (1988) 13. D. Freude, H. Pfeifer, M. Hunger, Proc. 7th Intl. Zeolite Cont., 1986, p. 107: J. Dwyer, Stud. Surf. Sci. and Catal. 37, 333 (1988) 14. X. Rozanska, R.A. van Santen, in preparation 15. P. Sinclair, P. de Vries, P. Sherwood, C.R.A. Catlow, R.A. van Santen, J. Chem. Soc. 94, 3401 (1998) 16. C. Lee, D.J. Parillo, R.J. Gorte, W.E. Farreth, J. Am. Chem. Soc. 118, 3262 (1996) 17. a. S.R. Blaskowski, R.A. van Santen, Topics in Catal. 4, 145 (1997) b. A.M. Righby, G.J. Kramer, R.A. van Santen, J. Catal. 170, 1 (1997) c. S.P. Bates, R.A. van Santen, Adv. Catal. 42, 1 (1998) d. M.V. Frash, R.A. van Santen, Topics in Catal. 9, 191 (1999) e. P.E. Sinclair, C.R.A. Catlow, J. Chem. Soc. Faraday Trans. 92, 2099 (1996) f. P.E. Sinclair, C.R.A. Catlow, J. Phys. Chem. B 101,295 (1997) g. M. Krossner, J. Sauer, J. Phys. Chem. 100, 6199 (1996) h. F. Haase, J. Sauer, J. Hutter, Chem. Phys. Lett. 266, 397 (1997) 18. a. W.O. Haag, Abstracts of Papers of the American Chemical Society 205, 46 (1993) b. Th. Narbeshuber, F. Vinek, J.A. Lercher, J. Catal. 157, 388 (1995)

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19. R.A. van Santen, H. Niemantsverdriet, Chemical kinetics and Catalysis, Plenum Press (1995), p. 252, 253 20. a. P.B. Weisz, Adv. Catal. 13 (1962) 137 b. M.L. Cornradt, W.E. Garwood, Ind. Eng. Chem. Prod. Res. Dev. 3, 38 (1964) 21. F. Eder, Thermodynamics and Siting of Alkane Sorption in Molecular Sieves, Ph. D. Thesis, Twente University (1996)

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Studies in Surface Science and Catalysis 130 A. Corrna, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Engineered Solid Catalysis for Synthesis of Fine Chemicals in Multiphasic Reactant Systems Dirk E. De Vos, Bert F. Sels, Wim M. Van Rhijn and Pierre A. Jacobs* Center for Surface Science and Catalysis, K.U.Leuven, Kardinaal Mercierlaan 92, 3001 Leuven, Belgium

An overview is given of reactions with a solid catalyst and two other immiscible liquid (or solid) phases. Polarity matching between the catalyst and the biphasic liquid system is often required to optimize activity or selectivity. Some future challenges are defined.

I. Introduction Three phase catalytic reactions with a gaseous reactant, a liquid reaction mixture and a solid catalyst are relatively common. However, there are less examples of two liquid phases that react under the influence of a solid catalyst. There are several cases where a liquid must react with another intrinsically immiscible phase, for instance - epoxidation of a hydrophobic olefin, such as methyl oleate or related fatty esters with H202; - nucleophilic substitution of (cheap) chloro compounds with other halides such as F-; - halogenation of aromatic or unsaturated compounds with aqueous hypohalite solutions. In such cases, all reactants may be dissolved in a single liquid phase by addition of a large amount of an appropriate solvent. For instance, it is known that acetonitrile/CHC13 mixtures can dissolve H202 and apolar olefins such as cyclooctene. However, the strong dilution intrinsically decreases reaction rates, and the use of a lot of solvent is economically unattractive. Often, the catalyst is dissolved in one of the liquid phases, which is a supplementary problem. Alternatively, one may design a solid that not only catalyzes the reaction at the molecular level, but also helps in achieving an intimate contact between the two reactant phases, without an excess of solvent. Essentially, this amounts to triphasic catalysis: the solid material accelerates the reaction between one liquid phase and another liquid (or even solid) phase. This paper discusses about ten examples of solid heterogeneous catalysts that were designed to effect a reaction with two immiscible liquid phases. This will help to appreciate the problems inherent to such triphasic catalysis, and to bring forward some solutions. Two major approaches to catalyst design are discussed: - modification of silica or siliceous materials (MCM-41) by covalent linking of functional groups [ 1-3]; - use of inorganic ion exchangers as host for a catalytic species.

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The supported aqueous phase (SAP) approach of Davis therefore falls outside the scope of this overview [4]. SAP catalysis is triphasic in the sense that reactants, dissolved in an apolar liquid, react with a catalyst that is dissolved in a polar phase, supported by a solid material. However, the volume of the polar supported film is in this case far too small to contain a reagent (e.g. an aqueous oxidant). 2. Zeolites and Covalently modified Silica materials

Acid Catalysis in Esterification and Etherification. We have designed silica and MCM-41 type materials with covalently anchored propylsulfonic groups (-(CH2)3SOaH) (1 in Scheme 1). The materials may be prepared via several routes. One may functionalize an existing silica surface with a precursor to a sulfonic acid, e.g. mercaptopropyltrimethoxysilane (MPTS). Alternatively, this MPTS may be mixed with TEOS or TMOS and the mixture can be used in a sol-gel process. These materials are not only strong acids; they are also considerably more hydrophobic than for instance zeolites [5]. In this respect, they behave not as much as mineral acids, but rather as organic sulfonic acids (para-toluenesulfonic acid, or the Twitchell reagent). In the case of the Twitchell reagent, which is used in homogeneous esterification, it is known that the catalyst is not only acid, but has also pronounced emulsifying properties.

'1 ~ i ~ S O 3 L a

H

I

O

I

~

ODHQD

I

n, ODHQD R~O/~"-..~~ DHQD= 9-O~ihydr~uinidinyl R = H or Si(CH2)3-

oPPh3 [

+Br

~PPh3+Br

Scheme 1. Functional groups on SiO2 surfaces: (1) sulfonic acid, (2) bis(9-Odihydroquinidinyl)pyrazinopyridazine for Os-catalyzed dihydroxylation, (3) polyethylene glycol and polypropylene glycol tails on a surface, (4) tertiary amine, (5) bicipital phosphonium group.

The most relevant direct esterification is that between glycerol and fatty acids. These two compounds are immiscible, and several solvents have been proposed to perform the reaction,

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such as dioxane and o-cresol. However, the solvent dilution retards reaction, and it is not obvious to remove these toxic solvents to the extent that the monoglycerides are suitable for human consumption. Zeolites have been used as catalysts for the solventless esterification of glycerol with lauric acid [6]. The monoglyceride selectivity is excellent, as the polar zeolite is fully immersed in the polar polyol layer. Consequently, the polyol / acid ratio is high at the active sites on and in the zeolite, and this favors formation of monoglycerides only. However, these sorption effects also decrease the rate to the extent that the zeolite-catalyzed reaction is only a few times faster than the uncatalyzed reaction at the same temperature. With sulfonic acids heterogenized on SiO2 or MCM-41, the reaction of glycerol with lauric acid is much faster than with a H-Y zeolite [7]. A remarkable feature is that at a fatty acid conversion of about 15 %, the reaction rate suddenly increases. Control experiments have proven that this is not due to leaching of active species. Rather, the fact that the biphasic mixture becomes visually homogeneous suggests that the monoglyceride concentration has reached the CMC (critical micelle concentration), which means that one liquid phase is very finely dispersed in the other phase. The rate acceleration with the solid sulfonic acids at the CMC, means that these materials are catalytically active at the interphase between the polar glycerol and the apolar fatty acid phases. Moreover, the selectivity for the monoglyceride is better with the solid sulfonic acid than with a dissolved acid (e.g. p-toluenesulfonic acid). The latter soluble acid is present in the apolar layer, and thus promotes reaction between monoglyceride and excess acid, which are both in the apolar layer. This entails formation of diglycerides, and decreased selectivity: + RCOOH - H20 ~

CHEOH CHOH I CH2OH

-

["O~c(o) R CHOH I CH2OH

+ RCOOH - HzO _ -

sO'~c(O)R

~O'~c(O)R

CHOH

+

L. -o ~ C ( O ) R

~O'~c(o) R CH2OH

Such problems are less encountered with the solid sulfonic acid, which is exposed at the interphase to the polyol as well as to the fatty acid. Thus the maximum yield of the monoglyceride with the solid sulfonic acid (53 %) is significantly higher than with paratoluenesulfonic acid (44 %), at a 1:1 polyol : acid ratio. Multiphase problems are also relevant for the etherification of sugars, as the sugars hardly dissolve in typical apolar alcohols. In this case, solid catalysts are sought for a reaction between a liquid alcohol and a solid sugar. Zeolites have been used in these reactions, but rates are rather small, particularly when a sugar such as glucose must react with an apolar fatty alcohol [8-10]: ~H2OH

glucose + 1-octanol

H+ ---

~ O H ~ OC,

CH2OH

+

Of 8 OH

OH

/ OH

Altemative solutions have been proposed, such as a two-step process, in which the glucose is first reacted with n-butanol; in a second step, the butylglycoside is transacetalated with longer

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fatty alcohols. For the one-step reaction, zeolite H-Beta seems most suitable. In this case as well, the initially formed product, viz. the higher alkylglucoside, accelerates the reaction due to its emulsifying properties. Enantioselective Epoxidation. The asymmetric olefin epoxidation with Jacobsen's homogeneous catalyst is normally a liquid biphasic reaction, with the olefin in the apolar layer and hypochlorite in the aqueous layer. The homogeneous catalyst resides in the organic layer, and is thus protected from exposure to an excess of the oxidant. Several attempts have been made to heterogenize this Jacobsen catalyst on inorganic supports. A specific problem is that the immobilized catalyst may 'choose' the wrong phase; if the solid is mostly in contact with the aqueous oxidant phase, quick deactivation is expected. Immobilization of the Jacobsen or related complexes in zeolite topologies such as FAU and EMT apparently leads to catalysts that function even with hypochlorite [11-12]. Even if it is well known that cation-rich zeolites such as NaY or EMT are hydrophilic, it must be considered that the adsorption of aromatic compounds inside the cages raises the organophilicity of the material. However, it was not attempted to relate the lifetimes of the catalysts to their hydrophobicity. With other immobilized Jacobsen catalysts, only reactions with PhlO as the oxidant have been reported, implying that the use of these materials in triphasic mode with C10- was not successful. This holds e.g. for Mn salen complexes covalently anchored to a silica via hydrosilylation, or for cationic Mn salen complexes that were exchanged on laponite [ 13-14]. Enantioselective dihydroxylation. A chiral dihydroquinidine ligand has been immobilized on silica, and has been employed in the Sharpless enantioselective cis-dihydroxylation of olefins with OsO4 (Scheme 1, structure 2) [15]. The catalytic cycle comprises the addition of OsO4 to the olefin, the hydrolysis of the diolate complex and finally, reoxidation of the Os v~ compound to OsO4. It is well known that the choice of oxidant in the latter step has a decisive effect on the enantiomeric excess of the dihydroxylation. H202, organic peroxides and Oz all reoxidize Os v~, but the best ee's are obtained with N-methylmorpholine-N-oxide or potassium ferricyanide. In the latter case, the reaction is conducted with a biphasic tBuOH-H20 solvent mixture: the olefin dissolves in the tBuOH-rich phase, while the inorganic oxidant resides in the water phase. The cinchona alkaloid was bound to the silica via large aromatic linker groups. Hence it can be expected that the alkaloid is embedded in the organic layer, where the enantioselective addition takes place. However, a major shortcoming of this catalyst is that the Os itself is not sufficiently retained on the heterogenized ligand. Thus, the repeated use of this ligand requires that Os is added in each recycle. It is at present unclear to what extent this impairs the applicability of the immobilized ligand for large scale reactions. Epoxidation with W or Re. An apparently widely applicable approach to immobilization of Lewis acid epoxidation catalysts comes from the group of Neumann [16-17]. They attached polyether tails to the surface of silica. By mixing polypropylene glycol and polyethylene glycol tails, the polarity of the surface can be adjusted to the specifics of any reaction (Scheme 1, structure 3). This approach relies in part on the knowledge that polyether solvents are to some extent capable to dissolve H202 and apolar olefins. Several homogeneous catalysts were

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immobilized in the polyether surface layer, including CH3ReO 3 and W-containing polyoxometallates. The good retention of these catalysts on the solid in reaction conditions is remarkable, since these compounds are hard to immobilize by simple physisorption [18]. At least in the case of the polyoxometallates, the ether oxygen atoms may coordinate on the complex, and thus improve the immobilization.

Catalysis with Immobilized Phosphines. Organometallic catalysis has mainly pursued liquid biphasic approaches in order to facilitate recuperation of the generally expensive ligands. Nevertheless, there are increasingly solid materials with covalently anchored phosphine ligands [3]. These are of course applied in heterogeneously catalyzed hydrogenations, hydroformylations or enantioselective variants thereof. Within the class of coupling reactions, some attention has also been devoted to Heck reactions, which are catalyzed by Pd, deposited on supports, or chelated by immobilized phosphines [19,20]. In such reactions, a base is needed to quench the acids (HBr, HI,...) which are generated. Typical bases are largely insoluble salts such as NaOAc, or soluble bases which form salts upon neutralization (e.g. Et3N --~ Et3N.HX). Thus, one encounters again a reaction of a liquid with a solid catalyst, and a third, solid phase. Little is known about the fate of the salts in the presence of a heterogeneous catalyst, or about their effect on the catalyst performance, and this aspect of triphasic catalysis deserves more attention.

Immobilized phase transfer catalysts. Silica surfaces have been decorated with typical phase transfer catalysts, such as quaternary ammonium or phosphonium salts. Quaternary groups can be used to exchange OH- ions, and this leads to useful materials for base catalysis [21]. However, even the quaternary group as such presents opportunities in triphasic catalysis. Quarternary phosphonium groups can be obtained by Ni- or Pd-catalyzed addition of triphenylphosphine to an immobilized bromophenyl group [22-24]. Apart from isolated tetraphenylphosphonium groups, bicipital (or 'two-headed') phosphonium groups have been prepared on the surface of silica with different pore diameters (Scheme 1, structure 5). The materials were compared in the reaction of aqueous KI with 1-bromooctane, dissolved in various solvents: KI + ~

B

r

--

KBr + ~

i

Good rates and satisfactory reusability were obtained in toluene, with a silica with 10 nm average pore diameter. The solid catalyst is even active in solvents such as cyclohexane; unsupported tetraphenylphosphonium bromide is hardly soluble in such apolar solvents and is therefore not effective. A particular advantage of the 'bicipital' material is that the two reaction partners (R-Br and I-) are simultaneously activated on a single site: while the C-Br bond is polarized at one 'head', an iodide ion is supplied by the adjacent phosphonium group, and this results in superior rates. As an alternative to direct esterification for the production of monoglycerides, fatty acids may react with the epoxide glycidol. Even if lauric acid is more miscible with glycidol than with glycerol, the reaction still involves components with a widely differing polarity. Brunel and coworkers have performed these reactions in toluene, with various amine and ammonium functionalized MCM materials as catalysts [1, 25]. Anchored tertiary amines, such as the

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piperidine 4 (Scheme 1), are more effective catalysts than primary amines. Significantly, treatment of the tertiary piperidine with methyl iodide, and exchange of the iodide with laurate anions, results in an even more active material:

~O + RCOOH

Silica~N+~ RCOO-

CH2OH

.~

sO~c(O)R CHOH I CHEOH

This indicates that not only the basicity but also the phase transfer properties of the surface are important. A mechanism was proposed, involving attack of the laurate anion on the epoxide, which is activated on the ammonium group. The approach can be extended to other fatty acids, such as undecylenic and ricinoleic acid. It would be interesting to evaluate these catalysts with lower amounts of solvents; in such conditions, the surface-promoted contact between the polar and apolar reactants will probably be even more important. 3. Multiphase Catalysts based on Ion exchanging Materials

Both classic, cation-exchanging clays (e.g. montmorillonites), as well as their anion exchanging counterparts (layered double hydroxides) have been used in triphasic catalysis. A major advantage of these materials is that their polarity can be varied from very hydrophilic (in the assynthesized state) to very hydrophobic (when exchanged with organic ions) [26]. The dramatic impact of such a polarity twist has been demonstrated for anion exchanging layered double hydroxides (LDHs), which were exchanged with WO42- and with a hydrophilic (NO3-) or a more hydrophobic anion (pTos-, para-toluenesulfonate). Comparison of the two resulting catalysts in olefin epoxidation with H202 shows that LDH-(WO42-, C1-) is the better catalyst with polar substrates such as allylic alcohols, while with apolar alkenes, better yields are obtained with the LDH-(WO42-, pTos-) catalyst. Such results can be interpreted based on the relative surface concentrations of H202 and olefin [27].

Halide exchange reactions.

Pinnavaia and Li used a quaternary ammonium exchanged montmorillonite to catalyze halide exchange reactions with an aqueous and an organic phase [28], much like those mentioned for the heterogeneous phosphonium catalysts (vide supra). The quaternary ammonium compounds are not only present at the periphery of the material, but are intercalated between the individual lamellae of the clay, as indicated by X ray diffraction studies. This means that a relatively large amount of quaternary compound is required to modify the clay. Visual inspection of a non-stirred liquid biphasic mixture shows that the clay is located at the interphase, which proves its amphiphilic nature. Upon stirring, the Quat-clay forms a stable emulsion, which is apparently easily broken by centrifugation.

Oxidative transformations with Mo and W exchanged LDHs: release of reactive particles (102, Br§ We have used tungstate or molybdate exchanged LDHs in oxidative catalysis. In interaction with aqueous H202, these anions are transformed into peroxoW and peroxoMo ions respectively. While e.g. peroxoW can directly epoxidize olefins (vide supra), these peroxo complexes can also generate small reactive particles. The latter can diffuse from the hydrophilic,

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polar surface where they are formed, to other, more hydrophobic compartments of the reaction system, where they react with organic molecules such as olefins. More specifically, the LDH-immobilized peroxoMo species generate IO2. This dioxygen in its excited singlet state is a selective oxidant for olefins [29]. The singlet peroxidation of e.g. 2,3dimethyl-2-butene with LDH-MoO41 and H202 in MeOH was the subject of a detailed kinetic study. The results were interpreted with a model which divides the reaction suspension into two compartments: a polar aqueous zone close to the surface, where 102 is generated by exchanged molybdate; due to the surface hydrophilicity, this compartment hardly contains any olefin substrate; the less polar bulk of the solvent contains the olefin substrate. Diffusion of IO2 from the surface to the olefin molecules in the less polar solvent compartment leads to peroxidation of the olefin substrate. Importantly, even if a solid catalyst is used for 102 generation, there is no need for adsorption of the olefin on the surface. The fast diffusion of 102 from one compartment to another is in this case the key element for successful reaction between reactants of different polarities, namely MOO42"/I'-I202and an apolar olefin. PeroxoW on LDH is capable of oxidizing bromide anions to 'Br +'. The latter reactive particle is able to react with olefins [30]. Not only dissolved olefins, but also emulsified olefins react smoothly. This implies that Br § is formed at the catalyst's surface, and subsequently crosses the liquid-liquid phase border to react with the olefin in the droplets. A typical example is the reaction with carveol, a C10 alcohol which is relatively easily dispersed in even highly polar continuous media. In the case of carveol, a triphasic reaction with a solid LDH-WO42-, a polar phase containing water, H202 and NH4Br, and a third, apolar carveol phase, leads to bromideassisted epoxidation.

W042" I H202 L

W

, Scheme

0 2-

Br-

aqueous phase

@ droplets withapolar, emulsifiedsubstrate

'Br§

@

2. Reactive 'Br § particles are generated at the LDH surface, and diffuse to the substrate

droplets.

4. C o n c l u s i o n ;

Future Challenges

The table gives an overview of what has been accomplished so far with solid catalysts in reactions between two immiscible reactants. These reactions (with the exception of entry 7) are all truly triphasic. In some cases (entries 1, 5, 6 and 8), the surface polarity of the material has been explicitly adapted to enable catalysis at an interphase. In other cases (entry 4), the material has been rendered hydrophobic on purpose in order to promote partition of the material to the

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organic layer; or the material may be fully in the aqueous phase (entry 9). As noted above, a particular situation arises when one of the products of the reaction has emulsifying properties (entries 1 and 2). Table. Reactions between compounds of widely different polarity, catalyzed by solid materials. Polar reactant

Catalyst

Apolar reactant

Ref.

1

Glycerol

-SOaH on SiO2

Lauric acid

7

2

Glucose

H-Beta

1-octanol

9

3

OC1-(aq.)

Jacobsen complex in FAU, EMT

Olefin

11-12

4

K3Fe(CN)6(aq.)

Quinidine-derived ligand on SiO2 + Os

Olefin

15

5

H202

Polyether tails on SiO2 + W, Re

Olefin

16-17

6

I-(aq.)

(bicipital) R4P+ on SiO2

Alkyl halide

23

7

Glycidol

Quaternized piperidine on MCM-41

Lauric acid

25

8

Br (aq.)

R4N+ exchanged on clay

Alkyl halide

28

9

H202,Br- (aq.)

WO42 exchanged on LDH

Olefin

30

Nevertheless, the domain of triphasic catalysis with engineered solid catalysts is still in its infancy. Some of the important challenges that remain are enumerated below: Techniques are needed to study the location of a heterogeneous catalyst in a biphasic reaction medium: does the catalyst largely reside in the polar or in the apolar layer? Apart from catalyst density, surface polarity is a crucial parameter. This question resembles the classic contact angle problem. However, contact angle measurements normally are performed on large, smooth surfaces. Application to a highly dispersed catalyst material is all but straightforward. Location (phase partition) of an immobilized catalyst may drastically alter the stability, selectivity, etc. in comparison with the corresponding homogeneous catalyst. These effects are till now only fragmentarily understood. It is known that some materials (e.g. Quat-exchanged clays) can stabilize liquid-liquid emulsions. However, at least as much attention should be devoted to breaking such emulsions - an essential stap in reaction workup. In this case, the statement may hold that 10 % of the time is devoted to making a formulation, and 90 % to breaking it [31 ]. Many organic reactions that are nowadays performed with homogeneous catalysts, use or produce salts, which precipitate or are insoluble. What is the role of a solid catalyst in such a medium ? We need new concepts, (cfr. the generation of active particles on a solid and transport to another liquid phase).

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Acknowledgements Our efforts in catalyst preparation are supported by the Belgian Federal Government in the frame of an I.U.A.P. program on Supramolecular Chemistry and Catalysis. We are indebted to FWO (DDV) and IWT (BFS, WVR) for fellowships. References 1 D. Brunel, Microporous Mesoporous Materials, 27 (1999) 329. 2 K. Moller, T. Bein, Chem. Mater., 10 (1998) 2950. 3 E. Lindner, T. Schneller, F. Auer, H.A. Mayer, Angew. Chem. Int. Ed. Engl., 38 (1999) 2154. 4 K. Wan, M. Davis, Nature 370 (1994) 449. 5 W.M. Van Rhijn, D. De Vos, B. Sels, W. Bossaert, P. Jacobs, Chem. Commun. (1998) 317. 6 E. Heykants, W.H. Verrelst, R.F. Parton, P.A. Jacobs, Stud. Surf Sci. Catal. 105 (1996) 1277. 7 W. Bossaert, D. De Vos, W. Van Rhijn, J. Bullen, P. Grobet, P. Jacobs, J. Catal. 182 (1999) 156. 8. A. Corma, S. Iborra, S. Miquel, J. Primo, J. Catal. 161 (1996) 713. 9. A. Corma, S. Iborra, S. Miquel, J. Primo, J. Catal. 180 (1998) 218. 10. J.F. Chapat, A. Finiels, J. Joffre, C. Moreau, J. Catal. 185 (1999) 445. 11. S.B. Ogunwumi, T. Bein, Chem. Commun (1997) 901. 12. M.J. Sabater, A. Corma, A. Domenech, V. Fom6s, H. Garcia, Chem. Comm. (1997) 1285. 13. K.B. Janssen, Ph.D. thesis, K.U.Leuven (1999). 14. J.M. Fraile, J. Garcia, J. Massam, J.A. Mayoral, J. Mol. Catal. A: Chem. 136 (1998) 47. 15. C. Bolm, A. Maischak, A. Gerlach, Chem. Commun. (1997) 2353. 16. R. Neumann, T. Wang, Chem. Cornmun. (1997) 1915. 17. R. Neumann, M. Cohen, Angew. Chem. Int. Ed. Engl. 36 (1997) 1738. 18. Obviously, anionic W complexes and polyoxometallates can be immobilized by ion exchange on for instance resins: see A. Villa, B. Sels, D. De Vos, P. Jacobs, J. Org. Chem. 64 (1999) 7267. 19 C.P. Mehnert, D.W. Weaver, J.Y. Ying, J. Am. Chem. Soc. 120 (1998) 12289. 20 J.Y. Ying, C.P. Mehnert, M.S. Wong, Angew. Chem. 38 (1999) 58. 21 I. Rodriguez, S. Iborra, A. Corma, F. Rey, J.L. Jorda, Chem. Commun. (1999) 593. 22 J.H. Clark, S.J. Tavemer and S.J. Barlow, J. Mater. Chem. 5 (1995) 827. 23 J.H. Clark, S.J. Tavemer and S.J. Barlow, Chem. Commun. (1996) 2429. 24 J.H. Clark, D.J. Macquarrie, Chem. Cornmun. (1998)853. 25 A. Cauvel, G. Renard, D. Brunel, J. Org. Chem. 62 (1997) 749. 26 F. Cavani, F. Trifir6, A. Vaccari, Catal. Today 11 (1991) 173. 27 B. Sels, D. De Vos, P. Jacobs, Tetrahedron Lett. 37 (1996) 8557. 28 Li, Pinnavaia, Chem. Materials 3 (1991) 213. 29 B. Sels, D. De Vos, P. Grobet, F. Pierard, A. Kirsch-De Mesmaeker, P. Jacobs, J. Phys. Chem. B, (1999) 11114. 30 B. Sels, D. De Vos, M. Buntinx, F. Pierard, A. Kirsch-De Mesmaeker, P. Jacobs, Nature 400 (1999) 855. 31 J.M. Aubry, personal communication.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

NEW SOLID ACID BASED BREAKTHROUGH TECHNOLOGIES Stanley A. Gembicki UOP LLC 25 East Algonquin Road, Des Plaines, Illinois 60017-5017, U.S.A. Recent advances in catalyst science and process technology have led to the commercialization of a number of new solid acid-based technologies. These technologies not only represent the first examples of a new process class, but also emphasize, once again, that technology renewal of our mature refining and petrochemical industries is still possible. The fundamental work in the discovery and characterization of new materials continues to make these advances possible and ultimately drives commercial innovation. I.

INTRODUCTION

The history of the refining and petrochemical industries is a story of technical accomplishment and innovation. These innovations have led to both tremendous growth and a high level of technical sophistication. But where will all of us go for the new ideas necessary to insure continued growth and success? For the ideas necessary to deal with inevitable societal change? And, how does catalysis fit into this broad picture of developing new technologies? This is the topic of my address and I will attempt to review not only where our ideas have come from historically but also where we will seek them out in the 21 st century. It is clearly relevant to consider economic issues, such as cheaper and more plentiful feedstocks, reduction of the number of process steps - both physical and chemical. In addition, one must consider relevant progress in science, specifically catalytic chemistry, in our case, as well as progress in process design. Both catalyst science and process innovation have provided the basis for discontinuousbreakthrough- progress in petroleum and petrochemical technologies. If we look at the catalytic chemistry applied in the past half century in motor fuels technology, we see that acid catalysis has been the key catalytic step for hydrocarbon conversion, not only for cracking but for almost all hydrocarbon conversion steps: isomerization, alkylation and aromatization. Liquid acid catalysts that led the way to new chemistry decades ago, have been mostly replaced by environmentally more desirable, economically superior solid-acid catalysts. Recent years also have shown important progress in solid acids. After the technology revolution created by acidic zeolite catalysts, we have seen lately the emergence of a new class of solid acids, the solid superaeids, with a stability suitable for use in hydrocarbon processing. So far, this new trend is largely due to a need to convert the relatively inert saturated hydrocarbons more efficiently. Considering the fact that the raw material resource balance in the future will further emphasize the use of small, saturated and increasingly "inert" molecules, the current trend toward the development and use of strong and super-acid catalysts, will no doubt put the "solid superacid theme" as one of the centerpieces of future catalysis research and catalysis technology.

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2. BRIEF H I S T O R Y OF C O M M E R C I A L L Y I M P O R T A N T SOLID ACID CATALYSTS

Solid Acid Catalyzed reactions and processes are of key importance in the refining and petrochemical industries, and are used in a wide range of hydrocarbon conversion reactions ~5. Solid acids 6'7'8 are generally regarded as the most important class of industrial catalysts. In recent years, new breakthrough catalysts and technologies have been developed such as optimized fluid catalytic cracking catalysts, new dewaxing catalysts, new solid catalysts 5 for biodegradable linear alkyl benzene, and sulfated metal oxides for paraffin isomerization. Solid acid based isobutane alkylation technology has moved from the development to the demonstration stage indicating new breakthrough process opportunities in the near future. A number of important inventions in the solid acid catalyst area are summarized in Table I. Table I Important Inventions in Solid Acid Catalyst Area INVENTION ( catalyst or process) 1928 CatalyticCracking 1932 Alkylation 1935 Butane lsomerization 1940 Platforming 1948 SyntheticZeolites 1949 Ether synthesis using acid ion exchange resin, e.g. MTBE 9 1957 Acidic-Y zeolites 1964 ZSM-5 1962 Sulfated Metal Oxide 1968 Shape Selective Catalysis 1982 Aluminophosphates 1997 SAPO-34for MTO Solid Acids for I-C4 Alkylation

INVENTOR E.J. Hardy Ipatieff and Pines lpatieff and Pines Haensel Milton, D. Breck, E. Flanigen

COMPANY Houdry Process Co. UOP UOP UOP UCC Anic-Snamprogetti9~

Rabo et al. ~2 Argauer et al. ~3

UCC Mobil

Holm/Philips Petroleum~4 Weiss, Csicseri E. Flanigen, S. Wilson ~6 Kaiser, Lewis ~7 Hommeltofi ~8 UOP 19

UOP~5 Mobil UCCAJOP UOP Norsk-Hydro

Haldor Topsoe UOP

Year of Commercialization 1936 1938 1941 1949 1962 for Catalytic Cracking 1973 1960 1962 MTG xylene isomerization 1997 CSIRO 1992 In progress In progress In pro~ess

3. SOLID ACID C A T A L Y Z E D N E W T E C H N O L O G I E S In this talk I will cover a few of the major new commercial and near-commercial processes with emphasis on paraffin activation and new solid acid catalysts. The development of these processes depends not only on new material discoveries, but a combination of several key enablers: the discovery of new inorganic compositions; the fundamental understanding of new compositions which drives material modifications and results in useful solid acid catalysts; and innovative process development and engineering design.

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3.1. Paraffin alkylation ( New UOP's Alkylene Process 19) Traditionally, alkylation process technology options have been limited to using either hydrofluoric or sulfuric liquid acids. Each option has proven itself for well over 50 years as a reliable and economical choice for alkylate production in hundreds of applications. Even though operations of liquid-acid alkylation plants are among the safest of all refinery plant operations, the safety of these plants and the transportation of the liquid acids continues to come under increased scrutiny. So at a time when the importance of alkylate in the gasoline pool is growing, refiners are having difficulty adding alkylation capacity because of the worldwide public concern over the potential hazards of liquid acids. To be viable, any new alkylation process must be safe, reliable, cost-effective, and economical to operate. In the past couple years, a number of companies have announced the development of solid acid-based processes. Table II summarizes the current process developments at the Pilot Plant stage. Table II Current Process Developments at the Pilot Plant Stage

Catalyst HETECAT BF3 on Alumina SbF5on Silica Triflic acid on silica Proprietar/catal~,st

Company Boreskov Inst./Russia Catalytica, Conoco,Nestle Chevron, CR&L HaldorTopsoe UOP

Reactor and Capacity Fixed bed, 3.5 ct~. SlurryCSTP,7 BPD 10 BPD 0.5 BPD =55kg/d Transport Reactor

Refs 2o 21 22 23 19

In 1994, UOP began an extensive process development and engineering effort to develop a new alkylation technology that would be economically competitive with the existing liquid-acidbased processes. The development of this technology involved innovations in both catalyst and process design. The Alkylene catalyst has been designed with sufficient acidity to promote the desired alkylation reactions, along with an optimum particle size and pore distribution to ensure good mass transfer. These characteristics provide the catalyst with the necessary activity to alkylate isobutane at a high reaction rate, yet allow the alkylate product to be rapidly transported away from the catalytic sites. Although many catalytic materials have shown promise as solid catalysts, most have short lives, measured in hours or even minutes, and these materials cannot be easily reactivated. The primary deactivation mechanism is the blocking of active sites on the catalyst surface by high molecular weight, unsaturated hydrocarbons. A unique and important aspect of the Alkylene catalyst is that these heavy hydrocarbons are easily removed from the catalyst surface. Reactivation of this catalyst is accomplished by contacting it with hydrogensaturated isobutane after the reaction step. The hydrogen saturates the heavy hydrocarbons, which then readily desorb and are removed from the catalyst surface. Fluidized-bed reactor technology is utilized to provide a means of overcoming the deactivation problems observed in fixed-bed reactors. Using a riser, or transport reactor in which the catalyst and feed move concurrently, ensures short contact time, which minimizes the degradation of the initial alkylate product and reduces the extent of catalyst deactivation per pass.

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化

Figure 4 - Alkylene Process Flow t

A/ky/ene '

~

End.~ LPG

Fr~tmnmnon s~tk).

Re~K:twilion Was. Zone

ReactlvaOofl ....

A

Integration of the technology, as illustrated by the Alkylene process, Figure 4, is accomplished in three steps: Feed training section; reactor section; and fractionation section

o~r~ Feed

,L

Fq.~ rmlcmlnt

Isobutane Recycle

3.2. Isomerization of Paraffins ( UOP's Par-lsom Process Is) The isomerization of hydrocarbons has become increasingly important mostly for automotive fuel applications and petrochemical applications. UOP has been the leading licensor of light naphtha isomerization technology since 1958, when the first Penex unit was commissioned. Over 90% of the world's total installed or committed naphtha isomerization capacity utilizes UOP technology. Improvements in technology have progressed from the introduction of zeolitic catalysts and the TIP TM process in the 1970's, through the introduction of the high activity I-8 catalyst and Hydrogen-Once-Through (HOT) Penex TM process in the 1980's, to the recent commercialization of the LPI-IO0 catalyst and the Par-Isom process in 1996. Figure 5 - lsomerization Catalyst Performance

NO

7 74-

9

Zeolitic

Metal Oxide

,

Chlorided Alumina

With the commercialization of the LPI-100 catalyst, there are now three types of catalysts available for isomerizing light naphtha. The product octanes resulting from processing a typical light naphtha feedstock over these three catalysts (Zeolite, Chlorinated Alumina, Sulfated Metal Oxides) are compared on a once-through basis in Figure 5.

Sulfated metal oxide catalysts can be considered to be strong acids and exhibit high activity for paraffin isomerization reactions. There has been active scientific publication on the acidity of sulfated metal oxides and their activity in paraffin isomerization. These catalysts are most commonly tin oxide (SnO2), zirconium oxide (ZrO2), titanium oxide (TiO2), or ferric oxide (Fe203) that have been sulfated by the addition of sulfuric acid or ammonium sulfate. The first sulfated metal oxide catalyst has now been commercialized with the introduction of UOP's LPI-100 catalyst. Activity of this new catalyst is considerably higher than traditional zeolitic catalysts, equivalent to about 80~ lower reaction temperature. The lower reaction temperature allows for significantly higher product octane, about 82 RONC for a typical feed or

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3 numbers higher than a zeolitic catalyst. The sulfated metal oxide catalysts are robust, and are not permanently deactivated by water or oxygenates in the feedstock. These catalysts are also fully regenerable using a simple oxidation procedure that is comparable to that practiced for zeolitic catalysts. Figure 6 - Par-Isom Flow

The unique performance advantages represented by the LPI-100 catalyst were recognized early in the development effort Ends Io~-and resulted in a parallel development l program culminating in the Par-lsom process, Figure 6. Extensive process development efforts were undertaken by Cs-Ce .= Reactor Feed UOP, aimed at optimizing the performance of the new catalyst over a wide range of process conditions and different feedstock compositions. Studies were also conducted Isomerate to define the impact of different feedstock contaminants and develop procedures that could be used successfully for in-situ or ex-situ regeneration. Makeup H=

1.

J

Separator:~

Light

Stabilizer

3.3. Methanol to Olefins (MTO 24) The use of molecular sieves to catalyze the conversion of methanol (MeOH) to hydrocarbons dates back to at least 1977 when Chang I and coworkers at Mobil Oil used zeolites such as ZSM-5. During the early 1980s Anthony2 and coworkers were also particularly active in the use of zeolites to convert MeOH to olefin. Exxon has also done extensive work in this field and developed catalysts for this reaction. The conversion of MeOH over medium and large pore zeolites (such as ZSM-5) normally produces extensive amounts of aromatics and paraffins, and in the case of large pore zeolites, this results in rapid coke formation. Because small pore zeolites adsorb linear hydrocarbons but exclude the larger branched or aromatic hydrocarbons, these bulkier species, if formed internally, cannot diffuse out. By far the most extensively studied medium-pore zeolite for MeOH conversion reaction is still ZSM-5. Because ZSM-5 normally exhibits higher selectivities to aromatics and paraffins, both zeolite and the process conditions have been modified to improve selectivity to light olefins. UOP and Norsk-Hydro (Norway) have jointly developed the UOP/HYDRO MTO process, an economic process for the conversion of natural gas to ethylene and propylene. An integral part of this process is the catalytic conversion of MeOH to light olefins over the UOP proprietary SAPO-34 molecular sieve. 4. NEW APPROACHES TO ADVANCED CATALYSIS In order to make the step changes necessary to succeed in the 21 st century, the chemical industry needs to adopt new methodologies, which will accelerate catalyst discovery and

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commercialization. A key to success will be the ability to generate fundamental knowledge, which can be leveraged to invent breakthrough catalysts and processes. Surface characterization of catalysts as well as the ability to characterize catalysts in situ will be critical in developing the fundamental understanding necessary to invent in mature areas. Meanwhile, adoption of modeling tools, which can successfully predict targets for the synthetic chemist, will result in an increase in the probability of success and a reduction in cycle time. Underscoring all of this, is the revolution that will be created by the adoption of combinatorial and high throughput tools in the preparation and testing of novel compositions. In concert, the characterization, modeling and combinatorial methods hold the promise of making a step change in the productivity and inventiveness of the chemical laboratory.

CHARACTERIZATION A detailed, atomic-level understanding of catalyst structure has long been the goal of catalyst characterization. Moreover, the true "holy grail" has been to determine the actual atomic level structure of the active site of the catalyst while it is turning over, i.e. under in situ conditions. While this has effectively been achieved on model single crystal catalysts in UHV using the enormous battery of surface science techniques, but primarily recently using scanning tunneling microscopy 2s, the added complexity of both the industrial catalyst and the process conditions has made this task elusive to date. However, it has been recognized that for true breakthroughs in catalyst development it is paramount that in situ studies are performed. Moreover new techniques are required, especially for characterization at atomic resolution 26' 27. There is reason to be optimistic that significant progress towards this in situ characterization will be made in the next decade. The use of synchrotron radiation has had a major impact on catalyst characterization 2s. New third generation synchrotrons are just becoming available, and fourth generation machines are even in the design stage. The improved characteristics of these facilities with respect to intensity, brilliance and stability will create exciting new opportunities for studies such as time resolved studies (kinetics), increased detectability (study of dilute impurities/additives), and spatial resolution (distribution of chemical states). Coupled with the improved characteristics of the new synchrotrons are new techniques such as diffraction anomalous fine structure (DAFS), giving site specific local structural and chemical information 29, three-dimensional X-ray microtomography, giving 3D maps of structure 3~ In addition it is now practical to combine methods to provide a greater range of information simultaneously. Such examples are simultaneous QEXAFS/XRD 31 and DEXAFS/XRD 32 giving local and long-range structure simultaneously with time resolution, and SAXS/WAXS allowing the study of hydrothermal synthesis over a range of crystal growth dimensions 33. There are several other techniques applicable to in situ characterization (e.g. infrared and Raman spectroscopy) and the reader is referred to the literature for further discussion 34. Significant advancements are also being made in these techniques. Thus, the future does appear to be a rosy one for in situ catalyst characterization.

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MOLECULARMODELING

Molecular modeling can be used to predict properties of novel materials before they are made. These predictions can be used to focus the efforts of materials synthesis groups on the most promising candidate materials. For example, the modeling of adsorption on zeolites has been hampered by the lack of efficient and accurate methods to predict the electric fields inside zeolite pores. These electric fields play a major role in determining the selectivity and activity of zeolitic catalysts. Recently, Molecular Simulations Inc. 35 has developed the program DMOL to predict charge densities in solids. Figure 8 shows a map of the electrostatic potential in zeolite 5A due to the charge density predicted by DMOL. The charge density was used in a model of Nitrogen adsorption on several different zeolites. The comparison of the predicted and experimental loadings for Nitrogen on several different zeolites (see Figure 9) demonstrates that the molecular model reproduces the interaction of molecules with catalysts36. It requires 2-3 days to predict an isotherm for a novel zeolite phase. The synthesis of a novel zeolite requires several months. Therefore, molecular modeling increases the efficiency of novel materials effort by screening candidate materials before they are made.

.. >~!

....

Figure 8 - Electrostatic potential in zeolite 5A.

I

I

0.0

0.5

1.0

1.6

2.0

2.5

Exporimenta| Loading (mmolelg)

Figure 9- Comparison of predicted and experimental Nitrogen loadings for the zeolites Faujasite, Mordenite, and Linde Type A.

Combinatorial Chemistry The chemical industry must continue to improve productivity and cycle time in developing and successfully commercializing technically superior, economical products. Combinatorial chemistry has great potential to provide a step-change in the productivity of the chemical research laboratory. A combination of sophisticated statistical design, miniaturization, and parallel experimentation increases by several orders of magnitude the ability of the chemist to obtain chemical information. Particularly in the catalyst development area, where testing and evaluation are slow and costly, the successful application of combinatorial methods allows highthroughput screening to select the most-promising candidates for further development.

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The primary application of combinatorial chemistry has been in the pharmaceutical industry. The potential of this method for accelerating drug discovery is so compelling that it has been rapidly embraced by pharmaceutical companies 37. Using combinatorial methods outside of the pharmaceutical industry is in its infancy. The extension of these methods to inorganic materials has been reported 38,39. During the next few years, the application of combinatorial methods to the discovery of inorganic oxides and heterogeneous catalysts will become a reality. Successful implementation of these methodologies requires that a link from the combinatorial scale to the commercial scale be created. This means that both the preparation and testing of catalyst libraries needs to be done under pertinent reaction conditions and provide predictive performance information. A key enabler will be the ability to tie the combinatorial data together via informatics platforms. A number of labs are working diligently towards this goal. Success is in the near future. 5.

FUTURE TRENDS

Chemistry and chemical technology have been at the heart of the revolutionary developments of the 20 th century. As the scale of chemical enterprise grew and as chemistry and chemical engineering became more sophisticated disciplines, the frontiers of innovation passed to the corporations where R&D organizations risked millions of dollars on new products and processes. There is a long tradition in the chemical industry4~ of combining theory and practice, science and engineering, technology and business to improve the lot of humankind. While this central tradition remains unchanged, the methods of achieving it have become more complex and have required the utilization of new advanced methods and tools, including: new materials, improved catalyst design, rapid kinetic analysis, high throughput evaluation of ideas, miniaturization, increased emphasis on modeling, and in-situ characterization. However, innovation will still depend on creative chemists and chemical engineers that connect ideas in the "fight" sequence. REFERENCES 1. J. M. Thomas and W. J. Thomas, Principles and Practice of Heterogeneous Catalysis, 1-669, Winhein: VCH, 1997 2. B. C. Gates, Catalytic Chemistry, New York: Wiley, 1992 3. L. L. Hegedus, Catalyst Design, Progress and Perspectives, New York: Wiley, 1987 4. George A. Olah and ,Z,rp~id Moln~ir, Hydrocarbon Chemistry, New York: Wiley-Interscience , 1995. 5. J. Ertl, J. Weitkamp and H. Knozinger, Handbook of Heterogeneous Catalysis, Weinheim: VCH, 1997 6. Avelino Corma, Inorganic Solid Acids and Their Use in Acid-Catalyzed Hydrocarbon Reactions, Chem Re. (Washington, D.C.) 95, no.3 (1955):559-614 7. Kozo Tanabe et al, Study in Surface Science and Catalysis: New Solid Acid and Base, Elselvier, 1989 8. Avelino Corma, Inorganic Solid Acids and Their Use in Acid-Catalyzed Hydrocarbon Reactions, Chem. Rev. (Washington, D. C.) 95, no. 3 (1995): 559-614. 9. A. Stuve, C.P. Halsig, H. Tschorn, Handbook of Heterogeneous Catalysis, Synthesis of MTBE, J. Ertl, J. Weitkamp, H. Knozinger, ed., 1986. Weinheim: VCH, 1997.

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10. Ivano Miracea and Giorgio Fusco, Integrated Process for Producing Isobutene and Alkyl TertButylethers, Italy Snamprogetti S.p.A., Eur. EP 502265 A2 19920909. 11. Franeo Morandi, MTBE and MAS. Snamprogetti Technologies in the Oxygenated Products Field, Pet. Teeh. 332, (1987): 14-18. 12. J.A. Rabo, P.E. Pickett, D.N. Stamires, J.E. Boyle, Acidity of Zeolite X and Y Proc. 2nd Intern. Congr. Catalysis, Edition Tech. 1960. 13. Robert J. Argauer, David H. Olson, and George R. Landolt, Molecular Sieves, MOBIL OIL, USA, GB, etc. 14. Vernon C. F. Holm, Grant C. Bailey, Sulfate Treated Zirconia-Gel Catalyst, Philips Petroleum Company, USA 3,032,599. 1 May 1962. 15. C. Gosling, R. Rosin, P. Bullen T. Shimizu and T. lmai, Revamp Opportunities for Isomerization Units, Petroleum Technology Quarterly (1997-1998): 55. 16. Stephen T. Wilson, Brent M. Lok, Celeste A. Messina, Thomas R. Carman, and Edith M. Flanigen, Aluminophosphate Molecular Sieves: a New Class of Microporous Crystalline Inorganic Solids, J. Am. Chem. Soc. 104, no. 4 (1982): 1146-7. 17. Lewis, Jeffrey Michael Owen, and William Howard. Henstock, inventors, Process and Catalysts for the Manufacture of Alkenes From Alcohols and Alkyl Ethers, S. African, and USA Union Carbide Corp., assignees. ZA. 8807237. A. 1989. 74 pp. 18. Hommeltoft, Sven Ivar, and Haldor Frederik Axei. Topsoe, Preparation of Alkanes by Alkylation Process, Eur. Pat. Appl., and Den. Haldor Topsoe A/S, EP. 433954. A1. 1991. 11 pp. 19. C.D. Gosling, J. C. Sheckler P. T. Barger H. U. Hammershaimb D. J. Shields S. J. Frey, The Alkylene Process: Innovative Technology Using a Solid Catalyst, JPI Petroleum Refining Conference Tokyo 1998, 1998. 20. V.K. Duplyakin, G. A. Urzhuntsev, N. M. Ostrovskii, V. N. Parmon, and A. I. Lugovskoii, Alkylation of Isobutane by Butylenes Over Solid Superacid Catalyst, Book of Abstracts, 212th ACS National Meeting, Orlando, FL, August 25-29, Boreskov Institute Catalysis Novosibirsk 630090 Russia Federal Research Center the Russian Federation, and Washington D American Chemical Society, PETR-0361996. 21. David L. King, Michael D. Cooper and William A. Sanderson, Improved Lewis Acid-Promoted Transition Alumina Catalysts and Isoparaffin Alkylation Processes, PCT Int. Appl., and Inc. USA Catalytica, WO. 9402243. A 1.1994. 63 pp. 22. Crossland, Clifford Stuart, Elliot George Pitt, Alan Johnson, and John. Woods, Process and Apparatus for Paraffin Alkylation and Catalyst for Use Therein, Eur. Pat. Appl., and USA Chemical Research and Licensing Co., EP. 495319. A2.1992. 21 pp. 23. Sven I. Hommeltoft and Haldor F. A. Topsoe, Supported Liquid Phase Alkylation of Aliphatic Hydrocarbons Using a Fluorinated Sulfonic Acid Catalyst, U.S., and S. A. Den. Haldor Topsoe, US. 5245100, A. 1993, 5 pp. Cont.-in-part ofU.S. Ser. No. 626,956. 24. P. T. Barger and S. T Wilson, Converting Natural Gas to Ethylene and Propylene by the UOP/HYDRO MTO Process, Proc. Int. Zeolite Conf., 12th, 567-731999. 25. For review of surface science techniques and catalysis see, "Introduction to Surface Chemistry and Catalysis", G.A. Somorjai, Wiley, 1994. For STM imaging of surface reactions see e.g.X.-C. Gua, R.J. Madix, Surf. Sci., 387, 1, (1997) and W.W. Crew, R.J. Madix, Surf. Sci. 356, 1, (1996). 26. Catalyst Technology Roadmap Report in Technology Vision 2020. The US Chemical Industry, CCR/DOE/ACS Workshop, April 1997 27. Catalysis Looks to the Future, National Academy Press, 1992. 28. X-ray Absorption Fine Structure for Catalysts and Surfaces, Ed Y.Iwasawa, World Scientific, 1996 29. H. Straiger, J.O. Cross, J.J. Rehr, L.B. Sorensen, C.E. Bouidin, C.E. Woicik, Phys. Rev. Lett. 69, 3064, (1992)

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30. H.W. Deekman, J.H. Dunsmuir, K.L. D'Amico, S.R. Ferguson, B.P. Flannery, Mater. Res. Soc. Symp. Pror 217, 97,(1991). 31. B.S. Clausen, L. Grabaek, G. Steffensen, P.L. Hansen, H. Topsoe, Catal. Lett., 20, 23 (1993). 32. J.W. Couves, J.M. Thomas, D. Waller, T.H. Jones, A.J. dent, G.E. Derbyshire, G.N. Greaves, Nature, 354, 465, (1991). 33. P.-P. E.A. de moor, T.P.M. Beelen, B.U. Komanschek, O. Diat, R.A. van Santen, J. Phys. Chem. B, 101, 11077, (1997). 34. For a recent review see, J.M. Thomas, Chem. Eur, J., 3, 1557, (1997). 35. Computational results obtained using software programs from Molecular Simulations Inc., density functional calculations were done with DMOL program, and graphical displays were printed out from the Cerius2 molecular modeling system. 36. A. Gupta, R. Snuff, J. J. Low, unpublished results 37. Borman, S. "Combinatorial Chemistry," C&EN, 47, April 6, 1998 38. Danielson, E.; Devenney, M.; Giaquinta, D.M.; Golden, J.H.; Haushalter, R.C.; McFarland, E.W.; Poojary, D.M; Reaves, C.M.; Weinberg, W.H.; Wu, D.X., Science, 279 (6), 837, 1998. 39. Akporiaye, D.E.; Dahn, I.M.; Karlsson, A.; Wendelbo, R.; Angew. Chem. Int. Ed., 37 (5), 609, 1998. 40. Bowden, M.E.; Smith, J.K.; American Chemical Enterprise, Chemical Heritage Foundation, (1994).

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化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Applications of t i t a n i u m oxide p h o t o c a t a l y s t s and u n i q u e s e c o n d - g e n e r a t i o n TiO2 p h o t o c a t a l y s t s able to operate u n d e r visible light irradiation for the r e d u c t i o n of e n v i r o n m e n t a l toxins on a global scale M. Anpo Department of Applied Chemistry, Osaka Prefecture University Sakai, Osaka 599-8531, J a p a n The p r e s e n t investigation deals with practical applications of t i t a n i u m oxide photocatalysts and the development of unique secondgeneration t i t a n i u m oxide photocatalysts by applying advanced ionengineering techniques, enabling effective and efficient reactions not only under ultraviolet (UV) but also under visible light irradiation. Solar beams are absorbed up to 30-40% more efficiently, allowing the large scale use of these t i t a n i u m oxide photocatalysts for the reduction of environmental toxins. 1. I N T R O D U C T I O N Environmental pollution and destruction on a global scale have drawn attention to the vital need for totally new, safe and clean chemical technologies and processes, the most i m p o r t a n t challenge facing chemical scientists for the 21st century. Strong contenders as e n v i r o n m e n t a l l y - h a r m o n i o u s catalysts are photocatalysts which can operate at room t e m p e r a t u r e in a clean and safe m a n n e r while applications of such photocatalytic systems are urgently desired for the purification of polluted water, the decomposition of offensive atmospheric odors as well as toxins, the fixation of CO2 and the decomposition of NOx and chlorofluorocarbons on a global scale [1-4]. To address such enormous tasks, photocatalytic systems which are able to operate effectively and efficiently not only under UV but also under the most environmentally ideal energy source, sunlight, must be established. And to this end, we are moving in a very positive direction with the development of unique titanium oxide photocatalysts which work under solar beam or visible light irradiation [3,4]. The present paper deals with 1) various practical applications of t i t a n i u m oxide photocatalysts and 2) the design and development of unique second-generation t i t a n i u m oxide photocatalysts which operate effectively u n d e r visible and/or solar b e a m i r r a d i a t i o n for the applications in improving our environment on a large global scale.

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2. VARIOUS P R A C T I C A L A P P L I C A T I O N S OF TITANIUM OXIDE PHOTOCATALYSTS

W h e n titanum oxides are irradiated with U V light that is greater than the bandgap energy of the catalyst (about ~, < 380 nm), electrons (e" ) and holes (h+) are produced in the conduction and valence bands, respectively. These electrons and holes have a high reductive potential and oxidative potential, respectively, which together cause catalytic reactions on the surfaces, namely, photocatalytic reactions are induced. Because of its similarity with the m e c h a n i s m observed with photosynthesis in green plants, photocatalysis m a y also be referred to as artificial photosynthesis [1-4]. As will be introduced in the later part, there are no limits to the possibilitiesand applications of titanium oxide photocatalysts as "environmentally harmonious catalysts" and/or "sustainable green chemical systems". In the presence of 02 and H20, the photo-formed e" and h + easily react with these molecules on the titanium oxide surfaces to produce 02and O H radicals, respectively. These 02-and O H radicals have very high oxidation potential, inducing the complete oxidation reaction of various organic compounds such as toxic halocarbons, as shown in equation (1) [1]: CnHmOzCly

> nCO2 + yHC1 + wH20

(1)

Such high photocatalytic reactivities of photo-formed e- and h + can be expected to induce various catalytic reactions to remove toxic compounds and can actually be applied for the reduction or elimination of polluted compounds in air such as NOx, cigarette smoke as well as volatile compounds arising from various construction m a t e r i a l s , oxidizing them into CO2. In water, such toxins as chloroalkenes, specifically trichloroethene and tetrachloroethene as well as dioxins can be completely d e g r a d a t e d into CO2 and H 2 0 [1,2]. Such highly photocatalytically reactive systems are also applicable in protecting the lamp-covers and the walls in tunnels from becoming dark and sooty by emission gases. Soundproof highway walls coated with titanium oxide photocatalysts have been constructed on heavily congested roads for the elimination of NOx (Fig. 1) [5]. The reactivity of photo-formed 02- and OH radicals is high enough to decompose or kill bacteria so that new cements and tiles mixed or coated with t i t a n i u m oxides have been commercialized and are already in use in the operation rooms of hospitals to keep it sterile and bacteria-free [4]. Furthermore, titanium oxide thin films have been found to exhibit a unique and useful function, i. e., a super-hydrophilic property. Usually, the contact angle of the water droplet and surface is 50-70 degrees, therefore, metal oxide surfaces become cloudy when water is dropped on them or if there is moisture in the atmosphere. However, under UV light irradition of the titanium oxide surfaces this contact angle of the water droplets becomes smaller, even reaching zero (super-hydrophilicity), its extent depending on the UV irradiation time. Thus, under UV light irradiation, titanium oxide thin film surfaces never become cloudy, even

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Fig. 1. View of the soundproof highway walls for the elimination of NOx (the walls were constructed in Osaka, April in 1999) (20 m 3 of the polluted air was purified at the rate of I m2 photocatalyst/h)

TiO2thin film

-...,

glass[._

~0=40-60

9

water droplet

9

0=0

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0 (degree); contact angle of water

Fig. 2a. Anti-fogging effect of TiO2 thin film coated surface. The glass mirror, whose right side was coated with TiO2 thin film, exhibits a clear image even in high water moisture like in a bath room. (b) Decrease in the contact angle under UV irradiation of the TiO2 thin film, leading an super-hydrophilic property of the mirror. in the rain. This r e m a r k a b l e function can also be applied for the production of new mirrors which can be used even in bathrooms and side mirrors for cars to protect against rain (Fig. 2) [6]. Some practical applications of t i t a n i u m oxide photocatalysts in J a p a n are as follows: 1) Air cleaners containing t i t a n i u m oxide photocatalysts White paper containing titanium oxide photocatalysts 2) Antibacterial textile fibers containing t i t a n i u m oxide 3) photocatalysts Systems for the purification of polluted air, e. g., the elimination of 4) NOx Super-hydrophilic, self-cleaning systems and coating materials for 5) cars Soundproof highway-walls covered with titanium oxide 6) photocatalysts Lamp-covers coated with titanium oxide thin film photocatalysts 7) Cements containing t i t a n i u m oxide photocatalyst powders 8) Architectural materials using t i t a n i u m oxide photocatalysts 9) Coating m a t e r i a l s using t i t a n i u m oxides for a r c h i t e c t u r a l walls I0) Self-cleaning tents II) Glass tablewares 12) 13) Outdoor a n t e n n a s coated with titanium oxide thin films

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3. D E V E L O P M E N T OF S E C O N D - G E N E R A T I O N TITANIUM OXIDE PHOTOCATALYSTS BY A P P L Y I N G A METAL IONIMPLANTATION METHOD

3. I. E X P E R I M E N T A L The main characteristics of various titanium oxide catalysts have been summarized in Table 1. Titanium oxide thin film photocatalysts were prepared using an ion cluster beam (ICB) method [7,8]. Using ICB, the titanium metal target was heated up to 2200 K in a crucible and Ti vapor was introduced into the high vacuum chamber to produce Ticlusters. These clusters then reacted with 02 in the chamber and stoichiometric titanium oxide clusters were formed. Ionized titanium oxide clusters formed by electron beam irradiation were accelerated by high electric field and bombarded onto the glass substrate to form titanium oxide thin films.

Table 1 Characteristics of titanium oxides used in this study Catalysts

Percent of BET surface anatase areas, m2/g

Particle size nm

Purity as TiO2

Bandgap eV

F-2 F-4 F-6 P-25 S-1

72.3 87.5 81.0 70.9 86.1

23.4 15.0 9.30 18.6 30.2

99.97 99.97 99.99 99.54 99.90

3.250 3.251 3.262 3.250 3.252

27.1 54.2 102 50.2 30.6

The metal ion-implantation of the catalysts was carried out by using an ion-implanter consisting of a metal ion source, mass analyzer, high voltage ion accelerator (50-200 keV), and a high vacuum pump [9]. The metal ions were expected to be injected into the deep bulk of the catalyst when high accelaration energy was applied to the metal ions. When high voltage as the accelaration energy is used, the metal ions are implanted deep inside the bulk of the catalyst. In fact, as was expected the method, SIMS analyses using a Shimadzu/Kratos SIMS1030 clearly showed t h a t the metal ions implanted into the titanium oxide catalyst exist in a highly dispersed state and are injected into the deep bulk of the catalyst, exhibiting a distribution maximum at around 1000-3000/~ from the surface and zero distribution at the surface [10-12]. Although such distribution depends on the acceleration energy and the kind of catalysts, it is one of the most significant advantages in using the metal ion implantation method to modify the bulk electronic properties of a catalyst. The metal ion-implanted catalysts were calcined in 02 at around 725-823 K for 5 h. Prior to UV-VIS diffuse reflectance, SIMS, XRD, EXAFS, ESR, and ESCA measurements as well as invesigations on the photocatalytic reactions, both metal ion-implanted and unimplanted original pure photocatalysts were heated in 02 at 750 K and then

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degassed in cells at 725 K for 2 h, heated in 02 at the same t e m p e r a t u r e for 2 h, and finally outgassed at 473 K to 10-6 Torr [10-12]. UV light i r r a d i a t i o n of the photocatalysts in the presence of r e a c t a n t molecules such as NOx was carried out using a high-pressure Hg lamp (Toshiba SHL-100UV) through water and color filters, i. e., ~, > 450 nm for visible light irradiation and )~ < 380 nm for UV irradiation, respectively, at 275-295 K. The reaction products were analyzed by GC and GC-MASS. The UV-VIS diffuse reflectance spectra were measured using a S h i m a d z u UV-2200A speetrophotometer at 295 K. The ESR spectra were recorded at 77 K with a Bruker ESP300E and a JEOL RE2X s p e c t r o m e t e r (X-band). The binding energies and the e l e m e n t composition of the catalysts were m e a s u r e d using a Shimadzu ECSA3200 electron spectrometer. The XAFS (XANES and FT-EXAFS) spectra were m e a s u r e d at the BL-7C facility of the Photon Factory at the National Laboratory for High-Energy Physics, Tsukuba. 3. 2. R E S U L T S AND D I S C U S S I O N

When t i t a n u m oxide photocatalysts are irradiated with UV light t h a t is greater t h a n the bandgap energy of the catalsyst (about ~, < 380 nm), electrons and holes are produced in the conduction and valence b a n d s , respectively. These electrons and holes t o g e t h e r induce photocatalytic reactions on the surfaces. However, as can be seen in Fig. 3-a and unlike photosynthesis in green plants, the t i t a n i u m oxide photocatalyst in itself does not allow the use of visible light and can make use of only 3-4% of solar beams t h a t reach the earth. Therefore, to establish clean and safe photocatalytic reaction systems, it is vital to develop t i t a n i u m oxide photocatalysts which can absorb and operate with high efficiency under solar and/or visible light irradiation. 0.4

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We have applied the metal ion implantation method to modify the electronic properties of titanium oxide photocatalysts by bombarding them with high energy metal ions and discovered t h a t metal ion implantation with various transition metal ions such as Cr, V, Co, Fe and Ni accelerated by high voltage enables a large shift in the absorption band of the titanium oxide catalysts toward visible light regions with differing levels of effectiveness. However, Ar, Mg, or Ti ion-implanted titanium oxides exhibited no shift, showing that such a shift is not caused by the high energy implantation process itself, but to some interaction of the transition metal ions with the titanium oxide catalyst. As can be seen in Fig. 3-(b-d), the absorption band of the Cr ion-implanted titanium oxide shifts smoothly to visible light regions, the extent of the red shift; depending on the amount and type of metal ions implanted, with the absorption maximum and minimum values always remaining constant. Such a shift; allows the metal ion-implanted titanium oxide to use solar beams more effectively and efficiently, at up to 20-30% [4, 12]. Furthermore, as shown in Fig. 4, such red shifts in the absorption band of the metal ion-implanted titanium oxide photocatalysts can be observed for any kind of titanium oxide except amorphous types, the extent of the shift changing from sample to sample. Also, it was found t h a t such shift in the absorption band can be observed only after calcination of the metal ion-implanted titanium oxide samples in 02 at around 723-823 K. Therefore, calcination in 02 in combination with metal ion implantation was found to be instrumental in the shift of the absorption spectrum toward visible light regions. All these results clearly showed that such a shift in the absorption band of titanium oxides by metal ion implantation is a general phenomena and not a special feature of a certain kind of titanium oxide catalyst. Figure 5 shows the absoprtion bands of the titanium oxide photocatalysts impregnated or chemically doped with Cr ions in large amounts as compared with those for Cr ion-implanted samples. The Cr

hv

T hLImpuritylevel

~~,,V/S- 1

~

~ ~ ~ V/I~-4 V/P-25

I

300

I

I

'

400

I

I

500

I

600

Wavelength /nm

Fig. 4. Shifts in the absorption spectra of various types of TiO2 photocatalysts implanted with the same amounts of Cr ions. (Cr ions: 6.6 x 10-7 mol/g)

300

,

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,

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600

/ nm

Fig. 5. Absorption spectra of TiO2 chemically doped with Cr ions. (Cr ions doped in 10-6 mol/g, a: TiO2, b': 1.6, c': 20, d" 100, e': 200)

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ion-doped titanium oxide catalysts show no shift in the absorption band, however, a new absorption shoulder appears at around 420 nm due to the formation of the impurity energy levels within the bandgap, its intensity increasing with the amount of Cr ions chemically doped. Such results indicate t h a t the method of doping causes the electronic properties of the titanium oxide catalyst to be modified in completely different ways, and thus confirming t h a t only metal ion-implanted t i t a n i u m oxide catalysts show shifts in the absorption band toward visible light regions. W i t h u n i m p l a n t e d or c h e m i c a l l y doped t i t a n i u m oxide photocatalysts the photocatalytic reaction does not proceed under visible light irradiation (~ > 450 nm). However, we have found that visible light irradiation of metal ion-implanted titanium oxide photoeatalysts leads to various significant photocatalytie reactions. As shown in Fig. 6, visible light irradiation (~, > 450 nm) of the Cr ion-implanted titanium oxide in the presence of NO at 275 K leads to the decomposition of NO into N2, 02, and N20 with a good linearity against the irradiation time. Under the same conditions of visible light irradiation, the u n i m p l a n t e d original pure t i t a n i u m oxide photocatalyst did not e x h i b i t any photoeatalytic reactivity. As can also be seen in Fig. 3 (open circles), the action spectrum for the reaction on the metal ion-implanted titanium oxide is in good a g r e e m e n t with the absorption spectrum of the photoeatalyst, indicating t h a t only metal ion-implanted t i t a n i u m oxide photoeatalysts were effective for the photoeatalytie decomposition reaction of NO. Thus, metal ion-implanted titanium oxide photoeatalysts were found to enable the absorption of visible light up to a wavelength of 400-600 nm and were also able to operate effectively as photoeatalysts, hence their name, "second-generation titanium oxide photoeatalysts" [4, 11]. It is important to emphasize t h a t the photocatalytic reactivity of the metal ion-implanted titanium oxides under UV light irradiation (k < 380 nm) retained t h e same photocatalytic efficiency as the unimplanted original pure titanium oxides. When metal ions were chemically doped into the titanium oxide photocatalyst, photocatalytic efficiency decreased 1.5 C:D m

O

E

off

on Fig. 6. Photocatalytic decomposition of NO into N2 and 02 as well as N20 on the Cr ion-implanted TiO2 photocatalyst under visible light (~ > 450 nm) irradiation at 295 K. Un-implanted original pure TiO2 photocatalyst did not show any photocatalytic reactivity under the same condition.

rD O

(Crfrio2)

O f:l. t__

*O 0.5 {D

light

>o m

(TiO2) 4 6 8 10 Reaction time / h

12

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dramatically under UV irradiation due to the effective recombination of the photo-formed electrons and holes via the impurity energy levels formed by the doped metal ions within the bandgap of the photocatalyst. These results clearly suggest that metal ions physically implanted do not work as electron and hole recombination centers but only work to modify the electronic property of the catalyst. We have conducted various field work experiments to test the p h o t o c a t a l y t i c r e a c t i v i t y of t h e newly developed t i t a n i u m oxide photocatalysts under solar beam irradiation. As can be seen in Fig. 7, under outdoor solar light irradiation at ordinary temperatures, the Cr and V ion-implanted t i t a n i u m oxide photocatalysts showed three and four times h i g h e r photocatalytic reactivity as compared with the unimplanated original pure titanium oxide photocatalyst. These results c l e a r l y show t h a t by u s i n g s e c o n d - g e n e r a t i o n t i t a n i u m oxide photocatalysts developed by applying the metal ion implantation method, we can utilize visible and solar light energy more efficiently. Figure 8 shows the relationship between the depth profiles of the V ion in the V ion-implanted t i t a n i u m oxide photocatalysts having the same numbers of V ions and their photocatalytic efficiency under visible light irradiation. As can be seen in Fig. 8, when the V ions are implanted in the same amounts into the deep bulk of the catalyst by applying high voltage as acceleration energy, the photocatalyst exhibits a high photocatalytic efficiency under visible light irradiation. On the other hand, when a low voltage is applied, this photocatalyst exhibits a low efficiency under the same conditions of visible light irradiation. It was found that increasing the numbers (or amounts) of V ioni m p l a n t e d into the deep bulk of the t i t a n i u m oxides caused the photocatalytic efficiency of these photocatalysts to increase under visible light irradiation, passing through a maximum at around 6x1016 V/cm 2 of the catalyst, and then decreased with a further increase in the number of V ions implanted. Only on samples implanted with an increased number of V ions could the presence of V ions at the near surfaces be observed by E

t,"..,... o,i

0

0 I=

6

500

1.0

400

0.8

"0

C

~0

_r

n O

Q.

=-9 4

.E| 300

E

0

> 200 .c_

0.4

"o

TiO2 Cr/TiO2

V/TiO2

Photocatalysts

Fig. 7. Effect of metal ion-implantation on the photocatalytic reactivity of TiO2 under solar beam irradiation at 295 K. (solar beams: 38.5 mW/cm2)

.o 100

0.2

~0

n

a.

E

sO "O

e

U~ r

0

.~ 0

0.6

e-

C~ z 2

O I=

0 30 keY

70 ReV

150 keY

Fig. 8. Effect of the depth profile of V ions in V ion-implanted TiO2 on the photocatalytic reactivity for the decomposion of NO at 295 K.

o W O

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ESCA measurements. Thus, these results clearly suggest that there is an optimal condition in the depth and number of metal ions implanted to achieve a high photocatalytic reactivity under visible light irradiation. The ESR spectra of the V ion-implanted titanium oxide catalysts were measured before and after calcination of the samples in 02 at around 723-823 K, respectively. Distinct and characteristic reticular V 4+ ions were detected only after calcination at around 723-823 K. It was found that when a shift; in the absorption band toward visible light regions was observed, the reticular V4§ ions could be detected by ESR. Such reticular V ions nor such a shift in the absorption band have never been observed with titanium oxides chemically doped with V ions. Figure 9 shows the XANES and FT-EXAFS spectra of the titanium oxide catalysts physically implanted with Cr ions (b and B) and chemically doped with Cr ions (a and A), respectively. Analyses of these XANES and FT-EXAFS spectra showed t h a t in the titanium oxide catalysts chemically doped with Cr ions by an impregnation or sol-gel method, the ions are present as aggregated Cr-oxides having octahedral coordination similar to C r 2 0 3 and tetrahedral coordination similar to CrO3. On the other hand, in the catalysts physically implanted with Cr ions, the ions are present in a highly dispersed and isolated state in octahedral coordination, clearly suggesting t h a t the Cr ions are incorporated in the lattice positions of the catalyst in place of the Ti ions. (a) ~ , , ~

t--

J

l :5.

(A)~ ~

Cr 6+, Cr 3+ aggregate

Cr / TiO2

t~ c~ t_ O {D c~

~I

<

!~

Cr-TiO2 6()10

0

6050

Energy / eV

~ highly ! ~ dispersed ~ ? ~S~~

'

2 4 Distance / A

0 / \O + ~,,,~.,~. t. 0 0 )n tetrahedral octahedral Cr ion-doped TiO2

o O O " IO/ I~O" I"O t. O 0 n 6

Cr ion-implanted TiO2

Fig. 9. XANES (left) and FT-EXAFS spectra (right) of Cr ion chemically doped TiO2 ((a) and (A)) and Cr ion implanted TiO2 catalysts ((b), (B)), and their local structures. Thus, these results clearly show that modification of the electronic state of titanium oxide by metal ion-implantation is closely associated with the strong and long distance interaction which arises between the titanium oxide and the metal ions implanted and not by the formation of impurity energy levels within the bandgap of the titanium oxides which is often observed in the chemical doping of metal ions. 4. C O N C L U S I O N S Various applications of titanium oxide photocatalysts to better our environment were introduced, especially successful developments in the

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purification of the polluted atmosphere and water. The advanced metal ion-implantation metod has been successfully applied to modify the electronic properties of the titanium oxide photocatalysts, enabling the absorption of visible light even longer than 550 nm. All results obtained in the photocatalytic reactions and various spectroscopic measurements of the photocatalysts indicate that the implanted metal ions are highly dispersed deeply inside the bulk and work to modify the electronic nature of the photocatalysts without any changes in the chemical properties of the surfaces, thus enabling the titanium oxides to absorb and operate effectively not only under VU but also under visible light irradiation. As a result, under outdoor solar light irradiation at ordinary temperatures, V or Cr ion-implanted titanium oxide photocatalysts showed three and four times higher photocatalytic efficiency as compared with the unimplanated original pure titanium oxide photocatalyst. Thus, the present research has opened the way to many innovative possibilities, significantly to address urgent environmental issues, and the design and development of such unique titanium oxide photocatalysts can also be considered an important breakthrough in the utilization of solar light energy which will advance research in sustainable green chemistry for a better environment.

A C K N O W L E D G M E N T : The present work is partly supported by the NODO Grant and the 1998 Mitsubishi Foundation. The author thanks Prof. H. Yamashita and Dr. M. Matsuoka and also Prof. M. Che for their contributions and useful suggestions. REFERENCES

1.

"Photocatalysis,", Eds. by N. Serpone and E. Pelizzetti, John Wiley & Sons, New York (1989). 2. M. Anpo, Catal. Surveys Jpn., 2, 167 (1997). 3. M. Anpo, Proc. 1st Int. Conf. Protect. the Environ., Roma, 75 (1998). 4. M. Anpo, in "Green Chemistry", Eds., P. Tundo and P. Anastas, Oxford University Press, 1 (2000). 5. K. Takami, N. Sagawa, H. Uehara, and M. Anpo, Shokubai, 41,295 (1999). 6. W. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, and T. Watanabe, Nature, 388, 431 (1997). 7. H. Yamashita, M. Honda, M. Harada, Y. Ichihashi, and M. Anpo, J. Phys. Chem. B, 102, 10707 (1998). 8. M. Harada, A. Tanii, H. Yamashita, and M. Anpo, Zeitschrift Physik. Chem., Bd., 0000, (1999) (in press). 9. M. Anpo, H. Yamashita, and Y. Ichihashi., Optronics, 186, 161 (1997). 10. M. Anpo, Y. Ichihashi, M. Takeuchi, and H. Yamashita, Res. Chem. Intermed., 24, 143 (1998). 11. M. Anpo, Y. Ichihashi, M. Takeuchi, H. Yamashita, Stud. Surf. Sci. Catal., 121, Eds., H. Hattori and K. Otsuka, Kodansha-Elsevier, 305 (1999). 12. M. Anpo, M. Takeuchi, S. Kishiguchi, and H. Yamshita, Surf. Sci. Jpn., 20, 60 (1999).

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

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C a t a l y s i s for fine c h e m i c a l s

An 9 industrial p e r s p e c t i v e

Pascal Metivier, Rhodia, Centre de Recherches de Lyon 85 avenue des Freres Perret 69192, Saint Fons Cedex, France 1. INTRODUCTION 1.1 Business considerations Research and development in the field of fine chemicals differs considerably from bulk chemicals. Requirements for industrial success of a new process are different from bulk chemical where the most important feature is cost performances. If this feature is of course important for development of fine chemicals, two other main parameters are to be considered. First of all, time to market: One must be ready to manufacture the product at the right time and for a limited period of time. The lifetime of most fine chemicals is much more shorter than for bulk chemical where 20 to 50 years is standard. Second, possible R&D expenses are much more lower than for bulk chemicals. Compared prices and productions characteristics of fine chemicals versus bulk chemicals product Price ~lk

lO0$1kg

lO$1kg

1$/kg

,~,, 1MT

100 kt 10 kT Production per year

1 kt

100 t

10 t

Very few fine chemicals products can be treated as bulk chemicals, nevertheless a few products such as vanillin (aroma), menthol (perfume), ibuprofen (pharmaceutical)... are relevant of the bulk chemicals R&D methodology, because of their high overall turnover and of their probable long lifetime.

CHo 30

OH

I

I ~ O H

~

CO2H

vanillin menthol ibuprofen Scheme 1 : examples of fine chemicals that can be <
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Most of new developments in fine chemicals will come from small turnover products with low lifetime and so low R&D development time and low investment possibilities. c o m p a r e d lifetimes and sales turnovers o f f i n e chemicals versus bulk chemicals

Sales A turnover

BULK

100M$ FINE CHEMICALS 10 MS

1M$ 1

2

3 4 5 R&D Characteristic time (years)

6

7

8 ~l~

1.2 Scientific considerations If business considerations implies a more strained research, fine chemicals are generally a long way from theory and even from bulk chemicals, regarding yields, productivity and effluents. Important improvements in competitivity could be brought by development of new catalysts. Due to complexity, development of new catalysts generally requires time, and the industry generally prefers the use of old classical methods that are well known and requires small development time. The three technical requirements for a new catalytic process to be industrialised are the following : 1/the chemistry must be already well advanced at the start of the project so to fit with the timetable, 2/The chemistry is easily adaptable to the industrial existing tool (with little modifications), 3/The chemistry must bring a breakthrough in term of economics and effluents compared to existing processes. The industrial approach chosen by Rhodia is to develop new technologies applicable to a wide range of products so that chemical feasibility is already demonstrated when the market need for the product arises. Careful choice of reactions to study and correct evaluation of breakthrough is key. Examples of this type of research will be exemplified with different types of reactions. For each reaction, its interest will be discussed as well as the technology that has been developed. The different reactions that will be discussed are the following 9 II FriedeI-Craft reactions: what possible technology to replace the old stoechiometric Lewis acid route ? The use of acido/basic (particularly zeolites) materials to perform this reaction will be discussed as well as the industrial aspects such as recycling, regeneration, and process simplification. II Carbonylation : How to avoid the use of CO to perform this extremely versatile reaction ? The use of this extremely versatile reaction has been very limited up to now because of the use of <<non friendly >>CO reagent. Replacement of this reagent will be discussed as well as the classical issues of recycling regarding homogeneous chemistry. II Reduction of acids to aldehydes : 1 step from the acid, how to avoid the Rosenmund reaction or the reduction to alcohol followed by oxidation ? Aldehydes are very important reagent for fine organics, the access to those compounds is always

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169 multistep. Development of a catalyst able to reduce and acid function to the aldehyde without reducing a double bond in the framework of the molecule will be presented and discussed. 9 Oxidation of alcohol's and ketones : Replacement of undesirable reagents such as Jone's, permanganate or hypochlorite ? Oxidation of alcohol's is one of the most used oxidation technology, nevertheless, in most cases stoechiometric, non friendly reagents are required to perform those reaction. Heterogeneous solutions developed in Rhodia using air or oxygen and classical catalysts will be presented and discussed.

2. The FriedeI-Crafts acylation of aromatics. The FriedeI-Crafts reaction as well as the related fries rearrangement of aromatics are the most important methods in organic chemistry for synthesizing aromatic ketones ~, which are important intermediates for the production of fine chemicals". O + RCOX (Y

~

+

H

(Y)

The conventional method for preparation of these aromatic ketones involves reaction of the aromatic hydrocarbon with a carboxylic acid derivatives using a Lewis acid (AICI3, FeCI3, BF3, ZnCI2, TiCI4) or Bronsted acids (polyphosphoric acid, HF). The major drawback of the FriedelCrafts reaction is the need to use a stoichiometrical quantity of Lewis acid relative to the formed ketone. This quantity is required due to the fact that the ketone (product of the reaction) forms a stoichiometrical stable complex with the Lewis acid. The decomposition of this complex is generally carried out with water, leading to total destruction and loss of the Lewis acid. This reaction has been discovered in the end of the nineteenth century, and despite intensive studies this initial drawback has not found a general solution "~. Studies have been realized to find mild Lewis acid forming less stable complexes with the ketone an still able to catalyze the reaction. Some successes have been obtained using rare earth triflate ~v or bismuth III salts v. These methods, if they proved to be catalytic still requires important quantities of catalysts (a few percent) and the recycling of the catalyst is not simple. Heterogeneous catalysis appears to be an interesting alternative technique to the homogeneous reaction. The physical and chemical competitive adsorption parameters can on the principle be tuned so to favor displacement of the product by one of the reagent, reactivity can be promoted using <<shape ~ factors to compensate the use of mild acidity of the catalyst. Important work has been carried out in the past 10-15 years to find new heterogeneous catalysts for this reaction. Systematic work was then realized in Rhodia company v~to try to rationalize the observed effects. Results obtained using the same reaction conditions changing only the catalyst are depicted in table 1.

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Entry

Catalyst

Yield (%)b

Selectivity (para)

1

HZSM5

12

> 98

2

H Mordenite

29

> 98

3

HI3

70

> 98

4

HY

69

> 98

5

exchanged clay

14

> 98

6 7

AI clay H2PW6Mo604o

16

> 98

21

> 98

Table 1 acetylation 9 of anisole with acetic anhydride with various heterogeneous catalysts (8 h, 90~ b yield 9 on acetic anhydride (initial ratio anisol/anhydride) = 5 All tested catalysts are active in this reaction and they all show a very high para selectivity. The Rhodia company is operating an industrial process for acylation of anisole to paraacetoanisole using zeolite v". This process using a fixed bed technology is a breakthrough in this field. It allows a considerable simplification of the process as well as an increase in para selectivity, and so a reduction of its operating cost and a dramatic reduction of the effluents. Scheme 2 and 3 illustrate the simplification brought by the zeolite process just comparing the two block diagrams.

DCE

;nisole

AICI3

~ Reaction preparation I

AcCI

Reaction

water i~! 9 9recycled

Hydrolysis

I Liquidlliquid separation I water water

I

I

'T

1

Washing 1

9

"1

I

9effluent rAI(OH) 3 HCI I I

4,

Washing 2

I Distillation/dehydration I I

Final distillation acetoanisole

Scheme 2 simplified 9 flow chart for the conventional aluminum dichloride process using DCE (1,2 dichloroethane) as solvent

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化

anisole

Ac20

J

"1

.,aceticacidi

Reaction(fixed bed)

I

Distillation

'j

I

I

'v

acetoanisole

Scheme 3 : simplified flow chart for the new zeolite fixed bed process using no solvent This process is a good example of how a new process can at the same time be cost efficient and environmentally friendly. This technology has now been adapted to the industrial synthesis of acetoveratrole using the same type of process.

3. Carbonylation without the use of CO The carbon monoxide chemistry has been extensively studied, leading to a wide range of methods used in small scale organic syntheses up to industrial processes v"~. Despite the versatility of carbonylation reactions, carbon monoxide suffers from major drawbacks that restricts its utilisation. From an industrial point of view, the cumbersome handling of this toxic gas necessitates very expensive facilities which prevent its use for most of fine chemical productions. An alternative process equivalent to a carbonylation reaction which avoids carbon monoxide introduction into the reactor and that can be used in standard polyvalent type units would be of great interest. Of course, catalyst cost, stability and productivity should also fulfil economical requirements. Previous studies realised in carbonylation, indicated that Iridium based catalysts are active in carbonylation under low CO partial pressure. Further more this catalyst is active for isomerisation of methyl formate with no initial CO pressure, but precipitates during reaction ~x. After more precise investigations of formates isomerisation processes we found that the hydroxycarbonylation of any carbonylable function with formic acid using an iridium based catalysts was possible without the use of CO gas. Alkenes and alcohols can be transformed to the corresponding hydroxycarbonylated products with good yields, n-hexene is carbonylated in 92% yield with a standard selectivity to linear acids neighbouring 70%. We compared this reaction with the hydroxycarbonylation of n-hexene with CO gas using the same catalyst and found similar results (Table 2). Entry

Substrate

CO source HCOOH

Pressure [bar] Autogeneous

Yield [%] 92 (68)

1

n-hexene

2

n-hexene

gas

5

86 (63)

Table 2. Hydroxycarbonylation of n-hexene. The scope of this reaction appears to be very large and examples are depicted in scheme 4.

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172

lineadties

O

~

C

r

O

O

~

2

o O

H

CO2H

yield> 97

O

O

H ---~H O / ~ O H

0

o HO/LV~ O 'H 0

O

o

9 OH

o

HO/LV~OH

9

~OH_. ~CO2. Scheme 4

+ 30 %

~,CO2H

OH

H

O 70 %

CO2H

~

H

Examples 9 of reactions performed using formic acid and an Iridium based catalysis.

Two possible mechanism pathways may be involved for this reaction, Either a common pathway for all reactants passing through the alkene formation followed by a <<standard )) hydroxycarbonylation of the substrate with CO formed in situ by decomposition from formic acid, either through oxidative insertion of iridium to an iodoakyl intermediate, corresponding to the carbonylation mechanism of an alcohol. These two possibilities are depicted in scheme 5.

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~O

H + AcOH

Y

0

..

_

---

"-.

,

.." ss

I \ ..CO

i/Irxco

..

HI=

CH3

+ HCO2 H

0

]- [//..,".co

,

i.,,,.!rXco

//-,,,H•co /Ir\ CO insertion reductive elimination

+

Y

s

,~

CH2 CH3 CO2 H

~ O

,-,,

co.

+ I-

(Ir 120 0 2 ) -

carbonylation of alcohol type mechanism

hydroxycarbonylation of olefin type mechanism

Scheme 5 : Possible mechanisms Using the formic acid/iridum catalysed procedure

4. Reduction of acids to aldehydes Access to aldehyde function has always been very important in organic chemistry. The two main transformations to access to this function are oxidation of primary alcohol to the aldehyde or reduction of an acid derivatives to the aldehyde. This second reaction has not found any general simple solution. If some progress have been made the main methods used today are either the Rosenmund x reaction on the acid chloride, reduction of the ester using a <~sophisticated )~ hydride, or reduction of the ester to the alcohol followed by re-oxidation of the alcohol function to the aldehyde. We decided to look for a <<more simple )) catalytic system able to reduce an acid function into an aldehyde function. Moreover, we had identified selective reduction using hydrogen of an acid without reduction of a double bond situated in the framework of the molecule as a breakthrough target. Screening of a large variety of catalysts in different conditions, lead to the selection of one active and selective catalyst and this using gas phase technology. A Ru/Sn supported type catalyst can be very active and selective toward this reactionXL Results obtained with this type of catalyst are depicted in table 3.

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1?4

Reagent

product

Senecoic acid

Conversion

Selectivity

40%

80%

80%

90%

65%

65%

85%

90%

85%

68%

prenal

CF3CO2H

CF3CHO

trifluoro acetic acid

OH [~CO2H

fluoral

OH ~CHO

salicylic acid

salicylic aldehyde o

~ C O 2 H Nonaoique acid

nonanal

o

Undecylenic acid

o

undecenal

Table 3 : Examples of results obtained for reduction of acids to aldehydes using Ru/Sn type catalyst Careful investigation of the catalyst indicates that The active phase is an Ru3Snz alloy phase. Ru3Sn7 is a cubic structure of the Im3m spatial group, which contains tin antiprisms as characteristic structure.

5. Oxidation of alcohols Oxidation of alcohol's to aldehyde or ketones has always been considered as a difficult reaction by organic chemists, they generally prefer to avoid the use of oxidation reaction during a synthesis. Most of the common reactions involve stoechiometric, environmentally unfriendly reagents with names such as Jones, Collin's, and others x". In Rhodia we perform on an industrial basis the oxidation of alcohol to the corresponding aldehyde using oxygen as the reagent and precious metal as the catalyst x~"promoted with Bi III salts. This industrial reaction is carried out in batch conditions and could be adapted to a wide variety of products. We decided to investigate the use of such reaction for difficult cases that could occur.

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化

OH

OH

OH

O

Pt/C -Bi

First careful studies on benzylic alcohol indicates that it is possible to oxidise stepwise with high selectivity to the aldehyde and then to the ketone. For example oxidation of bishydroxymethylgaiacol (precursor for trimethoxybenzaldehyde) can be selectively stopped at the bis-aldehyde stage or pursued to the carboxyvanillin as depicted in scheme 6x~V.

MeO~

OH OH

MeO~

OH O

H

MeO~

OH O

OH

r

intermediateto trimetho~benzaldehyde

MeO

Y OMe

carboxyvanillin

OMe

Scheme 6 example 9 of selective oxidation's, that can be obtained This ortho to para selectivity has been extended to intramolecular reaction, and using this technique ortho-vanillin can be selectively oxidise to ortho-vanillic acid in the presence of para vanillin xV(scheme 7)

O~gH MeO.~

OH MeO~~ +

~ ~

OH O MeO\.~

OH

MeO\

OH

+

~OH Scheme 7 example 9 intermolecular selectivity using Pd or Pt on charcoal promoted with Bi catalysts

6. Conclusions and perspectives Research to develop new specific catalysts for fine chemicals must be applicable to a range of products and cannot be limited to only one compound. Furthermore, since development time is limited, the chemical feasibility of the reaction must be demonstrated before. Since synthesis of fine chemicals has not been fully studied up to now, there is a unique opportunity to review classical organic chemistry and to find and develop new selective catalysts for key reactions. Since the reactions must be general careful choice of reactions to investigate is key to success.

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We have demonstrated that for important reaction such as FriedeI-Crafts, Carbonylation, Reductions and oxidations, it is possible to develop new catalysts for the selective synthesis of fine chemicals.

REFERENCES Oiah G.A. in <>, Wiley interscience, Vol I-IV, New-York (19631964) Olah G.A. in ~ Friedel-Craft chemistry >>, Wiley interscience, New-York (1973) ~ Szmant H. ~>, Wiley, New-York (1989) "' Friedel C. Crafts J.M., BulI.Chem.Soc.Chim.Fr., 2_._7.7482 , (1877) Friedel C., Crafts J.M., Bull.Chem.Soc.Chim.Fr., 2.J.7,530 (1877) ~vKawada A., Mitamura S., Kobayashi S., J.Chem.soc., Chem.Commun. 1157 (1993) Hachiya L., Mariwaki M., Kobayashi S., Tetrahedron Lea., 3__.fi6,409 (1995) v Desmurs J.R., Labrouillere M., Dubac J., Laporterie A., Gaspard H., Metz F., in <>,vol 8, p 15-25, Elsevier (1996) v~Spagnol M., Gilbert L., Aiby D. in ~ industrial chemistry library >>,vol 8, p 29-38, Elsevier (1996) "~ Spagnol M., Gilbert L., Benazzi E., Marciily C. Patent to Rhodia WO 96/35655 (1996) v,~ Colquhuon H.M., Thompson D.J., Twigg M.V. in carbonylation 9Direct Synthesis of carbonyl compounds, Plenum Press, New York (1991) ~xPatois C., Perron R., Thiebaut D, Patent WO 97/35828 priority 27/03/1996, to Acetex Chimie van Bekkum H., Peters M., Recl. Trav. Chim. Pays-Bas, 90, 1923, 1971, Burgstahler, Weigel, Shaefer, Synthesis, 767, 1976 ~ Jacquot R., Ferrero R.M., Patent EP 0539274, Priority 24/I 1/1991, To Rhodia, Jacquot R., Patent EP 0874687, priority 08/11/1995 to Rhodia ~ March J. in Advanced Organic Chemistry, Wiley-lnterscience, New york 4 th edition,, pl167 (1992) ~ Le Ludec J., Patent DE 2612844, Priority 07/10/1976, To Rhodia ~.v M6tivier P., Patent WO 96/32454, Priority 24/05/1995, to Rhodia xv Denis P., Maliverney C., M6tivier P., WO 98/16493, Priority 14/10/1996, to Rhodia .

.

.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Oxidative methanol reforming reactions for the production of hydrogen J.L.G. Fierro Instituto de Catalisis y Petroleoquimica, CSIC, Cantoblanco, 28049 Madrid, Spain [Fax: +34 91 585 4760; E-mail: jlgfierro 9

This key note focuses on the oxidative reforming reaction of methanol over ZnOsupported copper or palladium catalysts. For these systems, high hydrogen yields can be obtained over prereduced catalysts under O2/CH3OH feed ratios ranging from 0.5 to 0.5 and reaction temperatures from 480 to 560 K. Hydrogen selectivity was found to show a strong dependence on CH3OH conversion, which suggests that oxidation and reforming reactions take place consecutively. Some insight into the rate limiting step of the oxidation reaction was derived from the study of the kinetic isotope effect (KIE). KIE data and selectivity to major products in the partial oxidation (POM) reaction obtained at 488 K over pre-reduced C u o . a Z n 0 . 6 catalyst showed that H2 production is quite similar for both CH3OH and CH3OD, and apparently no KIE is observed. This indicates that the O-H bond in the CH3OH molecule does not participate in the rate-limiting step of CH3OH conversion into H2. For the same catalysts, the rates of CH3OH oxidation and of the formation of different products were found to depend strongly on the oxygen partial pressure. There is an optimum 02 pressure at which the H2 formation rate reaches a maximum. However, at higher oxygen pressures catalysts deactivate because the copper surface becomes oxidized, as confirmed by photoelectron spectroscopy. Better rates of hydrogen production can be obtained by combining POM and steam reforming (SRM) by feeding H20/O2/CH3OH mixtures simultaneously over prereduced Cu/ZnO(A1) catalysts prepared from hydrotalcite-like precursors. Under appropriate experimental conditions, H2 selectivity approaches 100% and no CO is observed in the reactor outlet. Additionally, the overall reaction heat is nearly neutral, which means that the heat necessary to maintain SRM is supplied by the POM reaction (autothermal reactor).

I. Introduction Hydrogen gas, whether used directly as a fuel in combustion engines [1] or indirectly to supply electricity using fuel cells [2,3], is inherently a clean and highly efficient energy vector. In car engines, the compression ratio can be markedly V and because of its wide combustion limits hydrogen can be burnt in mixtures leaner than is possible in conventional petrol/air mixtures. The second option concerns fuel cells and, more specifically, the proton exchanged membrane (PEM) fuel cell, which uses the chemical energy stored in the H-H bond to produce electricity. Because of safety, on-board

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H2 generation by the reformation of a liquid fuel is currently undergoing extensive studies for commercial. Among the liquid fuels to be reformed, methanol remains prominent because of its ease of handling, its high energy density and its low cost [4]. Methanol is the third commodity chemical atter ethylene and ammonia, with a production capacity above 25 million tons, far greater than current overall demand [5,6]. Hydrogen can be obtained from methanol via to three different processes: (i), steam reforming (SRM) (reaction 1); (ii), partial oxidation (POM) (reaction 2); and (iii), decomposition (MD) (reaction 3): CHaOH + H20 r 3H2 + CO2 CH3OH + 8902 r 2H2 + CO2 CH3OH r 2 Hz + CO CO + H20 r CO2 -k-HE

(AH = +49.4 kJ/mol) (AH =-192.2 kJ/mol) (AH = +92.0 kJ/mol) (AH = - 41.0 kJ/mol)

(1) (2) (3) (4)

While POM is exothermic, SRM is endothermic and produces more favourable H2/CO2 ratios. So far, the SRM reaction has been the only process used for hydrogen production for fuel cell applications [7,8]. The reaction, however, produces a considerable amount of CO as a by-product which irreversibly poisons the RuPt electrocatalyst in polymer electrolyte methanol fuel cells and dramatically diminishes their performance [9]. A second-stage catalytic reactor has been proposed to remove the CO poison via the water gas-shift reaction (WGS) (reaction 4) or even CO oxidation [10]. On the other hand, hydrogen containing lower amounts of CO can be obtained by the POM reaction. For fuel cell technology, the POM reaction becomes advantageous in comparison with SRM because it uses air instead of steam and is exothermic, which does not require an external heat supply. In a previous work [ 11 ], we examined the POM reaction over Cu/ZnO(A1203) catalysts in the absence of steam and found that CO selectivity was substantially higher than that observed for SRM and that the rate of POM was strong dependent upon the copper content. Similarly, ZnO-supported Pd [12,13] and ZrO2-supported Pd catalysts [14] were also found to be highly active and selective for POM to H2, CO2, and CO. Another possibility is to combine reactions (1) and (2) by simultaneously cofeeding oxygen steam and methanol via oxidative methanol reforming (OMR). In this process, the ratio of the three reactants can be chosen such that the overall reaction heat will be nearly neutral, which means that the heat necessary to maintain SRM is supplied by the POM reaction. While the SRM for hydrogen production has been extensively investigated [8,15,16], there have been fewer reports for OMR [7,17]. Huang et al. [7,17] studied the effect of the addition of oxygen to the SRM reaction and determined the kinetics of OMR over reduced Cu/ZnO(AI203) catalysts. These authors proposed that the rate of the combined OMR process could be modeled by considering a two-step sequence in which the overall reaction rate can be calculated from the sum of the rates of the POM and SRM reactions. Accordingly, our efforts were concentrated in two directions: (i) a detailed analysis of catalytic performances in the POM reaction; and (ii), a combination of both the POM and SRM reactions on the same catalyst bed in order to avoid the hot-spot operation which usually accompanies the POM reaction and to enhance the production of a CO-flee H2 fuel stream.

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methanol decomposition over ZnO deposited on Cu(110) single crystals pretreated with oxygen and concluded that ZnO promotes the oxidation of methoxide species generated on the copper surface to a formate intermediate, which undergoes decomposition into H2 and C02. O x y g e n partial pressure

The rates of CH3OH oxidation and of formation of different products were found to depend strongly on the oxygen partial pressure. At constant temperature (488 K), the methanol conversion and the rate of H2 and CO2 formation on the Cu0.~Zn0.6 catalyst increased upon increasing the 02 partial pressure from 0.0 to 0.05 bar. When oxygen pressure was further increased (0.05
v

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2. Partial Oxidation of Methanol

2.1. Cu/ZnO based catalysts

In a previous contribution we observed that Cu/ZnO-based catalysts exhibit very high activity for the POM reaction, with high selectivity to hydrogen [11 ]. As illustrated in Fig. 1, at 488 K the reaction takes off and the rates of CH3OH and 02 conversion increase sharply with temperature to yield H2 and CO2, while the CO production rate is very low throughout the temperature range studied and the H20 formation rate decreases for temperatures higher than 488 K. The apparent activation energy for the POM reaction on CuxZnl.x (x=0.30-0.70) was found to depend on the Cu content. While the activation energy is high for Cuo.3Zn 0.7 catalysts (482 kJ/mol), it decreases drastically upon increasing the copper content and almost levels off in the 71-117 kJ/mol range for CuxZnl-x catalysts (x=0.5-0.7). These values are comparable to those found for other methanol reactions over Cu-based catalyst with different supports [15,16,18,19]. From the specific area of copper, estimated by N20 adsorption-decomposition, turnover frequency (TOF) values were calculated. TOF numbers for the catalyst series CuxZnl-xat 497 K decreased from TOF=4.0 h -l for Cuo.zZno.8 to approximately TOF=2.6 h 1 for CuxZn l-x compositions (x=0.5-0.7). The exceptional ability of Cu/ZnO in the POM reaction might be due to the synergic effect between copper and ZnO [20-23]. Some of the proposals include: (i) a modification of the electronic properties of copper due to the contact of copper particles at the ZnO interface; (ii) the catalytic site is located at the Cu-ZnO interface; (iii) the stabilization of active crystallographic planes of Cu ~ metal; and (iv) the stabilization of Cu + species and a spillover phenomenon between Cu and ZnO. Evidence of the cooperative effect between Cu and ZnO also comes from model catalysts. Fu and Somorjai [24] reported results on 0.4 H2

~. 0"3 Io

CO2

0.2-

CH3OH

0.1 CO 0.0470

480

490

500

Temperature (K)

Figure 1. Partial oxidation of methanol on catalyst Cu0.4Zn0.6oxygen.

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Kinetic Isotope Effect

The kinetic isotope effect (KIE) was studied with a view to gaining insight the rate limiting step of the reaction. KIE data and selectivity to major products in the POM reaction obtained at 488 K over pre-reduced Cu0.aZn0.6 camJyst are summarized in Table 1. For conversion levels below 4%, H20, H2 and CO were the major products, while CO was detected only at trace level. It was observed that CH3OH conversion markedly decreased when CH3OH was replaced by CH3OD in the feed. The ratio of the rates of conversion for CH3OH and CH3OD, represented by the ratio of the specific rate constants (kH/ko), and defined as KIE, is rather large to suggest that this observation would be due to a primary KIE. CH3OH also produced H20 with high selectivity (70% at a rate of 0.0511 mol/g.h. From these values, the KIE for H20 formation is (kH/ko)H20 ,.~ 2.0. However, H2 production proved to be quite similar for CH3OH and CH3OD, and no apparent KIE was observed. This result indicates that the O-H bond in the CH3OH molecule does not participate in the rate-limiting step of CH3OH conversion into Hz. It is likely that the C-O bond is not broken

Table 1 Kinetic isotope effect and selectivity in the POM reaction on catalyst Cu0.4Zn0.6

S.2 (%)

XCH3OH(%)a

S.2o (%)

70 (0.0511) 3.87 (0.073) 30 (0.0219) 50 (0.0245) 2.60 (0.049) 50 (0.0245) 1.49 kH/kD ~2.0 ~0.9 (a), estimated error is + 5%; Values in parenthesis are rates (mol/gcat.h) CH3OH CH3OD

during the reaction because no CH4, even at trace level, was detected in the products. Therefore, it is likely that the rate determining step of the reaction is the breakdown of the C-H bond in the methyl group. With respect to the KIE observed in H20 formation, two possibilities could be envisaged. Water could be formed by the combination of oxygen on the surface via a Langmuir-Hinshelwood (LH) mechanism:

LH mechanism:

H

(0)

H

OH

H

I

I

I

I

I

lllllCu/ZnO/////

~

~

H20

//CulZnO//

According to the LH mechanism, the rate of reaction is controlled by the reaction of adsorbed species while the adsorption and desorption processes are in equilibrium. Thus, when hydrogen is replaced by deuterium a KIE appears. The second possibility for water formation could lie in the activation of methanol through the OH bond over a surface oxygen species via an Eley-Rideal (ER) mechanism:

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化

CH3OH--- (0)

H

OH

H

I

I

I

I

ER mechanism:

~

~

H20

////Cu/ZnO////

////Cu/ZnO////

According to the ER mechanism, the reaction would take place between chemisorbed oxygen species and the CH3OH molecule in the gas phase or weakly adsorbed onto the surface [25]. Therefore, whether the methanol molecule is CH3OD, the first step in the ER mechanism must show a KIE. Further insight into the LH and ER mechanisms can be obtained through Temperature-Programmed Desorption (TPD) experiments. The isotopic distribution of hydrogen and water molecules from the Cu0.aZn0.6 catalyst pretreated in a O2/CH3OD mixture are shown in Fig. 3. For the sake of simplicity, only masses rn/z = 2,3 and 4, corresponding respectively to H2, HD and D2, and 18, 19 and 20 corresponding respectively to H20, HDO and D20, have been plotted in this figure. The TPD profiles reveal two desorption peaks: one at low temperature -ca. 450 K- and the other at high temperature, at approximately 565 K. Although catalyst Cu0.4 Zn0.6 displays both peaks, the peak at 450 K is absent in the TPD profiles of the Cu and ZnO samples (not shown). Thus, the origin of this peak probably lies in the Cu+ZnO combination or even in a spillover effect, as already observed in low temperature methanol synthesis catalysts [26]

reduced catalyst m/e

1

19 x200

r

~o

o~ rar~

f/3

20 x2500 18 xl00 , 400

~ 500

600

700

2xl 3~ 4 x15 800

T (K). Figure 3. Temperature-programmed desorption profiles of hydrogen and water isotopes from catalyst Cuo.4Zno.6 pretreated in a O2/CH3OD flow.

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and Ni/A1203 catalysts [27]. Moreover, the second peak at 565 K is close to that observed on pure ZnO (575 K), suggesting that this peak might well be associated with processes taking place at the ZnO surface. The ratios of the mass signals (2:3:4) and (18:19:20) recorded in Fig. 3 are shown in Table 2. Despite the experimental error, the relative intensity of the hydrogen isotopes H2:HD:D2 = 64:32:4 does not differ substantially from that of the water isotopes H20:HDO:D20 = 78:19:3. These isotopic distributions are close to that expected from a random combination of H and D isotopes at a proportion of 3 to 1 (H:D = 3:1), which is 56:38:6 (HH:HD:DD).

Table 2 Relative intensity of mass signals for hydrogen and water isotopes Relative intensity (%)

m/z values

64" 32" 4 78" 19"3

2 (H2)" 3 (HD)" 4 (D2) 18 (H20) 919 (HDO) 920 (D20)

Accordingly, this isotopic distribution suggests that H2, HD and D2 molecules are formed through a random combination of hydrogen atoms at the catalyst surface, as depicted in the following scheme"

H2 HD D2 /\ / N/ N H H D D

~1

~1

~,1~,1~

///////////////// Cu/ZnO ////////////////

Statistical calculations reveal that if the catalyst surface contains a concentration of hydrogen atoms H:D =3:1 (in CH3OD there are 3 H atoms and 1 D atom), the statistical probability of combining hydrogen in H2 is 56%, whereas it is 38 and 6% of combining in HD and D2 molecules, respectively. The same considerations are valid for H20, HDO and D20 formation because statistical combinations of hydrogen and deuterium are also involved.

2.2. Palladium Catalysts ZnO-supported Pd catalysts also display excellent performance in the POM reaction. The behaviour of 1% Pd/ZnO catalyst was examined under O2/CH3OH ratios of 0.3 and 0.5 and reaction temperatures ranging from 503 to 543 K. Under these conditions, 02 consumption was complete, while methanol conversion reached 42-80%. When temperature was increased, CH3OH conversion also increased, with a simultaneous increase

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in H2 selectivity, while that of H20 decreased. Since 02 Was completely (a) consumed, this trend suggests some 80 contribution of the SRM reaction O= produced by the water product. The 60 product distributions exhibited a _.e Q marked dependence on CH3OH 40 conversion. The data summarized in Fig. 4, obtained by changing the 20 reaction temperature, revealed that very H=O, high H2 selectivity can be attained at 40 60 80 100 high CH3OH conversion. This is clear 20 for a ratio of O2/CH3OH = 0.3 in the CH30H Conversion (%) feed , with which a H2 selectivity of 96% can be attained at a CH3OH 100 conversion of 70%. From the data C02 reported in Fig. 4 it becomes apparent (b) 80 that, upon increasing 02 in the feed stream, larger CH3OH conversions are 60 required to obtain the same H2 o o selectivity. The less pronounced effect m 40 on carbon oxides points to a faster oxidation of H2 than CO. As stated 20 CO above, the SRM (Eq. 2) can be 0 I I considered as a combination of MD (Eq.3) and WGS (Eq. 4). The H20 (or 4o 6o 8o the CO2) required for the WGS comes CHaOH Conversion (%) from the deep oxidation of a fraction of CH3OH fed: CH3OH+O2 r CO2+ H20. Figure 4. Product distributions as a function of According to this reaction, if the CO CH3OH conversion on 1% Pd/ZnO catalyst for produced via MD is consumed in the O2/CH3OH ratios of 0.3 (a) and 0.5 (b). WGS, a coincidence in the proportion of products by SRM and by WGS-MD reactions could be expected. Although the WGS-MD scheme has been proposed for the SRM over copper catalysts (12,15,16), in this case the observation of a certain proportion of CO and H20 among the products indicate that the rate of the WGS reaction is not fast. This finding also supports the hypothesis that the POM reaction might well occur through the reforming of CH3OH, at least under the conditions of this study. 100

H2

3. Oxidative Methanol Reforming On the basis of the above reaction scheme and considering the exothermicity of the POM reaction, the next step was to feed HaO/O2/CH30H mixtures simultaneously in order to examine the beneficial effect of water on H2 production. For this purpose, Cu/ZnO(A1) catalysts prepared from hydrotalcite-like precursors were selected and tested under H20/O2/CH30H 1.3:x:l (x=0.2-0.5) molar ratios and reaction temperatures ranging from =

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520 to 600 K. The plot of selectivity to different products versus CH3OH conversion for several pre-reduced catalysts (ex-hydrotalcite-like precursors) was constructed under the two different regimes, namely POM and OMR. The behaviour of these systems in the POM reaction is similar to that of Cu/ZnO catalysts (11,15,16). At a given temperature, CH3OH conversion in the OMR operation is lower than in the POM reaction. Simultaneously, CO selectivity, which attains levels above 3% under the conditions of the POM reaction, is strongly inhibited in the OMR process. Since excess of water is present in the OMR reaction, the conditions are favourable for the WGS reaction to transform CO, if produced via the POM reaction, into H2 and CO2. This would account for the absence of CO in the outlet stream, even when CH3OH conversion approaches 100%. Finally, the important difference in the performance of the same catalyst under the conditions of POM and OMR can be related to a change in the nature of the copper sites involved in these reactions. Measurement of the Auger CULMMparameter of used catalysts in both reactions revealed that Cu + is the only species present under the OMR reaction while Cu ~ dominates under the POM reaction.

Conclusion ZnO-supported copper or palladium catalysts are very active and selective catalysts for the production of H2 in the POM reaction. The mechanism of this reaction can be very complex due to the possibility of many reactions occurring simultaneously. The possible reactions primarily taking place are combustion, forming CO2 and H20, and decomposition to CO and H2. Tracer isotope experiments with CH3OH and CH3OD revealed a kinetic isotope effect (KIE) for the formation of water (ki-i/kD ~ 2) but not for that of H2, which means that O-H bond in CH3OH does not participate in the rate-limiting step. The role of oxygen is to increase CH3OH conversion via the POM reaction, although H20 formation becomes more favourable as compared to that of H2. At high Oz and high temperature Cu/ZnO catalyst deactivate due to copper oxidation. Cu/ZnO(A1) catalysts derived from hydrotalcite-like precursors were found to perform very well in the OMR reaction. Higher CH3OH conversions (ca. 100%) can be obtained at temperatures of around 580 K, which is slightly higher compared with that of the POM reaction. The CO level in the outlet of reactor is almost zero, even at high CH3OH conversions suggesting that the WGS reaction operates simultaneously. Therefore, the OMR reaction is the technology of choice for COffee hydrogen fuel stream generation.

Acknowledgements Financial support from the European Union (Project: JOE3-CT97-0049) and the CICYT (Project: QUI98-1655-UE) is acknowledged. Thanks are due to Drs. M.A. Pefia, L. Alejo, R. Lago, L. Gomez Sainero and S. Murcia Mascaros for valuable comments and suggestions during the preparation of this work.

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References .

2. ~

4. ~

6. .

8.

9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.

Y. Jamal, N. Wyszynski, Int. J. Hydrogen Energy 19 (1994) 557. J.C. Amphlett, K.A. Creber, J.M. Davis, R.F. Mann, B.A. Peppley, D.M. Stokes, Int. J. Hydrogen Energy 19 (1994) 131. M.A. Pefia, J.P. Gomez, J.L.G. Fierro, Appl. Catal. A: General 144 (1996) 6. R. Kumar, S. Ahmed, M. Krumpelt, 1996 Fuel Cell Seminar Program and Abstracts (1996) 750. R. Kumar, S. Ahmed, M. Yu, Am. Chem. Soc. Div. Fuel Chem. 38 (1993) 1741. W. Cheng, H.H. Kung, Methanol Production and Use, Marcel Dekker, New York, 1994. T.J. Huang, S.L. Chren, Appl. Catal. 40 (1988) 43. B.A. Peppley, J.C. Amplett, L.M. Keams, R.F. Mann, Appl. Catal. 179 (1999) 21. H.F. Oetjen, V.M. Schmidt, U. Stimming, F. Trila, J. Electrochem. Soc. 143 (1996) 3838. K. Sekizawa, S. Yano, K. Eguchi, H. Arai, Appl. Catal. A: General 169 (1998) 291. L. Alejo, R. Lago, M.A. Pefia, J.L.G. Fierro, Appl. Catal. 162 (1997) 281. M.L. Cubeiro, J.L.G. Fierro, J. Catal. 179 (1988) 150. N. Takezawa, N. Iwasa, Catal. Today 36 (1997) 45. M.L. Cubeiro, J.L.G. Fierro, Appl. Catal. 168 (1998) 307. C.J. Jiang, D.A. Trimm, M.S. Wainwright, N.W. Cant, Appl. Catal. 93 (1993) 245. C.J. Jiang, D.A. Trimm, M.S. Wainwright, N.W. Cant, Appl. Catal. 97 (1993) 145. T.J. Huang, S.W. Wang, Appl. Catal. 24 (1983) 287. E. Santacesaria, S. Carra, Appl. Catal. 5 (1983) 365. H. Kobayashi, A. Hirose, M. Simokawabe, K. Takezawa, Appl. Catal. 4 (1982) 127. G.C. Chinchen, K. Mansfield, M.S. Spencer, Chemtech (1990) 692. V. Ponec, Surf. Sci. 272 (1992) 111. K. Kanai, T. Watanabe, T. Fujitani, M. Saito, Catal. Lett. 27 (1994) 67. J. Nakamura, I. Nakamura, T. Fujitani, M. Saito, T. Uchijima, Catal. Lett. 31 (1995) 325. S.S. Fu, G.A. Somorjai, J. Phys. Chem. 96 (1992) 4542. M.J. Philling, P.W. Seakins, in "Reaction Kinetics", Oxford University Press, 1995. R. Burch, S.E. Golunski, M.S. Spencer, Catal. Lett. 5 (1990) 55. B. Chen, J.L. Falconer, J. Catal. 144 (1993) 214.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Supported metaUocenes- monosite and multisite catalysts for olefin polymerization F. C iardelli a, A. Altomare ~ and S. Bronco a' b aDipartimento di Chimica e Chimica Industriale, Universi~ di Pisa, Via Risorgimento 35, 56126 Pisa, Italy blNFM, (Istituto Nazionale Chimica della Materia) ABSTRACT The main problems are presented regarding the transfer of the active species from the solution to the support in case of metallocene /MAO polymerization catalyst. Different approaches are discussed with references to examples reported in the literature from several laboratories and authors' laboratory. In particular, evidences are reported about variation of catalytic performances induced by the fixation to the support with particular reference to conventional amorphous silica and to crystalline supports such as Y-zeolites and MCM-41 mesomorphic silica. The effect of the chemical treatment of the support surface and the role of the cocatalyst are also described. Catalyst preparation and reaction parameters are analyzed in terms of productivity and molecular features of homopolymers and copolymers obtained in the presence of supported metallocenes. KEYWORDS Polymerization, metallocenes, silica, zeolites, olefins. 1. INTRODUCTION Ziegler-Natta catalyst from their origin to the most advanced supported industrial catalysts were costantly characterized by the presence of different families of active sites (multisite catalysts) [1]. Accordingly, they were not only heterogeneous from the physical view point but also chemically heterogeneous. As a consequence, very broad molecular weight distributions were obtained in case of olefin homopolymers

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and very dishomogeneous sequence distributions were obtained in case of copolymers [2]. These two problems were overcome by the discovery of metallocene based homogeneous catalysts [3]. Indeed these systems activated with MAO gave initially soluble catalyst where all sites had the same structure (monosite catalysts). Accordingly, molecular weight dispersity index about 2 were obtained, while stereochemistry and sequence distribution of copolymers could be controlled by varying the type of ligands around the metal in the catalyst precursor. For several well known reasons, physically heterogeneous catalysts offer some technical advantages with respect to their soluble analogs. Therefore, several attempts have been made in order to fix the above chemically homogeneous catalysts on solid surfaces with the aim of ending up with heterogenized monosite catalysts [5]. In the present work, some contributions are described where either this objective was at least partially achieved or evidences at molecular level are reported allowing to understand modification and structural change responsible for the different catalytic behavior of the heterogenized catalyst with respect to the same precursor in solution. In this connection, the present paper is not devoted to cover exhaustively all the literature data on supported metallocenes, but rather will focus on results which allow to relate the catalytic behaviour to some insight about the structure of the active sites. Also silica type-inorganic supports will be discussed in general as investigation was mainly concerned with this type of supports widely used in other type of catalytic processes.

2. GENERAL PROBLEMS HETEROGENIZATION

RELATED

TO

METALLOCENES

While metallocenes are well defined transition metal complexes they need a cocatalyst to be activated and therefore the active species structure can in general only be speculated on the basis of more a less direct evidences [6]. This aspect becomes even more complicated if activation takes place with supported metallocenes. Indeed the various roles of the cocatalyst, namely alkylation, cation formation, and active species protection against scavengers, are substantially affected by the presence of the solid support, by the surface treatment [7], and by the order of reagents mixing. Different approaches have been used in the many examples reported in the relevant literature as described in a simplified manner in Scheme 1, which can be of help in analyzing the results obtained in various laboratories.

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Scheme 1. Typical procedures for preparing soluble and supported polymerization catalysts derived from metallocenes. +M A O . , SOLUTION

I I

+ SUPPORT

]

[Cp2MtX2]e + [MAO]e

vl MONOSITE CATALYST

(A)

SUPPORTED METALLOCENE

+ MAO

"~

(B) + PRETREATED SUPPORT

J SUPPORTED ., CATALYTIC SPECIES (D)

.i CATALYTIC SPECIES

(c) + MAO

MORE ACTIVE SUPPORTED CATALYTIC SPECIES (E)

It is not strightforward that catalytic species (C), (D), and (E) are structurally the same as for the monosite catalyst (A); indeed several structural changes can occur during the fixation to the not inert solid surface and even the reaction with the cocatalyst may take different routes in the presence of the support. Moreover, the geometry around the transition metal can be varied due not only to possible partial ligand removal, but also to the steric constrains imposed by the tridimensional structure of the support. These aspects can have a significant influence not only on the stereochemistry of the polymerization but also on the propagation and transfer rate constants; moreover, shape selectivity can occur during copolymerization of monomers with different bulkyness. The above preparation complexity can be partially overcome either by weak binding of the catalyst precursor to the support or by covalently attaching cyclopentadienyl ligands to the support followed by the complexation of proper transition metal species. This time consuming procedure (Scheme 2) however, cannot strictly grant the formation of monosite systems and can give multisite supported catalysts as generally occurs in case of direct supportation of the catalytic precursor [8]. A combination of the above methods has been recently reported as based on the synthesis of metallocenes with functional chemical groups and their supportation by reaction with the carrier surface. The metallocenes can then be anchored covalently to the carrier allowing for different spacing from the surface and possible leaching. A large versatility of catalyst modulation is then possible in terms of accessibility of the species and polymer morphology [9].

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Scheme 2. Schematic representation of the formation of different sites during complexation of a zirconium derivative to a support functionalized with cyclopentadienyl groups.

I SUPPORT ~-X + YCp----~[ SUPPORT~-Cp + ZrLn

•

SUPPORT~--Cp + ZrLn-1 + SUPPORT

3. METALLOCENES ON AMORPHOUS SUPPORTS Three different procedures were suggested to support a metallocene catalyst on silica, namely: a) absorption of MAO on silica followed by the addition of the metal complex; b) absorption of the metallocene on silica and successive activation with variable amounts of MAO; c) as in b) but with a final filtration to remove MAO species from the solution (single phase catalyst). During propylene polymerization with the catalysts obtained through the above procedures and starting with Et(Ind)2ZrCl2. the following results were obtained: catalyst a) behaves as the same catalyst in solution while b) and c) give lower activity, higher molecular weight, higher isotacticity and melting point. The formation of a stabilized structure on the surface of the solid support is postulated, but no direct evidences are reported. An increase of catalytic activity was observed [ 11 ] by treating the silica surface with C12Si(CH3)2 which acted as a spacer allowing for the release of the steric constrains of the support. However, no characterization of catalyst species and polymer structure was described. The comparison of the isotactic specific Et(Ind)2ZrCl2 and the aspecific (Ind)2ZrCl2 complexes after anchoring to silica suggests that the same active species as in solution are formed by the preparation method a), whereas method b) gives multisites catalyst according to formation of different species depending on the precursor complex structure [ 12]. A high activity catalyst for use in slurry and gas phase ethylene polymerization was recently prepared [ 13] through supportation on hydroxylated silica or alumina reacted with MAO. In particular, the authors showed that proper preparation of the support limits the leaching of metallocene species and allows for a narrow composition distribution of copolymers of ethylene with 1-hexene or 1-0ctene. The need of pretreating the silica surface with MesS iC1 or A1R3 to remove silanol groups

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was found necessary for silica similarly to what reported by us for zeolites [ 15]. In the former case with MAO as the cocatalyst the polymer morphology is similar to that obtained with the homogeneous system, thus suggesting that MAO detaches the metallocene from the support. Silica surface was also modified by aminopropyltrimethoxysilane [16], and Cp(CHz)3Si(OCHzCH3)3 [17]. In this last case an accurate characterization of the supported complex structure was attempted in order to relate it to the catalytic behaviour. (Butylcyclopentadienyl)2ZrCl2 was supported on silica from a solution of the metallocene and MAO in liquid 1-hexene or styrene or 1,7-octadiene by the "incipient wetness" method. The preformed polymer provides a protective layer to the supported complex thus improving its stability and exerting an interesting effect on catalytic activity [ 18] which is still observable alter exposure to air for 5 hours. The active sites uniform character is in generally tested more significantly by the analysis of copolymers formed in their presence. Several detailed studies dealing with 1-hexene/ethylene copolymerization in the present of silica supported zirconocenes were recently reported [19-21]. GPC and copolymer fractionation indicated respectively narrow molecular weight distribution but broad, sometimes bimodal, comonomer sequence distribution. This result was taken as an indication that even with bridged metallocenes the supportation provides catalysts with more than one type of active sites [20]. An attempt to prepare a structurally well defined ethylene polymerization catalyst was recently based on the supportation of tetrakis(dimethylamino)titanium on silica, whose surface was modified with Cp(CH2)2 Si(OC2H5)3. A detailed structural analysis of the supported catalyst as compared to a homogeneous model is reported on the basis of IR and NMR spectroscopies. The conclusion was that, when all free silanol groups are eliminated by additional methyllithium treatment, the titanium amide species are bonded to either one or two cyclopentadienyl groups of silane coupling agent bonded to the surface [22]. The steric effect exerted by the SiO2-surface was furtherly confirmed by anchoring cyclopentadienylindenylzirconium dichloride on silica with a trisiloxane or a pentamethylene spacer. Both spaced supported catalyst showed higher activity than the same complex directly supported on silica. However the activity remained lower than in solution. Molecular weight and melting point of the polymer were similar in all cases with a larger MWD for the three supported catalysts [23]. Several bridged zirconocenes supported on silica were prepared and used for the copolymerization of ethylene with olefin bearing hydroxyl or dialkylamino functional groups spaced from the double bond. Disactivation both with homogeneous and supported catalysts was avoided by pretreatment of the functional comonomers with trisisobutylaluminium (TIBA). Diffusion retardation of the TIBA protected comonomers was observed when the S iO~ supported catalyst was used [24].

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4. METALLOCENES ON CRYSTALLINE SUPPORTS

Two different types of crystalline solids were used as support for metallocene based polymerization catalysts, namely Y-zeolite with variable S iOz/AllO3 ratio and mesomorphic MCM-41 silica. In addition to the different chemical composition, the above systems are characterized by different pore diameter, respectively 8.0 and 29.7 A in Y-zeolite and MCM-41. As in case of silica, the presence of flee silanol groups can produce significant modification of the supported metallocene [15]. Thus in this section the discussion will be essentially based on results obtained by supporting the transition metal complex on supports previously treated with MAO. The pretreatment of HY-zeolite (Si/AI = 7.25) with AIMe3 or C1SiMe3 allows to support Cp2ZrMe2 without methane evolution. This catalyst shows lower initial activity, but better time stability as compared to the same system in solution [25]. NaY-zeolite pretreated with either MAO or AIMe3, gave with Cp2ZrCI2 a supported catalyst having lower activity than in solution, particularly in the first minutes of the polymerization. According to extraction experiments, it was concluded that the metallocene (CplZrCI2) was confined inside the supercage of NaY-zeolite pretreated with MAO without any leaching under polymerization conditions [26]. NaY-zeolites after pretreatment with MAO were also used as support in another laboratory. Again lower activity and higher molecular weights were observed. The authors, by accurate chemical analysis, suggest that the concentration of framework aluminium atoms is the dominant factor but also the external surface affects the catalytic behavior [27]. The homopolymerization of both ethylene and propylene with Cp2ZrCI2 and various ansametallocenes supported on HY-zeolites has shown a marked decrease of activity and a remarkable increase of molecular weight in case of Cp2ZrCI/. These effects are less evident with ansametallocenes which because of their shape cannot enter the pores of the zeolite structure and are therefore supported on the external surface of the zeolite. These last catalysts give the same steric and stereochemical effect as in solution, but the productivity is lower as observed with the same complexes on amorphous silica. On the other side the effective confinement of Cp2ZrCI2 in the pores (Figure 1) could be furtherly substantiated by the copolymerization of ethylene with {x-olefins having variable shape [28].

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O O O O O 9

Aluminum Carbon Chlorine Oxigen Silicon Zirconium

Figure 1. Cp2ZrCl2confined in the pores of MAO-modified HY-zeolite (according to Ref 28). Indeed with different a-olefins as copolymerization solvents, the homogeneous Cp2ZrCI2/MAO catalyst gives oligomers with almost 1/1 ~t-olefin/ethylene composition, shoving that practically only initiation by ethylene followed by prompt transfer to the ct-olefin occurs. With the supported catalyst the access to the active sites by the a-olefins is hindered by their confinement inside the zeolite pores, thus incorporation of the ct-olefins is lower but the transfer rate is depressed and a higher molecular weight is obtained. This result provides a clear indication of the shape selectivity exerted by the superstructure of the inorganic ligand. This concept has been used to produce ethylene/1-hexene copolymers with 5% content of aolefins but with a very uniform distribution, high molecular weight and a single melting point [28]. The larger diameter of the channels of mesomophous silica allow to obtain supportation without remarkable steric effects by the support structure with both biscyclopentadienyl and ansametallocenes. Et(Ind)2ZrCl2 was supported on the surface of MCM-41 which had been pretreated with MAO. After activation with additional MAO the heterogeneous complex showed activity only 30% lower than the homogeneous analog in propylene polymerization, formation of higher molecular weight chain and slightly improved isotacticity [29]. Consistent results were also obtained in ethylene and propylene polymerization with complexes with different steric hindrance and stereospecific control, confined in the pores of MCM-41 silica. Direct grafting of Cp2TiCI2 onto the inner walls of this mesoporous silica has been reported to give olefin epoxydation catalyst with a

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large concentration of accessible, well spaced and structurally well defined active sites [30]. _

0 0

oi O(

O0 9

Figure 2. Cp2ZrCl2 species at the internal surface of channels of mesophorous silica MCM-41. In a similar manner Cp2ZrCl2 was confined [31 ] in the inner surface of MCM-41 pretreated with MAO (Figure 2). Apart of a partial reduction of productivity, the MCM-41 supported zirconocene complexes behave as in solution. The reduction of productivity is lower than for HY-complexes which however give rise to a marked increase of molecular weight. Even the copolymerization of ethylene with 1-hexene in the presence of both homogeneous and MCM-41 heterogeneized catalysts give copolymers with comparable composition, for all examined metallocene complexes

[32].

The rigidly shaped support appears to exert a physical deactivation by partially hindering or slowing down the access of cocatalyst to the metallocenes thus reducing number of active sites and of the monomer thus reducing the rate propagation constant. However the supportation of the substantially unmodified original metallocene is possible with very limited leaching when the precursor can be confined into the pores of the crystalline support. The diameter of such pores plays a determining role on productivity and shape selectivity.

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4. CONCLUDING REMARKS

The fixation to a solid surface of well defined catalytic precursors, such as metallocenes, can provide a realistic route to the preparation of physically heterogeneous but chemically homogeneous polymerization catalysts (monosite "heterogeneous" catalysts). However the above preparation, as shown by the examples described in this paper, is not trivial and several problems need further consideration before a definitive clear picture of the situation can be reached. The bonding to the support surface and its stability to polymerization conditions, and particularly to the cocatalyst, is one of the critical factors. Strong covalent bonding can be achieved by using functionalized metallocenes or by attaching the ligand to the support followed by metal complexation. These approaches need sophisticated expertise and may give some structural alteration on the active species if compared to the solution. Transfer of the unaltered metallocene from the solution to the solid phase can be achieved by weak cooperative bonding in case of surface modified carriers. Porosity of crystalline supports can help to reduce leaching in this last situation. Even assuming that the support is chemically inert, its effect on the catalysis can still be significant. Some undesired effects, particularly lower productivity, can be solved by using proper spacers and controlling pore diameter. While characterization of the structure of the active species on the surface is far from being exhaustive, polymer and copolymer analysis allows to show that substantially monosite "heterogenized" catalyst for olefin polymerization allowing extended control of primary structure, stereochemistry and morphology are within reach. ACKNOWLEDGMENT Financial support by the M U R S T - Roma is gratefully acknowledged. REFERENCES

1. G. Maschio, C. Bruni, L. De Tullio, F. C iardelli, Macromol. Chem. Phys., 199 (1998)415. 2. P. Pino, U. Giannini, L. Porri, in Encyclopedia of Polymer Sci. & Eng., Ed. J.I. Kroschwitz, Wiley, New York, 1985, Vol. 8, p. 147. 3. H. Sinn, W. Kaminsky, Adv. Organomet. Chem., 18 (1980) 99. 4. W. Kaminsky (ed.), Metallorganic Catalysts for Synthesis and Polymerization, Springer, Berlin, Heidelberg, 1999. 5. M.R. Ribeiro, A. Deffieux, M.F. Portela, Ind. Eng. Chem. Res., 36 (1997) 1224.

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6. H.H. Brintzinger, D. Fischer, R. MOlhaupt, B. Rieger, R. Waymouth, Angew. Chem., Int. Ed. Eng., 34 (1995) 1143. 7. V.N. Panchenko, N.V. Semikolenova, I.G. Danilova, E.A. Pankshtis, V.A. Zakharov, J. Mol. Catal. A Chem., 142 (1999) 27. 8. F. Ciardelli, A. Altomare, M. Michelotti, Catalysis Today, 41 (1998) 149. 9. A. Mufioz-Escalona, L. Mendez, J. Sancho, P. Lafuente, B. Pefia, W. Michiels, G. Hidalgo, F. Martinez-Nufiez, in Ref. 4, p. 381. 10. W. Kaminsky, F. Renner, Makromol. Chem., Rapid Commun., 14 (1993) 239. 11. K. Soga, T. Shiono, H.J. Kim, Makromol. Chem., 194 (1993) 3499. 12. M.C. Sacchi, D. Zucchi, I. Tritto, P. Locatelli, Macromol. Rapid Commun., 16 (1995)581. 13. D. Harrison, I.M. Coulter, S. Wang, S. Nistala, B.A. Kuntz, M. Pigeon, J. Tian, S. Collins, J. Mol. Catalysis A: Chemical, 128 (1998) 65. 14. B.L. Moroz, N.V. Semikolenova, A.V. Nosov, V.A. Zakharov, S. Nagy, N.J. O'Reilly, J. Molecular. Catal. A: Chem., 130 (1998) 121. 15. F. C iardelli, A. Altomare, G. Conti, G. Arribas, B. Mendez, A. Ismayel, Macromol. Symp., 80 (1994) 29, and references therein. 16. T. Uozumi, T. Toneri, K. Soga, Macromol. Rapid Commun., 18 (1997) 9 17. E.I. Iiskola, S. Timonen, T.T. Pakkanen, O. H~rkki, P. Lehmus, J.V. Sepp~l~, Macromolecules, 30 (1997) 2853. 18. T. Kamfjoral, T.S. Wester, E. Rytter, Macromol. Rapid Commun., 19 (1998) 505. 19. C. Przybyla, B. Tesche, G. Fink, Macromol. Rapid Commun., 20 (1999) 328. 20. J.D. Kim, J.B.P. Soares, ibidem, 20 (1999) 347. 21. K.J. Chu, L.P. Shan, J.B.P. Soares, A. Penlidis, Macromol. Chem. Phys. 200 (1999) 2372. 22. S. Timonen, T.T. Pakkanen, E.I. Iiskola, J. Mol. Catal., 148 (1999) 235. 23. D. Lee, K. Yoon, S. Noh, Macromol. Rapid Commun., 18 (1997) 427. 24. R. Goretzki, G. Fink, Macromol. Chem. Phys., 200 (1999) 881. 25. F. Ciardelli, A. Altomare, G. Arribas, G. Conti, F. Masi, F. Menconi, in Catalyst Desigh for Tailor-Made Polyolefins, K. Soga, M. Terano (eds.), Kodansha, Tokyo, 1994, p. 257. 26. S.I. Woo, Y.S. Ko, T.K. Han, Macromol. Rapid Commun., 16 (1995) 489. 27. M. de F~tima, V. Marques, C.A. Henriques, J.L.F. Monteiro, S.M.C. Menezes, F.M.B. Coutinho, Macromol. Chem. Phys., 198 (1997) 3709. 28. M. Michelotti, A. Altomare, F. C iardelli, E. Roland, J. Mol. Catal., A Chem. 129 (1998) 241. 29. Y.S. Ko, T.K. Han, J.W. Park, S.I. Woo, Macromol. Rapid Commun., 17 (1996) 749. 30. T. Maschmeyer, F. Rey, G. Sanchez, J.M. Thomas, Nature, 378 (1995) 159. 31. L. Ouderrahmania, PhD. Thesis, University of Pisa, December 1999. 32. F. Ciardelli, A. Altomare, M. Michelotti, G. Arribas, S. Bronco, in Ref. 4, p. 358.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

A new method for preparing nanometer-size perovskitic catalysts for CH4 flameless combustion R.A.M.Giacomuzzi, M.Portinari, I.Rossetti and L.Fomi* Dipartimento di Chimica fisica ed Elettrochimica e Centro CSRSRC CNR, Universit~ degli Studi di Milano, Via C.Golgi, 19 1-20133 Milano, Italy Fax: +39-02-70638129 e-mail: [email protected] A novel method based on flame hydrolysis of aqueous solutions of precursors has been set up for preparing perovskite-type mixed oxide catalysts in nanometer size particles of high surface area and high thermal stability. After optimising the shape and size of the various parts of the equipment and all the operational parameters, the method has been applied to the preparation of a series of samples of general formula Lal.xMxCoO3+~, with x - 0, 0.1 and M - Ce, Eu. Perfectly crystalline powders, made of 200-600/~ microspherical particles, generally clustered into 100-500 nm conglomerates, with SaET typically of 20-30 mE/g, were obtained. The catalysts proved highly thermally resistant and up to one order of magnitude more active than samples of identical composition, but prepared through traditional methods, allowing to attain 100% methane conversion at temperature as low as 560~ I. INTRODUCTION Perovskite-like mixed oxides are currently widely studied and proved to possess a comparable activity as substitutes of noble metal catalysts for the abatement of CO, HC and NOx from fossil fuel combustion exhausts and for the catalytic flameless combustion (CFC) of hydrocarbons [ 1]. Particularly, being carded out at considerably low temperature, CFC allows a substantial reduction or even elimination of harmful combustion products, especially NOx. The usual calcination-milling (CM) route [2] for preparing such materials in a thermally stable form, involving several high-temperature CM cycles of the precursor powder mixture, ends in a lowsurface-area, coarse-size particles of relatively low catalytic activity. Then several techniques have been proposed for preparing higher surface area perovskites [3,4], the most commonly used being the so-called sol-gel-citrate (SGC) method [5]. However, the final calcination in SGC being made usually at T<800~ the material so obtained rapidly sinterises under the CFC conditions. Silica can be prepared in very fine particles by flame hydrolysis (FH) of SIC14. However, to our knowledge, so far the FH process has been applied to monocomponent oxides only [4,6,7], possessing at least one volatile precursor. The aim of the present work was then to prepare perovskitic catalysts in nanometer-size particles of high surface area, by extending the FH method to a multicomponent aqueous solution of precursors, hydrolysed in a high-temperature flame. After characterisation by several techniques, the catalytic activity of each sample was tested for the CFC of CH4.

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2. EXPERIMENTAL

The nitrates, acetates or citrates of precursors in the desired ratios were dissolved in water. When necessary, the dissolution was aided by employing 10 % aqueous HNO3 as solvent, instead of pure water. In some cases (vide infra) an aqueous solution of citric acid (1/1 or 0.5/1 molar ratio with respect to the sum of metal salts) was added to the metal precursors. The resulting solution was nebulised by means of a pyrex glass nozzle (fed with 0.150 to 0.225 m3/h of air) into a H2+O2 flame. The nanometric particles of perovskites so forming were collected by means of an electrostatic precipitator. BET surface area (SBET), elemental composition, particle size and nature of phase(s) have been determined by means of a Micromeritics ASAP 2010 apparatus, a Jordan Valley EX-310 Xray fluorescence (XR~) spectrometer, a Cambridge Stereoscan 150 scanning electron microscope (SEM) and a Philips PW 1820 powder diffractometer, respectively. The XRF quantitative elemental analysis was done after calibration by means of standard samples of known composition. The XRD analysis was made by using the Cu Kt~ radiation (~,=1.5418/~). Catalytic activity tests have been carded out by means of a bench-scale apparatus, centred around a quartz-tube continuous microreactor (I.D. = 0.6 cm, length = 45 cm). The catalyst (ca. 200 mg) was activated in situ overnight at 650~ in 40 cm3/min flowing air. After cooling down to 250~ the air flow was switched to a gas mixture composed of 20 cm3/min of 1 vol% CH4 in N2 and 20 cm3/min of air. A 4~ temperature ramp was then begun, up to 650~ while analysing the outcoming gas every 12 min by gas chromatography. An extended time-on-stream run (80 h) was also performed at 560~ on the most active catalyst. 3. RESULTS AND DISCUSSION Besides flow rate of H2 and 02 to the burner, the size and shape of the latter and of the nebuliser and the value of at least four parameters can be varied to control the FH process, so to get the desired sample composition, structure and particle size. The four parameters are: i) nature of the precursor salts, ii) concentration of the latter in the aqueous solution, iii) flow rate and iv) humidity degree of the air fed to the nebuliser.

3.1. Equipment Our goal was the preparation of crystalline Lal.xAxCoO3~:~ samples, with A = Ce, Eu, x = 0, 0.1 and SBETat least one order of magnitude higher than that (0.5 to 3 mE/g) usually obtained by preparation through the usual CM process. The optimisation of both the nebuliser and electrostatic precipitator was then performed by preparing a series of Ce-doped (x = 0.1) samples (Table 1, samples 1 to 4). Characterisation of these catalysts allowed defining the best geometrical and working parameters of our equipment, in order to maximise surface area, phase purity and crystallinity and to minimise particle size. 3.1.1. Nebuliser The shape and dimensions of the nebuliser govern productivity and influence the physical-chemical properties of the samples obtained. Indeed, the smaller the orifice of the nozzle, the smaller are the droplets formed and consequently the smaller is the size of the particles produced. However, the smaller the orifice, the higher is the pressure drop through the nozzle. Furthermore, the higher the flow rate of air through the nozzle, the lower becomes the hydrolysis temperature. Moreover, the higher is reflux rate within the nebuliser, the lower is

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productivity. In addition, properly moistened air is needed to feed the nebuliser, to avoid the progressive change in concentration of the solution during nebulisation. In fact, the higher the concentration of the solution, the higher is productivity, but the higher is the particle size and consequently the lower is the BET surface area of the powder obtained. Table 1. Main characteristics of the catalysts prepared Sample no.

1 2

3 4 7 8

Catalyst

Precursors"

Lao9CeolCoO3+a La(NO3)3 Ce(NO3)3 C0(NO3)3 LaogCe01CoO3+8 La(NO3)3 Ce(NO3)3 Co(NO3)3 LaogCeolCoO3+a La(Ac)2 Ce(Ac)2 Co(Ac)2 Lao9CeolCOO3+8 La(Ac)2 Ce(Ac)2 Co(Ac)2 LaogCe01CoO3+8 La(Ac)2 Ce(NO3)3 Co(At)2 LaCoO3+a La(Ac)2 Co(Ac)2

Solution conc. (wt.%)

Additives

11.58

none

SBET

(m2/li) 39.7

Citric acid 1: l(mol)

18.7

HNO3

27.0

HNO3 Citric acid 1"1(mol) HNO3 Citric acid 0.5:l(mol) HNO3

17.5

6.96 5.80 5.75

6.78 5.83

Citric acid

24.0

18

0.5:l(mol) 9

a)

Lao9EUolCOO3+a La(Ac)2 Eu(Ac)2 Co(Ac)2 Ac = acetate

HNO 3

5.85

18.3

Citric acid 0.5:1 (mol)

3.1.2. Electrostatic precipitator A first type of multipin effluviator, made of chromium-plated brass, proved unsatisfactory, since the pins were slowly corroded, due to the relatively high temperature of the discharging tips and to partial condensation of chemicals on them. As a consequence, undesired Cr, Cu and Zn oxide impurities were found in the samples produced. An AISI 316 stainless steel effluviator was then designed and mounted, which allowed overcoming the problem. The optimisation of both nebuliser and electrostatic precipitator led to a substantial improving of the overall process yield, from the very low initial value (5%) to over 80%. 3.2. Influence of flame temperature and of process parameters Two different fuel/oxygen (F/O) combinations were tried, both in stoichiometric F/O ratio. When natural gas was employed, the flame temperature hardly attained 1600~ during the FH process. As a consequence, the degree of XRD crystallinity of the resulting powder was very low. By substituting hydrogen for natural gas the flame temperature was much higher (>2000~ and the XRD pattern matched perfectly the typical reflections (20 = 23, 40, 46, 54, 59 ~ of the perovskite-type structure [8]. The nature of the precursor salts on one hand may hinder the formation of a clear solution of the desired composition. On the other hand it may help in keeping the FH process to a sufficiently high temperature. Several salts with different anions were then tried, both inorganic

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and organic, for preparing the solution to be nebulised. Organic anions, acting as additional fuels, allowed a higher hydrolysis temperature. The further addition of an equimolar amount of citric acid led to a better crystallinity (Fig.la-c) and to a more uniform distribution of particle size, however accompanied by a lower surface area (Table 1, samples 1-4). Moreover, the complexing action of the citrate anion helped in obtaining a perfectly clear and stable feeding solution. As for the overall concentration of the latter, too high values (> 10 wt%) led to a higher surface area, but also to XRD amorphous powders, while too low values (<3 wt%) depressed unacceptably productivity. The best compromise was then found for concentration around 6 wt%.

f

e

d

a

b

b

8

2O

4O 2O

6O

Fig. 1. XRD patterns of the powders obtained, a-c) Samples 2-4, respectively; d-J) samples 7-9, Tab. 1

c

0 Fig.2. (a-d) Typical SEM micrographs of samples 3, 7, 8, 9, Table 1.

All the samples so prepared showed a high phase purity and good crystallinity. This confirms that there is not a preferential decomposition of anyone of the precursors with respect to the other ones, the FH process taking place almost instantaneously, as expected. This is due to the effect of both high flame temperature and very tiny size of the nebulised solution droplets. Table 1 data show also that higher values of SBET(up to ca. 40 mE/g) were obtained in the absence of organic salts (sample 1) and with more concentrated precursor solution. However, this was accompanied by a low degree of crystallinity. Conversely, when employing organic salts (acetates) as precursors, SBETdropped to less than 30 mE/g (sample 3), but the size of the powder particles was more uniform (Fig.2a).The addition of an equimolar amount of citric acid (sample 4) further lowered SBET, down to less than 20 mE/g, accompanied by a considerable increase of the degree of crystallinity (Fig.lc). However, when the molar ratio of citric acid to the metal precursors was halved (sample 7) the degree of crystallinity practically did not change (Fig.ld), but the SBET value (Table 1) increased to 24 mE/g and the particle size of the powder became much more uniform. Indeed, SEM micrographs (Fig.2b-d) showed that the powder is composed

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of microspherical particles 200 to 600 A in diameter, generally clustered into 10 to 500 nm conglomerates. Finally, a proper degree of humidity of the air sent to the nebuliser allows to keep constant the concentration of precursors' solution, so keeping constant the particle size of the powder and the production rate of the process.

3.3. Preparation of the optimised samples of catalyst The equipment so optimised was then employed for the preparation of a series of samples with general formula Lal.xMxCoO3§ with x = 0, 0.1 and M = Ce, Eu, whose characteristics are shown in Table 1 (samples 7, 8 and 9) and in Fig.s 1 and 2. The composition of all three catalysts, checked by XRF analysis, coincided within the experimental error (+2%) with the nominal one, calculated from the weighed amount of precursor salts. 3.4. Activity and durability tests Activity comparison tests have been carried out on the series of three catalysts (samples 7-9, Table 1) prepared as described. A first aim of these tests was to compare the performance of the catalyst (sample 8, Table 1) prepared by the present FH method with catalyst samples of identical composition, but prepared by the SGC method [5] or by the CM method [2], respectively. The results are shown in Fig. 3. It can be noticed that the sample prepared by the FH method is about four times as active than that prepared by the SGC method and one order of magnitude more active than that prepared by the CM method. Another aim of the activity comparison tests was to investigate the effect of the partial substitution of Ce or Eu for La in our perovskite-type La-cobaltite. The results are given in Fig. 4. The Ce-promoted catalyst confirmed [9] to be the most active with respect to both the unpromoted and to the Eu-promoted samples, which showed a comparable activity. 100

:

A

--

(

C

o o

i

,,'J

50

i

~r

tO

r tO

0

250

400 T

550 /

~

Fig.3. Activity comparison tests for CFC of methane over the La0.9Ce0.1CoO3+8catalyst, prepared by: ( , ) the present FH method, ( l ) the SGC method and ( A ) the CM method. Catalyst loading: 0.2 g, reactants feeding rate: 40 cm3/min of a 0.5 vol% CH4, 10 % 02, balance N2, gas mixture.

0 250

400 T/~

550

Fig.4. Effect of 10 mol % substitution of Ce or Eu for La on the activity of LaCoO3+~ catalyst for CFC of CH4: ( , ) sample 7, ( A ) sample 8, (m) sample 9, Table 1. Reaction conditions as for Fig.3.

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化

100

$

***

===

c O O

50

I O ,

0

i

t

30

Time-on-stream

60

90 / h

Fig.5. Extended time-on-stream activity test. Catalyst 7, Table 1. Reaction temperature increased by 4~ and then maintained to 560~ Other reaction conditions as for Fig.3. At last, the durability test was carded out on the most active catalyst (sample 7) only, under the usual reaction conditions. Reaction temperature was increased by 4~ up to 560~ the lowest value at which this catalyst attained 100% conversion and then kept to such a value for ca. 80 hours on-stream. The results are given in Fig. 5 and confirm the very good thermal resistance of the present catalysts. 4. CONCLUSIONS i) A FH preparation method, starting from aqueous solutions of precursors and leading to nanometer-size perovskite-type transition metal oxide catalysts has been set up successfully. ii) Crystalline perovskites, with high surface area and very good thermal resistance can be easily prepared by this method. iii) These catalysts proved up to one order of magnitude more active than traditionally prepared catalysts of the same composition, for the flameless combustion of CH4, down to reaction temperature as low as 560~ ACKNOWLEDGEMENT The financial aid of Italian National Research Council (CNR) through the PF MSTA2, Contract no. PF34.115.05320, is gratefully acknowledged. REFERENCES 1. Y. Teraoka, H. Fukuda, S. Kagawa, Chem. Lett., (1990) 1. 2. M.F.M.Zwinkels, S.G.J~fis, P.G.Menon, T.A.Griffin, Catal. Rev. Sci. Eng., 35 (1993) 319. 3. L.G.Tejuca, J.L.G.Fierro, Properties and Applications of Perovskite-type Oxides, Dekker, New York, 1993. 4. W.R.Moser, J.D.Lennhoff, J.E.Cnossen, K.Fraska, J.W.Schoonover and J.R.Rozak, in Advanced Catalysts and Nanostructured Materials, W.R.Moser, ed., Academic Press, New York, 1996, p.535. 5. M.S.G.Baythoun and F.R.Sale, J. Mater. Sci., 17 (1982) 2757. 6. J. Long et S. J. Teichner, Rev. Hautes Temper. et Refract., 2 (1965) 47. 7. W.R.Moser, ed., Advanced Catalysts and Nanostructured Materials, Academic Press, New York, 1996. 8. Selected Powder Diffraction Data, J.C.P.D.S. Swarthmore,PA, Files no. 40-1100, (1992). 9. D.Ferri and L.Fomi, Applied Catal., B: Environmental, 16 (1998) 119.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Experimental Investigation and Modeling of Platinum Adsorption onto Ion-modified Silica and Alumina W. Spieker, J. Regalbuto, D. Rende*, M. Bricker*, Q. Chen* Department of Chemical Engineering, University of Illinois at Chicago, 810 S. Clinton, Chicago, Illinois 60607 USA *UOP Research Center, 50 E. Algonquin Road, Des Plaines, Illinois 60017 USA In this work, we determine whether a change in the PZC of alumina or silica by ion doping can be used to control the platinum adsorption properties of these supports, and how the Revised Physical Adsorption (RPA) model can be used to simulate platinum adsorption over such modified supports. We investigate the adsorption of cationic (TAPC) as well as anionic (CPA) platinum onto ion modified silica and alumina supports. Dry impregnation with an ammonium chloride solution is used to stepwise lower the PZC of alumina from 7.7 to as low as 4.8. Potassium nitrate is used to increase the PZC of silica from 3.8 up to 8.8. The oxide PZC is measured using the EpHL ("equilibrium pH at high oxide loading") method proposed by Park and Regalbuto [ 1]. It was found that, even though the PZC of the studied oxides can be altered by several pH units, modified alumina and silica exhibit the same platinum adsorption characteristic as the unmodified supports. The results suggest that the modified supports can in practical catalyst preparation as well as in RPA simulations be treated as pure oxide supports. However, since the adsorption characteristics are not affected, ion doping cannot be used to "engineer" a catalyst preparation by alteration of the pH range in which an oxide support is capable of absorbing a specific metal complex. Adsorption over the unmodified supports is reasonably well simulated by the RPA model.

1. INTRODUCTION A first step in the preparation of many heterogeneous catalysts is the process of adsorption, where metal complexes dissolved in aqueous solution are contacted with a porous oxide catalyst support. A schematic of such an adsorption system is comprised of three distinct regimes. The first is the bulk liquid solution in which the metal is present in different ionic species (possibly pH dependent). The second is the oxide surface that is populated with hydroxyl groups which get, as a function of the solution pH, deprotonated or protonated, leading to an overall negative or positive charge of the oxide surface. The third is the oxidewater interface that is described by surface adsorption constants for each of the species in the solution. It is the nature of this adsorption step that distinguishes most of the adsorption models in the literature. Our recent work indicates that the adsorption process of the chloroplatinate ion over alumina is physical in nature rather than chemical or coordinative [2]. According to the Revised Physical Adsorption (RPA) model [3], electrostatic interaction between ionic metal complexes in solution and a charged oxide surface determine if and to

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what degree the metal will be deposited on the oxide surface. Opposite charges lead to adsorption while like charges result in the repulsion of the metal. The point of zero charge, PZC, is the solution pH at which the overall charge of the oxide surface is zero. This is an essential parameter as it determines the transition of the surface charge from positive to negative as a function of the pH, and thus the oxide's ability to absorb anionic or cationic metal complexes from solution. The RPA model can be used to quantitatively simulate all CPA/A1203 adsorption data currently available in the literature to a reasonable degree with no adjustable parameters [2-4]. Lycourghiotis' and Schwarz' group [5-9] have demonstrated that the modification of silica and alumina with ions such as chloride, fluoride, sodium or lithium significantly changes the PZC, the surface ionization constants and the number of surface hydroxyl groups, all of which are important parameters in physical adsorption. Doping with cations, for example, results in an increase of the PZC value, while doping with anions leads to its decrease. As seen in figure 1, the application of the RPA model to ion-doped alumina and silica suggests that both supports can be modified such that they exhibit very similar CPA and TAPC adsorption characteristics. In this light we are investigating how the RPA model can be applied to platinum adsorption over oxides with such ionic impurities. If a change in the PZC proves to alter the platinum adsorption characteristic of the oxide, ion doping may be a potentially powerful tool in catalyst preparation as it possibly allows for a considerable extension of the pH range in which an oxide is capable of absorbing a particular metal complex. '-" ~

-6

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Figure l: RPA simulation of Pt adsorption over silica and alumina as a function of the PZC On the other hand, unchanged adsorption characteristics would imply that the RPA model can be used to simulate platinum adsorption regardless of possible ionic impurities of the support. Especially in adsorption over mixed oxides, where the properties of the single phases may be unknown due to prior treatment of the support, the RPA model could be employed in an effort to selectively direct the noble metal onto any one phase. As an example consider the preparation of a zeolite supported platinum catalyst, bound with a binder oxide (such as silica or alumina) in an extrusion process for mechanical strength, which is to be loaded with the noble metal only after the extrusion process. Knowing the adsorption characteristics of the oxide binder phase will enable one to selectively direct the platinum onto the zeolite rather than onto the binder, given the adsorption characteristics of the zeolite are known.

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2. EXPERIMENTAL The specimen were prepared by dry impregnation of the oxide supports with solutions of potassium nitrate (for silica) and ammonium chloride (for),-alumina). An amorphous silica with a surface area of 160 mZ/g and a ),-alumina with a surface area of 147 mZ/g were used. The amount of liquid required to fill the pore volume of the supports (which corresponds to the state of incipient wetness) was about 1.2 ml/g alumina and 5.5 ml/g silica. The ion loading required to alter the PZC to a specific target value cannot be calculated a priori but must be determined by a trial and error procedure. The RPA model predicts very similar results for adsorption of platinum over silica and alumina if the PZC values of these supports are changed from their normal values to 6 (alumina), 5 (silica, TAPC) and 7 (silica, CPA). Accordingly, for the samples to be used in the adsorption study, the concentration of the solutions were adjusted so as to achieve a dopant loading of about one chloride ion per 20 surface hydroxyl groups on the alumina surface and one potassium ion per 8.2 (for CPA) or 20.2 (for TAPC) hydroxyl groups on the silica surface. Table 1: Ion loading and PZC values of the unmodified and modified supports support alumina alumina alumina alumina silica silica silica silica silica

OH groups per ion . . 24 20 4.6 . . 20.2 8.2 5.5 1.9

.

.

dopant loading (wt%) (mmol/g sup.)

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7.66 6.15 5.91 4.78 3.8 7.03 7.78 8.04 8.83

5 5 5 5 7 7 7 7 7

OH groups 2 t~er nrn 8 8 8 8 5 5 5 5 5

After impregnation, the samples were dried in a muffle furnace in air at 110~ for 3 hours following drying over night under ambient conditions and were then calcined for 5 hours at 500~ in a muffle furnace in air. The point of zero charge was determined employing the EpHL ("equilibrium pH at high oxide loading") technique introduced by Park [3]. Using a special spear-tip semisolid electrode, the equilibrium pH of a thick slurry of the oxide at incipient wetness is measured over a wide range of initial pH values. The PZC is marked by the point where initial and equilibrium pH are equal. The powders were placed in 120 ppm (Pt) solutions of tetraammineplatinum chloride (TAPC, cationic) or hexachloroplatinic acid (CPA, anionic) at a surface loading of 1000 m 2 oxide surface per liter solution and agitated on a shaker for 1 hour. The samples were then filtered and the filtrates analyzed for platinum using ICP. Disappearance of platinum from liquid solution indicates uptake onto the support. Adsorption from both platinum precursors over both modified supports, silica and alumina, was monitored as a function of the equilibrium solution pH in a range predicted by the RPA model to be favorable for adsorption.

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3. RESULTS AND DISCUSSION The following EpHL plots show that the PZC of silica was increased to values as high as 8.8, while aluminas were produced with PZC values as low 4.8. As seen from figure 3 and also reported by Lycourghiotis' and Schwarz' group, the PZC changes sharply at low dopant loading levels and approaches a saturation value at which further loading has only little effect on the PZC. Mieth [9] found this value to be about 4.8 for alumina, which is in agreement with our data. Akratopulu [7], however, reports a relatively moderate increase in the PZC of silica from 3.25 to a maximum of 4.8 after doping with sodium. This is in contrast to the remarkable shift by 5 pH units to 8.8 after doping silica with potassium at comparable loading levels seen in this study. 14

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Figure 2: Effect of ion doping on the PZC of oxides shown using the EpHL technique o

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14

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Figures 4a and b show platinum adsorption over modified as well as unmodified silica and alumina from CPA and TAPC as a function of the solution pH. A comparison is shown between experimental data and RPA simulations. Within experimental error, adsorption onto modified and unmodified supports appears identical. This is true for both supports, silica and alumina as well as for both precursors, CPA and TAPC. Adsorption over the unmodified oxides generally follows the expected trends: Silica absorbs TAPC at solution pH values beyond its PZC of about 3.8 while it absorbs only small amounts of CPA in a small pH window below the PZC. Alumina absorbs the cationic TAPC beyond and the anionic CPA below its PZC and shows the characteristic suppression of adsorption at the pH extremes. The RPA model simulates adsorption over the unmodified supports reasonably well without adjustable parameters, while it predicts a distinctly different uptake over the ion modified oxides that is not seen experimentally. For the simulations it was assumed that the ApK value (difference of the logarithms of the surface ionization constants) and the number of surface hydroxyl groups were not altered through the doping. This may seem to be an oversimplification, however, in the model these parameters only have a marginal influence on the range of pH in which adsorption will occur and can therefore not have caused the discrepancy between experimental and simulated adsorption data for the modified supports.

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Figure 4a: Experimental and simulated platinum adsorption from CPA solution over ion doped and unmodified silica and alumina

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Figure 4b" Experimental and simulated platinum adsorption from TAPC solution over ion doped and unmodified silica and alumina 4. C O N C L U S I O N S

The results show that even though the PZC of the studied oxides can be altered by several pH units, the platinum adsorption characteristics are not affected. This suggests, that the RPA model can be successfully used to simulate platinum adsorption even over oxides with unknown impurities. Especially for mixed supports like zeolite extrudates that had undergone some processing prior to the adsorption step, this may serve as an important tool in catalyst preparation. A study on selective partitioning of platinum in zeolite-SiO2 extrudates is currently being conducted. On the other hand, ion doping cannot be used to "engineer" a catalyst preparation by alteration of the pH range in which an oxide support is capable of absorbing a specific metal complex. REFERENCES:

1. Park, J., and Regalbuto, J. R., J. Colloid Interface Sc. 175 (1995) 239. 2. Regalbuto, J. R., Navada, A., Shadid, S., Bricker, M. L. and Chen, Q., J. Catal. 184 (1999) 335. 3. Agashe, K. and Regalbuto, J. R., J. Colloid Interface Sc. 185 (1997) 174. 4. Regalbuto, J. R., Agashe, K., Navada, A. and Bricker, M. L., Studies in Surface Science and Catalysis, 118 (1998) 147. 5. Vordonis, L. et al, J. Catal., 98 (1986) 296. 6. Vordonis, L. et al, J. Catal., 101 (1986) 186. 7. Akratopulu, K. Ch., Vordonis, L. and Lycourghiotis, A., J. Catal. 109 (1988) 41. 8. Vordonis, L. et al., Preparation of Catalysts IV, Elsevier Science Publishers, (1987) 309. 9. Mieth, J. A., Schwarz, J. A., Huang, Y-J. and Fung, S. C., J. Catal. 122 (1990) 202.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Nanocrystalline Thin Films as a Model System for Sulfated Zirconia F.C. Jentoft, A. Fischer, G. Weinberg, U. Wild, and R. Schl6gl Fritz-Haber-Institut of the Max-Planck-Society, Department of Inorganic Chemistry Faradayweg 4-6, 14195 Berlin, Germany We are presenting a model system for sulfated zirconia- a material interesting for the catalysis of light alkane isomerization - in the form of a silicon-supported nanocrystalline thin film. The goal is to obtain a conducting, non-porous material which allows the successful application of surface science techniques. The paper describes the optimization of the wetchemical preparation method of such films, and characterization data are shown to demonstrate that these films are comparable to sulfated zirconia powders. 1. INTRODUCTION Discovery of the extraordinary activity of sulfated zirconia for n-butane isomerization below 373K [ 1] has led to numerous efforts to investigate and correlate structure, acidity, and reactivity of this material. Although a large data set has been obtained from powdered sulfated zirconia, no consistent picture has evolved that satisfactorily describes acidity and reactivity [2]. In a new approach to better characterize sulfated zirconia we have developed a model system which consists of a nanocrystalline zirconia film supported on silicon. A preparation procedure for the deposition of various oxide films on various substrates from aqueous solution has been described in the literature [3]. A key element in this preparation is the use of a self-assembled monolayer (SAM), an ordered array of long chain surfactant molecules which are anchored to the oxidized surface of e.g. a silicon wafer. The polar terminating groups of the monolayer are believed to play an 2essential role in attaching the film to the substrate. ZrO 2 / S O The layer structure is shown schematically in Fig. SAM 1. Film thickness depended on deposition time, e.g. 9iO 25 nm after 4 h, and 39 nm after 24 h [3]. $i Calcination at 773K resulted predominantly in the Figure 1. Schematic representation of tetragonal phase of zirconia [3]. For simplicity, we layered model system structure. refer to these films as "zirconia films", although they contain additional species. 2. E X P E R I M E N T A L SECTION

2.1. Thin Film Preparation Substrate preparation: Pieces 1-3 cm 2 of single-crystal (100) silicon wafers (CZ, 750 ~m thick, p-type, polished on both sides) were cleaned sequentially with chloroform, acetone, and ethanol; and then were oxidized for 25 min with a 30:70 mixture of 30% H202 and conc. H2SO4 ('piranha'-solution) at 353K. The SAM was formed by immersing the wafer in a solution of 50 ~1 ethanethioic acid S-[ 16-(trichlorosilyl)hexadecyl]ester in 5 ml bicyclohexyl

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for 5 h under Ar atmosphere (glove bag). The desired hydrophilic sulfonic acid terminating group was introduced through oxidation of the hydrophobic thioacetate group by immersion into an oversaturated solution of KHSO5*KHSO4*K2SO4for 5-17 hours. After this oxidative treatment the wafers were rinsed with distilled water and immediately transferred into the deposition medium. Deposition of zirconia films: The deposition medium was a 4 mmol solution of zirconium (IV) sulfate tetrahydrate (97%, Alfa, Karlsruhe, Germany) in 0.4 N hydrochloric acid. Glass vials with the wafer and 50 ml of deposition solution were placed into a bath which was then heated to the desired temperature. Depositions were performed at two different temperatures: (i) at 343K, with deposition times from 4 to 17 h. Under these conditions, a precipitate was formed in the liquid phase after approx. 30 min. The wafers were either just rinsed with distilled water after removal from the deposition medium or they were washed by wiping the surface with lint-free tissues and distilled water. (ii) at 323K, with deposition times up to 48 h. The deposition medium remained clear for 24 h under these conditions. The specimens were only rinsed but not wiped. All samples were blown dry with Ar under ambient conditions. Thermal treatment: Annealing was performed in stagnant air or in a flow of Ar (125 ml/min). Samples were heated at a rate of 5K/min to 773K and held at 773K for 2 h.

2.2. Characterization Scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) were performed with a Hitachi S-4000-FEG / EDAX DX4 using 5 kV acceleration voltage and secondary electron mode. Atomic force microscopy (AFM) images were taken in contact mode (Si3N4 tip) with a Burleigh Instruments ARIS 3300 using a small range head ARIS 3005 with a horizontal resolution of 1.0 nm and a vertical resolution of 10 pm. X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) and ion scattering spectroscopy (ISS) were performed with a modified Leybold ESCA 100 in the FAT mode; with the following instrument settings: XPS: MgKcx excitation (1253.6 eV) at 12kV/14 mA, pass energy 48 eV; UPS: He I (21.22 eV) or He II (40.82 eV) excitation, pass energy 6 eV; ISS: 1 keV He +, pass energy 192 eV. Dried films were introduced into the UHV system. Thermal treatment was performed in a separate preparation chamber (base pressure 1"10 .8 mbar) adjacent to the analysis chamber (base pressure 1"10 -l~ mbar) and specimens were transferred back and forth without exposure to air. The position in the analysis chamber can be well reproduced, allowing good comparison of absolute intensities within different treatment steps of a single specimen. Samples were heated in 1.013 bar of 20% 02 in N2 to 773K at 20K/s, held at 773K for 1 h, and cooled before evacuation.

3. RESULTS 3.1. Substrate The surfaces of cleaned or oxidized Si-wafers appeared smooth and homogeneous in SEM and AFM images. Techniques probing the chemical rather than the morphological properties of the surface, such as streaming potential measurements [4], suggested some scattering in the isoelectric points and the hydrophilicity among different sets of wafers. SAM-coated oxidized Si-wafers before oxidation of the terminating thioacetate group to a sulfonic acid were hydrophobic (no wetting with water) and appeared homogeneous (SEM). The oxidation of the thioacetate to the sulfonic acid group was identified as a critical step itself and may also reveal deficiencies in previous steps. Wafers with sulfonic acid terminated

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SAMs were characterized by an inhomogeneous appearing surface (SEM) - caused either by contaminants or by reactions other than just oxidation of the terminating group which was sometimes incomplete (C, S oxidation states, XPS). Zirconia film deposition was always possible but the quality, i.e. continuity and adherence of the zirconia films was susceptible to changes in the substrate preparation. With otherwise identical preparation conditions, an additional cleansing [5] of the Si-wafers in NH3/H202 and then HC1/H202 (343K, 20 min each) allowed the deposition of zirconia films whose XP-spectra showed significantly less Si (sum of Si(+IV) and Si(0)), indicating fewer defects or a thicker film.

3.2. Zirconia Films from Deposition at 343K Freshly deposited films: A SEM image of a typical fresh film after rinsing is shown in Fig. 2. The surface is characterized by the following features: (i) mainly smooth and homogeneous sections, with EDX (detection depth ca. 0.35 lain) analysis giving Zr, O, S, C, and Si, (ii) spherically shaped particles (EDX: Zr, O, S) of 200 nm size and agglomerates thereof adsorbed on the film surface or embedded into the film, Figure 2. SEM image of a zirconia film, deposited in 4 h at 343K from 4 mmol Zr(SO4)2 and (iii) cracks of 0.5-1 lain length and 100 nm width. The deposited particles in 0.4 N HC1; rinsed and dried. were often associated with the cracks (see inset). The surface of washed films, Fig. 3, exhibited very few, small deposited particles and numerous 0.5 -1 lure sized circular defects [6,7]. On a scratch-marked specimen the same area could be investigated before and after washing and "holes" were found exactly in the locations where particles had been attached previously, suggesting that the film had been removed along with the particles. The roughness, Rrms, was about 3.5 nm in areas without holes or Figure 3. SEM image of a zirconia film, adsorbed particles. deposited in 4 h at 343K from 4 mmol Zr(SO4)2 Thermal treatment: Thermal in 0.4 N HC1; washed and dried. treatment in air increased the number of cracks considerably while treatment in Ar did not change the number of cracks significantly. The films were always smoother after thermal treatment, with roughnesses (AFM) Rrms= 2.4 nm (air) and Rrms = 1.3 - 1.9 nm (Ar). Deposition medium: Most of the films' defects could be associated with the presence of precipitate particles in the deposition medium. The particle growth kinetics in the deposition medium were investigated with analytical ultracentrifugation and dynamic light scattering and

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the rate of formation of particles with a hydrodynamic radius r > 5 nm was found to increase with increasing temperature, increasing zirconium concentration, and decreasing HCIconcentration [8]. At 323K, a 4 mmol Zr(SO4)2 in 0.4 N HC1 solution remained clear to the eye for 24 h. Ultracentrifugation experiments indicated the presence of structures of about 1.5 nm in diameter; with the same size distribution profile at 6, 12, and 24 h.

Figure 4. SEM image of a zirconia film deposited in 24 h at 323K from a 4 mmol Zr(SO4)2 in 0.4 N HC1; rinsed, dried, and calcined at 773K in air.

3.3. Zirconia Films from Deposition at 323K Film deposition: Although conditions at 323K do not favor nucleation in the liquid phase, a film is still formed on the substrate. XPS experiments showed an increasing zirconium signal with deposition times of from 15 min to 24 h. Depositions at 323K yielded films more suitable for our purposes, i.e. free of adsorbed precipitate particles and holes, and mainly crack-free. The film surfaces were fairly smooth (AFM: Rrms = 1.3 nm). According to cross-section analysis with transmission electron thick with 5 nm zirconia crystals after a with the presence of the tetragonal phase

microscopy (TEM) [9], the films were about 11 nm 24 h deposition. Diffraction patterns were consistent but not always unambiguous. Thermal treatment: The films did not crack during thermal treatment at 773K, neither in Ar nor in air (Fig. 4). Films deposited at 323K are thinner than films deposited at 343K (= 25 nm [3]), and thus may be less stressed. After thermal treatment, the zirconia crystals were 5-6 nm, and Rrms was = 0.6 nm. XPS, UPS, and ISS measurements: XP spectra of fresh films showed signals of Zr, ......... dried ,; 8000 O, S, C, often S i, and sometimes calcined : w of impurities (typically N). t o 6000 Signals of the Si substrate were i4 not always detectable, suggesting J 4000 either a contiguous zirconia film with a thickness in/just beyond oe 2000 the range of the electron escape depth or a thicker film with very few flaws. The maximum of the 290 288 286 284 282 Si 2p (2p3/2 + 2pl/2) signal was Figure 5. C ls signal of a zirconia film from 24 h detected at 99.7 eV, consistent deposition at 323K: dried (dotted line); calcined at 773K with Si(100), and 103.4 eV, (solid line). consistent with oxidized Si. Signals arising from the elements

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in the zirconia film were slightly shifted towards higher binding energy due to charging of the A 3,1 0 4 film; the shift was about 2.0 eV 0 (dried) or 1.7 eV (calcined films) which was much less than the ~- 2 , 1 0 4 W shift observed for sulfated zirconia powders (almost 7 eV). c 1.10 4 Binding energies of the shifted signals were corrected using Zr 3d5/2 = 182.2 eV of ZrO2 [ 10] as internal reference. Using this 534 532 530 528 calibration, the main Cls signal Binding Energy (eV) of fresh samples was located at Figure 6. O 1s of a zirconia film from 24 h deposition at 284.5 eV. Calcination removed 323K: dried (dotted line); calcined at 773K (solid line); most of the carbon from the after He + sputtering (line + circles). surface (Fig. 5); a concomitant increase in the intensity of Zr, O . . . . . . . . . dried Zr 3d I and S signals was observed. The calcined 4"104 -S2D I ,~ O ls signal of the fresh film can ---o--- sputtered | W be distinguished into at least two o. 3,104 0 z 1169.01 peaks, i.e. at about 531.5 eV and 530.0 eV (Fig. 6). The lower 04 binding energy species is assigned to the 0 2of ZrO2, _= 1,10 4 xlo 1 consistent with observations on powders [11]. Heating in air 0 reduced the fraction of high 185 180 175 170 165 160 binding energy oxygen and this Binding Energy (eV) peak was also slightly shifted to even higher binding energy. Figure 7. Zr 3d and S 2p signals (after X-ray satellite substraction) of a zirconia film from 24 h deposition at After sputtering the sample with 1 keV He + (fluence: = 3"1016 323K: dried (dotted line); calcined at 773K (solid line); He+/cm2), this oxygen species after He + sputtering (line + circles). had almost disappeared. The maximum of the S 2p (2p3/2 + 2pl/2) signal was detected at a binding energy of 168.4 eV before and 169.0 eV after calcination, consistent with the presence of S(+VI). After sputtering the signal was shifted to 161.2 eV (Fig. 7, with Zr3d), consistent with S(-II) [10]. The reduction of sulfur along with the disappearance of one oxygen species strongly suggests that this particular O ls peak at 531.8 eV belongs to the sulfate species, as has been proposed for sulfated zirconia powders [ 11]. ISS data showed peaks originating from O, S and Zr. In UP spectra, typically two features could be distinguished, however, their position was difficult to determine due to charging effects. XP-spectra after adsorption of ammonia: XP spectra taken of the N ls signal after exposure to NH3 (10000 L) at room temperature showed two peaks at 402.1 eV and 400.0 eV

4,104

i m

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(Fig. 8). The higher binding energy species was significantly weakened upon heating to 473K in vacuum while the lower binding energy species remained largely unaffected.

4. DISCUSSION & SUMMARY A consistent picture evolves when combining data from the different W microscopic techniques with the D.. O information from XPS: Contiguous films with the chemical composition Zr, O, W and S can be deposited on the G conducting silicon substrate. Film thickness can be controlled by varying the deposition time, e.g. about 11 nm after 24 h deposition at 323K in a 4 405 400 395 390 mmol Zr(SO4)2 solution in 0.4 N HCI. Binding Energy (eV) The films contain zirconia crystals (= 5 Figure 8. N 1s signal of a zirconia film from 24 h nm), presumably the tetragonal phase. deposition at 323K; dried and calcined at 773 K The films do not crack during (bottom); after exposure to 10000 L NH3 (top). calcination, and their surface is very smooth. Sulfate species are present, suggesting that the films resemble "sulfated zirconia". Ammonia can be adsorbed, and similar to what has been observed previously for the adsorption of pyridine on sulfated zirconia [ 11 ], the N ls signal reveals two species. The two peaks are tentatively assigned to the adsorption of ammonia on two different acidic sites. We are able to prepare sulfate-containing nanocrystalline zirconia films which appear to be a promising model system for sulfated zirconia catalysts. UP spectra which contain information on the valence levels of the catalyst could be collected. We expect the films to be suitable to acquire interpretable temperature-programmed desorption (TDS) data to further characterize acid sites. A

v

m

REFERENCES 1. M. Hino, K. Arata, J. Chem. Soc. Chem. Comm., (1980) 851. 2. X. Song, A. Sayari, Catal. Rev. - Sci. Eng., 38 (1996) 329. 3. M. Agarwal, M.R. De Guire, A.H. Heuer, J. Am. Ceram. Soc., 80 (1997) 2967. 4. A. Bismarck, J. Springer, A. Fischer, F.C. Jentoft, R. Schl/3gl, unpublished results. 5. W. Kern, J. Electrochem. Soc., 137 (1990) 1887. 6. A. Fischer, F.C. Jentoft, G. Weinberg, R. Schl/3gl, T.P. Niesen, J. Bill, F. Aldinger, M.R. De Guire, M. Rtihle, J. Mater. Res., accepted. 7. T.P. Niesen, M.R. De Guire, A. Fischer, F.C. Jentoft, J. Bill, R. Schltigl, F. Aldinger, M. RiJhle, J. Mater. Res., accepted. 8. H. Schnablegger, H. Ctilfen, M. Antonietti, A. Fischer, F.C. Jentoft, R. Schltigl, in preparation. 9. A.D. Polli, T. Wagner, M. Rtihle, A. Fischer, F.C. Jentoft, R. Schl/3gl, unpublished results. 10. J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin Elmer Physical Electronics Division, 1992. 11. M. Johansson, K. Klier, Topics in Catal., 4 (1997) 99.

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Studies in Surface Science and Catalysis A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Nanoparticle arrays as model catalysts: Microstructure, thermal stability and reactivity of Pt/SiO2 and Ag/AI203 fabricated by electron beam lithography G0nther Rupprechter*, Aaron S. Eppler, Armen Avoyan and Gabor A. Somorjai

Department of Chemistry, University of California, Berkeley and Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Electron beam lithography was utilized to produce well-ordered two dimensional arrays of Pt and Ag nanoparticles on silica and alumina. The mean size of the particles was varied between 20 and 50 nm, with interparticle distances of 100 and 200 nm. Due to the narrow size distribution of the particles, these arrays are ideal models for supported metal catalysts. The thermal stability of the arrays and the effect of annealing in different gases were examined by HRTEM and AFM. Pt nanoparticle arrays were stable up to 973 K, which is the onset temperature for recrystallization. Reaction studies of ethylene hydrogenation on Pt nanoparticle arrays yielded rates that are comparable to conventional catalysts. Ag nanoparticles were stable in vacuum and hydrogen up to 773 K, while rapid evaporation occurred above 873 K. In oxygen, the Ag arrays remained intact up to 623 K but sintered at higher temperature. 1.

INTRODUCTION

The application of microfabrication technology to catalysis research was pioneered by H. Saltsburg [1], and followed by E.E. Wolf [2], B. Kasemo [3] and the Berkeley group [4,5]. Lithographic methods were utilized to produce well-defined model catalysts in the form of layered metal-oxide towers or supported nanoparticles. Electron beam lithography (EBL) allows nanoscale control of the catalyst particle size, height, interparticle distance, composition, and metal-support interface [6]. The recent advent of EBL-equipment with electron beam sizes down to 2.5 nm (,,nanowriters") allows to fabricate nanoparticle arrays with metal particles as small as 10 nm [7]. These model catalysts are ideal candidates to study particle size effects, e.g. in the ethylene epoxidation on Ag particles where a sharp increase in rate was observed between 30 and 50 nm particle size [8]. We have employed electron beam lithography to microfabricate well-ordered square arrays of uniform platinum and silver nanoparticles on fiat silica and alumina supports, respectively. The mean size of the particles was varied between 20 and 50 nm, with interparticle distances of 100 and 200 nm. Due to the flatness of the support these catalyst are fully accessible to surface characterization techniques and nanoscale information on the structure, composition, and oxidation state of the catalyst can be obtained (EBL-samples can be handled in a way similar to single crystals). Since EBL-catalysts are exposed to solvents and air during their preparation, they need to be cleaned before reactivity data can be measured. This is achieved by annealing in oxygen and hydrogen or by ion bombardment and subsequent annealing in vacuum. The activation treatments will not only remove contaminants but they will also change the particle microstructure. Structural changes are also likely to occur during reactions in oxidizing or reducing environments. Because the activity and selectivity of supported metal *corresponding author; present address: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D- 14195 Berlin, Germany Fax: +49 30 8413 4105 Mail: [email protected]

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catalysts in structure-sensitive reactions critically depend on the type of surface sites available, it is inevitable to study the thermal and chemical stability of nanoparticle arrays in order to determine the particle surface structure during a catalytic reaction.

2. EXPERIMENTAL

The EBL fabrication process consists of several steps (a schematic diagram is shown in ref. [5]): The oxide supports were prepared on a Si(100) wafer either by surface oxidation to form SiO2 or by deposition of an alumina film onto the Si substrate (both oxides were about 10 nm thick). The next step is to spin-coat an 80 nm thick electron sensitive-polymer (polymethylmethacrylate, PMMA) onto the support surface. The desired pattern is subsequently "written" into the polymer layer by a highly collimated electron beam (o 4 nm, 100 kV) produced by the nanowriter (Leica Corp.). This is followed by the selective dissolution of the polymer chains damaged by the electron exposure in methyl-isobutyle ketone/isopropyl alcohol. This procedure produces a mask of periodically-spaced holes in the PMMA layer. A thin film of metal is then deposited on this mask and after the remaining polymer resist is removed completely by dissolution in acetone, metal nanop2articles of the prescribed pattern remain on the support. The model catalysts, typically 1 cm in size, were characterized by high resolution transmission electron microscopy (HRTEM) including in-situ heating experiments, electron diffraction, atomic force microscopy (AFM) in constant force and constant height mode, Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and temperature programmed desorption (TPD). For electron microscopy examination, the samples were thinned from the back to form an electron transparent window. 3. RESULTS Figure la shows a medium-magnification TEM micrograph of a Pt/SiO2 model catalyst with a mean particle size of 40 + 1 nm, and an interparticle distance of 200 + 2 nm (resulting in 2.5 x 109 particles cm2). A second Pt/SiO2 array, with a mean particle size of 28 + 2 nm and the same interparticle spacing, is presented in Fig. 2a. Microdiffraction patterns (Fig. l b) and HRTEM images (Fig. l c) of individual particles revealed that the Pt clusters were polycrystalline with domain sizes of about 5 nm (Moir6 fringes of 0.44 nm spacing are due to double diffraction of overlapping crystalline domains). The FFT in Fig. l c" reveals a (110) orientation of the particle area marked by a square in Figure l c. The rounded profiles of the particles in TEM images suggest that their surfaces consists mainly of stepped high Millerindex facets. The height of the Pt particles as determined by AFM was 20 nm + 0.5 nm and in agreement with the deposited film thickness as measured by a quartz crystal oscillator. The thermal stability of the arrays was examined b~r in-situ electron microscopy. The 28 nm Pt/SiO2 catalyst was annealed in high vacuum (5xl 0 mbar) up to 1173 K using a Gatan TEM heating holder (Fig. 2). The temperature was initially increased to 773 K within 2 hours and then increased hourly at increments of 100 K (rate of 10 K/min), with images and a video recorded throughout the heating process. Dark field microscopy allows to monitor the polycrystalline structure of the particles by imaging individual grains. The Pt array remained unchanged upon heating to 773 K and the particles maintained their polycrystalline structure (Figs. 2a,b). No changes were evident after heating to 873 K. After heating for 1 hour to 973 K, dark field images showed that the crystalline domains in the particles grew in size and some of the particles recrystallized completely to become single crystals (Figs. 2c,d). The reduction of the number of grains per Pt particle is also indicated by the reduced number of particles that are imaged in dark field. The metal crystallization continued until the f'mal temperature of 1173 K was reached. Although the individual particles did not migrate over the support surface at any

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Figure 1 (a) Transmission electron micrograph of a Pt nanoparticle array on SiO2; (b) microdiffraction pattern of an individual Pt particle (spots originating from a (110) oriented crystalline grain within the polycrystalline Pt particle are marked by circles); (c) HRTEM micrograph and (c') FFT of a Pt model catalyst particle; (d) AFM image of a Pt nanocluster array after several reaction-cleaning cycles. temperature, particle rounding and some new features were observed after heating at 1173 K for 1 hour (see arrows in Figs. 2e,f). According to dark field microscopy, these features are small Pt aggregates that are probably formed by nucleation of Pt atoms migrating across the support at high temperature. The mobility of Pt at 1173 K also leads to a change in the particle size distribution to 23 + 4 nm (Fig. 2e). At 973 K the Pt atom mobility seems to be restricted to an intraparticle diffusion, since small aggregates were never observed between the

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773 K

973 K

1173 K

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Figure 2 Bright field and dark field micrographs of a 28 nm Pt array on silica, taken in-situ during annealing in high vacuum: (a,b) 773 K, (c,d) 973 K, (e,f) 1173 K. Pt particles. To investigate the possibility that Pt diffused across the support surface even at 973 K, energy dispersive X-ray analysis (EDX) was performed in the TEM. Positioning an electron beam of 30 nm diameter close to a particle or at the spaces in between particles yielded no Pt emission. This data does not fully rule out the possibility of Pt diffusing across the surface, but it renders significant Pt diffusion at 973 K unlikely. Fig. 3 shows HRTEM images of a recrystallized Pt particle that shows distinct low Miller index surface facets, together with the corresponding fast Fourier transform (FFT) indicating a [110] zone axis orientation. Two driving forces can be considered for particle recrystallization: (1) Pt diffusion within one particle, and (2) melting of the particles. Recently, Wang et al. [9] observed that surface melting occurred on 8 nm Pt particles above 873 K. However, the Pt particles in our study are much larger and we have not seen any clear evidence for surface melting. Therefore, we rather favor intraparticle-diffusion as mechanism for recrystallization. The microscope used in this study was not equipped with an environmental cell. To anneal the arrays in hydrogen and oxygen, a reactor cell had to be used, with subsequent transfer to the microscope. The array of 40 nm Pt particles on SiO2 was heated to 673 K in 1 bar H2 for 3 hours. A following TEM inspection showed no changes in the array or particle microstructure. Finally, the same sample was heated at 973 K in H2 for 3 hours. As in vacuum, under 1 atm H2 the particles began to form larger crystalline domains, evidenced by dark field and HRTEM. An untreated array of 25 nm Pt particles was then heated for 4 hours in 1 bar 02 at 673 K. Significant differences to the vacuum and hydrogen treatment could not be detected. Studies of

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....................::::::::::::::::::::::::::::::::::::::~::~:::::::;:~:!~

Figure 3 (a) HRTEM micrograph of a recrystallized Pt nanoparticle on SiO2 after annealing in vacuum to 1173 K; (b) higher magnification and FFT of the top right surface region. Pt particles on alumina and ceria [10] and on silica [11] indicate that a oxygen treatment between 673 and 923 K should increase the surface step density. However, since the EBL particles have rough surfaces already after preparation, surface roughening is difficult to detect. Sputtering with 500 V Ne § ions is an alternative way to clean the particles. Of course, some Pt is also removed when the sample is sputtered. Under our conditions, the sputter rate was approximately 0.1 nm/min (as determined by measuring the nanoparticle height at different sputtering times by AFM). The sputter process can be useful to deliberately change the height of the Pt particles to see what effect this has on catalytic activity (e.g. by increasing the ratio between sites at the metal-support phase boundary and metal terrace sites). Kinetic measurements on the Pt arrays were performed in a high pressure cell attached to an ultrahigh vacuum system. Ethylene hydrogenation at 298 K was used as test reaction (15 mbar C2H4 and 1000 mbar H2) and a steady state turnover number of 30 molecules ethane (Pt site) -1 sec -I was obtained, comparable to rates reported for conventional catalysts. Figure l d shows the AFM image of a model catalyst array after several cycles of reaction and cleaning. The presence of hydrocarbons at temperatures up to 673 K had no effect on the catalyst morphology. The arrangement of the array was maintained and only the height of the particles decreased slightly, due to repeated sputtering. Studies of the isomerisation and hydrogenation of 1-butene on Pt arrays and Pt single crystals are in progress now. The morphology and microstructure of 20 nm Ag particles on A1203 was similar but, as expected, their thermal stability range was smaller. The catalysts were stable up to 773 K in vacuum and hydrogen, but around 873 K the Ag particles evaporated within minutes without preceding sintering (silver sublimation from polycrystalline foil in vacuo starts at-~850 K [12]). We did not observe spreading of Ag particles, however, surface diffusion (spillover) of Ag atoms from particles to the support on the sub-monolayer scale can not be ruled out completely. In 02, the Ag particles were oxidized below 473 K but the arrays were stable up to 623 K. Higher temperatures induced particle migration and sintering. Fig. 4 shows a scanning electron micrograph of Ag nanoclusters on an alumina-covered silicon wafer. The particles are approximately 20 nm in diameter, with 100 nm interparticle distance and a height of 15 nm.

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Figure 4 Scanning electron micrograph of a Ag nanocluster array on alumina: Mean particle size 20 nm, particle height 15 nm (measured by AFM), interparticle distance 100 nm.

4. CONCLUSIONS Electron beam lithography permits to fabricate supported metal catalysts with nanoscale control, i.e. the metal particle size and height, the interparticle spacing, and the particle composition can be adjusted with high precision. Pt nanocluster arrays on silica were stable up to 973 K in different environments. The EBL process produces polycrystalline metal particles with high index facets but their structure can be transformed to single crystals with lower index surfaces, e.g. by annealing at 973 K in vacuum or hydrogen. Arrays of Ag particles on alumina were stable up to 773 K in vacuum and hydrogen, while sintering occurred above 623 K in oxygen. Studies are in progress to explore reaction selectivity as a function of particle size and spacing in the 20-100 nm range. The variation of the interparticle distance should effect diffusion-controlled reactions, e.g. when selectivity is determined by secondary reactions of surface intermediates. In the future we plan to extend our studies to titanium oxide supports. REFERENCES 1. 2. 3. 4. 5. 6.

I. Zuburtikudis, H. Saltsburg, Science, 258 (1992) 1337. A.C. Krauth, K.H. Lee, G.H. Bernstein, E.E. Wolf, Catal. Lett., 27 (1994) 43. K. Wong, S. Johansson, B. Kasemo, Faraday. Discuss., 105 (1996) 237. P.W. Jacobs, F.H. Ribeiro, G.A. Somorjai and S.J. Wind, Catal. Lett., 37 (1996) 131. A.S. Eppler, G. Rupprechter, L. Guczi and G.A. Somorjai, J. Phys. Chem. B, 101 (1997) 9973. Note: Laser interference nanolithography is another promising route but it will not be discussed here; for details see e.g.: M. Schildenberger, Y. Bonetti, R. Prins et al., Catal. Lett., 56 (1998) 1. 7. O. Dial, C.C. Cheng, A. Scherer, J. Vac. Sci. Technol. B, 16 (1998) 3887. 8. V.I. Bukhtiyarov, I.P. Prosvirin et al., J. Chem. Soc., Faraday Trans., 93 (1997) 2323. 9. Z.L. Wang, J.M. Petroski, T.C. Green, and M.A. E1-Sayed, J. Phys. Chem. B, 102 (1998) 6145. 10. G. Rupprechter, J.J. Calvino, C. L6pez-Cartes, M. Fuchs, J.M. Gatica, J.A. P6rez-Omil, K. Hayek, and S. Bemal, this conference. 11. A.S. Ramachandran, S.L. Anderson, A.K. Datye, Ultramicroscopy, 51 (1993) 282. 12. A. Avoyan, S.H. Kim, G.A. Somorjai, unpublished results.

ACKNOWLEDGMENT Electron microscopy was performed at the National Center for Electron Microscopy, Lawrence Berkeley National Laboratory. EBL was carried out at the Center of X-ray optics at LBNL.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Epoxidation of olefins on M-SiO2 (M=Ti, Fe, V) catalysts with highly isolated transition metal ions prepared by ion beam implantation Qihua Yang, Can Li*, Suli Wang, Jiqing Lu, Pinliang Ying, Qin Xin and Weidong Shi State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China Tel: 86-0411-4671991(ext. 728) Fax: 86-0411-4694447 Email: canli@ms, dicp. ac. cn M-SiO2 (M=Ti, V, Fe) catalysts with highly isolated transition metal ions were prepared by ion beam implantation and the catalysts were tested for the epoxidation of styrene, 1octene and cyclohexene using dilute H202 (30%) as oxidant. Ti-SiO2 shows the highest epoxide selectivity, while Fe-SiO2 exhibits the highest activity among the three catalysts. Epoxide selectivity can approach 100% for the epoxidation of styrene on Ti-SiO2 catalyst at 298 K. The nature of active sites in the catalysts was significantly changed after the calcination. The activities and selectivities of the catalysts are mainly due to the isolated surface transition metal ions derived from the ion beam implantation. It is assumed that there are at least two types of active sites, M (ct) (isolated M "+) and M (13) (aggregated metal oxide nanoparticles) on the catalyst surface, and the good activity and selectivity are attributed mainly to the M (ix) sites. 1. INTRODUCTION Conventionally, the production of fine chemicals has been carried out by stoichoimetric chemistry and many wastes are produced accompanying the procedure. However, strong concerns on environment ask for clean catalytic procedures. A selectively clean process for a number of selective oxidations could be realized on using the TS-1 like catalysts and dilute H202 as oxidant. The isolated transition metal ions substituted in the framework of zeolites (e. g. TS-1) show excellent activity and selectivity for some selective oxidations [1-4]. Nevertheless it is difficult to prepare oxide-supported catalysts with highly isolated transition metals ions by conventional chemical methods, as the transition metal ions on catalyst surface easily aggregate together to form oxide particles which are usually not good for selective oxidation. Ion beam implantation, a physical method, has been extensively used for material modification [5,6], which has, however, not been well recognized as a useful method for catalyst preparation [7]. This method, from its principle, can not only implant various metal ions onto the surface of a support (or a matrix), but also make the metal ions highly isolated because the ions forming the beam are positively charged and repelled each other. In this paper, we present a new class of catalysts, M-SiO2 (M=Ti, Fe, V), prepared by ion beam implantation and their catalytic activity and selectivity were tested with the epoxidation of olefins such as styrene, 1-octene and cyclohexene, using the dilute H202 (30%) as oxidant.

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2. EXPERIMENTAL

2.1. Preparation of catalysts A Mevva-80-10 implantation system was used for the catalyst preparation. Silica powder of 40-60 mesh and with a BET surface of ca 300 m2g1 was chosen as matrix or support. The purity of the metal implanted is better than 99.9%. The ion beam implantation was carried out at 40 KeV. The vacuum system during the implantation was maintained at 1.07 x 10.3 N m 2. The silica powder was kept rotating in a vessel during the implantation to ensure a uniform distribution of the transition metal ions on the silica surface. The catalysts implanted with Ti, V, Fe were referred to as Ti-SiO2, Fe-SiO2 and V-SiO2, respectively. Because the instrument was designed for the modification of semiconductor surfaces, the dosage of ion beam is very small. So it usually takes more than 24 h to prepare an M-SiO2 sample with weight percent about 0.1 wt %. 2.2. Catalytic tests In a typical experiment, 0.4 g catalyst, 18 mmol olefin and 4 ml CH3CN (for 1-octene 10 ml CHaCN was added) were mixed in a 50 ml round flask equipped with a condenser and magnetic stirrer. After heated to a desirable temperature, 18 mmol H202 (30%) was added. The sample was filtered and analyzed by MS-GS chromatograph using a capillary column (OV-101, 30-m length) and a flame ionization detector (FID). The consumption of hydrogen peroxide was determined by iodometric titration. TOF is defined as moles of styrene converted per mol of M in the catalyst per hour and all M ions are assumed to be on the surface of the catalyst. Peroxide efficiency is referred to as mol substrate converted per mol of hydrogen peroxide consumed. 2.3. Characterization UV Raman spectra were recorded on a homemade Raman spectrometer, XPS was determined on VG-ESCALAB MK-2 X-ray Photoelectron Spectrometer and ESR was recorded on JEOL ES-EDX3 Electron Paramagnetic Resonance Spectrometer. Transition metal content was determined by ICP-AES. The transition metal content is Ti, 0.28 wt % in Ti-SiO2; V, 0.16 wt % in V-SiO2 and Fe, 0.10 wt % in Fe-SiO2. 3. RESULTS AND DISCUSSION

3.1. Characterization XPS and UV Raman spectroscopy didn't detect the signals related to the transition metal for M-SiO2 catalysts because the contents of the transition metal ions are extremely low. ESR signal at a g value of 2.003 was observed for all the catalysts. This ESR signal can be assigned to the trapped single electron and some isolated M "§ with unpaired electron stabilized in the SiO2 substrate. 3.2. Epoxidation of olefins Table 1 summarizes the epoxidation of styrene on all the M-SiO2 (M=Ti, Fe, V) catalysts at 348 K. Two main products, epoxide and benzaldehyde, were formed on these catalysts. TiSiO2 is found to be the best catalyst for epoxide selectivity and the epoxide selectivity can approach 100% when the reaction was performed at 298 K. Fe-SiO2 shows the highest activity among the catalysts. The conversion of H202 exceeds 60% on V-SiO2 and Fe-SiO2

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Table 1 The epoxidation of styrene on M-SiO2 (M=Ti, Fe, V) prepared by ion beam implantation Selectivity (mol %) Catalyst

Ti-SiO2 Ti-SiO2a Fe-SiO2 V-SiO2 SiO2 TS-1 tTl

TOF (hl) a

Conv Styrene (mol%)

12.3 4.7 44.2 22.5 5.9

4.8 1.8 5.3 4.7 0.4 35.4

Effec.H2O2

Epoxide

Benzald ehyde

Phenylaceta ldehyde

others

55.5 100 31.6 32.8 0 2.0

44.8 0 68.4 67.2 100 22.8

0 0 0 0 0 74.2

0 0 0 0 0 1.0

(%) 37.8 46.3 5.8 7.7 -

Reaction condition: catalyst, 0.4 g; styrene, 18 mmol; H202, 18 mmol; solvent CH3CN, 4 ml; reaction temperature, 338 K; reaction time, 3 h; The reaction was performed at 298 K. a

Phenylacetaldehyde was not detected for the catalysts prepared by ion beam implantation, while it was the main product when using TS-1 as the catalyst [8]. The activity is considerably high in terms of TOF although the total conversion is low because the surface concentration of the implanted ions is very low. Table 2 The epoxidation of cyclohexene on M-SiO2 (M=Ti, Fe, V) prepared by ion beam implantation Selectivity (mol %) Catalyst

TOF (h-T)a

Ti-SiO2 Fe-SiO2 V-SiO2 Si02

15.3 20.9 12.9 0

Conv cyclohexene Effec.mo2 Epoxide (mol%) (%) 6.1 2.5 2.7 0

40.2 8.2 10.1 0

60.4 48.7 50.0 0

Cyclohex-2en- 1-ol

cyclohex-2en- 1-one

32.9 43.5 42.5 0

6.7 7.8 7.5 0

Reaction condition: catalyst, 0.4 g; cyclohexene, 18 mmol; H202, 18 mmol; solvent CH3CN, 4 ml; reaction temperature, 338 K; reaction time, 3 h. The results of epoxidation of cyclohexene and 1-octene are listed in Tables 2 and 3 respectively. The Ti-SiO2 exhibits the highest epoxide selectivity and conversion. The epoxide selectivity approaches 60% in epoxidation of cyclohexene on Ti-SiO2, while the FeSiO2 shows almost the same catalytic activity and selectivity as that of V-SiO2. Interestingly the main side product is cyclohex-2-en-l-ol. The epoxide selectivity can reach 80% in epoxidation of 1-octene on Ti-SiO2. But almost no products are detected for Fe-SiO2 and VSiO2 in epoxidation of 1-octene probably because of the inactivity of 1-octene and the low contents of the transition metals in the catalysts.

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For Ti-SiO2, the catalytic activity for different kinds of olefins increases in the order: cyclohexene>styrene> 1-octene, which is the same order for the nucleophilicity of the olefins. As the catalysts prepared by ion beam implantation have the open surface, the steric limitation and the diffusion have only minor influence on the activity and the relative reactivities of olefins depend mainly on the nucleophilicity character of the olefins [9]. But for TS-1, sometimes the differences in sorption strength and diffusion rates are thought to cause the difference [ 10, 11 ]. Table 3 The epoxidation of 1-0ctene on M-SiO2 (M=Ti, Fe, V) prepared by ion beam implantation Catalyst

TOF (hq) a

Ti-SiO2 Fe-SiO2 V-SiO2 SiO2

13.3 0 0 0

Conv

1-oetene

5.3 0 0 0

(mol%) Effec.H2O2(%) Epoxide selectivity (mol%) 39.8 0 0 0

80.5 0 0 0

Reaction condition: catalyst, 0.4 g; 1-octene, 18 mmol; H202, 18 mmol; solvent CH3CN, 4 ml; reaction temperature, 348 K; reaction time, 12 h. The different activity of M-SiO 2 (M=Ti, Fe, V) can be explained by the oxidative ability of Ti, Fe and V. The activity of the catalysts decreases in the same order as the standard electrode potential: E (Fe3+/Fe2+) > E (vo2+/v3+) > E (TiO2+/Ti3+).The low epoxide selectivity for Fe-SiO2 and V-SiO2 may be due to the further oxidation of epoxide because of the high oxidative ability of iron and vanadium ions in SiO2. The activity and selectivity in epoxidation of olefins on M-SiO2 can be attributed to the unique active sites derived by ion beam implantation, since the content of M is very low, less than 0.28 % and the specific surface area of SiO2 is over 200 mRg-~, the possibility of aggregated metal ions is small. 3.3. Epoxidation of styrene on M-SiO2 calcined in air The calcination temperature of the catalysts has a big effect on the activity and selectivity of the catalysts. The activity and epoxide selectivity usually decrease for all catalysts when they are calcined at high temperatures except for V-SiO2 (Figures 1. and 2). This implies the coordination structure of the active sites in the catalyst is significantly changed after the calcination treatment. ESR signal of the catalysts becomes gradually weaker with the increasing of calcination temperatures and finally disappears for the catalysts calcined at 773 K. During the calcination, the oxygen in air oxidizes the transition metal ions with lower valence to higher one and the trapped single electron disappears gradually in the presence of oxygen. When the calcination temperature reaches 773 K, all the metal ions may be oxidized into metal oxide with the highest valence, such as Ti 4§(TiO2), Fe 3§(Fe203) and V 5§ (V205), meantime, the agglomeration of some isolated metal oxide may take place. There are at least two kinds of active sites on MSiO2, the isolated M n+ referred to as M (~) and the aggregated oxide clusters referred to as M ([3). According to the correlation between catalytic data and calcination temperature, it can

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化

8O 70 " 9

~Ti-SiO2 -I-Fe-SiO2 -i-V-SiO2

60

_"" 50 ~" 40

2

-O-Ii-SiO2 -I-Fe-SiO2 "-nl~V-SiO2

80 "6 60

m

4o

3o 20 10 0 250

,

,

350

450

,=~ 550

650

20 0 750

Calcination temperature (K) Fig. 1. Activity of styrene epoxidation on M-SiO2 catalysts calcined at different temperatures.

250

350

450

550

650

750

Calcination temperature (K) Fig. 2. Selectivity of styrene epoxidation on M-SiO2 (M=Ti, Fe, V) catalysts calcined at different temperatures.

be assumed that M (a) sites contribute mainly to production of epoxide, while M (13) sites lead mainly to side product. The activity and selectivity decreases significantly with calcination temperature for Ti-SiO2 while the catalytic properties of Fe-SiO2 and V-SiO2 change very differently from Ti-SiO2. This may indicate the different nature of transition metal ions and the less agglomeration of the transition metal oxides in Fe-SiO2 and V-SiO2 compared with Ti-SiO2. No obvious loss of the activity and selectivity is observed after the Ti-SiO2 catalyst is reused five times in epoxidation of styrene. This result confirms that the leaching of transition metal ions from the M-SiO2 catalyst is negligible, at least for the first several times. This may be due to the strong chemical bonding of transition metal ions with the SiOz. 4. CONCLUSION M-SiO2 (M=Ti, Fe, V) catalysts prepared by ion beam implantation show good activity and selectivity for the epoxidation of olefins using dilute H202 (30%) as oxidant. The activity and selectivity can be attributed to the highly isolated transition metal sites on the implanted silica surface. Ti-SiO2 exhibits the highest epoxide selectivity among the catalysts prepared. It is assumed that there are at least two types of active sites, M (tx) (isolated M n§ and M (~) (aggregated metal oxide) on the catalyst, and the formation of epoxide and side product is catalyzed mainly by the two kinds of active sites, respectively.

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5. A C K N O W L E D G E M E N T

The authors would like to thank the State Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology for supplying the instrument for the ion beam implantation. This work was financially supported by the National Nature Science Foundation of China for Distinguished Young Scholars (Grant No. 29625305). REFERENCES

1. D.R.C. Huybrechts, L. De Bruycker and P. A. Jacobs, Nature, 345 (1990) 240. 2. A. Corma, M.A. Camblor, P. Esteve,, A. Martinez and Perez-Pariente, J. Catal., 145 (1994) 151. 3. E. Jorda, A. Tuel, R. Teissier and J. Kervennal, J. Catal., 175 (1998) 93. 4. C. Cativiela, J. M. Frail, J. L. Garcia and J. A. Mayoral, J. Molecular Catal. A, 112 (1996) 259. 5. J. Leon Shohet, IEEE Trans. Plasma Sci., 19 (1991) 725. 6. D.M. Comes and S. L. Kaplan, MRS Bull., (1996) 43. 7. Q.H. Yang, C. Li, S.D. Yuan and Q. Xin, J. Catal., 183 (1999) 128. 8. S.B. Kumar, S. P. Mirajkar, C.G. Godwin, P. Kumar and R. Kumar, J. Catal., 156 (1995) 163. 9. J. Sobczak, J.J. Ziolkowski, J. Mol. Catal., 13 (1981) 11. 10. U. Romano, A. Esposito, F. Maspero, C. Neff and M.G. Clerici, Stud. Surf. Sci. Catal., (1990) 33. 11. J.C. Van der Waal, M.S. Rigutto and H. Van Bekkum, Appl. Catal., A., 167 (1998) 331.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Cyclohexane ring opening on metal-oxide catalysts L.M. Kustov a, T.V. Vasina a, O.V. Masloboishchikova a, E.G. Khelkovskaya-Sergeeva a, and P. Zeuthen b aN.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky prosp. 47, Moscow, 117334 Russia, e-mail" [email protected] bHaldor Topsoe A/S, Nymollevej 55, Lyngby, DK-2800 Denmark Ring opening of cyclohexane on various metal-containing oxide catalysts is studied in detail. Rhodium supported on neutral alumina is shown to exhibit the highest selectivity toward n-hexane, whereas the formation of side products of cracking, isomerization and dehydrogenation (CI-C4 hydrocarbons, isohexanes + methylcyclopentane, and benzene, respectively) is essentially suppressed.

1. INTRODUCTION Dearomatization of gasoline and diesel fuel is a very important problem of oil refining for the nearest future. Hydrogenation of aromatic compounds present in the fuels can be used to transfer aromatics into naphthenes but the latter can be converted again into the aromatic products and soot particulates in the engine. The most appropriate solution of the problem would be ring opening of aromatics and naphthenes into paraffins that cannot be further transformed into undesirable products. In the case of diesel fuel, ring opening occurring without skeletal isomerization can also result in the gain in the cetane number, while ring opening of C 6 - C 9 cyclic components of gasoline with simultaneous skeletal isomerization can be helpful in boosting the octane number of the motor fuel. Cyclohexane may be considered as a suitable model compound for ring opening. It is well known [1, 2] that the main transformations of cyclohexane over metal-supported catalysts involve dehydrogenation into benzene, hydroisomerization into methylcyclopentane and hydrogenolysis or cracking into low-molecular hydrocarbons, such as CI-C4 light products. Thus, although the reaction of cyclohexane ring opening theoretically could produce n-hexane with a 100% probability via single-step hydrogenolysis of one of the C-C bonds, the real pattern is much more complicated and includes the whole range of possible reactions as shown in the Scheme. The formation of n-hexane from cyclohexane was also studied in [3, 4]. The most interesting and commercially viable catalytic system based on l%Pt/H-Beta zeolite was developed by Mobil Company [5]. This catalyst exhibits rather high activity in ring opening of C6 cyclic compounds and is used in a combination with the isomerization catalyst to improve the C6 downstream feed. In this case, however, a 1 : 3 mixture of normal and isoparaffins was formed at conversions of-80% at 230-270~ under high-pressure conditions.

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REACTION SCHEME

H2

H

G

--,,. / C 1 -C5

Another catalyst proposed by the same authors for ring opening of C6 cyclic compounds comprises a Pt/WO3/ZrO2 system [6]. The performance of this catalyst is similar to that of Pt~-Beta zeolite, but the reaction conditions are different. The directions of cyclohexane transformations should be obviously dependent on the nature of the supported metal (Pt, Ru, Rh, Ni, etc.) and on the reaction conditions. Therefore, the goal of this paper was to study the influence of different factors (nature of the metal and loading, type of the carrier, reaction temperature and pressure, space velocity, hydrogen-tohydrocarbon ratio) on ring opening of cyclohexane on oxide catalysts promoted with noble metals. 2. EXPERIMENTAL

Neutral 7-alumina, fluorinated 7-alumina, and silica were used as catalyst supports. Platinum, rhodium, ruthenium, and nickel were supported by incipient-wetness impregnation of the carrier with aqueous solutions of [Pt(NH3)6](HCO3)2, [Rh(NH3)sC1]C12, [Ru(NH3)sC1]C12, and Ni(NO3)2; the metal loading was varied from 0.5 to 3 wt. %. Prior to testing in cyclohexane ring opening, the catalysts were calcined in an air flow at 500~ for 2 h and reduced in hydrogen at 350~ for 2 h. Tests were carried out in a flow catalytic unit at T=210-400~ P = 1-5 MPa, H2 : C6 = 5-20 (vol), VHSV = 2 h -1. 3. RESULTS AND DISCUSSION

The data on the performance of oxide-based catalysts in cyclohexane ring opening under high-pressure conditions are summarized in the table. Rh-containing systems were shown to be the most active and selective catalysts for the ring opening of cyclohexane (Fig. l a). The reaction proceeds at substantially lower temperatures compared to Pt-catalysts and dehydrogenation to benzene is suppressed to a considerable

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Table. Ring opening of cyclohexane on metal-containing oxide catalysts Catalyst

P, MPa

T, ~

Conv. %

S(n-

C6H14), CI-C5 %

l%Pt/A1203

2%Rh]AI203 1%Rh/SiO2 l%Ru/A1203

2 3 2 3 5 5 5 1

3%Ni/AI203

2

1%Pt/A1EOa-F 1%Rh/A1203

400 400 300 280 280 320 210 230 370 400

49.6 41.4 93.9 91.0 55.9 83.0 92.7 29.4 100.0 12.7 45.8

25 42 26 65 88 85 56 53 13 6

0.6 1.2 20.5 31.1 6.9 12.2 40.8 13.4 100 5.3

Yield, wt. % inC6H14 C6HI4 1.6 12.4 3.3 17.5 40.1 24.7 0.6 59.3 49.0 0.3 70.5 51.9 0.3 15.7 . . . 1.7 2.8

MCP

C6H6

2.3 3.7 4.0 -

32.7 15.7 4.6 -

2.2 4.4

8.8 33.3

.

extent. Thus, at 300~ and 3 MPa, the 1%Rh/A1203 catalyst provides the yield of n-hexane 59.3 wt.% at a selectivity of 65%. No skeletal isomerization (isohexanes, methylcyclopentane) or dehydrogenation (C6H6) products were identified under these conditions. The highest n-hexane yield of 70.5 wt.% was achieved on 2%Rh/A1203 (280~ 5 MPa) at the ring opening selectivity of 85%. The 1%Rh/SiO2 catalyst manifested lower activity in cyclohexane conversion (Fig. 1b) as compared to the l%Rh/Al203 system, and the ring opening selectivity on Rh/SiO2 did not exceed 80% at moderate yields of ring opening products at 300~ It should be also noted that the n-hexane yield on the l%Rh/SiO2 catalyst is significantly dependent on the pressure: at low pressures (up to 2 MPa), the yield is below 15%, whereas an increase in the pressure up to 5 MPa causes the enhancement in the activity and the n-hexane yield reaches 52% (320~ The temperature and pressure dependence of the n-hexane yield and selectivity for the 1%Rh/A1203 and 1%Rh/SiO2 catalysts are plotted in Fig. 1a, b. Thus, the best yield of n-hexane (50-70 wt. %) at the selectivity of 65-85% was obtained on Rh/A1203 catalysts. Dehydrogenation and cracking (the main side reactions) were shown to be almost completely suppressed at elevated pressures. Figure 2 shows the effect of the reaction temperature on the yield and selectivity of ring opening products formed on the Rh/A1203 catalyst. An increase in the reaction temperature up to 320~ results in a gradual increase in the yield of n-hexane up to -~35%. However, at 350~ crackingbecomes the predominant pathway and the yield of C1-C5 products reaches 100%. For Pt/A1203 catalysts, the main reaction route in the transformation of cyclohexane is the conversion into benzene. However, formation of aromatic hydrocarbons substantially decreases at high pressures, but still high reaction temperatures are required to provide a reasonable conversion (350-400~ Also, formation of methylcyclopentane is significant on this catalyst. For Pt supported on the neutral A1203 support, the highest n-hexane yield

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reaches 17.5 wt.%. The ring opening selectivity is 50% and n-hexane is the main product (the selectivity is 42%). The fluorination of

90 80 70 60 50

_S (280~ it"

,.

s (300%.)

I-'""

I------

,..--

,,,

I--....

f(300~

9

" ~ i

a

A1203

~

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the

acidity and, as a 40 result, the cracking 3O and isomerization r,j 20 - Y (28o%) processes are predominant over the Pt/A1203-F catalysts. Although the ring 10 20 30 40 50 60 opening process is r~ _ 9 b 80 also intensified, the =" " " S (300~ 970 / yield of n-hexane under the optimum / / S (320%) 60 ring opening 50 conditions (400~ 2 / / I~~Y(320~ 40 MPa) reaches 24.7 wt. %, while the yield 30 of isohexanes is even 20 higher (40 wt. %). It is noteworthy that the conversion on the 10 20 30 40 50 60 fluorinated catalyst is P, atm about two times higher than that on the neutral Pt/A1203. Fig. 1. Influence of the pressure and reaction temperature on the Obviously, this effect cyclohexane ring opening on can be explained by (a) 1%Rh/A1203 and the progressive (b) 1%Rh/SiO2 catalysts contribution of cracking processes at elevated temperatures. The ring opening selectivity to n-hexane + isohexanes is close to 70%, but the yield of isoparaffins is twice that of the normal product. A very similar pattern was observed for Ni/A1203 catalysts, but the contribution of the cracking and hydrogenolysis reactions was even more significant as compared to the Pt/AI203 systems, while the yield of ring opening products did not exceed 3 wt. %. For the bimetallic Pt-Rh/AI203 catalysts with Pt : Rh ratios varying from 25 : 75 to 75 : 25, the yield of n-hexane is higher than on the monometallic Pt/AI203 system, but the catalyst requires higher reaction temperatures as compared with the monometallic Rh/AI203 catalyst to provide about the same yields of ring opening products. r,

I

,

I

,

I

,

I

i

I

,

I

,

I

,

o

I

,

I

,

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,

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,

I

,,-

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1 0 0

-

o

80-

n-C6H14 -t-i-C6H 14 -~ C1-C5 o S n-C6H14

"~ 60-~ >. 1

4020-

0

i

220

i

- -

260

300

1

340

T~ Fig. 2. Temperature dependences of the yields of main products in ring opening of cyclohexane on 1%Rh/A1203 The conversion of cyclohexane on a Ru/A1203 catalyst is rather high (up to 30%) at as low temperatures as 200-210~ (Fig. 3), i.e. at temperatures when both Pt- and Rh-catalysts are inactive. However, at temperatures above 230~ the Ru-containing catalysts provide exclusively the formation of the cracking products, and the temperature interval favorable for ring opening on Ru/A1203 is very narrow. The highest yield of n-hexane on this catalyst (about 12%) is achieved at-260~ Ruthenium supported on fluorinated A1203 is more active in ring opening as compared to the non-fluorinated system: the maximum in the formation of ring opening products on the acidic catalyst is attained at about 200~ and the yield of n-hexane approaches 22%.

I00

-

80- * - - conversion % - I - C1-C5

60tl) >-

40-

-" n-C6H14 o i-C6H14

20-

0~180

220

260

300

T~ Fig. 3. Performance of the 1%Ru/AI203 catalyst in cyclohexane ring opening.

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Thus, cyclohexane transformations on noble metals supported on oxide carriers include the following processes in agreement with the scheme shown above: (1) dehydrogenation to benzene; (2) cracking and hydrogenolysis of cyclohexane and isomerization products leading to light products; (3) ring opening of cyclohexane (and MCP) resulting in the formation of n-hexane (and/or isohexanes); (4) simultaneous skeletal isomerization of n-hexane into methylpentanes and dimethylbutanes; (5) isomerization of cyclohexane to methylcyclopentane. Let us analyze this scheme with the purpose to find the factors favorable for the formation of ring-opening products, preferentially n-hexane. In addition to the nature of the metal and the carrier, as was discussed above, a very important factor that influences the reaction pattem is the reaction temperature. There must be an optimum on the dependence of the activity in ring opening versus temperature because of the interplay of the kinetic and thermodynamic factors. With decreasing temperature, the contribution of cracking and dehydrogenation processes should be suppressed. The value of the optimum temperature is obviously a function of the carrier and metal nature. Another factor affecting the catalyst performance is the hydrocarbon-to-hydrogen ratio. It is quite clear that this ratio should be high enough in order to prevent dehydrogenation of cyclohexane. On the other hand, very high hydrogen-to-substrate ratios are favorable for the occurrence of hydrogenolysis reactions. Finally, the reaction pressure seems to be a key factor determining the product distribution and other important characteristics. Indeed, by increasing the pressure, we can suppress both dehydrogenation and cracking and shift the equilibrium toward C6 paraffins. 4. CONCLUSIONS

The data obtained show that noble metals supported on oxides are efficient catalysts for ring opening of cyclic hydrocarbons. Cyclohexane can be converted with a high selectivity (up to 80-85%) into n-hexane. The use of acidic carriers causes the formation of isomeric ring-opening products and methylcyclopentane. The reaction temperature and pressure are essential parameters determining the catalyst performance. REFERENCES

1. G. A. Somorjai, Introduction to Surface Chemistry and Catalysis, New York, Wiley, 1994. 2. Y. L. Lam, J. H. Sinfelt, J. Catal., 42 (1976) 319. 3. L. Liberman, O. V. Bragin, B. A. Kazansky, Dokl. Akad. Nauk, SSSR, 156 (1964) 5. 4. Ger. Patent No. 2 127 624 (1971). 5. US Patent No. 5 382 730 (1995). 6. US Patent No. 5 382 731 (1995).

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Olefins from Chlorocarbons" Reactions of 1,2-Dichloroethane Catalyzed by Pt-Cu Lalith S. Vadlamannati, David R. Luebke, Vladimir I. Kovalchuk and Julie L. d'Itri ~ Department of Chemical Engineering, University of Pittsburgh, Pittsburgh, PA 15261 The effect of (Pt+Cu)/SiO2 catalyst composition on the activity and selectivity in the reaction of 1,2-dichloroethane dechlorination in a H2 containing atmosphere has been investigated. For monometallic Pt catalysts and those with Pt/Cu atomic ratio >_ 1, the reaction products are primarily ethane and monochloroethane. However, decreasing the Pt/Cu ratio increases the selectivity towards ethylene. A selectivity toward ethylene of nearly 90% is obtained for catalysts with Pt/Cu ratio < 1/3. Infrared investigations of CO adsorption indicate that Pt and Cu form bimetallic particles during pretreatment and that besides the dilution effect, Cu changes electronic properties of Pt atoms. 1. INTRODUCTION There is an increasing demand for technology that will convert chlorocarbons as byproducts of industrial processes into more useful or environmentally benign products. Hydrodechlorination of chlorocarbons to form paraffins has been widely investigated [ 1]; however, there are economic incentives to produce olefins rather than paraffins. To this end, bimetallic catalysts offer great potential. Addition of a second metal to a noble metal has been shown to significantly alter catalytic performance [2,3-6]. A possible explanation for this behavior is a change in the electronic properties of each metal component because of alloying. In addition, the simple dilution of one metal by another that occurs during alloying may effect catalytic performance by changing the geometry of active sites [7]. Alteration of either the electronic or geometric properties of an active site can result in changes in adsorption energetics of reactants and reaction products resulting in higher activity [8] or suppression of side reactions resulting in higher selectivity [9,10]. While abundant literature is available for dehydrogenation reactions catalyzed by bimetallic catalysts, there are relatively very few studies which deal with bimetallic catalysts in dechlorination reactions [ 11 ]. Earlier work has shown that carbon supported Pt-Cu catalyzes the conversion of 1,2-dichloroethane in excess H2 to ethylene with a selectivity greater than 90% [12]. However, the influence of the Pt/Cu atomic ratio on product selectivity in the dechlorination reaction selectivity is not yet well understood. The work here presents results from a combination of a kinetics and IR investigations to understand the effect of Cu on Pt's activity and selectivity towards the dechlorination of 1,2dichloroethane over (Pt+Cu)/SiO2 catalysts.

"Financial support from the Department of Energy- Basic Energy Sciences (DE-FG02-95ER14539)and The Dow Chemical Companyare gratefully acknowledged.

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2. EXPERIMENTAL

2.1. Catalyst Preparation and Characterization The catalysts were prepared by pore volume co-impregnation of SiO2 (Aldrich) by aqueous solutions of H2PtCI6"6H20 (Alfa, 99.9%) and CuC12"2H20 (MCB, 99.5%) followed by drying at 100~ for 2 h in the dynamic vacuum of ~25 Torr. The catalyst nomenclature is defined according to the Pt to Cu ratio. For example, a catalyst with Pt to Cu ratio of 1:3 is referred to as Ptl Cu3. CO chemisorption measurements were carried out using a volumetric sorption analyzer ASAP 2010 Chemi (Micromeritics | to determine the fraction of Pt atoms exposed, as described elsewhere [12].

2.2. Catalytic experiments The CH2C1CH2CI dechlorination was performed on catalysts reduced with H z at 493 K in a quartz microreactor at 473 K and atmospheric pressure. The reaction mixture consisted of 7,000 ppm CH2C1CH2C1, 36,600 ppm H2 and He as the balance. Details of the reaction system have been described elsewhere [12]. The activity and selectivity are reported as a function of time on stream (TOS) and at steady state with the conversion maintained in the range of 1.0-2.5%.

2.3. Infrared experiments The infrared spectra were recorded on a Mattson Research Series II FTIR spectrometer equipped with a MCT detector with a nominal resolution of 4 cm 1. The CO adsorption investigations were performed at room temperature in a conventional vacuum apparatus equipped with a standard IR cell and two leak valves to introduce 12CO (PraxAir, 99.99%) and ~3CO (Isotec Inc., 98%). Both gas lines connected to the leak valves had a trap cooled with liquid N2 to condense moisture and other high-boiling gaseous contaminants. No other purification was used. The catalyst wafers for the IR transmission experiments were prepared by pressing the powder (< 60 mesh) at a pressure of 15,000 pounds for 3 min using a hydraulic press. A typical wafer weighed about-~20 mg/cm 2. In-situ activation of the sample included drying under dynamic vacuum at 403 K for 1 h and reduction in 100 Torr of H 2 at 673 K for 1.5 h followed by evacuation at 673 K for 1 h and cooling down to room temperature. 3. RESULTS

3.1. Catalyst dispersion Based on the CO chemisorption measurements, 60% of the Pt atoms were exposed with the reduced silica-supported monometallic Pt catalyst. The bimetallic PtlCul and PtCu2 had Pt dispersions of N35%, and for the other PtCu catalysts that had a lower Pt/Cu atomic ratio the dispersion was approximately 10%, independent of the catalyst composition.

3.2. Kinetics Experiments Both monometallic Pt/SiO2 and bimetallic (Pt+Cu)/SiO2 catalysts were active for the dechlorination of 1,2 dichloroethane at 473 K to form ethane and ethylene (Table 1). However, under the same conditions the CH2C1-CH2C1 conversion for Cu/SiO2 was below the detection limit (0.1%). For each catalyst the conversion dropped by 1-2% during the first 0.5 h of reaction after which the rate decreased -~0.05% every 5 h (Fig. 1, 2). All of the catalysts containing Pt exhibited similar steady-state TOFs, based on CO chemisorption (Table 1). The highest TOFs were observed for Pt and PtlCul catalysts.

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Table 1.

Catalyst

235 Steady state kinetics parameters of (Pt+Cu)/SiO2 catalysts for the hydrodechlorination of 1,2-dichloroethane TOS for ste-

Conversion

ady state, h

%

C2H4

C2I-I6 C2H5C1

20 21 37 42 32 20 60 -

2.7 1.9 2.0 1.3 1.8 1.6 1.6 0

0 0 0 67.1 86.5 96.2 93.7 -

82.8 87.6 92.2 32.9 13.5 3.8 6.3 -

Pt PtlCul PtlCu2 PtlCu3 PtlCu6 PtlCu9 PtlCul8 Cu

Selectivity, mole %

17.2 12.4 7.8 0 0 0 0 -

Activitya

TOF xl03,

sl 60.9 31.2 10.0 4.8 4.5 4.7 5.4 0

4.0 3.5 11 1.9 1.8 1.8 21

anmol e S"1 geat"1.

The catalyst selectivity strongly depended on Pt/Cu atomic ratio (Table 1). For the monometallic Pt/SiO2 the major product was ethane (-~83%) and the second major product was monochloroethane (-~17%). With Ptl Cu 1, ethane and monochloroethane were also the major products, with an ethane selectivity o f - 8 8 % and a chloroethane selectivity of-~12%. The characteristic feature for the catalysts with a Pt/Cu atomic ratio _> 1/2 is the absence of ethylene in the reaction products. However, as the Cu content increased the selectivity towards ethylene increased considerably, ranging from 67% on PtlCu3 to above 93% on Ptl Cu 18 (Table 1). It is worth noting that the catalysts with Pt/Cu ratio < 1/2 did not produce monochloroethane. Thus, there is a marked difference in the selectivity of the monometallic Pt catalyst, bimetallic Pt/Cu catalysts with atomic ratio _ 1/2 and catalysts with Pt/Cu < 1/2.

3.3. Infrared experiments The infrared spectra of CO adsorbed on Pt/SiO2 and PtlCu3/SiO2 catalysts at saturation coverage are shown in Fig. 3 as spectrum 1 and 2, respectively. In spectrum 1 the band at 2078 cm 1 is characteristic of linearly adsorbed 12CO on Pt ~ [13]. The bands at 2130 and 2031 cm -1 in spectruim 2 have been assigned to linearly adsorbed CO on Cu ~ and Pt ~ respectively [13]. As the v(CO) adsorbed on Cu ~ and Cu 1+ are very close, the band assignment was confirmed by the fact that the band at 2130 cm 1 disappeared when gas phase CO was evacuated. Unlike CO-Cu ~ the CO-Cu 1§ adsorption complexes are known to be stable and do not decompose upon evacuation at room temperature [ 14]. The position of absorption band of 12CO on Cu ~ for all of the catalysts was independent of the composition of gaseous ~2CO+~3CO mixture used in the adsorption experiments and was the same as for the Cu/SiO2 catalyst. The frequency of 12CO vibration on Pt for Pt/SiO2 catalyst shifted from 2078 to 2052 cm 1 when the 12CO concentration of the 12CO+~3CO mixture was decreased from 100 to 0%, but remained almost constant at around 2030 cm 1 for the Ptl Cu3/SiO2 catalyst (Fig. 4). It is noteworthy, that the v(12CO) band of CO adsorbed on Pt for the Ptl Cu2/SiO2 catalyst was close to that of Pt/SiO2 for pure lZCO and to that for the Ptl Cu3/SiO: catalyst at infinite dilution of 12CO with 13CO (Fig. 4).

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100

6 5

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30

Fig. 2. Selectivity vs. time on stream for Ptl Cu3/SiO2 at 200~ 1 -ethane, 2 - ethylene, 3 - conversion.

4. DISCUSSION 4.1. N a t u r e o f active sites in ( P t + C u ) / S i O 2 catalysts

Through the analysis of the performance of (Pt+Cu)/SiO2 catalysts the nature of active sites for the hydrodechlorination of 1,2-dichloroethane has been clarified. Even though C-CI bond scission in vicinal dichlorohydrocarbons occurs readily on a Cu surface to form an olefin [15,16], silica-supported Cu did not show dechlorination activity at 200~ (Table 1). This is most likely because dissociative H2 chemisorption is activated on Cu surfaces [17]. A slower rate of H2 dissociation on a Cu surface than the rate of C-C1 bond cleavage of a halocarbon would result in a Cu surface covered with C1 atoms. Thus, the catalytic activity is extensively suppressed due to poisoning by C1. However, the active sites for 1,2-dichloroethane hydrodechlorination cannot consist of solely Pt atoms. Monometallic Pt catalyst is unselective toward ethylene. Only the mixed PtCu sites in (Pt+Cu)/SiO2 catalysts form ethylene during 1,2-dichloroethane hydrodechlorination.

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The catalytic activity of (Pt+Cu)/SiO 2 per gram of catalyst does not depend on Cu content for samples with Pt/Cu atomic ratio _< 1/3 (Table 1). These catalysts contain the same fraction of Pt atoms exposed. Even though the fraction of Cu atoms exposed in the catalysts is unknown, it is reasonable to conclude that the catalysts with higher Cu loading contain larger number of exposed Cu atoms. Therefore, even if some elementary steps take place solely on Cu, the step which determines the overall rate of 1,2-dichloroethane dechlorination would occur on Pt. To this end, the calculation of TOFs of (Pt+Cu)/SiO2 catalysts on the basis of the fraction of Pt atoms exposed appears to be quite reasonable.

4.2. Origin of (Pt+Cu)/SiO 2 catalysts' selectivity toward ethylene Platinum itself is a very active and stable catalyst for halocarbon hydrodehalogenation to form paraffins (Table 1, [4]). However, the key question to be answered is why this reaction does not proceed on Pt sites of (Pt+Cu)/SiO2 catalysts? It is known that the addition of a second metal to a Group VIII noble metal alters the catalytic properties in a number of hydrocarbon reactions [7]. Such results are commonly interpreted in terms of changes in the electronic properties of active metal or in the geometry of active sites [7]. Differentiating between the individual impact of electronic and geometric effects on the overall performance remains a challenge [7]. CO chemisorption is a widely used tool to probe electronic structure of metals because v(CO) is very sensitive to the electron density of the adsorption sites [13]. Previous studies by Ponec et al. showed that for PtCu catalysts an electronic effect, if it existed at all, was below the detection limit [18]. However, the results of the present investigation provide evidence that an electronic interaction between Pt and Cu in the (Pt+Cu)/SiO2 catalysts may exist. The singleton frequencies of CO adsorbed on Pt determined by isotopic dilution method [18] were 2052 cm -~ for Pt and 2030 cm ~ for PtlCul and PtlCu3 catalysts (Fig. 4). While the PtlCul catalyst is not selective toward ethylene in 1,2-dichloroethane hydrodechlorination, the ethylene selectivity of the Ptl Cu3 catalyst is 67% (Table 1). As the singleton frequency of the Ptl Cul and Ptl Cu3 are the same, the specific catalytic behavior of (Pt+Cu)/SiO2 catalysts in halocarbon dechlorination reaction is probably not caused by an electronic structure change of Pt. More likely, the higher selectivity of catalysts with a Pt/Cu atomic ratio _< 1/3 results from the dilution of Pt ensembles and particular elementary steps that occur only on Cu. Indeed, results presented in Fig. 4 suggest that the geometry of Pt sites in PtlCul and PtlCu3 is quite different. The first two catalysts have similarv(~2CO), around 2080 cm ~ when pure ~2CO is adsorbed, but the band position of adsorbed ~2CO shifts to lower wavenumbers as 12CO is diluted with ~3CO reaching a value characteristic of the PtlCu3 catalyst at infinite dilution (Fig. 4). This decrease in the v(12CO) is due to elimination of dipole-dipole coupling between 12CO molecules as the concentration of ~3CO increases [19]. However, no change in v(~2CO) occurs when the composition of 12co+i3co mixture is varied for the PtlCu3 catalyst (Fig. 4). Thus, if Pt ensembles are relatively large in the PtlCul catalyst, they consist of practically isolated Pt atoms in the Ptl Cu3 catalyst. Hence, the dramatic difference in selectivity toward ethylene between the catalysts with Pt/Cu atomic ratio < 1/3 and _> 1/3 can be understood in terms of the C-CI bond cleavage being size demanding elementary step [2,20]. If hydrogenolysis of C-CI bond requires larger Pt ensembles, the high selectivity toward ethylene of PtCu catalysts with Pt/Cu atomic ratio < 1/3 must be due to deficiency of sufficiently large Pt islands for the bond scission. In this case the role of Cu is to catalyze the C-C1 bond dissociation of adsorbed 1,2-dichloroethane to

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form surface -CH2-CH2" species that subsequently desorbs as ethylene. This hypothesis is consistent with the fact that the heat of ethylene adsorption on Cu is much less that that on Pt (18 kcal/mol [21 ] and 22-30 kcal/mol [22], respectively). Thus, if ethylene forms on Cu, it is more likely to desorb than hydrogenate, given also that Cu is not a good hydrogenation catalyst. Pt is necessary to provide a source of dissociating hydrogen which upon spilling over from Pt sites reduces surface CuC1 species to form HC1 regenerating catalytically active sites. CONCLUSION

The addition of Cu to Pt supported catalyst alters its selectivity towards ethylene in the dechlorination of 1,2-dichloroethane in the presence of hydrogen. Results obtained from infrared investigations using isotopic mixtures of nCO and ~3CO suggest that Cu has both a geometric and an electronic influence on Pt. Of the two effects, dilution of Pt ensembles by Cu appears to dominate in terms of the selectivity behavior of the bimetallic catalysts. A size demanding C-C1 bond scission is consistent with the experimental observations. As well, it is possible that Pt acts only as a source of dissociated hydrogen while dissociation of C-C1 bond followed by the desorption of ethylene occurs on Cu. REFERENCES

1. 2. 3. 4. 5.

D.I. Kim and D. T. Allen, Ind. Eng. Chem. Res., 36 (1997) 3019 and references therein. P. Bodnariuk, B. Coq, G. Ferrat, and F. Figueras, J. Catal., 116 (1989) 459. J.H. Sinfelt, Adv. Catal., 23 (1973) 91. J.S. Campbell and C. Kemball, Trans. Faraday Soc., 57 (1961) 809. J.J. Burton and R. L. Garten, in: Adv. Mater. Catal. (Burton, J.J. and Garten, R.L., Eds.), Academic Press, New York, 1977. P. 33. 6. Yu. I. Yermakov, B. N. Kuznetsov, and V. A. Zakharov, Catalysis by Supported Complexes, Elsevier, Amsterdam, 1981. 7. W . M . H . Sachtler and R. A. van Santen, Adv. Catal., 26 (1977) 69. 8. C.H.F. Peden and D. W. Goodman, J. Catal., 104 (1987) 347. 9. F.M. Dautzenberg, J. N. Helle, P. Biloen, and W. M. H. Sachtler, J. Catal., 63 (1980) 119. 10. J. L. Carter, G. B. McVicker, J. Weissman, W. S. Kmak, and J. H. Sinfelt, Appl. Catal., 3 (1982) 327. 11. B. Heinrichs, P. Delhez, J. Schoebrechts, and J. Pirard, J. Catal., 172 (1997) 322. 12. L. S. Vadlamannati, V. I. Kovalchuk, and J. L. d'Itri, Catal. Letters, 58 (1999) 173. 13. N. Sheppard and T. T. Nguyen, Adv. in Infrared and Raman Spectr., 5 (1978) 67. 14. A. Dandekar and M. A. Vannice, J. Catal., 178 (1998) 621 and references therein. 15. M. X. Yang, S. Sarcar, and B. E. Bent, Langmuir, 13 (1997) 229. 16. W. K. Walter, R. G. Jones, K. S. Waugh, and S. Bailey, Catal. Lett. 24 (1994) 333. 17. J. R. Anderson and N. R. Avery, Catal., 5 (1966) 446. 18. F. Stoop, F. J. C. M. Toolenaar, and V. Ponec, J. C. S. Chem. Comm. (1981) 1024. 19. M. Primet, J. Catal., 88 (1984) 273 and references therein. 20. P. Fouilloux, G. Cordier, and Y. Colleuille, Stud. Surf. Sci. Catal., 11 (1982) 369. 21. B. M. W. Trapnell, Proc. Soc. London A, 218 (1953) 566. 22. F. B. Passos, M. Schmal, and M. A. Vannice, J. Catal., 160 (1996) 118.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Artificial Control of Selectivity by D y n a m i c Lattice D i s p l a c e m e n t of A c o u s t i c Wave Effects: D e c o m p o s i t i o n and Oxidation of A l c o h o l on Ag and Pd Catalysts N. Saito, H. Nishiyo_ma, K. Sato, and Y. Inoue* Department of Chemistry, Nagaoka University of Technology, Nagaoka 940-2188 Japan A poled ferroelectric z-cut LiNbO3 crystal was employed as a support, on which 100 nm Ag or Pd thin film metal catalyst was deposited, and the effects of resonance oscillation (RO) generated by a piezoelectric effect on the selectivity in the decomposition and the oxidation of alcohol have been examined. For ethanol decomposition of a Ag catalyst, RO increased the catalytic activity for ethylene production with little enhancement of acetaldehyde production, thus causing a significant increase in the ethylene production selectivity. For the same reaction on a Pd catalyst, RO increased the ethylene production activity remarkably. The selectivity changes were also observed for methanol oxidation on a Pd catalyst, in which the selectivity for formaldehyde production increased with a decrease in the selectivity for methyl formate production. A laser Doppler method showed that the lattice displacement vertical to the surface had an irregular pattern of standing waves up to 40 nm at 2 W. The mechanism of selectivity changes with the RO effects is discussed. 1. INTRODUCTION The design of a metal catalyst with artificially controllable functions for chemical reactions has been among interesting issues in heterogeneous catalysis, but only a few studies have been done so far. Since the catalysis is governed by the arrangement of surface metal atoms and their electron densities, a promising approach could be artificial changes in these factors. We have employed acoustic wave effects which are generated on catalyst surfaces by a piezoelectric phenomenon of a ferroelectric crystal with spontaneous polarization. We have demonstrated that Rayleigh and shear horizontal leaky surface acoustic wave generated by radio frequency electric power have time-dependent lattice displacement and are useful to activate Pd, Ni, and Ag thin film catalysts deposited on the propagation path: the activity for ethanol oxidation of Pd increased several folds[ 1-4]. Recently, we have used resonance oscillation (RO) which is a bulk acoustic wave

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generated at a definite frequency. In a poled polycrystaUine ferroelectric Pbo.9~Sro.o5 Z~o.saTio.47 Oa (PSZT) substrate, the RO mode had significant effects on catalytic activity for ethanol oxidation of a Pd thin film catalyst [5]. In a single crystal of zcut lithium niobate, the RO gave rise to nearly 1900 fold enhancement of the catalytic activity for the same reaction on Pd [6]. It is strongly desirable to reveal the RO effects on the reaction selectivity for various kinds of catalytic reactions on different metals. In the present study, the RO effects were examined on selectivity changes in the production of ethylene and acetaldehyde in the ethanol decomposition over a Ag and a Pd catalyst and furthermore in the production of formaldehyde, methyl formate and carbon dioxide in the methanol oxidation over a Pd catalyst. Lattice displacement caused by RO was investigated by a laser Doppler method. 2. E X P E R I M E N T A L S Figure 1 shows the structure of a RO-generating catalyst. A poled ferroelectric single crystal of z-cut LiNbO3 (referred to as z-LN) with 1.0 m m in thickness was employed as a substrate. This crystal has a polarization axis normal to the surface, thus exposing a positively polarized surface at one plane and a negatively polarized surface at the other. Both positive and negative planes were covered with a catalytically active Ag or Pd thin film which was deposited at a thickness of 100 nm by resistance-heating of a pure metal in high vacuum. Thus fabricated catalysts are referred here to as Ag/z-LN and Pd/z-LN, respectively. Radio frequency (rf) electric power was generated from a network analyzer (Anritsu 3603B), amplified (Kalmus, AS225LC ), and then introduced to a catalyst after impedance adjustment. Catalyst temperature was monitored by two methods: one was to measure a surface temperature by a radiation thermometer (JSC,TSS-5N) through a BaF2 window of a reaction cell in which a catalyst was placed and the other was to detect the shift of resonance frequency, a method which made use of the linear relationship between the frequency shift and ferroelectric sample temperature change. The temperature was controlled by an outer electric furnace so as to m a i n t a i n the same temperature during catalytic reactions irrespective of presence and absence of RO. The decomposition of ethanol and the oxidation of methanol were carried Metal out i n a gas-circulating apparatus, fiii!:i:i:i:i:!:i:i:i:i:iiiii:i:i:!:iiiiiii:!:i:il Positive plane and the products were analyzed by a ~ ~. | Ferroelectric gas chromatograph connected to the ~" (~ z-cut LiNbO 3 reaction system. The selectivity for one component product, i, Si is ~ t!!i!iiiiii!!!i!!iiii!!!!!!!ii!ii!!!!!!!!i!!ii~!t Negative plane Metal defined as the ratio of i production rate to the sum of rates of Fig. 1 Structure of a RO-generating catalyst constituent products, t h a t is, S~ = Metal : Ag,Pd Vi / EVj x 100.

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Lattice displacement vertical to the surface was monitored with a laser Doppler method: a catalyst surface was irradiated by a He-Ne laser beam, and a reflected beam signal was received by a laser vibrometer ( Ono sokki LV-1300)[7]. 3. R E S U L T S

Figure 2 shows the detailed characteristics of the first resonance line of z-cut LN. Impedance, admittance and capacitance provided the s a m e frequency of 3.5 MHz. Thus, this frequency was defined as resonance frequency and used for the experiments. In ethanol decomposition on a Ag catalyst, the gaseous products were ethylene and acetaldehyde which were produced in nearly proportion to reaction time. Upon the introduction of a RO power of 3 W at 3.5 MHz, the ethylene production increased immediately, and the increased activity was m a i n t a i n e d until the power was turned off. Figure 3 shows the activity of ethylene and acetaldehyde production at 633 K. Without RO, the activity of ethylene production was 3.5 times larger t h a n t h a t of acetaldehyde production. By the generation of RO at 3 W, the ethylene production activity increased by a factor of 2.4, whereas acetaldehyde production activity remained nearly unchanged. The value of Se for ethylene production was 78% without RO and increased to 88 % with RO-on. The activity for ethylene production increased in proportion to rf power raised to the power of 2.8. Figure 4 shows the ethanol decomposition on a Pd catalyst at 573 K. Ethylene and acetaldehyde were produced in the gas phase: the production of acetaldehyde

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Fig.3 Effects of RO on catalytic activity for ethanol decompositionover a Ag catalyst RO power J=3W, f=3.5MHz, T=633K

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Fig.5 Effects of RO on selectivity of methanol oxidation over a Pd catalyst

RO power J=3W, f=-3.5MHz, T=573K

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was remarkably larger than t h a t of ethylene production. With RO-on, the activity of ethylene production increased by a factor of 70, whereas that for acetaldehyde remained with a small extent ( 5 0 % ) enhancement. The activation energy for ethylene production decreased from 264 to 0 kJmol 1 with RO-on. Figure 5 shows the RO effects on the selectivity of methanol oxidation over a Pd catalyst. Major products were three components of methyl formate formaldehyde and carbon dioxide. The generation of RO increased the activity for the three products, but activation efficiency was different among the products. 50 The RO decreased the selectivity for methyl formate production from 45 to 31% which was compensated by a selectivity 30 increase in the formaldehyde production from 29 to 43%. Figure 6 shows a line profile of lattice displacement of a Pd/z.~ lO LN catalyst which was measured by a laser Doppler 0 method. The pattern obtained at 0 2000 4000 6000 8000 a power of 2 W was composed of Distance/lam an irregular form of standing Fig.6 Lattice displacement measured by waves. The highest magnitude laser Doppler method reached as high as 40 nm. RO power J=2W, f=3.5MHz

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4. DISCUSSION

The resonance line calculation on the basis of a piezoelectric equation and the material constants of z-cut LN showed that a resonance line appeared at 3.4 MHz by assuming that the vibration has a thickness extensional mode [8]. There was good agreement between the observed and calculated frequency, which indicates that the vibration has thickness extensional mode. As shown in Fig. 6, laser Doppler measurements showed that the RO generated large thickness mode vibrations vertical to the surface which were composed of standing waves with different magnitudes. The vibration pattern is characteristic of thickness extensional mode vibration. This gives experimental evidence to thickness extensional mode RO. The RO effect on ethanol decomposition over a Ag catalyst was the promotion of ethylene production without significant influences on acetaldehyde production. The selectivity for ethylene production at 633 K increased from 78% to 88 % with a power of 3 W. The same reaction on a Pd catalyst showed that the RO effect was a remarkable increase in ethylene production activity with a slight enhancement of acetaldehyde production. For methanol oxidation on a Pd catalyst, RO increased the selectivity for formaldehyde production with a decrease in methyl formate production. Note that the selectivity changes were observed for different metal catalysts and different reactions, which demonstrates that RO has a high capability to control reaction selectivity of heterogeneous catalysts. In previous measurements of contact potential difference, it has been shown that the thickness extensional mode RO of a Pd/z-LN catalyst caused the appearance of opposite charges with respect to the polarized surfaces [9]. This indicates that the direction of spontaneous polarization axis (Fig. 1) of z-LN plays an important role in the movement of electrons, thus suggesting that electron enriched and deficient states are generated on the positive and negative surfaces, respectively. Our recent study using x-cut LiNbO3 instead of z-cut LN has shown that this ferroelectric crystal generates extensional shear mode RO, which is characterized by lattice displacement parallel to the surface, and causes little increase in the catalytic activity of a deposited Ag catalyst for ethanol decomposition [10]. This different effects between thickness and shear mode vibrations have shown that lattice displacement vertical to the surface is important for catalyst activation and selectivity changes. Furthermore, a study using low energy photoelectron spectroscopy for a polycrystalline PSZT ferroelectric substrate has shown that electron emission from the surface of a Ag/PSZT catalyst by UV irradiation is influenced by the thickness extensional mode RO of PSZT: for a positive PSZT plane, a threshold energy, which is a work function-relating parameter, shifted monotonously to lower energy side with increasing rf power and decreased by 0.12 eV at a power of 3 W [11]. Since work function is an important factor of determining the interactions between the adsorbed species and metal surfaces [12], it is evident that its changes with the RO have a significant effect on the adsorption of reactants

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and their reactivity. This effects are likely to exist for the thickness extensional RO of z-LN. In fact, in a kinetic study on ethanol oxidation over a Pd/z-LN catalyst, the RO was found to cause a remarkable decrease in the activation energy and changes in the reaction order with respect to oxygen pressure from 0.5 to -0.1 and ethanol pressure from 0.2 to 1.0 [9]. These results demonstrate two things. One is that the changes in the kinetic parameters are the results of the RO-induced work function changes which possibly occur for z-LN analogous to PSZT. The other is that the RO induces the stronger adsorption of oxygen and weaker adsorption of ethanol. Provided that similar situation holds for ethanol decomposition on Ag and Pd catalysts, the stronger interactions with oxygen are invoked by the RO to such an extent to which the abstraction of oxygen from the ethanol is accelerated. This leads to preferable promotion of dehydration reactions. For methanol oxidation on a Pd catalyst, a mechanism is considered in which methyl formate and formaldehyde are produced via hydrogen abstraction by an adsorbed oxygen from two adjacent adsorbed methanol molecules. It is plausible that the strongly adsorbed oxygen promotes hydrogen abstraction from the CH2 group of two adjacent adsorbed methanol and hence enhances the selectivity for formaldehyde production. In conclusions, the present study showed that RO, characterized by large dynamic lattice displacement vertical to the surface, has a remarkable effect on catalytic reactions and permits to artificially control the selectivity. REFERENCES

1. Y. Inoue, M. Matsukawa, and K. Sato, J. Am. Chem. Soc., 111 (1989) 8965. 2. Y. Inoue, M. Matsukawa, and IC Sato, J. Phys. Chem., 96 (1992) 2222. 3. Y. Inoue, Y. Watanabe, and T. Noguchi, J. Phys. Chem., 99 (1995) 9898. 4. H. Nishiyama, M. Shima, N. Saito, Y. Watanabe, and Y. Inoue, Faraday Discussion 107 (1997)435 5. Y. Ohkawara, N. Saito, and Y. Inoue, Surf. Sci., 357/358 (1996) 777. 6. N. Saito, Y. Ohkawara, Y. Watanabe, and Y. Inoue, Appl. Surf. Sci., 121/122 (1997) 343 7. N. Saito, H. Nishiyama, K. Sato, and Y. Inoue, Chem. Phys. Lett., 297 (1998) 72. 8. T. Ikeda, "Fund_~__m__entals of Piezoelectricity" Oxford Univ. Press. (1990) p 117. 9. N. Saito, I~ Sato, and Y. Inoue, Surf. Sci.,417 (1998) 384. 10. N. Saito, N. Nishiyama, K. Sato, and Y. Inoue, submitted to Chem. Phys. Lett.. 11. Y. Ohkawara, N. Saito, K. Sato, and Y. Inoue, Chem. Phys. Lett.,286 (1998) 502. 12. R. I. Masel, "Principles of Adsorption and Reaction on Solid Surfaces (John Wiley, New York, 1996) p 181.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

The Effect Of Hydrogen Concentration On Propyne Hydrogenation Over A Carbon Supported Palladium Catalyst Studied Under Continuous Flow Conditions D. Lennon a* , R. Marshall a, G. Webb a and S.D. Jackson b aDepartment of Chemistry, University of Glasgow, Glasgow, G12 8QQ, Scotland. bIcI Synetix, PO Box l, Belasis Avenue, Billingham, Cleveland TS23 1LB, England. The rate of propyne hydrogenation at 298K is shown to be critically dependent on hydrogen concentration, with a discontinuity apparent when the hydrogen:propyne concentration exceeds 6:1. This step change is attributed to hydrogenation of some of the hydrocarbonaceous overlayer present on the working catalyst that exposes metal sites, which are then active for dissociative adsorption of hydrogen. 1.

INTRODUCTION The vast majority of work on the catalytic hydrogenation of alkynes has concentrated on ethyne hydrogenation [1-3], with surprisingly few studies examining higher molecular weight alkynes [4,5]. This study examines propyne hydrogenation over a carbon supportrt palladium catalyst and concentrates on how catalyst activity and selectivity is affected by hydrogen concentration in the reactant stream. Previous work by this group on silica supported Pt [6] and Pd [7] catalysts performed under pulse-flow conditions clearly demonstrates the importance of hydrogen concentration on affecting catalytic performance. This investigation is performed under continuous flow conditions, which more closely mimic the conditions encountered in an industrial environment, and aims to define the reagent concentrations that favour propene selectivity. 2.

EXPERIMENTAL The catalyst containing 1.0% palladium was prepared by incipient wetting of carbon with palladium nitrate. BET analysis revealed the catalyst to have a surface area of 1118 m 2 g-I and a pore volume of 0.56 cm 3 g-l. Carbon monoxide chemisorption was performed on a volumetric chemisorption apparatus and revealed 6.2 x 1018 surface Pd a t o m s g-i (cat), assuming a Pd:CO stoichiometry of 2 : 1 [4], which corresponds to a metal dispersion of 11.0 %. Samples of the catalyst were examined by transmission electron microscopy using a Joel 1200 FX electron microscope where a mean metal particle size of c a . 10 nm was observed. Prior to reaction testing, catalyst pellets were crushed and sieved to a grain size of 250 500 l.tm. Approximately 5 mg of the catalyst was packed into the 2 mm internal diameter stainless steel reactor that was housed in a programmable furnace. Mass flow controllers (Brooks 5850S) ensured reproducible gas concentrations. Helium and hydrogen (BOC, 99.99% purity) were purified using in-line deoxygenating and drying traps. The propyne (BDH, 97% purity) was used without further purification. Propane and propene (Linde, 99% purity) were used for gas chromatography calibration. The catalyst was activated following a ICI Lecturer in Heterogeneous Catalysis, corresponding author

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similar procedure to that adopted by Jackson and Casey [4]. In the presence of a flowing 5% hydrogen~elium mixture, the sample temperature was increased from 298 to 473K at 3K m i n l then maintained at 473K for 30 minutes. The flow gas was then switched to pure helium and the sample allowed to return to 298K with the helium continually flowing over the catalyst. All reactions were performed at 298 K and 2 bar pressure and a fixed propyne flow rate of 10 ml min l , which equates to 4.5 x 1018 propyne molecules sl and a propyne:Pd(s) ratio of c a . 144:1. The hydrogen concentration was varied with respect to the helium diluent stream to maintain a total flow rate over the catalyst of 100 ml min l , which corresponds to a gas hourly space velocity of 400,000 h 1. Product distributions were analysed by on-line GC-MS (Hewlett Packard GC6890 fitted with a 25m x 4ktm CP-A1203/KC1 column and equipped with a Hewlett Packard 5973 mass selective detector) sampling via an automated gas sampling valve. Varying the catalyst mass and gas linear velocity had no effect on the observe,l reaction rate, confirming the absence of diffusional effects under operating conditions. Measurements were performed in triplicate and confirm the trends presented to be representative of the adsorption system. 3.

RESULTS The conversion of propyne as a function of time on stream is presented in Figure 1. Throughout the run, the propyne flow rate was kept constant but the hydrogen flow rate was progressively increased, in proportion to decreases in the helium diluent gas stream, so a constant overall flow rate was maintained. In this manner, the hydrogen:propyne ratio was increased from an initial value, post-activation, of 2:1 up to 10:1. Thereafter, the hydrogen concentration was reduced to a ratio of 6:1. Clearly, as demonstrated previously for propyne hydrogenation over silica-supported Pt and Pd catalysts [6,7], increasing hydrogen concentration results in increased conversion. Three distinct stages are identified. First, for a

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Time on s t r e a m / m i n u t e s Fig. 2. Variation of product distribution as a function of time on stream, where the hydrogen:propyne ratio is increased from 2"1 up to 10"1 and then decreased to 6:1. hydrogen:propyne ratio of 2:1, a modest conversion of ca. 2% is observed. The second stage is defined by hydrogen:propyne ratios of 4:1 - 6:1, where small positive steps in conversion are observed on increasing the hydrogen concentration but with the overall conversion not exceeding ca. 7%. The final stage is much more dramatic. On increasing the hydrogen:propyne ratio to 7:1, the conversion rapidly increases to 100% and remains consta,t up to the highest hydrogen concentration studied of hydrogen:propyne = 10:1. No deactivation was observed for these conditions over the period of time studied. The product distribution as a function of time on stream and hydrogen:propyne ratio is presented in Figure 2. Throughout Stage 1 (H2:C3H4 = 2:1), apart from unreacted propyne, propene is the only material detected in the product stream, resulting in a propene selectivity (defined as [propene/(propene + propane)]) of 100%. The amount of alkene increases in Stage 2 (H2:C3H4 = 4:1-6:1), with a small amount of the alkane now detected resulting in selectivities for propene and propane of ca. 90 and 10%. Although once the Stage 3 threshold level of H2:C3H4 = 7:1 is exceeded the activity is insensitive to hydrogen concentration (Figure 1), marked changes in the product distribution are observed throughout this region. The level of propene decreases as a function of increasing hydrogen coverage, whilst the propane concentration in the eluant stream increases by a corresponding amount. This results in propene and propane selectivities at the highest hydrogen concentration studied (H2:C3H4 = 10:1) of ca. 35 and 65% respectively. These results show high alkene selectivities require relatively low hydrogen concentrations, that are associated with low conversions. Previo,,,; work on ethyne hydrogenation on polycrystalline Pd has shown selectivity to be little influenced by hydrogen partial pressure [1 ], whereas recent studies by Tysoe et al on Pd foil report some sensitivity to hydrogen concentration [3]. However, in addition to ethane, Tysoe et al also report the detection of C4 hydrocarbons and benzene. This suggests differences in the reactivity of the Pd foil and the high surface area catalyst studied here, given that our work detects only C3 products. Alternatively, in addition to being a consequence of different

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化

]

100-1.

9

-

-,-.,

.,-

9

9

9

100 80 60

,~,

:E o r.~

40

~

j~

--

.~/," 0

40

20

Propene

A-- Propane '

I

25

'

= o

--o" - C~176

20///' I"

-~-- --

I

50

'

Time on stream / minutes

I

75

6 100

Fig. 3. Propyne conversion and selectivity towards propene and propane as a function of time on stream for a fixed hydrogen:propyne ratio of 10: 1. reaction conditions, the characteristics of the two ethyne studies [ 1,3] could reflect differences between ethyne and higher molecular weight alkynes, as reported elsewhere [5]. To our knowledge, the results presented here represent the first study of hydrogen effects in propyne hydrogenation over supported palladium catalysts operated in a continuous flow environment. Stepping the hydrogen concentration down from H2:C3H4 = 10:1 to 6:1, the conversion remains unaltered at 100%, and the production of propene relative to propane increases initially and, on increasing time, the amounts become equivalent. These conditions are more comparable to the case for H2:C3H4 = 7:1 (Stage 3), rather than the Stage 2 values seen for a 6:1 mixture. This result shows there to be a degree of hysteresis inherent in the system, consistent with the catalytic system exhibiting a relaxation time that is relatively long compared to the time in which the gas compositions can be switched and stabilised (ca. 1 0 , :. Given the sensitivity of these systems to catalyst conditioning [6,7], this is not unexpected. In order to determine if the ultimate performance of the catalyst observed at the higher hydrogen concentrations was a function of the conditioning process, an experiment was performed at a fixed hydrogen:propyne ratio of 10:1. The results are shown in Figure 3. At steady state the fixed hydrogen concentration results essentially reproduce the results obtained after ramping up the hydrogen concentration over a period of time. Figure 3 shows a propyne conversion of 100% with propene and propane selectivities of ca. 40 and 60 % respectively.

4.

DISCUSSION Figures 1 and 2 show that hydrogen concentration plays a major role in determining catalytic yield. However, this work shows a non-classical hydrogen dependence. A previous study on propyne hydrogenation over a variety of supported Pd catalysts showed the reaction order, determined over a hydrogen:propyne range of 0.5 - 3:1, to be 0.6 - 1.3 in hydrogen [4]. Figure 1 shows this is not the case when a wider range of hydrogen concentrations are examined, with the rate of reaction showing a clear discontinuity on increasing the hydrogen concentration. This type of behaviour has been reported previously for ethyne hydrogenatic,l on supported metal catalysts. Moyes et al [8] showed that, as a function of increasing ethyne

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pressure (decreasing hydrogen concentration), the initial rate of reaction first increased, then dropped sharply, and finally declined slowly. Figure 1 essentially reproduces this trend, including the gradual decrease in rate throughout Stage 2, as the propyne ratio is increased (decreasing hydrogen concentration). The discontinuity was attributed to two different mode s of ethyne molecules on the palladium surface, having different heats of adsorption. Thus, population of the strongly bound ethene reduces the surface hydrogen coverage which will attenuate the rate of reaction [8]. Analogous results have also been reported for ethene hydrogenation [9,10]. According to Langmuir-Hinshelwood kinetics and assuming dissociative hydrogen adsorption, the rate of reaction is given by d[C3H4] dt

oc 0C,H. 0n 2

(1)

Pulse studies of propyne hydrogenation on supported metal catalysts confirm that under operational conditions the working catalysts form hydrocarbonaceous overlayers which play a major role in controlling catalytic performance [6,7]. Now, increasing the hydrogen concentration to a specific level will permit hydrogenation of some of the overlayer, which will leave increasing areas of bare metal sites that, in the dominance of hydrogen in the gas stream, will be available for the dissociative adsorption of hydrogen. This mechanis,,-, therefore leads to an increased hydrogen supply at the surface and as equation (1) shows the rate to be dependent on the square of the hydrogen coverage, a substantial increase in the rate of propyne consumption will result. Thus this dramatic increase in conversion, characterised by Stage 3 in Figure 1, is a kinetic effect. The increased supply of adsorbed hydrogen will then increase the probability of fully hydrogenating the adsorbed alkyne/alkene moieties and so an increase in alkane production is expected, as is observed experimentally. Considering the lower hydrogen concentrations, this hypothesis shows the reaction to be surface hydrogen limited and it is under these conditions that maximum propene selectivity is achieved. These suggestions are consistent with those discussed by McLeod and Gladden, though it establishes that such discontinuities are not restricted to just C2-molecules [ 10]. The steepness of the discontinuity indicated by the eluant propyne concentration in Figure 2 shows hydrogenation of the overlayer to be a fast process, occurring in seconds rather than tens of minutes. However, the propene and propane profiles show more gradual changes, with systematic changes on concentration occurring over periods of at least 30 minutes. This contrast in response of the system to a perturbation (stepping of hydroge'~ concentration) indicates that selectivity changes are occurring on a longer timescale than that seen for the overall activity. The more gradual changes in the product distribution are attributed to progressive changes in the hydrocarbonaceous overlayer. Additional products, arising from hydrogenation of parts of the hydrocarbonaceous overlayer, might be expected at the discontinuity and yet Figure 2 shows no additional material, other than that supplied to the system at that time, is detected about this transition. This can occur for two reasons. First, the number of molecules involved are small. The catalyst charge corresponds to 3.1 x 1016 Pd(s) atoms and so hydrogenation of a (small) fraction of this value would lead to a minor pressure burst, not detectable against such a large presence of background gas (pressure=2 bar). Secondly, product stream sampling is performed periodically, not continuously, and is not synchronised with the discontinuity, making detection of such a transient stage unlikely under the experimental conditions used here.

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The fact that Figures 1 and 2 show that on reducing the hydrogen concentration from a Stage 3 value to a Stage 2 value leads to a product distribution characteristic of Stage 3 suggests that, under these circumstances, the surface hydrogen supply is maintained at a Stage 3 level. The reaction profile for the excess hydrogen mixture, Figure 3, highlights a further characteristic of the dynamics of the hydrocarbonaceous overlayer in these reactions. The initial low activity noted at short run times shows 100% selectivity to the alkene. In the earlier stages of reaction the overlayers will not have formed completely and the metal behaves as a poor hydrogen transfer agent [2]. This results in low activity, with the additicl of only one HE unit possible. However, once the overlayers are fully formed, the ability of the catalyst to transfer hydrogen to the adsorbed alkyne dramatically increases leading to an increase in activity. Under these conditions, addition of two HE units becomes possible and selectivity to the alkane also increases. 5.

CONCLUSIONS From the results for propyne hydrogenation over a Pd/C catalyst at 298 K the following conclusions may be summarised as follows: 1. The rate of reaction and product distribution are critically dependent on hydrogen concentration. 2. High hydrogen concentrations cause removal of a fraction of the hydrocarbonaceous overlayer present throughout operation of the catalyst. This results in an increase in surface hydrogen which leads to increases in catalytic yield. 3. At low hydrogen concentrations the rate of reaction is surface hydrogen limited and alkene selectivity is favoured. 4. Catalyst activity and selectivity may be controlled by optimisation of the hydrogei-t dissociation site (metal) and the hydrogen transfer site (hydrocarbonaceous overlayer). 6.

ACKNOWLEDGEMENTS ICI are thanked for provision of an ICI Lectureship in Heterogeneous Catalysis (DL). This work was performed on equipment supported as part of the SHEFC funded Catalyst Evaluation and Optimisation Service, who also supported a postdoctoral fellowship (RM).

REFERENCES 1. C.M. Pradier, M. Mazina, Y. Berthier and J. Oudar, J. Mol. Catal., 89 (1994) 211. 2. G. Webb, Catalysis Today, 7 (1990) 139. 3. H. Molero, B.F. Barlett and W.T. Tysoe, J. Catal., 181 (1999) 49. 4. S.D. Jackson and N.J. Casey, J.C.S. Faraday Trans., 91 (1995) 3269. 5. S.D. Jackson and G. Kelly, Current Topics in Catalysis., 1 (1997) 47. 6. D.R. Kennedy, B. Cullen, D. Lennon, G. Webb, P.R. Dennison and S.D. Jackson, Ed. G.F. Froment & K.C. Waugh, Studies in Surface Science and Catalysis, 122, Elsevier, (1999) 125 7. D. Lennon, D.R. Kennedy, G. Webb and S.D. Jackson, 8th International Symposium On Catalyst Deactivation, Brugge, 1999, Ed. G.F. Froment and B. Delmon, Elsevier, Amsterdam, accepted for publication. 8. R.B. Moyes, D.W. Walker, P.B. Wells and D.A. Whan, Appl. Catal., 55 (1989) LS. 9. S.D. Jackson, G.D. McLellan, L. Conyers, M.T.B. Keegan, S. Mather, S. Simpson, P.B. Wells, D.A. Whan and R. Whyman, J. Catal., 127 (1996) 10. 10. A.S. McLeod and L.F. Gladden, Ed. G.F. Froment and K.C. Waugh, Studies in Surface Science and Catalysis, Vol 122, Elsevier, (1999) 167.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

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Suggestion on a New Alternative Mechanism of the Sulfuric Acid Catalyzed Isioparaffin-Olefin Alkylation. V.B.Kazansky, T.V.Vasina N.D. Zelinsky Institute of Organic Chemistry of Russian Academy of Sciences, Leninsky prospect 47, Moscow B-334, Russia.

ABSTRACT 13C NMR study of the two-step isoparaffin-olefin alkylation indicated that alkyl carbenium ions are formed instead of protonation of olefins by protonation of alkylsulfates. In the case of sulfuric acid catalyzed alkylation of isoparaffins with propylene the low yield of propane indicates only a minor role in alkylation of a hydride transfer. Both those findings are in contradiction with the classical mechanism by Schmerling. Therefore, a new alternative mechanism of the two-step isoparaffinolefin alkylation is proposed. It involves a direct alkylation of isoparaffins by alkylsulfates.

1. INTRODUCTION The sulfuric acid catalyzed isoparaffin-olefin alkylation is traditionally considered as a classical carbenium ion reaction [1]. The mechanism of alkylation has been originally proposed by Schmerling almost fifty years ago [2]. Since that time it remained unchanged. For the isobutane-butene alkylation the most important reaction steps are the following: 1) C4H8 +

H + ---* C4H9+

2) C4H9+ +

(1)

C4H8 ---* t-C8H17+

3) t-C8H17+ +

i-C4Hlo

4) t-C4H9+ +

C4H8 - 4

--~

i-C8H18

+

t-C4H9 +

t-C8H17+ etc.

The main features of this chain mechanism is formation of alkyl carbenium ions by protonation of olefin and a subsequent cationic oligomerization via interaction of these species with the next olefin molecule. To explain the high selectivity in formation of C8 paraffins, it is also postulated that reaction of hydride transfer from

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isoparaffin to alkyl carbenium ion (3) is very fast. In addition, the hydride transfer also explains propagation of the reaction chains. One of the most important recent achievements in understanding of a more realistic mechanism of alkylation is a discovery by Albright et. al. that isoparaffinolefin alkylation can be carried out in two separate steps via intermediate formation of alkylsulfates [3-5]. Indeed, it is well known from the general organic chemistry that interaction of olefins with 95 % sulfuric acid results in formation of esters instead of alkyl carbenium ions [6]. The alkylsulfates with primary and secondary alkyl fragments are relatively stable compounds. They also don't react with isoparaffin unless the sulfuric acid is in excess. On the other hand, in the presence of excess of 95 % sulfuric acid the esters readily react with isoparaffins yielding products of alkylation. Their composition mainly corresponds to recombination of two alkyl fragments: that one resulting from the olefin with another one resulting from isoparaffin: RSO4H +

i-RiH

~

R-RI

+

H2SO4

(2)

Thus, the two step alkylation is rather a reaction of isoparaffin with the ester than with olefin. We explained the role of excess of 95% acid in Refs. [7-8]. It was suggested that similar to self-protonation of sulfuric acid the protonation of the ester with sulfuric acid results in formation of alkyl carbenium ions weakly solvated with the acid: RSO4H + H2SO4 ~

R+H2SO4 + HSO4-

(3)

Such possibility was confirmed by the 13C NMR study of mixtures of penthyl sulfates with excess of 95 % sulfuric acid and by the ab initio quantum chemical calculations [8]. Moreover, we estimated the concentration of the resulting alkyl carbenium ions from the experimentally observed ~3C chemical shifts with account of the rapid proton exchange between the protonated and non-protonated species. The obtained results indicated that amount of alkyl carbenium ions does not exceed only few percents of the total concentration of the esters. The present paper is a continuation of those works. It is devoted to the study of the two step alkylation of 3-methyl pentane and 2-methyl butane with propylene. The choice of these reagents allowed a better understanding of the role in alkylation of hydride transfer from isoparaffins to carbenium ions. Indeed, for propyl carbenium ions, the hydride transfer should result in formation of propane. The latter by-product of alkylation could be easily discriminated from other hydrocarbons. Therefore, we used the yield of propane as a measure of the rate of hydride transfer. 2. E X P E R I M E N T A L Di-isopropylsulfate or mixtures of di- and mono-isopropylsulfates were prepared at 0 C by interaction of propylene with 95% sulfuric acid. This reaction was either carried out at the pressure of 1-2 atm in a glass autoclave with a constant stirring of solution or by passing of a propylene flow through the 95% acid. The amount of sulfuric acid was equal to 5-6 g. The reaction was finished in approximately 30 min. In the static experiments this was indicated by stoichiometry of propylene absorption by the acid at the end of reaction or by a strong decrease of the reaction rate.

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Aikylation of 3-methyl pentane and 2-methyl butane with isopropyl esters of sulfuric acid was carried out in a closed glass system at 0 C with a constant stirring of the reaction mixtures. The 4-5 fold molar excesses of isoparaffins and of the 95% acid relative to the esters were used. The resulting gaseous hydrocarbons were collected in a gas burette and analyzed with a capillary gas chromatograph at the end of the reaction. The chemical analysis of the final products of alkylation was also carried out with the same gas chromatograph. The 13C NMR spectra of mono- and dipropylsulfates were measured with a Varian "Gemini 300" NMR spectrometer operating at the frequency of 300 Ml-lz fbr protons.

3. RESULTS AND DISCUSSION Practically quantitative formation of di-isopropylsulfate was proven by the proton decoupled high-resolution 13C NMR spectrum depicted in Fig. 1. The lines at 79.5 and 77.3 ppm correspond to the carbon atoms in the CH-O-SO4 fragments of the di- and monoester, respectively. This assignment was based on the nondecoupled J3C NMR spectrum of diester and on the NMR spectra of the reaction mixtures with a lower amount of absorbed propylene, when the yield of the monoester was considerably higher. The weakest line at 70.8 ppm most likely belongs to isopropyl oxonium ions.

80

70

60

50

40

30

20 ppm

Fig. 1. The proton decoupled ~3C NMR spectrum of di-isopropylsulfate. In the case of the lower amount of propene absorbed by the acid the reaction products represent a mixture of monoesters and diesters while amount of the former predominates. The composition of the liquid products of alkylation of 2-methyl pentane with diisopropylsulfate is shown in Table 1. The conversion of di-isopropylsulfate as estimated from these data was about 50%. Only a very little amount of propane was formed at the end of the reaction. As one can see from Table l, the yield of nonane isomers is close to 70 %. About 10 % of" lower C7 - Cs paraffins are formed in parallel, while the heavier paraffins are

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mainly represented by Ci0 and Cli isomers (about 15 %). The most surprising result was a practical absence of propane formation. This obviously indicates only a minor role in alkylation of hydride transfer, since otherwise the reaction of isopropyl carbenium ions with isohexane would result in formation of propane: i-C3H7+ +

i-C6Hl4 ~

C3H8 +

t-C6Hi

(4)

This is in an obvious contradiction with the above-mentioned classical mechanism of isoparaffin-olefin alkylation by Schmerling, where a fast hydride transfer explains both the propagation of the reaction chains and a high selectivity in formation of C9 isoparaffins. TABLE 1

Composition of alkylate resulting from reaction of 4.2 g of di-isopropylsulfate with 11 g of 3-methyl pentane at 0 C in presence of 10.5 g of 95% sulfuric acid. Fraction

C7

C8

C9

Clo+

Hydrocarbons 2,4-dimethylpentane 2-methylhexane 2,3-dimethylpentane 3-methylhexane 2-methylheptane 2,4-dimethylhexane 2,2,5-trimethylhexane 2,4-dimethylheptane 2,6-dimethylheptane 2,5-dimethylheptane 2,3-dimethylheptane 3,4-dimethylheptane Mainly Clo and Cil

Yield in weight % 2.2 5 1.5 1.2 Z=9.9% 5 1.6 Z= 6.6 % 2.5 9.9 9.3 32.2 12.4 1.2 Z= 67.5 % 16.5 % Total:

100.5 %

The results obtained for alkylation with isopropylsulfates of 2-methylbutane were quite similar. The yield of propane was also practically negligible while the main product of alkylation was the mixture of C8 isoparaffins (Table 2). This also confirms only a minor importance of hydride transfer. Another contradiction with the classical mechanism of alkylation consists in the absence in the reaction mixture of propylene. The latter was neither detected in our experiments by ~3C NMR at the beginning, nor at the end of the reaction. This is, however, quite natural since a concentrated sulfuric acid has been for a long time used

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in gaseous analysis for absorption of olefins. Therefore, in the presence of an excess of 95% acid the concentration of propylene in the reaction mixture certainly is very low. Thus, the two step reaction definitely represents a direct alkylation of isoparaffin with alkyl sulfate instead of its alkylation with olefin as it was postulated by the classical mechanism by Schmerling. This conclusion is also in line with our previous finding that alkyl carbenium ions are formed by protonation of alkyl sulfates by excess of the 95% acid instead of a direct protonation of olefins.

TABLE 2

Composition of liquid alkylate resulting from reaction of 3 g of isopropylsulfate with 7 g of 2-methyl butane at 0 C in presence of 7.05 g of 95% sulfuric acid. Fraction C3

Hydrocarbons

Yield in weight %

C4

propane isobutane

C6

2-methylpentane 3-methylpentane

C7

2-methylhexane

C8

C9

Cio

2,5-dimethylhexane + 2,4-dimethylhexane 2,3,4-trymethylpentane 3,4-dimethylhexane 2,5-dimethylheptane 2,3-dimethylheptane isoparaffins

2.5 5.4 2=7.9 3 2.6 2=5.6% 1 27.3 46.5 1.4 Z = 75.2% 5.1 2.7 Z =7.8% 2.6 Total: 100.1%

To explain the above-presented experimental results, and th contradictions with the classical mechanism of alkylation, we propose the following alternative new mechanism of a direct alkylation of isoparaffins with protonated esters. It includes two new reactions. First of them is formation of the nonclassical carbonium ion from isoparaffin and the protonated ester: i-RiH + R+H2SO4 ~

[R! H R] + H2SO4

(5)

This is an analog of the well-known reaction of carbenium ions with isoparaffins in the gas phase [9,10]. Normally this results in a hydride transfer upon dissociation of the nonclassical carbonium ion:

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R! + + HR

~

[RI

H R] + ~

RIH

+

R+

(6)

In the case of carbonium ions solvated with sulfuric acid the reaction of hydride transfer is slow. This indicates a relatively high stability of the nonclassical carbonium ions. Therefore, they are involved in the second reaction of recombination with HSO4anions. This results in a direct alkylation of isoparaffin with alkylsulfate: [Rl H R] § H2SO4

+

HSO4 ~

RIR

+ 2 H2SO4

(7)

Reaction (7) is a reverse of the well-known protolytic cracking of paraffins that has been well proven both for liquid superacids and for zeolites [11- 12]. Thus, the central point of a new alternative mechanism of the two step isoparaffinolefin alkylation is a suggestion that this reaction involves a direct alkylation of isoparaffins by protonated esters via intermediate formation of nonclassical carbonium ions. This explains the composition of reaction products of alkylation without assumption on a predominant role of the reaction of carbenium ions with olefin and on the fast hydride transfer. It is also most likely that the similar alternative mechanism could better explain the conventional isoparaffin-olefin alkylation as proceeding via following sequence of elementary steps: olefin--, the mixture of alkyl sulfates-.~ the protonated esters ---* nonclassical carbonium ions ---~- alkylate.

Acknowledgements: The authors express their best thanks to Prof. Avelino Corma for the interest to the present work and a fruitful discussion of the obtained results. REFERECES 1. A.Corma, A.Martinez, Cat. Rev. Sci. Eng. 35(1993)483. 2. L.Schmerling, Ind. Eng. Chem.45(1953)1447. 3. L.F.Albright, M.A.Spalding, J.S.Nowinsky, R.M.Ybarra and .E.Eckert, Ind. Eng. Chem.Res. 27(1988)381. 4. L.F.Albright, M.A.Spalding, C.G.Kopser, R.E.Eckert, Ind. Eng. Chem. Res. 27(1988)391. 5. L.F.Albright, M.A.Spalding, J.Faunce, R.E.Eckert, Ind. Eng. Chem.Res. 27(1988)381. 6. V.N.Ipatiev, r162 Reactions at High Temperatures and Pressures~, Academy of Sci. USSR Pub., Moscow-Leningrade 1936. 7. V.B.Kazansky, R.A.van Santen, Catal. Lett., 38(1996) 115. 8. D.A.Zhurko, M.V.Frash,V.B.Kazansky, Catal.Lett.55(1998)7. 9. T.F.Magnera, P.Kebarle, in ~donic Processes in the Gas Phase~, ed. by M.A.Almoster-Ferreira, Reidel, Dortrecht, 1984, p,135. 10. M.V.Frash, V.N.Solkan, V.B.Kazansky, J. Chem. Soc. Far. Trans. 93(1977)515. 11. M.Brower, H. Hogeveen, Prog. Phys. Org. Chem., 9(1972)179. 12. Haag, R.M.Dessau in Proceedings of 8th International Congress on Catalysis, Dechema, Frankfurt-on-Main, 1984, Vol. 2, p.305.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Hydroisomerization of n-decane in the presence of sulfur and nitrogen L.B.Galperin UOP LLC P.O. Box 5016, Des Plaines, Illinois, USA Isomerization of n-decane in the presence of 1000 ppm H2S and up to 770 ppm nitrogen was studied on Pt bifunctional catalysts (MAPSO-31, SAPO-11, SAPO-34, SAPO-41, SAPO-5, MFI and amorphous silica-alumina). The addition of nitrogen in the presence of sulfur allowed control of isomerization selectivity. Correlation of catalyst performance with material structure and acidity was established. Isomerization mechanism over bifunctional catalysts in the presence of sulfur and nitrogen was discussed.

1. Introduction Hydroisomerization of pure n-decane is widely used as a model reaction to study the mechanism of hydrocracking-isomerization of n-paraffins on bifunctional catalysts as well as for characterization of material (catalyst) structure [ 1-5]. A lot of attention was paid during the last years to isomerization of paraffins on SAPO family zeolite catalysts [6-9] and recently SAPO-11 has found commercial application in lube dewaxing [ 10]. All these studies have been focused on activity and shape selectivity of the medium pore molecular sieves and used pure hydrocarbons as a feed. Commercial feed is often contaminated with sulfur and nitrogen, which could have a dramatic effect on performance of bifunctional catalysts by poisoning active metal and acid sites. The objective of this study was to investigate the effect of sulfur and nitrogen in feed on catalyst behavior and develop an isomerization catalyst that is not sensitive to sulfur impurities. Specifically, it was demonstrated how to control n-paraffin isomerization selectivity in the presence of sulfur by the addition of nitrogen compounds.

2. Experimental Isomerization of n-decane was conducted in a fixed catalyst bed reactor at 500 psig, H2/HC = 1.4 (molar), WHSV = 5hr -~, temperature 300-460~ n-C~0 conversion range 20-80%. Catalyst loading 5g (40-60 mesh) was diluted with 5 g of quartz of the same size. Sulfur was introduced as H2S with hydrogen. Concentration of H2S in hydrogen was 1000 + 50 ppm (Matheson), while concentration of nitrogen (in the range 10-770 ppm) was controlled by the addition of tertiary butylamine to n-decane. All products (liquids and gases) were analyzed with on line GC. Conversion was calculated as disappearance of n-C~0, while isomerization selectivity was defined as yield of iso-component wt% divided by n-Cl0 conversion. Catalyst preparation included coextrusion of the corresponding materials (zeolites) with 20 % A1203 followed by calcination in air at 500~ for 4 hours. Calcined extrudates (1/16") were impregnated with H2PtCI6 (MAPSO-31 was impregnated with (NH3)aPtC12), targeting 0.4 % wt platinum on the finished catalyst and then

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dried and calcined in air at 500~ for 4 hours. All catalysts were reduced in-situ at 400~ for 4 hours with H2-1000 ppm H2S mixture. Catalysts were characterized by XRD (to control zeolite crystallinity after catalyst finishing), NH3 TPD (acidity), N2 adsorption (surface area) and H2/O2 titration (platinum dispersion). MAPSO-31, SAPO-11, SAPO-41, SAPO-34, SAPO-5, amorphous silica-alumina (25%SIO2/75%A1203), as well as MFI with variable Si/A1 ratios (from 38 to 400) were studied. Properties of materials used in this study are in Table 1. Table 1 Catalyst properties. Composition: 80% zeolite/20% A1203 SA,m2/g BET

Pt, wt%

Pt Dispersion

mmol NH3/g catal (% of total) 200300~

SAPO-34

355

0.40

0.86

SAPO-11

166

0.45

0.94

SAPO-41 MAPSO-31

218 151

0.45

0.80

0.39

0.56

SAPO-5

220

0.42

0.88

Amorphous

352

0.39

75%Si25%A1203 MFI-38

308

0.48

1.03

MFI-IO0

303

0.40

0.78

MFI-400

369

0.39

0.92

i

0.95

0.085 (27.3) 0.063 (26.4) 0.082 (26) 0.063 (24.7) 0.088 (26.9) 0.068 (24.6)

300400~ 0.142 (45.6) 0.112 (47) 0.141 (44.7) 0.113 (44.3) 0.137 (41.9) 0.114 (41.3)

0.058 (12.3) 0.054 (26.8) 0.023 (28.4)

0.137 (29.2) 0.096 (47.7) 0.034 (41.9)

400500~ 0.07 (22.5) i 0.05 (21) 0.072 (22.8) 0.062 (24.3) 0.084 (25.7) 0.072 i (26)

500550~ 0.014 (4.5) 0.013 (5.4) 0.02 (6.3) 0.017 (6.6) 0.018 (5.5) 0.022 (8.0)

200550~ 0.311

0.204 (43.4) 0.04 (19.9) 0.019 (23.4)

0.07 (14.9) 0.011 (5.4) 0.005 (6.1)

0.469

|

|

|

|

|

|

0.238 0.315 0.255 0.327 0.276

0.201 0.081

3. Results and discussion 3.1 Effect of sulfur and nitrogen addition To demonstrate the effect of sulfur and nitrogen on catalyst activity and selectivity 0.4% PtMAPSO-31 was tested (Fig. 1, 2) with pure feed (H2 and decane), with 1000 ppm H2S in H2 (pure decane), with 770 ppm nitrogen in decane (pure H2) as well as in the presence of both H2S and nitrogen (1000 ppm H2S in H2 and 770 ppm nitrogen in decane). Catalyst activity decreased according to feed contamination in order: pure feed> 1000 ppm H2S>770 ppm nitrogen > 1000 ppm H2S+770 ppm nitrogen. To achieve the same conversion in the presence of sulfur and nitrogen as for pure feed the reaction temperature was increased by about 100~ As to isomerization selectivity (total iso-Cl0), the lowest selectivity (50-55%) was observed in the presence of 1000 ppm H2S, while in the presence of 770 ppm nitrogen selectivity was aroflnd 70% and increased to 80-85% (1000 ppm H2S+ 770 ppm) nitrogen and 85-87% for pure feed. It was remarkable that at 100~ higher reaction temperature in the presence of sulfur and nitrogen, the catalyst still

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maintained very high isomerization selectivity. It is speculated that nitrogen neutralized the catalyst strong acid sites leading to the low catalyst acidity and activity. To compensate for the low activity, the reaction temperature must be significantly increased. At this temperature, the weak and intermediate strength acid sites will become active enough to achieve high paraffin conversion. On the second hand, metal activity will be significantly higher at high temperature and the perfect balance between the weak-intermediate catalyst acidity and strong metal function will be established, resulting in the high isomerization selectivity. Composition of the cracked and isomerized products reflected the proposed mechanism. If the pure n-decane used, the main isomerization products are methylnonanes, while formation of dimethyloctanes was low due to the strong steric constrains imposed by the MAPSO-31 structure and slightly increased with conversion. The addition of sulfur strongly suppressed the metal function and decreased the amount of methylnonanes relative to the pure feed run, while in the presence of nitrogen, the methylnonane selectivity was restored to the high level. Selectivity to ethyloctanes at all conditions was low and increased slightly with conversion from about 3% to 5%. At the middle conversion, the contribution of C type [3-scission is responsible for relatively low iso-normal ratio of C5-C7 (0.5). Addition of sulfur suppressed significantly metal activity (weak metal function and strong acid function) and required about 40-50~ higher reaction temperature. In spite of the fact that catalyst acidity was not affected by sulfur, the [3-scission at this high temperature is not the major cracking mechanism any more but interfered with Me hydrogenolysis, resulting in the high yield of methane. In addition, the suppressed metal function resulted in the shift to B type of I]-scission (iso/normal was close to 1). Nitrogen addition (no sulfur, weak acid function and strong metal function) deactivated the strong acid sites and decreased catalyst activity. To maintain the same conversion as for pure feed, the reaction temperature was increased about 100~ At that high temperature, the contribution of Me hydrogenolysis and type D hydrocracking was very high, methane yield was increased and iso/normal ratio dropped down to 0.2. Addition of sulfur and nitrogen to the feed affected both metal and acid functions. The metal hydrogenolysis activity was suppressed, resulting in the decreased methane formation, while isomerization selectivity was increased (iso/normal came back to 0.5). 3.2. Effect of N concentration

The effect of nitrogen concentration on performance of 0.4%Pt-MAPSO-31 catalyst in the presence of 1000 ppm HzS was studied in the range 10-770 ppm nitrogen (Fig. 3, 4). Up to nitrogen concentration of 10 ppm, no change in catalyst performance was found. The further increase of nitrogen concentration (50, 100,770 ppm) progressively suppressed catalyst activity without significant effect on total catalyst isomerization selectivity. The increase of nitrogen concentration corresponded with the increased reaction temperature and suppressed the formation of dimethyloctanes. This is in line with thermodynamics, which are favorable to methylnonanes at high temperature. Methane formation was increased with nitrogen concentration and reaction temperature.

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3.3. Effect of zeolite structure

SAPO-34 (8 member ring), SAPO-11 and SAPO-41 (10 member ring), SAPO-5 and MAPSO31 (12 member ring), MFI (10 member ring, Si/A12=38; 100; 400) as well as amorphous silicaalumina (25%A1203-75%SIO2) were studied as supports. All catalysts contained around 0.4% Pt and have been tested in the presence 1000 ppm H2S and 770 ppm nitrogen in temperature range 380-440~ (Fig. 5, 6). SAPO-34 showed poor activity which is presumably due to very small pore opening (3.8A*3.8A, diffusion limitation). It is likely that only the external surface of SAPO34 crystals was available for reaction, which affected the number of acid sites and catalyst activity. Catalyst isomerization selectivity was low, approximately 60%. SAPO-11 and SAPO-41 with the similar pore opening (3.9A*6.3A and 4.3A*7A,respectively) had excellent selectivity (90-95%) at conversions from 20-80%. 12 member ring SAPO-5 (pores 7.3A*7.3A) showed poor selectivity and stability while MAPSO-31 (pore opening 5.4A*5.4A) had performance close to SAPO-11 and SAPO-41. Product distribution corresponded with material structure. DM-C8 selectivity was highest for SAPO-5 and increased from MAPSO-31 to SAPO-41 according to pore opening. The same trend was observed for ethyloctane formation. On the other hand, SAPO-11, SAPO-41 and MAPSO-31 gave basically monomethyl isomers indicating to the steric constrains imposed by the pore structure of these materials. Silica-alumina catalyst also showed high isomerization selectivity and activity comparable to the activity of SAPO materials but with lower stability. The open structure of amorphous silica-alumina was favorable for high formation of dimethyl isomers and ethyl octanes (higher than SAPO-5). It was speculated that multibranched isomers preferably formed inside the open space of silica -alumina are not stable at high temperature and could be easily cracked to give olefins and coke, which are the cause of catalyst deactivation. Iso/normal ratios of the cracked product were around 0.5 for S APO materials, about 1 for silica-alumina and 2 for SAPO-5 and supported the above conclusions. 3.4. Correlation between catalyst performance and NH3 TPD

All SAPO family catalysts had acidity in the same range (0.25-0.33 mmol NH3/g) with similar strength distribution. Activity of SAPO-34 was discussed above and depended first on its geometric structure. As to the other SAPO materials, the range of activity (acidity) was SAPO-41 (0.315 mmol NH3/g)>SAPO-11 (0.238 mmol NH3/g)>SAPO-5 (0.327 mmol NH3/g)>(amorphous SIO2/A1203 (0.276 mmol NH3/g) > MAPSO-31 (0.255 mmol NH3/g). This range of activities was defined by the combination of many catalyst variables such as, for example, material structure, defect structure and preparation and finishing conditions, which were unique for each type of material and did not correlate with the total number of acid sites by NH3 TPD. To get a better understanding of the acidity effect, it was desirable to have a set of materials with the same structure and variable number of acid sites. To meet this requirement, the series of MFI catalysts with variable Si/A12 ratio (38; 100; 400) were tested (Fig. 7, 8). The increase of Si/A12 from 38 to 400 resulted in decreased catalyst activity and increased isomerization selectivity. MFI-38 (high acidity) behaved like a good cracking catalyst due to the high number of acid sites available for reaction even at 770 ppm nitrogen in feed. On the other hand, MFI-400 (low acidity) required much higher temperature to maintain the same conversion. At this temperature, the strong metal function and the low catalyst acidity led to the high isomerization selectivity. The decreased Si/AI2 ratio affected not only the number of acid sites but also their distribution. MFI-38 had a maximum

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on the acid site distribution curve corresponding to the strong acid sites (400-550~ while MFI100 and MFI-400 had acidity pattern similar to SAPO materials, with maxima corresponding to intermediate acid sites (300-400~ In addition, MFI-100 which had the total number of acid sites close to SAPO materials was active in the same temperature range and had significantly improved isomerization selectivity relative to MFI-38. Based on these data it was concluded that a good correlation between acidity and catalyst performance exists inside of the same class of materials (MFI in our case). It was also concluded that the strong metal function and mild acidity are required to provide the best isomerization performance in the presence of sulfur and nitrogen. Good correlation between acidity and catalyst performance was established for MFI containing catalysts. 4. Conclusions

1. Isomerization of n-decane in the presence of 1000 ppm H2S and up to 770 ppm nitrogen was studied on Pt bifunctional catalysts (MAPSO-31, SAPO-11, SAPO-34,SAPO-41, SAPO-5, MFI and amorphous silica-alumina). 2. The effect of nitrogen addition on activity and selectivity of isomerization catalysts in the presence of sulfur was established. The addition of nitrogen suppressed catalyst activity but allowed the control of isomerization selectivity. In the presence of 1000 ppm H2S and up to 770 ppm nitrogen, isomerization selectivity of SAPO-11, SAPO-41, MAPSO-31 was close to the catalyst selectivity with no sulfur and nitrogen added (pure feed). 3. Catalyst activity and selectivity correlated with catalyst acidity (NH3 TPD) within the same class of materials (MFI). 4. The mechanism of isomerization in the presence of sulfur and nitrogen over bifunctional catalysts was discussed. The carbenium ion isomerization mechanism (well-established for pure hydrocarbon feed) followed by ~-scission of the produced intermediate is consistent with the reaction products. In the presence of nitrogen (due to low activity and high reaction temperature), the contribution of metal hydrogenolysis to the cracking product distribution became significant. 5. Strong metal function and mild acidity are required for bifunctional catalysts to conduct selective isomerization of paraffins in the presence of both sulfur and nitrogen. References

1. J.Weitkamp, Appl. Catalysis, 8 (1983) 123 2. P.A.Jacobs, M.A.Martens, J.Weitkamp, H.K.Beyer, Faraday Dis. Chem Soc.,72 (1981) 353 3. J.A.Martens, P.A.Jacobs, J.Weitkamp, Appl Catalysis 20 (1986) 239 4. J.A.Martens, P.A.Jacobs, J of Catalysis 124 (1990) 357 5. J.A. Martens, R. Parton, L. Uytterhoeven, P.A.Jacobs, Appl Catalysis 76 (1991) 95 6. P. Meriaudeau, V.A.Tuan, V.T.Ngheim, C.Naccahe, G.Sapaly, Catal. Today 49 (1999) 285 7. J.M.Campelo, F.Lafont, J.M.Marinas, Appl Catalys 152 (1997) 5 8. A.K.Sinha, S.Sivasanker, P.Ratnasamy, Ind.Eng. Chem.Res. 37 (1998) 2208 9. J.M.Campelo, F.Lafont, J.M.Marinas, Appl. Catalysis 170 (1998) 139 10. S.J.Miller, 206 ACS mtg., Symp New Catal. Chem Utilizing Mol Sieves, Prepr.,(1993) 788

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Fig.1. Effect of S and N on catalyst activity

100

J

Fig. 2. Bfect of S and N on r

100

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90

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320

360

400

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.

.

.

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Fig.3. Effect of N concentration on catalyst activity,H 2S=1000ppm

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Fig.4. Effect of N concentration on catalyst selectivity,H2S=1000ppm 100 80 o ~ . 60 ._

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n-C10 conversion,wt% -~-- noS,noN -a--noN --e-- 50ppmN --*--100ppmN --)K--770ppmN ~ 1 0 p p m N F~6. Effect of SAPO stricture on catalyst ~,H2S--t000m~N=770mm

F~5. Elfect of SAPO stnclum on catalyst ~,H2S=1000ppmN=770m~

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Fig.7. Effect of Si/AI2 on MFI activity,H2S=1000ppm,N=770ppm

100

Fig.8. Effect of Si/AI2 on MFI e lectivity_,H2S_,~!_000p p m,N =770 p pm

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化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

State of metals in the supported bimetallic Pt-Pd/SO42-/Zr02 system A.V. Ivanov, A.Yu. Stakheev, and L.M. Kustov N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninskii prosp. 47, Moscow 117334, Russia, e-mail: [email protected] The behavior of the metals in Pt-Pd/ZrO2 and Pt-Pd/SO42-/ZrO2 was studied by DRIFT spectroscopy and XPS. In Pt-Pd/ZrO2 partial formation of the P t ~ P d alloy takes place after reduction at 200~ For Pt-Pd/SO42-/ZrO2 the effects of alloying and interaction of the metals with the surface SO4 groups superimpose. 1. INTRODUCTION Catalytic systems based on sulfated zirconia (SO42-/ZRO2) promoted by platinum are very active in n-paraffin isomerization [1]. However, rapid deactivation of the catalyst is the main drawback that deteriorates the performance of the Pt-catalyst in the isomerization process. One of the reasons behind the deactivation of the Pt/SO42-/ZrO2 catalyst is poisoning of the platinum surface by sulfur compounds formed due to reduction of SO4 groups in a hydrogen hydrocarbon media. The enhancement of the resistance of the catalyst to sulfur should suppress deactivation. The introduction of palladium in various Pt-catalysts is known to increase sulfur tolerance as compared with monometallic systems 12]. The aim of this work is the study of the interaction of metals in the Pt-Pd/SO42-/ ZrO2 system in order to estimate the influence of palladium on the state of platinum and vice versa. The influence of the support acidity and surface sulfur species on the state of metals was also of interest. 2. EXPERIMENTAL

Pt-Pd/SO42-/MxOv and sulfiar-free systems have been synthesized by coimpregnation of S042-/MxOy(where M - Zr, Ti, A1, Si) prepared according to [3] or parent oxides (ZrO2, TiO2, A1203, and SiO2) with solutions of H2PtC16 and PdC12. The overall metal (Pt + P d ) loading was 0.5 wt. %. The atomic percentage of Pt (%Pt) in the Pt-Pd bimetallic composition was 100, 60, 50, 30, 15, or 0 at. %. Samples were reduced in a hydrogen flow at 100~300~ The electronic state of metals was investigated by DRIFT spectroscopy using CO as a probe-molecule with a Nicolet Impact 410 spectrophotometer and by XPS using a XSAM-800 Cratos spectrometer (Mg/~l,2). 3. RESULTS AND DISCUSSION

MonometaUic systems Platinum. Only Pt ~ particles characterized by the IR bands at 2070 and 1855 cm -1 corresponding to linear and twofold bridging forms of adsorbed CO are

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t

i

0.5

i

'2070

a

'

'2100

'

'

'

b

~Pd ~

0.5

1

0.4 "E

2

0.3

Pt ~* 2115

o.2

2160

0.4 2095Pt~ 0.3 0.2

0.1 0.0

0.1

1855 2200

~

,

0.0

.

' 2 oo 2100 2000 1900 Wavenumbers, cm -1

'

21oo

'

26oo

'

1 oo

'

Fig. 1. DRIFT spectra of CO adsorbed on (a) (1) Pt/ZrO2 and (2) Pt/SO42-/ZrO2; (b) (1) Pd/ZrO2 and (2) Pd/SOa2-/Zr02 reduced at 200~ observed in the Pt/ZrO2 system after reduction at 200~ (Fig. la). The position of these maxima of absorption bands are typical of the state of Pt in the systems with weak metal--support imeraction. Adsorption of CO on the Pt/SO42-/ZrO2 sample makes it possible to distinguish Pt ~+ (2115 cm -1) and Pt ~ (2095 cm -1) species as the main states of platinum. Pt + ions that can exist as near-surface ( O - - P t ) - - C O complexes were also found in a minor concemration. Two reasons can be proposed to explain the formation of the positively charged metal species in the Pt/SO42-/ZrO2 sample: (1) the modification of the Pt electronic state by adsorption of S-containing moieties and (2) the direct interaction between the acidic protons and the metal particles yielding [Pt-H] ~+ adducts [3, 4]. With the purpose to discriminate between these two effects the state of Pt supported on SO42-/TIO2, SO42-/A1203, and SO42-/SIO2 possessing different acidic properties was also studied. The addition of the SO42- anions induces the shift of the maxima of the absorption band corresponding to CO adsorbed in the linear form on Pt particles to higher frequencies for all sulfated systems and stabilizes positively charged platinum species (Pt + and Pt *+) during the reduction in the case of SO42-/ZRO2 and SO42-/TIO2 catalysts (Table). The value of the high-frequency shift of the maximum of the absorption band of CO adsorbed in the linear form on platinum particles diminishes with decreasing acid strength of the sulfated systems: SO42-/ZRO2 > SO42-/TIO2 ~ SO42-/A1203 >> SO42-/SIO2 [5]. Hence, the mechanism of the formation of positively charged forms of platinum presumably includes the interaction of the acidic protons of Bronsted acid sites (BAS) with the metal particles. Palladium. The spectrum of CO adsorbed on palladium in Pd/ZrO2 (Fig. lb) shows the presence of only Pd ~ particles with characteristic bands at 2100 cm -1

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Wavenumbers of C O (cm -1) adsorbed on Pt supported on System

Pt +

Pt 8+

Pt/ZrO2 Pt/SO42-/ZrO2 Pt/TiO2 Pt/SO42-/TiO2 Pt/AI203

2160 2150 -

2115 2110 -

Pt/SO42-/m1203

Pt/SiO2 Pt/SO42-/SiO2

-

-

-

-

MxOyrx SO42-/MxOy

Wavenumbers, (cm-1) Pt ~ A Vco~,,+ _r~0) 2040 2095 2060 2095 2040 2080 2070 2075

75 50 -

A Vcoo%, _~0) 55 35 40 5

(CO adsorbed in the linear form) and 1970 and 1920 c m -1 with a shoulder at 1830 cm -1 (polycarbonyl bridging, monocarbonyl bridging, and three-centered forms of CO, respectively) [6]. The stability of the metal toward reduction increases u p o n modification with SO4 groups. For the Pd/SO42-/ZrO2 system, the metal particles with a partial positive charge (Pd ~+) characterized by the absorption band at 2120 cm -1 are revealed. At the same time, the formation of the polycarbonyl bridging complexes of CO is suppressed. One of the possible reasons of a decrease in the concentration of polycarbonyl bridging complexes is a decrease in electronic density on the Pd particles due to formation of Pd ~+ species. The other reason is the formation of new complexes of palladium due to the partial decoration of the Pd surface by the products of interaction with surface sulfur species [7]. These complexes are characterized by the narrow band at 1900 cm -1 in the IR spectra of CO.

Bimetallic systems

Pt-Pd/Zr02. After reduction of P t - P d / Z r O 2 (atomic ratio Pt : Pd = 1 : 1) at 100~ (Fig. 2), the main states of the metals are Pt ~ and Pd ~ with a low concentration of positively charged forms of Pt + or Pd 2+. Analysis of the spectra makes it possible to distinguish the regions of vibrations of bridging and linear carbonyl groups. Deconvolution of the first region gives the bands at 1830, 1880-1900, 1930, and 1970 cm -1 that can be assigned to Pd3(CO), PcI2(CO), Pd2(CO)2-3, and <&utterfly~> Pd2(CO)2 [6, 8]. In the second region, absorption bands characterizing adsorption on the individual Pt ~ (at 2060 cm -1) and Pd ~ (at 2095 cm -1) sites can be distinguished. All these data can indicate the existence of separate Pt ~ and Pd ~ species, i.e., give no evidence for the formation of a uniform P t - - P d alloy or for any significant mutual influence of the metals. An increase in the reduction temperatures to 200~ results in a decrease in the concentration of the Pd3(CO) complexes and an increase in the concentration of <,butterfly,> Pd2(CO)2 complexes. It may indicate the diminution of the area of Pd islands due to dilution of the Pd phase by Pt. The bands characterizing the linear forms of C O adsorption become more uniform as compared with the sample reduced at lower temperatures. In this case, partial formation of the P t - - P d alloy is possible.

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266

0.5

'

0.4

'2100

'

1

Pd 0

'

b

a

Pt ~

~9 0.3 i0.2

2095

1930

Oil/2/ 12ix60///l f~./

0.4

0.3 1

2065 75 1900

0.2

\ 1830 0.1

0.1

0.0

0.0 2200

2000

1800 2200 2000 Wavenumbers, cm- 1

1800

Fig. 2. IR spectra of CO adsorbed on Pt-Pd/ZrO2 reduced at (a) 100 and (b) 200~ (1) after adsorption and after evacuation at (2) 20 and (3) 100~ Pt-Pd/SO42-/ZrOe . The behavior of the metals in the Pt-Pd/SO4/ZrO2 systems was studied at different Pt/Pd ratios and reduction temperatures. Deconvolution of the IR spectra of CO adsorbed on the Pt-Pd/SO42-/ZrO2 sample ( P t Pd = 1 " 1) reduced at 100~ allows one to reveal two components (at 2115 and 2080 cm -1) which can be assigned to Pd ~+ and Pt ~ respectively (Fig. 3). The behavior of platinum is close to that in sulfur-free Pt/ZrO2. At the same time, the state of palladium is similar to that in Pd/SO42-/ZrO2. An increase in the reduction temperature to 200~ leads to the formation of more uniform adsorption sites, although they differ from those formed under similar conditions in the PtPd/ZrO2 system. One can assume that processes of alloy formation start in this system; however, they are overlapped by the action of surface active sites formed by SO4 groups. A further increase in reduction temperature to 300~ results in the formation of the charged forms of the metal, presumably Pd 2+, characterized by the band at 2155--2160 cm -1. The effect observed can be explained by redox processes involving metal atoms and surface SO4 groups. In the monometallic system promoted by SO42- anions, the phenomenon of metal re-oxidation was not observed and, hence, is characteristic behavior of the bimetallic system. An increase in the relative concentration of Pt in the P t ~ P d mixture results in the disappearance of the Pd ~ islands (bridging CO complexes) during the reduction even at 100~ An increase in the relative concentration of Pd leads to an increase of the intensity of the bands attributed to bridging complexes of CO adsorbed on Pd. However, reduction at 200~ causes the intensity of these bands to decrease. The behavior of the bands and during reduction resembles that in the monometallic Pd/SO42-/ZrO2 system. The main state of platinum in the systems

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0.3

267

' 21i0

2105

.~0.2

2115~

1905 2085

80

2135 IJ

( 1950~1\

~,

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o.o,

b

a

21

I

2200

,

/

i

2000

,

1

1800 2200 Wavenumbers, cm -1

2000

1800

Fig. 3. IR spectra of CO adsorbed on Pt-Pd/SO42-/ZrO2 reduced at 100~ (a) the concentration of Pt is (1) 60, (2) 50, (3) 30, and (4) 15 %Pt; (b) deconvolution of spectrum 2. with different Pt/Pd ratios is Pt ~ Presumably, Pd plays the main role in surface interaction in the PtmPd mixture. The surface SO4 groups seem to interact mainly with palladium and their influence on platinum is insignificant. The additional argument in favor of the participation of SO4 groups in the metalmsupport interaction was obtained by XPS. According to XPS data, the main part of sulfur is present on the surface as sulfate groups (Fig. 4). However, the apparent shift if the XPS line is observed compared to the position of the line in bulk zirconium sulfate [9]. The XPS line of sulfur in Pt-Pd/SO42-/ZrO2 is shifted to the position close to the binding energy for organic sulfates [9]. These data are supported by S2~69.4 IR data. Hence, the metal support interaction results not only (RO)2SO 2 - 169.8 [9]/ i in modification of the properties of Zr(SO4) 2 - 1 the metal but also alters the properties of the surface sulfur groups. 9 ,,,-i r~

To describe the interactions in the Pt-Pd/SO42-/Zr02 system the following model could be proposed. The surface acid sites formed by SO4 groups are known to be responsible for the unique properties of the S042-/Zr02 system [1]. In accordance with the

160

,

I

a

I

~

165 170 Binding energy, eV

I

175

Fig. 4. XP spectra of S2p line of the Pt-Pd/SO42-/ZrO2 system.

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MonometaUic systems

0

II

S

u

//

Zr

.~

II

o- S

"-~ Zr ~",

Lr

Bimetallic systems O

0

~

o . Z

217

Zr

M0

Zrf~ ) ~ Zr Zr

II

o S. o zr

, Zr

Zr ~"\ 0=-=-~ @ Zrf~ Zr

/ Pd~+

Scheme previous studies, these sites combine the acidic and redox properties (see Scheme). Acidic protons of BAS interact with the metal particle to form a [M--H] ~+ adduct (M = Pt or Pd). The value of the partial positive charge depends on the polarization extent of the M-.-H..-OSO3 bond and the shift of the maximum of the CO absorption band can serve as a measure of the strength of BAS. Protons can be abstracted from the acid sites to migrate over the metal surface. The OSO3groups can further react with the metal particles to form compounds of the IM + OSO3] type in which the metal atoms are positively charged. The formation of [M+--OSO3] complexes with Pt--O bond lengths which are different from those in conventional platinum oxides Pt--O--Pt was confirmed by EXAFS data [ 10]. Thus, in the bimetallic system the interaction of protons and sulfur species with metal depends on the nature of the metal. Palladium exhibiting higher reactivity interacts with the sulfate groups more easily as compared with platinum and, hence, platinum atoms are essentially not subject to the influence of surface active sites. This leads to the stabilization of the neutral state of platinum. ACKNOWLEDGMENTS

We gratefully acknowledge Dr. N.S. Telegina for XPS measurements. REFERENCES

1. 2.

X. Song and A. Sayari, Catal. Rev. - Sci. Eng., 38 (1996) 329. H. Yasuda, N. Matsubayashi, T. Sato, and Y. Yoshimura, Catal. Lett., 54 (1998) 23. 3. A.V. Ivanov, L.M. Kustov, Russ. Chem. Bull., 47 (1998) 1061. 4. C. Morterra, G. Cerrato, S. Di Ciero, M. Signoretto, F. Pinna, and G. Strukul, J. Catal., 165 (1997) 172. 5. H. Matsuhashi, H. Motoi, and K. Arata, Catal. Lett., 26 (1994) 325. 6. T. Sheppard and T.T. Nguen, Adv. Infrared Raman Spectr., 5 (1978) 67. 7. A.V. Ivanov, L.M. Kustov, Russ. Chem. Bull., 47 (1998) 57. 8. L.-L. Sheu, H. Knozinger, and W.M.H. Sachtler, Catal. Lett., 2 (1989) 129. 9. V.I. Nefedov, Rentgenoelektronnaya spektroskopiya khimicheskikh soedinenii (X-ray Spectroscopy of Chemical Compounds), Khimiya, Moscow, 1984. 10. T. Shishido, T. Tanaka, and H. Hattori, J. Catal., 172 (1997) 24.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

化

269

Dehydroisomerization of n-butane silicoaluminophosphates.

over

Pt

promoted

Ga-substituted

A. Vieira, M. A. Tovar, C. Pfaff, P. Betancourt, B. M6ndez, C. M. L6pez, F. J. Machado, J. Goldwasser and M. M. Ramirez de Agudelol. Centro de Cat/disis, Petr61eo y Petroquimica, Escuela de Quimica, Facultad de Ciencias, Universidad Central de Venezuela, Apartado Postal 47102, Los Chaguaramos, Caracas 1020A, Venezuela. INTEVEP, S.A, Apartado 76343, Caracas 1070-A, Venezuela. The catalytic transformations of n-butane were performed over a Pt-promoted Gasubstituted silicoaluminophosphate molecular sieve (Pt/GaAPSO-11 ), over a Pt-promoted Gasupported silicoaluminophosphate molecular sieve, and over a Pt-promoted SAPO-I 1 solid. The results showed similar yields for the formation of isobutane + isobutene, lower yields for the formation of C l-C3 hydrocarbons, for the Ga containing samples, particularly for the Pt/ Ga/SAPO-11 catalyst The latter also showed higher yields for the formation of dehydrogenated products. Differem techniques were used to characterize the metallic and acid functions. The possible formation of a Pt-Ga alloy and/or the formation of discrete Pt particles decorated by metallic Ga, are invoked to explain the higher dehydrogenation and lower hydrogenolysis activity shown by the Pt/Ga/SAPO-11 sample. 1. INTRODUCTION Metal-substituted aluminophosphates (MeAPO's) and metal-substituted silicoaluminophosphates (MeAPSO's) molecular sieves (Me: Cr(III), Mn(II), Zn(II), Fe(II), Co(II)), have been reported to be highly active and selective for the skeletal isomerization of nbutenes [ 1,2]. The importance of isobutene as a valuable feedstock is well documented due to it usefulness in the petrochemical industry. It can also react with methanol, producing methyl tert-butyl ether (MTBE), an important octane booster oxigenated fuel additive. Recently [3,4], we explored the possibility of producing isobutene from n-butane in a single process, thus avoiding the costs of two separate independent reactors (dehydrogenation and acid skeletal isomerization), which are presently used for the manufacture of isobutene. In addition, the direct transformation of n-butane can yield appreciable amounts of isobutane, a valuable feedstock used in the production of isooctane (by reaction with n-butenes). The reaction was performed over Pt-promoted Mn-AIPO4-11 and over Pt-promoted Mn-SAPO-11 molecular sieves. The results showed that the Pt/MnAPO-11 and Pt/MnAPSO-11 solids were more selective towards the formation of isobutene and isobutane than the Pt-promoted manganese supported counterparts. Other recent results [5] obtained over Ga silicoaluminophosphate molecular sieves, revealed that Ga(III) supported on SAPO-11 showed a much higher activity (particularly, at low times-on-stream (TOS)) towards the production of

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isobutene and isobutane, using n-butane and H2 as the reactants, than the G a - substituted counterpart. Extraframework gallium species (EFGS) species were invoked to explain the results The main purpose of the present work is to carry out the direct transformation of n-butane over a platinum promoted - gallium substituted silicoaluminophosphate molecular sieve (Pt/ GaAPSO-11) and over a platinum promoted - gallium supported silicoaluminophosphate molecular sieve (Pt/ Ga/SAPO-11). X-ray diffraction, acidity measurements (irreversibly chemisorbed pyridine, followed by infrared spectroscopy (IR)), H2 uptake at 773 K and metallic dispersion measurements (H2 chemisorption), were used as characterization techniques. 2. EXPERIMENTAL

2.1.Catalysts The synthesis procedure for the parent SAPO-11 has been reported elsewhere [5]. The specific surface area (SSA) was 190 m2/g. The GaAPSO-11 solid was synthesized according to ref 5. A final crystallization temperature of 473 K and a crystallization time of 90 h were employed. The solid was first washed with distilled water and dried at 353 K for 16 hours. The catalyst was then calcined under dry air at 773 K for 15 hours in order to remove organic residues. The molar composition formula TO2 for the calcined was (glo.42Po.42Sio.13Gao.03)O2. The SSA was 182 m2/g. A supported Ga/SAPO-11 catalyst was prepared by impregnating a sample of SAPO-11 with the same Ga promoter, using the incipient wetness technique (2.2 wt. % Ga). This sample was then dried and calcined following the same procedure as for the GaAPSO- 11 sample. The SSA for the Ga-supported catalyst was 152 m2/g. The Pt promoted catalysts were prepared by impregnating the different calcined solids with [Pt(NI-I3)4(NO3)2] (BDH, reagent grade). The experimental details have been reported previously [3,4]. The Pt loading was 0.5 wt. % Pt for all the Pt promoted solids. 2.2. Procedures Specific surface areas were determined on a commercial Micromeritics ASAP 2400 surface area analyzer at liquid nitrogen temperature. X-ray diffractograms were recorded with a Philips diffractometer PW 1730 using Co-KGt radiation (K= 1.790255A) operated at 30 KV, 20 mA and scanning speed of 2 ~ (20/rain). The H2 consumption experiments were performed in a conventional BET system identical to that used in Ref. 4 at 773 K. The experimental procedure has been outlined previously [4]. H2 chemisorption experiments were performed in the BET system mentioned above. The irreversibly held H2 was calculated using the double isotherm method. A 1:1 H:Pt stoichiometry was assumed to calculate the metallic dispersion (H/Pt). Infrared spectra were recorded at room temperature using a Perkin-Elmer 1760X FTIR spectrometer with a resolution of 2 cm1. The IR cell has been described previously [3, 4]. Before the addition of pyridine, the samples were treated with pure 02 (60 cm3/rain) at 773 K for 2 h (oxidic samples) or with pure H2 (60 cm3/rain) (reduced samples) at 773 K for 2 h. The catalysts were then evacuated (P<_2.10 .5 Torr) for 1 h at the same temperature and cooled to room temperature under vacuum. The pyridine chemisorption experiments were carried out by exposing the pretreated sample to 5 Torr of pyridine vapor for 1 h at 363 K. The wafer was then evacuated for 1 h at different temperatures before the spectra were recorded. The outgassing temperatures used in the present work were: 443 and 623 K. The reason to use

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these temperatures is to arbitrarily define the following acidity regions: moderate + strong (sites retaining pyridine at 623 K) and total (weak + moderate + strong) (sites retaining pyridine at 443 K). Integrated intensities were normalized to unit wafer thickness (mg/cm~). n-Butane catalytic transformations were performed in a conventional continuous flow system operated at atmospheric pressure at 773 K. The experimental conditions, as well as the data treatment have previously been reported [3, 4]. The catalytic experiments were performed after reducing the catalysts with pure H2 (60 cm~/min) for 2 h at 773 K. 3. RESULTS and DISCUSSION

The as-synthesized solids were all highly crystalline with AIPO4-11 (AEL) structure, in agreement with the literature [1-5]. The major XRD changes observed for all the calcined solids have previously been reported [3-5]. They have been associated with a change in the crystal symmetry from a body-centered to a primitive unit cell as a result of water adsorption after calcination. The diffraction patterns observed for the platinum promoted solids were very similar to that of the calcined SAPO-11 solid, reported previously [4]. This result supports the fact that the addition of platinum does not change the crystallographic structure of the calcined SAPO-11, GaAPSO-11 and Ga/SAPO-11 catalysts. Similar results were obtained with the Pt/ MnAPSO-11 and MnAPSO-11 solids [4]. No Pt or Ga diffraction peaks were apparent in any of the solids studied in the present work. The IR spectra for chemisorbed pyridine was obtained for all the solids after evacuation at 443 and 623 K. Two bands observed at ca. 1450 cm1 and ca. 1548 cm1 were used to estimate Lewis and Br6nsted acidity, respectively. The former corresponds to the 19b vibration of the Lewis bound pyridine. The band at ca. 1548 cm 1 has been assigned to pyridine adsorbed on Br6nsted acid sites. Table 1 summarizes the normalized integrated intensities for the IR bands due to Br6nsted and Lewis bound pyridine for the solids used in this work. The (moderate + strong) Br6nsted acidity was similar for the oxidic SAPO-11 and GaAPSO-11 systems. This result is not a surprise in terms of the substitution of AI(III) by Ga(III) ions in the silicoaluminate framework [5]. This substitution does not create a disruption in the net charge of the AEL structure, thus not yielding significant changes in Br6nsted acidity. A reduction (--30 %) in the (moderate + strong) Br6nsted acidity was observed for the Ga/SAPO-11 solid. This solid, however has ~-20 % less SSA than the others. It can also be seen that the addition of Pt causes a slight decrease in the integrated intensity of the IR bands corresponding to Lewis and Br6nsted sites, for the Pt/SAPO-11 and Pt/GaAPSO-11, in agreement with reports on related systems [3, 4]. For the Pt/Ga/SAPO-11, however, the addition of Pt does cause an increase in the Lewis acidity, while the Br6nsted acidity decreases somewhat. Upon reduction, the (moderate + strong) Br6nsted acidity for all the Pt promoted catalysts is slightly reduced (---10%) compared with the oxidic samples. A drastic decrease in the Lewis acidity was observed for the reduced Pt/Ga/SAPO-11 solid. Table 2 shows the hydrogen consumption values (773 K) obtained for the oxidic catalysts used in the present work. As observed the experimental values are very similar to those expected for complete reduction to metallic Pt and metallic Ga for the Pt/SAPO-11 and Pt/ Ga/SAPO-11 solids. For the Ga/ SAPO-11 sample the consumption was -~ 94 % of the theoretical value. In contrast, for the GaAPSO-11 and Pt/ GaAPSO-11 samples, the corresponding experimental and theoretical 1-12consumption values were very different. For the ~SO-11 solid, only a--- 25 % of the theoretical H2 consumption was observed. It should be

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noted that the reduction of Ga203 to metallic Ga is thermodynamically favored at 780 K for [H20]/[H2] ratios lower than 1.10.5 [6].

Table 1 Normalized integrated intensities for the 1548 cm"l IR band (BrOnsted bound pyridine) and for the 1450 cm q band (Lewis bound pyridine). EVACUATION TEMPERATURE / K Catalyst SAPO-11 Pt/SAPO- 11 (02) Pt/SAPO- 11 (H2) CraAPSO- 11 Pt/GaAPSO-I 1 (02) P t / G a ~ S O - 1 1 (H2) Ga/SAPO- 11 Pt/Ga/SAPO- 11 (02) Pt/Ga/SAPO-11 (H2)

443 1548 cm~ 3.1 2.8 2.5 3.2 2.8 2.4 2.2 2.1 1.9

623 1450 cmq 8.9 3.4 2.7 8.1 4.1 3.8 3.5 6.8 1.7

1548 cm"l 3.0 2.7 2.4 3.0 2.7 2.4 2.1 1.8 1.7

1450 cm~ 1.6 1.5 1.3 1.2 1.1 1.0 1.2 5.6 0.8

Table 2 Results for the H2 consumption experiments at 773 K. (nm/g.,)/10 4 b Catalyst (nm/g~.at)/10 -4 a Pt/SAPO-11 0.50 0.51 C_raAPSO- 11 1.60 6.24 Pt/GaAPSO-11 2.11 6.75 Ga/SAPO-I 1 4.47 4.74 Pt/Ga/SAPO-11 5.27 5.25 a experimental values, b theoretical values for complete reduction to the metallic state. These results would indicate that the extra framework Ga species present in the Gasupported catalysts are much easier to reduce than those present in GaAPSO-11 and Pt/ GaAPSO-11 samples, reinforcing our previous conclusion [5] that for the GaAPSO-11 solid, a majority of Ga (III) ions were incorporated into the molecular sieve framework. Table 3 shows the HE uptakes and the Pt dispersion measurements. As observed the Pt dispersion sequence is Pt/SAPO-11 > Pt/Ga/SAPO-11 > Pt/GaAPSO-11. This result is in agreement with that obtained with the Pt-Mn analogues [4]. The catalytic results are shown in Tables 4-6. In order to estimate the role of the acid and metallic functions on the catalytic results shown, we have included data (Table 7) obtained using a 0.5 wt. % Pt supported over a non-acidic AIPO4-11 catalyst [3]. The Pt dispersion for the latter was 68 %, the SSA for the support was 162 m2/g, and the catalytic reaction was carried out under identical experimental conditions than those used in the present work.

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Table 3 H2 uptakes and Pt dispersion measurements H/Pt Catalyst H2 uptake/ ~tmoles/g SAPO-I 1 Not detected 0.83 Pt/SAPO-I 1 10.6 GaAPSO- 11 0.73 0.39 Pt/GaAPSO-11 5.11 Ga/SAPO-11 1.36 0.62 Pt/Ga/SAPO-11 8.02

H/Ga

0.0034 0.0083

Table 4. Product distribution for the transformations of n-butane over Pt/SAPO-11

TOS 30 60 90 120 180 300

X 87.4 79.0 77.0 73.3 68.7 64.4

Siso-C4 Siso-C4= Sn-c4=

S
17.6 19.7 19.8 19.9 20.8 21.9

72.5 64.5 62.9 61.4 57.5 51.9

5.5 7.5 7.2 7.8 8.9 10.3

4.5 7.2 8.8 10.9 12.8 15.9

Yiso-C4 Yiso-C4= Yn-c4= 15.4 15.6 15.2 14.6 14.3 14.1

4.8 5.9 5.6 5.7 6.1 6.7

3.9 5.7 6.8 8.0 8.8 10.3

Y
As observed, isobutane, isobutene and
TOS

X

30 60 90 120 180 300

62.2 62.1 58.7 58.5 54.0 50.7

Siso-C4 Siso-C4= Sn-c4= S
Y
24.7 23.7 24.4 24.6 26.5 25.8

31.2 30.8 27.0 26.7 22.4 19.2

9.4 9.9 9.9 10.2 10.4 10.9

15.8 16.9 19.7 19.6 21.6 25.4

50.1 49.5 46.1 45.6 41.5 37.9

15.4 14.7 14.3 14.4 14.3 13.1

5.9 6.1 5.8 6.0 5.6 5.5

9.8 10.5 11.5 11.5 11.6 12.9

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Table 6 Product distribution for the transformations of n-butane over Pt/Ga/SAPO-11.

TOS 30 60 90 120 180 300

X

Siso-c4

39.9 38.0 37.0 34.8 36.4 33.8

27.8 25.9 25.3 22.0 22.5 24.2

Siso-C4= Sn-c4= 23.1 24.0 24.3 25.0 25.4 24.6

37.7 40.2 41.2 44.9 44.0 44.2

5
Yiso-c4 Yiso-c4= Yn-c4= Y
9.2 9.1 9.0 8.7 9.2 8.3

Table 7 Product distribution for the transformations of n-butane over Pt/AIPO4-11. TOS X Siso-C4 Siso4?4= Sn-c4= S
15.0 15.3 15.2 15.6 16.0 14.9

4.6 3.8 3.4 2.8 3.0 2.4

Y
The reduction studies shown before indicate complete reduction of Pt and Ga to their metallic state, for the Pt/Ga/SAPO-11 solid, probably forming a Pt-Ga alloy [7] or discrete Pt particles "decorated" by metallic Ga. The formation of such an alloy could explain the drop in activity observed for the hydrogenolysis reaction due to the high degree of surface sensitivity reported for this reaction (see Table 2). The drop in the hydrogenolysis activity can also be accounted in terms of the "decoration" effect of the Pt particles by metallic Ga. The latter is supported by previous results [5] which have shown an important initial dehydroisomerization activity for the reduced Ga/SAPO-11 solid, despite the low H2 uptake shown by this solid (Table 3). It is also clear that the Pt/ Ga/SAPO-11 solid shows a higher hydrogenationdehydrogenation activity than those observed for the other solids studied in this work. REFERENCES 1. L. H. Gielgens, I. H. E. Veenstra, V. Ponec, M. J. Haanepen and J. H. C. van Hooff, Catal. Lett., 32 ( 1995) 195. 2. D. Arias, I. Campos, D. Escalante, J. Goldwasser, C. M. Lopez, F. J. Machado, B. Mendez, D. Moronta, M. Pinto, V. Sazo and M. M. Ramirez de Agudelo, J. of Mol. Catal. A: Chemical, 122 (1997) 175. 3. A. Vieira, M. A. Tovar, C. Pfaff, B. Mendez, C. M. L6pez, F. J. Machado, J. Goldwasser and M. M. Ramirez de Agudelo, J. Catal., 177 (1998) 60. See references therein. 4. A. Vieira, M. A. Tovar, C. Pfaff, P. Betancourt, B. M6ndez, C. M. L6pez, F. J. Machado, J. Goldwasser, M. M. Ramirez de Agudelo and M. Houalla, J. Molec. Catal. A: Chemical, 144 (1999) 101. See references therein. 5. F. J. Machado, C. M. L6pez, J. Goldwasser, B. M6ndez, Y. Campos, D. Escalante, M. Tovar and M. M. Ramirez de Agudelo, Zeolites, 19 (1997) 387. 6. G. D. Meitzner, E. Iglesia, J E. Baumgarten and E. S. Huang, J. Catal., 140 (1993) 209. 7. N. Iwasa, T. Mayanagi, N. Ogawa, K. Sakata and N. Takezawa, Catal. Lett., 54 (1998) 119

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化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

P o r e m o u t h catalysis o v e r acidic zeolites. N a t u r e of active s p e c i e s

P. Magnoux =, M. Guisnet =, I. Ferino ~ = UMR 6503 - Catalyse en Chimie Organique, 40 av. du Recteur Pineau, 86022 Poitiers- France- e.mail : r~li~:t~_cl~t!i~!~ct~i~ii?gd!!lt~u.~ue!;i~i:t~,i~ic~ ~i

b Dipartimento di Scienze Chimiche, Universida di Cagliari, Italy Transalkylation reactions were shown to occur between polyalkylated aromatic compounds entrapped in the micropores of large pore zeolites during liquid phase alkylation of naphthalene with isopropanol and of toluene with long chain linear alkenes. In the first case, these reactions were responsible for the production of isopropylnaphthalene, in the second for catalyst regeneration. As the entrapped compounds blocked the access of nitrogen hence of the reactant molecules to the micropores, transalkylation reactions necessarily occurred at the vicinity of the external surface of zeolite crystallites (pore mouth catalysis). I-INTRODUCTION

Pore mouth catalysis has been recently proposed to explain the selective isomerization of long chain alkanes over Pt/ZSM22 catalysts [1], the active sites being Bronsted acid sites on the external surface of the crystallites. Furthermore, pore mouth catalysis could also be responsible for the selective isomerization of nbutene into isobutene observed over aged HFER samples. However, in this latter case, carbonaceous compounds trapped in the pores at the vicinity of the external surface could be the active species [2]. Two other examples are given here, confirming that carbonaceous compounds (called coke for sake of simplification) may participate in pore mouth catalysis processes. Both examples concern liquid phase alkylation of aromatic hydrocarbons over large pore zeolites. 2 - ISOPROPYLATION OF NAPHTHALENE WITH ISOPROPANOL

The isopropylation of naphthalene with isopropanol was investigated over zeolites of different structural types, with as objective to design a selective catalyst for the synthesis of 2-alkyl and 2-6 dialkylproducts i.e. of precursors of high-value aromatic chemicals:dyes, pharmaceuticals, perfumes, specialty polymers [3]. We show here that the formation of isopropylnaphthalenes over HFAU and HBEA zeolites with a stabilized activity occurs through a complex pathway and not through the simple alkylation of naphthalene [4-6].

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The alkylation of naphthalene was carried out under the following conditions : fixed bed reactor, liquid phase, 350~ 40 bar, the reactant mixture being composed of naphthalene, isopropyl alcohol and decaline as solvent (1 : 2 : 6 mol). Mono, di and tri-isopropyl naphthalenes referred as IPNs, DIPNs and TIPNs were formed in amounts depending on catalyst and on time-on-stream. Various by products (propene, propane, Cs-Cs aliphatic hydrocarbons etc) were also observed, these by products resulting mainly from isopropanol transformation. With most catalysts, the apparent conversion of naphthalene (determined from the composition of the naphthalene, IPNs, DIPNs and TIPNs mixture) is initially low and increases with time on stream (Figure 1). This low initial conversion can be related to limitations in the desorption of the alkylated products from the zeolite pores. A large amount of products is furthermore retained on the zeolite during the first hour reaction : 16 wt % on HFAU 4 (framework Si/AI ratio of 4) and 12-14 wt % on HBEA 10, 20 and 70 (Figure 2). It should be remarked that no activation period and practically no coke formation are observed with an amorphous silica alumina sample [4] confirming that these phenomena are related.

Ir 0

"~ Q

t-

O

80

16

60

12

40 4) O

20

o

i+s'~

8 4

--m-9 BEA 20

,~

0

100

200

300

400

T i m e on stream (min)

Fig. 1. Conversion of naphthalene over HBEA 10 as a function of timeon-stream

0

100

200

BEA 70 300

400

Time on s t r e a m (rain)

Fig. 2. Percentage of coke deposited on BEA samples during naphthalene isopropylation as a function of timeon-stream

The non desorbed products (" coke ") were recovered in CH2CI2 after dissolution of the zeolites in a hydrofluoric acid solution [7]. With HFAU 4, after lh on stream, up to 80% of coke is soluble in CH2CI2 whereas after 6h only 35% of coke is soluble. Soluble coke is mainly composed by polyisopropyl (2-4 alkyl groups) naphthalene compounds. Polyisopropyl pyrenic and indenopyrenic compounds are also observed. With HBEA 10 and 20 samples, practically all the coke is soluble in methylene chloride after dissolution of the zeolite in hydrofluoric acid. It is mainly composed of polyisopropyl (2-4 isopropyl groups) naphthalenes. C4-Cs naphthalenes are also observed but no polyaromatic compounds. Nitrogen adsorption experiments on coked samples of HFAU 4 show that after l h there is a complete blockage of the access to the micropores.

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Therefore two proposals can be made to explain the high activity of the coked zeolite samples : -Alkylation of naphthalene occurs on a small number of acid sites located on the outer surface of the crystallites or at its close proximity. -Alkylates result from transalkylation between polyisopropyl aromatic compounds entrapped in the zeolite pores and naphthalene (reaction 1), the loss of an isopropyl group being rapidly compensated by realkylation of the entrapped compound with isopropanol (reaction 2). Both reactions require the participation of zeolite protonic sites.

O)

~(iPr)3

~ ~ (LPr)4 +

CH 3CHOHCH 3

~

+

H20

(2)

The second proposal seems more likely. Indeed it provides an easy explanation of the activation period (initial formation of polyisopropyl derivatives which remain blocked inside the pores), which is not the case for the first proposal. Moreover, the possibility of reaction 1 is clearly demonstrated by substituting naphthalene for the naphthalene-isopropanol reactant mixture : with both HFAU 4 and HBEA 10 zeolites (Figure 3), naphthalene continues to be transformed into desorbed isopropyl products. 100

100

HFAU 4

80

80

.~ 6O x

60

A

=-..7. Z

Napht

40 20

x

i

Z

~

'S'''i"o'

40 20

Napht + isopropanol 0

HBEA 10

i

Napht

I

100 200 300 Time on stream (min)

400

0

100 200 300 Time on stream (rain)

40o

Fig. 3. Naphthalene conversion, XN, over HFAU 4 and HBEA 10, vs time-onstream for naphthalene reaction with isopropanol and after interruption of isopropanol feed.

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As the access of nitrogen, hence of the reactant molecules (naphthalene and isopropanol) to the micropores is completely blocked by coke, it can be proposed that both reactions 1 and 2 occur at the pore mouth between entrapped and liquid phase molecules. 3 - A L K Y L A T I O N OF T O L U E N E WITH 1 A L K E N E S

Carbonaceous compounds blocked in the pores of HFAU zeolites during alkylation of toluene with 1-alkenes were also shown to react with liquid phase molecules. This reaction was chosen as a model reaction for investigating over zeolites the synthesis of linear monoalkylbenzenes (LABs) which are used in the production of biodegradable surfactants [8]. Indeed, the substitution of solid acid catalysts such as zeolites for the corrosive and polluting commercial catalysts (HF, AICla) is a very important objective. The liquid phase alkylation of toluene was firstly investigated over HFAU zeolites in a batch reactor at 90~ and with a molar toluene/alkene ratio = 3 [9, 10]. Monoalkyltoluenes are rapidly formed whereas bialkyltoluenes appear in low amounts at high conversion. A large amount of alkyltoluenes is trapped inside the zeolite pores, suggesting that alkylation is limited by product desorption. The amount and the composition of the non desorbed products (" coke ") were determined as a function of time in the simple case of toluene alkylation with 1heptene over HFAU 6 [10].Whatever reaction time, coke is composed of mono, bi and triheptyltoluenes. Curiously, after total consumption of heptene, there is still an increase in the amount of coke suggesting that then coke formation does not involve heptenes. There is also a significant change in the composition of coke: increase with time in monoheptyltoluenes and decrease in bi and triheptyltoluenes. These observations can be explained both by transalkylation between bi and triheptyltoluenes trapped in the zeolite micropores and toluene in liquid phase and by a slow diffusion of monoheptyltoluenes from the liquid phase to the micropores [10]. Transalkylation between polyaikylated products trapped in zeolite micropores and liquid phase molecules was also demonstrated in toluene alkylation with 1dodecene carried out in a fixed bed reactor at 90~ over a HFAU 25 zeolite [11]. Monododecyltoluenes (MDT) are selectively formed, bidodecyltoluenes (BDT) appearing only in low amounts (2-4 mol %) at a complete conversion of dodecene. Tridodecyltoluenes (TDT) are also formed, but very slowly (< 0.1 mol %) and inside the zeolite supercages only. These bulky (TDT) molecules (" coke ") which cannot escape out of the zeolite are responsible for its deactivation by pore blockage. Indeed no nitrogen adsorption can be observed on the totally deactivated sample which contains 10 wt % coke. Regeneration of this sample by treatment under nitrogen or n-decane flow was attempted but without success, which confirmed that TDT molecules were sterically bloked in the zeolite micropores. On the other hand very good results are obtained by treatment of the deactivated sample under toluene flow. After lh, the

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coke content is 6.5 wt % (instead of 10 wt %) and after 12h all the coke is removed.

The activity of the catalyst is then completely restored and the change in conversion with time-on-stream is the same as with the fresh sample. On the other hand the initial conversion is only 50% with the sample treated for I h (Figure 4).

A lO0

o~

c

_o 8o

Fig. 4. Dodecene conversion into alkylation products as a function of time on stream over fresh (O), partially (e) and totally regenerated (r-I) HFAU 25 samples.

q) c> 0 Q

:

Q Q "0

o r.t

60

40

2o

0

100

200

300

400

500

Time on stream (rain)

Obviously, this removal of coke cannot be explained by desorption of the bulky TDT molecules from the zeolite micropores. Transalkylation reactions between these TDT molecules and toluene molecules of the liquid phase are responsible for this removal. Indeed analysis of coke shows that the treatment under toluene flow for lh causes not only a decrease in the amount of TDT blocked in the pores (from 8.8 to 1.5 wt %) but also an increase in those of BDT (from 0.9 to 2.5 %) and of MDT (from 0.3 to 2.5 %). Furthermore MDT and BDT (90% and 10%) but no TDT can be found in the toluene used for treating the deactivated sample. The removal of TDT from the zeolite pores can therefore be schematised as follows: CH 3

01

CH3

O12

C12 (TDT) Z

~ 3

"~ L

012/,,,~-012 (BDT) Z

0[~3 O12

(MDT) L

Part of BDT molecules may diffuse in the liquid phase (L), the other part reacting with toluene with formation of two MDT molecules, one in the liquid phase, the other one in the pores desorbing slowly from the zeolite ( Z )

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280

Cl

C12 (BDT) z

+

~ L

C12 (MDT) Z

+

012 (MDT) L

As nitrogen, hence reactant molecules, can no more accede to the pores of the completely deactived sample, at least the first alkylation steps occur at the pore mouth. However after a certain degree of regeneration it is most likely that toluene molecules may enter the zeolite pores and react inside these pores with BDT and even TDT molecules. 4 - CONCLUSION

Transalkylation reactions between polyalkylated aromatic compounds trapped in the micropores (coke) of large pore zeolites and reactant molecules in liquid phase play a significant role in the isopropylation of naphthalene as well as in the regeneration of "coked" alkylation catalysts. As the access of nitrogen hence of reactant molecules to the micropores is completely blocked by coke these reactions occur necessarily at the pore mouth. This pore mouth catalysis with participation as active species of entrapped molecules adsorbed on protonic sites could also play a role in gas phase reactions over medium pore size zeolites such as FER and TON from the micropores of which the desorption of little bulky products is difficult. REFERENCES

1. J.A. Martens, R. Parton, L. Uytterhoeven, P. Jacobs, G.F. Froment, Appl. Catal. 76 (1991) 95 2. P. Andy, N.S. Gnep, M. Guisnet, E. Benazzi, C. Travers, J. Catal. 173 (1998) 322 3. H.G. Franck, J.W. Stadelhofer (eds). Industrial aromatic Chemistry, SpringerVerlag, Berlin, Heidelberg, 1987 4. G. Colon, I. Ferino, E. Rombi, E. Selli, L. Forni, P. Magnoux, M. Guisnet, Appl. Catal. A: General 168 (1998) 81 5. G. Colon, I. Ferino, E. Rombi, P. Magnoux, M. Guisnet, React. Kinet. Catal. Lett. 63 (1998) 3 6. I. Ferino, R. Monaci, E. Rombi, V. Solinas, P. Magnoux, M. Guisnet, Appl. Catal. A : General 183 (1999) 303 7. M. Guisnet, P. Magnoux, Appl. Catal. 54 (1989) 1 8. B.V. Vora, P.R. Palado, J.B. Spinner, T. Imai, Hydrocarbon Proc. 63 (1984) 86 9. A. Mourran, P. Magnoux, M.Guisnet, J. Chim. Phys. 92 (1995) 1394 10.Z. Da, P. Magnoux, M. Guisnet, Appl. Catal. A: General 182 (1999) 407 11.Z.Da, P. Magnoux, M. Guisnet, Catal. Letters, 61 (1999)203.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

R e d i s c o v e r y of the P a r i n g R e a c t i o n : The C o n v e r s i o n of 1,2,4T r i m e t h y l b e n z e n e over H Z S M 5 at E l e v a t e d T e m p e r a t u r e H.P. ROger*, W. Btihringer, K.P. MOiler and C.T. O'Connor Catalysis Research Unit, Department of Chemical Engineering, University of Cape Town, Rondebosch 7701, South Africa The paring reaction was first described 4 decades ago as an acid catalysed sequence of reactions to apparently "pare" methyl groups from polymethylated cyclic hydrocarbons thereby joining the methyl groups to longer side chains and subsequently splitting off these chains to form low aliphatics. Since then the paring reaction of polymethylated aromatics was seldom reported in literature, presumedly because it hardly plays a role as a side reaction with the frequently studied isomerisation and transalkylation of low aromatics (such as xylenes) below 400~ In contrast, the paring reaction was found to play a decisive role during the conversion of 1,2,4-trimethylbenzene over HZSM5 at elevated temperature (450~ 1,2,4-Trimethylbenzene disproportionates to form xylenes and tetramethylbenzenes. Over amorphous silica-alumina the reaction almost terminates at the disproportionation step. Over HZSM5 the tetramethylbenzenes almost completely undergo the classical paring dealkylation to produce benzene. The benzene rapidly transalkylates with other feed molecules and eventually goes into an interconverting C 6 to C 9 pool. Product distributions from conversions over CVD treated HZSM5 and the effect of temperature indicate, that both the transalkylation of 1,2,4-TMB and the paring dealkylation of the tetramethylbenzenes take place inside the pore system. The diffusion resistances for these bulky intermediates inside the zeolite crystals decisively promote the paring reaction. I. INTRODUCTION 40 years ago Chevron researchers [1,2] observed a surprising product distribution when hydrocracking hexamethylbenzene and various C9+-naphthenes over a bi-functional sulfided Ni/silica-alumina catalyst. The observation of low molecular weight methylbenzenes and naphthenes respectively, and a light aliphatic co-product, predominantly consisting of isobutane, led to the proposal of a special, novel mechanism which was termed the "paring reaction". It was found also to occur over the mono-functional acidic silica-alumina support itself, in absence of any nickel and hydrogen, then yielding the primary olefinic co-product (i.e. predominantly isobutene) [ 1]. A carbenium ion mechanism was proposed which involves the repeated contraction of a protonated six-membered ring to a five-membered ring followed by re-expansion, eventually to produce side chain growth joining methyl groups. In its apparent effect the paring reaction peels or "pares" methyl groups from the ring of polymethylated cyclic hydrocarbons and combines them to longer, preferentially branched side chains. Such branched alkyl groups are excellent leaving groups, what makes isobutene/isobutane the predominating aliphatic product. Ethyl and propyl groups are cleaved off as well, however, to a much less extend. Correspondingly * Present address: Siad-Chemie AG, Division Catalysts, Research & Development, Waldheimer StraBe 13, 83052 Bruckm0hl, Germany

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the paring reaction of C9-aromatics was found to occur much slower than with hexamethylbenzene [2]. In the mid 80s the naphthene hydrocracking route of the paring reaction over bifunctional catalysts was studied with reference to the introduction of the "Spaciousness Index" for characterizing zeolites [3,4]. It was found, that the paring reaction also occurs over shape selective zeolites such as HZSM5. The paring reaction of the polymethylaromatics, in contrast, was seldom reported in literature since the early work. This presumedly is due to the fact, that it hardly plays a role with the frequently studied small aromatics (e.g. xylenes) and at temperatures below 400~ Over Ni/silica-alumina "no conversion" of o-xylene to ethylbenzene was detected under conditions at which the paring reaction occured "readily" with hexamethylbenzene [2]. The cyclopentadienyl cation intermediate, which was postulated in the paring reaction of the polymethylaromatics [ 1,2], was derived from a mechanism, which had been suggested for the formation of ethylbenzene from xylenes [5] and is still the accepted mechanism [6]. This step also appears in mechanisms suggested for the mono-molecular 1,2-methyl-shift reaction [7] or the ionic aromatisation of olefins as in the methanol-to-gasoline conversion [8]. A 6/7-ring expansion mechanism for the methyl/ethyl-isomerisation is currently under debate [6]. 2. E X P E R I M E N T A L

1,2,4-Trimethylbenzene (1,2,4-TMB) was converted over HZSM5 and amorphous silica-alumina in the gas phase at 450~ and low partial pressure of 3.5 kPa (0.58 mmol/l). A tubular flow through reactor was used. The conversion was varied between 1 and 90% changing 1/WHSV between 1.7 and 7000 min. Product samples were taken at steady state after 120 min time on stream and analysed by capillary gas chromatography. HZSM5 was synthesised as described elsewhere [9] but with a synthesis time of 24 hours. The molar Si/A1 ratio was 45. Silica-alumina was obtained from Kali-Chemie (Si/A1 = 10). 1,2,4-TMB (98%) was obtained from Aldrich (reactive feed impurities such as butyl- and propylbenzenes were found to form an "initial cracking product", which needed to be considered thoroughly with regard to kinetic studies). 3. RESULTS AND DISCUSSION a) Silica-alumina

~

b) HZSM5

0.4

1= 0.3 .=_

.s ~ 0.2

~ 1 1,2,3 + 1,a,5-TMB

C Q U

"0.1 0 o 0 0

200 400 600 800 0 1 / W H S V in rain

2000 4000 6000 8000

1 / W H S V in min Fig. 1. Product composition from the conversion of 1,2,4-TMB over amorphous silica-alumina and HZSM5

3.1. 1,2-Methyl-shift isomerisation 1,2-methyl-shift yields the 1,2,3- and 1,3,5-TMB isomers. It was found to be the most rapid reaction of the feed molecule over both silica-alumina and HZSM5 (Figs. la, lb, full symbols) with about 90% selectivity at low conversion. This 1,2-methyl-shift was shown to take place on the external surface of the zeolite crystals [10,11]. The thermodynamic equilibrium of the TMBs was established after about 40% conversion (Fig. 2). In the

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

A .2

283

A

.o m l._

tn 0 . 8 l_

,_ 15 tn

o

E = 0.6

o

v

E

v

in I-

0.4

Ik=

Q

E 0 ._= 0.2 =E P"

.

.

.

.

.

.

.

.

.

.

.

.

w X

Pm

0

Fig. 2. Isomers distribution in the

TMB fraction from 1,2,4-TMB conversion over HZSM5. Horizontal lines: thermodynamic equilibrium

5-

~ m i n a 0

---

1.0-

0 50 100 Conversion of 1,2,4-TMB in %

0 50 1O0 Conversion of 1,2,4-TMB in %

_~ 0.08 o

10

q} I--

In

~

following, the lump of the TMBs is treated as a pre-equilibrium and termed the "TMB pool".

20

1

Fig. 3. Molar ratio of BTX aromatics to TeMBs from 1,2,4TMB conversion over HZSM5 and amorphous silica-alumina

O.8

Disproportionation only

0 HZSM5 [] Silica-alumina

0.6 o

0.04

"

"

--

\,,0 ",

1,2,4,5-TeMB TeMBs

g

~

0.02 -

o o

0 t 0 50 1O0 Conversion of the TMB pool in %

Fig. 4. Concentration of TeMBs in the product of 1,2,4-TMB conversion over HZSM5 and amorphous silica-alumina

0.2 ~

=--'- ' ~ "

[ p-Xylene 0 I Xylenes , 0 50 100 Conversion of 1,2,4-TMB in %

Fig. 5. "p-Selectivity" in the xylene and the TeMB fraction from 1,2,4-TMB conversion over HZSM5 and amorphous silicaalumina. Horizontal lines: thermodynamic equilibrium

3.2. D i s p r o p o r tionation Over silica-alumina, xylenes and tetramethylbenzenes (TeMBs) were the major products from the TMB pool (Fig. la). TeMBs and xylenes (resp. BTX aromatics) were formed in almost equimolar amounts (Fig. 3). The concentration of the TeMBs (Fig. 4) corresponded to the stoichiometry of disproportionation of the feed molecules. On HZSM5, TeMBs and BTX aromatics were formed as well, but no longer in equimolar amounts. Over ZSM5, the most slim isomer dominated the xylene and the TeMB fraction at low conversion (Fig. 5) indicating, that the disproportionation of 1,2,4-TMB takes place inside the pore system.

3.3. Dealkylation Over HZSM5, only minor amounts of TeMBs were obtained (Fig. l b). As compared to the stoichiometry of the simple disproportionation reaction, there was only a small amount of TeMBs (Fig. 4) but a surplus of BTX aromatics (Figs. lb, 3). Additionally, high amounts

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

284

of C 2- to C4-aliphatics were observed (Fig. lb). In Fig. 6 the bottom curve refers to the axis"

carbon in aromatic methyl groups El ~

in molexocyclic carbon / m~

carbon in aromatic rings / 6

rings

The bottom curve shows a balance over the methyl groups and the rings in the aromatic product (almost exclusively BTX aromatics and TeMBs). In this product the value of Y! is about 2, whereas in the feed the value of Y1 equals 3 (tri-methylbenzene). The missing carbon appears in the low aliphatics formed, as indicated by a balance over both the methyl groups and the C 2- to C4-hydrocarbons. This balance refers to the axis:

carbon in aromatic methyl groups and aliphatics Y2 ~

carbon in aromatic rings / 6

in molnon_cyclic carbon/mOlaromatic rings

and is shown in the top curve. The balance meets the number of 3 carbon atoms per aromatic nucleus when extrapolated to zero conversion. In Fig. 7 a carbon balance (bottom curve) and a ring balance (top curve) are shown. The declining curve of the former indicates coking (supported by the formation of traces of methane [12] and the colour of the used catalyst). The ring balance shows, that there was no loss of aromatic nuclei (except for coking) and confirms, that the C 2- to C4-aliphatics originate from the dealkylation of higher alkyl aromatics. With increasing conversion the curves in Fig. 6 decline, a surplus of aromatic nuclei appears (Fig. 7, in relation to the carbon balance) and the composition of the C 2- to C4-fraction turns from olefins to paraffins (not shown). This indicates, that the split off alkyl groups eventually form additional aromatics (co-product paraffins [8]). 110

m i,,,,,,,i

r,.-i

4-

+ 2

X +

+

I-

I-, . . m+

2

.=

+ !+

r =

•

+ I-

1

+

+ In

E - 100 ._=

6 loo

IZl

X + X

|

o

3

IZl I n

,r

110 .~

_~

.8

o c m

90

90

C

c 0

,,Q

,r

== ,Q

80

80

._u m

E o! _

0

0 0

50

+

Conversion of the TMB pool in %

70

!

0

100 L--.J

Fig. 6. Balances over exocyclic carbon and aromatic rings (bottom curve) and total noncyclic carbon and aromatic tings (top curve) in the product from 1,2,4-TMB conversion over HZSM5 (definition of Y-axes see text)

5O

70 100

<

Conversion of 1,2,4-TMB in %

Fig. 7. Carbon balance (bottom curve) and ring balance (top curve) in the product from 1,2,4-TMB conversion over HZSM5

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

O .m

A

L_

M 0

m

E

O r Q rN

285

105 - Conversion in % / ...... 8 9 /I . . . . . 75 1/ I ........... 37 ...... 20 104 " ---

4.3

2.8

~9 10 3

r /

w 0 M

E -9 l 02 m

I

I

.

**********

I

0

," ! ~ /

/

/

m

>, ,,C Q

//

E -o = 1 01 I) r

~

N r Q

m 10 0

//

"%.

:.~-'

i

"~0 i

8

9

.

i 6

i 7 Carbon

number

Fig. 8. C 6- to C9-methyl aromatics distribution in the product from 1,2,4-TMB conversion over HZSM5 at different conversion of the TMB pool. Equilibrium distributions have been calculated for mixtures with the very same average number of methyl groups as in the related product fraction

3.4. Paring reaction It was shown, that the C 2- to C4-aliphatics originated from side chains. Methyl groups, however, do not dealkylate readily over acid catalysts. Moreover, this step needs hydrogen and forms methane (which was only observed in traces). But aromatics with 4 and more methyl groups are able to undergo rapid paring dealkylation, thereby forming C4-olefins. It is concluded, that the disproportionation of 1,2,4-TMB over HZSM5 is followed by rapid paring dealkylation of the TeMBs. Both the steps are taking place inside the pore system as indicated by the increased p-selectivity (Fig. 5). In Fig. 8 the carbon number distribution of the C 6- to C 9methyl aromatics is shown at different conversion of the TMB pool, relative to benzene. At high conversion thermodynamic equilibrium is obtained, indicating, that the aromatics are interconverting via transalkylation. At medium conversion the curves separate and at low conversion a striking difference appears in shape and height (see solid lines). There are less C 7- to C9-aromatics than in equilibrium, indicating, that the dealkylation of the TeMBs indeed yields benzene and follows the classical paring mechanism as outlined above.

3.5. The effect of pore mouth narrowing Why does the conversion of 1,2,4-TMB over silicaalumina terminate more or less after the disproportionation step but continue via paring reaction over the medium pore zeolite? In the reaction sequence the TeMBs are intermediates. In such consecutive reactions the intrusion of diffusional resistances accelerates the conversion of intermediates [ 13-15]. We have demonstrated this recently by means of the crystal size effect on the methanol-todimethylether-to-olefins reaction over HZSM5 [16,17]. O - - - experimental Chemical vapour deposition (CVD) of alkoxy silanes calculated [ 18,19] forms an inert surface layer which reinforces the diffusional resistances by pore mouth narrowing. A novel technique of repeated low temperature CVD enables a well controlled stepwise growth of this diffusion barrier [ 19]. The conversion of 1,2,4-TMB over such CVD treated HZSM5 showed an increase in p-selectivity over several steps (and a decline again when the pores became inaccessible to the feed molecules) [10,11]. Fig. 9 shows the ratio of TeMBs to BTX aromatics, i.e. the ratio of the intermediates to the final products. Over the shape selective zeolite this ratio was already lower than over silica-alumina and decreased further through repeated CVD or through a non-active Silicalite shell. The increasing diffusional resistances extended the residence time of the TeMBs inside the zeolite crystals and accelerated the paring dealkylation. As TeMBs are comparably bulky species, the effect is significant. An increase of the reaction temperature (i.e. rate constants) also had a strong effect (Fig. 10), and supports the above conclusion.

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286

1. ~" Amorphous/ =-~0.8 silica-alumina

~0.6

-

Silicalite

coated

= 0.4 I~

Silanised,v,,,O

|

0

0

---

5 10 15 Number of CVD steps

Fig. 9. Effect of increased extemal diffusion resistances on the molar ratio of TeMBs to BTX aromatics from 1,2,4-TMB conversion. Conversion kept at ca. 10%

~o o0"08 n ] lDiY spioporti 0 o. nati0on6 =

/

=!

~ 0.04 8 ~ 0.02 oo

" 300~ 350~

/ ff~ / ~

400~

0

0 50 100 Conversion of the TMB pool in % Fig. 10. Concentration of TeMBs in the product of 1,2,4-TMB conversion over HZSM5 at different reaction temperature

REFERENCES 1. R.E Sullivan, C.J. Egan, G.E. Langlois, R.E Sieg, J. Amer. Chem. Soc. 83 (1961) 1156. 2. G.E. Langlois, R.E Sullivan, ACS Adv. Chem. Ser. 97 (1970) 38. 3. J. Weitkamp, S. Ernst, R. Kumar, Appl. Catal. 27 (1986) 207. 4. J. Weitkamp, C.Y. Chen, S. Ernst, Stud. Surface Sci. Catal. 44 (1988) 343. 5. F.E. Condon in "Catalysis", P.H. Emmet (ed.), Reinhold, New York, Vol. IV (1958) 111. 6. D.S. Santilli, B.C. Gates, in "Handbook of Heterogeneous Catalysis", G. Ertl, H. Kn/3zinger, J. Weitkamp (eds.), VCH, Weinheim, Vol. 3 (1997) 1123. 7. P.A. Martens, J. Perez-Pariente, E. Sastre, A. Corma, P.A. Jacobs, Appl. Catal. 45 (1988) 85. 8. C.D. Chang, Catal. Rev.-Sci. Eng. 25 (1983) 1. 9. R.W. Weber, J.C.Q. Fletcher, K.P. M/311er, C.T. O'Connor, Microporous Materials 7 (1996) 15. 10. H.P. ROger, K.P. M/511er, W. Btihringer, C.T. O'Connor, Proc. DGMK/AFTP/IP Conf. on "The Future Role of Aromatics in Refining and Petrochemistry", Oct. 1999, Erlangen, DGMKTagungsbericht 9903, G. Emig, M. Rupp, J. Weitkamp (eds.), DGMK, Hamburg (1999) 161. 11. H.P. ROger, K.P. M/311er,W. B/3hringer, C.T. O'Connor, Stud. Surface Sci. Catal., this volume. 12. H. Schulz, S. Zhao, W. Baumgartner, Stud. Surface Sci. Catal. 34 (1987) 479. 13. A. Wheeler, Adv. Catal. 3 (1951) 249. 14. M. Kotter, L. Riekert, Verfahrenstechnik 17 (1983) 639. 15. L. Riekert, Appl. Catal. 15 (1985) 89. 16. K.P. Mt~ller, W. Bi3hringer, A.E. Schnitzler, E. van Steen, C.T. O'Connor, Microporous Materials 29 (1999) 127. 17. K.P. MOiler, W. B/Shringer, A.E. Schnitzler, E. van Steen, C.T. O'Connor, Proc. 12th Int. Zeolite Conf., Baltimore, July 1998, M.M.J. Treacy, B.K. Marcus, M.E. Bischer, J.B. Higgins (eds.), Materials Research Society, Warendale, Pa. (1999) 591. 18. M. Niwa, H. Itoh, S. Kato, T. Hattori, Y. Murakami, Chem. Commun. 15 (1982) 819. 19. H.P. RSger, M. Kramer, K.P. M/511er, C.T. O'Connor, Microporous Materials 21 (1998) 607.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

SAPO-34 Catalyst For Dimethylether Production Grigore P O P , Cristian T H E O D O R E S C U S.C. ZECASIN S.A. Splaiul Independentei 202, Sector 6. 77208-Bucharest Romania Thermodynamic analysis of dimethylether (DME) formation in single or two-stage processes shows high methanol conversions at low pressure and a better equilibrium transformation of syngas to DME in comparison with methanol synthesis. For large capacity DME production plants, a SAPO-34 zeolite in alumina matrix has been synthesized. Methanol dehydration to DME was used as a test reaction. Over this catalyst, thermodynamic equilibrium is virtually reached at 250~ and practically nr, side reactions occur. The catalyst is used in the industrial unit set up by ZECASIN in Brazi, Romania. 1. INTRODUCTION For many years, dimethylether (DME) had very limited uses (e.g., methylation agent in organic chemistry [1], cold start fuel for Diesel engines or as additive to coal gas to reach higher calorific value [2]). Due to its physical, chemical, toxicological properties and environmental friendliness, now DME is considered one of the best substitutes for CFC's and is used as propellant in aerosol industry [3] and refrigerant in cooling devices [4]. The main features of DME as Diesel fuel [5] are very low emissions, smokeless operation, quiet combustion, high Diesel thermal efficiency, low pressure, low noise, low cost fuel injection and turbo-charged. Diesel engines that run on DME fully comply to the drastic California's ULEV regulations for commercial trucks and buses [6]. Recent analyses [7] have also revealed that DME is an economically alternative to LNG, natural gas conversion to methanol or Fischer-Tropsch-like syntheses and even for turbine power generators. For energetic uses, DME must be produced cheaply in large capacities. For this goal, two technological approaches may be considered:- Single stage conversion of syngas to DME - Two-stage process, syngas to methanol conversion followed by a subsequent dehydration to DME. These alternatives involve three equilibrium reactions, so two main conditions are critical for an economical successful process: - Establishing and select the thermodynamically feasible domains - Synthesis of an active and selective catalyst that can operate very close to thermodynamically limitations. DME synthesis catalyzed by strong mineral acids and their Zn, Fe, Cu, Mn, Sn salts is well known for many years [8] and was patented as early as 1932 [9]. These technologies need high pressure, involve high corrosive and polluting media and cannot be used as dehydration components in single-stage process catalysts. In gas-solid catalysis, alumina, modified alumina, silica-alumina, thoria, zirconia, titania have been tested [9,10], as well as alumina treated with HF, H2SO4, HNO3 [11,12,13]. Alumina or silica-alumina with or without doping have been

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288

recommended and used in the first step (methanol dehydration to DME) of methanol to gasoline (MTG) technology [14,15,16]. DME as primary product in methanol conversion to hydrocarbons is mentioned in almost all studies concerning these processes. Over gallosilicates [17] and Cr-silicates [18] with pentasil structure, DME yields increase with Ga and Cr content as well as the aromatic hydrocarbon formation. Methanol TPD studies on SAPO-34 [19] and SAPO-44 [20] zeolites have shown DME formation at 200~ and DME, olefins and coke formation at temperatures over 290~ In the single-stage process starting from syngas (CO+H2) dual-function catalysts are used. They consist of a methanol synthesis component and a dehydration component. The two components must be compatible and sustain the conditions of both reaction steps, methanol synthesis and methanol dehydration. Alumina, boron modified alumina [21,22] and H-ZSM-5 zeolite [23] were used as methanol dehydration components in dual catalysts. Nowadays, industrial units for DME manufacture via gas-solid catalysis use alumina or slightly modified alumina. Their catalytic activity and selectivity in the reaction of methanol dehydration to DME, especially in dual function catalysts are poor. The aim of the present work was to synthesize an active and selective catalyst, based upon SAPO-34 zeolite for large-scale production of DME close to thermodynamic equilibrium, both in single and in two-stage technologies. 2. THERMODYNAMICAL ANALYSIS Known values of AGr and temperature-dependence of molar heat capacities for each specie in the system are used to estimate the integration factor/, which is then used to infer the temperature dependence of AG;. Alternatively, the Temkin-Schwartsman equation was also used - with similar results - in order to verify the results given by the Gibbs-Helmholtz equation: AG~ = 5J-I; - TAS; = 5J42~ +

CpdT - T 5S~8 + z~8 T

dT

2

Single stage process. It consists of the overall chemical reaction:

2 CO + 4 HE r CHa-O-CH3 + H20 + (AHR) (1) The process takes place with volume contraction and therefore should be enhanced by high operating pressure. The thermodynamic equilibrium curves in Fig. 1 show a sigmoid shaped variation, having an inflexion point corresponding to relatively low temperatures and low conversions. Two-stage process. This alternative follows the subsequent chemical reactions: I. CO + 2H2 r CHaOH + (AHR) (2) II. 2CH3OH r + H20 + (AHR) (3) The methanol synthesis being a well-known industrial process, the thermodynamic analysis was focused on the second stage of the DME synthesis only (methanol to DME conversion). The reaction takes place without volume change so it can be conducted at moderate pressures, to limit the reactor volume. The thermodynamic equilibrium curve in Figure 1 indicates that methanol conversion as large as 90% could be attained even at 500K. It follows that the twostage process could be a feasible industrial alternative. In single stage process at 527K, 50-70% conversion to DME can be obtained, much higher than in modem low-pressure methanol synthesis.

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3. EXPERIMENTAL

SAPO-34 catalyst synthesis. SAPO-34 zeolite has been obtained according to the previously described method [24] and is only summarized here. Hydrated alumina (65% wt AL203) is suspended in demineralized water and is charged in the crystallization vessel over tetraethylammonium phosphate (25% wt in aqueous solution). After thoroughly mixing, 28% SiO2 silica sol is added. The pH of the resulted suspension is 6.3 - 6.5. Zeolitization process takes place at 200~ in several steps, with periodic pH correction. After each 15 hours period, the reaction autoclave was cooled at room temperature, the pH of the reaction mixture was corrected to the initial pH value by adding H3PO4 85% and the crystallinity of the formed zeolite was measured. The effective duration of the crystallization process was 100 - 140 hours. The crystallization process was conducted in three different capacity levels, 10 ml o 0.8 ~ .... i (metallic autoclave, static conditions- no mixing), 750 ml (teflon-lined autoclave, 9 0.6 stirring) and 3500 1 (industrial autoclave, O o 0.4 continuous stirring). In the small 10 ml 0.2 autoclave, thecrystallization process was I| 0 studied for different initial reaction 0 I mixture compositions, obtained by o 300 400 500 600 700 T,C modifying the molar ratio r = A 1 2 0 3 / P 2 0 5 Xe, 1 Bar; . . . . . . . . Xe, 20Bar; between 0.6 and 2, the other ratios being ...... MeO H to DME set constant, 1.0 P205: (0.6-2) A1203: Figure 1. Equilibrium in DME synthesis 0.35 SiO2 : 1.1 TEA-OH : 130H20. After crystallization, the reaction mixture was cooled, centrifuged, the zeolite washed with demineralized water and finally dried for 4 hours at 110~ The optimum composition of the SAPO-34 zeolite was reproduced at the other 2 scale levels (750 ml and 3500 1 autoclaves). Dried SAPO-34 zeolite was mixed with hydrated alumina in different concentrations and after peptization with HNO3, was extruded to pellets 1.5 mm diameter, dried for 6 hours at 110~ and calcined for 8 hours at 550~ SAPO-34 zeolite was characterized by chemical analysis, XRD, SEM, thermogravimetric analysis [25], NH3-TPD and n-hexane molecular traffic [26]. For crystallinity degree and SAPO-34 yield, 5 characteristic XRD peaks were considered. -

4. RESULTS AND DISCUSSION As a test reaction, the methanol conversion to DME was used. The experimental method is described in [27]. For large-scale demonstration of the catalyst performances, an isothermal, fixed-bed reactor filled with 100 1 catalyst was operated for hundreds of hours. The reaction products were analyzed by gas-chromatography, after condensing and separating the process water and the unconverted methanol, using a coupling of two columns (1 st column: 6 m length, 4 mm inner diameter, filled with bis-2-metoxy-ethyladipate on Chromosorb P 60-80 mesh; 2 nd column: 3 m length, 4 mm inner diameter, filled with tris-ciano-etoxy propane on Chromosorb P 60-80 mesh). Unreacted methanol, condensed in the process water and dissolved DME were analyzed separately on a column identical to the 2 nd one described above. The reaction used 99.9% wt methanol feed.

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Figure 2a presents the evolution of the crystallization process in the 10 ml autoclave and static conditions, for 5 different chemical composition of the initial amorphous gel obtained by modifying the r molar ratio in five steps: - 0.6, 1.0, 1.4, 1.65 and 2.0. For all compositions, an important crystallization inhibition time-lag is indicated by a relatively small pH increase and low crystallinity after 50 hours effective crystallization duration. The pH vs. time curve shows a Gaussian shaped enveloping curve but the crystallinity increases continuously except for the cases of the extreme r molar ratio samples (0.4 and 2.0) where crystalline degree greater than 50% could not be obtained. 8.9

=7.9 '

/ =

.4f

6.9

//:, / / y

6.4

0

50

lOO

~75

///:

100

"E 50

~5 ~ o

150

T i m e , hours

a

-,--r=0.6;

=

r=l.0;

--o-- r=1.65;

=

r=2.

40

9 r=1.4; - - - envelope

Figure 2. Time evolution of

(a)

90

"lqrr~h

140

b --,-r-~.6; --*-FI.0; --,-r=1.4; --o-r=1.65; --,-r~

pH and (b) crystallinity during SAPO synthesis (200~ autoclave).

10 ml

In addition, in these cases, after 100 hours, an important decrease of the crystallinity degree is observed (Figure 2b). All the compositions have a similar evolution of the crystallinity if no pH correction is performed. The final pH difference considered after 140 hours reaction time, have also extreme values for extreme r ratios, namely ApH = 0 for r = 0.6 and ApH=I.5 for r = 2.0. -4

100

9~ INXI?

-3

95

E

~- 90

E

2 ~

dEE

-~9 85

t--

~ 8o

!

-

0

i

i

10

20

i

30lqme, min

Figure 3. NH3-TPD curve for SAPO-34 zeolite

1

._~

0

75

E

.g

0

200

400

600 Temperature, ~

Figure 4. Thermogravimetric analysis of the assynthesized SAPO-34 zeolite.

The compositions with r = 1.65 and 1.4 show the best crystallization process evolution. It can be inferred that for these compositions, the sum of Si and P atoms nearly equals the number of A1 atoms so the optimum atom space arrangement of SAPO-34 network would be: [Si-AI-(P-Si)]n. For confirmation of this hypothesis, more experimental work has to be made, especially NMR studies. In our set of experiments, the optimum initial gel composition is, SiO2: A1203:P205 = 0.35: 1.35: 1.0.

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This composition has been used for synthesizing SAPO-34 in the 750 ml and 3500 1 autoclaves and for catalyst preparation in a pilot plant and commercial scale. For near 100% crystallinity 70 hours effective reaction time proved enough. XRD spectra of the synthesized SAPO-34 zeolite correspond to the data published [24,28]. SEM micrographs exhibit uniform cubic shaped crystals of ca. 2 lxm dimension. Chemical analysis showed practically the same composition as the initial gel. NH3 - TPD curve (Figure 3) is similar with the known literature data [26,29]. It shows the following number of acid centers/g zeolite: 1.9xl 020 weak, 6.0xl 020 medium and 4.9xl 02~ centers. Thermogravimetric curves of the as-synthesized zeolite presented in Figure 4 show 18.78% zeolite total weight loss. It is interesting to observe that the derivative TG curve has a similar shape to that of the NH3 TPD curve, with better-defined peaks. So, when the template is a basic chemical like tetraethylammonium hydroxide, the differential curve of the thermogravimetric analysis indicates also the acid center distribution in zeolite. In the present case, the peaks indicating TEA-OH loss shift towards higher temperatures in opposition with NH3 desorption peaks on the NH3-TPD curve. Our SAPO-34 zeolite has only 42.4% n-hexane free access in its pores, 57.6% being physically adsorbed on the surface. 50 4O "~30 E o~ 20

n

10

~

...=

....

~

.=..

w

. H2Q BNE, eq. - - - MeO-I, ec~ A H20, I:XVE,SAPO-34 o Me(~ SA1:~34 ,, 1-120,DIVE, alumna x ~ akn'ina

0 100

150

200

250 Terq3e'atLre, C.

Figure 5. Temperature and catalyst influence on composition. Some preliminary tests at 200~ 250~ and methanol liquid hourly space velocity (LHSV) 1.0 h l with catalysts containing 10 - 50%wt SAPO-34 zeolite in alumina matrix showed that catalytic activity does not increase for catalyst compositions containing more than 30% zeolite. The catalyst with 30% zeolite and 70% alumina was tested in methanol conversion to DME at normal pressure, LHSV 1.80 h 1 and temperature between 150 - 280~ The results were compared with alumina matrix activity. As can be seen from Figure 5, over SAPO-34 catalyst, equilibrium values of methanol conversion and reaction product composition are reached at about 250~ At this value, practically only DME and H20 result. In the reaction product, less than 0.5% methane + ethylene were analysed. The catalyst produced at industrial scale has been tested in a pilot-scale fixed-bed reactor filled with 100 1 catalyst with same performances. After 300 hours on stream the catalyst held its initial activity. 5. CONCLUSIONS a. A very pure SAPO-34 zeolite can be synthesized by autogeneous pressure, 70 hours at 200~ from a gel with composition expressed in mole ratios, 1.0P2Os:l.35A1203:0.35SiO2:l.10 TEA-OH: 130 H20. The zeolite network seems to have a distribution like [Si-AI-(P-Si)]n. In any case, the optimal crystallization conditions were obtained when the number of Si+P atoms in the initial gel have been set equal to that of the A1 atoms.

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b. A very active and selective catalyst for methanol dehydration to DME has been prepared by including SAPO-34 zeolite in a 70% alumina matrix. Over this catalyst, methanol equilibrium conversion and equilibrium reaction product composition are virtually reached at 250~ Its performances are demonstrated at pilot and industrial scales. e. The catalyst is suitable for both single-stage and two-stage dimethylether production technologies d. The process based on this catalyst is patented [30] and used in ZECASIN industrial plant in Brazi, Romania for DME manufacture from methanol. REFERENCES

1. H.J. Derdulla, I. Hacker, M. Hemke and K. Rebbe, Chem. Techn., 29 (1977), p. 145. 2. J. Dippo, M.Karpuk, and S. Cowley, in IX Intl Syrup on Alcohol Fuels, ISAF, Firenze, 1991, p. 580. 3. R. Koene, Aerosol Report, 30 (1) (1991), p. 26. 4. Gr. Pop, T. Cherebetiu, R. Boeru, Gh. Ignatescu and V. Albulescu, in Proc. 1~t Trabzon Int. Energy Environ. Syrup., Trabzon (Turkey), July 29-31, 1996, p. 899. 5. T.H. Fieisch, C. McCarthy, A. Basu, C. Udovich, P. Charbonneau, W. Slodowska, S.F. Michelsen and J. McCandless, in SAE Intl Congress and Exhibition, Detroit (USA), March 1995, SAE paper 950061. 6. A. Maureen Rouhi, Chemical & Engng. News, May 29, 1995, p. 37. 7. Yoshitsugi Kikkawa, Ichizo Aoki, (1998), Oil & Gas Journal, April 6, p. 55. 8. Ullman's Encyclopaedia of Industrial Chemistry, 5th Edition, VCH Verlag, Vol A8, p. 541. 9. R.L. Brown and W.W. Odels, US Pat 403412 (1932) 10. H. Dornhagen, H. Hammer, B. Haas and E. Meisenburg, US Patent 4 885 405 (1989). 11. L.I. Ali and N.N. Amin, Egypt J.Chem, 34 (40), 1990, p. 445. 12. J. Ogonowski and E. Seikors, Przem. Chem., 75(4), 1998, p. 135. 13. Mitsui Toatsu Chemical Inc., J.P. Patent 0285244 (1990). 14. F.G. Dwyer and A.B. Schwartz, US Patent 2818831 (1978). 15. W.K. Bell and C. Chang, Ger. Patent DE-OS 3201155 (1981). 16. N.Y. Chen, W.G. Garwood and F.G. Dwyer, Shape Selective Catalysis in Industrial Application, Marcel Dekker Inc., 2nd Ed., 1996, p. 124. 17. E. Lalik, J.Phys.Chem., 96 (2), 1992, p. 805. 18. Sugimoto, Michio, Appl. Chem., 80 (1), 1992, p. 13. 19. S. Hocevar and J. Lever, J. Catalysis, 135 (1992), p. 518. 20. S. Hocevar, J. Batista and V. Kancic, J. Catalysis, 139 (1993), p. 351. 21. K. D. Vollgraf, A. Zahn and H. Hoffmann, Chem Ing.Techn., 66 (10), 1994), p. 1378. 22. G. Pagani, RO Patent 65 198 (1979). 23. A. Seidel-Morgenstern, K.D. Vollgraf, H. Zammer and H. Meyer, in 4th Congress of Chemical Engineering,, Karlsuhe (Germany), June 16-21, 1991, p. 61-64. 24. Gr. Pop, R. Ganea, D. Ivanescu, Gh. Ignatescu, R. Boeru and R. Birjega, P. C. T. Appl. R099/00001 25. Gr. Pop, R. Ganea, R. Birjega and S. Serban, Progress Catal., 1, 1993, p. 1. 26. P. Tomi, E. Pop, C. Craiu, G. Musca, Gr. Pop, Progress Catal., 1, 1992, p. 1. 27. Gr. Pop, G. Musca, D. Ivanescu, E. Pop, G. Maria, E. Chirila and O. Muntean, in Chemical Industry, L.F. Albright, B.1. Crynes and S. Newark Eds, Marcel Dekker Inc, New-York, Vol 46, 1992, p. 443. 28. B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannon and E.M. Flanigan, US Patent 4440871 (1984). 29. Liang Juan, Li Hongyuan, Zhao Suquin and Guo Wengui, in Preprint Poster Paper. The 7th Intl Conf., Tokyo (Japan)Aug 17-22, 1986, 3D-16. 30. Gr. Pop, RO Patent 107 249 C (1993).

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化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Catalytic performances of Pillared Beidellites compared to Ultrastable Y zeolites in Hydrocracking and Hydroisomerisation Reactions J.A. Martens a, E. Benazzi b, J. Brendl6 c, S. Lacombe b and R. Le Dred c a Centrum voor Oppervlaktechemie en Katalyse, K.U. Leuven, Kard. Mercierlaan 92, B-3001 Heverlee, Belgium b

IFP, 1 & 4 avenue de Bois-Pr6au, 92852 Rueil-Malmaison Cedex, France

c Laboratoire de Mat6riaux Min6raux, UPRES-A 7016, 3 rue A. Werner, 68093 Mulhouse Cedex, France The catalytic performances of a USY and of three pillared beidellites (pillars as Si, Si/A1 and A1) were compared using as model reactions the n-decane and the n-heptadecane hydroconversion. It was found that the beidellites have an activity on catalyst mass basis similar to that of ultrastable Y zeolite and lead to a higher yield of skeletal isomers. The product patterns show some clear differences, which may attributed to the peculiar pore structure of the clays. 1. INTRODUCTION The zeolites are well known for theft hydrocracking and hydroisomerisation performances, due to their high acidity and shape selectivity. Dealumination is often performed in order to optimize the acidity, stabilize the zeolite and/or enhance the hydrocarbon diffusion. An increasing interest is put on pillared clays which may present a larger pore diameter than the large pore zeolites, and therefore lead to a different shape selectivity. In this paper the catalytic performances of a USY and of pillared beidellites were compared using as model reactions the n-decane and the n-heptadecane hydroconversion. The purpose of the catalytic work was first to characterize the pore architecture of these beidellites at catalytic temperatures (the n-decane test [1] and the n-heptadecane test [2]), and second, to evaluate the potential of these catalysts for application in refining processes. 2. E X P E R I M E N T A L The host clay used to prepare the Si and Si/A1 pillared beidellites (samples B-Si and B-SiAl respectively) was a H-beidellite obtained after calcination of a NH4-beidellite synthesized in acidic fluoride medium [3]. An hydrogel of the molar composition 1 SiOz ; 0.44 A1203 ; 0.25 NH4F; 0.15 HF ; 48 H20 was matured at room temperature before being heated in a PTFE lined steel autoclave at 493K under autogeneous pressure for 2 days. After cristallization, the autoclave was cooled to room temperature. The product was filtered, washed with distilled water and dried at 333K overnight. The chemical formula per half a unit cell is:

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(NH4)o.78(Sia.22Alo.7$)(A12)Olo(OH1.5Fo.5)1.7H20. To obtain the H-form beidellite, this product was calcined under air at 773K during 4 hours. 4g of H-beidellite were then fully dispersed in 115 mL of a 0.1 M aqueous solution of hexadecyltrimethylammonium chloride. After stirring at room temperature for 1 h, the exchanged beidellite was filtered, washed with distilled water and air-dried overnight at 333 K. 4 g of this organo-beidellite derivative were then dispersed in a mixture containing 2.24 g of octylamine and 30.16 g of tetraethylorthosilicate (sample B-Si). To obtain the Si/A1 pillared beidellite (sample B-SiAl), 1.48 g of alumina isopropoxide was added [4]. After stirring at room temperature during 30 min, the resulting products were filtered and directly air dried at 333K without washing to further promote intragallery tetraethylorthosilicate hydrolysis. The basal spacing of the solids were respectively equal to 3.34 and 3.00 nm. The Alia pillared beidellite (sample B-A1) was prepared according to Miehe et al [5]. In this case, the host material was a synthetic Na-beidellite with the following chemical formula per half a unit cell: (Na)o.5o(Sia.5oAlo.5o)(A12)Olo(OH1.58Fo.42)2H20. 3 g of Na-beidellite were added to 56 mL of a pillaring solution prepared following Urabe et al [6]. After 8 min of stirring at room temperature, sample C was filtered, washed with distilled water before being air dried at 333K. The basal spacing of the air dried product equalled 1.79 rim. Sample B-Si, B-SiAl and B-A1 were calcined under air at 823K for 4 hours. The sample B-A1 was finally ammonium-exchanged under reflux conditions using NH4C1 solution. The BET surface areas of the three materials were around 340 m2.g-1. The USY samples with code names CBV-712, CBV-720 and CBV-760 were from Zeolyst International. The acidity of these zeolites with bulk Si/A1 ratio of 5.8, 13 and 30, respectively, has been characterized in detail [7]. The acid site concentration determined with ammonia TPD decreases in the order CBV-712 (691 gmol/g) > CBV-720(620 gmol/g) > CBV-760 (148 gmol/g). The acid solids were converted into bifunctional catalysts by impregnation with platinum (II) tetramine chloride, calcination and reduction at 400~ The final catalysts contained 0.5wt% Pt. The performances in hydroisomerisation and hydrocracking were evaluated in a lab scale reactor at a hydrogen pressure of 0.45 MPa and a hydrocarbon pressure of 1 kPa. The space time of hydrocarbon was 0.5 and 0.2 kg.s.mo1-1 for n-decane and n-heptadecane, respectively. 3. RESULTS AND DISCUSSION

3.1. Hydroisomerisation and hydrocracking of n-decane The evolution of the n-decane conversion with temperature, the corresponding yields of total isomers, monobranched isomers and multibranched isomers and the yield of the cracked product fractions at 35% cracking yield on the beidellite and the USY catalysts are shown in Fig.1. The beidellites are more active than the USY samples, for which the activity decreases with decreasing acid site concentration. The beidellite with Si pillars is more active than the mixed SiAl and the A1 pillared versions. The yield of isomerisation curves are higher with the beidellites, and especially the Si and SiAl versions, compared with the USY samples. These higher yields are found in the monobranched as well as the multibranched isomer fractions. It is concluded that the preparation method involving the organo-beidellite intermediate leads to better catalysts compared to the method using the more classic All3 pillars.

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100n IB-Si90

.B.SiA1

80

1

~

/

=I~I . . i ~ '

/

~k"A

~CBV-720/ 1 ~ v ~CBV_760j i ~t~O

60

g

#

O m -~

1o'

=~nP1

"~ tD 50 40 o 9-~ 30

"#o 20 I.

IIB-Si oB_SiAI A,B-A1 mCBV_712 •CBV-720

90 ~, ~ 80 = 70 9 .~ 60

A, ~

~c.v-71z['B-AI'- - l [ ~

~.'70

100

a~

/k 10 O--

0 375 400 425 450 475 500 525 550 575

0

Temperature (K)

10 20 30 40 50 60 70 80 90 100 Conversion ( % )

D

C 50

IIB-Si ] *B-SiAl I AB-AI I []CBV-7121 ~ OCBV-7201 ~ ~ ACBV-

~, 45 ~ 40 r O 35 ,~ 30 tD a= 25

~

~

~

~ ~

25 -9 .~ 20

N 2o

~15

x~ o 15 o 10

"~ 10 ~

~ 5 0-0

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8oli

70 ~ 60

% 0

10 20 30 40 50 60 70 80 90 100 Conversion (%)

F 80 m / ~ ~ ~ ,,~

% 50

F

i~iCBV-712I

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20

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10

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~ 10

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3

4

5

6

7

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8

9

......=...-:~--....

60 •CBV-7601

~ 50 o

40

1

~~t~l

5

10 20 30 40 50 60 70 80 90 100 Conversion (%)

liB-Si *B-SiAl "B-A1

liB-Si *B-SiAl AB-A1 []CBV-712 o CBV-720 &CBV-760

30

_II. r . B

o

,

1

~./

~

2

I

.,,,,

g\ =

.

3

4

9

5

|

6

|

7

'i~m ~

8

m ==

9

Carbon number Carbon number Fig 1. Hydroconv. of n-Clo on pillared beidellites and USY catalysts. A, n-Clo conv. against T; B, yield of isom. versus n-C~o conv.; C, yield of monobranched isomers versus n-Clo conv. ; D, yield of multibranched isomers versus n-Clo conv.; E and F, yield of cracked products per carbon number fraction and per 100 mol of n-Clo cracked at 35% cracking conv..

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The distribution curves of the molar yields of cracked products per carbon number, expressed as mol per 100 mol of cracked n-decane at 35% cracking display a maximum at C5 for the beideUite as well as for the USY samples. Methane and ethane pointing to hydrogenolysis were not formed, excepted in the B-A1 sample. The presence of hydrogenolysis has previously been encountered with All3-pillared beidellites synthesized in alkaline medium [8]. The B-Si and B-SiAl samples show almost symmetric cracked product distributions. The occurrence of one single cracking event in each Clo chain is characteristic of 10- and 12-ring zeolites with intersecting channels [9]. The cracking pattern of n-decane suggests that the gallery between the pillars of the beidellite has similar properties. On the beidellites, the yields of propane and heptane are relatively low, as on the USY samples. Such patterns are typical for bifunctional catalysis in a large pore zeolite [ 10]. The B-Si and B-SiAl samples favour the central cracking of the chain even somewhat more than the USY samples. The n-decane test [1] is based on product criteria from hydroisomerisation and hydrocracking of n-decane and can be handled to rank materials according to their shape selective properties (Table 1). Table 1. N-decane test on pillared beidellites and USY zeolites criterion B-Si B-SiAl B-A1 CBVCBVCBV- 12-MR 10-MR 712 720 760 zeolites zeolites CI ~ 1.6 1.5 1.9 1.4 1.3 1.5 1-2.2 >2.7 EC8 b 12.2 12.2 10.5 11.7 11.1 10.7 >5 <2 3/4EC8 c 0.7 0.6 0.6 0.6 0.6 0.6 PC7 c 1.7 2.0 1.5 1.1 1.1 1.2 >0 0 iC5 a 56 50 37 55 54 46 > 35 a, refined constraint index, corresponding to the ratio of the rates of formation of 2methylnonane/5-methylnonane at 5% isomerisation yield. b, content of 3- and 4-ethyloctanes in the monobranched isodecanes at 5% isomerisation yield. c, content of 4-propylheptane in the monobranched isodecanes at 75% conversion. d, yield of isopentane expressed in mol/100 mol n-decane cracked at 35% cracking yield.

The beidellites and USY samples have a similar refined constraint index, CI ~ situating both materials among the 12-ring zeolites. There is a considerable formation of the bulky ethyloctanes. The EC8 criterion is presented for a wider selection of zeolites in Fig.2. According to the EC8 criterion, the beidellites are situated between the larger pore omega and L zeolites and the USY zeolites. Other 12-ring zeolites like Beta and Offretite have much smaller pores. According to the EC8 criterion, the B-Si and B-SiAl samples have slightly larger pores than the USY samples. The B-Al sample has slightly narrower pores. Another criterion concerning the ethyloctanes is the ratio of 3-ethyloctane over 4-ethyloctane at 5% isomerisation yield. Kinetically, the formation of 3-ethyloctane is favoured, whereas thermodynamically, 4-ethyloctane is the favoured isomer. The ratio of 3-ethyloctane to 4ethyloctane at thermodynamic equilibrium is ca. 0.6. The equilibration of the EC8 fraction has to occur via ethyl shifts, which can be sterically hindered. The 3/4EC8 ratio of beidellites and USY zeolites close to the value of 0.6 shows that there is no sterical suppression of the ethyl shift. The bulkiest isomer of n-decane is 4-propylheptane. The same tendencies as for the ethyloctanes are observed, namely that the B-Si and B-SiAl samples have slightly wider pores

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than the USY samples. The conclusion from the n-decane test is that the beidellites with Si and SiAl pillars have slightly wider pores than in fauiasites. The same beidellite pillared with Alia has slightly narrower pores. This is consistent with the order of the basal spacing values. The pore architecture of A1 pillared beidellite synthesized in fluoride medium is very similar to that of beidellite synthesized in alkaline medium [8].

ECe 15

(%)

omega I"

/L

I~B-SiAI

12 pBV.712-~BV.720 / CBV-760 ~B-AI

offretite beta

SAPO-5

phi

~

I~

NU-13

ZSM-12

SAPO-11__} MCM-22

ZSM-22 ZSM-23 I

85

90

ferrierite

..1..O..-.M._.R....Z.eO..!!t.OS.... I

I

95

100

MC9 (%)

Fig. 2. Content of ethyloctanes (EC8) versus methylnonanes (MC9) in monobranched decane isomers at 5% isomerisation.

3.2 Hydrocracking and hydroisomerisation of n-heptadecane A catalytic experiment was performed with the most active beidellite catalyst B-Si. The maximum isomerisation yield was 35%, which is lower than with USY zeolites [11]. The nheptadecane test [11,12] was applied to compare the shape selective properties of the B-Si sample with those of large pore zeolites (Table 2). Two criteria are handled in the n-heptadecane test. The 2,3DMC4 and C5/C6 product characteristics allow to probe the cage and window structure. When the cages are too small to trap the entire molecule, part of the carbon chain has to be located in the window separating the two cages. Table 2. Content of 2,3-dimethylbutane (2,3DMC4) in the C6 cracked product fraction (mol%) and C5/C6 cracked product molar ratio at 35% cracking conversion (data from refs.11 and 12) zeolite topology 2,3DMC4 C5/C6 USY FAU 3.7 0.98 Y FAU 3.8 1.02 EMC-2 EMT 5.0 1.08 B-Si 6.0 1.17 Beta BEA 9.3 1.17

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In the window, cracking is suppressed. A high C5/C6 ratio and a high content of 2,3-dimethylbutane in the C6 fraction indicate that the individual cavities, or channel intersections are too small to accommodate the C17 molecule. This is clearly the instance in the B-Si sample. Its cavities are smaller than the supercages of faujasite, and are situated between the hypocages of the EMT structure and the intersections of a beta zeolite (Table 2).

4. CONCLUSIONS Very active beidellites were synthesized in fluoride media under relatively mild synthesis conditions and pillared with Si and SiAl according to an original method. This method leads to superior catalysts with respect to activity and pore size compared to traditional pillaring with Al13. According to the n-decane and the n-heptadecane tests, the B-Si and B-SiAl samples are quite similar. They have multidimensional cage and window structure, with cages that do not impose sterical limitations on the reaction of n-decane. A cage and window effect is present with n-heptadecane, suggesting that the size of the cavities is comparable to that of hypocages of the EMT structure or an intersection of a beta zeolite. REFERENCES

1. J.A. Martens, M. Tielen, P.A. Jacobs and J. Weitkamp, Zeolites, 4 (1984) 98. 2. J.A. Martens, G.M. Vanbutsele and P.A. Jacobs, Proceedings 9th IZC, Montreal, II (1992) 355, R. von Ballmoos, J.B. Higgins and M.M.J. Treacy (eds.), Butterworth-Heinemann, London, 1993. 3. L. Huve, R. Le Dred, J. Baron and D. Saehr, in Synthesis of Microporous materials II: Expanded Clays and Other Microporous Solids, M.L. Occelli and H. Robson (eds), Van Nostrand, New York, 2 (1992) 207. 4. J. Baron, E. Benazzi, J. Brendl6, N. Marchal-George, S. Lacombe, R. Le Dred, D. Saher, European Patent EP 908233 A1 (1999). 5. J. Miehe, R. Le Dred, D. Saehr and J. Baron, in Synthesis of Porous Materials, Zeolites, Clays and Nanostructures, M.L Occelli, and H. Kessler (eds.), Marcel Dekker, NewYork, 1996, p.491. 6. K. Urabe, H. Sakurai and Y. Izumi, J. Chem. Soc. Chem. Commun., 110 (1991) 1520. 7. M.J. Remy, D. Stanica, G. Poncelet, E.J.P. Feijen, P.J. Grobet, J.A. Martens and P.A. Jacobs, J. Phys. Chem., 100 (1996) 12440. 8. R. Molina, S. Moreno, A. Vieira-Coelho, J.A. Martens, P.A. Jacobs and G. Poncelet, J. Catal., 148 (1994) 304. 9. J.A. Martens and P.A. Jacobs, Zeolites, 6 (1986) 334. 10. J. Weitkamp, Erdoel, Kohle, Erdgas, Petrochem. Brennst.-Chem., 31 (1978) 13. 11. J.A. Martens, M. Tielen and P.A. Jacobs, Stud. Surf. Sci. Catal., 46 (1989) 49. 12. E.J.P. Feijen, J.A. Martens and P.A. Jacobs, Stud. Surf. Sci. Catal., 101 (1996) 721.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Effect of Ni on K-Doped Molybdenum-on-Carbon Catalysts: TemperatureProgrammed Reduction and Reactivity to Higher-Alcohol Formation a E.L. Kugler, L. Feng b, X. Li b and D.B. Dadyburjor c Department of Chemical Engineering, CEMR, West Virginia University, P.O. Box 6102, Morgantown WV 26506-6102, USA Temperature-programmed reduction (TPR) runs were carried out on activated carbon, a series of Mo/C with different Mo loadings, Mo-K/C with different K/Mo ratios, and Mo-NiK/C with different preparations. The data, after accounting for the C support, could be deconvoluted to two low-temperature peaks and one high-temperature peak. Reducibility was quantified in terms of Mo valence. The materials were also used as catalysts for the conversion of synthesis gas to high-molecular-weight ("higher") alcohols (HA). Calcination of the catalysts and the order of impregnation play a role in the Mo valence and in the performance of the catalysts. Lower values of Mo valence correspond to improved catalysts for HA synthesis. 1. INTRODUCTION Mo-based catalysts promoted by K have been used by us [ 1] and others for the synthesis of mixed alcohols from syngas. The catalytic performance depends, to a certain extent at least, on the support used, since the surface properties of the catalyst vary with the support. A carbon support possesses advantages over oxide supports for some reactions because of its inactive surface. Further, specially pretreated carbon supports can prevent coke formation on the surface of catalyst so that the life of the catalyst increases. Addition of Ni to these catalysts improves their performance considerably. These catalysts generally need to be reduced or sulfided prior to reaction. We have carried out a detailed investigation on the processes occurring during reduction of these catalysts, and their effect on the performance of the catalysts. In particular, we have investigated the effects of the order of component addition, and calcination on the reducibility and performance of the catalysts.

a Financial support from U.S. Department of Energy under Contract No. DE-AC2291PC034 is gratefully acknowledged. We acknowledge Eric Zubovic, Liu Zhenyu and Zhong Bing for their help with the sample preparation, and Ramesh Subramaniam for his help with GC/MS analyses. b Present Address" College of Chemistry and Chemical Engineering, Ocean University, Qingdao, Shandong 266003, China. c To whom correspondence should be addressed.

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2. EXPERIMENTAL

2.1 Catalyst Preparation Activated carbon was obtained from Aldrich. Ammonium heptamolybdate (AHM), potassium nitrate (KN) and nickel nitrate (NN), obtained from Fisher Scientific, were used as the sources of Mo, K and Ni respectively. Gases were obtained from Matheson. All materials were used as received. The sequential incipient-wetness method was used to prepare all samples. Mo/C samples were prepared by impregnating the carbon with an aqueous solution of AHM, drying in air at 100~ overnight, and calcining in flowing N 2 at 500~ for 2h. K/C samples were prepared by impregnating the carbon with KN solution, drying overnight and calcining in flowing N 2 at 300~ for 2h. Mo-K/C samples were prepared using KN to impregnate Mo/C. Both K-Mo-Ni/C and Mo-Ni-K/C materials were prepared, with NN used as the source for Ni: here, elemental order indicates the order of impregnation, either K first or Mo first. Some samples were calcined; this process was carried out under flowing N 2 at 300~ for 2h. Typically, 0.5 g of material was used in the reactivity runs, and 120 mg for the TPR runs.

2.2 TPR The sample was placed in a quartz U-tube reactor and surrounded with quartz chips. The reactor was heated in an external furnace with a temperature programmer. Either high-purity Ar or an Ar-H 2 mixture flowed through the reactor. The exit stream passed through a cold trap (dryice/acetone) to a TCD in a HP5890 GC so the H 2 content of the stream could be monitored. A HP PC and HP ChemStation Software were used for data acquisition and processing. In the actual run, the sample was first preheated in Ar at 250 ~ for 1h, to remove volatiles. After the sample was cooled to room temperature (again in flowing Ar), TPR was begun with the flow of the Ar-H 2 mixture. The temperature was increased to 850~ at 10~ and held at 850~ for 30 min. TPR profiles were analyzed using PeakFit software from Jandel Scientific.

2.3 Reactivity Reactivity studies were carried out in a computer-controlled plug-flow microreactor system. Details can be found in [1]. Catalysts were reduced in flowing H 2 at atmospheric pressure and 400~ for 12-15h before reaction. Typical g~as flows were 25 scc/min for H 2 and 25 scc/min for CO, corresponding to GHSVs of 6 - 15 sm-'/h/kg catalyst. The pressure was kept constant, typically 750 psig. The reactor could be set up to run either isothermally or in a temperature-ramping mode from 200 ~C to 400 ~C and back at 10 ~C/min. Products were typically analyzed on-line every 2h using a GC containing two columns, each leading to either a TCD or a FID. For the Ni-containing catalysts, off-line analysis by GC/MS was needed as well. 3. RESULTS In preliminary TPR runs [2], CuO was used to calibrate peak areas with H 2 consumption. This allows us to estimate metal valence from the area of the peaks, and thus to quantify the reducibility of the Mo. In other preliminary results, TPR from calcined AHM was identical with

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TPR using commercial MoO 3, indicating that calcination results in Mo with a valence of +6. Further, TPR on the activated-carbon alone shows only a small TPR peak associated with surface oxidation of impurities, and no peaks after additional runs, implying that reduction is complete after a single TPR sequence. Also observed is an apparent negative peak coinciding with the end of the ramp and the start of the soak process, clearly unrelated to Mo and TPR. Preliminary reactivity data [ 1] from temperature-ramping runs indicate that the decreasing-temperature leg represents the stable behavior of the catalyst, so only this leg is reported below. 3.1 Mo/C and Mo-K/C

TPR and reactivity runs were made on a series of Mo/C catalysts for which the Mo loading was increased from 0-24 wt% of the activated carbon support. For 18 wt% Mo, runs were made on a series of catalysts for which the K dopant was increased from K/Mo = 0 to 1.6 (molar). A few runs were carried out with K/C samples as well. Results on these series of materials are reported here only briefly, for space reasons. Details can be found elsewhere [ 1,2]. For TPR on the Mo/C species, three major peaks are observed, in addition to the peak attributable to impurities in the support. Two of the major peaks, located around 350~ and 440~ are attributed to the reduction of octahedrally coordinated Mo (Mo[O]). A third peak, located around 740~ is attributed to the reduction of tetrahedrally coordinated Mo (Mo[T]). TPR runs on K/C indicate that K is not itself reduced, but it may modify the reducibility of other metal compounds. Small amounts of K, when added to the Mo/C, increase the area of the low-temperature (Mo[O]) peaks at the expense of the high-temperature (Mo[T]) peak. Quantitatively the reducibility of Mo can be obtained by calculating the average Mo valence. The addition of small amounts of K can be said equivalently to decrease the valence of Mo after low-temperature reduction from around 5.0 to around 3.0, much greater than the decrease noted in the valence of Mo after high-temperature reduction, from around 2.4 to around 1.2. Larger amounts of K decrease the Mo[T] peak area and cause less reduction in both MolT] and Mo[O], i.e., valences are reduced by smaller amounts. K and Mo interact in K-promoted Mo/C catalysts, with the interaction being most pronounced for K/Mo between 0.2 and 1. Reactivity runs with Mo/C catalyst (i.e., in the absence of K or Ni) result in no alcohol formation, only hydrocarbons (HCs) and CO 2. Addition of even a small amount of K decreases the overall CO conversion, decreases the space-time yield (STY) of HCs, increases the STY of total alcohols (TA), and increases the selectivity of higher alcohols (HA). The TA STY is maximum around K/Mo = 1-1.3, depending upon the temperature, while the selectivity to HA is greatest between K/Mo = 0.5-3. With changing temperatures, there are characteristic maxima in TA STY and in HA selectivity. The optimum temperatures vary with K/Mo, but are typically 300-350~ Comparing these results with those from TPR, there is clearly a relationship between the low-temperature TPR peak Mo[O] (low Mo valence) and the preferred catalytic behavior. 3.2 K-Mo-Ni/C and Mo-Ni-K/C

TPR and reactivity data were obtained for a series of C-supported catalysts containing Ni as well as K and Mo. Figure 1 shows raw TPR data for the catalyst containing 18 wt% Mo, as before, and with K/Mo = 1.2 (molar) and Ni/Mo = 0.6 (molar). Data for the activated-carbon support alone are shown as curve (a). As mentioned earlier, the curve shows the small TPR peak

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due to the surface reduction of impurities and also the apparent negative peak coinciding with the end of the ramp and the start of the soak process. As for Mo/C and Mo-K/C, the spectra from the Mo-containing material are fitted to two low-temperature peaks Mo[O], and one high-temperature peak, Mo[T]. Assuming that Ni is unreducible allows (a lower bound of) values of the corresponding Mo valences to be obtained, assuming that the initial value is +6; see Table 1. The values for the corresponding Mo/C and Mo-K/C materials are shown also. Table 1 Mo valences (calculated) in C-supported Mo catalysts containing K and Ni (18 wt% Mo) K/Mo Ni/Mo F_~ After Low-T Redn. After High-T Redn. Mo/C, calcined [2] 0 0 -4.99 2.35 Mo-K/C, calcined [2] 1.2 0 -3.45 2.93 K-Mo-Ni/C, calcined 1.2 0.6 1b 4.39 3.01 Mo-Ni-K/C, calcined 1.2 0.6 1c 1.66 0.23 Mo-Ni-K/C, uncalcined 1.2 0.6 1d 1.62 1.32 From Table 1, the addition of Ni increases the Mo valence (relative to the Mo-K/C catalyst) when K is impregnated before Ni. In other words, Mo is more difficult to reduce in the K-Mo-Ni/C catalyst than in the catalyst without Ni at all. Under these conditions, the addition of Ni is expected to confer no particular catalytic advantage. In fact, when the K-Mo-Ni/C material is used in reactivity studies, the products between 200 ~ and 400 ~ are exclusively HCs (and CO2) -- no alcohol is produced at all. In other words, the K-Mo-Ni/C material behaves similar to Mo/C. This is consistent with the similarities of the values of the valence calculated for K-Mo-Ni/C and for Mo/C. The effect of impregnation order can be seen by comparing curves (b) and (c). In the latter case, the Mo valence is decreased significantly when Ni is added but is impregnated before K. Under these conditions, we should have an effective catalyst for the production of HA. Further, the effect of calcination can be seen by comparing curves (c) and (d). The calcined catalyst has approximately the same valence as does the uncalcined catalyst after lowtemperature reduction. However, after high-temperature reduction, the Mo valence of the calcined catalyst is much lower than that of the uncalcined one. Hence, the calcined Mo-Ni-K/C catalyst should be a superior HA-synthesis catalyst. This is in fact observed. Figure 2 shows how STY and selectivity vary with temperature for the uncalcined and the calcined catalyst. While calcining the catalyst leads to an increase in the STY of HCs, it significantly increases the HA STY. More importantly, there is a significant increase in the HA selectivity. Figure 3 shows the performance of the calcined catalyst at a constant temperature of 355 ~C. Ethanol is the predominant product, and the amounts of C 1-, C2and C 3- alcohols are each greater than the amount of total hydrocarbons formed. 4. S U M M A R Y AND CONCLUSIONS TPR and reactivity data are consistent with the hypothesis that highly reduced Mo species are required for HA synthesis. Addition of K to Mo/C aids the reduction of Mo and converts a HCsynthesis catalyst to an alcohol-synthesis catalyst. Addition of Ni and K leads to further

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reduction of Mo and significantly improves the catalyst for HA synthesis. REFERENCES

1. X. Li, L. Feng, Z. Liu, B. Zhong, D.B. Dadyburjor and E.L. Kugler, I & E C Res 30 (1998), 3853. 2. L. Feng, X. Li, D.B. Dadyburjor and E.L. Kugler, J. Catal. submitted (1999). t (min) at 850~ 0

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Quantitative Determination of the Number of Active Surface Sites and the Turnover Frequencies for Methanol Oxidation over Metal Oxide Catalysts Laura E. Briand a and Israel E. Wachsb aCentro de Investigaci6n y Desarrollo en Procesos Cataliticos, Universidad Nacional de La Plata, (1900) La Plata, Buenos Aires, Argentina bZettlemoyer Center for Surface Studies and Department of Chemical Engineering, Lehigh University, 7 Asa Drive, Bethlehem, PA 18015, USA Methanol chemisorption was successfully developed to quantify the number of surface active sites (Ns) and the methanol oxidation turnover frequencies (TOF) of metal oxide catalysts (supported molybdenum oxides, bulk metal molybdates and bulk metal oxides). This is the first example in the catalysis literature where the Ns and TOF values of different types of metal oxide catalysts have been quantitatively compared for an oxidation reaction. Comparable values of Ns and TOF were obtained for the corresponding molybdate catalysts (e.g., MoO3/A1203 and A12(MoO4)3), which reflect the influence of the cation ligands in these catalysts (e.g., A1). Further comparison with the pure metal oxides (e.g., MoO 3 and A1203), suggests that the bulk metal molybdate catalysts are enriched in surface Mo and may actually possess a surface molybdenum oxide overlayer. These new insights account for the special properties of bulk metal molybdates during catalytic oxidation reactions. 1. INTRODUCTION Several approaches have previously been proposed to quantify the number of surface active sites on metal oxide catalysts (oxygen chemisorption atler pre-reduction in H 2, reaction of NO and NH 3, etc. [1-3]). However, these methods suffer because (1) the number of surface active sites is determined with a different probe molecule than the actual reactant, (2) the oxide catalysts are treated with H 2 and the reduction stoichiometry of the catalyst is usually not known, and (3) the chemisorption and reduction temperatures are far removed from the actual reaction temperatures. The present study reports on a novel chemisorption method that employs the dissociative adsorption of methanol to surface methoxy intermediates in order to quantitatively determine the number of surface active sites on metal oxide catalysts. The commonality of the surface methoxy intermediate during dissociative chemisorption of methanol and methanol oxidation on oxide catalysts overcomes the limitations of the previously proposed techniques to quantify the number of surface active sites on metal oxide catalysts and their turnover frequencies during oxidation reactions [4]. In addition, the methanol oxidation product distribution at low conversions reflects the nature of the surface active sites since redox sites yield H2CO, acidic sites yield CH3OCH3

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and basic sites yield CO2 [5]. Thus, methanol is a "smart" probe molecule that can provide fundamental information about the number of surface active sites, the TOF values and distribution of different types of surface sites of metal oxide catalysts.

2. EXPERIMENTAL

2.1. Catalyst Synthesis Three different types of metal oxide catalysts were employed in the present investigation: supported molybdenum oxide catalysts (e.g. MoO3/A1203), bulk metal molybdate catalysts (e.g., A12(MoO4)3) and bulk metal oxide catalysts (e.g., MoO 3 and A1203). The supported molybdenum oxide catalysts were prepared via incipient wetness impregnation of the oxide supports by aqueous solutions of ammonium heptamolybdate [6]. The bulk molybdates were formed by coprecipitation of aqueous solutions of the metal salts (e.g., ammonium heptamolybdate and the corresponding metal nitrate). The bulk metal oxide catalysts were purchased as high purity commercial chemicals. 1.2. Catalyst Characterization The various metal oxide catalysts were extensively characterized to obtain their physical and chemical properties: BET surface area, X-ray Photoelectron Spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, and X-ray Absorption Near Edge Spectroscopy (XANES). Experimental details about the various characterization instruments can be found in prior publications [6,7]. This combination of characterization techniques provided detailed information about the bulk structure/composition and surface structure/composition of the various metal oxide catalysts. 2.3. Methanol Chemisorption and Oxidation The experimental conditions required to quantify the number of surface active sites via methanol chemisorption were determined over a wide range of temperature and methanol partial pressure in a Cahn TGA microbalance (Model TG-131). Methanol adsorption was performed in a flowing helium stream. Adsorption temperatures below 100 ~ resulted in the coadsorption of surface methoxy intermediates and physically adsorbed molecular methanol, and adsorption at temperatures significantly higher than 100 ~ resulted in the decomposition of the surface methoxy intermediates. Thus, 100 ~ was chosen as the adsorption temperature for methanol since it was above the desorption temperature of physically adsorbed molecular methanol, at the temperature where methanol readily dissociatively adsorbed as surface methoxy intermediates and below the decomposition temperature of the surface methoxy intermediates. The methanol partial pressure also influenced the amount of physically adsorbed molecular methanol that condensed on the catalyst pores, and 2,000 ppm of methanol in helium was found to essentially eliminate the condensation of molecular methanol in the pores of the oxide catalysts at 100 ~ The methanol oxidation steady state kinetics were obtained in a fixed-bed catalytic reactor under differential conditions and the product formation was determined with an on-line gas chromatograph [7].

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3. SUPPORTED MOLYBDENUM OXIDE CATALYSTS

Model supported molybdenum oxide catalysts (e.g., MoO 3 supported on ZrO2, TiO2, A1203, Nb205, etc.) were initially investigated for methanol chemisorption since such catalysts (1) possess 100% dispersion of the active molybdenum oxide component, (2) the number of surface active sites are independently known from other characterization experiments (e.g., Raman spectroscopy) and (3) their surface structures are known from in situ characterization studies (e.g., Raman and X-ray Absorption Near Edge (XANES) spectroscopy) [6]. The surface Mo coverages employed in the present investigation corresponded to about monolayer surface Mo coverage in order to minimize adsorption of methanol on exposed oxide support sites (-~7 umol of Mo/m2). The surface MOs densities and the methoxy chemisorption stoichiometries (CH3OaJMOs) are presented in Table 1. The methoxy chemisorption stoichiometry was found to be about 1 CH3Oaas per 3-4 Mo s for the chosen adsorption conditions and reflects the presence of lateral Table 1 Methanol chemisorption stoichiometry and methanol oxidation tumover frequencies for supported molybdenum oxide catalysts Catalyst 3% MoO3/ZrO 2 6% MoO3/Nb205 6% MoO3/TiO 2 2% MoO3/MnO 3% MoO3/Cr203 18% MoO3/AI/O 3 4% MoO3/NiO

Surface Mo (umol/m2) 5.31 7.64 7.64 7.50 7.42 6.97 7.32

N s (CH3Oads)

(umol/m2) 1.25 2.10 3.07 1.14 2.87 2.78 1.47

CH3Oads/MO s

0.24 0.28 0.40 0.15 0.39 0.40 0.20

TOF (sec "l) 0.47 0.14 0.18 0.44 0.08 0.04 0.04

interactions in the surface methoxy overlayer. Methanol oxidation over these supported MoO 3 catalysts primarily yielded redox products with H2CO as the primary product. The methanol oxidation turnover frequencies were calculated by determining the production rate of redox products at 280 ~ and normalizing the rate to the number of surface sites available for adsorption of CH3Oaas (Ns), and the resulting TOF values are also presented in Table 1. The methanol oxidation TOF was found to vary over an order of magnitude for the supported MoO 3 catalysts. The TOF variation was not found to correlate with the coordination of the surface molybdenum oxide species, which was independently determined with in situ Raman and XANES [6]. However, the TOF variation was found to correlate with the characteristics of the bridging M-O-Support bond rather than the terminal Mo=O bond. In summary, the methanol chemisorption studies with the model supported molybdenum oxide catalysts confirm that methanol chemisorption can be employed to quantitatively determine the number of active surface sites of metal oxide catalysts, which also allows for the quantitative determination of the TOF values for oxidation reactions such as methanol oxidation.

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4. BULK METAL MOLYBDATE CATALYSTS

A series of bulk metal molybdate catalysts (NiMoO4, A12(MoO4) 3, Fe2(MoO4) 3, etc.) were synthesized and structurally characterized by XRD and Raman spectroscopy to confirm their crystalline structures as well as the absence of excess crystalline MoO 3 (see Table 2). Unlike the model supported metal oxide catalysts where the number of Mos were independently known (see section 3), the number of Mo~ for the bulk metal molybdate catalysts is not known. Thus, methanol chemisorption was employed to determine the number of CH3Oads sites, Ns, which are presented in Table 2. Comparison of the number of CH3Oa& , Ns, for the bulk and supported molybdate catalysts gives very similar adsorption Table 2 Methanol chemisorption stoichiometries and methanol oxidation turnover frequencies for bulk metal molybdate catalysts Catalyst

Zr(MoO4) 2 TiMoO 5 MnMoO 4 Cr2(MoO4) 3

A12(MoO4)3 NiMoO4 CoMoO4 CuMoO 4

FeE(MoO4)3

N s (CH3Oads) (umol/m 2) 1.0 1.5 3.1 12.6 5.0 2.8 4.1 23.9 7.6

TOF (sec 1) 24.8 0.7 6.6 3.1 1.8 0.9 0.9 1.1 0.8

Selectivity to partial oxidation products 65.2 100.0 99.8 91.0 26.1 100.0 88.2 100.0 61.0

surface densities for most of the catalysts, - 3 umol CH3OaJm 2, which reflects the very similar adsorption stoichiometries for these different types of metal oxide catalyst systems. Two of the bulk metal molybdate catalysts, however, exhibited somewhat higher values, which may be due to special structural or compositional properties of these specific catalysts. Nevertheless, it appears that comparablesurface densities of CH3Oaas are obtained for most bulk and supported molybdate catalysts. Methanol oxidation over the bulk metal molybdate catalysts primarily yielded redox products with H2CO as the primary product (see Table 2). The methanol oxidation TOF values were calculated by determining the production rate of redox products at 380 ~ and normalizing the rate to the number of surface sites available for adsorption of CH3Oads (Ns), and the resulting TOF values are also presented in Table 2. Comparison of the TOF values with the known bulk structures of these bulk metal molybdates reveals that there is no correlation between the bulk Mo cation coordination and the TOF for methanol oxidation. The lower surface areas of the bulk metal molybdate catalysts, 5-10 m2/g, compared to the supported molybdenum oxide catalysts, 20-200 m2/g, required that the bulk metal molybdate catalysts be studied at a higher reaction temperature of 380 ~ In order to compare the TOF

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values of both the bulk metal molybdate and supported molybdenum oxide catalysts at comparable temperatures, the TOF values for the supported molybdenum oxide catalysts were extrapolated from 250 to 380 ~ from the well known activation energy of this reaction o f - 2 0 Kcal/mol [8]. Comparison of the methanol oxidation TOF values for the corresponding bulk and supported molybdate catalysts at 380 ~ reveals that they are very comparable, usually within a factor of--2-3 and sometimes even identical. Thus, the very similar number of surface active sites (Ns) and methanol oxidation TOF values for bulk metal molybdate and supported molybdenum oxide catalysts reveals the significant influence of ligands in oxidation catalysis (bridging Mo-O-M, where M is either an oxide support or a bulk metal oxide cation). This is the first example in the catalysis literature where the TOFs of bulk mixed metal oxide and supported metal oxide catalysts have been quantitatively compared. The new findings demonstrate that the same factors control the oxidation catalysis of bulk and supported metal oxide catalysts. 5. BULK METAL OXIDE CATALYSTS A series of pure metal oxides (NiO, A1203, Fe203, etc.) was also examined with methanol chemisorption and methanol oxidation in order to better understand the role of the metal components in the bulk metal molybdates, and the results are presented in Table 3.

Table 3 Methanol oxidation turnover frequencies and selectivities over pure metal oxides Catalyst

Cr203 NiO CoO CuO Fe203 MoO 3

Ns(CH3Oad~) (umol/m 2)

TOF (sec l)

H2CO

12.4 4.37 2.23 7.03 3.24 0.8

79.6 53.1 526.8 55.6 26.9 5.3

15.5 27.3 23.2 32.6 13.9 84.1

Selectivity CO2 Others 79.7 67.6 69.1 64.0 83.7 00.0

4.8 5.1 2.2 0.9 2.4 12.0

Again, the adsorption of methanol mostly corresponds to -3 CH3Oads umol/m 2. The higher N s for CuO is due to the lower adsorption temperature required for this sample, 50 oC, because it was reduced at the typical adsorption temperature required of 100 ~ Similar to bulk Cr2(MoO4)3, pure Cr203 exhibited a somewhat high value of Ns. The low value ofN s for pure MoO 3 reflects the platelet morphology of this oxide and the preferential adsorption of methanol on the edge sites [4]. The TOF values for the pure metal oxides were found to be orders of magnitude greater than the corresponding bulk metal molybdates at 380 ~ (see Table 2). Furthermore, the high selectivity towards CO2 formation during methanol oxidation, at low conversions, for the pure metal oxides (e.g., Cr203, NiO, CoO, CuO and Fe203) revealed the presence of significant amounts of basic surface sites, which were not significant for the bulk metal molybdate catalysts. In contrast to these pure metal oxides, pure MoO 3 exhibits a low TOF and a high selectivity of H2CO. These observations suggest

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that bulk metal molybdate catalysts are enriched in surface Mo and may actually possess a surface molybdenum oxide overlayer (analogous to the supported molybdenum oxide catalysts). Such a scenario would account for the somewhat low TOF values and high H2CO values of bulk metal molybdate catalysts during methanol oxidation. Indeed, XPS surface analysis of the bulk metal molybdates confirmed that the bulk metal molybdate catalysts were surface enriched with Mo. Thus, the catalytic properties of bulk metal molybdates are dominated by the surface Mo sites. 6. CONCLUSIONS

Methanol chemisorption was successfuly developed to quantify the number of surface actives sites in metal oxide catalysts (supported molybdenum oxides, bulk metal molybdates and bulk metal oxides). For the model supported molybdenum oxide catalysts, it was found that one CH3Oads surface intermediate occupies about 3-4 surface Mo sites due to lateral interactions in the surface methoxy overlayer. This fundamental information allowed for the quantitative determination of the TOF for the methanol oxidation reaction. The variation in TOF was not related to the Mo s coordination or the terminal Mo =O bond, but was related to the characteristics of the bridging Mo-O-Support bond. For the bulk metal molybdates, the surface densities of CH3Oad s w e r e mostly comparable to the supported molybdenum oxide catalysts, and the methanol oxidation TOF variation did not correlate with the bulk Mo cation coordination. Comparison of the methanol oxidation TOF values of the corresponding bulk and supported molybdate catalysts reveals that they are comparable. This suggests that the same factor controls the oxidation catalysis of bulk and supported metal oxide catalysts: the nature of the bridging Mo-O-M bond, where M is either an oxide support or a bulk metal oxide cation. Comparison of the bulk metal molybdates with the corresponding pure metal oxides, suggests that bulk metal molybdate catalysts are enriched in surface Mo and may actually possess a surface molybdenum oxide overlayer (analogous to the supported molybdenum oxide catalysts). These new insights account for the special properties of bulk metal molybdates during catalytic oxidation. The support of CONICET (Argentina) and the Office of Basic Energy Sciences at the U.S. Department of Energy are gratefully acknowledged (DE-FG0293ER14350).

ACKNOWLEDGMENTS:

REFERENCES 1. S.W. Weller, Acc. Chem. Res., 16 (1983) 101. 2. A.N. Desikan, L. Huang, and S.T. Oyama, J. Phys. Chem., 95 (1991) 10050. 3. M. Inomata, A. Miyamoto and Y. Murakami, J. Catal., 62 (1980) 140. 4. W.E. Fameth, F. Ohuchi, R.H. Staley, U. Chowdhry and A.E. Sleight, J. Phys. Chem., 89 (1985) 2493. 5. J.M. Tatibouet, Appl. Catal. A: General, 148 (1997) 213. 6. H. Hu, I.E. Wachs and S.R. Bare, J. Phys. Chem., 99 (1995) 10911. 7. X. Gao, S.R. Bare, J.L.G. Fierro and I.E. Wachs, J. Phys. Chem. B., 103 (1999) 618. 8. G. Deo and I.E. Wachs, J. Catal., 146 (1994) 323.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Metal particles on oxide surfaces: structure and adsorption behaviour M. B~iumer*, M. Frank, R. K'tilmemuth, M. Heemeier, S. Stempel and H.-J. Freund Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany Although deposited metal particles play an important role as heterogeneous catalysts, there is only limited fundamental knowledge about the relationship between their structure, their adsorption behaviour and their catalytic activity. In this article, we describe a strategy giving access to suitable model systems which can be studied with most surface spectroscopic and microscopic techniques. These systems are based on thin oxide films as supports, onto which metal particles of controllable size are grown by vapour deposition. As concrete examples, the preparation of Ir, Pd and Rh aggregates as well as their interaction with CO and C2I-L will be discussed. 1. INTRODUCTION Deposited metal particles play a central role in heterogeneous catalysis. Many catalysts consist of an active component, such as a transition metal, which is dispersed over a suitable support material- usually an oxide, such as alumina or silica. In the first place, this is done in order to achieve the highest possible surface area of the active phase. Because of the high degree of dispersion, however, particle size effects and metal substrate interactions can influence the catalytic behaviour significantly. Of course, it has always been an important question in catalytic research how these effects can be exploited to improve the activity or the selectivity of a supported catalyst [ 1]. Nevertheless, on a fundamental level, there is still only very limited knowledge about these relationships. In order to contribute to a better understanding, we have investigated a number of model systems based on thin, well-ordered oxide films [2,3]. From an experimental point of view, such supports have the advantage of good electrical and thermal conductivities as compared to oxide bulk materials. Hence, many powerful surface analytical tools, i.e. electron spectroscopies and scanning tunneling microscopy (STM), can be applied without charging problems [2-4]. The films can be either prepared by oxidation of a suitable metal substrate or by deposition of the corresponding element onto a host crystal followed by oxidation in an oxygen ambient [2-4]. An extremely well-ordered alumina film, for example, can be grown on a NiAI(110) surface [5], whereas a thin silica film is obtainable by the second technique using a Mo(112) support [6]. In the case of the alumina film which originally contains no hydroxyl groups, it has furthermore been possible to introduce OH groups by subsequent treatment with aluminium and water [7]. Onto the thin alumina film, we have vapour deposited a large number of metals including Ag, Pt, Pd, Rh, Ir, Co and AI [2]. After characterising their structure and morphology in detail with STM and electron diffraction, the adsorption and reaction behaviour has been studied using suitable probe molecules, such as CO and C2H4. In this article, we will focus on three examples, namely Pd, Rh and Ir, for which we will discuss the nucleation and growth behaviour under different preparation conditions first. These data show that it is possible to obtain a large spectrum *

Corresponding author. Fax: +49 30-8413-4312. E-mail address: [email protected]

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of different particle sizes and morphologies in a controlled state. The remaining part of the paper deals with adsorption experiments revealing interesting dependencies on cluster size and type of metal. 2. EXPERIMENTAL The experiments were conducted in ultrahigh vacuum systems, equipped with STM instruments, photoemission and electron diffraction facilities and all instruments necessary for the preparation of the surface. The infrared spectra have been taken in situ with a Bruker vacuum IR spectrometer in reflection geometry. The ordered Al203film was obtained as previously described in the literature, i.e. by oxidation of a NiAI(110) single crystal surface and subsequent annealing [2,5]. The metals (> 99.9 %) were evaporated with commercial evaporators based on electron bombardment. Their flux was calibrated by a quartz microbalance and checked via STM. The deposition rates varied between 0.1 - 0.6 ML min -~ (ML: monolayer). The sample temperature during deposition was either 90 K or 300 K. 3. THE SUPPORT: AI203/NiAI(110) As already mentioned, the support for all systems discussed in this article is a thin alumina film which can easily be prepared on the (110) surface of a NiA1 alloy single crystal. The most prominent feature of this film is its high degree of long-range order setting it apart from the amorphous alumina films grown on A1 single crystals. Another advantage is the excellent reproducibility of its structure, thickness and defect density [2]. Without going into details, a few features should be mentioned here [2]: 9 The thickness of the film is 5 A, corresponding to two layers of oxygen (and aluminium). 9 The oxide overlayer contains no Ni ions and is free of hydroxyl groups. 9 According to the phonon spectrum, the film has a structure which is similar to T-A1203. 9 The oxygen ions form a quasi-hexagonal structure on the surface. 9 The band gap (~ 8 eV) is in reasonable agreement with the bulk material. 9 Most simple molecules, including CO, do not adsorb on the pristine film at and above 90 K. 9 The film exhibits a characteristic defect structure which is dominated by a network of antiphase and reflection domain boundaries [5]. While the first type is the result of a lateral displacement between two adjacent oxide areas, the latter separates domains with two different azimuthal orientations (result of the twofold symmetry of the NiA1 support). Apart from the line defects, a certain concentration (~ 1013 cm-2) of point defects is present on the surface. 4. NUCLEATION AND GROWTH BEHAVIOUR OF Pd, Rh AND Ir Figure 1 contains a series of STM images taken (at 300 K) after deposition of Pd, Rh and Ir onto the thin alumina film at 90 K and 300 K. Considering the 90 K deposits first, a relatively isotropic arrangement of particles on the surface is obtained in all cases. (Note that neither the domain boundaries, which are visible as protruding lines in the pictures, nor the steps play a dominant part). Since the particle densities found for these and various other metals are nearly the same, it can be concluded that all metals follow a common nucleation behaviour under these conditions. In order to gain more information on this question, experiments at different evaporation fluxes have been conducted [8]. They revealed that a homogeneous nucleation mechanism can be ruled out, i.e. the point defects of the film act as nucleation centres at 90 K.

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a) 90 K %-

~.

0.024 ML Pd

~

~.~

0.028 ML Rh

0.0~6 ML Ir

b) 300 K

::,

0.023 ML Pd

:i:::

~

~

0.23 ML Rh

0.0511 ML Ir

Fig. 1. Series of STM images (CCT, all pictures: 1000 x 1000/~) taken after deposition of small amounts of Pd, Rh and Ir on the thin alumina film at (a) 90 K and (b) 300 K (data acquisition at 300 K).

Comparing the situation at 90 K with the one at 300 K, differences between the three metals can clearly be detected. Rh and Pd apparently prefer different nucleation sites now, while Ir largely reproduces its behaviour at 90 K. In the first case, an almost exclusive decoration of the domain boundaries, especially of the curved boundaries, and steps is observed. So, heterogeneous nucleation still governs the growth, but the line defects are the primary nucleation centres at 300 K. Evidently, they offer sites with a higher adsorption energy, which, however, only come into play if the thermal mobility is sufficient to reach them. An explanation for the deviating behaviour of Ir could simply be that it interacts less strongly with the line defects. More likely, though, is an increased metal support interaction resulting in a decreased mobility on the surface. This interpretation is in line with the higher oxygen affinity of Ir. As shown in Fig. 2, a large spectrum of particle sizes can be prepared by taking advantage of different preparation conditions. At 90 K, very small clusters consisting of just a few atoms can be grown in the low coverage regime. At higher coverages, particles up to several hundred atoms are accessible as well. For Rh and Pd, somewhat larger aggregates may be obtained by switching to 300 K. In the case of Pd, a large number of these aggregates are even crystalline with clearly developed facets (mainly (111)) [2,9]. 5. CO ADSORPTION In Fig. 3, infrared spectra of Pd, Rh and Ir deposits of different size are presented which have been taken after CO saturation at 90 K. From the wealth of information which can be extracted from these data, only a few aspects should be referred to in this context.

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10000

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Fig. 2. Average number of atoms per particle for Rh (left) and Ir (right) deposits as determined by STM and SPA-LEED (prof'de analysisof LEED spots [2]). Full (open) circles: deposition at 90 K (300 K). In the regime of very low coverages (Fig. 3 (a)), several sharp bands can be discerned in the case of Rh and Ir clearly pointing to the formation of small, well-defined carbonyl species upon CO adsorption. The signal at 2117 cm -1 in the Rh spectrum, for instance, has been assigned to a Rh(CO)2 species resulting from single Rh atoms trapped at the point defects of the support [ 10]. For Ir, the corresponding absorption peak can be found at 2107 cm -~, but, as deduced from the other sharp bands at lower wavenumbers, other types of carbonyls exist here as well. Pd, on the other hand, neither seems to form a dicarbonyl nor other distinct carbonyl species. Of course, the question arises whether the Rh and Ir species develop during CO adsorption (due to the disruption of larger particles, e.g. [11]) or originate from aggregates already existing. In fact, the first alternative has been excluded by annealing experiments ensuring that the experiments probe the original particle ensemble [10]. Turning to larger particles, broader bands can be detected which are due to CO adsorbed on bridge/hollow sites (Pd: < 2020 cm -1, Rh: < 1970 cm -1, Ir: not detectable) and on-top sites. Considering all three sets of spectra in Fig. 3, it turns out that the portion of the first species decreases in the series Pd > Rh > Ir, in agreement with measurements on single crystals. For Pd, it is furthermore obvious that the portion of terminally bonded CO increases as the particle size decreases ((a) ~ (c)). This change is also accompanied by a weakening of the Pd-CO bond, as deduced from photoelectron spectroscopic measurements [2,12]. Not surprisingly, the best ordered particles are obtained at 300 K. Here, the bands are clearly sharper than at 90 K. Especially, note the difference in the regime of terminally bonded CO for Ir and Rh. Only for the 90 K deposits, a shoulder signal at 2050 cm -1 appears. Since such a feature can be attributed to defect sites, i.e. kinks and steps [10], the data prove that the particles grown at 90 K exhibit a much higher defect density than the 300 K deposits. As mentioned, the Pd particles grown at 300 K are even crystalline. In fact, the corresponding bands can be assigned to specific facets on the aggregates [13]. The signals at 2108, 1956 and 1892 cm -~ are, for example, due to CO on (111) facets, whereas the peak at 2002 cm -~ is likely to be caused by CO on (100) facets and CO adsorbed on edge sites. Summing up, the results demonstrate on the one hand that IR spectroscopy with a suitable probe molecule can be a powerful tool to trace particle morphologies. On the other hand, they reveal interesting dependencies of the CO adsorption site on particle size and the position of the metal in the periodic table.

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a_)Low metal exposu.~__m_m at gO t(

b) High metal exposures at 90 K

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.

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2200 2100 2000 1900 1800 Energy [cm-1]

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2100

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Fig. 3. IR spectra of Ir, Rh and Pd particles of different size and order saturated with CO at 90 K (data acquisition at 90 K). Averageparticle sizes are given next to the spectra. 6. C2H4 ADSORPTION AND REACTIVITY Molecularly adsorbed C2H4 may be present on transition metal surfaces in two forms [14]. A weakly bound species, which is thought to be the primary intermediate in ethylene hydrogenation, is usually referred to as n-bonded ethylene. It is only weakly perturbed upon adsorption. The formarion of the second type, di-o-bonded ethylene, involves a stronger rehybridisation of the carbon atoms, increasing their sp 3 character. Upon heating, it dehydrogenates to ethylidyne, C2H3. Both, di-o-bonded ethylene and ethylidyne are regarded as spectator species in the hydrogenation reaction. Ethylene rehybridisation upon adsorption results in a downshift of the strongly coupled C-C stretching and CH2 scissoring modes from their gas phase frequencies of 1623 and 1342 cm -1. As a semiquantitative measure of this perturbation, a no parameter has been proposed [ 15], defined by no = [(1623-band I)/1623) + (1342-band II)/1342] 10.366. Here, band I' refers to the higher and band II' to the lower frequency of the C-C stretch - CH2 scissors coupled pair. A higher no parameter indicates a larger extent of ethylene rehybridisation. Upon saturation of Pd, Rh, and Ir particles containing about 200 metal atoms with C2H4 at 90 K, features typical for n-bonded ethylene are observed in the infrared spectra (Fig. 4). Signatures of molecules in the di-o state are not so readily discerned. In the case of Pd, however, a weak band at ~1115 cm -~ and a C-H stretch signal at 2924 cm -~ may point to the presence of such species [16]. A comparatively high proportion of n-bonded ethylene may be due to the abundance of step and defect sites on small metal particles. Such sites have been suggested to favour the formation of the n-bonded form [17]. From the observed trend in vibrational frequencies, which decrease according to the sequence Pd > Rh > Ir, we may infer that the interaction of n-bonded ethylene with the metals increases from Pd across Rh to Ir. Making use of the n~ parameter to quantify these observations, we obtain values of 0.40 for Pd, 0.48 for Rh and approximately 0.55 for Ir. These findings agree well with results obtained on n-bonded ethylene on Me/A1203 catalysts [16,18]. This indicates that the

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Rh

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o.o5 %

1600

i

I

1400

i

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1200

Energy [r -1]

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model systems are well suited for more extensive studies on hydrocarbon reactivity which are currently under way. ACKNOWLEDGEMENTS W e are grateful to a number of agencies for financial support: Deutsche Forschungsgemeinschaft, Bundesministerium ftir Bildung und Forschung, Fonds der Chemischen Industrie and N E D O International Joint Research Grant on Photon and Electron Controlled Surface Processes. This work has also been supported, in part, by Synetix, a m e m b e r of the ICI group, through their Strategic Research Fund. M.F. thanks the Studienstiftung des deutschen Volkes for a fellowship. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

M. Che, C.O. Bennett, Adv. Catal. 20(1989) 153. M. B~iumer, H.-J. Freund, Prog. Surf. Sci. 61 (1999) 127. H.-J. Freund, Angew. Chem. Int. Ed. Engl. 36 (1997) 452. D.W. Goodman, Surf. Rev. Lett. 2 (1995) 9; Surf. Sci. 299/300 (1994) 837. J. Libuda, F. Winkelmann, M. B~iumer, H.-J. Freund, Th. Bertrams, H. Neddermeyer, K. Miiller, Surf. Sci. 318 (1994) 61. Th. Schr6der, M. Adelt, M. Naschitzki, M. B~iumer, H.-J. Freund, in preparation. J. Libuda, M. Frank, A. Sandell, S. Andersson, P.A. Brtihwiler, M. B~iumer,N. M~u'tensson, H.-J. Freund, Surf. Sci. 384 (1997) 106. M. B~iumer, M. Frank, M. Heemeier, R. Ktihnemuth, S. Stempel, H.-J. Freund, submitted to Surf. Sci. K.H. Hansen, T. Worren, S. Stempel, E. l_~gsgaard, M. B~iumer, H.-J. Freund, F. Besenbacher, L Stensgaard, submitted to Phys. Rev. Lett.. M. Frank, R. Ktihnemuth, M. B~iumer, H.-J. Freund, Surf. Sci. 427-428 (1999) 288; submitted to Surf. Sci. F. Solymosi and M. P~ztor, J. Phys. Chem 89 (1985) 4789. A. Sandell, J. Libuda, P.A. Brtihwiler, S. Andersson, M. B~iumer, A.J. Maxwell, N. M~trtensson, H.-J. Freund, Phys. Rev. B. 55 (1997) 7233. K. Wolter, O. Seiferth, H. Kuhlenbeck, M. B~iumer, H.-J. Freund, Surf. Sci. 399 (1998) 190. P.S. Cremer, G.A. Somorjai, J. Chem. Soc. Faraday Trans. 91 (1995) 3671. E.M. Stuve, R.J. Madix, C.R. Brundle, Surf. Sci. 152/153 (1985) 532. S.B. Mohsin, M. Trenary, H.J. Robota, J. Phys. Chem. 95 (1991) 6657. P.A. Dilara, W.T. Petrie, J.M. Vohs, Appl. Surf. Sci. 115 (1997) 243. Y. Soma, J. Catal. 59 (1979) 239.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

An Atomic XAFS study of the metal-support interaction in Pt/SiO2AI203 and Pt/MgO-AI203 catalysts: an increase in ionisation potential of platinum with increasing electronegativity of the support oxygen ions D.C. Koningsberger a, M.K. Oudenhuijzen a, D.E. Ramaker b and J.T. Miller e a Department of Inorganic Chemistry and Catalysis, Debye Institute, Utrecht University, PO Box 80083, 3508 TB Utrecht, The Netherlands b Chemistry Department, George Washington University, Washington, DC 20052, USA c Amoco Research Center, E-IF, 150 W. Warrenville Rd., Naperville, IL 60566, USA

The neo-pentane hydrogenolysis tum-over frequency (TOF) of platinum on macroporous acidic SiOa-A1203 is about 500 times higher than that on basic MgO-A1203 hydrotalcite clay. The TOF increases with increasing electronegativity of the support oxygen ions similar to that found for Pt dispersed in microporous supports, such as LTL and Y zeolite. In addition, for Pt on silica-alumina, the intensity of the Fourier transform of the atomic XAFS oscillations, which were isolated from the XAFS spectra is larger and shifted to lower radius. A general model for the metal-support interaction is proposed, where a Coulomb attraction causes an increase in ionisation potential of the metal d-valence orbitals with increasing electronegativity (i.e. lower electron density) of the support oxygen ions. 1.

INTRODUCTION

Numerous studies have reported enhancements in the specific reaction rates for benzene hydrogenation [1], propane hydrogenolysis [2] and neo-pentane hydrogenolysis and isomerization [3,4,5,6], on acidic supports compared to neutral supports. Several explanations for the metal-support interaction have been proposed in literature (i) Formation of a metalproton adduct [3,7], (ii) Charge transfer between the metal atoms and the nearest neighbour zeolite oxygen atoms [8,9] and (iii) Polarisation of the metal particles by nearby cations [10,11]. For each explanation, however, there is experimental evidence, which is inconsistent with the proposed model [12]. Systematic experiments carried out by our group have shown that the catalytic activity and spectroscopic properties of supported noble metal catalysts are greatly affected by H + and K + in LTL zeolite [13] and by H +, La +3, different Si/A1 ratio and non-framework A1 in Y zeolite [12]. A newly developed tool called atomic XAFS (AXAFS), which is obtained from the X-ray Absorption Fine Structure (XAFS) spectra, can provide electronic information on metalsupported catalysts [14]. The intensity of the Pt AXAFS peak increases with increasing electronegativity of the support oxygen ions. The changes in the AXAFS data could be explained by an increase in the ionisation potential of platinum with increasing electronegativity of the zeolite oxygen ions

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In this paper the metal-support interaction for platinum particles supported on high surface area, macroporous supports is further investigated. The electronegativity of the support oxygen ions was varied by using an amorphous acidic SIO2-A1203 and a basic MgO-A1203 hydrotalcite clay. The influence of the support on the neo-pentane hydrogenolysis TOF of Pt was determined. XAFS spectroscopy (EXAFS and AXAFS) was used to investigate the structural and electronic properties of the supported Pt particles.

2.

EXPERIMENTAL

2.1. Catalysts preparation The supports were commercially available: SIO2-A1203 (denoted by Si(Al)O), Davison grade 135, 510 m2/g and 0.67 cc/g) and magnesium-alumina, hydrotalcite clay MgO-A1203 (denoted by Mg(A1)O), La Roche Ind, Inc, Table 1. Dispersion and TOF values 166 m2/g and 0.18 cc/g). The supports were Catalysts Dispersion~ TOF2 calcined at 500~ and 550~ respectively, SIO2-A1203 0.85 2.7x 10 -2 for 16 hr. Platinum was added to each MgO-AI203 0.26 5.3 x 10.5 support by impregnation with [Pt(NHa)4](NO3)2 (Pt loading 2 wt%). The ~Determined by volumetric HE chemisorption catalysts were dried at 120~ calcined at assuming 1 H/Pt atom. 2Molecules/sec-surfacePt atom. 250~ and reduced in flowing hydrogen at 350~ The Pt dispersions were determined by hydrogen chemisorption after reduction at 350~ and are given in Table 1.

2.2. Neo-pentane hydrogenolysis Neopentane hydrogenolysis was conducted in a fixed bed reactor at 325~ using 0.99 vol.% neo-pentane in H2. The catalysts were re-reduced at 325~ for 1 hour, and conversion was adjusted to values less than 2.0% by varying the space velocity. The TOF was calculated based on H2 chemisorption. The analysis of the reaction products was carried out using the Delplot method [15], which gives by extrapolation to zero conversion the primary product distribution.

2.3. XAFS data collection X-ray absorption spectra have been collected at station 9.2 of the Daresbury SRS. The samples were pressed into self-supporting wafers and were then mounted in an in-situ cell equipped with Be windows. The EXAFS samples were reduced in flowing hydrogen at 400~ (heating rate 5~ for 1 hour and evacuated at 200~ for 1 hour. XAFS spectra were recorded at liquid nitrogen temperature maintaining a vacuum of better than 2xl 0 -~ Pa.

2.4. XAFS data-analysis methods By using newly defined criteria the smooth atomic background and multi electron excitations are isolated from the AXAFS and EXAFS contributions (for further details see [16]). Theoretical phase and backscattering amplitudes for the Pt-Pt and Pt-O absorberbackscattering pairs were generated utilising the FEFF7 [17] code and calibrated against reference compounds [16]. The new references can be used from a k-value of about 2.5 A -l, significantly lower than the previously used experimental references. The result is a much

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better isolation of the AXAFS peak at low R. Fitting is done in R-space, without Fourier filtering of the data. To analyse the metal-oxygen contribution to the spectra the difference file technique was used [12]. After subtracting the first metal-metal and metal-oxygen contributions, the remaining signal will be the AXAFS together with the higher order shells.

2.5. AXAFS The AXAFS, ZAx(k), is caused by the scattering of the photoelectron off the deep valence electrons in the periphery of the absorbing atom [14]. The well-known muffin-tin approximation can be used to approximate the embedded atom potential. As illustrated in Figure l, the muffin-tin approximation "clips" the exact potential at the muffin-tin radius Rmt and sets it equal to the interstitial potential Via t [ 14]. Inside Rmt the potential is assumed to be spherical, outside it is assumed to be flat and zero (i.e. no forces are exerted on the particle in the interstitial region). Vint is determined by averaging the potential at Rmt of all the atoms in the cluster, and this determines the zero of energy or the effective bottom of the conduction band. A phase corrected and k weighted Fourier transform of Zgx(k) leads to [14]" [ FT(ke2i6 ~, ZAX)[ = AV*F where AV = Vemb- VTFA with Vemb the embedded atom potential, VTFA the truncated free atom potential, and F a broadening function due to the limited Fourier transform range. The embedded potential reflects the electron distribution after embedding the free atom into its chemical environment and allowing interaction with its neighbours. This means that the FT directly reflects this chan~e in the chemical environment. More specifically, the shape and intensity of the [FT/ can be represented by the shaded area between Vfree and Vemb and below Vcut (Vcut = 2• + IEfl) as illustrated in Figure 1 (For further details see [ 12,14]). 3.

RESULTS

3.1. Neo-pentane hydrogenolysis The results for conversion of neo-pentane are given in Table 1. Analysis of reaction products by the Delplot method indicated that methane, iso-butane (hydrogenolysis), and isopentane (isomerization) were primary products. It can be seen in Table 1 that the TOF of Pt particles supported on the acidic Si(A1)O is about 500 times higher than for Pt supported on the basic Mg(A1)O.

Ev

-.-

EF

~

Vint

Vfree :

~

Vemb A

UUUUU~

Vcut

"ber

Fig. 1. Illustration of the muffin tin approximation to the interatomic potentials showing locations of Ev, EF,Vint

9

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3.2. EXAFS

In Figure 2 the Fourier transform (k2, Ak = 2.5 - 14 A "l) of the raw XAFS data (solid line) of the Pt/Si(A1)O sample is displayed. The shoulders at both the low and high R side of the first Pt-Pt peak in the Fourier transform are due to the non-linear Pt-Pt phase shift and the k dependence of the backscattering amplitude. Fitting of the experimental spectra was done in R-space over the range R - 1.6 to 3.1 A using a k2 weighted Fourier transform over the range A k - 2.5 to 14.0 Al.The results of the fit are shown in Figure 2 (dotted line). The EXAFS coordination parameters for both catalysts are given in Table 2.

0.50

0.04

0.25

0.02

~

I--: 0.00 ~ -0.25

-0.50

-0.02

..e

0

1

2 R (A)

0.00

3

4

Fig. 2. Fourier Transform (k 2, Ak=2.5-14A-~)of raw EXAFS data of Pt/Si(AI)O (solid line) and fit (dotted line).

A -0"00.0

,

i 0.5

,

i 1.0

,

R

i

1.5

2.0

Fig. 3. Fourier Transform (k1, Ak=2.5-8 Al) of AXAFS data of Pt/Si(AI)O (solid line) and Pt/Mg(AI)O (dotted line).

3.3. AXAFS It can clearly be seen in Figure 2 that at low values of R differences are present between fit and experimental data. These differences are due to the AXAFS contribution. Subtracting the calculated Pt-Pt and Pt-O contributions from the raw XAFS produces a difference file, which contains the AXAFS contribution. The k I weighted Fourier transform of this difference file is given in Figure 3 for Pt/Si(A1)O (solid line) and for Pt/Mg(A1)O (dotted line). The amplitude of the AXAFS peak is larger and the centroid is at lower values of R for the Pt/Si(A1)O sample.

4.

DISCUSSION

4.1. Neo-pentane hydrogenolysis Since neo-pentane can not form an alkene intermediate, hydrogenolysis is dependent on only the catalytic activity of the metal [ 18,19] as confirmed by the primary reaction products: methane, iso-butane (hydrogenolysis), and iso-pentane (isomerisation). Moreover, neopentane does not undergo protolytic cracking at the temperatures used for the catalytic reaction (325~ The protons present on the Si(A1)O support, therefore, do not contribute to the neo-pentane conversion. Hydrogcnolysis reactions are dependent on the metal particle

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Table 2. EXAFS co-ordination parameters Coordination

Pt-Pt

Pt-O

Parameters

N

R (A)

Aft2(A 2)

E0 (eV)

N

R (A)

Aft2 (A 2)

E0 (eV)

Pt/Si(Al)O

6.7

2.67

0.007

3.1

0.1

2.13

0.000

3.1

Pt/Mg(Al)O

7.3

2.71

0.006

2.2

0.1

2.07

-0.003

5.3

size, generally, decreasing with increasing particle size: TOF identical for catalysts with a dispersions from about 0.1 to 0.7, but a factor 2 decrease as the dispersion increased to 1.0. Thus, at an equivalent dispersion, the TOF of Si(A1)O would increase to about 1000 times higher than that of Mg(Al)O. While the particle size does influence the rate, it is not sufficient to account for the large differences in TOF in these catalysts. We conclude that the change in TOF is primarily due to the metal-support interaction consistent with previous studies [12,14].

4.2. Structure of the Pt particles. The metal particles in both Pt/Si(A1)O and Pt/Mg(A1)O catalysts have first shell Pt-Pt coordination numbers around 7; i.e. the average metal particle consists of approximately 40 atoms assuming a spherical particle morphology. A Pt-Pt coordination number of 6.7 for Pt/Si(AI)O is slightly larger than expected for a catalyst with a dispersion of 0.85 and may be due to the higher reduction temperature in the EXAFS measurements. A small interfacial oxygen contribution (only Pt atoms in the metal-support interface have oxygen neighbours) is detected within the first co-ordination sphere of Pt, but neither silicon, nor aluminum ions were found within 3 A of the platinum particles for either catalyst. Because to the small Pt-O contribution to the EXAFS, the differences in the Pt-O distances are likely not significant. Since the Pt particles in both catalysts have the same structural properties, we conclude that the change in the catalytic properties of the metal particles is due to a change in their electronic properties induced by a Coulombic interaction with the oxygen ions of the support. 4.3. Nature of the metal-support interaction Basic support Vfree"",~...

i-"

~,, Mint

Pt

R

max

Acidic support .....,

z_

Pt

R

max

Fig. 4. Schematic potential curves for a basic (a) and acidic support (b) assuming polarisation of the cluster by the support.

For Pt/Si(A1)O, the intensity of the AXAFS peak is larger and the centroid is shifted to lower R in comparison to Pt/Mg(A1)O (see Figure 3). These results are consistent with the results recently published by our group for Pt/LTL [6] and Pt/Y [12]. The support properties (acidity, basicity) determine through the Madelung potential the electronegativity of the support oxygen ions. Figure 4a and b compare the difference in the Pt-O interatomic potential as the charge (electronegativity) on

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the oxygen ion increases. The AXAFS peak in the Fourier transform of the Pt/Mg(A1)O data is schematically represented on the right side of Figure 4a with the black area and is determined by the difference between Vfree and Vemb: the black area on the left side of Figure 4a. Increasing the charge on the oxygen (8+: higher electronegativity) will change the shape of the potential of platinum since the interaction with oxygen will move platinum electrons nearer to the oxygen. This is illustrated in Figure 4b by the larger Coulomb tail on the O atom with increased charge, and hence more "roll over" of the interatomic potential and the lowering of Vcut. This causes a larger difference between Vemband the free atom potential as is shown on the left side of Figure 4b. The original difference (black area, in size and position) for the Pt/Mg(A1)O sample is given as a comparison. This larger difference causes an increase in the amplitude of the Fourier transform of the AXAFS oscillations and a shift of the centroid to lower R values as shownon the left side of Figure 4b. At the same time the platinum valence d orbitals are moved to higher binding energy; i.e. the ionisation potential of Pt is increased. Moreover, the Pt d-orbitals are radially contracted and the width of the d-band is reduced resulting in less "metallic" character. This is also reflected in the XPS 3d core level shift of Pd particles dispersed in LTL [6] and in the increase in the linear/bridge ratio of the CO FTIR spectra of Pd/LTL, Pt/LTL and Pt/SiO2. [5]. The AXAFS studies on Pt/LTL [13], Pt/Y [12] and the results presented in this paper on Pt/Si(A1)O and Pt/Mg(al)O suggest a general model for the metal-support interaction, which is based upon a Coulomb attraction between the support oxygen ions and the platinum metal particles. An increase in electronegativity of the support oxygen ions leads to an increase in the ionisation potential of the platinum metal atoms. This change in binding energy of the metal valence orbitals alters the adsorptive, catalytic and spectroscopic properties of the metal particles. This model also implies that there is no transfer of electron density between the support and the metal particles.

REFERENCES 1. 2. 3. 4. 5 6.

S.D. Lin, M.A. Vannice, J. Catal. 143 (1993) 539. J.T. Miller, F.S. Modica, B.L. Meyers, D.C. Koningsberger, Prep. ACS Div. Petr. Chem. 38 (1993) 825. Z. Karpinski, S.N. Gandhi, W.M.H. Sachtler, J. Catal. 141 (1993) 337. S.T. Homeyer, Z. Harpinski, W.M.H. Sachtler, J. Catal. 123 (1990) 60. B.L. Mojet, M.J. Kappers, J.T. Miller, D.C. Koningsberger, Proc. of the 15th Int. Cong. Catal., Baltimore, MD, (1996). B.L. Mojet, M.J. Kappers, J.C. Meyers, J.W. Niemantsverdriet, J.T. Miller, F.S. Modica, D.C. Koningsberger, Stud. Surf. Sci. Catal. 84 (1994) 909. 7. Z. Zhang, T.T. Wong, W.M.H. Sachtler, J. Catal. 128 (1991) 13. 8. G. Larsen, G.L. Hailer, Catal. Lett. 3 (1989) 103. 9. A. de Mallmann, D. Barthoumeuf, J. Chem. Phys. 87 (1990) 535. 10. A.P. Jansen, R.A. van Santen, J. Phys. Chem. 94 (1990) 6764. 11 E. Sanchez-Marcos, A.P.J. Jansen, R.A. van Santen, Chem. Phys. Lett. 16 (1990) 399. 12 D.C. Koningsberger, J. de Graaf, B.L. Mojet, D.E. Ramaker and J.T. Miller, Appl. Catal, in print. 13 B.L. Mojet, J.T. Miller, D.E. Ramaker and D.C. Koningsberger, J. Catal. 186 (1999) 373. 14 D.E. Ramaker, B.L. Mojet, W.E. O' Grady and D.C. Koningsberger, J. Phys. Condens. Matter. 10 (1998) 1. 15 N.A. Bhore, M.T. Klein, K.B. Bischoff, Ind. Eng. Chem. Res. 29 (1990) 313. 16 G.E. van Dorssen, D.E. Ranaaker, D.C. Koningsberger, submitted Phys. Rev. B 17 S. I. Zabinsky, J. J. Rehr, A. Ankudinov, R. C. Albers, M. J. Eller, Phys. Rev. B 52 (1995) 2995. 18 S.M. Davis, G.A. Somorjai, The chemical physiscs of solid surfaces and heterogeneous catalysts (D.A. King, D.P. Woodruff, eds.), Elsevier Publishers Amsterdam, 4 (1982) 271. 19 J.R. Anderson, N.R. Avery, J. Catal. 16 (1967) 315.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Transition state and diffusion controlled selectivity in skeletal isomerization of olefins L. Domokos a, M.C. Paganini a, F. Meunier a'b, K. Seshan a and J.A. Lercher b aCatalytic Processes and Materials, Faculty Chemical Technology, University of Twente, P.O. Box 217, 7500 AE, The Netherlands bTechnische UniversiRit Mtinchen, Institut ftir Technische Chemie, D-85749 Garching, Germany Microkinetic and spectroscopic analysis of skeletal isomerization of linear butenes were performed over H-FER. The effect of butene partial pressure and the rate of formation of isobutene and major byproducts are reported. Results show zero order dependence of isobutene production and first order dependence of byproduct formation with butene pressure. Biphenyl type molecules are proposed as coke species and related to byproduct formation. 1. INTRODUCTION In the last decade skeletal isomerization of light olefins has seen intense efforts in research and development, most notably for n-butene isomerization, due to new legislations on gasoline composition. Although industrially applicable processes have been developed for skeletal isomerization, a unified view on the possible active site and the mechanistic pathway is still lacking. Mechanisms proposed in the literature for butene skeletal isomerization include (i) monomolecular isomerization [1], (ii) dimerization, isomerization and selective cracking (bimolecular pathway) [2] and (iii) addition of butene to carbonaceous species, subsequent isomerization and selective cracking to isobutene (pseudo-monomolecular pathway) [3]. High selectivity to isobutene is found preferentially with medium pore zeolites [4]. The role of the acid site strength seems to be quite subtle [5]. For the most successful catalyst, H-ferrierite, selectivity to isobutene was seen to improve with time on stream, while substantial coke deposition occurred. This led to the suggestion that steric constraints caused by coke helps to enhance isobutene selectivity [3]. The present contribution aims at combining microkinetic and spectroscopic evidence to demonstrate that under usual process conditions not one of the three reaction routes prevails, but that a complex network of mono- and bimolecular reactions determines the catalytic activity and selectivity. 2. E X P E R I M E N T A L

2.1. Catalyst preparation Ferrierite (FER) was obtained from Tosoh with a nominal Si/A1 ratio of 8 and in the NaK exchanged form. Successive ion exchange with 1M NH4NO3 was carried out (three times for 4 h each) in order to obtain the ammonium form of the catalyst. Samples were successively washed with deionized water and dried at room temperature in air.

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2.2. Infrared .pectro.copy The infrared measurements were carried out in a BRUKER IFS-88 spectrometer equipped with a flow cell. The spectra were recorded in the transmission absorption mode. The zeolite was pressed as a self-supporting wafer (2-5 mg/cm 2) and placed in the cell. An activation procedure similar to the kinetic measurements was used. Activated H-FER showed a small peak attributed to terminal OH groups at 3741 cm -l and an intense band at 3584 c m "1 assigned to Bronsted acidic hydroxyl groups. 2.3. Catalytic measurements The catalytic tests were conducted in a tubular continuous flow system. All measurements were carried out at temperature ranging between 250-450~ (typically 350~ with total pressure between 1.0-1.3 bar. Catalysts were activated in situ in the reactor in large excess of dry argon (99.995%) flow at 400~ for 1 hour. Subsequently the temperature was switched to reaction temperature and then the pure argon flow was switched to a mixture of 1butene and argon. The effluent stream was analyzed in regular intervals by a CG equipped with an HPPLOT/A1203 column ("S" deactivated) and FID detector. Carbon balance was close to 98% from initial time on stream on. Conversion of 1-butene, yield and selectivity of any product was calculated according to the literature. Since linear butenes are expected to be in equilibrium at temperatures at which skeletal isomerization occurs, all linear butenes are lumped together and not counted for conversion and yields. Conversion and yield are expressed in terms of mol% on a carbon basis. 3. RESULTS AND DISCUSSION Kinetic measurements at a reaction temperature of 350~ and a partial pressure of 100 mbar butene led to 30% selectivity to isobutene (22 mol% yield, Table 1) the remaining products being mainly propene and pentene (Fig. 1). Under these conditions, the yield of propene and pentene decreased while the yield of isobutene remained constant at short times on stream (TOS). After 10h TOS a significant increase in isobutene yield occurred (up to 37%) while conversion of butene stabilized around 50 mol%. This lead to an increase in isomerization selectivity (Table 1, Fig. 1). Note that the amount of hydride transfer products (ethane, propane) was negligible after the initial period of 1Oh TOS. Table 1 Selectivities to various components at different partial pressures of 1-butene (WHSV=2h ~) Selectivity (mol%) Component C1 C2 C3i-C4 = C5-~sum) C5 + Conversion Yield of i-C4-

p(C4 =) = 1O0 mbar 3 min 50h 0.2 0.0 4.1 0.1 32.4 5.5 30.2 84.5 17.3 4.9 10.9 4.0 73.1 44.6 22.1 37.8

p(C4--) =- 5 mbar 3 min 50h 0.0 0.0 0.0 0.0 3.3 3.0 93.4 93.9 2.6 2.5 0.3 0.1 36.7 31.7 34.0 29.7

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化

mol%

100

~^^^^^^^^^^^,vuxzxzxL~.~

80

,,^zxzxzxz~

60 s

^^^^^^~

-

i-C4= selectivity

a atxaaatxtxtxtxa~'-

4O 20

ilL

:E hydride transfer

0

20

+

40

60 80 Time on stream (h)

100

120

Fig 1. Skeletal isomerization of 1-butene using H F E R as catalyst. Product yields, conversion and selectivity to isobutene vs time on stream.

When the butene pressure was lowered to 5 mbar from 100 mbar, the initial selectivity to isobutene was 93% (yield 33%) and remained stable with TOS. It is important to note that the yield of isobutene changed relatively little, when the pressure of n-butene was lowered by an order of magnitude, while the rate of formation of propene and pentene decreased by approximately a factor of 10 (Table 1). The same observation was made during a pressure transient experiment shown in Fig. 2. Monitoring the reaction with in situ infrared spectroscopy showed that most of the carbonaceous species was deposited at short TOS (Fig. 3). In contrast, selectivity to isobutene increased gradually with TOS over 100 hours (Fig. 1). In addition, at low butene pressure skeletal isomerization was very selective (Fig. 2, 0-5 hours, Table 1) from short time on stream. This indicates that high selectivity to isobutene can be reached in the absence of significant fractions of coke deposited. Additionally, during reaction a fast but only partly (approximately 40%) coverage of

60{

mol%

4

50 mbar

Iiiii 0

20

5 mbar

~

-i ..........

0

i--

-

100

20000 c ,-; 15000

~C43yielZ~__ ~

-Q

i

v

60 "~

c ~o 5000 i

20

Fig. 2. Pressure transient experiment: 5 to 50 mbar partial pressure of butene

"0 Ol t~

r-

0

d cO 0

ffl

t-

1

V

80

"5 ":. 10000

C5+ yield

5 10 15 Time on stream (h)

A

~_J

"{3:3

[]

'

'

'

0

50

100

150

40 200

Time (min) Fig. 3. Coke deposition during reaction monitored by in situ i.r. spectroscopy

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3741 cm-, 9 terminal OH groups

hydrocarbon i stretching i vibration s

3584cm., ' Bronsted ~

'

1513

1504cm-, l aromatic coke species

1

overtones ofla~ice I

8h

1504 3500

3000 2500 2000 Wavenumber (cm-,)

1500

Fig. 4. Butene isomerization reaction monitored by in situ infrared spectroscopy at 350~ and 50 mbar butene partial pressure

1700

1600

1500 1400 Wavenumber (cm-1)

1300

Fig. 5. Evolution of the infrared band of different coke species at 5 mbar butene pressure at 350~

acid sites was observed. The profiles shown in Fig. 3 indicate the diffusion plays an important role governing the molecular transport inside of the zeolite channels during butene isomerization. As n-butene tends to polymerize on acidic zeolites under these conditions, the observation that during reaction of n-butene over H-FER only 40% of the acid sites were interacting with hydrocarbon suggests that a bulky sorbate must have been formed at the pore entrance hindering further intrusion of reactant or product molecules. The nature of the coke deposited on the catalyst (main peak at 1504 cm -~) seemed to have a distinct structure typical of aromatic rings interconnected by short aliphatic chains, most likely to have a poly-4,4'-biphenyl-like skeleton (Fig. 4). This is in contrast to Guisnet et al. [3] and Andy et al. [6] who reported mostly condensed aromatic structures as coke in HFER after few hours TOS. After deactivation, low and steady amount of butadiene was found in the effluent stream. During in situ infrared measurements o overall performed in the presence of only butadiene, E 6.0 the same band at 1504 cm -1 attributed to the =,o 9.,J biphenyl structure was again observed (Fig. 4) = 4.0 "01 indicating the origin of the coke. After the initial formation of the 2.0 biphenyl species, a small shift from 1504 cm 1 to 1513 cm -1 in the corresponding band (Fig. 5) indicates a probable intramolecular 0.0 ~ rearrangement of these species driving to a 0 20 40 60 more complex structure. Furthermore, during pressure of 1-butene (mbar) purging after reaction with inert gas, it was observed that the latter species (1513 cm -1) did Fig. 6. Effect of butene pressure at 350~ not desorb from the catalyst, while the species WHSVbutene=2h1 attributed to the 1504 cm -1 band could be O .1

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removed completely. This is in a good agreement with refs. [2] and [6] suggesting a more graphitic and less reactive state of the carbonaceous deposit located probably at the outer surface of the particle. The selective isomerization reaction of butene to isobutene showed zero order dependence upon butene pressure, while the production of propene and pentenes increased in first order with butene pressure (Fig. 6). This indicates that isobutene formation occurs probably monomolecularly on acid sites that are fully covered at the lowest n-butene partial pressure. Note that the rate of conversion of n-butene depends upon the concentration of acid sites with an apparent reaction order of 0.7. Furthermore, it implies too that yield of isobutene is limited by the desorption capabilities of the molecule from the pores and not by the intrinsic rate of formation. Consequently, at higher partial pressures, the secondary reactions originating from the more reactive adsorbed isobutene will be enhanced. The Arrhenius plot of isobutene formation indicates diffusion limitation above 325~ at short times on stream (Fig. 7). The two straight sections of the graph transformed to a curve with time on stream in 3 hours. This might indicate [7] a coverage dependence of the reaction due to surface poisoning by spectator molecules. Independently, the coverage of acid sites in the catalyst was not complete, and leveled off at approximately 40% (Fig. 3) indicating the possibility of a more complex diffusion phenomenon. Using the coverage of different kind of species observed during in situ i.r. experiments, it was possible to linearize the Arrhenius plot (Fig. 8). It is interesting to note that isobutene production could be normalized by the coverage of CH3 species (mainly represents isobutene adsorbed on the surface) and not by the coverage of the aromatic deposits. This confirms again that isobutene production is limited by its concentration on the surface. The linearity of the Arrhenius plot indicates that no significant change occurred in the nature of isobutene production in the temperature region of 250-450~ This is in contrast to the literature [4-5] where a gradual shift in the mechanism of selective isobutene production from a bimolecular to a monomolecular pathway was proposed with increasing temperature. The fact that aromatic species did not provide a straight line implies that these species may not play a significant role in the selective isomerization process. Both the kinetic and i.r. 2 0

O

1.0

O

u.

0.0

1.1_ v

v t-

-1.0

-i-after 3 h

-2.0 1.3

kI

I

I

I

1.5

1.7

1.9

1/'!" (10 "3, l/K) Fig. 7. Arrhenius plot of isobutene production. Initial and 3 h TOS data represented.

-5

I --B- CH3 cov. - I - aromatic

2.0

-6 -

-7

-8

-4

-9 -10

-6

1.3

1.5

1.7

1.9

1/T (10 3, l/K)

Fig. 8. Concentration of aromatic species and aliphatic CH3 vibration over H-FER during reaction/adsorption of 1-butene

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experiments suggested that after the initial period of coke formation, only minor changes occur in the deposited species. In Fig. 1 the total amount of hydride transfer compounds decayed to zero after 10 hours TOS. Subsequently (as shown in Fig. 3) significant increase in the amount of aromatic species was not observed after 3 hours TOS. These results strongly suggest (in agreement with J. Houzvicka et al. [8]) that the isobutene production enhancement from 22% to 37% is not induced by coke formation. Indeed, this loss of approximately 15 mol% of isobutene to carbonaceous deposit or byproduct formation can be related to consecutive reaction originating from isobutene. With time on stream, as the coke is transformed into a less reactive, more bulky state (Fig. 5 after 8 h), this consecutive reaction pathway decayed and the isobutene yield increased again (Fig. 1, after 10 h TOS). This implies that the active coke species are more related to byproduct formation (first order of butene pressure) than to selective isomerization (zero order of butene pressure). We, therefore, suggest that byproduct formation occurs primarily via reaction of adsorbed isobutene and weakly adsorbed n-butene. Together with the in situ infrared spectroscopic measurements, these results suggest that the reaction occurs on Bronsted acid sites at pore entrance. The desorption of the so-formed isobutoxy species seems to be the most difficult reaction step. The selectivity and stability of the catalyst is attributed to the relative ease with which the isobutoxy species can desorb compared to the addition of n-butene to the isobutoxy group leading to byproduct formation. ACKNOWLEDGEMENT Financial support of STW/NOW under the project number of 349-3797 is gratefully acknowledged. This work was performed under the auspices of NIOK, The Netherlands Institute for Catalysis. REFERENCES 1. P. Meriaudeau, R. Bacaud, L. Ngoc Hung and Anh.T. Vu, T., J. Mol. Catal. A, 110 (1996) L 177-L 179 2. H.H. Mooiweer, K.P. de Jong, B. Kraushaar-Czarnetzki, W.H.J. Stork, and B.C. Krutzen, in: Weitkamp, J., Karge, H. G., Pfeifer, H., and H61derich, W., (Eds.)/LZeolites and Related Microporous Materials; State of the Art 1994", Stud. Surf. Sci. Cat., 84 (1994) 2327-2334 3. M. Guisnet, P. Andy, Y. Boucheffa, N.S. Gnep, C. Travers, and E. Benazzi, Catal. Letters, 50 (1998) 159-164 4. J. Houzvicka, S. Hansildaar and V. Ponec, J. Catal., 167 (1997) 273-278 5. C-L. O'Young, R.J. Pellet, D.G. Casey, J.R. Ugolini and R.A. Sawicki, J. Catal., 151 (1995) 467-469 6. P. Andy, N.S. Gnep, M. Guisnet, E. Benazzi and C. Travers, J. Catal., 173 (1998) 322-332 7. D.W. Goodman and M. Kiskinova, Surf. Sci., 105 (1981) L265-L270 8. J. Houzvicka, S. Hansildaar, J.G. Nienhuis and V. Ponec, Appl. Catal. A: General, 176 (1999) 83-89

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Development and Application of 3-Dimensional Transmission Electron Microscopy (3D-TEM) for the Characterization of Metal-Zeolite Catalyst Systems A.J. Koster a, U. Ziese a, A.J. Verkleij a, A.H. Janssen b, J. de Graaf b, J.W. Geus b, K.P. de Jong b aMolecular Cell Biology, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands bDepartment of Inorganic Chemistry and Catalysis, Debye Institute, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands; e-mail: [email protected] With electron tomography (3D-TEM) a 3D-reconstruction is calculated from a series of TEM images taken at a tilt angle range (tilting range) of +70 ~ to -70 ~ The reconstruction can be visualized with contour surfaces that give information about the surface of the sample as well as with slices through the reconstruction that give detailed information on the interior of the sample. Electron tomography gives much more information than Scanning Electron Microscopy (SEM), since SEM gives only information about the surface of a sample. As a case study, the imaging of silver clusters on zeolite NaY is given. The reconstruction shows silver particles at the external surface as well as a silver particle in a mesopore of the zeolite crystallite. It is concluded that 3D-TEM comprises a breakthrough in the characterization of nano-structured solid catalysts. 1. INTRODUCTION Solid catalysts are of tremendous importance for economy and environment. The drive towards clean and efficient technology calls for precise design and characterization of catalysts. Today, many solid catalysts can be considered as sophisticated, three-dimensional nano-structured materials [1,2]. Especially zeolites and mesoporous materials are well known for their three-dimensional structures. To date, however, no method has been reported that is able to provide structural information in three dimensions with 1-30 nm resolution. In two dimensions, Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM) can provide a surface image of a material at atomic resolution. Scanning Electron Microscopy (SEM) can provide a high-resolution image of a surface in three dimensions (topography), but the material below the surface is not imaged. Transmission Electron Microscopy (TEM) does give high-resolution information of a sample, but the three-dimensional information is projected into a 2D image. The information in the third dimension is lost. Figure 1 shows a TEM image of a Pt/NaY catalyst, which illustrates the absence of three-dimensional information. From Figure 1 it cannot be determined where the Pt-particles are located: inside the zeolite crystal or at the external surface. However, with the development of electron tomography (3D-TEM) it has become possible to get a 3D image of both the surface and the interior of a sample.

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During the last five years automated electron tomography has been developed in the field of biology [3,4], although the theory has been developed much earlier. Due to the automation of the data collection and the increased performance of personal computers, electron tomography can now be applied for practical assays with reasonable investments in time and hard- and software. In this paper the first application of electron tomography in material science is presented. A concise introduction in the theoretical and practical aspects of 3D-TEM is described. As a case study, the 3D imaging of an Ag/NaY catalyst is given. This system is of fundamental interest to assess metal mobility under reducing and oxidizing environment in zeolites as is apparent from the work of Beyer et al. [5]. In this paper, however, we will restrict ourselves to the study of a freshly reduced Ag/NaY sample.

iiliii;;i:i~!~i!i!ii,i~;iiiil;il ~;;iiii 84:ii!;i:;,i~:~:iiiili,i~+!~i ;;!i!!!;~!ii!~:: !!iiiii!i:iiii: l ~:iiliii!iiiii!;~~!~i~ili;iii~: i~ ii~i~!!!iilliiii~il;iiiiiii:i!!!i:iiiiiiiiiii~ iiii'i;iiiiii!iii~iii;!ii!!!i;iiii! ,iiil;iiiiiiiiiiiii~!:;;il !i;iiii::iiii!i:i;;;i,!!i;i~!i;~, i i i~i,~i;i, ;i; :~,~iiiiiiiiiiiiii!;, iiiiiii!i~ii:iiii!~ii~!!ii ii iii!iiiiiiiii~,::'iiiiii!iiii!iiiil iliiiiii!iiiii a

Figure l a and b: (2D)-TEM image of Pt/NaY. Figure l a is taken at a magnification of 27.5k, Figure lb is an enlargement of the left side of Figure la. 2. THEORY OF 3D-TEM A TEM image is in good approximation a projection of the 3D structure of the sample. This causes that information about the 3D ordering of the structure is lost. This is shown in Figure 2, where the projections of several 3D structures are depicted. Although stereo images of a sample can contribute to the understanding of the 3D ordering of the sample, electron tomography is the only technique that is able to provide a 3D electron microscope image of a sample. With electron tomography a 3D image is reconstructed from a series of (2D) TEM images, taken at different tilt angles. The resolution of a 3D reconstruction is approximately given by the relation: Resolution = 7t * thickness of the sample / number of images, assuming that a tilt series is taken over the full tilt range (+ 90 ~ with a constant tilt increment [3,4]. For example, when 150 images are taken of a 100 nm thick object, a resolution of 2 nm is obtained. In practice, however, the tilting range is limited to about +75 ~, due to physical limitations of the sample holder. This causes that the resolution of the 3D reconstruction is direction

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dependent and that structures are slightly elongated in the direction of the angular gap. Therefore, the tilting range should be chosen as large as possible. However, at high tilt angles the travelling path through a non-spherical sample may increase, thus causing loss of contrast due to multiple scattering of the electrons. For example, the path through a 200 nm thick sample will be 580 nm at 70 ~ tilt. 9

i

9

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El~:trons . :. . :

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9

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/1

~

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.~-..i.~.~.;'...?

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Figure 2: Loss of information when projecting 3D structures into 2D images. Changing the specimen tilt angle causes a change of focus and shifting of the sample. These changes have to be corrected in order to obtain a pre-aligned data set. Correcting these changes manually is tedious, time-consuming and prune to error. Fortunately, with automated electron tomography [3], these corrections are carried out automatically. An essential aspect of this method is that the electron microscope images are collected digitally with a slow scan CCD camera 9 The digital images are used for the automatic compensation of image shift and focus change. After the acquisition of a pre-aligned data set, the data series has to be aligned more accurately 9 This can be done with the help of fiducial markers. Gold beads sprinkled on the grid or metal particles in/on the sample can serve as markers. By least-squares fitting of the positions of these fiducial markers, the data series is aligned. After the alignment of the data series the 3D reconstruction has to be computed 9The basics of the method were already proposed in 1917. It was stated that the projection of a 3D object is equal to a central section of the Fourier transform of that object. A data series thus provides many different central sections of the Fourier transform of the sample, thus filling the 3D Fourier space. By inverse Fourier transform of the obtained 3D Fourier space a 3D image of the original object is obtained. An algorithm that is often used for the computation of the 3D reconstruction is resolution-weighted back-projection [4]. Finally, the 3D image can be visualized in different ways. One way is to build a contour model in which the outer surface of the object is visible. Another possible way of visualization is cutting the 3D image in thin, nm-thick, slices. By looking at the individual slices one can exactly locate metal particles inside zeolite crystals in three dimensions with high resolution. If the sample is dose-sensitive, methods are available to collect datasets under low-dose conditions and at cryo-temperatures.

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~--t40 ~

Figure 3" Contour surface of an Ag/NaY crystal. Silver particles are coloured pink, the zeolite is coloured green. On the blue surface a black shadow-projection of the zeolite with silver particles is shown. The white arrow indicates the shadow of a silver particle that is located inside the zeolite.

80nm Figure 4" Imersection of Ag/NaY showing a silver particle (red) at or near the surface of the zeolite (yellow).

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A drawback of 3D-TEM Oust as is the case with 'normal' TEM) is that one investigates only a very small part of the sample. Therefore, additional (macroscopic) characterization techniques, such as XPS, are needed in addition to electron microscopy. 3. APPLICATION TO METAL/ZEOLITE SYSTEMS

Ag/NaY was made by suspending 500 mg NaY (LZY 54 from UOP, Si/AI ratio is 2.5) in 100 ml 4.0E-4 M AgNO3. An exchange efficiency of 100% results in a 0.9 wt% Ag catalyst. The suspension was stirred overnight at room temperature. After centrifugation from the solution, the loaded zeolite was washed and centrifuged two times with de-mineralised water and dried at room temperature. The material was dried at 150~ in argon and subsequently reduced at 150~ in hydrogen. A tilt series of Ag/NaY was taken at a magnification of 11.5k on a Philips CM 200 FEG microscope with a 1024 x 1024 CCD camera (pixelsize 1.12 nm). From a representative Ag/NaY crystal 143 images were taken from +70 ~ to -72 ~ with 1 degree intervals. For alignment purposes 7 dark features (silver particles) that could be followed throughout the whole tilt series were chosen as fiducial markers. The 3D reconstruction contains a volume of 1150xl 150xl 150 nm and has a resolution of 11 nm. X

Z

Y

X

Z

Figure 5: Slice through the reconstruction of Ag/NaY showing a silver particle of 10 nm inside the zeolite (arrow).

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In Figure 3 a contour model of the Ag/NaY sample is given. The colours of this model were obtained by selecting several bands of grey-values (e.g. the grey-values corresponding to the silver particles and the grey-values corresponding to the zeolite crystal) and assigning different colours to the different bands. The silver particles (shown in pink) are located at or near the external surface of the zeolite. The right-hand side of the image shows a shadow projection of the crystallite (black). In this shadow projection the shadow of a silver particle that is located inside the zeolite is also visible (white arrow). In Figure 4 an intersection of the same crystallite is shown. The placement of a silver particle (red) at or near the surface of the crystallite is clearly visible. The precise placement of the silver particles is best observed when the reconstruction is presented as a stack of thin slices. In Figure 5 one of these slices (X-Y) is given, showing a silver particle inside the zeolite. The orthogonal slices (X-Z and Y-Z) further support this conclusion. 4. CONCLUSIONS AND OUTLOOK To date, no techniques were available that could characterize an individual solid catalyst structure in three dimensions at high resolution. With electron tomography (3D-TEM), however, it is possible to obtain a 3D high-resolution image of a sample. From a series of 2DTEM images at different tilt angles a 3D reconstruction is calculated. The reconstruction can be visualized with contour surfaces and with slices through the reconstruction to investigate if particles are located inside or outside the porous material. Future work will involve the further development of 3D-TEM by combination with element analysis (EDAX). The Ag/NaY material will be studied more extensively to assess metal mobility under oxidizing and reducing atmosphere. Other systems currently under study are metal-loaded carbon nanotubes and mesoporous materials. ACKNOWLEDGEMENTS This work has been carried out under the auspices of NIOK, the Netherlands Institute for Catalysis Research, Report No. UU-99-3-02. The authors would like to thank the Netherlands Organization for Scientific Research (NWO) for financial support, grant 98037. The research of one of us (AJK) has been made possible by a fellowship of the Royal Netherlands Academy of Arts and Sciences REFERENCES

[ 1] K.P. de Jong, CATTECH 2 (1998) 87-94. [2] K.P. de Jong, Current Opinion in Solid State & Materials Science 4 (1999) 55-62. [3] A.J. Koster, R. Grimm, D. Typke, R. Hegerl, A. Stoschek, J. Walz, W. Baumeister, J. Struct. Biol. 120 (1997) 276-308 [4] J. Frank, Electron tomography, 1992, Plenum Press, New York [5] H. Beyer, P.A. Jacobs and J.B. Uytterhoeven, J. Chem. Soc., Faraday Trans. I 72 (1976) 674.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Interrogative Kinetic Characterization of Active C a t a l y s t Sites Using TAP Pulse Response Experiment. J. T. Gleaves a, G. S. Yablonsky ~ S. O. Shekhtmana, P. Phanawadee b "I)pt. of Chemical Engineering, Washington University, Campus Box 1198, One Brookings Drive, St. Louis, MO 63130, USA bDpt. of Chemical Engineering, Kasetsart University, Bangkok 10900, Thailand The approach for interrogative characterization of catalyst active sites using a combination of TAP pulse experiment is presented. This approach includes two principal steps: a state-defining experiment is used to determine the apparent kinetic parameters related to a given catalyst state, and a state-altering multi-pulse experiment is used to determine the number of active sites related to the same catalyst state. Using momentbased analysis, analytical expressions that correspond to both steps are obtained. The thin-zone TAP reactor that minimizes the influence of concentration gradient on observed kinetic characteristics and can be used to obtain information about very fast catalytic reactions is presented. I.

INTRODUCTION

The "interrogative kinetic" (IK) approach [1] that is an alternative to the traditional kinetic approach in heterogeneous catalysis. IK combines two types of nonsteady-state kinetic experiments, called "state-defining" and "state-altering" experiments. In contrast to traditional steady-state kinetics that attempts to obtain kinetic parameters for a well-determined "steady-state" of a catalytic reaction, IK attempts to systematically probe a variety of different states of a catalyst and to understand how one state evolves into another. An important dement of this approach is the rapid feedback between experiment and analysis that earl be likened to a "dialogue" between the researcher and the catalyst sample. The set of experiments that form an IK sequence represents a "question" (e. g. how does a change in the oxidation state of a catalyst changes its selectivity, or the activation energy of hydrocarbon conversion) that when answered leads to another question, and another IK sequence. A state-defining TAP experiment is one that does not significantly perturb the chemical state of a catalyst. The transient response data obtained in a state-defining experiment is a characteristic of that state. State-defining experiments involve reactions of probe molecules with a catalyst, and provide kinetic parameters such as rate

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parameters of adsorption and desorption. One example of a "probe reaction" is the irreversible chemisorption of a hydrocarbon on a metal oxide catalyst. A state-altering experiment is one in which the catalyst compositien is significantly changed. One type of a state-altering experiment is a TAP multi-pulse experiment in which the catalyst is exposed to a long series of reactant pulses. A statealtering experiment perturbs the catalyst and changes its composition or structure in some predetermined fashion. To complete an IK sequence, another state-defining experiment is performed to characterize the new state of the catalyst. 2. TAP-REACTOR CONFIGURATIONS The theoretical analysis of TAP pulse response data is based on the reactor model used to describe the catalyst bed through which the gas pulse travels. The simplest model is the "one-zone" reactor model in which the reactor is assumed to be uniformly packed with catalyst particles, and is heated uniformly over its entire length. An extensive theory for a one-zone TAP pulse response experiment has been developed and has been discussed in detail in a number of papers [ 1, 2]. A second type of reactor is the so-called "three-zone" reactor. In a three-zone reactor the catalyst zone is sandwiched between two beds of inert particles, called inert zones. Experimentally, the three-zone reactor has several advantages, and is the most commonly used reactor in TAP experimental studies. The main advantage of a three-zone reactor is that the catalyst zone can be more easily maintained in an isothermal condition. However, it is difficult to maintain uniform surface coverage in the catalyst zone because of the gas concentration gradient, which causes diffusion. It is also difficult to theoretically analyze three-zone TAP model. Currently, curve-fitting is the main approach used to describe three-zone experimental data. Recently, a new TAP reactor model that is a limiting case of a three-zone model and is called a "thin-zone" model was introduced [4]. In a thin-zone reactor, the thickness of the catalyst zone is made very small compared to the whole length of the reactor. The advantage of this configuration is that diffusional transport can be separated from chemical reaction, and influence of concentration gradients across the catalyst bed on the observed kinetic characteristics can be neglected. 3. MOMENT-BASED ANALYSIS A general theoretical approach that has been applied to an analysis of all three TAP reactor models is a moment-based approach. The feature which distinguishes the moment-based approach is that the set of TAP model equations uses moments as functions of axial coordinate rather than concentrations. The moments are related to the nature of a TAP experiment and describe the propagation of gas mixture throughout the reactor. The mathematical basis of these models is derived from the special initially and boundary conditions that reflect the "on-off" behavior of a TAP experiment. Moments have a clear physico-chemical meaning, (e.g. the zeroth and first moments are directly

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related to conversion and residence time, respectively) and can be readily measured. An important advantage of moment-based analysis is that in the case of linear or pseudolinear TAP models, moments can be calculated analytically in a compact form. The quantities of interest, e. g. kinetic parameters, can then be expressed analytically as functions of the moments of the exit flow. The results presented below are obtained using the moment-based approach. 4. STATE-DEFINING-ONE-PULSE TAP EXPERIMENT 4.1. Primary characterization of catalyst activity. Three-zone TAP reactor. The primary characterization of catalyst activity can be considered the first important step of an IK approach [2]. This characterization of catalyst activity should satisfy the following experimental and theoretical requirements: 1) insignificant change of the chemical composition and structure of the catalyst during the experimem (that is realized in a TAP state-defining experiment); 2) assumption of a first order reaction; 3) general analytical expression that relates catalyst activity and observed characteristics (e.g. conversion) The three-zone TAP reactor is the most general TAP reactor configuration, and one-zone and thin-zone TAP reactors are particular limiting cases. For the three-zone reactor, a general expression for irreversible catalytic processes on non-porous and porous materials has been obtained, and is given by:

l-X=

cosh(+) + a + sigh(V)

+=/t~Dgtkapp, where X is the conversion; D e is the effective Knudsen diffusivity; lcat is the length of the catalyst zone; r is the reactor parameter related to the geometry and transport properties (in a typical case, a=l); kopp is the apparent kinetic parameter that can be obtained from the experimental data using this expression. One- and two-step irreversible catalytic reactions on porous and non-porous catalysts were considered. 4.2. Thin-zone TAP reactor

The thin-zone TAP reactor model simplifies the interpretation of TAP data. The key idea of the thin-zone TAP reactor is to make the thickness of a catalyst zone very small compared to the length of the reactor. The rigorous mathematical basis of the thin-

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zone approach has been discussed [4]. A unique feature of a thin-zone reactor is that the influence of concentration gradients in the catalyst zone on the observed kinetic characteristics is insignificant. As a result, a thin-zone TAP reactor can be viewed as a "diffusional CSTR" For example, the conversion for irreversible adsorption/reaction in a thin-zone reactor is governed by the same relationship a for a first-order reaction in a CSTR, that is given by: kat,p "d~ff

X

~res,cat

~ .

b

~.diff

1 + ,~app ~res,cat

and ~ff tree,cat = 6b

where r,~.c~ ~ is the residence time for diffusion throughout the catalyst zone; lcat and lj,,2 are lengths of the catalyst zone and the second inert zone, respectively. The thin-zone reactor is particularly useful for investigating fast chemical reactions since the extent of reaction can be controlled by the thickness and position of the catalyst zone. 5.

STATE-ALTERING EXPERIMENT- MULTI-PUSLSE TAP EXPERIMENT. 5.1. Problem of active site number determination.

The concept of an "active site" has been a pivotal idea for revealing relationships between structure and activity in heterogeneous catalysis. The determination of the number of active site is a primary problem in heterogeneous catalysis [5, 6], and there is an important need for new techniques that can reveal the number of active sites on complex catalytic materials. Traditional methods of active site number determination use chemisorption, and determine the total amount of the reactant that can be adsorbed on the catalyst per unit surface area. However, the conditions of a complex catalytic reaction are very different from the adsorption measurement conditions. Moreover, the number of active sites is very dependent on catalyst preparation conditions, and composition/structure changes that occur during the catalytic process.

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5.2. A new method to determine a number of active sites based on TAP multi-pulse experiment.

To determine the number of active sites using TAP pulse response experiments a combination of state-defining and state-altering experiments, are used. As previously discussed, in a state-defining experiment, the chemical state of the catalyst, particularly surface coverage (number of unoccupied active sites), changes insignificantly. In contrast, in a state-altering experiment that uses a long sequence of pulses, the surface coverage (number of unoccupied active sites) as well as the quantities observed in a onepulse experiment gradually change as a function of the pulse number. Monitoring the change of the observed quantities, the number of "working" active sites at the beginning of a series of pulses can be determined. An important advantage of this method is that number of active sites can be found in a short series of pulses that produces a detectable change in observed quantities, and provides the number of active sites for a particular catalyst state. This method can be viewed as a "differential" method. In TAP studies, it is also possible to realize an "integral" method that can be likened to traditional adsorption methods. Using perturbation theory, analytical expressions for determining the number of active sites and kinetic characteristics per one site have been developed for the three main TAP-reactors types, i.e. one-, three-, and thin-zone TAP reactors. For example, in the case of irreversible adsorption, for a one-zone reactor, the following analytical expression can be obtained:

Mo(m) = 1 - X = M0(0)[1

Scatas

m

(1-- Mo(O))Mo(O)(Mo(O) 2 + 1)]

3Ml(O)

or

a'X c~

Np ( 1 - Mo(O))Mo(O) 2(Mo(O) 2 + 1) Scatas

3Ml(O )

And, for a thin-zone reactor, the expression is given by:

Mo(m ) = 1 - X = Mo(O ) 1 + Scatas m ( I - Mo(O)) 2 or

_~gX= Np c~

- - - - - Mo (0)(1- Mo (0)) 2 ,

Scatas

]

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where M~(m)is the n.th moment of the exit flow time dependence as a function of a pulse number, m=0, 1, 2, .... ,Np is the number of molecules in the inlet pulse, $r is the surface area of the catalyst, and a, is the number of active sites per unit area (mole/cm2). The number of active sites per unit area, a,, can be determined from the above equation using the experimentally measured characteristics, particularly moments, as functions of pulse number. 6.

CONCLUSIONS

The IK approach for characterization of catalyst active sites using a combination of TAP pulse experiment has been developed. A theoretical approach using a momentbased analysis has been developed and applied to all three TAP reactor models. The IK characterization approach includes two principal steps: 1) a state-defining experiment is used to determine apparent kinetic parameters that are related to a given catalyst state, 2) a state-altering multi-pulse experiment is used to determine the number of active sites related to the same catalyst state. A special attention has been paid to the thin-zone TAP experiment that minimizes the influence of concentration gradient on the observed kinetic characteristics and can be used to obtain information about very fast catalytic reactions. REFERENCES

1. I.T. Gleaves, I. R. Ebner, T. C. Kuechler, Catal. Rev. Sci. Engng., 30 (1988) 49. 2. I. T. Gleaves, G. S. Yablonskii, P. Phanawadee, Y. Schuurman, Appl. Catal., A: General, 160 (1997) 55. 3. G.S. Yablonsldi, S. O. Shekhtman, S. Chen, G. T. Gleaves, Ind. Eng. Chem. Res., 37 (1998) 2193. 4. S. O. Shekhtman, G. S. Yablonsky, I. T. Gleaves, S. Chen, Chem. Eng. Sci., 54 (1999) 4371. 5. M. Boudart, Chem. Rev. 95 (1995) 661. 6. F.H. Ribeiro, A. E. Schach yon Wittenau, C. H. Bartholomew, G. A. Somorjai, Catal. Rev.- Sci. Eng., 39 (1997) 55.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

UV resonance Raman spectroscopic identification of transition metal atoms incorporated in the framework of molecular sieves Guang Xiong, Can Li*, Zhaochi Feng, Jian Li, Pinliang Ying, Hongyuan Li and Qin Xin State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics Chinese Academy of Sciences, P. O. Box 110, Dalain 116023, China Fax: 86-411-4694447; Email: [email protected] The transition metal 'atoms in the framework of molecular sieves are characterized by the UV resonance Raman spectroscopy. An UV laser line (244 nm) is chosen to excite the charge-transfer transition between the framework oxygen and transition metal atoms in TS-1, Fe-ZSM-5, V-MCM-41, etc. The new Raman bands at 490, 530 and 1125 cm -~ are observed for TS-1, and the bands at 515, 1017 and 1170 cm l are detected for Fe-ZSM-5. V-MCM-41 also gives a new Raman band at 1070 cm l. The appearance of these Raman bands is due to resonance Raman effect since the laser line at 244 nm locates in the UV-visible absorption of transition metal atoms in the framework. Therefore the characteristic Raman bands solely associated with Ti, Fe and V atoms in the framework are selectively enhanced by resonance Raman effect, and the transition metal atoms in the framework are definitely identified by the UV resonance Raman spectroscopy. 1. INTRODUCTION Molecular sieves with framework atoms substituted by transition metal atoms in their framework have been considered as a new class of catalysts showing remarkable activity and selectivity for a number of oxidation reactions using dilute H202 as the oxidant [1]. The catalytic property is mainly attributed to the isolated transition metal atoms in the framework of the molecular sieves. Therefore, characterization of the transition metal atoms incorporated in the framework is the most important issue of the study. However, it remains difficult to know how and whether the transition metal atoms are incorporated into the framework of a molecular sieve, despite the extensive efforts using many techniques, such as XRD, FT-IR, UV-visible, NMR, ESR and Raman spectroscopy. In the present work, a new approach, UV Resonance Raman spectroscopy, is used to identify the transition metal atoms in the framework of molecular sieves based on resonance Raman effect since there are chargetransfer transitions between the framework oxygen and transition metal atoms. The characteristic Raman bands solely associated with the framework transition metal atoms were selectively enhanced, so that the transition metal atoms in the framework of a molecular sieve

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can be definitely identified. Among the examples, titanium atoms in TS-1, iron atoms in FeZSM-5 and vanadium atoms in V-MCM-41 were successfully identified. Therefore UV resonance Raman spectroscopy has opened up the possibility to identify the framework transition metal atoms in molecular sieves. 2. E X P E R I M E N T A L TS-1, Fe-ZSM-5 and V-MCM-41 were synthesized following the methods reported in the literature [2, 3]. UV Raman spectra were recorded on a home-made UV Raman spectrometer including four main parts: a UV cw laser, a Spex 1877 D triplemate spectrograph, a CCD detector, and an optical collection system. A 244- nm line from an Innova 300 FRED laser and a 488 nm line from Spectra Physics were used as excitation sources. The laser powers at the samples were kept below 2.0 mw and 100 mw for 244nm and 488 nm, respectively. The acquisition time was usually less than 5.0 min. The spectral resolution was estimated to be 1.0 cm 4. UV-visible Diffuse Reflectance spectra were recorded on a Shimadzu UV-365 UVVIS-NIR Recording Spectrophotometer. The molecular sieves were also characterized by XRD and FT-IR spectroscopy. 3. RESULTS AND DISCUSSION As shown in Fig. 1, the electronic transition absorption of transition metal atoms substituted molecular sieves frequently appears in the UV region. The absorption is assigned to the charge transfer transition between transition metal and oxygen atoms in the framework. This transition involves the excitation of an electron from a 7t bonding molecular orbital consisting of oxygen atomic orbital to a molecular orbital that is essentially a titanium atomic d orbital. This offers an opportunity to characterize the transition metal atoms substituted molecular sieves by resonance Raman effect. When the frequency of the laser is close to and/or within the electronic absorption band of the transition metal atoms in the framework,

/ o\ / \ o

/Si\/

G o M\ /

O Si \

\

f r a m e w o r k sites 0 ( 2p ) 220 nm Y i ( 3d ) 0 ( 2p ) 250 nm F e ( 3d ) O(2p)

280 nm ~ V(3d)

Fig. 1. Charge transfer transition between oxygen and transition metal atoms in the framework of molecular sieves.

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~~

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l l. c a l l t. e

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Fig. 2. UV-visible diffuse reflectance spectra (A) and UV resonance Raman spectra (B) of Silicalite- 1 and TS- 1. the intensity of the Raman bands related to this charge transfer transition will be increased by several orders of magnitude. As a result, not only the sensitivity is increased but also the local structure of the complicated molecular sieves can be characterized based on the resonance Raman effect. Such a possibility occurs when the laser line at 244 nm is chosen to excite the charge transfer transition between transition metal and oxygen atoms in the framework of TS1, Fe-ZSM-5, V- MCM-41, and so on. Figure 2A shows the UV-visible diffuse reflectance absorption spectra of TS-1 and silicalite-1. An absorption band centered at 220 nm is observed for TS-1 while no electronic transition absorption is found for silicalite-1. The band at 220 nm is attributed to the charge transfer of the pn-dTr, transition between titanium and oxygen atoms in the framework of TS-1 [4, 5]. The laser line at 244 nm was chosen to excite the electronic transition absorption of the titanium species in the framework of TS-1 zeolite. Figure 2B compares the UV Raman spectra of TS-1 and silicalite-1 excited by 244-nm line. The bands at 380, 815, 960 cm 1 are observed for both TS-1 and silicalite-1, indicating that these bands are not resonance Raman bands but are the characteristic bands of silicalite-1 zeolite [6-8]. The UV Raman spectrum of TS-1 is different from that of silicalite-1. It is clearly observed that the new bands at 490, 530 and 1125 cm ] are observed in the UV Raman spectrum of TS-1. These bands are never distinguished from the visible Raman spectra of both TS-1 and silicalite-1. These three peaks are not associated with the titanium species in the extra-framework, since these bands are totally different from the characteristic bands of TiO 2 (anatese) at 144, 390, 516 and 637 cm l [9]. In particular, the intensity of the band at 1125 cm ~ is tremendously strong while the Raman bands in the 1000 cm l region for

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silicalite-1 are very weak. Obviously the appearance of these bands is due to the resonance Raman enhancement since the laser line at 244 nm excites the electronic transition absorption of titanium species in the framework. This is the direct evidence for the presence of framework titanium atoms in TS-1 because only those bands associated with the chargetransfer transition between framework titanium and oxygen can be greatly enhanced by the resonance Raman effect. The bands at 490, 530 and 1125 cm "~ are attributed to the local vibrations of the tetrahedral unit of Ti(OSi)4, namely bending, symmetric stretching and asymmetric stretching vibrational modes of Ti-O-Si species, respectively. The relative intensities of the resonance Raman bands at 490, 530 and 1125 cm ~ are significantly increased with the crystallization time of TS-1, while the other bands are only slightly changed. This indicates that more titanium atoms are incorporated into the framework during the synthesis and crystallization. The band at 960 cm ~ was used to indicate the presence of the transition metal atoms in the framework. It was believed that the intensity of the band at 960 cm ~ of transition metal atoms substituted molecular sieve was stronger than that of its siliceous host, and the intensity of the band at 960 cm ~ increased with the increasing of transition metal content [10-12], but some authors hold opposite opinions [ 13]. As shown in Fig. 2B, the band at 960 cm ~ is also detected but not enhanced by the resonance Raman effect. Hence this band is not directly related to the framework titanium, but from the SiO4 unit of silicate-1 with defect sites or adjacent titanium ions that may induce symmetry changes of the local structure. Fig. 3 shows the UV-visible diffuse reflectance absorption spectra of Si-MCM-41 and VMCM-41. No electronic absorption bands is observed in the UV-visible absorption spectrum for Si-MCM-41, while the bands at 270 and 340 nm are found for V-MCM-41. The bands at 270 and 340 nm are assigned to the charge transfer transition between the tetrahedral oxygen ligands and the central V 5§ ion with tetrahedral coordination in the framework [13-15]. However, the UV-visible electronic absorption of polymerized vanadium oxides supported on SiO2 also appears in the 250-350 nm region [16]. The bands at 250 and 320 nm are observed in the UV-visible diffuse reflectance spectrum of supported vanadium oxides. Hence the broad bands at 270 and 340 nm are the overlap of the UV-vis bands of both isolated tetrahedral (framework) and polymerized octahedral (extra-framework) vanadium species. A laser line at 244 nm is close to the electronic absorption of the vanadium species in both the framework and the extra-framework simultaneously. Fig. 4 shows the UV Raman spectra of Si-MCM-41 and V-MCM-41 excited by 244 and 488-nm line. The bands at 490, 610, 810 and 970 cm -~ are detected in the visible and UV Raman spectra of Si-MCM-41. These bands are attributed to the fourfold siloxine, siloxane bridges and silanol groups [14]. Visible Raman spectrum of V-MCM-41 shows similar Raman bands like those of the Si-MCM-41, and no Raman bands associated with the vanadium species in the framework and extra-framework are detected. In the UV Raman spectrum of V-MCM-41 two additional bands at 930 cm ~ and 1070 cm l are detected as shown in Fig. 4. The band at 930 cm ~ is assigned to the V=O symmetric stretching mode of the polymerized vanadium oxides in the extra-framework [ 17]. The band at 1070 cm ~ is

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A lam

o o o~. I I / O . . I 1 . 1 0 v v

244 nm

i

81

270

I,,

<

488 nm

10

v

4,* m

(/}

0 c mi

.....

4,J

200

~:~'~,

E=

340

v

,,

o II V

300

400

500

!"t~ i i

', , ,,

--1 /

,'

I

a

600 500

W a v e l e n g t h / nm

Fig. 3. UV diffuse reflectance spectra of Si-MCM-41 and V-MCM-41. a. Si-MCM-41, b. V-MCM-41

Rarnan

1000 shift

1500

2000

/ cm-1

Fig. 4. UV and visible Raman spectra of Si-MCM-41 and V-MCM-41. a. Si-MCM-41,488 nm b. V-MCM-41,488 nm c. Si-MCM-41,244 nm d. V-MCM-41,244 nm

assigned to the V-O symmetric stretching mode of the vanadium atoms in the framework [18,19]. Considering that the 244-nm line is close to the charge transfer absorption of the vanadium atoms, it is the Raman resonance effect that makes the bands at the 930 cm -~ and 1070 cm 1 considerably enhanced. UV Raman spectrum of Fe-ZSM-5 also exhibits three new bands at 515, 1017 and 1170 cm ~ owing to the resonance Raman effect. These bands are observed because the chargetransfer transition between the framework iron and oxygen excited by the UV laser line at 244 nm. Accordingly these bands can be assigned to the local vibrations of framework Fe in the Fe-ZSM-5. The fact that these bands are different from those of TS-1 reflects the different nature of Fe and Ti and also the difference between ZSM-5 and silicalite-1. Transition metal atoms substituted in the other molecular sieves, such as TS-2, Ti-13 and VSAPO, have been also successfully identified by UV resonance Raman spectroscopy. Furthermore, this method can be also applied to characterizing the isolated transition metal atoms in other catalysts. 4. CONCLUSIONS The UV Raman spectra of TS-1, Fe-ZSM-5 and V-MCM-41 are obtained for the first time, and the transition metal atoms in the framework are definitely identified by the resonance Raman effect. It is also shown that the band at 960 cm ! often appearing in Raman spectrum of TS-1 is not directly associated with framework titanium species but with Si-O-Si species induced by the framework titanium species or by any other defect sites nearby. This

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study shows that UV Resonance Raman spectroscopy is a powerful approach for the characterization of the transition metal atoms substituted in the framework of molecular sieves. 5. ACKNOWLEDGEMENT

This work was supported by the National Natural Science Foundation of China (NSFC) for Distinguished Young Scholars (Grant No. 29625305). REFERENCES

1. 2. 3.

G. Bellussi and M. S. Rigutto, Stud. Surf. Sci. Catal., 85 (1994) 177. M. Taramasso, G. Perego and B.Notari, U. S. Patent No. 4 410 501 (1983). K.M. Reddy, I. Moudrakovski and A. Sayasi, J. Chem. Soc., Chem. Commun., (1994), 105. 4. D. Scareno, A. Zecchina, S. Bordiga, F. Geobaldo and G. Spoto, J. Chem. Soc., Faraday Trans., 89 (1993) 4123. 5. A. Zecchina, G, Spoto, S. Bordiga, A. Ferrero, G. Petrini, G. Leofanti and M. Padovan, Zeolite Chemistry and Catalysis, Elsevier Science Publishers B. V., Amsterdam, 1991. 6. E. Astorino, J. B. Peri, R. J. Willey and G. Busca, J. Catal., 157 (1995) 482. 7. C.U. Ingemar Odenbrano, S. Lars T. Andersson, Lars A. H. Andersson, J. G. M. Brandin and G. Busca, J. Catal., 125 (1990) 541. 8. G. Busca, G. Ramis, J. M. Gallarado Amores, V. S. Escribano and P. Piaggio, J. Chem. Soc., Faraday Trans., 90 (1994) 3181. 9. I.R. Beattie and T. R. Gilson, Proc. Roy. Soc., A 307 (1968) 407. 10. T. Sen, V. Ramaswamy, S. Ganapathy, P. R. Rajamohanan and S. J. Sivasanker, J. Phys. Chem., 100 (1996) 3809. 11. G. Deo, A. M. Turek, I. E. Wachs, D. R. C. Huybrechts and P. A. Jacobs, Zeolites, 13 (1993) 365. 12. W. Pilz, Ch. Peuker, V. A. Tuan, R. Fricke, H. Kosslick and Ber. Bunsenges, J. Phys. Chem., 97 (1993) 1037. 13. Z. H. Luan, J. Hu, H. Y. He, J. Klinowsli and L. Kevan, J. Phys. Chem., 100 (1996) 19595. 14. K. J. Chao, C. N. Wu, H. Chang, L. J. Lee and S. F. Hu, J. Phys. Chem., 101 (1996) 6341. 15. T. Sen, V. Ramaswamy, S. Ganapathy, P. R. Rajamohanan and S. Sivasanker, J. Phys. Chem., 100 (1996) 3809. 16. G. T. Centi, S. Perathoner, F. Trifir, A. Aboukais, C. F. Aissi and M. Guelton, J. Phys. Chem., 96 (1992) 2617. 17. C. Cristiani, P. Forzatti and G. Busca, J. Catal., 116 (1989) 586. 18. G. T. Went, S. T. Oyama and A. T. Bell, J. phys. Chem., 94 (1990) 4240. 19. H. Selig and H. H. Claassen, J. Chem. Phys., 44 (1966) 1404.

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化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Surface Mobility of Oxygen Species on Mixed-Oxides Supported Metals C. Descorme a, Y. Madier a, D. Duprez a, T. Birchem b a Laboratoire de Catalyse en Chimie Organique (LACCO) - UMR CNRS 6503 - Universit6 de Poitiers - 40, avenue du Recteur Pineau - 86022 POITIERS CEDEX - FRANCE. b RHODIA - CRA - 52, rue de la Haie Coq - 93308 AUBERVILLIERS CEDEX - FRANCE. Cerium-zirconium mixed oxides-supported metals (Rh, Pt, Pd) were prepared by impregnation of CexZr(~.x)O2 (x = 0, 0.63 and 1) supports with the salt of the corresponding metal. These materials were studied on the basis of their Oxygen Storage Capacity (OSC) and activity in the isotopic exchange of oxygen. For all of them oxygen as been found especially reactive and mobile above 400~ Introduction of a metal markedly enhances the OSC of the supports and their activity in the exchange of oxygen (Rh>Pt>Pd). The OSC at 400~ of Ce0.63Zr0.3702 is multiplied by a factor of almost 4 in the presence of 0.3%Rh. Metal particles are assimilated to portholes for the subsequent migration of oxygen on the support. In the same way, the activity at 350~ of 0.3%Rh/CeO2 in the isotopic exchange of ~802 is 3 orders of magnitude larger than in the absence of rhodium. 1. INTRODUCTION As pollution control problems appeared, more and more work has been done in the field of environmental catalysis. Especially for the reduction of automotive pollution, many studies dealt with the optimization of three-way catalysts (TWC). One of the clue parameters in developing such catalysts was the control of the oxygen mobility. In fact surface mobility is involved in many catalytic processes, like oxygen reversible storage or oxygen transfer in oxidation catalysts. Thus many studies have been devoted to the synthesis and subsequent characterization of new cerium-based mixed oxides supports [ 1-12]. Cerium-zirconium solid solutions were shown to be good candidates with enlarged Oxygen Storage Capacity (OSC) [2,3,5,8,9] and improved redox properties [2,12]. However, only a few studies have been devoted to the quantification of oxygen surface diffusion on cerium-zirconium mixed oxides supported metals. In addition to OSC measurements, isotopic exchange is an adequate technique for the study of chemisorbed species mobility [13]. This paper is concerned with the characterization of CexZr(l.x)O2 solid solutions based catalysts (x = 0, 0.63 and 1). Three noble metals are under study : Rh, Pt and Pd. The influence of the metal is reviewed on the basis of OSC measurements and isotopic exchange experiments.

2. EXPERIMENTAL Oxides, calcined at 900~ were directly provided by Rhodia Terres Rares (La Rochelle, France). Rh, Pt and Pd catalysts were prepared by impregnation of the oxides with aqueous

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solutions of Rh(NO3)3, Pt(NH3)4(OH)2 and Pd(NO3)2 respectively. All samples were pretreated under flowing air (30 ml.mn"l) at 450~ for 4 hours (fresh catalysts). To study their stability, catalysts were also aged at 900~ for 4 hours under flowing dry air (aged catalysts). The oxides structure was investigated by XRD using a Siemens D500 diffraetometer. Crystalline phases were identified by comparison of experimental diffractograrns with ICDD files. Crystallite sizes determination was based on the Debye-Scherrer relation. Surface areas were measured by adsorption of N2 at -196~ with a Micromeritics Flowsorb II. This apparatus uses the single point method. The metal dispersion was calculated from HE chemisorption experiments. The procedure of these measurements had to be optimized in order to prevent hydrogen spillover onto the support [ 14]. Oxygen Storage Capacities measurements, first introduced by Yao [15], were carried out on an home-made apparatus previously described [16]. Two types of information may be obtained 9the relative kinetics of the reduction-oxidation process (OSCC) and the amount of oxygen "immediately" available in the material (OSC). Isotopic exchange experiments consist in monitoring, by mass spectrometry, the oxygen isotopomers (1602, 160180, 1802)partial pressure. Exchange proceeds between labeled oxygen atoms, initially introduced in the gas phase, and oxygen atoms of the oxide support. The rate of exchange (Re) may be determined as well as the mechanism of the reaction (simple or multiple heteroexchange) depending on the relative partial pressure in 1602 and 160180 [13, 17-21]. IR studies of 02 adsorption were performed using self supported oxides wafers. Samples were pretreated in situ in flowing 02 at 400~ over 12h. Spectra were collected at a resolution of 4crnq on a Nicolet Magna 550 FT-IR spectrometer. 3. MAIN RESULTS

3.1. Materials The main characteristics of the materials used in this study are summarized below. Table 1 Main physicochemical properties of M/CexZrt~.x)O2 samples (M = Rh, Pt, Pd - x = 0, 0.63, 1) Support

0.3 o'ARh/CeO2 0.3%Rh/Ceo.63Zro.3702 0.3 % Rh/Z rO2 l%Pt/CeO2 1%Pt/Ceo.63Zro.3702 1%Pt/ZrO2 0.5%Pd/CeO2 0.5%Pd/Ceo.63Zro.3702 0.5%Pd/ZrOz

Fresh Catalyst

Structure

Crystallite size(A)

Surface area (m2.gq)

Dispersion (%)

cubic cubic mo noc linic cubic cubic monoclinic cubic cubic monoclinic

260 110 23 0 260 110 230 260 110 230

28 41 12 28 40 12 27 41 11

56 85 44 53 70 34 56 60 49

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All oxides were found to be purely monophasic. Ceria and cerium-rich mixed oxides are cubic while zirconia is monoclinic. Zirconium has a stabilizing effect on the structure. In fact, even after calcination at 900~ mixed oxides maintain a surface area of about 40 m2.g"~ compared to approximately 10 and 30 m2.g~ in the case of zirconia and ceria respectively. Looking at the metallic phase, it also appears that the dispersion is favored on mixed oxides.

3.2. Oxygen Storage Capacities OSCC measurements confirmed that the limiting step in the redox process of these bare oxides is the reduction [22]. This may be seen from the kinetics of the process during the measurements 9the reduction by CO is slow while the re-oxidation by 02 is total and instantaneous. In the case of oxides-supported metals, the reduction is still the limiting process but the re-oxidation is not complete. This observation evidences a strong metalsupport interaction, modifying the redox properties of the oxides. In close interaction with a metal, cerium could be irreversibly over-reduced. Table 2 shows the effect on the OSC of the introduction of Zr in the ceria lattice. As seen previously an optimum exists for Ce0.63Zr0.3702 [22,23]. This solid has an OSC four times larger than the one measured for ceria. Table 2 Oxygen Storage Capacity (OSC) at 400~ of M/CexZq~.x)O2 samples (M = Rh, Pt or Pd - x = 0, 0.63 and 1) OSC at 400~ (lxmolCO2.g"l) * x=0 Ce~Zro.x~O 2 0.3%Rh/CexZro.x~O 2

l*/oPtlCexZro.~02 0.5*/.Pd/Ce~Zro_~O2

X = 0.63

x= 1

Fresh

Aged

Fresh

Aged

Fresh

Aged

101 63 57

0 32 46 42

755 716 727

202 543 615 576

132 115 105

48 147 96 113

* amount of C02 produced after the first pulse of CO during alternate CO and 02 pulses. The presence of metallic particles on the surface of these oxides also modifies their OSC. All studied metals (Rh, Pt, Pd) approximately have the same effect on the increase of the OSC. The most efficient is Rh, especially in the case of the mixed oxide where an increase by a factor of almost 4 is observed. Ageing at 900~ does not considerably affect the OSC of those catalysts. However, Pt and Pd seem to be the best promoting metals for oxygen storage on aged M/Ce0.63Zr0.3702 catalysts.

3.3. 1SO2Isotopic Exchange The whole process of exchange was described, step by step, in an earlier publication [24]. Three steps may be separately studied. Direct exchange with the support, occurring at relatively high temperature, was investigated during preliminary studies on bare oxides. The

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results of these studies are represented in Figure 1 by line D. Informations on the oxygen activation on the metal particles may be accessed from homoexchange measurements (line A). Finally, a thorough study of the heteroexchange gives informations on the limiting step at a given temperature. In fact, during the exchange, two regimes can be differentiated 9line B = exchange is limited by the adsorption/desorption on the metal particle, line C = exchange is controlled by the diffusion on the oxide surface. The influence of the metal on the oxygen mobility at the ceria surface was investigated as a function of temperature. Results are presented in Figure 1. 4.5 3.5

Fig. 1 9Arrhenius plot of 1802 exchange on M/CeO2 samples (M = Rh, Pt or Pd) [from isotopic exchange (full symbols) and equilibration (open symbols) results, with I = Rh, & = Pt, @ = Pd, ~ = ceria]. A, B, C and D 9see text.

A

Z5 A

E

c

1.5

9 0.5 iv,

=: -0.5

B

_1

-1.5 -2.5 -3.5

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2

l O 0 0 f f (K "1) I

550

I

I

450

350

,

t

250

T (~

For all M/CeOz systems, the same behavior is observed (Figure 1). The change in the rate limiting step is observed at 300, 420 and 480~ for Rh, Pt and Pd respectively. Rhodium is the most active metal for the activation of oxygen (equilibration) and oxygen exchange. For Rh/CeO2 the oxygen surface diffusivity may be accessed between 310~ and 370~ In that temperature range, direct exchange with the support may be neglected and oxygen adsorption-desorption is fast. The surface diffusion coefficient (Ds) on ceria was calculated to be 3.10~Sm2.s l at 350~ The activation energy of the process is 48kJ.mol ~. On the opposite, palladium is practically inactive for 02 exchange : all measurements are perturbed by direct exchange with the support. The modification of the surface mobility of oxygen on various oxides was also examined by looking at 3 differem systems" Rh/CeO2, RIgCeo.63Zro.3702 and Rh/ZrO2. The activity of these systems was simply studied in the heteroexchange of oxygen. At low temperature, as previously, exchange is limited by the adsorption-desorption of oxygen on the metal particles. In fact, as we can see on Figure 2, all lines merge whatever the support is. No influence of the support is observed. At higher temperature, oxygen surface diffusion is rate limiting. In that case, the influence of the oxide is clear : oxygen mobility is higher on Ceo.63Zro.3702 than on ceria and

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than on zirconia. The rate of exchange on Rh/Ceo.63Zro.3702 at 350~ is about 2 and 9 times larger than on ceria and zirconia respectively.

A

E2

Fig. 2 9Arrhenius plot of lgo 2 exchange on Rh/CexZr(~.,,)O2 samples (x = 0, 0.63, l) [from isotopic exchange results, I

C

Q 1 iv =0 ,.J

=

Ce0.63Zr0.3702, ~

=

CeO2,

A = ZrO2]

3

i

1.2

1.4

i

1.6

l

1.8

i

i

2

2.2

IO001TIK") 3.4. Surface oxygen species

In a study conducted in parallel, the mechanism of exchange was also shown to vary depending on the nature of the oxide [22]. Multiple heteroexchange was related with the presence ofbinuclear oxygen species on the surface [25]. The specific behavior of mixed oxides was then tentatively correlated with the population in superoxides at the surface. These species, identified by FT-IR, are characterized by a single band at 1126 cm -l. 12

250

A ,t--

A

d

tO w o

10~~

200

150

E

6 _~ ~

o

.~

E

to

tn 0

'E

100

50

2 ~ Q. Z =

Fig. 3 :Correlation between the OSC of CexZr(l.x)02 samples (x = O, 0.63, 1) and the amount of superoxides formed on the surface upon adsorption of oxygen (0.5mbar) at room temperature.

o zirconia

mixed-oxide

ceria

The amount of superoxides was estimated from the integrated area of the band Vo-o at l126cm "1 and normalized to lg of sample. As shown in Figure 3, a good agreement is

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observed. The high oxygen mobility at the surface of mixed oxides seems to be closely related to the presence of binuclear species at the surface. Then, oxygen could be "transported" as superoxides species at the surface of these materials.

4. CONCLUSIONS Mixed oxides impregnated metals were shown to have very large Oxygen Storage Capacities and interesting activities in the isotopic exchange of oxygen. The OSC is multiplied by a factor of 4 in the presence of a metal and the mobility of oxygen on Ce0.63Zro.3702 is 9 times greater than on zirconia. Ageing does not drastically affect the OSC of these materials. Pt and Pd catalysts are the most stable systems. Their specific behavior towards oxygen was correlated with the surface population in dioxygen species (superoxides, peroxides). In fact, oxygen could be mobile on these oxides as superoxides entities. REFERENCES

1. M. Yashima and K. Morimoto, J. An~ Ceram. Soc., 76 (1993) 2865. 2. P. Fornasiero, R. Di Monte, G. Ranga Rao, J. Ka~par and A. Trovarelli, J. Catal., 151 (1995) 168. 3. P. Fornasiero, G. Balducci, J. Kagpar and M. Graziani, Catal. Today, 29 (1996) 47. 4. P. Fornasiero, R. Di Monte and J. Ka~par, J. Catal., 162 (1996) 1. 5. A. Trovarelli and F. Zamar, J. Catal., 169 (1997) 490. 6. P. Vidmar, P. Fornasiero, J. Ka~par and G. Gubitosa, J. Catal., 171 (1997) 160. 7. A. Suda, T. Kandori and Y. Ukyo, J. Mat. Sci. Lett., 17 (1998) 89. 8. D. Terribile, A. Trovarelli and J. Llorca, Catal. Today, 43 (1998) 79. 9. S. Rossignol, Y. Madier and D. Duprez, Catal. Today, 50 (1999) 261. 10. A. Trovarelli, F. Zamar and S. Mashio, Chem. Comm., 9 (1995) 11. 11. P. Fomasiero, G. Balducci, R. di Monte, J. Ka~par, V. Sergo, G. Gubitosa, A. Ferrero and M. Graziani, J. Catal., 164 (1996) 173. 12. G. Vlaic, P. Fomasiero, S. Geremia and J. Ka~par, J. Catal., 168 (1997) 380. 13. D. Martin and D. Duprez, J. Phys. Chem., 100 (1996) 9429. 14. Y. Madier, Ph-D Thesis, University of Poitiers (1999). 15. H.C. Yao and Y.F. Yu Yao, J. Catal., 86, (1984) 254. 16. S. Kacimi, J. Barbier Jr., R. Taha and D. Duprez, Catal. Lett., 22 (1993) 343. 17. E.R.S. Winter, Adv. Catal., 10 (1958) 196. 18. K. Klier, J. Novfikovfi and P. Jiru, J. Catal., 2 (1963) 479. 19. G.K. Boreskov, Adv. Catal., 15 (1964) 285. 20. E.R.S. Winter, J. Chem. Soc., 1 (1968) 2889. 21. J. Novfikovfi, Catal. Rev., 4 (1970) 77. 22. Y. Madier, C. Descorme, A.M. Le Govic and D. Duprez, J. Phys. Chem., 103(50) (1999) 10999. 23. A. Trovarelli, F. Zamar, J. Llorca, C. de Leitenburg, G. Dolcetti and J.T. Kiss, J. Catal., 169 (1997) 490. 24. D. Duprez, Stud. Surf. Sci. Catal., 112 (1997) 13. 25. S. Rossignol, F. G6rard and D. Duprez, J. Mat. Chem., 9 (1999) 1615.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

SO2-promoted propane oxidation over Pt/AI203 catalysts Adam F. Leea, Karen Wilsonb and Richard M. Lambertb aDepartment of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom. bDepartment of Chemistry, University of Cambridge, Cambs. CB2 1EW, United Kingdom. The origin of SO2-promoted propane oxidation over Pt/alumina catalysts has been elucidated utilising spectroscopic and kinetic measurements over dispersed Pt/A1203 and model y-AI203/Pt(111) systems. HR ELS, NEXAFS, TPRS and XPS studies show that under oxidising conditions SO2 promotes the dissociative chemisorption of C3H8 at both Pt and support sites via surface sulphation. Interfacial sulphoxy species play a key role in activating propane for subsequent combustion at metal sites via a bifunctional spillover mechanism. The loading dependence of the promotional effect is rationalised in the light of X-ray and TEM measurements that reveal catalyst sulphation is accompanied by the reduction and concomitant sintering of small (<20 A) PtO2 particles to form large, active Pt metal clusters. I. INTRODUCTION Pt/alumina is arguably one of the most important and widespread catalyst system in commercial use today, finding application in pollution abatement and large-scale hydrocarbon reformation [1]. Despite their socio-economic impact, many aspects of the complex interdependent interactions between Pt-alumina catalysts and poison/promoter species, including sintering and aluminide formation, remain poorly understood. Sadowski and Treibmann first reported the irreversible SO2-promoted catalytic oxidation of n-alkanes by Pt/AI203 in 1979 [2]. Early automotive emission control studies by Gandhi et al also showed that trace SO2 levels in simulated exhaust feeds promoted the Pt/AI203 catalysed oxidation of propane [3]. Catalyst sulphation by aqueous H2SO4 or gaseous SO2/O2 mixtures induces the same irreversible rate-enhancements. The advent of lean-burn and diesel vehicles and associated emissions control efforts has refocused attention on the role of SO2 in hydrocarbon chemistry [4]. Lean-burn vehicles may run for extended periods at low temperatures, with the consequent accumulation of high sulphate levels. We have thus undertaken to derive both a microscopic and macroscopic understanding of SO2/propane chemistry over Pt/AI203 catalysts. The genesis of ultrathin alumina films on a Pt(111) single-crystal substrate and their reactivity in SO2-promoted propane oxidation has been followed by high-resolution electron energy loss spectroscopy (HREELS), low energy electron diffraction (LEED), scanning tunneling microscopy (STM), temperatureprogrammed reaction spectroscopy (TPRS) and X-ray photoemission spectroscopy (XPS). These measurements have allowed us to eliminate complicating particle size effects and focus on the chemistry of the metal and metal/oxide interface. Complementary X-ray absorption (XAS), X-ray diffraction (XRD) and transmission electron microscopy (TEM) experiments on dispersed Pt/AI:O 3 catalysts have rationalised the structure-sensitivity of SO: promotion.

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2. EXPERIMENTAL

Photoemission, ttREELS and TPRS measurements were performed in a VSW ARIES ultrahigh-vacuum (UHV) chamber operated at base pressures of lxl0 "l~ Torr. The Pt(111) sample could be resistively heated to 1200 K and cooled to 150 K. Cleaning was achieved by cycles of Ar+ sputtering and annealing at 1000 K, followed by oxygen treatment (800 K, l xl0 "s Torr) until LEED/AES and TPD analysis indicated a clean well-order surface. Scanning tunneling microscopy (STM) was performed in an Omicron UHV VT-STM system equipped with LEED and Auger capabilities. STM images were acquired at 300 K in constant current (topographic) mode with an electrochemically etched W tip, biased between +100 mV and +_2Vwith tunnel currents from 0.5 to 5 nA. AI (Goodfellows 99.99%) was deposited as described previously [5] with a deposition rate of--0.15 monolayers (ML) per minute. A monolayer is defined as the number of adatoms required for completion of a continuous overlayer film, i.e. lxl015 atomscm"2. TPRS experiments were performed using a capillary doser and multiplexed quadrupole mass spectrometer with 02 (99.995 %), SO2 (99.98 %) and C3Hs (99.95 %), all gases supplied by Distillers. A linear heating ramp of 17 Ks~ was employed. Oxidation of AI was performed at pressures of lxl 0.7 Torr 02. 3. RESULTS

3.1 Preparation of Al2Oa/Pt(lll)surfaces AI overlayer growth on clean Pt(111) has been followed by XPS and STM at 300 K as a function of film thickness. STM reveals the formation of 2D dendritic AI islands for submo no layer coverages (Figure l a). At 160 K both STM and XPS (not shown) indictate AI islands continue to grow in a layer-by-layer fashion beyond the monolayer for at least 5 ML. In contrast at 300 K, completion of the first monolayer is followed by 3D island growth and surface roughening (Figure l b). Multilayer growth is accompanied by interfacial AI/Pt surface alloying and concomitant shifting of the Pt 4f core-level photoemission peaks. This exothermic process is facilitated by the lifting of surface energy restrictions for underlaying AI residing at the AI/Pt(111) interface beneath an A1 cap [5].

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Fig. 1. a) 0.25 ML and b) 1.5 ML of Al on Pt(111). (2000 A x 2000 A; +l V, lnA)

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The oxidation of these ultrathin AI films was subsequently investigated as a function of temperature and film thickness. Room temperature oxidation by molecular oxygen + 5MLAI results in a slight increase in surface roughness, and CO titrations confirm that even for films >3 ML, -20-30 % of surface Pt(lll) sites 300 K 0 2 remain exposed. The saturation oxygen uptake over AI/Pt(111) surfaces vastly exceeds that 8 0 0 K (3= possible over clean Pt(lll). Photoemission measurements also reveal a large chemical shift in the A1 66 eV Auger (Figure 2) and AI 2s transitions following high O2 exposures at 300 K. These shifts are further enhanced upon annealing above 800 K, and their magnitude and sign are consistent with A1203 formation. Multilayer films retain some interfacial AI in . , '_ ., I i' metallic form within a disordered alloy phase 9 " do ' 6o ' 40 of approximate composition Pt2Al3 [5]. In Kinetic Energy (eV) contrast, oxidation of a monolayer Pt3AI Fig. 2. AES spectra following 300 K growth surface alloy efficiently pulls all surface A1 into and oxidation of 5 ML of AI on Pt(111) an overlayer oxide. High temperature oxidation also induces the appearance of well-defined vibrational losses at 450, 670 and 870 cm~. These modes are characteristic of crystalline yalumina, and indeed coincide with the observation of a sharp (4~/3x4~/3)R30~ LEED pattern consistent with an expanded A1203 overlayer [6].

化

|,

C l e a n

(xl/s)

Pt

3.2 R e a c t i v i t y o f 7-A1203/Pt(I 11) s u r f a c e s

The subsequent exposure of Pt-supported alumina films towards SO2 was explored under oxidising conditions. Over clean Pt(111), XPS, HREELS and NEXAFS show such treatments result in weakly-bound surface SO4, prone to electron-stimulated decomposition. In the ].2 I Inteffacial / ~ presence of Al203 films ESD is greatly reduced, and both the thermal stability and -~ ] concentration of SO4 are increased. This = stabilisation of surface sulphoxy species, "~ 0.8 particularly located at the Pt-A1203 interface, has a profound impact on the .~ 0.6 surface reactivity towards alkane activation (Figure 3). The sticking probability of both =o0.4 bare Pt [7] and A1203/Pt(111) [6] surfaces / for dissociative C3Hs chemisorption is ~0.2 essentially zero under UHV. Although adsorbed SO4 facilitates propane oxidation 0 0 1 2 3 over Pt(lll) alone, interfacial sulphate Alumina Film Thickness / ML species promote a further five-fold rise in Fig. 3. Temperature programmed reaction of C3H8 over SO4/AI203/Pt(111) surfaces r~

~

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356

the yield of oxidation products. This unique enhancement is most likely mediated by a reverse spillover process, in which propane, dissoeiatively chemisorbed at sulphated interfacial Pt-alumina sites, migrates onto neighbouring metallic Pt centres where it undergoes reaction with coadsorbed atomic oxygen (Schemel).

. 'k...-"

,y~.. i~.,,,

so:

:'~ "~ It Q~O/d

I

Platinum

O H %

IH20 /

.,H

i1~~~1 o ? I -~ I_ P ! ~ ~ - I ..........................

~o:

/fl

)

~-

AT

Scheme 1. Mechanism for SO4-promoted C3I-Is chemisorption and oxidation at the AI203/Pt(111) interface

3.3 Structure-reactivity of dispersed Pt/Ai203 catalysts

Though the preceding single-crystal studies provide an explanation for the role of SO2 in enhancing propane chemisorption over Pt/AI203 catalysts, they do not explain the structuresensitive nature of this promotion. Light-off measurements reveal low loading (<1 wt% Pt) catalysts exhibit far greater promotion (ATs0 = -140 K) following exposure to SO2/O2/C3Hs mixtures than their higher loading counterparts (ATs0 = -50 K) [3]. We thus examined the structural and electronic properties of flesh and sulphated 0.05, 3 and 9 wt% Pt/AI203 catalysts in order to identify differences and correlate these with their associated reactivity. Figure 4 shows X-ray diffraction patterns of flesh and sulphated 3 wt% Pt/AI203 catalysts. (

9 Pt O

Al2(SO4)3.nH20

LX13-PtO2 9

A1203

o

~

I A~.^

,

Fresh ....

25

45

2~g (o)

65

__..-:-_, ~ , ~ .

-- _.... - _ ~

85

Fig. 4. X-ray diffractograms of fresh and sulphated 3 wt% Pt/A1203 catalysts

-,

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357

The fresh 3 wt% catalyst resembles that of the lower loading 0.05 wt% sample, predominantly exhibiting reflections from the alumina support. Weaker peaks at 30, 38 and 56~ can be attributed to a PtO2 phase, although the presence of other minority platinum oxide phases cannot be discounted. Sulphation attenuates the alumina support phase and induces the appearance of intense peaks arising from mixed aluminium-sulphate hydrates [8] (principally AI2(SO4)3.16H20 and AI2(SO4)3.14H20). Reflections attributable to metallic Pt also emerge at 39.4, 46.4, 67.7, 81.3 and 85.7 ~ following sulphation. These metallic features are clearly resolved in the fresh 9 wt% catalyst, wherein sulphation promotes peak growth and narrowing. Particle size analysis confirms these trends correlate with a rise in mean Pt diameter from--50 A in the fresh sample to -100 A_at~er sulphation. Additional insight into this sintering process was obtained using X-ray absorption spectroscopy (EXAFS). Ex-situ Pt Lm-edge spectra are presented in Figure 5a-b for fresh and sulphated Pt/AI203 catalysts. For the fresh samples, the white-line intensity decreases with increasing Pt loading while the energy range of EXAFS oscillations simultaneously extends, such that the 9 wt% Pt/AI203 catalyst resembles bulk Pt. This reflects a rise in Pt particle size with loading from-15 A to >100 A between the 0.05 and 9 wt% samples. These trends coincide with changes in the near-edge (XANES) region consistent with a transformation from oxidic to metallic Pt. Sulphation lifts this loading dependence, reducing the 0.05 and 3 wt% samples' white-lines and inducing XANES features representative of their higher loading and bulk Pt counterparts. Sulphated 0.05 wt%

9 wt% Pt foil

b !

-40

60 160 Energy above edge(eV)

260

-40

!

60 160 Energy above edge(eV)

|

260

Fig. 5. Normalised Pt Lin-edge XAFS spectra of a) flesh and b) sulphated Pt/Al203 catalysts as a function of Pt loading. The corresponding fitted Pt local coordination spheres, shown in Figure 6a-b, confirm the presence of a PtO2 phase within the fresh 0.05 wt% Pt/A1203 catalyst, which is reduced to Pt metal upon sulphation. These observations may be rationalised in terms of the simultaneous sulphate-induced reduction and sintering of oxidic Pt present within fresh, low loading Pt/AI203 catalysts. Oxidic Pt surfaces exhibit lower oxidation activity than metallic Pt [9,10].

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/

~ , ,

~

Fresh

~

0

2

4

6

Interatomic Distance (A)

8

0

/

~

2

,

Sulphated

\

~

4

3wt%

6

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Interatomic Distance (A)

Fig. 6. Pt Lni-edge pseudo-radial distribution functions of fresh and sulphated Pt/A1203 catalysts as a function of Pt loading Our investigations suggest that SO2 promotes propane oxidation over Pt/ml203 catalysts in three ways. First, sulphation modifies the alumina support, generating crystalline aluminium sulphate and surface sulphoxy groups. Sulphated alumina surfaces greatly enhance the dissociative chemisorption and subsequent oxidation of propane on neighbouring partiallyoxidised Pt sites. Second, surface sulphate enhances direct dissociative propane chemisorption onto metallic Pt clusters in the absence of alumina mediation. Both these factors contribute to the small enhancement in oxidation rate (ATs0 = -50 K) observed for high loading (9 wt%) Pt/AI203 catalysts. Third, we have demonstrated the sulphate-induced reduction and sintering of highly-dispersed platinum oxide particles, predominant for low loading (0.05 and 3 wt%) Pt/AI203 catalysts. The resultant large metallic platinum particles exhibit higher activities towards propane activation. The principal promotional effect of SO2 upon low loading Pt/AI203 catalysts is thus the reduction of PtO2 to Pt. This eliminates the structural differences with their higher loading counterparts, thereby lifting the dependence of propane oxidation activity on Pt concentration observed for unsulphated samples. REFERENCES 1. R.W. McCabe and J.M. Kiesenyi, Chem. Ind., (1995) 605. 2. G. Sadowski, and D. Treibmann, Z. Chem., 19 (1979) 189. 3. H.C. Yao, H.K. Stepien, and H.S. Gandhi, J. Catal., 67 (1981) 23 7. 4. K.M. Adams, J.V. Cavataio, and R.H. Hammerle, Appl. Catal. B, 10 (1996) 157. 5. K. Wilson, A.F. Lee, J. Brake and R.M. Lambert, Surf.Sci., 387 (1997) 257. 6. K. Wilson, A.F. Lee, C. Hardacre and R.M. Lambert, J. Phys. Chem. B, 102 (1998) 1736. 7. K.Wilson, C. Hardacre and R.M. Lambert, J. Phys. Chem., 99 (1995) 13755. 8. H. Moselhy, G. Pokol, F. Paulik, M. Arnold, J. Kristof, K. Tomor, S. Gal and E. Pungor, J. Therm. Anal., 39 (1993) 595. 9. K. Otto, J.M. Andino, and C.L. Parks, J. Catal., 131 (1991) 243. 10. R. Burch and M.J. Hayes, J. Molec. Catal. A: Chem, 100 (1995) 13.

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Studies in Surface Science and Catalysis 130 A. C o r n , F.V. Melo, S. Mendioroz and J.L.G.Fierro (Editors) 0 2000 Elsevier Science B.V. All rights resewed.

Selective Oxidation of Toluene to Benzaldehyde: Investigation of StructureReactivity Relationships by in situ-Methods A. Briickner, U. Bentrup, A. Martin, J. Radnik, L. Wilde and G.-U. Wolf

Institut fir Angewandte Chemie Berlin-Adlershof e.V., Richard-Willstatter-Str. 12, D- 12489 Berlin, Germany (VO)2P207and potassium-doped V205catalysts have been studied in the selective oxidation of toluene to benzaldehyde by in situ-EPR, -FTIR, -XRD, -UVNIS and -XPS. In (V0)2P207, Brensted surface sites formed under reaction conditions favour strong product adsorption and, thus, total oxidation. They can be blocked by adding pyridine, which improves catalytic performance. K-V205 catalysts are markedly reduced during reaction. Both V" and v4'species are likely to be active in the catalytic redox cycle. Crystalline Ko.5V205formed on the catalyst surface under feed probably lowers the catalytic performance due to structural reasons. 1. INTRODUCTION

From the industrial point of view, benzaldehyde is the most important aromatic aldehyde. It can be obtained by the selective gas-phase oxidation of toluene using catalysts based on vanadia. In general, low conversion rates are preferred to avoid deeper oxidation. However, the benzaldehyde selectivities obtained so far do not exceed 40 - 60 % [I]. Thus, improvement of the catalytic performance by rationalized catalyst design is of considerable industrial interest. This approach requires detailed knowledge on structure-reactivity relationships which can best be obtained by investigating the catalysts under working conditions. In this work, catalysts based on vanadium oxide were studied during reaction using in situ-EPR, -FTIR, -XRD, -UVMS and -XPS. The spectroscopic results are compared to the catalytic performance elucidated in catalytic tests. 2. EXPERIMENTAL

Catalysts: Crystalline (V0)2P207 was obtained by calcination of the precursor VOHP04. 0.5 H20 (prepared in aqueous medium [2]) in N2 atmosphere at 873 K (sample VPP = 4.7 m2 g-'). Potassium doped V ~ O S 1, SBET= 6.4 m2 g-l) or 1023 K (sample VPP 2, SBET catalysts were obtained i) by the incipient wetness method using 20 g V2O5and 25 g of a 10 % aqueous solution followed by drying for 8 h at 403 K (VOK 1) and ii) by impregnation of 18.19 g V205with 50 rnl of a 1.7 % aqueous K2S04 solution, followed by evaporation at 343 K and drying at 403 K (VOK 2). These catalysts are characterized by the following properties: W S N = 0.06910.03411 (VOK 1) and 0.0510.02511 (VOK 2); SBET= 4.0 m2 g-l (VOK 1) and 4.5 m2 g-l (VOK 2); mean vanadium valence state determined by potentiometric titration [3] 4.947 (VOK 1) and 4.858 (VOK 2). Methods: In situ-EPR measurements were performed in X-band (ELEXSYS 500- 10112, Bruker) using a home made fixed-bed reactor [4]. The product stream was trapped in ethanol and analyzed by off line-GC (Shimadzu GC 17AAF). Transmission in sztu-FTIR spectra (Bruker

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IFS 66) were recorded from self-supporting disks mounted in a heatable flow cell. In situ-XRD analyses were performed in a XRK reactor chamber (A. Paar). Quasi-in situ-XPS spectra were recorded at 293 K using a MgKa-source (ESCALAB 220 iXL, VG Instruments) after treating the catalysts under working conditions in a reaction cell installed in the lock to the analysis chamber. Peak positions were corrected with respect to the C Is signal at 284.5 eV. In situW M S measurements were performed using a Cary 400 W M S spectrometer (Varian) equipped with an in situ-difise reflectance accessory (praying mantis, Harrick). Catalysts were diluted with a-A1203 (calcined at 1473 K for 4h). The reaction cells used with the different methods were connected to a gadliquid dosing apparatus. Feed composition and contact time were chosen so as to be similar to the conditions of the catalytic tests performed separately using a fixed-bed U-tube quartz reactor. If not stated otherwise, a reactant mixture of molar ratio aidtoluene = 100 (total flow: 34 ml min-') was used. 3. RESULTS AND DISCUSSION 0 2 P 2 0 7 : The crystal structure of (VO)2P207 contains intinite double chains of VOa octahedra which are coupled by effective spin-spin exchange interactions giving rise to a narrow EPR singlet (Fjg. I ) . The stronger the exchange interaction the smaller the line width and, thus, the higher the signal amplitude. Under reaction conditions, a temporal decrease of the signal intensity due to line broadening is observed which arises from a perturbation of the spin-spin exchange between neighbouring VO" ions. A similar behaviour of the EPR signal has been observed, too, for a number of different v4?0 catalysts during the ammoxidation of toluene to benzonitrile [5, 61. The perturbation results from changes in the electron density at surface vo2' centres which are caused on the one hand by alternating reduction and re-oxidation steps according to a Mars-van Krevelen mechanism and on the other hand by the adsorption of the basic aromatic ring system [5]. The intensity loss is most pronounced for high conversion ~ ' are involved in the reaction cycle and, indicating that in this case more surface ~ 0centres thus, contribute to the spin-spin exchange perturbation (Fig. 2).

N,

toluene

N,

50

- 100

30

-90 ,

co I 0

I

I

fig. 1 EPR spectra of ~ p 2pat 658 K in N~ and under feed (time interval between spectra: 10 min)

00

70 20

80 40 60 conversion I %

a

100

. -

-60

Fig. 2 Relative EPR intensity of VPP 2 under feed (0, Inl(N2) = 100 %) and benzaldehyde selectivity ( 0 )versus toluene conversion

On switching again to N2 atmosphere, the EPR signal returns only slowly to its initial state (Fig. 1) indicating that aromatic products are strongly adsorbed on the surface. This is in line with in situ-FTIR results in which, besides benzaldehyde, cyclic anhydride intermediates were detected on the catalyst surface [7]. These intermediates are precursors for total oxidation products and, thus, responsible for the low aldehyde selectivities (Fjg. 2). FTIR data also

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revealed that strong product adsorption is due to the interaction of benzaldehyde with Bransted surface sites generated under reaction conditions. Therefore, pyridine which is not oxidizable under these reaction conditions has been continuously dosed to the feed (molar ratio: air/toluene/water vapour containing 4 wt.-% pyridine = 100/1/1) to block acidic sites and prevent strong product adsorption. In situ-EPR spectra reveal no change of the signal when only water vapour is added to the feed while its intensity starts to increase again as soon as pyridine is present (Fig. 3). In the latter case a roughly threefold increase of the benzaldehyde selectivity was observed at the same conversion rate (Table I). In situ-FTIR spectra indicated the preferred adsorption of pyridine on Brernsted surface sites, thus, displacing benzaldehyde from the latter and preventing deeper oxidation [8]. Table 1Maximum area specific rate of benzaldehyde formation, RBA,and reaction temperature, TR,derived from catalytic tests.

Catalyst

RBA1 pm01 h-' mV2 TR/ K i

VPP 1

VPP 1 + pyridine

VOK 1

VOK 2

29 648

76 665

120 660

110 66 1

toluene j toluene air i air

:

N,

N,

time on stream I min

Fig. 3 Relative double integrated EPR intensity of VPP 1 at 658 K as a function of feed composition and reaction time

toluene

N2

time on stream I min

Fig. 4 Relative double integrated EPR intensity of fresh (0) and conditioned sample VOK 1 ).( at 623 K as a function of feed composition and reaction time

XPS data of sample VPP 1 before and after treatment in the reaction chamber are listed in Table 2. Since the binding energies of the V 2p312peak in vanadium phosphates with equal vanadium oxidation state differ slightly [9, 10, 111, reliable identification of the vanadium oxidation state is not straightforward. Therefore, the difference between the binding energies of the 0 1s peak assumed to be constant and the V 2 p 3 ~peak has been used to account for small deviations of the vanadium valence state [9, 121. Values of 15.8 eV, 14.3-14.4 eV and 12.8-12.9 eV have been found for pure v3', v4+and containing phases, respectively [9, 131. Thus, the value of 14.2 obtained for the fresh sample VPP 1 points to a slight oxidation of the catalyst surface. After reaction, the V 2p3n peak is slightly shifted to lower binding energy suggesting that partial reduction of the catalyst surface occurred. The 0 1s peak in fresh VPP 1 appears at 532.5 eV. In contrast to some literature data in which two lines are fitted to the experimental peak (531.2 eV for lattice oxygen and 533.2 eV for OH-species [9, 14]), the peak in fresh VPP 1 can well be fitted with only one component assigned to 0'- ions of the lattice. This is in good agreement with the FTIR measurements in

v5'

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which no Brransted OH-sites could be detected for fresh VPP 1. After reaction, two 0 1s peaks appear at 531.7 and 528.5 eV. The former one arises from lattice oxygen and is virtually not influenced by the catalytic reaction. The new peak at 528.5 eV is assigned to OH groups generated by hydrolysis of V-0-P andlor P-0-P bonds. This is supported by the FTIR results indicating the creation of Brransted sites during reaction. In comparison to literature data [9, 141, this OH-peak is shifted to lower binding energies. The shift might be caused by the strong adsorption of reaction products (benzaldehyde and cyclic anhydrides) on the catalyst surface still persisting after evacuation at room temperature. Recently we have shown that the surface OH-groups in (V0)2P20, generated during reaction interact with the C=O group of benzaldehyde via hydrogen bonding [8]. Thus, the effective negative charge of the OH-oxygen atom is assumed to be enhanced giving rise to the shift of the binding energy. Table 2 XPS data of samples VPP 1 and VOK 1 before and after reaction

sample

signal

binding energy /eV assignment

VPP 1 (before)

0 1s V2p3 P 2p 0 1s

532.5 518.3 135.1 528.5 53 1.7 517.5 130.9 134.2

VPP 1 (after)

V2p3 P 2p VOK 1 (before)

VOK 1 (after)

a

surface percentage "

02v4', v5+ (trace) ~ 2 0 7 "

OH02'

v4+

P-OH P~O?

0 1s V 2p3 s 2~ K 2p3 0 1s V 2p3

Difference to 100 %due to carbon

The position of the P 2p peak in fresh VPP 1 (135.1 eV) is in good agreement with values found in a number of VPO compounds containing mono-, di-, poly- and hydrogenphosphate anions, respectively (133.2 - 134.5 eV [9,10, 141). After reaction, an additional P 2p peak occurs which is well below these values. Accordingly, it is assigned to P-OH groups interacting with adsorbed reaction products. K-V20s: In the EPR spectrum of the as-synthesized sample VOK 1 a small signal with partially resolved hyperfine structure is evident which indicates the presence of some isolated and weakly interacting tetravalent vanadium ions. This is in line with the mean vanadium valence state of 4.947. The intensity of this signal at 293 K increases by a factor of = 2.5 after heating in nitrogen for 1 h at 623 K and, additionally, by a factor of = 3 after treatment under feed for 1 h at 623 K indicating partial reduction of v5' to V4'. This is confirmed, too, by XPS measurements before and after reaction (Table 2).

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300

400

500

660

Nanometers

760

800

fig- 5 DRS Spectra of VOK 1 at 293 K: a) fresh sample; b) after heating in N2 to 653 K;C) after lh reaction at 653 K.

&,O,

2 Theta

Fig. 6 In situ-X-ray diffmctograms of VOK 2 in N2 (,) and underfeed (b),

Under feed, the conditioning process seems to be finished after = 30 min when the EPR intensity keeps constant (Fig. 4, open circles). This equilibrated catalyst has been used for another experiment (Fig. 4, filled circles). In this case, no temporal decrease of the EPR double integral is observed under feed as for (V0)2P207. Instead, slightly more intense EPR signals appear under reaction conditions which decrease again on switching to nitrogen. This can be understood assuming that both V4' and v'' surface species are involved in the catalytic reaction. In the case of EPR silent v", changes of the electron density should temporarily enhance the EPR intensity since EPR active v4'is created by the reduction step of the Mars-van Krevelen redox cycle. On the other hand, temporal intensity loss is expected in case of v4'due to spinspin exchange distortion observed similarly for (V0)2P207. It is likely that both effects contribute to the overall EPR intensity. However, the fact that V" is in excess (Table 2) could explain the observed net intensity increase (Fig. 4). The UVMS-DRS spectra of the fresh samples VOK 1 and VOK 2 are rather similar being dominated by the well known charge-transfer bands of crystalline V205 between 300 and 500 nm (Fig. 5 a) [IS]. However, in contrast to pure VzOs, these bands decrease markedly in the potassium doped samples VOK 1 and VOK 2 upon thermal treatment in N2 atmosphere. Simultaneously, a very broad appears above 600 nm which arises from d-d transitions of v4' (Fig. 5 b). In agreement with the EPR results, this indicates easy reducibility of the VOK samples. Switching from nitrogen to feed at 653 K creates a rather intense band below 300 nm which persists also after evacuation at room temperature (Fig. 5 c). Comparison with reference spectra of a number of different tetravalent vanadium phosphates all showing a similar absorption suggests, that this band arises from charge-transfer transitions of v4'.It is, however, not visible in the spectra of samples VOK 1 and VOK 2 after catalysis and storage in air for several weeks suggesting rapid reoxidation. In contrast to (VO)2Pz07,no adsorbed product molecules could be detected by FTIR. The reason may be a lower surface acidity caused by enrichment of potassium in the surface as evidenced by XPS (Table 2). The most obvious difference between (VO)zP207 and the K-V205 samples arises from the catalytic performance which is markedly higher for VOK 1 and VOK 2 (Table I). In situ-XRD measurements revealed the presence of crystalline KV3Os in fresh VOK 2 which transforms immediately into &,5Vz05 in contact with the reactant gas mixture suggesting that the crystalline vanadates are located on the surface of the catalyst (Fig. 6). For VOK 1 no reflections besides those of V205 were visible during the in situ-XRD experiment. However, the surface atomic ratios of K N = 0.34 and v5'fV4' = 3.37 detected by

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XPS in VOK 1 after reaction (Table 2) are rather similar to the values resulting from the

composition of Ko.5V20s. Presumably, the surface of VOK 1 is covered by a similar potassium vanadate phase which is, however, microcrystalline or amorphous and, thus, not detected by XRD.The beneficial catalytic effect of K-V205 in comparison to (V0)2P207 could be due to structure of the &.5V205 surface phase [16] in which the K' ions are located between the [V205In layers. Thus, they could partially block acidic sites and prevent too strong product adsorption. 4. CONCLUSIONS

In (V0)2P207 catalysts, in situ-formation of Brnrnsted sites under reaction conditions hinders product desorption and causes oxidative damage of the aromatic ring leading via cyclic anhydride intermediates to COX.This undesired process can partially be suppressed by blocking the acidic sites by pyridine which improves the benzaldehyde selectivity at constant conversion rates. K-V205 catalysts show even higher catalytic performance for which at least two reasons can be discussed: i) Marked reduction to ? occurs during conditioning which lowers the oxidation potential and improves the selectivity in comparison to a pure v5' surface. ii) Modification with potassium leads to the formation of a potassium vanadate surface phase. The enrichment of potassium in the surface in turn reduces the surface acidity, prevents strong product adsorption and, thus, diminishes total oxidation. ACKNOWLEDGEMENT

This work has been supported by the Federal Ministry of Education and Research (Germany), project no. 03C0280. REFERENCES [l] F. Briihne, E. Wright: in Ullmam, 6th Edition 1998 Electronic Release (benzaldehyde entry). [2] K. Schlesinger, M. Meisel, G. L a h g , B. Kubias, R. Weinberger and H. Seeboth, German Patent No. DD WP 256659 Al(1984). [3] M. Niwa and Y. Murakami. J. Catal., 76 (1982) 9. [4] a) A. Briickner, B. Kubias, B. Liicke and R. StOkr, Colloids and Surfaces, 115 (19%) 179; b) H. G. Karge, J. P. Lange, A. Gutze and M. Laniecla, J. Catal., 114 (1988) 144. [5] A. Briickner, A. Martin, B. Liicke and F. K. H~MOUT, Stud. Surf. Sci. Catal., 110 (1997) 919. [6] A. Briickner, A. Martin, B. Kubias and B. Liicke, J. Chem. Soc., Faraday Trans.. 94 (1998) 2221. [7j A. Martin, U. Bentrup, A. Briickner and B. Liicke, Catal. Lett., 59 (1999) 61. [8] A. Martin, U. Bentrup, B. Liicke and A. Briickner, Chem. Commun., 1999, 1169. [9] F. Richter, PhD Thesis, University Leipzig (Germany), 1998. [lo] L. M. Cornaglia, E. A. Lombardo in H. Hattori, M. Misono, Y. Ono (eds.), Acid Base Catalysis 11, Elsevier Science Publ., Amsterdam, 1994. p.429. [Ill M. Abon, K. E. Bere, A. Tuel and P. Delichere, J. Catal., 156 (1995) 28. [12] F. Garbassi, J. C. J. Bart, F. Montino and G. Petrini, Appl. Catal., 16 (1985) 612. [13] G. W. Coulston, E. A. Thompson and N. Herron, J. Catal., 163 (1996) 122. [14] S. Albonetti, F. Cavani, F. Tdir6, P. Venturoli, G. Calestan, M. Mpez Granados and J. L. G. Fierro, J. Catal., 160 (19%) 52. [15] G. Centi, S. Perathoner andF. Trifir6, J. Phys. Chem., 96 (1992) 2617. [16] J.-M. Savariault and J. Galy, J. Solid State Chem., 101 (1992) 119.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

化

365

Observation of unstable reaction intermediate infrared laser pulses

by picosecond

tunable

K. Domen*, K. Kusafuka, A. Bandara, M. Hara, J. N. Kondo, J. Kubota, K. Onda, A. Wada, and C. Hirose Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan. * CREST, JST (Japan Science and Technology) The picosecond tunable infrared laser pulses enabled us to carry out the vibrational spectroscopic measurements of the species adsorbed on surfaces with picosecond timeresolution. The irradiation of near-infrared pump pulses causes thermal excitation of the surface, and the change of adsorbates was probed by sum-frequency generation (SFG) spectroscopy. We applied this method to the direct observation of unstable species of formate on NiO(111) surface. The stable bidentate formate on NiO(111) was transformed to monodentate by the irradiation of pump pulses, and the dynamic behavior of the newly observed monodentate formate was followed as a precursor for the decomposition of formate. The application of the present method has extended to the study of methoxy species on Ni(111).

1. Introduction Time-resolved spectroscopy using ultrashort laser pulses is one of the powerful methods to study the reaction dynamics of molecules on surfaces [1,2]. Irradiation of near-infrared pump pulses causes thermal excitation at surfaces and the temperature jump of the surface at 100---1000 K in a few tens picosecond is possible to populate the adsorbed molecules imo an unstable state. Although the laser heating of surface has been utilized for the analysis of desorbing molecules in the laser induced thermal desorption (LITD) method [1-5], little attemion has been given to direct observation of the surface during the reaction of molecules due to the temperature jump. Sum-frequency generation (SFG) is a nonlinear vibrational spectroscopy using ultrashort laser pulses [ 1,6,7], which enable the time resolved observation of the surface in the heating period. Formation and decomposition of formate on various catalyst surfaces are key steps in many catalytic reactions such as water-gas shift reaction, methanol symhesis, etc. [8]. We have studied decomposition of formic acid on a NiO(11 l) surface which is prepared by the epitaxial oxidation of a N i ( l l l ) crystal [9-11]. The formic acid dissociates to bidemate formate on NiO(111) at-160 K and the formate species decomposes to form H2 and CO2 at 340-390 K and H20 and CO at 410 K [10]. Two types of formates, bidentate and monodemate, have been idemified by infrared reflection absorption spectroscopy (IIL&S) under the catalytic decomposition of formic acid in a flow of formic acid gas at > 102 Pa [11 ]. However, the dynamic behavior of bidemate and monodentate formates has still not been understood.

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In this study, we demonstrate by ultrashort laser method the stable bidentate formate on N i O ( l l l ) is transformed to monodentate before the decomposition [12-14]. The monodentate formate was regarded as an intermediate species for the decomposition of formate. Recently, our application of this method has been carried out for the study on the methoxy species on Ni(111), and the tentative results is shown in the last section of this article.

2. Method

The optical setup of generation of frequency-tunable infrared pulses, generation of SFG signals, and irradiation of pump pulses is shown in Fig. 1 [12-14]. A mode-locked Nd:YAG laser (1064 nm, 35 ps pulse width, and 10 Hz repetition rate) was used as the source of light pulses. The frequency-tunable near-infrared (NIR) pulses were generated by an optical parametric generator/amplifier (OPG/OPA) using 13-BaBO4 (BBO) crystals pumped by the second harmonic generation (532 nm) output of a KDP crystal. Tunable picosecond infrared (IR) pulses (60 ~tJ/pulse and 3 cm t FWHM at 2200 cm -~) 1064 1064 for SFG were obtained from the Nd:YAGlaser NIR and 1064 nm pulses by a5 s 532 1064 nm nm difference frequency generation NIR,,~ n ~ , , 532 (DFG) using an AgGaS2 (AGS) j,'."....~~ .. crystal. A portion of the grating IIq'" "'%: 1064 nm pulses was used to p: generate the visible (532 nm, 200 [ : ~tJ/pulse) light to be used for SFG. The remaining 1064 nm ODL pulse (-~10 mJ/pulse) was used as a pump pulse after passing PMr S ~ f ' " - ' ~ , ~ . ~ l ~ ~ through a variable optical delay "L.__~ -t'"--ll__ "~ sample ] UHV line (ODL). The IR and ~ chamber visible beams for SFG and the 1064 nm beam for pumping were Fig. 1 Optical setup for time-resolved SFG crossed at the surface at an angle measurement with 1064 nm pump beam. of incidence of about 75 ~. The diameters of the IR, visible and 1064 nm beams were ca. 2, 3 and 5 mm, respectively, at the sample surface. The generated SFG photons were detected with a photomultiplier tube after passing through optical filters and a monochromator. For the experiments of methoxy species on Ni(111), we used another OPG/OPA system which generates IR pulses between 2500-4000 cm ~ [15]. The experiments were performed in an ultrahigh vacuum (UHV) chamber equipped with an Ar-ion bombardment gun, low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) devices. A Ni(111) piece could be cooled to 130 K by liquid nitrogen and heated to 1300 K by resistive heating. Clean Ni(111) surfaces were obtained by repeated cycles of Ar ion bombardment followed by annealing at 1100 K. In the preparation of a NiO(111) surface [9], the cleaned Ni(111) surface was oxidized by the repeated cycles of exposure to oxygen at 1000 L (1 L = l x l 0 6 Torr s, 1 Torr = 133 Pa) and 570 K and annealing at 650 K in vacuum.

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The schematic drawing for the sequence of the time-resolved experiment is shown in Fig. 2. The surface is heated t o - 3 0 0 K above the initial temperature by the irradiation of pump pulses. The magnitude of the temperature jump has 9 0.2 0.3 been experimentally estimated from the time / a _ pressure" 10-5 ~ 10 -4 Pa change of vibrational temperature at } OD/NiO(11 l) surface by the irradiation [ 13]. The surface temperature is cooled ooo down within several hundreds picosecond ~r I==AT following the classical heat diffusion -300 K equation [4]. The surface molecules are pump ulse transformed to another species or SFG ~ ~-~.~ii.l-.- pu,~ decomposed to desorb from the surface by the temperature jump. When a small -100 0 100 200 part of surface molecules leaves from the delay time / pa surface by the irradiation, the initial Fig. 2 Sketch of the time flow of timecoverage is reproduced before the next resolved experiment. The initial surface is irradiation of pump pulse by the flowing of recovered before next irradiation of pump the formic acid gas at 10.5 - 10.4 Pa. In pulses, so that the signals for a number of the heating and cooling period, the gas pulses are able to be averaged over. phase molecule does not come into collision with the surface at the gas pressure of 10.5 Pa, and change of the surface in this period is free from the interaction with gaseous molecules. The advantage of this method is that the surface covered with the molecules at the extremely high temperature (AT - 300 K) is able to be observed. If the surface is kept at such high temperature for a long period (>ps), all of adsorbates should be completely decomposed and desorbed from the surface. The amount of decomposition is kinetically limited by the short period in the high temperature, and the behavior of adsorbates at the high temperature is able to be monitored by SFG. v

3. Results and discussions

(1) Formate/NiO(111) system The SFG spectra obtained at various delay times from the irradiation are shown in Fig. 3. Two polarization combinations were examined. For C-D stretching mode of formate, the signal obtained by the s-polarized visible and p-polarized infrared pulses is enhanced when the C-D axis is tilted from the surface normal. In the spectra at -100 ps, the peak assignable to bidentate formate was observed at 2160 c m "1 and this peak was assigned to the C-D stretching mode of formate in a bidentate configuration. When the surface was irradiated by the pump pulses, the 2160 cm ! peak weakened and new peak appeared at 2190 cm 1. At -20 ps delay from the peak top of the pump pulse, the surface temperature takes maximum and the temperature has been estimated as 300 K higher than the initial temperature. The 2190 cm ~ peak disappeared after 100 ps. The newly observed peak was assigned to monodentate formate [11]. The intensity of the 2190 cm ~ peak for the polarization combination of s-polarized visible and p-polarized infrared was consistent with the structure of monodentate in which the C-D axis tilts away from the surface normal. We have

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poralization poralization actually found that vis. 9 s vis. 9 p monodentate formate is ir 9 p ir : p present on NiO(111) under 2160 the catalytic decomposition ;~, 2190 of formic acid at 102 Pa. 2160. delay S ' ,;~j time The present study using .~ 2190 er ultrashort pulse laser clearly - 1 0 0 ps shows that monodentate :2, ] ,'.1,', formate was originated from ~20 ps bidentate formate directly but not from gaseous formic ; I acid. The monodentate 100 ps formate was equilibrated i 9 i ; , 9 I 2000 2100 2200 2300 2000 2100 2200 2300 with bidentate formate on wavenumber / c m "1 w a v e n u m b e r / c m -1 the surface at the high temperature induced by the Fig. 3 SFG spectra of formate on NiO(111) at 400 K with irradiation. pump pulses at various delay times from the pump pulse. The time profiles of the The surface was in the flow of DCOOD at 10.5 Pa. SFG signals at 2160 and 2190 cm -~ are shown in Fig. 4. The changes of bidentate and monodentate signals were compensated for each other indicating that monodentate formate comes from bidentate formate and that the two species are in equilibrium. The change of bidentate formate was not completely recovered at 300 ps delay in spite of the fully disappearance of monodentate signal. This indicates that a small part of bidentate was decomposed by the 5 irradiation. Intensity of SFG signal is known to ~i 1.0 be proportional to the square of the number of ~ . ~ 0 molecules. Thus, the 7% decrease of the SFG ~t-g*** [ 2160 cm-1 signal of bidentate suggests that only 3---4% 9 C bidentate formate was transformed to monodentate -~ 0.95~,~, bidentate C formate. The SFG susceptibility of monodentate ._~ . formate was thus quite higher than that of bidentate 5 formate because such small amount of 1t l d ~ ~ 2190cm-1 monodentate gave clear SFG peak in the spectra. monodentate ~ .D We assume the elemental reaction step as i

=Jj I

"

I

"

I

"

I

I

K

DCOQ

_

OCOOm

32 + CO _,D O + CO,

C

.

~C m

"

c

O"

I

I

. . . . . . . . . . . . . . . .

.m

where K is the equilibrium constant between bidentate and monodentate species which is expressed as K = 0,n / Oh = exp(- AE / RT) and k is the rate constant defined as dOm/dt =-kO m and

k=Aexp(-Ea/RT).

0b and

0m

are

the

-100

I

0

I

100

I

200 300

delay time / ps Fig. 4 Time profiles of SFG signal at 2160 and 2190 cm t for formate on NiO(111) at 400 K.

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coverages of bidentate and monodentate formatcs, and AE, Ea, R, A, and T arc the potential energy difference between bidentate and monodentate, activation energy for decomposition of monodentate formate, gas constant, pre-exponential factor, and surface temperature, respectively. The values of AE = 19 kJ mol "~, Ea - 30 kJ mol ~, and A = 5x10 ~ s"~ were estimated from fitting with all of the experimental results for various initial temperatures [ 1214]. We reasonably reconfirmed that the monodentate species was a precursor for the decomposition, and the obtained kinetic values approximately agree with those derived from kinetic analysis under the catalytic conditions [ 11 ]. (2) Methoxy/Ni(111 ) system Mcthoxy on catalyst surfaces is also one of the important species in various catalytic reactions and numerous studies have been reported on this species. The SFG spectra of methoxy species on Ni(100) was reported [16], however no dynamical aspect has been investigated. Our application of the time-resolved SFG experiments has extended to the study of methoxy species on Ni(111). The primary results on the methoxy/Ni(111) are shown produced on Ni(111) by introducing methanol at 200 K. before irradiation of pump pulses is shown in the top trace in Fig. 5a. Two peaks were observed at 2825 and 2923 cm ~, which were assigned to the symmetric C-H stretching mode and combination mode of methoxy group. The assignment of 2923 cm -~ peak has controversy at present; one is due to asymmetric C-H mode [17] and the other is due to combination mode of deformation mode [18]. On the basis of recent studies on photoelectron diffraction [19], it seems more reasonable that the methoxy species stands perpendicular on the surface and that the peak at 2923 cm -~ corresponds to the combination band. When the surface was irradiated by the 1064 nm pump pulses, the two peaks weakened as shown in the bottom trace. The transient change of peak intensity at 2923 cm -~ is shown in Fig. 5b, which corresponded to the change of surface temperature. We considered the reason of the weakening of SFG peaks caused by temperature jump. Our tentative interpretation is that the methoxy species changed its structure with the transient heating. The detailed experiments suggest that the change is not explained by the thermal broadening of SFG peaks. The temperature jump in the methoxy/Ni(11 l) system was estimated as 150-200 K. The surface was thus heated to 350-400 K by the irradiation which was-~100 K higher than decomposition temperature of methoxy species. Thus, the methoxy species on Ni(111) was found to change the structure above decomposition

in Fig. 5. Methoxy species was SFG spectrum of methoxy species

~

(a)

2923

~

'

A ,

delay time

~"~-150

~,

ps

-35 ps

27:00 " 28t)0" 2<JO0 " 3000 " 31()0 wavenumber / cm -1 5 (b) ,/ ~ 1.0-

c o.9-

_~ ~0.ae"~ 0.7 -200 "-1()0"

() " 1 ~ ) " 2()0'300

delay time / ps

Fig. 5 Time-resolved SFG spectra of methoxy species on Ni(111) at 130 K (a), and transient signal change of methoxy at 2923 cm l by the irradiation of pump pulses at 110 K (b).

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temperature. The methoxy species may tilt from the surface normal or migrate to another adsorption site at such high temperatures. The study of methoxy/Ni(111) system will be published with more detailed results. In conclusion, we demonstrate the direct observation of reaction intermediate in the formate decomposition. The laser induced temperature jump technique was found to be one of the powerful tools for identification of precursor molecules prior to the activation barrier in the reaction. Monodentate formate was an intermediate in the decomposition of bidentate formate on NiO(111). This technique has been also applied to the study of methoxy species on Ni(111). References

1. Laser Spectroscopy and Photochemistry on Metal Surfaces Part I, eds. H. L. Dai and W. Ho, (World Sci. Pub., Singapore, 1995). 2. Laser Spectroscopy and Photochemistry on Metal Surfaces Part II, eds. H. L. Dai and W. Ho, (World Sci. Pub., Singapore, 1995). 3. R. B. Hall and A. M. DeSantro, Surf. Sci. 137 (1984) 421. 4. D. S. King and R. R. Cavanagh, Molecule Surface Interactions, Advances in Chemical Physics, Ii"ol. 76, ed. K. P. Lawley, (John Wiley & Sons., New York, 1989) p. 45. 5. Z. Rosenzweig and M. Asscher, Surface Science of Catalysis: In Situ Probes and Reaction Kinetics, eds. D. J. Dwyer and F. M. Hoffmann, (American Chem. Soc., Washington, 1992), Chap. 22. 6. Y. R. Shen, The Principles of Nonlinear Optics (Wiley, New York, 1984). 7. T. F. Heinz, Nonlinear Surface Electromagnetic Phenomena, eds. H. -E. Ponath and G. I. Stegeman, (Elsevier, Amsterdam, 1992), Chap. 5. 8. G. A. Somorjai, Introduction to Surface Chemistry and Catalysis, (John Wiley & Sons., New York, 1994). 9. H. -J. Freund, Angew. Chem. Int. Ed. Engel. 36 (1997) 452. 10. A. Bandara, J. Kubota, A. Wada, K. Domen and C. Hirose, J. Phys. Chem. 100 (1996) 14962. 11. A. Bandara, J. Kubota, A. Wada, K. Domen and C. Hirose, J. Phys. Chem. B 101 (1997) 361. 12. A. Bandara, J. Kubota, A. Wada, K. Domen and C. Hirose, J. Phys. Chem. B 102 (1998) 5951 13. C. Hirose, A. Bandara, S. S. Katano, J. Kubota, A. Wada and K. Domen, Appl. Phys. B, 68 (1999) 559. 14. K. Domen, A. Bandara, J. Kubota, K. Onda, A. Wada, S. S. Kano and C. Hirose, Surf. Sci. 427-428 (1999) 349. 15. T. Yuzawa, T. Shioda, J. Kubota, K. Onda, A. Wada, K. Domen and C. Hirose, Surf. Sci. 416 (1998) L1090. 16. J. Miragliotta, R. S. Polizzotti, P. Rabinowitz, S. D. Cameron and R. B. Hall, Chem. Phys. 143 (1990) 123. 17. R. Zenobi, J. Xu, J. T. Yates Jr., B. N. J. Persson and A. I. Volokitin, Chem. Phys. Letters 208(1993)414. 18. J. P. Camplin and E. M. McCash, Surf. Sci. 360 (1996) 229. 19. O. Schaff, G. Hess, V. Fritzsche, V. Fernandez, K.-M. Schindler, A. Theobald, Ph. Hoffmann, A. M. Bradshaw, R. Davis and D. P. Woodruff, Surf. Sci. 331-333 (1995) 201.

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化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Changes in Morphology of 7-Al203-Supported Pt Clusters under Reaction Conditions: Evidence from In-Situ EXAFS Spectroscopy Oleg Alexeev and Bruce C. Gates Department of Chemical Engineering & Materials Science, University of California, Davis, CA 95616 USA Pt clusters on 7-A1203, MgO, and SiO2, with average diameters of 11, 20, and 45 A, were characterized by EXAFS spectroscopy in the presence of H2, ethene, propene, 02, and ethane as well as ethene + H2 and propene + H2 (undergoing catalytic reaction). Adsorption of alkenes (but not alkanes) or oxygen led to flattening of the smallest clusters (on 7-A1203) but to essentially no changes in the larger clusters (on MgO or SiO2); the changes were reversible. The morphology of the smallest Pt clusters depends on the gas composition in alkene hydrogenation catalysis. 1. INTRODUCTION UHV experiments have demonstrated the influence of adsorbates on single-crystal metal surface structures, leading us to expect that morphologies of supported metal clusters could change as a result of adsorption. Our goals were to test this expectation by measuring EXAFS spectra of supported Pt clusters of various average sizes in various gas atmospheres, including those undergoing catalysis of alkene hydrogenation.

2. EXPERIMENTAL METHODS The powder supports 7-A1203 (Degussa), MgO (EM Science), and SiO2 (Degussa), with BET surface areas of 100, 47, and 250 m2/g, respectively, were calcined and evacuated at 400~ prior to use. n-Pentane solvent was purified by refluxing over Na/benzophenone ketyl and deoxygenated. Pt/y-A1203, Pt/MgO, and Pt/SiO2 were prepared by slurrying [PtC12(PhCN)2] (Strem) with the corresponding support in n-pentane, with amounts giving samples containing 1 wt% Pt after removal of the pentane by evacuation. Schlenk lines and a N2-filled drybox were used to exclude air. Hydrogen chemisorption measurements were made, as before [ 1 ] . In-situ EXAFS experiments were performed at X-ray beamline X-11A of the National Synchrotron Light Source, Brookhaven National Laboratory. Each sample was reduced with H2 at 400~ for 2 h to remove organic ligands and form supported Pt clusters. Each powder sample was then placed into a sample holder and secured by Kapton tape. The sample mass was chosen to give an absorbance of about 2.5 at the Pt LIII absorption edge (11563.7 eV). The sample holder was placed in an in-situ cell connected to a gas distribution system that allowed flow of gases through the sample. H2, N2, He, ethene, ethane, and propene (Matheson, UHP grade) were purified by passage through traps containing reduced Cu/A1203 and activated zeolite to remove traces of 02

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and H20, respectively. elsewhere [ 1].

Data were analyzed with XDAP software [2]. Details are as stated

3. RESULTS 3.1. Dispersion of Pt The values of average Pt cluster size and dispersion (Table 1) were determined from hydrogen chemisorption data and the model of Kip et al. [3] relating the average cluster size to the Pt-Pt first-shell EXAFS coordination number (Npt-Pt). When Pt/~'-Al203 was exposed to H2 at 400~ Pt clusters with an average diameter of about 11 A (incorporating about 25 atoms each) formed. Similar treatment of Pt/MgO and Pt/SiO2 led to larger clusters, with average diameters of about 20 and 45 A, respectively (Table 1). Table 1 Dispersions of supported Pt catalysts after treatment in H2 at 400~ Sample Dispersion, Pts/Ptt Average Pt cluster diameter (A) from chemisorption from EXAFS Pt/~,-AI203 1.00 11 Pt/MgO 0.54 21 18 Pt/SiO2 0.25 45 -

3.2. EXAFS data characterizing interaction of H2 with supported Pt To determine the influence of H2 on the Pt morphology, samples were investigated by EXAFS spectroscopy under vacuum and in flowing H2 (Table 2). The A1203-supported clusters under vacuum are characterized by Npt-Pt of 6.4 and a Pt-Pt distance (Rpt-Pt) of 2.70 A. After exposure to flowing H2 at 25~ Npt-Pt remained unchanged, but Rpt-Pt increased to 2.76 A. After subsequent evacuation at 25~ the EXAFS data matched those observed initially for Pt/7A1203 under vacuum, showing the reversibility of the changes induced by H2. Table 2 EXAFS results at the Pt LIII edge characterizing the interaction of Pt clusters with H2 a Sample Conditions First-shell Pt-Pt contributions during scan N R (A) 103 . A~2 (A2) AE0 (eV) Pt/7-AI203 vacuum 6.4 2.70 6.77 3.5 Pt/7-A1203 H2 flow 6.5 2.76 4.87 0.1 Pt/MgO vacuum 8.3 2.75 2.00 0.0 Pt/MgO H2 flow 8.2 2.75 1.50 0.1 Pt/SiO2 vacuum 11.2 2.76 0.80 -0.9 Pt/SiO2 H2 flow 11.0 2.76 0.50 -0.7 aNotation: N, coordination number; R, distance between absorber and backscatterer atoms; A• 2, Debye-Waller factor; AE0, inner potential correction. Estimated accuracies: N, + 20%; R, + 1%; Ao2, + 30%; AE0, + 10%. Samples were scanned at 25~

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In contrast, the morphologies of the larger clusters in Pt/MgO and Pt/SiO2 were not influenced significantly by H2.

3.3. Influence of 0 2 on structure of Al203-supported Pt clusters When completely reduced Pt/qt-Al203 was exposed to doses of 02 in N2 at 25~ partial and then complete oxidation of Pt occurred (Table 3). The values of Npt-pt (4.2) and Rpt-Pt (2.68 A) represent partially oxidized Pt, indicating a decrease of the Pt cluster size as a result of 02 addition. The fully oxidized sample gave no indication of Pt-Pt contributions, indicating complete conversion of the Pt to platinum oxide. 3.4. Interaction of completely oxidized Pth/-Al203 with H2 The interaction of these supported platinum oxide clusters with H2 was tracked in a series of EXAFS experiments (Table 3). When the first portion of H2 was dosed into the N2 flowing through the completely oxidized sample, Npt-Pt increased to 2.7 and Rpt-Pt became 2.75 A, indicating conversion of parts of platinum oxide clusters to Pt clusters. After an additional dose of H2, the respective values were 3.0 and 2.77 A. Further dosing of H2 led to a gradual increase of Npt-Pt to 6.5 with the value of Rpt-Pt remaining essentially unchanged. These data correspond to complete conversion of platinum oxide clusters into Pt clusters in their original statewthe process was reversible. 3.5. Interaction of supported Pt clusters with alkenes and alkanes When propene or ethene flowed through the completely reduced Pt/)'-A1203, the value of RetPt (2.71 A) was the same as that characterizing the sample under vacuum (Table 4). The values of Npt-Pt representing the ~/-Al203-supported clusters with adsorbed ethene or propene were 3.0 or 4.1, respectively, indicating that the alkenes caused changes in the Pt cluster morphology. The decreases in Npt-Pt were accompanied by increases in the Debye-Waller factors. The adsorption of ethane on this sample led to much smaller changes in Npt-Pt, which

Table 3 EXAFS results demonstrating the influence of O2/H2 treatments on structure of ~/-A1203supported Pt clusters a Sample conditions during First-shell Pt-Pt contributions scan N R (A) 103 . Ao2 (A2) ALE0 (eV) 02 dosed in N2 flow 4.2 2.68 9.90 -2.2 02 dosed in N2 flow 0.0 0.00 0.00 0.0 H2 dosed in N2 flow 2.7 2.75 1.80 1.0 H2 dosed in N2 flow 3.0 2.77 1.95 -1.5 H2 dosed in N2 flow 3.7 2.77 2.00 -2.4 H2 dosed in N2 flow 4.4 2.77 2.29 -2.1 H2 dosed in N2 flow 4.8 2.77 2.45 -2.0 H2 dosed in N2 flow 5.9 2.77 3.00 -1.8 H2 dosed in N2 flow 6.5 2.76 4.87 -0.1 aNotation as in Table 2. See text for explanation of sequence of experiments.

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were directionally the same as those caused by the alkenes. These changes were reversed by exposure of the sample to H2. When Pt/MgO was exposed to flowing ethene, Rpt-Pt remained the same, and Npt-Pt became only slightly less (7.5) than that observed after treatment with H2 (8.3) (Table 4). Exposure of Pt/MgO to ethane led to no changes in the EXAFS parameters. The structure of Pt/SiO2 underwent no observed change as a result of interaction with ethene or ethane (Table 4).

3.6. EXAFS of supported Pt clusters during propene hydrogenation catalysis The samples were characterized by EXAFS spectroscopy during ethene hydrogenation and propene hydrogenation catalysis at 25~ with the EXAFS cell serving as a flow reactor. The data (Table 4) show that structural parameters characterizing the relatively large Pt clusters on MgO (or SiO2) remained essentially unchanged under catalytic reaction conditions. In contrast, the smaller Pt clusters on ~/-A1203 underwent structural changes depending on the composition of the reactant mixture. When H2 in the feed was not enough for complete hydrogenation of ethene (or of propene) and alkene was the predominant gas-phase component, Npt-Pt and Rpt-Pt were the same as when ethene (or propene) alone flowed through the sample. In contrast, when the H2 in the feed was enough for complete conversion of ethene to ethane, Npt-Pt and Rpt-Pt matched the values for the sample in ethane alone. Thus, the data demonstrate dynamic changes in the morphology of small A1203-supported Pt Table 4 EXAFS results at the Pt LIII edge characterizing the interaction of Pt clusters with alkenes and alkanes a Sample Conditions Detected First-shell Pt-Pt contributions during scan product N R (A) 103 . Act2 (A2) AE0 (eV) H2 flow 6.5 2.76 4.87 -0.1 C3H6 flow C3H6 4.1 2.71 5.30 4.8 2.71 5.30 4.8 C3H6/H2 (2:1) C3H6/C3H8 (1:1) 4.0 H2 flow 6.6 2.76 4.90 -0.1 C2H4 flow C2H4 3.0 2.71 5.50 3.6 H2 flow 6.5 2.76 4.70 0.1 Pt/7-A1203 C2H4/H2 (5:1) C2H4/C2H6 3.0 2.70 5.30 3.8 C2H4/H2 (1:1) C2H6 5.7 2.75 5.50 0.0 H2 flow 6.5 2.76 4.87 0.3 C2H6 flow C2H6 5.6 2.75 4.20 -0.1 H2 flow 8.3 2.75 2.00 0.0 Pt/MgO C2H4 flow C2H4 7.5 2.74 2.00 0.4 C2H6 flow C2H6 8.2 2.75 1.88 0.5 H2 flow 11.0 2.76 0.50 -0.7 Pt/SiO2 C2H4 flow C2H4 11.3 2.76 0.80 -0.8 C2H6 flow C2H6 11.0 2.76 0.50 -0.6 aNotation as in Table 2; flow rate ratios are molar.

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clusters, depending on the composition of the reaction mixture. The changes were reversible m subsequent treatment with H2 at 25~ led to values of Npt-Pt and Rpt-Pt close to those characterizing the initially reduced sample. ....

3.7. Analysis of XANES region The XANES data provide evidence of the influence of hydrogen, oxygen, alkenes, and alkanes on the electronic structure of the Pt (Figures 1 and 2). When relatively large Pt clusters were present on the support (MgO or SiO2), interaction with H2 or hydrocarbons led to no substantial changes in the XANES region. When A1203-supported Pt clusters were exposed to H2, neither the edge position nor the area under the white line changed relative to the values characterizing the sample scanned under vacuum, but interaction with H2 changed the position of the first inflection point (Figure 1). Exposure of the sample to 02 led to a change in the position of the first inflection point and a substantial increase in the area under the white line, which indicates that the sample had a lower electron density than the reduced sample. In contrast, the position of the first inflection point did not change when Pt/y-A1203 was exposed to ethene or to ethane (Figure 2). After adsorption of ethene, the white line area increased relative to that of the evacuated sample (Figure 2). Addition of H2 to the ethene changed the white line area, depending on the amount of H2 in the feed. When the H2 was sufficient to convert all the ethene to ethane, the white line area was the same as that after adsorption of ethane.

1.5

8 r

1.5

1.0

o r ..Q

._

i ~ \\

C2H4~

Complete

oxidation- - - - - - - - ~

Partial oxidation

8

1.0

o

Vacuum

Inflection point

..Q

Hydrogen flow 0.5 0.5

o Z

0.0 11540

11550

1156(

0.0 11570

11580

11590

11600

Energy, eV

Fig. 1. Comparison of XANES regions characterizing Pt/y-A1203 scanned in vacuum, H2, and after exposure to 02.

11610

11540

11550

11560

11570

11580

11590

11600

11610

Energy, eV

Fig. 2. XANES data characterizing Pt/y-A1203 sample scanned in vacuum and after interaction with hydrocarbons.

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4. DISCUSSION

The data demonstrate that although the effects of adsorbates on the structures of the larger Pt clusters (on MgO or SiO2) were negligible, the effects of adsorbates on the smaller Pt clusters (on A1203) were substantial. The adsorbates formed from H2, alkenes, and 02, which we infer were strongly bonded to Pt, caused a reversible flattening of the clusters, as indicated by the increasdd values of Npt-Pt and Npt-o. In contrast, the effect of adsorbed alkanes was very small, consistent with weak adsorption of these compounds. Changes depending on the composition of the reacting mixture of alkene and H2 imply that there were clusters of different morphologies at different positions in the flow reactor operating at steady state; catalysts near the inlet are expected to have had flattened clusters and those near the outlet (where the conversion was high and most of the gas was alkane) not being flattened. Clearly, it is an oversimplification to model the catalyst as a single Pt species. The results are contrasted with those representing Ir4/7-A1203 and Ir6/7-A1203 in propene (sometimes with H2) under the same conditions, which indicate the lack of changes of the clusters relative to those present in H2 [4]. We infer that It4 tetrahedra and It6 octahedra on 7A1203 are more resistant to morphological change in propene than the approximately 11 ,A, Pt clusters on 7-A1203. It is not clear whether the difference is associated with just the metal or whether the nearly uniform It4 and It6 structures may be more resistant to morphological change than (nonuniform) clusters of Pt containing about 25 atoms each. The results are consistent with the earlier inference [5] that the supported Ir4 and It6 clusters are different from larger clusters or particles that can be classified as (more nearly) metallic, as indicated by their catalytic activities for toluene hydrogenation. The data suggest a refinement of the classification of supported noble metals according to the average cluster or particle size (large particles which show almost no size effect on catalytic activity (turnover frequency); intermediate-sized clusters that show cluster size effects; and the smallest (nearly molecular) clusters that are chemically distinct and not metallic in character [6]. The morphological changes indicate changes of the cluster energy (with the interacting adsorbate and support ligands), and the evidence of changes with only the smallest Pt clusters suggests that the difference may represent kinetics rather than thermodynamics. ACKNOWLEDGMENT This work was supported by the National Science Foundation (grant CTS-9615257). REFERENCES

1. O. Alexeev, D.-W. Kim, G. W. Graham, M. Shelef, and B. C. Gates, J. Catal., 185 170. 2. M. Vaarkamp, J. C. Linders, and D. C. Koningsberger, Physica B 209 (1995) 159. 3. B.J. Kip, F. B. M. Duivenvoorden, D. C. Koningsberger, and R. Prins, J. Catal., 105 26. 4. G. Panjabi, A. M. Argo, and B. C. Gates, Chem. Eur. J., 5 (1999) 2417. 5. F.-S. Xiao, W. A. Weber, O. Alexeev, and B. C. Gates, Stud. Surf. Sci. Catal., 101 1135. 6. Z. Xu, F.-S. Xiao, S. K. Purnell, O. Alexeev, S. Kawi, S. E. Deutsch, and B. C. Nature, 372 (1994) 346.

(1999)

(1987)

(1996) Gates,

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

In situ F T - I R i n v e s t i g a t i o n o f h y d r o c a r b o n reactions o v e r z e o l i t e b a s e d bifunctional catalysts M. H6chtl a, Ch. Kleber a, A. Jentys b, H. Vinek a aVienna University of Technology, Institute for Physical and Theoretical Chemistry Getreidemarkt 9/156, A-1060 Wien, Austria bTechnical University of MUnich, Institute of Technical Chemistry II, Lichtenbergstr. 4, D-85748 Garching, Germany SUMMARY

The transport properties and the hydroconversion of n-heptane and 2-methyl-hexane over Pt loaded HZSM-5 and HBETA were investigated by in situ FTIR-spectroscopy combined with the evaluation of the kinetic data. Propane and i-butane were the main products observed on the catalyst surface as the temperature increased, while the number of interacting BrCnsted acid sites was decreasing. The diffusivities of n-heptane and 2-methyl-hexane were in the same range for HBETA, but a significant diffusion restriction for the monobranched isomer was observed on HZSM-5, which was reflected in a lower reaction rate. 1. I N T R O D U C T I O N Isomerization of linear C5-C8 alkanes, the main constituents of gasoline obtained from crude oil refining, is of great economic importance as branched alkanes have a considerably higher octane number than their linear counterparts. Therefore, alkane isomerization is an attractive process route to produce both, high-octane gasoline and environmentally acceptable gasoline components. Typically, alkane isomerization is performed over bifunctional catalysts, where the hydrogenating/dehydrogenating function is provided by noble metals like Pt, and the acid function by zeolites. According to the generally accepted bifunctional reaction mechanism, alkanes are dehydrogenated on the metal sites and the alkenes formed diffuse to the acid sites, where they are protonated yielding carbenium ions. After isomerization and B-cracking the products are hydrogenated on the metal sites. The rate-determining step depends on the kind and number of metal and acid sites. For metals with a low hydrogenation activity or catalysts with a low number of metal sites, the rate-limiting step is the hydrogenation/dehydrogenation. If sufficient metal sites are present or for high hydrogenation activity, isomerization of the carbenium ion on the acid sites is rate limiting. The overall reaction rate can be determined by the transport to and from the active sites, by adsorption - desorption or by the reaction itself. Therefore, the investigation of the nature, number and strength of the sites responsible for the transformation of hydrocarbons would appear to be critical for the understanding of the catalytic properties and for designing new catalysts. Furthermore, detailed experimental studies of the mechanism of catalytic reactions on a molecular level are of crucial importance.

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In situ infrared spectroscopy is certainly one of the most powerful techniques for obtaining information on the concentration of reactants, intermediates and products at the surface of the catalysts. The aim of this paper is to examine the hydrocarbon conversion over bifunctional catalysts by the determination of kinetic parameters combined with in situ FT-IR investigations.

2. EXPERIMENTAL The Pt containing HZSM-5 (2 %wt Pt, Module 28) and HBETA (2 %wt Pt, Module 27) were prepared by incipient wetness impregnation with [Pt(NH3)4]C12. The catalysts were dried for 4 h at 70~ and subsequently at 100~ for 12 h. Prior to reaction the samples were treated in flowing oxygen and reduced in hydrogen at 475~ Temperature programmed desorption (TPD) of ammonia was used to determine the concentration of strong acid sites. The platinum dispersion was obtained from a hydrogen adsorption isotherm measured in a volumetric adsorption apparatus. The morphology and the crystallite size of the catalysts were revealed by scanning electron microscopy (SEM). Infrared (IR) spectra were recorded on a Bruker IFS 28 FTIR - spectrometer, which was equipped with (i) a transmission vacuum cell and (ii) a reaction cell for in situ investigations under reaction conditions. The catalysts were pressed into self-supporting wafers that were placed in a ring shaped furnace inside the cell. To observe the molecular transport in the catalyst pores, n-heptane (n-C7) and 2-methylhexane (2-MC6) were adsorbed at 40~ and 10-i mbar in the vacuum ir-cell. Spectra were recorded every 10 seconds until the equilibrium was reached. The integrated C-H stretching vibrations were taken as measure of the amount of hydrocarbon adsorbed and the diffusion coefficients were estimated as presented in [ 1]. The in situ FTIR hydroconversion experiments were carried out in a reaction cell similar described by Mirth et al. [2] approximating a continuously stirred tank reactor with a volume of 1.5 cm 3. The effluent gas stream was sampled and subsequently analyzed by means of gas chromatography (HP 5980 II) equipped with a flame ionization detector. The hydrocarbon reactions were investigated at temperatures between 150~ and 350~ at 1.5 bar total pressure. The alkane partial pressure in the H2 stream was 25 mbar. To obtain the difference spectra of the in situ measurements, the spectra of the activated catalysts recorded at corresponding temperatures were subtracted from the spectra recorded during the reaction. 3. RESULTS In Table 1 the crystallite size, the platinum dispersion, the concentration of strong BrCnsted acid sites and the acid strength are given. Both molecular sieves exhibited similar acid strength, characterized by the frequency shift of the SiAl-OH groups after adsorption of benzene. The HZSM-5 crystallites consisted of inhomogeneous agglomerates of prism shaped particles with an average size of 0.9 ~tm, while HBETA showed homogeneous spheres with a diameter of 0.6 ].tm. The IR spectrum from HZSM-5 exhibited OH bands at 3610 cm -I and 3740 cm -I assigned to bridged SiAl-OH groups and terminal Si-OH groups, respectively. In the hydroxyl region of the IR spectrum of the activated HBETA five absorption bands were observed. The bands with the highest intensity at 3741 cm -I and 3610 cm -1 were

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assigned to terminal silanol and SiAl-OH groups, respectively. Two bands with a lower intensity at 3785 cm -l (terminal A1-OH groups), at 3668 cm l (extra framework A1) and a broad band around 3530 cm -~, attributed to internal bridging SiAl-OH groups were observed [3]. Table 1. Crystallite size, Pt dispersion, concentration of strong BrCnsted acid sites and SiAl-OH frequency shift after adsorption of benzene

HZSM-5 (M 28) HBETA (M 27)

Pt-dispersion crystallite size [%] [nm] 15 0.9 14 0.6

SiAl-OH Avon (benzene) [mmole g-l] [cm_l] 0.37 354 0.61 346

During the n-heptane conversion over HZSM-5 (Figure 1) in the IR-reaction cell the band at 3610 cm -~ decreased and the perturbed OH-groups gave rise to a band at 3447 cm -1. New bands appeared due to adsorbed n-C7 between 2700 and 3000 cm -l, which were assigned to the stretching modes of Vasy (C-H) in CH3, (2958 cm -l) Vasy(C-H) in CH2 (2939 cm -1) and fused bands of the stretching modes of Vsym(C-H) in -CH3 and -CH2- (2866 cm-l). Interaction of the alkane with the silanol groups was only found at low temperatures and high gas pressures. The formation of coke precursors could be excluded, as the spectra after reaction did not show bands at 1600, 1554, 1500 and 1343 cm -~, due to carbonaceous species [4]. Furthermore no CH-stretching vibrations above 3000 cm -~, resulting from unsaturated hydrocarbons, could be observed. At 210~ n-C7 was the main species adsorbed on the catalyst. With increasing reaction temperature the surface coverage, represented by the decrease of the SiAl-OH band and the intensity of the C-H stretching area, decreased. 1~~210~ Simultaneously, the dominating ~." surface species changed from n-C7 to reaction products as indicated by the change of the IR spectra in the f / / ~ ~ 285~ C-H stretching domain. The CH2 stretching vibration at 2939 cm -~ significantly decreased relative to a sharp band at 2965 cm -1, that can be attributed to the CH3 vibrations in i-Ca and C3, paralleled by the 3800 3400 3000 2600 decrease of the 1375 cm -~ band in wavenumber [cm-l] the CH deformation region. Hence, an increase in concentration of CH3 groups at the expense of CH2 Figure 1. Difference spectra of the in situ investigation during the n-heptane conversion over Pt/H-ZSM5. groups during reaction was shown. At 350~ only low intensities of the bands attributed to adsorbed molecules were observed. At this temperature the strong acid sites seemed to be uncovered, suggesting that only a low number of sites was actually contributing to the n-C7 conversion. Note that the reactant or product molecules occupied only 34% and 7% of the BrCnsted acid sites at 210~ and 285~ respectively.

/

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To identify the nature of the surface species, the IR spectra during the conversion of n-C7 over Pt/HZSM-5 were compared to the mathematically added spectra of the substances in the gas stream after reaction. The adsorption of the pure hydrocarbons (C3, i-Ca, n-C4, n-C7 and 2MC6) was recorded at 250~ on HZSM-5 (Module 1000), which possesses only a minor ,--, 80

,--, 40

O

O

o

60

30

o

40

20

~

20

_~

10

o,...i

~"

0

0 0

20 40 60 n-C~ conversion [%]

80

0

20 40 n-C 7 conversion [%]

60

Figure 2. Conversion/Yield dependencies of the n-C7 conversion over (a) Pt/HZSM-5 and (b) Pt/HI3 9 C3, A i-C4, V n-C4, O methyl-C6, ~di-methyl-C5 catalytic activity, in order to obtain spectra on a surface and conditions comparable to the reaction over HZSM-5 (M 28). The spectra were added according to the selectivities obtained from the analysis of the reaction products, which revealed that the main products of the n-C7 conversion reaction were C3 and i-Ca over both, Pt/I-IZSM-5 and Pt/HBETA (Figure 2). On the latter, a higher concentration of monomethyl-hexanes and dimethyl pentanes was found, which appeared as primary products. As shown in Figure 3 the C-H stretching vibrations of the "7. added spectra of propane and iso-butane (Fig. 3(a)) perfectly matched with the spectra of the n-C7 reaction over Pt/HZSM-5 at J (a) 350~ and 80% total conversion (b_. . ) ) j (Fig. 3(b)). The spectra of the (c) conversion of n-C7 over J Pt/I-IBETA (Fig. 3(c)) showed a higher concentration of heptane 3100 3000 2900 2800 2700 2600 isomers on the surface of the wavenumber [cm-1] Figure 3. catalyst, indicated by the CH2 (a) Addition of the of propane and iso-butane spectra adsorbed stretching vibrations at 2935 cm -~, on HZSM-5 (M 1000) confirming the results of the (b) IR-spectra during the n-heptane conversion over Pt/HZSM-5 product analysis. (M 28), T = 350~ total conversion = 80% In Figure 4 the time resolved (c) IR-spectra during the n-heptane conversion over Pt/H[~ (M IR spectra of the in situ 27), T = 350~ conversion = 52% measurements at 250~ are shown. When the hydrocarbon flow was stopped, the concentration of propane and iso-butane on the surface increased rapidly and the number of interacting BrCnsted acid sites decreased with the time of reaction.

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Table 2. Relative diffusivities and rates of n-C7 and 2-MC6 D(n-C7/2-MC6) Pt/HZSM-5 Pt/HBETA

100 1

rates [mol.s-~.g -l] n-C7 2-MC6 2.9 10-5 1.5 10-5 1.5 104 1.6 10-5

n-C7 22 11

iso/n-C4 2-MC6 24 12

In Table 2 the rates at conversions below 10% and the relative diffusion coeff'lcients of n-C7 and 2-MC6 are given. The diffusivity of n-C7 was the same over both catalysts, while for 2-MC6 great differences were observed. On HZSM-5 the ratio of the diffusion coefficients of n-C7 and 2-MC6 was 100, whereas on HBETA it was 1, which was reflected in the activity for the respective reactants. 4. DISCUSSION The hydroconversion of n-alkanes over bifunctional catalysts is supposed to proceed via a mechanism that was suggested by Weisz et al. [5]. In this reaction scheme the isomerization of alkenes on the acid sites is assumed to be the rate determining step. Isomerized and cracked products are hydrogenated and desorbe as saturated hydrocarbons. For the reaction step on the acid sites, S. Tiong Sie introduced the protonated cyclopropane structure as intermediate for o isomerization and cracking [6]. According 3800 3400 3000 2600 ~' to this mechanism, cracking reactions wavenumber [cm -1] should occur parallel with isomerization. Figure4. Time resolved IR spectra of the n-C7 As shown by adsorption measureconversion over Pt/HZSM-5, T = 250~ ments, all acid sites were accessible for nheptane and 2-methyl-hexane in P t ~ Z S M - 5 and P t ~ B E T A . The adsorption kinetics under non-reactive conditions (40~ 0.1 mbar) indicated that HZSM-5 imposed sterical restrictions on the transport of 2-methylhexane whereas no restrictions were observed for the large pore HBETA zeolite. This difference was reflected in the diffusion coefficients, as the same values for the diffusion coefficient were determined for n-heptane in both zeolites, while the value for 2-methylhexane was much lower on HZSM-5 than over HBETA. At 200~ the IR spectra showed n-heptane adsorbed on the Br6nsted acid sites (Fig. 1) The sorbed molecule is able to interact with the zeolite lattice (pore walls), the acid and metal sites and with other sorbed molecules. If enough metal sites are present on the catalyst the heptane-heptene equilibrium will be established. However the heptene concentration is extremely small. Therefore IR bands corresponding to alkenes could not be observed in the spectrum. At 250~ the number of Br6nsted acid occupied by the reactant decreased and the intensities of the stretching vibration bands of CH3 and CH2 changed indicating an increase of CH3 groups. Reactant or product molecules were interacting with only a small part of the acid

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sites at 300~ In agreement with the results of Mirth et al. [7] for xylene isomerization and Jolly et. al. [8], who investigated n-hexane cracking over HY zeolites, the intensity of the hydroxyl bands of the catalysts showed no decrease at 350~ compared to the spectra of the activated surface and thus a great part of the acid sites was uncovered. When the continously stirred tank reactor was converted to a batch reactor by turning off the hydrogen flow, the number of free acid sites increased as the reaction proceeded, due to the lower enthalpy of adsorption of the reaction products [9]. In the gas phase mainly cracking products, propane and i-butane, besides monomethylhexanes over Pt/HZSM-5 and monomethyl-hexanes and dimethyl-pentanes over Pt/HBETA were found. In the conversion-yield plots both, the cracked and isomerized products seem to be formed in a primary reaction. This is in agreement with Guisnet et al. [10], who found cracking as the main primary reaction for the conversion of n-heptane over Pt/HZSM-5. However, cracking products cannot be the result of the direct scission of n-heptane since ibutane and not n-butane is produced. Therefore i-butane can be only the product from cracking via a cyclopropyl-alkyl intermediate suggested by Sie. Otherwise, i-butane formed by l-cracking of dimethyl-pentanes would appear as secondary products. Molecular diffusion plays an important role in the reaction over microporous catalysts. Experimentally determined diffusion coefficients may contribute to further theoretical understanding of the molecular processes taking place on the catalyst surface. At lower temperatures cracking rates were not diffusion limited and the reaction rates showed a stronger temperature dependence than the diffusivities. At higher temperatures the activity is mainly governed by the diffusion of the products, as indicated by their increasing surface concentration, suggesting that their initial rate of formation must be higher than the transport out of the pore system. This clearly indicates that the diffusion of the products out of the zeolite pores strongly affects the observed product distribution in the gas phase.

ACKNOWLEDGEMENTS This work was gratefully supported by the ,,Fond zur F6rderung der wissenschaftlichen Forschung", Proj. Nr. 11749 and the "Oesterreichische Nationalbank" Proj. Nr. 5410. REFERENCES

1. 2. 3.

J. Crank, Mathematics of Diffusion, p. 62 ff, Oxford University Press, London 1956. G. Mirth, F. Eder and J.A. Lercher, Appl. Spectrosc., 48, 2 (1994) 194. I. Kiricsi, C. Flego, G. Pazzuconi, W.O. Parker jr., R. Millini, C. Perego and G. Bellussi, J. Phys. Chem., 98 (1994) 4627. 4. J.F. Joly, N. Zanier-Szydlowski, S. Colin, F. Raatz, J. Saussey and J.C. Lavalley, Catal. Today, 9 (1991) 31. 5. P.B. Weisz and E.W. Swegler, Science, 126 (1957) 31. 6. S. Tiong Sie, Ind. Eng. Chem. Res., 32 (1993) 403. 7. G. Mirth, J. Cejka and J.A. Lercher, J. Catal., 139 (1994) 24. 8. S. Jolly, J. Saussey and J.C. Lavalley, J. Mol. Catal., 86 (1994) 401. 9. F. Eder, Thesis, University of Twente, Enschede, The Netherlands (1996). 10. M. Guisnet, F. Alvarez, G. Giannetto and G. Perot, Catal. Today, 1 (1987) 415.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

A STEADY STATE ISOTOPIC TRANSIENT KINETIC ANALYSIS OF THE F I S C H E R - T R O P S C H S Y N T H E S I S R E A C T I O N O V E R A C O B A L T BASED CATALYST H.A.J. van Dijk, J.H.B.J. Hoebink, J.C. Schouten Laboratory for Chemical Reactor Engineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands A transient kinetic analysis is presented of the Fischer-Tropsch synthesis reaction using the SSITKA technique in combination with a GCMS analysis of the 13C-labeled hydrocarbon reaction products. By using the complete MS fragmentation pattern, both the total fraction labeled components as well as the transient responses for each C1 to C5 isotopomer are obtained. The experimental data give rise to a mechanism with a parallel route towards both methane and C2§ hydrocarbons or a buffer step for the Cl,ads species. 1. INTRODUCTION The Fischer-Tropsch synthesis, i.e. the conversion of synthesis gas into hydrocarbons, is an alternative route for the production of transportation fuels and for the liquefaction of natural gas. Although the synthesis is applied on industrial scale [ 1,2], the reaction mechanism is not yet fully understood. Early steady state kinetic modeling led to overall mechanistic models with only one kinetic parameter: the chain growth probability ot [3]. Recent more careful examinations of the product spectrum under different reaction conditions have led to more sophisticated models accounting for the readsorption of reactive olefins [4,5,6]. However, transient experiments are more promising for enraveling the reaction mechanism. With the Steady State Isotopic Transient Kinetic Analysis (SSITKA) technique, the catalyst is kept under steady state conditions and an isotopic transient is introduced by abruptly replacing one reactant by its isotope. Compared to other transient techniques, the surface composition of the catalyst does not change during SSITKA because of the overall steady state [7]. This technique has been widely used to study methane formation on Fischer-Tropsch catalysts [8,9]. This paper expands the technique towards the C1 to C5 hydrocarbons [10], both paraffins and olefins, while accounting for the degree of labeling of the reaction products. 2. EXPERIMENTAL SET-UP The SSITKA experiments are performed on an experimental set-up consisting of feed, reactor, and analysis sections. Mass flow controllers for every component in the feed section provide two feed streams with identical composition and total flow, which is necessary for the

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generation of an isotopic step-change without pressure effects. One feed stream contains a maximum of 5 vol% Ne as inert tracer to monitor both the quality of the isotopic step and the gas hold-up in the reactor. The other feed stream contains ~3CO instead of ~2CO. The synthesis gas is diluted with 40 vol% He or He plus Ne. A stainless steel tubular fixed bed reactor with 7 mm internal diameter and 40 mm length is used which is free of dead volumes, eliminating axial gas mixing. All product containing lines and valves are heated up to 388 K. The steady state performance of the catalyst is monitored by on-line gas-chromatography (GC), quantifying the C~ to Ct0 product spectnma. On-line quadrupole mass-spectrometry (MS) with reactor sampling via heated capillaries is used to monitor the transient responses of Ne, ~3CO, and ~3CI-h. Finally, a gas-chromatograph-mass-spectrometer (GCMS) equipped with a 12-loop sample valve is used for the determination of the transients of the C2 to C5 carbon labeled hydrocarbons. This particular analysis will be discussed in detail below. Because of the heavy overlap of the fragmentation patterns of hydrocarbons in massspectrometry, on-line MS cannot provide the transient responses for these products. For this reason a 12-loop sampling valve collects samples during the isotopic transient, which are sequentially analyzed by GCMS. The SSITKA experiment produces a hydrocarbon product mixture, containing not only different hydrocarbon types, but also the non-, partially and fully labeled variants for every hydrocarbon type. First, each sample is separated into the different hydrocarbon types. Second, the fragmentation pattern of each hydrocarbon type is recorded for the C1 to C5 range. Third, this fragmentation pattern is used to calculate the isotopic composition of the mixture of different isotopic variants for the particular hydrocarbon type. The observed fragmentation pattern can be considered as a linear combination of the fragmentation patterns for the individual isotopic components. By minimizing the difference between the observed and the calculated value for every m/z position in the fragmentation pattern, the contribution of each isotopic variant can be quantified. This requires the fragmentation pattern of each isotopic hydrocarbon to be known. These can be determined from the pattern of the non-labeled component when assuming that (i) peak intensities do not depend on the presence of 13C, and (ii) secondary fragmentations can be neglected. This only holds for symmetrical hydrocarbons (n-paraffins, ethene and 2-butenes). For asymmetrical hydrocarbons (e.g. propene), the fragmentation patterns for the different isotopomers cannot be determined from the non-labeled compound. Then, only the overall fraction labeling can be obtained by using the m/z region corresponding to the non-fragmented molecule. All experiments are performed on a Co/Ru/TiO2 catalyst with an average pellet size of 0.23 mm of which the composition is based on the work of Iglesia et al. [11]. Experiments are performed at low conversions and at intrinsic kinetic conditions. A combined set of steady state and transient data is obtained at 498 K and 1.2 bar at different bed residence times and H2:CO feed ratios. At these conditions considerable amounts of 2-olefins, 1-methyl-l-olefins, and 1-methyl-paraffins are produced besides the desired 1-olefins and n-paraffins. Low amounts of alcohols but no aromatics are detected. Typical c~4-t0 values ranging from 0.3 to 0.6 are obtained. 3. EXPERIMENTAL RESULTS Typical transient responses following a Ne/He/12CO/H2 ---> He/13CO/H2 step change are presented in figures 1 to 4. The chromatographic effect for CO, i.e. the fast exchange between adsorbed CO and CO in the gas phase, is responsible for the fast but delayed ~3CO response in figure 1. The delay is a measure for the surface concentration of adsorbed CO. Furthermore, it

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~1,o 9 "

"~,~...' 1.o

Ne

m I~ 0.6

13CH4

Q. I~ 0.6 "O QN0.4

~' NQ0.4 0.2 9

.............................................................................................................................................................. ~~ ~ 1 2 C 2 H 6 "

,s"

-~'-" 12C2H4

~ 0"2 iI~~" ~" " " ~"l b ~ , ~ ~

o.o

0

,

,

50

100

o

o.o-.

150

~

0

o.-l~

L ....................................................................................................................................................................................

~

1-'-

.

--

200

300

Time / s

Figure 1" Normalized responses for Ne, 13CO, and t3CH4 at H2/CO= 1 and W/F = 44.8 mol kgcat"l s"l.

o 0.8 m "on" 0.6

"~"~ 13C2H412C3'CH4

100

Time / s

9

""4)- 12C13CH6 -41- '3CzH6

-

Figure 2" Normalized responses for ethane ( ~ )

and ethene ( ...... ), conditions as in figure 1. l

~ 0.8 ~Q"

12C3Hs nCz13CHs

z 0.0

......................................................................................................................................................................................

I1

It

12 ,o -!1- lzCz13CzHlo

z 0.0 0

100

200 Time I s

Figure 3" Normalized responses for propane.

300

0

100

200

300

Time I s

Figure 4" Normalized responses for n-butane.

is observed that 13CH4 already evolves before 13CO is observed, indicating the absence of a strong interaction of CH4 with the catalyst. The similarity of the shapes of the transient responses for the C2 to C5 hydrocarbons (e.g. figures 2 to 4) illustrates the common surface chemistry of these products, characteristic for a polymerization-like process. From the transients for each isotopic variant in figures 2 to 4, the transient for the overall fraction labeled can be calculated for each hydrocarbon. The shapes of these responses can be compared to the methane response. They also appear to be similar in shape. The consequences of this observation are highlighted in the modeling section. The 1-olefin responses show a time lag compared to the paraffinic counterparts, as shown in figure 2 for ethane and ethene, and this lag increases with bed residence time. Similar to the reasoning of the chromatographic effect for CO, this delay without deformation of the shape is the result of a reversible process, i.e. the olefin readsorption. However, this lag is absent for the 2-olefins and the 1-methyl-l-olefins of the Ca hydrocarbons (not shown). Together with the observation that the changes in the selectivity of these latter products with bed residence time are similar to the behavior of the n-paraffins, we conclude that internal and iso-olefins may undergo readsorption, but much less than the 1-olefin. Although readsorption occurs, the Anderson-Schulz-Flory-plots for the C3 to C~0 products are essentially straight lines, indicating the absence of a strong chain length dependence of the olefin readsorption process under the applied conditions. This is also illustrated by the relative

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insensitivity of the chain growth probability with the bed residence time under the current experimental conditions, although it is sensitive to the H2:CO ratio in the feed gas.

4. M O D E L I N G The experimental results are modeled under the application of a plug-flow reactor model. It allows the incorporation of fast reversible reaction steps in the mechanism [7,12], like the CO chromatographic effect and the readsorption of olefins. When a CSTR reactor model is applied, concentration gradients over the catalyst bed due to these reversible reactions are neglected, which may lead to erroneous results. Tested kinetic models are based on a carbene mechanism, assuming a CH2,aas species as monomeric building block. A first consideration of methanation alone shows that models with a parallel route towards methane rather than a single route describe the responses significantly better. The best fit for models 1 and 2, both having a single route towards methane, is represented in figure 5. A better fit, as illustrated in figure 6, is obtained for models 3 through 6, having a parallel route towards methane or a buffer step. It is not possible to discriminate between models 3 through 6, since they result in identical fits. Model 1

Model 2

CH4,g COads-'--~ Cads

i~

COads"-'-~ Cot,ads

%

Model 4

Model 3

i~ g / CH41'g i~ g COads--'-~ Cot,ads COads-'-~

CH4 T'~ C~,ads Model 6

Model 5

CH41'g i~ g Cot,ads

> C~,ads

Cox,ads

COads

\

> C~,ads

COads Cox,ads

%

> C~,ads

> C~,ads

, 1.0 ~ 0.8

~ o.8

n,

~0.6

0.6

.~ 0.4 Z

0.2 0.0

j/

9.'7

0

~" 0.4

0.0 50

100 Time

150

0

i

50

Is

Figure 5: Fit for ~3CO (4,) and 13CH4(A) according to models 1 and 2. Symbols: data; lines: models,

100

150

Time/s

13CO(,)

Figure 6: Fit for and 13CH4(~k) according to models 3 through 6. Symbols: data; lines: models.

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On the basis of these 13CO SSITKA experiments alone, the chemical background of the Ca,ads and C~,ads species cannot be assessed. Moreover, the surface concentrations of adsorbed atomic hydrogen and of vacant sites need to be lumped into the reaction rate coefficients. The methanation models 3 through 6 are used as a basis for the formulation of models for the Fischer-Tropsch reaction. They can again be divided into models possessing a single or a parallel route towards the C2+ hydrocarbons, with examples of both types presented below. Model 3.1 contains a parallel route towards methane, but only one C-species contributes to chaingrowth, whereas in model 4.1 both C-species contribute to methane formation and to chaingrowth. The best fits for both models are presented in figure 7 and 8, where for clarity only the methane and ethane transients are displayed. Only models of type 4.1 can describe the similarity in shape of the ethane transients with the methane transient mentioned in the previous section. This implies that both types of C-species must contribute to methanation and to C2+ hydrocarbon formation. The hypothesis that one C-species leads preferably to methane and the other to C2+ hydrocarbons is therefore not supported by the data. Using model 4.1, typical values for surface coverages are calculated: the surface coverage for COads amounts to 70%, for Ca,ads to 20%, for CI3,ads to 7%, and for C2+,ads, to 1%. The surface is therefore mainly covered with CO and C1 species. Consequently, the surface concentrations of growing hydrocarbon chains are very small. Furthermore, the formation of C~,ads and C~,ads from CO are the slowest steps in the mechanism. From C~,ads onwards all processes are relatively fast, in agreement with the findings of Mims and McCandlish [ 13].

i~g

/

CH4 l ,g

C Oads--'-~ Cox,ads

co 0.8 Q. n,,

/ ~

~ oO.-""

u) 9 0.6

--

T

Model 4.1: Two paths towards methane and C2+.

:

1.0

o

I

l/,/

>

Model 3.1" Two paths towards methane, one path towards C2+.

o

c2n6,g c2n4,g

COads-''-~ Ca,ad s

I

,

CH4,g

C2H l 6,g C///2H4,g

. 1.0 q) w c 0.8 I~ 0.6

O.4

o O.2 Z 0.0 0

.....

- ..........

5O Time

/s

.

i

100

150

Figure 7: Transient responses for 13CH4 ( " and +), nCI3CH6 (& and ~ ) , and 13C2H6 (ll and El) according to model 3.1. Data: ... ; calculations: m

0.0

0

50 Time

/s

100

150

Figure 8" Transient responses 13CH4 (~l~ and +), 12C13CH6 (A and ~ ) , and 13C2H6 (Ira and I-'1) according to model 4.1. Data: .... calculations:

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Besides model 4.1, the experimental results can be equally well described by a similar scheme based on model 6. This scheme possesses a buffer step for the C l,ads species, affecting both methane and the C2+ hydrocarbons. Incorporation of a depolymerization reaction, as frequently observed by olefin cofeed studies [ 14], provides a buffer with only a small capacity since the surface concentration of growing hydrocarbon chains is small. The introduction of a surface depolymerization step in Fischer-Tropsch schemes based on models 3 and 5 does therefore not alter the responses for the C2+ hydrocarbons in the way the buffer step in model 6 does, and results in transient responses similar to those presented in figure 7. 5. CONCLUSIONS The combination of the transient SSITKA technique with a GCMS analysis of the C-labeled hydrocarbon reaction products is a powerful tool for a mechanistic investigation of the Fischer-Tropsch synthesis reaction. Only the 1-olefins display considerable readsorption onto the Fischer-Tropsch chain growth sites, whereas for iso- and 2-olefins this readsorption occurs in far less extent. The presence of a second route towards both methane and C2+ hydrocarbons or the presence of a buffer step for the Cl,ads species is required to model the experimental results. CO and C l,ads predominantly cover the catalyst surface and after the initialization of chaingrowth the surface reactions become relatively fast. ACKNOWLEDGEMENT

The authors gratefully acknowledge the financial support provided by the Commission of the European Union in the framework of the DG XII-JOULE program, subprogram Energy from Fossil Sources, Hydrocarbons (JOF3-CT95-00016). REFERENCES

10 11 12 13 14

M.E. Dry, Catal. Today, 6 (1990) 183 M.M.G. Senden, A.D. Punt, A. Hoek, Stud. Surf. Sci., 119 (1998) 961 B.W. Wojchiechowski, Catal. Rev.-Sci. Eng., 30 (1988) 629 E. Iglesia, S.C. Reyes, R.J. Madon, S.L. Soled, Adv. Catal., 39 (1993) 221 E.W. Kuipers, I.H. Vinkenburg, H. Oosterbeek, J. Catal., 152 (1995) 137 G.P. van der Laan, A.A.C. Beenackers, Ind. Eng. Chem. Res., 38 (1999) 1277 J. Happel, Isotopic assessment of heterogeneous catalysis, London, Academic Press, 1986 A.R. Belambe, R. Oukaci, J.G. Goodwin, J. Catal., 166 (1997) 8 S. Vada, E. Blekkan, A. Hilmen, A. Hoff, D. Schanke, A. Holmen, J. Catal., 156 (1995) 85 T. Komaya, A.T. Bell, J. Catal., 146 (1994) 237 E. Iglesia, S.L. Soled, R.A. Fiato, G.H. Via, J. Catal., 143 (1993) 345 R.H. Nibbelke, J. Scheerova, M.H.J.M. de Croon, G.B. Marin, J. Catal., 156 (1995) 106 C.A. Mims, L.E. McCandlish, J. Phys. Chem., 91 (1987) 929 D.S. Jordan, A.T. Bell, J. Phys. Chem., 90 (1986) 4797

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化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Use of membranes in Fischer-Tropsch reactors R.L. Espinoza a , E. du Toit a, J. Santamaria b, M. Menendez b, J. Coronas b and S. Irusta b a Sasol Technology, P.O.Box 1, Sasolburg 9570, South Africa b Dpt of Chemical and Env. Eng., University of Zaragoza, 50009 Zaragoza, Spain

Water is one of the primary products in the Fischer-Tropsch (FT) process for the conversion of coal or natural gas derived synthesis gas to hydrocarbons. This reaction water oxidizes the FT active sites, thereby shortening the catalyst life. For iron based catalysts it has the additional negative effect of inhibiting the reaction rate. A family of membranes has been developed for the highly selective in-situ removal of the FT reaction water. These membranes have proven to be effective at operating conditions typical of commercial fluidized and slurry bed FT reactors. The use of these membranes will not only result in longer FT catalyst life but also in a better reactor utilization. 1. INTRODUCTION In spite of its long life, the Fischer-Tropsch (FT) process for the conversion of coal or natural gas derived synthesis gas to hydrocarbons is still a very dynamic technology. Continuous improvements are being made in areas such as natural gas reforming, reactor technology and catalyst development. In the reactor technology front, some of these developments include the Slurry Phase Fischer-Tropsch reactor [e.g. 1,2] and the Sasol Advanced Synthol (SAS) process (fluidized bed reactor) [e.g. 3], while in the catalyst development front there has been numerous patents dealing with cobalt based catalysts for use in low temperature FT [e.g. 4], although the efforts placed towards the improvements of iron based low temperature FT catalysts [e.g. 5] did not receive the same degree of attention. A common feature of all FT catalysts, whether cobalt or iron based, or for low or high temperature FT application, is that they are reduced and carbided. Under these conditions, it is obvious that oxidation of the working catalyst has to be avoided or minimized. Under typical FT conditions, this oxidation can be caused by water. This water is produced directly by the hydrocarbon synthesis, according to equation 1. CO + (1 + x ) H 2 --~ CH2x + H 2 0

(1)

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2. S O M E N E G A T I V E E F F E C T S OF THE PARTIAL PRESSURE OF W A T E R ON THE FT PROCESS AND E F F E C T OF ITS IN-SITU REMOVAL

Although the effect of water on the deactivation of iron and cobalt based catalysts at low temperature FT [e.g. 2,6,7] and high temperature FT [e.g. 8] have been previously described, some comments are still necessary in order to have a clear picture. It is generally accepted that the rate of oxidation for iron and cobalt based catalysts increases with the water partial pressure. It is also accepted that the rate of oxidation is higher for iron based catalysts as compared to cobalt based ones. This means that an in-situ removal of the FT reaction water will result in a decrease of the oxidation rate, the decrease being more pronounced the higher the amount of water removed. In addition, water has an inhibiting effect on the rate of reaction for iron based FT catalysts. Therefore, for iron based catalysts, the removal of the reaction water will result in higher per pass conversions, due to a more favorable kinetic environment. In conclusion, and due to kinetic and\or oxidation considerations, the in-situ partial removal of the reaction water will result in an increase in the maximum practical per pass H2+CO conversions, therefore lowering the cost of the FT commercial plants based on both iron or cobalt FT catalysts. 3. USE OF MEMBRANES F O R IN-SITU REMOVAL OF FT REACTION WATER Several zeolite membranes and supports were tested (9) under a wide range of operating simulating conditions typical of SAS reactors (gas phase, 300 to 350 ~ 17 to 23 bar) and slurry bed reactor conditions (gas-liquid, 200 to 250 ~ about 20 bar) Mordenite, ZSM-5 and silicalite membranes were deposited on a stainless steel support. In addition, a mordenite membrane was deposited on an alumina support. They had a permeation area of about 8 to 9 cm 2. The mordenite membranes were prepared by in-situ hydrothermal synthesis onto a porous stainless steel support or onto a commercial c~-alumina support obtained from Societe des Ceramiques Techniques of France, following a procedure described by Salomon et al [10]. Significant amounts of ZSM-5 and chabazite were also found to be present in the zeolite material. The ZSM-5 membranes were deposited on a porous stainless steel tubular support following a procedure described by Coronas et al [11] while a silicalite membrane was deposited on a stainless steel support following a procedure described by Jia et al [ 12]. 4. EXPERIMENTAL RESULTS The conditions inside both the fluidized (SAS) and slurry bed FT reactors were simulated by feeding the main components inside each reactor at its typical operating

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temperature and pressure. hydrodynamic constrains.

No solids were used to simulate the catalyst due to

4.1 Gas Phase experiments The aim of the study was to evaluate the system capability to separate water (W) from other species (e.g. H2, CO2, CH4 and nCs as an example of higher hydrocarbons) at conditions similar to those found in SAS reactors. The zeolite membrane was placed in a stainless steel module and sealed by means of graphite gaskets. The gaseous components were fed as a multicomponent mixture (CO was added in some experiments) to the membrane tube side. The water and nC8 were fed as liquids by means of two mass flow controllers and passed through two evaporators operating at 400 ~ The mixed feedstream entered the tube side of the membrane module, while the shell side was swept with a flux of N2. Gas chromatography (TCD and FID) was used to analyze the composition of the permeate and retentate streams. The permeance for each component ( P e r i ) was calculated in the standard manner, using the molar flow rates at the permeate side, the partial pressure differences between the permeate and retentate side and the available permeation area. The water/speciesi selectivity (Swi) was calculated as the ratio of the permeances (equation 2), while the separation factors (Otw/i), were calculated as the quotient between the ratio of the molar fractions of water and any species "i" in the permeate and retentate side (equation 3).

Sw/i

=

Perw , i ~ W Peri

Yw / Yi Otw/i = ~ ,

i~ W

(2)

(3)

XW / Xi

Some of the data obtained for the gas phase reactor are shown in Table 1

4.2 Slurry phase experiments To simulate the conditions inside a typical commercial slurry FT reactor, the experimental set-up had to be modified in order to have a liquid phase inside the membrane tube. This was achieved by means of adding a stainless steel vessel containing nC8 to simulate the liquid hydrocarbons inside a FT slurry bed reactor. The level of liquid nC8 inside the tubular zeolitic membrane was controlled by means of a line between the nC8 vessel and the retentate side, therefore equalizing their pressure. The feed lines had to be modified accordingly. The N2 sweep gas was fed on the shell side of the tubular membrane while the water and other species (H2, CO, CO2 and CH4) were fed (bubbled) in the inside section of the tubular membrane. Some of the results obtained for the slurry bed FT reactor are shown in Table 2.

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Table 1 Results for the gas phase experiments Feed

Pt

T

P.

Memb [*C] [bar] [bar] A A A A A B B B B B B B B* B*

150 196 355 349 350 353 367 298 345 343 350 346 244 359

20.9 19.5 19.7 22.3 22.4 18.7 20.0 22.3 20.1 18.8 18.0 18.5 17.2 20.8

3.8 3.3 7.9 6.9 9.4 7.3 7.5 8.3 1.9 2.1 2.8 3.0 7.9 4.4

W

C8

H2

6.9 7.4 16.7 12.5 25.2 21.0 18.4 22.5 13.9 22.9 15.3 16.6 24.0 17.0

0.6 0.7 3.4 3.3 2.3 1.8 1.5 2.8 .1.2 1.2 0.9 1.3 0.5 0.4

13.0 14.0 12.7 10.8 14.6 13.4 12.4 13.7 47.4 71.5 24.7 26.8 14.0 32.0

Water Flux

6.4 5.9 12.8 9.1 22.4 8.3 5.4 8.4 3.1 3.3 4.2 4.1 9.5 11.7

Pt

CH 4 CO 2 CO

[bar]

N2

11.0 13.0 13.2 12.1 15.2 14.5 14.3 13.7 37.0 55.0 15.7 17.3 9.3 18.0

20.0 18.7 18.6 21.2 20.7 18.5 18.7 19.8 18.2 18.3 16.5 16.8 16.6 19.7

47.0 45.0 14.8 15.4 9.2 10.0 13.8 12.1 26.7 27.2 24.9 23.6 45.0 45.0

6.5 7.6 2.6 2.0 2.5 3.2 2.5 2.4 3.0 16.1 20.7 5.8 8.8 4.6 11.8 4.9 12.0 -

Separation Selectivity

Memb [kg .m-2.h-1] W/C8 A A A A A B B B B B B B B* B*

Permeate

Component flow [mmol/min]

80.0 63.0 9.7 8.6 1.4 8.7 4.1 5.5 12.7 20.0 11.7 9.8 25.4 23.5

72.1 44.5 5.4 5.1 1.0 7.0 4.3 4.1 14.4 16.5 6.3 5.3 41.5 25.5

58.3 10.7 10.0 5.9 1.3 3.8 3.7 4.5 4.8 5.1 10.6 5.3 13.8 36.6

W

Cs

4.7 4.3 9.7 6.9 16.9 7.8 5.1 6.0 2.3 2.4 3.1 3.0 14.0 8.6

0.0 0.0 0.6 0.7 1.4 0.1 0.1 0.1 0.0 0.0 0.2 0.3 0.0 0.0

H2

CH. CO 2 CO

0.4 0.3 0.3 0.5 3.7 3.6 3.2 3.6 10.1 10.1 3.2 1.4 2.2 1.2 1.2 0.6 1.6 0.6 1.4 0.5 1.8 0.7 1.8 0.7 0.4 0.3 2.9 1.2

0.6 0.1 0.6 0.5 1.6 0.4 0.2 0.2 0.7 0.5 0.1 0.2 0.5 0.6

Separation Factor

W/H2 W/CH W / C O W/CO 76.0 66.0 4.5 5.2 0.8 2.2 1.9 2.1 7.0 7.7 3.8 3.4 48.4 18.3

II

Component flow [mmol/min]

3.7 -

W/C 8 W/H 2 W/CH W/CO W/CO 69.5 51.7 3.7 4.7 1.3 6.8 3.7 4.6 9.8 14.9 9.8 8.5 22.4 13.0

72.9 54.1 2.0 2.9 0.9 1.8 1.8 1.8 5.5 5.9 3.3 3.0 42.9 10.2

66.0 36.6 2.3 3.8 1.0 5.4 3.9 3.5 11.1 12.4 5.4 4.6 36.7 14.1

21.0 8.7 3.8 3.3 1.2 3.0 3.4 3.8 3.8 4.0 8.9 4.6 12.1 20.3

A: Mordenite on alumina support B" ZSM-5 on stainless steel support B*" ZSM-5 on stainless steel support, sweep gas inside of membrane tube

3.0 -

2.2 0.4 0.6 -

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Table 2 Results for the slurry phase (simulation) experiments Permeate

Feed

T

Pt

Memb [~ B B C

C C C D D D

250 248 248 254 257 253 237 258 240

Pw

W

18.2 21.5 20.5 21.8 22.3 22.1 19.0 20.1 20.9

7.5 7.0 0.4 0.6 2.0 1.8 1.2 0.9 1.9

3.6 4.0 0.3 0.3 1.1 1.0 0.8 0.6 1.4

Water Flux

C s Flow

Memb [kg. m'2.h'l] [kg -m'2-h'l] B B C C C C D D D

4.9 6.1 0.2 0.7 2.0 2.2 1.1 0.6 0.8

Pt

Component flow [mmol/min]

[bar] [bar]

2.6 2.4 5.8 1.8 2.1 2.5 10.1 7.2 5.1

Ca -

-

H2

CH 4 CO 2 CO

12.0 13.0 11.9 12.8 18.3 4.5 22.5 5.4 24.6 4.1 22.5 5.0 10.4 13.0 16.2 7.8 18.3 7.4

5.4 5.7 0.4 0.4 0.3 0.4 5.3 1.7 0.4

-

0.6 -

-

[bar] 17.5 20.8 18.8 19.2 20.2 19.7 18.1 18.9 19.2

Component flow [mmol/min] N2

W

50.4 3.6 45.7 4.5 45.7 0.1 37.3 0.5 41.9 1.5 50.0 1.6 46.7 0.8 49.9 0.4 46.0 0.5

C8

H2

CH4 CO2 CO

0.3 0.3 0.6 0.2 0.2 0.3 1.1 0.8 0.6

2.3 0.7 0.4 6.8 4.0 8.1 0.0 0.4 0.0

1.8 2.0 0.0 1.7 0.5 0.7 0.0 0.1 0.0

0.5 0.2 0.0 0.1 0.0 0.1 0.0 0.4 0.0

Separation Factor W/H 2 W/CH W / C O W/CO 36.0 25.0 23.0 28.0 13.6 22.0 353.0 37.0 226

B- ZSM-5 on stainless steel support D: Mordenite on stainless steel

43.0 8.9 127.0 25.0 67.0 72.0 308.0 171.0 273.0

35.0 16.0 9.5 24.0 42.0 44.0 116.0 82.0 15.5

4.8 -

C" Silicate on stainless steel

5. DISCUSSION The following observations can be made with respect to the experimental data" The membranes are able to selectively separate the water from other species, operating at a wide range of temperature (i.e. 150 to about 360 ~ The membranes were effective with both a gas phase and two phases (liquid and gas) in the retentate side, therefore simulating a fluidized bed and a slurry bed FT reactor. The water flux tends to increase with the partial pressure of water in the feed, while the separation selectivities and separation factors tend to increase with the flow of sweep gas, in this case nitrogen.

2.4 -

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At low values (e.g. 150 and 196 ~ runs, table 1) temperature tends to increase the separation factors and the selectivity. This effect tends to diminish at temperatures of 240 ~ and higher.

For an industrial application, it would be preferable for the zeolitic membrane not to be in contact with the catalyst particles. This is to avoid possible attrition of the zeolitic layer by contact with the catalyst particles. The experiments performed by feeding the water and other species to be separated on the shell side, and the sweep gas on the zeolitic membrane side (internal part of the tubular membrane), have shown that there is no loss of water flux or separation selectivity when operating in this manner. 6. CONCLUSIONS The results obtained show that these membranes can be used for the highly selective, in-situ removal of the FT reaction water at operating conditions typical of those encountered in commercial fluidized bed and slurry bed reactors. The zeolitic membranes can be successfully deposited on a porous stainless steel support, which will facilitate their industrial application. Further developments may include the increase of the water flux rate, since this will have a direct impact on the number of tubes (membranes) to be fitted inside the FT reactor. 7. R E F E R E N C E S

1. B. Jager, R.C. Keltkens and A.P. Steynberg, Natural Gas Conversion II, H.E. Curt'y-Hyde and R.F. Howe (eds), Elsevier Science B.V. (1994) 419. 2. B. Jager and R.L. Espinoza, Catalysis Today, 23 (1995) 17. 3. B. Jager, M.E. Dry, T. Shingles and A.P. Steynberg, Catal. Lea., 7 (1990) 293. 4. R.L. Espinoza, J.L. Visagie, P.J. van Berge and F.H. Bolder, RSA Patent 952903 (1995). 5. R.L. Espinoza, P. Gibson and J.H. Scholtz, RSA Patent 982737 (1998). 6. R.L. Espinoza, A.P. Steynberg, B. Jager and A.C. Vosloo. Applied Catal., A: General 186 (1999) 13-26. 7. D.J. Duvenhage, R.L. Espinoza and N.J. Coville, Catalyst Deactivation 1994, B. Delmon and G.F. Froment (eds), Studies in Surface Science and Catalysis, Vol 88, Elsevier Science B.V. 8. A.P. Steynberg, R.L. Espinoza, B. Jager and A.C. Vosloo, in print, Applied Catal., A: General 186 (1999). 9. R.L. Espinoza, J. Santamaria, M Menendez, J. Coronas and S. Irusta, PCT/IB 99/01043 10. M.A. Salomon, J, Coronas, M. Menendez and J. Santamaria, Chem. Com., (1998) 125. 11. J. Coronas, J.L. Falconer and R.D. Noble, AIChE J., 43 (&) (1997) 1797. 12. M.D. Jia, B. Chen, R.D. Noble and J.L. Falconer, J. Membrane Sci., 90 (1994) 1.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Egg-Shell Catalyst for the S y n t h e s i s of Middle Distillates C.Galarraga b , E.Peluso b and H.de Lasa a aCREC, University of Western Ontario, N6A 5B9, London, Ontario, Canada bpDVSA-INTEVEP, Apdo 76343, Caracas 1070, Venezuela This study demonstrates that various preparation parameters of the eggshell catalyst affect the evolution of the eggshell thickness and consequently the metal distribution, the metal morphology and the metal crystallite size. In addition, it is shown that preparation conditions influence in a Co-Zr catalyst supported on silica, the production of middle distillates and particularly the C10-C20 hydrocarbon fraction. On this basis, it is established that an optimum eggshell catalyst should have 10wt% Co deposited in about the half radius of a 1.81mm diameter particle. This eggshell catalyst displays, under reaction conditions, encouraging selectivity, yielding 65wt% hydrocarbons in the diesel range. 1. I N T R O D U C T I O N The synthesis of middle distillate hydrocarbons via Fischer-Tropsch synthesis (FTS) is a process strongly influenced by intra-catalyst mass transport limitations. These mass transport limitations are due to the relatively slow diffusion of high-molecular weight paraffins inside the catalyst pores. To solve these problems eggshell catalysts have been proposed [1]. These eggshell catalysts are engineered with an active phase deposited in the outer region of the catalyst pellet and they can provide a suitable solution to overcome the diffusional problems commonly encountered in the Fischer-Tropsch synthesis. Even more, better understanding of the various preparation parameters can allow to take full advantage of the eggshell catalysts reducing intra-particle mass transport and achieving high yields of desired middle distillate paraffinic hydrocarbons.

2. EXPERIMENTAL PROCEDURES To prepare the eggshell catalysts a silica support (DAR-240 from UOP) with a 372m2/g specific surface area and a 1.81mm average particle diameter was selected [2]. The following steps were adopted: a) the desired amount of support was placed into a fritted glass funnel mounted on a flask connected to a vacuum system; b) the impregnating solution was poured onto the support in a volume ratio solution/support of about 5, c) after the desired impregnation time was reached (between 5 to 60 seconds) the excess

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solution was rapidly evacuated from to the flask by connecting the vacuum to the system. While for dry impregnation, the support was directly used as received from the silica manufacturer, for the case of wet impregnation the support was prewetted with water. No excess of water was allowed on top of the particles (or in between particles). In addition and to change the viscosity of the solution from 2 cP to 40 cP (0.1 g Co/ml solution), hydroxyethylcellulose (in a concentration of 1 wt %) was added in the water solution as viscosifying agent. The impregnating solution was constituted by a cobalt nitrate solution with close to 5 wt% zirconia promoter content (based on the combined Zr+Co metal content). Once the support impregnated it was transferred to a fluidized bed made out of sand particles (60 microns average size), and kept at 90~ The goal of this operation was to "freeze", at this temperature, the movement of the solution inside the pores of the support. This was achieved given the excellent heat transfer conditions in the fluidized bed enhancing fast drying. As well, the drying under fluidized conditions prevented the collapse of the catalyst porous structure given the close control of temperature in this unit. Completed this step the impregnated catalyst samples were calcined at 400~ for about 5 hours with the temperature being increased at the rate of 5~ This methodology was adopted following preliminary studies showing that at these conditions there is complete decomposition of the cobalt precursor [2]. Also under these conditions little interactions between the cobalt and silica are expected and as a result it is believed that most of the impregnated cobalt should be available as active species for the hydrocarbon synthesis reaction [4]. The adequacy of this method was also confirmed, in this study, using TPR and showing a high degree of reducibility of the cobalt species (72-98%). In the present study, changing various preparation conditions (refer to Table 1) five series of catalysts composed of 25 samples were prepared. In this respect, various impregnation times, viscosities of the impregnating solution, solution concentrations and state of the support before impregnation (dry or wet) were changed systematically in a quite wide range. Table 1. Preparation Conditions. Eggshell catalysts. Solution State of the Series Impregnation Viscosity of Concentration Support The solution Time (s) (gr cobalt/ml) (centipoises) Dry 0.10 4~5~6~7~8~12~16 2 A Wet 0.10 4-5~8,12,16,25 2 B Dry 0.20 4~5~12~20~40 5 C 0.20 Wet 4~10~20~40 5 D Dry 0.10 40 4,20,60 E Optical microscopy, SEM, nitrogen adsorption, hydrogen chemisorption and temperature programmed reduction were employed for catalyst characterization. The viscosity of the impregnating solution was measured using a falling ball viscometer Type 2[2].

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In addition, the prepared catalyst samples were evaluated under reaction conditions using a Berty laboratory scale reactor [3]. The experimental set-up included gas-pressurized cylinders, molecular sieves to trap gas impurities and low and high temperature separators to achieve adequate separation of the gas and liquid hydrocarbon fractions. Before the reaction experiments, the catalyst was activated under hydrogen flow with the temperature being progressively increasing as described in [2]. Following this the reactor was cooled down to 180~ The temperature was then, increased to 210~ under H/CO flow. Fortyeight hours of operation were allowed under these conditions to reach steady state. Additional details about the experimental procedures, used in this study, are reported by Galarraga [2] and Peluso[3]. 3.1. R E S U L T S AND DISCUSSION 3.1. Eggshell Formation Experiments Regarding the results obtained the following observations were established: a) Fig.1 reports the evolution of the eggshell thickness with impregnation time for selected cases of the catalysts listed in Table 1. It is shown that that the impregnating solution penetrates faster in the dry support than in the prewet particles. Thus, the state of the support (dry or wet) is of significance on the characteristics of the external particle film formed. In addition, as reported in Fig. 1, it is demonstrated that the viscosity and the cobalt concentration of the solution are two key parameters affecting the evolution of the cobalt film. Low concentration High concentration .0 AIA 1.07-0

~0.8r

=

0.6-

r

~0.4ho

0.2-

'•//

/

V /

~0.8-

I,

0

b/~ /

0

/

~0

.m HI/ / Hi

~0.6 ~0.4-

f

/vi

r

.2cP-01-D o2cP-01-W Y40cP-01-D

"~0.2-

/ /

m5cP-02-D [::]5cP-02-W

O.Q~. , , 0 20 40 60 Impregnation time (s) Fig.1. Evolution of the eggshell catalyst thickness with impregnation time. Codes: a) first digits" solution viscosity in centipoises, b) second digits" solution concentration in g Co/ml, c) D or W symbol: dry or wet support. b) Hydrogen chemisorption demonstrated that metal dispersion was in all cases smaller than 5% with wet impregnation giving the higher metal dispersions" 2-4% for wet support versus 0.3-1.1% for dry support. Crystallite sizes were in the 25-100nm range and this suggested the formation of metal 0.0,"

0 20 40 60 Impregnation time (s)

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agglomerates. Agglomeration was also confirmed w i t h SEM w i t h cobalt a g g l o m e r a t i o n being more i m p o r t a n t for c a t a l y s t s p r e p a r e d by dry impregnation versus the ones prepared via wet impregnation. c) EDX analysis was employed to confirm the distribution of Co and silicon across the spherical pellet. For catalysts prepared using wet impregnation a quite uniform metal profile with a diffuse ring of metal, gradually decreasing with the pellet radial position was observed. On the other hand, for the dry impregnated samples, the metal ring displayed a more irregular Co distribution. d) EDX was also used to confirm that the zirconium promoter was placed in the outer particle region and it was closely associated with cobalt.

3.2. Catalytic Activity and Selectivity in Eggshell Catalysts Experimental runs were developed in a Berty reactor to evaluate the catalytic activity of the prepared eggshell catalysts as follows: a) CO turnover frequency (TOF) for various impregnation times, b) product distribution for various cobalt contents. Typical operating conditions considered during these experiments were as follows: a) temperature:210 ~ Total Pressure:l.5MPa, GHSV=350h ~, H2/CO=2. These operating conditions were selected as being representative of suitable conditions to establish the effects of catalyst preparation on the CO conversion, the chain growth probability and the product distribution [3]. Table 2. Catalytic activity and selectivity for selected catalysts. Catalyst TOF Selectivity Codes

~mol CO converted]s/g Co

HC

H20

CO s

Reference

13.5

39.7

57.7

2.6

4s-2cP-01-D 12s-2cP-01-D 4s-2cP-01-W

16.7 18.7 13.3

43.7 43.6 43.3

55.3 54.2 56.8

1.0 2.2 0.0

20s-2cP-01-W

18.9

42.2

57.8

0.0

5s-5cP-02-D

3.8

42.2

57.8

0.0

25s-5cP-02-D

13.7

43.9

53.4

2.8

4s-5cP-02-W

6.1

41.4

53.8

4.8

20s-5cP-02-W

8.7

42.7

57.3

0.0

40s-5cP-02-W 6.8 42.8 57.2 0.0 Note: For catalyst codes refer to the caption of Fig.1. Regarding the CO turnover frequency, based on the unit weight of cobalt loaded, it was observed (Table 2) that the "reference" catalyst (12wt%Co) converted 13.5 ~Lrnoles of carbon monoxide per second per gram of cobalt. The observed selectivity for this catalyst was as follows: 39.9 % of hydrocarbons, 57.7 % of water and 2.6 % carbon monoxide.

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It was also noticed that the eggshell catalysts exhibited different activity trends depending on the preparation methodology and the degree of coverage (eggshell thickness). In fact those catalysts prepared from low concentration solution of cobalt (series 2cP-01) were the most actives. These catalysts had an activity comparable to the one of the "reference" catalyst (uniformly impregnated) and even more in some cases superseded the turnover frequency (TOF in ~moles]gCo.s) for the reference catalyst. In fact, the most active eggshell catalysts were those prepared with a low concentration of cobalt in the impregnating solution: series 2cP-01, wet (W) and dry (D) impregnation (refer to Table 3). In fact, for short impregnation times (4s) both catalysts (series 2cP-01) prepared from dry and wet impregnation converted as much carbon monoxide as the reference catalyst did. However, when the impregnation time was increased from 4s to 12s and 20s for both dry and wet impregnation the TOF activity was respectively improved up to 30%. Regarding the eggshell catalysts prepared from high concentrated solutions (0.2 g Co/ml) they did not compare well in terms of TOF with the reference catalyst or with the eggshell catalysts impregnated with low concentrated solutions. Generally, for series 5cP-02-D and series 5cP-02-W the TOF activity was about half of that observed, at the same impregnation time, for catalysts prepared with low concentrated solutions (0.1 g Co/ml). In addition, it appears that there is an optimum time for impregnating the eggshell catalysts since it appears there is a maximum TOF and this for both catalysts impregnated with both low and high viscosity solutions (or low and high concentration of cobalt in solution). It was also observed that the eggshell catalysts with the higher Co content exhibited a reduced surface area (about 15%) with respect to those with lower Co content. Consistent with this lower TOF were observed for the catalyst with higher Co levels. Regarding the hydrocarbon selectivity for the eggshell catalysts, it was found that the productivity of hydrocarbons ranged from 41 to 44 % while compared to about 40 % for the reference catalysts. This hydrocarbon selectivity improvement can be traced to an increased accessibility of active sites, smaller diffusivity constraints with more opportunity given to the reactants to reach active sites and to the reaction to be completed. Fig. 2 reports the performance of selected eggshell catalyst in terms of product distribution versus the reference catalyst. The following can be concluded: a) Eggshell catalysts prepared impregnating a dry support (Fig.2a), gave 5564% product yield in the Clo-C~o range versus 42% for the reference catalyst. b) Eggshell catalysts, prepared impregnating a wet support (Fig.2b), gave 4557% product yield in the C10-C~0range versus 42% for the reference catalyst. Furthermore, the C10-C20selectivity trends of Fig. 3 demonstrate that there is an optimum catalyst formulation to produce hydrocarbons in the C~o-C~0 range. This optimum meets a compromise between cobalt content (close to 10 wt%) and eggshell thickness (close to 0.5). On the other hand, when comparing the

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100-

400 different crystallite sizes it is also noticeable that to maximize the productivity of the Cto-C2o hydrocarbons, there is an optimum value of crystallite size (in the range of 67 - 89 nm), and consequently an optimum also for metal dispersion.

Product percentage, wt (%) a

80 60

t

'

40 20 0

CI

C 2 - C 4 C5-C9 C10-C20 C21+

CI

C2-C4 C5-C9 C10-C20 C21+

Fig.2. Product distribution for eggshell catalysts: a) dry impregnation, (b) wet impregnation. For codes refer to Fig.1. l~14s-2cP-01-D, r-112s-2cP-01-D,mReference

Productivity Clo-C2o range

100 80 60 40

-

(0.41) (0.47) ~, 67nm~-89nm (0.38) (0.~7) ~ ; ~ " " ~98nm 36nm~/ /P~ ~-, N[-~. ",~(0.75) (0.27) 27nm

20 I

(0.10) J ~ 50nm (1"00) 94nm

I

I

0 5 10 15 20 Cobalt content in the catalyst (wt%) Fig.3. C~o-C2oas a function of Co content. Crystallite sizes in nm. Eggshell thickness in between brackets. (.) 2cP-01-W.r ~n)5cP-02-D

REFERENCES

1. E. Iglesia, S. Soled, J. Baumgartner, and S. Reyes, J. Catal., 153, (1995) 108. 2. C. Galarraga, Master ofEngng. Sci., Univ. of Western Ontario, Canada (1998). 3. E. Peluso, Master of Engng.Sci., Univ. of Western Ontario, Canada (1998). 4. S. W. Ho, M. Houalla, and D. Hercules, J. Phys. Chem., 94 (1990) 6396.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

化

401

Computer-Aided Design of Novel Heterogeneous Combinatorial Computational Chemistry Approach

Catalysts

-

A

Kenji Yajima, Yusuke Ueda, Hirotaka Tsuruya, Tomonori Kanougi, Yasunod Oumi, S.Salai Cheettu Ammal, Seiichi Takami, Momoji Kubo and Akira Miyamoto Department of Materials Chemistry, Graduate School of Engineering, Tohoku University, Aoba-yama 07, Sendai 980-8579, Japan Combinatorial chemistry is an efficient technique for the synthesis and screening of large number of compounds. Recently, we have proposed a combinatorial computational chemistry approach and have applied it to design of deNOx catalysts. Various ion-exchanged ZSM-5 are good candidates for the removal of nitrogen oxides (NOx) from exhaust gases in the presence of excess oxygen. In the present study we investigated the adsorption energies of the NO and SOx on various ion-exchanged ZSM-5 by using a combinatorial computational chemistry. It was found that the Cu§ Fe2., Co2§ Irz* and TI3§ ion-exchanged ZSM-5 catalysts have a high resistance to SOx poisoning during the deNOx reaction. 1. INTRODUCTION Combinatorial chemistry has been developed as an experimental method which make possible to synthesize hundreds of samples at once and to examine their properties in detail. Originally the combinatorial chemistry was proposed and applied mainly to the synthesis of organic compounds. Recently it was introduced also into the inorganic chemistry fields such as thin films [1], luminous bodies [2], and magnetoresistances [3]. Combinatorial chemistry is expected to be as a highly efficient screening method even in the inorganic material synthesis. Computational chemistry is mainly used to elucidate the mechanism of catalytic reactions including also catalytic activity, deactivation, and so on. In addition to such investigations at atomic and electronic levels, computational chemistry is expected to have an important role to predict new catalysts with high activity, high selectivity, and high resistance to poisons.

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Recently we introduced the concept of the combinatorial approach to the computational chemistry for a catalyst design and proposed a new method denoted "a combinatorial computational chemistry" [4]. In this approach, the effects of a large number of metals, supports, and additives on the catalytic activity can be calculated systematically using computer simulation techniques, which can predict the best element for each catalytic reaction. Removal of nitrogen oxides (NOx) from exhaust gases in the presence of excess oxygen is a global problem. Much effort has been done on this process in the past decade. It has been reported that the selective catalytic reduction of NOx species by hydrocarbons can be catalyzed by various ion-exchanged ZSM-5 [5-13]. However, almost all ion-exchanged ZSM-5 were found to be poisoned by the water or SOx [13]. Moreover, various exchange cations presented in ZSM-5 exhibit a different activity, selectivity and durability, and one cannot compare those results directly, since the experimental conditions are often not the same. We have applied computational chemistry to the deNOx reaction on some ion-exchanged ZSM-5 [14-17]. Recently we applied combinatorial computational chemistry to search for novel effective exchange cations in ZSM-5 with high resistance to water poisoning for deNOx reaction and proposed new candidates [4]. In the present study we tried to propose novel effective exchange cations in ZSM-5 with high resistance to SOx using a combinatorial computational chemistry. The activity and durability of numerous ion-exchanged ZSM-5 were investigated and discussed. 2. METHOD OF CALCULATION AND MODELS Molecular Dynamics (MD) calculations were carded out with the MXDORTO program developed by Kawamura [18] to determine the structure of the zeolite framework. The Verlet algorithm was used to calculate the atomic motions, while the Ewald method was applied to calculate the electrostatic interactions. All MD calculations were performed under the following conditions: a temperature of 300 K, a pressure of 0.1 MPa, a time-step of 2.5 x 10-~s s, and a simulation time of 1000020000 steps. Quantum chemical calculations were based on Density Functional Theory (DFT) and were performed using the Amsterdam Density Functional (ADF) program [19] which employs a quasi-relativistic spin-unrestricted frozen-core mode. Basis sets were represented by the atomic Slater-type orbital and corresponded to a double zeta plus polarization basis. Geometry optimizations were carried out at the local density approximation (LDA) level with the VVVN exchange correlation functional.

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The corrections to the overestimated adsorption energies have been accounted for by a gradient correlation in terms of the Becke88-Perdew functional. The framework structure of the MFI-type silicalite without AI incorporation was obtained from the X-ray diffraction (XRD) data. In order to determine the framework structure of ZSM-5 with AI incorporation, one should to calculate the unit cell of ZSM5 containing all 288 atoms under the three-dimensional periodic boundary conditions. MD calculations were carried out in order to determine the framework structure of ZSM-5. The T12 sites were considered for the aluminum substitution. Eadier quantum chemical studies [20] have also reported that T12 site was energetically favorable for the incorporation of the aluminum. Various cations were selected as exchanged cations. We selected K*, Cu § Ag*, and Au § as monovalent cations, Fe2., Co2., Ni2., Cu 2., Zn 2., Pd2., and Pt2. as divalent cations, and AI3., Sc3., Cr3., Fe3., Co 3., Ga 3., In3., Ir3. and TI3. as trivalent cations. From experimental results [21-23] we assumed that monovalent cations existed as M§ (M = metal), divalent cations as M2*OH and trivalent cations as M3§ 2. During the MD calculations, the exchange cations were attached to the AI tetrahedral site. MD calculations have been performed on the framework structures of M§ M2*-ZSM-5 and M3*-ZSM-5, where M§ = Cu*; M 2. = Cu2*; M3* = Ga 3*. From the optimized structures, an AIO4-X (X -- exchange cation) cluster was extracted. Four hydrogen atoms were added to saturate the dangling bonds, so the cluster became as AI(OH)4-X. This system was used as a model for the active site in our DFT calculations. The positions of each distinct exchange cation were optimized. During the DFT calculations, AI(OH)4 fragment was fixed at the geometry of the framework structure of ZSM-5. 3. RESULTS AND DISCUSSION Many high active catalysts could not be industrialized because of their short lifetime. One of the important masons for the deactivation of catalysts is their poisoning by co-existent gases such as water and SOx (SO2, SO3). Hence, the investigation on adsorption of poisoning gases will provide an important information for the design of catalysts with high resistance to poisons. Experimentally it is well known that an ion-exchanged ZSM-5 can be easily deactivated in the presence of SOx. Hence, we calculated the adsorption energies of the NO and SOx molecules on various ion-exchanged ZSM-5. Here the adsorption energy (E,=) was defined according to the following equation:

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E,~ = E ( ~ , m , m ) -[E(zsM-s)+ E(mo~uk,)] Therefore a large negative H O value of E,~ indicates that the Ga S molecule strongly adsorbs on ZSM5. Fig.l shows the optimized configuration for the SO2 molecule on Ga3*-ZSM-5. In order to investigate the effect of exchanged cations on the Fig.l Optimized configuration for the adsorption of SO2, we analyzed the SO2 molecule on Ga3+-ZSM-5 charge on exchanged metal cations before adsorption. Fig.2 shows the correlation between the charge on an exchanged metal cation and the adsorption energy of SO2 molecule on various ion-exchanged ZSM-5. The exchange metal cations with a large positive charge attract more strongly the SO2 molecule. In order to evaluate the ability of different exchanged cations exhibiting resistance to SOx poisoning, the difference in the adsorption energies (A E) of the NO and SOx molecules on various ion-exchanged ZSM-5 were calculated and plotted in Fig.3. The positive value of A E indicates that the SOx molecule strongly adsorbs on the exchanged cation as compared to the NO molecule. Hence, the exchanged cation with the positive value is readily deactivated by SOx molecules. On the contrary, the

D

~-O

E

..,...

m

O

u

20

10

-

0

pd 2+ 9 9 N i 2+

-10

~ID - 2 0 ID " c. o

9C u 2+

9

2+ F e 2+ *

It3+ *

, Zn2+

pt 2*~

-30

o -40

Fe~+

Cr3+ 9

Co3+ 9

,..,.

g

O "~o

<

Tp +

-50 -60

In3+ 9 Ga3+

. S c 3+

A13+ 9

-70

0.5

1

1.5

Charge of exchange metal cation Fig.2 Correlation between the charge on an exchanged metal cation a n d the adsorption energy of SO2 on various ion-exchanged ZSM-5

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negative value A E indicates that the NO molecule strongly adsorbs as compared to the SOx molecule. Hence, the larger negative value is favorable to the deNOx catalysts. Among the monovalent cations, Cu § shows a strong resistance to SOx poisoning. Among the divalent cations, Fe2§and Co 2., and among the trivalent cations, I# § and TP* were found to exhibit a high resistance to SOx poisoning. Recently, the experimental studies have showed that Fe- and Co-ZSM-5 have a high resistance to SO2 [12], which is in a good agreement with our simulation results. Based on these results we proposed that the Ir3§ and TP* cations are new candidates for deNOx catalysts with a high resistance to SOx.

rl [-1 A Eads(NO-SO2)

50 -

I I A Eads(NO-SO3)

I

40 30 -

i~ 20 g

o

|c uJ

10

~

0 -10 -20

I

-30 t -40 Cu+

AlP

Au+

Fe2+ Co2+ C,u2+ Zn2+ Pd2+

Pt2+

AB+

Cr3+ Ga3+

In3+

Ir3+

TI3+

Exchange Cation

Fig.3 The difference in the adsorption energies of NO and SOx on various ion-exchanged ZSM-5 4. CONCLUSIONS Combinatorial computational chemistry method for a catalyst design. In the present resistance to SO2 and SOz were designed chemistry. From the calculation results, the Cu §

was found to be a novel innovative study deNOx catalysts with a high by a combinatorial computational Fe2., Co 2., Ir3. and TP* were found to

have a high resistance to SOx poisoning. Especially, our calculations predict that the Ir3. and TP* can serve as good catalysts for deNOx reactions with a high SOx resistance, which are not reported experimentally.

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REFERENCES [1] R.B. van Dover, L.F. Schneemeyer, R.M. Fleming, Nature 392 (1998) 162. [2] E. Danielson, J.H. Golden, E.W. McFarland, C.M. Reaves, W.H. Weinberg, X.D. Wu, Nature 389 (1997) 944. [3] G. Briceno, H. Chang, X. Sun, P.G. Schultz, X.D. Xiang, Science 270 (1995) 273. [4] K. Yajima, Y. Ueda, H. Tsuruya, T. Kanougi, Y. Oumi, S.S.C. Ammal, S. Takami, M. Kubo, A. Miyamoto, Appl. Catal. A, in press. [5] M. Iwamoto, H. Yahiro, Catal. Today 22 (1994) 5. [6] T. Tabata, M. Kokitsu, O. Okada, Appl. Catal. B 6 (1995) 225. [7] X. Feng, W.K. Hall, J. Catal. 166 (1997) 368. [8] Y. Li, P.J. Battavio, J.N. Armor, J. Catal. 142 (1993) 561. [9] M. Ogura, M. Hayashi, E. Kikuchi, Catal. Today 45 (1998) 139. [10] H.K. Shin, H. Hirabayashi, H. Yahiro, M. Watanabe, M. Iwamoto, Catal. Today 26 (1995) 13. [11] C. Yokoyama, M. Misono, J. Catal. 150 (1994) 9. [12] M. Iwamoto, A.M. Hernandez, T. Zengyo, Res. Chem. Intermed. 24 (1998) 115. [13] J.N. Armor, Catal. Today 26 (1995) 147. [14] A. Miyamoto, H. Himei, Y. Oka, E. Maruya, M. Katagiri, R, Vetrivel, M. Kubo, Catal. Today 22 (1994) 87. [15] H. Himei, M. Yamadaya, M. Kubo, R. Vetrivel, E. Broclawik, A. Miyamoto, J. Phys. Chem. 99 (1995) 12461. Y. Ueda, H. Tsuruya, T. Kanougi, Y. Oumi, M. Kubo, A. Chatterjee, K. Teraishi, [16] E. Broclawik, A. Miyamoto, ACS Symp. Series, p.321. [17] T. Kanougi, H. Tsuruya, Y. Oumi, A. Chatterjee, A. Fahmi, M. Kubo, A. Miyamoto, Appl. Surf. Sci. 130-132 (1998) 561. [18] K. Kawamura, in: F. Yonezawa (Ed.), Molecular Dynamics Simulation, Springer, Berlin, 1992, p. 88. [19] E.J. Baerends, D.E. Elis, P. Ros, Chem. Phys. 2 (1973)41. [20] E.G. Derouane, J.G. Fripiat, Zeolite 5 (1985) 165. [21] J.W. Ward, J. Phys. Chem. 72 (1968)4211. [22] G.D. Meitzner, E. Iglesia, J.E. Baumgartner, E.S. Huang, J. Catal. 140 (1993) 209. [23] X. Zhou, Z. Xu, T. Zhang, L. Lin, J. Mol. Catal. A 122 (1997) 125.

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化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Applications of density functional theory to identify reaction pathways for processes occurring in zeolites and on dispersed metal oxides Jason A. Ryder ~' 3, Mark J. Rice ~' 3, Francois Gilardoni L 3, Arup K. Chakraborty 2' 3.4, and Alexis T. Bell ~'3 Chemical Sciences Division ~ and Materials Sciences Division 2, Lawrence Berkeley National Laboratory and Departments of Chemical Engineering 3 and Chemistry 4, University of California, Berkeley, CA 94720, USA The use of quantum chemical calculations to represent elementary processes involved in catalyzed reactions is illustrated through several examples. Density functional theory is applied to obtain information about the dynamics of proton mobility in H-ZSM-5, the activation barriers for H202 synthesis over Pd-ZSM-5, and the initial steps in the oxidative dehydrogenation of C3H 8 over V205. 1. I N T R O D U C T I O N There is an increasing interest in the use of quantum chemical methods to describe the local structure and electronic properties of catalytic active centers as well as the energetics of adsorbate interactions with such centers (1). More recently, efforts have been undertaken to identify reaction pathways, and in particular the activation barriers for elementary processes involved in catalyzed reactions. There is also a growing interest in the use of quantum chemical methods to identify the most likely positions for free radical, electrophilic, and nucleophilic attack of either an active site or an adsorbed species by a gas-phase reactant (2). In the present study, density functional theory is used to examine three problems: the dynamics of proton migration in H-ZSM-5, the reaction pathways involved in the formation of hydrogen peroxide from hydrogen and oxygen over Pd-ZSM-5, and the pathway for initial proton transfer during the oxidative dehydrogenation (ODH) of propane over dispersed vanadia. 2. T H E O R E T I C A L APPROACH All calculations were carried out using a cluster to represent the catalytically active site or sites. For the studies of proton mobility in H-ZSM-5, the cluster contains 5-T atoms with AI located in the center of the cluster. The O atoms located at the edge of the cluster are constrained to their crystallographic positions in H-ZSM-5. These O atoms are terminated by H atoms located 1 A from the O atoms along a vector pointing in the direction of the next O-Si bond. A 1-T atom cluster is used for the initial studies of hydrogen peroxide formation; however, this work is being extended currently using a 5-T atom cluster. A Pd:* cation is bound to two of the four O atoms connected to the A1 atom. Dispersed vanadia is described by a cluster containing four V atoms situated in a suaacture representative of a single layer of V205. All of the calculations were carried using the density functional algorithm contained in the 3.5 release of the Jaguar code (Schrodinger, Inc.) or Gaussian 94. Results were obtained with the B3LYP, BH&HLYP, and BPW91 functionals. A 6-31G(d, p) basis set was used for most calculations.

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3. R E S U L T S AND D I S C U S S I O N

3.1

Proton

Motion

in H - Z S M - 5

~H NMR studies of dehydrated H-ZSM-5 indicate that at elevated temperatures the acidic protons hop between the four oxygen atoms bonded to each A1 site. The activation barrier for proton hopping was determined from DFT calculations the associated frequency factor was determined from absolute rate theory. Movement of the Bronsted acid proton from one O atom to another connected to the central A1 atom of the 5-T atom cluster occurs with a considerable change in the O-A1-O bond angle. As seen in Fig. 1, the equilibrium the O-A1-O bond angle of 96.9 ~ decreases to 83.3 ~ when the proton moves to the Transition State. The calculated activation barrier is 27 kcal/mol for the B3LYP functional and 32 kcal/mol for the BH&HLYP functional. These values are considerably higher than the value of 13 kcal/mol reported by Sauer et al. (3) based on HF calculations carded out with a fully relaxed 3-T cluster. The principal reason for the higher activation barrier reported here is the use of a partially constrained cluster. Calculations conducted with no constraints on the 5-T cluster but with all other aspects of the calculation the same, lead to a significant lowering of the activation barrier over that observed when the O atoms at the edge of the cluster are constrained to their crystallographic positions. The frequency factor for proton hopping estimated from absolute rate theory is 1.1xl0 ~3s ] at 600 K. 14.0

~- 9 II I

lm

12.0

99

9BH&HLYP 9B3LYP

r

~= 10.0 o

~ ~,,,i

o

~

9 1,,,,i

8.0

<= 6.0 o

m

9 < d

4.0 o 9

i

2.0-

011

0.0 0.0

0.2

0.4 0.6 Reaction Coordinate,

0.8

1.0

Fig. 1. Variation in the O-A1-O bond angle as a function of reaction coordinate occurring during the transfer of a proton from one to another O atom bonded to an AI atom in the T12 site of ZSM-5. The interaction of molecular water with the Bronsted acid proton of H-ZSM-5 facilitates proton transfer. For a single water molecule the activation barrier decreases to 6.9 kcal/mol. Here too it is observed that the calculated barrier is larger than the value of 2.0 kcal/mol reported by Krossner and Sauer (4) for DFT calculations conducted with a fully relaxed 5-T cluster,

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using the B P86 functional. In contrast to what is observed in the absence of water, the O-A1-O bond angle remains unchanged when proton transfer occurs in the presence of water. The effects of a small amount of adsorbed water on the apparent activation barrier for proton hopping were estimated using calculated enthalpies and entropies of adsorption. These calculations show that as little as few ppm of water will reduce the apparent activation energy from 32 kcal/mol for completely dry H-ZSM-5 to ~ 7 kcal/mol. The latter figure is within the range of activation energies measured experimentally on H-ZSM-5 dehydrated under vacuum at 773 K. What is implied by these results is that very small amounts of residual water can lower the apparent activation energy measured for proton hopping in H-ZSM-5. The process ZH + RH' = ZH' + RH (R = H, C H 3, C2H 5, etc.) involves both proton exchange between the O atoms bonded to an AI atom and proton exchange between the hydrocarbon and the zeolite. The relationship between the activation barrier for this process and the dissociation energy for are illustrated in Fig. 2. It is evident that to a first approximation the activation barrier increases with an increase in the energy for the heterolytic dissociation of the interacting species (RH). 42.0 02H6

40.0

o

"/

OH4

9 C3H8

38.0

~) 36.0 <

34.0 32.0

-

H2

30.0 400.0

I

I

I

410.0

420.0

430.0

440.0

Hetero]ytic Dissociation Energy, kcal/mo|

Fig. 2. Correlation of the activation energy for proton transfer in the process ZH + RH' = ZH' + RH (R = H, C H 3, C2H 5, etc.) with the energy for the heterolytic dissociation of RH. 3.2 Synthesis of H20 2 over Pd-ZSM-5

Recent studies have shown that the epoxidation of propene will occur over TS-1 containing Pd 2§ cations when a mixture containing C3H 6, H 2, and 02 is passed over the catalyst (5)~ Since propene can be epoxidized by hydrogen peroxide over TS-1, it is hypothesized that Pd§ is responsible for in situ formation of H202 from H 2 and 05. H202 formation from H 2 and 05 over Pd-ZSM-5 has also been reported. The present calculations were conducted to determine whether H202 could be formed on Pd 2§ cations in Pd-ZSM-5. A possible sequence of elementary steps for the formation of H202 and the competing reaction, the formation of H20, is shown in Fig. 3.

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410

~

.....

H

+I-Iz/-I-I20

:; ~

AGsoo -- -12.5

AE=-ll.9

ZPd(OH)

'~

"li

.. + 0 2

A AE=-38 5 ~ "

:

ill - ~ AGsoo = -4 7

~ ~t= +11i,m., ZPd(OOH) t~ =+8.o. . . . .

ZPd(H)

+Hz/-H20

t~t= +9.4 ( e ~ , _ . ~ AE =-53.8

y

ZPd(OH) Fig. 3. Calculated values of the internal energy change (AE), the Gibbs free energy change (AG), and the activation barrier (AE*) for elementary reactions involved in the reactions of H~ and O 2 over Pd-ZSM-5 to form H202 and H20. For Pd 2+exchanged into ZSM-5, the reaction is envisioned to initiate with the reduction of a Pd-OH group to form a Pd-H group, with then reacts with O 2 to form a Pd-OOH group. This latter group can react with H 2 for form either H202 or H20. Values of the change in the intemal energy (AE), the change in the Gibbs free energy change at 500 K (AG), and the activation barrier (AE*) are shown in Fig. 3 for each elementary step. There is no barrier (N.B.) for the first reaction in the sequence, and the barriers for the remaining steps are not exceptionally large. It is evident, as well, that the barriers for the formation of H20~ from H 2 and O 2 differs from that for the combustion of H 2 to H~O by less than 2 kcal/mol, even though the values of DG differ by nearly 50 kcal/mol. Assuming equivalent exponential preexponential values, the selectivity to H202 at 523 K would be approximately 16%. Thus, formation of H202 from H 2 and O 2 over individual Pd 2§ cations in Pd-ZSM-5 seems feasible and supports the experimental findings reported in ref. (5). 3.3 Oxidative Dehydrogenation of C 3 H s o v e r V20 s The oxidative dehydrogenation of propane to propene occurs over supported vanadia (6). Isotopic tracer studies have shown that lattice oxygen is involved in the dehydrogenation process (7). It is, therefore, of interest to identify which of the three generic types of oxygen in dispersed vanadia is involved in catalysis. It has been shown that there is a link between the sensitivity of the chemical system to reaction and the perturbation in the electron density of the system (2, 8, 9). The chemical perturbation theory is based on a density functional linear response formalism (8). Since the Kohn-Sham orbitals give the total ground state electron density of the system, they also provide a measure of both the global and local chemical reactivity of the system. The Fukui functions (FF) and related hardness/softness characteristics constitute the major chemical descriptors of molecular species as open systems, in which the number of electrons represents the continuous equilibrium variable. In the frozen core approximation FF represents the density of the relevant frontier molecular orbital involved in electron receiving (nucleophilic attack) or withdrawing (electrophilic attack) processes. Thus the local and condensed FF provide information on both the local reactivity and the hypothetical path for a reactant to interact with a substrate. In the present work, FF have been calculated for a molecular cluster representing the basal surface of V20 s. Experimental studies of propane ODH indicate that such a surface exhibits an catalytic activity similar to that of supported vanadia (10). Calculations of the condensed FF reveal that the apical (V=O) oxygens are

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expected to be the most likely centers for attack by an H atom of propane, followed by the bridged oxygen atoms (V-O-V). Exploration of the reaction pathway suggests that two V=O groups connected by a V-OV bridge are required for the initial activation of propane in good agreement with experimental observation (7). Figure 4 illustrates the reaction sequence calculated for the dissociative adsorption of propane to form isopropoxide and hydroxyl species. Notice that the initial interaction of C3Hs with a V=O group involves simultaneous interaction of C atom and one of the H atoms of the methylene group in CsH 8 with the apical O atom. This step is followed by the formation of a C-O bond and migration of an H atom to a second O atom associated with a second V=O group. The energy of reaction of propane to form normal and iso-propoxide species have been determined. AE = -28.4 kcal/mol for the formation of normal propoxide species and AE = - 36.8 kcal/mol for the formation of isopropoxide species. This result is in complete agreement with recent isotropic tracer studies which show that a kinetic isotope effect is observed when the rate of propane ODH measured using CH3CH2CH 3 is compared with that using CHaCD2CH 3. By contrast, no isotope effect is observed when the rate of propane ODH measured using CH3CH2CH 3 is compared with that measured using CD3CH2CD 3 (10). Since the dissociative adsorption of C3Hs has been shown to be irreversible and kinetically significant, both theory and experiment confirm that this first critical step occurs via release of an H atom from the 13-C atom of propane. The present results also explain why polvanadate groups, rather than monovanadate groups are more active for ODH over supported vanadia (7). As seen in Fig. 4, the initial activation of C3H 8 requires the presence of V=O groups in proximity.

... ~-. :

-

"

'

~ , .,.,.. , . . H ,~]!~

C

1,o H

~

.

~:~

......... il.

.

.

.

.

.

~

0 ~i:.:.:./ . . . . . . . . .

.

...

%..

~..o

::...;::.~.c ~-:"

H~

~-.-:-.-::C.!~

0

.

~

0

Fig. 4. An illustration of the interactions of C3H 8 with V---O groups located at the surface of V205.

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4.0 C O N C L U S I O N S

The results presented here demonstrate that density functional theory can be used to gain insights into the energetics of elementary processes occurring during catalysis over zeolites and metal oxides. Proton hopping within H-ZSM-5 involves considerable distortion of the O-AI-O bond angle. The activation barrier for proton hopping is reduced significantly by the presence of water and even trace amounts of water can influence the apparent activation energy for proton hopping. The activation barrier for proton transfer in the process ZH + RH' = ZH' + RH (R = H, CH 3, C2H 5, etc.) correlates with the energy for the heterolytic dissociation of RH. Calculations of the energetics of H~O2 synthesis via the reaction of H 2 with O 5 over Pd-ZSM-5 show that the precursor to H202 is Pd-OOH. Reaction of this species with H 2 can produce either H202 or H20. Since the activation barrier for the former process is only 1.7 kcal/mol lower than that for the former, it is estimated, assuming equivalent preexponential factors, that the selectivity to H202 will be 16% at 523 K. Condensed Fukui functions have been calculated for a cluster representation of V~O5 to determine the reactivity of different forms of oxygen to proton abstraction from propane. The O atoms associated with V---O groups are determined to be the most reactive. Analysis of the interactions of C3H 8 with the surface of V205 shows that two proximate V=O groups are required to form a propoxide and a hydroxyl group. The formation of an isopropoxide group is favored over the formation of a normal propoxide group energetically. These findings are in good agreement with those of recent experimental studies. ACKNOWLEDGMENTS This work was supported by the Director of the Office of Basic Energy Sciences, Chemical Sciences Division, of the U.S. Department of Energy under Contract DE-AC0376SF0098. REFERENCES

1. J.B. Moffat, Ed., Theoretical Aspects of Heterogeneous Catalysis, Van Nostrand Rheinhold, New York, 1990 2. H. Chermette, J. Computational Chem. 20, 129 (1999). 3. J. Sauer, C. Komel, C. M. Hill, and J. R. Ahlrich, Chem. Phys. Leit. 164, 193 (1989). 4. M. Krossner and J. Sauer, J. Phys. Chem. 100, 6199 (1996). 5. R. Meirs, U. Dingerdissen, and W. Holderich, J. Catal. 176, 376 (1998). 6. T. Blasko and J. M. Lopez Nieto, Appl. CataL A: General 157,157 (1997). 7. A. Khodakov, J. Yang, S. Su, E. Iglesia, and A. T. Bell, J. Catal. 177, 343 (1998). 8. R.G. Parr and W. Yang, Density Functional Theory of Atoms and Molecules, Oxford University Press, New York, 1989. 9. K.D. Sen, (ed.), Structure and Bonding, Vol. 80: Chemical Hardness, Springer-Verlag, Berlin, 1993. 10. K. Chen, A. T. Bell, and E. Iglesia, J. Catal., in press.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

化

413

Electrocatalytic Synthesis of Ammonia at Atmospheric Pressure . . .

George MameHos, G. Karagiannakis, S. Zisekas and Michael Stoukides Chemical Engineering Department and Chemical Process Engineering Research Institute, University Box 1517, University Campus, Thessalonild 54006, Greece Ammonia was successfully synthesized from nitrogen and hydrogen at atmospheric pressure in a solid-state proton-conducting cell-reactor. Two types of reactors were used, one double-chamber and one single-chamber cell. In the form of protons (I-F), hydrogen was electrochemically transported through the solid electrolyte, from anode to cathode (palladium) over which ammonia was synthesized. This novel process eliminates the thermodynamic requirement for high-pressure operation. I. INTRODUCTION In the last fifteen years, materials that exhibit protonic (H +) conductivity in the solid state have been discovered and introduced into heterogeneous catalytic systems [1]. These H + conductors are particularly useful in catalysis because they can operate at temperatures in which many industrial hydro- and dehydrogenation reactions take place [2]. One such reaction of major industrial importance is the synthesis of ammonia. Today, the dominant process for ammonia synthesis is the Haber process, which was developed at the beginning of the twentieth century and involves reaction of gaseous nitrogen and hydrogen on a Fe-based catalyst at high pressures (15 - 30 MPa):

N2+3H:

~

NH3

(1)

The conversion to ammonia is limited by thermodynamics. The gas volume decreases with reaction. Hence, very high pressures have to be used in order to push equilibrium to the fight according to the Le Chatelier principle. Also, the reaction is exothermic and therefore conversion increases with decreasing temperature. In order to achieve, however, industrially acceptable reaction rates, the reaction temperature must be high. The trade-off solution is to operate at temperatures in the range of 450 - 500~ at which the equilibrium conversion is of the order of 10 - 15%. In the present communication, an alternative route to ammonia synthesis at atmospheric pressure is presented. Solid-state proton (H +) conductors are used and hydrogen is supplied electrochemically. A model process using solid-state proton conductors to obtain conversions higher than those predicted by the reaction equilibrium, has been proposed in the past [3]. The

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principle is as follows. Gaseous H2 passing over the anode of the cell-reactor, will be converted to IT:

3H2

> 6H + +6e

(2)

The protons (H+) are transported through the solid electrolyte to the cathode where the halfcell reaction:

>2NH3

N2 + 6 H + + 6 e "

(3)

takes place. Thus, reaction (1) is again the overall reaction. Preliminary experimental verification of these model predictions has already been presented [4]. The present communication contains results obtained with both, a regular double-chamber and a single-chamber cell-reactor. 2. EXPERIMENTAL The experimental apparatus employed for the catalytic and electrocatalytic measurements has been described in a previous communication [4]. It consisted of the feed unit, the cellreactor and the analysis system. The reactants, N2 and H2 and the diluent He were of 99.999% purity. Schematic diagrams of the two cell-reactors are shown in Figures 1 and 2 respectively. The reactor of Figure 1 consisted of a non-porous ceramic tube (18 cm long, 1.25 cm inside diameter and 1.55 cm outside diameter) closed at the bottom end. The ceramic material was a strontia-ceria-ytterbia (SCY) perovskite of the form: SrCe0.95Yb0.0503. This solid has good mechanical strength'and high protonic conductivity [5]. The ceramic tube was enclosed in a quartz tube. Two porous polycrystalline palladium films were deposited on the inside and outside walls of the SCY tube and served as cathodic and anodic electrodes respectively.

I H2

rea~

j /""ILOO'Ok'VL

"/;

m

oTII +I,N2

H2

=_

]_

Cathode m

/

Anode .....

Cathode

"~":'~Anodr i

Figure 1: Schematic diagram of the Double-Chamber Reactor

Figure 2: Schematic Diagram of the Single-Chamber Reactor

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415

The single-chamber reactor-cell is presented schematically in Figure 2. The SCY disk (1-2ram thickness and 1.9 em diameter) was suspended into a quartz tube (18 em long, 2.75em inside diameter and 3.16 em outside diameter). Two porous polyerystalline palladium films (cathode and anode) were deposited on the two sides of the disk. The Pd electrodes were prepared from an Engelhard palladium paste (A2985), followed by calcination at 800 ~ for 2 hours. The superficial surface area of each electrode was 1.3 cm: (80 cm2 catalytic surface area) and 1 em2 (60 cm: catalytic surface area) for the double and single chamber reactor eortfigurations, respectively. Details on the catalyst preparation, construction of the cellreactors and analysis of reactants and products can be also found in previous communications [4, 6]. 3. RESULTS AND DISCUSSION 3.1. Double c h a m b e r reactor-cell

The rate of NH3 formation and the conversions of hydrogen and nitrogen, are shown in Figures 3a and 3b respectively, as a function of I/2F, where I is the imposed current and F is Faraday's constant. According to previous studies [7, 8], under the experimental conditions of the present study, the proton transference number is very close to unity; therefore, the ratio I/2F is equal to the electrochemical molar flux of hydrogen through the solid electrolyte. The cell of Figure 1 was kept at 570~ A mixture of 1.8% N2 in He was passed over the cathode at a volumetric rate of 8.3 " 10.8 m3/s and atmospheric total pressure. A flow of 5 . 0 10.7 m3/s of 100% hydrogen at atmospheric pressure was maintained over the anode. At I=0, no products were formed. Upon imposing a current through the cell, NH3 appeared at the cathode and, after a transient period of 2 to 6 min, a steady-state rate of NH3 formation was established. The two dotted lines in Fig. 3a are based on thermodynamic calculations and compare the present results with those that would have been obtained in a conventional catalytic reactor (CCR) with gaseous H2 equal to I/2F. It is also assumed that in the CCR, the

102

m

"6 E

z~

L z

~ , ~ . ~ p ~ . . . . 4 p . . , ~

10 -9

/

10"11

"16

-

A

lOS a"0 ~

1 oO

=

10.2

A

J

PCR

"---. . . . .

10 "13 1 0

~

lOS'~

,~176

,~w

,.--" CCR

e :

....

, ....

1

, ....

2

, ....

3

, ....

4

, ....

5

, . . . . . . . . .

6

7

8

112F (1 0 4 mol H21s)

Figure 3a: Dependence of the ammonia production rate on the electrochemical hydrogen pumping rate I/2F (T = 570~ PN2 = 1.8 kPa)

"~ >

8

= H2 =

: ..,

J

H (CCR) . . . . . . . . ....... "

104

104~

=

""'"

I

,

N2 (CCR) ,

,

' ....

.... 1 .... 2 .... 3 .... 4 .... 5

. '

6

....

' ....

7

8

112F ( 1 0 ~ m o l H 2Is )

Figure 3b: Dependence of the hydrogen and nitrogen conversion on the electrochemical hydrogen pumping rate I/2F (T=570~ PN2=l.8 kPa)

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residence time is so long that the exit molar flow rate of ammonia corresponds to the thermodynamic equilibrium of reaction (1) at 570~ and 1 bar. Hence, the CCR curve represents the maximum rate of NH3 production attainable in a conventional reactor. The NH3 production rates attained in the present experiments exceed the CCR rates by at least four orders of magnitude. Similarly, the curve denoted as PCR (pressure in a conventional reactor) represents the total pressure at which a CCR should operate so that the NH3 conversion reaches the values reported here. It can be seen that the calculated pressure values are of the order of 105 bars. In Fig. 3b, the percent conversions of 1-12 and N2 are plotted versus the rate of electrochemical supply of hydrogen, I/2F. It can be seen that 1-12 was almost completely converted into NH3. The corresponding conversion for N2 was considerably lower. This difference, however, cannot be attributed to low reactivity of N2. In these experiments, hydrogen was the limiting reactant and therefore, the rates of ammonia formation were very close to two-thirds the rates of hydrogen supply, according to the stoichiometry of reaction of ammonia synthesis.

3.2. Single chamber reactor-cell Figures 4a and 4b show the effect of electrochemical hydrogen pumping on the rate of ammonia synthesis and on the conversions of hydrogen and nitrogen respectively. The cell was kept at a temperature of 750~ A stoichiometric mixture of hydrogen and nitrogen was introduced into the reactor (2% nitrogen - 6% hydrogen) and helium was used again as diluent. Figure 4a shows that the rate of ammonia formation increases by increasing the current imposed to the cell, i.e., by increasing the electrochemical hydrogen-pumping rate l~om the anodic to the cathodic electrode. The ammonia rates experimentally achieved were at least 50 times greater than the corresponding CCR rates. Again, as denoted by the PCR curve, the total pressure at which a conventional catalytic reactor should operate in order to attain the current ammonia production rates is very high (of the order of 102-103 bar). 10 -r .=.

10 .8

E

10.8

10 s .....----'_....-..-----

104 "0

~

10 3 ~

,,m

(~

102 ;~

3:

~.z 10-10 10 "11q'

101

u"

"-I CCR ....

=

101 ~ s~Ioo

I/2F (10~mol H21s ) Figure 4a: Dependence of the ammonia production rate on the electrochemical hydrogen pumping rate I/2F (T = 750~ PN2 = 2 kPa, Pea = 6 kPa)

N=...~p.....~

10 0

Co In 10-1 c

o

0

10-2

N2 (CCR)

10.3~ . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

1

2

3

4

5

H~ (CCR)

6

7

8

9

II2F 110 ~ mol H21s) Figure 4b: Dependence of the hydrogen and nitrogen conversion on the electrochemical hydrogen pumping rate I/2F (T = 750~ P~2 =2kPa, P m = 6 kPa)

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Figure 4b shows the dependence of the percent conversions of H2 and N2 on the rate of electrochemical hydrogen supply, I/2F. The bottom dotted line corresponds to the maximum (equilibrium) hydrogen or nitrogen conversion that could be attained in a CCR. Similar to the case of the double chamber reactor-cell, the conversions achieved under closed circuit are much higher than those attained in a conventional catalytic reactor. The difference here is that the nitrogen and hydrogen conversions are equal to each other and this is expected because stoichiometric mixtures of these gases were introduced. 3.3. Discussion It is relatively well established that in the typical catalytic system for ammonia synthesis fie-based catalyst, pressures of the order of 100 bar, 450-500~ the rate-determining step is the dissociative adsorption Of the gaseous molecular nitrogen onto the catalyst's active sites [9]. The reaction conditions in the present work, were quite different: Pd catalyst, atmospheric pressure, temperatures between 570~ and 750~ and most important, the reactant hydrogen was supplied electrochemically either partly (in the single chamber reactor) or wholly (in the double chamber reactor). Hence, it is reasonable to expect either the reaction mechanism to be different or at least the rate-determining step to be different. In a number of experiments conducted in the double chamber reactor, it was found that the rate of ammonia formation was essentially independent of the partial pressure of N2 [4]. This observation indicates strongly that the reaction rate is not limited by the dissociative adsorption of N2. It is possible that the electrochemical transport of protons towards the cathode, induces an increase of the work function of the palladium electrode and therefore the dissociation of molecular nitrogen is facilitated. In the double chamber experiments and for all N2 pressures examined, nearly 75-80% of H2 was converted into NH3. At the temperatures of the experiments, ammonia decomposition should be expected. To quantitatively determine the degree of NH3 decomposition, experiments were done in which gaseous NH3 was introduced together with N: at flow rates equal to those used in the NH3 synthesis experiments. Indeed, it was found that NH3 decomposed into H2 and N2. Nevertheless, the measured decomposition was 20% or less. It is possible that the hydrogen found unreacted at the cathode had formed from the decomposition of NH3. The limiting reactant in the double chamber experiments was the electrochemicallysupplied hydrogen. It was found that a 6-fold increase in N2 partial pressure had essentially no effect on the rate of NH3 formation [4]. In order to s~dy intrinsic kinetics, the current density had to increase considerably. Higher I-I+ fluxes could be achieved at higher temperatures but this would also increase the rate of NH3 decomposition. On the other hand, in the single chamber experiments where hydrogen was not the limiting reactant, it was found that the rate of consumption of hydrogen to produce ammonia could be as much as double the rate of electrochemical transport of hydrogen through the solid electrolyte [10]. This indicates that in order to form one NH3 molecule, it is not necessary to supply all the H atoms electrochemically. Based on the above experimental observations, one possible reaction pathway at the cathodic electrode can be the following:

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H2 ~

2 Haas

(4)

N2 ~

2N.as

(5)

N.a, + H +

~- ~

NH~ds

NH~ds + e" ~

(rate determining step)

Nl-I,a,

NH3,~d~

(7)

(8)

NH~ds + Haas NH2,aa~ + Haas ~

(6)

NH3,ads

.~ ~ NH3

(9) (10)

This process offers an alternative route that permits operation at desired pressures and temperatures without the thermodynamic restrictions imposed on conventional catalytic reactors. This does not mean that the present data violate thermodynamics in any aspect. Simply, the final state of high conversion to NH3 is achieved by the consumption of electrical work by the system. This situation is similar to the case of H20 dissociation into H2 and 02. If this reaction were carried out at 25~ pressures of the order of 103o bar would be required to achieve quantitative conversion. Nevertheless, water can be quantitatively dissociated into its elements if electrical work is offered (electrolysis) at atmospheric pressure.

3.4. Acknowledgements We gratefully acknowledge financial support of this research by the General Secretariat of Science and Technology (PAVE 1997 Program) and by the Chemical Process Engineering Research Institute. REFERENCES 1. H. Iwahara, H. Uchida and S. Tanaka, J. Appl. Electrochem., 16 (1986) 663. 2. M. Stoukides, J. Appl. Electrochem., 25 (1995) 899. 3. E. Panagos, I. Voudouris and M. Stoukides, Chem. Eng. Sci., 51 (1996) 3175. 4. G. Marnellos and M. Stoukides, Science, 282 (1998) 98. 5. H. Iwahara, T. Esaka, H. Uchida and N. Maeda, Sol. St. Ionics, 3/4 (1981) 359. 6. C. Athanasiou, G. Marnellos, P. Tsiakaras and M. Stoukides, Ionics, 2 (1996) 253. 7. S. Hamakawa, T. Hibino and H. Iwahara, J. Electrochem. Soc., 140 (1993) 459. 8. P.-H. Chiang, D.Eng and M. Stoukides, Sol. St. Ionics, 61 (1993) 99. 9. C.N. Satterfield, Heterogeneous Catalysis in Practice, McGraw-Hill, N. Y., 1980. 10. G. Marnellos, PhD Thesis, Aristotle University, 1999.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Electrocatalysis at a Pt electrode surface in a fuel cell as observed in sire from Pt-H and Pt-O shape resonances and EXAFS scattering in Pt L2~ XANES D.E. Ramaker a'b and W.E. O'Grady b aChemistry Department, George Washington University, Washington, DC 20052 bChemistry Division, Naval Research Laboratory, Washington, DC 20375 Pt-H and Pt-O shape resonances are identified in Pt L23 XANES data obtained in situ at a Pt electrode surface. Unprecedented new information on the bonds formed by adsorbates at electrode surfaces, and their modification as a function of the electrode potential in a working fuel cell, is obtained. The data presented reveal that H remains near the surface well after it has been oxidized, and doesn't leave the surface until the hydrogen bonded water in the double layer region is disrupted enabling SO4= ions to directly adsorb. 1. INTRODUCTION The operation of a fuel cell depends critically on the electrocatalytic oxidation of hydrogen and reduction of oxygen at the respective Pt electrode surfaces. Comprehensive kinetic measurements on the oxygen reduction reaction have only recently been obtained for the low index planes of Pt [1]. However, the detailed mechanism for this electrocatalytic reaction is still not known. The rate is modified by the adsorption of anions, but how the anions interact with the surface or how these species effect the mechanism is not known. Is it a simple geometric blocking of the surface or is there some electronic effect? In the case of the catalytic oxidation of hydrogen, the mechanism on single crystal Pt surfaces was elaborated much earlier. However, even on well-defined surfaces it is not known how an adsorbed H atom or an H2 molecule interacts with the Pt surface prior to its oxidation or exactly how or when it leaves the surface. In a recent x-ray absorption (XANES) study of carbon supported Pt electrodes [2] an increase in the white line intensity was observed in the potential region where H is electrochemically adsorbed. Similar increases in the whiteline areas have been observed after gas phase adsorption of H2 by a number of workers. Of course changes are also seen when Pt is oxidized or oxygen is chemisorbed. Most often these effects are attributed to changes in the occupancy of the Pt d-orbitals at the Fermi level. We show in this work, that these changes can be attributed to the formation of shape resonances between an adsorbate and Pt. These shape resonances provide an important tool to obtain unprecedented new detail on the chemisorption bond between H, O, H2, 02 and SO4- and a Pt electrode surface obtained in situ in an electrochemical cell or working fuel cell.

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6 4 2

E4 -6 -10

1.4

Pt/C electrode in 0.5 M H2SO 4 H2 H

SO4=

L3

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O

~ O

I I 0.0

I 0.2

II 0.4

I I

I

I

I

89

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0.2 . J 0.0 11560

-.

"'

,,I

H/Pt (0.0 V)

-i

i

i

,

-

,i

~ (o.~,i v) o/Pt (1.14 v ) i

i

i

11565 11570 11575 Photon Energy (eV)

I~so,

0.6 0.8 1.0 1.2 Volts vs. RHE

f

/

0.6

E 0.4

z

/

11580

,

'1.6

Figure 1 Current vs. voltage curve for a Pt electrode in 1 M H2804. Also shown are the expected regions where H, O, SO4= direct anion adsorption, and H2, adsorption would dominate. Finally the applied voltages where the L2 and L3 absorption edges were measured in perchloric acid, and the L3 edges in sulfuric acid electrolyte are indicated.

._~ 1.0 _.=

~

L2

~--..

0.8 0.6

/ l

o 0.2 J z 0.0

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13260

'

/

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H/Pt (0.0 V)

-

Pt

-

o/pt (1.14 V)

I

i

(0.54 v) J

13265 13270 13275 Photon Energy (eV)

i

13280

Figure 2 Comparison of the L3 and L2 edges of the XANES spectra for a Pt electrode in HC104 electrolyte, at the indicated potentials relative to the RHE.

2. EXPERIMENTAL In this work, the Pt electrode was formed of highly dispersed 1.5-3.0 nm particles supported on carbon prepared as described previously [3]. The electrochemical cell measurements were performed in HCIO4 and H2SO4 electrolytes with the XAS data collected at room temperature in the transmission mode on beamline X-11A of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory. The NSLS storage ring operated at 2.52 GeV beam energy with ring currents between 40 and 200 mA. The m situ fuel cell data utilized 85% phosphoric acid as the electrolyte held in a 0.42 mm thick porous matrix. The fuel cell data were collect in the normal transmission mode at the Stanford Synchrotron Radiation Laboratory (SSRL) on beam line 1-5 with the beam energy at 3.0 GeV and operating current between 90 and 40 mA. These in situ measurements were taken with the fuel cell operating at different voltages by changing the external load on the cell. Figure 1 shows the cyclic voltamogram for a Pt/C electrode recorded in 1M H2SO4. We have labeled the regions where H2, H, O, and anion adsorption should occur. The potentials where the XAS data were recorded in the H2SO4 and in the HCIO4 are also indicated. The L3 edges were recorded in both electrolytes at all the potentials while the L2 edges were only recorded in the HCIO4. Fig. 2 shows the L2 and L3 edge regions at 0.0, 0.54 and 1.14 V, revealing the changes which occur due to the adsorption. 3. T H E O R Y A N D A N A L Y S I S

Adsorbate (Ad = H, O, H2, OOH, 8042. etc.) chemisorption induces a bonding and an anti-bonding orbital as reported by Hammer et al. [4] for Pt-H. The Pt-Ad bonding orbital is

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primarily localised on the Ad atom, and the anti-bonding state (AS) is localised on the surface Pt atoms. The anti-bonding state can be viewed as a localised state degenerate with a continuum state, here described by the Pt-H XAFS final state wave function. The outgoing photoelectron will reside temporarily in the potential well determined by the AS state and can escape producing a resonance with the Pt-H XAFS final state wave function. This process produces a resonance with the well known Fano-like profile. Its effect on the scattering cross section c(E) can be related to an EXAFS function x(E) via the normal expression 6(E) = vt(E)(I+ z(E)). It can be shown [5] that: z(E) = 1/k 9Asin tO 9[(1-qe)/(l+e2)],

(1)

with A an amplitude factor, q = cot cOand e = (E-F_~s)/F. cOcan be related to the usual total phase found in EXAFS containing the 2kr term and the phase from the absorber and backscatterer, e is the normalised energy scale relative to the resonance energy (Er~s), and F represents the resonance width. A fit to the experimentally observed AS lineshape gives values for F_~s,F, A and ~p.

,

Table 1 Principle Sam.pie Ad/Pt Pt ,

information present in the Pt L3 and L,2 near edge SlY.ctra ...L3edge . . . . L2 edge REF + AXAFS + AVB + AS REF + AXAFS REF + AVB REF . . . . . . .

,

,,

,

Table 1 summarises the important contributions that will be visible, based on the assumptions and theory described previously. At 0.54 V (vs RHE) the Pt electrode lies in the double layer region where there is no adsorption of the C104-anion. Therefore, the L2 edge spectrum at this voltage was used as a reference (REF), since this spectrum arises solely from the "free" atom absorption and the EXAFS contributions. The L3 spectrum of the clean Pt cluster contains in addition the electronic (empty valence band: AVB) contribution. The L2 spectrum for the Ad-Pt sample is different from the REF spectrum, became of changes in geometry of the cluster induced by chemisorption (changes in XAFS scattering: AXAFS). Finally the L3 spectrum for the Ad/Pt sample contains both the geometric (AXAFS) and electronic (AVB + AS) changes from the reference. By taking the difference between the spectra at 0.0 and 0.54 V for example, the Pt-H antibonding state (electronic effect) is isolated The difference between the spectra at 1.14 and 0.54 V allows isolation of the Pt-O antibonding state and the Pt-O EXAFS scattering. Similar difference spectra allowed Pt-O22, Pt-OOH-, Pt-OH, Pt-PO43 or Pt-SO42 and Pt-O "= shape resonances to be observed in situ in an electrochemical cell and even in a working fuel cell at different external loads. 4. RESULTS Figures 3 and 4 show the H and O adsorption results in HC104. Figures 3a and 4a show in each case AL2 and AL3 obtained by taking the difference in the aligned spectra at the potentials indicated. The AL2 spectra are dominated by the change in EXAFS contributions

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化

HCIO4

HCIO 4

0.09

0.2

0.07

~ 0.1

0.05 0.03

u

0.01 C

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/~

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(a)

AL2=aXAFs

0.0 -0.1

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0.01

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0.00

-0.03

O/Pt -Pt (1.14-0.54)

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~

~--~s

30

40

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N,3-~~ as (b)

0 10 20 30 40 Energy (eV rel. L 2 edge)

50

60

-0.05 -20 -10

0

10

20

Energy (eV rel. I~ edge)

50

60

Fig. 4 Similar spectra as in Fig. 3a and 3b, but for the O/Pt case.

Fig. 3 a) Comparison of the difference spectra H/Pt- Pt at potentials 0.0-0.54V and 0.240.54 V for a Pt electrode in HCIO4 electrolyte. AL2 = AXAFS (light solid) and AL3 = AXAFS + AS (heavy solid) differences are compared with the Pt-H Z (dashed line) calculated with FEFF7. The larger amplitude in each case comes from the larger voltage difference. The AL2 data and Pt-H XAFS have been offset by 0.03 for clarity. b) Comparison of the Fano resonance lineshape obtained from the experimental difference AL3 - AL2- AS (heavy solid lines for both potential differences) and the least squares fit of the Fano expression in eq. (1) (light solid). The data and fit for 0.24-0.54 have been offset by 0.01 for clarity. (AXAFS), the largest component due to the H or O adsorption. This is clear after comparing the experimental AL2 spectra with the Pt-H or Pt-O Z function calculated with the FEFF7 code (Fig. 3a and 4a respectively). Because of the different backscattering phases, the Pt-H and Pt-O EXAFS functions are very different. Agreement between theory and experiment is very good, considering the sensitivities of Z to the chosen potential parameters in FEFF7. Major differences between theory and experiment allow us to identify other contributions in the spectra. Within 5 eV of the edge, the shape resonance or anti-bonding state, AS, contribution dominates. The AS contribution can be isolated by taking the difference, AL3 - AL2, as shown in Figs. 3b and 4b. For both the H and O adsorbates, the AS contribution appears very close to and even below zero. The most significant disagreement between theory and experiment above 10 eV comes around 25 eV for both O and H adsorption. This feature arises from the change in the Pt-Pt bond distance as O or H is adsorbed (i.e. it is the second component in the AXAFS contribution). It is known that the Pt-Pt distance in small particles like those used here decreases due to the lack of full coordination of the surface Pt atoms [5]. This distance relaxes back to normal when the H or O is adsorbed. In Figs. 3b and 4b, we

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423

also show the fit of the Fano line shape to the experimental line shape. The Fano parameters obtained from the fits are indicated in Table 2. The Fano line shapes for O and H have a totally different shape because the background EXAFS phase q0 (Eq. 1) is very different, as mentioned above. Another significant point is the large difference in the Gaussian line broadening required for H compared to O. We attribute this to the much larger DebyeWaller factor expected for the very light H atoms compared to the much heavier O atoms. Table 2 Summary of Fan0fit parameters 0btained..by the fit of eq. (4) to the data for HCIO4,. ....... A F_~s(eV) F (eV) (1) Gausian broadening (eV) (+ 0.03) (_+0.03) (_+0.4) (_+0.1) H/Pt ( 0.0 -0.54) 0.07 -2.0 3.0 -0.71 5 H/Pt (-0.24-0.54) 0.05 -1.6 2.9 -0.90 5 O/Pt (1.!4-0.54) 0.21 2.2 0.8 0.06 1.5 ,

,

H

i

H

H

1.2

t4

1.0

~

,

,

H

H

H

H res. scat.

I

H

0.8

I

H

H

H i

H

!

H

"O

| 0.6 .N

Pt at 0.0 V

m

F- 0.4

o ~" 0 . 2

0.0

_,--,-:--: "

.

,

.

.

,

.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Potential vs RHE (Volts)

Fig. 5 Comparison of H reduction, resonance scattering intensity in HC104 (open squares) and H2SO4 (closed squares) electrolyte, and direct sulfate adsorption [6] with change in potential (all normalized to one).

H H H H H H \O / ~O / \O / '%H H / %' J %' H H "r H H H H \o / ~o/ \o / ! II ! I H* H* H H*

P t at 0.3 V

Fig. 6 Illustration of the double layer and hydrogen bonding at the surface, with the H § being held in place.

The amplitude of both the AS and the AXAFS increases with the coverage. Fig. 5 compares the normalized AS amplitude in HCIO4 and H2804 along with the H-coverage from anodic charge, and the direct adsorption of SO42- [6]. These results reveal that the H is reduced to I-I+ (as indicated by the anodic charge) well before it leaves the surface (as followed by the AS resonance scattering intensity). Further, it appears that when the H § leaves the surface, SO42 ions directly adsorb. These data suggest that hydrogen doesn't totally leave the surface as illustrated in Fig. 6 until the hydrogen-bonded water in the double layer region is disrupted. Once the double layer water is disrupted, SO4- ions can directly adsorb on the surface. This is also consistent with the recently reported disruption of the double layer at 0.35 V as seen via the H20 absorption bands in FTIR spectroscopy [7].

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5. SUMMARY

From these studies [8], most of which are not reported here because of space limitations, we have found the following: 1. The Pt-O and Pt-H resonance energies are found to move upward exactly as expected with increasing applied potential. The resonance energy for the Pt-H bond is found to be negative, similar to that found elsewhere for H chemisorbed on large Pt clusters supported on zeolites or amorphous supports in the gas phase[5]. The Pt-O resonance is positive reflecting the stronger Pt-O bond. 2. The Pt-H2 interaction in this electrochemical environment involves H2 bonded with its internuclear axis perpendicular to the surface with a Pt-H distance of 1.9 A and the H-H internuclear bond distance nearly unchanged from that in the gas phase. 3. As shown here, the intensity of the Pt-H resonance enables us to "watch" the H leave the surface with increasing anodic voltages. Comparison of this coverage with the anodic charge reveals that the H remains near the surface well after it has been oxidized, and doesn't leave the surface until the hydrogen bonded water in the double layer region is disrupted enabling SO4- ion to directly adsorb. 4. The energy and line shape of the Pt-oxygen shape resonance has a ve~ strong dependence on the nature of the oxygen species; for example chemisorbed 02 , OOH, OH, or 02. These changes can be correlated with the strength of the Pt-O interaction and the electron affinity of the adsorbate species. 5. The most stable species found at the surface is strongly dependent on the adsorbate coverage, which strongly determines the efficiency of the fuel cell (e.g. whether the 2 or 4 electron transfer mechanisms are preferred). 6. Perhaps, most important, the species found at different fuel cell potentials, although consistent with some previous 2 and 4 electron transfer mechanisms, point to a very strong electronic structure effect at the Pt electrode. The resonances and XAFS contributions found in this work have provided important and unprecedented new information on the bonds formed at electrode surfaces, and how they are modified as a function of the electrode potential in a working fuel cell. REFERENCES

1. N. Markovic, H. Gasteiger, and P.N.Ross J. Electrochem. Soc. 144 (1997) 1591. 2. S. Mukerjee, S. Srinivasan, M. P. Soriaga, and J. McBreen, J. Electrochem. Soc. 142 (1995) 1409; S. Mukerjee and S. Srinivasan J. Electroanal. Chem. 357 (1993) 201. 3. J. McBreen, W.E. O'Grady, K.I. Pandya, R.W. Hoffman, and D.E. Sayers, Langmuir 3 (1987) 428. 4. B. Hammer and J.K. Norskov, Nature 376 (1995) 238; Surf. Sci. 343 (1995) 211. 5. D.E. Ramaker, B.L. Mojet, J.T. Miller, and D.C. Koningsberger, Phys. Chem. Chem. Phys. 1 (1999) 2293. 6. M.E. Bamboa-Aldeco, E. Herrero, P.S. Zelenay, and A Wieckowski, J. Electroanal. Chem. 384 (1993) 451. 7. T. Iwasita and X. Xia, J. Electroanal. Chem. 411 (1996) 85. 8. W.E. O' Grady and D.E. Ramaker, J. Synchrotron Rad. 6 (1999) 599; also to be published.

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Studies in Surface Science and Catalysis 130 A. Con'na, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

In situ Catalytic and Electrocatalytic studies of Internal Reforming in Solid Oxide Fuel Cells running on Natural Gas C.M. Finnerty, R.H. Cunningham and R.M. Ormerod* Birchall Centre for Inorganic Chemistry and Materials Science, Department of Chemistry, Keele University, Staffordshire, ST5 5BG, United Kingdom * to whom correspondence should be addressed

Abstract

A test systembased around a thin-walled extruded solid electrolyte tube has been developed which enables the fuel reforming catalysis and surface chemistry occurring within solid oxide fuel cells to be studied under real operating conditions. It permits simultaneous monitoring of the catalytic chemistry and fuel cell performance, allowing a direct correlation between the cell output and the anode reforming characteristics. Using this system internal methane reforming over nickel/zirconia cermet anodes has been studied. The influence of the anode formulation, anode pretreatment, operating temperature and methane/steam ratio on the methane reforming characteristics, the nature and level of carbon deposition, and durability have been investigated under actual operating conditions. Pre-reducing the anodes in H2 at 1173 K leads to a more active reforming catalyst. Carbon deposited during reforming is removed from the anodes in two processes during temperature programmed oxidation. There is an increase in the temperature of both carbon removal processes and an increase in the population of the higher temperature state with increasing reforming temperature.

1. INTRODUCTION Fuel cells offer tremendous potential as a more efficient and cleaner alternative method of electricity generation than conventional methods. Solid oxide fuel cells (SOFCs) offer advantages over other fuel cell systems because the high operating temperatures allow flexibility in the choice of fuel, and the possibility of running the cell directly on natural gas or other liquid hydrocarbon fuels, internally reforming the fuel within the fuel cell [1,2]. This also leads to increased operational efficiency through chemical recuperation of waste heat into the fuel supply and simplifies the balance of plant. However, there are several major problems associated with internal fuel reforming in SOFCs, in particular the problem of carbon deposition on the anode at the high operating temperatures involved, which leads to deactivation and a loss of cell performance and poor durability. The deposition of carbon on the fuel reforming anode in SOFCs running on natural gas is thus an important area requiring further study, and there is much interest in developing optimised anode formulations for internally reforming SOFCs [3-6]. The SOFC anode is invariably nickel-based and can be considered to be somewhat analogous to supported nickel methane steam reforming catalysts, where the formation of carbon deposits has been extensively studied [7-9]. It is thus possible to study the catalytic behaviour of nickel-based anodes in powder form inside a conventional catalytic reactor;

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this approach has been demonstrated by several research groups, including ourselves [2,10,11]. There are a genuine lack of studies in which the fuel reforming catalysis has been studied in an actual SOFC. We have developed an SOFC test system based on a small diameter, thin-walled, extruded yttriastabilised zirconia tubular reactor which can be used to study the internal reforming catalysis and surface chemistry in SOFCs, in particular the problems of carbon deposition on the anode and poor durability, as well as the electrical performance of the SOFC [4,12-14]. This allows a direct correlation between the cell performance and the internal reforming characteristics of the cell. Hence different anode formulations and pre-treatment procedures can be readily evaluated over a range of operating conditions [3-6,15] In this paper we describe how this system has been used to study internal reforming in SOFCs, reporting bdefly the results of a detailed study of direct internal reforming over nickel/zirconia cermet anodes, investigating the influence of anode composition, anode pre-treatment, steam/methane ratio and cell operating temperature. 2. EXPERIMENTAL

All the experiments described here were carded out using the SOFC test system developed in our laboratory [4,12-14,16]. Briefly the apparatus consists of a linear temperature controlled furnace. The test cell inlet is linked to a stainless-steel gas manifold which allows complete flexibility in gas handling, gas composition, the choice of fuel and fuel/steam ratio. Thus evaluation is possible over a full range of operating conditions and fuel compositions. The reactor outlet is linked via a heated gas sampling system to an on-line mass spectrometer (Leda-Mass Satellite) which permits fuel processing in the actual SOFC to be continuously monitored under operating conditions and the chemistry occurring at the anode surface to be investigated using temperature programmed spectroscopy. A particular advantage of the tubular SOFC design is that it can be heated in the furnace in the same way as a conventional catalytic reactor. As zirconia is a good thermal insulator the ends of the electrolyte tube which project beyond the furnace remain sufficiently cool for a gas tight seal to be made, even when the inside of the furnace is at elevated temperatures. The system enables either a conventional reactor or an extruded solid electrolyte reactor to be used. The SOFC anodes used in this work were prepared by physically mixing nickel oxide (Alfa Chemicals) with 8 mol% yttria-stabilised zirconia (Unitec-FYT11), and adding this to a solvent. The resultant slurry was milled for three hours, with a small quantity of binding agent added at the end of the milling period. The anode sample can then be studied in the powder form, following firing, in a conventional reactor [11]. However, in this case the anode slurry is coated onto the inside of the fired zirconia tube. Following drying in air the coated zirconia tubes were fired to 1573 K. Two nickel Ioadings were used in the anodes, corresponding to 50 vol% NiO and 90 vol% NiO. Strontium-doped lanthanum manganite was used as the cathode. Following firing the tubular SOFC was sealed into the test system. Pre-reduction of the anode was carded out in situ at 1173 K for 30 minutes in a 10% H2/He stream. Steam reforming was carded out by passing methane/steam mixtures over the fired and pre-reduced anodes at reaction temperature. Postreaction temperature programmed oxidation (TPO) was used to characterise the nature and amount of carbon deposited during each reforming experiment. TPO measurements were carded out in a 10% O2/He mixture using a heating rate of 10 K min-1. 3. IN SlTU CATALYTICAND ELECTROCATALYTICMEASUREMENTS

The system thus allows the catalytic performance of the fuel reforming anode of a solid oxide fuel cell to be continuously monitored under real operating conditions. The real-time sampling of the on-line

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mass spectrometer enables transient phenomena to be observed, and allows temperature programmed reaction measurements to be carded out on the cells, providing information on the anode reduction characteristics, methane activation and steam reforming, as well as enabling carbon deposition to be studied. The identical experimental arrangment can be used to carry out electrochemical measurements on the same cell, allowing a direct correlation between the anode reforming characteristics and the fuel cell performance. Furthermore, the test system allows cell performance and electrocatalytic measurements to be made simultaneously on a working SOFC [12-14].

4. RESULTS 4.1 Influence of methane steam reforming temperature The influence of operating temperature on the methane steam reforming activity and carbon deposition was studied by passing a 5:1 methane/steam mixture over each anode, which had been prereduced in H2 at 1173 K, at reaction temperatures of 1023 K, 1073 K, 1123 K and 1173 K; these temperatures covering the most likely operating temperature range of zirconia-based SOFCs. These results are summarised in Table 1. Table 1. Methane conversion over 50 vol% and 90 vol% nickel cermet anodes as a function of reforming temperature (methane/steam ratio = 5:1). Reforming Tem~rature/K 1023 1073 1123 1173

i

' ' % Methane Conversion ................ 50 vol% Ni/zirconia ' '. 90 vol% Ni(zirconia 31.6 31.1 34.0 35.5 34.5 39.9 40.9 49.2 ,

,

,,

,,

The most noticeable differences occur at the higher reforming temperatures, where the 90 vol% Ni cermet shows higher activity; at lower reforming temperatures the anodes show very similar activity. Under these conditions both anodes show very little degradation with time, indeed the 90 vol% anode shows a small increase in methane conversion over one hour at 1123 K and 1173 K suggesting steadystate has not been reached. There are also noticeable differences in the product selectivities between the two anodes. Table 2 shows the CO selectivity for both anodes over the temperature range 1023 K to 1173 K for the 5:1 methane/steam reforming mixture. Both anodes show very high CO selectivity, and water conversion, under these conditions. However, it can be seen that the CO selectivity is always higher on the 90 vol% nickel cermet; the H2/CO ratio is also always higher for this anode. The CO selectivity of both anodes increases with reforming temperature. TPO measurements were carded out after each reforming experiment to determine the quantity of carbon deposited, as well as the strength of interaction of the carbon with the anode. Under these conditions both anodes showed a minimum in carbon deposition at a reforming temperature of 1073 K, with no carbon deposition detected on the 90 vol% Ni anode. Carbon deposition is similar on both anodes, though at all temperatures less carbon deposition occurs on the 90 vol% Ni anode. The nature of the carbon deposited also changes with reaction conditions (temperature, methane/steam ratio).

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Table 2 CO selectivity over 50 vol% and 90 vol% nickel cermet anodes as a function of reforming temperature (methane/steam ratio = 5:1). Reforming Temperature/K , 1023 1073 1123 1173 i

i

' CO selectivity,,/% . . . . . . . . 50 vol% Ni/zirconia 90 vol% Ni!zircon'ia' 95.4 96.7 96.2 97.6 97.3 98.2 98.1 98.8 i

|

i

i

Significantly increased carbon deposition occurs on the 50 voi% Ni anode following reforming at 1173 K compared to 1123 K. At both reaction temperatures the carbon is removed by temperature programmed oxidation in two processes. The carbon deposited during reforming at 1123 K is removed at 876 K and 927 K, whilst the carbon deposited during reforming at 1173 K is removed at 903 K and 950 K. In addition to the carbon being removed at higher temperatures following reforming at 1173 K, indicating greater stability, the higher temperature carbon species predominates. Following reforming at 1173 K on the 90 vol% Ni anode, carbon is again removed in two stages, the CO2 desorbing principally in a peak at 927 K, with a small low temperature shoulder at 895 K, whilst at 1123 K, two CO2 desorption peaks are observed at 870 K and 902 K. Thus, although both anodes show similar carbon removal temperatures, carbon is removed at lower temperature on the 90 vol% Ni anode, and the higher temperature form of carbon is relatively more prevalent. 4.2 Influence of methane/steam ratio The influence of methane/steam ratio on the methane reforming characteristics of the two anodes, in particular the carbon deposition, was studied. Methane/steam ratios ranging from 5:1 to 15:1 were studied at a reaction temperature of 1173 K. Table 3 summarises the methane conversion over both anodes after 30 minutes reforming at 1173 K. A clear correlation between methane conversion and methane/steam ratio can be seen, with activity decreasing with decreasing water content. The decrease in activity over the 90 vol% Ni cermet is considerably more marked than that over the 50 vol% Ni cermet. At the highest methane/steam ratio of 15:1 the initial methane conversion over both anodes is similar, ~25%. There is then a small loss of activity over both anodes over the first 10 minutes. However, thereafter there is no further decrease in activity over the 50 vol% Ni anode, whereas the activity continues to decrease over the 90 vol% anode, indicating that deactivation is rapidly occurring.

Table 3 Influence of methane/steam ratio on methane conversion over 50 vol% and 90 vol% nickel cermet anodes during reforming at 1173 K. Methane/steam ratio

i

i

7.5 10 15

i

i

i

% Methane Conversion 5()'vo1% Ni/zirconia ' 90 voi~o Ni/zirc0nia...... 38.0 45.0 23.6 24.9 22.1 21.8 21.5 14.1 i

i

ii

ii

i

i

i

i|

i

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The nature of the carbon deposited and the relative amounts of the two carbon species present alter as the methane/steam ratio is increased. At low methane/steam ratios only the low temperature species is present. As the methane/steam ratio is increased the higher temperature species is also formed, and at the highest methane/steam ratio this is the only carbon species formed, being removed at 950 K from both anodes.

4.3 Influence of reforming time TPO measurements carried out following reforming suggest that the low temperature carbon species can be aged or transformed into the higher temperature species with increased reforming time [17]. Similar behaviour has been observed for supported nickel steam reforming catalysts [9]. Extended reforming experiments were carried out at 1123 K and 1173 K using a methane/steam ratio of 5:1. Under these conditions the 90 vol% Ni anode shows excellent stability with only a very small decrease in methane conversion over a 48 hour pedod. In contrast the 50 vol% anode shows an initial substantial decrease in activity, which is then observed to vary considerably with time, suggesting anode deactivation and regeneration is occurring. This is supported by the variation in water conversion and H2/CO ratio which follow and mirror the methane conversion, respectively. Post-reaction TPO reveals only one carbon species, removed at 927 K from the 90 vol% Ni cermet and at 940 K from the 50 vol% Ni cermet, indicating that after a certain reaction time only one form of carbon exists. Under these reforming conditions ten times less carbon is deposited on the 90 vol% Ni cermet than on the 50 vol% anode. Thus the 90 vol% anode is both more stable for steam reforming and has a higher resistance to carbon deposition under these conditions. 4.4 Influence of pre-reduction of the anode The effect of running the anode directly in the methane/steam fuel mix without pre-reducing the anode in H2 was investigated. Experiments were carded out at 1173 K using a methane/steam ratio of 5:1. After initially very high methane conversion corresponding to reduction of the nickel oxide, steadystate steam reforming is observed. Under these conditions conversions of 41.9 % and 33.0 % were observed over the 90 vol% and 50 vol% Ni anodes, respectively, compared to 49.2 % and 40.9 % conversions on the same anodes following pre-reduction in H2 at 1173 K. Thus pre-reducing the anode produces a more active reforming catalyst. Interestingly, following reforming over the unreduced anodes at 1173 K in a 5:1 methane/steam mixture, ten times more carbon is deposited on the 50 vol% Ni anode than on the 90 vol% Ni anode, whereas the amount of carbon deposition is only slightly less on the prereduced 90 vol% anode; the major difference being on the 90 vol% Ni anode where six times less carbon is deposited on the unreduced anode. In both cases the carbon deposited on the unreduced anodes is removed at lower temperatures than on the pre-reduced anodes. Again both carbon species are observed, with the higher temperature state dominating. 5.

SUMMARY

The methane reforming characteristics of two different nickel/zirconia cermet anodes, with 50 vol% Ni and 90 vol% Ni, have been studied over the temperature range 1023 K to 1173 K, and a range of methane/steam ratios. The 90 vol% anode shows higher activity than the 50 vol% anode at higher reforming temperatures, whereas both anodes show very similar activity at lower reforming temperatures. As expected the activity increases significantly with increased reforming temperature. Carbon is removed from the anodes in two processes, suggesting two types of carbon species. As the reforming temperature is increased carbon deposition increases, with an increase in the temperature of both carbon removal processes and an increase in the population of the higher temperature state.

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As the methane/steam ratio is increased the reforming activity decreases, with a much more significant drop in activity occurring over the 90 vol% anode. At higher methane/steam ratios only the high temperature carbon species is formed. Following extended reforming the higher temperature state is also the only carbon species formed, suggesting that the lower temperature carbon species can be aged or transformed into the higher temperature species with increasing reforming time. Compadson of anodes pre-reduced in H2 and unreduced anodes shows that anode pre-treatment is very important. Although the unreduced anodes are active for reforming pre-reducing the anode leads to a more active reforming catalyst. However, significantly less carbon deposition occurs over the non prereduced 90 vol% Ni anode. In summary it can be concluded that the factors influencing methane conversion and carbon deposition are complex, and that the precise operating conditions, in terms of temperature and fuel/steam ratio, have a very significant beadng on the optimum choice of anode formulation and anode pre-treatment. 6. ACKNOWLEDGEMENTS

This work was supported by the UK Engineedng and Physical Sciences Research Council under grant GR/K58647. The EPSRC Clean Technology Programme is also acknowledged for the award of a studentship to CMF. 7. REFERENCES

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

A.L. Dicks, J. Power Sources, 61 (1996) 113. E. Achenbach and E. Riensche, J. Power Sources, 52 (1994) 283. C.M. Finnerty, N.J. Coe, R.H. Cunningham and R.M. Ormerod, Catal. Today, 46, 1998, 137. R.M. Ormerod, Stud. Surf. Sci. Catal., 122 (1999)35. C.M. Finnerty, R.H. Cunningham and R.M. Orrnerod, Proc. 3rd European SOFC Forum, 1998, 217. C.M. Finnerty, R.H. Cunningham and R.M. Orrnerod, Proc. 6th Int. Symp. on SOFCs, The Electrochemical Soc., 1999, in press. S.C.Tsang, J.B. Claddge and M.L.H. Green, Catal. Today, 23 (1994) 3. O. Yamazaki, K. Tomishige and K. Fujimoto, Appl. Catal. A:, 136 (1996) 49. J.R. Rostrup-Nielsen and L.J. Christiansen, Appl. Catal. A:, 126 (1995) 381. R.T. Baker and I.S. Metcalfe, Appl. Catal. A:, 126 (1995) 297. N.J. Coe, R.H. Cunningham and R.M. Ormerod, Catal. Lett., 49 (1997) 189. C.M. Finnerty, R.H. Cunningham, K. Kendall and R.M. Ormerod, J. Chem. Soc. Chem. Commun., (1998) 915. C.M. Finnerty, R.H. Cunningham and R.M. Ormerod, Proc. 3rd European SOFC Forum, 1998, 227. C.M. Finnerty, R.H. Cunningham and R.M. Ormerod, Solid State lonics, 1999, in press. R.H. Cunningham, C.M. Finnerty and R.M. Ormerod, Proc. 5th Int. Symp. on SOFCs, The Electrochemical Soc., 1997, 973. R.H. Cunningham, C.M. Finnerty, K. Kendall and R.M. Ormerod, Proc. 5th Int. Symp. on SOFCs, The Electrochemical Soc., 1997, 965. C.M. Finnerty and R.M. Orrnerod, in preparation.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Effects of Gd2Oa d o p i n g and s t e a m / c a r b o n ratio on the activity of the c a t a l y s t for i n t e r n a l s t e a m r e f o r m i n g in m o l t e n c a r b o n a t e fuel cell Young Jun Shin a, Hyeung-Dae Moon a, Tae-Hoon Lim b, and Ho-In Lee a aSchool of Chemical Engineering, Seoul National University, San 56-1, Shinlim-dong, Kwanak-ku, Seoul 151-742, Korea bBattery and Fuel Cell Research Center, Korea Institute of Science and Technology, P. O. Box 131, Cheongryang, Seoul 130-650, Korea The effect of Gd203 doping on the catalyst Ni/MgO was studied. The Gd doping suppressed coke formation, and enhanced catalytic activity for steam reforming reaction. The Gd doping seemed to activate the dissociative adsorption of water to increase OH concentration on the catalyst as if steam feed was increased. BET surface area and Ni dispersion were increased with the increase of Gd addition till 10 wt% referenced to Ni weight, and then decreased due to the segregation of Gd203 from the mixed compound of Gd and MgO. The increase of Ni dispersion was one of the reasons to suppress coke formation. 1. I N T R O D U C T I O N A catalyst in the molten carbonate fuel cell of direct internal reforming type (DIR-MCFC) should be both active for the steam reforming of hydrocarbons at the working temperature of MCFC and substantially resistant to deactivation in the atmosphere of hot electrolyte vapor inside the cell. However, the activity of such a reforming catalyst usually reduces with the lapse of time due to the contact with the vapors of alkali metal carbonates and/or alkali metal hydroxides produced from electrolyte components, K2CO3 and Li2CO3 [1] and also due to coke formation [2]. Even though the coke formation on the catalyst Ni/MgO is generally suppressed by increasing the content of steam in the feed [2], the ratio of steam/carbon (S/C) can not be increased over a proper level in DIR-MCFC because the more electrolyte loss happens with the higher S/C ratio through the form of hot alkali hydroxide vapor [1]. So, the coke formation must be suppressed by the addition of promoter. For current commercial nickel catalysts, alkali or alkali earth element is adopted as a promoter to depress carbon deposition, but these additives also decrease the catalytic activity [3]. The purpose of the present study is to investigate the effect of Gd doping to suppress coke formation without additional steam feed.

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2. E X P E R I M E N T A L

The catalyst of 25 wt% N i ~ g O designated to '25N-M' was doped with lanthanide element (Ln), and was prepared through such a procedure as impregnation of Mg(OH)2 to a mixed solution of Ni and Ln nitrates followed by drying, liquid phase oxidation [4], and final reduction under H2 atmosphere. The amount of added Ln element (La, Ce, Pr, Nd, Gd, and Ho) was evaluated by wt% on the basis of Ni loading, and the Gd-added catalysts were designated as xG25N-M, where x is wt% of Gd referenced to Ni loading, and x = 0, 1, 5, 10, 50, and 100. The catalytic activity for steam reforming of methane was measured in a fixedbed continuous-flow reactor. The standard experimental conditions were 650 ~ of temperature, 20 cc/min of CH4 feed, 2.5 of S/C, and 25 mg Ni of catalyst amount. To observe the effect of Gd addition on coke formation, the S/C was varied from 2.0 to 3.5 by step of 0.5. Chemical and physical properties of the catalysts were analyzed by XRD, CHN, TPR, BET, and He chemisorption. 3. R E S U L T S AND D I S C U S S I O N The carbon deposition process has been widely studied. It is considered that the tendency of carbon deposition depends on the nature of surface carbon species and crystalline plane of surface Ni. The carbon species can either react with water to form products (hydrogen, carbon monoxide, carbon dioxide and methane) or pass through a series of steps leading to carbon deposition. So the carbon deposition process can be regarded as a competitive reaction with steam reforming. A general idea to minimize coke deposition requires minimal coke formation and maximum coke removal by gasification of carbon or intermediates which can lead to carbon. Table 1. Steam reforming activities of catalysts Ni/MgO doped by LneO~ Doped element (Ln)

H2 yield (~mol/g. sec)

CH4 conversion (%)

H2 selectivity (H2/CH4)

La Ce

432 419 402

75 72 70

3.73 3.81 3.76

Pr

424

74

3.75

Nd Gd Ho

429 461 433

74 75 75

3.78 3.90 3.68

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The doping effect of 10 wt% Ln203 (Ln : La, Ce, Pr, Nd, Gd, and Ho) on the catalyst 25N-M for the internal steam reforming in a DIR-MCFC was studied. The activities of catalysts 10Ln25N-M were summarized in Table 1. The Gdadded catalyst (10G25N-M) showed the highest values of both H2 yield (461 ~mol/g. sec) and H2 selectivity (3.92). A commercial Ni/MgO catalyst for DIRMCFC showed much lower H2 yield (319 ~mol/g. sec) and H2 selectivity (3.42) than the catalyst 10G25N-M. So, Gd was selected as a promising co-catalyst for the internal steam reforming catalyst, Ni/MgO, in molten carbonate fuel cell. Fig. 1 shows the variation of 100, 4.0 catalytic activity with the increase of 90 1 Gd amount doped. Both H2 yield and 3.8 H2 selectivity exhibited volcano 80 1patterns with the increase of Gd 3.6 addition giving maximum values of 60H2 yield (461 pmol/g, sec) and H2 3.4 selectivity (3.92) at 10 wt% of Gd. 40 Fig. 2 shows the XRD patterns of 30 3.2 various Gd-doped Ni/MgO catalysts. 0 1 5 10 50 100 Only for the catalysts whose amount Amount of Gd (wt%) of Gd was beyond 10 wt%, Gd in the Yield of H2 (10 ;amol/g x sec) catalysts existed as a separated ,~-,~ Conversion of CH 4 (%) Gd203 phase. The added Gd element . Selectivity for H 2 (H2/CH4) initially reacted with Ni or MgO upto some saturation extent (about Fig. 1. Activity vs. amount of Gd doped 10 wt% for Ni), and the excess Gd for catalysts xG25N-M. element was separated as Gd203 on the surface of the catalyst Ni/MgO. It seems that the separated Gd203 phase does not O contribute to the activity, but i 100G25N-M:! O Q ~.0 9 rather blocks the catalytic active sites resulting in the decrease of v activity. Zhuang et al. [5] reported that 50G25N-M~_.~.j tO cerium oxide decreases the carbon deposition on the nickel catalyst, and increases the catalytic steam 10G25N-M reforming activity in contrast with alkali or alkaline earth elements, I I 1 I I l and claimed t h a t lower-valence10 20 30 40 50 60 70 80 state cerium(III) oxide produced 2 Theta during s t e a m reforming reaction Fig. 2. XRD patterns of various catalysts may adsorb dissociatively water, xG25N-M. (O; Gd203, e; MgO, m; Ni) and the r e s u l t a n t s p e c i e s - 0 o r OH may be t r a n s f e r r e d to nickel

5~

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and reacted with surface carbon species to form carbon monoxide, carbon dioxide and hydrogen. Similar results were observed with a lanthanide promoter [6]. Rosynek [7] reported t h a t although lanthanide oxide itself can oxidize hydrocarbons its activity is several orders lower t h a n those of other transition metal oxides. Then, how can the promoting action of Ga doping in the catalyst Ni/MgO take place? To elucidate the effect of Gd addition, the S/C ratio in the feed was reduced from 3.5 to 2.0. If m e t h a n e and steam are converted completely to H2 and CO2 without any side reaction including coke formation, the H2 selectivity has an ideal value of 4.0 (4 moles of H2 per mole of CH4). If m e t h a n e is partially converted to coke, the H2 selectivity shows lower value t h a n the theoretically ideal one. Fig. 3 shows the effects of both steam and Gd addition on the "-~ ~ 8/C=2 0 carbon formation. The H2 selectivity -1- 4.2 ! S/C=2.5 o i for the catalyst Ni/MgO containing "--. ~ -v-S/C=3 0 4.0 ~ S/C=3.5 no Gd was increased in proportion to H20 feed (S/C). The self-made --9 i catalyst 25N-M had higher H~ o 3.8 selectivity t h a n the commercial Ni/MgO catalyst due to the difference ~ 3.6 of preparation method. The reduction "T" , I I I of carbon deposition by increase of 0 10 20 30 40 50 steam feed is generally observed, and Amount of Gd (wt%)

it is convinced that the increase of surface OH concentration induced by H20 addition diminishes the coke formation by enhancing the rate of gasification of carbon [5]. Such a kind of tendency dependent on the increase of S/C was the same for the Gd-added catalysts. However, for the catalyst 10G25N-M, the H2 selectivity (HJCH4) was over 3.9 even at the smaller steam feed (S/C = 2.5), which is the same H2 selectivity as that observed in 25N-M at high steam feed (S/C = 3.5). Even the lowest H2 selectivity (3.54) at 2.0 of S/C for the 10G25N-M was higher than that (3.42) of a commercial Ni/MgO catalyst at 2.5 of S/C, proving that the catalyst 10G25N-M is a good catalyst for internal steam reforming in a DIRMCFC. On the other hand, the H2 selectivity was increased proportionally with the amount of Gd addition at the same S/C condition. The catalyst 10G25N-M showed the highest value (3.98) at 3.0 of S/C suggesting t h a t m e t h a n e was almost ideally converted to H2 without coke accumulation. Doping of Gd had the same effect as the increase of steam feed did, that is, the increase of OH concentration on the catalyst. So, it is confirmed t h a t Gd element will inhibit the coke formation by activating the dissociative adsorption of water to increase the surface OH concentration [8] like cerium oxide [5]. In summary, the coke formation in the fuel reforming process of MCFC could be suppressed Fig. 3. H2 selectivity (H2/CH4) vs. amount of Gd doping at various steam/carbon ratios (S/C).

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by introducing Gd element to the catalyst NYMgO without any additional H20 feed. Beyond 10 wt% Gd, Gd addition showed a little enhancing effect or a slightly negative effect depending on the value of S/C, suggesting the separation of Gd203 or too much high OH concentration. So, the optimum values of Gd addition and S/C for the catalyst xG25N-M are suggested to be 10 wt% and 3.0, respectively. Table 2. Physical properties of various Gd-doped catalysts, xG25N-M Catalyst 0G25N-M

Amount of coke (wt%) 0.271

TPR peak temperature 442

Ni dispersion (%) 3.6

1G25N-M 5G25N-M 10G25N-M 50G25N-M 100G25N-M

0.27G 0.252 0.241 0.221 0.21,~

440 453 455 468 480

4.4 4.6 8.1 4.6 1.3

BET surface area (m2/g) 81 80 87 107 84 63

To study the reason why the Gd addition less than 10 wt% enhanced the catalytic activity and suppressed the coke formation with the increase of Gd amount, various analyses including TPR were performed, and the physical properties of the catalysts xG25N-M were summarized in Table 2. The amount of carbon deposited during 6 hrs operation at S/C=2.0 was decreased with the addition of Gd (from 0.28 to 0.22 wt% of C), and the amount of carbon was inversely proportional to the H2 selectivity (H2/CH4). Simultaneously, the reduction temperature of NiO which was directly contacted with MgO support was increased from 442 (0G25N-M) to 455 (10G25N-M) in cases of no segregation of Gd203, suggesting that Gd doping increases the bond strength between NiO and MgO support to resulting in the increase of Ni dispersion shown as in Table 2. It was reported that Ni(111) plane mostly observed in large Ni particles was the active sites for coke formation [9]. And Yamazaki et al. [10] claimed that the reduction of nickel particle size can suppress the coke formation. This suggests that the other function of Gd addition is the reduction of Ni particle size resulting in the increase of Ni dispersion. The diminution of catalytic activities beyond 10 wt% Gd (cf. Fig. 1) is well explained by the reductions of both BET surface area and Ni dispersion (cf. Table 2) caused by the segregation of Gd203 from the Gd-doped Ni/MgO catalyst (cf. Fig. 2). Fig. 4 presents the pore size distributions of the catalysts xG25N-M. Average pore diameter was enlarged with the amount of Gd doping. It was revealed from the observation that Gd doped on the catalyst Ni/MgO not only affects the bond strength between NiO and MgO, but also reacts with MgO support to form an uniform solid solution below saturation limit (cf. Fig. 2) and changes pore structure (cf. Fig.4 and Table 2).

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| E _=

0.08

0

>

0.06

o~ o.~

0.04

9

0.02

E 0 b c

0G25N-M 1G25N-M 5G25N-M ; 10G25N-M 50G25N-M - - o - - 100G25N-M --

0.00 10

100

1000

Average pore radius (A)

Fig. 4. Pore size distributions of xG25N-M catalysts. 4. CONCLUSIONS Gd element reacts with MgO below the saturation limit of 10 wt% referenced to Ni to increase Ni dispersion, bond strength between NiO and MgO support, and OH concentration on the catalyst surface as if steam feed were increased, which resulted in suppression of coke deposition. So, Gd was a promising cocatalyst for the internal steam reforming catalyst, Ni/MgO, in MCFC. ACKNOWLEDGEMENT This work was financially supported by R&D Management Center for Energy and Resources (RACER), The Korean Energy Management Corporation. REFERENCES

1. A. L. Dick, J. Pow. Sour., 71 (1998) 111. 2. M. C. Demicheli, D. Duprez, J. Barbier, O. A. Ferreti, and E. N. Ponzi, J. Catal., 145 (1994) 437. 3. J. R. Rostrup-Nielsen, in J. R. anderson and M. Boudart (Editors), CatalysisScience & Technology, Vol. 5, Springer-Verlag, Heidelberg, 1984, p. 1. 4. K. S. Jung, B.-Y. Coh, and H.-I. Lee, Bull. Korean Chem. Soc., 20 (1999) 89. 5. Q. Zhuang, Y. Qin, and L. Chang, Appl. Catal., 70 (1991) 1. 6. A. Slagtern, U. Olsbye, R. Blom, I. M. Dahl, and H. Fjellvag, Appl. Catal. A, 165 (1997) 379. 7. M. P. Rosynek, Catal. Rev. Sci. Eng., 16 (1977) 111. 8. I. Alstrup, B. S. Clausen, C. Olsen, R. H. H. Smits, and J. R. Rostrup-Nielson, Stud. Surf. Sci. Catal., 119 (1998) 5. 9. R. T. Yang and J. P. Chen, J. Catal., 115 (1989) 52. 10. O. Yamazaki, K. Tomishige, and K. Fujimoto, Appl. Catal. A, 136 (1996) 49.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

A Microstructured Catalytic Reactor/Heat Exchanger for the Controlled Catalytic Reaction between H2 and 02 M. Janicke a, A. Holzwarth a, M.Fichtner b, K. Schubert b, F. Schtith a Max Planck Institut ftir Kohlenforschung, P.O. Box 10 13 53, 45466 MUlheim, Germany b Forschungszentrum Karlsruhe, P.O. Box 3640, 76021 Karlsruhe, Germany

a

A microstructured catalytic reactor/heat exchanger has been developed, based on the microstructured heat exchanger of the Forschungszentrum Karlsruhe. One channel set of a crossflow device is coated with a platinum catalyst supported on A1203, the other channel set is used for heat removal, either by a gaseous or a liquid fluid. Stoichiometric mixtures of hydrogen and oxygen can be safely combusted in this device, while heat can be transferred either to a heat exchanger oil or used to evaporate water or methanol. Such a system can be useful as part of the fuel processor for fuel cell driven cars. 1. I N T R O D U C T I O N Microstructured reactors are gaining increasing interest in different fields of chemistry [ 1]. Such microstructured reactors are characterized by a high surface-to-volume ratio, small internal dimensions and well defined flow conditions. This suggests applications in several fields, i.e. reactions which need to be quenched rapidly to prevent subsequent steps [2,3], highly exothermic or endothermic reactions to prevent hot or cold spots [3], on demand production of chemicals needed only in small quantities or on the spot synthesis of highly toxic chemicals in order to avoid transportation of such substances [4], or reactions in the explosive regime [5,6]. Such a reaction is the combustion of H2 with 02, since hydrogen and oxygen are explosive in mixtures of almost any concentration. If dimensions of the structure, in which the reaction proceeds, are sufficiently small, the homogeneous chain reaction cannot propagate, since the quench distance is longer than the reactor dimensions. A microstructure as used in this work has been tested as an explosion barrier for this reaction. It was found to prevent flame propagation even at elevated pressure, if two vessels filled with H2/O2 are connected by such a structure and the gas in one of the vessels is electrically ignited [7]. In addition, since heat can be removed extremely efficiently in microstructures, overheating of the reactor can be avoided and the heat generated can be used for other purposes. Such applications of microstructured reactors can be envisaged in a fuel processor system for a fuel cell driven car [6]. The approach chosen by most car manufacturers for hydrogen fuel cell technology is the production of hydrogen on board from methanol or gasoline by steam reforming. Such a system requires many components, which have to be small and lightweight, i.e. this is the ideal application field for microstructures. The catalytic hydrogen combustion/heat exchanger unit would be used for the rapid and efficient evaporation of the liquid fuel before passing into the reformer stage. Such a system must efficiently evaporate methanol or gasoline with a very short warm up period. One possibility is electric heating, but

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such a system would be relatively heavy. An alternative is using a hydrogen sidestream from the main hydrogen stream and using the heat generated by the combustion for methanol evaporation. Another possible use of this reactor is the coupling of the exothermal combustion reaction with the endothermal steam reforming reaction itself.

2

Experimental

2.1

Reactors/Heat Exchanger

The catalytic reactor/heat exchanger is based on the microstructured crossflow heat exchanger designed and built by the Forschungszentrum Karlsruhe [8]. It consists of stainless steel plates with micromachined channels which are stacked in an alternating manner, whereby subsequent plates are perpendicular to each other. This creates a crossflow channel system. The plates are diffusion bonded to each other, providing a pressure tight fit between the channel sets. A Fig. 1: SEM of part of the microstructured stainless steel housing provides reactor, looking on one edge. Entrance to connections to the periphery via reaction channels upper left, entrance to cooling standard tube fittings. The channels channels lower right, section. used for cooling have a cross section of 70 tam (width) x 100 tam (height), the reaction channels are 140 tam x 100 tam. The channels in both sets are l0 mm long. A SEM of the comer of the reactor showing part of both sets of channels is shown in fig. 1.

2.2

Catalyst Preparation

In order to increase the surface area in the catalytic channels, a coating of A1203 has been deposited in these channels. There are alternative pathways to produce such coatings, i.e. deposition from a sol, or chemical vapor deposition (CVD). The best method in our case proved to be CVD, since it allows the formation of relatively homogeneously coated channels. For the CVD of A1203 we used aluminum isopropoxide as the precursor. It was kept molten in a loading vessel at 160~ and carried into the 140 tam x 100 tam channel set via a stream of ll/min of nitrogen bubbling through the melt. This alkoxide loaded stream was mixed with 7 1/min of oxygen before entering the reactor. The oxygen facilitates the decomposition of the precursors and prevents build-up of carbon. The reactor was kept at 300~ by passing hot nitrogen gas through the channel set later used for cooling of the reactor. The deposition process was carried out for one hour, then the reactor was cooled down and the process repeated for another hour with the alkoxide flow entering from the opposite direction in order to obtain a homogeneous coating. To produce deposits of reproducible quality, the deposition system had to be operated for several days to reach a stable state. Following this procedure, typically coatings of 5 - 10 tam thickness could be prepared. The alumina is present in the gamma phase, krypton adsorption isotherms revealed, that the

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surface area is increased typically by a factor of 100. In a test system with removable individual foils which were not diffusion bonded, the coating was shown to be fairly homogeneous over the length and width of one foil. Platinum loading was achieved by passing hexachloroplatinic acid solution through the reaction channel system repeatedly, typically five times. After the last step, the channels were left filled with the platinum solution. Excess solution at the inlet and outlet was removed with filter paper, and the whole reactor calcined at 570~ The final reduction of the catalyst was carried out at 350~ under flowing nitrogen/hydrogen (10:1). If higher platinum loadings were desired, the loading procedure was repeated up to three times. Elemental mapping of the removable foils in the test reactor showed a homogeneous distribution of the platinum in the alumina-coated channels.

2.3

Catalytic experiments

Flows of HE, O2 and optionally N2 in different ratios - stoichiometric, net oxidizing or net reducing - and at different flow rates were mixed in a T-element, using mass flow controllers, about three centimeters before entering the reactor. The temperature of the gas leaving the reactor was measured with a thermocouple placed immediately at the outlet. Cooling was either achieved by a strong flow of nitrogen through the cooling channels or by pumping a heat exchanger oil through the cooling channels. The temperature of the exiting heat transfer medium was also measured immediately at the outlet. Conversions were determined by collecting the water formed in a cooled molecular sieve trap and weighing after a predetermined time. Since strong thermal gradients were observed over the reactor, especially when using the heat exchanger oil, the temperature profile on the external surface of the reactor was visualized using an IR thermographic imaging system. Due to the reflectivity of the reactor and problems compensating for this, the readout of the imaging system could not be fully calibrated and thus only qualitatively shows the temperature distribution on the external surface of the reactor. For high loadings of platinum (repeated impregnation) the catalytic reaction ignited already at room temperature and no external heating was necessary. At lower platinum loadings, the reactor had to be preheated to about 70~ Once the reaction had started, though, the reactor could be operated autothermally. 3

RESULTS

Before discussing the results obtained under different conditions, first some remarks about safety of the reactor operation are necessary. In numerous experiments, only one explosion occurred, however, not in the reactor, but in the bubbler through which the offgas was led to the vent. This explosion was due to an ill prepared catalyst coating, which did not well adhere to the wall. During the experiments, the catalyst coating was carried out of the reactor and accumulated in the bubbler. When the amount of catalyst in the reactor was not sufficient any longer to reach full conversion, the explosive gas mixture could ignite on the catalyst in the bubbler. Due to the low volumes handled, however, no damage except to the bubbler itself occurred. This single explosion, on the other hand, clearly shows the potential of the microstructured reactor itself to efficiently prevent the homogeneous ignition of the explosive mixture.

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3.1

Operation with gas cooling

The first experiments were carried out with a reactor loaded only once with platinum (low loading) and gas cooling of the whole setup. Since the reaction did not start at room temperature at low platinum loadings, it had to be heated to 80~ by two heating element on the top and bottom of the reactor. Once the reactor reached this temperature, the reactor could be operated autothermally. Fig. 2 shows the temperature response at the reactor outlet during such an experiment. The first data points do not appear at the preheating ~ ' 226 0.2 I/min 02 ~,~,,,~ temperature of 80~ because 0.4 I/min H2 j ~ .o 20o data were only recorded from the time when the final H2 flow had & 181J been reached. This flow was E 160 increased gradually due to safety 0.1 I/min 02 reasons, so that the temperature 9 14() 0.2 I/min H2 == already started to rise before the o 12t) final settings were adjusted. As one can see, the gas composition g 100, (u entering the reactor can be used "80 0 2 4 6 8 ~0 to control the temperature level. time [min] The cooling gas exited the reactor at the same temperature as the Fig. 2: Reactor response at different inlet gas reaction gas. For the low reactant compositions. Cooling gas: 3 1/min N2, reaction gas flows, the conversion had not diluted with 1 1/min N2. reached 100 % after 10 min. At 0.2 1/min of oxygen and 0.4 1/min 400" hydrogen, however, full _ . . . . ,, ~ ~-~..~.------ of conversion had been achieved. After 5 runs n m 0 The still rising temperature at 300" Q that point is due to the slow heat up of the reactor housing. m i 0 In a reactor impregnated three 200" 9 Q E " e First run times with platinum solution, the Ireaction started already at room 100" " I temperature at the same inlet gas J m flows. In this case, the temperature of the whole reactor was controlled by changing the 0 20 40 60 80 coolant gas flow. With coolant Time [rain] gas flows of 4 1/min, 5 1/min and Fig. 3" Reactor response for higher catalyst loading. 7 1/min steady state temperatures Inlet 0.2 1/min 02, 0.4 1/min H2, 1 1/min N2, cooling 3 of 315~ 280~ and 230~ were 1/min N2. reached. Under these conditions, a power of about 70 W is produced, of which about one third is carried out of the system via the reacting and cooling gases, the rest is lost via the reactor housing and the fittings. One should note, that the gas o L----,

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mixture corresponds roughly to the composition of air, so that air would be sufficient to start the reaction at room temperature if the microstructured reactor would be used in a fuel processor system. With a freshly prepared catalyst in the reactor, the reactor needed about 30 min before the reaction ignited. However, a very substantial shortening of this induction period to about one minute was observed after several runs. Fig. 3 shows the response of the fresh catalyst and after five runs under identical conditions. Similar observations, i.e. that exposing a catalyst to hydrogen/oxygen mixtures leads to more active materials compared to reducing in pure hydrogen, have been reported before [9,10,11]. Different explanations have been put forward in these publications, but due to the fact that the catalyst can not be analyzed under reaction conditions or after the reaction due to the inaccessibility of the reactor interior, we do not have evidence for or against either of the explanations. Experiments were also carried out without diluting the reaction gases and using different mixtures of hydrogen and oxygen. The highest flows were adjusted in an experiment with 0.8 1/min of both hydrogen and oxygen, i.e. in the middle of the explosive regime. Under such conditions, even with 14 1/min of nitrogen passing through the cooling channels, the maximum which could be used with the equipment installed, the reactor temperature rose to 300~ in five minutes and kept rising with a steep gradient. Since the homogeneous reaction could have ignited in the hydrogen/oxygen mixture before entering the reactor, this experiment was terminated before a steady state could be reached. Instead, we investigated more efficient means of heat removal.

3.2 Operation with liquid cooling Using a heat transfer oil is such a more efficient method. The oil (Haake Synth 260) was pumped through the reactor using a peristaltic pump. In a typical experiment a feed composition of 1.2 1/min H2, 0.6 1/min 02 and 2 1/min N2 as carrier gas was used. Typical oil flows were about 30 ml/min. The reaction ignited at room temperature in a reactor with a high catalyst loading, which generates a total power of approximately 250 W. Under these conditions, the oil exits the reactor at a temperature of 207~ while the reaction gas exits at a temperature of only 70~ This on first inspection surprising behavior can be attributed to the cross flow design of the reactor/heat exchanger. The reaction is so fast, that most of the hydrogen is consumed immediately after entering the reactor. At this point, most heat is transferred to the oil. When passing further through the reactor, the reaction gas is efficiently cooled by the cold oil, exiting at low temperature. The exit temperature of the oil is the average of very hot oil . . . . . . . . . . generated near the reactor entrance and colder oil Fig.4" Temperature profile of the which passed through the cooling channels near the reactor. H2/O2 entering from exit. The temperature profile of the reactor recorded bottom, cold oil entering from the with an IR imaging system is shown in fig. 4. The right. Hottest temperature at the dark colors correspond to the hottest regions. It can reactant gas entrance.

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clearly be seen, that the hottest section of the reactor is immediately at the reactor entrance, where the surface temperature measured with a thermocouple is around 260~ The heat is very efficiently removed by the coolant oil, so that the temperature drops to about 65~ near the reactor exit. Due to the high viscosity of the oil, the flow could not be increased further with the equipment available. Although it seems possible to generate much more power in the reactor by passing higher H2/O2-flows through the system, this was not possible, since the temperature limit of the oil is 270~ which was already exceeded under the conditions used near the reactor entrance, so that some decomposition of the heat transfer oil occurred which was indicated by gas bubbles present in the oil after leaving the reactor. Initial experiments were also conducted with water entering the cooling channels which is evaporated in the heat exchanger. About 10 g/min of hot steam could be generated in this fashion. However, the conditions in the reactor are fairly ill defined in this mode of operation and proper energy balancing becomes difficult. One possibility to solve these problems is the construction of a counterflow device. Work to this effect is in progress. With such a device the generation and transfer of 1 kW in the cubic centimeter sized device seems to be readily achievable, which would be sufficient to evaporate close to 4 1 of methanol per hour if fed in at room temperature which is close to the demand of a small size fuel cell in a car.

4

CONCLUSION We have demonstrated that the catalytic combustion of hydrogen can be safely run in a microstructured reactor/heat exchanger. Such a device could find use for the evaporation of methanol in the fuel processor of fuel cell powered cars. With a counterflow design, a thermal power generation by catalytic combustion and transfer of more than 1 kW in a cubic centimeter sized device seems possible. REFERENCES See the Proceedings of the International Conferences on Microreaction Technology 1-3 R. Srinivasan, I.M. Hsing, P.E. Berger, K.F. Jensen, S.L. Firebaugh, M.A. Schmidt, M.P. Harold, J.J. Lerou, J.F. Ryley, AIChE J. 43 (1997) 3059. 3. D. HOnicke, G. WieBmeier, in: Microsystem Technology for Chemical and Biological Microreactors, DECHEMA Monograph Vol. 132, VCH, New York (1995), p.93 4. J.J. Lerou, M.P. Harold, J. Ryley, J. Ashmead, T.C. O'Brien, M. Johnson, J. Perrotto, C.T. Blaisdell, T.A. Rensi, N. Nyquist, ibid., p.51. 5. U. Hagendorf, M. Janicke, F. Schtith, K. Schubert, M. Fichtner, in: Topical Conference Preprints: 2nd International Conference on Microreaction Technology, AIChE 1998, p. 81. 6. A.L.Y. Tonkovich, D.M. Jimenez, J.L. Zilka, M.J. LaMont, Y. Wang, R.S. Wegeng, ibid., p. 186 7. Fraunhofer Institut fur Chemische Technologie, unpublished 8. W. Bier, G. Keller, G. Linder, D. Seidel, K. Schubert, in DSC-19, Microstructures, Sensors and Actuators. Cho, D et al. (Eds.), The American Society of Mechanical Engineers, Book No. G00527 (1990). 9. S.J. Gentry, J.G. Firth, A. Jones, A., J. Chem. Soc. Faraday Trans. 70 (1974) 600. 10. F.V. Hanson, M. Boudart, J. Catalysis 53 (1978) 56. 11. G. Pecchi, P. Reyes, I. Concha, J.L.G. Fierro, J. Catalysis 179 (1998) 309. 1. 2.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Catalytic Water Denitrification in Membrane Reactor O.M.Ilinitch (a), F.P.Cuperus (b), L.V.Nosova (a) and E.N.Gribov (a) (a)Boreskov Institute of Catalysis, Novosibirsk 630090, Russia* (b)Agrotechnological Research Institute, NL-6700 AA Wageningen, The Netherlands* Mono- and bimetallic catalysts with Pd and/or Cu supported over y-A1203 were investigated in respect to reduction of aqueous nitrate and nitrite ions by hydrogen. Composition of the supported catalysts was analyzed using XRD, SIMS and H2-O2 chemisorption techniques. Pronounced limitations of catalytic performance due to intraporous diffusion of the reactants were observed in the reaction. Catalytic membrane containing Pd-Cu active component supported over macroporous membrane-support was prepared and investigated. Forced flow of the reaction solution through the membrane was revealed to increase the effective catalytic activity.

1. INTRODUCTION In three-phase catalytic processes, the reactants diffusion in the catalyst pores is often a ratelimiting factor. To minimize the internal diffusion limitations, such processes are typically performed in slurry reactors with powdered catalysts that must be separated from the mixture when the reaction is completed. Our approach, reducing the negative influence of the internal diffusion and avoiding the need to separate the catalysts, presumes the use of porous membranes as a catalyst support of a specific type. The catalytic membranes with an active component deposited on the pore walls can assure more intensive intraporous transport of reactants to the catalytic centers compared to the conventional solid catalysts. Intensification of the intraporous mass transfer for the reactions limited by the internal diffusion can result in the improved catalytic activity and selectivity [ 1, 2]. These improvements were suggested to be due to a different character of mass transfer in the pores of a macroporous membrane and catalyst particles OCorcedflow in a membrane vs. diffusion-drivenflow in a catalyst). In this study, interrelations between the catalytic behavior and intraporous mass transfer were explored for the macroporous catalytic membrane in the process of nitrate ions reduction by hydrogen in water. The reaction occurring at ambient temperatures in the presence of palladium-containing catalysts [3] is a potential method of purifying drinking water from toxic nitrates that are increasingly produced by the industrial and agricultural activities worldwide. An incentive to applying the macroporous catalytic membranes for water denitrification stemmed from observations of their superior catalytic behavior in three-phase reactions and techno-economic assessments of the process [4]. In the present study the catalytic and Support of this work by the grant No. 047-010-100-96 from the Netherlands Organization for Scientific Research (NWO) is gratefully acknowledged.

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structural characteristics of Pd-Cu catalysts were investigated. The macroporous catalytic membrane was employed to facilitate the intraporous mass transfer and to increase on this basis the effective activity of Pd-Cu active component.

2. EXPERIMENTAL 2.1. Preparation of catalysts and catalytic membranes The series of mono- (Pd or Cu) and bimetallic Pd-Cu catalysts was prepared by (co)impregnation of 7-A1203 granules (SaEv=197 m2/g, mean pore diameter 15 nm) with hydrochloric solution of the catalyst precursors [PdC12 and/or Cu(NO3)2]followed by oxidation in air at 300~ and reduction in aqueous solution of NaBH4 at room temperature. The samples of catalytic membrane were prepared via the same procedure. The ceramic membrane (SaET=0.9 mE/g, pore diameter lktm) developed at BIC was employed at this stage as the catalyst support. The metal content was ca. 5 wt.% for Pd-containing catalysts and 1.7 wt.% for Cu/A1203; the catalytic membrane contained 1.6 wt.% of Pd and 1.2 wt.% of Cu. 2.2. Catalytic runs The catalysts were tested at 298K in a glass apparatus with the stirred reactor described in detail elsewhere [5]. In experiments with the catalytic membrane, in-house built membrane reactor of 100 cm 3 volume equipped with a magnetic stirrer was used. Concentrations of NOaand NO2- anions in the catalytic experiments were monitored using a liquid ion chromatograph "Tsvet-3006" (Russia) with electroconductivity detector. NH4+ cations were analyzed with an ion-selective electrode ELIT-51 (Russia). Reaction rates were determined by differentiating the "concentration vs. reaction time" dependencies at low (_<10%) conversions of the initial anion. 2.3. Characterization techniques Analysis of catalysts and membranes: Dispersion and composition of metal particles in the catalysts and membranes were studied by X-ray diffraction (XRD) with HZG-4C instrument (Freiberger Pr~izisionmechanik) in CuKGt radiation. Metal loadings were determined by atomic absorption spectroscopy (AAS 1N, Carl Zejss - Jena). Secondary_ Ion Mass Spectroscopy (SIMS): The vacuum apparatus equipped with MC-7201 mass spectrometer (Russia) sensitive in the range of mass numbers 1-250 was employed. Argon ions of 4 keV energy at current density 0.12-0.63 A/m 2 were used for the bombardment of the samples. Chemisorption measurements: Palladium dispersion was measured by H2-O2-H2 titration in a pulsed flow setup with on-line gas chromatographic analysis [6]. The catalysts loaded into the adsorber were heated in vacuum at 373K and residual pressure ca. 1.10 3 Torr (1Torr = 133.3 Pa) for 1 hour to drive off adsorbed water. The outgassed samples were cooled to room temperature and titrated first with hydrogen, then with oxygen and finally with hydrogen a second time. Other experimental details can be found in [7].

3. RESULTS AND DISCUSSION 3.1. Reduction of NO3" and NO2" over Pd, Cu and Pd-Cu catalysts In the experiments performed for the series of 7-A1203 supported mono- (Pd or Cu) and bimetallic Pd-Cu catalysts (slma3, reactor, mean grain size 15 ~tm) at the conditions where the re-

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action performance was not affected by diffusion of the reactants, the highest catalytic activity in the reduction of 200 mg/1 NaNO3 aqueous solution was found for the sample of the atomic ratio Pd:Cu=1:0.8 (Fig. 1). Selectivities of these catalysts at various NO3 conversions towards side products - NO2 and NH4+ ions - are listed in Table 1. As can be seen, Pd-Cu catalysts exhibit similar selectivities for ammonium with a minor decrease upon decrease in Pd:Cu ratio. On the contrary, selectivities for nitrite ion rise with decreasing Pd:Cu ratio. In reduction of NO2- however copper has no beneficial influence at the catalytic activity of palladium. In the same series of catalysts the activity decreases with an increase in Cu content (Fig.2).

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Fig. 2. Activities of y-AI203 supported catalysts in reduction of aqueous nitrite ions at 298K, P.2=l bar, CNO2-=50mg/l, pH=6.0

Table 1. Selectivities of Pd/Al203 and Pd-Cu/A1203 catalysts in reduction of NO3Reaction conditions: temperature 298K, PH2 = 1 bar, CNO3~--200mg/1, pH=6.0 Atomic ratio of XNo3-= 50% XNO3 --- 75% XNO3= 25% XNO3-= 95% metals in catalyst SNO2-,% SNH4",% SNO2-,% SNH4-,% SNO2-,% SNH4-,% SNO2-, o~ SNH4-,o~ Pd Pd:Cu = 1:0.33 Pd:Cu = 1:0.8 Pd:Cu = 1:2.25

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3~. Characterization of the catalysts and catalytic membrane

3.2.1. XRD investigation No peaks of palladium, of copper or of the combinations of the two known to form in Pd-Cu system were detected in the XRD spectra of Pd/A1203, Cu/AI203 and Pd-Cu/A1203 catalysts. This is most likely due to the pronounced line broadening typical of the nanosized metal particles, along with the overlapping of the resulting halo and the peaks of the ~-A1203 support. XRD spectrum of the catalytic membrane contains a superposition of sharp peaks belonging to the membrane-support fabricated of the natural silica-alumina mineral and a broad peak with the maximum at 20~,~,41o characteristic of nanosize metal clusters. The XRD patterns of the metallic particles in the catalytic membrane are similar to those reported for the silica-supported Pd-Cu catalysts in [8], where formation of palladium-copper alloys has been suggested. Mean size of the metal particles in the membrane calculated according to the Scherrer's approach is ca. 3 nm. 3.2.2. SIMS investigation ~u

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According to the SIMS results (Fig. 3), the topmost layer of the metal particles in Pd-Cu/A1203 catalysts with the ratios Pd:Cu=1:0.33 and Pd:Cu=1:0.8 is strongly enriched with Cu compared to the integral composition. Higher copper content characteristic of the third catalyst and the membrane favors more homogeneous distribution of the metals. In the porous grains of Pd:Cu=l:0.33 and Pd:Cu=l:0.8 catalysts the ratio Pd:Cu is the lowest on the outside, gradually increasing to reach the steady level at the granule depth of ca. 0.1~tm and 0.03~tm respectively. Essentially homogeneous spatial distribution of the metals was registered for the catalyst Pd:Cu = 1:2.25 and the membrane.

3.2.3. H2-O2 titration The results of hydrogen titration were used to determine the palladium dispersion in Pd/A1203 and Pd-Cu/A1203 catalysts. The uptake of hydrogen and oxygen by the catalysts under the experimental conditions employed was assumed to result from chemisorption by the surface atoms of the metal particles without formation of bulk phases of both hydride and oxide [9]. It was also assumed that hydrogen titration involves only oxygen species preadsorbed over palladium atoms in the course of oxygen titration [6]. The values of hydrogen titration HT/Pdt, expressed as the number of gas atoms adsorbed relative to the total number of palladium atoms in a given catalyst, are listed in Table 2. Divided by the stoichiometric coefficient of hydrogen titration 3 [10], these numbers give dispersion D of palladium in the supported metal particles, i.e. fraction of the surface atoms Pds relative to the total amount of palladium atoms Pdt : D - Pds/Pdt = (HT/Pdt)/3.

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As one can see, relatively high palladium dispersion was achieved for Pd/AI203 catalyst. The average size of palladium particles calculated by the ratio dpd = 0.9/D [ 10] is 1.7 nm. Addition of copper decreases the amount of H atoms chemisorbed per one Pd atom. The resulting values of palladium dispersion are ca. 4-6 times lower for PdCu/A1203 catalysts than those for Pd/AI203. This suggests a marked decrease in the number of the surface Pd sites accessible to hydrogen adsorption in the bimetallic catalysts. The latter can be caused by screening of palladium atoms by copper on the surTable 2. Hydrogen titration of Pd/A1203 and face of the bimetallic particles Pd-Cu/A1203 catalysts at 298K and/or by increase in the size of the particles. Taking into account H2 Pd Catalyst composition titration dispersion, the above results of XRD and SIMS studies and evidence existMetal content, atomic ratio HT/Pdt D=Pds/Pdt ing in literature [11 ], it can be conwt. % Pd:Cu cluded that the most probable rea1.56 0.52 4.1 Pd son for the observed sorption be0.40 0.13 5.2 Pd + 1.1 Cu 1:0.33 havior of Pd-Cu catalysts is the 0.38 0.13 5.2 Pd + 2.5 Cu 1:0.8 screening of palladium by copper. 0.28 0.09 5.0 Pd + 6.8 Cu 1:2.25

3.3. Catalytic membrane and internal diffusion hindrance In our catalytic experiments the activity of Pd-Cu/A1203 catalysts was found to increase with decrease in the grain size down to as low as 10-20 pin, revealing a pronounced influence of the intraporous diffusion which dictates the usage of fine catalyst powders in this process [12]. As an alternative means to minimize the internal diffusion hindrance, the catalytic membrane was employed in this study. The disk-shaped membrane (diameter 45 mm, thickness 4 mm) was installed in the reactor shown schematically in Fig. 4, and comparative catalytic runs were performed at the identical experimental conditions (temperature, NO3-concentration, stirring speed). In the first run the valve at the reactor exit was closed thus preventing the reaction solution from flowing through the membrane (flow rate "0" in Fig. 5) and making the membrane

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porous space accessible for the reactants only via diffusion. In the following runs ("flowthrough" mode) the valve was opened and the solution circulated through the membrane. In accordance with our hypothesis which predicts that the forced flow of the reaction mixture in the pores can assure more intensive mass transfer compared to the diffusion-driven flow, an increase in the reaction rate was registered in the "flow-through" mode (Fig. 5). Furhermore, higher flow rates of the solution circulating through the membrane were revealed to entail a higher effective catalytic activitiy, which apparently reflects intensification of the intraporous mass transfer with an increasing flow rate. 4. CONCLUSION

Catalytic behavior of mono- and bimetallic catalysts with Pd and/or Cu supported over 7-A1203 in reduction of aqueous nitrate ions by hydrogen was investigated. Activity of the catalysts was found to reach maximum at the atomic ratio Pd:Cu~l. In the reduction of nitrite ions over the same catalysts, maximum activity was registered for Pd/A1203 catalyst. Composition of the supported metal catalysts was analyzed using XRD, SIMS and H2-O2chemisorption. Diffusion of reactants in the pores of Pd-Cu catalysts impairs the reaction performance. Multifold increase in catalytic activity was achieved with the catalytic membrane containing Pd-Cu active component deposited over the macroporous membrane-support. The concept of catalytic membrane explored in this study offers new means of improving catalytic performance in the processes where internal diffusion hindrance must be minimized while the use of finely grained catalysts is undesirable. REFERENCES

1. O.M.Ilinitch and Yu.S.Vetchinova, Catalysis Today 25 (1995) 423. 2. O.M.Ilinitch, P.A.Simonov and F.P.Cuperus, in B.Delmon and J.T.Yates (Eds.), Preparation of Catalysts VII, Elsevier, 1998, p. 55. 3. K.-D.Vorlop und T.Tacke, Chem.-Ing.-Tech. 61 (1989) 836. 4. O.M.Ilinitch, F.P.Cuperus, V.V.Gorodetskii, M.Yu.Smirnov, O.P.Burmatova and I.O.Ilinitch, in Proc. 4th Workshop Catal. Membr. React., Oslo, 1997, p. 89. 5. E.N.Gribov, M.Sc.Thesis, Novosibirsk, 1998. 6. L.N.Nosova, V.I. Zaikovskii and Yu.A.Ryndin, React. Kinet. Catal. Lett., 53 (1994) 131. 7. O.M.Ilinitch, F.P.Cuperus, L.V.Nosova and E.N.Gribov, Catalysis Today 1999 (to appear). 8. A.J.Renouprez, K.Lebas, G.Bergeret, J.L.Rousset and P.Delichere, in J.W.Hightower et al. (Eds.), Studies Surf. Sci. Catal., v. 101, Elsevier, 1996, p. 1105. 9. C.A.Leon y Leon and M.A.Vannice, Applied Catalysis 69 (1991) 269; ibid., 69 (1991) 291. 10. J.E.Beson, H.S.Hwang and M.Boudart, J. Catal. 30 (1973) 146. 11. B.E.Nieuwenhuys, Surface Review and Letters, 3, Nos. 5&6 (1996) 1869. 12. K.-D.Vorlop, T.Tacke, M.Sell and G.Strauss, Process for Removing the Nitrite and/or Nitrate Content in Water, US Patent No. 4 990 266 (1991).

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Reactivity and thermal profile of methane partial oxidation at very short residence time. F, Basilr G. Fomasari, F. Trifir6 and A. Vaccari Dipartimento di Chimica Industriale e dei Materiali, Universith degli Studi di Bologna, Viale del Risorgimento 4, 40136 Bologna, Italy The catalytic partial oxidation (CPO) of methane was investigated in autothermal conditions as a function of the residence time, using a Rh/Mg/A1 (1:71:28, as atomic ratio) catalyst obtained by calcination at 900~ and following reduction of a hydrotalcite-type (HT) precursor. A CH4/O2/He = 2:1:4 (v/v) reaction gas mixture was fed, while the gas phase temperature was monitored by a chromel-alumel thermocouple put inside the catalytic bed and the catalyst surface temperature and thermal profile distribution were followed by IR thermography. Comparing the thermal and the catalytic data, the two possible reaction pathways in the formation of synthesis gas (indirect or direct) was critically discussed. 1. INTRODUCTION The use of natural gas as raw material is a goal of strategic relevance, which needs economic routes to liquid fuels and petrolchemical intermediates. The activation of methane, its main component, is still performed with very expensive processes, such as the steam reforming or the non catalytic partial oxidation [ 1]. The catalytic partial oxidation (CPO) of methane at low residence time has mainly focused the attention of academic and industrial researchers as more economical process to convert methane to liquid products [2,3]. Nevertheless many scientific and technical problems are still open and need to be solved before a possibile industrial application. The temperature control, the heat production and distribution along the catalytic bed as well as the reaction kinetics are still under debate [3]. Two possible reaction mechanisms are reported in the literature for the CPO of methane: i) CO and H2 are produced by steam and dry reforming reactions, which take place after a total oxidation step [3,4]; ii) the methane is directly transformed to CO and H2 [3,5]. These reaction pathways have been supported or refused on the basis of gas-temperature evaluations by means of thermocouples located in the catalytic bed. Moreover, the presence of hot spots on the catalyst surface has also been claimed, although they have not been clearly evidenced [6]. Aim of this work is to shed light on the actual temperature and thermal profile on the catalyst surface as a function of the residence time, checking also at very short values, at which unconverted oxigen is present in the outfeed. Moreover, the correlation between the catalytic data and the surface temperatures can contribute to clarify the possible reaction pathway. 2. EXPERIMENTAL The catalyst was obtained by calcination and reduction of a hydrotalcite-type (HT)

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[Rh3+0.01Al3+0.2sMg2+0.71(OH)2 ] (CO3)2"0.145mH20 were prepared by coprecipitation at constant pH, pouting a solution containing the nitrates of the metal ions in a solution containing a slight excess of Na:CO3, keeping the pH - 10.0 by dropwise NaOH addition. The precipitate was kept in suspension under stirring at 60~ for 40 min, then filtered and washed with distillated water till a Na20 content lower than 0.02 wt %. Then it was dried overnight at 90~ calcined at 900~ and reduced at 750~ for 5h in equimolar H2/N 2 flow of 7 L/h [7, 8]. XRD powder analyses were carried out using a Philips PW1050/81 diffractometer equipped with a graphite monochromator and controlled by a PW1710 unit (~, - 0.15418 nm). A 2 0 range from 10 to 80 was investigated at a scanning speed of 70~ The surface area was determined by N 2 adsorption using a Carlo Erba Sorpty model 1700. The catalytic tests were performed in autothermal conditions, after ignition of the reaction by means of a thermal gun, feeding at 25~ a CH4/OE/He = 2/1/4 reaction gas mixture and using a quartz reactor (i.d. 6 mm) filled with 0.2g or 0.05g of catalyst (30-40mesh) in order to check residence times in the ranges 7.2-24.0ms and 1.0-9.0ms, respectively. The lenghts of the catalytic bed were 9 or 3mm, respectively. The gas phase temperature was measured by a chromel-alumel thermocouple sliding in a quartz wire put inside the catalytic bed. The surface temperature was measured with IR thermography equipment (AGEMA) collecting emitted radiation with ~. in the 2-5 ~tm range. The IR camera was equipped with two zoom to improve the spatial resolution on the catalyst surface. The vertical thermal profile has been graphed plotting the maximum temperature of horizontal lines drawn on the thermography images. The reaction products were analysed on-line, after condensation of the water formed, by two gas chromatographs equipped with HWD and carbosieve SII columns, using as carrier gas He in the analysis of CH4, O2, CO and CO2 and N 2 in the analysis of H 2.

3. RESULTS AND DISCUSSION The Rh-containing HT precursor (Fig. 1A) shows a surface area of 79m2/g and by calcination at 900~ gives rise MgA104 9 MgO phase 9 I~ "* Spinel phase ~ II 9 and MgO phases (Fig. 1B), in which Rh 3§ 9 9 ions are homogeneously distributed. The catalyst exhibits a surface area of 98m2/g, notwithstanding the high temperature of calcination, that can be attributed to the specific features of the HT precursor [9]. By reduction, small Rh-containing clusters are obtained, highly dispersed in the Mg and A1 oxide or mixed oxides [8]. The 9 characterisation of the sample after the 99 catalytic tests shows the presence of the (A) ~ ~ o same crystalline phases, without any zo 40 60 ' evidence of sintering phenomena and/or 2o segregation of Rh metallic particles (Fig. Fig. 1. XRD powder patterns of the Rh/Mg/A1 1C). Furthermore, no significant change in (1:71:28 as atomic ratio) catalyst: A) dried at the surface area value of the catalyst was 90~ B) calcined at 900~ C) after the detected. The homogeneity of the catalyst catalytic tests, obtained from a HT precursor, allows to 9 HTphase

~

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850

--~

a~0"C

--*-24 ms --4-12 ms

750

~9

=.... I- 650

4~

550 450 0

t 2

I 4

I 6 bed length (mm)

I 8

3 ~ 0%

10

Fig. 2. Thermal profiles of the catalyst surface in the CPO reaction as a function of residence time, using 0.2g of catalyst and the CHa/O2/He =2/1/4 (v/v) gas mixture.

Fig. 3. IR image of the surface temperature during the CPO test with 0.2g of catalyst [residence time = 12 ms; CHa/O2/He =2/1/4

v/v)]

compare the data obtained with the two different amounts of catalyst. Furthermore, these results confirm the stability of the Rh-containing particles during the reaction also at high temperature, allowing to evaluate the catalytic behaviour in different reaction conditions regardless of phase segregations and/or catalyst modifications. The thermal profiles collected using 0.2 g of catalyst show a hot zone on the catalyst surface at the beginning of the catalytic bed (Fig.s 2 and 3), while the temperature of the gas phase remains at significantly lower values (Figure 3). The thermal behaviour in this first zone can be explained considering the presence on the catalyst surface of highly exothermic total and partial oxidation reactions and heat transport limitations between the catalyst surface and the gas phase, that do not allow to equalize their temperatures. In the following part of the catalytic bed, the gas phase temperature increases reaching a maximum, with a corresponding decrease of that on the surface. Near to the end of the catalytic bed, the temperature of the catalyst surface is lower than that of the gas phase. The decrease of the surface temperature below the value in the gas phase allows to exclude significant contributions in this zone by the exothermic reactions, suggesting on the contrary a relevant role of the endothermic reactions, as steam and dry reforming. By decreasing the residence time, i.e. increasing the flow rate, it may be observed (Fig. 2): i) an increase of the surface temperature in the whole catalytic bed; ii) a widening of the zone at highest temperature, with the detection of a second peak. While the general increase of surface temperature can be associated to the increasing amounts of methane fed, consequently, to the increase of the heat generated by its exothermic oxidations, the enlargement of the hot zone may be attributed to increasing amounts of catalyst in 02 rich atmosphere, due both to the increase of the amount of oxygen fed and of the flow rate, which lead to a significant oxygen diffusion trasfert from the particles on the top of the catalytic bed to those immediately below. The catalytic data reported in Table 1 show a total oxygen conversion in all the tests performed with 0.2g of catalyst, while the conversion of methane and the selectivity in synthesis gas increase by decreasing the residence time from 24.0 to 7.2ms, attributable to the increase of the temperature in the whole catalytic bed and, consequently, to the higher weight of the steam and dry reforming reactions, with production of hydrogen and carbon monoxide.

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Table 1:

Catalytic activity as a function of residence time for the Rh/Mg/A1 (1:71:28, as atomic ratio) calcined at 900~ feeding the reaction gas mixture: CH4/O2/He =2/1/4 (v/v). Residence time 02 conversion CH 4 conversion CO selectivity n 2 selectivity Tempm~x(gas) ms 1.0# 1.4~ 1.8 ~ 3.0 ~ 6.0 ~ 9.0 ~ 7.2* 12.0" 24.0*

% 86.7 93.5 96.9 99.5 100 100 100 100 100

% 54.5 58.9 62.9 64.7 62.1 53.3 70.6 65.6 58.2

% 87.2 87.3 87.5 85.8 76.1 62.5 80.8 76.0 64.6

% 77.0 77.0 79.2 80.3 81.4 74.0 88.8 85.8 81.8

~ 655 700 717 705 646 600 710 670 621

Catalytic tests with 0.05 g of catalyst * Catalytic tests with 0.2 g of catalyst Furthermore, it is worth noting the decrease of the H2/CO ratio due to the effect of the temperature on the water gas shift reaction, that at temperatures higher than 800~ favours the formation of CO. However, in these tests the complete oxygen conversion reached in the first part of the catalytic bed and the sharp temperature profiles did not allow to collect useful informations on the possible pathway in the CPO reaction. To evidence the reactions occurring in the first part of the catalytic bed a lower amount of catalyst (0.05 g) was used, thus reaching either partial or total oxygen conversion as a function of the residence time adopted and, therefore, also operating in conditions far from the thermodynamic equilibrium (Table 1). The catalyst surface in the tests performed with 0.05 g of catalyst shows an increase of the temperature at the beginning of the bed only till a residence time of 1.8ms, while for lower values the temperature at the beginning of the bed remains constant and the maximum temperature shift towards the middle of the catalytic bed (Fig.s 4 and 5). These phenomena may be attributed to a saturation of the Rh-containing active sites of the first particles and, consequently, to the presence of an high amount of unconverted oxygen in the second part of the catalytic bed, where it gives rise to a second peak and a corresponding smoothing of the thermal profiles. In the tests performed using 0.05g of catalysts (Table 1) the conversion of methane increases by decreasing the residence time till the oxygen conversion is complete, while at residence time _< 3.0ms, it decreases in agreement with the lowering of the oxygen conversion. The selectivity in synthesis gas is very high in all the catalytic tests performed, also when the oxygen conversion decreases below 100%. This high selectivity in syngas is in contrast with the proposed indirect reaction pathway (i.e. the total oxidation of part of the methane fed, followed by dry and steam reforming reactions) and suggests that syngas is mainly produced by the direct reaction pathway. The selectivity in CO increases by decreasing the residence time, reaching a constant value; its extrapolation for a residence time equal to zero confirms that CO has to be considered a primary product. On the other hand, the selectivity in H2 as a function of the residence time shows a maximum in correspondence of the maximum in the methane conversion. At low residence times, the selectivity in H2 decreases at values lower

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6:~l s]

1000

800 T(~

"............................................................................... i

1.8 ms "-~--l'4ms ]

~

'~

~ . , _ . . "q

600 l l~' ~ 2 ~ - ~ t ~ % g ~ e ~

ing

~

400 ~ - - - f 8 9 I ,~ ~5 Reactor length (mm) Fig. 4. Thermal profiles of the catalyst surface in the CPO reaction as a function of residence time, using 0.05g of catalyst and the CH4/OE/I-Ie =2/1/4 (v/v) gas mixture.

Fig. 5. IR image of the surface temperature during the CPO test with 0.05 g of catalyst [residence time = 6 ms, CH4/O2/He =2/1/4 (v/v)].

than those in CO, since the presence of the fast water gas shift reaction, that strongly depends on the temperature and affects the H2/CO ratio. The comparison of the tests performed using 0.05 or 0.2g of catalyst, shows that at the same flow rate (i.e. comparing the tests at 1.8, 3.0 and 6.0ms of Fig. 4 with those at 7.2, 12.0 and 24.0ms of Fig. 2) the surface temperatures in the tests carried out with 0.05g of catalyst are much higher not only at the end of the catalytic bed but also in the hot zone at the beginning. It may be hypothesized that the quartz wool putted at the end of the catalytic bed, much more close to the hot zone in the tests with 0.05g of catalyst than in those with 0.2g, reduces the heat dispersion, increasing the temperature in the whole catalytic bed through a possible complex mechanism [10]. The comparison of the methane conversion values for the tests performed with the two different amounts of catalyst (Table 1) enlightens the role of the reforming reactions occurring in the second part of the catalytic bed: in fact, both methane conversion and syngas selectivity are higher using 0.2g of catalyst, notwithstanding the lower surface temperatures. A comparison of the values of temperature on the surface at the end of the catalytic bed and those of equilibrium for CO2- and steam-reforming, calculated on the basis of the conversion and selectivity data (Teq-cr and Teq-sr for CO2 and steam-reforming, respectively) (Fig. 6), shows significant differences at high residence time values, at which the last part of the catalytic bed can be assumed as almost inactive since the very low temperatures. For intermediate residence times, the differences become smaller and the values of conversion and selectivity are those expected on the basis of the equilibria values. Finally, at very low contact times the surface temperature is significantly higher than the values calculated on the basis of the equilibria for CO2- and steam-reforming, since in these conditions the system is very far from the equilibrium and the partial oxygen conversion increases the amount of unconverted methane, affecting the values of the corresponding Teq. 5. CONCLUSIONS Starting from a Rh/Mg/A1 (1:71:28, as atomic ratio) HT precursor, it is possible to obtain by calcination and reduction a catalyst containing Rh metal particles well dispersed in a high-

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surface area mixed oxides matrix. The stability of this catalyst also in the hard 750 ~ 9 Teq-SR reaction conditions of the CPO of methane, allows to study a wide range of residence 650 9 Teq-CR times, without any interference by T (*C)~ 9 9 ~ --!1-Ts-out OQ 9 dishomogeneities and/or phase 550 - lilt.." ~ segregations. A detailed investigation of the temperature on the catalyst surface by 450 IR thermography shows that in the catalytic I 10 100 tests the exothermic reaction are confined Log(contact time) (ms) in the first part of the catalytic bed, where oxygen is fully converted. Fig. 6. Comparison of the temperature values By decreasing the residence time (and at the end of the catalytic bed (Ts-out) with consequently increasing the flow rate) those of equilibrium for CO2- and steam- oxygen saturates the sites of the first part reforming, calculated on the basis of the of the catalytic bed and the hot zone is widened. At very short residence times, it catalytic data. was observed a not complete conversion of the oxygen, with the appearance in the thermal profile of a second peak in the middle of the catalytic bed. The selectivity values in CO and H2 at very short residence times evidence that both have to be considered mainly as primary products. However, the presence of a maximum as function of the contact time in the selectivity in H 2 suggests a significant role also of the water gas shift reaction. The role of the reforming reaction is detected for values of the residence time in the range between 6ms to 12ms, at which surface temperatures at the end of the catalytic bed is sufficiently high to favour the further conversion of the methane till the equilibrium value by the reforming reactions. On the other hand, at higher residence times (i.e. > 12ms) the second part of the catalytic bed is almost inactive, due to its low temperature. Finally, the length of the catalytic bed affects the catalyst surface temperature as a function of changes in heat dispersion phenomena at the end of the bed, modifying the temperature of the whole catalyst. REFERENCES

1. J.R. Rostrup-Nielsen, Catal. Today, 18 (1993) 305. 2. D.A. Hickman and L.D. Schmidt, J. Catal., 136 (1992) 300. 3. G.A. Foulds and J.A. Lapszewicz, in Catalysis, Vol 11 (J.J. Spivey and S.K. Karval, Ed.s), The Royal Society of Chemistry, London 1994, p. 413. 4. W.J.M. Vermeiren, E. Blomsma, P.A. Jacobs, Catal Today, 13 (1992) 427. 5. D.A. Hickman and L.D. Schmidt, J. Catal., 138 (1992) 267. 6. D. Dissanayake, M.P. Rosinek and J.H. Lunsford, J. Phys. Chem., 97 (1993) 3644. 7. F. Basile, G. Fomasari, E. Poluzzi and A. Vaccari, Appl. Clay Sc., 13 (1998) 329. 8. F. Basile, L. Basini, G. Fomasari, M. Gazzano, F. Trifir6 and A. Vaccari, Chemm. Commun. (1996) 2435. 9. F. Cavani, F. Trifirb and A. Vaccari, Catal. Today, 11, 173 (1991). 10. L. Basini, A. Guarinoni and K. Aasberg-Petersen in Studies and Surface Science and Catalysis - Natural Gas Conversion V, Vol. 119 (A. Parmaliana, D. Sanfilipo, F. Frusteri, A. Vaccari and F. Arena Ed.s) Elsevier Amsterdam 1998, p. 699.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

化

455

A New Catalyst for Environmental Issues

an

Old

Process

Driven

by

L. Abrams a, W. V. Cicha b, L. E. Manzer *a, and S. Subramoney a aDuPont Central Research and Development, Experimental Station, Wilmington, DE 19880-0262, USA, b942 Tudor Ave., North Vancouver, BC, V7R 1X4 Canada The large-scale commercial production of phosgene for the manufacture of pharmaceuticals, agrochemical, polyurethane and polycarbonates occurs over carbon catalysts and has been practiced worldwide for more than 70 years [1]. These catalysts are traditionally derived from conventional carbon materials, such as coconut shells. Although carbon as a catalyst is extremely selective, 100-1,000 parts per million of carbon tetrachloride are produced as a byproduct in this process. DuPont now has developed a new commercial catalyst that reduces the carbon tetrachloride level by an order of magnitude. The catalyst has been scaled up from the laboratory to commercial production without any problems. Lifetime of the new catalyst is also 5 to 10 times longer than that of the conventional coconut based carbons. This presentation will outline the experimental program, some characterization details and a possible mechanism for CC14 production. This is a rare example of how good science has resulted in the discovery of a new catalyst for an old process that eliminates an environmental issue with no investment. 1. I N T R O D U C T I O N Although phosgene is extremely toxic and hazardous, it is widely used in the production of polycarbonates, polyamides, pharmaceuticals and agrochemical products. Global production exceeds 4000 kilotons per year. Phosgene was first prepared in 1812 by John Davy [2], from carbon monoxide and chlorine in the presence of sunlight. Its initial catalytic preparation using carbon as the activator was reported in 1878. The reaction was carried out at ambient temperature and gave a 100% yield [3]. In today's commercial process, high purity carbon monoxide and chlorine are passed through a tubular reactor, over a high surface-area carbon catalyst, at a temperature of about 80~ and a pressure of 70 psig. Typical carbon catalysts have surface areas of >1000 m~/gm and are obtained from

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natural sources such as coconut trees. While the yield is close to the 100% reported in 1878, small amounts of other chlorocarbons such as tetrachloroethylene and carbon tetrachloride are produced at levels in the vicinity of 500 ppm (by wt.). Due to increasing concern over the production of ozone-depleting chlorocarbons such as CC14, these low levels become quite significant, and consequently a catalyst program was initiated at DuPont to see if this byproduct could be reduced or eliminated. 2. D I S C U S S I O N 2.1 E x p e r i m e n t a l

To study the chlorination of carbon monoxide, a 1/4" Inconel reactor with on-line GC-MS capability was built and operated at 1 atmosphere pressure. Typical contact times were 0.9-12 seconds (corrected for temperature). Two modes of operation were used: a low temperature (LT) mode (125~ to evaluate relative activities for COC12 production rates and a high temperature (HT) mode (300~ to rank the relative levels of CC14 generation. In the HT mode, CC14 was determined quantitatively by mass spectroscopy using the selective-ion technique for chlorine isotopes. 2.2 S o u r c e o f t h e CCI 4

Before any m e a n i n g ~ l investigation could be commenced, it was critical to determine the source of CC14 in the process. We considered four likely distinct possibilities: 1) chlorination of trace quantities of methane in the CO feed; 2) overchlorination of COC12 to CC14; 3) disproportionation of COC12 to CC14 and CO2; and finally 4) direct chlorination of the carbon catalyst. Since the level of CH 4 in our ultra-pure plant CO gas is much less than 100 ppm, methane chlorination was ruled out as a principal source. Naturallyoccurring coconut carbons are well known to contain high levels of impurities such as P, Fe, K, Na Si and so on, which can cause undesirable side reactions. Disproportionation of COC12 due to some of these impurities, like Fe, in the carbon was a real possibility, although washing the carbon with HC1 and HF to remove trace metal impurities did not produce the desired effect of lowering CC14. Furthermore, the level of CC14 generated by the numerous and diverse carbons tested in the lab reactor did not correlate well at all with the respective carbons' metal content. We were thus left with the possibility of direct carbon catalyst chlorination as the source of the CC14. To prove this hypothesis, we purchased 13C-labeled carbon monoxide with a composition of 10% 13CO/90%x2CO. Using this feed, in the HT mode, we carefully analyzed the reaction products and found that the ~3C content of the CC14 thus generated, was no greater than the

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natural abundance of ~3C (1.1%) in carbon. The accompanying ~3C level i n t h e COC12 produced (-10%) was expectedly very close to that of the labeled feed gas. This result ruled out both the disproportionation of phosgene and the overchlorination of phosgene as possible sources of the CC14 and conclusively established direct chlorination of the carbon catalyst as the only significant source of carbon tetrachloride.

2.3 Catalyst Scouting Protocol Chlorination of carbon to produce e e l 4 obviously is an oxidation process. Therefore, we reasoned that a carbon which was oxidatively stable at high temperatures also could be an excellent catalyst for the production of phosgene, without generating any CC14. We needed a method to screen high surface area carbons for their oxidative stability and subsequently developed a rapid-screening technique using thermal gravimetric analysis (TGA). This avoided the need to test an exceedingly wide variety of catalysts in a toxic and hazardous environment. The protocol for the scouting was to evaluate, by weight loss, the stability of a catalyst in air over a wide temperature range. The TGA analysis was done using a TA Instruments analyzer with an air flow rate of 80 ml/min over the carbon sample. Each sample was heated in air for exactly the following times and temperatures; 1) 125 ~ for 30 minutes 5) 400 ~ for 45 minutes 2) 200 ~ for 30 minutes 6) 450 ~ for 45 minutes 3) 300 ~ for 30 minutes 7) 500 ~ for 30 minutes. 4) 350 ~ for 45 minutes The weight loss was measured at each interval and finally after completion of the heating cycle. Not unexpectedly, many carbons suffered severe weight loss due to combustion; several, however were found to be quite stable. As shown in Table 1, only 2.4% of the commercial phosgene catalyst remained aider the oxidative screen. In contrast, the most oxidatively stable carbon came from the Boreskov Institute of Catalysis and is trademarked Sibunit [4,5]. These carbons are prepared at high temperature through a coking process, then converted, also at high temperature, under an oxidative environment to an altered carbon with high area and macroporosity but with little or no microporosity. A shot-coke carbon from Conoco also showed remarkable stability but, owing to its low surface area, was found to be inefficient as a phosgene producing catalyst.

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Table 1. Percent of Carbon Remainin~ vs Temperature in TGA Screenin~ Test ........ Carbon 125~ 200~ 300~ 350~ 400~ 450~ 500~ Plant Coconut 92.24 91.7 88.67 81.14 57.19 6.456 2.408 Sibunit-2 99.52 99.48 99.46 99.42 99.35 99.16 98.37 Conoco Shot Coke 99.99 99.99 99.99 99.97 99.81 98.74 92.45 We next tested select carbons from the TGA study for phosgene production rates in the LT mode and CC14 generation in the HT mode. Interestingly, a remarkably good correlation resulted between the weight loss in the TGA and the CC14 levels measured. This data is plotted in Figure 1. The CC14 levels generated by the Sibunit carbon in the lab reactor were reduced by an order of magnitude compared to that of the commercial coconut carbon (Fig. 1), to about 50 ppm, which was the goal of our research. 350

[

Fresh Coconut Carbon

300

Levels 250

J

of

CC!4 200 (ppm) 150

jz

100

J

J

J

U u d Co(mnut Cmbon

" J

50

22

0 0

20

40

60

80

100

% Wt. Loss Between 125 and 500~

Figure 1. Carbon Tetrachloride Production vs TGA Stability With these promising results, a commercial charge of the Sibunit carbon was obtained from Siberia, Russia and used in the plant. This was a huge scaleup factor since no pilot plant test was run. Fortunately, the plant started up very well; phosgene production levels were comparable to the laboratory results. After 1 year of production, the catalyst continued to perform like new. Regular analysis of the commercial reactor offgas indicated that the carbon tetrachloride level was indeed below 50 ppm, by weight, as predicted in the small laboratory reactor. After 2 years of full-scale operation, the catalyst continued to perform very well with a total reduction of about 95% in the CC14 levels. The commercial operation actually exceeded our laboratory performance. 3. C H A R A C T E R I Z A T I O N Following its six-month use in the plant reactor, the coconut carbon displays dramatic change in microstructure, as indicated by the highresolution TEM image in Figure 2. Compared to the fresh catalyst, the used

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coconut carbon also appears to have relatively few micropores. It is likely that the micropores in the starting material are preferential locations for diatomics such as chlorine to condense into a more ordered liquid state under the plant's reaction conditions. Literature studies have shown that O 2 arranges itself into ordered clusters in carbon micropores at low temperatures [ 6 ] . Theoretical studies of CC14 in a graphitic micropore environment predict that it should exist as an ordered "plastic crystal" already at 303K, whereas bulk CC14 does not exhibit such ordering at temperatures in excess of 253 K [7]. In general, the overlap of energy potentials from opposite walls of a given micropore serves to enhance the adsorbed molecule-surface interaction, via the increased ordering of the adsorbent molecules. This general tendency analogously helps explain not only the liquefaction of the chlorine on our plant carbon's micropores, but also the diatomic's resultant oxidative reaction with the carbon atoms surrounding it to form chlorocarbons such as the observed CC14. It is therefore expected that the micropore volume of the carbon would decrease after its use as a catalyst. This loss of micropores and the resultant loss in volume were confirmed by pore volume measurements. Comparison of the pore volumes of the fresh, 0.50 cc/gm, and used, 0.43 cc/gm, coconut carbons indicates a 15% loss in pore volume. As expected, the reduction occurs exclusively in the micropore size range of <20~. Elemental analysis via energy dispersive spectroscopy (EDS) of the "darker" appearing regions in the used coconut carbon (see Fig. 2) confirms that it contains a significant quantity of adsorbed but "unreacted" chlorine. Qualitative analysis of various EDS spectra indicates that the material contains approximately one atom of chlorine for every 4 - 5 atoms of carbon.

Figure 2 Electron Micrograph and EDS of Used Plant Coconut Carbon Catalyst

Figure 3. Electron Micrograph and EDS of Fresh Plant Coconut Carbon Catalyst

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The Sibunit carbon, on the other hand, appears to undergo no significant change in its microstructure after use in the D u P o n t plant environment over the same time period as the original coconut carbon (see Figure 3). Elemental analysis by EDS in the TEM also confirms t h a t there is no detectable uptake of CI~ by the Sibunit carbon during the time on-line. This finding is also s u b s t a n t i a t e d by macropore volume m e a s u r e m e n t s of fresh and used Sibunit catalyst samples, yielding values of 0.58 cc/g and 0.63 cc/gm respectively. The a p p a r e n t slight increase in non-microporous volume of used catalyst, without a concurrent increase in byproduct CCI,, suggests t h a t some of the carbon reacted to form phosgene, and t h a t this reaction results in the formation of new macroporous pits. 4. C O N C L U S I O N S We have developed and commercialized a new catalyst for the production of phosgene from carbon monoxide and chlorine. Levels of byproduct carbon tetrachloride are reduced by an order of m a g n i t u d e relative to a previously employed conventional coconut based carbon catalyst. Scaleup to a commercial u n i t from a small laboratory unit was accomplished without additional investment. Uptime and reactor performance also have significantly improved. P a t e n t s [8] are in the process of being issued and global licensing of the technology has begun as well. REFERENCES 1 T. A. Ryan, C. Ryan, E. A. Seddon, and K. R. Seddon, Phosgene and Related Carbonyl Halides, Elsevier, 1996, pg. 167 2. J. Davy, Philos. Trans.R. Soc. Londonl02 (1812) 144-151. 3. E. Paterno, Gazz. Chim. Ital., 8 (1878) 233-234. 4. V. F. Surovikin, G. V. Plaxin, V. A. Likolobov and I. J. Tiunova, Porous Carbonaceous Material, US Patent No. 4,9789,646 (1990) 5. V. A. Likholobov, V. B. Fenelonov, L. G. Okkel, O. V. Goncharova, L. b. Avedeeva, V. I. Zaikovskii, G. g. Kuvshinov, V. A. Semikolenov, V. K. Duplyakin, O. N. Baklanova, and G. V. Plaksin, React. Kinet. Catal. Lett. Vol 54, No. 2, (1995) 381-411. 6. H. Kanoh and K. Kaneko, J. Phys. Chem. 99 (1995) 5746. 7. T. Suzuki, K. Kaneko, and K. E. Gubbins, Langmuir 13 (1997) 2545 8. W. A. Cicha and L. E. Manzer, Carbonaceous Materials in Phosgene Manufacturing Process, International Publication Number WO 9730932; and W. A. Cicha and L. E. Manzer, Manufacture of phosgene having low carbon tetrachloride from carbon monoxide and chlorine, International Publication Number WO 9800364.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

S u p e r c r i t i c a l S y n t h e s i s o f D i m e t h y l C a r b o n a t e f r o m CO 2 a n d methanol Tiansheng Zhao a, Yizhuo Han b, and Yuhan Sun b aKey Laboratory of Energy Sources & Chemical Engineering, Mailbox 114, Ningxia University, China 750021 bState Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan, China 030001 Dimethyl carbonate could be directly synthesized as the u n i q u e product from supercritical CO2 and m e t h a n o l at 305K using nickel acetate as the catalyst. The synthesis was sensitive to the pressure and showed a m a x i m u m for DMC yield at 9.3MPa. The concentration of m e t h a n o l showed an obvious influence on both yield and selectivity of DMC. Ni acetate a p p e a r e d to be the precursor of the catalyst. The formation m e c h a n i s m of DMC in supercritical phase was proposed.

I. INTRODUCTION Much attention h a s been paid to the production of dimethyl carbonate (DMC) due to its environmentally benign behavior as organic synthesis intermediates and promising gasoline additive 1-2. DMC has mainly been synthesized via non-phosgene route (oxidative carbonylation of methanol) since 1980's 3-s. However, direct synthesis from CO2 and methanol is most attractive due to the low-cost of CO2. Unfortunately, the synthesis is limited by t h e r m o d y n a m i c equilibrium, a n d favorable only at low t e m p e r a t u r e (See Fig. 1). Bu2Sn(OMe)26, metal(IV) tetra-alkoxide 7, m a g n e s i u m dialkoxide s a n d other organometallics have been reported as catalysts for the direct synthesis. But the activity was low even in the presence of dehydrates a n d promoters. The catalysts were also easily decomposed by water, a n d considered not to overcome limits of the reaction 6. In addition, the reaction was in most cases conducted above 423K, which was far from the critical t e m p e r a t u r e of CO2. On the other hand, coupling reaction processes via epoxides, a m m o n i a , alkyl

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A10 8

C3 0 250 300 350 400 450 500 550 Temperature(K) Fig. 1. Calculated equilibrium conversion of m e t h a n o l (CO 2 pressure: 7.0MPa). chlorides 9, trimethyl orthoacetatel~ or tert-amines + acetylenell have been developed for the synthesis of DMC utilizing CO2 a n d methanol. In this p a p e r was reported t h a t n e a r supercritical CO2 conditions for improving direct synthesis of DMC via the reaction of CO2 with methanol. 2. E X P E R I M E N T A L 2.1. Reaction procedure In a typical experimental, m e t h a n o l (100mmol) a n d catalyst (4mmol) were p u t into a n 80mL-stainless steel autoclave. After CO 2 was i n t r o d u c e d to the prescribed p r e s s u r e at 285K, the autoclave was h e a t e d u p to the prescribed t e m p e r a t u r e with magnetically stirring, a n d m a i n t a i n e d d u r i n g 2 h o u r s reaction. The p r o d u c t s were analyzed by gas c h r o m a t o g r a p h y with a t h e r m a l conductivity detector. 2.2 XRD measurement Powder X-ray diffraction p a t t e r n s of the catalysts were recorded on Rigaku Dmax-rA X-ray diffractometer with CuK~ radiation at a s c a n n i n g rate of 6 ~ / min. 2.3. EXAFS measurement EXAFS m e a s u r e m e n t s of the catalyst were carried on the s y n c h r o t r o n radiation facility at the National Laboratory for High Energy Physics (Beijing) with EXAFS analysis package.

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3. R E S U L T S AND D I S C U S S I O N

3. I. I n f l u e n c e o f p r e s s u r e o n t h e s y n t h e s i s At non-supercritical conditions, the p r e s e n t catalyst showed low yield and selectivity of DMC as the literatures reported. After reaction at 413K and 7.0MPa for 12-hour, only 62.0% DMC was produced with a MeOH conversio~ of 2.1%, a n d at the same time 76.1% of methyl acetate produced as by-product (see Table 1). However, the yield of DMC was highly improved at near supercritical CO2 conditions (305K for 2-hour), a n d DMC was the unique product. As the p r e s s u r e changed from 7.4 to 10.3 MPa (correspond to the reduced p r e s s u r e 1.1N 1.4), the yield of DMC reached the m a x i m u m at 9.3Mpa, which was 13 times higher t h a n t h a t at non-supercritical conditions, showing t h a t the production of DMC was p r e s s u r e sensitive. S u c h p h e n o m e n o n was also observed in DMC synthesis from CO2 a n d trimethyl orthoacetate (dehydration p r o d u c t of methanol) at 423K for 24h 10. It should be noted t h a t this improvement could be attributed to the supercritical p h a s e behavior itself r a t h e r t h a n high CO2 concentration 12. The p r e s s u r e d e p e n d e n c e of reaction rate could be a consequence of CO2 compressibility in near-critical region, which led to very large and negative partial molar volume of solutes 13. F u r t h e r m o r e , the results suggested t h a t a different reaction p a t h w a y might occur at the supercritical conditions. Table I Influence of p r e s s u r e on DMC synthesis at near-supercritical conditions a

Pressure (MPa)

DMC Co n c. (Wt. %)

7.0 b 7.4 8.3 9.3 10.3

1.62 5.91 6.04 8.03 3.23

Yield %/mol-cat. D MC M e thylace tate 62.0 569 587 796 350

76.1 0 0 0 0

a Ni acetate 4H20/MeOH=1:25(molar); 305K;2h. b413K;12h.

3.2. Influence of methanol concentration on the synthesis It was interesting t h a t with the rise of the a m o u n t of methanol, the yield of DMC decreased while methyl acetate was produced (see Fig. 2). In this case, the increase in the concentration of methanol likely c a u s e d the shift of CO2

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supercritical point a n d t h e n resulted in the change of reaction. Thus, the supercritical p h a s e composition was i m p o r t a n t to the control of p r o d u c t distribution. 600,,

200

40ok- e ylace t

"

/

o~~300' ~

100~ 50 ~

0.1

0.2 0.3 0:4 0.5 Amount of methanol/mol

0 0.6

Fig.2. Influence of m e t h a n o l c o n c e n t r a t i o n on DMC synthesis Ni acetate 4 H 2 0 / M e O H = I " 25(molar), 8.5 MPa, 305K, 2h.

3.3. S t r u c t u r e of the c a t a l y s t and r e a c t i o n m e c h a n i s m The c h a n g e of reaction mixture from green into bright jade green, a n d the d i s a p p e a r a n c e of XRD p e a k of nickel acetate after the reaction (see Fig.3), s u g g e s t e d the change of the s t r u c t u r e of acetate nickel. EXAFS m e a s u r e m e n t s (see Fig.4) also indicated t h a t the coordination n u m b e r of nickel r e d u c e d from 6 to 4.56 a n d t h a t the average coordination distance of Ni-O s h o r t e n e d from 2 . 0 9 A I141 to 2.067A after the reaction. Therefore, acetate nickel was very likely the p r e c u r s o r of the catalyst u n d e r the supercritical conditions. Considering t h a t the oxygen a t o m in CO2 could show Lewis acidity, the formation of DMC might u n d e r g o three steps. Nickel methoxide was firstly formed a n d t h e n nickel methoxide c a r b o n a t e by "normal" insertion of CO2. The f u r t h e r action between methyl acetate a n d c a r b o n a t e species led to the production of DMC. The d i s a p p e a r a n c e of methyl acetate at supercritical CO2 conditions suggested t h a t the formation of methyl acetate be s u p p r e s s e d while the formation of DMC a n d its selectivity were improved. 4. CONCLUSION In conclusions, DMC could be directly synthesized in high yield a n d 100% of selectivity from supercritical CO2 a n d m e t h a n o l u s i n g nickel acetate as the catalyst at 305K. This provided a new clue for DMC s y n t h e s i s at low t e m p e r a t u r e via the reaction of CO2 with methanol.

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(b) ~'0

~5

2'0

2'5

Jo

20 Fig.3. XRD p a t t e r n s of Ni acetate before (a) a n d after supercritical reaction (b)

I t .

0

1'2

2'4 Distance(~)

Fig.4. EXAFS p a t t e r n s of nickel acetate before ( reaction ( ...... ).

0 ) a n d after supercritical

The a u t h o r s t h a n k the financial s u p p o r t s from the NSF of C h i n a a n d the Natural Science foundation of Shanxi province. REFERENCES

1. Y. Ono., Appl. Catal. General A:, 155 (1997) 133. 2. M. A. Pacheco, C. L. Marshall, Energy & Fuels, 11 (1997) 2.

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3. U. Romano, R. Tesei, M. M. Mauri and P. Rebora, Ind. Eng. Chem. Prod. Res. Dev., 19 (1980) 396. 4. S. T. King, J. Catal., 161 (1996) 530. 5. K. Nishihira, T. Matsuzaki, and S. Tanaka, Shokubai, 37 (1995) 68. 6. N. Yamazaki & S. Nakahama, Ind. Eng. Chem. Prod. Res. Dev., 18 (1979) 249; J. Kizlink, Collect. Czech. Chem. Commun., 58 (1993) 1399; K.Ko, O.Fujimaro, J p n KoKai Tokkyo Koho 95,224,011(1995). 7. J. Kizlink, I. Pastucha, Collect. Czech. Chem. Commun., 60 {1995) 687. 8. X. Gui, F. Cao, D. Liu, D. Fang, J o u r n a l of Chemical Engineering of Chinese Universities, 12 (1998) 152. 9. W. McGhee and D.Riley, J. Org. Chem., 60 (1995) 6205. 10. T. S a k a k u r a , Y. Saito, M. Okano, J. Chio, and T. Sako, J. Org. Chem., 63 (1998) 7095. 11. Y. Sasaki, Chem.Lett., (1996) 825. 12. P. G. Jessop, T. Ikariya & R. Noyori, Nature, 368 (1994) 231. 13. C. A. Eckert, B. L. Knutson & P. G. Debenedetti, Nature, 383 {1996) 313. 14. J. N. V. Niekerk and F. L. Schoening, Acta Crystallographica, 6 (1953) 609.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

M e c h a n i s m of H D N over Mo and Nb-Mo Carbide Catalysts Viviane Schwartz a, V.L. Teixeira da Silva b, Jingguang G. Chen c, and S.Ted Oyama a aDepartment of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0211, USA bInstituto Militar de Engenharia, Pra~a General Tibfircio 80, Praia Vermelha, Rio de Janeiro, RJ 22290-270, Brasil CDepartment of Materials Science and Engineering, University of Delaware, Newark, DE 19716, USA Comparison of the hydrodenitrogenation rate of a series of isomeric amines was performed over M02C, NbC, NbM02-O-C and MoS2/SiO2 catalysts. It is shown that the reaction occurs mainly through an elimination mechanism and that the order of reactivity does not correlate with the surface acidity of the catalysts. 1. INTRODUCTION More stringent requirements in reducing the content of sulfur, nitrogen, oxygen and aromatics in crude oil have stimulated increasing attention on new catalysts for hydrotreating processes. Transition metal carbides and nitrides have shown excellent potential for use in hydrodenitrogenation (HDN) and hydrodesulfurization (HDS) reactions [1]. Bimetallic compounds formed from two different transition metals have been shown to have enhanced HDN and HDS activity over their monometallic counterparts and a commercial sulfide catalyst [2,3]. In this work, we investigate the activity of a Nb-Mo bimetallic carbide catalyst. The major advantage of this bimetallic carbide over the commercial sufide catalyst resides on its higher activity towards the HDN reaction. Therefore, this investigation focuses on the study of the mechanism of the C-N bond cleavage during HDN on this bimetallic carbide, and a comparison of its action with the monometallic carbides and a sulfide catalyst. For this purpose, the reactivity of three isomeric amines, which have different structures and different number of hydrogen atoms on the carbon atoms in the cz and 13 positions with respect to the nitrogen atom, were tested in the presence of a sulfur compound. This method was developed by the group of Breysse [4] and aimed to distinguish between the two mechanisms most often proposed: 13-elimination and nucleophilic substitution. Acid properties of the surface, which are expected to influence the mechanism, were determined by temperature programmed desorption (TPD) of the gaseous base, NH3. In order to better determine the effect of acidity, comparisons of the carbides were made with MoS2 supported on neutral SiO2. 2. E X P E R I M E N T A L The synthesis of the bimetallic carbide involved two stages. First, the bimetallic oxide precursor was prepared by the solid-state fusion of two monometallic oxides with a metal ratio (Mo/Nb) equal to 2.0. Second, the precursor material was carburized by temperature-

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programmed reaction using a reactive gas flow of 20% cn4/I--I2 up to a temperature of 1063 K. For the preparation of the monometallic Mo2C and NbC, only the second step applied, with the corresponding metal oxides being used during the carburization. The MoS2/SiO2 catalyst used as the sulfide reference, was prepared in situ by flowing 102 ~tmol sl (150 cm 3 min "l) of 10% H2S/H2 over 5%MoO3/SiO2 for 2 h at 673 K. The catalysts were characterized by CO chemisorption (02 chemisorption for the sulfide catalyst), N2 physisorption, X-ray diffraction (XRD), and TPD of NH3. The values of CO and 02 chemisorption were used for the calculation of turnover rates and proper comparison of catalytic activity. Surface areas were determined by N2 physisorption, while XRD patterns were acquired using the powder method. TPD of NH3 following saturation and purging in a He stream, was employed to determine the strength of the acid sites and their quantity. The bimetallic carbide was also characterized by near-edge X-ray absorption fine structure (NEXAFS) and the reactivity of all catalysts were tested for their HDN of quinoline (QNL) and HDS of dibenzothiophene (DBT) in a three-phase, trickle-bed reactor at 3.1 MPa and 643 K. The NEXAFS and reactivity experiments are described in detail elsewhere [3]. The reactivity of the three different amines (n-pentylamine, tert-pentylamine and neopentylamine) was tested in the same trickle-bed reactor, employing liquid phase conditions at 3.1 MPa and various temperatures. The feed composition consisted of 2000 ppm of nitrogen (amine), 2000 ppm of octane (internal standard) and 3000 ppm of sulfur (dimethyl disulfide) in a tetradecane solvent. The H2 flow rate was 110 ~tmolsn (150 cm3/min, NTP) and the liquid feed was introduced at a rate of 5 cm 3 h -l. The liquid samples were analyzed off-line with a gas chromatograph using a fused silica capillary column (CPSIL-5CB) and a flame ionization detector. 3. RESULTS AND DISCUSSION The XRD pattern of the bimetallic oxycarbide (Fig. 1) reveals a single phase material with peaks indicating a facecentered cubic metallic arrangement, similar to one of its parents (NbC), but different from the other (Mo2C), which presented an hexagonal closed-packed structure. The line-broadening of the X-ray peaks of the bimetallic eoxycarbide indicates the presence of small crystallites. C The surface composition h .... Nbc of the bimetallic oxycarbide was also characterized by NEXAFS [3]. The 10 20 30 40 50 60 70 90 Angle 120 monometallic carbides are denoted here as oxycarbides to Figure 1" X-ray diffraction pattern of Nb-Mo oxycarbide indicate some residual oxygen and its parent monometallic carbides. content. The main features of the C K-edge of the monometallic oxycarbides as well as of the bimetallic carbide (Fig.2) m

,

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.

I

,

I

.

I

,

I

I 0

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indicate the presence of surface carbides. These features consist of two peaks at low energy (286 and 289 eV) and one peak at higher energy (298 eV) [5]. The presence of some graphitic carbon on the surface is revealed by a broad feature at 292.5 eV. Besides the general [C K-Edge( Oxycarbide similarity, the bimetallic 289 oxycarbide shows differences in i Graphite the line shape and relative peak Oxycarbide 292 Oxycarbide 298 intensities when compared to either NbOC and Mo2OC indicating the formation of an ~I I I I Bulk alloy with unique electronic properties. A summary of the CO tO uptakes, surface areas and /I ' b-~_ I Bulk reactivity of each catalyst is m presented in Table 1. It can be observed that Mo2C presents the ; highest site density and that NbC shows very low surface areas and CO uptake. The reactivity of the monometallic NbC and the 280 290 300 310 320 sulfide catalyst for both HDN and Incident Photon Energy (eV) HDS are very low. The bimetallic carbide presents higher Figure 2: NEXAFS C K-edge features of Nb-Mo levels of activity than the oxycarbide and its parem monometallic carbides. corresponding monometallic carbides, although its level varies with the ratio Mo/Nb [3]. ....

I ....

; ....

I ....

' ....

I ....

' ....

I ....

~ ....

I ....

Table 1

Characteristics and reactivity of catalysts Mo2C

NbC

NbMo2-O-C

MoS2/SiO2

SA (m 2 g-i)

67

23

94

88

CO uptake (l.tmol g-l)

106

5

25

17c

Site density (x 1015cm"2)

0.095

0.013

0.016

0.016

% HDN (QNL) a

47

5

57 b

2

% HDS (DBT) a

43

4

59 b

3.4

aBased on a catalyst amount equivalent to 70 l,tmol of CO uptake (carbide catalysts) and 56 ~tmol of 8902 uptake (sulfide catalyst) loaded in the reactor. For NbC the values correspond to 30 m 2 loaded in the reactor (20 l~mol) [3]. bReactivity of a bimetallic with a composition of NbMo~.75-O-C [3], which we believe has a similar crystal structure to the bimetallic prepared in this study. CValue based on the 02 uptake.

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The amount of NH3 desorbed during TPD from each of the carbide catalysts gives an estimate of the number of acid sites present on the surface of these catalysts (Fig.3). The sulfide NbMo2-O-C catalyst MoS2/SiO2 shows 9.73 mmol goat"1 the lowest acidity after subtraction of the amount Mo2C of H20 desorbed from the 6.87 mmol goat-~ support material, which is responsible for the NbC appearance of a second 3.73 mmol geat "! peak at higher temperatures (900 K). The low CO uptake indicates that the H20 MoS2/SiO2 3.26 mmol gcat"1 NbC surface is covered by graphitic carbon formed at the high temperature 200 400 600 800 1000 1200 1400 required for the synthesis of Temperature / K this catalyst. It is the likely Figure 3" Temperature-programmed desorption of NH3. reason this catalyst presents a lower acidity. The bimetallic material has a higher acidity compared to Mo2C due to the presence of niobium on the surface. An important step during HDN reactions is C-N bond cleavage. Several mechanisms have been proposed [6], which consist of either a [3-elimination (E2) or a nucleophilic substitution (SN2). Both mechanisms are related to the acid-base properties of the catalyst. In an elimination reaction, a base pulls a hydrogen atom from the carbon in the fl position and a carbon-carbon double bond is formed; in a nucleophilic substitution reaction, the base acts on the carbon in the ot position with the subsequent creation of a bond without intervention of any hydrogen atom. Monomolecular mechanisms (El and SN1) are also a possibility where the transition state corresponds to the formation of a carbocation. In order to differentiate between the possible mechanisms, we employed a method developed by the group of Breysse [4,7], which makes use of the relationship between the reactivity and the structure of a group of isomeric amines. The reactivities of n-pentylamine (2 H on the Ca and 2 H on the Cp), tertpentylamine(no H on Ca and 8 H on Cp), and neo-pentylamine (2 H on Ca and none on Cp) should depend on the operative mechanism (Table 2). Our work can be distinguished from that of Breysse, from the case of liquid-phase conditions and higher pressures. The nature of the products and intermediates obtained for all catalysts are consistent with the mechanisms proposed: saturated or unsaturated hydrocarbons, resulting from an elimination reaction, or diamines and thiols, resulting from a nucleophilic reaction. The values of conversion observed for NbMo2-O-C (Fig. 4) using different temperatures shows that the formation of HDN products is almost zero when no hydrogen atoms are present on the C~ and increases with the number of these hydrogen atoms. These results suggest that the removal of the hydrogen on the C~ is the key step and that the 13-elimination mechanism should be responsible for this reaction. |

I

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I

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I

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Table 2 Reactivities of different amines [7] Possible mechanisms

Molecules

F

X- = strong base or nucleophile C = amine carbon I)

~ + X-/.-~C~

SN1 "

|s~2

x-/--~c~

_El E2

X-

*

Cp~C,~

et

f~ ~NH2

SN2 > E2 > > SN1 ~ E1

~ N H 2 II)

III)

~

N

H

E1 > E2 ~ SN1 > > SN2

2

SN2 > SN1

i00 CH3 I CH3--CH2- C -- NH2

80

!

CH3 r 60

CH3-- (CH2)4--NH2

O

=m

ID O

U

40 20 ~~~_.~.~ir~.,,,~

~

CH3_C_CH2_ NH2 CH3

420

I

440

'

I

460

'

I

480

'

I

500

'

I

520

'

I

540

'

I

560

'

580

Temperature / K

Figure 4: Amines conversion over NbMo2-O-C. The overall rate of amine conversion for all catalysts tested (Fig. 5), shows exactly the same trend as the one obtained for the bimetallic carbide. MoS2/SiO2 is the most active and, likewise, elimination is the main reaction path. NbC shows the same levels of conversion as a blank. An interesting observation is that the activity sequence for the C-N bond cleavage exhibited by the catalysts shown in Figure 5 is the opposite to the sequence obtained for the

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HDN of quinoline (Table 1). This is because deamination is easier than denitrogenation, as indicated by the lower temperatures required for the former (550K vs. 643 K). It is likely that the 2.5 carbides (particularly NbMo2C) are more 2.0 ~n effective than the sulfide at 1.5 breaking C-N bonds in heterocyclic structures like 1.0 quinoline. The expected ~.5 correlation between acidity ).0 (Fig. 3) and reactivity (Fig. 'oVs,o 5) was not found when the C results obtained for the carbide and sulfide 2 catalysts are compared. This may indicate that acid sites alone are not sufficient Figure 5" Overall turnover rates at 500 K of tert- for this reaction, but that pentylamine, n-pentylamine, and neo-pentylamine on adjacent vacancies (base) NbM02-O-C, Mo2C, and MoS2/SiO2. and acid sites are required.

/

CONCLUSION Our results suggest that a 13-elimination mechanism is the main reaction path for the amine bond cleavage over the bimetallic Nb-Mo oxycarbide, and the same is true for the monometallic Mo2C and the molybdenum sulfide catalyst. However, the involvement of acidity in C-N bond cleavage remains an open question, since no correlation was found between acidity and reactivity.

Acknowledgement: This paper was written with the support from the U.S.Department of Energy, Office of Basic Energy Sciences, Grant No DE-FG26-97FT97265. V. Schwartz is grateful to CAPES (Brazil) for the scholarship received during the development of this work. REFERENCES 1. S.T. Oyama in The Chemistry of Transition Metal Carbides and Nitrides; Oyama, S.T., Ed., Black Academic and Professional, London, 1996, p. 1 2. S.Ramanathan, C.C.Yu, and S.T.Oyama, J. Catal., 173 (1998) 10-16 3. C.C.Yu, S.Ramanathan, B.Dhandapani, J.G.Chen, and S.T.Oyama, J.Phys.Chem. B, 101 (1997), 512-518 4. J.L.Portefaix, M.Cattenot, M.Guerriche, and M.Breysse, Catal. Lett., 9 (1991) 127-132 5. J.G.Chen, Surf. Sci. Rep., 30 (1997) 1 6. N.Nelson and R.B.Levy, J.Catal., 58 (1979) 485 7. M.Cattenot, J.L. Portefaix, J.Afonso, M.Breysse, M.Lacroix, and G.Perot, J.Catal., 173 (1998) 366-373

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Comprehension of the promoting effect in the MCr2S 4 mixed sulfide catalysts. P. Afanasiev, A. Thiollier, P. Delichere, and M. Vrinat. Institut de Recherches sur la Catalyse, 2 Avenue A. Einstein, 69626, Villeurbanne, C6dex, France. e-mail [email protected] 1.fr

The sequence of mixed sulfides MCr2S 4 (M = Mn, Fe, Co, Ni, Cu, Zn, Cd, 2Na) was prepared using different chemical methods including sulfidation of mixed oxide dispersions and pyridinium dichromate complexes or reactions in the polysulfide containing ionic fluxes. The solids were characterized using X-ray diffraction, BET surface measurements, X-ray photoelectron spectroscopy (XPS), low energy ion scattering spectroscopy (LEIS), and temperature programmed reduction (TPR). The catalytic properties of mixed sulfides were determined in the model reactions of tetraline hydrogenation and thiophene hydrodesulfurization. The variations of specific activity of MCr2S 4 solids as a function of M in both reactions were similar, having the maxima for the NiCr2S 4 and CoCr2S 4 compounds. The differences in the catalytic activities were in good correlation with the reducibility, determined as the amount of sulfur removed from the catalysts by hydrogen at 573 K. The reducibility of MCr2S 4 solids is related to the electronic structure, as illustrated by the Extended Htickel calculations. 1. INTRODUCTION The insight on the catalytic properties of sulfides, the nature of active sites and promotion mechanisms still contains a matter of controversy despite a great effort on this topic. The studies of simple model systems such as non supported binary and ternary sulfide dispersions can be helpful to understand the rules determining catalytic properties of these systems. The sequences of catalytic activity of binary sulfides have been obtained in previous works [1, 2]. Correlations were built between the hydrogenation (HYD) or hydrodesulfurization (HDS) activity and some electronic and/or thermodynamic properties [3-5]. However, comparison of catalytic properties is more straightforward for the rows of similar chemical compounds, where the elements have similar coordination numbers and oxidation states. That can be done using several mixed sulfides of Cr(III) of general formula MCr2S 4 where M is a bivalent metal, which often adopt thiospinel structure. This structure is versatile in being able to accommodate a wide range of metal cations, presenting therefore a suitable model system for the fundamental studies of the promoting effect or for attempting to optimize catalytic performance [ 6 ]. This paper describes synthesis of dispersed MCr2S 4 sulfides and study of their properties as model hydrotreating catalysts.

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2. EXPERIMENTAL

The sequence of the mixed sulfides MCr2S4 (M = Mn, Fe, Co, Ni, Cu, Zn, Cd, 2Na) was prepared using different methods including sulfidation of mixed oxide dispersions by H2S at 673 or 873 K (eq.1), from pyridinium dichromate complexes at 673 K (eq.2), or by means of the reactions in the polysulfide- containing ionic fluxes at 573 K (eq.3). MCr204 +4H2S => MCr2S4+4H20

(1)

MCr207 2Py +7H2S => MCr2S4+3S + 2Py + 7H20

(2)

3Na2CO 3 + 3S + CrC13 => 3NaC1 + NaCrS2 + Na2SO4 + 2CO2 + CO

(3)

Hydrous oxide precipitates were obtained by adding aqueous ammonia to the solutions of mixtures of 0.2 M. Cr(NO3) 3. 9H20 and 0.1 M of the hydrated chloride or nitrate precursor of the second metal. Dichromate pyridinium complexes were precipitated after addition of 50 ml of pyridine to the solutions of 0.01 mol (NH4)2Cr207 and 0.01 mol of the second metal nitrate in 100 ml of distilled water. The precipitates were aged for 2 days in the solution, then filtered, washed with distilled water and dried at room temperature. Sulfidation was done in a Pyrex reactor at 673 K for 4 h under a flow of 15%H2S in N2 mixture. Preparation of NaCrS2 compound was done in the molten flux of elementary sulfur (0.2 mol) Na2CO 3 (0.04 mol) and CrC13 (0.01 mol). After the reaction at 623 K for 2 h, the product was consequently extracted with toluene and water. The solids were characterized using chemical analysis, X-ray diffraction, BET surface measurements, X-ray photoelectron spectroscopy (XPS), and low energy ion scattering spectroscopy (LEIS). Thermoprogrammed reduction (TPR) was carried out in a quartz reactor under a flow of hydrogen. Samples of mixed sulfides were linearly heated from 293 to 1073 K (5 K /min). Hydrogen sulfide evolved upon reduction was detected by means of a HNU photoionisation detector. Thiophene hydrodesulfurization (HDS) and tetraline hydrogenation (HYD) reactions were chosen as model reactions for the comparison of the catalytic properties of the solids. Thiophene HDS was carried out in the vapor phase in a fixed bed microreactor operating in the dynamic mode at the atmospheric pressure of hydrogen without addition of H2S (thiophene pressure: 2.4 KPa, total flow : 6 l/h). A catalyst charge of about 0.1 g was employed. For the tetraline gas phase HYD, the experimental conditions were so chosen to avoid thermodynamic equilibrium that would favors dehydrogenation to form naphthalene. The range of temperatures studied was 523 - 573 K, the hydrogen pressure 4.5 MPa, the tetraline vapor partial pressure 8.9 KPa and the H2S pressure 84 KPa. Extended Htickel calculations were done using BICON - EDIT program package [ 7 ]. Density of states (DOS), crystal orbitals overlap population (COOP), Fermi level and electronic stabilization energy were calculated for several mixed sulfides. 3. RESULTS AND DISCUSSION

The properties of the sulfides obtained are listed in Table 1. As follows from the characterizations, the target mixed sulfides were prepared with specific surface areas varying from 13 to 110 m2/g, depending on the preparation technique. Most of the solids had the cubic spinel structure. Na and Ni compounds had different lattice symmetry. In the case of Co and

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Ni, compound prepared from pyridinium complexes, metastable solid solutions with the pyrite structure were probably formed at 673 K but they were transformed into the stable spinel and monoclinic lattices at 873 K. Table 1. Preparation conditions, Precursor Used Mn-Cr hydrous oxide MnCr207 -Py Fe-Cr hydrous oxide

Cofr20 7 -Py CoCr207- Py CoCr204 Ni-Cr hydrous oxide NiCr207 -Py NiCr207 -Py CuCr204 ZnCrzO 4 -Py ZnCr204 CdCr204 NaCrS2

composition and Sulfidation Temperature, K 873 673 673 673 873 873 873 673 873 673 673 873 873 Molten salt

surface areas (Ssp) of mixed MCr2S4 sulfides. Structure Obtained Chemical S so m2/g (XRD) Composition Spinel MnCr2.03Sn.01 13 Spinel MnCr2.Sn.01 63 Spinel FeCrl.8783.74 50 Pyrite CoCri.99S4.01 96 Spinel CoCrl.99S4.21 22 Spinel CoCrl.99S4.01 16 Monoclinic NiCr2.07S4.15 13 Pyrite NiCr2.00S4.25 110 Monoclinic NiCr2.00S4.15 31 Spinel CuCr2.00S4.0~ 48 Spinel Znfrl.9784.02 78 Spinel ZnCrl.99S4.oo 55 Spinel CdCr2.o2S4.o2 14 Trigonal NaCrS2 51

The variations of specific catalytic activity of MCr2S 4 solids as a function of M in both HDS and HYD reactions were similar, having the maxima for the NiCr2S4 and CoCr2S4 compounds (Fig.l). These latter systems showed enhanced hydrogenating properties, specific activities being comparable to those of Mo and NiMo sulfides (1.03xl0-6.mol.min-'.m -2 in HYD of tetraline ). The HYD/HDS activity ratio was higher for the MCr2S 4 solids than for the molybdenum sulfide reference, in agreement with our previous finding on the enhanced hydrogenating activity of the chromium - based sulfide catalysts [8].The variations of both HYD and HDS catalytic activities as a function of M in MCr2S4 were in good correlation with the reducibility of MCr2S4, determined as the amount of sulfur removed from the catalysts by hydrogen at the temperature of the catalytic tests (573 K) (Fig.l). Therefore we suggested that sulfur vacancies created by reduction with hydrogen are the catalytic centers in both reactions. To clarify further the structure of active surfaces of MCr2S4 solids we performed XPS and LEIS studies. XPS measurements demonstrated that surface layer of mixed sulfide crystals was enriched with Cr, relative to the bulk composition. The only exception was Fe compound where the surface composition was close to that of the bulk. LEIS measurements carried out as a function as sputtering time suggest that the crystalline planes which are preferentially exposed to the surface of MCr2S4 dispersions, contain mostly Cr species (Fig. 2).

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化

10

(a) A

-r.

0

0.75

60 -50

A

a

0.50

-40

-30,.

-r

0.25

~

> 0

E

a) I_

-20"~ 10

0.00

0

Fig. 1. (a) - catalytic activity (10 .7 mol/min.m 2) of the MCr2S 4 solids at 573 K in the reaction of HDS of thiophene. Circles - samples prepared from the mixed oxides, triangles- samples prepared from the pyridinium complexes. (b) - catalytic activity (10 -6 mol/min.m 2 ) of the MCr2S4 solids at 573 K in the reaction of HYD of tetraline (circles); and the amount of sulfur removed by hydrogen at 573 K , (~tmol/m2 ) (triangles). Both reduction by hydrogen and LEIS sputtering lead to the removal of the top layer of sulfur and exposure of coordination unsaturated Cr atoms which are supposed to be active centers.

ta]

(b)

Fig. 2 L o w - index planes of thiospinels 9(111) (a), and (110) (b). Large hollow circlessulfur; black circles- chromium; gray circles - the second metal.

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The reducibility of MChS4 solids is related to their electronic structure, as illustrated by the results of EH calculations, used to determine cohesion energy of the MCr2S4 sulfides. The electronic stabilization energies obtained from EH calculations are listed in Table 2. Pronounced minimum of the cohesion energy is obtained for Co and Ni compounds. Table 2. EH electronic stabilisation energies and interatomic distances (crystallographic data) in some MCr2S4 sulfides M AE stab d(M-S) .A d(Cr-S), ,~ 2Na -558 2.798 2.454 Mn -485 2.241 2.497 Fe -451 2.209 2.461 Co -449 2.191 2.448 Ni -431 2.388 2.424 Cu -481 2.17 2.418 Zn -489 2.211 2.463

The inverse general correlation between the sulfur binding energy and the M-S or Cr-S bond lengths was observed, i.e. more ionic bonds are stronger than short (more covalent) ones. However, Cu compound having the shortest bonds was not at the minimum of stabilization energy so the correlation is not complete. The results of EH calculations can be qualitatively explained in terms of relative positions of the elements valent orbitals.. In the NaCrS2 compound, the presence of the alkali metal leads to the increase of bonds ionicity, i.e. lowering of the S 3p states. Therefore, interaction between S3p and Cr 3d t2g levels decreases i.e. the bond covalence decreases. For the transition metal sulfides, when in MCr2S4 M goes from the left to the fight of the periodic table, the electronic effects observed can be realized as the competition of two opposite trends - that of filling of the d-band of M (so that the energy the 3d electrons provided by M increased), and that of increase of M nuclei charge, shifting the band of the same 3d electrons down.

Cr3d eg P

Cr3d t 2g

Co, Ni Fe, Mn Cu, Zn

Fig 3. Schematic energy diagram, showing the relative bands positions in the MCr2S 4 compounds.

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For the chromium neighbors in the periodic table, Mn and Fe, the d - levels of the second metal are close to those of Cr (Fig 12) and the sign of the effect is not clear, at least in the frames of the EH method. Then, going to Co, we see that band filling prevails over the band shifting down, and the HOMO states of the mixed sulfide CoCr2S4 are probably those of Co 3d. At the same time the S-Cr and S-Co antibonding orbitals become populated due to the increased d - electrons count. The same situation was observed for Ni, the effect being the decrease of sulfur binding energy. However in the case of Cu, the d- band, though almost filled is already placed too low, and the result of Cu introduction into the mixed sulfide is that of the increased sulfur bonding. Indeed, CuCr2S4 was less reducible than Co and Ni compounds. 4. CONCLUSIONS The MCr2S 4 systems, being chosen rather as a model solids for basic research purposes showed enhanced hydrogenating properties, specific activities being comparable to those of Mo and NiMo sulfides. By contrast to the industrial sulfide catalysts, which are extremely difficult to characterize, MCr2S 4 mixed sulfide dispersions are stable, and have well-defined surface and bulk properties. In this case the trends of catalytic activity could be easily explained from comparison with TPR and surface characterizations data. The idea about dynamically created active centers [5] get an additional support in this work. Though different elementary steps of HYD and HDS catalytic reactions were not considered here, sulfur lability which depends on the nature of metal M, appears to be a key parameter determining the catalysts performance in both processes. REFERENCES 1. Pecoraro, T.A., Chianelli, R.R., J. Catal., 67, (1981) 430. 2. Lacroix, M., Boutarfa, N., Guillard, C., Vrinat, M., Breysse, M., J. Catal., 120, (1989) 473. 3. Raybaud, P., Kresse, G., Hafner, J., Toulhoat, H., J. Phys, Condens. Matter, 9 (1997), 11085. 4. Harris, S., Chianelli, R.R., J.Catal., 86, (1984), 400. 5. Byskov, L.S., Hammer, B, Norskov, J, Clausen,B.S., Topsoe,H., Catal. Lett., 47 (1997) 177. 6. Bacon, G.E., Zeit. Crystallogr 82 (1932) 325. 7. Brandle, M., Rytz, R., Calzaferri, G., BICON - CEDiT Extended Hiackel Band Structure and Crystal Electronic Dipole induced transitions calculations. Bern, 1997. 8. Thiollier, A., Afanasiev, P., Cattenot, M., Vrinat. M, Catal. Letters 55(1) (1998) 39.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

In situ characterization of transition metal sulfide catalysts by IR probe molecules adsorption and model reactions. G. Berhault 1 M. Lacroix 1 M. Breysse 2, F. Maug63 and J-C. Lavalley 3

1Institut de Recherches sur la Catalyse, CNRS UPR 5401, 2, Avenue Albert Einstein, 69626 Villeurbanne Cedex, France. 2Laboratoire de R6activit6 de Surface, CNRS UMR 7609, Universit6 Pierre et Marie Curie, 4, place Jussieu, Casier 178, 75252 Paris Cedex 05, France. 3Laboratoire Catalyse et Spectrochimie, CNRS UMR 6506, ISMRA-Universit6, 6, Boulevard du Mar6chal Juin, 14050 Caen Cedex, France. This work reports a detailed characterization of reduced states of RuS2/SiO2 catalyst by combining catalytic activity measurements and IR probe molecule adsorption. Depending on the solid composition monitored by a progressive reduction these surfaces gradually moves from an acid-base character to a metallic one. Both Lewis and BrCnsted acidic sites are created in mild reduction conditions and the Lewis acidic sites play an important role in the activation of sulfur containing molecules and subsequently on their transformations. The hydrogenation properties are related to Ru sites with a low sulfur coordination. 1. I N T R O D U C T I O N Transition Metal Sulfides are efficient materials for catalyzing several reactions such as the C-X (X=S, N, O, Metal) bond hydrogenolysis, hydrogenation, the selective transformation of organic disulfides into the corresponding thiols as well as the aromatization of cyclic thioethers and the selective ketones amination. Among these solids, RuS2 is one of the most active TMS [1-2]. This indicate that its surface is flexible enough to adapt the proper configuration site required to catalyze this large variety of reactions which demand different intrinsic properties. Theoretical and experimental studies have ascribed the high activities of RuS2, as for RhzS3 and PtSx to their weak metal-sulfur bond energy [3]. This property is propitious to the formation of a large number of coordinatively unsaturated sites (CUS) whose properties may be regarded as a Lewis-type center interacting with electron-donating organic substrates [4]. Beside this CUS, the surfaces of TMS also contains some sulfur anions and SH groups which simultaneously co-exist depending on the nature and on the composition of the surrounding atmosphere. However, there is a lack of characterization of the acid-base properties of these surfaces which are supposed to play an important role in the successive elementary steps involved in the above mentioned reactions. Besides this acidic-base character, Moraweck et al have demonstrated that for small RuS2 clusters encaged into a Y zeolite, some metallic Ru microdomains may co-exist at the surface of the sulfided particles leading to a metal-sulfide type interface [5]. Accordingly, the surface of a RuS2 particle may

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behave as a metal or as an acid-base material depending on reaction conditions in agreement with the highly reducible character of such a sulfide. The aim of this work was to develop the required tools for characterizing the modification of the surface properties of a silica supported RuS2 induced by a progressive reduction. For this purpose, we used a silica supported RuS2 catalyst as model system because silica is relatively neutral and does not interact too much with the supported sulfide phase. Solid characterizations were performed by combining catalytic measurements with in situ probe molecule adsorption (CH3SH, CO, pyridine and lutidine). Pyridine was used to detect the Lewis acidity while lutidine was preferred for dosing the BrCnsted acidity because of its higher basicity and CO was selected because its wavenumber is sensitive to the CUS environment. The catalytic properties were determined in two model reactions suspected to reflect different surface properties i.e. the lbutene hydrogenation and the condensation of CH3SH into CH3SCH3.

2. EXPERIMENTAL 2.1. Catalyst preparation The silica-supported RuS2/SiO2 was prepared by the pore filling method using RuC13 aqueous solutions. The impregnated and dried solid was sulfided at 673 K with a 15%H2S85%N2 mixture. After this activation procedure, the solids were cooled down to room temperature in the presence of the sulfur-containing atmosphere, flushed with an oxygen free nitrogen flow and stored in sealed bottles. The Ru loading was 7.5 weight %, the S content corresponds to RuS2.7 and the residual chlorine content was lower than 0.1%.

2.2. Catalyst reduction and catalytic properties These experiments were performed in situ in the same flow microreactor equipped with two parallel detectors, a Flame Photometric Detector (FPD) and a Flame Ionization Detector (FID) in order to detect respectively H2S and the hydrocarbons. The H2S released upon hydrogen reduction was quantified by calibrating the detector with a known concentration of H2S (573 ppm) diluted in hydrogen. The degree of reduction ot was defined by the ratio of the amount of H2S eliminated from the solid to the total sulfur content. The reduced catalysts were then contacted at 273 K with a mixture of H2(93.4%)-l-butene(6.6%) or at 473 K N2(94.4%)-CH3SH(5.6%). For both reactions, conversions were kept lower than 10% in order to avoid mass transfer limitations.

2.3. FTIR spectroscopy FTIR characterization was performed using self-supporting discs of pressed samples. The catalysts were resulfided in situ in the infrared transmission cell according to the procedure already described[4]. Solid reduction was performed with 200 Torr of hydrogen at various temperatures. Several reduction-evacuation cycles were done in order to remove the H2S formed upon reduction. Then, the samples were evacuated at 393 K for 30 min prior to molecule adsorption. Probe molecule adsorptions were performed at 100 K for CO or at room temperature for the others probes. The reduced catalysts were contacted with 1 Torr of CO, 2 Torr of pyridine (Py) and 2-6 dimethylpyridine (DMP) or 4 Torr of CH3SH and then evacuated. The IR spectra were recorded using a Nicolet 60SX FTIR spectrometer. Band intensities were corrected from slight differences in catalyst weight and adjusted to 10 mg.

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3. RESULTS 3.1 Solid reduction and catalytic properties

The starting point of this work was to examine how the RuS2/SiO2 catalyst behaves towards a hydrogen treatment. Preliminary TPR experiments have evidenced that the silica support sulfided in the same experimental conditions does not retained any detectable amount of H2S. It was also observed that over the RuS2/SiO2, H2S is mostly removed upon heating and then the solid rapidly equilibrates when treated in isothermal conditions. Accordingly all solids were reduced at the desired temperature and left in isothermal conditions for only 2h 12 ~o

100

Deg.of Red.

[-~

2 ~

N

80

"~

8

N'~~

60

o

6

~~~ 0

40

",,..a

CH3SH (10 -8 mol.s- I .g- 1)

10

I

4 1-Butene (lamol.s- 1.g- 1) w

|

,

i

,

,

,

298 423 473 523 573 623 673 Reduction Temperature (K) Fig. 1. Evolution of the degree of reduction and of the mean particle size as a function of the temperature of reduction.

0

20

40

60

80

100

Degree of Reduction (%) Fig. 2. Evolution of the catalytic properties as a function of the degree of reduction

Figure 1 shows the evolution of the degree of reduction versus the reduction temperature. At 673 K the amount of H2S released from the solid corresponds to that determined by chemical analysis indicating that the solid is entirely reduced. This figure also reports the mean particle size determined by HREM. The non-reduced solid could be considered as an assembling of spherical particle with a mean diameter of circa 35/k with a narrow distribution since the standard deviation was about 8/k. Neither the particle size nor the distribution width were affected up to a reduction temperature of 573 K. At 623 K both parameters increase and the XRD patterns reveal the concomitant presence of the RuS2 and the metal Ru phases while only the latter is detected when the solid is reduced at 673K. These data indicate that the pyrite phase preserves its morphology up to a reduction temperature of 573 K. Figure 2 shows the variation of the catalytic activities as a function of the degree of reduction. The non-reduced solid is already active for the condensation reaction. As far as sulfur is removed from the catalyst the activity increases, reaches a maximum for ot = 20% and then continuously decreases for further sulfur removal. By contrast, the butane formation follows a distinct trend i.e. the non-reduced solid is inactive and the activity increases up to = 40% and then stabilizes. The different comportment of the catalyst towards both reactions strongly suggest that they require different type of sites.

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3.2 Pyridine (Py) and 2-6 dimethyipyridine (DMP) adsorption.

Py interaction with the silica support gives rise to several bands characterizing Lewis and BrCnsted sites. However, this interaction is weak because a low signal is detected after desorption at 423 K (Fig. 3). By contrast, these bands remain on the Ru catalyst. The spectra recorded on the non-reduced sample (NR) exhibits intense bands at 1602 and 1444 cm -I involving Lewis (L) acidity as well as some weaker ones in the range 1500-1580 and 16101660 cm l characterizing the BrCnsted (B) sites while the band at 1485 cm -I arises from both L and B sites. Solid reduction up to 473 K brings about an increase of band intensities without any change in their positions suggesting an increase in the number of acidic sites without a large modification of their strength. The diminution of band intensities for a reduction temperature of 573 K is rather surprising since the amount of H2S removed has drastically increased. This unexpected behavior suggests a strong modification of the surface properties because particule size remains unaffected. ---,

1602 L 1636 B. 9

~

9

1485 1444 L L+B " 9 1540 B 9 9 ~

9

,

9

~

773K

,.Q O ,.Q

<

~ 9

I

1700

,

9

I

1600

,.

~

9

I"

.

,

.

1500

J

473K 423K 393K NR SiO 2

1400

Wavenumbers (cm- 1) Fig. 3. IR spectra of adsorbed pyridine for various reduced Ru catalyst.

1603 1583

tD

573 K

,.Q

473 K

O

423 K 393 K

<

NR SiO 2 1700

I

I

I

J

1650

1600

1550

1500

Wavenumbers (cm- 1) Fig. 4. IR spectra of adsorbed DMP for various reduced Ru catalyst.

As evidenced in Fig. 3 solid reduction also modifies the concentration of B sites. However the band related to pyridinium ion is too weak for estimating the variation of their concentration 9 Figure 4 shows, the IR spectra of adsorbed DMP recorded after an evacuation at RT. In the 1550-1700 cm ~ range, four main bands are detected. The most intense ones located at 1583 and 1603 cm -~, whose intensities vary little with the pretreatment, correspond to DMP adsorbed on both L and B sites 9 The two others at 1629 and 1644 cm -l agree fairly well with those reported in the literature for protonated species and they may be respectively ascribed to v8b and v8a vibration modes of DMPH § species [6]. These bands are not present on the silica support treated in the same conditions and their intensities slightly varies with the solid reduction and a maximum emerges at a reduction temperature of about 423 and 473 K.

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3.3. CH3SH and CO adsorption

Adsorption of CH3SH on silica leads to four IR bands located at 3012, 2948, 2858 and 2590 cm -I which corresponds respectively to the va(CH3), vs(CH3), 2&I(CH3) and to the v(SH) stretching mode of the free CH3SH molecule. The presence of the latter suggests that the probe is only physisorbed on the support. This assumption is confirmed by the flat spectrum observed after evacuation at RT (Fig. 5). 2033

92915 cm -1 .~,,.,--.~,~------- . . . . . . 673K ~

~ - ~ ~ ' ~ 6 2 3 K 0.0 l t ~ , r

r

523K

<

"~[I 1 t:;

473K

723 K

d:) O ,.O

573 K

<

473 K 423 K NR

,,~'~/~"~~--,393K

3000

2800

2600

Wavenumbers (cm- 1) Fig. 5. IR spectra of adsorbed CH3SH for various reduction temperatures (Evacuation at RT). NR" non-reduced solid

2200

2100

2000

1900

Wavenumbers (cm- 1) Fig. 6. CO adsorption spectra for various reduction temperatures (Evacuation at RT). NR 9non-reduced solid

On the Ru catalysts the va(CH3) bands are shifted towards lower wavenumbers and the v(SH) stretching band was no longer observed. Band intensities slightly decrease upon evacuation and these bands are still detected after evacuation at 393 or 473 K. These data suggest that CH3SH heterolytically dissociates leading to the formation of thiolate species linked to a Lewis type center whose IR signature corresponds to the intense band observed at 2915 cm -l. The amount of such a species increases upon reduction and its concentration exhibits a maximum for Tr = 473 K in agreement with the evolution of L sites previously determined using Py as probe molecule. CO adsorption leads to several bands and a shift of band positions towards lower wavenumber appears when the reductive treatment becomes more severe (Fig.6). This is in favor of a progressive sulfur depletion around Ru. By comparison with literature data, the band at 2093 cm -! indicates the presence of CUS in a highly sulfided environment while part of the band at 2033 cm -~ with the band at 2016 cm -l could be related to the formation metallic Ru [7]. The bands at 2073, 2056 and part of that at 2033 cm -I characterized Ru cations in a lower sulfur coordination.

3.4. Relationship to catalytic activities Figure 7 shows the evolution of the intensities of the thiolate and Py-L bands with the degree of reduction follows the same trend that the condensation reaction suggesting an adsorption of the S containing molecules on an electron accepting site. The similar behavior of the intensity of the high frequency CO bands indicates that these Ru sites are still in a rich

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sulfur environment. The smoother variation in the relative amount of Br0nsted sites and the lower temperature at which the maximum is observed shows that these sites do not play an important role in this reaction. ^ ii~

~ --

c

Thiolate species CO at 2056 cm l m l

v v

9

CO at 2016 cm:ll CO at 2033 cm 1-Butene Hydrogenation V

, , _, ~ . - - - o - ~----------_~~Y

0

CILI3SH~kctiv B acidity

I

I

I

I

I

20

40

60

80

100

Degree of Reduction (%) Fig. 7. IR data and CHaSH activity relationship

0

20

40

60

80

100

Degree of Reduction (%) Fig. 8. IR data and hydrogenation activity relationship

By contrast, highly depleted Ru sites are required to performed the hydrogenation reaction as shown in Fig. 8. Indeed, a nice correlation was found between the 1-butene hydrogenation activity and the low CO bands. 4. CONCLUSIONS These data evidence that the properties of the RuS2 surface progressively moved from an acid-base character to a metallic one. The properties of the several surface species whose concentration and nature are strongly dependent on the sulfur to metal ratio have been characterized by means of IR spectroscopy and model molecules conversion. The modification of the surface properties of transition metal sulfide catalysts as a function of the surrounding atmosphere and particularly of the relative H2-H2S partial pressure should be taken into account when these solids are used in complex reaction such as dibenzothiophene conversion. This approach developed on a model catalyst can be now applied to more complex catalytic systems. REFERENCES

1. T.A. Pecoraro and R.R. Chianelli, J. Catal., 67 (1981) 430. 2. M. Lacroix, N. Boutarfa, C. Guillard, M. Vrinat and M. Breysse, J. Catal., 120 (1989) 473. 3. J.K. NCrskov, B. S. Clausen and H. TopsCe, Catal. Lett., 13 (1992) 1. 4. G. Berhault, M. Lacroix, M. Breysse, F. Maug6, J.C. Lavalley, H. Nie and L. Qu, J. Catal., 178 (1998) 555. 5. B. Moraweck, G. Bergeret, M. Cattenot, V. Kougionas, C. Geantet, J.L. Portefaix, and M. Breysse, J. Catal., 165 (1997) 45. 6. P.A. Jacobs and C.F. Heylen, J. Catal. 34 (1974) 267. 7. K. Kostov, H. Rauscher, and D. Menzel, Surf. Sci., 278 (1992) 62.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

化

485

Potassium excitation

at catalytic

surfaces

-

stability,

electronic

promotion

and

A. Kotarba'*, G. Adamski a, Z. Sojka a, S. Witkowski a, G. Djega-Mariadassou b aFaculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Cracow, Poland bUniversit6 P. et M. Curie, 4 Place Jussieu 75252 Paris, Cedex 05, France Thermal desorption of potassium atoms, ions and Rydberg atoms from K-doped transition metal carbides and nitrides were investigated. The results were rationalized in terms of Schottky cycle. The status of the promoter was evaluated in three aspects: stability at the surface, electronic promotion and non-equilibrium excitation. Their relevance to catalysis is briefly discussed. 1. INTRODUCTION Alkali metal compounds are widely used as promoters in many catalytical processes. Addition of alkali affects the efficiency in many different ways, increasing the activity, selectivity or prolonging the lifetime [ 1]. However, despite that the beneficial action of alkali additives is experimentally well established in catalysis, an understanding of their action is still incipient and alkali promotion often reveals its 'alchemical' nature. One of the most striking features of alkali promotion is their beneficial role in a number of various types of distinct reactions, including such commercially important processes as styrene production, ammonia synthesis, methanation, WGS, ethylene epoxidation or FT. Among the alkali metals, Li, Cs, Na and K, typically applied as promoters, potassium is the most frequently used. For several years, molecular beam techniques have been employed to study desorption of neutral and ionic alkaline species from solid surfaces, including commercial iron catalysts [2, 3, 4, 5]. The activation energies of potassium atom desorption from fresh and spent catalysts were determined. In general, the activation energy of K desorption has a value characteristic of the phase it desorbed from. Thus, the equality of desorption energies for the industrial catalyst and the reference compound may provide a hint about surface phase composition of the catalysts, even if for conventional XRD they appear essentially amorphous. In an attempt to elucidate the promoting effects of alkali and identify the decisive parameters responsible for their action we have undertaken potassium thermal desorption studies in three different aspects: (i) stability at catalytic surfaces, (ii) electronic promotion and (iii) non-equilibrium excitation. All these issues will be addressed using selected examples of K-doped transition metal nintrides and carbides, coming from our laboratory.

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2. EXPERIMENTAL

2.1. The materials Transition metal carbides and nitrides promoted with potassium investigated in this study were prepared from the corresponding oxides (Nb205 or MOO3) in a temperature programmed reaction [6] by flowing NH3 or CH4/H2 mixture. The potassium promotion was effected in two different ways: a nascent doping when the KOH was added before and a post doping when it was added after the TPR synthesis. Potassium nominal loading was 2 and 0.2 wt%. The resultant materials were characterized by XRD, XPS, SEM and BET methods. A complete list of the samples is given in Table 1. 2.2. The Measurements For the thermal desorption experiments all samples were pressed to form wafers of 10 mm in diameter and typical mass of 100 mg. The experiments were carried out in a vacuum apparatus with the background pressure of 10-7 mbar. The chamber was equipped with a rotatable manipulator with a sample holder, which could be heated up to 900 ~ Surface- [5] and field-ionization [3] detectors, and a quadruple mass spectrometer (SRS RGA 200), were used. In the QMS studies potassium was detected as the peak at 39 unit only. Desorption of potassium in molecular forms like KO, K20, KO2 etc. was below the detection limits. The desorbing fluxes of potassium atoms and ions were monitored simultaneously. The ionic flux of K § was measured directly as a current by an ion collector in the field ionization detector. The atomic flux of K was determined by means of the surface ionization detector, while the excited potassium atoms K* were pre-ionized with the field gradient of 1050 Vcm-~ and then measured as ionic current. In all cases, the current to the collector was directly measured with the digital electrometer Keithley 6512 connected on-line to the computer. Typically 1000 points were collected and averaged for each temperature. For all investigated samples the thermal desorption measurements were carried out in the temperature range 600 - 1000 K. From the part of the Arrhenius plot, where the correlation coefficients were better than 0.999, the activation energies for K desorption were determined. The XPS valence band (VB) measurements were carried out with a VSW spectrometer using Mg Kct radiation (1253.6 eV and 200 W of irradiation power), a hemispherical analyzer of 150 mm in diameter and a pass energy 12 eV. The background pressure in vacuum chamber was 10s mbar. Binding energy was referenced to the carbon C ls peak (284.6 eV).

3. RESULTS AND INTERPRETATION 3.1. Stability Since the activation energies represent the strength of a surface chemical bond, which brakes during the desorption process they serve as a parameter gauging the surface stability of potassium. The observed energies ranges from 2.30 to 3.00 eV for K + and from 1.00 to 3.40 eV for K depending on the catalyst nature and especially on the way of doping (Table 1). The latter can be best illustrated taking K-doped niobium oxynitride catalysts as an example.

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Table 1 Potassium desorption from investigated catalysts Sample

work function/eV

K-desorption energies/eV

K-doping;

*

wt. %

method

K§

K

Nb205 NbN NbN NbN NbC NbC

2 0.2 2 2 0.2 0.2

post nascent nascent post nascent post

2.60

2.90

4.60

2.76

2.34

3.88

3.00

2.50

3.80

Mo2C

0.2

nascent

K

2.30

1.00

2.64

3.40

3.26

2.73

3 12

4.11

2.59

3.07

2.50

3.00 m

Potassium introduced via nascent-doping and post-doping shows distinctly different stability. As can be seen from the Table 1, the measured activation energies are equal to 2.99 and 2.35 eV for ions and 2.50 and 1.00 eV for atoms, for nascent- and post-doped samples, respectively. While the former values are typical energies of bond breaking (for instance the rupture of K-O bond in K20 requires about 3 eV), the latter one is unexpectedly small approaching potassium sublimation barrier (0.99 eV). The energies reflect apparently different status of the surface potassium. Indeed, the complementary XPS depth profiling experiments revealed the accumulation of potassium at the surface of post-doped samples and more homogenous distribution in the case of nascentdoping.

3.2. Electronic promotion Owing to their high electron donating ability alkali metals are efficient electronic promoters used for catalyst work function optimization. Using K desorption data the work function of the surface can be evaluated from the Schottky energy cycle:

=E.~-E~+I where Ea and Ei are the energies for K and K § desorption, respectively while I is the first ionization potential of K (Fig. 1). The method is applicable provided that potassium desorbs only as ions and ground state atoms. As implied by the figure the difference ~ - 1 controls the relative intensities of K and K § fluxes. The highest work function is observed for the K-doped parent Nb205. This value falls to 3.88 - 3.00 eV upon nitridation. The results obtained indicate that the shift in the Fermi level depends both on K loading and the way of doping. It appears that the post-doping method is more effective ( ~ = 3.0 eV) than the nascent one ( ~ = 3.8

eV).

The changes in work function were corroborated by XPS measurements in the valence band region. A distinct shift of the top edge of the valence band towards lower binding energies was observed. In accordance with the thermal desorption studies the shift is more pronounced in the case ofpost-doped samples.

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488

.~!~:, e~ + ~ : , ~

dk

Ei

~~

~ ....:::ii!i!i:i~i~: .....

~ii!iiilKii::iiiii: .....~iliiii!:ii!!:.>

~

:'~i~"' :~i~"

:~i!~ilili~+: e; I | *ll

Fig. 1 Schematic representation of Schottky cycle. El, Ea represent measured energies for K § and K, ~ is the calculated work function, I tabulated potassium ionization potential, e~ and ev refer to electron at Fermi level and vacuum respectively. 3.3. Excitation

The most spectacular are undoubtedly the results obtained for carbides. In this case not only emission of mere K and K § species was observed but additionally a presence of highly excited potassium K* (Rydberg atoms) was detected. From the applied field gradient Fc in the detector the threshold value of their principal quantum number n > 25 was estimated using the formula [7]: FJ(Vcm -l) = 3.2 • 108/n4. Since the intensity of Rydberg atom emission strongly depends on the surface voltage [8] thermal desorption studies were conducted as a function of the sample potential Vs. The results are shown in Figs. 2a and 2b. The desorption bands of K* exhibit clear fine structure, which varies with the temperature and voltage. Following the literature we assigned this structure to K*n clusters with various numbers of potassium atoms. In general, low voltages favors larger clusters while at high field due to fragmentation smaller clusters are produced. In the Fig. 2b the same data are presented in the Arrhenius coordinates. For each peak a straight line was obtained, which means that they correspond to distinct processes. The activation energies for the K* desorption at V~ >25 V vary from 1.04 to 1.81 eV. Since the values are significantly smaller than the K desorption energies for carbides, they rather reflect the process, in which K* merge into clusters of the size governed by the magic numbers. Such clusters of Rydberg matter K, (n = 7, 14, 19, 61) were previously observed by Holmlid et al. [9] using time-offlight spectrometry. 4. DISCUSSION Surface stability of potassium and its electronic promotion appear to be inimical phenomena. Although the post doping has the greatest impact on the electronic properties of the catalyst it results in lower potassium stability compared with the nascentdoping. The work function can also be modified by varying the K loading. Thus, there are at least two variables, which can be exploited during catalyst preparation for adjustment of the Fermi edge in order to optimize particular catalyst performance.

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6

-26

1 /aI/

~

W

~ ~ / I/ ~------~/'~ ~ 4U 3 ~ 6 4

(j$/]/,] 10 (a)

/

0

7"- 720 7--700 -7" 680 660 0fO X,,\

0 620

/

-28 / -29

1.

/'

1 .12

~/

-30" 4 0 " ~ 1 . 0 4 (b)

~ ,~/k

Us /V

Fig. 2. Rydberg state signal as a function of sample voltage and temperature (a) and the corresponding Arrhenius plot (b). The mechanism, along which K atoms can be brought into Rydberg states at the catalytic surfaces still remains unclear. From our experiments it seems that the Rydberg state can be considered as an intermediate in surface ionization of potassium: K ---> K* ---> K § + e-. The formation of K* is observed when the bonding of K is stronger than K § (Table 1). This suggest that the potential energy diagram given in [3], where the minimum of potential for the surface K should be deeper and closer to the surface, while that for K § is flatter and more distant is applicable also for this case. The potential curve for K* is located higher in energy approximately by the ionization potential of potassium lowered by the cluster formation energy. If the crossing locus of K and K § curves occurs above the minimum of K* then the excited Rydberg atom may appear as an intermediate in the potassium ionization pathway (stepwise mechanism). However, if the crossing occurs below the minimum then the surface K atoms are directly ionized into K § (concerted mechanism). What is actually observed depends on the binding energies of K and K § work function of the catalyst and the potential applied to the surface, which shifts the relative position of the curves. The existence of Rydberg atoms at the surface leads to a large decrease in the work function value, due to their very low ionization potential. In this case the Schottky cycle is invalid. The relevance of Rydberg atoms to catalysis is a challenging question. An original concept of the alkali action at catalytic surface was proposed by Pettersson et al. [10]. The model includes the reaction between a Rydberg state of an alkali atom and a reactant molecule at the surface, which was proposed to explain the increase in sticking probability of the reacting molecules upon alkali doping. Along this line, the transfer of excitation energy from K Rydberg

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species to reacting molecules has recently been proved experimentally [ 11]. Emission of highly excited potassium species (K*) from industrial iron catalyst for ammonia and styrene production were experimentally observed [12, 2]. Moreover, the flux of K* was shown to correlate with the catalyst activity [5]. The emission of Rydberg atoms was high for active catalysts and low for the spent one, although the total flux of potassium in the latter case increased due to surface segregation. 4. CONCLUDING REMARKS Different aspects of the alkali promotion can be successfully studied by a simple thermal desorption method. As shown, the chemical status of alkali metals on catalytic surfaces involves several effects and the complex approach is necessary to evaluate their role in heterogeneous catalysis. In particular, the investigation on excited states of alkali metals may provide a sensitive starting point for unraveling the mechanism of promoting action. A working hypothesis has been advanced to account for the formation of Rydberg atoms at the catalytic surfaces. ACKNOWLEDGEMENTS A.K. is grateful for a "Poste Rouge" (PICS 508 - C.N.R.S.) at the Laboratoire de R6activit6 de Surface, Universit6 P. et M. Curie, Paris. REFERENCES

1. W.D. Mross, Catal. Rev.- Sci. Eng., 25 (1983) 637. 2. A. Kotarba, K. Engvall, J.B.C. Pettersson, M. Svanberg, L. Holmlid, Surf. Sci., 342 (1995) 327. 3. K. Engvall, A. Kotarba, L. Holmlid, J. Catal., 181 (1999) 256. 4. A. Kotarba, M. HagstrOm, K. Engvall, J.B.C. Pettersson, React. Kinet. Catal. Lett., 63 (1998) 219. 5. K. Engvall, L. Holmlid, A. Kotarba, J.B.C. Pettersson, P.G. Menon, P. Skaugset, Appl. Catal. A, 134 (1996) 239. 6. C. Sayag, PhD Thesis, P. M. Curie University, Paris VI, 1993. 7. R.F Stebbings, F.B. Dunning, Rydberg States of Atoms and Molecules, Cambridge University Press, Cambridge 1983. 8. J. Wang, K. Engvall, L. Holmlid, J. Chem. Phys., 110 (1999) 1212. 9. J.Wang, L.Holmlid, Chem. Phys. Lett., 295 (1998) 500. 10. J.B.S. Pettersson, K. MOiler, L. Holmlid, Appl. Surf. Sci,. 40 (1989) 151. 11. J.Wang, L.Holmlid, submitted to Chem. Phys. Lett. 12. K. Engvall, A. Kotarba, L. Holmlid, Cat. Lett., 26 (1994) 101.

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化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Heterogeneous catalysis of aldolisations on activated hydrotalcites Joseph Lopez, Roland Jacquot* and Frangois Figueras Institut de Recherches sur la Catalyse. 2, Avenue Albert Einstein. 69626 Villeurbanne Crdex, France and *Rhodia, Centre de Recherche des Carrirres, 85 Avenue des frrres Perret, 69192 StFons Cedex, FRANCE. The aldolic condensations of benzaldehyde on acetone and acetophenone have been investigated on hydrotalcites, KF and KNO3 supported on alumina, La203, X zeolites containing Cs and Mg clusters. A Langmuir Hinshelwood kinetic mechanism with competitive adsorption of the reactants is observed. Changing the polarity of the solvent induces large differences of rate. Hydrated hydrotalcite is the best catalyst at 273 K, reaching 70% conversion with 90% selectivity to aldol. Supported KF catalyses these reactions at 273 with lower selectivity. The different solid bases have been compared at a reaction temperature of 423 K: KF treated at 723 K is the most active, reaching 90% selectivity in chalcone at 80% conversion. INTRODUCTION Aldolisation reactions are of interest in fine chemistry because they allow the formation of C-C bonds. These reversible reactions usually show an unfavorable thermodynamic equilibrium, and are exothermic which means that the equilibrium conversion decreases when the reaction temperature increases [ 1]. In most cases the aldol can be easily dehydrated so that the reaction at high temperature yields the unsaturated ketone. They can carried out by acid or basic catalysis, and the investigation of the effect of substituents showed that activated hydrotalcites (HDT) catalysed the reaction by a basic mechanism [2]. Several attempts have been performed either in the gas [3] or the liquid phase [4]. Recent work on the activation of HDT demonstrated that the aldolisation of acetone [5,6] or the aldolic condensation of benzaldehyde on acetone [7] can be performed selectively on BrOnsted bases obtained by rehydrating thermally decarbonated HDT. In addition of the basic properties of the solid the nature of the solvent and the kinetics can condition the yield of the reaction, and we report here the results of this investigation. Moreover, several basic catalysts have been described such as over exchanged zeolites [8,9], supported potassium fluoride [ 10] or nitrate [ 11 ] which relative basicity is not known. They have been compared in order to establish a pattern of basicity from their catalytic properties. EXPERIMENTAL METHODS The preparation of HDT was made by coprecipitation as reported by Miyata [12]. Aqueous solutions containing the first one 0.75 mol/L of MgNO 3 and 0.25 mol/L of A1NO3, the second one 2 mol/L of KOH and 0.5 mol/L of KzCO3, were introduced by two electric pumps to a 4 L flask and mixed under vigorous stirring at constant pH = 10, controlled by a pHstat. The mixture was aged under stirring at 338 K for 18 hours. The precipitate w a s washed several times until the solution was free of chloride ions (AgNO 3 test) then dried at 383 K. For activation, the catalyst (about 0.15 g) was first heated in a flow of nitrogen at a rate of 10 K per minute up to 723 K maintained for 8 h. The solid was then cooled in dry ni-

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492

trogen and contacted with a flow of nitrogen (61/h) saturated with the vapor pressure of water for 6 h at this temperature. KFla was supported on SPH 512 or-alumina from Rh6ne Poulenc (surface area 10.5 m2/g). 15 g of alumina were poured into 150 mL of water containing the desired amount of KF (1 mmol KF/g of support). Water was evaporated at 323 K then the solid was dried at 383 K, and calcined at 723 K just before use. A commercial sample from Aldrich denominated KFA (40 wt% KF, 14.9 m2/g) was used for comparison. A sample of KNO3h/AI203 (41% KNO3) was prepared using the procedure reported by Zhu et al. [ 11 ] on SCP 350 of Rh6nePoulenc (surface area 400 m2/g) as support. 3.5 g of nitrate and 5 g of alumina were dried at 373 K, then mixed by grinding in a mortar for 15 min, then 0.5 ml of water were added. Atter mixing the resulting solid was dried at 383 K overnight. Lanthanum oxide was precipitated at pH 9 from a solution of La(NO3)3 using NH4OH. The solid was washed twice, dried at 383 K and activated by slow calcination (1K/min) at 923 K just before use.The surface area of the resulting La203 oxide is 12.4 m2/g. The basic zeolites were prepared from NaX (CECA) by exchange with Cs acetate (CsAc) at room temperature: 20 g of NaX were contacted with 1 L of a 0.1 M solution of CsAc and strirred for 48 h. A second exchange was performed, then the solid was washed 3 times and dried at 353 K: the degree of exchange of Na + by Cs + was 50.4%. This NaCsX zeolite was further impregnated by AcCs: 5g of zeolite were contacted with 12.8 mL of water containing 0,736 g of AcCs, then water was evaporated at 308 K. The resulting solid contains 14.9 Cs per unit cell. A NaMgX zeolite was prepared from NaX contacting 10 g of zeolite with 150mL of ethanol containing 5.216 g of Ac2Mg, 4H20, then ethanol was evaporated at 323 K. The resulting solid contains 32 Mg per unit cell. These basic zeolites were activated by slow calcination (1K/min) at 823 K. The characterisation consisted in X Ray diffraction, XPS, DTA-DTG and nitrogen adsorption to measure the surface area and porosity. The reactions at low temperature were investigated in batch conditions using a three neck glass reactor equipped with a condensor. The solvent and the substrates were mixed at 273 K. When the temperature was stabilised the freshly activated catalyst was rapidly introduced and the measurement started. For the reactions of benzaldehyde-acetophenone performed at 423 K the solid was introduced in the solvent (12 ml of DMF), the reactor was closed, purged with nitrogen and heated to the reaction temperature, then the reactants (0.89 ml of acetophenone and 0.76 ml ofbenzaldehyde in 6.4 ml of DMF)were introduced from a vessel connected to the reactor. The standard amount of catalyst was 0.15 g. Reactants and products were analysed by gas chromatography using a polar capillary column. RESULTS

1) Characterisation of the samples. The original sample is a pure HDT. No change of the surface composition could be observed upon activation by XPS, therefore the solid is assumed to be homogeneous. Hydrotalcite treated at 723 K in nitrogen or air is converted to a mixed oxide of high surface area, which can be reversibly rehydrated to a layered structure by contact at room temperature with a stream of nitrogen saturated with water (Fig. 1). This rehydration also corresponds to large changes in surface areas as illustrated in Table 1. From XRD analysis, KFA is a mixture of K3AIF6 and KF, while the pattern of KFla contains only very weak reflexions of K3AIF6 and or-alumina. KFla looses water below 373 K, but is stable in the range 373-723 K, suggesting that fluorine is retained. The two samples

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493

show similar surface areas (8.5 m2/g for KFla and 14.9 m2/g for KFA), and the main difference is then a higher dispersion of KF in KF 1a. KNO3/TAI203 appears as an XRD amorphous solid of surface area 15.8 m2/g after aetivatinn at I~9.~1~ 8 800 , .~, 600

5

.~- 400 "~ r"

HT

5h

~ 200

o

.7"

7

,~

6

,

5

~

4

~

3

2 % -.~ 1 10 20 30 40 50 60 70 80 Angle 2Theta(~

Fig. 1. XRD patterns of hydrotalcites after synthesis, calcination and rehydration for different periods of time.

N o

0 1 2 3 4 5 15 7 Molar ratio acetophenone/benzaldehvde Fig.2. Kinetics of condensation ofbenzaldehyde with benzophenone at 298 K on rehydrated HDT

Table 1. Surface areas and porosities of a synthetic hydrotalcite after calcination at 723 K, and further rehydration at room temperature for different periods of time. HT1A Treated 723 K Rehydration 5h 15h 48h SBET [rn2/g) 95.3 265.2 50.2 19.1 11.7 Pore vol (ml/g) 0.415 0.857 0.261 0.0991 0.0636 The basic properties of HDT have been described elsewhere [13]: judged from the changes of the enthalpy of adsorption of CO2, the basic strength does not change much upon rehydration. Two types of solid can then be obtained acording to the pretreatment: a Lewis base by decarbonation and a Bronsted base by further rehydration. 2) Reaction of benzaldehyde on acetone. At 273 K in acetone as solvent, the reaction gives the aldol but also some aldolisation of acetone to diacetone alcohol. It was attempted to suppress this bimolecular side reaction by diluting the system, but this raises the problem of the choice of the solvent. Solvent effects were investigated on this reaction using activated HDT as catalyst. The results reported in Table 2 show that the initial reaction rate is related to the polarity of the solvent. The effect of acido-basic character of the solvent can be estimated using the parameters Aj and Bj related to the ability of solvation of anions and cations [14]. Ethanol and THF show the same polarity but ethanol is more acidic and less suitable as solvent. THF appears as a good solvent, less toxic than DMF and has been used for standard measurements. The kinetics of this aldolisation determined in THF obeys a competitive mechanism with a rate going through a sharp maximum for a ratio acetone/benzaldehyde =12.6. This maximum was not noticed in absence of solvent [7] which gives evidence of a competition between the reactants and the solvent for adsorption at the surface.

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494

Hydrating the sample increases activity and selectivity for the aldol of HDT but increases activity and decreases selectivity of KFA (Table 3) suggesting that the sites for aldolisation are different in the two cases.

Table 2. Effect of the solvent on the initial rate of the aldolic condensation of benzaldehyde on acetone in the liquid phase at 273 K. Solvent Polarity Aj Bj Aj+Bj Initial Rate * 106 heptane toluene anisole THF EtOH DMF water

(D) 0 0.3 1.3 1.7 1.7 3.2 1,8

0 0.13 0.21 0.17 0.66 0.30 1

0 0.54 0.74 0.67 0.45 0.93 1

0 0.67 0.96 0.84 1.11 1.23 2

Table 3. Effect of hydration for HDT and KFA catalysts. Solid KFA calc. KFA + 501aL H20 r0 (mol.g-l.s -1) % conv. benzaldehyde (lh) % selectivity aldol (lh)

1.3 10-6 40.9 76.8

1.2 10-5 28.8 31.5

(mol.8_l.sec_l) 0.31 0.34 1.3 5.2 2.85 16.4 0

HDT calc

HDT calc + r6hydr.

7.5 10-7 2.2 56.6

3.5 10-5 72.7 83.8

For hydrotalcites prepared in identical conditions, the rate of reaction goes through a maximum at a ratio Mg/A1 close to 3, thus a maximum of basicity is observed for isolated sites.

3) Condensation of benzaldehyde on acetophenone: At 298 K the equilibrium conversion from thermodynamic values in aqueous phase is 89.1% for an equimolar mixture of the reactants [ 1]. The main product of the reaction on HDT is the unsaturated ketone (chalcone) resulting from the dehydration of the aldol. HDT is the only solid base selective in these reaction conditions. KF/alumina produces large amounts of 1,3,5-triphenylpentan-l-5-dione, by Michael addition of acetophenone on chalcone. Since this reaction is more difficult, the kinetic study of aldolisation on HDT was performed in DMF in order to have shorter reaction times at 298 K. Here also a sharp maximum is observed for the initial rate as a function of the concentration of both reactants (Fig. 2). The optimum ratio is here close to 1, which illustrates the differences in acidity of the two ketones. Using the optimum ratio of reactants the yield reaches 82% after 20 h at 310 K. At higher reaction temperatures, the selectivity to chalcone decreases at the expense of the Michael addition. 4) Comparison of different solid bases The different solid bases were compared at 273 K for benzaldehyde-acetone. The only solids showing good activitiy at this temperature are HDT and supported KF, which reach complete conversion in less than 6 h. The pattern selectivities of these catalysts is: HDT calcined < KF/alumina rehydrated
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化

are active at room temperature, and reach about 80% yield in <3 h (Fig. 3) but over exchanged zeolites and La203 are inactive and require a temperature of 423 K to reach similar conversions. At 423 K the activities reported in Fig. 4a, give a pattern expressed on a weight basis HT calcined rehydrated<MgNa X< La203 < CsNaX< KNO3/AI203< KFla but when the activity is expressed per unit surface area, La203 is most active. The final selectivity for the unsaturated ketone changes in a different manner (Fig. 4b): HT (sel 100 %)> La203 (96)> KF1 a (94)> KNO3/AI203(92)> MgX (89) > CsNaX (82). The lower selectivity of microporous zeolites, connected to a lower activity, is attributed to diffusional limitations which favour consecutive reactions. The low activity of HDT at 423 K is due to the dehydroxylation of the solid: for a reaction at 298 K, the conversion after 3 h decreases from 70% for the hydrated form to 4.6 % after dehydration at 423 K, the selectivity being unchanged. 0.4

0.4 i~. ~ ~

0.3

g

0.3

=

"~k - ~

0.2

= 0.2 o

0.1

g 0.1 . / / "

~ ~

benzaldehyde acetophenone olefinic ketone

/

0.0

HT21

0.0 0

60

120

180

240

300

0

60

120

Time (min)

180

240

300

Time (min)

Fig. 3. Aldol condensation of acetophenone and benzaldehyde at 273 K on KF1 (left) and rehydrated HDT (fight)

~ 100

100

9O 8O

g

7O e

~m

o

l"

c

e

c: ~ 70

* =

o u

20

"O3 0

10 0

8o

(D 09 (~

,-

,

|

,

,

,

0

60

120

180

240

300

360

r 60

-

0

60

120

KFla KNO3/TAI203 NaCsX-AcCs La20 3 NaX-(Ac)2Mg II

180

240

300

360

Time (min)

Time (rrin)

Fig.4. Conversion ofbenzaldehyde at 423 K as a function of time (let~) and selectivities into ehalcone (fight) for a series of solid bases: a) HDT calcined, b)NaMgX, c) La203, d) HDT calcined then rehydrated, e) NaCsX, f) KNO3/A1203 and g) KF1 a.

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

496

DISCUSSION In spite of a rather mild basic strength hydrated HDT appear as a good aldolisation catalyst. The degree of hydroxylation of the surface is a crucial parameter. Aldolisation can be performed at 273 K either by hydrated HDT or supported KF.. The most active sample KFla is supposed to be supported KF with good dispersion. In that case the active sites are

proposed to be F- anions of low coordination. Both OH- or F- sites can catalyse aldolisation with comparable rates, but the selectivity is higher on hydrated HDT. The importance of the consecutive Michael reaction when using supported KF is attributed to a higher basicity of the solid. The comparison of the different solids in the aldolic condensation of acetophenone with benzaldehyde shows that comparable conversions can be obtained at 423 K with many solid bases. Microporous systems are not suitable as expected for this type of reactions due to the possibility of consecutive steps. However selectivities for chalcone higher than 90% can be reached with mesoporous systems. The bimolecular Langmuir-Hinshelwood mechanism has been observed in many other cases [15, 16] and requires that the reactants compete for the same sites. The hypothesis that acetone or acetopheneone forms the carbanion which reacts at the surface on benzaldehyde adsorbed by an hydrogen bond could account for the experimental observations. In this case, the position of maximum rate is related to the ratio of the adsorption coefficients of the reactants which in turn is related to the basicity of the surface. It is expected that these curves are shitted when the basic strength increases, as reported earlier [ 16]. With hydrated hydrotalcite the higher activity is observed for an equimolar mixture of reactants, and HDT is therefore an interesting catalyst from the practical point of view. It can also be recycled without loss of activity. In conclusion the success in the application of solid bases is controlled by several factors such as activation, nature of solvent and composition of the reaction medium. When these are optimised very good yields can be reached with high selectivities, close to 100%. References. 1. J. P. Guthrie, Can. J. Chem. 56 (1978) 962. 2. Tichit, M.H. Lhouty, A. Guida, B. Chiche, F. Figueras, A. Auroux, E. Garrone, J. Catal. 151 (1995) 50-59. 3. W. T. Reichle, US Patent 4.458.026 (Jul. 3, 1984) to Union Carbide. 4. M.J. Climent, A. Corma, S. Iborra and J. Primo, J. Catal. 151 (1995) 60 5. F. Figueras, D. Tichit, M. Bennani Naciri, R. Ruiz, in "Catalysis of Organic Reactions" (F. E. Herkes ed) Marcel Dekker Inc, New York 1998, p37-49. 6. R. Tessier, D. Tichit, F. Figueras, and J. Kervenal French Patent 95 00094, 1995. 7. K. Koteswara Rao, M. Gravelle, J. Sanchez Valente, F. Figueras, J. Catal. 173 (1998) 115. 8. H. Tsuji, H. Hattori, H. Kita, Proc. 10th Inter. Congr. Catalysis (1992) 1171. 9. M. Lasperas, H. Cambon, D. Brunel, I. Rodriguez, P. Geneste, Micropor. Mat. 1 (1993) 343. 10.J.H. Clark, Chem Rev. 80 (1980) 429. 11 .J.H. Zhu, Y. Wang, Y. Chun, X. S. Wang, J. Chem. Sot., Faraday Trans., 94 (1998), 1163. 12. S. Miyata, Clays & Clay Miner. 23 (1975) 369. 13. J. Sanchez Valente, F. Figueras, M. Gravelle, P. Kumbhar, J. Lopez, J-P Besse, in press. 14. C. Reichardt, "Solvents and solvent effects m Organic Chemistry" 2nd edition, VCH (1990) 402. 15. A. Aguilera, A R. Alcantara, J. M Marinas, J. V. Sinisterra Can. J. Chem. 65 (1987) 1165. 16. A. Guida, M. H. Lhouty, D. Tichit, F. Figueras, P Geneste Appl. Catal 164 (1997) 251.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

The Influence of Metal-Support Interactions During Liquid-Phase Hydrogenation of an ~, [3-Unsaturated Aldehyde over Pt Utpal K. Singh and M. Albert Vannice Department of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802 Liquid-phase hydrogenation of citral was investigated over SiO2- and TiO2- supported Pt catalysts in the range of 298 - 423 K, 7 - 41 atm H2 pressure, and 0.5 - 6.0 M citral in hexane. The initial rate of citral hydrogenation over Pt/SiO2 catalysts exhibits an activity minimum with respect to temperature accompanied by an increase in selectivity for hydrogenation of the C=O bond with increasing reaction temperature. Furthermore, the initial rate of citral disappearance is strongly influenced by metal particle size since the rate of citral disappearance at 373 K decreased 20-fold from the 5 nm SiO2-supported Pt crystallites to the 1 nm SiO2-supported Pt crystallites. This difference is suppressed at 298 K and only a five-fold decrease in the rate is observed during this change in Pt crystallite size. Pt/TiO2-LTR (low temperature reduced - 473 K) and Pt/SiO2 catalysts exhibited zero- and first-order dependencies on citral concentration and hydrogen pressure, respectively, at 373 K. In contrast, the Pt/TiO2-HTR (high temperature reduced - 773 K) catalyst exhibited negative first- and zero-order dependencies on citral concentration and hydrogen pressure, respectively. The TOF on the Pt/TiOz-HTR catalyst was more than an order of magnitude greater than that on Pt/SiO2 and Pt/TiO2-LTR. In addition, the Pt/TiO2-HTR catalyst exhibited a marked enhancement in selectivity towards hydrogenation of the C=O bond. 1. INTRODUCTION It has been stated that approximately 50-100 kg of by-product are produced per kg of product in the fine chemicals and pharmaceutical sectors of the chemical industry [1]. Therefore, in light of the increased environmental awareness, it is of interest to develop heterogeneous catalysts for synthesis of pharmaceuticals and fine chemicals In the present work we report the influence of metal-support interactions (MSI) during the selective liquidphase hydrogenation of citral (3,7-dimethyl-2,6-octadienal), which contains three unsaturated bonds including a conjugated system of C=C and C=O bonds and an isolated C=C bond. From a thermodynamic perspective, the isolated C=C bond is the most favorable to hydrogenate followed by the conjugated C=C bond and lastly the C=O bond [2]. However, kinetic control of the reaction can be induced to yield high selectivity for hydrogenation of the C=O bond alone. In the present paper we examine the influence of reaction parameters and support effects on selective hydrogenation of citral.

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498

2. EXPERIMENTAL

The details of catalyst synthesis, characterization, and hydrogenation reaction procedures are described in detail elsewhere [3]. Briefly, the catalysts were prepared via incipient wetness or ion exchange using H2PtC16 or Pt(NH3)4CI2, respectively, as the precursor. SiO2 (Davison Grade 57 silica gel - 220 m2/g) and TiO2 (Degussa P25 - 47 m2/g) were dried and calcined at 773 K for four hours prior to catalyst synthesis followed by drying at 393 K overnight. The catalysts were characterized using H2 and CO chemisorption at 300 K to evaluate dispersion and average particle size. Pt/SiO2, Pt/TiO2-LTR (low temperature reduced), and Pt/TiO2-HTR (high temperature reduced) were reduced in situ at 673 K, 473 K, and 773 K, respectively. Nitrogen was bubbled through both hexane and citral prior to their addition into the reactor to remove trace quantities of oxygen from the liquid phase. The reaction progress was monitored by GC analysis of liquid samples periodically withdrawn into a N2 purged vessel as well as by the instantaneous rate of H2 uptake [3]. 3. Results and Discussion 3.1 Citral Hydrogenation over Pt/SiO2

The kinetic data obtained with Pt/SiO2 catalysts was previously shown to be free of transport limitations and poisoning effects as verified by the Madon-Boudart test [3, 4]. The influence of reaction temperature on rate and product distribution is displayed in Figures 1 and 2, respectively, for reaction over a 1.44% Pt/SiO2 catalyst at 20 atm hydrogen pressure and 1 M citral in hexane in the range of 298 - 423 K [3].

0.6 t[

-~

i 0.4 + i

~"

~ ,-

.......

L

-""

/

-- - 2 9 8 K 1

', '~ ~-

o

9

.

i i

,

0

80

| ,

'

.... 373K ~ 423 K i

i

"~ ~

"",.,.,ih,... i."

~ - ~ ~

20

:~" i /-~ 40 ~-~",~

~

7 40

60

Citral Conversion (%)

Figure 1:Temporal H2 uptake profiles for reaction at 298 K, 373 K, 423 K with 1 M citral in hexane at 20 atm H2 pressure.

20 t

.

.

.

.

.

I

....

-0 ....

,i

I o Conjugated C=C Bond (298 K);; o Conjugated C=C Bond (373 K)':'

~ 1 ~ - - - - '

O~ L

t

0

20

~ ---

40

~

---

60

I "~

80

100

Citral Conversion (%)

Figure 2" Selectivity for hydrogenation of conjugated C=O and C=C bonds at 298 K, and 373 K with 1 M citral in hexane at 20 atm H2 pressure.

It is apparent from Figure 1 that there is an activity minimum in the rate with respect to temperature. Furthermore, significant deactivation is observed at 298 K in contrast to the negligible deactivation exhibited at 373 K and 423 K. The product distribution is dramatically altered by reaction temperature as seen in Figure 2. The selectivity towards

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499

hydrogenation of the conjugated C=C bond is greater than that for the C=O bond at 298 K. This trend is reversed at 373 K where hydrogenation of C=O bond is the primary reaction pathway. This unusual kinetic behavior was rationalized by utilizing a conventional Langmuir-Hinshelwood-type model for each of the hydrogenation steps along with a concurrent inhibiting decarbonylation reaction. The activity minimum has been explained based on the lower activation barrier for the decomposition reaction yielding chemisorbed CO as compared to that for CO desorption [3, 5, 6]. Such a trend was shown to have three consequences including: strong deactivation during reaction at low temperatures i.e., 298 K, an activity minimum at approximately 373 K, and negligible deactivation at 373 K and 423 K [3]. The complex kinetics observed during liquid-phase hydrogenation is also manifested in the apparent structure sensitivity of the reaction at 373 K with 20 atm H2 pressure and 1 M citral in hexane as shown in Table 1. The differences in the rate of citral hydrogenation among catalysts with different dispersion was suppressed at 298 K in contrast to the behavior at 373 K. Due to the strong deactivation behavior at 298 K, there is significant uncertainty present in reporting a single value for the rate at this temperature. Therefore, Figure 3 displays the temporal H2 uptake profile for reaction over Pt/SiO2 catalysts with varying metal dispersion at 298 K, 20 atm H2 pressure, and 1 M citral in hexane. It should be noted that in spite of the dramatic differences in the initial rate of citral hydrogenation for catalysts with different particle sizes, the selectivity vs. conversion behavior was similar for all the catalysts, within experimental uncertainty, as shown in Figure 4. Table 1 Effect of average Pt particle size, determined from H2 chemisorption at 300 K, on the initial rate of citral hydrogenation at 373 K, 20 atm H2 pressure, with 1 M citral in hexane. Dispersion H/Pt

Metal Particle Size

Initial TOF

Pt/SiO 2

(nm)

(s")

3.59% Pt

0.04

28

0.120

6.65% Pt

0.09

2.65% Pt

0.22

5.1

0.092

3.80% Pt (sintered)

0.37

3.1

0.040

1.44% Pt

0.41

2.8

0.017

0.49% Pt

0.45

2.5

0.012

3.80% Pt

0.66

1.7

0.010

1.1

0.005

Catalyst

0.77% Pt

0.065

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化

0.8

r~

..............................................................................................................

0.6

o H / p t = 0.09

,., e~

9 H / Pt =

0.4

o H / Pt = 0.4

\',,,

~.

\o

~ H / Pt =0.7

0.2

~o

" H/Pt

i o~....'-~___~---~__ [-

0

0.2

-----I_

~---~_---~___^

0

= 1.0

- ....... o ~ - - - - o _ o ~ _ o

60

"-=---u--~--~~---o 120

..............

!

240

300

180

Time (min)

Figure 3" Influence of metal dispersion on the rate of H2 uptake during reaction at 298 K and 20 atm H2 with 1 M citral in hexane. 100 -;. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -=

80

-

60:

L

.<

,,j

o

40

"" '=

~

20

0-

9

,, H / P t = 0.41

o

+

121

9

oH/Pt=0.22

1

oH/Pt=0.09

i t I

9 H/Pt=0.06

i 0

10

20

30

40

Citrai Conversion (%)

Figure 4" Influence of metal dispersion on selectivity towards hydrogenation of the C=O bond at 373 K 20 atm H2 with 1 M citral in hexane. Lercher and coworkers have previously alluded to a particle size effect during the liquid-phase hydrogenation of crotonaldehyde [7]. However, in contrast to their results, the selectivity towards hydrogenation of the C=O functionality in the present study is not affected significantly by particle size. Furthermore, the 20-fold enhancement in rate with 5 nm Pt particles compared to 1 nm Pt particles observed in the present study is significantly greater

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

501

than the four- to five-fold rate increase over a similar range of particle size reported by Lercher and coworkers for crotonaldehyde hydrogenation [7]. The apparent structure sensitivity observed during citral hydrogenation may be related to an inhibiting reaction proposed earlier to explain the observed activity minimum with respect to temperature [3]. It is well known that highly dispersed metal catalysts have a larger fraction of coordinatively unsaturated atoms which may facilitate reactions involving C-C bond scission. Furthermore, the results of Barteau and coworkers suggest that alcohol decomposition reactions over Pd surfaces is structure sensitive [8]. Therefore, the lower rate exhibited by the highly dispersed catalyst at 373 K may be due to a larger fraction of sites being poisoned by CO from the decarbonylation reaction. The suppression of an apparent particle size effect at 298 K is in general agreement with the results of Galvagno and coworkers which suggest that citral hydrogenation is structure insensitive during reaction at low temperatures over Ru catalysts [9]. The suppression of apparent structure sensitivity at 298 K is not entirely clear but may be attributable to a lower rate of the alcohol decomposition reaction as compared to the hydrogenation reaction, thus reducing the influence of the structure sensitive decomposition reaction on the overall hydrogenation reaction kinetics. 3.2 Citral Hydrogenation over Pt/TiO2 The kinetic results reported in the present study for Pt/TiO2 catalysts are free of transport limitations as verified by the Madon-Boudart test and the Weisz-Prater criterion. The behavior of Pt/SiO2 catalysts is similar to that for Pt/TiO2-LTR catalysts but is in sharp contrast to that for Pt/TiO2-HTR catalysts. Table 2 compares the TOF for citral disappearance and selectivity towards hydrogenation of the C=O bond at 373 K and 20 atm H2 with 1 M citral in hexane. A comparison of reaction orders with respect to citral and hydrogen for Pt/SiO2, Pt/TiOz-LTR and Pt/TiO2-HTR catalysts is also shown in Table 2 for reaction at 373 K, concentrations of 1 - 6 M citral in hexane, and hydrogen pressures from 7 41 atm. Dramatic differences are apparent in the kinetics with the Pt/TiO2-HTR catalyst compared to that with Pt/SiO2 and Pt/TiOz-LTR catalysts. The TOF for the Pt/TiOz-HTR catalyst, based H2 chemisorption on a HTR catalyst, is around two orders of magnitude higher than that for Pt/SiO2 (H/Pt=I.0) and Pt/TiOz-LTR catalysts. Even when the initial rate on the Pt/TiOz-HTR catalyst is normalized with respect to H2 chemisorption on the LTR catalyst, the rate is still three-times larger than that for the Pt/TiOz-LTR catalyst, indicating a greater activity per gram of catalyst, and it is 15-fold higher than SiO2-supported Pt.

In addition to the rate enhancement, the reaction orders exhibited by the Pt/TiO2-HTR catalyst are significantly different from those observed with the Pt/SiO2 and Pt/TiO2-LTR catalysts. Approximately zero-order kinetics are observed with respect to citral concentration with Pt/SiO2 and Pt/TiO2-LTR at 373 K, 20 atm H2 and 1.0 - 6.0 M citral in hexane. This is comrasted with the -0.86 order observed with Pt/TiO2-HTR. Furthermore, in contrast with the first-order dependency of rate on hydrogen pressure observed with Pt/SiO2 and Pt/TiO2LTR, Pt/TiO2-HTR exhibits a near zero-order dependence on H2. The Pt/TiO2-HTR catalyst also exhibits enhanced selectivity towards hydrogenation of the C=O functionality as compared to Pt/SiO2. For example, selectivity towards

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

502

hydrogenation of the C=O bond at 373 K and 20 atm H2, extrapolated to zero time, is 40%, 87%, and 93 % over the Pt/SiO2, Pt/TiOa-LTR, and Pt/TiO2-HTR catalysts, respectively.

Table 2 Initial rate of citral disappearance and reaction orders with respect to citral and hydrogen over Pt/SiO2, Pt/TiO2-LTR, and Pt/TiO2-HTR at 373 K. Pt/SiO 2

Pt/TiO2-LTR

Pt/TiO2-HTR

TOF*

0.004

0.02

0.06 (1.0)**

S (C=O Bond)***

40

87

90

Reaction Order in Citral

0.02

-0.21

-0.86

Reaction Order in H2

0.91

1.1

0.16

* TOF normalized to H2chemisorption on LTR catalyst ** TOF normalized to Hz chemisorption on HTR catalyst *** Extrapolated to zero citral conversion

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.

Sheldon, R. A., J. Mol. Catal. A." Chem 107 75 (1996) Patil, A. Banares, M. A., Lei, X., Fehlner, T. P., and Wolf, E., J. Catal. 159 458 (1996) Singh, U. K., and Vannice, M. A. J. Catal., Submitted for publication Madon, R. J. and Boudart, M., Ind. Eng. Chem. Fundam. 21 438 (1982) McCabe, R. W., and Schmidt, L. D., Surf Sci., 66 I 01 (1977) Davis, J. L., and Barteau, M., J. Am., Chem. Soc., 111 1782 (1989) Englisch, M., Jentys, A., and Lercher, J. A., J. Catal. 166 25 (1997) Shekhar, R., Barteau, M., Cat. Lett. 31 221 (1995) Mercadante, L. Neri, G., Milone, C., Donato, A., and Galvagno, S., J. Mol. Catal 105 93 (1996)

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Tailoring of acido-basic properties and metallic function in catalysts obtained from LDHs for the hydrogenation of nitriles and of a,]5unsaturated aldehydes. B. Coq, S. Ribet, R. Durand and F. Medina # Laboratoire de Mat6riaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRS/ENSCM 8, Rue Eeole Normale 34296 Montpellier Cedex 5 France. Ni 2+-, Co 2+-, Ru 3+-, and Mg2+/A13+ layered double hydroxides have been used as precursors of catalysts for the hydrogenation of acetonitrile, valeronitrile and citral. The acido-basieity and the metallic function of these materials are finely tuned up through the nature and the amount of the cations in the layers, and the activation conditions thus leading to active and selective catalysts. 1. INTRODUCTION Layered Double Hydroxides (LDHs)of general formula [Ml.x2§ Mxs+ (OH)2] [Ax/n[L']mH20 contain divalent and trivalent cations in the layers, compensating anions in the interlayer spaces and lead to mixed oxides of the Ma(MIa)o type upon calcinations [1]. These materials may exhibit strong basic properties specially those containing Mg 2+ [2]. Further reduction of LDHs, or mixed oxides, containing reducible cations yields supported metal catalysts. In these materials both the metallic function and the acido-basic properties can be tailored through several parameters such as nature and amount of the cations in the layers, and the activation conditions. On that account, these materials are of special interest for hydrogenation reactions in which the catalytic properties are promoted when the support exhibits basic sites and/or "electron donor" properties. Exemples of those can be found in the hydrogenation of nitriles into primary amines and of ct,13-unsaturated aldehydes. The selectivity in the former reaction is indeed increased with basic supports, and preferably small metallic particle sizes, in order to inhibit the condensation reactions between multibonded reactive intermediates leading to by-products [3]. The condensations occur either on the acid sites of the support and the metal through a bifunctional mechanism, or on the metal particles alone [4]. Selectivity in the hydrogenation of r aldehydes is also very sensitive to the nature of the support [5]. In this work the tailoring of acido-basic properties and metallic function of dedicated LDHs is shown, and related to their catalytic behaviours in the hydrogenation reactions of acetonitrile, valeronitrile and citraI.

# on leave from Departamentd'EnginyeriaQuimica,UniversitatRovira i Virgili, Carreterade Salou,43006 Tarragona, Spain

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

504

2. EXPERIMENTAL

LDHs have been prepared by precipitation at pH = 9 of a solution containing a mixture in the desired amount of the metal nitrate salts (Mg, AI, Ni, Co, Pd) with a NaOH solution. The precipitated gel was aged overnight in air at 330 K, filtered and washed several times [6]. Some Pd-based catalysts have also been prepared by exchange of Mg/A1 LDHs with Pd acetylacetonate (Pd(acac)2) in toluene or PdCI2 in aqueous solution. These materials were characterized at different stages of the preparation by XRD and N2 sorption at 77 K. The acidbase sites were studied by chemisorption of CD3CN followed by FTIR spectroscopy and by differential microcalorimetry of ethylamine (MEA). The metal function was studied by temperature programmed reduction (TPR) by H2/Ar (3/97) from room temperature to 1073 K at 5 K min -1 and by chemisorption of CO followed by FTIR spectroscopy and calorimetry. The vapor phase hydrogenation of acetonitrile was performed with a H:/acetonitrile molar ratio of 6.8 in a microflow reactor at atmospheric pressure. The liquid phase hydrogenations ofvaleronitrile and citral were studied in a 100 cm 3 batch reactor at 363 K under 1.6 MPa H2 in cyclohexane solution. 3. RESULTS 3.1. Modification of the acido-basic properties Ni 2§ and Co2+ containing LDHs are the most suitable for the hydrogenation of nitriles into primary amines and the tuning of the acido-basicity would result from several parameters, mainly (i) the presence of Mg2+ in the layers [7], and (ii) the activation procedure, i.e. the calcination and reduction temperatures of Ni or Co/Mg(Al)O obtained from the LDHs precursors [8]. The influence of the Mg2+ amount is clearly shown using Ni/Mg/Al LDHs with Mg/(Mg+Ni) ratio ranging from 0 to 0.86. Remarkably, the differential heat of MEA adsorption, measured by calorimetry, is 40 kJ mo1-1 lower for the mixed oxide obtained with Mg/(Mg+Ni) = 0.23 compared to the Mg-free LDH (Ni/AI). This accounts for a net decrease of the acidity in the former. Accordingly the MEA selectivity goes through a maximum value of 92.6 % at 99 % acetonitrile conversion for the catalyst with Mg/(Mg+Ni)= 0.23 at a reaction temperature of 403 K. In the same way the selectivity into pentylamine (MPA) in the hydrogenation of valeronitrile increases with the Mg content of the LDHs. It reaches 95.5 % MPA at 99 % conversion, instead of 87.6 % with the Mg-free catalyst. Concerning the influence of the activation procedure, interesting features came from IR study of adsorbed CD3CN [8] on the Ni/Mg/AI LDH with Mg/(Mg+Ni)= 0.23 activated in a wide range of calcination (393-773 K) and reduction (573-873 K) temperatures. In the spectral region 2000-2400 cm "1, mainly concerned by the vCN and vCD vibration modes of CD3CN adsorbed on both surface Brmlsted sites and basic sites, two bands appearwith vCN at ca 2295 and 2170 cm "1 as shown in Figure 1. The former correspond to CD3CN species interacting with very weak Mg 2+ Lewis sites [9] and/or Ni ~ [10]. The later corresponds to CD2CN" carbanions formed by interaction of CD3CN with highly basic O2" sites of the mixed oxides. The intensity ratio I2295/I2170 of the IR bands is the lowest aider calcination at 623 K and reduction at 723 K. Accordingly, as reported previously, the selectivity to MEA was the highest, reaching 92.6 % at 99 % conversion with the sample activated in these optimum

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in

. . . .

I

. . . .

I

'

'

'

'

'

I

'

'

'

I

'

'

'

I

"

'

'

I

'

9

1 O

-t2300

2200

21

Wavenumber (cm -1)

b

O0

..

Fig. 1. IR spectra of adsorbed CD3CN on Ni/Mg(AI)O, (a) calcined at 393 K and reduced at 723 K, (b) calcined at 623 K and reduced at 723 K, (c) calcined at 623 K and reduced at 673 K, (d) calcined at 773 K and reduced at 723 K

300

500

700

900

Temperature (K)

11 O0

Fig. 2. TPR profiles of (a) Ni/Mg/AI, Co/Ni/MgAI with: (b) Co/Ni = 0.33, (c) Co/Ni=l, (d) Co/Ni=3 and (e) Co/Mg/A1 samples

conditions. TPR experiments show that Ni 2§ whose reducibility decreases with the Mg 2§ content of the samples, is not fully reduced in these conditions. Therefore the optimum Mg/(Mg+Ni) ratio and activation conditions accounted for a balance between the decrease of the acid character and the Ni 2§ reducibility. On that account, the increase of MEA selectivity with catalysts of lower acidity is due to inhibition of condensation reaction between protonated imine and amine catalyzed by acid sites [4,7]. 3.2. Modification of the metallic function

The modification of the metallic function in catalysts obtained from LDHs can be achieved through bimetallic formulations using two reducible cations (e.g. NiCo, NiCu,..) and/or metalsupport interactions. The former refers to the general "bimetallic effect" in catalysis by metals, and was not really explored in the case of multicomponent LDHs till now [11]. In contrast recents reports deal with the later effect and provide evidences that significant modifications of the electronic properties of nano-sized noble metal (Pt, Pd .... ) particles can be achieved by support from LDHs precursors [12-15]. In this context, the tentative introduction of small amounts of noble metal through the direct synthesis of LDHs seems attractive [ 15,16]. We will describe hereafter these two aspects, i.e. metal-support interaction and bimetallic effect, of the tuning of metallic function properties through he use of LDHs precursors. The modification of electronic properties of Ni particles formed from Ni/Mg/AI LDHs has been proved on Ni/Mg/A1 LDHs by the adsorption of CO followed by calorimetry and IR

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spectroscopy. In the later experiments a special attention was paid in studying the bands at ca 2030-2065, 1935-1960 and 1890 cm -1 assigned to linear, bridged and multicentred (Ni)xCO species respectively. Indeed when comparing the behaviour of Ni/Mg(AI)O with Mg/(Mg+Ni) = 0.23, calcined at 673 K and reduced at 723 K, to that of a Mg-free sample it has been established that (i) CO desorbed at higher temperature [8], (ii) the heat of CO adsorption was larger by 20 kJ mo1-1 [7], and (iii) vco of (Ni)xCO shifted by 40 cm -1 to lower frequencies [6], on the former sample. This is due to an increased back donation of Ni towards the 2 ~. antibonding orbital of CO, thus reinforcing its bonding with Ni. On the other hand, the dilution of metallic function in smaller ensembles are expected to reduce the reactions involving multibonded species, e.g. the condensation reactions leading to byproducts. This could be achieved by the addition of a second metal to the active one. With this aim, multicomponents Ni/Co/Mg/AI LDHs precursors have been prepared by varying the ratio Co2+/Ni2+ between 0.33 and 3. All samples exhibit the XRD pattern of a unique lamellar structure. When the samples are calcined at 773 K the Co and Ni cations respectively segregate in mixed oxides with spinel and rock salt-like structures, and the XRD profiles exhibit the characteristic patterns of the most abundant phase in the mixture. TPR profiles show a reduction in two steps as shown in Figure 2. The species reduced below 753 K are assigned to Co304 type phases. Above 753 K, the profiles have an intermediate shape between those of Co/Mg/AI and Ni/Mg/A1. However they are not their convolution and the temperature of the maximum rate of H2 consumption does not vary linearly with the Co 2§ /Ni2+ ratio. These features suggest that among the different spinel phases reduced above 753 K, shown by XRD, at least one of them accomodate both Co2+ and Ni 2+ in the same lattice. The adsorption of CO, followed by IR spectroscopy (Figure 3), on these samples calcined at . . . .

I

. . . .

I

. . . .

I

. . . .

~

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'

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eo

-

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, 2100

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1900

1800

17110

W a v e n u m b e r ( c m ~)

Fig. 3. IR spectra in the CO stretching domain of adsorbed CO on Co/Ni/Mg/A1 LDHs with (a) Co/Ni = 3, (b) Co/Ni = 1, (c) Co/Ni = 0.33, (d) Ni/Mg/A1

0

0.0

0.2

0.4

0.6

0.8

1.0

Ni/(Co+Ni)

Fig. 4. Selectivity in (EEI+DEA) as a function of the Ni/(Ni+Co) ratio of the Co/Ni/Mg/A1 samples

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393 K and reduced at 893 K shows that the bridged and multicentered species formed on Nirich samples vanish progressively as the Co content increases [11]. This accounts for a dilution of the Ni ensembles by Co. Accordingly the selectivity to condensed by-products (Figure 4), diethylimine (DEI) and diethylamine (DEA) mainly, goes through a minimum value when Ni/(Ni+Co)=0.5 in the hydrogenation of acetonitrile at 10% conversion, providing evidence of a synergy between Ni and Co. This synergy comes from the dilution of Ni by Co in small ensembles less able to promote the condensation between neighboring multibonded adsorbed species. " Table 1. Some caracteridtics, and catalytic properties in the hydrogenation of citral, of Pd catalysts from LDHs precursors Sample Pd Precursor wt% Pd Red. temp. (K) H/Pd TOF a (s-1) Selectivityb PdHT3 Pd(NO3)2 1.68 673 0.09 0.3 69 PdHT2 Pd(NO3)2 2.74 573 0.34 0.07 72 PdeHT5 Pd(acac) 2 0.50 823 0.75 0.52 50 PdeHT4 PdCl2 0.94 723 0.72 0.19 75 Pd/C 54 afrom initial rate; bselectivity to citroneUal at 80% citral conversion

Interesting properties can also be achieved from nano-sized noble metal particles prepared from LDHs with incorporation of the noble metal (Ru3+, Pd 2+, ...) within the brucite layer [ 15,16]. We have explored this route for Pd/Mg/A1 LDHs and compared these materials with classical Pd/Mg(AI)O prepared by exchanged of various precursors with the Mg/Al LDHs. The materials were compared in the hydrogenation of citral to citronellal, reaction chosen for the low selectivity generally exhibited by Pd-based catalysts (see Pd/C in Table 1); Table 1 reports the main results. A higher temperature of reduction reflects a lower reducibility of Pd put in evidence by TPR experiments. In this respect, one can observed a strong interaction between the Pd precursor and the LDHs framework, and this is particularly true at low Pd content. There is no clear relationship between the Pd particle size and the rate (TOF) or the selectivity to citronlllal. This reaction has been indeed claimed to be structure-insensitive on Pd- and Ru-based catalysts [ 17,18] for samples prepared from the same precursors. However, in the present work the samples were prepared according to very different protocols and metal precursors. This is very likely the reason for some extend of structure-sensitivity. The catalyst which exhibits the highest selectivity to citronellal are those showing the lowest TOF. Studies are in progress to identify which factors, geometric or electronic, are mainly affected as a result of this interaction. 4. CONCLUSIONS

Catalystsobtained from LDHs containingNi 2+,Co 2§ and M g 2+ in controlledamounts, give selectivitieshigher than 90 % into primary amines in the hydrogenation of nitriles,and of 80 % for the hydrogenation of the C=C bond of ct,l$-unsaturatedaldehydes. The former results from a decrease of the acid character when M g 2+ is introduced into the LDHs layers. An optimum Mg/(Mg+Ni) ratio accounted for a balance between the acidity and the Ni 2+

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reducibility with the Mg 2§ content. The acido-basicity of the support and the electron density at the metal sites are also determined by the calcination and reduction temperatures. Catalysts obtained from multicomponents Ni/Co/Mg/Al LDHs are the most selective into MEA in the hydrogenation of acetonitrile. A synergy results from the dilution of Ni by Co giving smaller ensembles, and an electronic enrichment of Ni. These effects are able to reduce the transmination reactions. REFERENCES

1. F. Cavani, F. Trifiro and A. Vaccari, Catal. Today 11(2) (1991) 173. 2. W. T. Reichle, J. Catal. 94 (1985) 547. 3. C. De Bellefon and P. Fouilloux, Catal. Rev. Sci. Eng. 36(3) (1994) 459. 4. M. J. F. M. Verhaak, A. J. Van DiUen and J. W. Geus, Catal. Lett. 26 (1994) 3 7. 5. P. Gallezot, A. Giroir-Fondler and D. Richard, in "Catalysis of Organic Reactions", Marcel Del&er, New-York, 1991, p. 1. 6. S. Ribet, D. Tichit, B. Coq, B. Ducourant and F. Morato, J. Solid State Chem. 142 (1999) 382. 7. F. Medina Cabello, D. Tichit, B. Coq, A. Vaccari and N. T. Dung, J. Catal. 167 (1997) 142. 8. N. T. Dung, D. Tichit, B. Huong Chiche and B. Coq, Appl. Catal. A 169 (1998) 179. 9. A. G. Pelmenschikov, G. Morosi, A. gamba, S. Coluccia, G. Martra, E. A. Paukshtis, J. Phys. Chem. 100 (1996) 5011. 10. S. G. Yim, D. H. Son and K. Kim, J. Chem. Soc., Faraday Trans. 89 (1993) 837. 11. B. Coq, D. Tichit and S. Ribet, J. Catal., submitted. 12. R. J. Davis and E. G. Derouane, Nature 349 (1991) 313. 13. Z. Gandao, B. Coq, L. C. de Menorval and D. Tichit, Appl. Catal. A: 147 (1996) 395 14 S. B. Sharma, J. T. Miller and J. A. Dumesic, J. Catal. 148 (1994) 198. 15. F. Basile, L. Basini, G. Fomasari, M. Gazzano, F. trifiro and A. Vaccari, Chem. Commun. (1996) 2435. 16. S. Narayanan and K. Krishna, Catal. Today 49 (1999) 57. 17. M. A. Aramendia, V. Borau, C. Jimenez, J. M. Marinas, A. Porras and F. J. Urbano, J. Catal. 172 (1997) 46. 18. S. Galvagno, C. Milone, A. Donato, G. Ned and R. Pietropaolo, Catal. Lett. 18 (1993) 349.

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Studies in Surface Science and Catalysis 130 A. Conrm, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Chemical v e r s u s enzymatic catalysis for the regioselective synthesis of sucrose esters of fatty acids M. Ferrer a, M.A. Cruces a, F.J. Plou a, E. Pastor a, G. Fuentes a, M. Bernab6 b, J.L. Parra e and A. Ballesteros a a Instituto de Cat/disis, CSIC, Campus UAM Cantoblanco, 28049 Madrid, Spain*. b Instituto de Quimica Orgfinica, CSIC, Juan de la Cierva 3, 28006 Madrid, Spain. c Centro de Investigacirn y Desarrollo, CSIC, Jorge Girona 18-26, 08034 Barcelona, Spain. In this work we present two processes for the regioselective synthesis of sucrose monoesters. The first is enzymatic, and is carried out in a medium constituted by two miscible solvents, namely tert-amyl alcohol and dimethylsulfoxide. The second is chemical, and is based on the use of simple basic catalyts in dimethylsulfoxide. The regioselectivity of both methodologies will be compared. 1. INTRODUCTION Sucrose esters of fatty acids are biodegradable non-ionic surfactants that can be synthesized from renewable and readily available sources (sucrose is the organic chemical obtained in higher quantity in the world). They are particularly useful in the food industry-as emulsifiers, foaming agents, viscosity reducers, anticaking agents, fruit coatings, etc.- [1]. Due to their surfactive, anti-bacterial and anti-mycotic properties, sucrose esters are also currently being used in personal-care formulations [2]. In addition, their anti-tumoral [3] and insecticidal properties [4] have been reported, thus indicating their great versatility. The specific properties of a sucroester depend mainly on the fatty acid, the degree of substitution and the positions of acylation. Sucroesters are synthesized in the industry by transesterification of a methyl ester of a fatty acid with sucrose in the presence of a basic catalyst, at high temperature (more than 100~ and reduced pressure. These processes involve a high energy cost and the formation of undesirable byproducts but, more importantly, the resulting esters are heterogeneous (i.e. they are mixtures of compounds differing in the degree of esterification and the position of the acyl groups on the sugar moiety). The regioselective acylation of sucrose is difficult to achieve due to the similar reactivity of the eight hydroxyl groups (3 primaries and 5 secondaries) and to the existence of intramolecular migration processes. For that reason, multi-step synthesis based on protection/deprotection reactions is required. However, several one-step procedures for the * This research was financed by the European Union (project BIO4-CT98-0363) and by the Spanish CICYT (project BIO98-0793). We thank Comunidad de Madrid, Fundacirn Caja de Madrid and Ministerio de Educacirn y Ciencia for research fellowships.

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regioselective acylation of sucrose using chemical catalysts, based on the used of extreme reaction conditions and/or sterically-hindered acyl donors have been described [5]. The discovery that enzymes are able to work in non-aqueous media, functioning in the reverse direction that in Nature, has made possible that biological catalysts are being applied to regioselective transformations of mono- and disaccharides, including transesterifications of sucrose using activated esters in anhydrous organic solvents [6-7]. Enzymatic synthesis of sucrose fatty acid esters is limited by the fact that many biological catalysts are inactivated by the polar solvents (DMSO, DMF, DMA) where sucrose is soluble. Only the proteases of the subtilisin-family have shown to be active in this type of solvents. Most attempts to acylate sucrose either in media where the disaccharide is poorly soluble [8] or in absence of organic solvent [9] have resulted in very low yields. In this work we present two methodologies (enzymatic and chemical) that try to overcome the limitations of the procedures exposed above. 2. MATERIALS AND METHODS 2.1. Materials

Lipase from Humicola lanuginosa (Lipolase 100L) was kindly donated by Novo Nordisk (Denmark). Controlled pore glass (CPG, 7.4 m2/g), molecular sieves (3 A), Bial's reagent and 2-methyl-2-butanol were purchased from Sigma. Celite (diatomaceous earth, 30-80 mesh) was from BDH. v-alumina (145 m2/g) was from Rhone-Poulenc. Eupergit C was from Rh6m (Germany). Sucrose, silica gel 60 for column chromatography and dimethylsulfoxide were supplied by Merck. Vinyl laurate was from Fluka. Di-sodium hydrogen phosphate (anhydrous) was from Panreac (Spain). All other reagents were of the purest grade available. All solvents were stored over molecular sieves (3 A), at least 24 h prior to use. 2.2. Enzymatic synthesis of sucrose monolaurate

Sucrose (0.15 retool) was dissolved in 1 ml of DMSO. Then, 2-methyl-2-butanol was slowly added to a final volume of 5 ml. The biocatalyst (500 mg) and molecular sieves (500 mg) were added, and the mixture stirred for 30 rain at 40~ Vinyl laurate (1.5 mmol) was then added. Aliquots were withdrawn at different times, centrifuged using 0.45 lain filters, and analyzed by TLC and HPLC. The mixture was cooled and filtered, and the monoester isolated and purified as described elsewhere [6]. 2.3. Chemical synthesis of sucrose monolaurate

Sucrose (1.5 mmol) and vinyl laurate (6.0 retool) were added to 5 ml of DMSO. Then, the catalyst (750 rag) was added and the suspension stirred at 40~ Aliquots were withdrawn at different times, centrifuged using 0.45 ~tm filters, and analyzed by TLC and GLC. Once the reaction was finished, the catalyst was removed by filtration, and the monoester isolated and purified as described elsewhere [ 10]. 2.4. Lipase immobilization

For immobilization of lipase from H. lanuginosa onto Celite, the pH of commercial lipase solution (100 ml) was adjusted to 7.0. The support (8 g) was added and the suspension stirred for 30 rain at 4~ Then, 200 ml of cold acetone (0~ were slowly added with stirring. The immobilized enzyme was filtered, washed with acetone and dried in vacuo.

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2.5. Thin-layer chromatography Analytical TLC was performed on silica gel 60 plates (Merck) with chloroform/methanol 4:1 as eluent. Spots were detected by inmersion in a solution of orcinol/ferric chloride (Bial's reagent) diluted with ethanol, drying and heating at 120~ for 5 min.

2.6. HPLC analysis The analysis of enzymatic reactions was carried out by reverse-phase high-performance liquid chromatography (HPLC) using a system equipped with a Spectra-Physics pump, a Sugelabor Nucleosil 100-C18 column (250 x 4.6 mm) and a refraction-index detector (Spectra-Physics). The temperature of the column was kept constant at 40~ Methanol:water 85:15 (v/v) was used as mobile phase (flow rate 1.5 ml/min).

2.7. GLC analysis The analysis of chemical reactions was carried out by capillary gas-liquid chromatography with a 5% diphenylsilicone column (15 m x 0.25 mm) using flame-ionization detection. Injector and detector were at 290~ Helium was used as cartier gas. All reaction mixtures were subjected to precolumn derivatization with 1-(trimethylsilyl)imidazole. 3. ENZYMATIC PROCESS IN TWO-SOLVENT MIXTURES In a recent work, we observed that the lipase from Humicola lanuginosa immobilized on Celite was able to catalyze the acylation of sucrose with vinyl laurate in 2-methyl-2-butanol [6]. The use of tertiary alcohols for the enzymatic acylation of sucrose is particularly attractive. Firstly, the toxicity of the solvent is notably reduced; secondly, the inactivation processes of lipases are minimized, expanding the range of potential biocatalysts.Vinyl laurate was chosen as acyl donor, since the equilibrium can be shifted towards the ester formation (the vinyl alcohol formed during the process tautomerizes to the low-boiling-point acetaldehyde). Interestingly, we observed that pre-dissolving sucrose in a minumum amount of DMSO and then adding 2-methyl-2-butanol as bulk solvent, a 4-7 fold increase of reaction rate was produced. The final concentration of DMSO has a major influence on the ratio monoester/diester. At 5-10% DMSO, the acylation proceeds very quickly, but after 24 h all the initial sucrose has been transformed into diesters. At 20% DMSO, although the reaction is not so fast, the formation of diesters is almost negligible. This allows to control the production of monoester and makes the purification of products more simple. A drawback of enzymatic processes is the cost of the biocatalyst. This can be overcome if the enzyme can be reused. We analyzed the stability of the lipase from H. lanuginosa under reaction conditions. Once the reaction was stopped, the mixture was filtered, the solid dried with acetone, and the enzyme separated from the molecular sieves using stainless steel sieves (0.84 mm). The biocatalyst was tested again in the synthesis of sucrose monolaurate under the same reaction conditions. Fig. 1 shows the residual activity in 6 cycles; more than 60% of its initial synthetic activity was kept in 5 cycles. This result contrasts with the rapid inactivation of lipases in pure dimethylsulfoxide. In order to optimize the production of sucrose monolaurate, a process was developed where sucrose was added at different stages during the synthesis (to consume the acyl donor, present in a molar excess 10:1). As shown in Fig. 2, the productivity was notably high (46 g/1 monoester). The conversion (referred to the total sucrose added to the system) was 82%.

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100

,

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Fig. 1. Operational stability of H. lanuginosa lipase/Celite in tert-amyl alcohol:DMSO (4:1 v/v). Reaction conditions as described in Experimental Section. Cycles were of 24 h. ~ =

~

15

'

15

Reaction time

Cycle

100

i

10

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,~"

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Fig. 2. Effect of the addition of consecutive sucrose loadings on productivity. Addition of 0.26 g sucrose (0.76 mmol) was performed at 4 hours intervals (1"). Reaction volume: 25 ml. J

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20

25

(h)

Fig. 3. Effect of water content of the biocatalyst (w/w) on the acylation course: 2% (0); 2.7% (O); 3.8% (,). Reaction conditions as described in Experimental Section.

20

0 5

10 Reaction time

15

20

25

(h)

Fig. 4. Effect of the ratio biocatalyst/molecular sieves (w/w)" 1:2 (0); 2"1 (O); 1"1 (,). Reaction conditions as described in Experimental Section.

The amount of water is a key parameter in enzyme-catalyzed acylations in organic media. The effect of the water content of the biocatalyst (determined by the Karl-Fisher method) was studied. Although Fig. 3 illustrates that the higher the water content the higher the conversion, it is important to point out that with the biocatalyst containing 3.8% water, the acyl donor is greatly hydrolyzed to lauric acid and cannot be recovered. In this context, the ratio biocatalyst/molecular sieves was varied and found to be critical in the course of the acylation (Fig.4, using an immobilized lipase with 2.7% water). It is shown that in presence of an excess of molecular sieves, the medium is probably too anhydrous, having a negative effect on the performance of the lipase. Using the biocatalyst in an excess 2:1, the formation of lauric acid was significant (data not shown). We found that a ratio 1:1 was optimum for the transesterification process.

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4. CHEMICAL PROCESS IN DIMETHYLSULFOXIDE

When studying the acylation of sucrose with vinyl laurate in pure DMSO in presence of the lipase from H. lanuginosa, we obtained a substantial conversion (50%) in 24 hours. However, we found in a routine control that the support used, Celite, was able to catalyze the transesterification, giving a conversion close to 30% in 24 h (Fig. 5). We assayed other inorganic carriers commonly used in biotransformations, such as controlled-pore glass (CPG) or 7-alumina, but the acylation of sucrose was almost negligible. Phosphate salts are widely used to adjust the pH of enzyme solutions before immobilization or lyophilization. Interestingly, we found that di-sodium hydrogen phosphate catalyzed efficiently the transesterification process in DMSO. We optimized this reaction varying several parameters and, as a result, we patented a procedure for the selective synthesis of sucrose monolaurate using this cheap and available basic catalyst [ 11 ]. II Monoester [] Diester /

"~ 30

o lo2oi 0

=

"=i

/

9

Celite NaH2PO4 7-AI203 Celitel Na=HPO4 Lipase/CPG

Fig. 5. Acylation of sucrose with vinyl laurate in DMSO using different catalysts. Experimental conditions: 0.30 M sucrose; 1.20 M vinyl laurate; 150 mg catalyst/ml; 40~ Na2HPO4 was used in a ratio 20 mg/ml.

When lipase from H. lanuginosa was immobilized on CPG, the reaction did not take place. That suggested that the process we initially thought as enzymatic was being chemically catalyzed by the support and the sodium phosphate used to adjust the pH of lipase solution.With this in mind, we went further and assayed different organic carriers. Although some of them, such as polypropylene or activated Sepharose, gave rise to negligible yields of monoester, Eupergit C (an acrylic polymer containing oxirane groups) also catalyzed the acylation (data not shown).

5. ENZYMATIC VERSUS CHEMICAL R E G I O S E L E C T I V I T Y The monoesters obtained both by chemical and enzymatic means were isolated and purified. The enzymatic process gave rise to the regioselective formation of 6-0lauroylsucrose, whereas by chemical catalysis (using NazHPO4, Celite or Eupergit C) the monoester in the secondary hydroxyl 2-OH was the major product (about 80%, see Scheme 1). The monoester in the secondary hydroxyl 3-OH was obtained in approx. 20%. It has been described that the chemical acylation of sucrose takes place with the preference 6-OH > 6'-OH > I'-OH > secondary-OH. However, under the conditions described above, the secondary hydroxyl 2-OH was the most reactive. An explanation for this regioselectivity is probably to be the activation, by general-base catalysis, of the most acidic hydroxyl of sucrose (the 2-OH) to a more nucleophilic alcoxide [5]. On the contrary, the enzymatic acylation of sucrose has been reported to occur at the I'-OH primary hydroxyl

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group (on the fructose ring) using subtilisin-like proteases [7] or at the primary 6-OH with different lipases [8-9].

HO H

4

-OH

6 ~

1

%-

Vinyl ,aurate /

HO'~I"~O " ~-,"~OH

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Lipase from Humicola I

lanuginosalCelite

I

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Na2HPO4 Celite, Eupergit C DMSO 40~

Le(-OH O

o/

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~

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/o

%-

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2-O-lauroylsucrose

Scheme 1. Enzymatic (left) and chemical (right) acylation of sucrose with vinyl esters. In conclusion, different regioisomers may be obtained with an appropriate election of the catalyst. This is remarkable since the properties of different monoesters have been reported to be significantly different [ 1]. Both methods we propose here are rather simple and suitable for the synthesis of sucrose monoesters with high regioselectivity.

REFERENCES 1. F.A. Husband, D.B. Sarney, M.J. Barnard and P.J. Wilde, Food Hydrocolloid, 12 (1998) 237-244. 2. Ver H. Baal, G. Vianen, Cosmetics News, 20 (1997) 33-37. 3. Y. Nishikawa, M. Okabe, K. Yoshimoto, G. Kurono and F. Fukuoka, Chem. Pharm. Bull., 24 (1976) 387-393. 4. O.T. Chortyk, J.G. Pomonis and A.W. Johnson, J. Agric. Food Chem., 44 (1996)1551-1557. 5. C. Chauvin, K. Baczko, D. Plusquellec, J. Org. Chem., 58 (1993) 2291-2295. 6. M. Ferrer, M.A. Cruces, M. Bernab6, A. Ballesteros and F.J, Plou, Biotechnol. Bioeng., 65 (1999) 000-000. 7. S. Riva, M. Nonini, G. Ottolina, B. Danieli, Carbohydr. Res., 314 (1998) 259-266. 8. M. Woudenberg, F. Van Rantwijk, R.A. Sheldon. Biotechnol. Bioeng., 49 (1996) 328-333. 9. J.E. Kim, J.J. Han, J.H. Yoon and J.S. Rhee, Biotechnol. Bioeng., 57 (1998)121-125. 10. F.J. Plou, M.A. Cruces, E. Pastor, M. Ferrer, M. Bernabe and A. Ballesteros. Biotechnol. Lett., 21 (1999) 635-639. 11. F.J. Plou, E. Pastor, M.A. Cruces, M. Ferrer and A. Ballesteros. Spanish Patent No. 9802086 (1998).

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

The H i g h l y Selective Conversion of Toluene into 4-Nitrotoluene and 2,4-Dinitrotoluene Using Zeolite H-Beta D. Vassena, A. Kogelbauer, and R. Prins Laboratory for Technical Chemistry, ETH-Ztirich, CH-8092 Ziirich, Switzerland The nitration of toluene was studied in the vapour and in the liquid phase to assess the potential of various zeolites and solid acids for replacing sulfuric acid. Zeolite beta in its proton form provided a higher para-to-ortho ratio in the nitration of toluene to nitrotoluene (NT) compared to other catalysts. Enhanced para-selectivity was also observed for the formation of dinitrotoluene (DNT) using H-beta and acetyl nitrate in the liquid phase. Characterisation and adsorption studies suggest that the high para-selectivity originated from steric hindrance of the ortho position induced by adsorption rather than from classical transition state selectivity. 1. INTRODUCTION Nitrotoluenes are important intermediates in the chemical industry. The typical product composition that is obtained in the industrially applied mixed-acid nitration favours the formation of the less desired ortho product [1]. A substantial number of studies has been reported trying to overcome these limitations and aiming at a higher fraction of parasubstituted products [2-7]. Mainly zeolites such as mordenite [2,3], ZSM-5 [2-4], ZSM-11 [5], beta [2,6] and Y [3,5] have been tested as catalysts, but also clay-supported metal nitrates [8] and SiO2 or A1203 impregnated with H2SO4 or H3PO4 [7] have been investigated. In the majority of studies organic nitrating agents were applied such as various acyl nitrates [5,6] or alkyl nitrates [2,4]. Few studies on nitrogen dioxide (dinitrogen tetroxide) [2,3] or nitric acid [5,7] have been done. The use of zeolites seems to be the most promising route so far and the concept of shape selectivity has been commonly invoked to explain the enhanced paraselectivity [4,5]. In the current paper we present results obtained for the nitration of toluene and 2-nitrotoluene (2-NT) using H-beta zeolite under various reaction conditions suggesting reasons other than classical shape selectivity being responsible for this unique behaviour. Our earlier work has shown that zeolites were catalytically inactive in the nitration of aromatics in the liquid phase with nitric acid as the industrially preferred nitrating agent because of the poisoning of the acid sites by water [9]. One crucial task was therefore the removal of water from the acid sites in order to keep the catalyst active. This was achieved by raising the reaction temperature and thereby evaporating the water [10-13] or by chemically trapping it with acetic anhydride [6].

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2. EXPERIMENTAL

The acid form of the different zeolites was obtained by repeated ion exchange of the parent Na zeolites (Chemie Uetikon) with 1 M aqueous NH4NO3 solution and subsequent calcination in static air. For comparison, the non-microporous solid acid Deloxan, a polysiloxane bearing alkylsulfonic acid groups (Degussa AG) was used as received. The Si and AI concentration of the zeolites was determined by AAS analysis; nitrogen adsorption at 77 K was carded out on a Micromeritics ASAP 2000M volumetric analyser. The catalysts were degassed prior to analysis under vacuum at 400~ The specific surface area was evaluated using the BET method, the external surface area is given as the difference between the BET surface area (N2 adsorption) and the micro pore surface area which was determined according to the t-plot method [14]. The average pore diameter of pores bigger than 17 .A was estimated from the desorption branch of the isotherm using the BJH method [15]. The resulting zeolite characteristics are presented in Table 1. The vapour phase nitration was achieved in a flow reactor system at 158~ and atmospheric pressure over a period of 26 h reaction time. The catalysts were pretreated in flowing N2 at 158~ during 1 h. Equimolar amounts of toluene and 65% nitric acid were fed using N2 as carder. The HNO3 conversion was determined by back-titration of the unreacted nitric acid. The organic products were collected in dichloromethane and analysed off-line by gas chromatography using a HP 5890 gas chromatograph equipped with a HP-1 fused silica capillary column and 1,3-dinitrobenzene as integration standard. An alternative method for sustaining the activity, the trapping of water by chemical reaction, was attempted using acetic anhydride [6]. Acetyl nitrate, the nitrating agent, was generated in situ from 90 wt % nitric acid and acetic anhydride. For reaction experiments 35 mmol 90 wt % nitric acid and 1.0 g of dried catalyst (130~ overnight) were mixed and stirred at 0~ 53 mmol of acetic anhydride were added, corresponding to the stoichiometric amount needed to convert nitric acid into acetyl nitrate and the water present in nitric acid into acetic acid (AcOH). 35 mmol of toluene or 3.5 mmol of 2-NT were then added dropwise in order to keep the temperature below 20~ After addition of the substrate, the mixture was stirred for 30 minutes. The organic products were separated by extraction with methylene chloride and analysed in the same way as for the vapour phase nitration. Infrared spectra of self-supporting wafers of H-beta and H-ZSM-5 were recorded at ambient temperature on a Mattson Galaxy 6020 IR spectrometer equipped with a MCT detector at a resolution of 4 c m "1. Prior to analysis, samples were degassed at 500~ for 10 h Table 1 Characteristics of the catalysts Si/AI BET surface area (m2/g) Deloxan H-beta H-ZSM-5 H-mor

11.5 16.9 4.6

external surface area (m2/g)

pore volume (cm3/g)

aver. mesopore diam. (A)

micro pore volume (cm3/g)

642 199 64 6

1.11 0.90 0.51 0.27

51 102 98 64

0.21 0.16 0.22

642 690 433 544 HH

,H...

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at a pressure below 10"4 Pa. Adsorption experiments using 2-NT and 4-NT were carded out at ambient temperature at a pressure of 10 Pa. The stepwise desorption of NT was followed by IR in a temperature range between ambient temperature and 500~

3. RESULTS AND DISCUSSION In both reaction regimes enhanced formation of 4-NT was observed using H-beta while non-microporous solid acids and also other zeolites gave product compositions similar to mixed acid. In the vapour phase nitration (Figure 1) zeolite H-beta exhibited a para-to-ortho ratio of nitrotoluenes of more than 1.1 during the first hours on stream. Selectivity to 4-NT decreased over a period of about 10 hours on stream due to pore filling/blockage by strongly adsorbed products/byproducts [10]. Another zeolite with a 12-membered ring as pore opening, Hmordenite, did not exhibit enhanced para-selectivity compared to the reaction without zeolite. Using H-ZSM-5 and Deloxan the p/o ratio of product NT was slightly higher than that observed in the absence of a catalyst (p/o = 0.7). It remained at the somewhat higher value for the whole duration of the experiment with Deloxan whereas a rapid decrease was observed

1.2 1.1 Z

r

!

Iz

,r I

9

1 09

H-mor

0.9

0.8

v

H-ZSM-5

x

Deloxan 9

0.7 0.6

H-beta

~

o

'

~

g

~ -

'

'0

'1

'5

Blank

-

2'o2'5ao

Time-on-stream / h

Fig. 1. Time-on-stream behaviour of various solid acids during vapour phase nitration (158~

65 % wt HNO3, HNOa/Tol = 1, pTol=13.2 kPa, W/F = 5 g h moll).

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Table 2 Nitration of toluene with acetyl nitrate (toluene/HNO3 = 1) NT Yield (mol %) DNT Yield (mol %)

none H-beta H-ZSM-5 H-mor

76 76 73 83

0.0 1.4 0.0 0.0

4-NT/(2+3+4)-NT 0.38 0.72 0.40 0.39

when H-ZSM-5 was used. Judging from the kinetic diameters of the nitrotoluenes (4NT = 5.2 A, 2-NT = 6.7 A [4]) in combination with the irreversible nature of the nitration reaction [16] one may conclude that 2-NT can be formed in the channel intersections of HZSM-5 but remains virtually trapped. This would lead to rapid deactivation of H-ZSM-5 and the non-selective formation of products by the homogenous vapour phase reaction. The nitration of toluene in the liquid phase with acetyl nitrate demonstrated clearly that only with H-beta a higher selectivity to 4-NT was observed (Table 2). Beta was also the only catalyst that gave a small yield of DNT. We have shown previously that the selective formation of 2,4-DNT using H-beta was not only linked to the enhanced formation of 4-NT but also to the highly selective conversion of 2-NT into 2,4-DNT [17]. The results in Table 3 show that this is also a unique property of beta zeolite. H-beta, having pore openings of 7.6 x 6.4 A, should not impose steric restrictions upon the penetration by nitrotoluenes. This was confirmed by in situ infrared measurements of the adsorption of 2-NT and 4-NT which showed that all acid sites were rapidly covered after the exposure of H-beta to 10 Pa NT (Figure 2). On the contrary, incomplete coverage of the Bronsted acid sites was observed for H-ZSM-5 during 2-NT adsorption while 4-NT was able to adsorb on all acid sites. These results are in line with the behaviour of H-ZSM-5 during nitration reactions where initially some enhanced formation of 4-NT was observed followed by a rapid decline of activity and selectivity. Since re-equilibration of products does not occur, selectivity regarding the transition state may be invoked for explaining the high para-selectivity. The size of the critical transition state, the Wheland intermediate, is very similar to that of the product molecule NT. A severe restriction regarding the formation of the Wheland intermediate that yields 2-NT should therefore also be manifested in a significant blockage of the adsorption of 2-NT. This, however, was not observed. Even more, high selectivity toward the para product (2,4-DNT) Table 3 Nitration of 2-NT with acetyl nitrate (2-NT/HNO3 = 0.1) DNT Yield (mol %) 2,4/(2,4+2,6)-DNT none H-beta H-ZSM-5 H-mor

0.2 89.6 0.8 1.3

0.94 0.60 0.60

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519

~

b ,

a

3800 3600 3400 32'o0 3o'00 2800 Wavenumber

/ c m -~

38'oo36'oo'34'oo'32'oo 30'00 2800 Wavenumber

/ c m -~

Fig. 2. IR spectra of H-beta (left) and of H-ZSM-5 (right) at ambient temperature after degassing at 500~ (a), after 30 min equilibration with 10 Pa 4-NT (b), after subsequent evacuation at ambient temperature for 24 h at a pressure below 10-4 Pa (c); after 30 min equilibration of a fresh zeolite with 10 Pa 2-NT (d), after subsequent evacuation at ambient temperature for 24 h at a pressure below 10-4 Pa (e).

was observed when 2-NT was nitrated. The steric requirements for these two reactions, nitration of toluene and nitration of 2-NT are significantly different. It is our conclusion therefore that classical transition state selectivity can not explain the observed results. The possibility that the bulky acetyl nitrate induces selectivity by not being able to approach the ortho position can be ruled out for the following reasons. Acetyl nitrate formed in situ from acetic anhydride and nitric acid gave high yields of NT even without any zeolite, however, in the typical product ratio with about 60% 2-NT. Furthermore, carrying out the nitration with simultaneous removal of the water by distillation, 2-NT washighly selectively nitrated to 2,4-DNT using H-beta and nitric acid. H-beta is unique in its behaviour because the use of other large pore zeolites such as mordenite did not give enhanced formation ofpara-substituted products. While H-beta has a high external surface due to the small crystallite size, H-mor is characterised by a small external surface and big crystallite size. Given the fact that the non-selective homogenously occurring nitration reaction is always in competition with the heterogenously catalysed reaction on the zeolites, the crystallite size is expected to play an important role regarding the overall selectivity. The smaller crystallite size of beta might therefore be beneficial by providing shorter diffusion paths and thereby enabling a larger contribution of the heterogenously catalysed selective nitration. Further experiments with a mesoporous mordenite and a macrocrystalline beta are in progress to test this hypothesis. Another peculiarity of beta is the high concentration of silanol groups. During the adsorption studies we observed strong interaction of the non-acidic silanol groups up to the temperature of desorption of NT. Similar observations have been reported by others using NMR

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spectroscopy [18]. It is evident that these silanol groups participate in the bonding and therefore conceivably influence the steric arrangement in the adsorbed state. At present, we tentatively ascribe the observed selectivity effects to an adsorption-induced steric blockage of the ortho position of the substituted aromatics. 4. CONCLUSIONS

H-beta has been shown to be unique among solid acids with respect to the high para selectivity obtained during nitration of substituted aromatics. The enhanced para-selectivity seemed to originate from sites located inside the micropore system of H-beta and is most probably linked to steric hindrance induced by adsorption on a rigid solid surface. REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

Ullmann's Encyclopedia of Industrial Chemistry, A17, VCH, Weinheim, 1991. J.M. Smith, H. Liu and D.E. Resasco, Stud. Surf. Sci. Catal., 111 (1997) 199. D.B. Akolekar, G. Lemay, A. Sayari and S. Kaliaguine, Res. Chem. Intermed., 21 (1995) 7. T.J. Kwok and K. Jayasuriya, J. Org. Chem., 59 (1994) 4939. S.M. Nagy, K.A. Yarovoy, V.G. Shubin and L.A. Vostrikova, J. Phys. Org. Chem., 7 (1994) 385. K. Smith, A. Musson and G.A. DeBoos, J. Org. Chem, 63 (1998) 8448. H. Schubert and F. Wunder, US Patent No. 4 112 006 (1978). L. Delaude, P. Laszlo and K. Smith, Acc. Chem. Res., 26 (1993) 607. A. Kogelbauer, D. Vassena, R. Prins and J.N. Armor, Catal. Today, submitted. D. Vassena, D. Malossa, A. Kogelbauer and R. Prins, in: M.M.J. Treacy, et al. (Eds.), Proceedings of the 12th International Zeolite Conference, Vol II, Materials Research Society, 1999, p. 1471. L. Bertea, H.W. Kouwenhoven and R. Prins, Stud. Surf. Sci. Catal., 78 (1993) 607. L. Bertea, H.W. Kouwenhoven and R. Prins, Stud. Surf. Sci. Catal., 84 (1994) 1973. L. Bertea, H.W. Kouwenhoven and R. Prins, Appl. Catal.: A, 129 (1995) 229. B.C. Lippens and J.H. de Boer, J. Catal., 4 (1965) 319. E.P. Barret, L.G. Joyner and P.P. Halenda, J. Am. Chem. Soc., 73 (1951) 373. A. Germain, T. Akouz and F. Figueras, J. Catal., 147 (1994) 163. D. Vassena, A. Kogelbauer and R. Prins, Proceedings of the First International FEZA Conference, accepted for publication. M. Hunger and T. Horvath, Catal. Lett., 49 (1997) 95.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Catalytic Asymmetric Heterogeneous Aziridination and Epoxidation of Alkenes using Modified Microporous and Mesoporous Materials Graham J. Hutchings, a Christopher Langham, ~ Paola Piaggio," Sophia Taylor," Paul McMorn," David J, Willock, a Donald Bethell, b Philip C. Bulman Page, c Chris Slyf Fred Hancock e and Frank King e aDepartment of Chemistry, Cardiff University, P.O. Box 912, Cardiff CF1 3TB, UK. bLeverhulme Centre for Innovative Catalysis, Department of Chemistry, University of Liverpool, Liverpool L69 3BX, UK. CDepartment of Chemistry, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK. dRobinson Brothers Ltd., Phoenix Street, West Bromwich, West Midlands B70 0AH, UK. eSynetix, R&T Division, P.O. Box 1, Billingham, Teeside TS23 1LB, UK. Copper-exchanged zeolite Y is a highly active catalyst for the aziridination of alkenes. Modification using bis(oxazolines) leads to the formation of an enantioselective aziridination catalyst. Using a similar approach, manganeseexchanged MCM-41 modified with a chiral salen ligand is found to be an effective enantioselective heterogeneous epoxidation catalyst for cis-stilbene. 1. INTRODUCTION The design of asymmetric catalysts is of intense current interest and procedures describing the use of chiral transition metal complexes as homogeneous catalysts have been described for epoxidation, cyclopropanation, aziridination and hydrogenation of alkenes. There is an increasing awareness that heterogeneous catalysts can offer significant advantages over homogeneous catalysts and this has prompted research activity in this field. To date, three experimental approaches have been used in the design of enantioselective heterogeneous catalysts: (i) the use of a chiral support for an achiral metal catalyst; (ii) the immobilization of an asymmetric homogeneous catalyst onto an achiral support; and (iii) modification of an achiral heterogeneous catalyst using a chiral cofactor. The first approach was pioneered by Schwab in 1932 [1]. Using Cu, Ni, Pd and Pt supported on enantiomers of quartz he demonstrated low enantioselection in the dehydration of butan-2-ol. A number of other chiral supports have been examined, e.g. natural fibres and chiral polymers. Most recently, attention has focused on using zeolite [3, since it is possible that a chiral form of this zeolite could be synthesised [2]. The second approach has been particularly effective for enantioselective hydrogenation reactions using zeolites as supports for asymmetric Ru and Rh catalysts [3]. The third approach,

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involving the creation of a chiral catalyst surface by the adsorption of a chiral modifier onto an achiral catalyst, has been successful in a number of studies, again particularly for enantioselective hydrogenation. For example, the modification of platinum catalysts with cinchona alkaloids for the hydrogenation of prochiral 0~-ketoesters [4] have been extensively studied. We have also used the third approach, and we have concentrated our design efforts on zeolite Y since we consider that, for the optimal catalyst design, the achiral catalyst should have a well defined structure. In our initial proof of concept studies we studied the modification of zeolite H-Y with chiral dithiane 1oxides and we have shown that this approach can give catalysts that are capable of enantioselection for the dehydration of butan-2-ol in the temperature range 110-150 ~ [5]. We have now extended this generic approach and have designed catalysts for the enantioselective aziridination and epoxidation of alkenes. 2. E X P E R I M E N T A L

The zeolite HY used in this study was supplied by Union Carbide (LZY 82). A1MCM-41 was synthesized according to literature methods [6], and prior to use was calcined at 550 ~ for 4hrs in nitrogen, followed by 16hrs in air. Cu-exchanged zeolite (CuHY) was prepared using a conventional ion-exchange method in which zeolite H-Y was treated with aqueous Cu(OAc)2 solution, the concentration of which was chosen so as to obtain the required exchange level (ca. 50-60%). The solids were then washed with distilled water until all u n b o u n d Cu 2. was removed and dried at 110 ~ in air. An identical method was used for the preparation of CuA1MCM-41. The aziridination of alkenes was carried out in a batch reactor using (N-(p -tolylsulfonyl)imino)phenyliodinane (PhI=NTs) and (N-(p-nitrobenzylsulfonyl)imino)phenyliodinane (PhI=NNs) as the nitrene donors. The alkene, nitrene donor and solvent were stirred together in a flask under controlled temperature in the presence of the Cu catalyst. The nitrene donor reagents are relatively insoluble under the chosen conditions and the reaction was monitored by observing the dissolution of this reagent; when the dissolution was complete the reaction was considered to be complete. In a typical experiment, styrene (500 ~tl) was reacted with PhI=NTs (0.3g) in acetonitrile together with CuHY catalyst (0.3g) at 25 ~ for 2 h. The products were isolated by column chromatography and product identification was confirmed by NMR, elemental analysis, GCMS and infra red spectroscopy. When enantioselective aziridinations were carried out, the CuHY catalyst was pretreated with a chiral modifier prior to reaction, and the products were analysed using chiral HPLC. The method described above was used to prepare MnHY and MnA1MCM41 via ion exchange with aqueous manganese acetate (0.2M, 25 ~ 24 h), followed by filtration, washed with water and vacuum dried. This procedure was repeated twice and the material was calcined (550 ~ 24 h). The calcined Mn-exchanged materials were refluxed with the chiral salen ligand, (R,R)-(-)-N,N'- bis(3,5-ditert-butylsalicylidene)-l,2-cyclohexanediamine, in CH2CI 2 (24 h, metal:salen = 1:1), cooled to 0 ~ and washed with CH2C12. In the case of MnA1MCM-41 this procedure resulted in 10% of the salen ligand being incorporated (determined by TGA and solution analysis). The Mn-exchanged material:salen catalyst was then

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investigated for the epoxidation of cis -stilbene using iodosylbenzene as oxidant in a batch reactor. 3. RESULTS A N D DISCUSSION

3.1. Heterogeneous asymmetric aziridination of alkenes with CuHY CuHY is found to be an effective catalyst for the aziridination of a range of alkenes when using PhI=NTs and PhI=NNs as the nitrene donors (Table 1). To confirm the process was wholly heterogeneous, following the reaction the zeolite catalyst was recovered by filtration and another aliquot of reactants was added to the recovered filtrate and no reaction was observed. Further, the r e m o v e d catalyst was reused with fresh reagents and solvent and the catalyst gave the same reactivity as that observed initially. It is observed that the catalyst gives best results with phenyl-substituted alkenes, and although lower yields are observed with cyclohexene and trans-hex-2-ene the reaction is still observed. Interestingly, for the aziridination of trans-stilbene no product could be observed. We consider this to be due to the relatively bulky aziridine product being too large to be accommodated within the small pores of CuHY and is further evidence that the reaction proceeds inside the pores of the zeolite. This interesting result illustrates the potential for a heterogeneous catalyst to possess size-specificity to a precise degree. Such a property could be exploited by making use of zeolites with a range of pore sizes, and could also be developed to achieve regioselectivity in a reagent containing two or more double bonds. Modification of the CuHY with chiral bis oxazoline ligands (e.g. 2,2-bis-[2-((4R)-(1-phenyl)-l,3-oxazolenyl)]propane) leads to the aziridine being obtained with up to 75% enantiomeric excess in these initial experiments (Table 1). We have found that a temperatures in the range of -10 to 20 ~ provide the highest combination of enantioselectivity, yield and reaction time when using acetonitrile solvent. In these experiments, the chiral modifier is simply added to the reaction mixture, and no special pretreatment of the catalyst system is required. To demonstrate that alternative types of silicate framework can be used for this reaction, experiments were carried out with copper-exchanged MCM-41. Yields of up to 87% of the aziridine with e.e. of 37% were obtained. Using this type of mesoporous catalyst system greatly enhances the versatility of the heterogeneous aziridination reaction. The major advantage of the use of CuHY as a catalyst for this reaction is the ease with which it can be recovered from the reaction mixture by simple filtration if used in a batch reactor (alternatively it can be used in a continuous flow fixed bed reactor). We have carried out the heterogeneous asymmetric aziridination of styrene until completion, filtered and washed the zeolite then added fresh styrene, PhI=NTs and solvent, without further addition of chiral bis(oxazoline), for several consecutive experiments. The yield and the enantioselectivity decline slightly on reuse; we have found that adsorbed water can build up within the pores of the zeolite on continued use and we believe that this is the cause of loss of activity and enantioselection. However, full enantioselectivity and yield can be recovered if the catalyst is simply dried in air prior to reuse, or alternatively the catalyst can be recalcined and fresh oxazoline

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ligand added. We are therefore confident that this catalyst system can form the basis of a commercial heterogeneous catalyst for the aziridination of alkenes. Table 1 CuHY-Catalysed Aziridination of Representative Alkenes. ....................................................................................... Bis-oxazoline Alkene a Sytrene cz-Methylstyrene Cyclohexene Methyl cinnamate trans-Stilbene trans-Hex-2-ene Sytrene Styrene Trans-~Methylstyrene trans-f~Methylcinnamate

None

Me Me ~O r ~ /~_ p, (1) Me

~ n l ~ Me~r

~M%

Styrene Styrened

Op~~/1~ h~nhO,,~Pt,,,,,,,Nr (2)

PhINTs T e m p .....~'ieid 6e.e. c ~ % % 25 90 (92) 25 33 25 50 (60) 25 84 (73) 25 25 25 -10

0 (52) 44 87 82

29 44

-10

74

36

-10 25 -20 -20 0

8 (21)

61 (70)

64 15 (89)

0 18 (63)

Styrene

25

78 (75)

10 (10)

Styrene

25

73 (74)

0 (15)

Styrene

-10

4

61

PhINNs Yield 6 e.e. c % % 93 (97)

69 100e

52 75e

30 ~ 87

59 f 64

100

34

(3)

Ph

,mPh

(4)

(5) a

b

Solvent CH3CN, styrene: PhI=NTs = 5:1 molar ratio; Isolated yield of aziridine based on PhI=NTs. Values in parentheses indicate yields obtained from homogeneous reactions; CEnantioselectivity determined by chiral HPLC; dstyrene was used as solvent, e0 ~ f25~ Absolute configurations of major products, determined by optical rotation, are (S) for trans~- and trans-~-methylcinnamate, (R) for styrene.

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3.2. Asymmetric epoxidation using modified Mn-exchanged materials.

To demonstrate the flexibility of the approach to catalyst design that we set out in this paper, the epoxidation of alkenes using iodosylbenzene has also been studied. Initial studies focused on MnHY:salen catalysts for the epoxidation of styrene, however, the reaction was slow, and low yields of styrene oxide were observed. Analysis of the reaction mixture revealed the breakdown of the salen ligand within a few turnovers. Subsequently Mn-A1MCM-41 was used with iodosylbenzene as the oxygen donor and cis -stilbene was used as substrate, and the results, together with those of control experiments, are shown in Table 2. Mn(OAc)2 in the absence of A1MCM-41 or salen ligand is not particularly reactive, and only 1.5% yield of the epoxide was formed after reaction for 24 h at 25 ~ Modification of Mn 2. in solution by the salen ligand, as expected, leads to a significant rate enhancement, and both the cis-epoxide and the trans -epoxide were formed, the latter with 78% e.e. Interestingly, immobilization of the Mn 2. within A1-MCM-41 leads to an increase in reactivity, and the epoxide is formed with an enhanced cis/trans ratio to the homogeneously catalysed Mn:salen catalyst. This effect suggests that the A1-MCM-41 is occupying part of the Mn 2. coordination sphere, restricting the cis ---~trans transformation. Further modification of the Mn-exchanged A1-MCM-41 with salen leads to a further enhancement in reactivity, and the trans epoxide is formed with an 70% e.e., very similar to that observed for the equivalent homogeneous reactions, trans Stilbene is found to be a significantly less reactive substrate, and the e.e. of the resultant trans-epoxide is significantly decreased with the salen modified Mnexchanged A1-MCM-41. The use of Mn-exchanged A1-MCM-41:salen catalyst for this epoxidation does not result in the formation of significant levels of byproducts as has been observed when manganese bypiridyls have been used as catalysts, and typically only deoxybenzoin is observed at low levels (ca. 5-10%). A further set of experiments was carried out to examine the reusability of the Mnexchanged A1-MCM-41:salen catalyst. Following the reaction, the Mn-exchanged A1-MCM-41:salen catalyst was recovered by filtration and the solid was reused in a new catalytic reaction; although the reactivity and enantioselectivity had declined, epoxide was still formed and the cis/trans ratio was unchanged. Recalcination of the recovered material and addition of new salen ligand essentially restored both the reactivity and the enantioselection. Use of the solution following the filtration did not give any activity, and furthermore this solution contained no Mn 2§ These experiments demonstrate that the reaction occurring with Mn-exchanged A1-MCM-41:salen is wholly heterogeneously catalysed. At this stage we have made no attempt to optimise the catalytic performance, but we anticipate that appropriate modification of the chiral salen ligand and the reaction conditions will lead to enhanced reactivity and e.e.. CONCLUSIONS In this paper we have described a design approach for heterogeneous enantioselective catalysts. The approach is based upon modification of the counter-cation of zeolites or mesoporous alumino-silicates with a suitable chiral

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Table 2 Epoxidation of stilbene at 25~ usin~Mn-exchan~ed MCM-41 Entry Catalyst Time Conv. Epoxide Selectivity (%) Ha Yield (%)f cis trans 1 2

None Mn(OAc)2

3

Mn(salen) complex

4

c,d

A1MCM-41 MnMCM-41

5 6 7

solution

9

25 24

0 100

0 1.5

0 0

0 100

0 0

1

100

86

29

71

78

0

0

0

0

0

2

45

3

0

100

0

ce

2 26

100 100

69 35

58 0

42 100

70 25

2

0

0

0

0

0

c

2

37

18

51

39

30

c

MnMCM-41+salen

(%)g

24

c

MnMCM-41+salen

8

c

e.e. trans

c

~

MnMCM-41 reused 10 recalcined +salen c,h 2 100 52 63 37 54 a reaction time, ~ as determined by decomposition of iodosylbenzene to iodobenzene, US~lr~ HPLC, Csolvent CH2CH 2 with molar ratio of cis-stilbene:catalyst:iodosylbenzene=7:1:0.13, reaction conducted m CH3OH, trans-stilbene used as substrate, Conversions, yields a d selectivity determined by HPLC, g enantiomeric excess determined by chiral HPLC., "MnA1MCM-41 from entry 6, recalcined and refluxed with salen ligand. 9

9

e

9

f

"

"

n

ligand. We have demonstrated the approach with two examples: (a) enantioselective aziridination of alkenes using Cu2*-exchanged zeolite Y modified with chiral oxazolines and ( b ) t h e modification of manganeseexchanged A1-MCM-41 by a chiral salen ligand for the enantioselective epoxidation of alkenes. Since there is a broad range of zeolites and m e s o p o r o u s materials available as catalytic materials, it is anticipated that the approach described in this paper can form the basis of a generic design of new enantioselective catalysts. Acknowledgements

We thank Synetix, Robinson Brothers and EPSRC for financial support. REFERENCES

1. G.M. Schwab and L. Rudolph, Naturwiss., 20 (1932) 362. 2. M.E. Davis and R.L. Lobo, Chem. Mater., 4 (1992) 756. 3. A. Corma, M. Iglesis, C. del Pino and F. Sanchez, Stud. Surf. Sci. Catal., 75 (1993) 2293. 4. G. Webb and P.B. Wells, Catal. Today, 12 (1992) 319. 5. S. Feast, D. Bethell, P.C.B. Page, M.R.H. Siddiqui, D.J. Willock, F. King, C.H. Rochester and G.J. Hutchings, J.Chem.Soc., Chem. Comm. (1995) 2409. 6. J.S. Beck, J.C. Vartuli, w.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.-W. Chu, D.H. Olson, E.W. Shepherd, S.B. McCullen, J.B. higgins and J.L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Catalytic Hydrogenation of Nitriles to prim., sec. and tert. Amines over Supported Mono- and Bimetallic Catalysts Yin-Yan Huang* and Wolfgang M.H. Sachtler N.V. Ipatieff Laboratory, Center for Catalysis and Surface Science Department of Chemistry, Northwestern University, Evanston, IL 60208, USA ABSTRACT The selectivity of nitrile hydrogenation to prim., sec. and tert. amines is dominantly controlled by the transition metal, similar selectivities are observed in gas phase flow and liquid phase batch runs. All amines are formed during one residence at the catalyst surface. Isotopic labeling in acetonitrile hydrogenation and co-hydrogenation of acetonitrile and butyronitrile show that the hydrogen atoms in the amine groups of the product are not provided by Ha~ at the metal surface, but by the methyl group of other acetonitrile molecules. It is concluded that N-bonded surface complexes are likely intermediates for the formation of prim., sec. and tert. amines 1. INTRODUCTION Hydrogenation of unsaturated compounds over transition metal catalysts dominated the list of intensely studied catalytic reactions for much of the 20th century. Work by Sabatier and Senderens on the hydrogenation of unsaturated acids opened the list in 1902.[ 1] In 1934 Polanyi and Horiuti proposed the mechanism for ethylene hydrogenation[2] which is still considered an adequate description of the shortest route from reactant to products. Bond and Wells showed that alkynes are hydrogenated first to alkenes, while the subsequent step of alkane formation has to wait until alkynes no longer dominate the metal surface.[3,4] In the second half of the century evidence was obtained that the hydrogenations of CO or N2 were no hydrogen additions to a double bond, but dissociative chemisorption of N2 [5] and CO [6,7] precedes the reaction of the fragments with adsorbed H atoms to form ammonia or Fischer-Tropsch products. The world-wide research effort also showed that in hydrogenation catalysis other reactions take place. Polymerization and" coke" formation are often inevitable side reactions; surface science studies also showed that adsorbed alkyl groups split offH atoms forming alkylidene and alkylidyne groups[8], while work with labeled molecules proved that allylic adsorbates occur.[9]. In view of the impressive mechanistic knowledge accumulated during a century of hydrogenation catalysis, it is surprising that the hydrogenation of a nitrile, such as acetonitrile:

*Present address: Prototech Co, 32 Fremont Street, Needham, MA 02194, USA

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CH3CN + 2H2 = > CH3CH2NH 2

(1)

presents a number of perplexing features. No catalyst is known which forms exclusively primary amines, but secondary and tertiary amines are major co-products, often they are the dominant products. Minor side products are Schiff bases, enamines and alkanes. If the analogy of adding H atoms to C - N or C - C groups were realistic one would expect formation of an aldimine such as CH3-CH=NH from CH3CN, but in reality no aldimine has been observed in nitrile hydrogenation. In the research described in this paper, the following problems will be addressed: ,

2.

Selectivity: Are sec. and tert. amines prinmry or secondary products? Alloying: Does alloying of an active transition metal with a less active metal decrease or increase the specific activity? Can selectivity changes be predicted on the basis of an ensemble effect?

.

4.

Mechanism: Are chemisorbed H atoms added in steps to the C---N group? Overlayer effects Are H atoms from the metal surface directly transferred to an adsorbed nitrile molecule, or does indirect H transfer take place via an overlayer?

Zeolite NaY supported transition metal catalysts were prepared by ion exchange; for amorphous supports impregnation was used. Catalysts were tested either in a microflow reactor at atmospheric pressure with a H 2 or D 2 flow first passing through a saturator, or in a stirred autoclave with an initial H2 pressure 0f24 bar. The hydrogenation of acetonitrile, AN, was studied in the gas phase, butyronitrile, BN, was hydrogenated both in the gas and the liquid phases. Products were analyzed by GC. For experiments with D2, GC-MS was used; moreover certain product fractions were condensed and subjected to a secondary exchange with liquid D20. As only the amine hydrons are exchanged in this process, the mass difference before and after the second exchange directly shows the number of D atoms bonded to N atoms. Deutero-AN was used in cohydrogenation with BN + H 2 in order to identify the source of hydrons in the resulting amines. Experimental details are given in references [ 10-13] 2. SELECTIVITY If aldimines and primary amines were formed in successive steps and both molecules were desorbed from the catalyst surface into a surrounding liquid, one might expect that they react with each other, forming a larger molecule that can subsequently be hydrodeamminated to a sec. anfme. Its reaction with another aldimine could lead to a tert. arrflne. Such reaction steps have been proposed for hydrogenation of nitriles in a liquid phase.[ 14] Our work shows, however, that also in the absence of a liquid phase higher amines are formed with high selectivity; even at very low conversion sec. and tert. amines are formed during a single residence of the reactant at the catalysis surface. The selectivity for a particular amine depends mainly on the nature of the metal. Comparison of different metals at the same temperature in liquid and gas phase operation leads to very similar selectivity patterns; no liquid is required for the formation of higher amines. Over Ru the prim. anfme prevails, over Pd, the sec. and over Pt the tert. anfme. The selectivities can be correlated with another catalytic parameter characterizing transition metals, viz. the multiplicity in the H/D exchange of alkanes. Among the Pt group elements the selectivity towardprim, amines is highest over Ru, which has the highest propensity for catalyzing C-C fissions.

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Adding pentylamine to liquid B N in the presence of H 2 at high pressure and a Pd/NaY catalyst strongly retards the reaction rate of BN hydrogenation, indicating that the amine is more strongly ehemisorbe~ than the nitrile. The amine participates in the reaction, dibutylamine and butyl-pentyl amine are fonmd in equal quantities. The selectivity for the sec. amines is roughly 75% of that in the absence of added pentylamine.[ 12] 3. ALLOYING Previously we showed that bimetallic clusters are formed when two transition metal precursors are co-reduced with hydrogen inside the same zeolite. The temperature at which the less noble element can be reduced is lowered by the presence of the more easily reducible rnetal.[15] Alloying Ni with Ru, Rh, Pd or Pt increases the activity for BN hydrogenation beyond the amount attributable to the enhanced Ni reduction. Mixed surface ensembles are thought to be responsible. Addition of Sn to Pt lowers the activity for AN hydrogenation but improves the selectivity for the sec. anfme at high Sn contents. Addition ofRu, Rh, Pd or Pt to Cu enhances the reduction of Cu 2+ ions and results in active catalysts with high selectivity towards the sec. anfme. For instance, at 125~ the selectivity of PdCu/NaY for this amine is 84%, while it is 64% over Pd/NaY and Cu/NaY is inactive. 4. MECHANISM

Kinetic analysis of BN hydrogenation in the gas phase reveals a positive order in H 2 of 1.2 and in BN of 1.35 and an apparent activation energy of 68.3 kJ/mole.. Upon directing a flow of AN + D 2 over NaY supported Pt, Pd or Ru at 75~ part of the AN exchanges H atoms against D and leaves the surface as partially deuterated AN, while another fraction is converted to (partially deuterated) amines. Conclusions of relevance to the mechanism are drawn by analyzing both the H/D exchange and the "hydrogenation" more carefully. The H/D exchange is of the stepwise type over Pt and Pd, the molecule with one D atom, d~, prevails after short and moderate exchange times. In contrast, multiple exchange is significant over Ru. When a mixture of ethylamine + D2 is led over R u ~ a Y the d2 product prevails. Remarkably, this H/D exchange is highly regiospecific: the H atoms in the methylene group are easily exchanged, exchange of methyl hydrons is slower but, amazingly, almost no H/D exchange occurs with the hydrons in the amino group. NMR and MS analysis after secondary exchange with D20 unambiguously show that after the primary exchange the amino group is mainly -NH 2. This apparent lack of exchangeability of the hydrons in the amino group is also confirnaed with (CH3CH2)2NH: over Ru/NaY: all 10 hydrons bonded to C atoms are readily exchanged, but the H atom bonded to the N atom is not. The primary exchange product contains all isotopomers from d~ to d~o, but virtually no dn.. Accordingly, secondary exchange with D20 causes a shift of the MS peaks by one m/e unit. These results could still be interpreted in two ways: either the N-H bonds are never broken when amines are chemisorbed at the metal surface, or they are broken and strong N-M bonds are formed with the M atoms of the surface, but desorption requires H transfer from another amine molecule. Which of these models is correct, can be decided on the basis of the data obtained by

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"hydrogenating" AN with D2. Again, dramatic deviations from the simple D addition model are observed. Initially, a large fraction of the amines contain no D atoms at all This is largely due to the interference of the overlayer, see the next Section. But even after extended reaction time, when the overlayer is in isotopic equilibrium with the adsorbates, the isotopic composition of the reaction product remains markedly different from that predicted by a simple D addition model Detailed analysis of the isotopomers shows that almost no D atoms are present in the amine group of the reaction product. As with H/D exchange of diethylamine, there is a precipitous decrease in abundance from d~0 to d~ with all catalysts.[ 11] Any interference by OH groups of the support has been excluded as a possible source of the H atoms which become attached to the N atom; only methyl groups of the AN reactant appear to act as H donors.

To check this conclusion, BN was hydrogenated with H2 over Ru/SiO2 and Pt/SiO2 in the presence of CD3CN at an AN/BN ratio ~1. It was found that all prim. and sec. amines had predominantly D atoms in their amine groups.[ 13] The results thus indicate that a precursor of the amine is bonded to the metal surface via the N atom; these M-N bonds are predominantly broken in a concerted mechanism when AN molecules transfer some of their H atoms to this precursor. The adsorbed AN will replace the lost H atoms by D atoms from the catalyst surface and leave it as a partially deuterated AN molecule. 5. OVERLAYER EFFECTS When the nitrile containing gas flow is directed over the catalyst, neither nitrile nor amines are observed in the first few minutes, indicating that an overlayer is built up. Over Pt/NaY, for instance, the first amine is detected only after 5 min TOS. Overlayer formation is confirmed by thermal gravknetry. Previously, Thomson and Webb had presented arguments that hydrogenation of ethylene occurs by H transfer between an C2Hx overlayer and the adsorbed olefin, rather than by direct addition of chemisorbed H atoms.[ 16] Indeed, the initial ethylamine product of AN + D 2 runs contains much do, the predominant isotopomer is d2. After 4 h on stream, a steady product composition is observed with 30% d 3 as the dominant compound. These findings confirm the presence of a large concentration of strongly adsorbed species and their participation in the catalysis by H donation to or exchange with molecules which are desorbed and detectable in the gas phase. For further details we refer to reference [ 17].

6. DISCUSSION AND CONCLUSION The resuks with labeled molecules clearly prove that an intermolecular H transfer takes place to the N atom of the chemisorption complex, both in the H/D exchange of amines and the "deuteration" of acetonitrile to the prim. and sec. amines over Group VIII catalysts. Apparently, this "indirect" process is more efficient than direct addition of adsorbed D atoms to the N atom.. Although the release of a strongly held intermediate from the surface will be rate limiting, this desorption is assisted by the interaction with another impinging or weakly adsorbed nitrile molecule in accordance with the positive reaction order in nitrile. This kinetics is typical for the "adsorption assisted desorption phenomenon" as studied by Yamada and Tamaru, for CO on single crystal faces of several Pt group metals. In generalizing their findings, these authors

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conclude: "'Reactant molecules can enhance the desorption of tightly bonded product molecules in certain kinds of catalytic processes".[ 18] This expectation appears to be confirmed by the present results on nitrile hydrogenation and H/D exchange of amines. In both cases a reactant molecule donates H atoms to a chemisorbed entity, assisting its desorption. A simplified scheme for this unconventional chemistry is shown below. Ruthenium is used as an example, its propensity to form strong M-N bonds is at the base of its ability to catalyze ammonia synthesis from N2 + H2. Scheme 1:

/CH3 CN CD2 I I CH 3 + N ,,~ II Ru

/CH3 CN CD2 I I I-I2C tIN \

/ Ru

~

CN I HC + II Ru

/ CH3 CD2

I HNH

During nitrile hydrogenation the metal surface will become densely populated with N bonded adsorbates Under the conditions of AN + D 2 the CH3-CD2-N=Ru surface complex will interact with a CH3CN molecule; H transfer leads to CH3-CD2-NH2 and Ru=CH-CN. The =CH-CN group will pick up two adsorbed D atoms and leave the surface as CHD2CN, so that the overall reaction, in this example, becomes: 2CH3CN + 2D 2 = CHD2CN , + CH3CD2NH 2

(2)

The CH3-CH2-N=Ru surface complex can be considered as the N analogue of a propylidene complex. Alkylidene complexes have been identified on metal surfaces (8). It is easy to see, that sec. or tert. amines can be formed on transition metals by addition of alkyl groups to the N bonded intermediates. This mechanism for the formation of prim., sec. or tert. amines appears to be in accordance with the experimental data known at present, whereas an earlier proposal, postulating condensation reactions of a hypothetical aldimine intermediate with an amine in solution or on acid sites, fails to explain the present findings. 7. A C K N O W L E D G M E N T We thank the management of Air Products and Chemicals for sponsoring this research and their kind permission to publish the results.

REFERENCES 1 P. Sabatier, and J.B. Senderens, Comptes Rend. 135 (1902) 87. 2 M. Polanyi and J. Horiuti, Trans. Far. Soc. 30 (1934) 1164. 3 G.C. Bond, P.B. Wells, Adv. Catalysis 15 (1964) 92.

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4 V. Ponec, G. C. Bond, "Catalysis by Metals and Alloys" Vo195 of"Studies in Surface Science and Catalysis" Elsevier, Amsterdam, 1995. 5. P. Mars, J. J. F. Scholten and P. Zwietering in The Mechanism of Heterogeneous Catalysis J. H. de Boer et al. (eds.) Elsevier, Amsterdam, 1959. 6. M. Araki and V. Ponec, J. Catal. 44 (1976) 439. 7. P. Bilocn and W. M. H. Sachtler; Adv. Catal. 30 (1981) 165. 8. G. A. Somorjai, "Chemistry in Two Dimensions: Surfaces". Cornell University Press Ithaca, London, 1981, p. 278. 9. S. Naito and M. Tanimoto, J.Chem Soc. Chem Commun. 1987, 363. 10. Y. Y. Huang and W. M. H. Sachtler, J. Phys. Chem. B. 102 (1998) 6558. 11. Y. Y. Huang and W. M. H. Sachtler, J. Catal. 184 247 (1999) 12. Y. Y. Huang and W. M. H. Sachtlcr, Appl Catal A 182 365 (1999). 13. Y. Y. Huang and W. M. H. Sachtlcr, J. Catal. (subm.) 14. J. Volf, and J. Pa~ek in "Catalytic Hydrogenation"; L. (~erven~, (ed.), Elsevier, Amsterdam, 1986, p 105. 15. J. Feeley, and W.M.H. Sachtler, J. Catal. 131 (1991) 573. 16. S. J. Thomson and G. Webb, J.C.S.,Chem.Comm., (1976) 526. 17. Y.Y. Huang, and W. M. H. Sachtler, Appl. Catal. (in press). 18. T. Yamada, K. Tamaru; Surf. Sci. 146 (1984) 341.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

ALKALI P R O M O T E D REGIO-SELECTIVE HYDROGENATION OF STYRENE OXIDE TO [3-PHENETHYL A L C O H O L C.V. Rode*, M. M. Telkar and R.V.Chaudhari Homogeneous Catalysis Division, National Chemical Laboratory, Pune 411 008, India. Fax: +91 20 5893260, e-mail: [email protected] The selective hydrogenation of styrene oxide to 2-phenyl ethanol (~-Phenethyl alcohol) has been investigated using different catalysts and supports. The effect of reaction conditions such as H 2 pressure, agitation speed, concentration of substrate and temperature on the initial rate of reaction was investigated. The complete conversion of styrene oxide was obtained using 1% Pd/C, as a catalyst, under milder temperature (313K) and pressure (2.048 MPa) conditions. 2- phenyl ethanol was selectively formed when alkali was used as a promoter. A plausible mechanistic pathway has also been proposed for the hydrogenation of the styrene oxide to 2- phenyl ethanol. 1. INTRODUCTION 2-Phenyl ethanol (13-Phenethyl alcohol, PEA) is extensively used in perfumery and deodorant formulations as it possesses faint but lasting odour of rose petals[ 1]. The conventional synthetic methods for PEA involve Grignard synthesis starting from ethylene oxide and Friedel craft alkylation of benzene in presence of A1C1312]. Both these processes are multistep and suffer from the following drawbacks: Formation of side products (biphenyl) leading to poor selectivity of PEA Handling of hazardous chemicals like diethyl ether, ethylene oxide Tedious work up and recovery of pure PEA which is critical for perfumery applications. Generation of appreciable quantities of wastes due to use of A1C13. PEA can also be prepared by reduction of styrene oxide using different reducing agents in stoichiometric quantities and major side product formed in such reduction processes is secondary alcohol (phenyl carbinol)[3-6]. Recently, single step catalytic hydrogenation of styrene oxide has been reported using Raney Ni and other metal catalysts in a temperature range of 120150 oC with selectivity of PEA in the range of 60-87%. Thus, all of these routes give one or the other side products along with PEA, posing serious problems in the recovery of pure PEA which is crucial for the perfumery applications. In this paper we report a single step catalytic hydrogenation of styrene oxide using alkali promoted supported metal catalyst which gives complete conversion of styrene oxide with PEA selectivity as high as 99.9 %. The aim of our work was to screen various transition metal catalysts on different supports and study the effect of temperature, hydrogen pressure and concentration of substrate on conversion of styrene oxide and selectivity of PEA. 2. E X P E R I M E N T A L 2.1 Materials All the chemicals were procured from Aldrich, Co Ltd, USA, and the various catalysts were prepared by the procedure given elsewhere[7]. Hydrogen gas of > 99.9% purity was supplied by Indian Oxygen Ltd., Bombay.

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2.2 Experimental set-up and procedure All the hydrogenation experiments were carried out in a 300 ml capacity SS-316 autoclave (Parr, USA) the details of which are described else where [8]. In a typical experiment, known quantities of styrene oxide, solvent, catalyst along with the promoter were charged into the autoclave and the contents were flushed twice with nitrogen and then the system was pressurised with H2 to the required pressure. The reaction was then continued at a constant pressure by supply of hydrogen from the reservoir vessel. The consumption of H2 was recorded as a function of time. The liquid samples were analysed by GC for reactant and products. 3. RESULTS AND DISCUSSION

Some initial experiments on hydrogenation of styrene oxide using 1% Pd/C catalyst showed that the selectivity of PEA was only 51% due to the formation of side products such as 1-methoxy ethyl benzene, dimethoxy ethane and 2-methoxy benzene ethanol. These side products were identified by GCMS and GCIR. In order to enhance the selectivity of PEA, a systematic study on catalyst screening, role of support, and promoter was undertaken. Table 1: Screening of catalysts Catalyst Conversion Used (%) I%Pt/C 70 1%Pd/C 100 10%Ni/C 60 10%Ni/HY 10 2%Ru/C 82

Selectivity (%) 88 99 85 -87

3.1 Screening of Catalyst

Several transition metal catalysts such as Pd, Pt, Ni, Ru were tested for their activity and selectivity for hydrogenation of styrene oxide at 313 K and 2.048 MPa pressure in presence of NaOH as a promoter and the results are presented in Table 1. It can be seen from this Table that I%Pd/C catalyst selectively gives Temp :313 K, Pressure : 2.048 MPa, Solvent: PEA as the product, with 100% conversion of MeOH, Conc of Catalyst:0.375 Kg/m~, Conc of styrene oxide. In case of other catalysts, the Styrene oxide: 0.4166 Kmol/m3, Conc of NaOH : conversion of styrene oxide was much less than 0.013 kg/m~ that for 1%Pd/C with low selectivity to 2- phenyl ethanol. The other side product formed was found to be 1- methoxy ethyl benzene which, was identified by GCMS. Pt/C and Ni/C catalysts showed almost comparable activity (70% and 60% conversion respectively), while Ni/HY catalyst showed lowest activity (10% conversion). In case of Ni/HY catalyst, from the consideration of pore size of support and the particle size of supported metal, almost all metal is expected to be on the outer surface of HY zeolite, leading to a very small surface area for the supported metal causing the lowest catalyst activity [9]. It is interesting to note that the formation of neither deoxygenated (e.g. ethyl benzene) nor any isomerisation products (e.g. 1phenethyl alcohol) was observed in the present work. The formation of such products has been reported in earlier work for other epoxy compounds [10] in which mostly the gas phase hydrogenation experiments were conducted in the temperature range of 120-180~ The absence of any deoxygenated product, in this work suggests that the metal-styrene oxide interaction is weaker particularly, for the Pd catalyst. Isomerization products were also not observed because of addition of alkali, which neutralises the acid sites responsible for the isomerisation which is said to be a parallel reaction with hydrogenation of epoxy compounds [ 11 ]. 3.2 Effect of supports In this work the role of support such as carbon, alumina, silica and zeolite-ZSM-5, was investigated for 1% Pd catalyst and the results are shown in Figure 1. For all the supports

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studied, 100% selectivity to 2- phenyl Cony ethanol was obtained in presence of NaOH looSel. while the catalytic activity varied in the order C >A1203 >SiO2 >ZSM-5. It is known that in 0~80a basic medium, (pH range of this work was 11-12) the activated carbon support is stable ~> but alumina and silica are likely to dissolve ~ 60undergoing structural changes[ 12]. This may be the reason for decrease in activity of the fi 40 catalysts when supported on alumina and silica [13]. When zeolite was used as a support, the channel dimension of ZSM-5 20 (5.4 x 5.6 and 5.8 x 5.2 A ~ does not allow the penetration of styrene oxide due to its 0 Carbon Alumina silica ZSM-5 larger diameter (7.32A ~ to get adsorbed on Fig 1 Effect of supports on conversion and selectivity the entire surface (external + pore surface) of Temp: 313 K, Pressure: 2.048 MPa, Solvent: MeOH, the catalyst[13]. This is a probable explanation Conc. of Styrene oxide: 0.4166 kmol/m3, Conc. of catalyst: 0.375 Kg/m3,Conc.ofNaOH:0.013Kg/m3 and more work is required for a clear understanding of the observed trends. Since 1%Pd/C was the most active catalyst, detailed investigation on the effect of solvent, promoter, its concentration, temperature, hydrogen pressure, etc. was carried out using this catalyst, and the results are discussed in the following sections. .,I

O

3.3 Effect of solvents Solvents such as methanol, hexane and 1-4 dioxane were screened for hydrogenation of styrene oxide. In a protic solvent, methanol the conversion obtained was 100% while, in aprotic solvents such as hexane and 1-4 dioxane the conversion obtained was 75% and 33% respectively. This can be explained in two ways, i) Solubility of hydrogen is higher in methanol hence, highest conversion was obtained in methanol ii) and the protonated diol gets attacked by hydride to give 2-phenyl ethanol. As the protonation increases the hydride attack is easier therefore, leading to highest conversion of styrene oxide in methanol. 3.4 Effect of Promoters Nucleophilic promoters are believed to play a key role in the hydrogenation of epoxides. The role of various organic and inorganic promoters was investigated and the results are given in Table 2. It was found that in absence of a promoter and methanol as a solvent though, the conversion of styrene oxide obtained was 99%, the selectivity of PEA was only 51%. Besides PEA, other side products obtained were 1-methoxy ethyl benzene and 1,2- dimethoxy ethyl benzene. For all the promoters studied in this work the selectivity to 2-phenyl ethanol achieved was above 95% and in some cases even >99% however, the level of conversion of styrene oxide varied, giving complete conversion with only sodium hydroxide as a promoter.

3.5Effect of H2 Pressure Figure 2 shows the effect of pressure of hydrogen on the initial rate of reaction, for different temperatures, I%Pd/C catalyst and NaOH as the promoter. It was observed that initially as the pressure increases the rate of reaction also increases to a maximum (3.44 MPa) and then

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drops down with further increase in H 2 pressure indicating the possibility of hydrogen inhibited kinetics at higher pressure. At low pressure (< 3.44 MPa) both styrene oxide and H2 would be chemisorbed on the catalyst surface with some free active sites also available. As the H2presure increases the rate would increase until all surface sites are occupied by hydrogen. Further increase in the H 2 pressure, would cause the adsorbed styrene oxide to be swept away which 4 would result in decrease in hydrogenation rate. "7 9 40~ O t~

Table 2: Screening of promoters Promoter used

Conv.

Selec.

(%)

(%)

NaOH

99.9 Na2CO 3 47.0 Quinoline 36.8 Pyridine 64.3 Triethylamine 70.2 Diethylamine 47.5 Dimethylamine 55.0 Without 99.0 promoter . . . . . . . . . . . . . . .

50~

x

9

-'63

......

99.9 97.6 96.9 94.3 99.6 99.9 99.8 51.2

Temp. 313 K, Pressure : 2.048MPa, Solvent : MeOH, Conc. of styrene oxide : 0.4166 Kmol/m 3 Conc. of catalyst: 0.375 Kg/m 3, Conc. of promoter :0.013 Kg/m 3.

X

0

i lib

0

,

i

1

9

i

9

i

9

i

2 3 4 Pressure, MPa

9

i

,

5

6

Fig 2 Effect of pressure on initial rate of reaction. Temp. 313 K, Solvent : MeOH, Conc. of styrene oxide : 0.4166 Kmol/m 3 Conc. of catalyst : 0.375 Kg/m 3, Conc. of NaOH :0.013 Kg/m 3.

3.6 Effect of substrate concentration The effect of concentration of substrate on the initial rate of reaction was studied and the results are shown in Figure 3. Initially, the rate of reaction increases as the concentration of styrene oxide increases upto 1.2 x 1 0 - 4 K m o l / m 3 beyond which the rate decreases, with further increase in substrate concentration. This effect is more pronounced at higher temperature (333K). Similar observation made for H2 effect on rate of hydrogenation indicate that the adsorption of both styrene oxide as well as Hz is important and need to be considered. 3.7 Effect of Temperature The effect of temperature on both selectivity of PEA and the rate of hydrogenation was studied in a temperature range of 313-333 K. The selectivity of PEA was found to be unaffected at all the temperatures

.7~ I .~E3

o

0

~~~,,~

B 40~ X 50~ 60~

1 2 Concentration of substrate, kmoCm3

3

Fig 3 Effect of concentration of substrate on initial rate of reaction. Temp.: 313 K, Pressure: 2.0148 MPa, Solvent: MeOH, Conc. of catalyst: 0.375 kg/m 3, con. of NaOH: 0.013 kg/m 3

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while, the initial rate of hydrogenation increased with increase in the temperature and the activation energy evaluated from the Arrhenius plot was found to be 55.4 KJ/mol. 4. P R O P O S E D M E C H A N I S M

The reactions of epoxy compounds with H 2 in the presence of supported metal catalysts are known to give deoxygenated, isomerised and hydrogenated products. In our work, formation of ethyl benzene or styrene (deoxygenated products) was not observed hence, the strong adsorption of oxygen to the catalyst surface is not expected. Notheisz, et al. also reported that the metal-epoxy oxygen interaction was found to be weaker in case of Pd catalysts [14]. Moreover, the presence of NaOH on the catalyst surface decreases the adsorptivity of the epoxy oxygen resulting in higher selectivity of the desired alcohol (PEA). The absence of isomersied products is due to the neutralisation of acid centres (if any) by added alkali. Many authors have described mechanism of ring opening of oxiranes. Among them Bartok described the opening ofoxacycloalkanes in acidic medium to give different products with secondary alcohol as a major product [ 10]. Mitsui, et al. have explained deoxygenation of styrene oxide involving the radical cleavage reaction [15]. They have suggested two different mechanisms for explaining the formation of PEA, one on the basis of radical cleavage and the other involving SN 2 mechanism. In their work none of the intermediates could be separated or characterised and also the role of NaOH as a promoter in the reaction mechanism was not clearly understood. In our work the formation of only 2- phenyl ethanol indicates the regio selective opening of the C-O bond which is less hindered i.e. distant from the subsituents which is normally observed in the case of Pd and Pt metal catalysts [ 16]. The addition of NaOH is also responsible for the formation of PEA, because it neutralises the acidic sites reponsible for isomersization products (ketone in this case) which after hydrogenation give secondary alcohol. The regioselective formation of 2- phenyl ethanol can be explained based on two different reaction pathways as shown in schemes I and II. Scheme I, involves formation of n benzyl complex from the adsorbed styrene oxide. The rc benzyl complex (2) yields an alkoxide ion (3), which is stabilised by NaOH. The alkoxide ion on protonation with a solvent like methanol gives selectively 2-phenyl ethanol. According to this mechanism, the cleavage of C-O bond is postulated to be from the more substituted side, which is normally not the case for Pd catalyst. However this has been proposed by Mitsui, et al [ 10]. Scheme I

(1)

(2)

(3)

In scheme II S N 2 attack of OH- is proposed, leading to the cleavage of C-O bond from the less hindered side. The secondary alkoxide ion (4) formed in this ease then yields an intermediate 2-phenyl ethane diol (5) which on hydride attack gives selectively 2- phenyl Scheme II ,.

~

,

H

(4) (5) ethanol. Both the mechanistic pathways may contribute simultaneously to regioselective formation of PEA. However, the probability of Scheme II operating seems to be more because

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i) the cleavage of C-O bond is from the less hindered side. ii) C2 carbon of 2- phenyl ethane diol is more electropositive than C, carbon atom due to the electronegativity of phenyl ring hence, the hydride attack on C2 atom is favored to give selectively PEA. iii) In a separate experiment, styrene oxide was refluxed in aqueous sodium hydroxide for 3 hours to give 2- phenyl ethane diol (5) which was separated and well characterized. This diol was isolated and then further hydrogenated using 1%Pd/C catalyst, in methanol as a solvent to give 2-phenyl ethanol. 5. CONCLUSIONS The hydrogenation of styrene oxide in presence of Pd/C catalyst and NaOH as a promoter under very mild conditions was found to be regioselective to give only 2- phenyl ethanol as the product. A systematic study on screening of catalysts, promoters, solvents and the effect of major reaction parameters such as H z pressure, substrate concentration and temperature on the catalyst activity and selectivity was carried out. Speculative reaction pathways have been proposed for the regioselective formation of 2- phenyl ethanol. REFERENCES

1. B. D. Mookherjee and R. A Wilson, Kirk othmer (eds.) Encyclopedia of chemical technology, 4 ed, John Wiley, New York, Vol. 4, 1996. 2. E.T. Theimer, Fragrance chemistry, Academic Press, New York, 1982. 3. E. L. Eliel and D.W. Delmonte, J. Am. Chem. Soc.,78 (1956) 3226. 4. M. L Mihailovic, V. Andrejevic and J. Milovanoic, Helv. chim. Acta., 69 (1976) 2305. 5. A. Okawa and H. K. Soai, Bull. Chem. Soc. Jpn., 60 (1987) 1813. 6. S. Krishnamurthy, R. M. Schubert and H. C. Brown, J. Am. Chem. Soc., 95 (1973) 8486. 7. R. Mozingo and E. C. Homing, (eds) Organic Synthesis collective volume, 3, John Willey, London, 1956. 8. C. V. Rode, S. P. Gupte, R. V. Chaudhari, C. D. Pirozhkov and A.L Lapidus, J. Mol. Cat., 91 (1994) 195. 9. M. V. Rajshekharam, C.V. Rode, M. Arai, S.G. Hegde and R.V. Chaudhari, Appl. Cat. A: General 195 (2000) 1. 10. M. Bartok, F. Notheisz, A. G. Zsigmond and G.V. Smith, J. Cat. 100 (1986) 39. 11. H. Davidova and M. Kraus, J. Cat. 61 (1980) 1. 12. M. Bartok, Catalyst supports and supported catalyst: Theoretical aspects and applied concepts, (eds). Alvin Stiles Butterworth, New York, (1987). 13. R. Augustine, Heterogenous catalyst for synthetic chemist, Marcel Dekker, New York, 1996. 14. R.E. Malz and H. Heinemann (eds), Marcel Dekkar, New York, 1996. 15. S. Mitsui, S. M Imaizumi and Y. Sugi, Tetrahedron, 29 (1973) 4093. 16. F. Notheisz, A. Molnar, A.G. Zsigrnond and M. Bartok, J. Catal, 131 (1986) 98.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Production of Fatty Alcohols by Heterogeneous Catalysis at Supercriticai Single-Phase Conditions Sander van den HarkS, Magnus H~trrOd~ and Poul M~ller2 ~Chalmers University of Technology, Department of Food Science c/o SIK, Box 5401, SE-402 29 G6teborg, Sweden Fax: +46-31-83 37 82 Email: [email protected] /Augustenborggade 21B l a, DK-8000 Aarhus C, Denmark Fatty alcohols can be produced by catalytic hydrogenation of fatty acid methyl esters. This heterogeneous catalytic reaction, traditionally performed in a multi-phase system, is limited by the mass transport of hydrogen to the catalyst. To overcome this limitation vve have used the unique properties of supercritical fluids, properties which are in between those of liquids and gases, making them a very suitable medium for reactions. By adding propane to the reaction mixture of hydrogen and fatty acid methyl esters we have created supercritical single-phase conditions. These single-phase conditions eliminate the transport resistance for hydrogen and create the possibility to control the concentration of all the reactants at the catalyst surface independently of the other process settings. In this way, extremely rapid hydrogenation can be combined with a high product selectivity. In our lab-scale experiments the catalyst activity was studied as a function of hydrogen pressure, substrate concentration and temperature. The catalyst activity was extremely high compared to the multi-phase hydrogenation. Complete conversion of the liquid substrate was reached in a few seconds. The high catalyst activity results in reaction rates which are comparable with similar gas-phase hydrogenation reactions of much smaller molecules (e.g. methylacetate). As long as single-phase conditions remain-in our experiments we have tested up to 15 wt.% substrate- the gas-phase-like activity can be maintained. Our results prove that performing hydrogenation at supercritical single-phase conditions is beneficial for this and other heterogeneous catalytic processes which are limited by mass transfer. 1. INTRODUCTION Fatty alcohols (FOH) and their derivatives are one of the major oleochemicals and widely used as surfactants. They can be produced from natural fats and oils by catalytic hydrogenation of fatty acids or fatty acid methyl esters (FAME). When dealing with such low volatile substrates, one is confronted with a multi-phase system consisting of a liquid (substrate and product), a gas (hydrogen) and a solid (catalyst). The gas liquid binary "subsystem" is a result of the low solubility of hydrogen in these liquids, especially in relation to the stoichiometric hydrogen requirement (often above 50 mol%). The hydrogenation of FAME requires severe process conditions, typically: hydrogen pressures between 200 and 300 bar, temperatures ranging from 200 to 300 ~ (1, 2). Even at these conditions the solubility of hydrogen in the liquid phase (i.e. in the FAME) is low, as can be

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Hydrogen

Supercritical

Catalyst

FAME

Catalyst

|

Substrate

o -~,

r.~

Hydrogen

o t~

o

o

t~ o

ll'

E ~. Hydrogen

:

O

ixM.

Substrate m

A)

Distance

B)

Distance

Fig. 1. General concentration profile over the reactor for the substrate and hydrogen in: (A) a multi-phase system (C *= solubility of H2 in FAME. See also on the base-line, representing this binary gas-liquid system, in Fig. 2). (B) Homogeneous supercritical phase.

Propane

FAME

C

,

Hydrogen

Fig. 2. Phase diagram for the system FAME, propane and hydrogen, ~ the estimated singlephase region at 100 bar and 200 ~ [4]. (---) indicates the stoichiometric amount of hydrogen needed. (--) the composition of the reaction mixtures used in the experiments. seen in the concentration profile over the reactor (Fig. 1A). As a consequence there is a lack of hydrogen at the catalyst surface. Hence, the reaction rate is low and limited by the mass transfer of hydrogen. Also, as can be seen in Fig. 1A, there is an unfavorable ratio of FAME and hydrogen in the liquid phase surrounding the catalyst. To overcome these restrictions we have added propane to the reaction mixture. Supercriticalpropane (Pc=42.5 bar and Tc=96.6~ is completely miscible with hydrogen and FAME. By choosing the fight conditions, at the given reaction temperatures, a substantially homogeneous supercritical phase can be formed, with almost unlimited a c c e s s to the catalyst surface for all the reactants (3, 4). The single-phase area, based on literature data (5, 6, 7) and our own experiments, was estimated as shown in the ternary phase diagram (Fig.2). The concentration

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profiles over the reactor can now be illustrated by Fig 1B; note that the ratio of FAME to hydrogen is inverted compared to the traditional process. Very high reactions rates are possible under these conditions, e.g. the partial hydrogenation of methylated rapeseed oil, a 400 fold increase in LHSV (substrate volume processed per hour by a unit volume reactor, m3substratc/m3reaetorh.) was obtained (8). A high selectivity is possible because the concentrations and other process conditions can be set independent of each other. Selectivity is of major interest for the product quality. In our case, besides the main reaction (i.e. the formation of FOH), "overhydrogenation" can lead to the undesired formation of alkanes. Other side products are e.g. aldehydes and wax esters (wax) which are reaction intermediates. This study demonstrates that the reaction rate is enhanced in a single-phase system. Furthermore, experiments were performed to investigate the influence of the hydrogen concentration, temperature and substrate concentration on the catalyst activity and selectivity at supercritical single-phase conditions (for details see 9, 10). 2. MATERIALS AND METHODS

A CuCr catalyst (Cu-1985 P, Engelhard, the Netherlands) with a particle size of 32-71 gm was used. Further, propane (Instrument Quality, AGA, Sweden), hydrogen (Hydrogen Plus 99.995%, AGA, Sweden) and methylated sunflower oil (C18:0-2) were used in the reaction mixtures. An experimental space was created by varying temperature, residence time, and hydrogen level (as molar ratio of H2 to FAME), according to a central composite design, see Table 1 (11). All other factors were kept constant; the total pressure was 150 bar, the FAME concentration was 0.3 mol% (~2.3 wt%), and the total flow rate was 60 retool/rain (1.40 l/rain NTP). The experiments were performed on the same equipment and analyzed as reported in earlier work (8,10). Table 1. Experimental space for the investisation, variables and levels. Variables Experimental space Low High Temperature (~ 260 300 H2:FAME 1 4 (1.8) 64 (30) Residence time (s) , 0.1 0.9 1) The correspondinghydrogenpressure is given in brackets. In each experiment the reactor productivity was measured in terms of conversionrA~ and yieldFoH. Models (i.e. goal functions) for these two dependent variables were constructed to describe the correlations with the independent variables in the experimental space. ConversionFAME was defined as the decrease in FAME concentration in the reaction mixture, in mol%. (FAMEi, = start concentration, FAMEout= concentration after the reactor)

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conversion

-- FAME~, - FAME ,u, , 100% FAME

FAMEin

(1)

The products from this conversion can be aldehydes, FOH, alkanes or wax esters. The actual yield of FOH, including conversion and selectivity, was therefore defined as the ratio between the amount of produced stearyl alcohol (FOH) from FAME: FOH , 100% yield FOH = FAME--------~

(2)

The catalyst activity, in terms of reaction rate, is expressed as the consumption of FAME at a given time (i.e. at a given point in the catalyst bed). It can therefore be calculated as the derivative of the conversionF~ with respect to time. 3. RESULTS AND DISCUSSION

The used catalyst both hydrogenates the carbon-carbon double bonds and the carboxyl groups in the fatty acid chains. Hence only saturated FOH are produced (1). Hydrogenation of carboncarbon double bonds was very fast at the applied reaction conditions. In the following, "hydrogenation" refers only to the slower reaction involving the carboxyl group. Totally 3.4 mol H2/mol FAME are needed to complete both reaction steps. The catalyst activity was very high, high conversion levels of FAME were reached with residence times below 1 s. Temperatures up to 250-260 ~ accelerate the catalyst activity, while higher temperatures only lead to a slight additional increase in the activity (9). Thus, the reaction temperature is not changed by the supercritical conditions (10). By calculating the reaction rate at a constant FAME concentration (i.e. a fixed level of conversionFAME) the effect of the ratio H2:F ME on the catalyst activity can be studied (see Fig. 3). The linear dependency of the reaction rate on the H2:FAME ratio, indicates first order kinetics with respect to hydrogen. This is in agreement with the first order in hydrogen concentration found in gas-phase kinetics (12, 13). As stated earlier, the catalyst activity should be combined with a high selectivity (i.e. mainly the suppression of "overhydrogenation"). Both these objectives are included in yieldFoH (eq. 2). Fig. 4 shows a contour plot of this yield as function of the ratio of H2:FAME and residence time. A contour plot can be regarded as a two dimensional projection of a continuous response surface. In the experimental space the selectivity was around 95%. The yield decreases from 100% to 90% in the upper-fight comer of Fig. 4. This is a result of a lower conversion not due to overhydrogenation of the product. The conversion decreases as a consequence of a large pressure drop over the catalyst bed, leading to a multi-phase system when long residence times (i.e. long catalyst beds) were used (9). All our successful experiments were performed in the single-phase region with an excess of hydrogen (see Fig. 2). As a consequence the reactor productivity was large and ranges from LHSV of 10 to 100 ( i.e. 300 to 2600 ktmol/g cat min). This can be compared with a LHSV of

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900

A

C

I

I

I

I

I

I

=m

E 2000-

A

/

"~ 1500

v

E

im

I-.

1000 0 ,,=m

,,i,,a

o 0

IZ:

E O

E

E =

U~

= 300 tJ

t.19

500

"10

0 0

I

i

i

i

10

20

30

40

H2:FAME

(mollmol)

Fig. 3. Reaction rate (pmol/g=t, ly,tmin) versus H2:FAME ratio (280~ and FAME=0.09 mol%).

(R 19

IZ

N

100 4

8 16 32 H 2" FAM E (mollmoi)

Fig. 4. YieldroH at 280 ~ FAMEi, =0.3 mol%

64

(1.5*SEE=9.1).

0,2-0,4 for the traditional fatty alcohol process (1). The catalyst activity reached at supercritical conditions with these large molecules (MW=300 g/mol) is comparable to that found for gas-phase reactions of methyl- and ethyl acetate (MW=100) under similar temperatures (12, 14). In such gas-phase reactions there is no external diffusion limitation. Hence, at supercritical single-phase conditions we have achieved the state where the catalyst activity is controlled by the "surface" kinetic also for large molecules. Only the single-phase region to the fight of the stoichiometric hydrogen demand (see Fig.2) can give these high activities. Hydrogen is an antisolvent and reduces the maximum concentration of FAME in the singlephase at a given set of conditions. Furthermore, in a future process where propane is recycled, excess hydrogen should also be recycled. Hence, a low excess of hydrogen and high substrate concentrations are favorable and were therefore tested in additional experiments. Experiments no. 1 to 3 in Table 2 verify the trends from Fig. 4; a lower hydrogen excess reduces the catalyst activity due to a lower hydrogen concentration at the catalyst surface and should be compensated by longer residence times to maintain the conversion. With the fight balance between residence time and hydrogen a high selectivity can be achieved, resulting in high yields. Table 2. Additional hydro~;enation experiments with sunflower FAME over CuCr cata!yst 1) Run H2/FAME Time ConversionF~tE YieldFoH Side-products 2) FAMEm (%) (mol%) (wt.%) (moVmol) (s) (%) Wax 66 1 0.5 ~3 10 2 94 Alkane 67 2 0.5 ~3 10 8 100 90 3 0.5 ~3 4 8 100 48 Wax 4 2 13 4 8 72 91 5 2 15 12 8 100 1,Conditions: 150 bar, 280 ~ 2~Alkaneindicates "overhydrogenation",Wax is a reaction intermediate to FOH

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When we increased the FAME concentrations in our process, we could maintain the high catalyst activity, as long as single-phase conditions remained (see Table 2). Under the applied conditions (280 ~ and 150 bar) we have measured a homogeneous phase with up to 15 wt% FAME and 24 mol% hydrogen in the reaction mixture. Unforttmately, exact phase behavior data for the reaction mixture in the region of interest are missing. Current research is focussed on this topic.

The exothermic reaction in combination with the high reaction rates would cause a temperature rtmaway in a traditional reactor. In the supercritical process the propane present as a solvent also acts as a "cooling medium" and the temperature rise can be limited. 4. CONCLUSIONS For the hydrogenation of a liquid substrate to fatty alcohols in a homogenous supercritical phase, created through the addition of propane, extremely high catalyst activities were reached. This activity is strongly influenced by the ratio of hydrogen to substrate in the reaction mixture. The selectivity could be maintained at a high level. With more solubility data and more knowledge about the influence of the substrate concentration on the catalyst activity the process can be further optimized. A pilot plant will be constructed together with an industrial partner.

Acknowledgement This work was financially supported by Daka, a.m.b.a., Losning, Denmark REFERENCES 1. F. Ullmann, Ullmann's Enzyclopaedie der technischen Chemie, 4th edn., Verlag Chemie, Weinheim, 1976. 2. M. Hoffman, K. Ruthardt, Oils and Fats International, 9 (1993) 25. 3. M. Hiirr6d, P. Moiler, PCT patent application, WO 96/01304, (1996). 4. M. Hiirr6d, M. -B. Macher, S. van den Hark, P. Moiler, In: Proc. 6 th Meeting on Supercritical Fluids, ISASF, Nancy (1999) 253. 5. H. Schiemann, PhD Thesis, Universit~it Erlangen-Numberg, Erlangen, 1993. 6. H.G.A. Coorens, C.J. Peters, and J. De Swaan Arons, Fluid Phase Equilibria, 40 (1988) 135. 7. E.Weidner, D. Richter, In: Proc. 6th meeting on supercritical Fluids, ISASF, Nancy (1999) 657. 8. M. -B. Macher, M. H~irr6d, P. Moiler and J. H6gberg, Fett/Lipid,101 (1999) 301. 9. G.A. Camorali, MSc Thesis, Chalmers Univeristy of Tecnology, Sweden, 1999. 10. S. van den Hark, M. H~irr6d, M., P. Moller, J. Am. Oil Chem. Soc., 76 (1999) 1363. 11. G. Box, W.G. Hunter, J.S. Hunter, Statistics for Experimenters. An Introduction to Design, Data Analysis and Model Building, John Wiley & Sons, New York, NY, 1978. 12. J. Evans, M. S. Wainwright, N. W. Cant, D. L. Trimm, J Catal, 88 (1994) 203. 13. N. Chikamatsu, T. Tagawa, S. Goto, J. of Chemical Eng. of Japan, 21 (1991) 604. 14. P. Claus, M. Lucas, B. Lticke, Applied catalysis A, General, 79 (1991) 1.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Regioselective oxidation of primary hydroxyl groups of sugar and its derivatives using a new catalytic system mediated by TEMPO. H. Kochkar a, M. Morawietz b and W. F. Hrlderich ~* aDepartement of Chemical Technology and Heterogeneous Catalysis, University of Technology, Rgrl'H-Aachen, Worringerwegl, 52074 Aachen-GERMANY. Phone 949 241 80 65 60/61, Fax 949 241 88 88 291, e-mail [email protected] bDegussa-Hfds AG, Hanau, GERMANY.

Primary hydroxyl groups were oxidized regioselectively using organic oxoammonium salts generated on supported silver catalysts, which promote disproportionation of 2,2,6,6tetramethylpiperidinyl-1-oxy (TEMPO) in aqueous solution. The oxidation reactions were performed at pH 9.5 in a batch reactor at RT using heterogeneous silver catalysts and peroxides as primary co-oxidants. 99 mol.% selectivity to methyla-D-glucopyrasiduronic acid was obtained at 90 % conversion of the pyranoside over a silver-celite catalyst. The activity was increased by adding carbonates to the silver catalysts. This resuk can be explained by the increase of the electron charge deficiency on silver in presence of carbonate, which accelerates the nucle0philic attack of TEMPO and/or hydroxyl groups. This result was proved using TPD of ammonia in the case of Ag-AI203 catalyst. The observed regioselectivity is due to the sterical hindrance caused by the four methyl groups in TEMPO. 1. INTRODUCTION Metal-catalyzed oxidation of alcohols to carboxylic compounds, in conjunction with cooxidants, is an important step for synthesis of fine chemicals 1. Particularly, the oxidation of sugar and its derivatives such as starch and cellulose is important. The oxidized carbohydrates can be used as thickening, gelling agents, in paints, resins, detergents co-builders and super absorbers have an important economic impact. There are many methods for the selective oxidation of secondary hydroxyls groups in the presence of primary ones 2, but few suitable reports describe procedures for the oxidation of primary hydroxyl groups that leave the secondary hydroxyl groups still intact. Semmelhack et al3, reported that electrooxidation as well as autoxidation of alcohols, mediated by 2,2,6,6tetramethylpiperidinyl- 1-oxy (TEMPO), shows this matter of regioselectivity. The corresponding nitrosonium ion of the nitroxyl radical, which is a powerful oxidizer of alcohols 4, can be obtained with a hypochlorite/bromide system5, sodium bromite and calcium hypochlorite6, copper(i)chloride.oxygen 7, p-toluenesulfonic acid s, electrochemically 9. Depending on the reaction conditions and the substrate used, aldehyde 1~ o r carbox-ylate 11 were obtained. The drawback of this method is a large accumulation of waste salts and contamination with chlorinated compounds. Furthermore, the new European laws will not accept these "conventional" environmental unfriendly process technologies anymore. Therefore, the above cited process should be replaced by heterogeneous catalysis.

* To whomthe correspondence shouldbe addressed.

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We report here about a new method for the regioselective oxidation of primary hydroxyl groups over a silver catalyst/peroxodisulfate system mediated by TEMPO. The oxidation was performed first with methyl-a-D-glucopyranoside since it has been used as a model molecule. 2. E X P E R I M E N T A L

2.1. Catalyst preparation Ag-Na-Y, Ag-AIPO4 and Ag-AI203 were prepared by incipient wetness impregnation with silver nitrate by stirring at 25 ~ during 15 h. After filtration, washing and drying at 373 K. Then, the catalyst was calcined under air at 773 K for 6 h. The materials were characterized by ICP-AES and nitrogen sorptlon at 77 K (see Table 1). Silver carbonate celite catalyst was prepared according to M. F&izon et a112, the catalyst contains 0.17 mmol of Ag per gram of celite and a BET surface are of 6 m2.g-1. Table 1" Characterizations of the catalysts. Catalyst/support

Ag ~ (Wt %)

Surface area (m2.g -1)

Pore diameter b <~>A

A1203 A B C D

1.1 2.0 2.8 5.0

253 247 249 241

65 65 64 64

NaY E

5.8

573

< 12

AIPO4

4.5 41 F a 9determined by ICP-AES. b: determined by using BJH method.

20<~<40

2.2. Oxidation procedure 5 mmol of substrate was dissolved in 100 ml of water. Then 30 mg (0.19 mmol) of TEMPO and 10 mmol of co-oxidant e.g. ammonium peroxodisulfate (PDS) were added. The system reached equilibrium at 298 K and pH 9.5. The catalyst was instantaneously added to the mixture (time zero). The pH of the solution during the reaction was kept constant at 9.5 by automatic titration with 1M KOH solution. At the end of the reaction, the pH was adjusted at 8. Then the catalyst was recovered by filtration, water and TEMPO were removed under vacuum at 313 K. The product mixture was dried overnight under vacuum at RT. The oxidation degree of the substrate was determined by gas chromatography after silylation. For silylation, e.g. 10 mg of the oxidized sugar and 600 ~tL of pyridine stirred until the mixture was totally solubilized. Then 600 ~tL of hexamethyldisilazane and 150 ~tL of trifluoroacetic acid were introduced. The mixture was placed in an oven at 363 K for 30 mins. Then n-decane was added as internal standard. The products were analyzed by GC.

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3. RESULTS AND DISCUSSIONS

The oxidation was performed first with methyl-c~-D-glucopyranoside (a-MDG) since it has been used as a model molecule (Table 2). Table 2: Oxidation of a-MDG over metal catalysts mediated by TEMPO, at pH=9.5, 298K, molar ratio cz-MDG/TEMPO = 26. ( ): molar ratio of substrate to silver. Catalyst Ag20 (0.5) No Ag-A1203 (108)

(NH4)2S2Os/a-MDG Conversion 0 2 2

(%) 49 9 70

Selectivity (mol %) 95 86 97

The selectivity of the catalyst for the oxidation of cz-MDG to form methyl-cc-Dglucopyrasiduronic acid was good, ranging from 86 to almost 97 mol %. The oxidation reaction was found to be stoichiometric when Ag20 was used in absence of the oxidant. Using the PDS in combination with TEMPO but in absence of Ag20, a low conversion of 9 % was obtained. However, when Ag-A1203 was used with a molar ratio of cz-MDG to silver = 108, a very high selectivity to methyl-cz-D-glucopyrasiduronic acid (97 mol %) was obtained at a conversion of 70 %. In this procedure the silver works as a catalyst. In the absence of TEMPO radicals but with Ag-A1203 and PDS the conversion was lower than 2 %. This allows to state that the absence of TEMPO radicals in this type of reaction makes it ineffective and non selective, while the presence of a catalytic amount of radical increases the regioselectivity of the oxidation to a very high degree. These materials are good catalysts for the regioselective oxidation of carbohydrates: they give high selectivities for the carboxylic acid sak without cleavage of the pyranoside ring. For the oxidation of cc-MDG, Van Bekkum et a113 reported a conversion about 60 % after 20 h using [Ti]-MCM-41 and H202, but the selectivity to the hydroxyl-group in C6 position is low due to decomposition of H202 and oxidative cleavage of the vicinal diol groups. Earlier experiments 14-15 of our group showed V-Y and Co-Y gave 15 % and 60% conversion respectively with 99% selectivity to the acid. The comparison with the present results shows that silver catalyst gives better results using PDS as co-oxidant, cz-MJ)G oxidation was also conducted versus the silver content on alumina, using a molar ratio aM D G : PDS: TEMPO = 1:2:0.1. These results show that the conversion of the pyranoside increases with silver amount and reaches a plateau at 2 wt%. A catalytic sequence for the oxidation by silver is proposed in Scheme 1. Silver ions effect a two-electron oxidation of 1 to nitrosonium ion 2 (Eqn. 1). The alcohol is then oxidized by 2 (Eqn. 2), generating the carboxylic acid and hydroxylamine 3, rapid syn proportionation of 3 with 2 regenerates 1 (Eqn. 3). Finally, Ag is regenerated by peroxodisulfate, giving Ag (II) and sulfate (Eqn. 4). The net reaction is the alcohol oxidation by silver and peroxodisulfate to carboxylic acid and water.

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Ag(II)~ 2 KOH

(1)

O" 1

2 H20

R'CH2OH §~ ~ 0 H _

+ H+ (2)

--~ R-COOH + ~ " ~ r ' ~ OH 3

2 2

~

+H §

(3)

O" 1

S20$2" + Ag ~ 2 SO42"+ Ag(II) (4) Scheme l" Proposed mechanism for the regioselective oxidation of primary hydroxyl groups.

3.1. Effect of peroxodisulfate/TEMPO ratio.

The molar ratio PDS/TEMPO was varied between 25 and 250 (Table 3). No conversion was observed in absence of PDS. By contrast, the conversion reaches a maximum at approximate ratio PDS/TEMPO close to 50 and decreases at higher ratios. This result can be explained by a complex formation between the cationic oxidizing agent and sulfate anion (Scheme 2). The hydrogen bond is expected to lower the positive charge on the nitrogen, which would be the driving force in this reaction pathway. T. Miyazawa et al. 16 demonstrated that the nucleophilicity of the counter anion of this cationic oxidizing agent has an important effect on the catalytic properties : e.g. secondary alcohols are oxidized faster than primary alcohols in presence of Br-. The opposite results were observed in presence of C1-. On the other hand, the adsorption of polar substrate or products can modify this charge 17. The selectivity to the acid decreases at high ratio of PDS, and this might be explained by the sulfatation effect, the acidity of the catalyst increases and resuks in a ring opening of the pyranoside. Table 3" Effect of the PDS/TEMPO PDS/TEMPO

conversion (%)

selectivity (mol.%)

25 50 75 125 250

27 70 42 30 19

98 97 96 87 70

O O I ~,'/I. O ~..~)I ~ .. O Scheme 2"

3.2. Effect of the support

In order to improve the silver dispersion four supports with different texture were checked : A1203, NaY, celite and AIPO4 (Figure 1). The conversions and activities obtained varies in the order: Ag-carbonate-celite >> AgY > Ag-A1PO4 > Ag-A1203, with selectivities to methyl-mD-glucopyrasiduronic acid of 99 tool %. High conversion is obtained using Ag-carbonatecelite catalyst, which has the smallest surface area. Here probably all the silver ions are on the surface. We suggest that the presence of carbonate modified the electron density on silver and therefore the catalyst became more active.

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In order to verify this hypothesis ot-MDG oxidation reaction were conducted on Ag-A1203 (2 wt%) and Ag-A1203-(CO32"). Initial rate increases by a factor of 10 in the case of AgA1203-(CO32). This is consistent with the idea that the active site for oxidation are weak Lewis sites. This hypothesis was verified by TPD of ammonia. 100

~

"~

80 9 Conversion (%)

60

[] Selectivity (mol. %)

40 20

0

Ag-celite

Ag-Y Ag-A1203

I

Ag-AIPO4

Figure 1: Effect of the carrier. 3.3. Substrate effect We were especially interested in the influence of the ring size because it was found that primary hydroxyl groups in pyranosides were oxidized more selectively than those of the furanosides 1~. The model substrates 1,2-propandioL c~-MDG and saccharose were oxidized (see Table 4). Obviously, primary alcohols are oxidized more rapidly than the secondary ones. With primary/secondary polyol (1,2-propandiol), the selectivity for the primary hydroxyl groups is high and reaches 75 mol.% at 90 % conversion. For pyranosides ring, the selectivity depends on the sterical demand of the primary hydroxyl groups. In general, the observed regioselectivity for different substrates depends on the accessibility of the alcohols (Scheme 3), which would favour the more sterically confining transition state. Primary hydroxyl groups of a-MDG were oxidized more rapidly than those of the saccharose. In the oxidation of various pyranosides it appeared that the size of the ring play an important role. The oxidation of starch was not successful. May this can be explained by diffusion limitations. For comparison reasons, oxidation of starch was performed in a fluidised bed using NO: as an oxidant. By means of ~3C NMK, UV-visible and FT-IR spectroscopy, we concluded that oxidation takes place regioselectively in primary hydroxyl groups. Table 4: Substrate effect. Substrate

Conversion (%)

1,2 propandiol a-MDG saccharose starch starch (NO2 as oxidant)

90 78 20 < 1 20

Selectivities acid 75 99 100 99

(mol.%) others 25 1 100 1

O

(]

r,,, ~I----R I-I"

Scheme 3"

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4. CONCLUSIONS

As compared with the conventional methods, the process of the present work has the following merits" 1. Extremely high activity and selectivity are obtained in the oxidation of primary alcohols or primary hydroxyl groups e.g.. in the case of carbohydrates. More particularly, primary hydroxyl groups of methyl-c~-D-glucopyranoside are oxidized regioselectively in presence of secondary ones. Side reactions are diminished, 2. The hypochlorite/bromide system used before and well described in the literature 4-5 w a s replaced by an environmentally benign catalytic system

Acknowledgements This work was carried out in the flame of European project (BRITE/CARBOPOL-CT961208). The authors thank Dr. R. Vanheertum (Degussa-Hiils) for his steady interest and discussions.

REFERENCES 1. IL A. Sheldon, J. K, Kochi., Metal-catalyzed oxidations of organic compounds, Academic Press., New York ( 1981). 2. G.H. Posner, 1LB. Perfetti, A.W. Runquist, Tetrahedron. Lett., (1976) 3499. 3. M.F. Semmelhack, C.1L Schmid, D. A. Cortes, C.S. Chou, J. Am. Chem_ Soc.,105 (1983) 4492. 4. S.P. Chang, J. F. Robyt, J. Carbohydr. Chem., 15(7) (1996) 819. 5. A.E.J. de Nooy, A. C. Besemer, H. Van Bekkum, Carbohydr. Res., 269 (1995) 89. 6. T. Inokuchi, S. Matsumoto, T. Nishiyama, S. Torii., J. Org. Chem., 55 (1990) 462. 7. M. F. Semmelhack, C. R. Schmid, D. A. Cortes, C.S. Chou~ J. Am, Chem. Soc.,106 (1984) 3374. 8. Z. Ma, J. M. Bobbitt, J. Org. Chem., 56 (1991) 6110. 9. I~ Ito, J. M. Bobbitt, J. F. Rusling, Proc. Electrochem. Soc., 93 No. 11 (1993) 260. 10. T. Miyazawa, T. Endo, S. Shihashi, M. Okawara, J.Org. Chem., 50 (1985)1332. 11. A. E. J. de Nooy, A. C. Besemer, H. Van Bekkum, Recl. Trav. Chim. Pays-Bas., 113 (1994) 165. 12. V. Balogh, M. F6tizon, M. Golfier, J. Org. Chem, 36 (1971)1339. 13. E. J. M. Mombarg, S. J. M. Osnabrug, F. van Rantwijk, H. van Bekkum, Heterogeneous Catalysis and Fine Chemicals IV., (1997) 385. 14. H. Kochkar, W. F. Hrlderich, in preparation. 15. H. Kochkar, W. F. Hrlderich, DE. Patent, application submitted (1999). 16. T. Miyazawa, T. Endo, S. Shiihashi, M. Okawara, J. Org. Chem., 50 (1985) 1332. 17. A. E. J. de Nooy, A. Besemer, H. van Bekkum, Tetrahedron., 50 No. 8 (1995) 8023.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Aspects of regioselective control in the hydroformylation of methyl methacrylate with the in situ formed (o-thiomethylphenyl)diphenylphosphine rhodium complex H.K. Reinius a*, R.H. Laitinen b, A.O.I. Krause a and J.T. Pursiainen b a Department of Chemical Technology, Helsinki University of Technology, P.O. Box 6100, FIN-02015 HUT, Finland b Department of Chemistry, University of Oulu, Linnanmaa, FIN-90570 Oulu, Finland The regioselective control of the in situ formed (o-thiomethylphenyl)diphenylphosphine (oSP) rhodium catalyst was studied in methyl methacrylate (MMA) hydroformylation by changing the functionality of the triphenyl phosphine based ligands by systematically adding the possibility for multidentate binding, increasing the steric stress and modifying the electronic properties. Chelation control has a significant effect on the regioselectivity. 1. INTRODUCTION The most active research area in hydroformylation is ligand synthesis and their coordination chemistry. Chelating phosphines have proved especially useful in modifying the electronic and steric properties of reactive metal centres, offering considerable advantages in the design of new homogeneous catalysts. However, despite extensive screening of different phosphines the mechanism through which they control regioselectivity remains obscure [ 1,2]. In the widely accepted Wilkinson's mechanism the aldehyde regioselectivity is determined in the hydride addition step that converts a five-coordinated H(alkene)-Rh(CO)L2 into a primary or secondary four-coordinate (alkyl)Rh(CO)L2. Two monodentate ligands or one bidentate ligand might occupy either two equatorial sites or one apical and one equatorial site in the trigonal bipyramide intermediate. Casey et al. [3] showed that in the hydroformylation of 1-hexene there is a strong correlation between regioselectivity for n-aldehyde formation and natural biting angle of the ligand. Ligands like 2,2'-bis[(diphenylphosphino)methyl]-l,1 'biphenyl having a wide natural biting angle (120 ~ prefer diequatorial chelation and favour nisomer formation (n/i=66), whereas ligands like 1,2-bis(diphenylphosphino)ethane having a narrow biting angle (85 ~) prefer apical-equatorial coordination and result in lower n-isomer selectivity (n/i=2.6). However, neither the steric nor the electronic properties of the ligands alone could adequately explain the observed n/i ratios [ 1, 3]. Van der Veen et al. [2] recently showed with thixantphos ligands in the hydroformylation of 1-octene and styrene that even though the coordination mode p e r se did not explain the regioselectivity of the reaction the natural biting angle did correlate with it. They further suggested that the four-coordinate L2Rh(CO)H complexes could offer better explanation for the regioselectivity of the hydroformylation reaction than the five-coordinate ones. Corresponding author

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Under hydroformylation conditions, MMA is mainly converted into branched methyl-c~formylisobutyrate (tx-MFIB) or the linear isomer, methyl-13-formylisobutyrate (13-MFIB). In our previous work [4] it was shown that the potentially bidentate oSP ligand exhibited considerably higher selectivity to methyl-tx-formylisobutyrate ( i / n - 27) than did 1,2-bis(diphenylphosphino)ethane and 1,4-bis(diphenylphosphino)butane, which are known to be very effective in the or-selective hydroformylation of tx,13-unsaturated esters. We also showed that even though the regioselectivity of the reaction is known to be highly dependent on process variables [5], neither the pressure increase [4] nor the variation of temperature [6] had any effects on the regioselectivity of the oSP ligand. Therefore, the mechanism explaining the regioselective control of the oSP ligand is studied in this work by changing the functionality of the triphenyl phosphine based ligands by systematically adding the possibility for multidentate binding, increasing the steric stress and modifying the electronic properties. 2. EXPERIMENTAL All the hydroformylation experiments were carried out in a 250 ml autoclave (Berghof) equipped with a sampling system. The experimental procedure is explained in detail elsewhere [4]. In a typical experiment the autoclave was charged with Rh(NO3)3 (6.5 mg), methyl methacrylate (5 g), toluene (18 g), internal standards decane (1 g) and cyclohexane (1 g), and respective phosphine. The system was first flushed with nitrogen and heated to reaction temperature with continuous stirring, and then pressurised to reaction pressure with a desired molar ratio of H2 and CO. Four samples were taken for analysis in each experiment: one immediately after pressurising with H2 and CO, which was considered as the starting point of the reaction, and one after every first, third and fifth hour. The products were analysed with a Hewlett Packard 5890 GC equipped with a capillary column (HP-1, 1.0 ~m x 0.32 mm x 60 m) and a flame-ionisation detector. Products were quantified by the internal standard method. In addition, the aldehydes that formed were identified by GC-MS analysis, and after fractional distillation by 1H NMR spectroscopy. The ligands were synthesised and identified with published methods: (o-thiomethylphenyl)diphenylphosphine (oSP), (o-methylphenyl)diphenylphosphine (oMeP)[4]; (p-thiomethylphenyl)diphenylphosphine (pSP), bis(o-thiomethylphenyl)phenylphosphine (oSSP), tris(o-methoxyphenyl)phosphine (oOOOP)[7]; (p-thiomethylphenyl)bis(o-pyridyl)phosphine (pSPpyr2); (o-thiomethyl)bis(o-methoxyphenyl)phosphine (oSoOOP), (o-thiomethylphenyl)bis(p-thiomethylphenyl)phosphine (oSpSSP), (o-thiomethylphenyl)bis-l-naphtylphosphine (oSPnaf2)[8]. The purity of the ligands (> 95%) was checked with IH-NMR. 3. RESULTS Plain rhodium nitrate strongly favoured the formation of 13-MFIB i/n ratio being 0.2 (Figure 1). The adequate addition of the oSP ligand (L/Rh higher than 2) promoted the formation of t~-MFIB, but the decrease in conversion was rather severe; about 35 percentage points. However, the Figure 1 shows that at ligand to rhodium ratios equal to or higher than four the c~-MFIB was favoured and the conversion as well as the i/n ratio stabilised to a rather constant level. Even though the initial a-selectivity was quite high already with L/Rh=3, the decrease in c~-selectivity during the run was rather severe (6% after 5 hours), whereas with higher L/Rh ratios the decrease in a-selectivity was almost unnoticeable.

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553 40

100 80

30 o

~6o >~40

10

o 20 0

l

1 1

0

2

4

L/Rh

!

!

!

6

8

10

0

Figure 1. Effect of L/Rh on conversion and selectivity. (100 ~ reaction time 5h)

60 bar, MMA/Rh=2500,

To enlighten the coordination mechanism of the oSP ligand we synthetised a family of tertiary phosphines that differed in stereoelectronic properties (Scheme 1), thus in possibility for multidentate binding, in steric requirements as well as in the electronic state of the phosphorous atom.

[~SCH3 oSP

SCH3 OCH3~SCH3

oSoOOP

S.CH3

oSPpSSP

~~j~C H3CH3~OCH3

oOOOP

S.CH3~SCH3

oSSP

pSP

oSPnaf2

PPh3

SCH3 '

oMeP

pSPpyr2

Scheme 1. Triphenyl phosphine based ligands differing in stereoelectronic properties. The ligands differing in electronic and steric properties had a profound effect on the regioselectivity (Table 1). All the triphenyl phosphine type ligands containing one thiomethyl group in ortho-position (oSP, oSoOOP, oSpSSP) indeed gave high a-selectivity (i/n=14-30) regardless of other substituent groups in the two phenyl rings. The other substituent groups had, however, considerable effect on the activity of the catalyst, the conversions after one hour being 17, 4 and 15%, respectively. When the thiomethyl group was moved to para-position (pSP, pSPpyr2) or replaced with a substituent group unable of forming a chelate (oMeP) the regioselectivity of the catalysts

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changed significantly (i/n=1.7-3.1) resembling that of PPh3. The oOOOP ligand gave no reaction and the reaction with the oSSP ligand yielded only trace amounts of products. Increasing the steric hindrance of the ligand by replacing phenyl tings with naftyl ones (oSPnaf2) resulted in the poorest a-selectivity and low conversion. The 31p-NMR chemical shifts reflecting the electronic state of the phosphorus atom did not correlate with the obtained selectivities or conversions.

Table 1. Conversions and regioselectivities obtained with ligands differing in stereoelectronic properties. (I O0~ 60bar, MMA/Rh=2500, L/Rh=4, reaction time 1 h) Ligand

3~p-NMR chemical shift [8]

Conversion, %

MIB

Selectivity, % c ~ - M F I B 13-MFIB

oSP -12.9 17 7 86 3 oSoOOP -34.1 4 12 82 6 oOOOP -37.1 n.r. n.a. n.a. n.a. oSSP -22.0 t.a. n.a. n.a. n.a. pSP 33 3 67 28 oSpSSP -13.9 15 4 92 4 oMeP -10.7 39 2 62 36 pSPpyr2 - 1.6 11 7 68 22 oSPnaf2 -29.7 12 2 56 37 PPh3 -3.3 47 2 70 27 MIB = methylisobutyrate, n.r. = no reaction, n.a. = not applicable, t.a. = trace amounts

other 3 0 n.a. n.a. 2 0 0 3 4 2

4. DISCUSSION The addition of the oSP ligand into the reaction mixture slowed the reaction rate immediately. The addition of the oSP ligand does not, however, result to the formation of aselective catalyst before the L/Rh ratio of 3 (Figure 1). Rather, is seems that at ligand to rhodium ratios 1 to 2 the addition of oSP ligand deactivated the activity of the unmodified rhodium because conversion was lower than without the ligand. Apparently, the oSP ligand formed at low ligand to rhodium ratios a non-reactive complex with rhodium decreasing the yields, whereas at ligand to rhodium ratios equal to or higher than three the a-selective catalyst was formed. Thus, at low ligand concentrations the reaction rate is negatively dependent on the ligand concentration (during which the selectivity changes), whereafter it becomes independent of the ligand concentration. The hydroformylation results with the oSP, oSoOOP and oSpSSP ligands (Table 1) clearly show that when the thiomethyl group is in ortho-position the selectivity of the reaction changes, strongly favouring the formation of a-MFIB. Instead, in the case of the pSP and pSPpyr2 ligands where the thiomethyl group is located in the para-position the selectivity to I3-MFIB remains rather high as with the PPh3 ligand. Therefore, the effect of the thiomethyl group cannot only be an electronic one. Moreover, in the case of the oMeP ligand in which the methyl group simulates the plain steric effect of the substituent group in ortho-position the product distribution again resembles that of PPh3. This suggests that the sulphur atom indeed coordinates to rhodium so that the ligand acts as a bidentate one. Indeed, metal complexes in which the oSP ligand is bound to the metal centre through both phosphorus and sulphur atoms

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have been presented with rhodium [9]. The coordination ability of the thiomethyl group to rhodium was strongly influenced by the other groups in the phosphorus atom e.g. in the case of the oSPnaf2 ligand the naphtyl groups increased the steric hindrance and thus decreased the flexibility of the ligand disabling the coordination of the thiomethyl group. As a general trend there is a severe decrease in the conversion with all the ligands that contain the thiomethyl group in ortho-position compared to that of PPh3. The decrease is less severe when the thiomethyl group is in the para-position. In asymmetric hydroformylation with sulphur containing alkenes Nannon et al. [10] showed that the reaction rate is slower than the rate of a respective non sulphur containing alkene suggesting strong interaction between rhodium and sulphur. With the oSSP ligand the interaction is even stronger which explains the inactivity of the ligand. The ligand containing three methoxy groups also gave no reaction during five hours of testing. This is consistent with our previous findings with a ligand that contained only one methoxy group (oOP). However, interestingly the oSoOOP ligand was highly a-selective even though the structure contains two methoxy groups in ortho-position. If it is assumed that the reaction mixture contained roughly equal amounts of four different conformational forms of the in situ formed oSoOOP rhodium catalyst - one where only sulphur was coordinated to rhodium, three where sulphur was coordinated but one or two methoxy groups were blocking the reaction- the activity and selectivity were obtained through the oSP-type conformation. In fact, the activity of the oSoOOP ligand is approximately one fourth of the activity and the selectivity practically the same as obtained with the oSP ligand. Thus, the high ot-MFIB selectivity was obtained through the effect of the thiomethyl group, whereas the effect of methoxy groups was purely inhibitive for the activity. Eventhough there is not a linear correlation between the 31p NMR chemical shifts and the activity of a ligand, among the triphenyl type ligands having the possibility for bidentate binding (oSP, oSoOOP, oOOOP, oSSP and oSpSSP) there seems to be a trend: the activity decreases with the decreasing 31p NMR chemical shift.

0

~ 0

0

,H CH:~_/ Rh- H

H2

CH

0

0

CO

I / ~ SRh - B CO

O Rh/-- CO CH3-s/I

-o5CO CH3~ S-- Rh-- CO

-.... O b ~

CO y O ~Rh--CO CH3-s/I~

-CO +CO

-

CH3~

HI S--Rh--CO

0

H

~major path 0 i

CO I ~.~-O CH3~ S~ iRh---~

NN~nor path n-aldehyde

Scheme 1. The proposed reaction mechanism explaining high a-selectivity.

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Campi et al. [11] concluded that the chelation of thioether during the catalysis was the explanation of the high selectivity to branched isomer in case of alkenes containing sulphur. In case of bidentate binding the small biting angle of the oSP ligand favours equatorial-apical coordination to five-coordinated rhodium which enhances the formation of branched product [3, 12]. Scheme 1 shows the path to c~-MFIB formation based on Wilkinson's dissosiative mechanism. In case of equatorial-apical binding of the oSP ligand to the five-coordinated rhodium the sulphur and phosphorus atoms can occupy either apical or equatorial sites. Moreover, as a bidentate ligand the oSP five-coordinated rhodium complex is highly unsymmetrical. Therefore, the oSP ligand forms two different highly unsymmetrical equatorial-apical-coordinated complexes. We propose that the complex in which the sulphur atom is in equatorial and the phosphorus atom in apical position creates a stereoelectronically favourable coordination site for MMA so that only the formation of ct-MFIB is possible. The formation of the other equatorial-apical complex also occurs, but due to unfavourable stereoelectronic coordination does not lead to reaction partly explaining the substantially lower reaction rate observed with oSP when compared to that of PPh3. 5. CONCLUSIONS We have shown that the in situ formed oSP rhodium catalyst is highly selective catalyst forming the branched isomer in the hydroformylation of MMA. We have also shown that chelation control has a significant effect on the regioselectivity in MMA hydroformylation with triphenylphospine type ligands containing thiomethyl group in ortho-position. The high regioselectivity observed was explained by the stereoelectronic properties of the formed fivecoordinated H(alkene)Rh(CO)L2 intermediate. Work is continuing to evaluate the effect of substrate on regioselectivity and chelation control. Acknowledgements. The authors would like to thank Ms. Salla Tuulos-Tikka and Ms. Paula Sommarberg for performing part of the activity tests. REFERENCES 1. C.P. Casey, E.L. Paulsen, E.W. Beuttenmueller, B.R. Proft, L.M Petrovich, B.A. Matter and D.R. Powell, J. Am. Chem. Soc., 119 (1997) 11817. 2. L.A. van der Veen, D.K. Boele F.R. Bregman, P.C.J. Kamer, P.W.N.M. van Leeuwen, K. Goubitz, J. Fraanje, H. Schenk and C. Bo, J. Am. Chem. Soc., 120 (1998), 11616. 3. C.P. Casey and L.M. Petrovish, J. Am. Chem. Soc., 117 (1995), 6007. 4. H.K. Reinius, R.H. Laitinen, A.O.I. Krause and J.T. Pursiainen, Catal. Lett. 60 (1999), 65. 5. C.U. Pittmann, W.D. Honnick and J.J. Yang, J. Org. Chem., 45 (1980) 684. 6. H.K. Reinius and A.O.I. Krause, J. Mol. Catal. A: Chem., submitted. 7. D.W. Meek, G. Dyer and M.O. Workman, Inorg. Synth., 16 (1976) 168. 8. R.H. Laitinen, Acta Univ. Oul., A 324 (1999). 9. J.R. Dilworth, J.R. Miller, N. Wheatley, M.J. Baker and J.G. Sunley, J. Chem. Soc. Chem. Comm., (1995) 1579. 10. T. Nannon, N. Sakai, K. Nozaki and H. Takaya, Tetrahedron: Asymmetry, Vol 6 10 (1995) 2583. 11. E.M. Campi, W.R. Jackson, P. Perlmutter and E.E. Tasdelen, Aust.J. Chem., 46 (1993) 995. 12. P.C.J. Kamer, J.N.H. Reek and P.W.N.M. van Leeuwen, Chemtech, (1998) 27.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and LEG. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Highly Efficient Synthesis of N,N-Dialkylformamides from Carbon Dioxide and Dialkylamines over Ruthenium-Silica Hybrid Gels L. Schmid and A. Baiker Laboratory of Technical Chemistry, Swiss Federal Institute of Technology, ETH Zentrum, CH-8092 Zerich Fax: +41 1 632 11 63; e-mail: [email protected] N,N-dialkylformamides have been synthesized from 002, H2 and dimethyl- or diethylamines using homogeneous bidentate ruthenium phosphine complexes as well as silica hybrid gels containing these complexes. Hybrid xero- and aerogels were prepared via a sol-gel route employing ruthenium complexes functionalized by silylether groups and tetraalkoxysilane as precursors. Xerogels possessed pronounced microporosity and exhibited much lower activity than corresponding aerogels with mesoporous structure, indicating that beneficial use of the outstanding activity of the bidentate ruthenium complexes in a hybrid gel requires a suitable mesoporous structure. Bidentate ruthenium phosphine complexes as well as corresponding xero- and aerogels afforded 100% selectivity to the desired N,N-dialkylformamides. 1. INTRODUCTION

Carbon dioxide is gaining growing attention as a Cl-building unit in chemical synthesis [1, 2]. This attention is mainly spurred by the abundance, non-toxicity, and easy handling of CO 2. Catalysis, either homogeneous or heterogeneous, is a promising approach to CO 2 utilization in chemical synthesis. Hybrid gels, consisting of metal complexes or organometallic compounds which are anchored on an oxide network, allow the merging of advantageous features of both homogeneous and heterogeneous catalysts. Recently we have made use of this approach for designing novel catalysts for the synthesis of N,N-dimethylformamide (dmf) from CO 2, H2 and dimethylamine [3-7]. Monodentate ruthenium-phosphine complexes of the type RuCI2(PR2(CH2)2Si(OEt)3) 3 (R=methyl, phenyl) incorporated into a silica xerogel material proved to be excellent catalysts for dmf synthesis showing the highest activity achieved so far for heterogeneous catalysts. However, further improvement of these xerogel catalysts regarding stability of the active complex and suitable pore structure seemed to be possible. This prompted us to explore the catalytic activity of more stable bidentate ruthenium phosphine complexes and their potential for use in hybrid gel catalysts. Here we report a comparison of the catalytic properties of homogeneous bidentate ruthenium phosphine complexes with their corresponding hybrid gels and extend the previous studies of dmf synthesis to N,N-diethylformamide (def) synthesis.

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2. EXPERIMENTAL

2.1 Synthesis Bidentate ruthenium phosphine complexes RuCI2(dppm)2 (1, dppm= Ph2PCH2PPh 2, Fig. 1) and RuCI2(dppp) 2 (2, dppp=Ph2P(CH2)3PPh2) were prepared according to the procedure described in [8]. The synthesis of the corresponding functionalized sol-gel precursors, 1' and 2', and their ligands has been described in detail in [9]. Preparation and characterization of the hybrid xerogels derived from these precursors is also reported in [9]. Gels were prepared using different molar ratios ruthenium:tetraalkoxysilane, 1:50 (la, 2a), 1:100 (lb, 2b) and 1:200 (lc, 2c). For the synthesis of the aerogel 2d from 2', a modified procedure had to be applied. To a mixture of 15.22 g (0.10 mole) tetramethylorthosilicate (TMOS) and 7 ml tetrahydrofuran, 1.03 g 65% HNO 3 in 8.81 g H20, and 27 ml tetrahydrofuran was added dropwise. Subsequently, 0.727 g (5.10 -4 mole) of 2' in 20 ml tetrahydrofuran was slowly added. After stirring the homogeneous mixture overnight, 4.05 g (0.015 mole) trihexylamine in 15 ml tetrahydrofuran was given dropwise to the solution during 10 minutes. Within one hour the solid gel had formed. It was aged for 7 days. During this time the tetrahydrofuran which was released from the gel was replaced by ethanol. The gel was dried either by extraction of the solvent with supercritical CO 2 at 22.0 MPa and 41 ~ affording aerogels or by slow evaporation of the solvent, producing xerogels. Finally, the gels were washed with acetone and dried in vacuum at 100 ~ All steps, including aging, crushing, washing, and drying, were carried out under argon atmosphere.

2.2 Characterization and catalytic tests Solid state 31P-NMR spectra of the hybrid gels were measured with a Bruker AMX 400 at an operating frequency of 162.0 MHz, using 85 % orthophosphoric acid as external standard. CP/MAS technique was applied and 2048 scans were accumulated to obtain good spectra within reasonable time. 29Si NMR measurements were carried out on a Bruker AMX 400 at 79.5 MHz applying TMS as internal standard. Magicangle spinning was routinely used for all solid samples at 4 kHz spinning rate. For all samples 48 scans were accumulated. The repetition time was 200 s for single-pulseexcitation (SPE) with high-power decoupling. Nitrogen physisorption measurements of the specific surface area and mean pore diameters of the gel catalysts were performed at 77 K using a Micromeritics ASAP 2010 instrument, as described previously [6]. Catalytic tests were performed as previously described in [7] using a 500 ml stainless steel reactor. Experimental conditions of catalytic tests are specified in caption of Fig. 2. The products were analyzed by GC equipped with a TCD and a fused silica capillary column (Supelco SPB-1).

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3. RESULTS 3.1 Bidentate ruthenium phosphine complexes

The synthesized bidentate ruthenium phosphine complexes (1 and 2) used as active components are presented in Fig. 1. For incorporation into the silica matrix the

Ph2CI Ph2

Ph2CI Ph2 /!

P I P Ph2 Cl Ph2

~/h2C~l

1 (EtO)3Si"-~

(EtO)3Si

Ph2

2 fSi(OEt)3

Ph CI Ph ~Si(OEt)3 1'

(EtO)3Si\ ~

Ph Cl P ~ I .....

Si(OEt)3

P~" I "~P ~'-/---'~/h Cl P h ~ (EtO)3Si'--..~/ ~.i"Si(OEt)3 1

Fig. 1" Bidentate ruthenium phosphine complexes (1 and 2) and corresponding complexes functionalized with silylether groups (1' and 2'), which were used as precursors in the sol gel preparation of the xero- and aerogels. complexes were functionalized by introducing silylether groups affording 1' and 2', respectively. Fig. 2 compares the activities expressed as turnover frequencies (TOFs) of the non-functionalized (1 and 2) and corresponding functionalized complexes (1' and 2'). The turnover frequency of the non-functionalized ruthenium complexes measured in dmf synthesis was 190,000 h1 for 1 and 2,650 h-1 for 2. The functionalized ruthenium complex 1' showed a dramatic loss in activity with a TOF of 3,030 h-1, whereas 2' exhibited similar activity (2,300 h 1) as the analogous non-functionalized complex 2 (2,650 h-l). Precursor 2' was also tested in the synthesis of diethylformamide from CO 2, H 2 and diethylamine. TOF achieved under similar reaction conditions as applied for the synthesis of dmf amounted to 3,130 h -1 .

3.2 Hybrid gel catalysts Results of the physico-chemical characterization of the hybrid gel catalysts l a-lc and 2a-2c have been reported in [9]. Pore size distribution measurements of the synthesized aerogel 2d and xerogel 2e prepared from 2' are shown in Fig. 3. Note that 2d, which was dried by substitution of the solvent with supercritical CO 2 exhibited mainly mesopores with a maximum pore diameter between 30 and 40 nm. In contrast, conventional evaporative drying (2e) led to much smaller pores with a great amount of micropores. Aerogel 2d possessed a BET surface area of 670 m2/g, whereas for xerogel 2e it was 1,000 m2/g. The cumulative pore volume of the aerogel 2d, 1.74 cm3/g,

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for pores between 1.7 and 300 nm was much higher than that for the xerogel 2e with 0.44 cm3/g. 200 180 160 ~,~ ,-

3

1'

2

2'

1 o

Fig. 2: Activity of bidentate ruthenium phosphine complexes in the synthesis of dmf from CO 2, H 2 and dimethylamine. Black bars represent results for the nonfunctionalized complexes 1 and 2, whereas the open bars show the activity of the corresponding complexes, 1' and 2', with functionalizing silylether groups. Reaction conditions: T=100 ~ P[H2]=8.5 MPa, p[CO2]=13.0 MPa, stirring rate=300 min 1. Note the different scale for 1. Solid 31p CP-MAS NMR measurements were carried out for 2d and 2e. The spectra showed only one prominent peak, which was also the case for liquid 31p NMR of 2' [9] supporting the non-destructive incorporation of the metal complexes within the silica matrix. The degree of condensation (cross linking) of the aerogel 2d was examined by 29Si SPE NMR. Three different species of Si atoms with four oxygen neighbours were detected: fully condensed (Q4, 47%) moieties, species with one hydroxy group (Q3, 47%) and species with two hydroxy groups (Q2, 6%). Due to the low metal complex content and the noise of solid state NMR, Si atoms with three oxygen neighbours directly connected to the complex ligands were not discernible. Catalytic tests for the synthesis of dmf (Fig. 4) were carried out for hybrid gel catalysts la-r and 2a-r Both types of catalysts exhibited similar activity with TOFs between 650 (la, lb) and 970 h-1 (2r A decrease in metal loading generally afforded an increase in activity. In the synthesis of diethylformamide from CO 2, H 2 and diethylamine aerogel 2d and xerogel 2e were tested. Repetitive experiments with 2d showed a reproducibility of 3%. After a parametric sensitivity investigation a reaction temperature of 110 ~ a total pressure of 18 MPa, and a stirring rate of 1,000 rpm were applied. The results are shown in Fig. 5 together with the cumulative pore volumes of 2d and 2e. The activity of aerogel 2d with a TOF of 18,400 h1 exceeded that of the xerogel (TOF

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2,210 h"1) by a factor more than 8. A clear relation between the pore volume and the activity is discernible.

12. p- 1200 CO ";-9 1000.

•E'-•

+ Adsorption o Desorption

C~

4

03

E 8o0 .o. "10 ..Q i_ O

e}

"{3

E o

>

Eo 2

600 400,

0

200 0

Aerogel 2d

oo

o'2

o'4

o'6

o'8

9 . . ~f~---A....., _erogel 2d

"0

1.0

0

1

R e l a t i v e P r e s s u r e (P/Po)

9

..Q i_

o

"10

<

200-

50 0

[nm]

~1.0-

/

.0.6 ~0.4 -

100

E

1000

"TE 1.2"

/

2501

>O

100

1.4

3001

150

"1'0

Pore Diameter

13_ 400 I-Or) 350 ~r E o

.......

. . . . . . .

+ Adsorption

o Desorption

0.0

o'2

0.2-

Xerogel 2e

o'4

o'6

o'8

R e l a t i v e P r e s s u r e (P/Po)

.0

0.0

Xerogel 2e . . . . . . . .

i

10

9

Pore Diameter

-!

100

. . . . . . . .

1000

[nm]

Fig. 3: Isotherms and pore size distributions resulting from different drying method of the gels derived from precursor 2'. Top: aerogel 2d, bottom" xerogel 2e. 4. DISCUSSION Bidentate ruthenium phosphine complexes as catalysts in the synthesis of dmf and def from CO2 were found to be very active with a selectivity of 100% to dmf and def, respectively. The loss of activity in dmf synthesis by the complexes with functionalized ligands can be explained by electronic influences of the ligands on the metal atom. However, the activity is still higher compared to functionalized monodentate Ru complexes [6]. In addition, bidentate ruthenium phosphine complexes are stable in air. Non-destructive immobilization of the functionalized bidentate ruthenium complexes was supported by 31p NMR. The active complexes are incorporated as single entities within the silica matrix, as proven by EXAFS measurements for hybrid gels synthesized from monodentate Ru precursors [6]. A significant amount of incompletely condensed Si species exist in the hybrid gels. The texture of the hybrid gels has a strong influence on the catalytic activity, as

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demonstrated in diethylformamide synthesis. The aerogel with pronounced mesoporous structure derived from 2' exhibits a much higher activity than the corresponding xerogel with mainly micropores. The catalytic performance, of the xerogel is affected by the non-accessibility of active sites and intraparticle diffusion limitation, whereas the aerogel possesses pores are large enough for efficient diffusional transfer. 1000 800-

la

lb

lc

2a

2b

2c

600,_..= LL 400O 1-2000

20 1816,--~ 14~,.~_, 12108-

I-1:50

1:100 1:200 1:50 1:100 1:200 molar ratio c o m p l e x : T E O S

Fig. 4: Activity in dmf synthesis of hybrid gel catalysts containing different loading of bidentate complexes 1' (left) and 2' (right).

420

2d

.-

~--.

s S 2'

2e

,x Precursor

Xerogel

Aerogel

2.0 1.8 1.6 1.4 1.2 .1.0 -0.8 -0.6 -0.4 -0.2 o.o

"~ "-" ~ "~ ~ ~. >~ "5 E o

Fig. 5: Activity in def synthesis and cumulative pore volume of precursor 2' and xero- (2e) and aerogel (2d) derived from 2'.

5. CONCLUSIONS

The synthesis of N,N-dimethylformamide from 002, H2 and dimethylamine with hybrid gel catalysts containing bidentate ruthenium phosphine complexes has been successfully extended to that of N,N-diethylformamide. It seems that by applying different dialkylamines, the scope of possible N,N-dialkylformamide syntheses can be greatly extended. Mesoporosity has been shown to be a crucial requirement for achieving high activity of hybrid gel catalysts. REFERENCES

[1] [2] [3] [4]

[5] [6] [7] [8] [9]

A. Behr, Angew. Chem. Int. Ed. Engl. 27 (1988) 661. A. Baiker, in "Greenhouse Gas Control Technologies", (B. Riemer, B. Elliason and A. Wokaun, Eds), Elsevier, 1999, 391. O. Kr6cher, R. A. K6ppel and A. Baiker, in "3rd International Symposium on High Pressure Chemical Engineering", (P. R. von Rohr and C. Trepp, Eds), Vol. 12, Elsevier, 1996, 91. O. Kr6cher, R. A. K6ppel and A. Baiker, J. Chem. Soc., Chem. Commun. (1996) 1497. O. Kr6cher, R. A. K6ppel and A. Baiker, J. Chem. Soc., Chem. Commun. (1997) 453. O. Kr6cher, R. A. K6ppel, M. Fr6ba and A. Baiker, J. Catal. 178 (1998) 284. O. Kr6cher, R. A. KSppel and A. Baiker, J. Mol. Catal. A: Chem. 140 (1999) 185. R. Mason, D. W. Meek and G. R. Scollary, Inorg. Chim. Acta 16 (1976) L11. L. Schmid, O. KrScher, R. A. K6ppel and A. Baiker, Micropor. Mesopor. Mater. (1999), in press.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Metal complex catalyzed functionalization of naturally occurring monoterpenes: oxidation, hydroformylation, alkoxycarbonylation § E. V. Gusevskaya*, E. N. dos Santos, R. Augusti, A. O. Dias, P. A. Robles-Dutenhefner, C. M. Foca and H. J. V. Barros Dept. de Quimica, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte, Brasil Selective oxidation, hydroformylation and alkoxycarbonylation of monoterpenes, i.e., limonene, cz-pinene, 13-pinene, and camphene, via homogeneous catalysis by metal complexes were studied. "Pd(II)+reversible co-oxidant (CuCI2 or LiNO3)" systems were applied for the oxidation of monoterpenes with dioxygen. Hydrogen peroxide was also used as a final oxidant in the presence of the catalytic amounts of Pd(OAc)2 or Pd(OAc)2/benzoquinone. The Rh and Pt/Sn systems with various achiral and chiral diphosphines as ligands were applied for the hydroformylation of monoterpenes and the Pd/Sn/phosphine(diphosphine) systems for the alkoxycarbonylation, which provided the carboxylic acid derivatives. It was found that in all systems the starting monoterpenes as well as the primarily formed products can undergo various concurrent transformations, such as isomerization, hydrogenation, cyclization, addition, with the balance between the reaction pathways being delicate. Efforts were made to investigate the effects of the reaction variables and the catalyst composition on the product distribution in order to find the most favorable conditions for selective syntheses. Stereochemistry of the products was also studied. I. INTRODUCTION Naturally occurring monoterpenes are a useful source of inexpensive olefins. Selective functionalization of these olefins can provide oxygenated derivatives which are commercially important materials for perfumery, flavor, and pharmaceutical industries. Optically pure terpenes containing prochiral centers are frequently easily available and their stereoselective functionalization could be useful for the production of chiral synthetic intermediates. We have recently been involved in selective oxidation and carbonylation of monoterpenes, such as limonene (1), ct-pinene (2), 13-pinene (3), and camphene (4), via homogeneous catalysis by metal complexes. Although the metal complex catalyzed reactions of olefin oxidation and carbonylation have been developed into important synthetic methods [1], there is little information in the literature concerning their application to natural product synthesis. In the present paper the new and recently published results in this field are reviewed. +Supportedby FAPEMIG, CNPq, PADCT "To whomcorrespondence shouldbe addressed

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2. EXPERIMENTAL

All chemicals were purchased from commercial sources. R-(+)-Limonene, (-)-a -pinene, (-)13-pinene, and (-)-camphene were distilled before use. The synthesis of the Pt and Pd complexes were described in [2, 3]. The Rh complexes were synthesized by analogy with a method described in [4]. The oxidation reactions were carried out in a stirred glass reactor equipped with a sampling system and connected to a gas burette to monitor the gas uptake. The hydroformylation and alkoxycarbonylation reactions were performed in a glass lined stainless steel reactor. Typical runs were described in [2, 3, 5, 6]. The products were analyzed by GC (Shimadzu 14B instrument, Carbowax 20M capillary column) and identified by GC/MS (Hewlett-Packard MSD 5890/Series II instrument), IR (Mattson FTIR 3000/Galaxy Series spectrophotometer), ~H and ~3C-NMR (Brucker CXP-400 spectrometer) spectroscopy. Stereochemistry of some products was determined using NOESY experiments. 3. RESULTS AND DISCUSSION

3.1. Palladium catalyzed oxidation of monoterpenes by dioxygen The applications of the catalytic systems "Pd(II)+reversible co-oxidant (CuCI 2 or LiNO3)" for the oxidation of monoterpenes with dioxygen were investigated. Acetic acid solutions of limonene (1) containing PdCI2, CuCI2 and LiCI rapidly consume dioxygen (60-80~ 0.1 MPa) [5]. Allylic oxidation into carveyl acetate (5, 85% trans) and carvone (6) is the main reaction (Scheme 1), along with the HOAc and 1-120 additions resulting in ot-terpenyl acetate (7) and otterpineol (8) as well as isomerization into tx-terpinolene (10) and 7-terpinene (11) (Scheme 2).

PdCI2/CuCI2/UCI 1

5 (85% trans)

6

Scheme 1. Oxidation of limonene by dioxygen Both addition and isomerization reactions are reversible, with no allylic oxidation of 7, 8, 10 and 11, which contain no terminal double bond, being observed. A 7r-allyl Pd complex was suggested as a reaction intermediate, with the coordination of the exocyclic double bond on the same Pd atom being of a critical importance for its reactivity: e.g., carvomenthene (9) undergoes no allylic oxidation even at high temperature [7, 8]. The isomerization and addition equilibria are shifted towards the formation of I at higher conversions, thus, the selectivities for 5 and 6 rise significantly with the reaction time. Under the optimized conditions (PdCI20.01 equiv., CUC12-0.05 equiv., LiCI-0.19 equiv., 80~ 4 h) 90% conversion and the following product distribution were obtained: 5 (87% based on reacted 1), 6 (9%), 7+8 (3%), 10 (1%). The reactions of a-pinene (2) and 13-pinene (3) under similar conditions result in a mixture of 5 (ca. 25%), 7 (ca. 25%), bornyl chloride (12) (ca. 30%), and fenchyl chloride (16) (ca. 15%) [5]. Camphene (4) also undergoes a skeletal rearrangement and an acetic acid/water addition resulting in bornyl acetate (13) and borneol (14) as major products [6]. The results obtained show that the application of the PdCI2-CuCI2 system for the oxidation of bicyclic monoterpenes is limited, since CuCI2, acting as a Lewis acid, promotes the undesirable skeletal

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isomerizations. We concentrated our efforts on developing a CuCl2-free catalyst. The activity of the Pd(OAc)2-LiNO 3 system has been studied [5, 6]. Although nitrate ions readily oxidize the reduced Pd species in HOAc solutions and are reoxidized back by dioxygen, no formation of the oxidation products in significant amounts are observed for all examined monoterpenes.

2 ~OH

7 X=OAc 8 X=OH 9 X=H ~

10

11

12 X=CI 13 X=OAc 14 X=OH 15 X=OMe

16 X=CI 17 X=OAe

\ 23

24

26

Me

27

28

31

Scheme 2

3.2. Palladium catalyzed oxidation of monoterpenes by hydrogen peroxide Hydrogen peroxide is a cheap and strong oxidant, with water being formed as the only byproduct, which permits to develop environmentally friendly processes. Recently, a Pdcatalyzed allylic acetoxylation of internal and simple cyclic olefins using H202 has been described [9, 10]. We have found that some monoterpenes can be selectively oxidized by H202 using Pd(OAc)2 as a catalyst in HOAc/H20 solutions in the absence of halogens and co-metals, with the olefin structure greatly influencing the product nature [6]. 13-Pinene gives the allylic oxidation products, i.e., pinocarveol (18), pinocarveyl acetate (19), and myrtenyl acetate (20), with selectivity up to 75% at virtually complete conversion of 3 (Pd(OAc)z-0.02 equiv., 1-12023 equiv., 60~ 40 min) (Scheme 3). The competing skeletal isomerization and solvent addition yield various by-products, the major of them being 7, along with 13, 8, and bornyl acetate (17).

H202/HOAc

18 X=OH 19 X=OAc

3

20

Scheme 3 Oxidation of 13-pinene by hydrogen peroxide

~::: 4

Pd(O,ac)2 H20-#HO.ac

21

O~Ol-i O,~c

Scheme 4. Oxidation of camphene by hydrogen peroxide Camphene (4), which has the only and not easily abstractable allylic hydrogen at a bridgehead position, gives camphene glycol monoacetate (21) (Scheme 4) with selectivity up to 90% at 80% conversion (Pd(OAc)2-0.02 equiv., benzoquinone (BQ)-0.2 equiv., 1-1202-2 equiv., 60~ 120 min). The introduction of BQ in catalytic amounts up to BQ/Pd=10 exerts a beneficial effect on the catalyst stability and selectivity. However, no selective oxidation of limonene and ct-pinene was observed under similar conditions [6].

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3.3. Rhodium and platinum/tin catalyzed hydroformylation of monoterpenes

A number of monoterpenes have been hydroformylated in the presence of Co and Rh complexes [11-14], which are most commonly used in industrial processes The alternative Pt/Sn catalysts allow high regioselectivity and are especially efficient in asymmetric reactions, however, their applications to monoterpenes are scarcer [2, 15, 16]. We studied the hydroformylation of 13-pinene (Scheme 5) using Rh complexes and various P(III) compounds as auxiliaries (Table 1). For all catalyst precursors used the activity and product distributions are similar (2(o~-pinene)/22(10-formylpinane, 75% trans)~l/1) indicating that the same active species operate in all unpromoted systems (runs 1-4). Higher temperature favors the formation of the thermodynamically more stable trans isomer 22b. The addition of PPh3 or diphosphines, i.e., 1,2-bis(diphenylphosphino)ethane (dppe), 1,3-bis(diphenylphosphino)propane (dppp), and (+)-2,3-O-isopropylidene-2,3-dihydroxy- 1,4-bis(diphenylphosphino)butane ((+)-diop), decreases the isomerization of 13-pinene into less reactive c~-pinene and switches the hydroformylation selectivity from trans to cis isomer 22a (runs 5-8). These effects depend on the auxiliary nature, the temperature, and the Rh/P ratio (selected runs are given in Table 1). ,O COIH2 .. catalyst Pt/SnCI2 catalyst

H _ ~O

+

Rh

3

benzene

22a

22b

Scheme 5. Hydroformylation of 13-pinene Table 1 Hydroformylation of 13-pinene catalyzed by Rh complexes Run Precursor Auxiliary Conver Product distribution (%) sion (%) Aldehyde 22 (224/22b) ct-pinene (2) 1 [Rh(COD)CI]2 79 52 (23/77) 48 2 [Rh(COD)2]OTs 98 53 (25/75) 47 3 [Rh(COD)2]PF6 97 54 (24/76) 46 4 [Rh(COD)OAc]2 97 54 (24/76) 46 5 [Rh(COD)OAc]2 PPh3 62 88 (85/15) 12 6 [Rh(COD)OAc]2 dppe 9 83 (80/20) 17 7 [Rh(COD)OAc]2 dppp 30 79 (74/26) 21 8 [Rh(COD)OAc]2 (+)-diop 53 74 (63/37) 26 aReaction conditions: substratr (10.5 mmol), precursor(0.015 mmol in Rh), auxiliary (if any) (0.03 mmol in P) benzene (10 mL), 9 MPa (CO/H2= 1/1), 100~ 4 h. (COD-l,5-cyclooctadiene;OTs-p-toluenesulfonate) Limonene (1), 13-pinene (3), and camphene (4) were hydroformylated regiospecifically resulting exclusively in the linear aldehydes in the presence of the PtCI2P2/SnCI2/PPh3 systems (P=PPh3, P2 = dppe, dppp, and 1,4-bis(diphenylphosphino)butane (dppb)) (Schemes 5-7, Table 2). The hydroformylation of 3 yields trans-isomer 22b with a 98% diastereoisomeric excess (d.c.), while I and 4 give the diastereoisomeric mixtures of the aldehydes 29 and 25 (d.e. of ca. 10 and 15%, respectively). Differently from most of the Rh and Co catalysts, the isomerization of 13- into tx-pinene is slow (1-5% on reacted 3). Concurrent transformations of monoterpenes and the primarily formed aldehydes occur: hydrogenation of substrates (into pinane (24),

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isocamphane (28), and 9) and aldehydes (into alcohols 23 and 26), substrate isomerizations which can be accompanied by a nucleophilic addition (giving 2, tricyclene (27), 10-12, and 16), etc. The product distribution depends on the ligand nature and the Sn/Pt ratio (selected runs are given in Table 2). The use of PPh3 as the only P-containing ligand (run 2) and the excess of SnCI2 (Sn/Pt >1, run 3) promote monoterpene isomerizations. The system with dppp (run 4) causes excessive monoterpene hydrogenation. Under optimized conditions, chemoselectivities for aldehydes of near 90% have been attained for all monoterpenes studied, however, d.c. for 25 is low in all systems with achiral ligands. Asymmetric hydroformylation of 4 in up to 60% d.c. has been achieved with PtClz(PhCN)z/SnCl2/diphosphine catalysts containing chiral diphosphines such as (R)-(+)-binap (2,2'-bis(diphenylphosphino)-1, l'-binaphthyl) (run 6).

34a

4

CO0

OO/I-12

CO/MeOH

~"

COOMe PdCl2L21SnCl2 34b

benzene

+

CHO

PtCl2L21SnCl2 4

benzene

25a

CHO 25b

Scheme 6. Hydroformylation and alkoxycarbonylation of camphene ~COOMe 32

~

CO/MeOH

ooo2;C':n : snc'' 33

PtCl2t21SnCl2 1

HO

benzene

29

H 30a -(1 R, 2R, 4S, 5R) 30b -(1R, 2R, 4R, 5R)

Scheme 7. Hydroformylation/cyclization and alkoxycarbonylation of limonene Table 2 Hydroformylation of monoterpenes catalyzed by PtCI2L2/SnCIz Run SubL Conver Product distribution (%) strate sion Hydroformylation products Other products (%) Aldehydes {a/b }g Alcohols 1a 3 dppb 31 22 (85) {3/97} 23 (5) 24 (5) 12 (3) 2 (1) 2a 3 PPh3 25 22 (51) {3/97} 23 (30) 10+11 (3) 12+16 (11) 3 a'c 3 dppb 25 22 (33) {3/97} 23 (6) 10+11 (18) 12+16 (38) 4a 3 dppp 45 22 (75) {2/98} 23 (6) 24 (14) 12 (4) 5a 4 dppb 49 25 (92) {57/43 } traces 27 (4) 12 (2) 28 (2) 6b 4 PhCN 92 25 (84) {80/20} 26 (6) 28 (8) 27 (2) 7 a'd 1 dppb 10 29 (89) traces 9 (7) 10+11 (3) 8a'~ 1 dppb 19 29 (52) 30 (38) 9 (6) 10+11 (4) 9 a'f 1 dppb 95 29 (3) 30 (82) 9 (11) 10+11 (4) Reaction conditions: substrate (5 mmol), SnCI2-2H20 (0.05 mmol), benzene (5 mL), 9 MPa ( C O / I ' - I 2 = 1/1), 100~ 45 h. a PtCI2L2(0.05 mmol), PPh3 (0.1 mmol), b PtCI2(PhCN)2(0.05 mmol), R-(+)-binap (0.05 mmol). ~SnCIz-2H20- 0.25 mmol. d 4 h. e 20 h. f 130~gRatio of diastereomers. Under hydroformylation conditions limonene can be converted in one pot into bicyclic alcohol 30, useful as perfume, employing a PtCl2(dppb)/PPh3/SnCl2 bifunctional catalyst, which promotes both the hydroformylation of I and then the stereospecific intramolecular cyclization of menthene (29) [2]. At 130~ 30 becomes the dominant product (82% selectivity at 95%

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conversion, 30a/30b~l/1, run 9). High selectivities for 29 can be attained at lower conversions (runs 7, 8). In the absence of either a Pt complex or SnCl2 no cyclization of 29 is observed, thus, the heterobimetaUic Pt-Sn species seems to be involved in this reaction.

3.4. Palladium/tin catalyzed alkoxycarbonylation of monoterpenes Metal complex catalyzed alkoxycarbonylation represents another valuable route for the functionalization of monoterpenes which provides the carboxylic acid derivatives. We used the PdCI2P2/SnCI2/PPh3 systems as catalysts for alkoxycarbonylation of monoterpenes (P=PPh3, P2 = dppe, dppp, dppb) [3]. Limonene gives mainly the linear ester 32 (ca. 90%) along with 10, 11 and the methanol addition product 31, while in the absence of SnCI2 the branched ester 33 is also formed (ca. 10%) (Scheme 7) [3, 17]. The reaction of 13-pinene yields exclusively the products of the Lewis acid catalyzed skeletal rearrangement/nucleophilic addition (1, 2, 9-12, 16). No products of CO incorporation were observed. Camphene can be converted into the linear ester 34 (344/34b~1/1) with a chemoselectivity of 90% and virtually 100% regioselectivity (linear/branched esters) (Scheme 6). Methyl bomyl ether 15 is the only major by-product. SnCI2 and PdCI2(PPh3)2 exhibit a strong synergetic effect on the camphene alkoxycarbonylation. PdCI2(PPh3)2 alone shows a very low catalytic activity promoting the predominant formation of the thermodynamically more stable e x o isomer 34b (34a/34b ~1/2). REFERENCES 1. B. Cornils and W.A. Herrmann (eds.), Applied Homogeneous Catalysis with Organometallic Compounds, VCH, Weinheim, 1996. 2. A.O.Dias, R.Augusti, E.N.dos Santos and E.V.Gusevskaya, Tetrahedron Lett.,31(1997) 41. 3. L.L. Rocha, A.O. Dias, R. Augusti, E.N. dos Santos, and E.V. Gusevskaya, J. Mol. Catal., 132(1998)211. 4. R. R. Schrock and J. A. Osbom, J. Am. Chem. Soc., 93 (1971) 3089. 5. E.V. Gusevskaya and J.A. Gonsalves, J. Mol. Catal., 121 (1997) 131. 6. E.V.Gusevskaya, V.S. Ferreira, and P.A. Robles-Dutenhefner, Appl. Catal, 174 (1998) 177. 7. L.EI Firdoussi, A. Benharref, S. Allaoud, A. Karim, Y. Castanet, A. Mortreux, and F. Petit, J. Mol. Catal., 72 (1992) L 1. 8. L.EI Firdoussi, A. Baqqa, S. Allaoud, B.A. Allal, A. Karim, Y. Castanet, and A. Mortreux, J. Mol. Catal., 135 (1998) 11. 9. B. Akermark, E.M. Larsson, and J.D. Oslob, J. Org. Chem., 59 (1994) 5729. 10. C. Jia, P. Muller, and H. Mimoun, J. Mol. Catal., 101 (1995) 127. 11. A. J. Chalk, in Catalysis of Organic Reactions, P. N. Rylander, H. Greenfield, and R. L. Augustine (eds.), Marcel Dekker, Inc., New York, 1988, Vol. 22, p. 43. 12. S. Sirol and Ph. Kalck, New J. Chem., 21 (1997) 1129. 13. F. Azzaroni, P. Biscarini, S. Bordoni, G. Longoni, and E. Venturini, J. Organomet. Chem., 508 (1996) 59. 14. K. Soulantica, S. Sirol, S. Koinis, G. Pneumatikakis, and Ph. Kalck, J. Organomet. Chem., 498 (1995) El0. 15. L. Kollhr, J. Bakos, B. Heil, P. S~,ndor, and G. Szalontai,, J. Organomet. Chem., 385 (1990) 147. 16. L. Kollhr and G. B6di, Chirality, 1 (1995) 121. 17. T. Chenal, I. Cipres, J. Jenck, and Ph. Kalck, J. Mol. Catal., 78 (1993) 351.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Adsocat: Adsorption/catalytic combustion for VOC and odour control E. Kullavanijaya a, D.L. Trimm a and N.W. Cant b aSchool of Chemical Engineering & Industrial Chemistry, University of New South Wales, Sydney NSW 2052, Australia bDepartment of Chemistry, Macquarie University, Sydney NSW 2109, Australia

Abstract The concept of concentrating odours by adsorption coupled to desorpfion and catalytic combustion has been studied. Dilute streams of cyclohexene, thiophene, diethylamine and methyl methacrylate were concentrated by adsorption. For moist gases, activated carbon was found to be the best adsorbent with an adsorption capacity of 1 to 3x10 -3 mol g-~. Desorption of all adsorbed gases could be achieved by heating to 270~ or above. The catalytic combustion of desorbed gases was examined over Pt, Pd and Rh based catalysts. Of these, the Pd based systems offered the best compromise with all pollutants being destroyed at temperatures below 380~ The combined system is shown to be an efficient means of control of VOCs and odorous pollutants. 1.

INTRODUCTION

The use of catalytic combustion to control the emissions of volatile organic compounds (VOC) is now well established [1,2]. The wide range of fuel: air ratios and the lower operating temperatures of oxidation offer significant advantages over direct combustion. Various aspects of the technology have been reviewed [1,2,3], to show that precious metals or metal oxides are the preferred catalysts. Depending on the nature of the VOC, total conversion can be achieved between 140~ and 360~ [3]. A combination of VOCs or the presence of water can have a significant effect on catalyst performance [3]. The necessity to treat gas streams containing small amounts of odorous compounds or VOCs introduces added economic liability, in that the catalytic combustor temperature has to be maintained. To counter this, the concept of concentrating odours by adsorption coupled to intermittent desorption/catalytic combustion has been studied (Adsocat [4]). The limitations to the technology depend on the adsorption capacity of the adsorbent and/or the allowable emission limits of oxidised species such as SOx or NOx produced during combustion. Initial studies of Adsocat were focused on the control of hydrogen sulphide in sewage gases [4]. The present studies are concerned with the more general application of the technology to the control of organic, sulphur containing and nitrogen containing pollutants. The concentration by adsorption, the desorption and the catalytic combustion of cyclohexene, thiophene, diethylamine and methyl methacrylate have been studied. The adsorption capacities of activated carbon, 13X zeolite, KL, and HL zeolite and ~,-alumina for the various VOCs have been compared. Catalytic combustion over Pd, Pt and Rh based catalysts has been investigated.

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2.

EXPERIMENTAL

A flow system was used for all measurements. Activated carbon was obtained from Wright & Company Inc. 13X zeolite was obtained from Aldrich Chemical Company and zeolite KL powder from Engelhard Corporation. HL zeolite was prepared by ion exchange with nitric acid solution (2 mol/L). Ca. 0.5-1 g of adsorbent (particle size 200-300 lxm) was mounted in a 0.8 cm i.d. reactor suspended in a controlled temperature oven. Gas flows were controlled using Brooks 5850E mass flow controllers, and gas analysis was carried out gas chromatographically. VOCs were introduced by passing nitrogen through a controlled temperature bubbler to give concentrations of cyclohexene 3000 ppm, thiophene 2800 ppm, diethylamine 7000 ppm and methyl methacrylate 2000 ppm. A similar bubbler was used to produce a concentration of water (0.9%). Adsorbents were first dried (120~ overnight: 100 ml min -1 N2) and then cooled to 22~ A stream of VOC/nitrogen (100 ml min -1) was then admitted and the breakthrough curves [5] recorded. After equilibration, the temperature was ramped (5~ min 1) until no more VOC could be desorbed. The adsorption-desorption cycles were repeated until reproducible results were obtained. Catalytic combustion was then examined using precious metals suspended in a ceriaalumina washcoat on a cordierite monolith. A sample 1 cm diameter, 1.5 cm length was used. The catalysts (Johnson Matthey CSD Australia) contained 1.82 g L -1 Pd, 1.4 g L -1 Pt and 0.22 g L -1 Rh respectively. All catalysts were reduced before use (25% H2: 300~ 12 hours). Mixtures of individual VOCs (6500 ppm cyclohexene, 7000 ppm diethylamine, 2200 ppm methyl methacrylate or 5600 ppm thiophene) with 20% oxygen in helium were oxidised in the absence and presence (2%) of water. The total flow was 100 ml min -1. The temperature was increased at 5~ min -~ until the system lit off [6]. Conversion of VOCs was confirmed by gas chromatographic analysis, exit gases being separated on a CTR-1 column. 3.

RESULTS AND DISCUSSION

3.1 Adsorption-desorption Typical adsorption-desorption curves are shown in Figure 1. Adsorption capacities were measured easily from these curves. Desorbed gas often came off in a series of peaks, reflecting different strengths of adsorption on the various surfaces. Measurement of the adsorption capacities of dry gas showed that activated carbon and 13X zeolite were the preferred adsorbents (Table 1). However, 13X, KL and HL zeolites showed evidence of declining adsorption capacities with successive adsorption-desorption cycles. In addition, the amounts desorbed were found to decrease. Basic molecules such as diethylamine were irreversibly adsorbed on the acidic zeolites: polymerisation and coking were found to occur with methyl methacrylate and cyclohexene. Both effects reduced desorbed gas. The adsorption measurements were repeated in the presence of 0.9% water. The adsorption capacities of all zeolites for VOCs was found to decrease significantly, as a result of the preferential adsorption of water in the pore system [4]. Even the use of high silica zeolite did not improve the adsorption capacity to any large extent. The adsorption capacities of activated carbon were much less affected by moisture than the other adsorbents, and the bulk of the experiments were carried out using this adsorbent. Not all of the adsorbed gas could be recovered, but reproducible desorbed volumes were achieved over several cycles (Table 2). The temperature at which desorption took place was quite similar between VOCs.

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化

1 O

o~.

]tr- . . . . 9

0.8

0.6 0.4

>

a)

0.2 0 0

50

100

150

Time (min) 4000 3000 I 0

=s 2000 i0

\

1000

b)

~"~,

0 0

100

200

300

400

Temp ~ Figure 1 a) Adsorption of methyl methacrylate under dry conditions at 22~ b) Temperature programmed desorption of methyl methacrylate under dry conditions. 9 Activated carbon, 9 13X, 9 KL, o HL Table 1 Adsorption capacity at 22~ in the absence of water Adsorbents Apparent Adsorption capacity (mmol/g) surface area Methyl Cyclohexene Thiophene (m2/g) methacrylate Activated 764 2.6 2.9 3.3 carbon 13X zeolite 470 1.8 1.5 1.7 KL zeolite 152.5 1.5 1.1 1.1 HL zeolite 171 1.2 1.5 1.1 Gamma 258.9 1.1 0.3 0.5 alumina

Diethylamine 2.9 1.7 0.7 0.8

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Table 2 Amounts adsorbed and desorbed and temperatures at which desorption was complete, in the presence of water (0.9%) on activated carbon. Methyl Cyclohexene Thiophene Diethylamine methacrylate 1 st Adsorption (mmol/g) 4.0 3.7 3.3 2.1 cycle Desorption (mmol/g) 3.5 3.5 3.1 1.4 Temperature (~ 267 260 249 253 2 no Adsorption (mmol/g) 4.5 3.9 3.3 1.9 cycle Desorption (mmol/g) 4.0 3.5 3.1 1.4 Temperature (~ 265 263 252 256 3r~ Adsorption (mmol/g) 4.5 3.5 3.1 2.1 cycle Desorption (mmol/g) 3.8 3.1 3.0 1.3 Temperature (~ 268 265 256 253

3.2 Catalytic oxidation

Catalytic oxidation of the four VOCs was examined over the Pd, Pt and Rh catalysts with ceria-alumina as the support. Typical conversion versus temperature curves comparing the three catalysts for the oxidation of a single VOC (cyclohexene), and showing the effect of 2% water on the oxidation of another (thiophene), are shown in Figure 2. The temperatures required for 20% and 100% conversion are summarised in Table 3 Pd is considerably more active than Pt for the oxidation of cyclohexene under dry conditions. This conforms to the observations of Yu Yao [7] for three different unsaturates, propene, toluene and 1-hexene. In that study Rh was only slightly, if at all, less active than Pt whereas the difference is much greater here. However two factors need to be borne in mind. Firstly, the Rh catalyst used here contained only one-seventh as much metal as the Pt catalyst which is equivalent to a change in the temperature needed for 20% conversion of ca 40~ assuming an activation energy of 90kJ/mol [7]. Secondly, the present measurements used 20% 09 compared to 1% in the study of Yu Yao [7]. Higher 02 concentrations will magnify differences in the activity of Rh relative to Pt since the kinetic order in Oa is positive for Pt but negative for Rh [7,8]. These two factors are probably sufficient to explain the lower activity of Rh seen here. The inclusion of water improves the activity of both Pt and Rh for the oxidation of cyclohexene (Table 3) possibly due to some steam reforming for which Rh is particularly active [9]. On the other hand, water inhibits the activity of palladium. Burch et al. [10] observed the same trend during methane oxidation and attributed it to conversion of an active layer of palladium oxide to inactive palladium hydroxide. The same explanation could apply for the present situation if the Pd surface was oxidised during the course of reaction by the large excess of O2 present and hydrolised in the presence of water. The same order of activity amongst the present catalysts (Pd > Pt > Rh) also applies for the oxidation of methyl methacrylate and diethylamine under dry conditions (Table 3). Oxidation of the ester over Pt and Rh showed some evidence of polymerisation/coking with formation of carbon dioxide falling short of that expected from loss of reactant. The large improvement in the activity of Pt when water is present is probably due to an inhibition of these processes or gasification of polymers once formed. In the case of diethylamine the activity order is changed when water is present. The Rh catalyst becomes at least as active as Pt and would be much more

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so if the comparison was made on an equal mass of metal basis. There is no obvious reason for the large effect of water with Rh alone. All the catalysts were substantially less active for the oxidation of thiophene than the other VOCs, with temperatures of over 360~ required for complete removal under dry conditions (Table 3). Palladium is now the least active catalyst, in conformity with its sensitivity to sulfur poisoning as seen in the methane/H2S system [11]. Water improves the activity of all three systems with the largest effect for Pd and the least for Pt, suggesting that it reduces the influence of sulfur poisoning probably by competing for active sites.

1O0

~, =

=

/

/

~ 9

60

~

_..

O

_

/

80

~

0

r

,"

I - -

100

_-,.,

~'

0

O

.,-.q ra~

>

/

/

.

,.-I

/

i

A

80

!

/ / /

/

'

/

m

.

/

! g 20

/

100

y-

o o

a)

i

6040-

b)

20-

/

I

200 Temp ~

300

400

0

r-

i

200

400

600

Temp ~

Figure 2 The dependence of conversion of temperatures for a) cyclohexene oxidation, no water, 9 9 Pt, 9 Rh and b) thiophene oxidation over Pd, 9 in the absence of water, o in the presence of water. Table 3 Comparison of precious metal catalysts for the oxidation of dry and wet (2% moisture)VOCs. VOC Catalyst Temperature ~ Dry Temperature ~ Wet 20% 100% 20% 100% conversion conversion conversion conversion Methyl Pt 183 226 131 187 methacrylate Rh 227 290 214 280 Pd 130 175 130 175 168 260 157 225 Cyclohexene Pt Rh 262 360 232 335 Pd 80 158 125 220 190 315 205 340 Diethylamine Pt Rh 264 354 204 312 Pd 140 266 125 280 Pt 309 360 296 360 Thiophene Rh 337 368 326 356 Pd 340 380 300 340

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Thus it is clear that the application of Adsocat to the control of odorous impurities in the gas phase is practicable. Activated carbon is the best adsorbent in moist streams, giving an adsorption capacity of ca. 1 to 3x10 -3 mol g-1. Desorption can be achieved by heating the carbon to above 270~ and catalytic combustion, preferably over a Pd based catalyst, occurs readily at temperatures above 360~ to completely destroy the pollutant. Continuous tests based on hydrogen sulphide and sewage gas have shown no loss of activity over 2 years operation [5]. Similar long term tests for odour control are in progress.

Acknowledgment Financial support by Mahanakorn University of Technology, Bangkok, Thailand is gratefully acknowledged. The authors also acknowledge Johnson Matthey Catalytic Systems Division for providing catalyst samples.

REFERENCES 1.

R.M. Heck and R.J. Farrauto, Catalytic Air Pollution Control, Van Nostrand Reinhold (1995). 2. J.J. Spivey, Ind. Eng. Chem. Res., 26 (1987) 216. 3. C. Lahousse, A. Bernier, P. Grange, B. Delmon, P. Papaefthimoiu, T. Ioannides, and X.E. Verykios, J. Catal., 178 (1998) 214. 4. V. Meeyoo, J.H. Lee, D.L. Trimm, and N.W. Cant, Catalysis Today, 44 (1998) 67. 5. V. Meeyoo, D.L. Trimm, and N.W. Cant, J. Chem. Tech. Biotechnol., 68 (1997) 411. 6. J.H. Lee and D.L. Trimm, Fuel Processing Technol., 42 (1995) 339. 7. Y-F Yu Yao, J. Catal., 87 (1984) 157. 8. N.W. Cant and W.K. Hall, J. Catal., 16 (1970) 220. 9. J.Rostrup Neilson, Steam Reforming Catalysts, Danish Technical Press, Denmark (1975). 10. R. Burch, F.J. Urbano and P.K. Loader, Appl. Catal. A., 123 (1995) 173. 11. J.H. Lee, D.L. Trimm, and N.W. Cant, Catalysis Today, 47 (1999) 353.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Structure sensitivity of the hydrocarbon combustion reaction over aluminasupported platinum catalysts T.F. Garetto and C.R. Apesteguia* INCAPE (UNL-CONICET), Santiago del Estero 2654, (3000) Santa Fe, Argentina The reaction kinetics, structure sensitivity, and in-situ activation of cyclopentane and methane combustion on Pt/A1203 catalysts of different metallic dispersion were studied. The reaction orders in oxygen were 1 (cyclopentane) and zero (CH4). Methane oxidation turnover rates did not change significantly by changing the metallic dispersion but the cyclopentane combustion activity increased dramatically with increasing Pt crystallite size. On both reactions, the activation energies did not change by changing the Pt dispersion. Results are interpreted in basis of two different reaction mechanisms over the metallic Pt active sites. I. INTRODUCTION Platinum-based catalysts are highly active for oxidative removal of small amounts of hydrocarbon from gaseous or liquid streams. The effect of varying the platinum particle size on the catalytic combustion of different hydrocarbons has been extensively studied [1-4], but the results obtained are conflicting, probably because correlation between catalytic activity and metallic dispersion depends on the type of hydrocarbon to be abated. Several papers on light alkanes combustion, namely methane [ 1], propane [5], and butane [4] have reported that alkane oxidation turnover rates increase with increasing platinum particle size. In contrast, in a recent study on the C2H4 combustion over platinum-supported catalysts Pliangos et al. [6] proposed that turnover frequency changes, which cannot be explained by structure sensitivity considerations, are caused by interactions between the metal crystallites and the carrier. Similarly, Papaefihimiou et al. [7] reported that the benzene oxidation turnover rate on Pt/A1203 strongly increases with increasing Pt particle size but does not change by changing the Pt dispersion on Pt/SiO2 and Pt/TiO2 catalysts. Palladium and platinum catalysts are often activated on stream, ab-initio of the hydrocarbon combustion reaction [5,8,9]. This phenomenon has been widely studied on palladium-based catalysts; in the case of methane oxidation, several authors have proposed that the initial activation period is caused by reoxidation from Pd metal or oxygen-deficient PdOl.x to more active steady state PdO species [8]. In contrast, very few papers have been published using platinum-based catalysts [5] and the causes of induction periods on platinum remain unclear. In a recent paper [9], we studied the structure and reactivity of Pt/A1203 catalysts for benzene oxidation at low temperatures. In this work we report results on the oxidation of cyclopentane and methane over a set of Pt/AI203 catalysts of different metallic dispersion and chlorine concentration. Our goal was to obtain further information on the catalyst activation phenomenon and on the sensitivity of hydrocarbon oxidation turnover rates to Pt crystallite size. "Corresponding author. Email: [email protected],fax: 54-342-4555279

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2. EXPERIMENTAL

A Pt(0.3%)/AI203 catalyst (catalyst A) was prepared by impregnation at 303 K of a highpurity 7-A1203 powder (Cyanamid Ketjen CK303) with an aqueous solution of H2PtCI6.6H20 and HCI. After impregnation, samples were dried 12 h at 393 K and heated in air stream to 773 K. Then the chlorine content was regulated using a gaseous mixture of HC1, water and air. Finally, the sample was purged with N2 and reduced in flowing H2 for 4 h at 773 K. A set of three catalysts with different Do (Pt dispersion) was prepared by treating catalyst A in a 2% O2~2 atmosphere at 848, 873 and 903 K for 2 h (catalysts B, C, and D, respectively). The Pt dispersion was measured by 1-12 chemisorption by using the double isotherm method and a stoichiometric atomic ratio H/Pt~=I, where Pts implies a Pt atom on surface. The characteristics of catalysts A, B, C, and D are shown in Table 1. Tabla 1 Characteristics of the catalysts used in this work Catalyst Pt loading CI concentration (wt.-%) (wt.-%) A 0.30 0.95 B 0.30 0.61 C 0.30 0.58 D 0.30 0.60

Pt dispersion Do (%) 65 38 24 15

Hydrocarbon oxidation reactions were carried out at 1 atm in a fixed-bed tubular reactor. Cyclopentane (0.65%) or methane (2%) were fed in a 10% O2/N2 mixture. On-line chromatographic analysis was performed using a gas chromatograph equipped with a flame ionization detector and Bentone 34 or Porapak Q packed columns. Before gas chromatographic analysis, the reaction products were separated and carbon dioxide converted to methane by means of a methanation catalyst (Ni/Kieselghur) operating at 673 K. Two experimental procedures were used for catalyst testing. The complete oxidation of hydrocarbons was studied by obtaining curves of hydrocarbon conversion (X) as a function of temperature (light-off curves). The temperature was raised by steps of about 23 K, from 25 to 673 K (cyclopentane) or 913 K (methane). More fundamental differential reactor experiments (less than 10% conversion) were performed at constant temperature. 3. RESULTS AND DISCUSSION 3.1. Catalytic tests: Light-off curves Fig. 1 shows the X vs T curves obtained on catalyst A in two consecutive catalytic tests. The cyclopentane combustion started at about 473 K in the first run and the conversion increased then dramatically at ca. 553 K reaching a value of X ~ 100% between 663 and 673 K. The reaction was maintained at 673 K for 2 h and then the catalyst was purged and cooled down in nitrogen to 373 K. Subsequently, a second catalytic test was carried out. As shown in Fig. 1, the X vs T curve corresponding to the second run was clearly shifted to lower temperatures as compared to that obtained in the first run. Such a displacement of the light-off curves typically illustrates the catalyst activation phenomenon in hydrocarbon combustion reactions. To compare catalyst activities, we measured from light-off curves the value of the

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577 temperature at X = 50 %, T i,j, 5~ where i identifies the catalyst and j indicates first (1) or second (2) runs. The difference ATSO = TSO 50 is a measure of the i,l - Ti,2

100 80

-~

.

Methane

60

activation phenomenon on catalyst i. Table 2 shows that the AT5~ value for cyclopentane combustion was about 80 K. The CH4 combustion on Pt occurs at temperatures (T5~ = 823 K) significantly

4o

r

20 9 n

o =

!

1st. run 9 2nd. run

I

higher as compared with cyclopentane combustion (TS~ = 593 K). The consecutive

=

light-off curves for CH4 combustion were similar (Fig. 1), thereby suggesting that catalyst A is not activated ab-initio of this Fig. 1" Light-off curves on catalyst A reaction. On the other hand, we measured the Pt dispersion on catalyst A atter the second runs (D2, Table 2). By comparing the Do and D2 values in Table 2 it is inferred that the metal was severely sintered in both reactions after two consecutive catalytic tests. Similar experiments were carried out on catalyst C. For cyclopentane combustion, the light-off temperature in the first run (T c5~ = 518 K) was clearly lower than that obtained on 400

600

Temperature

8O0

1000

(K)

catalyst A; the initial catalyst activation was negligible (ATc5~ 5 K) as well as the metal sintering at~er two consecutive runs (Table 2). In contrast, for methane combustion the T~5~ and AT5~ values obtained on catalyst C were similar to those found on catalyst A. Tabla 2 Catalytic activity and Pt dispersion in two consecutive catalytic runs Catalyst Reactant Temperatures at X = 50% (K) T~~ T25~ AT 5~ A A C C

Cyclopentane Methane Cyclopentane Methane

593 823 518 823

513 818 513 823

80 5 5 0

Pt dispersion (%) Do D2 65 65 24 24

15 18 20 17

3.2. Low-conversion catalytic tests In order to establish the effect of the Pt crystallite size on catalyst activity, additional kinetically-controlled catalytic tests were performed In all the cases, the initial conversion was lower than 10%. The oxidation reactions were performed over catalysts A, B, C, and D at 443 K (cyclopentane) and 713 K (methane). For cyclopentane combustion, the activity of welldispersed catalyst A slowly increased with time on stream, but the turnover frequency (TOF, sl) on sintered catalyst D was constant along the 20 hour run. Over all the catalysts, methane oxidation rates did not change with time on stream. In Fig. 2 we have plotted the initial turnover frequencies as a function of the metallic dispersion. Cyclopentane combustion turnover rates

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increased drastically with the Pt particle size; the TOF value on catalyst D (Do = 15%) was about 40 times higher than that measured on catalyst A (Do = 65%). This result shows that cyclopentane combustion on Pt/Al203 catalysts is a structure-sensitive reaction preferentially promoted on larger Pt crystallites. On the contrary, for methane combustion the effect of Pt dispersion on the catalytic activity was rather weak and the turnover rate does not change significantly with Do (Fig. 2). All these results suggest that the existence of initial activation periods is related to the sensitivity of

~

n

e

.L_._

0.1 v

I.i_

o

0.01

I

6O

o0 (%) Fig. 2: Turnover frequency (TOF) vs Do the combustion turnover rate to the Pt crystallite size. 3.3. Kinetic studies

Kinetic data were interpreted by considering a power-law rate equation: #

r o = k(P~ )~ (P~2~, where r0 (mot HC/hg Pt)is the initial reaction rate. In Figs. 3 and 4 the ro values obtained on catalyst A for the oxidation of cyclopentane and methane were represented in logarithmic plots as a function of P~c and p00 2 respectively. Reaction orders cx and 13 were determined graphically from Figs. 3 and 4. The reaction orders for cyclopentane Pg= 0.126 atm Methane

~ o

o

,_o r -1 o

Cyclopentane

-2

443 K

-3 -4

o

_~

'

713 K

Methane

e-

-2

PHc = 0.02 atm,

Cyclopentane~._~

-3

_15

'

_'4

'

_~3

In Pnc Fig. 3" Reaction orders in the hydrocarbon

I

I

-2.4

PHC= 6.5 10 .3 atm, 443 K i

In P0 2

I

-2.0

Fig. 4: Reaction orders in oxygen

combustion were ot _-- 0 and J3 -= 1 while values of cz _=_1 and J3 --- 0 were determined for CH4 combustion. Similar values for ct and 13 were measured on catalyst C. On the other hand, we plotted the In TOF values as a function of 1/T for calculating the apparent activation energy (Ea) and the preexponential factor A of both reactions on catalysts A, C and D via an Arrhenius-type function. The apparent activation energies were 11 + 1 kcal/mol (cyclopentane) and 17 + 1 kcal/mol (CH4), irrespective of the mean Pt crystallite size of the sample. For

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cyclopentane combustion, we measured a A D/A A ratio of about 60. These results suggested that increasing the Pt particle size increases the density of active sites available for the ratedetermining step but does not modify the cyclopentane oxidation mechanism. The kinetic results show that CH4 and cyclopentane are oxidized by different mechanisms. The reaction orders obtained for cyclopentane combustion are well interpreted by considering that the reaction occurs via a Mars-Van Krevelen type mechanism [ 10], being the dissociative adsorption of oxygen on Pt the rate determining step. For cyclopentane oxidation on Pt, this mechanism may be represented by the following elementary steps: 02(g) + 2 L CP(g) + L

CP.L + O.L (CP...O).L

k~ > k2 >

20.L

k3 >

(CP...O).L

O.L

>

CP.L

CO2(g)+ H20(g)

where L represents the vacant active sites. The expression of initial rate r0 results: pO o klk3 o~PcP r0 = k,po ' + vik3PO p w h e r e v i is the stoichiometric coefficient of oxygen in the overall reaction.

(1) If k I < < k 3

Eq. (1) reduces to: klP~ 2 ro = - Vi

(2)

and the orders with respect to cyclopentane and oxygen predicted by Eq. (2) are 0 and 1, respectively, which are the approximate orders determined from our experiments. According to Eq. (2), any increase in rate constant k 1 accelerates the cyclopentane oxidation rate. The observed turnover rate increase with increasing Pt particle size would reflect therefore an increase in the density of reactive Pt-O species resulting from higher Pt oxidation rates. This assumption is consistent with previous work which showed that the number of Pt-O bonds of lower binding energy, i.e. the site density of more reactive surface oxygen, increases on larger Pt particles [2]. The initial activation of well-dispersed Pt catalysts in cyclopentane combustion would be caused by sintering of the metallic phase, which occurs in reaction conditions even if the cyclopentane the combustion reaction is performed at low-temperature and low-conversion regimes. The reaction is highly exhotermic and the Pt crystallite temperature is significantly increased in reaction conditions. Hot-spots on the metallic particles together with the presence of gaseous water cause the metal phase sintering at mild reaction conditions and the formation of larger, more active, Pt particles. The methane combustion has been interpreted by considering a Langmuir-Hinshelwood mechanism, where the rate-determining step is the abstraction of the first hydrogen on the

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

580

adsorbed methane molecule and oxygen chemisorption steps are not kinetically significants [8]. The proposed reaction pathways"

202+4.L

". K, > 4 0 . L

CI-I4+L

< K2 )

k

CH4.L + O.L CH3.L + 3 0 . L

>

CH4.L CH3.L+ OH.L

". K3 > C.L+3OH.L K4 < > C O2.L+2L K5 < > 2 H20(g) + 20.L + 2 L K6 < )' C02(g)+ L

C.L+20.L 40H.L

C02.L

leads to a complex kinetic rate expression: k K34K,K2 Pc., [Po~1,2

r= ,

+4K,K

,

+I

+2 K6

qKKK6 [p02]1/2Pc0212

When hydroxyl groups are the most abundant species, the initial rate expression becomes:

~ r0 -

PH2o

(3)

which is consistent with the observed experimental rate equation.

REFERENCES 1. K. Otto, Langmuir, 5 (1989) 1364. 2. P. Briot, A. Auroux, D. Jones and M. Primet, Appl. Catal., 59 (1990) 141. 3. M. Kobayashi, T. Kanno, A. Konishi and H. Takeda, React. Kinet. Catal. Lett., 37 (1988) 89. 4. R.F. Hicks, H. Qi, M.L. Young and R.G. Lee, J. Catal., 122 (1990) 280. 5. P. Mar6cot, A. Fakche, B. Kellali, G. Mabilon, M. Prigent and J. Barbier, Appl. Catal. B: Environmental, 3 (1994) 283. 6. C. Pliangos, I.V. Yentekakis, V.G. Papadakis, C.G. Vayenas and X.E. Verykios, Appl. Catal. B: Environmental, 14 (1997) 161. 7. P. Papaefihimiou, T. Ioannides and X.E. Verykios, Appl. Catal B: Environmental, 15 (1998) 75. 8. K. Fujimoto, F. Ribeiro, H.M. Avalos Borja and E. Iglesia, J. Catal., 179 (1998) 431. 9. T.F. Garetto and C.R. Apesteguia, J. Catal., in press. 10. P. Mars and D.W. van Kravelen, Chem. Eng. Sci., 3 (1954) 41.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Ceria-zirconia-supported platinum catalyst for hydrocarbons combustion : low-temperature activity, deactivation and regeneration Christine Bozo, Edouard Garbowski, Nolven Guilhaume* and Michel Primet Laboratoire d'Application de la Chimie/L l~Environnement (UMR 5634), Universit~ Lyon 1 (B~.t. 303), 43 Boulevard du 11 Novembre 1918, F-69622 Villeurbanne Cedex, France ABSTRACT A Ce0.67Zr0.ssO2 solid solution has been prepared by coprecipitation, and used as support for platinum, to investigate the effect of this support on the total oxidation of hydrocarbons. The total oxidation of C3I-I6 (in the presence of an excess 02) occurs at low temperature (120180~ but the catalyst deactivates rapidly when kept isothermally at 155~ This deactivation is reversible, and the activity is fully restored after decomposition of large amounts of surface carbonates. Methane conversion occurs at higher temperatures, but strong deactivations under isothermal conditions (especially at 350~ are also observed. Ex-situ characterizations of the deactivated catalyst as well as in-situ study of the effect of oxidizing and reducing treatments suggest that the deactivation in methane combustion is related to the presence of oxidized species linked to the support and/or to the metal. 1. INTRODUCTION Ceria-zirconia solid solutions tend nowadays to replace ceria as noble metals supports in three-way catalysis, because they show improved thermal stability and oxygen storage capacity compared to pure ceria [ 1]. The introduction of zirconia in the ceria lattice leads to a higher mobility of surface and bulk lattice oxygen [2], and to the enhancement of the catalytic activity under reducing conditions and after ageing at 1000~ [3]. Since the ceria-zirconia support seems to play an important role in oxidation reactions, we investigated its effect on the activity of a Pt/Ceo.67Zro.3302 catalyst for the combustion of two hydrocarbons: propene and methane. 2. RESULTS AND DISCUSSION

2.1. Experimental part The Ce0.67Zr0.ssO2 support was prepared by coprecipitation of an aqueous solution of cerium nitrate and zirconyl nitrate in ammonia. The precipitate was washed with water, dried and calcined at 700~ for 3 hours in an air flow. Pt(NH3)4(NO3)2 was impregnated from an aqueous solution, followed by calcination at 400~ for 12h under 02. The catalyst was then reduced at 300~ under hydrogen. The Ce0.67Zr0.s302 formula was deduced from chemical analysis. Temperature-programmed reduction (TPR) of the catalysts was performed under 1 vol.% H2 in argon, with a temperature ramp of 20~ 1, from ambient to 1000~ The solids were pretreated at 400~ in flowing air for 1 hour, then flushed with argon at the same * Corresponding author. Fax: (33) 4 72 44 81 14, E-mail: [email protected]

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temperature (1 h.), and cooled to room temperature under argon. Hydrogen consumption was measured with a TCD detector. 0.50 g catalysts were used in the activity measurements. Methane combustion was measured under isothermal conditions, between 200 and 800~ the temperature being increased by steps of 50~ and the catalysts kept at each temperature for 3 hours. The feed consisted in 1 vol.% CI-I4, 4 vol.% 02 and balance nitrogen (total flow: 6.4 l.h-1). The analysis of the products and unreacted methane was performed as described in ref. [4]. In the propene combustion experiments (0.3 vol.% C3I-I6, 2 vol.% 02, balance N2, total flow 12 l.h-1), infrared analyzers were used for the detection of C3H6, CO and CO:, while O: was analyzed with a paramagnetism analyzer. 2.2. Preparation and characterisation of the solids

The X-ray diffraction of the support alone after calcination at 700~ shows the diffraction lines corresponding to cubic ceria, with a slight displacement of the lines due to the incorporation of the smaller Zr4§ cation (0.84 A in cubic coordination) in place of Ce4§ (0.97 A in cubic coordination) [5]. The support pattern is not modified after impregnation. The amount of platinum, determined by chemical analysis, is 1.6 wt.%. The dispersion calculated from 1-12chemisorption isotherms is 40 %, which should correspond to an average particle size of 2.8 nm.. The surface area of the ceria-zirconia support (70 m2.gl aider calcination at 700~ is not modified aider Pt impregnation and subsequent thermal treatments (69 m2.gl). Temperature programmed reduction profiles of the ceria1200 zirconia support alone and Pt140 loaded are shown in Fig. 1. - 1000 Like pure ceria [6], Ceo.67Zr0.3302 is reduced in 800 90 t~ two peaks under hydrogen : 600 the first peak (maximum at 680~ is attributed to the 400 reduction of "surface" cerium, N 40 d while the second one 200 (~1000~ corresponds to the -j reduction of bulk Ce4+. -10 Actually, the hydrogen 0 5000 10000 consumption of the first peak Time (s) corresponds to the reduction of ~ 4 surface layers of Fig. 1: H2-TPR profiles of the Ceo.67Zro.3302 support (thin cerium, and to ~38 % of the total Ce4+ amount in the line) and Pt/Ce0.67Zr0.3302 catalyst (thick line). sample. The total H2 consumption in the TPR experiment represents the reduction of 62 % of the total cerium. Balducci et al. [7] obtained a similar reduction of cerium after H2-TPR of a Ce0.sZr0.502 sample of comparable surface area. The TPR profile of the Pt/Ceo.67Zr0.3302 catalyst is different : a very important hydrogen consumption occurs at ambient temperature, when the H2 / Ar mixture is sent on the catalyst. At the beginning of the temperature ramp, a small negative peak is due to the desorption of hydrogen chemisorbed on platinum. Two reduction

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peaks are seen at 150 ~ and 565~ (weak), while the reduction of bulk cerium at 1000~ is not modified compared to the support alone. The important feature is that the 1-12consumption at room temperature corresponds not only to the reduction of surface platinum, but also to that o f surface Ce 4+ (1.6 to 1.9 layers according to the hypotheses made for taking into account the hydrogen consumption at room temperature, i) surface reduction of Pt particles, ii) reduction of bulk PtO aider the oxidizing pretreatment). At the end of the TPR, more than 70 % of the total cerium is reduced into Ce ~+. Recently, Fornasiero et al. [8] studied the redox behavior ofRh-, Pt- and Pd-loaded Ce0.5Zr0.502 catalysts of high surface areas (53, 42 and 35 m2.g~ respectively) by TPR. No hydrogen consumption at room temperature was mentioned in the work of Fornasiero et al. and the amount of Ce 3§ was estimated to 6 1 % after the TPR. This difference may be ascribed to several points : the surface area of our Pt/Ce0.67Zr03302 catalyst is larger (69 m2.gl), and the Pt loading ~3 times higher. Furthermore, the Pt/Ce0.sZr0.502 catalyst (0.5 wt.% Pt) in ref. [8] was prepared from a H2PtCI6 precursor, and it has been shown for Rh/CeO2 catalysts that the use of RhC13 as precursor leaves a significant amount of chloride species on the support [9], which affects the chemisorption and redox properties of rhodium [9, 10], but also strongly disturbs the redox behavior of ceria [ 11 ]. The presence of platinum, nevertheless, promotes the support reduction in two ways : the surface cerium is reduced at a much lower temperature, and the overall reduction of cerium occurs in a larger extent. 2.3. Activity in propene combustion The low-temperature activity of the P t / C e 0 . 6 7 Z r 0 . 3 3 0 2 catalyst was tested in the oxidation of propene. Fig. 2 shows the evolution of the C3H6 conversion during a light-off experiment (temperature ramp 5~ Followed by stabilization of the temperature at 155~

The propene conversion increases rapidly between 130 and 155~ 100 180 (typical "S" conversion curve), where it reaches 77% (the catalyst temperature is shown on 120 o the figure, and it slightly r~ 50 temperature because of the o t~ 0 60 .-.o exothermicity of the reaction). :ff After the temperature ramp, the temperature was stabilized at 155~ The conversion decreases then very quickly, and is only 0 30 60 90 10% after 30 minutes dwell. It Time (min) further decreases and is only 7% at the end of the experiment. Since ceria is a basic oxide and Fig 2: Propene conversion in a light-off experiment carbonates rapidly in the (5~ followed by a temperature dwell at 155~ presence of CO2, the catalyst was heated under nitrogen between 20 and 300~ 9a CO2 desorption peak appears, which is maximum at 270~ This peak corresponds to ~ 250 ~tmol. CO2 per gram catalyst (or 3.6 l,tmol. CO2 per m2). Table 1 shows

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the results of a series of light-off measurements, followed by an isothermal deactivation at 155~ and separated by intermediate desorptions under nitrogen. The amount of CO2 desorbed is reproducible, and the light-off activity is fully restored after each desorption. In order to correlate these results with a poisoning of the catalyst by carbonates, the adsorption and desorption behavior of CO2 was studied by infrared spectroscopy. CO2 adsorbed at ambient temperature gives bands at 1587 and 1298 cm1, which can be attributed to bidentate carbonates [12, 13], and two broad bands in the 1550-1480 and 1415-1330 em~ domain, which probably correspond to bulk carbonates. Table 1 : Light-off activity and CO2 desorption from Pt/Ce0.67Zr0.3302 catalyst. Conversion temperature Desorption Experiment T2o (~ Ts0(~ CO2 desorbed Temperature of peak n~ (l.tmol/g. catalyst) maximum (~ 1 153 167 2 244 270 3 160 171 4 246 271 5 130 157 6 252 267 7 136 159 Upon desorption under vacuum at increasing temperatures, the bands due to surface bidentate carbonates decrease progressively between 100 and 300~ At 400~ only bulk carbonates (1460, 1400 cm 1) are present, and at 500~ nearly all the carbonate species are eliminated. This strongly suggests that the low-temperature poisoning of the propene oxidation reaction could be related to the formation of surface carbonates which remain adsorbed at 155~ Aider desorption of these species, the activity is fully recovered. No deactivation was usually observed for propene oxidation over Pt supported onto alumina. The loss of activity here observed is due to a poisoning of active sites present of the ceria-zirconia support. This conclusion stresses the participation of the CeO2 - ZrO2 support in the combustion process. 2 . 4 . A c t i v i t y in m e t h a n e c o m b u s t i o n

The activity of the Ce0.67Zr0.3302 support and of the Pt/Ce0.67Zr0.3302 catalyst for methane total oxidation is shown on Fig. 3. The solids were kept at each temperature for 3 hours. The most important feature to notice is that the ceria-zirconia solid solution is active for methane combustion : the conversion starts at 400~ and is total at ca. 800~ This means that the Ce0.67Zr0.3302 support presents active oxygen species which can oxidize methane. Addition of platinum leads to a very active catalyst (the conversion starts at 200~ which shows important deactivations under isothermal conditions, particularly between 350 and 450~ Since the deactivation is the most pronounced at 350~ we chose this temperature for in situ study of the parameters influencing it, and characterizations of the deactivated catalyst were performed at~er isothermal tests at the same temperature for 12 hours.

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100 Fig. 3 9Methane conversion on (ZX) Ceo.67Zro.3302 and (0)

~0 r~ l,,,

,~ 50

Pt/Ce0.67Zr0.3302.

O

0 200

400 600 Temperature (~

800

Because of the carbonatation of the support observed at low temperature, we tested the effect of a nitrogen purge at 500~ which should remove surface carbonates, as well as the effect of CO2 addition in the feed-steam (Fig. 4). A nitrogen treatment of the deactivated catalysts at 500~ has no effect on catalytic activity in methane oxidation. Similarly the introduction of 1 vol. % CO2 for three hours in the feed of reactants at 350~ does not modify the conversion 1 0 0 -~

1

5000 4000

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.o

3000

50 r~

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2000

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1000

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4000

6000

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Fig. 5: Effect of intermediate catalyst Fig. 4: Effect of CO2 addition (1 vol.%) in oxidation (thick line) and reduction (thin the feed (e), and of intermediate N2 purge line) on the deactivation at 350~ at 500~ (/x ) on the deactivation at 350~ of methane. Thus the deactivation observed at 350~ is not connected with a poisoning by surface carbonates. The catalyst was characterized in its deactivated state. The surface area of the deactivated solid is not different than that in the flesh state. The platinum accessible area measured by 1-12 chemisorption is also unmodified (40 %) for a sample aged at 600~ under reactants and subsequently hydrogen reduced. This suggests that the loss in the activity is not related to the catalyst sintering, or to metal encapsulation, as proposed for Pd/CeO2 upon reduction [14], and for Pd/Ce0.5Zr0.502 [15,16] and Pd or Rh/Ce0.vZr0.302 [17] upon high temperature redox aging. The effect of in-situ oxidation or reduction of the catalyst on the activity at 350~ is shown on Fig. 5. Starting from a deactivated state, in which the methane conversion is ~ 20%, the catalyst was first treated under 4 vol. % 02 for 2 hours, then the CH4 + 02 mixture was sent again. The conversion remains the same as before the treatment under oxygen. When an

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intermediate reduction under H2 for 1 hour is performed at 350~ the catalyst is strongly activated : the conversion recovers the value observed at the beginning of the deactivation, before decreasing again slowly. Finally the activity in CH4 combustion of the Pt/Ce0.67Zr0.3302 catalyst initially preoxidized under oxygen at 350~ is very similar to that of the reduced sample and the same deactivation on stream occurs. 3. CONCLUSION The ceria-zimonia support modifies the properties of platinum in oxidation reactions " it is active for methane oxidation, and probably participates to the reaction in the presence of Pt. At low temperature (ca. 200~ the support is quickly carbonated, which leads to a rapid but reversible deactivation. At temperatures higher than 300~ the carbonates species are not longer adsorbed. Nevertheless a loss in activity for CI-I4 total oxidation is observed. The activity is only recovered by a reduction step under hydrogen at the same temperature. The deactivation in isothermal conditions is due to the formation of poisoning species linked to the support and/or to the platinum particles, such species are reducible in the conditions of the combustion reaction. ACKNOWLEDGMENTS

The financial supports of GAZ DE FRANCE and ADEME are gratefully acknowledged. REFERENCES 1. J.P. Cuif, G. Blanchard, O. Touret, A Seigneurin, M. Marczi and E. Qu6m6r6, SAE paper 970463. 2. P. Fomasiero, R. Di Monte, G. Ranga Rao, J. Kaspar, S. Meriani and A Trovarelli, J. Catal., 151 (1995) 168. 3. J.G. Nunan, W.B. Williamson and H.J. Robota, SAE paper 960798. 4. N. Guilhaume and M. Primet, J. Chem. Soc. Faraday Trans., 90 (1994) 1541. 5. R.D. Shannon, Acta Cryst., A32 (1976) 751. 6. V. Perrichon, A. Laachir, G. Bergeret, R. Frdy, L. Toumayan and O. Touret, J. Chem. Soc. Faraday Trans., 90 (1994) 773. 7. G. Balducci, P. Fomasiero, R. Di Monte, J. Kagpar, S. Meriani and M. Graziani, Catal. Lett., 33 (1995) 193. 8. P. Fomasiero, J. Kagpar, V. Sergo and M. Graziani, J. Catal., 182 (1999) 56. 9. D.I. Kondarides and X. Verykios, J. Catal., 174 (1998) 52. 10. D.I. Kondarides, Z. Zhang and X. Verykios, J. Catal., 176 (1998) 536. 11. S. Bemal, J.J. Calvino, G.A. Cifredo and J.M. Rodriguez-Izquierdo, J. Phys. Chem., 99 (1995) 11794. 12. A. Trovarelli, Catal. Rev. Sei. Eng., 38 (1996) 439. 13. C. Binet, A. Jadi and J.C. Lavalley, J. Chim. Phys., 89 (1992) 1779. 14. A. Badri, C. Binet and J.C. Lavalley, J. Chem. Sor Faraday Trans., 92 (1996) 1603. 15. G.W. Graham, H.W. Jen, W. Chun and R.W. MeCabe, Catal. Lett., 44 (1997) 185. 16. J.C. Jiang, X.Q. Pan, G.W. Graham, R.W. MeCabe and J. Sehwank, Catal. Lett., 53 (1998) 37. 17. G.W. Graham, H.W. Jen, W. Chun and R.W. MeCabe, J. Catal., 182 (1999) 228.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Characterisation of a ~(-MnO2 catalyst used in VOC abatement C. Lahousse, C. Cellier, B. Delmon, P. Grange* Unit6 de Catalyse et Chimie des Mat6riaux Divis6s, Universit6 catholique de Louvain, 2/17 P1. Croix du Sud, B 1348 Louvain-la-Neuve, Belgium, Fax: 32.10.47.36.49 This study presents the results of a comprehensive characterisation of a very efficient VOC abatement catalyst. The changes suffered by a ~,-MnO2 catalyst during VOC oxidation are determined and their impact on the catalytic activity is discussed. Sintering and partial reduction are detected. But they respectively have only a limited and no effect on the stability of the catalytic activity. Conversely, water vapour adsorption appears to cause very long (12h) stabilisation delays. 1. I N T R O D U C T I O N In a recent paper [ 1], we have shown that the nsutite (T) form of MnO 2 is a very promising VOC removal catalyst which is more active and in many respects superior to conventional catalysts based on noble metals. A 150-hour test showed that this catalyst was able to maintain its activity over a long period of time. However, it sometimes presented an important decrease of activity during the first hours of operation. In some conditions T-MnO_~ activity was decreasing during several hours before stabilising while in other conditions, the activity was stable after a few minutes. The aim of this paper is to characterise the modifications suffered by this catalyst as a function of the reaction conditions in order to determine the origin of this decrease of activity. Surface area, XPS and IR measurements are performed to detect and evaluate possible sintering and reduction phenomena. Specific tests are performed to determine the effects of reduction and reactant adsorption on the catalytic activity.

2. EXPERIMENTAL 2.1. XRD Changes in the T-MnO 2 lattice parameters were assessed using a Siemens D5000 powder diffractometer operating at 20 kV with copper Ka wavelength. The shift of the XRD pattern is quantified in table 1 by pointing the position of the most intense peak.

* Financial support by the Commission of the European Union (Contract No. EV5V-CT93025) is gratefully acknowledged. Special thanks are also due to Mrs Piton of SEDEMA-SADACEM Belgium for providing the catalyst. The authors also thank the R6gion Wallonne is for funding on-going research in this area (Convention 971/3667).

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2.2. N 2 physisorption The textural characteristics of fresh and used catalysts were measured on a Micromeritics ASAP 2000 sorptiometer. The measurement was performed on samples outgassed at 150~ As manganese oxides are known to be sensitive to outgassing [2], the applicability of this treatment was checked. A measurement on a sample outgassed at 80~ which gave consistent results confirmed that the treatment at 150~ was still appropriate. 2.3 XPS XPS was used to detect possible changes in the catalyst surface composition (e.g. coke deposition or alkaline segregation) and to evaluate the modifications of the oxidation state of the surface Mn ions. As indicated by the literature [3,4], the most sensitive measurement of Mn oxidation state is given by the distance in eV between the Mn3s main peak and its shakeup satellite. This is the value reported in table 1. The apparatus used is a Surface Science Instrument spectrometer (SSI 100) working with monochromatised AI ka radiation(1486,6 eV). 2.4 FT-IR The lattice vibration of fresh and used catalysts were studied using IR spectroscopy. The results were interpreted using the work of Potter and Rossman [5]. For this study, 1 mg of catalyst was diluted in 120 mg of dried KBr. The diluted powders were pressed into cardboard supported pellets and placed in a small MIDAC FT-IR spectrometer.

2.5 Catalytic activity measurement In this work, 2 types of tests are presented. The variation of conversion as a function of time is measured either using a low concentration mixture of ethylacetate (ea) and n-hexane (hex) (250 ppm each) or with a very high concentration of n-hexane (20 000 ppm). Low concentration tests are performed at 150~ (unless otherwise specified), high concentration ones at 220~ For the evaluation of water effect, 20 000 ppm of water vapour were added to the low concentration stream. The VVH is, in all cases, 72 000 h -l. Catalyst activation and reactivation were performed using a flow of O 2 (or when specified of N2) for 30 minutes. For some of the tests, conversion was first measured as a function of time, then as a function of temperature from 0 to 100 % conversion. These tests are marked as "final conversion 100%" in table 1 as this has an influence on the characterisation results. More details concerning test procedure can be found elsewhere (1,6). 3. R E S U L T S The characteristics of the used catalysts are presented as a function of the reaction conditions in Table 1. This table shows that : - Surface area decreases (from 100 m2/g to 80 m2/g) in the presence of added water vapour or when the VOC concentration is very high (20 000 ppm). - The XRD pattern of catalysts shifts when the samples are coming from high - concentration experiments finished below 100% conversion. - A reduction of the average oxidation state of Mn ions at the surface is detectable by XPS. This reduction is observed in all conditions as long as the test is stopped below 100% conversion. XPS and IR characterisations also showed that no coke formation occurred whatever the conditions. XPS carbon content does not increase and no new band is observed when

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performing the IR analysis of used samples. IR spectra nevertheless change. The IR lattice vibration bands are significantly broadened in the spectra of the sample with a high VOC concentration. The initial spectrum is recovered after catalyst reactivation with 0: but not when reactivation is performed with N 2. As underlined in the introduction, once stabilised, ?-MnO~ is able to maintain its activity over a long period of time but an important decrease of activity was observed at the beginning of the test. The length of this stabilisation period varies enormously (between l0 minutes and more than 12 hours) and depends upon the reaction conditions. Our experiments showed that : the lower the VOC concentration, the longer the stabilisation the higher the conversion, the quicker the stabilisation the addition of water in the inlet stream dramatically shortens the stabilisation time. -

-

-

4. DISCUSSION The surface area was measured at each step of a typical reaction procedure with a high concentration. The results are presented in figure 1. The variation of surface area as a function of time shows that sintering, when it happens, mostly occurs during the activation and at the beginning of the experiment. The surface area rapidly reaches 80 m:/g and seems very stable afterwards. The catalytic activity diminishes accordingly. The stabilisation period can be observed in the absence of sintering (low VOC concentration, no water addition) or with presintered (reactivated) catalyst.

Hexane conversion on a fresh sample and after 1,2 and 3 reactivations 70% m2/g

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The shift of the XRD pattern of ?-MnO: has been observed previously in the literature [7]. It happens when this oxide gets partially reduced. This partial reduction is also detectable by IR and results in the broadening of the IR structural vibration bands. When working with nonrealistic concentrations of VOC, partial reduction of the catalyst occurs. However, the nsutite structure is always conserved. ?-MnO 2 is not reduced to lower oxide (Mn203) and keeps its activity. Partial reduction of the bulk does not impair long term stability. As shown by IR and XRD, bulk reduction is reversible. Treating a used catalyst for 30 minutes under oxygen (or letting it work 1 hour at 100% conversion) is enough to recover the original XRD pattern and IR spectrum. XPS shows that bulk reduction begins by a surface reduction. Unlike bulk reduction, surface reduction takes place whatever the concentration. It remains a reversible phenomenon since it can not be observed any more on catalysts used in experiments finished at 100% conversion. In order to determine the effect of reduction on the stabilisation period, the stabilised catalyst was reactivated by a flushing with either 02 or N 2. Figure 2 compares the results of the two reactivation methods. The reactivation proves to be equally efficient with either gas. Although XRD, XPS, and IR indicate that N 2 reactivated samples are as reduced as the original used ones, they regain as much activity as the reoxidised ones. This clearly indicates that the long stabilisation period is not due to the reduction of the catalyst surface or the consumption of MnO 2bulk oxygen. By elimination, it is thus possible to attribute the appearance of the very long stabilisation period to the adsorption of one of the reaction products, namely water vapour. This conclusion is in agreement with all our observations and is particularly confirmed by the fact that, as shown in figure 3, water vapour addition leads to a very rapid stabilisation.

Catalyst reactivation with oxygen I initial activity

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fresh 1 2 3 number of reactivations N 2

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conversion of 250 ppm n-hexane as a function of time

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Fig. 3. Effect of water vapour on the duration of the stabilisation period. 5. CONCLUSIONS The modifications of our catalyst in different conditions have been determined. Sintering takes place at the beginning of the experiments and does not impair the long term stability of the catalyst. A reversible partial reduction of ~,-MnO2 occurs but seems to have no effect on the catalytic activity. The decrease of activity recorded during the first minutes or day is certainly due to the hydration of the catalyst surface. This communication provides an example of the unusual phenomena encountered when dealing with low temperature processes in environmental catalysis. In these types of application, air moisture is almost always present and begins to significantly affect the catalyst behaviour when the reaction temperature approaches 100~ REFERENCES

1. 2. 3. 4. 5. 6.

C. Lahousse, A. Bernier, P. Grange, B. Delmon, J. Catal., 178 (1998) 214. F. Kapteijn, L. Singoredjo, A. Andreini, J.A. Moulijn, Appl. Catal. B., 3 (1994) 173P. D.A. Shirley, Phys. Scripta, 11 (1975) 117. B.W. Veal, P. Paulikas, 51, 21 (1983) 1995. A.M. Potter, G.R. Rossman, Am. Mineralogist, 64 (1979) 1199. C. Lahousse, A. Bernier, E. Gaigneaux, P. Ruiz, P. Grange, B. Delmon, in " 3 rd World Congress on Oxidation Catalysis" (R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons, Eds.), Stud. Surf. Sci. and Catal., 110 (1997) 777. 7. R. Ruetschi, J. Giovanoli, J. Electrochem. Soc., 135-11 (1988) 2663.

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Table 1 Conversion and characterisation results as a function of reaction conditions test conditions: catalyst m a s s d , l g ; VOC=n-hexane, VOC concentration= 20000 ppm (reactivation experiments) or 250 ppm (TvaFiation experiments) ; canier flow=l00 cmYmin ; carrier nature=air +(when specified) 20000 ppm H 2 0 .

procedure 220°C, activation & reactivation 0 2 22OoC, activation & reactivation 0 2 220°C. activation 0 2 & reactivation N2 220°C, activation 0 2 & reactivation N2 220°C. activation & reactivation N2 220°C. activation & reactivation N2 220°C, activation & reactivation 0 2 220°C, activation 0 2 & reactivation N2 220°C. activation & reactivation N2 stabitisation 16h 150°C + T variation stabilisation 16h 150°C + T variation + H 2 0

conversion % after 1 minute I h 30 12h 5 minutes I hour after reactivation

63 59 57 100 94

45 38

38 79 22

52 46 46 37 20

41

37 36

sample catalyst changes Remarks characterised S BETl(m2tg) XRD dhkl1A XPS DMn3sIeV after before after before after before after 95 2.42 2.41 4,9 4,8 activation 100 reactivation 100 84-80 2.42 2,41 4,9 4,7 activation 100 95 2.42 2,41 4.9 4.8 reactivation 100 80 2.42 2.44 4.9 5.2 activation 100 2.42 2.42 4.9 4.9 reactivation 100 80 2.42 2,44 4,9 5,2 full test 100 80 2.42 2.44 4,9 5,2 full test 100 80 2.42 2.44 4.9 5,2 full test 100 80 2.42 2,44 4,9 5,2 full test 100 100 2.42 2,42 4.9 4,9 final conversion 100% full test 100 80 2.42 2.42 4,9 4,8 final conversion 10096

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

New alumina/aluminium monoliths for the catalytic elimination of VOCs N. Burgosa, M. Paulisa, A. Gilb, L.M. Gandia b and M. Montes a aGrupo de Ingenieria Quimica, Dto. de Quimica Aplicada, Fac. de C. Quimicas, UPV/EHU, Apdo 1072, E-20080 San Sebasti~in, Spain bDto. de Quimica Aplicada, Universidad POblica de Navarra, Campus de Arrosadia, E-31006 Pamplona, Spain Metallic monolithic catalysts have been prepared and tested for the abatement of VOCs. The monoliths are based on a A1203/A1 cermet produced by the controlled anodization of aluminium foils, and have been impregnated either by a noble metal (Pt or Pd) or by a transition metal oxide (Mn203). Both kinds of monoliths presented high activity for the complete oxidation of toluene, above that of the powder catalysts. I. INTRODUCTION Catalysts for VOCs oxidation are usually prepared over monoliths. The advantages of monolithic catalysts are the very low pressure drop, the high external surface area, the uniformity of the distribution of the flow within the honeycomb matrix to improve the pollutant-active site contact, etc. Ceramic monoliths are most commonly obtained by extrusion, and the cost of largescale production is very low [1 ]. The manufacture of metallic monoliths is easy and cheap for small series. However, the fixation of a porous catalyst on the surface of the metal is not an easy matter. With regard to this adhesion problem, the surface oxidation properties of aluminium offer a very interesting choice to grow up a porous and adherent layer of alumina, by a controlled anodization process. This work deals with the different variables of the anodization process, to produce a catalytically suitable support. Monoliths prepared with these alumina/aluminium cermets were used to prepare catalytic devices by impregnation with noble metals or manganese. The samples were characterised by different physico-chemical techniques and the catalytic activity was measured in the complete oxidation of toluene.

Financial support by MEC (CICYT-QUI97-1040-CO3), UPV/EHU, Gobierno Vasco, Departamento de Educaci6n y Cultura del Gobierno de Navarra (Ordenes Forales 557/1996 y 143/1998) and Universidad Pfiblica de Navarra are gratefully acknowledged. Aluminium sheets supply by INASA is acknowledged.

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2. EXPERIMENTAL PROCEDURE 2.1 Preparation of the monolithic substrate: anodization Four variables were chosen to study the anodization process: electrolyte concentration, current density, anodization time and temperature. The choice of the electrolyte can vary the A1203 final properties: surface area, porosity, thickness of the layer, etc. Based on literature data [2] H2SO4 has been chosen as the electrolyte in order to obtain satisfactory A1203 layers. An air bubble assisted cooling system was used to eliminate the local temperature rises produced by the anodization process [3]. The study of the influence of the variables on the properties of the alumina produced was carried out by N2 adsorption, gravimetry and SEM. The amount of formed A1203 was calculated by gravimetry, dissolving the A1203 layer with a phosphoric-chromic mixture. The process yield was calculated as the ratio between the experimentally measured layer of alumina and the theoretical one calculated using Faraday's rule, and the dissolved amount of A1203 was calculated as the difference between those values. Figures l, 2 and 3 show the amount of A1203 formed per m E of aluminium foil, the amount of A1203 dissolved per mE of aluminium and the specific surface area of the alumina formed during the anodization process as a function of: time, current density, and electrolyte concentration. The effect of the temperature follows the same trend as the electrolyte concentration does.

40

200~ m

~, 120

30 -]

i

I

o,,.~

"~..~ 80

~

o

....

9

-

/il

20

f

//

o o

o

Fig.2. Amount ofA1203 generated (O), A1203 dissolved (ll) and m ~ A1203/g A1203 formed ( , ) for varying current densities.

50

4o i, 30 ~ >

0

~1o~>

60

20 ~ lO

o

I- --"----"~

current density (A/dm2)

L

.-----'.120

-I !s

20

~ .... ' .... 1 . . . . . . .

60

Fig.1. Amount of A1203 generated (O), A1203 dissolved (11) and m" A1203/g A1203 formed (0) for varying anodization times.

"~i so 2_

..... o--

io2 o

-

40 time (minutes)

i I o ilOO

1 2 3 electrolyte concentration (mol/1)

Fig.3. Amount of A1203 generated (0), A1203 dissolved ( I ) and m~ A1203/g A1203 formed (0) for varying electrolyte concentrations.

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The properties of the alumina/aluminium cermets obtained are the result of two opposite processes: the anodization of the aluminium that increases the alumina layer from the top of the aluminium surface to the inside, and the dissolution of the alumina formed inside the pores of this layer that reduces and modifies the texture of the oxide layer. Then, as anodization time increases, the amount of A1203 generated increases but the amount of A1203 dissolved increases too (Figure 1). In this case the generated alumina has higher specific surface area at higher anodization times. However, the higher the current density, the lower the specific surface of the generated alumina (Figure 2), due the generation of a less porous alumina layer. On the other hand the A1203 production decreased on increasing electrolyte concentration (Figure 3) due to an enhanced dissolution process. A global view of the result of these analysis, together with the decrease of the process yield with the increment of all the variables led as to choose the following conditions as a balance between the best alumina properties and the lowest process cost" anodization time, 50 minutes; current density, 2.06 A/din; electrolyte concentration 1.64 mol H2SO4 per litre; temperature, 303K. Once the aluminium sheets were anodised, they were rolled together with alternate corrugated sheets to prepare the monoliths. The properties of the monoliths prepared in this way, similar to that of the commercial ones, are presented in Table 1. Table 1 Structural properties of the AI/O3/A1 monoliths prepared by anodization Geometric volume 6 cm 3 Cell area Total exposed surface 40 m 2 Surface to volume area ratio Number of cells 355 cell/in2 Empty fraction Specific surface area/cell 0.36 m2/cell Wall thickness

1.9 10-4 mZ/cell 1900 m -l 81% O.lmm

2.2 Impregnation of the active phase The impregnation of the monoliths with platinum or palladium was done from a (NH4)2PtCI6 (Fluka, puriss) or Pd(NO3)2 (Jonhson Matthey, Alfa) solution over 70 minutes. The noble metal impregnated monoliths were dried at 393K for 2h and calcined at 723K for 2h. In the case of manganese, the monoliths were dipped in a solution containing Mn(NO3)2 (Merck, PA) and citric acid (Panreac, PA) for 30 minutes. The dipping process was carried out one to three times to follow the catalytic properties of the monolith after several impregnations. Afterwards they were dried in air for 30 minutes, dried in vacuum at 343 K for 4 hours and calcined at 723 K for 2 hours. 2.3 Characterisations and catalytic tests Textural characterisation by N2 adsorption at 77K (Micromeritics ASAP2000) and metallic dispersion by chemisorption of H2 pulses (Micromeritics PulseChemisorb 2700) were carried out using the complete monoliths and not small specimens. The catalytic activity was measured in the complete oxidation of toluene in air obtaining both, the ignition curves at increasing temperature (2.5 K/min) and isothermal tests at high conversion. Catalytic tests were carried out in a plug flow reactor, using mass flow controllers to control the feed mixture. A He stream was bubbled through two thermostated

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and pressurised saturators containing toluene. This stream, fimher diluted with air, was passed through the monolith placed inside a furnace with temperature controller. The temperature of the reactant mixture was continuously monitored by a thermocouple placed at the inlet of the monolith. Conversion was calculated by three ways: by the disappearance of toluene and the appearance of water followed by GC-TCD containing a semicapilar column (TR-WAX, 30m), and by the appearance of carbon dioxide followed using a specific IR detector (SENSOTRANS, IR). The adsorption-desorption of reactants and products was studied by TPD and Temperature Programmed Surface Reaction (TPSR) using a MS detector (Omnistar, Balzers). 3. RESULTS AND DISCUSION N2 adsorption analysis of the AI203/A1 monoliths prepared with the chosen anodization conditions, showed the formation of a porous layer of A1203 with specific surface areas from 35 to 50m2/g and very homogeneous mean pore diameters of about 18nm. The alumina thickness is 15 to 20 lam allowing to obtain more than 3500m2 of surface area per m2 of aluminium sheet. The toluene ignition curves of the Pt and Pd monoliths are presented in Figure 4, together with the toluene ignition curves of powder Pt/A1203 and Pd/A1203, all of them pretreated in air at 573K before the reaction. Both noble metals, Pt and Pd, monoliths present excellent activity showing ignition temperatures (Ts0, temperature at which conversion is 50%) between 455 and 485K, and complete conversion of toluene below 530K. These temperatures are similar or lower than those corresponding to the conventional powder catalysts presenting comparable surface area and metal content.

.,o 0.8 r~

O

Fig. 4. Toluene ignition curves for Pt and Pd supported on powder A1203 (~ Pt and Q Pd) and A1EO3/A1 monoliths (O Pt and O Pd) (225ppm of toluene).

0.6

O

= 0.4 O

_

9

9

0.2 0

350

,

400

450

,

I

,

500 550 T (K)

, ~ _ ,

I

,

600

,

,

650

In spite of the low metal content of the samples (2 to 10 mg of noble metal per monolith) the metallic dispersion was moderate to low (<5%). Nevertheless, the activity of the big particles seems to be higher than that of small crystallites, as can be deduced by the comparison with powder samples presenting high dispersion (35% for Pt/A1203 and 32% for Pd/A1203). The positive influence of the prereduction of the powder samples confirms that the reduction degree of the noble metal (higher in the big crystallites) plays an important role in the oxidation activity. The influence of the reaction conditions was studied by varying the toluene

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concentration between 112 and 756 ppm and the space velocity between 2025 and 10100 11"1. Complete oxidation was obtained always at temperatures lower than 530K. Furthermore catalytic tests at high conversions were carried out over 40 hours, as shown in Figure 5 for a Pt impregnated monolith. No deactivation occurred, showing the high catalytic stability of the monoliths. 1 .or~

0.8

o

480

0.6

o o

~ o.4

460 ~

_= 0.2 k OF

....

0

i .....

10

I .....

i ....

20 3O Time (h)

40

Fig. 5. Toluene oxidation conversion followed by the CO2 signal (o) for Pt/AI203/A1, at constant temperature (D) over time.

440

Figure 6 shows the toluene conversion followed by the C02 signal, for the manganese loaded monoliths compared with the Pd/AIEOa/A1 monolith. Monoliths loaded with manganese oxide presented also high activity. On the other hand, a CO2 peak occurs for the manganese loaded monoliths, surpassing 100% of conversion above 525K. Furthermore these monoliths, in the same way as some powder samples, present disagreements between the conversion calculated by the toluene, water and carbon dioxide signal. 2j~15 o

o O

[]

&,

, j

Gq-+ '~,

1

O

--= o 0.5

Fig. 6. Toluene ignition curves following the toluene, H20 and C02 signals for reduced Pd/AI203 (m toluene, • H 2 0 and D CO2) and for the three times impregnated MnzO3/AlzO3/AI monolith (e toluene, + H/O and O CO2).

300 350 400 450 500 550 600 650 700 T (K) In both cases conversion reaches approximately 200% following the C02 signal, and 150% following the H20 signal. Moreover there is a clear disagreement between the conversion followed by the toluene signal and the rest. So, at low temperature, toluene disappears in higher amount than that corresponding appearance, and at high temperature, the contrary happens.

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To clarify these points a complete series of experiments was carried out including repeated tests at increasing and decreasing temperature and MS-TPD of reactants and products at experimental conditions similar to those of the catalytic tests. In Figure 7, the TPD of toluene and CO2 over A1203 followed by mass spectrometry are presented. As it can be seen in Figure 7, toluene has the maximum of desorption at 450K, and its desorption finishes at 540K. CO2 has two desorption maxima, one at 460K and a smaller one at 580K.

EO

[]

[]

[] 9

300

400

9

[]

500

600 700 800 900 T (K) Fig. 7. Toluene (r-l) and CO2 (0) TPD over A1203.

At the temperature ranges at which the oxidation reaction starts (370K over reduced Pd/A1203 and 400K over Mn203/A1203/A1) some toluene will remain adsorbed on the support. Due to the high activity of the supported phases the oxidation of toluene will start at low temperature. But at those temperatures, 400K in the case of reduced Pd/AI203 and 450K in the case of Mn203/AI203/AI, part of the CO2 formed will remain adsorbed on the support. However, the toluene combustion is an exothermic reaction, and as the reaction proceeds the generated heat of reaction will produce the desorption of toluene from the catalyst surface, which will react producing CO2 and more heat. As a result, a kind of chain reaction will occur bringing about the desorption and reaction of toluene and the desorption of CO2 and H20, leading to CO2 and H20 peaks above the ignition temperature. As a conclusion, the analysis of the TPD results and the ones of reactions at increasing and decreasing temperatures, allowed us to discard several hypotheses corresponding to sample preparation problems, partial oxidation products appearance or experimental artefacts, and confirmed the important role played by the adsorption-desorption of reactants and products over the active phase and the support. 4. CONCLUSIONS The controlled anodization of aluminium sheets to produce A1EO3/A1 monoliths has been proved to be an easy and reproducible method for the preparation of catalysts supports. The production of around 4000 m E of A1203 per mE of A1 has allowed us to impregnate noble metals, Pt and Pd, and manganese oxide on the metallic monoliths. The resulting impregnated monoliths have shown high activity in the complete oxidation of toluene, even higher than similar powder catalysts. REFERENCES

1. A. Cybulski and J.A. Moulijn, Structured Catalysts and Reactors, Marcel Dekker Inc., New York, 1998. 2. N.P. Fedotiev and S.P. Grilijes, Electropulido y Anodizaci6n de Metales, Gustavo Gili, S.A, Barcelona, 1972. 3. G. Patermarakis and N. Papandreadis, Electrochimica Acta, 38 No. 15 (1993) 2351.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Hydrotaicite-Derived Catalysts for Removal of Nitrogen-Containing Volatile Organic Compounds J. Habera , K. Bahranowskib, J. Janas", R. Janika, T. Machej", L. Matachowski", A. Michalik", H. Sadowskaa, E.M. Serwiekaa

"Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, 30-239 Krak6w, ul. Niezapominajek 1, Poland, bFaculty of Geology, Geophysics and Environmental Protection, Academy of Mining and Metallurgy, 30-059 Krak6w, al. Mickiewicza 30, Poland, The catalytic properties of mixed oxides derived from the multi-metallic hydrotalcites (HTs) containing Cu, Cr, V, A1 and Zn were tested in the total oxidation of dimethylformamide with particular stress on to the role of the precursor composition and the temperature of thermal decomposition (400~ and 550~ Optimum performance for the 400 series was given by the material obtained from the Cu,Zn, Cr,A1,V-HT, while in the 550 series the catalyst derived from the Cu, Cr,V-HT precursor gave best results. The observed activity and selectivity patterns are discussed in terms of the catalysts physico-chemical properties. 1. INTRODUCTION Recent studies show that exposure of human beings to air contaminated with N-containing organic vapours induces genotoxic consequences and may increase the cancer risk in the affected population [1,2]. While abatement of hydrocarbons by total oxidation has been widely studied [3,4], the catalytic conversion of nitrogen containing organic compounds remains, as yet, an unexplored field. The process requires oxidation of the organic part to CO2 and reduction of the possibly evolving NOx to molecular nitrogen. Thus the catalyst must be able to perform two functions, oxidative and reductive, both in the presence of excess oxygen. Our experience with design of HT-derived catalysts for the removal of volatile oragnic compounds [5] prompted us to research the possibility of using HT precursors for preparation of mixed-oxide catalysts capable of efficient combustion of N-contaning organics. In view of the fact that Cu-Cr oxides are known to perform well as total oxidation catalysts [3-7], while vanadium is the major component of oxide catalysts used for selective reduction of nitrogen oxides, Cu, Cr and V were chosen as main elements constituting the HT structure with A1 and Zn acting as moderating agents. Dimethylformamide (DMF), commonly used industrial solvent, was used as a probe molecule.

2. EXPERIMENTAL HTs containing Cu, Cr, Zn and A1 as layer-forming elements, were pepared by a copreciptation method at constant pH from the solutions of respective nitrates, described in detail in [5]. Vanadium was introduced into the interlayer as decavanadate or pyrovanadate anions by

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means of anion exchange, according to the procedure described in [6]. Samples of theoretical general formula [•M(II)0.67ZM(III)o.33(OI-I)2]An'o.33/n, where M(II)=Cu, Zn, MOII)=Cr,AI, and An--NO3", V1oO286"or V2074", were obtained and labeled in a manner showing the atomic ratios of the metal elements in each material: Cu2Cr (nitrate form), Cu2CrVo.5 (pyrovanadate form) Cu2CrVI.7, Cu2Cro.5A]o.5Vi.7, CuZnCro.5A]o.5V1.7 (all decavanadate forms). The parent HTs were calcined for 3 h in air at 400~ and 550~ The calcination products obtained at lower temperature were designated Cu2Cr-400, Cu2CrVo.5-400, Cu2CrV~.7-400, Cu2Cr0.sAlo.5V~.7400, CuZnCro.sAlo.sV~.7-400, those prepared at 550~ were referred to as Cu2Cr-550, Cu2CrV0.5-550, Cu2CrVl.7-550, Cu2Cr0.5Alo.5V1.7-550,CuZnCr0.sAlo.5V1.7-550. Samples were characterized with PXRD (DRON-3 diifractometer, Ni filtered Cu Kcx radiation), BET (nitrogen adsorption, outgassing at 473 K, Micromeritics ASAP 2000V apparatus), and chemical analysis (ICP-AES Plasma 40 Perkin-Elmer spectrometer). The reaction was carried out in a flow system at atmospheric pressure, in the temperature range 175-500~ The catalyst volume was 0.5 cm3 (0.7_+0.05g), particle size 0.2-0.5 ram. DMF concentration was 1.5 g/m3 (~1000 ppm), G.H.S.V.=10000 hq. 3. RESULTS AND DISCUSSION

The real atomic ratios of the metal elements in the synthesized smiles are given in Table 1. Table 1

S~le doo3 (A) Cu2Cr 8.99 Cu2CrVo.5 7.79 Cu2CrVI.7 11.79 Cu2Cro 5Alo5V1.7 11.61 CuZnCro.sAlo.sV1.7 11.61

Cu 2.05 1.99 1.96 1.89 1.02

Zn 0.88

Cr 1.00 1.00 1.00 0.50 0.50

AI 0.49 0.54

V 0.57 1.78 2.01 2.13

A higher than intended vanadium contem is seen in two decavanadate-exchanged samples. It may be due either to the presence of impurity phases (see below) or to the enhanced loading with decavanadate anions caused by partial protonation of the intercalated species. The XRD patterns of fresh samples confirm that the structures expected for the given anions have been obtained [8] (Fig. 1). In the Cu2Cr0.5Al0.5V~.7sample, of poorest crystallinity, a small amount of an impurity, identified as copper orthovanadate, is visible. The PXRD patterns of solids obtained after calcination at 400~ and 550~ are presented in Fig. 2 a and b respectively. Heat treatment at 400~ transforms all decavanadate forms of hydrotalcites into predominantly amorphous solids, while in the Cu~Cr-400 and Cu~CrV0.5-400 samples crystalline phases appear. In the former the reflections of copper oxide CuO, copper chromite CuCr~O4 and copper chromate CuCrO4 can be identified, in the latter the pattern is dominated by several modifications of copper vanadate Cu5V20~0 accompanied by copper chromite. After calcination at 550~ all previously amorphous solids reveal the presence of crystalline phases. The patterns of Cu2Cr-550 and Cu~CrV0.5-550 are similar to those observed after calcination at 400~ except that in the former the chromate phase disappears and in the Cu~CrV0.5-550 a strong peak emerging around 20=40 ~ points to the enhanced crystallization of CuO and the CusV20~0 modification known as stoiberite. The patterns of all other samples are dominated by

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CuZnCr0.5AI0.5V1.7

Cu2Cr0.5AI0.5Vl.7

Cu2CrV1.7

Cu2CrV0.5 Cu2Cr

0 .

.

I0

;;0

.

.

.

20

30 40 50 Cu Kct [deg] ~

..

60

70 I

Fig. 1 XRD patterns of HTs precursors reflections characteristic of various vanadates. Cu2CrV1.7-550 contains mainly Cu2V2OT, CuVO3, CuCr204, and CrO3. Cu2Cr0.sA10.sV1.7-550 shows reflections characteristic of Cu2V2OT, CuV206, CuVO3 and a mixed oxide Cr0.07V1.9304. CuZnCr0.sA10.sV1.7-550 is composed mainly of CUEV207, Zn3(VO4)2, and Cr0.07Vl.9304. It is worthwhile to note that in the Al-containing samples of the 550 series no crystalline CuCr204 is formed.

0

10

20

30

40-

50

60 0 1"0 20 Cu K(x [deg]

2"0

N

Fig. 2 XRD patterns of HTs samples calcined at: a) 400~ b) 550~

40

5"0

60

70

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BET specific surface areas of all calcined samples are presented in Table 2.

Table 2 ,BE T ~ . c ~ C . . . s ~ c e ~ea of.c.flc~cd.,.~s Calcination temperature Cu2Cr Cu2CrVo.5

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

(~

Cu2CrV1.7 Cu2Cro.sAlo.sVx.7CuZnCro.sAlo.WL7

..................................................................................................................................................................................................

400 550

43 20

25 19

54 19

28 8

25 3

All catalysts proved very active in the total oxidation of DMF, reaching 100% conversion in the temperature range between 200~ and 350~ depending on the sample composition and on the temperature of calcination. The order of activities was similar for both series: Cu2Cr-4~>Cu2CrV~.7-4~Cu~CrV~.5-4~>Cu2Cr~.5A~.5V~.7-4~>CuZnCr~.5A~.5V~.7-400 and Cu~Cr-55~>Cu2CrV~5-55~>Cu2CrV~7-55~>Cu2Cr~5A~.5V~.7~55~>CuZnCr~.5A~5V~7~550. All samples which as 400 series displayed amorphous character showed an important loss of activity as 550 catalysts, understandable in view of the decrease in specific surface areas. On the majority of catalysts the organic component was converted exclusively to CO2. Only on samples with half of chromium replaced by aluminium CO could be also seen before the conversion reached 100%. The effect was more significant for CuZnCr0.sAl0.sV1.7-400 and CuZnCr0.sA10.W~.7-550 samples having additionally half of copper replaced with zinc. At low temperature range the nitrogen part of DMF was converted to N2 with 100% selectivity. At higher temperatures nitrogen oxides started to evolve (Fig. 4). The ordering of catalysts according to the increasing temperature of the onset of NOx evolution follows exactly the pattern of activities, i.e. the more active catalyst the sooner NO~ appear in the reaction products. Analysis of the influence of sample composition on the selectivity to NO~ shows that, as expected, addition of increasing amounts of vanadium to the copper-chromium catalyst 100

r'

--~=.-~f~Z-.-.---~.~,~..~~

i9

,, 9

oj

9

,,

8O

z-

O o0 n, LM > Z

o o

60

"

: l~

40 //

20

/

,, 9

--

9-- Cu~Cr-400

,'

--

!i!_cucrv

,,'

9- - Cu2Cr-550

- - A - - Cu2CrVl.7-550

.C

--- Cu2Cro ~,1o5Vl 7-400

r

o - - Cu2CrosAIo5V~7-550 ---A--- CuZnCro ~.1o5V1 r400 --

200

o,,OO

Cu2CrVo s-550

/ ] 6~~

150

~= --=

./

js S

250

300

9

350

TEMPERATURE, ~

Fig. 3 DMF conversion over calcined HTs.

CuZnCrosAIosV~r550 400

450

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化

100

o~

80

~-" >

60

r UJ ._1 LLI o~

40

b)

20 m

150

200

250

300

350

400

450 150

E

200

~/

250

i

300

350

400

450

TEMPERATURE, *C Fig. 4 Selctivity to NOx over HTs calcined at: a) 400~

Cu2CrVo.5, A -

Cu2CrV1.7, O - Cu2Cro.sAlo.sV1.7,

and b) 550 ~ C;

9 - Cu~Cr,

9-

A - CuZnCro.sAlo.sVi.7.

results in a gradual retardation of NOx evolution. Similar observation was reported recently for the non-hydrotalcite derived Cu-Cr-V catalyst [9]. Interestingly, the spectrum of various nitrogen oxides is affected in a different way depending on the V content in the sample. Thus, initially the later onset of NOx formation is due to the reduced evolution of N20 while higher amount of V reduces the evolution of NO and NO2. Further modification by replacing half of chromium with aluminium lowers evolution of N20 and so does subsequent replacement of part of copper with zinc. Varying shapes of the conversion and selectivity curves cause that in each of the series different ~nples show best characteristics. Table 3 compares the temperature ranges and the temperature windows in which the catalysts show the activity higher than 80% and the yield of NOx lower than 20%. Table 3 Temperature ranges and temperature windows of>80% conversion and <20% NOx selectivity for calcined HTs. Calcination temperature CuECr CuECrV0.5 Cu2CrV1.7 Cu2Cr0.sAl0.sV1.7CuZnCr0.sAlo.sV1.7 ...................( 0 c )

400 550

.....................................................................................................................................................................

184-192~ 8~ 180-203~ 23~

208-238~ 30~ 216-280~ 64~

208-262~ 54~ 242-291 ~ 49~

246-313~ 67~ 285-356~ 63~

264-360~ 96~ 310-350~ 40~

In the case of the catalysts calcined at 400~ the temperature window increases with enrichment of the composition, simultaneously being shifted towards higher temperatures. The best of the series is CuZnCr0.sAl0.sV~.7-400 which shows good performance over almost 100~ There is no clear trend in the catalysts calcined at 550~ Samples Cu2CrV0.s-550 and Cu:Cr0.sA10.sV~.7-550 show both temperature windows of over 60~ but the former has to be regarded as superior because its maximum falls at lower temperature range. The calcination at

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550~ worsens the properties of previously amorphous solids by shifting their maximum performance towards higher temperatures and reducing the temperature window. The latter is particularly significant for CuZnCr0.sA10.sVL7-550 catalyst. In this sample the crystallization of new phases is most pronounced, as seen by narrow XRD reflections and a strong drop in the specific surface area. On the other hand, the Cu2Cr-550 and Cu2CrV0.5-550 samples, both crystalline already at 400~ improve their characteristics after heat treatment at higher temperature. This effect may be tentatively attributed to the changes in phase composition of the catalysts when passing from one temperature of calcination to another. In the first case the phase of copper chromate disappears, in the second CuO and the stoiberite modification of CusV2O10 can be identified in significant quantities. It is conceivable that the presence of the latter phases makes the Cu2CrV0.5-550 catalyst superior with respect to the richer in vanadium Cu2CrV~.7-550 sample. Investigations into these effects are now in progress in our laboratory. 4. CONCLUSIONS The appropriate choice of elements constituting the hydrotalcite precursors and adjustement of the calcination temperature allow to optimize the balance between the oxid'ging and the reducing functiom of the resulting mixed oxides. The catalysts of higher activity show enhanced evolution of NO~ in a degree depending on the catalyst composition and temperature of calcination. As a result, the ordedering of catalysts according to the best performance differs for the two series derived from the same precursors but calcined at different temperatures. In the 400 series it is the amorphous material doped with AI and Zn, and thus having the lowest oxidizing power. In the 550 catalysts the best is the sample with no A1 and Zn in the structure, revealing the presence of CuO and a significant amount of the stoiberite modification of Cu5V201o.

Acknowledgement: TI~ work was financially supported by the Polish Committee for Scientific Research within the research projects 6 P04D 040 14 and 7 T08D 008 17. REFERENCES

1. J. Major, A. Hudak, G. Kiss, M.G. Jakab, J. Szaniszlo, M. Naray, I. Nagy, A. Tompa, EnvironrrL and Mol. Mutagenesis 31 (1998) 301. 2. R. Wrbitzl~, Int. Arch.ives of Occupational and Environm. Health 72 (1999) 19. 3. J.J. Spivey, Ind. Eng. Chem.. Res. 26 (1987) 2165. 4. J.J. Spivey, J.B. Butt, Catal. Today 11 (1992) 465. 5. K. Bahranowski, E. Bielafiska, R. Janik, T. Machej, E.M. Serwicka, Clay Miner.34 (1999) 67. 6. K. Bahranowski, G. Bueno, V. Cort6s-Corberfin, F. Kooli, E.M. Serwicka, R.X. Valenzuela, K. Wcisto, Appl. Catal. (1999) in print. 7. T. Machej, J. Janas, H. Sadowska, J. Haber, S. C~ckiewicz, L. Matachowski, PL Patent No 166598 (1992) 8. C. Dep6ge, L. Bigey, C. Forano, A. de Roy, J.P. Besse, J. Solid State Chem. 126 (1996) 314. 9. J. Haber, T. Machej, H. Sadowska, J. Janas, L. Matachowski, 16th Meeting of the NAM Catalysis Society, Boston 1999, Book of abstracts, PI-002.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Meio, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Surface Catalytic Reactions Assisted by Gas Phase Molecules on Supported Co-ensemble Catalysts Aritomo Yarnaguchi a, Takafiuni Shido a, Kiyotaka Asakurab and Yasuhiro Iwasawa~* a Department of Chemistry, Graduate School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan b Catalysis Research Center, Hokkaido University, Kita-ku, Sapporo 060-0811, Japan This paper presents a new aspect of surface catalytic reactions assisted by gas phase molecules in the catalytic NO reduction with CO on supported CoZ+-ensemble catalysts prepared by chemical vapor deposition of Co2(CO)8 on A1203 and TiO2. NO molecules adsorbed on Co 2§ sites to form Co dinitrosyls which were surface reaction intermediates as characterized by FT-IR, whereas no CO molecules adsorbed on the surfaces. CO molecules undetectable at the catalyst surfaces promoted the NO adsorption, the reactivity of Co dinitrosyls and a change in the dinitrosyl structure. A possible mechanism for the observed phenomenon is proposed. 1. INTRODUCTION The longer-term challenge to surface chemistry of supported metal catalysts is to address important issues in activity and selectivity in catalytic reactions, in particular the principles of activating adsorbed species and reaction intermediates at catalyst surfaces [1 ]. The activation processes are regulated by several mechanisms involving geometrical control and/or electronic modification of active sites/species. Gas phase molecules can modify catalytic performance and reaction path through adsorption at surfaces [2]. There are many reports on adsorptionmediated surface restructuring and catalytic performance [2-6]. The rate and selectivity of water-gas shitt reactions on MgO, ZnO and Rh/CeO2 were dramatically enhanced and regulated by coexisting 1-120 molecules[7]. Reaction path for ethanol transformation on Nb/SiO2 was switched over from dehydration to dehydrogenation by the presence of ethanol in the gas phase [8]. In these cases adsorption of the gas phase components is a key issue and readily detectable at the catalyst surfaces. Recently we have fotmd a new aspect of surface catalytic reactions assisted by gas phase molecules in NO-CO reaction on a Co/A1203 catalyst [9]. The surface processes such as NO adsorption, reaction of the adsorbed NO and a structural change in the reaction intermediate (Co dinitrosyls) were promoted by gas phase CO molecules which were undetectable at the catalyst surface. In this paper we report the detailed phenomenon, characterizing the behavior of Co dinitrosyls ha situ by FT-IR, XANES and TPD.

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2. E X P E R I M E N T A L

AI~O3- and TiO2-supported C02+-ensemble catalysts were prepared by chemical vapor deposition of C02(CO)8 on A1203 and TiO~, respectively, followed by stepwise transformations as shown in Scheme 1 [9, 10]. The Co 2+ species were characterized by EXAFS, UV-VIS, TPD and Raman. A SiO2-supported Co 2+ catalyst with isolatedly distributed Co 2+ sites was also prepared [9, 10]. For comparison an impregnated C0/A1203 catalyst was also prepared by a traditional impregnation method using an aqueous solution of Co nitrate. Catalytic NO-CO reactions were carried out in a closed circulating system equipped with a gas chromatograph and a quadruple mass spectrometer. FT-IR spectra were measured in an IR cell with two CaF2 windows combined in a closed circulating system. 3. RESULTS AND DISCUSSION 3.1. Catalytic NO-CO reactions Figure 1 shows performances o f several supported Co 2+ catalysts and an impregnation

catalyst for NO-CO reactions at 463 K; 2 NO + CO ---> N20 + CO2. At the higher temperatures the subsequent reaction, N~O + CO ---> N2 + CO2, proceeded. The first reaction has been investigated from the mechanistic interest in this paper. The catalytic activity of C0/A1203 was higher than those of Co/TiO~ and Co/SiO2. The activation energy for Co/A1203 was 14 kJ mol-' which was much lower than 25 and 23 kJ mol ! for Co/TiO~ and Co/SiO2, respectively. The activity o f the Co/AI~O3 catalyst was also much higher than that of

the traditional impregnation C0/A1203 catalyst (46 kJ mol-'). The reaction rate on the C0/AI203 catalyst was expressed by v=[NO]' 6[CO] ~ in the pressure range 0.26-4.0 kPa. The reaction rate was nearly proportional to CO pressure. The initial rate for the catalytic NO-CO reaction OO

_oc .~c.%

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.0

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Reaction time / min Figure 1. The amount of N~O formed in NO-CO reactions on (C)) Co/A1203, ([3) Co/TiO2, (O) Co/SiO2, and (/%) impregnated

Scheme 1. Structural transformations of Co2(CO)s on SiO2, AlsO~ and TiO2 surfaces.

Co/A12Oa, and (B) by NO decomposition on

ColAl203.

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was 23 times larger than that for the decomposition of NO alone at 463 K. If CO alone was admitted to the fresh Co/AI203 catalyst, no CO2 was produced, indicating that there existed no reactive oxygen species on the catalyst. It seems that the catalytic NO-CO reaction on Co/AI203 can not be explained by redox mechanisms that have often been reported.

3.2. TPD of adsorbed NO Figure 2 depicts TPD spectra after NO adsorption of different periods on Co/AI203 in the absence and presence of gas phase CO molecules. The TPD spectra in the absence of gas phase CO showed only NO desorption, where little N20 product was evolved. The higher temperature peak around 450 K became larger with increasing NO adsorption period. On the other hand, N20 formation was observed above 325 K in the presence of gas phase CO and simple NO desorption was negligible. The TPD data revealed that the reactivity of adsorbed NO was remarkably promoted by the gas phase CO molecules. Note that CO molecules which promote the surface NO conversion were volumetrically and spectroscopically undetectable as adsorbates on the Co/Al~O3 catalyst surface. 3.3. Behavior of adsorbed NO species at room temperature As the adsorbed state of NO seemed to change during adsorption, the behavior of adsorbed NO species was monitored by in-situ FT-IR and XAFS at room temperature. NO adsorbs as dinitrosyls Co(NO)~ on the Co/Al203 catalyst as characterized by the isotope peaks at 1831 and 1776 cna-I for Co(14NO xs2, 1772 and 1701 crn"l for Co(15NO)2 and 1828 and 1735 cm ! for Co(~4NO)(~sNO) [9, 11 ]. The intensity of the syrmnetric and antisymmetric stretching peaks at (a)

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Figure 3. The amount of adsorbed NO and

Co/A12Oa under vacuum (a) and in the

the bond angle of

presence of gas phase CO (b). (0,0,@) ([],

estimated by IR spectra.

E,[~) and (/k,A,A) represent the amount of desorbed NO, N~O, and CO2, respectively. The sample was exposed to 1.3 kPa of NO for 3 rain (O, [:], A), 30 rain (Q, E, A), and 5 h (@, I~, A).

Co(NO)~ on Co/A1203

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1831 and 1776 crn~, respectively, changed with NO exposure time and also by evacuation. The bond angle (0) between the two NO ligands on a Co atom (NO-Co-NO) has been demonstrated to be related to the ratio of the antisymmetric stretching peak intensity (la,) to the symmetric stretching peak intensity (Is) by the equation, l,dls = tan 2 (0/2) [12]. The amount of adsorbed NO (MNo) can also be estimated by the equation, MNo = a l J sin2 (0/2) (a: constant). The calculated 0 and MNo are plotted against exposure time and evacuation time in Figure 3. The change in MNo coincided with the change in the amount of adsorbed NO determined gravimetricaUy in a separate experiment. Figure 3 shows that there are two kinds of dinitrosyls (species A and B); the dinitrosyl with a small bond angle (species A) is rapidly formed upon NO adsorption and easily desorbed by evacuation, while the dinitrosyl with a large bond angle (species B) is slowly formed and not desorbed by evacuation at room temperature. We estimate the ratio of species A to species B to be 1-1.9 at room temperature. As the 0 of species B was 110~ and the 0 for a mixture of species A and B at 120 min was 97~ Figure 3, the 0 for species A was estimated to be 71 ~ Reintroduction of NO nearly recovered the amount of adsorbed NO and the bond angle to the original values at 120 min before evacuation. Figure 4 (a) shows a change in the Co K-edge XANES spectra for Co/A1203 and Figure 4 (b) shows a time profile of the edge peak intensity. When NO adsorbed on the catalyst, the edge peak decreased in agreement with the IR peak profile. The edge intensity was not recovered by evacuation, instead the larger decrease occurred by evacuation. The peak intensity decreased further by NO reintroduction. The results imply that an irreversible change in the Co state occurred geometrically and probably also electronically. This is contrasted to the reversible feature for the NO species in Figure 3. Thus the XANES spectral change during NO adsorption and desorption may also reflect an irreversible structure change at the interface between the Co2+ ions and the A 1 2 0 3 surface. (a)

c-

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3.4. FT-IR spectra during the catalytic NO-CO reactions To clarify the role of gas phase CO molecules in the catalytic NO-CO reactions, Co dinitrosyls were monitored by FT-IR. Figure 5 shows typical FT-IR spectra for adsorbed NO on Co/A1203 at 463 K, where the intensities of the two peaks and their relative ratio were changed by CO introduction to the system. Figure 6 shows the changes of 0 and MNO at different temperatures as a function of time. The bond angle of the dinitrosyls on Co ~+ ions increased by the admission of gas phase CO molecules undetectable at the surface. Degree of the 0 expanding was larger at higher temperatures. The amount of the dinitrosyls also increased by CO admission. Again, degree of the MNO increase was larger at higher temperatures. At 463 K, the MNO increased by the presence of gas phase CO about twice as compared to the MNO in the case of NO alone. The effects of the ambient CO were large at the temperatures where the catalytic NO-CO reactions proceeded, whereas the changes of 0 and MNO by CO were negligible below 323 K where the catalytic NO-CO reaction did not take place significantly. The gas-phase assisted surface processes were not observed with isolatedly distributed Co 2+ sites in Co/SiO~. 3.5. Possible mechanism The observed surface catalytic NO-CO reactions assisted by gas phase CO molecules undetectable at the catalyst surface are summarized as follows. (1) The amount of adsorbed NO (Co dinitrosyls) increased by the presence of gas phase CO. (2) The bond angle of the dinitrosyls increased by the presence of gas phase CO. (3) The reactivity of the (a) 120 ~ 110

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1900

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Figure 5. IR spectra for adsorbed NO on

the amount of adsorbed NO on Co/Al~O:~as

Co/A1203at 463 K. The sample was exposed

a function of exposure time to NO (1.3 kPa),

to 1.3 kPa of NO for 5 min (A), and 20 rain

at 323 K (O), 373 K ([:]), 423 K (A), 448 K

(B), and the mixture of NO (1.3 kPa) and

(O), and 463 K (@). CO (1.3 kPa) was

CO (1.3 kPa) for 5 rain (C) and 30 min (D)

added to the system at the time indicated

after 03).

by arrows.

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dinitrosyls was promoted by the gas phase CO. The phenomena are not relevant to simple collision-induced surface processes because Ar and N2 never showed similar promoting effects. The detection limit for CO adsorption is of the order of less than 5x10 "3 CO per Co. If spectator CO molecules below the detection limit at the surface are assumed to play a role in the phenomena, one CO molecule must activate more than 200 Co-dinitrosyls. We can not find the origin of this peculiar interaction of undetectable CO with strongly adsorbed surface species. It may be possible to assume the presence of short-lived adsorbed CO molecules at the catalyst surface. The collision fi-equency for impinging CO on the Co 2+ sites is roughly 5x107 s"!. If they stay at the surface for 10~~ s on average, the total life-time of surface CO molecules corresponds to the amount of 5x10-3 Co in agreement with the detection limit for CO adsorption in the present study. This means that 99.5% of Co 2+ sites on average are always free without interaction with CO. Thus we propose an irreversible memory effect of a short life-time CO species on the surface dinitrosyls. The irreversible effect may be supported by the fact that the NO decomposition activity was maintained after evacuation during the course of the catalytic NO-CO reaction. This may also be related to the irreversible feature in the XANES data. The spectator CO molecules activate the Co(NO)2 species, changing the structure from the small bond angle to the large bond angle during the total life-time. The large-bond angle Co(NO)2 induced by impinging CO molecules under the catalytic reaction conditions may be more stable and reactive than the small-bond angle Co(NO)2, which leads to the increase in the amount and reactivity of adsorbed NO at the reaction temperatures. Further investigation is necessary for deeper understanding of this catalytic aspect. Phenomenon described in this paper may be extended to many catalytic systems. The concept of surface catalytic reactions assisted by undetectable adsorption may provide an approach to design new catalytic systems through activation of unreactive surface sites in situ under catalytic reaction conditions. The work has been supported by CREST (Core Research for Evolutional Science and Technology) of the Japan Science and Technology Corporation (JST). References

1. Y.Iwasawa, Stud.Surf.Sci.Catal. (Proc. 11th Int.Congr.Catal.), Elsevier, 101 (1996) 21. 2. Y.Iwasawa, Acc.Chem.Res., 30 (1997) 103. 3. G.A.Somorjai, MRS Bulletin, 23 (1998) 11; "Introduction to Surface Chemistry and Catalysis", Wiley, NY, 1994. 4. G.Ertl, Science, 254 (1991) 1750; J.Mol.Catal., 74 (1992) 1. 5. M.L.Burke and R.J.Madix, J.Am.Chem.Soc., 113 (1991) 3675. 6. K.Takahashi, E.Miyamoto, K.Shoji and K.Tamaru, Catal.Lett., 1 (1988) 213. 7. Y.Iwasawa, "Elementary Reaction Steps in Heterogeneous Catalysis (R.W.Joyner and R.A.van Santen eds.), NATO ASI Ser.C. 398 (1993) 287. 8. M.Nishimura, K.Asakura and Y.Iwasawa, Proc.9th Int.Congr.Catal., Vol.4, 1988, p. 1842. 9. A.Yamaguchi, K.Asakura and Y.Iwasawa, J.Mol.Catal., in press. 10. Y.Iwasawa, M.Yamada, Y.Sato and H.Kuroda, J.Mol.Catal., 23 (1984) 95. 11. L.Portela, P.Grange and B.Delmon, Catal.Rev., 37 (1995) 699. 12. P.S.Braterman, "Metal Carbonyl Spectra", Academic Press, 1975, p.43.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Fresh and used V2Os-WO3/TiO2 SCR EUROCAT standard catalyst: an european collaborative characterisation Jacques C. V6drine On the behalf of all contributors from 30 European laboratories Leverhulme Centre for Innovative Catalysis, Department of Chemistry, The University of Liverpool, Oxford Street, P.O. Box 147, L69 7ZD, Liverpool, United Kingdom I. INTRODUCTION Fundamental research in heterogeneous catalysis in most academic or industrial laboratories is usually performed on catalysts prepared on a very small scale and difficult to reproduce and to characterise fully. It therefore appeared in the early seventies that there existed a need for standard or reference catalysts to be available to the world catalysis community. A research group of European scientists working in Catalysis and later incorporated in a charity group called European Association of Catalysis (EUROCAT) decided in 1976 to embark on a collaborative programme on characterisation of supported metals and, later on, on zeolites, sulfides and oxides to compare experimental procedures, and to interpret the results. A review of catalyst standardisation programmes was published in the "Hanbook of Heterogeneous Catalysts" [1]. The EUROCAT oxide group produced in 1994 a first work on V2OJTiO2 catalysts, which was published as a special issue of Catalysis Today [2]. At present the EUROCAT oxide group works are part of the activities of the European Federation of Catalysis Societies (EFCATS) created in 1990. The Selective Catalytic Reduction (SCR) of NOx by NH3 in presence of oxygen represents an important catalytic process [3] for the combined removal of N Ox and SOx in thermal power plants and industrial boilers and for the abatement of N Ox and dioxins in incinerators. The deNOx reaction is operated industrially over catalysts made of homogeneous mixtures of TiO2, WO3 (or MOO3) and V 2 0 5 [4,5]. TiO2, in anatase form, is used as high surface area support. V205 is responsible for the activity for both SCR of NOx and SO2 oxidation. Silico-aluminates or fibreglass are used as binders to improve catalyst strength and commercial SCR catalysts are used in the form of honeycomb monoliths or plates.

2. EXPERIMENTAL 2.1 Catalyst preparation For the purpose described above, a large batch of a SCR catalyst was prepared by Austrian Energy and Environment Co. from Wien, Austria and provided to us in monolith form in its starting state, designated "fresh" and after nine thousands hours on stream in an industrial power plant, designated "usear'. Both samples and the starting WO3/TiO2 support were distributed per 100 g and characterised by 30 different European laboratories. Catalyst characteristics and catalytic conditions in industrial plant, as supplied by the industrial partner, were as follows

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Catalysts parameters: wall thickness: 0.58 mmm; channel width: 3.58 mm; fraction of accessible channel cross sections: 69.8 % and surface/volume catalyst: 779 m "1 Worlang conditions: 9,000 hours at 280 ~ with 1 nag SO3/Nm3 in the exit, with NH3/NO=I.0, H20=10 vol %, GHSV/spec. surface: 12.8 Nm h "1 (Nm3hlm2), linear velocity: 2.3 Nm s"1, Ko = 45 Nm 3 h "1 m"2

2.2 Catalyst characterisation The main techniques used were F T I ~ UV-vis, Raman, XPS, SIMS, NMR and ESR spectroscopies, XRD, BET, porosimetry, electrical conductivity measurements and thermal techniques (DSC, TPR/TPO, TPD, DTA, TG) and several rnicroscopies such as AFM, SEM, EDX-SEM, HRTEM and EDX-TEM The equipments were different from one laboratory to the others and all experimental conditions and data are given in detail in ref 6 Catalytic testing was performed for Selective Catalytic Reduction of N Ox; SO2 to SO3 oxidation; oxidative dehydrogenation of ethane and propane; propene, 1- and 2-chloropropane oxidation and isopropanol conversion reactions (chap. 11 and 12 in this issue of Catal Today). 3. EXPERIMENTAL RESULTS

3.1 Main characteristics The V2Os-WO3/TiO2 EUROCAT SCR catalyst corresponds to anatase Ti02, the unit cell of which is not perturbed by V or W cations as shown by XRD and HRTEM. This presumably arises from the very closely similar cation radii of 58 and 60 pm for V5§ and W6§ respectively, versus 60.5 pm for Ti 4§ and from the fact, which will be discussed below, that V 5§ cations concentration is high in the first one or two layers of the catalyst particles. Some V205 crystallites could scarcely be detected by XRD, indicating that, if there exist such particles, only a few of them are big enough to be detected by XRD. The surface area of the samples are equal to respectively 47 and 46 m2g-1 for the fresh and used samples, which exhibit a mesoporosity in the region of 10-20 nm in diameter. The catalyst does not show significant macroporosity, which would have modified crushing strength characteristics. The catalysts are composed of particles of ovoid form, 20-30 nm in size as shown both by X-ray line broadening and SEM and TEM observations; therefore the mesoporosity is certainly created by the interparticle void volume. Chemical compositions of the different samples are given in Table 1. The V205 loading is relatively large for a SCR catalyst, which should lead to high SO2 oxidation activity. This is probably motivated by the extremely low content of SO2 expected to be present in the flue gases in the range 5-10 ppm, and to burn any VOCs present. Table 1. Chemical compositions of the samples from chemical analysis in wt % and atomic ratios Samples Ti W V W/Ti V/Ti Support: WO3/Ti02 44 6.5 0.1 0.039 0.002 Fresh 43.2 6.3 1.9 0.038 0.041 Used 43.5 6.9 2.1 0.042 0.045 Chemical attacks of the samples, in order to dissolve selectively V and W cations, were performed with a 1M ammonia aqueous solution at 20 ~ by flowing the solution at a rate of

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0.3 crn3 min "l for a kinetic study (up to 3 h) or at 60 ~ in static conditions (12 or 24 h) for a stronger attack. It appears (table 2) that, after a fast attack, one tends to asymptotic values. Table 2 Amounts of V and W in atoms % dissolved by chemical attack at 20 ~ in dynamic flow for up to 1 h and at 60 ~ in static from 3 to 24 h by a 1M aqueous ammonia solution versus time Etching time 15 min 30 min [ 1h 3h 12 h 24 h Element V W V W ,V W V W V W V W Fresh 34 6.5 40 9 47 13 59 18 72 88 25 Used 30 42 61 It appears that large amounts of vanadium and, to a much smaller extent, tungsten ions are chemically dissolved very rapidly, as if they were only weakly bound to TiO2 or at the surface of the particles. The used sample gave smaller values which is coherent with XPS data (see below), showing that V ions are more incorporated in TiO2 after industrial operation. Note also that majority of W cations but minority of V cations are deeply embedded in TiO2 particles, since, even after a 24 h chemical attack, 75 mol % of W and 12 mol% V were not dissolved. If V and W oxidic species were only deposited at the TiO2 surface as polymeric bidimensional (-O-V-O-V-) moieties, one may estimate the surface coverage, taking V-O bond length as in V205 [6], corresponding to a O-V-O distance of ca. 0.38 nm and a surface area of ca 0.11 nm 2 per VOx unit [ref.2, p. 171 ] and the same value for WOx unit, the W radius being very close to that of V. It follows that taking a surface area equal to 47 m2g-1 and an atom wt% equal to 0.19 and 0.63 for V and W respectively, one gets 52.5% and 48.5% theoretical coverages for VOx and WOx polymeric species, i.e. the TiO2 particles may be, theoretically, totally covered by the VOx and WOx polymeric species if they were located at the surface.

3.2 Spectroscopic studies IR studies of adsorbed probe molecules as NH3, NO, NO+NH3, CO, pyridine, B(OCH3)3 were carried out to characterise acid-base properties. Both Bronsted and Lewis acid sites were identified for fresh and used samples, in agreement with previous findings in the literature. However, note that for the used sample the strength of the Lewis sites was slightly weaker, while nucleophilic (basic) sites appeared, presumably due to small poisoning by basic compounds such as alkaline or earth alkaline cations. Both V 5+ and W 6+ (as V=O at 2v =2048 cm 1 and W=O at 2v =2014 cm 1) acted as Lewis centres of higher strength than the Bronsted centres (as shown by elution of ammonium and pyridinium cations upon outgassing at increasing temperatures). The scheme of the particle surface can be presented as follows: v=3750cm 1 Si-OH ~O 2

2v = 2 0 1 4 c m 1 W=O

2v=2048cm l V=O

v = 1 3 7 5 c m -1 S=O

TiO2

NH3 was found by FTIR to displace water from Lewis sites, while in the co-adsorption of NH3 and NO, N20 was detected. Molecular spectroscopies (FTIR and Raman) showed also that all hydroxyl groups of TiO2 disappeared due to their neutralisation by VOx and WOx species, indicating a good dispersion on the surface and that both fresh and used samples were similar, with only minor differences. FTIR, Raman and UV-vis spectroscopies showed that the

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samples contained vanadyl and wolframyl species chiefly in pseudo tetrahedral symmetry and, to a minor extent, agglomerated in V205 and WO3 2D and 3D structures. When m situ FTIR experiments were done in SCR conditions at 525 K on V205-WO3/TiO2 fresh and used catalysts, ammonia was observed not to neutralise all Br~nsted sites, as it does at RT, i.e. that part of Bronsted sites remained on the surface under reaction conditions. It is clear, and this has been already well established [7], that NO adsorbs weakly on the catalysts, while NH3 adsorbs strongly, displaces adsorbed water from the Lewis sites and reduces the catalysts upon heating. The MAS-NMR study could identify three V5§ distorted octahedral environments, different from simply grafted V5§ cations; the major one at -612 ppm is similar but corresponds to a slightly more distorded octahedral environment than in V205, the second one at -664 ppm is obtained only when WO3 is present, and the last one at - 553 ppm, detected b y high speed MAS-NMR experiments (35 kHz), can be assigned to polymeric VOx species in a distorted octahedral environment, as proposed for V2Os/TiO2 catalysts [ref. 2, p.77)], and strongly bound to TiO2 support. Such a species, in relatively low amount, could be related to the -~12% V 5§ cation which is insoluble by strong chemical attack.

3.3 Electrical conductivity measurements Electrical conductivity measurements on EUROCAT WO3/TiO2 and W2Os-WO3/TiO2 catalysts were performed at 300~ under 400 Torr (0.53 105 Pa) 02. It was observed (Table 3) that electrical conductivity was much higher for WO3/TiO2 support than for a classical TiO2, which was interpreted as due to the incorporation of W 6+ into TiO2, causing a n-type doping effect. Each dissolved W 6+ cation shares 4 electrons with 4 02. nearby anions, whereas the fifth and sixth electrons are delocalized around the positive W6+ impurity. Taking a formula established for standard EUROCAT V2OJTiO2 catalyst [ref. 2, p. 135]], the amount of W 6+ ions dissolved can be calculated: it corresponds to 1.07 x 1020 W 6+ ions dissolved/cm3, i.e. 9.5% of W. Moreover, the electrical conductivity values for the fresh and used catalysts are still higher (Table 3). Using the same calibration curve and assuming that the same amount of W 6+ ions as in WO3/TiO2 support were dissolved, it was calculated: 6.61x102~ M 5+ ions/cm3, i.e. 4.47 xl02~ V5+ ions (or 38% of V) and 1.07x102~ W6+ ions (9.5 % W) for the fresh sample and 9.56x102~M 5+ ions/cm 3, i.e. 7.42x102~V5+ (57 % V) and 1.07x102~ W6+ ions (9.5 % W) for the used sample. As the technique concerns primarily surface conductivity, it can be concluded that part of V5+ and W6+ ions are dissolved in the surface layers of TiO2 and that the amount of V5+ dissolved is larger for the used sample than for the fresh one.

Table 3 Electrical conductivity of some samples under 53 kPa (400 Torr) 0 2 at 300~ Catalysts o (ohm "1 cm-1) x 1010 TiO2 (EUROCAT) 10m2~1 EL10V1 D 10m2g1 WO3/TiO2 V2Os-WO3/TiO2 Q~esh) V2Os-WO3/TiO2 (used)

2.4 24. 74. 216. 311.

ref. 2 (p. 135) 2 (p. 135) this work this work this work

Electrical conductivity values were also measured in presence of 2,000 ppm NO; 2,000 ppm NH3; 2,000 pppm NO + 2,000 ppm NH3 and 2,000pppm NO + 2000 ppm NH3 +500

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ppm 02 at 300 ~ They were found to decrease by about half for NO alone and to increase up to 2; 30 and 8 10-7 ohmlcm ~, in presence of NH3 respectively, showing that these compounds reacted with TiO2 presumably by oxidation by NO and reduction by NH3, even more if NO was also present. The change in electron conductivity is usually assigned to the creation of doubly ionised vacancies Vo2§ presumably by dehydroxylation, which captured one electron each to form the thermodynamically stable singly ionised anionic vacancy (Vo2+ + e ~ Vo+), reduction step, which could be replenished by oxygen (or NO) according to : Vo§ + 1/2 O2 + e 02- (reoxidation step). It was concluded that the activity of V2Os-WO3/TiO2 catalyst in SCR reaction is a redox process assisted by Bronsted acid sites necessary to a dissociative chemisorption of ammonia. A reaction mechanism was proposed for the SCR reaction [6], close to that suggested by Topsoe et al [8], but where the redox process is ensured by the anionic vacancies created in TiO2 by reduction and dehydroxylation of surface V-OH groups. 3.4 XPS, SIMS, AFM, IIRTEM and SEM studies XPS analyses have shown that the surfaces of fresh and used samples are enriched by a factor of about three in V and W, which presumably comes from the preparation procedure. However, several points are worth reporting: (i) more W is embedded in TiO2 than V as could be seen from Ar and He ion sputtering in XPS and SIMS and in good agreement with the chemical attack described above; (ii) surface V/Ti ratios are lower for used than for fresh samples, both in monolith and in crushed forms, as if more V cations were embedded in the TiO2 matrix in the former case (this result is also in good agreement chemical attack data, see above); (iii) Ti, V and W cations at the particle surface are principally in + 4, + 5 and + 6 oxidation states, respectively. Note that UV-vis spectroscopy, which is sensitive to the bulk of the material and not only to its surface layers, also showed that these cations are at their highest oxidation state; (iv) XPS indicated also that after strong Ar § sputtering the V/Ti atomic ratio was much less (0.005 to 0.02) than the chemical composition (0.041), indicating that a large part of V cations is located at or near the crystallite surface. This was not observed for W/Ti. All these data indicate that W is more incorporated in the bulk of TiO= particles than V. Analyses by SIMS, technique sensitive to the first surface layer, and Ar § sputtering at increasing times showed that V/Ti ratio is much more sensitive to sputtering than W/Ti ones, indicating that V is located more at the particle surface than W and that V/Ti ratio is larger for the fresh sample by a factor of two with respect to the used sample. EDX-SEM analyses on monoliths showed that Ti and W are homogeneously distributed in the monoliths and V slightly less. EDX-TEM analyses on small particles and ensembles of particles of the ground catalysts showed that only a very few particles contained only Ti, i.e. V and W species are really incorporated in TiO2 particles and/or deposited at their surface. High resolution electron microscopy analyses showed that the catalyst particles are well crystallized with the anatase lattice fringes, whereas their surface (one or two layers maximum) appears amorphous.Taldng all data above into consideration, it can be suggested that this amorphous surface is composed of V and, to a lesser extent W, embedded in TiO2 matrix. It is this amorphous surface which, very probably, was so rapidly chemically dissolved. Note that if one takes arbitrarily the values of 40 atom % V and 9 atom % W chemically dissolved after 30 min (Table 1) as the end of the fast chemical attack, it would correspond to ca 50% theoretical coverage in one surface layer deduced from chemical analysis (see above) and obviously less two times for two layers. This is in good agreement with electrical conductivity measurements and XPS data for the fresh sample but not so well for the

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used one. Note that the used catalyst exhibits a more ruguous surface than the fresh one, as shown both by SEM and AFM analyses, which may correspond to a thicker amorphous layer.

3.5 Catalytic properties For the SCR reaction, it was found that the used catalyst was slightly more active with an activation energy of 20 to 30 kJ mol~ indicating a diffusion-controlled reaction. An analysis of data collected over honeycomb catalysts by means of a 2-D single-channel model of the SCR monolith reactor has allowed us to evaluate the intrinsic kinetic constant of the deNOx reaction. The results well compared with those obtained on crushed samples. For SO2 to SO3 oxidation the catalytic activity was found quite higher than those usually measured on other SCR catalysts, as could be expected for higher vanadia loading of the present catalysts. For light alkane (C2 and C3) oxidative dehydrogenation (ODH), both fresh and used catalysts behaved similarly, although minor differences were observed, and are quite active but rather unselective in ODH, at comparable conversion levels but at quite lower temperatures, with respect to other V205 supported catalysts. This is attributed to the stronger acidity of V2Os-WO3/TiO2 catalysts. The conversion of chloropropanes was complete at 300 ~ for the 1-isomer and at 200 ~ for the 2-isomer giving propene and COx following an E 1 mechanism. Isopropanol decomposition under air, as carrier gas, showed that ODH properties of both fresh and used samples are similar and comparable to a standard EUROCAT VzOs/TiOz catalyst, while its dehydration (acid) properties are similar for both samples but very much enhanced with respect to the standard EUROCAT VEOs/TiO2 catalyst. This is consistent with a higher acidity due to the presence of both V and W oxidic species. Acetone, di-isopropyl ether and propene were obtained as products, demonstrating the presence of both acid-base and redox sites at the surface, in agreement with IR, TPR/TPO and electrical conductivity data. 4 CONCLUSION As a general conclusion, it can be said that such materials, which do not need an activation period and are similar after 9,000 h on stream can be used as standard catalysts not only for SCR and SO: oxidation reactions but also for many other partial oxidation reactions. One can then fruitfully take profit from the stronger acidic properties of such samples with respect to the EUROCAT standard VzO~/TiO2 (EL-l) catalyst [2].

REFERENCES G.C. Bond, in "Handbook of Heterogeneous Catalysis", G. Ertl, H. Kni3zinger and J. Weitkamp, ed., VCH, Weinheim, vol.3 (1997) 1489 EUROCAT oxide, J.C. Vedrine (Ed.), Catalysis Today, vol. 20(1) 1994. H. Bosch and F. Janssen, Catal. Today, 2 (1988) 369; P. Forzatti and L. Lietti, Heter. Chem. Rev., 3 (1996) 33. M. Kunichi, H. Sakurada, K. Onuma and S. Fujii, Ger. Often., 2.443,262 (1975); F. Nakajima, M. Takeuchi, S. Matsuda, S. Uno, T. Moil, Y. Watanabe and M. Inamuri, US Patent 4,085,193 (1978) 12 L Casagrande, L. Lietti, I. Nova, P. Forzatti and A. Baiker, Appl. Catal. B: Environmental, 22 (1999) 63 H.G. Bachmann, F.R. Ahmed and W.H. Barnes, Z. Krist., 115 (1961) 110 G. Ramis, G. Busca, F, Bregani and P. Forzatti, Appl. Catal. 64 (1990) 259 N.Y. Topsoe, H. Topsoe and J.H. Dumesic, J. Catal., 151 (1995) 226 and 241

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Lean NO~ reduction over Snl.xZrxO 2 solid solutions Jun Ma, Yuexiang Zhu, Jiying Wei, Xiaohai Cai, Youchang Xie" Institute of Physical Chemistry, Peking University, Beijing, 100871, P. R. China SnO2-ZrO2 binary oxide solid solution was found to be a novel and good catalytic system for the selective reduction of NO by propene in the presence of oxygen. The NO conversion over SnO2-ZrO2 binary oxides changed with SnO2 content, and was the highest at about 60wt% SnO2. At 350~ 77% conversion of NO to N2 was obtained for a feed of He stream with 1186ppm NO, 948ppm propene and 2.23% 02 at a space velocity of 15,000h"~. XRD analysis showed that uniform solid solutions with cassiterite phase were found for the SnO2 rich Snl.xZrxO2 samples, while monoclinic and tetragonal ZrO2 phases were formed for the ZrO2 rich samples. TPR measurements suggest the introduction of Zr ions into SnO2 lattice can lead to the decease of hydrocarbon oxidation capacity and the formation of new easier reduction oxygen, which may be beneficial to the reactivity enhancement of NO conversion. The sample with composition in the intersection point of the two kinds of solid solutions (Sn0.ssZr0.4502) presented as an amorphous phase and showed the highest activity. The solid solution catalysts could sustain moderate activity after a calcination at 800~ in the presence of high O2 partial pressure or in the presence of 10% water.

1. INTRODUCTION Reduction of nitrogen monoxide by hydrocarbons in an oxidizing atmosphere is the most promising way to eliminate nitrogen oxide r~.21.Metal-ion exchanged zeolite had been reported as good catalysts for the reaction. But their hydrothermal stability is not good enough for real application. One alternative to the zeolite-based catalysts is to make non-zeolite based catalysts such as metal oxide with good activity and durability for the reaction. In 1993, Teraoka et al. t31 reported that tin oxide showed catalytic activity for the selective reduction of nitrogen monoxide with hydrocarbons in the presence of excess oxygen and its activity was not so much decreased even after heat treatments in dry air at 900~ as well as in humid air in 800~ Recently, Tabata et al./41 reported that the addition of tin promoted the catalytic activity of alumina for the NO selective reduction by methanol at low temperature and it could maintain its activity even in the presence of SO2. More recently, M. C. Kung et al. t51 reported Sn/y-Al203 were effective and highly stable catalysts for NO reduction with propene under high partial pressures of oxygen and water. In this work, a series of SnO2-ZrO2 binary oxides prepared by two-flux solution co"Corresponding author, e-mail" [email protected]

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precipitation method, have been demonstrated to be uniform solid solutions and the solid solutions performed significantly higher activities than that pure SnO2 and ZrO2 did on the selective reduction of NO with propene in the presence of 02. And more important these catalysts showed moderate activity even in the presence of 10% water, high 02 partial pressure or after a calcination at 800~ 2. EXPERIMENTAL

2.1. Catalyst preparation SnO2-ZrO 2 binary oxides were prepared by adding ammonia solution and a mixed solution of SnCl 4 and ZrOC12 simultaneously to certain amount of water, keeping at pH 6 to 7. The precipitate was suction filtered, washed with distilled water until the filtrate was free of Cl ions as tested with 0.5M AgNO3. Then the product was dried at 1l0 ~ followed by calcination at 500~ in air for 4h and some samples were then calcined at 800~ for 4h.

2.2. Catalytic activity measurements The catalytic activity was measured with a fixed-bed flow reactor under atmospheric pressure. A gas stream of helium with 1186 ppm NO, 948 ppm C3H6, 2.23% 02 and when desired, 10 vol.% H20, was fed to 0.4ml catalyst at a rate of 100 cm3/min, resulting in a gas hourly space velocity GHSV-15,000h ~. The analysis of the effluent gas was made by a gas chromatograph equipped with a TCD detector and a twocolumn system (Molecular Sieve 5A column and Porapak Q) for separation and analysis of N 2, CO, CO2 and C3H6.

2.3. Catalyst characterization Polycrystalline X-ray diffraction patterns were recorded with a BD-86 X-ray diffractometer using Cu Kct radiation at 40 kV and 20 mA. The BET surface areas of all the samples were measured by using nitrogen adsorption data at 77K. TPR (temperatureprogrammed reduction) measurements were carried out to estimate the reduction behavior of the samples. 5 vol% H2-Ar gas mixture was used as a reducing gas with a flow rate of 20 ml min ~ and the temperature of the catalyst bed was raised at a heating rate of 10 ~ min ~. 3. RESULTS AND DISCUSSION

3.1. Effect of composition To study the effect of composition on catalytic properties, a series of SnO2-ZrO2 binary oxide catalysts with SnO2 concentration ranging from 0 to 100 wt%were prepared and tested. Table 1 summarizes the activity of SnO2-ZrO 2 binary oxides for NO reduction by propene in the presence and absence of H20. As for the reaction in the absence of H20, with the increase in S n O 2 content up to 60wt% in the binary oxides, the maximum conversion of NO to N 2 increases simultaneously. The sample with 60wt% SnO2 exhibits the highest activity among the SnO2-ZrO 2 binary oxides, the maximum conversion of NO to N2 in the presence of 02 reaches 77% at 350~ However, when SnO2 content further increases to 100wt%, the

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maximum NO conversion over SnO2-ZrO2 binary oxides decreases. But for all the SnO2-ZrO 2 binary oxides tested here, the NO conversions are much higher than that on pure SnO2 or ZrO 2 alone. When 10vol% H 2 0 w a s introduced into the reaction gas, the activities of SnO2-ZrO 2 binary oxides are depressed considerably at low temperature, the temperature windows become narrow and shitt to higher temperature region. However, similar to the results in dry condition, the NO conversions on binary oxides are higher than that on pure S n O 2 or Z r O 2 alone. The sample with 60wt% SnO 2 which shows the highest activity in the absence of H20, still exhibits a 60% conversion of NO to N 2 in the presence of water vapor. Table 1 Activities of SnO2-ZrO2 catalysts for the selective reduction of NO with propene in the presence and absence of H20* SnO 2 content

(wt%) 0

Surfacearea (m2/g) 60

10

71

20

70

30

59

40

78

50

71

60

136

70

66

80

62

90

59

100

35

H20(% )

0 10 0 10 0 10 0 10 0 10 0 10 0 10 0 10 0 10 0 10 0 10

250~ / / 14 / 15 / 25 / 25 / 12 / 14 / 16 / 16 / 14 / 7 /

NO conversion to N z (%) 300~ 350~ 400~ 450~ / 17 22 36 / / / / 30 42 57 44 / / 10 43 23 48 50 41 / 9 28 39 36 60 53 38 / 9 38 38 41 63 49 41 / 13 52 41 45 71 57 30 / 19 56 41 54 77 54 34 / 23 60 31 41 74 49 38 / 17 52 28 39 62 50 48 / 20 50 42 43 52 44 39 / 30 54 37 40 39 27 18 9 32 41 27

500~ 43 20 26 38 24 28 28 26 29 16 14 18 21 15 20 24 29 33 24 18 10 15

* Reaction conditions: Atmospheric pressure, He stream containing l l86ppm NO, 948ppm C3H6, 2.23% 02, 0 or 10% H20, GHSV: 15,000hq

3.2. Phase analysis of SnO2-ZrO2 binary oxides by XRD Figure 1 shows the XRD patterns of SnO2-ZrO2 binary oxides with various composition calcined at 500~ for 4 hours. The ZrO2 obtained by calcining pure Zr(OH)4 at 500~ for 4h is a monoclinic with trace of tetragonal(t) phase. With the increase in SnO2 content, the fraction of tetragonal ZrO2 in the binary oxides increased obviously. As the content of SnO2

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reaches 40wt%, all the oxides become tetragonal ZrO 2 phase. Since no diffraction peaks of SnO2 can be detected in these samples and the peaks of monoclinic or tetragonal ZrO2 shift gradually with the composition, it indicates that SnO2 has been doped into ZrO2 lattice to form uniform solid solution. As more SnO 2 is added, the composite oxides with 60wt% SnO2 is changed into an amorphous phase. When the SnO2 content further increases to h 70-100wt%, the oxides present as g cassiterite phase. The X-ray diffraction peaks of crystal plane (110) and (101) in cassiterite have been selected to determine the b lattice parameter of SnO2-ZrO2 binary oxides. The lattice 1'0' 20' 30' 40' 5 0 " 60 70 parameters, cell volumes and crystal size of SnO2-ZrO2 binary oxides 2 theta/~ with cassiterite phase are Fig. 1. XRD pattems of SnO2-ZrO2 binary oxides summarized in Table 2. According SnO2 content (wt%): a.0; b.10; c.20; d.30; e.40; to Table 2, the cell volume and f.50; g.60; h.70; i.80; j.90; k. 100 lattice parameter (a, b and c) generally increase with increase in zirconium content in the SnO2-ZrO2 binary oxides. It indicates uniform solid solutions were formed. The reason for the change of lattice parameters is because the ionic radius of zirconium ions (0.79 A) is larger than that of tin ions (0.71 A). In summary, Sn4+ ions can be easily doped into ZrO2 lattice to form solid solution in the SnO2 rich samples, and on the other hand, for the SnO2 rich samples, Zr 4+ ions can also be easily inserted into SnO2 lattice to form uniform solid solutions. It is also noteworthy that the sample with composition in the intersection point, SnO2 content of 60wt%, presents an amorphous phase, and give a maximum BET surface area of 136m2/g as well as a highest conversion for NO reduction (see table 1,350~ data). G

II

i

.

i

Table 2 Lattice parameters, cell volume and crystal size of Snl.~Zr~O2 c(A) cell vol (A 3) Crystal size Snl.xZrxO2 SnO2 a=b (A) (I) content(wt%) (nm) X=0.00 100 4.7308 3.1792 71.152 14.1 X=0.12 90 4.7379 3.1796 71.375 7.8 X=0.23 80 4.7554 3.1798 71.907 8.9 X=0.34 70 4.7625 3.2042 72.676 8.6 (1) Lattice parameters were determined from the XRD peaks of crystal plane (110), (101). (2) Crystal size was determined from the XRD peak broadening with Scherrer equation.

3.3. TPR results of SnOz-ZrOz binary oxides Figure 2 shows the result of H2-TPR measurements carded out to clarify the tread of hydrocarbon oxidation reactivity of the solid solution. Pure SnO2 as a standardsample showed

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only one reduction peak between 500~ and 700~ On the other hand, pure ZrO2 didn't show clear reduction peaks in the temperature region from room temperature to 800~ Apparently, with the increase of zirconium substitution in the solid solutions, the reduction peak ~ - _ ..... --."/r 610"C ~__. ~::123 between 500~ and 700~ decreased and ~ X=0.34 shifted to high temperature region X=0.45 slowly. The Sn~.xZrxO2 solid solution ZrO~ sample with x equaled to 0.45 only 0 ' 200 ' 400 ' 600 ' 800 showed a small reduction peak at 537~ This fact suggests that for the sample with composition in the intersection point, SnO2 content of 60 wt%, has a Fig. 2. TPR profiles of Sn~_xZrxO2 mild hydrocarbon oxidation capacity Conditions" Catalyst weight=0.02 g, gas flow and also some easier reduction oxygen rate = 20 cm 3 min~, heating rate=10~ min~, species were formed. On the basis of reducing gas: 5% H2-Ar these results, we concluded that the introduction of Zr ions into SnO2 lattice can lead to the decrease of hydrocarbon oxidation capacity and the formation of new easier reduction oxygen which may be benefit to the reactivity enhancement of NO conversion.

Temperature(~

3.4. Effect of oxygen concentration on the NO conversion over Sn,.xZrxO2 solid solutions Fig.3 shows the effect of oxygen concentration on the NO conversion over Sn0.55Zr0.4502 (SnO2: 60wt%) solid solution. The presence of 02 is found to be essential for the NO reduction on the solid solution. In the absence of oxygen, the NO conversion is quite low at all the reaction temperatures. At 450~ the NO conversion increases steeply to a maximum with 02 addition up to 0.5%, followed by a slight decrease with further addition of 02. Similarly, at 350~ and 300~ the NO conversion rises to a maximum with adding 02 up to 2%. The dependence of the lean NOx reduction activity of Sno.55Zr04502 on the oxygen concentration is quite different with that of Cu-ZrO2 catalyst reported in the literature t61.For Cu-ZrO2 catalyst, the addition of up to 1% 02 was found to increase the rate of NO conversion to N2, but further additions of 02 sharply decreased the rate. For the Sno.55Zr0.4502 solid solution catalyst in our study, the NO conversion only decreased a little at high 02 partial pressure. It indicates that Snl.xZrxO2 solid solution catalysts can sustain the activities at high 02 partial pressure.

3.5. Effect o f calcination solutions

temperature

on the catalytic activity of Sn~.lZrxOz solid

To study the effect of calcination temperature on the catalytic activity of Sni.xZrxO2 solid solutions, the Sn0.55Zr0.4502sample was calcined at 500~ and 800~ then tested respectively. The results are shown in fig.4. Although at lower reaction temperature (<400~ region, the catalyst calcined at higher temperature (800~ gives NO conversion lower than that of catalyst calcined at lower temperature (500~ at higher reaction region (>400~ the catalyst calcined at higher temperature gives higher NO conversion. This result may be

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~

so 70

S

60

o~

40

-'

/o~

80~ 70:

o

so

-

~

60"

r

50

AN

_r1__500Oc

--o~ooOc

.[ 40

30 2o ~

~ 30 ~2o

~

~ 1O

010

210

410

610

0 2 partial pressure(%)

810

Fig. 3. Effect of 02 concentration on NO conversion over Sno 55Zr04502solid solutions (l-q: 350~ (O: 300~ (A: 450~

Z

0

O

O

250' 360 350 460 450 560 Temperature (~

Fig. 4. Effect of calcination temperature on NO conversion over Sn0,Zr0.4502 catalyst. Reaction conditions are same as in table 1.

attributed to the lower propene combustion (a reaction competed with NO reduction) activity for the catalyst calcined at higher temperature. 4. CONCLUSION In this study, a series of novel solid solution catalysts Sn~.xZrxO2were prepared by a twoflux solution co-precipitation method. XRD results show that for the SnO2 rich samples, Zr4§ ions are inserted into SnO2 lattice and for the ZrO2 rich samples, Sn4§ ions are doped into ZrO2 lattice to form uniform solid solutions. The Sn~.xZrxO2 solid solutions show much higher activities than that of pure SnO2 or ZrO2. A Sno55Zr04502(SnO2: 60wt%) catalyst gives the highest activity among the Snl.xZrxO2 solid solutions. For the catalyst, the maximum conversion of NO to N2 in the presence of excess 02 reached 77% at 350~ When adding more oxygen or 10% H20 to the reactant mixture or after high temperature calcination, Sno 55Zr04502can still sustain moderate activity. ACKNOWLEDGEMENT The authors acknowledge the National Science Foundation of China for financial support (Project No: 29803001 and 29733080) REFERENCES [1]V. I. Parvulescu, P. Grange, B. Delmon, Catal. Today 1998, 46, 233-416 and references therein [2] M. Shelef, Chem. Rev., 95 (1995) 209-225 [3] Y. Teraoka, T. Harada, T. Iwasaki, T. Ikeda and S. Kagawa, Chem. Lett. 1993, 773-776 [4] M. Tabata, H. Hamada, F. Suganuma, T. Yoshinari, H. Tsuchida, Y. Kintaichi, M. Sasaki and T. Ito, Catal. Lett. 1994, 25, 55-60 [5] M.C.Kung, P.W.Park, D.-W.Kim, H.H.Kung, J.Catal., 181 (1999) 1-5 [6] K.A.Bethke, M.C.Kung, B.Yang, M.Shah, D.AIt, C.Li, H.H.Kung, Catal. Today, 26 (1995)169-183

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Concentration Programmed Adsorption-Desorption / Surface Reaction study of the SCR-DeNOx reaction I. Nova, L. Lietti, E. Tronconi, P. Forzatti Dipartimento di Chimica Industriale e Ingegneria Chimica "G.Natta", Politecnico di Milano, P.zza L. Da Vinci 32, 20133 Milano, Italy 1. INTRODUCTION Transient methods in heterogeneous catalysis are being more and more applied to kinetic studies and mechanistic investigations. These methods consist in imposing perturbations to the reacting system (e.g. step changes in the inlet reactant concentrations), while analysing the outlet transient response. Such techniques were extensively applied in our labs to the study of the Selective Catalytic Reduction (SCR) of NO by NH3 [1-3]: by imposing step-wise changes in the inlet NO and NH3 concentration, the dynamics of the SCR reaction could be investigated and valuable mechanistic and kinetic data on the reaction could be collected. However, the investigation was carried out under ideal conditions (model catalyst, H20- and SO2-free feed stream). In this work the unsteady analysis of the SCR reaction has been extended to more representative conditions by investigating among others the effects of H20 and SO2 content in the feed. Furthermore, a commercial catalyst sample has been used. A modification of the classical transient response method has been developed for this purpose, wherein the step changes of the inlet reactant concentration are replaced by linear variations of the inlet gas composition. In principle, this technique (hereafter called Concentration Programmed Adsorption-Desorption, CPAD, or Concentration Programmed Surface Reaction, CPSR) is more informative if compared to the usual transient response method based on step (discrete) changes in the inlet reactant concentration. Indeed, by imposing a finite rate of change of the operating variables, the system dynamics can be analysed over the full range of intermediate conditions. This point will be demonstrated in the following, where the application of the CPAD - CPSR methods to the study of both the adsorption-desorption of the SCR reactants and of the NH3 + NO surface reaction is reported. 2. EXPERIMENTAL The experiments were performed on a V205-WO3/TiO2 commercial catalyst containing 0.62 % w/w V205 and 9 % w/w WO3, loaded in a flow-microreactor system operated at atmospheric pressure and at GHSV - 1.4 105 h "1. The flow rates of the feed gases, coming fi'om calibrated cylinders, were measured and controlled with mass flow controllers. The reactor consisted in a quartz tube (6 mm ID) with a tapered exit (2 mm ID) connected by a silica capillary to a mass spectrometer. This arrangement, which minimises the dead volumes, allows for fast transfer of the products to the analysis section of the apparatus. A layer of carborundum over the catalytic bed assured the good mixing of the reactants in the feed stream. The catalyst temperature was measured by means of a K-type thermocouple directly immersed in the catalytic bed. In each run 120 mg of catalyst were used: in order to minimise diffusion resistance without exceedingly high-pressure drop, the catalyst was powdered in

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particles of calibrated diameter (75-100 ~tm). Data obtained from the mass spectrometer were quantitatively analysed by taking into account the response factors and the overlapping cracking pattern of the different species. Nitrogen balances, estimated at steady-state, always closed within +5%. In order to verify the response of the system in absence of the catalyst in the reactor, blank experiments were also performed. In all cases the system response closely resembles that of the inlet concentration, and no evidence of any contribution of homogeneous gas-phase reactions was detected.

3. RESULTS AND DISCUSSION 3.1. NH3 and NO dynamic adsorption-desorption experiments (CPAD) a) N H 3 - C P A D - The NH3 adsorption/desorption process was investigated by means of the CPAD method by imposing linear variations of the inlet NI-I3 concentration (0 --> 840 --> 0 ppm) in flowing 02 (2%) + balance He in the temperature range 550-680K. A typical result of a NH3-CPAD experiment is shown in Figure 1, where the dotted line and the symbols represent the inlet and the outlet NH3 concentration, respectively. Fig. 1 shows that upon linearly increasing the inlet N[-I3 concentration, the outlet concentration exhibits a dead time of about 400 s and then a rapid increase with a knee near 500 s. During the whole rise phase (0-1500 s) the outlet NH3 concentration is lower than the inlet one, due to the adsorption of NH3 on the catalyst surface. Only during the phase at constant inlet NH3 concentration (15003400 s) the outlet NH3 concentration equals the inlet NH3 concentration because the system has reached a steady state. On the other hand, upon decreasing the NH3 inlet concentration (3400-3800 s), the outlet NH3 concentration is higher than the inlet one since adsorbed NH3 is being released from the catalyst surface. Notably, ' ' ' ' ' ' ' ' " ' i the area between the inlet 800 and the outlet NH3 concentration curves is -~ smaller than in the rise o. 600 phase, thus indicating = that some NH3 remains ._o adsorbed on the catalytic 400 surface at the end of the ,-~ exper~ent (4500 s) ..

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upon subsequent heating of the catalyst in He + 2 0 ] % 02 (TPD exper~ent 0 1000 2000 3000 4000 5000 6000 up to 823 K at 15 K/min), Time (s) indicated by an arrow in Figure 1- NH3-CPAD experiments performed at 573 K. Dotted the figure. line: inlet NH3 concentration; circles: outlet NH3 concentration; NH3-CPAD solid line: model fit (k%= 33.87 m3/mol s, k%=2.2 10 6 l/s, E~ = experiments were 22.0 kcal/mol, a=0.256, ~ = 2 7 0 mol/m3). The arrow indicates performed at different the start of the TPD run. temperatures (in the range 573-673 K) and results similar to those shown in figure 1 were always obtained. _o

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However the initial dead time progressively decreases upon increasing the temperature, in line with an exothermic adsorption phenomenon. Furthermore, at temperatures above 630 K, the formation of N2 is also observed, due to catalytic NH3 oxidation by oxygen. No formation of nitrogen oxides is observed at any investigated temperatures. The effect of H20 on the NH3 adsorption-desorption characteristics has also been investigated. For this purpose, experiments were performed in the presence of 0.8 % v/v H20 in the feed. The results, not reported for the sake of brevity, indicated that no significant changes occur in the NH3 adsorption/desorption characteristics, in spite of the fact that H20 content is nearly 1 order of magnitude higher than that of NH3. This indicates that for the investigated H20 level water does not appreciably compete with NH3 for the adsorption on the catalyst surface. In order to gain quantitative information on the NH3 adsorption-desorption characteristics, the experimental data obtained at different temperatures have been analyzed according to a dynamic one-dimensional isothermal heterogeneous PFR model of the test reactor. On the basis of diagnostic criteria, the influence of both intraparticle catalyst gradients and external mass transfer limitations were found negligible. Under these hypotheses, the unsteady mass balance of NH3 on the catalyst surface and of NH3 and NO in the gas-phase were written [1]. The following kinetic expressions for the NH3 adsorption/desorption processes (ra and rd) and for NH3 consumption by oxidation to nitrogen (ro~) were used: ra=k~ CNH3 (1-0NH3),

rd=k~ exp (-Ed(0NH3)/RT) 0NH3,

rox= kox 0NH3

where k~ k~ and kox are the kinetic rate constant for NH3 adsorption, desorption and oxidation, respectively, Ed is the activation energy for NH3 desorption and 0NH3 is the NH3 surface coverage. A non-activated NH3 adsorption process has been considered, on the basis of preliminary results. Different dependencies of EO on 0NH3 have been used, including a Langmuir (Ed=constant) and a Temkin-type (Eo=Ed~ dependency, this latter taking into account the catalyst surface heterogeneity. When the dynamic kinetic model was fitted to the data, the Langmuir-type kinetics failed in describing the results, whereas both the NHaCPAD data and the final TPD experiment could be satisfactorily represented (solid line of Figure 1) by a Temkin-type desorption process with a value of the activation energy for desorption at zero-coverage (E~ of 22 kcal/mol. Estimates of the other kinetic parameters are given in the caption. Notably, the NH3 oxidation occurring at high temperatures (and the corresponding N2 formation) could be nicely fitted by the dynamic model as well. b) N O - C P A D - No adsorption-desorption of NO was observed in this case, thus suggesting that NO, as opposite to NH3, does not appreciably adsorb on the catalyst surface. 3.2. NO-NI-Ia dynamic surface reaction experiments (CPSR) The study of the SCR reaction under unsteady-state conditions was carded out in flowing 02 (2%) + balance He by performing" i) linear variations of the NH3 inlet concentration (0 -~ 840 ppm -~ 0) in constant NO (750 ppm) (NH3-CPSR); and ii) NO variations (0 -~ 750 ppm -~ 0) in constant NH3 (840 ppm) (NO-CPSR). a) N H j - C P S R - A typical result of a NH3-CPSR experiment is shown in Figure 2 where the NH3 (circles), NO (up triangles) and N2 (squares) outlet concentrations are shown as a function of time. The NH3 inlet concentration (dotted line) is also reported. During the rise phase, the NH3 outlet concentration shows a long dead time (about 750 s),

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and then an increase till the end of the transient phase, reached at t=1500 s. On the other hand, the outlet NO concentration shows completely different dynamics: in fact it immediately decreases upon admission of NH3, it shows a weak minimum near 750 s and then it slightly increases up to the end of the NH3 rise phase (t=1500 s). The concentration of molecular nitrogen (squares in fig. 2), formed in the reaction along with water (not reported the figure), is specular to that of NO, thus suggesting 800 that neither NO nor N2 adsorption is 700 involved in the SCR reaction. The different NH3 and NO dynamics observed during the NH3 rise phase is in v500 line with a mechanism involving the e~9 400 reaction between adsorbed NH3 and gaseous NO. As a matter of facts, NO is ca00 ID o consumed as soon as NH3 is fed to the o 200 f catalyst, thus showing that the adsorption of NH3 is a very fast process. Notably, the outlet NO concentration (and hence the 0 1000 2000 3000 4000 5000 6000 NO conversion as well) shows a complex Time (s) dependence on the NH3 inlet Figure 2 - NH3-CPSR experiment at 573 K. Dashed lines: inlet NH3 concentration; symbols: concentration, being nearly linear with the experimental data (a: ammonia, b" NO inlet NH3 concentration for low NH3 levels and showing a weak inhibiting effect at concentration); solid lines" model fit (k~ high NH3 concentrations. This l0 s l/s, E~ = 19.2 kcal/mol, 0~3=0.06, other phenomenon, which has been reported by oarameters as in fie. 1). other authors [4], has also been confirmed by steady-state experiments, and accordingly it is not related to a transient feature. It is also noted that this inhibiting effect, weak at the lowest investigated temperatures, vanishes upon increasing the reaction temperature. The data obtained during the NH3 decrease phase (t>3300 s) confirm the results obtained upon the rise phase. Indeed also in this case the NO consumption shows a weak inhibiting effect by NH3 at high N H 3 concentrations and a nearly linear dependence with the inlet NH3 concentration for low NH3 levels. To quantitatively describe the transient reactivity data, the dynamic reactor model used to fit the CPAD experiments was modified by including a term accounting for NH3 consumption by the SCR surface reaction (rNO): I

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0~3 rso = k so C so 0 ~ , (1 - e ) In line with the results shown in Figure 2, this kinetic expression accounts for the complex dependence of the rate of NO consumption on the NH3 surface coverage, but not for the observed weak inhibiting effect of NH3. The solid lines shown in Figure 2 represent the model fit, based on the parameter estimates reported in the caption. A good agreement between experimental data and model fit has been obtained: in particular, the initial dead time in the concentration of NH3 and the levelling off of the NO conversion during the rise phase are well represented. Notably, in the fit of the data shown in figure 2 the parameters for the NH3 adsorption-desorption dynamics obtained during the NH3-CPAD experiments (figure 1) have been used. This confirms the adequacy of the adopted model for the description of the

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transient adsorption-desorption and reaction kinetics, as well as the virtual superposition of 90o the two processes. ' ' ' ' ' ' ' ' ' ;' ~ !_. b) N O - C P S R - A t y p i c a l r e s u l t o f a 8oo NO-CPSR experiment is shown in ~" Figure 3 The NH3 and NO outlet ~. 700 600 concentrations (circles and up triangles, "o 500 respectively) are reported as a function "~ 400 ~' ' " of time, along with that of the NO inlet 300 " concentration (dotted line). The NI-'I3 200 " and NO outlet concentration curves o 100 exhibit very different features from those observed during the NH3-CPSR

~

~

0

..........

! .............

1...

1000 2000 3000 4000 5000 6000-7000

experiment shown in figure 2. Indeed in

Time (s) Figure 3 - NO-CPSR experi~nent performed at 573 K. Symbols: experimental data. a): NO, b): ammonia concentration. Dotted line: inlet NO concentration. Solid lines: model predictions, Kinetic parameters as in figure 2.

this case the NH3 consumption and the N2 production (not reported in the figure) start as soon as NO is fed to the

0

reactor and they continue to increase during all the rise phase. The curve of the NO outlet concentration does not show any dead time, thus indicating that NO does not appreciably adsorb on the catalyst surface. The curves of the different species are symmetrical, pointing out a direct dependence of the reaction rate on the gaseous concentration of NO. The kinetic model used to fit the NH3-CPAD and NH3-CPSR experiments was also used to analyze the NO-CPSR experiments on a purely predictive basis. It appears that these experiments are nicely described by using the kinetic parameter estimates obtained in previous fits (see Figure 3, T=573K). c) Effect of H20 and S02 in the feed stream - To ~100 ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' investigate the reactivity of the commercial catalyst ~v 80 .:"" -.. NO under more represemative operating conditions, NH3- and NO-CPSR experiments were performed 8 60 b ......... ::~..in the presence of H20 (0.8 and 5 % v/v) and of E 4o SO2 (500 ppm) in the feed stream. As an example, o 2o figure 4 compares the results of NH3-CPSR tO experiments performed in the absence of 1-I20 and 0SO2 (curves a), in the presence of H20 (5% v/v, ' ' ' ' ' . . . . . . . ' ' ' " .. curves b), and in the presence of H20 (5% v/v) and ~= 80 SO2 (curves c). In all cases similar dynamics are t::: observed, i.e. the presence of water and SO2 in the .o 60 feed does not modify the dynamic features of the 40 reaction. On the other hand, figure 4 points out that to water and SO2 addition to the feed stream strongly 20 tO affects the reactivity of the catalyst, since very different NO conversion levels are attained at 0 50 100 150 200 250 300 350 400 steady-state in the presence of water and water + Time (a.u.) SO2. In particular, water inhibits the SCR reaction, Figure 4 - NH3-CPSR experiment at 593 K. Dashed lines: inlet NH3 concentration; whereas the presence of SO2 enhances the catalyst curves a: NH3 in NO; curves b: NH3 in activity. Notably, the presence of SOz in the feed NO+ H20 (5%); curves c: NH3 in NO+ H20 overcomes the inhibiting effect of water, since the (5%) + SO2 (500 ppm) .,- ...................

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reactivity in the presence of H20 + 502 is higher than that measured in the absence of both species (compare curves a and c). It is also noteworthy that a strong inhibiting effect of water can be observed already for low water contents, e.g. 0.8 % v/v (data not reported). The well known inhibiting effect of water on the SCR reaction has been interpreted in different ways, e.g.: i) competition of H20 with NH3 on the adsorption on the active sites; ii) modification of the structure of the active sites (e.g. conversion of Lewis into Bronsted acid sites [5]); and iii) retention of high catalyst oxidation state [6]. As a matter of facts, the NH3-CPAD data reported in fig. 1 indicate that the presence of H20 in the feed (0.8 % v/v) does not appreciably modify the adsorption-desorption characteristics of N H3: this apparently rules out any competition effect of H20 with NH3 on the adsorption on the catalyst surface. The beneficial effect of SO2 on the catalyst activity is also well documented in the literature, and it has been associated with the strengthening of the Lewis acidity of the vanadyl sites [7], or to the formation of new acid sulfate sites (either Lewis or Bronsted) [8] close-by to the vanadyl sites, that has been suggested to favor the SCR reaction [9]. Work is currently in progress to arrive at a quantitative analysis of the effect of H20 and SO2 on the SCR reaction aiming at a better understanding of the related mechanistic implications. 4. CONCLUSIONS Our work demonstrates the potential of the CPAD/CPSR technique in evaluating kinetic and mechanistic aspects of the SCR process. In particular, it has provided a way to study the adsorption-desorption of reactants separately from their surface reaction, thus allowing separate investigation of the sequence of steps of the reaction. The data confirmed that over the investigated V205-WOa/TiO2 commercial catalyst NI-I3 is stored on the catalyst surface, and that the reaction occurs between adsorbed NH3 and gaseous or weakly adsorbed NO. The dynamic study clearly showed that H20 does not compete with NH3 in the adsorption on the surface acid sites at any surface coverage, but significantly inhibits the SCR reaction; an inhibiting effect of adsorbed NH3 on the reaction has also been pointed out. SO2, on the other hand, enhances the reactivity of the catalyst: in all cases similar dynamics are observed, i.e. the presence of H20 and SO2 in the feed does not modify the dynamic features of the reaction but affects the reactivity of the catalyst. It is worth of note that such aspects could not have been established so conclusively neither based on steady-state techniques nor on the usual transient step response methods, since by imposing a finite rate of change of the operating variables the system d ~ ~ c s can be analysed over the full range of intermediate conditions. The overall set of data could be nicely described according to a dynamic kinetic model of the SCR reaction which superimposes the reaction to the NH3 adsorption-desorption processes. A complex dependence of the rate of NO consumption on the ~"I3 surface coverage has been established, and the related mechanistic implications will be addressed. REFERENCES 1. L. Lietti, I. Nova, S. Camurri, E. Tronconi, P. Forzatti, AIChE Journal, 43 (1997) 2559. 2. E. Tronconi, A. Cavanna and P. Forzatti, Ind. Eng. Chem. Res., 37 (1998) 2341. 3. E. Tronconi, C. Orsenigo, A. Cavanna, P. Forzatti, Ind. Eng. Chem. Res, in press. 4. M. Koebel, M. Elsener, Ind. Eng. CherrL Res., 37 (1998) 327 5. G.Ramis, C.Cristiani, P.Forzatti and G. Busca, J. Catal. 124 (1990) 574 6. S.A. Selim, Ch.A.Philip and R.Mikhail, Sh. Thermochirn. Acta, 36 (1980) 287 7. G. Ramis, G. Busca and F. Bregani, Catal. Lett., 18 (1993) 299 8. J.P. Chen and R.T. Yang, J. Catal 139 (1993) 277 9. C. Orsenigo, L. Lietti, E. Tronconi, P.Forzatti and F. Bregani, Ind. Eng. Chem. Res., 37 (1998) 2350

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Bifunctional Nature of Reduction of NO~

SnO2/Y-Al203Catalysts in the Selective

A. Yezerets*, Y. Zheng*, P.W. Park #, M.C. Kung* and H.H. Kung*. * Center for Catalysis and Surface Science, Northwestern Univ., Evanston, I1 60208, USA. # Current address: Caterpillar Inc., Peoria, IL, USA. ABSTRACT In lean NOx reduction by C3H6 over SnO2/A1203, both SnO2 and A1203 participate in the reaction. C3H6 is activated on SnO2 active sites to form oxygenated intermediates such as acrolein and acetaldehyde. The oxygenates subsequently react with NOx on A1203 to yield N2. 1. INTRODUCTION The push for more fuel-efficient vehicles and the increasingly stringent environmental regulations have spurred intensive research in lean NOx catalysis. Lean NOx reduction is challenging because the selective reduction process (Eq. 1) has to compete with the combustion reaction (Eq.2). 2C3H6 + ((12+y)/x)NOx ~((12+y)/2x) N2 +yCO2+(6-y)CO+6H20 C3H6 + 3(1+x)/202 ~ 3COx+ 3H20

(1) (2)

The selective reduction ofNOx (Eq.1) is a multi-step process involving the activation of the hydrocarbon and its subsequent reaction with NOx to yield N2. It has been proposed that the activated hydrocarbon may be an oxygenated one [ 1,2]. Hamada et al., have demonstrated that oxygenates, like alcohol, are very effective reductants of NOx over inert oxides such as A1203 [2]. SnO2/A1203 is one of the most active and stable lean NOx catalyst reported in the literature [3]. Extensive characterization shows that both amorphous and crystalline SnO 2 co-exist on the A1203 support [4]. The size distribution of the amorphous oxo-tin clusters is very broad and the ratio of the amorphous and crystalline SnO2 changes with Sn loading. Surprisingly, the maximum NOx conversions over a large range ofSn loadings (1-10 wt.%) are very similar [4]. Since for most lean NOx catalysts, the maximum NOx conversion occurs when hydrocarbon consumption is near completion, its value obtained under similar reaction conditions reflects how effective an active site is in promoting the selective NOx reduction. Thus, it appears that the ability to promote selective NOx conversions over the combustion reaction is not sensitive to the dispersion of SnO2. This is in contrast to many other A1203- supported lean NOx catalysts, where the best performance occurs at relatively low metal loadings of 2 wt.% or less [5,6]. The objective of this study is to understand this unusual feature of SnO2/A1203 catalyst via careful delineation of the roles of SnO2 and A1203 in the lean NOx reduction process.

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2. EXPERIMENTAL

2.1. Catalyst Preparation The supported SnO2 catalysts were prepared by incipient wetness impregnation with an ethanolic solution of SnC12 on y-A120313] or SIO217], and calcined in air at 800~ for 2h. XRD showed that on the both supports SnO2 exists in rutile form. The catalysts are labeled according to the nominal Sn loading and support. Thus, Sn5/A1203 is a catalyst with 5wt% Sn loading, supported on A1203. 2.2. Catalytic Reaction The catalysts were tested in a flow of 200 cc/min gas feed composed of 15% 02, 10 % H20, 0.11% C3H6,0.1% NO and balance He. Two designs of the reactor assembly were used: 1. single-bed configuration with the option to reverse flow direction through the reactor. 2. double-bed: two identical reactors were placed in series so that the feed can pass through one or both of the reactors. The temperature of each reactor was controlled separately. The void space of the fused silica microreactor was packed with quartz chips to minimize gas phase reactions. Gas phase reaction is greatly promoted by the presence of 0.1% NO and the extent of gas phase reaction in an empty reactor as a function of temperatures, detected as C3H6 conversions, are as follows: 1% at 525~ 6% at 550~ and 40% at 575~ No N2 production was observed accompanying gas phase reaction of C3H6. The reaction products were analyzed using a HP 6890 gas chromatograph equipped with two parallel columns packed with Haysep Q and molecular sieve 5A. The exit gas from the former column was analyzed with a HP5973 massselective detector. NO, concentration was measured using a Beckman 951 NOx analyzer.

3. RESULTS AND DISCUSSION 3.1 Evidence of Bifunctional nature of SnO2/Al203 Figure 1 compares the NOx conversions over 0.04 g of Sn5/AI203 (curve a) and 0.2 g of Snl/A1203 (curve b). Although the same weight of SnO2 was used for both experiments, the latter catalyst was much more effective in NOx reduction. This is unexpected, as the ability to promote

60

%

Figure 1. Conversion of NOx b

50-

C

4030-

d

20-

a

100 400

,

450

,

500

~.

#

#

e

550 600 Temperature,~

% 80 70 60 5O 40 30 20 10 0 400

Figure 2. Conversion of C3H6

450

500

550 600 Temperature,~

Figures 1 and 2: Catalytic Performance of a) 0.04 g Sn5/A1203, b) 0.2 g Snl/A1203, c)physical mixture of 0.04 g Sn5/A1203 + 0.16g A1203, d) 0.16 g A1203, and e) no catalyst.

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selective NOx conversions over the combustion reaction of supported SnO2 appears to be independent of its dispersion [4]. However, the performance of 0.04 g Sn5/A1203 can be elevated almost to the level of0.2g Snl/A1203 by simply mixing it with 0.16g A1203 (curve c). The NOx conversions of the physical mixture of 0.04 g Sn5/A1203 and 0.16 g A1203 were substantially higher than the sum of the conversions of the individual components (curves a and d). Figure 2 shows the corresponding C3H 6 conversions over these catalysts. Supported SnO2 readily promotes the activation of C3H6as significant C3H 6 conversions were observed using only 0.04 g Sn5/A1203 (space velocity of 160,000 h-l). It is interesting to note that the C3H6conversion over A1203 at 575~ was lower than the gas phase reaction. This is because gas phase reactions are free radical reactions and the presence of catalysts effectively arrests the chain propagation. The importance of A1203 for NOx reduction is further demonstrated when the NOx and C3H 6 conversions were compared over Sn5/A1203 and Sn5/SiO2 (Table I). NOx conversions were observed only for the Sn5/A1203 catalysts, although C3H 6 was effectively converted on the Sn5/SiO2 catalyst as well. Table I. Comparison of NOx and c 3 n 6 conversions (%) over 0.2g Sn5/A1203 and Sn5/SiO2 Catalyst

NOx Conversion

NOx Conversion

C3H 6 Conversion

C3H 6 Conversion

Sn5/A1203

54 (450~

52 (450~

48 (525~

88 (525oc)

Sn5/SiO2

0.8 (450~

16 (450~

3 (525oc)

92 (525~

The above results indicate that the activation of C3H 6 occurs on the SnO 2 site and the production of N2 occurs on A1203 sites. To ensure that the synergistic effect observed for the physical mixture (Fig.l) was not due to migration of SnO2 from Sn5/A1203 to A1203, an experiment was performed in which 0.1 g Sn5/SiO/was physically separated from 0.1 g A1203by a layer of quartz wool (Fig 3). The flow direction of the reaction feed was controlled such that it could pass from the layer of Sn5/SiO2 to A1203 or vice versa In the absence of any synergistic Fig. 3a:NO x Conversions

30 25

Figure 3b: C3H6 Conversions

60 o 50

40

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440 460 480 Terroerature (~

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420

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Figure 3" NOx and C3H 6 conversions over physically separated Sn5/SiO2 and A1203:" a) Flow direction from Sn5/SiO2 to A1203; b) feed from A1203 to Sn5/SiO2.

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effect between the two beds, the overall conversions should be similar regardless of the direction of the flow. This was seen for the C3H6 conversion (Fig.3b). However, significantly higher N2 yield was observed with the feed flowing from Sn5/SiO2 to A1203 catalyst than when the flow was reversed (Fig.3a). That a synergistic effect was observed even when the beds were physically separated suggests that some stable intermediates are generated on the SnO2 site and these intermediates can react over A1203 to form N2.

3.2 Cause of Synergistic Effect To uncouple the catalytic characteristics of SnO2 and A1203, a two bed configuration was employed and the compositions of the exit gas from different catalysts were tabulated in Table II. Significant C3H6conversions was observed over 0.1 g Sn5/SiO2 or 0.04 g Sn5/A1203, but not over 0.1 g A1203. C3H6conversion was stable with-time-on stream over 0.04g Sn5/A1203. The catalyst was white after reaction and within experimental uncertainties the carbon appeared balanced. However, C3H6 conversion over Sn5/SiO2 decreased slowly with time. The decrease was faster for C3H6 conversion and CO2 production than for the generation of acetaldehyde and acrolein. Deactivation of Sn5/SiO2 may in part be due to coking as the catalyst was grey after reaction and there was a carbon imbalance which decreased with time-on-stream. Trace amounts of products are not listed in Table II and these included acetonitrile, propenenitrile, nitromethane and acetone. Table II. GC-MS analysis of exit gas from reactors containing 0.1 g Sn5/SiO2 or 0.04 g Sn5/A1203 at 500~ and 0.1 g A1203 at 475~ or combination of these two. Product Concentrations in ppm C2H4 C3H6 Ac a

% Conv.

Acl b

HCN

N2

C3H6

NO

42

120

0

0

43

0

894

27

89

0

0

23

0

8

809

29

2

22

85

30

17

0

1

1122

2

0

0

19

3

4

497

434

35

699

0

0

0

106

40

21

Sn5/SiO2 (50 h ) +A1203 c

264

531

12

843

4

0

0

85

27

17

Sn5/A1203 + A1203c

463

690

10

731

2

0

0

180

37

36

Catalysts

CO 2

CO

Sn5/Si02 (flesh)

421

0

26

662

Sn5/Si02 (after 50 h)

134

0

14

Sn5/A1203

402

391

A1203

20

Sn5/SiO2 (flesh) +A1203 c

a. acetaldehyde; b. acrolein: c. supported SnO2 was upstream of A1203 and in separate reactors The distributions of carbon containing products differed significantly for Sn5/SiO2 and Sn5/A1203. CO2 was the only combustion product for Sn5/SiO2, whereas comparable amounts of CO and CO2 were produced over Sn5/A1203. Besides CO2, acrolein was the major product detected over Sn5/SiO2. However, it was not detected over Sn5/A1203. On the other hand, HCN was detected from Sn5/AI203 but not from Sn5/SiO2.

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The absence of acrolein in the product stream from Sn5/A1203 catalysts might be due to its facile reaction over A1203. Indeed, when the product stream from the Sn5/SiO2 catalysts was passed through a second reactor containing only A1203, the exit gas no longer contained acrolein, This ease of reaction of acrolein is in agreement with reports that oxygenated compounds usually react at significantly lower temperatures than alkene [2] and proposals that oxygenated hydrocarbons are important intermediates in the pathway towards N2 production [1]. The observation of N2 yield on Sn5/A1203 and not on Sn5/SiO2 is consistent with the proposal that acrolein is able to react with NOx to produce N2 on A1203. Furthermore, with time-on -stream over Sn5/SiO2, although C3H 6 conversion declined by as much as 47%, acrolein production only dropped by 26% and concomitantly N2 yield over Sn5/SiO2 + A1203 combination only decreased by 20% (Table II). Acrolein reaction with NOx on A1203 appeared to result in HCN formation, as the latter was detected in the product stream of 0.04 g Sn5/AI/O3 but not Sn5/SiO2. In the 2-bed configuration of Sn5/SiO2 and A1203, HCN was detected when a reduced amount of AlzO3 (0.01 g) was used, but was absent with 0.1 g A1203 present in the second bed. Since the TCD sensitivity factor of HCN is not known at present, its concentration was estimated by assuming a sensitivity factor identical to water. It is possible that HCN reacts with NOx on A1203 to form N2, as higher NOx conversion was observed with 0.1 g than 0.01 g of A1203. However, this reaction can only be confirmed with further experiments using isotope labeling. Baiker, et al. [9] have proposed formation ofallyl oxime from propene, and that trans-elimination reaction of the oxime can result in the formation of acetaldehyde and HCN. HCN has also been observed in the exit gas of some lean NOx catalysts such as Cu-ZSM-5 and Cu/A1203 [9,10] and thus may play an important role in the selective reduction of NOx by hydrocarbon. The other major product from the reaction ofC3H 6 on supported SnO2 is acetaldehyde. As discussed, acetaldehyde can also be produced from allyl oxime on A1203, and at the same time, being an oxygenate, it can readily react with NOx or 02. That acetaldehyde was being produced and consumed at the same time when the exit gas from supported SnO2 was passed over a bed of A1203 was demonstrated by changing the temperature of A1203. When A1203 was at 475 ~ the acetaldehyde produced on the SnOz site was consumed over A1203, but at 400~ the acetaldehyde concentration at the exit of the A1203 bed was higher than at the inlet. Thus, a number of potential reaction intermediates were detected in these experiments. They include various oxygenates such as acrolein and acetaldehyde, as well as HCN. All of these have been shown to react on A1203 and with their consumption, concomitant increases in the production of N2 was observed. Additional experiments will be performed to further identify their detailed role in the NOx reduction process. For many bifunctional catalytic systems, enhanced N 2 yield is achieved by using one component to generate NO2 and a second component to facilitate the reaction of N O / w i t h hydrocarbon to produce N2 [8]. In this study, the contribution of NO2 to the observed synergistic effect was minor since its concentration measured after Sn5/SiO2 was the same as the background level and that measure aider Sn5/A1203 was only between 20 and 30 ppm above that.

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4.

CONCLUSIONS

Figure 4 shows a possible reaction scheme for the selective reduction of NOx over SnO2/A1203 catalyst. The detected molecules are depicted in bold. This is a scheme that applies to the observed synergistic effect in experiments where the supported SnO2 and A1203 are separated. However, it may not be a complete picture as it is not understood why in the doublebed configuration, NOx conversions are higher for the SnO2/A1203 + A1203 configuration than the SnO2/SiO2 + A1203 one. However, the presence of synergistic effect is very promising. It suggests that tremendous flexibility in the design of lean NOx catalysts can be introduced by separating the hydrocarbon activating function from the N2 forming function. The optimization of the former function may be achieved by designing a good partial oxidation catalyst.

CH3_CHO NOx 02, NO~ ~ CH2=CH-CHO ...................> CH2=CH-CH=N-OH---->

C3H~

'~

A1203

SnO2 L

HCN

>N~

AI203

NOx CH3_CHO

'~

A1203

Figure 4. Proposed reaction scheme for the selective reduction of NOx over SnO2/AI203. ACKNOWLEDGMENT This work was supported by the Department of Energy, Basic Energy Sciences. Y. Zheng acknowledged partial support from the EMSI program of the National Science Foundation and the Department of Energy [CHE-9810378] at the Northwestern University Institute for Environmental Catalysis. REFERENCES AND .

2. .

4. 5. 6. 7. .

9. 10.

NOTES

T. Truex, Autocatalyst News, No 8 (1991) ( Johnson-Matthey Plc, Royston, U.K.) H. Hamada, Y. Kintaichi, T. Yoshinari, M. Tabata, M. Sasaki, and T. Ito, Catal. Today, 17,111 (1993). M.C. Kung, P.W. Park, D.-W. Kim, and H. H. Kung, J. Catal., 181, 1 (1999). P. W. Park, H. H. Kung, D.-W. Kim, and M. C. Kung, J. Catal., 184, 440 (1999). J.Yang, M.C. Kung, W.M.H. Sachtler, and H. H. Kung, J. Catal., 172, 178 (1997). K. A. Bethke, and H. H. Kung, J. Catal., 172, 93 (1997). Davidson-62 silica (250 m2/g) was washed with 4MHNO3. dried at 120~ and then calcined in air at 450~ for 12h. C. Yokoyama, and M. Misono, Catal. Lett., 29,1 (1994). F. Radtke, R.A. Koeppel, and A. Baiker, J. Chem. Soc. Chem. Commun., 427 (1995). F. Radtke, R.A. Koeppel, E. G. Minardi, and A. Baiker, J. Catal., 167, 127 (1997).

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Studies in Surface Science and Catalysis 130 A. Corrna, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

SO2 resistant Fe/ZSM-5 catalysts for the conversion of nitrogen oxides G. Centi, G. Grasso, F. Vazzana and F. Arena Dip. Chim. Ind. ed Ing. Materiali, Univ. Messina, Italy. Salita Sperone 31, 98166 Messina, Italy. Phone: +39-090-393134, fax: +39-090-391518, e-mail: [email protected] Fe/ZSM-5 catalysts prepared by CVD (chemical vapor deposition) show a stable activity in the selective catalytic reduction of N20 and NO with propane in the off-gas from chemical processes. These catalysts show a better propane economy and a considerable higher resistance to deactivation by SO2 especially at lower reaction temperatures than using Fe/ZSM-5 catalysts prepared by ion-exchange or impregnation, especially when the parent zeolite is pretreated to create structural defects. The peculiar activity and stability characteristics of these catalysts are suggested to be related to the presence of iron-oxide nanocluster, whereas with other methods highly clustered Fe 3+ species and Fe203 particles also form.

Introduction The requested reduction of greenhouse gas emissions (Osaka agreement) will lead soon to new regulation limits on the emission of N20, a powerful greenhouse. Therefore, efficient technologies for its removal, in particular in the off-gas of the production and use of nitric acid and the industrial combustion of waste, must be developed. [ 1]. Current catalysts are not stable and active enough, and the process cannot be applied economically, when N20 is present in a diluted concentration (typically below 0.1%) and in the presence of H20, 02 and poisoning agents such as SO2 and NOx. Recent data, however, have shown that Fe/MFI catalysts may be successfully applied for the reduction of nitrogen oxides (N20 and NO) in such a type of conditions, when an hydrocarbon is cofeed [1-6].

Experimental Fe/ZSM-5 catalysts were prepared by impregnation (FeIMP/ZSM-5), ion-exchange (FelE/ZSM-5) and CVD (Chemical Vapour Deposition) using a parent ZSM-5 zeolite unpretreated or pretreated to create structural defects in the zeolite (FecvD.UNP/ZSM-5 and FecvDp/ZSM-5, respectively) are studied. The parent zeolite for the preparations was the Na-form of a commercial ZSM-5 sample synthesized with a template-free method (SN27 from ALSIPenta, SIO2/A1203 = 27). The zeolite pretreatment prior to Fe addition (FecvD_p/ZSM-5) was a hydrothermal treatment (6h at 650~ in a flow of N2 containing 3% steam) followed by washing with ammonium acetate aqueous solution to remove extra-lattice aluminium. 3+ The impregnation was made using incipient wet impregnation method and Fe -nitrate as the salt. The ion-exchange method was made using an 0.02 N aqueous solution of ironammonium-sulphate heated to 80~ The CVD method was made in a N2 flow after anhydrification of the zeolite and using FeCl3 as the reactant. Further details on the preparation were reported previously [6]. The catalytic behavior was studied in a flow reactor apparatus with on-line mass quadru-

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Table I Characteristics of the Fe/ZSM-5 samples used for the catalytic tests. (wt.) ~

Si/Al molar ratio I

Surface area, me/g

Crystallinity, % (from IR2; ._.+10)

phases 3

FelMv/ZSM-5

2.27

27

286

100

MFI

FeIE/ZSM-5

3.75

27

237

100

MFI

Fecvo.tmv/ZSM-5

0.81

27

362

100

MFI

35

306

90

Sample

%Fe

XRD

MFI 0.63 FecvD.v/ZSM-5 1 determined from atomic adsorption (AA) spectroscopy 2 crystallinity of the zeolite estimated from the intensity ratio of the bands at 450 and 550 cm~ in the infrared (IR) spectrum with respect to the parent Na-ZSM-5 zeolite. 3 No phases associated to iron were detected by X-ray diffraction (XRD). pole analysis. Tests were made using 0.2g of zeolite in the form of pellets with 40-60 mesh of dimension and a gas hourly space-velocity (GHSV) of 23.000 h 1. The typical gas composition for the tests was 0.05% N20, 2% 02, 3% H20, 0.1% C3H8 and the remaining helium. The same composition, but in the presence of also 0.05% SO2, was used for the tests of durability. The analysis of the composition of the inlet and outlet gas streams of the reactor was made by an on-line mass quadrupole apparatus, after correction of the mass intensities to consider multiple fragmentations, when necessary. The line from the reactor to the mass quadrupole was heated at 150~ to prevent the condensation of the products. The conversion of the reactants (N20 and C3H8) was estimated on a molar basis. Samples were characterized by AA, IR, XRD, UV-Visible-diffuse reflectance spectroscopy, temperature programmed reduction and electron microscopy analysis (SEM-EDX). Results and Discussion

The catalysts characteristics are summarized in Table 1. Three methods of loading the iron were used, by (1) incipient wet impregnation (FelMp/ZSM-5) using iron-nitrate, (2) ionexchange (FelE/ZSM-5) using a very soluble salt (iron-ammonium-sulphate), and (3) chemical vapour deposition with FeC13 on the anhydrified zeolite. In the latter case, either the zeolite as such in the sodium form (FecvD-LrNp/ZSM-5) or the zeolite partially dealuminated by hydrothermal treatment (FecvD.p/ZSM-5) was used. No residual C1 in the samples prepared by CVD was detected after calcination. In all cases, XRD show a good crystallinity and the absence of crystalline phases other than the zeolite itself (MFI structure). IR analysis in the region around 500 cm l confirms that, within the experimental error, a loss of crystallinity does not occurs, apart a slight decrease for FecvD_p/ZSM-5. All the samples show a larger surface area than the parent zeolite in the sodium form (196 m2/g), probably due to the removal of some clustered sodium. The analysis of the pore distribution in the samples do not shows significant changes in the micro-meso porosity distribution, in particular in the samples prepared by CVD. Electron microscopy with electron dispersion X-ray (EDX) analyses confirm the absence of modification in the morphology of zeolite crystallites (mean dimension 1-2 lam) and of segregation of iron out of the zeolite crystallites, the latter apart for the sample prepared by impregnation. Activity in N20 reduction. The activity curve for N20 reduction to Nx of the various Fe/ZSM5 catalysts is reported in Figure 1. All samples show a good activity in the selective reduction of N20 to N2 with conversion above 80% at low reaction temperatures (below 400~ not-

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80

Z

60

~

4o

g

8

-41-- FezE/ZSM-5 - O - Fecvo.,JZSM-5 - - ~ - FeMJZSM-5

2O

Fecvo.uNP/ZSM-5

0

200

~

,

300

i

,

i

,

|

500

400

Reaction temperature, *C

100

80 td

"1" e,)

o c ._o r

> t-

60

40

O

o

o 200

300

~

~

~

Fecvo.uNP/ZSM,

400

5

500

Reaction temperature, ~

Fig. 1 Catalytic behavior of Fe/ZSM-5 catalysts in the reduction of N20 with propane/O2 in the presence of steam in the feed. Fig. 2 Turnover frequency at 350~ of of Fe/ZSM-5 catalysts in N20 and C3H8 depletion.

I--.~o II r~C3H8 < o o

z~ 14.

o

I-

II CVD-P Fex/ZSM-5

IMP

CVD-UNP

9

withstanding the low concentration of propane in the feed (1000 ppm). All the samples show also a comparable activity, apart from Fecvo.tmv/ZSM-5 slightly less active. The activity in N20 conversion does not parallel that in propane conversion, where two group of samples can be identified: (i) samples prepared by ion-exchange or impregnation and (ii) samples prepared by CVD. The latter show a lower activity in propane conversion. Based on a first order rate equation that was found to satisfactory model the kinetic of the reaction and a plugflow reactor model, it is possible to estimate the turnover frequency (TOF) assuming that all iron ions are available for the catalytic activity. The resuits for a reaction temperature of 350~ are shown in Figure 2. The sample prepared by CVD on the pretreated zeolite (FEcvD_p/ZSM5) shows the highest specific activity in N20 reduction. Furthermore, differently from the other samples, the rate of N20 depletion is higher than the rate of propane depletion. This indicates that on this catalyst it is possible to use very low amounts of propane as selective reductant, without affecting the rate of nitrogen oxides selective reduction, an important effect in terms of process economics. Durability in the presence of SO_2. The Fe/ZSM-5 catalysts prepared in the different modes showed a different durability in the presence of SO2 especially at temperatures below about 450~ which are the more interesting for practical applications. Reported in Figure 3 is the comparison of FelMP/ ZSM-5 and FecvD.p/ZSM-5 durability in accelerate tests using a high SO2 concentration in the feed (500 ppm). FeIMP/ZSM-5 catalyst shows a very

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=~

F%

80

G) C

|~

u~

60

._

U

400"C

o

,",

60 ~

u"~

o

~

c

o

40

433~ (+ 430 ppm NO)

"~

>e

,

~8

>

9 FeuJZSM5 (NzO) O Fecvo.p/ZSM-5(N20) 9 Fec~JZSM5 (NO)

20

0

i

0

200

,

,

.

,

,

400

800

600

Time on stream, h

Fig. 3 Comparion of the conversion of N20 of FeIMp/ZSM and Fe CVD-P/ZSM-5 during durability tests in the presence of a high SO2 conc. (500 ppm). Other conditions as indicated in the experimental section. In the case of Fe CVD-P/ZSM-5 aider 600h of time on stream 430 ppm NO were also added to the feed. 120

t 100~___

9 Conv.N20 O Conv.C3H8

|

fast deactivation at 380~ Increasing the reaction temperature to 410~ the rate of deactivation decreases, but still a large deactivation could be observed in about 100 h of time on stream. Only at much higher reaction temperatures (above 480~ a nearly constant activity could be observed for over 500 h of time on stream. A different behavior was detected for FecvD.p/ZSM5. In this case, even at low temperature (400~ a constant catalytic behavior is observed after an initial minor decay of the activity occurring approximately during the first 100 h of time on stream. Then a constant activity was observed up to 600 h of time on stream. Increasing the reaction temperature to about 430~ a constant conversion of N20 of over 80% is observed.

A temperature dependence of the durability behavior in the presence of SO2 was observed also for .o_ 60 500"C FecvD.tmp/ZSM-5 and FeIE/ZSM-5. T= 480 *C c The behavior of the latter which is o 0 40 T= 450 *C similar to that of FecvD_UNP/ZSM-5 is reported in Figure 4. At 500~ a 20 stable catalytic activity is observed, both in N20 and propane conver0 20 40 60 80 100 sion. Decreasing the reaction temTime, h perature to 450~ a progressive decrease of the activity is detected. Fig. 4 Durability tests in the presence of 802 (500 ppm) at A stable behavior requires a reacdifferent reaction temperature of Fe~E/ZSM-5. Other tion temperature higher than about conditions as in Fig. 3. 470~ Therefore, although these two samples show a lower deactivation rate by 8 0 2 than FelMP/ZSM-5, they have a stable activity at temperature only above about 470~ Only FecvD.p/ZSM-5 show a stable activity in the presence of SO2 at reaction temperatures lower than 450~ ~

8o

0

,

,

,

,

,

,

i

,

,

i

,

,

,

,

,

,

,

,

,

l

l

,

,

,

Conversion of both N20 and NO. The simultaneous conversion of NO and N20 was tested in the aged FecvD.p/ZSM-5 sample by adding 430 ppm NO to the feed (Figure 3). Although activity of the catalyst in converting NO is worsen with respect to N20 (at 430~ about 46% with respect to over 80% conversion of N20), a stable activity in converting NO in the presence of 500 ppm SO2, H20 and 02 is also shown, indicating the applicability of these catalysts for the simultaneous conversion of N20 and NO in industrial emissions.

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8

,

,

Nature of the iron species. Figure 5 show UV-Visible 2 - - - - FemlZSM-5 after cat tests diffuse reflectance (UV-Viscalc 6 / "~'": 3 ....... Fecvo-,JZSM-5 DR) spectra (normalized to L/ .:'~. 4-Fecvo..*/ZSM'5 after cat tests ~" " ~, 5 .... Fe,Mp/ZSM-5 calc the same Fe content) of ~" 6 ~ Fecvo-u. #ZSM'5 calc Fe/ZSM-5 samples before "10 ' "5 '~ | . ,~ .N_ 4 ' . and after the catalytic tests. The spectra are characterized by two intense bands at about 210 and 260 nm due to ligand to metal Fe 3§ charge L . . . . ~.~.~.~..;.~ ~ ~ I transfer in isolated species in o [ . . . . . . . , ~ ' " = ....... ~ " ~ ........... distorted octahedral coordi200 400 800 800 nation [7]. These bands are not specific of a type of iron Fig. 5 UV-Visible diffuse reflectance spectra (normalized to the species and are detected in a same iron content) of Fe/ZSM-5 samples before or after catanumber of compounds, such 3+ lytic tests in N20 reduction with propane/O2 (absence of SO2). as Fe 2§ or Fe-sulphate, Fe 3 + -hydroxide or oxide, etc. 1000 The latter two compounds show in addition a band at 2 ,ecv. S.-s !i about 380 nm which shifts 3 ~ Fecvo.u~p/ZSM-5 750 4 Fe,E/ZSM-5 ! i. to higher frequencies (330 nm) in nanosized crystallites [7]. A broad diffuse absorption between 400-600 nm with a maximum at about 250 550 nm could be also detected upon partial reduction, due to intervalence 0 ' ' charge transfer. 200 300 400 500 600 700 800 The spectra after calcinaTemperature, *C tion (Figure 5) show all a Fig. 6 H2-TPR (temperature programmed reduction) curves (norbroad absorption in the 300malized to the same iron content) for calcined Fe/ZSM-5 sam600 nm region, apart from ples. 80 mg sample, 6% HE in He, 20~ FecvP_p/ZSM-5 for which only a shoulder centred at near 340 nm could be detected. In the other samples, the broad tail can be roughly deconvoluted in two peaks centred at about 380 and 550 nm. This suggests that while in FecvP.p/ZSM-5 small clustered Fe 3§ in (hydr)oxide-type species is present, larger, partially reduced iron-oxide species are present in the other samples. Note, however, that the intervalence band indicates the presence of Fe 2+ ions in an Fe3+-oxide matrix and not the presence of reduced iron-oxide species. In samples prepared by impregnation and ionexchange the 260 band is more intense than the 210 nm band, differently from CVD sample, indicating a different coordination of iron ions, in agreement with the presence of iron-oxide nanoparticles in the latter sample. After catalytic tests, the spectra remain nearly unchanged (for clarity, only spectra of FeIMP/ZSM-5 and FecvP.p/ZSM-5 after catalytic tests have been reported in Figure 5), although in some cases spectra may result apparently more intense (compare, for example, spectra 3 and 4) due to a darkening of the sample related to trapped 1 ~

9

. . . . .

nm

0 r-

|

FemlZSM-5 calc

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electrons. The same effect prevented the analysis of the samples after durability tests. Figure 6 show hydrogen temperature programmed reduction (H2-TPR) experiments. The curves have been normalized to the same iron content. The sample prepared by impregnation shows the typical H2-TPR behavior of supported iron-oxide [3]. The two peaks correspond to the reduction of Fe 3+ to Fe2+ (max. at 373~ and Fe 3§ to Fe ~ (max. at 534 ~ In the sample prepared by ion exchange the start of the reduction shifts to higher temperatures, with two reduction peaks, the first of which is the more intense and the second corresponds to the higher temperature peak of Fe~Mp/ZSM-5. The total area of the two peaks roughly corresponding to that of FeIMP/ZSM-5. The change in the H2-TPR curve indicates the presence of smaller ironoxide crystallites (in agreement with UV-Vis-DR spectra; Fig. 5) reasonably inside the zeolite crystallites, as confirmed by SEM-EDX analysis, and differently from FeIMP/ZSM-5 where part of iron is also outside the zeolite crystals. In the samples prepared by CVD on the unpretreated zeolite the lower temperature peak is also absent, but the reduction occurs only at high temperature, in coincidence with the higher temperature peak of FelMP/ZSM-5. The area of the curve is around 40% of that of the previous two samples, and being the H2-TPR curves in Figure 6 normalized to the same iron content, this indicates that part of the iron could be not reduced. The sample prepared on the pretreated (partially dealuminated) zeolite shows a lowering of about 100~ in the maximum of the reduction curve. The area of the reduction curve also decreases, indicating a further lowering of the fraction of reducible iron species. Relationship between nature of iron species and catalytic behavior. Figure 2 shown that all Fe/ZSM-5 samples, apart from FecvD.p/ZSM-5 show a higher rate of propane depletion than that of N20. This indicates that two parallel pathways of propane conversion are present, one which leads to the selective reduction of N20 forming N2 + CO2 as the final products and a second of direct oxidation of propane to CO2. The comparison of these data with the UV-VisDR spectra (Figure 5) suggests that the species responsible of the direct propane oxidation and of the lowering of TOF (Figure 2) is the iron-oxide species having larger crystal dimensions (UV-Vis-DR band above 350 nm) and higher temperature of reduction (580~ The more selective iron species shows a similar TPR curve (a single TPR peak centred at about 480~ of the dinuclear oxo-hydroxo iron species suggested by Sachtler et al. [3,4] to be the active species in Fe/ZSM-5. UV-Vis-DR spectrum and the role of zeolite pretreatment in the formation of this iron species, however, suggests that a small nanosized iron-oxide is a preferable interpretation [8]. Iron ions during CVD treatment probably initially react with the zeolite defects created during pretreatment (or by the HC1 generated during CVD method itself) forming pseudo-framework ions which probably are not active (a pure iron-silicalite was found inactive in the reaction), but acts as the nucleation center for the iron-oxide nanocrystals. This species also shows a higher resistance to deactivation by SO2 during durability tests, especially at reaction temperatures below 450~ (Figures 3 and 4), and is active in both N20 and NO conversion (Figure 3). [1] [2] [3] [4] [5] [6] [7] [8]

G. Centi, S. Perathoner, F. Vazzana, CHEMTECH, 29(12) (1999) 48. M. Krgel, V.H. Sandoval, W. Schwieger, A. Tissler, T. Turek, Catal. Lett., 51 (1998) 23. H.Y.Chen, W.M.H. Sachtler, Catal. Today, 42 (1998) 73. T.V.Voskoboinikov, H.Y. Chen, W.M.H. Sachtler, Appl. Catal. B, 19 (1998) 279. W.K.Hall, X. Feng, J. Dumesic, R. Watwe, Catal. Lett., 52 (1998) 13. F. Vazzana, G. Centi, Catal. Today, 53 (1999) 683. S. Bordiga, R. Buzzoni, F. Geobaldo, C. Lamberti, E. Giamello, A. Zecchina, G. Leofanti, G. Petrini, G. Tozzola, G. Vlaic, J. Catal., 158 (1996) 486. R.W. Joyner, M. Stockenhuber, Catal. Lett., 45 (1997) 15.

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Studies in Surface Science and Catalysis 130

A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

化

641

M E C H A N I S T I C STUDIICS O F T H E N O x

R E D U C T I O N BY H Y D R O C A R B O N IN O X I D A T I V E

ATMOSPm~RE

St6phanie Schneider, Sandrine Ringler, Paule Girard, Gilbert Maire, Frangois Garin, Dominique Bazin * LERCSI, UMR 7515, ECPM-ULP, 25, rue Becquerel, 67087 Strasbourg Cedex 2, France Tel : 33 (0)3 88 13 69 44, Fax : 33 (0)3 88 13 69 68, e-mail: [email protected] *LURE, Centre Universitaire Paris-Sud, Bat 209 D, 91405 Orsay France This paper gathers an ensemble of experiments realized on platinum-based catalysts for the reaction of reduction of NOx by hydrocarbons in the presence of an excess of oxygen. We have analyzed the influence of two parameters on the reactivity : the metal loading of the catalyst and the acidity of the support. Through the combined used of labeled compounds and characterization techniques, we were able to explain differences observed in the various mechanisms.

1. INTRODUCTION The removal of NOx from exhaust gases has been widely studied since several years. The catalytic post-combustion treatment represents the only available solution to diminish the NOx emissions [1,2]. Despite the numerous catalysts tested, none satisfies fully the required conditions in the selective catalytic reduction (SCR) of NOx by hydrocarbons. In order to improve their compositions, it is necessary to understand the reactional mechanisms which govern the SCR by hydrocarbons. Several propositions have been made [3-6] but it appears that the mechanism(s) are not fully understood. Indeed, the various pathways implicated in the reaction make it very difficult to understand. Platinum-based catalysts represent one of the most promising materials to achieve the reduction of NOx by propene and propane in the presence of an excess of oxygen [6]. To bring some new responses on the mechanism(s), we have engaged a comparative study on a set of platinum-based catalysts deposited either on alumina or on zeolites and used labeled compounds such as 15NO and 1802 in order to follow potential intermediates. Among the work performed with the platinum catalysts on the various SCR mechanisms, still remain several aspects concerning the influence of i) metal loading, i.e. particle size and shape and ii) support acidity; for such reactions.

2. EXPERIMENTAL PROCEDURE 2.1. Catalysts used: preparation and principal characteristics The studied catalysts are composed of platinum deposited either on alumina or on zeolites. They are listed in Table 1.

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2.1.1. Alumina- based catalysts: The preparation method was described earlier in a previous paper [7]. What is important to note is the difference in the heating mode during the reductive treatment between the two 0.2wt% Pt/A1203 : one was heated following a conventional manner with a Joule effect heating (0.2%AL), the other one was heated by the use of microwaves (0.2%ALMO).

2.1.2. Zeolites based catalysts: The catalysts were prepared by ionic exchange of the zeolite with a solution of hexachloroplatinic acid complexed with NH3. The catalyst is calcined under air at 450~ for 2 hours before the experiments. Table 1 : Catalyst description: platinum percentage, support used, specifications in function of their coded name. Specifications Coded name Support Ptwt% I%AL H2 reductive treatment-Joule effect heating 1 )' A1203 H2 reductive treatment-Joule effect heating 0.2%AL 0.2 ]I A1203 0.2%ALMO H2 reductive treatment-MICROWAVE heating 0.2 7 A1203 EMT 02 oxidative treatment NaEMT 0.5 0.5 H-ZSM5 02 oxidative treatment ZSM

2.2. Characterization techniques used 9 The mean particle sizes of platinum were determined either with TEM measurements on pre-oxidized samples or with CO or H2 chemisorptions on reduced samples. 9 The 1%AL catalyst was analyzed by in situ EXAFS measurements at the platinum Lm edge under a reactional atmosphere composed of NO (500 ppm), C3H6 (500 ppm) and 02 (14%). The experiments were done at the LURE synchrotron (XAS 4 station). The catalyst was first reduced under pure hydrogen at 450~ for 2 hours before being cooled down at room temperature. The gaseous mixture was then introduced and, at three temperatures: 100, 200 and 300~ the XAS spectra were collected. 9 The 0.2%AL and 0.2%ALMO were analyzed by in situ Infrared experiments realized under CO. Infrared spectra were recorded on a Nicolet 5 DCX Fourier transform apparatus with a 4 cm ~ resolution and 35 scans. The catalyst was a pellet of 13 mm diameter. 2.3. Catalytic tests: experimental set-up The experiments were performed in a catalytic reactor working under recirculation conditions directly coupled to a magnetic mass spectrometer, as analyzer. With this set up, we were able to follow the catalytic activity versus the time at various constant temperatures between 150 and 250~ under 550 torr. The gaseous mixture was composed of 3.6% 1802, 727 ppm 15NO, 727 ppm C3H8 and 363 ppm C3H6 with He as balance. The quantity of catalyst used for the experiments was 100 and 300 mg. Before each test, the catalysts were pre-treated in situ under a flow of ~602 at 350~ for 3 hours. 3.

RESULTS

3.1. Initial rates and NO conversion: general remarks The analysis of the initial rates, at each temperature, reveals how complex the mechanism of NO reduction is. Indeed, if we consider that total oxidation reactions such as the oxidation

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of propene by 1802 or 15NO only occurs, we should obtain an equality between the disappearance rates of the reactants and the formation rates of the products concerned, which is never the case. This implies that several reactions occur simultaneously at the same temperature [7]. In Table 2 are listed the initial rates in mol.(gpt.S)"1 for 15N2formation and for 15NO, C3H6 and 1802 disappearance at 200~ for each catalyst. Table 2 : Comparison of initial rates ('104) of 15NO, C3H6 and 1802 disappearance (minus sign) and 15N2 formation with NOx conversion and 15N2selectivity at 200~ Catalyst 15NO 1802 C3H6 15N2 Conversion NOx % Selectivity 15N2 % 1%AL -5.6 -9.3 -4.1 +0.2 75 30 0.2%AL -2.8 -14.4 -4.6 +0.5 68 99 0.2%ALMO -24.4 -2.6 +0.6 67 1011 EMT - 7. 3 -3.3 -4.5 +0.2 97 24 ZSM -5.6 -3.3 -10.0 +0.4 78 53

We may notice important differences in reactivity on one hand, between alumina supported catalysts and zeolite supported ones and, on the other hand, between the two 0.2%Pt catalysts which differ from their heating mode during the H2 reductive treatment. The most striking points which arise from these experiments are : -The initial rate of 15NO disappearance is a function of platinum content, i.e. of metallic dispersion. -The selectivity in 15N2 is high for the microwave catalyst. At the opposite, the EMT catalyst exhibits a poor selectivity in 15N2. -A high consumption of 1802 for the microwawe treated catalyst is linked with a high 15N2 formation. -The presence of zeolite enhances the initial reactivity of propene. These above points developed in the following parts. 3.2. Influence of platinum loading Three important effects related to platinum loading seem to influence the reactivity of the catalysts: i) the dispersion, ii) the growth of the particles during the catalytic test, and iii) the influence of the particle shape. 3.2.1. Influence of the metallic dispersion on the initial rates The catalysts used for the DeNOx reaction exhibit different platinum particle sizes as shown in Table 3. Table 3 9Particle size values and dispersion for each catalyst. Catalyst 1%AL* 0.2%AL 0.2%ALMO EMT* ZSM* Particles size (A) 70 13 18 110 80 Dispersion (%)** 16 90 60 10 14 9The TEM measurements were done on pre-treated samples under 10% O2 (balance He) for 2 hours at 200~ (0.2%AL and 0.2%ALMO were reduced before the TEM). 9* Dispersion was estimated from the mean particle diameter with the spheric model. Even if the particle sizes are high, we do not exclude the presence of small particles, with a diameter lower than 10 A which could not be detected by TEM. In Table 4 are reported the evolutions of the initial rates of disappearance of 15NO and formation of 15N2 and C1802 at different temperatures in function of the platinum dispersion.

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Deliberately we did not express the rates in TOF because we do not know about the state of the active platinum sites: are they metallic, oxidized or both of these two cases? However, it appeared that the rates of 15NO reaction is greatly enhanced when the particle sizes are important as was already mentioned in the literature [7,8]. The EMT catalyst exhibits the highest initial rates for lsNO disappearance whereas the initially well dispersed 0.2% Pt/Alumina catalyst leads to the lowest ones. A similar tendency is observed for the formation rates of C 1 8 0 2 . Furthermore, we have also to take into account the influence of the nature of the support. Table 4 : Initial rates of disappearance of 15NO and formation of 15N2 and CISo2 between 180 and 250~ in mol.(gPt.S) 1.104in function of the dispersion in perce atage. T~ Catalysts EMT ZSM 1%AL 0.2%ALMO 0.2%AL

Dispersion 180

200

250

'~No

15N2 C1802 15NO 15N2 C1802 15NO

15N2 C1802

10% -6.7 +0.7 +0.5 -7.3 +0.2 +1.1 -10 +0.6 +16.0

14%

16%

60%

90%

-0.8 +0.7 +0.8

-5.6 +0.4 +2.1 -5.4 +0.2 +10.0

-1.3 +0.3 +1.4 -5.6 +0.2 +4.6 -6.5 +1.3 +11.9

-1.6 +0.2 +0.9 -2.8 +0.5 +2.1 -3.2 +0.9 +3.2

+0.6 +1.4

-8.2 +2.3 +7.2

A question may arise : do the mean particle sizes change during the catalytic reaction? We noticed that particles sintered after a DeNOx reaction. For example, platinum particles dispersion has decreased from 60 to 18% for the 0.2%ALMO and from 90 to 21% for the 0.2%AL after an experiment realized at 250~ This sintering was pointed out through XAS experiments. 3.2.2. EXAFS results: The experiments were done on the I%AL catalyst following the procedure described previously. In Figure 1 are given the evolution of the Fourier transform moduli associated to the first coordination shell of the platinum. By comparison with Pt-Pt and Pt-O reference moduli, oxygen neighbors appear in the first co-ordination shell of platinum. The results of the numerical simulations (not given here) confirm this qualitative observation. Moreover, even if oxygen neighbors are present, still platinum-platinum bonds remain when the temperature increases. In fact, the particles sinter. This led us to suggest that platinum particles are constituted of a metallic core surrounded by one or more platinum oxide layer as represented in Scheme 1. At this stage, we cannot deduce if the oxygen atoms are chemisorbed or take part entirely to a platinum oxide phase [ 14]. We have already observed by in situ XAS the sintering of platinum particles for the same catalyst when it was submitted to NO only [10]. However, in this latter case we have not noticed the formation of an oxidic phase. Moreover, even in presence of an excess of oxygen, when the gaseous flow contains either NO or propene in low quantity, the particles are not oxidized in the bulk.

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Pt

0

I

2

3

4

5

6

O

(c) co) (a)

7R( ~

Figure 1: Fourier transform moduli for the reduced catalyst at 25~ (a), under NO+ C3H6+O2 at 100~ (b), 200~ (c) and 300~ (d)

Scheme 1 : Evolution of the reduced platinum when the catalyst is submitted to a flow of NO+C3H6+O2

3.2.3. Influence of the particle shape Another point to put emphasis on is the particular behavior of the microwave catalyst: indeed, it appears that this catalyst leads to a poor formation of 15N20 between 150 and 250~ which means that the selectivity in 15N2 is high. In fact, compared to the "classical" catalyst containing 0.2 wt %Pt, the metallic cristallites of the catalyst present a more important proportion of comer and edge atoms. This result was pointed out through measurements by infrared spectroscopy of CO adsorption [11]. Ertl and coll. [12] have identified the active sites responsible for NO dissociation on a (0001) ruthenium surface. They concluded that NO dissociates on the step atoms of the metal. This result could explain the better reactivity of NO and the better selectivity in N2 for the microwave catalyst. 3.3. Influence of the support Not only the general morphology of the platinum aggregates plays a key role in the DeNOx reactions. Indeed, we have noticed that the presence of Br6nsted acid sites has a favorable effect on the catalyst efficiency. Experiments were done on the 0.2%AL, ZSM and EMT catalysts in a continuous flow reactor in the same conditions as for the recirculation reactor. In Table 5 are gathered the evolution of the temperature range of "NOx mildconversion", which represents the width at half of the height of the conversion peak of NO versus temperature, and the quantity of Br6nsted and Lewis acid sites. These latest results were obtained by pyridine adsorption measured by IR at 150~ [ 13]. Table 5: Temperature range of "NOx mild-conversion", and quantity of Br6nsted and Lewis acid sites. Temp. range of"NOx Catalyst Br6nsted acid sites Lewis acid sites mild-conversion" ~tmoles/g 9moles/g ZSM 160-335~ 1231 80 EMT 205-240~ 250 10 0.2%AL 210-310~ 0 116 It appears that the ZSM catalyst shows a high quantity of Br6nsted acid sites when the alumina catalyst exhibits none acidic sites in the conditions of the measurement. EMT is in between these two catalysts. The ZSM catalyst gives the largest temperature range of "NOx mild-conversion",. This could be linked to the propene reactivity. In fact, the propene remains adsorbed in the pores of the ZSM zeolites, which allowed to enlarge the NOx conversion

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temperature ranges. Moreover, less the catalyst has Br6nsted sites, more rapidly the formation of C1SO2 occurs, which characterizes the oxidation of propene with 1802, as shown in Table 4.

3.4. Apparent activation energy determination The analysis of the apparent activation energy (Ea) allowed us to notice differences between the catalysts. Indeed, the Ea values vary in function of temperature :there is a break in the slope of the Arrhenius plots. One can distinguish the low temperature range and the high temperature range. These domains are quite similar for all the catalysts. From Ea values, one may deduce that several mechanisms occur on these catalysts in these two domains. For I%PTAL, 0.2%PTAL, EMT and ZSM, at low temperature, an additive process between propene and 15NO takes place giving nitroso and oxime intermediates. By oxidative degradation, the formation of 15N20 is favored. At high temperature, for alumina SU~sPOrted platinum, a partial oxidation of propene occurs, giving a ketone, before reacting with 5NO to form preferentially 15N2. For EMT, this reaction does not seem to take place. This explains why we observed a poor formation of 15N2 for this catalyst. For the microwave catalyst, these two mechanisms occur following parallel routes for the whole temperature range. The formation of 15N2 is however favored on the microwave catalyst. This could be due to the special shapes of the platinum aggregates which exhibit different crystallographic orientations [ 16, 20]. 4. CONCLUSION From this study, we pointed out that large particles with the presence of defects favor the reaction NOx to N2 in presence of light hydrocarbons. Moreover, such reaction leads to a sintering of the Pt particles. Two mechanisms were proposed an additive one and a "partial oxidation" one which proceed consecutively on quite all catalysts except on the microwave treated one where these mechanisms occur in parallel. REFERENCES: A. Fritz, V. Pitchon, Appl. Catal. B, 13 (1997) p. 1 V.I. Parvulescu, P. Grange, B. Delmon, Catal. Today 46 (1998) p. 233 A.P. Walkers, Catal. Today, 26 (1995) p. 107 M. Shelef, Chem. Rev., 95 (1995) p. 209 F. Acke, B. Westerberg, L. Eriksson, S. Johansson, M. Skoglundh, E. Fridell, G. Smedler, CAPOC IV, April 1997, Stud. Surf. Sci. Cat., 116, Eds N. Kruse, A. Frennet, J-M. Bastin, Ed. Elsevier (1998) p. 285 [6] R. Burch, T.C. Watling, Catal. Lett., 37 (1996) p. 51 [7] S. Ringler, P. Girard, G. Maire, S. Hilaire, G. Roussy, F. Garin, Appl. Catal. B, 20 (1999) p. 219 [8] R. Burch, P.J. Millington, A.P. Walker, Appl. Catal. B, 4 (1994) p. 65 [9] G.C. Bond, "heterogeneous Catalysis : Principles and Applications, Clarenton Press, Oxford (1987) [10] S. Schneider, D. Bazin, F. Garin, G. Maire, G. Meunier, M. Capelle, R. Noirot, Appl. Catal. A, 189 (1999) p 272 [ 11] S. Ringler, Ph. D Thesis, University Louis Pasteur, Strasbourg, 1998 [12] T. Zambelli, J. Winterllin, J. Trost, G. Ertl, Science, 273 (1996) p. 1688 [ 13] Professor Michel Guisnet, personnal communication

[1] [2] [3] [4] [51

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

NO Reduction in Presence of Methane and Ethanol on Pd-Mo/AI203 Catalysts L.F. de Mello% M.A.S. Baldanza a, F.B. Noronha b and M. Schmal a* aNUCAT/COPPE-PEQ - Universidade Federal do Rio de Janeiro Caixa Postal 68502 - CEP 21945-970 - Rio de Janeiro - RJ, Brazil The reduction of nitric oxide by methane and ethanol on Pd/ml203 and Pd-8%Mo/AI203 was studied. TPSR analyses of NO + CH4 revealed that above 560 K methane reacted with NO on palladium sites. On Pd-8Mo/alumina, NO decomposition was also observed. The NO+CH4 reaction at 723 K on both catalysts showed high NO conversion and selectivity for N2. The presence of water did not affect the NO conversion, however, selectivity towards N2 was lower on the Pd-8Mo/alumina catalyst. The NO + ethanol results suggested that ethanol and NO competed for the same active sites. However, the presence of MoO3 improved the selectivity for N2 formation during reaction at 593 K. I. INTRODUCTION The catalytic reduction of NOx produced by both stationary and automotive combustion processes is of great importance, due to severe restrictions for NOx emissions. The automotive three-way catalysts have in their basic formulations noble metals (rhodium, platinum or palladium) dispersed over washcoated 7-A1203 and are used for controling NOx, CO and hydrocarbon emissions. Earlier studies [ 1,2] have shown that molybdenum oxide when associated to Pd and Pt presented good NOx reduction activity and high selectivity for N2 formation. This behavior evidenced that molybdenum oxide might substitute Rh, in view of its high cost and scarce resources. Recent papers confirmed the high selectivity for N2 in the CO+NO reaction on PdMo/AI203 catalysts [3]. Based on TPD analysis of NO and CO adsorption, a redox mechanism for NO reduction to N2, involving partially reduced molybdenum oxide was proposed [3]. However, it is interesting to study the reduction of NO using different hydrocarbons as reductants. Methane is considered a convenient reductant because of its low costs and it is also the main alkane in lean bum exhaust gases and emissions from natural gas powered vehicles. It is noteworthy studying oxigenated organic compounds as reductants, since alcohols and ethers are widely used as fuel additives. Therefore, this work focuses attention on the surface reactivity and adsorption capacity of NO in presence of CH4 and ethanol. The NO + CH4 and NO + ethanol reaction, in presence and absence of 02 and water, on a Pd/Al203 and Pd8Mo/AI203 catalysts were studied. TPD and TPSR analyses were the main techniques used.

Corresponding author. Tel. 55-21)590-2241, fax. (55-21)290-6626, e-mail: [email protected]

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2. EXPERIMENTAL

The 8%Mo/AI203 catalyst was prepared by A1203impregnation with aqueous solution of (NH4)6Mo7024.4H20. The sample was dried at 383 K and calcined under air flow at 773 K for 213. Pd/Al203 and Pd-Mo/AI203 samples were obtained by impregnation of A1203 and Mo/A1203, respectively, with a solution of Pal(NO3)2 (Aldrich). Catalytic tests, TPD and TPSR analyses were performed in a multi-purpose apparatus coupled to a quadrupole mass spectrometer and on-line gas chromatography. Prior to all experiments, the catalysts were purged under helium flow (50 cnaa/min.) at a heating rate of 10 K/rain from room temperature (RT) up to 823K The catalysts were cooled and then reduced at 773K for 2h, with pure H2 (3 0cna3/min.). Following reduction, the system was outgased with helium flow at the reduction temperature for l h and cooled. All samples were characterized using TPD analysis atter NO or ethanol adsorption. The adsorption of NO or ethanol was performed in the same way as discussed elsewhere [4]. The TPSR experiments for NO+CH4 were performed similarly, however, alter adsorption of NO, the samples were heated under a flow of 3.94% CH4/He mixture. In the TPSR of NO+ethanol, the adsorbed gas was ethanol and the samples were heated under a flow of 1% NO/He mixture. The catalytic reduction of nitric oxide by methane was carried out at 723 K using a feed mixture consisting of 0.5% CH4 and 0.3% NO. After reaction, oxygen at 0.8% was added to the feed, then removed and followed by adding 10% water. A catalyst weight of 100 mg and a total flow rate of 150 cc/min were used. For the reduction of nitric oxide by ethanol, the feed mixture consisted of 0.2% ethanol and 0.3% NO. Oxygen (0.6%)was then added. The reaction was carded out at 593 K and the total flow rate and catalyst weight were 250 cc/min and 140 nag, respectively. 3. RESULTS AND DISCUSSION

3.1. Temperature Programmed Desorption (TPD) After adsorption of ethanol on alumina, TPD analyses showed ethanol desorption at 395 and 500 K and a great formation of ethylene around 550 K. No dehydration to acetaldehyde was detected. The Pd/AI203 and Pd-8Mo/AI203 catalysts exhibited similar desorption profiles. There was a large decrease of ethylene production together with the formation of CO, CH4 and H2 around 495 K, which is attributed to the decomposition of ethoxy species adsorbed on alumina, diffusing to palladium particles [4, 5]. On the Pd/Al203 catJyst, ethanol underwent dehydrogenation to acetaldehyde at 530 K, while on Pd8Mo/AI203 (figure 1), two desorption peaks of acetaldehyde at 466 and 516 K were observed. The peak at 466 K is due to the oxidative dehydrogenation of ethanol on partially reduced molybdenum oxide. Further desorption of CO, HE and CO2 was observed on both catalysts above 723 K, which was associated to the decomposition and/or reaction with surface hydroxyls of an intermediate carbonaceous species [4]. The 8Mo/AI203 catalyst showed desorption of ethanol at 386 and 476 K, ethylene formation at 540 K and also two peaks corresponding to acetaldehyde at 455 and 530 K. Unlike the other catalysts, no CO, CH4 and HE were observed.

3.2. Temperature Programmed Surface Reaction (TPSR)

The TPSR profiles of NO + CH4 on the Pd/Al203 catalyst are displayed in figure 2. Nitric oxide desorption was observed up to 570 K with no decomposition products. This initial

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behavior was very similar to the TPD profile of adsorbed NO [3]. At 570 K, however, NO desorption (m/e = 30) decreased drastically with simultaneous formation of N~ (m/e = 28 ) and CO2 (m/e = 44) besides methane (m/e = 16) consumption. It indicates that at 570 K, methane is activated on palladium, starting to react with NO. Above 720 K, CO2 formation decreased, exhibiting mainly CO and H2. Thus, methane probably reacted with adsorbed NO up to 720 K, when NO was totally consumed. After that, methane probably decomposed and/or reacted with surface hydroxyls yielding CO and H2.

' m./o=28

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~9

_ CH ! H ~ ((m/e=l m / e = ! 6 6) )]

~

~ y l e n e -

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m/e = 16 (x 0 . 5 ) ~ ~ ' - " ~ f r o / ( = 2 (x 0.25)

.__.....~dehyde(m/e=29 t ~ ~. . . . C O (m/e=44)

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3 00

5 00

7 00

|

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Temperature (K) 823 Figure 1: TPD of adsorbed ethanol on Pd-8Mo/A1203 catalyst.

. ~ j

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473

,

I

673 [ isothermie

Temperature / K

Figure 2: (a) NO + CH4 TPSR on Pd/Al203 (b) amplified detail from (a).

The TPSR lineshapes for Pd-8Mo/AI203 are displayed in figure 3. Initially, nitric oxide was desorbed around 395 and 555 K (m/e=30), with simultaneous formation of N2 (m/e=28) and N20 (m/e=44) at 555 K (figure 3 b). Once again, the initial behavior was similar to the TPD profile aider adsorption of NO [3]. NO desorption and decomposition decreased sharply above 560 K, together with methane (m/e=l 6) consumption. As for the Pd/A1203 catalyst, at 560 K, methane is activated on palladium, beginning to react with NO. However, at this temperature NO was already being decomposed on the MoOx surface. Therefore, it seems that NO decomposition decreased, prevailing the reaction of NO with CI-h on Pd. Above 560 K, TPSR profile for both catalysts were very similar, indicating that methane probably reacted with adsorbed NO until 730 K, when NO was totally consumed and CO2 formation decreased. Then, methane was decomposed and/or reacted with surface hydroxyls forming CO and H2. The 8Mo/A1203 catalyst did not evidence any reaction between the adsorbed nitric oxide and flowing methane. The TPSR and TPD profiles were very similar [3], exhibiting NO desorption at 375 and 580 K with small formation of N20 and N2 between 340 and 700 K. This is attributed to the decomposition of NO on the partially reduced molybdenum oxide.

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.,~

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o~,,~

m ' ~ 2'x I

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.

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Temperature / K Figure 3" (a) NO + CH4 TPSR on Pd-8Mo/Al203. (b) amplified detail from (a).

~~ ~

_

~N~,~(m~-12 ~ x4)

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_ethylene(m/e=27)

l__ ~~,_#cetaldehy de ina/e=29i -

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Temperature / K Figure 4' NO + ethanol TPSR on Pd8Mo/A1203.

The TPSR of NO + ethanol on Pd/AI203 catalyst showed that at low temperature the profile was very similar to the TPD profile of ethanol. However, during TPSR two ethanol desorption peaks were observed around 400 and 495 K. NO consumption was observed only at 590 and 695 K, with simultaneous increase of signals m/e= 28, 44 and 12, that may be assigned to CO (m/e= 28 and 12), CO2 (m/e=44, 28, and 12), N2 (m/e=28) and/or Y20 (m/e = 44). This suggested that NO probably reacted on Pd particles with carbonaceous species deriving from ethanol decomposition at lower temperatures, as discussed previously [4]. The TPSR results of Pd-8Mo/AI203 are shown in figure 4. Ethanol desorbed around 400 and 495 K. Ethylene was observed at 550 K and acetaldehyde at 480 and 540 K, quite similar to the TPD profile after adsorption of ethanol (figure 1). The NO consumption was observed only above 565 K, in a wide temperature range, showing one peak at 695 K and shoulders at 645 and 600 K. However, the TPD profile of NO on Pd-8Mo/AI203 [3], showed that NO decomposed below 565 K. This suggests that ethanol and NO are competing for the same active sites and that the NO decomposition occurred only aider the desorption and/or decomposition of ethanol. The NO consumption around 600 and 695 K coincide with the consumption peaks on the Pd/AI203 catalyst and are probably deriving from the reaction of NO with the carbonaceous species on Pd particles. In fact, the signal for the m/e=12 fragment showed two peaks at 600 and 695 K (figure 3), indicating CO and/or CO2 formation. The NO consumption around 645 K was only observed on the Pd-8Mo/A1203 sample, probably due to the NO decomposition on reduced molybdenum oxide, since N2 formation (m/e=28) was also observed at 645 K. This is reinforced by the fact that at this temperature the signal (m/e=12) did not increase. Results on 8Mo/AI203 showed that below 575 K the profile was very similar to the TPD of adsorbed ethanol. Above that NO was decomposed and N2 and N20 were observed.

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3.3. Catalytic Activity

The catalytic results of the NO reduction by methane in the absence and presence of water are presented in table 1. The 8Mo/AI203 did not show any activity for this reaction at 723 K. For the Pd-containing catalysts, NO was completely reduced to N2. Methane conversion was higher on Pd/AI203, although the Pd-8Mo/AI203 catalyst showed better selectivity for CO2 production. In a previous work [3], the NO + CO reaction was studied and a redox mechanism was proposed. It was shown that NO decomposed on reduced molybdenum oxide and the oxygen was available at the palladium sites to oxidize CO to CO2. Therefore, for the Pd-8Mo/AI203 catalyst, a similar mechanism may occur and a greater amount of oxygen is available for palladium to oxidize methane. That would explain the higher selectivity of this catalyst for CO2 formation. In fact, the main difference observed on the TPSR analyses of CI-I4 + NO on Pd/AI203 and Pd-8Mo/AI203 was that, besides the reaction with methane on Pd particles, NO was also decomposed on the Pd-8Mo/AI203 catalyst, in contrast with the Pd/AI203 catalyst, where no decomposition was observed. The reaction feed containing water was passed through the catalyst bed for 1 hour before data acquisition. The presence of water did not change the activity and selectivity for NO conversion to N2 on the Pd/ml203 catalyst. However, methane conversion increased and the catalyst was more selective for CO2 when compared to the reaction feed without water. This is probably due to the reforming of methane on Pd particles. On the Pd-8Mo/AI203 catalyst, N2 selectivity dropped while N20 was formed. Methane conversion did not increase as much as for the Pd/AI203, although CO2 selectivity was further enhanced. This indicates that water may be competing with NO for adsorption sites on the reduced molybdenum surface, decreasing NO decomposition and, hence, the selectivity for N2. However, the oxygen from water and also from nitric oxide was still available for palladium to oxidize methane, showing a higher selectivity for CO2 production (table 1). Table 1 - NO reduction by CH4 in absence and presence Absence of water Conv. (%) Selectivity (%) NO CH4 Nspecies C species Catalyst N2 N20 CO CO2 Pd/A1203 100 33 100 0 47.5 52.5 Pd-8Mo/Al203 100 21 100 0 36.2 63.8

of water at 723 K Presence of water Conv. (%) Selectivity (%) NO CH4 N species C species N2 N20 CO CO2 100 52.5 100 0 21 79 100 25 84 16 9 91

In the presence of oxygen, NO reduction ceased, prevailing methane reaction with oxygen, forming CO2 as the only product. For the Pd/A1203 catalyst, methane conversion was higher in the presence of oxygen (61% conversion) than in the absence of oxygen. However, the Pd-8Mo/AI203 catalyst was less active for oxidation of methane by oxygen (11% conversion) than for the oxidation of methane by nitric oxide (21% conversion). The results for the reduction of nitric oxide by ethanol in the absence of oxygen are presented in table 2. Nitric oxide and ethanol conversion were approximately the same for both Pd/AI203 and Pd-8Mo/AI203 catalysts. However, the Pd-8Mo/AI203 catalyst showed a much higher selectivity for N2 formation than the Pd/AI/O3 catalyst which, on the other hand, was more selective for CO2 production. Ukizo et. al. [6] studied the reaction of NO+ethanol+O2 on alumina-supported silver catalysts and through IR analyses they observed the appearance of

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characteristic bands of isocyanate species adsorbed on m1203 and Ag, above 573 K These species were assigned to be intermediates of the NOx reduction. In this work, it was observed through IR analyses of adsorbed ethanol and NO (not shown), that the presence of Mo covered great part of the alumina surface area. Although this would hamper the formation of isocyanate species on alumina, the NO conversion was the same on both catalysts. As seen from the TPSR analyses, NO not only reacted on the Pd particles but also decomposed (forming mainly N2) on the MoOx surface. Hence, this would explain the similar activity on both catalysts and also the higher selectivity for N2 formation on the Pd-8Mo/AI203 sample. The 8Mo/AI203 catalyst showed lower activity and 100 % selectivity for N20 and acetaldehyde formation, which is most likely due to the decomposition of ethanol and NO on the surface of the catalyst, and not the reaction between ethanol and NO. Table 2 - NO reduction by ethanol in absence of 02 at 593 K Conversion (%) Selectivity (%) NO Ethanol N species ,,, C species Catalyst N2 N20 CO CO2 Acetaldehyde Pd/AI203 54.1 89.9 47.9 52.1 23.8 76.2 0 Pd-8Mo/AI203 55.8 85.7 83.2 16.8 6 8 . 3 31.6 0 8Mo/AI203 a 19 83 0 100 0 0 100 a activity measuremems for this catalyst were taken at 623K In presence of oxygen, the Pd-containing catalysts were not active for the reduction of NO. Instead, all ethanol reacted with oxygen to form carbon dioxide. For the 8Mo/AI203 catalyst there was still some formation of N20, while part of the ethanol was now oxidized to CO2 (12% CO2 and 88% acetaldehyde). 4. SUMMARY The effect of MoO3 on the Pd/Al203 catalyst for the reduction of NO by CH4 and ethanol demonstrated that the selectivity towards N2 was enhanced due to the promoting effect of NO decomposition on reduced MoOx surface. The presence of oxygen decreased drastically the conversion of NO, affecting the selectivity. On the other hand, water did not affect the reduction of NO by methane. Indeed, it increased the conversion of methane and the selectivity towards CO2 as well. The redox mechanism proposed for the CO+NO reaction [3] also plays an important role for the NO reduction by methane, however the reduction by ethanol was accomplished by an important promotion of the NO decomposition. REFERENCES 1. H.S. Ghandi, H.C. Yao and H.K. Stepien, "Catalysis Under Transient Conditions" (Bell, A.T. and Hegedus, L.L., Eds.), ACS Symposium Series No. 178 (1982) 143. 2. I. Halasz, A. Brenner, M. Shelef and Ng. Simon, Applied Catal. A : General 82 (1992) 51. 3. M. Schmal, M.A.S. Baldanza and A. Vannice, Journal of Catalysis 185 (1999) 138. 4. M.A.S. Baldanza, L.F. Mello, F.B. Noronha, A. Vannice and M. Schmal, submitted to Applied Catal. B : Environmental (1999). 5. E.M. Cordi and J.L. Falconer, Journal of Catalysis 162 (1996) 104. 6. Y. Ukisu, T. Miyadera, A. Abe and K. Yoshida, Catal. Letters 37 (1996) 265.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Fe-vanadyl phosphates/TiO2 as SCR catalysts G. Bagnasco l, p. Galli 2, M. A. Larrubia3, M. A. Massucci 2, P. Patron04, G. Ramis 3, M. Turco ~ ~Dipartimento di Ingegneria Chimica, Universith "Federico II", P.le Tecchio 80, 80125 Napoli, Italy. 2Dipartimento di Chimica, Universith "La Sapienza", P.le Aldo Moro 5, Roxlm, Italy. 3Dipartimento di Ingegneria Chimica e di Processo "G.B. Bonino", Universifft di Genova, P.le J.F. Kennedy 1, Genova, Italy. 4IMA[ CNR, Via Salaria Km 29.600, 00016 Monterotondo Stazione, Roma, Italy. Fe-vanadyl phosphate (FeVOP) was precipitated in the presence of different amounts of TiO2. The samples also contained Ti(HPO4)2"H20 and probably amorphous FePO4. In the catalysts heat treated at 450~ besides TiO2 and anhydrous FeVOP, amorphous layered TiP207 was likely present. NH3 adsorption sites with medium and high strength were detected, that were related to Bronsted acidity of a pyrophosphate phase and Lewis acidity of FeVOP. The catalysts were noticeably more active and selective than pure FeVOP. NO conversion was increasing with FeVOP content, reaching 90% value at 400~ with unit selectivity to N2 and NH3/NO reaction ratio close to 1. 1. INTRODUCTION VOPO4 phases have been widely investigated, due to their role in VPO oxidation catalysts (1). Recently new materials have been obtained by isomorphous substitution of VO groups of VOPO4-2HzO with a trivalent metal such as A1, Cr, Fe, Ga, Mn (2). Such substitution modifies the adsorption properties of VOPO4 phase (3, 4). Moreover Fe-vanadyl phosphate gave high activity for NO reduction by NH3 (SCR process) if compared with conventional SCR catalysts (5). The activity of this compound can be related to the dehydrogenation properties of Fe, promoting the formation of species like amide, that reacts with gaseous NO (2,5). However the catalytic activity of Fe-vanadyl phosphate is limited by its low surface area (5 m2/g).Therefore these systems could be improved by dispersing the active phase on a suitable material. In this work, we have studied catalysts obtained by precipitating Fe-vanadyl phosphate in the presence of titanium dioxide. Such catalysts that were never reported before, are not simple supported systems, due to the presence of other phases formed during the preparation. These systems were studied for SCR catalytic properties and characterized for physical and chemical properties by means ofEDS, XRD, NH3 TPD and FT-IR techniques.

2. EXPERIMENTAL [Fe(H20)]o.2(VO)o.sPO4"2.25H20 (FeVOP) was prepared by refluxing for 16 h a suspension of V205, Fe(NO3)3"9H20 in H3PO4 3.3 M, with a yield of 60% (2). Materials A, B and C were

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prepared by refluxing for 16 h the above suspensions in the presence of 12, 6 or 3 g of TiO2 (s. a.=125mEg ~) respectively. The catalysts A-450, B-450 and C-450 were obtained by treating the materials at 450~ for 12 h in He flow. A reference material TiP-TiO2 (s.a.-35mEg 1) was prepared by refluxing TiO2 with HaPO4 3.3 M in the same conditions. Elemental analysis was effected by EDS on a Philips XL30 apparatus. BET surface areas were measured on a Quantachrom Chembet 300. XRD measurements at room temperature (r. t.) and at 450~ were performed by Philips diffractometers PW 1100 and 1710 (HT-A.Paar diffraction camera) respectively. NI'-I3 temperature programmed desorption (TPD) was carried out in a flow apparatus at a rate of 10~ min -~. FT-IR spectra were recorded with a Nicolet Proteg6 460 instrument, using conventional IR cells with evacuation-gas manipulation apparatus. Catalytic activity tests were carried out in a flow apparatus with a fixed bed reactor at T=200-450 ~ contact time=8 9103 s. The feed mixture contained 700 ppm of NO and NH3, 27000 ppm of O2~He as balance. NO and NH3 were measured by continuous analyzers, N2 and N20 by gaschromatography. The nitrogen balance was verified within 5% error.

3. RESULTS AND DISCUSSION Table 1 Composition and surface areas of the materials Sample

FeVOP

P Ti mol% b) mol% b) A 10 34.0 61.7 B 20 30.7 64.4 C 40 27.0 56.2 a) nominal, assuming 60% yield in FeVOP; wtO~ a)

V mol% b) 2.12 2.10 11.5 b) EDS analysis,

Fe Surface area mol% b) m2g-~ 2.09 56 2.80 59 5.21 61 on oxygen free basis

Composition and surface areas of the samples are reported in Table 1. Surface areas are markedly higher than pure FeVOP and are unchanged after treatment at 450~ The vanadium content of the samples A and B would correspond to about 7wt% FeVOP, that of C to about 30wt% FeVOP. These percentages, quite lower than the nominal ones, suggest that the presence of TiO2 hinders in some way the formation of FeVOP, as more as higher is the TiOz amount in the preparation mixture. The phosphorous content is always largely exceeding the amount corresponding to FeVOP and the Fe/V ratio is higher than 1/4, that is the value usually obtained (2,5). These data indicate that the samples cannot be described by a simple FeVOP/TiO2 composition, and that other phases are produced when FeVOP is precipitated in the presence of TiO2. The XRD patterns are reported in Fig. 1. TiO2 shows the characteristic signals of the anatase and brookite phases. TiP-TiO2 shows the reflexions of layered cz-Ti(HPO4)z-H20 (TIP) (6), besides weak signals of TiO2. This indicates that refluxing TiO2 with H3PO4 leads to partial conversion of TiO2 into TIP. A similar behaviour was found in the treatment of TiO2 supported catalysts with H3PO4, leading to formation of titanium hydrogenphosphate (7). FeVOP shows its characteristic pattern (3). Different phases are detected in the XRD of A, B and C. The signals of TiO2 and TIP are always present, the intensity of TiP signals decreasing from A to C. The signal with d-7.11 A of FeVOP is not observable in the pattern of A, while it appears as a shoulder in B and is clearly evident in C that shows also other reflexions of FeVOP. Thus on going from A to C the amount of the FeVOP phase increases while that of

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TiP decreases. This confirms that decreasing the amount of TiO2 in the preparation mixture

.•3.55 A

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Fig. 1. XRD patterns of A, B, C and reference materials.

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Fig. 2. XRD patterns of A-450, B-450, C-450 and reference materials.

favours the precipitation of FeVOP in respect to the formation of TiP. The presence of TiP in all samples explains why the phosphorous amounts exceed those corresponding to FeVOP (Table 1). Moreover the high Fe/V ratio suggests the presence of other Fe containing phases, such as FePO4, not detected by XRD because in too low amount or in an amorphous state. Fig. 2 reports the XRD patterns of the materials heated at 450 ~ After this treatment TiO2 shows an unchanged pattern, while FeVOP transforms into a well crystalline anhydrous phase (3). The pattern of TiP-TiO2 treated at 450~ (TiP-TiO2-450), shows the signals of TiOz and of layered titanium pyrophosphate (L-TIP207), formed by condensation of the TiP phase. In the XRD of the catalysts, besides the signals of TiO2, the strongest signal of anhydrous FeVOP with d=4.21 ,~ is well evident, while the reflexion with d=3.08 A becomes evident only in C-450. This suggests an increase of the amount of anhydrous-FeVOP from A-450 to C-450, that agrees with the increase of hydrated FeVOP in the corresponding precursors. It is worthnoting that the reflexions of L-TiP207 are not observable in the XRD of the catalysts, although they are present in the pattern of TiP-TiO2-450, suggesting that the condensation process of TiP in the catalysts is influenced by the presence of FeVOP (or other unidentified phases). It can be supposed that some reaction between TiP and FeVOP (or other phases) can occur during heat treatment, leading to formation of a disordered phase, not detected by XRD. NH3 TPD spectra of the catalysts and reference materials treated at 450~ are reported in Fig. 3. TPD spectrum ofTiO2-450 shows two bands due to Lewis acid sites of different strength,

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due to coordinatively unsaturated Ti4+ ions (8). FeVOP-450 exhibits a broad band due to superficial Fe 3§ and VO 3+ ions acting as .~ ._ Lewis acid sites, with a large variety of acid strength (3). The TiP-TiO2-450 sample 't1 'r'= 1 ~ gives a spectrum very different from that of -~ TiO2. However the spectrum is very similar ~ to that reported for TiP phase treated at ~, • 450~ that is partially transformed into ~ L-TiP207 with signals due to adsorption g on Bronsted acid sites (9). This suggests ~, ~. that TiP-TiO2-450 has adsorption properties similar to L-TiP207. It can be g supposed that the TiO2 particles are ~0 o=~ z . , . z surrounded by TiP phase, formed by the reaction of T i O 2 with H3PO4, and 0 200 400 600 transformed into L-TiP207 during heat Temperature, ~ treatment. TPD spectra of the catalysts show broad desorption peaks due to NH3 Fig. 3. NH3 TPD of A-450, B-450, C-450 and adsorbing sites with strength from medium reference materials (right axis for FeVOPto very high. By taking into account surface 450) area values, the amoums ofdesorbed NH3 correspond to similar concemrations of surface sites (abt. 2-1014 cm2). The shape of the curve of A-450 is very similar to that TiPTIO2-450, suggesting the presence of a titanium pyrophosphate phase in the catalysts. XRD failed to detect this phase probably because it was amorphous. The shape of the curves gradually changes from A to C, as the 400~ component increases, while that at 550~ decreases. XRD shows that the content of FeVOP increases from A-450 to C-450. Therefore the increased intensity of the signal at 400~ can be related to an increase of the amount of FeVOP, since this phase shows a noticeable concentration of sites desorbing NH3 at 350400~ The decrease of the 550~ signal can be related to a decrease of the amount of the amorphous pyrophosphate phase. The nature of surface acid sites of the catalyst has been investigated by FT-IR technique (Fig. 4). According to (10,11) the band observed at 1605 cm1 is assigned to asymmetric deformation mode (Sas NH3) of ammonia coordinated to Lewis acid sites; the corresponding symmetric deformation (Ssym NH3) is not detectable because obscured by the cut-off of the transmittance of the sample due to the absorption of the bulk. Moreover, bands of NH4+, due to the adsorption of ammonia over Bronsted acid sites, can be observed at near 1680 and 1440 cml; these bands are respectively due to symmetric (Ssym NH4) and asymmetric (Sas NH4) deformation mode and the associated stretching. The relative intensities of the 8as NH4 with respect to the 8as NH3 seems higher in the spectrum of TiP-TiO2-450 and lower in the spectrum of pure FeVOP-450; in the case of C-450 and A-450 an intermediate trend is observed. These data indicate that ammonia adsorbs over all the catalysts in the form of molecularly coordinated species and of ammonium ions. The former are due to Fe 3§ and VO 3§ groups, the latter to HPO4 groups present on the surface of L-TiP207 (6). Moreover protonation of NH3 by water molecules coordinated to Fe 3+ ions cannot be excluded. The results of catalytic tests for the SCR reaction are reported in Fig. 5. The catalysts are very active, giving NO conversions up to 90%. TiO2 is inactive, FePO4 (5) and TiP-TiO2-450 have

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very low activity, appreciable only at temperatures higher ), 13" than 300~ Thus the (/) 0 catalytic activity must o" I1} be related to the -.1 t") anhydrous FeVOP I11 phase. The catalysts r appear more selective than pure FeVOP. In fact the NH3/NO reaction ratio is close to 1 and no formation of N20 is observed in all conditions, except Wavenumbers (cm -1) for C-450 that gives conversion to N20 of Fig 4. FT-IR spectra of adsorbed species arising from contact of about 15% at 450~ NH3 over pure FeVOP-450 (a), C-.450 (b), A-450 (c) and TiPOn the other hand TIO1-450 (d). with pure FeVOP conversion to N 2 0 was observed starting fi'om 300~ (5). The catalytic activity towards NH3 oxidation has also been investigated, in the same conditions as SCR tests, but in the absence of NO. NH3 oxidation activity is negligible up to 300~ and markedly increases at higher temperatures (Fig. 6), giving N2 as the only product (traces of NO are produced at 450~ with 100

100

80.m L_

60-

E:

o r

40-

806040-

Z

20-

o'-9. 200

300

400

20-

50(:

200

Temperature, ~

Fig. 5. SCR reaction: NO conversion on A-450 ( O ) , B - 4 5 0 (E3), C-450 (A) and FeVOP-450 (V)..

300 400 Temperature, ~

500

Fig. 6. N H 3 oxidation: N H 3 conversion on A-450 (O) ,B-450 ([2) and C-450

(a).

C-450). However under SCR conditions, such NH3 oxidation activity is ineffective suggesting that NH3 reacts preferentially with NO rather than with 02. The activities of the three catalysts can be directly compared, since their surface areas are almost the same. The catalytic activity increases from A-450 to C-450, suggesting that it is related to the content of FeVOP phase.

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However, since the catalysts are more selective than pure FeVOP, it can be supposed that either the FeVOP phase is someway modified by interaction with titanium phosphate, or other phases are involved in the catalysis. It can be supposed that a modified pyrophosphate phase, in which some V4+ ions replace T1.4+.ions, catalyzes NO reduction with higher selectivity, taking into account the catalytic properties observed for V4+ species in V2OJTiO2 catalysts (12). REFERENCES

1. G. Centi, Catal. Today, 16 (1993) 5. 2. K. Melanov~, J. Votinsky, L. Bene~ and V. Zima, Mat. Res. Bull., 30 (1995) 1115. 3. G. Bagnasco, L. Bene~, P. Galli, M. A. Massucci, P. Patrono, M. Turco and V. Zima, J. Therm. Anal., 52 (1998) 615. 4. G. Bagnasco, G. Busca, P. Galli, M. A. Larrubia, M. A. Massucci, P. Patrono, G. Ramis, M. Turco, J. Therm. Anal., in press. 5. G. Bagnasco, G. Busca, P. Galli, M. A. Massucci, K. Melanova, P. Patrono, G. Ramis, M. Turco, submitted to Appl. Catal. B: Envir. 6. G. Alberti, P. Cardini-Galli, U. Costantino and E. Torracca, J. Inorg. Nucl. Chem., 29 (1967) 571. 7. J. Blanco, P. Avila, C. Barthelemy, A. Bahamonde, J. A. Odriozola, J. F. Garcia de la Banda, H. Heinemann, Appl. Catal. 55 (1989), 151. 8. N. Y. Topsoe, J. Catal., 128 (1991) 499. 9. G. Bagnasco, P. Ciambelli, A. La Ginestra, M. Turco, Thermochim. Acta, 162 (1990) 91. 10. A. A. Tsyganenko, D. V. Pozdnyakov, V. N. J. Filimonov, Mol. Struct., 29 (1975) 299. 11. K. Nakamoto, in "Infrared and Raman Spectra of Inorganic and Coordination Compounds", 4 th ed., Wiley, New York (1986). 12. H. Bosch, F. Janssen, Catal. Today, 2 (1988) 369.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Photoindueed non-oxidative methane coupling over silica-alumina Hisao Yoshida, a Yuko Kato, a Tadashi Hattori b a Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan

b Research Center for Advanced Waste and Emission Management, Nagoya University, Nagoya 464-8603, Japan* Photoinduced non-oxidative methane coupling proceeds around room temperature any oxidant molecules on silica-alumina. No oxygenated products are formed. dispersed aluminum species, which are generated on the surface of silica-alumina dehydration at high temperature and provide the characteristic phosphorescence function as the active sites for the photoinduced non-oxidative methane coupling.

without Highly through spectra,

I. INTRODUCTION In order to convert natural gas into useful chemicals, the oxidative methane coupling is an expedient reaction. However, it is quite difficult to obtain the coupling products in high selectivity, because the oxidation of coupling products to COx proceeds more selectively than the coupling reaction. If the oxidant molecules are removed to avoid complete oxidation, the reaction requires very high temperature [1]. Photoinduced reaction are one of the most available reactions taking place at low temperature where complete oxidation could be minimized. Actually, it was reported that photoinduced methane coupling proceeded at 373-473 K in the presence of oxygen over TiO2 [2]. However, the selectivity of COx was very high and the yield of coupling products was only ca. 0.5%. The possibility of the photoinduced non-oxidative coupling, where no oxidant molecules are employed, was examined by using transition metal oxides. However, the highest yield was on Mo/SiO2 only ca. 0.007 % in gaseous phase and less than 0.4% even after forced desorption by heating or by admission of water vapor [3]. It seems that employment of transition metal oxides is not the only solution. Recent years, some kinds of photore act ions were successively discovered on silica and silica-based materials, e.g., photometathesis of alkene over amorphous silica or mesoporous silica [4], photooxidation of propene over mesoporous silica, SiO2 and Mg/SiO2 [5]. In addition, silicaalumina was found to be photoactive; it exhibited a characteristic phosphorescence emission spectra [6]. In this study, photoinduced non-oxidative methane coupling was examined on silica, * Present address is the same as that of the other authors.

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silica-alumina and alumina and it was found that it occurs on silica-alumina in yields as high as 5 % without formation of CO and CO2 [7]. The active sites on silica-alumina were assigned to be the phosphorescence sites.

2. EXPERIMENTAL

The silica sample was prepared from Si(OEt)4 by sol-gel method followed by calcination in dry air at 773 K [8], and its specific surface area of was 679 m2g"1. The alumina sample was a reference catalyst of the Catalysis Society of Japan, JRC-ALO-4 (174 m2g"1) [9]. The silicaalumina samples employed, SiO2-A1EO3(L) and SiO2-AIEO3(H), were also JRC samples, JRCSAL-2 (560 m2g"1) and JRC-SAH-1 (511 m2g'l), respectively. The alumina contents were 13.8 wt% and 28.6 wt%, respectively [9]. Phosphorescence spectra were recorded at 77 K with a Hitachi F-4500 fluorescence spectrophotometer using a UV filter (transmittance X > 330 nm) to remove scattered light from UV source, where the excitation light was cut off by chopper and emission light was recorded after a time lag to collect phosphorescence emission which was free from fluorescence. Before measurement of spectra and the reaction test, the sample was heated in the presence of oxygen and evacuated at desired temperature. The reaction test was carried out under irradiation by using a 250 W Xe lamp for 3, 18 or 30 h. No oxidant molecules were introduced into the reaction system. Under photoirradiation, the temperature of sample bed was measured to be around 310 K. Products in the gaseous phase were collected with a liquidN2 trap and analyzed by GC. Then adsorbed products were thermally desorbed by heating (573 K 15 min), collected, and analyzed. 3. RESULTS AND DISCUSSION 3.1. Activities in photoinduced non-oxidative methane coupling Table 1 shows the product yields in photoinduced non-oxidative methane coupling in the absence of gaseous oxidants over silica, silica-alumina and alumina [7]. Note that no oxygenates such as MeOR HCHO, CO and CO2 were detected in the absence of oxygen molecules (runs 1-7). For the empty reactor (run 1), only a trace amount of C2H6 was formed upon photoirradiation. On the silica sample (run 2), a small amount of CEH6 and C3H8 were obtained in the gaseous phase, and a trace amount of C2H4 and C3H6 were observed as the thermally desorbed products. On the alumina sample (run 3), the conversion was obviously much higher than that over silica; total yield was 5.33%. In the dark, alumina exhibited no activity (run 6), which indicates that photoirradiation is necessary for the reaction over alumina. However, the products upon irradiation were dominantly obtained through thermal desorption. It would be important point to release products without heating for lowtemperature methane coupling. Over the silica-alumina samples (runs 4, 5), the coupling products were also obtained as high yields as on alumina. In contrast to the case of alumina, however, a large amount of gas phase products of C2-C 4 alkanes were obtained without heating while smaller amount of C2-

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C 6 alkanes

and alkenes were desorbed upon heating. Among the thermally desorbed products, alkenes were the major products [7]. In the dark at 473 K (in an electric furnace, run 7), no products were detected, clearly indicating that photoirradiation is necessary for the above reaction. On SiO2-A12Oa(L), total yield went up to 5.90 % and even the yield of gaseous phase products reached 4.53 %. They are extremely higher than those in any other reports on photoinduced coupling of methane [2,3]. It should be noted that no oxidant molecules were introduced in the reaction system and that the temperature of the sample bed, measured by a the rmocoupl e, was 310 K. In the presence of oxygen (run 8), though photoinduced coupling reaction of methane also occurred, a large amount of CO2 (11.5 C%) was mainly produced. Table 1 Results of photoinduced non-oxidative methane coupling over the samples pretreated at 1073 K a Run Sample Surface Gaseous phase products Desorption Total area b (C%) c products (C%) c

(m2g"1) C 2 H 6 C 3 H 8 C4Hlo Total (C%) c 1 (blank) trace 0.00 0.00 trace trace 2 SiO2 526 0.08 0.01 0.00 0.09 trace 0.09 3 A1203 134 0.48 0.02 0.00 0.50 4.83 5.33 4 SiO2-A1203(L) 223 3.54 0.85 0.14 4.53 1.37 5.90 5 SiO2-A1203(H) 273 1.82 0.27 0.03 2.12 0.90 3.02 6 d A1203 134 0.00 0.00 0.00 0.00 0.00 0.00 7 d SiO2-A1203(L) 223 0.00 0.00 0.00 0.00 0.00 0.00 8 e SiO2-A1203(L) 223 0.24 0.01 0.00 11.75 f 1.30 13.05 f a Reaction temperature was ca. 310 K. Reaction time was 18 h. Initial amount of CH4 was 100 lamol. Production of H2 was not monitored in this case. b The specific surface area was measured after reaction test. c Based on the initial amount of CH4. d Reaction at 473 K without UV-irradiation. e Photoinduced oxidative coupling (CH4 = 100 ~tmol, 02 = 30 ~tmol). y CO2 of 11.5 C% was included. Time course of the reaction was recorded for 30 h (initial amount of methane was 300 lamol) on SiO2-A1203(L) evacuated at 1073 K (Fig. 1). Amount of produced H2 increased linearly with time. Amount of alkane in the gaseous phase, such as C2H6, C3H8 and C4Hl0, also increased, but not linearly. Fraction of higher hydrocarbons also increased with time; after 30 h, even 0.5 mol% of C5-C7 fractions were detected as thermal desorption products. Figure 2 shows Schultz-Flory plot of the products in this coupling reaction. The results after irradiation for 18 h (Fig. 2a) gave a good straight line as expected from the equation (1) with the assumption that the reaction proceeds successively with the same probability for coupling reaction though the number of data is not enough for strict discussion: log (Nx/No) = log (1-p)2/p + x log p

(1)

where x is number of carbon in the hydrocarbon, Nx is amount of hydrocarbon which has x of

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8

0.0-

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Fig. 1 Time course of produced amount of

H2 (a), C2H6 (b), C3Hs (c) and C4Hlo (d) in photoinduced non-oxidative methane coupling on SiO2-A1203(L) evacuated at 1073 K. The unit is ,umol for (a) and C% for (b)-(d).

o

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I

1

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2

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3

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Fig. 2 Schultz-Flory plot derived from the results in Fig. 1. The reaction time was 18 h (a) and 30 h (b).

carbons, No is amount of the converted methane, p is the probability for coupling of methane with another hydrocarbon. This linearity of the plot suggests that the photoinduced nonoxidative coupling proceeds successively with the same probability. However, the results after 30 h gave the non-linear line (Fig. 2b), while C5-C7 fractions were produced . Coupling reaction of higher hydrocarbons might predominantly occur when the reaction proceeds. 3.2. P h o t o a c t i v e sites on s i l i c a - a l u m i n a

Fig. 3 shows the dependence of the total products yield in this reaction on the pretreatment temperature of SiO2-A1203(L). It is clear that the products yield was affected by the pretreatment temperature; when the pretreatment temperature was high such as 1073 K, high products yield was obtained. Probably dehydration occurring on the surface of the silicaalumina at high temperature would produce the active sites. Phosphorescence spectra of SiO2-A1203(L) evacuated at various temperature are shown in Fig. 4. The characteristic spectrum which has the free structures [6] appeared when evacuated at higher temperature such as 873 or 1073 K, its intensity increased with an increase of pretreatment temperature. This dependence of phosphorescence intensity seems to be related to the dependence of activity for the photoinduced reaction on pretreatment temperature (Fig. 3). Thus, the luminescence sites, which are generated through dehydration at high temperature and provide the characteristic phosphorescence spectra, are proposed as the active sites for the photoinduced non-oxidative methane coupling. The luminescence sites of silica-alumina which exhibit the characteristic phosphorescence spectra have been proposed to be highly dispersed aluminum species on the surface of dehydrated silica-alumina [6]. The results on the reaction listed in table 1, i.e., the

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Fig. 3 Total yield in photoinduced nonoxidative methane coupling on SiO2A1EOa(L) evacuated at various temperature. Irradiation time was 3h. Initial amount of methane was 200 ~tmol.

400

500 600 Wavelength /nm

7t

Fig. 4 Phosphorescence emission spectra of SiO2-A1203(L) evacuated at 473 K (a), 673 K (b), 873 K(c) and 1073 K(d). Excitation wavelength was at 300 nm.

total yield on SiO2-A1203(L) was higher than that on SiO2-A1203(H), also suggested that the highly dispersed aluminum species are responsible for the activity in the photoinduced reaction. Therefore, the highly dispersed aluminum species on the silica-alumina would be the active sites for both the phosphorescence and the photoinduced reaction. In order to confirm that the luminescence sites correspond to the active sites for this reaction, interaction between the photoexcited sites and methane molecules was studied. In the presence of gaseous molecules the luminescence would be quenched if the r~ ID molecules interact with the photoexcited sites. Fig. 5 shows quenching effect by methane on phosphorescence spectra of SiO2-A12Oa(L) evacuated at 1073 K. The intensity was obviously reduced with 400 500 600 700 Wavelength /nm increase of methane pressure, indicating that methane interacted with the photoexcited sites on the surface of Fig. 5 Phosphorescence spectra of S iO2SiO2-A1EOa(L). Introduced methane of A1203(L) evacuated at 1073 K in vacuo (a) only 2 Torr was enough to quench a hal f and in the presence of methane: 1.0 Torr (b), of the phosphorescence intensity, while 3.1 Torr (c) and 4.0 Tort (d).

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nitrogen (inert gas) of 15 Torr was required. This indicates that methane molecules interact with the photoexcited sites and receive the excitation energy from the photoexcited sites. Probably this step is the key step for the activation of methane molecules. From the above results, the conclusion of this section is that the highly dispersed aluminum species on the surface of silica-alumina evacuated at high temperature act as not only the luminescence sites for the characteristic phosphorescence spectrum but also the active sites for the photoinduced non-oxidative methane coupling.

4. CONCLUSION It was found that the coupling of methane proceeds successively upon photoirradiation over the silica-alumina evacuated at high temperature without using any oxidant molecules. No oxygenated products such as MeOH, HCHO, CO and CO2 are observed in this system. The silica-alumina exhibits the high products yield in the gaseous phase in contrast to the alumina. The active sites for this photoinduced reaction are identical to the phosphorescent luminescence sites, which are highly dispersed aluminum species on the surface of the silicaalumina evacuated at high temperature. The energy transfer to methane molecules from photoexcited sites on the silica-alumina, which would be the key step for methane activation, is observed by phosphorescence study. REFERENCES

1 L. Guczi, R. A. Van Santen and K. V. Sarma, Catal. Rev. Sci.-Eng., 38 (1996) 249. 2 K. Okabe, K. Sayama, H. Kusama and H. Arakawa, Chem. Lea., 1997, 457. 3 W. Hill, B. N. Shelimov and V. B. Kazansky, J. Chem. Soc., Faraday Trans. 1, 83 (1987) 2381. 4 (a) H. Yoshida, T. Tanaka, S. Matsuo, T. Funabiki and S. Yoshida, J. Chem. Soc., Chem. Commun., 1995, 761. (b) T. Tanaka, S. Matsuo, T. Maeda, H. Yoshida, T. Funabiki and S. Yoshida, Appl. Surf. Sci., 121/122 (1997) 296. (c) H. Yoshida, K. Kimura, Y. Inaki and T. Hattori, Chem. Commun., 1997, 129. 5 (a) H. Yoshida, C. Murata, Y. Inaki and T. Hattori, Chem. Lea., 1998, 1121. (b) H. Yoshida, T. Tanaka, M. Yamamoto, T. Funabiki and S. Yoshida, Chem. Commun., 1996, 2125. (c) H. Yoshida, T. Tanaka, M. Yamamoto, T. Yoshida, T. Funabiki and S. Yoshida, J. Catal., 171 (1997) 351. 6 H. Yoshida, T. Tanaka, A. Satsuma, T. Hattori, T. Funabiki and S. Yoshida, Chem. Commun., 1996, 1153. 7 Y. Kato, H. Yoshida and T. Hattori, Chem. Commun., 1998, 2389. 8 S. Yoshida, T. Matsuzaki, T. Kashiwazaki, K. Mori and K. Tarama, Bull. Chem. Soc., Jpn., 47 (1974) 1564. 9 (a) Y. Murakami, Stud. Surf. Sci. Catal., 16 (1983) 775. (b) T. Uchijima, Catalytic Science and Technology, (eds.) S. Yoshida, N. Takazawa and T. Ono, Kodansha, VCH, Tokyo, vol. 1, (1991) 393.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Photocatalytic oxidation of gaseous toluene on polycrystalline TiO~: FT-IR investigation of surface reactivity of different types of catalysts G. Martraa*, V. Augugliarob, S. Colucciaa, E. Garcia-L6pezb, V. Loddo b, L. Marchese c, L. Palmisa._~oband M. Schiavellob di Chimica IFM, Universit~ di Torino, Via P. Giuria 7, I-10125 Torino, Italy ~ ipartimento ipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universit~ di Palermo, Viale

_ delle Scienze, 1-90128 Palermo, Italy ~Dipartimento,, di Scienze. e Tecnologie Avanzate, Universit~ del Plemonte" Orientale" "A. Avogadro, C.so Borsalino 54, I-15100 Alessandria, Italy

Commercial TiO2 Merck and TiO2 Degussa P25 powders were employed as the catalysts for the photo-oxidation of toluene. By using TiO2 Merck benzaldehyde was found in gas phase as the main product of the toluene partial oxidation. After an initial transient period, this catalyst exhibited a high stability in the presence of water vapour in the gaseous mixture, whereas the photoproduction of benzaldehyde sharply decreases after removal of H20 fi'om the feed. IR spectra of the used catalyst revealed that in the absence of water vapour benzaldehyde is molecularly held on the catalyst surface. This feature was confirmed by co-adsorbing benzaldehyde and water on the fi'esh catalyst. By contrast, when toluene photo-oxidation was carried out on TiO2 Degussa P25 no products in the gas phase were detected. In this case benzaldehyde adsorption, monitored by IR spectroscopy, mainly resulted in the formation of benzoate-like species, strongly adsorbed on the catalyst surface. 1. INTRODUCTION Heterogeneous photocatalysis by semiconductors is a fast growing field of basic and applied research, especially for the case of the oxidation processes of organic pollutants in waste waters [1-3], or in air [4-6]. Among the photocatalytic processes in gas-solid regime, the photo-oxidation of gaseous toluene (the most abundant aromatic volatile organic pollutant in indoor and industrial emissions) to benzaldehyde on TiO2 powders has been recently reported as an effective method to transform noxious C6HsCH3 molecules in a valuable chemical [7]. However, the product distribution and the catalyst stability strongly depend on the nature of the catalyst and the experimental conditions. Ibusuki and Takeuchi [8] obtained the complete photo-oxidation of toluene on TiO2 in the presence of water vapour carried out at room temperature. They used four photoreactors in series and they found that the presence of water was beneficial in order to achieve the almost complete mineralisation of toluene, benzaldehyde having been detected only in very small amounts. Furthermore, Obee and Brown [9] studied the photooxidation of toluene and other organic pollutants in gas-solid regime by using polycrystalline TiO2 as catalyst, evaluating the influence of the competitive adsorption of water and toluene vapours on the photo-oxidation rate.

*Author to whom correspondence shouldbe addressed; tel.: +39-011 670 7538; FAX: +39-011 670 7855; e-mail: [email protected]

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In a previous paper we reported a combined catalytic and FT-IR study of the partial photooxidation of toluene in gas-solid regime by using a commercial anatase TiO2 Merck powder as catalyst [7]. In that case benzaldehyde in gas phase was obtained as the main product, and a strong dependence of the photocatalytic behaviour on the presence of water vapour in the gaseous reaction inlet was observed. In the present study we investigate more deeply the phenomena occurring during the process on the surface of the semiconductor particles by FT-IR spectroscopy, focusing on the role of molecular water in the interaction between photo-oxidation products and the catalyst surface. Furthermore, the fate of the photo-oxidation products in dependence of the strface features of the catalyst was also studied, carrying out the toluene photo-oxidation on a different type of TiO2 powder (i.e. TiO2 Degussa P25). 2. EXPERIMENTAL Photo-oxidation runs were carried out by using polycrystalline TiO2 Merck (anatase phase, BET specific surface area 10 m2.gq) as catalyst. Some selected rims were carded out by using TiO2 Degussa P25 (80% anatase, 20% rutile, BET specific surface area 50 mE'g-l). The reactant mixture was obtained by bubbling air through saturators containing bidistilled water and toluene (Carlo Erba, RS) at room temperature. The gas flow rate was 0.42 cma.s-1 and the toluene and water molar fractions were 1.3x10-2 and 2.5xl 02 respectively. Water vapour was kept in the feed until steadystate condition of toluene fractional conversion to benzaldehyde was attained. After that, water was removed fi'om the inlet gaseous mixture and then readmitted in the final part of the run. A 400 W medium pressure Hg lamp (Polymer GN ZS, Helios Italquartz) was used to irradiate a continuous fixed-bed photoreactor consisting of a Pyrex cylinder described in ref. 2, thennostatted at 140 ~ For the IR measurements (Bruker IFS 48, 4 cm -1 resolution), the TiO2 powders were pressed in form of self supporting pellets, and then placed in a IR quartz cell equipped with KBr windows, permanently connected to a conventional vacuum line (residual pressure: 1.0xl0 ~ Torr: 1 Tow= 133.33 Pa) allowing all thermal and adsorption-desorption experiments to be carded out in situ. 3. R E S U L T S and D I S C U S S I O N 3.1. Photoreactivity results

The UV irradiation of the toluene\air~I20 mixture flowing through a catalytic bed of TiO2 Merck mainly resulted in the production of benzaldehyde, but also benzene, benzyl alcohol and traces of benzoic acid were detected. In Figure 1 the toluene fractional conversion to benzaldehyde versus the irradiation time is reported for consecutive runs carded out in the presence ofH20 vapour (Fig. 1A) and after its removal (Fig. 1B) and re-admission (Fig. 1C).

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By using a wet feed, steady state conversion of toluene to benzaldehyde was achieved after a transient period of 70 h of irradiation (Fig. 1A). After 340 h no decrease of the benzaldehyde photoproduction was observed, showing that, in the presence ofH20 vapour, TiO2 Merck behaves as a highly stable photocatalyst. By contrast, a sharp decrease of toluene conversion to benzaldehyde occunvxt after removal of water from the feed, followed by a further slight loss in activity for longer time of irradiation (Fig. 1B). When water vapour was readmitted in the reaction mixture, a limited increase of the amount of benzaldehyde in gas phase was observed, but it quickly turned down to the level achieved in the absence of H:O (Fig. 1C). By contrast, for the runs carded out by using TiO2 Degussa P25 as photocatalyst, no significant amount of products of toluene conversion was detected in the gas phase during the irradiation time. Moreover, the catalyst became brown coloured after the runs, indicating that toluene photo-oxidation products remained adsorbed on the catalyst surface. In order to determine the nature ofthese species, the catalyst was stirred in acetonitrile for 24 hours at the end of the run and, after its separation, the liquid phase was analysed by HPLC. Benzaldehyde, benzyl alcohol and benzoic acid were found. In a similar experiment on TiO2 Merck much less benzoic acid was found. 3.2. FTIR studies 3.2.1. Nature of the reaction products adsorbed on the surface of the TiO2 Merck catalyst In order to mtionalise the catalytic behaviour commented on above, a study of the nature of species present on the catalyst surface at different stages of the run was performed, comparing the IR spectra of a sample atter a photocatalytic nm in the presence of H20 vapour (Fig. 1, stage A) with the spectra of a sample after irradiation in the absence of H20 (Fig. 1, stage B). The results are reported in Figure 2, where the spectrum of fresh TiO2 Merck powder simply outgassed at r.t. (dotted lines) is also reported to better evidence the spectral features related to the species adsorbed on the used catalysts. This spectrum simply exhibits a broad band at ca. 1630 cm-~, due to the bending vibration of adsorbed molecular water, and weak components in the 1500-1400 cm -~ range, due to carbonate-like groups [7].

10,

// ./

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Fig:l 2. IR spectra in the 1800-1300 cm range of the TiO2 Merck catalyst used in toluene photooxidation runs lasted at steady state conditions in the presence (curve a) and in the absence (curve b) oi water vapour in the gaseous feed. Dotted lines are the spectrtma of the fresh catalyst outgassed at r.t..

180016'5015'0013'50 wavenumber (cm)

In the case of the catalyst employed in the run in the presence of water vapour a broad and complex absorption is present in the 1650-1400 cm"1 (Fig. 2, a). The various com~nents appeared too poorly resolved to allow the recognition of a pattern assignable to defined molecular structures. However, HPLC analysis of acetonitrile solutions obtained by washing the catalyst after the photocatalytic run indicated that several product, i.e benzyl alcohol, phenol and benzoic

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acid are present on the surface of the catalyst. The irreversible adsorption of these species under the reaction conditions used could be responsible of the partial deactivation of the catalyst observed during the first hours of the run. By contrast, a series of bands at 1690, 1645, 1600 and 1580 cm~ is present in the spectrum of the catalyst atter the run in the absence of water vapour (Fig. 2, b). These components should be assigned to benzaldehyde molecules adsorbed on the catalyst surface [vide inca and ref. 7], indicating that, in the absence of water, some of these photoproduced molecules remain adsorbed on the catalyst surface. 3.2.2. Benzaldehyde adsorption on TiO2 Merck

To investigate in more detail the nature of the interaction of benzaldehyde with the catalyst surface and the role played by water in this process, benzaldehyde adsorption and desorption experiments were performed in a model system, by admitting benzaldehyde at r.t. in the IR cell containing a fresh TiO2 Merck sample pre-outgassed at 140 ~ (the temperature of the catalytic run). In this way, a higher intensity of the bands due to adsorbed benzaldehyde was obtained, and their spectral features were not perturbed by the presence of co-adsorbed species. By adsorbing benzaldehyde at r.t. on the catalyst, a main peak at 1700 cm~ and bands at 1657, 1601, 1586, 1447 and 1312 cm -1 are observed in the IR spectrum (Fig 3A, a). This spectrum is very similar to that exhibited by benzaldehyde in CC14 solution, suggesting that aldehyde molecules are adsorbed on the surface of the TiO2 Merck catalyst essentially in an unperturbed form. As a consequence of the benzaldehyde adsorption, the peak at 3665 crn1 observed in the spectrum of the catalyst preoutgassed at 140 ~ due to the stretching modes of surface OH groups [7] (inset in Fig. 3A, curve a) completely disappears, being transformed into a broad and complex band in the 3650-3200 crnl range (Fig. 3A, inset, curve b). 1.00

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0.00 t 17'0016'0015'0014'0013'00 wavenumber (cm -l)

1800 1700 1600 15'001,~0~1300 wavenumber (cm "1)

Fig. 3. IR spectra ofbenzaldehyde adsorbed on TiO2 Merck preoutgassed at 140 ~ Section A: a) after admission of 3 Torr benzaldehyde; b-f) after 0.5, 1, 3, 10, 30 min outgassing at r.t.. Inset: spectra of the catalyst: a) before and b) after benzaldehyde adsorption. Section B: a) after 30 min outgassing at r.t. (as curve f in Sect. A); b) after outgassing 30' at 140 ~ c) after admission of 18 Torr H20 at r.t.; d) after subsequent outgassing at r.t. for 30 min. Spectra in the main frames are reported in Absorbance, having subtracted the spectrum of the sample before adsorption as background.

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This behaviour clearly indicates that hydroxyl groups act as effective adsorption sites for benzaldehyde, through hydrogen bonding between the oxygen atom of the carbonylic group and the H atom of the OH species [10]. Furthermore, by pre-outgassing the catalyst at 140 ~ some coordinatively unsaanv, d Ti4+ ions are exposed at the catalyst surface, but they were found to behave as weak Lewis acid centres, like the hydroxyl groups [11]. The amount of adsorbed benzaldehyde was then progressively decreased by pumping off at r.t., and a progressive decrease in intensity of the adsorbed benzaldehyde was observed (Fig. 3A, b-f). Bands due to adsorbed benzaldehyde were still observed atter outgassing at 140 ~ (Fig. 3B a, b), although significantly reduced in intensity, confirming that a fraction of the product of the toluene photo-oxidation can be adsorbed on the catalyst surface at the reaction temperature in the absence of water. To evaluate the influence of water on the interaction between benzaldehyde and the catalyst surface, water vapour was then admitted onto the sample, resulting in the appearance in the IR spectrum of a dominant band at c a . 1640 crn~ due to the bending vibration of physisorbed H20 molecules. Features of pre-adsorbed benzaldehyde appear as ill-defined shoulders of the overwhelming H20 absorption (Fig. 3B, c). Noticeably, after a subsequent outgassing at r.t., not only the band associated with water molecules disappears, but also the bands due to benzaldehyde are much less intense (Fig. 3B, d). This behaviour indicates that water promotes the desorption of aldehyde molecules. On this basis, the observed sharp decrease of toluene fractional conversion to benzaldehyde after removal ofH20 vapour from the gaseous feed (Fig. 1B) and the transient limited recovery of the amount of benzaldehyde in gas phase after water vapour readmission (Fig. 1C) can be satisfactorily explained in terms of adsorption and desorption of the photoproduced benzaldehyde respectively. However, it must be noticed that an almost irreversible deactivation of the catalyst occurred by nmning the photo-oxidation test in the absence of water vapour (Fig. 1B,C). As previously reported [12], this behaviour can be mainly ascribed to the photodesorption of hydroxyl groups, which are the main active centres in the photoproduction of reactive species. In the absence of water vapour, benzaldehyde molecules are adsorbed on the catalyst surface for long time, and then their oxidation to strongly adsorbed species (e.g benzoate-like species) may occur, further promoting the progressive poisoning of the catalyst. 3.2.3. Benzaidehyde adsorption on TiO2 P25 As reported in section 3.1, no toluene photo-oxidation products were detected in gas phase in reaction tests performed by using TiO2 P25 as photocatalyst, while mainly benzoic acid adsorbed on the catalyst surface was found. However, on the basis of the reaction mechanism previously proposed [7], benzaldehyde is reasonably expected to be the first species produced by the toluene photo-oxidation. An IR study of benzaldehyde adsorption on TiO2 P25 pre-outgassed at the reaction temperature (140 ~ was then performed, aimed to obtain insights on the peculiar behaviour of this catalyst. Interestingly, the spectrum ofbenzaldehyde adsorbed on TiO2 P25 (Fig. 4, a) appeared significantly different with respect to that observed in the case of the TiO2 Merck catalyst (Fig. 4, b). In particular, the main peak at 1650 cm~ and bands at 1518, 1495, 1451, and 1413 cm1 can be tentatively assigned to benzoate-like species formed by nucleophilic attack of basic O2 centres to the carbon atom of the carbonyl group of an aldehyde molecule adsorbed on a neighbour Lewis acid Ti4§ cation through its oxygen atom Benzaldehyde molecules resulting from the photo-oxidation of toluene on TiO2 P25 should be then transformed in benzoate-like species, strongly adsorbed onto the catalyst surface, without the release of any product in the gas phase.

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0.6

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Fig. 4. IR spectra of benzaldehyde (3 Tort) adsorbed on: a) TiO2 Degussa P25 and b) TiO2 merck (the same as curve a in Fig. 3), both preoutgassed at 140 ~ The spectra are reported in Absorbance, having subtracted the spectrum of the corresponding sample before adsorption as background

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1800"1700"16'00"15'00"14'00"13'00

wavenumber (cml) Apparently Ti4+- 0 2- acid-base pairs responsible for the formation of benzoate-like species are much less effective on the surface of the Merck catalyst and, as a consequence, photoproduced benzaldehyde is desorbed from the catalyst surface in the gas phase. REFERENCES

1. M. Schiavello (ed.), Photocatalysis and Environment. Trends and Applications, Kluwer, Dordrecht, 1988. 2. E. Pelizzetti and N. Serpone (Editors), Photocatalysis. Fundamentals and Applications, Wiley, New York, 1989. 3. F. Ollis and H. A1-Ekabi, (Editors), Photocatalytic Purification and Treatment of Water and Air, Elsevier Science Publisher, New York, 1993. 4. V. Augugliaro, L. Palmisano, M. Schiavello and A. Sclafani, J. Catal., 99 (1986) 62. 5. L. A. Dibble and G. B. Raupp, Environ. Sci. Technol., 26 (1992) 492. 6. J. Fan and J. T. Yates Jr., J. Am, Chem. Soc., 118 (1996) 4686. 7. V. Augugliaro, S. Coluccia, V. Loddo, L. Marchese, G. Mama, L. Palmisano and M. Schiavello, Appl. Catal. B: Environ., 20 (1999) 15. 8. T. Ibusuki and K. Takeuchi, Atmos. Environ., 20 (1986) 171. 9. T.N. Obee and R. T. Brown, Environ. Sci. Technol., 29 (1995) 1223. 10. M. Allian, E. Borello, P. Ugliengo, G. Spanb and E. Garrone, Langmuir, 11 (1995) 4811. 11. M. G. Faga, Thesis, University of Turin, 1999. 12. G. Martra, S. Coluccia, L. Marchese, V. Augugliaro, V. Loddo, L. Palmisano and M. Schiavello, Catal. Today, in press.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Investigation o f E n v i r o n m e n t a l Photocatalysis by Solid-State N M R Spectroscopy

D. Raftery, S. Pilkenton, C. V. Rice, A. Pradhan, M. Macnaughtan, S. Klosek and T. Hou H.C. Brown Laboratory, Department of Chemistry, Purdue University, West Lafayette, IN 47907 Abstract

Solid-state NMR methods are applied to study the detailed surface chemistry of a number of promising semiconductor and zeolite based photocatalysts. Emphasis is made on the direct detection of reaction intermediates in the degradation of environmental pollutants. The development of new and efficient UV or visible light activated photocatalytic systems using coated optical microfiber catalysts is also discussed. I. I N T R O D U C T I O N The photocatalytic activity and surface chemistry of TiO2 is currently of significant interest owing to its potential for light harvesting and pollutant remediation applications. Semiconductor photocatalysts can convert environmental contaminates to benign species using UV light and oxygen. Metal oxide semiconductors such as TiO2, V205 and WO3 are frequently employed in photocatalytic applications due to their stability, non-toxicity and readily accessible band gap energies. Much is know about TiO2 photocatalysis at the liquid/solid interface whereas the gas/solid interface is less well understood [ 1-3]. A variety of experimental techniques have been applied to study the surface photochemistry of TiO2, including GC-MS [4,5], IR [6,7], and XPS [8]. We have recently introduced a new approach to the study of photocatalytic surface chemistry consisting of an in situ magic angle spinning (MAS) NMR probe and sample preparation methods that allow us to monitor photocatalytic reactions as they transpire in the NMR magnet [9-11]. The broad range of detailed structural and dynamical information available with solid-state NMR is well known, and our approach creates an opportunity to investigate a number of fundamental issues involved in effective semiconductor photocatalysis. In their powdered form, photocatalysts are efficient light scatterers, a fact that can complicate the surface studies. By simply packing the catalyst into a transparent sample rotor, the exterior portions of the catalyst bed will be illuminated while the interior will remain dark. This can lead to the formation of long-lived surface species that are difficult to remove [10]. Therefore, we have employed two methods to circumvent these problems. First, chemical vapor deposition methods are used to create TiO2, V205 and WO3 monolayer catalysts on porous Vycor glass (PVG). These catalysts allow the homogeneous UV irradiation of the entire sample. Alternatively, photocatalytic particles can be attached chemically to micro-optical fibers [ 12,13]. A bundle consisting of approximately 40,000 of

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these fibers can be loaded into an NMR sample tube along with reagents and studied using in situ NMR techniques. We have recently shown that this coated fiber methodology can be extended to examine intrazeolite photochemistry [14-16] by attaching zeolite crystallites to the micro-optical fibers [17]. Example in situ experiments detailed below follow the photocatalytic degradation of ethanol, trichloroethylene (TCE), and chloroform and show that this methodology has significant promise towards the further understanding and development of photocatalytic surface chemistry. 2. EXPERIMENTAL 2.1 C a t a l y s t P r e p a r a t i o n

Monolayer catalysts were prepared by vapor deposition of TIC14, VO(acac)2 in toluene, or WC16 in CC14 onto previously calcined PVG rod (3.6 mm dia, 12 mm long) following methods established by Anpo and co-workers [18]. The catalysts were then hydrated and calcined to form the oxides before use. Figure l(a) shows the gas phase synthesis of the monolayer photocatalyst, and 1(b) shows the absorbance spectra of PVG and the monolayer TiO2 catalyst. Samples were prepared by introducing controlled amounts of ethanol or TCE, and 02 to the catalysts (TiO2 powder, catalyst/PVG, or TiOz/optical fibers), and then sealing them inside 5 mm NMR tubes for the in situ studies. UV-VIS Absorbance Spectra 3.5 3.0

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,

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(a)

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0.5

(b)

0.0 200

i

300

i

400

Wavelength (nm)

500

Figure 1. (a) Schematic illustration of the gas phase synthesis of PVG supported monolayer photocatalysts. (b) UV-VIS absorbance spectra of pure PVG and a TiO2/PVGphotocatalyst. TiO2-coated optical fibers were prepared from 9 ~tm fibers (Quartz Products Company) that were calcined at 400 ~ then boiled in H2SO4 to remove the polyimide cladding. The fibers were then dipped into a suspension of TiO2 particles (10 g, Degussa) in water (16 ml). Addition of 2,4 pentanedione promoted dispersion of the particles. After dipping, the fibers were heated to 500 ~ for 30 min, then cooled and rinsed under flowing water to remove any excess TiO2. The final surface area w a s - 7 m2/g (from BET analysis). Figure 2 shows a SEM image taken of the coated optical fibers.

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673

Figure 2. SEM image showing a 9 mm diameter TiO2-coated optical microfiber photocatalyst.

2.2 NMR Measurements All spectra were obtained on a Varian Unity-plus 300 spectrometer operating at 75.4 MHz for 13C and using a homebuilt optical NMR probe. The UV source was a 300W Xe arc lamp, and roughly 5mW of near UV light was delivered over the surface of the samples under magic angle spinning (MAS) and in situ conditions using sealed 5 mm NMR tubes. The reaction studies were carried out under batch conditions.

3. RESULTS AND DISCUSSION

3.1. Vycor Supported Monolayer Catalysts The adsorption of ethanol on the different catalysts was first examined using IH-13C cross polarization (CP). Short CP contact times of 50 Its ensured that only strongly adsorbed ethanol molecules would be observed. Figure 3 shows proton-decoupled 13C CP/MAS NMR spectra of ethanol loaded onto the different photocatalysts. It is evident from these spectra that the NMR shift of the methylene carbon (-CH2-) is very sensitive to the nature of the adsorption site on different materials (peaks at 58-83 ppm) while that of the methyl carbon (CH3-) shows only a single resonance at 15.3 ppm. With the exception of PVG (Figure 3 (a)), the other photocatalysts (Figure 3(b)-(e)) provide ethanol

(e)

Figure 3. Solid-state 13C NMR spectra of ethanol adsorbed to the surface of different metal oxide photocatalysts. (a) PVG, (b) TiO2 powder (Degussa P25), (c) TiOjPVG, (d) VzOJPVG, (e) WO3/PVG. The methylene resonance (6085 ppm) is sensitive both to the type of adsorption (chemisorbed or physisorbed) and to the nature of the metal oxide.

(d)

(c)

(b) (a) 150

100

50 Chemical Shift (ppm)

0

-50

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with at least two different adsorption sites. The peaks located around 60 ppm are characteristic of hydrogen-bonded ethanol molecules, and their chemical shifts are insensitive to surface metals. However, the down field peaks (70-83 ppm) depend on the specific metal oxides, which we assigned as metal-bound ethoxide (M-OCH2CH3) species. The formation of ethoxide may originate from the coordination of ethanol molecules into surface defect sites or oxygen vacancies on metal oxide surfaces. Thermal desorption experiments (using solid-state NMR as a detector) confirmed the presence of the two different surface ethanol species [ 13]. Among the PVG-supported photocatalysts studied, TiO2/PVG was observed to be the most active in oxidizing ethanol to CO2. The formation of acetaldehyde, acetic acid, formic acid, and formaldehyde on this catalyst was observed using NMR. The V2Os/PVG catalyst was also active, producing acetaldehyde as an intermediate and ultimately CO2. However, WO3/PVG was inactive for the degradation of ethanol. 3.2 TiO2-Coated Fiber Optic Catalysts

Photocatalytic oxidation of TCE and ethanol were carried out on the optical microfiber catalyst to follow the degradation chemistry. These reaction studies also allowed an evaluation of the effectiveness of the microfibers for homogeneous irradiation of the entire catalyst bed. Figure 4 shows the n3C solid-state NMR spectra taken during irradiation of 50 lamol TCE and 100 l.tmol oxygen loaded on the microfiber supported TiO2 catalyst. From 9 lamQuartz .dmlp~ Fiber ~ Ti02 Coating

a\

~hY ~ .... f;l\ Side Irradiation' ----"=~'~-'-:~'~~ ""~ H--G-- G. oo I o-~., 9

ii

~-'~ l"

I

....H

a/~.'=cxc~

"~" 0 minutes i

I "~ l

~

o ~.

30 m i n u t e s

~

~ j

75 minutes

CO~,

r_;i"" " e l

~--~ 255 minutes .......~i~"'"ii0"i'i0"ii6"i~'"i'~'0"i~ ......~ii......i~'"'ilill.

r

Figure 4. ~H decoupled ~3C spectra acquired during the in situ UV irradiation of TCE and oxygen loaded onto a TiO2-coated optical microfiber photocatalyst. The formation of dichloroacetal chloride and oxylyl chloride (30 min), dichloroacetic acid (75 min), and final products phosgene and CO2 (255 min) was observed. The cross polarization experiment after irradiation showed no strongly surface bound species, due to the homogeneous light exposure.

]:~

H---c .... 260

2OO

.15O. . . .! O0 . . . . SO . . . . .0

,3CCPMAS -SO

pp m

afterthe

experiment

a/

%0

ilBIliililm S u r f a c e B o u n d Acetate

this series of spectra it is possible to identify and quantify the formation and degradation of reaction intermediates and products. The formation of dichloroacetal chloride, oxylyl chloride, phosgene and CO2 was observed. At the end of the reaction, a CP/MAS spectrum was acquired to probe for surface bound dichloroacetate. We have shown that the presence of this chemisorbed species formed after continued irradiation indicates that dark regions exist in the catalyst, and we observed this species on packed YiO2 powder samples previously [9]. However, there is no signal in the CP/MAS spectrum for this microfiber catalyst that would

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indicate the presence of the acetate species, which confirms our previous observation of light penetration to the sample interior. The photocatalytic oxidation of ethanol over the TiO2-supported optical fibers resulted in complete degradation of ethanol, forming acetic acid and CO2 within 3 hrs. For comparison, the PVG supported TiO2 monolayer catalyst showed much less conversion. In experiments using multiple monolayers of ethanol, a new intermediate, acetal (CH3CH(OCH2CH3)2) was observed. Acetal could be oxidized to acetic acid under further UV irradiation.

3.3 Zeolite-Coated Fiber Optic Photocatalysis The use of micro-optical fiber supports can also be extended to follow intrazeolite photocatalysis. Currently, there is a growing interest in the use of zeolites to carry out highly selective partial oxidation [16] or shape-selective photochemistry [14]. Ion exchanged zeolites, such as Ba-Y can be chemically attached to the optical fibers using a sol-gel process to generate robust catalysts with much improved efficiency. For example, the degradation of methylene chloride using visible light occurs at a rate that is 20 times faster than that observed in the powder alone [17]. In Figure 5(a), a SEM image of Ba-Y zeolite coated onto a micro-optical fiber is shown. This catalyst is effective for the degradation of a number of chlorinated organic species. For example, selective oxidation of chloroform to form phosgene is observed using in situ NMR (Figure 5(b)) with irradiation with visible light (>400 nm). Chloroform degradation on the Ba-Y powder alone occurs much more slowly.

660 rain.

HCI 3

0 rain.

200

(a)

160

120 80 Chemical Shift (ppm)

40

0

(b) Figure 5. (a) SEM image of a 9 gm diameter zeolite-coated microfiber. (b) In situ t3C NMR spectra showing the selective intrazeolite photooxidationof cholorform to phosgene.

4. SUMMARY In situ solid-state NMR methods described above provide detailed surface information on photocatalytic oxidation reactions, including the differentiation of adsorption sites and their respective reactivities, as well as the identification of long-lived surface intermediates. NMR studies of ethanol adsorption and photooxidation on several PVG supported semiconductor catalysts indicate that the formation of surface ethoxide species is important for high activity. Grafting of semiconductor photocatalysts and zeolite crystallites to the optical microfibers provides a highly efficient methodology for the photocatalytic degradation of organic

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pollutants using either UV or visible light. In particular, zeolite/microfiber photocatalysts represent a new and efficient approach for partial oxidation or shape selective photochemical reactions. Solid-state NMR studies are expected to contribute significantly to this area.

ACKNOWLEDGMENTS

Support for this work from the National Science Foundation and the A. P. Sloan Foundation is gratefully acknowledged.

REFERENCES

1. M.A. Fox and M. T. Dulay, Chem. Rev., 93 (1993) 341. 2. M.R. Hoffmann, S. T. Martin, W. Choi, and D. W. Bahnemann, Chem. Rev., 95 (1995) 69. 3. A.L. Linsebigler, G. Lu, and J. T. Yates, Jr., Chem. Rev., 95 (1995) 735. 4. M. R. Nimlos, W. A. Jacoby, D. M. Blake and T. A. Milne, Environ. Sci. Technol., 27 (1993) 732. 5. Y. Lou and D. F. Ollis, J. Catal., 163 (1996) 1. 6. J. Fan and J. T. Yates, Jr., J. Am. Chem. Soc., 118 (1996) 4686. 7. M. D. Driessen, A. L. Goodman, T. M. Miller, G. A. Zaharias, and V. H. Grassian, J. Phys. Chem. B, 102 (1998) 549. 8. S.A. Larson and J. L. Falconer, Appl. Catal. B: Environ., 4 (1994) 325. 9. S.-J. Hwang, C. Petucci, and D. Raftery, J. Am. Chem. Soc., 119 (1997) 7877. 10. S.-J. Hwang, C. Petucci, and D. Raflery, J. Am. Chem. Soc., 120 (1998) 4388. 11. S.-J. Hwang and D. Rafiery, Catal. Today, 49 (1999) 353. 12. C. V. Rice and D. Raftery, Chem. Commun., (1999) 895. 13. S. Pilkenton, S.-J. Hwang, and D. Raftery, J. Phys. Chem. B, in press. 14. N. Turro, Pure Appl. Chem., 58 (1986) 1219 15. V. Ramamurthy, P. Lakshminarasimhan, C. P. Grey and L. J. Johnston, Chem. Commun. (1998)2412. 16. F. Blatter, S. Vasenkov, and H. Frei, Catal. Today, 41 (1998) 297. 17. A. Pradhan, M. Macnaughtan, and D. Raflery, J. Am. Chem. Soc., in press. 18. M. Anpo, M. Aikawa, Y. Kubokawa, M. Che, C. Louis and E. Giamello, J. Phys. Chem. (1985) 89, 5017.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

A Study of CH4 Reforming by C02 and H20 on Ceria-Supported Pd S. Sharma, S. Hilaire, and R. J. Gorte* Department of Chemical Engineering, University of Pennsylvania, Philadelphia, PA 19104 CI-I4 reforming by CO2 and 1-120 was studied on Pd/ceria catalysts using steady-state, rate measurements and pulse-reactor studies. Reaction rates for both reactions on Pd/ceria are several orders of magnitude higher than rates on either Pd/silica or ceria by itself, demonstrating that contact between the precious metal and ceria is crucial. Pulse-reactor studies show that reduced ceria can be oxidized by either CO2 or H20, which suggests that the reforming reactions involve a redox process. The implications of these results for oxygen storage in three-way, automotive catalysis are discussed.

1.

INTRODUCTION

Ceria is widely used in automotive, emissions-control catalysis as an oxygen-storage component (OSC) because of its ability to accept and release oxygen [1]. Since oxygen desorption from ceria occurs at temperatures which are much higher than that used in the catalytic converter [2,3], oxygen release from ceria requires contact between ceria and a precious metal and involves reaction of oxygen from ceria with a reductant on the metal. Indeed, the reaction of oxygen from ceria with CO adsorbed on supported precious metals has been observed in temperature-programmed-desorption (TPD) studies carried out in ultra-high vacuum [4]. This ceria-mediated mechanism is also important under normal reaction conditions. The kinetics for CO oxidation are observed to be zero order in CO under conditions for which the metal is saturated with CO [5,6]. The zero-order kinetics can be understood according to the following mechanism, where o represents an adsorption site on the precious metal: CO + 0 -~ CO.~d 1/'202 -I- Ce203 --~ 2 Ce02 COad + 2 Ce02 ~ C02 + Ce203 + t~

(1) (2) (:3)

Oxidation of Ce203 by H20 and CO2 is also energetically favorable, and the above mechanism implies that one should be able to carry out oxidation with these reactants, as well as with O2. Considering the water-gas-shift reaction to be the oxidation of CO by H20, the fact that precious metals and ceria, individually, are much less active than ceria-supported catalysts supports this redox reaction picture [7]. Obviously, the implication that H20 and CO2 could be oxidants is very important for automotive, three-way catalysis since both species are present in high concentrations in the exhaust environment. In the present study, we extended our investigations of ceria-supported metals through measurements of CO2 and steam reforming of methane, using steady-state reaction studies and

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pulse-reactor studies on Pd/ceria catalysts. The results are consistent with a redox process in which CO2 and H20 can be considered oxidants.

2. EXPERIMENTAL Steady-state rates for CO2 and steam reforming of methane were measured by placing 100-rag, catalyst samples into a 1A-inch, quartz, tubular reactor which has been described in previous publications [8,9]. The total pressure in the reactor was maintained at one atmosphere, but the partial pressures of CH4, CO2, and either He or N2 could be controlled by the flow rates to the reactor. Water could be introduced to the reactor by bubbling the He or N2 carder through distilled, deionized water. Differential conversions were maintained for all measurements. PrOduct analysis (CO2, CO, and CH4) was performed with an on-line HewlettPackard 5730A gas chromatograph, equipped with a methanator and an FID detector. All rates have been normalized to the weight of the catalyst. The transient, pulse experiments were performed on a system, similar to that described in the literature [10], for which the product gases could be analyzed by an on-line, quadrupote mass spectrometer. Reactant gases were passed over a 280-mg sample in a ~A-inch, quartz tube. Computer-controlled switching valves allowed the composition of the gases to undergo step changes. In all cases, He was the major component of the gas phase, with a flow rate of 30 cm3/min, and the active component (CO, 02, and CO2) was chosen to be ~ 10% of the total flow rate. This ensured that the step changes in composition did not dramatically change the flow rate. The pulse reactor data were used primarily to determine the amount of oxygen which was added or removed from the catalyst during each oxidation and reduction cycle. Integration of the partial pressure as a function of time allowed accurate determination of the amounts of CO2 formed during a CO pulse. The catalysts were prepared in our laboratory. For all of the catalysts, Pd was added by aqueous wet impregnation of Pd(NH4)a(NO3)2. After impregnation, each catalyst was dried, calcined in flowing air for 2 h at 673 K, and then pressed into wafers. Prior to reaction experiments, the wafers were reduced in a stream of 10% CO in He for 1 h, then cooled in flowing He. A 1% by weight loading of Pd was used for all of the catalysts in the pulse experiments, while a 10% wt loading was used for tlae steady-state reaction measurements. Ceria was used as received from Johnson Matthey. BET analysis indicated that the surface area was 33 m2/g. The silica support was obtained from Sigma. 3. RESULTS 3.1 Pulse Experiments The results for the pulse measurements with CO and 02 on Pd/ceria at 723 K are shown in Fig. 1. In this experiment, the sample was exposed to a train of CO and 02 pulses while the concentration of the gas phase was monitored with the mass spectrometer, looking for CO (m/e=28), 02 (m/e=32), and CO2 (m/e--44). The results shown in the figure are representative of the entire train. Following each CO pulse, a significant amount, 450 lxmol/g, of CO2 (m/e = 44) was formed at the leading edge of the pulse. For comparison purposes, complete reduction of PdO would result in the production of--94 vxnol/g of CO2 and complete reduction of ceria from CeO2 to Ce203 would provide an additional 2900 lxmol/g. Therefore, the majority of the oxygen from this catalyst must originate from the ceria support but reduction of ceria is obviously incomplete.

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C02 mCO !

|

|

i

|

I

1

r

a m

i

t~

,4-1

ilmm

J~ i._

<

CO Flow

"02 Flow"

CO Flow

Time(s)

v

!

2500

Fig. 1. Pulse-reactor results for CO and 02 pulses over Pd/ceria at 723 K. The exposure of the reduced sample to 02 led to a very sharp CO2 spike corresponding to 400 to 500 ~mol/g. (Note that the small peak at m/e = 28 is part of the fragmentation pattern of CO2 in the mass spectrometer.) Using the BET surface area of the catalyst and 400 ~tmol/g for the amount of CO2 formed, the surface coverage of CO2 was estimated to be 6x10 ~8 molecules/m 2, which is close to the coverage expected for one monolayer. Therefore, one possible explanation for the CO2 peak is that CO2 formed during the initial CO pulse adsorbs on Ce § sites on the reduced ceria support. This CO2 is then displaced when the ceria is reoxidized during the 02 pulse. We reject the alternative explanation as unlikely, that the sharp CO2 peak is due to oxidation of CO or carbon adsorbed on the Pd. First, the amount of CO2 is too large for the carbon to be only on the Pd since it would require approximately four carbons per metal atom. Second, the CO2 formation during the O2 pulse is only observed with ceria and not with the other supports. Third, we would not expect the reaction to form CO2 from carbon to be so rapid. To test whether reduced ceria could be oxidized by CO2, we examined a series of COCOa pulses, with the results shown in Fig. 2. Again, the results are representative of the entire train of pulses, showing that the catalyst could be cycled reproducibly. Since both CO and CO2 contribute an m/e=28 signal in the mass spectrum, one must consider the ratio of the peaks at 28 and 44 in order to unambiguously interpret the data. The catalyst is clearly reduced by each CO pulse. The amount of CO2 formed during the CO pulse is --120 ~tmol/g, which is significantly less than the amount of CO2 formed during the CO-O2 pulse cycle. It is obviously not surprising that CO2 is not as efficient at oxidizing ceria as O2. Additional evidence that

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ceria is being oxidized by the CO2 is observed during the CO2 pulse. Most of the peak at m/e=28 is part of the fragmentation patter of CO2 and follows the peak at m/e=44. However, the leading edge of the leading edge of the m/e=28 peak indicates that a significant amount of CO is formed by the reduction of CO2. Indeed, the amount of CO formed during the CO2 pulse is approximately equal to the amount of CO2 formed during the CO pulse, within experimental error. That the reduction of CO2 involves ceria was demonstrated by the fact that we found no evidence for oxidation of either Pd/7-A1203 or Pd/silica by CO2 at temperatures up to 723 K [9].

,, - - C O 2

*'=

i-co

CO Flow ............

C 0 2 Flow

CO Flow

T i m e (s)

i

i

I

2500

Fig. 2. Pulse-reactor results for CO and CO2 pulses over Pd/ceria at 723 K.

3.2 Steady-State Rate Measurements Rate measurements for both steam-reforming and COz-reforming of methane are shown in the Arrhenius plot in Fig. 3 for 10% Pd/ceria and Pd/silica catalysts, as well as for pure ceria. The rates for both reactions were measured under differential conditions. The steamreforming reaction was carried out in 15 torr H20 and 5 torr CH4 and the CO2-reforming was carried out in 15 torr for both CI-I4 and COz. Considering the steam-reforming reaction first, it is clear that the activity of Pd/silica is negligible. We were unable to observe any conversion at the temperatures used for the rate measurements on Pd/ceria; reasonable conversions could only be achieved by working at higher temperatures. By extrapolating the data for the 10% Pd/ceria catalyst, we estimate that the Pd/ceria catalyst had an activity that was at least 105 times that of the Pd/silica, demonstrating that Pd, by itself, is not an active catalyst for this reaction. While ceria by itself showed some activity, the rates were again orders-of-magnitude lower than we observed for Pd/ceria. Clearly, reaction on Pd/ceria catalysts must occur through a ceria-mediated mechanism.

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化

10000

.u

1000

m

=I

100

m

O

10 +

1 El

O I.

0.1 0.01 0.001

I

1

1.2

I

I

1.4 1.6 1/T (10 "3 K "1)

I

I

1.8

2

Fig. 3. Steady-state reaction rates for CO2 reforming of CH4 on Pd/ceria (,,), ceria (+), and Pd/silica (t~) and for H20 reforming of CH4 on Pd/ceria (.) and Pd/silica (0). The conclusions for the CO2-reforming reaction are similar. While the absolute rates for CO2 reforming are significantly lower than for steam reforming, the Pd/ceria catalyst does show significant activity. As discussed in a previous paper [8], the rates on Pd/ceria are higher than those observed on Pd/zirconia, which has been reported in the literature to be a very active catalyst [11-13]. Again, the Pd/silica catalyst had essentially no activity and it was not possible to measure rates in the same temperature region used for Pd/ceria. Extrapolating the data for Pd/ceria to the same temperature as that used for Pd/silica, the Pd/ceria catalyst was estimated to be a factor of ~ 104 times more active. 4.

DISCUSSION

Based on the results discussed in this paper, both the CO2-reforming and the steamreforming of methane on Pd/ceria occur through a redox mechanism in which CO2 and H20 should be considered oxidizing agents. It appears that ceria-supported precious metals are active catalysts because oxygen transfer is facile between ceria and the precious metals, Therefore, in these reactions, ceria is oxidized by CO2 or H20 in a reaction that is probably catalyzed by the precious metal. The catalyst is then reduced by dissociative adsorption of CH4 in a separate step. These results have important implications for three-way, automotive-exhaust catalysis. The primary role of ceria is that of OSC, which requires a cycling of the oxidation state of

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ceria. Obviously, the exhaust environment contains significant amounts of CO2 and H20; and, based on our present results, these species strongly affect the oxidation state of the catalyst. This, in turn, will affect the extent to which oxidation state can be cycled. Recognition of this point may well change the manner in which one tests catalysts and should receive additional consideration. It is well known that deactivation of automotive, emissions-control catalysts can occur through the loss of the oxygen-storage capacity [10]. The deactivation has usually been explained as being due to a loss in the contact between ceria and the precious metal. However, recent work in our laboratory has suggested that the reducibility of ceria is strongly structure sensitive, so that deactivation is frequently due, at least in part, to the loss of ceria reducibility [6,7]. The present results confirm the importance of the redox properties of ceria, demonstrating that it is crucial to have facile exchange of oxygen between the precious metal and ceria. Ceria and Pd, separately, exhibit no activity for the reforming reactions. Finally, we note that the pulse-reactor studies described in this paper show great promise for studying catalytic chemistry. In addition to allowing one to determine the oxidation state of the catalyst, the data provide information on adsorbate coverage under working conditions. For example, the observation that significant amounts of CO2 are present on the reduced catalyst may well have important consequences for understanding the water-gas-shift reaction on ceria-supported catalysts. We are continuing our investigations in this area. 5. CONCLUSIONS CO2 and H20 able to partially oxidize reduced Pd/ceria catalysts. This redox process i s responsible for the high catalytic activity of Pd/ceria for CH4 reforming by CO2 and H20 and may be important in controlling the oxygen stoichiometry of three-way automotive catalysts. ACKNOWLEDGMENTS This work was supported by the DOE, Basic Energy Sciences, Grant #DE-FG03-8513350. REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

R.W. McCabe and J. M. Kisenyi, Chemistry & Industry, 15 (1995) 605. E.S. Puma, J. M. Vohs, and R. J. Gorte, Catalysis Letters, 45 (1997) 143. E.S. Puma, J. M. Vohs, and R. J. Gorte, J. Phys. Chem., 100 (1996) 17862. G.S. Zaftris and R. J. Gorte, J. Catal., 139 (1993) 561. G.S. Zafu'is and R. J. Gorte, J. Catal., 143 (1993) 86. T. Bunluesin, R. J. Gorte, and G. W. Graham, Appl. Catal. B, 14 (1997) 105. T. Bunluesin, R. J. Gorte, and G. W. Graham, Appl. Catal. B, 15 (1998) 107. R. Craciun, B. Shereck, and R. J. Gorte, Catal. Lett., 51 (1998) 149. S. Sharma, S. Hilaire, J. M. Vohs, R. J. Gorte, and H.-W. Jen, J. Catal., in press. H.-W. Jen, G. W. Graham, W. Chun, R. W. McCabe, J.-P. Cuif, S. E. Deutsch, and Touret, Catalysis Today, 50 (1999) 309. 11. M . C . J . Bradford and M. A. Vannice, J. Catal., 173 (1998) 157. 12. J.H. Bitter, K. Seshan, and J. A. Lercher, J. Catal., 183 (1999) 336. 13. S.M. Stagg, E. Romeo, C. Padro, and D. E. Resasco, J. Catal., 178 (1998) 137.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Catalytic behaviour of Ni containing catalysts in vaporeforming of methane with low H20/CH4 ratio and free carbon deposition. H. Provendier, C. Petit and A. Kiennemann LERCSI-ECPM UMR 7515, 25, rue Becquerel 67087 Strasbourg C6dex 2 France An active system for vaporeforming of methane with low H20/CH4 ratio has been investigated starting from a LaNixFe(l_x)O3 perovskite (0 < x < 1) (96 % CH4 conversion, 96 % CO selectivity) at 800 ~ On most active systems (0.2 ___x < 0.4), La(NiFe)O3 structure, LaFeO3 perovskite and well dispersed nickel particles are present after reaction without carbon deposition. Presence of nickel in the bulk and of the surface of the perovskite leads to strong interaction between small metal nickel particles and crystal lattice explaining the good ageing of this kind of catalysts. 1. I N T R O D U C T I O N The main key to improve the C O 2 o r steam reforming of CH4 is to expand the room for operation without free carbon formation [1]. Design of improved methane reforming is dictated by the need of avoiding carbon formation and using lower steam to carbon ratios. Limited coke formation depends on the competition between dissolution of surface carbon into the bulk of nickel particle (carbon formation) and the reaction of surface carbon or CHx with oxygenated surface species (OH group) to form CO. Modified nickel catalysts can favour the reactivity of the oxygenated surface group by increasing the adsorption of water vapor and/or CO2, by enhancing the reaction rate of surface carbon and/or CHx species with the oxygenated surface entities, or by modifying the activation mode of methane [2]. According to literature, rare earth oxide promoters or supports can play this role but also diminish carbon formation by decorating the metal particles [3]. We will report in the present work the results obtained in steam reforming of CH4 with a H20/CH4 ratio = 1 on an active nickel system whose precursor is a LaNixFel_xO3 perovskite (0 _<x < 1). We have previously shown the interest of this system (in particular for x = 0.3) in dry reforming (CO2 + CH4) [4] where the perovskite structure is maintained with a Ni-Fe alloy formation. 2. E X P E R I M E N T A L

2.1 Catalysts preparation The mixed LaNixFel_xO3 catalysts have been prepared via a sol-gel related method with x values varying from 0 to 1 with a 0.1 step. A complete description has been given in [5]. Briefly, lanthanum nitrate, La(NO3)3.6H20, nickel nitrate Ni(NO3)2.6H20 and iron nitrate Fe(NO3)3.9H20 were respectively dissolved in hot propionic acid with the respective amounts for the desired x values (0 < x < 1). Iron and nickel propionic solutions were mixed and rapidly added to the lanthanum one. After a 30 min stirring, the resulting solution was

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evaporated until a formation of resin or a foam that is calcined at increasing temperature (3~ min -~) until 750~ and maintained at 750~ for 4 h.

2.2 Characterisation of the formed perovskites Powder X-ray diffraction (XRD) on a Siemens D-5000 diffractometer using CuKo~ radiation gives the nature of the phases and the lattice parameters. Transmission electron microscopy (TEM) were performed on a Topcon-EM002B apparatus coupled to an energy dispersive X-ray spectrometer (EDS) in order to determine the nanoscopic state, the homogeneity and the lattice parameters. La, Ni, Fe, C were measured by elemental analysis in the CNRS Centre in Vernaison. Temperature programmed reduction (TPR) measurements were performed on a 50 mg sample placed in a U-shaped quartz reactor (6.6 mm ID) using a heating slope of 15~ rain -~ from 25 up to 900~ The reducing gas used was a mixture of He and 3% H2 (50 ml rain -l) and the hydrogen consumption is quantify by a TC detector. Magnetic susceptibility was performed on a susceptometer DSM-8 Manics apparatus, with a mixture of 5% H2 in nitrogen as reducing agent and a heating slope of 25~ -~ from 25~ to 920~ The measurements were obtained under a magnetic field of 1T.

2.3 Reaction test The operating conditions were the following : fixed bed quartz reactor (6.6 mm I.D.), inlet temperature 9400-800~ feed flow rate 9 C H 4 9 1.5 Lh-tgcat -1, H20 " 1.5 or 3L h-~gcat -j (ratio H20/CH4 = 1/1 or 3/1), Ar : 12 Lh-~gcat -t The outlet gas was analysed by two G.C.. The program was composed first of a cycle including or not a reduction under hydrogen (from room temperature to 600~ (10~ min-t)), then after an argon purge, the gas mixture is admitted into the reactor and the temperature is increased until 800~ with steps of one hour each 50~ (G.C. analyses). For ageing test the activity is maintained at 800~ 3. RESULTS AND DISCUSSION

3.1 Characterisation of the LaNixFe~.xO3 catalysts before catalytic test Characterisations of LaNixFel_xO3 before test have been described in [4]. Briefly, XRD diagrams of the catalysts show that only one perovskite phase is obtained. A zoom on the 20 diagram area between 32.0 and 33.5 (most intensive diffraction peak) shows a regular shift of the structure peak between those of LaNiO3 and LaFeO3. The lattice parameter for a assimilated pseudocubic structure varies linearly between 3.92 A for LaFeO3 and 3.84 A for LaNiO3. This two results confirm the formation of a La(NiFe)O3 solid solution [6]. EDS coupled with TEM carried out on different areas of the samples with a large (200 nm diameter) or a fine (14 nm) focussed beam confirms the sample homogeneity [7,8].

3.2 Reducibility of the catalysts As the reduced form is the active phase, the reducibility has been studied by TPR. LaFeO3 is almost irreducible and LaNiO3 presents two reduction peaks (at 420~ and 520~ respectively corresponding to the reduction into La2NiO5 (Ni HI to Ni II) then Ni n to Ni ~ (La2NiO5 into Ni ~ on La203) (XRD). This second temperature peak regularly increases from 550~ to 900~ when nickel content decreases. Concerning LaNiFe catalysts, the reduction of a part of iron is assisted by nickel and a Ni-Fe alloy is formed at the end of the reduction, as confirmed by M6ssbauer and magnetism. With M6ssbauer, we can find a gap of 33.0 T

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for pure t~-Fe at 32.7 T then at 29.4 T for Fe ~ phases respectively formed from reduced LaNi0.3Feo.703 and LaNi0.TFe0.303. Magnetic susceptibility of an in-situ reduced catalyst indicates a single Curie temperature between iron and nickel ones for each X value. F o r example, starting from LaNi0.3Fe0.703 catalyst, the Curie temperature is situated at 580~ and the Ni-Fe alloy contains 30% of nickel which corresponds to initial structure. A size particle profile (several micrographs, more than 500 particles) shows a repartition between 2 and 28 nm with a maximum at 4 nm for LaNi0.3Fe0.703. EDX analysis (200 and 14 nm zones) show some parts of the catalyst that are only formed of La203 and some metallic parts formed of Ni-Fe alloy whose respective proportions correspond to LaNixFel_xO3 catalysts composition.

3.3 Catalytic reactivity Catalytic tests have been realised with a H20/CH4 ratio equal to 1 (figures 1 and 2) and equal to 3. In the last case, a preliminary hydrogen reduction until 600~ is necessary. Three kinds of behaviour can be observed on these curves : - at low nickel content (0 < x < 0,1), systems have a weak activity and are little CO selective. - the intermediary systems (0.2 _< x _< 0.9) are all very active (CH4 conversion > 95 %) and very selective (CO selectivity > 94 %) at 800~ H2/CO ratio is close to 3 which means that water gas shift reaction is little active and confirms that the reduced iron located outside the perovskite is linked to nickel (cf. TPR) or with lanthanum as LaFeO3 (XRD).Yet, those systems can become differentiated for each others when studying their ageing (figure 3). LaNi0.3Fe0.7 is very stable and after a slight initial decrease, the catalyst keeps an yield over 90% after 200 working hours. At the opposite, CO yield decreases with time (from 95% initially to 40% after 200 hours) on catalyst that contains more nickel (LaNi0.TFe0.303). - LaNiO3 that rapidly forms a carbon deposit in our procedure test. H20/CH4 ratio value is a crucial point. With a H20/CH4 = 3, even if water excess allows the limitation of carbon formation [8], the selectivity turns from CO to CO2 (71% conversion, 60% CO and 40% CO2 selectivity) at 800~ for LaNi0.3Fe0.703. 3.4 Characterisation of LaNixFel.xO3 after catalytic test Table 1 gathers the crystalline phase detected by XRD, and the percentage of free nickel (XRD) and the ratio NirejNifree calculated from magnetism measurement. On studying XRD data, it can be concluded that poor nickel content systems (0.1 _< x _ 0.3) keep their LaNiFe trimetallic structure even if the nickel ratio strongly decreases. The absence of free nickel characterisation (Ni ~ or NiO) shows its high dispersion. On the nickel rich systems, the LaFeO3 perovskite is the only one to be conserved with an increasing formation of La203. Whereas nickel is on a NiO form for x = 0.5 and 0.6, nickel is on a Ni ~ form for 0.7 _< x _ 1. For x = 0.9 and 1, we can observe carbon deposition. The regular increase of specific area with x from 3.2 m2.g -1 to 37.6 m2.g -1 (from 4.2 to 5.5 m2.g -~ for the catalysts before reactivity) is mainly due to carbon formation. This specific area increase becomes important from x _> 0.6 (9.4 m2/g for 0.5; 15.9 for x = 0.6). An elementary analysis of carbon on all the catalysts after reaction gives a carbon percentage below 0.3% for x _< 0.5 and 13.4% for x = 0.9.

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~

100 . .. -A ....

80

e~

I

._o 60

/ ~- , -

I,..

>

40

cO

0

0 9C H 4

ta

_ _,_

~.--~

t

d

t

2On

/

1

\

0 "-0

X a H20/CH4

" - ~--

.ItI

- - e - - CO2 selectivity

i

. "~--.----o--0.2 0.3 0.4

" Cselectivity CO selectivity

~i

~

~-:--_...-~.,,----*--~ _ 0.5 0.6 0.7 0.8 0.9 1

Fig. 2 : Selectivity into CO, CO2 and carbon at 800~

ratio = l.

'~!

i\"

" 0.1

800oc

01

k "\

700oc

~ ' ~ ,

/~"\

'\

_

-

',,

\,,/

40 / 9

-i\

650oc

e--

---A--- 750oc

conversion at various temperature versus x with

60 i_ ~

--,,.--*..

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

801~ \,

.~

_. ~_. 600~ -.

"'-.,I ~ '.. ;

./.--7 .." . ~...... . ~ ,........ . ~ ., . ~ . ,. ~ . , ,~: ;'._~.-.~,

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.~,

-A . . . .

/."':"" ,,.- ., - . _ , ~ - , ~ - , .". ',, /3./,\ ~:'/

~ 20

~

."

A...

X

versus X (H20/CH4 = 1).

100 80 60

---n-LaNio.3Feo.703

"~,

40

~LaNio.7Feo.303

O O

20

"o (1}

0 0

50

100

150

200

T i m e (h)

Fig. 3 : CO yield versus time for LaNio.3Feo.703 and LaNio.7Feo.303.

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Table 1 Crystalline phases detected by XRD after test : Crystalline phases detected by XRD after test

X 0 0.1 0.2 0.3 0.4 0.5 0.6

Nired/Nifree

50 65 73 100 100 100

15 19 28 43 47 57

100 100

64 67

100

74

100

85

(%)

LaFeO3

LaFeO3 LaFeO3 LaFeO3 0.7 LaFeO3 0.8 LaFeO3

LaNio.05Feo.9503 LaNio.ovFeo.9303 LaNio.08Feo.9203

La203 La203 La203 La203 La203

NiO NiO NiO NiO*

Ni* Ni

La203

Ni

La203 1.0 La203 * Corresponds to phases present as traces.

Ni Ni

0.9

% of free Ni

C C

On a micrograph after reactivity and EDS analyses of LaNi0.3Fe0.703, we confirm the presence of LaNiFeO3 perovskite (figure 4a). Some NiO particles (several nm) have also been evidenced (figure 4b). For the rich nickel systems (x = 0.7), some 10 to 20 nm particles of NiO can be seen on the edge of perovskite structure. LaFeO3, La203 and turbostatic carbon around big nickel particles can also be observed. The study of the elementary distribution (EDS) of these three elements for x = 0.3 shows that La/Fe ratio is constant (perovskite structure is preserved), and presence of nickel enriched zones (free nickel). For x _> 0.7, catalysts are much more heterogeneous with a variable La/Fe ratio and some very rich nickel zones. These observations can explain the catalysts ageing (figure 3). As CH4/H20 is more oxidant than CH4/CO2 it must be noticed that unlike TPR and dry reforming [4] results, in the reaction conditions no Ni-Fe alloy formation can be found and evoked for restricted the carbon formation. For the nickel poor systems (0.2 _< x ___0.4), the presence of nickel in the bulk and on the surface of the perovskite structure leads to a strong interaction between the small metal particles and the crystal lattice. This strong interaction results in a slow progressive formation of active species decreasing the sintering and hence the carbon deposition. The carbon deposit is activated in presence of the LaFeO3-La203 oxidizing system in vicinity of the metal particles and its accumulation will be limited. Further, the oxidizing character of LaFeO3 near the metal particles lowers the sticking coefficient of CH4. The strong interaction of Ni with the crystal lattice permits the regeneration of the system by a mere calcination. For the nickel rich systems, such an interaction is not observed, and the systems are not sufficiently reducible to form a Ni-Fe alloy as with CH4/CO2. The higher the nickel content, the more nickel particles without strong interaction will be formed and the more carbon will be deposited with time on stream leading to deactivation. These latter catalysts are not regenerated by reconstruction of the initial perovskite.

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[~

Fig. 4 : HRTEM micrographs of LaNio.3Feo.703 after vaporeforming (a) : mixed La, Ni, Fe perovskite (b) : NiO particles on La,Ni,Fe perovskite 4. CONCLUSION Systems active for vaporeforming of CH4 with low H20/CH4 ratio have been obtained starting from a trimetallic LaNixFel_xO3 perovskite and then characterised before and after reactivity. The system are particularly efficient for x=0.3 which the trimetallic structure is preserved together with LaFeO3 perovskite and small Ni particles. By this way, carbon deposition is limited. ACKNOWLEDGEMENTS

The authors thank the European Community for financial support of the JOULE contract JOR3-CT95-0037. REFERENCES

1. J.R. Rostrup-Nielsen, , Catalytic Steam Reforming >>in J.R. Anderson and M. Boudart (Editors), Catalysis, Science and Technology Vol. 5, Springer Verlag, Berlin (1983). 2. Alstrup, B.S. Clausen, C. Olsen, R.H.H. Smits and J.R. Rostrup-Nielsen, Stud. Surf. Sci. Catal., 119, (1998) 5. 3. Z. Zhang and X.E. Verykios, Catal. Lett. 38 (1996) 175. 4. H. Provendier, C. Petit, C. Estourn6s and A. Kiennemann, Stud. Surf. Sci. Catal., 119 (1998), 141. 5. H. Provendier, C. Petit, A.C. Roger and A. Kiennemann, Stud. Surf. Sci. Catal., 118 (1998) 285. 6. J. Garcia, J. Blasco, M.G. Proietti and M. Benfatto, Phys. Rev. B. Condensed Mat., 52 (1995) 15823. 7. M . Houalla, F. Delannay, I. Matsuurai and B. Delmon, J.C.S. Faraday 176 (1980) 2128. 8. H. Provendier, C. Petit, C. Estourn~s, S. Libs and A. Kiennemann, Appl. Catal. A, 180 (1999) 163. 9. D.L. Trimm, Catal. Today, 37 (1997) 233.

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化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Carbon deposition and reaction steps in CO2/CH 4 reforming over NiLa203/SA catalyst J.Z. Luo", L.Z. Gao "'b, Z.L. Yu a'b, and C.T. Au a'* "Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Kowloon, Hong Kong, China. Emaii: [email protected] bChengdu Institute of Organic Chemistry, Chinese Academe of Sciences, Chengdu, Sichuan, China The cause of carbon deposition and the reaction pathways for COJCH4 reforming over NiLa203/5A have been investigated. XRD results revealed that due to the formation of perovskite-like La2NiO4 in Ni-La203/5A, the small-size (ca 9 nm) Ni ~ crystallites formed in H2 reduction remained unsintered during 48 h of on stream reaction. The accumulation of carbon on the active sites is the main reason for catalyst deactivation. The detection of ~3CO2 and CO2 in 02 pulsing onto a sample pretreated with 13CH4/CO2 confirmed that the deposited carbon was from both CH4 and CO2. In CO and COJCH4 atmospheres, we observed similar TGA patterns and identical TEM images (carbon nanotubes) of deposited carbon; we propose that carbon deposition is mainly via CO disproportionation. The observation of CD3COOH and CD3CHO in CD3I chemical trapping experiments and the detection of formate/formyl bands in DRIFT suggested that HCOO and HCO were intermediates. The amount of CO2 converted was roughly proportional to the amount of H present on the catalyst. These results indicated that CO2 activation could be H-assisted. Pulsing CH4 onto a H2-reduced sample and a similar sample pre-treated with CO2, we found that CH4 conversion was higher in the latter case. Hence, the idea of oxygen-assisted CH4 dissociation is plausible. As for the rate of methane conversion, a kH/kD ratio of 1.2 and 1.1 was observed at 600 and 700~ respectively, implying that C-H cleavages are slow kinetic steps. Based on these experimental results, we have derived reaction pathways for CO2/CH 4 reforming. In the proposed mechanistic model, CHxO (x-1 or 2) decomposition is considered to be a rate-determining step. 1. INTRODUCTION CO2 reforming of methane for the production of syngas with HJCO ratios suitable for F-T synthesis has aroused renew interest in recent years. Many transition metal-based catalysts have been investigated [1]. It has been reported that catalyst deactivation due to carbon deposition is a serious problem [2,3]. Noble metals such as Rh and Ru can reduce carbon deposition [3]. However, considering the high cost and limited availability of these precious metals, it would be more attractive to develop a nickel-based catalyst. In general, carbon deposition via CH4 decomposition or CO disproportionation could be reduced if fine nickel particles are supported on a metal oxide of strong basicity [4-6]. In the present work, COJCH4

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reforming over Ni-La203/5A was studied by in situ TGA, pulse experiments, chemical trapping, TEM, and in situ DRIFT.

2. EXPERIMENTAL The Ni-La203/5A catalyst was prepared by adopting the citric acid complexing method. The loading of Ni was 8 wt%. The catalytic activity was measured in a fixed-bed quartz-reactor under reaction conditions of temperature = 800 ~ GHSV = 48,000 mL h ! g~, and CO2/CH 4 molar ratio = 1 / 1. The phase structures were determined by X-Ray Diffractrometry (XRD, Rigaku D-MAX). Ni particle size was estimated according to the half height width of the Ni (111) peak obtained in XRD investigation. The deposition and reactivity of carbon on the catalyst were examined using a Thermal Gravimetric Analyzer (TGA, Shimadzu DT-40). TEM images of deposited carbon were taken by means of a JEM-1000CX (JEOL) equipment operated at 100 kV. Pulse and CD3I chemical trapping experiments were performed in a micro-reactor equipped with a mass spectrometer (HP G1800A). In order to identify the products correctly, we have taken into account the interference due to isotopes and fragments. High purity He (99.99%) was used as carried gas. 13CH4 (>99.9%) and CD4(>99%) were employed for the investigation of isotope effect. For the pulsing experiments, the volume of each pulse was 67.5 pL. As for the in situ DRIFT experiments, a Nicolet Magna 550 FT-IR Spectrometer was used. 3. RESULTS AND DISCUSSION

3.1. Phase structure and catalytic performance According to the XRD patterns, strong signals of La2NiO4 and 5A phases as well as weak signals of A1203 and SiO2 phases were observed. After reduction in H 2 at 500~ nickel existed mainly as Ni ~ particles, with diameter estimated to be ca 9 nm. The formation of La2NiO4 in the fresh catalyst indicated that with the aid of complexing ability of citric acid, Ni 2§ and La 3§ ions could be evenly distributed. The size (ca 9 nm) of Ni ~ particles generated in H2-reduction is much smaller than that (> 100 nm) observed over Ni/La203 [5]. It indicates that La2NiO4 is a good means for producing fine Ni ~ particles. Supports which are basic have been reported to be capable of activating CO2 and are favourable for the elimination of deposited carbons [ 1,6]. 5A molecular sieve can adsorb and activate CO2 due to its great affinity towards the gas. At 800~ the Ni-La2OB/5A catalyst showed 90.0% CO2 conversion and 91.7% CH 4 conversion. With the advance of time from 10 rain to 48 h, CO2 and CH 4 conversions decreased from 90.0% and 91.7% to 82.2% and 82.0%, respectively. These catalytic performances are comparable to those observed over the 1 wt% Ir/A1203 catalyst [3]. Generally speaking, there are two reasons for the deactivation of nickel-based catalysts in CO2/CH4 reforming: (i) the blocking of active sites by carbonaceous deposits; (ii) the sintering of nickel particles. Since the size of nickel particles remained unchanged in 48 h of on-stream reaction, we suggest that the main cause of catalyst degradation is carbon deposition. 3.2. Carbon deposition Figure 1 shows the TGA profiles of carbon deposition as related to reaction time at various temperatures in a c a 4 , CO or CO2/Ca 4 (molar ratio - 1:1) atmosphere. In ca4, the amount of

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carbon deposited increased when the temperature was changed from 700 to 800~ However, in a CO/N 2 or CO2/CH4 atmosphere, the extent of carbon deposition decreased with the same change in temperature. The morphology of carbon deposited on the catalysts at 800~ in different atmospheres was investigated by TEM. The carbon coming from CH4 was mainly the encapsulated type and EDX demonstrated that it was amorphous. The deposited carbon formed in CO or CO2/CH 4 existed mainly as carbon nanotubes. These carbon nanotubes were more or less twisted with an outer diameter of 10-~20 nm and a length of up to 10 ~tm; EDX revealed that they were mainly graphitic carbon. We purpose that CO disproportionation is the main cause for carbon deposition in CO2/CH4 reforming. 150 100 50

~o 150

E 100 ~

~

50

.,~

15o 100

a)CH~

.,,__,

// / m _ _ m - ~

m--'m

/A~x/X-~--'x~X--X

~m~

14 =.12

10

(b) CO

_•AX.I.I--i, ~ x

.

'1--1

x- - x

.&--&

(c) COdCH4

~

8

0 ~

6

y-

(a) / -

(b)

o o

<E

50

0

20 40 60 80 100 On-stream time (min)

Fig. 1. TGA profiles of Ni-La203/5A versus time on stream at 600~ (=), 700~ (• and 800~ (A) in various atmospheres.

(c)

2 A~A~A_---------&

~

&

Ordinal number of 13CH4/CO2 pulse Fig. 2. The amount of 13CO2 generated versus 13CH4/CO2 pulse number at (a) 600~ (b) 700~ and (c) 800~ over Ni-La203/5A.

In order to investigate the origin of deposited carbon and the reaction mechanism of CO2/CH 4 reforming, we pulsed 13CH4/CO2 (molar ratio = 1:1) at a desired temperature onto a Ni-La203/5A sample which had been Hz-reduced at 500~ for 1 h. After 5 pulses of 13CH4/CO2, the deposited carbon was treated with 4 pulses of 02 at the same temperature. Throughout the experiment, the 13COJCO2 ratio and the amount of '3CO2 generated were monitored. With the increase of 13CH4/CO2 pulse number, the amount of 13CO2 generation increased; such an increase was the most obvious at 600~ (Fig. 2). The generation of 13CO2 affirms the accumulation and oxidation of surface 13C species. When 02 was then pulsed onto the sample, both ~3COz and CO2 were detected. We conclude that both CH4 and CO2 contribute to carbon deposition. In the 4 pulses of 02, the total amount of 13CO2 produced and the '3COJCO2 molar ratio decreased with the increase in temperature. It is clear that carbon deposition above 700~ is mainly due to CO2 dissociation and CO disproportionation.

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3.3. Reaction mechanism for syngas formation

3.3.1. CO2 and C H 4 activation The amount of C H 4 converted in 5 pulses of C H 4 at 600~ over a H2-reduced Ni-La203/5A sample was 1.3 pL (Table 1) and there was no generation of C2H6, C2H4, or C O x. It indicated that the carbon generated in CH4 dissociation remained completely on the catalyst. When CH4 was pulsed onto a H2-reduced sample pretreated with 5 pulses of CO2 at 600~ (Table 2), CO was detected, indicating that there was interaction between C H 4 and the oxygen released in CO2 dissociation. Taking into consideration that the amount of converted C H 4 (8.8 pL) over the CO2-treated catalyst was much more than that (1.3 ~tL) over the reduced catalyst, we propose that surface oxygen species such as O and OH promote the decomposition of CH4. Similar trends were observed at 700 and 800~ Hence, we advocate the idea of oxygenassisted CH4 dissociation.

Table 1 The amounts of converted CH 4 and CO2 and that of CO generated in first 5 pulses of CH4 followed by 5 pulses of CO2 over a H2-reduced Ni-La203/5A sample. Temp. (~

CH 4 pulse C H 4 (~L)

600 700 800

1.3 67.5 113.4

CO (~tL) 0.0 0.0 4.2

CO 2 pulse C02 (~tL) 13.0 101.2 303.8

CO (~tL) 14.0 114.8 336.8

Table 2 The amounts of converted CO2 and CH 4 and that of CO generated in first 5 pulses of CO2 followed by 5 pulses of CH4 over a H2-reduced Ni-La203/5A sample. Temp. (~ 600 700 800 * produced from

CO2 pulse C H 4 pulse CO2 (~tL) CO (~tL) CH 4 (ktL) CO (~L) 8.1 6.0 8.8 1.4 58.7 55.3 114.3 36.5 293.0 283.5 308.5 219.4 5 pulses of C O 2 / C H 4 (molar ratio = 1/1) at 600~

C H 4 / C O 2 pulse*

CO (~tL) 140 240 307

When CO was pulsed over a H2-reduced Ni-La203/5A sample at or above 600~ CO2 and detected in the effluent. The generation of CH4 was due to CO methanation, a result of CO interaction with the hydrogen adsorbed during the reduction of the catalyst. We envisioned that the amount of C H 4 generated should be proportional to the amount of H species present. In order to vary the concentration of H adspecies, we purged the H2-reduced sample with He for 10 min at 600, 700, and 800~ respectively. We then cooled the sample to 600~ in He and pulsed CO onto the catalyst until there was no observable change in CO peak intensity. Table 3 shows the amount of CH4 produced during CO pulsing. One can observe that with the rise in purging temperature, the amount of CH4 generated decreased. Similarly, we observed that the amount of CO2 consumption during CO2 pulsing at 600~ over the HeCH 4 were

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purged samples decreased with the rise in purging temperature. It was a clear indication that surface hydrogen promoted the dissociation of CO2. Following the introduction of 5 pL of CD3I onto a sample pretreated with CO2/CH4, w e detected CD3COOH. Formate signals at 2915 and 2885 cm -~ were observed in DRIFT studies. These results suggested that HCOO was an intermediate in C O 2 / C H 4 reforming. The existence of HCOO on the sample indicated that CO~ reacted with the adsorbed H on nickel. From Table 2, it can be seen that the total amount of CO (7.4 ktL) formed in the first 5 pulses of CO2 and in the following 5 pulses of C H 4 w a s much smaller than that (140 pL) generated in 5 pulses of C O 2 / C H 4 at 600~ Similar trends were obtained at 700 and 800~ Therefore, we suggest that C H 4 and CO2 can mutually activate each other. _

Table 3 The amounts of CH 4 formed in CO-pulsing and that of CO2 consumed in CO2-pulsing over a H2-reduced Ni-La203/5A sample. He purging Temperature (~ Amount of C H 4 formed (~tL)* Amount of CO2 consumed (pL)* * detected at 600~

600 4.7 9.0

700 2.7 5.2

800 0.5 2.0

3.3.2. Reaction intermediates Deuterium isotope effect on methane conversion was observed at 600 and 700~ but not at 800~ There was no obvious isotope effect on the conversion of CO2. The ratios of C H 4 and CD4 conversions were 1.2 and 1.1 at 600 and 700~ respectively. The results are in accord with the expected CH4/CD 4 isotope effect [7]. At 800~ C H 4 and CO2 conversions over NiLa2OJ5A were, respectively, 91.7% and 90.0%, suggesting that the reaction was close to thermodynamic equilibrium. In other words, the reaction was controlled by thermodynamics rather than by kinetics and n o C H 4 / C D 4 isotope effect was observed. The isotope effect observed at 600 and 700~ indicated that C-H cleavages are slow steps. There are three possible ways for C-H cleavage in C O 2 / C H 4 reforming: C H 4 ---) C x +

4H

(i)

CHx + O --) CO + xH (ii)

CH•

--) CO + xH (iii)

Among them, step (i) was believed to be reversible [1,8]. As described above, direct C H 4 dissociation could take place at 600~ over a reduced Ni-La203/5A sample. After CO2/CH4 exposures, no IR bands at around 3000 cm ~ were observed, indicating the absence of CHx adspecies; this could be a result of fast CHx interaction with surface oxygen species. The 13CH4/C02and C H 4 - C O 2 pulsing experiments also demonstrated that the deposited carbon deriving from C H 4 reacts with CO2 quite readily. Thus we deduced that step (ii) was a combination of two elemental steps: CHx + O --) CHxO and CHxO --) CO + xH. Since the former elemental step is rather facile, we propose the latter to be rate-determining. The observation of CD3CHO in CD3I chemical trapping indicated that there was HCO on the sample. The broad and weak IR bands in the range of 2700-3000 cm ~ could be due to CHxO (x = 1-3) species [7,9]. Since no CD3OCH3 was detected in CD3I trapping and there was no ethane detected in C O 2 / C H 4 reforming, we speculated that there was no methoxy or CH3 on the surface during the reforming reaction. It was reported that formate dissociated quite

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readily to formyl and formaldehyde in H 2 at high temperatures [9]. Also, IR studies demonstrated that H2CO adsorption would give rise to formyl [9]. Based on these understandings, we suggest that there were HxCO (x = 1 or 2) species on the sample and the decomposition of HxCO is rate-determining.

3.3.3 Mechanistic model Based on the above results and discussion, we propose the following mechanistic model for C O J C H 4 reforming (s, surface; g, gas phase):

(1) (2) (3) (4) (5) (6)

CH4, g + CH4, s CO2, g "-) CO2, s CH4,., "-) CH,,, s + (4-x)H, s c o . , , , --> CO, s + O,s CO2, s + H,~ + HCOO, s HCOO, s ---) HCO, s + O,s

(7) (8) (9) (10) (11) (12)

HCOO, s --> CO, s + OH, s CHx, s + O,~ --) CH,,O, ~ CH4, s + O, s '--) CHxO + (4-x)H, s CHx, s + OH, s "--) CHxO, s + H,s CHxO, s "-> CO, s + xH, s H,s + OH, s "-> H20, s

In this model, CH4 decomposition on Ni ~ is assisted by the oxygen generated in CO2 dissociation via CHxO (x = 1 or 2) formation. The CO2 adsorbed on basic sites dissociates with or without the aid of H species originated in CH4 decomposition to give CO and O or CO and OH. The rate-determining step is the decomposition of CHxO to CO and H. ACKNOWLEDGMENT

The work described above was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administration Region, China ( Project No. HKBU 2053/98 P). REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9.

M.C.J. Bradford and M.A. Vannice, Catal. Rev. -Sci. Eng., 41 (1) (1999) 1. T. Nishiyama and K.I. Aika, J. Catal., 122 (1990) 346. A.T. Ashcroft, A.K. Cheetham, M.L.H. Green, and P.D.F. Vernon, Nature (london), 35 (1991)225. O. Yamazaki, Y. Nozaki, K. Omata, and K. Fujimoto, Chem. Lett., (1992) 1953. Z.L. Zhang, X.E. Verykios, S.M. Macdonald, and S. Affrossman, J. Phys. Chem., 1O0 (1996) 744. G.J. Kim, D.S. Cho, H.H. Kim, and H.J. Kim, Catal. Lett., 28 (1994) 41. H. Burghgraef, A.P.J. Jansen, and. R.A. van Santen, J. Chem. Phys., 101 (1994) 11012. C.T. Au, M.S. Liao, and C.F. Ng, J. Phys. Chem., A 102 (1998) 3959. J.F. Edwards and G.L. Schrader, J. Phys. Chem., 89 (1985) 782.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

The Autothermal Partial Oxidation Kinetics of Methanol to Produce Hydrogen E. Newson, P. Mizsey, T. Truong and P. Hottinger General Energy Deptartment, Paul Scherrer Institute, 5232 Villigen-PSI, Switzerland, Fax: +41 56 310 2199, Email: [email protected] The kinetics of autothermal methanol partial oxidation are investigated to produce hydrogen for fuel cell systems. Two reactor systems are used to determine the kinetic parameters under isothermal conditions. The originally supposed six-reaction system (dimethyl ether formation, methanol decomposition, water gas shift, steam reforming, methanol partial oxidation (POX), hydrogen total oxidation) could be simplified, because the water gas shift reaction is slow in comparison to the others and the total oxidation of hydrogen is mass transfer limited with the commercial copper/alumina catalyst used. Previously determined kinetic data for methanol decomposition [7] were also used to facilitate the evaluation of the kinetic data. Respective activation energies in kJ/mol are 117, 76, -, 81 and 65 (POX), with the standard deviations of 6-24%. Turnover frequencies at 250~ for the POX reaction were calculated from copper surface area measurements. They were the same order of magnitude (460 min -1) as literature values [ 1,11 ]. Under non-isothermal "hotspot" operation, hydrogen production rates were 10000-13000 litresH2/hour/ litre reactor volume(lrv), which is equivalent to 30-39 kWth/lrv, providing significant power densities from the fuel processor. Hydrogen yields of 72% or 2.2 moles of hydrogen per mole methanol feed, with 1-2% CO in the exit gas, were measured. 1.

INTRODUCTION

The autothermal partial oxidation of methanol or hydrocarbons is being intensively studied on the catalytic [ 1,2] and reaction engineering scale [3,4] to produce hydrogen for stationary and mobile fuel cell systems. The mobility aspect requires reaction engineering of the exothermic partial oxidation for fast startup and endothermic steam reforming for higher hydrogen concentrations and system efficiencies [3]. Kinetics for the latter reaction have been published [5] and apparent activation energies for the POX reaction (482-71 kJ/mol) were dependent on Cu-Zn catalyst composition [ 1]. The valence state of Cu-Zn catalysts is critical to maximise hydrogen yields [1] whilst high methanol conversions and hydrogen selectivities were obtained with zinc oxide supported palladium catalysts using sub-stoichiometric oxygen/methanol feed ratios [2]. Targets for the catalyst performance are suggested by analysis of full fuel cycle efficiencies (ffc) from well to wheel [6]. The fuel processor-fuel cell subsystem must reach an energy efficiency of 40% so that the ffc is 23%. This exceeds comparable values of 18% for mobile systems using internal combustion engines. With a 50% efficiency for the polymer electrolyte fuel cell, an 80% efficiency of the fuel

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processor is required which is determined primarily by catalyst kinetics and subsequent reaction engineering. This paper develops an applied kinetic model derived initially on the basis of six kinetic constants based on isothermal measurements in two laboratory reactor systems. Non-isothermal measurements were also made to illustrate the power densities of the commercial catalyst. 2.

REACTION STOICHIOMETRY

The reaction system based on product analyses from preliminary work, is limited initially to six simultaneous reactions with equilibrium limitations as shown, together with the corresponding heats of reaction (AHR) at standard conditions. Table 1 Reactions considered for autothermal methanol partial oxidation Nr.

3.

Name

Reaction

1.

DME formation

2.

Me decomposition

2 CH3OH r

3.

Water gas shift (WGS)

4. 5.

Steam reforming (SR) Partial oxidation(POX)

6.

Hydrogen burning

AHR (kJ/mol)

CH3OCH3 + H20

CH3OH r

CO + 2H2

CO + H20 r

-21 99

CO2 + H2

-39

CH3OH + H20 r CO2 + 3 H2 CH3OH + 0.5 02 ~ CO2 + 2H2

60 -184

H2 + 0.5 02 ~ H20

-245

EXPERIMENTAL

The laboratory microreactor system is shown in Figure 1 as three major parts: feed preparation, reactor, analytics and PC control. The system is computer controlled for unattended, continuous operation with safety features for temperature and pressure. The control system monitors and saves the data on process parameters every minute for

gt7

.... ~

~ :

Balance

n

.!.

o

n

r

o

cco~

Fig. 1. Laboratory microreactor system for autothermal methanol partial oxidation.

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subsequent analysis. Methanol feed quality was analysis grade (99.8%) and gas purities were 99.995%. The feed mixture is vaporised and contacted in a static mixer with nitrogen, hydrogen or oxygen as required. The reactor has an internal diameter of 4 mm with a catalyst bed length of 14-120 mm depending on catalyst dilution and an internal thermowell for temperature measurements. 100 mg of commercial copper-alumina catalyst was used in the 250-500~tm particle size range. Catalyst activation began with outgasing in nitrogen at 300~ followed by air oxidation at 450~ a nitrogen flush, and reduction in hydrogen for one hour at 450~ GC analysis utilised three different columns and two detectors (FID, TCD). Condenser water content was analysed by a 737KF Coulometer. Reactor temperatures were between 220300~ weight hourly space velocities (WHSV) based on methanol were between 5-50 h -l, pressures between 102-103 kPa and data were taken continuously for up to 300 hours. Only data with elemental mass balances for C,H,O _+10% were considered. 4.

RESULTS AND DISCUSSION

For the kinetic model, each reaction is first studied to determine if equilibrium limitation is significant. Based on Gibbs free energies the equilibrium constants are determined. Reaction 1" Dimethyl ether formation (DME) Keq,DME =0.106243 exp(21858.37/R*T)

(1)

Reaction 2: Methanol decomposition (Me) Keq,Me =1.71791e14 exp(-95417.89/R*T)

(2)

Reaction 3" Water gas shift (WGS) Keq,WGS =9.54335e-3 exp(39876.31/R* T)

(3)

Reaction 4: Steam reforming (SR) Keq,SR =1.8493e10 exp(-56087.19/R* T)

(4)

The kinetics are significantly influenced by the WGS equilibrium, less so for DME and Me, and insignificant for SR. Reactions 5 and 6, partial oxidation of methanol and hydrogen burning, are not equilibrium limited. The determination of the kinetics of the 6-reaction-system were facilitated by investigating the reactions individually. Considering the complexity of the system only reactions 4 and 6 can be studied alone. To simplify the simultaneous solution of kinetic parameters for the six parallel reactions, auxiliary isothermal experiments were made for these reactions (WGS and hydrogen burning) in a microreactor system. 4.1 Kinetics of WGS A reformate gas mixture (2% CO, 10% CO2, 20% H2, 68% N2) is used to determine the kinetics of the water gas shift (WGS) reaction. The measurements at different temperatures, water compositions (water contents: 4.2, 20, and 30% of the total mixture), and conversion rates show that the rate of reaction of WGS depends both on the water ( PH20 ) and the carbon

monoxide composition ( "~ ) and can be described by the following equation:

rWGS=2"25682e-3exp ( 50,000 / R * T PCO * PH20 * Eqco

[

mol ] gcat sec

(5)

where the partial pressures are in kPa, and Eqco is:

Eqc 0 = 1- PCO~ * PH2/Keq,WGS * PCO * PH20

(6)

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During the evaluation of the experimental data for the whole 6-reaction-system, it was found that the WGS reaction rate is 1-3 orders of magnitude less than those of the other reactions and therefore this reaction can be neglected at the conditions studied.

4.2 Kinetics of hydrogen burning The hydrogen burning reaction was also individually studied. Isothermal kinetic measurements were made in the microreactor system, the results being evaluated in the temperature range 25-65~ At higher temperatures (200-300~ required for autothermal pox, this reaction is so fast that mass transfer is the rate limiting step. The rate on the pellet surface can be written: rH2 : k m a m ( C b - C s )

(7)

where km is the mass transfer coefficient, am is the external unit surface area per unit mass of pellet, Cb and cs are the concentrations in the bulk gas and at the pellet surface, respectively. Because the reaction rate at 200-300~ derived from the kinetics is much greater than the mass transfer rate, the concentration at the surface (cs) approaches zero. Ruthven [8] recommends a method to estimate the mass transfer coefficient which is dependent on the flows and the external surface area per unit mass of pellet, but the temperature dependence (< 5%) is neglected in the temperature range investigated.

4.3 Utilisation of the results of methanol decomposition kinetics Previous investigations of the kinetics of methanol decomposition [7] showed that the rate equation for dimethyl ether formation could be expressed as second order with equilibrium limitations, while the remaining methanol decomposition and steam reforming could be assigned their stoichiometric order with equilibrium terms being necessary for reactions 2 and 4 [5,6]. The relative rates of the four reactions are rDME > rSR > rMe >> rWGS . The rate of the WGS reaction is significantly lower than those of the others and can be neglected. 4.4 Kinetics of autothermal methanol partial oxidation (WPOX) The measured isothermal data points were evaluated with the SIMUSOLV software package. Arrhenius temperature dependence is assumed and the activation energies and preexponential factors are determined by the multidimensional optimisation method of the gradient type. As starting values, the parameters obtained for the kinetics of methanol decomposition are used. The multidimensional optimisation showed, that the activation energies of the previously studied DME formation, methanol decomposition, and steam reforming reactions are practically the same as those for the autothermal methanol partial oxidation (WPOX), Table 2. However, the pre-exponential factors for WPOX are different from those obtained for methanol decomposition kinetics. In the second step of the optimisation, the activation energies obtained earlier are accepted and only the pre-exponential factors and the activation energy of the methanol partial oxidation are determined. The results with standard deviations are shown in Table 3. Figure 2 shows the Arrhenius plot of the reactions. Figure 3 shows a typical rate of reaction profile at 250~ in the isothermal microreactor, WGS is neglected.

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Table 2 Comparison of activation energies Reaction Activation energies [J/mol] MeOH decomposition system WPOX data DME formation 117,000 116,157 MeOH decomposition 76,000 75,800 Steam reformin~ 81,000 81,100 Table 3 Pre-exponential factors and activation energies for WPOX Reaction Activation energy [kJ/mol] Pre-exp. factor [mol/gcat sec kPa x] DME 117 130.0 (17%) MeOH decomp. 76 1.138 (24%) SR 81 34.5 (12%) POX 65 (6%) 0.466 (11%)

The commercial catalyst used did not contain zinc oxide, which could explain the DME found in the reaction products. The estimated activation energy of 117 kJ/mol is similar to values obtained (95kJ/mol) with gamma alumina [10]. The values estimated for methanol decomposition (76kJ/mol) and steam reforming (81kJ/mol) are close to values found for copper-zinc catalysts, 77 and 78 kJ/mol, respectively [ 11 ]. Turnover frequencies at 250~ for the POX reaction were calculated from surface copper area measurements by the N20 pulse technique and rates of reaction, Figure 2. Values of 460 min -~ were estimated compared to 250 min -~ for experimental Cu-Zn catalysts [ 1,11 ].

....

DME

MeOH . . . . . . WGS . . . . SR POX H2 burning

-10

-15

CI-I3OH <=> DME

In(k)

+ 20

CI-I3OH <=> CO + 2 Id CO + I-IO <=> CO + I-Iz CI-I3OH + H20 <=> CO2+ 32

---..~ ~....: : . . . . . . . . . . . .

-20

CI-I3OH + 0.5 Q => CO2 + 2 Ial -25 0.0017

0.0018

300~

0.0019

0.002

1 IT

0.0021

0.0022

200~

Fig. 2. Arrhenius plot of the 6-reaction-system, k [mol/gcat sec kPa x]

H2 + 0.5 Q => I~O at G=0.08 kg/m 2 sec

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8.00E-04

5.00E-05 r~

l.d

=1 6.00E-04 ,.=

~

@

4.00E-04

2.00E-04

3.75E-05

1"'"'""

~

~ 0

2.50E-05

~

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......

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7"=''''=" 50

75

burning

DME

....

25

"

H2

t/

0.00E+00

l= r

MeOH SR

@

O.OOE+O0 1O0

Length of reactor [ %] Fig. 3. Rates of reactions of the 6-reaction-system [mol/g sec] at 250 ~ 5. NON-ISOTHERMAL OPERATION TO PRODUCE HYDROGEN Under nonisothermal, "hot spot" operation, hydrogen production rates were 10000 - 13000 litres H2/hour/litre reactor volume (lrv), which is equivalent to 30-39 kWth/lrv, providing significant power densities. In a single reactor tube, methanol conversions were about 90%, yields to hydrogen 72% or 2.2 moles hydrogen per mole methanol feed. Scale-up on a catalyst weight basis of a factor of 25 has been achieved with a small loss in hydrogen yields due to the presence of both a hotspot and a coldspot in the same reactor. ACKNOWLEDGEMENTS

The project was financially supported by the Swiss Federal Office of Energy (BFE), commercial catalysts were supplied by Johnson-Matthey plc (UK) under a confidentiality agreement, P. Binkert was responsible for constructional work. REFERENCES

1. L. Alejo et al., 3 rd World Congress on Oxidation Catalysis, Ed. R.K.Grasselli et al., Elsevier (1997) 623. 2. M.L. Cubeiro and J.L.G. Fierro, J.Catal. 179 (1998) 150. 3. E. Edwards et al., J. of Power Sources, 71 (1998) 123. 4. W.L. Mitchell et al., SAE 1999-01-0535. 5. B. A. Peppley, Ph.D. Thesis, RMC Kingston, Ontario, Canada, May (1997). 6. B.L. H6hlein, lEA Adv. Fuel Cell Workshop, Wislikofen, Switzerland (1997) 43. 7. E. Newson, P. Mizsey, T. Truong, P. Hottinger, EUROPACAT-IV, Rimini (1999),P/II/114. 8. D. M. Ruthven, Chem. Engng Sci., 23 (1968) 759. 9. SIMUSOLV, Version 3.0-150, Dow Chemical Company (1993). 10. M. Wittman, Diss. Techn. Univ. Mtinchen, Germany (1991). 11. J. L. G. Fierro, Pers. Commun (1999)

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Studies in Surface Science and Catalysis 130 A. Corrna, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

New Reaction Mechanism for Methane Formation in CO Hydrogenation over Pd/CeO 2 Shuichi Naito*, Shigeru Aida and Toshihiro Miyao Department of Applied Chemistry, Faculty of Engineering, Kanagawa University, 3-27-1, Rokkakubashi, Kanagawa-ku, Yokohama, 221-8686, Japan. [email protected] Drastic changes in the activity and selectivity of methane and methanol formation were observed in CO-H2 reaction over Pd/CeO2 catalysts by high temperature reduction (SMSI effect). A new reaction mechanism for methane formation is proposed, which involves (CO)2(a) and /or (HCO)2(a) species as intermediates for isotopic exchange and methane formation processes, respectively. I. INTRODUCTION It has been well known that the selectivity for the formation of methane or methanol in CO hydrogenation over palladium catalysts is highly sensitive to the composition of supports[I,2], as well as the particle sizes of Pd[3,4] and the presence of promoters[5,6]. The structural difference of active sites for both products is debatable, and the mechanism of this reaction is still controversial, especially concerning the question of whether non-dissociative CO(a) is associated with the formation of methane or not. Cerium oxide is an important support for group 8-10 metal catalysts, because of their application in automotive pollution control and in syngas conversion. The promotive SMSI effect has been reported for hydrogenation of CO2 to methanol over Pd/CeO2 under high pressures[7]. We have also reported that the selectivity of CO-H 2 reaction over Pd/CeO2 changes drastically by higher temperature reduction, which is restored to its original state by lower temperature reduction atter oxidation (SMSI phenomena) [8]. In this paper we have studied the influence of SMSI effect to the sequential elemental steps involving CO-H2 reaction over Pd/CeO2 by isotopic exchange reactions as well as infrared spectroscopic investigation during the reaction. We propose a new reaction mechanism for methane formation, which may involve (CO)2(a) and/or (HCO)2(a) type intermediates at the interface of Pd and reduced ceria. We also propose the transformation of active sites during CO-H2 reaction is the key step for the acceleration of methanol over these catalysts. 2. EXPERIMENTAL Ceria (20 m2/g) supported catalysts (0.5 and 4 wt %) were prepared by a conventional impregnation method, employing (NH4)2PdC14 as a precursor (Pd/CeO2). In order to investigate the effect of residual chloride ions on the reaction, some of the catalysts employed in this study were washed thoroughly by distilled water to remove a trace amount of chlorine (Pd(w)/CeO2). The reaction was carried out in a closed gas circulation system (CO:H2 = 1:3, total = 120 Torr). Before each run the catalyst was reduced by hydrogen at 573K (low temperature reduction: LTR) or 773K (high temperature reduction: HTR). To gather primary

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products, a liquid nitrogen cold trap was employed in the circulation system, and the products were analyzed by gas chromatography as well as mass spectroscopy. 3. RESULTS AND DISCUSSION 3.1 C h a r a c t e r i z a t i o n of the catalysts

Table 1 summarizes the results of H 2 and CO adsorption at room temperature over 4 wt % Pd/CeO 2 after various sequential pretreatments, as well as the particle sizes and dispersions estimated by XRD line width and TEM photograph. It was confirmed from XRD and TEM as well as the amount of sorbed hydrogen H(b) into Pd bulk, that these treatments did not affect Pd particle sizes. But the amounts of adsorbed H(a) or CO(a) on the Pd surface decreased drastically by Table 1. Amount of Sorptionand Particle Size of4wt%Pd/CeOEat 300K HTR treatment, and Redt~on Amount of Sorpfion Particle Size were restored to their original values by 02 T ~ (xlO-Smol.g.cat-') (A) treatment at 723K (K) H(a) (H/M) H ( b ) CO(a) XRD TEM followed by LHR. 790 30(0400 573 8.1 (025) 12 8.0 These results clearly 790 5(g)-6~ 773 3.0(0.08) 13 1.7 indicate the modification atter oxidation at 723K of reduced ceria(Ce203) 573 8.2 (0.25) 12 onto the top layer of 773 2.9(0.08) 11 surface Pd atoms (SMSI effect) by HTR, and its removal by re-oxidation. The occurrence of SMSI was confhxned by the infrared spectra of adsorbed CO, whose intensity decreased drastically by HTR(see Fig. 5). High resolution TEM photographs as well as EDX analysis represent that particle sizes of Pd increased slightly (from 300 to 500 A) by HTR, and their surface seems to be covered by thin layers of reduced ceria. It is worth noticing that the dispersion determined from hydrogen adsorption (H/M) was much larger than that estimated from XRD or TEM particle sizes, which suggests a spillover of H(a) or CO(a) onto ceria support. 3.2 Kinetic study of C O - H 2 reaction

When a mixed reaction gas (CO:H2=1:3, total pressure = 120 Torr) was introduced at 413K onto 4 wt % Pd/CeO2 which was freshly reduced at 573k (LTR), both methane and methanol were formed from the beginning of the reaction as shown in Fig. 1. However, the rate of methanol formation was slow and exhibited an induction period of one hour. After the induction period, its formation rate increased drastically and was accompanied by a decrease in the rate of methane formation. After 12h, the gas phase was evacuated briefly and the same mixed reaction gas was reintroduced. No induction period was observed this time and --

I

Ii

I

~

30 i !

I

CH3OH(LTR) I CH4(HTR) I

.

I

I

I

I

/I / +

I

/

-

"

s

C, <

0

2

~

~

~'v~

4

6

8

i

~

I

10 12 14 Reaction Time / h

I

9

16

18

I

I

20

22

Fig: I Time courses of CO-H reaction over 4wt% CeO2 at 413K

24

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morethan 99% of the products was methanol as shown in the latter part of Fig.

1. On the other hand, the situation was completely different and the selectivity changed drastically under HTR. The rate of methane formation increased four times at the initial stage of the reaction, and the length of the induction period for methanol formation became longer(from 1 h to 5h). Accordingly, the selectivity for methanol formation was only 40% even after 12h. These results indicate that the active sites for methane formation are created by HTR treatment, which are transformed into methanol formation sites during the prolonged reaction time. The acceleration effect by SMSI was much more pronounced in the case of Pd(w)/CeO2, which did not have any residual chlorine, as shown in Fig. 2. The rate of methane formation increased 30 times, with a longer induction period for methanol formation (from 3 h to 13h). It is reported that chloride ions occupy the electron vacancy formed by the hydrogen reduction and suppress the SMSI effect[9]. Similar time courses were obtained in the case of 0.5 wt% Pd/CeO2 catalysts both for LTR and HTR. But the acceleration effect of SMSI was much larger for methane formation(80 times), and the formation of methanol was not detected within 25 h under HTR. Temperature dependencies of the initial rates of methane formation under LTR and HTR conditions are demonstrated on the Arrhenius plots in Fig. 3. Under HTR conditions, 0.5 wt % Pd/CeO 2 seems to have a higher turnover frequency than 4 wt % catalysts, even though it is considered that Pd particle sizes are smaller in the former catalyst. However, the activation energy of methane formation on these catalysts were almost the same (85-90 kJ/mol), which did not change with particle sizes of Pd metal or by HTR treatment. These results strongly suggest that the active sites are located at the interface of Pd and ceria. -- 3001--1

~

II

o

"~ -~

II

= =

r-~ 250~-[

20o~-i

[]

II

CH4(LTR)

CHaOH(LTR) CH4(HTR) CHBOH(HTR)

"~150 t-~ ~

I

I

/

/

jp -

5O

-

<

I 0

10

20 30 Reaction time / h

o

40

Fig.2 Time courses of CO-~ reaction over 4wt% Pd(wYCeO'at 413K %

J

2

50

~

~ _ -9 - ~ - ~ ~

I I

= []

0"5~%'HTRI

4wt%,LTR I

-11! -12

I

2.1

2.2

2.3

1FF / xl0 -3 K -I

2.4

2.5

Fig.3 Arrhenius plots of methane formation over 0.5 and 4wt% Pd/CeO2

3.3 InfraredspectroscopyduringC O - H 2 reaction

Fig. 4 shows the infrared spectra of adsorbed CO, as well as adsorbed species during COH2 reaction over 4 wt% Pd/CeO2 under LTR. When only CO was introduced onto the freshly reduced (LTR) surface at 300K, strong peaks at 2076 and 1958 cm 1 were observed, which can be attributed to the linear CO(a) on smaller Pd crystallites and bridged CO(a) on the Pd(100) plane, respectively (spectrum (a)). In addition, broad peaks emerged at around 1550-1350 cm -l, which may be assigned to some carbonate species formed at the interface of Pd and ceria. Two processes may be considered for the formation of carbonate only from CO in the gas phase, (1) its disproportionation or (2) reaction with OH group on ceria. At elevated temperatures, a weak band emerged at around 1750 cm 1, which may be attributed to adsorbed CO whose bond was weakened by electronic interaction with vicinal ceria. When H2 was introduced to this surface and CO-H2 reaction was carried out at 413K, broad peaks at around 1850-1900 cm -~ and 1600-1300 cm ~ and three sharp peaks at around

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0.4

I

I

~

I

I

.... ~

I

1

I

.,. JN.

I I I I I ! I 2200 2000 1800 1600 1400 1200 1000 WaveNumber/cml

Fig.4 FT-IR Spectra of Adsorbed Species over 4wt% Pd/CeO2 (LTR) (a)CO ads.(300K), (b)CO-n2(413K;0.5h) (c)CO-m(413K;6h),(d)H2(453K;2h)

l

0.4

2200 2000 1800 1600 1400 1200 1000 WaveNumber / cm "l

Fig.5 FT-IR Spectra of Adsorbed Species over 4wt% Pd/CeO2 (HTR) (a)CO ads.(300K), (b)CO-H2(4~3i~2h) (c)co-m(413K;Sh),(d)CO-I-t2(413K; 13h)

1095, 1035, and 1005 cm -~ emerged gradually during and after the induction period of methanol formation (spectra (b) and (c), respectively). The first broad peak may be attributed to bridged CO(a) on Pd(111) plane. It is interesting to note that the Pd(11 l) plane is reported to be more easily recovered from SMSI decoration than Pd(100)[10], and is more active for methanol formation in C O - H E reaction[11 ]. The second i.r. peaks can be assigned to surface formate together with carbonate, and the third ones to three different methoxide species, whose peak positions were almost the same as those on reduced ceria itself reported in the literature[12]. After the reaction, only n 2 w a s introduced to remove these adsorbed species, but no change was observed at 413k. Adsorbed CO(a) and methoxide species began to disappear at around 453K as shown in the figure (spectrum (d)), but a much higher temperature was required to remove surface formate and carbonate species (around 513K). These results strongly suggest that formate, carbonate, and methoxide species observed by i.r. were mainly adsorbed on ceria support and do not participate in the methane or methanol formation directly. Fig. 5 represents the infrared spectra corresponding to Fig.4 in the case of Pd/CeO 2 under HTR. When only CO was introduced onto the freshly reduced (HTR) surface at 300K, the amount of adsorbed CO decreased drastically and only a very weak band was observed at 2056 cm 1 attributable to linearly adsorbed CO (spectrum (a)), which clearly shows the occurrence of the SMSI. When C O - H E reaction was carried out over this surface, a small amount of formate and carbonate emerged at around 1600-1300 cml(spectrtun(b)), whose intensities did not change much after the induction period of methanol formation(spectra (c) and (d)). In contrast, two sharp methoxide peaks were observed at 1035 and 1005 cm -1, and the intensity of the former peak increased significantly after the induction period of methanol, which may indicates the partial oxidation of C e 2 0 3 [ 1 0 ] . It is worth noticing that the amount of adsorbed CO(a) did not change at all even after a prolonged reaction period, indicating that the SMSI state was not destroyed by C O - H E reaction and higher temperature O2 treatment was required to remove the thin layer of ceria from the surface of Pd metal.

3.4 Isotopic exchange reactions To clarify the influence of SMSI effect for the dissociation steps of H 2 and CO (if exists), HE-DE and ~2C~80-13C160 isotopic exchange reactions were compared between LTR and HTR treatments. As summarized in Table 2, the HE-DE exchange reaction proceeded even at 77K after LTR treatment of Pd/CeOE, and was retarded approximately to one fourth by HTR treatment. Accordingly, SMSI seems to reduce the active sites for hydrogen dissociation. In the presence of gaseous CO, HE-D2 exchange reaction was also retarded to one fourth in

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both eases, indicating stronger adsorption of CO than hydrogen on this catalyst. On the other hand, 12C180-13C160 isotopic exchange reaction took place at around 240270K, and was enhanced about four times by HTR treatment without changing the activation energy, which is quite similar to the enhancement effect of methane formation by SMSI. To clarify the mechanism of this isotopic exchange process, we studied ~2C180-~3C~60 isotopic exchange reaction over Pd black at elevated temperatures up to 573 K, but no detectable exchange was observed. We also studied the oxygen exchange reaction between C~80 and Cel602, which only took place slowly above 400K. Consequently, we could exclude the possibility of C-O dissociation on Pd metal or CO2-1ike species formed with Ceria-oxygen for the intermediate of this isotopic exchange reaction. The extremely high activity with low activation energy in this exchange reaction strongly suggests that this process did not proceed through dissociative adsorption of CO, but through a geminal dicarbonyl adsorbed species,(CO)2(a), as proposed by Bossi et al. in the ease of Ru/AI203113]. It is worth noticing that the exchange reaction was strongly retarded by the presence of H2, although Table 2.Various isotopic exchange reaction over 4wt% Pd/CeO 2 adsorption of CO is much stronger Reaction Treat React. Initial Activat. than hydrogen as mentioned Temp. Rate* Enelgy already. Therefore, in the (kJ/mol) presence of hydrogen, (HCO)2(a) type intermediate may be formed, H2-D2 LTR 77 24x!0-5 which inhibits the reverse reaction HTR 77 6x10-5 to dicarbonyl species. Moreover, H2-D2-CO LTR 77 0.Sx 10.5 this could be the intermediates for HTR 77 0.2x10-5 methane formation by its CO-H 2 LTR 413 3.3xl fill 87 decomposition. As shown in the HTR 413 10.6x10-ll 85 fourth column(CO-H 2 treat.), the C 180-13CO LTR 243 0.8xl 0"4 19 exchange reaction did not proceed HTR 243 3.2x104 21 at all over the surface atter CO-H 2 CO-H 2 273 0.0xl0 "4 reaction for a prolonged period, C180 -13CO-H2 LTR 273 0.3xl 0"~ suggesting that these active sites Ce16OE-ClSO LTR 413 3.6xl 04 are the same as methane formation HTR 413 10.6x10~ sites, which are transformed into methanol formation site during the *CO-H2 : mol/s.g-cat., Isotopicexc. : composition/s.g-cat. induction period. 3.5 Reaction mechanism of C O - H 2 reaction

Two reaction mechanisms have been proposed for methane formation in CO-H 2 reaction as follows: (1)dissociation of CO(a) to form C(a) and O(a), followed by their hydrogenation to CH(a) and water, (2)hydrogenation of CO(a) to form HCO(a) or HECOH(a), followed by its dehydration to CH(a) and water. Methane may be formed by the successive hydrogenation of CH(a) species. As mentioned already, dissociative adsorption of CO is unlikely over Pd metal itself, but support-assisted dissociation of C-O bond at the interface of Pd and ceria is probable. In fact, we could observe a low frequency IR band around 1750 cm 1, which can be attributed to a precursor CO(a) for dissociation, and may have some connection with the slow oxygen exchange process between CtSO and Ce1602 at higher temperatures. At elevated temperatures(higher than 400-450K), we could detect the formation of CO2 by circulating only CO over the catalyst. In order to investigate the role of the carbon species, the disproportionation of CO was carried out on the freshly reduced Pd/CeO 2 at 413K, and then c o - n 2 reaction was conducted on this carbon accumulated surface. However, the rate of methane formation was not affected by these accumulated carbon species. From these experimental results we like to propose a new reaction mechanism for methane

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formation in CO-H 2 reaction over Pd/CeOE, which does not involve the direct CO dissociation process. As summarized in Fig. 6, the active sites for these reactions are located at the interface of Pd and reduced ceria (probably Ce203), where the isotopic exchange of CO takes place through dicarbonyl-like species below room temperature. At higher temperatures, hydrogenation of the dimer intermediate gives (HCO)2 species, which decomposes into CH(a) and HCOO(a), and then is hydrogenated into methane and H20. In fact, we could observe a IR spectra of HCOO(a) species by the hydrogenation of CO(a) at room temperature. At the same time, some Ce203 may gradually be oxidized to CeO2.y by H20 formed during the induction period of CO-H2 reaction, and the methane formation sites are transformed into methanol formation sites by this process. The following isotopic tracer experiments were carried out to prove the above mentioned mechanism. First of all, 13CO-H2 reaction (2:10 Torr) was carried out at 353K for 1 h over freshly reduced Pd/CeO2. No methane formation was detected at this temperature but we may expect the formation of adsorbed (HUCO)2 species on the surface. After the temperature was lowered to 273K, the gas phase was replaced with a large amount of 12CO (100 Torr) in order to exchange all the adsorbed uCO into 12CO on the surface. Again after the evacuation of the gas phase, 5 Torr of H2 was introduced in the gas phase at 273K and gradually raised the reaction temperature. We could observe the formation of a certain amount of 13CH4, together with a large amount of 12CH4 by mass spectroscopy. These results may suggest the existence of a (H13CO)2 intermediate during 13CO-H2 reaction. - - , . . / ~ N . Below Ce~O. Isotopic E x c h a n g e R.T. Oxygen Reaction of CO -" Vacancy ~+~ CeO2.y Fig. 6

Proposed reaction mechanism

Methane Formation

Td:;h~:or~aFotiOr~at~-on Site QCHs~H ~

~-~

~HO 0

4. CONCLUSION A new reaction mechanism for methane formation was proposed for CO-H E over Pd/CeO 2. Active sites for methane formation are located at the interface of Pd metal and reduced ceria, which are produced by SMSI interaction. Methane is formed not through the direct dissociation of CO but through geminal dicarbonyl intermediates and (HCO)E(a) on these vicinal sites, which are transformed into methanol formation sites by the oxidation of CeEO3 into CeOE.y. REFERENCES 1. R.F.Hichs, Q-J.Ycn, and A.T.BeII, J.Catal.,89 (1984) 498. 2. C.Sudhaker, and M.A.Vannice, J.Catal., 95 (1985) 227. 3. S.Ichikawa, H.Poppa, and M.Boudart, J.Catal., 91 (1985) 1. 4. K.P.Kelly, T.Tatsumi, T.Uematsu, ct.al., J.Catal., 101 (1986) 396. 5. Y.Kikuzono, S.Kagami, S.Naito, T.Onishi, and K.Tamaru, J.Chem.Soc.,FaradayDiss.,72 (198 l)135. 6. J.M.Ddessen, E.K.Poels, J.P.Hindcrmann, and V.Ponec, J.Catal., 82 (1983) 26. 7. Li Fan, and K. Fujimoto, J.Catal., 150 (1994) 217. S.Naito, S.Aida, T.Tsunematsu, T.Miyao, Chem.Letts. (1998) 941. 8. L.Kepinski, M.Wolcyrz, and J.Okal, J.Chem.Soc.Faraday Trans., 91 (1995) 507. 10. Badri, C.Binet, and J-C.Lavalley, J.Chem.Soc., Faraday Trans., 92 (1996) 1603. 11. R. F. Hicks, and A. T. Bell, J. Catal., 91 (1985) 104. 12. C.Li, K.Domen, K.Maruya, and T.Onishi, J.Catal., 141 (1993) 540. 13.A. Bossi, G.Carnisio, F.Garbassi, G. Giunchi, G.Petrini, J.Catal., 65, (1980) 16.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Methane Coupling over SrCoO3-Based Perovskites in the Absence of GasPhase Oxygen Yu.I.Pyatnitsky, N.I.Ilchenko, L.N.Raevskaya, L.Yu.Dolgikh and N.V.Pavlenko L.V.Pisarzhevsky Institute of Physical Chemistry, Prosp.Nauki 31, 252039 Kiev, Ukraine. Methane coupling reaction over SrCoO3-based perovskites has been studied in a periodic regime where a catalyst was alternatively supplied with methane and oxygen. The correlation between catalytic activity and catalyst oxygen reactivity measured with temperatureprogrammed reduction method has been found. Experimental and simulation study of reaction kinetics has been performed, and the probable heterogeneous-homogeneous reaction mechanism has been discussed* I.INTRODUCTION The oxidative coupling of methane aimed at the production of higher hydrocarbons is usually carded out in the presence of catalysts in a continuous flow set-up. It is generally accepted that a catalyst generates the formation of free methyl radicals which initiate the gasphase branched chain reaction. Methyl radicals play key role in formation of the target higher hydrocarbons. However, they easily interact also with molecular oxygen as well as some intermediates of the gas-phase reaction producing carbon oxides. To avoid the homogeneous formation of carbon oxides, the methane coupling reaction can be carded out in the periodic regime where methane and oxygen alternatively interact with a catalyst. Producing of C2 hydrocarbons during the methane passing over a series oxide systems at 973-1173K has been demonstrated in the works [ 1-5]. In the present study, methane coupling reaction has been performed in the unsteady-state regime using the SrCoO3 and AxSrl.xCoO3 (A = Li, Na, K) perovskites as solid oxidants. The perovskites belong to oxide systems that can lose relatively large amounts of oxygen without destroying its crystal structure. It is necessary property for the catalysts operating in the alternative redox regime. Modification of oxide catalysts with alkali metals often facilitates methane activation and, in that way, increases the reaction rate. 2. EXPERIMENTAL The ABO3 type perovskite catalysts have been prepared by evaporation of aqueous solutions comprising of metal nitrates and ammonia citrate with final calcination at 1123 K. The resulting products were SrCoO3.x, Lio.25Sro.75CoO3-x,Nao.25Sro.75CoO3-x, Ko.25Sr0.75CoO3-x and Nao.125K0.125Sr0.75CoO3-xmixed oxides. The catalytic properties of the perovskites were determined in a flow type quartz reactor (i.d. 21 mm, volume 27 cm 3) by passing automatically switched methane, He and 02 pulses * The financial support of the INCO-CopemicusGrant (IC15-CT97-0709) is gratefullyacknowledged.

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through a catalyst bed at given conditions and periods of the pulses. Before catalytic runs, a catalyst was pre-oxidized by oxygen. To determine an initial catalytic activity of an oxidized samples, catalytic cycles consisted of the consecutive pulses of He (5 min), CH4 (1.5 min), He (5 min) and 02 (1.5 min) with gas velocity 100cm3/min have been repeatedly performed. Temperature-programmed reduction (TPR) measurements were performed with a quartz tube reactor (i.d. 8 mm), H2-argon mixtures flowing at 40 cm3/min, and heating rate of 10K/min in the temperature range of 300-1100K. Kinetic modelling of methane conversion to C2 hydrocarbons has been made using onedimensional reaction model including heterogeneous step of formation of methyl radicals and gas-phase chain reaction initiated by methyl radicals. The model simulates process carrying out in a perfect plug flow reactor. 3. RESULTS AND DISCUSSION

3.1. Catalytic properties The reaction products were mainly ethane, ethylene and CO2. In relatively small amounts, propane, propene and C4 hydrocarbons were also detected. Table 1 demonstrates initial catalytic activity and selectivity of the perovskites measured at the same reaction conditions. Table 1. Initial activity and selectivity of the perovskites (1023 K, catalyst bed volume 3.0 cm 3) Catalyst Surface Selectivity, % Methane C2-4 Reaction area, conv., yield, rate, % % mole/m2.s m2/g C2H4 C2H6 C2-C4 CO2 SrCoO3 1.42 33.7 40.2 76.3 23.8 4.9 3.8 3.6-10-07 LiSrCoO3 1.28 42.8 43.4 90.9 9.1 10.9 9.9 9.5-10-07 NaSrCoO3 1.93 57.0 30.0 93.2 6.8 16.4 15.3 8.4.10 -07 KNaSrCoO3 0.75 54.9 34.8 96.8 3.2 12.9 12.5 1.7.10-06 KSrCoO3 0.78 35.3 49.1 89.1 10.9 8.2 7.3 1.2.10-06 As seen from Table 1, partial substitution of strontium with alkali metals increases the specific catalytic activity as well as selectivity towards higher hydrocarbons. The largest selectivity and specific catalytic activity were observed for the substituted KNaSrCoO3 perovskite. During first 20-30 catalytic runs, activity of the fresh KNaSrCoO3 sample was increased. Then, the catalyst kept high stability and selectivity at temperature of 1023K and lower.

3.2. Temperature-programmed reduction The TPR spectra of the catalysts are shown in Fig.1. These spectra have been measured after a consecutive catalyst exposition into Ar (973K/30 min) and O2 (973K/30 min) with subsequent catalyst cooling in oxygen atmosphere. No significant change in TPR spectra of KSrCoO3 and KNaSrCoO3 was observed after pre-oxidized catalyst treatment by Ar at 1023K with subsequent catalyst cooling in Ar atmosphere [6]. Therefore, it is probable that oxygen detected by TPR method participates in the interaction of methane with perovskites under chosen reaction conditions.

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Fig. 1. TPR spectra of the perovskite catalysts. The TPR spectra of the catalysts have a complex pattern indicating non-uniformity of the catalyst oxygen. Introduction of alkali metals into original SrCoO3 leads to a shift of the peaks of the most reactive oxygen towards lower temperature (Fig.l). The oxygen reactivity increases with increasing the basicity of alkali metal introduced into the SrCoO3 catalyst in the order SrCoO3 < LiSrCoO3 < NaSrCoO3 < KNaSrCoO3 < KSrCoO3. This order is in an approximate correlation with the order of the catalyst specific activity under the unsteadystate conditions (Table 1). Thus, there is a certain correlation between catalyst oxygen reactivity (reducibility of an oxide catalyst) and catalyst activity in methane conversion into higher hydrocarbons. 3.3. R e a c t i o n k i n e t i c s

Fig.2 demonstrates the dependence of the rate of CH4 interaction with the pre-oxidized KNaSrCoO3 catalyst on amount of consumed oxygen expressed in terms of oxygen monolayer number (it was assumed that the catalyst surface area of 1 m 2 contains 10 ]9 oxygen atoms). 12 1-.1 o __A~k- 4 k - - 9 O

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Fig.2. Dependence of the specific reaction rates of the product formation on amount of consumed oxygen (1023K, Feed" 100% CH4,).

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As seen from Fig2, the reaction rate is constant up to more 5 consumed monolayers; after that, the rate falls linearly. Total amount of consumed oxygen achieves 25 monolayers. It can be supposed that removal of the first portions of oxygen is limited by some chemical reactions in surface and nearest subsurface layers of the catalyst, while more deep catalyst reduction depends on the rate of oxygen diffusion from remote catalyst layers to a catalyst surface. It was also found that the higher hydrocarbon formation is the first order reaction with respect to methane, and the CO2 formation is the zero order reaction (Fig.3). 14 i,.12 o ,t,,,,

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0

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Fig.3. Dependence of the specific rates of formation of C2-C4and CO2 on methane concentration in reduction pulse at 1023K. The reaction kinetics of methane interaction with oxidized catalyst can be explained as follows. At the first reaction time, the consumption of the surface oxygen is quickly compensated by oxygen diffusion from the subsurface layers. As a result, the catalyst surface remains almost completely oxidized and the reaction of methane conversion proceeds with a constant rate. According to the zero order of CO2 formation with respect to methane, reaction rate is apparently determined by desorption or decomposition of some surface intermediates. The first order of methane conversion to C2 hydrocarbons is most probably due to the impact methane interaction with surface oxygen in the reaction rate-determining step. To agree with reaction kinetics, both reactions of CO2 and C2 hydrocarbons formation must proceed on different kinds of the surface sites (in opposite case, the reaction order must be zero for the both reactions). The same conclusion on different reaction sites for CO2 and higher hydrocarbon formation over SrCoO3.~ catalyst based on results of oxygen thermoprogrammed desorption was made in [4]. 3.4. Kinetic simulation Taking into account obtained results, it can be assumed that the reaction mechanism includes the following heterogeneous steps: CH4 + ZO -~ ZOH + CH3; 2ZOH -~ ZO + Z + H20. Methyl radicals can initiate the gas-phase formation of higher hydrocarbons. To verify this reaction network, the kinetic simulation has been made using the gas-phase reaction model presented in Table 2.

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Table 2. Kinetic model of the gas-phase chain reaction No.

Reaction

(k=A 7" exp(-E/RT)

A, ma/mole-s; s 1

1 CH4+H--~CH3+H2 2 CH3+C2H6--~CH4+C2H5 3 2CH3--~CEH6 4 C2Hs-~C2H4+H 5 C2H5+CH4---~C2H6+CH3 6 C2Hs+CH3---~C3H8 7 C2Hs+C2Hs---~CaHI0 8 C2Ha+CH3--~n-C3H7 9 C3Hs+CH3---~n-C3H7+CH4 10 C3Hs+CH3---~i-C3H7+CH4 11 n- C3H7---~ C2H4+CH3 12 n- C3H7+ CHa---~C3Hs+ CH3 13 n- C3H7+C2H6---~ C3Hs+C2H5 14 n- C3H7+ CH3----~n-CaH10 15 n- C3H7+ CH3---~CHa+C3H6 16a) n- C3H7+ C2H4----~i-C3H7+C2H4 17 i- C3H7+ CH4---~ C3H8+ CH3 18 i- C3H7+C2H6---~C3H8+ C2H5 19 i- C3H7+ CH3---~i- CaHI0 20 i- C3H7§ CH3----~CH4+C3H6 21 i- C3H7+ C2H4---~ C3H6+ C2H5 22 CaHl0+ CH3----~CHa+CaH9 23 C4H9---~C3H6+ CH3 a)kl6=A16exp(-3500/T) exp(-1000/T)/(1+exp(-1000/T))

2.25 102 5.48 10-7 1.01 109 4.9 109 8.61 108 2.83 107 1.08 107 3.31 105 9.03 10"7 1.51 10-6 1.2 1013 2.41 10-8 2.53 10-7 1.93 108 1.14 107 4.82 104 7.22 10-l~ 8.43 10-9 2.83 107 4.53 106 2.65 104 1.35 10-6 2 1013

n

E/R, K

3 4 -0.64 1.19 4.14

4406 4169

3.65 3.46 4.02 3.82 -0.32 -0.32 4.4 4.2

3.65

18722 6322 (300/T) ~ 3877 3600 2758 15249 5473 4550

5434 4386 (300/T) ~ (300/T) ~ 3324 3600 15075

Rate constants of the gas-phase reactions selected from the kinetic database [7] were used without any optimization. The rate constant of heterogeneous initiation of the gas-phase process CH4 + ZO ~ CH3 + Z, khet, was derived from the experimental results obtained in the present work. A one-dimensional kinetic model simulated a process in a perfect plug flow reactor was used. Fig.4 presents the simulated temperature dependence of ethane/ethylene distribution in C2 fraction and methane conversion in a comparison with their experimental temperature dependencies. The simulation was made using the following parameters: CH4 concentration 11.9 mole/m 3; catal~cst surface area/free space volume ratio 1.87 106 ml; residence time 0.25 C'~ khet-- 1.33 10 exp(23670/T) m/c (the residence time was calculated as ratio of experimental gas flow velocity at 1023K to reaction volume in which gas-phase reaction proceed; it was assumed to be 1.3 10-6 m3). As seen from Fig.4, there is reasonable agreement between simulated and experimental data. It gives an evidence that accepted heterogeneous-homogeneous mechanism of higher hydrocarbons formation reflects the principal features of the real mechanism.

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ff ~

80

C2H6

5 -I-

O

4 a,o

o 60 .m

0

D

3 <

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o 40

,m,

2 o

O

m ~L

20

0

-

1

C2H4Ar~960 970 980 990 1000 1010 1020

N

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Temperature, K

Fig.4. Experimental (points) and simulated (curves) temperature dependencies of ethane/ethylene distribution in C2 fraction and methane conversion. 4. CONCLUSIONS The SrCoO3-based perovskites are selective and stable catalysts of the methane coupling reaction in the periodic regime. The partial substitution of strontium with alkali metals increases the catalyst oxygen reactivity. Parallel with an increase of oxygen reactivity, strontium substitution leads to an increase of catalytic activity and selectivity of the explored perovskites. Thus, there is a certain correlation between catalyst oxygen reactivity and catalyst activity in methane conversion into higher hydrocarbons. The rate of methane interaction with a pre-oxidized perovskite is constant up to more five monolayers of catalyst surface oxygen consumed in the reaction. In this region, the higher hydrocarbon formation is the first order reaction with respect to methane, while the C O 2 formation is the zero order reaction. It can be supposed that formation of C2 hydrocarbons and CO2 takes place with participation of different kinds of surface oxygen. Experimental and simulation studies of reaction kinetics agree with the heterogeneoushomogeneous reaction mechanism. The mechanism consists of the heterogeneous formation of free methyl radicals via methane interaction with oxidized catalyst sites and homogeneous reactions initiated by methyl radicals. REFERENCES 1.T.Fang and C.T.Yeh, J.Catal., 69 (1981) 277. 2. G.F.Keller and M.Bhasin, J.Catal., 72 (1982) 9. 3. J.A.Sofranko, J.A.Leonard and C.A.Jones, J.Catal., 103 (1987) 302. 4. K.Omata, O.Yamazaki, K.Tomita and K.Fujimoto, J.Chem.Soc., Chem.Commun., No. 14 (1994} 1647. 5. O.V.Buevskaya, Rothaemel, H.W.Zanthoff and M.Baerns, J.Catal., 150 (1994) 71. 6. L.Yu.Dolgikh, N.V.Pavlenko, N.I.Ilchenko, L.A.Zuzya, and Yu.I.Pyatnitsky, Theoret. Experim.Chem.., 35 (1999) (in press). 7. W.Tsang and R.F.Hampson, J.Phys.Chem. Ref.Data,15 (1986) 1087; 17 (1988) 887; 19 (1990) 1; 20 (1991) 221.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Novel Conception of the Methanol Synthesis Mechanism on Conventional Cu-containing Catalysts G.I. Lin, K.P. Kotyaev, A.Ya. Rozovskii Topchiev Institute of Petrochemical Synthesis of the Russian Academy of Sciences, Leninsky prospect 29, 117912, Moscow, Russia Novel mechanism of methanol synthesis on conventional Cu-containing catalysts is proposed and proved. It includes a conjugation of methanol synthesis and WGSR and involves the transformations of strongly adsorbed species. The mechanism is based on the data of tracer technique (use of ~4C, 180, D) and different variants of kinetic studies, including transformations of surface species in non-stationary systems, as well as various chemical treatments of catalyst surface followed by TPD. Individual steps of the mechanistic scheme have been proven in independent experiments. Theoretical kinetic model of the whole process (including methanol synthesis and WGSR) based on the mechanistic scheme is proposed and computer simulation is made. It describes all known experimental data (for the pressures from 0.1 up to 10 MPa). 1. Macroscopic mechanism of methanol synthesis Methanol synthesis is one of the few industrial processes, amount of knowledge on which allows to build substantiated mechanistic scheme and then, on its basis, theoretical kinetic model. The foundation of the contemporary understanding of methanol synthesis macroscopic mechanism was laid by the studies involving ~4C, made for the stationary process on industrial catalysts as early as 1975 ([ 1, 2], see also [3]). It was shown that only CO2 (and not CO) is the source of methanol origin in the case of Cu-containing, and also Zn-Cr catalysts; on the other hand, in alcohols synthesis on iron catalyst methanol is obtained from CO almost exclusively. Let us consider the data of typical experiment with labeled CO2, conducted in stationary system at 250~ under pressure about 5 MPa with gas circulation and continuous freezing of methanol and water from the cycle, with simultaneous uninterrupted gas sampling for comparative analysis. For the mixture containing excessive CO (initial content of ~4CO2- 4%, C O - 30%) the isotopic portion of ~4C in various components of resulting mixture was (in arbitrary units): 430 for CO (as the result of exchange), 1480 for CO2, 1500 for methanol. Similar results were obtained later by Chinchen et al. [4]. Even for syngas containing very small amount of CO2 (perhaps, below the level of detection) they found the content of ~4C in CO2 to be very close to that in methanol and much higher than that in

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CO under conditions of stationary synthesis of methanol. It follows that methanol synthesis is the system of reactions: CO2+3H2=CH3OH+H20; CO+H20=CO2+H2. Studies involving lSo have led the authors [5,6] to opposite conclusions. Klier et al. [5] by addition of H21sO in syngas have obtained the same label of oxygen in methanol and water at the reactor exit in stationary methanol synthesis conducted in flow reactor. They have concluded that methanol is formed from CO and H20. Nevertheless, the real reason behind the content of 180 in CO2 and H20 becoming equal is large contact time. We have calculated the isotopic fraction of 180 in the products exiting the reactor in experiments [5]: CO - 0.6%, CO2 - 3.6%, H20 - 3.3%, methanol- 3.4%. It shows that the conclusion of Klier et al. is invalid, and by the results of [5] both H20 and CO2 can be the source of oxygen atom in methanol. Kung et al. in their elegant investigation [6] have shown (using flow reactor) that 180 label is significantly lower in methanol than in CO2, to conclude that both CO and CO2 participate in synthesis. This conclusion was also confusing due to non-stationary behavior of the system relating to the processes of label transfer. Fig. 1 shows the dynamics of 180 isotopic fraction in CO2 0,8 Z 1 and methanol, calculated by us using data 9 0,6 [6] (the initial label was C1802). It is seen o o o that the part of 180 increases in methanol, o ~ 0,4 0 o o o and decreases in the source of 180 (CO2), 2 0 o along with time. It means that there is no 9 o,2 direct label transfer into methanol. It I I I I proceeds via some oxygen-containing "buffer" on the surface. Thus, the 0 3 6 9 12 15 experimental results show only the lower IM , min. limit of the label in methanol. The veritable Fig. 1. Dynamics of oxygen label in value of the label can be obtained after the CO2 (1) and methanol (2), stationary label flow is established. It is calculated from data [6]. seen from the figure that the curves of 180 content in methanol and CO2 become closer with time, so this experiment also shows that at least the main amount of methanol forms from CO2. nn

nn

ID

i

2. Detailed mechanistic scheme

Our own further investigation [3] has shown that CO2 and H20 are strongly adsorbed on active centers of methanol synthesis, so that their characteristic times of desorption (> 1 h) are much greater than characteristic time of methanol synthesis (-~1100 s). So, from the viewpoint of usual adsorption-desorption mechanisms methanol synthesis should not proceed at all, for water molecule forms in each cycle of synthesis. In reality, the exchange between surface and gas phase is provided by adsorption substitution reaction [3], which occurs via two-particle complex involving both CO2 and H20 (probably, carbonate or bicarbonate species with hydroxyl):

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zH20+CO2=z.CO2.H20=zCO2+H20 (z is Cu-containing active site). Kinetics of this reaction were measured in non-stationary experiments. They can be observed directly by TPD-spectra after sequential treatments of catalyst with water and then with CO2 (Fig. 2). Two CO2 peaks are seen. Low temperature CO2 peak and the shoulder on H 2 0 peak correspond to the species z.CO2.H20. These two peaks are observed also after methanol adsorption. Hydrogenation of strongly adsorbed CO2 leads to methanol formation, its rate being in good correlation with that of stationary synthesis. In principle, both carbon-containing species can produce methanol upon their hydrogenation. Discretion of methanol precursor can be made using deuterium label. If 1,4

~'

0,8 O FR 0,6 AC

0,7

II

0

TI 0,4 ON

i100

I 300

1

III

0000

18 0,2 I

500

T,~

II

0

3

I 0

2

0

I

I

I

6

9

12

O

15

TIME, rrin. Fig. 2. TPD-spectra of common catalyst SNM after treatment at 100~ with vapor (1, 3), then CO2 (1, 2): 1 - H20, 2, 3 - CO2.

Fig. 3. Dynamics of deuterium accumulation in hydrogen (empty symbols) and methanol (blackened symbols) at 240~ 106 h 1, 5 MPa: 1 -CO-CO2-H2, 2-CO2-H2 mixtures.

methanol forms upon hydrogenation of z.CO2.H20 species, then the appearance of D in CH3-group of methanol can be expected. It was found by Klier et al. [5]: CH2DOH was obtained using mixtures of D20 and CO in stationary synthesis. However, too high contact times (which favor secondary reactions) were used in experiments [5]. That is why we have reproduced these experiments under close conditions and then conducted analogous experiments at much smaller contact times (by two orders of magnitude). Dynamics of deuterium accumulation in methanol and hydrogen (gas circulation, 240~ 5 MPa) are shown in Fig. 3. It is seen from the plot that at small contact times the isotopic fraction of D in methanol and hydrogen is the same and much lower than that in water, which changed in these experiments from 100% at the reactor inlet to 40% (CO-CO2-H2 mixture) or to 75% (CO2-H2 mixture) at the reactor outlet. Thus, Klier's conclusion was not proved and it can be concluded that under the conditions of methanol synthesis hydrogenation of z.CO2.H20 species does not proceed. Obtained information alone (considering that the order of methanol synthesis reaction on hydrogen is close to 1, so the first step of hydrogenation appears to be the rate-limiting one) allows to make relatively simple scheme of surface species

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transformations, leading to methanol formation. All the steps of this scheme were proven (and realized separately in independent experiment). Kinetic model based on this scheme was in +C02 satisfactory agreement with zH20 < " zCO2H20 < zC02 experimental data. However, +HE0 continuation of our study of reaction kinetics became the reason for reconsideration of zCO2.H2 this mechanistic scheme. (macro step)l[+2H2 zH20+CH3OH 3. Kinetics and advanced mechanistic scheme

Experiments with partly deactivated catalysts for mixtures enriched with CO have shown anomalous kinetics: observed rate of methanol synthesis reaction in integral reactor increased along with contact time. This effect was reproduced for fresh catalysts at very small contact times. This result is all the more "exciting" in that the products of reaction do not accelerate the process (water retards it greatly). Fig. 4 contains kinetic curves of methanol synthesis in integral flow reactor at 240~ under 5 MPa on partly deactivated industrial catalyst 51-2 for the mixture CO2H2 (normal kinetics, curve 1) and for the mixtures with excessive CO (anomalous kinetics, curves 2, 3). For the mixture CO2-H2 reaction rate rapidly decreases along with contact time due to strong retardation of methanol synthesis by forming water. For mixtures enriched with CO, normal behavior of kinetic curves can be observed only for the small range at very small contact times. Otherwise, methanol r/) "~ 4,0 synthesis rate in integral reactor grows with contact time and decreases only at large 1 2 0 3,0 comact times, out of scope of this plot. This ,~" 2,0 effect could be caused by "re-reduction" of o ! catalyst surface. To check it, 0.3 vol. % of 1,0 oxygen was added to the gas mixture. 0,0 Kinetic curve for this mixture is also given 0,0 0,2 0,4 0,6 in Fig. 4 (curve 3). It is seen that the effect remains intact, and additional water, CONTACT TIME, s forming in hydrogen oxidation, only retards Fig. 4. Kinetics of methanol synthesis on catalyst 51-2 (240~ 5 MPa): 1 - from CO2 +H2, 2 - from CO-enriched gas, 3 - same as 2, + 0.3 vol. % 02.

methanol synthesis further. The composition of reaction mixture in minimums of kinetic curves roughly corresponds to WGSR equilibrium: at smaller contact times reverse WGSR dominates; at larger times - direct WGSR. Then direct correlation between the

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reaction rates of methanol synthesis and WGSR can be expected. The plot in Fig. 5 shows such correlation: the higher is the WGSR rate, the higher is the increase in methanol synthesis rate. The data given here allow to make definitive, , as it seems to be, conclusion: that methanol 3 synthesis reaction is conjugated with WGSR, i.e. CO transformation into CO2 favors the formation -9o 2 of zCO2 species, which are responsible for methanol synthesis. Such transformation cannot H G 1 -'k proceed as direct oxidation of CO into CO2 with m o [formation of z.CO2, because 14C label would be 0 transferred then from CO into methanol, which has not been observed in experiments. Therefore, CONTACT TIME, s WGSR results either in formation of free center, or in that CO2 molecule remaining on the center Fig. 5. Kinetics of WGSR and undergoes quick exchange with CO2 from the methanol synthesis (51-2, 220~ gas phase. 5 MPa) from CO-CO~-Hg. Then the methanol synthesis mechanistic scheme above should be modified: +H2 zCO.H20

zH20 <

<

+CO~

)

z[ ]CO2 <

zCO2.CO2

) zOO2[ ]

)' z C O 2 H 2 0

zCO2.H2

ma ro zH20+CH3OH

, where zCO2[ ] and z[ ]CO2relate to two different types of CO2 species. It is in agreement with the conclusions by authors [7], who have proposed in their FT-IR study that methanol synthesis and WGSR occur over Cu-Zn-A1 catalyst via the same species. Theoretical kinetic model of the whole process has been also developed. As an example of the possibilities proposed by this model, we can consider the data given in Fig. 4. A plot in Fig. 6 demonstrates good agreement of calculations by theoretical model and these experimental results. It should be noted that both curves 1 and 2 of Fig. 4 (normal and anomalous kinetics) were interpreted using the same set of kinetic parameters. The differences between these two cases are seen: for the normal kinetic

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718

--~ 5,0 o

13

-30

1

2

[..

3

Z9 1,0 [.-

"

0,3

~-1,0

4

o16

CONTACT TIME, s Fig. 6. Calculation of data from Fig. 4 (curves 1, 2) using theoretical kinetic model: 1, 2 methanol synthesis; 3, 4 - corresponding WGSR curves (symbols - experimental data).

curve, no CO transformation into C02 takes place; for the anomalous one, this transformation proceeds rapidly. Thus, the novel mechanistic scheme of methanol synthesis and the derived theoretical kinetic model of the whole process (including methanol synthesis and WGSR), as well as computer simulation of the process have been developed. The model successfully interprets all available experimental data, obtained so far by the authors and other researchers in very broad intervals of conditions.

SUMMARY

Novel mechanistic conception of methanol synthesis coupled with WGSR is proposed and proved. It produced a new theoretical kinetic model that successfully interprets all available experimental data, including anomalous kinetics for CO-enriched feed. Acknowledgments This work was supported by Russian Foundation of Fundamental Researches (Grant # 98-03-32186) and New Energy and Industrial Technology Development Organization (NEDO) of Japan (Grant # 98EA4).

REFERENCES 1. Yu.B. Kagan, L.G. Liberov, E.V. Slivinskii et al., Doklady AN SSSR (Rus.), 221 (1975) 1093. 2. Yu.B. Kagan, A.Ya. Rozovskii, L.G. Liberov et al., Doklady AN SSSR (Rus.), 224 (1975) 1081. 3. A.Ya. Rozovskii and G.I. Lin, Methanol Synthesis Fundamentals, Khimiya, Moscow, 1990. 4. G.C. Chinchen, P.J. Denni, D.G. Parker, K. Waugh, Appl. Catal., 30 (1987) 333. 5. C.A. Vedage, R. Pitchai, R.G. Herman, K. Klier, in" Proc. of 8th Int. Congr. on Catalysis, Berlin, 1984, Vol. 2, 47. 6. G.I. Liu, D. Willcox, M. Garland, H.H. Kung, J. Catal., 96 (1985) 251. 7. F. Lepeltier, P. Chaumette, J. Saussey, M.M. Bettahar, J.C. Lavalley, J. Mol. Catal. A-Chem., 132 (1998) 91.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

"Real" and "inverse" model catalysts for studies of metal-support interactions: CO hydrogenation on titania and vanadia supported Rh Wolfgang Reichl and Konrad Hayek Institut ffir Physikalische Chemie, Universit~t Innsbruck Innrain 52a, A-6020 Innsbruck, Austria In view of the well-known promoting effect of reducible oxides on the performance of noble metal catalysts it was attempted to elucidate the kinetics of the reaction of hydrogen and CO on clean and oxide-modified Rh surfaces, and in particular to asses the effect of the metaloxide boundary. Inverse supported metal catalysts were prepared by UHV deposition of Ti and V metal on the Rh surface in the presence of oxygen. After characterisation by AES and reduction in hydrogen the methanation kinetics was measured in an externally attached high pressure cell. The reaction is promoted by submonolayer quantities of titania or vanadia, but the reaction rates depend on the extent and temperature of hydrogen reduction. This dependence can be correlated with concomitant changes of the layered structure of the VOx deposits, and finally with the partial dissolution of V in the bulk, if the reduction temperature is increased. I. INTRODUCTION It is well-known that reducible transition metal oxides may have a strong influence on the catalytic properties of group VIII metals. In particular, a high-temperature reduction suppresses.the catalytic activity in hydrocarbon reactions [1,2], while reduction at lower T usually promotes reactions of hydrogen with CO or with molecules containing C-O bonds [3,4]. Different types of model catalysts have been adapted to study this kind of metal-support interaction. In models for "real" catalysts noble metal particles may be either deposited on a thin film of the oxide [5], or regular nanoparticles may be partly embedded in the respective oxide film [6,7]. "Inverse" models for supported catalysts are obtained by depositing submonolayers of reducible oxide on a single crystal or polycrystalline surface of the metal. It is generally assumed that in these metal-oxide systems the phase boundary plays a dominant role as site of the catalytic activity. This implies that the activity must be directly related to its dimensions, either to the perimeter of the metal particles on the oxide support, or to that of the oxide islands on the metal support, respectively [8,9]. In this paper we want to present an approach to study metal-support interactions with emphasis on the interface, attempting to assess the effect of metal-boundary sites by comparing reaction rates on "real" and "inverted" models of a catalytic system. Model samples of identical geometry were prepared and characterised under ultrahigh vacuum conditions, and their catalytic activity was investigated in a specially designed high pressure cell providing a clean environment [ 10,11 ]. Additional single crystal studies were performed in a molecular beam reactor, and the effect of oxide coverage on the reactivity of Rh (111) surfaces could be determined in parallel [12].

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The reaction of CO and hydrogen to methane is an excellent test reaction because it has been studied on numerous catalysts of various structure, morphology, surface composition and reduction state, and a great amount of kinetic data is therefore available [e.g. 3,4,13-15]. On the other hand, it is well-known that the most probable initial step, the dissociation of molecular CO, is very unlikely on the densely packed Rh faces [16]. The rate enhancement by reducible oxides will therefore be primarily due to promotion of CO dissociation, probably near the metal-oxide interface. A quantitative assessment of interface contributions is complicated by the fact that surface restructuring may affect real and inverse model catalysts in a different way, even under identical experimental conditions. After treating the restructuring of TiOx-supported Rh particles extensively in previous work [6,9] we want now to present the first results of a study of the changes of inverse catalysts under hydrogen reduction, and of their effect on the CO hydrogenation. Following in principle the procedure of Somorjai et al. [3,4], a Rh metal surface was partly covered with submonolayers of titania and vanadia. Thereafter, the surface composition and structure and the catalytic activity were determined as a function of reduction conditions. 2. EXPERIMENTAL The kinetic experiments were carried out in a pyrex glass high pressure reactor externally attached to an ultrahigh vacuum (UHV) chamber. The reaction cell has a volume of about 65 ml and experiments were performed at 1 bar total pressure. The reactants were introduced into the preheated reactor and circulated by means of a metal bellows pump. Samples were taken periodically via a sample loop valve (200pl). The products were separated and analysed by gas chromatography with a mass selective detector. The UHV system is equipped with two evaporation sources, a quartz microbalance, a sample heating stage and an Auger electron spectrometer. The apparatus is designed to transfer the catalyst sample between UHV chamber and reaction cell without any connected holders or wires. Due to this fact and to the all-glass construction of the reactor, background reactions were completely excluded, and the effect of impurities like sulphur could be controlled very carefully. The experimental setup is described in detail in refs. [ 10] and [ 11 ]. The samples were ultrapure metal sheets (Rh foil, 99,999%, 0.1 mm thick and 2x2cm 2 in area). Both sides of the sample could be prepared as catalyst surfaces, yielding a relatively large surface area of uniform composition. The metal foil was cleaned by cycles of Ar + sputtering and annealing in oxygen before the transition metal oxide was deposited. Titania and vanadia overlayers were prepared by evaporation of the respective metal wire (99,9%) in 2"10 -7 mbar of oxygen, followed by annealing in oxygen at 623 K. Sample cleanliness and composition were checked by Auger electron spectroscopy. Prior to every kinetic test the catalysts were subjected to a 10 min. oxidation, either at 200 mbar 02 in the high-pressure cell or at 2"10 -7 mbar 02 in the UHV chamber. This oxidation was then followed by a hydrogen reduction for 10 minutes, also either at 200 mbar or in the 10-7 mbar range, with the reduction temperature varied between 373 and 953 K. It is interesting to note that the catalytic activity was almost independent of the hydrogen partial pressure (UHV or "high pressure"), but it was strongly influenced by the reduction temperature, as shown below.

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d (TiOx) =

13

4I

IAI

& e~

i 6

E

{

2

1

~ O

2

0

0,5

I

1

I

1,5

~o

I

2

9 this work

1 r

Fig. 1. Effect of titania coverage on the rate of methane formation at 573 K, from 40 mbar CO and 120 mbar H2, He added to 1 bar. The results are compared to those of Levine et al. [3].

i

i

1

i

,

0,1

0,2

0,3

0,4

0,5

0,6 nominal metal coverage [ML]

2,5

0 ( T i O 0 IML] zx Levine et al.[3]

0 [ 1 ! [

[] bare Rh 9 VOx - "as grown" 9 VOx - prereduction @373K 9 VOx - prereduction @673K

Fig. 2. Effect of V coverage on the rate of methane formation. Reaction cond. as in Fig. 1. Hydrogen reduction (200 mbar, 10 min) at 373 or 673 K was performed after oxidation at 673 K.

3. RESULTS AND DISCUSSION A survey of the experimental results is presented in Figs. 1 and 2, where the initial reaction rates are plotted as a function of nominal metal oxide coverage. The titania coverage was based on the assumption of a stoichiometry of TiO2 [3,4], whereas the stoichiometry of vanadium oxides is more variable under reduction, and therefore the metallic state was chosen as a reference ("1 ML" vanadium corresponds to 1,58" 1015 surface metal atoms / cm2). Plots of the Auger signal versus deposition time indicate a 2D growth mode of the oxide in the submonolayer range. (For VOx on P d ( l l l ) 2D growth has been verified by STM measurements [17]). The H2/CO ratio for the kinetic tests was 3:1. On the titania covered surface the results were in good agreement with those of Levine et al. [3] under identical hydrogen pretreatment. Changing the pretreatment conditions led to changing reactivity on either Rh/oxide system. In the as-grown state the deposit was less active than the bare Rh surface, but already upon prereduction at 373K an increase of the initial reaction rate could be observed, as seen in Figs. 2 and 3 for the VOx/Rh system. The maximum rate increase (about 2.5 times that of the clean Rh surface) was observed at a V coverage between 0.25 and 0.3 ML. This is in good agreement with the observations of Boffa et al. [4], because 0.28 ML V corresponds about to half a monolayer of vanadium oxide. At 0.5 ML V coverage the activity was somewhat smaller than that of the bare surface. Increasing the reduction temperature to 473 and 573 K (always subsequent to oxidation at 673 K, as mentioned before) did not induce further activity changes, but upon further reduction at 673 K the activity was again found to increase. In Figs. 3 and 4 the methane yield at 573 K is plotted vs. time for the bare metal surface and for a V/Rh surface ratio of 0.13 and 0.5. On the bare surface the reaction rate was almost independent of conversion, and no deactivation was observed with reaction time.

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722

3 J Iv -013!

o (7)- prereduction @473K 9 (6)- prereduction @893K

2

5

9 (5)- prereduction @773K

3

9 (4)- prereduetion @373K

1

2,4

1

zx(3)- prereduction @673K o (2)- prereduction @373K m (1)- bare Rh

0 0

20 40 time [mini

60

Fig.3 Methane yield from 40 mbar CO and 120 mbar H2, He added to 1 bar, T=573K.V/Rh surface ratio is 0.13. Hydrogen reduction steps (2,0"10 -7 mbar H2, 10 min) are numbered consecutively and were always performed after oxidation. On the contrary, the activity of oxide covered rhodium decreased with time, depending on the hydrogen excess in the reacting gas. This deactivation was connected with carbon deposition on the surface, implying that more CO was dissociated than carbon was consumed by the reaction. On the bare surface carbon buildup during the reaction was almost negligible in high excess of hydrogen (Fig. 5). On Figs. 3 and 4 the sequences of reduction steps (always separated by oxidation at 673 K) are consecutively numbered. The reaction rates could be well reproduced even if two identical low-temperature reductions were separated by one or more reduction steps at higher temperature (below 673 K). The variation of the intensity of the vanadium and rhodium Auger signals due to reduction up to 673 K was not significant while the intensity of the oxygen signal was somewhat decreased (Fig. 6). Increasing the reduction temperature above 673 K, e.g. to 893 K in Fig. 3 and up to 953 K in Fig. 4, both under 10-7 mbar hydrogen pressure, led to a further strong increase of 1,5

1

I-0.51

M

o w

A:6

9 (6) - prereduction@ 953K [] (1) - bare Rh 9 (5) - prereduction@ 673K

m o~

0,5

~

3,5 2,4

zx(3) - prereduetion@ 673K 9 (4) - prereduction@ 373K o (2) - prereduction@ 373K

o

0

20

40

time [mini

60

Fig. 4. Methane yield from 40 mbar CO and 120 mbar H2, He added to 1 bar, T=573K. The V/Rh surface ratio is 0.5. Hydrogen reduction steps (2,0" 10.7 mbar H2, 10 rain) are consecutively numbered and were always performed after oxidation at 673 K.

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

723

~

b

~

e

W

Y

\1 \

W

Rh 9

16o

'

260

'

V

\Rh

C

36o

'

46o

'

=

VandO

s6o

'

I 0ll\0

V

V

400"489 440"460"480 560"589 5,~0 E [eV]

e

E [eV]

Fig. 5. Auger spectra taken after 90 min. reaction: (a) bare Rh,(b) Rh + 0.13 ML V, (c) Rh + 0.28 ML V, (d) Rh + 0.5ML V, after 12 min, (e) Rh+0.5 ML V after 90 min.

IT V

Fig. 6. Auger spectra of the O and V region taken after reduction in 2,0" 10.7 mbar H2 for 10 min at (a) 373 K, (b) 673 K, (c) 953 K.

the initial activity at a coverage of 0.13 (Fig. 3, up to about 5.7 times that of the bare Rh surface), while the "0.5 ML" catalyst regained the activity of the bare surface (Fig. 4). After this high temperature reduction no oxygen could be detected on the surface, and the vanadium Auger signals were strongly reduced, as is seen in Fig. 6. This indicates the formation of a Rh-V alloy with vanadium partly diffused into the bulk. After a HTR above 773 K the vanadia coverage and the catalytic activity were not fully reversible under subsequent oxidation and low-temperature reduction, i.e. the original V coverage could not be reestablished by simply re-oxidising the sample at 673 K. Furthermore, in the highly reduced state deactivation by carbon was found to be less effective than after low-temperature reduction. We therefore conclude that upon reduction below 673 K the promotion of the reaction occurs indeed at the perimeter of the vanadium oxide islands (probably V203 [17,18]), as previously assumed [3,4], and that the rate depends on the effective length of the boundary. However, after reduction above 773 K the reaction is probably promoted by small amounts of vanadium metal present in the first or second Rh surface layer. Table 1 Comparison of some rhodium model catalysts: activity (R/R0) and selectivity. catalyst

R / R0 a

Selectivity [%]

CH4

C2

C2-

C3

C3-

Rh bare 1 98 2 Rh + 0.7 ML TiO2 2.5 82 6 10 2 Rh + 0.28 ML V 2.8 72 14 3 4 7 Rh + 0.5 ML V 0.75 55 13 15 2 15 a methane rate compared to bare rhodium; reaction conditions: see Fig. 1; reduction @ 673 K in 2* 10.7 mbar H2.

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724

Finally, as seen in Table 1, the selectivity towards production of higher hydrocarbons has increased as a function of titania coverage, and even more so of vanadia coverage. At "0.5 ML V" and a CO/H2 ratio of 3, the proportion of alkanes and alkenes (ethane, ethene, propane and propene) in the products has already increased to 45%, thereby proving the applicability of vanadia promoted rhodium to Fischer-Tropsch catalysis. 4. CONCLUSIONS The promotion of the CO hydrogenation on polycrystalline rhodium by submonolayers of reducible oxides was examined in a clean environment under atmospheric pressure and carefully controlled reduction conditions. The reaction rates were a reproducible function of the reduction state of the coveting vanadium oxide, but promotion of the reaction was not necessarily confined to oxidic vanadia. The highest rate enhancement was observed upon reduction above about 773 K, with only metallic vanadium present in the near-surface layers of rhodium. A further systematic study of the respective alloy surfaces is probably needed to understand this promotional effect which may be a promising topic for future work. ACKNOWLEDGEMENT

This work was supported by the Joint Research Project "Gas Surface Interactions" (S 8105) of the Fonds zur F6rderung der wissenschaftlichen Forschung of Austria. REFERENCES ~

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

S. J. Tauster, S.C. Fung, J. Catal., 55 (1978) 29 G. L. Haller, D. E. Resasco, Adv. Catal., 36 (1989) 173. M.E. Levine, M. Salmeron, A.T. Bell, G.A. Somorjai, J. Catal., 106 (1987), 401. A.B. Boffa, C. Lin, A. T. Bell, G.A. Somorjai, Catalysis Letters, 27 (1994), 243. H. Glassl, K. Hayek, R. Kramer, J. Catal., 68 (1981) 397. G. Rupprechter, K. Hayek, H. Hofmeister, J. Catal. 173 (1998), 409. G. Rupprechter, G. Seeber, H. Goller, K. Hayek, J. Catal. 186 (1999), 201. J.D. Bracey, R. Burch, J. Catal., 86 (1984), 384. K. Hayek, R. Kramer. Z. Pafil, Applied Catal. A: General, 162 (1997), 1. K. Hayek, M. Fuchs, B. K16tzer, W. Reichl, G. Rupprechter, Topics in Catalysis, in press. W. Reichl, G. Rosina, G. Rupprechter, C. Zimmermann, K. Hayek, Rev. Sci. Inst., in press. B. K1ftzer, K. Hayek, this conference. D.W. Goodman, R.D. Kelley, T.E. Madey, J.T. Yates, J. Catal. 63 (1980) 226. R.D. Kelley, D.W. Goodman, Surface Sci. 123 (1982) L743. V. Ponec in Handbook of Heterogeneous Catalysis, G. Ertl, H. Kn6zinger, J.H. Weitkamp Eds., Verlag Chemie, Weinheim 1997, Vol.4, p. 1876 ft. J.T. Yates, E.D. Williams, W.H. Weinberg, Surface Sci.91 (1980) 562. F.P. Leisenberger, S. Surnev, L. Vitali, M.G. Ramsey, F.P. Netzer, J. Vac. Sci. Technol. A 17 (1999) 1743. T. Hartmann, H. Kn6zinger, Z. Phys. Chem. 197 (1996) 113.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Agglomeration of Pt Particles and Potential Commercial Application for Selective Hydrodechlorination of CC14 Z. C. Zhang*, B. C. Beard Akzo Nobel Chemicals, Inc., 1 Livingstone Ave., Dobbs Ferry, NY 10522, USA Tel: (914) 674-5034; Email: [email protected] Supported Pd and Pt catalysts were evaluated for hydrodechlorination (HDC) of CC14. A durable and selective catalyst is desired for converting CC14 to CHC13. Pd/C showed low selectivity for CHC13, with CH4 as the main product, in addition to C2C16. While commercially available Pt/A1203 egg-shell catalyst showed encouraging selectivity for CHC13, it became rapidly deactivated within 1/2 hr on stream. A novel catalyst pretreatment with an aqueous NHaC1 solution was developed that induced long term (>2,000 hr) durability of the Pt/A1203 catalyst for selective HDC of CC14 to CHC13. Platinum clusters in commercial 0.3wt% Pt/A1203 catalyst are predominantly smaller than 1.5 nm and are stable against thermal sintering at temperatures as high as 500~ in H2. These clusters exhibited a high degree of electron deficiency. After treating it with NH4C1, however, the agglomeration of Pt primary particles took place in H2 at temperatures as low as 320~ as a result of de-anchoring of Pt clusters from Lewis acid sites. The small Pt clusters are highly susceptible to HC1 poisoning. On the other hand, the agglomerated large Pt particles exhibited high resistance toward HC1, therefore affording a stable catalyst for the HDC of CC14. The selectivity for CHC13 over the supported large Pt particles was also improved. A commercial viable process for the selective HDC of CC14to CHC13 was developed.

Introduction The production and use of carbon tetrachloride (CC14) has been phased out according to the Montreal Protocol effective as of January 1, 1996, due to its high potency in depleting the stratospheric ozone layer [ 1]. However, in the production of CH2C12 and CHC13, CC14is still formed as a major by-product. The disposal of this CC14waste by-product, typically by incineration, has become an environmental challenge and a major economic burden to manufacturers of CH2C12/CHC13. As CHC13 is much less destructive than CC14 for stratosphere ozone layer and is used as a precursor for the manufacture of replacement HCFC's and fluorinated polymers, an attractive alternative to CC14 disposal is to convert it to CHC13 by a catalytic HDC process. However, the selectivity to CHC13 can be limited due to a high exotherm toward CH4 in the process. In addition, a catalyst has to be resistant to HC1, a byproduct of the HDC process. *Author to whomcorrespondence on this paper shouldbe addressed.

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Previous researches on the subject dating back to the early seventies as well as recently indicated prolonged catalyst life only at a rather low conversion level and at a high H2/CC14 ratio [2,3,4]. Such conditions would preclude commercial viability of this process. Supported Pt catalysts were reported with reasonable selectivity to CHC13 in liquid phase (typically under pressure) [5,6] and vapor phase [3,4,7] CC14 HDC. However, heavier byproducts (C2C14 and C2C16) are usually produced. In addition, the catalysts in vapor phase reaction typically deactivated over a short period. Alumina supported Pt catalysts have been extensively used in many important industrial processes including the abatement of automotive and industrial plant emissions, naphtha reforming [8,9], etc. Techniques for preparing supported noble metal catalysts are typically chosen to achieve high metal dispersions in order to obtain high activity for a given metal loading. For various applications, the sintering of metal particles has been a major cause for catalyst deactivation [ 10,11 ]. Therefore, re-dispersion of sintered metals on deactivated catalysts such as Pt/A1203 has been the theme of many studies [ 12,13,14]. In this paper, we report the performance of several commercial noble metal catalysts for CC14 HDC. In particular, a commercial 0.3wt% Pt/A1203 egg-shell catalyst was studied that led to intriguing discoveries on the reductive agglomeration of Pt in the presence of NHaC1 and on the durability and selectivity of the catalyst as a result of the Pt particle agglomeration.

2. Experimental 2.1.

Catalysts and Pretreatment A 0.5% Pd/C and a 1.0% Pd/C were from Engelhard Inc. No additional pretreatment was performed on these catalysts except standard activation in H2 prior to catalyst evaluation as described below. A Pt/A120 3 pellet (1/8") catalyst with 0.3 wt% Pt was obtained from Johnson Matthey (JM). Platinum is predominantly distributed at the exterior of the pellets as a thin dark layer, and therefore they are often referred to as an "egg-shell" type catalyst. The fresh catalyst is also referred to as "untreated catalyst" to distinguish it from the NHaC1 treated catalyst as described below. NHaC1 treatment was performed on the JM Pt/A1203 catalyst by soaking it in a saturated solution of NHaC1 at ambient temperature for 89hr. The excess solution was drained, followed by drying at 100~ over night in air. The catalyst pretreated with NHaC1 is hereat~er referred to as "treated catalyst" or "NHaC1 treated catalyst". 2-2. Activation and Reaction Catalyst were evaluated in a fixed bed glass reactor with a frit to support 1.0 g of Pt/A1203 catalyst (for Pd/C catalyst, 0.5g) and a concentric thermocouple to measure the catalyst bed temperature. Prior to catalytic evaluation, each catalyst was treated with H2 at a flow rate of 20 ml/min at 350~ for 2 hr and then cooled to 90~ in H2. The HDC reaction was started at 90~ and 1 atm by passing a mixture of H2 with vaporized CC14 (25 ml/min in total) at a H2/CC14 molar ratio of 7.0 through the reactor. An on-line gas chromatograph (Varian 3400) with a DB-1 capillary column and flame ionization detector was used to analyze the downstream gases.

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2-3. X-ray Photoelectron Spectroscopy (XPS) Surface compositional analysis of the catalyst pellets was performed with a Physical Electronics 5600ci x-ray photoelectron spectrometer. Due to overlap of the strong Pt(4f) peaks with those of Al(2p) from the support, the Pt(4d,5/2) peak was used to evaluate the concentration and chemical state of the active Pt phase. Sample charging correction was performed based on the C(1 s) peak from adventitious hydrocarbon at 284.4 eV. Charge correction consistently yielded an Al(2p) binding energy of 74.6 + 0.1 eV, as expected for alumina. In-situ activation of the pellets (2 hrs, 350~ was performed in a side chamber attached to the XPS instrument, equipped with a sample heater, manifold, thermocouple, and vacuum pumps. Details in sample handling can be found eslewhere [ 15].

3. Results 3.1. Pd/C catalysts The results obtained with Pd/C catalysts for CC14 HDC are shown in Figures 1 and 2. The conversion of a 1% Pd/C at 90~ decreases rapidly on stream due to catalyst deactivation. The activity of a 0.5% Pd/C is less than 5% under similar conditions and CH4 was the only product detected. The selectivity to CHC13 on the 1.0% Pd/C was 20% with 80% CH4. Conv (%) 100

Selectivity

100 d/C 1%Pd/C -o-

8O

80

6O

60

4O

40

20

20

00

,

t

-----

~

,

~

,

~

~

t

,

~

,

~

,

20 40 60 80 100 120 140 160 Time on Stream (min)

Figure 1. Conversion of Pd/C catalysts for CC14 HDC

(%)

II

CHCl3

0.5%Pd/C 1%Pd/C 9

CH3Cl Products

CH4

Figure 2. Selectivity of Pd/C catalysts for CC14 HDC

3.2. Pt/A1203 catalysts 3.2.1. Untreated Pt/A1203 catalyst The fresh commercial 0.3% Pt/A1203 egg-shell catalyst that was activated at 350~ in H2 showed an initial conversion of CC14 over 90%. But the catalyst was rapidly deactivated; the conversion dropped to <2% within an hour (Fig. 3). The initial selectivity to CHCI3 was 60%, with 30% to CH4 and 10% to C2C16(Fig. 4). 3.2.2. NH4C1 treated Pt/A1203 catalyst Atter treating the commercial 0.3% Pt/A1203 catalyst with NH4C1, the catalyst showed remarkable durability for CC14 HDC. As shown in Fig.5, the conversion maintained at over

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80% for 70 hour without deactivation. The selectivity on the treated catalyst was 80% for CHC13 and 20% for CH4. As has been reported elsewhere, the catalyst was even stable for over 2000 hr in both laboratory and pilot plant scale tests [ 15]. Conversion (%)

Selectivity (%)

70

100

60

80

50

6O

40

40

,o

2O

O0

,==

20

40 60 80 Time on Stream (min)

100

Figure 3. Conversion of untreated Pt/A1203 catalysts for CC14 HDC at 90~

20 10 0

CHCI3

Products

C2CI6

Figure 4. Selectivity of untreated Pt/A1203 catalysts for CC14 HDC at 90~

Conversion (*/,)

Catalyst R(4d,5/2) Binding Energies

100 80

II

CH4

> 315.3 ~'~"~'~'-

............................ * - ........... 9

m

~ 314.9 L~ 314.5

60

~

m

314.1

40

i~ 313.7

20 O0

,

1

,

I

,

1

20 40 60 Time on Stream (hour)

J

80

Figure 5. The conversion of NH4C1 treated 0.3% Pt/A1203 catalyst for CCI4 HDC at 90~

a_

a_a~

I-ae

Figure 6. Pt(4d, 5/2) binding energies of Pt/A1203 catalysts

3.2.3. Pt particle size and Pt (4d, 5/2) binding energy The TEM results as reported earlier [ 15] showed that Pt particles in the untreated Pt/A1203 catalyst are predominantly smaller than 1.5 nm, even after activation in Hz at temperatures as high as 500~ On the NHaC1 treated catalyst after activation at 320~ the Pt particles are narrowly distributed at 5-8 nm range. It was concluded that Pt particle agglomeration in the presence of NHaC1 was due to the de-anchoring of primary Pt particles from the strong Lewis acid sites and the formation of mobile chlorinated Pt precursors on the catalyst surface. XPS data of the untreated and NH4C1 treated Pt/A1203 catalysts are shown in Fig. 6. For the untreated Pt/A1203, the binding energy of Pt after activation in H2 is 314.4 eV. In comparison, the binding energy of Pt on the NHaC1 treated Pt/A1203 catalyst is 313.9 eV, which is identical to that of bulk Pt metal.

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4. Discussion 4.1. Catalyst performance As shown in Fig.2, the Pd/C catalysts produce mainly CH4 for CC14 HDC. In addition, the more active 1.0% Pd/C catalyst was found to rapidly deactivate during reaction. The activity of the 0.5% Pd/C was considerably lower than that of the 1.0% Pd/C. A selectivity of 60% to CHC13 on the untreated commercial 0.3% Pt/A1203 catalyst indicated that Pt/A1203 is a better catalyst for the desired product. However, it became rapidly deactivated (Fig. 3). In addition, C2C16was produced as a byproduct, as well as CH4. Most strikingly, the NHaC1 treated Pt/A1203 catalyst not only became stable, it was also more selective for CHC13. The selective for CHC13 reached 80% on this catalyst. In addition, no heavy byproduct, such as C2C16, was produced. While Pt on different support materials showed different rates of deactivation, [2] the NHaC1 treated Pt/A1203 catalyst represents the most selective and stable catalyst ever reported for the HDC of CC14 to CHC13. The dramatic difference in catalytic performance for CC14 HDC between the untreated and NHaC1 treated Pt/A1203 catalysts is mainly due to Pt particle size effect. While small Pt particles smaller than 1.5 nm are readily poisoned by HC1 that was produced from the reaction, the metal-like large Pt particles of 5-8 nm are highly resistant to HC1. 4.2. Agglomeration of primary Pt particle in the presence of NH4C1 Sintering of Pt particles of 4 nm size on A1203 support in pure oxygen was reported at a higher temperature (600~ after a longer heating period [ 16]. In this work, it is found that thermal sintering in pure H2 did not take place even after treatment at 500~ The stability of the small Pt clusters at this high treatment temperature is attributed to their strong interaction with the predominantly Lewis surface acid sites. Such interaction is proposed to be the cause for the strong anchoring of the Pt clusters onto the alumina surface. Compared to bulk Pt metal, the small Pt clusters are highly electron deficient, as indicated by its higher electron binding energy from XPS measurement. The electron deficiency may be ascribed to two sources. (1) Strong interaction with Lewis acid sites leading to a profound electronic polarization of the Pt clusters, forming a stable Ptn+AI(O)3 local structure, analogous to Ptn-H + adduct as proposed for electron deficient Pt particles in zeolites [17]. And (2) the intrinsic electronic character of small metal particles, which is correlated to electron binding energy [18,19,20]. It is particularly significant that Pt in the NHaC1 treated Pt/A1203, prior to activation, has the highest BE ( 315.3 eV). This increase of Pt BE as a result of NHaC1 indicates that an interaction of small Pt clusters with NHnC1 has already taken place. Such an interaction may have set the stage for Pt agglomeration during subsequent thermal treatment in H2. It is possible that some PtnClx, e.g. in the form of Cl'Ptn + Al(O)3 was formed. The negatively charged chloride ions may induce the deanchoring of Ptn clusters, which is responsible for the agglomeration of primary Pt clusters [21 ]. Upon activation at 320~ the Pt particles agglomerated to a narrow distribution of 58 nm in the presence of NH4C1. The agglomerated 5-8 nm Pt particles exhibit purely metallic Pt characteristic, as indicated by XPS results. At the initial low temperature, during the agglomeration process, ammonia could play a role in neutralizing the acid sites. However, no significant Pt agglomeration was observed

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by treating the Pt/A1203 catalyst with NH4OH and NH4NO3. The experimental results indicate that ammonium ions alone are insufficient to cause Pt cluster agglomeration. The fact that LiC1 can also result in Pt cluster agglomeration (results not shown in this paper) is a further confirmation on this point.

5. C o n c l u s i o n s HDC of CC14 on Pd/C catalysts produced mainly CH4. In addition, an active 1.0% Pd/C catalyst deactivates rapid on stream. A batch of typical commercial 0.3wt% Pt/A1203 egg-shell catalyst was found to deactivate rapidly due to HC1 poisoning. C2C16 was produced as a byproduct. The Pt particles on the catalyst are predominantly smaller than 1.5 nm. The high stability of the small Pt particles on this catalyst against thermal sintering at 500~ suggests the formation of stable Ptn+AI(O)3 local structure as the cause for the strong anchoring of small Pt clusters at the A1203 surface. Treatment with NHaC1 resulted in highly stable catalyst for CC14HDC with 80% selectivity for CHC13, balancing CH4. The increased durability is ascribed to the increased Pt particles in 5-8 nm due to NHnC1 induced agglomeration of Pt at 320-350~ under a reducing environment (H2). The agglomeration is ascribed to the deanchoring of Ptn clusters from surface Lewis acid sites by chloride ions.

6. REFERENCES 1 United States Environmental Protection Agency Air and Radiation Stratosphere Division, Section 606 of the Clean Air Act Amendments of 1990 (1993) 2 H.C. Choi, S. H. Choi, J. S. Lee, K. H. Lee, Y. G. Kim, J Catal., 166 (1977) 284 3 A.H. Weiss, B. S. Gambhir, R. B. Leon, J. Catal., 22 (1971) 245 4 A.H. Weiss, S. Valinski, J. Catal., 74 (1982) 136 5 S. Morikawa, M. Yoshitake, S. Tatematsu, US Patent 5,334,782 (1994) 6 E.T. Miquel, J. M. S. Gimenez, A. C. Arroyo, X. L. S. Gomez, A. A. Martin, US Patent 5,208,393 7 S.Y. Kim, H. C., Choi, O. B. Yanga, K. H. Lee, J. S. Lee, Y. G. Kim, 3. Chem. Soc., Chem. Comrnun., 2169 (1995) 8 J.P. Brunelle, A. Sugier, and J. F. le Page, J. Catal., 43 (1976) 273 9 C. Corolleur, S. Corolleur, D. Tomanova, and G. Gault, J. catal., 24 (1972) 385 10 G.A. Mills, S. Weller, E.B. Comelius, Proc. 2nd Int. Congr. Catal., p.2221 (1960) 11 S.F. Adler, J. J. Keavney, J. Phys. Chem., 64 (1960) 208 12 I. Guo, T. T. Yu, and S. E. Wanke, in Catalysis, J. W. Ward ed., p21 (1987) 13 E. Ruckenstein, Y. F. Chu, J. Catal., 59 (1979) 109 14 E. Ruckenstein, M. L. Malhotra, 41 (1976) 303 15 Z.C. Zhang, B. Beard, Applied Catalysis, 174 (1998) 33 16 A. Bellare, D. B. Dadyburjor, and M. J. Kelley, J. Catal., 117 (1989) 78 17 W.M.H. Sachtler, A. Y. Stakheev, Catalysis Today, 12 (1992) 283 18 R.C. Baetzold, M. G. Mason, and J. F. Hamilton, J. Chem. Phys 72(1) (1980) 366 19 M.G. Mason, Phys. Rev., B, 27(2) (1983) 27 20 J. Goetz, M. A. Volpe, A. M. Sica, C. E. Gigola, and R. Touroude, J. Cat., 153 (1995) 96 21 Z.C. Zhang, B. C. Beard, Appl. Catal., 188 (1999) 229

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Catalytic Diesel Soot Elimination on Co-K/La203 Catalysts: Reaction Mechanism and the Effect of NO Addition E.E. Mir6, F.Ravelli, M.A.UIIa, L.M.Cornaglia, C.A. Querini Instituto de Investigaciones en Cat/disis y Petroquimica- INCAPE- Fac.Ing. Quimica (FIQUNL, CONICET) - Santiago del Estero 2654 - (3000) Santa Fe - ARGENTINA Catalysts containing Co and/or K supported on La203 have been studied for diesel soot catalytic combustion. While supported Co provides redox sites for the reaction, potassium and the support itself contribute to create additional sites for soot consumption by forming carbonates intermediates. The formation of a perovskite structure after high temperature treatment leads to the lost of activity. The presence of NO in the gas phase improves the catalytic activity for soot elimination. NO is oxidized to NO2 on the catalyst surface, and NO2 is a stronger oxidizing agent than 02, therefore decreasing the temperature needed to burn the soot. 1. INTRODUCTION Particulate matter and NOx are the main pollutants in diesel engine emissions. The combination of traps and oxidation catalysts appears to be the most plausible after-treatment technique to eliminate soot particles (1). Since the temperature of typical exhausts is 400~ or below in light duty applications, and diesel stack temperatures can exceed 600~ at full load even for a turbocharged/afiercooled engine, a potentially useful catalyst has to operate efficiently at low temperatures and be thermally stable. Studies with a large number of formulations have been reported during the last few years. However, the reasons for the better or poorer performance of a given material have been scarcely studied. We have previously reported studies with Co-K/MgO catalysts, establishing a relationship between the catalytic behavior for soot combustion and the properties as determined by a variety of characterization techniques (2,3). It is the objective of this work to establish such relationship for Co-K/La203 catalysts, in order to determine the mechanisms of catalytic soot combustion. 2. EXPERIMENTAL Catalysts were prepared from a La203 suspension in water, to which C o ( N O 3 ) 2 and/or KOH were added, in order to obtain 12wt.%Co and 4.5 and 7.5wt.%K. After evaporation and drying, the samples were calcined at 400 and at 700~ The soot was obtained by burning commercial diesel fuel (YPF, Argentina) in a glass vessel, as described in (2). TPR experiments were carried out in an Okhura TS-2002 instrument. The XPS spectra were obtained at room temperature with a computer-controlled Shimadzu ESCA 750 instrument, using MgK~ radiation. X-ray diffractograms were obtained with a Shimadzu XD-D1 instrument with monochromator using CuK~ radiation. In order to study the interaction of the

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catalyst surface with CO2, pulse experiments were carried out. At selected temperatures, consecutive pulses (0.25 cc) of 1.2% of CO2 were sent to the reactor. The catalytic activity for the combustion of soot was determined from TemperatureProgrammed Oxidation (TPO), of carefully prepared mixtures of catalyst and soot (20:1). To study the effect of nitric oxide, some experiments were carried out at constant temperature. These were performed by heating the sample under helium flow until the desired temperature was reached. Then, the feed was switched to the oxygen-nitric oxide-helium reacting mixture. Samples were taken and stored in a sixteen-loop Valco valve for chromatographic TCD analysis. 3. RESULTS AND DISCUSSION

Table 1 shows activity results measured at 350~ hydrogen consumption during TPR experiments (carded out up to 600~ and XRD main phases obtained with catalysts calcined at 400 and 700~ (number between brackets). All the samples calcined at 400~ have poor crystallinity, showing only La202CO3 and La(OH)3 as the main XRD crystalline phases. This is not the case for samples calcined at 700~ which show better crystallinity. La203 and La(OH)3 peaks are seen in XRD spectra of K/La203 samples; and La203 , C0304 (trace level) and LaCoO3 (perovskite) in the case of Table 1 Soot burning rate, H2 consumption during TPR, and XRD main phases. Catalysts

K(400)

Reaction Rate b lamol/s, gsoot 1.33

K(700)

1.56

Co (400) Co (700)

Co-K(4.5) (400)

0.78 0.00 1.31 0.63

Co-K(4.5) (700) Co-K(7.5) (400) 1.29 Co-K(7.5) (700) 068 Co-K(4.5) (400) a 4 00 awith 0.5 % of NO in the feed. bmeasuredat 350~

H2 Consumption mmol/g 0.0 0.0 4.9 19 4.9 2.9 5.4 2.0 4.9

XRD main phases La202CO3, La(OH)3 La202CO3, La(OH)3 La202CO3, La(OH)3 La203, La(OH)3, LaCoO3 La202CO3, La(OH)3 La203, La(OH)3, LaCoO3 La202CO3, La(OH)3 La203, La(OH)3, LaCoO3 La202CO3, La(OH)3

Co,K/La203 samples. In the latter case, the crystallinity of the perovskite phase is favored by the increase of potassium content. Pure La203 burns the soot starting at 450~ The catalysts containing K are more active than Co/La203 catalysts. When the catalysts are treated at high temperature (700~ a decrease both in activity and in the amount of Co that can be reduced is observed, except in the case of K/La203. This catalyst displays a very good thermal stability. A shift in the TPR profiles (not shown) to higher temperatures is also observed upon the high temperature treatment. As said above, the XRD diffraction patterns of the catalysts shown in Table 1

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indicate that after the treatment at 700~ a perovskite-type structure is developed in those catalysts containing Co. Therefore, the lower amount of Co304 observed by TPR, is due to the formation of this mixed oxide compound, which does not reduce below 600~ This is the reason why Co,K/La203 deactivates while K/La203 does not, upon high temperature treatment. When Co is present, it reacts with the support forming perovskite with a decrease in the amount of surface La203 which as will be shown below plays a role during the soot burning reaction. We have previously suggested that one of the roles of K in Co-K/MgO catalysts is to consume C from the soot, to form an intermediate carbonate which then decomposes regenerating the K active site (3). La203 at 400~ interacts with CO2 similarly to CoK(4.5)/MgO and Co-K(4.5)/CeO2. Figure 1 shows results obtained in the CO2 pulse experiments. The highly distorted pulses of CO2 coming out from the cell containing Co/La203 or Co,K/La203 catalyst at 400~ indicate a strong interaction between CO2 and the catalyst surface. More likely, a carbonate type compound is formed, and as CO2 is leaving the reactor, the carbonate decomposes generating a broad peak. This behavior is not observed at low temperature. Since this behavior is also observed in absence of K (Fig. 1C), an important role of the support (La203) is to provide more active sites for soot consumption through the formation of a carbonate, which is a different role from that we have previously observed for the CeO2 supported catalyst. The role of the latter, is to provide oxygen for soot combustion, through a redox mechanism (4).

A

B

5

C

5

~ 3

25~

,ooocI 2- Cl

jl\

,ooocl

40

80

TIME,S EC

120

oooc/

C

3oocI

- ~I

~

54

0

i 40

TIME,S EC

" ~~176 80

120

3

25oc

~IJl \~----

2000c

/;

~I/-------~

I-------J

0L

40

....

i

80

....-

4-0-~c i~

12D

TIME,S EC

Figure 1:CO2 pulses at different temperatures on La203 supported catalysts. A: Co/ La203 ;B: Co,K/La203 ; C: La203.

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Table 2 shows the surface composition of each catalyst, as measured by XPS. The highest K/La ratio is found on K/La203, which has an activity similar to the Co-K catalysts (see Table 1). On the other hand, these catalysts have C0304 Which reduces with a main peak below 400~ The high temperature treatment leads to the deactivation of the La203 supported Co-K catalysts. The XPS results indicate that an enrichment in Co and K occurs alter such treatment. According to the XRD results, this is due to the formation of a Table 2 XPS signal intensity ratios for La203 supported catalysts Catalysts a

Calcination Temperature

Co/La

K/La

O/La

400 400 400 400 700 700 700

0.49 -0.16 0.10 0.42 -0.24 0.17

-0.75 0.09 0.12 -0.90 0.50 0.90

6.3 5.5 5.1 5.7 5.9 6.3 5.5 6.1

(~

Co/La203 K(4.5%)/La203 Co, K(4.5%)/La203 Co, K(7.5%)/La203 Co/La203 K(4.5%)/La203 Co, K(4.5%)/La203 Co, K(7.5%)/La203 "Co/La)bulk = 0.40

700

K/La)bulk = 0.23

perovskite oxide, which is not as active for this reaction as cobalt oxideand lanthanum oxide. The high temperature treatment increases the amount of potassium on the surface but due to the formation of the mixed oxide, it decreases the amount of active Co and La203. The final result is a loss in activity. It has to be emphasized that the support (La203) in this case plays an active role during the 0,02 reaction. Therefore, the formation of the perovskite NO 0.5% decreases the amount of free La203 and also the amount of C0304, decreasing both the 23 redox property and the ability O 0,01 to form carbonate-type O intermediates. Since nitrogen oxides and particulate matter emissions are the main pollutants present in diesel engine emissions, it is interesting to explore the 0,00 25O 300 350 400 450 500 550 60 possibility of the simultaneous Temperature (C~) NO and soot removal reaction. Figure 2: TPO analyses of mechanical mixtures of Shangguan et al (5) reported Co,K(4.5%)/La203 catalyst and soot. II soot +02 (6%)that perovskite-related oxides and spinel-type oxides are +NO(0.5%) reaction. D soot+O2 (6%) reaction. active for the above mentioned

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reaction. On the other hand, Mul et al. (6) reported that the addition of NOx enhances the catalytic carbon black oxidation rate, which is attributed to bifunctional catalysis: NO2 reacts with carbon black yielding NO and NO is reoxidized to NO2 over a transition metal oxide. In this work, we have found that the La203 - supported Co-K catalyst is effective for the simultaneous elimination of soot and NO. In Figure 2 it can be seen that there is a decrease of c.a. 25~ in the combustion temperature when NO is introduced in the feed, if compared with the soot combustion with oxygen. Additionally, the TPO profile becomes sharper. In the last row of Table 1 it can be seen that when 0.5% of NO are added to the feed, the soot combustion reaction rate increases three times, the NO being partially converted into N= and N=O. Figure 3 shows the evolution of the soot conversion under isothermal conditions when temperature or reactant concentrations are changed. As it can be seen, an induction period is present in most of the cases, suggesting the m situ formation of new active sites, probably from the interaction of NOx and the soot particles. In the same figure it is observed that soot conversion increased when either NO or O= were increased. It can also be seen ( Fig. 3B) that when oxygen is not present in the feed, the soot conversion is very low, indicating that NO is not a good oxidant and the formation of NO= through the NO + 89O= ~ NO= reaction is necessary. A fundamental role of the catalyst surface would be to increase the oxidation rate of NO to NO=, as suggested by Teraoka et al. (7). These authors also found that the doping of potassium to CuFe204 was effective in promoting the catalytic performance for the reaction under study. The catalytic performance of Cul.xKxFe204 significantly depends on the doping amount of potassium, and the highest activity and selectivity were simultaneously attained at 0,8

400 ~

/

Lt'r (D > r 0,4 o

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time (sec.)

Figure 3 : Isothermal combustion of soot. Mechanical mixture of Co,K/La203 and soot. (A) Effect of temperature. 6% 0 2 , 0.5% NO. (B) Effect of oxygen concentration. T=400~ 0.5% NO. (C) Effect of NO concentration. T=400~ 6% 02.

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736

x=0.05. From Fig. 2 it can be seen that our catalyst shows similar soot ignition temperature (c.a. 250~ if compared with the best potassium promoted CuFe204 studied by Shangguan et al. (7). According to this study, La203 supported catalysts have good activity for both soot and soot+NO elimination. Therefore, additional studies are now being carried out in our laboratory in order to obtain more information about the thermal stability of these catalysts and the influence of the potassium content on the catalytic properties. 4. CONCLUSIONS

La203 is a good support for Co and K to be used both for soot oxidation and for the simultaneous NO and soot removal. This support provides additional sites for soot consumption, by forming carbonates intermediates. Potassium, due to its high mobility, is much more efficient than La203 for improving the rate through this path. Cobalt provides additional oxygen by a redox mechanism. However, since all the support surface plays a role by forming carbonate-type intemediates, the influence of Co on the activity for soot combustion with oxygen is very low, K/La203 having as good activity as Co,K/La203 . Moreover, the formation of a perovskite structure LaCoO3 after high temperature treatment leads to activity loss. Then cobalt accelerates the deactivation of the La203 supported catalysts, and should not be present in an optimal formulation.

Acknowledgments The authors wish to thank Universidad Nacional del Litoral, CAID program and to ANPCyT (PICT N ~ 14-00000-00720 ) for its support to this project. Thanks are also given to Prof. E. Grimaldi for her help in the edition of the English manuscript, and to Mr. Claudio Maitre for his technical assistance. The experimental work and technical assistance of Ms Maria L. Pisarello and Clara Saux are also acknowledged. REFERENCES 1. J.Widdershoven, F.Pischinger, G.Lepperhof, SAE paper 860013, 1986.

2. C. Querini,M.Ulla, F.Requejo,J. Soria,U. Sedran,E.Mir6,Appl. Catal.B :Environmental 3. 4. 5. 6. 7.

11(1997)237 C.Querini, L.Cornaglia, M.Ulla, E.Mir6, Appl. Catal B :Environmental 20(1999)165. F.Ravelli, C.Querini, M.Ulla, L.Cornaglia, E.Mir6, Catalysis Today, in press. W.Shangguan, Y.Teraoka, S.Kagawa, Appl.Catal.B:Environmental 12 (1997) 237. G.Mul, W.Zhu, F.Kapteijn, J.Moujlin, Appl.Catal.B:Environmental 17(1998)205. Y.Teraoka, K.Nakano, S.Kagawa, W.F.Shangguan, Appl. Catal. B:Environmental 5(1995)L181.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Environmentally Benign Carbonylation of Nitrobenzene and Aniline Steven S. C. Chuang', Mahesh V. Konduru, Yawu Chi, and Pisanu Toochinda Department of Chemical Engineering, The University of Akron, Akron, OH 44325-3906

Abstract Carbonylation of nitrobenzene and aniline on supported metal catalysts provides an environmentally benign route to isocyanate synthesis. Infrared studies suggest that both oxidative and reductive carbonylation proceed via the insertion of adsorbed CO into adsorbed nitrene. Adsorbed nitrene is formed from the deoxygenation of nitrobenzene in the reductive carbonylation pathway and from the dehydrogenation of aniline in the oxidative carbonylation pathway. With appropriate catalysts, oxidative carbonylation can take place at significantly milder conditions than reductive carbonylation. I.

INTRODUCTION

Carbonylation is the process in which a CO molecule is attached onto a parent molecule as a carbonyl functional group. The interest in carbonylation stems from the fact that the reaction provides a pathway for the conversion of organic substrates to high-added value products. Advances in catalyst development have allowed the use of carbonylation to bypass a number of traditional synthesis pathways for increased yields and reduced environmental risks. One notable example, illustrated in Figure 1, is the use of the reductive carbonylation of nitrobenzene and oxidative carbonylation of aniline to replace phosgene as a feedstock for isocyanate synthesis (1). The reaction of phosgene with a primary amine is currently the most important process for the synthesis of isocyanates (1). The environmental impact of the process arises from not only the highly toxic and corrosive nature of phosgene but also the use of chlorinated hydrocarbons as a solvent for the reaction. The catalytic carbonylation of nitrocompounds and amines provides a direct route to isocyanate without the use of phosgene. This reaction process has been considered as the most promising environmentally benign process to replace the conventional method of isocyanate synthesis (2). Extensive efforts have been directed toward development of effective catalysts for oxidative and reductive carbonylation (1,2). Depending on catalysts and the type of reactants used, the process can lead to either isocyanate or carbamate in a single step. Carbamate synthesis from the carbonylation of nitrocompounds and amines has been the most attractive for its high selectivity, high "To whom all the correspondence should be sent

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stability, and low toxicity of the carbamate products as well as selective conversion of carbamate to isocyanates. In contrast, the direct isocyanate synthesis from carbonylation of nitrocompounds suffers from the polymerization side reaction (1). Extensive carbamate synthesis studies have been carried out in liquid media with the metal-complex catalyzing the reductive carbonylation of nitrobenzene and the oxidative carbonylation of aniline under high-pressure conditions. Although some of these studies have shown high product yields, these reaction processes not only require the use of high pressure but also involve significant catalyst recovery and product separation problems.

a. Synthesis of Isocyanate from Aniline and Phosgene RNH2 + COC12 ~ RNCO + 2 HC1

R = alkyl or phenyl group

b. Reductive Carbonylation of Nitrobenzene to Carbamate RNO2 + R'OH + 3 CO ~ R-NH-CO-O-R' + 2 CO2 c. Oxidative Carbonylation of Aniline to Carbamate RNH 2 +

R'OH + CO + 890

2 ---'> R-NH-CO-O-R' + H 2 0

Fig. 1. Isocyanate and Carbamate Synthesis The ability to catalyze carbonylation on the surface of heterogeneous catalysts under low pressure conditions provides a novel route for eliminating the catalyst recovery and solventuse problems, simplifying the synthesis process, and reducing environmental risk. The present study was aimed at determining the reactivity of adsorbed CO in reductive carbonylation ofnitrobenzene with Ce-Pd/A1203 at 373 K and 0.101 MPa and oxidative carbonylation of aniline with CuC12-PdC12/ZSM-5 at 0.101 MPa and 438 K (mild reaction condition compared with those reported in literature).

2.

EXPERIMENTAL

2.1

Catalyst preparation

The Ce-Pd/A1203 was prepared by sequential impregnation of PdC12 on to the Ce/A1203 catalyst that was prepared by the incipient wetness impregnation method. The ratio of Ce to Pd was 20 to 1. The CuC12-PdC12/ZSM-5 was also prepared by the sequential impregnation of PdC12 onto the CuC12/ZSM-5 that was prepared by the incipient wetness impregnation of

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CuC12into H-ZSM-5. The ratio of Cu to Pd was 1.4. Both catalysts were dried in air at 298 K for 24 h and the Ce-Pd/Al203 was further calcined at 673 K for 4 h. 2.2

In situ IR studies

A self-supporting disk of the catalyst (Ce-Pd/A1203 or CuC12-PdC12/ZSM-5) weighing 25 mg, was pressed by a hydraulic press at a pressure of 4000-4500 psi. The disk was placed in the IR beam path of the reactor cell (3). Prior to the injection of the organic reactants into the reactor cell, the Ce-Pd/AI203 was pretreated in flowing H2 in the in situ IR cell. The catalysts were exposed to 5% CO at the reaction temperature and allowed to stand for 30 min. The surface was flushed with He and the volatile organic reactants such as nitrobenzene, aniline, ethanol, and methanol were injected into the reactor with the aid of a liquid syringe through a septum present at the reactor inlet. IR spectra were collected at regular time intervals.

3.

RESULTS AND DISCUSSION

3.1

Reductive Carbonylation

Figure 2 shows the IR spectra taken during the addition of CO (g), nitrobenzene, ethanol, and aniline on Ce-Pd/A1203 at 373 K and 0.101 MPa. CO adsorbed as linear CO (Pd-CO) at Pd 2071 cm ~ and bridged CO ( Pd > CO ) at 1905 cm -~ (4) during CO flow and these species were retained on the surface even after flushing with He. The strongly adsorbed CO species disappeared on nitrobenzene addition (325 ~tl) while bands due to vibrations of the NO2 species of nitrobenzene appeared at 1344 and 1526 cm ~ (5). Subsequent addition of ethanol (50 lal) led to the appearance of ethanol bands at 1071 cm -~ and did not cause any significant change in the 1344 and 1526 cm ~ bands. Further addition of 500 lal of aniline caused an increase in the nitrobenzene species (1344 and 1526 cm-'), appearance of aniline species at 1605 cm ~, and formation of methyl phenyl carbamate (C6Hs-NH-COOC2Hs) as observed by the presence of a weak band for the carbonyl species (-C=O) at 1740 cm -~ (5).

3.2

Oxidative Carbonylation

IR spectra of CuC12-PdC12/ZSM-5 taken during the addition of CO (g) and 200 ~tl of aniline/methanol/NaI (17.1/79.3/3.6) at 438 K and 0.101 MPa is shown in Figure 3. Addition of the reactants led to the appearence of linear CO at 2082 cm ~ and 2054 cm l, NH stretching of aniline at 1618 cm ~, C=C aromatic stretch of the aromatic species at 1502 cm ~, C-H bending at 1454 cm ~, C-N aromatic stretch at 1345 cm l, and C-N stretches at 1279 and 1268 cm ~. Increase in reaction time from 10 to 27 min led to a decrease of all the adsorbed reactant species and formation of CO2 (g) as well as increase in the C=O stretching

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740

~'~ ~ ~

~" ~

, ~

~,~, ; ~

NH i: fillil COCHj lil II 2 ~, hi

O

"

1

"-~

[il it

CH / 2 OH ,

I!1 :I

Ng u --~"O ~ p/d~d o,ii ,~j[I~x~fI~COI~NO2 ~ ~ + t~

::

r

'.'.

'

:

'.

'.

~: " ~ i____~:~,(~"1 -He flus'~(x3)~_ N ~

{~

"-"~

C2 H OH i~,-,,-, .

CO flow (x3) I

2267

11i33 Wavenumber (cni l) IR spectra of Ce-Pd/A1203 during reductive carbonylation at 373 K and 0.101 MPa ~

Fig. 2

1r

~

r

~

oO

_'2 ~

ee~

ee~

'

:

,

:

',

"Iime (min)

V i

?

t'q

~ "

::

~

anol ,r~

Fig. 3

2400

2200

2000

:

I

I

I

I

1800 1600 1400 1200 Wavenumber (cm l) IR spectra during oxidative carbonylation on CUC1E-PdCIJZSM-5 at 438 K and 0.101 MPa

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( 1765 cm~) of the methyl phenyl carbamate species suggesting that the carbamate species can be produced via the oxidative carbonylation of aniline at 0.101 MPa.

3.3

Reaction Pathways

Decrease in IR intensity of adsorbed CO accompanied by the increase in IR intensity of the carbamate species observed in Figures 2 and 3 suggests the involvement of adsorbed CO in both oxidative and reductive carbonylation. Figure 4 provides a schematic of the proposed mechanism which delineates the steps required for carbamate synthesis. In reductive carbonylation, the reaction begins with N-O bond dissociation for deoxygenation of nitrobenzene, CO insertion into adsorbed nitrene, dehydrogenation of alcohol to form the alkoxide, and O-C bond formation between the carbonyl species and alkoxide to form carbamate. In oxidative carbonylation, the reaction involves dehydrogenation of aniline and alcohols to produce the same type of adsorbates postulated for reductive carbonylation. a. Reductive carbonylation pathway

N02~

CH

CO----~*CO + N

~

H

~

CO + H OCH3

-

NHCOOCH3

///////

b. Oxidative carbonylation pathway

CH

CO----~*CO + N

Fig. 4.

~

I

CO +

H

OCH3

~

Reaction pathways during the carbonylation reactions

NHCOOCH3

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The rapid growth of the carbamate band in Fig. 3 demonstrates the high activity of C u C I 2PdCl2-NaI catalyst in oxidative carbonylation. The excellent activity of CUCIE-PdCI2NaI/ZSM-5 catalysts for oxidative carbonylation can be attributed to the high dehydrogenation activity of C u C I 2 and PdC12 and promoter effect of NaI. The redox cycle of C u C I 2 and PdCl2 observed in Wacker catalysis (6) may play a role and remains to be investigated.

4.

CONCLUSIONS

This study demonstrates that the adsorbed CO is involved in both reductive carbonylation on Ce-Pd catalyst and oxidative carbonylation on CuC12-PdC12-NaI catalysts to produce carbamate. The latter provides a potential route for an environmentally benign synthesis of isocyanate under mild reaction conditions. The absence of harmful organic solvents in this reaction process not only reduces the environmental risk but also eliminates the catalyst recovery step. The catalyst remained to be tested for a large number of analogus oxidative carbonylation reactions.

ACKNOWLDEGMENT

This work has been supported by the United States National Science Foundation under Grant CTS 9816954. The authors also wish to thank Dr. Khalid A1-Musaiteer for providing the catalyst.

REFERENCES

.

.

~

5. ~

Cenini, S., Pizzotti, M., and Crotti, C., "Metal Catalyzed Deoxygenation Reactions by Carbon Monoxide of Nitroso and Nitro Compounds" in Aspect of Homogeneous Catalysis: A Series of Advances, vol. 6, Ugo, R., Ed.; D. Reidel Publishing, Holland, 1988; pp 97-198. Parshall, G. W., and Lyons, J. E., "Catalysis for Industrial Chemicals" in Advanced Heterogeneous Catalysts For Energy Applications; U. S. Department of Energy, 1994; Chapter 6. Chuang, S. S. C., Brundage, M. A., Balakos, M., and Srinivas, G., Appl. Spectrosc. 49 (8), 1151 (1995). Grill, C. M., and Gonzalez, R. D., J. Phys. Chem. 84, 878 (1980). Silverstein, R. M., and Webster, F. X., in "Spectrometric Identification of Organic Compounds", John Wiley and Sons, Inc., New York, 1998. Park, E. D., and Lee, J. S., J. Catal. 180, 123 (1998).

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

化

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N i t r o u s O x i d e ( N 2 0 ) - W a s t e to V a l u e Anthony K. Uriarte Solutia Inc., P.O. Box 97, Gonzalez, FL 32560-0097, USA

Waste N20 from adipic acid process is used a feedstock for novel single-step technology of manufacturing phenol from benzene (AlphOx process). The AlphOx process is discussed both as an integral part and key step of the new adipic acid manufacturing concept and as a stand-alone technology. Major developments in N20 recovery and purification from the adipic process waste stream, as well as in N20 on purpose manufacturing are presented. 1. INTRODUCTION

The worldwide adipic acid industry capacity is approximately 2000k mt/yr with almost all produced by the nitric acid oxidation of cyclohexanone (A) and/or cyclohexanol (K), KA-oil, (eq. (1)). OH

0 HNO3

HO2C(CH2)4CO2H + NOx + N20 (1)

The HNO3, NOx and N20 stoichiometry is complex and dependent upon the reaction conditions and feedstock composition. In general, 0.8 to 1 mole of N20 is produced per mole of adipic acid. This equates to a potential emission total of 600k mt/yr. A few adipic producers employ thermal reduction furnaces to decrease NOx emissions. The furnaces coincidentally destroy 99+% of the N20 contained in the waste offgas from these adipic plants. However, this represented only about 35% of the total adipic process generated N20. In 1991, Thiemens and Trogler recognized the magnitude of adipic-related N20 emissions and reported that adipic acid manufacture might contribute up to 10% of the growth of atmospheric N20, a potent greenhouse gas and suspect ozone depleter [ 1 ] . Although not regulated at the time, the heightened awareness of the environmental impact, prompted the major adipic acid and catalyst manufacturers to form a consortium to develop N20 abatement

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technologies [2]. By 1998, Asahi, Bayer, BASF, Rhone-Poulenc and DuPont were implementing various forms of abatement from catalytic decompostion to N2 and 02 to thermal oxidation for nitric acid recovery value [3]. R&D and capital investments for implementation of this effort have been over $40M.

Solutia, at their Pensacola adipic plant, was already practicing complete N20 abatement in a thermal reduction unit. In the early 90's, the decision was made to pursue a more desirable path of value-added utilization of N20. Over 100k mt/yr was available. The direct hydroxylation of benzene to phenol by N20 over ZSM-5 catalysts was receiving much attention at this time [4]. This appeared to be good candidate process. Particularly, when the phenol process was incorporated into overall new adipic acid process (eq. (2)).

3H2~ C

~O2) ~../COOH N20

............................................................................................ J

(2)

Uniqueness of nitrous oxide is determined by several factors the most important being the fact that it is a carrier of a single oxygen atom and its metastability. When contacted with the catalyst, N20 decomposes giving N2 and highly active form of oxygen bound to the catalyst, called a-oxygen, a-Oxygen, upon contact with organic molecules, inserts into C-H bonds giving products of selective partial oxidation. In collaboration with the Boreskov Institute of Catalysis (BIC), a program was initiated in 1994 to commercialize the direct benzene to phenol process. The two major steps: (1) the hydroxylation reaction and (2) the N20 recovery and purification were developed in parallel. All aspects of the hydroxylation process, including all gas and liquid recycle effects, were successfully demonstrated in Solutia's pilot facility at Pensacola [5]. 2. ADIPIC ACID PROCESS DESCRIPTION AND N20 SPECIFICATIONS Figure 1 shows a simplified diagram of a typical adipic acid process. In the reactor, KA-Oil and excess HNO3 are intimately mixed in the presence of a soluble Cu and V catalyst system. A large recycle of the HNO3 liquid stream improves product selectivity and temperature control. A side stream is

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w i t h d r a w n from the recycle stream and sent to a crystallization train for purification. The off-gas from the reactor [A] is sent to a water and air scrubber to convert the NOx to recoverable HNO3. The waste gas from the scrubber [B] is sent to a NOx a b a t e m e n t system. Table 1 shows the approximate compositions of the two gas streams from which N20 could be recovered. NOx Abaterr~nt

H20

,

I?

Air

)

hllgtllP

~

HNO3 recovery r

KA-Oil

:l

NO3

..-

~

~ Adipic acid purification

Fig. 1 Table 1. Approximate compositions of the gas streams in process streams [A] and [B] (see Fig.l), their temperatures, and pressures Parameter

Stream A

Stream B

NeO, vol.%

75

35

Ne, vol.%

4

61

02, vol.%

0

3

CO2, vol.%

6

3

CO, vol.%

0.2

0.1

NOx, vol.%

5

0.3

HeO, vol.%

5

3

VOC, ppm

300

100

Temperature, oC

75

40

Pressure, psia

45

40

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Despite its high reactivity, the N 2 0 - Fe-ZSM-5 system is very delicate and sensitive to impurities that may be present in the feed stream. Thorough studies of the influence of these impurities on the hydroxylation of benzene have revealed that such contaminants as NOx, ammonia, and N-containing compounds are catalyst poisons. Others, like CO, although not catalyst poisons, can compete for a-oxygen, thus reducing the selectivity of the process. O2 was found to lead to complete benzene combustion. Therefore, the nitrous oxide purification requirements are stringent with respect to these species. However, there are many compounds that are inert under the reaction conditions (N2, CO2, etc.). Solutia's new phenol process, in fact, requires that inert components be present in the feed stream to avoid the flammable region. This leads to a unique nitrous oxide purification process - while some of the components have to be removed completely, it is desirable to leave the others. In summary, the poisons should be reduced by 99% whereas the CO and O2 by 95%. 3. NaO RECOVERY AND PURIFICATION OPTIONS

A large-scale adipic acid plant (> 200 mt/yr) generally has multiple reactor and purification trains. Depending upon the adipic acid plant configuration and purification/recovery scheme chosen, the use of one stream will have overall cost advantages over the other. The more concentrated stream [A] is more amenable to absorption, membrane, and/or cryogenic separation technologies. Catalytic purification is more suited for the dilute stream [B]. Fig. 2 shows the schematic of the catalytic process. Nitrous oxide stream similar in composition to stream B in Table 1 is processed in the first reactor using low temperature selective catalytic reduction (SCR) with low ammonia slip to remove NO and NOz. Because the feed stream to this reactor is coming from multiple adipic acid trains operating under varying conditions, the system is designed for rapid response to fluctuations in NOx levels as high as +20%. It is also highly resistant to catalyst poisons.

Gas From Adipi~~ Acid Adsorber

1

SCR

DeOx

T NH3

l H2

Fig. 2

N20 to AlphOx

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The gas s t r e a m leaving the SCR unit is fed to the DeOx unit to remove oxygen. This is a special case of oxygen removal using a noble m e t a l catalyst differing from traditional DeOx applications in t h a t the s t a r t i n g levels of oxygen are higher (2-4% vs. usual 100 ppm) and oxygen is selectively removed in the presence of a n o t h e r oxidant - N20. There is an additional benefit in using the DeOx system in this a p p l i c a t i o n - it destroys VOCs in the feed stream.

Both SCR and DeOx systems have to operate below 350~ to prevent N20 loss. The Pilot P l a n t using the actual adipic acid off-gas has been in operation for more t h a n 1.5 years and d e m o n s t r a t e d t h a t fast-response control logic systems are critical for stable and efficient purification of N20. The Pilot P l a n t P r o g r a m established a n u m b e r of i m p o r t a n t process features: 95 to 97% recovery of the N20, 90% destruction of the organic carbon, and 98% destruction of the NOx. It also showed how well the technology could ride out u p s t r e a m upsets and led to the evaluation of non-traditional regeneration, startup, and shutdown procedures.

4. ON-PURPOSE N20 One should not be mislead to assume t h a t the novel hydroxylation technology should only be of interest to the parties having access to large a m o u n t s of waste N20. On the contrary, our studies have shown t h a t competitive processes can be designed even if one has to make N20 on purpose. Traditional route for m a n u f a c t u r i n g N20 - decomposition of a m m o n i u m nitrate - would require 100 mt of molten a m m o n i u m nitrate (!) to be handled at any time to supply 90K mt/year N20 sufficient to make 150K mt/year phenol. Direct oxidation of a m m o n i a by oxygen is practiced by Mitsui Toatsu Chemicals to m a k e 450 mt of medical grade nitrous oxide per year [6]. The direct oxidation reaction shows the most promise for economically viable route, especially t a k i n g into account t h a t the hydroxylation process does not require neat N20. Solutia and BIC t e a m e d up on the N20 on-purpose project with the goal of developing a stand-alone hydroxylation process independent of adipic acid off-gas. Oxidation of a m m o n i a to N20 is a strongly exothermic reaction producing ca. 80~ of adiabatic t e m p e r a t u r e rise per 1 mole % of converted NH3 in feed (eq. (3)). The process t e m p e r a t u r e needs to be kept below 400~ to minimize N20 decomposition. These constraints make fluidized bed reactor design the preferred technology. 2NH3 + 202 -~ N20 + 3H20

(-180 kcal/mole)

(3)

The program, t h a t included both catalyst and process development, has had some significant advances and it now entering a Pilot P l a n t stage. Ammonia oxidation to nitrous oxide is accomplished using a Mn/Bi/A1 oxide catalyst [7] and selectivity to N20 as high as 92% have been achieved at almost complete conversion of ammonia.

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5. CONCLUSION

What started as a search for an innovative way to deal with a waste steam evolved into a collection of novel technologies with great potential. Single-step hydroxylation of benzene to phenol provides a commercially viable and environmentally friendly alternative to the traditional cumene process. It also serves as the basis for a step change in adipic acid manufacture, which allows to convert waste N20 to valuable product. The need to provide N20 of certain specifications has led to the development of new efficient purification technique. Other hydroxylation processes using N20 are being developed and will emerge in the near future. The general attractiveness of partial oxidation using nitrous oxide dictated the necessity to produce it in the most economical way possible and this need was met by the development of direct ammonia oxidation process. Success in one field opens opportunities in other fields, and we believe that the industrial chemistry of nitrous oxide as reagent is just beginning. The fact that the cost of active oxygen in N20 is about 88of its cost in hydrogen peroxide should provide significant driving force for search for new applications in all fields of chemical manufacturing from commodity to fine chemicals.

REFERENCES 1. M.H. Thiemens and W.C. Trogler, Science, No. 251 (1991) 932 2. R.A. Reimer, C.S. Slaten, M. Seapan, M.W. Lower and P.E. Tomlinson, Environmental Progress, No. 13 (1994) 134. 3. Chemical Week, No. 160 (1998) 37. 4. (a) E. Suzuki, K. Nakashiro and Y. Ono, Chem. Lett., (1988) 953; (b) M. Gubelmann and P.J. Tirel, US Patent No. 5 001 280 (1991); M. Gubelmann, J.M. Popa and P.J. Tirel, US Patent No. 5 055 623 (1991); (c) R. Burch and C. Howitt, Appl. Catal., A, No. 86 (1992) 139; (d) A. S. Kharitonov, T.N. Aleksandrova, L.A. .Vostrikova, K.G. Ione and G.I. Panov, USSR Patent No. 4 445 646 (1989); A. S. Kharitonov, G. I. Panov, K. G. Ione, V. N. Romannikov, G. A. Sheveleva, L. A. Vostrikova and V. I. Sobolev, US Patent No. 5 110 995 (1992). 5. A.K. Uriarte, M.A. Rodkin, M.J. Gross, A.S. Kharitonov and G.I. Panov, In Proc. 3rd Intern. Congress on Oxidation Catalysis, R.K. Grasselly, S.T. Oyama, A.M. Gaffney and J.E. Lyons, Stud. Surf. Sci. Catal., Elsevier Science B.V., No. 110 (1997) 857. 6. Jpn.Chem. Week, No. 35(1758) (1994) 9. 7. V.V. Mokrinskii, E.M. Slavinskaya, A.S. Noskov and I.A. Zolotarskii, PCT Int. Appl. WO 9825698 (1998), priority: RU 96-96123343.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Catalytic W e t Peroxide Oxidation over mixed (AI-Fe) Pillared Clays J. Barraulta, C. Bouchoule a, J.-M. Tatibouet a, M. AbdellaouP, A. Majest6 a, I. Louloudi b, N. Papayannakos b and N. H. Gangas b a LACCO, ESIP, 40, Av. Recteur Pineau, 86022 Poitiers cedex, France, e-mail [email protected], fr b Dept. of Chem. Eng., NTUA, 9 Heroon Politechniou, GR 15780 Zografou, Athens, Greece

Mixed (AI-Fe) pillared clays, now prepared on pilot scale are very efficient solid catalysts for the oxidation of organic compounds with hydrogen peroxide in water. It is demonstrated in this study that in rather mild experimental conditions (atmospheric pressure, T _< 70~ and with a low excess (20 %) of hydrogen peroxide, phenol was highly converted to CO2 without catalyst leaching. Indeed the (AI-Fe) pillared clay (called FAZA) was recycled and used several times without any change of their catalytic properties. Iron strongly bonded to aluminium in the pillars characterized by M6ssbauer spectroscopy were suggested as the active sites in the phenol oxidation. I. INTRODUCTION Numerous wastewater streams containing organic pollutants are generated by many industrial processes and agricultural activities. As environmental regulations and health quality standards are becoming more restrictive there is a need [1 ] for defining different strategies in order: (i) to develop new "clean" technologies; (ii) to improve existing processes; and (iii) to build closed industrial water purifying and recycling systems. With respect to toxic and refractory pollutants~ catalytic wet oxidation process (CWO) employing either oxygen, ozone, hydrogen peroxide (CWPO) or a combination thereof could prove useful, since the CWO process appears to be more suitable for achieving high conversion or/and high rates at lower temperature and pressure [2]. Homogeneous catalysts are in general very effective but their recovery from the treated ettluents requires additional separation costs. This drawback can be overcome by using heterogeneous catalysts if they are stable. Among all the possible processes for catalytic wet oxidation, depending on the oxidising agent, the CWPO process using hydrogen peroxide as oxidant was chosen and investigated in our laboratory. This choice is based on the idea of a porous catalyst having active centres in order either for inducing the homolytic activation of hydrogen peroxide, followed by a reaction initiated by radical species, or for forming hypervalent species useful perhaps in a follow up non-radical mechanism. Up to now very different solids were proposed as catalysts for the oxidation of various organic compounds in water [3-7] but one often observes a declining catalytic activity and a

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concomitant leaching of the catalytically active elements. Previous studies with some clay based catalysts showed that these materials could represent an alternative if their activity could be improved [3, 4]. In the present study, the characteristics and the catalytic properties of natural clays intercalated or pillared with aluminium or mixed (aluminium-iron) polyoxocations, now prepared on pilot scale [8], are reported and phenol oxidation is chosen as a model reaction.

2. EXPERIMENTAL 2.1. Phenol oxidation Phenol oxidation was carried out in a thermostated pyrex [3] or in a stainless steel reactor at a constant pH. A 100 ml phenol solution containing 5 10-5 M of phenol was placed over the solid catalyst in the presence of hydrogen peroxide generally at atmospheric pressure and at 293 K. Hydrogen peroxide was added either at the beginning of the reaction or continuously. At the end of the experiment a molar ratio of 1.2 was reached for H20~ added versus carbon of phenol; [H~O2] / 6 [phenol]. Samples of the reaction medium were analysed in order to obtain the phenol and the hydrogen peroxide conversion. Total organic carbon (TOC) was measured with a DC 190 Dohrmann equipment. Phenol and products of phenol conversion were analysed with a Waters HPLC equipped with a Cls column. The main products detected were 9catechol, hydroquinone, quinones, muconic acid, maleic acid, oxalic acid and acetic acid. The H202 concentration was obtained by a colorimetric method with a UV/Visible DMS 90 Varian spectrophotometer. The content of metal ions in the solution was determined from atomic absorption analysis (PerkinElmer 2380). 2.2. Catalysts preparation and characterisation The catalyst FAZA was a mixed AI-Fe pillared clay. The starting clay was a Greek bentonite with the commercial name Zenith-N. The preparation route for FAZA was developed at laboratory scale and was further scaled-up to 1 Kg/batch quantity [8, 9]. It comprised the following steps : A 2 % wt Zenith-N clay suspension in water was prepared. An intercalant solution was prepared by titration of an A13+/Fe 3+. cationic solution with 0.2 M NaOH. The cationic solution consisted of 0.18 M AICI3 and 0.02 M FeC13. The addition of the NaOH solution to the cationic solution was attempted at a controlled rate and at 70~ while the amount of the NaOH solution added was such that the final OH/cation ratio was equal to 1.9. The intercalant solution was added to the clay suspension under stilxing. The final (Al+Fe)/clay ratio was equal to 3.8 mol/Kg dry clay. The pillared clay precursor was then washed till chloride free, dried at 60-70~ and calcined at 500~ for 2 h to receive the FAZA catalyst pillared clay. The do0~-spacing of FAZA was determined as 1.84 + 0.02 nm by XRD analysis. Nitrogen adsorption data were used to estimate a BET specific surface area equal to 240 + 10 m2/g. The pore volume of FAZA was estimated as 0.14 cm3/g. The composition of FAZA on dry basis is : SiO2 : 52.50 wt %, A1203 : 27.56 wt %, Fe203 : 7.02 wt ~ CaO : 0.35 wt %, MgO : 2.50 wt %, K20 : 1.38 wt %, Na20 : 0.27 wt %, TiO2 : 0.17 wt % and LOI : 7.50 wt %.

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3. RESULTS

3.1. Phenol oxidation in presence of a homogeneous [Fen, Fe m, 02, H202] system Before studying clays based solid catalysts, the homogeneous phenol transformation in the presence of soluble iron species was investigated and the results are presented in figure 1. In agreement with previous works [10, 11] a maximum rate of phenol elimination is obtained for a pH value between 2.5 and 3.5 which corresponds to the pH range where the rate of H202 decomposition is minimum [ 10]. Indeed for such pH values, iron is in a Fe(OH) ++ complex form [ 12] while (i) for a more acidic pH, the Fem content is more important increasing the consumption of HO" and HO2" radicals; (ii) for a more basic pH the ionic decomposition of H202 is preponderant and produces molecular oxygen without radicals formation. H202 + O H ~ HO2 + H20 (1) HO2- + H202 --~ 02 + OH" + H 2 0 (2)

10

100

-*- Phenol

8 .o 6

.or/3 60

4

=~ 40

r,r l-q O

/~

80

-*- Phenol

o

o

20 0

2

3

4

pH

5

Fig. 1. Phenol (100 mE, 5 10.5 M) oxidation by H202 (2 mE.h "l, 0.01 M) with iron (5 mg.L -l) and a nitrogen flow (2L.h l) at 25~ at 60 min

,

2

3

4

pH

5

6

Fig. 2. Phenol (100 mL, 5 10.5 M) oxidation by H202 (2 mL.h l, 0.01 M) with iron (5 mg.L l) and an air flow (2 L.h~) at 25~ at 60 min

In the presence of oxygen the reaction rate is much more important either for phenol elimination or TOC conversion (figure 2). In fact in agreement with work of Uri [13] and Debellefontaine [ 14], the (Fe 2+, H202) system initiates the reaction via the formation of radicals and oxygen could increase the propagation steps via the following and simplified reaction scheme, Fe 2+ + H202 --~ H O ' + Fe 3+ + O H (3) Fe 3+ + H 2 0 2 ~ Initiation steps HO2"+ Fe 2+ + H + (4) RH + HO" ~ R" + H20 (5) RH + HO2" ~ R" + H202 (6) Propagation steps

~ ~

R" + 02 ROO" + RH -~ ROOH + Fe 2§ ~ ROOH + Fe 3+ ~

ROO" ROOH + R" RO" + Fe 3+ + O H ROO'+ Fe 2§ + I-I+

(7) (8) (9) (10)

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In such a way, at room temperature, the phenol is completely eliminated from the solution and mainly transformed into catechol, hydroquinone and maleic acid, the TOC conversion being of 15 % and 35 % after respectively 60 and 120 min of reaction. 3.2. Phenol oxidation over clay based catalysts Preliminary experiments carried out with (AI-Cu) or (AI-Fe) pillared clays [3, 4, 15] showed that such solids catalysed the phenol oxidation in similar conditions with a TOC conversion greater than that obtained with homogeneous catalysts. It was also demonstrated that the final result is greatly dependent on the preparation process of the catalyst and on the metal content. Moreover all these variables also influence the metal leaching during the reaction which is one of the main problems to be solved. From these researches we discovered that mixed (A1-Fe) pillared clays prepared according to the procedure described in the experimental section are among the more stable catalyst in aqueous phase. 3.3. Phenol oxidation over (AI, Fe) pillared clays Table 1 and results presented in figures 1 and 2 show that phenol and specially TOC conversion in the homogeneous catalysis experiments (Fenton - type reaction) are lower than the values obtained in heterogeneous catalysis with FAZA solids. Table 1 Phenol oxidation with hydrogen peroxide over mixed (A1-Fe) pillared clays catalysts Experiment Catalyst T (~ P (MPa) Phenol TOC Soluble (g) conversion (%) conversion iron (ppm) (time) (%) 1 0 25 0.1 0 0 0 2 Homog. 25 0.1 100 (2h) 10 (2h) a 2 3 0.5 25 0.1 100 (90mn) 50 (2hf 0.4 (4h) 4 0.5 40 0.1 100 (90mn) 70 (2h) a 0.8 (4h) 5 1 70 0.1 100 (15mn) 80 (90mnf 0.2 (2h) The experiment was done in a batch reactor (100 ml of phenol, 5 10.5 M) with a continuous introduction o f H202 (2 mL.h -1 , 0.01 M) for 4h, an.air flow of 2 L.h1 and a pH = 3.5 - 4 . a The main products obtained at the end of the experiment were oxygenated (oxalic, acetic and formic acids). Moreover, in the presence of iron homogeneous catalysts, the amount of iron in the phenol solution was 2 or 5 ppm, i.e. about 4 to 30 times higher than the amount detected in the experiments with the FAZA samples. Indeed from experiment 3 carried out at 25~ one can observe that besides the total conversion of phenol in less than 60 min, the TOC conversion reaches 50 % over the pillared clay. In similar conditions and with an iron homogeneous catalyst the TOC conversion was only of 10 %. If the experimental conditions are modified i.e. by increasing the temperature (experiment 4) and the catalyst weight (experiment 5), the TOC conversion is higher than 80 % after 90 min. This result means that in such conditions, the more resistant light acids are at least partially transformed into CO2. The comparison of theoretical and experimental TOC conversions seems to indicate that these results are not the consequence of a lack of hydrogen peroxide

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even though some direct transformation of hydrogen peroxide to oxygen occurs during the reaction.

3.4. Recycling of FAZA sample used in the phenol oxidation with H20, A 1 g of FAZA sample was used in 3 consecutive experiments and the results are reported in Table 2. It is rather evident that there is neither catalyst deactivation during the reaction nor catalyst leaching. Indeed the iron analyzed in the solution after the phenol oxidation represents less than 0.05 % of the total iron content of the FAZA catalyst. Free iron oxides on the clay outer surfaces are most probably the main source of the iron leached in the above experiments. Table 2 Recycling of FAZA sample in the phenol oxidation Experiment Time Phenol TOC convers~n Solub~ kon (min) convers~n (%) (%) (ppm) 15 3 3 1 60 55 20 0.2 120 100 60 15 30 5 2a 60 100 25 0.25 120 100 60 15 50 7 3a 60 100 35 0.35 120 100 60 The experiment was done in conditions presented in table 1 at a temperature of 25~ with a catalyst weight of 1 g. Before reuse, the sample was dried in an oven at 80~ for 12 h The low iron leaching shown by FAZA sample is in line with the conclusion of a study by Komadel et al [16] that iron in this material is strongly fixed with aluminium in the pillars. This conclusion was based on measurements of the amount of iron extracted upon treating two different AI-Fe pillared clays with acids, and complexing and/or reducing agents. More specifically, it was found that less iron was extracted from FAZA than from the other AI-Fe pillared clay that contained about 25 wt % of iron. The existence of strong bonding of iron to aluminium in the pillars was also concluded by Bakas et al [ 17] in a M~Jssbauer study of an A1Fe pillared clay containing-20 wt % of iron. In the case of the FAZA samples used in this study M~ssbauer and chemical data have revealed that - 60 % of the iron is bonded in the pillars together with aluminium [9]. 4. CONCLUSION In this paper it is shown that [Iron-Aluminl"um] oxides pillared clays are very efficient catalysts for the total conversion of phenol to carbon dioxide in rather mild conditions (atmospheric pressure, temperature _< 70~ Of course, the use of hydrogen peroxide as oxidant favours the oxidation reactions in such experimental conditions, but one must notice that it is the first time that so signifieative results are obtained with a so low excess of hydrogen peroxide (1.2) and without catalyst leaching.

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ACKNOWLEDGEMENTS

The authors gratefully acknowledged the European Community, (the HYDROCONV network) for financial support. REFERENCES

1. Y.I. Matatov-Meytal and M. Sheintuch; Ind. Eng. Chem. Res., 1998, 37, 309. 2. F. Luck, Chem. Biochem. Q., 1996, 27, 195. 3. J. Barrault, C. Bouchoule, K. Echachoui, N. Frini-Srasra, M. Trabelsi and F. Bergaya; Appl. Catal. B - Envi, 1998, 15, 269. 4. M. Abdellaoui, J. Barrault, C. Bouchoule, N. Frini-Srasra and F. Bergaya, J. Chim. Phys., 1999, 96-3, 419. 5. M. Falcon, K. Fayerwerg, J.N. Foussard, E. Puech-Costes, M.T. Maurette and H. Debellefontaine; Environmental Technology, 1995, 16, 501. 6. K. Fayerwerg and H. Debellefontaine; Appl. Cat. B. Env., 1996, 10, L229. 7. C. Hemmert, M. Renz and B. Meunier; J. Mol. Cat. A. Chem., 1999, 137, 205. 8. V. Kaloidas, C.A. Koufopanos, N.H. Gangas, N.G. Papayanakos, Micr. Mat., 1995, 5, 97. 9. N.H. Gangas and N.G. Papayannakos; Sections 111.4 to 111.6 in the Final Report (1993) of Feasibility Award: FA- 1010-91; Contract Number: BRE2-063. 10. D.F. Bishop, G. Stern, M. Fleischmann and S. Marshall, Ind. Eng. Chem. Process Res. Dev., 1968, 7-1, 110. 11. H.R. Eisenhauer, J. Wat. Poll. Control. Fed., 1964, 36-9, 1116. 12. M. Pourbaix, "Atlas d'~quilibres 61ectrochimiques", Gauthier- Villars Eds, Paris, 1963. 13. M. Uri in "Autooxidation and antioxidants", W.M. Lundberg Ed, Interscience, New York, 1961. 14. H. Debellefontaine, M. Chakchouk, J.N. Foussard, D. Tissot and P. Striollo, Env. Poll., 1996, 92-2, 155. 15. M. Abdellaoui, J. Barrault, C. Bouchoule, M. Touchard and F. Bergaya, submitted. 16. Komadel. D. H. Doffand J.W. Stucki; J. Chem. Soc. Chem. Commun., 1994, 1243. 17. T. Bakas, A. Moukarika, V. Papaefthymiou, A. Ladavo, N.H. Gangas, Clays and Clay Minerals, 1994, 42-5, 634 and 1996, 44-6, 851.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

A n e w iron p h o s p h a t e c r y s t a l l i n e p h a s e a n d its c a t a l y t i c a c t i v i t y in o x i d a t i v e d e h y d r o g e n a t i o n o f l a c t i c a c i d a n d glycolic acid M. Ai ~ and K. Ohdan b ~Department of Applied Chemistry and Biotechnology, Niigata Institute of Technology, 1719 Fujihashi, Kashiwazaki 945-1195, Japan bUbe Laboratory, UBE Industries Ltd., 1978-5 Kogushi, Ube 755-8633, Japan Iron phosphates are effective as catalysts for oxidative dehydrogenation of lactic acid and glycolic acid. The structure of iron phosphate is transformed into a new crystalline phase during the use as catalyst in these reactions. This induces a marked increase in the catalytic performances. The formation, characteristics, and catalytic properties of the new iron phosphate phase were studied.

1. INTRODUCTION Phosphates of vanadium and molybdenum, such as (VO)2P207 and H3PM012O40, are widely used as oxidation catalysts for producing carboxylic acids and anhydrides. Indeed, a great number of studies concerning them have already been reported. On the other hand, little attention has been paid to iron phosphate which possesses both acidic and redox properties like (VO)2P207 and H3PMo12040, though iron phosphate catalysts have been known to be effective for the oxidative dehydrogenation of isobutyric acid to form methacrylic acid. Recently, it was found that iron phosphates show a unique catalytic performance for several oxidative dehydrogenation reactions, such as formation of pyruvic acid from lactic acid [1], glyoxylic acid from glycolic acid [2], and pyruvaldehyde (2-0xopropanal) from hydroxyacetone (acetol)[3]. More recently, it was also found that a part of iron (III) orthophosphate (FePO4) is transformed into a new crystalline phase during the use as catalyst in the oxidative dehydrogenation of both lactic acid [4] and glycolic acid, and that the performances of catalyst are markedly enhanced by the change in structure. XRD patterns of the new phase are very similar to those of clay minerals such as kaolinite, halloysite, dickite, and nacrite. From the studies on DTA/TG, FT-IR, oxidation states of iron in the bulk and on the surface, XRD patterns, and elementary analysis, the chemical composition of the new crystalline phase was estimated as FePO4-0.5H20 or Fe2P2OT(OH)2 [5]. The aim of this paper is to get more insight into the new crystalline phase concerning the formation, the characterization, and the catalytic properties.

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2. EXPERIMETAL

2.1 C a t a l y s t The original iron phosphate sample was prepared according to the procedures described previously [3,6,7]. The P/Fe atomic ratio was in the range of 1.05 to 1.2. It was calcined in a s t r e a m of air at 400~ for 8 h. It consisted of tridymite-type FePO4. The presence of quartz-type FePO4 was not detected in the XRD patterns. 2.2 Reaction procedures The reaction was carried out with a continuous-flow system at atmospheric pressure. The reactor was made of a stainless steel tube, 50 cm long and 1.8 cm I.D., m o u n t e d vertically and immersed in a lead bath. The iron phosphate sample was placed n e a r the bottom of the reactor and porcelain cylinder, 3 m m long and 1.5 m m I.D./3.0 mm O.D, were placed both under and above the sample. Air, oxygen, nitrogen, a mixture of oxygen and nitrogen, hydrogen, or helium was fed in from the top of the reactor; an aqueous solution containing a fixed a m o u n t of an organic compound was introduced into the p r e h e a t i n g section of the reactor by a syringe pump. The effluent gas from the reactor was led successively into four chilled scrubbers to recover the w a t e r soluble compounds. The products were analyzed by GC and LC. 2.3 Characterization XRD p a t t e r n s were studied using a Shimadzu 6000 diffractometer with Cu I ~ radiation. FT-IR spectra were recorded from 4000 to 400 cm 1 with a PerkinE l m e r 1700, using KBr disk technique. E l e m e n t a r y analyses were performed with a Yanagimoto CHN corder MT-5. Surface areas were m e a s u r e d by the BET method using nitrogen as adsorbate a t - 1 9 6 ~ The amounts of Fe 2§ and Fe 3§ ions in the bulk were determined by the redox titration method [8]. 3. RESULTS AND DISCUSSION 3.1 F o r m a t i o n o f n e w iron p h o s p h a t e phase The effects of reaction variables on the formation of the new iron phosphate crystalline phase were studied. The typical procedures were as follows. A 5 g portion of the tridymite-type FePO4 sample was placed in the stainless steel reactor. A mixture of nitrogen and oxygen was fed in the reactor with the feed rates of 200 and 5.0 ml/min, respectively. An aqueous solution containing about 10 wt% of an organic compound was introduced into the reactor with a feed rate of 35 ml/h. The t e m p e r a t u r e was 220~ and the reaction time was 24 h. 3 . 1 . 1 E f f e c t s o f organic c o m p o u n d Vapors of various kinds of organic compounds were passed over the tridymitetype FePO4 sample in the presence of a large a m o u n t of steam and a limited a m o u n t of oxygen. The results are s u m m a r i z e d Table 1

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Table 1 Effects of organic compound Organic compound n-propanol Propylene glycol Formic acid Isobutyric acid Pyruvic acid Oxalic acid Lactic acid Glycolic acid Hydroxyacetone (vs) = very small

24. 5 ~

Phase observed in XRD 12.2 ~ (new p - new phase) Quartz + new p(vs) 32.4 ~ Quartz + new p(vs) "Quartz + new p(vs) Quartz + new p(s) Tridymite + new p(vs) New p 10 15 20 25 30 35 New p + Fe2P2OT(s) 2 theta / o New p + Fe2P207 Fig. 1 XRD p a t t e r n s of new phase. Fe2P207 amount; (s) = small a m o u n t c-

t

The best results were obtained from oxalic acid; the new phase was the sole crystalline compound observed. The next best results were obtained from lactic acid; a small a m o u n t of Fe2P207 was always formed together with the new phase. A mixture of the new phase and Fe2P207 was obtained with glycolic acid. In the case of hydroxyacetone (acetol), the iron phosphate sample was reduced totally to Fe2P207 and the new phase was not obtained. On the other hand, the new phase was scarcely obtained with n-propanol, propylene glycol, formic acid, isobutyric acid, and pyruvic acid. The transformation from tridymite-type to quartz-type is ascribable to the steam [9], but not to the organic compounds. Typical XRD p a t t e r n s of the new phase obtained using oxalic acid are shown in Fig. 1. The relative intensities of the three main peaks at 20 of 12.2 ~, 24.5 ~, and 32.4 ~ were 60, 100, and 65, respectively. No clear peaks assigned to any known iron phosphate crystalline phases were detected in the spectra.

3.1.2 Effects of temperature The effects of reaction t e m p e r a t u r e were studied using oxalic acid as the organic compound. The optimum t e m p e r a t u r e was found to be in the range of 210 to 240~ At a t e m p e r a t u r e below 200~ the vaporization of oxalic acid became difficult. The FePO4 sample was not transformed into the new phase at a t e m p e r a t u r e above 300~ This finding may be consistent with the fact t h a t the new phase contains crystalline water [5]. 3.1.3 Effects of oxygen The effects of oxygen in the feed were studied by changing the concentration from zero to 20 mol% using oxalic acid as the organic compound. In t h e absence of oxygen, a small amount of crystalline Fe2P207 was obtained beside the new phase at 220~ While at 200~ the new phase was the sole phase observed in XRD patterns. Moreover, when the oxygen concentration was more t h a n 1 mol%, the new phase was the sole phase observed even at 220~

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3 . 1 . 4 Effects o f reaction period When the amount of sample used was 5 g, the whole sample was transformed within 5 h on stream. With further time on stream, no change in the structure was observed. However, when the amount was 40 g, a reaction period of more t h a n 24 h was required to transform the whole sample into the new phase.

3 . 1 . 5 Effects o f starting iron p h o s p h a t e The oxalic acid solution was passed over different iron phosphate samples prepared from the original tridymite-type FePO4, such as amorphous FePO4, tridymite-type FePO4, quartz-type FePO4, Fe3(P2OT)2, Fe2P2OT, and Y-phase. The Y-phase was obtained by reoxidation of Fe2P207 in air at 400~ for 8 h [10,11]. It was found that, independent of the change in the structure, all of the FePO4 samples can be transformed into the new phase, while that the Fe3(P2OT)2, Fe2P2OT, and Y-phase samples cannot be transformed into the new phase. 3.2 C h a r a c t e r i s t i c s o f new phase 3 . 2 . 1 T h e r m a l stability The new phase was found to be stable enough up to 300~ in both air and nitrogen. However, it decomposed gradually at 400~ in both air and nitrogen; within 4 h the majority was decomposed to form amorphous phase with a small amount of either Y-phase (in air) or Fe2P207 (in nitrogen). 3 . 2 . 2 R e g e n e r a t i o n of new phase The sample consisting of the new phase was calcined in nitrogen at 440~ for 46 h. The peaks assigned to the new phase were disappeared. Then, a mixture of nitrogen and steam was passed over it at 250~ for 23 h. The new phase was regenerated in a small part and another unknown phase, which is characterized with two clear peaks in the XRD patterns at 20 of 16.3 ~ and 23.4 ~ was formed. 3.3. Catalytic a c t i v i t y Activity of the new phase was studied in the following two reactions; CH3-CH(OH)-COOH + 0.502 (HO)CH2-COOH + 0.502

~

-~

CH3-CO-COOH + H20

O=CH-COOH + H20

(1) (2)

3 . 3 . 1 O x i d a t i o n of lactic acid to pyruvic acid The catalytic performances of the new phase were compared with those of various other iron phosphates prepared from the original tridymite-type iron phosphate. The results and the reaction conditions used are shown in Table 2. The catalyst consisting of the new phase was clearly higher than that consisting of the original tridymite-type FePO4 in both the activity and selectivity. The Fe3(P207)2 and Y-phase iron phosphate show a higher activity than the new phase does, but the new phase is higher in both the yield and selectivity t h a n the Fes(P207)2 and Y-phase iron phosphate.

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Table 2 Comparison of the catalytic performances of various iron phosphates in the oxidation of lactic acid to form pyruvic acid * Catalyst Temperature = 225~ Temperature = 230~ Iron phosphate Cony/% Yield/% Selct/% Cony/% Yield/% Select/% Tridymite-type FePO4 29.5 19.6 66. 41.1 25.5 62 New phase 53.2 40.0 75. 62.3 45.0 72 Y-phase 58.0 34.8 60. 67.0 35.5 53 Fe3(P207)2 61.0 37.8 62. 78.0 42.1 54 Fe2P207 32.4 25.0 77. 42.2 31.5 75 *Feed: lactic acid/air/steam = 19.2/350/962 mmol/h; contact time = 1.6 s.

3 . 3 . 2 O x i d a t i o n o f glycolic acid to glyoxylic acid The catalytic performances of the new phase in the oxidation of glycolic acid was studied under the following reaction conditions; feed rates of glycolic acid/oxygen/water/nitrogen = 12.3/25/480/500 mmoYh, temperature = 240~ amount of catalyst used = 5.0 g (contact time = 1.25 s). It was found th at the catalyst consisting of the new phase is higher t h a n that consisting of freshly prepared tridymite-type FePO4 in both the catalytic activity and selectivity as in the case of the oxidation of lactic acid. On the other hand, it has been reported t ha t the partially and fully reduced iron phosphates are clearly lower in both the catalytic activity and selectivity t h a n the iron phosphate consisting of tridymite-type FePO4 [2]. It is important to note that the structure and the oxidation states of iron phosphate change during the use as catalyst in the oxidation of glycolic acid much as in the case of the oxidation of lactic acid. That is, after 20 h on stream the original tridymite-type FePO4 is transformed into quartz-type FePO4 and after 100 h on stream all of the quartz-type FePO4 is transformed into the new phase. The variation in XRD patterns are shown in Fig. 2. Figure 3 shows the variations in the conversion, the yield of glyoxylic acid, and the oxidation states of iron in the bulk as a function of the time on stream. As the time on stream increases, the catalytic activity increases, shows a maximum at around 20 h on stream, and then falls with a further increase in the reaction time. After about 110 h on stream, the catalyst was calcined in a stream of air at 400~ for 1 h. It was found that the calcined catalyst consisting of the new crystalline phase, shows a markedly higher catalytic activity. The presence of a large amount of steam is essential to every oxidative dehydrogenation reaction performed with iron phosphate catalysts [7,12,13,14]. This may be related closely to the finding that the new phase containing crystalline water shows a performance better t han that of the iron phosphates without crystalline water. Similar view has been proposed in the case of oxidation of isobutyric acid by Millet et al. [12]. It is considered t ha t as the time on stream increases the catalyst is deactivated gradually due to a poisoning by deposit of some nonvolatile products on the

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surface. Possibly, iron phosphate catalysts cannot burn out the nonvolatile products at low temperatures used. This may be the reason why the catalytic activity was totally regenerated by the calcination in air at 400~ 9

-=O

~

|

1

S t r u c t u r e change t r idymi r e - > q u a r t z ~ new phase

I

90

'/!

GA conv GXA y i e l d

"O

-~ ,4~

0

Ca I c i nat ion at 4()0~

,

_=

Fe2+/(Fe 2+ + Fe 3+) 10

15

20

25

30

35

0

0

2 theta / o 9T r i d y m i t e FePO. o New phase

20

I

,

I

40

60

80

100 120

Reaction time / h zl Ouartz FeP04 0 Fe2P207 Fig. 3

Fig. 2 Variation of structure during

Variation of catalyst performances

and oxidation states during the oxidation

the use in the oxidation glycolic acid. of glycolic acid. REFERENCE

1. M. Ai and K. Ohdan, Appl. Catal. A, 150 (1997) 13. 2. M. Ai and K. Ohdan, Stud. Surf. Sci. Catal., 110 (1997) 527. 3. M. Ai and K. Ohdan, Bul. Chem. Soc. Jpn., 72 (1999) in press. 4. M. Ai and K. Ohdan, Appl. Catal. A, 165 (1997) 461. 5. N. Ishizawa, A. Saeki, K. Ohdan, and M. Ai, Powder Diffract., 13 (1998) 246. 6. M. Ai, E. Muneyama, A. Kunishige, and K. Ohdan, Bul. Chem. Soc. Jpn., 67 (1994) 551. 7. E.Muneyama, A.Kunishige, K.Ohdan, and M. Ai, J. Mol. Catal., 89 (1994) 371 8. R.A.J. Day and A.L. Underwood, "Quantitative Analysis," 4th ed. PrenticeHall, Englewood-Cliffs, N.J. (1980). 9. M. Ai and K. Ohdan, Appl. Catal. A, 180 (1999) 47. 10. J.M.M.Millet, J.C.V~drine, and G.Hecquet, Stud.Surf.Sci.Catal.,55(1990) 833. 11. E. Muneyama, A. Kunishige, K. Ohdan, and M. Ai, J. Catal., 158 (1996) 378. 12. J.M.M. Millet, D. Rouzies, and J.C. V~drine, Appl. Catal., A, 124 (1995) 205. 13. M. Dekiouk, N. Boisdron, S. Pietrzyk, Y. Barbaux, J. Jrimblot, Appl. Catal. A, 90 (1992) 61. 14. E.Muneyama, A.Kunishige, K.Ohdan, and M. Ai, Catal. Lett., 31 (1995) 209.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Dynamics of the oxidative dehydrogenation of propane over VMgO catalysts studied by in situ electrical conductivity and step transients H.W. Zantho~, J.C. Jalibert a, Y. Schuurman a, P. Slamaa, J.M. Herrmann b and C. Mirodatos a a Institut de Recherches sur la Catalyse, 2, ave.nue Albert Einstein, F-69626 Villeurbanne C6dex, France, Fax : 33 (0)4 72 44 53 99 / E-mail :[email protected] b U.R.A., C.N.R.S., Photocatalyse, Catalyse et Environnement, Ecole Centrale de Lyon, BP 163, F-69131, Eeully, France The oxidative dehydrogenation of propane (ODHP) over redox VMgO catalysts was studied under non-steady state conditions using in-situ electrical conductivity and steptransient kinetic experiments. It was found that higher yields to propene can be obtained compared to steady state operation. The reaction of propane over these catalysts occurs by a fast reduction of the surface VO34 polymers and slow reduction of the bulk Mg3V2Os. Important kinetic parameters for the bulk diffusion of oxygen necessary for a possible scale up of transient operation can be obtained from the modelling of the conductivity and kinetic transient experiments. 1. INTRODUCTION The oxidative dehydrogenation of alkanes is an attractive alternative way for obtaining olefins, as compared to the applied non oxidative processes : cheap and abundant feedstock, relatively low temperature and almost no deactivation. However, no process has been developed industrially so far, mostly due to yield limitation. The lack of product selectivity at reasonable conversion levels is due to parallel and consecutive formation of undesired products, i.e., carbon oxides. A typical example is the oxidative dehydrogenation of propane over VMgO catalysts which are active and selective for properle production [ 1,2,3]. The reaction occurs via a Mars-van Krevelen mechanism that implies surface and bulk reactions in which catalytic redox cycles participate. The selective surface involves V 5§ cations, lattice oxygen and ionic vacancies [2,3]. However, in the steady-state mode where hydrocarbon and oxygen are co-fed the undesired total oxidation can hardly be suppressed due to the presence of loosely bonded oxygen species [4,5]. Therefore, it has been recently proposed [5] and demonstrated [6] that higher yields and selectivities can be obtained if the reaction is carried out in anaerobic transient operation mode, e.g. in a riser-regenerator or moving bed reactor. Carrying out a reaction in transient operation, however, requires a more detailed knowledge on the surface and bulk kinetic processes compared to steady state operation. In the present study we demonstrate how a combination of transient in-situ electrical conductivity experiments and conventional transient kinetic studies lead to improved understanding and important kinetic parameters for possible scale-up of ODHP in the non-steady state regime.

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2. EXPERIMENTAL

2.1. Catalyst preparation and characterisation VMgO catalysts with vanadium contents from 0 to 25 wt.% (referred to as 0, 2, 5, 10, 14 and 25V/VMgO in the text) were prepared from [Mg(OR)2]4 and [VO(OR)312 alkoxides with R = CH(CH3)CH2OCH3 in 1-methoxy 2-propanol and calcined up to 1073 K, as described in [7]. The catalysts were crushed and sieved to 200 to 300 ~tm for the experiments. At steady state conditions (C3Hs/O2/He = 64:90) these catalysts give optimised propene yields from about 10 % to 13 % at 823 K in a micro-catalytic fixed bed reactor. BET surface areas range from 88 to 37 m2.g~, decreasing with increasing vanadium content. Detailed physico-chemical bulk and surface characterisation using HREM, XPS, UV-vis and XRD techniques (as discussed in [3,9]) reveals that the activated catalysts are essentially biphasic with i) platelets of MgO covered with an amorphous scattered overlayer of VO34 units as isolated and polymeric species and ii) crystalline Mg3V208 large particles in lower amounts. Increasing vanadium loading favours the fraction of the crystalline Mg3V2Os phase [8]. Under reducing atmosphere the amorphous layer can easily and reversibly polymerise and form precursors of spinel-like surface structures with MgO. In the mean time, the Mg3V208 phase also transforms slowly to spinel but less reversibly [9]. 2.2. In-situ electrical conductivity measurements A static conductivity cell described elsewhere in detail [10] was used. Step changes in electrical conductivity were followed m situ by contacting the fully oxidised samples (treated under oxygen atmosphere to reaction temperature for at least 30 min) with a reducing propane atmosphere (Pc3a8 = 45.6 Torr). Prior to propane admission, the solids were kept under vacuum for 1 min. Temperatures between 723 and 773 K were applied. 2.3. Transient kinetic experiments Non steady-state step-response experiments simulating a circulating bed were carried out in a micro catalytic fixed bed reactor fed by automated valves allowing to switch abruptly from an oxidising mixture (O2/He) to a reducing one (C3Hs/He) and vice-versa with a continuous analysis of the effluent by on-line mass spectrometry. Intensity of the masses 4, 28, 29, 32, 41, 43, and 44 were used for quantification. Conversion was calculated from the sum of products formed. The reaction was carried out at TR = 823 K. The exposure time for each gas amounted to three min before switching to the other gas mixture. 2.4. Modeling of the transient conductivity and catalytic experiments The electrical conductivity step changes with time (reduction and oxidation) were modelled by accounting for i) the primary reducing process of propane oxidation to propene (rrcd) ii) the surface reoxidation by bulk oxygen diffusion (Do) and gaseous oxygen activation (rox) to replenish the created anionic vacancies (Fig. 1). The kinetic step transient experiments were modelled assuming two parallel propane-topropene reactions corresponding to i) the fast consumption of the initially present surface oxygen (from amorphous VOx units) and ii) the diffusion limited consumption of bulk oxygen provided by the orthovanadate phase. The ratio between these two pools of oxygen was varied according to the vanadium content.

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Call8 + Os ~ CsFI6 + H20 + Ds 02 + 2Ds -4 2 0 s Oi + [--]i-I ~

O = kr

1

L

Oi-I + [--]i

i

~ [ 1-0i]/n

i+l

?/

dOs d20s - DO dL2 - k~ed0sPCSH8 + 2. kox[lr _0~]2.1 PO2 dt dOi andfori~s:-~-

_

Ir

n

d20 i D O dL2

Subscript s represents the surface layer. where Oi = concentration of lattice oxygen and [ l-0i] = concentration of vacancies, in layer i.

Fig. 1. Mechanistic scheme and continuity equations used for modeling step changes in electrical conductivity. 3. RESULTS AND DISCUSSION In oxygen atmosphere the initial conductivity values range between In o = -9.8 ~'2l-cmq and -8.5 f~q-cm1. Under reducing propane atmosphere, o instantaneously increases by several orders of magnitude (proportionally to the V content) as typical for n-type semi-conductors [2,10]. Figure 2a reports the relative changes in electrical conductivity obtained in propane atmosphere at 773 K for the applied V/VMgO catalysts. A very rapid increase occurs during the first minutes of propane introduction and then the conductivity increases only slowly, the slope being a function of the catalyst V content. On pure MgO no significant change in the conductivity is observed on introducing propane. A similar fast primary increase is observed if performing the step responses at different temperatures (Fig. 2b), but the second slow process becomes more important with increasing temperature. According to the theory of n-type conducting materials the strong increase in conductivity can be assigned from the formation of anionic vacancies V0 in place of surface lattice oxygen anion 0 2- due to the reduction of the metal ions. Paraffin + 0 2- -40lefin + H 2 0 + V 0 The anionic vacancies can easily release free electrons into the conduction band, via:

(1)

v0 <=>v~ + e(2) which are responsible for the increase in conductivity. In the presence of an oxidising agent, e.g. oxygen, the number of vacancies is reduced, due to: 1 / 2 0 2 + V0 r 0 2(3) and the conductivity decreases again. For numerous oxide catalysts it was observed that more than one monolayer of oxygen is participating in the redox process, i.e. bulk lattice oxygen is able to diffuse to the surface to participate in the reaction [ 11]. In turn, this means that surface oxygen defects (anionic vacancies) "diffuse" into the bulk of the catalyst. In consequence, the observed change in conductivity during a redox transient is due to both surface and bulk phenomena. If the kinetics of the limiting steps of the bulk and surface processes are different they can be observed separately. From the occurrence of two regimes with different time constants, we assume that the primary fast increase in conductivity is due to surface

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phenomena, i.e. the formation of surface anionic vacancies, whereas the second slower increase is due to bulk effects, i.e. the diffusion of oxygen vacancies into the volume of the catalyst (that means reoxidation of surface active sites by bulk lattice oxygen ions). These two phenomena may also be related to changes in phase structure. 9

!

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~EB~

'

'6-1

V content 1%

/ - - o - 475"C

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5

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- - A - - 10 - - e - - 14 - - 0 - - 25 r

1.o i 1.0

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9~

"1~0"1~0

at

20

time I Bin

_Lx~

'

;0

'

~0

'

~0

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Fig. 2. Relative changes in electrical conductivity during a step transient of propane (a) for different V-content (TR = 773 K) and (b) for a 10V/VMgO catalyst at different temperatures For low V contents (up to 5 wt.%), which corresponds to samples with mainly surface polymer VO34 and only minor bulk Mg3V2Os, only the primary increase in conductivity is revealed (Fig. 2a). According to our characterisation study [3,9] this fast change in conductivity can be ascribed to the reduction of the amorphous VS+Oxsurface polymers into a V3§ phase which is completely reversible. For higher V contents (>5 wt.%), which corresponds to samples still containing surface polymer VO34 but with much larger amounts of bulk Mg3V2Os, the reduction of both surface and bulk V ions occurs together, but at distinct rates. The fast increase of conductivity (surface reduction) is now followed by a slower one corresponding to the reduction of the Mg3V208 phase to a bulk V 3+ spinel phase. Since oxygen/vacancies diffusion increases with increasing temperature, the second slow part becomes faster at elevated temperatures (Fig. 2b). Log (~

-6 Fig. 3. Experimental and calculated electrical conductivity transient response when switching from oxygen to propane over a 14V/VMgO catalyst (TR = 773 K; Pc3m = 45 Torr)

9

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20 30 40 time (min)

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50

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Using a model that accounts for reactive surface oxygen and bulk oxygen diffusion (see experimental part) the modelled change in electrical conductivity during a propane step transient gives a diffusion coefficient of the lattice oxygen close to 1021 m2.s"~(Fig. 3). This is actually an average value of surface and bulk diffusion ions. It is comparable with literature data in oxide materials [12]. In the transient kinetic step responses carried out in anaerobic conditions, C3I-I6, CO and CO, are observed as products, similar to steady state conditions (i.e. cofeeding C3I-h and O2) but with distinct yields. In a 3 min pulse (TR = 823 K; C3Hdinert = 0.05; ptot= 0.1 MPa) over the 14V/VMgO catalyst the propene, CO2, and CO yields decrease from 18.5 % to 2.4 %, from 7.0 % to 0.5 % and from 7.9 to 2.2 %, respectively. The conversion therefore drops during the experiment from 33.4 % to 5.1%. In a steady state operation experiment over the same catalyst a conversion of 30.5 % is obtained. Yields towards C3I-E, CO2 and CO amount to 15.1%, 8.9 % and 6.5 %, respectively. These experiments clearly show that in an anaerobic step transient a significantly higher selectivity and yield towards propene can be obtained initially, compared to steady state operation. The advantage of the transient operation, however, depends on the exposure time of the transient which should be lower than approx. 2 min. Similar data were obtained by Creaser et al. [6]. 9

1.0

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0.8

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s'0

' 160

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Fig. 4. Response signals for C3H6in a step transient of propane over a (i) 5V/VMgO and a (ii) 14V/VMgO catalyst. (solid line: experimental; dashed line: simplified model) Like for the electrical conductivity experiments the shape of the anaerobic step transient responses depends on the catalyst V content (Fig. 4). With a low V content (2 to 5 wt.%), a sharp initial peak of propene formation occurs after propane admission, exhibiting a low time constant, followed by a period with a slow decrease with an approximately 10 times larger constant. On a catalyst with a higher V loading (14V/VMgO) the sharp decrease of the initial propene peak is overlayed by a broad slow decrease with a larger time constant. Again, the sharp peak in the beginning of the step response is attributed to the fast reduction of the

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surface V 5§ polymers and the formation of the V 3§ spinel phase whereas the broad tailing is related to the reduction of the bulk Mg3VEOs phase and the respective bulk spinel formation. As shown in Figure 4a and b the simplified model based on two distinct pools of oxygen (see experimental part) is able to describe satisfactorily the propene formation in the propane transient. For low (5 wt.%, Figure 44) and high (14 wt.%, Figure 4b) V content, ratios of bulkto-surface oxygen of 60 and 150 have been found, respectively. These values are quite consistent with the advanced characterisation of these biphasic materials [3,9]. A complete set of kinetic parameters (rate constants, diffusion constants, amount of reactive oxygen available) was derived, but still has to be confirmed in a full model accounting for all products. 4. CONCLUSIONS In-situ electrical conductivity and step transient experiments of the ODHP over VMgO catalysts reveal that the reaction occurs in a redox cycle in which different surface and bulk oxygen species participate. For low V content (up to 5 wt.%), surface Vs+Ox species forming an amorphous adlayer can be rapidly and reversibly reduced into a spinel-like structure. At higher V contents also a slower reduction process takes place which can be attributed to the conversion of the MgaVEOs phase into a bulk V 3+ spinel phase. Very close experimental and modelled data were obtained for the non steady-state kinetics experiments simulating the circulating bed. They demonstrated that such a process would both lead to improved propene yields and decrease the costly step of propene/COx separation which is required for any conventional co-feed process. Optimized ODHP catalysts for non-steady state operation would consist in a well dispersed ovedayer of amorphous VO4 3" units. Bulky mixed phases should be avoided.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8 9. 10. 11. 12.

M. Chaar, D. Patel and H.H. Kung, J. Catal., 109 (1988) 463. A. Pantazidis, A. Auroux, J.M. Herrmann and C. Mirodatos, Catal. Today, 32 (1996) 81. A. Pantazidis, A. Burrows, C.J. Kielly and C. Mirodatos, J. Catal., 177 (1998) 325. A. Pantazidis, S.A. Buchholz, H.W. Zanthoff, Y. Schuurman, and C. Mirodatos, Catal. Today, 40 (1998) 207. H.W. Zanthoff, S.A. Buchholz, A. Pantazidis, and C. Mirodatos, Chem. Eng. Sci., 54 (1999) 4397. D. Creaser, B. Andersson, R.R. Hudgins and P.L.Silveston, Chem. Eng. Sci., 54 (1999) 4437. L. Albaric, N. Hovnanian, A. Julbe, C. Guizard, A. Alvarez-Larena, and J. Piniella, Polyedron, 16 (1997) 587. A. Pantazidis, and C. Mirodatos, ACS Symp. Series "Heterogeneous Hydrocarbon Oxidation" ed. B. Warren and T. Oyama, 638 (1996) 207. J.C. Jalibert, PhD thesis, Lyon 1 University, 1999. J.M. Herrmann, in: Catalyst Characterization- Physical Techniques for Solid Materials; Eds. J.C. Vedrine and B. Imelik, Plenum Press, London, (1994) 559. A. Bielanski and J. Haber, Oxygen in Catalysis, Dekker, New York, USA, 1991. D. Martin and D. Duprez, J. Phys. Chem., 100 (1996) 9429.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Effect of Potassium on the Structure and Reactivity of Vanadium species in VOx/Al203 Catalysts P. Concepci6n l, S. Kuba ~, H. Kn6zinger l, B. Solsona2, J.M. L6pez Nieto 2 Institut f'tir Physikalische Chemie, LMU. Miinchen, 80333 Mtinchen, (Germany). 2 Instituto de Tecnologia Quimica, UPV-CSIC, 46022 Valencia, (Spain). Undoped and potassium doped (K/V atomic ratio of 0.7) alumina-supported vanadia catalysts (3.5 wt% of V-atoms) have been characterized by laser Raman spectroscopy, FTIR-spectroscopy of adsorbed CO at low temperature, temperature-programmed reduction (TPR) and electron paramagnetic resonance (EPR). All results suggest a higher dispersion of the vanadium species on the A1203 support by addition of potassium. This, together with the possibility of the presence of small amounts of K20, leads to a decrease in the amount of A13+ Lewis acid sites and a lower reducibility of the vanadium species. Moreover, the reoxidation ability of the vanadium species seems to be reduced by the presence of potassium. All these results can explain the higher selectivity to oxydehydrogenation products observed over K-doped catalysts in the oxidation of C4 hydrocarbons. 1. INTRODUCTION Alkali metals have been used as promoters of supported vanadia catalysts [ 1, 2]. This is the case of Na-doped [3-5] or K-doped [6-8] supported vanadia catalysts. Recently, it has been observed that the incorporation of potassium strongly modify the catalytic behaviour of VOx/A1203 catalysts [ 1,8]. There is little structural information available on the vanadium species formed in the presence of potassium additives [6, 7]. The aim of this paper is to determine the influence of potassium on the nature of the vanadium species in VOx/AI203 catalyst. The catalyst were characterized by laser Raman spectroscopy, temperature-programmed reduction (TPR) and electron paramagnetic resonance (EPR). In addition, the acid properties of the catalysts were studied by FTIR-spectroscopy of adsorbed CO at low temperature (77K). Carbon monoxide is one of the best probe molecules for sensitive determination of the nature and strength of Lewis acid sites [9]. 2. EXPERIMENTAL 2.1. Catalyst preparation. VOx/A1203 (3.5 wt% V) catalyst was prepared by impregnation of 7-A1203 support with an aqueous ammonium metavanadate solution. The VOx-K/A1203 catalyst (KJV atomic ratio of 0.7) was prepared by impregnation of the alumina-supported vanadia catalyst with an aqueous solution of potassium nitrate. The catalysts were calcined at 600~ for 6h.

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2.2. Catalyst Characterization. The laser Raman spectra were measured with an OMARS 89 triple monochromator spectrometer (Dilor) using the 488nm line of a spectra physics 2020 Ar + laser for excitation. A laser power of 50mW and a spectral resolution of 10 cm -I was used. IR spectroscopy studies were carried out with a Bruker IFS-66 spectrometer at a spectral resolution of 1 cm l and a number of 128 scans. Before the measurements the samples were activated for lh in a flow of oxygen at 773 K, followed by lh evacuation at the same temperature. TPR experiments were carried out with a Micromeritics apparatus loaded with 20 mg of catalyst and an H2/Ar mixture (H2/Ar molar ratio of 0.15 and a total flow of 50 ml min -l) and heated at a rate of 10 ~ min -! to a final temperature of 1000 ~ EPR spectra were recorded on an Varian E9 spectrometer using the X band. The spectra were obtained at 80K and with a power of 10mW. 2.3. Catalytic test. The catalytic test were carried out in a fixed bed tubular reactor, and the analysis of reactant and products were achieved by gas-chromatograhy [8, 10]. An alkane/oygen/ helium molar ratio of 4/8/88 and different contact times were used in order to obtain similar alkane conversions.

3. Results and Discussion 3.1 Catalytic tests Figure 1 shows the selectivity to oxidehydrogenation products obtained during the OXDH of C2-C4 alkanes on undoped and K-doped catalysts. On undoped catalyst, the longer alkane chains gave the lower selectivity to OXDH products. In contrast, the opposite trend is observed on K-doped catalysts. Thus, the higher selectivity to olefins on ] 1 C-2 I-1 C-3 ~ C-4 ] the undoped catalyst was obtained from ! J ethane, while the better selectivity to olefins on K-doped catalyst was t obtained during the OXDH of n-butane. Si

(%

1

u

undoped

K-doped

Figure I. Selectivity to oxidehydrogenation products during the OXDH of ethane, propane and n-butane on undoped and K-doped VOx/AI203 catalysts at 550~

3.2. Raman laser results Raman spectra of VOx/A1203 and VOx-K/A1203 catalysts are shown in Figure 2. Some differences in the molecular structures of the surface vanadium oxide species on unpromoted and K-promoted catalysts are observed in the hydrated and dehydrated states, respectively. Bands at 823, 890, 997 and 1023 cm -I can be observed on the dehydrated VOx/Al203 sample (Fig. 2a). The sharp band at 823 cm ~ is attributed to isolated tetrahedral vanadium species such as

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orthovanadate (VO4) 3", and the band at 1023 cm l is assigned to isolated vanadyl species (OaV=O). Raman bands in the region of 870-1000 cm ! characterize polyvanadate species, where a shift to higher frequencies is related to an increase in the average chain length and coordination number of the vanadium species [ 12, 13 ]. Thus, the band at 890 cm ~ can be attributed to dimeric pyrovanadate-type species and the band at 997 c m -1 t o octahedral polyvanadate species. Previously reported 51V-NMR results [11] show the presence of mainly polymeric tetrahedral species. On the other hand, from the FT-IR spectra no bands associated with octahedral polyvanadate ~o o are observed. Therefore, we may infer that the band at 997 cm -! could also be due to a perturbation of the V=O bond of the isolated vanadyl species by the coordination of small amounts of readsorbed water. Adsorption of water (Fig. 2b) leads to a shift of the band at 1023 cm -~ to lower frequencies and to the appearance of a broad band near 950 cm -~ due to coordination of water to the vanadium species.The Raman spectra of the dehydrated K-doped VOx/A1203 sample shows two strong bands at 823 and 997 cm -I (Fig. 400 600 800 1 0 0 0 1200 2c). As indicated above, the band at 823 cm ~ c,m-1 characterizes isolated tetrahedral vanadium species. The band at 997 cm -~ is assigned to octahedral polyvanadate species but can also be attributed to the V - O stretching mode of the vanadyl species (1023 cm -I) shifted to lower frequencies due to an interaction with the potassium ions. Otherwise no band at 890 cm -I characterizing dimeric vanadate species cannot be observed. These results suggest a higher dispersion of the vanadium species in the presence of potassium, which has also been observed on samples with a higher amount of vanadium [14]. Addition of water lead to the appearance of a 400 600 800 1 0 0 0 1200 strong band at 945 cm ~ and a decrease of the cr.n-1 intensity of the band at 823 cm l. This may be due to a solvation of the vanadium species with an Figure 2. Raman spectra of dehydrated (a, c) and rehyincrease in the coordination number and/or chain drated (b, d) samples of length. It must be noted that the hydratationundoped (a, b) and K-doped rehydratation process is completely reversible. VOx/AI203 (c, d) catalysts.

3.3. Acid properties of cata!ysts IR spectroscopy of CO adsorbed at low temperature has been employed in order to determine the influence of potassium on the number and nature of acid sites in relative to undoped VOx/A1203 catalysts. The IR spectra in the CO stretching region of the A1203, VOx/A1203 and K-doped VOx/A1203

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samples, obtained after adsorption of CO at low temperature (equilibrium pressure of 2-0.01 hPa CO and evacuation of the sample for increasing times) are shown in Fig. 3. 0.0

57

A

0.05

I

0.05

2157

C

v-

_J 2250

2200

2150

2100

Wavenumber (cm-1)

2050

2250

22'00 ' 21's0 ' 21'00 ' 2 0 5 0

Wavenumber (cm-1)

2250

2200

2150

2100

2050

Wavenumber (cm-1)

Figure 3. IR spectra of CO adsorbed on A!203 (a), undoped (b) and K-doped (c) VOx/Al203 catalysts.

Three bands, at 2204, 2195 and 2157 cm -~ are observed on pure A1203 support after adsorption of CO (Fig. 3A). According to the literature [10] and in agreement with a simultaneous shift of the OH groups, the band at 2157 cm l is due to CO interacting with OH groups. The bands at 2204 and 2195 cm ~, shifted to lower frequencies (2189 cm -l) by increasing amount of adsorbed CO, are characterizing to the adsorption of CO on A13+ Lewis acid sites. Chemical inductive effects are responsible for this kind of shift. Adsorption of CO at low temperature on undoped VO• sample (Fig. 3B) shows two bands, at 2204 cm 1 and 2195 cm ~. Both bands can also be assigned to V 4+ species [15]. Therefore an assignement of both bands to A13+ or V 4+ is controversial, taking into account a similar stability of both bands towards evacuation. A band at 2157 cm -~, associated with CO interacting with hydroxyl groups, is also observed. The addition of potassium leads to a strong decrease of the Lewis acid sites (Fig. 3C). Only a very small broad band associated with V 4+ and A13+ species is observed. The decrease in the amount of Lewis acid sites could be due to a higher dispersion of the vanadium species as observed from the Raman results and/or to the presence of small amounts of K20 species blocking the Lewis acid sites. Changes are also observed in the v(OH) region of the A1203 sample after addition of vanadium and potassium (spectra not shown) [ 14]. According to the literature five different OH groups are observed on the A1203 sample [10]. Addition of vanadium (VOx/A1203 sample) leads to the dessapearance of the more basic OH bands (3792 and 3770 cm-l). Moreover, addition of potassium leads to a decrease of the intensity of the more acidic OH bands (3729, 3683 and 3670 cm l ) probably due to ion exchange of the proton by potassium cations. 3.4. TPR and EPR results TPR results of VOx-K/AI203 catalyst show the presence V-species with a different reducibility as on VOx/A1203 catalysts (Figure 4)" (i) vanadium species with

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lower reducibility than in VOx/AI203 samples, probably due to an interaction of K § with the V-O-A1 anchoring bond, and (ii) vanadium species with higher reducibility due to a more dispersed vanadium species, as observed by Raman results. It has been observed that the addition of Na § on VO• catalysts leads to a dramatic change in the shape of the EPR spectrum of V 4§ spectrum [4]. In order to obtain more information about the interaction of the K § cations .1-, with the vanadium species a serie of Q. :3 EPR experiments was carried out on the t" unpromoted and K-promoted catalysts. 0 i."0 The behaviour of catalysts after >., -1addition of 1-butene at 200~ have also been studied. The EPR spectra on unpromoted and K-promoted catalysts are shown in Figure 5. 2C)0 ' 4C)0 ' 6C)0 ' 8C)0 Only a slight difference in the Temperature, ~ nature of the V 4§ species due to the presence of potassium was observed on Figure 4. TPR patterns of undoped (a) oxidized samples (spectra _a in Fig.5B). and K-doped (b) alumina-supported After addition of 1-butene at 200 ~ an vanadia catalysts. increase in the amount of V 4§ species is observed (Spectra b, in Fig. 5B). The nature of these species is similar in the undoped and doped samples. Addition of oxygen at r.t. on the reduced samples and subsequent evacuation leads to a slight broadening and a decrease of the V 4§ signal on

.w~-~-~

~

b

(x30)

200000

300000

400000

(xlO)

200000

i

a

300000

400000

B B Figure 5. EPR experiments of un-doped (A) and K-doped (B) catalysts: a) oxidized; b) after adsorption of 1-butene at 200~ c) reduced in H2 at 480~ d) reoxidized at 25~

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the VOx/AI203 sample (Fig. 5A, spectrum c), whereas no changes in the shape and intensity of the signal in the promoted catalyst has been observed (Fig. 5 B, spectrum c). 0 2. species were not observed on neither the unpromoted nor the promoted catalysts. This confirms that lattice oxygen ions participate in the reaction, whereas adsorbed oxygen is not involved. An increase of the V 4+ concentration in the sample is induced by the presence of potassium while the reoxidation ability (at 25~ of the vanadium species seems to be suppresed by the presence of K+. 4. CONCLUSIONS

Thus the results presented here seem to indicate a higher dispersion of the vanadium species on the A1203 support by addition of potassium. This, together with the possibility of the presence of small amounts of K20, leads to a decrease in the amount of A13+Lewis acid sites. An interaction of K+ with the V=O bond of the vanadyl species and with the V-O-AI anchoring bond is proposed leading to a lower reducibility of the vanadium species. The reoxidation ability of the vanadium species seems to be reduced by the presence of potassium. All these results are in accordance with the higher selectivity to oxydehydrogenation products observed over K-doped catalysts in the oxidative dehydrogenation of both n-butane and 1-butene [1, 8, 14]. Thus, both the absence of acid sites and the presence of V-species with a low oxidation state appear to be important aspects in the selective oxidehydrogenation of Ca-hydrocarbons. REFERENCES

1. 2. 3. 4.

T. Blasco, J.M. L6pez Nieto, Appl. Catal. A, 157 (1997) 117. G. Deo, I.E. Wachs, J. Haber, Crit. Rev. Surf. Chem. 4 (1994) 141. C. Martin, V. Rives, A. R. Gonzalez-Elipe, J. Catal. 114 (1988) 473. R. Ficke, W. Hanke, H.-G. Jerschkewitz, 13. Parlitz and G. Ohlmann, Appl. Catal. 9 (1984) 235. 5. a) G.T. Went, S.T. Oyama and A.T. Bell, J. Phys. Chem., 94 (1990) 4240; b) S. Irusta, A.J. Marchi, E.A: Lombardo, E.E. Mir6, Catal. Lett. 40 (1996) 9. 6. a) J. Zhu, S.L.T. Andersson, J. Chem. Soc., Faraday Trans.1, 85 (1989) 3629; b) G. Deo and I.E. Wachs, J. Catal. 146 (1994) 335; 7. G. Ramis, G. Busca, F. Bregani, Catal. Lett. 18 (1993) 299; 8. A. Galli, J.M. L6pez Nieto, A. Dejoz and M.I. V~izquez, Catal. Lett., 34, 1995, 51. 9. H. Kn6zinger, "Handbook of Heterogeneous Catalysis" (G. Ertl, H. Kn6zinger and J.Weitkamp, Edts.), Wiley-VCH, Weinheim, p.707 (1997). 10. T. Blasco, A. Galli, J.M. L6pez Nieto, F. Trifir6, J. Catal. 169 (1997) 203. 11. J.M. L6pez Nieto, R. Coenraads, A. Dejoz, M.I. Vfizquez, Stud. Surf. Sci. Catal. 110 (1997) 443. 12. a) G.T. Went, S.T. Oyama and A.T. Bell, J. Phys. Chem., 94 (1990) 4240; b) J.M. Jehng, G. Deo, B.M. Weckhuysen and I.E. Wachs, J. Mol. Catal. A., 110 (1996) 41. 13. a) G.T. Went, L.J. Leu and A.Bell, J. Catal, 134, 1992, 479; b) J. Haber, A. Kozlowska and R. Kozlowski, J. Catal., 102 (1986) 52. 14. J.M. L6pez Nieto, P. Concepci6n, A. Dejoz, F. Melo, H. Kn6zinger and M.I. Vfizquez, Catal. Today (2000) in press. 15. P. Concepci6n, K. Hadjiivanov and H. Kn6zinger, J.Catal., 184 (1999) 17.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

STRATEGIES FOR COMBINING LIGHT PARAFFIN DEHYDROGENATION (DH) WITH SELECTIVE HYDROGEN COMBUSTION (SHC)

Robert K. Grasselli *a, David L. Stem b and John G. Tsikoyiannis b a Department of Chemical Engineering, University of Delaware, Newark, DE 19716, USA; and Institute of Physical Chemistry, University of Munich, Butenandtstr. 5-13 (Haus E), D-81337, Munich, Germany b Mobil Technology Company, Strategic Research, 600 Billingsport Road, Paulsboro, NJ 08066-0480, USA

By combining catalytic dehydrogenation (DH) with selective hydrogen combustion (SHC), higher than equilibrium olefin yields can be obtained from the corresponding light paraffins. Towards this goal, two different process approaches were explored: A cofed DH->SHC->DH method with three reactors interconnected in series, neat paraffin fed to the first reactor and oxygen or air cofed to the second reactor; and a redox DH+SHC method using a single reactor containing a mixture of both catalysts, with no oxygen or air cofed, where the lattice oxygen of the SHC catalyst is the sole oxidizing agent for the conversion of the in situ produced hydrogen. Periodic regeneration with air is required in the latter method. In microreactor cofed operation, (0.7 wt% Pt-Sn-ZSM-5 DH catalyst and 10 wt% In203/ZrO 2 SHC catalyst) with propane as feed, propylene yields between 29.7% (97% sel.) and 33% (89% sel.) were obtained at 550~ compared to an equilibrium yield of 25% at 99% selectivity; i.e., an olefin yield improvement of 19 to 32% over equilibrium. Similarly, an 18% yield improvement over equilibrium was realized for isobutylene from isobutane. In redox mode operation (0.7 wt% Pt-Sn-ZSM-5 DH catalyst mixed with 42 wt% Bi203/SiO z SHC catalyst) initial propylene yields of 48.2% at 90% selectivity were obtained from propane at 500~ compared to an equilibrium yield of 20% at 95% selectivity. The resulting olefin yield enhancement is 140%. Although the olefin yield declines with on stream time, it is still in excess of 65% over equilibrium after 10 redox cycles. More stable SHC catalyst systems are suggested for future consideration. 1. INTRODUCTION Conventional catalytic dehydrogenation (DH) of paraffins is equilibrium limited, while oxidative dehydrogenation (ODH) is not. Catalytic dehydrogenation is well established and practiced commercially on large scale. Best known are the Oleflex (UOP) , STAR (Phillips Petroleum Co.) , Catofin (Houdry United Catalysts Inc. - ABB Lummus Crest), Linde-BASF and FBD (Snamprogetti Yarsintez) processes, which have been recently reviewed in depth by Buonomo, et. al., [1 ]. * Corresponding Author: Fax: + 1-302-831-2085 (USA)/+49-89-2180-7568 (Germany) e-mail: rkgrasse @ olymp.cup.uni-muenchen.de

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Oxidative dehydrogenation of light paraffms has recently received considerable attention. However, olefm yields obtained by ODH are still halting, because the selectivity to olefins declines precipitously as conversion is increased. Propylene yields as high as 35% at about 80% selectivity have been claimed by some investigators [2,3], however, a more realistic level is a 20% yield at 70% conversion [4-6]. An alternative approach to DH and ODH is a combination process of dehydrogenation with selective hydrogen combustion (DH/SHC), a concept pioneered by Imai, et.al., [7,8]. In this latter process, a paraffin is converted in a first reactor by conventional dehydrogenation to the corresponding olefin in an equilibrium process, the effluent therefrom led to a second reactor containing a selective hydrogen combustion catalyst, to which oxygen or air is cofed. The effluent from the second reactor is then fed to a third reactor which contains again a conventional dehydrogenation catalyst. In this manner higher than equilibrium yields of olefins are attained. We have substantially expanded on this latter concept and summarize our findings here. 2. E X P E R I M E N T A L

2.1 Catalyst Preparation The SHC catalysts were prepared by well known methods described earlier [9], entailing in general the refluxing of given metal-nitrates in the presence or absence of molybdenum heptamolybdate and/or silica sol (Ludox AS-40). The resulting slurries were refluxed for 16 hours, dried at 120~ pulverized, air dried at 290~ for 4 hour, and calcined in air at 600~ for 4 hours. The calcined solids were crushed, pelletized and sized to 20-40 mesh, before use. The 0.7 wt% Pt-Sn-ZSM-5 DH catalyst was prepared according to known procedures [10]. 2.2 Gravimetric Experiments A Cahn 1000 Vacuum Electrobalance with 1/100 mg sensitivity was employed for the gravimetric experiments. Typically, the oxide catalysts were reduced in hydrogen, propane, propylene or H2/C3 mixtures at 1 atm and 500~ and were then reoxidized in air at the same temperature. Further details of operation have been reported elsewhere [9].

2.3. Experimental Setup for Catalyst Evaluation The cofed DH->SHC->DH and redox DH+SHC experiments were carried out in automated microreactor units described earlier [ 11]. For the DH->SHC->DH experiments a three reactor setup was used (lg DH->0.1g SHC, diluted with quartz->lg DH; Fig. 1), for the DH+SHC experiments a single reactor (lg DH + l g SHC catalysts mixed; Fig. 2). 3. RESULTS and DISCUSSION

3.1. DH->SHC->DH (Cofed) and DH+SHC (Redox) Process Modes In the DH->SHC->DH cored process mode (Fig. l), conventional dehydrogenation catalysts are placed into the first and third reactor, where equilibrium dehydrogenation takes place. Into the second reactor a selective hydrogen combustion catalyst is placed to which oxygen or air is cored to selectively combust the hydrogen produced in the first reactor. Herewith, the hydrogen content of the feed entering the third reactor is reduced, allowing the dehydrogenation catalyst of the third reactor to produce overall olef'm yields in excess of equilibrium. The DH catalysts are generally Pt/AI203 based systems, the SHC catalysts Cs/Pt/(Sn)/AI203 [7,8] or Cu, Fe, Ni-ZSM-5 systems.[ 12]. We studied a 0.7 wt% Pt-Sn-ZSM5 composition as the DH catalyst [9] and selected metal oxides as the SHC catalysts [9, 11].

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Fig. 2. Schematic of DH+SHC redox process mode [9].

In the DH+SHC redox process mode, the DH and SHC catalysts are commingled in a single reactor (Fig. 2). No oxygen or air is cofed to the reactor. The lattice oxygen of the metal oxide SHC catalyst is the sole oxidizing agent to combust the hydrogen produced in situ by the commingled DH catalyst. The catalyst mixture must be periodically reoxidized to replenish the lattice oxygen of the reduced SHC catalyst. This process concept is operable with SHC catalysts of the redox type containing removable lattice oxygen which preferably oxidizes hydrogen over the paraffin feed and olefin product. Such redox SHC catalysts have recently been identified by us [9, 11]. Because of the combined redox and SHC 101 ._= requirements, Pt-based SHC catalysts used '-E 0 2 by Imai, et. a1.,[7,8] and the Cu,Fe,Ni-ZSMpane "= o ~o 10" 3 5 catalysts used by Lin et.al.,[12] in the DH0-3 VzOs >SHC->DH process, cannot be used in the "67' redox DH+SHC process. O O1

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Our gravimetric studies using a Calm balance show large differences in the SHC ,.-,.. behavior of the different metal oxides. For .__. 10 example the preference of metal oxides such cE O as V205 to combust propane over hydrogen ~x,~ydrogen = 1 contrasts the performance of Bi2Mo3Ot2 BizNIo3Ot2 which combusts hydrogen with great ~ ' O lO-1 opane preference to propane (Fig. 3). tr,~, The reducibility of several metal oxides m 1 0.a E 0 0.'05 0.1 0.15 is intercompared in Table 1. In this table the ' ' Fractional Weight Loss percent of the lattice oxygen removed by hydrogen is compared to that removed by propane after exposure of a given metal Fig. 3. Reduction behavior of V205 and Bi2Mo3Ot2 oxide to neat hydrogen or propane for five at 500~ (Calm Balance). minutes at 500~ The selectivity for hydrogen combustion (SH) in preference to propane combustion is also recorded. It is seen that compositions such as Bi203, In203, Bi2M%O,2 and In2M%O,2 are both, very selective towards preferred hydrogen combustion (SHC selectivities ranging from 95.3 to 99.0 %) and 0.05 0.1 0.15 Fractional Weight Loss

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are very active as well (47.7 to 85% of the lattice oxygen being removed in 5 minutes). Compositions such as A12Mo3O12, Fe2MO3Ol2 and Cr2Mo3012 are of intermediate SHC selectivity (88.4 to 95.5%) and are substantially less active (10.2 to 21.2% lattice oxygen removal) than the first group. La2Mo3Ol2, Ce2Mo3Ol2 and MoO 3 are less selective (Sn ranging from 78.3 to 85.7 %) and much less active (3.0 to 4.1% oxygen removal). In contrast to the above stands the poor hydrogen selectivity of V205 which is only 6.1%, and its rather high propane activity (in a five minute experiment 30.9% of the lattice oxygen is removed by propane, while only 2% is removed by hydrogen). Table 1 SHC behavior of some metal oxides over a five minute reduction span at 500~ [9] Catalyst BizMo~Ou In2Mo~Oi2 InzO~ Bi20~ AlzMo~Oiz Fe2MoaOtz CrzMo~O~2 LazMo~Otz CezMo~Olz MoO~ V20~

Percent Lattice Oxygen Removed By Propane [O]p By Hydrogen [O]H (in 5 min) (in 5 min) 0.47 47.7 47.7 0.78 82.6 3.70 85.9 4.27 14.9 0.70 10.2 0.65 21.2 2.77 4.0 0.67 4.1 1.07 3.0 0.85 2.0 30.9

SH 99.0 98.4 95.7 95.3 95.5 94.0 88.4 85.7 79.3 78.3 6.1

These gravimetric experiments suggest that Bi203, In203 , Bi2Mo3012 and In2Mo3O12 are good SHC catalysts and should be applicable as such in DH-SHC processes, while the remaining systems are less well suited, with V205 being totally unsuitable as an SHC catalyst. As an aside, all good redox SHC catalysts identified thus far [9,11] contain elements in their prevailing oxidation state during reaction having a lone pair of electrons.

3.3 DH->SHC->DH Cofed Process Approach As shown in Fig. 1 a three reactor arrangement, placing DH catalysts into the first and third reactor and an SHC catalyst into the second reactor constitutes the simplest cored arrangement of combining conventional catalytic dehydrogenation with selective hydrogen combustion. In our study we employed a 0.7 wt% Pt-Sn-ZSM-5 as the DH catalyst and a 10 wt% In203/ZrO2 as the SHC catalyst. The performance of this assembly, using neat propane as feed to the first reactor and cofeeding various amounts of air to the second reactor are shown in Fig. 4. At 550~ atmospheric pressure and WHSV of 2h l, propylene yields of 29.7% at 97% selectivity (0.3 air/propane in SHC reactor) and 33% at 89% selectivity (0.6 air/propane ratio in SHC reactor) were obtained. These results compare favorably to the equilibrium yields of 25% at 99% selectivity when only the DH catalyst was used under identical operating conditions. The yield improvements over equilibrium are 19 and 32%, respectively. Similarly, under the same operating conditions, but with isobutane as feed (Fig. 5), higher than equilibrium yields are also obtained using the DH->SHC->DH process arrangement.

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DH->SHC->DH cofed process mode. DH catalyst = 0.7wt%Pt-Sn-ZSM-5, SHC catalyst = 10wt%In203/ZrO2 550~ atm pressure,WHSV=2h", feed = neat paraffin to reactor 1, cofed artificial air (21%O,./79%He) to reactor 2 [11]. With a 0.2 air/isobutane ratio in the SHC stage, the isobutylene yield from isobutane was 47.5% at 99+% selectivity, compared to a 40% yield at 99+% selectivity when the DH catalyst was used by itself. The yield improvement over equilibrium is 18%. 3.4. D H + S H C Redox Process Approach As already mentioned, it is also possible to operate a version of the DH/SHC processing in a redox mode (Fig. 2) where the DH and SHC catalysts are commingled in a single reactor, and where the SHC catalyst must be a reducible and readily reoxidizable (i.e., redox) catalyst. The catalysts reported earlier [9,11] and mentioned above in section 3.2 fulfill these requirements. The lattice oxygen of the metal oxide is the sole oxidizing agent for the in situ generated hydrogen. A demonstrative example of this redox DH+SHC DH + SHC process configuration was carried out in a 5 cc microreactor containing a physically well DH + SHC .................................. commingled mixture of a 0.7 wt% Pt-Sn-ZSM-5 as 50the DH catalyst and a 42 wt% Bi203/SiO2 as the SHC 40" catalyst. The test conditions were 540~ WHSV of 2 hr ~, neat propane as feed and a cycle time of 2.8 minutes between regenerations. The results of the 20first cycle are shown in Fig. 6. It is seen, that an 10initial propane conversion of 59.9% is obtained using the DH+SHC redox process method, which is I . . . . . . . i. . . . . ! ! ....... substantially higher than the 27.5% conversion Conversion Yield obtained with the DH catalyst alone. Similarly, the Fig. 6. Comparison of DH and DH+SHC propylene yield is 48.2% for the DH+SHC method redox operations. Initial propylene yields. while it is 20.0% for the DH alone. The propylene Neat propane as feed, 540~ atm pressure, yield increase is a respectable 140% over 2h~ WHSV, 2.8 min run time [11]. equilibrium. ...........

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While the observed initial results are extremely encouraging, it must be mentioned, that the high yields are not sustainable with time on stream using the catalyst mixture studied. Over a period of ten consecutive redox cycles, the propylene yields stay still well (i.e., 65+%) above the equilibrium yield obtained using DH alone; i.e., the propylene yield declines from the initial 48.2% to 33.0% after 10th cycles, or about 2.2%/cycle. The decline is attributed to a loss of Bi203 dispersion on the SiO2 support, rather than Bi poisoning the Pt of the DH catalyst [ 11 ]. In order to overcome this observed decline in performance, it will be necessary to identify more rugged, redox SHC catalysts. 4. CONCLUSIONS

Catalytic dehydrogenation (DH) of light paraffins can be combined with selective hydrogen combustion (SHC) in various process modes to obtain higher than equilibrium yields of olefins. In a DH->SHC->DH cofed process mode arrangement, using a 0.7 wt% Pt-Sn-ZSM-5 DH catalyst and a 10 wt% In203/ZrO2 SHC catalyst, yield improvements of 19 to 33% over equilibrium were realized for propylene from propane, and 18 % for isobutylene from isobutane at 550~ In a DH+SHC redox process mode arrangement, using the same 0.7 wt% Pt-Sn-ZSM-5 DH catalyst and a 42 wt% Bi203/SiO2 SHC catalyst, propylene yield improvements over equilibrium as high as 140% were realized at 540~ These initial high yield improvements cannot be sustained over time and decline to about 65% (i.e., still well above equilibrium) after 10 redox cycles. In order to overcome this observed decline in performance with cycle time, more rugged redox SHC catalysts need to be identified. Possible candidates are complex mixed metal Bi-molybdates, -tungstates or-phosphates. Such catalysts are extremely rugged in various catalytic processes, e.g., (amm)oxidation of olefins and can be reoxidized rather readily, even if severely reduced [13, 14]. Once stability and compatibility problems of catalysts are worked out, it is believed that the redox process mode will win out over the cofed mode. REFERENCES

[1] [2] [3]

[4] [5] [6] [7] [8] [9] [10] [ 11] [12] [13] [14]

F. Buonomo, D. Sanfilippo and F. Trifiro, "Dehydrogenation of Alkanes" Handbook of Heterogeneous Catalysis (G. Ertl, H. Knoezinger, J. Weitkamp, Eds.), 4 (1997) 2140. V.D. Sokolovskii, Catal. Rev.-Sci. Eng., 32 (1990) 1. B. Delmon, P. Ruiz, S.R.G. Carrazan, S. Korili, M.A. Rodriguez, and S. Sobalik, Catalysis in Petroleum Refining and Petrochemical Industries, M. Absi-Halabi et. al., (Eds), Elsevier, Amsterdam (1996) S. Albonetti, F. Cavani and F. Trifiro, Catal.Rev.-Sci. Eng. 38 (1996) 413. J.C. Vedrine, J.M. Millelet and J.C. Volta, Catal. Today, 32 (1996) 115. D.L. Stern and R.K. Grasselli, Stud. Surf. Sci. Catal., 110 (1997) 357; ibid. J. Catal. 167, (1997) 550; ibid., J. Catal. 167 (1997) 560. T. Imai, U.S. Pat. 4,435,607 (1984). T. Imai and D.-Y. Jan, US. Pat. 4,788,371 (1988). J.G. Tsikoyiannis, D.L. Stern and R.K. Grasselli, J. Catal. 184 (1999) 77. R.M. Dessau and E.W. Valyocsik, U.S. Pat. 4,868,145 (1989). R.K. Grasselli, D.L Stern and J.G. Tsikoyiannis, Appl. Catal. 189 (1999) 1 and 9. C.-H. Lin, K.-C. Lee and B.-Z. Wan, Appl. Catal. 164 (1997) 59. R.K. Grasselli and H.F. Hardman, U.S. Patent 4,503,001 (1985). D.D. Suresh, M.S. Friedrich and M.J. Seely, U.S. Pat. 5,212,137 (1993).

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Oxidative Dehydrogenation over Promoted Chromia Catalysts at Short Contact Times D. W. Flick and M. C. Huff Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, DE 19716, USA The oxidative dehydrogenation of ethane and propane at millisecond contact times was studied using Cr203 and Pt coated ceramic foam monoliths. The supported Cr203 catalyst was able to achieve a higher selectivity to C2H4 at higher conversion of the hydrocarbon feed than the Pt coated monolith. I. I N T R O D U C T I O N The selective dehydrogenation of alkanes still remains a formidable obstacle in the wider use of lower alkanes as feedstocks for large industrial processes. Currently, olefins are used as important chemical intermediates for a large number of industrial processes. Thermal dehydrogenation of light alkanes to olefins is thermodynamically favorable at high gas temperatures (800-900~ but often leads to high yields of smaller hydrocarbons and coke [ 1]. In the industrial cracking process, steam is added to retard the deposition of coke on the walls of the reactor tube that eventually require the periodic shut-down of the process for decoking [ 1]. As a result, there has been much interest recently in trying to find a catalytic alternative to the current industrial steam cracking process. The catalytic oxidative dehydrogenation of hydrocarbons offers a promising alternative to thermal pyrolysis and catalytic dehydrogenation. In this study, we investigate C2H4 and C3H6 production by catalytic partial oxidation of C2H6 and C3H8 over chromium oxides supported on cx-A1203 and Mg-stabilized ZrO2 foam monoliths at millisecond contact times. In addition to identifying optimum feed conditions, the study examines the effect of the support on the activity of the metal oxide catalysts. The performance of these metal oxide catalysts is compared to the results from oxidative dehydrogenation over Pt coated monoliths which have been examined for the oxidative dehydrogenation of C2-C6 hydrocarbons [2-11 ].

2. EXPERIMENTAL 2.1 Reactor Configuration The reactor is essentially identical to that previously described for the production of syngas [ 12] and oxidative dehydrogenation of light alkanes [2]. The reactor consists of a quartz tube with an inner diameter of 20 mm. The catalyst is sealed in the tube with high temperature silica-alumina felt which prevents the bypass of gases around the catalyst. To reduce the radiation heat loss in the axial and radial directions and to better approximate adiabatic operation, inert foam monoliths are placed in front and behind the catalyst as heat shields, and the reaction zone is externally insulated. The reaction temperature was measured

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by type K (chromel/alumel) thermocouples placed at the center of the reactor tube between the catalyst and the heat shield. The gas mixture for the reactor consists of C2H6 and 02 with N2 as the diluent, and are controlled by electronic mass flow controllers. The level of dilution ranged from 20 to 50%. The total feed flow rate to the reactor ranged from 1 to 3 SLPM which corresponds to an approximate contact time of < 10 milliseconds for the monolith catalyst. For all the experiments, the reactor pressure is maintained at 1.2 atm (18 psi). The autothermal reaction takes place over the catalyst around 900~ and a sample of the product gases is fed to a HP 6890 Gas Chromatograph (GC) through heated stainless steel lines. While the reaction operates autothermally at steady state, an external heat source is necessary to initially ignite the reaction. Over the Cr203 catalyst, the reaction ignites at approximately 350~ which is significantly higher than the 220~ preheat needed to ignite that reaction over a Pt coated monolith catalyst under the same conditions. After ignition, the external heat source is removed, the composition is adjusted to the desired value and steady state is established (<10 min.) before analysis of the reaction products by the GC where the N2 was used as an internal calibration standard. The atom balances, carbon and hydrogen, closed to within _+10%.

2.2 Catalyst Preparation In this study, the ceramic foam monoliths from Vesuvius Hi-Tech Ceramics with 45 pores per linear inch (ppi) are impregnated with a 0.25 M aqueous solution of Cr(NO3)3, and then calcined in air at 600~ for at least four hours. After calcination, the Cr203 coated monoliths are bright green in color. Several monoliths were further reduced in H2 at 600~ but the additional reduction step did not have any effect upon the color or reactivity of the catalyst. The chromium oxide was supported on monoliths made from 0;-A1203 (92%, A1203, 8% SIO2) and Mg-stabilized ZrO2. This process results in loadings of approximately 5-6 wt. % Cr203 on the 0:-A1203 monolith and 3 wt. % Cr203 on the ZrO2 monolith. Lower weight loadings are obtained by using a more dilute solution of Cr(NO3)3, and higher weight loadings could be achieved by repeating the process of impregnation and calcination. Pt was deposited on the monoliths similarly using a saturated solution of H2PtC16 [2]. The monoliths used measured 10 mm thick and between 18-20 mm in diameter. 3. RESULTS

3.1 Catalyst Screening The oxidative dehydrogenation of ethane was examined over a variety of catalysts supported on ceramic foam monoliths at a flowrate of 2 standard liters per minute (SLPM) which corresponds to a catalyst contact time of approximately 10 ms. The catalysts studied included 10 wt. % Cr203/t~-A1203, 9 wt. % Cr203/ZrO2, 3 wt. % Pt/t~-A1203, and Pt (0.05 wt. %) modified 10 wt. % Cr203/tx-A1203 catalysts. The 3 wt. % Pt/ct-A1203 and 9 wt. % Cr203/ZrO2 catalysts yield significantly higher selectivity to C2H4 than either the Cr203/(tA1203 or Pt modified Cr203/o~-A1203 catalysts. In general, the Cr203 catalysts show a slightly lower selectivity to CO than the Pt catalyst with somewhat more CO2 produced. The CO and CO2 selectivity for the Pt-modified Cr203 catalyst falls direct between the two single component catalysts.

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Figure 1 illustrates the reaction results for the chromium oxide catalyst supported on a ZrO2 monolith (solid line) and the 3 wt. % Pt-coated t~-A1203 monolith (dashed line) for the oxidative dehydrogenation of ethane as a function of C2H6/O2 ratio in the feed with 20% N2 dilution at a flowrate of 2 SLPM. The catalysts show similar product selectivity trends with increasing ethane concentration in the feed. The C2H6 conversion and C2H4 yield is higher over the 9 wt. % Cr203/ZrO2 catalyst than over the Pt catalyst; the difference increases with increasing ratios of C2H6/O2 in the feed. At C2H6/O2 ratios of 1.2 and 1.5, the C2H4 selectivity for the two catalysts is essentially the same. But at the higher C2H6/O2 ratios, the selectivity to C2H4 is higher over the Cr203 catalyst at -70% compared to <65% over the Pt catalyst. Additionally, the CO selectivity is lower for the chromia-zirconia catalyst; at the higher C2H6/O2 ratios the CO selectivity is approximately half of the CO selectivity for the Pt catalyst with the selectivity to CO2 making up the difference. Even though the Cr203/ZrO2 has a higher C2H6 conversion and higher selectivity to CO2, the lower selectivity to H20 causes the reaction temperature of the Cr203/ZrO2 catalyst to be lower than the temperature of the Pt catalyst by approximately 100~ 3.2 Catalyst Lifetime

In addition to product selectivities, the lifetimes of the Cr203 and Pt catalysts under reaction conditions also differ. For the Pt-coated monolith, reaction has been shown to be

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Table 1 catalyst lifetime (hrs.) as a function of C21-LJO~ratio in the feed ~th 20% I~ dilution with a total feed flow rate of 2 SLRVI in an autothen~ reactor at a pressure of 1.2 arm. The catalyst lifetime is the length of time the reaction remainsignited over the catalyst at each specific flow coition before the reaction

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sustained for very long periods with no apparent deactivation or product selectivity changes for a wide range of C2H6/O2 ratios. The supported Cr203 catalysts in this study, however, have been shown to have a limited lifetime which is defined as the length of time the reaction remains ignited over the catalyst at specific flow conditions before the reaction extinguishes. In our investigation, the lifetime of the catalysts was shown to depend on the C2H6/O2 ratio, the support material, the weight loading of the catalyst, the flowrate, the N2 dilution, and the preheat of the feed. The lifetime of the various catalysts are shown in Table 1 as a function of C2H6/O2 ratio with 20% N2 dilution at a flowrate of 2 SLPM. The Pt/ff-A1203 and 9 wt. % Cr203/ZrO2 catalysts are much more stable than the other catalyst compositions examined. For example, the 10 wt. % Cr203/A1203 catalyst at a C2H6/O2 ratio of 1.2 sustained reaction for very long periods of time with no signs of deactivation. However as shown in Table 1, at higher C2H6/O= ratios, the reaction is sustained for < 1.6 hours. Before extinction of the reaction, the product selectivities and reactant conversions are fairly constant over the entire time period. In contrast to the Cr203/ff-AlaO3 catalyst, the reaction lifetime for 9 wt. % Cr203/ZrO2 catalyst showed no deactivation for C2H6/O2 ratios up to 2.4. For example at a C2H6/O2 ratio of 1.8, the 9 wt. % Cr203/ZrO2 monolith catalyst did not show any deactivation for over 5 hours of continuous operation, while the reaction quickly extinguished under these condition for lower Cr203 loadings on ZrO2 and on all Cr203/ff-A1203 catalysts. In addition to the support material, it can also be seen in Table 1 that the Cr203 loading of the monoliths can significantly affect the lifetime of the catalysts.

3.3 Limited Catalyst Lifetime and Reaction Front As described above, in some cases the supported Cr203 catalyst extinguished rapidly. When the catalyst extinguished, the process could be followed by observing the temperatures at the front and back faces of the monolith. Initially, the temperature measured at the front of the monolith is higher than the back temperature. With increasing reaction time, the front temperature would initially fall rapidly from 900~ to around 600~ and then the temperature

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usually less than 20~ Finally, the back temperature quickly falls as the reaction extinguishes. Since the temperatures can be used to follow the deactivation of the catalyst, the reaction was stopped at various times and the catalyst was removed and examined. For all cases, the front face of the catalyst was green (Cr203), while the rear section was black (coke and possibly CrO). There was a very sharp line between the green and black sections. The longer the reaction time the further the green section proceeded down the length of the catalyst. After the catalyst extinguished, the monolith was completely green. At this point, the monolith could be reignited by applying external heating, and the catalyst would go through the same cycle of slow deactivation and finally extinction. Thus, the reaction front on the catalyst can be located by observing the line between the green and black sections. It is widely known that the oxidation state of chromium oxide can be changed fairly easily and rapidly. Therefore, the transition on the catalyst between the green section and black section marks the transition in the chromium oxide catalyst between an oxidizing environment and a reducing environment. The supported Cr203 catalyst acts essentially as two catalysts in the two reaction environments. In the oxidizing environment near the front face of the catalyst, the supported Cr203 acts as a relatively active and selective oxidative dehydrogenation catalyst. After the majority of the oxygen has been consumed at the front end of the catalyst, the supported Cr203 catalyst, as well described in the literature, acts as a highly active and selective dehydrogenation catalyst by utilizing the heat generated by the highly exothermic oxidation reactions at the front of the monolith. Therefore, the

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unreacted C2H6 in the reducing environment is dehydrogenated by the reduced chromium oxide surface. Thus at the higher C2H6/O2 ratios where there is more unreacted C2H6, the Cr203 monolith should and does show higher C2H6 conversion and C2H4 selectivity than the Pt monolith alone.

3.4 Oxidative Dehydrogenation of Propane The oxidative dehydrogenation of propane was examined over Cr203 and Pt supported on ceramic foam monoliths at a flowrate of 2 SLPM. Figure 2 illustrates the reaction results for the Cr203 on a ZrO2 monolith (solid line) and the 3 wt. % Pt-coated ~-A1203 monolith (dashed line) as a function of the C3H8 to 02 ratio with 50% N2 dilution. The product selectivities and reactant conversion, as in the oxidative dehydrogenation of ethane, follow the same trends with changes in the fuel-to-oxygen ratio. The C2H4, total olefin, and C3H8 conversion are higher over the Cr203 catalyst than over the Pt catalyst. The selectivity to C3H6, however, is higher over the Pt catalyst. Similar to the ethane case, the CO selectivity is lower and the CO2 selectivity is higher for the Cr203 catalyst. Additionally at the higher C3H8/O2 ratios, the Cr203 catalyst has a slightly lower reaction temperature than the Pt catalyst. 4. CONCLUSION It has been found that supported Cr203 is a good oxidative dehydrogenation catalyst. With a Cr203 catalyst supported on a ceramic foam monolith at contact times on the order of milliseconds, ethylene is produced with high conversion of the hydrocarbon feed and complete consumption of oxygen at atmospheric pressure. The Cr203/ZrO2 catalyst had the highest C2H4 selectivity along with the lowest CO selectivity of all the catalysts examined. We propose that the improved C2H4 selectivity and fuel conversion results from the catalytic nature of the supported Cr203. In the front section of the reactor where 02 is present, supported Cr2Os is an active oxidative dehydrogenation catalyst. After the 02 has been consumed, the supported Cr203, which is a well-known active and highly selective dehydrogenation catalyst, reacts with the unreacted fuel by utilizing the heat generated by the highly exothermic oxidation reactions. Under some operating conditions, the Cr203 catalysts have a limited active lifetime, unlike the Pt/c~-A1203 catalyst which can operate for very long periods of time with no deactivation. The lifetime of the supported Cr203 catalysts depends primarily on the feed conditions, support material, and catalyst loading.

References (1) (2)

(3) (4)

(5) (6) (7)

(8) (9)

(10) (11) (12)

Pyrolysis: Theory and Industrial Practice; Albright, L. F.; Crynes, B. L.; Corcoran, W. H., Eds.; Academic Press: New York, 1983. Flick, D. W.; Huff, M. C., Catal Lett 47 (1997) 91-97. Flick, D. W.; Huff, M. C., J. Catal. 178 (1998) 315-327. Dietz III, A. G.; Carlsson, A. F.; Schmidt, L. D., J. Catal. 176 (1998) 459-473. Huff, M. C.; Schmidt, L. D., J. Catal. 149 (1994) 127-141. Astbury, C. J.; Griffiths, D. C.; et.al., US Patent 5382741 (1995), to British Petroleum Company p.l.c. Huff, M.; Schmidt, L. D., J. Phys. Chem. 97 (1993) 11815-11822. Heck, R. M.; Flanagan, P., US Patent 4844837 (1989), to Englehard Corporation. Font Freide, J. J.; et.al., US Patent 5105052 (1992), to British Petroleum Company p.l.c. Golunski, S. E.; Hayes, J. W., US Patent 5593935 (1997), to Johnson Matthey PLC. Witt, P. M.; Schmidt, L. D., J. Catal. 163 (1996) 465-475. Hickman, D. A.; Haupfear, E. A.; Schmidt, L. D., Catal Lett 17 (1993) 223-237.

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化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Propane Oxidative Dehydrogenation by continuous and periodic operating flow reactor with a nickel molybdate catalyst C. Mazzocchia a, A. Kaddouri a, E. Tempesti b and R. Del Rosso a aChemical Engineering and Industrial Chemistry Department, Politecnico di Milano, P.zza L. Da Vinci, 32 20133 Milan, Italy. bDepartment of Chemistry and Physic for Material Science, University of Brescia, Via Valotti 9, 25123 Brescia, Italy NiMo04 phases have been tested in the propane oxydehydrogenation in either continuous flow system and periodic operating system. The lattice oxygen is proposed as active center for selective propene formation. [3-NiMoO4 was found to be the most selective phase. 1. INTRODUCTION The increasing market demand for light alkanes and particularly lights olefms such as propene and isobutene cannot be satisfied with the traditional sources of production and so alternative ways to produce these important feedstocks have to be developed. Considering this aspect, the oxidative dehydrogenation of alkanes represents a valid alternative to the widely used dehydrogenation processes, allowing to overcome their thermodynamic limitation (high temperatures required for obtaining a negative AGo for the specific reaction) and the rapid coking and short life of the catalyst used. Several catalysts and processes have been reported so far for the oxidative dehydrogenation of propane to propene [ 1]. Among them 13-NiMoO4 is one of the best. Moro-oka et al [2] studied various metal molybdates among whom cobalt molybdate showed the highest catalytic performances. The influence of the catalyst preparation and the consequent physico-chemical properties of c~ and 13phases of NiMoO4 on the obtained results have been widely showed by the authors in some of their previous publications [3-7]. Referring to the obtained results the aim of this paper is to confirm that lattice oxygen plays a key role in the propene formation while the gas oxygen causes total oxidation. 2. EXPERIMENTAL

2.1 Catalyst Preparation The stoichiometric catalyst NiMoO4 is prepared [6-7] by coprecipitation from an equimolar molybdic acid and nickel nitrate solution (0.25M), at 85~ with the pH adjusted at 5.25 by addition of ammonia. The precipitate obtained is hot filtered and dried at 120~ for 15 hours, then heated 2 hours at 550~ [5]. The catalyst particle size is 200 - 325 mesh.

2.2 Catalytic runs a) Continuous Operating system (COS) Catalytic experiments in Continuous Operating System are achieved in a quartz tubular reactor (total volume -- 30 cm 3, length of the catalytic zone L = 10 cm, diameter - 1 cm). A

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mass of 0.5 g of catalyst is mixed with about 10g of silicon carbide powder of similar size in order to avoid severe temperature gradients within the catalytic bed. Preliminary tests have been run to verify the absence of diffusion limitations 9Several runs were performed by varying both the flow rate and the catalyst weight in order to maintain the same contact time. Other tests were carded out by using the same catalyst with a different particle size. These tests have shown that the propane conversion and the product distribution was unvaried. Before the experiments, the reactor is heated at 550~ under a mixture of oxygen (18% volume) and helium for one hour in order to stabilise the NiMoO4 catalyst, then the reactor is cooled up to the working temperature and propane is introduced after thirty minutes.

b) Transient Operating System In the tests performed under continuous flow in the absence of oxygen the mixture was composed of 3% propane and 97% helium, maintaining the same contact time as in the tests performed in the presence of oxygen. Sampling of the reaction products was performed using a Valco 16 positions valve along a period starting aider 4 minutes and ending after 16 minutes from the moment in which the oxygen feed was turned off.

c) Periodic Operating System (POS) The fldw reactor apparatus [6] (Fig. 1) consisted of four quartz tubular reactors containing 0.5 g catalyst each. Each reactor was first exposed to a feed containing propane for an assigned time period, followed by a same period of helium washing; during the third period oxygen containing gas re-oxidises the catalyst surface. A f'mal helium washing period precedes the reintroduction of propane and the complete cycle is continuously repeated. For each period the same gas volumes were introduced in the four reactors and the same contact times were used to keep the pressure drop constant. A rotating automatic distribution valve with a variable rotation frequency has been especially designed to operate according to the described four step sequence. The system is designed in such a way that while two reactors are exposed to the helium washing one is reacted with propane and the other re-oxidised by oxygen. Periods were varied between 1 and 180 seconds. C3, He, 02, He, C3, ... b (T = .540~ period 60 s)

Delta T= 30~

a (2" = 465~ period 10 s)

DeltaT= I~

1 Fig. 1 Experimental apparatus with the periodic sequence of feed gas

Fig. 2 Temperature differences between POS reactors in phase opposition

[

t

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2.3 Analysis The reaction products (COx, C2H4, C3H6 and C3H40) are analysed by gas chromatography: oxygen and carbon monoxide by a molecular sieve 5A column connected to a Thermal Conductivity Detector, while ethylene, propylene, propane and acrolein are separated and analysed by a Porapak QS column linked both to a F.I and to a T.C. Detectors.

2.4 Physical-chemical Characterisation The evolution of the decomposition of the precursors has been studied under both air and nitrogen using a thermogravimetric analyser Seiko instrument (TG-DTA-DTG) coupled to Mass Spectrometry. The samples, weighing approximately 20 rag, were placed in quartz crucible and heated up to 500~ (5-10~ The gas feed (air or nitrogen) was 61/hr. B.E.T surface measurement have been performed on a Quantasorb (Quantachrome Co) instrument. H.T. X ray diffraction patterns of the samples have been recorded using a Siemens D 5000 diffractometer with filtered CuKr radiation (count time of 1s in the range 2 0 10-70~ The samples have also been characterised by FT-IR (Perkin Elmer Mod. 1760, KBr pellets), XPS, SEM, Atomic Absorption Spectroscopy conductivity and surface acidity measurements. Table 1 reports some physical-chemical properties of cz and [3 NiMoO4 Table 1. Physical-chemical characteristics of NiMoO4 or-phase NiMoO4 33 m2/g Surface area ionise twice Oxygen vacancies acid charactedstic(isopropanol decomposition) 11 (C3H6~tmol./h.m2)

[3-phase 13 m2/g ionise once 20

The acid and BET surface area properties of the [3-phase are assumed to be equal to the one cooled to ambient temperature. The cz'phase is obtained after cooling the [3-phase until ambient temperature. The test on pure [3 phase was not possible to perform due to the low reaction temperature at which the [3 --~ cz' transition occurs (cz --, [3 -~ cz' ) 3. RESULTS AND DISCUSSION Kinetic measurements were carried out on NiMoO4 phases with continuous flow system at atmospheric pressure and kinetic parameters were calculated using a pseudo homogeneous equation rate: Rc3Hg = k [Pc3H6]m[Po2]". Obtained data showed that while on the a-NiMoO4 phase the partial order m is equal to 1.2 + 0.01 and n is equal to 0.04 • 0.01, in the case of the more selective fl-NiMoO4, the reaction orders with respect to propane and oxygen are 0.95 and 0.03 respectively [8]. These values are based on an average propylene formation rate, calculated from the results of several tests, with a maximum standard deviation of 10%. It can be concluded that with both phases the change in the oxygen partial pressure does not affect propylene production, which is instead dependant on the propane partial pressure. The effect of oxygen partial pressure on propylene selectivity is reported in Table 2. The decrease in the oxygen partial pressure improves the propylene selectivity (from 77.6% to 88.7% with only 1.7% loss in conversion) confirming the importance of the oxygen feed for CO and CO2 formation. We can stress that rate of formation of propylene is constant. On the other hand additional tests performed with NiMoO4 catalyst under continuous flow in absence of oxygen, indicate that the lattice oxygen plays a crucial role in the formation of propylene. At the beginning of the exposure of the catalyst to the propane feed, no cracking products (ethylene, methane) were indeed detected, and the only reaction products are propylene and traces of CO and CO2. Thus it may be assumed that gaseous oxygen is not

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directly responsible for the selective oxidative dehydrogenation and a red-ox type mechanism can be proposed. Table 2. Reactivity of [3-NiMoO4 phase as a function of oxygen partial pressure % Oxygen O2/C3H8 Cony.(%) Selectivity(%) CO CO2 C3H6 18 1.20 11.5 9.2 9.0 77.6 7 0.44 11.0 7.0 6.6 83.5 1.3 0.32 10.1 5.6 4.1 86.5 0.6 0.19 9.8 4.3 2.6 88.7 T= 500~ W/F = 0.033g.lq.h.

Acrolein 2.8 1.9 3.7 4.3

In order to confirm the lattice oxygen involvement, the catalytic behaviour of both a and 13NiMoO4 phases was investigated using also the Periodic Operating System described in the experimental section. The AT existing between two of the four reactors operating in phase opposition (reduction vs. re-oxidation and washing vs. washing), measured by the aid of two thermocouples placed within the catalytic bed, is continuously recorded. The constant amplitude of the cyclic temperature oscillation indicates that the system is working under stationary conditions. At low conversion (< 2%) the constant amplitude of the cyclic temperature oscillation AT is small (Fig. 2 i.e. T = 465~ AT ~ I~ while for a higher conversion the temperature difference becomes larger (Fig.2 i.e. T=540~ ATmax ~ 30~ In this case the catalyst deactivation immediately begins, and a progressive reduction of the AT amplitude is monitored indicating that the re-oxidation step may not be capable to restore the catalytic surface. Metallic Ni is irreversibly formed and carbonaceous products can be indeed observed on the surface of the exhausted catalyst. Cracking products such as ethylene, methane and hydrogen were indeed detected. Table 3 reports the conversion and the propylene selectivity as a function of the period under stationary conditions. The data were taken for each different period after two hours. Propylene selectivity is improved by increasing the period. This can be explained in terms of the degree of catalytic reduction of the system in agreement with Silveston et al [9]. Table 3. Reactivity of [3-NiMoO4 phase as a function of period T = 480~ Period = 30s 40s 50s Convpropane 1.54 1.32 1.17 Selco . . . . Selco2 5.18 4.26 3.9 Selethylene

.

Selpropene

94.82

.

.

95.74

60s 1.16 3.86

.

96.10

96.14

Table 4 reports conversion and propylene selectivity obtained in either COS and POS as a function of the period under stationary conditions. Considering the kinetic data and the results obtained by decreasing O2/C3H8 molar ratio (Table 2), the passage from the continuous mode to the cyclic operating mode causes an abrupt increase in the propylene selectivity. As soon as the adopted period becomes as long to assure a good separation between the gas containing the hydrocarbon and the one containing oxygen, the conversion decreases and the propylene selectivity improves. The data obtained at iso-conversion reported in Table 5 indicate, that the increase of propylene selectivity is not

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only due to a decrease of propane conversion but also depends on the separation of oxidationreduction steps. This fact indicates that the red-ox mechanism is the one leading to propylene, while the reactions occurring in presence of gaseous oxygen and propane, are mainly responsible for the pronounced carbon oxides formation. Table 4 Conversion and Selectivities obtained with c~NiMoO4 catalyst. % COS Period = 10 s 30 s 60 s 90 s Convpropane 9.6 2.90 1.59 1.42 1.28 Selco 23.0 3.68 . . . . Selco 2 21.4 14.10 9.04 8.32 8.25

180 s 1.03 8.20

Selethylene

2.0 . . . . . Selpropene 53.6 82.22 90.96 91.68 91.75 91.80 POS system" alternance of set of 30 cc/min C3Hs ,30cc/min 02, 30 cc/min He; T = 465~ COS system" 37.5 cc/min C3H8,44.5 cc/min 02, 168 cc/min He, Total feed 250cc/min, T= 430~ Table 5. selectivity data obtained on ~-NiMoO4 phase at various period and temperatures C3H8 conversion (%) Temperature(~ Period (s) C3H6 selectivity (%) 1.16 480 60 96.1 1.13 380 0 91.6 a-13 NiMoO4_comparison The a-phase selectivity in iso-conversion tests (COS) is much lower and that is confirmed by the results achieved by Moro-oka et al. [2]. The same authors have stressed that f~-CoMoO4 is the most selective of the various molybdates studied. Actually 13-NiMoOa has proved highly selective (Table 2). The differences between the two NiMoO4 phases concerning SBET, acidbase properties and conductivity (Table 1) prove their different propene selectivity, being these properties essential for both gas oxygen activation and propene desorption. The data reported in the Tables 3 and 4 confirm the involvement of the lattice oxygen. The existing different selectivities are much less evident in periodic operating system owing to the oxygen environment which is not the same, thus determining non equivalent oxygen centers. The different co-ordination of oxygen sites must result in different local properties and hence in their different catalytic activity. Pietrzyk et al. working in transient reaction conditions [ 10] have confirmed our results obtained on a-NiMoO4 and have found that : a) lattice oxygen can also destroy slightly the propylene formed. b) when the reacted mixture corresponding of oxygen re-oxidation catalyst steps was analysed only small peaks of CO and CO2 were observed. The corresponding amounts being of the order of 1% reacted propane. Therefore referring to the CO2 formation, it was observed that for periods longer than 30 seconds the CO disappears and only CO2 is formed. It follows that CO2 production is due to a consecutive reaction both with lattice oxygen and gas oxygen. Monolayer calculations The conversions reported in the Tables 3 and 4 for the Periodic Operating System are consistent with an oxygen consumption corresponding to a small percentage of the mobile oxygen content of the catalyst monolayer. To calculate the total catalyst oxygen contained in the monolayer it is necessary to consider the monolayer depth and the exposed area of the NiMoO4 cell. Crystallographic data reported in the literature [11] and the theoretical surface area of the NiMoO4 cell are resumed in Table 6 according to the different planes considered as exposed.

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For example for the 13=NiMoO4 phase the amount of exposed elementary cells is evaltmted according to the surface area of the catalyst (13 m2/g) and to the mean exposed area of the NiMoO4 cell (_= 75 A2). It is thereforepossible to estimate the values of 16 x 1018 elementary cell/g of the catalyst and of 1.3 x 102~for the mobile oxygen atom/g. If it is assumed that 5 cell layers may represent the so called monolayer region, the total mobile oxygen atoms are 6.4 x 102~which means 1.05 x 10.3 oxygen g-atom/g catalyst. Table 6. Cell parameters of NiMoO4 catalyst 1 Catalyst Cell dimensions (A) Angle 13 a B c (deg) tx NiMoO4 9.555 8 . 7 4 5 7.693 113.62 13NiMoO4 1 0 . 1 3 9.280 7.020 107.20 1Nmnberof molecules/cell Z= 8

Cell volume (A3) 588.7 630.0

Cell surface area (A2) Sab Sbc Sac 83.55 63.80 67.35 94.00 62.23 67.93

With a conversion less than 2%, only ca. 1% of the oxygen of the monolayer is consumed, and the reaction is controlled by monolayer oxygen diffusion within the catalyst. 4. CONCLUSION On the base of the obtained results we can state the following: - the lattice oxygen of NiMoO4 is extremely selective in the propane oxydehydrogenation. [3-NiMoO4turned out to be the most selective in either continuous flow system and periodic operating system. - NiMoO4 catalyst can be successfully used in a circulating bed reactor provided that: a) the reduction and re-oxidation conditions are carefully controlled, b) any microstructural modification and mechanical strength of the catalyst are checked. -

REFERENCES 1. S. Albonetti, E Cavani and E Tdfirb Cat. Rev. Sci. Eng. 38, n~ (1996) 413. 2.Y.S. Yoon, N. Fujikawa, W. Ueda, Y. Moro-oka, K. W. Lee, Cat. Today, 24 (1995) 327. 3. C. Mazzocchia, R. Anouchinsky, A. Kaddouri, M. Sautel and G. Thomas, J. Therm. Anal. 40 (1993) 1253. 4. C. Mazzocchia, A. Kaddouri, R. Anouchinsky, M. Sautel and G. Thomas, Sol. State Ion., 63-65 (1993) 731. 5. C. Mazzocchia, Ch. Aboumrad, E. Tempesti, J. M. Hermann, and G. Thomas, Catal. Lett.10 (1991) 181 6. R. Del Rosso, C. Mazzocchia, P. Gronchi, P. Centola, Proc. X Congr. Catal., L'Aquila, Italy, 8-11/9, 1996, p. 143. 7. C. Mazzocchia, Ch. Aboumrad and E. Tempesti, Eur. Patent, 90-400137, 1990, US Patent 5086032, 1992 8. M. Sautel, G. Thomas,A. Kaddouri, C. Mazzocchia and R. Anouchinsky, Appl. Catal., 155 (1997)217. 9. D. Creaser, B. Anderson, R. R. Hudgins and P. L. Silveston, J. Catal. 182 (1999) 264. 10. S. Pietrzyk, M. L. O. M. Mahmoud, T. Rembeczky, R. Bechara, M. Czemicki and N. Fatah, Dynamics of Surfaces and Reaction Kinetics in Heterogeneous Catalysis G. E Froment, K. C. Waugh (eds) Elsevier Science, 1997, p. 263. 11. A. W. Sleight, B. L. Chamberland, J. F. Weiher, Inorg. Chem. 7 (1968) 1672.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

A Novel Catalyst of Copper Hydroxyphosphate (Cu2(OH)PO4) with High Activity in Hydroxylation of Phenol by Hydrogen Peroxide Feng-Shou Xiao*, Jianmin Sun, Ranbo Yu, Xiangju Meng, Hongming Yuan, Dazhen Jiang, Ruren Xu Department of Chemistry, Jilin University, Changchun 130023, P. R. China Fax: (0431) 5671974, Email: [email protected]

A novel catalyst of copper hydroxyphosphate (Cu2(OH)PO4) that has not microporous and mesoporous pores (surface area <0.01 m2/g) has been successfully synthesized from hydrothermal method by using ethylenediamine, phosphoric acid, and copper acetate. Catalytic data in hydroxylation of phenol by hydrogen peroxide as a model reaction for oxidation catalysis showed that the copper hydroxyphosphate is very active catalyst, and its activity is even higher than that of microporous TS-1 catalyst that is known as one of the most effective catalysts. Furthermore, we observed that the Cu2(OH)PO4 catalyst is readily regenerable to its active state by recalcining the expired form in air. Comparison of various catalysts suggests that the unusual catalytic activity on the Cu2(OH)PO 4 catalyst may be related to unique structure of as-synthesized Cu2(OH)PO4. Characterization of catalytic process by ESR method gives very strong signals assigned to radical .OH species, showing their possible catalytic mechanism. 1. INTRODUCTION Because catalysis by metal or by metal oxide is a surface phenomenon, many technological catalysts have very high metal dispersion. Catalytic oxidation, as an important industrial reaction for production of fine chemicals, has been investigated extensively recently [1-3]. In catalytic oxidation reactions a large amount of catalysts are focused on microporous and mesoporous materials such as TS-1, TS-2, and Ti-MCM-41, because of their large surface area with high metal dispersion. But some disadvantages of these catalysts will limit their applications as a popular tool for oxidation reactions in organic chemistry. For examples, the synthesis of microporous and mesoporous titanosilicates is somewhat complicated in order to avoid precipitation of TiO2 as a separate phase, which often acts as a catalyst poison in the subsequent oxidation reactions by hydrogen peroxide. Small crystal size (0.1-10 ~tm) of microporous and mesoporous titanosilicates has a trouble in separation of catalysts mixed with

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the reaction products. Small pore size of microporous titanosilicates restricts their use in oxidation reactions for larger organic molecules. Thus new materials with much higher reaction rate have always been sought [3]. Notably, although a series of microporous and mesoporous catalysts in oxidation reactions have been investigated intensively, catalysts with small surface area have not been studied yet, which were usually considered as low catalytic conversion or inactive catalysts in oxidation reactions [1-3]. Here we reported a novel catalyst of copper hydroxyphosphate (Cu2(OH)PO4) that has not microporous and mesoporous pores with very small surface area (< 0.01 m2/g) in hydroxylation of phenol by hydrogen peroxide as a model reaction for catalytic oxidation. Catalytic data show that the Cu2(OH)PO4 catalyst is very active with advantages of simple preparation method, high reaction rate, large crystal size, suitable solvent, and good catalyst regeneration. Furthermore, catalytically active sites are discussed. 2. EXPERIMENTAL The copper hydroxyphosphate of nominal composition Cu2(OH)PO 4 was hydrothermally synthesized using ethylenediamine (H2NCH2CHRNH2), phosphoric acid (H3PO4, 85 mass%), copper acetate (CuAc2) in a Teflon-lined stainless-steel autoclave for 3 days at 140-170~ The crystalline product was filtered, washed with distilled water and dried at ambient temperature, the final product being deep green with crystal size of very uniformed 900 ~tm. The X-ray powder diffraction patterns of the copper hydroxyphosphate, recorded with CuKa radiation, are similar to those of Libethenite mineral [4]. Single crystal diffraction data were collected with a Siemens P4 with Mo(Kct) radiation. All computations were carried out using the SHELXTL program (Version 5.0) [5]. Differential thermal analysis (DTA) and thermogravimetry (TGA) were performed on a Perkin-Elmer DTA 1700 analyzer and TGA7 analyzer with heating rate of 20K/min, respectively. Nitrogen isotherm was performed on ASAP 2010M at 77 K after degassing at 673 K. Catalytic phenol hydroxylation experiments were run in a 50 ml glass reactor and stirred with a magnetic stirrer. In a standard run, 1.6 g of phenol, 0.08 g of catalyst, and 15 ml of solvent were mixed, followed by addition of 0.58 ml of H202 (30% aqueous, molar ratio of phenol/H202=3.0, weight ratio of catalyst/phenol =0.05). After the reaction for 4 h at 80~ the products were hydroquinone (HQ), catechol (CAT), 1,4-benzoquinone (BQ), and tar. The products of hydroquinone, catechol, 1,4-benzoquinone were analyzed by GC, and the amount of tar was determined on products from which the catalyst was removed by filtration and the volatile products by evaporation in vacuo [the amount of tar in various runs was at 8-18 wt.-% (weight percentage of tar with hydroquinone, catechol, and 1,4-benzoquinone)]. Electron spin resonance (ESR) spectra were recorded on Bruker RE 200D spectrometer with microwave frequecy of 9.77 GHz and microwave power of 6.5 mW, equipped with 100 KHz modulation. 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was chosen as a spin-trap reagent. To determine relative intensity of radicals quantitatively, all reagents in this study were calculated, and a quartz tube with diameter of lmm was used. While the reactants were added

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into the reaction quartz tube, the ESR spectra were measured.

3. RESULTS AND DISCUSSION 3.l.Structure of Copper Hydroxyphosphate The structure of copper hydroxyphosphate was resolved by direct method and refined by full-matrix least squares to R=0.043 and Rw=0.060. The deep green copper hydroxyphosphate crystallizes in the orthorhombic space group Pnnm with a=8.058(2), b=8.393(2), c=5.889(2) A. The structure consists of PO4 tetrahedra, Cu(1)O5 pentahedra, Cu(2)O6 octahedra, and OH group between two Cu species, in which oxygen atoms are shared each other but no P-O-P chains (Fig. 1). In Cu(1)O5 pentahedra, two oxygen atoms are close to the Cu atom, giving Cu-O distances at averaged 1.932 A, and three oxygen atoms are far from the Cu atom, giving Cu-O distances at averaged 2.05 A. In Cu(2)O6 octahedra, four oxygen atoms are closed to the Cu atom, giving Cu-O distances at averaged 1.970 A, and two oxygen atoms are far from the Cu atoms, giving Cu-O distances at averaged 2.399 A. As a result, the Cu(2)O 6 octahedra becomes longer due to John-Teller distortion [6]. In PO 4 tetrahedra, P-O distances vary from 1.517-1.568 A (averaged 1.538 A), and O-P-O angles are within range 106.6-110.5 ~ suggesting that the tetrahedra is highly symmetrical.

01B

04A

OIA

4..a ~ r~ 4-a

04

F/////~ Cu2

O3D

03A

02A 03B

03C

I 5

10

I

I

20

30

40

20 (degrees) Fig. 1. Asymmetric unit of Cu2(OH)PO 4.

Fig.2. XRD patterns of (a) as-synthesized, (b) calcination at 550~ for 2h, (c) 850~ for 6h of the as-synthesized Cu2(OH)PO4.

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3.2.Characterization of Copper Hydroxyphosphate

Isotherms of nitrogen on the sample show no adsorption, suggesting that the framework structure is not microporous. DTA curve of the copper hydroxyphosphate does not exhibit any peaks in the ranges of 25-650~ and at the same range of temperatures TGA curve has not any weight loss. These results suggest that the copper hydroxyphosphate is thermal stable during 25-650~ When the temperature is over 650~ the curve exhibits a peak at 690~ which is assigned to a formation of new copper compound. Furthermore, XRD patterns (Figure 2) of the samples have confirmed the same assignment. Figure 2a shows a characteristic of assynthesized copper hydroxyphosphate. However, when the copper hydroxyphosphate is calcined at 850~ for 6 h, the calcined sample XRD pattern gives quite different peaks with that of the as-synthesized sample, as shown in Figure 2c. These results have demonstrated that calcination at 850~ for the copper hydroxyphosphate leads to a formation of new copper compound (Cu40(PO4)2) [7].

3.3.Catalytic Phenol Hydroxylation Catalytic activities of various samples in phenol hydroxylation are presented in Table 1. Na3PO4 is catalytically inactive. However, a series of copper compounds such as CuC12, CuO, Cu2(OH)PO4, Cu40(PO4)2, Cu(OH)2 are catalytically active, giving the conversion in the range of 7-28%. These results suggested that copper species are catalytic active sites in the catalysis. In particular, the phenol conversion on the CUE(OH)PO4 catalyst prepared from hydrothermal crystallization is highest (28%), which is even slightly higher than that of TS-1 catalyst that was well known as an effective catalyst in catalytic hydroxylation of phenol [ 1-3]. It is very interesting to note that the best solvent on the Cu2(OH)PO4 catalyst is water. In contrast, the suitable solvents on a series of microporous titanosilicate catalysts are methanol, acetone, and acetonitrile [1-3]. Furthermore, we observed that the crystal size of the Cu2(OH)PO4 catalyst is very large as compared with that of TS-1 catalyst, indicating that the separation from the mixture of catalyst and products is relatively easy. Additionally, the preparation of the CUE(OH)PO4 catalyst with a pure phase is relatively simple and easy. In contrast, the synthesis of titanosilicate catalysts such as TS-1 is somewhat complicated in order to avoid the formation of TiO2 as a separate phase. Moreover, the Cu2(OH)PO4 is prepared by using ethylenediamine as an organic alkali. In contrast, TS-1 catalyst is usually synthesized in presence of organic templates such as TPAOH which are very expensive. Consequently, the price for preparation of the CUE(OH)PO4 is much lower than that of TS-1 catalyst. The Cu2(OH)PO4 catalyst is readily regenerable to its active state by recalcining the expired form in air at 550~ for 2h. The X-ray diffraction and infrared spectroscopy, as well as the catalytic characterization of the recycled catalyst, show neither detectable structural degradation nor loss in activity of any significance. The unusual catalytic activity in hydroxylation of phenol on the Cu2(OH)PO4 catalyst may be related to unique structure of as-synthesized CUE(OH)PO4. Notably, the catalytic activities of CuCI 2, CuO, and Cu40(PO4)2 are very similar, giving conversion at 7.5, 7.9, and 11.8%, respectively (Table 1). The structure of Cu40(PO4) 2 prepared from calcination of Cu2(OH)PO4

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Table 1 Catalytic activities in hydroxylation of phenol by H202 over various catalysts Catalyst Crystal Surface area, Solvent Phenol H202 efficiency, % size4tm m2/g conv., % Na3PO 4 . . . . . . water . . . . . . CuCI 2 . . . . . . water 7.5 28.1 Cu2(OH)PO4 900 2.4x 10.3 water 28.3 82.0 Cu2(OH)PO4 900 2.4x 10.3 acetonitrile 5.9 20.5 Cu2(OH)PO4 900 2.4x 10.3 acetone . . . . . . Cu2(OH)PO4 s 900 2.4x 10.3 water 16.1 51.2 Cu2(OH)PO4s~ 900 2.4x 10.3 water 28.2 81.6 Cu40(PO4)2" 50 2.4x 10.3 water 11.8 39.6 CuO 5 0.44 water 7.9 30.5 Cu(OH)2 5 18.6 water 9.2 35.3 TS-1 0.4 380 acetone 25.5 72.3 s Used catalyst. s, Regeneration of used catalyst by calcination at 550~ for 2h. Prepared by calcination of Cu2(OH)PO4 at 850~ for 6 h. (Figure 2) also consisting of CuO5 and C u O 6 units, is similar to that of Cu2(OH)PO 4. Only difference in structure between Cu2(OH)PO4 and Cu40(PO4)2 is the OH species attached on Cu sites [7]. Relating to a large difference in catalytic conversion for Cu2(OH)PO4 and Cu40(PO4)2 catalysts (Table 1), we suggest that the OH species attached on Cu sites play an important role for the catalysis. However, Cu(OH)2 catalyst with both OH and Cu sites shows low catalytic conversion (9.2%) as compared with Cu2(OH)PO4 catalyst (Table 1), suggesting that the conventional Cu(OH)2 sample is not good catalyst. Therefore, we proposed that the high activity for Cu2(OH)PO4 in the catalysis could be mainly attributed to synergetic effect of OH and Cu species of Cu2(OH)PO4 catalyst with unique structure.

3.4.ESR Spectra Figure 3 shows ESR spectra of H202 on CuO and Cu2(OH)PO 4 with DMPO at room temperature, respectively. Notably, very strong signals (aN= a.=1.49 mT) appeared on Cu2(OH)PO4 catalyst, which are assigned to the adduct of hydroxyl radical with DMPO (DMPO-OH) [8]. On the CuO catalyst, the signals were much weaker, suggesting much lower concentration of hydroxyl radicals. In the contrast, the ESR spectrum of H202 on TS-1 with DMPO shows typical signals assigned to DMPO-O2, which are well consistent with those of reported mechanism [2]. Comparison of the intensity of hydroxyl radicals with catalytic activity on various catalysts, it was reasonably suggested that hydroxyl radicals may be important intermediates in the catalysis. The easier the production of .OH, the faster the production of diphenols. Possibly, OH species in Cu2(OH)PO4 catalyst promote production of hydroxyl radicals (.OH) in the catalysis.

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15 Gauss

_

-

-

.

.

.

.

-

f

_

Figure 3. ESR spectra of H202 on the samples with DMPO: (a) CuO & (b) Cu2(OH)PO4 ACKNOWLEDGEMENTS: The financial support of this research by the National Natural

Science Foundation of China, the Education Ministry, and National Advanced Materials Committee of China (NAMMC) is acknowledged. REFERENCES

1. M. Taramasso, G. Perego, and B. Notari, US Patem No. 4410501 (1983). 2. D.R.C. Huybrechts, L. De Bruycker, and P. A. Jabobs, Nature, 345 (1990) 240. 3. R. Murugavel, and H. W. Roesky, Angew. Chem. Ira. Ed. Engl., 36 (1997) 477. 4. Natl. Bur. Stand. (US) Monogr., 25 (1980) 30. 5. G.M. Sheldrick, SHELXTL PLUS, Program packed for structure solution and refinement, version 4.2, Simens Analytical Instruments Inc., Madison, WI, 1990. 6. F.A. Cotton and G. Wilkinson (eds.), Advanced Inorganic Chemistry, John Wiley and Sons, 1988, 54 ed., p766-767. 7. M. Brunel-Laugt, A. Durif, and J. C.Guitel, J. Solid State Chem., 25 (1978) 39-47. 8. H, Xiao, W. Fu, B. Zhao, F. Yang, and W. Xin, Acta Biophysica Sinica, 8 (1992) 334.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

化

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Oxidation of alkanes with hydrogen di-iron-substituted inorganic synzyme ~

peroxide

catalyzed

by

Noritaka Mizuno, Yoshiyuki Nishiyama, Ikuro Kiyoto, and Makoto Misono Department of Applied Chemistry, Craduate School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan The Keggin-type di-iron-substituted silicotungstate, y-SiW~0{Fe(OH2)}203s6, can catalyze the selective oxidation of various alkanes including methane with hydrogen peroxide. The tetrabutylammonium salt catalytica~ oxidized cyclohexane, n-hexane, n-pentane, and adamantane in acetonitrile. Even lower alkanes such as methane, ethane, propane, and n-butane were catalytically oxidized. It is remarkable that the efficiency of hydrogen peroxide utilization to oxygenated products reached up to ca. 100% for the o~genation of cyclohexane and adamantane. It was also demonstrated that the water-soluble potassium salt catalytically, oxidized the lower alkanes with hydrogen peroxide in water. I. INTRODUCTION The d/nuclear iron-oxo centers ofhemerythrin [ 1], ribonucleotide reductase [2], and the purple acid phosphatases [3] catalyze not only the oxidation of methane to methanol but also the oxygen atom transfer from 02 into alkanes, alkenes, ethers, and so on [4,5]. While a number of structural models with di-iron center have been reported, functional models with di-iron center for oxidations of alkanes are fewer in numbers mainly because of the instability of organic ligands towards oxygen donors. [Fe(salen)]:O (salen = N, N'-ethylenebis (s alicylideneaminato)) [6], Fe20[HB(pz)3]2(OAc)2 (pz = 3,5-bis(isopropyl}pyrazolyl) [7], [(PA):Fe]:O (PA = 2,6dicarboxylatopyridine) [8], Fe20(OAc 2(bpy):C12 (bpy = 2, 2'-bipyridine) [9,10], and [Fe2(TPA)20(OAc)](C104)3 (TPA = tris 2-pyrldylmethyl)amine) [11]are examples. Various elements can be introduced into polyoxometalates and the countercations and the numbers and structures can also be controlled. The additional attractive aspects of polyoxometalates in catalysis are their inherent stability towards oxygen donors such as molecular oxygen and hydrogen peroxide. For example, high stability towards hydrogen peroxide has been reported for manganese- or ironsubstituted polyoxometalates [12-19]. Therefore, polyoxometalates are useful catalysts for liquid-phase oxidations of various orgamc substrates with hydrogen peroxide. There are several examples of iron-containing polyoxometalates, mono and tnironsubstituted Keggin-type silicotungstates [16,17], Fe2Ni-substituted Keggin-type phosphotungstate [18], and tetrairon-containing D a w s o n - a n d Keggin-derived sandwich comvounds [13,14]. However, d/ir0n-substituted silicotungstate, ~,-SiW10{Fe(OH:)}:O3s6 has never been reported. Recently, we have reported the s y n t h e s i s ofy-SiWlo{Fe(OH2)}EO3s ~ [19].

"

Here, we demonstrate that inorganic methane monooxygenase analogue,

f This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan. To whom all correspondence should be addressed.

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),-SiWl0{Fe(OH2)}203s6" silicotungstate, of which the structure of iron center is s i n ~ r to that of methane monooxygenase, can catalyze oxidation of various alkanes not only in an organic solvent but also in water. 2. EXPERIMENTAL

2.1. Preparation ofpolyoxome~alates

The y-SiWlo{Fe(OH2)}203s polyoxometalate was sy_nthesiz~d as reported previously [ 1 9 ] . Tetrabutylhmn~nium salt of ~,-SiWl0{Fe(OH2)}203s ~ was synthesized as follows. Stoichiometric amounts of Kg[~,-SiWloO36]" 12H20 (1 mmol) and Fe(NO3)3-9H20 (2 mmol) were mixed under acidic conditions. After the solution had been stirred for 5 min, the addition of an excess tetrabutylammonium nitrate (10 mmol)resulted in a white-yellow precipitate. The precipitate was fdtered offand purified by twice dissolving it in acetonitrile (15 mL) and then adding water (300 mL) to reprecipitate the product. The resulting tetrabutylamn~nium salt was purified by reprecipitation from acetonitrile / water. The yield was 37 %. Elemental anal.: Found (calcd) for [(C4H9)4N]3.sH2.5[y-SiW lo{Fe(OH2)}203s] 9 H20: C, 19.08 (19.26); H, 3.63 (3.88); N, 1.58 (1.40); Si, 0.80 (0.80); Fe, 3.20 (3.20). Infrared spectrum (cm-'): 1025 (w), 1002 Fi~. 1. Polvhedral representation of (w), 961 (s), 903 (s), 886 (s), 797 (s), 755 ~,-giWlo{Fe((~H2)}20386 Keggin-type (s, br), 547 (w). Figure 1 shows the polyoxometalate. Iron atoms are polyhedral representation of resented by shaded octahedra. octahedra occupy the white y-SfWlo{Fe(OH2)}203s 6" confn'med by the various spectroscopic data [19]. octahedra and an SiO4 group is shown as the internal black tetrahedron. 2.2. Titration of hydrogen peroxide The titration of hydrogen peroxide was carried out according to ref. [20]. 1 - 2 g of the reaction solution was accurately weighed and quickly dissolved in 200 mL water. The solution was stirred with a magnetic stirbar at 296 K. Titration data were obtained with HM-30 pH meter (TOA Electrochemical Measuring Instruments). Thepotential was monitored as a solution of Ce(NH4)4(SO4)4" 2H20 in water (0.1 M) was added with a buret into the solution in 0.1 mL intervals.

2.3. Catalytic reaction

The oxidation reactions of cyclohexane, n-hexane, n-pentane, and adamantane were carried out in a glass vessel by mixing of 1.0 mn~l of substrate, 1.0 mn~l of 30% hydrogenperoxide, and 8 ~tmol polyoxometalate with acetonitrile (6 mL)under Ar. The oxidation reactions of methane, ethane, propane, and n-butane were carried out with an apparatus, where the wall inside the stainless tube was coated with Teflon to avoid the progress of oxidations on the wall. The standard procedure was as follows. Specified amounts of salts of~,-SiW~o{Fe(OH2)}2Oas6 (5 pmol) and hydrogen peroxide (1.0 or 2.4 mmol) were dissolved in 1.8 or 1.9 mL of water in a Teflon vessel. Next, this Teflon vessel was quickly attached inside an autoclave. The gas phase above the liquid was removed by evacuation and then the autoclave was pressurized with alkanes. The autoclave was heated up to 305 or 353 K in an oil bath for 24 h. After the reaction, the autoclave was cooled to room temperature.

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The reaction solution was analyzed on a gas chromatography with Porapak QS and HayeSep DB colurrms or with TC-WAX and FFAP capillary columns. For the calculation oI the efficiency of hydrogen peroxide utili~tion, epoxides, alcohols, ketones / aldehydes, and acids were counted as requiring one, one, two, and three oxidizing equivalents, respectively. The gas phase analysis was carried out by TCD C~ with Porapak QS and Molecular Sieve 5A columns. 3. RESULTS AND DISCUSSION

The oxidation of various alkanes with hydrogen oeroxide was carried out in the presence of tetrabutylammonium salt of y-SiW10{Fe(~)H2)}20386 in acetonitrile at 305 K. The results are shown in Table 1. The main products were the corresponding alcohols, ketones, aldehydes, and acids. The tumover numbers (estimated by moles of oxidizing equivalent in all products per mole of catalyst) for the oxidation of methane, ethane, propane, n-butane, n-pentane, n-hexane, cyclohexane, and adamantane were 25, 64, 42, 36, 19, 33, 53, and 57, respectively. This shows that the reactions are catalytic. To our knowledge, such high turnover number of 25 has never been reported for oxidation of methane with hydrogen peroxide by d#-iron containing model complexes having organic ligands. It is remarkable that the efficiencies of hydrogen peroxide utili~tion to oxidized products for the oxidation of cyclohexane and adamantane were aln~st 100%. Such high efficiency in the oxidation of cyclohexane has never been reported: For example, Table 1 Oxidation of various alkanes with hydrogen peroxide catalyzed by v-SiW~n[Fe(OH2)}20.t86 in acetonitrile at 305 K Substrate Conversiona/% Product Selectivity/% Efficien~ mcyO/% Methane r 0.4 Methanol 73 20 Carbon dioxide 27 Ethane r 1.2 Acetaldehyde 57 52 Acetic acid 40 Carbon dioxide 3 Propane r 4.0 Acetone 86 33 Acetaldehyde 12 Carbon dioxide 2 n-Butane ~ 11.0 Methylethylketone 82 28 Acetaldehyde 17 Carbon dioxide 1 n-Pentane d 9.0 Pentanols ~ 33 74 Pentanone, s~ 67 n-Hexaned 15.0 Hexanols" 25 83 Hexanones f 75 Cyclohexane d 25.0g Cyclohexanol 32g 99 Cyclohexanone 68 g Adamantane h 42.0 1-Adamantanol 20 99 2-Adamantanol 71 2-Adamantanone 9 Catalyst, 8 ~tmol; H202, 1 mmol; reaction time, 96 h; acetonitrile, 6 mL. a(MO1 of products/mol of substrate used) x 100. ~162 100 (%), where [H202]~ is the concentration of H202 consumed. The pressures of~ methane, ethane, ~mpane, and n-butane were 50, 28, 6, and 2 atm, respectively. "Substrate, 1 mmol. 1-o1:2-o1:3-ol = 5:27:68; 1-one:2-one:3-one = trace:35:65, q-o1:2-o1:3-ol = 11i35:54; 1one:2-one:3-one- trace:47:53, gConversion to dicyclohexyl was < 1%. Solvent, acetonitrile/benzene = 6 mL/3 mL.

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the efficiency was higher than those in the cyclohexane oxidations with hydrogen peroxide on the catalysts (efficiency m the parentheses), Fe(PA)2 (PA, picolinic acid; 85%), FeO3/py/Ph2S/picolinic acid system (79%), Mn(TDCPP)CI (H2TDCPP, 5,10,15,20-tetrakis-2',6'-dichlorophenyl porphyrin; 52%), TS-! (32%),

~ = -~ N 100 @ "-" "=~ .~ 80 "~

[PWgO37{Fe3.xNix(OAe)3}] C~§

~,

(x

=

predominantly 1) (14%); [Fe20(bipy)4(OH2)2] [CIO414 (8%), or Fe30(OAC)6(H20)3 (4%). The dependency of the efficiency on reaction temperatures and concentrations of hydrogen peroxide was investigated. The efficiency was slightly decreased to 95% by increasing reaction temperature from 305 K to 356 K and still high. The efficiency was kept almost 100% below the amount of H202 of 1 mmol and decreased to 48 % at the amount of H202 of 2 mmol as shown in Fig. 2. The catalytic activity and selectivity of v-SiW10{l~e(OH2)}20386 were solventdependent as shown in Table 2. The oxMation hardly proceeded in dimethylformamide and methanol, while the oxidation of cyclohexane proceeded

A

~ 60 ~=

-~ 40 .=~ ~ 20 ~= ~ 0 [-rd

I

I

I

I

0 1 2 Amount of hydrogen peroxide / mmol

Fig. 2. Change in efficiency of hydrogen peroxide utifization with the amount of hydrogen peroxide for the oxidation of cyclohexane. t.;atalyst, 8 lunol; reaction time, 96 h; acetonitrile, 6 mL; reaction temp., 305 K.

Table 2 Effect of solvent on oxidation of cyclohe~ane with hydrogen peroxide catalyzed by v-SiW~n{Fe(OH2)}20~6 at 305 K Solvent Conversiona/% Selectivity/% Efficiency"/% Cyclohexanol Cyclohexanone Acetonitrile 25.0 32 68 99 1,2-Dichloroethane

2.0

45

55

3

Methanol

0.3

0

100

<1

trace

0

100

<1

Dimethylformamide

Catalyst, 8 ~unol; H202, 1 mmol; reaction time, 96 h; solvent, 6 mL. a'~SeeTable 1. in acetonitrile and 1,2-dichloroethane. The highest conversion was obtained for acetonitrile. The initial rates (i.e., intrinsic activities) much changed with the structures of iron centers: The initial rate of oxidation of cvclohexane catalyzed by 7-SiW10{Fe(OH2)}20386 was 101 - 102 times higher than tliose of n o n - , m o n o - and triiron-substituted s~cotungstates, showing that the di-iron site in ~,-SiWlo{Fe(OH2)}2038 i s an effective center for the oxidation. Such a structure

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Table 3 Oxidation of lower alkanes with hydrogen peroxide catalyzed by v-SiW~o[Fe(OH2)]20~s6 in water at 353 K Substrate Conversion'/% Product Selectivity/% Efficiency~ M ethane ~'~ 0.1 M ethylformate 54 5 Methanol 1 Formic acid 1 Carbon dioxide 44 EthaneC'+ 0.3 Acetic acid 77 16 Acetaldehyde 10 Carbon dioxide 13 Propane ~'+ 1.1 Acetone 82 12 Propionic acid 11 Carbon dioxide 7 n-Butane ~'~ 0.6 Methylethylketone 52 6 Carbon dioxide 48 Catalyst, 5 iraqi; reaction time, 24 h; water, 1.8 or 1.9 mL. a'"See Table 1. ~The pressures of methane, ethane, prot)ane, and n-butane were 50, 28, 6, and 2 atm, respectively, dH202,2.4 mmol. +ff202, 1.0 mmol. dependency of the catalysis is noticeable and the catalytic performance of di-ironcontaining polyoxometalate is possibly related to the catalysis by methane monooxygenase. The oxidation of lower alkanes with hydrogen peroxide in water by water-soluble potassiumsalt ofT-SiW~0{Fe(OH2)}203s6 was camed out since the use of water as a solvent in homogeneous transition-metal catalysis has been of growing importance because of its environmentally friendly nature and catalytic oxidation of alkanes in water is of great interest on the standpoint of enzymatic catalysis. The data are shown in Table 3. In the case of oxidation of methane, hydrogen peroxide was completely consumed and the amounts of respective products little changed after 24 h. T h e resulting hydrogen peroxide decomposed into molecular oxygen, of which the amount was also quantitatively confilmed. The main products were methylformate (yield, 10.3 ~nol) and carbon dioxide (17.8 ~anol). The total turnover number reachedto 23 (8.4 to selective oxidation products) after 48 h, showing that ~,-SiWlo{Fe(OH2)}20386is catalytically active. Such catalytic oxidation of methane by di-iron containing model complex catalysts under environmentally fliendly conditions (i.e., with hydrogen peroxide in water) has never been reported. The amounts of products significantly increased with the increase in the reaction temperature from 303 K to 353 K, and decreased by further increase in the reaction temperature to 403 K, where the decomposition of hydrogen peroxide to form dioxygen and water was dominant at 403 K. Therefore, reaction was carried out at 353 K. The other lower alkanes were also catalytically oxidized to the corresponding ketones, aldehydes, and acids. Solubilities of methane (50 atm) in acetonitrile and water were 510 + 50 and 70 + 10 mmol/L, respectively. The lower conversions in water than those in acetonitrile probably results from the lower solubility. REFERENCES 1. D.M. Kurtz, Jr., D. F. Shriver, and I. M. Koltz, Coord. Chem. Rev., 24 (1977) 145. 2. T. Joelson, U. Uhlin, H. Eklund, B.-M. Sj6berg, S. Hahne, and M. Karlsson, J. Biol. Chert, 259 (1984) 9076; B.-M. Sj6berg, T. M. Loehr, and J. Sanders-Loehr, Biochemistry, 21 (1982) 96; L. Petersson, A. Gr/islund, A. Ehrenberg, B.M. Sj6berg, and P. Reichard, J. Biol. Chent, 255 (1980) 6706; C. L. Atkin, L. Thelander, P. Reichard, and G. Lang, J. Biol. Chent, 248 (1973) 7464; P. Nordlund, B.-M. Sj6berg, and H. Eklund, Nature, 345 (1990) 593; R. Davydov, S.

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Nordlund, B.-M. Sj6berg, and H. Eldund, Nature, 345 (1990) 593; 1L Davydov, S. Kuprin, A. C~slund, and A. Ehrenberg, J. Am. Chem. Soc., 116 (1994) 11120. 3. J.W. Pyrz_, J. T. Sage, P. G. Debrunner, and L. Que, Jr., J. Biol. Chelrt, 261 (1986) 11015; B. A. Averill, J. C. Davis, S. Burman, T. Zirino, J. Sanders-Loehr, T. M. Loehr, J. T. Sage, and P. G. Debrunner, J. Am Cherrt Soc., 109 (1987) 3760; B. C. Antanaitis, T. Strekas, and P. Aisen, J. Biol. Chem_, 257 (1982) 3766; 1L B. Lauffer, B. C. Antanaitis, P. Aisen, and L. Que, Jr., J. Biol. Cherrt, 258 (1983) 14212; D. F. Hunt, J. R. Yates, III, J. Shabanowitz, N.-Z. Zhu, T. Zirino, B. A. Averill, S. T. Daurat-Larroque, J. G. Shewale, 1L M. Roberts, and K. Brew, Biocherrt Biophys. Res. Commun., 144 (1987) 1154; J. B. V'mcent, M. W. Crowder, and B. A. Averill, Biochemistry, 30 (1991) 3025. 4. J. Colby and H. Dalton, Biocherrt J., 157 (1976) 495; J. Colby, D. I. Stifling, and H. Dalton, Bioche~ J., 165 (1977) 395; J. Colby and H. Dalton, Biochent J., 171 (1978) 461; J. Green and H. Dalton, Biochent J., 236 (1986) 155; J. Green and H. Dalton, J. Biol. Chent, 263 (1988) 17561; D. D. S. Smith and H. Dalton, Eur. J. Biochem., 182 (1989) 667. 5. B.G. Fox and J. D. Lipscomb, Biochem. Biophys. Res. Commum., 154 (1988) 165; B. G. Fox, W. A. Froland, J. E. Dege, and J. D. Lipscomb, J. Biol. Chem_, 264 (1989) 10023. 6. D.H. 1L Barton, J. Boivin, M. Gastiger, J. Morzycki, 1L S. Hay-Motherwell, W. B. Motherwell, N. Ozbalik, and K. M. Schwartzentruber, J. Cherrt Soc., Perkin Trans. 1, (1986) 947. 7. N. Kitajima, M. Ito, H. Fukui, and Y. Moro-oka, J. Che~ Soc., Chero_ Commun., (1991) 102. 8. C. Sheu, S. A. Richert, P. Cofr6, B. Ross, Jr., A. Sobkowiak, D. T. Sawyer, and J. R. Kanofsky, J. Am. Chem. Soc., 112 (1990) 1936. 1L H. Fish, M. S. Konings, K. J. Oberhausen, 1L H. Fong, W. M. Yu, G. Christou, J. B. Vincent, D. K. Coggin, and 1L M. Buchanan, Inorg. Chem., 30 (1991) 3002. 10. J.B. V-recent, J. C. Huffman, G. Christou, Q. Li, M. A. Nanny, D. N. Hendrickson, IL H. Fong, and R. H. Fish, J. Am. Chem. Soc., 110 (1988) 6898. 11. R.A. Leising, J. Kim, M. A. P6rez, and L. Que, Jr., J. Am, Che~ Soc., 115 (1993) 9524. 12. M. B6sing, A. NOh, I. Loose, and B. Krebs, J. Am. Chem. Soc., 120 (1998) 7252. 13. X. Zhang, Q. Chen, D. C. Duncan, C. F. Campana, and C. L. Hill, Inorg. Cherrt, 36 (1997) 4208. 14. X. Zhang, Q. Chen, D. C. Duncan, R. J. Lachicotte, and C. L. Hill, Inorg. Cherrt, 36 (1997)4381. 15. 1L Neumann and M. Gara, J. Am. Chem. Soc., 116 (1994) 5509. 16. F. ZonnevijUe, C. M. Tourn6, and G. F. Tourn6, Inorg. Chem., 21 (1982) 2751. 17. J. Liu, F. Ort6ga, P. Sethuraman, D. E. Katsoulis, C. E. Costello, and M. T. Pope, J. Chem. Soc., Dalton Trans., (1992) 1901. 18. N. Mizuno, T. Hirose, M. Tateishi, and M. Iwamoto, J. Mol. Catal., 88 (1994) L125. 19. N. Mizuno, C. Nozaki, I. Kiyoto, and M. Misono, J. Am Cherrt Soc., 120 (1998) 9267; J. Catal., 181 (1999) 171; J. Catal., 182 (1999) 285, J. Catal., 184 (1999) 550. 20. A. I. Vogel, A Textbook of Quantitative Inorganic Analysis Including Elementary Instrumental Analysis, Longman, New York, 1978. .

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Vanadyl-Aluminum Binary Phosphate: Effect of Thermal Treatment over its Structure and Catalytic Properties in Selective Oxidation of Aromatic Hydrocarbons F.M. Bautista*, J.M. Campelo, A.Garcia, D. Luna, J.M. Marinas, A.A. Romero and M.T. Siles Departamento Quimica Org~.nica, Facultad de Ciencias, Universidad de C6rdoba, Avda. San Alberto Magno, s/n, E-14004 C6rdoba, Espafia.**

Aluminuna-vanadium-phosphoms catalysts calcined over the range 450-750~ were synthesized and characterized by using TGA, XRD, DRIFT and DR UV-V spectroscopes, 27A1, 5~V, and 31p MAS NMR, TPR and nitrogen adsorption. Decomposition of 2-propanol and the selective oxidation of benzene, toluene and o-xylene were used as catalytic tests. Those catalysts calcined at 450-550~ showed the best catalytic performances 1. INTRODUCTION Vanadium-phosphorus mixed oxides (V-P-O) have been the subject of extensive research, due to their wide use as catalysts for the oxidation of n-butane to maleic anhydride [e.g. (1)]. Several phases have been identified in different oxidation states of vanadium, depending on the formation of a specific phase mainly in the preparation method [2]. On the other hand, aluminum phosphates, A1PO4, have also been intensively studied as catalyst and catalyst support in a great variety of reaction processes including the oxidative dehydrogenation of ethylbenzene [3] and it is also a good support for VPO catalysts in the selective oxidation of 1-butene to maleic anhydride [4]. Here we study several systems based on aluminum-vanadiumphosphorus (A1VPO) reporting on the preparation, characterization and catalytic behavior of the selective oxidation of benzene, toluene and o-xylene The materials have been characterized by several physical methods. Furthermore, decomposition of 2-propanol was selected as a probe reaction for the characterization of solid acid and basic (redox) sites. 2. EXPERIMENTAL The A1VPO system (P/V/A1 molar ratio = 1/0.5/0.5) was prepared from A1C13.6H20, VOC13 and H3PO4 (85 wt-%) by precipitation with aqueous ammonia and stirring at 0~ until pH=4. After filtration the solid was calcined in air at 450, 550, 650 y 750~ for 3 h to obtain four catalysts: A1VPO-450, A1VPO-550, A1VPO-650 and A1VPO-750. As a reference, a VPO system (P/V molar ratio = 1/1.3) was also obtained [5] and calcined at 450 and 550~ "Corresponding author. E-mail: qo [email protected] **This work was subsidized by a grant from DGESIC (PB97-0446) and Consejeria de Educaci6n y Ciencia.

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The final A1, V and P contents of the systems determined by inductively coupled plasma atomic emission spectrometry were practically coincidental with the theoretical amounts, Table 1. Textural properties were obtained by using a Micromeritics ASAP 2000 apparatus with nitrogen as the adsorbate. Thermogravimetric analysis (TGA) was obtained using a Cahn 2000 thermobalance in the presence of static air at 10~ min ~ (30-800~ X-ray diffraction (XRD) patterns (20 = 10 to 50 ~ were carried out using a Siemens diffractometer (Ni-filtered CuKct radiation, 35 kV and 20 mA). 27A1(pulse: 2 ~ts; recycle delay: 2 s), 31p (pulse: 2.6 ~ts; recycle delay: 6 s) and 5~V (pulse: 5 kts; recycle delay: 5 s) MAS NMR spectra were recorded at 104.26, 161.98 and 105.25 MHz, respectively, with a Bruker ACP-400 spectrometer. Diffuse reflectance infrared (DRIFT) spectra (4000 to 400 cm 1, 256 scans, resolution: 8 cm ~) were recorded on a FTIR instrtmaent (Bomen MB-100). All the calcined catalysts (diluted to 15 wt% in KBr) were previously dried at 120~ for 24 h under vacuum. Diffuse reflectance UV-V (DR UV-V) spectra (200-800 nm) were collected with a Variant Cary 1E UV-Vis spectrophotometer. Temperature programmed reduction (TPR) was obtained in a Micromeritics apparatus. Samples of 50 mg were first treated in Ar at 100~ for l h. and then contacted with an H2/Ar mixture (10% mol of H2 ; total flow of 50 cm 3 min ~) and heated (5~ min ~) to a final temperature of 800~ 2-Propanol reactions were studied with a microcatalytic pulse reactor by using: 2-propanol size 2 ~tL; reaction temperature, 170-250~ catalyst weight, 20 mg; flow rate of N 2 carrier gas, 10 cm 3 min ~. The reaction products were analyzed by GC with FID. Hydrocarbons oxidations were carried out in a fixed-bed tubular reactor with a continuous-flow system, and on-line GC analysis of reagent and product composition. Catalyst (ca. 100 mg, <0.149 mm) was pretreated at the reaction temperature (310-450~ for 1 h in a N2 flow (30 c m 3 min-~). Reagents were fed at 0.6 mL h-~ (W/F = 0.19 h) and the flow of 0 2 and He gas were 30 and 80 c m 3 min -~, respectively. Products were characterized by GC-MS. Conversion (C) and selectivity to products (S~) are expressed as mol% on a C atom basis. The carbon balances were within 100+ 5%. 3. RESULTS AND DISCUSSION

The nitrogen isotherms obtained for A1VPO and A1VPO-450 samples were type IV of the BDDT classification, exhibiting H1 hysteresis loops which correspond to mesoporous solids. Both AIVPO as well as VPO systems displayed reversible Type II isotherms after heating at 450~ Table 1 compiles the values of BET surface area, SBET,and the pore volumes, Vp, and mean pore radii, ro, for those solids with porosity. The SBETvalues decreased as calcination temperature increased, indicating a sintering process in the A1VPO solid beyond 550~ Besides, A1VPO catalysts showed SBEThigher than VPO catalysts. A1VPO and VPO TG profiles were very similar and both were characterized by two weight losses up to 400~ The first one, <200~ can be attributed to the desorption of physically adsorbed water and 2-propanol. The second one, between 200 and 400~ is the highest (10,3 wt% and 15.9 wt% for A1VPO and VPO, respectively) and it can be associated with the removal of NH4C1. At temperatures >400~ a slight weight loss (<2 wt%) was observed for A1VPO that could mainly be due to the condensation of surface hydroxyls as occurred in A1PO4-A1203 and A1PO4-B203 systems [6,7] and/or to dehydration of the vanadyl hydrogenphosphate hydrates [2]. XRD pattern of uncalcined A1VPO, Fig. 1A, corresponds to an amorphous solid. The weak reflections observed could be related to VOHPO 4. 4 H20 and/or VOHPO 4. 0.5 H20 [2]. After calcination at 450~ weak reflections related t o ~ - V O P O 4 [JCPDS 27-948] and to A1PO4

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Table 1.- Chemical composition and textural properties of different systems and hydrogen consumption in TPR, H2. Chemical composition A1 V p

Catalyst

rp

SBET (m2/g)

Vp (mE/g)

(nm)

H2* 10 3 (mol/g)

A1VPO

26.4

24.9

48.7

12.6

0.07

11.1

2.1

A1VPO-450

26.4

24.9

48.7

10.8

0.06

12.5

2.7

A1VPO-550

26.3

24.9

48.8

2.9

-

-

1.5

A1VPO-650

27.4

24.6

48.0

3.1

-

-

1.1

A1VPO-750

27.0

25.1

47.9

0.9

-

-

1.1

VPO

-

57.1

42.9

2.0

-

-

1.2

VPO-450

-

56.4

43.6

1.8

-

-

4.9

VPO-550

-

57.5

42.5

0.8

-

-

4.9

0

0

O0

_

0

0

J

vpo .

10

20

.

30

.

.

.

40

.

.

.

.

10 2 THETA

20

30

40

Fig. 1. Powder XRD profiles ( C u K a , ?~=1.5405/k): (O) 13-VOPO4; (A)A1PO 4 Table 2.-27A131p and 5~V chemical shifts Catalyst A1VPO-450

6v (ppm) -

6p (ppm) -11.9 ; -31.5

6~ (ppm) 37.5

A1VPO-550

-760

-11.7 ; -29.0

38.8

A1VPO-650

-760

-11.6 ; -28.6

39.1 ; 28.7

AIVPO-750

-450 ;-760

-29.1

38.6 ; 33.1 ; 28.7

VPO-450

-760 ;-777

- 10.6 ; -20.2

-

VPO-550

-760;-777

-10.4 ;-20.2

-

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crystallized in the ct-cristobalite form [JCPDS 11-500] were obtained. Thermal treatment at 550~ and 650~ resulted in the formation of two crystallized compounds I3-VOPO4 and AIPO4. Finally, from the XRD pattern of the A1VPO system calcined at 750~ AIPO4 was clearly identified but I3-VOPO4 was not observed. Thus, the sample showed new lines which can not be assigned to any vanadyl phosphate phase described. With respect to VPO samples, Fig. 1B, it can be seen that uncalcined sample showed a very low degree of crystallinity as revealed by the two broad lines at 20=14 ~ and 20=28.6 ~ which could be related to VOHPO4" 0.5 H20. In addition, VPO samples calcined at 450 and 550~ turned into fully crystallized [3-VOPO4. The temperature treatment of the VPO in oxygen-containing atmosphere would explain the presence of 13-VOPO4 instead of vanadyl pyrophosphate due to its oxidation [8]. 5IV MAS spectra allow us to obtain accurate information about isotropic chemical shifts (Siso) of the vanadium species (VS+). The spinning frequencies used in the present work, 3.5, 5.3 and 6 kHz, let us determine the unshifted spinning sidebands (SSB) and, accordingly, the 8iso values [9,10], Table 2. However, the A1VPO-450 8~o value was not obtained since its spectra showed an intense and broad band between-200 and -500 ppm, indicating vanadium ions in a disorganized environment (or amorphous character) in agreement with XRD results. All the other systems exhibited one set of SSB with 8~so= -760 ppm that could be adscribed to I3-VOPO4 according to XRD results and to the very close value assigned to VOPO 4 compound, ~i~so= -734 ppm [9]. A1VPO-750 spectra also showed an additional SSB pattern of high intensity with 8~so= -450 ppm, that suggests the formation of new vanadium sites with a local symmetry differem than I3-VOPO4. In VPO spectra a second set of small intensity SSB with 8iso= -777 ppm, which could be associated with a VOPO4 phase similar to [3-VOPO4. A1VPO and VPO alp MAS spectra of both uncalcined systems showed a very broad signal that can be related to the presence of some vanadyl phosphate phase containing V 4§ given the paramagnetic character of this cation [11]. In addition, the uncalcined A1VPO spectrum showed a peak at about -31.5 ppm which was also displayed in the spectra of calcined A1VPO systems (see Table 2) that is characteristic of phosphorous in a P(OA1)4 environment [6,7]. The resonance was narrowed during thermal treatment of the samples and beyond 550~ it shifted to -29 ppm due to the crystallization of A1PO4 into ct-cristobalite polimorphs [6]. When A1VPO samples were calcined up to 750~ another resonance at around -11.5 ppm, Table 2, was obtained. This resonance is also present in VPO spectra and can be attributed to 13-VOPO4 [ 11]. The highest peak intensity due to this vanadyl phosphate indicating the whole formation and crystallization of the phase was obtained in sample A1VPO-550. The intensity of this peak, in fact, was comparable to that of A1PO4. On the contrary, in the A1VPO-650 spectrum, its intensity decreased while in the A1VPO-750 spectrum it disappeared completely. Furthermore, in both samples the behaviour of this peak due to [3-VOPO4 was related to the presence of a small shoulder in the resonance of A1PO4 at around -25 ppm. Calcined VPO sample spectra showed one second resonance, at around -20 ppm, of very low intensity especially in the VPO550, which could be due to ctn-VOPO4 [11]. 27A1MAS spectra of the calcined A1VPO samples were mainly formed by a resonance, between +37 and +40 ppm depending, in each case, on the calcination temperatures, Table 2. This is a typical behaviour of A1 sharing oxygen atoms with four tetrahedra of P like in a-cristobalite [6]. The minor additional resonances, 28.7 and 33.1 ppm, observed in A1VPO-650 and A1VPO-750 samples, could also be assigned to tetrahedral A1 but with a change in their electronic environment.Complementary DRIFT spectra obtained for A1VPO systems (not shown) confirmed the highest degree of crystallization and the

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presence of both aluminum and vanadyl phosphate phases in the A1VPO-650 and AIVPO-550 samples as well as structural changes in the A1VPO-750. Thus, the spectra of those catalysts calcined below 650~ showed broad and simple bands at 820-1300 cm ~ region assigned to P-O stretching vibration of (PO4) -3 and to V=O stretching vibration [5,6]. On the other hand, according to the XRD and 31p MAS results, the presence of some absorptions above 550 nm in the DR UV-V spectra of uncalcined A1VPO and VPO samples demonstrated that V 4+ ions were present [11,12]. These bands disappeared after calcination, indicating that V 4+ has been mostly oxidized to V 5+. Spectra of calcined AIVPO and VPO samples were all very similar with chargetransfer band maxima at 220, 300 and 450 nm, like those present in the V205 spectrum [12]. In Fig. 2A we can see that TPR profiles in the A1VPO systems show remarkable differences, which reflect the structural changes obtained by all the characterization techniques studied. Thus, TPR-peaks shifted to higher temperatures and the total amount of hydrogen consumed in the reduction process decreased, Table 1, in the catalysts calcined up 550~ TPR profiles of A1VPO systems are also clearly different than of VPO system profiles, Fig. 2B, indicating higher reduction temperatures than the former systems. Decomposition of 2-propanol: At different temperatures studied, propene was obtained as the main reaction product. Propanone and diisopropylether in small amounts was only evidenced in the A1VPO-450 and A1VPO-550 systems. Kinetic parameters for conversion of 2-propanol have been obtained by analysis of the data through the Bassett-Habgood equation [13]. Table 3 collects the apparent rate constants (kK) for propene and propanone formation, at 210~ The 2-propanol conversion apparent activation energies, Ea, are also shown. Both A1VPO-450 and A1VPO-550 systems showed the highest activities in the dehydration (related to acid sites) and dehydrogenation (related to basic/redox sites) reactions [13,14]. Hydrocarbon oxidation: results obtained in the oxidation of benzene, B, (at 450~ toluene, T, and o-xylene, X, (at 350~ are also shown in Table 3. A1VPO-450 and AIVPO-550 catalysts showed the best catalytic performance in all oxidation reactions studied. Secondary

A

i

752

.... 11 1o4

Z9

[..-,

7701 B

,2104

A1--YP O-65"0-'-"~558

P0-5.50

[

-_

~

]

ra~

ZO A 1 V P O - 5 5 0 ~ l

4

--

ILVPO-450

.

~:

663

100

300

500 T (~

700

I

100

I

3'00

Fig. 2. TPR profiles of catalysts

i

I

500 T (~

i

7'00

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products were: hydroquinone and C02 in the benzene oxidation and several coupling products [15] and phtalic anhydride in toluene oxidation. Methylbenzoic acid and methylbenzaldehyde as well as coupling products were obtained in the oxidation of o-xylene. Table 3.- Catalytic performance of catalysts in 2-propanol reaction as well as in oxidation reactions of different aromatic hydrocarbons.

Catalyst

KKc3H6 KKc3H60 (~mol/atm g s)

Ea (kcal/mol)

Ca SMA (mol%)

CT SaA+BAc Cx SpA+PhD (mol%) (mol%)

A1VPO-450

4.4

0.6

22.7_+0.8

42

92

5

58

71

83

A1VPO-550

1.5

0.2

19.3-+0.5

26

86

11

54

61

75

A1VPO-650

0.2

-

22.0-+0.6

8

67

<2

100

28

71

A1VPO-750

0.1

-

25.2-+0.6

.

8

34

VPO-450

0.2

-

23.1_+0.9 <2

16

70

.

. 100

. <2

83

58 VPO-550 0.1 28.0_+2.0 <2 63 19 C3H6: propene; C3H60: propanone; MA: maleic anhydride; BA: benzaldehyde; BAc: benzoic acid; PA: phtalic anhydride; PhD: phthalide. In conclusion, calcination of AIVPO catalysts above 550~ promote an important decrease on catalytic activity, probably related to the observed structural changes. The presence of A1 along the VPO synthesis also develops the formation of A1PO4 which promote its catalytic action by forming binary systems with higher activity and selectivity than VPO alone. REFERENCES

1. "Vanadyl Pyrophosphate Catalysts, Catal. Today", ed. G. Centi, Elsevier, Vol. 16, 1993. 2. E. Bordes, Catal. Today, 1 (1987) 499. 3. F.M. Bautista, J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas and R.A. Quir6s, Studies in Surface Science and Catalysis Vol. 82, p. 759. Elsevier, 1994. 4. P.S. Kuo and B.L. Yang, J. Catal., 117 (1989) 301. 5. M. Nakamura, K. Kawai and Y. Fujiwara, J. Catal., 34 (1974) 345. 6. F.M. Bautista, J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas and A.A. Romero, Appl. Catal. A, 96 (1993) 175. 7. F.M. Bautista, J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas, M.C. Moreno, A.A. Romero,.A. Navio and M. Macias, J. Catal., 173 (1998) 333. 8. F. Trifir6, Catal. Today, 41 (1998) 21. 9. O.B. Lapina, V.M. Maskhin, L.G. Simonova, A.A. Shubin, V.N. Krasilnikov and K.I. Zamaraev, Prog. NMR Spectrosc., 24 (1992) 457. 10. C.Femandez, Ph.Bodart, M.Guelton, M.Rigole and F.Lefebvre, Catal. Today, 20 (1994) 77. 11. N.H. Batis, H.Batis, A. Ghorbel, J.C. Vedrine and J.C. Volta, J. Catal., 128 (1991) 248. 12. G. Lischke, W. Hanke, H.-G. Jerschkewitz and G. Ohlmann, J. Catal., 91 (1985) 54. 13. F.M. Bautista, J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas, A.A. Romero and M.R. Urbano, Catal. Lea., 35 (1995) 143. 14. B. Grzybowska-S, G. Coudurier, J.C. Vedrine and I. Gressel, Catal. Today, 20 (1994) 165. 15. J. Zhu and S.L.T. Andersson, J. Catal., 126 (1990) 92.

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Studies in Surface Science and Catalysis 130 A. Corrna, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Direct Synthesis of Phenol from Benzene with 02 over VMo-Oxide/SiO2 Catalyst I. Yamanaka, M. Katagiri, S. Takenaka, and K. Otsuka* Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan Direct partial oxidation of benzene to phenols (phenol, hydroquinone, and benzoquinone) with 02 was achieved over VMo-oxide/SiO2 catalysts by co-feeding H20 vapor at 823K. A synergism of V and Mo on the partial oxidation of benzene was observed. The maximum yield of phenols of 4.3% (40% selectivity) was obtained for VaMos-oxide(5wt%)/SiO2 and VsMo4-oxide(4wt%)/SiO2 catalysts. Water vapor (>10kPa) strongly inhibited the formation of COx and enhanced the formation of phenol for both catalysts. Quite different kinetic results, different isotope effects in oxidation of benzene, and different product distributions in oxidation of toluene were observed between the two catalysts. These observations suggest that the nature of active oxygen species and the reaction mechanisms are different between the two catalysts. 1. INTRODUCTION The one-step synthesis of phenol from benzene with 02 is one of the most attractive subject in the field of oxidation chemistry today. At present, phenol is manufactured mostly by Cumene Process. The Cumene Process requires multi-step operations and inevitably produces acetone as a co-product which is oversupplied in the market. Therefore, it is expected to find a new catalytic system which enables one step synthesis of phenol. So far, two methods for direct hydroxylation of benzene have been reported. The one is to apply N20 as the oxidant over V205/SIO2, H-ZSM-5 or Fe-ZSM5 catalyst at >600K [ 1]. The other one is to apply a mixture of 02 and H2 over Pd-Fe-zeolite, CuSOa-Pd/SiO2, or Pt-V205/SiO2 catalyst under ambient temperature [2]. However, N20 is expensive, thus can be used only under the conditions where N20 is supplied as a by-product [3]. The use of a mixture of 02 and H2 requires a special care for the explosion. Therefore, hydroxylation of benzene by using only 02 or air is most desirable. However, catalytic systems targeted the direct synthesis of phenol by using only 02 as the oxidant did not achieve the yield of phenol more than 1% [4]. The purposes of this work are to investigate the direct partial oxidation of benzene to phenols with 02 over different VMo-oxide/SiO2 catalysts and to clarify the catalytic functions of the oxides.

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2. EXPERIMENTAL VMo-oxide/Si02 catalysts were prepared by a conventional impregnation method from SiO2 and aqueous solutions of NHaVO3 and (NH4)6Mo7024. The slurry was dried on a hot plate and calcined at 373K for 5h and at 873K for 5h. The weight percent of the VMo-oxide on SiO2 were evaluated on the assumption that vanadium and molybdenum oxides were supported as V205 and MOO3, respectively. The weight of VMo-oxide was calculated from the amounts of NH4VO3 and (NH4)6Mo7024 impregnated in the support of SiO2. Oxidation of benzene was carried out at atmospheric pressure using a conventional gas-flow system with a fixed-catalyst bed in a quartz tube reactor. The standard reaction conditions were as follows; T (823 K), catalyst (0.2 g), C6H6 (5.6 kPa), 02 (11.3), H20 (75.6), He (8.8), total gas-flow rate (63 ml min-1). Products were analyzed by HPLC with a UV detector and by GC with TCD and FID detectors. 3. RESULTS AND DISCUSSION 3.1. Oxidation of benzene with 02 over different VMo-oxide/SiO2 catalysts Figure 1 shows the oxidation of benzene with O2 as functions of the quantity of V in VnMoiz_n-oxide/SiO2 catalyst [V : Mo = n : (12-n), n=0-12]. Products were phenol (PhOH), hydroquinone (HQ), benzoquinone (BQ), maleic acid (MA), and carbon oxides (COx). The MOl2-Oxide/Si02 was not active for 5 the oxidation of benzene though the [ Phenols(PhOH+HQ+BQ) selectivity to Phenols (= sum of 4 ~ 60 PhOH, HQ, and BQ) was high (55%). ~ ................ o" [] o . . -~ On the other hand, the Vl2-Oxide/Si02 ~ r ~= ~ 3 40 .~ showed a high conversion of benzene "~ "~ but the selectivity to phenols was low ~ "'. ."" []""a s as 22%. The maximum yields of ~ 2 ~ 'o;" "-.. PhOH and Phenols were obtained at ~ 20 "7, n=4 (V4Mos-oxide/SiO2) and n=8 1 ~_] (VsMoa-oxide/SiO2), which suggests 0 0"r ' ' ' the synergism between V and Mo for 4' 8' 120 the catalytic formation of Phenols. n in VnMo12-n-oxide/SiO2 The yield and the selectivity to Fig. 1. Oxidation of benzene with 02 over Phenols were greater than 4 % and various composition of VMo-oxide/SiO2. 40 %, respectively, for both catalysts. Figure 2 shows the effect of the loading VsMo4-oxide on the formation rates of products at 823K. The oxidation activity of the VsMon-oxide/SiO2 appeared when the amount of VsMo4oxide on the SiO2 increased above l wt%. The maximum yield of Phenols was obtained at around 4wt%. Further increase in the amount of VsMoa-oxide decreased the yield of Phenols, but, in contrast, the formations of M A and COx were accelerated. A mixed oxide of VsMo4 without SiO2 support did not give Phenols but only COx. The similar results of the effect of

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loading were observed for the V4Mos12 ",e~0 1.5 oxide/SiO2 catalyst. The maximum "7, MA yield of Phenols was obtained at O i around 5wt% for V4Mos-oxide/SiO2. 8 ~ The catalytic tests of H-ZSM-5 ........ . C 9 ~176176176176176176 _ and Fe-ZSM-5 under the standard 9 reaction conditions revealed that both o O catalysts did not produce Phenols but _ 4 N o 0.5 just burned benzene. We have 9O ~,,,i confirmed that the H-ZSM-5 and FeO ZSM-5 catalysts are active for the 0 ~ hydroxylation of benzene with N20 0 0 2 4 6 8 as reported in the references [1]. Loading of V8Mo4-oxides / wt% However, these catalysts did not give Fig. 2. Effect of the loading of V8Mo4-oxides Phenols in the oxidation of benzene on the formation rates of prodcuts. with 02. As far as we know, VsMo 4oxide/SiO2 and V4Mos-oxide/SiO2 were the most active and selective catalysts for the synthesis of phenols from benzene using 0 2 as an oxidant.

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3.2. Role of H 2 0 vapor on the oxidation of benzene

Figure 3 shows the effect of P(H20) on the oxidation of benzene catalysed by VsMo4oxide(4wt%)/SiO2. When H20 vapor was not cofed in the reaction mixture, deep oxidation of benzene to COx was the major reaction. In this case, the estimated partial pressure of H20 produced from the burning of benzene was ca. 3 kPa. When a higher partial pressure of H20 (> 10 kPa) was cofed, the formation rate of COx was drastically reduced. In other words, deep oxidation activity of the 80 " VsMoa-oxide(4wt%)/SiO2 decreased ., 1.2 with addition of an excess H20 go t 9 O vapor. Thus, the conversions of .~ )".... o . . . . BQ+HQ -60 benzene and 02 decreased with -6 N increasing P(H20). In contrast to ~ 0.8 4o 8 COx, the formation rate of PhOH r "... ~ MA O increased linearly with increasing N P(H20). While, the formation rates .~ 0.4 - " " ' Y "'~ ..... 0 COx 20 = r PhOH ...................... /.. ~ of (HQ + BQ) and of 1VIA did not change with P(H20). I I I I I I I Very similar effect of H20 vapor, 0I 0 ~ 0 20 40 60 80 i.e., inhibition of COx formation and P(H20) / kPa acceleration of PhOH formation, Fig.S. Effect of P(H:O) on the oxidation of was observed also for the VaMosC6H6with O: over V8Moa-Oxide/SiO2. oxide(4wt%)/SiOa catalyst.

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3.3 Kinetic studies for the oxidation over VMo-oxide/SiO2 catalyst

Figure 4 shows the effect of P(O2) 1.5 20 on the oxidation of benzene over VsMo4-0xide(4wt%)/SiO2. Oxidation of .." .... "0--. "6 benzene proceeded selectively at a low .~ " -15~ partial pressure of 02 (1 kPa). The "~ 1 N 9 formation rate of PhOH gradually 10 N increased with increasing P(O2) at > 1 N kPa and reached the maximum at = 0.5 16kPa. On the other hand, the .9 5 .9 formation rates of COx, M A and (HQ+BQ) were accelerated at P(O2) > ~, o O~ i i i I 0 5 kPa. Thus, the selectivity to phenol 0 5 10 15 20 25 decreased at P(O2) > 5 kPa. A higher P(O2) / kPa P(O2) than 5 kPa was not favorable for Fig.4. Effect of P(O2) on the oxidation of C6H6 the selective formation of phenols over over V8Mo4-Oxide/SiO2. the VsMo4-oxide/SiO2 catalyst. Very similar dependence of P(O2) on the (a) VaMo4-Oxide/Si02 COx ....."0 oxidation of benzene was also observed 2i ..... - 20 for the VaMos-oxide/SiO2. Figure 5 shows the effects of P(C6H6) on the formation rates of . ~ " ~ 10~ products for (a) VsMo4-oxide ~ 1 o (4wt%)/SiO2 and (b) V4Mos-oxide ~ (4wt%)/SiO2. Over VsMoa-oxide (4wt%)/SiO2, the formation rates of ~ 0 9 15rj PhOH, HQ, and BQ increased linearly ~ 1.5 (b)V4Moa-Oxide/Si02 a_ o with increasing P(C6H6). The = hOH conversion rate of benzene over "-. 10 . 9= VsMoa-oxide/SiO2 depended on the ~ 1 c o x first order with regard to P(C6H6). In the case of V4Mos-oxide/SiO2 o catalyst, the formation rate of PhOH 0.5 linearly increased with P(C6H6) at < 8kPa, but reached a plateau above 8 MA 0, , i , , 0 kPa. In contrast to the PhOH 0 5 10 15 20 25 formation, the formation rates of P(C6H6) / kPa (HQ+BQ), MA, and COx showed Fig.5. Effect of P(C6H6) on the oxidation of C6H6. maxima at P(C6H6)~7 kPa. These formation rates decreased at P(C6H6) > 8 kPa. The conversion rates of benzene and 0 2 also showed maxima at P(C6H6) ~ 7kPa and decreased sharply at higher P(C6H6). In order to get information about the reaction paths for the formation of products, the effect of contact time (W/F) on the oxidation of benzene was studied for the two catalysts. I

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Figure 6 shows the effects of W/F on 60 [ I the selectivities of products for VsMo4-.. (a) VaMo4-Oxide/SiO2 oxide(4wt%)/SiO2 (a) and V4Mos40 1 ~~,,,Phenols oxide(4wt%)/SiO2 (b). In the case of VsMo4-oxide/SiO2, the selectivities of phenols increased with decreasing contact time. The selectivities to PhOH, HQ, and BQ were extrapolated to 30, ~ 0 100 , , , 15, and 15%at zero contact time. The (b) V4Moa-Oxide/Si02 selectivities to COx and M A were '-o 8 0 " ' extrapolated to 35 and 5 %. These results suggest that each product is 60 ' ,,'~ PhenoIs produced via parallel reaction paths at the initial stage of the oxidation of benzene. In the case of V4Mos-oxide/SiO2 catalyst (Fig. 6(b)), the selectivity to 0 BQ I I " i --~ PhOH remarkably increased with 0 0.2 0.4 0.6 0.8 W/F / sec g cm-3 decreasing W/F. The selectivities to Fig.6. Effect of W/F on the selectivities of PhOH, HQ, and BQ were extrapolated products. to 85, 10, and 5% at zero contact time. In contrast to these Phenols, the selectivities to COx and M A were extrapolated to zero. These results suggest that phenols were selectively produced on the V4Mos-oxide/SiO2 catalyst at the initial stage of the oxidation of benzene. The COx and M A are the secondary oxidation products from the Phenols. The results described above strongly suggest that the reaction paths for the oxidation of benzene are very different between the two catalysts. The apparent activation energies for the oxidation of benzene were evaluated from the effect of temperature on the oxidation rates of benzene for the two catalysts. Good straight lines were obtained for the plots of the formation rates of products versus 1/T (T=723-823K) for both catalysts. The apparent activation energies for the conversion of benzene and the formation of phenols were evaluated from the slopes and they are shown in Table 1. The apparent activation energies are quite different between the two catalysts. The quite different kinetic results obtained for the two catalysts suggest different reaction mechanisms or different active oxygen species responsible for the hydroxylation of benzene on the two catalysts. I

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3.4 Mechanistic information for benzene oxidation In order to get more information about the reaction mechanism, the oxidation of a mixture of C6D6 and C6H6 were carried out at 823K to estimate kinetic isotope effect (kH/kD). The isotope effect of 1.3 was observed for the formation of PhOH over the VsMo4-Oxide/SiO2, suggesting that the cleavage of C-H bond of benzene might be the rate-determining step. In

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contrast, the kinetic isotope effect was 1.0 for V4Mos-oxide/SiO2 catalyst, which suggests that the cleavage of C-H bond is not the rate-determining step. These results also support the idea that nature of the active oxygen species or the reaction mechanisms are very different between the two catalysts. The nature of the active oxygen species can be discussed on the

Table 1. Nature of catalysisofVMo-Oxide/SiO2.

basis of the regioselectivity in the oxidation of toluene. The oxidation of toluene was studied at 723K. The

VMo-Oxide /SiO2

Ea / kJ mol 1 C6H6 Phenols

benzene toluene kH/kD Cr/Bz

ratio of the amount of the phenyl V4Mos145 111 1.0 1.6 ring oxygenated products (o, m, pcresols) and that of the Me-group VsMo477 33 1.3 0.5 oxygenated products (benzaldehyde and benzyl alcohol) was defined as Cr/Bz for the evaluation of the regioselectivity. As shown in Table 1, Cr/Bz value of 1.6 was observed for the V4Mos-oxide/SiO2, suggesting that the active oxygen species on this catalyst prefers the oxidation at the stronger C-H bond (111 kcal mol l ) in phenyl ring. Probably addition of oxygen species to the aromatic C=C bond proceeds at the initial stage of the oxidation. On the other hand, Cr/Bz ratio for the oxidation over VsMo4-oxide/SiOz indicates a preferential attack of the active oxygen at the weaker C-H bond (88 kcal mol "l) of Me-group, which results in the abstraction of hydrogen atom from the Me-group. The discussion on the basis of the regioselectivities in the toluene oxidation suggested the reactivities of the active oxygen species, i.e., the oxygen on V4Mo8-oxide/SiO2 is susceptible to add on the aromatic ring, but that on VsMoa-oxide/SiO2 has a strong ability to abstract hydrogen. If the rate determining step is an electrophilic addition of oxygen to the aromatic ring, we do not observed the isotope effect (kH/kD~l.0) as this was the case for the hydroxylation over V4Mos-oxide/SiO2 catalyst. On the other hand, if the rate determining step is the abstraction of hydrogen atom from the phenyl ring by the active oxygen having a radical character on V8Mo4-oxide/SiO2, the experimentally observed isotope effect (kH/kD=1.3) for the hydroxylation of benzene may be well explained. REFERENCES 1. M. Iwamoto, et al., dr. Phys. Chem., 87 (1983) 903; E. Suzuki, K. Nakashiro, Y. Ono, Chem. Lett., (1988) 953; G.I. Panov, et al.,Appl. Catal. A, 98 (1993) 1; Chemical Week, 159, (1997) 11. 2. N. Herron and C.A. Tolman, J. Am. Chem. Soc., 32 (1987) 200; K. Sasaki et al., Bull Chem. Soc. Jpn, 62 (1989) 2613; T. Miyake, et al.,Appl. Catal. A, 131 (1995) 33. 3. A.K. Uriarte et al., Stud. Surf. Sci. Catal., 110 (1997) 857. 4. W.Ueda, et al., Ind. Eng. Chem. Res., 28 (1989) 1587.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

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Eu-Ti-Pt-Catalytic System for Direct Hydroxylation of Benzene by 02 and Hz under Mild Conditions I. Yamanaka*, T. Nabeta, S. Takenaka and K. Otsuka Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan One step synthesis of PhOH from benzene was performed by Eu-Ti-Pt-catalytic system with 02 and H2 at atmospheric pressure and 40~ The best combination of the elements for the hydroxylation of benzene was Eu(OTf)3-TiO(acac)2-Pt-oxide/SiO2. The Eu3§ 2+ redox catalyses the reductive activation of 02, which is enhanced by TiO(acac)2. Pt-oxide/SiO2 works as a catalyst for the reduction of Eu 3+ to Eu 2+ by H2. The reductively activated oxygen by the concerted actions of the catalytic components is responsible for the hydroxylation of benzene to phenol. 1. INTRODUCTION In the current chemical industry, a large amount of PhOH is manufactured by the Cumene Process. The Cumene Process requires multi-step operations as well as a process for the utilization of co-product of acetone. Therefore, a new catalytic system for direct hydroxylation of benzene to PhOH has been desired. Under these circumstances, the direct hydroxylation of benzene to phenol (PhOH) by 02 and H2 with catalysts such as Pd-Fezeolite, Pd-CuSO4/SiO2, Pt/V205/SiO2, etc. [ 1-3] attracted much attention. We have recently reported a new catalytic system based on Eu salts, Zn powder and acetic acid for the partial oxidation of hydrocarbons by 02 [4]. This Eu-catalytic system can oxygenate benzene to PhOH, propene to propene oxide, and cyclohexane to RH ROH cyclohexanol and cyclohexanone at room Zn0 eu 3 temperature. If we use CF3CO2H for the Eucatalytic system instead of MeCO2H, methane can be hydroxylated to MeOH at C02 room temperature. As shown in Scheme 1, we Zn2+ consider that Eu 2§ species formed by the reduction of Eu 3+ with Zn powder (step 1) o2 MeCCH activate 02, forming active oxygen species Scheme 1. Reaction scheme for the oxidation (step 2) which oxidize hydrocarbons to of hydrocarbon with a system of EuC13oxygenates (step 3). One of the problems of MeCO2H-Zn powder. this Eu-system is to use an expensive

step21~E-u2+~'~

Eu2+f ; (

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reducing agent of Zn powder. If 1-12,a cheaper reducing agent, can be used for the Eu-catalytic system instead of Zn powder, a new process of the direct PhOH synthesis might be brought into an industrial application. The purpose of this work is to demonstrate hydroxylation of benzene by 02 and 1-12with Eu compounds-based catalytic system, and to get informations about the catalytic functions of each catalyst component for the hydroxylation. 2. EXPERIMENTAL The standard experimental procedures were as follows; Eu(OTf)3, (120 ~tmol) and bis(2,4pentanedionate)TiO (TiO(acac)2, 20 ~tmol) was dissolved in a mixture of benzene (4 ml) and MeCO2H (16 ml) in a round bottom flask with a reflux condenser. After Pt/SiO2 or Ptoxide/SiO2 (lwt%Pt, 0.1 g) was added, a gas mixture of 02, H2 and Ar (1:1:1, 30 ml min-l) were introduced to the reaction mixture. The reaction was carried out for 2h by stirring the reaction mixture with a magnetic spin-bar at 40 *C. Products were analyzed by HPLC with a UV detector and GC with TCD and FID detectors. Eu(OTf)3 was prepared from Eu203 and CF3SO3H. Pt-oxide/SiO2 was prepared by an impregnation method from H2PtCI6 aq. and SiO2 (SIO-8, supplied from Catalysis Society of Japan). A precursor of the catalyst was calcined in air at 573K for 2h (Pt-oxide/SiO2). Ptoxide/SiO/was reduced by H2 at 673K for 2h. Other reagents (special-grade) were used without purification. The H2-efficiency used for the hydroxylation of benzene was defined as Eq. 1. H2-Efficiency = [sum of hydroxylated products (mol) / H20 formed (mol)] x 100% (1) Eu and Ti species in the reaction mixture were quantitatively analyzed by an UV-visible spectrometer after the mixture had been reduced by H2 with PtJSiO2 catalyst and after subsequent oxidation of the reaction mixture by 02. 3. RESULTS AND DISCUSSION

3.1. Hydroxylation of benzene by 02 with Eu-catalytic system As described above, EuC13 catalyst can hydroxylate benzene to phenol (PhOH) with 02 in the presence of Zn powder (e- donor) and MeCO2H (H§ donor and solvent) under mild conditions [4]. When a gas mixture of 02 and H2 was introduced to the Eu-catalytic system in the absence of Zn powder, the oxidation of benzene did not take place at all. We considered that the absence of oxidation was due to the inability of the system for the activation of H2. Therefore, several catalysts (Pt, Pd, Rh, Ir and Ni supported on SiO2, A1203, active carbon, etc.) for the activation of H2 were added to the reaction mixture, and tested for the hydroxylation of benzene by 02 and H2. The hydroxylation of benzene took place with EuC13-catalytic system by addition of Pt/SiO2 at atmospheric pressure and 40~ Hydroxylated products were PhOH > hydroquinone (HQ) > catechol (CT). Other byproducts were cyclohexane due to the hydrogenation of benzene. CO2 was also produced from oxidation of MeCO2H.

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Although the EuCI3-Pt/SiO2-MeCO2H-O2EuCl3, Pt/SiO2 1-12 system could hydroxylate benzene 9 R1OH directly, the yield of the sum of PhOH, HQ Eu(OT03, Pt/SiO~ m and CT was very low, turnover number less Eu(ClO4)3, Pt/SiO 2 than 1 in 2h. In order to enhance the productivity of phenols, the catalytic system Eu(acae)3, Pt/SiO2 has been developed further as follows. First, the effects of various Eu compounds Eu(TFA)3, Pt/SiO2 on the hydroxylation were studied. Figure 1 Eu(OTf)3, Pt-oxide/SiOz shows the product yield for several Eu-salts Eu(OTf)3, TiO(acac)2 and complexes with Pt/SiO2 for the oxidation Pt-oxide/SiO2 of benzene by 02 and H2. Eu(OTf)3, TiO(acach, Eu(CIO4)3, and EuC13 were active for the Pt-oxide/SiO 0 i i hydroxylation. However, when Eu(acac)3, 5 100 150 2()0 Eu(TFA)3 and Eu(NO3) 3 were used, the Product yield //tmol oxidation of benzene did not take place. Fig. 1. Hydroxylation of benzene by 02 Second, the preparation conditions of and H2 with various catalytic systems at 1 atm and 40 ~ Pt/SiO2 (calcination and reduction temperatures, amount of Pt loading, etc.) 200 were important. We found that Pt-oxide/SiO2 -" Jl~l~ II phenols o II ,., ~. o PhOH showed better performance for the ~-150 zx HQ hydroxylation of benzene than Pt/SiO2. The results for different Pt loading (1 to 5wt%) on ~1oo SiO2 indicated that the yield of phenols did not change appreciably. The combination of o Eu(OTf)3 and Pt-oxide/SiO2 showed a better 50 results, as shown in Fig. 1. Third, enhancing effect of the addition of o TiO(acac)2 to Eu(OTf)3-Pt-oxide/SiO2 was 0 60 120 180 240 Amount of TiO(acac)2 added //tmol examined because TiO(acac)2 worked as a cocatalyst for the previous Eu-catalytic system Fig. 2. Effect of TiO(acac)2 on the benzene using Zn powder [4]. The yield of phenols hydroxylation by O2 and H2. remarkably increased with the addition of TiO(acac)2, as shown in Fig. 1. The TiO(acac)2-Pt-oxide/SiO2 in the absence of Eu(OTf)3 was inactive for the hydroxylation of benzene. These results suggest that TiO(acac)2 works as cocatalyst of Eu(OTf)3 for the hydroxylation of benzene by 02 and H2. Figure 2 shows the effect of the amount of TiO(acac)2 added to the Eu(OTf)3-TiO(acac)2Pt-oxide/SiO2 catalytic system on the oxidation of benzene. When a small amount of TiO(acac)2 (10~tmol) was added to the catalytic system, the hydroxylation activity was enhanced considerably. The maximum yield of phenols was obtained at 20~mol addition of TiO(acac)2. This quantity was just 1/6 of that of Eu(OTf)3. An excess addition of TiO(acac)2 decreased the catalytic activity of the Eu(OTf)3-TiO(acac)2-Pt-oxide/SiO2 system.

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Figure 3 shows the yields of phenols as 10 ...--m--..../--'>" O functions of the partial pressure of H2 6~ (P(H2)) for the oxidation of benzene with 6 ID the Eu(OTf)3-TiO(acac)E-Pt-oxide/SiO2 4"~ "7, 4 catalytic system. These experiments were 9 =72 Eft performed without an inert gas of Ar 0, o (P(H2) + P(O2) = 101kPa). The yields of 400 cyclohexane, CO2 and H20, and the HE--,9 9 phenols Q E~ [] CsH12 ~ efficiency estimated from Eq.1 were also :r 3 0 0 a 002 / -- " ~ plotted in Fig. 3. The results in this figure u indicate that the oxidation of benzene did not take place at all at P(H2)-0 by Eu(OTf)3-TiO(acac)E-Pt-oxide/SiO2 N100 ~ ~ catalytic system at 40~ Yield of phenols (PhOH + HQ + CT) increased with 0~ ' ~-"~ ' ' ' ' ' 0 ' 20 40 EF- 60 80 100 increasing P(H2) and reached to the P(H2) / kPa maximum at around 67kPa. Main product Fig. 3. Effect of P(H2) on the benzene was PhOH at all P(H2), and the hydroxylation by 02 and H2. selectivities to HQ and CT were lower than 5%. A considerable amount of cyclohexane, that is a hydrogenation product of benzene was produced at higher P(H2). The yield of cyclohexane increased with increasing P(H2), decreasing the PhOH selectivity. Thus, higher P(H2) was not favorable on the view point of selective hydroxylation. The formation of H20 was accelerated with increasing P(H2) and reached the maximum at P(H2)=50 kPa. The maximum efficiency of H2 was obtained at 75 kPa. Under the experimental conditions in this work, a P(H2)~(O2) ratio of 3 was the most suitable gas composition from the view point of the effective and selective hydroxylation of benzene. Kinetic curves for the oxidation of benzene by 02 (75kPa) and H2 (25kPa) with the Eu(OTf)a-TiO(acac)2-Pt-oxide/SiO2 catalytic system were studied. Hydroxylation of benzene took place for more than 8h. The formation rate of phenols was decelerated after 2 h due to the accumulation of H20 in the reaction mixture. Effects of total amount of catalysts with different ratios of each catalytic component, concentration of benzene, and reaction temperature on the formation of phenols were studied for the Eu(OTf)a-TiO(acac)2-Pt-oxide/SiO2-system. The maximum formation rate of phenols was 1.5 (mmol g-cat. "1 h "l) with a H2-efficiency of 10% at atmospheric pressure and 40 ~ This is fairly good productivity of phenols comparable to that obtained with Pt/V2Os/SiO2catalyst [3]. i

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3.2. Character of active species of Eu-Ti-Pt-catalytic system As mentioned above, Eu(OTf)3-TiO(acac)2-Pt-oxide/SiO2 catalytic system was the most active one for the hydroxylation of benzene by 02 and H2. Under the standard reaction conditions, the yield of phenols for each Eu-catalysts was Eu(OTf)3 (310~tmol in 2h) > Eu(CIO4)3 (260)> EuC13 (150)>> Eu(acac)3, Eu(TFA)3, Eu(NO3), Eu(OAc)3 (-10).

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Therefore, the three Eu-compounds, Eu(OTf)3, Eu(CIO4)3, and EuCI3, were chosen as the Eu(OTf)3, catalysts for further studies on the nature of TiO(acac)2 active species. First, the kinetic isotope effect on the Eu(CIO4)3, TiO(acac)2 formation of PhOH from a mixture of C6D6 m-Cr and C6H6 (1:1 mol) was measured for the three EuC13, catalysts. The isotope effect on the formation TiO(acac)2~ of PhOH (kH/kD) were 1.0 for Eu(OTf)3, 1.1 for Eu(C104)3, and 1.2 for EuCI3. These small I , I , I , I , 0 200 400 600 kH/kD-values suggest that a cleavage of C-H Products Yield //tmol bond of benzene may not be involved in the Fig. 4. Oxidation of toluene by 02 and H2 rate determining step of the hydroxylation of with (Eu-compounds + TiO(acac)2 benzene for the three catalytic system. + Pt-oxide/SiO2). In order to get information for the reactivity of active oxygen species, oxidation of toluene was carried out with the three Eu-components. Figure 4 shows the product yields of toluene oxidation by 02 and H2 with the Eu-compounds-TiO(acac)2-Pt-oxide/SiO2. Products were cresols (o-, m-, p-Cr), benzyl alcohol (BzOH) and benzaldehyde (BzO). Product yields and distributions were very different among the three Eu-compounds. In the case of Eu(OTf)3, the major product was cresols (o:m:p = 3:1:5). The ratio of the benzene ring oxidized products (o-, m-, p-Cr) and the Me-group oxidized products (BzOH, BzO) was 7.0 (Cr/Bz). The active oxygen species preferes to attack at benzene-ring rather than Me-group. This result suggests that active oxygen species has electrophilicity. In contrast to Eu(OTf)3, the Me-group oxygenated products (BzOH, BzO) were the major ones for Eu(C104)3 and EuC13 catalysts. The ratios of Cr/Bz were 0.6 and 0.2 for Eu(C104)3 and EuC13, respectively. The small Cr/Bz values for these two Eu-catalysts suggest that active oxygen species is favor for the abstraction of H. from the Me-group rather than it's addition to benzene ring. Thus, this oxygen species may have a radical property.

/L~

3.3 Oxidation state of Eu and Ti species

Figure 5 shows the UV-visible spectra of Eu species after reduction by H2 with Pt/SiO2 and after oxidation by 02. Spectrum (i) is a UV spectrum of solution of EuC13 in MeCO2H. Spectrmn (ii) has been measured after reduction of the solution of EuC13 (6 mmol ll) and MeCO2H by H2 with Pt/SiO2. Spectrum (ii) shows the broad absorption bands having two peaks at 270 and 320 nm. These absorption bands were identified as UVabsorption of Eu2+ (f-d) [5]. Very similar spectrum to that of (ii) was observed for the

(i) before reduction (ii) afterreductionby H2 (iii) after oxidation by 0 2

(ii)

O) o

(ii

300 400 ' 5()0 ' Wavelength / nm Fig. 5. UV spectra of Eu species.

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solution of EuCI3 and MeCO2H after reduction by Zn powder. Spectrum (iii) was measured after the oxidation of the solution (ii) by 02. The spectnma (iii) is almost same as the spectrum (i). The changes as spectra (i) to (iii) were observed repeatedly by the reduction and the oxidation. These results strongly suggest that Eu 3+ is reduce to Eu 2§ by H2 in the presence of Pt/SiO2 and the Eu 2§ species is oxidize back to Eu 3+ by 02. Similar UV measurements were performed for the co-catalyst TiO(acac)2. The UV spectra were not changed before and after the reduction treatment by 1-12 with Pt/SiO2 and the oxidation treatment by 02. These results suggest that Ti 3§ (or Ti 2§ species is not produced during the hydroxylation. Thus, the redox of Ti4§ 3§ (or Ti 2§ may not be involved in the hydroxylation.

3.4 Model of reaction scheme As described above, Eu 3+ is reduced to Eu 2+ by 1-12with Pt/SiO2 . This Eu 2+ species is oxidized by 02 to Eu 3+, which suggests that 02 is reduced by Eu2+, generating active oxygen species. While, Ti 4§ (TiO(acac)2) is not reduced by 1-12with Pt/SiO2. Ti 4§ must enhance the oxygenation by keeping its oxidation state (IV). On the bases of the above experimental facts in this work, we propose a tentative model of reaction scheme in Scheme 2. The Eu 2§ generated by reduction with H2 on Ptoxide/SiO2 (or Pt/SiO2) reductively activates 02, which is assisted by TiO(acac)2. The active oxygen species may \ [Eu,tT! I be generated by a concerted action of Eu 3+ Eu3+ and Ti 4§ The reactivity of the active ~,~ Eu 2§ \ H+ H2 oxygen species is strongly affected by the ~Ti4+ ligands such as TfO-, C104-and C1-. Further studies are needed to clarify the state of the active oxygen species and the SiO 2 role of the ligands. For the application of the catalytic systems in this work, we have Scheme 2. Model of the hydroxylation of benzene to improve remarkably the catalytic by 0 2 and H2 with Eu-Ti-Pt system. activity and, especially, the H2-efficiency for the production of phenol.

Q

( ou

o2 [..-~

REFERENCES 1. N. Herron and C.A. Tolman, J. Am. Chem. Soc., 32 (1987) 200. 2. K. Sasaki et al., Bull Chem. Soc. Jpn, 62 (1989) 2613. 3. T. Miyake, et al., Appl. Catal. A, 131 (1995) 33. 4. I. Yamanaka, K. Otsuka, et al., J.C.S., Perkin 2, (1996) 2551; Chem Lett., (1996) 565. 5. G. Adachi, J. Shiokawa, et al., Inorg. Chim. Acta, 113 (1986) 87.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

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High yield butane to maleic supported catalysts.

anhydride

direct oxidation

on new

M. J. LEDOUX a V. TURINES a, C. CROUZET a, K. KOURTAKIS b, P. L. MILLS b and J. J. LEROU b a Laboratoire de Chimie des Mat4riaux Catalytiques GMI-IPCMS, UMR7504 du CNRS, ECPM, Universit6 L. Pasteur 25 Rue Becquerel, 67085 STRASBOURG Cedex 2 (France) bE.I. du Pont de Nemours and Co, Experimental Station, Route 141 WILMINGTON, Del. 19880-0262, USA The preparation and the characterization of VPO supported on a new SiC support are described. This catalyst provides a very significant gain in maleic anhydride yield by reacting air and butane, because of the control of the surface temperature. I. I N T R O D U C T I O N The direct oxidation of butane by oxygen (air) into maleic anhydride (MA) is a wellestablished industrial process (1,2) using vanadium pyrophosphate (VO)2P2OT, (VPO), in its bulk form as a catalyst. Hundreds of papers and patents have addressed the method of preparation and the characterization of the active site(s) (3-6), but very few deal with the process itself. Two different configurations of reactor are used: - fixed-bed, cheap in capital investment and easy to run, however, strongly limited by the release of a large amount of heat (a very exothermic reaction), and consequently only able to work at a low butane concentration ; - transported-bed with a very short contact time between the catalyst and the feed mixture in a riser tube, costly in capital investment and tricky to run. Because of the very short contact time, the reoxidation step of the catalyst taking place outside the reactor, and the efficient heat transport by the outlet gases, it is possible to work at high MA yield provided by a high concentration of butane. The main inconvenience of these systems is the poor ability of bulk VPO, because of its low thermal conductivity, to conduct heat outside the reactor, and also to absorb the heat generated at the surface to avoid hot spots. Many attemps to disperse VPO on different conventional supports (alumina, silica, TiO2) were not very successful because of the strong interaction (7) between the oxygen atoms of the support and the oxygen atoms of the active phase, especially during the activation step transforming the hemihydrate precursor into VPO. In addition, the thermal conductivity of these oxidic supports is quite low and large improvements in heat transfer were not expected (see Table 1). Table 1 Thermal conductivity of different materials used as catalyst supports Material Thermal Conductivity in W.moll.K "1 SiO2 A1203 SiC

0.015 - 1 1- 8 146 - 270

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A new generation of supports based on ceramics with high thermal conductivity and without oxygen atoms has been prepared and tested in our laboratory. The best example is a new relatively large specific surface area SiC (> 20 m2/g) prepared via the "shape memory synthesis"(8-10). Conventional SiC (carborandum) of low specific surface area (< lmE/g) prepared via the Acheson process was patented a long time ago as support for VPO (11,12) but it does not appear to disperse the active phase because of too little interaction between the two solids. In addition, because of this lack of interaction the supported catalyst very quickly demixes under heating, and VPO and SiC are irreversibly separated after a short time. The objective of this article is firstly to describe the preparation and the characterization of VPO supported on the new SiC and secondly to show the very significant gain in MA yield obtained on this catalytic material because of the control of the surface temperature. 2. E X P E R I M E N T A L

SiC was prepared in different shapes; fine powder, grains (diameter 0.5-1 mm) and extrudates (1 cm long, 2 to 4 mm diameter), according to the two methods already described and referred to by the genetic term "shape memory synthesis". The first method (8-10) is based on preformed activated charcoal reacted with SiO vapor generated in a different chamber of the reactor, the second (13) being based on a mixture of carbon, oxygen containing polymer and Si powder, preshaped and heated under a neutral atmosphere. Because of the relatively low temperature of the reaction, between 1200 and 1300~ compared to the 1800~ which is necessary for the synthesis of carborandum or a-SiC, the product obtained is mainly I$-SiC with a surface area varying between 20 and 180 m2/g according to the reaction conditions and the doping agents, with no microporosity. The profile of the pore distribution is centered around 80-100 A (10) and the surface is made either of pure SiC outside the pores or a thin layer (<20 A)of a mixture of Si oxides and oxycarbides inside the pores where the density of defects of the SiC crystals is high. (10,14). VPO was prepared in a conventional way according to the recipe published elsewhere (15) where V205 is reduced to V204 by a mixture of benzyl alcohol and isobutanol while the water formed is constantly extracted out of the solution. Phosphoric acid is then added after the solution has been cooled down and this mixture is subsequently heated to 120-130~ to form the hemihydrate (VO)HPO4, 0.5 n20. At this stage (16) under very specific conditions of temperature and stirring, SiC is added to the solution, and the drying process is conducted under partial vacuum to obtain a "mud". This mud is washed many times in hot water in order to extract the unwanted VO(H2PO4)phase. The mixture is then dried at 200~ and crushed to recover the initial shape of the support. Only the sample containing 27%w. of VPO on SiC will be used for this article but the concentration of VPO can be varied from 0 to 50%w. Fig la shows the XRD of the hemihydrate/SiC obtained after these operations. XRD was performed on a Siemens D5000 diffractometer, using the Cu (Kct) radiation over a large range of 20 angles in a step-scan mode. The peaks corresponding to the hemihydrate and to B-SiC are easily detected and recognized but three unknown peaks are also observed. For many reasons, the species at the origin of these peaks is considered as the precursor of the glue phase which will be discussed later. The hemihydrate/SiC sample is then subjected to activation according to the process described in Ref (15) i.e. many steps of thermic treatments under air, different mixtures of butane and air to accelerate the formation and the stability of VPO/SiC, the total time necessary for this procedure being slightly above 100 hours. The longer activation/stabilisation time for VPO bulk is due to the fact that it has to be performed at a lower temperature. At this stage, the catalyst is extremely stable and can be used for a long time without any apparent deactivation or activation. Fig l b shows the XRD diagram of this stable catalyst. Only the peaks corresponding to (VO)2P207 and to SiC are observed.The anisotropic crystallinity observed on active bulk VPO and demonstrated by a variation in the width of the peaks corresponding to different planes of diffraction is also observed on VPO/SiC. This phenomenon will not be discussed here but

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shows that the active supported VPO seems to be very close to the active bulk VPO in terms of crystallinity. It should also be underlined that the background noise observed on the XRD diagram, which was higher than with the hemihydrate, could reflect the presence of a noncrystalline phase, attributed to the glue phase binding VPO and SiC (see later). Fig lc shows the XRD diagram of the sample after more than three months (>2000 hours) of reaction under different conditions of temperature and butane/air flows and ratios. There are no significant changes between this diagram and the preceeding one, in agreement with the fact that the catalyst was stabilized after 100 hours of activation, but a slight increase in the background noise should be noted. 700

1

600

1

0

Figure la

2

0" VO(H2P O4)2 1 " VO(HPO)0.5 H20 2" SiC

500

l

400

0

100 0

~22

9

m

30

.

m

50

.

n

' .

20

|

.

n

.

70

l

2

3 9(VO) 400

0 30 50 20 70 l0 Fig. 1b. VPO/SiC after 100h XRD

90

2P207

2 : SiC

300

3

3

2 3

200 3

0 I

2

Figure le

500

100

2

3

0

.

90

Fig. l a. Hemihydrate/SiC XRD 600

3 3

0

~~.~,~-'_x~:

10

3 9(VO)2P207 2 9SiC

0

unknown l 1~2 2 12

Figure lb

0

300 200

2

0

2 3

23

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30 50 20 70 10 Fig. 1c. VPO/SiC after 2000h XRD

i""

,

n

.

i

90

Fig. 2. HRTEM of VPO/SiC

In order to obtain more information on the nature of the interaction between VPO and SiC, HRTEM was performed using a Topcon Model EM200B operating at 200kV with a point-topoint resolution of 1.7 A. The sample of catalyst was deposited on a carbon grid and introduced into the analysis chamber without specific precautions as all these samples seem to be stable for a short time in air, otherwise they were kept under dry nitrogen for long periods. VPO is very sensitive to the energy of the microscope beam and cannot be observed more than 10s without starting to decrystallize. For this reason the pictures were blindly taken by switching the beam on and off for the time of the photo and moving the grid in different directions after the beam had been focused on one spot of the sample followed by a partial destruction of the crystal. Luckily 2 to 10% of the resulting pictures were usable and could provide excellent information. For EDS analysis, when an area was selected, the subsequent decrystallization was not detrimental as only the elemental composition was sought. Fig 2 shows a microcrystal of VPO

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in contact with the SiC support. At the junction, the crystal order of VPO progressively disappears into an amorphous phase (between arrows) which contains V, P, O, Si and C elements as found by EDS. This phase does not have the same appearance as the amorphous layer found on the top edge of the crystal which only contains V, P and O. It is suggested that the interphase is a solid solution of VPO in a silicon oxycarbide acting as glue. On a different support this glue phase can be observed by XRD because it is partly crystallized (16). The same sort of interface cannot be observed on carborandum or conventional ct-SiC and explains the very poor mechanical strength of the VPO supported on this ct-SiC. An improvement of the attrition resistance when compared to bulk VPO reinforced by a coating of silica (17) was researched with the SiC impregnation. This was not obtained, VPO/B-SiC presented a poorer attrition resistance than the V P O + S i O 2 c o a t . Because of the relatively small amount of oxygen contained in the upper layer of the SiC pores and probably because of the peculiar chemical property of this oxycarbide, it seems that there is the fight balance of interaction, strong enough to form the glue phase, but weak enough to avoid oxygen interference during the formation of the VPO phase from the hemihydrate precursor.

3. R E S U L T S AND D I S C U S S I O N The catalysts were tested in a micropilot already described (18). Each gas of the feed was independently monitored before mixing, i.e. O2, butane, He ( N 2 w a s replaced by He in the air mixture to ease the analysis) and a known amount of CH 4 was injected after the reactor to be used as standard to connect the two chromatographic analyses made on-line. The mass balance in oxygen and carbon was always better than 99%. The reactor was of a fixed-bed type with a thermocouple measuring the temperature inside the catalyst bed and another the temperature outside the reactor wall at the heart of the oven. The temperature measured in the bed was used as reaction temperature. At steady state and with the bulk VPO reference catalyst the difference in temperature between the two thermocouples could be as high as +30~ while with the SiC supported sample, the difference was found to be generally much lower, between 0 and +5~ the temperature inside always being higher than outside. The reference bulk catalyst was a state of the art industrial sample. The standard conditions of reaction were: O2/butane volume ratio (or mole)=l.4-1.5/1, partial pressure: 12 to 14 % of butane, total flow: = 40 cc/min for a catalyst weight o f - 0.5 g giving WHSV = 1.7 h -~ (all these figures were very accurately measured for each experiment). In Fig 3 the specific rate of MA formation on each catalyst (bulk VPO and VPO/SiC) as a function of the reaction temperature per gram of VPO and not of total catalyst is reported. At low temperature, below 350~ where the selectivity in MA is very high (see Fig 4), but the conversion very low, the rate of reaction seems to be equivalent on the two catalysts. However at 370~ for the same selectivity of 79%, the rate on VPO/SiC (3.10- 6 m/g.s) is almost 2 times faster than on bulk VPO (1.7.10 .6 m/g.s). This could be attributed to a dispersion effect due to the support. At higher temperature the comparison is impossible as a sharp drop in selectivity is observed on the bulk VPO. Conversions and yields in MA are reported in Figs 3 and 4 as a function of the temperature. The main by-products are CO 2 and CO and to a minor extent some acrylic acid and methacrolein. If the yield of MA at 370~ reaches 24% on the bulk and only 78% on the supported catalyst catalyst, this is only due to the fact that VPO/SiC contains 27% of VPO instead of 100%. At 420~ the two catalysts produce the same yield (16%), above this temperature most of the products found on bulk VPO are CO and CO 2 while the selectivity in MA remains relatively stable on VPO/SiC, above 70%, up to 470~ which explains the yield of 38% observed at this temperature. The simplest hypothesis to explain these results is to consider that the temperature of the surface of the bulk catalyst does not correspond to the temperature measured in the bed, and that the MA formed after a certain limit of temperature is burnt into CO and CO 2, while on the supported catalyst the surface temperature is quickly equilibrated by the heat sink effect of the conductive SiC support. If one assumes that the surface temperature equals the temperature of

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the full bed with the supported catalyst, Fig 4 explicitly shows that MA is not extensively burnt even at a temperature as high as 470~ which means that on bulk VPO the surface temperature should be much higher considering the very poor selectivity in MA observed above 420~ It was interesting to see if higher yields could be reached on the supported catalyst. In the standard conditions all oxygen was consumed at 4700C and the limitation in yield could only be due to this shortage in oxygen. To check this, a series of measurements were made on a slightly more active catalyst than the preceeding series by increasing the O2/He f l o w , keeping constant the butane flow, thus working at constant butane WHSV but at higher total flow. The results are presented in Fig.5. It is clear that the reaction is only limited by the amount of oxygen available for the reaction; the selectivity remains more less constant, the conversion increases faster at the beginning but quite linearly with the 02 avaibility. The maximum yield obtained for a ratio O2/butarie of 10 reaches 55% for a butane conversion of 70% and it is probably possible to go higher, but the micropilot was not equipped to do so. Many other parameters have been studied and will be published in a longer article later. 50

1 10.5

40

~ ;;

~VPO/SiC - - I - - - ' ~ - V P O bulk

/ I1r

O

8

100

10. 6

90

t~

30

.-.

6 10-6

80

20

~_.

4 10-6

70

2 10-6

60

0 ld'

50

10 Figure 3

"

0 300

350

400 T (~

-o--~vPo/sic

450

500

Figure 4 ~

300

350

400 T (~

450

500

Fig. 3. Compared reaction rates on VPO bulk Fig. 4. Compared MA selectivity on VPO bulk and VPO/SiC and VPO/SiC 90

Figure 5 Selectivity

80 70 6050" 40 30

Air/butane flow ratio 15

I

20

I

25

I

30

I

35

I

40

I

45

I

50

55

Fig. 5. Maleic anhydride yield and selectivity 4. C O N C L U S I O N The new [3-SIC with a large mesoporosity is an excellent support for VPO. The interaction between the active phase and the support is well balanced, not too strong which would perturb

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the formation and the selectivity of the active phase, not too weak in order to keep a good mechanical strength. The high thermal conductivity of SiC allows an homogeneisation of the temperature in the catalyst bed, particularly by strongly decreasing the surface temperature at steady-state. Consequently the total oxidation can be better controlled and very high yields can be reached in a fixed-bed configuration twice as high as for the conventional bulk catalyst- i.e. 55% instead of 24% with a partial pressure of butane of about 14%. REFERENCES

1. J.T. Wrobleski, J.W. Edwards, C.R. Graham, R.A. Keppel and M. Raffelson, US Patent 4562 268 (Monsanto) (1985). 2. R.M. Contractor, H.E. Bergna, H.S. Horowitz, C.M. Blackstone, B. Malone, C.C. Torardi, B. Griffiths, U. Chowdhry and A.W. Sleight, Catal. Today, 1 (1987)49. 3. E. Bordes, Catal. Today, 3 (1988) 163. 4. G.J. Hutchings, Applied Catal., 72 (1991) 1. 5. F. Cavani and F. Trifiro, in Catalysis, The Royal Society, Vol. 11,246 (1994). 6. G. Centi (Eds), Catal. Today, 16 (1993) 5. 7. M. Ruitenbeek, A.J. Van Dillen, D.C. Koningsberger and J.W. Geus, in Prep. of Catalysts VII, Elsevier Sciences B.V., Studies in Surface Science and Catalysis, Vol 118, 549 (1998). 8. M.J. Ledoux, S. Hantzer, C. Pham-Huu, J. Guille and M.P. Desanaux, J. Catal., 114 (1988) 176. 9. M.J. Ledoux, S. Hantzer, J. Guille and D. Dubots, Eur. Patent 0 313 480 (1992) and US Patent 4 914 070 (P6chiney) (1990). 10. N. Keller, C. Estoumes, C. Pham-Huu, S. Roy, J. Guille and M.J. Ledoux, J. of Mat. Sc., 34, 3189, (1999). 11. R.O. Kerr, US Patent 3 156 705 (Petro-Tex Chem. Corp.) (1964). 12. R.A. Schneider, US Patent 3 864 280 (Chevron) (1975). 13. B. Grindatto and M. Prin, Eur. Patent 0 543 752 A1 (Pechiney) (1992). 14. N. Keller, C. Pham-Huu, M.J. Ledoux, J.B. Nougayrede, S. Savin-Poncet and J. Bousquet, Catal Today (in press). 15. K. Katsumoto, D.M. Marquis, US Patent 4 132 670 (Chevron) (1979). 16. M.J. Ledoux, C. Crouzet, C. Bouchy, K. Kourtakis, P.L. Mills, J.J. Lerou, a patent application is in progress. 17. H.E. Bergna, US Patent 4 677 084 (Dupont) (1987). 18. F. Meunier, P. Delporte, B. Heinrich, C. Bouchy, C. Crouzet, C. Pham-Huu, P. Panissod, J. J. Lerou, P.L. Mills and M. J. Ledoux, J. Catal., 136 (1997) 33.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Propylene Epoxidation over Gold-Titania Catalysts Eric E. Stangland, Kevin B. Stavens, Ronald P. Andres, and W. Nicholas Delgass School of Chemical Engineering, Purdue University, West Lafayette, IN 47907 We present here kinetic isotope effect support for a hydroperoxy- intermediate, data that suggest consideration of oxidized Au in the active site, and XPS evidence for extended TiO2 as sites for propylene oxide oligomerization in the direct epoxidation of propylene to propylene oxide over Au/TiO2 catalysts in H2/O2 mixtures. 1. INTRODUCTION Haruta et al. have shown that deposition-precipitation (DP) gold-titania catalysts produce greater than 90% selectivity to propylene oxide (PO) from propylene, 02, and H2 at temperatures of less than 400 K [ 1,2]. Their suggestion that a peroxy- intermediate is a likely source of this unexpectedly high selectivity in the presence of allylic hydrogens [2] is supported by Sellers et al. who compute that gold is the best metal for producing peroxy radicals from H2/O2 mixtures [3]. In our work, gas phase propylene epoxidation using O2/H2 has been performed over a wide variety of gold-titania catalysts resulting in TOFs that vary over 4 orders in magnitude. A maximum in PO TOF is observed for most catalysts at 413 K as higher temperatures promote further oxidation products. XPS suggests that alkali promoters may control some of these further reactions, particularly PO oligomerization, at the catalyst surface. A decrease in the steady state PO TOF when D2 is substituted for feed H2 suggests that hydrogen from H2 is present in the complex involved in the rate limiting step and supports the assumption that peroxy intermediates are involved. Kinetic studies involving Au(OH)3 show that oxidized gold species stabilized by titania may play a role in creating the active site for epoxide formation. 2. EXPERIMENTAL METHODS Gold-titania catalysts were prepared and characterized by a variety of spectroscopic techniques [4]. Titanium tetra-isopropoxide (TTIP)-modified-Au(OH)3 (Alfa Aesar) was prepared by impregnating Au(OH)3 with TTIP in pentane, which then evaporated. Propylene oxidation was carried out differentially in a stainless steel 1/2" packed bed reactor over the temperature range of 323-473 K using an atmospheric 10/10/10/70% mixture of H2 (or O2), O2, hydrocarbon, and He at a flow rate of 35 cc (STP) min -l. Ethanal, PO, acetone, propanal, acrolein, propane, and CO2 were analyzed in the product stream by gas chromatography. XPS analysis was performed using a Mg anode powered with 15 keV at 300 W. A reaction chamber connected to the XPS UHV chamber allowed samples to be exposed to the normal propylene oxidation reaction conditions mixture and then analyzed without being exposed to air. A saturator was also used to expose catalysts to a similar number of PO molecules to that which they produce in 6 hours of propylene oxidation. Additionally a

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heating stage could warm samples in v a c u o during analysis if desired. XPS of Au(OH)3 and Au powder was performed by burnishing samples into Ag foil, charge compensating with a low energy electron gun, and referencing all binding energies to Ag 3d5/2 at 368.0 eV. Silica supported catalysts were referenced to the Si 2p in at 103.4 eV. Further details concerning the experimental methods can be found in ref. [4].

3. PREVIOUS RESULTS A detailed analysis was performed on catalysts prepared by a variety of techniques, seen in Table 1 [4]. This work has shown that PO activity can be generated from almost every Au-TiO2 preparation method and Au particle size above 2 nm in diameter. The PO selectivity for the representative Au/TiO2 catalysts, Figure 1, clearly shows there are optimal preparation methods that maintain better selectivity over the temperature range studied.

Table 1: Characterization results for gold-titania catalysts [4]. Abbreviations: Titanium tetra-isopropoxide (TTIP), Distributed Arc Cluster Source (DACS), Degussa P25 TiO2 (P25), deposition-precipitation (DP). The ratio in parenthesis, for the DACS Au-Ti cluster, e.g. (Au:Ti, 4:1), corresponds to the Au:Ti atomic ratio as determined by XPS. Catalyst Preparation Method Gold Loading Dp (wt. %) (nm) T/Au TTIP on Au Powder 99.5 -1000 (Au)/P25 DACS Au clusters on P25 1.7 7.6+3.1 Au(DP)/P25 DP on P25 1.1 7.4+2.3 Au(DP)/A DP on anatase 0.2 4.5+1.0 Au(DP)/T-S DP on TiO2-modified-SiO2 0.5 4.2+1.0 (Au:Ti, 4:1)/S DACS Au-Ti clusters on SiO2 0.3 11.1+10.0 (Au:Ti, 200:1)/S DACS Au-Ti clusters on SiO2 0.2 3.0+2.0 (Au:Ti, >300:1)/S DACS Au-Ti clusters on SiO2 0.6 5.0+2.8 Pure gold or pure titania have no intrinsic activity, but Au powder modification with TTIP creates a few PO active sites. The catalyst created by supporting gold DACS particles onto P25, (Au)/P25 had similar selectivity and TOF. Better gold-titania contacting, and therefore better PO activity and selectivity, was created by using the DP method of Haruta [1]. It is interesting to note, as did Haruta [2], that DP of Au onto three different titania supports, P25, anatase, and titania-modified-silica, did not produce materials of equivalent activity and selectivity at temperatures above 413 K. It appears the type of titania Support has dramatic effects on the formation of active Au-Ti interfaces. The most active catalysts in this study included Au(DP)/T-S and (Au:Ti, 200:1)/S with TOFs of 0.016 s 1 and 0.005 s 1 respectively at 413 K. Nijhuis et al. have suggested that PO yields are capped over Au/TiO2 catalysts at about 2% as a result ofPO oligomerization and rate inhibition [5]. While PO oligomerization may be responsible for some loss of PO yield, the sequential oxidative cracking of PO to ethanal and CO2 represents a critical loss of PO selectivity at temperatures higher than 373 K. A similar effect is observed with decreasing weight-hourly-space velocity [4]. Both PO formation and its oxidative cracking apparently occur at the Au-Ti interface, suggesting that maximizing PO formation sites may not be the true solution to a high yield catalyst. Further

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PO oxidization must also be controlled. Higher sustained PO selectivities with increasing temperature are achieved b y depositing Au onto a welldispersed, non-crystalline TiO2 phases, either as DP Au on TS-1 [5], T-S supports, or by using certain DACS bi-metallic Au-TiO 2 clusters on SiO2 [4]. Figure 1 shows even for the DACS bimetallic catalysts a variation in selectivity is observed with Au:Ti cluster composition. The lack of extended TiO2 phase on both DACS and TS-1 catalysts help control further reactions of PO, demonstrating the importance of not only the type, but amount, of titania in contact with Au.

829

1 ~ ~

Temperature (K)

500

455

2.0

2.2

417

385

357

2.4 2.6 IO00/T (K1)

2.8

>

Q~

0

Figure 1. PO selectivity over various Au/TiO2 catalysts. Reaction conditions were C3H6/O2]H2/ He= 10/10/10/70% at 35 STP cc min ~ ( O ) Au(DP)/T-S, ( A ) Au(DP)/A, ( + ) Au(DP)/P25, ( V)(Au)/P25, ( 0 ) (Au:Ti, 4:1)/S, ( N ) ((Au:Yi, 200:1)/S, ( X ) (Au:Yi, >300:1)/S, ( O ) T/Au. Data taken from ref. [4]

4. RESULTS AND DISCUSSION 4.1 Control of PO oligomerization Oligomerization may be responsible for loss of PO yield and catalyst deactivation over many DP Au/TiO2 with bulk titania phases [5]. Figure 2 shows the C:Ti or Si atomic ratios from XPS spectra of carbon deposition over Au(DP)/T-S during steady state propylene oxidation and PO exposure. Column a) shows the normal amount of carbon deposited during six hours of propylene oxidation in O2/H2. Column b) shows that a fresh catalyst exposed at 473 K to a similar number of PO molecules as in a) results in a larger amount of carbon deposition. Bulk gold foil gave no carbon retention, but the T-S support used to create Au(DP)/T-S not exposed to the DP method, and therefore having no

1 0.9 0.8

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0.4 0.3 0.2 0.1 0

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b)

c)

~cq

I

d)

Figure 2. XPS of C ls region after PO exposure over Au(DP)/T-S, the untreated T-S support, an Au foil, and (Au:Ti, 200:1)/S. Layer models were used for calculation of the atomic ratio, Nc/Nyi/si. Black bars are C ls < 289 eV; white bar C ls > 289 eV.

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surface alkali, collects about 1 carbon atom per Ti atom as shown in Column c). XPS detects that the Au(DP)/T-S catalyst in b) has about 0.1 Na atoms per surface Ti atom. It would appear that alkali promoters may block TiO2 surface sites responsible for oligomerization. The reactivity of Au(DP)/T-S, however, suggests that sites for the sequential PO oxidative cracking reactions are much less affected. Studies on the effects of surface promoters, which are known to alter the reactivity of Ag for gas phase propylene epoxidation [6,7], are just beginning for AuffiO2 materials [8]. The C:Si atomic ratio on (Au:Ti, 200:1)/S in column d), calculated assuming a similar number of Si atom as Ti atoms in the T-S monolayer, shows without an extended TiO2 phase, carbon deposition from PO is reduced.

4.2 D2 Kinetic Isotope Effect (KIE) Temperature (K) Upon the substitution of D2 for H2 in the feed, the rates to all 500 455 417 385 357 -1 products except acrolein were 10 suppressed. This is clearly shown for PO in Figure 3 for the Au(DP)/T-S "7 and (Au:Ti, 200:1)/S catalysts. The u_l 0.2 o PO rate maximum with temperature is preserved with H2 or D2, but the O 13.. differences in TOFs diminish as temperatures approach 473 K. The 10-3 observation that D2 affects the 210 212 2.'4 216 2.'8 addition rate of one oxygen atom to 1000/T (K"~) propylene forming PO is strong evidence that hydrogen from H 2 is Figure 3. Effect of D2 substitution on PO involved in the formation of the formation rate for Au(DP)/T-S and (Au:Ti, critical reactive intermediate, perhaps 200:1)/S. Reaction conditions were C3H6/02/I--I2 or through a hydroperoxy- species. In a Dz/He=10/10/10/70% at 35 STP cc min -1. related system, Laufer et al. have Au(DP)/T-S: ( O ) H2, ( 9 ) D2 (Au:Ti, 200:1)/S: concluded that the in situ generation ( D ) H2,( m ) D2. of H202 from gas phase H2 and 02 at the Pd site in liquid phase epoxidation of propylene with Pd/TS-1 is the rate determining step [9]. The lack of a similar isotope effect on acrolein production during propylene oxidation suggests that it occurs by a different route; perhaps one that starts with loss of allylic hydrogen from the propylene and subsequent oxidation by adsorbed O atoms rather than the selective peroxy- intermediate. Acrolein, however, makes up a just a small portion of the overall product distribution at these reaction conditions. Further evidence for a possible PO selective hydroperoxy-- intermediate is offered by results in our laboratory that show the Au(DP)/T-S catalyst epoxidizes ethylene in a gas phase reaction with H2/O2 at a TOF (413 K) of 0.003 s ~ with a selectivity of 84%. This parallels TS-1 which utilizes H202 in the selective epoxidation of both ethylene and propylene [10]. The different temperature sensitivity of the KIE between Au(DP)/T-S and (Au:Ti, 200:1)/S suggests that these catalysts differ by more than just the number of PO active sites. While there are small changes in apparent activation energies (Ea) with D2 substitution for reactants and most products, a large change in Ea is observed for ethanal, from 18.9+0.7 to 24.8+1.8 kcal/mol with H2 and D2 respectively, over (Au:Ti, 200:1)/S. A smaller 1.5 kcal/mole difference for ethanal is observed over Au(DP)/T-S. Since ethanal formation I

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appears to occur at the Au-Ti interface [4], this suggests that there are distinct differences in the energetics at the active site for different catalyst preparations. 4.3 Studies with Au(OH)3

100 DP Au/TiO2 catalysts, prepared by the deposition and 80 ~--I subsequent reduction of Au(OH)x C o 3 species at the catalyst surface, 6 0 "o {1:) generally produce very selective catalysts for PO formation [ 1,2,4,5 ]. Dehydration and reduction of Au(OH)3 occurs 0 .... ,'" 20 0 through a gold oxyhydroxide, Au(O)OH, phase [11] which has 0 0 been linked to enhanced low 0 2 4 6 8 10 temperature CO oxidation activity Time (hours) [12]. Although a direct link between the sites active for low Figure 4. Au(OH)3 reaction profiles at 35 total STP temperature CO and propylene (balance He) cc min l. Symbol Key: ( O ) PO, ( ~ ) epoxidation over Au catalysts has CO2, (0) propane, ( ..... ) temperature profile. not yet been established, we have Reaction conditions: a) 0.3 lg Au(OH)3 with 10% probed the Au(OH)3 surface for C3H6, b) 0.34g Au(OH)3 with 0% C3H6/10%O2/ POactivity. Figure 4b) shows 10%H2, c) 0.30g 1.0 wt.%-TiO2/Au(OH)3 with that the propylene/Oz/H2 mixture 10%C3H6/10%O2/10%H2. reacts with Au(OH)3 to form PO and other oxygenates (not shown) at temperatures as low as 298 K. The initial PO selectivity was about 60%. The PO formation rate decreases as the reaction mixture eventually reduces the hydroxide to metallic gold, resulting in subsequent hydrogenation activity to propane. The amount of PO produced integrated over the entire duration of the experiment results in a total turnover number (TON) of about 0.08 PO molecules produced per atom of Au in the reactor, or about 10 turnovers per Au surface atom as measured by BET. With propylene only, Figure 4a), no propylene oxidation is observed at 298 K. Slightly higher temperatures are necessary to form reaction products which are sustained for a shorter time than with 02/H 2. A slightly lower PO TON of 0.06 is observed. Figure 4c) shows that modification of the Au(OH)3 with TTIP, despite an initial PO selectivity of only 30%, maintains PO activity over the entire duration of the experiment in contrast to the non-modified Au(OH)3. The PO TON in this case is 0.1. The formation of large amounts of propanal over the modified-Au(OH)3 accounts for the primary difference in initial selectivity between Figure 4b) and 4c. Since isomerized PO is the likely source of the propanal in 4c) [4], it is reasonable to believe that a combined PO and propanal total TON of 0.2 is a more appropriate value. While the estimations of these TONs still suggest thatthe reaction with the Au(OH)3 is largely stoichiometric and not catalytic, the presence of TiO2 appears slightly enhance the reactivity before ultimate reduction of the gold. Most interesting, however, is the ability of pure Au(OH)3 to produce PO in the absence of Ti, suggesting a role for oxidized Au in the active site. XPS of model Au(OH)3 compounds and the Au(DP)/T-S catalyst shows that TiO2 may help stabilize oxidic gold states. In Figure 5a) we see the typical 4f lines expected for

~

t

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Heating the Au(OH)3 to 373 K in vacuo results in partial gold reduction and a complex mixture of states that eventually fully reduce Au ~ When a sample of TTIP-Au(OH)3 is studied in >, a similar fashion, the time required for w reduction of the Au species is extended c beyond the expected reduction time c based on pure Au(OH)3. Figure 5d) shows a sample of modified-Au(OH)3 at a heating time one hour longer than 96 94 92 90 88 86 84 82 80 the Au4f spectrum for 5c), a time at Binding Energy (eV) which complete reduction was expected. Figure 5e) shows that fresh Figure 5: Au 4f XPS of a) Au Powder, b) Au(DP)/T-S vacuum dried overnight at Au(OH)3 at 298 K, c) Au(OH)3 at 373 K, d) 393 K retains some unreduced gold TTIP-modified-Au(OH)3 wafer at 373 K, e) A species unlike catalysts were gold has 1.0 wt.%. Au(DP)/T-S. The Sicts.6 intensity been deposited by AuC13 [13]. While has been subtracted from the spectrum in e). the presence of oxidic gold species have not been detected after calcination over Au/TiO2 prepared by DP [2,4] or as precipitated Ti(OH)4 supported Au(PPhB)(NO3) [14], the selective catalysts formed by deposition of Au(OH)x species may retain as yet undetected amounts of non-metallic gold species stabilized on the catalyst surface as those observed for Au on rI-A1203 [15]. Alternatively, the oxidized gold could form the Au-O-Ti linkages that are the precursors to the active site. A

====

l

REFERENCES

M. Haruta, CataL Today, 36 (1997) 153. T. Hayashi, K. Tanaka, and M. Haruta, J. Catal. 178 (1998) 2. P. Paredes Olivera, E. M. Patrito, and H. Sellers, Surf Sci. 313 (1994) 25. E. E. Stangland, K. B. Stavens, R. P. Andres, and W. N. Delgass, J. Catal., submitted. T. A. Nijhuis, B. J. Huizinga, M. Makkee, and J. A. Moulijn, Ind. Eng. Chem. Res. 38 (1999) 884. 6. A. Gaffney, A. Jones, R. Pitchai, and A. Kahn, US Patent 5,698,719. 7. G. Lu and X. Zuo, CataL Lett. 58 (1999) 67. 8. B.S Uphade, M. Okumura, S. Tsubota, and M. Haruta, App. Catal., A 190 (2000) 43. 9. W. Laufer, R. Meiers, and W. H61derich, J. Mol. Catal., A 141 (1999) 215. 10. B. Notari, Adv. Catal. 41 (1996) 253. 11. R. J. Puddephatt, "The Chemistry of Gold", Elsevier, Amsterdam, 1978, 38. 12. R. M. Finch, N.A. Hodge, G. J. Hutchings, A. Meagher, Q. A. Pankhurst, M. R. H. Siddiqui, F. E. Wagner, and R. Whyman, Phys. Chem. Chem. Phys. 1 (1999) 485. 13. Y. -S. Su, M. -Y. Lee, and S. D. Lin, Catal. Lett. 57 (1999) 49. 14. H. Liu, A.I. Kozlov, A.P. Kozlova, T. Shido, K. Asakura, and Y. Iwasawa, J. Catal. 185 (1999) 252. 15. W.N. Delgass, M. Boudart, and G. Parravano, J. Phys. Chem. 72 (1968) 3563.

1. 2. 3. 4. 5.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Vapor-Phase Epoxidation of Propene using H2 and 02 over Au/Ti-MCM-41 and Au/Ti-MCM-48 B.S. Uphade, M. Okumura, N. Yamada, S. Tsubota and M. Haruta Osaka National Research Institute, AIST, Midorigaoka 1-8-31, Ikeda 563-8577, Japan Tel.: +81-727-51-9550, Fax: +81-727-51-9629, E-mail: [email protected] Vapor-phase epoxidation of propene using H 2 and 0 2 o v e r homogeneously dispersed gold particles deposited by deposition-precipitation (DP) on the supports Ti-MCM-41 (hexagonal) and Ti-MCM-48 (cubic) is studied at a space velocity of 4000 h-l.cm3/g(cat.) and 100~ or 150~ Better performance of Ti-MCM-48 over Ti-MCM-41 in the propene epoxidation reaction was observed. Influence of the various parameters were investigated: in the case of Ti-MCM-48 support, Si/Ti ratio, precipitating agent for Au deposition, Au loading, calcination temperature and in the case of Au/Ti-MCM-41 the influence of CsC1 addition as a promoter. GC-MS investigation of the desorbed species from the used catalyst revealed the presence of acidic as well as oligomeric species accumulated on the catalyst surface responsible for the catalyst deactivation. Based on the above experimental results, a probable reaction mechanism is explained. 1

INTRODUCTION

2

EXPERIMENTAL

2.1

Synthesis of Ti-MCM-41 and Ti-MCM-48 Supports and Au deposition

2.2

Catalytic Activity Measurements

The existing two commercial processes for the production of PO, using either chlorohydrin or hydroperoxide, have many shortcomings [1]. Our research work on the catalysis of gold [2] has opened a new stage for the direct epoxidation of propene using hydrogen and oxygen. We have reported recently in a series of papers the vapor-phase epoxidation of propene over highly dispersed nanosize Au particles supported on TiO 2 [3, 4], TiO2/SiO 2 [3, 4], and titanosilicates such as TS-1, TS-2, Ti-~ and Ti-MCM-41 [5-7]. This finding is being followed by few other researchers [8]. We also reported very recently ~e formation of propanal from propene, H 2 and 02 at high temperatures (_> 200~ [9]. Here, we attempt to compare two different titanosilicate supports in the epoxidation of propene over Au supported on Ti-MCM-41 and Ti-MCM-48.

The mesoporous supports were prepared under hydrothermal conditions at 100~ in a static Teflon bottle for 10 days according to the literature procedures [ 10, 11] and were finally calcined at 540~ for 6h. They were characterized by XRD, UV-Vis, FT-IR, and specific surface area measurements. Finely dispersed gold was deposited on the supports by deposition-precipitation (DP) method [12] and by calcination at 300~ tbr 4h. TEM observations were made to know the Au particles size and its distribution. Actual Au and Ti contents in the catalysts were analyzed by ICP. Catalytic tests were carried out in a vertical fixed-bed quartz reactor (i.d. 10 mm) using a feed containing 10 vol% each C3H 6, H2 and 02 diluted with Ar at a space velocity of 4000 h

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~.cm3/g.cat. passed over the catalyst (0.5 g) bed. As a promoter, CsC1 was impregnated or physically or mechanically mixed. Prior to testing, the catalysts were first pretreated at 250~ for 30 min in a stream of 10 vol% H 2 in Ar, followed by 10 vol% 02 in Ar. The feed and products were analyzed by using three on-line GCs equipped with TCD (AC and porapak Q columns) and FID (HR-20M column) detectors.

3

R E S U L T S AND D I S C U S S I O N

3.1

Characterization of Supports and Supported Au Catalysts

XRD, UV-Vis, FT-IR and very high surface area (970 m2.g -1 and 1170 m2.g ~ for TiMCM-41 and Ti-MCM-48, respectively) confirmed the structural integrity and mesoporous nature of the supports [13]. UV-Vis confirmed the absence of segregated TiO 2 phase at ca. 330 nm. However, the presence of anatase TiO 2 at >__4 wt.% Ti in mesoporous supports was observed. FT-IR spectra showed the presence of sharp peak at 963 cm ~ indicative of Ti in its tetrahedral coordination. This peak increased in intensity with an increase in Ti content and disappeared for MCM-41 and MCM-48 when calcined at 1000~ ICP analysis gave an actual Au loading of about 1 wt.% for most of the samples. TEM observations showed a homogeneous dispersion of Au with an average Au particle size of 2.0 nm and 2.1 nm for Au/Ti-MCM-41 and Au/Ti-MCM-48, respectively.

3.2 Epoxidation of propene

The results of propene epoxidation at 150~ over Auffi-MCM-41 and Auffi-MCM-48 are compared in Fig. 1. For both the catalysts initial propene conversion is about 5 mol% and it decreases with time due to catalyst deactivation. Superior performance of Auffi-MCM-48 in terms of activity, PO selectivity and also the H 2 efficiency than Auffi-MCM-41 is attributed to its three-dimensional pore system which can be more resistant to blockage by extraneous materials like oligomers of PO than the one-dimensional pore system of Ti-MCM-41. Koyano and Tatsumi [ 11] also have shown in the liquid phase epoxidation of cyclododecene with H202 the higher activity of Ti-MCM-48 as compared to Ti-MCM-41. The results of the influence of Si/Ti ratio in Ti-MCM-48 on the epoxidation of propene over 8%Auffi-MCM-48 at 100~ are given in Fig. 2. Conversions of H 2 and 02 increases whereas PO selectivity decreases with an increase in Si/Ti ratio. This can be ascribed to the increase in Au loading due to higher Ti content. The propene conversion is maximum at Si/Ti ratio of 50. Further increase in Si/Ti ratio causes a decrease in propene conversion, which is most probably due to the presence of polymeric anatase TiO 2 phase as observed in UV-Vis and TEM observations. We have also found that the best results in the propene epoxidation are obtained when NaOH is used as a precipitating agent for depositing gold on the supports. The results are presented in Fig. 3. One probable reason for this could be uniform and high dispersion of Au nanoparticles coupled with traces of alkali content. More basic nature of Na than Cs may also be the other reason for higher PO selectivity. The results of the influence of Au loading in the catalyst, Au/Ti-MCM-48 (Si/Ti - 50), on the epoxidation of propene at 150~ are given in Fig. 4. With an increase in Au loading propene, H 2 and 02 conversions increase whereas PO selectivity decreases. Propene conversion almost levels off at the 16% Au loading in the solution. At the Au loading below 2% in solution formation of propane was observed. We have reported earlier [4] that at the lower gold loading of < 0.2 wt.% propane formation takes place over AuffiO 2 due to the formation of gold particles smaller than 2 nm in diameter. The results of the catalyst calcination temperature on the epoxidation activity are presented in Table 1. Propene conversion passes through maximum for the catalyst calcined at 300~ The increase in PO selectivity with an increase in catalyst calcination temperature is expected due to the stronger contact between the support, Ti-MCM-48, and the gold particles.

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Fig. 1 : Influence of support (Ti-MCM -41 vs Ti-MCM-48) in the epoxidation of propene [Au in solution: 12wt.%, Temp. 150~

30

40 50 60 70 80 sifri ratio in support

90

100

Fig.2 : Influenece of Si/Ti ration in Ti-MCM-48 on the epoxidation of propene over 8wt.%Au/Ti-MCM-48 at I(X)~

The surprising fact is that even the catalyst calcined at 150~ also produce PO with a propene conversion of about 1.7%. At such a low calcination temperature the reduction of Au(OH~3 into its metallic state is highly unlikely. Our attempt to detect the presence of Au + and Au " species for the catalyst calcined at 150~ were unsuccessful due to highly dispersed Au particles mostly inside the channels eventhough the Au loading was very high. The results in the epoxidation of propene, however, may indicate that the reduction of Au(OH) 3 can take place easily during pretreatment and/or reaction. Table 1: Influence of catalyst calcination temperature on the epoxidation of propene [catalyst: 16 wt.%Auffi-MCM-48 (Si/Ti = 50), Reaction Temp.: 150~ Temp. (~ for Conversion (%) off Selectivity (%) for calcination a pretreatment b C3H 6 Ha 02 PO CO2 150/4h 150/4h 1.73 27.3 200/4h 200/4h 2.86 26.9 300/4h 250/4h 3.03 21.8 400/4h 250/4h 1.87 17.3 500/4h 250/4h 1.50 15.5 ain air, Din 10vol% 02 in Ar, Cafter 20 min of reaction

19.5 15.2 13.5 11.6 10.0

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Au (wt. %) in solution

Fig.3 :Influence of precipitationg agent during Au deposition on TiMCM-48(16wt. % Au in solution, Si/Ti=50) on the epoxidation of propene at 150~

Fig.4 :Influence of Au loading in the catalyst Au/Ti-MCM-48 (Si/Ti=50) on the epoxidation of propene at 150~

3.3

Drastic Depression in H z Consumption An important parameter for the feasibility of application o f these catalysts in the epoxidation of propene is the efficiency for H 2 and 02. At present though the propene conversion is around 3-4%, the conversions of H~ and O~ are very high, usually above 50%. The addition of CsC1 successfully lowers the consumption of H~_ and 02 by about 90%, while maintaining the propene conversion above 1.7%. Among the methods for adding the promoter, the best way is to just mix physically (simple mixing) the supported gold catalyst with a finely crushed CsC1 powder. The optimum loading of CsC1 as a promoter is found to be about 1.0 wt.% (Fig. 5). An appreciable increase in the size of Au particles was noticed by TEM analysis when CsC1 content was more than 1.0 wt.%. Therefore, even in the case of physical mixing, the Au particles tend to agglomerate in the presence of C1- anions. It was observed in H 2 oxidation experiments that the Au/Ti-MCM-41 with CsC1 require 350~ for quantitative H 2 oxidation as compared to 150~ Auffi-MCM-41 alone (Fig. 6). The results, therefore, indicate that the presence of CsC1 markedly depressed the reaction of H 2 with 0 2, while giving little influence on the reaction of propene with selective oxygen species. _

3.4 Catalysts deactivation and their regeneration

Another major problem associated with this system is the fast catalyst deactivation. The experimental results of the preadsorbed PO or propene on the catalysts before the start of the reaction suggests that the catalyst deactivation occurs mainly due to the adsorption of PO but not of propene. GC-MS study indicated the presence of large number of organic compounds adsorbed on the catalyst surface. These are: acetic acid, propanoic acid, 1acetoxy-2-propanol, 2-acetoxy-l-propanol, 1-hydroxy-2-propanone, propylene glycol, propylene carbonate, 2,5-dimethyl-l,4-dioxane, etc. We believe that these species are formed due to initial PO adsorption on the catalyst surface and its further oligomerization,

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rearrangement, cracking, coupling etc. on the free titanium sites. Acidic species such as acetic acid, propanoic acid may also contribute to the cause of catalyst deactivation. Our attempts to regenerate the deactivated catalysts by thermal treatment at > 250~ in the oxygen stream and also by dissolving organic moieties from the catalyst surface in organic solvent such as ethanol and acetone failed to achieve original activity of the catalysts.

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Fig.6 : Oxidation of l vol% H 2 in air over a) Au/Ti-MCM-41 and b) lwt.% CsC1 + Au/Ti-MCM-41 (a physical mixture) catalysts

Fig.5 :Influence of promoter CsC1 content physically mixed with Au/TiMCM-41 in the propene epoxidation at I(X)~ 3.5

Probable Reaction M e c h a n i s m Based on the theoretical prediction by Olivera et.al. [14] that the surface of gold is capable of generating H202 in-situ, we have proposed the following reaction mechanism which is different from that we proposed for Au/TiO 2 [4].

C3H6 + [S]

~

~-- [S]- - .C3JH6 S: the surface of Au and [Ti4+-SiO2] H2 + 02

H202 + [Ti4+-SiO2] [Ti4+-SiO2]- - -C3H6

[Aul

~ H202

~- HOO- - -[-Ti4+-SiO2] + HOO- - -[-Ti4+-SiO2] H3C---~"--CH2 ",O /

+ H20 + 2[Ti4+-SiO2]

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CONCLUSIONS 1 Superior performance of Au/Ti-MCM-48 than Au/Ti-MCM-41 in the vapor-phase epoxidation of propene using H 2 and 02 is attributed to its three-dimensional (cubic) pore system which can be more resistant to blockage by extrarieous materials like oligomers of PO and bulkier organic compounds as evidenced in GC-MS study than the unidimensional pore system of hexagonal Ti-MCM-41. 2 Physical mixture of CsC1 and Auffi-MCM-41 brings about 90% decrease in H~ consumption; however, the presence of c r anions also causes agglomeration of gold particles thereby also reducing propene conversion to certain extent. 3 Catalyst deactivation takes place not only due to the large number of organic compounds on the catalyst surface but also due to acidic compounds such as acetic acid.

REFERENCES 1. S.J. Ainsworth, Chem. Eng. News. (1992) 9 2. M. Haruta, Catalysis Surveys of Japan, 1 (1997) 61; Catal. Today, 36 (1997) 153 3. T. Hayashi, K. Tanaka and M. Haruta, Shokubai, 37 (1995) 72 4. T. Hayashi, K. Tanaka and M. Haruta, J. Catal., 178 (1998) 566 5. Y.A. Kalvachev, et.al., Stud. Surf. Sci. Catal., 110 (1997) 965 6. M. Haruta, B.S. Uphade, S. Tsubota and A. Miyamoto, Res. Chem. Internaed., 24 (1998) 329 7. B.S. Uphade, M. Okumura, S. Tsubota and M. Haruta, Appl. Catal. A: Gen. (in print) 8. T.A. Nijhuis, et.al., Ind. Eng. Chem. Res., 38 (1999) 884 9. B.S. Uphade, S. Tsubota, T. Hayashi and M. Haruta, Chem. Lett., (1998) 1273 10. K. Watanabe, Y. Onoda, H. Tsuneki and T. Tatsumi, Jpn. Patent App. H07-300312 (1997), to Nippon Shokubai Co. Ltd. 11. K.A. Koyano and T. Tatsumi, Stud. Surf. Sci. Catal., 105 (1997) 93 12. M. Haruta, et.al., J. Catal., 144 (1993) 175 13. A. Corma, N.T. Navarro and J. Perez-Pariente, J. Chem. Commun., (1994) 147 14. P.P. Olivera, E.M. Patrito and H. Sellers, Surf. Sci., 313 (1994) 25

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Ring-opening reactions of ethyl- and vinyloxirane on HZSM-5 and CuZSM-5 catalysts A. FfisP, I. Pfilink6 b and I. Kiricsi c* aChemical Research Center, Institute of Chemistry, Hungarian Academy of Sciences, Pusztaszeri t~t 59-67, H-1025 Hungary bDepartment of Organic Chemistry, J6zsef Attila University, D6m t6r 8, Szeged, H-6720 Hungary CDepartment of Applied and Environmental Chemistry, J6zsef Attila University, Rerrich B. t6r 1, Szeged, H-6720 Hungary

Transformations of ethyloxirane, vinyloxirane and their mixture were studied over HZSM-5 and CuZSM-5 zeolites in the presence of hydrogen at 363 K. Ethyloxirane underwent single ring-opening and deoxygenation, while ring expansion was the main transformation pathway for vinyloxirane. Reactions of the mixture revealed that the active sites for adsorbed oxygen mediated ring transformations can be separated on HZSM-5 from those, where the adsorption of the vinyl group and the ring oxygen is a requirement. This cannot be done on CuZSM-5.

1. INTRODUCTION Epoxides, due to their strained three-membered ring, can undergo easy ring opening, making these compounds versatile intermediates e n r o u t e of preparing more complicated organic molecules. The ring may be opened both stoichiometrically and catalytically. Catalytic ring opening may be performed by a nucleophile in the presence of a Lewis acid activator [3], on solid acids [4] and on supported metals in the presence of added hydrogen as well [5]. The electrophilic catalytic way of ring opening may involve solid acids of various kinds. These acids may contain Bronsted or Lewis acid centres or their combination. Both types of sites may be involved in the reactions, however, evaluating their specific roles may not be an easy exercise. Zeolites can be used as effective solid acid catalysts for promoting ring opening and the same parent material can be transformed to a form containing appreciable amount of Bronsted sites or overwhelmingly Lewis acid centres, therefore their role may be studied more or less separately. In this contribution the ring-opening reactions of ethyloxirane and vinyloxirane are described on HZSM-5 prepared by wet ion-exchange or CuZSM-5 zeolites prepared by ion exchange in the solid state.

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2. EXPERIMENTAL

Ethyloxirane and vinyloxirane were commercial products (Fluka) and were used as received, except for some freeze-evacuation-thaw cycles before reactions. The parent zeolite was NaZSM-5. The H-form was prepared via NH4ZSM-5 by de-ammonization at 873 K for 6 hours in vacuum. CuZSM-5 was made from HZSM-5 using the solid-state ion exchange method [6]. Certain amount (5 mol %) CuC12 was intimately mixed with well-powdered HZSM-5 in an agate mortar. The mechanical mixture was heat-treated at 873 K for 8 hours in air. The product was cooled to ambient temperature and washed free of chloride, then dried at 373 K. The zeolites were characterized by X-ray diffractometry and BET measurements and the metal content was determined by X-ray fluorescence spectroscopy. X-ray diffractograms were registered on well-powdered samples with a DRON 3 diffractometer in order to check crystallinity. BET measurements were performed in a conventional volumetric adsorption apparatus at the temperature of liquid N2 (77.4 K). Prior to measurements the samples were pretreated in vacuum at 573 K for 1 hour. The ratio of Bronsted to Lewis acid sites was determined by pyridine adsorption followed by FT-IR spectroscopy. Self-supported wafers were pressed and degassed in situ in the optical cell at 573 K for 1 hour. Then, they were cooled to 473 K and pyridine was loaded. The wafers were kept in pyridine for 1 hour followed by evacuation at the same temperature. For determining crystallinity from IR measurements the ratio of bands in the range of 440-480 crn-1 and 550-650 cm-~ was used [7]. For samples of good crystallinity the value should be around 0.72. FT-IR measurements were performed with a Matson Genesis spectrometer and 128 scans for collected for one spectrum. Characteristic data on the catalysts collected by the methods described above are displayed in Table 1. The reactions were run a static closed circulation reactor in the presence of hydrogen to slow down deactivation. A mixture of 1.33 kPa of the respective oxiranes and 20 kPa of H2 was prepared and allowed to react on 10 mg of the dehydrated zeolite (1-hour evacuation at 573 K). The reaction temperature was 363 K. In a series of runs the mixture of ethyl and vinyloxiranes (0.67 kPa of each) was allowed to react in the presence of 20 kPa H2 over the usual 10 mg zeolite at 363 K. Analysis was performed by the GC-MS method (Hewlett Packard (HP) 5890 gas chromatograph equipped with a HP 5970 quadrupole mass selective reactor; 50-m long HP-1 capillary column, 523 K and 423 K as the temperature of the injector and the oven, respectively) on samples withdrawn at certain time intervals. Table 1 Characteristic data on the catalysts HZSM-5 a

CuZSM-5 a

-

2.7

BET surface area/m2g -1

336

318

Crystallinityb/%

89

83/79

Bronsted/Lewis c

0.88

0.09

100

46

Characteristics C u 2+

content/weight %

Degree of exchange/%

a - Si/AI= 13.8 b - by XRD/IR c - by pyridine adsorption

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3. RESULTS 3.1. Reactions of ethyloxirane Ethyloxirane underwent ring opening on both catalysts either by single or double C-O scission (Scheme 1).

~/

~~/CHO 2

0

0 1

1 Scheme 1. Transformations of ethyloxirane on HZSM-5 and CuZSM-5 On the whole, CuZSM-5 was more active than HZSM-5. Double C-O cleavage, which means deoxygenation, was an important transformation pathway on both catalysts, but more significant on CuZSM-5 than on HZSM-5. Out of the two ways of single C-O rupture, the one leading to butanal formation was predominant on both catalysts, however, 2-butanone was also found. Selectivity towards butanal formation was higher on CuZSM-5 than over HZSM-5, nevertheless, on the absolute scale more 2-butanone was formed on CuZSM-5 than on HZSM5. For details, see Table 2. Table 2 Product distribution (mol %) in the reactions of ethyloxirane on HZSM-5 and CuZSM-5 zeolites (10 ms zeolite, 363 K, 20 kPa Hz) Comp.

HZSM-5

CuZSM-5

0 min

5 min

15 min

25 min

5 min

15 min

25 min

1

100

96.5

93.8

92.1

95.2

89.1

85.3

2

0

1.7

3.7

5.0

2.2

5.7

7.1

3

0

0.5

0.8

1.0

0.7

1.2

1.5

4

0

1.3

1.7

1.9

1.9

3.0

6.1

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3.2. Reactions

of vinyloxirane

More complicated, but in a way more selective reactions took place with vinyloxirane (Scheme 2 ). CHO

\__/

~ C H O

/

6

7

0 5

O Scheme 2. Reactions of vinyloxirane on HZSM-5 and CuZSM-5 zeolites As far as the overall ring opening is concerned, the HZSM-5 catalyst was more active than CuZSM-5. Single C-O cleavage was the only transformation route on HZSM-5, while double C-O bond rupture was also observed on CuZSM-5. It led to 1,3-butadiene. Single C-O scission provided with 2,5-dihydrofurane, trans-2-butenal and cis-2-butenal in this order of decreasing significance on both catalysts. Furane formation was faster on HZSM-5 than on CuZSM-5, while the other two products formed with similar rates and relative selectivities. For details see Table 3. Table 3 Product distribution (mol %) in the reactions of vinyloxirane on HZSM-5 and CuZSM-5 zeolites (10 mg zeolite, 363 K, 20 kPa Hz) Comp.

HZSM-5

CuZSM-5

0 min

5 min

15 min

25 min

5 min

15 min

25 min

5

100

49.2

20.3

13.2

54.2

35.1

25.7

6

0

2.1

10.8

11.9

10.9

14.4

14.7

7

0

2.0

2.9

3.5

2.1

2.9

3.4

8

0

46.7

66.0

71.4

32.7

47.5

56.0

9

0

-

-

-

0.1

0.1

0.2

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3.3. Reactions of ethyloxirane - vinyloxirane mixture

When a 1-1 mixture of the oxiranes was allowed to react, new product was not formed compared with the reactions of the neat compounds on any of the catalysts. The relative rate of transformations on HZSM-5 was not altered either. On CuZSM-5, however, the activities and selectivities of product formation were modified (Table 4). Table 4 Relative concentrations at various sampling times in a 1"1 mixture of ethyloxirane and vinyloxirane to the neat ethyloxirane or vinyloxirane on CuZSM-5 in the presence of hydrogen (20 kPa H2 at 363 K; data and the notations for the oxiranes are in italics) t/min

c(mixture)/c(neat) 1

2

3

4

5

6

7

8

9

5

0.9

0.9

0.8

0.7

1.1

0.4

0.8

1.3

1.2

15

0.6

0.7

0.7

0.6

1.1

0.5

1.1

1.3

1.2

25

0.6

0.3

0.7

0.4

1.1

0.6

1.1

1.3

1.2

Ethyloxirane transformed slower in the mixture than alone, while vinyloxirane reacted somewhat faster. Each product from ethyloxirane in the mixture was formed slower than in the neat form, while for vinyloxirane in the mixture the picture is mixed. Ring expansion accelerated in the greatest extent, while the proportion of the single ring opening route became smaller. Within this reaction pathway the formation of trans 2-butenal decelerated the most.

4. DISCUSSION Products from the ring opening of vinyloxirane are formed by the scission of the sterically more hindered C-O bond (or with double C-O rupture when 1,3-butadiene, a reaction pathway only existing on CuZSM-5, but even there of minor importance). This means more selective transformations than those of ethyloxirane. However, ethyloxirane underwent simpler single C-O scission reactions, since there were no secondary reactions after isomerization. In vinyloxirane two functionalities, the olefinic double bond and the C-O bond in the highly strained ring are combined. Both can be attacked by acid centres. Therefore, parallel with ring opening double bond migration also takes place. Beside the formation of internal olefinic bond either cis-trans isomerization or, as the more important transformation pathway, ring enlargement occurs. Each reaction occurring via single C-O scission with vinyloxirane and the one producing butanal from ethyloxirane can be considered typical acid-catalyzed transformations. The intermediates are the most stable carbenium ions the system allows, secondary carbenium ions, those are. Interpreting 2-butanone formation is more problematic. Since the reactions were run under hydrogen atmosphere to slow down deactivation, certain copper ions may have reached metallic state. In HZSM-5 similar things might have happened with metal ion impurities present in minute amounts. These centres transiently being in zero oxidation state may be responsible

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for 2-butanone formation. Comparison of products and their distribution on HZSM-5 and CuZSM-5 catalysts reveal, that both types of acid centres catalyze single as well as double C-O scission. Lewis acid sites alone, however, seem to be less active in the ring expansion reaction leading to 2,5-dihydrofurane than the mixture of Bronsted and Lewis sites (Scheme 3). @

@

(~

(~0"" s'o

///

ili

O-/~1--=-

,

o

/// "-

e

~

~

, ,

,

,~ eili

, eH

.

e

,

~

e

~

Ill

Scheme 3. The formation mechanism of the ring expansion product on a combination of Bronsted and Lewis sites Experimental results indicate that the active sites necessary for the reactions of the two oxiranes can be separated when HZSM-5 is applied. The mixture of the oxiranes reacts as if in their neat forms. Since this catalyst contains about equal amounts of Bronsted and Lewis sites, because of the availability of two nearly equally good choices, the molecules do not disturb each other. When practically only Lewis sites are available (either in the form of Cu § ions due to partial reduction by, e.g., hydrogen, or Cu 2§ ions due to the oxidizing ability of either oxirane when they are adsorbed through their ring oxygen, or extraframework AIO+), the adsorption of vinyloxirane suppresses somewhat the adsorption of vinyloxirane, accelerating especially the route leading to the ring expansion product. Conclusions

Although HZSM-5 and CuZSM-5 zeolites showed similar performance in the reactions of ethyloxirane and vinyloxirane in their neat forms, the reactiviy and selectivity of transformations of a 1:1 ethyloxirane-vinyloxirane mixture revealed that there was no competition between the two oxiranes when Bronsted and Lewis sites are equally available, however, when Lewis sites predominate, the active sites overlap, thus competition between the two oxiranes occurs. REFERENCES 1. W.F. H61derich, in "Catalytic Science and Tehnology", (S. Yoshida, N. Takezawa, and T. Ono, eds.), Kodansha/VCH, Weinheim/New York/Cambridge/Basel, 1991, Vol. 1, p. 31. 2. B. T6rSk, Gy. Sz6116si, M. R6zsa-Tarj~ni, M. Bart6k, Mol. Crys. Liq. Crys., 311 (1998) 289. 3. S.E. Denmark, P.A. Barsanti, K.-T. Wong, R.A. Stavenger, J. Catal., 63 (1998) 2428. 4. M. Bart6k, in "The Chemistry of Functional Groups." Supplement E2" The Chemistry of Hydroxyl, Ether and Peroxide Groups: (S. Patai, Ed.), Chap. 15, p.843. Wiley, Chichester/ Brisbane/New York/Toronto/Singapore, 1993. 5. A. F~isi, I. P~link6, J. Catal., 181 (1999) 28. 6. H.G. Karge, H.K. Beyer, Stud. Surf. Sci. Catal., 69 (1991) 43. 7. G. Coudurier, J.C. Naccache, J.C. Vedrine, J. Chem. Soc., Chem. Commun., (1982) 1413.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Controlling the Distribution of Framework Aluminum in High-Silica Zeolites D. F. Shantz, a R.F. Lobo, a C. Fild, b H. Koller b a Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, DE 19716, U.S.A. b Institut for Physikalische Chemie, Westf~lische Wilhelms Universit~it MOnster, Schlosspaltz 7, D-48149 MOnster, Germany

Using CP/MAS, REDOR, and heteronuclear correlation NMR spectroscopy we have investigated the geometrical relationships between the positive charge of organic structuredirecting agents and the negative charge in the framework of zeolite ZSM-12. In samples of all-silica and aluminum containing ZSM-12 prepared using benzyltrimethyammonium cations as structure-directing agents (SDA), we have found that the positively charged segment of the SDA is preferentially ordered near the negative framework charge. This result implies that the distribution of A1 sites in the zeolite is distinct from the random distribution often assumed for high-silica zeolites. Our results suggest that the distribution of acid sites can be controlled using SDAs with different charge distributions. 1. INTRODUCTION Control of the distribution of acid sites in microporous materials is an important objective in the synthesis of more selective catalysts as well as in the preparation of model catalysts. Controlling the average concentration of acid sites in zeolites (i.e., the Si/A1 ratio) by synthetic or postsynthetic methods (or both) can be done in a straightforward manner. However, obtaining materials with identical concentrations of catalytic sites, while at the same time containing a different spatial distribution of sites is still a synthetic objective that has not been demonstrated. The catalytic properties of zeolite samples with, e.g., individual, isolated aluminum sites could be contrasted to other samples with 'pairs' of adjacent acid sites. In this way, materials with similar Si/A1 ratios could be used to determine if postulated mechanisms of catalytic reactions (like relative rate of cracking vs. hydride transfer) depend not only on acid strength but also on acid site distribution. How could we systematically control the distribution of acid sites? One path towards achieving this goal was recently suggested by an investigation of the clathrate nonasil[ 1]. We have shown that small trimethylalkylammonium molecules, molecules that are not hindered by the cage to rotate, are permanently 'attached' to the negative charge of the cage walls up to at least 100 ~ In cases where the conformation of the organic structure-directing agent is

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fixed because of steric constraints, the location of the negative charge should be severely limited by the geometry of the organic molecule. The objective of this work is to determine if the location of negative charges in the framework is controlled by the location of the positive charge of the structure-directing agent. To answer this question we have used samples of the one-dimensional large-pore zeolite ZSM-12 (MTW, see Fig. 1) in its all-silica form and in its Al-containing form (with an A1/R+-I where R += benzyltrimethylammonium). This latter organic molecule has the advantage that it can be selectively deuterated in different locations (either on the methyl, phenyl, or methylene groups or combinations thereof). Subsequently, the dipolar coupling between different hydrogen nuclei (1H) and different nuclei (298i, 27A1) in the zeolite framework can be studied to estimate relative distances. From these data we can infer the local geometry of the organic molecule. We will use cross-polarization (CP) and rotational echo double resonance (REDOR) to investigate our samples. The strong spatial dependence on the rate of CP (~l/r 6) makes CP/MAS spectroscopy a useful qualitative tool for studying solids. REDOR is in contrast a difference experiment that gives rise to a curve for each signal of the 1D NMR spectrum. The stronger the dipolar interaction between the coupled nuclei (i.e., the closer they are to each other), the steeper the initial slope of the REDOR curve. The interpretation of CP/MAS and REDOR experiments depends on the mobility of the nuclei of interest, so we have also performed 2H NMR experiments on the samples to quantify the relative mobility of the different segments of the SDA. 2. EXPERIMENTAL The samples of ZSM-12 are prepared as reported elsewhere[2,3]. Benzyltrimethylammonium chloride is prepared by the reaction of trimethylamine with benzyl chloride. The selectively deuterated molecules are prepared from suitably labeled precursors (d9trimethylamine and/or d2, dT, ds-benzyl chloride). For A1-ZSM-12, the chemical analysis gives a Si/Al=28, consistent with one framework aluminum per structure-directing agent.

1-13

// Fig. 1. Projection of the structure of the zeolite ZSM-12 (left) and the benzyltrimethylammonium SDA.

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1H MAS spectra were acquired using a Bruker MSL 300 spectrometer, a spinning rate of 10 kHz, 5 s recycle time and a 5/2s 90 ~ pulse. 2H static and MAS spectra were acquired using the quadrupole echo pulse sequence. 29Si-{1H} CP/MAS spectra are recorded using a 6 ,as 1H 90 ~ pulse, a spinning rate of 3 kHz, 5 s recycle delay and high-power decoupling. 29Si-{1H} CP-REDOR experiments are performed using a 5 s recycle delay, a 6 ,as 1H 90 ~ pulse length and a contact time of 10 ms. The spinning rate was 3 kHz with a 32 step phase cycling routine. More experimental details can be found in the references[2,3]. 3. RESULTS AND DISCUSSION

3.1. Mobility of the Structure-Directing Agent 2H NMR of d9-ZSM-12 shows the methyl groups have a quadrupole coupling constant of 15.9 kHz, consistent with methyl groups that undergo two rapid rotations; one about the methyl C3 axis and one about the nitrogen C3 axis. An additional smaller narrow signal is observed indicating the presence of molecules with isotropic motion. The cause of this small component is not known at this time and continues to be investigated; but it is not due to decomposition of the SDA or chemical exchange during the synthesis process. 2H NMR shows the d2- and ds- samples have quadrupole coupling constants of 110 and 100 kHz, which are consistent with essentially immobile deuterons. The small reduction of the QCCs from the static values are due to small angle 'wobbling' motions because the organic groups do not fit tightly in the pores of ZSM-12. 3.2. Siliceous ZSM-12 Figure 2 shows the ~H MAS spectra of all-silica ZSM-12 and A1-ZSM-12. The major difference is the presence of a line at 10.2 ppm that has been assigned to silanol groups that are strongly hydrogen bonded to siloxy groups (SiO--..HO-Si). Also, there is a line at 7 ppm

~

.

.

.

.

.

.

.

.

.

,

20

.

.

.

.

.

.

.

.

.

,

10

.

.

.

.

.

.

.

.

.

,

.

.

0 d/1H (ppm)

.

.

.

.

.

.

.

.

.

-10

.

.

.

.

.

.

.

.

,

-20

Fig. 2.1H MAS NMR of as-made A1-ZSM-12 (top) and all-silica ZSM-12 (bottom).

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due to the aromatic protons, and the resonances between 4.5 and 3 ppm are due to the aliphatic protons. 29Si MAS NMR of several as-made all-silica zeolites has shown the positive charge of the structure-director is balanced by a siloxy group coordinated to two or three silanol groups[4]. Heteronuclear correlation spectra of selectively deuterated samples of ZSM-12 made with a different SDA as well as samples of nonasil[2] show that the methyl groups of the structure-directing agent are in close proximity to the defect site composed of one siloxy group and three silanol groups. We have recently confirmed several aspects of this defect model using multiple quantum NMR. The results obtained with a sample prepared with a perdeuterated SDA show that indeed the defects consist of one siloxy group and three silanol groups. Double and triple quantum NMR experiments performed are consistent with a model where three silanol protons are engaged in strong hyrdogen bonding to the charge compensating siloxy group. These three hydrogen bonded silanol protons are responsible for the resonance observed at 10 ppm. Also, the double quantum NMR experiments performed show that the methyl and methylene protons of the SDA are in close proximity to the defect site, consistent with heteronuclear correlation spectroscopy results for several as-made all-silica zeolites[2]. 3.2. AlUminum-containing ZSM-12 As observed in Fig. 2, our samples of A1-ZSM-12 contain no defects within the detection limits of NMR since there is no signal intensity at 10 ppm. Single pulse 27A1 MAS NMR reveals that all the A1 atoms are in tetrahedral conformation: only one sharp and slightly asymmetric signal is observed at 56 ppm. We have obtained 27A1 - {IH} REDOR curves for samples of A1-ZSM-12 prepared with do, d7, d9, and d14-SDAs. Not surprisingly, the initial slope of the REDOR curve scales with the number of protons present in the SDA. That is, no structural information can be obtained from these experiments. Figure 3 shows the 29Si-{1H} CP/MAS spectra obtained with a 400/ts contact time for

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

-60

-70

-80

-90

-100 -110 6' 29Si / (ppm)

-120

-130

-140

Fig. 3 29Si- {1H} CPMAS NMR of (a) d7-, (b) dg-, and (c) d14- AI-ZSM-12

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samples prepared with dT, d9 and dI4-SDAs. For the dI4-ZSM-12, we see a clear difference between the relative intensity of the Si(1A1) signal and the Si(0A1) signal as compared to the other samples. The Si(1A1) signal is substantially enhanced when only the methylene protons are not labeled (Si(1A1)/Si(0A1)=0.98 as compared to the one-pulse 29Si MAS spectrum value of 0.17). We conclude that the methylene protons are preferentially ordered near silicon atoms adjacent to the framework aluminum. We have obtained 29Si- {1H} CP/REDOR curves (see Figure 4) for the same set of materials and the results are fully consistent with the interpretation obtained from the 29Si-{1H} CP/MAS experiments[3]. The CP/MAS and CP-REDOR results both show that there are preferential dipolar interactions between the methylene protons of the SDA and the Si(1A1) silicons which are adjacent to the framework aluminum. Because the dipolar interaction is related to the internuclear distance, the results show that the methylene protons of the SDA are preferentially ordered with respect to the Si(1A1) silicons. The preferential ordering observed is consistent with coulombic forces between the cationic SDA and the framework aluminum. These results, therefore, indicate that there is a direct spatial association between the charge center of the SDA and the framework aluminum atoms. While the SDAs are not ordered in the crystallographic sense, for one-dimensional materials they are packed in close contact down the length of the pore. We believe these results demonstrate that is possible to exhibit some control over the distribution of catalytic sites in high-silica zeolites based on the synthesis conditions. Further, we believe that a combination of altering the charge distribution of the SDA and a basic knowledge of the SDA's organization in the zeolite micropores should be a useful starting point for a more rational approach to altering the distribution of framework heteroatoms via synthesis. In the context of zeolite synthesis, this is another positive step towards the goal of tailoring the physical properties of microporous

0.8

0.6

9

A

r./3

<1 0.4

0.2

0

I

I

I

i

I

1

2

3

4

5

N "L'R,m s

Figure 4. 29Si - {1H} CP-REDOR curves of d14-Al-ZSM-12. Dark spheres and clear triangles are the Si(1A1) and Si(0A1) REDOR curves, respectively.

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Figure 5. Illustration of a plausible distribution of aluminum sites in zeolite A1-ZSM-12 that is consistent with the NMR data shown in Figures 3 and 4. The aluminum atoms have been represented as spheres for clarity. materials by design. It is important to note that as the amount of alkali cations used in the synthesis mixture is increased that competing effects between the SDA and alkali will complicate this issue. Also, calcining these materials in the presence of water could lead to migration of the A1 sites. 4.

ACKNOWLEDGEMENTS

This work was funded from NSF grant CTS-9713516. R. F. L. and D. F. S. acknowledge an NSF Intemational Travel Award to support collaborative work at Westfalische Wilhelms Universit/~t M~nster in the group of H. Eckert (INT-9725941). We thank H. Eckert for use of his laboratory facilities and helpful discussions. We are grateful to the Department of Chemistry and Biochemistry at the University of Delaware for use of the NMR facilities, and C. Dybowski for useful discussions. REFERENCES

1. 2. 3. 4.

D.F. Shantz and R. F. Lobo, J. Phys. Chem. B, 102 (1998) 2339. D.F. Shantz and R. F. Lobo, Chem.Mater., 10 (1998) 4015. D.F. Shantz, C. Fild, H. Koller and R.F. Lobo, J. Phys. Chem. B, 103 (1999) 10858. H. Koller, R. F. Lobo, S. L. Burkett and M. E. Davis, J. Phys. Chem., 99 (1995) 12588.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

TOLUENE DISPROPORTIONATION CATALYSIS USING EUO TYPE ZEOLITES: INITIAL OPTIMISATION AND D E V E L O P M E N T J.L. Casci a and A. Stewart b

a- Synetix, RT&E Department, PO Box 1, Billingham TS23 1LB, UK. b - ICI Technology, Science Support Group, PO Box 90, Wilton, TS90 6JE, UK. This paper is a combined preparation and catalysis study, which explores the relationship between catalyst preparation and performance, in particular how the conditions of manufacture impact on catalyst deactivation. The commercially important reaction of toluene disproportionation (TDP) was studied using catalysts based on zeolite EU-1. This report describes initial results from a screening study, where it was found that different samples of EU-1 decayed at markedly different rates; these were samples of different compositions synthesised at different temperatures for different periods of time. The decay characteristics are rationalised in terms of synthesis reaction over-run during which there is a marked variation in both the pH of the reaction mixture and the surface composition of the isolated material, as determined by x-ray photoelectron spectroscopy (XPS). This over-run phenomena, not previously reported for high-silica zeolites, is related to synthesis temperature (and composition) and is likely to be important for other zeolite catalysts. I. Introduction

The preparation and manufacture of (Zeolite- based) a catalyst is multi-stage process comprising zeolite synthesis, activation (typically calcination and ion exchange) and forming (e.g. extrusion after addition of a binder). All stages can have a profound influence on the catalyst performance. Transalkylation reactions are widely used in petrochemical industries where they are used to upgrade the economic value of, chemical feedstocks. An example is toluene disproportionation 1 (TDP) in which toluene is reacted to generate benzene and (ortho-, meta- and para-) xylene: 2C7H 8 ~ C6H6 + C8H10 schemel Thus a material, toluene, which has only "fuel value" can be upgraded to the more valuable chemical feedstocks benzene and para-xylene, key intermediates (for example) for nylon and polyethyleneterephthalate respectively. In addition this process has the advantage of generating a low ethylbenzene C 8 stream for further processing. The reaction, however, is not quite as simple as scheme 1 suggests with a series of side and/or consecutive reactions yielding light gas (by dealkylation) and heavier products (C 9, C 10 etc. by further reaction of xylenes). There are numerous reports of zeolite-based catalysts for TDP with most referring to either MOR or MFI materials 1, with a modified MFI also being reported 2, for Selective TDP in which a p-xylene rich product is generated rather than the thermodynamic distribution of xylene isomers. While MFI consists of intersecting 10-ring channels 3, the MOR framework is based on unidimensional 12-ring channels linked by 8-rings 3. Based on these features alone one might consider the EUO framework type to be of interest in

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TDP since it is based on 10-ring channels with deep 12-ring side-pockets 3'4. The EUO framework was first discovered as EU-15, 6 by workers at Edinburgh University and ICI then later as TPZ-3 (Teijin) and ZSM-50 (Mobil) - see reference 3 for further details. This paper (which is the first in a series which will examine the development and optimisation of a TDP catalyst based on the EUO, and related, frameworks) will concentrate on the relationship between the synthesis conditions and catalyst decay. It will explore, it is believed for the first time, the effect of crystallisation over-run on catalytic performance and surface composition; these effects are likely to have profound implications for other materials and other reactions. 2. Experimental

Samples of EU-1 were synthesised using fumed silica (Cab-o-Sil, M5, BDH Ltd.), solid sodium aluminate (approximate composition 1.35 Na20 A120 3 5H20, BDH Ltd.) and sodium hydroxide pellets (BDH Ltd.). The organic template used was hexamethonium bromide: hexane-l,6-bis(trimethylammonium bromide)(Fluka). Demineralised water was used for all solution make-up. The reaction mixture was prepared by dividing the required water into three approximately equal portions. The sodium aluminate and sodium hydroxide were dissolved in the first portion, hexamethonium bromide was dissolved in the second and the third aliquot used to disperse the fumed silica. The solutions of hexamethonium bromide and sodium aluminate/sodium hydroxide were mixed then added, with stirring, to the silica dispersion. Stirring was continued until a smooth gel resulted (about 5 minutes) following which the mixture was transferred to the reactor. Syntheses were carried out in 1 or 2 litre stirred (4-blade, 45 ~ pitched-paddle impellers operating at 300rpm) stainless steel autoclaves. The reactors were electrically heated with temperature control to + 2~ Reaction times were taken from when the autoclave contents reached operating temperature. The autoclaves could be sampled. All samples and final products (in this study) were activated by calcination (static air: 450~ 48 hours; 550~ 24 hours) followed by ion-exchange (1M HC1, 2 x 4 hours, 60oc, 50cc per g zeolite). After exchanged materials were filtered, washed and dried before a final calcination (static air; 550oc, 24 hours). Materials were characterised by powder xrd (Philips APD 1700 diffractometer, Cu Ka radiation), chemical analysis (AAS + ICP) and XPS (Kratos XSAM 800 spectrometer using Mg Kct radiation (hv = 1253.6 eV). Catalytic evaluation was carried out on 0.5-1.0g fractions of catalyst (undiluted zeolite) in a 3mm I.D. stainless steel tube with quartz wool used to maintain the bed in the isothermal zone. TDP was carried out in the absence of H 2 and at atmospheric pressure, at temperatures between 300 and 500~ and at a WHSV of about 0.4 (see later for exact temperatures and space velocities). Products (and unreacted toluene) were analysed by GC. 3. Results and Discussion

Initial screening of a series of EU-1 samples in TDP revealed a spread in catalytic properties. Figure 1 presents some typical data for toluene conversion against time-on-line.

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Only four sets of data are shown to avoid confusion and these samples have been chosen simply to exemplify the variability observed. Large differences in decay rate can be clearly seen. Detailed examination suggested that there are two types of behaviour: EU-1 types I and II. Three examples of type I materials (A, B and C in the figure) are shown as solid symbols and display the very rapid decay in activity. The type II material, displayed in open symbols, decays in "two stages": an initial rapid decay (c.f. type I) then a long period of slow decay. This paper will rationalise the variability between the type I materials; subsequent papers will describe the differences between types I and II (composition, crystal size etc.) and a detailed analysis of the decay kinetics. 9O r.~ ;~

m

70

~

60

~

so

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o

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u

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o

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A

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0

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100

C rystallisation

Figure 1.

150

200

Tim

250

300

e (hours)

Toluene Conversion against Time-on-Line (hours) for a series of EU-1 samples Temperature = 330~ WHSV = 0.4

It must be stressed that the designations, Type I and II, used here are employed simply to distinguish between the different catalytic behaviours observed. Examination, by XRD, of all samples ( A - D) in Figure 1 showed that the materials were EUO types. Some minor differences are observed (e.g. in xrd peak width) but these were fully consistent with "simple" differences in crystallite size. All three type I materials (A, B and C in the figure) were activated under identical conditions, but there were some significant differences in the method of crystallisation see Table 1. A typical reaction mixture composition was: 60 SiO 2 x A1203 10 Na20 10 HexBr 2 3000H20 (where HexBr 2 is the template hexamethonium bromide referred to above) Table 1 Comparison of EU-I Type I samples in Figure 1 Type I materials in Fig 1 SiO 2/A120 3 Temperature/ o C 200 60/0.77 180 160 60/1

Time / hours 51.5 166 163

All three materials had similar sized crystals: ellipsoidal aggregates approximately 1- 5 microns in length which have been described previously 7 although sample C also contained some smaller crystals. The significance of these smaller crystals and the (slight)

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differences in SiO2/A120 3 will be described in subsequent papers. External surface areas for samples A-C (typical of Type I materials) were about 1-10 m2gn. One of the most notable differences in Table 1 is the crystallisation temperature. It can be noted that the materials' decay in activity (Figure 1) follows this temperature i.e. sample A which is crystallised at the highest temperature decays more rapidly than B, than C. Thus the lower the crystallisation temperatures the more stable the resulting catalyst. All three materials had xrd patterns (see above), which were virtually identical, but it was known 7 that after maximum crystallinity was reached there were some changes in reaction mixture pH. An example of this and, it is believed the reason behind the variation in performance is shown in Figure 2.

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Time (hours) Figure 2. Plot of % Crystallinity (XRD), pH and surface composition Vs Time

200

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Figure 2 is based on an EU-1 Synthesis of composition: 60 SiO 2 0.77 AI20310 Na2010 HexBr 2 3000H20 carried out at 180~ It can be seen that maximum crystallinity is reached at about 48 hours and thereafter remains constant. The pH, however, after reaching a maximum at a similar time to xrd thereafter shows a marked decline - it was this phenomenon which first suggested that the reaction was not "static" in this period and that further changes were taking place. There are a number of possibilities for the change (decline) in pH but it is believed the most likely is the (Hofmann) degradation of the template which consumes hydroxyl ions and liberates trimethylamine (and generates an olefinic species). Regardless of its origin, the decline in pH will have a marked influence on the silica that remains in solution. If it is assumed that at full crystallisation an equilibrium is established between the crystalline EU-1 and the silica remaining in solution, then a decrease in pH will perturb the equilibrium and generate a solution supersaturated in silica (most or all of the alumina will have been consumed). This supersaturation will be discharged by the deposition of silica: either as an amorphous phase or as a coherent, crystalline silica-rich "over layer" - in both cases effectively "coating" the EU-1. It is believed that it is this coating which is responsible for the more rapid deactivation of certain EU-1 samples (Type I) in TDP. At any given crystallisation temperature "over-running" the reaction will lead to this decline in pH and the resulting loss in performance. The effect is purely a time based phenomenon which is most pronounced at higher crystallisation temperatures, that is, EU-1 samples of similar surface composition and "integrity" can be made at all temperatures. However, the "time window" for this high integrity EU-1 is much reduced at higher temperatures: it is only a few hours at 200~ This is illustrated by Figure 3, which shows the variation in surface composition (SiOJA1203 ratio determined by XPS) as a function of time for series of reactions carried out at different temperatures.

90

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80 70

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D-2-0OCm180C ,A 160C

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A

A

50

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150

200

250

300

Crystallisation Time (hours) Figure 3. Plot of surface SIO2/A1203 against crystallisation time for a series of EU-1 samples crystallised at different temperatures. (Reaction mixtures had similar compositions to those shown in Figure 2.)

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Measurements were made on isolated samples taken after 100% crystallisation had been reached, that is, measurements were only carried out on samples of fully crystalline zeolite EU-1 (as determined by xrd analysis). It can be seen that both "initial" and "final" compositions (SIO2/A1203 ratios) are similar for all preparations (about 30 and 90 respectively) regardless of crystallisation temperature and that the composition increases steadily with t i m e - although the rate of increase is more marked at the higher temperatures. The minimum value of the surface SIO2/A1203 ratio (30) represents the maximum aluminium loading that can be obtained for EU-1 crystallised using the hexamethonium template. At this value it would be expected that catalysts would have maximum activity since acid site density is at its highest- this presupposes that all sites have similar intrinsic acidities. What is less obvious is why such materials also have the greatest stability. This topic will form the basis of subsequent papers.

Conclusion The "optimum" EU-1, type I materials (for TDP catalysis) are prepared by isolating the sample at maximum pH (as soon as full crystallinity is reached) and this is most readily accomplished at lower (say 160~ crystallisation temperatures. A similar surface composition (SIO2/A1203 ratio) can be obtained from reactions carried out between 160 and 200~

Acknowledgements The authors gratefully acknowledge the technical expertise of Mr N. Malone (XPS), Mr S. Huntley (zeolite synthesis and activation) and Mr P.J. Hogan, M.A. Martin and S. Johnson (TDP catalysis) and thank Synetix and ICI Technology for permission to publish. References 1 A. Azzouz,V. Hulea,B. Zaoui,M. Attou and E. Dumitriu,J. Soc. Alger. Chim., 3 (1993) 38. 2 For example, J.S Abichandani, J.S. Beck, R.H. Fischer, I.D. Johnson and D.L.Stern, U.S. Patent 5625103. [See also Chemical Week, August 10, 18 (1994).] 3 W . M . Meier, D.H. Olson and L. McCusker, "Atlas of Zeolite Structure Types" Fourth Revised Edition, Elsevier, 1996. 4 N.A. Briscoe, D.W. Johnson, M.D. Shannon, G.T. Kokotailo and L.B. McCusker, Zeolites, 8 (1988) 74. 5 J.L. Casci, Ph.D. Thesis, University of Edinburgh, 1982. 6 J.L. Casci, B.M. Lowe and T.V. Whittam, E.P. 42226. 7 J.L. Casci, B.M. Lowe and T.V. Whittam, Proc. VIth Int. Zeolite Conf., 894, Butterworths, 1984.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Influence of the aluminium content on the acidity and catalytic activity of MTW-type zeolites A. Katovica, B.H. Chiche b, F. Di Renzo b, G. Giordano a and F. Fajulab aDipartimento di Ingegneria Chimica e dei Materiali, Universittt della Calabria, 1-87100 Rende, Italy t'Laboratoire de Mattriaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 ENSCM/CNRS, Ecole Nationale Suptrieure de Chimie, 8 rue d l'Ecole Normal, 34053 Montpellier, France Zeolite with MTW topology have been synthesized from gels containing different amounts of aluminium. The lowest value of Si/AI reached in the crystals, near 20, confirms the limits to the incorporation of aluminium. H-MTW-type zeolites contain highly acidic and readily accessible structural BrCnsted sites and very strong Lewis sites associated with extra-framework species. In the cracking of n-hexane the materials show little deactivation with time-on-stream and an activity level directly proportional to the framework aluminium content. 1. INTRODUCTION Zeolites of the MTW structural group (ZSM-12, TEA-silicalite, CZH-5, Nu-13 and TPZ-2) [1-3] possess a monodirectional channel system limited by twelve-membered tings of tetrahedra with apertures of 5.6 x 7.7 A. The synthesis of MTW-type zeolites is well documented in the open and patent literature and has been reviewed by Jacobs and Martens [4]. MTW zeolites can be obtained in a broad range of aluminium content (15 < Si/AI < oo) using several organic templates, among which tetraethyl and methyltriethylammonium cations (TF_A and MTEA) are the most commonly used. Literature reports [5-7] on the catalytic behaviour of MTW-type materials indicate good activity and stability with time-on-stream and selectivities intermediate between those of medium (MFI) and large (BEA) pore zeolites. Recently, acidity studies using infrared spectroscopy [7] demonstrated that all the bridging hydroxyl groups in the lattice exhibited high acid strength and are easily accessible to pyridine molecules. Since the acid and catalytic characteristics of zeolites are directly related to their aluminium content, this work was intended to investigate and compare the properties of MTW-zeolites with varying compositions. A series of materials have been therefore crystallized and characterized. MTEA cations, which allow to enlarge the synthesis domain and favour aluminium incorporation into the framework [8,9], have been used as templating agents.

2. EXPERIMENTAL PART 2.1. Synthesis and activation of the parent MTW zeolites

The zeolites were crystallized at 170~ without stirring, in Teflon-lined Morey-type autoclaves (20 ml) for a predetermined time, varying from 4 to 12 days depending on the aluminium content, using the hydrogel composition: 5 Na20 : 10 MTEABr : x AI203 : 50 SiO 2 : 10001-120 with x values of 0, 0.5, 1.25 and 2.25 [9]. The reaction mixtures were prepared by dispersing under stirring the silica source (precipitated silica form BDH) into the alkaline solution containing the sodium and aluminium (when present) hydroxides (from Aldrich) and the organic salt (from

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Fluka). After the formation of a homogeneous paste, the autoclaves were sealed and placed in an oven. The products, recovered by filtration, were washed with deionized water until pH 8 and ovendried at 100*C overnight. H-forms were obtained by an overnight calcination in flowing dry air at 600"C, two ion-exchange treatments in 1M ammonium chloride solution (100 ml/g of zeolite, reflux temperature) and a final calcination in flowing air at 5500C. In the following, the various samples will be denominated MTW (x), where x refers to the aluminium fraction introduced in the synthesis gel.

2.1. Characterization, acidity and catalytic procedures

The activated zeolites were characterized by x-ray diffraction(XRD, CGR Theta 60), 27Al NMR spectroscopy (AM 400 Bruker, 104.2 MHz), scanning electron microscopy (SEM, Stereoscan Cambridge 360) equipped with energy dispersive X-rat chemical analysis (EDX) and Fourier transform infrared spectroscopy (FTIR, Nicolet 320). Nitrogen adsorption-desorption isotherms were recorded using an automated porosimeter (Micromeritics Asap 2000), while the n-hexane sorption capacities at room temperature and a relative pressure of 0.15 were measured by gravimetry (Setaram B 85 Balance). Pore volumes were , calculated assuming normal liquid density of the adsorbed phase. The acidity of the surface was probed using adsorption of deuterated acetonitrile and infrared spectroscopy on self-supported wafers (20 mg/cm2) of zeolite. The catalytic activity was evaluated by measuring the apparent fist order rate constant (k= -WHSV In [ l-x]) where x is the conversion) for the cracking of n-hexane at 350~ at atmospheric pressure with a nitrogen to hydrocarbon ratio of 17 and space velocities (WHSV) between 2 and 4 h ~.

3. RESULTS AND DISCUSSION 3.1. Aluminium incorporation and physicochemical properties

All syntheses produced XRD highly crystalline MTW-zeolites free from extra crystalline or amorphous phases. The crystals appeared as rods with irregular surfaces and an average size of 5 ~tm for the most siliceous sample (MTW(0)) and around 1 ~tm for the aluminium-containing ones. The 27Al NMR spectra of the as-synthesized products showed a single signal at 56 ppm characteristic of tetrahedrally coordinated aluminium atoms. As shown in Table 1, increasing the aluminium content, s, in the starting hydrogel from 0 to 1.25 led to a parallel increase in the incorporation of aluminium in the crystals. A further increase of the aluminium content in the synthesis mixture (x= 2.25) did not produce Al richer crystals. On the conWary, AI incorporation in MTW(2.25) was less effective than for the other syntheses. These observations are in line with the published data [4] that show that beyond Si/AI= 15 in the synthesis gel, MTW-zeolites can be produced with a composition nearly identical with that of the gel. Below this threshold, aluminium incorporation in the crystals is more difficult. This has been attributed to a location of the Al atoms in the four-membered tings (4-MR) of the structure and a maximum occupancy of one Al atom each of the four 4-MR of the unit cell (56-tetrahedra) [4]. Table 1 Composition of initial gels and crystals of MTW-zeolites. sample gel composition composition of crystalsa Si/Al

Na/A1

Si/A1

Na/AI

MTW(O)

oo

oo

~o

oo

MTW(0.5)

50

10

40

state of AIb

Alu.c.

Td

0.42

1.36

100

Oh

MTW(1.25)

20

4

20.8

0.26

2.57

90

I0

MTW(2.25)

11.1

2.2

22

0.18

2.43

80

2O

a: obtained by EDX analysis; b: evaluated by ZVAlNMR on the H-forms

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Table 2 Main physicochemical characteristics of the H-MTW-zeolite catalysts sample infrared frequencies of micropore volume (rrd/g) framework vibrations (crn~)

accessible to

n-hexane

cracking activity

nitrogen

n-hexane

k (g/gxh)

MTW(0)

1098

792

0.09

0.08

< 0.01

MTW(0.5)

-

790

0.11

0.07

0.32

MTW( 1.25)

1094

788

0.10

0.07

0.56

MTW(2.25)

1096

788

0.09

-

0.45

The activation treatment led to the formation of hexacoordinated extra-framework aluminium evidenced by 27A1 NMR sio~al around 0 ppm in the case of samples MTW(1.25) and MTW(2.25). The amounts of octahedral Al in these samples were estimated (assuming comparable sensitivities for the two types of species in the 27Al NMR spectra) to 10 and 20% of the total A1 content, respectively. The dealumination of the framework did not lead however to the creation of mesopores. This is understandable in view of the high silicon content of the parent materials [ 10]. The very similar framework Al content of the two samples was confirmed by the position of the infrared frequencies characteristic of T-O bonds in the tetrahedra (Table 2). All samples exhibited nitrogen and n-hexane sorption capacities (Table 2) corresponding to those expected for zeolites of the MTW family [4,11]. Extra-framework phases, when formed, did not seem therefore to affect notably the accessibility of the whole free pore volume of the structure to the sorbates.

3.2 Acidity and cracking activity

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Fig.1. Infrared spectra of acetonitrile adsorption and desorption on MTW(1.25). Clean surface (a), adsorption of 0.23 Torr of base at room temperature (b), adsorption of 0.47 Torr (c), adsorption of 1.6 Tort (d), desorption at 90~ (g), at 150~ (f) and at 180~ (e).

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Fig 2. Infrared spectra of acetonitrile adsorption and desorption on MTW(2.25). Clean surface (a), adsorption of 0.07 Torr of base at room temperature (b), adsorption of 0.20 Torr (c), adsorption of 0.97 Torr (d), resorption at 900C (g), at 1500C (f) and at 180~ (e). As already reported recently [7], the infrared spectrum of protonic forms of MTW-zeolites after outgassing the zeolite wafer at 450~ under vacuum exlfibits up to five hydroxyl stretching vibrations at 3780, 3747, 3660, 3610 and 3580 cm -1 assigned to extra-framework aluminium containing species (EFAI, dislodged or partially attached to the lattice, signals at 3780 and 3660 ern ~), isolated silanol groups (signal at 3747 cm") and framework AI-OH-Si acidic groups pointing towards the 12-membered ring channel (signal at 3610 cm 1) and the smaller (6- or 8- membered) tings (si gnal at 3 580 cm -~). The IR spectra of Al-rich MTW samples are reported in the curves a in Figures 1 and 2, related to MTW(1.25) and MTW(2.25), respectively. The examination of the curves reveals the absence of the 3780 cm -~ signal and a higher intensity of the 3660 cm ~ signal for the latter. Since these two signals have been tentatively attributed to isolated A1OOH species for the former and to alumina-like phases for the latter [12, 13], our infrared results could indicate a higher degree of polymerization of the EFAI in the case of MTW(2.25) due to a higher amount of dislodged aluminium. Although comparison between NMR and infrared data is complicated by the different degree of hydration of the samples, the two techniques lead to self-consistent conclusions. Acetonitrile adsorption at room temperature was used to probe the acidity of the surface of MTW-zeolites and typical results are reported in Figures 1 and 2. On both samples, dosing increasing amounts of base led to the development of signals characteristic of interactions with Brc~nsted (signal at 2300 em ) and Lewis ( signal at 2327.5 era ) sites with parallel decrease of the intensity of all signals of the hydroxyl groups (curves b-d in fig. 1 and 2). A signal at 2115 cm ~, due to the bending vibrations in CD 3 groups, was also present. The introduction of base was stopped just before the appearance of the physisorbed acetonitrile. At this stage the signals of the 1

I

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hydroxyl groups had almost totally vanished, confirming the full accessibility of the acid centers in the structure; regarding the nature of the acid sites present in the samples MTW (1.25) and MTW(2.25) it may be seen from the figures that, irrespectively of the surface coverage in aeetonitrile, the ratio between the intensities of the BrCnsted and Lewis signals is about 2 for the former and about 0.5 for the latter. Such a situation has probably to do with the different content and nature of the EFAI species present in the two samples. Precise determination of the extinction eoeftieients will be necessary, however, in order to quantitatively relate the two facts. Spectra obtained by desorbing aeetonitrile at increasing temperatures under vacuum are shown by curves e-g in the Figures 1 and 2. Contrary to what was observed during the adsorption step, thermal desorption revealed the same behaviour for the two samples, namely an easier desorption of the base interacting with the BrCnsted sites. Actually, even after heating the samples under vacuum at 180"C, a significant amount of acetonitrile was still retained on the Lewis sites. It has to be noted that thermal desorption allowed to recover the intensity of the infrared signals due to the structural hydroxyl groups but not that of the signals associated with extra-framework species. The latter must exhibit therefore a strong Lewis character, as it has been observed many times. The percentage of conversion as a function of time on stream obtained in the cracking of nhexane at 350"C with a space velocity of 2 h ~ is given in Figure 3 for the three aluminiumcontaining samples (under these conditions MTW(0) was inactive). Using the very same experimental conditions and catalysts with Si/AI ratios around 20, the activity loss after two hours on stream was 5% for ZSM-5 and 80% for beta [7]. It can be, therefore, stated that in spite of a unidirectional porosity delineated by the 12-membered tings, MTW-type zeolites exhibit good resistance to deactivation by coking. This contrasts with what is known, for instance for mordenite, where deep dealumination and formation of a secondary mesopore network is required in order to improve catalyst lifetime [ 14]. The apparent first order rate constants in n-hexane cracking measured at conversions around 10% are listed in the last entry of Table 2 and plotted versus the aluminium content per unit cell of the catalysts in Figure 4. For MTW(1.25) and MTW(2.25), the global and framework AI contents, based on the NMR results, have been considered in the figure. The data conclusively show that all framework aluminium atoms generate active catalytic sites and are equally effective in the reacion. 25 20

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5 .....

I .... 50

I .... I . . . . . . . . : ....... 100 150 200 Time (min)

:

250

Fig.3. Conversion as a function of time-on-stream for n-hexane cracking on MTW-zeolite catalysts.

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0,6

t

0,4

"~ 0,2

4- total AI content El framework AI

0

1 2 AI per unit cell

Fig.4. n-Hexane cracking activity of MTW-zeolites as a function of the aluminium content. 4. CONCLUSIONS Acid catalysts prepared from the MTW-type zeolites exhibit good n-hexane cracking activity with reasonable stability with time-on-stream. The synthesis of the zeolite precursors leads to an upper limit of the aluminium incorporation corresponding to Si/AI ratios near 20. Activation generates minor amounts of extra-framework phases which do not reduce sorption capacity and accessibility to the acid sites. Some specific characteristics in the catalytic behaviour of the MTWtype materials are intermediate between those of medium and large pore zeolites providing potential interest in shape-selective catalysis. REFERENCES 1. W.M. Meier and D.H. Olson, Atlas of Zeolite Structure Types, Butterworth-Heinemann, 1992 2. R.B. Lapierre, A.C. Rohrman Jr, J.L. Schlenker, J.D. Wood, M.K. Rubin and W.J. Rohrbaugh, Zeolites 5 (1985) 346. 3. C.A. Fyfe, H. Gies, G.T. Kokotailo, B. Maler and D.E. Cox, J. Phys. Chem. 94 (1990) 3718. 4. P.A. Jacobs and J.A. Martens, Stud. Surf. Sci. Catal. 33 (1987) 297. 5. J. Weitkamp, S. Ernst and R. Kumar, Appl. Catal. 27 (1986) 207. 6. J.A. Martens, M. Tielen, P.A. Jacobs and J. Weitkamp, Zeolites 4 (1984) 98. 7. B.H. Chiche, R. Dutartre, F. Di Renzo, F. Fajula, A. Katovic, A. Regina and G. Giordano, Catal. Lett. 31 (1995) 359. 8. A. Katovic and G. Giordano, Chem. Express 6 (1991) 969. 9. A. Katovic and G. Giordano, in "Synthesis of Microporous Materials: Zeolites, Clays and Nanostructures", M.L. Occelli and H. Kessler, Eds., Marcel Dekker (1997) p.127 10. B. Chauvin, P. Massiani, R. Dutartre, F. Figu6ras, F. Fajula and T. Des Couri&es, Zeolites 10 (1990) 174. 11. E.J. Rosinski and M.K. Rubin, U.S. Pat. 3,832,449 (1974) 12. E. Bourgeat-I.ami, P. Massiani, F. Di Renzo, P. Espiau, F. Fajula and T. Des Couri6res, Appl. Catal. 72 (1991) 139. 13. E. Loeffler, U. Lhose, Ch. Peuker, O. Oehlmann, L.M. Kustov, V. L. Zholobenko and V.B. Kazansky, Zeolites 10 (1990) 266. 14. B.L. Meyers, T.H. Fleisch, G.J. Ray, J.T. Miller and J.B. Hall, J. Catal. 110 (1988) 82.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Synthesis of ethylbenzene from 1,3-butadiene using basic zeolite catalysts J. Ackermann, E. Klemm and G. Emig Institute of Technical Chemistry I, Friedrich-Alexander-University of Erlangen-Niirnberg, Egerlandstr. 3, D-91058 Erlangen, Germany The vapour phase synthesis of ethylbenzene from 1,3-butadiene was investigated using FAU-type zeolites with basic properties. The influence of basicity was studied by: (1) variation of the Si/Al-ratio, (2) ion-exchange with cesium and (3) impregnation with sodium or cesium salts. One-step direct aromatization of 1,3-butadiene is possible by using these zeolites as bifunctional Lewis-acid~ase catalysts but suffers from strong deactivation. Consequently the reaction should be performed in two separate steps. For the second reaction step - the vapour phase conversion of 4-vinylcyclohexene (which can be easily synthesized from 1,3butadiene) to ethylbenzene - basic zeolites represent useful catalysts, especially those obtained by impregnation with alkali metal salts. Applying molecular modelling techniques to study the sorption at reaction conditions was (in combination with experiments) very helpful to demonstrate that deactivation results from product inhibition. 1. INTRODUCTION Ethylbenzene, the well-known precursor in the styrene synthesis, is industrially produced by the alkylation of benzene with cost-intensive ethylene. Currently, 1,3-butadiene is highly available on the petrochemical market due to increasing worldwide overcapacities. Its upgrading by aromatization in the direction of ethylbenzene seems therefore to be profitable [1], especially if it would be possible to use the crude C4-stream (containing 45 - 50 wt.-% of

1,3-butadiene).

In a consecutive reaction 1,3-butadiene (1,3-BD) first undergoes a cyclodimerization ([4+2]-eycloaddition, Diels-Alder reaction) to form the intermediate 4-vinylcyclohexene (4-VCH). Ethylbenzene (EB) is then obtained thereof by dehydroisomerization. The first reaction step is known to be catalyzed by Lewis-acidity [2], while the second reaction occurs preferably in the presence of basic sites [3]. Both properties are given in FAU-type zeolites: extra-framework cations act as Lewis-acid sites and framework oxygen acts as Lewis-basic sites. The acid-base properties of these zeolites can be controlled by variation of nature and number of extra-framework cations [4]. Intensification of the basic character can be achieved by encapsulation of alkali metal oxides [5]. The aim of our work was to investigate the synthesis of ethylbenzene from 1,3-butadiene in the vapour phase favourably as a one-step process on faujasitic zeolites with basic properties. For post-synthetic generation of basicity, alkali metal ion-exchange or impregnation with alkali metal salts followed by thermal decomposition to the corresponding oxide were applied. Additionally, we tried to usecomputer simulations in the sense of molecular modelling to improve our knowledge on structure-reactivity relations, especially with regard to sorption phenomena.

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2. E X P E R I M E N T A L

Commercial FAU-type zeolites with different Si/Al-ratios were used in their sodium form as starting material (NAY: nsi/nA! = 2.69 from Grace; NaX: nsi/nAI = 1.26 from Fluka; NaLSX: nsi/nAi = 1.0 kindly provided by H. Toufar/W. Schwieger, Halle/Erlangen, Germany). Cesium ion-exchange was carded out according to the corresponding ion-exchange isotherm [6]. Following the method of HATHAWAY AND DAVIS [7], NaX was threefold exchanged with 0.1 N solution of cesium chloride at 75 ~ tbr 3 h. The sample was filtered and washed chlorine free after each exchange. Finally, the catalyst was dried at 100 ~ in air. According to LASPI~RASET AL. [8], alkali metal oxide added zeolites NaX were prepared by impregnation with an aqueous solution of sodium acetate or cesium hydroxide of appropriate concentration and subsequent calcination at 550 ~ (heating rate: 1 ~ for 1 h in air. Different loadings of n = 4 - 5 additional sodium or cesium atoms per super cage (corresponding to 8-n sodium or cesium atoms per unit cell) were obtained by this method. Catalytic measurements were carried out using an isothermal fixed-bed reactor (inner diameter: 20 mm) operated at atmospheric pressure. The catalyst screening was performed under the following standard reaction conditions: T = 400 ~ W/Ftot. = 800 (g-min)/mol, mcat = 2 g, dparticle= 1.1 - 2 mm, Pnitrogen:Pl,3-BD= 4:1 or Pnitrogen:P4-VCH= 9:1. Analysis of organics was done by on-line gas chromatography equipped with a FID-detector. Prior to use, catalysts were activated in flowing nitrogen at 450 ~ for 1 h. The textural data (BET surface area and micropore volume) of the catalysts were measured by N2-physisorption. For determining the chemical composition of unmodified and ionexchanged samples XRF or ICP-AES were used. The composition of impregnated samples was calculated from their synthesis mixture. Computer simulations were performed on the basis of molecular modelling techniques with the commercial software Cerius 2 (version 3.5, developed by MSI). 3. RESULTS AND DISCUSSION 3.1. Characteristics of Zeolites

The nomenclature as well as some physico-chemical characteristics of the tested zeolites are given in table 1. Ion-exchanged samples show the expected reduction in micropore volume due to the partial replacement of Na + by the larger and less electronegative Cs +. The decrease in micropore volume with growth in loading of alkali metal cations reflects the succesful formation of intrazeolitic metal oxide species. It is important to mention that the two impregnated samples have different loadings but are nearly equal in micropore volume. Table 1" Characteristics of the tested zeolites: nomenclature, textural data and chemical composition

micropore volume surfacearea molarNa content molarCs content catalyst

[cma/g]

SBET

[m2/g]

nNa/nAi [-]

NaY NaX NaLSX CsNaX

0.32 0.25 0.27

660 526 566

1

NaX+4CsOH NaX+5NaOAc

0.19

0.10 0.09

399

Vmicropore

202 191

ncs/nAI [-]

loading mprecursor/mzeol.,dn/ [wt.-%]

0

0

1

0

0

0.38 1 1.47

0.62 0.38 0

35.8 24.5

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3.2. Computer Simulation of the Sorption Computer simulations on the basis of molecular modelling techniques were employed to study the sorption of reactants inside the intrazeolitic micropores at reaction conditions. First, a simulation method was developed considering: (1) L6wenstein's rule, (2) energy minimization of the structure using the augmented CVFF force field [9], (3) calculation of the charges on each atom of the sorbate molecules using the semiempirical method AM1 [ 10], (4) assuming both sorbate molecules and zeolitic framework to be rigid, (5) calculation of van der Waals energy using Demontis force field [11], (6) calculation of long-range Coulomb interactions by an Ewald summation, (7) a Monte-Carlo method working at fixed pressure (grand canonical ensemble). The simulation was tested on the well studied problem of the adsorption of benzene in NaX. Because of the good agreement with experimental data like e.g. the isosteric heat of adsorption published by RUTHVEN AND GODDARD [12], the simulation method was applied to the sorption of 4-vinylcyclohexene and ethylbenzene in NaX and CsNaX. The calculated values of the isosteric heats of adsorption AHads. of reactants (which is a measure for the strength of adsorption) are shown in table 2. Obviously, on both samples ethylbenzene is stronger adsorbed than 4-vinylcyclohexene. Comparing NaX with CsNaX a weaker adsorption of 4-vinylcyclohexene and a stronger adsorption of ethylbenzene is found on the more basic sample. The latter can be explained by the stronger interaction between the aromatic n-electron system and the more electropositive extra-framework cesium cations [ 13]. Additionally, the value for AHads. of ethylbenzene on NaX is nearly the same as the one measured by AVST ET AL. [ 14], showing also the qualitiy of the simulation. Table 2: Isosteric heats of adsorption AHads. of reactants in different zeolites calculated by molecular modelling (in kJ/mol) catalyst NaX CsNaX

I

4-vinylcyclohexene 65 59

I I I

ethylbenzene 73 81

3.3. One-Step Synthesis (direct conversion of 1,3-butadiene to ethylbenzene) First, we investigated the aromatization of 1,3-butadiene as a one-step synthesis using zeolites as bifunctional Lewis-acid/base catalysts [15]. The effect of varying Si/Al-ratio on catalytic activity and selectivity for different FAU-type zeolites in the sodium form can be seen from table 3. Generally, the intrinsic Lewis basicity which is induced by the framework oxygen sites increases with decreasing Si/Al-ratio, while the number of extra-framework cations increases. The more Lewis-acid sites are available, the higher is the conversion of 1,3butadiene. More basic frameworks are reflected by a growing selectivity to ethylbenzene while selectivity to the intermediate 4-vinylcylohexene decreases. The influence of the different post-synthetic modifications on the catalytic activity and selectivity is exemplary shown for a X-type zeolite in table 3 as well. Both modified samples show a decreasing conversion of 1,3-butadiene possibly due to the loss of micropore volume and for CsNaX due to the negative change in Lewis-acidity. Cs-exchange causes higher formation of the intermediate 4-vinylcyclohexene while impregnation with sodium oxide (equivalent to a greater number of Lewis-basic sites) favours the production of ethylbenzene. However, the yield of ethylbenzene decreases in any case.

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Table 3: Catalytic activity and selectivity of various (unmodified and modified) FAU-type zeolites in the conversion of 1,3-butadiene (after 15 min time-on-stream) conversion of 1,3-BD

selectivity to 4-VCH

selectivity to EB

yield of EB

deactivation (Xt=5h'Xt= 15min)/Xt = 15rain

NaY NaX NaLSX

57.2 67.9 69.2

10.3 4.9 3.3

12.4 55.6 66.5

7.1 37.8 46.0

- 89.5 - 89.0 - 88.2

CsNaX NaX+5NaAc

56.0 45.9

10.5 3.4

43.1 61.5

24.1 28.2

- 85.5 - 81.5

catalyst

[%]

[%]

[%]

[%]

[%]

The main disadvantage of the one-step process is the strong deactivation of all catalysts (see table 3). For example, on zeolite NaLSX activity drops from initially 69 % conversion of 1,3-butadiene to less than 10 % within 5 h time-on-stream under parallel loss of selectivity. Using DRIFT-spectroscopy accompanied by elemental analysis and thermogravimetric determination of the coke content, hydrogen-rich oligomers of 1,3-butadiene were identified as coke deposits. Linear as well as cyclic oligomers are conceivable to represent these carbonaceous compounds [16]. However, repeated regeneration by buming-off the coke at 450 ~ in air is possible.

3.4. Two-Step Synthesis (esp. conversion of 4-vinylcyclohexene to ethylbenzene) Because of the above-mentioned strong deactivation the process should preferably be performed in two separate steps. Since 4-vinylcyclohexene can be produced in high yield from 1,3-butadiene using e.g. homogeneous catalysts [17] or Cu(I)-Y zeolite in the liquid phase [18], we focused on the use of basic zeolites in the dehydroisomerization of 4-vinylcyclohexene to ethylbenzene in the vapour phase. The catalytic activity and selectivity as well as the deactivation behaviour (observed within 6 h time-on-stream and given as relative loss of initial activity) of tested zeolites are presented in table 4. On unmodified zeolites conversion as well as selectivity to ethylbenzene increases with decreasing Si/Al-ratio due to the growth of the instrinsic basicity of the framework. The main side-product p-xylene as well as all other side-products (the isomers of 4-vinylcyclohexene and traces of o-xylene) diminish with decreasing Si/Al-ratio. On all these samples deactivation occurs and gets stronger with decreasing basicity of the framework and the entailed increasing number of extra-framework cations which represent the sorption sites for aromatic systems. Table 4: Catalytic activity and selectivity of various (unmodified and modified) FAU-type zeolites in the conversion of 4-vinylcyclohexene (after 200 min time-on-stream) conversion of 4-VCH [%]

selectivity to p-xylene [%]

selectivity to EB [%]

yield of EB [%]

(Xt=6h-Xt= 20min)/Xt= 20min [%]

NaY NaX NaLSX

57.9 81.2 94.8

4.6 0.4 0.5

30.8 74.1 89.4

17.8 60.2 84.8

- 41.3 - 22.9 - 5.5

CsNaX

46.0

7.5

63.6

29.3

NaX+4CsOH NaX+ 5NaOAc

99.9 99.9

none none

96.8 94.6

96.7 94.5

catalyst

deactivation

- 33.8 I [

none none

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Compared to the unmodified sample, the Cs-exchanged X-type zeolite shows a reduction in conversion. This might be caused by the reduced micropore volume which affects diffusional problems and the lower affinity for the sorption of the reactant 4-vinylcyclohexene as it is derived from the computer simulations (see table 2). Although CsNaX has an enhanced basicity of the framework, the selectivity to ethylbenzene is reduced while the formation of side-products is favoured (esp. for p-xylene). Figure 1 shows the time-on-stream behaviour of NaX in comparison with CsNaX at different reaction temperatures. While on NaX conversion of 4-vinylcyclohexene remains constant on the high level of approx. 100 % at 450 ~ significant deactivation occurs at 400 ~ and lower temperatures. The identical behaviour is observed on zeolite CsNaX. Comparing these two zeolites at the same temperature of 400 ~ the Cs-exchanged sample shows a lower initial activity and deactivation runs faster. Additional measurements on the same initial activity level (adjusted by varying residence time and keeping temperature constant) lead to the same result: namely a stronger deactivation on CsNaX. As already described, the computer simulation studies of the sorption indicate that a strong adsorption of the product ethylbenzene might be responsible for the deactivation (see table 2) in the sense of product inhibition. To check this assumption, we performed some special experiments: at fixed 4-vinylcyclohexene concentration (10 vol.-%) ethylbenzene was added in different ratios to the feed as part of the inert. The results in figure 2 show, that the more ethylbenzene is present, the stronger is the catalyst deactivation. In comparison to unmodified and ion-exchanged samples, alkali metal oxide loaded X-type zeolites show outstanding results (see table 4). They distinguish theirselves by a high conversion and selectivity to ethylbenzene, both existing over a broad range of varied reaction conditions. In addition, no deactivation can be observed over a period of 6 h time-on-stream. The more basic cesium oxide containing sample shows a higher yield of ethylbenzene despite the lower loading of additional extra-framework basic oxygen sites. A remarkable effect is observed exclusively on this zeolite: during an initial period, toluene and cumene are produced in a 1:1 ratio (with a considerable quantity of initially up to 12 % yield of each toluene and cumene) at the expense of ethylbenzene. Both the variation of residence time and the utilization of ethylbenzene as feed indicate the unusual base-catalyzed side-chain alkylation of ethylbenzene by itself. The reason for the suppression of this reaction with timeon-stream remains still unclear. 100

100

i--i

i

i

i

~ 8o u >, ,q, 60 C

.2 411

40

> e~ 2O 0 U

20 .............~

'

!

1

0 0

50

9 NaX (400~ o NaX (450~ [] NaX (400~

100

150

200

250

time-on-stream [min] W/F = 800) 9CsNaX (400~ W/F = 800) C) CsNaX (450~ W/F = 200)

300

350

W/F = 800) W/F = 800)

Figure 1: Time-on-stream behaviour in the conversion of 4-vinylcyclohexene

0

50

100

150

200

250

time-on-stream [min]

9O 9 [] 9C)

300

350

p(4-VCH):p(FB) - 2:1 p(4-VCH):p(EB) = 1:1 p(4-VCH):p(EB) = 1:2

Figure 2: Influence of adding ethylbenzene to

the feed for zeolite NaX and CsNaX

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By using the impregnated zeolites and decreasing the residence time, a space time yield of more than 5.5 kgEB/(kgcat.'h) at total conversion can be obtained and thus makes them attractive for industrial application. 4. CONCLUSIONS In our investigations concerning the synthesis of ethylbenzene starting from 1,3-butadiene by means of heterogeneous catalysis in the vapour phase we found the following essential results: 9 a one-step process by using FAU-type zeolites as bifunctional Lewis-acid/base catalysts is in fact possible, especially by using the sodium form and adjusting a low Si/Al-ratio, but it is negatively affected by strong deactivation; 9 within the scope of a two-step process, faujasitic zeolites, especially those having a low Si/Al-ratio and being post-synthetically modified by impregnation with alkali metal salts, represent promising catalysts for the base-catalyzed dehydroisomerization of 4vinylcyclohexene; 9 computer simulation studies on the basis of molecular modelling techniques can be a powerful tool to gain more knowledge on structure-reactivity relations, especially relating to deactivation caused by sorption phenomenas. REFERENCES

[1]

[2]

[3] [4]

[5]

[6] [7]

[8]

[9] [10] [ 11] [12] [13] [14] [15] [16] [17] [18]

M.L. Morgan; DGMK Tagungsbericht 9705 (1997), 9 R.M. Dessau; J. Chem. Soc., Chem. Commun. (1986), 1167 G. Ruckelshauf5, K. Kosswig; Chem.-Z. 101 (1977) 2, 103 D. Barthomeuf; Catal. Rev.-Sci. Eng. 28 (1996), 521 H. Tsuji, F. Yagi, H. Hattori; Chem. Lett. (1991), 1881 H.S. Sherry; J. Phys. Chem. 70 (1966) 4, 1158 P.E. Hathaway, M.E. Davis; J. Catal. 116 (1989), 263 M. Lasp6ras, H. Cambon, D. Brunel, I. Rodriguez, P. Geneste; Micropor. Mater. 1 (1993), 343 P. Dauber-Osguthorpe, V.A. Roberts, D.J. Osguthorpe, J. Wolff, M. Genest, A.T. Hagler; Proteins: Structure, Function and Genetics 4 (1988), 31 M.J.S. Dewar, E.G. Zoebisch, E.F. Healey, J.J.P. Stewart; J. Am. Chem. Soc. 107 (1985), 3902 P. Demontis, S. Yashonath, M.L. Klein; J. Phys. Chem. 93 (1989), 5016 D.M. Ruthven, M. Goddard; Zeolites 6 (1986), 275 S.M. Auerbach, L.M. Bull, N.J. Henson, H.I. Metiu, A.K. Cheetham; J. Phys. Chem. 100 (1996), 5923 E. Aust, W. Hilgert, G. Emig; Stud. Surf. Sci. Catal. 46 (1989), 495 J. Ackermann, E. Klemm, G. Emig; Proc. 12th Int. Zeolite Conf., Mater. Res. Soc., 1999, 2727 T.V. Voskoboinikov, B. Coq, F. Fajula, R. Brown, G. McDougall, J.L. Couturier; Micropor. Mater. 24 (1998), 89 H.A.M. Duisters, J.G.D. Haenen; US-Patent No. 5 545 789 (1995) I.E. Maxwell, R.S. Downing, S.A.J. van Langen; J. Catal. 61 (1980), 485 R.S. Dixit, C.B. Murchison; Proc. Ethylene Prod. Conf. 3 (1994), 118

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Activity~selectivity

and iigation of the Co ions in zeolites in ammoxidation of ethane to acetonitrile

B. Wichteflovh a, Z. Sobalik', Y. Lib, and J.N. Armor c aj. Heyrovsk~ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolej~kova 3, CZ-182 23 Prague 8, Czech Republic* bEngelhard Corporation, Iselin, NJ 08820 USA CAir Products & Chemicals, Inc., Allentown, PA 1819-1501, U.S.A.

The reactivity of the Co ions exchanged in pentasil-ring zeolites, BEA, MFI and MOR and six-ring zeolites, Y and dealuminated USY differing in Si/A1 and Co ion coordination was studied with respect to the activity/selectivity in the ammoxidation of ethane to acetonitrile, and strength of bonding of the Co ions with reactants, intermediates and products. It has been shown that a decisive factor controlling the activity of the Co ions is negative charge of the zeolite framework, and the accessibility of the reactants to the Co ions. The strength of Coligand bonding was estimated from the extent of the shift of the antisymmetric vibration of the framework T-O bonds adjacent to the cation-ligand complexes vs. the dehydrated sample: ethylamine > acetonitrile > ammonia >> ethylene. This also correlated well with the temperature for the decomposition of the individual Co-ligand 1:1 complexes. This highlights the importance of stronger ammonia bonding compared to weakly bound ethylene and the stronger bonding of ethylamine for achieving high acetonitrile selectivity in the ethane/NH3/O2 reaction. 1. INTRODUCTION Co ions planted by ion exchange into BEA and MFI structures have been reported as highly active in selective ammoxidation of ethane to acetonitrile [ 1-3] The reaction pathway suggested [3] includes ethane oxidative dehydrogenation to ethylene, which in the presence of ammonia, is in further steps selectively oxidized into acetonitrile via ethylamine. However, if ammonia is not present in the gas phase, ethane is oxidized non-selectively to carbon dioxide. Therefore, the Co ion activity/selectivity has been assumed to be strongly modified by its ligation with ammonia. Recently, we have shown for Co-FER [4,5], that antisymmetric T-O vibrations of the framework together with the characteristic vibrations of the adsorbate molecules can be used to monitor ligand bonding and its strength to the exchanged metal ions.

Acknowledgement. B. W. and Z. S. appreciate financial support for this study from Air Products & Chemicals, Inc, and J.N.A. thanks to the same company for allowance to publish these results.

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This study deals with a comparison of the function of the Co ions in BEA, MFI, MOR and in Y-USY zeolites in ammoxidation of ethane to acetonitrile, i.e. those with different topology and content of aluminum in their framework. Bonding of the Co ions in the ligand field of polyoxoanion-cation-extraframework ligand complexes, relevant to the ethane/NH3/O2 reaction, at a temperature close to that of the catalytic reaction, is described. The effect of AI related acidic Broensted and Lewis sites on the reactant complexation and reaction performance is given as well.

2. E X P E R I M E N T A L NH4-BEA, NHn-MFI, NH4-MOR, NH4-Y, dealuminated NHa-USY, highly differing in content of aluminum in their framework and topology, were Co(II) ion exchanged with 0.01 M Co acetate solutions refluxing at 80~ to reach near complete ion exchange, i.e. Co/AI = 0.5; Co levels, and bulk and framework Si/A1 molar ratios are given in Table 1. Some of dealuminated USY zeolites were treated in 0.3 M HCI solution to remove extraframework aluminum, and then transferred into their NHa-ion forms. Dehydration of Co-zeolites took place in situ in an oxygen stream or under vacuum at 480~ before catalytic activity or IR spectra measurements. It has been proven elsewhere [6] that divalent Co ions, if placed at cationic sites of zeolites, preserve their divalent state at < 500~ treatment in both oxygen and vacuum. Catalytic activity of ethane ammoxidation to acetonitrile was carried out in a throughflow microreactor at steady-state conditions at 450 or 475~ Reactant feed consisted of 10 % ethylene, 5 -10 % NH3 and 6.5 % 02, balance as helium, at total flow-rate of 100 ml/min. Catalyst weight was 0.2 - 0.3 g. Turn over frequency, TOF (sec'~), represents number of acetonitrile molecules formed per Co atom per second. The data come from Refs. 1-3. IR spectra of self-supported pellets (5 mg.cm 2) were recorded at the 4000-400 cm-1 region at RT on a Magna-IR System 550 FTIR (Nicolet) spectrometer using MCT-B liquid nitrogen cooled detector and equipped with a heated cell up to 500~ connected to a vacuum/gas system. A single spectrum was obtained with resolution of 2 crn-~ and 200 scans. Time-resolved measurements with up to 15 spectra per second were done at 8 cm -1 resolution. Adsorption of ethylene (0.35 kPa), ammonia (3.3 kPa), ethylamine (3.3 kPa) and acetonitrile (0.6 kPa) took place for 30 min at RT on dehydrated Co-zeolites, followed by time-resolved desorption under evacuation (up to 102 kPa) for 30 min at temperature steps up to 480 ~

3. RESULTS and DISCUSSION 3.1 Catalytic activity of Co-zeolites in ethane/NH3/O2 to acetonitrile reaction Catalytic activity of Co-zeolites, with Co/AI values of about 0.5, but differing in topology and in a broad range of Si/A1 framework ratio are given in Table 1 and Figure 1. The yields of acetonitrile (per catalyst weight) increased with increasing Si/A1 values, although the content of Co decreased, being the highest with Co-BEA and Co-MFI. In contrast, the lowest activity exhibited Co-zeolites with high content of aluminum in their frameworks (Si/A1 < 4), and thus high cobalt concentration. TOF values (per Co ion) clearly indicates that with

decreasing content of aluminium in the zeolite framework, the activity~selectivity of the Co ions dramatically increases. However, in the series of the studied zeolites, the Co ions are located at different cationic sites, with different coordinations, as reported in the previous studies. In MFI, MOR and BEA [7-9] three typical Co coordinations were found, with most of the Co ions (60-70 %) bound to six framework oxygens of the deformed six-ring in BEA and

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MFI, and of the twisted eight-ring in MOR. While this Co site is at the channel intersection in MFI framework and very open in BEA, in MOR it is accessible only from the inter-connected narrow channels. Moreover, all these zeolites exhibit Co ions (ca 30%) coordinated to four oxygens in their main channels, weakly bound to the framework, and therefore, potentially very reactive, because of the coordinative unsaturation (details on the coordination and reactivity see Ref. 8). A series of Co-Y and dealuminated Co-USY zeolites with the same -faujasitetopology, provides the Co ions with the same coordinations, i.e. at SII/SII'/SI' sites coordinated to three oxygens in C3v symmetry of the regular six-ring; SI site in the center of hexagonal prism is not occupied by the Co ions. At SII/SII' sites the Co ions are accessible to reactants, but SI' site Cannot be reached by ethane, oxygen and acetonitrile. Under the assumption that the population of the cationic sites by the Co ions does not differ dramatically for dealuminated zeolites, then the catalytic activity/selectivity of the dealuminated Co-Y zeolites (Table 1, Figure 1) indicates that a decisive effect controlling the activity/selectivity of the Co ions is the negative framework charge balancing the cobalt ions.

3.2 Co-ligand complexes relevant to ethane/NH3/O2 to acetonitrile reaction Complexation of the Co ions in a polyoxoframework-Co-extraframework ligand complexes, such as those assumed to take place during the catalytic reaction, is demonstrated for Co-BEA (Figure 2). Bare Co ions bound only to framework oxygens induce perturbation of the adjacent framework T-O bonds, yielding a characteristic shiit of the antisymmetric T-O vibration to lower frequency into a skeletal transmission window (BM band). This effect quantitatively mirrors the presence of the Co ions at cationic sites (see Refs. 4 and 9). By adsorption of ethylene, ammonia, ethylamine or acetonitrile on Co-BEA, 1:1 complexes were detected by a relaxation shiit of the BM band into a BML band (of higher frequency) with isosbestic points in the time-resolved spectra, as illustrated in Figure 2 for ammonia adsorption-desorption. The relaxation shifts characteristic for the individual adsorbates bound to the Co ion in 1:1 complexes, and temperatures for the existence of dehydrated Co ion and Co-ligand 1:1 complexes are given in Table 2. The values of relaxation shifts (Ar) together with the temperatures indicate weakening of the Co ion bonding to the framework oxygens due to the ligand, and thus are a measure of the strength of the Co-ligand bond. The strength of adsorbates bonding to the Co ion (Table 2) is following: ethylene << ammonia < acetonitrile < ethylamine.

This sequence indicates the importance of the presence of ammonia for selective transformation of ethylene on the Co ions. Ammonia is much more strongly bound to the Co ions compared to ethylene, resulting in limited adsorption of ethylene on the Co ions. C2H4 adsorption might induce its oligomerization and easy oxidation to carbon dioxide. Instead, ethylene interacts with Co-NH3 complex with formation of the Co-ethylamine complex, which was suggested to be a reaction intermediate. As a ligand, ethylamine is strongly held with the highest stability among possible ligands for the Co ions in the ethane/NH3/O2 reaction, while acetonitrile is less strongly bound. It implies that once formed, acetonitrile preferably desorbs from the Co ions as a final product. If low Co loaded Co-BEA also contained Al-Lewis and Broensted sites, then oligomerization of ethylene on these acidic sites could take place as documented by IR studies (10). At reaction temperature (450~ these sites are not covered by ammonia, like Co ions, and represent highly exposed strong electron acceptor and proton donor sites appropriate for catalyzing ethylene oligomerization, instead of its reaction with the Co-NH3 complex. This results in decreasing acetonitrile yields as well as deactivation of the zeolite.

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10

化

20

18

"-~-rl

8

16

-q

o

"13 X

14 "5, a) e-o

tD o

.e""

12

.

.....-'"

......o

.......:]il]ol]i ...o ~ , /

"to 8

6 --" O

/ - ii-i//

O O

4

.o :;:""" 7"

i

2

'

I

4

~

i

6

'

1'

'

8

1;

'

'

lJ2

14

Si/A]

Fig. 1. Dependence ofacetonitrile yield and TOF on Si/A1 framework ratio for Y - USY (o and o, resp.) and zeolites of various topologies (O and I, resp.), see Table 1.

f___ 3 - - ' - - '~ - - ' g 0 a0. ~

3(X~ '

o5 '

950 '

S '

g30 '

'

850 '

wavenumber, cm I Fig. 2. FTIR spectra of desorption of ammonia from Co-BEA (Co/A1 0.16) with Co-NH3 complex at RT (a), 100 (b), 250 (c), 300 (d), 350 (e) and 480 ~ (f).

ol r t">.

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Table I Composition and activity of Co zeolites in ethane/NHj/02 reaction catalyst

SilAl~: (molar)

Si/Alu (molar)

Co (wt %)

conversion

(%I

selectivity (%)

acetonitrile yield (%)

TOFx 1000 (sec-I)

CO-USY 7.1 5.3 6.60 8.0 1.8 12.7 12.7 36.1 4.6 0.8 2.5 2.5 CO-Y 13.3 15.3 45.4 6.9 1.1 3.9 2.6 CO-USY 6.60 20.5 56.0 11.5 3.8 7.1 5.3 CO-USY~ 9.8 6.9 6.70 24.6 58.8 14.5 4.7 CO-USY~ b areaction temperature 450°c, catalyst weight 0.2 g, 5% ethane; reaction temperature 475OC, catalyst weight 0.3 g, 10% ethane; reaction temperature 450°c, catalyst weight 0.3 g, 10% ethane Table 2 Co-L complexes in Co-BEA. Relaxation shifts (A r) in antisymmetric T - 0 vibration and temperature for existence of bare Co ion (TM)and Co-L 1:1 complexes (ThL) under the presence of adsorbates. Bhlband for bare Co cation in dehydrated zeolite is at 916 cm-I.

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4. CONCLUSIONS

The following factors have been indicated to control the ethane/NH3/O2 reaction over Cozeolites (i)

(ii)

(iii)

the cation should be accessible to reactants, with a possibility for an optimum approach of reactants to the cation, controlled by the cation coordination and the environment of the cationic site, given by the framework topology; a decisive effect of negative framework charge for the Co ions activity~selectivity was shown; low negative framework charge given by low content of aluminum in the zeolite framework leads to a high Co ion reactivity. Generally, weak bonding of the Co ions to framework oxygens controlled by their low negative charges and cation coordination provide high activity of the Co ions; stronger bonding of ammonia compared to ethylene avoids ethylene oligomerization on the Co ions and enables interaction of ethylene with Co-NH3 complex to form ethylamine, an intermediate for acetonitrile formation. If alumina originated acid sites (protonic or Lewis) are present in the zeolite besides the Co ions, ethylene oligomerization occurs, and decreases performance of the Co zeolites in acetonitrile formation.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.

Y. Li and J.N. Armor, J. Chem. Sot., Chem. Commun. 1997, 2013. Y. Li and J.N. Armor, J. Catal. 173 (1998) 511. Y. Li and J.N. Armor, J. Catal. 176 (1998) 495. Z. Sobalik, Z. Tvarfi~kovh and B. Wichterlov/l, J. Phys. Chem. 102 (1998) 1077. Z. Sobalik, Z. Tvarfi~kov/L and B. Wichterlov/l, Microp. Mesop. Mater. 25 (1998) 525. J. D~de6ek, D. Kauck~, and B. Wichterlovb., Microp. Mesop. Mater. (1999) in press. J. D6de6ek and B. Wichterlov/t, J. Phys. Chem. 103 (1999) 1462. B. Wichterlovfi, J. D6de6ek and Z. Sobalik, Proc. 12th Int. Zeol. Conf., Baltimore, 1998, Eds. M.M.J. Treacy, B.K. Marcus, M.E. Bisher, J.B. Higgins, Material Research Society, 1998, p.941. 9. Z. Sobalik, Z. Tvarfi~kov/l and B. Wichterlovb., Microp. Mesop.Mater. 25 (1998) 225. 10. Z. Sobalik, A.A. Belhekar, Z. Tvarfi~kov/l and B. Wichterlov/l, Appl. Catal.A: 188 (1999) 175.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 92000 Elsevier Science B.V. All rights reserved.

R o o m - t e m p e r a t u r e o x i d a t i o n of h y d r o c a r b o n s o v e r FeZSM-5 zeolite Mikhail A. Rodkin *~, Vladimir I. Sobolev, b Konstantin A. Dubkov, b Noel H. Watkins ~, and Gennady I. Panov b Solutia Inc., P.O. Box 97, Gonzalez, FL 32560-0097, USA b Boreskov Institute of Catalysis, 630090 Novosibirsk, Russia

SUMMARY Interaction of a variety of organic molecules with a-oxygen formed by N20 decomposition over FeZSM-5 zeolites leads to products of selective hydroxylation. O-insertion occurs rapidly at room temperatures and affects both aromatic and aliphatic C-H bonds.

1. INTRODUCTION N20 decomposition over FeZSM-5 zeolites is accompanied by the formation of a specific oxygen form called a-oxygen [1,2]. This oxygen exhibits a very high reactivity similar to that of monooxygenases and mimics their unmatched ability for selective oxidation of hydrocarbons at room temperature [3] The unique chemistry of a-oxygen has found practical application in the catalytic selective oxidation of benzene to phenol by nitrous oxide [4]. The process is being developed by Solutia, Inc. in cooperation with the Boreskov Institute of Catalysis and was successfully pilot-tested at Solutia's facilities in Pensacola [5]. To expand the application scope, room-temperature oxidations of various organic compounds by (z-oxygen using single-turnover experiments have been investigated and are reported in the present work.

2. EXPERIMENTAL The FeZSM-5 zeolite (SiOJA1203 = 72; 0.56 wt.% Fe) used in this work was prepared by hydrothermal synthesis with the introduction of iron in the form of FeC13 to the starting gel. The zeolite was converted into H-form by exchange with ammonia buffer and subsequent air calcination at 550~ To increase the concentration of a-sites, on which a-oxygen is formed, sample activation in vacuum at 900~ was used [6]. The experiments included three steps that were performed in the following sequence: 1. a-oxygen loading,

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2. interaction of a-oxygen with organic substrate, 3. extraction of the product and its analysis.

The first and the second steps were performed in a static setup equipped with an on-line mass-spectrometer. A catalyst sample (~1 g) was placed in a microreactor, which could be isolated from the rest of the setup. Before each experiment the sample was pretreated consecutively in vacuum and in oxygen at 550~ a-Oxygen was loaded by N20 decomposition at 250~ as described m reference [2]. N20 decomposition at this temperature is accompanied by stoichiometric binding of oxygen atoms to the active sites and the evolution of N,, into the gas phase (eq. (1)): N20+(

)a --> ( 0 ) a + N2

(1)

The n u m b e r of loaded oxygen atoms was determined from the amount of N2 evolved, as well as from the isotopic exchange with 1802, and found to be 2.0 2.2• m oxygen atoms per gram of catalyst in all cases. After a-oxygen loading the microreactor was isolated from the rest of the reaction volume and cooled to room temperature. Gas in the reaction volume was replaced with organic vapor which contacted with the catalyst sample for 10-15 min. The excess organic starting material was removed by evacuation, the sample was then taken out of the reactor and extracted with 2 ml of aqueous solution of acetonitrile (CH3CN : H20 = 1 : 1). The composition of the extract was determined using GC and GC/MS methods. The overall reaction is selective hydroxylation described by eq. (2). R-H + N20

-~ R-OH +N2

(2)

Additional information on the experimental details can be found in references [7, 8].

3. RESULTS AND DISCUSSION In our previous w o r k [7, 8] a similar technique has been used for detailed studies of the reaction of benzene and methane with a-oxygen at room temperature. This interaction has been shown to yield selectively phenol and methanol, respectively. The amounts of detected products within experimental error matched the amounts of reacted starting materials. Accurate quantitative t r e a t m e n t of the results of such studies is quite challenging. Zeolites are known to strongly adsorb both the reaction products and the starting materials. The main goal of this work was to expand the scope of compounds that can react with a-oxygen, which inevitably led us to focus on the qualitative rather than quantitative information. Thus, we did not optimize the extraction procedure for every case. The extracted products accounted for 515 ~mole/g of catalyst, which enabled us to reliably identify them and determine

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their relative ratios. However, taking into account the possibility of incomplete extraction for different products, the ratios of the products reported below should be treated as preliminary results, that will be refined in our more detailed studies. 3.1 Oxidation of alkanes and alkenes

In view of our earlier results with methane [7], it was no surprise that oxidation of ethane yielded ethanol as the only product. When more complex alkanes are subjected to interaction with a-oxygen, secondary alcohol formation is predominant over the hydroxylation of the primary C-H bond. Thus, when propane is reacted with a-oxygen, 1-propanol and 2-propanol are formed m 1:2 ratio. When the reaction was performed with n-hexane only secondary positions of the molecule have been affected, giving an approximately equimolar mixture of 2-hexanol and 3-hexanol (eq. (3)). In most cases, the secondary alcohols can be oxidized further and the corresponding ketones were also observed, usually in small quantifies. OH

a-oxygen

:

1

OH

(3)

In the case of branched 2-methylhexane, the hydroxylation exhibits high sensitivity to steric hindrances: hydroxylation affects neither the tertiary C-H bond, nor the secondary C-H bond next to the branching. Hydroxylation attack on position 7 to branching is much easier than that on the ~-position (eq. (4)). OH or-oxygen

1

:

4

(4)

Similar observations were made for cydoalkanes. Cyclohexane gives cydohexahol exclusively. In the case of methylcydohexane, having a methyl substituent on the cyclohexane ring led to almost complete blocking of positions 2 and 3 of the ring leaving position 4 as the only available for attack on the ring (2and 3-methylcydohexanols are formed in trace quantities). Cydohexanemethanol is another major product, the formation of which is probably also determined by specific steric environment of methylcyclohexane in the zeolite micropore space (eq. (5)).

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?CH3

ct-oxygcn HOX

?CH3

?CH2OH

3 : 1 (5) The reaction of a-oxygen with olefins was studied using cyclohexene as the starting material. Allylic oxidation was found to be the predominant process with 2-cyclohexene-l-ol as the major product (along with minor amounts of the corresponding a,~-unsaturated ketone- 2-cyclohexene-l-one, the formation of which can be explained by enhanced reactivity of allylic alcohol towards oxidation). 3-Cyclohexene-l-ol was also detected; the ratio of the products of allylic and homoallylic attack being ca 4:1 (eq. (6)). OH

O

0

OH

(~-oxygen

4

:

1

(6)

3.2 Oxidation of aromatics

As has been shown previously [8], the interaction of benzene with ~oxygen resulted in selective hydroxy]ation of the aromatic nucleus. The competition between the hydroxy]ation of aIiphatic and aromatic carbons as we]]

as the balance of electronic and steric factors influencing these transformations was studied using alkylaromatic compounds as starting materials. In all cases, hydroxylation of the aromatic nucleus was observed to be much more facile than hydroxylation of the side chain. Interaction of toluene with a-oxygen leads to both the products of benzylic and aromatic hydroxylation: benzyl alcohol and cresols (o:m:p = 1:1:2.5) (eq. (7)). CH2OH a-oxygen + 1

:

H 2.5

(7)

The oxidation of ethylbenzene (eq. (8)) and isopropylbenzene (eq. (9)) shows that an increase in the bulk of the substituent strongly suppresses hydroxylation of both ortho- and meta-positions. For ethylbenzene, ortho- and

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meta-ethylphenols are found in trace quantities, whereas for isopropylbenzene, para-isopropylphenol is the only ring-hydroxylated product observed.

Hydroxylation of the side chain of alkylaromatic compounds provides another good example of steric influences in a reaction happening in a zeolite micropore space. Hydroxylation of the side chain of ethylbenzene leads predominantly to ~phenetol. With the increase of the steric bulk of the alkyl substituent in isopropylbenzene, a-hydroxylation is strongly suppressed and 2-phenyl-1propanol is the predominant product of side chain hydroxylation, even though the tertiary C-H bond in the alkyl substituent of cumene is generally the most reactive. Even with severe steric restrictions, its reactivity is sufficient enough to make formation of 2-phenyl-2-propanol the only example in this study of the hydroxylation of the tertiary sp3-carbon. HO a-oxygen

O~

k

1

OH

9

3

(8)

OH

a-oxygen +

+

~" 1

OH 9

4

(9)

Hydroxylation of halobenzenes was also studied in the reaction with ~oxygen at room temperature (eq. (10)). In the case of fluorobenzene (X= F), fluorophenols were the major products (o:m:p = 1:2.6:5.1). When chlorobenzene (X=C1) is hydroxylated by a-oxygen, ortho- and para-chlorophenols in a 1:5.1 ratio are the major products. X

X a-oxygen v

(io)

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In a,a,a-trifluorotoluene (X=CF3), the CF3 group, being a substituent with a strong -I effect and large steric bulk, directs hydroxylation predominantly in the meta-position (m:p = 1.5:1).

4. CONCLUSIONS A screening study of alkanes, alkenes and aromatic compounds in room temperature reactions with a-oxygen formed by N20 decomposition on the surface of Fe-containing zeolites is reported. Though the results are of qualitative nature, they significantly expand our knowledge of the chemistry of (z-oxygen and provide leads to other catalytic oxidations by N20. N20/FeZSM-5 zeolite was shown to be a versatile system for selective hydroxylation of a variety of organic compounds, effecting O-insertion into both aromatic and aliphatic C-H bonds. The reactivity of aromatic nucleus was found to be higher than that of the aliphatic substituents in all the studied alkylaromatic compounds. The regioselectivity of hydroxylation is determined by steric and electronic factors and is strongly influenced by the constraints imposed by the micropore space of the zeolite matrix. The reported results demonstrate the possibilities of novel routes to alcohols and phenols, most of which currently are manufactured via multi-step processes. To make these transformations commercially viable catalytic versions of these reactions need to be developed, a formidable challenge. We hope that the reported results will inspire researches to take a closer look at this novel and exciting field of oxidative catalysis.

REFERENCES 1. G.I. Panov, A.K. Uriarte, M.A. Rodldn and V.I. Sobolev, Catalysis Today, No. 41 (1-2) (1998) 365. 2. G.I. Panov, V.I. Sobolev, and A.S. Kharitonov, J. Mol. Catal., No. 61, (1990) 85. 3. G.I. Panov, V.I. Sobolev, K.A. Dubkov and A.S. Kharitonov in Proc. 1 lth Intern. Congr. Catal. J.Hightower, W.N. Delgass, E. Iglesia and A.T. Bell (eds.), Stud. Surf. Sci. Catal., Baltimore, 1996, Elsevier Science B.V. No.101 (1996) 493. 4. G.I. Panov, A.S. Kharitonov and V.I. Sobolev, Appl. Catal. No. 98 (1993) 1. 5. A.K. Uriarte, M.A. Rodkin, M.J. Gross, A.S. Kharitonov and G.I. Panov m Proc. 3rd Intern. Congress on Oxidation Catalysis, R.K. Grasselly, S.T. Oyama, A.M. Gaffney and J.E. Lyons (eds.) Stud. Surf. Sci. Catal., Elsevier Science B.V., No. 110 (1997) 857. 6. V.I. Sobolev, K.A. Dubkov, Ye.A. Paukshtis, L.V. Pirutko, M.A. Rodkin, A.S. Kharitonov and G.I. Panov, Appl. Catal. No. 141 (1996) 185. 7. K.A. Dubkov, V.I. Sobolev and G.I. Panov, Kinet. Katal., No. 39 (1998) 79. 8. V.I. Sobolev, A.S. Kharitonov, Ye.A. Paukshtis and G.I. Panov, J. Mol. Catal., No. 84 (1993) 117.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

S e l e c t i v e o x i d a t i o n of h y d r o c a r b o n s by a c t i v e o x y g e n f o r m e d f r o m N 2 0 on ZSM-5 at m o d e r a t e t e m p e r a t u r e s S.N.Vereshchagin a, N.P.Kirik a, N.N.Shishkina a, S.V.Morozov b, A.I. Vyalkov b and A.G.Anshits a a Institute of Chemistry and Chemical Engineering SB RAS, K.Marx st. 42, Krasnoyarsk, 660049, Russia; e-mail: [email protected] b Institute of Organic Chemistry SB RAS, Novosibirsk, 630090, Russia Conversion of methane, ethane, propane and benzene on HNaZSM-5 in the presence of N20 at 350-450~ was studied. The reaction rate was shown to be determined by the strength of C-H bond attacked and zeolite acidity had no influence on methane and benzene reactivity. Oxidative methylation of aromatic ring with methane occured under reaction condition giving toluene from benzene. Hydrogen form of zeolite favored the reaction of oxidative methylation whereas reaction of deep oxidation prevailed on non-acidic samples. 1. INTRODUCTION It is known that the interaction of N20 with ZSM-5 (or their iron analogs) produces surface oxygen [1] called a-oxygen [2]. This active oxygen species shows an extraordinary ability for selective oxidation: at moderate temperatures it converts ethane to ethylene or benzene to phenol with 96-98% selectivity. Though the process of benzene to phenol hydroxylation with N20 was demonstrated by Solutia on a pilot plant scale, the mechanism of these reactions is still unknown and there is a lack of data on the chemistry of a-oxygen. Despite of the high cost of N20 for selective oxidation of the most hydrocarbons the understanding of regularities of a-oxygen formation and its interaction with different hydrocarbons could be very useful for general concept of selective conversion of low alkanes and design of active catalysts. The objective of the present paper is to consider the peculiarities of light alkane transformation over ZSM-5 in the presence of nitrous oxide to reveal the chemistry of the oxygen species produced from N20 under condition of catalytic reaction.

2. EXPERIMENTAL Zeolites H-Na-ZSM-5 (SIOJA1203=33, the contents of iron 0.06 wt.%, sodium 0.01-2.5 wt.%) were prepared from NaZSM-5 by triple ion exchange with 1M NH4C1 followed by impregnation with NaOH solution to reach desired content of sodium. Catalytic runs were carried out at atmospheric pressure and 350-450~

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in a conventional set-up with stationary catalyst bed as described elsewhere [3]. GC-MS analyses of the liquid products of 13CH4+C6H6+N20 conversion were performed on capillary column HP-5MS (5% phenyl methyl siloxane). 13C isotope distribution and location of labeled atoms were calculated from mass spectra (GC-MS data) and from 1H and 13C NMR spectra [4].

3. RESULTS AND DISCUSSION 3.1 Reactivity of hydrocarbons in respect of oxygen species Three distinct types of transformation were found for catalytic conversion of methane, ethane, propane and benzene in the presence of N20 on ZSM-5 (Table 1). Methane undergoes basically deep oxidation to CO2 and CO with a small amount of aromatics; ethane and propane are selectively dehydrogenated to olefins, whereas phenol is formed from C6H6, the selectivity of dehydrogenation and hydroxylation being very high (90-98%). The conversion of the hydrocarbons drops rapidly with time on stream but the selectivity remains almost constant. It is interesting that the highest rate of activity decrease is observed for methane (about 4 times for the first 20 min of run), the lowest- for ethane. Table 1 Main routs of transformation of RH-N20 feeds on HZSM-5 at 380-420~ 15%vol, CN2o=3-10%vol, He balance, RH conversion less than 10%). RH

C2H6 C3Hs C6H6

main product (selectivity, %) > > >

C2H4 (>90) C2H4 (>80) C6HsOH (95-98)

(CRH--3-

minor products (selectivity, %)

+ + +

C3-C6aliphatics+aromatics(5-10), COx (1-5) C4-Csaliphatics+aromatics (5-15), COx(l-5) CO2(1-5)

To understand the differences in catalytic behavior we have studied a reactivity of hydrocarbons towards surface oxygen formed from N20. The values of relative reactivities expressed as krd=kRH/kCH4 (where kcH4, kRH- kinetic constants of reactions (1) and (2)) are calculated from equation (3), where P~ PORH, PCH4, PRH, - partial pressures of methane and R H before and after reactor, provided that feed contains simultaneously methane and R H (e.g. C2H6-CH4N20) [3]. The values of krel calculated in such a way decrease in the order C3Hs>C2H6>CH4~H2~C6H6. High selectivity to principal products (Table 1) indicates that a-oxygen attacks only C-H bond of hydrocarbon molecule (H-H for hydrogen), therefore, the properties of C-H bond can influence the course of the reaction. From the experimental data one can estimate the reactivity of the single C-H bond as k'rel- k,.el/N, where N-number of equivalent H atoms. CH4 + [O] ~kcH4--~ products (1) R H + [O] mkRH---->products, where R H - C3H8, C2H6, H2 or C6H6 (2)

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PCH4 kcH4 ln( PRH ) In( POH4)- kRH POH

(3)

An excellent linear correlation between energy of C-H bond and corrected relative reactivity k'rel (Fig.l) 1,6 indicates that it is the strength of C-H bond (H-H for hydrogen) that determines 1 2 1,0 the rate of alkanes conversion. The reactivity of benzene is found to be close 0,4 v to that of methane and fits in with the correlation line for alkanes though the -0,2 3 value of k'c6H6 is slightly less than predicted. The data presented do not -0,8 allow to distinguish between C-H and -1,4 C=C double bond attack for hydroxy92 96 100 104 lation of C6H6. In our case the low reactivity of benzene can be explained E, kcal/mole not only by the distinction of reaction Fig. 1. Variation of k'rel a s a function of mechanisms but also by transport energy of C-H bond E for: 1-propane, 2- limitation that decreases the actual ethane, 3-methane, 4-benzene and 5- concentration of C6H6 in the vicinity of hydrogen. active centres inside the channels. 2,2

14

-0,02

12 :5 'O O

or}

n,"

- b

10

-1-

d

8

o I1.

"I-

6

E

4

200

@-

-

-

-0,10

_m -0,14

2 0

. . . . . .

-0,06

300

400 T, C

500

600

-0,18 -0,12

-0,08

-0,04

0,00

Ig(Pc H4/P~ H,=)

Fig 2. (a)- NH3 TPD profile (Tads=200~ ~=10~ and (b)- relative reactivity 0 of methane and benzene for: $ - HZSM-5; D- 1.5)ANaHZSM-5; A- 2.5%NaZSM-5 Influence of acidity on the relative reactivity of methane and benzene was studied over three samples of ZSM-5 possessing different amount of sodium and, therefore, distribution of acid sites (as can be seen from TPD profiles of ammonia, Fig.2a). It is evident that the experimental points for all the samples satisfy the same correlation line in coordinates ln(PRH/pORH)-ln(Pcm/pOcHr(Fig.2b). Therefore, the relative reactivity of methane and benzene does not depend on acidity and no

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884

additional activation on acidic site is necessary for the first step of the interaction of surface oxygen with the molecule of a hydrocarbon. It is reasonably to suggest t h a t the different product composition (Tabl.1) is caused not by peculiarities of aoxygen interaction with hydrocarbon, but by subsequent transformation of activated surface hydrocarbon species.

3.2 R o u t e s o f h y d r o c a r b o n t r a n s f o r m a t i o n in the p r e s e n c e of N20 Though preferred formation of COx occurs over HZSM-5 for neat m e t h a n e (Tabl.1) only traces of deep oxidation products are found for CH4-C6H6-NeO feed. Instead the enhanced amount of toluene (Tol) and diphenylmethane (DPhM) are observed (Table 2), xylenes (Xy), cresols (Cre), indene (Ind) and methylated naphtalene (Naph) are also formed in minor amount. Composition of the products obtained over 2.5% NaZSM-5 differs Table 2. Amount of products formed on HZSM-5 from t h a t on HZSM-5. For acidic at 418~ with different reaction feeds. sample the formation rates of phenol Time of s t r e a m 2000s, GHSV=26500 h -1. and toluene are about equal (Table 2), but for non acidic zeolite the rate Reaction feed (vol.%, He balance) of toluene formation is more t h a n an CH4 14 14 order of magnitude lower (Figs.3a) C6H6 3.5 3.5 and only traces of aromatic NeO 5.7 5.8 5.8 compounds are found. The A m o u n t of products (mmole/g) conversion of m e t h a n e remains the COx <0.01 0.57 <0.01 same but COe, CO and coke are toluene 0.58 0.049 predominantly formed instead of xylenes 0.036 0.063 methylated aromatic compounds. phenole 0.63 0.004 0.6 Intermediate activity of Tol and COe indene 0.08 0.014 formation is found for 1.5% Nacresols 0.23 0.008 HZSM-5. Therefore, acidity appears DPhM 0.19 to play key role in the reaction of methylated aromatics formation. An additional indication of the importance of acid centres for the reaction is a sharp reversible decrease of the rate of toluene formation when a pulse of NH3 is added to reaction feed (Fig.3b). Addition of a m m o n i a does not influence the rate of phenol production, whereas the loss of activity for toluene is about one order of magnitude. It was assumed t h a t on acidic samples benzene is able to intercept surface species formed from CH4 preventing from deep oxidation, the reaction resulting in aromatic ring methylation. To clarify the origin of methylated compounds liquid products of conversion of 13CH4-C6H6-NeO feed were collected and subjected to GC-MS, 1H and 13C NMR analyses. The n u m b e r of 13C atoms in a molecule can be calculated from the position of molecular ion in the mass spectrum. If (for a product) one expects m 13C atoms to be present in the molecule then a set of molecules with 0,1..m+2 atoms of 13C (binomial distribution) can be found as the content of 13C label in initial CH4 is 72%

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and benzene contains 1.1% of ~3C. Table 3 shows the theoretical and experimental distributions of label in some products of 13CH4-C6Hs-N20 transformation. a

,

,.

lO-1

~ 50..1

le-7

Nt3

0

e..l

E d n,

le-8

NH3

le-9 100

1000 Time,s

Fig.3. (a) - formation rates of phenol, toluene and T=418~ feed (vol.%) CH4-14, N20-5; C6H6-3.5; He toluene formation as a function of time of stream. HNaZSM-5; A- 2.5% NaZSM-5; 9 - HZSM-5 + pulses of

CO2 over H-Na-ZSM-5. - balance; (b) - rate of O - HZSM-5; 0 - 1.5% NH3

Table 3. Theoretical (1) and experimental (2) distributions of labeled molecules containing n atoms of 13C in the products of CH4-C6H6-N20 conversion. 1.5% NaHZSM-5, T=418~ feed (vol.%) 13CH4 (72% 13C) 13%, C6H6 (1.1% 13C) 3.5%, N20 9%, He balance; reaction time 2000 s. RH

Total Fraction with n 13C atoms 13C frac. 0 1 2 3 4 Tol*(1) 1 0.11 0.252 0.700 0.046 0.001 2 0.12 0.26 0.69 0.03 + + Xy*(2) 1 0.191 0.068 0.372 0.525 0.034 0.001 2 0.23 0.06 0.27 0.48 0.15 0.04 Ind*(3) 1 0.251 0.018 0.150 0.413 0.393 0.025 2 0.20 0.124 0.236 0.353 0.259 0.029 Naph*(4) 1 0.299 0.005 0.054 0.221 0.408 0.294 2 0.24 0.08 0.20 0.31 0.23 0.15 DPhM*(1) 1 0.06 0.244 0.671 0.083 0.002 2 0.06 0.291 0.611 0.097 + Cre*(1) 1 0.11 0.252 0.700 0.046 0.001 2 0.12 0.26 0.65 0.08 + *- the figure in parenthesis denotes the expected n u m b e r of 13C atoms

5 +

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

+ 0.001 + 0.019 0.03

The data presented points to the presence of only one 13C atom in molecules of Tol, Cre and DPhM; xylenes have mainly two labels per molecule.

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According to NMR the label is located in methyl or methylene groups. It means that formation of methylated derivatives occurs via methylation of aromatic ring with methane. The CO2, CO, H20 following reaction sequence Scheme I for alkylated aromatics +N20 I([O]) formation may be supposed CH4 + [O] [CH4..O] (Scheme 1). Methane reacts -H20 with surface a-oxygen to +[0]~ -H20 give active hydrocarbon coke species [CH4...O] which can be further oxidized by excess of oxidant to COx. It can lose hydrogen via either oxidative dehydrogenation or hydrogen redistribution and forms coke precursor. Additional route exists on acidic zeolites - methylation of benzene or toluene releasing toluene and xylene. Number of labeled carbon atoms in the molecules of Ind and Naph is found to be less than theoretically expected (Tabl.3). Substantial number of non labeled molecules and those, containing only one 13C atom points to the notable contribution of benzene to the formation of Ind and Naph, the reaction is accompanied with scrambling of 13C label. Therefore condensed hydrocarbons are formed most probably by means of surface hydrocarbon's residue transformation. 4. C O N C L U S I O N S Rate of oxidative conversion of hydrocarbons on HNaZSM-5 in the presence of N20 is determined by the strength of C-H bond. Zeolite acidity has no influence on methane and benzene reactivity. Oxidative methylation of aromatic ring with methane occurs under reaction condition. Surface acidity favors the reaction of oxidative methylation whereas reaction of deep oxidation prevails over non-acidic samples. ACKNOWLEDGMENTS

The authors thank V.I.Mamatyuk and M.M.Shakirov for conducting NMR experiments and spectra interpretation. REFERENCES

1 S.N.Vereshchagin, L.I.Baikalova and A.G.Anshits, Izv.Akad.nauk, Ser.khim. 8 (1988) 1718, in Russian. 2 V.I.Sobolev, A.S.Kharitonov, Ye.A.Paukshtis and G.I.Panov, J.Molec.Catal. 84 (1993) 117. 3 S.N.Vereshchagin, N.P.Kirik, N.N.Shishkina and A.G.Anshits, Catal. Lett. 56 (1998) 145. 4 S.N.Vereshchagin, N.P.Kirik, N.N.Shishkina, S.V.Morozov, M.M.Shakirov, V.I.Mamatyuk and A.G.Anshits, J. Molec. Catal., submitted.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

化

887

Redox molecular hydrocarbons

sieve catalysts

for the aerobic

selective

oxidation

of

John M e u r i g T h o m a s * , R o b e r t Raja, G o p i n a t h a n S a n k a r and R o b e r t G Bell The Royal Institution of Great Britain, Davy Faraday Research Laboratory, 21 Albemarle Street, London W IX 4BS, United Kingdom We report the design and perforrnance of redox molecular sieve catalysts, containing isolated, tour-coordinated transition metal ions that are substitutionally incorporated into the framework and act, in concert with the surrounding framework structure, as shape-selective and regiospecific catalysts for the oxidation of linear and cyclic alkanes, using molecular oxygen as the oxidant. These materials have also proved effective in the Baeyer-Villiger oxidation of cyclic ketones to lactones and alkenes to epoxides, using a sacrificial aldehyde and air as an oxidant. 1.

INTRODUCTION

Devising inorganic catalysts for the shape-selective and regiospecific oxidation of saturated hydrocarbons using either dioxygen or air under rather mild conditions, with an efficiency that rivals that of natural enzymes such as cytochrome-P-450 and non-heme iron o)hydroxylase, has proven to be extraordinarily difficult [1]. Yet trying to do so is of considerable merit since such partial oxidation would generate a source of important feedstocks for the fine chemicals, polymer and pharmaceutical industries. For example, the selective oxidation of cyclohexane to a mixture of cyclohexanol and cyclohexanone (the so-called K-A oil) under mild conditions is of prime importance in the manufacture of Nylon-6 and Nylon-6-6, and the regiospecific oxidation of linear paraffins generates functionalised derivatives for the production of surfactants and detergents.

2.

EXPERIMENTAL

Taking cues from the general field of redox metallo-enzymes that catalyse the shape- and regio-selective oxidation of hydrocarbons, we have designed aluminophosphates (A1PO's), into the framework tetrahedral sites of which a few atom percent of various divalent metal ions (such as cobalt, iron or manganese) may be readily accommodated as isomorphous replacements for the AI(III) ions. Details of the synthesis and structure of these materials have been previously reported [2,3]. Such solids are exceptionally good solid acid catalysts when operated under conditions that maintain the transition metal in its +2 oxidation state. But, when the metal ions are raised to a higher oxidation state, whilst still remaining intact in the framework, they function in air (see below) as active sites for the catalytic oxidation of linear and cyclic alkanes. The catalytic reactions were carried out in a high-pressure reactor (Cambridge Reactor Design) and dry air was pressurised into the PEEK lined reaction vessel. In-situ studies on the catalysts were carried out using combined extended X-ray absorption spectroscopy and X-ray powder diffraction using the Daresbury Synchrotron Radiation Source [4,5]. 3.

RESULTS AND DISCUSSION

3.1 Structural Charaeterisation Experiments involving combined in situ X-ray absorption spectroscopy and X-ray diffractometry [4] plus ex situ scanning transmission electron microscopy, have established that, in 02 or air, at temperatures up to c~. 550 ~ C, the Co(II) ions (like those of isomorphously

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888

substituted Mn(II)) are converted to the (III) oxidation state while remaining tetrahedrally coordinated (i.e. coordinatively unsaturated) without loss of structural integrity of the AIPO host or exsolution from it of its embedded transition-metal ions [5]. For example, Fig. l(a) shows the

1 (b) MnAIPO- 18 As-prepared

1.4.

1.2. 1.0.

1 (a)

0

0.8. 0.6.

=

0.4.

m

d

0.2. 0.0-0.2

6520 6540 6560 6580 6600 6620

2.o

.,x 2~10 20 30 40 ~ Two-Theta (Degrees)

Energy (eV)

1.5

1 (c) t~

~o

_.= Fig- 1 9Structural characteristics of MnAIPO- 18

o.s ) 480

J ~ ' " (MnO2) -_

. 6520

.

.

.

6560

, 6600

.

,

.

6640

Energy (eV)

stacked plots of X-ray diffractograms (recorded during the calcination process), showing that the integrity of the framework structure remains intact, when Mn(II) framework ions are oxidised to their Mn(III) state, in MnA1PO-18. In AIPO-18, the Co(II) or Mn(II) ions may be completely converted in dry air or 02 to the Co(Ill) or Mn(III) states; however, only a fraction of these isomorphously incorporated ions in AIPO-36, A1PO-11 and A1PO-5 are convertible to their (III) oxidation state by a similar treatment in air or 02 [6]. While the Co(II) ions embedded in the framework of AIPO-5 are extremely resistant to oxidation to their Co(Ill) state, Fe(II) ions in AIPO-5, on the other hand, are readily raised to their Fe(III) state upon similar treatment in 02. Fig. l(b) shows the X-ray absorption near edge spectra at the Mn K-edge for the MnAIPO-18 sample and, for comparison, the corresponding spectrum for MnO2 is shown in l(c). Both this and those of MnO and Mn203 are very different from those of the MnAIPO catalyst [5].

3.2

Oxidation of alkanes

3.2.1 Oxidation of cyclohexane The catalytic results (Table-1) show that MnAIPO-36 (and CoAIPO-36) are indeed the most active catalysts, for the oxidation of cyclohexane, compared to either Co/Mn/FeA1PO-5 or Co/MnAIPO-I 1; and as expected in view of its pore dimension Co/MnAIPO-18 shows no activity. These results are not surprising in that Co/MnAIPO-36 possesses not only the appropriate pore dimension (6.5 x 7.5/k) for cyclohexane to diffuse in to the channels and react with the catalytically active centres, but also this catalyst has a higher fraction of oxidisable cobalt/manganese centres compared to Co/MnA1PO-11 and Co/MnAIPO-5. Although Co/MnA1PO-18 have the highest fraction of oxidisable metal centres, because of its smaller pore openings the cyclohexane molecule does not gain access to its interior active sites and no catalytic activity ensues. The results clearly indicate that framework-substituted cobalt (iron or manganese) are responsible for the catalytic activity, and there is a correlation between the degree of activity and the amount of oxidisable cobalt in the framework (Fig. 2).

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Table-1 9

Aerobic oxidation of cyclohexane*

Catalyst CoAIPO-36 MnAIPO-36 FeAIPO-5 CoAIPO-5 MnAIPO-5 C o A I P O - 11 MnAIPO-I 1 C o / M n A I P O - 18

t (h) 8 24 8 24

T O N '~

8 24 8 24 8 24 8 24 8 24 24

45 113 29 36 47 107 47 63 81 136 0

126 188 157 233

Conv. mol % 7.3 9.9 8.6 12.8 2.5 6.6 1.4 2.0 2.7 6.2 2.6 3.4 4.7 7.9

Product distribution m o l e % A -

B

C

D

E

F

G

51.2 21.7 19.5 14.6

41.0 55.3 56.2 47.2

6.3 18.5 19.2 33.0

-

-

1.7 4.6 5.3 5.5

44.1 13.5 24.2 7.5 11.2 -

41.1 36.2 35.0 15.0 36.9 18.5 28.6 22.3 48.4 26.2

17.9 26.2 15.5 31.0 21.7 . 67.8 56.0 5.9 42.8 31.5 44.7 61.2 2.1 36.9 58.4 4.2 N o reaction

5.4 . -

1.9 2.7

13.5 9.2 . . -

3.4 1.2"* 7.9** 2.6 7.1 3.6** 10.9"*

Reaction conditions : * C y c l o h e x a n e = 48.6 g; Catalyst = 0.5g; pressure (air) =1.5 M P a ; t e m p = 403 K; A = c y c l o h e x y l h y d r o p e r o x i d e ; B = cyciohexanol ; C = c y c l o h e x a n o n e ; D = adipic acid ; E= v a l e r a l d e h y d e ; F = valeric acid; G = probably CO2, CO, water and traces of l o w e r olefins in the gas phase. * * m a i n l y glutaric + traces of succinic acid. w T O N = m o l e s of substrate c o n v e r t e d per m o l e of metal (Co, Mn or Fe) in the catalyst ~::i~

120 Fig-2: Bar chart representation of the

100

percent o f M e ( I I I ) f o r m e d (as d e d u c e d f r o m E X A F S ) and catalytic activity for c y c l o h e x a n e oxidation at 403 K (see Table- 1 for details)

80

60

~.

:, .........~ii!i~..... :!~.

)

= M

[] % M(III) formed (EXAFS) [] T O N

:i!il

[] c H H P

,-.;;!.:.: ;;:b:'

[] c y c l o h e x a n o l

40

[] c y c l o h e x a n o n e

2o

[]

i

0 FeAIPO-5

3.2.2

(

i~-

MnAIPO-5

adit~ic acid

[] valerald + valeric acid CoAIPO-5

Oxidation of linear aikanes

T h e results f r o m the oxidation of n-pentane, n-hexane, n-octane and n - d o d e c a n e are s u m m a r i s e d in Table-2, and, as indicated by the p r i m a r y selectivity index [7], the overall p e r f o r m a n c e o f the C o A 1 P O - 1 8 and M n A I P O - 1 8 catalysts, is superior to that o f all p r e v i o u s l y reported inorganic catalysts for the regioselective oxidation of linear alkanes. T h e p e r f o r m a n c e

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Table 2: Oxidation of n-alkanes* using molecular oxygen: Comparison of catalysts and product selectivity

alkane

Catalyst

Conv mol 8 n-C5HI2 CoAlPO-18 8.8 n-C5HI2 CoAlPO-36 5.5 n-CSHI2 MnAlPO-18 8.2 n-C5H12 MnAlPO-36 5.9 n-CsH14 CoAlPO-5 2.4 3.6 n-C6HI.t CoAlPO-I I n-C6H,j COAIPO-I8 7.2 n-C6H14 CoAlPO-36 5.2 n-C6HI4 MnAIPO-18 8.7 n-C6HI4 MnAIPO-36 5.4 n-CsH18 CoAIPO-18 6.2 n-C8HI8 CoAIPO-36 5.8 n-C8Hls MnAIPO-18 6.8 n-C8Hls MnAIPO-36 5.9 5.5 n-CI2H2h MnAIPO-18 n-CI2H26 MnAIPO-36 5.2 2.3 n-CIZH26 MnAIPO-I I n-C12H26 MnAIPO-5 4.7 n-C12HL6 CoAlPO- 18 6. 1 n-C12H26 CoAlPO-36 3.9

=

TON 175 109 151 116 41 62 121 89 149 82 80 75 83 72 50 45 20 39 57 38

CI-01: CI-al 32.5 5.3 38.6

acid

-

-

1.3 7.4 4.5 8.7 9.9

4.1 7.8 3.3 3.8 12.5

-

30.5 1.7 10.3 3.6

-

-

6.0 17.3 13.9 9.2 8.4 7.9 12.6 5.5 12.8 3.5 27.8 6.3 13.3

3.2 4.2 53.5 10.2 43.1

33.9

15.3

26.0

-

-

14.4 4.9 9.4 2.5 7.9 3.1 3.5

18.8 7.5 22.1 3.1 29.3

14.7 -

C1-ol

27.1 -

Product distribution (mole 8 ) C2-on Cj-ol C;-on C4-ol 43.7 21.1 33.3 59.9 48.3 11.8 21.5 76.7 10.1 41.0 30.4 17.1 13.9 29.9 22.2 19.8 14.5 30.6 23.3 19.3 31.5 39.7 18.2 6.4 9.9 8.4 16.5 17.9 23.5 5.0 12.5 19.9 18.4 20.2 8.4 9.0 9.5 17.3 7.8 11.0 17.4 8.5 16.6 14.3 15.1 7.5 6.2 4.8 11.7

P.S. I" C4-on Cj-01 Cs-on others" 2.7 1.5 1.3 1.8 3.9 2.4 2.6 3.2 2.8 1.6 2.5 27.0 2.4 1.7 0.9 32.5 4.5 13.6 11.0 14.4 4.0 15.1 6.3 13.3 3.8 20.6 11.6 17.9 3.9 2.7 19.0 23.5 31.9 3.1

0.48 0.06 0.63 0 0.13 0.32 2.1 0.39 2.5 0 3.O 0.28 3.2 0.15 3.O 0.24 0.12 O 5.9 0

*substrate 50 g; catalyst = 0.5 g; oxidant (air) = 1.5MPa; temp = 373 K; time = 24 h; t C1-ol = I-alcohol; CI-a1 = I-aldehyde; Cz-01 = 2-alcohol; Cz-on = 2-ketone; C3-01 = 3-alcohol; C3-on = 3-ketone; C e o l = 4-alcohol; C4-on= 4-ketone; $ others = mainly COz, CO, water and lower olefins/hydrocarbons in the gas phase. %C P.S.= primary selectivity index is defined as the ratio of concentration of primary products to secondary /tertiary products normalized for the respective number of hydrogens in the alkane molecule

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and various degrees of regioselectivity of the MnAIPO-18, CoALPO-36 and CoAIPO-5 catalysts for the oxidation of n-octane are shown in Fig. 3. It is very clear from Fig. 3, that by choosing the appropriate molecular sieve, only end-on entry of the linear alkanes into the cavities that contain the active sites is possible. This steric constraint predisposes the oxyfunctionalisation to be favoured at the terminal CH3 and penultimate CH2 groups. A bound octane (or any other linear alkane) molecule, with its slightly bent end (proven by MD calculations), gains ready access to the active sites, which selectively favour oxidation at the terminal carbons. We believe that the overall selective oxidation of the alkanes reported here generally involve the participation of free radicals [8] and it is certain that framework-isolated Co(III) or Mn (III) 35 30 Fig-3 : Regioselectivity plot for the oxidation of n-octane at 373 K using Co and Mn substituted molecular sieve catalysts (see Table-2 for reaction conditions)

25 .84

O ~ Conversion

2015

selectivity

[] C 2

selectivity

[] C3 s e l e c t i v i t y

10

MnALPO-18

[] C 1

[] C4 s e l e c t i v i t y

CoALPO-36

CoALPO-5

ions are vital for the catalysis. While the prior addition of flee-radical initiators, such as tert-butyl hydroperoxide, greatly diminish the induction period and accelerate the overall rate of reaction, addition of free-radical scavengers, such as hydroquinone, greatly hinder the oxidation reaction. Further in the oxidation of cyclohexane, the presence of cyclohexyl hydroperoxide (which subsequently decomposes to cyclohexanol and cyclohexanone) has been directly detected. 3.3 Baeyer-Villiger oxidation of alkenes and ketones Baeyer-Villiger oxidations have been conventionally carried out using stoichiometric oxidants such as CrO3, KMnO4, RuO4, Pb(OAc)3, Ag20 and Co(acac)3. In 1991, Mukaiyama and coworkers [9] reported an efficient method for epoxidising olefins using various aldehydes that can be smoothly oxidised to their corresponding carboxylic acids using homogeneous transition metalbased catalysts. Transition metal-ion substituted aluminophosphates are effective catalysts for the aerobic selective oxidation of hydrocarbons and can convert alkanes and aldehydes to their corresponding carboxylic acids [8]. Taking into account the redox properties and pore dimensions of a range of metal ion substituted AIPO' s, we have identified three different structures of potential catalysts that contain small amounts of either Co or Mn redox (framework) cations, that can oxidise a number of alkenes and ketones, selectively, to their corresponding epoxides and lactones (Table3) in the presence of a sacrificial aldehyde, using molecular oxygen as an oxidant. The reasons for the superior performance of these redox catalysts are the active Co In (or Mn In) ions that line the micropores. Benzaldehyde molecules may freely enter the large internal surfaces of these catalysts, and in the presence of 02, generate first PhCO 9and then the PhCOOOo radicals inside the micropores (Figure-4). This peroxy acid is then involved in a nucleophilic attack at the substrate, leading to the production of epoxides and lactones.

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Table 3 : Baeyer-Viiliger oxidation of ketones (A) and alkenes (B) Substrate

Catalyst CoA1PO-36 MnAIPO-36 MnAIPO-5 CoAIPO-36 MnAIPO-36 MnAIPO-5 C o A I P O - 36 MnAIPO-36 CoAIPO-36 MnAIPO-36 MnAIPO-5 CoAIPO-36 MnAIPO-36 MnA1PO-5

T (K) 323 323 323 323 323 323 353 353 323 323 323 323 323 323

t (h) 6 6 6 6 6 6 5 5 8 8 8 8 8 8

Conv. mol% 71 78 64 58 61 50 80 87 54 62 44 54 59 42

TOF h -i 250 257 207 238 246 187 220 224 184 203 136 111 127 78

cyclohexanone cyclohexanone cyclohexanone cyclopentanone cyclopentanone cyclopentanone adam antan-2-one adamantan-2-one cyciohexene cyclohexene cyclohexene oc-(+)-pinene ot-(+)-pinene ot-(+)-pinene (R)-(+)-limonene (R)-(+)-limonene (R)-(+)-Iimonene styrene styrene styrene

CoA1PO-36 MnAIPO-36 MnA1PO-5 CoAIPO-36 MnAIPO-36 MnA1PO-5

323 323 323 323 323 323

8 8 8 4 4 4

46 51 30 46 55 32

94 114 56 247 282 169

Product selectivity, mol % L E D O 98" 98" 82" 15 92 b 7 94 b 5 77 b 20 99 99 69 27 4c 77 19 5c 77 13 10 r 84 16 d 91 9d 66 34 d -

78 87 51 34 27 49

59 61 60

23 e 13 e 49 e 8 12 -

Reaction conditions: (A) substrate ~ 20 g; catalyst _-- 0.15 g; (B) substrate _= 35 g; catalyst --_ 0.25 g;substrate : benzaldehyde = 1 : 3 (mol/mol); 02 (air) = 30 bar; L=iactone; E=epoxide; D=diol; O=others; Products : a = e-caprolactone; b = 5-valerolactone; c = c y c l o h e x - 2 - e n - l - o l & cyclohex2 - e n - l - o n e ; d = verbenol & verbenone; e = carveol, carvone & trans-carveyl acetate. 02

O C

Fig-4 : Formation of the perbenzoic acid intermediate effected by f r a m e w o r k M e Ill ions and O2, inside the micropores of A1PO-36 molecular sieve.

AI e

O Mn

@o O

......

C

f\ < .j./...'i. H

REFERENCES

,

5. 6. 7.

8. 9.

C.L. Hill (ed.), Activation and Functionalisation of Alkanes. Ch. 6-8, Wiley, 1989. J. Chen and J.M. Thomas, J. Chem. Soc., Chem. C o m m u n . , (I 994) 603 P.A. Wright, S. Natarajan, J.M. Thomas, R.G. Bell, P.L. Gai-Boyes, R.H. Jones and J.S. Chen, Angew. Chem. Intl. Ed. Engl., 31 (1992) 1472. J.M. T h o m a s and G.N. Greaves, Science, 265 (1994) 1675. G. Sankar and J.M. Thomas, Top. Catal., 8 (1999) 1. P.A. Barrett, G. Sankar, C.R.A. Catlow and J.M. Thomas, J. Phys. Chem., 100 (1996) 8977. B.R. Cook, T.J. Reinert and T.S. Suslick, J. Am. Chem. Soc., 108 (1986) 7281. J.M. T h o m a s , R.Raja, G. Sankar and R.G. Bell, Nature, 398 (I 999) 227. T. Yamada, T. Takai, O. Rhode and T. Mukaiyama, Chem. Lett., (1991) 1.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Catalytic b e h a v i o u r of H-type z e o l i t e s in the d e c o m p o s i t i o n of c h l o r i n a t e d VOCs R. LSpez-Fonseca, P. Steltenpohl$, J.R. Gonz~lez-Velasco, A. Aranzabal, and J.I. Gutidrrez-Ortiz Department of Chemical Engineering, Faculty of Sciences, University of the Basque Country/EHU, P.O. Box 644, E-48080 Bilbao, Spain. ABSTRACT Several H-type zeolites were investigated for their activity and selectivity during TCE oxidative decomposition in dry and humid conditions. These catalysts showed high activity resulting H-MOR to be the most active. The catalytic behaviour appeared to be determined by strong Bronsted acidity. The main oxidation products were CO, CO2, HC1 and C12. Water enhanced the catalytic activity at lower temperatures and also promoted the selectivity to deep oxidation products generation (CO2 and HC1). 1. I N T R O D U C T I O N Chlorinated volatile organic compounds (VOCs) are emitted to the atmosphere in flue gases from a variety of industrial processes. Chlorinated hydrocarbons have enjoyed widespread acceptance as solvents in the chemical industry because of their relative inertness in chemical processes and their ability to dissolve many organic compounds. Chlorocarbons are among the most widespread and persistent noxious pollutants which are hazardous to h u m a n health or the environment. Hence, public awareness of the harmful effects of chlorinated VOCs emissions has led to increasingly stringent environmental regulations. Trichloroethylene (TCE) is a toxic solvent widely used in dry cleaning and degreasing processes and is also present in air stripping and soil venting remediation off-gases [1]. Among the methods adopted for its abatement from waste streams, catalytic oxidation over zeolites has recently gained interest as an effective and advantageous alternative to the noble and metal oxide catalysts employed in most commercial applications. The objective of this work is to evaluate the deep catalytic oxidation of trichloroethylene over H-Y, H-ZSM-5 and H-MOR zeolites, at conditions of lean chlorocarbon concentration (1000 ppmv) in air between 200 and 550~ The effect of water addition (15000 ppmv) has also been studied. Present address: Department of Chemical and Biochemical Engineering, Slovak University of Technology, 812 37 Bratislava, Slovak Republic.

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2. EXPERIMENTAL

2.1. Materials The zeolites H-Y (CBV400), NHa-ZSM-5 (CBV5524G) and Na-MOR (CP500C11) were supplied from PQ Corporation. The H-Y zeolite was used as received. The H-ZSM-5 zeolite was obtained by calcining the NHa-ZSM-5 zeolite in air at 550~ for 3 h. The mordenite sample in the NHa-form was prepared by two successive ion-exchanges with 3M NHaNO3 solution at room temperature for 24 h. The NHa-form was then transformed to the H-form by calcining in air at 550~ for 3h. The zeolites were pelletised using methylcellulose as a temporary binder which was removed by calcination in air. Then pellets were crushed and sieved to grains of 0.3-0.5 mm in diameter. 2.2. Catalysts characterization The compositions and Si/A1 atomic ratios measured by XRF and BET areas obtained from N2 adsorption-desorption isotherms for the H-type zeolites tested in this work are given in Table 1. The water content was determined as the percentage difference between the weight of water satured samples and calcined ones at 900~ The samples were also characterized by XRD analysis. Table 1 Physicochemical properties of the zeolite catalysts. SiO2 (%) A1203(%) Na20 (%) H20 (%) H-Y 74.6 21.6 2.8 12.3 H-ZSM-5 97.1 2.8 0.I 7.2 H-MOR 91.0 8.8 0.2 5.2

Si]A1 2.4 27.3 5.2

SBET(m2g -') 730 425 425

2.2.a. Temperature programmed desorption ( T P D ) o f pyridine The acid strength of the zeolite catalysts were determined by temperature programmed desorption (TPD) of pyridine. The overall process involved heating of the sample at 500~ for 15 h in He flow, pyridine adsorption at 250~ removal of physisorbed pyridine by passing He (80 cm 3 min -1) over the sample at 250~ for 4 h, and finally, desorption of chemisorbed pyridine from the sample by heating from 250~ to 500~ at a rate of 5~ min 1.

2.2.b. Diffuse Reflectance FTIR Spectroscopy Diffuse reflectance (DRIFT) spectra of pyridine adsorbed on the zeolite samples were obtained with a Nicolet Proteg6 460 ESP spectrometer, equipped with a Spectra-Tech high-temperature chamber and a nitrogen-cooled MCT detector. All spectra were recorded in the range 4000-400 cm 1 averaging 200 scans with a 4 cm -1 resolution and analysed using OMNIC software. After the sample was evacuated at 500~ at 10 .5 Torr for 1 h, pyridine was admitted at 200~ at the equilibrium pressure of 2 Tort and the spectra of adsorbed pyridine were then measured. Difference spectra were obtained by subtracting the spectrum of the dehydrated zeolite sample from the spectra obtained after pyridine adsorption.

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2.3. Catalytic activity measurement

Oxidation reactions were carried out under atmospheric pressure in a fixed bed tubular reactor. Liquid reactants were injected into a dry, off-free compressed air stream by two syringe pumps. The flow rate through the reactor was set at 500 cm 3 min -1 and the gas hourly space velocity was maintained at 15000 h -1. Reactor effluent was analysed on line by a Hewlett Packard 5890 Series II gas chromatograph equipped with an electron capture detector (ECD) and a thermal conductivity detector (TCD). Operation conditions and reaction product analysis were described in detail elsewhere [2]. 3. R E S U L T S AND D I S C U S S I O N H-type zeolites tested in this work exhibited high activity in the oxidative decomposition of trichloroethylene. TCE was chosen for this study as a model compound for the general category of hydrogen deficient, chlorinated unsaturated hydrocarbons. It was found that the conversion hierarchy was H-MOR>H-ZSM5>H-Y (Fig. 1). H-MOR showed a Ts0 (temperature at which 50% conversion is achieved) of 450~ compared to 470 and 480~ obtained by H-ZSM-5 and H-Y zeohtes, respectively. Above 500~ the difference in activity among the three zeolites became less noticeable, and at 550~ the level of conversion was quite similar (99%). Previous studies have also reported that significant high temperatures are required for the TCE deep oxidation in comparison with saturated chlorinated hydrocarbons [2,3]. It is believed that the relatively large size and high electronegativity of the chlorine atom can produce severe steric and electronic hindrances to the adsorption of chlorinated ethylene molecules [4]. Tajima et al. [5] examined the decomposition of chlorofluorocarbons over a variety of solid acids catalysts. H-type zeolites showed a higher activity resulting H-MOR the most active catalyst. 100

100 __--

80

=~ t-

=

H-Y

____

H-ZSM-5 H-MOR

80

60

=~ t-

.o ..

H-ZSM-5 H-MOR

60

.O ..

|

O

__.

H-Y

E c

40

0

o

40

o 20

0 200

20

250

300

350

400

450

500

550

200

2so

300

3so

400

450

5~0

sso

Temperature, ~

T e m p e r a t u r e , =C

F i g u r e 1. TCE oxidation light-off cmwes over

Figure 2. TCE oxidation light-off curves over H-zeolites in humid conditions.

H-zeohtes in dry conditions.

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TPD of pyridine data (Fig. 3) indicated that the total number of acid sites was larger for H-MOR and H-Y compared to H-ZSM-5, as expected from the higher aluminium content. However, acid sites in H-ZSM-5 resulted to be markedly stronger t h a n in H-Y. Diffuse-reflectance infrared spectra of pyridine adsorbed on the three H-type zeolites are shown in Fig. 4. The absorption band around 1545 cm 1 can be related to pyridine adsorbed on Bronsted sites and the absorption band around 1455 cm -1 is attributed to pyridine adsorbed on Lewis sites [6]. The H-MOR and H-ZSM-5 samples exhibited predominately Bronsted acidity while H-Y contained both Bronsted and Lewis acidity. It was established that strong Bronsted acidity played a major role in determining the catalytic activity of the H-type zeolites. Additionally, adsorption of TCE in He atmosphere on the zeolite samples was carried out in a thermobalance. As a result, a positive correlation could be established between activity and adsorption capacity of the H-type zeolites. Thus, the adsorption capacity was in the order: H-MOR>H-ZSM-5>H-Y. 0.7

":' oD o E E n

0.6

9

I[

HY

H-ZSM-5

0.10 ~

/~

Ii

1450

1400

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0.5 0.4

o w -oo

0.3

o ,o_. "o

0.2

t_ n~

A = =

0.1

0

250

300

350

400

Temperature, "C

450

500

Figure 3. TPD profiles of pyridine desorbed from H-type zeolites.

1700

1650

1600

1550

1500

Wavenumbers (cm "1)

Figure 4. Diffuse-reflectance infrared spectra of pyridine adsorbed on H-type zeolites.

TCE conversion was extremely slow below 350~ in dry conditions. As the temperature exceeded 400~ the conversion rapidly increased and CO, CO2, HC1 and C12 were promoted in quantity. Trace amounts of perchloroethylene (C2C14) were also produced and their concentration peaked (110-140 pppmv) at 500~ independent of the zeolite. This by-product was generated by chlorination of the TCE and was partially destroyed at higher temperatures. Bickle et al. [7] and Wang et al. [8] also observed the chlorination of the TCE to C2C14 over Pt supported catalysts. As far as CO and CO2 formation was concerned, the formation of CO2 was relatively favoured as temperature increased. Therefore, at 550~ H-MOR showed the greatest selectivity towards CO2 formation (68%) whereas selectivity values for H-ZSM-5 and H-Y resulted about 64%. The formation of carbon monoxide over H-type zeolites reflects the difficulty of its oxidation over these catalysts. Likewise, van der Brink et al. [9,10] found 7-

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alumina greatly selective towards CO formation in the oxidation of several chlorocarbons. An important observation is that HC1 and C12 were not generated in an equimolecular ratio when TCE was completely decomposed, as it would be expected according to the stoichiometry. It is suggested t h a t the H § ions in the zeolite structure promotes HC1 formation by the Deacon reaction (2HCl+ 89 as the water content in the dry feed stream was below t h a n 1 ppmv. H-MOR was found more selective to the production of desirable HC1, since it is less toxic and can be easily removed by caustic absorption from the effluent stream. Figure 2 shows the light-off curves of the TCE oxidation conducted in humid conditions. It was noticed t h a t the addition of water to the feed stream did not alter the trend of the activity of the zeolites. The presence of water vapour enhanced the zeolite activity at lower temperatures, but at higher temperatures water reduced slightly catalytic activity. This effect was less noticeable in H-Y zeolite as the activity was only improved at low levels of conversion (<15%). However, Ts0 values decreased from 450 to 400~ over H-MOR and from 470 to 450~ over H-ZSM-5. It could be concluded that both hydrolysis and oxidation accounted for TCE destruction in the presence of water vapour over H-type zeolites. Lester [11] and R a m a c h a n d r a n et al. [12] also reported t h a t catalytic hydrolysis is an important pathway for the catalytic destruction of halogenated compounds with a H/C1 ratio <1. The significant decrease in TCE conversion over H-Y zeolite in humid conditions was due to its hydrophilic character related to the high aluminium content. It is thought that water molecules create a diffusion blockage for TCE molecules resulting in low activity even at high temperatures. Moreover, the presence of water in the TCE decomposition changed markedly the reaction product distribution. Water was found to promote the oxidation of CO, by the water-shift reaction, resulting in selectivity to CO2 higher t han 80% over the three zeolites. The formation of HC1 was greatly enhanced (>90%), thus the generation of undesired C2C14 and C12 was almost completely inhibited leading to selectivity values <5%. Carbon balances closed above 90% in most cases. No coke formation was observed during reaction. However, it is worth pointing out t h a t the chlorine balance Was in the range of 75-90%. It is known t h a t A1 atoms can be easily removed from the zeolite framework as volatile A1C13 leading to partial collapse of the zeolite structure [13]. Dealumination was slightly enhanced in moisture conditions. XRD patterns of the used samples were analysed in order to study the resistance of the zeolites to dealumination which results in a loss of cristallinity. Relative cristallinity was calculated based on changes in peak heights from XRD data. The cristallinity of deactivated H-Y and H-MOR zeolites diminished to 7580% whereas no significant loss in cristallinity was found in H-ZSM-5 zeolite sample. This can be related to the low aluminium content of H-ZSM-5 zeolite in comparison with H-Y and H-MOR zeolites, thus indicating zeolites with high Si]A1 ratios are stable catalysts to oxidise chlorinated VOCs.

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4. C O N C L U S I O N S

H-type zeolites studied in this work showed a good activity in the TCE decomposition. H-MOR zeolite was found to be the most active catalyst. Temperature-programmed desorption of pyridine and diffuse reflectance FT-IR measurements of adsorbed pyridine revealed that strong Bronsted acidity plays an important role in controlling the oxidative activity of the H-type zeolites. The main oxidation products were CO, CO2, HC1 and C12. Small amounts of perchloroethylene were also detected as a result of the chlorination of the feed molecule. H-MOR appeared to be the most selective zeolite towards the formation of deep oxidation products (CO2 and HC1). The presence of water in the feed stream did not alter the activity trend of the zeolites. It was concluded that both hydrolysis and oxidation accounted for the TCE destruction in humid conditions. It was found that water promoted the formation of CO2 and limited the generation of undesired chlorinated by-products (C12 and C2C14). XRD analysis determined that H-ZSM-5 was the most resistant zeolite to dealumination by chlorination. ACKNOWLEDGEMENTS The authors wish to thank University of the Basque Country/EHU (UPV069.310-G40/98) and Spanish Education and Culture Ministry (QUI960471) for the financial support, and Basque Government for the Researchers Mobility Grant for one of the authors (P.S.)' REFERENCES

1. A.R. Gavaskar, B.C. Kim, S.H. Rodansky, S.K. Ong and E.G. Marchand, Environ. Progress, 14 (1995) 33. 2. J.R. Gonz~lez-Velasco, A. Aranzabal, J.I. Gutidrrez-Ortiz, R. LSpez-Fonseca and M.A. Gutidrrez-Ortiz, Appl. Catal. B, 19 (1998) 189. 3. S. Chatterjee and H.L. Greene, Appl. Catal. B, 13 (1992) 179. 4. H. Windawi and M. Wyatt, Platinum Metals Rev., 37 (1993) 186. 5. M Tajima, M. Niwa, Y. Fujii, Y. Koinuma, R. Aizawa, S. Kushiyama, S. Kobayashi, K. Mizuno and H. Ohfichi, Appl. Catal. B, 9 (1996) 167. 6. G. Busca, Catal. Today, 41 (1998) 191. 7. G.M. Bickle, T. Suzuki and Y. Mitarai, Appl. Catal. B, 4 (1994) 141. 8. Y. Wang, H. Shaw and R.J. Farrauto, ACS Syrup. Ser., 495 (1992) 125. 9. R.W. van den Brink, P. Mulder, R. Louw, G. Sinquin, C. Petit and J-P. Hindermann, J. Catal., 180 (1998) 153. 10. R.W. van den Brink, R. Louw and P. Mulder, Appl. Catal. B, 16 (1998) 219. 11. G.R. Lester, Air&Waste Management Association Meeting, Anaheim, CA, 1989. 12. B. Ramachandran, H.L. Greene and S. Chatterjee, Appl. Catal. B, 8 (1996) 157. 13. KSnya, I. Hannus and I. Kiricsi, Appl. Catal. B., 8 (1996) 391.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Coupling Alkane Dehydrogenation with Hydrogenation Reactions on Cation-Exchanged Zeolites Wei Li, Sara Y. Yu, and Enrique Iglesia Department of Chemical Engineering, University of California at Berkeley, Berkeley, CA 94720 Thiophene desulfurization was achieved using hydrogen surface species formed during dehydrogenation of alkanes on H-ZSM5 modified by exchanged cations (M = Co, Mo, Zn) with the selective formation of H2S without direct involvement of the H2 formed in alkane reactions. Thiophene reactions in the absence of a hydrogen source (H2, C3H8) lead to significant formation of benzothiophene and dibenzothiophene by-products. Propane markedly increases desulfurization rates and H2S selectivities, while inhibiting catalyst deactivation on all catalysts. Rates, selectivities, and deactivation rates using propane (20 kPa) as a direct hydrogen source are equivalent to those obtained at >200 kPa H2 on H-ZSM5 and to 50-100 kPa H2 on Co/H-ZSM5 and much higher than at the H2 pressures prevalent during alkane reactions (1-2 kPa). Alkane reactions on M/H-ZSM5 are limited by hydrogen desorption bottlenecks that lead to surface hydrogen chemical potentials higher than those in the contacting H2 gas phase. The addition of these hydrogen species to thiophene leads to the observed desulfurization, which provides an alternate path for hydrogen removal and leads to higher alkane dehydrogenation rates. This synergistic coupling occurs on H-ZSM5, but cations appear to provide a promotional effect. Cations provide recombinative desorption sites that decrease the availability and chemical potential of adsorbed hydrogen, but they also appear to be involved in providing alternate binding sites for hydrodesulfurization reactions. 1. INTRODUCTION Sulfur compounds in crude oil often remain in fuels, leading to their release and to the poisoning of exhaust catalysts. Environmental concems and legislation will lead to a significant decrease in the sulfur content of diesel and gasoline fuels (400 to <40 ppm) [1]. Catalytic cracking (FCC) naphtha contributes most of the sulfur compounds in gasoline [ 1]. Some desulfurization via hydrogen transfer to form H2S occurs during FCC, but thiophene and alkylthiophenes remain largely unreacted and can adsorb on FCC catalysts, leading to SOx release during regeneration [ 2 ] . Sulfur removal from FCC naphtha requires hydrodesulfurization processes that lead to significant hydrogenation of high-octane alkenes and aromatics. Thiophene desulfurization without significant saturation and H2 requirements would provide alternate process options and mechanistic details of largely unexplored hydrogen transfer reactions. This study explores one example of such chemistry- the desulfurization of thiophene with selective formation of H2S using alkanes as direct hydrogen sources on cation-exchanged zeolites.

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Alkane dehydrogenation and dehydrocyclodimerization on cation-modified H-ZSM5 lead to hydrogen removal bottlenecks that limit reaction rates and lead to the formation of hydrogen-rich cracking products [3, 4]. This bottleneck causes the chemical potential of adsorbed hydrogen to be significantly higher than that of H2 in the contacting gas phase, leading to the hydrogenation of adsorbed species that can cause the undesired formation of hydrogen-rich light alkanes as a side product of dehydrogenation and aromatization steps [5]. These observations led to the use of concurrent desired hydrogenation reactions to remove the desorption bottleneck and to upgrade unsaturated molecules such as O2, CO, and CO2 [6]. Here, we report the hydrodesulfurization of thiophene using surface hydrogen formed in-situ during propane dehydrogenation reactions on H-ZSM5 modified by exchanged Co, Zn, and Mo. 2. EXPERIMENTAL METHODS

2.1. Catalyst Preparation and Elemental Analysis Na-ZSM5 (Zeochem, Si/A1 = 14.5) was converted to the ammonium form (NH4ZSMS) by exchange with a 0.16 M NH4NO3 solution (Fisher, Certified ACS, >98.0%) at 353 K for 16 h four times using fresh NH4NO3 solutions to ensure complete exchange. The zeolite sample was filtered and washed with deionized water. Samples were dried in air at 398 K for 20 h and calcined at 773 K for 20 h in flowing dry air to convert to the proton form of the zeolite (H-ZSM5). Co/H-ZSM5 was prepared by ion exchange of H-ZSM5 at 353K for 16 h in a 0.05 M Co(NO3)2 solution (Aldrich, 99%). The sample was filtered, washed with deionized water, dried in air at 398 K for 20 h, and then treated in flowing dry air at 773 K for 20 h. Atomic absorption spectroscopy (Galbraith Laboratories, Inc.) measured a Co content of 0.91 wt% (Co/A1 - 0.15). Co§ isolated cations are located at the exchange site and replace two Bronsted acid sites during exchange. These characterization data will be reported elsewhere [7]. Zn/H-ZSM5 was prepared by aqueous exchange methods that lead to the predominant formation of Zn+2 cations bridging two A1 sites [8]. Mo/H-ZSM5 was prepared by solid-state exchange methods using MoO3/H-ZSM5 physical mixtures, which lead to the formation of (Mo205) +2 dimers interacting with two A1 sites [9].

2.2. Catalytic Thiophene Desulfurization Thiophene desulfurization studies were performed in a tubular reactor with plug-flow hydrodynamics. Rates, selectivities, and deactivation behavior were measured at 773 K using C3H8 (20 kPa)/C4H4S (1 kPa), H2 (0-200 kPa)/C4H4S (1 kPa), or pure C4H48 (1 kPa) reactants. C3H8(Praxair, >99.5%) was purified using O2/I-I20 traps (Matheson) and C4H4S (Aldrich, >99%) was used without purification. HE (Praxair, UHP) and He (Praxair, UHP) were purified using 02 and 13X sieve traps (Matheson). Reactant and product concentrations were measured by gas chromatography (Hewlett-Packard 6890) using capillary (HP-1 Crosslinked Methyl Siloxane, 50m x 0.32mm, 1.05 ~t film) and packed columns (Hayesep-Q, 80/100 mesh, 1 0 ' x 0.125") and flame ionization and thermal conductivity detection. Thiophene reaction rates are reported as molar thiophene conversion rates per A1 or per residual OH group after exchange. Isotopic exchange (DE-OH) experiments were used to determine the number of residual OH groups [8, 9]. Sulfur selectivities are reported on a sulfur basis as the percentage of sulfur from the converted thiophene that appears in each sulfur-containing product. Propane conversions were <15% in all experiments. Thiophene conversions varied over a much wider range and, when required, were converted to rate

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constants using the first-order rate expression obtained in independent kinetic experiments. The rate constants correspond to rates expressed as mole thiophene/mole OH-s. The deactivation behavior of the catalysts was studied at a constant space velocity. 3. RESULTS AND DISCUSSION 3.1. Effect of Propane on Thiophene Desulfurization Thiophene desulfurization rates (per OH group) on H-ZSM5, Co/H-ZSM5, and Zn/HZSM5 are shown in Figure 1 with and without C3H8 as a co-reactant. In the absence of C3H8, desulfurization proceeds slowly and leads to H2S, benzothiophene (BT), and dibenzothiophene. The larger sulfur compounds can form via thiophene desulfurization to H2S and subsequent reaction of thiophene fragments with another thiophene or via bimolecular Diels-Alder reactions of two thiophene molecules [10, 11]. Propane (20 kPa) markedly increases thiophene desulfurization rates on all three catalysts, apparently by increasing the availability of hydrogen adatoms required for C-S bond activation and for the hydrogenation of thiophene fragments. The rates reported in Figure 1 were obtained at high but similar C4H48 conversions (65-85%), which are not limited by thermodynamics [ 12], and low C3H8 conversions (<15%). Thus, they reflect the relative catalytic activities of these materials in spite of the integral reactor conditions imposed by the high thiophene conversions. A kinetic analysis using the measured first order rate dependence on C4H4S concentration was used to rigorously calculate reaction rate constants (k). For example, the addition of propane increases the value of k from 1.55 to 26.2 cm3/mole OH-s on H-ZSM5 and from 6.45 to 39.9 cm3/mole OH-s on Co/H-ZSM5. 1.5

80 M/AI

o_,-',

0

0.15

0.14

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~,0

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I With Propane

(2o kea)

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H

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Co Zn M/H-ZSM5 Figure 1" Comparison of thiophene desulfurization rates with and without propane [773 K, 1 kPa C4H4S, 0 or 20 kPa C3H8, balance He]

0.15

0.14

O .,..~

60 t~ O

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i.i, ,i,

40 ,.~ O ....,

v.., rl,

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20 ~ vl,

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H

r/,

|

The availability of reactive adsorbed hydrogen species with C3H8 as a co-reactant also influences thiophene desulfurization selectivity (Figure 2). The (H2S/BT) ratio is significantly higher when C3H8 is present; hydrogen species are used to hydrogenate unsaturated thiophene fragments and to decrease the probability of direct or indirect thiophene self-reactions that lead to more refractory organosulfur compounds. In addition, the coupling of propane dehydrogenation with thiophene desulfurization not only increases the rate of thiophene desulfurization, but also increases the rate of propane

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conversion based on our preliminary results using 13C-labeled propane and unlabeled thiophene reactants, in agreement with the concept of kinetic coupling.

3.2. Effect of Cations on Thiophene Desulfurization

The effect of Co, Mo, and Zn cations on the rate of propane-thiophene reactions is shown in Table 1 for a set of samples with similar M/A1 ratios. Also reported in Table 1 are the OH densities of each sample, obtained from D2-OH isotopic exchange measurements after cation exchange. H-ZSM5, which contains predominantly Bronsted acid sites associated with A1 sites (OH/AI--1), catalyzes propane-thiophene reactions, suggesting that such sites can be used to activate both propane and thiophene reactants. This is in agreement with a recent theoretical study which showed that thiophene desulfurization occurs on acidic zeolites in the presence of H2 [10]. Thiophene desulfurization was proposed to proceed via a two-step pathway: opening of the thiophene ring on the Bronsted acid site, and removal of sulfur as H2S and formation of butadiene in the presence of H2. Butadiene is expected to be too reactive to be isolated and is rapidly converted to aromatics on H-ZSM5. Reaction rates (normalized by the number of A1 atoms) are slightly higher on Co- and Zn-exchanged samples than on H-ZSM5, even though each divalent cation has replaced about two Bronsted acid sites during exchange. In the case of Mo/H-ZSM5, the severe conditions required for solid-state exchange lead to significant dealumination [9], and desulfurization rates are lower than on H-ZSM5 because of their very low OH/A1 ratios (Table 1). Reaction rates normalized instead by the number of residual OH groups show a much greater effect of cations, suggesting that cations enhance the activity of the catalysts, either by providing parallel desulfurization pathways or by introducing bifunctional desulfurization pathways (Table 1). Cations appear to have two counteracting effects on propane-thiophene reactions. Cations increase the rate of recombinative hydrogen desorption during alkane reactions and thus decrease the availability of surface hydrogen for desulfurization reactions [4-6]. On the other hand, cations can provide binding sites for thiophene adsorption and reactions, which can intercept recombinative desorption steps and scavenge hydrogen before desorption as H2. Na and K cations in H-ZSM5 have been shown to bind sulfur compounds, such as thiophene and H2S [13, 14]. Table 1: Comparison of thiophene desulfurization rates on H-ZSM5 and M/H-ZSM5 (M=Co, Mo, Zn) [773 K, 1 kPa C4H4S,20 kPa C3H8,balance He] Catalyst M/A1 O H / A 1 Desulfurization Rate Desulfurization Rate Ratio Ratio (per A1, 10-3, s-1) (per OH group, 103, sl) H-ZSM5 Co/H-ZSM5 Mo/H-ZSM5 Zn/H-ZSM5 * measured at 3 kPa;

0 0.15 0.18 0.14 corrected to

0.97

0.62

0.64

0.71 0.79 1.11 0.25 0.22* 0.88* 0.72 0.91 1.26 1 kPa thiophene using measured rate expression

3.3. C3H8 and H2 as Hydrogen Sources on H-ZSM5 and Co/H-ZSM5

C3H8 and H2 were compared as hydrogen sources by measuring thiophene desulfurization rates and selectivities using C3H8/C4H4Sand also HE/C4H4Sreactant mixtures with varying HE concentrations on H-ZSM5 and Co/H-ZSM5 (Table 2).

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Table 2: Comparison of thiophene desulfurization rates and (H2S/BT) molar ratios in the presence of H2 or propane on H-ZSM5 and Co/H-ZSM5 [773 K, 1 kPa C4H4S, 0-200 kPa H2 or 20 kPa C3H8, balance He] H-ZSM5 Co/H-ZSM5 Gas Phase Thiophene (H2S/ Gas Phase Thiophene (H2S/ Pressure of Desulfurization BT) Pressure of Desulfurization BT) Hydrogen Rate (per residual Molar Hydrogen Rate (per residual Molar Source (kPa) OH group, 103, sl) Ratio Source (kPa) OH group, 103, sl ) Ratio 0 0.067 3.45 0 0.26 5.56 100 (H2) 0.13 5.67 25 (H2) 0.83 27.9 200 (H2) 0.28 6.57 50 (H2) 1.01 43.0 20 (C3H8) 0.62 31.3 100 (H2) 1.29 129 20 (C3H8) 1.08 71.4 At the pressures and conversions of our study, C3H8 reactions lead to gas phase H2 pressures of 0-0.5 kPa on H-ZSM5 and 0-2 kPa on Co/H-ZSM5. The higher H2 pressures on Co/H-ZSM5 reflect the effect of cations on the rate and H2 selectivity of propane reactions, as previously reported also on Ga- and Zn-exchanged H-ZSM5 [4, 15]. The direct use of hydrogen from alkanes without the involvement of H2, is demonstrated by the higher desulfurization rates obtained with C3H8 (at 20 kPa) than with H2 at pressures even higher than those existing during propane reactions on both H-ZSM5 and Co/H-ZSM5 (Table 2). On H-ZSM5, desulfurization rates and (H2S/BT) ratios using 20 kPa propane exceed those obtained with 200 kPa H2 (Table 2). Increasing H2 pressures slightly increase desulfurization rates and H2S selectivities; these effects are directionally similar to those obtained when propane is introduced as a hydrogen source and they suggest that H adatoms formed from propane and H2 are kinetically similar. Clearly, the H2 available during propane-thiophene reactions cannot account for the high desulfurization rates obtained using propane as the hydrogen source. The (H2S/BT) ratio also reflects hydrogen availability and the effectiveness of the hydrogen source. (H2S/BT) ratios are higher with C3H8 (20 kPa) than with 200 kPa H2 (Table 2) confirming our conclusion that propane reactants lead to hydrogen surface concentrations that exceed those that would exist in equilibrium with 200 kPa H2 on H-ZSM5. A hydrogen desorption bottleneck on H-ZSM5 leads to high chemical potentials of adsorbed hydrogen species, which are effectively used in hydrogenation reactions of thiophene species with the selective formation of H2S and aromatics via pathways similar to those involved using H2 as the hydrogen source. On Co/H-ZSM5, desulfurization rates with 20 kPa C3H8 are similar to those achieved with 50-100 kPa Hz (Table 2). (H2S/BT) ratios increase with increasing H2 pressure and reach values similar to values obtained with C3H8 co-reactants also at 50-100 kPa H2. This equivalent H2 pressure (50-100 kPa) is lower on Co/H-ZSM5 than on H-ZSM5 (>200 kPa), because Co cations catalyze hydrogen adsorption-desorption steps. As a result, the availability of surface hydrogen is lower than on H-ZSM5 and H2 can be more effectively used as a source of hydrogen than on H-ZSM5. Thiophene desulfurization rates for H2/CaHaS mixtures with H2 pressures (1-2 kPa) typically present during propane reactions, however, were lower than for C3H8/C4HaS mixtures; therefore, desulfurization does not require gas phase H2 on Co/H-ZSM5. Zn cations in H-ZSM5 show similar effects on the rate of hydrogen removal and on the equivalent H2 pressure obtained during propane-thiophene reactions [ 12].

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3.4. Effects of C3Hs and H2 on Catalyst Deactivation Rates 0.5 Catalyst deactivation rates also depend on

A

the availability of surface hydrogen species that decrease the level of unsaturation and increase ~ , -~, , '~ ~ H 2 pressure during the reactivity of strongly adsorbed carbonaceous "'' C3H8 reaction deposits. Deactivation rates are proportional to ~ 0.3 the concentration of undeactivated catalytic sites. r176~ Treatment in 20% 0 2 at 773 K restores initial ~o 0.2 desulfurization rates, suggesting that deactivation w can be reversed by combustion of carbonaceous C3H8 / 0.1 C4H4S A H 2/C4H4S deposits. Deactivation rate constants (kd) with propane-thiophene reactants (0.014 h 1) are much l ~ 0.0 lower than with pure thiophene (0.49 hl). H2 coo 2'5 50 75 125 reactants also decrease deactivation rate H E Pressure (kPa) constants, approaching the values obtained with Figure 3: Comparison of first order deactivation rate propane at 50-100 kPa of H2 (0.060-0.021 h -1) constant (kd) in the presence of propane or H 2 on (Figure 3). Thus, reactions of propane-thiophene Co/H-ZSM5 [773 K, 1 kPa C4H48, 20 kPa C3Hs mixtures on Co/H-ZSM5 show deactivation or 0-100 kPa HE, balance He, Co/Al=0.15] behavior that reflects a surface with hydrogen chemical potential similar to that present with 50-100 kPa HE in the contacting gas phase. 0.4

A

REFERENCES

.

4. 5. 6. 7. .

9. 10. 11. 12. 13. 14. 15.

EPA Staff Paper on Gasoline Sulfur Issues, 1998, U.S. Environmental Protection Agency Office of Mobile Sources. Wormsbecher, R.F., Weatherbee, G.D., Kim, G., and Dougan, T.J., National Petroleum Refiners Association Annual Meeting 1993, San Antonio, TX. Biscardi, J.A. and Iglesia, E., J. Phys. Chem. B, 102 (1998) 9284. Biscardi, J.A. and Iglesia, E., J. Catal., 182 (1999) 117. Yu, S.Y. and Iglesia, E., J. Phys. Chem., manuscript in preparation. Iglesia, E. and Baumgartner, J.E., Catal. Lea., 21 (1993) 55. Li, W., Yu, S.Y., Meitzner, G.D., Yu, G. and Iglesia, E., J. Phys. Chem., manuscript in preparation. Biscardi, J.A., Meitzner, G.D., and Iglesia, E., J. Catal., 179 (1998) 192. Borry, R.W., Kim, Y.-H., Huffsmith, A., Reimer, J.A., and Iglesia, E., J. Phys. Chem. B, 103, (1999) 5787. Saintigny, X., van Santen, R.A., Clemendot, S., and Hutschka, F., J. Catal., 183 (1999) 107. Benders, P.H., Reinhoudt, D.N., and Trompenaars, W.P., "Cycloaddition Reactions of Thiophenes, Thiophene 1-Oxides, and 1, 1-Dioxides." Thiophene and Its Derivatives, ed. S. Gronowitz, Wiley, New York, 1985. Yu, S.Y., Li, W., and Iglesia, E., J. Catal., 187 (1999) 257. Garcia, C.L. and Lercher, J.A., J. Phys. Chem., 96 (1992) 2669. Weitkamp, J., Schwark, M., and Ernst, S., J. Chem. Soc., Chem. Comm., (1991) 1133. Meitzner, G.D., Iglesia, E., Baumgartner, J.E., and Huang, E.S., J. Catal., 140 (1993) 209.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Water vapor tolerance of the Pd/HZSM-5 in the NO-methane-O2 reaction: its relation with very strong solid acidity of zeolite support Jiro Amano, Osamu Shoji, Kazu Okumura, and Miki Niwa 1 Department of Materials Science, Faculty of Engineering, Tottori University Koyama, Tottori 680-8552 Japan Pd/HZSM-5 with very strong acid site due to the extra-framework AI has the high water vapor tolerance in the NO-methane-O2 reaction. The very strong solid acid site is required to prevent the active species of highly dispersed Pd-O from agglomeration into the large particle. I. INTRODUCTION Methane is the most preferable reductant, because it is contained in natural gas. However, catalyst with a high activity is required for the reduction of NO with methane due to the low activity of methane. From this viewpoint, much attention has been paid to the catalysis of Pd loaded on the zeolite ZSM-5 and mordenite, because these are active and selective for the reduction of NO with methane in the presence of 02, as first reported by Nishizaka and Misono [1,2]. Acid site of support zeolite is inevitable for the selective reduction of NO on the Pd catalyst, and without the acidity, only the combustion of methane occurs. The catalysis of the acid site in the reaction is however not clarified, and a possible role of the acid site as for oxidation of NO was reported [2]. Another problem to the catalytic system is the loss of activity by water vapor, although the Pd/zeolite has the relatively high stability against the water vapor. In general, enhancement of the water vapor tolerance is important for development of the environmental catalyst, because it will be used at the atmospheric conditions. We have already proposed that the chemical vapor deposition of silica on the external surface on mordenite [3] and ZSM-5 [4] is effective in obtaining the durable activity in the presence of water vapor. Silica deposited on the external surface of zeolite suppressed the sintering of Pd: i.e., the deposited silica not only affords the hydrophobicity, but also retards the stabilization of Pd large particles on the external surface. Rh- [5] or Co [6] -modified Pd/zeolite were reported for the same strategy, but these modifiers were added in order to enhance the oxidation of NO into NO2. In this investigation, various ZSM-5 zeolites with 25 of Si/AI2 molar ratio were synthesized under the different conditions for synthesis, and the proton-type ZSM-5 were used as a support of Pd. The activity of NO-CH4-O2 was measured on these Pd/HZSM-5 with and without added water vapor, and the dependence of the activity and stability upon the characterized property, particularly the solid acidity will be studied.

l Corresponding author: e-mail, [email protected]

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Table 1. Preparation conditions and fundamental data of the selected ZSM-5 species No 1-120/ Na § Time Rotate Size Si/Al2 Pd SiO 2 mol h rpm ~tm wt % 0 Provided by Tosoh 1.4, 0.7 23.8 0.20 1 50 0.095 120 500 0.3 23.2 0.34 3 60 0.07 168 100 0.5 26.3 0.29 4 60 0.05 168 100 0.6 23.9 0.02 6 80 0.07 168 100 0.36 25.1 0.20 8 120 0.046 168 100 2.3, 1.1 23.4 0.28 12 60 0.09 168 100 0.6 23.2 0.31

2. EXPERIMENTAL Zeolites ZSM-5 were prepared under various hydrothermal conditions. Colloidal silica Ludox HS-40, A12(SO4)3, and template molecule tetrapropylammonium bromide were used, and the molar ratio of Si/AI2 was adjusted to 25; on the synthesis, such conditions as temperature, H20/SiO2, and rotate rate were varied. Six species of them were identified as the highly crystallized ZSM-5, and these and one more sample supplied from Tosoh Co (named, No. 0) were used. After it was calcined to remove the template molecule, the sample was ion-exchanged with NH4NO3, and calcined to form the proton-type ZSM-5. [Pd(NH3)4] 2+ cation was then exchanged. Finally, the sample was calcined in a N2 flow at 773 K to remove ammonia. Preparation conditions and fundamental data of the selected fine species were shown in Tab. 1. The catalytic reaction of NO-CH4-O2 (NO and CH4, 1000 ppm, 02, 1%, and He balance) with 0 or 10 % H20 was performed at 473 - 823 K, and the outlet gas was analyzed by GC and NOx meter. Temperature programmed desorption (TPD)of ammonia was measured using all-glass apparatus with a mass spectrometer detector. After ammonia adsorption, water vapor was admitted at 373 K, and water was adsorbed in place of removable ammonia. After the water vapor treatment, the sample bed temperature was raised from 373 K in a ramp rate of 10 Kmin ~. Desorbed ammonia was detected using 16 of m/e. Detailed method was described in our review [7]. Pd K-edge EXAFS was measured at BL01B1 station of Japan Synchrotron Radiation Research Institute (SPring-8). The storage ring was operated at 8 GeV with a ring current of 44-65 mA. The sample was transferred to glass cells with two Kapton windows connected to a flow reaction system without contacting air. For the curve fitting analysis, the empirical phase shift and amplitude functions for Pd-O and Pd-Pd were extracted from the data for PdO and Pd foil, respectively. 3. RESULTS AND DISCUSSION 3.1. Reduction of NO The activity of the catalysts in the reaction of N O - C H 4 - O 2 w a s measured, as shown in Fig. 1. The reduction activity showed the maximum against the reaction temperature at 423 - 473 K. N2 and CO2 were products all over the catalysts, and neither N20 nor CO was formed. The degree of conversion of NO were different from species to species, and the sequence of the catalyst with respect to the reduction activity was observed, i.e., No. 3 > Nos. 0, 12 > Nos. 6, 8 > No. 1.

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9

8C QNo.1 ANo.3 iNo.6 <>No.8 ANo.12

-

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0~ O

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ANo.3

iNo.6 ~No.8

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/ f' _.~ \. ///~ \

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o 4C

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-

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500

7t~0 ' 81~0 Temp/K

Fig. 1. Activity in NO-CH4-O 2 reaction.

Temp/K Fig. 2. Activity in NO-CH4-O2-H20 reaction.

No. 4 showed the least activity due to the low Pd loading, and this catalyst was skipped in following experiments. On the other hand, the addition of 10 % water vapor to the reaction system resulted in the decrease of activity, and the different sequence of the activity was obtained for the reaction of NO-CH4-OE-H20, as shown in Fig. 2. No. 3 > No. 0 > No. 8 > No. 6 > No. 1 > No. 12. One can identify, from the comparison between them, that the loss of activity due to the water vapor was outstanding on the samples No. 12 and 6. To the contrary, the loss of activity of Nos. 3 and 0 was small; and, these catalysts showed the highest reduction activity in the presence of water. In our previous paper [4], two kinds of deactivation behavior due to the added water were found, i.e., reversible and irreversible decrease in the catalyst activity. From the study of transient experiment, the irreversible change of activity was outstanding on the sample No. 12. Therefore, the loss of activity due to the added water vapor was caused by the irreversible change of activity, and this was the serious problem encountered on this catalytic system. 3.2. C h a r a c t e r i z a t i o n a n d correlation with the activity

These catalysts were characterized by SEM, infrared spectroscopy (OH profile), and N 2 adsorption. The size of zeolite crystal was 0.3 - 2.3 ktm, as shown in Table 1. Infrared spectra showed the usual acidic hydroxide and isolated silanol at 3600 and 3745 cm ~, respectively. Nitrogen adsorption showed the typical type I isotherm with adsorbed amount of ca. 85 to 110 cmS/g at p/p0, 0.5. These characterization data were not related with the reduction activity shown above. Acidity of these Pd loaded catalysts was then characterized by TPD of ammonia. In the present study, the water vapor treatment was performed after the adsorption of ammonia, and the low temperature peak (/-peak) which was weakly adsorbed ammonia was completely removed. Because only ammonia molecule adsorbed on acid site is desorbed after the water vapor treatment, the number of acid sites is measured quantitatively. Even with this technique, however, two desorption peaks were observed in the TPD of ammonia, as shown in Fig. 3. One was a typical desorption peak of HZSM-5 at about 600 K, and ascribable to the

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0.01

g

z

0.

Temp/K

Fig. 3.

TPD of ammonia from various Pd-loaded zeolites

acid site due to framework AI (h-peak). One more desorption peak at about 800 K was identified as the very strong acid site due to the non-framework AI in combined with the framework AI (h+-peak) [8]. A weak shoulder at 480 K was observed on a few catalysts, however, the assignment of the weak peak was ambiguous. Distribution of the two desorption peaks depended on the Table 2 sample, as shown in Fig. 3. Such a difference in the Total amount of acidity distribution was found to be closely related with the desorbed NH~ activity and the decay due to the added water vapor. The No Acid site samples No. 12 and 6 on which the activity was most mol/kg severely suppressed by the water vapor were characterized to 0a) 0.44 have the high concentration of acid site of h-peak. On the 1~) 0.39 other hand, the samples Nos. 3 and 0 with a small loss of 3~) 0.47 activity showed the high intensity of h+-peak. Thus, a 6~) 0.75 strong relation was observed between the loss of activity due 8~) 0.57 to the added water and the distribution of acid sites. The 12a) 0.63 HZSM-5 with the usual acid site of the framework AI is 0b) 0.48 therefore not enough for the support of Pd. The very strong a)Pd/HZSM-5 acid site of h+-peak is required to obtain the active and b) HZSM-5 durable catalyst. The strengths of acid sites of h- and h+-peaks were calculated to be 130-135 and 180-190 kJ/mol, respectively, based on the theoretical equation derived by us [7]. Total desorbed amount of ammonia was small on the samples with the high intensity of h+-peak, as shown in Tab. 2. This can be explained by the stoichiometry that two A1 cations are required to form the very strong acid site. 3.3. Change of Pd and acid site due to the water added Furthermore, we characterized the change of activity of Pd/HZSM-5 using EXAFS and TPD. Fourier transforms of the k3~(k) EXAFS of Pd on HZSM-5 (No. 0) were shown in Fig.

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4. Atter calcination in N 2 at 773 K, only Pd metal was observed at 0.26 nm / ' ~ Pd-Pd(PdO)z.a ~ (Fig. 4-a). Pd amine complex was, therefore, so easily reduced by the high 40~,u) temperature calcination in N2. _ (c) However, the Pd metal was completely 30 oxidized upon oxidation at 773 K. After the oxidation (Fig. 4-b), Pd-O was ~ ' (b) 20 observed at the distance of 0.16 nm with few of neighbor Pd atoms. Therefore, the PdO species was dispersed so well in 10 al) the zeolite framework. Because the PdO could be regarded as a basic metal oxide [9], the PdO could be interacted 0 1 2 3 4 5 6 with acid sites. The difference in Distance/0.1 i n chemical properties is a driving force to keep the highly dispersed PdO. One Fig. 4. EXAFS of Pd on HZSM-5 (No.0) taken can indicate the interaction with the PdO atter: (a), calc in N2, (b), oxidized (c); used for species as the important role of the solid NO-CH4-O2; (d), used for NO-CH4-O2-H20. acid site in the present catalyst. The observed EXAFS of PdO could not be interpreted with the exchanged Pd cation in the zeolite as reported previously [ 10,11 ], because the distance of Pd-O was in good agreement with that of a standard PdO, and coordination number of oxygen around the Pd was indeed 4. These parameters obtained from EXAFS measurement are elucidated with the structure of dispersed Pd-O species in the zeolite framework [ 12]. Atter used for the reaction of NO-CH4-O2 for 3 h, it seemed that the coordination number of Pd decreased somewhat (Fig. 4-c). However, the Pd-Pd bond appeared, atter it was used for the reaction of NO-CH4-O2-H20 (Fig. 4-d). Therefore, the PdO was agglomerated into 50

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TPD of ammonia on samples No. 0 and 12 before and after the reaction.

1000

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the large particle after the reaction with 1-120. In other words, the sintering of PdO occurred during the reaction with added water, thus resulting in the irreversible deactivation of the catalyst activity. Recent study using Raman spectroscopy [13] also showed the agglomeration of PdO during the reaction with water, which was in consistent with the present observation. On the other hand, TPD of ammonia was measured atter the reaction, and compared with that before the reaction. As shown in Fig. 5, the TPD of sample No. 12 changed drastically atter the reaction. On the other hand, the change of TPD was not significant on the sample No. 0. The decrease of h-peak intensity on the No. 12 was obvious, whereas that of h+-peak was not outstanding. Change of solid acidity suggests the restructuring of aluminum cation in the zeolite. The decrease in acid site during the reaction seems to indicate the dealumination of aluminum in the framework. It could be therefore indicated that the stronger acid site of h+-peak is stabilized more than that of usual h-peak. Usual acid site of H-ZSM-5 is not sufficient to stabilize the PdO highly dispersed and active for the reduction of NO. In conclusion, very strong acid sites due to the non-framework AI are required to prevent the active PdO from sintering under the humidified conditions. REFERENCES

1. Y. Nishizaka and M. Misono, Chem. Lett. (1993) 1295. 2. M. Misono, Catteeh, 2 (1998) 183. 3. M. Suzuki and M. Niwa, Chem. Lett. (1996) 275. 4. M. Suzuki, J. Amano, and M. Niwa, Mieroporous Mesoporous Mater., 21 (1999) 541. 5. M. Misono, Y. Nishizaka, M. Kawamoto, and H. Kato, Stud. Surf. Sei. Catal., 105 (1997) 1501. 6. M. Ogura, Y. Sugiura, M. Hayashi, and E. Kikuehi, Catal. Lett., 42 (1996) 185. 7. M. Niwa, and N. Katada, Catal. Surveys Jpn., 1 (1997) 215. 8. T. Kunieda, N. Katada, and M. Niwa, "Proceedings of the 12th International Zeolite Conference", Vol. 4, M. M. J. Treaey, B. K. Marcus, M. E. Bisher and J. B. Higgins, Eds., Materials Research Society, Warrendale, p 2549 (1999). 9. R. T. Sanderson, "InorganicChemistry"; Reinhold Publishing Corporation: New York, 1967. 10. A. Ali, W. Alvarez, C. J. Loughran, and D. E. Resaseo, Appl. Catal., B: Environmental, 14 (1997) 13. 11. A. W. Aylor, L. J. Lobree, J. A. Reimer, and A. T. Bell, J. Catal., 172 (1997) 453. 12. K. Okumura, J. Amano, and M. Niwa, manuscript in preparation. 13. H. Ohtsuka and T. Tabata, Appl. Catal. B: Environmental, 21 (1999) 133.

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个

911

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Simultaneous NO and N20 Decomposition on Cu-ZSM5 R. Pirone a, P. Ciambelli b, B. Palellac and G. Russo a aIstituto di ricerche sulla combustione, CNR, P.le Tecchio 80, 80125 Napoli (Italy) bDipartimento di ingegneria chimica e alimentare, Universit~ di Salerno, via Ponte Don Melillo, 84084 Fisciano, SA (Italy) CDipartimento di ingegneria chimica, Universi~ di Napoli "Federico II", P.le Tecchio 80, 80125 Napoli (Italy) The mutual interaction between NO and N20 on Cu-ZSM5 has been studied in experimental conditions in which both decomposition reactions occur. The oscillating behaviour observed in N20 decomposition is strongly influenced by the presence of NO, as the latter quenches the oscillations and stabilises the highest active state of the catalyst. A reaction mechanism is proposed to explain the experimental observations, based on the assumption that NO reduces the catalyst sites from Cu +2to Cu +, via the formation of adsorbed species and gaseous NO2. 1. INTRODUCTION Nitrogen oxides, mainly produced in combustion processes, are very dangerous pollutant gases for the environment. One of the most attractive ways to solve the problem of NOx atmospheric pollution is their catalytic decomposition to molecular nitrogen and oxygen. CuZSM5 is a very active catalyst in both NO and N20 decomposition [ 1,2]. Many efforts have been devoted to the study of each decomposition reaction on this catalyst [1,2], but at the moment few investigations exist on their simultaneous proceeding on Cu-ZSM5 [2-7] and report contrasting results. Kapteijn et al. [2] showed that the presence of NO does not affect N20 decomposition on Cu-ZSMS, although it is able to increase N20 consumption rate on Co- and Fe-ZSM5 catalysts via the formation of NO2 and N2 according to the following stoichiometry: NO + N20 ~

NO2 + N2

(1)

On the other hand, very different results have been reported by Lintz and Turek [4] and Turek [5]. They found an oscillating behaviour of N20 decomposition rate on Cu-ZSMS, quenched by the presence of 20 ppm of NO. The activity of the catalyst towards the decomposition of nitrous oxide was also found to be increased by the presence of NO, due to the stabilisation of the most active state of catalyst sites. Moreover, in a further paper, Turek [6] hypothesised the occurrence of the reaction (1) also on Cu-ZSMS, as proposed by Kapteijn for Fe- and Co-ZSM5 [2], in order to explain the high consumption observed for N20.

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We found a different effect of NO, that is a decrease of N20 decomposition rate on CuZSM5 with high Si/A1 ratio (80) as a consequence of NO addition to the feed [3]. Similar results were reported by Mascarenhas and Andrade [7] in the investigation of the effects of several additional gases on the catalytic decomposition of N20 over Cu-ZSM5. We also observed the presence of self-sustained and isothermal oscillations of N20 and 02 outlet concentrations during the decomposition of N20 on over-exchanged Cu-ZSM5, disappearing in the presence of NO [3]. The genesis of the dynamic behaviour was assumed kinetic, i.e. related to a particular reaction mechanism [8]. More specifically, we have proposed that periodic changes of copper oxidation state occur on the catalyst surface, due to the interaction of Cu sites with N20, according to the following reaction steps [8]:

N20 + Cu + --~ [N20"-Cu +2] [N20"-Cu +2] + Cu + ~ [Cu+2-O'2-Cu +2] + N2 N20 + [Cu+2-O'2-Cu +2] "-~ N2 + 02 + 2 Cu + [Cu+2-O'2-Cu +2] ~ Oads + 2 Cu + 20,ds ~ 02

(2) (3) (4) (5) (6)

The reaction mechanism hypothesised by us is based on the existence of two different decomposition steps, i.e. N20 reaction with either Cu § (2) and Cu § sites (4). Moreover, a kinetic model was formulated, involving a heterogeneous description of the step (3) kinetics, in which the activation energy is a linear function of the Cu § molar fraction on the catalyst surface [9]. The kinetic model is able to describe the behaviour of the system in different experimental conditions and, in particular, to reproduce the effect of varying N20 inlet concentration and contact time or adding 02 to the feed [10], but the behaviour of the system in the presence of NO remains unknown. In this paper, we have studied the mutual interaction between NO and N20 when simultaneously fed to a Cu-ZSM5 catalytic bed, in conditions in which both decomposition reactions occur. The aim of the work is to give a further insight into the reaction mechanism of these reactions. 2. EXPERIMENTAL The Cu-ZSM5 catalyst (Si/AI = 25) was prepared by ion-exchanging H-ZSM5 powder (Zeolyst) with copper acetate aqueous solution to 114% exchange (Cu/A1 atomic ratio = 0.57). Catalytic activity tests were carried out with a fixed-bed flow microreactor heated by an electrical oven. The reactor temperature was monitored by a 'K' thermocouple, placed in a quartz tube, coaxial and internal to the reactor. The gas analysis was carried out with continuous analysers (Hartmann & Braunn) to measure the concentrations of NO, NO2, N20 and 02. Brooks mass flow controllers were used to maintain the flow rates of high purity gases: NO (1% vol.) + He, N20 (1% vol.) + He, He (99.995%) and 02 (99.7%), allowing to feed a gas mixture with controlled composition. Before each test, the catalyst has been treated in He (35 N1/h) for 2 h at 550~ to obtain a reduced form of Cu-ZSM5.

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3. RESULTS AND DISCUSSION

The measurements of catalytic activity were carried out at two different temperatures (343 and 394~ by feeding a gas mixture containing NO (0-5000 ppm) and/or N20 (0-2000 ppm) diluted by He. When simultaneously fed, both nitrogen oxides are converted to N2 and 02 at 394~ although a very different catalytic activity for the two decomposition reactions is exhibited by Cu-ZSM5. Fig. 1 shows the influence of the presence of N20 on the conversion of NO to N2 at different values of NO/N20 ratio in the feed.

o,,0. 4( & Z o .~ 3

o

9-

l,

0

500

,

~

,,I,,

IF

1000

1500

2000

N20 inlet concentration, ppm Fig. 1. NO conversion to N2 as a function of N20 inlet concentration at 394~ (a, 5000; b, 2000; c, 1000 ppm), N20 and balance He. W/F = 0.02 g-s-Ncm"3.

Feed: NO

In the absence of N20, NO is decomposed to N2 and 02 at 394~ with a very low conversion, due to the low activity exhibited by Cu-ZSM5 for this reaction in the experimental conditions investigated [ 11]. Furthermore, the co-presence of N20 reduces NO conversion, probably due to the competitive adsorption of the two species. Therefore, it is possible to exclude the occurrence of reaction (1) in these experimental conditions, since the consumption/production ratios between the four species involved are very different from those expected by the stoichiometry of the reaction. In Fig. 2 the conversion of N20 to N2 is plotted as a function of the co-fed NO concentration for different values of N20 inlet concentration, reaction temperature and contact time. In the absence of NO, the decomposition of N20 is a much faster reaction than that of NO and shows an oscillating behaviour, in agreement with previous observations [3-6,8]. The copresence of NO in the feed, even at very low amount, results in two main effects: it (i) quenches the oscillations of both reactant and product concentrations, and (ii) dramatically increases the conversion of N20 to N2. With respect to the first effect, similar results were previously reported by us [3] and Lintz and Turek [4]. They showed that at 450~ the

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oscillations are still observed in the presence of 10 ppm of NO, but not yet at higher NO concentrations, and proposed that NO, even fed in low amount, stabilises the catalyst in its most active state for N20 decomposition. 100

"

// A

80 60 40 0 r~

t

~ w...!

20

I

i

I

I

//

80

0 0

0 Z

C,,1

60 40

~

~--._____

6

20

I

I

I

J

//

0

500

1000

1500

2000

/

/

i

5000

NO inlet concentration, p p m Fig. 2. N20 conversion to N2 as a function of NO inlet concentration at 394~ (A) and 343~ (B). Feed: N20 (O, 1000; El, 2000 ppm), NO and balance He. W/F - 0.02 (A) and 0.073 (B) g.s.Ncm"3. In the absence of NO, the time-averaged conversion has been reported. Closed symbols represent the maximum and minimum values attained in the presence of oscillations. Fig. 2A proves that at 394~ N20 conversion is higher at lower N20 inlet concentration, in agreement with our previous results [8]. Moreover, with both values of N20 concentration in the feed (1000 and 2000 ppm), in the presence of NO, N20 conversion is enhanced above the maximum value attained in the oscillating regime which establishes in the absence of NO. By increasing NO concentration, N20 conversion tends to decrease, but at 5000 NO ppm, it is still higher than in the absence of NO for both values of N20 inlet concentration. A slightly different behaviour is observed at lower temperature. Fig. 2B shows that at 343~ the addition of NO still results in increasing N20 decomposition rate, but less strongly than at 394~ In the presence of 5000 ppm of NO, the conversion of N20 reached a quite similar value to that measured in the absence of NO. An inhibiting effect of NO on N20 decomposition rate, likely due to the different experimental conditions and catalyst involved in the measurements, has been reported by

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Mascarenhas and Andrade [7] and by us [3] at 370~ In particular, in our previous experimental investigation, N20 inlet concentration was 300 ppm, while the range of NO concentration investigated was 500-5000 ppm, i.e. the feed ratio NO/N:O was much higher than that involved in the present investigation and in the results of Turek [4-6]. Owing to these considerations, it is possible that at 370~ the positive effect of the presence of NO is not observable at high NO/N:O ratio, but only the second decreasing part of the conversion plot shown in Fig. 2 is detected. However, the presence of different copper species in the highly overexchanged Cu-ZSM5 catalyst at Si/AI = 80 could also explain the different experimental results obtained. Since contrasting results have been reported on the effect of the co-presence of NO in the feed on N20 decomposition rate, no clear explanations still exist about the phenomena involved when both nitrogen oxides are fed to Cu-ZSM5. Ochs and Turek [12] hypothesise a relation between the genesis of oscillations and the byproduction of NO measured during N20 decomposition above 400~ They propose a reaction mechanism in which N20 can react with either Cu + and Cu+2-O2-Cu +2 sites, as in our mechanism (step 2 and 4) [8], and the product of N20 reaction with the oxidised sites is NO. Consequently, the role hypothesised for NO is to restore catalyst active species Cu+ by reducing the catalyst through the formation of surface nitrate species. Moreover, Turek [6] also proposes that the increase in N20 conversion in the presence of NO is due to the occurrence of the redox reaction (1) between NO and N20, following the experimental results of Kapteijn et al. in similar conditions [2]. We do find the formation of NO2 in our experimental conditions, which increases by increasing NO concentration in the feed, but it is lower than that expected by the stoichiometry of reaction (1). In particular, the excess of N20 converted is much higher than the amount of NO reacted or NO2 produced in all conditions. Actually, we explain the appearance of NO2 in the reaction products as result of NO oxidation with 02 produced by NO and N20 decomposition reactions: 2 NO + 02 ~--- 2 NO2

(7)

Our previous results, which evidenced the high activity of Cu-ZSM5 catalyst in this reaction [11] allow us to conclude that the rate of NO oxidation is so high that the equilibrium condition between NO, 02 and NO2 is reached in these experimental conditions. Due to the proven occurrence of reaction (7) on Cu-ZSM5, we believe that the formation of NO2 could be related to the catalyst reduction, as proposed by Ochs and Turek [12], but through the following simple reaction step: NO + [Cu+2-O'2-Cu+2] ~ NO2 + 2 Cu+

(8)

Our previous results [ 11], proving that Cu-ZSM5 is very active in reaction (7), support the hypothesis that the catalyst is reduced by NO, reaction (8) being a step of NO oxidation to NO2. Furthermore, the above assumption is congruent with the "complex" role played by NO when interacting with the catalyst. Indeed, it is known that in the absence of O2, NO could be strongly adsorbed on both oxidised and reduced sites, and at low temperature oxidises Cu + to Cu +2 [ 13-15]. Finally, by assuming the reducing ability of NO towards Cu +2 sites, the experimental results here presented could be explained as follows:

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i. the stabilising effect of NO on N20 decomposition rate is justified, having attributed the occurrence of oscillations to the capability of N20 to react with either Cu + and Cu +2 sites, but with different rates. In fact, in the presence of NO, copper sites mostly exist as Cu + if reaction (8) rate is very fast. This phenomenon is observed even with very low NO concentrations, when the rate of reaction (8) is much higher than those catalyst of the oxidation steps in the reaction mechanism (2-6), in agreement with our previous results [9-11 ]; ii. the enhancing effect of NO on N20 decomposition rate is likely due to the increased concentration of Cu + sites. This effect is highly effective already at low NO concentration, having assumed a very fast kinetics for reaction (8); iii. at higher concentrations of NO, the inhibiting effect of nitric oxide on N20 decomposition can be expected. In fact, an increase of NO concentration increases the amount of NO adsorbed on the catalyst surface, resulting in the reduction of the fraction of the active sites available for N20 decomposition.

4. CONCLUSIONS Decomposition into elements of N20 is much faster than that of NO over Cu-ZSM5. This difference is dramatically increased when both nitrogen oxides are fed to the catalytic reactor, since the conversion of NO is decreased by the presence of nitrous oxide, while N20 decomposition rate is strongly enhanced by the presence of nitric oxide. The oscillating behaviour shown by N20 decomposition on Cu-ZSM5 is not observable in the presence of NO. Even 50 ppm are enough to quench the oscillations and stabilise the highest active state of the catalyst. These phenomena have been explained by assuming that NO quickly converts the less active Cu § species into reduced copper sites (Cu+), which are the most active for N20 decomposition, through the formation of gaseous NO2. REFERENCES

1. V.I. Pftrvulescu, P. Grange and B.Delmon, Catal. Today, 46 (1998) 233. 2. F. Kapteijn, G. Marb/m, J. Rodriguez-Mirasol and J. A. Moulijn, J. Catal., 167 (1997) 256. 3. P. Ciambelli, E. Garufi, R. Pirone, G. Russo and F. Santagata, Appl. Catal. B, 8 (1996) 333. 4. H.-G. Lintz and T. Turek, Catal. Lea., 30 (1995) 313. 5. T. Turek, Appl. Catal. B, 9 (1996) 201. 6. T. Turek, J. Catal., 174 (1998) 9808. 7. A.J.S. Mascarenhas and H.M.C. Andrade, React. Kinet. Catal. Lett., 64 (1998) 215. 8. P. Ciambelli, A. Di Benedetto, E. Garufi, R. Pirone and G. Russo, J. Catal., 175 (1998) 161. 9. P. Ciambelli, A. Di Benedetto, R. Pirone and G. Russo, Chem. Eng. Sci., 54 (1999) 2555. 10. P. Ciambelli, A. Di Benedetto, R. Pirone and G. Russo, Chem. Eng. Sci., 54 (1999) 4521. 11. R. Pirone, P. Ciambelli, G. Moretti and G. Russo, Appl. Catal. B, 8 (1996) 197. 12. T. Ochs and T. Turek, Chem. Eng. Sci., 54 (1999) 4513. 13. R. Pirone, E. Garufi, P. Ciambelli, G. Moretti and G. Russo, Catal. Lett., 43 (1997) 255. 14. E. Giamello, D. Murphy, G. Magnacca, C. Morterra, Y. Shioya, T. Nomura and M. Anpo, J. Catal., 136 (1992) 510. 15. Y. Li and J. N. Armor, Appl. Catal., 76 (1991) L 1.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Catalytic Properties of Silica-Supported Tantalum and Tungsten Hydrides in the Cleavage and Formation of C-C Bonds of Alkanes. O. Maury, L. Lefort, G. Saggio, C. Cop6ret, M. Taoufik, M. Chabanas, J. Thivolle-Cazat* and

J.M. Basset*

Laboratoire de Chimie Organom6tallique de Surface, UMR CNRS-CPE 9986 43 Bd du 11 Novembre 1918, 69616 VILLEURBANNE, C6dex, France

Heterogeneous catalysts are widely used in industrial applications; they are often easily prepared at a low cost and can be conveniently separated from the reaction medium. However it is always difficult to define and control the active site and then to determine the various elementary steps of a catalytic reaction. On the contrary, homogeneous catalysis has been based on the rules of molecular organometallic chemistry, which affords a better understanding of what are the active species and the elementary steps of a process. Surface Organometallic Chemistry (SOMC) is aimed at the preparation of a new type of heterogeneous catalysts for which the concepts and the rules of molecular organometallic chemistry could be transposed. The reaction of molecular complexes with functional groups of a surface affords the formation of highly reactive well-defined supported organometallic species, usually unprecedented in solution. 1. INTRODUCTION Various silica-supported transition metal hydrides of Ti,[ 1] Zr,[2] Hf,[3] Ta,[4] Cr and W[5] have been prepared in the laboratory. In particular, the Ta(-CH2CMe3)3(=CHCMe3) complex reacts with the OH groups of a silica dehydroxylated at 500~ to form a mixture of two surface species: (-Si-O-)xTa(-CH2CMe3)3_x(=CHCMe3) (x=l: =65%; x=2: =35%). Upon treatment under hydrogen at 150~ overnight, these two complexes lead to a surface tantalum(Ill) monohydride (=Si-O-)2TaH, which has been fully characterised by IR spectroscopy, EXAFS, elemental analysis and quantitative chemical reactions.[4] Silicasupported tungsten hydride can be obtained similarly from the reaction of W(-CH2CMe3)3(-CCMe3) with the surface OH groups, followed by a treatment under hydrogen at 150~ overnight.[5] These surface metal hydrides are strongly anchored to the silica surface, coordinatively unsaturated, and highly electron deficient; thus showing unprecedented properties in the electrophilic activation of alkanes either in a stoichiometric or a catalytic way.

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2. EXPERIMENTAL

2.1. Preparation of the silica-supported tantalum hydride (-=Si-O-)2TaH Tris(neopentyl)neopentylidene

tantalum Ta(-CI-I2CMe3)3(=CHCMe3) is prepared

according to the literature description.[6] Silica (Aerosil Degussa 200m2/g) can be used as a compacted powder or pressed into a wafer for IR studies; it is treated under vacuum at 500~ overnight; Ta(-CI-LzCMe3)3(=CHCMe3) is reacted with silica either via sublimation or impregnation from a solution in pentane previously degassed and dried over Na/K; the excess of complex is desorbed via back sublimation or washing in pentane. The grafted complexes (---Si-O-)xTa(-CI-I2CMe3)3_x(=CHCMe3) are then treated under hydrogen (600 torr, 150~ 15h) to form the tantalum hydride (-Si-O-)2TaH.

2.2. Stoichiometric and catalytic experiments (-Si-O-)2TaH can be prepared in situ in IR glass cell on self-supported silica disk, or used as powder in glass reactor. After evacuation, gas reagents are introduced at the desired pressure through a zeolithe-deoxo purifying trapp. Liquid hydrocarbons are degassed and kept on zeolithe in small glass containers; they are transferred as a vapour into reactor. The reactor is then heated at the desired temperature in an oven; during the reaction, aliquots are expended in a small volume, brought to atmospheric pressure and analysed by GC or GC-MS (A1203/KC1 on fused silica column-50m x 0.32mm). 3. RESULTS AND DISCUSSION

3.1. Stoichiometric C-H bond activation of cycloalkanes. The (-Si-O-)2TaH complex can activate a C-H bond of cyclic alkanes (cyclopentane to cyclooctane) at room temperature to stoichiometrically form the corresponding tantalum(m)cycloalkyl species along with the evolution of one equivalent of hydrogen: (-Si-O-)2Ta-H + (cyclo-RH)

> (-Si-O-)2Ta-(cyclo-R) + H 2

(1)

3.2. Metathesis reaction of acyclic alkanes. The surface tantalum and tungsten hydrides catalyze at moderate temperature (25200~ a novel reaction: the metathesis of acyclic alkanes, which transforms an alkane into its higher and lower homologues.[5] In the case of ethane, there is formation of propane and methane in comparable amounts and traces of butane and isobutane (figure 1); this process implies the transfer of a methyl fragment from one molecule of ethane to a second one. A distribution of higher and lower homologues is also obtained from propane, butane, pentane as well as branched alkanes such as isobutane or isopentane according to the general equation: 2 CnH2n+2 > Cn+iH2(n+i)+2 + Cn-iH2(n-i)+2 where i = 1, 2 .... n-l, but with i = 1 generally favoured.

(2)

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Ia-

919

40

Methane

100--

.,.30

0 G

o 30 E Propane

o

80

m,,~

(b)

20 *

(a)

10 ~

E: 20 60 o 10

Butane + Isobutane

o

~

0

24

48

72

96

d 4O

0

24

Time (h)

48

72

96

0 120

Time (h)

Fig. 1. Metathesis of ethane catalyzed by (=Si-O-)2Ta-H (550 Torr, 150~ CEH6]Ta = 500)

Fig.

2.

Degenerated Metathesis of (120 Torr, 150~ laCH3-CHa/Ta- 120).

lacHa-CH3

Mechanistic investigations with 13C monolabelled ethane showed the occurrence of a degenerated process along with the formation of metathesis products;[7] indeed, during the metathesis reaction, production of non- and dilabelled ethane was observed (figure 2) (degenerated metathesis) together with that of methane, propane and butanes (productive metathesis). This detectable degenerated metathesis is necessarily accompanied by an undetectable process (fully degenerated metathesis) where two 13C monolabelled ethane molecules exchange their methyl groups with each other to form two different lac monolabelled ethane molecules (scheme 1):

Scheme1 *CHa-CHa *CHa_CHa

*CHa ~ H3

*CHa-CHa

*CHa CH a

CHa-*CHa

Detectable degenerated metathesis

*~H3 ~ H3 CH a *CHa

Fully degenerated metathesis

The total scrambling process of 13C methyl groups can be written as follows (eq. 3) and proves to be at least 5 times faster than the productive metathesis:

4 13CHa-CHa

> CHa-CH3 + 2 13CHa-CHa + 13CHa-13CH3

(3)

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[Ta]s-H CH3-CH 3

Scheme 2

H2

[Ta] s-CH2-CH3 CH 4

CH3-CH 3

A Productive Metathesis

CH3-CH3

CH3-CH2-CH3

[Ta] s- CH 3 CH3- CH3

B

CH3-CH3

Degenerate Metathesis CH3-CH3

CH3-CH 3 [Ta] s- CH 3

Productive and degenerated metathesis both require the cleavage and the formation of a C-C bond. The productive reaction is assumed to involve first a C-H bond activation of ethane to form a surface (-Si-O-)2Ta-Et species similarly to the activation of cycloalkanes. In a second step a new molecule of ethane may react with this surface species to transfer a methyl group affording the liberation of propane and the formation of a (-Si-O-)2Ta-Me species. This last species can be displaced by an incoming molecule of ethane to liberate methane and regenerate the (-Si-O-)2Ta-Et species (scheme 2 A); however, the (-Si-O-)2Ta-Me species which is very similar to the (-Si-O-)2Ta-Et species can also be involved in a C-C bond cleavage process, thus giving rise to the degenerate metathesis (scheme 2 B).

3.3. H/D exchange reaction on (-=Si-O-)2TaH. Since a scrambling reaction involving C-C bonds has been evidenced during the metathesis reaction, it was interesting to verify if a similar process could occur in the case of C-H bonds. The study was performed with methane in order to avoid a metathesis reaction of C-C bonds. Different mixtures of C I ~ C D 4 were heated with (-Si-O-)2TaH at 150~ and the formation of the different isotopomers of methane (dl, d2, da) was monitored by IR spectroscopy (figure 3); GC/MS was also used at the end of the reaction to confirm the composition of the gas mixture. Formation of CHaD and CHD3 was first observed before that of CH2D2 suggesting that the H/D exchange proceeds stepwise with only one H or D atom exchanging at a time. After about 10 h, a new stable composition was obtained corresponding

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to the statistical distribution given by the coefficients of the polynom (do + d4)4. This exchange process proves to be approximately 10 times faster than the productive metathesis. , cx 4

6O

= CH3D

A

w I_ G

50

E

40 -=

o

30

0

" 0

9 CH2D 2 x CHD 3 x CD 4

O o

z

o

(4

|

.

c

=:9 q)

0

=

10 0

II -

0

x

0

|

.

9

9 I

200

I

400

I

600

I

800

I

I

1000

1200

I

1400

T i m e (min)

Fig. 3. Evolution of the different isotopomers of methane during the heating of a 53/47 mixture of CH4/CD4 with (-Si-O-)2TaH at 150~ An H/D exchange reaction can also be performed with a CD4/I-I2 mixture to produce a similar distribution. During the reaction with CH4/CD4 at 150~ the IR spectra of the catalyst showed the decrease of the v(Ta-H) band at 1830 cm -~ for the benefit of a new band at 1323 cm -1 consistent with a v(Ta-D) vibration mode. No band for the methyl groups is observed, but the IR intensity of such a band is usually very weak. Three different mechanisms can be proposed for the H/D exchange between CH4 and CD4 depending on whether (-Si-O-)2TaH or (-Si-O-)2TaMe or both species are the active intermediates. 3.4. Hydrogenolysis of alkanes. Surface tantalum and tungsten hydrides also catalyze the hydrogenolysis of light alkanes. Our laboratory has previously reported that silica-supported group 4 metal hydrides catalyze the hydrogenolysis of neopentane, butane, isobutane, propane into a final mixture of ethane and methane; ethane was not cleaved by these metal hydrides. The mechanistic pathway which fits nicely with these observations is based on the key step of 13-alkyl tranfer, which corresponds to the microreverse process of a double C=C bond insertion into a metalcarbon bond: R R \

\1M

\ R'

-

--

- - R'

This key step may logically occur on a metal-alkyl species after a C-H bond activation of the alkane by any of these metal hydrides; since a metal-ethyl species arising from the activation

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of ethane does not contain any alkyl group in 13-position, no C-C bond cleavage can occur. In the opposite, tantalum and tungsten hydrides catalyze the hydrogenolysis of the abovementioned alkanes and also ethane; a [3-alkyl tranfer process can still be invoked to explain the cleavage of all the alkanes except ethane for which another mechanism has to be proposed.

4. CONCLUSION Surface tantalum and tungsten hydrides present unsual properties in the activation of alkanes. The metathesis reaction of alkanes is an unprecedented process affording the formation of higher and lower homologues; it is accompanied by degenerated processes leading to the scrambling of carbon and hydrogen atoms in the molecules; these hydrides also catalyse the hydrogenolysis of ethane. None of these two reactions is catalysed by the group 4 metal hydrides; this tremendous difference can be related to the electronic structure of these surface hydrides which is do for group 4 metals and d2 for tantalum or tungsten.

REFERENCES 1. C. Rosier, G.P. Niccolai and J.M. Basset, J. Am. Chem. Soc. 119 (1997) 12408. 2. J. Corker, F. Lef~bvre, C. IAcuyer, V. Dufaud, F. Quignard, A. Choplin, J. Evans and J.M. Basset, Science 271 (1996) 966. 3. L. D'Ornelas, S. Reyes, F. Quignard, A. Choplin and J.M. Basset, Chem. Lett. (1993) 1931. 4. V. Vidal, A. Th6olier, J. Thivolle-Cazat, J.M. Basset and J. Corker, J. Am. Chem. Soc. 118 (1996) 4595. 5. V. Vidal, A. Th6olier, J. Thivolle-Cazat and J.M. Basset, Science 276 (1997) 99. 6. R.R. Schrock and J.D. Fellmann, J. Am. Chem. Soc. 100 (1978) 3359. 7. O. Maury, L. Lefort, V. Vidal, J. Thivolle-Cazat and J.M. Basset, Angew. Chem. Int. Ed. 38 (1999) 1952.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Characteristics of ethylene polymerization catalyzed over Ziegler-Natta/ Metallocene hybrid catalysts: Comparison between silica-based and magnesium-based supports Han Seock Cho a, Dae Jung Choi a, In Kyu Song b, and Wha Young Lee a* a Division of Chemical Engineering, College of Engineering, Seoul National University, Shinlim-Dong, Kwanak-Ku, Seoul 151-742, Korea* b Department of Industrial Chemistry, Kangnung National University, Kangnung, Kangwondo 210-702, Korea Two types of inorganic supports, silica-based and magnesium-based, were prepared by the sol-gel and the recrystallization method, respectively. The polyethylene produced by the Ziegler-Natta/Metallocene hybrid catalysts showed two melting temperatures and a bimodal MWD, corresponding to products arising from each of the catalysts. This suggests that the hybrid catalysts acted as individual active species and produced a blend of polymers. 1. INTRODUCTION Metallocene catalyst systems hold great promise as the next generation of catalysts for olefin polymerization[l]. Such systems show a high activity and are capable of producing polymers which possess special properties. Although metallocene catalysts have the advantages of high activity and the capability to produce polymers with special properties, the polymers produced via these catalysts have a very narrow molecular weight distribution (MWD) which affects the processability of polymers. In polymer processing, molecular weight (Mw) and MWD are important factors, since they play a major role in mechanical and rheological properties, respectively. On the one hand, polymers with a narrow MWD lead to products with high impact resistance and a higher resistance to environmental stress-cracking, while, on the other hand, polymers with a broad MWD show greater melt flowability at high shear rates. These properties are important for blowing and extrusion techniques[2]. The narrow MWD of polymers produced via metallocene catalysts can be broadened by preparing Ziegler-Natta/Metallocene hybrid catalysts because of the relatively higher Mw of polymers

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derived from Ziegler-Natta component. Such hybrid catalysts can take advantage of the

properties of both metallocene and Ziegler-Natta catalysts. In addition, hybrid catalysts would be expected to enhance polymer processability and to be utilized in prevailing processes without significant process modifications. In this study, two types of supports, silica-based and magnesium-based, were prepared for the impregnation of hybrid catalysts, and ethylene polymerization was carried out to investigate the characteristics of the hybrid catalysts. 2. EXPERIMENTAL

2,1. Materials Toluene and ethylene were further purified by usual procedures. CpEZrCI2 (Strem Chem), TiCl4 (Aldrich Chem), MAO (methylaluminoxane, type 4, Akzo Chem), TiBAL (triisobutylaluminum, Aldrich Chem), MgCI2 (Aldrich Chem), colloidal SiO2 (LUDOX HS-40, Dupont), CH3OH (Farmitalia Carlo Erba) were used without further purification. 2.2. Preparation of magnesium-based support and catalysts 0.10 mol of MgCI 2 was introduced into a reactor and 100 mL of methanol was then added. 100 mL of n-decane was then added to this solution and the mixture was stirred at 2000 r.p.m. under vacuum at 80 ~ This recrystallized support (MgC12 94CH3OH) was pretreated with MAO according to the methanol content (18mmol MAO/g-Support). 2g of the MAO-treated supports were suspended in 100 mL toluene, and reacted with 0.10 g of Cp2ZrCI2 at 50 ~ for 2 h (Cp2ZrCIE/MAO/MgC12, Zr:0.14 wt%). 3mL of TIC14 was introduced into this supported catalyst and the mixture was stirred at 70 ~ for 2 h, and then washed (TiCI4/Cp2ZrCIE/MAO/ MgC12, Zr:0.12, Ti:2.30 wt%). The support was reacted with 3 mL of TiCl4 at 70 ~ for 2 h and then washed (TiC14/MAO/MgC12, Ti:2.78 wt%). 2.3. Preparation of silica-based bisupports and catalysts 0.035 mol of MgC12 was dissolved in 20 mL of distilled water (pH 6.70), and was then introduced into 1.4 L of corn oil media. It was uniformly dispersed and colloidal

SiO 2 was

then added to initiate gelation. The particles were separated, washed, and dried at 80 ~ for 24 h. The bisupport (MgCI2/SiO2) was suspended in toluene and MAO was added according to the hydroxyl content (10mmol/g-Support). 2g of the MAO-treated bisupport was suspended in 100mL of toluene and reacted with 0.10g of CpEZrC12 at 50"C for 2 h and then washed (CPEZrCIE/MAO/MgCI2/SiO2, Zr:0.10wt%). This catalyst was reacted with 3mL of TiCl4 at 70~

for 2 h

(TiC14/CpEZrCIE/MAO/MgCI2/SiO2,Zr:0.06,

Ti:0.71wt%). The bisupport was

reacted with 3 mL of TiC14 at 70 ~ for 2 h and then washed (TiCI4/MgCI2/SiO2, Ti:0.70 wt%).

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2.4. Characterization and ethylene polymerization The morphology of the support was observed by SEM (JEOL JSM-840A). The elemental content ratio on the surface of support was measured by SEM-EDS (JEOL JSM-840 A). The surface area and pore volume were determined by N2-BET (Micromeritics ASAP-2000). The Ti and Zr content of the catalysts were determined by ICP (VG PQ2-Turbo, VG elemental). Polymer analysis by DSC (Dupont V 4.0B) was carried out at a heating rate of 10 ~ The Mw and MWD were determined by GPC (PL-210, Polymer Lab. Ltd.). 300 mL of toluene and a cocatalyst were introduced into a 1 L reactor and the reactor was then evacuated to remove N2. Ethylene was fed into the reactor at a constant pressure of 1.3 atm, which contains hydrogen partial pressure of 0.2 atm. The polymerization reaction was initiated by introducing the catalyst suspension into the reactor at 700(]. The polymerization was terminated after 50 min by adding an excess of dilute hydrochloric acid solution, and the resulting polymer was isolated and dried. The polymerization rate was determined from the amount of consumed ethylene, as measured with a mass flowmeter. 3. RESULTS AND DISCUSSION

3.1. Characteristics of the bisupport (MgCI2/SiO2) Fig.1 shows a proposed mechanism for the formation of the bisupport. Particles of the colloidal sol are approximately 12 nm in diameter. These particles contain negative charges on their surfaces, which serve to suppress the gelation of particles. When a MgCl2 solution is added to this stable sol, the dissolved magnesium salt neutralizes the negative charge on the surface of the stable sol and the silica undergoes gelation via dimerization, trimerization and further polymerization. Since the agglomerated bisupport is formed in a dispersed media, the particle growth is limited, resulting in the formation of solid particles within a few minutes. As shown in Fig.2, the bisupport has a spherical morphology and the particle size varies with agitation speed. This morphology and the size of the support are important in heterogeneous catalysis because the produced polymers replicate that of the supported catalyst. Table 1 shows the characteristics of the bisupport. The difference in the relative weight ratio of Mg/Si Cl

Si(

__OH O'N~ I~ 0 jOH HO ..

Stable Sol

~.-"

--~ ( SiO2 MgC,2 ~ O H

Solution

~'~

H IVkJ H 0 ~. 0' 0 " - ~

SiO2 '~

etc.

H O ~

Mg

Fig. 1. Proposed mechanism of the reaction between colloidal Si02 and MgCI2 solution.

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.

.

Table 1. Characteristics of the bisupport with respect to agitation speed Agitation Average Particle Surface Pore Si,Mg Contents Speed Size Area Volume in Bisupport (r.p.m.) (~tm) (m2/g) (cc/~) (wt%) 1000 61.7 138 0.229 Si:43.1 Mg:l.34 2000 39.6 135 0.217 Si:43.9 Mg:l.47 3000 13.5 134 0.238 Si:44.5 Mg:l.39 .

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SurfaceRatio Si: Mg: CI (wt%) 89.1" 1.3:9.6 89.4: 1.7:8.9 87.2: 1.7:11.1 .

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between the bulk and on the surface of the bisupport was about 50 (wt%/wt%), suggesting that magnesium is reasonably well distributed throughout the interior and exterior portions of the bisupports. In addition, the relative mole ratio of Si: Mg: C1 on the surface of the bisupport indicates that hydroxyl groups on the surface of colloidal silica interact with Mg +2 during the formation of the bisupport, thus generating magnesium oxide (-Si-O-Mg-CI).

3.2. Characteristics of the recrystallized MgCI2 In the case of recrystallized MgC12, considerable amount of methanol (ca. 55wt%), which was employed as a solvent, exists. The methanol content of the support is important because the chemical complexes between methanol and alkyl aluminum create the impregnation sites for the metallocene catalyst[2]. The methanol in the support also serves as a deactivation material, that is Lewis base, for the metallocene catalyst, and, therefore, should be eliminated. An alternative method is to heat the support, in order to reduce the methanol content. In this case, however, the methanol serves as a source of methoxy groups, which do not serve as impregnation sites, as shown in equation (1)[3]. Thus, stoichiometric amounts of MAO were reacted with the recrystallized MgC12 to create the impregnation sites. MgC12 * 4CH3OH

~ MgClx(OCH3) v + mHC1 + nCH3OH

(n+m=4, X+Y=2)

A

(a) (b) (c) Fig.2. SEM photographs of the bisupport with respect to the agitation speed: (a) 1000 r.p.m. ; (b) 2000 r.p.m. ; (c) 3000 r.p.m.

(1)

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3.3. Ethylene polymerization over hybrid catalysts Tables 2 and 3 show the results of ethylene polymerization over the metallocene supported catalyst, the hybrid catalysts, and the Ziegler-Natta supported catalyst. The activity decreases, compared between the metallocene supported catalyst and the Ziegler-Natta supported catalyst, due to the relatively low activity characteristics of Ziegler-Natta catalysts. The polymers produced via the hybrid catalysts with MAO cocatalyst showed different patterns for each support. The polymers produced via the hybrid catalysts supported on MgCI2 showed one melting temperature (Tm), which was mainly produced via Ziegler-Natta catalyst. In contrast to this pattern, the polymers via the silica-supported hybrid catalysts showed two Tm. In this case, the metallocene portion is more dominant than that of Ziegler-Natta catalyst, and the bimodality could be adjusted by varying the cocatalyst ratio, TiBAL to MAO. With corresponding to TiCl4,

increasing amounts of TiBAL, the peak intensity around 140~ increased, and the peak around 130~

corresponding to Cp2ZrCI2, decreased, as shown in

Fig.3 (A). This is due to the fact that aluminum alkyls form complexes with the active zirconium, thus reducing the number of active sites[4]. Two characteristic peaks can be clearly observed in Fig.3 (A), suggesting that the polymers are composed of two lamellar structures, each of which are polymerized by one of the catalysts. The variation of GPC profiles is similar to that ofDSC thermograms as shown in Fig.3 (B). The positions of the two peaks via the hybrid catalysts are consistent with that of polymers produced by metallocene and Ziegler-Natta supported catalysts, respectively. It is noteworthy that this variation in modality affects the molecular weight as well as the molecular weight distribution, and can be cotrolled by varying the cocatalyst ratio. 4. CONCLUSIONS Hybrid catalysts appear to be compatible with the supports examined herein, and produce a blend of polymers. The produced polymers showed bimodal patterns in DSC and GPC analyses, suggesting that the hybrid catalysts acted as individual active species on the support. Table 2. Analytical data on polyethylene via various catalysts supported on magnesium-based support Mole ratio Tm Xc b Mw Catalysts Cocatalyst (Al/[Metal]) Activitya (~ (%) (x 105) MWD Cp2ZrCIJMAO/MgC12 .

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MAO

TiCI4/Cp2ZrCI2/ MAO/MgCI2 .

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0.48

3.8

MAO A1/Zr=3000 6.21 137.0 57.2 6.40 "-- M.~t3-- - - -/(1/ZIL--~3()()(~. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.4 137.1 60.2 6.09 TiBAL AI/Ti=25 TiBAL A1/Ti=100 9.1 137.3 59.8 5.88 TiBAL A1/Ti=100 12.5 137.3 61.4 5.45

8.6

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TiCI4/MAO/MgCI2

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A1/Zr=3000

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a Activity: kg-HDPE/g-[Metal].atm.hr,

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38.8 .

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129.5 .

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70.8 .

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7.4

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Crystallinity: Xc(%)= 100(AHm/AHm*); AHm*=282.84J/g.

6.4 5.9

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Table 3. Analytical data on polyethylene via various catalysts supported on silica-based support Catalysts Cp2ZrCIJMAO/

Mg_Cb_/Si9~_

TiCI4/Cp2ZrCI2/ MAO/MgCI2/SiO2

TiCI4/MgCI2/SiO2

Cocatalyst

Mole ratio (Al/[Metal])

Activity~

Tm (~

Xc b (%)

MW (x 10-5)

MWD

MAO

AI/Zr=3000

20.2

131.0

60.1

0.51

2.3

MAO

AI/Zr=3000

4.43

59.6

2.10

3.6

MAO TiBAL MAO TiBAL TiBAL TiBAL

Al/Zr=3000 Al/Ti= 100 Al/Zr=3000 Al/Ti=300 AI/Ti=100 AI/Ti=100

44.3

5.84

8.1

40.4

8.33

42.0

39.8 42.5

7.34 7.60

5.9 6.3

a Activity: kg-HDPE/g-[Metal].atm.hr,

b

3.58 2.92 2.50 1.97

Crystallinity: Xc(%) = 100(AHm/AHm*); AHm*=282.84 J/g.

f A

131.0 137.8 128.3 139.5 127.9 139.5 139.7 139.0

0.8 I1

e

=1

0.6 o .~ 0.4

~ r~

0.2 0.0 100

120

140

160

2.5

3.5

4.5

Temperature (~

5.5

6.5

7.5

logM

(A) (B) Fig. 3. (A) DSC thermograms: (B) GPC profiles; Cp2ZrCIJMAO/MgCI#SiO2: (a) MAO; TiCL/ Cp2ZrCI2/MAO/MgCIJSi02: (b) MAO; (c) MAO & TiBAL (AI/Ti=100); (d) MAO & TiBAL (AI/Ti= 300): (e) TiBAL (AI/Ti=100); TiCL/MAO/MgClJSiO:: (f) TiBAL (AI/Ti=100). ACKNOWLEDGMENT

The authors wish to acknowledge financial support from the Korea Science and Engineering Foundation (KOSEF) and the Korea Institute of Industrial Technology (KITECH). REFERENCES

1. W. Kaminsky and H. Sinn, Adv. Organomet. Chem., 18 (1980) 99. 2. H. S. Cho, J. S. Chung, J. H. Han, and W. Y. Lee, J. Appl. Polym. Sci., 70 (1998) 1707. 3. D. N. T. Magalhaes, O. D. C. Filho, and F. M. B. Coutinho, Eur. Polym. J., 27 (1991) 1093. 4. D. Fischer and R. Mtilhaupt, Makromol. Chem. Makromol. Syrup., 66 (1993) 191.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Ethylene polymerization with zirconocene-MAO supported on molecular sieves Icaro S. Paulino § Antonio Pedro de Oliveira Filho, Jos6 Luis de Souza and Ulf Schuchardt* Instituto de Qufmica, Departamento de Quimica Inorg~nica, UNICAMP P.O. Box. 6154, 13083-970, Campinas, SP- Brazil. The catalytic activity of Cp2ZrC12 supported on three types of molecular sieves (MCM41, VPI-5 and Y Zeolites) was evaluated in the polymerization of ethylene. The supports were dehydrated, pre-treated with methylaluminoxane (MAO), and then reacted with Cp2ZrCI2. After heterogenization of Cp2ZrC12 on the molecular sieves, the MAO concentration could be reduced without significant effect on the catalytic activity. The polymers obtained with the heterogeneous catalysts showed higher melting points and molecular weights, as well as narrower polydispersion than those obtained with the homogeneous catalyst precursor. 1. INTRODUCTION In the presence of methylaluminoxane (MAO), zirconium metallocene dichloride catalyses the polymerization of olefins with high activity [ 1]. However, the industrial use of this catalyst is hampered by the large quantities of MAO needed. Furthermore, it is not possible to promote gas phase polymerization reactions [2]. In order to reduce the amount of MAO used, the immobilization of metallocene on the surface of oxides such as alumina and silica gel has been attempted by Soga et al. [3] and by Sacchi et al [4]. A different approach involves the incorporation of the catalyst and co-catalyst in the channels of molecular sieves. Molecular sieves are promising supports for catalysts because of their large surface area and well defined pore diameter. Furthermore, deactivation of zirconocene by dimerisation is avoided because the narrow channels of the molecular sieves do not allow the formation of dinuclear species. Molecular sieves are available with one, two and three-dimensional structures and different pore sizes. In Zeolite Y the pore diameter is 7.4 .A and the supercage is 13.7/k wide [5]. A1PO-VPI-5 is an aluminophosphate containing onedimensional channels with a diameter of 12.4 ik [6]. MCM-41 is a silicate with hexagonal array of one-dimensional mesopores, whose diameters can be tuned from 20 to 100 A [7]. In this work, the polymerization of ethylene catalyzed by bis(cyclopentadienyl)zirconium(IV) dichloride supported on three molecular sieves (Zeolite Y, A1PO-VPI-5 and MCM-41) was studied. 2. EXPERIMENTAL

2.1. Synthesis of the Catalytic Precursors The zirconocene (Cp2ZrC12) was prepared by reaction of cyclopentadienylsodium with +Correspondingauthor: [email protected]

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zirconium(IV) chloride, in freshly dried tetrahydrofuran [8]. AIPO-VPI-5 was synthesized according to the method described in the literature [9]. MCM-41 was synthesized according to a method developed in our laboratory using sodium metasilicate Na2SiO3 and Aerosil 200 as silica sources. Tetramethylammonium hydroxide (TMAOH) was used as a mineralizer and cetyltrimethyammonium bromide (CTMABr) as a template. The reaction gel was prepared by suspending the silica sources in an aqueous solution of TMAOH (25%) and combining this suspension with CTMABr dissolved in water, obtaining a final gels with the following composition: 1 SiO2 : 0.15 Na20 : 0.08 TMAOH : 0.21 CTMABr : 100 H20. The gel was refluxed in a glass flask for 16 h.. The obtained solid was filtered off, dried at 100~ and finally calcined at 540~ for 1 h under a flow of nitrogen and subsequently for 6 h under a flow of synthetic air. Zeolite Y samples were provided by Degussa and used as received.

2.2. Preparation of the Supported Catalysts The catalysts were prepared by stirring the molecular sieves and MAO (10% in toluene, Akzo Nobel) in toluene for 4 to 20 h, filtering and washing with toluene. The slurry obtained was dried in v a c u u m (10 "2 mmHg) and the resulting solid impregnated with bis(cyclopentadienyl)zirconium(IV) dichloride, under vigorous stirring in toluene. The catalyst was then washed and dried in vacuum. Zr and Al contents of the supported catalyst were determined by inductively coupled plasma (ICP) spectroscopy. 2.3. Polymerization The polymerization experiments were carried out in a 1 L Biachi reactor at 70~ and 2 bar of ethylene using 50-100 mg of catalyst and 0.5-2.0 mL of MAO in 200 mL of toluene. Polymerization was also accomplished in homogeneous phase using 4.8 mmol of Cp2ZrC12, 3.0 mL of MAO and 200 mL of toluene. The polymerization temperatures were varied at 70~ After 30 min, the polymerization reactions were interrupted with the addition of ethanol. The polymers were filtered, washed with ethanol and dried in a stove at 60~ for 4 h. 2.4. Determination of the Melting Point The thermal characteristics of the polymers were examined using a DSC 4 (Perkin Elmer) with a heating rate of 10~ in the range from 50 to 200~ Melting points (m.p.) are related to the 2 nd heating process. 2.5. Determination of the Molecular Weights The molecular weight averages, Mw and Mn, of the polyethylene samples were determined by gel permeation chromatography (GPC), using 1,2,4-trichlorobenzene as a solvent at 140~ in a Waters 150C chromatograph. Viscosity-average molecular weights (Mv) of the polymers were measured in decalin at 135~ using the equation of MarkHouwink [ 10]. 2.6. Analysis of the Polymer Morphology The morphology of the polymers was analyzed by scanning electron microscopy using a JEOL model JSM-T300 microscope, with a tension of 25 kV and amplification of 1500 to 7500 times. Qualitative measures of distribution of the metals in the samples were made with a Noram dispersive energy X-ray spectrometer (EDS), coupled to the electron microscope. Larger particles of the polymer were analyzed with a Micronal-CBA-K optical microscope.

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3. RESULTS AND DISCUSSION The amounts of Zr and AI supported on the molecular sieves were measured by ICPOES. The results are shown in Table 1. Table 1. Zr and A1 contents in the catalysts as determined by ICP-OES. Catalyst A1PO-VPI-5/MAO/Cp2ZrC12 NaY/MAO/CpzZrCI2 MCM-41/MAO/CpzZrCI2

btmol Zr/g cat. 41 65 140

mmol Al/g cat. 2.92 a) 2.95 a) 4.02

AI/Zr 75 45 29

a) Aluminum belonging to MAO only. The amount of Zr and Al supported on the different molecular sieves varied from 41 to 140 ~tmol Zr/g cat. and 2.92 to 4.02 mmol A1/g cat. To determine the amount of aluminum belonging to MAO in zeolite Y and in AIPO-VPI-5, an analysis of the pure molecular sieves was done previously. 0.26 mmol A1/g was found in zeolite Y, and 6.80 mmol Al/g in A1POVPI-5. The largest amount of MAO immobilized in MCM-41 can be attributed to the larger number of hydroxyl groups present in this molecular sieve. This phenomenon was also observed by Tait et al. [ 11 ] in the reaction of MAO with alumina and with silica. They found that alumina adsorbs twice as much MAO as silica does, because of the higher concentration of superficial hydroxyls in alumina. With an increased amount of MAO on the support, it is possible to form a large number of the complex zirconocene/MAO inside the pore system. Furthermore, larger pore diameters would allow zirconocene to diffuse more freely into the molecular sieve.

3.1. Polymerization Reactions Typical results obtained in the polymerization reactions are shown in Table 2. With the heterogenization of CpEZrC12 on the molecular sieves, the amount of MAO used to promote the polymerization could be reduced without loss of activity, since the immobilization of zirconocene on the molecular sieves hampers its deactivation by bimolecular processes [ 12]. Table 2. Typical results obtained in the ethylene polymerization using the catalyst Cp2ZrC12 in homogeneous phase and supported on A1PO-VPI-5, MCM-41 and zeolite y.a) A1/Zrb) Activityc) Prod. a) Mv e) Catalyst NaY/MAO/Cp2ZrC12 423 2300 298 182 VPI-5/MAO/CpzZrC12 1000 2470 201 216 MCM-41/MAO/CpzZrC12 425 2560 704 350 CpzZrClz/MAO 2000 2980 ..... 86 a) The polymerization was carried out at 70~ b) Rate Al/Zr during the polymerization. c) kg PE/molZr.bar.h. d) Productivity- g PE/g catalyst. e) kg/mol. f) polydispersity Index Mw/~n.

Mn e) 269 363 539 126

Mw e) PDi t) m.p.(OC) 553 2.1 139 753 2.1 141 1070 2.0 145 280 2.2 135

2 bar of ethylene, for 1 h.

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The polymers synthesized with the heterogeneous catalysts present a polydispersity similar to that obtained with the homogeneous catalyst. This indicates that zirconocene molecules are homogeneously distributed on the support, forming active sites with similar steric and electronic characteristics. The homogeneous distribution is probably due to the regularity of the pore system in the molecular sieves, which promotes the formation of roughly uniform catalytic sites inside the channels. The opposite is observed when supports with wide distribution of pore size are used, such as silica or alumina. The polydispersity of polyethylene produced with these catalytic systems range between 3 and 5 [ 13]. The heterogeneous catalysts produce polymers having viscosimetric molecular weights between 180 and 350 kg/mol, which are 2-4 times higher than those obtained with the homogeneous system. In a similar fashion, the melting points of the polyethylene obtained with the heterogeneous catalysts were found to be 139-145~ while the homogeneous system produces polyethylene with melting points around 135~ This increase in the melting point is related to the increase in the molar mass of the polymers. MCM-41 proved to be the best support for Cp2ZrC12, showing the highest values for activity and productivity. Additionally, polyethylene obtained with this catalytic system presented better properties such as larger molecular weight and higher melting point.

3.2. Polymer Morphology The morphology of the polyethylene obtained with the catalysts was analyzed by scanning electron microscopy and optical microscopy. Figure 1 shows micrographs of the polymers and of the respective supports.

. . . . . . . . . . . . . .iI!. . . . .

9

. .i!.::,,,,. iL

!

................ ..L..s ................................................................ (a~

(b)

(c)

(d) (r (f) Figure 1. Scanning electronic micrographs of the supports: MCM-41 (a), Zeolite Y (b) e A1PO-VPI-5 (c). Optical micrograph of the polyethylene obtained with the catalysts MCM41//MAO/Cp2ZrC12 (d), NaY/MAO/Cp2ZrC12 (e) e VPI-5/MAO/Cp2ZrC12 (f).

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One of the most important characteristics of olefin polymerization using heterogeneous catalysts is the replication of the polymer particles following the original morphology of the particles of the support. The replication rate (rate between the medium diameter of polymer particles and the medium particle diameter of the catalyst) for third generation Ziegler-Natta catalysts varies between 40 and 50 [ 14]. With the catalysts supported on the molecular sieves studied in this work, it was possible to obtain polymers with up to 4 mm diameter and replication rates equal to 600, which are much higher than those obtained with the conventional Ziegler-Natta catalysts. The polymers obtained with the heterogeneous catalysts present a granulated form, unlike those obtained with the homogeneous catalyst, which produces powdered polyethylene. The granulated morphology is of great industrial interest because it facilitates the processing of the material. Assuming that the polymerization reactions occur inside the pores of the supports, the particles of the polymer formed will fill these spaces gradually. The tension inside the support increases due to the accumulation of polymeric chains in the pores, eventually causing the structure of the catalyst to collapse. Niegisch et al. [ 15] suggested that the fragmentation happens inside due to the build-up of hydraulic force of the pores of the support in the beginning of the polymerization, but it is difficult to examine the remains of the support, since the small fragments of the support remain in the large particles of the polymer. To overcome this problem, the polymerization was interrupted after five minutes, with the objective of obtaining small particles of the polymer that could be analyzed in the electron microscope. When the particles of the polymer were examined by backscattering, fragments of the support A1PO-VPI-5 were found dispersed on the polymer, confirming that the catalyst is broken into fragments during the polymerization. Figure 2a shows the micrograph of a polyethylene particle, where the points in white are fragments of the support. Figure 2b is a amplification of one fragment of the support (detail A). The presence of a fragment of the support was confirmed by the analysis of EDS.

(a)

(b)

Figure 2. Micrography of the Polyethylene (a) and fragments of the support A1PO-VPI-5, detail A (b). 4. CONCLUSION The molecular sieves used in this work proved to be good supports for the catalyst Cp2ZrC12, since they led to catalytic activities similar to those obtained with its homogeneous precursor, using a lower ratio AI/Zr. Additionally, the polyethylene obtained with the

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supported catalysts presented higher melting points and molecular weights, still maintaining a narrow distribution of the molecular weight. With the heterogenization of the zirconocene it was possible to obtain polyethylene granulated, which is not possible with the homogeneous catalysts. A small decrease in the catalytic activity was observed probably because of the difficult access to the active sites, which at least in case of zirconocene seems to be confined in the inner pores of the molecular sieves. Particularly, MCM-41 was shown to be the best support for Cp2ZrCI2, leading to larger activity and higher productivity. Additionally, the polyethylene obtained with this catalytic system presented better properties such as larger molecular weight and higher melting point. ACKNOWLEDGEMENTS The authors would like to thank FAPESP and CNPq for financial support and Petroquimica Uni~o, Degussa AG, Akzo and Condea for supplying the starting materials. REFERENCES

1. H. Sinn and W. Kaminsky, Adv. Organomet. Chem., 99 (1980) 18. 2. S. S. Reddy and S. Sivaram, Prog. Polym. Sci., 20 (1995) 309. 3. K. Soga and M. Kaminaka, Makromol. Chem., 194 (1993) 1745. 4. M. C. Sacchi, D. Zucchi, I. Trito and P. Locatelli, Macromol. Rapid Commun., 16 (1995) 581. 5. M. M. J. Treacy, J.B. Higgins e R. von Ballmoos, Zeolites, 16 (1996) 323. 6. M. E. Davis, C. Saldarriaga e C. Montes, Zeolites, 8 (1988) 362. 7. S. Bis e M. L. Occelli, Catal. Rev., 40 (1998) 329. 8. G. Wilkinson and J.M. Birminghan, J. Am. Chem. Soc., 76 (1954) 4281. 9. M. E. Davis, C. Montes, P. E. Hathaway and J.M. Garces, Stud. Surf. Sci. Catal., 42 (1989) 199. 10. R. B. Seymor and C. E. Carraher Jr., "Polymer Chemistry- An Introduction", Marcel Dekker, New York, 1981. 11. P. J. T. Tait, A. I. Abozeid and A. S. Paghaleh, Proceedings of the Worldwide Metallocene Conference, 1995. 12. W. Kaminsky and F. Renner, Macromol.Chem, Rapid Commun., 14 (1993) 239. 13. M. R. Ribeiro, A. Deffiex and M. F. Portela, Ind. Eng. Chem. Res., 361 (1997) 1224 14. H. F. Mark, N. M. Bikales, C. G. Overberger and G. Menges, Encyclopedia of Polymer Science and Engineering, vol. 13, John Wiley, New York, 1988. 15. W. D. Niegisch, S. T. Crisafulli, T. S Nagel and B. E. Wagner, Macromolecules, 25 (1992) 3910.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

In situ and ex situ study of propylene polymerization with a MgCI2-

supported Ziegler-Natta catalyst V. Oleshkoa, P. Crozier a, R. Cantrell b and A. Westwood ~ aCenter for Solid State Science, Arizona State University, Tempe, AZ 85287-1704, USA. bpolypropylene Catalyst R&D, Unipol | Systems Department, Union Carbide Corporation, Westhollow Tech. Ctr., PO Box 4685, Houston, TX 77210, USA. CAnalytical Technology Section, R&D, Union Carbide Corporation, PO Box 670, Bound Brook, NJ 08805, USA. In situ light and TEM microscopy observations of polypropylene formation over a TiC14MgC12 supported Ziegler-Natta catalyst have been performed by video recording in real time. Polymer formation was confirmed by TG-DTA. The morphologies of the catalyst and polymer particles were characterized by the combination of ex situ light microscopy, SEM/EDX, conventional and high-resolution TEM, and STEM/PEELS/EDX.

1. INTRODUCTION The large-scale production of polyolefins (polyethylene, polypropylene) continually stimulates the development of high activity heterogeneous Ziegler-Natta catalysts with the third and fourth generation systems in production, such as, high surface area defective MgC12 with TiCI4 chemisorbed on to the surface. Currently, Ziegler-Natta polymerization is the only major process for producing highly stereospecific polypropylene. However, in spite of intensive research on the kinetics and mechanisms of the process, the present level of understanding of the catalyst is still incomplete because of its complex composition, comprising multiple components (i.e., TiCI4, AIR3, electron donors) leading to a multitude of local active site environments [ 1]. To our knowledge, this paper presents the first in situ video microscopy study of propylene polymerization over a MgC12-supported Ziegler-Natta catalyst combined with ex situ characterization by light microscopy (LM) and advanced electron microscopy techniques.

2. Experimental 2.1. Catalyst preparation and characteristics The catalyst was prepared in accordance with the procedures outlined in an earlier patent [2]. As prepared, the procatalyst contains 19% magnesium, 2.9% titanium and 12.4% diiso-

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butylphthalate. Catalyst performance was evaluated in a standard one gallon LIPP (liquid propylene polymerization) reactor. Reaction conditions were as follows: Ti - 0.01 mmol, H2 45 mmol, triethylaluminum (AIEt3) - 1.0 mmol, a silane donor- 0.25 mmol, temperature 65~ and duration - one hour. Polymer yield was 28.8 kg polypropylene/g of the catalyst.

I

Color TV-camera VK-C370 i I Video recorder I I IA/I HS-U760 ~ 1_.___[Infini--var

Color monitor

PVM-1351Q a

Lig~V ~V5

~3

~nI-I: V4~

IRP!

Fig. 1. (a) Setup for in situ video recording of catalytic polymerization of propylene: V 1-5 valves, P- manometer, RP - rotary pump; (b, c) instantaneous growth of polymer; arrows indicate growing polymer beads, the two images were recorded within 1/34 s interval; (d) the same area after 29 s; (e) the same area after 129 s. The catalyst specimens were stored in a glove box under a He atmosphere (< lppm H20/O2). All sample preparation for microscopic analysis was conducted in the dry box under dry conditions. Samples were transferred to specimen holders in the dry box and then transferred into the microscopes under high purge N2 conditions to prevent poisoning of the catalysts by air and moisture.

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2.2. In situ video recording of polymerization

A schematic of the experimental setup used for the in situ study of propylene polymerization is shown in Fig. 1. The procatalyst was combined with the co-catalyst, A1Et3, and gas phase catalytic polymerizations were run at 22-25~ in a glass cell with an input of dry propylene at a pressure of~20 psi. Monomer was periodically added to the cell to keep the same reaction conditions. The process was monitored by an InfiniVar-ZOOM video microscope at a magnification of x80-160. Recently direct real-time observations of the polymerization were also performed using a Philips 430 transmission electron microscope (TEM) at 300 kV accelerating voltage fitted with an environmental cell at gas pressures of 0.5-1 torr.

2.3. Ex situ characterization Thermogravimetric and differential thermal analysis (TG-DTA) was carried out on a SETARAM-TG-DTA 92-16 derivatograph with an alumina tube furnace and a graphite heater at a heating rate of 2~ min -1 over the temperature range 20-200~ in an Ar atmosphere (Fig.

2). 40 56t ~ - ~

60

80

100

~

120

140

^ [I

4-1

~

160

200

Mass Loss, mg (1,2) -6.5 Heat Flow, ~tV (3,4)_ f

- -

II

180

~

~

/

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..3~

E 3 o

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o

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9

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60

80

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120

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160

180

200

Temperature, ~

Fig. 2. TG-DTA of the catalyst and polymer in argon. TG, left axis" 1 - heating cycle, 2 cooling cycle; DTA, right axis: 3 - heating cycle, 4 - cooling cycle. LM was performed on a Mitutyo Ultraplan FS-110 video microscope equipped with a long focus lenses in bright- and dark-field transmission, reflection and differential interference contrast modes at working magnifications from x100 to x1000. Images were recorded by a color video TV-camera and digitized on-line. Scanning electron microscopy (SEM) was carried out at 15 kV accelerating voltage on a JEOL JSM-840 scanning electron microscope equipped with an EmiSPEC Vision digital data acquisition system and a Noran energy-dispersive x-ray (EDX) analyzer. For SEM, catalyst powders were deposited onto conductive supports and carbon coated. For TEM, the material was dry crushed in an oxygen-free He atmosphere and deposited onto lacey carbon films. Samples were analyzed using a JEOL-4000EX microscope operating at 400 kV accelerating

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voltage with a point-to-point resolution of 0.17 nm. Scanning transmission electron microscopy (STEM) and compositional nanospectroscopy by parallel electron energy-loss spectroscopy (PEELS) and windowless EDX analysis were performed on a VG HB-501 STEM operating at 100 kV accelerating voltage with resolution of 0.4 nm.

3. RESULTS AND DISCUSSIONS

Figs. l b and 1c present successive images of the reacting catalyst recorded with an interval of 1/34 s, which revealed growing polymer beads 9-10 ~tm in size. Primary beads could then aggregate forming much larger conglomerates of 20-40 ~tm in size. The amount of polymer increased drastically within the localized area during the next 129 s as shown in Figs. 1d and le.

z0~m

...~i . ii :z

Fig.3. Morphology of initial (al, bl, el) and "working" catalyst (a2, b2, c2): al, a 2 - LM, bright-field, transmission mode; b 1, b 2 - SEM, SE; c 1, c2 - SEM, BSE, "compo" mode.

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As a result of the polymerization, the volume of material increased by a factor of three during the reaction. The formation of polypropylene was confirmed by TG-DTA on the appearance of a narrow exothermic peak corresponding to polypropylene melting at 133~ [3] (Fig. 2). Fig. 3 compares the morphology of the initial and "working" catalyst as revealed by LM (al and a2) and by SEM in secondary electrons (SE, b l and b2) and in backscattered electrons (BSE, c l and c2), respectively. The average particle size in the initial catalyst is about 9 lam. Polypropylene growth increases the particle size in the "working" catalyst to 3050 lam. Fig. 3c2 shows 2-6 lam-sized bright features (higher backscattering coefficient) on the surface of large catalyst/polymer particle of 40 ~tm in size suggesting a partial fracturing of the catalyst in the course of polymer growth. The microscopic observations indicate a complicated fragmentation of the catalyst and formation of aggregated hierarchical structures exhibiting a variety of morphologies [4].

Fig.4. Low-magnification TEM (a, b) and HRTEM (c, d) of catalyst (a, c) and polymer/catalyst (b, d) particles. The TEM image and diffraction pattem of the catalyst particles in Fig. 4a shows crystalline domains of 2-5 nm in size and amorphous areas. The combination of ring reflections in the

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diffraction pattern (insert) was assigned to a hexagonal structure of MgCI2. This is in agreement with high-resolution TEM (HRTEM) observations (Fig. 4c) which exhibit partially disordered lattice fringes with spacings of between 0.26-0.31 nm and 0.56-0.66 nm. On the contrary, the diffraction pattern of a supposed polymer particle (Fig. 4b, insert) revealed only an amorphous halo and the HRTEM image in Fig. 4d showed no evidence for crystallinity. As one can see from the images b and d, this large particle consists of many randomly aggregated polymer globules of 20-40 nm in diameter. 150[

CK

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I I

250

300

10001[

-~ 350

Tin23 400

Energy Loss, eV

450

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Ib

,,.o

Fig. 5. (a) PEEL and (b) EDX spectrum of catalyst/polymer particles. Lines of interest are marked. Combined PEELS/windowless EDX analyses in the STEM mode (Fig. 5) confirmed the presence of carbon rich polymer particles. PEELS/EDX data point to that polymer and catalyst are often present in all parts of the analyzed particles. CONCLUSIONS We have observed for the first time in situ gas phase polymerization of propylene over a TiCIa-MgCla supported Ziegler-Natta catalyst by video LM and TEM microscopy. Real time observations show that 9-10 ~tm-sized polymer beads appear to form rapidly (<1/30 s) on randomly distributed active sites following fast formation of a larger amount of polypropylene within localized reaction areas. Catalyst particles initially contain crystalline domains of 2-5 nm in size, which during polymerization are subjected to complicated transformations, including fracturing and fragmentation that form polymer/catalyst aggregated hierarchical structures of 30-50 ~tm in size with varying morphologies. Acknowledgements

The work was supported by the Union Carbide Corporation and the Industrial Associates Program of Arizona State University. References

1. J.J. Dusseault and C.C. Hsu, J. M. S.-Rev. Macromol. Chem. Phys., C33 (1993) 103. 2. R.C. Brady, F.G. Stakem, H.T. Liu and A. Noshay, US Patent 5 093 415 (1992), to Union Carbide Corporation. 3. J. Boor, Ziegler-Natta Catalysts and Polymerizations, Academic Press, NY, 1979, p.50. 4. M.A. Ferrero, E. Koffi, R. Sommer, and W.C. Conner, J. Polym. Chem., A30 (1992) 2131.

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Studies in Surface Science and Catalysis 130- Part B 12th INTERNATIONAL CONGRESS ON CATALYSIS

ICC ~Zeee

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Studies in Surface Science and Catalysis

A d v i s o r y E d i t o r s : B. Delmon and J.T. Yates Vol. 130

12th INTERNATIONAL CONGRESS ON CATALYSIS PART B Proceedings of the 12th ICC, Granada, Spain, July 9-14, 2000 Editors

Avelino Corma Francisco V. Melo

Instituto de Tecnologia Qu/mica, UPV-CSIC, A vda. de los Naranjos s/n, 46022- Valencia, Spain

Sagrario Mendioroz Jos~ Luis G. Fierro

/nstituto de Cat~/isis y Petro/eoqu[mica, CS/C, Campus UAM Cantob/anco, 28049- Madrid, Spain

2000 ELSEVIER Amsterdam - Lausanne - New York - Oxford - Shannon - Singapore - Tokyo

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ELSEVIER SCIENCE B.V. Sara Burgerhartstraat 25 P.O. Box 211, 1000 AE Amsterdam, The Netherlands

9 2000 Elsevier Science B.V. All rights reserved.

This work is protected under copyright by Elsevier Science, and the following terms and conditions apply to its use: Photocopying Single photocopies of single chapters may be made for personal use as allowed by national copyright laws. Permission of the Publisher and payment of a fee is required for all other photocopying, including multiple or systematic copying, copying for advertising or promotional purposes, resale, and all forms of document delivery. Special rates are available for educational institutions that wish to make photocopies for non-profit educational classroom use. Permissions may be sought directly from Elsevier Science Global Rights Department, PO Box 800, Oxford OX5 1DX, UK; phone: (+44) 1865 843830, fax: (+44) 1865 853333, e-mail: [email protected]. You may also contact Global Rights directly through Elsevier's home page (http://www.elsevier.nl), by selecting 'Obtaining Permissions'. In the USA, users may clear permissions and make payments through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA; phone: (978) 7508400, fax: (978) 7504744, and in the UK through the Copyright Licensing Agency Rapid Clearance Service (CLARCS), 90 Tottenham Court Road, London WlP 0LP, UK; phone: (+44) 171 631 5555; fax: (+44) 171 631 5500. Other countries may have a local reprographic rights agency for payments. Derivative Works Tables of contents may be reproduced for internal circulation, but permission of Elsevier Science is required for external resale or distribution of such material. Permission of the Publisher is required for all other derivative works, including compilations and translations. Electronic Storage or Usage Permission of the Publisher is required to store or use electronically any material contained in this work, including any chapter or part of a chapter. Except as outlined above, no part of this work may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without prior written permission of the Publisher. Address permissions requests to: Elsevier Science Global Rights Department, at the mail, fax and e-mail addresses noted above. Notice No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Because of rapid advances in the medical sciences, in particular, independent verification of diagnoses and drug dosages should be made.

First edition 2000 Library of Congress Cataloging in Publication Data A catalog record from the Library of Congress has been applied for.

ISBN:

0 444 50480 X

The paper used in this publication meets the requirements of ANSI/NISO Z39.48-1992 (Permanence of Paper). Printed in The Netherlands.

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PREFACE The twelfth Congress on Catalysis was held in Granada (Spain) under the auspices of the International Association of Catalysis Societies and the Spanish Society of Catalysis. Those Proceedings are the expression of the Scientific Sessions which constituted the main body of the Congress.

They include 5 plenary lectures, 1 award lecture, 8 keynote lectures, 124 oral presentations and 495 posters. The oral and poster contributions have been selected on the basis of the reports of at least two international reviewers, according to standards comparable to those used for specialised journals, among 1045 two-page abstracts received from 53 countries. The submitted camera-ready manuscripts were then evaluated by the International Scientific Board. Fortunately, most of the corrected manuscripts were received in due course and have been included as such in the Proceedings; however, in a few exceptions, no answer was obtained from the authors; in those cases, a first version of the manuscript appears in the Proceedings. In order to accommodate all these contributions, the Congress was divided in four parallel sessions and three additional sessions in which all the posters were displayed. The management of this fantastic volume of work forced us to take several decisions. As the contributions are published prior to the meeting for distribution to all delegates who attend the Congress in Granada, no discussions at the meeting have been included. Besides this, for space reasons we were restricted to expand the works to only six-page text.

Financial contribution from the Ministry of Education and Culture, the National Council of Scientific Research, other local and national institutions or corporations, chemical, refining and petrochemical companies made it possible to balance the budget of the Congress. Allowance for young students to pay a reduced registration fee was also possible from this income.

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We are grateful to the outstanding scientists, expert in different fields of catalysis, who

accepted our invitation to overview vital research areas in plenary lectures, the 1998 awardee by his illustrative conference and the keynote lecturers that introduce the various topics of the sessions covered by the Congress. The Organisers are indebted to all the scientists who accepted our invitation to come to Spain and made this meeting another outstanding success in the 44 year tradition of this event. We hope everybody will enjoy the meeting and will find these Proceedings a useful book to be added to the catalysis library. We are also grateful to Drs. A. Jongejan, managing director, Dr. P.S. Jackson, publishing director, and specially to Drs. Huub Manten of Elsevier Science Publishers for the guidance and co-operation provided in getting these four volumes printed before the Congress.

Granada, July 2000 The Editors

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LIST OF SPONSORS (by May 17, 2000)

PA TR ONS 1- Ministerio de Ciencia y Tecnologia (Spain) 2- Junta de Andalucia (Spain) 3- Ayuntamiento de Granada (Spain) 4- Consejo Superior de Investigaciones Cientificas (Spain) 5- Universidad de Granada (Spain)

DONORS

_

CEPSA (Spain)

7- REPSOL- YPF (Spain) 8- UOP (USA) 9- DEGUSSA HILLS AG (Germany) 10- PROCATALYSE (France) 11- INSTITUT FRAN(~AIS DU PETROLE (France) 12- ENGELHARD (The Netherlands) 13- NOVARTIS AG (Switzerland) 14-SHELL International Chemicals B.V.A. (The Netherlands) 15-BP AMOCO Chemicals Co. (UK) 16- CHEVRON (USA) 17- EXXON-MOBIL Research and Engineering (USA) 18- GRACE Co. (USA) 19- DSM Research (The Netherlands) 20- SI]D-CHEMIE Inc. (USA)

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o,, VIII

EXHIBITORS 21-HALDOR TOPSOE (Denmark) 22-SE Reactor, Inc. (USA) 23- ENGELHARD (Italy) 24-ACADEMIC PRESS, Inc. (USA) 25-VINDUM ENGINEERING, Inc. (USA) 2 6 - P S R - SOTELEM (The Netherlands) 27-SPRINGER-VERLAG Ib&ica S.A. (Spain) 28-CHAMBERS HISPANIA S.L. (Spain) 29-JOHNSON MATTHEY (USA) 30-GENERAL ELECTRIC Plastics S.A. (Spain) 31- HIDEN ANALYTICAL (UK) 32-KAISER Optical Instruments Industries. (France) 33-THERMO QUEST- CE Instruments (Italy) 34-AIR LIQUIDE S.A. (Spain) 35-IBERFLUID Instruments S.A. (Spain) 36-ARGONAUT Technologies AG (Switzerland) 37- IN-SITU Research Instruments (USA) 38-VINCI Technologies (France) 39-MEL Chemicals (UK) 40- ISCOA Inc. (USA) 41- CRI Katalema (UK) 42- PARR Instrument (Germany) 43-ELSEVIER SCIENCE (The Netherlands)

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ix

TABLE OF CONTENTS

PLENARY LECTURES

PL-1. In Situ Characterization of Catalysts H. Topsere .

1

PL-2. Pollution Abatement through Heterogeneous Catalysisfor Preserving Clean Air M.Iwamoto .

23

PL-3. Molecular Design of Heterogeneous Catalysts M.E. Davis .

49

PL-4. Millisecond Chemical Reactions and Reactors L.D. Schmidt .

61

PL-5. Catalysisfor Oil Refining and Petrochemistry, Recent Developments and Future Trends G. Martino .

83

AWARD CONFERENCE New Catalysisfiom Metal Oxide Surface Science M. A. Barteau .

105

KEYNOTE LECTURES KN- 1 From Unit Operations to Elementary Processes: A Molecular and Multidisciplinary Approach to Catalyst Preparation M. Che

115

KN-2 Acidity in Zeolite Catalysis R.A. van Santen and F.J.M.M. de Gauw.

127

.

KN-3 Engineered Solid Catalysisfor Synthesis of Fine Chemicals in Multiphasic Reactant System D.E. De Vos, B.F. Sels, W.M. Van Rhijn and P.A. Jacobs .

137

KN-4 New Solid Acid Based Breakthrough Technologies S.A. Gembicki

147

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KN-5 Application of Titanium Oxide Photocatalysts and Unique Second-Generation Ti02Photocatalysts able to Operate under Visible Light Irradiationfor the Reduction of Environmental Toxins on a Global Scale M.Anpo .

KN-6 Catalysis for Fine Chemicals: an Industrial Perspective P. Mktivier .

KN-7 Oxidative Methanol Reforming Reactionsfor the Production of Hydrogen J.L.G. Fierro . KN-8 Supported Metallocenes: Monosite and Multisite Catalystsfor OIefn Polimerization . F. Ciardelli, A. Altomare and S. Bronco ORAL COMMUNICATIONS Session A Preparation of Catalysts A New Methodfor Preparing Nanometer-size Perovskitic Catalystsfor CH4Flameless Combustion R.A.M. Giacomuzzi, M. Portinari, I. Rossetti and L. Forni .

Experimental Investigation and Modelling of Platinum Adsorption onto Ion-modified Silica and Alumina . W. Spieker, J. Regalbuto, D. Rende, M. Bricker and Q. Chen Nanocrystalline Thin Films as a Model Systemfor Sulfated Zirconia F.C. Jentoff, A. Fischer, G. Weinberg, U. Wild and R. Schlogl . Nanoparticle Arrays as Model Catalysts: Microstructure, Thermal Stability and Reactivity of Pr/Si02 Fabricated by Electron Beam Lithography . G. Rupprechter, A.S. Eppler, A. Avoyan and G.A. Somorjai Epoxidation of Ole$ns on M-SiO2 (M=Ti, Fe, Catalysts with Highly Isolated Transition Metal Ions Prepared by Ion Beam Implantation Q. Yang, C. Li, S. Wang, J. Lu, P. Ying, Q. Xin, W. Shi . Catalysis on Metals and Metal Oxides

Cyclohexane Ring Opening on Metal-Oxide Catalysts L.M. Kustov, T.V. Vasina, O.V. Masloboishchikova, . E.G. Khelkovskaya-Sergeeva and P. Zeuthen

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OleJinsfiom Chlorocarbons: Reactions of l,2-Dichloroethane Catalyzed by Pt-Cu L. Vadlarnannati, D. Luebke, V. Kovalchuk and J.L. d'Itri . Artijkial Control of Selectivity by Dynamic Lattice Displacement of Acoustic Wave Effects: Decomposition and Oxidation of Alcohol on Ag and Pd catalysts N. Saito, H. Nishiyama, K. Sato and Y. Inoue . The Effect of Hydrogen Concentration on Propyne Hydrogenation over a Carbon Supported Palladium Catalyst Studied under Continuous Flow Conditions D. Lemon, R. Marshall, G. Webb and S.D. Jackson Refining and Petrochemistry Suggestion of a New Alternative Mechanism of the Sulfuric Acid Catalyzed Isoparafln-OleJin Alkylation V.B. Kazansky and T.V. Vasina . Hydroisomerization of n-Decane in the Presence of Sulfur and Nitrogen L.B. Galperin . State of Metals in the Supported Bimetallic P ~ - P ~ / s o ~ ~ -System -z~o~ A.V. Ivanov, A.Yu. Stakheev and L.M. Kustov . Dehydroisomerization of n-Butane over Pt Promoted Ga-Substituted Silico-aluminophosphates A. Vieira, M.A. Tovar, C. Pfaff, P. Betancourt, B. Mtndez, C.M. Lbpez, . F.J. Machado, J. Goldwasser and M.M. Rarnirez de Agudelo Pore Mouth Catalysis over Acidic Zeolites. Nature of Active Species P. Magnoux, M. Guisnet and I. Ferino . Rediscovery of the Paring Reaction: The Conversion of l,2,4-TrimethylBenzene over HZSMS at Elevated Temperature H.P. Roger, M. Bohringer, K.P. Moller and C.T. O'Connor SAPO-34 Catalystfor Dimethylether Production Gr. Pop and C. Theodorescu . Catalytic Performances of Pillared Beidellites Compared to Ultrastable Y Zeolites in Hydrocracking and Hydroisornerization Reactions J.A. Martens, E. Benazzi, J. Brendlt, S. Lacombe and R. le Dred .

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xii

Catalyst Characterisation Effect of Ni on K-Doped Molybdenum-on-Carbon Catalysts: TemperatureProgrammed Reduction and Reactivity to Higher-Alcohol Formation E.L. Kugler, L. Feng, X. Li and D.B. Dadybujor . Quantitative Determination of the Number ofActive Surface Sites and the Turnover Frequenciesfor Methanol Oxidation over Metal Oxide Catalysts L.E. Briand and I.E. Wachs . Metal Particles on Oxide Surfaces: Structure and Adsorption Behaviour M. Baumer, M. Frank, P. Kiihnemuth, M. Heemeier, S. Stempel and H.-J. Freund . An Atomic X4FS Study of the Metal-Support Interaction in Pt/Si02-A1203 and Pt/MgO-A1203 Catalysts: An Increase in Ionization Potential of Platinum with Increasing Electronegativity of the Support Oxygen Ions D.C. Konigsberger, M.K. Oudenhuijzen, D.E. Ramaker and J.T. Miller . Transition State and D f i s i o n Controlled Selectivity in Skeletal Isomerization of Olefins L. Domokos, M.C. Paganini, F. Meunier, K. Seshan and J.A. Lercher

.

Development and Application of 3-Dimensional Transmission Electron Microscopy (3D-TEM)for the Characterisation of Metal-Zeolite Catalyst Systems A.J. Koster, U . Ziese, A.J. Verkleij, A.H. Janssen, J. de Graaf, J.W. Geus and K.P. de Jong . Interrogative Kinetic Characterisation of Active Catalyst Sites Using TAP Pulse Experiment J.T. Gleaves, G.S. Yablonsky, S.O. Shekhtman and P. Phanawadee . W Resonance Raman Spectroscopic Identijcation of Transition Metal Ions Incorporated in the Framework of Molecular Sieves G. Xiong, C. Li, Z. Feng, J. Li, P. Ying, H. Li and Q. Xin . Surface Mobility of Oxygen Species on Mixed-Oxides Supported Metals . C. Descorme, Y. Madier, D. Duprez and T. Birchem SOz-Promoted Propane Oxidation over Pt/Alz03 Catalysts A.F. Lee, K. Wilson and R.M. Lambert . Selective Oxidation of Toluene to Benzaldehyde: Investigation of StructureReactivity Relationships by in situ-Methods A. Briickner, U . Bentrup, A. Martin, J. Radnik, L. Wilde and G.-U. Wolf

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xiii

Observation of Unstable Reaction Intermediate by Picosecond Tunable Intared Laser Pulses K. Domen, K. Kusafika, A. Bandara, M. Hara, J.N. Kondo, J. Kubota, K. Onda, A. Wada and C. Hirose . Changes in Morphology of y-Al2O3-Supported Pt Clusters under Reaction Conditions: Evidence @om In-Situ EXAFS Spectroscopy 0. Alexeev and B.C. Gates . In Situ FT-IR Investigation of Hydrocarbon Reactions over Zeolite Based Bifunctional Catalysts M. Hochtl, Ch. Kleber, A. Jentys and H. Vinek . Session B Fischer-Tropsch Synthesis A Steady State Isotopic Transient Kinetic Analysis of the Fischer-Tropsch Synthesis Reaction over a Cobalt-Based Catalyst . H.A.J. van Dikj, J.H.B.J. Hoebink and J.C. Schouten Use of Membranes in Fischer-Tropsch Reactors R.L. Espinoza, E. du Toit, J. Santamaria, M. MenCndez, J. Coronas and S. Irusta . Egg-Shell Catalystfor the Synthesis of Middle Distillates C . Galarraga, E. Peluso and H. de Lasa . Application of Theoretical Methods to Catalysts Computer Aided Design of Novel Heterogeneous Catalysts A Combinatorial Computational Chemistry Approach K. Yajima, Y. Ueda, H. Tsuruya, T. Kanougi, Y. Oumi, S.S.C. Ammal, . S. Takami, M. Kubo and A. Miyarnoto Applications of Densily Functional Theory to Identrh Reaction Pathways for Processes Occuring in Zeolites and on Dispersed Metal Oxides . J.A. Ryder, M.J. Rice, F. Gilardoni, A.K. Chakraborty and A.T. Bell Fuel Cells and Electrocatalysis Electrocatalytic Synthesis of Ammonia at Atmospheric Pressure . G. Marnellos, G. Karagiannakis, S. Zisekas and M. Stoukides Electrocatalysis at a Pt Electrode Surface in a Fuel Cell as Observed In Situpom Pt-H and Pt-0 Shape Resonances and E U F S Scattering in Pt Lr3 XA NES D.E. Ramaker and W.E. O'Grady .

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In situ Catalytic and Electrocatalytic Studies of Internal Reforming in Solid Oxide Fuel Cells Running on Natural Gas . C.M. Finnerty, R.H. Cunningham and R.M. Ormerod Efectof Gd2O3 Doping and SteamKarbon Ratio on the Activity of Catalystfor Internal Steam Reforming in Molten Carbonate Fuel Cell Y.J. Shin, H.-D. Moon, T.-H. Lim and H.-I. Lee . Reactor Engineering and Catalytic Processes

A Microstructured Catalytic Reactor/Heat Exchanger for the Controlled Catalytic Reaction between H2 and 0 2 M. Janicke, A. Holzwarth, M. Fichtner, K. Schubert and F. Schiith Catalytic Water Denitrification in Membrane Reactor O.M. Ilinitch, F.P. Cuperus, L.V. Nosova and E.N. Gribov . Reactivity and Thermal ProJle of Metahne Partial Oxidation ar Very Short Residence Time F. Basile, G. Fomasari, F. TrifirC, and A. Vaccari . Supercritical Synthesis of Dimethyl Carbonate@om C 0 2and Methanol T. Zhao, Y. Han and Y. Sun . A New Catalystfor an Old Process Driven by Environmental Issues L. Abrams, W.V. Cicha, L.E. Manzer and S. Subramoney . Catalysis on Sulfides, Nitrides and Carbides

Mechanism ofHDN over Mo and Nb-Mo Carbide Catalysts V. Schwartz, V.L. Teixeira da Silva, J.G. Chen and S.T. Oyarna . In Situ Characterisation of Transition Metal Suljde Catalysts by IR Probe Molecules Adsorption and Model Reactions G. Berhault, M. Lacroix, M. Breysse, F. MaugC and J.-C. Lavalley Comprehension of the Promoting EfSect in the MCr& Mixed Suljde Catalysts . P. Afanasiev, A. Thiollier, P. Delichere and M. Vrinat Potassium at Catalytic Surfaces - Stability, Electronic Promotion and Excitation A. Kotarba, G. Adamski, Z. Sojka, S. Witowski and G. Djega-Mariadassou

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Catalysis for Fine Chemical Synthesis Heterogeneous Catalysis of Aldolisations on Activated Hydrotalcites J. Lhpez, R. Jacquot and F. Figueras .

49 1

The Influence of Metal-Support Interactions During Liquid-Phase Hydrogenation of an &PUnsaturated Aldehyde over Pt U.K. Singh and M.A. Vannice .

497

Tailoring of Acido-basic Properties and Metallic Function in Catalysts Obtainedfrom LDHs for the Hydrogenation of Nitriles and of ~pUnsaturated Aldehydes D. Tichit, B. Coq, S. Ribet, R. Durand and F. Medina . Chemical versus Enzymatic Catalysisfor the Regioselective Synthesis of Sucrose Esters of Fatty Acids M. Ferrer, M.A. Cruces, F.J. Plou, E. Pastor, G. Fuentes, M. Bernabe, J.L. Parra and A. Ballesteros . The Higly Selective Conversion of Toluene into 4-Nitrotoluene and 2,4Dinitrotoluene Using Zeolite H-Beta D. Vassena, A. Kogelbauer and R. Prins . Catalytic Asymmetric Heterogeneous Aziridination and Epoxidation of Alkenes using ModiJied Microporous and Mesoporous Materials G.J. Hutchings, C. Langharn, P. Piaggio, S. Taylor, P. McMorn, D.J. Willock, D. Bethell, P.C. Bulman Page, C. Sly, F. Hancock and F. King

521

Catalytic Hydrogenation of Nitriles to prim., sec. and tert. Amines over Supported Mono- and Bimetallic Catalysts Y.-Y. Huang and W.M.H. Sachtler .

527

Alkali Promoted Regio-Selective Hydrogenation of Styrene Oxide to Phenethyl Alcohol C.V. Rode, M.M. Telkar and R.V. Chaudhari

533

Production of Fatty Alcohols by Heterogeneous Catalysis at Supercritical Single-Phase Conditions S. van den Hark, M. Harrod and P. M ~ l l e r .

539

Regioselective Oxidation of Primary Hydroxyl Groups of Sugar and its Derivatives Using a New Catalytic System Mediated by TEMPO H. Kochkar, M. Morawietz and W.F. Holderich . Aspects of Regioselective Control in the Hydroformylation of Methyl Methacrylate with the in situ Formed (0-Thiomethylphenyl) diphenylphosphine Rhodium Complex H.K. Reinius, R.H. Laitinen, A.O.I. Krause and J.T. Pursiainen .

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Highly Eficient Synthesis of N,N-Dialky&ormamides@om Carbon Dioxide and DialRylamines over Ruthenium-Silica Hybrid Gels, L. Schmid and A. Baiker . Metal Complex Catalyzed Functionalization of Naturally Occurring Monoterpenes: Oxidation, Hydroformylation, Alkoxicarbonylation E.V. Gusevskaya, E.N. dos Santos, R. Augusti, A.O. Dias, P.A. Robles-Dutenheher, C.M. Foca and H.J.V. Barros . Session C Environmental Catalysis. Combustion VOCs

Ahocat: Ahorption/Catalytic Combustionfor VOC and Odour Control . E. Kullavanijaya, D.L. Trimm and N.W. Cant Structure Sensitivity of the Hydrocarbon Combustion Reaction over Alumina-Supported Platinum Catalysts T.F. Garetto and C.R. Apesteguia . Ceria-Zirconia-Supported Platinum Catalystfor Hydrocarbons Combustion: Low-Temperature Activity, Deactivation and Regeneration C. Bozo, E. Garbowski, N. Guilhaume and M. Primet .

Characterisation o f a y-MnO2 Catalyst Used in VOC Abatement C. Lahousse, C. Cellier, B. Delmon and P. Grange . New AluminaiAluminium Monoliths for the Catalytic Elimination of VOCs N . Burgos, M. Paulis, A. Gil, L.M. Gandia and M. Montes. Hydrotalcite-Derived Catalystsfor Removal of Nitrogen-Containing Volatil Organic Compounds J. Haber, K. Bahranowski, J. Janas, R. Janik, T. Machej, L. Matachowski, . A. Michalik, H. Sadowska and E.M. Senvicka Environmental Catalysis. NO,

Surface Catalytic Reactions Assisted by Gas Phase Molecules on Supported Co-ensemble Catalysts A. Yamaguchi, T . Shido, K. Asakura and Y. Iwasawa . Fresh and Used V205-WOfli02 SCR EUROCAT Standard Catalyst: An European Collaborative Characterization J.C. Vedrine .

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xvii

Lean NO, Reduction over Sn,,Zr,02 Solid Solutions J. Ma, Y . Zhu, J. Wei, X. Cai and Y. Xie .

617

Concentration Programmed Aakorption-Desorption / Surface Reaction Study of the SCR-DeNOx Reaction I. Nova, L. Lietti, E. Tronconi and P. Forzatti .

623

B~functionalNature of Sn02/y-A1203Catalysts in the Selective Reduction of NO, . A. Yezerets, Y. Zheng, P.W. Park, M.C. Kung and H.H. Kung SO2 Resistant Fe/ZSM-5 Catalystfor the Conversion of Nitrogen Oxides G. Centi, G. Grasso, F. Vanzzana and F. Arena . Mechanistic Studies of the NO, Reduction by Hydrocarbons in Oxidative Atmosphere . S. Schneider, S. Ringler, P. Girard, G. Maire, F. Garin and D. Bazin NO Reduction in Presence of Methane and Ethanol on Pd-Mo/A1203 Catalysts L.F. De Mello, M.A.S. Baldanza, F.B. Noronha and M. Schrnal . Fe- Vanadyl Phosphates/Ti02 as SCR Catalysts G. Bagnasco, P. Gilli, M.A. Larmbia, M.A. Massucci, P. Patrono, G. Rarnis and M. Turco . Photochemistry

Photoinduced Non-Oxidative Methane Coupling over Silica-Alumina H. Yoshida, Y . Kato and T. Hattori . Photocatalytic Oxidation of Gaseous Toluene on Polycrystalline Ti02: FT-IR Investigation of Surface Reactivity of Diferent Types of Catalysts G. Martra, V. Augugliaro, S. Coluccia, E. Garcia-Lbpez, V. Loddo, . L. Marchese, L. Palmisano and M. Schiavello Investigation of Environmental Photocatalysis by Solid-state NMR Spectroscopy D. Raflery, S. Pilkenton, C.V. Rice, A. Pradhan, M. Macnaughtan, S. Klosek and T. Hou . C1 Chemistry

A Study of CH4Reforming by C02 and H20 on Ceria-Supported Pd S. Sharma, S. Hilaire and R.J. Gorte .

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Catalytic Behaviour on Ni Containing Catalysts in Vaporeforming of Methane with Low H20/CH4 Ratio and Free Carbon Deposition H. Provendier, C. Petit and A. Kiennemann .

683

Carbon Deposition and Reaction Steps in C02/CH4Reforming over Ni-La20d5A Catalyst J.Z. Luo, L.Z. Gao, Z.L. Yu and C.T. Au .

689

The Autothermal Partial Oxidation Kinetics of Methanol to Produce Hydrogen E. Newson, P. Mizsey, T. Tmong and P. Hottinger .

695

New Reaction Mechanismfor Methane Formation in CO Hydrogenation over Pd/CeOr S. Naito, S. Aida and T. Miyao .

70 1

Methane Coupling over SrCo03-Based Perovskites in the Absence of Gas-Phase Oxygen Yu.1. Pyatnitsky, N.I. Ilchenko, L.Yu. Dolgikh and N.V. Pavlenko

707

Novel Conception of the Methanol Synthesis Mechanism on Conventional Cu-Containing Catalysts G.I. Lin, K.P. Kotyaev and A.Ya. Rozovskii

713

"Real" and "Inverse" Model Catalystsfor Studies of Metal-Support Interactions: CO Hydrogenation on Titania and Vanadia Supported Rh W . Reichl and K. Hayek .

719

Environmental Catalysis. Catalysis for Clean Processes and Fuels

Agglomeration of Pt Particles and Potential Commercial Application for Selective Hydrodechlorination of CC14 Z.C. Zhang and B.C. Beard . Catalytic Diesel Soot Elimination on Co-K/La203 Catalysts: Reaction Mechanism and the Effect of NO Addition E.E. Miro, F. Ravelli, M.A. Ulla, L.M. Cornaglia and C.A. Querini

725

.

73 1

Environmentally Benign Carbonylation of Nitrobenzene and Aniline S.S.C. Chuang, M.V. Kondum, Y. Chi and P. Toochinda .

737

Nitrous Oxide - Waste to Value A. K. Uriarte .

743

Catalytic Wet Peroxide Oxidation over Mixed (Al-Fe) Pillared Clays J. Barrault, C. Bouchoule, J.-M. Tatibouet, M. Abdellaoui, A. MajestC, I. Louloudi, N. Papayannakos and N.H. Gangas .

749

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Session D Selective Oxidation (ODH) A new Iron Phosphate Crystalline Phase and its Catalytic Activity in Oxidative Dehydrogenation of Lactic Acid and Glycolic Acid M . Ai and K. Ohdan .

755

Dynamic of the Oxidative Dehydrogenation of Propane over VMgO Catalysts Studied by In Situ Electrical Conductivity and Step Transients H.W. Zanthoff, J.C. Jalibert, Y. Schuurman, P. Slama, J.M. Herrmann and C. Mirodatos .

76 1

EfSect of Potassium on the Structure and Reactivity of Vanadium Species in VO/A1203 Catalysts P. Concepcion, S. Kuba, H. Knozinger, B. Solsona and J.M. Lopez Nieto .

767

Strategies for Combining Light Parafin Dehydrogenation (DH) with Selective Hydrogen Combustion (SHC) R.K. Grasselli, D.L. Stem and J.G. Tsikoyiannis . Oxidative Dehydrogenation over Promoted Chromia Catalysts at Short Contact Times D.W. Flick and M.C. Huff . Propane Oxidative Dehydrogenation by Continuous and Periodic Operating Flow Reactor with a Nickel Molybdate Catalyst C . Mazzocchia, A. Kaddouri, E. Tempesti and R. del Rosso

Selective Oxidation (General Papers) A Novel Catalyst of Copper Hydroxyphosphate (Cur(OH)PO4) with High Activity in Hydroxylation of Phenol by Hydrogen Peroxide F.-S. Xiao, J. Sun, R. Yu, X. Meng, H. Yuan, D. Jiang and R. Xu . Oxidation ofAlkanes with Hydrogen Peroxide Catalyzed by di-lronSubstituted Inorganic Synzyme N. M i m o , Y. Nishiyama, I. Kiyoto and M. Misono Vanadyl-Aluminum Binary Phosphate: Effect of Thermal Treatment over its Structure and Catalytic Properties in Selective Oxidation of Aromatic Hydrocarbons F.M. Bautista, J.M. Carnpelo, A. Garcia, D. Luna, J.M. Marinas, A.A. Romero and M.T. Siles . Direct Synthesis of Phenol ffom Benzene with 0 2 over VMo-Oxide/Si02 Catalyst I. Yamanaka, M. Katagiri, S. Takenaka and K. Otsuda .

809

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EM-Ti-Pt-CatalyticSystem for Direct Hydroxylation of Benzene by 0 2 and Hz under Mild Conditions . I. Yarnanaka, T. Nabeta, S. Takenaka and K. Otsuka High Yield Butane to Maleic Anhydride Direct Oxidation on New Supported Catalysts M.J. Ledoux, V. Twines, C. Crouzet, K. Kourtakis, P.L. Mills and J.J. Lerou Propylene Epoxidation over Gold-Titania Catalysts E.E. Stangland, K.B. Stavens, R.P. Andres and W.N. Delgass

.

Vapor-Phase Epoxidation of Propene using Hz and 0 2 over A m MCM-41 and Au/Ti-MCM-48 B.S. Uphade, M. Okumura, N. Yamada, S. Tsubota and M. Haruta Molecular Sieves Ring-Opening Reactions of Ethyl- and Vinyloxirane on HZSM-5 and Cu-ZSM-5 Catalysts A. Fasi, I. Palink6 and I. Kiricsi . Controlling the Distribution of Framework Aluminum in High-Silica Zeolites . D.F. Shantz, R.F. Lobo, C. Fild and H. Koller Toluene Disproportionation Catalysis Using EUO Type Zeolites: Initial Optimisation and Development J.L. Casci and A. Stewart . Injluence of the Aluminium Content on the Acidity and Catalytic Activity of MTW-Type Zeolites A. Katovic, B.H. Chiche, F. di Renzo, G. Giordano and F. Fajula . Synthesis of Ethylbenzene fiom 1,3-Butadiene Using Basic Zeolite Catalysts J. Ackermann, E. Klemrn and G. Emig . Activity/Selectivity and Ligation of the Co Ions in Zeolites in Ammoxidation of Ethane to Acetonitrile B. Wichterlova, Z. Sobalik, Y. Li and J.N. Armor . Room-Temperature Oxidation of Hydrocarbons over FeZSM-5 Zeolite M.A. Rodkin, V.I. Sobolev, K.A. Dubkov, N.H. Watkins and G.I. Panov Selective Oxidation of Hydrocarbons by Active Oxygen Formedfiom N2O on ZSM-5 at Moderate Temperatures S.N. Vereshchagin, N.P. Kirik, N.N. Shishkina, S.V. Morozov, A.I. Vyalkov and A.G. Anshits .

.

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Redox Molecular Sieve Catalystsfor the Aerobic Selective Oxidation of Hydrocarbons . J.M. Thomas, R. Raja, G. Sankar and R.G. Bell Catalytic Behaviour of H-Y Type Zeolites in the Decomposition of Chlorinated VOCs R. Ldpez Fonseca, P. Steltenpohl, J.R. Gomiilez Velasco, A. Aranzabal, and J.I. Gutitrrez Ortiz Coupling Alkane Dehydrogenation with Hydrogenation Reactions on Cation-Exchanged Zeolites W. Li, S.Y. Yu and E. Iglesia Water Vapor Tolerance of PdHZSM-5 in the NO-Methane-02 Reaction: Its Relation with Very Strong Solid Acidity of Zeolite Support J. Amano, 0 . Shoji, K. Okurnura and M. Niwa . Simultaneous NO and N 2 0 Decomposition on Cu-ZSM5 R. Pirone, P. Ciambelli, B. Palella and G. Russo . Polymerization and Organometallics Catalytic Properties of Silica-Supported Tantalum and Tungsten Hydrides in the Cleavage and Formation of C-C Bonds of Alkanes 0. Maury, J. Lefort, G. Saggio, C. Coptret, M. Taoufick, M. Chabanas, J. Thivolle-Cazat and J.M. Basset . Characteristics of Ethylene Polymerization Catalyzed over Ziegler-Natta / Metallocene Hybrid Catalysts: Comparison between Silica-Based and Magnesium-Based Supports H.S. Cho, D.J. Choi, I.K. Song and W.Y. Lee . Ethylene Polymerization with Zirconocene-MA0 Supported on Molecular Sieves I.S. Paulino, A.P. de Oliveira Filho, J.L. de Souza and U. Schuchardt In Situ and Ex Situ Study of Propylene Polymerization with a MgC12Supported Ziegler-Natta Catalyst . V. Oleshko, P. Crozier, R. Cantrell and A. Westwood

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POSTER SESSIONS Session P1 Preparation of Catalysts

PO0 1 Nanoheterogeneous Metal-Polymer Composites as the New Type of Eflective Selective Catalysts L.I. Trakhtenberg, G.N. Gerasimov, E.I. Grigoriev, S.A. Zavjalov, O.V. Zagorskaja, V.Yu. Zufman and V.V. Smimov.

94 1

PO02 Design of Heteropolyacid-Polymer Composite Films as Catalytic Materials for Heterogeneous Reactions G.I. Park, S.S. Lim, Y.H. Kim, I. K. Son and W.Y. Lee .

947

PO03 Catalyts Based on Heteropolyacids Supported on Zirconia L. Pizzio, P. Vkquez, C. Caceres and M. Blanco .

953

PO04 Dependence of structure andphysico-chemical properties of the catalysts containing heteropolyacids on the conjugatedpolymer matrix E. Stochmal-Pomarzanska and W. Turek .

959

PO05 Mesoporous Silica Supported Solid Acid Catalysts S. Choi, Y. Wang, Z. Nie, D. Kambapati, J. Liu and C.H.F. Peden .

965

PO06 Study of Acidic and Catalytic Properties of Sulfated Zirconia Prepared by Sol-Gel Process: Influence of Preparation Conditions L.B. Hamouda, A. Ghorbel and F. Figueras .

97 1

PO07 Synthesis and Characterization of Ta-Pillared Ilerite Y. KO, M.H. Kim, S.J. Kim and Y.S. Uh .

977

PO08 Al-Pillared Hectorite and Montmorillonite Preparedfiom Concentrated Suspensions: Structural,Textural and Catalyic Properties R. Molina, S . Moreno and G. Poncelet .

983

PO09 Synthesis of High Surface Area Transition Metal Carbide Catalysts A.P.E. York, J.B. Claridge, V.C. Williams, A.J. Brungs, J. Sloan, A. Hanif, H. Al-Megren and M.L.H. Green .

989

PO10 Use of Carbon Fabrics as Supportfor Hydrogenation Catalysts Usable in Polyphasic Reactors J.P. Reymond and P. Fouilloux .

995

PO 11 Preparation, Characterization and Reactivity of Activated Carbon Supported Platinum Catalysts by Fluidized Bed Organometallic Chemical Vapor Deposition (FBMOCVD) Ph. Serp, J-C. Hierso, R.Feurer, R. CorratgC, Y. Kihn and Ph. Kalck

1001

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xxiii

PO 12 Metal-Carbon Aerogels as Catalysts and Catalyst Supports F.J. Maldonado Hbdar, C. Moreno Castilla, J. Rivera Utrilla and M.A. Ferro Garcia .

1007

PO13 Preparation and Characterisation of Carbon Supported Pt-Ce02 Catalysts A. Sepclveda Escribano, J. Silvestre Alvero, F. Coloma and F. Rodriguez-Reinoso .

1013

PO 14 Highly Dispersed Pd Based Catalystsfor Selective Hydrogenation Reactions M. Delage, B. Didillon, Y. Huiban, J. Lynch and D. Uzio .

1019

PO15 Preparation of New Type of Supported Sn-Pt Bimetallic Catalysts Containig Lewis Acid Sites Anchored to the Platinun J.L. Margitfalvi, I . Borbith, M. Hegediis, S. GBbolos, A. Tompos, andF.Lonyi .

1025

PO16 Influence of Small Amounts of Rhodium on the Structure and the Reducibility of CdA1203 and CdCe02-Al203 Catalysts X . Courtois, V . Perrichon, M. Primet and G. Bergeret .

1031

PO 17 Synthesis, Characterization and Catalytic Properties of Chromium Silicate Xerogel R. Serpa da Cruz, M. de Magalhaes Dauch, U. Schuchardt and R. Kumar .

1037

PO 18 Thermal Chemistry of Oxide-Supported Platinum Catalysts: A Comparative Study J.F. Lambert, E. Marceau, B. Shelimov, J. Lehman, V. Le Be1 de Penguilly, X. Carrier, S. Boujday, H. Pernot and M. Che

1043

.

PO19 Geochemistry and Catalysts Preparation: Alumina as a Chemical Reactant in the Synthesis of MoOJA1203 and WOJA1203 Systems X . Carrier, J.F. Lambert and M. Che .

1049

PO20 Chemistry of the Preparation of Silica-Supported Cobalt Catalysts@om Co(I4 and Co(II4 Complexes: Grafting versus Phyllosilicate Formation . R. Trujillano, J . Grimoult, C. Louis and J.-F. Lambert

1055

PO2 1 New Method of the Preparation of Heterogeneous Ni(0) Complexesfor Ethylene Oligomerization M.K. Munshieva .

1061

PO22 Synthesis, Characterization and Catalytic Activity of [Cu(NH3)4l2' Supported on Hydrous Oxides D. Patel and A. Patel .

1067

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XXiV

PO23 Preparation and Characterization of CdZnO and PdZnO Catalysts for Partial Oxidation of Methanol. Control of Catalyst Surface Area and Particle Size Using Microemulsion Technique J. Agrell, K. Hasselbo, S. Jaras and M. Boutonnet .

1073

PO24 Mechanism of Silation of Alumina and Silica with Hexamethyldisilazane S.V. Slavov, A.R. Sanger and K.T. Chuang .

1079

PO25 Solvent Influence in Silica-Alumina Sol-Gel Synthesis A. Carati, C. Rizzo, M. Tabliabue and C. Perego .

1085

PO26 Ru, Co and RuCo/SiOz Catalyst Prepared by Sol-Gel Methods B. Botti, D. Cauzzi, P. Moggi, G. Predieri and R. Zanoni .

1091

PO27 Structure and Catalytic Properties of Co-Re Bimetallic Catalysts Prepared by SOIJGEL Method and by Ion Exchange in N a y Zeolite L. Guczi, G. Stefler Z. Schay, I. Kiricsi, F. Mizukami, M. Toba andS.Niwa .

1097

PO28 On the Effect of the Preparation Conditions on the Localization and the Size of Platinum Particles on Zeolite NaX L.V. Mattos, M.C.M. Alves, F.B. Noronha, B. Moraweck and J.L.F. Monteiro

1103

PO29 Hollow Metallic Particles Obtained by OxidatiodReduction Treatment of Organometallic Precursors . F.J. Cadete Santos, R. Darji, J.F. Trillat, A. Howie and A. Renouprez

1109

PO30 Synthesis, Characterization and Catalytic Application of Inorganic Nanotubes . I . Kiricsi, A. Kukovecz, A. Fudala, Z. Konyia and J.B. Nagy

1115

PO3 1 Nanostructuring Surfaces by Lithography: The Production and use of Model Catalysts M. Schildenberger, Y. Bonetti and R. Prins .

1121

PO32 New Catalyst Supports and Catalytic Systems on the Basis of Metallic Gauzes Coated with II-IV Group Element Oxides M.P. Vorob’eva, A.A. Greish, A.Yu. Stakheev, N.S. Telegina, A.A. Tyrlov, E.S. Obolonkova and L.M. Kustov .

1127

Fischer-Tropsch Synthesis PO33 Precipitated Iron Fischer-Tropsch Catalysts: The Effect of Surfde Ions . T.C. Bromfield, T.H. Dlamini, F. Marsicano and N.J. Coville

1133

PO34 The Nature of the Active Phase in Iron Fischer-Tropsch Catalysts A.K. Dave, Y. Jin, L. Mansker, R.T. Motjope, T.H. Dlamini and N.J. Coville

1139

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XXV

PO35 Selective Inhibition: The Intrinsic Feature of Fisher-Tropsch Synthesis: Transient Initial Episodes in FT Synthesis on Cobalt Catalysts H. Schulz and Z. Nie .

1145

PO36 Eflects of Support on Methane Selectivity in Cobalt-Catalyzed FischerTropsch Synthesis: A Kinetic Model C.H. Bartholomew and W.-H. Lee .

1151

PO37 Does Mono-Atomic Ru Catalyse the Fischer-Trosph Synthesis? . M. Claeys, M. Hearshaw, J.R. Moss and E. van Steen

1157

PO38 Fischer-Tropsch Synthesis Using Monolithic Catalysts A.-M. Hilmen, E. Bergene, O.A. Lindvag, D. Schanke, S. Eri and A. Holmen

1163

Application of Theoretical Methods to Catalysis PO39 The Design ifcatalysts SurJaces by the Use of Stochastic Optimisation Algorithms A.S. McLeod and L.F. Gladden .

1169

PO40 Modeling of Structure and Properties of Active Centers of Catalysts on the Base of Metalorganosiloxanes A.V. Nernukhin, I.M. Kolesnikov and V.A. Vinokurov .

1175

PO4 1 Theoretical Study on Active sites of Molybdena-Alumina Catalystfor Olefin Metathesis J. Handzlik and J. Ogonowski

1181

PO42 Modeling the Oxygen Activation on Dinuclear Iron MMO Mimics, a Quantum Mechanic Study P.-P. Knops-Gerrits, P.A. Jacobs and W.A. Goddard I11 .

1187

PO43 Quantum-Chemical and Experimental Study of an Interaction between CO, and Propylene over Rh-Co/AlIOj Catalysts L.B. Shapovalova, G.D. Zakumbaeva, A.V. Gabdrakipov, I.A. Shlygina and A.A. Zhurtbaeva .

1193

PO44 Ab-Initio Study of the Vacancy Formation in Keggin and Linqvisr Heteropolyanion J.F. Paul and M. Founier .

1199

PO45 Theoretical Studies of the Interaction of Butane and Butene Isomers in H-Ferrierite and H-Mordenite D.E. Galindo, M.J. Goncalves and M.M. Ramirez Corredores. .

1205

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xxvi

PO46 Theoretical Investigation of the Intrinsic Reaction Path (IRC) of the Methylation of mono-Substituted Aromatic Molecules over Faujasite Catalysts K.H.L. Nulens, A.M. Vos, G.O.A. Janssens and R.A. Schoonheydt

1211

Environmental Catalysis. Combustion VOCs

PO47 Catalysts or Combustion of Halogenated Volatile Organis Compounds . J. Trawczynski, J. Walendziewski and M. Kulazynski

1217

PO48 Non-Stoichiometry and Catalytic Activity in ABO3 Perovskites: LaMnO3 and LaFeO3 A.A. Barresi, D. Mazza, S. Ronchetti, R. Spinicci and M. Vallino

1223

PO49 Deep Catalytic Oxidation of Chlorinated VOC Mixturesfiom Groundwater Stripping Emissions A. A r d b a l , J.A. G o n d e z Marcos, R. Lopez Fonseca, M. A. GutiCrrez Ortiz and J.R. G o d l e z Velasco .

1229

PO50 Transformation of Chlorinated Compounds on Diferent Zeolites under Oxidative and Reductive Conditions I. Hannw, A. Tamhi, Z. Konya, S.4. Niwa, F. Mizukami, J.B. Nagy andI. Kiricsi .

1235

PO5 1 Biofiltration of Gasoline VOCs with Diferent Support Media I . Ortiz, M. Morales, C. GobbCe, S. Revah, V.M. Guerrero, I. Shifter, R. Auria, G.A. PCrez and L.A. Garcia

1241

Environmental Catalysis. NOx

PO52 An Unified View on Nitrogen Oxide Abatement J. Paul .

1247

PO53 Measuring the Adsorption of Reactive Probes as a Toolfor Understanding the Catalytic Properties of De-NO, Catalysts A. Gervasini and A. Auroux .

1253

PO54 Reactivity of the NO, Sugace Species Formed Afrer Co-adsorption of NO + 0 2 on a WO3/ZrO2Catalyst: An FTIR Spectroscopic Study S . Kuba, K. Hadjiivanov and H. Knozinger .

1259

PO55 The Mechanism of the Selective NO, Adsorption on 12- Tungstophoshoric Acid hexa-Hydrate (HPW) S. Hodjati, C. Petit, V. Pitchon and A. Kiennemann.

1265

PO56 Activity and DRIFT Spectroscopy Studies of NO Oxidation and Reduction by C3Ht in Excess 0 2 over A1203and AdA1203 G.R. Bamwenda, A. Obuchi, S. Kushiyama and K. Mizuno

1271

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XXVii

PO57 Transient Study of NO, N20, c3&Interactions over a Silica-Supported Pt Catalyst P. Denton, Y . Schuurman, A. Giroir-Fendler, H. Praliaud, M. Primet . and C. Mirodatos

1277

PO58 Electrochemical Promotion of NO Reduction by c3H6 on RWYSZ Catalyst-Electrodes and Investigation of the Origin of the Promoting Action Using TPD and WF Measurements C. Raptis, Th. Badas, D. Tsiplakides, C. Pliangos and C.G. Vayenas

1283

PO59 The Reaction of Propane with Mixtures of NO, N20 and 0 2 Over Platinum, Palladium and Rhodium Catalysts . D.C. Chambers, D.E. Angove and N.W. Cant

1289

PO60 Kinetics of NO Reduction by CO on Rh(1 I I): A Molecular Beam Study F. Zaera and C.S. Gopinah .

1295

PO61 The Surface Migration of NO, Adspecies as a Factor Determininig the Reactivity of Supported Pt Catalystsfor the Treatment of Lean Burn Engine Emissions G.E. Arena, A. Bianchini, G. Centi and F. Vazzana .

1301

PO62 PdSnO2 as deN0, Catalysts: Preparation, Characterization and Activity . D. Amalric-Popescu, J.-M. Herrmann and F. Bozon-Verduraz

1307

PO63 Decomposition of NO Over Copper-Manganese Oxide Catalysts at Room Temperature I. Spassova, M . Khrishtova, N. Nyagolova and D. Mehandjiev .

1313

PO64 Catalytic Performance of SrTi03-Based Catalystsfor NO Direct Decomposition Y. Yokoi, I. Yasuda, H. Uchida, 0. Okada, Y. Nakamura, H. Ishikawa and H. Kimura 1319 PO65 CO and NO Elimination over Pd-Cu Catalysts M. Fernhdez Garcia, A. Martinez Arias, J.A. Anderson, J.C. Conesa and J. Soria

1325

PO66 NO, Storage and Reduction over Pt/Ba/A1203 J.A. Anderson, A.J. Paterson and M. Fernhdez Garcia

1331

.

PO67 Nitrogen Emission Process in the Decomposition of Nitrogen Oxides on Pd(lI0): An Angular and Velocity Distribution Stua'y I . Kobal, K . Kimura, Y. Ohno, H. Horino, I. Rzeznicka and T. Matsushima

1337

PO68 Reduction of NO by Propylene over ModiJied RWTi02 Catalysts in the Presence of Oxygen. T.I. Halkides, D.I. Kondarides and X.E. Verykios .

1343

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xxviii

PO69 Efect of sul&r on the oxygen storagehelease capacity of RWCeO2 and RWCeOz-ZrOz model TWCs M. Boaro, C. de Leitenburg, G. Dolcetti and A. Trovarelli .

PO70 Thermal Stability and Oxygen Storage Capacity of Noble MetalICeriaZirconia Catalystsfor the Automotive Converters with the On-Board Diagnostics (OBD) P. Fornasiero, R. Di Monte, T. Montini, J. Kaspar and M. Graziani

1349

.

1355

PO7 1 N20 Formation over CeOz-ZrO2 Mixed Oxides Supported Metal (Pt, Pd, Rh), Catalystsfor Automotive Exhaust Control J.L Marc, J.R. G o d e z Velasco, M. A. Gutierrez Ortiz, J.A. Botas, M.P. G o d e z Marcos and G. Blanchard .

1361

PO72 Model Studies of Automobile Exhaust Catalysis Using Single Crystals of Rhodium and CeridZirconia C.H.F. Peden, G.S. Herman, S.A. Chambers, Y. Gao, Y.-J. Kim and D.N. Belton .

1367

PO73 Thermally Stable, High-Surface-Area, PrO,-CeOz-Based Mixed Oxides for Use in Automotive-Exhaust Catalysts A.N. Shigapov, H.-W. Jen, G.W. Graham, W. Chun and R.W. McCabe .

1373

PO74 Improvement of Three Way Catalytic Perfomance by Optimizing Ceria and Promoters in Pd only Catalyst Prepared bu Sol-Gel Method H.-S. So, 0.-B. Yang, D.H. Kim and S.I. Woo .

1379

PO75 Flue Gas NO, Removal by SCR with NH3 on CuO/AC at Low Temperatures Z.P. Zhu, Z.Y. Liu, S.J. Liu, H.X. Niu, T.D. Hu, T. Liu and Y.N. Xian .

1385

PO76 Low temperature Monolithic SCR Catalystsfor Tail Gas Treatment in Nitric Acid Plants J. Blanco, P. Avila, L. Marzo, S. Suarez and C. Knapp .

1391

PO77 Effect of La203 Concentration in LazO3-AI203 Supports and PdLa203-Al203 Catalysts in Reduction of NO by H2 N.E. Bogdanchikova, A. Barrera, S. Fuentes, G. Diaz, A. Gomez-CortCs, A. Boronin, M.H. Farias and M. Viniegra R..

1397

PO78 Molecular Basis for the Design of Transition-Metal Oxide Catalystsfor Selective Catalytic Reduction of NO by Hydrocarbons K. Shimizu, H. Maeshima, H. Kawabata, H. Yoshida, A. Satsuma and T. Hattori .

1403

PO79 Silver as Promoterfor the Catalytic Decomposition of NO, under Oxygen-Excess Condition: Evidence for Oxygen SpilloverJi.om Noble Metals to Silver W.X. Huang, J.W. Teng, T.X. Cai and X.H. Bao .

1409

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

PO80 Reactiviry of Surface Species in the Reduction of NO, by Ethanol on Ag/AI203 Catalyst T . Chafik, S. Kameoka, Y. Ukisu and T. Miyadera. .

1415

PO8 1 Modelling of Uncatalysed and Barium Catalysed NO Reduction by Activated Carbon S.A. Carabineiro, F.B. Fernandes, J.S. Vital, A.M. Ramos and I.F. Silva

1421

PO82 Selective Catalytic Reduction of NO, with C3Ha under Lean-Burn Conditions on Activated Carbon-Supported Metals J.M. Garcia Cortts, M.J. Illin Gbrnez, A. Linares Solano and C. Salinas Martinez de Lecea.

1427

PO83 Role of Acidic Sites in the DENO, Process on Supported Tin Oxides A. Auroux, D. Sprinceana and A. Gervasini .

1433

PO84 Sulphated-Zr02 Prepared by Impregnation with Ammonium, Sodium or Copper Sulphate: Catalytic Activity for NO Abatement with Propene in the Presence of Oxygen V. Indovina, M.C. Campa and D. Pietrogiacomi .

1439

PO85 Co.Based exTHlcfor the Decomposition of N20: Tailoring Catalystsfor Active and Stable Operation J. Perez Ramirez, G. Mul, X. Xu, F. Kapteijn and J.A. Moulijn .

1445

PO86 Cold Start Vehicle Emission Control Using Trapping and Catalyst Technology N.R. Burke, D.L. T r i m and R. Howe .

1451

PO87 Oscillation of the NO, Concentration in its Selective Catalytic Reduction on Platinum Containing Zeolite Catalysts Y . Traa, B. Burger and J. Weitkamp .

1457

PO88 Experimental and Theoretical Description of Transition Metal Ion Structures in Zeolites Relevant to DENO, Catalysis Z . Sobalik, J.E. Sponer and B. Wichterlova .

1463

PO89 Cooperation of Pt and Pd over H-Mordenitefor the Lean SCR of NO, by Methane C. Montes de Correa, F. Cordoba C. and F. Bustamante L. .

1469

PO90 The Selective Catalytic Reduction of NZO by NH3 on a Fe-BEA Catalysts M. Mauvezin, G. Delahay, B. Coq and S. Kieger .

1475

PO91 A New Mechanismfor the Selective Catalytic Reduction ofNOx by NH3 on Cu-zeolite Catalysts B. Coq, S . Kieger, G. Delahay, D. Berthomieu and A. Goursot .

1481

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PO92 In Situ FT-I. Stu& of the Selective Catalytic Reduction of NO by Propane on Cu-ZSM-5: Evidence of a Reaction Pathway by Oxygen Pulses F . Poignat, J.L. Freysz, M. Daturi, J. Saussey and J.C. Lavalley .

1487

PO93 Catalytic Removal of NO: the Eflect of Cu Loading and Type of Binder on Catalytic Properties of CdZSM-5 Catalyst V. Tomasic, A. Gerzina, Z. Gomzi and S. Zrncevic .

1493

PO94 Changes in Cation Coordination during Deactivation of Cu-ZSM-5 deNOx Catalysts S.A. Gbmez, A. Campero, A. Martinez Hernindez and G.A. Fuentes .

1499

PO95 On the Role of oxygen in the Selective Reaction of NO Reduction with Methane over Co/ZSM-5 Catalyst E.M. Sadovskaya, A.P. Suknev, L.G. Pinaeva, V.B. Goncharov, C. Mirodatos and B.S. Bal’zhinimaev

1505

PO96 Eflect of SiLA1 Ratio of Mordenite and ZSM-5 Type Zeolite Catalysts on Hydrothermal Stabilityfor NO Reduction by Hydrocarbons . S.Y. Chung, B.S. Kim, S.B. Hong, 1.-S. Nam and Y.G. Kim

1511

PO97 Precipitation of MnOz onto the External Surface of Zeolite Microcrystals. Structure of the Manganese Oxide and its Role in the Removal of NO, by NH3 M. Richter, H. Kosslick and R. Fricke .

1517

PO98 Catalytic Properties of Mesoporous Molecular Sievesfor the Reduction of NO, 1523 A. Jentys, W . Schieber and H. Vinek PO99 Catalytic Activity of High Surface Area Mesoporous Mn-Based Mixed Oxidesfor the Deep Oxidation of Methane and Lean-NO, Reduction V.N. Stathopoulos, V.C. Belessi, C.N. Costa, S. Neophytides, P. Falaras, A.M. Efstathiou and P.J. Pomonis .

1529

PlOO Characterization of the Role of Pt on Copt and InPt Ferrierite Activity and Stability upon the SCR of NO with CHd L. Gutierrez, L. Cornaglia, E. Mir6 and J. Petunchi .

1535

Environmental Catalysis. Catalysis for Clean Processes and Fuels P 101 Cu-Cr Oxide Catalystsfor Complete Oxidation of Aromatic Hydrocarbons V. Georgescu .

1541

P 102 A g / L a ~ . ~ r ~ . ~ M n O / yCatalyst - A l ~ O for ~ Complete Oxidation of Methanol at Low Concentration H.-B. Zhang, W . Wang, Z.-T. Xiong and G.-D. Lin.

1547

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P103 Treatment of Aqueous Solutions of Organic Pollutants by Heterogeneous Catalytic Wet Air Oxidation (CWAO) . M . Besson, J.-C. Beziat, B. Blanc, S. Durecu and P. Gallezot

1553

P104 Efect of the Supportfor Pt Catalysts on Soot Oxidation J. Obuchi, J. Oi-Uchisawa, R. Enomoto, S. Liu, T. Nanba and S. Kushiyama

1559

P105 Dimethyl Carbonate Synthesis From Carbon Dioxide and Methanol over Ni-Cw'MoSiO (VSiO) Catalysts . S.H. Zhong, J.W. Wang, X.F. Xiao and H.S. Li

1565

P106 Promotion of Support in Synthesis of Propylene Carbonatefiom COr and Propylene Oxide T. Zhao, Y . Han and Y. Sun .

1571

P107 A Novel Processfor the Removal of Nitratesfiom Drinking Water A. Pintar, J. Batista and G. Bercic .

1577

P108 Greenhouse Gases and Emissions Control by New Catalysts Free of Precious Metals V. Gryaznov and Y. Serov .

1583

P109 Study on the Initial Steps of the Polyethylene Cracking over DifSerent acid Catalysts D.P. Serrano, R. Van Grieken, J. Aguado, R.A. Garcia, C. Rojo and F.Temprano .

1589

P110 Catalytic Conversion of Traces in Biomass Gasijkation Fuel Gases with Nickel Activated Ceramic Filters D.J. Draelants. H.-B. Zhao and G.V. Baron .

1595

P l l l An Environmental Friendly Catalystfor the High Temperature Shift Reaction G.C. Araujo and M.C. Range1

1601

P112 Direct Catalytic Conversion of Chloromethane to Higher Hydrocarbons over Various Protonic and Cationic Zeolite Catalysts as Studied by In-Situ FTIR and Catalytic Testing D. Jaumain and B.-L. Su .

1607

P113 Catalytic Conversion of 2-Chloropropane in Oxidizing Conditions: A FT-IR and Flow Reaction Study C. Pistarino, F. Brichese, E. Finocchio, G. Romezzano, R. di Felice, M. Baldi and G. Busca

1613

P114 Catalyst Degradation of Polychlorinated Biphenyls at Low Temperature D.-K. Lee, E.-S. Byun, 1.-C. Cho and S.-W. Kim .

1619

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xxxii

P115 Microwave Eflects in Exhaust Catalysis M. Turner, R.L. Laurence, K.S. Yngvesson and W.C. Conner

.

1625

Selective Oxidation (General Papers) P116 A New Way to Ti-containing Zeolite Beta Catalystsfor Selective Oxidation F. Di Renzo, S. Gomez, R. Teissier and F. Fajula .

1631

P117 Synthesis of Organically ModJied Titania Silica Aerogels: Application for Epoxidation of Cyclohexenol A. Gisler, C.A. Muller, M. Schneider, T. Mallat and A. Baiker .

1637

P118 Characterization and Catalytic Properties of Titanium Pillared Clays in the Epoxidation of Allylic Alcohols . L. Khalfallah-Boudalli, A. Ghorbel, F. Figueras and C. Pine1

1643

P119 Epoxidation of Allylic Alcohol by Vanadium-MontmorilloniteCatalyst I . Kheder and A. Ghorbel .

1649

P120 Activity and Stability of Thermally Stable Polyimide Supported Mo(V.. Complexesfor Epxidation of Olefins J.H. Ahn, C.G. Oh, S.K. Ihm and D.C. Sherrington .

1655

P121 Epoxidation of Soybean Oil Catalysed by CHjReOdH202 H . Sales, R. Cesquini, S. Sato, D. Mandelli and U. Schuchardt

1661

.

P122 Epoxidation of Limonene with Layered Double Hydroxides as Catalysts M.A. Aramendia, V . Borau, C. JimCnez, J.M. Luque, J.M. Marinas, . F.J. Romero, J.R. Ruiz and F.J. Urban0

1667

P123 Epoxidation of Electron-Deficient Alkenes Using Heterogeneous Basic Catalysts J.M. Fraile, J.I. Garcia, D. Marco, J.A. Mayoral, E. Shnchez, A. Monzon and E. Romeo.

1673

P124 Hydroxylation of Benzene to Phenol with Nitrous Oxide on Fe-Silicalites R. Monaci, E. Rombi, M.G. Cutrufello, V. Solinas, G. Berlier and G. Spoto

1679

P125 Ammoxidation of Propane to Acrylonitrile over V-Sb-Al-Oxide Catalysts - Influence of Catalyst Preparation on Reaction Kinetics 0.Ratajczak, H.W. Zanthoff and S. Geisler .

1685

P126 Ammoxidation of Propylene to Acrylonitrile Catalyzed by Multimetal Molybdateand Iron Antimoniate-Based Active Compounds Dispersed in Oxidic Matrixes: The Efect of the Dispersing Oxide on the Catalytic Performance R. Catani, F. Cavani, U. Cornaro, A. Del Bianco, E. Frontani, D. Ghisletti, 1691 A. Tasso and F. Trifiri,

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

P127 Mastering of Chemical State of Mo at the Surface of Oxide Catalysts in the Selective Oxidation of Hydrocarbons: Towards a Fine Optimization of Catalytic Performances E.M. Gaigneaux, M-L. Naeye, E. Godard, H.M. Abdel Dayem, P. Ruiz and B. Delmon

1697

P128 Oligomerization of Higher Olejins and Oxidation in a Series of Isobutanol - Isobutyric Aldehyde - Isobutyric Acid in the Presence of Some Heteropoly Compounds S.M. Zulfugarova, M.K. Munshieva, D.B. Tagiev and P.S. Mamedova .

1703

P129 ModiJications of Vanadium Phosporous Oxides by Aluminium Phosphate for n-Butane Oxidation to Maleic Anhydride S. Holmes, L. Sartoni, A. Burrows, V. Martin, G.J. Hutchings, C. Kiely and J.-C. Volta

1709

P130 Selective Oxidation of n-Butane over Novel V-P-O/Silica-Composites Prepared Through Intercalation and Exfoliation of Layered Precursor N. Hiyoshi, N. Yamamoto, N. Terao, T. Okuhara and T. Nakato .

1715

P131 Molecular Structure-Reactivity Relationships in n-Butane Oxidation over Bulk VPO and Supported Vanadia Catalysts: Lessons for Molecular Engineering of New Selective Catalystsfor Alkane Oxidation . V.V. Giulants, J.B. Benziger, S. Sundaresan and I.E. Wachs

1721

P132 The Nature of the Cobalt Salt Affects the Catalytic Properties of Promoted VPO 1727 . L. Cornaglia, C. Cmara, J. Petunchi and E. Lombard0 P133 Oxidation of Toluene to Benzaldehyde over VSb,,Ti,Od Kinetic Studies A. Barbaro, S . Larrondo and N. Amadeo .

Catayst.

P134 Interaction of Surface- and Bulk-Oxygen in Mo/V-Mixed Oxides during the Partial Oxidation of an UnsaturatedAldehyde A Concentration-Programmed-ReductionStudy R. Bohling, A. Drochner, M. Fehlings, K. Kraub and H. Vogel .

1733

1739

P135 Steady State and Transient Kinetic Experimentsfor Rational Catalyst Design: Differences of Acrolein and Methacrolein Oxidation on Mixed Oxide Catalysts 1745 H . Bohnke, J.C. Petzoldt, B. Stein and J.W. Gaube . P136 The Effects of Particle Size on the Performance of Er203 and 30 mol% BaCI2/Er203 Catalysts in the Selective Oxidation of Ethane to Ethene W. Zhong, H.X. Dai, C.F. Ng, and C.T. Au .

1751

P137 Perovskite Type Chloro Oxide SrC003&1; A Novel and Durable Catalystfor the Selective Oxidation of Ethane to Ethene H . X . Dai, C.F. Ng, and C.T. Au .

1757

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XXXiV

P138 Preparation and Activity of Copper, Nickel and Copper-Nickel-A1Mixed Oxides via Hydrotalcite-Like Precursorsfor the Oxidation of Phenol Aqueous Solutions A. Alejandre, F. Medina, X. Rodriguez, P. Salagre and J.E. Sueiras

.

1763

P139 Influence of Temperature and Catalyst Loading on the Aqueous-Phase Catalytic Oxidation of Phenol . A. Santos, P. Yustos, B. Durbhn and F. Garcia Ochoa

1769

PI40 Glyoxal Synthesis by Vapour - Phase Ethylene Glycol Oxidation on a Silver and Copper Catalysts O.V. Vodyankina, L.N. Kurina, A.I. Boronin and A.N. Salanov .

1775

P141 Production of Alkenes through Oxidative Cracking of n-Butane over OCM Catalysts M. Nakamura, S. Takenaka, I. Yamanaka and K. Otsuka .

1781

P142 Selective Oxidation of Monosaccharides using Pt and Pd-Containing Cctalysts E. Sulman, N. Lakina, M. Sulman, T. Ankudinova, V. Matveeva, A. Sidorov 1787 and S. Sidorov PI43 Synthesis of New Neocarboxylic Acids via Rearragement and Oxidation of 1,3-Dioxanes J . Fischer and W F. Holderich

1793

P144 Low Temperature Radical Oxidation of Butane over In-Situ Prepared Silica Species A. Satsuma, N. Sugiyama. Y. Kamiya, T. Kamatani and T. Hattori

1799

P145 Mechanochemical Preparation of V-Ti-0 Catalystsfor o-Xylene Low Temperature Oxidation V.A. Zazhigalov, J . Haber, J. Stoch, A.I. Kharlamov, A. Marino, L. Depero and I.V. Bacherikova .

1805

P146 A Common Concept Accountingfor Selectivity in Mild and Total Oxidation Reactions: The Optical Basicity of Catalysts P. Moriceau, B. Taouk, E. Bordes and P. Courtine .

1811

P147 Copper Species in CuClZ/y-A1203 Catalystfor Ethylene Oxychlorination M . Garilli, D. Carmello, B. Cremaschi, G. Leofanti, M. Padovan, A. Zecchina, G. Spoto, S. Bordiga and C. Lamberti .

1817

PI48 A Study of the Main Path and of Side-Reactions upon Ethylene Oxychlorination over CUCIZ-AI~O~ Based Catalysts A. Marsella, D. Carmello, E. Finocchio, B. Cremaschi, G. Leofanti, M. Padovan and G. Busca .

1823

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

Selective Oxidation (ODH)

P149 Oxidative Dehydrogenation of Ethane on Lithium Promoted Oxide Catalysis S . Wang, K. Murata, T. Hayakawa, S . Hamakawa and K. Suzuki .

1829

P150 Oxidative Dehydrogenation of Ethane to Ethylene over NiO/AI*Oj Catalysts X. Zhang, G. Yu, Y. Gong, D. Jiang and Y. Xie .

1835

P151 Oxidative Dehydrogenation of Ethane over Catalysts Prepared Via Heteropolycompounds N . Haddad, C. Rabia, M.M. Bettahar and A. Barama

1841

P152 The Importance of Nonstoichiometric Oxygen in NiO for the Catalytic Oxidative Dehydrogenation of Ethane T. Chen, W. Li, C. Yu, R. Jin and H. Xu .

1847

P153 Oxidative Dehydrogenation of Ethane on Vanadium-Phosphorus Oxide Catalysts J.M. L6pez Nieto, V.A. Zazhigalov, B. Solsona and I.V. Bacherikova .

1853

P154 High Throughput Synthesis and Screening of Nb and Ta Containing Mixed Metal Oxide Librariesfor Ethane Oxidative Dehydrogenation Y. Liu, P. Cong, R.D. Doolen, H.W. Turner and W.H. Weinberg .

1859

P155 Chromium-Impregnated Mesoporous Silica as Catalystsfor the Oxidative Dehydrogenation of Propane J. MCrida Robles, M. Alchntara Rodriguez, E. Rodriguez Castellbn, J. Santaman’a Gonzdlez, P. Maireles Torres and A. Jimenez L6pez.

1865

P156 Transient Kinetic Studies of the Propane Oxidehydrogenation on V20dTiO2 Catalysts S . Pietrzyk and F. Genser . .1871 PI57 Vanadium-Doped Titania-Pillared Montmorillonites as Catalystsfor the Oxidative Dehydrogenation of Propane K. Bahranowski, R. Dula, R. Grabowski, E.M. Senvicka and K. Wcislo .

1877

P158 Oxidative Dehydrogenation of Propane over Alkali-Mo Catalysts Supported on Sol-Gel Silica-Titania Mixed Oxides R.B. Watson and U S . Ozkan.

1883

P159 Vanadium Oxide Supported on Gallium and Indium Oxides: Synthesis, Physicochemical and Catalytic Properties B. Sulikowski, A. Kubacka, E. Wloch, Z. Schay, V. CortCs Corberhn and R.X. Valenzuela .

1889

P160 Oxidative Dehydrogenation of Cyclohexane over Heteropolymolybdates . S . Hocine, C. Rabia, M.M. Bettahar and M. Fournier

1895

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m i

P161 Transient Response Studies of Isobutane Oxidative Dehydrogenation over Molybdenum Catalysts N.V. Nekrasov, N.A. Gaidai, Yu.A. Agafonov, S.L. Kiperman, V. Cort6.s Corberhn and M.F. Portela

1901

P162 Olefns Formation by Oxidative Dehydrogenation of Propane over Monoliths at Short Contact Times V.A. Sadykov, S.N. Pavlova, N.F. Saputina, LA. Zolotarski, N.A. Pakhomov, E.M. Morov, V.A. Kumin, A.V. Kalinkin, A.N. Salanov, I.G. Dalinova and E.A. Paukshtis .

1907

P163 Experimental and Theoretical Investigation on the Roles of Heterogeneous and Homogeneous Phases in the Oxidative Dehydrogenation of Light Parafins in Novel Short Contact Time Reactors A. Beretta, E. Ranzi and P. Forzatti .

1913

Fuel Cells and Electrocatalysis

PI64 Composite Nickel-Oxide Surfaces, Modified by Palladium R.G. Baisheva, Z.K. Kanaeva, K.A. Zhubanov and Zh.K. Kairbekov

.

1919

Photochemistry

P165 Photocatalyst-CoatedAcrylic Waveguidesfor Oxidation of Organic Compounds L.W. Miller, M.I. Tejedor Tejedor, M. PCrez Moya, R. Johnson 1925 and M.A. Anderson . P166 Preparation of Eflcient Titanium Oxide Photocatalysts by an Ionizer Cluster Beam Method and Their Applicationfor the Degradation of Propanol Diluted in Water H . Yamashita, M. Harada, A. Tanii, M. Honda, M. Takeuchi, Y. Ichihashi and M. Anpo .

1931

PI67 Thermal Treatment of Titanium Alkoxides in Organic Media: Novel Synthesis Methods for Titanium (Iv) Oxide PhotGcatalyst of Ultra-HighActivity H. Kominami, J. Kato, S. Murakami, M. Kohno, Y. Kera, S. Nishimoto, M. Inoue, T. Inui and B. Ohtani .

1937

P168 Photocataliytic Water Decomposition by Layered Perovskites T. Takata, A. Tanaka, M. Hara, J.N. Kondo and K. Domen.

1943

PI69 Catalyst Preparation and Reactor Designfor Gas-Phase Photocatalytic Oxidation of Trichloroethylene (TCE) Pollutant . A.J. Maira, K.L. Yeung, C.K. Chan, J.F. Porter and P.L. Yue

1949

P170 Formation of Ethylene Oxide by Photooxidation of Ethylene over Silica ModiJied with Copper Y. Ichihashi and Y. Matsumura .

1955

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xxxvii

P171 Photocatalytic Oxidation of n-Butane at a Steady Rate over Rb-Modjfied Vanadium Oxide Supported on Silica T. Tanaka, T. Ito, T. Funabiki and S. Yoshida .

1961

P172 Photocatalytic Oxidation of Toluene on PlatinudTitania Catalysts M.C. Blount and J.L. Falconer .

1967

P173 Photocatalytic Degradation of Toluene in Aqueous Suspensions of Polycrystalline Ti02 in the Presence of the Surfactant Tetradecyldimethylamino-Oxide V . Augugliaro, V . Loddo, G. Marci, L. Palmisano, C. Sbriziolo, M. Schiavello and M.L. Turco Liveri

1973

P174 Photooxidation of Benzene to Phenol by Ru Complex Occluded in Mesoporous FSM-16 K. Fujishima, A. Fukuoka and M. Ichikawa .

1979

Session P2 Catalysis on Metal and Metal Oxides

PI75 Hydrodechlorination of CC12F-CFj (CFC-114a) over Silica-Supported Noble Metal Catalysts T. Mori and Y. Morikawa .

1985

P176 Hydrodechlorination of CCI2F2 (CFC-12) by Carbon- and MgF2Supported Palladium and Palladium-Gold Catalysts A. Malinowski, W. Juszczyk, J. Pielaszek, M. Bonarowska, M. Wojciechowska and Z. Karpinski. .

1991

P177 C-C Bond formation during Hydrodechlorination of CCId on PdContaining Catalysts E.S. Lokteva, V.V. Lunin, E.V. Golubina, V. I. Simagina, M. Egorova and I.V. Stoyanova .

1997

P178 Hydrodehalogenation of Aryl Halides by Hydrogen Gas and Hydrogen Transfer in the Presence of Palladium Catalysts M.A. Aramendia, V . Borau, I.M. Garcia, C. JimCnez, J.M. Marinas, A. Marinas and F.J. Urban0 .

2003

P179 Carbon-Supported Palladium Catalystsfor Liquid-Phase Hydrodechlorination of Carbon Tetrachloride to Chloroform L.M. G6mez Sainero, J.M. Grau, A. Arcoya and X.L. Seoane

2009

P180 Dispersed Pd-Ag Alloys for Selective Production of 0leJnsJi.om Chlorinated Alkanes B. Heinrichs, J.-P. Schoebrechts and J.-P. Pirard .

.

2015

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Surface Structure and Methylcyclobutane Hydrogenolysis Activity of Pt/AlzO3 and Pt/CeOz a+ Reduction at Increasing Temperature (3 73 to 973 K) G. Rupprechter, J.J. Calvino, C. Lopez-Cartes, M. Fuchs, J.M. Gatica, J.A. PCrez Omil, K. Hayek and S. Bernal . Hydrogenation ofXylose to Xylitol :Three Phase Catalysis by Promoted Raney Nickel, Catalyst Deactivation and In-situ Sonochemical Catalyst Rejuvenation J.-P. Mikkola, T. Salmi, R. Sjoholm, P. M&i-Arvela and H. Vainio Immobile Silica Fibre Catalyst in Liquid Phase Hydrogenation T . Salmi, P. M&i-Arvela, E. Toukoniitty, A.Kalantar Neyestanaki, L.E Lindfors, R. Sjoholm and E. Laine . Lowering the Trans-Isomer Content in Hydrogenation of Triglycerides of Unsaturated Fatty Acids at Ambient Temperatures I.A. Makaryan, O.V. Matveeva, G.I. Davydova and V.I. Savchenko Size Particle Effect and Copper or Silver Addition Effect on Catalytic Properties of Rhodium Supported onto Amorphous Silica Z. Ksibi, A. Ghorbel, B. Bellamy and A. Masson . Kinetic Studies on the Hydrogenation of 1,3- Butadiene, I-Butyne and their Mixtures P. Shafer, N. Wuchter and J. Gaube . Selective Hydrogenation of Nitrobenzene in Phenylhydroxylamine on Silica Supported Platinum Catalysts L. Pernoud, J.P. Candy, B. Didillon, R. Jacquot and J.M. Basset . Hydrogenation of Carbonylic Compounds on Pt/Si02 Catalysts Modified with SnBu4 G.F. Santori, M.L. Casella, G.J. Siri, O.A. Ferretti, A. Moglioni and G. Moltrasio Iglesias . Catalytic Behaviour of Several Ni/NiAl20 Catalysts in the Hydrogenation of 1,2,4-Trichlorobenzeneand Benzene to Cyclohexane Y . Ceseros, P. Salagre, F. Medina and J.E. Sueiras . Catalytic Lifetime of Amine Metal Complexes Supported on Carbons in Ciclohexene Hydrogenation J.A. Diaz Auiion, M.C. R o m b Martinez, P.C. l'hgentiere and C. Salinas Martinez de Lecea. Promoting Effect of Ni in Semi-Hydrogenation of 1,3-Butadieneover Ni-Pd Catalysts A. Sarkany .

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P192 Influence of the Surface Structure of Co on the Selective Hydrogenation of Crotonaldehyde E.L. Rodrigues, C.E.C. Rodrigues, A.J. Marchi, C.R. Apesteguia and J.M.C. Bueno . P193 Hydrogenation of Olejns in Aqueous Phase, Catalized by LigandProtected and Polymer-Protected Rhodium Colloids A. Borsla, A.M. Wilhelm, J.P. Canselier and H. Delmas . P194 Preparation and Characterisation of Ni-Mg-A1 Hydrotalcites as Hydrogenation Catalysts E. Romeo, C. Royo, A. Monzbn, R. Trujillano, F.M. Labajos and V. Rives P 195 Hydrogenation of Acrylonitrile-Butadiene Rubbers with Palladium Loaded Mesopore-Size Controlled Clay Minerals M. Shirai, K. Torii and M. Arai . P196 Single-Stage Liquid Phase Hydrogenation of Maleic Anhydride to y-Butyrolactone, 1,4-Butanediol and Tetrahydrofurane on Cu/ZnO/Al203catalysts A. Kuksal, E. Klemm and G. Emig . PI97 Methanol Decomposition to Synthesis Gas Over Supported Pd Catalysts Preparedfiom Synthetic Anionic Clays T. Shishido, H . Sameshima, T. Hayakawa, S. Hamakawa, E. Tanabe, K. Ito and K. Takehira P198 Methanol Decomposition on Unpromoted and Zn Promoted Cu/Si02 Catalysts M . Clement, Y . Zhang, D.S. Brands, E.K. Poels and A. Bliek . P199 Comparative Study of P and Mn Mod$ed CuO/Si02Catalystfor Methanol Dehydrogenation R. Zhang, Y . Sun and S. Peng P200 Decomposition of Ethanol on Rh(100) and Rh(100)c(2x2)-Mn Surfaces . R. Zhai, Z. Tian, H. Luo, D. Liang and L. Lin P201 Use of New Tin orthophosphates as Catalystsfor the Gas Phase Dehydrogenation-Dehydrationof Alcohols M.A. Aramendia, V . Borau, C. JimCnez, J.M. Marinas, M.L. Ortega, F.J. Romero, J.R. Ruiz and F.J. Urbano . P202 Steam Reforming of Ethanol Using Cu-Ni Supported Catalysts . F. Man'fio, M. Jobbagy and G. Baronetti, M. Laborde P203 Activity and Stability of Single and Perovskite-Type Manganese and Cobalt Oxides in the Catalytic Combustion of Acetone A. Gil, N . Burgos, M. Paulis, M. Montes and L.M. Gandia.

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Efect of Copper Addition on the Activity and Selectivity of Oxide Catalysts in the Combustion of Carbon Particulate C. Pruvost, J.F. Lamonier, D. Courcot, E. Abi Aad and A. Aboukai's The Efect of Mn Substitution on the Catalytic Properties of Ferrites L.C.A. Oliveira, R.M. Lago, R.V.R.A. Rios, P.P. Sousa, W.N. Mussel and J.D. Fabris Oxygen Nonstoichiometry and Linear Deformation of La-Sr-Fe-Co-0 Perovskites within the Redox Cycle . L.A. Rudnitsky, V.V. Aleksandrovskii and S.Y. Stefanovich Molecular Mechanism of Surface Recognition during the Adsorption/ Degradation of Organic Compounds on Iron Oxides . J. Bandara, J. Mielzcarski and J. Kiwi Characterization and Catalytic Activity of Mixed Cu-Cr/Ce02 Catalyst W. Daniell, P. Grotz, H. Knozinger, N.C. Loyd, C. Bailey and P.G. Harrison . Acid Strength of Support Materials as a Factor Controlling Catalytic Activity of Noble Metal Catalystsfor Catalytic Combustion Y. Yazawa, H. Yoshida, N. Takagi, N. Kagi, S. Komai, A. Satsurna, Y. Murakami and T. Hattori . Structural Features and Activity for CO Oxidation of LaFeXNil,03+6 Catalysts H. Falcbn, J. Baranda, J.M. Campos Martin, M.A. Peiia and J.L.G. Fierro. Amorphous Alloys as Catalystsfor Hydrogen Oxidation M. Stancheva, St. Manev, D. Lazarov and E. Stancheva

.

Precursor of Iron Catalystfor Ammonia Synthesis: Fe3O4, Fel,O, Fe203 or their Mixture? L. Huazhang and L. Xiaonian Production of Alkenes over Chromia Catalysts: Effect of Potassium on Reaction Sites and Mechanism S.D. Jackson, I.M. Matheson, M.-L. Naeye, P.C. Stair, V.S. Sullivan, S.R. Watson and G. Webb . Acid-base Site Requerimentsfor Elimination Reactions on Alkali Promoted MgO V.K. Diez, C.R. Apesteguia and J.I. Di Cosimo . Structure-Reactivity Correlations Across the Pressure-Gap Studied on Epitaxial Iron Oxide Model Catalyst Films C. Kuhrs and W. Weiss .

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xli Catalytically-Mediated Generation of HNO2 in Highly HN03 Concentrated Media S. Guenais-Langlois, C. Bouyer, J.-C. Broudic, A. Ananiev and B. Coq Catalytic Nitrate Reduction: Kinetic Investigations . U . Priisse, J. Daum, C. Bock and K.-D. Vorlop Liquid Phase Isomerization of Propadiene to Methyl Acetylene on Modi3ed Alumina Catalysts R. Dotzel, M. Reif and E. Klernm . The Eflects of Raney Alloy Structure on the Activation Process and the Properties of the Resulting Catalyst S. Knies, M. Berweiler, P. Panster, H.E. Exner and D.J. Ostgard . Raney-Nickel-Iron Catalysts Obtained by Mechanical Alloying: Characterization and Hydrogenation Activity J. Salmones, B.H. Zeifert, J.A. Hernandez, R. Reynoso, N. Nava, J.G. Cabafias Moreno and G. Aguilar Rios . Reduction of Benzaldehyde on Copper Supported on SiO2. Effect of Method of Preparation . A. Saadi, M.M. Bettahar and Z. Rassoul Some Phenomenological and Mechanistic Aspects ofthe Use of Copper as Catalyst in Trichlorosilane Synthesis H. Ehrich, T . Lobreyer, K. Hesse and H. Lieske . The Role of Oxygen Vacancies in Zirconia on the Dispersion, Stabilisation and Reactivity in the Presence of 0 2 of Supported Rh Particles G. Centi, B. Panzacchi, S. Perathoner and F. Pinna . Interactions among Rh, Mn and Li Components in the Rh-Based Catalysts Y . Wang, Z . Song, D. Ma, H. Luo, D. Liang and X. Bao . Direct Synthesis of Propionamide and Propiononitrilefrom Ethylene Carbon Monoxide, and Ammonia Using Supported Ru Catalyst S.-P. Zhao, S.-I. Sassa, H. Inoue, M. Yamakazi, H. Watanabe, T. Mori . and Y. Morikawa Structure, Surface State and Catalytic Properties of a Model Platinum Catalyst J. Find, Z. Pail, H. Sauer, R. Schlogl, U. Wild and A. Wootsch . Structure of Ultradisperse Pt Powders and their Performance in the Partial Oxidation of C-H Bonds by Molecular Oxygen A.P. Suknev, V.B. Goncharov, V.I. Zaikovskii, A.S. Belyi, N.I. Kuznetsova, V.A. Likholobov and B.S. Bal'zhinimaev .

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xlii

P228 Eflect of Water Vapour on the Catalytic Activity of Supported Platinum Catalysts S. Lunzer and R. Krarner .

2303

P229 Stabilization of Molecular Pd Species in a Heterogenized Wacker-Type Catalystfor Low Temperature CO Oxidation E.D. Park and J.S. Lee

2309

P230 Surface Properties of Palladium Supported on Cerium Oxide and its Catalytic Activity for Methanol Decomposition Y . Matsurnura, Y . Ichihashi, Y. Morisawa, M. Okurnura and M. Haruta .

2315

P23 1 Deactivation of Palladium Catalysts Supported on Functionalized Resins in the Reduction ofAromatic Nitrocompounds M. Krhlik, V . Kratky, M. Hronec, M. Zecca and B. Corain .

2321

P232 Metal Palladium Dispersed Inside Macroporous Ion-Exchange Resins: Textural Characterization and Accessibility to Gaseous Reactants A. Biffis, K. Jerabek, A,A, D'Archivio, L. Galantini and B. Corain

2327

P233 Novel Supported Heterogeneous Palladium Catalystsfor Carbon-Carbon Forming Reactions E.B. Mubofu, J.H. Clark and D.J. Macquarrie .

2333

P234 Passivation of Nickel Activity in the Nickel-Nb205-Si02System at Low Metal Contents M.M. Pereira, E.B. Pereira, Y.L. Lam, L.T. dos Santos and M. Schmal

2339

.

P235 Gold Based Environmental Catalyst L.A. Petrov .

2345

P236 Evolution of a New Reaction Pathway in Propene-Deuterium Exchange Reaction by the Co-adsorption of CO over Rh, Ir, and 0 s Carbonyl Cluster Derived Catalysts Supported on Alumina S. Naito, K. Oguni, T. Naito and T. Miyao .

235 1

P237 Studies on a ChromidLanthanalZirconiaAromatization Catalyst D.L. Hoang, A. Trunschke, A. Briickner, J. Radnik and H. Lieske .

2357

P238 The Reforming Catalytic Properties of Bulk and Supported Molybdenum and Tungsten Dioxides, X02 (X = Mo, W). A. Katrib, L. Urfels and G. Maire .

2363

Refining and Petrochemistry

P239 Active Sites of the Isomerisation of n-Butane over Oxygen-Modijed Molybdenum Carbide and Molybdenum Oxides S . Liebig, T. Gerlach, Kh. Doukkali and W. Griinert

2369

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AlzOj-Promoted Sulfared Zirconia Catalystsfor the Isomerization of n-Butane R. Olindo, F. Pima, G. Strukul, P. Canton, P. Riello, G. Cerrato, G. Meligrana and C. Morterra . n-Pentane Isomerization over Mesoporous Sulfated Zirconia Catalysts M. Risch and E.E. Wolf . Bzfunctional Ni, Pt Zeolite Catalystsfor the Isomerization of n-Hexane M.H. Jordao, V. Simoes, A. Montes and D. Cardoso About the Importance of Surface Hydrogen Availability during n-Hexane Isomerization over Platinum on Tungsten Oxide Promoted Zirconia M.G. Falco, S.A. Canavese, R.A. Comelli and N.S. Figoli . Highly Dispersed Framework Zirconium Phosphates-Acid Catalysts of Hexane Isomerization S.N. Pavlova, V.A. Sadykov, G.V. Zabolotnaya, D.I. Kochubey, R.I. Maximouskaya, V.I. Zaikovskii, V.V. Kriventsov, S.V. Tsybulya, E.B. Burgina, A.M. Volodin, N.M. Ostrovskii, A.M. Simakov, T.A. Nikoro, V.B. Fenelonov, M.V. Chaikina, N.N. Kmetsova, V.V. Lunin, D. Agrawal and R.Roy . Zeolites and Natural Materials as Catalystsfor n-Heptane Hydroisomerization Reaction A. Brito, M.C. Alvarez, F.J. Garcia, M.E. Borges and M. Torres . Influence of the Zeolite Structure on the Hydroisomerization of n-Heptane A. Patrigeon, E. Benazzi, Ch. Travers and J.Y. Bernhard . Hydroisomerization of Octane on Pt/AI-Pillared Vermiculite and Phlogopite and Comparison with Zeolites F.J. del Rey P.C., M.L. Shchez Henao and G. Poncelet . Slurry Isomerization of n-Hexadecane over Moo3-Carbon-Mod$ed, Pt/PZeolite, Pt/ZSM 22 and Pt/SAPO 1 I Catalysts at Medium Pressure S. Roy, C. Bouchy, C. Pham-Huu, C. Crouzet and M.J. Ledoux . Zinc Oxide Modified by Alkylsilylation as an Efficient Catalystfor Isomerization of Hydrocarbon Y . Imizu, T. Narita, Y. Fujito and H. Yamada . Eflect of Redox Treatments on the Surface State of Pd- W-Based Catalysts and on the Skeletal Rearrangement of Hydrocarbons. C. Bigey, F. Garin, L. Hilaire and G. Maire .

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xliv

P25 1 Acid or Bzfunctional Mechanism in ParafJin Isomerization Reaction and Pt/W03-Zr02 Catalysts on P~/SO;--Z~O~ J.C. Yori, C.L. Pieck and J.M. Parera

P252 Catalytic Cracking of Heavy Oil Fractions over Natural Zeolite Contained Composites R.H. Ibrasheva and K.A. Zhubanov . P253 Evidence for DifSerent Reaction Mechanisms as the Cause of the Enhanced Hydrocarbon Cracking Activity of Steamed Y Zeolites . B.A. Williams, W. Ji, J.T. Miller, R.Q. Snurr and H.H. Kung P254 TPR and C02-TPDof Composite Titania (Anatase)-Alumina Systems as Potential Vanadium Trapsfor Fluid Catalytic Cracking (FCC) F. Hernimdez Beltrim, E. Mogica Martinez, E. L6pez Salinas and M.E. Llanos Serrano . P255 Formation of SulJitr-Containing Compounds Under Fluid Catalytic Cracking Reactions P. Leflaive, J.L. Lemberton, G. Perot, C. Mirgain, J.Y. Carriat and J.M. Colin P256 The Use of Catalytic Cracking Technologyfor the Creation of Novel Processes for Oil Refinery and Petrochemistry M.I. Levinbuk and N.Y. Usachev . P257 Transformation of Methylcyclohexane over Fresh HFA U and HMFI Zeolites: Reaction Scheme and Kinetic Modeling H.S. Cerqueira, M.G.F. Rodrigues, P. Magnoux, D. Martin and M. Guisnet P258 Mechanism of n-Hexane Cracking on MFI (Si/Al= 10) L. Isernia, A. Quesada, J. Lujano and F.E. Imbert . P259 Hydrocracking of Heavy Straight Run Naphtha with Pt Supported on Zeolites Y, USY, ZSM5 and Beta H. Ortiz Soto and L.J. Hoyos Marin . P260 Hydrocracking of Phenanthrene over Pt/Si02-A1203,Pt/H-Y, Pt/H-Beta and Pt/H-ZSM-5 Catalysts: Reaction Patway and Products Distribution L. Leite, E. Benazzi, N. Marchal-George and H. Toulhoat . P261 Hydrogenation and Dehydrogenation of Hydrocarbons over Ni Supported on Alumina- and Silica-Promoted Titania G. Ptrez and T. Viveros . P262 Dehydroisomerization of N-Butane on PT-ZN/H-Mordenite M.M. Ramirez-Coreedores, T . Romero and M. Gordlez .

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xlv Bimetallic Pt-M(M=Ga,ln,T1) Silica Supported Catalystsfor lsobutane Dehydrogenation J. Llorca, P. Ramirez de la Piscina, J. Lebn, J. Sales, J.L.G. Fierro and N. Homs . Aromatization of Ethane on Modijied Zeolites in the Presence of co-reactans . F. Roessner, A. Hagen and J. Heemsoth Effect of Barium and Lanthanum Oxides on the Properties of Pt/KL Catalysts in the n-Heptane Dehydrocyclization J.M. Grau, L.M. Gomez Sainero, L. Daza, X.L. Seoane and A. Arcoya Effect of Lanthanum Doped y-A1203 in the Selectivity of Reforming Pt-Pb Supported Catalysts. . G. del Angel, G. Torres and V. Bertin Regeneration and Oxidation-Reduction Cycles of Vapor Phase and Incipient Wetness Impregnation Pt/KL Catalysts F. Ghadiali, G. Jacobs, A. Pisanu, A. Borgna, W.E. Alvarez and D.E. Resasco Mechanism of Iso-butane Alkylation by Butenes over H3PWlr040, H3PTiW11039and Zr Supported Catalysts E.A. Paukshtis, V.K. Duplyakin, V.P. Finevich, A.V. Lavrenov, V.L. Kirillov, L.I. Kuznetsova, V.A. Likholobov and B.S. Bal'zhinimaev. Porous 12-TungstophosphoricAlkaline Salts for Isobutane/Butene Alkylation. lnfuence of Protonic Density and Surface Polarity of the HPA. Effects of the Supercritical Phase P.Y. Gayraud, N . Essayem and J.C. Vedrine Influence of the Metal Content on the Amount and the Nature of the Coke Formed during Isobutane/2-Butene Alkylation over Ni-Y Zeolite P.A. Arroyo, C.A. Henriques, E.F. Sousa Aguiar, A. Martinez and J.L.F. Monteiro . The Role of Hydride Transfer in Zeolite Catalyzed Isobutane/Butene Alkylation 2561 . G.S. Nivarthy, A. Feller, K. Seshan and J.A. Lercher Mechanistic Routes of Low Temperature Alkane Activation over Zeolites J.A.Z. Pieterse, K. Seshan and J.A. Lercher .

2567

Catalytic Conversion of n-Parafins in Supercritical Phase W. Wei, F. Li, J. Ren, Y. Sun and B. Zhong.

2573

Sulphur Resistant Palladium-Platinum Catalysts Preparedfiom Mixed Acetylacetonates A.J. Renouprez, A. Malhomme, J. Massardier, M. Cattenot and G. Bergeret .

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xlvi

P275 Production of High Cetane Diesel Fuels by Simultaneous Hydrogenation of Aromatics and Ring Opening of Naphthenes over Bzfimctional Molecular Sieves Based Catalysts M.A. Arribas and A. Martinez

P276 Formation of Diisopropyl EtherJLoin 2-Propanol Using Keggin-Type H3[W12P040]and H4[W12Si040]Heteropolyacids Supported on Zirconia E. Lopez Salinas, I.G. Hemiindez Cortez, J. Navarrete, M. Salmon and I. Shifter . P277 The Mechanism of Diisopropyl Ether Synthesispom a Feed of Propylene and Isopropanol over Ion Exchange Resin F.P. Heese, M.E. Dry and K.P. Moller . P278 The Conversion of 2,3-Butanediol to Methyl Ethyl Ketone over Zeolites J. Lee, J.B. Grutzner, W.E. Walters and W.N. Delgass . P279 Effect of Internal Dzffusion on Liquid-Phase Synthesis of MTBE R. Pla, J. Tejero, F. Cunill, J. Felipe Izquierdo, M. Iborra and C. Fit6 P280 Acid and Catalytic Properties of Supported Sulfpolyphenylketones in the Formation of DIPE, MTBE and TAME Ethers T. Jarecka, St. Miescheriakow and J. Datka . P28 1 Effect of the Addition ofAcidic and Basic Compounds on the Catalytic Behavior of Exchanged Zeolites in the Alkylation of Toluene with Methanol . A. Borgna, J. Sepulveda, S. Magni and C. Apesteguia P282 Zeolite Beta: Selective Molecular Sievefor Synthesis ofXylenesfiom Trimethylbenzenes J. Cejka and A. Krejci P283 Characterisation and Reactivity of Supported and Unsupported B/P/O Catalysts in the o-Alkylation of Diphenols with methanol F. Cavani, T . Monti and D. Paoli . P284 Oxidative Alkylation of Isobutene, Propene and Toluene with Methanol . I.P. Belomestnykh and G.V. Isaguliants P285 Side Chain Methylation of Toluene and Ethylbenzene with Dimethylcarbonate over Alkaline X-Zeolite R. Bal and S. Sivasanker . P286 The effect of the particle size on methanol conversion to ligh olefins De Chen, K. Moljord and A. Holmen

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xlvii

P287 Synthesis of N-Isopropylacrylamide Synthesis Acrylonitrile and Isopropyl Alcohol over Solid Acids X . Chen, H. Matsuda and T. Okuhara

2657

P288 Pumice as Catalyst in the Claus Reaction A. Brito, R. Larraz, R. Arvelo, M.T. Garcia and M.E. Borges

2663

.

P289 Supported Alkali Salt Catalysts Active for the Guerbet Reaction between Methanol and Ethanol K. Gotoh, S . Nakamura, T. Mori and Y. Morikawa .

2669

P290 Catalyst Porous Structure and Acidity Effects on Alkylphenol Transformations 2675 . J.C. Hernhdez, F.E. Imbert, P. Ayrault, M. Guisnet and N.S. Gnep Reactor Engineering and Catalytic Processes

P291 Natural Gas Utilization through CO2 Reforming in a Membrane Reactor T.M. Raybold and M.C. Huff.

268 1

P292 Activity and Selectivity of a Pd/y-A1203 Catalytic Membrane in the Partial Hydrogenation of Acetylene C. Lambert, M. Vincent, J. Hinestroza, N. Sun and R. Gonzalez .

2687

P293 Catalysis of Palladium Salt Reduction in a Gas-Liquid Membrane Reactor S. Miachon, A. Mazuy, J.-A. Dalmon .

2693

P294 Propane Aromatization in a Silicalite-1 Membrane Reactor W . Yang, P. Yang, X. Xu and L. Lin. P295 Effects of Operation Modes on the Oxidation of Propane to Acrolein in a Membrane Reactor W . Yang, P. Yang, W. Fang and L. Lin .

2705

P296 The Conversion of Methanol to Hydrocarbons over ZSMS: Fixed Bed vs. Recycle Reactor K.P. Moller, W . Bohringer, A.E. Schnitzler and C.T. O'Connor

.

271 1

P297 Catalytic Dehydrogenation of n-Butane in a Fluidized Bed Reactor with Separated Coking and Regeneration Zones C. Callejas, J. Soler, J. Herguido, M. Menendez and J. Santarnaria.

2717

P298 Study in a Riser Simulator Reactor of the Role of a HZSM-5 Zeolite as FCC Catalyst Additive J.M. Arandes, I . Abajo, I. Fernhdez, J. Bilbao, H.I. de Lasa .

2723

P299 Difision Analysis of Cumene Cracking over ZSMS Using a Jetloop Reactor P. Schwan, K.P. Moller and J.P. Henry .

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Monolithic Catalysts as more Eficient Reactors in Three-PhaseApplications T.A. Nijhuis, F. Kaptjein and J.A. Moulijn . Synthesis of Ethylene Oxide in a Catalytic Microreactor System H. Kestenbaum, A. Lange de Oliveira, W. Schmidt, F. Schiith, W. Ehrfeld, K. Gebauer, H. Lowe and Th. Richter Develoment of a Novel Structured CataIytic Reactorsfor Highly Exothermic Reactions . G. Groppi, C. Cristiani, M. Valentini and E. Tronconi Simulation of Alternative Catalyst Bed Configurations in Autothermal Hydrogen Production A.K. Avci, D.L. Trimm and Z.I. 0nsan . Development and Study of Metal Foam Heat-Exchanging Tubular Reactor: Catalytic Combustion of Methane Combined with Methane Steam Reforming Z.R. Ismagilov, O.Yu. Podyacheva, V.V. Pushkarev, N.A. Koryabkina, V.N. Antsiferov, Yu.V. Danchenko, O.P. Solonenko and H. Veringa . Optimization in Design, Testing and Reactor Use by Statistical Modeling of the Reaction Variables, Energy Input and Reaction Time E. Balanosky, F. Herrera and J. Kiwi Catalysis on Sulfides, Nitrides and Carbides

Synthesis of Highly Dispersed Molybdenum and Ruthenium Sulfides Using Tetraalkylammonium Surfactant P. Afanasiev, G.-F. Xia, B. Jouguet and M. Lacroix. Properties of Sonochemically Synthesized, Highly Dispersed MoS2/AI203 Catalysrsfor the Hydrodedulfurization of Dibenzothiophene and 4,6Dimethyldibenzothiophene J.J. Lee, C. Kwak, Y.J. Yoon, T. Hyeon and S.H. Moon . Structure and Catalysis of Intrazeolite Co-Mo Binary Sulfide Model Clustersfor Hydrodesuljkrization Y. Okamoto, H. Okamoto and T. Kubota . An Inelastic Neutron Scattering Study of the Interaction of Thiophene with a Hydrodesulfurisation Catalyst P.C.H. Mitchell, D.A. Green, E. Payen, J. Tomkinson and S.F. Parker Characterization of Oxide Catalysts Using Time-Resolved XRD and XANES: Properties of Pure and Sulfided CoMo04 and NiMoO4 J.A. Rodriguez, J.C. Hanson, S. Chaturvedi and J.L. Brito .

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E$ect of the Support Porosity on the Thiophene and Dibenzothiophene Hydrodesulfirization Reactions. A1203-Ti02Mixed Oxide Support T . Klimova, J. Ramirez, R. Cuevas and H. GonAlez . TPR-S and TPS Studies of CoMo and NiMo Catalysts Supported on Al203-Ti02 Mixed Oxides L. Cedefio, R. Zanella, J. Ramirez and A. L6pez Agudo . Characterization of Reduced and Sulphided Ru-V/A1203Catalysts C.E. Scott, J. Guevara, A. Scaffidi, E. Escalona, C. Bolivar C., M.J. PCrez Zurita and J. Goldwasser . Sulfir Effect on Mo2N/y-ALO3Catalyst Studied by In Situ FT-IR Spectroscopy Z. Wu, Y. Chu, S. Yang, Z. Wei, C. Li and Q. Xin . Concerted Mechanisms in the Heterogeneous Catalysis by SulJides A.N. Starsev . Selective Promoting Effect on Alkylbenzothiophenes Hydrodesuljimization Pathways F. Bataille, J. Mijoin, J.L. Lemberton, G. Perot, C. Berhault, M. Lacroix, . F. Mauge, S. Kasztelan and M. Breysse Deep Hydrodenitrogenation on Pt Supported Catalysts in the Presence of Hfi, Comparison with NiMo Sulfide Catalyst E. Peeters, C. Geantet, J.L. Zotin, M. Breysse and M. Vrinat . Hydrotreating of Heavy Gas Oil on Unsupportesd and Supported Mo-, W-, Nb-Nitrides J.A. Melo Banda, J.M. Dominguez and G. Sandoval Robles . Evidence for the Effect of Phosphorus on Molybdenum Oxynitrides and Oxycarbides, Activity and Selectivity in Propene Hydrogenation and n-Heptane Isomerization P. PCrez Romo, C. Potvin, J.-M. Manoli and G. Djeda-Mariadassou Degradation of a Spent Hydrotreating Cata1yst:interaction with Enviroment J.C. Afonso, T . Siqueira de Lima and P.C. Campos . Influence of Sulfur Containing Compounds on High Temperature Coke Formation on Hydrotreating Catalyst R. Lebreton, S. Brunet, G. Ptrot, V. Harle and S. Kasztelan Deactivation of MoS2 Cataysts during the HDS of Thiophene. Effect of Nickel Promoter F. Pedraza, S. Fuentes, M. Vrinat and M. Lacroix .

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P323 Deactivation and Testing of Molybdenum Nitride Catalysts in Hydrodesulfirization of Dibenzothiophene M. Nagai, M. Kiyoshi and S. Omi .

P324 Development of a Method for the S t u 4 of the Deactivation of HDS Catalysts J.M. Trejo and F. Albertos . P325 Deactivation by Oxygen and Subsequent Activation of Bulk Mo2C for Benzene Hydrogenation at 298 K J.-S. Choi, G. Bugli and G. Djega-Mariadassou . P326 Silicon Carbide Supported NiS2 Catalystfor the Selective Oxidation of H2S in Claus Tail Gas M.J. Ledoux, C. Pham-Huu, N. Keller, J.B. Nougayrede, S. Savin-Poncet and J. Bousquet . Molecular Sieves

P327 Novel Method of Preparing Zeolite Filmsfrom Colloidal Zeolite Nay H-Q. Lin, Z-S. Chao, T-H. Wu, G.-Z. Chen, F-X. Zhang, H-L. Wan And K.-R. Tsai P328 Evidence of the Supermicropores Creation in Zeolites. Dealumination of Narrow-Pore Zeolites P. Hudec, A. Smieskova, Z. Zidek and E. Rojasova . P329 Transition through Various Regimes of Reaction Kinetics and Selectivity Driven by Progressing Modification of the External Surface of Zeolites H.P. Roger, K.P. Moller, W. Bohringer and C.T. O'Connor P330 Polycarbonylic and Polynitrsylic Species in ~ u ' - ~ x c h a n ~ZSM-5, e d Beta, Mordenite and Y Zeolites: Comparison with Homogeneous Complexes G. Turnes Palomino, A. Zecchina, E. Giamello, P. Fisicaro, G. Berlier and C. Lamberti . P33 1 The Characterization of Zeolite Acidity by Dinamic Methods Gy. Onyesty&, J.Valyon and L.V.C.Rees . P332 Textural Characterization of Zeolites by Adsorption of Molecules of Different Size Followed by Adsorption Microcalorimetry J.M. Guil, R. Guil Lopez and J.A. Perdigon Mel6n . P333 Unusual bomerization Routes of n-Butenes on the Acidic OH(0D) Groups on Ferrierite Zeolite Studied by FT-IR J.N. Kondo, E. Yoda, M. Hara, F. Wakabayashi and K. Domen .

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P334 Use o f ' 2 9 ~N eMR Spectroscopyfor the Study of Gaseous Reactant Dzfision in a Fixed Bed of Zeolite; Application to Benzene Difision in HZSM-5 P. N'Gokoli-Kekele, M.-A. Springuel-Huet, J.-L. Bonardet and J. Fraissard

P335 Ti-0-Si and Fe-0-Si Valence Angles of Tetrahedral Ti and Fe Sites in Porous Silicas of Diflerent Structures F. BCland and B. Echchahed and L. Bonneviot . P336 FTIR Study of the Proton Transfer in Al-MCM-22 Zeolite to Insaturated Hydrocarbons B. Onida, F. Geobaldo, F. Testa, F. Crea and E. Garrone . P337 Sorption of Methanol in Alkali Exchanged Zeolites M. Rep, A.E. Palomares, J.G. van Ohrnmen, L. Lefferts and J.A. Lercher . P338 Structural Lewis Sites in Zeolite Beta - Role on Coking of the Catalyst A. Vimont, F. Thibault-Startzyk and J.C. Lavalley . P339 Mechanism of Coking and Regeneration of Catalysts Containing ZSM-5 Zeolite V.G. Stepanov, E.A. Paukshtis, V.V. Chesnokov and K.G. Ione . P340 Effect of Dealumination of Mordenite Catalystfor Amination Reaction of Ethanolamine K. Segawa and T. Shimura . P34 1 Synthesis and Characterization of Pd-Zr- and Pd-Rh-Beta Zeolite Catalystsfor Removal of EmissionsJi.om Natural Gas Driven Vehicles N. Kurnar, F. Klingstedt and L.-E. Lindfors . P342 Catalytic Behavior of the Microporous Hexagonal Zincophosphate CZP in Base-Catalyzed Reactions L.A. Garcia Serrano, J. PCrez Pariente, F. Shchez and E. Sastre . P343 Basic CsBeta - A New Support for Pt Nanoparticles Active in Aromatization of Parafins C. Jia, A.P. Antunes, J.M. Silva, M.F. Ribeiro, M. Lavergne, M. Kermarec and P. Massiani P344 Hydroformylation of Olefins Using Encapsulated HRh(CO)(PPhJ 3 in Nay Zeolite as a Catalyst K. Mukhopadhyay, V.S. Nair and R.V. Chaudhary . P345 Spectroscopic and Catalytic Investigation ofthe NO Reactivity on CoAPOs with Chabasite-Like Structure L. Marchese, E. Gianotti, B. Palella, R. Pirone, G. Martra, S. Coluccia . and P. Ciambelli

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P346 Bifinctional Zeolite Catalystsfor the Selective Synthesis in One Step of Various Ketones P. Magnoux, N. Lavaud, L. Melo, G. Gianetto, A.I. Silva, F. Alvarez and M. Guisnet P347 Considerations about the Bzfunctionallity and the Gallium Species Role in the Propane Aromatization Reaction over [Gal- and [Ga,AI]-ZSM-5 Catalysts A. Montes and G. Gianetto . P348 Novel Shape-Selective Catalystsfor Synthesis of 4, 4'-Dimethylbiphenyl J.-P. Shen, L. Sun and C. Song .

P349 Investigation of n-Pentane and Cyclohexane Total Oxidation on the Cu-Containing ZSM-5 Zeolites M.A. Botavina, M.-V. Nekrasov and S.L. Kiperman P350 Textural Properties, Stability and Catalytic Activity of Acidic Mesoporous Materials Obtained By Direct Incorporation of Aluminum Y.-H. Yue, A. GCdCon, J.-L. Bonardet, J.B. D'Espinose and J. Fraissard . P351 Templating Fabrication and Catalysis of Platinum Nanowires in Mesoporus Channels of FSM-I6 A. Fukuoka, N . Higashimoto, M. Sasaki, M. Harada, S. Inagaki, Y. Fukushima and M. Ichikawa . P3 52 Physico-Chemical and Catalytic Properties of Ni-Containing Mesoporous Molecular Sieves of MCM-41 Type M. Ziolek, I. Nowak, I. Sobczak and H. Poltorak . P353 Preparation of Highly Structured V-MCM-41 and Determination of its Acidic Properties S. Lim and G.L. Haller P354 Post-Synthetic Preparation of Ti-MCM-41 Oxidation Catalysts with Titanium Alkoxides A. Hagen, K. Schueler and M. Voskamp . P355 Fe-MCM-41 as a Novel Sulfuric Acid Catalystfor SO2 Rich Feeds A. Wingen, N . Anastasieviec, L. Hollnagel, D. Werner and F. Schiith Session P3 Catalyst Characterisation P356 In situ Inpared Spectroscopic Study of the Reaction between CO and Hz over an SmzO3 Catalyst . Y: Sakata, T. k i h o t o , H. Imamura and S. Tsuchiya

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P357 In Situ Inpared Spectroscopic Investigation of 12-Tungstphosphoric Acid and its Cesium Hydrogen Salts in Relation to Catalytic Activiry G. Koyano, T . Saito, M. Hashimoto and M. Misono

3077

P358 The Oxidation State of Ru Catalysts under Conditions of Partial Oxidation of Methane Studied by XPS and FTIR Spectroscopy C. Elmasides, D.I. Kondarides, S. Neophytides and X.E.Verykios .

3083

P359 Correlation between "In Situ " XPS Characterization and Catalytic Activity of Moo&-A1203 for Hexenes and Hexanes Isomerization. Mechanistic Approach Using Probe Molecules V. Keller, F. Barath, F. Garin and G. Maire .

3089

P360 In Situ Observation of a Surface Catalysed Chemical Reaction by Fast X-Ray Photoelectron Spectroscopy A.F. Lee, K. Wilson and R.M. Lambert .

3095

P361 The Structure of Vanadium Oxide on y-Alz03: An In Situ X-Ray Absorption Spectroscopy Study during Catalytic Oxidation M . Ruitenbeek, F.M.F.de Groot, A.J. Van Dillen and D.C. Koningsberger

3101

P362 Flux Response Analysis - A New In Situ Techniquefor Catalyst Characterization K. Hellgardt, B.A. Buffham, M.J. Heslop, G. Mason and D.J. Richardson

3107

P363 New Method of the Surface Characterisation of a Metal Catalyst under Real Reactor Conditions using Electron Microscopy W. Arabczyk, K. Kalucki, U. Narkiewicz, D. Moszynski and A.W. Morawski

31 13

P364 Direct Observation of Reduction of PdO to Pd Metal by In Situ Electron Microscopy P.A. Crozier and A.K. Datye .

31 19

P365 Identification and Roles of the Different Active Sites in Supported Vanadia Catalysts by In Situ Techniques M.A. Baiiares, M. Martinez-Huerta, X. Gao, I.E. Wachs and J.L.G. Fierro

3125

P366 Design of an SFG-Compatible Uhv-High Pressure Reaction Cell: Studies of CO and NO Adsorption on Ni and NiO(l00) by IR-Vis Sum Frequency Generation Vibrational Spectroscopy G. Rupprechter, T . Dellwig, H. Unterhalt and H-J. Freund .

3131

P367 The Combined Use of Acetonitrile and Adamantane-Carbonitrilefor FTIR Spectroscopic Characterization of Acidity in Zeolites C. Otero Areh, M. Rodriguez Delgado, M. Peiiarroya Mentruit, . F.X. Llabres i Xamena and C. Morterra

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P368 Infiared Spectroscopic Evidence for Two Ways of Adsorbed CO Coordination A.A. Tsyganenko, C. Otero Arein and E. Escalona Platero .

3143

P369 Study of Layer-by-Layer Growth of Silica on Alumina and Alumina on Silica Using IR Spectroscopy of Adsorbed CO A.A. Tsyganenko, O.V. Manoilova, K.M. Bulanin, P.Yu. Storozhev, S. Haukka, A. Palukka and M. Lindblad .

3 149

P370 Applications of Inelastic Incoherent Neutron Scattering in Technical Catalysis P. Albers, G. Prescher, K. Seibold and S.F.Parker .

3155

P371 Hydrogen Species in Cerium - Nickel Oxide Catalysts: Inelastic and Compton Neutron Scattering Studies C. Lamonier, E.Payen, P.C.H.Mitchel1, S.F. Parker, J. Mayers and J.Tornkinson .

3161

P372 Using Atomic Force Microscopy (AFW to Study the Surface Structure of Oxide and Metal-Decorated Oxide Particules E. Dokou, W.E. Farneth and M.A. Barteau .

3167

P373 Study of Catalyst Preparation Processes by Atomic Force Microscopy (AFM: Adsorption of a Pt Complex on a Zeolite Surface M.KomiyamaandN.Gu .

3173

P374 Chemistry of SO2 on Model Metal and Oxide Cata1ysts:Photoemission andXANES Studies J.A. Rodriguez, T. Jirsak, S. Chaturvedi, J. Hrbek, A. Freitag and J.Z. Larese

3 177

P375 Changes in the Electronic Structure of Pt/CeOz Based DENOXCatalysts as a Function of the Reduction Treatment. Detection of a Pt-H Anti-Bonding Shape Resonance and Pt-H EXAFS J.H. Bitter, M.A. Cauqui, J.M. Gatica, S. Bernal, D.E. Ramaker and D.C. Koninsberger .

3183

P376 Bimetallic Nano-Particle Formation in the Pt-Re Reforming Catalysts Revealed by STEWEDX XANES/EAYFS and Chemical Characterization Techniques. EfSects of Water and Chlorine T . Gjeman, M. Rmning, R. Prestvik, B. Tstdal, C.E. Lyman and A. Holmen

3189

P377 Hyperfine Interactions on Pt-In Catalysts F.B. Passos, P.R.J. Silva and H. Saitovich .

3195

P378 Oxygen Atom Radical Formation on the Sol-Gel Molybdenum-Silica Catalysts Characterized by X-Ray Absorption Fine Structure Spectroscopy Y . Izumi .

3201

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P379 Structural Characterization of Pd-Ag and Pd-Cu Bimetallic Catalysts by means of EX4FS WAXS and XPS A. Longo, A. Balema, F. Deganello, L.F. Liotta, C. Meneghini, . A. Martorana and A.M. Venezia P380 Super Acidity Confirmed on a Monolayer of Sulfare Species Loaded on Zirconia N. Kadata, J-I. Endo, K-I. Notsu, N. Yasunobu, N. Naito and M. Niwa

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P38 1 Characterization and Catalytic Properties of Aerogels Suyated Zirconia . M.K.Younes, A. Ghorbel, A. Rives, R. Hubaut P382 Structure and Surface Properties of Zr02-Supported W03Nanostructures C.D. Baertsch, R.D. Wilson, D.G. Barton, S.L. Soled and E. Iglesia . P383 Proton Afinity in Heterogeneous Acid Base Catalysis. Measurements and Usefor Analysis of Catalytic Reaction Mechanism E.A. Paukshtis P384 Basic Sites on Mixed Nitrited Galloaluminiophosphates "AIGaPON" Confrontation of Spectroscopic and Catalytic Data S.Delsarte and P. Grange . P385 Study of Relevant properties injluencing the catalytic activity of layered double hydroxides in the Meixnerite-likeform F. Prinetto, G. Ghiotti and D. Tichit . P386 Copper Redox Chemistry in CuZSM-5 Zeolites: EPR, IR and DFT Investigations B. Gil, J. Datka, S. Witkowski, Z. Sojka and E. Broclawik . P387 Dynamics of Adsorbed Benzene on Ag-Y Zeolite Studied by Solid-State NMR A. GedCon, D.E. Favre and B.F. Chmelka . P388 Interaction of CO and NH3 with Noble Metal Cations Dispersed in ZSM-5 Zeolites. Spectroscopic and Microcalorimetric Investigation V . Bolis, S. Bordiga, V. Graneris, C. Lamberti, G. Turnes Palomino and A. Zecchina . P389 The Use of NMR Imaging and Mercury Porosimetry in the Modeling and Measurement of Coke Profiles in Deactivated Catalyst Pellets S.P. Rigby . P390 Low-loaded Metal Pd-Au Supported Catalysts on Active Carbon. Recent Developments of the X-Ray Dzji-action Analysis to Detect Simultaneously Nanoclusters and Larger Particles . P.Riello, P. Canton, A. Minesso, F. Pinna and A. Bennedetti.

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P391 Characterisation of Aukorbates and Surface Functional Groups on Polycrystalline Oxides by UltravioletPhotoelectron Spectroscopy (UPS) M. Heber and W.Griinert .

P392 Evidence of an Alloying Eflect in Zeolite Supported Pt-Pd Systems as Seen by Hidrogen Chemisorptionand Proton NMR T . Rades, M. Polisst-Thfoin and J. Fraissard. P393 Catalytic Probe ofAg Venters on Silver Halide and Supported Silver 1.1. Mikhalenko and V.D. Yagodovskii . P394 Change in the Redox Properties of PdRWCe02-ZrO2 Catalysts afier Ageing at 1323 and 1423 K S. Salasc, V. Perrichon, M. Primet, M. Chewier and N. Mouaddib-Moral . P395 XPS Study of AdTiO2 Catalytic Systems J.W. Sobczak and D. Andreeva . P396 Procedure-Sensitive HTR for Au/TiOz M.-Y. Lee, Y.-J. Lee and S.D. Lin . P397 Characterization of Amino-ModiJied Silicate Xerogels Complexed with Copper (IQ, Cobalt (III) and Chromnium (IIQ W.K. Jozwiak, E. Szubiakiewicz, A.M. Klonkowski, T. Widemik, W. Ignaczak and T. Paryjczak . P398 Active Sites on Well-CharacterizedPd/Si02 Determined by (-)-Apopinene Deuteriumation, Cyclohexene Hydrogenation and CS2 Titration G.V. Smith and D.J. Ostgard . P399 Structural and Catalytic Properties of Zr-Ce-0 Mixed Oxides. Role of the Anionic Vacancies S. Rossignol, C. Micheaud Especel and D. Duprez . P400 An X-Ray Photoelectron Spectroscopy Investigation of a-Alumina-Supported Nickel Catalysts Preparaedfiom Nickel(I~Acetylacetonate R. Molina, M. Genet and G. Poncelet P40 1 The Role of Oxide Promoters in the Dissociation of CO and its Reaction with Hydrogen on Pd (111) and Rh (I 1I): A Molecular Beam Study B. Klozter and K. Hayek . P402 Characterization of the Active Sites of Ni-Si-A1 Sol-Gel Hydrogenation Catalysts C. Guimon, N. El Horr, E. Romero and A. Monzon.

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Catalysis for Fine Chemical Synthesis

P403 A New Type of Anchored Homogeneous Catalyst R.L. Augustine, S.K. Tanielyan, S. Anderson, H. Yang and Y. Gao

.

P404 Regioselective Homogeneous Hydrogenation of Heteroaromatic Nitrogen as the Precatalyst Compounds by Use of [OSH(CO)(NCM~)~(PP~~~)Z]BF~ M. Rosales, F. Arrieta, J. Castillo, A. Gonzalez, J. Navarro and R. Vallejo P405 Catalyst Selection and Solvent EfSects in the Enantioselective Hydrogenation of 1-Phenyl-l,2-propanedione E. Toukoniitty, P. M&i-Arvela, A.Kalantar Neyestanaki, T. Salmi, A. Villela, R. Leino, R. Sjoholm, E. Laine, J. Vayrynen and T. Ollonqvist P406 Catalytic Asymmetric Hydrogenation and Hydroformylation Reactions with Chiral N or N,S Ligands M.L. Tomrnasino, M. Casalta, J.A.J. Breuzard, M.C. Bonnet and M. Lemaire P407 Heterogeneous Asymmetric Hydrogenation of Ethyl Pyruvate on Chirally modified Pt/A1203Catalyst with Fixed-Bed Reactor X. You, Xi. Li, S. Xiang, S. Zhang, Q. Xin, Xu. Li and C. Li . P408 EfSect of Coacid Acidity on the Cinchona-Mod$ed Pt-Catalized Enantioselective Hydrogenations B. Torok, K. Balkzsik, K. Felfdldi and M. Bartok . P409 Lactones 8. [I] Enantioselective Hydrolysis of y-Acetoxy-&lactones T. Olejniczak and C. Wawrzenczyk . P410 New Environmentally Friendly Base Catalystfor Enantioselective Reactions. Heterogenisation of Chiral Amines to USY and MCM-41 Zeolites M . Iglesias Hemhndez and F. Shnchez Alonso . P411 Selective Enzymatic Production ofAmide EmulsifiersJi.om Ethanolamine and Fatty Acids M . Femhndez Ptrez and C. Otero . P412 Rapid Solvent-Free Esterification of Conjugated Linoleic Acid and Glycerol in a Packed Bed Reactor Containing an Immobilized Lipase J.A. Arcos and C.G. Hill Jr. . P4 13 Etherijkation over a Novel Acid Catalyst R.S. Karinen, A.O.I. Krause, K. Ekrnan, M. Sundell and R. Peltonen P4 14 Esterification of Lauric Acid with Glycerol Using Modified Zeolite Beta as Catalyst M. da Silva Machado, D. Cardoso, J. Perez Pariente and E. Sastre .

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P415 Preparation of Alkylglucosides using Al-MCM-41 Molecular Sieves as Catalysts H.M.C. Andrade, A.R.E. Gonzaga and L.G. Aguiar .

3423

P4 16 Novel Supported Solid Acid Catalystsfor Environmentally Friendly Organic Synthesis K. Wilson, J.K. Shorrock, A. Renson, W. Hoyer, B. Gosselin, D.J. Macquarrie and J. H. Clark .

3429

P417 Catalytic Activity of Polymer-Anchored Pd(I4 SchlflBase Complexes: Hydrogenation of Styrene and Oxidation of Benzyl Alcohol M.K. Dalal and R.N. Ram .

3435

P418 Highly Promising Activity of Na$on/Silica Composite Catalyst in Acylation Reactions A. Heidekum, M.A. Harmer and W.F. Hoelderich .

344 1

P4l9 Alcohol Coupling to Unsymmetrical Ethers over Solid Acid Catalysts K. Klier, H.-H. Kwon, R.G. Herman, R.A. Hunsicker, Q. Ma and S.J. Bollinger 3447 P420 Gas versus Liquid Phase a-Pinene Transformation over Acid Molecular Sieves of Different Topolog~and Composition C.M. Lbpez, F.J. Machado, K. Rodriguez, D. Arias and M. Hasegawa .

3453

P42 1 Ruthenium-Catalyzed [2+2] Cross-Addition of Norbornene Derivatives and Dialkyl Acetylenedicarboxilates H. Suzuki, I. Hashiba, T.-A. Mitsudo and T. Kondo.

3459

P422 Shape-Selective N-Alkylation of Melamine Using Alcohol as a Alkylating Agent with RdMordenite Catalyst in the Liquid Phase . S. Shinoda, K.-I. Inage, T. Ohnishi and T. Yamakawa

3465

P423 Diastereoselective Hydrogenation of a Prostaglandin Intermediate over Ru Supported on MCM-41 and MCM-48 S. Coman, R. Caraba, F. Cocu, V.I. Parvulescu, H. Bonnemann, B. Tesche, C. Danumah and S. Kaliaguine .

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P424 Generation of Uniformly Sized and Dispersed Copper Particles in New Cu-A1 Based Mesoporous Catalysts and their Role in Selective Hydrogenation of Conjugated Unsaturated Carbonyl Groups . S. Valange, Z. Gabelica, A. Derouault and J. Barrault P425 Selective Hydrogenation of Furfural on Ni-P-B Nanometals S.-P. Lee and Y.-W. Chen . P426 A Sulfur Based Catalytic Systemfor the Carbonylative Reduction and the Reductive Carbonylation . V . Macho and M. Kralik

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P427 Rapid Synthesis of Ketenes From Carboxylic Acids on Functionalized Silica Monoliths R. Martinez, M.C. Huff and M.A. Barteau .

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P428 Nitroaldol Condensation Promoted by Organic Bases Tethered to Amorphous Silica and MCM-41-Type Materials F. Bigi, S. Carloni, R. Maggi, A. Mazzacani and G. Sartori.

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P429 Alcoholysis of Ester and Epoxide Catalyzed by Solid Bases H. Hattori, M. Shima and H. Kabashima . P430 Beckmann Rearrangement over Anionic Clays L. Forni, G. Fornasari, R. Trabace, F. Trifiro, A. Vaccari and L. Dalloro .

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C1 Chemistry P43 1 Partial Oxidation of Methane over Supported Nickel Catalysts A.M. Diskin and R.M. Ormerod .

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P432 Partial Oxidation of CH4 into Synthesis Gas on Ni/Perovskite Catalysts Prepared by SPC Method K. Takehira, T. Shishido. M. Kondo, R. Furukawa, E. Tanabe, K. Ito, S. Harnakawa and T. Hayakawa .

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P433 Methane Partial Oxidation in New Iron Zeolite Topologies P.P. Knops-Gerrits and W.J. Smith .

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P434 Investigations of the Selective Partial Oxidation of Methanol and the Oxidative Coupling of Methane on Copper Catalysts H.-J. Wolk, A. Scheffler, G. Mestl and R. Schlogl .

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P435 Partial Oxidation and Chemisorption of Methane over Ni/A1203 Catalysts Ya. Chen, C. Hu, M. Gong, Yu. Chen and A. Tian .

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P436 Carbon Deposition on Ni/A1203 Catalyst during Partial Oxidation of Methane to Syngas Q.-G. Yan, Z.-S. Chao, T.-H. Wu, W.-Z. Weng, M.-S. Chen and H.-L. Wan

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P437 Mechanistic Studies of Methane Partial Oxidation to Synthesis Gas over Si02-Supported Rhodium Catalyst Z.-S. Chao, Q.-G. Yan, T.-H. Wu, W.-Z. Weng, H.-Q. Lin, L. Yang, J.-L. Ye, M.-S. Chen, H.-L. Wan and K.-R. Tsai .

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P438 Reaction Kinetics and Product Selectivity in the Oxidation of Methane over Pd/Si3N4 D. Whang, F. Monnet and C. Mirodatos .

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Sustainable Ni Catalystfor Partial Oxidation of CH4 to Syngas at High Temperature S. Liu, G. Xiong, H. Dong, W. Yang, S. Sheng, W. Chu and Z. Yu Improvements of Ceria Promoted Nickel Catalystsfor Natural Gas Oxidation to Syngas W. Chu, Q. Yan, S. Liu and G . Xiong . Partial Oxidation of Methane Catalyzed by Mo-Incorporated SBA-I . T. Tatsumi, N . Hamakawa and L.X. Dai Kinetic and Mechanistic Study of the Partial Oxidation of Methane to Formaldehyde on Silica Catalysts . F. Arena, F. Frusteri, A. Mezzapica and A. Parmaliana Two Step Synthesis of Methyl Formatefi.om CH4 and Air via Formaldehyde: Surface Reactivity of Oxide Catalysts towards HCHO G. Martra, F. Arena, S. Coluccia, F. Frusteri, A. Mezzapica and A. Parmaliana Oxidation of Methanol on Sodium Mod$ed Mo-Based Catalysts . K. Ivanov, S. Krustev, P. Litcheva and I. Mitov Novel Rhenium Based Catalystsfor Direct Dehydroaromatization of Methane with CO/C02 towards Ethylene and Benzene R. Ohnishi, K. Issoh, L. Wang and M. Ichikawa . H-ZMS-8 Supported Mo-Based Catalystsfor Methane Conversion under Non-oxidative Condition S . Li, C.-L. Zhang, Q.-B. Kan, D.-Y. Wang, Y. Yuan and T.-H. Wu Methane Dehydro-aromatization and NO Adsorption on Mo/HZSM-5 W. Liu and Y.-D. Xu . The Location, Structure and Role of MOO,and MoC, Species in Mo/H-ZSM5 Catalystsfor Methane Aromatization . W. Li, G.D. Meitzner, Y.-H. Kim, R.W. Borry and E. Iglesia Oxygen-Free Conversion of Methane in the Presence of Intermetallic Hydrogen ~ b c e ~ t o r A. Sauvage, M. Mercy, H. Amariglio, J.-C. Gachon and P. Pareja . Conversions of Substituted Methanes Over ZSM-Catalysts C.E. Taylor . A Study of Coke Formation Kinetics by a Conventional and an Oscillating Microbalance on Steam Reforming Catalysts R. Lerdeng, D. Chen, C.K. Jakobsen and A. Holmen

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Methanol Reforming over CuO/ZnO under Oxidizing Conditions T.L. Reitz, S. Ahrned, M. Krumplet, R. Kumar and H.H. Kung . Methane COz Reforming over a Stable Ni/Al203 Catalyst A. Castro Luna, A. Becerra, M. Dimitrijewits and C. Arciprete

.

Optimization of Ni and Ru Catalysts Supported on LaMn03for the Carbon Dioxide Reforming of Methane E. Pietri, A. Barrios, M.R. Goldwasser, M.J. Perez Zurita, M.L. Cubeiro, J. Goldwasser, L. Leclercq, G. Leclercq and L. Gingembre. Metal Support Interaction on Pt/ZrOz Catalystsfor the C 0 2 Reforming of CH4 S.M. Stagg-Williams, R. Soares, E. Romero, W.E. Alvarez and D.E. Resasco Methane Reforming with C02 over Ni/ZrO2-CeO2 and Ni/ZrO2-MgO Catalysts Synthesized by Sol-Gel Method J.A. Montoya, E. Romero, A. Monzon and C. Guimon . Support Effect over Rhodium Catalysts during the Reforming of Methane by Carbon Dioxide P. Ferreira Aparicio, M. Femindez Garcia, I. Rodriguez Ramos and A. Guerrero Ruiz . C 0 2Reforming of CH4over Mo Promoted Nickel-Based Catalysts C.E. Quincoces, S. Ptrez de Vargas and M.G. Gonzalez . Stable Ni/Zr02 Catalystfor Carbon Dioxide Reforming of Methane J.-M. Wei, B.-Q. Xu, Z.-X. Cheng, J.-L. Li and Q.-M. Zhu. Dependence ofActivity of ZrO2 Catalystsfor Isobutene Formation in CO Hydrogenation on the Phase Structure K-I. Maruya, T. Komiya and M. Yashima . Long-chain Alcohols fiom Syngas A. Frennet, C. Hubert, E. Ghenne, V. Chitry and N. Kruse . Preparation and Catalytic Properties of Copper-Ytterbium Oxide System for CO Hydrogenation Y. Sakata, S. Tsuchiya, N. Kouda, F. Takahashi and H. Imamura . Mechanistic Studies of CO and C02 Hydrogenation to Methanol over a 50Cd45Zd5Al Catalyst by In Situ FT-IR, Chemical Trapping and Isotope Labelling Methods L.Z. Gao, J.T. Li and C.T. Au Effect of Nb2Os Addition to Co/A1203 Catalyst on CO Hydrogenation Reaction . F.T.~ e n d e sF.B. , Noronha and M. ~chmal..

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P465 Investigation of Alternative Support Materials for Monometallic and Bimetallic Catalystsfor CO Hydrogenation T. Biilbiil, A.I. Isli, A.E. Alsoylu and Z.I. dnsan .

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P466 Isotopic Transient Inffared Study of Reaction Pathway and Intermediates for CO Insertion and Hydrogenation S.A. Hedrick and S.S.C. Chuang .

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P467 Relationship between Formation of DME, CH30Hand i-C4Hs in Isosynthesis . C.-L. Su, D.-H. He, J.-R.Li, B.-Q. Xu, Z.-X. Cheng and Q.-M. Zhu

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P468 CO Hydrogenation on RWAI-PILC: The Effect of the Metallic Precursor S. Mendioroz, B. Asenjo, P. Terreros, P. Salerno and V. Muiioz .

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P469 Enhancement of the Catalytic Performance to Methanol Synthesis @om C02/H2 by Gallium Addition to Palladium/Silica Catalysts A.L. Bonivardi, D.L. Chiavassa, C.A. Querini and M.A. Baltanas .

3747

P470 A Comparison of the Catalytic Performancefor Low-Temperature Methanol Synthesis in a Liquid Medium S. Ohyama .

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P471 A Novel Effect of Li Additive: Dynamic Control of Rh Mobility during C 0 2Hydrogenation Reaction . K.K. Bando, H. Arakawa, N. Ichikuni and K. Asakura

3759

P472 Structure and Characteristics of Supported Metal SPR Catalystsfor C02 Hydrogenation T. Uematsu, D. Li, N. Ichikuni and S. Shimazu .

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P473 The Nature of the Active Species on the Fe/MgO Used in the Combustion of Methane R. Spretz, S.G. Marchetti, M.A. Ulla and E.A. Lombardo .

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P474 Oxidation of Methane over Pt-Cr(Mo, W)/A1203Catalysts Z . Sarbak and S.L.T. Andersson . P475 Monolith Honeycomb Mixed Oxide Catalystsfor Methane Oxidation L.A. Isupova, V.A. Sadykov, G.M. Alikina, 0.1. Snegurenko, . S.V. Tsybulya, A.N. Salanov and V.A. Rogov P476 Catalytic Properties ofActive Phase of Glass Crystal Microespheres in the Reaction of Methane Oxidation A.G. Anshits, E.V. Kondratenko, E.V. Fomenko, A.M. Kovalev, O.A. Bajukov and A.N. Salanov .

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Catalytic Methane Combustion over Alumina Supported Palladium Catalysts Prepared by Sol-Gel Method: Investigation of the Activity Evolution S . Fessi, A. Ghorbel, A. Rives and R. Hubaut .

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Study of PdO/Pd Transformation over Alumina Supported Catalystsfor Natural Gas Combustion G.Groppi, C. Cristiani, L. Lietti and P. Forzatti .

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Catalytic Activity of Tungsten Phosphate ( I Q (V), (VI) at Carbon Monoxide Oxidation V.V. Lesnyak, N.S. Slobodyanik, V.K. Yatsimirsky and N.A. Boldyreva .

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A Study of Pt/ZrOz Catalystsfor Water-Gas Shift Reaction in Presence of H2S E. Xue, M. O'Keeffe and J.R.H. Ross

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Reduction Requirementsfor Ru(Na)/Fe203 Catalytic Activity in Water Gas Shifi Reaction W.K.Jozwiak, A. Basinska, J. Goralski, T.P. Maniecki, D. Kincel and F. Domka . Probing the Elementary Steps of the Water-Gas Shift Reaction over Cu/ZnO/AI203 with Transient Experiments . 0 . Hinrichen, T. Genger and M. Muhler Highly Selective and Highly Efficient Catalytic Conversion of Methane into Ethylene by the Oxidative Coupling . A. Machocki, A. Denis, J. Gryglicki and H. Mlynarska Oxidation of Methane to Methanol by Hydrogen Peroxide on a Supported Hematin Catalyst T.M. Nagiev and M.T. Abbasova . Polymerization and Organometallics

Investigation of a Cr/Silica Polyethylene Polymerisation Catalyst by In-situ Inpared Spectroscopy S.F. Parker, C.C.A. Riley and J.E. Baker . Supported (Cp)2 ZrCI2 as a Catalystfor Ethylene Polymerization S. Teixeira Brandao, M.L. dos Santns Correa, J.S. Boaventura, . and S. Carneiro Vianna On the EfSect ofAdding a Lewis Acid as a ModiJier of Metallocene Based Polymerization Catalysts P.G. Belelli, M.L. Ferreira and D.E. Darniani Surface Science Studies of Model Ziegler-Natta Polymerization Catalysts S.H. Kim, E. Magni and G.A. Somorjai .

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P489 Ethylene Polymerization Catalyzed over Single Site Ni Complex Catalysts Supported on Silica D.S. Hong, T.S. Seo and S.I. Woo .

P490 DRIFTS, TPRS and NMR Investigations of Unsupported and Supported Pentamethylcyclopentadienyldiallyl-Complexes-Catalystsfor the Polymerization of Butadiene . H. Landmesser, H. Berndt, D. Miiller and D. Kunath P491 SchrfJBases Containing Metal ComplexesAnchored on Aerosil as Catalysts of Low-Temperature Ozone Descomposition T.L. Rakitskaya, A.A. Golub, A.A. Ennan, L.A. Raskola, V.Ya. Paina, A.Yu. Bandurko and L.L. Ped P492 Carbon Nanotubes-Supported Rh-Phosphine Complex Catalystsfor Propene Hydroformylation . H.-B. Zhang, Y . Zhang, G.-D. Lin, Y.-2. Yuan and K.R. Tsai P493 Model Rhodium Organometallic Complexes as Catalyst Precursors for CO Hydrogenation P. Terreros, R. Fandos, M. L6pez Granados, A. Otero, S. Rojas and M.A. Vivar Cerrato . P494 An OrganometallicApproachfor Tin Promotion Enhancement over Pt/y-A1203 Alkane Dehydrogenation Catalysts G.F. Santori, M.L. Casella, G.J. Siri, O.A. Ferreti and J.L.G. Fierro P495 Il9sn Mhsbauer Study and Catalytic Properties of Magnesia-Supported Platinum-Tin Catalysts Prepared by Surface Organometallic Chemistry L. Stievano, F.E. Wagner, S. Calogero, S. Recchia, C. Dossi and R. Psaro. AUTHORINDEX

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SUBJECT INDEX

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

NANOHETEROGENEOUS M E T A L - P O L Y M E R C O M P O S I T E S AS A N E W T Y P E OF EFFECTIVE AND SELECTIVE CATALYSTS L.I. Trakhtenberg a, G.N. Gerasimov a, E.I. Grigoriev a, S.A. Zavjalov a, O.V. Zagorskaja b, V.Yu. Zufman b, V.V. Smimov ~ aKarpov Institute of Physical Chemistry, 10, Vorontsovo Pole, 103064, Moscow, Russia bDepartment of Chemistry, Moscow State University, 119899, Moscow, Russia The new catalytic effect was discovered in reaction of the C-CI bond metathesis. It was shown specific catalytic activity of Cu-nanoparticles immobilized in polymer matrix greatly increases with the nanoparticle content. This effect is most probably caused by the electron transfer between nanoparticles of different size resulting in their mutual charging. I. INTRODUCTION Polymer composites, containing immobilized nanoparticles of metals and semiconductors, attract the attention because of fundamental problems and important practical applications [1]. The size, electronic structure, and physical-chemical properties of nanoparticles depend largely on structure and properties of polymer matrix, and on the nanoparticle formation [ 1-4]. Of special interest is a novel vapor deposition cryochemical synthesis of nanocomposite polymer systems elaborated in [5-8]. It is based on the low temperature cocondensation of metal (or semiconductor) and monomer vapors followed by the low temperature solid state polymerization of obtained cocondensates. In this method the particle stabilization takes place without stabilizing compounds which are adsorbed on the particle surface. In this case the immobilization of particles and the restriction of their size are due to the rigidity of polymer lattice and do not need any specific coordinate bonds between particle surface and polymer environment. So the cryochemical solid-state synthesis allows to prepare nanoparticles within hydrophobic and even non-polar polymer matrix. The concentration of nanoparticles stabilized by the cryochemical synthesis can be varied over a wide range up to high values. This gives a possibility to elucidate important cooperative properties of such particles in polymer matrices of different structure. The preliminary results show, in particular, the cryochemically synthesized nanocomposites, which contain metal or semiconductor nanoparticles free from ionic or strongly polar screening caps, reveal the unique sensor properties [8,9]. The adsorption of even traces of some substances from atmosphere accompanied by the changes of conductivity of material on several orders of magnitude.

The work is supported by the Russian Fondation tbr Basic Research (project .No 98-03 32130a)

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The adsorption of chemical compounds at nanoparticles of these composites leads not only to electrical sensor effects, but can results in new catalytic processes, because of the surface properties of such nanoparticles [10]. The important catalytic peculiarity of the metal nanoparticle systems immobilized in matrix was discovered in [ 11,12]. It was shown, the ratio between cis- and trans-isomers of 1.4-diclorobutene-2, formed as a result of the 3,4diclorobutene isomerization on the Pd nanoparticles in poly-p-xylylene (PPX), depends on the concentration of nanoparticles, that is on the distance between them in polymer matrix. The data demonstrate the sufficient influence of the interaction between metal nanoparticles on the reaction. Most likely, such interaction leads to the formation of charged metal particles of opposite sign in dielectric polymer matrix. It was shown [6], in our metal-polymer particles, as well as in other systems of the same type [4], there is the rather wide size distribution of nanoparticles. The energy of Fermi level of small metal particles depends strongly on their size and form [13]. Electron transfer between particles of different size results to their mutual charging and equalization of the electrochemical potential of nanoparticles [ 13]. Such process takes place at the sufficient high metal content. It is believed, unusual catalytic properties of interacting metal nanoparticles are caused by the participation of charged particles in reaction [11,12]. This work is aimed at studying the influence of the interaction between metal nanoparticles on the catalytic activity of the Cu - containing PPX composite films in the reaction of the C-CI bond metathesis CC14 + CloH22 = CHCI3 + CIoH21CI.

(l)

For this purpose, the catalytic activity of films with different metal content was determined and the relation between the catalytic activity of the films and their conductivity was established. It was shown, the most activity is achieved at metal content close to the percolation threshold of conductivity, that is, at the most effective nanoparticle charging by electron transfer. 2. EXPERIMENTAL SECTION The vapor deposition cryochemical synthesis of Cu- PPX composite films was performed in the special vacuum apparatus described in [9]. Cu - vapors were produced by heating of Cu- plate by the high-power electron beam. p-Xylylene monomer was prepared by the vaporization of p-cyclophane and its following pyrolysis at 600~ The mixture of vapors of Cu and p-xylylene was deposited at 77 K on polished quartz plates with platinum contacts. This allows to measure the film conductivity in the course of deposition and, by this means, to control metal content in deposited film. The heating of obtained metal - monomer composite films results to the polymerization of p-xylylene. IR- spectroscopy data [6 ] shown, the thermal polymerization of such metal- monomer system proceeds in the temperature range 170 - 180 K, that is much below not only the glass temperature of PPX (around 290 K [ 14]), but the melting temperature of p-xylylene (around 220 K [ 15],). The scheme of synthesis is presented below:

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H2C,

CH2

H2

600 0 C pyrolysis

H2

p-cyclophane

2 CH2

===~

~:=

deposition together with Cu-vapors, 77K

CH2

solid-phase polymerization

p-xylylene

r

[-CH2~CH2-

]

n

poly-p-xylylene containing Cu-nanoparticles

The composition of film produced was governed by the ratio of the rate of p-xylylene deposition to that of the Cu one. These rates were measured by special calibration experiments. To determine the metal content in obtained Cu-PPX composite films, the X-ray fluorescence method was used. The film thickness, determined by using of a laser interferometer, is in the range 3 - 5 microns. Measurements of electrical resistance ( R ) of the composite films were performed by the methods described in [9]. The reaction (1) was performed in the absence of oxygen at 180~ during 4 hours at effective stirring of the reaction mixture. The molar ratio CCl4/n-decane - 4 : 1. The reaction mixture contained 0,3 weight % of Cu-PPS film in all experiments. After the reaction, the resulting mixture was analyzed by the gas chromatography method. 3. RESULTS AND DISCUSSION Specific catalytic activity of the Cu-PPX catalyst is defined as the number of molecules of chlorodecanes produced per one Cu-atom during one hour of the reaction. In the following table the data on the yield of chlorodecanes are related to the data on R of the composite films.

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Table. Dependence of electrical resistance and catalytic activity of nanocomposites Cu-PPX on Cu content

Content of Cu, vol %

R at 77K, Ohm

R at 298K, Ohm

Yield of chlorodecanes, mol.. %

Specific catalytic activity

1,3 3,5 5,0 7,0

oo 7,0. 108 10,2 10 3 3,3.103

oo 3,5.108 8,3.103 645

traces 4 15 20

traces 130 500 650

10,3 14

66 16,2-103

29 3,1.103

35 14

1150 450

At a very small concentration (around 1 vol %) of Cu (Xcu), R of composite film is close to that of pure PPX. In this case Cu nanoparticles, which are fully isolated in matrix, do not practically interact to one another and do not influence the composite conductivity. But as is seen from the table, the increase of Xc, even to 3.5 vol. % leads to the sharp fall of R as a result of the tunnel electron transfer between Cu nanoparticles [ 1]. The probability of such transfer rises exponentially with the decrease of distance between particles, that is, with Xc, increasing. It is important to note, the lowering of R with temperature in the range from 77 to 298 K evidences, even at the most Cu concentration in obtained films, metal nanoparticles do not form infinity cluster characterized by the metallic behaviour, that is by the increase of R with temperature [16]. Therefore, the tunnel electron transfer between nanoparticles governs the film conductivity in all investigated range of metal content. The data for R (Table) show, the rate of such electron transfer processes in the films reaches its maximum value at Xc. in the range 5 - 10 vol. %. The further increase of Xc, up to 14% leads to explosive aggregation of nanoparticles with the resulting formation of isolated large scale metal clusters: this process is accompanied by the substantial decrease of number of metal particles and the corresponding rise of R (Table). At 2-10 vol. % metal content metal-PPX films, synthesised by the vapour deposition cryochemical method, contain nanoparticles of mean size about 4 nm [5,6]. It can be easily shown, at metal content ~10 vol. %, the mean distance, d, between such particles, which are uniformly distributed in the matrix, should be about 3 nm. This value is sufficient for the rather fast tunnel electron transfer between particles which leads to the corresponding mutual charging of particles of different size. In actually, metal nanoparticles are predominantly located in the amorphous regions of semi-crystalline PPX, so that above estimation gives the upper limit of d. At very low Xc, catalytic activity of fully isolated and non-interacting Cu- nanoparticles in PPX (see Table) is very small. The increase of Xc, yields the strong gain in catalytic activity. It achieves the maximum value near the percolation threshold namely in conditions of the most effective interaction between metal nanoparticles isolated in the polymer matrix.. The metal aggregation accompanied by the decrease of the number of particle s leads to the m

m

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diminution of the catalytic activity. The maximal specific activity is 1150. It is much more than activity of all known catalysts of this reaction. For comparison, the same reaction of C-CI bond metathesis was investigated with the special prepared catalyst containing 1 weight % of high dispersed metallic Cu deposited on silica. In conditions analogous to those of the reaction with the nanocomposite catalyst, the specific activity of this catalyst is no more than 4. Moreover, it has low selectivity: in this case the reaction is accompanied by the side-product formation from condensation processes. At the same time Cu-PPX catalyst gives monochlorosubstituted decanes only. To understand catalytic properties of Cu containing PPX-nanocomposites the further investigations are necessary. Now one can only make some suggestions. The initiation of the reaction (1) proceeds most probably by the dissociation C-CI bonds with formation of initiating radicals CC13" CC14 ~

CC13 + CI.

(2)

At the proceeding of this reaction in the gas state (El v) and in the adsorption state on Cu (Ei ad) the energy required will be differ by the value Eiad _ Eiv = Qad(c1) + Qad(CCI3) -Qad(ccI4), where oad(x) is the heat of adsorption of corresponding molecule. Most likely the heat of adsorption of CC14 on Cu is less than that of adsorption of CI or radical CC13, so that Eiad - Eiv > 0, and adsorption of CC14 on Cu-particles should favor its dissociation and initiation of reaction. But obtained data (Table) lead to the conclusion this effect is rather insignificant for neutral particles. Presumably, it increases for negative charged panicles. Such particles form as a result of the mutual charging of Cu- particles in the composite (see above). Because CC14, CC13 and C1 have the significant electron affinity (EA is about 2eV for CC14, 1.5eV for CC13, and 3.6eV for CI), Qad should increase due to the very great EA of C1 atom. Consequently, one would expect the increase of the rate of CC14 dissociation. The effect of charge on the dissociation of CC14 will be especially strong on roughness of the nanoparticles surface where work function of electron decreases because of the sharp increase of charge density on such roughness. It should be noted that nanoparticles have the high percentage of atoms on or near the surface (for example, a 5 nm CdS- particle has about 15% of the atoms on its surface) [10]. The structure and properties of such vast interface between the particle and surrounding medium depend, to a large extent, on medium. Nanoparticles formed in the gaseous phase have quasi-equilibrium surface, which is nearly the same as that of bulk metal. At the same time the surface of nanopaticles produced and immobilized in solid or highly viscous matrix can be faulty, because matrix can hinder formation of equilibrium surface structure of a nanoparticle. As a result, catalytic properties of such nanoparticles can be greatly influenced by polymer matrix (cf., [ 17]).

4. CONCLUSIONS The vapor deposited cryochemical synthesis of metal- polymer films (the transfer an initial metal containing system from the gaseous state to the solid polymer one, bypassing

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liquid phase) permits to produce high efficient catalysts with a great concentration of metal particles. It should be noted this synthesis gives a possibility to vary the structure of polymer matrix in a rather wide range to tailor metal polymer systems with desirable catalytic properties. Not only PPX, but also its derivatives containing various substutients including strong polar ones [5], as well as other polymers (e.g., [18]) can be used as matrices in the synthesis. As a result of the performed investigation, the relationship between the specific catalytic activity of the metal containing nanocomposite catalyst and the content of metal particles within them was discovered at first. The results obtained testify that the interaction between immobilized metal nanoparticles plays a very important role in catalytic processes involving such nanoparticles. REFERENCES

1. L.I. Trakhtenberg, G.N. Gerasimov, E.I. Grigoriev, Russian J. Phys. Chem., 73 (1999) 209. 2.. D.Yu. Godovski, Adv. Polymer Sci., 119 (1995) 58. 3. F. Ciardelli, E. Tsushida, D. Woehrle (eds.), Macromolecule - Metal Complexes, Germany, Berlin, Heidelberg, 1996. 4. Pomogailo A.D, Russian Chemical Rev., 66 (1997) 750. 5. G.N. Gerasimov, V.A. Sochilin, S.N. Chvalun, L.V.Volkova, I.Ye. Kardash, Macromol. Chem. and Physics., 197 (1996) 1387. 6. E.V. Nikolaeva, S.A. Ozerin, A.E. Grigoriev, E.I. Grigoriev, S.N. Chvalun, G.N. Gerasimov, L.I. Trakhtenberg, Mater. Sci. & Eng. C, 304 (1999). 7. V.A. Sochilin, G.N. Gerasimov, I.Ye. Kardash, Polymer Sci.(Russia), Ser. B., 37(1995) 1938. 8. L.I. Trakhtenberg, G.N. Gerasimov, E.I. Grigoriev, A.E. Grigoriev, I.E. Kardash, S.V. Radzig, P.S. Vorontsov, S.A. Zavialov in J.R. Durig and K.J. Klabunde (eds.), Proceedings of the Second International Conference on Low Temperature Chemistry, USA, Kansas, 1996, 211. 9. G.N. Gerasimov E.I. Grigoriev, A.E. Grigoriev, P.C. Vorontsov, S.A. Zav'yalov, L.I. Trakhtenberg, Chem. Phys. Reports 17 (1998) 168. 10. Wang Y., Herron N., J. Phys.Chem., 95 (1991 ) 525. 11. P.C. Vorontsov, G.N. Gerasimov, E.N. Golubeva, E.I. Grigoriev, S.A. Zav'yalov, L.I. Zavyalova, L.I. Trakhtenberg, Russian J. Phys. Chem., 72 (1998) 1742. 12. S.A. Zavyalov, P.C. Vorontsov, E.I. Grigoriev, G.N. Gerasimov, E.N. Golubeva, O.V. ZagorsKaya, L.M. Zavyalova, L.I. Trakhtenberg, Kinetics and Catalysis, 39 (1998) 905. 13. E.L. Nagaev, Physics. Uspekhi, 162 (1992) 49. 14. D. E. Kirkpatrick, B. Wunderlich, Makromol. Chem., 186 (1985) 2595. 15. G. Treiber, K. Boehlke, A. Weitz, B. Wunderlich, J. Polym. Sci., Polym. Phys. Ed., 11 (1973) 1111 16. L.I. Trusov, V.A. Kholmyanskii, "Island" metal films, Russia, Moscow, 1973. 17. M.C.M. Alves, G. Tourillon, J. Phys. Chem., 100 (1996) 7566 18. G.N. Gerasimov, S.M. Dolotov, A.D. Abkin, Radiat. Phys. Chem., 15 (1980) 405.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Design o f heteropolyacid-polymer composite films as catalytic materials for heterogeneous reactions Gyo Ik Park a, Seong Soo Lima, Yong Heon Kim a, In Kyu Songb and Wha Young Lee** *Dept. Chem. Eng., Seoul National Univ., Shinlim-dong, Kwanak-ku, Seoul 151-742, Korea* bDept. Indust. Chem., Kangnung National Univ., Kangnung, Kangwondo 210-742, Korea Membrane-like H3PMol2040-polymer composite film catalysts were successfully prepared using homogeneous H3PMo~2040-polymer solutions by a membrane preparation technology, and they were applied as heterogeneous catalysts to the vapor-phase ethanol conversion and MTBE decomposition, and to the liquid-phase t-butyl alcohol (TBA) synthesis. Finely and uniformly dispersed H3PMol2040 catalysts supported on polymer materials were obtained. The catalytic properties of the composite film catalysts were affected by the nature of solvents and polymer materials used. The composite catalysts showed the enhanced catalytic activities in the model reactions compared to the mother catalyst. The modified catalytic properties of H3PMo~2Oa0-polymer composite film catalysts were confirmed by characterization. 1. INTRODUCTION Taking advantage of a property that heteropolyacids (HPAs) are highly soluble in some polar solvents [1-3], HPAs can be blended with polymer materials using suitable solvents to form homogeneous solutions [4, 5]. The homogeneous solutions in turn can be directly converted into membrane-like composite films by a membrane preparation technique, and they can be used as catalytic materials for heterogeneous reactions. Finely and highly dispersed HPAs supported on polymer materials can be obtained by this method. It is expected that the acid and redox catalytic properties of HPAs can be modified by the properties of solvents and polymer materials used to meet the need for desirable reactions. In this work, H3PMol2040-polymer composite films were prepared and tested as heterogeneous catalysts for the vapor-phase ethanol conversion and MTBE decomposition, and for the liquid-phase TBA synthesis. 2. EXPERIMENTAL

H3PMo12040 (PMo from Aldrich) was used as an active catalytic material. Polysulfone (PSF, Udel-1700 from Union Carbide), polyethersulfone (PES, Victrex 5200P from ICI), and polyphenylene oxide (PPO, poly-2,6-dimethyl-l,4-phenylene oxide from Aldrich) were used as blending polymers. A mixture of methanol(M)-chloroform(C) was used as a solvent for the blending. A homogeneous solution of PMo(1.22 wt%)-PSF (or PES or PPO) (6.90 wt%)M(4.41 wt%)-C(87.47 wt%) was casted on a glass plate with a constant thickness in ambient condition (56% relative humidity) to form a corresponding membrane-like film catalyst, and

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subsequently it was dried for 4-5 hrs under the same condition. The PMo-PSF-MC solution was also casted and dried at 85% relative saturation of methanol vapor to form PMo-PSFMC-M. The thickness of the composite film catalysts was 17 ~tm. The composite film catalysts were cut into small pieces (2 mm x 2 mm) to be used as heterogeneous catalysts. All the film catalysts were treated at 170~ for 1 hr by passing air (5 cc/min) before the reaction. Vapor-phase ethanol conversion and MTBE decomposition were carried out in a continuous flow fixed-bed reactor. Liquid-phase TBA synthesis from isobutene and water was also investigated in a semi-batch reactor. The composite catalysts were thermally treated at 170~ before characterization. 3. RESULTS AND DISCUSSION Fig. 1 shows the SEM images of PMo-PSF-MC, PMo-PPO-MC and PMo-PSF-MC-M. No visible evidence representing PMo was found in PMo-PSF-MC and PMo-PSF-MC-M. It was also observed that PMo-PES-MC showed the same surface morphology with PMo-PSF-MC. Above results mean that PMo was not recrystallized into large particles but was physically dispersed as fine particles throughout PSF and PES matrix. However, PMo in PMo-PPO-MC existed as large particles having diameters of 1 lam or less. This might be due to the different blending pattern of PMo-PPO-MC from PMo-PSF(or PES)-MC. Although PMo-PSF-MC was a non-porous film, PMo-PSF-MC-M had well-developed macropores (porosity: 81.5%, average pore diameter: 5.48 ~tm, total pore area: 14.9 m2/g).

Fig. 1. SEM images of (a)PMo-PSF-MC, (b)PMo-PPO-MC and (c)PMo-PSF-MC-M. The pore characteristics of PMo-PSF-MC-M were intentionally and successfully controlled through a phase inversion method [6, 7] by modulating the preparation conditions in this work. Three components were considered in the phase inversion process; polymer, solvent and nonsolvent for polymer. It was observed that the miscibility of solvent with non-solvent for polymer played a key role for the pore formation throughout the composite film catalysts. The macropores of PMo-PSF-MC-M film catalyst were formed due to the miscible nature of chloroform (a solvent for PSF and major component of the mixed solvent) with methanol vapor (a non-solvent for PSF) under the controlled preparation condition. The non-porosity of PMo-PSF-MC was attributed to the immiscible nature of chloroform with water vapor (a nonsolvent for PSF) in the air. It is believed that the majority of PMo in PMo-PSF-MC-M exists on/near the pore walls because PMo migrates to the PSF-lean phase together with the migration of chloroform during the phase inversion process, as illustrated in Fig. 2.

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chloroform evaporation

methanol diffusion

T :migr~fi ~ i ~ ~ ~ ~

~: : ....:.: ::i:!..!i.i] P Mo on pore wall (a) PMo migrates to the droplets of PSF-lean phase together with chloroform migration (b) Droplets of PSF-lean phase grow further (c) Pores are formed by gelation

PSF-rich phase

PSF-lean phase

Fig. 2. Model for pore formation and PMo distribution throughout PMo-PSF-MC-M. Fig. 3 shows the XRD pattems of PMo-polymer composite film catalysts. PMo in PMoPPO-MC formed a certain crystal structure, while PMo in PMo-PSF-MC and PMo-PES-MC showed no characteristic XRD patterns. This means that PMo in PMo-PSF-MC and PMoPES-MC film catalysts did not exist as a crystal form but as an amorphous state. Contrary to PMo-PSF-MC, PMo-PSF-MC-M had a certain crystal structure although the same mixed solvent was used for the preparation of the composite catalysts. This might be due to the different effect of non-solvent during the phase inversion process. A series DSC analyses revealed that the glass transition temperature of PSF (or PES) was decreased after the blending with PMo while that of PPO was increased after the blending with PMo. This result means that PMo acted as an impurity for PSF (or PES) and that there was neither interaction nor physicochemical bonding between PMo and PSF (or PES). This implies that PMo was physically blended with PSF (or PES). The DSC result also means that PMo was not an impurity for PPO anymore and that there was a certain interaction between PMo and PPO. The physicochemical interaction of PMo with PPO was well confirmed by the solid-state 31p-MAS NMR analyses. As shown in Fig. 4, bulk PMo, PMo-PSF-MC and PMo-PES-MC showed the singlet while PMo-PPO-MC showed the doublet. The 31p chemical shift for bulk PMo was -3.9 ppm, and this was in good agreement with the reported value [1]. The 31p chemical shifts for PMo-PSF-MC and PMo-PES-MC were a little different each other, although both samples recorded the singlet. It is also known that the 31p chemical shift for HPA in solid-state NMR is strongly dependent on the identity and the number of coordinated organic molecule [1]. Therefore, the differences in 31p chemical shift for PMo-PSF-MC and PMo-PES-MC from bulk PMo were believed to be due to the different effect of solvent and polymer during the composite film fabrication. It is clear from the above results that PMo was physically blended with PSF (or PES). It is also noticeable that there were different kinds of PMo in PMo-PPO-MC. Together with the DSC results, this result strongly indicates that PMo was physicochemically blended with PPO.

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Fig. 4.31p-MAS NMR spectra of (a)PMo, (b)PMo-PSF-MC, (c)PMo-PES-MC, and (d)PMo-PPO-MC with respect to H3PO4.

Table 1 shows the typical catalytic activities of the composite film catalysts for the vaporphase ethanol conversion. The ethanol conversions were in the following order ; PMo-PSFMC > PMo-PES-MC > PMo-PPO-MC > PMo. The enhanced conversions over the composite catalysts were due to the enhanced PMo dispersion. PMo-PPO-MC showed the smallest conversion among the film catalysts. This may be partly resulted from partial agglomeration of PMo throughout PPO matrix. PMo-PSF-MC and PMo-PES-MC showed both enhanced oxidation and acidic catalytic activities compared to the bulk PMo. The selectivity to oxidation over PMo-PPO-MC was three times or more compared to the other film catalysts, but that to dehydration was 50% or less. The structural flexibility of PMo in PMo-PSF-MC and PMo-PES-MC may be also responsible for the enhanced catalysis of non-porous composite film catalysts. The macroporous PMo-PSF-MC-M composite catalyst showed the higher ethanol conversion than the non-porous PMo-PSF-MC. This might be due to the welldeveloped macropores of PMo-PSF-MC-M providing a reduced mass transfer resistance. Table 1. Catalytic activity of composite film catalyst for the ethanol conversion at 170~ Catalyst

EtOH conversion

Bulk PMo a) PMo-PSF-MC b) PMo-PES-MC b) PMo-PPO-MC b) PMo-PSF-MC-M c)

(%)

6.9 39.5 33.7 13.4 46.0

Amount of EtOH converted to product (xl04 moles/g-PMo-hr)(carbon selectivity)

CHaCHO

C2H4

0.52(12.8) 4.67(20.0) 1.79(9.0) 4.71 (59.4) 2.95(10.8)

0.34(8.4) 3.76(16.1) 6.46(32.4) 0.78(9.8) 9.67(35.5)

C2HsOC2H5 3.22(78.8) 14.93(63.9) 11.68(58.6) 2.44(30.8) 14.60 (53.7)

W/F=169.1 g-PMo-hr/EtOH-mole ; air=5cc/min ; film thickness=17 ktm ; a)bulk heteropolyacid treated at 300~ ; b) casted and dried in ambient condition (56% relative humidity) ; r casted and dried at 85% relative saturation of methanol vapor

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The composite film catalysts were also applied to the liquid-phase TBA synthesis from isobutene and water in a semi-batch reactor. As shown in Fig. 5, all the composite film catalysts showed the enhanced TBA yields compared to the homogeneous PMo. The catalytic activities were in the following order; PMo-PPO-MC > PMo-PES-MC > PMo-PSF-MC > homogeneous PMo. In order to confirm such a catalytic behavior of the composite film catalysts, the absorption amounts of isobutene in/on the composite film catalysts were measured at room temperature, as shown in Fig. 6. The PMo-PPO-MC showed the highest absorption amounts of isobutene. It was also observed that PMo-free PPO-MC film showed the highest absorption amounts of isobutene among the PMo-free polymer films. This indicated that polymer matrix was not a simple support for PMo but an active carrier for the reaction. It is believed that the high absorption capability of PMo-PPO-MC for isobutene played an important role in enhancing the isobutene concentration in/on PMo-PPO-MC composite catalyst in the reaction medium. The absorption capability of PMo-PPO-MC was high enough to overcome the low solubility of isobutene in water, which was encountered in a normal liquid-phase TBA synthesis 3

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Time (hr) Time (hr) Fig. 5. TBA yields at 70~ with reaction time; Fig. 6. Absorption amounts of isobutene into O=PMo-PPO-MC, O=PMo-PSF-MC, the composite catalysts at room temperature; O=PMo-PES-MC, II=homogeneous PMo, O=PMo-PPO-MC, E3=PMo-PSF-MC, isobutene pressure=2.0 kgf/cm2, H20=50 ml. O=PMo-PES. Stability of PMo-PPO-MC was also confirmed by observing the surface morphology of the composite film catalyst after 20-hr reaction, as shown in Fig. 7. No great difference in surface morphology was found in PMo-PPO-MC before and after TBA synthesis, as shown in Fig. 1 and Fig. 7. This is because PMo was physicochemically blended with PPO unlike PSF. However, a great difference in surface morphology was observed in PMo-PSF-MC before and after TBA synthesis. PMo-PES-MC showed the same behavior with PMo-PSF-MC. The above result implies that PMo in PMo-PPO-MC was more stable than that in PMo-PSF-MC and PMo-PES-MC during the reaction. It is concluded that the high catalytic activity of PMoPPO-MC in the TBA synthesis was attributed to its high absorption capability for isobutene as well as its stability in the reaction medium. It is expected that the composite film catalysts have many advantages over the homogeneous catalyst in terms of handling and recovery.

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Fig. 7. SEM images of (a)PMo-PSF-MC and (b)PMo-PPO-MC after 20-hr TBA synthesis. A composite catalyst was also prepared using homogeneous solution of PMo(2.33 wt%)PSF(23.29 wt%)-methanol(M, 6.15 wt%)-benzene(B, 68.23 wt%) for the vapor-phase MTBE decomposition. The PMo-PSF-MB was a non-porous film catalyst and PMo was physically blended with PSF matrix like PMo-PSF-MC composite film. The PMo-PSF-MB film catalyst showed the higher catalytic activity than the mother catalyst in the MTBE decomposition. The enhanced activity of the composite film catalyst was due to the fine dispersion and structural flexibility of PMo throughout PSF matrix. Neutral property of benzene also played an important role for the maintenance of acid catalytic activity of PMo in PMo-PSF-MB. 4. CONCLUSIONS Membrane-like PMo-polymer composite film catalysts were successfully prepared using homogeneous PMo-polymer solutions by a membrane preparation technology, and they were applied as heterogeneous catalysts to the vapor-phase ethanol conversion and MTBE decomposition, and for the liquid-phase TBA synthesis. The composite catalysts could be regarded as finely and uniformly dispersed PMo catalysts supported on polymer matrix. The catalytic activities and characteristics of the composite film catalysts were affected by the nature of solvents and polymer materials used. The composite catalysts showed the enhanced catalytic activities in the model reactions compared to the mother catalyst. REFERENCES 1. T. Okuhara, N. Mizuno and M. Misono, Adv. Catal., 41 (1996) 113. 2. M. Misono, Catal. Rev.-Sci. Eng., 29 (1987) 269. 3. I. V. Kozhevnikov, Catal. Rev.-Sci. Eng., 37 (1995) 311. 4. J. K. Lee, I. K. Song, W. Y. Lee and J. J. Kim, J. Mol. Catal., A104 (1996) 311. 5. G. I. Park, S. S. Lim, J. S. Choi, I. K. Song and W. Y. Lee, J. Catal., 178 (1998) 378. 6. R. Fay, R. Jerome and P. Teyssie, J. Polym. Sci. Polym. Lett., 19 (1991) 79. 7. M. Xanthos, Polym. Sci. Eng., 23 (1988) 1392.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

C A T A L Y S I S BASED O N H E T E R O P O L Y A C I D S S U P P O R T E D O N Z I R C O N I A Luis Pizzio, Patricia Vfizquez, Carmen C~iceres and Mirta Blanco Centro de Investigaci6n y Desarrollo en Procesos Cataliticos (CINDECA), UNLP-CONICET, 47 N ~ 257, 1900 - La Plata, ARGENTINA. e-mail: hds@ dalton.quimica.unlp.edu.ar The preparation and characterization of acid catalysts based on TPA using ZrO2 as support are presented. ZrO2 was prepared using different gelation agents and temperatures of pretreatments. Two impregnation techniques were used to obtain ZrO2 supported TPA. The catalytic activity for isopropanol dehydration and esterification of acetic acid with isoamyl alcohol is measured. 1. INTRODUCTION The strong acidity ofheteropolyacids (HPA) make them suitable as catalysts for many acidcatalyzed reactions. Since HPA are less corrosive and produce lower amount of waste than conventional acid catalysts, they can be used as replacement in environmemally benign processes. The use of a support allowing the tungstophosphoric acid (TPA) to be dispersed over a large surface must result in an increase of its catalytic activity. The latter, for supported HPA, depends on the carrier, the HPA loading, conditions of pretreatment, among other variables [1]. Acidic or neutral substances such as active carbon, acidic ion-exchange resin, SiO2 and ZrO2 are suitable as supports [2]. Nevertheless, ZrO2 has been scarcely used as a HPA support. There are a number of papers dealing with tungstate supported on ZrO2 using isopolytungstates as active phase precursors, in which their characterization, specially the acidic properties, and in a lesser extent their use as catalysts in acid reactions were studied [3,4]. At the present time, only a report about ZrO2 supported heteropolyacids was found [5]. We present here the preparation and characterization of solid acid catalysts based on TPA using ZrO2 as support. Zirconia was obtained in the laboratory using differem gelation agents and two temperatures of calcination. Zirconia supported TPA was prepared by two impregnation techniques. The catalytic activity of the solids is measured in the dehydration of isopropanol and the esterification of acetic acid with isoamyl alcohol.

2. EXPERIMENTAL Support preparation. Zirconium hydroxide gels were prepared from 0.07 M zirconium oxychloride aqueous solutions by adding drop wise either an ammonium hydroxide solution (25 % ammonia) up to pH-9.5 (sample ZN) or a 1M sodium hydroxide solution up to pH-6.8 (sample ZNa). The hydrogels were aged at room temperature for 72 h and then filtered and washed with deionized water. Both gels were dried in air at 110 ~ for 24 h (ZN110 and ZNa110 supports), ground to proper particle size and finally calcined during 24 h at 310~ (ZN310 and ZNa310 supports). Catalyst preparation. Supports ZN 110, ZNal 10, ZN310 and ZNa310 were impregnated with ethanol-water (e-w) solutions (50 % v/v) of TPA (H3PWl2040.nH20), in order to obtain 10 %

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W (w/w) in the final catalysts, using the incipient wetness method. Then, they were dried at 70 ~ for 24 h (named as ZN110D, ZNa110D, ZN310D and ZNa310D catalysts). A portion of the four dried catalysts was calcined at 310 ~ for 24 h (named as ZNll0C, ZNall0C, ZN310C and ZNa310C catalysts). The ZN110 and ZN310 supports were also impregnated in equilibrium at 20 ~ by contacting 1 g of support with 4 ml of 60 gW/l TPA solution in e-w, under constant stirring during 4320 min. This time was high enough to attain equilibrium of the adsorption-desorption processes. The solids were separated from the solutions by centrifugation for subsequent analysis and then were dried at 70 ~ for 24 h (named as ZN110ED and ZN310ED catalysts). Solution Characterization. The UV-visible spectra of TPA solutions were obtained through a Varian Super Scan 3 UV-visible double beam spectrophotometer with built-in recorder. The tungsten concentration in solutions was determined by atomic absorption spectrophotometry. The equipment used was an IL model 457 double beam single channel spectrophotometer. Supports and Catalyst Characterization. The differential scanning calorimetry (DSC) was carried out With a Shimadzu DT 50 thermal analyser in argon, using 25 mg samples and a heating rate of 10 ~ Textural properties were determined by nitrogen adsorption/desorption using a Micromeritics Acccusorb 2100E. The X-ray dif~action (XRD) patterns were recorded with a Philips PW-1732 equipment in the 5-60 ~ 20 range, using Cu Ka radiation and scanning speed of 1~ per minute. Fourier transform infrared (FT-IR) spectra of the solids were obtained in the 400-1500 crnq wavenumber range using a Bruker IFS 66 FT-IR spectrometer. Diffuse reflectance (DRS) spectra of the solids were recorded through a UVvisible Varian Super Scan 3 equipment, fitted with a diffuse reflectance chamber, which presents an integrative sphere with internal surface of barium sulphate. Catalytic activity. Both the isopropanol dehydration test reaction and the esterification of acetic acid with isoamyl alcohol were carried out in a conventional flow fixed bed reactor, operating at atmospheric pressure, with a solid mass of 0.5 g. The gas employed as carrier was He. For dehydration, the isopropanol volumetric flow was of 0.44 ml/min in a total flow of 50 rnl/min. For esterification, the acetic acid-isoamyl alcohol mixture (1"1 molar ratio) volumetric flow was of 0.13 ml/min in the same total flow. The temperature was set at 180 ~ and the reaction products were quantified by gas chromatography, using a thermal conductivity detector.

3. RESULTS AND DISCUSSION According with DRX, FT-IR and DCS results, ZN110 (SBFr=248 m2/g), ZNa110 (SBrr=214 m2/g), ZN310 (SBEr=181 rn2/g) and ZNa310 (SBE~=137 m2/g) supports have a poor crystallinity, neither the tetragonal nor the cubic and monoclinic crystalline phases of ZrO2 are present. TPA based catalysts obtained by impregnation of zirconia support can be prepared by the incipient wetness method or equilibrium impregnation. In the first method, the solution transport into the porous support is only performed by capillary action while, in the second technique, the solute transport is the result of convective flow till the pores are filled up and, then, by diffusive mechanism if there exists a solute-support interaction. In the preparation of supported TPA as acid catalyst, the identification of the species present in the impregnating solutions is particularly important. In aqueous solutions, the [PW12040]3 anion has a limited stability range; at pH 1.5 - 2, it is reversibly and quickly transformed into the lacunar species [PWll O 39] 7-9 The aqueous solutions of [PW~ O 39] 7- ion are stable for pH values in the range 2 - 6. Thus, owing to the restricted stability in aqueous

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a.- initial solution b.15 min c.35 min d.55 min e.145 min

=. m v

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200

225

250

275 300 X (nm)

325

350

Figure 1.- UV-visible spectra of TPA solutions in ethanol-water for different contact times with ZNa310.

solutions and bearing in mind previous reports indicating that the addition of an organic solvent has a stabilizing effect on the Keggin type heteropolyanions in solution [1], TPA solutions in e-w 50% (v/v) were used. The UV-visible spectrum of the 60 g W/l TPA solution in e-w before the contact with the supports shows a band at 265 ran, which is the characteristic charge transfer band of the [PW12040]3heteropolyanion [6]. The UV-visible spectra for different impregnation times of ZNa310 support with this solution, are shown in Figure 1. All spectra are similar because everyone presents the characteristic band of the [PWI2040]3species. The intensity of this band decreases gradually with time, in parallel with the decrease of solution TPA concentration. A similar behavior was verified for the other supports and implies that the main species present in the TPA solutions is the tungstophosphate anion, which gradually disappears from the

solution as it is adsorbed on the support surface. The low tungsten concentration in the catalysts prepared using the equilibrium impregnation method (Table 1) could be due to the low solution concentration, the low adsorption of TPA on zirconia and also, the pore blockage. The diameter of [PWl2040] 3- anion is 1,2 nm and the supports have mainly microporous structure, with pore diameters of less than 2 nm. So, the penetration and later adsorption of [PWI2040] 3- anion into the pores could be affected. The specific surface area of ZN110ED and ZN310ED are significantly lower than those of the supports (Table 1). The decrease is attributed to the obstruction, either total or partial of support pores, due to the pore filling with TPA solutions. On the other hand, the specific surface area and the total pore volume of the supports also decrease when they are impregnated using the incipient wetness but the decrease is less marked. This is due to that the lower impregnation time and the smaller volume of concentrated solutions involved in this preparation method disfavour the penetration of [PWl2040] 3" anion into the pores. As a consequence of the steric hindrance, there is a low penetration of the [PW12040]3- anion, which is agglomerated in the surroundings of pore mouths. The XRD patterns of supports impregnated using the incipient wetness method and dried at 70 ~ are shown in Figure 2. They present the peaks corresponding to the stable phase H3PW]2Og0.6H20 [7]. On the other hand, in the XRD patterns of samples calcined at 310 ~ (Figure 3) neither peaks corresponding to H3PW12Oa0.6H20 nor to H3PWI2040 phase are

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0C

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Figura 3. DRX patterns of bulk TPA and

different catalysts at 70 ~

different catalysts at 310 ~

present. According with these results, after the calcination at 310 ~ the H3PWl2Oa0 is present as a non-crystalline phase. The characteristics of XRD patterns of ZN110ED and ZN310ED catalysts are similar to those of the corresponding supports, so neither the diffraction lines of tungstophosphoric acid nor those of other crystalline phases were detected. From these results it can be inferred that the species are deposited as separate entities, a reasonable assumption bearing in mind the low concentration of TPA on solid and the surface area of the supports. Thus, the species present onto the support surface must be highly dispersed as a non-crystalline form, because of the interaction with the support. The FT-IR spectra of ZN110D and ZN310D (Figure 4), as well as the ones of ZNal 10D and ZNa310D, show the characteristic bands of the TPA, although some of them overlap with those of the supports. For ZN110ED and ZN310ED catalysts, the FT-IR spectra (Figure 4) show small bands at 1081 and 980 crn~ assigned to [PW12040]3- anion and two weak bands at 1060 and 960 cm~ which can be ascribed to [PWIIO39] 7" species. Then, the study of the catalysts by FT-IR allowed us to verify that the species present in the solid coincided with those in the impregnating solution in contact with the support only when the wetness impregnation method was used. During the equilibrium impregnation, the interaction degree between the [PW12040]3" anion and the support in greater, as result of the higher contact between the TPA and the support. Due to the higher interaction degree, the [PW12040]3 anion is transformed into the lacunar species [PWI1039] 7". On the other hand, the FT-IR study

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TPA

8

1400

1200 1000 800 600 wavenumber (era~)

400

Figure 4. FT-IR spectra of different catalysts, bulk TPA and bulk NaTPWllO39.

allowed us to determine that both the [PW12040]3 and [PWllO39] 7" species does not undergo irreversible degradation during the thermal treatment until 310 ~ because the spectra are similar to those of dried catalysts. The DRS spectra of the catalysts using the incipient wetness method present a band at 220-225 nm and another broad band that extend from 260 to 390 m~ Its characteristics resemble to those of the spectrum of bulk H3PW12040.6H20. The intensity of the band in 260-390 nm increases, in the spectra of the catalysts calcined at 310 ~ due to the reduction of [PWI2040] 3- species during the calcination. The DRS spectra of the catalysts prepared by equilibrium impregnation and dried at 70 ~ let us to observe that the broad band extends from 260 to 350 ran, mainly, as a result of the low amount of [PWl2040]apresent in these solids. It is difficult to confirm by this technique that the species [PWllO39] 7- is also present. The DSC diagrams of catalysts prepared using

the incipient wetness method and dried at 70 ~ show endothermic peaks at temperatures under 300 ~ and one exothermic peak at 590 ~ The endothermic peaks are associated with the dehydration of H3PWI2040.6H20 phase and the exothermic one are assigned to the Keggin's anion transformation into bronze compound PW8026. The DSC diagrams of catalysts prepared using the equilibrium impregnation method dried at 70 ~ are similar to that of the supports, due to the low amount of heteropolycompounds present in these samples. These results complement those obtained by FT-IR and confirm that the [PWl2040]3-anion does not degrade up to 310 ~ On the other hand, the thermal behaviour observed for the supported TPA was similar to those of the corresponding bulk acid. Table 1 presents the results of the isopropanol decomposition reaction obtained with the catalysts prepared by the incipient wetness or equilibrium adsorption methods, dried at 70 ~ together with the value obtained for bulk TPA. In the working conditions used, it was found that isopropanol does not react on the bare supports and it decomposes completely to propene on the TPA supported catalysts, though traces of acetone and diisopropylether were detected by means of GCMS. It can be observed that specific conversions are higher for supported TPA than they are for bulk TPA. Above 150 ~ the reactions catalyzed by supported HPA take place mainly on the surface or in the outer layers of the HPA crystals. As they retain little amount of water, so a too rigid structure is formed that isopropanol molecules cannot penetrate. Bulk TPA has a surface area of 2 m2/g, but when impregnated on the different supports the acid becomes dispersed, so increasing the catalytically active surface. The specific conversion decreases

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Table 1. Isopropanol conversion Catalyst CT Isopropanol conversion (mgW/gZrO2) (%) Bulk TPA 40.0 ZN 110D 97.7 26.5 ZN310D 97.7 20.5 ZNa110D 97.7 23.0 ZNa310D 97.7 19.2 ZN 110ED 32.2 2.9 ZN310ED 23.8 2.7

SBET

(m2/g) 2 214 151 194 118 40 50

Specific conversion (1/mg W) 0.11 0.60 0.46 0.52 0.43 0.25 0.23

gradually with the decrease in the SaET. It also decreases when the catalysts are calcined at 310 ~ The catalysts prepared by equilibrium impregnation present lower specific conversions than ZN110D and ZN310D catalysts. This is probably the result of the degradation of [PWl2040]3into [PWt1039]7 species in the solid, due to the interaction of TPA and the solid during the impregnation, and the lower surface acidity of the catalysts ZN110ED and ZN310ED. The catalytic activities of the supported TPA catalysts for the esterification of acetic acid with isoamyl alcohol are higher than bulk TPA. The relative specific conversions (RSC) calculated as the ratio of supported TPA and bulk TPA specific conversion for catalysts based on ZN110 and ZN310 are shown in Table 2. For this reaction, the ZN110D and ZN310D catalysts show lower RSC than the ZN110ED and ZN310ED. This behavior is mainly due to the steric hindrance of the two reactants to access the surface active sites, as a consequence of the lower species dispersion on the surface. The TPA supported catalysts showed high selectivity to the ester, though traces 3-methyl-l-butene, 2-methyl-l-butene and isovaleric aldehyde were detected by means of GCMS. The results show that equilibrium impregnation Table 2. Esterification specific conversion ZN310D Catalysts Bulk TPA ZN110D 5.19 RSC 1 2.66

ZN 110ED 8.80

ZN310ED 9.62

lead to HPA supported on ZrO2 acid catalysts with high activity and selectivity for the studied esterification reaction, thus obtaining an important product for flavour and fragance industries. ACKNOWLEDGEMENTS The authors thank L. Osiglio and G. Valle for their contribution and Project 01104 of ANPCyT and Project X224 of UNLP for financial support. REFERENCES 1. L. Pizzio, C. C~ceres and M. Blanco, Appl. Catal. 167 (1998) 283, and references included. 2. I. Kozhevnikov, Catal. Rev.- Sci. Eng. 37(2) (1995) 311. 3. R.A. Boyse and E. Ko, J. Catal. 171 (1997) 191. 4. B-Y. Zhao, X-P. Xu, H-R. Ma, D-H. Sun and J-M. Gao, Catal. Letters 45 (1997) 237. 5. J. Hern~dez-Cort6z, E. L6pez-Salinas, M. Manriquez-Ramirez, J. Navarrete-Bolafios, M. Llanos-Serrano and T. L6pez-Goeme, XVI Simp. Iberoam. Catal. Vol. II (1998) 875. 6. K. Nomiya, Y. Sugie, K. Amimoto, M. Miwa, Polyhedron 6 (1987) 519. 7. J.B.Mioc, Journal of Materials Science 29 (1994) 3705.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Dependence of Structure and Physico-Chemical Properties of the Catalysts Containing Heteropolyacids on the Conjugated Polymer Matrix E. Stochmal-Pomarzafska a and W. Turekb a Department of Materials Science and Ceramics, Academy of Mining and Metallurgy, AI. Mickiewicza 30, 30-059 Kraktw, Poland* b

Department of Chemi~qtry, Silesian Technical University, ul. Ks. M. Strzody 9, 44-100 Gliwice, Poland

Aromatic poly(azomethines) (PPI and PMOPI) have been used as a matrices to which selected Keggin-type heteropolyacids have been introduced. Heteropolyacids inserted into poly(azomethine) matrices preserve their structural identity. They are molecularly dispersed and do not form a separate phase. The incorporation of the dopants leads to the changes in the polymer chain conformation. The catalysts studied show enhanced redox activity in isopropanol conversion. The phenomenon of the diffusion of the catalyst active phase towards the catalyst surface is observed. 1. INTRODUCTION The application of conjugated polymers doping reaction to the preparation of a new genaration of conjugated polymer supported heterogeneous catalysts has been initiated in our research group. In particular heteropolyanions of Keggin-type have been successfully inserted to polyacetylene and polypyrrole via oxidative doping and to polyaniline via the protonation reaction (acid-base doping) [1-3]. In the present research we have used other conjugated polymers of poly(azomethine) type. Aromatic poly(azomethines) exibit extremely interesting properties which are associated mainly with their polyconjugated backbone. Thus they show good thermal stability [4] and interesting non-linear optical properties [5,6]. Poly(azomethines) contain basic sites of imine nitrogen type. Thus, similarly as polyaniline they can be doped via protonation reaction [7]. This acid-base type doping enables among others the introduction of anions of catalytic properties to the polymer matrix. 2. EXPERIMENTAL In this communication two types of polymer matrices were studied: the simplest unsubstituted aromatic poly(azomethine) (abbreviated PPI) and its dimethoxy ring substituted derivative (abbreviated PMOPI).

*This workwas financially supportedby the Polish Committeefor ScientificResearch (KBN) grant number 10.10.160.427.

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PPI was prepared from p-phenylenediamine and terephtaldehyde according to the method described earlier [8,9]. PMOPI was also obtained by polycondensation using the method described in [5,10] from p-phenylenediamine and 2,5-dimethoxyterephtaldehyde.

I

HaCO

1 OCH3

Jn

PMOPI

The doping (protonation) of the base form of the synthesized polymers was carried out at room temperature in acetonitrile solution. The following acids were used as the dopants in this study: H3PMo]2040, HaPW12040, H4SiW~2040. The doping level was conveniently varied by changing the ratio of poly(azomethine) to heteropolyacid. We express the doping level as the fraction of heteropolyacids molecules (HPA), y, per repeat unit i.e. (C14HIoN2)(HPA)y or in shortened version PPI(HPA)y and (C16H14N202)(HPA)y or in shortened version PMOPI0-IPA)y. In the catalytic studies isopropanol conversion has been used as the test reaction. Catalytic tests were carried out in a wide temperature range using a differential reactor. The reaction products were analysed chromatographically. Additionally the kinetics of isopropanol conversion was measured on catalysts previously used for the studies of propylene oxidation kinetics. In this case the catalysts containing PMOPI as a matrix were used. Both unprotonated and protonated PPI and PMOPI were characterized by FTIR, Raman spectroscopy, XPS studies, X-ray dif~action measurements and thermal studies. 3. RESULTS Catalysts containing n3PW12040, U4SiWl2040 exhibit high thermal stability (fig. 1). The temperature at which their degradation starts depends on the protonation level (the higher the protonation level the lower thermal stability). However, even at the highest . 2 . " ~ DTG.__G_~ ~ /\ ~o ~ ~. protonation levels the temperature of the thermal decomposition exceeds 400~ The catalysts containing H3PMo12040 are less thermally stable; .-7.o I / for high protonation levels their II.._ '~ / I\ '~ .lo.o / I~ decomposition starts at 250 ~ " K [=I.~o ~ / I~ 10 The protonation reaction leads to significant amortization of the "~176 t '-~176 sample. Basic PPI and PMOPI are t // \o_ " 50 170 1150 ~ 250 300 35=0 4?0, 46j0 u 15,?0 6m00 remarkable highly crystalline [ 11]. The Temperature ['C] degree of crystallization estimated Fig. 1. Thermogravimetric (TG), differential according to the method of Hindeleh gravimetric (DTG), differential scanning calorimetry and Johnson [12] is close to 30% for (DSC) set of curves for PPI(HaPW12040)o.104. both PPI and PMOPI. Increasing protonation level results in a gradual disappearance of Bragg reflections due to the undoped polymer with simultaneous appearance of a broad peak at d - 7-10 A (figs. 2, 3). X-ray

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diffraction patterns show that HPA introduced into the polymer matrices are molecularly dispersed and do not form a separate phase. Protonation of PPI and PMOPI with heteropolyacids is manifested in Nls XPS spectra by the appearance of a peak at 401.8 eV characteristic of charged nitrogen atoms. XPS studies show that protonation with

a

~ o

a3PWl2040,

e.

~

.... 3

, .........

6

, .... 9

12

, .... 15

, . . . . . . . . . . . . . . 18

21

24

~

A

c

, ......... 27

, ....

30

33

, .... 36

20

39

Fig. 2. X-ray dit~actograms of: a)unprotonated PPI, b)PPI(I-IPMo)o 009,c) PPI(HPMo)o 021,d) PPI(HPMo)o 066, e) PPI(HPMo)o 452,f) PPI(HPMo)o 709.

n4SiWl2040

is

a simple acid-base reaction and the incorporation of the dopants does not lead to the change in the oxidation level of tungsten. Doping with HaPMo12040 is a more complex process since it is accompanied by a redox-type side reaction which results in the reduction of some Mo(VI) atoms to Mo(V) [4].

The FTIR spectra of poly(azomethines) reacted with heteropolyacids contain the features characteristic of b spectrum of polyazomethine base and the spectrum of the corresponding acid [ 13,14]. Four strongest bands characteristic of Keggin-unit are present in spectra of polyazomethine reacted with heteropolyacids (fig. 4). This proves that during the reaction of ................................................. 7,, polyazomethine with heteropoly20 acids, heteropolyanions are Fig. 3. X-ray diffIactograms of: a)unprotonated PMOPI, incorporated into polymer matrix b) PMOPI(I-IPMo)o 018,c) PMOPI(HPMo)o 036, and the structural indentity of d) PMOPI(HPMo)o 068,e) PMOPI(I-IPMo)o 198, anions is preserved. The IR bands f) PMOPI(HPMo)o 431. characteristic of Keggin structural units grow with increasing protonation level and at higher protonation levels totally dominate the spectrum. Changes in some band positions indicate some structural deformations of heteropolyanions upon insertion to PPI or PMOPI matrices. FTIR studies prove that the structural identity of individual heteropolyanions is preserved upon introduction into the polymer matrices. However, protonation leads to the changes in the polymer chain conformation as well as in the force constants of some bonds which causes the appearance of the new bands in the Raman spectra of the protonated polymers and the shifts of some bands originating from the undoped polymers (fig. 5). 3

6

9

12

15

18

21

24

27

30

33

36

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Among others significant changes are observed in the bands corresponding to the protonation sites (CH=N groupings) and also bands due to C=C strechings in the aromatic ring, C-C~ strechings and C-H ring deformations. Contrary to FTIR studies no lines attributed to heteropolyanions are observed for heteropolyanions in the Raman spectra. As a result of the introduction of heteropolyanions, catalytically active centers are molecularly dispersed on the surface and strongly chemically bonded to the polymer. The surface of the active phase of poly(azomethine) supported catalysts exhibits significantly different properties as compared to unsupported cristalline heteropolyacids.

~t

~a~l

400

600

800

.~,~ "-

"

1000

1200

1400

"

1600

1800

W e v e n u m b e r e [ 9 "g]

Catalytic conversion of Fig. 4. FTIR spectra of: a) unprotonated PMOPI, isopropanol leads to two reaction b) PMOPI(HPMo)o.o36, c) PMOPI(HPMo)o.o6s, products: propylene, which is formed in d) PMOPI(HPMo)o.431, e) crystalline H3PMo1204o. the dehydration reaction on the acid-base centers and acetone, which is formed in the dehydrogenation reaction on the redox centers. The insertion of heteropolyacids into the polyazomethine matrix via the protonation reaction significantly changes the selectivity as compared to unsupported HPA. Since upon protonation of the polymer chain the heteropolyacid molecule is transformed into a weak base, a decrease in the number of acidic sites is expected and the catalyst shows predominantly redox activity with high selectivity towards acetone. However, with increasing HPA content in polyazomethine matrix the selectivity towards acetone formation decreases. Similarly with an increase of temperature the content of propylene in 600 800 1000 1200 1400 1600 1800 the reaction products increases (Table 1). W=venurnber= [om "~] The PPI(H3PW12Oao)y and Fig. 5. Raman spectra of: a) unprotonated PMOPI, PPI(H3PMol2040)y catalysts show similar b) PMOPI(HPW)o.o3s, c) PMOPI(HPW)o.o66, behaviour but in the case of these latter d) PMOPI(I-IPW)o.173, e) PMOPI(HPW)0.399. ones, the selectivity towards acetone is 9

"-

.

.

^.

_

_

~

=

_

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lower (for close protonation level values). This means that H3PM012040 is less efficient protonating agent than H3PW~2040. The PMOPI(H3PW~2040)y and PMOPI(H3PMo~2040)y catalysts exhibit similar properties in isopropanol conversion. Comparing catalytic properties of PPI-HPA with PMOPI-HPA systems containing close amount of the heteropolyacid and studied in the same experimental conditions in general PPI(HPA)y catalysts show higher catalytic activity, except those containing higher amounts of H3PW1204o. In this case both systems show close catalytic activity. Table 1 Selectivities of the catalytic isopropanol decomposition reaction mesured for: PPI(HPA)y and PMOPI(HPA)y catalysts.

Selectivity [%] Catalyst

HPA content in the sample [wt %] PPI(H3PMo1204o)o.o21 10.1 PPI(H3PMo1204o)o.o66 35.8 PPI(H3PMo1204o)o.452 68.7 PPI(H3PMo1204o)o.7o9 79.5 PPI(H3PW 1204o)o.o3s 33.2 PPI(H3PW1204o)o.lo4 56.6 PPI(HaPW1204o)o.468 85.9 PPI(H3PW1204o)o.8o4 89.7 PMOPI(H3PMo 1204o)o.o68 29.8 PMOPI(H3PMo 1204o)o.431 71.8 PMOPI(H3PW 12040)0.066 3 8.2 PMOPI(HaPWI204o)o.399 72.7

403 K Propylene 23.2 53.0 72.2 90.2 4.0 12.1 89.5 96.9 10.4 47.6 95.9

Acetone 76.8 47.0 27.8 9.8 96.0 87.9 10.5 3.1 89.6 52.4 4.1

456 K Propylene 78.6 89.9 91.1 96.1 18.0 38.4 97.2 99.3 67.6 79.9 2.4 97.8

Acetone 21.4 10.1 8.9 3.9 82.0 61.6 2.8 0.7 32.4 20.1 9 7.6 2.2

Conversion levels ranged from 5 to 20%. In cases of PMOPI(H3PW12040)y, PMOPI(H3PMo12040)y catalysts propylene oxidation reaction carried out prior to isopropanol conversion causes an increase of the catalyst activity in the dehydratation reaction and simultaneously a significant decrease of the activation energy of this latter reaction. This observation may suggest that some of the protons previously inactive are activated via propylene oxidation process. In the reaction of isopropanol conversion on freshly prepared polymer supported catalysts, containing different amounts of H3PMo1204o, the catalytic activity seems to be independent on the heteropolyacid concentration i.e. the activity normalized per l g of heteropolyacid is very similar for all catalysts tested. However for catalysts, used previously for propylene oxidation the measured activity is higher (fig. 6). This increase of the activity is especially pronounced in the case of the catalyst containing the highest amount of HPA i.e. PMOPI(H3PMo 12040)0.431. An increase in the activity of the catalysts used previously for propylene oxidation can be rationalized by the assumption of the diffusion of the heteropolyacid molecules from the bulk of the polymer to its surface.

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-6 -6.5

-6 ~ . q ~

-6,5

xA

-7 i

-7,5 i

-7,5

2&

-8,5 -9,5 ~ 1.9

2,1

i

2,3 tOZl T (K "1 )

2,5

2,7

-9

1,9

2,1

J

I

2,3 10 = I T (K "1)

2,5

9 2,7

Fig. 6. Arrhenius plots ofisopropanol decomposition over catalytic samples: ( a ) PMOPI(HsPMol:O40)0.o68; (b) PMOPI(H3PMo12040)0.43]. 1 - propylene; 2 - acetone.; A - freshly prepared catalysts, B - the catalysts used previously for the propylene oxidation reaction. The phenomenon of the diffusion of the catalyst active phase towards the catalyst surface induced by the catalyst has previously been observed for oxide as well as metal alloys catalysts [15]. This concept of the diffusion is additionally corroborated by the fact that the increase of catalytic activity is more pronounced for catalysts with higher content of the active phase. REFERENCES:

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

I. Kulszewicz-Bajer, M. Zag6rska, A. Profi, D. Billaud and J. Ernhardt, Mat. Res. Bull. 26 (1991) 163. J. Po/aficzek, I. Kulszewicz-Bajer, M. Zagtrska, K. Kruczata, K. Dyrek, A. Bielafiski and A. Profi, J. Catal., 132 (1991) 311. M. Hasik, W. Turek, E. Stochmal, M. Lapkowski and A. Profi, J. Catal., 147 (1994) 544. E. Stochmal-Pomarzafiska, Phi) Thesis, University of Minning and Metallurgy (AGH), Cracow Poland - unpublished. C.J. Yang and S. A. Jenekhe, Chem. Mater., 3 (1991) 878. C.J. Yang and S. A. Jenekhe, Macromolecules, 28 (1995) 1180. E. Stochmal-Pomarzafiska, M. Hasik, W. Turek and A. Profi, J. Mol. Catal. A: Chemical, 114 (1996) 267. J. Manasseh, S. Khalif, J. Am. Chem. Soc., 88 (1966) 1943. I. Kulszewicz-Bajer, I. Wielgus, J. Sobczak and A. Profi, Mat. Res. Bull., 30 (1995) 1571. P. W. Morgan, S. L. Kwolek and T. C. Plechter, Macromolecules, 20 (1987) 729. W. Lu~ny, E. Stochmal-Pomarzmiska, A. Profi, Synth. Met., 101 (1999) 69. A. M. Hindeleh, D. J. Johnson, J. Phys. D., 4 (1971) 259. A. Vogel, A Textbook of Practical Organic ChemLqtry - Polish Edition, WNT, Warsaw, 1984. C. Rocchiccioli-Deltche~ F. Thouvenot and 1L Franck, Spectrochim. Acta, 32A(1976) 587. J. Haber, E. Mielczarska and W. Turek, Z. Phys. Chem., 144 (1985) 69.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Mesoporous Silica Supported Solid Acid Catalysts Saemin Choi, Yong Wang t, Zimin Nie, Dev Khambapati, Jun Liu, and Charles H.F. Peden t Pacific Northwest National Laboratory*, P.O. Box 999, Richland, WA 99352, USA. In this brief proceedings paper, we summarize the results of recent studies of the preparation and characterization of mesoporous silica supported tungstophosphoric acid (TPA/MS) and Cs-substituted tungstophosphoric acid salt (Cs-TPA/MS). In particular, we demonstrate that we have synthesized MS-supported Cs-substituted catalysts having significantly improved dispersion of the active clusters compared to materials described previously in the literature. Transmission electron micrographs and the activity results for a model reaction, the alkylation of 1,3,5-trimethylbenzene by cyclohexene, are presented as evidence for the enhanced dispersion and performance. In addition, we demonstrate improvements in the physical and thermal stability of these materials with Cs substitution using various characterization techniques. Finally, we also briefly describe the characterization and catalytic activity of TPA/MS materials, providing evidence for shape selectivity that is likely imparted by the structure of the MS support. The promising results with TPA/MS and Cs-TPA/MS catalysts indicate that they have potential applications in a variety of acid-catalyzed organic reactions involving large-sized reacting, intermediate, and/or product molecules with desired shape selectivity to products and/or intermediates. 1. INTRODUCTION Currently, the chemical and petrochemical industries produce a wide range of organic compounds via alkylation and isomerization by homogeneous acid catalysts such as H2SO4, HF, and A1C13 [1-3]. Although current homogeneous catalysts are efficient, as a result of their corrosive and toxic nature, these materials pose potential environmental hazards and present operational problems, including difficulty in separation, recovery and reutilization, that leads in higher capital costs. Among many solid acid systems, heteropoly acids (HPA) with Keggin anion structures have received considerable attention due to their simple preparation and strong acidity [4,5]. Specifically, 12-tungstophosphoric acid (H3PWl2040), denoted as TPA hereafter, is among the most extensively studied [6-8] since it possesses the highest Br6nsted acidity [9]. However, to date, low efficiency due to low surface area, rapid deactivation and relatively poor stability are some of the major problems associated with these TPAs in conventional bulk acid forms. Attempts to improve the efficiency of these materials have been made by supporting TPA on various high surface area supports [ 10,11 ]. In part because a strong interaction between TPA and a support material likely results in the decomposition of the Keggin structure, "Pacific NorthwestNational Laboratory is a multiprogram national laboratoryoperated for the U.S. Department of Energyby Battelle Memorial Institute undercontract# DE-AC06-76RLO1830. tAuthors to whomcorrespondenceshould be addressedat: [email protected];[email protected].

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several reports in the literature have identified silica as a suitable support due to its intrinsic inertness [12-18]. Most recently, mesoporous silica (known as MCM-41), first developed by researchers at Mobil [19-20], has been used to support TPA clusters to take advantage of the uniform pore size and highly ordered structures available in these unique materials [ 12-15]. Another method that could possibly enhance stability of the active clusters in solution is to prepare catalysts in the form of TPA salts [8,21,22]. For example, _partial substitution of Cs + for protons render bulk TPAs with higher surface area (up to 150 m2/g compared to 5 m2/g), improved thermal stability, and they are known to be insoluble even in liquids as polar as water. Consequently, TPA salts should be better suited for practical applications that might involve polar reagents in harsh operating conditions. However, their small particle size (-ttm) limit their application for use as catalysts in commercial fixed bed or slurry type reactors. An obvious solution is to support these TPA salts on a larger particle size carder. Unfortunately, preparation of these catalysts in an engineered form is challenging since direct aqueous impregnation is not feasible. For example, materials synthesized by Soled, et al. [22] consisted of thin internal rings of Cs-substituted TPA salt (so-called egg-white distribution) within the silica extrudate suggesting nonuniform dispersion of the active clusters on silica. The premise of our work [23-25], some of which is described in this paper, is that dispersion of TPA can be manipulated by adopting appropriate grafting techniques (Figure 1). We have prepared a series of mesoporous silica supported TPA and Cs-TPA salts with highly dispersed and intact Keggin anions, and compare their structural and catalytic properties to conventionally prepared supported catalysts and their bulk counterparts.

F

i:'/. i,,

'i

i ~

4t ,t

P

(a)

(b)

(c)

Figure 1 Dispersion of tungstophosphoric acids on mesoporous silica: (a) TEM micrograph of hexagonal honeycomb structure of mesoporous silica with 30A pore size; (b) Keggin structure of the tungstophosphoricacid cluster;and (c) a well dispersedsolid acid catalyst. 2. EXPERIMENTAL MCM-41 type mesoporous silica with mono-dimensional pores of 18, 30, 50 and 100 A were synthesized using a protocol reported elsewhere [19,20,23,25]. Described elsewhere are the preparations of highly dispersed TPA [23-25] and Cs-TPA [24,25] on mesoporous silica, denoted as Csx-TPA/MS (x = Cs stoichiometry, MS = mesoporous silica) hereafter. For comparison, we prepared supported Cs-TPA samples using method reported by Soled, et al. [22]. Bulk Cs-TPA materials were prepared by previously published [21] procedures.

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Keggin structures of bulk and supported samples were examined using FT-IR spectroscopy and 31P NMR experiments. The physical properties of the catalysts were assessed with thermal analysis experiments, BET surface area and pore size distributions measurements, and transmission electron microscopy (TEM). The composition was also analyzed using the energy dispersive spectroscopy technique (EDS). Selected samples were examined for leaching of TPA by water after stirring vigorously for 2 hours at 50~ in a water bath. Thermal stabilities of the TPA/MS (x=0) catalysts were studied with in-situ x-ray diffraction. TPA/MS (x=0) catalysts were evaluated with a probe reaction, the alkylation of 4-tbutylphenol (TBP, Aldrich, 99%) with styrene (Aldrich, 99+%) [26]. The catalytic properties of the Csx-TPA/MS materials were evaluated using the liquid phase alkylation of 1,3,5trimethylbenzene (mesitylene, Aldrich, 98%) with cyelohexene (Aldrich, 99%) [22,27]. Specific experimental details are provided elsewhere [23-25]. 3. RESULTS AND DISCUSSION

3.1 Characterization and Reactivity of Supported TPA Catalysts Mesoporous silicas with three uniform pore size distributions (18A, 30A, and 100,~) were chosen as the supports for unsubstituted (i.e., x=0 in Csx-TPA/MS) tungstophosphoric acid (TPA) catalysts. For comparison purposes, a premium, amorphous Lyosil silica was also used as a catalyst support. Even at relatively high (70wt%) TPA loadings, supported catalysts still have rather large surface areas (> 150 m2/gm), although it appears that the rather largesized TPA clusters (12A) can readily clog the pores during solution impregnation when support pore sizes are smaller than 30A. Factors such as the surface properties of supports [5] and the pH of the sample preparation solution [28] play important roles in affecting the Keggin structure of TPA. 31p NMR is the most convenient and revealing method for the assessment of the phosphorous environment in the phosphorous-containing HPA compounds, and, in turn, the stability of the Keggin structure of TPA. Using this technique, we determined that impregnation of an untreated mesoporous silica leads to the partial decomposition of the Keggin-type structure of TPA [23]. The intact Keggin structure was preserved when TPA was supported on silica neutralized with 1N nitric at high TPA loadings although, again, decomposition was observed for lower loading levels. However when methanol is used to impregnate acid-treated supports with TPA, the intact Keggin structure was retained even at a 10wt% TPA loading. It should be noted that all catalysts with decomposed Keggin structures were found to be inactive in the probe reaction studied in this paper under the conditions investigated. Therefore, even with inert supports such as silica, neutralization of supports with acids and use of non-hydrolyzing polar solvents such as methanol are required to retain the Keggin structure when preparing supported TPA catalysts with a solution impregnation method. Importantly, we also found that that the thermal stability of TPA is enhanced by ~75~ when TPA is supported on 30A SiO2 [23]. The catalytic performance of supported TPA catalysts were evaluated using the probe reaction, alkylation of 4-t-butylphenol (TBP) by styrene. Both products, 2-(1-phenylethyl)-4t-butylphenol (product 1) and 2,6-bis-(1-phenylethyl)-4-t-butylphenol (product 2), are bulky molecules. Thus, shape selectivity to the products provided by mesoporous silica can be studied. Selectivities at 90% conversion of TBP are summarized in Table 1. Compared with a bulk TPA material, supported..TPA catalysts show at least a six fold enhancement in activity due to more accessible protons when TPA is supported on large surface area silica [23]. 30A and 100A silica supported TPA catalysts exhibit about twice as high activity as that of Lyosil

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silica supported TPA catalysts, again due to the huge surface area with mesoporous silica. The higher activity observed with 100A supported TPA material over that of 30A supported TPA catalyst is ascribed to the better dispersion of TPA due to less TPA clogging in large pores (100A) during sample preparation. On the other hand, the 30A silica supported TPA catalyst showed much improved shape selectivity to monoalkylated product 1 (Table 1), which is attributed to a steric hindrance to dialkylated product 2 (about 20A [12]) by its uniform and compatible pore size (30A). 18A silica supported TPA showed very low activity compared with other supported TPA catalysts, likely due to the poor dispersion of TPA as described above, or to the severe pore diffusion resistance to the bulky reactants.

Table 1. Selectivities at 90% Conversion of TBP Catalyst Product 1, % TPA 50wt%TPA/amorphous SiO2 50wt%TPA/30A SiO2 50wt%TPA/100A SiO2

46 54 80 61

Product 2, % 45 38 13 32

3.2 Characterization and Reactivity of Supported Cs-TPA Catalysts Primary structures of the supported, Cs-substituted (Csx-TPA/MS) catalysts were identified by comparing their FT-IR absorbance bands to those of bulk TPA, tungstophosphoric acid salt (Cs-TPA), and mesoporous silica [24,25]. Bulk TPA (HaPWl2040) and TPA salt (Cs2.5H0.5PW12040)show the characteristic IR bands at-1080 (P-O in the central tetrahedra), 984 (terminal W-O), 897 and 812 (W-O-W) cm l associated with the asymmetric vibrations in the Keggin polyanion. The same distinguishable features were observed for the 50 wt% TPA/MS and Cs-TPA/MS catalysts, indicating that the primary Keggin structure is preserved after supporting it onto mesoporous silica. Furthermore, 31 P NMR results have confirmed that the materials prepared in this study had intact polyanion structures on the silica surface. The dispersion of Cs-TPA on mesoporous silica can be inferred from the TEM results illustrated in Figure 2, and from EDS analysis (not shown). As mentioned above, direct impregnation using a Cs-TPA solution was not possible since Cs-TPA is not sufficiently soluble in any solvent. The material we prepared using a two-step method used previously by Soled, et al. [22] resulted in a segregated phase where Cs-TPA is not uniformly dispersed (Figure 2(a)). In contrast, the material newly prepared here (synthesis described in [24]) consists of uniformly dispersed Cs-TPA salt on mesoporous silica (Figure 2(b)). An important potential benefit of supporting TPA on oxide supports (including MS) is enhanced stability for the TPA salts. In Section 3.1, we report that the thermal stability of HPA is enhanced by 75 ~ (to 585 ~ when TPA is supported on mesoporous silica [23]. It is noteworthy that the thermal stability of the supported TPA is enhanced further even with a single Cs substitution. However, the enhancement in thermal stability for these catalysts comes at the expense of the catalytically active acid protons. Thus, the effect of Cssubstitution on catalytic activity must also be assessed (see below). The stability of the active species in solution has also been of concern for solid acids, specifically for the supported materials [8,23] because of a likely weak interaction of TPA species with silica. This can result in significant leaching of TPA in the presence of a polar solvent. Thus, it is significant that we find that supported catalysts show the same trend as the bulk TPA materials where resistance to leaching is improved considerably with increasing Cs stoichiometry.

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Figure 2 Transmissionelectron micrographsof (a) Cs2.5-TPA/MSprepared from a published method [22] and (b) our improved,newlysynthesizedCs2.5-TPA/MSmaterial. Catalyst activities were evaluated using the alkylation of trimethylbenzene (mesitylene) by cyclohexene as a model reaction. The 50 wt% Cs2.5-TPA/MS material synthesized in this study [24,25] was about five times as active as that made from a previously published method [22] and was also more active than a bulk Cs25-TPA material. The primary purpose for adopting the Cs - 2.5 for bulk materials is that this specific stoichiometry provides high surface area and mesoporous porosity giving rise to optimized activity [8]. Considering the fact that a high surface area carder with ordered structure is adopted here to support the I'PA clusters and that more Cs substitution leads to a reduction of acidic protons, we have prepared and examined supported Cs-TPA catalysts with a lower Cs stoichiometry. In this way, Cs substitution levels are optimized for both catalytic activity and stability to solvent leaching. In Figure 3, activities of MS-supported 50wt% Cs-TPA are plotted as a function of Cs stoichiometry. Typically, the Cs-TPA catalysts prepared by us [24,25] demonstrated superior catalytic properties than those prepared with the previously published method [22], regardless of the Cs stoichiometry. Even the supported TPA materials (without Cs substitution) were more active and therefore better dispersed than those prepared using the "conventional" [22] method. That is, the former materials were probably less susceptible to pore-clogging by poorly dispersed acid anion clusters. It is also noteworthy that supported CSl-TPA catalysts are more active than their supported parent acids despite lower quantities of active, acid proton sites. This result suggests that the dispersion of Cs-TPA clusters is improved regardless of the preparation method. It is important to note that these measurements may underestimate the enhanced activity of the novel, improved catalysts prepared in this study relative to those prepared by the "conventional" method. This is because conversions for the most active materials were near 100% under the conditions tested here. 5. ACKNOWLEDGMENTS The work described here was funded by the U.S. Department of Energy (DOE), Office of Science, Laboratory Technology Research Program. S. Choi thanks Associated Western Universities Inc. Northwest Division for the Fellowship awarded under Grant DE-FG0692RL-12451 with the DOE. We thank Professor William J. Thomson for the in-situ X-ray diffraction measurements performed at Washington State University.

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8O

.o

Figure 3: Alkylation of 1,3,5trimethylbenzene by cyclohexene at 80 ~ for (a) improved 50 wt% CsTPA/MS and (b) "conventional" [22] 50 wt% Cs-TPA/MS as a function of Cs stoichiometry.

6O

(b)

o 0

4O

0

I 0

I

! 1

I

I 2

I 2.5

Cs Stoichiometry

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.

J.M. Thomas, Sci. Am., 266 (1992) 112. M. Misono and T. Okuhara, ChemTech., 11 (1993) 23. A. Corma and A. Martinez, Catal. Rev.-Sci., Eng., 35 (1993) 483. M. Misono, Catal. Rev.-Sci. Eng., 29 (1987) 269. I.V. Kozhevnikov, Catal. Rev.-Sci. Eng., 37(2) (1995) 311. M. Misono and N. Nojiri, Appl. Catal., 64(1) (1990) 1. A. Corma, Chem. Rev., 95 (1995) 559. T. Okuhara, N. Mizuno and M. Misono, Adv. in Catal., 41 (1996) 113. M. Misono, N. Mizuno, K. Katamura, A. Kasai, Y. Konishi, K. Sakata, T. Okuhara and Y. Yoneda, Bull. Chem. Soc. Japan, 55 (1982) 400. 10. G.I. Kapustin, T.R. Brueva, A.L. Klyachko, M.N. Timofeeva, S.M. Kulikov and I.V. Kozhevnikov, Kinet. Katal., 31 (1990), 1017. 11. L.R. Pizzio, C.V. C~iceres and M.N. Blanco, Appl. Catal. A: General, 167 (1998) 283. 12. I.V. Kozhevnikov, A. Sinnema, R.J.J. Jansen, K. Pamin and H. van Bekkum, Catal. Lett., 30 (1995) 241. 13. I.V. Kozhevnikov, K.R. Kloetstra, A. Sinnema, H.W. Zandbergen and H. van Bekkum, J. Mol. Catal. A: Chemical, 114 (1996) 287. 14. T. Blasco, A. Corma, A. Martinez and P. Martinez-Escolano, J. Catal., 177 (1998) 306. 15. C.T. Kresge, D.S. Marler, G.S. Rav and R.H. Rose, US patent 5366945, 1994. 16. Y. Izumi, R. Hasebe and K. Urabe, J. Catal., 84 (1983) 402. 17. J.B. Moffat and S. Kasztelan, J. Catal., 109 (1988) 206. 18. C. Rocchiccioli-Deltcheff, M. Amirouche, G. Herve, M. Fournier, M. Che and J.M. Tatibouet, J. Catal., 126 (1990) 591. 19. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz and C.T. Kresge, J. Am. Chem. Soc., 114 (1992) 10834. 20. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710. 21. T. Okuhara, T. Nishimura, H. Watanabe and M. Misono, J. Mol. Catal., 74 (1992) 247. 22. S. Soled, S. Miseo, G. McVicker, W.E. Gates, A. Gutierrez and J. Paes, Catal. Today, 36 (1997) 441. 23. Y. Wang, A.Y. Kim, X.S. Li, L. Wang, C.H.F. Peden and B.C. Bunker, in Shape-Selective Catalysis: Chemicals Synthesis and Hydrocarbon Processing, C. Song, J.M. Garces, and Y. Sugi, Eds., American Chemical Society (Washington, DC) 1999, in press. 24. Y. Wang, S. Choi and C.H.F. Peden, patent pending, November, 1998. 25. S. Choi, Y. Wang, Z. Nie, J. Liu and C.H.F. Peden, Catal. Today (1999) in press. 26. I.V.Kozhevnikov, A.I.Tsyganok, M.N.Timofeeva, S.M.Kulikov, and V.N.Sidelnikov, React. Kinet. Catal. Lett. 46 (1992) 17. 27. T. Okuhara, T. Nishimura and M. Misono, Chem. Lea., (1995) 155. 28. G.B.McGamey and J.B.Moffat, J.Molecular Catal. 69 (1991) 137.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

STUDY OF ACIDIC AND CATALYTIC PROPERTIES OF SULFATED ZIRCONIA P R E P A R E D BY SOL-GEL PROCESS :INFLUENCE OF PREPARATION CONDITIONS L. Ben Hamouda* ; A. Ghorbel a and F. Figueras. b

a Laboratoire de Chimie des Mat6riaux et Catalyse, D6partement de Chimie, Facult6 des Sciences de Tunis. b Institut de Recherche sur la Catalyse, 2 Av. Albert Einstein 69626 Villeurbanne cedexFrance. Abstract

Two different processes were used in this study to prepare sulfated zirconia. The effect of both the hydrolysis ratio H20/Zr and S/Zr molar ratio were examined. Sample characterization was carried out using XRD, NH3-TPD, N2 adsorption-desorption and FT-IR spectroscopy using adsorbed probe-molecules ( C6I% and pyridine). Sulfur content before and after n-hexane isomerization reaction were measured. We found that samples obtained by solgel process retain high amount of sulfur and exhibit important BET surface areas. The decrease of the hydrolysis ratio improves considerably acidic and catalytic properties. Lewis and Br0nsted acid sites coexist but the Lewis ones are predominant. It was also found that the increase of the Pt lowding in mechanical mixture of Pt/AI203 and sulfated zirconia enhances the catalytic activity and selectivity at moderate activation temperature. 1. INTRODUCTION In recent years SO42-doped zirconia have attracted great interest as strong acid catalysts for isomerization, acylation and esterification reactions[ 1,2], however many research groups have found difficulties to produce an active catalyst. One reason advanced to explain this results was related to the presence of some water which seem essentiel to produce a very active catalyst[3]. Thus, the amount of water used must be controled. Among the different preparation methods, Sol-Gel synthesis seems to be an intersting way, because this method combines the two steps of zirconia support formation and sulfate promotion, and allows the control of the water and sulfate contents. It has been shown in several works that sulfates on sulfate doped zirconia, as well as on other sulfated oxide, can assume different structures depending on the degree of surface hydration [4]. Superficial concentration of sulfate species causes different kinds of acidity. Morterra et al.[5] indicated that only Lewis acidity is present if the concentration of the SO4~ groups corresponds to values lower than two sulfur atoms per nm 2. If the coverage increases over an average half-monolayer of SO42- Br0nsted type acidity appears[6].

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2. EXPERIMENTAL

2.1. Catalyst preparation Two different processes were used in this study to prepare sulfated zirconia. The first sample denoted ZSI was prepared as follows. ZrOC12.6H20 was dissolved in water and precipitated with NH3 solution at a fixed pH-10. The precipitate was washed with distilled water until elimination of CI and then dried at 393K. sulfation was carried out with H2SO4 1N solution. The second kind of samples (ZSH) were prepared by sol-gel technique as follows. Zr(OPr)4 was mixed with n-propanol and different amounts of sulfuric acid. The latter was adjusted in order to obtain a molar ratio x = S/Zr variable from 0.1 to 0.8. After stirring for lh different amounts of water were added to obtain hydrolysis ratio y - H20/Zr variable from 1 to 4. The resulting gels were oven dried at 393K for 24h and then calcined in air at different temperatures (T). The solids obtained were denoted ZSH-y-x-T.

2.2. Experimental techniques The effect of changing preparation method, hydrolysis ratio H20/Zr and S/Zr molar ratio during the sol-gel process, and activation temperature were studied using different equipments: XRD, chemical analysis, surface BET and porosity measurement, NH3-TPD, FT-IR spectroscopy using adsorbed probe-molecules(C6H6 and pyridine). XRD was used to study the structure of the samples after calcination. The diffractograms were recorded on a Philips equipments using CuKo~ radiation. Chemical analysis of sulfur were performed with a Coulomat 702. BET surface areas and pore diameters of the samples were determined by adsorption of N2 at 77K, the apparatus used is a Micromeritics ASAP 2000. IR measurements were performed using a Perkin-Elmer FT-IR spectrometer. Catalytic activity and selectivity for n-hexane isomerization reaction were measured in a tubular reactor in dynamic regime at 473K under atmospheric pressure. Sulfated zirconia was mixed with reduced Pt/A1203 0.35 wt% at 723K for 3h and then activated at 473K or 673K for 2h under H2 before test. Reaction was performed by feeding H2/n-hexane molar ratio of 17. Products were analyzed by on line FID gas chromatography. 3. RESULTS AND DISCUSSION

3.1. XRD analysis

Figure 1 give the diffractogram pattems corresponding to sulfated zirconia prepared by solgel process with differem amount of sulfate. We note that sulfation retards the cristallization and the crystal phase transition in agreemem with results reported by many research groups[7].The study of the effect of the amount of water added on structural properties shows that cristallization is improved when hydrolysis ratio increases which indicates that when the amoum of water is reduced sulfate groups were more bonded to the gel of zirconia during the preparation step. Furthermore, in this case the number of organic groups which leave at higher temperature remains important.

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化

T

0.5 ~2

20

I

I

I

60

50

40

i. . . . . . .

30

I

20

Fig. 1 9Diffractogram patterns corresponding to sulfated zirconia calcined at 773K. T" Tetragonal

M" Monoclinic

(0 ; 0.2 ; 0.5 = S/Zr)

3.2. BET analysis Textural properties are summerized in Table 1. We note that sulfated zirconia prepared via sol-gel process present higher BET surface areas than ZSI. Furthermore sulfation inhibits surface area decrease during heat treatment. Hydrolysis ratio appears to have great effect upon the BET surface area wich decreases considerably when the amount of added water is lowered (Fig. 2). This result is in good agreement with XRD analysis. Table 1 : BET surface area of sulfated zirconia. . . . . . . . . . . . . . .

BET surface area mZ/g

Sample

Calcination temperature (K) 393

623

773

873

ZSI

229

158

132

98

ZSH-4-0

226

63

42

10

ZSH-4-0.2

387

210

214

141

ZSH-4-0.5

320

207

117

145

100 80 60 40 20 0

3

2

1

Hydrolysis ratio (H20/Zr)

Fig 2 9Effect of the hydrolysis ratio on the surface area of ZSH-y-0.1-833K.

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3.3. Chemical analysis

Table 2 summerizes the sulfur content of different solids. We note that ZSH samples retain more amount of sulfur than ZSI. Investigation of the effect of S/Zr molar ratio during the preparation on the final catalysts shows that the decrease of this ratio improves the sulfur content after calcination at 833K. Furthermore the amount of added water influences the sulfate content. In fact, when the hydrolysis ratio decreases, the sulfur weight loss after catalytic reaction is reduced. This result agree with a better incorporation of sulfate groups to the gel if the amount of water is lowered. Table 2 : Sulfur content of calcined and used catalysts. wt % of S after calcination at 833K

Sample

wt % of S after reaction

ZSI

0.76

0.63

ZSH-4-0.1

2.50

1.15

ZSH-4-0.2

1.43

0.95

ZSH-4-0.5

1.32

1.05

ZSH-4-0.8

1.16

0.89

ZSH-1-0.1

1.12

1.09

ZSH-1-0.2

0.98

0.90

ZSH-1-0.5

0.93

0.90

ZSH-1-0.8

0.82

0.80

3.4. NH3-TPD Results of NH3-TPD are summarized in Table 3. We note that the amount of NH3 adsorbed increases with the hydrolysis ratio, whereas optimal desorption temperature decreases. This result can be interpreted on the basis of the enhancement of the acid site force on the surface of the sulfated zirconia for low amount of water used. It's worthy to note that the increase of the S/Zr molar ratio improves the amount of NH3 adsorbed and retards the temperature of its desorption. Table 3 : Amount of NH3 retained by samples and its optimal desorption temperature. Sample

Amount of NH3 retained (~tmol/g)

Optimal desorption temperature(K)

ZSH-4-0.2

0.336

653

ZSH-4-0.5

0.531

763

ZSH-4-0.8

0.591

783

ZSH-2-0.2

0.199

783

ZSH-2-0.5

0.461

803

ZSH-2-0.8

0.499

813

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3.5. FT-IR study in the presence of probe molecules FT-IR spectra of adsorbed pyridine on the different sulfated zirconia samples calcined at 833K show typical absorption bands. We note that Lewis and Br0nsted acid sites coexist at the ZSH surface as illustrated respectively by bands at 1440cmq and 1540cm"1(Figure 3). However in all samples thc amount of Br0nsted acid sites does not exceed 40%. The fraction of this typc of acid sites appears to decrease slightly with an increase in the initial S/Zr molar ratio as illustrated by Figure 4, and this result agree with those reported by Nascimento et al. [8] who found that the ratio of Br0nsted/Lewis acid sites increases with sulfur content. The shift of the sulfate bands after C6H6 adsorption indicates that the force of sulfate groups is rcduced when S/Zr increases. The decreas~ of the hydrolysis ratio improves the shift of the OH band which can be explained probably by the few number but strong hydroxyl groups generated at the surface of sulfated zirconia. .

.

.

.

.

.

.

.

.

1 ~0 9 ~ I00 "~ t;O 60 "~ 40 2o

1540 : .....

- r

.

0,2

.

.

.

.

.

.

.

t-

i

. . . . .

0,5

0,8

S / Z r molar ratio

~S-B HS-L Fig. 4. % of BrOnsted and Lewis acid sites as a ftmction of S/Zr molar ratio. cm

Fig. 3. FT-IR spectra of adsorbed pyridine 3.6. Catalytic activity

as a function of hydrolysis ratio

The catalytic activities of the different samples confirm that the preparation conditions are crucial in determining the catalytic properties of sulfated zirconia. We note that samples obtained via sol-gel process are more active than those prepared by imprecation in accordance with the sulfur content and the textural properties of these catalystg. In fact, ZSH samples exhibit higher BET surface areas and retain a larger amount of sulfur. Figure 5 shows the variation of the catalytic activity after 2h 30min in stream, with the hydrolysis ratio and the S/Zr molar ratio. We note that the decrease of the hydrolysis ratio improves considerably the catalytic activity. This result is in good agreement with the acidity of ZSH samples and the sulfur content before and aider reaction (Table 2).The selectivity to isomerization products is higher then 90% for the different catalysts. The investigation of the influence of the amount of Pt / A1203 added to sulfated zirconia on the catalytic properties shows that catalytic activity and selectivity are improved with Pt amount when catalysts were activated at 473K under H2. Furthermore an excess of Pt reduces the catalytic performance after activation at

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673K, and since sulfate groups are very stable after calcination of sulfated zirconia at 673K, this result could be assigned to reduction of sulfate groups in presence of Pt, which leave as H2S during activation of sulphated zirconia mixed with Pt/A1203 at 673K. 2,0 E -O5 J / J ~

1,6E-05 t 1,2E-05 t 8,0E-06 !

4,0E-06 9 0,0E+00 ", 9

o,z

S/Zr

0,5

0,8

Hydrolysis ratio

Fig. 5. Variation of the catalytic activity with hydrolysis ratio (H20 / Zr) and S/Zr molar ratio at 473K. 4. CONCLUSION The results of this study show that the acid and catalytic properties of sulfated zirconia are very sensitive to preparation conditions. The decrease of the hydrolysis ratio allows the retention of a high amount of sulfur at higher temperature and improves considerably the strength of acid sites, and as a consequence increases the catalytic activity. S/Zr molar ratio of 0.5 seems to give samples with the higher surface area and sulfur content. In the same way, acid strength and catalytic activity are much improved. Lewis and BrOnsted acid sites coexist at the sulfated zirconia surface but the Lewis ones are predominant. The increase of the amount of Pt added to sulfated zirconia enhances both catalytic activity and selectivity.

REFERENCES [1] S.R. Vaudagna, R.A. Comelli and N.S. Figoli, Catal. Lett., 47 (1997) 259. [2] S. Ardizzone, C.L. Bianchi, W. Cattagni and V. Ragaini, Catal. Lett., 49 (1997) 193. [3] S.X. Song and R.A. Kydd, J. Chem. Sot., Faraday Trans., 94 (1998) 1333. [4] C. Morterra, G. Cerrato, F. Pinna, M. Signoretto and G. Strukul, J. Catal., 149 (1994) 181. [5] C. Morterra, G. Cerrato, C. Emanuel and V. Bolis, J. Catal., 142 (1993) 349. [6] C. Morterra, G. Cerrato and V. Bolis, Catal. Today, 17 (1993) 505. [7] C. Morterra, G. Cerrato, F. Pinna and M. Signoretto, J. Catal., 157 (1995) 109. [8]P. Nascimento, C. Akratopoulou, M. Oszagyan, G. Coudurier, C. Travers, J. F. Joly and J.C. Vedrine, Catal. Today, (1993) 1185.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Synthesis and Characterization of Ta-Pillared llerite Younghee Ko, Myung Hun Kim, Sun Jin Kim and Young Sun Uh* Korea Institute of Science and Technology P.O. Box 131, Cheongryang, Seoul 130-650, Korea A new porous material, Ta-pillared ilerite is derived by pillaring tantalum oxide into silicic acid of ilerite. The new material was characterized through solid state 29Si NMR, IR, X-ray, TGA, UV-VIS, TPD and BET measurements. Upon pillaring of tantalum oxide, the basal spacing of ilerite is expanded to 37 A, and pillared structure of the ilerite is preserved even after calcination at 700~ The BET surface areas of the Ta-pillared ilerite after complete elimination of the surfactant are ranged between 378 m2/g and 400 m2/g. From the NH3-TPD profile of Ta-pillared ilerite, it is shown that the new material contains large number of acid sites of intermediate acidity. 1. INTRODUCTION Layered silicates [ 1-4], such as kanemite (NaHSi2Os3H20), magadiite (Na2SiNO29l 1H20), kanyaite ( K2Si200411H20 ), makatite ( Na2Si409SH20 ) and ilerite ( Na2SigO17xH20 ) have been successfully used as hosts for synthesizing of pillared material since their chemical compositions and building units of the structure are closely analogous to zeolites. The crystal structure of ilerite, which is identical with the structure of RUB-18, has been reported by Gies et al. [5,6] and the layer of ilerite shows remarkable similarities to a well-known feature of zeolite structures. The basic unit of the ilerite layers is a cage made of four five-membered rings, [54] cages, containing eight [SiO4] tetrahedra. This [54] cage is a well-known building unit from a number of zeolite structures such as zeolite 13, ZSM-5 and ferrierite etc. The proton-exchanged silicates (Silicic acids) have reactive silanol groups oriented in a crystallographically regular manner on their interlayer surface. The incorporation of polyhydroxy cation into the interlayer space of the layered silicates leads to generate a moderate acidity forming as bidimensional molecular sieves and to increase thermal stability of layered silicates. Comparing with the conventional impregnation method, pillaring with metal oxides has the advantage that it produces more uniform load of metal oxide species on support and yields more active and selective solid acid catalysts. Recently, several researchers have studied the pillared reactions of layered silicates including kanemite, magadiite and ilerite [7-11]. Agnes et al. synthesized new mesoporous materials by intercalation of silicate tubes analogous to the building blocks of MCM-41, between magadiite layers [7]. Kosuge et al. reported silica-pillared material prepared from the layered silicic acid ofilerite [8].

*Correspondingauthor. E-mail:uhyoung@,ksitmail.kist.re.kr

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Up to date most works on pillaring layered silicates have been limited to those only with silica or alumina pillars. Pillaring with other metal oxides has not been reported yet. In the present study, pillaring with tantalum oxide other than silica and alumina is used to derive a new porous material from ilerite. Tantalum oxide pillar is prepared by the hydrolysis of a tantalum alkoxide precusor. The synthesis and characterization of the new metal pillared ilerite, along with its acidic property are reported. 2. EXPERIMENTAL

2.1. Preparation of Ta-pillared ilerite Na-ilerite was readily synthesized hydrothermally at 100~ for 2 weeks from a suspension of Ludox-HS 40 and NaOH solution with a molar ratio of SiO2 : NaOH : 1-120 = 1 : 0.5 : 7. The optical microscopic images of the samples showed that the microcrystals were of thinplatelets form. The chemical composition of the sample was Na2SisO17 as determined by an Inductively coupled plasma (ICP) spectrometer and an Atomic absorption spectrometer. Silicic acid of ilerite (H-ilerite) was prepared by acid titration of Na-ilerite. A suspension of Na-ilerite in water was titrated with 0.1N HCI to a final pH of 2.0 and allowed to remain with stirring for an additional 24 hours. The H-ilerite was recovered by filtering, washing, then was dried at 40~ The sodium content of the samples was 0.11% by weight as determined by an Atomic absorption spectrometer. Octadecylammine intercalation was performed by allowing H-ilerite (lg) to react with excess octadecylamine in anhydrous ethanol solvent at room temperature. The tantalum pillaring solution was prepared by careful control of the hydrolysis of a tantalum penta etoxide (0.Sg) precusor. Small amount of octadecylammine was added to the solution to increase the degree of hydrolysis of tantalate species. The molar ratio of Ta(OC2Hs)5 : 1-120 : Octadecylammine was adjusted to 1 : 12 : 0.3. The pillar precusor, symbolized generally as TaxOy(OR)z, is transformed into tantalum oxide after calcination. The tantalum pillaring solution was droppwised to octadecylamine-ilerite suspension with continuous stirring. The final suspension was allowed to stand for 3 days with stirring at room temperature. The resultant product was washed with ethanol twice, filtered and air-dried at 100~ Octadecylammine from Ta-pillared ilerite was removed by refluxing with 1M NH4NO3 solution at 80~ After removal of octadecylammine, the sample was calcined at 700~ for 1 hour to eliminate residual-OR group. 2.2. Characterization Solid state Si29 NMR analyses were performed on a Varian 400 VXR solid state NMR spectrometer operated at 79.5 MHz, and thermogravimetric analyses were done using a Cahn TG system 121 thermogravimetric analyzer. All samples were heated to 1000~ at a heating rate of 10~ under nitrogen atmosphere. A Rigaku D/Max-RINT 2500 diffractometer with CuKct radiation was used for the initial XRD analysis of the samples. The IR spectra were recorded between 400-1400cm 1 on a Perkin Elmer 1600 M-80 spectrometer. BET surface areas were determined using a Micrometrics ASAP 2000 analyzer. All samples were outgassed at 200~ under vacuum for 2 hours. In chemical analyses, a Jarrell-Ash Poliscan 61E inductively coupled plasma (ICP) spectrometer and a Perkin-Elmer 5000 atomic absorption spectrophotometer were used. The NH3 TPD data were obtained on a Micromeritics TPD/TPR 2900 analyzer. Electron-probe microanalyses for the samples were carded out using Philips, CM 30 scanning transmission electron microscopy.

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3. RESULTS AND DISCUSSION 3.1. X-ray powder diffraction and IR data

Octylamine expanded-ilerite has an interlayer spacing much larger than those of Na- and H-ilerites, 11.1 A and 7.4 A, respectively. The XRD pattern of metal pillared ilerite is dramatically changed. Upon pillaring of tantalum oxide, the basal spacing of ilerite is expanded to 37 A. Although the basal spacing decreases gradually as the calcination temperature increases, pillared structure of the ilerite is preserved even after calcination at 700~ The broad peak at low angle indicates that a poorly ordered structure is formed due to the pillaring process. The IR spectrum of Ta-lJillared ilerite exhibits absorption bands at 960cm1 that does not exist in the spectrum of ilerite and related metal oxides. The intensity of the band increases with the metal contents of the samples. From the fact that the band 960cm ~ is caused by a SiO vibrational mode perturbed by the presence of heavy metal ions in a neighboring position, it can be deduced that the metal ions reside in the framework [ 12].

29Si MAS NMR spectra In Fig. 2, the solid state 29Si MAS NMR spectra of Na-ilerite, H-ilerite and Ta-pillared ilerite are shown. The 29Si MAS NMR spectra of Na- and H-ilerites are quite similar to the data reported in other literature [9]. The spectrum obtained for Na-ilerite shows doublet Q3 resonance in Si(OSi)30(H, or Na) units at near-100 ppm and Q4 resonance in Si(OSi)4 units at near-110 ppm. The Q3 resonance in the proton exchanged H-ilerite spectrum become singlet, which confirms the presence of only silanol groups between the layers. In the spectrum of Ta-pillared ilerite, the Q3 resonance is diminished, whereas Q4 resonance is enhanced. This furnishes the clear evidence for the formation of new Ta-O-Si bonds between the pillars and the layers. In addition, 29Si MAS ~ spectra of Ta-ilerite become broad, which indicates that the local atomic arrangement in the silicate network is not uniform. 3.2.

d=3.70nm .m

I

d=O.74nm

qa

Ta-Pillared

llerite_ ----

~

H~" "

-98

"

~ -110

~~ -.---------

gl ~mq

-

10

20

30 40 50 2 Theta Fig. 1. XRD patterns of (a) Na-ilerite, (b) H-ilerite and (c) Ta-pillared ilerite.

-80

,

-90

-1()0 - 1 1 0 ppm

-120 -i30

Fig. 2. 29Si MAS-NMR spectra

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化

100

o~ 90 ipJ

,,

,

,,

(b)

S,I

O O

~:

8o

<

(c)

(b) 70

0 "200"400"600"800"1000 Temperature(*C) Fig. 3. TGA data of (a) as-synthesized Tapillared ilerite containing octa-decylamine (b) NH4NO3 treated Ta-pillared ilerite.

250 300 350 "400 " 4 ; 0 " 5 ~ 0 Wavelength (nm) Fig. 4. UV-VIS diffuse reflectance spectra of (a) H-ilerite, (b) Ta-pillared ilerite and (c) Ta205 powder.

3.3. Thermal analysis Unlike the cases of silica pillared materials, the octadecylamine in Ta-pillared ilerite is hardly removed by simple calcination process. This implies a strong Ta-N interaction in the as-synthesized structure. To break Ta-N interaction and remove octadecylamine, a proton source is required. Octadecylamine removal from Ta-pillared ilerite is achieved by refluxing with 1M NH4NO3 at 80~ The octadecylamine removal was examined by weight loss in the thermal gravimetric analysis. Thermogravimetric analysis data of as-synthesized Ta-pillared ilerite and NH4NO3 treated Ta-pillared ilerite are shown in Fig. 3. The first weight loss below 100~ is mainly due to the evaporation of physisorbed water. The sharp weight loss in Fig. 3(a) between 100~ 200~ is caused by the elimination of octadecylamine of as-synthesized samples. The sharp weight loss between 100~176 is not detected in the NH4NO3 treated sample ( Fig. 3(b)). Continuous weight loss was detected in both samples between 200~176 due to the removal of residual alkoxide groups. NH4NO3 treated sample was stable up to 1000~ alter complete removal of organic phase ( Fig. 3(a)). However, without NH4NO3 treatment, sample shows an additional weight loss at around 600~ and this weight loss can be attributed to the dehydroxylation of ilerite layer ( Fig. 3(b)). And the dehydroxylation between the layers leads to collapse of the structure. The BET surface area of the sample calcined after refluxing in NH4NO3 solution are between 350 m2/g and 400 m2/g, but the samples calcined without treatment of NH4NO3 is only 40 m2/g. This suggests that the essence of tantalum pillaring into layered silicates is the removal of surfactant keeping the pillared structure intact. 3.4. UV-VIS Spectra The UV-VIS diffuse reflectance spectra of the H-ilerite, Ta-pillared ilerite and TazOs are shown in Fig. 4. The maximum absorption of Ta2Os is observed at 270 nm that is similar to

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the data reported in other literature [13]. The absence of this band in the Ta-pillared ilerite means that the samples do not contain occluded Ta2Os impurity. In the UV-VIS diffuse

reflectance spectra of the Ta-pillared ilerite, the maximum absorption occurs at a shorter wavelength, similar to titanium or niobium silicate-1 [12,14]. In titanium or niobium silicalite-1, a band shiR to a shorter wavelength is assigned to isolate metal species in the samples. H-ilerite shows no corresponding absorption. 3.5. BET surface area and Acidity measurement The BET surface areas of the Ta-pillared ilerite calcined after complete elimination of the surfactant varies from 378 m2/g to 400 m2/g much larger than the surface areas of the H- and Na-Iledtes, 20 m2/g and 40 m2/g, respectively. Acidic property of the Ta-pillared ilerite is studied to identify inherent acidity caused by incorporation of tantalum oxide between the layers of ilerite. Also, acidic property of Sipillared ilerites is investigated to compare inherent acidities caused by incorporation of Ta and Si between the layers of ilerite. Acidities of metal pillared ilerites were determined by carrying out NH3 TPD experiments. The TPD profiles of Ta-pillared ilerite exhibit a broad desorption peak with maximum occurring at 220~ This implies that the new material contains the large number of acid sites of intermediate acidity mainly due to the formation of a new hydroxyl group formed on the Ta-(OH)-Si linkage. According to Tanabe's theory [ 15], a proton must be associated with the six oxygen surrounding tantalum to keep the structure electroneutrality. On the other hand, Si-pillared ilerite shows no desorption peak as expected. 3.6. N2 Adsorption-Desorption Isotherm

The N2 adsorption-desorption isotherm of Ta-pillared ilerite after complete elimination of surfactant shows a combination of type 1 and type 4 behaviors according to the BDDT 800 9

700

9 Adsorption

o

~

"

o

o

Desorption

g o

6OO Im ,trot ~

..~ r~

O

O

"~ r

0 0

500

so

400

r

fJ2

300

wJ

z ~

Im

t-

(a) .

100

.

I

.

9

.

I

.

,

.

.

-

I

200 300 400 500 T e m p e r a t u r e (~

9

'--I

200 100 9

600

Fig. 5. NH3-TPD profiles of (a) Sipillared ilerite, (b) Ta-pillared ilerite.

-

|

-

I

9

I

0.0 0.2 0.4 0.6 Relative pressure

,

I

,

|

0.8 1.0 (P/Po)

Fig. 6. N2 adsorption-desorption isotherm of Ta-pillared ilerite.

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classification [16]. At low relative pressure, the isotherm shows a characteristic of type 1 isotherm, which indicates the presence of micropores within the Ta-pillared ilerite. At high relative pressure ( P/Po>0.6 ), the isotherm can be associated with a type 4 isotherm indicating the presence of mesopores within the Ta-pillared ilerite. The hystersis is observed in the relative pressure region of P/Po>0.6 due to the capillary condensation in mesopores, which originates pillaring layered silicates.

3.6. Chemical analysis The metal contents of the samples are ranged from 5 ~ to 3.8 % by weight as determined by an Inductively Coupled Plasma (ICP) spectrometer and an Atomic absorption spectrometer. The Si/Ta ratios in several points of the sample are checked by EPMA (electron probe micro analysis) method and the Si/Ta ratios are constant for the entire samples from same batch. This data indicates a homogeneous dispersion of Ta in the Ta-pillared ilerite samples. Acknowledgments This work is supported by the Korea Institute of Science and Technology under the contract 2E15210 and Brain Pool Program of Korean Government. REFERENCES 1. H. Annehed, L. Faith and F.J. Lincoln, Zeitschriftff~r Kristallographie, 159 (1982) 203G. 2. S. Inagaki, Y. Fukushaima and K. Kuroda, J. Chem. Soc. Chem. Commun., (1993) 680 3. S. Inagaki, A. Koiwai, N. Suzuki,Y. Fukushaima and K. Kuroda, Bull. Chem. Soc. Jpn., 69 (1996) 1449 4. Z. Johan, G.F. Maglione, Bull. Soc. Fr. Mineral Cristallogr., 95 (1972) 372 5. H. Gies, B. Marler, S. Vortmann, U. Oberhagemann, P. Bayat, K. Krink, J. Rius, I. Wolf and C. FyFe, Micropor. andMesopor. Mater., 21 (1998) 183 6. S. Vortmann J. Rius, S. Siegmann and H. Gies, J. Phys. Chem., B 101 (1997) 1292 7. F. Agnes, K. Imre, S.I. Niwa, M. Toba, Y. Kiyozumi and F. Mizukami, Applied Catalysis A: General, 176 (1999) L153 8. K. Kosuge and A. Tsunashima, J. Chem. Soc. Chem. Commun., (1995) 2427 9. J.S. daily and T.J. Pinnavania, Chem. Mater., 4 (1992) 855 10. R. Sprung, M.E. Davis, J.S. Kauffmann and C. Dybowski, lnd. Eng. Chem. Res., 29 (1990) 213 11. S.T. Wong and S. Cheng, Chem. Mater., 5 (1993) 770 12. A.M. Parkash and L. Kevan, Jr. Am. Chem. Soc., 120 (1998), 13148 13. Y.S. Ko and W. S. Ahn, Micropor. andMesopor. Mater., 30 (1999) 283 14. G.N.Vayssilov, Catal. Rev.-Sci. Eng., 39 (1997) 209 15. G. Guiu and P. Grange, J. of Catal., 156 (1995) 132 16. S.J. Gregg and K.S.W. Sing, Adsorption, Surface Area and Porosity, 2nd Edition, Academic Press, 1982

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Al-pillared hectorite and montmorillonite prepared from concentrated clay suspensions: structural, textural and catalytic properties. R. Molina a S. Moreno b and G. Poncelet I

l

Unit6 de Catalyse et Chimie des Mat6riaux Divis6s, Universit6 Catholique de Louvain, Place Croix du Sud 2/17. 1348 Louvain-la-Neuve, Belgium. Concentrated suspensions of hectorite and montmorillonite have been pillared with base-hydrolyzed aluminum solutions with different OH/A1 molar ratios. The pillared clays have been characterized, and their catalytic performances have been evaluated over Pt-impregnated samples using the hydroisomerization of heptane. 1. INTRODUCTION Pillared clays with different types of pillars have been abundantly investigated over the last 20 years. Al-pillared interlayered days (PILCs) prepared in dilute clay suspensions are the most documented ones. However, in industrial conditions, preparations in diluted systems are uneconomical [1]. In recent years, a few studies on similar materials obtained in concentrated medium have been reported [2-6]. Schoonheydt et al. [2] described the preparation of Al-pillared saponite using 6-10 % clay suspensions. Previous works have shown that Al-pillared clays obtained from concentrated suspensions exhibited structural, textural, and catalytic properties similar to those prepared in dilute conditions [6]. In this study, suspensions of commercial hectorite (6 %) and montmorillonite (2 and 40 %) have been pillared with partially hydrolyzed aluminum solutions. The main structural and textural characteristics of the pillared clays have been determined. The influence of the clay concentration and the OH/A1 molar ratio of the pillaring solution on the catalytic performances has been examined in the isomerization of heptane. Present address: aDepartamento de Qufmica, Facultad de Ciencias, Universidad de Antioquia, Medellfn, Colombia. bLaboratorio de Cat~ilisis, Facultad de Ciencias, Universidad Nacional de Colombia. Ciudad Universitaria, Santa Fe de Bogot~i, Colombia.

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2. EXPERIMENTAL

2.1. Pillaring procedure

Pillaring experiments were carried out with commercial hectorite and Westone-L montmorillonite. They were used as received without previous fractionation. Partially hydrolyzed solutions were prepared by slow addition of 0.5 M sodium hydroxide to 0.2 M aluminum nitrate to achieve OH/A1 molar ratios of 1.2, 1.6, and 2.0 for the hectorite, and 1.6 for the montmorillonite. The concentration of A13+ in the final solution was adjusted to 0.1 M. Vigorous stirring was maintained throughout the hydrolysis. The pillaring solution was aged for 24 h at room temperature. Dialysis bags containing weighed amounts of 2 and 40 % clay slurries of WLmontmorillonite, and 6 % clay slurry of hectorite (the highest possible concentration for this clay) were dipped in glass vessels containing the required volumes of the pillaring solution in order to supply 50 mequiv. A13+per g clay. Each system was aged under mild stirring for 48 h at room temperature. Washing was performed by dipping the dialysis bags containing the clay slurry in distilled water, renewing the water until the conductivity was reduced to its initial value. 2.2. Characterization methods

The basal spacings (001 reflection) of the pillared materials were obtained from the X-ray patterns recorded with a Philips equipment fitted with copper anticathode (Cu-Ko~ radiation, Ni-filtered). The specific surface areas and micropore volumes were determined from the nitrogen adsorption isotherms established at the boiling point of nitrogen, using an ASAP 2000 Sorptometer from Micromeritics. The samples were previously calcined at 400 ~ and degassed in situ at 200 ~ for 6-8 h prior to the measurements. The micropore volumes were determined with a method recently proposed [7]. The catalytic performances of the pillared clays were evaluated in the hydroisomerization reaction of heptane over samples previously impregnated with 5 10-3 M tetrammineplatinum(II) chloride solution in order to load the solid with I wt % Pt. The pretreatment and reaction conditions were similar to those detailed elsewhere [8,9]. The catalytic tests were performed in a continuous flow micro-reactor operated at atmospheric pressure. The catalyst (200 mg) was activated in situ in flowing air for 2 h at 400 ~ followed by helium purge, and reduction of the metal in flowing H 2 for 2 h at 400 ~ The reactor was then cooled to 150 ~ and stabilized at this temperature. A stream of hydrogen saturated with heptane vapor was generated by passing hydrogen through a glass saturator containing heptane thermostated at 27 ~ The total flow was 10

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ml min 1, and the WHSV was 0.9 h -1. The reaction was carried out in temperature-programmed mode, with a ramp of 1.7 ~ min "1. On-line gas phase analysis was done in a HP-5890 gas chromatograph provided with TC detector and 50 m long CPSil-5 capillary column (Chrompack). The treatment of the results (conversion, selectivity) was done as detailed elsewhere [9]. 3. RESULTS AND DISCUSSION. 3.1. Characterization of the pillared clays

The interlayer distances (001 reflections), the specific surface areas, and the micropore volumes of the A1-PILCs previously calcined at 400 ~ are given in Table 1. The two pillared montmorillonites exhibited stable spacings of 17.7 ,/k, and between 16.1 and 17.7 A for the hectorites. These spacings are in the range of values usually found for adequately pillared materials. The hectorite pillared with a solution with OH/A1 ratio of 1.2 had a smaller interlayer distance (16.1 A.). The XRD patterns of the pillared hectorites showed a reflection at ca. 10 ,/k, indicating that a fraction was not pillared. Table 1. Spacings (d), surface areas (SBET), and micropore volumes (V~) Sample (%) Mont.-2% Mont.-40% Hect. (1.2) Hect. (1.6) Hect. (2.0) * shoulder

d (001) (400 ~ (A)

SBET

V~

(m 2 g-l)

(cm 3 g-l)

17.8

263

0.074

17.7 16.1 - (10.0)* 17.7- (10.1)* 17.4 - (9.8)*

277 97 222 164

0.079 0.040 0.076 0.070

The surface area and micropore volumes of the pillared clays determined from the nitrogen sorption isotherms are shown in Table 1. The BET surface areas and the micropore volumes of the montmorillonites prepared with concentrated suspensions were similar to those of the sample pillared in dilute suspension. The textural characteristics of the pillared hectorites differed according to the OH/A1 ratio of the pillaring solution. As expected from the d spacings, a smaller micropore volume was obtained for the hectorite treated with the solution with OH/A1 ratio = 1.2. The sample prepared using a pillaring solution with OH/A1 of 1.6 exhibited similar surface area and micropore volume to those found for the montmorillonites pillared both in diluted or concentrated suspension.

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3.2. Catalytic properties

Hydroisomerisation-hydrocracking of n-paraffins has been largely used for the evaluation of activity of acid solids. Three types of reaction products are obtained: isomerization, cracking and cyclization produts. In these bifunctional catalysts, the role of the metal is to dehydrogenate the paraffin and rehydrogenate the iso-olefins. Isomerization to mono- and di-branched C7 occurs on the protonic sites via protonated cyclo-propane intermediates. 100

5O

9 A 9

80

[]

0

~ 0

U

9

60

4ot

Hect (2.0) Hect (1.6) Hect(1.2) W L - 40~ WL-2%

30

40

20

20

10

0

9 Hect (2.0) 9 Hect (1.6) 9 Hect (1.2) o W L - 40% 9 WL-2%

0 150

200

250

300

350

Temperature

Fig. 1. Variation of total conversion vs reaction temperature

400 (~

150

200

250

300

350

Temperature

400 (~

Fig. 2. Variation of isomerization vs reaction temperature

Figure 1 shows the variation of the conversion of heptane vs reaction temperature obtained over the Al-pillared montmorillonites and hectorites. The results obtained over the two samples of Al-pfllared montmorillonite were consistent with previous studies [8-11]. None of these catalysts reached total conversion. A higher conversion was achieved over the sample prepared with the diluted clay suspension. For the Al-pillared hectorites, differences were observed according to the OH/A1 ratio of the pillaring solution. The clay prepared with the solution with OH/A1 = 2 ratio was the most active while the lowest conversion was found for the day obtained with a solution with OH/A1 = 1.6. The decreasing conversion noticed above 350 ~ reflected the loss of stability of the acid sites. This effect was not found for the Al-pillared montmorillonites. The two montmorillonites and the hectorite prepared with the pillaring solution with OH/A1 = 1.6 were less active than the pillared hectorites (1.2) and

(2.0).

The isomerization vs reaction temperature curves obtained over these samples is shown in Fig. 2. This figure is another illustration of the influence of the OH/A1 molar ratio of the pillaring solution. The two montmorillonites and

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the hectorite (1.6) exhibited similar results, while hectorites (1.2) and (2.0) produced more C7 isomers, particularly the later one. Note that slightly more isomers were produced over the montmorillonite pillared in concentrated suspension. Characteristic values taken from the experimental results have been compiled in Table 2, where T10iso is the temperature at which the yield of heptane isomers reached 10 %. It provided an estimation of the relative activities. At this conversion, cracking and cyclization products were not formed. The values in the following columns were taken at maximum isomerization conversion, namely, the temperature (TMax.), the conversion of heptane (Conv. %), the yield of isomers (Yt~o,% ), and cracking products (Ycr, %), essentially C3 and iso-C4, and the selectivities to C7 isomers (S, %). Table 2. Catalytic results at maximum isomerization Clay Mont.-2 % Mont.-40 % Hect. (1.2) Hect. (1.6) Hect. (2.0)

T10Iso TMax. Conv.

(oc)

(oc)

(%)

265 268 262 267 262

335 327 325 341 312

54 44 66 56 63

YIso

YCr

(%)

(%)

34 36 39 35 47

14 5 22 18 2

63 82 59 63 75

(%)

S

With respect to the yields of isomers, the Al-montmorillonite prepared with 40% clay slurry exhibited a significantly higher selectivity (82 %) than the sample prepared with a 2 % suspension (63 %). At the opposite, higher yields of cracking products (14 %) were formed over the sample Mont.-2 %, than for the sample Mont.-40 % (5 % cracking). In this Table, the difference between the conversion percentage and the sum of the percentages of the isomerization and cracking products represents the percentages of cyclization products (mainly benzene and toluene). For the Al-pillared hectorites, Table 2 clearly shows the influence of the OH/A1 ratio used in the preparation of the catalysts. For the higher OH/A1 ratio (2.0), both higher yields and selecfivities of C7 isomers were obtained compared with the other two samples. Conversely, the lower yield of cracking products (only 2 %) was observed for hectorite prepared with the higher OH/A1 while the others two samples exhibited significantly higher yields of these products (18 and 22 %). This last result should be interpreted as a direct influence of the OH/A1 ratio used in the preparation of catalyst, as it has been reported [12]. This effect is attributed to the higher proportion of Al137§ species with Keggin structure in solutions with OH/A1 = 2.0 than in those with lower molar ratios (1.2 and 1.6) assuming, of course, that this polymeric species is the one constituting the pil!ar precursors [12].

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4. CONCLUSIONS

Al-pillared clays prepared from concentrated suspensions exhibited heat resistant basal spacings, high surface area and micropore volume that did not differ from similar materials obtained in dilute conditions. Hectorite and montmorillonite pillared with hydroxy-aluminum solutions with OH/A1 molar ratio of 1.6 showed similar catalytic performances in the isomerization of heptane. The OH/A1 molar ratio of the pillaring solution plays a role on the catalytic properties. The yields of C7 isomers were significantly improved when using a pillaring solution with OH/A1 = 2.0. The Bronsted acid sites were less stable in M-pillared hectorite than in A1pillared montmorillonite. REFERENCES

1. D.W.E. Vaughan, Catal. Today, 2 (1988) 187. 2. R.A. Schoonheydt and H. Leeman, Clay Miner., 27 (1992) 249. 3. R. Molina, A. Vieira-Coelho and G. Poncelet, Clays Clay Miner., 40 (1992) 480. 4. W. Diano, R. Rubino and M. Sergio, Microporous Mater., 2 (1994) 179. 5. R.A. Schoonheydt and H. Leeman, Clays Clay Miner., 41 (1993) 598. 6. S. Moreno, E. Gutierrez, A. Alvarez, N.G. Papayannakos and G. Poncelet, Appl. Catal., 165 (1997) 103. 7. M. Remy, A. Vieira-Coelho and G. Poncelet, Microporous Mater., 7 (1996) 287. 8. S. Moreno, R. Sun Kou, R. Molina and G. Poncelet, J. Catal., 182 (1999) 174. 9. S. Moreno, R. Sun and G. Poncelet, J. Catal., 162 (1996) 198. 10. A. Schutz, W.E.E. Stone, G. Poncelet and J.J. Fripiat, Clays Clay Miner., 35 (1987) 251. 11. R. Molina, A. Schutz and G. Poncelet, J. Catal. 142 (1994) 79. 12. S. Moreno, R. Sun, and G. Poncelet., J. Phys. Chem. 101 (1997) 1569.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Synthesis of high surface area transition metal carbide catalysts A.P.E. York, J.B. Claridge, V.C. Williams, A.J. Brungs, J. Sloan, A. Hanif, H. AI-Megren and M.L.H. Green

Wolfson Catalysis Centre, Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QR, U.K. The synthesis of molybdenum and tungsten carbide from their oxides using the temperature programmed reaction method with methane and ethane is presented. Lower reaction temperatures are required for metal carbide formation with ethane, and the highest surface areas materials are also prepared by this method. In addition, the use of the different hydrocarbons enables synthesis of various phases of molybdenum or tungsten carbide. Thermogravimetric analysis studies demonstrate that the carbide formation mechanisms for the two transition metal carbides in ethane differ. Finally, a similar technique has been employed for the synthesis of uranium monocarbide with moderately high surface area. I. INTRODUCTION High surface area transition metal carbides are active catalysts for a wide range of reactions, including hydrodesulphurisation (HDS), hydrogenation, Fischer-Tropsch synthesis, hydrocarbon isomerisation and methane oxyforming [1-5]. Synthesis of these potentially important materials has attracted considerable attention over the years, and a number of procedures have now been described for the production of carbides with surface areas suitable for catalyst materials (i.e. > 30 m E g-l) [6]. The most commonly employed method is temperature programmed reaction (TPRe) of metal oxide with a hydrocarbon (usually methane), which was developed by Boudart and co-workers [7-9]. In spite of all the interest in these materials, comparatively few detailed studies of the material synthesis have been reported. Indeed, it may be possible to prepare carbides with improved properties by modification of already known methods. For example, replacement of methane by ethane for TPRe should lead to higher surface areas due to the more reactive nature of the latter hydrocarbon; the carbon activities can be expressed by the following equations, respectively (1 and 2) [ 10]. dC --

ar

(1)

)2

=I I___L__.

)3

(2)

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The reaction with an oxide is more complicated, but relationships between methane activity and gas composition for higher hydrocarbons can still be obtained (3) [ 11 ].

C2H6tg) + H2(g) ---> 2CH4(g)

acH ' = Ke(cH,).

Ke(c~H,)

(3)

In this paper, we will demonstrate that molybdenum and tungsten carbides with higher surface areas can be synthesised using ethane as the reactant in place of methane, and also that the choice of hydrocarbon determines which carbide phase is produced. This may be important for the application of the carbide materials as catalysts. In addition, the synthesis of uranium carbide using methane will be presented; these materials have been almost neglected outside of the nuclear industry, while uranium shows some similarities to the group VI metals. 2. EXPERIMENTAL 2.1. Synthesis of molybdenum and tungsten carbides The apparatus used in this work was an up-graded version of the commercial Labcon microreactor which has been described in detail previously [5]. The TPRe metal carbide syntheses were carried out as follows: 100 mg of oxide was heated at 1 K min -~ from room temperature to a final temperature, chosen according to the hydrocarbon and metal oxide being used (see Table 1). The reactant gases, 20 vol.% CH4/H2 or 10 vol.% C2H6/H2, were passed over the oxide at 250 cm 3 min ~. After reaction the samples were quenched to room temperature under argon. Before exposure to the atmosphere the carbide samples were passivated in flowing 1 vol.% O2/Ar. Material surface areas were then determined using nitrogen BET. Methane (Union Carbide, >99.95%), ethane (BOC, C.P. grade), hydrogen (BOC, C.P. grade) and argon (BOC, C.P. grade) were used as received without further purification. MoO3 (Johnson Matthey, Puratronic| 99.998 %) and WO3 (Johnson Matthey, Puratronic| 99.998 %) were used as supplied, and had surface areas less than 1 m 2 g~. High-resolution analytical electron microscopy (HRTEM) was carried out using a JEOL-4000FX microscope, XRD on a Phillips PWl710, and thermogravimetric analysis (TGA) using a Rheometric Scientific PL-STA.

2.2. Synthesis of uranium carbide A small piece of uranium metal was cleaned and placed into the furnace tube. This was hydrided under a low flow of hydrogen and then heated under vacuum at 623 K, affording a fine pyrophoric uranium metal powder. The surface area of this powder was not determined because crushing caused it to ignite under an atmosphere of purified nitrogen gas and so handling it in a nitrogen filled glove-box was problematic (02 < 10ppm). The uranium powder was carburised at 903 K under methane (50 cm 3 min l) for 6 h and then allowed to cool to room temperature.

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3. RESULTS AND DISCUSSION 3.1. Molybdenum and tungsten carbides Table 1 summarises the surface areas and final synthesis temperatures employed in this study. Ethane TPRe gave materials with surface areas much higher than from the methane method for both the molybdenum and tungsten carbides. In addition, the reaction temperatures when ethane was employed were much lower than those with methane, as expected from the thermodynamics Table 1 discussed in the introduction. The Surface areas and final reaction temperatures for the surface areas obtained here for TPRe synthesised metal carbides. ethane TPRe synthesised CH4 TPRe C2H6 TPRe molybdenum carbide are as high as S~ (m2 g~) T t (K) S~ (m2 g~) Tt (K) any reported previously [8,12]. 91 1020 174 900 MoO 3 TGA studies of the 39 1150 71 900 W03 synthesis of W2C and M02C from t Final synthesis temperature. the reaction of their respective oxides with ethane are shown in Figure 1. The traces for the two systems are clearly very different, with three peaks in the tungsten trace, compared to the molybdenum system which displayed only one weight loss. This indicates a different carburisation reaction mechanism for the two metal oxide-carbide transformations. 22~ 21

,m0;I

23 (mg) 22

181

17t b)

a) 20

500

600

700 800 Ol)0 Temperature (K)

1000

11oo

16

,

soo

6;0

700 8;o 9;o Temperature (K)

lo'oo

11oo

Fig. 1. TGA of the ethane TPRe of a) WO3 (10 K min -~) and b) MoO 3 (13 K min~). Oyama has proposed that the route to niobium carbide is via an oxycarbide [ 13]. This may also be the ease for the synthesis of molybdenum carbide presented here (reaction (4)), and indeed Ledoux and co-workers have shown that for higher hydrocarbons (C4§ an oxycarbide can be formed under certain conditions [ 14,15]. For example, using hexane as the carburising gas, Ledoux showed that molybdenum oxycarbide was formed at 623 K, while the carbide was favoured at slightly higher temperature, 673 K. For the tungsten system, we propose that the most likely route is as follows; the first step is the reduction of tungsten (VI) oxide to a shear phase oxide, followed by further reduction to WO2, and finally carburisation to W2C (reaction (5)). MoO 3 ~ [MOOxCy] --9, Mo2C WO3 ~ WO3.x ~ WO2 ~ W2C

(4) (5)

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a)

MoO3

C2HdI-I29 fl- Mo2C(P631mmc) /

~

b)

~/'O3

21020K fl- Mo2C(P631mmc)

2II50K

C2H6~I2

-

/,.,~c)

~-

wc(:6~2)

/ / NH3980K W2N(Fm3m)

I

_~.-~-.~

fl- WC,_x(Fm'3m) CH4/H2 1050K

Fig. 2. Routes to phases of a) molybdenum carbide and b) tungsten carbide via TPRe. In addition, it should be noted that, where several different phases can be formed, the choice of route can allow the synthesis of comparable surface area systems, e.g. 13-Mo2C and [3-MoCk.x, and the various tungsten phases. Figure 2 shows the phases that are formed by TPRe with methane or ethane, and also via a two step route involving conversion of the oxide to the nitride, by ammonia TPRe, and then carburisation to the carbide under methane. This may be important since the phase formed may effect the catalytic properties of these materials. From the above results, it can be seen that higher hydrocarbons should be promising reagents for the conversion of transition metal oxides to the high surface area carbides under comparatively mild conditions for application as catalysts. 3.2. Uranium carbide

Figure 3a shows the XRD pattem of the material formed after the reaction of uranium hydride with methane. Uranium dioxide was rapidly formed on contact with air, water or even carbon dioxide, severely hampering data collection. Despite this, however, comparison of the pattern with standard XRD data indicates that uranium monocarbide has been synthesised; this is the favoured phase under the conditions used (i.e. 903K) [16]. Uranium carbide only exists over a narrow range of compositions; the C/U ratio is 1.00 + 0.03, and any deviation outside this stoichiometry is due to either residual metal or free carbon. The UC material synthesised here is f.c.c., with a lattice parameter of 4.962 A, in close agreement with the published value of 4.9605(2) A for a pure, highly crystalline material. However, the relatively broad line widths in the XRD pattern indicate that the crystallite size is rather small (r~), 530 A). The surface area of the uranium carbide measured using N2 BET was 5 m 2 g~, and the particle size calculated assuming spherical particles was 880 A. When the high

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density of UC is taken into account (13.6 g cm3), this surface area, although not exceptional, is certainly moderately high. A small sample of the synthesised uranium carbide was carefully passivated so that it could be briefly handled in air, allowing study by HRTEM. In Figure 3b a typical micrograph of the UC material is shown. The UC synthesised here is made up of micrometre sized particles, composed of aggregates of smaller particles, consistent with the XRD and BET measurements.

a)

I00

l

l

' ~./~

:''

,

. 'l

i:ii

b)

l

~. ,, ~. ,.. ,:.;:,,: ..l.O-nm I,!,!)

(2,0,0)

6O

(2,2,0)

(3,1,I)

(2,22)

'L

30

9

3'5

9 4~

'

45

,

,

!

,

!

5 !0

Fig. 3. Characterisation of the uranium monocarbide by a) XRD and b) HRTEM. 4. CONCLUSIONS TPRe is a versatile method for producing high surface area early transition metal, or uranium, carbides, and reactions with ethane tend to proceed at lower temperatures than is the case with methane. The potential of molybdenum and tungsten carbides as future industrial catalysts has already been demonstrated. However, using the ethane TPRe method shown here it may now be possible to extend the catalytic applications to a wider variety of molybdenum and tungsten phases. The suitability of these materials for the important industrial reactions mentioned earlier is currently being determined. REFERENCES

1. J.S. Lee and M. Boudart, Appl. Catal., 19 (1983) 207. 2. G.B. Raupp and W.N. Delgass, J. Catal., 53 (1979) 361. 3. M.J. Ledoux, C. Pham-Huu, A.P.E. York, E.A. Blekkan, P. Delporte and P. Del Gallo, in "The Chemistry of Transition Metal Carbides and Nitrides", S.T. Oyama (ed.), p.373, Blackie Academic and Professional, Glasgow, 1996. 4. A.P.E. York, J.B. Claridge, A.J. Brungs, S.C. Tsang and M.L.H. Green, Chem. Commun., (1997) 39. 5. J.B. Claridge, A.P.E. York, C. M~irquez-Alvarez, A.J. Brungs, J. Sloan, S.C. Tsang and M.L.H. Green, J. Catal., 180 (1998) 85.

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6.

7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

S.T. Oyama in "The Chemistry of Transition Metal Carbides and Nitrides", S.T. Oyama (ed.), p.1, Blackie Academic and Professional, Glasgow, 1996. L. Volpe and M. Boudart, J. Solid State Chem., 59 (1985) 332. L. Volpe and M. Boudart, J. Solid State Chem., 59 (1985) 348. J.T. Wrobleski and M. Boudart, Catal. Today, 15 (1992) 349. M. Katsura, J. Alloys Comp., 182 (1992) 91. J.B. Claridge, A.P.E. York, A.J. Brungs and M.L.H. Green, Chem. Mater., submitted. M.J. Ledoux and C. Pham-Huu, Catal. Today, 15 (1992) 263. V.L.S. Teixeira da Silva, M. Schmal and S.T. Oyama, J. Solid State Chem., 123 (1996) 168. E.A. Blekkan, C. Pham-Huu, M.J. Ledoux and J. Guille, Ind. Eng. Chem. Res., 33 (1994) 1657. S.T. Oyama, P. Delporte, C. Pham-Huu and M.J. Ledoux, Chem. Lea., (1997) 949. E.K. Storms, The Refractory Carbides, Academic Press, New York, 1967, Vol. 2.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Use of carbon fabrics as support for hydrogenation catalysts usable in polyphasic reactors. J.P. Reymond and P. Fouilloux L.G.P.C.; E.S.C.P.E.; 43 Bd du 11 Novembre ; F-69616 Villeurbanne cedex; France. In order to improve the performances of fixed bed reactors or slurry reactors in polyphasic catalytic hydrogenation of organic molecules, routes allowing to deposit noble metals on bidimensionnal supports, which could transformed further in structured catalytic systems (monolithic reactors), have been studied in this paper. Carbon fabrics have been selected as bidimensionnal support and the obtained catalytic systems have been tested in two triphasic reactions 9hydrogenation of acetophenone and hydrogenation of 4-chlorophenol. 1. INTRODUCTION A great number of catalytic conversions of organic molecules take place in triphasic reactors (gas-liquid-solid). Compared to conventionnal fixed bed or slurry reactors, the monolithic reactor is an interesting alternative [1 ]. Due to its particular structure, constituted of a great number of small parallel channels (up to 100 per cm2), a monolith presents some advantages compared to the usual reactors : mass and heat transferts are more effective; pressure drops are smaller; as the active phase is anchored on the channel walls the liquidcatalyst particles separation is eliminated. Typically, the channel walls of a monolith are covered by a porous oxide layer (washcoat), over which the catalytic phase is deposited. It is also possible to deposit the metal rightly on the channel walls. We have studied two routes to deposit metal clusters on a bidimensionnal support, support which could be further shaped into monolith, making up the channel walls. Use of stainless steel grids, or sheets, has been described elsewhere [2], use of carbon clothes is described in this paper. 2. E X P E R I M E N T A L

2.1. Preparation of catalysts Two routes of deposition of active metal have been investigated (the amount of metal precursor is calculated to lead to catalysts containing 5 wt% of metal). - the well known impregnation method has been used to deposite ruthenium. Four fabrics pieces (each of 4x4 cm, total weight ~ 0.45g) are immersed in an aqueous solution of RuC12 for 15 hours, drained, dried (3 hours at 443 K under nitrogen flow). An activation treament, reduction under H2 flow, is necessary to reduce the deposited metal complex and generate the zerovalent active metal. - the autocatalytic deposition method [2], deriving from the electroless plating method [3] has been used to deposit palladium. This process comprises four steps 9 a- cleaning of carbon fabrics pieces (4 pieces; each of 4x4 cm; 0.45g), which is operated in a Kumagawa apparatus with acetone

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b- preparation of an aqueous PdCI2 solution : adding of hydrochloric acid (0.5 to 1 g HCI 36 wt% in 25 cm 3 of water) is necessary to dissolve PdC12 c- deposition of the metal : the carbon clothes are immersed in the PdC12 solution. A given volume of a reductant solution (sodium hypophosphite) is then added in well controlled conditions (temperature, reactant concentrations and hypophosphite adding rate). d- drying : 2 hours in air at 298 K. This process is similar to a galvanic process (anodic and cathodic reactions take place in the mechanism) in which the electrons are provided by the chemical reductant [3]. It leads to the zerovalent metal which is catalytically active without activation treatment. 2.2. M a t e r i a l s

9

Carbon clothes (CECA and Actitex) are obtained from carbonization (hydrogen consumption) and activation (porosity formation) treatments of viscose fabrics (rayon). The diameter of carbon fibers is 20 ~tm and cloth thickness is ~ 0.5 mm. Some textural characteristics of three carbon clothes and of an active carbon in grains are given in table 1. Table 1 Characteristics of carbon supports. Support (type and manufacturer)

SBET

(m2/g)

Smicr~176 T~ Vp~ (m2/g) (cm3/g)

Vmicr~176 d micropore (cm3/g) (nm) 0.215 0.7 0.475 0.5

TE80 : Active carbon in grains (Degussa)

1098

495

0.550

IS : woven carbon cloth (CECA)

1250

948

0.505

RS 1301 : woven carbon cloth (Actitex)

1348

1004

0.731

0.462

0.5

FC 1201 : non woven carbon cloth (Actitex)

1000

889

0.444

0.410

0.6

1.3. C a t a l y t i c r e a c t i o n s :

The best way to evaluate the catalytic efficiency of a solid is to study its activity in a test reaction. Two reactions have been used : hydrogenation ofacetophenone (palladium and ruthenium catalysts) and hydrogenation of 4-chlorophenol (ruthenium). Catalytic tests take place in a 125 c m 3 stainless steel stirred reactor (semi-batch). In the case of acetophenone hydrogenation, catalyst samples are immersed in cyclohexane, hydrogen pressure is 2.5 MPa and reaction temperature is 393 K. Extent of reaction is evaluated from hydrogen and acetophenone consumption and product formation. Catalyst activity is characterized by initial activity and selectivity values. Acetophenone (AC), has an aromatic ring with a ketone function and reaction products depend on the nature of the active metal [4, 5]. On ruthenium catalysts the aromatic cycle is hydrogenated, leading to the formation of methylcyclohexylketone (MCC) and cyclohexylethanol (CE). On palladium the ketone function is hydrogenated in phenylethanol (PE) and ethylbenzene(EB). Hydrogenation of 4-chlorophenol is part of a two steps process of depolluting waste water which has been developped in our laboratory [6]. The first step is the adsorption of chlorophenol on the carbon support of the catalyst (Ru/carbon). The second step is the catalytic dechlorination by hydrogenation of the adsorbed chlorophenol, in an aqueous basic media (soda) and under mild operating conditions (313 to 353 K, and hydrogen pressure 0.3 to 0.5 MPa) [6]. The reaction products are cyclohexanol and hydrochloric acid (neutralized by soda).

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3. RESULTS AND DISCUSSION

997

3.1. Catalysts based on ruthenium deposited on carbon clothes: 3.1.1. Hydrogenation of acetophenone :

The IS carbon cloth (see table 1) has been used as support to prepare ruthenium catalytic systems, following the impregnation method. Figure 1 depicts the conversion of reactants and products (Ru/carbon cloth; 393 K; 2.5 0,3MPa H2; A i is 7.29 molH2/g.h), as a function of time. Acetophenone is first hydrogenated in 0,2 methylcyclohexylketone (MCC) which, in g turn, is hydrogenated in cyclohexylethanol e(CE) which is the main (or only) product at 9 0,1 0 tthe end of the reaction (no more hydrogen O 0 consumption). Light formation of phenyl 0~ ethanol (PE) and ethyl benzene (EB) is 0 20 40 60 noted, indicating hydrogenation of the Time (mn) ketone function of acetophenone. Fig. 1. Product and reactant conversion during The influence of several operating acetophenone hydrogenation over Ru/C cloth. parameters of preparation on the activity of Ru/Carbon cloth have been evaluated. Table 2 depicts some results. For example, there is an optimal reduction temperature (773 K). Above 773 K the support reacts with hydrogen and sintering of ruthenium particles probably occurs, leading to a poor catalytic activity. ~nH2

---.Zk--AC

t'-

Table 2 Effect of preparation operating parameters of Ru/carbon cloth catalysts on initial activity. Reduction temperature (K) Reduction time (h) mass (g) Activity (moIH2.glRuh-l) 573 573 573

3 20 6

693 773 873

6

0.46 0.45 0.9 0.45 0.22 0.1 0.5 0.235 0.09 0.47 0.44

5-9 7-9 5.5 6.5 6.1 5.9 12.15 13.14 12.2 21-25 7

As it is observed in table 2 the initial activity does not depend on the catalyst mass. Moreover, the activity of Ru/carbon cloth is egal to that of the same catalyst finely grinded. It can be concluded from the experiments that there is no internal diffusion problem with the catalytic system, that the reactor is perfectly stirred and that the reaction occurs in chemical regime. Carbon cloth appears as a convenient support to prepare catalytic systems.

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However, it has to note that in same conditions of activation and reaction, a commercial catalyst (Ru over carbon powder, from Degussa), is twice more active (up to 53 molm.gtRu hl) than our catalysts but presents similar selectivities. 3.1.2. Adsorption and hydrogenation of 4-chlorophenol on Ru/Carbone: The depolluting process, mentioned above, was first operated over a conventional catalyst : ruthenium deposited on active carbon in grains (Pica, TE 80). To improve this process, the properties of two carbon fabrics, RS1301 (woven cloth) and FC1201 (non woven), towards adsorption of 4-chlorophenol have been evaluated and compared to that of the carbon in grains. Adsorption isotherms of 4-chlorophenol on the supports, presented on figure 2, are similar (in fig.2, Q is the amount adsorbed at a given time and Qmaxis the maximal amount adsorbed). The adsorption rates, determinated in a stirred batch reactor, are also similar while adsorption rate of 4-chlorophenol is slightly greater on FC 1201.

1

1 0,8

0'8 t E0,4--O--FC1201 ---i~--TE80 ---X-- RS1301

0,2 C

f

o,6

.~ o,6 E0,4

0,2

I

0~ o

750 1500 2250 Equilibrium concentration (ppm)

Fig.2. Adsorption isotherms of chlorophenol on adsorbents (293 K).

- -

--~---TE 80 ~FC 1201 ---X'-- RS 1301

I

2000 4000 Time (s)

6000

Fig.3. Breakthrough curves of adsorbents (3g; 1l/h; initial concentration" 4000 ppm; 293 K)

Two important parameters, Q~ and t~, are determined from the so-called breakthrough curves which are presented on figure 3. These curves, obtained from in-line adsorption experiments (column), give the pollutant concentration in the outlet flow versus time. t~ is the critical time, time for which the pollutant concentration becomes higher than the maximun value allowed for pollutant rejection (0.1 ppm for 4-chlorophenol) and Qr is the corresponding critical amount of adsorbed pollutant. Table 3 Comparison of adsorptive properties of carbon supports. Qmax tc (s) Qc (g/g) FC1201 RS1301 TE80

0.489 0.577 0.455

0.5 1/h 2070 1710 840

1.5 1/h 614 175 37

0.5 1/h 1.43 1.04 0.48

1.5 1/h 0.43 0.26 0.06

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The values of Qmax, tc and Qc are given for two flow rates in table 3. For FC1201 the outlet flow remains below the limit value for a longer time and then rises more stiffly than for the other adsorbents. With FC 1201 the in-line adsorption of 4-chlorophenol could be carried out with a higher flow rate. From these results it can be concluded that adsorption properties of carbon fabrics are slightly better than that of carbon grains. Compare to active carbon grains, carbon fabrics present improved characteristics : greater adsorption capacity and stiffness of the breakthrough curves. Moreover, the packing of an adsorption column with carbon cloth disks leads to an ordered structure presenting a lower pressure drop than it is in the case of carbon grains (the same result will be obtained in an inline reactor). Then, ruthenium has been deposited by the impregnation method on the RS 1301 cloth which presents the higher adsorption capacity. The resulting catalyst has been tested in the hydrodechlorination of 4-chlorophenol and its activity has been compared to that of the conventional catalyst (Ru/TE80). Catalytic activities (transformation rates of 4-chlorophenol in cyclohexanol) are very similar over the two tested catalysts : 0.4molH2gRu-lhI in each case. As evidenced by batch and in-line adsorption experiments carbon clothes show interesting performances for chlorophenol adsorption. They are also useful as support to prepare effective catalysts in the further hydrogenation of the adsorbed 4-chlorophenol.

3.2. Catalysts based on palladium: hydrogenation of acetophenone. The autocatalytic deposition method, usable on all kinds of support, has been used to deposit palladium on the IS carbon fabrics (see table 1) instead of the impregnation method. The influence of several preparation operating parameters on the activity of obtained catalytic systems has been studied in the hydrogenation of acetophenone. As evidenced from chemical analysis and X-ray emission analysis [2], the autocatalytic deposition process leads to the formation of a layer of palladium-phosphorus alloy coating the whole surface of the support and to the formation of discrete particles of the same alloy. If sodium borohydride is used as the reducer, instead of sodium hypophosphite, the deposit is an alloy of palladium and boron. Mechanism of phosphorus and palladium codeposition in electroless plating baths is not really elucidated, it could occur via interaction of hypophosphite ion with atomic hydrogen or with hydride ions or with palladium metallic surface [7]. Catalytic properties of Pd/carbon fabrics have been compared to that of industrial hydrogenation catalysts (Degussa and Doduco) in which palladium (5 wt%) is supported on powdered active carbon (SBET'~1000 m2g~). Results are presented in table 4. Table 4 Comparison of initial activity and selectivities of industrial and laboratory Pd/carbon catalysts. Catalyst Reduction Ai Phenylethanol Ethylbenzene treatment mo1H2g-lpd hl % % Pd/C (Degussa) N 52 99 1 Pd/C (Doduco) N 123 18 82 Pd/C cloth N 68 0 100 Pd/C (Degussa) Pd/C cloth

Y Y

219 45

76 15

24 85

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All catalysts have the behaviour of palladium : they catalyze the hydrogenation of the ketone function. However, our catalytic system appears quite different to industrial catalysts. When catalysts are not prereduced by hydrogen, Pd/C-fabrics and Pd/C-Degussa have quite similar initial activity (hydrogen consumption rate: ~ 60 molH2gpdl.h-~) while Pd/C-Doduco is twice more active (~120 molH2gpdl.h~). On the other hand, selectivities at the end of the reaction are more different. Pd/carbon cloth leads only to ethylbenzene while industrial catalysts have a more lighter hydrogenolysis activity, producing only phenylethanol (Degussa) or phenylethanol and ethylbenzenze (Doduco). A reducing pretreatment (H2, 2h, 493 K) decreases slightly the initial activity of Pd/C fabrics (45 molH2gpd~.h~), while initial activity of industrial catalyst (Pd/C Degussa) is strongly enhanced (220 molH2gpdl.hl). This is not surprising because the autocatalytic preparation method leads to the deposition of zerovalent palladium which is catalytically active (the influence of phosphorus is not elucidated). On the contrary, impregnation of a support from a metal salt solution leads to the formation of a surface metal complex which must be decomposed in order to generate the zerovalent active metal. Nevertheless, reduced Pd/C-Degussa has a great hydrogenation activity (formation of phenylethanol) and a weak hydrogenolysis action (conversion of phenylethanol in ethylbenzene). Selectivities observed with our catalyst are different, it leads mainly to the formation of ethylbenzenze, its hydrogenolysis action is greater, but phenylethanol is not fully conversed. Combined to the autocatalytic deposition process carbon clothes allow to prepare interesting bidimensionnal catalytic systems which could shaped as structured catalyst (monoliths and column packings are intended). 4. CONCLUSION

Compare to carbon powders or granulates the implementation of carbon fabrics in polyphasic reactors present several improved caracteristics : good textural characteristics and adsorptive properties, possibility to prepare structured catalytic systems. Carbon clothes have been used as support to prepare efficient hydrogenation catalysts. Nevertheless, the activity of actual catalytic systems on carbon fabrics has to be improved to reach the activity level of industrial catalysts. REFERENCES 1. A. Cybulski and J.A. Moulijn, in "Structured Catalysts and Reactors" (A. Cybulski J.A. Moulijn Eds., M. Dekker, Amsterdam, 1995), 1. 2. J.P. Reymond, D. Dubois and P. Fouilloux, Studies in Surface Science and Catalysis, (1998), 63-72. 3. D. Dukes in "Electroless Plating : fundamentals and Applications" (G.O. Mallory, Hadju eds., A.E.S.F., Orlando, 1992), 511. 4. N.S. Barinov, D.V. Mushenko and E.G. Lebeva, Zh. Prikl. Khim., 39 (1966), 2599. 5. J. Mason, P. Cividino, J.M. Bonnier and P. Fouilloux in "Heterogeneous Catalysis Fine Chemicals II" (1991), 245. 6. V. Felis, C.de Bellefon, P.Fouilloux and D. Schweich, Appl. Catal.,20 (1999), 91. 7. G. Salvagno and P.L. Cavalotti, Plating, 59 (1972), 665.

and 118 J.B.

and

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Characterisation and Reactivity of Activated Carbon Supported Platinum Catalysts Prepared by Fluidised Bed Organometallic Chemical Vapour Deposition ( F B O M C V D ) Ph. Serp", J-C. Hierso a, R. Feurerb, R. Corratg6 ~, Y. Kihn~ and Ph. Kalcka a) Laboratoire de Catalyse, Chimie Fine et Polym~res b) Laboratoire de Cristallographie, R~activitO et Protection des Mat~riaux Ecole Nationale Sup~rieure de Chimie de Toulouse, 118 route de Narbonne, 31077 TO UL0 USE Cedex, FRANCE. c) C.E.M.E.S.-C.N.R.S. no. 8011, 2, Rue Jeanne Marvig, 31055 Toulouse, FRANCE

A.E. Aksoylu, J.L. Faria, A.M.T. Pacheco and J.L. Figueiredo Laborat6rio de Catdlise e Materials, Faculdade de Engenharia, Universidade do Porto, Rua dos Bragas, 4050-123 Porto, PORTUGAL

I. INTRODUCTION In the course of developing new methods for the preparation of heterogeneous catalysts, we have recently shown that the combination of organometallic chemical vapor deposition (OMCVD) and the fluidizati0n of a bed of porous particles is a powerful method to prepare supported catalysts [1]. Highly dispersed deposits of rhodium, palladium and platinum on metal oxide supports were obtained in a single step using a fluidized bed reactor [2]. The method of chemical vapor deposition (CVD) allows direct deposition of the active phase onto the catalyst support by means of the reaction between surface sites containing oxygenated groups and the vapor of a suitable organometallic compound. Using silica or alumina with large specific areas- around 200 m2g1- the presence of hydroxyl groups on the surface allows the facile grafting of small and dispersed particles of the noble metal (2-5 nm). On such supports, the high concentration of anchoring groups presumably permits the maintenance of a high nucleation rate which favors the growth of the particles. For carbon supports the situation is somewhat different, since less active sites are available and these have a larger diversity in their chemical nature. Through careful oxidation processes, it is possible to control the nature and concentration of the oxygenated functions on the surface. As carbon supported catalysts play an important role in many catalytic reactions, we have undertaken a study in which we have examined the suitability of the FBOMCVD method for the direct elaboration of such catalysts. The present report concerns the deposition of platinum on various carbon types. Atter preliminary OMCVD experiments on planar model supports, various activated carbon powders were examined for the elaboration of platinum supported catalysts. The activity of these catalysts was compared with that of conventional wet impregnated catalysts in benzene hydrogenation.

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2. EXPERIMENTAL

2.1- Preparation of carbon supports - Planar supports: highly oriented pyrolytic graphite (HOPG) was used as received or oxidised in refluxing 5N HNO3. Rough planar graphitic carbon disks were obtained by cutting high purity carbon rods for spectrography and were used directly or alter the same oxidative treatment as previously. - Powder supports a commercial activated carbon, Hydraffin, was ground and sieved to 200300 gm particle size prior to its use. Then the following pre-treatment procedures were applied" (i) Hydraffin was washed with 2N HCI solution for 12 h, and then washed with distilled water under reflux for 6 h. Then they were dried overnight at 383 K (AC1). HC1 washed activated carbons were used as such and also taken as the base for the subsequent oxidation treatments as follows: (ii) AC1 was oxidised in 5 % 02-95 % N2 mixture at 723 K for 10 h (AC2) and (iii) AC1 was directly oxidised in 5 N HNO3 solution for 3 h and washed with boiling distilled water till the pH of the rinsed solution reached 5.5. These treatments were followed by overnight drying at 383 K (AC3). 2.2- Preparation of platinum catalysts - CVD catalysts: the deposition of platinum on planar supports has been performed in a classical horizontal hot-wall CVD reactor. Vapours of PtMe2(COD) were carried on by a 3% molar H2/Helium mixture, under reduced pressure, onto the support heated at 110~ For the powder supports, the previous conditions were adapted to a vertical fluidised bed CVD reactor. Impregnated catalysts: three 1 wt% Pt/AC catalysts were prepared on AC1, AC2 and AC3 supports by incipient wetness impregnation of aqueous solution of hexachloroplatinic acid. The impregnation was conducted under vacuum with ultrasonic mixing. The precursor solutions (2 ml.g'lsupport) with calculated concentration were added via a peristaltic pump. After impregnation step, the samples were dried overnight at 373 K. Transmission electron microscopy (TEM) analyses (Philips CM12, 120 kV) and scanning electron microscopy (SEM) observations (Jeol JSM-35 C)were performed on both CVD and impregnated catalysts. Atomic force microscopy (AFM) was performed on a Nanoscope II Scanning Tunneling Microscope (Digital Instrument Inc). Temperature-programmed desorption (TPD) profiles were obtained in a home maid set-up consisting of a U-shaped tubular reactor mounted inside a furnace. The amounts of CO and CO2 desorbed from the samples were monitored by mass spectometry (Spectramass Dataquad quadrupole). 2.3- Catalytic experiments All six catalysts were tested in benzene hydrogenation at 393K with a H2:C6I-I6 molar ratio of 11, and 17 gl/min Cd-I6 flow in the feed. All gas flows were controlled by using mass flow controllers (Bronkhorst) and the flow of benzene was controlled by an HPLC pump (Gilson 305) connected to an Autoclave Engineers BTRS reactor. Prior to reaction, impregnated samples were reduced at 623 K for 14 h under pure hydrogen. In reaction tests, 250 mg fresh sample was placed in a tubular microreactor whose temperature was controlled within a range of + 0.1 C by a temperature controller. 3. RESULTS AND DISCUSSION

3.1- Platinum deposits on model supports

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Figure 1: AFM image of Pt deposited by CVD on oxidised HOPG On the untreated HOPG support, the surface of which does not contain any oxygenated functions nor physical defects (e.g. steps or edges), no deposition was observed by SEM (xl 0 000) and AFM, whatever the duration of the runs (3 to 20 minutes). Deposition was noted on the reactor walls. However, oxidation of the HOPG support by HNO3 solution provides anchoring sites, which allows the deposition of platinum particles. SEM examination reveals the presence of particles, some of them isolated on planar surfaces, others laid out presumably on crystal defects. Analysis by AFM of the deposits obtained aRer 3 min confirms this observation and reveals the presence of particles (roughly 50-60 nm in diameter after 3 min of deposition) localised on surface defects created by the oxidation (Figure 1). On the unoxidised graphitic disks, deposits are observed mainly on the defects induced by sawing. After nitric acid oxidation a greater number of particles are deposited on the whole surface and the crystallite size distribution is more homogeneous (centred around 50 nm for both samples). 3.2- Platinum deposits on activated carbon powders

The results displayed in Table 1 show that air oxidation treatment led to a slight increase in the BET surface area, whereas HNO3 oxidation led to a significant decrease. The decrease in the BET surface area aRer HNO3 oxidation can be explained by the collapse of the pore walls due to the attack of highly concentrated nitric acid during oxidation. As a consequence, the microporous volume (Wo) of the nitric acid oxidized sample, AC3, dropped by35 %. Table 1: Characteristics of the supports Support AC1 AC2 AC3

BET (m2/g) 1415 1589 845

Adsorption Wo (cm3/g) 0.597 0.689 0.399

Smeso (m2/g) 40.3 89.0 32.0

CO 1325 4315 4616

TPD (~tmol/8) CO2 CO/CO2 393 577 2732

3.37 7.48 1.69

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The results indicated that air oxidation doubled the mesoporous surface area (Smeso) of the hydraffin support. On the other hand, AC3 (HNO3 oxidised) showed a mesoporous surface area 20% smaller than that of AC 1. The concentration of oxygenated groups, which release CO during the TPD run, is almost four times higher for AC2 than for AC 1. In the TPD of AC2, the CO-type groups which decompose above 800 K are mainly carbonyl, phenol, ether and quinone. The CO~ release curves indicate that, although AC 1 has carboxylic groups, AC2 does not present the carboxylic acid, function, but instead it has carboxylic anhydride groups which are indicated by the high temperature CO2 release. The concentration of oxygen containing groups is greater for the oxidized samples, as shown by larger amounts of CO and CO~ released compared to AC1. In particular, the carboxylic acid groups (CO~-type groups that decompose at around 523 K) are predominantly formed by HNO3 oxidation. For AC3, a decrease of the CO/CO~ ratio was observed when compared to AC1. Since CO~ releasing groups have more acidic nature than CO releasing surface groups, CO/CO~ ratios point out that air oxidation led to a more basic support, whereas HNO3 oxidation led to a relatively more acidic surface, compared to AC 1. HCl-washed activated carbon powders (AC1) submitted to CVD in a fluidised bed, give rise to a loading of 1% of platinum aggregates whose sizes range mainly from 5 to 20 nm. A similar distribution was observed for the catalysts prepared by impregnation (Figure 2). Thus, whatever the technique used, the distribution appears to be equally poor in the absence of any oxidative treatment of the support. For the two techniques, and aider an oxidising treatment in air of the support (AC2), the particle size distribution appears narrower (1-9 nm, Figure 3). It is worth to mention that only a few large particles were detected by TEM analyses. Thus the use of the combined technique FBOMCVD for the preparation of highly dispersed small particles appears very suitable. Finally, nitric oxidation of AC1, providing more oxygen-containing surface groups, particularly carboxylic ones (AC3), results in a satisfactory platinum particle size distribution (Figure 4) since they have a homogeneous morphology, their size is centred on 3-5 nm (CVD) or 2-4 nm for impregnation and no large particle was detected. Examination of the catalysts on AC2 and AC3 shows mainly that the CVD method affords a roughly Gaussian distribution of the particle size whereas for the impregnation method, the number of particles of size higher than 5 nm increases significantly and regularly.

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10-20 20-30 30-50

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Figure 2: Particle size of Pt/AC1 prepared by CVD (black) and impregnation (white).

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Figure 4: Particle size of Pt/AC3 prepared by CVD (black) and impregnation (white). 3.3- Catalytic tests

The six supported platinum catalysts prepared on AC1, AC2 and AC3 by both methods were tested in the hydrogenation of benzene. The experiments, performed at 120~ C, reveal that roughly the same activity was observed for the two AC 1 catalysts (Figure 5), but the productivity is very low when compared to the oxidised supports. ~

140

~"

120

"~

100

8o

9Pt/AC 1 (CVD) r-I Pt/AC 1 (imp)

60 40 20 o 15

30

60

90

120 180 240

rxn. time (mini

Figure 5 Activities of Pt/AC1 prepared by CVD (black) and impregnation (white).

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However, the air oxidation of the support affords the most impressive differences between the OMCVD/AC2 and impregnated/AC2 catalysts (Figure 6). Indeed, the former is four times more active (-~1150 gmol.gPtls "l) than the latter. For the AC3 support (Figure 7), the catalyst prepared by OMCVD has roughly the same activity as the one classically obtained (around 900 gmol.gPt is'l), although a slight deactivation occurs with time. ,:.,~ 1200 ..~" 9 1000 ,,,.., ....:

800

II Pt/AC2 (CVD)

600

[] Pt/AC2 (imp)

400 ~.,

200 0 15

30

60

90

120 180 240

rxn time (min)

Figure 6: Activities of Pt/AC2 prepared by CVD (black) and impregnation (white). 1200 1000 ....:

800

9Pt/AC3 (CVD)

600

13 Pt/AC3 (imp)

400 ~,

tO

200 0 15

30

60

90

120 180 240

rxn time (min)

Figure 7 Activities of Pt/AC3 prepared CVD (black) and impregnation (white). 4. CONCLUSION These results demonstrate the viability of the FBOMCVD process for the production of well-defined platinum/carbon catalysts. It was shown that, for the best results, the active metal species need to be bound in a controlled manner, indicating the importance of the interaction metal-support in the elaboration of catalysts by FBOMCVD. It must be also mentioned that OMCVD has pronounced advantage over impregnation especially when AC surface is rich in basic groups like in the case of air oxidised (AC2) support. 5. R E F E R E N C E S

1. Serp Ph., Feurer R., Morancho R. and Kalck Ph., Jr. Catal., 157 (1995) 294. 2. Hierso J-C., Serp Ph., Feurer R. and Kalck Ph., Appl. Organomet. Chem., 12 (1998) 161. 3. Serp Ph., Hierso J.-C., Feurer R., Kihn Y., Kalck Ph., Faria J., Aksoylu E., Pacheco A. M. T and Figueiredo, J.-L. ,Carbon, 37 (1999) 527.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Metal-carbon aerogels as catalysts and catalyst supports F.J. M a l d o n a d o - H 6 d a r , C.Moreno-Castilla*, J. Rivera-Utrilla and M.A. FerroGarcia. Departamento de Quimica Inorg~ica, Facultad de Ciencias, Universidad de Granada, 18071 Granada. Spain. Several transition-metal-containing organic aerogels (Ti, Cr, Mo, W, Fe, Co and Ni) were obtained by polymerization of resorcinol and formaldehyde. The metal-carbon aerogels were obtained by pyrolysis of these aerogels in an inert atmosphere. The porous texture of the samples was characterized by different techniques (mercury porosimetry, He and Hg density and gas adsorption) as well as their surface chemistry and morphology (XPS, XRD, DTG, TEM, SEM). Some of the samples were tested as catalysts in reactions such as the conversion of alcohols and the skeletal isomerization of 1-butene.

1. INTRODUCTION Materials prepared by the sol-gel approach have rapidly become a fascinating field of research in material science. The most common aerogels are inorganic, usually derived from the solgel polymerization of metal alkoxides. However, Pekala (1) synthesized organic aerogels by polycondensation of certain organic monomers. Carbon aerogels and activated carbon aerogels can be obtained by carbonization and activation, respectively, of the corresponding organic aerogels (2,3). These materials present very large meso and microporosity. Recently (3,4), we have demonstrated that transition-metal-containing carbon aerogels can also be prepared by adding the adequate metallic salt to the original recipe of the aerogels. These materials present also a well developed porosity depending on the nature of the metal. The aim of this work is to study the synthesis and characteristics of transition-metalcontaining carbon aerogels and xerogels. These materials can have the advantage against other supported catalysts, obtained by impregnation, that the metal is trapped in the porous carbon matrix and then its distribution would be better. Moreover, the porosity of the carbon matrix within which is trapped the metal can control the selectivity of the reaction. The purpose, in the case of TiO2-containing carbon aerogels and xerogels is to obtain composite materials with appropriate acidity, and more developed porosity and surface area than the TiO2, which is widely used as support for catalysts.

2. EXPERIMENTAL Transition-metal-containing aerogels were prepared by dissolving the metal precursor in an aqueous solution of formaldehyde and resorcinol. The metals studied were: Cr, Mo, W, Fe, Co and Ni (1% by weight in the starting solution). A blank, without any metal in the starting solution was also prepared. The mixtures were cast into glass molds (0.5 cm internal

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diameter) and cured, after that, the gels were cut into pellets and supercritical dried to obtained the aerogels. These aerogels were carbonized at both 500 and 1000 ~ in N2 flow to obtain the corresponding carbon aerogels, which will be referred to in the text as AM-500 and AM-1000 series (M indicate the metal present), respectively, some of the samples obtained at 1000 ~ were steam activated at 900 ~ (S series). The TiO2- organic xerogels and aerogels were prepared by copolymerization of the tetrabutil orthotitanate with the resorcinol-formaldehyde solution. These samples were pyrolyzed at 900 ~ in N2 flow after supercritically drying the gels (aerogels) or not (xerogels). Textural characteristics of the samples were studied by N2 and CO2 adsorption at 77 and 273 K. Mercury porosimetry was also used in order to know the pore size distribution with diameter greater than 3.7 nm, the particle density (pp), macropore volume (V3, pores with diameter greater than 50 nm) and accessible mesopore volume (V2, pores with diameter between 3.7 and 50 nm). Other techniques used to know the morphology of the solids prepared and the state of the metals and dispersion were: XRD, XPS, SEM, TEM and DTG. The catalytic activity of some samples in the decomposition of isopropanol and the skeletal isomerization of 1-butene were studied. Catalytic tests were carried out in a glass microreactor at atmospheric pressure with c a . 0.2 g catalyst. Analysis of the reaction products was done by on-line gas chromatography. The samples were heated in He flow at 400 ~ for 4 h prior to the catalytic test.

3. RESULTS AND DISCUSSION 3.1. Metal-containing carbon aerogels The metal-containing aerogels, carbon aerogels and activated carbon aerogels present a large variety of textural characteristics depending on the metal present, thermal treatment and activation processes. The textural characteristics of the organic aerogels were strongly influenced by the metal present. Table 1 shows, as an example, the textural characteristics of some of the aerogels obtained. Table 1.- Textural characteristics of some of the aerogels obtained. Aerogel A Afe Ani AW

lop

V2

0.69 0.59 0.62 0.47

0.000 0.000 0.647 0.000

g cm-3

V3

Sext

0.695 1.968 0.607 1.429

4 7 128 11

cm 3 g-1

m 2 g-1

SBET 19 284 35

The blank aerogel had an important macropore volume, but it had no mesopores and consequently showed a low external surface area. Aerogels containing Cr, Mo, W and Fe were essentially macroporous, while both Co and Ni-containing aerogels were highly meso and macroporous samples. Metal-carbon aerogels and activated metal-carbon aerogels were prepared from the above aerogels. The release of volatile matter during the aerogel carbonization, produced in all cases

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weight loss (WL) around 50 %. Previous thermogravimetric experiments (4) showed that the main WL occured at around 550 ~ and that no important WL took place at temperatures above 750 ~ Obviously, the textural characteristics of the parent aerogels strongly influenced the characteristics of their derivatives. Table 2 shows, as an example, the textural characteristics of some carbon aerogels. Table 2.- Textural characteristics of some of the metal-containing carbon aerogels.

WL % 54 43 54 50 54 41 51

Sample A-1000 AFe-500 AFe-1000 ANi-500 ANi- 1000 AW-500 AW-1000

pp g cm-3 0.72 0.44 0.39 0.57 0.56 0.43 0.54

V2

V3 cm 3 g-i 0.000 0.628 0.000 1.516 0.060 1.936 0.819 0.032 0.205 0.873 0.047 1.525 0.011 1.320

Sext

m 2 g-I

18 7 30 176 80 29 11

SBET 470 482 306 669 400 528 610

Comparing the results of Table 1 and 2, it is observed that carbonization of the aerogels at 500 ~ produced, in all cases, an increase in the micropore range, and consequently, an increase of the surface area (BET) of the samples. Carbonization at 1000~ made to decrease, in general, the SBET value. The meso and macropore volumes, although in general tended to increase, presented smaller variations. The pore size distribution (PSD) obtained by mercury porosimetry (pores greater than 3.7 nm in diameter) was strongly influenced by the pyrolysis temperature. Except the Cr-containing carbon aerogels, the PSD became narrowest after cabonization and presented an unimodal distribution, showing only one maximum whose position depended on the metal present. Metal-carbon aerogels obtained at 500 ~ presented smaller pores than those obtained at 1000 ~ Thus, Co and Ni-containing carbon aerogels were mainly mesoporous materials when were obtained at 500 ~ while all the samples prepared at 1000 ~ were essentially macroporous. Figure 1 show the evolution of the PSD of Co-containing carbon aerogels. ACo n,

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300

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450

A

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[]

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600 T e m p e r a t u r e (K)

750

Figure 2. Isobutene yield as a function of the reaction temperature.

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The blank and the Fe, Co and Ni-carbon aerogels obtained at 1000 ~ were subsequently steam activated. Their textural characteristics are compiled in Table 3. In general, activated carbon aerogels were essentially macroporous materials. The presence of the metal favored an opening of the porosity. Thus while the metal-containing activated carbon aerogels showed greater macro and mesopore volumes, the A1000-S sample showed the highest microporosity (SN2 values)

Table 3.- Textural characteristics of some of the metal-containing activated carbon aerogels. Sample A1000-S AFe-1000-S ACo-1000-S ANi1000-S

BO % 25 28 52 37

lop

g cm-3 0.55 0.29 0.34 0.34

V2

9V3

cm 3 g-I 0.035 0.984 0.210 2.602 0.326 2.076 0.248 2.234

Sext 35 66 103 119

m 2 g-I

SBET 1085 377 431 550

The chemical state and dispersion of metal after the carbonization process were determined by XRD, XPS or TEM. TEM micrograph showed well and homogeneous catalyst dispersion. In the case of Cr, Mo, W, Co and Ni-containing carbon aerogels, no XRD peaks corresponding to the metal phase were observed in samples obtained at 500 ~ indicating that metal was well dispersed into the carbon matrix. Fe-containing carbon aerogel showed the diffraction peaks of Fe203. At 1000~ the metal oxides particles of Fe, Co and Ni were reduced to the corresponding metal by the organic matrix, showing particle size between 15-20 nm. The sample containing tungsten showed the formation of tungsten carbide, while those containing Cr and Mo did not present any diffraction peak, indicating that even after treatment at high temperature the metals were well dispersed. XPS showed a mixture of oxides with the metal in different oxidation states. The metal/C ratios indicated a loss of dispersion when the carbonization temperature increased from 500 to 1000 ~ Both ACr-500 and ACr-1000 showed a mixture of 72 % Cr(III) and 28% Cr(VI). AMo-500 showed a mixture of 82% Mo(V)-18% Mo(III) whereas in AMo-1000 metal was 45% Mo (III)-55% Mo(V). In the case of AW-500 100% of W(VI) was detected and in AW-1000 appeared W(VI) (96%) and 7% of tungsten carbide was detected, which was also detected by XRD. The acid-base character of the active sites of the catalyst can be determined by means of inderect determinations, such as the measurements of the catalytic activity for the decomposition of isopropanol, as proposed by Ai (5). The acidity is related to the reaction rate of dehydration to obtain propene, rp, and the basicity to the ratio of the reaction rates for dehydrogenation (to acetone), ra, and dehydration (ra / rp). Table 4 summarizes the results obtained with Cr, Mo and W catalyst prepared at 500 ~ These results show that the acidity increased in the order ACr-500<
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Table 4. Acidity and basicity of the catalysts evaluated through the catalytic decomposition of isopropanol at 403 K Catalyst

rp

ACr-500 AMo-500 AW-500

0.0006 0.0515 0.2689

~tmol goat-1 S-1

ra

rp / ra

0.0348 0.0285 0.0055

58.0000 0.5534 0.0205

The greatest catalytic activity of AW-500 with respect to the other samples is due to its highest acidity and a more developed porosity. Isobutene and trans-2-butene were the main reaction products. C3-C5 and C2-C6 by-products were not observed, likely due to an effect of the porosity, and indicates that the reaction must follow a monomolecular mechanism. The catalyst AW773 showed an isobutene yield of 40% at 448 K which was higher and obtained at a lower reaction temperature than in the case of other tungsten oxide catalysts described in the literature (4).

2.2. TiO2- carbon aerogels and xerogels. The synthesis of titania-based aerogels has been recently reviewed (6). Three TiO2-carbon samples were synthesized according to the recipes showed in Table 5. Table 5. Recipes formaldehyde, W C02. Sample R B 0.056 C 0.056 D 0.056

of the TiO2-carbon samples (amount in moles). R = resorcinol, F = = water, TBOTi = Tetrabutil orthotitanate, SCD = supercritical drying in F 0.112 0.112 0.112

W 0.782 0.782 0.782

TBOTi Solvent 0.056 The proper water 0.056 500 ml ciclohexane 0.196 500 ml ciclohexane

drying 1 day 80 ~ 3 days 110 ~ SCD + 3 days 80~ SCD+ 3 days 80~

The textural characteristics of samples from Table 5 are compiled in Table 6. The carbon dioxide surface area of these samples, seem to be more or less independent of the drying method or the TiO2 content. However, when comparing sample B and C, it is observed that the supercritical drying preserved a large meso and macropore volume, leading to a lower density and greater external surface area. The increase in TiO2 content made to decrerase the V2 and V3 values and therefore the Sext. After the heat treatment in N2 flow up to 900 ~ (samples B-900, C-900, D-900), SCO2 increased. This increase was lower in sample D, probably due to its high TiO2 content. If the heat treatment at 900~ is carried out in air to burn all the carbonaceous matrix, pure TiO2 with a SCO2 of only 13 m2gl , was obtained. XRD of these samples showed that TiO2 remained amorphous or forming small microcrystallites when the heat treatment was carried out in N2, because no diffraction peaks were observed. Howerver, after heat treatment in air at 900 ~ a mixture of rutile and anastase were detected.

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Table 6. Textural characteristics of TiO2-carbon aerogels and xerogel and their carbonized derivatives at 900 ~

Sample B C D B-900 C-900 D-900

TiO2 (%) 29.7 29.6 49.0 -

W.L. (%) 39 44 62

13~ 3 g cm"

V2

1.22 0.54 1.19 1.02 0.67 1.64

0.004 0.411 0.133 0.028 0.380 0.069

V3

Sext

0.210 0.688 0.040 0.249 0.519 0.052

2 137 63 12 101 20

cm 3g-1

m2g-i

SCO2 137 161 128 382 429 184

The catalytic behaviour of these samples in the decomposition reaction of isopropanol was measured. The pure TiO2 obtained after treating at 900 ~ in air the sample B, and the carbon aerogel A1000 were used as reference materials. The results obtained are compiled in Table 7. Both TiO2 and A1000 were less actives than any other sample. A1000 was practically inactive bellow 300 ~ and only produced propene. TiO2 was more active, and high acetone selectivity was observed at low conversion values, decreasing as conversion increased. Samples B-900 and C-900, with similar TiO2 content, showed similar activity but the aerogel (C-900) presented higher acetone selectivity than the xerogel (B-900). Sample D-900, with higher TiO2 content, showed a higher conversion value as well as a greater acetone selectivity than the other TiO2-carbon aerogel (sample C-900). Table 7. Catalytic activity of TiO2-carb0n samples in the isopropanol decomposition reaction.

Sample A1000 TiO2 B-900 C-900 D-900

Temperature oC 240 240 180 180 180

Conversion (%) 0.3 1.6 4.2 4.1 5.6

Selectivity (%) Propene Acetone 100 74.6 25.4 62.1 37.9 54.1 45.9 43.2 56.8

4. REFERENCES

1. R.W. Pekala, J. Mater. Sci., 24 (1989) 3221. 2. Hanzawa, K.Kaneko, R.W. Pekala, M.S. Dresselhaus, Langmuir, 12 (1996) 6167. 3. F.J. Maldonado-H6dar, M.A. Ferro-Garcia, J. Rivera-Utrilla, C. Moreno-Castilla, Carbon 37(1999) 1199. 4. Moreno-Castilla, F.J. Maldonado-H6dar, J. Rivera-Utrilla, E. Rodriguez-Castell6n, Appl. Catal. A:\ General 183 (1999) 345. 5. L.H. Gielgens, M.G.H. van Kampen, M.M. Brock, R. van Hardelveld, V. Ponec, J. Catal. 154(1995)201. 6. M. Schneider, A. Baiker, Catal. Today 35 (1997) 339.

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Studies in Surface Science and Catalysis 130 A. Corrna, F.V. Melo, S. Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

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Preparation catalysts

and

characterisation

of

carbon-supported

Pt-CeO2

A. Sepflveda-Escribano, J. Silvestre-Albero, F. Coloma and F. Rodrlguez-Reinoso Departamento de Quimica Inorgfinica. Universidad de Alicante. Apartado 99. E-03080 Alicante, Spain Activated carbon-supported cerium dioxide (CeO2/C) has been prepared by impregnation of the support with a solution of cerium (III) nitrate and further decomposition in helium. Then, platinum has been deposited on CeO2/C by impregnation with solutions containing H2PtC16 or [Pt(NH3)4](NO3)2. Sample characterisation has been carried out by thermogravimetry (TG), temperatureprogrammed decomposition (TPD) and reduction (TPR), X-Ray diffraction and temperature-programmed desorption of adsorbed hydrogen (H2-TPD). The interaction between cerium oxide and platinum after different reduction treatments has been checked in two test reactions carried out in the gas phase: hydrogenation of benzene and hydrogenation of crotonaldehyde (2-butenal). Results show that ceria is very well dispersed on the carbon support, this enhancing its interaction with the active metal after high temperature reduction treatments (773 K). 1. INTRODUCTION The use of cerium oxide as promoter in heterogeneous catalysts is continuously increasing because of its important role in industrial catalytic processes such as three way catalysts (TWC) and fluid catalytic cracking (FCC) [1]. It is also used in several other reactions; for instance, it has been reported that ceria addition to Pd and Rh catalysts favours the formation of oxygenates in the hydrogenation of CO [2-4], and enhances the production of long chain hydrocarbons and alkenes in the same reaction over Ru and Co catalysts [5]. In these applications, ceria is usually spread over a thermally stable support (alumina, silica), given its low textural stability under the reaction conditions at which the catalysts work. This fact, together with its cost, encourages to prepare catalysts with improved ceria performance. The aim of this work is to enhance the effectiveness of ceria in its interaction with platinum through its dispersion over a high surface area and relatively inert support such as activated carbon.

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2. EXPERIMENTAL 2.1.

Catalyst preparation

The support was an industrial activated carbon prepared from olive stones by direct steam activation. It was washed with diluted H2SO4 to remove the main part of the ashes and then with distilled water until no sulphates were detected. The particle size was comprised between 0.5 and I mm. Its BET surface area (N2, 77K) was 1370 m2g 1 and its micropore volume (N2, 77K, Dubinin-Radushkevich equation) was 0.55 cm3g ~. CeO2 supported on the activated carbon was prepared by impregnation of the previously dried support (373 K, overnight) with a solution of Ce(NO3)3-6H20 (99.99%, from Ventron) in acetone. Then, the excess of solvent was removed by flowing N2 until dryness and the sample was kept in a dessicator until use; it will be referred as Ce(NO3)3/C. The nominal CeO2 loading is 20 wt%. The decomposition of the cerium precursor to obtain CeO2/C was accomplished under flowing helium (50 cm3-min -1) at 623 K for 5 h, with a heating rate of 5 Kmin 1. For the preparation of Pt(C1)-CeO2/C a given amount of CeO2/C was impregnated with an ethanolic solution of H2PtC16 of the appropriate concentration as to load I wt% Pt. After 24 h, the slurry so formed was faltered, washed with ethanol and dried with air. A monometallic Pt(C1) catalyst was prepared in a similar way for the sake of comparison. Also, a chlorine free Pt(N)-CeO2/C catalyst was prepared by a similar impregnation of CeO2/C with an aqueous solution of [Pt(NH3)4](NO3)2.

2.2.

Catalyst characterisation

When one intends to prepare ceria dispersed on a ceramic support such as silica or alumina, the decomposition of the cerium precursor is usually carried out by calcination in air. This step can be problematic when a carbonaceous support is used, as it is the case here. Thus, the thermal decomposition in an inert atmosphere of activated carbon-supported cerium (III) nitrate has been investigated. The weight loss during the heat treatment of the fresh impregnated Ce(NO3)3/C sample was determined in a TG-DSC92 system (Setaram). The thermal decomposition was also analysed by mass spectroscopy, by following the evolution of signals corresponding to m/z = 12, 18, 28, 30 and 44 as the temperature was increased at 10 K.min -1. XRD profiles of samples were obtained with a JSO Debye-Flex 2002 system, from Seifert, fitted with a Cu cathode and a Ni filter, using a 2~ -1 scanning rate. Temperature-programmed reduction measurements were carried out in a U-shaped quartz cell, using a 5%H2/He gas flow of 50 cm3"min -1 and about 0.15 g of sample. Hydrogen consumption was followed by on-line mass spectroscopy. Desorption of hydrogen retained in the catalysts after the reduction treatments was accomplished in the same experimental device. The sample was reduced under a hydrogen flow (50 cm3"min -1) at a given temperature for 4 h, with a heating rate of 5 K-min -1. Once room temperature was reached, hydrogen was replaced by helium and, after

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waiting for the hydrogen signal to level out, the sample was heated at 10 K.min -1 up to 1350 K.

2.3.

Catalytic activity

The catalytic behaviour of the samples in the gas phase hydrogenation of benzene and crotonaldehyde were carried out in a microflow reactor at atmospheric pressure under differential conditions, as described previously [6]. Catalysts (about 100 mg) were previously reduced in situ in flowing hydrogen at 573 or 773 K for 4 h, with a heating rate of 5 K.min -1.

3. RESULTS AND DISCUSSION

3.1. Catalyst characterisation The TG profile obtained upon the heat treatment in helium of the freshly prepared Ce(NO3)3/C sample shows several temperature ranges with different slopes. The first weight loss stage on Ce(NO3)3/C, from room temperature up to about 440 K, can be ascribed to the evolution of hydration water from the cerium precursor. The sharpest weight loss takes place from 440 K to 460 K, and corresponds to the evolution of mainly NO, as deduced from mass spectroscopy analysis. N20 is also produced at this step, but it is the main product evolved between 460 and 550 K. It is likely produced from the reaction between NO and the carbon support. From 550 K up, there is still a slow weight loss that continues up to 1025 K, which is related to the gasification of the carbon support through the evolution of carbon monoxide. Following these results, it can be concluded that the decomposition of the cerium precursor (nitrate) finishes at about 550 K. The nature of the cerium species formed upon the heat treatment in helium of the carbon supported cerium nitrate has been assessed by temperatureprogrammed reduction (TPR). A freshly prepared sample was treated in helium at 573 K for 4 h; after cooling, its TPR profile was obtained. The decomposition treatment was applied to another fresh sample which, after cooling, was exposed to air overnight at room temperature and, then, its TPR profile was obtained. Both profiles were coincident; furthermore, XRD analysis of the air exposed sample showed peaks corresponding to the fluorite structure of CeO2. Thus, it has been evidenced that thermal treatment of activated carbon supported cerium (III) nitrate in an inert atmosphere already produces supported cerium (IV) oxide. After these previous results, the CeO2/C sample was prepared by thermal treatment of Ce(NO3)3/C at 623 K for 5 h in flowing helium. Figure 1 compares the TPR profiles corresponding to samples CeO2/C and Pt(C1)-CeO2/C. It has to be noted that the TPR profile of bulk CeO2 usually includes two peaks, centred at about 850 and 1200 K, which correspond to the surface reduction and to the bulk reduction of ceria, respectively [6]. Indeed, the ratio between the amount of hydrogen consumed in both peaks has been used by some authors to estimate the surface area of ceria [7].

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1016

I (a.u.) 800 i

mmol/min/g "-~ CeOEC

0.04

Pt-CeOz/C 0.02

600 _ m

400 -0.02 200

-0.04

-0.06

300

i

1

500 700 900 1100 1300 Temperature (K)

Figure 1. TPR curves for CeO2/C and Pt(C1)-CeO2/C.

0 25

35

45 2el ~

55

65

Figure 2. XRD profiles of samples CeO2/C (a) and Pt-CeO2/C reduced at 623 K (b), 773 K (c) and 973 K (d).

The TPR profile of CeO2/C shows only one band centred at 800 K, with a small shoulder at about 900 K. This indicates the high surface area of ceria obtained, in such a way that it is completely reduced to Ce203 at temperatures corresponding to the surface reduction of pure ceria. The TPR corresponding to Pt(C1)-CeO2/C shows a first sharp peak centred at 500 K and a broad band centred at 800 K. The first one is assigned to the surface reduction of ceria in close contact with platinum, with a contribution due to the reduction of the metal precursor. The band at higher temperature represents a hydrogen consumption higher than that obtained in the Pt-free sample, and seems to be formed from the overlapping of several contributions. The reduction of ceria more or less close to the metal particles and hydrogen spillover to the carbon support can account for this band. A strong hydrogen evolution is observed at temperatures higher t h a n 1000 K, which is related to the decomposition of functionalities in the surface of the carbon support. Figure 2 compares the XRD profiles of samples CeO2/C and Pt(C1)-CeO2/C, the latter reduced at 623, 773 K and 973 K. The high degree of ceria dispersion over the carbon support is evident from the broad peaks in the XRD spectrum of CeO2/C. A mean crystallite size obtained from the (111) line broadening of ceria gave values of 5.4 nm for CeO2/C, and 4.2 nm, 8.3 nm and 10.1 nm for Pt(C1)CeO2/C after reduction at 623, 773 and 973 K, respectively. No peaks corresponding to platinum can be observed in the XRD spectra of Pt(C1)-CeO2/C, this being indicative of the presence of very small metal particles.

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0.025

1017 mmollminlg

80

0.02

60

a)

0.015

Y

0.01 0.005 0

300

X CROALC (%)

/ I

f

i

500 700 900 Temperature (K)

40

20

v 1100

Figure 3. H2-TPD spectra of Pt(C1)CeO2/C reduced at 573 K (a) and 773 K (b), and Pt(C1)/C reduced al 573 K

(c).

I

0

20

I

I

40 60 ]]me (min)

i

80

100

Figure 4. Selectivity to Crotyl alcohol of selected samples as a function of time-on-reaction at 333K.

The H2-TPD spectra shown in Figure 3 show the effects of the presence of ceria and the reduction temperature. A much higher platinum dispersion is obtained in the ceria containing catalyst, as evidenced by the H2 desorption peak at 380-404 K, which is hardly detectable for Pt(C1)/C. The amount of hydrogen retained by the support is also higher in the ceria containing catalyst, and increases with the reduction temperature.

3.2. Catalytic activity Catalytic activity data for benzene hydrogenation at 373 K are reported in Table 1 for two selected catalysts as a function of the reduction temperature. After reduction at 573 K, when no SMSI effect is expected, activity is much higher for the ceria containing catalyst, which is related to the lower size of the platinum particles. Activity decreases for both catalysts after reduction at 773 K. The decrease for Pt(C1)/C can be related to some degree of metal sintering; however, the strong Pt-ceria interaction induced upon the reduction treatment at this high temperature mainly accounts for the decrease in activity of Pt(C1)CeO2/C. This SMSI effect can be better evidenced by the change in selectivity obtained in the selective hydrogenation of crotonaldehyde [6]. The surface reduction of ceria creates sites at the neighbourhood of the platinum particles which are able to activate the carbonyl bond, this facilitating its hydrogenation over that of the C=C bond. Figure 4 shows the selectivity to crotyl alcohol of

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selected catalysts as a function of time-on-stream at 333 K. It can be seen that the selectivity towards the hydrogenation of the carbonyl bond increases drastically after reduction at 773 K in the ceria containing catalysts, irrespective of the platinum precursor used, yielding up to 75 % crotyl alcohol in the first stages of the reaction. Similar values have been obtained with a Pt/Ce02 catalyst [6]. This selectivitydecreases quickly under the reaction conditions used, maybe due to the re-oxidation of the ceria surface and/or the coverage of active sites by reaction products. However, it is completely recovered after re-reduction under hydrogen. Table 1. Benzene hydrogenation at 373 K. Catalyst

Reduction temperature (K)

Pt(C1)/C

573 773 573 773

Pt(C1)-CeO2/C

Activity (~mol/s-gPt) t = 5 min t = 90 rain 260 113 170 130 1500 270 470 24

4. CONCLUSIONS Cerium dioxide highly dispersed on activated carbon has been prepared from cerium (III) nitrate. Its small particle size, favoured by the high surface area of the carbon support, enhances the interaction with platinum after high temperature reduction (773 K), its intensity becoming comparable to that obtained in platinum supported on bulk CeO2. However, a lower amount of ceria is used and the textural stability is also improved.

REFERENCES

1. A. Trovarelli, C. Leitenburg, M. Boaro and G. Dolcetti, Catal. Today, 50 (1999) 353. 2. C. Diagne, H. Idris, J.P. Hindermann and A. Kienneman, Appl. Catal., 51 (1989) 165. 3. J.C. Lavalley, J. Saussey, J. Lamotte, R. Breault, J.P. Hindermann and A. Kiennemann, J. Phys. Chem., 94 (1990) 5941. 4. M.L. Turner, P.K. Byers and P.M. Maitlis, Catal. Lett., 26 (1994) 55. 5. A. Guerrero-Ruiz, A. Sepfilveda-Escribano and I. Rodrlguez-Ramos, Appl. Catal. A: Gen., 120 (1994) 71. 6. A. Sepfilveda-Escribano, F. Coloma and F. Rodrriguez-Reinoso, J. Catal., 178 (1998) 649. 7. M.F.L. Johnson and J. Mooi, J. Catal., 103 (1987) 502.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Highly dispersed Pd based catalysts for selective hydrogenation reactions M. Delage, B. Didillon, Y. Huiban, J. Lynch, D. Uzio IFP, 1-4 Av Bois Pr6au, 92852 Rueil Malmaison cedex France A new method of preparation of Pd supported catalysts is presented allowing to prevent the electron deficient character of highly dispersed metallic particles. Starting with a stabilised aqueous soluble Pd complex and under well-chosen activation procedure, ultradispersed Pd alumina supported particles obtained exhibit similar diene hydrogenation activity expressed by surface exposed atoms than bigger particles obtained by classical dried impregnation methods. Characterisation techniques (EXAFS, XPS, TPR, TEM) clearly evidence the non electro deficient character of these small particles. Catalytic performances (orthoxylene and butadiene hydrogenation) are discussed in relation with characterisation results.

1. INTRODUCTION Supported Pd catalysts are known to be amongst the more active and the most selective for the purification of feedstocks containing highly unsaturated compounds like alkynes or alkadiene. These reactions are usually included in the family of structure sensitive reactions so that average particle size of such catalysts is carefully adjusted by varying with the preparation parameters in order to achieve the particle dimension corresponding to optimum activity and selectivity [1]. Nevertheless, such sizes (30-50 A) lead to very low accessible metal atom fraction, the major part of the metal being located in the bulk and, as a consequence, catalytically inactive. With this in mind, it appears of great interest to increase the accessibility of palladium particles by preparing highly dispersed catalysts. The main difficulty consists in retaining the metallic character of palladium atoms forming the small particles, one of the key conditions for maintaining high catalytic activity. Indeed, highly dispersed metallic particles, under an unstable thermodynamic state, are generally obtained from metallic species stabilised via strong interaction with the support. This interaction lead to electron deficient atoms as demonstrated in theoretical and experimental studies [2-6]. As a consequence, the catalytic activity will be perturbed because of the changes in adsorption strength of reactants and products. In this paper, we present a method of preparation which enables very small particles and high catalytic performances for alkadiene hydrogenation to be obtained simultaneously. This starting point is the utilisation of a Pd complex prepared from Pd nitrate and sodium nitrite solution. Catalysts have been characterised in terms of accessibility, reducibility and electronic properties using physical techniques (EXAFS, XPS, TEM, sorption, and reducibility measurements) as well as by catalytic test (orthoxylene and butadiene hydrogenation) in comparison with catalysts prepared from more conventional methods.

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2. EXPERIMENTAL

2.1 Preparation of the catalysts Two types of preparation method have been employed: the first uses the well-known wet impregnation techniques of an alumina support with solutions of palladium bisacetylacetonate [7]. The desired amount of Pd(C5H702)2 is dissolved in 300 ml of toluene and added to 20 grams of A1203 for 72 h at room temperature under stirring. The mixture is then filtered and washed 3 times with 100 ml of distilled water. After drying at 393K during 12h, the catalyst is calcined in an air flow at 623~ during 2 h. This catalyst is referred to here as Pd[acac]. For the second catalyst, a volume of an aqueous solution containing a mineral Pd precursor corresponding exactly to the porous volume of the support is impregnated using a rotating beaker. After drying at 393K, the catalysts is calcined under air flow. This catalyst is referred to here as Pd[X]. A third catalyst (referred as Pd[NO3]) has been prepared starting from a palladium nitrate solution impregnated using the same procedure as Pd[X]. After drying overnight at 393K, the sample has been calcined in air flow. For all the catalysts, the palladium loading was fixed at 0.3% wt and the support used was a 7A1203 with a specific surface area of 130 mE/g and a porous volume of 1.04 cc/g.

2.2 Characterisation The initial aqueous solutions were characterised by UV-visible spectrophotometry PerkinElmer. Lambda 11. Spectra are recorded in the 200-700 nm wavelength range. Temperature Programmed Reduction measurements were carried out using a K-Sorb| apparatus. Samples (circa 2 g) are submitted to hydrogen flow (5% H2 with At) whilst raising the temperature from 298 to 773K at a heating rate of 5K.mn -1. CO chemisorption were performed using a dynamic method: several pulses of a precise volume of CO are injected into the reactor until no consumption is observed by gaschromatography. Samples are previously reduced under hydrogen flow at temperatures selected on the basis of TPR results. From assumptions on the adsorbate/Pd stoechiometry, particle size and accessibility are obtained. The particle size was determined by transmission electron microscopy (TEM) using a JEOL2010 microscope working at 200keV. The Pd catalysts were crushed in ethanol and deposited on a perforated carbon film supported by a Cu grid. EXAFS experiments were carried out in the Laboratoire pour l'Utilisation de Rayonnement Electromagn6tique (Orsay) at the EXAFS4 beam line using synchrotron radiation from the DCI storage ring running at 1.85 GeV with an average current of 250 mA. X-ray absorption spectra were obtained in the transmission mode through a double crystal monochromator (Ge(400)) using two ion chambers as detectors. The samples were reduced in situ using a temperature ramp (298-623 K, 5FUmin). After reduction the specimens were cooled to room temperature and full X ray absorption spectra (24250 - 25000 eV acquisition time about 40 min) were acquired. The Pd K-edge region of the spectra was analysed using a standard data analysis package (SIMPLEX software from ref [8]). Amplitude and phase functions for fitting Pd-O and Pd-Pd shells were obtained respectively from Pd[acac] and Pd metal foil references. XPS measurements were carried out at the Institut de Recherches sur la Catalyse (Lyon) and performed with a VG Escalab 220 spectrometer using A1 Kct radiation at 1486.6 eV. The

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pass energy of electron was 50 eV with a triple Channeltron analyser. Samples were analysed under oxidised and reduced form.

2.3 Catalytic evaluation Orthoxylene hydrogenation tests were carried out in a quartz reactor at atmospheric pressure. The reactor is operated isothermally in dynamic mode with a concentration gradient. 2 g of powdered catalyst are contacted with the feed (10% vol orthoxylene in nC7) at 373K, HE/oXylene=2.3 l/l, oXylene-30 ml/h, atmospheric pressure. Buta-1.3-diene hydrogenation tests were performed in liquid phase using a laboratory scale batch stainless steel reactor working under static conditions with variation of the concentration of reactants and products over time. 2g of reduced catalyst is contacted with buta-l,3-diene diluted in n-C7 at 10 bar, 290K, under high stirring velocity. For both catalytic tests, experimental conditions were previously selected in order to avoid mass transfer limitations. Liquid and gaseous effluents are analysed by GC analysis. 3. RESULTS AND DISCUSSION 3.1 Characterisation of the catalysts In the case of Pd[X], and Pd[acac], the fraction of metal exposed determined by chemisorption is near 100% in agreement with TEM analysis showing particle size below 7A (limit of detection). The Pd[NO3] catalyst exhibits a much lower dispersion corresponding to a 35A average particle size. Temperature Programmed Reduction profiles of the different samples are presented fig. 1. The TPR profile of the Pd[X] catalyst only exhibits one very narrow peak at 430K. Ha consumptions are very high in the case of Pd[X] and Pd[NO3] due to the co-reduction of nitrite and nitrate with palladium atoms present in their neighbourhood. TPR of the Pd[acac] presents clearly two reduction peaks around 393 and 623K ascribed respectively to palladium species in low and high interaction with the support. In the case of Pd[NO3], the profile is more heterogeneous indicating probably different types of Pal-support interactions. ]:12 consumption (ua) _

-

Pd[acac]

.

0

100

.

.

200

.

.

edIX]

300 400 500 Temperature (~ Figure 1" TPR profiles for the Pd[X], Pd[NO3], and Pd[acac] catalysts.

,

600

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3.2 EXAFS and XPS measurements

Aider reduction, the sample Pd[X] shows an EXAFS spectrum very similar to Pd metal foil, but of reduced intensity. This is illustrated in figure 2-a, showing the corresponding RDF functions. Modelling of the filtered data (inverse Fourier transform window: 1.5 to 3.0A) allows the average number of first nearest neighbours to be estimated as 5 +/-1. This coordination number is consistent with the existence of three-dimensional particles less than one nanometer in diameter. The RDF of the Pd[acac] sample is compared with that of Pd[X] in figure 2-b. The Fourier transformed data clearly shows, in addition to Pd-Pd neighbours, a peak at low distances (less than 1.5A uncorrected for phase shift) for Pd[acac]. Modelling of the filtered data (inverse Fourier transform window: 0.95 to 3.0A) reveals this peak to be due to low atomic number neighbours, probably oxygen atoms. On the assumption that these neighbours are oxygen atoms, the average Pd-O co-ordination number is three at a distance of 1.95A. Pd-Pd co-ordination is similar so that particle size can be considered practically unchanged. The X-ray absorption results for the Pd[X] sample show the palladium in reduced samples to be in an environment close to that of bulk metallic palladium. Although the Palladium palladium co-ordination number is reduced due to the small particle size, no evidence of a well-defined particle-support interaction is observed. It is clear on the contrary that the palladium in the Pd[acac] sample is in stronger interaction with the support. This is shown by the higher temperatures required for the white line intensity to reach that of Pd metal and by the existence of Pd-O bonds in the reduced sample. The EXAFS result averages over all environments of palladium atoms in the sample. The Pd-O bonds are probably located at the particle/support interface and palladium atoms at the interface will have a number of oxygen neighbours larger than the average of three. Despite the fact that EXAFS reveals the presence of oxygen neighbours, the edge spectra show the Pd[acac] sample to be reduced after the treatment at 623K. As pointed out above, the Pd K edge is due to transitions from a deep electronic level. Although the white line allows an easy distinction between Pd n and Pd ~ the edge is likely to be insensitive to the detailed electronic properties of the palladium atoms. FFT kax(k)

0,45

I 5-a

I 5-b

0,15

I

0,40

I

/~

Pd metal

0,35 0,30

,

/

~

Pd[acac]

0,10

0,25

Pd[X]

0,20 0,15 0,10

,

0,05 0,00

..

i/'\ ,

0

1

2

3

4

5

0,00 0

.

.

i

1

.

.

.

J

2

.

.

.

r(A)

i

3

.

.

.

i

4

.

.

,

i

5

r(A) Figure 2" a) RDF of the Pd[X] catalyst compared to that of a Pd metal foil. b) Comparison of RDFs of the Pd[X] and Pd[acac] catalysts. XPS results are presented in table 1. Binding energy measured by XPS of the reduced Pd[X] and Pd[acac] are in good agreement with literature data [2] for such particle size. The similarity of the binding energy is not in contradiction with a lower metal-support interaction

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for the Pd[X] since the shift of metal core level to higher energies is not necessary due to electron transfer but to an "intrinsic property of the metal particle" [9]. For the oxidised Pd[X], the E13ds/2 value is very high and closed to the PdO2 binding energy (338 eV) [10]. This result can be interpreted as follow: the low calcination temperature treatment does not decompose the Pd complex. This decomposition occurs during the reduction step, with high H2 consumption as observed during TPR (high H2/Pd ratio), leading to formation of Pd ~ particles (decrease of the % atomic Pd). Table 1 XPS results

Pd[X] Pd[acac] Pd[NO3]

Binding energy Pd 3d5/2eV + 0.2 Oxidised Reduced 338.1 335.6 335.8 335.7 336.8 335.1

% atomic Pd Oxidised Reduced 0.08 0.05 0.09 0.07 0.11 0.10

3.3 Catalytic properties Table 2 summarises the catalytic data for orthoxylene hydrogenation. This reaction is reported in literature as an indicator of electronic properties of supported metal catalysts. Indeed, the hydrogenation of orthoxylene lead to isomers cis and trans 1.2 dimethyl cyclohexane (DMCH) which are favoured respectively by high and low electron density of the metallic centre [11-12]. In our conditions, it has been checked that the trans isomer selectivity is not dependent on the conversion so that the discrepancy between the catalysts cannot be related to a kinetic effect. The data obtained are consistent with other characterisation methods if one considers the accepted interpretation: the Pd particles of Pd[acac] which have the lowest electron density lead to the highest trans-selectivity. The particles of Pd[X] have the same dimensions but lead to a strong decrease of the trans selectivity more closed to that obtained for Pd[NO3] catalysts, in agreement with a high electron density due to low metal/support interaction (no Pd-O bounds) as defined by the Schwab effect [ 13]. Concerning buta-l.3-diene hydrogenation, Pd[X] exhibits the same TOF than Pd[NO3] giving a particularly high activity expressed per unit weight of metal. Table 2 Catalysis results for orthoxylene and buta-1.3-diene hydrogenation. . Pd[NO3] Pd[X] Activity (mole.minl.~g Pd l) orthoxylene hyd. 2.09 1.45 Selectivity for trans DMCH % 46.4 46.9 Activity (mole.mnl.g Pd 1) buta-l.3-diene hyd. 7.6 14.6 TOF (mole.mnl.g at Pdsl) 1684 1547 Selectivity But- 1-ene- 1 for Buta- 1.3-diene conversion=80% 61 55

Pd[acac] 1.41 69.4 1.2 127 60

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4. CONCLUSION

As a conclusion, this work shows how it is possible to tune the metal-support interaction of Pd particles deposited on alumina controlling the synthesis parameters. The strong complexation of the nitrite ligand prevents the hydrolysis of the complex by alumina surface hydroxyl group, and low temperature treatments avoid the formation of anchored particles to the support. Nitrite complexes allows to produce hyperdispersed palladium particles (size below 7A) without the electron deficient character usually observed for supported particles of such a size. Moreover, it has been shown that these small particles may have the same activity for diene hydrogenation expressed by surface atoms than bigger ones. The well-known structure sensitive reaction phenomenon observed for metallic particles of different sizes is attributed here to a metal/support imeraction sensitivity, which governs the adsorptiondesorption constant of reactant and products.

Authors thanks P. Delich6re from the Institut de Recherches sur la Catalyse, Villeurbanne (France) for XPS analysis and fruitful discussion.

REFERENCES

1 JP. Boitiaux, J. Cosyns., E. Robert., Appl. Catal. 35 (1987) 193-209. 2 Z. Karpinski, Adv. Catal., 37, 45-100. 3 D.C. Koningsberger and B.C. Gates Catalysis Letters 14 (1992) 271-277. 4 R. Bouwman, P. Biloen, J. Catal., 48 (1977) 209. 5 J.R. Katzer, G.C.A. Smith, J.H.C. van Hoof, J. Catal., 59,(1979) 278. 6 J.C. Bertolini, P. Delich6re, B.C. Khandra, J. Massardier, C. Noupa and B. Tardy, Catalysis Letters 6 (1990), 215-224. 7 JP. Boitiaux, J. Cosyns., M. Derrien., AIChE National. Meeting., Houston, March 24-28, 1984. 8 A. Michalowicz, "M6thodes et Programmes d'Analyse des Spectres d'Absorption des Rayons X", PhD Thesis -Universit6 Paris, Val de Marne, 1990). 9 A. Masson, B. Bellamy Y. Hadj. Romdhane, M. Che, H. Roulet. Surf. Sci 173 (1986). 10 Kim KS, Gossmann AF, Winograd N, Analytical Chemistry Vo146(2) (1974) 197-200. 11 Gomez R, Novaro O., React. Kinet. Catal. Lett., 45 (2) (1991) 251-256. 12 M. Viniegra, G. Cordoba, M. Gomez, J. Mol. Catal., 58 (1990) 107. 13 Schwab G.M., Block J., Schultze D. Angew. Chem. 71 (1958) 101.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Preparation of new type of supported Sn-Pt bimetallic catalysts containing Lewis acid sites anchored to the platinum J. L. Margitfalvi, I. Borb/lth, M. Heged0s, S. G~bOl6s, A. Tempos and F. L6nyi Institute of Chemistry, Chemical Research Center, Hungarian Academy of Sciences, 1525 Budapest, POB. 17, Hungary Methods of Surface Organometallic Chemistry (SOC) were used to prepare new type of supported Sn-Pt catalysts. In the new approach the focus was laid on the stabilization of tin in the form of SnOx anchored directly to the platinum. The new surface species have been characterized by different methods. The results indicate the platinum nano-clusters are decorated by SnO• surface entities with Lewis-acid character. The new Sn-Pt/AI203 catalysts were tested in hydrocarbon reactions above 500 ~ Due to the presence of new acid sites in the atomic closeness to the platinum the new Sn-Pt catalysts showed decreased aromatization and increased isomerization selectivities. 1. INTRODUCTION Tin modified supported bimetallic catalysts have been widely used both in the hydrocarbon technologies [1,2] and in the production of fine chemicals [3]. These catalysts are prepared using different methods [1-3], however the exclusive formation of well-designed surface entities can only be achieved by using Controlled Surface Reactions (CSRs) [3-9]. The combination of CSRs with methods of Surface Organometallic Chemistry (SOC) appeared to be a very powerful tool to prepare Sn-Pt, Sn-Rh bimetallic catalysts with metalmetal interaction [6,7]. In these catalysts the formation of alloy-type nano-clusters takes place in a hydrogen atmosphere above 200 ~ In this contribution we shall report on the use of CSRs and SOC to prepare new type of supported platinum catalysts in which Lewis-acid type surface entities are anchored to supported platinum nano-clusters.

1.1. Surface chemistry CSRs between Sn(C2Hs)4 and adsorbed hydrogen has been first described in 1984 [4]. This reaction provided direct tin-platinum interaction what was maintained upon decomposition of the formed primary surface complex in a hydrogen atmosphere [5,7]. This approach led to the monolayer coverage of platinum by tin organometallic species [7]. Recently we have demonstrated that upon modification of a Pt/SiO2 catalysts with Sn(C2H5)4 high Sn/Pts ratios (Sn/Pts=3) can be obtained without introduction of tin into the support [9]. The new results indicated the formation of multilayer organometallic complexes (MLOC) at the Pt surface. In the formation of MLOC the following reactions are involved [9]: PtHads. Pt-SnR3 Pt-SnR(3.x)

+

+ +

SnR4 x Ha n SnR4

.... > Pt-SnR 3 + RH .... > Pt-SnR(3.x) + x RH .... > Pt-{ SnR(3.x)-(SnR4)n}

(1) (2)

(3)

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Pt-{ SnR(3.x)-(SnR4)n} + n x y Ha .... > Pt-{SnR(3.x)-(SnR(4.y))n} + ny RH (4) Pt-{ SnR(3.x)-(SnR(4.y)),l} + m SnR4 .... > Pt-{ SnR(3.x)-(SnR(4.y))n-(SnR4)m} (5) MLOC Reaction (1) and (2) describe the formation of coordinatively saturated and unsaturated primary surface complexes (PSC), respectively. The coordinatively unsaturated PSC interacts with SnR 4 used in large excess (reaction (3)) resulting in surface species in the second layer (SSSL). Similar surface species have been suggested when supported rhodium was modified with tin tetrabutyl [6]. In the presence of excess hydrogen the SSSL is partially hydrogenolyzed resulting in coordinatively unsaturated species in the second layer (reaction (4)), which also interacts with SnR4 (reaction (5)). The net result is the formation of slab-like MLOC on the Pt surface. Reactions (1) - (5), which take place in the presence of a solvent, are referred as tin anchoring reactions. The decomposition of MLOC is accomplished in a gas-solid reaction. The decomposition in a hydrogen atmosphere leads to the formation of alloy-type bimetallic surface entities [6,7]. Recent M6ssbauer results conformed the formation of Sn-Pt alloys even at Sn/Pts = 2.5 [ 10]. However, the decomposition of MLOC can also be carried out in the presence of oxygen. The aim of this study is to demonstrate that the decomposition of MLOC in the presence of oxygen results in ionic type of surface species containing SnOx entities anchored to the platinum. Further goal of this study was the characterization of the new bimetallic catalysts and their test in hydrocarbon reactions. 2. EXPERIMENTAL 2.1. Catalysts and materials The Pt/SiO2 catalyst (3 wt % Pt, Incat Pt-1, provided by Boreskov Institute of Catalysis) has been used in our earlier studies to obtain Sn-Pt/SiO2 catalysts with high Sn/Pts ratios [9]. The 0.3 wt % Pt/AI203 catalyst was prepared by impregnation of alumina by H2PtCI6 in 1 M HCI. The support surface area (S = 135 m2/g), total pore volume (Vp= 0.66 cm3/g) and average pore radius (R = 8.4 nm) was determined by N2 adsorption and mercury porosimetry, respectively. The catalyst was reduced under hydrogen for 4 hours at 500 ~

2.2. Catalyst modification The catalysts were re-reduced in hydrogen prior to the tin anchoring step at 400 ~ for 4 hours. The tin anchoring process was carried out in a hydrogen atmosphere at 40-65 ~ for 1.5 - 6 hours in benzene. The decomposition of surface organometallic complexes in a hydrogen atmosphere was carried out as described elsewhere [9], while the decomposition in the presence of oxygen was accomplished using 10 vol. % oxygen in nitrogen. In both cases temperature programmed reaction (TPR) technique with 5 ~ heating rate was used. The tin content of the catalysts was determined by AAS. 2.2. Catalyst characterization ASDI RXM 100 equipment (Advanced Scientific Designs, Inc.) was used to determine the H/Pt and CO/Pt ratios. The adsorption isotherms were always corrected for the amount of physisorbed hydrogen and CO, respectively. FTIR spectra of adsorbed pyridine and CO were measured on Nicolet 400 spectrometer.

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2.4. Hydrocarbon reactions The transformation of n-hexane was carried out under atmospheric pressure at 510 ~ in a continuous-flow reactor operated in a periodic mode [4]. Continuous-flow high pressure isotherm reactor was used to test the catalysts in n-octane conversion.

3. RESULTS AND DISCUSSION 3.1. Study of the tin anchoring reaction Upon modification of the 3 % Pt/SiO2 catalyst 95 - 100 % of tin tetraethyl reacted was anchored to the platinum when the tin anchoring reactions were carried out at 50 ~ and the concentration of tin tetraethyl is lower than 2.5 x 10-] M [9]. The modification of Pt/AI203 catalyst by tin tetraethyl resulted in more complex surface chemistry. Parallel to reactions (1)(5) surface reaction (6) with the involvement of the support OH groups takes also place. Consequnetly, on alumina supported Pt the formed surface organometallic species (SOMS) contains MLOC and (I). [-OH + Sn(C2Hs)4 ..... --) I-O-Sn(C2Hs)3 + C2H6 (6)

(I)

In the presence of hydrogen the decomposition of surface species formed in reactions (1) (5) gives ethane. However, the decomposition of (I) formed in reaction (6) gives ethane and ethylene in 1:1 ratio. The decomposition of SOMS formed upon modification of Pt/AI203 catalyst with Sn(C2H5)4 is given in Figure 1. This figure shows the formation of ethane and ethylene in the presence of hydrogen using TPR technique. 50

40

A

~ ' 40

x 30

x

ol

01 30

o

m

E 20

o

E 20

I=..i

c LU

10 o

B

r--i

tD C

_.~ lo

;,.,,cA 0

100

200

Temperature [*C]

era

300

0

~- .

0

.

.

.

.

.

100

!

II

200

I J%.J

Temperature [*C]

300

Figure 1. Decomposition of SOMS formed during tin anchoring. A-formation of ethane, B formation of ethylene. Pt/AI203 catalyst; temperature of tin anchoring, ~ 9O - 40, !"7 _ 50, x - 55, 9 - 65; [Sn(C2Hs)4]o = 5 x 102 M; amount of tin anchored (Snanch/Pts): O - 1.4, 1"7 - 2.3, x - 3.2, 9 - 4.3. Results given in Figure 1 indicate that the introduction of tin to platinum is exclusive when the tin anchoring reaction has been carried out at 40 ~ The formation of ethylene indicates that the increase of the temperature of tin anchoring leads to partial introduction of tin into the alumina. These results show that the modification of Pt/AI203 catalysts with tin tetraethyl results in Sn-Pt bimetallic catalysts in which two forms of tin can be distinguished: (i) tin anchored to the platinum and (ii) tin anchored to the alumina. The formation of tin anchored

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to the platinum is exclusive up to Snanch/Pts=l.4-1.5. The decomposition of SOMS in the presence of oxygen resulted in mostly CO2, although trace of CO, ethane and ethylene was also detected. Catalysts prepared by decompostion in oxygen will be denoted as Sn-Pt (O), while catalysts prepared in hydrogen as Sn-Pt (H).

3.2. Characterization of catalysts prepared 3.2.1. Chemisorption results Chemisorption results obtained on tin modified samples are summarized in Table 1. Upon increasing the Sn/Pts ratio the amount of hydrogen and CO chemisorbed decreases. The decrease is more pronounced on (O) than on (H) type Sn-Pt catalysts. On both type of Sn-Pt catalysts the decrease of hydrogen chemisorption induced by tin is more pronounced than that of the CO (compare the B/A ratios in table 1). These results show that the introduction of tin strongly decreases the number of sites involved in the activation of dihydrogen. Table 1. Chemisorption properties of Sn-Pt203 catalysts prepared by decomposition of SOMS.

Catalysts

Sn/Pts*

H/Pt**

CO/Pt**

B/A

0.6 0.8 1.7 1.4 2.3 4.3

0.337 n.a. (0.225) 0.162 (0.129) 0.105 (0.000) 0.341 0.212 (0.174) n.a. (0.122) O. 169 (0.086)

0.361 n.a. (0.278) 0.245 (0.21 O) 0.168(0.157) 0.703 9 0.509 (0.439) n.a. (0.354) 0.486 (0.273)

1.07 n.a. (1.24)

Pt/SiO2 Sn-Pt/SiO2 Sn-Pt/SiO2 Sn-Pt/SiO2 Pt/AI203 Sn-Pt/AI203 Sn-Pt/AI203 Sn-Pt/AI203

(B)

(A)

1.60(oo) 2.06 2.40 (2.52) n.a. (2.90) 2.88 (3.17)

* calculated from AAS analysis and the overall material balance of tin anchoring, ** the first number: catalysts type (H), in parenthesis: catalysts type (O). 2075 'E .ca. 2070 9

E

+

2065

9 2060

2055 0

t

I

I

50

1O0

150

200

Temperature [~

Figure 2. FTIR spectra of chemisorbed CO on Pt/A1203 ( + ) and Sn-Pt/A1203 catalysts (D (H) type; 9 - (O) type), Sn/Pts= 2.6 (see catalyst Sn-Pt2 in tables 2 and 3). The CO chemisorption on Sn-Pt/A1203 catalysts was also followed by FTIR as shown in Figure 2. As emerges from these results in catalyst type (H) the introduction of tin has no influence on the CO vibrational frequencies. Contrary to that on catalyst type (O) the CO

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vibrational frequencies are significantly changed, probably due to the electronic interaction between platinum and anchored SnOx moieties. The pyridine adsorption on catalyst type (H) and (O) showed also a distinct difference. The band at 1625 cm "j appeared exclusively in (O) type Sn-Pt catalysts. This band was attributed to new type of Lewis-acid sites originated from tin [ 11 ]. The new Lewis-acid sites should be formed in the close vicinity of platinum.

3.3. Catalytic tests Some of the prepared Sn-Pt/AI203 catalysts were tested in the dehydrocyclization of nhexane and n-octane. These results are summarized in Table 2 and 3. Preliminary results indicated that catalysts with identical Sn/Pt ratio resulted in more pronounced suppression of the formation of benzene when the decomposition of SOMS was carried out in oxygen ((O)type Sn-Pt catalysts). For this reason in tables 2 and 3 results obtained on (O) type Sn-Pt catalysts are included.

Table 2. Summary of results obtained in n-hexane reaction under atmospheric pressure. Catalysts

Sn/Pt

Selectivities, %*

Initial rate, mol/sec goat

Benzene MCP iso - C6 ~'~' CI-s Pt/AI203 0.0 0.109 50.3 30.5 3.2 11.1 Sn-Ptl 1.8 0.102 41.3 32.0 3.7 18.4 Sn-Pt2 2.6 0.075 37.5 42.8 3.7 9.1 Sn-Pt3 4.3 0.042 30.2 50.7 3.7 6.8 * measured around 60 % conversion; ** 2-methyl pentane + 3-methyl pentane and 2,2dimethyl butane, MCP = methyl cyclopentane. Reaction conditions: T = 510 ~ Ptotal = 1 bar, Pn-c6 = 46 tort, catalyst (O-type), amount: 0.3 g.

Table 3. Summary of results obtained in n-octane reaction at high pressure. Selectivities, % Catalysts

Sn/Pt

Conversion, %

C8 aromatics

Pt/A1203 Sn-Ptl Sn-Pt2

0.0 1.8 2.6

82.7 82.6 78.2

Reaction conditions T = 510 ~ PH2 amount: 2.0 g. time on stream: 5 h.

43.8 25.4 21.7

=

Total aromatics

C8 isoparaffines and naphtenes

C1-5 hydrocarbons

49.0 31.6

45.6 62.9

3.1 3.6

25.5

67.7 3.2 7 bar, H/CH = 2.2, LHSV = 2.0, catalyst (O)-type,

Results obtained in n-hexane reaction at atmospheric pressure are shown in Table 2. As emerges from table 2 the introduction of tin slightly decreases both the initial rate and the selectivity of benzene formation. The loss of benzene formation is compensated by the increase of the selectivity of C6 isomers. The high-pressure results given in table 3 show similar trend, i.e. the introduction of tin decreases significantly the formation of aromatic hydrocarbons from n-octane. However,

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under this experimental condition the activities of tin modified catalysts (with Sn/Pts = 1.8 and 2.6) were very close to the activity of Pt/Al203 catalyst (see table 3, conversion values).

CONCLUSION Results obtained in this study indicated that the decomposition of tin containing SOMS in the presence of oxygen leads to the formation of a new catalytic system in which supported platinum is covered by tin in ionic form. In this new type of Sn-Pt/AI203 catalyst the metallic and Lewis-acid type active sites are in atomic distance from each other and the atomic closeness provides unique properties to these catalysts in hydrocarbon reactions.

Acknowledgement The authors acknowledge the research grant from OTKA (Grant No 23322).

REFERENCES Z. Pa6.1 in Catalytic Naphtha Reforming, G. J. Antos et al. (eds.), p. 19. Marcel Dekker, New York 1995. 2. L. Guczi and A. Sarkany: Selectivity of bimetallic Catalysts. Structure and Reactivity. J.J. Spivey and S.K. Catalysis, Special Periodical Report, Greval (eds.), Vol. 11, Chapter 8, p. 318. Royal Society of Chemistry, London 1994. 3. B. Didillon, A. El. Mansour, J.P. Candy, J. P. Bournonville and J. M. Basset: in Heterogeneous Catalysis and Fine Chemicals 11, Stud. Surf. Sci. Catal., M. Guisnet et al. (eds.), Vol.59, p. 137. Elsevier, Amsterdam 1991. 4. J. Margitfalvi, M. Hegedfis, S. G6b616s, E. Kern-Thlas, P. Szedlacsek, S. Szab6, F. Nagy: in Proc. 8 th Int. Congress on Catalysis, Vol.4, p. 903. Berlin (West) 1984. 5. Ch. Travers, J. P. Bournonville, G. Martino: in Proc. 8 th lnt. Congress on Catalysis, Vol. 4, p. 891. Berlin (West) 1984. 6. J. P. Candy, B. Didillon, E. L. Smith, T. B. Shay and J. M. Basset: J. Mol. Catal., 86, 179 (1994). 7. J.L. Margitfalvi, E. Tfilas and S. G6b616s: Catal. Today, 6, 73 (1989). 8. Cs. V&tes, E. Talas, I. Czak6-Nagy, J. Ryczkovski, S. GObOlOs, A. V6rtes, J. Margitfalvi: Appl. Catal., 68, 149 (1991). 9. J. L. Margitfalvi, I. Borbath, E. Tfirst and A. Tompos: Catal. Today, 43, 29 (1998). 10. J. L. Margitfalvi, Gy. Vank6, I. Borbath, A. Tompos, and A. V6rtes: J. Catal., (accepted for publication). 11. E.P. Parry: Jr. Catal., 2, 371 (1963). 1.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Influence of small amounts of rhodium on the structure and the reducibility of Cu/Ai203 and Cu/CeO2-Ai203 catalysts. X. Courtois a, V. Perrichon a*, M. Primet a and G. Bergeret b a Laboratoire d'Application de la Chimie/t rEnvironnement (LACE), UMR 5634 CNRS/Uoiversit6 Claude Bernard Lyon 1, 43 boulevard du 11 Novembre 1918, 69622 Villeurbanne Cedex, France. b Institut de Recherches sur la Catalyse, CNRS, 2, avenue Einstein, 69626 Villeurbanne Cedex, France. The influence of small amounts of rhodium on the structure and the reducibility of copper-based catalysts deposited on alumina and celia-alumina supports has been studied by XRD and H2-TPR techniques. For both supports, the addition of rhodium does not affect the reduction mechanism of copper, which has been shown to involve small clusters/isolated ions at lower temperatures and large particles at higher temperatures. However, rhodium addition inhibits the formation of large size copper particles in favor of small ones, thus making easier the reduction of the catalyst at low temperature. 1. INTRODUCTION In the general framework of studying new types of three-way catalysts, copper has been studied as a substitute for automotive post-combustion [1] but seems not active enough without promoter. Considering that rhodium is a key component for the NOx elimination, we have looked for the promotional effect of rhodium on the catalytic properties of copper. This study reports on the characterization of supported copper-alumina catalysts and specially on the influence of small amounts of rhodium on their reducibility by hydrogen. Since rhodium is easily reduced, with a reduction temperature lower than that of copper, it is expected to favor the reduction of copper. The influence of a redox support was also investigated by replacing alumina by celia-alumina, since the surface of ceria is known to be reduced at low temperature in presence of metallic rhodium [2] and consequently may also favor the reduction of copper. 2. EXPERIMENTAL The catalysts were prepared by co-impregnation with nitrate precursors of alumina (A1, 115m2/g) and ceria(20%)-alumina (CeAI, 95mZ/g) supports supplied by Rhodia, and calcination at 400~ for 6 h under air. The copper content was 4.75 wt% and that of rhodium was 100, 500, 1000 and 2000 ppm. The corresponding Cu/Rh atomic ratio for the bimetallic samples were 760, 150, 76 and 38 respectively. * Corresponding author

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For the temperature programmed reduction (TPR) experiments, 50 mg of calcined sample were put in a quartz reactor and pretreated in situ under air at 400~ for 60 min and then under Ar at 500~ for 90 min, and finally cooled to room temperature (RT) under Ar. The reduction was then carried out under a 1% H2/Ar mixture (total flow rate: 1.2 L.hl), using a 20~ heating rate. H2 consumption was detected by thermal conductivity. Before X-ray diffraction (XRD) experiments, samples were prereduced under H2 at 500~ for 6 h, cooled under H2 to RT, then put under N2 and finally slowly exposed to air. Diffraction patterns were obtained with a D500 Siemens goniometer equipped with a diffracted-beam monochromater selecting the Cu Ka radiation (~,= 1.5406/k). The evolution of the crystalline phases during the reduction was followed by in situ XRD. The experiments were performed on a D500 Siemens diffractometer equipped with a high temperature camera using Mo Ka Zr-filtered radiation (X=0.7107 2,) and a positive sensitive proportional detector (Raytech). After the standard oxidizing pretreatment, the sample was heated at 20~ with a 5% H2/He mixture flowing through the catalyst from RT to the first reduction temperature (total flow rate : 2.4 L.h-~). After reaching the reduction temperature, the gas flow was changed to He and the sample was allowed to cool to RT. The diffractogram was then recorded. This reduction procedure was repeated on the same sample, increasing the reducing temperature stepwise for subsequent diffractograms. 3. R E S U L T S Figures 1 and 2 show the XRD patterns for both series of catalysts. The peaks observed at 20 = 45.5 ~ 47.3 ~ and 67.2 ~ are characteristic of the alumina support and those at 20 = 33.0 ~ and 56.5 ~ are due to the cubic CeO2 phase. Metallic copper can be detected at 20 = 43.3 ~ (Cu (111)) and 50.4 ~ (Cu (200)). Rh and Cu addition induces no detectable change in the structure of the supports but for each type of support the intensity of the two main Cu peaks decrease when Rh content increases. On A1 supported catalysts, the Cu peaks were also clearly detected but only for Rh < 500 ppm. The TPR profiles of the CuRh/A1 catalysts are given Fig. 3. The Cu/A1 catalyst exhibits two main peaks at 270~ and 375~ followed by a small shoulder at about 500~ The calculation of the H2 consumption up to 800~ shows a complete reduction of the deposited copper. The same type of profile was obtained with the Cu/CeA1 catalyst with two main modifications (Fig. 4) : i) the first peak is broader, because the CeO2 surface in contact with reduced copper is simultaneously reduced [3] and ii) a low H2 consumption is observed from approximately 550~ until the end of the test at 800~ which is attributed to the reduction of the bulk ceria [4]. As for Cu/A1, a complete reduction of copper may be assumed on Cu/CeA1, although the contribution of the ceria reduction makes difficult a precise calculation. For both supports, the presence of rhodium does not seem to lower the temperature of the beginning of the reduction and does not change the temperature of the peaks. Its main effect is clearly to increase the intensity of the first TPR peak at about 270~ whereas the intensity of the second peak at 375~ decreases with the Rh content (Figs. 3 and 4). With the CeA1 support, the additional broadening of the first peak on the low temperature side can be attributed to the reduction of both rhodium oxide and the ceria surface in contact with rhodium [4]. Note however that for the CuRh/A1 catalysts where no ceria surface reduction occurs, the contribution of Rh to the H2 consumption is too low to be detected with our

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apparatus. The small H2 consumption obtained at about 500~ for Cu/A1, which decreases with the Rh content, could be attributed to the reduction of a surface Cu aluminate [5]. Finally, with the CuRh2000/Al catalyst, a broad but limited H2 consumption is observed near 600~ Its origin is not elucidated yet, but the experiments indicate that it could be linked to the Rh content. .

.

.

.

.

.

.

.

.

.

.

Cu(111)

_

/

~e ~

d

-~

r

~

.

,

,

Cu(111)

Cu (20o)

Cu(200)

~a

l-

a

,

,

I

I

30

40

.. I

50 2O

9

60

30

70

:::3

50 20

60

70

Fig. 2. XRD patterns of the CuRh/CeAI catalysts 9 (a) Cu, (b) CuRh l00, (c) CuRh500, (d) CuRh 1000, (e) CuRh2000.

Fig. 1. XRD patterns of the CuRh/AI catalysts" (a) Cu, (b) CuRhl00, (c) CuRh500, (d) CuRh 1000, (e) CuRh2000.

CuRh2000

40

~.1 CuRh2000 j ~

Cu

Cu

tO o. E

c

..,._

CD c 0

ro 32

LI/i

CuRhS00

Od

CuRhS00

| -

-

~

|

-

-q~,wlRBnmlm~,.m=--

9

200 400 600 temperature (~

800

Fig. 3. TPR profiles of the CuRh/AI catalysts.

0

...

200 400 600 temperature (~

I

800

Fig. 4. TPR profiles of the CuRh/CeAI catalysts.

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Figure 5 shows XRD patterns during in-situ reduction of the Cu/AI sample. The quantitative evolution of the phases CuO and Cu are given Fig. 6. No change in the diffraction pattern is noted between the calcined sample and after reduction at 200~ with only CuO being detected. After reduction at 220~ Cu ~ peaks are coming out with CuO peaks still remaining but with a lower intensity. No formation of Cu20 is evidenced. After reduction at 250~ peaks of CuO totally disappear while those of Cu ~ increase. Cu20 is still undetected. Upon increasing the reduction temperature above 250~ only the increase of the Cu ~ peaks is observed, smoothly between 250 and 500~ more abruptly between 500 and 680~ This increase in the copper peaks intensity can be assigned to the sintering of metallic copper particles initially not detected. 4. DISCUSSION The presence of two peaks in the TPR profiles and their modifications upon addition of rhodium have to be discussed in relation to the XRD results and literature data. The in-situ XRD experiment has clearly shown that for the Cu/A1 sample, only the peaks of metallic Cu have appeared simultaneously with the vanishing of the CuO phase, without the formation of Cu20. Thus, the CuO particles which are large enough to diffract have a reduction process that occurs in a single step. The formation of Cu ~ species as an intermediate step has been proposed for Cu/zeolite solids [6], for some Cu/SiO2 [7] or Cu/AI203 catalysts [8]. It is not evidenced in the present study, although it is not possible to completely discard that highly dispersed copper not detected by XRD could be reduced through the formation of a Cu § intermediate. A two/three peaks reduction TPR profile has often been found in literature for supported copper catalysts and is generally interpreted as due to the successive reduction of small clusters and/or isolated ions at low temperature, followed by the reduction of larger particles at higher temperature [5, 9-11 and references therein]. Increasing the Cu content so that excess copper formed bulk CuO particles evidenced by XRD or STEM/EDX increased the high temperature peak intensity [10,11 ]. We can interpret our results in the same way by attributing the second TPR peak at 375~ to the reduction of bulk CuO whereas the first one at 270~ corresponds to the reduction of small CuO clusters and isolated Cu 2§ ions, undetectable by XRD. This attribution is in agreement with the results of Dumas et al. who observed on a 6 wt% Cu/alumina catalyst a similar profile with peaks at 242 and 312~ [5]. Increasing the Cu loading to 10% was accompanied by a much higher reduction peak at 312~ [5]. The fact that higher temperatures are found in the present study may be attributed to a higher heating rate during the TPR (20~ min -t instead of 5~ min-~). The differences in the operating conditions used can also explain the shift in the temperature range observed for the in situ XRD reduction of CuO particles which occurs at about 250~ (H2 partial pressure five times higher, double flow rate and cumulated heating ltimes above 200~ corresponding to several minutes, whereas the heating rate was 20~ m i n in the TPR experiments). The attribution of the second peak to the reduction of CuO particles and its decrease when the rhodium content increases is supported by XRD data on the initial calcined samples. As shown in Fig.7, a CuO phase could be evidenced on the Cu/CeA1 while it could not be detected with the CuRhl000/CeA1, showing that rhodium addition limits the formation of CuO particles. This is also in line with the XRD spectra of the reduced catalysts. As evidenced in Fig. 8 for the CuRh/CeA1 catalysts, there is a linear relationship between the intensity of the (111)

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,,-:,

100

>,

80

-~=

o~

Y.

60

0~

100 0 80 -~ 6 0 "E 40

(e)

(d) (c) (b) (a) ,

14

~

,

16

18 2 O

40 20

20 _=_ 0 U

,

20

2"~

Fig. 5. hz-situ XRD patterns of the Cu/A1 sample reduced at (a) 200~ (b) 220~ (c) 240~ (d) 260~ and (e) 680~

0 0 200 400 600 reduction temperature (~

Fig. 6. Cu/AI" evolution of the CuO/initial CuO (calcined sample) (D) and the Cu~ Cu ~ (sample reduced at 680~ (,) determined by integration of the XRD peaks of CuO and Cu ~

CuRhlOO/CeA!

"~" "~

CuO (1"11) and (002)

/

Cu/CeAI

CuO (111) and (200)

CuRh2000/CeAI

v,%Cu Rh500/Ce AI !

30

40

!

50 2e

!

60

70

Fig. 7. XRD patterns of the calcined CuRh/AI catalysts" (a) Cu, (b) CuRh 1000.

I ( ~'CuRhl000/CeAI

TPR 2nd 9 peak intensity (a.u.)

Fig. 8. Relationship between the height of the (111) Cu diffraction line (CeO2 as internal standard) and the intensity of the 2 nd TPR peak for the CuRh/CeA1 catalysts.

o o

E =

.::2.

U

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Cu diffraction line and the intensity of the second TPR peak at 375~ The same tendency is observed with the CuRh/AI catalysts, for which no Cu diffraction peaks are detectable for [Rh] >I00 ppm in agreement with the low intensity of the second TPR peak observed for these catalysts. Since no variation of the breadth of the XRD Cu peaks could be measured, this decrease of the Cu diffraction peaks indicate a decrease in the number of large Cu particles. These results clearly show that addition of small amounts of rhodium inhibits the formation of CuO particles expely to give large size copper particles by reduction. It favours the formation of small clusters which are more easily reduced at lower temperatures.

4. CONCLUSION The addition of small amounts of rhodium to a copper-alumina catalyst does not basically modify the reduction mechanism based on the successive reduction of i) the isolated ions and small clusters and then ii) the bulk copper oxide. However, the structure of the catalyst is deeply changed since the presence of rhodium decreases the number of large size copper oxide particles. In this sense, it can be said that rhodium assists in the reduction because it favors the formation of small clusters which are reducible at lower temperature. This effect of rhodium addition resulting in a more dispersed structure of copper is more easily observed when copper is supported on ceria-alumina than on alumina. The redox nature of ceria does not deeply modify the reduction process at low temperature.

Acknowledgements: Rhodia is gratefully acknowledged for supplying the alumina and ceriaalumina supports.

REFERENCES I. P.Y. Lin, M. Skoglundh, L. Lowendahl, J.E. Dahl, K. Jansson and M. Nygren, Appl. Catal. B: Environmental, 6 (1995) 237. 2. S. Bernal, J.J. Calvino, G.A. Cifredo, J.M. Rodriguez-Izquierdo, V. Perrichon and A. Laachir, J. Catal., 137 (1992) 1. 3. Lj. Kundakovic and M. Flytzani-Stephanopoulos, J. Catal., 179 (1998) 203. 4. E. Rogemond, R. Frety, V. Perrichon, M. Primet, S. Salasc, M. Chevrier, C. Gauthier and F. Mathis, J. Catal., 169 (1997) 120. 5. J.M. Dumas, C. Geron and J. Barbier, Applied Catalysis 47 (1989) L9. 6. C. Torre-Abreu, M.F. Ribeiro, C. Henriques and G. Delahay, Appl. Catal. B: Environmental., 12 (1997) 249. 7. E.D. Guerreiro, O.F. Gorriz, J.B. Rivarola and L.A. Arrua, Applied Catal. A: General, 165 (1997) 259. 8. H. Praliaud, S. Mikhailenko, Z. Chajar and M. Primet, Appl. Catal. B" Environmental, 16 (1998) 359. 9. W.P. Dow, Y.P. Wang and T.J. Huang, J. Catal., 160 (1996) 155. 10. R.X. Zhou, X.Y. Jiang, J.X. Mao and X.M. Zheng, Appl. Catal. A: General, 162 (1997) 213. 11. Lj. Kundakovic and M. Flytzani-Stephanopoulos, Appl. Catal. A: General, 171 (1998) 13.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Synthesis, characterization and catalytic properties of a chromium silicate xerogel Rosenira Serpa da Cruz a'+, Martin de M. Dauch a, Ulf Schuchardt a and Rajiv Kumar b alnstituto de Quimica, Universidade Estadual de Campinas- CP 6154- 13083-970-CampinasSP, Brazil bCatalysis Division, National Chemical Laboratory- Pune 411008- India A mixed oxide xerogel, Cr203-SiO2, was prepared by the sol-gel method, characterized and investigated in cyclohexane oxidation. According to diffuse reflectance UV-vis, IR, DRX, TPR and EPR measurements, Cr 3+ ions are located in octahedral sites in the silicate framework, with no evidence for a separate Cr203 phase. Leaching experiments suggested that the catalytic activity in cyclohexane oxidation is due to the leaching of chromium. 1. INTRODUCTION In order to meet the increasing ecological and economic regulations for the development of chemical processes, more attention is being paid to the use of heterogeneous catalysts. Controlled synthesis of the catalytic material and appropriate tuning of the catalytic site is necessary to achieve the high selectivity required in chemical production processes. Sol-gel chemistry has emerged as the most important and versatile method for the preparation of inorganic and mixed organic-inorganic materials [ 1]. The sol-gel approach to material synthesis is based on hydrolysable molecular precursors. Hydrolysis and polycondensation reactions lead to the formation of oxo polymers or metal oxides[2]. These fundamental chemical processes are influenced by several parameters which, once they are understood for a particular chemical system, allow the control of the homogeneity (or controlled heterogeneity) and nano- and micro-structure of the derived material. Chromium is widely used as catalyst in oxidation reactions. Up to now traditional oxidation methods employing stoichiometric or even excess quantities of inorganic oxidants such as potassium dichromate have been predominantly applied both in industry and in small scale. Although chromium substituted molecular sieves have been used to catalyze the oxidation of different substrates, leaching of chromium to the liquid phase during oxidation reactions has been observed [3, 4]. Another much less studied approach to redox metal containing heterogeneous catalysts is the use of amorphous metallosilicate aerogels and xerogels prepared by sol-gel methods. In this work we present our results on the synthesis and

+Present address: Departamento de Ci~ncias Exatas e Tecnol6gicas, Universidade Estadual de Santa Cruz, Rodovia Ilh6us-Itabuna-Km 16, 45650-000, Ilh6us-Ba, Brazil

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characterization of an Cr203-SiO2 xerogel. This material.was investigated in the oxidation of cyclohexanol and cyclohexane with t-butyl hydroperoxide (TBHP) as the oxidant.

2. EXPERIMENTAL 2.1. Synthesis Tetraethyl orthosilicate, 98% (TEOS), Aldrich; chromium (III) acetylacetonate, Alfa Inorganics; ethanol, 99,8% and hydrochloric acid, 37%, Merck were used as received. The catalysts were prepared by a single step acid-catalyzed sol-gel method [5] which permits structural control of the mixed oxides during the preparation. The amount of (TEOS + metal source) was typically 50 mmol. If not mentioned explicitly, the standard procedure was the following: TEOS is placed in a 100 ml PP beaker equipped with a magnetic stirring bar and mixed with the corresponding amount of the metal precursor compound dissolved in ethanol. Subsequently, the water and acid is introduced by adding aqueous HCI (8 mol L "l) dropwise to the stirred solution at room temperature. The solution was stirred for 5 minutes and then placed in the hood to allow for a slow evaporation of the volatiles. Gelation usually occurred after 4-5 days. After gelation was completed, the sample was heated in an oven in the following way: room temperature up to 223 K (24 h at 223 K); up to 423 K (12 h) and up to 523 K (12 h), always with a 15 K min 1 heating rate. Finally the samples were cooled down to room temperature, crushed and sieved ( 100 mesh).

2.2. Characterization The chemical composition of the samples was determined by X-ray fluorescence spectroscopy (Spectrace TX-5000), using mechanical mixtures of SiO2 and chromium (III) acetylacetonate for the calibration curves. The carbon and hydrogen contents of the final samples were determined with a Perkin Elmer 2400 C/H/N analyser. FT-IR measurements were performed on a Perkin Elmer 1600 instrument. X-ray diffraction patterns were measured using a Shimadzu XD-3A diffratometer with CuK~ radiation in the range of 2e = 10-50 ~ with a scanning rate of 2~ min -l. The UV-vis spectra in the DRS mode were obtained on a Varian Cary 5 spectrometer in the range of 200 to 700 nm, on powdered samples. The spectrum of SiO2 was taken for background subtraction. The surface areas, the micropore volume and the pore size distributions were calculated from the N2 adsorption-desorption isotherms at liquidN2 temperature in a Omnisorb 100 CX (Coulter Corporation, USA) instrument. Prior to the adsorption measurements, the samples were activated at 473 K for 8 h in high vacuum. The ESR spectra were measure at room temperature using a Bruker ESR spectrometer(200D). Temperature-programmed reductions of the samples were carried out in a homebuilt instrument using 3 vol% H2-N2 mixture at a heating rate of 10 Kmin l. 2.3. Catalytic experiments The catalytic oxidation reactions were carried out in a three-necked flask equipped with a condensor and magnetic stirring. The temperature of the reaction was maintained using an oil bath. In a typical experiment, the solid catalyst was added to the substrate in the specified solvent followed by the addition of TBHP in cyclohexane.

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Atter reaction, the mixture was quenched to room temperature and the catalyst was separated by filtration. The products were analyzed by GC. Cyclohexane, cyclohexanone and cyclohexylhydroperoxide (CHHP) were quantified using calibration curves. As CHHP is partially decomposed upon injection, its concentration was estimated by double analysis, before and after reducing CHHP to cyclohexanol with PhaP. The oxidation products were identified using an HP 5980 gas chromatograph coupled to an HP 5970B mass spectrometer. To verify if acids were formed, the reaction mixture was esterified before analysis. Two leaching experiments were performed to determine if the activity observed for the catalysts in the liquid phase oxidation of cyclohexane and cyclohexanol with TBHP was due to heterogeneous or homogeneous catalysis. In the first experiment, the solid catalyst was initially stirred with a solution of TBHP, filtered at reaction temperature after 4 h reaction time and the substrate was added to the filtrate. This solution was allowed to react at the same temperature for another 20 h. The second experiment consisted in the filtration of the catalyst from the reaction mixture at 80~ after 4 h reaction time, then a fresh portion of TBHP was added to the filtrate which was allowed to react for another 20h. 3. RESULTS AND DISCUSSION The physicochemical properties of the samples are given in Table 1. Table 1. Physicochemical properties of the samples. Material

metal (mol %)

SiO2 Cr203-SiO2

1.18

BET surface area ( m 2 g-l) 364 450

240

ft. I,U)

200-

,,~ 160-

_f

~ 120-

~

0.06

< ~

|

0.04

80--

>

40--

E

e Io.o~ 0 0

0

0

o!1

a 0.4

i 0.5

.

.

2

4

.

i 0.6

. ti

. 8

. 10

m,(A)

i 0.7

.

.

12

o!.

. 14

. 111 18 20

o79

Relative P r e s s u r e (PIPo)

Fig 1. N2 adsorption isotherm of Cr203-8iO2 and the micropore distribution.

average poresize diameter (nm) 1.1 1.6

C (%)

H (%)

0.9 1.1

1.2 2.1

The samples showed a narrow monomodal pore-size distribution and a type I isotherm in the N2-adsortion. Figure 1 shows the isotherm and pore size distribution of Cr203-SiO2. The organic residue resposible for the carbon and hydrogen contents could be originated for realkoxylation of the hydroxyl groups in the surface and/or by incorporation of the alkoxide ligands as well as the solvent in the silica matrix [6]. All the samples were X-ray amorphous, showing only broad reflections.

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Since the metallosilicates contain- 98-99% SiO2, no large differences between their FT-IR spectra and that of SiO2 were expected. Only the bands common to all silicate structures at-~1080, 790 and 450 cm "1, ascribed to the strectching and bending vibrations of Si-O bond were observed. Of special interest is the band at ~ 930-960 cm "l, which has been attributed to one of two vibrations: (SiO3)Si-OH units in the xerogel and associated to Si-OH vibrations due to the hydroxylated surface [7]; or the Si-O stretching in the polarized Si-O-M bond. In our case, this band at 945 cm "1 is also present in Cr-free xerogels, indicating that at least for the most part it is due to the hydroxylated surface in the xerogel. Then, this band cannot be an indication of isomorphic Cr-substituion in mixed oxide xerogels. The IR spectrum also shows that the acetylacetonate ligand is clearly lost. The DR-UV-vis spectra of the Cr203-SiO2, Cr203 and a mechanical mixture of Cr203 + SiO2 are given in Figure 2. The spectra of Cr203+SiO2 and C[203 show two bands at 600 and 466 nm, assigned to 4TE---~4T5 and 4T2----~4T4[F]

70 60

i \ i \ i \

/'''/ L/I

50 ~Z 40

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~3

300

400

.

,5O3

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,

600

.

I

700

transitions of chromium(Ill) in an octahedral coordination, respectively [8]. The spectrum of Cr203-SiO2 indicates the presence of chromium in two different coordination spheres. The absence of peaks of reduction by H2 in the temperature-programmed reduction thermogram indicates the absence of a separate Cr203 phase. Additionally, evidence for this hypothesis was obtained from UV-vis diffuse reflectance spectroscopy and XRD. The EPR spectrum of Cr203-SiO2 shows a broad peak at g= 1.975, which is attributed to small clusters of Cr 3+ invisible to XRD [9] or associated with Cr3+in the framework, irrespective of coordination (octahedral or tetrahedral)[8].

Fig. 2. Diffuse reflectance spectra of (a) mechanical mixture of Cr203 + SiO2, (b) Cr203-SiO2 and

(C) Cr203.

The results of cyclohexane oxidation with TBHP in cyclohexane catalyzed by Cr203SiO2 are presented in Table 2.

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Table 2 Cyclohexane oxidation over Cr203-SiO2 a Product distribution (mol%) Temperature (~

TON b Cyclohexanone

Cyclohexanol

CHHP

70 95 85 3 80 120 75 2 100 185 69 0 aReaction contitions: cyclohexane, 95 mmol, TBHP in cyclohexane 9.5 2,3 10-2 mmol Cr, 8h. b Moles of oxygenated products per mol of chromium. c The major products are bicyclohexyl and adipic acid.

Others r

10 2 6 17 0 31 mmol, Cr203-SiO2'

Data of Table 2 show that the total yield of oxidation products increases as the temperature is increased. The ratio cyclohexanone/cyclohexanol increases when the temperature increases from 70 to 100~ implying that cyclohexanol is continually oxidized at higher temperatures. In order to confirm this hypothesis, a parallel experiment using cyclohexanol as the substrate was performed at 80~ The oxidation of cyclohexanol, giving 46% conversion and cyclohexanone in 100% selectivity was indeed observed. The selectivity of CHHP decreases and becomes 0 at 100~ suggesting that CHHP is an unstable intermediate.

......-I1""

Cr203-SiO2

.U

m .....

mmov~

~4o L..

I1) >

E 0

2

0

I

'

'

'

I

'

'

0

reaction time (h)

Fig. 3. Effect of catalyst separation on the cyclohexane oxidation at 80~ Reaction conditions as in Table 2.

In order to determine if the catalysis was indeed heterogeneous, a few leaching experiments were performed following the procedure described by Lempers and Sheldon [10] and discussed in detail in a recent review [11]. The activity of the liquid phase of the reaction after removing the solid catalyst was examined and showed variable catalytic activities, sometimes higher (o) sometimes lower(A) than the ones in the presence of the solid (m), e.g., cyclohexane was continuously converted to cyclohexanone (Figure 3), strongly suggesting that chromium species leached from Cr203Si02 are responsible for the catalysis.

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In an additional experiment, the catalyst was stirred with the substrate at the reaction, temperature, filtered off and TBHP added to the filtrate. Since no catalysis was observed in the filtrate even after 24h, one cannot say that leaching of the chromium was promoted by substrate/solvent. As, however, leaching is observed in the presence of the oxidant, be do not agree with Neumann et al. [12] and Maier et al. [13] who reported that mixed oxides prepared by sol-gel method are true heterogeneous catalysts in oxidation reactions. The used catalysts were dried at 120~ for 2 h, and then used for recycling experiments. The conversion of cyclohexane in three consecutive reactions was very closed to that of a fresh catalyst. This result can give erroneous conclusions about leaching because minimal amounts of leached Cr can account for the observed catalysis as observed by Sheldon et al. [ 14] during liquid phase oxidations using CrAPO-5, CrAPO-11 and CrS-1 as catalysts. 4. CONCLUSIONS The study of the oxidation of cyclohexane in the presence of a Cr203-SiO2 xerogel as catalyst has shown that the catalytic activity is due to the chromium leached into the reaction medium. In spite of the very small amount of dissolved chromium, a high conversion catalysis was observed. ACKNOWLEDGMENTS Financial support by FAPESP and CNPq and supply of raw material by NITROCARBONO S.A. are gratefully acknowledged. We also thank Dr. D. Srinivas, NCL-Pune, India for the EPR spectra and Dr. R. Buffon for helpful discussions and revision of the manuscript. REFERENCES 1. A. Baiker, Stud. Surf. Sci. Catal, 101 (1996) 51. 2. U. Schubert, J. Chem. Soc., Dalton Trans., (1996) 3343 3. R.A. Sheldon, Stud. Surf. Sci. Catal, 110 (1997) 151 4. W.A. Carvalho, P.B.Varaldo, M. Wallau and U. Schuchardt, Zeolites, 18 (1997) 408 5. S. Klein, S. Thorimbert and W.F. Maier, J. Catal, 163 (1996) 476 6. A.Baiker, R.Hutter, M.Schneider and D.C.M. Dutoit, J. Catal., 161 (1996) 1651 7. F.Boccuzzi, A.Chiorino, G. Ghiotti, C. Morterra and A. Zecchina, J. Phys. Chem., 82 (1978) 1278 8. J.S.T. Mambrim, E.J.S.Vichi, H.O. Pastore, C.U. Davanzo, H. Vargas, E. Silva and O. Nakamura, J. Chem.Soc.,Chem. Commun., (1991) 922 9. P.G. Harrison, N.C. Lloyd and W. Daniell, J. Phys. Chem. B, 102 (1998) 10672 10. H.E.B. Lempers and R.A. Sheldon, Stud. Surf. Sci. Catal., 105 (1997) 1061 11. R.A. Sheldon, M. Wallau, I.W.C.E. Arends and U. Schuchardt, Acc. Chem. Res, 31 (1998) 485 12. M. Rogovin and R. Neumann, J. Mol. Catal. A: Chemical, 138 (1999) 315 13. S. Klein, J.A. Martens, R. Parton, K. Vercruysse, P.A. Jacobs and W. Maier, Catal. Letters, 38 (1996) 209 14. H.E.B. Lempers and R.A. Sheldon, J.Catal., 175 (1998) 62

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Thermal Chemistry of Oxide-supported Platinum Catalysts: A" Comparative Study J.F. Lambert, E. Marceau, B. Shelimov, J. Lehman, V. Le Bel de Penguilly, X. Carrier, S. Boujday, H. Pemot and M. Che Laboratoire de R6activit6 de Surface (UMR-CNRS 7609), Universit6 Pierre & Marie Curie 4, Place Jussieu, 75252 Paris Cedex 05, France This communication discusses some of the phenomena occurring during the thermal transformation of oxide-supported platinum catalysts. Based on previously established molecular understanding of the deposition step, the reactivity of several surface platinum species is followed through the later steps of catalyst preparation : drying, calcination (or thermal activation under inert atmosphere) and H 2 reduction. The phenomena observed include Pt complex self-reduction, H spillover to various support sites and formation of strongly held "residual chlorine". Their consequences on metal dispersion and catalytic activity are briefly presented. 1. INTRODUCTION Pt/A1203 and, to a lesser extent, Pt/SiO 2, are among the most widely used heterogeneous catalytic systems [1]. In spite of this, many questions pertaining to the chemistry of their preparation remain unanswered. The interfacial coordination chemistry of Pt complexes during their initial deposition on oxide surfaces from aqueous solution has recently been studied in our laboratory for several closely related systems [2-7], but there remains a gap in our understanding between the deposition step, which implies a relatively well-understood chemistry at the oxide/water interface, and the final catalyst that has been submitted to thermal treatments. The purpose of the present communication is to determine in what way the initial adsorption mechanism and speciation of supported Pt affect its subsequent transformations on thermal treatments of calcination and reduction, until the final catalyst is obtained. Both square planar P t II and octahedral Pt TM complexes have been studied by macroscopic methods (adsorption isotherms, elemental analysis, TGA, TPR followed by MS) as well as by spectroscopic techniques sensitive to local environment (UV-visible, ~95ptNMR, XAS). 2. SPECIATION DURING THE DRYING STEP. On alumina, Pt complexes are often electrostatically adsorbed initially but they can become grafted through a thermally activated ligand substitution. Figure l a illustrates this transition for the hexachloroplatinate/alumina system which has been most studied. Here, the initial adsorption mechanism involves electrostatic adsorption together with "specific" adsorption, probably by hydrogen-bonding. Drying causes the replacement of two chloride ligands by surface hydroxyls of the alumina ; the driving force appears to be the elimination into the gas phase of HC1. More details can be found in ref. [4]. On silica, in contrast, grafting seems to occur on drying (at 90~ but is quickly reversed on exposure to room humidity. The successive transformations implied are illustrated in figure lb, where it is seen that digrafted species [(SiOH)2PtC14] ~ are transformed into free [PtCln(H20)2] ~ upon rehydration. In the same way, monografted [(SiOH)PtC15] are transformed into free [PtC15(H20)], as revealed by 195ptNMR (see below). Thus, the oxidic surface groups of alumina have a higher affinity for Pt than those of silica. However, it was found that previous modification of an alumina surface by specific adsorption of isopolytungstate ions effectively prevented the grafting of [PtC16]2-, even though the fraction of the surface physically occupied by the tungstate was only 10 to 25% [5].

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

lO44

a

AI~

+ 2 HCI

+H+ Drying

v

A

A b

B

B 'Drying~

C ~ Rewethng

/

+2H+ Fig. 1. (a) Change in the adsorption mode of Pt Iv in H2PtC16/AI203." A, after deposition, B, after drying ; (b) change in the adsorption mode of Pt ~v in HzPtC16/SiO2.: A, after deposition, B,. after drying, C, after rehydration (rewetting). It seems therefore possible to modulate the interaction between the support oxide surface and the transition metal precursor by an appropriate pretreatment. As we will see, this may provide a way to control the sequence of events occurring during subsequent thermal treatments. 3. SPECIATION DURING THE CALCINATION STEP; SELF-REDUCTION OF Pt =v Calcination under a neutral atmosphere, and even under 0 2 at moderate temperatures (under 500~ sometimes has the unexpected result of causing a partial reduction of Pt species. The reducing agent is necessarily constituted by the Pt ligands, and consequently the products of this redox reaction should be detected in the gas phase. In the case of silicasupported chloroplatinates, two successive steps may even be identified, as shown by the DTG trace of Figure 2. To ascertain the nature of thermal events 1 and 2 in DTG traces, the thermal treatment was interrupted at various points (labelled A, B and C in figure 2) and the nature of the existing phases was checked by XRD. At point A, only the diffractogram of the alumina support was apparent, meaning that Pt species were still molecularly dispersed. At point B, sharp diffraction lines were observed at positions corresponding to PtC12. At point C, these had disappeared but narrow peaks of metallic platinum were observed. This allows to interprete events 1 and 2 in DTG as corresponding to the following reactions, respectively: IV II HzPt C16 --->Pt C12 + 2 HC1 + C12 (1) ptncl2 --->pt ~ + C12 (2) The occurrence of these reactions is strongly detrimental to Pt dispersion as the volatile PtC12 intermediate forms very large crystallites, whose large size is maintained upon further transformation to Pt~

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700

(oc)

Fig. 2. DTG (first derivative of thermogravimetric curve) of a H2PtC16/SiO2 sample heated under N 2 at 10~ Labels A, B and C indicate points where TG analysis was interrupted to check the X-Ray Diffractogram. For alumina-supported chloroplatinates, on the other hand, self-reduction is definitely inhibited. We have suggested [4] that this inhibition is due to the nature of chloroplatinate interaction with the support surface. As explained in w1, at moderate temperatures (RT to 90 ~ depending on preparation conditions), the chloroplatinate species are irreversibly grafted onto the alumina surface to give [(A1OH)2PtC14] ~ The grafted species is much more stable than its hypothetical analogue on the silica surface and this effectively prevents platinum self-reduction. This model can be checked by using modified systems in which grafting will be inhibited by pretreatment of the alumina surface prior to Pt deposition, such as WOx/A120 3. As we have said earlier, metatungstate and paratungstate ions specifically adsorb on those sites that cause Pt grafting, effectively blocking them 9Pt then remains as [PtC16]2- and is self-reduced to Pt ~ at temperatures of the order of 500~ very much like on the silica support. Chlorination of the alumina support by treatment with dilute HC1 constitutes another way to inhibit Pt grafting, and it has the same result as polytungstate modification : self-reduction to Pt ~ NH 3 is a more efficient reducing agent than C1-. Samples containing NH4 + as a counterion ((NHn)2[PtC16] precursor) were reduced to Pt~ at least for treatments under neutral atmosphere at T_<400~ as observed on samples where ammonia was introduced as a ligand ([Pt(NH3)4] 2+ precursor). The mechanism of self-reduction by ammonia is still open to discussion. For instance, in the case of [Pt(NH3)4] 2§ one could write: 3 [Pt(NH3)4] 2+ ---->3 Pt ~ + 6H + + 10 NH 3 + N 2 (3) However, mass spectroscopy indicates that NH 3 and N 2 are not the only gases released : some NO is also observed, perhaps suggesting participation of adsorbed moisture in the redox reaction. Quite expectedly, treatments under 02 at higher temperatures cause a reoxidation of Pt to PtO and/or PtO 2. 4. EFFECTS OF SPECIATION ON H 2 REDUCTION AND CATALYTIC ACTIVITY

4.1.Selective reduction of platinic species in Pt/SiO 2 Even during this final preparation step, marked effects of the initial speciation during deposition and drying are still manifested. A clear example is provided by the [PtC16]Z-/SiOa system. The evolution of surface platinic species in this system is sketched in Fi~. lb, which shows how digrafted species exposed to humidity regenerate the [PtC14(H20)2] ~ complex. In the same way, monografted species regenerate [PtC15(H20)]-, Both these species, as well as untransformed [PtC16]2-, can be detected and quantified using 195pt NMR, as illustrated in figure 3a. Furthermore, we have demonstrated elsewhere that the distribution of Pt w between these different species (in other

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words, Pt TM speciation)can be controlled by a simple procedure (partial titration of the initial chloroplatinate solution with NaOH - see ref. [6] for details). If the samples obtained in this way are reduced under H 2, each surface species is reduced at a characteristic temperature: there is an obvious correlation between the ~95ptNMR spectra of Fi~. 3a and the TPR traces of Fig. 3b.

A

o, loll/

639

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B

0,.81

140 ~

10 ~

....................

0

800

400 ;5 (ppm~)

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Figure 3: 195Pt NMR spectra (A) and TPR traces (B) of dried and rehydrated [PtC16]2-/SiO2 systems, as a function of the amount of NaOH added to the precursor solution before deposition (expressed as the OH/Pt ratio). It is therefore possible to selectively reduce one ~!ven platinic species, say [PtCla(H20)2] ~ to the metallic state while leaving the others as Pt", as shown by NMR. This selective reduction, occurring at low temperatures, gives great flexibili~ for the control of Pt particle size. At any rate, direct reduction gives rise to particles with 10-25~ average diameters, as compared to over 100A when a calcination step is previously applied with occurrence of selfreduction (see w3). 4.2 Spillover hydrogen in Pt/AI20 3 The interpretation of TPR traces in Pt/A1203 is more difficult than in Pt/SiO 2. Several H 2 consumption peaks are usually apparent, even when it may be thought that only one Pt species is present. An example can be found in Figure 4a, corresponding to the TPR of a 1.5 % [PtC16] / A120~ sample dned at 90 C, and containing mostly or uniquely the dxgrafted [(A1OH)2PtC14]U species. No less than four peaks are apparent (labelled 1 to 4 in Figure 4). Figure 4b shows the TPR of the same sample calcined at 450~ under flowing 0 2 prior to reduction, a treatment which does not modify the nature of platinic species. The intensity of TPR peak 1 is unaffected, but all the others are much weaker than in the uncalcined sample. Since this kind of thermal treatment is expected to decrease the degree of surface hydroxylation, TPR peaks 2-4 are correlated with the presence of A1-OH groups. Most probably, they are in fact due to hydrogen spillover on the alumina surface and not to Pt x reduction, which is complete at 250~ 2-

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.

.

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~oo

~

uoom

100 200 300 400 500 600 700 800 T(~

% [PtC16]2-/A1203sample dried at 90~

(a) and calcined at 450~ (b).

It turns out that modification of the alumina surface by polytungstate adsorption has a similar effect to calcination as it strongly suppresses peaks 2-4. Probably, the A1-OH groups with which tungstates are in specific interaction are those that were able to promote H spillover.

='L =O

348~

El. [::: v-" 21 e--

t'M

40

530~

3 1'00

:~00

300

~,00

500

600 T (~

Fig.5. TPR of a 3 % [Pt(NH3)412+/A1203, (a) prepared by incipient wetness impregnation ; (b) prepared by "wet impregnation" with excess solution. Both procedures included a 24h ageing time before drying. Similar TPR traces are observable when other Pt precursors are used, as for instance in [Pt(NH3)a]E+/AI203 (Fig. 5). However, peaks 3 and 4 are only observed if the preparation procedure involves prolonged contact between the support and a basic, bulk aqueous phase (compare traces a and b). Characterization of the support in both cases revealed that such an ageing in the presence of the solution resulted in extensive restructuring, including formation of AI(OH)3 particles. Consequently, it is tempting to assign TPR peaks 3 and 4 to inter-particular spillover (possibly to neoformed AI(OH) 3) while peak 2, which is always present, would correspond to a spillover to less remote sites (possibly at the perimeter of Pt ~ particles). 4.3. Residual chlorine in Pt/AI20 a The fate of chlorine during thermal treatments of ptX/oxide catalysts is another open question. We have mentioned that Pt reduction was complete at 250~ and a naive view would be to consider that all of the chlorine is eliminated as HC1 at this step, along reactions such as:

[(A1OH)EPtC14] 0 + 2 H E --) Pt 0 + 4HC1 + 2 (A1OH) (4) In fact, significant amounts of residual chlorine are still present in the system after reduction at 450~ This was found to have an important influence on the properties of the reduced catalysts: for instance, reduced Pt/A1203 catalysts prepared by impregnation of HEPtC16, calcination in He

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and reduction, exhibit lower activity for total oxidation of methane than catalysts prepared from Pt(NH3)4(OH)2, despite identical particle size (15 A) [6]. The limited H 2 and O 2 adsorption and low oxidative activity of the ex-H2PtC16 catalysts were clearly related to the presence of residual chlorine (about 0.4 wt%). Therefore, additional experiments were carried out on chlorinated catalysts; some of the results are summarized in Table 1.

Table 1. Evolution of the conversion at 450~ and activation energy with the chlorine content and mode of introduction on Pt/A1203 catalysts for total oxidation of methane (Pt = 20 I.tmol ; He = 95 mol.L -t, C H 4 = 1 mol.L -~, O 2 = 4 mol.L ~, T=450~ Precursor salt Pt(NH3)4(OH) 2 Pt(NH3)4(OH)2 Pt(NH3)a(OH)2 Pt(NH3)4(OH) 2 Pt(NH3)4(OH) 2 Pt(NH3)4(OH) 2 Pt(NH3)4(OH)E

Additional treatment reference ref. + NH4C1 ref. + NH4C1 ref. + NH4C1 ref. + NH4C1 ref. + NH4C1 ref. + NH4C1

Atomic CI/Pt 0 0.10 0.25 0.35 0.80 1.40 2.70

Conversion 0.77 0.60 0.45 0.36 0.23 0.18 0.14

Ea (kJ.mol-') 64 70 74 77 85 103 93

HEPtC16 HEPtC16 H2PtC1 ~

reference ref. + dechlorination ref. + NH4C1

1.60 0.50 2.80

0.21 0.19 0.22

110 103 113

.......

The inhibiting role of chlorine toward methane combustion was evidenced by the decreasing catalytic activity observed after impregnation of reduced ex-Pt(NH3)4(OH) 2 catalysts by NH4C1 prior to reaction [7]. However, impregnation by NH4C1 or dechlorination in boiling water of eX-HEPtC16 samples did not alter significantly their activity, though their chlorine content was modified. It has been supposed that the catalytic properties of reduced ex-HaPtC16 catalysts are ruled mainly by a strongly held fraction of residual chlorine species in the vicinity of the platinum particles, able to mask the possible influence of other chlorine atoms fixed on the alumina at a longer distance. 5. CONCLUSION This presentation only gives a brief overview of the complexity of Pt/oxide catalytic systems (some of which is more fully treated in the quoted references). However, we hope that the few examples treated here may convince the reader that 1) phenomena occuring in the "black box" between the Pt deposition and the obtention of the final catalyst may indeed be understood at a molecular level, and 2) they are strongly directed by the initial speciation of the platinum precursors and the mechanism of their adsorption on the support surface, which may therefore provide ways of controlling the final catalyst properties.

REFERENCES 1) J. P. Boitiaux, J. M. Dev~s, B. Didillon and C. R. Marcilly, in Catalytic Naphta Reforming: Science and Technology, G. J. Antos, A. M. Aitani, J. M. Parera (Eds.), Marcel Dekker, New-York, 1994, pp. 79-111. 2) B. Shelimov, J. F. Lambert, M. Che and B. Didillon, J. Am. Chem. Soc., 121 (1999) 545. 3) B. Shelimov, J. F. Lambert, M. Che and B. Didillon, J. Catal. (1999) in the press. 4) B. N. Shelimov, J. F. Lambert, M. Che and B. Didillon, J. Mol. Catal. (1999) in the press. 5) V. Le Bel de Penguilly, Ph.D. Thesis, Universit~ Pierre et Marie Curie, Paris, 1998. 6) B. Shelimov, J. Lehman, J. F. Lambert, M. Che and B. Didillon, Bull. Soc. Chim. Fr. 133 (1996), 617. 7) E. Marceau, J. M. Tatibou~t, M. Che and J. Saint-Just, Catal. Today 29 (1996) 415. 8) E. Marceau, Ph.D. Thesis, Universit6 Pierre et Marie Curie, Paris, 1997.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Geochemistry and catalysts preparation" alumina as a chemical reactant in the synthesis of MoOx/Al203 and WO~/AI203 systems. X. Carrier a, J.F. Lambert a and M. Che a'b a Laboratoire de R6activit6 de Surface (UMR 7609 - CNRS), Universit6 Pierre & Marie Curie 4, Place Jussieu, 75252 Paris Cedex 05, France b Institut Universitaire de France

This study deals with the role of alumina dissolution on the speciation of Mo during the synthesis of MoOx/A1203 catalysts. Alumina dissolution leads to the formation of an aluminomolybdic species in solution and its deposition on the support. These phenomena, explained by using concepts developed in geochemistry, can have general implication in catalyst synthesis since the formation of mixed species is also shown to occur for WOx/A1203 preparation. Depending on the preparation method, these mixed species are either molecularly dispersed or precipitated as a 3D phase on the support. The influence of such species on the state of the catalyst obtained after calcination is discussed. 1. INTRODUCTION A molecular understanding of chemical processes involved in the successive steps of solid catalysts preparation is a necessary condition for an efficient fine-tuning of catalysts properties. To this end, we recently proposed an approach using concepts from various fields such as colloid science, electrochemistry, supramolecular chemistry, coordination chemistry and geochemistry [1,2]. This approach has been applied to the first preparation step, consisting in the deposition of transition metal precursors from aqueous media. It appears that the phenomenon with the most drastic consequences involves dissolution of the support material, followed by formation of mixed species by reaction with the transition metal precursors. This phenomenon has been evidenced in the Ni2+/SiO2 and Ni2+/A1203 systems, with Ni phyllosilicates and Ni hydrotalcites formed from cationic Ni precursor complexes [3,4]. There also was evidence that it could occur for the molybdate/SiO 2 system [5]. The purpose of the present communication is: i) to prove that increased support dissolution followed by mixed species formation does happen in systems with anionic transition metal precursor, e.g. in molybdates/A1203 and tungstates/A1203, ii) to indicate how the formation of mixed species can be explained using the concept of ligand-promoted support dissolution, imported from the field of geochemistry, and iii) to discuss the influence of preparation parameters, especially temperature and contact time, over the extent of support dissolution. 2. EXPERIMENTAL MoOx/A1203 catalysts were mostly prepared by equilibrium adsorption where 1 g of T-alumina (specific surface area = 189 m2.g1) is suspended in 100 mL of distilled water before

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adding ammonium heptamolybdate ((NH4)6Mo7024.4 H20, Merck, hereafter referred to as [AHM]) up to a Mo concentration of 6.4.10 .2 M. The suspensions were stirred at room temperature for variable times and the pH kept constant by adding either concentrated HNO3 or NH3. After filtration, the solid phases were washed three times with distilled water and left to dry under air at room temperature (RT). The liquid filtrates were kept for analysis. Some MoOx/A1203 samples were prepared by incipient wetness impregnation by dissolving the appropriate amount of AHM in water and adding this solution to T-alumina in an amount just sufficient to fill the pore volume (0.65 mL.gl). These samples were also left to dry under air at room temperature. Another set of catalysts was prepared by changing the drying procedure. In order to minimize alumina dissolution, the impregnation was carried out as quickly as possible (a few seconds) and the resulting sample immediately frozen in liquid nitrogen. It was then freeze-dried under vacuum and stored under partial vacuum in a dessicator over silica gel. Details on the synthesis and characterization of the aluminomolybdic reference compound (Al(OH)6Mo60183") as well as on experimental techniques used can be found elsewhere [6,7]. 3. RESULTS

3.1 MoOx/Al203 catalysts prepared by equilibrium adsorption 3.1.1 Characterisation of the liquid phase 27A1 NMR spectra of solutions recovered by filtration after equilibrium adsorption performed at different pH's are shown on Figure 1.

950 (x 6 ) ~ ~'-~JM/__. d) 565 38( (x 6) c)

910

898

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f)

.. .=

e!d)

c/u) 30

25

20

15 10 5 0 ~5(ppm) Fig. 1.27A1 liquid state NMR spectra of filtrates obtained after equilibrium adsorption (168 h) at varying pH: (a) pH 8, (b) pH 7, (c) pH 6.5, (d)pH6.3,(e)pH6.1,(f)pH5.1 and(g)pH4,

(x

b)

897 836 9b0

'

J a ) I

I

I

700 500 300 Wavenumber (cm 1) Fig. 2. Liquid state Raman spectra of filtrates obtained after equilibrium adsorption (168 h) at varying pH: (a) pH 6.3, (b) pH 5.1, (c) pH 4 and of (d) Al(OH)6Mo6O183-.

The appearance of the spectra depends on pH. For pH < 6.5, after an equilibrium time of 168 h, all filtrates show a single peak at 15.5 + 0.2 ppm. The integrated intensities of this peak for different filtrates are in agreement with the aluminum contents found by elemental analysis [7]. The spectra (not shown here) for shorter equilibration times [7] indicate that the peak at 15.5 ppm can be seen even after 1 h at pH = 4 and its intensity increases with time. It is also present at pH 6.1 after 24 h of equilibrium. The peak position is identical to the chemical shift

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found for an aluminomolybdic species of formula Al(OH)6Mo6OI83" [8] (hereafter referred to as [AIMo6]) with the Anderson structure made up of six MoO6 octahedra surrounding one AI(OH)6 octahedron. Thus [A1Mo6] is spontaneously formed within less than 1 h when AHM solutions are contacted with alumina at pH 4, and within less than 24 h at pH 6.5. To check the presence of heptamolybdates, Raman spectra of filtrates obtained after an equilibrium time of 168 h were recorded and are shown on Figure 2. The spectrum of the solution equilibrated at pH 4 for 168 h (spectrum c) is identical to [A1Mo6] (spectrum d). In particular, the features characteristic of the heptamolybdate and monomolybdate ions [9] are not observed. Therefore, the only molybdate species detected in solution is the [AIMo6] species. At pH 5.1 (spectrum b), the band at 565 cm l and the shoulder at 910 cm l typical of the Anderson-type species are still present but a new shoulder appears at 898 cm l typical of the heptamolybdate species [9]. The conclusion is that both [A1Mo6] and Mo70246" are present. In the solution at pH 6.3 (spectrum a), three bands at 897, 836 and 316 cm l can be assigned to MoO42 [9] while one band at 950 cm l of lower intensity than that at 897 cm l may be due either to [AIMo6] or to M070246". We know from 27A1 NMR that some [A1M06] species are indeed present. 3.1.2 Characterisation of the solid phase Having detected [A1M06] in solution, we next tried to determine its presence on the surface of alumina. This is a difficult task since we have to discriminate the weak signal arising from the central aluminum of a limited number of [A1M06] species adsorbed on 7-A120 3 from the strong signal of the aluminum atoms of the support. This led us to use Cross-Polarization (CP) MAS NMR where only alumintuns in dipolar contact with hydrogen will be observed, excluding most of the aluminums from the bulk of alumina [10]. Especially, CP should be very effective for the central A1 in [A1M06] since it is coordinated to six hydroxyls.

Fig. 3.27A1 CP-MAS NMR spectra of a) A1203, "~ b) MoOx/A1203 prepared at pH 4, 168 hours (5.8 Mo wt %), c) difference of (b)-(a) and -~ d) MoOx/A1203 prepared at pH 4, 672 hours (7.9 Mo wt %). -=

b)

40

30 20

a) 10 0 -10 -20 -30 -40 8(ppm)

The 27A1 CP-MAS NMR spectra of the bare alumina and MoOx/A1203 catalysts (5.8 and 7.9 Mo wt %) are shown on Figure 3. Besides one broad peak centered at 9 ppm assigned to surface octahedral aluminum from the support, the spectra from both catalysts (spectra b and d) show one peak at 15.6 ppm. This peak is better revealed in spectrum b) by subtracting the contribution from the support (spectrum c). The peak position suggests that [A1Mo6] is not only formed in solution but also is deposited on the support.

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3.2 MoOx/Al203 catalysts prepared by incipient wetness impregnation

Figure 4 shows the diffractograms of the 7 and 9 Mo wt % catalysts either air-dried at room temperature (4-b and 4-d) or freeze-dried (4-c and 4-e). The diffractograms of the samples dried at RT show that alumina dissolution and subsequent reaction with AHM to form [A1Mo6] did occur since diffractograms 4-b and 4-d are identical to that of the [A1Mo6] ammonium salt. The limiting step of this dissolution-complexation process is believed to be the dissolution of alumina since the complexation of A1nI by [AHM] is relatively fast (completed in a few hours [8]) while equilibrium aluminium concentrations are reached only in several days for aluminum oxides [11]. Interestingly, the data presented above suggest that alumina dissolution is much more rapid when [AHM] is present in solution, since both diffractograms (4-b and 4-d) were recorded only 1 h after the synthesis. This suggests that molybdate ions are able to kinetically promote alumina dissolution.

e) d) c)

>-,

~9

9

r

=

b)

c) b)

a) 10

20 30 20[Cu Kc~] (~

40

50

Fig. 4. X-Ray diffractograms before calcination of" a) [NH4]3AI(OH)6Mo6018 and of MoOx/A1203: b) 7 Mo wt % air-dried at RT, c) 7 Mo wt % freezedried, d) 9 Mo wt % air-dried at RT and e) 9 Mo wt % freeze-dried,

10

20

I

I

30 40 20[Cu Koq (o)

a) 50

Fig. 5. X-Ray diffractograms after calcination (400~ under dry oxygen) of: a) [NH4]3AI(OH)6Mo6018 and of MoOx/A1203 catalysts: b) 7 Mo wt % airdried at RT, c) 7 Mo wt % freeze-dried, d) 9 Mo wt % air-dried at RT and e) 9 Mo wt % freeze-dried.

Another striking result is that the heteropolyanions are not molecularly dispersed for impregnated samples, in contrast with catalysts prepared by equilibrium adsorption, but form a separate phase on the alumina surface. Diffractograms of the freeze-dried samples (4-c and 4-e) show no other diffraction peaks than those of alumina. This suggests that freezing the catalysts at 77 K a few seconds after impregnation at RT is sufficient for limiting alumina dissolution and subsequent reaction of solvated A1111with molybdates. The method used here, i.e., quenching alumina dissolution, allows to control the speciation of the molybdic species deposited on the support. The question now arising is what influence the initial molybdate speciation has on the final properties of the catalyst. Indeed, MoOx/A1203 systems are well-known precursors for HDS catalysts and significantly different behaviours can be expected for air-dried and freeze-dried catalysts. One answer to this question is given in Figure 5, which shows the effect of calcination on the structure of MoOx/A1203 catalysts. Diffractogram 5-a corresponds to the

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reference compound (NH4)3[A1Mo6] calcined at 400~ under dry oxygen, and mainly shows the diffraction peaks (stars) of MOO3. Catalysts prepared by classical incipient wetness impregnation also exhibit peaks (Fig. 5-b and 5-d) corresponding to MoO3 even for a weight loading as low as 7 Mo wt %. Catalysts prepared by freeze-drying with the same Mo loadings exhibit no such peak (Fig. 5-c and 5-e). These results show that the presence of (NH4)3[AIMo6] after air-drying the impregnated samples leads to the formation of bulk MoO3 after calcination. On the contrary, for freeze-dried samples with the same Mo loadings, there is no evidence for the presence of (NH4)3[A1Mo6] after freeze-drying and XRD does not show the presence of MoO3 peaks after calcination at 400~ either. This means that we are able to avoid the formation of MoO3 at 400~ or, at least, of MoO3 particles large enough to be detected by XRD after the freeze-drying procedure. 4. DISCUSSION

4.1 Ligand-Promoted Support Dissolution In geochemistry, there have been extensive studies on the weathering of minerals (i.e. the dissolution of a primary mineral such as feldspar, an aluminosilicate, and the precipitation of a secondary mineral, such as a phyllosilicate) in natural media [ 12]. The major lesson that can be drawn from geochemistry is that the oxide solubility can be dramatically increased by the presence of ligands (in geochemistry these are natural entities such as carboxylates) in solution. This promotion of oxide solubility can have a kinetic origin [13] but, in this work, we are dealing with thermodynamic promotion [14]: for aluminum oxides for example, the presence of aluminum-complexing ligands in solution leads to the complexation of dissolved aluminum which will in turn, enhance the total amount of aluminum in solution by shifting the dissolution reaction toward the formation of dissolved metal ions. Molybdic anions in solution are equivalent to aluminum-complexing agents since [A1Mo6] can be considered as the result of the complexation of the central A1m by polymolybdate ligands. The same type of conclusion has been reached for WOx/A1203 catalysts [15,16] where several Keggin-type species related to AIW120405 are formed. Thus, ligand-promoted dissolution (LPD effect) is a concept with rather general implications for catalysts synthesis. 4.2 Implications and applications: kinetic control of Mo and W speciation Initial differences in Mo speciation have lasting influence on the catalyst life: the formation of Mo-A1 mixed species is a crucial factor for the state of the calcined catalyst, since it determines the formation of MoO3 after calcination at 400~ and, thus, the final dispersion of molybdenum [6]. In addition, the differences in speciation resulting from the LPD effect may be controlled to a large extent via those parameters that control the kinetics of interracial phenomena, i.e. temperature and duration of the preparation steps. Figure 6 illustrates this idea, classifying various steps of catalysts preparation in a T = fit) plot (T is the temperature and t the time of the corresponding preparation step). It is expected that the LPD phenomena can be kinetically slowed down for low temperatures and their extent limited by short times and conversely accelerated for high temperatures and their extent increased by long times. As an application, we show that incipient wetness impregnation immediately followed by freezedrying effectively inhibits the formation of [A1Mo6], allowing to compare the evolution of supported iso- and hetero-polymolybdates.

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/: :.!::miri~: i:i i:1:::hoi!r: i: i: ::~y::: i: week time

Fig. 6: Various steps of catalysts preparation in a T = f(t) plot (see text)" FD" freeze-drying, Imp: impregnation, EA: equilibrium adsorption, DP: deposition-precipitation, Dr: drying, A g i n g (also k n o w n as maturation). 5. C O N C L U S I O N Although the MoOx/AI203 and WOx/A1203 systems have been extensively studied, both in their o w n fight and as precursors of HDS catalysts, their chemistry is still a matter o f discovery as illustrated here with the formation in solution of AI(OH)6Mo6Ols 3" during the initial preparation step of MoOx/A1203 and its subsequent deposition on the support. Complexation o f dissolved A1In by the polyoxometalate precursor to form [A1M06] results in extensive dissolution of alumina, even at p H ' s close to neutrality. The same type o f p h e n o m e n o n also occur for WOx/A1203. The formation of these new mixed species is shown to influence the final state of the catalysts. This can be achieved via the control of the parameters (temperature, duration of the preparation step) influencing the kinetics o f processes occurring at the oxide/water interface. These results call for a critical reevaluation of literature data on the speciation o f Mo and W species in the MoOx/A1203 and WOx/A1203 systems. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

J.F. Lambert and M. Che, Stud. Surf. Sci. Catal., 109 (1997) 91. M. Che, Stud. Surf. Sci. Catal., 75A (1992) 31. O. Clause, M. Kermarec, L. Bonneviot, F. Villain and M. Che, J. Am. Chem. Soc., 114 (1992) 4709. J.B. d'Espinose de la Caillerie, M. Kermarec and O. Clause, J. Am. Chem. Soc., 117 (1995) 11471. S. Kasztelan, E. Payen and J. B. Moffat, J. Catal., 112 (1988) 320. X. Carrier, J. F. Lambert and M. Che, Stud. Surf. Sci. Catal., 121 (1999) 311. X. Carrier, J. F. Lambert and M. Che, J. Am. Chem. Soc., 119 (1997) 10137. L.O. Ohman, Inorg. Chem., 28 (1989) 3629. W.P. Griffith and P. J. B. Lesniak, J. Chem. Soc. A., 7 (1969) 1066. H. D. Morris and P. D. Ellis, J. Am. Chem. Soc., 111 (1989) 6045. G. Furrer and W. Stumm, Geochim. Cosmochim. Acta, 50 (1986) 1847. A. E. Blum and L. L. Stillings, Rev. Mineral., 31 (1995) 291. W. H. Casey and C. Ludwig, Nature, 381 (1996) 506. P. Benezeth, S. Castet, J.L Dandurand, R. Gout and J. Schott, Geochim. Cosmochim. Acta, 58 (1994) 4561. 15. X. Carrier, J. B. d'Espinose de la Caillerie, J. F. Lambert and M. Che, J. Am. Chem. Soc., 121 (1999) 3377. 16. X. Carrier, J. F. Lambert and M. Che, Stud. Surf. Sci. Catal. 118 (1998) 469.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro(Editors) 9 2000 Elsevier Science B.V. All rightsreserved.

Chemistry of the preparation of silica-supported cobalt catalysts from Co(II) and Co(III) complexes: grafting versus phyllosilicate formation R. Trujillano*, J. Grimoult, C. Louis and J.-F. Lambert Laboratoire de R6activit6 de Surface, CNRS-UMR 7609, Universit6 Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05 (France). Fax: +33 1 44 27 60 33 Depending on the experimental conditions used for the preparation of C o / S i O 2 materials, the cobalt complexes interact with the oxide surface in different ways, so as to lead to: (i) grafted isolated Co species when silica acts as a bidentate ligand and (ii) cobalt phyllosilicates when silica acts as a reactant. 1. INTRODUCTION Co/SiO2 systems have been recently the object of renewed interest as efficient Fischer-Tropsch catalysts [1-3]. In spite of several studies on the preparation of such systems [4] some uncertainty remains on the basic mechanisms of m e t a l / s u p p o r t interaction during the initial steps. In comparison with the better-known Ni/SiO2 catalysts [5-7], the Co/SiO2 catalysts present the additional complication of possible redox equilibria involving the Co(II)/Co(III) couple. It is well known that the coordination chemistry of Co(II) is dominated by the formation of both octahedral and tetrahedral complexes. In addition, in basic solutions, Co(II) is readily oxidized to Co(III), especially when it is in the form of amine complexes [8]. In this work, we establish that the dominant phenomenon during the deposition step may be the formation of Inner-Sphere Complex (ISC) between surface ligands and Co(II) (grafting), or the neoformation of Co(II)phyllosilicates. The two phenomena may be in competition, depending on the deposition procedure and the parameters settings. 2. ADSORPTION OF [Co(en)x(H20)~i-2x]2+: Co (II) GRAFTING The first procedure used for Co/SiO2 synthesis was an equilibrium adsorption of specific cobalt complexes. 2g of silica (Degussa, Aerosil 380) were suspended into 50 mL of a thoroughly outgassed 0.2M Co(II) nitrate solution. Stoichiometric amounts of ethanediamine (en) were then added with molar ratios en/Co = 1, 2 or 3. The resulting suspension was left to equilibrate for 15 minutes under Ar. The solids were separated by centrifugation under air and washed several times with distilled water. This procedure was repeated until no more nitrate anions were observed in the supernatant water. The absence of nitrate ions was verified by the ferrous ammonium sulphate test [9]. The solids retained about 2 wt% of Co.

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Samples have been n a m e d as follows: C o e n x / w , C o e n x / R T and Coenx/100, where x = 1, 2, or 3 depending on the en/Co ratio. The samples were characterized wet (w), dried at room temperature (RT) or at 100~ for 24 hours. The samples C o e n l / w and Coen2/w are bright mauve-blue whereas C o e n 3 / w is "dirty-pink". All of the samples are mauve-pink after drying at RT and mauve after drying at 100 ~ The UV-Vis spectroscopy was used to follow the d-d transitions of supported cobalt complexes, which are highly sensitive to the oxidation state and molecular environment of the metal center. The spectra of Coenl and Coen2 are similar. Figs. la and lb show the UV-Vis spectra of the Coenl and Coen3 samples, respectively. The UV-Vis spectra of the Coenl (Fig. la) and Coen2 samples show the presence of a triplet of bands characteristic of the 4T 1 ~ 4A2 transition of Co(II) tetrahedrally coordinated that appears as multiple absorption [10]. This intense triplet allows to distinguish clearly tetrahedral from octahedral species. Coen3/w and Coen3/RT present bands at 365 nm that may be attributable to the 1Alg~lT2g transition of the Co(III) in octahedral coordination[10]. These samples were indeed observed to be oxidised by atmospheric 02 during the washing and centrifugation. Upon drying at 100~ the UV-Vis spectrum is deeply modified and the bands characteristic of tetrahedral Co(II) complexes, are now predominant, as they are for the Coenl and Coen2 samples. The TGA-DTA analysis show that the ethanediamine ligands can be eliminated by calcination at about 300 ~ The likely outcome of this procedure consists in isolated, grafted Co(II) with coordinative unsaturation.

/-

b

a

1

IIi

t~

f O

IOC

<

200

I

I

400

I

I

600 ~/nm

I

I

800

200

I

I

400

I

I

600 /nm

I

I

800

i

Fig. 1.-UV-Vis spectra of Coenl (a) and Coen3 samples (b) (15 min. of contact time)

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The FT-IR spectra of the three series of samples show the bands characteristic of silica. The shoulders at about 3250 and 3100 cm "1 are mostly attributable to the N-H and C-H vibrational modes of ethanediamine ligands. For all of the samples dried at 100 C, the UV-visible spectra show bands characteristic of Co(II) tetrahedrally coordinated. The FT-IR spectra and the thermal analysis confirm the presence of ethanediamine ligands. However some ethanediamine ligands are lost during washing [11]. The most likely assignment is [(SiO)2Co(II)(en)] ISC [11]. EXAFS experiments are in progress to check the presence of Si in the second coordination sphere. This indicates that the reaction of oxidation of Co(II) into Co(III) during washing/centrifugation in air, in the Coen3 series, is completely reversible on drying at 100 C. 3. ADSORPTION AT LONG CONTACT TIMES A second series of samples were prepared using the same procedure as in 2, but with a much longer contact time (one week). Samples were characterized after drying at 100 C. The Co loadings retained by these solids were 10, 18 and 8 wt% for Coenl, Coen2 and Coen3, respectively. The characterization of the solid shows that cobalt is almost quantitatively transformed to a trioctahedral Co(II) phyllosilicate with talc structure (T-O-T). A Co-talc synthesized by the deposition-precipitation method developed by Geus [12] was used as reference compound. Fig. 2 shows the FT-IR spectra of Coenl, Coen2 and Coen3, and Co-talc. 0.8

Coen3

--"-""-x

0.70.6

i

~

~,_./// Coen2

/'~

~~"~~f"~'~

1010/A ,

O5

~

["0.4 0.3-

~

/

1

.

2

-

3630

0.20.1

4000

I

I

I

3500

3000

2500

I CITI"

12000

I

I

I

1500

1000

500

Fig.2.-FT-IR spectra of Co-Talc, Coenl, Coen2 and Coen3 (long contact time)

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In the Coenl, Coen2 and Coen3 FT-IR spectra (Fig. 2), the symmetric stretching Si-O vibration of silica can be observed at around 1100 cm -1 with a shoulder at higher wavenumber. At lower frequencies the bands at 800 and 468 cm -1 are due to the asymmetric Si-O stretching and the Si-O bending modes of silica, respectively [13]. The Coenl and Coen2 FT-IR spectra also show the stretching vibration band, VOH, at 3633 cm -1, and the vsio and ~5OH vibrations characteristic of the Co-Talc at 1010 and 670 cm -1, respectively [14] (inset in Fig. 2). For sample Coen3, the peaks characteristic of phyllosilicates are much weaker. The XRD pattern of Co-Talc (inset in Fig. 3) shows the in-plane d(06,33) peaks at 1.52 ik, and the d(001) peak at 12.5 ~. The first peak permits the detection of very small amounts of the silicate phase. For the Coenl and Coen2 samples the reflection peak at 1.52 A is observed. For Coen3, this peak is much less intense. The (001) basal spacing does not appear probably because the stacking along the c axis is not regular [15].

i

Co-Talc

._

0

ll0

I

20

I

30

l

40 2O

I

50

I

I

60

70

I

80

s

.~,i

!

iO 2 0

10

' .... J~ ' " 'S

....

I

I

I

I

I

I

I

20

30

40 20

50

60

70

80

Fig. 3.-XRD of Co-Talc, Coenl, Coen2 and Coen3 (long contact time)

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1059

r

o oo

t~ @ ,.m

<

Coen3 Coen2 524 I

200

I

i

400

I

~nm

I

645

600

I

I

Coenl Co-Talc

800

Fig. 4.- UV-Vis spectra of Co-Talc, Coenl, Coen2 and Coen3 (long contact time) Fig. 4 shows the UV-Vis spectra of the Coenl, Coen2, Coen3, and the CoTalc samples. The spectrum of the Co-Talc presents a band at 524 nm and a shoulder at 645 nm. These bands can be assigned to the transitions v3 (4A2g 4Tlg ) and v2 (4Tlg (P)~ 4Tlg ), respectively, typical of Co(II) in octahedral coordination. The Coenl and Coen2 spectra (Fig. 4) also show these bands. In contrast, the Coen3 spectrum shows the three bands characteristic of tetrahedrally coordinated Co(II), already observed after adsorption during a short time (Fig lb). The characterization by IR, XRD and UV-Vis indicates that Cophyllosilicates are formed as a major species in the samples Coenl and Coen2, but only to a lesser extent for the sample Coen3. The number of en ligands per Co(II) complex in the solution is an important factor determining the formation of the phyllosilicates: the complexation with three chelating ligands inhibits the silicate nucleation. 4. CONCLUSIONS The formation of grafted Co(II) complexes is fast and probably reaches equilibrium in procedure 1 within less than 15 min. Contact time of 2 hours did not show any difference in the Co speciation or Co loading [11]. However, the formation of grafted Co(II) complexes is accompanied by a much slower reaction of the Co complexes with dissolved silica species to yield cobalt phyllosilicates. It

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is proposed here that the reacting Co species is the hexaaqua complex [Co(H20)6] 2+, which is present due to aquation equilibria (although as a minor species). Quantitatively, it should be formed to a higher extent in the Coenl and Coen2 samples than in the Coen3 sample. The kinetics of phyllosilicate formation would then follow the same trend. This example has the interest of raising the often-overlooked question of reaction kinetics in supported catalysts preparation [5]. Chemical phenomena with very different kinetics may be operating simultaneously, and this may be used as a tool for the fine-tunning of catalyst preparation. Work is currently in progress to assess the catalytic properties of the two different types of Co/SiO2 systems that have been synthesized.

Acknowdlegements: R. Trujillano thanks the European Commission for the TMR Marie Curie Training Grant ref. ERB4001GT974983 and the Ministerio de Educaci6n y Cultura (Spain) for the FPU Grant ref. EX9907848484 REFERENCES

1. Y. Nitta, K. Ueno and T. Imanaka, Appl. Catal., 56 (1989) 9. 2. L. B. Backman, A. Rautiainen, A. O. I. Krause and Linblad, M. Catal. Today, 43 (1998) 11. 3. L.M. Gandia, A. Diaz and M. Montes, J. Catal., 197 (1995) 461. 4. Y. Okamoto, K. Nagata, T. Adachi, T. Imanaka, K. Inamura and T. Takyu, J. Phys. Chem., 95 (1991) 310. 5. J.-F. Lambert and M. Che, In Dynamics of Surfaces and Reaction Kinetics in Heterogeneous Catalysis, G. F. Froment and K. C. Waugh, (eds.), Elsevier: Amsterdam. Stud. Surf. Sci. Catal., 109 (1997) 91. 6. M. Kermarec, J.Y. Carriat, P. Burattin and M. Che, J. Phys Chem., 98 (1994) 12008. 7. P. Burattin, M. Che and C. Louis, J. Phys. Chem. B, 102 (1998) 2722. 8. B.J. Hathaway and C.E. Lewis, J. Chem. Soc. (A), (1969) 1183. 9. A.F. Williams, In Classical Analysis: Gravimetic and Titrimetric Determination of the Elements, C. L. Wilson and W. W. Wilson, (eds.), Elsevier: Amsterdam, Vol. IC (1962) 207. 10. A.B.P. Lever, Inorganic Electronic Spectroscopy, Elsevier: Amsterdam, 1984. 11. R. Trujillano, J. F. Lambert and C. Louis, In preparation. 12. J.W. Geus, Production and Thermal Pretreatment of Supported Catalysts, Elsevier Science Publishers B. V.: Amsterdam, Vol. III (1983) 1. 13. E.R. Lippincot, A. Van Valkenberg, C.E. Weir and E.N. Bunting, J. Res. Nat. Bur. Std., A 61 (1958) 61. 14. V.C. Farmer, The Infrared Spectra of Minerals, Mineralogical Society: London, (1974). 15. N. Takahashi, M. Tanaka, T. Satoh and T. Endo, Chem. Soc. Jpn., 67 (1994) 2463.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

N e w method of the preparation of heterogeneous Ni(0) complexes for ethylene oligomerization M.K.Munshieva Institute of inorganic and physical chemistry Azerbaijan Academy of sciences, Baku, 370143, H.Javid avenue, 29, Azerbaijan Republic Heterogeneous metal-complexes of the zero-valent nickel have been synthesized by ligand exchange between triphenylphosphine or hexaethyltriamidophosphine Ni(0) complex and chemically modified by method of immobilization support- silica, containing complex forming- NPRE-group, where R is Ph or NEt2. The comparative characteristics of the activity of these catalysts and their homogeneous analogues in ethylene oligomerization are given. It is shown that various factors (pressure, temperature, molar correlation A1/Ni) do not significantly influence the selectivity of this process in the presence of investigated heterogeneous metal-complexes.

1. INTRODUCTION Available vast information in the scientific literature on synthesis and mechanism of heterogeneous metal-complex catalysts (HMC) action points to combination of advantages in both homogeneous and heterogeneous catalysts. The catalytic activity of HMC has been investigated in a series of processes, including oligomerization of lower olefins [1]. However, in connection with limitations of physicochemical methods of research of HMC, the data on formation of catalytic active centres on the surface of the support and mechanism of their action are not numerous and are contradictory. Therefore, the aim of present research is working out the method of stickiness of triphenylphosphine and first synthesized by us hexaethyltriamidophosphine complexes of Ni(0) to the surface of support- silica, study of process of forming the active centre on the case of homogeneous analogue and comparative study of catalytic activity of these HMC and their homogeneous analogues in ethylene oligomerization. 2. EXPERIMENTAL Silica with a specific surface- 120 m2/g was used as a support. Before modifying, the silica was calcinated by way of keeping it in vacuum (10 2 torr) at 600~ for 6 hours. The density of stickiness of modifier on the support is defined by the titrimetric method, processing it by (EtO)3Si(CHE)3NH2 [2].

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The heterogeneous Ni-complexes were obtained by the method of ligand exchange of phosphine groups stuck to the surface of the support and ligands in Ni(PR3)., where R is Ph or NEt2.

The amount of nickel in samples on the surface of studied supports was determined by method of polarography. It ranges within 0,75-0,8% wt. Ni(PR3)n with R-Ph was obtained by method [3] and with R- NEt2, according to [4]. IR-spectra of diffuse reflection were recorded on the spectrometer "Hitachi M-430" at the n)om temperature. ~H and 13C NMR spectra were measured on the spectrometer "Bruker WP- 200 SY" at 22~ in C6D6 or CDCI3 taken as a solvent. 31p-{IH} NMR spectra were registered on a "Bruker HX-90 (with 85% H3PO4 as an external standard) Ethylene oligomerization was conducted in the reactor, which served as a vessel of stainless steel in volume of 250 cm 3 tested for the pressure of 100 atm. Into the reactor filled with argon the certain amount of catalyst in corresponding solvent was introduced. Following that, the system while intermixing, was filled with ethylene up to the pressure of 10 atm and activator AIEt2CI was added. GLC analysis of the reaction products was carried out on LHM-8MD chromatograph, equipped with flame-ionizing detector, using a 2m• mm column (5% SE-chezasorb AWHMDS as filler and helium as carrier gas) with temperatures ranging from 50 to 250~ All the experiments were carried out under an argon atmosphere using absolute solvents, which were distilled under argon prior to use.

3. RESULTS AND DISCUSSIONS To obtain the chemically modified support the method of surface collection or immobilization is used. The first method consists of interaction of dehydrated silica with (EtO)sSi(CH2)3NH2 with further processing by benzene solution of PPh2CI or P(NEt2)2CI, from which it follows that on the surface of silica the heterogeneous compounds can be located due to its heterogeneity and taking into consideration the steric factors. Therefore, in our subsequent research we used only the method of immobilization, consisting of singlestage stickiness to the surface of silica of preliminarily prepared modifier- the product of interaction of (EtO)3Si(CH2)3NH2 with PPh2Cl or P(NEt2)2CI. For purpose of obtaining more full information on processes proceeding on the surface of the support, the attempt of carrying out the identical reaction with homogeneous analogues (contrary synthesis) has been undertaken. The contrary synthesis of the initial modifier for subsequent immobilization on silica has shown that reaction can proceed in two directions with formation of mono-(a) and twosubstituted (b) product: (EtO)3Si(CH2)3NH2 + PR2CI

[-

~ (EtO)3Si(CH2)3NHPR2

(a)

I..

~ (EtO)3Si(CH2)3N(PR2)2

(b)

where R is Ph or NEt2. The results of elementary analysis, NMR and IR spectroscopic studies witness formation of two-substituted product. Thus, the absence of bands of absorption at 3300-3370 cm ~

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points to absence of N-H bond in the synthesized compounds. Data of 1H NMR spectra confirm this conclusion that in the field typical for NH group the signals are not observed. In 13C NMR spectra of the obtained product (EtO)3Si(CH2)3N(PPh2)2 notable removal of the signals of -CH2- to side of the weaker field is observed. Attention is paid to the fact that the chemicals shiR of CH2N-group (56.45 ppm) in 13C NMR spectra approaches the chemical removal of CH20-group (58.38 ppm). The same spectra identified splitting the signal of this group on phosphorous atom with constant of splitting (11.0_+0.6 Hz). Identification of IH and ~3C N M R - spectra of (EtO)3Si(CH2)3N[P(NEt2)2]2 is somewhat difficult because the superposition of own vibrations of CH2N and CH3C-groups of ligand on vibrations of these groups in (EtO)3Si(CH2)3NH2 takes place (Table 1). -

Table 1. 13C and 31p_{IH} NMR chemical shifts of (EtO)3Si(CH2)3NH2,the synthesized product and relative compounds [5,6] 13C ~, ppm. 31P-{IH} CH3C CH20 CH2 -CH2- CH2N 5, ppm Substance (EtO)3Si(CH2)3NH2 18.48 58.31 8.01 27.31 45.44 (EtO)3Si(CHE)3N(PPh2)2 18.52 58.30 8 . 2 1 25.33 56.45 63.11 (EtO)3Si(CH2)2N[P(NEt2)2]2 18.51 58.0 8.2 23.17 51.62 119.74 Me(CHE)EN(PPh2)2 53.3 48.20 EtNHPPh2 45.0 40.04 EtN(PPh2)2 50.9 61.80

On 31p{IH} M R spectra of product of interaction of (EtO)3Si(CH2)3NH2 with P(NEt2)2CI one singlet signal is observed at 5 119.79 ppm which, taking into consideration the data of the work [6], should be refereed to the obtained compound (EtO)3Si(CHE)3N[P(NEt2)2]2. After 12 hours of keeping this compound at 80~ no change has been observed. It is a white gel like mass. The total data obtained when studying reaction of phosphining NH2-groups permit to draw a conclusion that, as a result of aforementioned reaction, substitution of both hydrogen atoms of NH2-group with creation of two-substituted compound takes place. By interaction of calculated amounts of preliminary pretreated at 600~ (10 2 torr) silica with (EtO)2Si(CH2)2N[PR2]2 in boiling benzene there was obtained chemically modified silica containing N[PR2]2 group, characterized by significant electron-donor capability. OEt ~ SiOH+(EtO)3 Si(CH2)3N[PR2]2

'

- EtOH

~

S i O - Si (CH2)3N[PR2]2

odt

From the known physicochemical methods of research restriction only by study of IRspectra of diffuse reflection we can obtain limited information on creation of the active centre on the surface of support. On IR-spectra of diffuse reflection of samples of the chemically modified by (EtO)3Si(CH2)3N[PPh2]2 silica the main changes are observed in the field of vibrations of CH-bonds. Two high frequency bands of absorption appear at 3055 and 3010 cm-1

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

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as well as at 2965, 2940 and 2860 cm "l, which belong to the aromatic rings. On these spectra of samples of chemically modified by (EtO)3Si(CH2)3N[P(NEt2)2]2 silica besides bands of absorption at 3750-3600 cm~, typical for OH-group of silica, there are observed the strong bands at 2970-2865 cml, belonging to the ethyl fragments, and at 2250 cm1,

typical f o r - N < . The suggested method of preparation of chemically modified silica helps us to obtain support, which contains the similar compounds and avoids the influence of the steric factors, because the preliminarily prepared modifier immobilizes on the surface of support. The obtained data have most important theoretical and practical significance, because of an opinion in the literature [7], that the phosphining of aminogroups contained in chemically modified silica with the creation of the two-substituted product is very difficult for steric reasons. To obtain the HMC as an active component zero-valent nickel complexes of the general formula- Ni[PRa]n (n=2-4), where R was Ph or EtEN, characterized by high activity in oligomerization of lower olefins in homogeneous conditions were taken. Heterogenization of these complexes was conducted by method of ligand exchange. Literature has examples of carbonyl complexes of palladium prepared by this method, which show activity in propylene dimerization [8]. However, we have failed to find data on such nickel catalysts in the literature. The ligand exchange has been carried out, according the following scheme: OEt I Si-O-S i(CH2)3N(PR2)2Ni(PR3)~_2 I OEt

OEt

I

~ Si-O-Si(CH2)3N(PR2)2 + Ni(PR3)n

I

OEt n=2-4

OEt I Si-O-Si(CH2)3N(PR2)2PR2Ni(PR3)n-, OEt

One or two PR2- ligand of Ni-complex is substituted for phosphine groups stuck to the surface of the support. Unfortunately, identification of nickel on the surface of support by the present spectral methods is not possible. In this connection determination of nickel and also the process of formation of Ni-HMC was conducted by method of polarography. For the control of completeness of ligand exchange proceeding the samples of the same series of support have been treatec[ by benzene solution of Ni-complex with identical nickel concentration during the different time. To achieve the equilibrium 3-4 hours are quite enough. It was shown by experiments these complexes totally collapsed at 80~ Therefore, the formation of Ni-HMC was carried out at the room temperature. For purpose of comparison of activity, selectivity and duration of work, the catalytic properties of Ni-HMC and its homogeneous analogues- (EtO)3Si(CHz)3N(PRz)2Ni(PR3)n-2 in the identical conditions in combination with activator A1EtzC1have been studied. (Table 2).

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Table 2. The activity and selectivity of Ni - HMC and their homogenous analogues in ethylene oligomerization Selectivity (%) Ni-complex A1/Ni Time ~ --~ (mol) (min) ~ ,~ Y~C4 ]~C6 d:~ Z ~: O ~r..) r/)

,,..~

Ni(PPh3). Ni[P(NEt2)3]2 Ni(PPha)n+ (EtO)3 Si(CHz)3N(PPh2)2 Ni[P(NEt2)3]2+ (Et 0)3 Si(C HE)3N [P (NEt2)2]2 Ni(PPh3). I SiO2-PPh2* Ni[al.]qEt2)3]2 I SiO2-P(NEt2)2 * _

n=2-4;

* Brief notation of

6.1 3.3 6.0

50 30 50

60 30 60

79.8 64.4 102.2

85.5 83.4 91.4

14.4 16.6 4.4

0.1 4.2

3.0

30

30

77.6

94.7

5.2

0.1

6.0 6.1

50 50

60 60

46.7 52.0

98.2 91.6

1.8 8.3

0.1

I

OSi (CHz)3N(PPh2)z.Ni(PPh3)._2

I

I

OSi (CH2)3N[P(NEt2)2]z-NiIP(NEt_03]2

I

The catalytic experiments have shown the essential differences both in productivity and selectivity of the process of oligomerization for catalysts of different kind. During the early stage of work homogeneous catalysts were more active. Their initial activity exceeds the heterogeneous analogues by 3-3.5 times and for systems on Ni[P(NEt2)3]2 base it reaches 11-16 kmol C2Ha/molNi.min. However, the time of their active work does not exceed 1015 min. At the same time the heterogeneous sample in one hour of work, in practice, has entirely preserved initial activity. The amount of ethylene entered in reaction on compared systems for 1 hour of work made up 102.2 and 46.7 g correspondingly, i.e. the difference in yields of product on this systems even in 1 hour of work is significantly less [9,10]. The compared catalytic systems show differences also in selectivity of the process. In case of homogeneous analogues the amount of dimers in products of reaction does not exceed 8385%. The heterogeneous samples are characterized by more high content of butene, which reaches 91-98%. In addition, selectivity, according butene-1 in C4-fraction, for homogeneous and heterogeneous catalysts makes up 15 and 49.5% correspondingly. It should be noted that the heterogeneous complexes on Ni[P(NEt2)3]2 base, showing at the early stage of work higher activity in ethylene oligomerization and better selectivity in respect to butene-1, are less steady in comparison with such complexes on Ni(PPh3), base. It was established that in the presence of investigated Ni-HMC various factors, such as pressure, tempcrature, molar correlation A1/Ni do not significantly influence the selectivity of oligomerization process, but the olefin conversion and the yield of product are very depended on these factors. (Table3).

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Table 3. The influence of various factors the

ethylene oligomerization in the presence of system

OSi (CH2)3N[P(NEt2)2]2-Ni[P(NEt2)3]2+AIEt2CI Selectivity (%) AI/Ni (mol)

r~

r

w o E O

o

~ N

Time (min)

m o,,.~ ~ o

"O

EC4

%C6

t~ O

z

"O

0,61

3,9

38

10

20

60

2,40

95

3,5

1,5

0,58 0,58 0,32 0,4 0,62 0,67

3,6 3,6 2,5 3,0 3,9 4,0

42 42 65 52 20,5 9

10 10 20 10 5 8

22 30 32 35 22 22

40 40 60 50 60 30

2,26 2,20 2,01 1,99 0,51 0,53

93 93 82 93 96 95

5,3 4,8 14,5 5,3 3 5

1,7 2,2 3,5 1,7 1,0 -

Thus, by modifying the Ni-HMC with specific exchangeable ligands its efficiency can be increased and the selectivity of reaction regulated.

REFERENCES

1 2. 3 4 5 6.

D.D. Whitehurst, Chem. Technol., No. 1 (1980), 44. G.E.Berendsen, K.A.Pikaart, L. de Galah, J. Liquid chromatogr., No.10 (1980), 1437. R.Wade, D.G.Holah, A.N.Hughes, B.C.Hui, Catalytic reviews, No.2 (1976), 211. D.B.Tagiev, D.B.Furman, M.K.Munshieva et al, Russ.Chem. Bull., No.3 (1991), 541. J.Schraml, Nguyen-Duc-Chuy, V.Chvalovsky et al. Org. Magn. Reson., No.7 (1975) 379. J.P.Van Linthoudt, E.V.Van der Berghe, G.P.Van der Kelen, Spectrochim. Acta. No.3 (1980), 315. 7. B.D.Dovganjuk, L.I.Lafer, V.I. Isaeva, V.Z. Shaft et al. Russ.Chem.Bull., No. 12 (1987),2660. 8. T.O.Mitchell, D.D.Whitehurst, Organic compounds conversion, US Patent No 4053534 (1977). 9. D.B.Furman, A.O.Ivanov, M.K.Munshieva et al. Russ.Chem.Bull., No.3 (1990),516. 10. M.K.Munshieva, Azerb. Chem.J., No.2 (1999), 104.

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Studiesin Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Synthesis, eharaeterizafion and catalytic activity o f [Cu(NH3)4] 2+ supported on hydrous oxides D. Patel and A. Patel* *Department of Chemistry, Faculty of Science, M. S. University of Baroda, Vadodara- 390 002, India. Hydrous oxides of Group-iv have been synthesized and used as supports on which [Cu(NH3)4] 2+ has been supported. The supports as well as supported materials have been characterized for Chemical analysis, f.t.i.r., e.s.r., reflectance spectra, t.g.a., d.t.a, and surface area measurement. The supported species are effective as catalysts for the decomposition of H202. The effect of concentration of H202, catalyst amount and temperature on the HEO2 d e c o m p o s i t i o n have been studied. The trend of the activation energy of systems has been explained by considering their structure, and the combined effect of surface area and the amount of the [Cu(NH3)4] 2+ supported on the hydrous oxides.

1. INTRODUCTION Catalysis by supported metal complexes is an attractive and interesting field of research, involve the dissolution of the traditional disciplinary barrier between homogeneous and heterogeneous catalysis. Transition metal complexes have been affixed to polymers, silica or alumina surfaces [1-3]. The encapsulation of Transition metal complexes in zeolite pores have been well established [4, 5]. It is a measure of the c o m p l e x i c t y and variety of chemical reactions in which H202 participates that they have been attracting attention for almost a hundred years. Especially because of its biochemical significance, there has been a good deal of interest in kinetic studies in formation, d e c o m p o s i t i o n and substract reaction of peroxo compounds of biologically important metal complexes such as catalase and peroxidase. The decomposition of H202 is catalysed by most of transition metal ions and their complexes. The catalytic activity of Fe(II), Co(II) and especially Cu(II), binary metal complexes in the dispropornatation of H202 in aqueous solution has been studied [6, 7]. The catalytic activity of Transition metal complexes supported on Dowex 50 [8, 9] and on insoluble acid salts of tetravalent metal ions[10] have also been studied. The term ' h y d r o u s - o x i d e ' has been used in it's widest sense to refer the insoluble materials with a metal oxide - water system. The amorphous form can be represented as MOb xH20 and surface hydroxyl groups are responsible for the ion exchange behaviour. Much of the work has been done on ion

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e x c h a n g e and sorption p r o p e r t i e s of h y d r o u s oxides [11, 12] of group iv since they are i n t e r e s t i n g from the v i e w point of their acid, base p r o p e r t y . Not m u c h work has been done on the c a t a l y t i c aspects of the same. It was t h e r e f o r e t h o u g h t of interest to make the use of the h y d r o u s oxides as the c a t a l y s t carriers. In this paper we report the synthesis and characterization of [Cu(NH3)4] 2+ supported hydrous oxides, namely hydrous zirconium (iv) oxide(Z), hydrous tin (iv)oxide (S) and mixed hydrous oxides of zirconium(iv) and tin(iv) (ZS) and their use in the oxidation of H202 decomposition. All studies have been carried out on amorphous materials.

2. EXPERIMENTAL METHODS 2.1. Preparation of the supports [Hydrous zirconium(iv)oxide(Z) ,Hydrous tin(iv) oxide (S),Mixed hydrous oxides (ZS)] Zirconium hydroxide and Tin hydroxide were prepared by adding an aqueous ammonia solution to an aqueous solution of ZrOC12.8H20 (0.31 M) or SnC14 (0.33 M) upto pH 8.8. The mixed hydroxide of Zr (iv) and Sn (iv) with atomic ratio of Sn to Zr equal to 1:9 was prepared by coprecipitation of a mixed solution of ZrOC1E.8H20 and SnC14 with aqueous ammmonia solution upto the pH 8.8. The resulting gel was aged at 100~ over a water bath for 1 hour., filtered, washed with conductivity water until free of chloride ions and dried at 100~ for 24 hours. The obtained gel was converted to the desired particle size (30-60 mesh) by sieving.

2.2. Preparation of the catalysts [CuTZ, CUTS, CuTZS] One gram of the each support was placed in contact with a known concentration of the complex ion [Cu(NH3)4 ]2+-(CUT), solution in flask for 24 hours with intermittent shaking. It was finally filtered, washed with conductivity water to effect complete removal of the adhering complex and dried at 100~ The amount of copper present on the hydrous oxide was calculated from the difference between the initial and final concentration of the complex ion solution by the Iodometric method [ 13].

2.3. Catalytic studies

A known weight of catalyst was stirred with 10 ml. (5.0 Vol.) of H2 02 in a thermostat at 25~ The decomposition was followed by measuring the evolution of 02 as a function of time in a gas burette [14]. The reaction temperature was varied between 25 to 40~ + l~ The influence of the amount of catalyst and concentration of H2 02 was also studied at 30~

2.4. Characterizations The samples has been analysed gravimetrically for Zr as zirconium oxide by cupferron method and for Sn as Tin oxide by sulphide method. Thermogravimetric analysis was performed on Shimadu DT 30 at a heating rate of 10~ min l. Differential thermal analysis were performed on SETARAM DTA A92 equipment with a heating rate of 5 K min -1 and an air flow rate of 20 cm 3 min "1. Platinum Crucible was used as a sample holder and ct - A1203 as the reference material. The f.t.i.r, spectra of the samples were obtained using KBr wafer on f.t.i.r. Model PARAGON 1000. Reflectance spectra of the samples using Barium Sulphate as reference were recorded on Shimadzu PR-1. The e.s.r, spectra (at room temperature) were obtained on a Varian E-15 spectrometer. The gav. values were reported relative to TCNE

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standard with g = 2.00277. Adsorption- desorption isotherms of nitrogen were recorded on a Carlo Erba Sorptomatic series 1800 at-196~ and surface area was calculated using Brunner Emmett Teller (BET) method.

3. RESULTS AND DISCUSSION The t.g.a, of Z and S indicates weight loss of 13% and 9% respectively within a temperature range of 100-180~ corresponding to the loss of hydrated water. Hence the single hydrous oxide can be formulated as ZrO2.H20 and SnO2.0.7H20. The t.g.a, of ZS shows weight loss of 13% corresponding to the loss of hydrated water. Chemical analysis indicates the composition of ZS, ZrO2SnO2 to be 6:1. Thus, from the chemical analysis and the t.g.a, data, formula for the mixed material, ZS, can be formulate as SnO26ZrO2.7H20. The number of H20 molecules was determined using Alberti- Torracca formula [ 15]. In all three cases no significant change in weight is observed upto 500~ The d.t.a, of Z, S and ZS shows exothermic peak at 404~176 and 445~ respectively. These could be due to phase transformation / crystallization. It is also interesting to note that the temperature for exothermic peak of ZrO2.SnO2 is higher than that of ZrO2. This indicates that the ZrO2.SnO2 is not simple physical mixture ofZrO2 and.SnO2, but it must be solid solution. The t.g.a, of CuTZ, CuTS and CuTZS shows a break within the temperature range of 300350~ corresponding to the decomposition of CuT from the surface of the supports. The d.t.a. of all three shows additional exothermic peak at 350~ in good agreement with the t.g.a, data. Thus, the supports are amorphous and decomposition of CuT takes place before the temperature require for phase transformation / crystallization. The f.t.i.r, spectra of all the samples show band in the region ~ 3400 cm -I attributed to the asymmetric and symmetric hydroxo -(OH) and aquo -(OH) stretches. Two type of bending vibrations at-~1620 cm 1 and ~ 1320 cm -1 are observed indicating the presence of- (H-O-H) bending and - (O-H-O) - bending, respectively. A weak bending band at-~ 850 cm -1 in Z and at ~ 1000 cm -1 in S is also observed attributing to the presence of Zr-O-H and Sn-O-H bond, respectively. In ZS, two weak bending bands are observed at-~ 850 cm -I and ~ 1000 cm l attributing to the presence of Zr-O-H and Sn-O-H bond respectively. The f.t.i.r, spectra of the supported species indicates the presence of the complex on the supports. However, a quantitative interpretation is not possible due to strong coupling between various modes. The f.t.i.r, spectra of CuTZ, CuTS and CuTZS shows broad bands at-~ 3300 cm -I and -~ 1600 cm 1 which may be attributed to merging of the - OH and -NH stretching and bending modes respectively. max for Cu (II) amine complex is 600 nm while that for all three samples is 800 nm. This shift in )~max may be attributed to a slight distortion in the geometry around the Cucentre, due to the sorption of CuT on to the surface of the supports [ 16, 17]. The e.s.r, spectra of the supported spicies are similar to that of the free complex in solid state. The gav values of CuTZ, CuTS and CuTZS (2.116) are slightly less as compared to gav value of free complex (2.126) indicating not much change in the geometry of the support bound complex. Similar observations have also been made earlier [ 16, 17].

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The surface area of supports as well as CuT bound to supports are represented in Table-1. Table- 1 Surface area of supports and CuT bound to supports. Supports SA (mZg"l) Supported SA (m2g "l) species Z S ZS

170 107 166

CuTZ CuTS CuTZS

200 121 170

In the present investigation, the kinetic analysis is based on the initial rate data, because the reaction rate approaches equilibrium after a lapse of time. It was found that, the rate was independent of initial concentration of H202 (from 5 Vol. to 10.0 Vol., the rate for Z, S and ZS was 6.61 x 10-4 k min l, 4.28 x 10-4 k min l and 5.70 x 10-4 k min l respectively). An increase in the amount of catalyst from 0.025 g to 0.075 g, increased the rate of reaction from 1.78 to 7.35 x 10-4 k min l, 1.57 to 6.01 x 10-4 k min l and 2.40 to 7.27 x 10-4 k min -l for Z, S and ZS respectively. An increase in the temperature of the systems increase the rate of the decomposition (Table- 2). Table-2 Influence of Temperature on decomposition of H202 [Catalyst = 0.05 g, Volume of H202 = 10 ml (5 Vol.) ] Materials Temp. (0~ 104 k (min l ) E (Kcal mol l ) CuTZ

25 30 35 40

4.30 6.57 9.48 14.39

CuTS

25 30 35 40

3.19 4.32 6.81 9.70

CuTZS

25 30 35 40

3.59 5.28 9.30 13.50

14.00

15.00

17.00

The colour of surface adsorbed complex ion was observed to change from blue to greenish yellow colour when it comes in contact with H202. The colour persisted as long as any residual H202 remained undecomposed. After complete decomposition of H202, the catalyst regained its original colour [10, 17] and this could be due to the formation of a peroxo species [ 19] as reported earlier [ 10, 18].

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It is well known that H202 dissociates to H202 --~ HO2" + H § (1) The surface [Cu(NH3)4 ]2+ may interact with HOE" ions to form an intermediate complex. (2) Surface [Cu(NH3)4 ]2+ + HOE" ~ Surface [Cu(NH3)4 (HOE)]+ A second molecule, of H202 may then interact with the intermediate complex to form the products (3) Surface [Cu(NH3)4 (HO2)] + + H202 ~ Surface [Cu(NH3) 4]2+ + H20 + 02 + O H During the decomposition of H202, a continuous increase in pH occurs confirming the liberation of O H ions as per suggested mechanisum[20]. The activation energy (E) calcualted from the Arrhenius plots (Fig.l) for all -2.4 three system is in the following sequence I , Zr02 and suggests that they are catalytically -2.6 , o Sn02 active. Mix CuTZ < CuTS < CuTZS -2.8 The activity of the catalyst may be " -3 explained on the basis of the formation of o a stable ternary complex of copper ligand -3.2 and hydrogen peroxide as an oxygen -3.4 ~ donor. The activity of surface complex is l dependent on the availability of sites -3.6 present on the surface of the support. 3.2 3.25 3.3 3.35 3.4 3.15 Thus, on increasing the amount of the catalyst, the rate should increased. The 1/T x 10-3 observation is in keeping with this Fig. 1. Arrhenius Rot expectation. It is known that the catalytic activity increases with increase in surface area. Higher surface area of catalysts as compared to that of the supports may be another reason for higher catalytic activity in the decomposition of H202. The observed trend can be explained by considering the structure [21 ] of Z and S. A comparison of structure (Fig.2) shows that the structure of Z is more open and consists of more number of hydroxyl groups exposed towards the surface. Thus, more number of protons are accessible to metal ions or metal complex ions for exchange Thus, although S is more acidic than Z, the Z can exchange more number of protons, with metal ions or metal complex ions and hence the amount of Cu(II) amine complex sorbed on Z as well as the surface are of CuTZ should be more as compared to that of S and CuTS respectively. The observation agrees with these expectations. (The amount of CuT sorbed on 1 g of each Z and S is 1.53 x 10 -l g and 1.078 x 10"1 g, respectively. The values forsurface area measurement for CuTZ and CuTS are 200 m2g-1 and 170 mEg-1, respectively). For mixed oxide, the amount of CuT sorbed on 1 g of ZS is 1.150 x 101 g. It is intermediate between the amount sorbed on individual hydrous oxides, i.e. Z and S. Accordingly the value of energy of activation for CuTZS can be expected to be between activation energy for CuTZ and CUTS. However, it is seen that the energy of activation for CuTZS is higher than for CuTZ and CUTS. It was shown earlier [22] that the amount of the catalyst and surface area, both are important factor responsible for catalytic activity. In case of CuTZS, the value of surface area

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is less where as amount of catalyst sorbed is more.

~

I-t20 H ~-.ra 9 o..H__d~

H

"" ~ ) - H

H

H" H-

h d~6"H

H

"H

I-~

H" H H H"o"H"O'~ HO-H

H

"H H

.

HI't3~ I-I20 ~ H ~'-~ I-I30+ ~

H

H 1LI

H20

I~O

OI5 H20

I--I30+ U

I-I20 H

H HH-

H, I-I20

]SnOa.nH20 ]

n H-

h

OH" n

H -H

H~ OH" I-t20 H H ~

I-IX) H-H,~H

H

ZrOi nH20

Fig.2. Structure of hydroxides The increase in energy of activation and hence decrease in catalytic activity is a combined effect of the two factors. REFERENCES 1. R.H. Grubbs, Chem. Tech., (1977) 512. 2. M. Jianbiao and D.C. Sherrington, J. Chem. Res., (1995) 4. 3. R.K. Bhatia and B. N. Rao, Synth. React. Inorg. Met. Org. Chem., 25 (1995) 781. 4. H. Diegruber, P. J. Plath and G. E. Schulz., J. Mol. Catal., 24 (1984) 115. 5. N. Herron and C. A. Tolman, J. Am. Chem. Soc., 109 (1987) 2837. 6. P. Waldmeier and H. Sigel, Inorg. Chem., 11 (1972) 2174. 7. E.N. Rizkalla, O. H. El. Shafey and N. M. Guindy, Inorg. Chim. Acta., 57 (1982) 109. 8. I.A.Salem, J. Mol. Catal., 80 (1993) 11. 9. I.A.Salem, Int. J. Chem. Kinet., 27 (1995) 499. 10. B.Pandit and U. Chudasama, Stud. Surf. Sci. Catal., 113 (1998) 865, Ref. there in. 11. A.Clearfield, Ind. Chem. Rev., 34 (8) (1995) 2865. 12. R.K.Ghatuary, A.K.Das, A.Ghatuary, R.N.P.Sinha, J.Inst. Chem.(Indian)68(6)(1996) 172. 13. A.I.Vogel, Textbook of Quantitative Inorganic Analysis, Longmans green, London (1987). 14. R. P. Gupta, R. N.Sharma and B. B. Prasad, J. Scient. Res., BHU, 23 (1972) 103. 15. G. Alberti and E. Torracca, J. Inorg. Chem., 30 (1968) 3075. 16. A.Clearfield and L. R. Quayle, Inorg. Chem., 21 (1982) 4197. 17. A.Shivanekar and U. Chudasama, J. Ind. Chem. Soc., 31A (1992) 771. 18. D.T. Gokak, B. V. Kamath and R. N. Ram, Ind. J. Chem., 23A (1986) 1143. 19. R. N. Ram, Ind. J. Chem. Tech., 23 (1985) 237. 20. Shivanekar and U. Chudasama, J. Mol. Catal., 55 (1989) 119. 21. A.Clearfield, Role of Ion Exchange in SolidState Chemistry, Chem.Rev., 88 (1988) 1253. 22. J. C. Vederine, E. G. Derovane and Y. Ben Tarrit, J. Phys. Chem., 78 (1974) 531.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

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Preparation and characterisation of Cu/ZnO and Pd/ZnO catalysts for partial oxidation of methanol. Control of catalyst surface area and particle size using microemulsion technique. Johan Agrell, Kristina Hasselbo, Sven J~irfis and Magali Boutonnet KTH - Royal Institute of Technology, Department of Chemical Engineering and Technology, Chemical Technology, Teknikringen 42, SE-100 44 Stockholm, Sweden

In this work, Cu/ZnO and Pd/ZnO catalysts for partial oxidation of methanol have been prepared by microemulsion technique. The microemulsion technique is a versatile preparation method, which enables control of catalyst surface area and particle size. The water-tosurfactant ratio (W0) governs the water droplet size in the microemulsion and the influence of W0 on catalyst properties was studied. It was found that CuO/ZnO surface areas as high as 87 m2/g could be obtained by varying W0 in the microemulsion. Furthermore, effective control of the Pd particle size (10-16 nm) was possible. The catalysts were characterised by TGA, BET, XRD and TEM. 1. I N T R O D U C T I O N Partial oxidation of methanol (Eq. (1)) for on-board hydrogen generation in fuel cell vehicles is an attractive alternative to conventional methanol steam reforming (Eq. (2)). The partial oxidation route is exothermic, requires no steam and is expected to display good response to transients. Cu and Pd based catalysts have been studied for methanol partial oxidation and the reaction rate has been found to be substantially higher than for the steam reforming route [ 1,2]. CHsOH + V202 ~ 2 H2 + CO2 CH3OH + H20 ----)3 H2 + CO2

AHr~ - 192 kJ/mol

AHr~

kJ/mol

(1) (2)

By using a liquid fuel, the problems of safety, handling and storage of hydrogen can be circumvented and the existing infrastructure for distribution of gasoline can be employed. Methanol is a fuel that has attracted increased attention. It has a high H/C-ratio, it is readily available and can be reformed into a hydrogen-rich gas at moderate temperatures, which makes it a suitable fuel for mobile fuel cell applications. This work deals with Cu/ZnO and Pd/ZnO catalysts prepared in water-in-oil (w/o) microemulsions. A w/o microemulsion is an optically isotropic dispersion of water in hydrocarbon, stabilised by surfactant molecules. The microemulsion is heterogeneous at molecular level, but thermodynamically stable [3]. By dissolving metal salts in the water domain of the microemulsion, in situ precipitation can be carried out in a controlled environment. This is known as microemulsion technique, a preparation method that generates

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particles of narrow size distribution and enables strict control of the particle size in the nanometer range [4]. In pioneering work by Boutonnet et al. [5], 3-5 nm particles of Pt, Pd, Rh and Ir were prepared by microemulsion technique. More recently, a wide range of materials has been prepared using this technique, e.g. metallic particles, metal oxides, semiconductors and magnetic materials [5-8]. The method is especially suited for preparation of catalysts and for studies on the effect of particle size and surface area on catalytic activity and product distribution. For instance, Kishida et al. [9] were able to vary the surface area of silicasupported Rh catalysts from 60 to 600 m2/g by using microemulsion technique and studied these catalysts for hydrogenation of carbon dioxide. 2. EXPERIMENTAL 2.1. Preparation of Cu/ZnO Cu/ZnO catalysts of 50:50 composition (wt.% Cu/Zn) were prepared by co-precipitation in microemulsion, using oxalic acid as the precipitating agent. A 0.65 M aqueous solution of Cu(NOa)2-2.5H20 (Riedel-de Hahn, 99%) and Zn(NO3)2"6H20 (Merck, 98.5%) was added to concentrations of 5-10 wt.% to a solution of 20 wt.% Berol 02 (nonylphenolethoxylate, Akzo Nobel Surface Chemistry) in cyclohexane (Merck, 99.5%). A second microemulsion containing a 0.80 M (25% excess) aqueous solution of oxalic acid (H2C2Oa'2H20, KEBO, puriss) was prepared and upon mixing of the two microemulsions, a precipitate consisting of Cu-Zn oxalate was formed. After stirring overnight, the precipitate was recovered by centrifugation and washed thoroughly with ethanol. The powder was dried at room temperature and subsequently calcined in air by increasing the temperature at 10 ~ to 350 ~ keeping it isothermal for 12 h. The microemulsion composition is generally expressed in terms of the molar ratio of water-to-surfactant (W0). 5-10 wt.% of aqueous phase correspond to Wo-ratios of 7.1-15. 2.2. Preparation of Pd/ZnO ZnO-supported Pd catalysts were prepared by first reducing Pd 2§ in AOT (sodium bis(2ethylhexyl) sulfosuccinate) microemulsions, using hydrazine as reducing agent. This was followed by destabilisation of the microemulsion and deposition of Pd ~ onto ZnO. In short, an isotropic microemulsion was prepared by adding a 0.22 M aqueous solution of Pd(NH3)4C12 (Alfa) to a solution of 15 wt.% AOT (Sigma, 99%) in iso-octane (Baker, 99.8%), followed by ultrasonication. W0 was varied from 1.8 to 3.7. Reduction of Pd 2§ was accomplished at ambient temperature by dropwise addition of hydrazine (N2H4"H20, Aldrich, 98%) in excess under vigorous stirring. Reduction was complete within minutes. 2 wt.% Pd was deposited onto the support by adding ZnO powder to the Pd suspension, followed by destabilisation of the microemulsion by dropwise addition of TI-IF (tetrahydrofuran) under stirring. The catalyst powder was recovered by centrifugation and washed thoroughly with ethanol. The ZnO powder was prepared according to the method described above.

2.3. Catalyst characterisation The thermal decomposition of Cu-Zn oxalate was studied using a Perkin-Elmer TGA 7 thermogravimetric analyser controlled by a Perkin-Elmer 7/DX thermal analysis controller. 20 mg of powder was placed in the sample holder and the temperature was elevated at 5 ~ in flowing synthetic air.

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The BET surface areas of ZnO and CuO/ZnO were obtained on a Micromeritics ASAP 2010 instrument by adsorption/desorption of nitrogen at liquid nitrogen temperature, using 0.164 nm 2 as the cross-sectional area of the nitrogen molecule. The samples were degassed at 250 ~ Powder X-ray diffraction (XRD) patterns were acquired on a Siemens D5000 instrument, using monochromatic CuK~ radiation and scanning 20 from 20 to 80 ~ The Pd particle size was determined using a JEOL 2000FX transmission electron microscope (TEM) operating at 200 kV. The Pd containing microemulsion was diluted with butanol before deposition onto the nickel grid. 3. RESULTS AND DISCUSSION

3.1. Thermogravimetric analysis (TGA) The TGA profile and its derivative (DTGA) for the thermal decomposition of Cu-Zn oxalate (W0=15) are shown in Fig. 1. There are three major decomposition peaks, located at approximately 120, 280 and 340 ~ The first peak is associated with removal of water from the oxalate hydrates. The main decomposition peak at 280 ~ corresponds to thermal decomposition of the oxalates. Carbon monoxide is one of the main decomposition products and since it is highly reductive under decomposition temperatures, the catalyst structure may be affected when it reacts with CuO [10]. The third peak may be related to the formation of CuO/ZnO. The TGA/DTGA profiles indicate that a temperature of 400 ~ is required for complete decomposition of Cu-Zn oxalate. In order to maintain the BET surface area as high as possible, the calcination temperature was fixed at 350 ~ while extending the calcination time to 12 h. 100

0

A

90

-5

80 A

70

r O~

"~

A

.-=

E

-lO .~

60

.,m i_

50

-15

40 30

. 0

. 100

.

.

. 200

.

. 300

.

. . 400

20 500

Temperature (~

Fig. 1. The TGA and DTGA profiles of Cu-Zn oxalate (W0=15). The temperature was elevated at 5 ~ in air.

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3.2. B E T surface area

The BET surface areas of the calcined CuO/ZnO powders were strongly affected by Wo (Table 1). When decreasing W0 from 15 to 11, the BET surface area increased from 36 to 40 mE/g. By further lowering W0 to 7.1, a surface area of 87 mE/g was obtained. Upon lowering of W0, the size of the water droplets in the microemulsion is reduced. Smaller Cu-Zn oxalate particles upon precipitation are the result and consequently, a higher surface area after calcination. Such strict control of the precursor size is not possible by using conventional coprecipitation. It is possible that even higher BET surface areas can be obtained by further lowering W0. However, by decreasing the water content, the production capacity of the microemulsion suffers severely. The pure ZnO prepared by microemulsion technique (W0=15) had a surface area of 47 mE/g. Table 1 The dependence of CuO/ZnO surface area on W0 in Berol 02 microemulsions Microemulsion composition (wt.%) W0 BET area (-) (m2/g) Cu+Zn H20 Berol 02 cyclohexane 7.1 87 0.21 5.0 19 76 11 40 0.31 7.5 19 74 15 36 0.42 10 18 72 3.3. X-ray diffraction (XRD) Powder XRD measurements were performed on Cu-Zn oxalate and CuO/ZnO samples (Fig. 2). In the oxalate sample, the hydrates (CuC204"H20 and ZnC2Oa-2H20) could be identified. After calcination, peaks assigned to CuO and ZnO were observed. The decrease in crystallite size caused by lowering W0 is clearly visible as broadening of the peaks for the sample prepared at W0=7.1. Cu ~ which is active in the methanol partial oxidation reaction, can be formed by reduction of CuO in hydrogen atmosphere.

1

~"

2

2 4

4

/

43

3

4 3

34

3 3 4

3

I.,,.

3

i

c J ~ l ~

20

2

1 1

2

1

!

(a):

i

i

i

I

i

i

30

40

50

60

70

80

2 theta

(~

Fig. 2. XRD patterns of (a) Cu-Zn oxalate (Wo=15), (b) CuO/ZnO (Wo=15) and (c) CuO/ZnO (Wo=7.1). (1-CuC204"H20, 2-ZnCzO4"2H20, 3=CuO, 4=ZnO.)

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3.4. Transmission electron microscopy (TEM) TEM microscopy was used in order to determine the Pd particle size after reduction. Fig. 3 shows Pd particles prepared at W0=3.7. The particles are mostly spherical and the average particle size was estimated at 16 nm. As shown in Table 2, the correlation between W0 and particle size is apparent. The particle size at constant Pd salt concentration in the aqueous phase (0.22 M) varied from approximately 10 to 16 nm, depending on W0.

Table 2 The dependence of average Pd particle size (do) on W0 in AOT microemulsions W0 dp Microemulsion composition (wt.%) (-) (nm) Pd H20 AOT iso-octane 1.8 10 0.026 1.1 15 84 2.9 13 0.039 1.7 15 84 3.7 16 0.055 2.2 15 83 3.5. Particle size The particle size after reduction in a microemulsion is determined not only by the water droplet size (governed mainly by W0), but also by the concentration of metal salt and the ratio of reducing agent (e.g. hydrazine) to metal. The particle size is generally known to increase with increasing metal salt concentration. Furthermore, the lower the initial hydrazine concentration, the larger the final particle size. When hydrazine is in large excess, nucleation is rapid and there are few unreduced species on the particle surface. Surfactant alkyl chains stabilise the particles and prevent further growth. As the hydrazine excess decreases, reduction may be incomplete on the time-scale of growth. Unreduced species on the surface prevent interactions with the anionic head groups of AOT, thus allowing continued growth [7]. In this work, hydrazine was always used in excess of stoichiometry, ensuring pseudo firstorder reduction kinetics. ,..~,.~,*~:*,

9 ".:'. L .

"'.

*...,

~%~g,2~.

. k ,T M . ' . ~ ' ~

,"

~.~,

,

~'~'

...,..~.'. , ~ i ~ ,.y.,. * . 9 ~ . , . , )~:

~,.. :,~.,,,

~ ~

.

~ 2

~

..

.~*...~0

....-..- .

,~ " ~ ~..

^ :

~

.

.

.

~

"

. . . . . . .

.

~

.

.

0

~..

..

9

,

.

, ...

9

9

,

9 ~ . . .

~ ~

,

...

.. .

.

.

.

..

.,

,,.

,, ,.

~

.

..

,.. 9 ~

,~ ,,

. .

9 ,~..

. ,.,

.-.

9

~

. . . ~

!

,.

Fig. 3. TEM micrograph of Pd particles prepared in AOT microemulsion (W0-3.7).

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The choice of microemulsion system influences the possible range of water droplet sizes. AOT is an anionic surfactant with two branched hydrocarbon chains. AOT microemulsions have a low solubilisation capacity for water, due to the bulky hydrophobic group. Therefore, this system is most suitable for preparation of small metallic particles. Berol 02, which was used for preparation of CuO/ZnO, is a nonionic surfactant with a single hydrocarbon chain, containing an aromatic ring. More aqueous phase can be dissolved in Berol 02 microemulsions, which makes it a suitable surfactant system when high production capacity is more crucial than small particle size. 4. CONCLUSIONS When using conventional catalyst preparation techniques, e.g. co-precipitation or impregnation, it is difficult to control the catalyst surface area and particle size and to study the effect of these parameters on catalytic behaviour. However, by using microemulsions as the synthesis and deposition medium, it is possible to effectively control both of these properties. The particle size of the precipitate is determined mainly by the water-to-surfactant ratio, which governs the water droplet size in the microemulsion. Thus, CuO/ZnO catalysts with BET surface areas as high as 87 m2/g could be prepared by adjusting W0 in Berol 02 based microemulsions. The size of Pd particles prepared in AOT microemulsions could be varied in the range 10 to 16 nm, depending on W0. ACKNOWLEDGEMENT The authors wish to acknowledge funding from the European Union (contract JOE3CT97-0049) and Kjell Jansson at Stockholm University (Inorganic Chemistry) for assistance with TEM microscopy. REFERENCES [1] L. Alejo, R. Lago, M.A. Pefia and J.L.G. Fierro, Appl. Catal. A, 162 (1997) 281. [2] M.L. Cubeiro and J.L.G. Fierro, Appl. Catal. A, 168 (1998) 307. [3] B. J/3nsson, B. Lindman, K. Holmberg and B. Kronberg, Surfactants and Polymers in Aqueous Solution, John Wiley & Sons, Chichester, 1998, p. 365. [4] K. Kon-No, in K. Esumi and M. Ueno (Eds.), Structure-Performance Relationships in Surfactants, Marcel Dekker, New York, 1997, p. 551. [5] M. Boutonnet, J. Kizling, P. Stenius and G. Maire, Colloids Surf., 5 (1982) 205. [6] M. Boutonnet Kizling and F. Regali, in Preparation of Catalysts VII, B. Delmon, P.A. Jacobs, R. Maggi, J.A. Martens, P. Grange and G. Poncelet (Eds.), Elsevier, Amsterdam, 1998, p. 495. [7] J.H. Clint, I.R. Collins, J.A. Williams, B.H. Robinson, T.F. Towey, P.C. Cajean and A. Khan-Lodhi, Faraday Discuss., 95 (1993) 219. [8] V. Pillai, P. Kumar, M.J. Hou, P. Ayyub and D.D. Shah, Adv. Colloid Interface Sci., 55 (1995) 241. [9] M. Kishida, T. Hanaoka, S. Tashiro and K. Wakabayashi, in Preparation of Catalysts VII, B. Delmon, P.A. Jacobs, R. Maggi, J.A. Martens, P. Grange and G. Poncelet (Eds.), Elsevier, Amsterdam, 1998, p. 265. [10]Y. Ma, Q. Sun, D. Wu, W.-H. Fan and J.-F. Deng, Appl. Catal. A, 177 (1999) 177.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Mechanism of Silation of Alumina and Silica with Hexamethyidisilazane Stefan V. Slavov, Alan R. Sanger, and Karl T. Chuang Department of Chemical and Materials Engineering University of Alberta, Edmonton, Alberta, Canada T6G 2G6 The silation of ~,-A120 3 and SiO2 with hexamethyldisilazane (HMDS) has been examined over the temperature range 150-450~ The products and sequence of the surface reactions have been determined, and a mechanism is proposed. Silation of Ale03. At all temperatures and feed rates of HMDS the initial gaseous product is methane. As the reaction progresses, hexamethyldisiloxane (HMDSO), ammonia, and nitrogen are formed. The quantity of each of these products decreases with increasing temperature. Initially, HMDS reacts with surface sites to generate pendant-O-SiMe3 and-NH-SiMe3 moieties. At temperatures over 300~ the predominant subsequent reaction is elimination of methane by reaction of the pendant silyl groups with acidic surface hydroxyls. At lower temperatures reactions between silyl groups to form HMDSO, protonation of amino groups to form ammonia, and redox reactions to form elemental nitrogen predominate over methane elimination. Silation of Si02. In contrast to A1203 the reaction of SiO2 produces ammonia and HMDSO with lesser amounts of methane and nitrogen. No methane is produced during reactions below 300~ Ammonia is the initial product detected, followed by HMDSO. The amount of methane and nitrogen produced increases, and the amount of ammonia and HMDSO produced decreases, with increasing temperature of reaction. The initial reaction forms NH 3 and pendant - O - S i M e 3 groups. The major subsequent reaction is the reaction of neighboring pairs of - O - S i M e 3 groups to form HMDSO, at all temperatures 150-450~ A second, minor, competing subsequent reaction is the reaction of neighboring hydroxyl and - O - SiMe 3 groups to eliminate CH 4 and form bridging - O - S i M e 2 - O - groups, at temperatures 300~ or higher. 1. INTRODUCTION Organosilanes are frequently used for modification of the surfaces of alumina and silica [ 1-3]. Chemically replacing or removing some or all of the surface hydroxyl groups renders the surface less hydrophilic [4-6]. The utility of the modified material for use in analytical, separations or catalytic processes can be changed by changing the surface properties in a controlled manner [7,8]. Recently, we have shown the Me3SiC1 reacts with the surfaces of either alumina or silica to strongly bind chloride and eliminate methane [9-11 ]. The initial reaction in the sequence is with coordinatively unsaturated ("cus") cationic and anionic sites, respectively, to bind chloride and form - O - SiMe 3 groups. The inductive effect of the bound chloride enhances the acidity of

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neighboring surface hydroxyls, which then react with - O - SiMe 3 groups to eliminate CH4 and form - O - SiMe 2 - O - bridged groups. It has been proposed that silation of surfaces with HMDS is effected by the reaction of HMDS with surface hydroxyls to form pendant - O - SiMe 3 groups and liberate ammonia (Eq. 1) [1214]. \ 27M-OH

+ (Me3Si)2NH

\ --~ 2 - T M - O - S i M e 3

+ NH 3

(1)

where M = A1, Si, Ti. Further, it has been suggested that ammonia is then bound wholly or in part in the surface of the silated oxide, and acts in a catalytic manner to promote further silation by HMDS. However, the interaction of ammonia or methyl amines with dehydroxylated alumina and silica provides no evidence for such an effect [15-16]. We have now studied the reactions of alumina and silica with HMDS to determine the nature of the surface reactions, the gaseous products and surface species formed. 2. EXPERIMENTAL Materials. Commercially available granulated alumina La Roche (BET surface area 206 m2g-I and total pore volume 0.386-0.390 cmag-l) and silica CS2040, PQ Corporation, (BET surface area 452 m2g- l and total pore volume 2.60 cmag-l) were used. HMDS (Aldrich, 99.9%) was analyzed using GC-MS, and was used as received. Equipment. Silation of alumina and silica with HMDS was studied using the dynamic method. The equipment comprised a gas handling system, a U-shaped glass reactor (100 cm 3) and movable furnace controlled within + 1~ and an exit gas control and monitoring system. Instrumental Methods. The surface area and pore volume for solid samples were determined using Omnisorp 360. Gas chromatographic analyses of the feed or exit gases were performed using a Hewlett-Packard 5890 series II GC with a GC column (5 f i x 1/8 in. o.d.) packed with Porapak T. FTIR spectroscopic data were collected using a Nicolet 730. Elemental analyses (C,H,N,Si) of solid samples were performed using a Carlo Erba CHNS-O Instrument with a GC TCD EA 1108 element analyzer and a JEOL Instrument JSM 6301 FXV scanning electronic microscope.

3. RESULTS AND DISCUSSION To enable direct comparisons of results of silation of 7-A120 3 and SiO 2 at different temperatures all other conditions were kept constant. Samples were sequentially dehydroxylated at 500~ for 24 h, cooled and weighed, heated to the selected temperature, and silated using either H M D S / N 2 or HMDS/He at a concentration of 9.7 x 10-4 mol (HMDS).L l and a flow rate of 13.6 cmamin-1. Thus the feed rate was 0.13 g (HMDS).h -l . Runs were performed within the range 150-450~ HMDS was fed continuously for 800-1200 min, to completion of each silation reaction as determined by the composition of the exit gases. It was determined that the rate of the reaction varied with the concentration of HMDS in the feed, but that the nature and amounts of each product and the nature of the reaction each depend upon the temperature of silation and the feed rate.

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3. 1. M e c h a n i s m of Silation of A1203. The major gas phase products are methane, ammonia, and HMDSO, with lesser amounts of nitrogen, and the amount of each produced increases with time of reaction to a maximum value, then decreases sharply. The time to maximum concentration of methane in the exit gases is inversely dependent upon concentration of HMDS in the feed, but the total amount of methane formed is independent of feed rate. The reaction of HMDS with the surface is the primary reaction, and methane formation is a subsequent and not a coincident step. Chemisorption. No HMDS is detected in the exit gases until after termination of the reaction, at each temperature. It is proposed that the initial reaction of HMDS with 7 - A 1 2 0 3 is dissociative chemisorption by reaction with coordinatively unsaturated A1+ and O- sites. Chemisorption of HMDS is not reversible. When a sample of 7 - A 1 2 0 3 which has been previously silated using HMDS at 200~ is then heated to higher temperatures the major product is methane and minor products are ammonia and HMDSO. It suggests that the HMDS is predominantly chemisorbed on the available cus A1+ and O - sites by a dissociative mechanism, as illustrated by reaction (2). Methane formation. The reaction of proximate acidic hydroxyls a n d - O S i M e 3 groups produces methane and forms bridging-OSiMe20- groups (eq 3). Formation of HMDSO. The reaction of ], - A120 3 with HMDS had previously been assumed

to be a metathetical reaction to form surface - OSiMe 3 groups and ammonia [12-14]. We have

/ o-

I

\

/

AI"

\

O

/

SiMe 3

HN

O

AI

AI

I

AI

\

O

/

+

(Me3Si)2NH

~

\

O

/

/

SiMe 3

I

\

O

/

(2)

\

O

/ O

now shown for the first time that significant amounts of HMDSO and small amounts of nitrogen are formed, especially at temperatures below 380~ To form HMDSO a silyl group must be transferred to an existing - O S i M e 3 moiety from another source, Me3SiX. The nature of the group X cannot be determined unambiguously from the available evidence, however the amount, H

/

\

/ O

/

SiMe 3

/

SiMe 2

\

O

O

O

I

I

I

I

AI

AI

AI

AI

\

/

\

O

/ O

--~

OH 4

+

\

/ O

O

\

/ O

(3)

\

/ 0

rate and timing of formation of HMDSO are consistent with reaction of neighboring surface - SiMe 3 groups (Eq. 4).

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化

\

/ 0

SiMe 3

/ x

0

I

I

AI

SiMe 3

.../

\

/ 0

AI

x-

\

/

--I~

(Me3Si)20

0

+

I

\

/ 0

AI

\

/ 0

AI*

\

/

(4)

0

(X : O, NH)

Formation of ammonia and nitrogen. Dissociative chemisorption of HMDS on the surface of 7 - A1203 will afford surface - N H S i M e 3 groups (Eq. 2). The formation of ammonia from a surface - N H S i M e 3 group, or HMDS, requires the transfer of two hydrogen atoms to the nitrogen center, as in Equation 1. We have seen that HMDSO and NH 3 are formed during essentially the same period of reaction at all temperatures. This observation suggests that amido groups from which the silyl group has been removed, as in Eq. 4 (X = NH), are readily converted to ammonia and liberated from the surface. Molecular nitrogen was detected in the exit gases, in molar quantities approximately corresponding to the difference between the HMDS consumed and other nitrogen-containing products. The oxidation of nitrogen compounds to elemental nitrogen by alumina was unexpected, but oxidative reactions at the alumina surface are precedented. When H2S is passed over ],-A1203 , initially a fraction of the H2S is oxidized to elemental sulfur by surface oxide species which cannot be readily removed using common methods [ 18]. 3. 2 . Mechanism of Silation of SiO2. The amount of HMDS consumed varies only slightly with temperature, indicating that the extent of the initial reaction of HMDS with the surface of SiO 2 depends only on the number of

available and active hydroxyl sites. The amounts of NH 3, HMDSO, N 2 , and CH 4 produced vary greatly with temperature of silation. The amounts of NH 3 and HMDSO each decrease with increasing temperature of reaction. The amount of N 2 increases slightly with temperature, as does the small amounts of CH 4 produced in reactions at 300~ or higher temperatures. The surface moieties formed in the initial reaction of HMDS with the surface of SiO x then react further by at least two competing pathways. In contrast to T -A1203, SiO2 has relatively few cus, and so silation occurs predominantly by reaction with surface hydroxyls [10]. An initial reaction of HMDS, either pre-adsorbed or from the gas phase, with the surface of SiO 2 results in formation of pendant - O - S i M e 3 groups (eq. 5). However, any Me3SiNH 2 so formed is then rapidly consumed by either of two rapid reactions. It is known that sterically unhindered aminosilanes, R3SiNH 2 , are inherently unstable to disproportionation (eq. 6), and also react readily with protonic reagents, which will include surface hydroxyls, to liberate NH 3 (eq. 7) [19]. Thus, as we have found, no Me3SiNH 2 appears in the exit stream, but copious amounts of ammonia are formed immediately after initiation of silation with HMDS.

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HMDSO is a major product of silation of SiO 2 with HMDS at all temperatures studied. However, the onset of formation of HMDSO varies with temperature. At 200~ HMDSO is

%

(Me3Si)2NH + ~ Si-OH 2R3SiNH2

~

R3SiNH 2 +

---> ; Si-O-SiMe 3 + "Me3SiNH2"

(R3Si)2NH + NH3

~ Si-OH

(5) (6)

---) ~ Si-O-SiR 3 + NH 3

(7)

formed essentially from the initiation of silation. At 400~ HMDSO is not formed until 4 h after initiation of silation. Further, the amount of HMDSO formed decreases with increasing the temperature of silation. Thus the reaction by which HMDSO is formed is not an initial reaction, and is not thermally promoted in a direct manner. It is also unlikely that HMDSO is formed by reaction of HMDS with pendant - O - S i M e 3 groups, as both are present from initiation of silation but HMDSO is not formed immediately for reactions at temperatures of at least 300~ It is noteworthy that the delay in formation of HMDSO coincides with the formation of small amounts of CH 4 , and the length of the delay increases with the amount of CH 4 formed. These data suggest that a competitive reaction is occurring. Acidic hydroxyls at the surface of oxides react with pendant - O - S i M e 3 groups to liberate CH 4 and form bridging - O - S i M e 2 - O - groups (eq. 8) [9-11]. /SiMe 3 O

si

Si

i

/

/H

O I

i

\

O

/

I

/ SiMe2\ ---)

O

O

Si

Si

i

/

I

t

x

O

/

+ CH 4

(8)

I x

The rate and extent of reaction 8 increases with increasing temperature. Any silyl group that so reacts will no longer be available to form HMDSO. These data suggest that, at temperatures of 300~ or higher, acidic hydroxyls on the surface of silated SiO 2 initially react rapidly with neighboring pendant - O - S i M e 3 groups to liberate CH 4 . Thereafter, the concentration of intact - O - S i M e 3 groups begins to increase, and neighboring - O - S i M e 3 groups are then able to react to form HMDSO (eq. 9). 2 ", ,S i - O - S i M e 3

--->

(Me3Si) 2 O +

,-

Si-O-Si

(9)

No CH 4 is produced during silation of SiO 2 by HMDS at 200~ and so HMDSO is produced by reaction 6 immediately after initiation of silation. At 300~ after formation of HMDSO has

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commenced, HMDSO and CH 4 are produced concurrently for a short time. HMDSO and CH 4 are produced concurrently for an extended period at 350~ and 400~ The rate of formation of CH 4 and HMDSO at 350~ and 400~ each decline coincidentally with the decline in rate of consumption of HMDS. These data suggest that reactions 8 and 9 each continue, in a competitive manner, at these temperatures until silation of SiO 2 by HMDS is completed. The net effect is that the amount of HMDSO formed, as a proportion of HMDS consumed, decreases with increasing temperature, and that the number of surface-bound silyl groups increases with increasing temperature. The surface silyl groups include both pendant - O - S i M e 3 groups, and bridging groups from which methyl groups have been removed by elimination of methane. The predominant nitrogen-containing product at all temperatures is NH 3 . The majority of the remaining N from HMDS forms N 2 , and a small amount of N remains bound to the surface after silation at temperatures 300~ or higher. The oxidative reaction to form N 2 involves surface oxide species, possible formed during calcination or dehydroxylation at 500~ which are not readily removed by thermal desorption or decomposition.

REFERENCES

Paul, D.K.; Ballinger, T.H.; Yates, J.T., Jr. J. Phys. Chem. 1990, 94, 4617. Kurth, D.G.; Bein, T. J. Phys. Chem. 1992, 96, 6707. Kang, H.-J.; Blum, F.D.J. Phys. Chem. 1991, 95, 939. Paul, D.K.; Yates, J.T., Jr. J. Phys. Chem. 1991, 95, 1699. Yates, J.T., Jr.; Paul, D.K.; Ballinger, T.H.U.S. Patent 5,028,575 (1991). 5. Armistead, C.G.; Hockey, J.A. Trans. Faraday Soc. 1967, 63, 2549. 6. Angst, D.L.; Simmons, G.W. Langmuir 1991, 7, 2236. 7. Gonzalez, R.D.; Miura, H. Catal. Rev. - Sci. Eng. 1994, 36, 145. 8. Zecchina, A.; Otero Arean, C. Catal. Rev. -Sci. Eng. 1993, 35, 261. 9. Slavov, S.V.; Chuang, K.T.; Sanger, A.R. Langmuir 1995, 11, 3607. 10. Slavov, S.V.; Chuang, K.T.; Sanger, A.R., J. Phys. Chem. 1995, 99, 17019. 11. Slavov, S.V.; Chuang, K.T.; Sanger, A.R.J. Phys. Chem. 1996, 100, 16285. 12. Barthel, H. Chemically Modified Surfaces,. Mottola, H.A.; Stteinmetz, J.R., Eds.; Elsevier, Amsterdam (1992), p. 243. 13. Proctor, K.G.; Blitz, J.P. Chemically Modified Surfaces, Mottola, H.A.; Stteinmetz, J.R., Eds.; Elsevier, Amsterdam (1992), p. 209. 14. Hancock, R.D.; Howell, I.V.; Pitkethly, R.C.; Robinson, P.J. Catalysis: Heterogeneous and Homogeneous, Delmon, B., Jannes, G., Eds.; Elsevier: Amsterdam, 1975; p. 361. 15. Morrow, B.A.; Cody, I.A.J. Phys. Chem. 1976, 80, 1998. 16. Blomfield, G.A.; Little, L.H. Can. J. Chem. 1973, 51, 1771. 17. Slavov, S.V.; Sanger, A.R.; Chuang, K.T.J. Phys. Chem. B 1998, 102, 5475. 18. Liu, C.L.; Chuang, K.T.; Dalla Lana, I.G.J. Catal. 1972, 26, 474. 19. Lappert, M.F.; Power, P.P.; Sanger, A.R.; Srivastava, R.C. Metal and Metalloid Amides; Wiley: Chichester, 1980.

1. 2. 3. 4.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

S O L V E N T I N F L U E N C E IN S I L I C A - A L U M I N A S O L - G E L S Y N T H E S I S A. Carati ~, C. Rizzo, M. Tagliabue, C. Perego EniTecnologie, Via Maritano 26, 1-20097 San Donato Milanese, Italy Fax 0039-02-520-56364: [email protected] Textural properties are often a target of heterogeneous catalyst design: indeed surface area and pore size determine the accessibility to active sites and this is o~en related to catalytic activity and selectivity in catalysed reactions. In this work amorphous silica-aluminas with different textural properties have been synthesised modifying the type and the amount of solvent. Information about the pore structure of these materials has been obtained by physisorption isotherms, that are able to discriminate between micro and mesoporosity and associated border line situations. Formation mechanism for micro- and mesoporous silica-aluminas is proposed. I. INTRODUCTION Amorphous silica-alumina materials represent an important class of porous inorganic solids which have not long-range order and usually have a wide distribution of the pore size, in the micro and mesopore region. Since early 90's a strong synthetic effort has been devoted to developing amorphous silica-aluminas with high pore volume, high specific surface area and pore size larger than that of zeolites: MCM-41 [1], MSA [2], ERS-8 [3], AI-HMS [4] and FSM-16 [5]. They show outstanding catalytic behaviours in several acid catalysed reactions [2, 61. Syntheses performed via sol-gel are particularly interesting, thanks to their versatility. In a typical sol-gel preparation the reagents are mixed to form a homogeneous solution that then gels to give a highly porous oxides and with good homogeneity. The type of precursor, the solvent, the gelation temperature and gelation catalyst can give rise to materials with different properties. Our group has recently claimed amorphous silica-aluminas, having acidic properties as catalysts [7], prepared from alkoxide precursors by sol-gel synthesis route [2, 3]. The use of gelling agents, as tetraalkylammonium hydroxide (NR4-OH), permits to control the porosity distribution. Depending on the N~-OH/alkoxides molar ratio and on the alkyl chain length of R groups two families of silica-aluminas have been obtained: 9 MSA is mainly mesoporous with a small contribution of micropores responsible of the adsorption observed at very low relative pressure, p/p~ < 0.1, in agreement with a Type IV + (I) isotherm [8]. 9 ERS-8 is a microporous solid, in agreement with a reversible Type I isotherm. A broad peak in the low angle region of XRD pattern is present, due to a very low structure order.

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In this work the influence of the alcoholic aliphatic chain and the H20/alcohol molar ratio has been investigated. The formation mechanism of different silica-aluminas is discussed on the base of their textural properties. 2. EXPERIMENTAL Samples were prepared via sol-gel in alkali-free medium using Si(OC2H5)4 (Dynasil-A, Nobel), Al(sec-OC4H9)3 (Fluka), tetrapropylammonium hydroxide (TPAOH, Sachem), alcohol (ROH) selected among C2HsOH, n-C3H7OH, n-C4H9OH, n-CsHllOH, n-C6H13OH (Fluka). All preparations were performed at the same molar ratio: SIO2/A1203=300, TPAOH/SiO2=0.09. The molar ratio H20/alcohol was changed between 0.5 and 5. When alcohols with a long aliphatic chain are used, only samples with low water amount can be synthesised, in order to obtain a homogeneous solution. In all syntheses the volume fraction of TPA § in the dried precursors is 40% (calculated by assuming a mass density of 0.9 g/cm 3 for TPA § and 2.0 g/cm3 for SiO2) [3]. A typical synthesis preparation is following described. ml(sec-OC4n9)3 was dissolved in Si(OC2H5)4 at 60 ~ The obtained homogeneous solution was cooled at room temperature, then the required alcohol and TPA-OH in aqueous solution were added in sequence. Monophasic clear solutions were obtained, then transformed in homogeneous compact gel without separation of phases. The molar compositions of reagent mixtures are reported in table 1. The sample acronyms indicate the number of carbonium atoms in the alcohol used and the molar ratio H20/ROH. Table 1 Molar compositions of reagent mixtures. SAMPLES

ET/C2-5 ET/C2-1 ET/C2-0.5 !ET/C3-5 ET/C3-1 ET/C3-0.5 ET/C4-0.5 ET/Cs-0.5 ET/C6-0.5

ROH

C2HsOH

Molar ratio HzO/ROH

Molar ratio H20/SiO2

Molar ratio ROH/Si02

5

10 8 2 10 8 2 2 2 2

2 8 10 2 8 10 10 10 10

1

0.5 5 n-C3H7OH

1

n-C4H9OH n-CsH~ 1OH n-C6HlaOH

0.5 0.5 0.5 0.5

After 15 hour ageing at room temperature, the gels were dried at 100 ~ and calcined 8 hour in air at 550 ~ The textural properties of all calcined samples were determined by nitrogen isotherms at liquid N2 temperature, using a Micromeritics ASAP 2010 apparatus (static volumetric technique). Before determination of adsorption-desorption isotherms the samples (-~ 0.2 g) were outgassed for 16 h at 350 ~ under vacuum. The specific surface area (SBET)was evaluated by 2-parameters linear BET plot in the range p/pO 0.01-0.2. The total pore volume (VT) was evaluated by Gurvitsch rule. Medium pore size (dDvr) and pore size distributions were calculated using DFT method for all materials.

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Indeed, DFT, based on molecular statistical approach, is applied over the complete range of the isotherm and is not restricted to a confined range of relative pressure or pore sizes. Pore size distribution is calculated by fitting the theoretical set of adsorption isotherms, evaluated for different pore sizes, to the experimental results. 3. RESULTS All samples synthesised by using ethanol (ET/C2) and propanol (ET/C3) are characterised by irreversible Type IV isotherms, with H2 Type hysteresis (Figure 1 and 2 respectively). Higher pore volume and surface area are always obtained for ET-C3 compared to ET-C2 samples, furthermore, a shift towards upper relative pressure of the hysteresis loop is generally observed. For each alcohol, samples prepared at H20/ROH = 1 show the highest oore volume and the highest surface area. 750

A

E v

"o o .Q L O I/) "o

<25O c

0

<E 0 QO

02

Q4

Q6

0.8

1.0

~F~u~~ Figure 1. N2 adsorption-desorption isotherms of: mET/C2-5, 9 ET/C2-1, 9 ET/C2-0.5 samples.

OI

0.0

. . . .

0.2

0.4

0.6

0.8

1.0

Figure 2. N2 adsorption-desorption isotherms of: 9 ET/C3-5, 9 ET/C3-1, 9 ET/C3-0.5 samples.

ET/C2 samples are characterised by similar shape of the isotherms. The pore volume, the surface area and the mean pore diameter of ET/C2-5 and ET/C2-0.5 are comparable and all lower than for ET/C2-1. ET/C3-5 shows an isotherm of similar shape with respect to ET/C3-1, but a lower pore volume is observed. A shift at lower relative pressure of the hysteresis loop is observed in the sample ET/C3-0.5, characterised by the lowest pore volume within ET-C3 samples. Increasing the alkyl chain of the linear alcohols, at molar ratio H20/ROH-0.5, a change from mainly Type IV to Type I isotherms is detected (Figure 3). Samples ET-C3 show the highest mean pore diameter. Further increase in the alkyl chain of alcohols gives rise to a strong reduction of mean pore diameter till typical values of microporous samples (Figure 4).

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Results are summarised in the table 2. Table 2 Textural pr aperties of synthesised materials. SAMPLES Isotherm SBET

Type

ET/C2-5 ET/C2-1 ET/C2-0.5 ET/C3-5

ET/C3-1 ET/C3-0.5 ET/C4-0.5

VT

(m2/g)

(ml/g)

686 808 715 749 1045 812 655 800 880

0.45 0.62 O.49 0.72

IV + (I)

IV + (I) IV + (I) IV + (I) IV IV + (I)

IV+I

ET/Cs-0.5 ET/C6-0.5

dDFT (A)

25 29 22 51 41 25 15 11 12

1.04

0.57 0.36 0.39 0.49

.....

4. DISCUSSION Solvent can affect sol-gel syntheses in several ways. Both alcohol (ROH) and H20 are able to react with alkoxides by two different reactions: alcoholysis and hydrolysis. Alcoholysis (or alcohol interchange) reaction occurs when alkoxide is dissolved in an alcohol with the alkyl groups different from the starting ones. Alcoholysis reaction is generally slow, but it is catalysed by acid or basic medium [9]. Alcoholysis gives rise to alkoxides with groups having a different hydrolysis rate. The hydrolysis corresponds to the nucleophilic substitution of alkoxy ligand by hydroxyl ligand,

m

a-C3H7OH

"o I. o "o r~

<

n-C4H9OH

o

n-CsHnOH

m

o o I

0

0.2

I

I

I

0.4

0.6

0.8

Relative Pressure ( p/pO ) Figure 3. N2 adsorption-desorption isotherms for samples prepared at H20/ROH = 0.5 molar ratio with different alcohols.

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0.6

化

n-C311701 CzHsOH

~0.4 -

.

.

.

.

.

.

.

.

O

n-CIH9OH tlOH

~0.2 -

i

0.0 1

10

100

1000

Pore Width (Angstroms)

Figure 4. DFT cumulative pore volume for samples prepared at H20/ROH = 0.5 molar ratio but with different alcohols leading to the monomeric units formation, which constitute the precursors for the following polymerization reactions. The H20/ROH molar ratio strongly affects the textural properties, probably influencing the hydrolysis and the successive polymerization/depolymerization reactions [ 10]. During all these reactions the reagent mixtures increase their complexity. Solvent mixtures are formed by water, added alcohol and alcohols formed by hydrolysis of alkoxide precursors, in particular hydrolysis of Si(OC2H5)4 gives rise to a relevant amount of ethanol. The composition during synthesis depends on the extent of alcoholysis, hydrolysis, polymerization. The simplest condition is when C2HsOH is used because the added alcohol is the same of that produced by alcoholysis and hydrolysis of the Si-precursor. The amount of C2H5OH gives a negligible influence on the shape of isotherms (Figure 1), nevertheless it affects the textural properties. When n-C3H7OH is used the system is complex due to the presence of H20, n-C3H7OH, and the C2HsOH. The amount of n-C3H7OH affects both the shape of the isotherms and the textural properties, as shown in Figure 2. As a general rule, hydrolysis is fast and depolymerization favoured when the H20/ROH molar ratio is high. By contrast, few Si(AI)OH groups are formed for gelation or crosslinking, when the ratio is low and there is a competition between alcoholysis and hydrolysis [ 10]. In the system studied, the middle conditions give rise to a materials characterised by the highest pore volume and the highest surface area (ET/C2-1 and ET/C3-1). As described [11, 12] the TPAOH role is to provide the pore formation through the TPA § clusters as well as to organise SiO2 and A102 units into polymeric structure. The TPA §

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fraction volume in the sol influences the formation of micro- or meso-porous materials [3]. Using C2HsOH, the threshold between range of existence of MSA and ERS-8 was identified around 50% volume fraction of TPA § in the dried precursors (calculated by assuming a mass density of 0.9 g/cm 3 for TPA § and 2.0 g/cm 3 for SiO2). In the present work the volume fraction of TPA § in the dried precursors is 40%. In agreement with previous results, MSA is obtained in presence of C2HsOH. Nevertheless by changing the kind of alcohol used several behaviours are observed. This can be explained by modification in the solvation of the TPA § clusters due to different alcohols. Indeed, the solvation sphere affects the real TPA § volume fraction, being larger when alcohol has a longer aliphatic chain. When C2HsOH or n-C3H7OH are used as solvents, the TPA § solvated clusters are completely surrounded by aluminosilicate oligomers, as for MSA proposed model. Increasing the alcoholic aliphatic chain (n-CsH~IOH or n-C6H13OH), the solvated clusters, due to the steric hindrance, do not permit a complete surrounding of aluminosilicates that grow as sheets, as proposed for ERS-8 formation. When n-C4HgOH is used a borderline situation is observed. According to this hypothesis, a change from mainly type IV (i.e. MSA) to type I (i.e. ERS-8) isotherms is detected. 5. CONCLUSIONS Solvent system is an important parameter in sol-gel syntheses. The molar ratio H20/ROH affects the relative rate of alcoholysis and hydrolysis reactions. The textural properties are strongly affected by synthesis parameters. A MSA with highest pore volume and highest surface area is obtained by using n-CaH7OH as a solvent. At the same H20/ROH and at the 40% of TPA + volume fraction in the dried precursors, both MSA and ERS-8 formations are observed. The threshold between range of existence of MSA and ERS-8 is affected by the kind of alcohol used. A change from mainly type IV to type I isotherms is detected increasing the alkyl group of alcohol. REFERENCES 1. J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt, C. TW. Chu, D. H. Olson, E. W. Sheppard, S. B. McCullen, J. B. Higgins, J. L. Schlenker; J. Am. Chem. Soc., 114 (1992), 10834. 2. G. Bellussi, C. Perego, A. Carati, S. Peratello, E. Previde Massara, G. Perego; Studies in Surface Science and Catalysis, 84 (1994) 85. 3. G. Perego, R. Millini, C. Perego, A. Carati, G. Pazzuconi, G. Bellussi; Studies in Surface Science and Catalysis, 105 (1997) 205. 4. A. Tuel, S. Gontier; Chem. Mater., 8 (1996) 114. 5. T. Shinoda, Y. Izumi, M. Onaka; J. Chem. Soc., Chem. Comm., (1995)1801. 6. M.J. Climent, A. Corma, S. Iborra, M.C. Navarro, J. Primo; J. Catal., 161 (1996) 783. 7. C. Perego, S. Amarilli, A. Carati, C. Flego, G. Pazzuconi, C. Rizzo, G. Bellussi; Microporous and Mesoporous Materials, 27 (1999) 345. 8. C. Rizzo, A. Carati, M. Tagliabue, C. Perego; Book of abstracts of 5th Int. Symp. on the Characterisation of Porous Solids COPS-V, Heidelberg, Germany (1999), 538. 9. M. Guglielmi, G. Carturan; J. Non-Crystalline Solids, 100, (1988), 16. 10. J. Livage; Stud. Surf. Sci. Catal., 85, (1994), 1. 11. M. Schneider, A. Baiker; Catal. Rev.-Sci. Eng., 37 (4) 515 (1995). 12. A.Corma, J. Prrez-Pariente; Appl. Catal., 63 (1990) 145.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Ru, Co and RuCo/SiO2 catalysts prepared by Sol-Gel methods B. Bottia, D. Cauzzi b, P. Moggia, G. Predieri b and R. Zanonic aDepartment of Organic and Industrial Chemistry, University of Parma, Parco Area delle Scienze 17/A, 43100 Pmana, Italy bDepartment of General and Inorganic Chemistry, Analytical Chemistry and Physical Chemistry, University of Parma, Parco Area delle Scienze 17/A, 43100 Parma, Italy CDepartment of Chemistry, University of Roma "La Sapienza", Piazzale Aldo Moro 5, 00185 Roma, Italy Ru, Co and RuCo/SiO 2 catalysts were prepared by sol-gel methods from Si(OMe)4 and Ru3(CO)I 2, Co2(CO)8 or mixed clusters HCoRu3(CO)I 3 and HRuCo3(CO)12, as metal precursors. The activated catalysts were characterized by FTIR and DRIFTS spectra, TEM and XPS analyses, and catalytic activity tests in the CO hydrogenation reaction at atmospheric pressure. 1. INTRODUCTION Ruthenium- and cobalt-based catalysts are very active in Fischer-Tropsch (FT) synthesis, the latter being industrially preferred as less expensive [ 1,2]. However, the performance of the catalysts strongly depends on the metal precursor and support used, on the preparation method, as well on the preactivation procedures [3-6]. The use of metal carbonyl clusters, such as Ru3(CO)12, rather than RuCl3, as Ru precursor on inorganic oxides as supports, provides higher metal dispersions and specific activities [7,8]. A similar effect was determined for Co precursors: the dispersion of the metallic species and the initial activity in CO hydrogenation were greater for Co2(CO)8 than Co(NO3) 2 derived catalysts [9]. Very high dispersed Ru metal particles on silica were obtained by sol-gel processing Si(OMe)4 and Ru3(CO)l 2, the resulting catalysts being extremely more active than the corresponding ones prepared by impregnation methods [10,11]. Finally, it was demonstrated that synergistic bimetallic interactions between Co and Ru increase FT synthesis rate and C5+ selectivity on supported cobalt catalysts [ 12]. Thus, it appeared very interesting to us to study the catalytic activity of sol-gel prepared Ru, Co and Ru-Co bimetallic systems in FT synthesis.

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2. EXPERIMENTAL 2.1. Preparation of catalysts Monometallic catalysts Ru/SiO 2 and Co/SiO 2 were prepared from Ru3(CO)l 2 and Co2(CO)8 respectively; bimetallic catalysts (RuaCo)/SiO2, (RuCo)/SiO 2 and (RuCo3)/SiO 2 were prepared either from bimetallic clusters, HCoRu3(CO)I 3 and HRuCo3(CO)I 2, or from mixures ofRu3(CO)l 2 and Co2(CO)8. The metal loading always was 5% by weight. Ru3(CO)I 2, Co2(CO)8 and Si(OMe)4 commercial products (Aldrich, Fluka) were used as received; mixed clusters HCoRu3(CO)I 3 and HRuCo3(CO)I 2 were prepared by selected literature ways [ 13-15]. The metal precursors were always completely dissolved in dry THF as solvent at room temperature, then Si(OMe)4, as SiO 2 precursor, was added, under a nitrogen stream. The homogeneous solutions were stirred for 1 h, then the appropriated amount of distilled water to induce the hydrolysis of the alkoxide was added dropwise, under continous stirring. No catalyst was added to favour the condensation step of the sol-gel process, but after the addition of water the solutions were transferred in a large vessel suitable for the evaporation of solvent and allowed to rest until the gelation occurred. The catalysts were generally activated as follows: a weighed amount (0.3-0.4 g) of solid was charged into a flow microreactor and gradually heated (2 K min -1) up to 623 K in a helium stream, then the reduction to dispersed metal particles was performed by feeding hydrogen (60 ml min -l) at 623 K for 3-4 h. 2.2. Activity tests All the sol-gel prepared catalysts were tested for their activity in the FT synthesis at atmospheric pressure, by feeding CO/H 2 l'l or 1"3 gas mixtures, at about 1500 h -l GHSV in the 473-573 K temperature range. 3. RESULTS AND DISCUSSION 3.1 Characterization of the catalysts Figure 1 and Figure 2 show the FTIR spectra, in the carbonyl region, of the gel obtained from Ru3(CO)I 2 and from HCoRu3(CO)I3, respectively. Both exhibit two bands (about 2060 e 2000 cm -l) assignable to Ru (II) dicarbonyl species anchored to silica by Ru-O-Si bonds, whose formation is also demonstrated by a strong band at about 995 cm -1. The Ru/SiO 2 catalyst, discharged after the activity tests, has been subjected to pulse CO chemisorption in a DRIFTS cell at 473 K. Figure 3 shows that the catalyst binds CO giving again twin carbonyl stretching bands (2058 and 1983 cm'l), suggesting that ruthenium is adequately dispersed being able to readsorb CO, affording dicarbonylated species. The discharged (Ru3Co)/SiO 2 catalyst has been analysed by transmission electron microscopy (TEM). The micrographs of the sample (see Figure 4) evidence the presence of metal particles, whose dimensions are lower than 3 nm, very homogeneously dispersed into the silica matrix.The X-ray fluorescence analysis, executed on a l0 nm diameter region, confirmed a mean Ru/Co 3:1 composition of the metal grains, the same of the precursor cluster, according to a possibly bimetallic character of any single particle.

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1093

r,.) I--, [-., r~

<-,,

i

2150

1

2100

2050

2000

'19'50'

'

1"900

Wavenumbers (cm- 1) Fig. 1. FTIR spectrum (carbonyl region) of the gel from Ru3(CO)I 2 and TMOS.

\

2~ < b-,

Z

7 ij84

[..,

,+ !

2200

v

,

,

2i00'

L

L/ '

' 20'00 . . . .

1900 '

'

Wavenumbers (cm- 1) Fig. 2. FTIR spectrum (carbonyl region) of the gel from HCoRu3(CO)I 3 and TMOS.

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

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1983

E .sD

(b) (a) 2150

!

i

9

f

I

....

2100

|

|

!

I

|

!

i

-

i

|

|

T

--F-----

J

20'50 '2000 19~50 Wavenumbers (cm-l) Fig. 3. DRIFTS analysis (carbonyl region of discharged Ru/SiO 2 catalyst: (a) before and (b) after CO saturation at 473 K.

Fig. 4. TEM image of discharged (Ru3Co)/SiO 2 catalyst prepared from HCoRu3(CO)I3.

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The XPS analysis has been used to investigate on the oxidation state of ruthenium and cobalt in the Ru/SiO 2 and Co/SiO 2 catalysts at~er the activity tests. For Ru/SiO 2, the bond energy value of the 3p peak resulted 461.5 + 0.2 eV, in good agreemem with Ru metal (461.8 + 0.1 eV), and the measured atomic Ru/Si ratio on the surface was 4.6-4.8%, near to theoretical 5.0%, according to the maintenance of the original high dispersion. However, for Co/SiO 2, the XPS analysis showed the presence of Co (II), oxide or hydroxide, and Co metal species, the former being prevalent (4.5:1), and the measured atomic Co/Si total ratio on the surface was 3.8%, indicative of more distant particles. Then, the observed poor catalytic activity of Co/SiO 2 is possibly attributable to an incomplete reduction to metal during the activation step, that confirmed by the deep blue colour of the discharged catalyst.

3.2. Activity tests Some results are reported in Table 1 and Table 2. Table 1 Catalytic activity tests in the Fischer-Tropsch synthesis reaction at 523 K (CO/H 2 1:3)

Catalyst Ru/S iO 2 (Ru3Co)/SiO 2 (Ru3Co)/SiO 2 (RuCo)/SiO 2 (RuCo3)/SiO 2 (RuCo3)/SiO 2 Co/SiO 2

Precursor Ru3 (CO) 12 HCoRu3(CO)I 3 Ru3(CO)12+Co2(CO)8 Ru3(CO)I2+Co2(CO) 8 HRuCo3(CO)I 2 Ru3(CO)12+Co2(CO)8 Co2(CO) 8

Conversion 9 (%) Product selectivity (%) CO Methane Ethane 58.3 90.9 7.8 11.9 77.3 22.7 11.4 77.8 15.6 11.9 70.0 15.0 10.4 57.7 19.2 10.8 70.6 15.3 0.0 ..........

Propane

6.7 15.0 23.1 14.1

Table 2 Catalytic activity tests in the Fischer-Tropsch synthesis reaction at 548 K (CO/H 2 1:1)

Catalyst Ru/SiO 2 (Ru3Co)/SiO2 (Ru3Co)/SiO2 (RuCo)/SiO 2 (RuCo3)/SiO2 (RuCo3)/SiO2 Co/SiO2

Precursor Ru3(CO)12 HCoRu3(CO) 13 Ru3(CO)I 2+Co2(CO)8 Ru3(CO) 12+Co2(CO)8 HRuCo3(CO)12 Ru3(CO)12+Co2(CO)8 Co2(CO) 8

Conversion (%) Product selectivity (%) CO Methane Ethane Propane 15.5 88.7 11.3 ..... 6.7 47.6 23.8 28.6 4.5 72.0 20.9 6.9 4.2 21.8 41.4 36.8 2.8 33.7 37.1 29.2 4.5 29.6 28.2 42.3 0.0

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The catalyst containing only ruthenium showed the highest activity, whilst that containing only cobalt was totally inactive. However, cobalt containing catalysts, both prepared from mixture of carbonyls or from mixed clusters, showed higher selectivities to ethane and propane than ruthenium alone, particularly with the CO-H2 1-1 gaseous feed. Ru/SiO2 is largely more active than RuCo/SiO 2 as catalyst in FT synthesis at atmospheric pressure, whereas bimetallic catalysts are more selective to higher hydrocarbons. The lower activity of Co containing catalysts is probably due to an incomplete reduction of cobalt (II), bonded to silica, to the metal state. In fact, Co/SiO2 was totally inactive. 4. CONCLUSIONS Very high dispersed metal Ru and bimetallic RuCo particles on silica can be obtained by using the above described sol-gel method, starting from Ru3(CO)I 2, Co2(CO)8 or mixed clusters HCoRu3(CO)I 3 and HRuCo3(CO ) 12, as metal precursors.

AKNOWLEDGEMENTS

We thank the University of Parma and MURST (Corm98) for a grant and dr. A. Armigliato, of the CNR LAMEL Institute of Bologna, for TEM analyses. REFERENCES

1. 2. 3. 4. 5.

E. Iglesia, S.C. Reyes, R.J. Madon and S.L. Soled, Adv. Catal., 39 (1993) 221. E. Iglesia, Appl. Catal. A: General, 161 (1997) 59. J.A. Mieth and J.A. Schwarz, J. Catal., I 18 (1989) 203. A.A. Adesina, Appl. Catal. A: General, 138 (1996) 345. M.K. Niemel/i, L. Backman, A.O.I. Krause and T. Vaara, Appl. Catal. A: General, 156 (1997) 319. 6. J.J. Kiviaho, M.K. Niemel/i, M.K.O. Reinikainen, T. Vaara and T.A. Pakkanen, J. Mol. Catal. A: Chemical, 121 (1997) 1. 7. P. Moggi, G. Albanesi, G. Predieri and G. Spoto, Appl. Catal. A: General, 123 (1995) 145. 8. V. Ragaini, R. Carli, C.L. Bianchi, D. Lorenzetti, G. Predieri and P. Moggi, Appl. Catal. A: General, 139 (1996) 31. 9. M.K. Niemel/i, A.O.I. Krause, T. Vaara, J.J. Kiviaho and M.K.O. Reinikainen, Appl. Catal. A: General, 147 (1996) 325. 10. F. Di Silvestri, P. Moggi and G. Predieri, Preparation of Catalysts VII, Elsevier Science B.V., (1998) 219. l l. P. Moggi, G. Predieri, F. Di Silvestri and A. Ferretti, Appl. Catal. A: General, 182 (1999) 257. 12. E. Iglesia, S.L. Soled, R.A. Fiato and G.H. Via, J. Catal., 143 (1993) 345. 13. J.E. Ellis and E.A. Flom, J. Organomet. Chem., 99 (1975) 263. 14. P.C. Steinhardt, W.L. Gladfelter, A.D. Harley, J.R. Fox and G.L. Geoffroy, Inorg. Chem., 19(1980)332. 15. P. Braunstein and J. Rose, Inorg. Synth., Vol. 26 (1989) 356.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

STRUCTURE AND CATALYTIC PROPERTIES OF COBALTRHENIUM BIMETALLIC CATALYSTS PREPARED BY SOL/GEL M E T H O D A N D B Y I O N E X C H A N G E IN N a Y Z E O L I T E L. Guczi l*, G. Stefler l, Z. Schay l, I. Kiricsi 2, F. Mizukami 3, M. Toba 3, S. Niwa 3 ~Department of Surface Chemistry and Catalysis, Institute of Isotope and Surface Chemistry, CRC, HAS, P. O. Box 77, H-1525 Budapest, Hungary 2Department of Applied Chemistry, Attila Jrzsef University, Rerrich B. trr H-6720, Szeged, Hungary 3Department of Surface Chemistry, National Institute of Materials and Chemical Research, Tsukuba, Ibaraki 305, Japan ABSTRACT The promotion of rhenium on cobalt catalysts prepared by various methods has been investigated in the CO hydrogenation. 10 at.% rhenium was added to Co/NaY sample and 10 at.% with 90 at.% Co bimetallic samples were prepared by sol/gel technique. The samples pretreated in various manner revealed an about five fold increase in activity and an increase in the chain length in the NaY supported sample while the activity and selectivity changed in various way in the sol/gel prepared samples. In the mechanism the rhenium provides an action preventing fast deactivation of cobalt. 1. I N T R O D U C T I O N

Among the new "gas-to-liquid" technologies the "Shell Middle Distillate" technology [ 1] and the cobalt based high efficiency EXXON process [2] are worth mentioning. Rhenium, being a well known additive to the catalysts applied in the reforming technologies, proved to be useful to modify the structure and activity of iron [3, 4] and cobalt [5] catalysts. Addition of 10 at.% rhenium to Fe/Cab-O-Sil catalyst increased the rate of CO hydrogenation by a factor of 1.5 order of magnitude leaving the olefin selectivity still high. This purely morphological effect was explained by stabilization of the small superparamagnetic iron particles [3, 4]. Similar, but not completely clarified phenomenon caused the activity to increase in the Re-Co/A1203 system [5]. In the present paper we aim to elucidate the effect of rhenium on the Co-based catalysts using various supports (SiO2 or NaY) and altering the way of preparation corresponding author, fax: +361-395-9001; e-mail: [email protected]

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(sol/gel method vs. ion exchange). We wish to obtain information about the effect of the valence state and particle size in the cobalt affected by rhenium and the effect of structural changes on the activity and selectivity revealed in the CO hydrogenation.

2. EXPERIMENTAL 2.1. Catalyst preparation The samples were prepared by sol/gel method denoted by Co/SG and by ion exchange technique denoted by Co/NaY and Re-Co/NaY. For preparation of the Re and Co bimetallic samples 4.94 g Co(NO3)36H20 and 0.455 g NH4ReO4 dissolved in ethyleneglycol/alcohol (77.2 g in 30 cm 3) were added to 64.8 g Si(OC2H5)4, and stirred at 353 K for 3 h. Then, H20/C2HsOH mixture was added and kept at 353 K overnight to obtain transparent monolythic gel. The gel was dried at 413 K in vacuum, powdered and activated in a stream of hydrogen at 673 K for 4 h (a), calcined at 823 K in air for 4 h followed by reduction in H2 at 673 for 4 h (b), calcined at 823 K in air for 4 h (c). Co/NaY and Re-Co/NaY samples were prepared by ion exchange method using Co(NO3)3 6H20 solution and ReC13 aqueous solution exchanging rhenium first then cobalt. The details for the exchange similar to that of Pt-Co or Ru-Co, have been given elsewhere [6]. The total metal loading is 5 wt % in both cases and in each bimetallic sample 10 at % Re is present. 2. 2. Sample characterization The samples were characterized by temperature programmed reduction (TPR) using 1 vol. % hydrogen/argon mixture with 20~ min ~ ramp rate. An apparatus type SORBSTAR was employed for the TPR measurements. As the samples prepared by sol/gel method were already reduced at 400~ for 4 h ((a) and (b)), the post-reduction TPR gives information only about the composition of the outer skin of the catalyst grains. The Co/NaY and CoRe/NaY samples after ion exchange were treated in oxygen and then TPR was performed. X-ray diffraction (XRD) data were collected by a Rigaku powder X-ray diffractometer. X-ray photoelectron spectroscopy (XPS) of the samples were performed on the as received, reduced and spent catalyst samples using an in situ catalytic reactor attached to an XSAM-800 cpi photoelectron spectrometer manufactured by KRATOS. A1Kcx characteristic X-ray lines were applied using an 80 eV pass energy to measure the cobalt and rhenium spectra. In calculation the Si 2p was used for reference in the binding energy scale (103.3 eV binding energy). 2. 3. Catalytic reaction The CO hydrogenation was carried out at 1 and 10 bar pressures at differential regime in the temperature range between 503-593 K. All catalyst samples prepared by SG methods were treated in hydrogen before reaction. The NaY exchanged samples were treated either in oxygen or in He followed by reduction at 670 K for 2 h. The products were analyzed by means of a gas chromatograph type CHROMPACK CP 9002 using a 50 m long plot fused silica column (0.53 mm I.D) with a stationary phase of CP-A1203/KC1 with a temperature programmed mode. The rate was measured using 100 mg catalyst at 15 cm 3 min ~ flow rate, and was calculated in ~tmol sl gcat~. Selectivity was calculated by (.Ci/C2+) x 100 from i = 1-8.

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"4. R E S U L T S A N D D I S C U S S I O N S

XPS results on catalysts CoRe/NaY and CoRe/SG are given in Figs. 1 and 2, respectively. In the as prepared and spent On CoRe/NaY catalysts (used at 10 bars) no Re can be detected, only Co 2§ at 783.4 • 0.2 eV binding energy. Co O at 778.8 eV binding energy could be seen

778 eV Coo

~ 9 7 7 8 =

eV Co N.

e--

} 9

790

=

|

780

b)

9

Binding Energy / eV

b)

=

770

t--

a) "

790

780'7"?0

Binding Energy / eV

xlO

b)

t~ t.m CO

a

~

o

i-

273

473

673 873 T/K

o

t')

~

_.a)

50 45 40 35 Binding Energy / eV

Fig. 2 XPS spectra for CoRe/SG samples: (a) as received, (b) H2/673 K. Upper Co, bottom Re

Fig. 1 XPS spectra for CoRe/NaY (a) as received, (b) H2/673K, (c) H2/CO/673K, (d) H2/CO/773 K

5 t~

t-

c) 1073

only after in situ reaction at atmospheric pressure at 773K, but even in this case no Re was detected. The Co concentration measured by XPS was 4 • 0.3 wt % and was independent of any treatments. On catalyst CoRe/SG both Co and Re were in oxidized states in the as prepared samples, however, the Re and a part of Co could be reduced by in situ hydrogenation at 673 K In Figure 3 the TPR results are presented for CoRe/NaY and CoRe/SG (b) and (c) samples. As shown from the top curve in

CoRe/NaY sample the reduction takes place to a very small extent (about 5 %). On the other hand, in the CoRe/SG sample (c) (bottom curve) the extent of reduction is about 35 %, which is significantly higher than that for CoRe/NaY catalyst. Both the XPS and the Fig. 3. TPR results on CoRe/NaY (a) and CoRe/SG (b) and (c) samples

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TPR well illustrate that although the same amount of rhenium is present in both samples, the rhenium is occluded in CoRe/NaY sample, while on the catalyst prepared by sol/gel method rhenium is located at the surface. In Table 1 the rates measured at 509 and 536 K and the energies of activation are presented for the Co-Re/NaY and Co/NaY samples. The samples were pretreated in oxygen (or He) at 673 K for 2 h followed by hydrogen treatment at 673 K for 4 h. For kinetic measurements the catalysts were stabilized by overnight treatment in the reaction mixture. As shown from Table 1 there is a threefold increase between the rates measured on Co/NaY and CoRe/NaY the least active being the Co/NaY. The energy of activation considerably increases when the samples He/H2 treated. Methane selectivity decreases, while the amount of higher hydrocarbons (C2- Ca and C5+) increases with addition of rhenium to cobalt. In Table 2 the results obtained on catalysts prepared by sol/gel technique well illustrate that the rates do not drastically change with various treatment conditions. In Fig. 5 the selectivity 9C l [] C2-C4 [] C5 values, measured in the CO 100% hydrogenation on the samples prepared by sol/gel method at 10 80% ._>, bar pressure at 551 K, are 60% .>_. presented. The methane select40% ivity at 10 bar pressure becomes G) (n higher, whereas on the zeolite 20% -~ supported samples the methane selectivity is significantly lower SG(a) SG(b) SG@ SGO and does not change at 10 bar CoRe/SG samples pressure. In discussion of the results, we Pig. 4. ~ielectlvlty o! I-t2/t, SU reaction measured over have to take into consideration SG samples at 10 bars and at 551 K the following points. In agreements with TPR data the reducibility of the sol/gel prepared samples is higher than that of the NaY exchanged ones. The XPS results also show in the NaY exchanged catalysts that during reduction using 1 bar hydrogen pressure only trace amount of metallic cobalt is formed at 673 K. 0 ,==,

0%!

r

|

i

- - i

Table 1. Kinetic parameters for H2/CO reaction over Co/NaY and Re-Co/NaY at 10 bar

Sample

Treatmen

T/K

Rate p.mols-~g

C~

0.035 0.14 0.11 0.54 0.07 0.31

27.4 27.0 22.7 24.3 28.2 30.2

1

Co/NaY

O2/H2

ReCo/NaY

O2/H2 He/H2

509 536 509 536 509 536

Selectivity C2-C4 p o

C5+

20.9 22.9 25.5 28.7 26.3 32.3

20.7 0.77 20.0 24.3 0.77 25.9 21.0 0.72 17.0

31.0 30.0 27.5 21.0 24.5 21.8

o~

Eact kJmol-~ 98 92 117

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The data also indicate that in presence of rhenium only a slight increase can be experienced in the degree of reduction.

Table 2. Kinetic parameters measured at 551 K for the various samples prepared by SG methods Sample Treatment Pressure Rate Eaet (bar) ~tmole s"l g-1 kJ mol -I Co/SG CoRe/SG(a) CoRe/SG(b) CoRe/SG(c)

O2/I-I2 O2/I-I2 O2/I-I2 O2/I--I2

10

0.63

113

10

0.63

132

10

1

115

10

0.63

105

The presence of rhenium measured by XPS is more "visible" in CoRe/SG than in CoRe/NaY catalyst. The factor affecting the appearance of rhenium lies in the difference of preparation: although the metal dispersion is high on the CoRe/SG sample, rhenium is located 9 e l [] (R-CA 9~ at the surface in the vicinity of cobalt. However, the Re 3+ ions 1010"/,first exchanged in NaY, 80*/, are mostly covered by the Co 3+ ions used in the "~" R}*/*t subsequent ion exchange process. As shown in Fig. 1, when the CoRe/NaY is 0%1 . . . . . . . . . . treated in the reaction mixture at higher temperature and/or higher r o~ ~ pressure, the extent of reduced cobalt increases. Fig. 5. Comparison of selectivities in H2/CO reaction Consequently, this is why measured over SG and NaY supported samples at 1 bar overnight stabilization is (1, 2, 5, 6 bar graphs) and at 10 bars (3, 4, 7, 8 bar required in the reaction graphs) mixture to stabilize the sample. On the other hand, in the CoRe/SG samples XRD shows amorphous cobalt-rhenium particle in agreement with TEM measurements which illustrate a highly dispersed metallic cobalt and bimetallic particles (EDX) the average diameter of the metal particles being about 2 nm [71. The slight increase in the rates compared to the SG and NaY prepared catalysts reflects the difference mentioned above. Characteristic difference can be seen in the selectivity values. As shown in Fig. 5 a distinct increase in methane formation on Co/SG as well as CoRe/SG pretreated in different ways can be observed (compare SG0, SG(a), SG(b) and SG(c)) indicating the increasing importance of Re. That is, calcination and reduction increases the size of metal particles containing rhenium,

"=o,,4o,/,t

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which promotes hydrogenation producing methane. This effect is more pronounced at higher pressure (see Fig. 5) at which the methane formation is further increased. For Co/SG and CoRe/SG compare bars 1 and 2 as well as bars 3 and 4 for 1 bar and 10 bars pressure, respectively. NaY supported cobalt rhenium behaves in a different way. XRF indicates that rhenium is present in the sample, but it is not detectable in a measurable quantity at the surface. However, as a result of its effect, the rate increased as shown in Table 1 and methane selectivity is diminished while the C2-C4 as well as C5+ selectivity increase. This effect is attributed to the rhenium retarding the carbonaceous deposition on the catalyst. Paraffin selectivity increases in the presence of rhenium, which is due to the increase of hydrogenating activity inside the supercage. To sum up, the results clearly illustrate the difference between the pure cobalt and the rhenium cobalt catalysts as well as among the ReCo bimetallic catalysts prepared in different ways. In the CO hydrogenation rhenium has a direct effect on cobalt prepared by sol/gel method indicated by the increase in methane selectivity due to the enhanced hydrogenating activity. The presence of rhenium at the surface provides also higher reducibility of cobalt ions shown by TPR and XPS. This is a direct effect of rhenium, which results always in enhanced methane selectivity and a decrease in the C5+ products measured both at 1 and 10 bars. Incontrast, in Re-Co/NaY the rhenium is not located at the surface of the metal particles (no XPS signal and small reducibility of cobalt). Still the rate of the CO hydrogenation increases and the methane and C2-C4 as well as C5+ fractions decreases and increases, respectively. This is why rhenium, similarly to other systems [3, 4, 8, 9], makes the catalyst more stable against deactivation and increases the activity. However, in the Re-Co/NaY sample an optimum performance for the formation of higher hydrocarbons is observed. ACKNOWLEDGMENTS

The work was supported by the Hungarian Science and Research Fund (T-022117) REFERENCES

1 2

3 4 5 6

7 8 9

V . M . H . van Wechem and M. M. G. Senden, Natural Gas Conversion II (Eds.: H. E. Curry-Hyde and R. F. Howe), Stud. Surf. Sci. Catal., Vol. 81, p. 43, Elsevier Sci. Publ. Co., Amsterdam, 1994 B. Eisenberg, R. A, Fiato, C. H. Mauldin, G. R. Say and S. L. Soled, Natural Gas Conversion V (Eds.: A. Parmaliana, D. Sanfilippo, F. Frusteri, A. Vaccari and F. Arena), Stud. Surf. Sci. Catal., Vol. 119, p. 943, Elsevier Sci. Publ. Co., Amsterdam, 1998 Z. Schay, K. L~tzhr, I. Bogyay and L. Guczi, Appl. Catal., 51 (1989) 33 Z. Schay, K. L~izhr, K. Matusek, I. Bogyay and L. Guczi, Appl. Catal., 51 (1989) 49 US Patent 4,801,573 (1989) Genmin Lu, Tamers Hoffer and L~szl6 Guczi, CataL Lett., 14, 207 (1992); L. Guczi, R. Sundararajan, Zs. Kopp~y, Z. Zsoldos, Z. Schay, F. Mizukami and S. Niwa, J. Catal., 167, 482 (1997) F. Mizukami, S. Niwa, M. Toba and L. Guczi, to be published V.K. Shum, J. B. Butt and W. M. H. Sachtler, J. Catal., 99, 126 (1986) Z. Schay, K. Matusek and L. Guezi, AppL Catal., 10, 173 (1984)

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

On the effect of the preparation conditions on the location and the size of platinum particles on zeolite NaX L.V. Mattos a, M.C.M. Alvesb, F.B. Noronha c, B. Moraweckd and J.L.F. Monteiro a ~NUCAT/PEQ/COPPE/UFRJ, CP: 68502-CEP:21945-970-Rio de Janeiro/RJ-Brazil, e-mail: [email protected] or monteiro@peq, eoppe.ufrj.br. ~t,aborat6rio Nacional de Luz Sincrotron (LNLS), CP: 6192-CEP: 13083-970-Campinas/SPBrazil, e-mail:[email protected]. qnstituto Nacional de Tecnologia (INT), Av. Venezuela, n~ Janeiro/RJ-Brazil, e-mail:[email protected].

de

dlnstitut de Recherche sur la Catalyse (IRC), 2 Avenue Albert Einstein, F 69626 Villeurbanne Cedex-France, e-mail: [email protected]. The effect of the calcination temperature, and of the reduction temperature on the location and the size of platinum particles on zeolite NaX was studied by EXAFS. The results showed that the tetramin platinum complex in the cationic sites of the zeolite is not completely decomposed atter calcination at 633 K for 2 h. For this sample, some amine groups are still present after reduction at 633 K for 2h. Increasing the reduction temperature from 633 to 773 K caused the growth of the particles, the larger ones probably located on the outer surface of the zeolite crystals. On the other hand, for the samples calcined at 773 K for 2 h, the complex is completely decomposed and the reduction temperature did not affect the particle size significantly. For these samples, most of the particles are probably located inside the zeolite supereages. I.INTRODUCTION Metals supported on zeolites are catalysts that usually combine highly dispersed particles of uniform size with the stereospecificity imposed by the molecular dimensions of the porous structure of zeolites. However, the determination of the appropriate conditions of preparation of these catalysts (ion exchange, calcination, dehydration and reduction) is important for obtaining the desired activity/selectivity. These conditions will control the location and size of metal particles within the zeolite cages [1,2]. Pt and Pd ions are preferentially introduced into zeolites X and Y by ion exchange from solutions of amminated metal complexes. The ligands of the precursor ions are removed during the calcination step. A secondary effect of this calcination is that the metal ions, alter deprived from their ligands, can migrate from the supercages to the smaller cages (sodalite cages or hexagonal prisms), which may be undesirable, unless precautions are taken so as make the metal particles to migrate back to the supercages during reduction. Finally, reduction conditions should be

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chosen so as to minimize the growth of the particles and their presence in the small cages. In the first ease, a small dispersion would result, and in the latter the particles would be inaccessible to most organic and inorganic molecules. Although there have been many reports on Pt-eontaining faujasites [2-7], particularly for the protonie form of the zeolite, data on EXAFS characterization for Pt particles on X zeolites could not be found. So, the goal of this work is to study the effect of the calcination temperature, and of the reduction temperature on the location and the size of the platinum particles on zeolite NaX by EXAFS. 2. EXPERIMENTAL SECTION The parent sample was a NaX zeolite with a Si/AI ratio of 1.25, provided by Instituto de Pesquisas Teenolrgicas (IPT)/Brazil. The metal ions were introduced by ion exchange from a 5x10 3 M Pt(NH3)4CI2 solution. The exchanged sample (Pt(NH3)4X) was calcined under pure O2 (1.0 l/g.min) at 1 K/min up to 633 K or 773 K and kept at the final temperature for 2 h. For the EXAFS measurements, the calcined samples were exposed to ambient air, dehydrated at 633 K under N2, reduced under 1-12at 5 K/min up to 633 K or 773 K and kept at the final temperature for 2 h. Then, the catalysts were transferred to a sample holder under N2 in a glove box and isolated with a Kapton film. The EXAFS measurements were done at the XAFS beamline [8] of LNLS/Campinas-Brazil at room temperature, using a Si (200) double crystal monochromator. The storage ring was operated with an electron energy of 1.37 GeV and a current of 100mA. The measurements were taken in transmission mode at the Pt Lmedge (11.5 keV). The EXAFS spectra were analyzed in a standard manner [9]. After background removal and normalization using the method by Lengeler-Eisenberger [10], the k 1- and k 3- weighted ~ (k) function was Fourier transformed from k=3-14 A using a Kaiser window (t=9). Thus, the peak corresponding to the first coordination shell was isolated and back Fourier transformed into the k-space. Platinum foil, PtO2 and Pt(NH3)4CI2 were used as references to calculate phase and amplitude functions for the first Pt~ ~ pt2§ and pt2§ coordination shells, respectively. The quality of the fit (Q) was determined according to [ 11 ].

9

3. RESULTS AND DISCUSSION

3.1. Atomic absorption The composition of the unit cell, as given by atomic absorption, for samples NaX and Pt(NH3)gX was, respectively, Na85A185Si1070384 and Pto,62(NH3)2,48Nas3,76A185Si1070384(0.89 wt% pt).

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3.2 EXAFS 3.2.1. Exchanged and Calcined Samples

The Fourier transforms fiTs) of P t ~ 3 ) 4 C | 2 and Pt(NH3)4X ale very similar, with a peak around 1.69 A. The fitting for this sample showed the presence of 4.5 N atoms at 2.02 A in the first Pt 2+ coordination shell (Table 1). This result agrees with those ofTzou et al [2]. The FTs of the samples calcined at either 633 or 773 K exhibit only a peak at 1.651.69/1, (without phase correction). Furthermore, there are no metallic particles in the calcined samples. For the sample calcined at 633 K, the best fit was obtained with 2.0 N atoms in the 1~t coordination shell of Pt 2+, besides the O atoms (Table 1). This shows that this sample still contains NH3 ligands. These results agree with those obtained by temperature-programmed decomposition of the ion-exchanged complex in the zeolite, which revealed the formation of NO at temperatures above 633 K. So, contrary to what has often been assumed [1,2], the tetramin complex is n o t completely decomposed at 633 K. This behavior could be attributed to the higher stability of some partially decomposed amine complex, which migrated into the sodalite cages, due to the high electronic density within these cages in zeolite X. On the other hand, for the sample calcined at 773 K, all NH3 groups were removed (Table 1).

3.2.2 Reduced Samples The FT of the sample calcined and reduced at 633 K shows a peak at 1.69A, assigned to 2+ Pt -N, indicating that the partially-decomposed complex located in the sodalite cages was still not completely decomposed after reduction at 633 K (as also confirmed by TPR). Moreover, two peaks were observed at 2.07 and 2.60 A. The peak at 2.60 A corresponds to the first Pt~ ~ shell and the peak at 2.07 A derives from both the contribution of the satellite peak of Pt~ ~ and the interaction pt0-O~ite [ 12,13]. This interaction is stronger the smaller the particles. The intensity of this peak made it impossible to obtain a good fit and indicates that the particles were very small. The mobility of these metal particles is low at 633 K and they remained in the supercages upon reduction. For the sample calcined at 633 K and reduced at 773 K , the reduction was complete, the intensity of the satellite peak was weak (Figure 1), and the fitting gave a coordination number of 10.9 (Table 1). The average particle size determined from the average coordination number [ 14] was 45 A, indicating that most of the particles were probably located on the outer surface of the zeolite crystals. Increasing the reduction temperature increases the migration to the outer surface and the sintering of the metal particles derived from the Pt 2+ ions originally in the supercages. The FTs of the samples calcined at 773 K and reduced at 633 K or 773 K are very similar (Figures 2 and 3). The reduction was complete at both temperatures. The coordination numbers obtained for the samples reduced at 633 and 773 K are 6.8 and 7.4, which corresponds to an average particle size of 11.3 and 12 A, respectively. These results indicate that the reduction temperature did not affect significantly the particle size, contrary to what was observed for the sample calcined at 633 K. For the samples calcined at the highest temperature, a larger fraction of Pt 2+ ions migrate into the sodalite cages. During the reduction, at either 633 or 773 K, the particles formed in the smaller cages migrate to the supercages where the particles already there act as nucleating sites. So, the particles grow up to a size that fits well into the supercage.

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化

Ii

9

2.56

experimental

fit

t l~

= 2.0

I

0

,

I

I

2

4

,

I

6

,

8

R(A)

0

4

8 k ( h -1)

12

16

Fig 1 FT and data fitting for the sample calcined at 633 K and reduced at 773 K.

experimental 2.58

0 "2

zi R(A)

6

8

0 T 4 ~

1'2

16

k (A-I)

Fig 2: FT and data fitting for the sample calcined at 773 K and reduced at 633 K.

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experimental 2.58

2.12

It

m

R(A)

0

4

~

8 k ( A -1)

12

16

Fig 3: FT and data fitting for the sample calcined at 773 K and reduced at 773K. Table 1 Fitted parameters (k3 weighted X (k) function and AR=1.8-3.0 A for reduced samples / k ~weighted X (k) function and AR=I.0-2.0A for exchanged and calcined samples) pt2+-O Pt2+-N Pt-Pt sample aN t'R (A) N R (A) N R (A) ~Q (%) Pt(NH3)aX 4.5 2.02 4.9 calc. 6 3 3 K 3.2 2.00 2.0 2.00 3.0 calc.773 K 3.9 2.00 . . . . 7.8 Calc. 633K/red.633K . . . . . . . Calc. 633K/red.773K . . . . 10.9 2.74 8.2 Calc. 773K/red.633K . . . . 6.8 2.72 4.3 Calc. 773K/red.773K . . . . 7.4 2.72 11.2 The Debye-Waller factor (Ao2) and IAE~ ] were smaller than 3.5 x 103 ti.2 and 4.0 eV, respectively, for all samples, aCoordination number, bBond distance. CQuality of the fit. 3. CONCLUSIONS The EXAFS results obtained for Pt-containing NaX zeolites show that the decomposition of the tetramin complex is not complete at 633 K after 2 h. The partiallydecomposed complex is probably located in the sodalite cages. For the samples calcined at

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633 K and reduced at 633 K, some amine groups are still present and the interaction Pt~ is strong, which suggests that the metal particles formed are very small. For the sample calcined at 633 K and reduced at 773 K, the complex is completely decomposed. Fitting of EXAFS data for this sample gave a particle size of 45 A, indicating that most of the particles are probably located on the outer surface of the zeolite. For the samples calcined at 773K, the reduction temperature did not affect the particle size appreciably and the average size of the particles show that they fit well into the zeolite supercages. Moreover, for the samples reduced at 633 K, the particles obtained from the sample calcined at 773K were larger than those from the sample calcined at 633 K. This can be ascribed to the migration of the Pt particles from the sodalite cages to the supercages, during the reduction of the samples calcined at 773 K, and their subsequent coalescence with the reduced particles already there.

ACKNOWLEDGEMENT We are grateful to LNLS for the beam time given to perform these measurements. One of the authors (L.V. Mattos) acknowledges the scholarship received from CNPq. REFERENCES

1. 2. 3. 4.

W.M.H. Sachtler and Z. Zang, Advances in Catalysis, 39 (1993) 129. M.S. Tzou, B.K. Teo and W.M.H. Sachtler, J. Catal., 113 (1998) 220. P. Gallezot, Catalysis Review-Sci. Eng. 20 (1979) 1335. R.A. Della Betta and M. Boudart, Proceedings of the 5th International Congress on Catalysis, (1972), 101. 5. M.G. Samant and M. Boudart, J. Phys. Chem., 95 (1991) 4070. 6. A. Mallmann, D. Barthomeuf, Stud. Surf. Sci. Catal., 46 (1989) 429. 7. K.I. Pandya, S.M. Heald, J.A. Hriljac, L. Petrakis and J. Fraissard, J. Phys. Chem., 100 (1996) 5070. 8. M. Tolentino, J.C. Cezar, D.Z. Cruz, V.C. Cailhol, E. Tamura and M.C. Alves, J. Synchrotron Rad., 5 (1998) 521. 9. B. Moraweck and A.J. Renouprez, Surf. Sci., 106 (1981) 35. 10. B. Lengeler and P. Eisenberger, Phys. Rev., B21 (1980) 4507. 11. A. Michalowicz, Phd. Thesis, Paris, 1990. 12. M. Vaarkamp, J.V. Grondelle, J.T. Miller, D.J. Sajkowski, F.S. Modica, G.S. Lane, B.C. Gates and D.C. Koningsberger, Catalysis Letters, 6 (1990) 369. 13. S.J. Cho, W-S. Ahn, S.B. Hong and R. Ryoo, J. Phys. Chem., 100 (1996) 4996. 14. R.B. Greegor and F.W. Lytle, J. Catal., 63 (1980) 476.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Hollow metallic particles obtained by oxidation/reduction treatment of organometallic precursors. F.J. Cadete Santos Aires a, R. Darji b, J.F. Trillat a, A. Howie b, A. Renouprez a aInstitut de Recherches sur la Catalyse (UPR 5401, CNRS), 2 Avenue Albert Einstein, 69626 - Villeurbanne, France bCavendish Laboratory, University of Cambridge, Madingley Road C B 3 0 H E , United Kingdom Oxidation and rapid reduction of supported organometallic precursors on silica yield large metallic particles (5 to 20 nm) that exhibit in transmission electron microscopy (TEM) a typical difference of contrast between the center and the borders. Z-contrast high angle annular dark field ( A A D F ) imaging in the scanning transmission electron microscope (STEM) has been successfully used to show that these particles are hollow in the case of Pd. PdMn particles have also been prepared and it is shown that they exhibit surface cavities instead of internal cavities. 1. INTRODUCTION Supported particles may be obtained by metal acetylacetonate deposition on silica [1]. Oxidation and subsequent reduction of such particles are necessary to obtain "organic shellfree" metallic supported particles suitable for catalysis. In the case of Pd, this generally yields collections of narrowly distributed (3-5 nm) supported particles. However, in some cases, larger particles (5-20 nm) may be formed that exhibit, in TEM images, a central core with a lower contrast than the shell surrounding it [2]. We have noticed that a rapid reduction following the oxidation of supported Pd acetylacetonate precursors on silica, at temperatures around 400~ yields collections of particles in which as much as 60% of them exhibit such a contrast [3]. Based on TEM bright field (BF) imaging alone this difference in contrast can not easily be attributed neither to a chemical difference (oxide, ...) between the core and the border of the particle nor to an internal cavity. The purpose of this paper is to show that such Pd particles are hollow using their characterization by Z-contrast HAADF imaging. We also extend this approach to bimetallic (PdMn) particles prepared under the same conditions. 2. PREPARATION METHODS

2.1. Metallic particles The particles were prepared by impregnation of silica (Aerosil 200, Degussa; 200 m2/g) with Pd bis-acetylacetonate and Pd and Mn bis-acetylacetonates, in the case of PdMn. The

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supported precursors were then oxidized, at 400 ~ under 02 for 4h, and reduced under H2 for lh, at the same temperature. Samples are 2% (wt) Pd and Pd90Mnl0.

2.2 Electron microscopy sample preparation Suitable electron microscopy samples were obtained by crushing the powder (Pd/SiO2, PdMn/SiO2) in ethanol which is then ultrasonically dispersed. A drop of the solution is then deposited on a holey-carbon film supported on a Cu microscopy grid (3.05 mm, 200 mesh).

3. CHARACTERIZATION TECHNIQUES 3.1. High angle annular dark field imaging in the STEM A schematic representation of a STEM is shown in Figure 1. A sub-nanometer electron beam is used to probe the sample. Following the probe-sample interaction, inelastically scattered electrons (at low angles) can be used to perform bright field (BF) imaging or electron energy loss-spectroscopy (EELS). Simultaneously, elastically scattered electrons (at

deflection coils quadrupoles

spectr

........~ ~ ~ .................. ~ "

~ '

1 7

~~001 ...........

CCD

BrightdetectorField

High Angle Annular Dark Field detector(50 - 300 mrad)

specimen

electron beam

Fig. 1. Schematic representation of a STEM with HAADF and BF detectors as well as the CCD parallel electron energy loss spectrometer developed at the Cavendish Laboratory [4].

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high angles) can be detected by a high angle annular detector in order to perform HAADF imaging in which the intensity strongly depends on Z. This imaging technique has been used to study supported catalysts [5-8] including the quantitative analysis of bimetallic supported catalysts (PdPt) [8]. By using this technique to study our particles we expect to see a depletion of the HAADF signal corresponding to the low-contrast central core observed in BF images if the particles happen to be hollow. Experiments were performed with VG HB501 STEM (100 kV) using a HAADF detector of 50 mrad inner, and 300 mrad outer, collection angle.

3.2. Transmission electron microscopy and energy-dispersive X-ray spectroscopy TEM observations and energy dispersive X-ray spectroscopy (EDX-S) linescans across the particles were performed with a JEOL JEM 2010-F TEM (200 kV) equipped with a high resolution pole piece and an EDX spectrometer (Pentafet-LinK ISIS, Oxford Instruments). An electron probe of 1 nm was used for the linescan EDX-S experiments. 4. RESULTS AND DISCUSSION

4.1. Pd hollow particles Pd particles exhibiting a lower contrast central core are shown in Figure 2. In some cases the particles seem to be single-crystalline with marked facets and the central core is also facetted (Figure l b). The latter can occupy up to two thirds of the volume of the particle. EDX-S linescans across the particles show that the Pd-L intensity decreases at the center (Figure 3), proving that there is a lack of Pd in the region corresponding to the lower contrast.

a

25 nm

~~;~'

~--~:

:'

5 nm

Fig. 2. TEM images of Pd particles exhibiting a lower contrast in the center. General view (a) and facetted particle (b).

Figure 4 shows that in the case of pure Pd particles the A A D F image exhibits a strong depletion at the center of the particle corresponding to the lower contrast central core in the BF image. Since the HAADF intensity depends on Z, this means that there is a pronounced lack of Pd as already observed by the EDX-S linescans. We have performed a quantitative analysis of the HAADF intensity in order to verify that this depletion can be attributed to a cavity. In Figure 5 we present the HAADF intensity across a particle (corresponding to the

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line in Figure 5a) together with the simulated HAADF intensity for a Pd particle, a hollow Pd particle and a PdO core surrounded by a Pd shell. The best quantitative fit with the experiment is obtained for the hollow particle.

60 50 40

~ ~

30 20 lO o

0 8.5 17 25.5 34 42.5 51 59.5 68 76.5 85 b

Distance (nm)

Fig. 3. TEM image (a) and EDX-S linescan (b) across a Pd particle. The linescan was performed along the black line drawn in (a).

50 n m

Fig. 4. STEM BF (left) and HAADF (right) images of Pd particles. (a) 0 ~ tilt. (b) 15 ~ tilt.

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EELS analysis has not shown the presence of any gas within the cavity and X-ray diffraction spectrometry of the same sample shows that we do not have palladium hydride. In HAADF imaging (Figure 4), when tilting the particles by 15 ~ the cavity is still centered meaning that that the particles are hollow. All observed particles show this behaviour. These Pd particles are clearly formed by a Pd shell surrounding an empty cavity.

350 300

.............................. B

b

.~ ~ 250

i ! 200 i~

150 a 100 50 0 -12,5 -9,375 -6,25 -3,125

0

D i s t a n c e across the

3,125 6,25 9,375 12,5 particle (am)

Fig. 5. (a) STEM HAADF image of a hollow Pd particle (scale bar is 25 nm). Line corresponds to the plot A of (b). (b) experimental HAADF intensity across the particle in (a) (A) and normalized HAADF intensity simulation for a normal Pd particle (B), a hollow Pd particle (C) and a PdO core surrounded by a Pd shell (D). 4.2. Surface cavities in PdMn particles In the case of bimetallic PdMn particles we have observed the same phenomenon. However, most of the particles appear to have surface defects (Figure 6) instead of lower contrast central cores. HAADF images (Figure 7) clearly shows these defects to be surface cavities. This is the first time that this has been observed for bimetallic particles. Since the presence of cavities has only been clearly observed for Pd (or Rh - an image of a Rh particle with the same kind of contrast has been published even if no mention of the presence of a cavity has been made [9]), it is reasonable to assume that in PdMn alloy particles the cavities are preferentially formed on Pd rich regions. Furthermore, in PdMn alloys, Mn is expected to strongly segregate to the surface [ 1], which means that the surface will be mainly poor in Pd. The formation of the cavities will then not be so easy which certainly explains the formation of surface cavities (probably on surface Pd "less-poor" regions) instead of internal cavities. 5. CONCLUSION This study clearly shows that supported Pd particles on silica obtained by oxidation/reduction treatment of organometallic precursors are hollow particles. It seems that the step of a rapid reduction following a normal oxidation increases the yield of such particles. For the first time, a similar phenomenon is reported for bimetallic (PdMn) particles presenting surface cavities. The understanding of the mechanism leading to the formation of

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such particles needs further systematic investigation even if recent in-situ TEM studies tend to indicate that the oxidation step is important [ 10]. It would also be interesting to obtain smaller particles (< 5 nm) exhibiting the same particularities which may have specific electronic, optical and catalytic properties. Z-contrast HAADF imaging has proved to be relatively easy to operate and a well adapted technique for this type of study and confirms electron holography studies already [2] performed.

Fig. 6. TEM image of a PdMn particle exhibiting a surface cavity (arrow).

Fig. 7. STEM BF (left) and HAADF (right) images of PdMn particles with surface cavities.

ACKNOWLEDGMENTS We thank the Alliance program for providing travelling expenses for this collaboration. REFERENCES

1. J.F. Trillat, Th~se de doctorat n ~ 29/97, Universit6 Claude Bernard Lyon I (1997). 2. L.F. Allard, E. Voelkl, D.S. Kalakkad and A.K. Datye, J. Mat. Sci., 29 (1994) 5612. 3. F.J. Cadete Santos Aires, R. Darji, J.F. Trillat, A. Howie and A. Renouprez, Electron Microscopy 98, Volume III, H.A. Calder6n Benavides and M. Jos6 Yacam~in (eds.), lOP Publishing, Bristol, 1998, p. 165. 4. D. McMullan, J.M. Rodenburg, Y. Murooka and A.J. McGibbon, Inst. Phys. Conf. Ser., 98 (1989) 55. 5. M.M.J. Treacy, A. Howie and C.J. Wilson, Phil. Mag. A, 38 (1978) 569. 6. M.M.J. Treacy and S.B. Rice, J. Microsc. 156 (1989) 211. 7. P.D. Nellist and S.J. Pennycook, Science 274 (1996) 413. 8. R. Darji, A. Howie, A. Bleloch, F.J. Cadete Santos Aires, J.L. Rousset, A.M. Cadrot and A. Renouprez, Electron Microscopy 96, Volume 2, CESM (ed.), Brussels, 1998, p. 66. 9. A.K. Datye and D.J. Smith, Catal. Rev. - Sci. Eng., 34 (192) 129. 10. P.A. Crozier and R. Sharma, Electron Microscopy 98, Volume III, H.A. Calder6n Benavides and M. Jos6 Yacam/m (eds.), lOP Publishing, Bristol, 1998, p. 499.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Synthesis, characterization and catalytic application of inorganic nanotubes I. Kiricsia*, tit. Kukovecz a,/~. Fudala a, Z. K6nya~, I. Willemsb, J. B.Nagy b aDepartment of Applied and Environmental Chemistry of J6zsef Attila University, H-6723 Szeged, Rerrich B61a t6r 1, Hungary, bNMR Laboratory, University Notre Dame de la Paix, B-5000 Namur, rue de Bruxelles 61, Belgium.

Nanotubes of inorganic compositions have been prepared and characterized by various techniques. Two main additives were used for the preparation. As template material carbon nanotubes were used for preparation of silica and alumino-silica tubes with diameter of nanometer scale. Optically active organic acids were used as additives for synthesis of silica nanotubes. In this case the structure of the tube showed that its geometry was angular rather then circular. For removal of organic material from the as prepared nanotubes two different methods were applied. Besides the conventional burning with oxygen, a novel method, applying ozone as oxidant, was also used. The inorganic nanotubes were suitable support for heteropoly acids as catalytically active components. 1. INTRODUCTION Intensive research activity has been invested in the preparation and characterization of mesoporous materials, such as MCM, HMS, FSM and MSU families, to clarify their applicability as catalysts in petrochemical processes and in the production of fine chemicals [1]. As a new challenge, the paper by Rao et al. appeared describing the preparation of nanometer size tubes from silica, alumina, molibdena, vanadia and various transition metal oxide containing silica using carbon nanotubes as template [2]. A new method has been suggested for the preparation of nanotubes from silica by Shinkai et al [3]. They applied organic gel fibers formed from cholesterol derivatives in various solvents as templates. Matsui's group reported a very simple method for the synthesis of silica nanotubes [4]. They used a simple organic molecule the DL-tartaric acid in ammonia containing solution. The average parameters of the silica tubes they obtained were as follows: inner diameter 20800 nm, outer diameter 0.8-1.0 ~tm, the length of the tubes were between 200-300 ~tm. Nanotubes from metal sulfides were described by Dorhout et al. [5]. The MoS2 nanotubes were prepared starting from ammonium tiomolibdate and its polimer. The product had 50 nm inner diameter, 10 nm wall thickness and 30 ~tm length. Mesoporous molecular sieves, MCM-41 with a hollow tubular morphology were also prepared [6]. Quaterner alkylammonium salts with long alkyl chain were used for the *This work was supportedby the FKFP 0486 and OTKA 25246 grants.

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preparation of MCM-41 material. The morphology of the product showed an ordered arrangement of the single MCM-41 tubuli forming a secondary tubular structure with a 10 ~tm outer and 900 nm inner diameter. Further development can be demonstrated by the paper of Han et al, who reported a novel preparation method for synthesis of micrometer sized carbon tubes [7]. Using our experience in the synthesis of carbon nanotubes [8] and preparation of mesoporous structures by pillaring layered silicates [9], we studied the synthesis of nanotubes from inorganic compounds. The nanostructured materials were characterized by various instrumental analytical methods and attempts were made for their application as catalysts and catalyst supports.

2. EXPERIMENTAL

2.1. Synthesis of nanotubes The template material carbon nanotubes were prepared by a catalytic method using acetylene as carbon source. Carbon generated on the catalyst surface at 1000 K was collected and the tubes were separated and cleaned by the method described elsewhere [ 10]. Tetraethylorthosilicate (TEOS) was the precursor of the silica nanotubes. TEOS was mixed with an appropriate amount of carbon nanotubes and was slowly hydrolised. After 12 h the solid material was separated from the solution and dried first at room temperature then at 373 K and finally at 773 K in vacuum. For the preparation of alumino-silicate nanotubes appropriate mixtures of TEOS and aluminum isopropoxide (Si/AI=40) were prepared in the presence of tetrapropylammoniumbromide and the carbon nanotubes were mixed into this dilute gel. After 2 h stirring the solid phase was separated and treated as described above. Besides the usual method of template removal (burning off the carbon nanotubes at 1023 K), a novel one (oxidation of carbon with an ozone treatment at 473 K) was used as well. In the latter case, the low temperature helped to protect the wall structure of the nanotubes during carbon removal. This was found to be particularly important for the alumino-silicate tubes, where the preservation of the tubular structure was extremely difficult. Silica nanotubes were prepared at ambient conditions by the modified sol-gel method described by Nakamura et al. [4]. We have found that it is possible to scale up the original process while maintaining a constant nanotube yield. The reaction was carried out as follows: 10.0 g DL-tartaric acid and 30.0 g water were dissolved in 2000 ml absolute ethanol. To this solution 365 g TEOS was added and the mixture was stirred for 5 minutes, while stirring with a magnetic stirrer. After that, the system was allowed to stand for 30 minutes and then 1000 ml of 25% NH3 solution was added, while stirring lightly by a glass rod. In about 15 seconds an opal white gel was formed, which was permitted to stand for 20 minutes and then was washed with water to remove excess NH3, dried at 378 K overnight and finally calcined in 02 flow at 873 K for 12 hours. The resulting material contained spherical SiO2 particles with a diameter of 0.3-0.6 ~tm and silica nanotubes with an outer diameter of 0.8-1.5 lam, inner diameter of 0.5-0.8 l.tm and length of 5-100 ~tm. The catalytically inert nanotubes were transformed into active heterogeneous catalysts by building a Keggin type heteropoly acid (H3[PW12040].-~19 H20 (PW12), chosen on the basis of its favorable thermal behavior [11]) into the tubes by the means of impregnation. Samples containing 5 w% and 20 w% PWl2 were prepared by mixing 3 g of calcined nanotube powder with a calculated amount of PW~2 dissolved in 500 ml water. The water was

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gradually evaporated under continuous stirring at 363 K and, finally, the samples were dried in air at 423 K overnight. All materials used for the preparation of nanotubes were of reagent grade and used without further purification. 2.2. Characterization of the nanotubes

High resolution transmission electron microscopy was used to identify the shape and structure of the synthesis products using Phillips CM 20 instrument. The nitrogen adsorption isotherms were measured at 77 K in a volumetric system and the BET specific surface areas were determined. The generally applied measurement of the acidity using pyridine adsorption by IR spectroscopic detection failed as the structure of the tubes collapsed upon pressing IR wafers. Catalytic activity of the samples was tested by the isomerization reaction of 1-butene to 2-butenes in a recirculatory batch reactor. In a typical experiment, 100 mg catalyst (outgassed for 1 hour at 673 K in a vacuum of 3* 10.2 torr) was placed in the reactor and 500 torr of 1-butene was introduced. The reaction was monitored on-line GC analysing the samples at preselected reaction times by a HP 5710A gas chromatograph. 3. RESULTS AND DISCUSSION Figure 1 shows the as synthesized material containing the carbon nanotube template in the tube (part A) and the template free silicate nanotube (part B). It is clearly seen that the resulting material possesses tubular structure and its wall thickness is also quite uniform. For this sort of nanotubes the wall structure is only slightly sensitive to the oxidation agent. Using either oxygen or ozone for the burning off the carbon inlay, a similar wall structure is obtained. However, some advantage of using ozone instead of oxygen or air was that the specific surface area of the nanotubes prepared by the former method proved to be significantly higher. The BET areas of the specimens were as follows: tube with carbon nanotube inside: 183 m2/g. Tube treated in oxygen: 536 m2/g and tube prepared with ozone removal of carbon inlay: 611 m2/g. From this we concluded that the increase in inner surface, which was observed for the ozone treated material, can be attributed to the more intact structure of the tubes.

A

B

Fig. 1. TEM images of silicate nanotubes prepared by using carbon nanotubes as template, A: as synthesized tube showing the carbon tube inside and the outer silicon cover, B: silicate tube after removal of the carbon nanotube template.

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~

f

.......

-~

Fieure 2/B

化

Fi~re.~2/A

,",

,~

:'~,~

,~ ......

, ,.,,.

Figure 2/C

Figure 2/D

Figure 2/E

Figure 2/17

,

~,,

ili

Fig. 2. SEM images of samples prepared with various gel compositions. A: original synthesis suggested by Nakamura et al. B: original composition, continuous mixing, C: synthesis with 2.5x amount of tartaric acid, D: synthesis at 348 K, E: synthesis with no extra water added to the system, F: synthesis using citric acid instead of tartaric acid.

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In Figure 2 silica nanotubes prepared in the presence of optically active organic acids are shown. As can be seen tubular structures are recognizable for each case, however, the shape of the tubes and the amount of amorphous material retained in the product atter the synthesis are different. Inspecting the shape of the inner and outer diameters of the tubes prepared in this way, we believe that the shape is angular rather than circular. The reason of this behavior is unknown yet. The role of the organic acid, DL-tartaric acid that is, is also unclear. The concentration of this compound in the synthesis mixture suggests that it does not perform as the classical template molecules. To get more insight into the detailed mechanism of the formation of these type of nanotubes requires further research effort. The role of carbon nanotubes in the formation of silicon nanotubes is clear. In this case it happens that the TEOS hydrolysis giving monomer or oligomer silicic acid species that cover the outer surface of the carbon tube. The only question arises is the thickness of the silica wall. It should depend on the TEOS concentration used for the synthesis. Our experience showed that the problem is much more complex. To influence the thickness of the wall is rather difficult. As far as the usefulness of these nanotubes in catalysis is concerned, it is difficult to give any prediction. The kinetic curves in Figure 3 show that the tubular materials can be used as support of catalytically active component, in this case as support for the heteropoly acids. As the data measured at different temperatures and heteropoly acid concentrations revealed, the rate of reaction is very sensitive to their change. This means that the higher the heteropoly acid concentration the faster is the reaction. The influence of the temperature follows the trend expected on the knowledge of basic reaction kinetics. 443

K

h

_

500

500 ~ - -

400 [-~~,,

400

i

0 I 0

i

I 25

i

I 50 t (rrin)

,

I 75

,

I 100

0

i

:

i

,

0

:

I 25

;

,

473 "

K

I

,

50 t (n'in)

9 ;

I 75

(a)

,

(C) l 100

Fig. 3. Conversion of 1-butene into 2-butenes at 443 K and 473 K in a fixed bed recirculatory batch reactor. Curves corresponding to the reaction over pure calcined sol-gel silica nanotube are denoted with (a), while (b) denotes reactions over nanotubes containing 5 w% PW12 and (c) denotes reactions over nanotubes containing 20 w% PW12

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The measurements performed to obtain information on the deactivation of the catalytic materials showed that the tubular structure of the samples was retained. The TEM images of the used samples were identical to those of the fresh ones. From this follows that the deactivation is influenced by the formation of clusters or small crystals from the well dispersed heteropoly acid ions rather than the destruction of the tubular structure of the silica. 4. CONCLUSIONS Nanostructured tubular shaped inorganic material was prepared with various compositions and characterized. Applying oxidation with ozone for burning the template off the structure proved to be advantageous on the nanotubes. Samples prepared by loading the tubes with heteropoly acids showed acidic catalytic activities. REFERENCES

1. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 395 (1992) 710; US Patent 5098684. 2. B.C. Satishkumar, A. Govindaraj, E.M. Vogl, L. Basumallick and C.N.R. Rao, J. Mater. Res., 12 (1997) 604.; C.N.R. Rao, B.C. Satishkumar and A. Govindraj, Chem. Commun., (1997) 158. 3. Y. Ono, K. Nakashima, M. Sano, Y. Kanekiyo, K. Inoue, J. Hojo and S. Shinkai, Chem. Commun., (1998) 1477. 4. H. Nakamura and Y. Matsui, J. Am. Chem. Soc., 117 (1995) 2651. 5. C.M. Zelenski and P.K. Dorhout, J. Am. Chem. Soc., 120 (1998) 734. 6. H.-P. Lin, S. Chemg and C.-Y. Mou, Chem. Mater. 10 (1998) 581. 7. C.-C. Han, J.-T. Lee, R.-W. Yang, H. Chang and C.-H. Han, Chem. Commun., (1998) 2087. 8. K. Hernadi, A Fonseca, P. Piedigrosso, J. B.Nagy, D. Bernaerts, J. Riga and A. Lucas, Catal. Lett., 48 (1997) 229. 9. A. Fudala, I. Kiricsi, S.-I. Niwa, M. Toba, Y. Kiyozumi and F. Mizukami, Appl. Catal. A, 176 (1999) L153. 10. K. Hernadi, A Fonseca, J. B.Nagy, A. Fudala, D. Bernaerts and A. Lucas, Zeolites, 17 (1996) 416. 11. M. Varga, B. TOrOk and A. Molnfi.r, J. Therm. Anal., 53 (1998) 207.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

N A N O S T R U C T U R I N G S U R F A C E S BY L I T H O G R A P H Y : THE PRODUCTION AND USE OF MODEL CATALYSTS Markus Schildenberger a, Yargo Bonetti b, and Roel Prins a aLaboratory for Technical Chemistry, Swiss Federal Institute of Technology (ETH), CH-8092 Zurich, Switzerland 2Laboratory for Micro- and Nanostructures, Paul Scherrer Institute (PSI), CH-5232 Villigen Nanostructured 4 inch Si wafers were prepared as new supports for model catalysts. A single wafer exhibits 109 to 10 l~ pits on an otherwise fiat silicon oxide surface. To produce these nanostructures, oxidized wafers were covered with photo resist material and exposed to laser interference patterns. Using hydrofluoric acid, wet-chemical etching of the SiO2 through the structured resist resulted in a pitted surface. The etched pits were loaded with metal particles by evaporation (palladium, silver) or by means of spin coating (copper). Optimizing the methods enabled exact deposition of single metal clusters exclusively in the pits. A fi~her model catalyst system was produced by sputter etching of a number of bilayers (Pd/SiO2), stacked on an oxidized Si wafer surface. This results in billions of isolated towers, consisting of disks of active metal layers, separated by inert substrate material. Sputter deposition was used to deposit the multi-layered material onto the wafer. The active species is present as the rims of the metal disks that make up the towers. The topography and chemical composition as well as their changes, induced by the reaction conditions applied, including stability and chemical behavior of the nanostructured systems, were investigated by means of AFM, SEM, temperatureprogrammed methods and XPS. To determine their usefulness in catalysis, especially designed reactors were developed for the catalytic investigations. Carbon monoxide and hydrogen oxidation have been used to test the catalytic activity of the systems. All palladium containing catalysts showed remarkable activity after cleaning and activation treatments. 1. Introduction

Lithography as a method for building model catalysts has been used since 1992 [1]. Research has been continued to improve this method for developing catalytic systems. Electron-beam lithography followed by metal evaporation is the most sophisticated method available nowadays [2]. The excellent features of the resulting systems (well-defined and separated nano-scale metal clusters, easy accessibility for surface science methods, sufficient active surface area and high reproducibility) are somewhat diminished by the considerable problems that remain to be solved. Beside the fact that e-beam lithography is a very time-consuming technique, removing the resist material often leaves the surface contaminated by carbonaceous species. Thus, the poisoned catalyst is fairly inactive and

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hardly suitable for reactivity investigations. Activating the system is a challenging task. Most of the cleaning techniques used in surface science have failed, thus far. Methods used in ,,real catalysis" often lead to a loss of the system's defined properties, due to the high temperatures and oxidizing conditions applied. Nevertheless, it is worth to try and solve these difficulties, because the well-ordered array of well-defined catalytically active metal clusters opens up numerous research possibilities. We present new model catalyst systems that are produced using a combination of methods [3]. Sputter deposition or evaporation is applied to create thin metal and insulator films. Laser interference lithography is the method of choice for lateral patterning of the photoresist and wet chemical etching is used to transfer the resist pattern to the catalyst material. Furthermore, impregnation is achieved by spin-coating of metal salt solutions.

2. EXPERIMENTAL AND RESULTS 2.1. Metal deposition by evaporation The substrates for all the systems presented here are industrially available wafers of single crystalline (100) silicon (development of catalysts with aluminum oxide wafers as substrates is in progress). The surface of the wafers is heavily oxidized and covered with a layer of photo-resist material. Double exposure to interfering laser beams and developing the resist leads to a sieve-like polymeric layer. Pits are etched into the silicon oxide through these holes using wet chemical methods. The patterned and etched wafers are then covered with a layer of noble metal. After removing the resist and, consequently, the metal on top of the resist (,,lift-off'), metal films of defined thickness remain inside the pits (Figure 1).

~

/-L ~'g~

~

resist oxide layer ~ Si wafer

~

. ~

1/

etching"-.....___~ oxide layer

m m m i ~ ~

metal evaporation

A

I

y 0xob ective

Beamsplitter top view of laser set-up

hole

--

--

/removing ~-'~xposed resist

pits loaded with metal films

on,y

cleaning

................ pure structured substrate

Fig. 1: Schematic presentation of the laser set-up for the interference lithography and steps in the preparation of the model catalysts.

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0

~J" "0

gm

0

Fig. 2 3D presentations of AFM images of a nanopitted sample. Images show the sample after etching and before Pd evaporation (a), with evaporated Pd in the pits before (b) and after (c) annealing at 700~ in UHV.

The metal films inside the pits then can be restructured to spherical particles by annealing in UHV or other atmospheres. The size of the spheres is controlled by varying the initial thickness of the metallic layer. Figure 2 shows a sample after the various preparation steps. Exposing the sample to temperatures above 500~ in UHV led to a strong decrease in intensity of the Pd 3d signal. At the same time, a slight shift in binding energy to lower values was observed (from 336.3 eV at room temperature to 335.6 eV at 500~ and above). This shift is explained by the thermo-induced desorption of carbon- and oxygen-containing species from the surface of the Pd particles. The decrease in intensity was accompanied by significant changes in the structure of the particles. Figure 2c shows an AFM image of a sample after UHV annealing at 700~ The formerly flat, disk-like films (Figure 2b) reshaped to hemispherical particles but were still trapped inside the pits. Therefore, even at the high temperatures applied, no particle-particle sintering was observed. The same results were observed after treating the samples at 400~ and 600~ in oxygen. Calculating the volumes of the disks before and those of the hemispheres after annealing results in constant values for both shapes. This leads to the conclusion that no material was lost as a result of evaporation or due to losses through the diffusion of material into the bulk. Therefore, reshaping the particles led to the decrease in the intensity of the XPS signal, because the hemispherical particles exhibited 40% less surface area than the initial disks after the annealing procedure. Starting with deposited films of 20 nm thick (Figure 2b), the annealed particles are typically 45 nm high and about 200 nm in diameter. Evaporation of smaller amounts of material leads to smaller clusters, as will be shown in the case of silver deposition.

2.2. Metal deposition by spin-coating The evaporation of metals under high vacuum conditions as described in section 2.1. is not necessarily comparable to the usual wet chemical impregnation techniques used in catalysis. This cannot be neglected, since structure, morphology, and catalytic properties are often strongly influenced by the method of preparation. However, the samples as shown

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in Fig. 2a can be used for metal deposition by the spin-coating method. A drop of metal salt solution is added to the rotating wafer [4]. The sample is then calcined and reduced according to synthesis of ,,real world catalysts". Optimizing the parameters of the production procedures results in a single metal cluster arrangement (Figure 3). The metal cluster at the bottom of the pit in Figure 3 has a diameter of 80 nm and a height of 10 nm.

0 Fig. 3:

Fig. 4:

AFM image (0.5 x 0.5 ktm2) of a sample, spin-coated with Cu(NO3)2 solution, after annealing at 350~ in UHV.

TEM image of etched multilayers on an oxidized Si wafer (bottom of the image). SiO2 is light grey, Pd dark grey.

2.3. Etching multilayered structures The system presented in Figure 4 ("Nanotowers") is produced by sputter-etching of stacked bilayers (Pd/SiO2), resulting in billions of isolated towers with submicrometer dimensions. The examples shown consist of 40 Pd/SiO2 bilayers. They exhibit catalytically active metal surfaces on the side walls, separated by support material. The altemating layers were made by sputter deposition and are from 3 to 10 nm thick. The diameter of the towers is about 200 nm, and their height ranges from 300 to 600 nm. The etching of the structures is done by argon ion milling. Since two different materials (silicon dioxide and palladium) have to be etched, wet chemical etching is not possible for the creation of the nanotowers. As can be seen in the image (at the left side of the towers), the towers are covered substantially by redeposited material, therefore blocking and poisoning the active sites. 2.4. Reactions carried out on the model catalysts. The palladium-based, pitted catalysts (with evaporated Pd) were active towards the oxidation of hydrogen during oxidation/reduction treatments carried out in a specially designed quartz glass reactor. Since some parts of the reactor could not be heated, the water that formed during the experiments was visible as a condensed water film on the inner walls of these reactor parts. More reliable and comparable measurements of the activity for the formation of water were not possible, because the stainless-steel capillary that was used for connecting the reactor to the mass spectrometer showed high catalytic activity, too.

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The pitted samples with palladium as the active material showed the best stability against oxidizing atmospheres at elevated temperatures. One of these samples was therefore used to test the catalytic activity for the CO oxidation. After several oxidation and reduction treatments at 400~ the sample was kept in a flow of 6 ml/min Ar and 2 ml/min CO at 105 Pa and 300~ When the flow of oxygen (2 ml/min) was started, no CO2 was formed at this temperature (Figure 5). Increasing the temperature to 350~ (dashed line) caused the catalytic reaction to start, showing oscillations. The temperature was then increased to of 475~ The maximum production of CO2 was achieved already at a temperature around 360~ with no major changes above this temperature. The empty reactor (without the catalyst but with the steel capillary) showed almost no activity towards CO oxidation. Therefore, the oxidation and reduction cycles are appropriate for cleaning and activating model catalysts that have been contaminated by large amounts of carboncontaining species. This is a typical problem of systems produced by lithography. 0 6-

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3. CONCLUSIONS AND O U T L O O K Due to the complexity of heterogeneous metal catalysts there is a strong need for the development of simplified model systems for basic catalyst research. Despite the simplifications, these models should be as close as possible to industrial catalysts, in order to deliver relevant data about the actual processes. Laser interference lithography is a promising tool for the production of new model catalyst systems. Three different systems were produced which provide an ideal combination of structured topography (mimicking porous and powdered supports) and easy access due to a

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variety of surface science methods. Controlled deposition of metallic particles onto a nanostructured silicon oxide surface was achieved. The evaporation of palladium through a patterned resist into pre-etched hollows as well as the wet chemical impregnation method led to perfectly ordered arrays of metal clusters with predictable sizes. The size of the clusters does not yet reach the lower nanometer region, as possible by means of electron beam lithography. However, compared to most model systems known thus far, these arrays have great potential because of the unique combination of uniformity, stability and large metal surface area (they can be produced with a large overall area on 4 inch wafers). Therefore, the catalysts provide sufficient metal surface area for reaction studies, even at atmospheric pressure. Optimizing the preparation parameters (especially reducing the thickness of the evaporated films) will help to decrease the size of the clusters. The large number of catalytically active sites as well as the mimicking of actual impregnation of such systems will help to bridge the material and pressure gaps in model catalysis. As with all lithographic methods, a wide variety of support materials and metals can be used after the process has been installed. Silica as a support material in combination with palladium, silver and copper as metals has been described. Current investigations focus on the use of alumina wafers as support material. Thus far, hydrogen and carbon monoxide oxidation have shown activity on the Pd systems after long activation treatments. Since these reactions and others such as hydrocarbon conversion occur on comparably prepared systems, they seem to be less insensitive to surface contamination by carbon-containing species, which can not be avoided on lithographically produced catalysts. The etching of multilayered structures led to isolated towers with high aspect ratios. This way, the amount of catalytically active area on a single 4 inch wafer can be increased significantly due to the extension of the system in the third dimension. However, the contamination problems due to redeposition of the sputtered material have to be solved before the towers can be used for catalytic investigations. The introduced systems offer many possibilities for research related to surface science, not only in the field of catalysis. The wetting behavior of various metals on a variety of support materials at extremely high temperatures and in different gas atmospheres can be studied in detail. The role of interfaces and contact regions can be determined by employing multi-layered systems with defined sequences of material (i.e., support/metal/promoter). The spin-coating method enables a more detailed investigation of the essential steps in catalyst production using wet chemical methods, without difficulties related to sintering at elevated temperatures. Finally, the mechanisms involved in alloying and the properties of the resulting composites can be investigated by evaporating two or more metals into the pits and then annealing them. Therefore, these samples may contribute to the ongoing research process in combinatorial chemistry. REFERENCES 1. I. Zuburtikudis and H. Saltsburg, Science 258 (1992) 1337. 2. M.X. Yang, D.H. Gracias, P.W. Jacobs and G.A. Somorjai, Langmuir 14 (1998) 1458. 3. M. Schildenberger, Y. Bonetti, M. Aeschlimann, L. Scandella, J. Gobrecht and R. Prins, Catal. Lett. 56 (1998) 1. 4. E.W. Kuipers, C. Laszlo and W. Wieldraaijer, Catal. Lett. 17 (1993) 71.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

New catalyst supports and catalytic systems on the basis of metallic gauzes coated with II-IV group element oxides +

M.P. Vorob'eva a , A.A. Grelsh, 9 a A.Yu. Stakheev a, N.S. Teleglna, 9 a A.A. Tyrlov, a E.S. Obolonkova b and L.M. Kustov a ~N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky prosp. 47, Moscow, 117334, Russia, Fax: 7095-135-5328, E-mail: [email protected] bInstitute of Synthetic Polymeric Materials, Russian Academy of Sciences, Profsoyuznaya str. 70, Moscow, 117393, Russia Preparation of catalysts and catalyst supports on the basis of metal gauzes by electrophoretic deposition was studied. Samples of stainless-steel gauzes with the thickness of wire of 50 ~tm coated with the A1203, ZrO2, SiO2 and zeolite layers from 1 to 10 ~tm were obtained. The properties of the prepared coatings were studied by SEM and XPS. Additionally, the catalytic properties of obtained by EPD zeolite catalyst were studied in the reaction of partial oxidation of benzene to phenol with N20. 1. I N T R O D U C T I O N Catalysts supported on the metal carriers such as monoliths, metal foams and wire grids are promising systems for various catalytic processes that require a low pressure drop. These catalysts possess a high mechanical strength, resistance to thermal shocks, high thermal conductivity and provide conditions for perfect heat and mass transfer. Catalyst supports on the basis of metal wire gauzes are especially perspective systems because of the higher accessible surface area and gas transfer compared to ceramic or metallic monoliths. The use of the metal wire gauzes as catalysts requires to solve the problem of deposition of a uniform secondary support layer with a high surface area and porosity onto the metal wire. The washcoating method is commonly used for preparation of the oxide layers on the ceramic or metallic monoliths. However, this method cannot be applied for deposition of coatings on the metal gauze with the wire thickness as low as 50 ~tm, because of the poor adhesion and nonuniformity of the resulting coating [1 ]. Based on a literature data, the electrophoretic deposition method (EPD) was chosen in this study for deposition of oxide layers on the metal gauzes in order to prepare new catalyst carriers. The EPD is based on the deposition of particles of a colloid suspension on the surface of an electrode under the influence of an electric field. The weight of the particles deposited on the unit of the electrode surface is a function of the amount of the charge passed through the colloid solution, concentration of particles, and deposition time. The EPD is an inexpensive and simple method characterized by fast coating formation. The mechanisms and kinetics of EPD and its applications for preparation of various ceramic materials were +M.P. Vorob'eva is grateful to "Haldor Topsoe" A/S for postgraduate student grant financial support.

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considered in the review [2]. In the present study, the oxide layers of A1203, ZrO2, SiO2 and the coating containing H-ZSM-5 zeolite microcrystallites supported on the stainless-steel gauze were prepared by the EPD method. In order to obtain the optimal coatings, the influence of the EPD conditions on the quality of the deposited layer was studied in detail. 2. EXPERIMENTAL

The alumina sol was prepared by hydrolysis of aluminum isopropoxide followed by aluminum hydroxide peptization by nitric acid at the molar ratio HNO3 : ( i s o - C 3 H 7 0 ) 3 A 1 = O. 1 : 1 [3]. The concentration of A10(OH) in the thus prepared sol was 33 g/l, pH=4.5. The zirconia sol was prepared by precipitation of zirconium hydroxide from a zirconium oxychloride solution followed by washing of the precipitate with water and peptization by nitric acid [4]. The sol with the concentration 60 g/1 was used. The SiO2 sol was prepared by passing a sodium silicate solution (14 g/l) through the column of ion-exchange resin at pH=2.5. Then a NaOH solution was added until pH=8 was reached and the obtained sol was heated to increase the size of SiO2 particles and the stability of the sol [5]. The gauze made from 50 lam thick stainless-steel wire was used as a metallic support for the deposition of coatings. To improve adhesion, the surface of the metal wire gauze was cleaned before the deposition by the electrochemical method and then additionally etched in the solution containing 100 g/1 HzSO 4 and 200 g/1 NaC1 [6]. For preparation of the oxide layer, the metal gauze was placed in a sol solution of the corresponding oxide as one of the electrodes and the direct current was passed. Deposition of alumina, zirconia and zeolites was performed on the cathode, whereas a silica layer was deposited on the anode. The EPD coated gauze samples were dried at room temperature for 24 h and then calcined at 550~ for 2 h (the rate of heating did not exceed 2~ SEM pictures were obtained with a JEOL JSM-5300LV scanning electron microscope. XPS spectra were measured with a XSAM-800 X-ray photoelectron spectrometer using Mg Ktx radiation. The surface area of samples was measured by nitrogen thermodesorption using the BET method. The deposition of coatings based on H-ZSM-5 zeolite was carried out from a suspension containing 8.25 g/1 of zeolite particles in the alumina sol diluted with ethanol in order to provide the A10(OH) concentration 8.25 g/1. The zeolite catalyst was tested in the reaction of partial oxidation of benzene to phenol with N20. The process of benzene oxidation was carried out in a flow reactor at 425~ The volume space velocity estimated on the basis of zeolite volume was 75 h ~ the ratio ofN20 : C6H6=1.5. Prior to testing, the zeolite catalyst was calcined at 900~ in an N2 flow. 3. RESULTS AND DISCUSSION 3.1. Pretreatment of the metal wire gauze surface The SEM picture of the stainless-steel gauze surface after the chemical etching is shown in Fig. 1a. The roughness of the metal surface generated by etching is clearly seen. The roughness is essential to improve the adhesion of oxide coatings. The increase in the ratio of Cr/Fe line intensities in XPS spectra from 0.32 for the initial to 0.59 for the etched support (Fig. 2a, b) provide an evidence that the roughness formation results from the leaching of iron from the steel surface.

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Fig. 1. SEM pictures of the stainless-steel gauze: (a) after chemical etching, (b) coated with alumina by EPD from aqueous alumina sol at 5 V. 3.2. Alumina coatings In order to obtain the optimum conditions of the deposition, the influence of the applied voltage on the thickness and quality of the coating was studied. The voltage was varied from 1 to 50 V. The quality of the obtained coatings was controlled by SEM and by measuring the A1/Fe atomic ratio calculated from the A1 2p and Fe 2p line intensities in XPS spectra. The dependence of the AI/Fe ratio on the applied voltage is shown in Fig. 3a. The increase in the A1/Fe ratio with increasing voltage indicates that the fraction of the uncoated surface characterized by the concentration of detectable iron decreases and the quality of the coating is improved. The SEM picture of the alumina layer on the stainless-steel gauze is presented in Fig. lb. The metal I Ols Cls AI surface is perfectly coated with a uniform oxide layer composed of fine particles (about hundred nanometers) without formation of large agglomerates. The I" Cr2p . . . . . . "~ intensity of the Fe 2p and Cr 2p lines in XPS spectra of gauze samples coated with alumina appreciably decreases, whereas the high-intensity of A1 2s and A1 2p lines appeared (Fig. 2c). The use of the voltage higher than I Fe2p L i 10 V leads to the formation of a thick layer of the oxide film composed of large agglomerates that can be easily peeled off after drying. The decrease in the coating quality at the higher voltage is probably 900 600 300 0 the result of both the intensification of formation of hydrogen bubbles and the Binding Energy, eV high rate of deposition resulting in an increase in thickness. The thick alumina Fig. 2. XPS spectra of stainless-steel layer tends to crack during drying and gauze: (a) initial sample, (b) after chemical calcinations because water removal is etching, (c) coated with alumina.

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1130

--

hindered by fine pores of the deposit. To prevent the crack formation, slow drying at a low ml/Fe temperature and at high humidity was atomic performed. The quality of the coating was ratio shown to be improved when larger particles of alumina (about 1 gin) were introduced into the sol. -, a 1V 2 V 3 v 5V To decrease the hydrogen evolution, the 10V 20V Voltage initial sol was diluted by ethanol in the volume ratio 1 : 3. The high A1/Fe atomic ratio for the Fig. 3. Dependence of the A1/Fe atomic samples coated in the water-ethanol solution ratio on the applied voltage at different (Fig. 3b) indicates that the alumina layer is more sol compositions: (a) aqueous sol, uniform than that obtained in the aqueous (b) sol diluted by ethanol. solution. Noteworthy, the structure of the A1203/gauze interface also appears to be a function of the solution composition. Thus, XPS analysis of the sample prepared in the aqueous solution testifies that iron on the AlzO3/gauze 3+ . . . . 2+ interface exists as Fe . In the sample obtained in the water-alcohol solution, a mixture of Fe and Fe 3+"ions was found. This effect may be accounted for by the better quality of the A1203 layer in the second sample, which effectively protects the gauze surface against oxidation under calcination. In turn, the presence of different forms of iron on the A1203/gauze interface can also result in the better adhesion of the A1203 layer, thus improving the quality of the coating. The stainless-steel gauze with the alumina layers with a thickness of 1-10 gm were prepared. The thickness of the coating L was evaluated from SEM pictures and calculated using the equation:

1 .....'

L=w/ps, where W is the weight of deposited alumina per gram of the gauze, p is the bulk density of deposit and S is the surface area per 1 g of the gauze. The bulk density of the alumina coating obtained by EPD is about 2.5 g/cm 3 [7]. The geometrical surface area of the stainless-steel gauze made from the 50 gm thick wire is about 0.01 m2/g. The thickness of the alumina coating as a function of the deposition time is shown in Fig. 4. At the initial deposition stage, the thickness of the coating increases proportionally to the deposition time. At longer deposition times, the rate of the deposition decreases due to the increase in the electrical resistance of the coating. 10 It should be pointed out that the specific surface area of the gauze coated %7.5 with alumina determined by BET was rather 5 high and increased with an increase in the thickness of the deposited layer. For the 2.5 sample with a coating thickness of 5 ~m, the 0 J I I I ~ surface area after calcination at 550~ was 0 60 120 equal to 40 m2/ggauze. The specific surface Time, sec area per gram of the deposited alumina was Fig. 4. Thickness of alumina coating as a about 450 mZ/g, i.e. two times higher than function of the deposition time (applied that of the bulk alumina sample prepared voltage, 5 V; aqueous alumina sol, 33 g/l). from the alkoxide sol.

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a

Fig. 5. The zirconia coating: (a) after drying, (b) after calcination (applied voltage 1 V, 6 min).

3.3. Zirconia coatings The SEM picture of the zirconium hydroxide coating after drying at room temperature is shown in Fig. 5a. The surface of the stainless-steel wire is completely coated with a thin and uniform zirconium hydroxide layer. However, during calcination, the coating is extremely prone to cracking and peeling off due to the shrinkage as a result of dehydration and hydroxide-to-oxide phase transformation. (Fig. 5b). The deposition of ZrO2 powder was performed to obtain the zirconia-based coating resistant to cracking. For preparation of the coating based on ZrO2, the suspension of zirconia particles in the alumina sol diluted with ethanol was used. The obtained coating is shown in Fig. 6. 3.4. Silica coatings Deposition of SiO2 layers was performed on the anode surface because the silica sol is stable in alkali solutions and silica particles bear a significant negative charge when neutral and basic conditions are used. The electrophoretic deposition was performed in the presence of sodium dodecyl sulfate (SDS) as the surfactant to prevent the anodic dissolution of stainless steel. The weight ratio of SDS to silica was about 1 to 15. The SEM picture of the obtained sample of stainless-steel gauze coated with a SiO2 layer of the thickness about 2 gm is presented in Fig. 7. Furthermore, the coating from the suspension of SiO2 powder in the alumina sol was prepared.

Fig. 6. Coating obtained from the suspension of Zr02 particles in the alumina sol (10 V, 15 sec).

Fig. 7. Coating based on Si02 (5 V, 5 min).

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6 r

-

,~ 4

i

20k U

:K 1 , 0 8 0

I O~m

808453

Fig. 8. Coating based on H-ZSM-5 zeolite

3.5. Zeolite coatings The deposition of the ZSM-5 zeolite layer was performed using a suspension of zeolite particles in the alumina sol. The obtained coating is composed of large zeolite particles with a size of about 1-5 pm bound together and with the metal surface by alumina nanoparticles (Fig. 8). The prepared catalyst was tested in the reaction of partial oxidation of benzene to phenol with N20. Before the catalytic tests, the zeolite catalyst was calcined at 900~ in an N2 flow and the stainless steel gauze was found to be not destroyed and zeolite remained fixed on the metal surface. The conversion of benzene was lower than that on the bulk zeolite catalysts. However, it should be noted that the heavy condensation products usually appear in the reaction products if the bulk zeolite catalyst is used. The local overheating of the catalyst may be responsible for the heavy by-product formation. When the reaction was carried out with the gauze-supported zeolite, the heavy condensation by-products were not detected. This fact provides an evidence for the more effective heat and mass transfer in the gauze-based catalyst. 4. CONCLUSIONS This study showed that uniform oxide coatings on the stainless-steel wire gauzes can be obtained by EPD. For alumina coatings, the use of the alcohol diluted sol is preferable to the aqueous sol. Slow drying at low temperatures and at high humidity should be applied to prevent the crack formation. It was shown that the microcrystalline powders of A1203, ZrO2, SiO2 and zeolites could be deposited on the metal surface simultaneously with the alumina sol. The oxide coatings obtained by EPD possess rather high specific surface areas and can be used as catalysts or catalyst supports.

REFERENCES 1. 2. 3. 4. 5.

P.G. Menon, M.F.M. Zwinkels, E.M. Johansson, S.G. Jaras, Kinet. Katal., 39 (1998) 670. P. Sarkar, P.S. Nicholson, J. Am. Ceram. Soc., 79 (1996) 1987. B.E. Yoldas, Am. Ceram. Soc. Bull., 54 (1975) 289. E.V. Gorokhova, V.V. Nazarov et al., Kolloid. Zhum., 55 (1993) 30. V.A. Dzisko, Osnovy Metodov Prigotovleniya Katalizatorov (Bases of methods of Catalyst Preparation), Nauka, Novosibirsk, 1983, 19. 6. P.S. Melnikov, Spravochnik po Galvanopokrytiyam (Handbook on Galvanodeposition), Mashinostroenie, Moscow, 1979, 54. 7. D.E. Clark, W.J. Dalzell, D.C. Folz, Ceram. Eng. Sci. Proc., 9 (1988) 1111.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Precipitated Iron Fischer-Tropsch Catalysts: The Effect of Sulfide Ions Tracy C. Bromfield, T. Humphrey Dlamini, Fabrizio Marsicano and Neil J. Coville Department of Chemistry, University of the Witwatersrand, Johannesburg, PO WITS 2050, South Africa. The addition of sulfide ions (500 - 20000 ppm) to iron/carbonate solutions over a range of pH values generates iron hydroxo precipitates containing S2. These materials show different physical parameters (pore characteristics, colour and particle size) as well as catalytic properties. Chemical speciation studies using MINTEQA2 reveal that the pH and sulfide concentrations influence the type and amount of the solution iron and sulfide species and an attempt has been made to correlate the solution species with the physical properties associated with the novel iron/sulfide complexes. 1. I N T R O D U C T I O N The synthesis of iron precipitated catalysts for use in the Fischer-Tropsch (FT) reaction is known to depend on a range of variables that include reagent type, pH, temperature, method of ingredient mixing etc. [ 1-5]. Once the precipitate has formed events such as aging, drying, calcination etc. also affect the final morphology and hence the catalytic activity of the precipitated material. Recently we reported on our attempts to modify the final properties of a precipitated iron catalyst by addition of sulfide ions during the precipitation reaction [6,7]. Our study revealed that while the addition of sulfide ions at levels greater than 5000 ppm poisoned the catalyst, smaller amounts resulted in enhancement of catalytic activity. In a more recent study we have observed that the pH of the iron/carbonate solution to which the sulfide ions are added also affects the catalyst [8]. To further explore this result we report here our attempts to correlate physical (colour, crystallinity, pore information) and chemical properties of the sulfided catalyst with the chemistry of the reactant solution. To this end we report our novel observations from chemical speciation studies. 2. E X P E R I M E N T A L

The catalysts were prepared by adding solutions of Fe(NO3)3.9H20 to a solution of Na2CO3 at 75 ~ as described previously [6,7]. Na2S (varying amounts) in water was added to

the reactant solutions over a range of pH values. After the precipitate was washed it was centrifuged until the solutions had a conductivity of less than 200 ItS (Na + concentration <30 ppm). The precipitates were then dried at 120 ~ in a fan-dried oven for 16 h, and calcined in air at 400~ for 16 h.

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SEM was performed on a Jeol (JSM-840) Scanning Electron Microscope with a LINK analytical dispersive X-ray analysis facility (EDX) at 1 0 - 20 keV. Samples were mounted on AI stubs and carbon coated. Porosimetry was performed on an ASAP 2010 porosimeter. The method of Barrett, Joyner and Halenda (BJH) was employed to determine the neck and cavity sizes of the pores.

Speciation modeling studies were performed on an aqueous system containing Fe(NO3)3.9H20, Na2CO3 and varying amounts of Na2S (0, 500, 2000, 5000 and 20000 ppm) at 75 ~ and a range of pH values. The system was modeled using MINTEQA2 (Version 3.10, developed by the US Environmental Protection Agency). The input file was created in PRODEFA2. The carbonate concentration represented the total inorganic carbon content. The ionic strength was computed as part of the model, and activity coefficients were computed using the Davies equation [9]. An Eh value of either 100 or 280 mV was specified to allow the model to reach convergence, and the system did not appear to be sensitive to Eh inside this range. The Eh values selected were estimated from an Eh-pH diagram of water. In the analysis the ferric species are determined as a percentage of the total added Fe 3+, ferrous species as a percentage of the Fe 2+, and sulfide species (even those containing Fe 2§ as a percentage of total added S2. 3. RESULTS AND DISCUSSION 3.1 SEM and Colour

Addition of sulfide ions to the iron/carbonate solutions resulted in the formation of precipitates with differing colours. In general the catalysts changed from dark red-brown (no S2) through red-brown to yellow as the S2- concentration increased. Further, the precipitates changed from light to dark colours as the pH at which the S2- was added, decreased. Changes in the morphology of the sulfided iron complexes were also detected by SEM (2000 x magnification). The unsulfided catalyst revealed a material with an irregularly shaped surface; after calcination the surface had a more regular appearance. By contrast the sulfided catalyst (2000 ppm, low pH) showed a material with randomly orientated crystals (equiaxed), suggestive of growth by heterogeneous nucleation. This would be consistent with growth from an iron-sulfide nucleus that had the correct crystallographic orientation to the iron-oxide precipitate [ 10]. At higher sulfide levels dendritic growth was observed. Dendritic growth is associated with increased sulfide content and increased crystallinity (see below). No sulfur was detected by EDX on the precipitates prepared from solutions containing less than 5000 ppm S2. Occlusions of sulfur were seen on the 20000 ppm loaded materials. 3.2 XRD XRD spectra were recorded on both uncalcined and calcined precipitates. All spectra revealed exclusively Fe203; no 'FexSy' was detected. The crystallinity of the precipitates was quantified (Scherrer equation) by measurement of the 33.1 ~ peak intensity of Fe203 and the crystallinity was found to increase with (i) sulfur content and (ii) decreasing pH (Figure 1).

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A-

I

20

t ' '

22

24

I

I

i

I

t

i

I

26

28

30

32

34

36

38

40

2O Figure 1- Stack-plot of diffraction patterns obtained from catalysts sulfided with 5000 ppm Na2S a) Catalyst sulfided at high pH b) Catalyst sulfided at intermediate pH c) Catalyst sulfided at low pH

3.3 Pore

measurements

The adsorption/desorption isotherms of the sulfided catalysts show Type IV behaviour [ 11 ] with hystereses indicative of mesoporous solids. Typical data are shown in Table 1. It is clear from the data that addition of sulfide to the catalyst results in the synthesis of a material with larger pores and pore necks. In general catalysts sulfided at the start of the precipitation reaction were found to exhibit pores with wider necks/cavity sizes than those to which sulfide was added later in the reaction, a result independent of the amount of sulfide added. Table l: Effect of pH and sulfidation level on the pore characteristics of calcined sulfided catalysts ''

Catalyst

Pore

size range

Average

pore

Average

pore neck

(.~)

cavity size (,~)

unsulfided

150'-800'

310

200

2000 ppm S pH 10.75

180-1000

400

450

2000 ppm S pH 6.9

150-950

525

310

20000 ppm S pH 10.75

130-950

600

550

150-1000

600

350

20000 ppm S pH 6.9 i

i

i

i

size

(/~)

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3.4 Speciation studies

Chemical speciation modeling is a procedure that enables the extent of aqueous metalligand interactions to be quantified as percentages of the total concentrations of any component present in solution [12]. Speciation refers to the chemical form of a given metal ion or ligand with respect to valence state(s), concentrations of complexes and free aquated metal ions in equilibrium with solid precipitates [12]. The solution chemistry computer simulation programs used are based on the Laws of Mass Action and Mass Balance and solve complex mass balance relationships in terms of the metal ions and ligands. In this study a geo-chemical equilibrium speciation modeling computational package called MINTEQA2 has been used. An important limitation of this type of speeiation study is that it assumes that all processes are at equilibrium i.e. kinetic effects are not taken into account.

Effect of sulfide loading

A typical plot of the effect of S2 loading on the species expected at a given pH is shown in Figure 2(a) (high pH). This plot shows the change of Fe species with S2 concentration. The change in [Fe(OH)] + and the increase in the [Fe(HS)2] and [Fe(HS)3]-with S2 concentration is to be noted. By contrast the Fe 3+ species remain constant, as do the sulfide species (S 2-, HS-- not shown). As the pH decreases (Figure 2(b); low pH) the distribution of most iron hydroxo species now become strongly dependent on the sulfide concentration. Further, a change in the solution concentrations of Fe(OH)4- and Fe(OH)2 + is to be noted after addition of between 2000 and 5000 ppm S2. The catalytic activity of the iron/sulfide catalysts was found to decrease after addition of this amount of S2- [6,7]. Interestingly the sulfide species ([HS]-, [Fe(HS)2], [Fe(HS)3], H2S and FeS(s)) remain constant as the S2concentration increases (not shown).

a)

b)

o o90

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.

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2000

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Figure 2: Influence of sulfide loading on Fe species at (a) high pH and (b) low pH. In the above figures, aqueous ferric species are expressed as a percentage of total Fe(III) in solution added, and similarly, ferrous species are expressed as a percentage of total Fe(II) (as determined by the Fe3+/Fe2+ redox couple). It is to be noted that [Fe(III)] >> [Fe(II)].

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Effect ofpH

The effect of the pH of the iron/carbonate solution on the iron species formed on the addition of 500 and 20000 ppm S2" is shown in Figures 3 (a) and (b). The data reveals that as the pH is varied, the concentrations of the various iron and sulfide species vary substantially. At high pH the addition of S2" results in formation of a high concentration of H S and [Fe(OH)4]" and very little Fe(HS)x formation. After sufficient [Fe(HS)2] has formed solid FeS begins to precipitate out of solution. The FeS species are assumed to provide nuclei for the adhesion of iron hydroxo specie and to allow the iron to precipitate. As the pH decreased the Fe 3§ was reduced to Fe 2§ The trends are maintained in the intermediate pH range. Here [Fe(HS):], FeS(s) and Fe(OH)3 formation reaches a maximum. Precipitation of iron hydroxo species as well as iron sulfides should occur. At low pH a strong sulfide dependency is noted for the ferric hydroxo species. Extensive precipitation of ferrihydrite occurs as the [Fe(OH)4] decreases. Also, at this point most of the S2- is complexed as Fe(HS)2 or FeS(s). Hence a high level of sulfidation (> 2000 ppm S) at low pH also serves to precipitate iron hydroxide/ferrihydrite. The iron sulfide species presumably aggregate the precipitated iron species, giving rise to the highly crystalline material indicated by XRD.

a)

b) 9O

oT'-

90

HS

.................

~70

/

....:::....

x

f

O

it) O

J

30

":::...

...""

"-..

~HSk(~) . . . . . -~;:=~

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t).

x.

,,~

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,

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i

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HIGH

ION

pH of s u l f i d a t i o n

Figure 3: Effect of solution pH on the iron species found in solutions containing different amounts of S2 (a) 500 ppm, (b) 20000 ppm.

In these figures sulfide-containing species are expressed as a percentage of total sulfide added, and ferric hydroxide species expressed as a percentage of total Fe 3+ added. Therefore, X refers to sulfide, ferrous or ferric. a

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4. CONCLUSION Chemical speciation studies permit for an understanding of the influence of S2" on the physical properties of precipitated iron oxide. Thus, both the concentration and the pH of addition of S2 to the iron/carbonate solutions have been shown to influence the crystallinity and pore properties of iron oxides. This study shows that it is possible to control the physical properties of the precipitated iron complexes by variation of the above parameters. The precipitation process also influences the position of the sulfide ions within the crystallites and this impacts on the catalytic behaviour of the iron catalysts.

A clmowledgements We wish to thank the University, the FRD/NRF and Sasol for financial assistance. REFERENCES

1. 2. 3. 4.

R.A. Diffenbach and D.J. Fauth, J. Catal., 100 (1986) 466. R.M. Cornell and R. Giovanoli, Clay and Clay Minerals, 33 (1985) 424. U. Schwertmann and E. Murad, Clay and Clay Minerals, 31 (1983) 277. D.R. Milburn, R.J. O'Brien, K. Chary and B.H.Davis, Studies in Surface Science and Catalysis, 87 (1994) 753. 5. M.E. Dry, Catalysis: Science and technology, eds. J.R. Anderson and M Boudart, Springer-Verlag, New York, 1 (1981) 159. 6. T.C. Bromfield and N.J. Coville, Appl. Surf. Sci., 119 (1997) 19. 7. T.C. Bromfield and N.J. Coville, Appl Catal, A: General, in press (1999). 8. T.C. Bromfield and N.J. Coville, to be published. 9. K. Lazar, W.M. Reiff, and L. Guczi, Hyperfine Interactions, 28 (1996) 87. 10. JCPDS (40832 FeS). 11. S. Yunes, Explanation and Application of the Physisorption and the Chemisorption techniques in the Characterisation of solids in general and catalysts in particular, Micromeritics (1996). 12. J.R. Duffield, F. Marsicano and D.R. Williams, Polyhedron, 10 (1991) 1105.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

The nature of the active phase in iron Fischer-Tropsch catalysts Abhaya K. Datye a, Yarning Jin a, Linda Mansker a, R. Thato Motjope b, T. Humphrey Dlamini b and Neil J.Coville b aCenter for Microengineered Materials and Dept. of Chemical & Nuclear Engineering, University of New Mexico, Albuquerque, NM 87131, USA. bDepartment of Chemistry, University of the Witwaterstrand, Johannesburg, South Africa. Working Fe Fischer-Tropsch (F-T) catalysts were studied using Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD) and M0ssbauer spectroscopy. The catalysts were removed from a slurry phase Fischer-Tropsch reactor. Instead of using Soxhlet extraction, which may expose the highly reactive catalyst to atmospheric oxidation, we have used an alternative pretreatment involving room temperature extraction that allows us to concentrate the catalyst, but at the same time protect it against atmospheric oxidation. Catalysts from two slurry reactor F-T runs were analyzed, one in its active state and the other after deactivation. Major changes in phase composition as well as in particle size can be seen in these two catalysts. In its active state, the Fe F-T catalyst contains highly dispersed carbide particles of z-FesC2 and Fe7C3. The deactivated catalyst, on the other hand, shows a different carbide structure that is indexed by MSssbauer as the E' carbide. The combination of XTEM, XRD and Mtissbauer spectroscopy helps resolve some of the discrepancies in the literature about the identity of the phases seen in a working F-T catalyst. 1. INTRODUCTION F-T Synthesis is recognized as a viable route for conversion of syngas to liquid fuels [1]. Due to their high water gas shift activity, Fe catalysts are the preferred choice for coal derived syngas having a H2/CO ratio <1. In order to get accurate phase information from working FTS catalysts, and to minimize problems with surface oxidation of the reduced iron phases, we have studied these catalysts while they are still protected by the hydrocarbon wax [2]. The low catalyst loading in the wax makes analysis difficult. In addition the hydrocarbon wax can be crystalline, interfering with the XRD peaks from the Fe phases of interest [3]. We have used an alternative procedure where most of the wax is extracted using a toluene solvent at ambient temperature. The catalysts were analyzed by XRD, cross-section transmission electron microscopy (XTEM) and Mtissbauer spectroscopy, allowing for the first time a direct correlation between these three techniques. 2. EXPERIMENTAL The catalyst samples studied in this paper had a chemical composition of 100Fe/4.4Si/1.0K (on a weight basis) and were provided by Dr. Burtron Davis at the Center of Applied Energy

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Research (CAER), University of Kentucky, after use in F r synthesis runs. In run LGX-171, the precipitated oxide precursor was pretreated with syngas, then underwent FI'S at 270~ 175 psig. The wax-mixed catalyst sample was removed from the slurry reactor after 3164 hours time-on-stream (TOS). The catalyst activity was high for the first 2800 hours of this run, but over the last few hundred hours there was a rapid deactivation and the CO conversion was 20% at the point where the catalyst was removed. On the other hand, in run LGX-175, the catalyst precursor was pretreated in CO and underwent F r s at the same conditions as used for LGX-171. The catalyst sample was removed after TOS 1160 hours while its CO conversion was 79%. Detailed reactivity data were reported elsewhere [4].

The M6ssbauer experiments were performed with a Co-57 (Rh) source. The spectrometer was operated in the symmetric constant acceleration mode with 100/us of dwell time per channel. The spectra were collected over 1024 channels in mirror image format. The amount of sample used for each analysis was optimized to give a Fe mass to area ratio of ca. 15 mg/cm ~ (ta = 2), when loaded into the sample holder. This gives a measurable M6ssbauer signal with minimal distortion due to secondary effects. The data was analyzed using a leastsquare fitting routine that models the spectra as a combination of quadruple doublets and sextuplets based on a Lorentzian line-shape profile. The spectral components were identified based on their isomeric shift (8), quadruple splitting (A) and hyperfine magnetic field (H). The isomeric shift values are reported relative to metallic iron (r and the iron content, on a mole basis, of each phase was determined from their relative peak areas. TEM sample preparation method was described in detail in reference [5]. The experimental conditions for XRD measurement can also be found elsewhere [3]. 3. RESULTS The XRD powder patterns of catalyst LGX-171 and LGX-175 with wax partly extracted are shown in figure 1. Both catalysts show sharp magnetite peaks from large, crystalline particles. Most recent work on Fe F-T catalysts shows that magnetite is not active for the synthesis reaction. These magnetite particles therefore must represent unreduced oxide coming from the catalyst precursor. We suspect that these perfect, faceted, magnetite particles are harder to reduce and hence they persist even after 3000 hours of reaction in syngas. The diffraction peaks in catalyst LGX-175 at 20=44.89 o and 39.97 ~ match the (121) and (120) diffraction lines of Fe7C3-carbide (the Adcock-EckstrSm carbide, labeled A in Fig. 1). In addition, we see diffraction peaks of the Fe5C2 (Z) carbide. It can be seen that the calculated pattern [6] does not match the diffraction peak locations seen in the patterns reported here. This is a result of lattice defects and carbon vacancies in the Fe5C2 structure, as determined by Reitveld structure refinement [7]. We have therefore labeled these peaks as C' in Fig. 1. The broad peaks of this carbide show it to be present in the form of poorly crystallized, highly dispersed particles. In contrast, the XRD spectrum of LGX-171 (the deactivated catalyst) shows much sharper carbide peaks. Besides, it is clear that the Adcock-Ekstr6m carbide (Fe7C3) is missing. Our preliminary analysis suggests that the carbides in LGX-171 (deactivated catalyst) comprise hexagonal Fe3C (bainite) and Fe5C2. There are also some peak offsets caused by packing errors, requiring further analysis via Reitveld methods [7].

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1141

?

SO0

-

m; c'

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~, c' r~, c'

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'

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I

].

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= P

: 25

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....

~1 . . . . 30

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|,

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.l

....

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i,

i

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I , ,k, , I . . . . 55

80

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65

70

20

Figure 1. X-ray diffraction patterns of the active catalyst (LGX-175) and the deactivated catalyst (LGX-171). The various phases have been labeled as follows: Magnetite (M), FesC2 (C), modified FesC2 (C'), Fe7C3 (A), Hexagonal Fe3C, Bainite (B).

Figure 2. Cross section TEM images of LGX-175 (the active catalyst), a. Low mag overview, the large magnetite single crystal are highlighted; b. High mag image of one of these faceted particles.

The picture obtained from XRD is corroborated by TEM. Figure 2a shows XTEM images of the sample from LGX-175. Large (around 50 nm) faceted particles are seen to be sparsely dispersed in this image. By analyzing the high resolution images, we can conclusively

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1142

identify these particles as magnetite single crystals, as shown in Fig. 2b (2.94/~ corresponds to the (220) plane of magnetite). The rest of the catalyst is a complex mixture of iron phases. A typical view in Fig. 3a shows three different types of iron phases to be present. These distorted particles (20 to 40 nm) show lattice spacings of 2.26/~ and 2.02/~ corresponding to the (120) and (121) planes of FeTC3-carbide. In addition to Fe7C3, the catalyst contains highly dispersed particles (usually <10 nm) as shown in Fig. 3a. Those without a surface layer are

50 nm

"

Figure 3. TEM images showing the complex microstructure of sample LGX-175. a. Low mag view of three distinct phases: large distorted Fe7C3 particles (some are highlighted), and highly dispersed magnetite particles; b. High mag view showing the coexistence of these three phases. .o

9. ~ . :k : ~ g ~ y

.

9

dispersed carbide

!. , . :..

':

:

,. . "

9

Figure 4. Comparison of carbide phases in LGX-171 (deactivated) and LGX-175 (active). a. Regular shaped carbide particles in LGX-171" b. Highly dispersed particles seen in LGX-175 (active catalyst) are missing in sample LGX-171.

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are identified as fine magnetite particles. The particles with a surface layer are identified as highly dispersed iron carbide. The surface layer represents active carbonaceous species as identified by electron energy loss spectroscopy and XPS [5].

In contrast to sample LGX-175 (the active catalyst), sample LGX-171 (the deactivated catalyst) only showed the presence of large carbide particles in addition to the magnetite, both by XRD and TEM. These carbide particles had smooth surfaces, and well-defined shapes, rectangular or spherical. M/Sssbauer spectra obtained from the catalyst samples are presented in Fig. 5. Hyperfine interaction parameters are listed in Table 1. At 298 K, these spectra consist of a complicated superposition of magnetic sextuplets and paramagnetic doublets. The spectrum of the active catalyst LGX-175 shows the presence of ~-Fe5C2, Fe7C3 and Fe304. In order to elucidate the composition of the phase associated with the paramagnetic doublet, the sample LGX-175 was also analyzed at 4.2 K. An increase in the amount of Fe304 upon analysis at 4.2 K indicates that the paramagnetic doublet observed at 298 K may result from the presence of small Fe304 particles with an average particle size of ca. 5 nm. Comparison of the spectra recorded at 298 K and 4.2 K also shows a bimodal particle size distribution of Fe304.

!

w

I

w

... ~-.-~_~.,

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Figure 5. MSssbauer spectra of L G X - 1 7 5 and LGX-171. A. LGX-175 at 298 K, B. LGX-175 at 4.2 K, and C. LGX- 171 at 298 K.

The deactivated catalyst sample LGX-171 shows a Mtssbauer pattern different from that observed with LGX-175. Although both samples show the presence of Fe304, LGX-171 contains a different carbide phase. We have tentatively assigned this carbide as e' carbide based on previous literature [8]. Furthermore the intense and well-defined peaks of both the carbide phase and Fe304 observed with LGX-171 indicate that this sample has larger crystallites than the active LGX-175. The less contribution of the paramagnetic doublet, compared to LGX-175 (298 K), observed with this sample further confirms that this catalyst consists of larger crystallites. These data suggest that the low activity observed with this catalyst sample could be associated with the poorer dispersion as well as the e' carbide phase. 4. CONCLUSIONS The correlation of Fe catalyst phase composition with Fisher-Tropsch reactivity has been an extensively studied subject for the past 50 years. By using careful sample preparation and the combination of XRD, TEM and Mtssbauer spectroscopy we have been able to derive a consistent picture of the working catalyst. These analyses show that the active catalyst contains a mixture of Fe7C3 and highly dispersed FesC2 carbides (<10 nm). On the other

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1144

hand, the deactivated catalyst contains crystalline carbides of larger particle size (30-50 nm). It is interesting to observe that the active catalyst is characterized by a bimodal carbide particle size distribution. We see highly dispersed carbide particles co-existing with larger, distorted particles indexed as the Fe7C3 phase. The co-existence of the Fe7C3 phase in the form of distorted particles with a highly dispersed FesC2 carbide seems to be characteristic also of other CO pretreated catalysts in their active state [9].

Table 1 Hyperfine interaction parameters for LGX - 175 and LGX- 171 Sample (Analysis Temp / K) LGX-175 (298)

LGX-175 (4.2)

LGX-171 (298)

IS (mmsl)Fe 0.31 0.25 0.30

QS (mms"l) 0.11 0.14 0.10

B (T) 11.8 ~ 17.1 $ 21.2 J

0.32

0.21

16.5 ll

0.33 0.30 0.30 0.67 0.40 0.31 0.36 0.37 0.28 0.24 0.34 0.36 0.93 0.27 0.33 0.65 0.40

0.09 -0.07 0.03 -0.01 1.12. 0.11 0.14 0.10 0.18 -0.01 -0.11 -0.02 0.03 0.06 0.01 0.00 0.50

18.4 22.8 49.0 45.8 22.1 18.1 12.7 18.6 21.7 25.8 49.7 48.5 16.6 48.4 45.3 -

/

~ 9

RA (Intensity) 14.5 28.5 23.8 33.2

~ /

14.8

/ /

28.4

/ I ~ ~

56.8 41.1 51.7 7.2

Compound z-Fe2.sC z-Fe~.5C z-Fe2.5C Fe7C3

FeTC3 FeTC3 Fe304 Fe304 Fe3§ z-Fe2.5C z-Fe2.5C z-Fe2.5C FeTC3 Fe7C3 Fe7C3 Fe304 Fe304

B-G Fe304 Fe304 Fe3§

ACKNOWLEDGEMENTS Financial support from the US Department of Energy, FETC university coal research grant DE-FG26-98FT40110. We thank Dr. Burtron Davis for providing the catalyst samples. REFERENCES 1. L. Xu, S. Bao, R.J. O'Brien, A. Raje and B.H. Davis, ChemTech, January (1998) 47. 2. M.D. Shroff and A.K. Datye, Catal. Lett., 37 (1996) 101. 3. L.D. Mansker, Y. Jin, D.B. Bukur and A.K. Datye, Appl. Catal., 186 (1999) 277. 4. R.J. O'Brien, L. Xu, R.L. Spicer and B.H. Davis, Energy and Fuels, 10(4) (1996) 921. 5. Y. Jin, Ph.D. Thesis, University of New Mexico, 1999. 6. M. Dirand and L. Afqir, Acta. Metall., 31 (1983) 1089. 7. L.D. Mansker, Ph.D. Thesis, University of New Mexico, 1999. 8. J.B. Butt, Catal. Lett., 7 (1990) 6 I. 9. Y. Jin and A.K. Datye, Proc. Intl. Congr. Electron Microscopy Cancun 1998, vol II, Inst. Of Physics Publishing, (1998) 379.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Selective inhibition, the intrinsic feature of Fischer Tropsch synthesis; Transient initial episodes in FT synthesis with cobalt catalysts Hans Schulz and Zhiqin Nie Engler-Bunte Institut, Universitiat Karlsruhe, Kaiserstrage 12, 76128 Karlsruhe, Germany Fischer Tropsch synthesis has been performed with cobalt catalysts promoted with zirconia and rhenia in an isothermal, "no mass transfer resistance" fixed bed reactor. Time resolution of activity and selectivity was obtained through ampoule sampling and analysis CI-C22 by gas chromatography. With the help of a kinetic model, results of product composition were transformed to data of chain growth probability, branching probability and olefinicity, each as dependent on carbon number and reaction time. It is shown how the steady state of FT synthesis develops with time. Apperantly selective inhibition of desorption and additional spatial constraints establish the polymerization nature of the Fischer Tropsch synthesis. 1. I N T R O D U C T I O N The Fischer synthesis can be mathematically treated as a polymerization reaction [1-5]. It appears that chains of CH2-groups are formed via repeated addition of a C l-species. However, to prolong the chain by one "CH2-monomer" comprises about 10 reaction steps (dissociative adsorption of two molecules hydrogen to form 4 chemisorbed H, one step of COchemisorption, 4 steps of addition of H to carbon and oxygen to produce CH 2 and H20, one step of C/O-bond cleavage and one step of C/C-bond formation). Thus FT synthesis is only in part of polymerization nature. Further reaction steps of heterogeneous catalysis are intergated. FT synthesis features through selection of reactions from the many possible steps. It has been suggested that the essential FT principle is the selective inhibition of desorption of products from the catalyst surface [6]. In this way the reaction of surface species with each other to form a new bigger surface species, becomes dominant and the polymerization nature develops. The "monomer" for polymerization is steadily supplied from CO and H 2. As a conclusion it appeares no longer contradictive when with different catalysts as iron and cobalt, on which the reaction rate is differently depending on the partial pressures of CO, H 2 and H20 [7, 8], a similar hydrocarbon product can be obtained. It is well known since the early days of FT synthesis, that it needs some time until the steady state of a reaction is established, but only recently tools are available - ampoule sampling and adapted GC programed from -- -100 to -- 270~ - which allow for high time resolution at detailed analysis of the product [9]. It has been the purpose of this investigation to measure the changes of reaction rate and product composition as a function of time from the beginning of the experiment. Three cobalt catalysts from which two of them were promoted with zirconium- respectively rhenium-oxide have been compared. Parameters for characterizing the FT regime - chain propagation, chain branching and olefinicity, each as a function of carbon number- have been calculated [ 1-5] and these are regarded in dependence of duration of the experiment. 2. E X P E R I M E N T A L The fixed bed reactor was developed for "no intra particle mass transfer limitation" and "isothermal" operation. Small catalyst particles (dp<0.1 mm) were applied as a thin layer

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1146

adhering on larger fused silica particles (dp=0.25-0.4 mm) in a weight ratio l'10 and these packed into a fused silica tube to form the catalyst bed, uniformly packed and exhibiting only minor pressure drop. Such provisions are of importance, because selectivity of the FT reaction is strongly temperature dependent and filling of pores with liquid product can strongly influence the kinetics when using larger catalyst particles. At zero time of an experiment the flow to the reactor was switched from argon to synthesis gas. A stream of cyclopropane in nitrogen was added to the product stream for determination of conversion and yields [10]. Sampling at 523 K was performed with small glass ampoules. GC analysis of the samples covered the range of compounds CI-C20. Precolumn hydrogenation of olefins was used for determining of chain branching [ 11 ]. The catalysts were prepared by quick precipitation from the hot solution of the nitrates of cobalt and promotor as adapted from the original procedure of Fischer et al. [12]. As a support Aerosil 200 (Degussa, Hanau) a finely powdered SiO2-material was suspended in the solution. The precipitate was washed, dried, milled and the powder-fraction dp<0 9l mm mixed with the fused silica particles (together with some isopropanol) to produce the layered diluted catalyst, which was placed in the reactor tube, calcined and reduced. Three catalysts with the compositions (weight ratios) 97Co- 100Aerosil- 12Zr (termed Co-Zr-SiO2), 104Co- 100Aerosil9Re (termed Co-Re-SiO2) and 97Co-100Aerosil (termed Co-SiO2) were made. The reaction conditions were kept the same for all experiments: Temperature 463 K, pressure 0.5 MPa, H2/ CO molar ratio 2, GHSV 300 h-1, amount of catalyst in the reactor 2 g (dried). Space velocity related to the synthesis gas at normal pressure and to the volume of the catalyst bed ( - l 0 ml) consisting of 20 g fused silica particles and 2 g of catalyst. For experimental details see ref. [ 10]. The products contained no CO 2, indicating the absence of any water gas shift reaction.

3. RESULTS AND DISCUSSION Composition of the Fischer-Tropsch product is presented in this paper as kinetic data which have been calculated with the help of a model [2-5]. Chain growth probability pg N For a surface species o f carbon number N C there are a distinct probability to grow (pg N) and a corresponding probability to desorb (Pd N) in the model, pg N and Pd N are related to each other by Pg N + Pd N = 1. Pg N is commonly termed "all'ha" in the FT literature and determined from the slope of the so called Anderson-Schulz-Flory (ASF) distribution curve. For an ideal polymerization the growth probability pg is carbon number independent. In Fischer Tropsch synthesis deviations from ideality are no-ticed. In this paper pg N is plotted for different reaction times as a function of carbon number N C (of the growing species) (diagrams left/above in the figures 1-3). Branching probability Pg, br This is the probability for a surface species to add a C1 monomer and thereby causing branching of the chain. Several branching reactions have been proposed [4]. It can be assumed that the transition state for a branching reaction is more demanding in space than the reaction for linear growth ane therefore branching probability can be regarded as an indicator for spatial constraints at the active sites. The probability for growth of a species SPN, Pg, N , is the sum of probabilities for linear "Pg,N,lin" and branched "Pg, N, br" growth of a species. (Pg,N = Pg, N,lin + Pg,N, br)" Branching probabilities Pg, N,br are presented in the above/fight diagrams of Figs. 1-3.

Olefinicity Olefinicity of the Vr-product reflects a mechanistically complex situation. There is a primary olefinicity of 70-80% corresponding to an about 4 times higher probability of a hydrocarbon molecule to desorb as olefin than as paraffin [5]. However, these primarily formed terminal olefins can undergo readsorption and further growth on Vr-sites which results in a

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Fig. 1 Fischer Tropsch synthesis on a Co-Zr-SiO2 catalyst. Influence of reaction time on chain growth, chain branching and olefinicity of the product. decline of olefinicity. The olefins also undergo secondary hydrogenation which reduces olefinicity of the product additionally. Double bond shift affects reactivity for olefin hydrogenation. Information about these reactions is to be drawn from the diagrams below/left and below/right of Figs. 1-3 where the olefin content in carbon number fractions of the product (left) and the olefin-1 content in linear olefin carbon number fractions are shown for series with different reaction times.

3.1. Chain growth For the three cobalt catalysts chain growth probabilities (Figs. 1-3, above/left) exhibit similar trends. At the steady state (long reaction time of 10.000 and more minutes, episode III) the carbon number dependence of Pg,N is only weak. Particularly for the Co-Re-SiO 2 catalyst, from NC=2 to NC=12, Pg,N is almost constant. With the other two catalysts Pg,N2 is significantly higher than Pg,N3" Then Pg,N increases for the next few carbon numbers. This phenomenon reflects olefin readsorption on FI' sites for further chain growth which is very strong at C 2 (ethene) as recently shown with olefin addition experiments and kinetic modelling [5, 13]. So we see olefin readsorption on FT sites for chain growth to be least with the rhenium promoted catalyst. And we see olefin readsorption for further chain growth to be favoured in the early episodes. As understood from the olefin-addition experiments and kinetic modelling [5, 13] the value of Pg,3 is the one which is least influenced by olefin readsorption and the change of Pg,3 therefore suited to see the influence of reaction time on the polymerization nature of the synthesis. We see the general trend of a low chain growth probability at the start of the synthesis. In Fig. 4 left Pd,3 is plotted against reaction time. For the catalyst Co-Zr-SiO 2 a decline of desorption probability from pd,3=0.35 at -10 min to ca. Pd,3=0.28 at >4000 min is noticed (with the rhenium promoted catalyst, the respective decline of desorption probability is ca. 0.35 to ca. 0.14). Thus we conclude: The polymerization nature of the FT synthesis

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Fig. 2 Fischer Tropsch synthesis on a Co-Re-SiO 2 catalyst. Influence of reaction time on chain growth, chain branching and olefinicity of the product. develops with time. This is equivalent to the statement, that the rate of desorption reactions is selectively reduced (inhibited) with increasing time on stream. Simultaneously the desorption probability at NC=I (methane) declines with time (see Fig. 4 fight), reflecting increasing inhibition of the only imaginable methane desorption reaction (CH3) a + (H)a ~ CH 4 when the regime of FT synthesis organizes itself. Inhibition of methane formation with time is very selective because with both the promoted catalysts Co-Zr-SiO 2 and Co-Re-SiO 2 (but particularly with Co-Re-SiO2) the rate of synthesis (rate of CO consumption) increases strongly in episode II (Fig. 5). Three episodes of FI'-synthesis have been distinguished: A first episode up to the 10th to 20st minute with very fast changes when probably a pseudosteady state is approached through fast adsorption events and surface coverage also with products and intermediates. A second episode in which the changes are slower. This episode can last even for several days as in the case of the Co-Re-SiO 2 catalyst where in episode II the CO-conversion increases from -25 to 80%. The slowness of these activating changes is regarded as indicative for slow catalyst solid state conversions (catalyst restructuring). Episode III then is the steady state. 3.2. Chain branching Chain branching shows high sensitivity against reaction time and promotor addition (Figs. 1-3, above/fight). In the steady state the common pattern of the Zr- and Re-promoted catalysts is a decline of Pbr with increasing carbon number from NC=4 onwards. This behaviour is consistent with increasing spatial demands for the branching reaction, as the size of the surface species increases. It is in accordance with this explanation that branching is favoured in the early episode of the synthesis - as observed with the Co-Zr-SiO 2 catalyst (Fig. l) and the unpromoted Co-SiO 2 (Fig. 3) - when the intensity of the surface coverage can be assumed low and the average size of the chemisorbed hydocarbon chains small. With the Co-Zr-SiO 2 catalyst the development of shape of the curve of Pg,br=f(Nc) with time is very interesting:

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Fig. 3 Fischer Tropsch synthesis on a Co-SiO 2 catalyst. Influence of reaction time on chain growth, chain branching and olefinicity of the product. Specifically in the early stages of the experiment (e.g. for 180 min. reaction time), amazingly, the branching probability increases from NC=7 onwards (Fig. 1 right/above). At this early stage of relatively low spatial constraints at the sites, the with chain length increasing spatial demands are overruled by the with chain length increasing adsorbability of product olefins and adsorption at the C-atom in position 2 is then possible and allows chain growth at the 2position of the chain - whereas in (steady state) FT regimes very exclusively chain growth of readsorbed olefins is only possible at the last C-atom of the chain. It is now remarkable that with the rhenium promoted catalyst (Fig. 2, right/above) essentially no influence of reaction time on branching probability is observed. From the very early beginning of the experimental run branching probability is as low as in the steady state. Thus rhenia promotion seems to constrain the spatial situation around the FT site. With the unpromoted poorly active Co-SiO 2 catalyst (Fig. 3 above/fight) the value of branching probability at NC=4 is exceptionally high. This is explained as reflecting the occurrence of an olefin side reaction with this poorer catalyst. 0.5

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3.3. Olefinicity Olefinicity in its time dependence is shown in the diagrams below/left and below/right of Figs. 1-3. It is known from literature, that in FT systems without olefin secondary reactions the primary olefin content as a function of carbon number represents an almost horizontal line at a level of about 70-80 mol-% [13, 14]. This knowledge is important for interpretation of the results in Figs. 1-3. The curves of olefin content over carbon number (Figs. 1-3, left/below) are all characterized by a low value at C 2 (due to easy ethene hydrogenation and also ethene consumption for readsorption for chain growth), a maximum at C 3, C 4 (least reactive olefins) and a decline with carbon number (due to with carbon number increasing readsorbability). There are some peculiarities with the individual catalysts, but the common trend is a low olefinicity at the beginning of the FT synthesis, when the constraints on chemisorption are not yet so serious. In accordance to this, the contents of olefins-I (which are regarded as primary products) among the straight chain olefins increase with time.

o

4. CONCLUSION The ordered multicompound composition of FT T I M E O F R E A C T I O N , tRCTN, MIN reaction products has been converted into kinetic data with the help of a polymerization model. On this basis Fig. 5 FT-conversion on three the transient initial episodes are characterized and this catalysts as a function of time. allows a better understanding of the FT-steady state. It is seen that selective inhibition of desorption develops with time and this stabilizes the polymerization nature of the FT synthesis. At the same time spatial constraints at the active sites become more stringent and restrict branching reactions. Zr- and Re-oxides as promotors in cobalt catalysts efficiently help to establish the unique regime of FT synthesis. O 6 0

0

'

101

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5. REFERENCES 1. E.F.G. Herington, Chem. Ind., 65 (1946) 346. 2. H. Schulz, K. Beck and E. Erich, Stud. Surf. Sci. Catal., 36 (1988) 457. 3. H. Schulz, K. Beck and E. Erich, Fuel Proc. Techn., 18 (1988) 293. 4. H. Schulz, K. Beck and E. Erich, in M. Phillips and M. Ternan (eds.), Proc. "9th Int. Congr. on Catalysis" Calgary, Vol. 2, 829, Chemical Institute of Canada, Ottawa, 1988. 5. H. Schulz and M. Claeys, Appl. Catal. A, 186 (1999) 91. 6. H. Schulz, E. van Steen and M. Claeys, Topics in Catalysis, 2 (1995) 223. 7. H. Schulz, E. van Steen and M. Claeys, Stud. Surf. Sci. Catal., 81 (1994) 455. 8. H. Schulz, M. Claeys and S. Harms, Stud. Surf. Sci. Catal., 107 (1997) 193. 9. H. Schulz and S. Nehren, Erd01 und Kohle-Erdgas-Petrochemie, 39 (1986) 93. 10. Z. Nie, Dissertation, Karlsruhe, 1996. 11. E. van Steen, Dissertation, Karlsruhe, 1993. 12. F. Fischer and H. Koch, Brennstoff-Chemie, 12 (1932) 61. 13. H. Schulz and M. Claeys, Appl. Catal. A, 186 (1999) 71. 14. H. Schulz and H. G0kcebay, in J. R. Kosak, M. Dekker (ed.), Catalysis of Organic Reactions, New York, 1984.

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化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Effects of support on methane selectivity in cobalt-catalyzed Fischer-Tropsch synthesis: a kinetic model Calvin H. Bartholomew a and Won-Ho Leeb aDepartment of Chemical Engineering, Brigham Young University, Provo, Utah 84602 bLucky Central Research Institute, Science Town, Dae Jeon, Korea Effects of metal loading and support (i.e., alumina, silica, and titania) on methane produced during Fischer-Tropsch synthesis on supported cobalt by primary reaction on the metal and by spillover/secondary reaction on acid sites of the support were studied by temperatureprogrammed hydrogenation of adsorbed carbon monoxide. A model was developed for predicting methane formed from these primary and secondary reactions which incorporates consensus kinetic parameters for primary Fischer-Tropsch and those from TPD for the secondary, via-the-support process. It is concluded that a large part of the greater-than-theoretical methane formed in FTS may be formed via the secondary spillover/support reaction path. 1. INTRODUCTION Recent interest in conversion of remotely-located natural gas to liquid fuels has stimulated renewed interest in improving cobalt catalyst technology for Fischer-Tropsch Synthesis (FTS). There is, for example, considerable economic motivation for reducing methane selectivity in FTS. A significant reduction is thought to be feasible, since methane selectivities are in general higher than predicted theoretically by Anderson-Schulz-Flory (ASF) kinetics, which govern this reaction. Several mechanisms have been postulated to account for greater than ASF methane selectivity: (1) hydrocracking of olefins on acid sites of the catalyst support [1], (2) hydrogenolysis of olefins and paraffins to methane on the metal surface [2-6], (3) reaction of steam and hydrogen with surface carbides [7] and (4) bifunctional catalysis involving spillover of reactive species to the support to form intermediates which then decompose either on the support or metal [8-12]. Experimental evidence [1-12] supports at least qualitatively the contribution of all of these mechanisms during Fischer-Tropsch synthesis on cobalt. For example, it is well documented [2-7] that (a) Group VIII metals catalyze the hydrogenolysis of olefins and paraffins during FT synthesis, (b) hydrogenolysis of olefins occurs on Co and Fe by successive demethylation, and (c) the greater than ASF yield of methane can be partly attributed to hydrogenolysis. Moreover, in our previous temperature programmed hydrogenation (TPH) study of adsorbed CO on cobalt alumina catalysts [12], we found evidence for two mechanistic routes to the formation of methane, the first involving termination to methane of the chain growth process on cobalt metal in accordance with classical ASF kinetics and the second involving spillover of hydrogen atoms and CO molecules to acid sites of the support, formation of a formate intermediate, diffusion of the formate species to the metal, and decomposition of the formate to methane. Kuipers et al. [6] developed a simple model to describe quantitatively the effects of olefin reinsertion and hydrogenolysis on FT product distributions for Co catalysts. However, models treating the production of methane by other routes have not been reported.

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This work was conducted to (1) ascertain if bifunctional production of methane occurs during FTS in cobalt-support systems other than Co/alumina, (2) determine the kinetics of the spilloversupport mechanism for methane formation on these catalysts, and (3) construct a kinetic model suitable for quantitatively predicting the effects of support on methane selectivity in FTS on cobalt.

2. EXPERIMENTAL AND THEORETICAL APPROACH Cobalt catalysts (3, 10 and 15 wt.% Co on A1203 and 10% Co on SiO2 and TiO2) were prepared by incipient wetness impregnation of the supports with an aqueous solution of cobalt nitrate. Metal surface areas of these same catalysts were measured by H2 chemisorption at 100~ and extents of reduction were measured by oxygen titration at 400~ in previous studies [ 13-16]. Specific activities in the form of turnover frequency (TOF) and product selectivities at 1 atm were likewise measured in these earlier studies. Table 1 summarizes the chemisorption, extent of reduction and activity-/selectivity properties of these catalysts. A TPD/TPSR mass spectrometer system was used in this study to determine the kinetics of methane formation for the different reaction paths on these same catalysts. This apparatus and the procedure for collecting TPH spectra are described elsewhere [12]. Table 1. H2 chemisorption, extent of reduction, and CO hydrogenation activity/selectivity data for cobalt catalysts a. Nco e x 10 3 CH4 sel. r O~g (~ (S"1, 473 K) (mole%) 100% Co 100 0.26 498 0.55 h 13 h 0.79 h 3% Co/A1203 28 15 473 1.4 16 0.70 10% Co/A1203 34 8.7 473 9.1 8.9 0.77 488 12 0.75 498 13 0.72 508 15 0.65 15% Co/A1203 37 44 6.6 473 17 9 0.85 10% Co/SiO2 82 92 10 498 2.0 29 0.75 10% Co/TiO2 34 47 8.6 498 15 16 0.80 a H2 chemisorption, extent of reduction, and activity/selectivity data compiled from Refs. 13-16. b Measured by total H2 chemisorption at 100~ c Percentage reduction of cobalt to the metal measured by oxygen titration at 400~ d Percentage dispersion or percentage exposed; %D = 1.18 X/(w * %R) where X is the H2 uptake, w the wt. fraction of cobalt in the catalyst, and %R the percentage reduction. e CO turnover frequency, i.e. the number of molecules of CO converted per second per site (measured by H2 chemisorption). f Methane selectivity in mole%, i.e. SCH4= N CH4/Ncoat Treactiongenerally after 20 h of reaction; however, data for 10% Co/TiO2 and 10% Co/SiO2 were obtained after only 3 h of reaction. g Propagation probability based on ASF kinetics; determined by plotting mole fraction of product versus carbon number; generally obtained after 20 h of reaction (data for 10% Co/TiO2 and 10% Co/SiO2 after 3 h of reaction). h Data from Rankin [16] after 20 h of reaction. Catalyst

H2 uptake b

% Redn. c

%D d

Treaction

(~tmol/g) 22 11 29

In our previous study of Co/A1203 [12], it was found that kinetics of primary FTS via CO dissociation and carbon hydrogenation on the metal surface could be separated from the kinetics

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1153

of the alternate (secondary) reaction path involving spillover to the support. Accordingly in our model, calculations of methane formation rates were conducted separately for each step. Kinetic parameters obtained from a comprehensive analysis of literature data [17] were used to calculate the CO turnover frequency on Co metal for each catalyst at a standard condition of 1 atm (total pressure), 473 K, and H2/CO = 2; adjustments were made for the observed [ 13-16] variations in TOF with metal loading and support. To obtain the theoretical methane selectivity for primary FTS, the CO TOF for each catalyst was then multiplied by the mole fraction of ASF methane, i.e., mn = (1-(x)2 ~-1 for n - 1. Values of cz were determined in our previous studies [13-16]. Kinetic parameters for the spillover/support path were determined from Redhead analysis of the secondary reaction peak in each TPH spectrum for hydrogenation of CO adsorbed at 298 K [12]; kinetics for 3, 10, and 15% Co/A1203 catalysts were reported earlier [ 12]. 3. RESULTS AND DISCUSSION Rates of methane formation versus temperature were obtained during TPH of the COcovered surface as shown in Figure 1 for 10% Co on SiO2, A1203, and TiO2. These spectra are super-imposed on the spectrum for unsupported (100%) Co (dotted curve). It is evident that each spectrum consists of two peaks (albeit overlapping peaks in the case of Co/SiO2) and that the peak at lower temperature (designated Peak A) is coincident with that of 100% Co. We demonstrated previously for 100% Co and Co/A1203 [12] that Peak A corresponds to formation of methane (and heavier hydrocarbons) on the metal surface via CO dissociation and carbon hydrogenation; we further demonstrated that the kinetic parameters for this peak are very similar to those derived from steady-state FTS kinetics. Evidence was also provided earlier [12] to support our conclusion that the peak at higher temperature (designated Peak B) corresponds to dissociation of a formate species; the formate intermediate is apparently formed by spillover of hydrogen atoms and CO to acid sites on the support; once formed it diffuses to the metal and is decomposed to methane. That a second peak (Peak B) is observed in the spectra for Co/SiO2 and Co/TiO2 (Fig. 1) suggests that methane is also formed in these catalysts via the formate/support path. This result is unexpected, since silica and titania are generally thought of as weakly acidic supports, i.e., they would probably not contain acid sites of sufficient strength to enable formation of the formate intermediate. Moreover, formation of formate was observed directly by in situ FTIR during FTS on Co/A1203 but was not observed under the same conditions on Co/SiO2 [ 18], suggesting that if a formate intermediate is formed in Co/SiO2, it is present in very low concentrations. Although silica by itself is weakly acidic, addition of a base metal oxide (by impregnation of the metal salt followed by drying and calcination) has been shown to substantially increases its acid strength [19]. For example, in the case of silica-supported iron oxide, iron cations are found in low coordination environments which are strongly acidic; indeed, incorporation of iron oxide into silica raises the heat of pyridine adsorption from 96 to 135-150 kJ/mol [19]. Similar sites of low Co 2+ coordination, e.g. CoO. SiO2, are observed to be present in typical Co/SiO2 FT catalysts [20, 21 ], and extrapolating the observations for iron oxides [ 18] to cobalt oxides, these sites would probably be strongly acidic. In the case of the 10% Co/SiO2 of this study (Fig. 1) Peak B is large and overlaps Peak A, a situation that would favor high rates of extra-ASF methane; this is consistent with the high methane selectivity of 29% observed for this catalyst (Table 1). On the other hand, the area of Peak B for Co/TiO2 is relatively small and wellseparated, a situation favoring low rates of methane formation, consistent with its lower observed

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methane selectivity (Table 1). Unfortunately, methane selectivity data for these two catalysts were measured after only 3 h and data are not available after 20 h of reaction when steady-state is typically observed; methane selectivities of supported cobalt usually decrease very significantly between 3 and 20 hours of reaction [13-16]. Kinetic parameters for supported-assisted formation of methane were obtained using Redhead and Arrhenius analysis of the high temperature B peaks for TPH of adsorbed CO (e.g. those in Fig. 1) for each of the supported catalysts, with the exception of 10% Co/silica for which the overlap of Peaks A and B prevented a quantitative analysis of the TPH data (see Table 2). The details of this analysis are provided elsewhere [12, 22]. Methane selectivities of Co/A1203 and Co/TiO2 catalysts calculated from the simple selectivity model of this study for ASF plus support contributions are listed in Table 3 and compared with available experimental values. For 10% Co/A1203 calculated and experimental methane selectivities are plotted as a function of temperature in Fig. 2. From these data it is evident that (1) methane formation from the support/formate route contributes very significantly to total methane, (2) calculated and experimental methane selectivities agree well for the Co]A1203 catalysts indicating that the support/formate route is the most important contribution to non-ASF methane, and (3) the trends of increasing methane selectivity with increasing temperature for 10% Co/A1203 and increasing cobalt loading for Co/A1203 catalysts are very similar for model and experiment. For 10% Co/TiO2, however, the model predicts much lower methane formation (i.e. 6.4%) than observed experimentally (16%), although the experimental value is artificially high, since the run time of 3 hours was inadequate to reach steady state. Nevertheless, the calculated selectivity agrees well with those of 5-10% reported for Co/TiO2 at 20 atm [23, 24], if such a comparison is valid. It is unclear how the kinetic parameters of the spillover/support/formate route are affected by variations in reactant partial pressure. In the case of unsupported cobalt, there is no contribution from the support; thus, the calculated theoretical value of 4.4% consists only of the ASF contribution. That the experimental methane selectivity of 13% is much higher than that calculated from ASF suggests that (1) the catalyst surface may have contained some unreduced cobalt oxide sites of strong acidity which contributed to methane make and/or (2) the cobalt surface was poisoned by contaminants such as carbon or sulfur which are known to be present in cobalt metal and cobalt salts in very low concentrations, but which

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Table 2. Kinetic parameters for support-assisted formation of methane (Peak B) from TPH of adsorbed CO on supported cobalt catalysts, a Catalyst

Peak T (~

Abx 10 -6 (s "l)

Eactc (kJ/mol)

Kinetic Parameters ka d qBe (s "l)

fBf

Ncrh(B) g 473 K (s "l)

3% Co/A120 3 544 28.2 95+3 9.1x10 4 2.8 0.77 8.6x10 5 10% Co/A1203 524 110 98+2 1.6xl 0 "3 16 0.66 2.9xl 0 -4 15% Co/A1203 493 400 97+2 7.8xl 0 3 33 0.62 8.9xl 0 -4 10% Co/TiO2 498 4.07xl 03 106 7.5xl 0 -3 7 0.26 2.8xl 0 -4 a Kinetic parameters for catalysts reduced at 648 K; values for 3, 10 and 15% Co/AI203 were reported earlier [ 12]. b Pre-exponential factors determined from Redhead's formula [25] for Peak B of spectra in Fig. 1. c Activation energy determined from Arrhenius plots of areas for Peak B of each spectrum in Fig. 1. d Rate constant; kB - A exp[Eact/8.314T]. e Coverage of species forming methane B peak in Fig. 1 in ~tmoles/g-catalyst; qB -- qco fB; qco is the CO TPD uptake. f Fraction of total methane observed as Peak I3 in Fig. 1. g. Turnover frequency for support-assisted methane formation; NCH4 (B) = ka qs/qM, qM = H2 uptake x 2.

Table 3. Methane selectivity data: (a) calculated from selectivity model and (b) measured at steady state in a differential fixed-bed reactor. Catalyst

Treaction Methane Selectivity, mole % (~ ASFacalc Supportbealr T o t a l C c a l c Totaldexptl unsupported Co 498 4.4 0.0 4.4 13 3% Co/A1203 473 10.4 4.7 15 16 10% Co/A1203 473 5.1 3.1 8.2 8.9 488 6.4 3.0 9.4 12 493 7.0 3.0 10 13 498 7.7 3.0 11 15 503 9.6 3.0 13 18 508 12.3 2.9 15 22 15% Co/A1203 473 2.3 5.4 7.7 9.0 10% Co/SiO2 493 6.2 29 e 10% Co/TiO 2 498 4.0 2.4 6.4 16 e a Predicted from ASF distribution, i.e. NCH4 -- Nco (1 -~ 002; SCH4 -" NcH4fNco; Nco - 1.05x109 exp [-102,000/(8.314 T) Prh ~ Pco ~ (pressures in atm.) based on data from Ref. 17. b Calculated from kinetic parameters obtained from temperature-programmed hydrogenation of adsorbed CO and TPD of CO. c Sum of methane calculated from ASF distribution and methane calculated from TPH kinetic parameters. d Generally measured after 20 h of reaction at temperature indicated at a CO conversion of about 5% and H2/CO = 2, except as noted below. e Data were obtained after only 3 hours of reaction; it is doubtful if steady state had been reached.

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could nevertheless contaminate the surface of a low surface area unsupported catalyst and which have been observed in some instances to increase methane make. The significantly lower activity (by a factor of 20-30) of unsupported cobalt relative to 10 or 15% Co/A1203 and its high extent of reduction (100%) (see Table 1) favor the latter hypothesis. The results of this study suggest the possibility of lowering methane make in supported cobalt FT catalysts by suitable modification of the support to reduce acidity and/or increasing the extent of reduction of cobalt to the metal, thereby reducing the fraction of cobalt oxide phases available to increase acid strength. REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

H. Beuther, C.L. Kibby, T.P. Kobylinski, and R.B. Pannell, U.S. Patent 4,413,064, Nov. 1, 1983. H. Pichler and H. Schulz, Chem. Ing. Tech., 42 (1970) 1162. H. Schulz, B. R. Rao, and M. Elstner, Erdoel Kohle, 23 (1970) 651. C.S. Kellner and A. T. Bell, J. Catal., 70 (1981) 418. K.R. Krishna and A. T. Bell, Catal. Lett., 14 (1992) 305. E.W. Kuipers, C. Scheper, J.H. Wilson, I.H. Vinkenburg, and H. Oosterbeek, J. Catal. 158, (1996) 288. A. Ekstrom and J.A. Lapszewicz, J. Phys. Chem., 91 (1987) 4514. K.B. Kester, E. Zagli, and J.L. Falconer, Appl. Catal., 22 (1986) 311. P.G. Glugla, K. M. Bailey, and J.L. Falconer, J. Catal., 115 (1989) 24. B. Sen and J.L. Falconer, J. Catal., 117 (1989) 404; 133 (1992) 444. Y.-J. Huang, and J.A. Schwarz, Appl. Catal., 30 (1987) 239; 32 (1987) 45; 36 (1988) 177. W.H. Lee and C.H. Bartholomew, J. Catal., 120 (1989) 256. R.C. Reuel and C.H. Bartholomew, J. Catal., 85 (1984) 78. L. Fu and C.H. Bartholomew, J. Catal., 92 (1985) 376. L. Fu, J.L. Rankin, and C.H. Bartholomew, C l Mol. Chem., 1 (1986) 369. J.L. Rankin and C. H. Bartholomew, unpublished data. F.H. Ribeiro, A. E. Schach von Wittenau, C.H. Bartholomew, and G.A. Somorjai, Catal. Rev.-Sci.Eng., 39 (1997) 49. G.R. Fredriksen, E. A. Blekkan, D. Schanke and A. Holmen, Chem. Eng. Technol. 18 (1995) 125. N. Cardona-Martinez and J.A. Dumesic, J. Catal., 127 (1991) 706. E. van Steen, G.S. Sewell, R.A. Makhoth, C. Micklethwaite, H. Manstein, M. de Lange, and C.T. O'Connor, J. Catal., 162 (1996) 220. B. Ernst, A. Bensaddik, L. Hilaire, P. Chaumette, A. Kiennemann, Catal. Today, 39 (1998) 329. W.H. Lee, Ph.D. Dissertation, Brigham Young University, 1988. E. Iglesia, S.L. Soled, and R.A. Fiato, J. Catal., 137 (1992) 212-224. R. Zennaro and C.H. Bartholomew, paper submitted, 1999. P.A. Redhead, Vacuum, 12 (1962) 203.

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Studies in Surface Science and Catalysis 130 A. Corma,F.V.Melo,S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 ElsevierScienceB.V.All rightsreserved.

Does mono-atomic Ru catalyse the Fischer-Tropsch synthesis? M. Claeys ~, M. Hearshaw 2, J.R. Moss 2, E. van Steen ~

Catalysis Research Unit, IDpt. Chemical Engineering, 2Dpt. Chemistry, University of Cape Town, Private Bag, Rondebosch 7701, South Africa The Fischer-Tropsch synthesis was studied using a Ru-dendrimer (Rp3G1C) supported on silica as a catalyst. The stability of the Ru-dendrimer catalyst was investigated using TGA in nitrogen, hydrogen, carbon monoxide and a hydrogen/carbon monoxide atmosphere. Under typical Fischer-Tropsch conditions the catalyst is meta-stable. The time-on-stream behaviour of this catalyst in the CO hydrogenation was compared with an impregnated Ru/SiO2 catalyst and showed no FT-specific product patterns. This seems to indicate that a single metallic site is not sufficient for the Fischer-Tropsch synthesis. This is substantiated by the rate of formation of some specific hydrocarbons. 1. INTRODUCTION Catalysts for the Fischer-Tropsch synthesis must possess multiple functionality, since the synthesis of higher hydrocarbons from syngas (CO + H2) require the adsorption of hydrogen and carbon monoxide, the cleavage of the H-H and the C-O bond, the hydrogenation of carbonaceous surface species, the removal of surface oxygen species, and the formation of C-C bonds [ 1]. Iron, cobalt, nickel and ruthenium metal are well known for their ability to catalyse the formation of higher hydrocarbons from synthesis gas [2]. Catalysts containing these metals, are usually prepared by (co)-precipitation or impregnation onto an inert support. In these cases the metal particles consist of a large number of surface metal atoms. The Fischer-Tropsch reaction can then be catalysed using an ensemble of the surface metal atoms. Of interest is of course, the minimal size required for such an ensemble. This study forms a part of our effort to estimate the minimal ensemble size for the synthesis of higher hydrocarbons. Mono-atomic ruthenium exists in homogeneous complexes such as Ru(CO)6. Ruthenium carbonyl can not be applied under typical Fischer-Tropsch conditions, due to its high volatility. Increasing the size of the ligand will decrease the volatility. Another approach would be to bind the complex to the support. In this study a hybrid approach was taken. A Ru-dendrimer, which contains mono-atomic Ru atoms on the periphery of an organic macro molecule, CH3 was synthesized (Rp3G1C; see figure 1), and then supported on an inert support. Since the stability of the o dendrimer at reaction conditions is unknown and its decomposition and/or leaching due to its slight solubility in apolar organic solvents such as liquid Fischer-Tropsch oc o ~ co products can be expected, the time-on-stream behaviour oc..~u-e" ~Z-RuCco cO rO during the CO hydrogenation must be studied. The obtained data can then be compared with the performance of a Figure 1" Structure of standard Ru-catalyst at similar reaction conditions. Rp3G1C (M = 2252 g/mol).

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2. EXPERIMENTAL

The Ru-dendrimer (Rp3G1C) was prepared according to the method described by Liao and Moss [3]. Silica gel (Davisil TM, grade 646, do = 200-250 l.tm, dr,ore = 150 A) was used as support material. The dendrimer (55 mg) was dissolved in CH2C12 and contacted with 0.445 g silica (resulting in a Ru loading of 3 wt.-%). The solvent was removed applying a N2 stream at room temperature. For comparison an impregnated Ru/SiO2 catalyst was prepared starting from an aqueous RuCI3 solution and the same silica using the incipient wetness method. In contrast to the dendrimer, the dried catalyst (drying in air at 110~ for 4 h) was pretreated in the FT-synthesis reactor (see below) via subsequent temperature programmed (4~ 300~ 4 h) calcination in N2 reduction in H2 respectively. Assuming complete reduction of ruthenium aider this the standard catalyst had a Ru content of 0.8 wt.-%. Prior to the CO hydrogenation experiments, the stability of the Ru-dendrimer catalyst in different environments was characterised using thermogravimetric analysis (Stanton Redcrot~ 780) under flow conditions applying different gases: N2, H2, CO and a H2/CO mixture. FTsynthesis was performed with 0.4 g of each catalyst in a U-tube plug flow reactor ( 88 stainless steel, internal volume = 1.5 ml) at 170~ a total pressure of 2 bar, ~ Syngas= 9 ml(NTP)/min and a molar H2/CO ratio of 2. Figure 2 shows schematically the experimental set-up for these studies. Gas flows were adjusted using mass flow controllers, the total reaction pressure was maintained by adding a pressure controlled argon stream before the pressure reducing valve. Samples of the product stream, to which a precisely measured reference gas stream (0.1 vol.-% cyclo-hexane in N2) was added, were taken using the ampoule sampling technique [4] and were later analysed gas-chromatographically (Varian 3400, flame ionisation detector, 50 m x 0.25 mm fused silica capillary column (Chrompack CP-Sil 5 CB), temperature program: -65 to 280~

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Figure 2. Simplied flow scheme of apparatus for CO hydrogenation. The set-up of the apparatus was specifically designed to enable us to detect the initial behaviour of each catalyst used and to separate FT-activity and decomposition of the Ru dendrimer, which had to be expected seeing results from thermo-gravimetric analysis (TGA) of the Ru dendrimer. This was achieved by ensuring a rapid heat-up of the catalyst bed in the syngas in less than 30 s, by dipping the small reactor, containing a small catalyst volume and a pre-heating zone for the syngas (0.5 ml SIC), in a pre-heated silicon oil bath. Rapid sampling was ensured using the ampoule sampling technique, which allows for the evaluation of mass balances and product spectra with high temporal resolution (possible sampling frequency <20s).

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3. RESULTS AND DISCUSSION

3.1 Characterisation of the Ru-dendrimer by TGA Figure 3 shows the results of thermo-gravimetric analysis of the Ru-dendrimer/SiO2 catalyst as obtained in different environments (conditions: msample = 25 mg, heating rate = 5~ gas flow rate = 30 ml(NTP)/min). In all cases two steps of weight 0.5 i i i i i i :: i i :: i i i i i : loss could be observed: the small i i i i ! i i i : : : : : i : weight loss at low temperatures ~ 0.0 " ~ ..... i....i....i....i....~.....i.....i....!....i.........i... i.....i.....i (<100~ can most probably be E i i~-" . . . . . . . i ~ ~ ~ attributed to evaporation of the ~'-0"5 i .................. ! ) i i solvent (CH2CI2) from the catalyst q 4.0 ~ i i -.-i.~.....N ~ ....i.-.~.0....E preparation, whereas the second step (>1800C) can be ascribed to the "~ decomposition of the dendrimer. It ~-2.0 ~ ~ ~ 9 ~ ~ ~ ~ ~ ~ ~ ~ ~CO~ ~ can clearly be seen, that the :: : : :: : : : : : :: ~ : :: : : dendrimer is less stable in hydrogen-2.5 ~ ~ ~ ~ ~ ; ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ rich atmospheres (H2 or H2+CO) 0 100 200 300 400 Temperature, ~ suggesting the involvement of hydrogen in the destruction of the dendrimer structure, e.g. by hydroFigure 3. Thermo-gravimetric analysis (TGA) genolytic reactions, of the Ru-dendrimer/SiO2 catalyst applying Based on these results the different gas environments. reaction temperature (170~ for the CO hydrogenation experiments was chosen to be slightly lower than the decomposition temperature of the dendrimer. However, a slow decomposition during the reaction had to be expected. Pre-runs of FT-synthesis with the standard Ru/SiO2 catalyst showed, that an even lower reaction temperature was not applicable in order to garantuee for reliable product data (GC detection limits) at the reaction conditions (flow rates and total pressure) used. .

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3.2 CO Hydrogenation with the standard Ru/SiO2 catalyst The activity (expressed as total formation rates or yields of all volatile organic compounds (voc) on a carbon basis) of the standard Ru/SiO2 catalyst as function of time on stream is depicted in figure 4. The catalyst was beginning to deactivate rapidly ca. 4 minutes after the start of the synthesis. Only after about one day time on stream almost steady state conversion levels (Yvoc<0.02 C-%) were reached. These very low yields of organic compounds, however, still allowed for a detailed quantitative evaluation of Fischer-Tropsch selectivities (see also

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chromatogram, fig. 6) A 2 o 4 min Figure 5 shows an Anderson-Schulz_='~e,' 925 min Flory chart (logarithmic plots of molar Eo 1 ,&19h product fractions versus carbon number), ,& 13 which can taken as a proof for the ,-X X existance of polymerisation kinetics [5]. -~ 0 & Indeed very straight lines from C3 to C7 or C9 respectively were obtained, their slopes --~ -1 allowing for the calculation of chain 0 5 10 growth probabilities of ca. 0.60+0.04 Carbon Number, Nc (decreasing with decreasing catalyst activity). Methane selectivities in the Figure 5. ASF-distributions at 3 reaction voc's ranged between ca. 20 and 31 C-% times. FT-synthesis with Ru/SiO2 catalyst. (increasing with time on stream). Olefin contents in the respective hydrocarbon fractions of the same carbon number amounted to ca. 70 mol-% and were carbon number independent, indicating the absence of secondary olefin hydrogenation, a consecutive reaction often occuring during FT-synthesis [6,7]. ,

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3.3 CO Hydrogenation with Ru-dendrimer on SiO2 In figure 6 a sector of a typical chromatogram as obtained from samples taken during the CO hydrogenation with the Ru-dendrimer/SiO2 is shown and put opposite to the products obtained with the standard Ru/SiO2 catalyst. The comparison of the two chromatograms reveals hardly any resemblance. ,..

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Figure 6. Gas-chromatogrammes of product samples after a reaction time of 4 min. CO hydrogenation with Ru/SiO2 (above) and Ru-dendrimer/SiO2 (below).

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Beside a number of identified compounds (such as propene and eyclopropane (main peaks) and traces of methane (also abundant in the CO used), ethene, ethane, propene, propane, eyelopropane, n-butene-1, butadiene, n-butane, n-butenes-2, methanol, acetaldehyde and nbutanal) a number of compounds, which have not been identified (main peaks XI and X2), could be detected in the spectra obtained with the dendrimer. Typical Fischer-Tropseh product patterns can not be found in the chromatogram of the experiment with the dendrimer suggesting that mainly products of the decomposition of the dendrimer structure were

monitored. Time resolved formation rates of the main products and the sum of all products during the CO hydrogenation with the Ru-dendrimer are shown in figure 7. The total product formation rate 2.5 passes through a maximum after ca. ~/ totalproduct .C m 4min time on stream. At this time E 2 more than 80 C-% of the product ~~' ~~2~ Pr~ O E 1.5 consists of propene plus cyclo~o propane. The maximal formation rates b 1 of the compounds X1 and X2 appear V jF~ Yt -~ Cyclo-Propane ~5 0.5 somewhat earlier, indicating their formation to be comparatively faster. The composition of the product and 1 10 100 1000 10000 0.1 the sharp drop in the formation rates Time on Stream, min of organic compounds as a function of reaction time are clear indications, Figure 7. Product formation rates (C-basis) as that the dendrimer decomposes at function of time on stream. CO hydrogenation reaction conditions. Propene, cyclowith Ru-dendrimer/SiO2. propane, X1 and X2 were also shown to be formed during thermal decomposition of the dendrimer in air. During the 24 h CO hydrogenation experiment the integration on a carbon basis of all products formed suggests that only ca. 7% of the dendrimer was decomposed. Assuming all product molecules bearing three carbon atoms (propene, propane, cyclopropane) to exclusively originate from C3-units in the dendrimer (see fig. 1) then a degree of decomposition of the peripheral structure of the dendrimer of ca. 20% can be estimated. Effects of leaching of the Ru-dendrimer by liquid reaction products can be excluded since no formation of liquids could be detected. In conclusion: although the dendrimer decomposes and part of the Ru could even have escaped the reactor as e.g. volatile Ru-carbonyls, the major fraction of the Ruthenium should still be available for any catalytic reactions (e.g. Fischer-Tropsch). .

.

.

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3.4 Comparison of"FT-activity" of Ru-dendrimer/SiOz and Ru-SiO2/catalyst Since the CO hydrogenation with the Ru-dendrimer showed no obvious FT-product patterns a direct comparison of FT-selectivities such as chain growth probability with the Ru/SiO2 catalyst is not possible. However, the GC-spectra obtained from samples of the experiment with the dendrimer contain compounds like ethene plus ethane and the linear C4-hydrocarbons that could theoretically be the result of FT-typical chain growth steps. For this reason, the molar formation rates of linear C4-hydrocarbons (excluding butadiene) per gram Ru versus time on stream are shown for the two "catalysts" in figure 8. (Remark: a blank FT-activity of the reactor without catalyst was not detectable.)

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Both formation rates pass a maximum after 4-5 minutes. At all stages 6 of the experiment the rates with the E: Ru/SiO2 catalyst are much higher (15-20 E times) as compared to the Ru-dendrimer. o 4 E Assuming an initial metal dispersion of 100% in the dendrimer and considering o 2 Ru-d end ri me r / S i O 2 1 " t - i l ~ an estimated Ru-dispersion of ca. 10% in o the fresh Ru-SiO2 catalyst the apparent ,_o 0 formation rates are even at least 2 orders 0.1 1 10 100 1 0 0 0 10000 of magnitude higher with the standard Time on Stream, min catalyst. This indicates that the monoatomic Ru in the dendrimer does not Figure 8. Formation rates of Ca-hydrodisplay any FT-activity from the very carbons (C-basis) during CO hydrogenation beginning of the CO hydrogenation with Ru/SiO2 and Ru-dendrimer/SiO2. experiments, and the formed C4 products are more likely due to hydroformylation of propene, which Ru is known to catalyse [8], and further hydrogenation of the so formed Ca-oxygenates (n-butanal was detected). tr

v

4. SUMMARY AND FUTURE WORK Results from time resolved CO hydrogenation with a standard Ru/SiO2 catalyst and a Rudendrimer on the same support strongly suggest the mono-atomic Ru in the dendrimer not to be able to catalyse FT-specific C-C bond formation. It is concluded that the minimal metal ensemble size must contain more than one metal atom. Future work aims at investigations with larger "cluster" sizes, e.g. bi-nuclear Ru-organic compounds, applying the same experimental set-up, which should enable us to detect any occuring FT-activity - even from the very beginning of an experiment.

References"

[1] Schulz, H, C1Mol. Chem. 1, 231 (1985) [2] Vannice, M.A., Jr. Catal. 50, 228 (1977) [31 Liao, Y.-H., Moss, J.R., Organometallics 15, 4307 (1996) [4] Schulz, H., BOhringer, W., Kohl, C., Rahman, N., Will, A., DGMK-Forschungsbericht [5] [6] [7]

[8]

320, DGMK, Hamburg (1984) Anderson, R.B., "Hydrocarbon synthesis, hydrogenation and cyclisation" in "Catalysis", vol. IV, (eds. Emmett, P., Reinhold Publ., New York, 1956) Schulz, H., Claeys, M., Appl. Catal. A: General, 186, 71, 91 (1999) Schulz, H., van Steen, E., Claeys, M., Proc. DGMK Conference "Selective Hydrogenations and Dehydrogenations", Kassel/Germany, 1993, (eds. Baerns, M., Weitkamp, J., DGMK-Tagungsbericht 9305, DGMK, Hamburg) 139 Likholobov, V.A., Moroz, B.L., "Hydroformylation" in "Handbook of heterogeneous catalysis", vol. 5, (eds. Ertl, G., KnOzinger, H., Weitkamp, J., Weinheim: VCH, 1997) 2231

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Fischer-Tropsch synthesis using monolithic catalysts A.-M. Hilmen 2, E.Bergene 2, O.A. Lindv~g2, D. Schanke 3 , S. Eri 3 and A. Holmen 1 IDept. of Chemical Engineering, Norwegian University of Science and Engineering (NTNU), N-7491 Trondheim, Norway, Fax. +47 73 59 50 47 2SINTEF Applied Chemistry, N-7765 Trondheim, Norway 3Statoil, R&D Centre, Postuttak, N-7005 Trondheim, Norway Cordierite monoliths have been washcoated with a 20 wt. % - 1 wt. % Re/T-alumina Fischer-Tropsch catalyst and tested for Fischer-Tropsch activity. The activity and selectivity were comparable to the corresponding powder catalyst at low washcoat loadings. Higher washcoat loadings resulted in decreased C5+ selectivity and olefin/paraffin ratios due to increased transport limitations. Impregnated alumina monoliths with a Co loading of 12 wt. % (and 0.5 wt. % Re) also showed similar C5+ selectivity as the small-particle powder catalyst. A large-particle powder catalyst (425-850 ~tm) showed very low C5+ selectivities due to severe intraparticle transport limitations. 1.

INTRODUCTION

Natural gas is becoming more attractive as an energy source and methods of upgrading the natural gas to higher hydrocarbon fuels are also increasing in importance. The FischerTropsch synthesis is a promising way for conversion of natural gas to liquid fuels. A key element in improved Fischer-Tropsch processes is the development of active catalysts with high wax selectivity. Supported cobalt are the preferred catalysts for the Fischer-Tropsch synthesis of long chain paraffins from natural gas due to their high activity and selectivity, low water-gas shift activity and a comparatively low price. Fixed-bed and slurry reactors have been the reactors of choice for low temperature Fischer-Tropsch synthesis. The large supported particles in fixed-bed reactors result in poor intraparticle mass transfer characteristics and the space-time yield is limited by heat transfer in the catalyst bed. The slurry system gives rise to significantly improved mass transfer characteristics within the catalyst particles, but requires separation of the catalyst from the product. The backmixing also makes the slurry reactor less efficient in terms of reactor volume than the plug flow reactor.

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Mass transfer effects are very important in Fischer-Tropsch synthesis [ 1]. Even though the reactants are in the gas phase the pores of the catalyst are filled with liquid products. The diffusion rates in the liquid phase are typically 3 orders of magnitude slower than in the gas phase, and even slow reactions may be diffusion limited in the liquid phase. C5+ selectivity will go through a maximum with increasing transport limitations within the catalyst pellet [ 1]. It will increase due to longer olefin residence times resulting in increased readsorption and thus decreased chain termination to olefins. But, increasing transport limitations will eventually result in CO depletion and enhanced hydrogenation reactions resulting in lower C5+ selectivity [1]. A decrease in olefin/paraffin ratio at a given conversion is therefore an indication of increased transport limitations. In a fixed-bed reactor the intraparticle diffusion resistance can be reduced by using catalyst pellets where the catalytic material is deposited in a thin outer layer (egg-shell catalysts) [2]. However, this means that only a fraction of the catalyst present in the reactor is participating in the reaction. In a slurry reactor, the selectivity problem is solved by using small catalyst particles. The application of monolithic reactors for other three-phase catalytic systems has been reported [3,4]. Recently, modelling of a monolithic reactor for the Fischer-Tropsch synthesis has shown promising results for this type of reactor compared to slurry reactors [5]. We have earlier reported characterisation and deactivation studies of cobalt FischerTropsch catalysts [6,7,8]. The present work deals with the use of a monolithic system for carrying out the Fischer-Tropsch synthesis. In a monolithic reactor, a short diffusion distance can be achieved while still maintaining a reasonable fraction of active material in the reactor since the catalyst is located on the thin walls of the monolithic structure.

2.

EXPERIMENTAL

Cobalt catalysts containing 20 wt. % Co, 1 wt. % Re o n A1203 were prepared by incipient wetness co-impregnation of the support with aqueous solutions of Co(NO3)2- 6H20 and HReO4. The catalysts were dried in air overnight at 393 K before calcination in air at 573 K for 2 h. The monoliths used in this work were all supplied by Corning: Cordierite (Coming) (400 cells/in 2) and y-alumina (based on Vista Catapal) (400 cells/in2). The monoliths were cut into cylindrical pieces with a diameter of 9 mm and length of 100 mm. A slurry containing the Co-Re/A1203 catalyst was grinded in a ball mill for lh before dipcoating the cordierite substrates. After washcoating, the monoliths were dried in air at 393 K overnight. In some cases the monoliths were dipped several times (dried at 393 K between dips) in order to increase the amount of catalyst on the monoliths. The y-alumina monoliths were prepared by impregnating the monolithic support with aqueous solutions of Co(NO3)2. 6H20 and HReO4. Drying and calcination procedures were the same as for the powder catalysts described above. The nominal Co loading of the alumina monoliths were 12 wt. % Co and 0.5 wt. % Re. The experiments were carried out in a conventional flow apparatus using a stainless steel reactor surrounded by an electrically heated aluminium jacket. The powder catalyst was

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diluted with an inert material (SIC) in a 1:5 weight ratio in order to minimize temperature gradients. The monolithic catalysts were used as prepared. All catalysts were reduced in flowing hydrogen at 1 bar and 623 K for 16 h. After reduction, the catalysts were cooled to 443 K in flowing hydrogen and purged with He before increasing the pressure to the desired and switching to a feed mixture consisting of synthesis gas (H2/CO=2.1) premixed with 3 tool% N2 as an internal standard. The reaction temperature was then slowly increased to the desired temperature (usually 483 K). The space velocity was varied to give 30-50 % CO conversion. The product gas was analysed for N2, CO, CO2, and C~+ hydrocarbons on a GC. The experimental data are averaged over a period of > 10 h and are representative of stabilised catalysts, i.e. after about 90 hours on stream. 3.

RESULTS AND DISCUSSION

Table 1 illustrates the effect of increasing intraparticle mass transfer restrictions for conventional (powder) catalysts. The table shows the selectivity for two powder catalysts with different particle size. Increasing the catalyst particle size from (53-751.tm) to (425-8501am) results in low C5+ selectivity and high CH4 and CO2 selectivities as a result of significant mass transfer restrictions. CO depletion leads to increased termination by hydrogenation and increased intraparticle residence time for produced water results in increased secondary watergas-shift reaction producing CO2. Due to the kinetic negative order dependency of CO, the effect of CO transport limitations on the overall reaction rate is delayed and therefore the presence of transport limitations will first be seen on the selectivity, due to the well known relationship between H2/CO ratio and selectivity [ 1]. The increase in particle size in Table 1 corresponds to a 10-fold increase in diffusion length. Even larger particles (>1000 lam) are necessary in commercial fixed-bed reactors to avoid unacceptable pressure drops. Table 1 Comparison between powder CoRe/A1203 catalysts of different pellet size. K, pressure 13 bar, H2/CO=2.1 CH4 C2-4 Catalyst Relative rate* sel. sel. Powder catalyst (53-751am) 1.00 9.0 7.4 Powder catalyst (425-8501.tm) 0.80 21.5 12.7 * Rate (gHc/gcat.'h) relative to the rate for powder catalyst (53-75 lam).

Temperature: 483 C5+ sel. 82.9 64.4

CO2 sel. 0.7 1.5

The thermal stability and the ability to withstand rapid temperature variations are of great importance in many monolith applications. Monolith structures are therefore usually made from a low-surface area ceramic material. The surface area can be increased by depositing a high surface area material such as y-alumina on the monolith surface and subsequently add the catalytically active material or the supported catalyst can be deposited directly on the monolith surface by washcoating.

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Table 2 Comparison between a conventional powder CoRe/A1203 catalyst and the corresponding monolithic catalysts. Temperature: 483 K, pressure 20 bar, H2/CO=2.1 Catalyst Relative CH4 C5+ C3JC3- CO2 Rate* sel. sel. ratio sel. 1.00 8.3 82.3 2.4 0.2 Powder catalyst (38-53 I,tm) Wascoated cordierite 0.92 8.9 82.5 1.9 0.3 (washcoat thickness 0.04mm) * Rate (gHc/gcat.'h) relative to the rate for powder catalyst (38-53 lttm). The performance of washcoated cordierite monoliths is shown in Table 2. Calculation of the relative rate to hydrocarbons shows that the washcoated monolithic substrates are as active as the conventional (powder) catalyst. They also have similar CH4 and C5+ selectivities, but a slightly lower olefin to paraffin ratio for the cordierite-based catalyst indicates higher diffusion resistance than for the powder sample. Figure 1 shows the effect of increasing the washcoat layer thickness on the cordierite substrate. The washcoat layer thickness was calculated assuming an even distribution of the washcoat on the monolith surface. The rate is relative to the rate of powder catalyst (38-53 ~tm), Table 2.

1,5

83 I1)

(~ 82 o

~

o.o--'" ~:'::,".... .o........

t~

~ 8~

.u> t~

>

~

-~ 80 o

0,5 n-

o,,

---.-- (35+ selectivity

',,

...o... Relative rate 79

0,00

i

I

i

0,05

O,10

O,15

0

0,20

appr. washc0at layer thickness [mm] Figure 1. Effect of washcoat layer thickness on the rate and C5§ selectivity of monolith catalysts (483 K, 20 bar, H2/CO=2.1).

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3 r

o. O o 2

~176

A

~

~,~_~176176176

O"

>-9 1

o 11) I1) r

o 0 0,00

.o.

""8

2

t__

o

"~176

"'k

---<~-- C3 olefin ...o... C3 paraffin ...A..- C3=/C3- ratio I

I

0,05

0,10

0,15

o

om

1 o

0,20

appr. washcoat layer thickness [mm]

Figure 2. Effect of washcoat layer thickness on the C3 olefin and paraffin selectivity of monolith catalysts (483 K, 20 bar, H2/CO=2.1). As the washcoat layer thickness on the cordierite monolith increases, C5+ selectivity and olefin/paraffin ratio decreases, while the rate remains relatively unaffected. The decrease in C3 olefin/paraffin ratio is a result of increased paraffin selectivity and decreased olefin selectivity (Figure 2). These observations can be explained by increased mass transfer restrictions as described in Introduction. Apparently, negative effects on C5+ selectivity appears already at a washcoat layer thickness between 0.05-0.1 mm at our operating conditions. The thermal stability is actually not a critical factor in slow reactions such as the Fischer-Tropsch synthesis. Monoliths can therefore be made directly from high surface area materials such as T-alumina. The catalytic material can then be incorporated into the total volume of the monolith, resulting in higher catalyst loading compared to washcoated lowsurface area monoliths. Table 3 shows the performance of an impregnated T-alumina monolith. The monolith has a nominal Co content of 12 wt. %. The performance of the monolithic catalyst is compared with a crushed and sieved monolith sample (38-53 ILtm). The impregnated alumina monolith shows C5§ selectivity similar to the selectivity of small-particle powder catalysts, Table 2, and the crushed alumina monolith. The relative rate is not directly comparable to the powder catalysts and the washcoated monoliths due to a lower Co loading, but the crushed monolith sample and the monolith show similar rates.

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Table 3 Selectivity and relative rate of 12% Co-0.5 % Re on ),-alumina monolith. Pressure: 20 bar, H2/CO=2.1 Catalyst T Relative CH4 C5+ C3-_.]C3_ CO2 [K] Rate* sel. sel. ratio sel. Alumina monolith, crushed (38-53~tm) 483 0. 39 9.8 81.5 2.0 0.5 Alumina monolith 479 0.35 8.6 83.2 2.4 0.4 Rate (gHc/gcat.'h) relative to the rate for the 20 % Co- 1% Re powder catalyst (38-53~tm), Table 2. The results presented here show that monlithic catalysts are as active as small-particle powder catalysts for the Fischer-Tropsch synthesis and are superior to large-particle powder catalysts that are characterised by low C5§ selectivities due to severe mass transfer restrictions. 4.

CONCLUSIONS

The activity and selectivity of washcoated cordierite monoliths were comparable to the corresponding powder catalyst at low washcoat loadings. Higher washcoat loadings resulted in decreased Cs+ selectivity and olefin/paraffin ratios due to increased transport limitations. Impregnated alumina monoliths with a Co loading of 12 wt. % (and 0.5 wt. % Re) also showed similar C5+ selectivity as the small-particle powder catalyst (38-531ttm). A largeparticle powder catalyst (425-850 ktm) showed very low C5+ selectivities due to severe intraparticle transport limitations. ACKNOWLEDGEMENTS Statoil and the Norwegian Research Council are acknowledged for financial support and the authors thank Corning for providing monolith samples.

REFERENCES [1] E. Iglesia, S. C. Reyes, R. J. Madon and S. L. Soled, Adv. in Catal. 39 (1993) 221. [2] E. Iglesia, S.L. Soled, J.E. Baumgartner and S.C. Reyes, J. Catal. 153 (1995) 108. [3] V. Hatziantoniou and B. Andersson, Ind. Eng. Chem. Fundam. 23 (1984) 82. [4] S. Irandoust and B. Andersson, Chem. Eng. Sci. 43(8) (1988) 1983. [5] V.D. Mesheryakov, V.A. Kirillov and N.A. Kuzin, Chem. Eng. Sci. 54 (1999) 1565. [6] D. Schanke, S. Vada, E.A. Blekkan, A.M. Hilmen, A. Hoff and A. Holmen, J. Catal. 156 (1995) 85. [7] D. Schanke, A.M. Hilmen, E. Bergene, K. Kinnari, E. Rytter, E. ,~dnanes and A. Holmen, Catal. Lett. 34 (1995) 269. [8] A.M. Hilmen, D. Schanke, K.F. Hanssen and A. Holmen, Catal. Lett. 186 (1999) 169.

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Studies in Surface Science and Catalysis 130

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A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

The Design of Catalyst Surfaces by the Use of Stochastic Optimisation Algorithms A.S. McLeod ~ and L.F. Gladden b School of Chemical Engineering, University of Edinburgh, Kings Buildings, Mayfield Road, Edinburgh, EH9 3JL, United Kingdom. b Department of Chemical Engineering, University of Cambridge, Pembroke Street, Cambridge, CB2 3RA, United Kingdom.

We describe the application of stochastic optimisation algorithms to heterogeneous catalyst design. We discuss the optimal design of a two-component catalyst for the diffusion limited A+B--+ 0 and A + B2-~ 0 reactions in which each species is adsorbed specifically on one of the catalytic sites. The geometric distribution of the sites that maximises the catalyst activity is determined by the use of a genetic algorithm and a simulated annealing algorithm. In the case of the A + B - ~ 0 reaction, it is found that the optimal active site distribution, that of a checkerboard, is approximately 25% more active than a random site distribution. While both the genetic and simulated annealing algorithms obtain identical optimal solutions for a given reaction, the simulated annealing algorithm is shown to be more efficient. 1.

INTRODUCTION

We consider the design of a two-component catalyst for the diffusion limited A + B--+ 0 and A + B e - + 0 reactions. The catalyst surface is modelled as a regular array of two distinct adsorption sites, each of which specifically adsorbs one of the two reactants. Possible applications of the method described may therefore lie in the optimisation of bimetallic or metal-metallic oxide composites for low temperature CO oxidation catalysts. Two stochastic optimisation methods are compared in this work, generic algorithms and simulated annealing. Stochastic optimisation algorithms find application in the field of combinatorial optimisation, where the number of variables to optimise is very large. The current problem, to determine the optimal distribution of a number of catalytic sites, is an example of a combinatorial problem. Genetic algorithms (GAs) are general purpose optimisation algorithms based on Darwinian evolution [1,2]. Although GAs draw upon a biological metaphor, they are finding increasing application in a wide range of disciplines, particularly the engineering and physical sciences [3]. Simulated annealing (SA) algorithms are a class of optimisation algorithms based on thermodynamic principles [4]. In the first section of this paper the optimisation methodology is outlined. The results of a comparative study of catalyst design using the GA and the SA algorithms are then

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presented. We conclude by discussing the extension of this work to more realistic catalytic systems. 2.

METHOD

In summary, there are two stages in the optimisation procedure: first the determination of the activity of a specific catalyst surface and, second, the use of this activity in guiding the optimisation algorithm toward an improved solution. In this section, the reaction model is described and a outline of the optimisation methodology presented. 2.1.

Reaction model

The reaction mechanism for the A + B-4 0 and A + B2 -4 0 reactions is A + S1 --+ A-S1 Bn + nS2 -4 Bn-nS2 A-$1 + Bn-nS2 -4 $1 + nS2

(1) (2) (3)

where n = I for the monomer-monomer reaction and n = 2 for the monomer-dimer reaction. As the catalyst surface is disordered it is necessary to use Monte Carlo (MC) simulations [5,6] of the reaction to determine the activity of the catalyst surfaces. The MC method employed here follows from that described in previous studies of the A + B-4 0 and A + B2 -4 0 reactions occurring on homogeneous surfaces [5,6]. The reacting system consists of an infinite bulk phase composed of the reacting species which can adsorb onto the discrete adsorption sites of the catalyst surface. During each simulation step, a reactant is chosen from the bulk phase and an attempt is made to adsorb the molecule on a randomly selected adsorption site. A particle adsorbs successfully only if the adsorption site is of the correct type and is not already occupied. The lattice sites neighbouring the adsorbed molecule are then checked for molecules of the opposite species. If any AB (or AB2) pairs are formed, a pair is selected at random and is removed from the surface. The reaction rate is obtained by counting the number of reactions occur. The catalyst activity is expressed as a turnover frequency, r = R/(tN), where R is the number of product molecules produced in t time steps. The simulation is conducted on a regular square lattice of N = 120x120 adsorption sites. The catalytic activity obtained by the simulation is then returned to the GA or SA algorithm as the value of the objective function that is to be optimised. The optimisation problem is simplified by describing the catalyst surface by a smaller sub-lattice that is then tiled to generate the larger simulation lattice. As the reactants only occupy one or two adsorption sites, the optimal site distribution is not expected to be described by a complex site pattern. A sub-lattice of 6 x 6 sites was therefore used with an 8 x 8 lattice used to verify the results. The optimisation problem to be addressed by the GA and the SA algorithm is therefore to determine the optimal 6 x 6 site pattern that describes the surface. Several examples of such sub-lattices are shown in Figure l(a).

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Application of a genetic algorithm to catalyst design

The principles of genetic optimisation can be only briefly summarised here, further details can be found elsewhere [1,2]. Borrowing concepts from biological evolution, the GA operates by the manipulation of a binary string, or chromosome, that represents an encoding of a particular catalyst surface, the corresponding phenotype. In the present case the relationship between these two representations is trivial- the $1 sites are represented by O's and the $2 sites by l's in the chromosome. The GA is initialised by generating a number of random binary strings to represent a population of chromosomes. In this work, a fixed population of 200 chromosomes was used. The chromosomes are manipulated by the GA using two generic operators: (i) crossover, where two chromosomes mate to form two new children by swapping subsections of their chromosomes and (ii) mutation, where one gene of the chromosome is flipped at random to assume the value of the alternative binary digit. During a single iteration of the algorithm, representing a single generation, each chromosome mates once on average. Evolutionary pressure is applied by selecting the chromosomes that encode the more active surfaces for crossover more often than those representing the less active surfaces. The probability of attempting to crossover any individual member of the population is determined by its fitness function. The fitness function, F(r), biases the crossover probability in favour of the more active members of the population. Although the linear function, F(r) = r, can be used a power law function was chosen for this work where F(r) = r ~. The exponent, 7, was chosen adaptively so as to maintain the ratio r'/Y = 3, where ~/= l n ( n - r'/Y) and r' is the activity of the most active member of the population [7]. Using this function, a chromosome encoding the most active surface is selected for crossover three times more often than a chromosome representing a surface with average catalytic activity. 2.3.

A p p l i c a t i o n of s i m u l a t e d a n n e a l i n g to catalyst design

Simulated annealing is an optimisation technique based on thermodynamic principles [4]. The principle can be illustrated by analogy with the phase transition undergone by a material as it is cooled. If a material is cooled slowly, the resulting solid phase is often stable and crystalline. If, however, the same material is quenched a meta-stable amorphous phase of higher energy is likely to form. It is the concept of energy minimisation by slow cooling that is exploited in SA algorithms. In the current problem, the "energy" of the system corresponds to a measure of the activity of the catalyst surfaces. A SA algorithm requires three functions to be specified: (i) a stochastic generating function, that determines the path taken through the search space of all possible catalyst surfaces, (ii) a probability density function to determine the probability of accepting each new surface selected by the generating function, and (iii) an annealing schedule to specify the rate at which the system cools. Annealing algorithms differ principally in the choice of the cooling schedule. In classical annealing, a logarithmic cooling schedule is used and the temperature after k iterations of the algorithm is given by Tk = To/In k, where To is the initial system temperature. More efficient cooling schedules can be used by choosing alternative site generating functions. In this work, the very fast simulated re-annealing (VFSR) algorithm has been used [8]. In the

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1

10

0.22

20

[] B e s t of p o p u l a t i o n

o Population average

~,o 0.21 0.2 30

40

50

z

>=

q~d~%Dd~q~

~_~~

o~

0.19

o

~_ 0 . 1 8 9

0.17

9

oo 0

20

40

6'0

80

Generations

(a)

(b)

Figure 1. (a) Evolution of the optimal solution to the A + B--+ 0 problem with an A:B ratio in the bulk ph~e of 1:1 using the GA. A adsorption sites are shown in white. (b) The evolution of the most active surface and population average for the A + B ~ 0 reaction. VFSR algorithm an exponential cooling schedule is used where Tk -- To exp(--ckUD). The parameter c specifies the rate of cooling. Starting with a randomly generated surface, the activity of the surface is first calculated by MC simulation. A new surface is then generated at random. The probability of accepting this new surface as the next step in the search is given by the generating function N

1

gk - iII= 1 2(lyil + Tk) ln(1 + 1/Tk)"

(4)

The parameter lYil is equal to the number of differences between the sites of the two surfaces. Small steps in search space are therefore more likely to be successful than large leaps. Note that gk and Tk must satisfy the condition Egk = c~. This condition requires all sites must at least be able to be visited, assuming that the search is ergodic. If the new surface is accepted as the next trial location, then the activity of the new surface is calculated. The probability, P, of accepting the new solution in place of the previous solution is given by the Metropolis condition [

p _ ~exp(AR/Tk)

(

1

ifAR < 0;

(5)

otherwise,

where A R is the difference in catalyst activity between the current catalyst surface and the new surface. Initially, at high system temperatures, most changes are accepted allowing for large areas of the parameter space to be searched. As the optimisation progresses, and the system cools, the acceptance criterion becomes more difficult to satisfy and only successive improvements are accepted. 3.

RESULTS AND DISCUSSION

The evolution of the solution to the A + B--+ 0 problem for an A:B ratio of 1"1 using the GA is shown in Figure 1. The A adsorption sites are shown in white in the figures or as the

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化

0.22 0.2

-'-"

0.21

.-

0.2

0.15 I--

E = Z

E

z

P

0.19

=._

> O

> O r

= I--

E

0.18 0.17

o Simulated annealing r~ Genetic algorithm 0

[]

0.1

5 10 15 F u n c t i o n E v a l u a t i o n s [x10 -3]

20

I-

0.05

o Simulated annealing

Genetic algorithm

0 F u n c t i o n E v a l u a t i o n s [x10 -3]

(b)

Figure 2. (a) A comparison of the GA and SA solutions to the A+B --+ 0 problem. (b) The corresponding data for the A + B2--+0 problem '0' sites in the matrix notation. The optimal solution with the most active catalyst surface, a checkerboard site distribution, is located after 50 generations. The spreading of the highly reactive [~ 0] group, which maximises the mixing of the reactants on the surface, illustrates the ability of the GA to identify and propagate the desirable characteristics of the solution through the population. A comparison with solution obtained by the SA algorithm for the A + B - + 0 problem is given in Figure 2(a). Although it may have been possible to predict the checkerboard solution, the solution for other bulk phase compositions is not as intuitive. As a further test of the methodology, the design of a catalyst surface for the A + B - + 0 reaction was repeated using a range of bulk phase compositions. Altering the bulk phase composition such that the A species becomes twice as likely to make an adsorption attempt leads to very different solution. The most active surface in this case is given by a tiling of the [ 0 ~ ] group. The site density changes as to increase the probability of adsorbing the 110 component that is deficient in the bulk phase. This diagonal pattern was maintained for increased A:B ratios of 3:1 and 4:1. Results for the A+B2--+ 0 reaction are shown in Figure 2 (b). In this case the optimal solution, obtained by both the GA and SA, is found to be similar to that obtained for the A+B--+ 0 reaction. The most active surface found was a checkerboard tiling of the [0 0 ~ 00] group. A comparison of the two algorithms demonstrates that, for both reactions, SA is the more efficient optimisation method. For the A+B--+ 0 reaction the first occurrence of the optimal solution required approximately 5 x 10a function evaluations, as compared to 1 x 104 evaluations for the GA. Similarly, for the A + B 2 ~ 0 reaction the SA algorithm required only 2.7 x 10 a evaluations as compared to the 1.1 x 104 required by the GA. Despite the efficiency of the SA algorithm, however, there is a compelling reason for using the GA for practical design problems, where the catalyst surfaces will be more complex that those discussed here. The advantage of using the GA is that the GA does not seek out a single solution, but instead aims to propagate specific site groups throughout the solution population. These groups correspond to the desirable traits of the catalyst surface. Thus, even if a single optimal solution is not obtained for a complex problem,

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the aspects of the catalyst surface that most influence the catalytic activity or selectivity may be identified. In the A + B - + 0 reaction for example, 50 generations are required to find the optimal site distribution. However, examination of the population after only a few generations revealed that the majority of solutions in the population had an $1 : $2 ratio of 1:1. Thus, the principle influence on the catalyst activity is the proportion of $1 :$2 sites rather that their geometric distribution on the surface. 4.

CONCLUSIONS

A rational methodology for the optimal design of simple catalyst surfaces at the molecular level has been introduced. The method has been applied to design of two-component catalysts for the A+B--+ 0 and A+B2--+ 0 reactions. For the A+B--+ 0 reaction, where the bulk phase is a mixture of equal amounts of each component, the surface yielding the highest catalytic activity is found to be a checkerboard distribution of the two active sites. Deviations from this configuration are observed for cases in which the bulk phase is not composed of an equal mixture of the reactants. As the relative proportion of one of the species increases, the number of adsorption sites for that species tends to a ratio of 2:1 in favour of the species that is deficient in the bulk phase. Extension to the more complex A+B2 ~ 0 reaction with equal amounts of each component in the bulk phase produces a similar checkerboard pattern consisting of a tiling of pairs of adjacent adsorption sites. While the SA approach has been found to be more efficient than the GA, the GA is expected to be more suited to practical catalyst design problems. The advantage of the GA is that the GA identifies the desirable characteristics common to a large population of solutions. For complex optimisation problems, where qualitative trends are more likely to be identified that unique optimal solutions, the GA is will be the preferred optimisation algorithm. 5.

ACKNOWLEDGMENTS

ASM thanks Peterhouse, Cambridge for the award of the Rolls Royce Frank Whittle research fellowship. LFG thanks the Innovative Manufacturing Initiative and EPSRC.

REFERENCES

[1] J.H. Holland, Adaptation in Natural and Artificial Systems, MIT Press, Cambridge, 1994. [2] D.E. Goldberg, Generic Algorithms in Search, Optimisation, and Machine Learning, AddisonWesley, New York, 1989. [3] L. Davis, Generic Algorithms and Simulated Annealing, Pitman, London, 1987. [4] S. Kirkpatrick, C.D. Gelatt and M.P. Vecchi, Science, 220 (1983) 671. [5] R.M. Ziff, E. Gulari and Y. Barshard, Phys. Rev. Lett., 56 (1986) 2553. [6] K. Fichthorn, E. Culari and Y. Barshard, Phys. aev. Lett., 63 (1989) 1527. [7] A.S. McLeod, M.E. Johnston and L.F. Gladden, J. Catal., 167 (1998) 279. [8] A.L. Ingber, Mathl. Comput. Modelling, 12 (1989) 967.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Modeling of structure and properties of active centers of catalysts on the base of metalorganosiloxanes

A.V.Nemukhin *~ I.M.Kolesnikov b and V.A.Vinokurov b a- Department of Chemistry, Moscow State University, Moscow, 119899, GSP, Russia b - Russian State University of Oil and Gas, Leninsky Prospect, 65, Moscow 117296, Russia Quantum chemical calculations provide support to the reaction mechanisms suggestinq chemisorption of the ethylene molecule on the A104 site of alumophenylsiloxane accompanying a n-bond rupture and a creation of cation-radicals. Studies of elementary chemical processes occurring at the alumosilicate compounds are of importance due to a great significance of these species as catalysts in the conversion of oil fractions~. Along with wellknown solid alumosilicates related soluble substances, in particular alumophenylsiloxane (APS) and metallphenylsiloxanex possess similar structural properties and are viewed as perspective catalysts as well 2. Recent experimental studies 3 show that in the presence of APS in a liquid reaction volume an propilene-benzene interaction occurs yielding propyl-benzene. It is suggested that one of the stages of the process may be related to the chemisorption of unsaturated hydrocarbons on APS ahd MPS accompanying a creation of cation- and anion-radicais. Considerable efforts are being performed to clarify mechanisms of catalytic activity of alumosilicates starting from different viewpoints 4. According to the theory catalysis by polihedra developed by one of the authors 5 active sites of alumosilicate compounds are the tetrahedrons {A104} and {SiO4}. This work presents a quantum-chemical contribution in some support to this idea taking the ethylene-APS system as an example.

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All calculations described here have been carried out with the GAMESS package 6. We have considered the interaction of the ethylene molecule with APS by means of a group of atoms representing the local chemisorption site. Namely, the {AIO4} fragment have been selected as an ac,tive species in environments simulating the original APS complex. A special attention has been paid to such parameters as partial charges on atoms, effective electronic configurations and compositions of bonding orbitals. At the preliminary stages the structure of APS, (C6Hs(HO)2SiO)2AI(OSi(OH)2C6Hs), has been studied by using molecular mechanics and semiempirical AM1 quantum chemistry techniques 7 followed by ab initio MO LCAO calculations with the 3-21G basis set. According to these approaches the central part of APS may be considered as a distorted AIO4 tetrahedron with two A1-O bonds directing to the Si(OH)2C6H5 fragments and two AI-O bonds forming a well-known aluminum-oxygen-silicon cycle with a bridged OH group 4. A partial electronic charge on aluminum is estimated as +2.0, on oxygen as-1.2 (on average), and on silicon as +2.4. A composition of the bonding A1-O orbital may be described in terms of natural bond orbitals 8 as 0.35 sp a3 (A1) + 0.95 sp 13 (O) indicating that the orbital is strongly polarized towards the oxygen hybrid. A strongly ionic character of the compound is consistent with the data common to aluminum oxide molecules 9'~~ To simulate an active site of APS, namely, the {A104} fragment, several reduced systems of APS have been considered: A104Li3, AI(OH)4, AI(OSiN)202Li, AI(OH)2(OSiH3). In all cases the tetrahedral arrangement of oxygen atoms placed at a distance 1.72 A from the aluminum center has been assumed. Detailed results will be presented below for the interaction C2H4 + AIO4Li3 although most impirtant qualitative conclusions are similar for all species. The lithium atoms play here the role of "pseudoatoms" often used in attempts to select a finite cluster from an extended system when studying complicated processes like chemisorption 4. In the A104Li3 moiety partial charges on atoms (+ 1.4 on A1 and -0.9 on O) resemble those calculated for APS (+2.0 and-1.2) as well as effective electronic configurations (3s ~ 3p ~~ in A104Li3, versus 3s ~ 3p ~ in APS for aluminum, and 2s19 2p5.0 in A104Li3 versus 2s 1"8 2p 5"4 in APS for oxygen). A composition of the bonding AI-O orbitals in A104Li3 (0.34sp z3 (AI) + 0.94sp 1"6(O)) also reproduces nicely that of APS. We conclude that the local electronic properties of the A104 site are reproduced successfully by the A1 O4Li3 molecule.

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Chart I shows geometry arrangements of the entire reactive complex C2H4 + AIO4Li3. The C2v symmetry has been assumed for the system. Other spatial configurations have been rejected since their energies are considerably higher. We have varied only one geometry parameter, a distance R between A1 and a center of the C-C bond in C2H4. Other parameters have been optimized with the RHF/3-21G (a restricted Hartree-Fock model within the 3-21G basis set) wavefunctions for C2H4, and A104Li3 in separate calculations.

H~

J

c

CL..

t

H j

H H

R

O Li /

O ~ ~

A1 ~ ~

/\ \/

O

Li

O

Li Chart 1

Within the natural borM orbital formalism 8 the electronic structure of C2H4 is described as a configuration with the doubly occupied bonding orbitals: [Core]cy2(C-C)[cya(C-H)]4n2(C-C). When interacting with the A104, site of APS or related reduced systems specific changes in the electronic structure of C2H4 are expected. For the goals of the present study the most important is a process of a n-bond rupture in C2H4 which should be reflected by a substantial reduction of population of the n (C-C) orbital. The data presented in Table I show the main result of this work, namely, the changes in total energy and in the electronic structure of the ethylene molecule during its interaction with the A104Li3 species. The numbers have been computed at the RttF/3-21G level of the theory.

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Table 1. Interaction energies C2H4 + A104Li3 (AE), total charge on the C2H4 fragment (Q), and population of the n (C-C) orbital (n~) versus intermolecular distance (R) calculated at the RHF/3-21G approximation.

R,A

AE, kJ/mol

Q

nn

50.0 5.0 4.0 3.5 3.0 2.75 2.50 2.25

0.0 +3.3 +26.8 - 114. -413. -527. -4601 +54.8

0.0 0.0 +0.194 +0.496 +0.638 +0.656 +0.668 +0.676

2.00 1.98 1.77 < 1.5 --'--

First of all we note that the interaction C2H4 + AIO4Li3 leads to a strongly bound system at the distance R about 2.75 A with a binding energy above 527 kJ/mol. A small barrier in the entrance channel (about 27 kJ/mol) may disappear and the well depth may increase if a geometry relaxation is taken 71-110 account in calculations. A bound C^H + A10 Li complex evidences that a chemisorption may take place when ethylene is contacting APS. A total natural charge on the ethylene fragment is increasing from zero to a value (+0.68) close to +1 indicating that reaction mechanisms which assume creation of cation-radicals gain some support by these quantum chemical simulations. We also see. that along the reaction pathway a population of the n-orbitaj of C2H4 is reducing from 2.00 to a boundary value of 1.50 generally accepted as a critical one for regularly occupied MO's 8 immediately after passing the potential barrier. We can interpret the result as a rr-bond rupture in this region. An analysis of electron density in terms of natural orbitals shows that .instead of the n(C-C) orbital the ~(C-O) orbitals are getting populated which results in a cycle extending over the entire complex.

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In conclusion the present study confirms that the {A104} tetrahedron may serve as an active site of the alumophenylsiloxane complex. According to the quantum chemical model calculations the ethylene molecule reacts efficiently with this site yielding a singly bonded cation-radical as an intermediate species. Acknowledgments The authors thank Prof. J.Almlof for the making these computations possible, Professors F.Weinhold for using their computer programs.

financial support M.Schmidt and

References 1. Kreking neftyanykh frakzyi na tseolitsoderzhschikh katalizatorakh (Cracking of oil fractions on ceolit containing catalysts), ed. S.N.Khadjiev, Khimia, Moscow, 1982, p.280 (in Russian). 2. I.M.Kolesnikov, G.M.Panchenkov and V.A.Tulupov, Zh.Phys.Khim., 1965, 39, 1869 (in Russian) 3. I.M.Kolesnikov and N.N.Belov, Zh.PrikIadnoy Khim., 1990, 63, 162 (in Russian) 4. G.M.Zhidomirov, A.A.Bagaturyanz and I.A.Abronin, Prikiadnaya kvantovaya khimia (Applied Quantum Chemistry), Khimia, Moscow, 1979, p.296 (in Russian) 5. I.M.Kolesnikov, Kinetika i kataliz v gomogennykh i geterogonnykh uglevodorodsoderzhschikh sistemakh (Kinetics and catalysis in homogeneous and heterogeneous hydrocarbon containing systems), Textbook, Moscow Institute of Oil and Gas, Moscow, 1990, p. 198 (in Russian) 6. M.W.Schmidt, K.K.Baldridge, J.A.Boatz, J.H.Jensen, S.Koseki,M.S.Gordon, K.A.Nguyen, T.L.Windus and S.T.Elbert, Quantum Chemistry Program Echange Bulletin, 1990, 52 7. M.J.S.Dewar, E.G.Zoebisch, E.F.Healy and J.J.P.Stewart, J.Amer.Chem.Soc. ,1985, 107, 3902 8. A.E.Reed, R.B.Weinstock and F.Weinhold, J.Chem.Phys., 1985, 83,737 9. A.V.Nemukhin and F.Weinhold, J.Chem.Phys., 1992, 97, 3420 10.A.V.Nemukhin and L.V.Serebrennikov, Uspekhi Khimii, 1993, 62, 566 (Russian Chemical Reviews, 1993, 62, 527)

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Studies in Surface Science and Catalysis 130

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A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Theoretical study on active sites of molybdena-alumina catalyst for olefin metathesis J. Handzlik and J. Ogonowski Institute of Organic Chemistry and Technology, Cracow University of Technology, ul. Warszawska 24, 31-155 Krak6w, Poland Theoretical investigations of ethylene and propene metathesis reactions proceeding on molybdenaalkylidene and molybdacyclobutane centres of the molybdena-alumina catalyst were done. The density functional theory in Gaussian 94 package was applied. The energetic effects of the reactions of the alkene molecule with Mo TM and Mo vI carbene centres were calculated. Reaction pathway of ethylene addition to Mo vl alkylidene complex was also investigated 1. INTRODUCTION It has been generally accepted that transition-metal-catalysed olefin metathesis proceeds according to carbene mechanism [1]. Metal carbene reacts with olefin, thus forming the metallacyclobutane complexes. Subsequent decomposition of the metallacyclobutane leads to formation of a new olefin and a carbene structure. Several theoretical studies on the olefin metathesis reaction and the structures of molybdenum homogeneous catalysts have been performed [2-4]. Those works concem Mo vI alkylidene and molybdacyclobutane complexes. However, we have not found examples of theoretical modelling of olefin metathesis active sites on heterogeneous molybdena catalysts. The active catalyst centres contain probably M o vI [ 5 - 6 ] , but other Mo valences are also possible (e.g., Mo TM [7]). In this work theoretical studies on possible ethylene or propene metathesis reactions proceeding on molybdenaalkylidene centres of molybdena-alumina catalyst are reported. Two variants of theoretical models of the active sites have been developed. In the first case, simple structures of the carbene and molybdacyclobutane complexes are proposed, where the bonds between molybdenum and the carrier are replaced by hydroxyl groups. In the second case, molybdenum is attached to a small cluster of formula A12(OH)6, which represents alumina. 2. COMPUTATIONAL Calculations were carried out with the GAUSSIAN 94 program [8], installed on SPP1600/XA computer in ACK CYFRONET AGH (grant No. KBN/SPP/PK/099/1998). Density functional theory was applied. Geometry of the structures was optimized with the Slater local exchange functional and the VWN5 local correlation functional [9], or, with the B3LYP nonlocal functional [10]. Harmonic vibration frequencies were calculated for each structure to confirm the potential energy minimum or the transition state involved, and, to obtain the thermal energy, enthalpy, entropy and Gibbs free energy. The electronic energy of

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each structure was calculated using B3LYP functional. Three basis sets were employed: basis A - LANL2DZ basis set (the Hay-Wadt effective core potential [11 ] plus double-zeta basis for molybdenum and aluminium atoms, Dunning-Huzinaga valence double-zeta basis set D95V - for other non-transition elements), basis BI and B2 - LANL2DZ basis set for molybdenum atom and D95V(d) or 6-31G(d) basis set for non-transition elements, respectively. Calculations of the alumina clusters alone were carried out with MOPAC 7 program using semiempirical AM1 Hamiltonian. 3. RESULTS AND DISCUSSION

3.1. Simple models of the carbene and molybdacyclobutane complexes The following Mo vl complexes: Mo(O)(CH2)(OH)2 (1), Mo(O)(CH2CH2CH2)(OH)2 (2a and 2b), syn-Mo(O)(cncn3)(on)2 (3), Mo(O)[CH(CHa)CH2CH2)(OH)2 (4), Mo(O)[CH(CHa)CH(CHa)CHE](OH)2 (Sa and 5b), and Mo Iv complexes: Mo(CHE)(OH)2 (6a and 6b), Mo(CH2CHECHE)(OH)2 (7a and 7b) were studied. Calculated geometry of 1 is shown in Fig. 1. In Table 1, structures of 1 obtained using different functionals and basis sets are compared. The results indicate that optimized geometry almost does not depend on used functional or basis set. Calculated structures of other Mo vx complexes and Mo TM structures, singlet (6a, 7a) and triplet states (6b, 7b), are shown in Fig. 2. 01 Hl Mo

~

C

-M 1

Table 1 Optimized structures of 1. Bond distances are in angstroms, angles are in degrees. SVWN5/A B3LYP/B1 B3LYP/B2 Mo-C 1.886 1.896 1.891 Mo-O l 1.716 1.691 1.696 Mo-O 2 1.880 1.916 1.917 C-H l 1.102 1.092 1.089 C-H 2 1.109 1.099 1.097 OI-Mo-C 103.0 103.8 103.9 Mo-C-H 1 124.9 126.6 127.0 Mo-C-H 2 119.3 118.0 118.0 O2-Mo-O 3 110.5 110.3 109.9

Fig. 1. Optimized geometry of 1. Four-coordinate molybdenum alkylidenes I and 3 have pseudotetrahedral structure with Cs symmetry. Mo vl molybdacyclobutanes have a square pyramidal (SP" 2a, 4, 5) or a trigonal bipyramidal (TBP: 2b) structure. Calculated structures of Mo vI complexes are in a very good agreement with other published results for Mo alkylidenes and molybdacyclobutanes [2-4]. SP molybdacyclobutane (2a) is predicted to be more stable than TBP structure (2b) by 66 kJ/mol with the B3LYP/B1 method and by 69 kJ/mol with the B3LYP/B2 method. For Mo TM methylidene complexes, B3LYP/B1 and B3LYP/B2 calculations give a 33 kJ/mol and 35 kJ/mol preference to the triplet state (6b), respectively. Triplet state of molybdacyclobutane 7b is more stable than the singlet state 7a by 57 kJ/mol and 60 kJ/mol, according to B3LYP/B 1//SVWN5/A and B3LYP/B2//SVWN5/A calculations, respectively.

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@ =Mo

O

=C

9 =H

~!~3 = 0

1.889

4+

I

C)

F+

6a

5b +..

.+__ 1.890

6b

i

7a

t ~ ' k ' - 1.868

~.~

/g+

~

,--.

7b

g (

Fig. 2. Optimized structures of Mo vl and Mo TM complexes. Mo=C bond lengths (in angstroms) were obtained from SVWN5/A calculations. Thermodynamic effects of the reactions involving Mo vl and Mo TM centres are given in Table 2. Reactions of alkene and Mo vI alkylidene complex leading to formation of SP molybdacyclobutane (I, III, IV, V) are exothermic with relatively small absolute value of the

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change of Gibbs free energy. On the other hand, the reaction VIII of Mo TM methylidene complex with ethene is very exothermic and exoergic. The enthalpy of formation of trigonal bipyramidal molybdacyclobutane from methylidene complex and ethene (reaction II) is close to zero, but the reaction is endoergic. Molybdacyclobutane leading to trans-2-butene has lower energy than the corresponding complex giving cis-2-butene (reactions IV and V). However, the decomposition of the latter structure is energetically preferred, in comparison to the decomposition of the former one (reactions VI and VII). Table 2 Calculated changes of thermal energies (AE~ kJ/mol), enthalpies (AH~ kJ/mol) and Gibbs free energies (AG~ kJ/mol) for the metathesis reaction of MoVl and Mo Iv complexes. Entry Reaction Method AE~ AH~ AG~ I 1 + C2H4 "-) 2a B3LYP/B1//SVWN5/A -69.1 -71.6 -16.8 B3LYP/B1 -65.6 -68.1 -14.3 II 1 + C2H4 --) 2b B3LYP/B1//SVWN5/A -4.9 -7.3 44.9 B3LYP/B1 0.7 -1.8 49.8 B3LYP/A 2.5 0.0 52.4 III 1 + C3H6 ") 4 B3LYP/B1//SVWN5/A -57.3 -59.8 -0.1 B3LYP/B1//SVWN5/A -52.2 -49.7 8.2 IV 3 + C3H6 ") 5a B3LYP/B1//SVWN5/A -40.2 -37.8 22.0 V 3 + C3H6 ") 5b VI 5a --) 1 + trans-2-butene B3LYP/B1//SVWN5/A 55.8 58.2 -0.1 VII 5b --) 1 + cis-2-butene B3LYP/B 1//SVWN5/A 49.0 51.5 -9.7 VIII 6b + C2H4 "-) 7b B3LYP/B1//SVWN5/A -130.1 -132.6 -79.7 Reaction pathway of ethylene addition to Mo vl alkylidene complex 1 was investigated. n-Complex 8 of ethene with the alkylidene complex and transition state 9, leading to TBP molybdacyclobutane, were localised (Fig. 3). The efforts to find a square-pyramidal transition state have failed. The thermodynamic parameters calculated for the reactions studied are given in Table 3.

9

=Mo

~

=C

. =0

Q =H (--~

8

9

"

C

~,..5

Mo

- C

1 --

2.744

Mo - C l = 2.306

Fig. 3. Optimized structures of zc-complex 8 and transition state 9. Mo - C distances (in angstroms) were obtained from B3LYP/A calculations.

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Table 3 Calculated changes of thermal energies (AE~ kJ/mol), enthalpies (AH~ kJ/mol), kJ/mol) for the reaction entropies (AS0298, J/(molK)) and Gibbs free energies (AG~ pathway of ethene addition to Mo vl methylidene complex. Entry Reaction Method AE0298 AH~ AS~ AG~ IX 1 + C2H4 --) 8 B3LYP/A 22.3 19.8 -148.0 63.9 X 8 --) 2b B3LYP/A -19.8 -19.8 -27.9 -11.5 XI 8 --) 9 B3LYP/A 12.7 12.7 -30.1 21.6 3.2. Model structures including alumina

The following Mo vl and Mo TM (triplet state) complexes: Mo(O)(CH2)(H4A1206) (10), Mo(O)(CH2CH2CH2)(H4A1206) ( l l a and lib), Mo(CH2)(H4A1206) (12) and Mo(CH2CH2CH2)(H4AI206) (13) were studied (Fig. 4). Their geometries are very similar to the corresponding geometries described in chapter 3.1. To verify correctness of the small alumina cluster of formula A12(OH)6, deprotonation energies and charges on the hydrogen atoms in this cluster were compared with the respective deprotonation energies and charges in a larger cluster of A1203 consisting of over one hundred atoms. 9 =Mo

9 =C

10

=O

lla

9 =H

Q

=A1

lib

Fig. 4. Optimized structures of Mo w and Mo TMcomplexes including alumina. Mo=C bond lengths (in angstroms) were obtained from SVWN5/A calculations.

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Thermodynamics of the metathesis reactions is given in Table 4. As in the case of the previous results, the SP molybdacyclobutane (lla) is predicted to be more stable than the TBP structure (lib). Formation of Mo Iv molybdacyclobutane 13 is exothermic and exoergic. Table 4 Calculated changes of thermal energies (AE~ kJ/mol), enthalpies (AH~ kJ/mol) and Gibbs free energies (AG~ kJ/mol) for the metathesis reaction of MoVl and Mo TM complexes. AE~ AH~ AG~ Entry Reaction Method XII 10 + C2H4 ") l l a B3LYP/B1//SVWN5/A -49.3 -51.8 1.5 XIII 10 + C2H4 "-) l i b B3LYP/B1//SVWN5/A 7.6 5.1 59.9 XIV 12 + C2H4 "-) 13 B3LYP/B1//SVWN5/A -132.4 -134.9 -82.2 4. CONCLUSIONS 1. SP structure of the molybdacyclobutane has lower energy than TBP structure. The formation of the former structure from molybdenaalkylidene complex and alkene is exothermic with relatively small absolute value of the change of Gibbs free energy. The formation of the latter structure is clearly endoergic. 2. During propene metathesis, the formation of the molybdacyclobutane leading to trans2-butene is energetically favoured, in comparison to the formation of the corresponding complex leading to cis-2-butene. On the other hand, decomposition of the molybdacyclobutane leading to cis-2-butene is more exoergic than the analogous process leading to trans-2-butene isomer. 3. Formation of the Mo TM molybdacyclobutane is very exothermic and irreversible. REFERENCES

1. J. L. Herisson, Y. Chauvin, Makromol. Chem., 141 (1970) 161. 2. T. R. Cundari, M. S. Gordon, Organometallics, 11 (1992) 55. 3. E. Folga, T. Ziegler, Organometallics, 12 (1993) 325. 4. Y.-D. Wu, Z.-H. Peng, J. Am. Chem. Soc., 119 (1997) 8043. 5. K. A. Vikulov, I. V. Elev, B. N. Shelimov, V. B. Kazansky, J. Mol. Catal., 55 (1989) 126. 6. W. Griinert, A. Yu. Stakheev, R.Feldhaus, K. Anders, E. S. Shpiro, K. M. Minachev, J. Catal., 135 (1992) 287. 7. K. Tanaka, in: Y. Imamoglu (ed.), Olefin Metathesis and Polymerization Catalysts, NATO ASI Series C326, Kluwer Academic Publishers, Dordrecht 1990, p. 303. 8. Gaussian 94, Revision E. 1, M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W. Gill, B. G. Johnson, M. A. Robb, J. R. Cheeseman, T. Keith, G. A. Petersson, J. A. Montgomery, K. Raghavachari, M. A. A1-Laham, V. G. Zakrzewski, J. V. Ortiz, J. B. Foresman, J. Cioslowski, B. B. Stefanov, A. Nanayakkara, M. Challacombe, C. Y. Peng, P. Y. Ayala, W. Chen, M. W. Wong, J. L. Andres, E. S. Replogle, R. Gomperts, R. L. Martin, D. J. Fox, J. S. Binkley, D. J. Defrees, J. Baker, J. P. Stewart, M. Head-Gordon, C. Gonzalez, and J. A. Pople, Gaussian, Inc., Pittsburgh PA, 1995 9. S. H. Vosco, L. Wilk, M. Nusair, Canadian J. Phys., 58 (1980) 1200. 10. A. D. Becke, J. Chem. Phys., 98 (1993) 5648. 11. P. J. Hay, W. R. Wadt, J. Chem. Phys., 82 (1985) 299.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Modeling the Oxygen activation on Dinuclear Iron MMO Mimics, a Quantum Mechanic Study Peter-Paul H. J. M. Knops-Gerrits a-b, Peter A. Jacobs b • William A. Goddard III a aMaterial & Process Simulation Center, Beckman Institute (139-74), California Inst. of Technology, Pasadena CA 91125, USA Tel 626-395-2731, Fax 626-585-0918, email [email protected] bCenter for Surface Science and Catalysis, Katholieke Universiteit Leuven, Kardinaal Mercierlaan 92, B-3001 Heverlee, Belgium, Tel: 32-16-32 1597; Fax : 32-16-32 1998, email [email protected] Methane Mono-Oxygenase (MMO) is a di-iron active site containing enzyme that catalyzes the dissociative binding of molecular oxygen. To mimic the MMO active site we chose to study the heptapodate coordinated binuclear iron (II or III)-complexes of N,N,N'N'tetrakis(2-benzimidazolylmethyl)-2-hydroxy- 1,3-diamino-prop ane (HPTB), N,N,N',N',Tetrakis(2-pyridylmethyl)-2-hydroxy-l,3-diamino-propane (HPTP) in experiments and their finite cluster model N,N,N',N',-Tetrakis(2-iminomethyl)-2-hydroxy-l,3-diamino-propane (HPTM) in theoretical calculations. These have active sites of the form [Fez(HPTL)(p~-OH)]4+ or 2+. Quantum Mechanic structures are compared with experimental EXAFS data. For the O2 binding on the reduced active site the g-q~:ql-O 2 mode seems to proceed formation of the O=Fe-O-Fe=O bis-ferryl active site that reacts exothermally with methane. The nature of the ferryl groups are these of a reactive two center three electron bond. 1. INTRODUCTION Methane Mono-Oxygenase (MMO) [1] and Deoxyhemerythrin [2] are examples of diiron enzymes that catalyze the dissociative and non-dissociative binding of molecular oxygen. Dissociative binding of oxygen via a peroxo intermediate to a diamond core structure [3] leads to a reactive species active in the oxidation of alkanes [4-5]. Non-dissociative binding of oxygen via a side-on peroxo intermediate such as in the active site of deoxy-hemerythrin does not allow the splitting and allows binding/release of oxygen as a function of the physiological conditions [2]. These active sites are among the growing list related to O- and OH-bridged di- or poly-iron cores in biological systems [8-9]. MMO has a binuclear iron active site with two histidines and four glutarates. Both iron ions are coordinated by a histidine, an oxygen from a bridging carboxylate and a ~-oxo bridge [5]. Theoretical modeling of such enzyme active sites has been recently reported [10-16]. Yoshizawa et al. studied the dioxygen cleavage and methane activation on diiron enzyme models with the extended Hfickel method, an approximate molecular orbital method, the ~t']]1:']]1-02 o r ~.~-']]2:']']2-0 2 binding modes are distorted to the corresponding dioxygen complex. The p-rl~:q~-O2 mode is more effective for electron transfer to the d-block orbitals. Regarding methane activation Crabtree [11] reviewed the recent data. According to Siegbahn et al. [12].

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the most significant structure is the FeH~-O-FeV=O oxo structure. The ground state of this structure is ~A and the iron spins are 4.00 and 2.94, the spin on the bridging oxygen is 0.76 and on the oxo ligand is as high as 1.13. In reactions with ethane there is 35% of inversion of configuration, the 65% that remains unchanged is difficult to realize if free radicals have more than a transient existence. The mimicking of MMO by immobilization of the model complexes in the voids of clays or mesoporous silica and silica-alumina has been our ongoing interest [21-23]. The characterization and theoretical quantum structure of [Fe2(HPTP)(~tOH)(NO3)2](C104)2 (1) and [Fe2(HPTB)(g-OH)(NO3)2] (C104) 2 (2) and their oxygen and methane activation, is investigated here. 2. E X P E R I M E N T A L 2.1.SYNTHESIS 2.1.1.[Fez(HPTB)(OH)(NO3)2](N03) z N,N,N',N'-tetrakis(2-benzimidazolylmethyl))-2-hydroxy-

1,3-diaminopropane (HPTB) is prepared [8-9]. To an ethanol solution of Fe(NO3)3.6H20 (0.31 g) the HL (0.30 g) is added and the precipitated complex is collected. 2.1.2.[Fez(HPTP)(OH)(NO3)z](C104) 2 N,N,N',N',-tetrakis(2-pyridylmethyl)-2-hydroxy -1,3diamino-propane H(HPTP)* as perchlorate is prepared from p-chloropicoline and 2-hydroxy1,3-diamino-propane after [10]. As in the previous synthesis, Fe(NO3)3.6H20 (0.31 g) and H(HPTP)(C104)2 (0.28 g) are solved in ethanol. The complex is washed with acetonitrile/diethylether and recrystallised in diethylether.

Figure 1. Structure of [Fe2(HPTM)(O2)] 3+optimized by Quantum Mechanics. 2.2.COMPUTATIONAL ANALYSIS The ab initio calculations used involve full geometry optimization of the clusters with density functional theory (dft) as implemented in Jaguar [15] (Jaguar 3.0, Schrodinger, Inc., Portland, Oregon, 1997) at the B3LYP method level (Becke3 hybridization functionals, Slater/Becke88 non-local exchange and Li, Yang, Parr local and nonlocal correlation corrections to the local potential energy functionals of Vosko, Wilk and Nusair ) using the Los Alamos effective core potential and valence double Zeta for iron ( LACVP** basis sets ). The molecular mechanics calculations involve a new molecular mechanics force field, the Universal force field (UFF) of Rapp6 et al. [13-14]. The force field parameters are estimated using general rules based only on the element, its hybridization and its connectivity. The force field functional forms, parameters, and generating formulas for the full periodic table have

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been published [13]. For charge equilibration used in molecular dynamics simulations the charges in the complexes were determined [16] to readjust charges based on geometry and experimental atomic properties. The initial structures were energy minimized using suitable sets of parameters appended to UFF, to better describe coordination around iron. The quantum mechanical study was performed at the ab initio level starting from a UFF optimized structure. UFF minimization is carried out on an Origin2000 machine. (16 MIPS R10000 (IP27) CPU's, 195 MHz w/4 MB secondary cache each, with an IRIX 6.4_$2MP + OCTANE operating system) using a Newton-Raphson minimization scheme with a norm of the gradient convergence criteria of 1 x 10-1~ k c a l / m o l / A . In order to accommodate iron in five coordinate form, according to EXAFS data, after UFF optimization a QM calculation is performed. Atom types for iron and other transition metals in the UFF is given with its symbol, hybridization geometry and valence state. UFF contains 126 atom types, the force constants are generated using Badgers rules as described, the van der Waals parameters are computed based on a Lennard-Jones type potential [ 13-14]. 3.RESULTS AND DISCUSSION 3.1. EXAFS S P E C T R O S C O P Y The Fe K-edge EXAFS-XANES analysis gives direct information of the coordination environment of the complexes as salts. The Fe K-edge XAS on [F%(HPTP)(pOH)(NO3)2](NO3)2 and [Fe2(HPTB)(p-OH)(NO3)](NO3)2 show pre-edge features indicative of a distorted five coordinated iron with a sixth Fe ligand, the edge-shifts of 13.1 eV and 15.5 eV are characteristic for its high spin ferric form. The Fe K-edge EXAFS indicates that the [Fe2(L)(p-OH)] 4+ gives a coordination number (CN) of 1 for the Fe-Fe bonding and an Fe-Fe distance of 3.020 A for L=HPTP and of 3.223 A for L=HPTB in accordance with the crystallographic data [18-19]. For complexes with less bulky ligands e.g. HPTP the two species in the Mossbauer spectra are observed, i.e. a p-OH and a p-O species. Shorter Fe-OH inter-atomic distances are seen of 1.92-1.99 A with EXAFS compared with computations and crystallographic structures [5]. The HPTP pyridine and amine groups give Fe-N bonds of 2.11 A~and 2.327 A~. The HPTB benzimidazole and amine groups give Fe-N bonds of 2.185 A and 2.384 A~. The large Debye-Waller factor arises from many different species in the samples.

3.2. C O M P U T A T I O N A L ANALYSIS 1. Modeling the Structures. The [F%(HPTP)(p-OH)] 4+(1), and [F%(HPTB)(p-OH)] '+ (2) complexes are used in catalysis. These are models that have a structure that combines all the features of the ligand and iron active site. The [Fe2(HPTM)(p-OH)] '+ complex was used in the calculations and an imine group is positioned where a pyridine or a benzimidazole occur in the actual ligands as seen in Figure 1. Both the ferric [Fe2(HPTM)(p-OH)] 4+and the ferrous [Fe2(HPTM)(p-OH)] 2+ cores are then optimized with Quantum Mechanics (QM). The QM optimized structure of [Fe2(HPTM)(p-OH)] 4+ is reacted with oxygen in an acidic medium to give intermediates P and Q . [21]. The charge of the [F%(HPTM)(p-OH)] •247cluster is +4 or +2, depending on the simulation of a Fe(III) or an Fe(II) active site and the energy levels of different multiplicity are studied. The relative energies of some important catalytic reactants were analyzed, as are the effects of their solvation again obtained by QM calculations. In UFF due to the change of a ferrous to a ferric type the decrease in the bond-length and the increase in the force constant for the Fe-X distances are seen and some changes in the angle bending parameters are observed. Only a slight increase is seen for the angles of the N_2 from 111.2 to

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111.3 o, these for the O R increase from the standard 110 to 128.0 and 126.4 o in the diferric core. Also, a decrease of the O_R Fe O_R angle can be seen from 90 to 74,5 ~ The N_2 Fe3+3 N_2 angles remain constant at 109.47 ~ The complex has C2h symmetry and both ferrous ions occur either in a high-spin quintet state, and intermediate-spin triplet state or a low-spin singlet state. When these states couple we obtain a nonuplet state if the two iron ions are high-spin, the quintet state Q if the two iron ions are intermediate-spin, or singlet S when the two iron ions are in low-spin form. N (dxz) 2 (dxy)~(dyz)l(dz2)l(dx2-y2)l Q (dxz) 2 (dxy)2(dyz)l(dz2)l S (dxz) 2 (dxy)E(dyz)2 The ferric ions occur either in a high-spin sextet state, and intermediate-spin quartet state or a low-spin doublet state. When these states couple we obtain the undecuplet state if the two iron ions are high-spin, the septet state SP if the two iron ions are intermediate-spin, or triplet T when the two iron ions are in low-spin form. U (dxz) ~ (dxy)l(dyz)~(dz 2) l(dx2-y2)~ SP (dxz) 2 (dxy)~(dyz)l(dz2)l T (dxz) 2 (dxy)2(dyz) ~ In the ferric case (Fe I") the septet (SP) and the undecuplet (U) are its ground state and low lying exited states. The equatorial (trans) and axial (cis) effect of the N atoms with respect to the bridging ~t-oxo groups dictate their bond lengths that are 2.03-2.04 ,~, 2.31 A and 1.951.99/~ respectively. The distances of 1.95 and 1.99/~ obtained from QM seen in the direction of the equatorial unprotonated and protonated O groups compared to the calculations, are accompanied by Fe-O-Fe angles of 90-93 ~ and an O-Fe-O bite angle of 74.5 ~ 2. Modeling the Oxygen Activation. Geometrical en electronic properties affect the relative catalytic properties such as the hydrogen bond abstraction energies of the binuclear cores of these iron complexes. In the QM optimized structure the iron core has two ~t-oxo and six terminating nitrogen atoms. In acid aqueous reactions, the bridging by a deprotonated ligand alcohol group remains strong, the bridging (kt-OH) can be protonated and removed as water. This helps the binding of molecular oxygen and the consequent transformation into a peroxo (022-) group (P) with formal change of charge of iron from +2 to +3. This leads to a diamagnetic singlet state S (dxz) 2 (dxy) 2 (dyz)~(O2rr) ~ (dz 2) ~(O2~) 1 here the dyz orbital is coupled to the two orthogonal three electron pi-system of the 02 ligands. An alternative is a paramagnetic quintet state Q (dxa-y2)~(dxz)a(dxy)l(dyz)l(OEr01 (dz 2) ~(O2~) ~ The two ferryl bonds become stronger by transfer of ~ bonding electrons between the two Oxygen atoms to their anti-bonding orbitals and the pairing of these with an extra electron from the iron ion. In a consequent step the peroxo (022-) group is transformed into two ferryl (02-) bound groups (intermediate Q), with the formal change of the charges of iron from +3 to +4. In an alternative step the peroxo (O22-) group of the complex can also transform to yield two bridging (~t-O) oxo groups. The superexchange coupling for these complexes is fairly small, this agrees with the experimental observation of a J value of 12 cm -~ for the diferrous OH bridged model compound [18-19]. The geometrical implications on these reactions are probed with QM analysis and the results are shown in Table 1 and 2. The Fe-O bond length of 1.21 ,~ increases to 1.31 ,~ on the model compound and is smaller than the 1.49 .~ distance in H202, consequently this bond is broken in the model compound. The Fe..Fe distances of 3.14/~ increases to 3.43 A in intermediate P and 3.63 ,~, in intermediate Q. Upon transformation into the bis (~t-O2-) oxo bridged dimer, it decreases to 2.65/~. The peroxo form P is slightly more stable than the ferryl form Q by about 18.9 kcal/mol. The formation of the bis (kt-O2-) oxo is unfavorable since it is 44.4 kcal/mol higher in energy.

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Table 1. Bond len[~ths (A) obtained b~, QM calculations ~li~and L= HPTM) bond L O(=O)2 L O(O2) L O3 L o(on)2 L o(on) (OCH~) 3.60 Fe Fe 3.628 3.433 2.647 3.639 Fe O 1.607-1.611 1.843-2.037 1.824, 1.909 1.724, 1.756 1.729, 1.747 Fe O (C) 1.971-1.982 1.876-2.039 2.071 1.958-2.008 1.993-1.993 O O 1.309 Fe a N(sp 2) 2.020-2.034 2.000-2.085 1.956-2.078 2.093-2.109 2.010-2.072 Fe b N(sp 2) 2.093-2.095 2.031-2.071 2.094 Fe a N(sp 3) 2.101-2.166 2.077 2.056 3.120 Fe b N(sp 3) 2.270 2.107 O1-H1 0.974-0.984 0.974-0.984 1.432 C1-O2 1.437 1.435 1.420 1.433 i

However, solvation stabilization is higher in complexes with increased charge transfer i.e. the Fe(IV) complexes are better stabilized by solvation than the Fe(III) complexes. Shaik et al. [17] showed that iron oxide cations have a high spin ground state and..,adjacent low spin excited state. The adjacency of the two spin states together with the poor bonding of the high spin state and the good bonding of the low spin state, leads to a spin cross-over along the reaction coordinate and opens a low-energy TSR (two state reactivity) path fo.r hydroxylation. Table 2. Bond an~;les (o) obtained b~, QM calculations bond angles HPTM HPTM HPTM O(:O)2 0(02) 03 Fe O Fe 133.2 122.4 79.4- 93.0 O Fe O 97.8-98.7 80.3-89.9 72.2- 78.2 Fe O O 119.9-123.7 H O Fe C O Fe 109.9-116.8 117.9-119.6 116.9 Ne-Fe-Ne 115.6-115.9 108.9-110.1 118.0 Na-Fe-Ne 81.0-81.3 81.7-83.4 80.6-83.9 Na" axial nitrogen, Ne 9equatorial nitrogen.

HPTM O(OH)2 133.1 93.2-101.9 113.0-117.1 112.2-114.5 116.9-119.2 80.0-80.7

HPTM O (OH)(OCH~) 133.7 95.9-99.4 112.2 112.7-113.3 117.6-117.8 80.2-81.3

3. Modeling the Alkane Activation. Upon interaction with C H 4 the geometry of the two ferryl (02-) bound groups (intermediate Q) to the ferric (FenI-OH, FenI-OCH3) groups does not change substantially, the Fe..O distances change, as the system becomes more asymmetric since one OH group that is formed will show hydrogen bonding with the neighboring methoxy group. For the ground state (multiplicity 7) the bridging Fe-O distances change from 1.97 and 1.96 ,~ to 1.993 and 1.993 A and the terminal Fe-O distances increase from 1.61 and 1.61 A to 1.747 (FeHIOH) and 1.729 (FemOCH3) /~, the CH and OH distances are 1.097, 1.096, 1.095 and 0.982 ,~, respectively. Overal the reaction with methane is exothermic by 50.56 kcal/mol, the consecutive substitution of the methoxy group by a hydroxo group is endothermic by 7.50 kcal/mol and the regeneration of the active site with H202 is again endothermic by 3.24 kcal/mol. The solvation calculations are very important to obtain good quantitative data. The solvation energy is about 150, 300 and 500 kcal/mol for the 2+, 3+ and

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4+ complexes, respectively. The active bis-ferryl ~-oxo-bridged site (intermediate Q) shows eight unpaired electrons. The localization of these free d-electrons occurs partially on the iron (Fe TM like with a spin density of 2.6 to 2.8) and on the oxygen (spin density of 0.80 to 0.85).

4. CONCLUSION. The reaction of the MMO binuclear heptapodate coordinated iron (III)-complexes of N,N,N',N'-tetrakis(iminomethyl)-2-hydroxy-l,3-diamino-propane model with methane is exothermic by 50.56 kcal/mol. The [Fe2(HPTP)(~t-OH)]4+ and [Fe2(HPTB)(~t-OH)] 4+ model complexes give a coordination number of 1 for the Fe-Fe and a distance of 3.02 A and of 3.22 A respectively, in accordance with the QM value of 3.135 A obtained on the [Fe2(HPTM)(~tOH)] 4+ model complexes. The o- and n-bonds of the ferryl Fe=O in the plane of the Fe-O-Fe bridge, have the properties of a two atom three electron bond. ACKNOWLEDGEMENTS

PPKG thanks the FWO-Flanders for a post-doctoral fellowship, A.Fukuoka & M.Ichikawa from the CRC, at Hokkaido University, Sapporo, Japan for a collaboration on EXAFS. PPKG and WAG wish to thank BP Amoco for financial support. REFERENCES

1. L., Que, Y., Dong, Acc. Chem. Res., 29 (1996) 190. 2. Loehr, T. M.Ed., Iron carriers and Proteins; VCH, Weinheim, 1989, 373. 3. L. Shu, J.C.Nesheim, K.Kauffrnann, E. Munck, J.D. Lipscomb, L. Que, Jr., Science, 275 (1997) 515.4. Y. Dong, S. Yah, V.G.Young Jr., L.Que Jr., Angew. Chem., 108 (1996) 674. 5. A.C., Rosenzweig, C.A., Frederick, S.J., Lippard, P., Nordlund, Nature, 366 (1993) 537. 6. K.E. Liu, D. Wang, B.Huynh, D. Edmonson, A. Salifoglou, S.J. Lippard, J.Am.Chem.Soc., 116 (1994) 7465.7. K.E. Liu, A.M. Valentine, D. Qiu, D.Edmonson, E.Appelman, T.Spiro, S.J. Lippard, J.Am.Chem.Soc., 117 (1995) 4997. 8. S.J. Lippard, Angew.Chem. Int.Ed. Engl., 27 (1988) 344. 9. D.M. Kurtz, Chem. Rev., 90 (1990) 585. 10. K. Yoshizawa, K. Ohta, T. Yamabe, R. Hoffmann, J.Am.Chem.Soc., 119(1997) 12311. 11. R.H., Crabtree, Chem.Rev., 95 (1995) 987. 12. P.E., Siegbahn, R.H., Crabtree, J. Am. Chem. Soc., 119 (1997) 3103. 13. A.K. Rappe, C.J. Casewit, K.S. Colwell, W.A.Goddard, W.M.Skiff, J.Am.Chem.Soc., 114 (1992) 10024. 14. A.K., Rappe, W.A., Goddard, J. Chem. Phys., 95 (1991) 3358. 15. References in the Jaguar User Guide. 16. P.P. Knops-Gerrits, F. Faglioni, W. Goddard III, J. Am. Chem. Soc., submitted. 17. S. Shaik, M. Filatov, D., Schrrder, H., Schwarz, Chem. Eur. J., 1998, 4, 193-199. 18. S. Menage, B.A.Brennan, C.Juarez-Garcia, E.Munck, L.Que, Jr, J.Am.Chem.Soc., 112 (1990) 6423.19. R.G.Wilkins, Chem. Soc. Rev., (1992) 171-178; P.C.Wilkins, R.G.Wilkins, Coord. Chem. Rev., 79 (1987) 195-214. 20. R.E.Norman, R.C.Holz, S.Menage, L.Que, jr., J.O'Connor, Inorg. Chem., 29 (1990) 4629. 21. P.P.Knops-Gerrits, A.Weiss, S.Dick, P.Jacobs, Stud.Surf.Sci.Catal., 1997, 110, 1061. 22. P.P.Knops-Gerrits, A.M.Van Bavel, G.Langouche, P.Jacobs, NATO ASI 3.44 (Derouane et al. Eds.), 1998, 215. 23. P.P.Knops-Gerrits, A.Verberckmoes, R.A.Schoonheydt, M. Ichikawa, P.A. Jacobs, Micro- and Mesoporous Mat., 1998, 21, (4-6) 475.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Quantum-chemical and experimental study of an interaction

between C02 and propylene over Rh-Co/AI2-Oa catalysts

L.B. Shapovalova, G.D. Zakumbaeva, A.V. Gabdrakipov, I.A. Shiygina and A.A. Zhurtbaeva The Institute of Organic Catalysis & Electrochemistry of the Ministry of Science and Higher Education of the Republic of Kazakstan, 142, Kunaev str., Almaty, 480100, Kazakstan

1. INTRODUCTION The studies of catalytic activation of CO2 and its interaction with hydrocarbons intensively increase at present time. Because it needs to decrease of carbon dioxide in atmosphere. Utilization of carbon dioxide can solve the problem of green-house effect and as well as the involving of carbon into circulation for production of artificial oil, different organic compounds and synthesis-gas (CO+H2). Recently the results of study of interaction between hexene-1 or propylene and carbon dioxide on Ru-Co/AI203 have been published [1,2]. In this paper the process of interaction between CO2 and propylene over Rh-Co/AI203 cluster type catalysts has been studied.

2. EXPERIMENTAL The process was carried out in flow type reactor at variable temperatures from 423 to 627K and pressure from 0.1 to 1.5 MPa. The space velocity was 100-120 hr -~. The mixture C3HJCO2=1/4 was used. Catalysts were prepared by impregnation of AI203 support with mixture of RhCI3 and Co(NO3)26H20 solutions. Then they were reduced by hydrogen at 773K during 3 hours, washed from CIx and NO3x ions and dried up in the air at 303-323K. Catalyst was additionally reduced directly in the reactor at temperatures from 473 to 673K during 1 hour before the reaction between CO2 and propylene. The reaction rate was controlled on propylene decrease by using a chromatographic analysis. IR-spectra of reactants adsorbed on catalyst surface were recorded in a UR-75 spectrometer in the 1200-3500 cm -~ range.

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Quantum-chemical calculations have been made on the basis of extended Huckel method complemented with Anderson core-core repulsion as item of total electron energy. Cluster approximation was used for calculations. Minimal quantity of metal atoms (2-7-13) was taken into account. This approach seams a reasonable, because, as is known, chemisorption is high Iocalizated phenomenon.

3. RESULTS AND DISCUSSION It has been established that the conversion of propylene and carbon dioxide depends on catalyst composition and pressure of mixture CO2+propylene. Propylene conversion on Rh-Co(I:I)/AI203 at Tex~=523K and P=I.8MPa is 25.0%. The reaction products are methanol (8.4%), formaldehyde (0.8%), acetone (10.3%), ethanol (8.7%), i-propanol (13.9%), buthanol (1.7%), butyric acid (37.3%), traces of butylaldehyde and other compounds. A decrease of pressure up to 1.5 MPa (Tex,=523K) implies the propylene conversion decreases to 8.3%. Also the product composition changes. The yield of methanol, formaldehyde and i-propanol increases up to 22.7, 3.4 and 31.0% consequently. The yield of acetone and butyric acid decreases to 5.4 and 20.3% at the same time. It was shown that the degree of propylene conversion is 7.9-9.3% when 523 to 673K (P=1.5 MPa). But with temperatures increase the ratio of formed products significantly changes. It is observed the tendency to decrease of yield of methanol from 22.7 to 18.4, formaldehyde from 3.4 to 1.7 and ethanol from 7.9 to 0.7%. The yield of butyric acid changes extremely. The maximum yield reaches 33% at Tex~=573K. Then the yield of butyric acid sharply decreases to 7.3.% at Texp=673K. In these conditions the yield of acetone and i-propanol increases from 5.4 and 31.0 (Tex~=523K) and to 17.5 and 38.3% (Texp=673K) consequently. Propylaldehyde are formed at 623-673K (2.8-4.0%).

Texp increase from

In IR-spectra absorption bands at 3500, 3300 (OH-groups), 3060, 3000 (=CH2- groups), 2980, 2920, 2850 (-CH, -CH2, -CH3 - groups), 2380, 2350 (CO2~g~), 2030 and 1900 cm ~ corresponding to linear and bridge forms of GOads on Rh n+ and Rh~ are presented at chemisorption of CO2 + C3H6mixture (T=473K) on Rh-Co(9:I)/AI203 catalyst. In addition it was observed the intensive absorption bands in the range 1700-1300, 1580 and 1440 cm -~. The absorption bands at 1580 and 1440 cm ~ can be correspond to chemisorbed CO2 molecule. With an increase of Co concentration in catalyst composition to 30-50 mas.% the intensity of absorption bands in the range of 2400-2000 cm ~ increases. The position of absorption bands varies slightly with the temperatures increase from 473 to 573K. However intensity of absorption bands at 1950, 2380-2350 and especially 1550 and 1430 cm -~ significantly increases at the

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adsorption of mixture on Rh-Co(I:I)/AI203 catalyst. These structures are sufficiently strongly bonded with catalyst surface and don't disappear under vacuum. The structure and state of surface of Rh-Co/AI203 catalyst were studied by physico-chemical methods of analysis. The size of metal particles in RhCo(9:1)/AI203 catalyst is 9-10 nm. With increase of cobalt concentration in catalyst composition the particle size decreases. The particle size of RhCo(1:9)/AI203 catalyst is 1.5-2.0 nm. It has been established that Rh and Co form the cluster structure. Their state and stability are determined by ratio Rh:Co.

By quantum-chemical method it has been shown that replacement of rhodium on cobalt in 13-atoms cluster results to increase of cluster stability. It grows with increase of quantity of cobalt atoms. The formation of bimetallic structures in catalyst is more energetically favourably than monometallic ones. The stability of 13-atoms Rh-Co-clusters decreases in the series: 12Co-Rh(~) < 3Co(~,4,7)10Rh <

12Co-Rh(1) < 3CO(ls,8)lORh 2Co(1,7)11Rh < 1Co(1)12Rh <

< 3Co(~,5,4)10Rh < 1Co(s)12Rh < 13Rh

Hexagonal cluster of Co is rather stable. At replacement of Co on Rh atoms the cluster stability is sharply increases. The results of calculations show that the possibility of formation of bimetallic clusters is higher as well as monometallic ones. It is especially characterized for catalysts with high Co content. It needs to note that at location of Rh atom in center of 13atoms Co cluster the maximal stabilization of cluster occurs. By quantum-chemical method the adsorption of carbon dioxide and propylene on 2 and 7-atoms mono- and bimetallic Rh, Co, Rh-Co clusters has been investigated. It has been shown that destruction of C=C bond occurs and CH2 and CH3-CH-fragments are formed at propylene adsorption on the monometallic Rh- centers. Type of adsorption on bimetallic Rh-Coclusters is determined by orientation of C3H~ molecule. It is considered two types of models. The first type of orientation is: C H : group of C3H6 molecule is adsorbed on Rh atom. The second one is: C H : group is adsorbed on Co atom. In the first case the associative adsorption of propylene molecule takes place. In the second case the destruction of propylene molecule with formation of C H : , CH2- and CH- fragments occurs. At propylene adsorption on mono-metallic Co-clusters the formation of weak covalent bonds of CoC3HBoccurs. C=C bond weakens without further destruction (Fig. 1).

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W

OC

LI

-H

Fig.1 The structures obtained at total optimization of propylene molecule geometry It has been observed the complicated dependence of CO2 molecule adsorption on Rh-Co atomic ratio in surface active centers (Fig. 2). Vertical chemisorption of CO2 molecule on the central Rh-atom in 7 atoms' Rh-Coclusters results to associatively adsorbed CO2 molecule. Chemisorption on one of external Rh atoms in such cluster results to the dissociation of CO2 with CO,d, and O,d, formation. Basically carbon dioxide adsorbs associatively on monometallic Rh and Co- clusters. .0

a

oC

b

c

OO

Fig. 2 Total optimization of CO2 molecule geometry on Rh-Co-clusters Carbon dioxide can adsorb on the surface of Rh-Co/AI203 associatively as well as dissociatively. Interaction of associatively adsorbed carbon dioxide with propylene and products of its decomposition results to acid formation. On some centers of bimetallic catalyst propylene is exposed to destruction with formation of C~- and C2-containing radicals. Their interaction with CO2~, CO~, Oa~ results to C~-C3- oxygenates formation. The results of IR-study of chemisorption of CO2 and propylene are correlated to quantum-chemical calculations.

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REFERENCES 1. G.D. Zakumbaeva, L.B. Shapovalova, Japan-FSU Catalysis. Seminar (1994). "Catalytic Science and Technology for 21 Century Life", Japan (1994) 28. 2. G.D. Zakumbaeva, L.B. Shapovalova, Advances in Chemical Conversions for Mitigating Carbon Dioxide. Studies in Surface Science and Catalysis. Elsevier Science B.V.V. 114 (1998) 171. 3. R. Hoffman, An Extended Huckel Theory. J.Chem.Phys. No 6 (1963) 1397

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Ab-initio study o f the v a c a n c y formation in K e g g i n and Linqvist heteropolyanion. J. F. Paul and M. Founier Laboratoire de catalyse de Lille, USTL/CNRS UPRESA8010, batiment C3, F-59655 Villeneube d'Ascq cedex, France.

Heteropolycompounds are interesting materials for the development of new catalysts. They present various redox and acidic properties that can be tuned in respect of the chosen reaction. Heteropolyanions (HPA), presenting important activity in the oxydeshydrogenation (ODH) of alkanes, have been described in the literature. In order to improve our knowledge of the mechanism of the reaction, we performed a first principle study and computed the proton affinity of two of these compounds : the Keggin PMol20403-and the Linqvist M060~92. In a second step, we investigated the possibility to creme defects in these structures. These studies showed that the acidic properties of the two anions are quite similar, while it is much more easy to remove oxygen atom from the Keggin structure than from the Linqvist one. 1 Introduction

Heteropolycoumponds, where the metal atom is generally molybdenum or tungsten, are now extensively used either in homogeneous or heterogeneous catalysis (1,2). Due to their redox and/or acidic properties, they can be used in mild oxidation or strong acidic process. More interesting is the possibility of their properties to be finely monitored in the well known dodecametalate family (XMoIEO40n'),related to the Keggin structure, by changing the central atom X or some of the metal M atoms by others such as vanadium, or more generally, any transition metal atom. These formulation variations lead to a possible adjustable bifunctional behavior. In some oxydehydrogenation reactions, particularly in alkane activation, the mechanism and, more precisely, the ability of oxocompounds to remove proton or hydride ion from the CH bond, remains under debate. The aims of this paper is to perform some theoretical investigations on compact isopolyanion (Linqvist MO60192") a n d on Keggin anion (PMo120403), in order to appreciate the results of oxo-vacancy creation or protonation process onto the stability of these oxoprecursors for new catalytic materials. 2 Computational details.

All the calculations presented in this study, have been performed with the density functional program ADF (3)(Amsterdam density functional). In order to improved the description of the system, we have included GGA corrections (4) in all the calculations. The frozen core

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approximation is used for the oxygen atom l s orbital and up to the 3d orbital for the molybdenum atom. The valence orbitals are described by triple~ basis set. We have added polarization function for the oxygen atoms. All the structures have been fully optimized. We did not include any symmetry constraint for the Linqvist systems. For the Keggin structures, we have conserved a plane symmetry in order to reduce the computational effort. 3 Geometrical results

Ot oxygen atom

gen

gen

Fig. 1 Representation of the Keggin unit. The experimental structures of the Keggin (fig. 1) and Linqvist (fig. 2) anions have been determined by X-ray and Neutron diffraction (5,6). The agreement between the calculated and the measured geometrical parameters is good (table 1). The main characteristics of the anions are reproduced. Detail analysis of the Keggin and Linqvist experimental structures suggest a periodic variation of the Mo-Ob distance into trimere units. This effect is also reproduced into the calculation when no or small symmetry restrictions are imposed. For example, in the Linqvist structure, the Mo-Ob distances in a Mo-Ob-Mo bridge are alternatively 1.91 and 1.95 A. A possible explanation of this distortion has been tracked back (6) to the quite important ionic character of the Mo-O bond. The charge of the various oxygen atoms are summarized into Table 2. The bridging oxygen atoms are always the most charged ones. Electrostatic potential calculation from the electronic density confirmed that the most important fields are located in the vicinity of these bridging oxygen atoms. Others experimental and theoretical results (7,8) confirm this tendency. Furthermore, the HOMO orbital of the two HPA is also mostly located on the bridging oxygen atoms. For the Keggin HPA, this orbital has no contribution on the terminal oxygen atoms.

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Table 1

Keggin (cat)

Keggin(exp.)

Linqvist(cat)

Linqvist(exp.)

Mo-Oa

2.42

2.43

2.33

2.35

Mo-Ob

1.92

1.91

1.93

1.91

Mo-Oc

1.91

1.91

Mo-Ot

1.71

1.66

1.72

1.66

Mo-Ob-Mo

125

124

117

119

Comparison between the computed and measured geometrical parameters. Distances in A and angles in degree. Ot oxygen atom ;en

Fig. 2 Representation of the Linqvist anion In conclusion, all these results indicate that the most basic site of the two HPA should be a bridging oxygen atom. Table 2

Keggin

Linqvist

Mo

2.27

2.21

Ot

-0.65

-0.67

Ob

-0.85

-0.85

Oc

-0.89

Electronic Mulliken charges of the various atoms in Keggin and Linqvist units.

4 Computation of the proton affinity. In order to confirm the previous prediction, we have computed the proton affinity (PA) (1)

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of the various oxygen atoms in the Linqvist and Keggin anions. The PA is the energetic differences of the two members of reaction (2). PA=E(HA) - E(H § -E(A)

( 1)

A + H += HA

(2)

4.1 The Linqvist anion.

Mo60192is a dibasic anion. The PA of the Ot oxygen atoms is -14.99 eV while it is -15.63 eV for the Ob oxygen atoms. The charges on the atoms that are not connected directly to the basic center are only slightly changed during the coordination of the proton. The variation of the electronic density on the Linqvist structure induced by the proton is localized to the first and second neighbors and has very small influence on the rest of the molecule. The coordination of the proton to the Linqvist structure induce small deformations of the anion. The energetical cost of the geometrical displacement are very similar for the two situations (+0.77 eV for the bonding to a Ot oxygen atom and +0.82 to a Ob one). However, these reorganizations have an important effect on the energy and on the shape of the HOMO orbital. This orbital is mostly localized on the oxygen atom that will create the bond with the proton. The bridging oxygen atom is more affected by this localization than on the terminal ones. We have also computed the PA for the second acidity of HMo6OI9. The results of the various possible isomers are summarized in table 3. The interactions between the two protons are small. The PA of a second bridging oxygen atoms is mostly the same whatever the distance between the two protons. The differences between the various tested situations, with at least one proton connected to one Ob oxygen atom, are smaller than 0.20 eV, except when the two protons are bound to the same oxygen atom. In this case, the second PA is reduced by 0.30 eV. The same effects arise when the two proton are connected to Ot oxygen atoms, but proton-proton interactions are even smaller. The variations between the isomers are smaller than 0.10 eV. Table3. 2 ~d P A

1 ~ H on O t a t o m

I n H on O b a t o m

2 "a H on Ot a t o m

-11.63

-11.57

2 "a H on Ob a t o m

-12.21

-12.01

2 nd PA, in eV, for the Linqvist structure. The most stable isomer is considered.

4.2 The Keggin anion. The Keggin structure also presents important acidic characteristics. We have computed the first Pa for the three types of oxygen atoms that are located on the outer shell of the anions (Table 4). The bridging sites are more basic than the top ones. Unfortunately, the energetic difference between the Ob and Oc sites is relatively small and the stability order may be changed by interactions with the cation or the solvent. The calculations have been carried on the whole structure. Previous calculations (7), on part of the Keggin anion lead also to the same order of oxygen basicity. Experimental NMR measurements of the P-H distance conclude

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that the proton is bonded to the Ob atom (9). Table 4

Site

Ob

Oc

Ot

PA

- 15.84

- 16.02

- 15.18

First PA, in eV, of the various oxygen atoms in a Keggin unit. In order to reduce the computational effort, we have only calculated the second and third PA of the Oc and Ot oxygen atoms. We have only tested positions of the hydrogen atoms that allow the conservation of the Cs symmetry. The second and third PA of the Ot oxygen atoms are respectively - 13.73 eV and-11.24 eV. The O-H bond distance is always the same and is not affected by the total charge of the Keggin HPA. The decrease of the PA reflects the diminution of the electrostatic interactions. The values of the PA on the Oc oxygen atom are 14.18 eV and -11.47 eV. The difference between the two sites is reduced as the number of protons increases All these results show, experimentally and theoretically, that the most basic oxygen atoms are the bridging one in Linqvist and Keggin polyoxoanions.

5 Vacancy creation. The behavior of the two HPA toward the acidic properties are very similar. Furthermore, the geometrical parameters are also similar. But it is known experimentally that Keggin HPA may be a good catalyst while the corresponding Linqvist structures are slightly inactive. One of the postulated mechanism of the alkane oxydeshygenation over metallic oxide catalyst involves the creation of vacancy into the catalyst and the mobility of the oxygen atoms through the solide phase (Mars and Van Krevelen mechanism).The activation of the catalyst implies the departure of water molecules and the creation of vacancy into the HPA without reduction. We have studied this part of the reaction in order to know if the structure of the catalyst can be conserved in the process and also to pinpoint differences between the two systems. This vacancy creation energy is related to the reaction (3). protonated HPA = Vacancy + HO

5.1 Vacancy creation into the

(3)

M060192"anion.

The removed oxygen atom may also be a bridging or a terminal one. In the gas phase, the calculation suggests that the Ob oxygen atom is the easiest to take away. The removal of one oxygen atom from the Linqvsit structure is an endothermic process. The calculated energy variations, according to the reaction (3) is 6.67 eV for the Ob oxygen atom and 7.60 eV for the Ot oxygen.,These values take into account the difference in the acidic properties of the two kinds of oxgen atoms. The Linqvist anion without one Ot oxygen atom is 1.57 eV less stable than the Linqvist anion without one Ob oxygen atom. The geometry of the Linqvist structure without one Ot atom is very similar to the geometry of the perfect Linqvist. There is only a small relaxation of the molybdenum atom toward the center of the anion. When one Ob oxygen atom is removed, the relaxation is more important. The oxygen atom in the center of the

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Linqvist moves into the vacancy. The energetic effects of this relaxation are not important, 1.11 and 1.39 eV respectively for Ot and Ob removal.

5.2 Vacancy creation into the Keggin structure. We have computed the energy of vacancy creation into the Keggin structure for the three kinds of oxygen atom Ob, Oc and Ot. The reaction, as in the Linqvist case, is endothermic. The cost for the oxygen removal is 5.30 eV for the Ot atom, 4.28 eV for the Oc atom and 4.29 eV for the Ob atom. It is much more easy to remove atoms from the Keggin structure than for the Linqvist one. The relaxation of the Keggin anion after the abstraction of the oxygen atom is relatively important and decreases the energy of the resulting anion by 1.54 eV for the Ot atom, 2.85 eV for the Ob atom and 2.55 for the Oc one. In this case, the molybdenum atoms move toward in order to adopt a classical geometry for a five coordinated atom: the triangle bipyramid. Indeed, the Keggin structure is much more flexible than the Linqvist one. The relative facility to remove oxygen atom and so, to create vacancy, seems to be due to this property.

6 Conclusion The calculations reported in this paper, show that the most reactive oxygen atoms of the Keggin and of the Linqvist heteropolyanions are the bridging one. Indeed, these oxygen atoms are the more basic ones in the two structures. However, for the Keggin structure the difference in the proton affinity between the various kinds of oxygen atoms decreases during the successive protonations. We could not exclude that the electrostatic effect or hydrogen bonding will not favor the coordination of the third proton on one Ot oxygen atom. Moreover, these bridging oxygen atoms are the easiest to remove. The important difference of the energy of abstraction between the Linqvist and Keggin HPA, may be related to the higher flexibility of the Keggin unit. So the reactivity in ODH catalyst, or at least the activation, of these compounds seems to the related to the vacancy creation. If only the acidic properties were involved, Keggin and Linqvist heteropolycompounds would have presented similar catalytic behavior. References. 1. T. Okuhara, N. Mizuno, M. Misono, Adv. In Catal. 41 (1996) 113 2. I. V. Kozhevnikov, Catal. Rev. Sci. Eng. 37 (1995) 311 3. E. J. Baerends et al., Chem. Phys. 2 (1973) 41 4. J. P. Perdew, Phys. Rev. B 46 (1992) 6671, 5. H. R. D'Amour, R. Allamann, Z. Krist. 143 (1976) 1 6. H. R. Allcock, E. C. Bissel, E. T. Shawl, Inorg. Chem. 12 (1973) 2963 7. B. B. Bardin, S. V. Bordawekar, M. Neurock, R. J. Davis, J. Phys. Chem. B. 102 (1998) 10817 8. A. Dolbecq, A. Guirauden, M. Fourmigue, K. Boubekeur, P. Batail, M.-M. Rohmer, M. Benard, C. Coulon, M. Salle,P. Blanchard, J. Chem. Soc Dalton Trans. 8 (1999) 1241 9. S. Ganapathy To be published

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Theoretical Studies of the Interaction of Butane and Butene Isomers in HF e r r i e r i t e and H-Mordenite D. E. Galindo, M. J. Goncalves and Maria M. Ramirez-Corredores. PDVSA-INTEVEP, S.A. Departamento de Procesos. Apdo. 76343. Caracas, 1070. Venezuela. e-mail: [email protected]. The conversion of light paraffins into reactive olefins is becoming more important in view of the increasing requirements of environmental rules. The study of their diffusion and sorption behaviour in zeolites, typically employed as catalysts, might contribute to the understanding of their transformation. We have approach this study by computational means. The six C4 isomers, n- and iso-butane, and the n- (1-, cis- and trans-) iso-butene in two protonated zeolitic structures: ferrierite (H-Fer) and mordenite (H-Mor) were considered. It was used a simulation strategy which combines molecular mechanics (MM) and dynamics (MD), to provide information about the energetics of sorption and the diffusion properties of the organic molecules. The results, when qualitatively compared, could be extrapolated to explain the catalytic properties of the two solids on the paraffin and olefin molecules. In particular, they provide a rationale for the experimentally observed very high selectivity of H-Fer for the skeletal isomerization of nbutenes, as well as its rapid deactivation. These findings are of relevance in understanding the considerable activity of zeolites in the conversion of paraffins and olefins. Both, sorption and diffusion are controlling factors in the final selectivity of the zeolite catalyst. 1. INTRODUCTION The vapour pressure of gasoline has to be kept as low as possible for environmental protection seasons. Its control requires to keep the light hydrocarbon concentration at low levels. However, withdrawing the C4-C5 cut from the gasoline will decrease the production volume. So, their transformation into value added products or reactive intermediates would be highly advantageous. Reformulated gasoline (RFG) production is one of the obvious alternatives. Branched ethers, such as MTBE and TAME, despite of being under questioning, are important components in the formulation of these products [1]. These oxygenated compounds are prepared from either isobutene or isopentene and methanol in a direct way. The availability of iso-olefins is a limitation for the expansion of the present production of MTBE and TAME. The skeletal isomerization of n-olefins to iso-olefins, is a necessary process for increasing the production of branched ethers. The commonly used, acid catalysis isomerization has the inconvenient that the undesired dimerization/oligomerization of butenes occur to a large extent, decreasing the yield and causing catalyst deactivation [2]. The improvement of catalyst selectivity for the skeletal isomerization reaction has to be, thereby, approached reducing the formation of dimers/oligomers. The H-Fer zeolite is well known as a highly selective catalyst for the skeletal isomerization of olefins whose selectivity has been shown [3] to depend on two factors: the availability of void space in the solid's channels

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(shape selectivity) and its moderate acidity. Both factors are believe to enhance skeletal isomerization while preventing the formation of oligomers. As an alternative to the direct isomerization of n-olefins, one could think on a two-step process where n-paraffins are firstly isomerized to iso-paraffins, followed by dehydrogenation with production of the iso-olefins. A convenient catalyst for isomerization of paraffins is the zeolite H-Mor [4]. The conversion of n-butane on H-Mor involves both isomerization, to produce isobutane, and disproportionation, to produce propane and pentane. The mechanisms for both reactions might proceed via dimeric intermediates. We have performed a computational study of the diffusion and sorption processes of both paraffinic and olefinic compounds on the two considered zeolites. The goal has been to provide insights of some of the experimental observations regarding catalytic activity and selectivity. The calculations are of a limited nature, given the complexity of these systems, and aim to generate information about the energetics of sorption and diffusion for both, reactants and products of the isomerization reactions into the channels and cavities of the microporous solids. 2. MODEL AND COMPUTATIONAL METHODS All calculations were performed on a SG-IndigoH using the Catalysis software from MSI [5] which provide codes for lattice energy minimisation and MM, MD and Montecarlo simulations. 2.1. Overview of the model The standard model for catalytic activity in a solid material assumes that the critical catalytic event takes place at specific reactions centres within the solid structure. In the case of a porous material it is therefore of paramount importance the determination of the threedimensional structure because the reaction centres are located in a very intricate network of channels and cavities that must be accessed by the reactant. Regarding zeolites, those centres are supposed to be adjacent or very close to the substitutional elements, typically aluminium. This consideration justifies the study of self-diffusion coefficients for the involved molecules as well as activation barriers for diffusion along specific paths, but also a crucial magnitude in pointing out suitable candidates for reaction centres is the sorption energy, i.e., the binding energy of the reactant to the specific location. Considering that higher interaction between the reactant and the solid, relates to large energy values, the possible reaction centres could be identified by screening the host-guess interactions throughout the channels. Furthermore, a reasonable assumption might then be made that larger binding energies translate into longer residence times, thereby enhancing reaction probability. Once the reaction has occurred, the products must undergo a desorption process and subsequent diffusion from the reaction centre into the pore or channel. Again, sorption energies and diffusivities give indication of the facility of removal of a given product. Large differences in sorption energies between different products can translate into large selectivity toward the product with the smallest binding energy, i.e., the one that would be most easily desorbed. On the other hand, relatively small diffusion coefficients for a given product would imply larger residence times and an increased probability for further reactions to occur, bringing about the occurrence of undesired reactions leading to deactivation processes, e.g., coke formation.

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2.2. Lattice energy minimisation

The initial structures for H-Fer and H-Mor were taken from previous work [6]. We tried three different structures for H-Mor and two for H-Fer by changing the AI siting within the framework and the H- position between the two possible bridging oxygen atoms. Finally, those having the lowest lattice energy were adopted. It is important to remark that the resulting protonated zeolite is a neutral structure.

2.3. Sorption energetics The location of sites with high sorption energy values for both, reactant and product molecules, within the cavities of the two zeolites was found with a methodology that combines MD, Monte Carlo and energy minimisation techniques [7]. A library of conformation of the sorbates was first generated with a high temperature MD and the resulting conformations are subsequently sampled in Monte Carlo docking. This procedure searches automatically for low energy host-guest structures according to an energy threshold. The dominant energy contributions are of the van der Waals type due to the neutral character of both the trial adsorbate conformation and the zeolite walls. Once the docking procedure is completed, the energy of each of the docked structures is minimised to allow the sorbate molecule to optimise its interactions with the zeolite framework. The molecule then reaches the nearest low energy sorption site while its energy is minimised with respect to the internal degrees of freedom and the short range non-bonding interactions with the pore wall, described by a potential energy function of the Lennard-Jones type. This procedure, which leads to the binding energy of the adsorbate to the reaction centre candidates e.g., the minimum energy site, uses the MSI MM code included in Discover [5] with the forcefield cff9 l_pcff[7].

2.4. Diffusion properties Self-diffusion coefficients are determined in the conventional way in a MD simulation. Minimum energy pathways for diffusion, also sample pre-determined paths in the zeolite structure, are determined by simulating the forced diffusion process. As a result, the energy variation'of the system during the diffusion of a guest in a given host can be determined, thereby rendering the average energy barriers involved in diffusion. In such a procedure, the sorbate molecule is forced to move stepwise along a given direction. At each point of the trajectory, the molecule is constrained to lie at a fixed distance from the selected origin, while its conformation and energy are minimised with respect to the internal degrees of freedom and to the short range non-bonding interactions, described by a Lennard-Jones potential, with the pore wall. The forcefield used in this procedure was the cvff [5]. 3. RESULTS AND DISCUSSION Two types of adsorption sites are found in H-Fer: one in the main 10-T channels and the other in cavities along the 8-T channels. The cavity associated with the smaller channel is suitable for lodging, both size- and energywise, the molecule of iso-butene, which is of relevance in connection with the mechanism of deactivation as discussed below. Table 1 contains the binding energies for the different sorbates and the two solids. As can be seen from there, on the basis stated in Section II, H-Mor does not seem to discriminate between the two types of adsorbates, paraffinic and olefinic species, whereas H-Fer shows a clear tendency to stabilise better paraffins than olefins. In the case of olefins, the binding energies found show the high selectivity of H-Fer to the isomerization of n-butene to isobutene, because the latter has a lower binding energy compared to the other isomers. This is

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plausibly related to the experimental observation that H-Fer tends to be more active for olefin isomerization than H-Mor [3,4]. Two classes of adsorption sites are found, in the main 10-T channels and in cavities along the 8-T channels, n-Butene, iso-butene, cis- and trans- butene are located in the 10-T channel, while the iso-butane goes to the 8-T channels and the nbutane to the intersection between the 10-T and 8-T channels. It is important to remark that for all the isomers the second minimum energy site is always located in the 8-T channel. In summary, regarding selectivity, the binding energy values shown in Table 1 suggest that of all the three different products considered for olefin isomerization, it is iso-butene the most likely candidate for the highest yield. This result apparently confirms the experimental observation about the high selectivity of H-Fer. No similar behaviour is suggested for H-Mor, where the isobutene has a higher binding energy value compare to the other isomers. Table 1. Binding energy (BE) and Energy barriers (EB) (kcal/mol) Isomer

H-Fer BE

n-butane iso-butane 1-butene iso-butene cis-butene Trans-butene

-14 -13 - 11 -5 -6 -8

EB 8T-10T 10T -8T 6 8 85 54 5 4 51 18 5 -4 -6 - 11

H-Mor BE EB -22 -25 - 19 -32 - 18 - 10

-3 2 -2 5 6 4

Table 1 contains the energy barrier values for the different sorbates interacting with the two solids. In the system H-Fer the chosen diffusion path was initiated from the minimum energy position in the 8-T channel. So, the first energy barrier which the molecule has to pass is from the 8-T to the 10-T channel and the second corresponds to passing backwards. These diffusion'path was taken on the basis that the two minimum energy host-guess configurations locate the sorbate molecules in the 10-T and in the 8-T channel, respectively. The energy barriers for the iso-molecules for passing from the 8-T to the 10-T channels are very high compared to those found for the other molecules. The steric hindrance for the branched molecules to leave the 8 member rings is larger than that of the linear molecules. Comparing the values of the energy barriers to that of kT, one might conclude that even at 500 ~ the kinetic energy of the molecules is not enough to overcome the energy barrier for the studied path. It is worth mentioning that this is one of the many possible paths through the H-Fer structure. If the isomerization of n-butane or 1-butene takes place in the confinement of the 8 members ring, the reaction products, iso-butane or iso-butene become trapped and probably secondary reactions could occur. The large energy diffusion barrier for the iso-molecules might be associated with a long residence time within the 8-T ring. So, the probability for a second molecule to entry the same ring might be high and side reactions such as dimerization and coke formation could take place. This fact offers an explanation for the quick deactivation of H-Fer during isomerization reactions. The same trends and behaviour can be extrapolated from the examination of the selfdiffusion coefficients showed in Table 2, for the different reactant and product molecules.

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Although the diffusion time (200 fs) is shorter than the usual values reported in the literature [9], the results seem to provide a simple explanation for the observed fast deactivation of HFer. As expected, linear molecules diffuse faster than branched ones. The fact that the most likely product, in isomerization with H-Fer, iso-butene, has one of the lowest diffusion coefficients points in the direction that dwelling times in the lateral channel are longer. This results in an increase probability for dimerization/oligomerization reactions which are the initial steps in coke formation that eventually leads to catalyst deactivation. The high energy barriers and the low self-diffusion coefficients for the iso-molecules seems to indicate that there is a high probability for these molecules to stay for long time in the 8-T ring which can provoke secondary reaction such as dimerization and oligomerization and can explain the quick deactivation of H-Fer during the isomerization reaction.

Table 2. Self-diffusion coefficients (10 -4 cm2/s), 500 ~ Isomer n-butane Iso-butane 1-butene Iso-butene Cis-butene Trans-butene

H-Fer 0,030 0,015 0,136 0,096 0,870 2,640

H-Mor 1,243 1,815 1,036 0,908 2,546 6,363

Paraffins and olefins did not show the significant difference in their interaction energetics with H-Mor, as that found in the case of H-Fer. Iso-butene interacts more strongly with the structure than the linear butenes, so one might expect H-Mor to be selective to cisand trans-isomers, which might leave the channel system faster. On the other hand, iso-butene might undergo oligomerization because its strong interaction with the structure, could be related to a longer residence time. In the same way, and for the same reason iso-butane might undergo "undesired cracking, probably leading to coke formation. Consequently, the acid reactions of light hydrocarbons on H-Mor should lead to fast deactivation by pore blocking, as has been reported experimentally [4]. Two minimum energy conformations were found for H-Mor, both located in the main channel. The selected diffusion path goes through the main channel, which contains two protons in it. The energy barriers associated to that path show no limitation for any of the molecules particularly as those found in H-Fer. Similarly of what has been suggested for the H-Fer case, on the basis of the selfdiffusion coefficients, for H-Mor the double bond shift would be the most preferred reaction of olefins, in agreement with the already mentioned results. Although, the computational study could only explain the selectivity of H-Mor towards light olefins conversion and not the observed one for paraffins, it could also explain the preponderance of undesired side reactions. 4. CONCLUSIONS Notwithstanding, only quantum chemical calculations are the valid vehicle for the study of reactivity, the use of sorption and diffusion energetics could also provide important insights for the understanding of other catalytic properties, such as stability and selectivity.

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We have demonstrated, how the combination of MM, MD and MC simulations can strategically be used in the explanation of selectivity and stability of zeolite catalysts. Particularly, the high selectivity and fast deactivation of H-Fer for the skeletal isomerization of olefins and also the deactivation of H-Mor during the transformation of light hydrocarbons. The methodology could not distinguish which could be the preferred selectivity of H-Mor in the conversion of n-paraffins, but it was good enough to predict the possibility of double bond shift for olefins. Using this methodology for screening reacting conformations is highly recommended, since large (more realistic) systems can be considered. The so selected conformers can then be used for more precise QM calculations, in a complete reactivity or mechanistic study.

Acknowledgement The authors acknowledge PDVSA-INTEVEP for permission to publish this work and to PDVSA for its finance support.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.

I.E. Maxwell; J. E., Naber, Catalysis Letters 12 (1992) 105. V.R. Choudhary, Chem. Ind. Dev. 8 (1974) 32. W. Q Xu; Y. G Yin; S. L Suib; C. L O'Young, J. Phys. Chem. 99 (1995) 758. R. Asuquo; G. Eder-Mirht; J. Lercher, Journal of Catalysis, 155 (1995) 376-382. Catalysis 4.00 and Discover 4.00 User Guide, September 1996. San Diego. Molecular Simulations Inc. (MSI), 1996. D. Galindo; M. Goncalves and M. M. Ramirez, INT-03262,96, INTEVEP S. A, 1996. C.M. Freeman; C. R. A Catlow; J. M Thomas and S. Brose, Chem. Phys. Lett. 186, 137 1991. J. R Hill. and J. Sauer, J. Phys. Chem. 98 (1995) 9536. L. Leherte; J. M. Andre; E. G. Derouane and D. P. Vercauteren, J. Chem. Soc. Faraday Trans. 87 (1991) 1959.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Theoretical Investigation of the Intrinsic Reaction Path (IRC) of the Methylation of Mono-substituted Aromatic Molecules over Faujasite Catalysts K.H.L. Nulens, A.M. Vos, G.O.A. Janssens, R.A. Schoonheydt Cenm~n voor Oppervlaktechemie en Katalyse (C.O.K.), K.U.Leuven, K. Mercierlaan 92, 3001 Leuven, Belgium

Abstract

In this work, we have investigated the methylation of benzene, toluene nitrobenzene. A methoxide, formed by dehydration of methanol, is the methylating agent. Semi-empirical method, AM l, was applied to calculate the entire iRC of the reactions over the cluster H3SiO(H)AI(OSiH3)3. In all cases one transition state was found along the reaction path, but bond breaking and bond forming do not occur simultaneously, it is possible to predict the order of the reactivity between the different sites in an aromatic molecule (ortho, meta or park) and it is suitable to compare the reactivity between different aromatic molecules (toluene and nitrobenzene), in the second part we used the Electronegativity Equalisation Method (EEM) to predict bond breaking and forming and to see if a carbenium ion is formed during the reaction.

1. Introduction In homogeneous conditions a two step mechanism is proposed for the Electrophilic Aromatic Substitution (EAS) catalyzed by e.g. metal halides [ 1]. The first step consists of the attack of the electrophile on the substrate and the second step is the departure of the leaving group. This requires two Transition States (TS). Acid zeolites are valuable alternatives for the environmentally unfriendly homogeneous catalysts. Although EAS is an industrially important reaction, theoretical investigations of EAS over acid zeolites are rare. Corma et al. [2,3] investigated the Intrinsic Reaction Coordinate (IRC) for the methylation of benzene over an acid zeolite. For the first step, the attack of the electrophile, a TS was found. After this TS a stable Wheland intermediate is formed but no second Transition State (TS), to expel the proton, was found. They also investigated the influence of other characteristics which influence the reactivity/selectivity of EAS such as the dimensions of the zeolite pores (confined spaces) [4] and the acid softness and hardness of the zeolite [5]. Blaszkowski and van Santen [6] investigated the methylation of benzene and toluene, over acid zeolites. As Corma et al. [4] they found one TS and the reactivity and selectivity follow the order ortho/para > meta for toluene, as was found experimentally. This work presents a theoretical investigation to obtain fundamental insight in the mechanism of EAS over acid zeolites, more specifically whether a one-step (concerted)

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or two-step mechanism is operative. To find this, the complete Intrinsic Reaction Co-ordinate (IRC) is calculated.

2. AMl-method The systems under investigation consist of a cluster and an aromatic molecule. The cluster is composed of 4 Si tetrahedra and one A1 tetrahedron. These tetrahedra are terminated with hydrogens. The bridging hydroxyl is replaced by a methoxide, which acts as the electrophile. The aromatic molecules under study are benzene, toluene and nitrobenzene. The adsorption of the aromatic molecules on the methoxide, the Transition State (TS) of the EAS reaction and the adsorption of the methylated aromatic molecules are studied at AMi level. These three states are linked via the intrinsic Reaction Coordinate (iRC). it is the coordinate of steepest descent in energy which links the TS with the reagents and products and is calculated with the TS as starting point.

2.1. Dehydration of Methanol The Transition State (TS) is CH3OH2+ with a longer C-O bond, 2.13 A, than in methanol. However there is no interaction between the oxygen of the cluster and the protons of the activated complex, CH3OH~+ (fig l a). This is not in agreement with the literature where a proton still has some interaction with an oxygen of the cluster [7]. After the TS the C-O bond breaks, water is expelled and a methoxide is formed. Fig.l b, which gives the energy change along the reaction path during the reaction, shows that this reaction occurs according to a one-step mechanism with one activation energy of 262.53 kJ/mol. There is only one maximum. Another interesting feature is the position of the bond breaking and formation along the reaction path (fig. 2a). Before the TS is reached, the Or bond is lengthened and the Hmah-Om~ bond is shortened, indicative of the jump of the acid proton of the bridging hydroxyl towards the methanol. After the TS the Cm~h-O~,~te~ distance is shortened and the Cmeth-Omethbond is lengthened, indicative of methoxide formation. As there is no simultaneous breaking and forming of bonds, it is - strictly speaking - not a concerted reaction. Fig. 2b presents the change of the sum of the three HCH angles of the methyl fragment: in the adsorption stage of the reagents and products, the sum equals +/- 327 ~ la

lb

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fig. 1: a; Transition State, b; energy for the dehydration of methanol

20

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fig2: a; bond distance, b; sum HCH-angles for the dehydration of methanol characteristic for sp3-hybridisation. Exactly after theTS, the sum is 360 ~ characteristic for sp2-hybridisation. The methyl has a planar geometry, typical for a carbenium ion. This carbenium ion forms a covalent bond with the lattice oxygen. 2.2. Methyiation of benzene The geometry of the TS of the methylation of benzene is given in fig. 3a and is similar

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80

length

[Bohr.am~]

41.311 -20

i

0

I

2O

t

4O

l

6O

l~th length[l~hr.~u]~ 1

fig. 3: a; geometry, b; energy, c; bond distance, d; charge for the methylation of benzene

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化

4a

01 0.1l 4b

~tra. .. ............... ortho '

.-.

-

--

::

' : : ; i

---

, ,

0.0

-, .,

-0.3

!l

,

|

,

|

,

o

20

40

6O

path length[

\'

-0.2

-20

I.....

6

L : : ........

_I-

i~o

3 4

~

_ ..............................

\,

.-0.2

~i'

~,-o.o5~

............ -~.I

i

.'

,

.~

~,

0.0

~0

"021f -0.3 -40

Bohr.~rnJ/~

I

-20

,

l

:[ ,

0

I

20

,

112

pathl~,th[Bohr.amu ]

I

40

fig. 4: charge distribution in toluene and nitrobenzene during the reaction with the one obtained by Blaszkowski et al. [6]. No interaction exists between the proton that will be expelled and an oxygen of the cluster (fig.3a). At the TS, the Cm~h-Octus and Cm~Cb~z~e are lengthened to respectively 2.29/~ and 1.87/~. Fig. 3b,c show the change of the energy and bonds during the methylation of benzene. They indicate a non-concerted mechanism with breaking and formation of bonds at two different positions on the reaction coordinate. First the Oeluster-fmahylbond is broken and the methyl jumps towards the benzene. In the second step, benzene expels a proton towards the cluster and restores its aromaticity. Fig.3a shows that at the TS, the leaving group, a proton, is not in the neighborhood of the bridging oxygen. The activated system has to rotate and translate before it can expel the proton. This explains the slow decrease of the energy exactly after the TS. During this process the molecule is a carbenium like ion. During the reaction, an ion pair is formed with a positively charged activated complex (methylated benzene). The charge is stabilized by the aromatic system, mainly by the carbon atoms in para and ortho positions (fig. 3d). After the reaction, these carbon atoms become negative again. This is due to the fact that methyl is ortho/para directing.

2.3. influence of the substituent Substituents, which activate the aromatic ring, e.g. CH3, or deactivate the ring, e.g. NO2, have no influence on the mechanism of the methylation. For both systems, the methylation occurs in the most favored position, respectively para and meta. Their influence on the charge during the reaction can be followed in fig. 4a and b. Comparing the methylation of benzene, fig.3c, to that of toluene and nitrobenzene, we see that the charge variation, on the carbon atoms, is similar for all the systems. For toluene, the differences are more pronounced due to the electron donating effect of the second methyl group. In the case of nitrobenzene, the difference is less pronounced due to the deactivation of the aromatic system by the nitro

table I: Activation Energy (IU/mol), EA, for the methylation of toluene and nitrobenzene toluene ortho 259

meta 266

nitrobenzene para 260

ortho 322

meta 302

para 308

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group. The effect of the -CH3 or -NO2 substituents diminish due to the positive charge of the methylated carbon during the reaction. It dominates the charge distribution in the aromatic system. Table I presents the influence of the substituents on the activation energy, EA. Comparing the results of benzene and toluene with those of Blaszkowski et al. [7] we can conclude that AMI overestimates the EA as expected. However the order is preserved. Although AM I is a quite modest level for calculations, the experimental trends are reflected well in the calculated activation energies. Thus the reactivity/selectivity follows the order ortho > para > meta for toluene ad meta > para > ortho for nitrobenzene.

3. EEM The intrinsic Reaction Coordinate (iRC) is transformed into an Electronic Reaction Coordinate (ERC) within the frame of the Electronegativity Equalization Method (EEM). This ERC is characterized by its principal hardness, h, and the corresponding Polarization and Charge Transfer Channel (P&CT) [8,9]. The CT&P channels describe the redistribution of the charge over the atoms. The principal hardness indicates the difficulty how such a redistribution can be achieved. The higher the principal hardness, the more difficult the ERC is and thus the more difficult the reaction is. The geometry, needed for the EEM calculations, is obtained from the IRC calculations. For each point of the ERC, three successive points of the iRC are required [ i 0]. Two points (first and third) are needed to calculate the charge difference, dq. From the second point, we can derive the principal hardness, h, and the relative changes in the electronic population, U. With these characteristics, the energy, needed for the charge redistribution, can be calculated (eq. i),

dE = 2 i~hi(dqU~)2

U~ the ith transposed vector of U

(1)

and the energetical contribution of each mode, eq. 2, at that moment in the ERC: lhi (dquir)2 dE i _ 2 ~lh~(dqU~) 2

(2)

EEM is restricted to atomic level [1 i, 12]. This implies that the number of modes equals the number of atoms. it is impossible to discuss all the modes in the system, so only the two most important are given. The energy, in %, of these modes, is given in fig.5 for the dehydration of methanol. The mode in fig. 5a presents the polarization channel in the O-H bonds of the system. This includes the polarization channel of the O-H bond in methanol as well as the one in the bridging hydroxyl. The position of these peaks in fig.5a is the same as the position of the jump of the acid proton towards methanol in fig.2b. The mode in fig.5b is important in two places during the reaction. The first peak is due to the polarization in Cmethanol'Omethanol and the second due to the P&CT in CH3. when CH3 has the characteristics of a carbenium ion. We can conclude that, it is possible with EEM to predict when a bond is formed or broken.

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100

100

5a

化

go-"

"--' >, OI =" 0r a)

5b

90-

so -"

80 -J

70 -"

70 -4

O0 -"

80

50 -

A

40

50 1

30

Q

20 -"

o-

-~o

~o~ ,oJ

I

lo-"

-;o

-;o -;o o io path length [Bohr.amu la]

30

2'o

.~o

_;o '-~o

-,'o

~

,'o

path length [Bohr.amu la]

2'0 ' 3~

fig.5: Charge Transfer and Polarisation channels f o r the dehydration o f methanol

4. Conclusion

Although AMi is a quite modest level of calculation, it is possible to investigate whether EAS occurs according a one- or two-step mechanism, it is also possible to predict the order of the reactivity between the different sites in an aromatic molecule (ortho, meta or para) and it is suitable to compare the reactivity between different aromatic molecules (toluene and nitrobenzene). EEM is capable to predict bond breaking and forming and it is possible to see if a carbenium ion is formed during the reaction. References

1. J. March, Advanced Organic Chemistry: reactions, mechanisms and structure, Wiley New York (N.Y.), i992 2. A. Corma, G. Sastre, P. Viruela, Stud. Surf. Sci. Catal., 84. 1997. 2171 3. A. Corma, G. Sastre, P. Viruela, J. Moi. Catai. A, 100, 1995, 75 4. A. Corma, H. Garcia, G. Sastre, M. Viruela, J. Phys. Chem., 101, i997, 4575 5. A. Corma, F. Llopis, P. Virueia, C. Zicovich-Wilson, J. Am. Chem. Sot., i i 6, i 994, 2171 6. S. R. Blaskowski, R. A. van Santen, D., Transition State Modeling for Catalysis, G. Truhlar and K. Morokuma, eds., Oxford University Press,1999,307 7. S.R. Blaskowski, R. A. van Santen, J. Phys. Chem. B, 10 i, 1997, 2292 8. B. G. Baekelandt, G. O. A. Janssens, H. Toufar, W. J. Mortier, R. A. Schoonheydt, R. F. Nalewajski, J. Phys. Chem., 1995, 99, 9784 9. B. G. Baekelandt, G. O. A. Janssens, H. Toufar, W. J. Mortier, R. A. Schoonheydt, Acidity and Basicity of Solids: Theory, Assessment and Utility, J. Fraissard and L. Ptralds, eds., Kluwer Academic Publishers, Nato ASi Series, voi C444, i 994, 95 10. G.O.A. Janssens, H. Toufar, B.G. Baekelandt, W.J. Mortier, R.A. Schoonheydt, Stud. Sci. Catal., 105, 1997, 725 11. W. J. Mortier, S. K. Ghosh, S. Shankar, J. Am. Chem. Soc., i 08, i 986, 43 i 5 12. K. Van Genechten, W. J. Mortier, P. Geerlings, J. C. S. ,Chem. Commun., 1986, i278

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Catalysts for combustion of halogenated volatile organic compounds J. Trawczyfiski, J. Walendziewski, M. Kuta~,fiski Institute of Chemistry and Technology of Petroleum and Coal Wrodaw University of Technology 50-344 Wrodaw, ul. Gdafiska 7/9, Poland, fax: (+48-71) 320-35-51 e-mail: [email protected] The effect of the support and active phase composition on catalyst activity in the reaction of trichloroethylene combustion was investigated. It was stated that silica-titania based cartier is the most resistant against destruction by hydrogen chloride and chlorine evolved in the course of the reaction. Application of chromium exchanged Y zeolite gives very active however unstable catalyst. Chromium oxides-containing catalysts quickly loss activity due to vaporisation of chlorine derivative of chromium. The most active and stable appeared to be palladium catalyst supported on the silica-titania carrier.

1. INTRODUCTION The ceaselessly increasing manufacture and consumption of halogenated (mainly chlorinated) volatile organic compounds (VOC's and HVOC's) is bringing about a growing concern for the proper disposal and destruction of these hazardous wastes. Among various methods of destruction applicable for halogenated VOC's, catalytic combustion has been more recently considered. Although total catalytic oxidation of VOC's is well known process, however such process for halogenated VOC's still is not well known and presents new problem. In the case of chlorinated VOC's combustion the desired reaction is the total oxidation to CO2, H20 and HCI without formation of by products including the toxic dioxins that can result from incomplete combustion of parent compound. However H-VOC's are usually more difficult to destroy than VOC's and HC1 (C12) c a n attack and deactivate the catalyst [ 1,2]. The paper describe the influence of a support as well as a active phase type on catalytic activity of chromium and noble metals based catalysts in combustion reaction of trichloroethylene (TCE). Effect of the reaction parameters on the catalytic activity those catalysts has been investigated. 2. EXPERIMENTAL Catalysts were prepared by conventional method, involving the deposition of the active components onto the supports by dry impregnation. The catalyst supports were prepared using natural clays (aluminosilicates) and mixed titanium and silicon oxides (SM carrier). Natural clays D, G and K (aluminosilicates) delivered from the selected Polish deposits were used as a binder for forming of the zeolite based catalysts as well as catalyst supports. Some catalysts were prepared from chromium exchanged Y zeolite. Chromium exchanging of Y zeolite (Slovnait-Vurup a.s., Bratislava) was carried out in two steps - raw

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zeolite with solution of ammonium salt and then NH4+-Y zeolite with diluted CrCI3 solution. Resulted Cr-exchanged zeolite contained 2.77 wt% of Cr. Following granular catalysts have been prepared: 9 CrY+K/600, (CrY+K/750). Mixture of 73% of chromium exchanged Y zeolite and K clay was peptised with nitric acid solution, extruded, dried and calcined 6h at 873K (1023K). 9 CrY+D/600, (CrY+D/750). Mixture of 73% of chromium exchanged Y zeolite and D clay was peptised with nitric acid solution, extruded, dried and calcined 6h at 873K (1023K). 9 CrY+G/600. Mixture of 73% of chromium exchanged Y zeolite and G clay was peptised with nitric acid solution (1%), extruded, dried and calcined 6h at 873K. 9 0.3Pt/SM, 0.3Pt~. 0.3% of platinum (from chloroplatinous acid) was supported respectively on the SM (D) carrier, dried and calcined 4 h at 723K. 9 0.3Pd/SM. 0.3% of palladium (from palladium (III) chloride) was supported on the SM cartier, dried and calcined 4 h at 723K. 9 Cr203/SM; (Cr203/D). 15% of Cr203 was supported on respectively the SM (D) carrier, dried and calcined for 4 h at 723K. Catalytic activity and stability of the prepared catalysts was determined in the test reaction of TCE combustion. Test was carried out in the flow micro-reactor under the following conditions: weight of catalyst: 0.30 g, fraction 0.3 - 0.5 mm, feed composition: 600 - 2 400 ppm TCE in air stream, GHSV: 15 000- 75 000 h l , temperature 523 - 823K. All activity measurements were carried out in a stainless steel, single-pass, fixed bed reactor. TCE was introduced to the reactor by saturation of an air stream in TCE containing saturator, temperature of which was stabilised by using of a cryostat. Then air stream saturated with TCE was mixed with the appropriate stream of clear air to achieve the desired feed concentration, preheated and introduced to the combustion reactor. Analyses of reactants and products were performed online using gas chromatograph (N-504) equipped with flame ionisation detector. Some experiments with empty reactors were also carried out. Results of the activity tests are presented in the Tables 1- 4. 3. RESULTS AND DISCUSSION Experiments made with using of empty reactor (without catalyst) showed that up to 773K and GHSV above 15 000 h~ no TCE thermal combustion was observed. The TCE conversion over investigated catalysts with the various active phase is shown as a function of reaction temperature in Table 1. Cr203 supported catalysts prepared with using of all the carriers used in this work gave catalysts of high activity in TCE combustion reaction. At the temperature range 623-673 K chromium based catalysts allowed to almost completely destroy TCE. However chromic oxide based catalysts quickly lost their activity during the stability test. It can be seen in Table 2 that activity of almost all chromia containing catalysts decreased during first 10-15 hours of test. It was also stated that the 15Cr203/SM catalyst after 20 h of testing at 823K contained only 39% of the initial chromium amount.

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Table 1 Conversion of TCE over prepared granular catalysts GHSV = 30 000 h"~, CTCE= 2400 ppm.

Temperature, K 523 573 623 673 723 773

Catalyst 15Cr203/SM- 1 17.2 76.7 99.3 99.5

0.3Pd/SM 0 6.5 11.6 33.7 51.3 62.7

0.3Pt/SM 7.6 10.6 18.7 26.5 48.0 61.7

CrY+D/600 7.8 10.6 14.1 42.0 81.9 95.6

Both platinum and palladium based catalysts were very active in the reaction of TCE combustion. However platinum supported on D clay based carrier was unstable and after 17 hours of the test it almost completely lost activity. It can be assumed therefore that D carrier was destroyed by hydrogen chloride (or chlorine) contained in the reaction products or active phase, platinum, was poisoned by them. It should be also noted that platinum deposited on chlorine or chlorine derivatives resistant titania-silica carrier exhibited high and stable activity (Table 2, SM). This last observation is confirmed by the fact that after break in the testing platinum based catalyst partially recovered previous activity. Palladium supported on the SM cartier was as active as Pt/SM catalyst in the test reaction. Platinum containing catalyst seems even to be more active than palladium containing ones at low temperatures (Table 1). However it is visible that the 0.3Pd/SM catalyst presented superior stability in comparison to the similar platinum catalyst (Table 2). As it was earlier stated CrY zeolite based catalysts were very active in TCE destruction. However this class of catalyst slowly loses activity during stability test and is characterised by the poorer mechanical properties than SM or natural clays-based catalysts. During 16 h of the test at 823K catalyst CrY+D lost 72% of the initial content of chromium. The decreasing in activity was independent on the type of used clay (as a binder) and on the calcination temperature. On the other hand it is visible that (Table 2) CrY based catalyst prepared with using of G type clay is quite stable during high temperature stability test. This result needs further investigations. High activity of chromium exchanged zeolite Y in combustion of chlorine contained organic compounds was observed by Chintawar and Greene [3]. They did not observe catalyst deactivation because they work under milder conditions. It should be pointed out that activities reported here are obtained at rather high catalyst loading. Stability tests of the prepared catalysts were carried out at the temperature 823 K with using the air containing 2400 ppm of TCE. Results of the tests are presented in Table 2. It can be seen that CrY-zeolite containing catalyst presented high activity and relatively poor stability in comparison to the noble based ones. Palladium supported on titania-silica carrier was appreciably more active than platinum catalyst and showed the highest stability. On the other hand chromium catalyst supported on natural aluminosilicate presented low stability. It seems that the main reason of the low stability of the latter catalyst was the lost of active phase, presumably as a volatile chromic chloride, during the test.

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Table 2 Results of the stability test of investigated catalysts - TCE conversion T = 823K, CTCE= 2400 ppm, GHSV = 30 000 h1

versus

test time, %,. ,

Time CrY+K/6 on strea 00 m, h 1 -~100 2 99.5 3 97.4 4 96.5 5 95.8 6 92.0 7 93.1 8 92.5 9 88.5 10 92.4 11 90.7 12 90.0 13 89.5 14 91.4 15 90.4 16 90.6 17 18 19

CrY+K/7 50

CrY+D/6 00

CrY+D/7 50

CrY+G/6 00

0.3Pt /D

0.3Pt/ 0.3Pd/ SM SM

99.7 98.6 96.6 94.8 93.0 92.0 87.7 83.5 84.9 83.0 83.8 81.5 82.5 83.5

99.1 98.6 96.4 93.9 90.2 91.1 89.6 86.2 85.0 85.6 83.6 84.4

98,9 98.8 97.7 95.0 94.2 94.0 88.8 87.5 86.5 85.9 82.9 95.4 88.2 82.0 82.2 84.6

97.3 97.6 97.2 97.8 97.7 98.4 98.1 97.6 97.6 96.6 97.7 97.1 96.4 97.1 97.0 97.5 97.4 96.3

53.7 55.5 55.5 53.9 54.2 51.0 Break 52.6 49.2 40.0 34.8 28.0 23.0 22.0 21.5

54.9 57.1 51.4 53.0 50.3 51.2 49.3 break 67.1 64.0 64.4 67.4 67.3 69.1 70.3 68.3

83.8 88.5 87.0 86.8 83.3 85.4 88.5 84.4 84.2 83.6 85.8 87.0 86.5

Cr203 C r 2 0 J /D SM

89.3 91.0 91.2 86.5 86.8 86.8 84.1 83.2 81.9 76.8 76.6 74.1 72.0 70.3 71.8

99.8 99.5 97.9 99.5 99.5 98.7 97.5 94.5 93.9 89.9 87.6 84.5 80.4 79.6 80.1 79.7 77.5 75.0 70.7

Chatterjee at al investigated deactivation of cobalt exchanged zeolite catalyst [4] during combustion of chlorinated hydrocarbons. They concluded that main cause of deactivation was coking. Probably under our study coking also could influence on the catalyst activity however it should be noted that vapour pressure of CrC13 is much higher than COC12vapour pressure. It can be stated that both investigated types of chromium containing catalysts exhibit very high activity in TCE combustion however they quickly diminish activity due active component lost as well as carrier damage by chlorine and chlorine derivative products of the reaction. Palladium (platinum) based catalysts shows lower activity but it is stable during the test (Table 1, 2). The most stable catalyst support is titania-silica based one. The main drawback of zeolite containing catalyst is not satisfied mechanical strength as well as low stability. It was stated that platinum (palladium at less extent) catalysts during the work is poisoned probably due to adsorption of chlorine derivative products of reaction. After the break in the work they recover partially the initial activity. Rossin and Farris [5] have demonstrated that oxidation of chloroform can accurately be described by a reaction rate expression which assumes reaction inhibition by the adsorption of the reaction product HC1. It seems that such mechanism can explain catalyst deactivation and that aiter the break, it recovery activity as we observed in our experiments of TCE combustion on Pt (Pd)/SM catalyst.

,,

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Influence of TCE concentration on the catalyst activity was also investigated. Results of the studies are shown in Table 3. The catalytic stability was found to diminish with increasing in TCE content in the feed - chromium containing catalyst lost it more quickly during work with feed containing higher TCE concentration. On the other hand Cr-Y zeolite catalyst exhibits lower activity when TCE concentration is lowered. Palladium containing catalyst is less sensitive on TCE concentration than Cr-Y based one. Table 3 Effect of the TCE concentration on its total conversion in combustion reaction (%). T = 823 K, GHSV = 30 000 h-1.

Time

CrY+K/600

0.3Pd/SM

on

stream, h

2400 ppm

1200 ppm

600 ppm

2400 ppm

1200 ppm

600 ppm

1

-~100 -100 98.3 96.5 95.8 94.0 93.1 92.5 88.5 89.4 90.4 90.0 89.5 87.4 86.4 85.6

-100 -100 ~ 100 -~100 -~100 99.0 98.7 96.6 96.6 94.7 93.1

98.5 98.8 97.4 96.8 97.2 96.5 95.8 96.2 95.5 95.6

83.8 88.5 87.0 86.8 83.3 85.4 88 5 84.4 84.2 83 6 85.8 87.0 86.5 83.8 84.5

84.9 83.5 85.4 85.0 85.9 86.6 87.2 86.5 87.2 86.7 87.6 86.3 85.9 85.8 86.1

84.8 84.2 83.2 83.9 84.3 83.6 85.1 85.5 86.4 85.2 84.6 84.2 84.0 85.2 86.2

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Effect of loading of the selected catalyst on their activity in TCE combustion reaction is shown in Table 4. Table 4 Effect of GHSV on the TCE conversion (%). T = 773K,

GHSV, h -~

Catalyst

75 000 60 000 45 000 30 000 15 000

0.3Pd/SM 51.3 51.9 57.4 63.8 78.4

CTCE --

0.3Pt/SM 45.7 46.0 49.3 54.6 82.9

600 ppm

CrY+K/600 74.7 79.6 83.1 90.0 98.6

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Application of Cr-Y catalyst even at the highest TCE flows through the catalyst bed, (GHSV = 75 000 h") allowed to obtain nearly 75 % of TCE conversion. These results confirm high initial activity of Cr-Y zeolite catalysts. Catalysts containing noble metals are less active at such high loading and allows to obtain about 50% (Pd containing one) or 45% (Pt based one) of TCE conversion at the GHSV = 75 000 h". 4. CONCLUSIONS On the basis of the obtained results it can be concluded that: 9 Mixed silica-titania systems are the most resistant materials against HCI and it is preferred for preparation of support for H-VOC's combustion catalyst, 9 Catalysts containing palladium (platinum) as a main active component are active and stable in the TCE combustion, 9 Chromia based catalysts are very active in the investigated reaction but they quickly loss chromium and should be excluded from further investigations, 9 Platinum (at less extent palladium) catalysts during the work are poisoned by products of the combustion reaction (chlorine or hydrogen chloride). After the break in the work they recover part of the previous activity. ACKNOWLEDGMENTS This work is supported by the COMMISSION OF THE EUROPEAN COMMUNITIES under Contract No. ICOP-DEMO-2094-1996. REFERENCES 1. 2. 3. 4. 5.

A.M. Padilla, J. Corella, J.M. Toledo; Appl. Catalysis B: Environmental, XXXX (1999) 1. S.K. Agarwal, J.J. Spivey, J.B. Butt; Appl. Catalysis A: General, 82 (1992) 259. P. S. Chintawar, H.L. Greene; Appl. Catalysis B: Environmental, 13 (1997) 81. S. Chatterjee, H.L Greene, Y.J. Park; Catalysis Today, 11 (1992) 569. J.A. Rossin, M.M. Farris; Ind. Eng. Chem. Res., 32 (1993) 1024.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Non-stoichiometry and catalytic activity in ABO3 perovskites: LaMnO3 and LaFeO3. "A.A. Barresi a, D. Mazza% S. Ronchetti", R. Spinicci b and M.Vallino" " Dip. Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy b Dip. Energetica, Universit/t degli Studi di Firenze, Via S. Maria 3, 50139 Firenze, Italy The possibility of altering ot and 13-oxygen equilibrium by altering the A/B ratio in the perovskite lattice, and the effect on the catalyst activity and selectivity in combustion of methane and chlorinated VOCs is investigated. LaMnO3 can accept structural underoccupation on the A as well as on the Bsites, while for the LaFeO3 apparent non-stoichiometry is caused by the presence of defective layers. 1. I N T R O D U C T I O N Perovskite-type oxides with general formula ABO3, with A = lanthanide element and B = transition metal, display prominent catalytic activity in many reactions, like NRO decomposition, SO2 reduction, partial or total oxidation of hydrocarbons, total oxidation of chlorinated hydrocarbons, and others. In the ABO3 perovskites the A ions are in general catalytically inactive, while the active ions in the B position are placed at such relatively large distance (ca. 0.4 nm) from each other that a reactant molecule interacts only with a single site. Total oxidation of hydrocarbons is supposed to occur on the perovskite surface by means of a intrafacial catalytic mechanism, in which the adsorbed oxygen is partly consumed and regenerated during a continuous cycle. On this basis we can explain why the transition-metal perovskites are particularly active in oxidative catalysis if the metal on the B-site can fluctuate between two stable oxidation states. By this way it is possible to electrically balance the insertion of O i o n s in the lattice from the gas-phase O2 or reversibly to bind O radicals to the hydrocarbon from lattice O-. Two types of oxygen are however considered to take part in the adsorption - desorption of oxygen from the outermost layers of the perovskite structure, which form the 'active surface' of the material: c~- and 13-oxygen [1-3]. a-oxygen is accommodated in the O- " vacancies formed by the partial substitution of A site cations by low valence ions or even by vacancies and involves diffusion of O--ions through the lattice with the formation of neighbouring high valent metal ions. This form is believed to be more active and reacts with hydrocarbons at lower temperatures than the 13-oxygen. 13-oxygen is observed for the substituted as well unsubstituted samples; the O diffusion inside the lattice is accompanied by the A and/or B ionic diffusion and is therefore activated at higher temperatures in respect to a-oxygen. In this communication we refer about the possibility of altering ot and 13-oxygen equilibrium not by aliovalent substitution on A or B sites, as proposed by us and other authors in previous works [4-8], but by altering the A/B ratio in the same perovskite lattice. Two cases are here chosen as

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representative, LaMnO3 and LaFeO3. LaMnO3 is a perovskite that can accept structural underoccupation on the A as well as on the B-sites up to a certain amount therefore altering the A:B ratio. For the same stoichiometric perovskite it has been demonstrated recently that the usual notation "LaMnO3+~", which would indicate excess (interstitial) oxygen in the lattice, does not represent the real unit formula of this hexagonal phase. T6pfer and Goodenough [9] have clearly demonstrated that the real formula should be written as Lal.~ Mnl_,O3 (with e = 6/(3+6)), a notation which indicates a fully occupied oxygen lattice, showing however cation vacancies. On the contrary LaFeO3 does not deviate from the 1:1 ratio; in this case, however, non-stoichiometry can be introduced only if the samples are prepared at relatively low (600 ~ C) crystallisation temperature from amorphous xerogels. The coupled evidence of X-ray powder diffraction and M6ssbauer spectroscopy indicate that the crystalline domains in Latl_~,)FeO(3_l.5~) correspond always to regular 1:1 stoichiometry (LaFeO3). The size of these domains is however lowered by the non-stoichiometry, as evidenced by the broadening of the XRD peaks, up to 130-140 A mean diameter for e=0.3. The excess iron oxide is most probably confined in defective layers of the perovskite lattice (like stacking faults) or in amorphous zones. Indeed the samples with e=0.2 - 0.3 display an additional M0ssbauer doublet centred around v=0.8 mm/s which indicates the absence of tridimensional ordering for a part of Fe. The phases are metastable; indeed they give rise to the equilibrium phases LaFeO3 and Fe203 (hematite) if heated at 900~ for some minutes [ 10]. Both non-stoichiometric perovskites were tested for their catalytic activity in the methane combustion and in the oxidation of chlorinated hydrocarbons. The only two other examples found in literature about catalytic activity of nonstoichiometric perovskites are NO reduction by La0.9~0.1MnO3 referred by Voorhoeve [ 1] and methane combustion over La-deficient LaCoO3 reported by O'Connell [11]. It was demonstrated that these materials can easily release oxygen and hence become more active than the stoichiometric. The reason for this is that surface oxygen (a-oxygen) may be activated by introducing La 3§ vacancies which alter also the electronic configuration of the BO6 octahedra by lowering the binding energy of oxygen.

2. EXPERIMENTAL 2.1. Catalyst preparation and characterisation The La-Mn perovskite samples were prepared by firing at 800~ for 15 hours amorphous xerogels prepared by the reductive decomposition of nitrates. Samples were prepared with - 0, 0.1, 0.2 (La under-occupation) and with ~' = 0, 0.1, 0.2 (Mn under-occupation). As it can be seen from Table 1, the only monophasic samples by X-ray diffraction resulted LaMn0.9~0.103, LaMnO3, La0.9~0.1MnO3; for this reason the catalytic tests were carried out only on these samples. The specific surface area shown was calculated with the Scherrer method. In the case of La-Fe perovskite, the samples were prepared by firing at 600~ for 24 hours the amorphous xerogels as obtained from reductive decomposition of nitrates. The prepared samples range from 30% La-underoccupation to 30% Fe-underoccupation. All these samples are monophasic if examined by X-ray diffraction; LaFeO3 is orthorhombic s.g. Pnma a=5.567 b=7.855 c= 5.553A. The non-stoichiometric compositions display pseudocubic symmetry, space group Pm3m a=3.929 A. Cell dimensions do not vary with composition, however the diffraction peaks broaden along with deviation from La/Fe = 1.

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Table 1. Crystallographic characteristics of prepared Mn-perovskites Nominal composition

Phases detected by XRPD

Cell dimensions (A)

Lattice symmetry

Space Group

Lao.sMnO3

perov. + Mn304

5.529 13.370

rhomboedral

R3c

La0.9MnO3

perov.

5.530 13.364

rhomboedral

R3c

LaMnO3

perov.

5.531 13.357

rhomboedral

R3c

LaMn0.903

perov.

7.796

pseudocubic

Pnma

LaMn0.803

perov. + La203

7.770

pseudocubic

Pnma

LaMn0.7503

perov. + La203

7.770

pseudocubic

Pnma

Specific surface (mZ/g) 7.5

14

2.2. Catalytic activity Features of stoichiometric and non-stoichiometric perovskites were examined in catalytic combustion of methane, which represents a good test for their activity, because of the difficulties in activating the C-H bond of this molecule. Methane combustion tests were performed by Temperature Programmed Reaction. After a 30 min pretreatment in air, the reaction mixture was fed (1.2 cm3.s-~) to a fixed-bed reactor; the mixture contained CH4= 1.5 %, 02 = 18 %, He = balance, in the tests of the La-Mn perovskites, but the oxygen/methane was lowered to 6/1 for the La-Fe catalysts. The reactor temperature was then lowered at a 3 ~ rate down to 300~ meanwhile monitoring methane conversion by analysing the outlet concentration of CO2 (the only carbon oxidation product) by use of a Non-Dispersive-Infra-Red photometer (Hartmann & Braun URAS 10E). The catalytic activity for destruction of chlorinated compounds was measured using a steady state tubular microreactor inserted in a tubular furnace equipped with temperature control. Blank runs in the same operative conditions but substituting the catalyst with inert silica sand have been carried out to evaluate the thermal destruction of the chlorinated VOCs; the residence time of the gases in the hot section of the line is about 2.5 s. 1,1,2trichloroethylene (trieline) in air (500 ppm by volume) was used as test compound, on the base of previous results on stoichiometric catalysts [8, 12]. The VOC-air mixture was fed at the flow rate 6.0 Nl/min, and saturated with water at 20~ water vapour slightly increases conversion, improves selectivity toward CO2, and avoids almost completely the formation of C12. The outlet concentration was measured by gas-chromatography, while carbon dioxide concentration was measured using a NDIR photometer connected on line. 1 gram of catalyst formed in 0.2-0.5 mm granules was used in all the runs. 3. RESULTS AND DISCUSSION 3.1. Combustion over Lao_~)Mnw~,)O3 In methane combustion the most active composition was LaMno.9[[]0.103 with a Ts0 as low as 494~ to be compared with a Ts0 of 612~ for La09[~0.1MnO3 and of 543~ for LaMnO3. The most active sample is pseudocubic (orthorhombic), and has 39% of Mn 4§ These activities can be interpreted in terms of the best compromise between factors positively affecting the activity,

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-F

化

100 80

E 0

60

>

40

C 0

tO

=~

//'/ & + + ...o..

20 0

' 200

300

400

~

8o

o

60

>

40

9

20

tO

La/Mn=0.9 La/Mn = 1 La/Mn = 1.11 thermal 500

100

ffl

Fig. 1. Trieline (500 ppm) in moistened air. Deep oxidation over La-Mn perovskite.

I

, , , , , , , ,"~", ther,,,,~,~,

u

200

600

Temperature, ~

,/ ! !iiiiii!ii

n

300

400

500

Temperature, ~

600

Fig. 2. Selectivity toward CO2 in trieline combustion over La-Mn perovskite.

like surface area, R-oxygen activation by increased Mn4+/Mn 3+ ratio and negative factors, like B sites underoccupation. A different behaviour was observed in trieline deep oxidation; La0.9MnO3 was the most active catalyst, and the increase in activity with respect to the stoichiometric one was significant, while the LaMn0.903 was equivalent, or even poorer than the stoichiometric one (Fig. 1). Considering the selectivity toward CO2, it is interesting to note that for both non-stoichiometric catalysts it was significantly higher than for the stoichiometric one: Lao.9MnO3 > LaMn0.903 > LaMnO3 (see Fig. 2). The non-stoichiometry influences also the formation of products of incomplete combustion (PIC); tetrachloroethylene is the main by-product, even if only a few ppm are generally observed. Over La0.9MnO3 larger quantities of this PIC are formed (3-4 times), and in addition other more chlorinated species, not found over the other catalysts are found: tetrachloromethane and pentachloroethane. The Mn-based perovskites favour the formation of by-products with higher chlorine content, if compared to the Fe and Cr containing ones, but this effect is enhanced by non-stoichiometry. 3.2. C o m b u s t i o n o v e r s t o i c h i o m e t r i c a n d n o n - s t o i c h i o m e t r i c L a F e O 3

The methane combustion tests have been carried out in the temperature range 300 - 480 ~ 15

~

100

~_

o-e E .9

10

>

= O

tO 1

200

400

600

Temperature, ~

Fig. 3. Oxygen TPD La0.9FeO2.85 perovskite.

curve

800

for

i

10

1

_

Lao99

_

E

1000

the

0.1 0.001

. . . .

'

. . . . .

0.0015 l I T , K "1

0.002

Fig. 4. Methane combustion over La0.9FeO2.85; contact time = 457 kg s m 3 (semi-log plot).

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with a reactant mixture characterised by a ratio oxygen/methane - 6/1; at these conditions in the entire range examined a selectivity of 100% towards the formation of CO2 was observed. A preliminary kinetic analysis, performed on these curves by means of non linear regression methods indicates that the reaction can be described, at least at the lowest temperatures of the interval, by a Rideal-Eley mechanism.

1227

o-.9. ff o P ,m

100

#

:

60

#

j P 'q

80

>

40

o

20 !

0

~

-- La/Fe=0--9

/'/'/

--c~ La/Fe = 1 i

0 ] ' ~ ~ ' '

200

..p.p.o--

r" ~";"" ;hermal

400

600

Temperature, ~

This result could strengthen the hypothesis that methane is not adsorbed and reacts from Fig. 5, Trieline oxidation. Thermal destruction is shown for comparison. the gas phase with surface oxygen species. This prompted us to perform experiments of 100 temperature programmed desorption with k .5 0 80 1 adsorbed oxygen [13]; Fig. 3 shows that o p oxygen becomes more and more mobile from 250 ~ determining the possibility of its 40 9 La/Fe = 0 9 abstraction from the surface. At 3 7 0 - 400 ~ --c3-La/Fe = 1 we observe no more desorption (except for a < ) - thermal r large peak at about 850 ~ and probably up (n 0 L to these temperatures a labile oxygen surface 300 400 500 600 species can be responsible for methane Temperature, ~ oxidation, while at higher temperatures the oxygen involved in the oxidation is strongly Fig. 6. Selectivity toward CO2 formation in bond to the bulk. It is interesting to note that trieline. plotting methane conversion (at constant contact time) versus 1/T on an Arrhenius-type plot, two different well defined temperature ranges can be evidenced, and the temperature which separates the two intervals is about 400~ this result seems to strengthen the hypothesis that at this temperature a change in the mechanism occurs. In Fig. 4 the data relative to La0.9FeOz85 are shown, but a similar behaviour, particularly evident for La0.sFeO2.v, is observed for the other two iron-based perovkites. La0.9FeO2.85(52 m2/g specific surface) has the greatest activity in methane combustion with a Ts0 as low as 385~ to be compared with 394~ for La0.sFeO2.7 (56 m2/g) and 425~ for stoichiometric LaFeO3 (28 m2/g). The underoccupation of the B site adversely affects this catalytic activity. Indeed Ts0 between 505 and 515~ were obtained for LaFe0.902.85 and LaFe0.802.v. The improved catalytic activity of La0.9FeO2.85 can be explained in terms of higher surface area induced during the synthesis by non-stoichiometry. La0.9FeO2.85 was also tested in the combustion of trieline; a slight increase in activity with respect to the stoichiometric case was observed, in the fresh catalyst, but the activity decreased quickly, approaching that of the stoichiometric sample; conversion curves obtained after different times on stream (differing of about 7 h each) are shown in Fig. 5. Previous work demonstrated that, on the contrary, the activity of LaFeO3 was quite stable over more than 200 h on stream [12]. ~

.j~r

o 60 i L; 20 1

....0

i

I

I

i

~

i

. ~176

''~

i

,

.L__

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The selectivity toward complete oxidation products is shown in Figure 6; differently from the conversion, the non-stoichiometric catalyst has a slightly lower selectivity toward CO2 than the stoichiometric one, the other main product being CO. It can be pointed out that even if the conversion over Lao.9FeO3 decreases with time on stream, the selectivity toward CO2 remains practically constant. As concerns the selectivity toward PICs, it must be said that the La-Fe perovskite is generally very good in this respect, as the selectivity toward by-products is always very low. At low temperature some trichloromethane is formed over the fresh stoichiometric catalyst, while this reaction does not occur over the aged one or over the non-stoichiometric one; a few ppm of tetrachloroethene is the only other compound observed in both cases. 4. C O N C L U S I O N S It has been shown that non-stoichiometry increases the catalytic activity in the total oxidation of methane and chlorinated hydrocarbons. In the case of structural nonstoichiometry, like that of LaMnO3, this effect could find explication in the activation of the lattice m-oxygen in the oxidation intrafacial redox cycle. In the case of defective non-stoichiometry, like that of LaFeO3 , the increase in catalytic activity is in our opinion mainly related to the wider surface area of the material. This wider area derives from the preparative route, which employs precursors with altered L a ~ e atomic ratio. It is not clear, up to now, if defective layers are exposed to surface and if in this case they can form active catalytic centres.

Acknowledgements - The authors wish to thank A. Delmastro for the preparation of some catalyst samples and M. Bengl for his contribution to the experimental work. Financial support from Italian C.N.R. (Project: Combustione catalitica a fini energetici e ambientali) is gratefully acknowledged. REFERENCES l 2. 3. 4. 5 6 7 8. 9. 10. 11. 12. 13.

Voorhoeve, R.J.K., Remeika, J.P., Freeland, P.E., & Matthias, B.T. Science, 177 (1972) 353. Seiyama, T., Yamazoe, N., & Eguchi, K. Ind. Eng. Chem. Prod. Res. Dev. 24 (1985) 19. Shimizu, T. C, tal. Rev.-Sci. Eng. 34 (1992) 355. Seiyama, T. Catal. Rev.-Sci. Eng., 34 (1992) 281. Ronchetti, S., Onida, B., & Mazza, D. Atti 3 ~ Cong. Naz. AIMAT, Napoli, Italy, 938 (1996). Saracco, G., Scibilia, G., Iannibello, A., & Baldi, G. Appl. Catal. B, 8 (1996) 229. Barresi, A.A., Delmastro, A., Piccinini, P., Paglicci, L., & Baldi, G. Atti Conv. "Materiali. Ricerca e prospettive tecnol, alle soglie de12000" (FAST), Milano, Italy, Vol. 1,775 (1997). Barresi, A.A., Delmastro, A., & Baldi, G., in: "Rbcents Progrbs en G~nie des Procbdbs", vol. 13, Nr. 65 (Proc. 2nd Europ. Congress of Chemical Engineering), p. 263. Paris: Ed. Lavoisier, 1999. T6pfer, J., & Goodenough, J.B.J. ,Solid State Chem., 130 (1997) 117. Mazza, D., Ronchetti, S., & Spinicci, R.. Proc. 4th Europ. Cong. Catalysis, Rimini, Italy, 860 (paper P/II/325) (1999). O'Connell, M., Norman, A.K., Htittermann, C.F., & Morris, M.A.. Catal. Today, 47 (1999) 123. Barresi, A.A., Ronchetti, S., Bengl, M., Mazza, D., & Baldi, G.. Pror ICheaP-4, Firenze, Italy, Vol. 1, 27 (1999). AIDIC Conf. Set., in press. Spinicci, R., & Tofanari, A.. Thermochim. Acta, 269-270 (1995) 677.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Deep catalytic oxidation of chlorinated VOC mixtures from groundwater stripping emissions A. Aranzabal, J.A. Gonz~lez-Marcos, R. L6pez-Fonseca, M.A. Guti6rrez-Ortiz and J.R. Gonz~lez-Velasco. Department of Chemical Engineering, Faculty of Sciences, Universidad del Pals Vasco/EHU, P.O. Box 644, E-48080 Bilbao, Spain. Phone: +344-46012681; Fax: +344-4648500; E-mail: [email protected] The total oxidation chlorinated VOCs mixtures, with composition similar to those expected from groundwater air strippers, over Pd/A1203 and Pt/A203 catalyst at temperatures from 200 to 550~ was investigated. 1,2-Dichloroethane showed greater oxidability, followed by dichloromethane and 1,1dichloroethylene and trichloroethylene. The addition of hydrocarbons to a chlorinated feed produced changes in the reactivity order of the chlorinated VOCs, indicating the existence of a complex reaction system due to the energetically non-uniform catalytic surface. 1. I N T R O D U C T I O N The accidental spills, leaking, storage tanks and past disposal practices of fuels, cleaning solvents and degreasers led to subsurface contamination. Air stripping has been shown to be an efficient and cost effective method for removing volatile organic contaminants from groundwater and soil. Unfortunately, these contaminants are transferred to the air where they may continue to pose environmental and health threats. Catalytic oxidation of chlorinated volatile organic compounds (CVOC) is currently receiving increased attention due to its low destruction temperatures and its excellent selectivity towards formation of harmless reaction products [1]. So far, catalytic oxidation studies have been based on well characterised single component feeds. However, in practical applications, gas streams containing mixtures of VOCs of variable composition make it difficult to predict the behaviour of the catalytic oxidation unit and to control the destruction efficiency, because the "mixture effect" may be significant, resulting in either inhibition or promotion of oxidation of one hydrocarbon in the presence of others. The scope of this research is to evaluate the "mixture effect" in the oxidation of several VOC catalysed by supported platinum and palladium and to elucidate the reaction mechanism by the competing-reaction method, using feedstreams with a similar composition to those obtained from the air-stripping of contaminated groundwaters.

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2. E X P E R I M E N T A L

Catalysts (Pt/A12Oa and Pd/A12Oa) were prepared by adsorption of H2PtC16 and PdC12 over T-A12Os provided by RhSne-Poulenc. Prior to reaction the catalysts were pre-treated by drying for 1 hour at 150~ in dry air, calcining at 550~ in air stream for 4 hours and finally reducing at 550~ in a 3:1 ratio nitrogen/hydrogen stream for additional 2 hours. The final catalysts resulted in 0.44 wt% Pt and 0.42 wt% Pd, measured by atomic absorption spectroscopy (AAS); the metal dispersion was determined by hydrogen chemisorption, resulting in 53% and 43%, respectively. Oxidation reactions were carried out under atmospheric pressure in a fixed bed tubular reactor. The flowrate through the reactor was set at 500 cm 3 min -1 that produced a space velocity of 15000 h -1. The feedstream to the reactor was prepared by delivering the liquid hydrocarbons by syringe pumps into a dry, oilfree compressed air, which was purified using a commercial PSA drier and metered by a mass flow controller. Reactor effluent was analysed o n l i n e by a Hewlett Packard 5890 Series II gas chromatograph equipped with an electron capture detector (ECD) and a thermal conductivity detector (TCD). Three different feed streams were used to simulate the gas streams from airstrippers used for groundwater clean-up. Table 1 summarises the composition of these feed streams. The first stream contained a total concentration of 1000 ppm in a mixture of 1,2-dichloroethane (DCE), trichloroethylene (TCE), 1,1dichloroethylene (DIC) and dichloromethane (DCM). The second and third feed streams were generated by adding 1000 ppm of non-chlorinated hydrocarbons, namely, hexane and toluene, to the chlorinated feed (feed 1). Table 1. Feed streams composition (ppmv) Compound

Feed 1

Feed 2

Feed 3

1,2-dichloroethane (DCE) trichloroethylene (TCE)

200 250

200 250

200 250

dichloromethane (DCM)

100

100

100

1,1-dichloroethylene (DIC)

450

450

450

hexane

-

1000

-

toluene

-

-

1000

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3. R E S U L T S AND D I S C U S S I O N Fig. 1 and 2 show the conversion data of the chlorohydrocarbons from the multicomponent feed 1 over 0.42 wt%Pd/A1203 and 0.44 wt%Pt/A1203, as a function of temperature. Conversion of DCE began at 250~ and increased quickly so that complete oxidation was reached at 375~ DIC also began to react at 250~ but its oxidation rate was slow till 60% DCE was converted (aprox. 325~ then reaction rate of DIC becomes faster and complete conversion of DIC was achieved at 425-450~ DCM showed practically the same behaviour than DIC. Combustion of TCE took place at higher temperatures than DIC and DCM, as it proceeds slower than the other chlorohydrocarbons, in such a way that at 550~ its conversion was not higher than 98%. Activity comparison between both catalysts, showed that oxidation of TCE occurred faster over Pd catalyst, while conversion of DIC and DCM was slightly higher over Pt. Oxidation of DCE proceed in the same extent over Pd and Pt catalysts. Windawi and Wyatt [2] investigated the oxidation of several chlorinated C1 and C2 hydrocarbons, including DCE, TCE and DCM over a supported Pt catalyst and they found that single bond C2 chlorohydrocarbons were the fastest destructed compounds, followed by chloromethanes and chloroethylenes, so that the reactivity order of the compounds of interest was DCE>DCM>TCE. Bond and Sadeghi [3], by using a 0.8 wt% Pt/y-A1203 catalyst, found that the reactivity order of the investigated CVOC was DIC>DCM>TCE>tetrachloroethylene (C2C14). The reactivity order for the individual compounds in the multicomponent

100

o~

-

--

:

--

100 f

80

80

60

o~ ~ 60

8 40

8

O

-~

IJ 200

/ 250

300

~

350

/ /

r

/

:o,c I 400

450

500

550

Temperature, ~

Fig. 1. Ignition curves for the oxidation of DCE, TCE, DCM y DIC in the feed 1 over 0,42 wt%Pd/A12Os.

t

/

40

,TO,

20

O

DCE

O

/

O

200

~

250

300

/

II

/

t,

TCE 'ooM

:o,c

...................

350

400

450

500

550

Temperature, ~

Fig. 2. Ignition curves for the oxidation of DCE, TCE, DCM y DIC in the feed 1 over 0,44 wt%Pt/A12Os.

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chlorinated feed (Fig. 1 and 2) (DCE>DIC>DCM>TCE) coincides with those reported in the literature for single component streams, indicating that the oxidation mechanism of each chlorohydrocarbon is not altered by the presence of the others.

Agarwal et al. [4], investigated the same chlorinated stream but half concentrated, over a Cr203-A1203 catalyst and found that the oxidability order was: DIC>TCE>DCE>DCM. This indicates the active sites of each catalyst shows different affinity to each compound, so that the destructibility of the hydrocarbons in the mixture is strongly influence by the catalyst type. The addition of non-chlorinated hydrocarbons, i.e. hexane and toluene to the multicomponent chlorinated feedstream, (feed 2 and 3), caused the change in the oxidability order of the chlororganics as shown in conversion vs. temperature curves in Fig. 3 to 6. As a result, DIC and TCE oxidation were promoted, while DCE oxidation was inhibited, resulting in the following destruction order: DIC > TCE > DCE > DCM. Ts0 (temperature for the 50% conversion) for DIC oxidation, in the presence of hexane over Pt/A1203, was reduced from 350~ to 250~ However, Ts0 for DCE increased from 300~ to 350~ while DCM decomposition curve was not affected by the presence of hydrocarbons. TCE experienced the major oxidability change, specially in the temperature range 200-400~ where Ts0 was reduced from 450~ to 290~ being the second compound with higher reactivity; however, above 350-400~ reaction got slower, becoming the compound with the lowest oxidability. This behaviour can be better analysed in the Fig. 4, reaction in the presence of hexane and catalysed by Pt/A1203. In general terms, DIC and TCE oxidation were the most promoted reactions (Fig 3

lOO

I ~ f

iOOb

l ,/li I1' II

8 20f

lOO

y

II

~

I I

60

:t.

9

8>:~ 7TCE

0 ~ ............. 200 250 300 350 400 450 500 550 Temperature, ~ Fig. 3. Ignition curves for the oxidation of DCE, TCE, DCM y DIC in the feed 2 over 0,42 wt%Pd/A12Os.

20[-~.

,,~/f

~'/

:-" TOE OCM

0 ~ 200 250 300 350 400 450 500 550 Temperature, ~ Fig. 4. Ignition curves for the oxidation of DCE, TCE, DCM y DIC in the feed 2 over 0,44 wt%Pt/A1203.

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100 f

100

80 f

80 c" 60 o

" f 0 .... 200

0

, -

250

), ~ /~

9 DCM

...................

3~o'3~o

400

Temperature, ~

4~o

~oo

/ /

20

_//'7/ . . . . . . . . . . . .

~5o

Fig. 5. Ignition curves for the oxidation of DCE, TCE, DCM y DIC in the feed 3 over 0,42 wt%Pd/A1203.

= TCE ,, DcM

;.,.D'.C,

O= 200 250 300 350 400 450 500 550 "

"

"

" m

,m

~,

Temperature, ~

Fig. 6. Ignition curves for the oxidation of DCE, TCE, DCM y DIC in the feed 3 over 0,44 wt%Pt/A1203.

and 5), whereas in the presence of toluene DCE oxidation is most inhibited (Fig 4 and 6). In both cases, using palladium catalyst higher conversion of TCE was achieved, specially in the temperature range between 300 and 550~ Apart of the inhibition or promotion, the CVOCs experienced faster ignition in the presence of hexane and toluene, specially DCE and DCM. Their conversion increased from 10 to 90% in a range of 10~ as a consequence, the DCM oxidation was completed at 400~ (Fig. 3 and 4) when hexane was present in the feed, requiring 50~ less than in the absence of the hydrocarbons (feed 1). The addition of toluene inhibited the oxidation of DCM at low temperatures, but due to the quick ignition, temperature for complete conversion was not altered. The results obtained with the feeds 2 and 3 indicate the complexity of the reaction mechanism: the presence of non-chlorinated hydrocarbons (hexane and toluene) modifies the work function of the catalyst, leading the different oxidability of the chlorohydrocarbons with respect to their oxidation in the absence of them. At first sight, the reduced DCE conversion suggests that competitive adsorption effects play the major role. Both hexane and toluene are more competitive for catalytic sites than DCE. This conclusion would also led to the inhibition of TCE oxidation, since in a previous research about the catalytic oxidation of two component ~ D C E and T C E - - feedstreams, inhibition for TCE was observed, concluding that TCE was competing for the same active centres than DCE. However, TCE and DIC oxidation are highly promoted when hexane and toluene are present in the feed. Therefore, it seems that the oxidation of nonchlorinated hydrocarbons changes the properties of the catalytic sites, which

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enables the chlorinated ethylenes (TCE and DIC) to be adsorbed faster in such sites than DCE and DCM. Nevertheless, when ignition of DCE and DCM started (350~ TCE oxidation was inhibited, requiring higher temperatures for its total oxidation. In such case, palladium catalyst was able to reach higher TCE conversion in the range 300-550~ than platinum catalyst. All these with promotion and inhibition effects, indicate that the catalyst surface is energetically non-uniform, leading to active sites with different affinity to each reactant depending on the reaction conditions, i.e. temperature and environment. Somorjai [5] reported the existence of different crystal faces, edges and corners demonstrating the heterogeneity of the metallic surface. He observed carbonaceous overlayer was formed over the faces of the platinum crystals reducing their activity, while the edges and the corners, much more reactive, remained clear. The heterogeneity of the catalytic surface was also demonstrated by variation of the chemisorption heat of oxygen [6,7], concluding that the rate of oxygen chemisorption was different on energetically different sites. Gangwal et al. [8] found different oxygen chemisorption constant (ko) values for the oxidation of benzene and n-hexane over Pt,NYT-A1203. Since ko is the kinetic constant for the oxygen chemisorption on the catalyst, it should be independent of the nature of the organic compound oxidised. Gangwal et al. explained this fact by assuming that the active catalyst species were in different oxidation sates. Likewise, Minot and Gallezot [9] reported that the selectivity to adsorption of a certain compound depended also on the electrodonating or electron-accepting species interacting with the metal surface, since they change the work function, leading to the "mixture effects" observed in this work. ACKNOWLEDGEMENTS The authors wish to thank Universidad del Pais V a s c o / E H U (UPV069.310G40/98) Gobierno Vasco (GV069.310-0111/94) and Ministerio de Educaci6n y Cultura (QUI96-0471) for the financial support. One of the authors (A.A.) acknowledges Gobierno Vasco for the grant BF194.010. REFERENCES 1. J.R. Gonz~lez-Velasco, A. Aranzabal, J.I. Guti6rrez-Ortiz, R. L6pez-Fonseca and M.A. Guti6rrez-Ortiz, Appl. Catal. B, 19 (1998) 189. 2. H. Windawi and M. Wyatt, Platinum Metals Rev., 37 (1993) 186. 3. G.C. Bond and N. Sadeghi, J. Appl. Chem. Biotechnol., 25 (1975) 241. 4. S.K. Agarwal and J.J. Spivey, Appl. Catal. A, 82 (1992) 259. 5. G.A. Somorjai, Adv. Catal., 26 (1997) 1. 6. G.L. Golodets, Heterogeneous Catalytic Reactions Involving Molecular Oxygen, Elsevier, Amsterdam, 1983. 7. P. Briot and A. Auroux, Appl. Catal., 59 (1990) 141. 8. S.K. Gangwal, M.E. Mullins, J.J. Spivey, P.R. Caffrey and B.A. Tichenor, Appl. Catal. A, 36 (1988) 231. 9. C. Minot and P. Gallezot, J. Catal., 123 (1990) 341.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Transformation of chlorinated compounds on different zeolites under oxidative and reductive conditions I. Hannus a, A. Tamfisi a, Z. K6nya a, S.-I. Niwa b, F. Mizukami b, J. B.Nagy c and I. Kiricsi a aDepartment of Applied and Environmental Chemistry, J6zsef Attila University, Rerrich t6r 1, H-6720 Szeged, Hungary* bNational Institute of Materials and Chemical Research, Department of Surface Chemistry, 1-1, Hagashi, Tsukuba, Ibaraki 305, Japan CLaboratoire de RMN, Facult6s Universitaires Notre-Dame de la Paix, Rue de Bruxelles 61, B-5000 Namur, Belgium

Decomposition of carbon tetrachloride was investigated over mono- and bimetallic forms of ZSM-5 zeolite catalysts. Independently on the reduction methods used for generating platinum-containing samples two main types of reactions were found to occur. Under inert or oxidative conditions a simple reaction takes place between the zeolite and the reactant leading to the destruction of zeolitic framework. In hydrogen flow, in reductive atmosphere hydrodechlorination took place and no detectable dealumination of zeolite skeleton occured.

1. INTRODUCTION Due to their high stabilities chlorofluorocarbon (CC12F2) and carbon tetrachloride (CC14) are suitable materials in great many applications. This character is advantageous in daily life, however, harmful from environmental point of view since they play significant role in ozone depletion. This fact gave rise to environmental concerns and eventually resulted in their negotiated phase-out under the terms of the Montreal Protocol [1] and its subsequent revisions. Nowadays, there is another problem, the storage, treatment and safe destruction or transformation of CC12F2 produced until 1996. Furthermore, the decomposition or transformation of chlorinated by-products, such as CC14, in manufacturing of methylene chloride (CH2C12) and chloroform (CHC13) are also urgent problems [2]. Two main potential ways are in attention: their transformation to potentially valuable chemical compounds by hydrodechlorination (HDC) [3] or their (oxidative) destruction to form environmentally safe products. This work was performed with the help of grants OTKA T 025248 and F25245, Hungary.

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In this paper we report on the investigation of the oxidative decomposition and reductive hydrodechlorination reactions of CC14 over modified ZSM-5 zeolites. 2. EXPERIMENTAL

Mono-ionic samples were prepared by ion-exchange of Na,HZSM-5 zeolite (Si/A1=32.95 and unit cell composition Na0.87Ha.13A13.0Si93.00192). The zeolite samples were exchanged in solutions of Co(NO3)2 or [Pt(NH3)4](NO3)2 with stirring for 24 h at room temperature to prepare the corresponding CoZSM-5 and PtZSM-5. For preparing two-ionic Pt,CoZSM-5 sample an equivalent amount of the Pt(NH3)4]2+ was dissolved in distilled water. While stirring, a solution containing the Pt(NH3)4]2+ ions was added dropwise to the filtered and dried CoZSM-5 solid suspended in water. After 24 h stirring the solution was evaporated. The metal loading of the samples are summarized in Table 1. Table 1 Meta!=!oad!ng and ac!d!~,of the s ~ p ! e s ........ ZSM-5 samples Metal loading (mmol/gzeolite) Reduction with Br6nsted/Lewis Co:~T Pt~-:~.............................................................................................................acidity: .... "................... ratios= ................. CoZSM-5 0.02 non 0.16 PtZSM-5 0.02 hydrogen 2.15 NaBH4 0.14 NaBH4+H2 0.1 Pt,CoZSM-5 0.02 0.02 hydrogen 0.27 NaBH4+H2 0.04 The PtZSM-5 and Pt,CoZSM-5 samples were divided into two parts for reduction. The first portion was suspended in water and 0.1 mol/dm 3 NaBH4 solution was added dropwise. The suspension was stirred for 8h filtered and dried. The samples obtained after such treatment are denoted as PtZSM-5(Na) and Pt,CoZSM-5(Na), respectively. The second part of the sample was reduced in hydrogen flow at 573 K. This way we received samples denoted as PtZSM-5(H2) and Pt,CoZSM-5(H2). In some experiments the PtZSM-5(Na) and Pt,CoZSM5(Na) specimens were reduced in hydrogen flow in order to check the changes in the acidities of samples. These samples marked as PtZSM-5(Na)(H2) and Pt,CoZSM-5(Na)(H2). Appropriate amounts of samples were put into the reactor followed by starting the decomposition experiments. Self-supporting wafer technique was employed for acidity and adsorption measurements. The wafers (10 mg/cm 2) were prepared from the powdered zeolites and placed into the sample holder of the i n s i t u IR cell. The pretreatment of the samples was as follows: The temperature of the wafer was slowly increased to 723 K under continuous evacuation of the cell. (When hydrogen was used for the reduction, 26.6 kPa (200 Torr) H2 was introduced in the cell and reacted with the wafer at 573 K, for 1 h, followed by degassing at the same temperature for an additional hour.) After this treatment the sample was cooled down to room temperature and the background spectrum of the zeolite was recorded by a Mattson Genesis Spectrometer. The resolution of the spectra was 1 cm -1 and 16 scans were accumulated for a spectrum.

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TEM images were made using HITACHI electron microscope in order to obtain information about the particle size distribution of platinum or bimetallic species burned in the zeolite samples upon reduction either with hydrogen or with NaBH4. The XRD patterns serving primary data on the crystallinity of the specimens were taken with a DRON 3 X-ray diffractometer. For the acidity measurements 1.33 kPa (10 Torr) pyridine was introduced into the cell and heated up to 473 K. After 1 h adsorption, the cell was evacuated for 1 h at the same temperature. After cooling the sample to room temperature spectra of the adsorbed pyridine were taken. Structural changes occurring in the zeolite framework upon reaction was investigated by IR spectroscopy (KBr pellet technique). Particularly the 800-1000 cm l range was monitored since the band generally appearing at 960 cm -~ was attributed to the structural vacancies generally caused by dealumination of the zeolite skeleton. Hydrodechlorination reactions of CC14 was carried out in the flow system applying IR spectroscopic product analysis. The catalysts placed in a glass reactor were pretreated as described above. After adjusting the desired reaction temperature (generally we started the reaction at room temperature) the carbon tetrachloride was fed using either nitrogen or nitrogen-hydrogen, or oxygen flow with a flow rate around 10 ml/min. The gas stream containing the products formed were passed through an IR gas cell and the spectra were taken at predetermined times (generally after 0.5 h reaction time). Then the temperature of the reactor was increased. Stepwise raising of the temperature was performed up to 573 K, while the products were analyzed at each step.

3. RESULTS AND DISCUSSION 3.1. TEM studies As the TEM images in Figure 1 and 2 show, the metal particles of Pt,CoZSM-5 zeolite samples are quite large. Independently of the reduction procedure (in NaBH4 solution at room temperature or in H2 stream at 573 K) their particle size distribution seems to be practically identical and homogeneous in both samples. This fact indicates that the hydrogenation capability of these samples is similar. 3.2. Infrared spectroscopy The results of acidity measurements performed with adsorption of pyridine revealed higher BrOnsted acidity of the samples reduced by hydrogen than that treated in basic NaBH4 solution. As data in Table 1 show insignificant change in Br~nsted acidity for samples first reduced by NaBH4 followed by reduction in hydrogen. This finding may play an important role when CC14 decomposition activities are compared in neutral or oxidative regime. The activity of zeolites in the conversion of chlorofluorocarbons leading to phosgene, carbon dioxide and HC1 depended on their acidities [4]. In Figure 3 IR spectra of gas phase products formed in the decomposition of carbon tetrachloride in neutral, oxidative and reductive atmosphere over PtZSM-5 zeolites reduced in hydrogen and by NaBH4 are depicted. It is seen that the products in the former two media are very similar, almost identical. Phosgene, CO2 and HC1 are the main products giving rise to bands at 1750, 2400 and 2900 cm "l. The concentration of these compounds increases with reaction temperature. This result is in accordance with those found by NMR spectroscopy [6].

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Figure 1. TEM image of Pt, CoZSM-5 sample reduced in NaBH4 solution.

Figure 2. TEM image of Pt,CoZSM-5 sample reduced in H2.

A

2500

B

1500 2500 Wavenumber (1/cm)

1500

Fig. 3. Transformation of CC14 over PtZSM-5 samples reduced in hydrogen (A) or in solution by NaBI-h (B) The reaction conditions were: temperature 523 K, time 1 h, media: oxygen (A), nitrogen (B) or hydrogen (C).

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The chlorine atoms of CC14 react with the AI of the zeolites coordinated tetrahedrally resulting in the formation of octahedrally coordinated extraframework AI. This transformation results in the formation of zeolite with vacancies in their skeleton. This has been proven by the 27A1NMR spectra [7] and the IR spectra by the band generally appearing near 930 cm l

[8].

As a consequence of these transformations, the crystal structure of the zeolite collapses for zeolites with low Si/A1 ratios, and the zeolite should be, therefore, regarded a reaction partner rather than a catalyst. Oxygen, present in the reacting mixture slows down the dealumination of zeolite framework [4]. Figure 4 shows the spectra of the products obtained over Pt,CoZSM-5 samples reduced in different ways and reacted in neutral or reductive conditions. The difference in the product distributions is clearly seen. While in neutral atmosphere phosgene, CO2 and HCI are the main products, in the experiments performed in reductive atmosphere methane, HC1 are the main products. From this follows that substantial difference should be in the mechanism

HCI

CO2

COC12

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i

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i

B

l

I

i

i

i

2000

i

I

i

i

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I

1000

Wavenumber (1/cm) Fig. 4. Products formed in the transformation of CC14 over Pt,CoZSM-5 samples, A: Pt,Co(Na)(H2), B: Pt, Co(H2)(H2), C: Pt, Co(Na)(N2), D: Pt, Co(Hz)(N2), at 523 K after 1 h reaction time.

i

i

I

i

i

1250

IAI

i

i

1000

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i

i

i

i

750

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i

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i

Wavenumber (l/cm)

Fig. 5. IR spectra of the spent catalysts, A: Na, H, B :Pt(H2)(N2),C: Pt(Na) (H2), D: Pt(H2)(H2), E: Pt(Na)(H2), F: Pt(H2)

(o~)

I,

500

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of the reaction taking place under various conditions. It is also remarkable, that the products are present in almost identical concentrations for both the hydrogen and the NaBH4 reduced samples. This result shows the negligible importance of the reduction method and acidity in the reaction performed in hydrogen. As we mentioned before, acidity played an important role at the simple decomposition reactions of chlorofluorocarbons. Here, in reductive atmosphere the importance of the acidity became insignificant. From this follows that the metal function (Pt or Pt,Co) is predominant in the hydrodechlorination reaction of carbon tetrachloride. IR spectra taken on the spent zeolite samples and depicted in Figure 5, and the XRD patterns showed unequivocally that no crystal destruction and/or significant dealumination of the zeolite framework occurred in the reactions performed in hydrogen atmosphere. No indication of the IR band attributed to framework vacancies is seen at 930 cm -l in the spectra presented. We can conclude that bimetallic zeolites prepared on the basis of high modulus materials such as ZSM-5 are promising specimens for application in hydrodechlorination reactions. The use of ZSM-5 type zeolites supplies a new, promising way for the preparation of catalysts with prolonged activity in hydrochlorination reaction, while Kim et al claimed fast deactivation of P t ~ a Y zeolite [9]. 4. CONCLUSION The completely different product distributions observed in the decomposition reaction of carbon tetrachloride in neutral, oxidative or reductive conditions suggest two different reaction mechanisms. Under neutral or oxidative regimes the carbon tetrachloride reacts with the zeolite irrespective its metal content. This reaction leads to fast dealumination of the zeolite resulting in the collapse of the crystal structure. Under reductive conditions, in hydrogen flow, hydrodechlorination takes place as the main reaction producing partially chlorinated methane derivatives. Under the experimental circumstances applied the main products were methane and HC1 being typical for the hydrodechlorination. The bimetallic ZSM-5 type zeolites proved to be promising catalysts for this transformation. REFERENCES

1. Montreal Protocol on Substances that Deplete the Ozone Layer, Final Report, United Nations Environment Programme, New York, 1987. 2. Z.C. Zhang and B.C. Beard, Appl. Catal., A General, 174 (1998) 33. 3. A. Viersma, E.J.A.X. van de Sandt, M. Makkee, C.P. Luteijn, H. van Bekkum and J.A. Moulijn, Catal. Today, 27 (1996) 257. 4. I. Hannus, Z. K6nya, P. Lentz, J. B.Nagy and I. Kiricsi, Proc. 12th Intern. Zeolite Conf., Baltimore, USA (J. Mater. Res. Soc.) Vol. IV. p. 2963 (1998). 5. A. Tam~isi, I. Kiricsi, Z. K6nya, J. Hal/tsz and L. Guczi, J. Mol. Struct., 482-483 (1999) 1. 6. I. Hannus, I.I. Ivanova, Gy. Tasi, I. Kiricsi and J. B.Nagy, Colloids Surfaces A: Physicochem. Eng. Aspects 101 (1995) 199. 7. I. Hannus, Z. K6nya, P. Lentz, J. B.Nagy and I. Kiricsi, J. Mol. Struct., 482-483 (1999) 359. 8. I. Hannus, Z. K6nya, T. Koll~r, Y. Kiyozumi, F. Mizukami, P. Lentz, J. B.Nagy and I. Kiricsi, Stud. Surf. Sci. Catal., 125 (1999) 245. 9. S.Y. Kim, H.C. Choi, O.B. Yanga, K.H. Lee, J.S. Lee and Y.G. Kim, J.Chem. Soc., Chem. Commun., 1995, 2169.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

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Biofiltration of Gasoline VOCs with Different Support Media I. Ortiz., M. Morales, C. Gobb6e, S. Revah, V.M. Guerrero 2 Shifter L. A. Garcia 4

12, R. Auria 3, G.A. P6rez4,

UAM- Iztapalapa, 09000, Mexico,, D. F 1, Mexican Petroleum Institute, Eje central lazaro Cardenas . 152 Mexico, D. F. 2; ORSTOM (Institut Francais de Recherche Scientifique pour le DOveloppement en Cooperation). Ciceron 609, Mexico, D.F. 3, University Valley Mexico 4.

BTX constitute an important part of the total VOCs in different types of commercial gasoline in Mexico. Evaporation of this Kind of compounds from storage provokes undesirable emissions to the atmosphere. Biofiltration has demonstrated be an effective control technology to eliminate these compounds. To increase the performance of biofilters it is important to understand the effects of the support on long-term operation. In this work, organic and inorganic supports were evaluated to remove VOCs from gasoline vapours in 4 liter bench-scale reactors inoculated with an adapted consortium. The supports were: peat, a mixture of vermiculite and activated carbon (V-AC), barks and porous 15 mm glass rashing rings. A mixture of benzene, toluene and xylenes (BTX) of 200 g C/m3/h was fed for more than 100 days to the 4 biofilters with an EBRT of 60 s. Removal efficiencies higher than 95% were obtained with V-AC, 85% for peat and barks and 65% for the rashing rings. In all cases, drying problems in beds were observed after several days of operation. Peat was the best material to support microbial growth without manipulation, however to restore its moisture content, it was necessary to perform mechanical mixing. In contrast, this problem was surmounted by direct water addition to the others supports. Addition of mineral media to the biofilters packed with barks and rashing rings was required until the establishment of an efficient microbial population. In steady state operation, experiment at loads from 50 to 400 g C/m3/h were carried out and the behaviour of each biofilter was studied.

1. INTRODUCTION Air pollution is a pressing problem in big cities. In Mexico City air, quality has decreased due to the emission of volatile organic compounds, nitrogen oxides and others compounds that contribute to the formation of high levels of ozone and smog. Emissions of hydrocarbons in Mexico City [1 ], derive mainly from storage evaporation (19%) and mobile

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sources (75%). The development of Air Pollution Control (APC) technologies is a response to these increased levels of pollution. The biological APC technologies have some advantages over others APC technologies [2] especially in treatment of gaseous flow with low concentration of pollutants. Biofiltration benefits from the capacity that some bacteria., fungi and yeast have to degrade a great variety of both, organic and inorganic compounds into H20, biomass and CO2. Support media has been reported [2], [3], as one of the essential factors in biofilter performance. Recently, new inert material or formulated supports have been successfully used in biofiltration. [4, 5, 6, 7 y 8]. The use of a appropriate media can help to reduce the problems associated with bed drying which has been reported to be the main cause of failure in biofilters. To increase the performance of biofilters it is important to understand the effects of the support on long- term operation. To study these effects natural and inert supports were selected and challenged against a mixture of aromatics (BTX). Inert supports used in this work were selected not only by their practical interest, but also, to look into fundamental processes that take place during the degradation. 2. EXPERIMENTAL SYSTEM Methods and materials supports. Four different packing materials were used: Peat (P), A mixture of vermiculite and activated carbon (V-AC), Pine three barks (B), porous 15 mm ceramic Siporax raching rings (GR). (Fig.l) shows a schematic representation of the experimental system used. The reactors were placed in a room with controlled temperature at 30+2~ Operating conditions were: Inlet gas concentration: 2.4 g C/m 3, EBTR: 60s, initial water content of supports: P: 65%; V-AC: 70%, B: 30%; GR: 40%, Measured parameters were: Inlet and outlet gas concentrations by FID chromatography and emitted CO2 by infrared analyser.

l

Figure 1. 1) Compressor, 2) Humidifier, 3) Water recirculation system, 4) Cyclone, 5) Chamber of solvent addition, 6) Temperature Controller, 7) Evaporator of Solvents, 8) Pump, 9) Mixing Chamber, 10) Flow distributor, 11) Sampling ports, 12) Biofilters, 13) Sampling system.

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3. RESULTS AND DISCUSSION

Biofilters were operated during for more than 100 days. (Fig. 2) shows an example of the performance of the V-AC packed biofilter. During the first 30 days of experiment, Elimination Capacity (EC) of the BTX was about 30 g C/m3h. To increase biofilter performance, nutrients were added at the 12th day. This supply of water and nutrient was correlated with an increment of CO2 production while EC remained almost constant. A lag phase of 15 days was observed before EC increased. After this period, only water additions were performed which corresponded to new EC increases. Between days 30 and 100, EC and CO2 concentration were around 125 g C/m3h and 4.5 g/m 3, respectively. This evolved CO2 corresponds to about 60% of theoretical considering total oxidation and may be explained as carbon retained for biomass, intermediates and carbonates [9]. (Fig. 3) represents EC variation for the BTX mixture (Benzene, Toluene and Xylenes). Changes of EC for each components followed the global behaviour of the gas mixture. After day 30, EC of toluene and xylenes were around 25g C/m3h and 125g C/m3h, respectively. Support:

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Due to the low inlet concentration of benzene (25 ppmv), EC of benzene was very low (1 g C/m3h). Toluene was the first compound to be degraded followed by xylenes. At this time para and meta isomers were completely degraded and ortho had xylenes. At this time para and meta isomers were completely degraded and ortho had an elimination of 85% and above.

Support: V-AC

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Figure 3 . - Individual degradation of BTX for the V-AC Biofilter MMA-Mineral Media addition, WA: Water Additon. Higher efficiency was obtained with V-AC support for the mixture gas and each one component. Peak EC values were 260g C/m3h, 300g C/m3h, 220g C/m3h and 130g C/m3h for V-AC, peat, barks and GR, respectively. These EC values were comparable with values reported previously. [8,5,10 y 11]. Although, higher punctual EC were obtained with peat, longer periods of high EC were found in V-AC reactor, which justifies an overall better performance. The low EC in GR could be explained by the large void space with this support in the reactor. The GR has also lower biofilm superficial area, which is about half of that found with vermiculite (600m2/m3). Moreover, GR support loses water easily, so frequent addition of water, which may bring about Elimination Efficiency (%) biomass washing. Barks

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(B) permit also a direct addition of nutrients and water but several weeks were necessary to attain a proper humidity of the support due to its hydrophobic composition. In all cases, it was observed that a restoration of water cotent of bed results in increased EC. Biofilter packed with peat had similar results.

-

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Load (g C/m3h) Figure 4. Integrated values until the first 92 days of BTX for the V-AC Biofilter. MMAMineral Media addition, WA-Water Addition However, humidity conditions were re-established by unpacking the bed, adding water and mixing it manually. (Fig. 4) shows variation of EC with load for the four supports. This results confirm that biofilter packed with V. CA support has the best performances to degrade the mixture of BTX. Maximum EC was about 260g C/m3h for this support.

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4. CONCLUSIONS

The main results obtained in this study were, microbial culture was appropriate to carry out degradation of the BTX mixture. Biofilter operation was strongly affected by the bed moisture content for all supports. Different strategies, depending on the support, were necessary to maintain the required operation conditions to sustain high EC. The mixture of Vermiculite and Activated Carbon (V-AC) had the best long-term results, but addition of nutrients and water was required. V-AC, GR and B required longer time for the acclimation of the population. Peat does not need additional nutrient sources as the rest of the supports. However, mixing and restoration of moisture were necessary to maintain good biofilter performance. REFERENCES

1 2 3 4 5 6 7 8 9 10 11

Riveros H. G., Tejada J., Ortiz L., Julian-Sanchez A. and Riveros-Rosas H., J. Air and Waste Manag. Assoc. 45 (1995) 973-980. Leson G. and Winer A.M., J. Air and Waste Manag. Assoc., 4 (8) (1991) 1045-1054. Bohn, H., Proc. 89th Annual Meeting of Air and Waste Manag. Assoc. Nashville, Tennessee, U.S.A. 1996. Groenestijn Van J., Harkes M., Cox H. And Doddema H., Proc. Air Pollution Control Tech., Los Angeles Cal. U.S.A. (1996) 317-323. Lith Van C., Proc. 89th Annual Meeting of Air and Waste Manag. Assoc., Nashville, Tennessee, U.S.A. 1996. Wang Z., Govind R. And Bishop D.F., Proc. 89th Annual Meeting of Air and Waste Manag. Assoc., Nashville, Tennessee, U.S.A. 1996. Stewart C.W., and Kamarthi R., Proc. 90th Annual Meeting of Air and Waste Manag. Assoc, Toronto, Can. 1996. Thompson D., Sterne L., Bell J., Parker W. and Lye A., Proc. 89th Annual Meeting of Air and Waste Manag. Assoc, Nashville, Tennessee, U.S.A. 1996. Acufia M.E., Auria R., Pineda J., Perez F., Morales and Revah S., Proc. 90th Annual Meeting of Air and Waste Manag. Assoc, Toronto, Can. 1996. Seed L. and Corsi R.L., Proc. 89th Annual Meeting of Air and Waste Manag. Assoc. Nashville, Tennessee, U.S.A. 1996. Kennes C., Cox H. H. J, Veiga M. C. and Doddema H.J., Proc. Air Pollution Control Tech., Los Angeles Cal. U.S.A.4 (1995) 2279-2284.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

A Unified View on Nitrogen Oxide Abatement Jan Paul"2

1. LuleA University of Technology, Physics, 971 87 LuleL Sweden [email protected] 2. Hacettepe University, Chemistry, 06532 Beytepe-Ankara, Turkey [email protected] A unified model for nitrogen oxide abatement is suggested. The model observes that nitrates are the most common surface species on oxide supported catalysts under literally all conditions below the light-off temperature. The three routes for nitrogen oxide reduction; (i) direct decomposition, (ii) selective catalytic reduction (SCR) with ammonia and (iii) hydrocarbon driven NOx reduction, merely become different ways to destabilize surface coordinated nitrates. Nitrates are decomposed by thermal excitations in direct NO decomposition and by reactions between NO 3 and NH4§ in ammonia SCR. Reactions between NO 3 and hydrocarbon derived ligands rationalize the importance of partial oxidation products for the third route. The model stresses the significance of Cu2§ n§ dimers for dual ion exchanged catalysts and lattice flexibility for direct decomposition over aluminum rich ZSM5 [TM = Transition Metal].

I. Introduction Nitrogen oxides can be reduced using different technologies. Three way conversion (TWC) of CO/NO proceeds via complete dissociation and reactions between atomic species at metal surfaces but few other routes are as well understood on a molecular level. Catalysts are often inhomogeneous and many reaction pathways have been suggested for direct NO decomposition and selective catalytic reduction (SCR) with ammonia or hydrocarbons in an oxidizing environment. The present text is an attempt to justify a 'unified' model for nitrogen oxide reduction based on our results and literature data. We discuss direct NO x decomposition and reactions with ammonia and hydrocarbons. TWC and NOx reactions with carbon supported metals are clearly excluded albeit we do acknowledge that both these paths are conceivable for hydrocarbon/NOx reactions under reducing conditions. Our model emphasizes the significance of nitrato groups at the catalyst surface, either as siteblockers or as intermediates. NO 3 is found in unidentate and bidentate coordination to single metal atoms (Fig. 1). Metal dimers and Me-O-Me pairs allow for bridge ligation. Finally, multiply coordinated or mineral nitrates with near C3v symmetry are observed but less likely to form on a surface unless strong compound formation occurs.

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2. Experimental

We use Ti doped aluminas and zeolite ZSM5, both ion exchanged with Cu 2§ or Cu 2§ in combination with ions of Ni or Pd. The materials where characterized with TGA including mass-spectrometry, temperature programmed XRD, ESCA and Infrared spectroscopy. Reactions of NO, NO + O2 or NO + 02 + CH4 where followed by in situ transmission and reflectance FTIR.

3. Results & Discussion Following exposure of a Cu-ZSM5 catalyst to NO or NO + 02 a vibrational band is observed below the light-off temperature and interpreted as a bridged nitrato group bound to Cu 2§ - O Cu 2§ dimers. This band disappears above the light-off temperature but the intensity below this temperature correlates with the catalytic activity [ 1]. These bridge coordinated nitrato groups block the active sites for NO conversion. The tentative reaction intermediate, N203, also binds in a bridge configuration to the same Cu 2§ - O - Cu 2§ dimers. A flexible lattice, such as low SiO2 : A1203 ratio ZSM5, is advantageous for direct decomposition, thermal motion in the support destabilizes ligated nitrates and opens the sites for adsorption of reaction intermediates [ 1].

NO.

Cu

N

Cu

0 - Cu

Cu

0 - Cu

Fig. 1 Unidentate NO 3 (right), bridge coordinated NO 3 (middle) and bridge N203 (right) The roles of the support in direct decomposition are twofold, to destabilize ligated nitrates by lattice vibrations at elevated temperatures and to conserve a high dispersion of metal dimers. Zeolite ZSM5 is the preferred carrier and ideally we choose a low SiO2:A1203 ratio. Ratio 48 gives maximum adsorption bond strength but higher A13§ concentrations give more flexible lattices and are thus often preferred. The significance of thermal lattice vibrations is illustrated by a comparison between hydroxyl group destabilization and NO conversion. Fig. 2 shows that water evolution from zeolite ZSM5

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with no metal ion exchange (H-ZSM5) has the same temperature dependence as the catalytic reaction over Cu-ZSM5. The common explanation is thermally activated vibrations in the lattice framework. The amplitude of these vibrations increase with the A1 content, eventually leading to dealumination unless the effect is countered and the lattice reinforced by di- or trivalent ions. 40

Cu-ZSM5-53-145 II

o~ 30 v

tO (/)

B

.,..,

G)

>

tO

0 0 Z

II

---\i\

I,.,.

2o

/ 10

OH removal Cu-ZSM5-53-O (arb. y-scale)

0

~

0

I

1O0

,

I

200

m

t

I

-

B

!

i

/"7

,

300

I

400

,

I

500

600

Temperature (C) Fig. 2 NO conversion over zeolite ZSM5 with SiO 2 "Al20 s ratio 53, ion exchanged with Cue+ to 145 %. The dotted lines shows the temperature dependence of water evolution from hydroxylgroup decomposition over Cu-ZSM5-53-O i.e. the same zeolite in its proton form.

Surface nitrates are destabilized by thermal activation but an asymmetry in the adsorption site by co-cations will translate into an asymmetry in the adsorbed N O 3, causing one N-O bond to weaken and eventual nitrate dissociation. This enables N203, the critical but short lived intermediate in direct decomposition, to form on the 'same' sites at a lower temperature for dual ion exchange zeolites than for single ion exchanged ZSM5. A destabilization of the nitrates translates into a shift to lower light-off temperature. Only small differences are observed between different co-cations which is logical given the above model. Spectroscopic data for bridge coordinated nitrates can provide information about the abundance and characteristics of the active site where N203 is formed. Other means to destabilize the surface nitrate is via ligation of a functional group derived from a reductant. This is the case for ammonia where NH4+ ideally coordinates to the same site as NO 3. Together they form a NOaNH 4 complex [2]. Ammonia is mixed with the oxygen rich exhaust over a catalyst bed of vanadia/titania in SCR units. Again abundant nitrates are descended by vibrational spectroscopy. Interaction with co-ligated NH4+ leads to N-O bond weakening in NO 3 and eventually a reaction between the two ligands [2]. Fig. 3 symbolically

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illustrates a mixed metaloxide site, without favouring a specific ligation for NO 3 and N H 4.

NH 4

(

T M n*-- 0

- T M r"§

Fig. 3 Surface bound N O 3 and N H 4 o n a mixed metaloxide site. TM ~+and TM ~+ mean different transition metal ions.

Similar to ammonia adsorption, partial dehydrogenation of a hydrocarbon leads to a surface bound species. For methane this activation occurs on a metal site. Fig. 4 illustrates a methyl species bound to a transition metal in the vicinity of a Cu 2. bound N O 3 group.

C u 2*-- 0

- T M n+

Fig. 4 NO 3 and C H 3 coordinated to different metal ions at a dual ion exchange site

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Following a familiar path this affects the nitrate which becomes partially reduced by oxygen transfer to the ligated hydrocarbon. This also explains the observed significance of partially oxidized hydrocarbons [3]. Adsorption sites capable of coordinating two ligands, NO 3 and NH4§ or NO 3 and a hydrocarbon derived species, are preferred for (ii) and (iii) and catalyst preparation should focus on this task. Slow 50/50 Cu-cocation exchange from dilute solutions at appropriate pH gives an even distribution of mixed Cu-O-Me dimers, the preferred active sites for direct decomposition [ 1]. 4. Conclusions We suggest that the key to nitrogen oxide reduction is to destabilize surface nitrates. These species readily form under oxidizing conditions albeit different coordination can give different vibrational fingerprints on different catalysts. We note that photoemission spectroscopy of the bonding orbitals provide a fingerprint less readily confused by an altered coordination. Thermal nitrate destabilization opens sites for adsorption of N203 during direct decomposition. Ammonia and hydrocarbon derived intermediates serve the same purpose for SCR reactions, both types of derivatives react with surface coordinated NO 3.

The similarity between the three ways of nitrogen oxide abatement, (i-iii) is logical when considering the similarities between the preferred catalysts. In all three cases metal ions bound to an oxidic carrier are the active material. An oxidic support does not provide any delocalized electron density for strong back-donation to ligated NO and NO will consequently not dissociate on these catalysts. Three-way-conversion (TWC) and NOx abatement over active carbon catalysts both proceeds via dissociative adsorption followed by desorption of N 2 and CO 2. TWC is active on metallic or bimetallic particles with the ability to promote dissociative adsorption. NO x abatement over carbonaceous materials are promoted by metal inclusions or metalcarbides, again with the ability to donate electrons. The latter mimics the reaction path of CO z during coal gasification. Oxide supported metal ions lacks this electron donating capacity but provide excellent supports for nitrate adsorption.

I kindly acknowledge discussions with Drs. E. Bjiirnbom, M. Kantcheva and R. Keiski REFERENCES 1. Ganemi, B. Bj6rnbom E., and Paul J., Appl.Catal.B Environmental 17 (1998) 293; Ganemi B., Bj6rnbom E., and Paul J., Micropor.Mesopor.Materials (submitted); Ganemi B., Rahkamaa K., Keiski R.L., 0hman L.O., Bj6rnbom E., Salmi T., and Paul J. (manuscript) 2. Sadykov V.A., Baron S.L., Matyshak V.A., Alikina G.M., Bunina R.V., R.V., Rozovskii A.Ya., Lunin V.V., Lumina E.V., Kharlanov A.N., Ivanova A.S., and Veniaminov S.A., Catal.Lett.37 (1996) 157; Kantcheva M., manuscript 3. Petunchi J.O., Sill G., and Hall W.K., Appl.Catal.B Environmental 2(1993)303; d.Itri J.L. and Sachtler W.M.H., Appl.Catal.B Environmental 2(1993)L7

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Measuring the adsorption of reactive probes as a tool for understanding the catalytic properties of de-NOx catalysts A. Gervasini ~ and A. Auroux b a Dipartimento di Chimica Fisica ed Elettrochimica, Universitb. di Milano, via Golgi 19, 1-2013 3 Milano, Italy b Institut de Recherches sur la Catalyse, CNRS, 2 avenue Einstein, F-69626 Villeurbanne Cedex, France The surface chemistry of three different copper based systems (Cu-ZSMS, Cu-ETS 10 and Cu-silica-alumina) prepared with different copper loadings has been studied by adsorption microcalorimetry of NO and CO. The reducibility and regeneration of the Cu2+ ions and copper oxide crystals were studied by TPR-TPO experiments. The catalytic activity for NO reduction with hydrocarbons in oxidizing atmosphere appeared to depend on the nature and dispersion of the copper species. 1. INTRODUCTION The negative impact of nitrogen oxides on human health and on the environment results in increasingly strict regulations on their emission levels. To match the severe standards, intense research in the area of catalytic reduction of NO to N2 with hydrocarbons (CrC4 species) has been developed (de-NOx catalysts) [1,2]. Copper exchanged ZSM-5 systems are widely used in the selective catalytic reduction of NOx with hydrocarbons while supported copper catalysts are active in the reduction of NOx with ammonia and with CO. The various results found in the literature imply that these reactions could proceed via different mechanisms and, probably, on different sites. The support of the active phase plays a determining role by ensuring a high dispersion of the active metal cations and by stabilizing these cations at low coordination. A lot of effort has been made to elucidate the mechanism of de-NOx reactions, mainly using kinetic evaluations and spectroscopic studies [3,4], whereas little is known about the properties of the active sites in relation with their activity. A full understanding of the NOx reduction reaction has not been yet completely achieved, in particular concerning the roles of the oxidation state of the metal, of its coordinative insaturations, and of the contribution of the support. As the adsorption of some reagent species should be among the steps that describe the catalytic reaction of NO reduction, the measurement of the adsorption properties, in terms of adsorption capacity and associated energetics, is of crucial importance to understand the catalytic performances of the active sites. In this perspective, we present a comparative investigation of various Cu based catalytic systems taking into account different host supports: an alumino-silicate, ZSM-5 (Si/Al = 15), a

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titanium silicate, ETS-10 (Si/Ti = 4.7) and a silica-alumina (Si/AI = 5.5). Our study was focused on the adsorption properties of some reactive probes (such as NO as well as CO and NH3) as determined by adsorption mierocalorimetry and on the redox properties as determined by TPRffPO cycles, in relation with the catalytic activity of the different samples in the NO reduction by C2I-I4 in oxidizing atmosphere (NO-C2H4-O2). 2. EXPERIMENTAL

The amorphous, on silica-alumina (SA), and crystalline, on ZSM-5 and ETS-10, catalysts were prepared by deposition of Cu ions from copper acetate precursor. The starting materials were the IT forms of SIO2-A1203 oxide (alumina content of 13.5 wt %, from Akzo Chemic) and of ZSM-5 zeolite (alumina content of 2.9 wt %, from ENI), and the Na§ + form of ETS10 titanium silicate (titania content of 18 wt %, from Engelhard). Conventional base exchange procedures were performed to prepare the samples with low copper amounts (3 to 11 wt %), while the more copper loaded catalysts were prepared by a two-step procedure of exchange plus impregnation. The samples were labelled as Cu-ETS-(02,03,04,1,5) and Cu-ZSM5-(02,5) depending on the amount of copper deposited. Details on their preparation and characterization can be found in [5] and [6]. The microcalorimetric studies were performed in a heat flow calorimeter (C80 from Setaram) at 150~ for NH3 adsorption and 30~ for CO and NO adsorptions. A detailed description of the technique has been reported elsewhere [7]. The microcalorimeter is linked to a volumetric adsorption line, equipped with a Barocel capacitance manometer for pressure measurements. The samples were outgassed at 400~ overnight prior to microcalorimetric measurements. The differential heats of adsorption were measured as a function of coverage by sending repeated doses of gas onto the samples until an equilibrium pressure of about 130 Pa was reached. Then the sample was evacuated for 1 h at the same temperature and a second adsorption was performed in order to allow the determination of chemisorption uptakes. Thermoprogrammed reductions (TPR) were carried out in a differential scanning calorimeter linked to gravimetry (TG-DSC 111 from Setaram) at a heating rate of 5~ Prior to TPR runs, the samples (ca. 35 mg) were pretreated at 100~ for 1 h and cooled to room temperature in a stream of helium. Then they were heated in a flow mixture of helium and hydrogen (80 vol % of H2) up to 650~ in quartz sample holders (total flow rate : 43 mL/min). The weight loss together with the heat flow signal were recorded as a function of temperature. The TPR experiments were followed by TPO runs in a flow mixture of helium and oxygen (60 vol % 02) with the same heating procedure. Catalytic activity in the NO reduction by C2H4 in oxygen-rich atmosphere (NO-C2H4-O2) was studied in a fixed-bed quartz tubular downflow reactor at atmospheric pressure. The flow of feed gases was controlled from independent mass flow controllers (Hi-Tec. from Bronkhorst). Fixed NO, C2H4 (0.4 % v/v) and oxygen (4 % v/v) concentrations in He were used. The gas flow was 5.5 L.h ~ corresponding to a space velocity of 7500 h "~ (GHSV). Temperatures in the interval 150-500~ were investigated. The reactor outflow was analyzed using a gas chromatograph equipped with a T.C.D. detector (C.P. 9000 from Chrompack) mounting a 60/80 Carboxen-1000 column (from Supelchem). The extent of NO reduction was controlled by determining the N2 production and that of C2H4 conversion by the CO2 production. Integral conversion rates of NO to N2 (TOF, in terms of mole of N2 formed per mole of Cu per second) were calculated for comparative purposes.

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Table 1 Physieo-ehemical characteristics of selected materials Sample Cu amount Exchange ETS10 Cu-ETS-02 Cu-ETS-04 Cu-ETS-1 Cu-ETS-5 ZSM5

Cu-ZSM5-02 Cu-ZSM5-5 SA Cu-SA

(wt%)

(%)

3.1 8.4 11.4 23.6

24 66 90 186 93 519 -

-

3.0 16.6 8.0

Surface Area (m2/g) 506 381 388 296 27 435 366 308 365 285

Acidity Qtmol NHdm 2) 1.68 1.15 0.87 2.22 nd 1.70 2.59 2.74 0.90 1.48

3. R E S U L T S A N D D I S C U S S I O N

A summary of the compositions, exchange levels and surface areas of the samples is presented in Table 1. Table 1 also gives the adsorbed ammonia uptakes, expressed in pmol/m2 for a given equilibrium pressure of 26.7 Pa. These values are representative of the acidity of the samples. The adsorption of NH3 on the surface of the sample is strongly influenced by the acidity of the support. The totally exchanged Cu-ETS-1 sample displays a much higher number of NH3 adsorption sites than the ETSI0 matrix. On the contrary, the low exchange ETS samples are less acidic. Both the low and over-exchanged Cu-ZSM5 samples display more NH3 adsorption sites than the corresponding ZSM5 host matrix. Thus ammonia adsorption on copper sites appears to be essentially dependent on their location in the matrix [5]. Although NO and CO adsorbates on the active surface may serve as active precursors involved in surface reactions for N2 and CO2 formation, few attempts have been made to correlate the bonding strength and concentration of adsorbates with the catalytic activity [8]. In the present work, adsorption calorimetry was used to determine the strength of active sites and reactivity of various adsorbates (CO, NO). Figs. 1 and 2 represent the differential heats of NO and CO adsorption respectively as a function of coverage for Cu-ETS, Cu-ZSM5 and Cu-SA samples. The ETS10 and ZSM5 supports do not adsorb significantly CO or NO [6]. The differential heats of NO adsorption on the Cu-ETS samples tend to increase with the copper amount for the samples prepared by single-step ion exchange, reaching a maximum for the totally exchanged Cu-ETS-1 sample, while the overexchanged Cu-ETS-5 sample displays low heats of adsorption, very close to those of the support, because of the very low surface area and partial collapse of the structure. The curves are continuously decreasing, which indicates a large heterogeneity of the systems under study. The Cu-ZSM5 samples display important NO adsorption properties compared to the host matrix and a greater homogeneity of sites, but the amount adsorbed does not vary significantly with the Cu amount. Finally, the Cu-SA sample shows the highest initial heats (ca. 130 kJ.mol1) but adsorbs slightly less NO than Cu-ETS-04 in spite of a similar copper content. The differential heats of CO adsorption (Fig. 2) show that the CO uptake on Cu-ETS samples also increases with the copper amount, reaching again a maximum for Cu-ETS-1.

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化

Q (kJ/mol)

2OO

140

CuZSM5-02

- -9- "

CuZSM5-02

"+ 9-

CuZSM5-5

--+-"

CuZSM5-5

--,V--

CuETS-02

----

Z

120 [

Q (kJ/mol)

CuETS-02

150

CuETS--04

100 - •

~"

-I-

~

CuETS-1

--E--

CuETS-04

m

CuETS-1 -5

--C .... C u E T S - 5

80~

100

6O

40

N

50

20

0 0

I

I

I

I

I

20

40

60

80

100

0

,

NO uptake (~mol/g) Fig. 1. Differential heats of NO adsorption versus coverage.

120

0

I

I

I

I

l

1

20

40

60

80

100

120

140

CO uptake (IJmol/g) Fig. 2. Differential heats of CO adsorption versus coverage.

Contrarily to the behavior with NO, the most loaded Cu-ZSM5 sample (Cu-ZSM5-5) adsorbs a much greater CO amount than the Cu-ZSM5-02 sample, and the shapes of the two curves differ by the presence of a large plateau around 140 kJ.mol 1 for Cu-ZSMS-5. CO adsorption on Cu(I) copper sites is known to be partly irreversible at room temperature, while CO stabilized on Cu(II) or Cu metal sites is almost completely depleted by briefly outgassing at room temperature [9]. So, assuming that CO adsorbs preferentially on Cu § sites, a direct correlation between the concentration of Cu § ions and the amount of CO adsorbed can be established. On the contrary, NO strongly adsorbs on Cu 2§ sites. These contrasted behaviors provide a useful tool to identify the oxidation state of matrix-encaged copper. The respective amounts of CO and NO adsorbed are similar for the Cu-ZSM5 samples, while the amount of NO adsorbed is approximately half of the amount of CO adsorbed for the Cu-ETS samples. Previous IR studies and TPR results on Cu-ZSM5 [ 10,11 ] have indicated that a significant fraction of the copper in over-exchanged Cu-ZSM5 is present in the form of dimeric Cu E+ complexes in addition to monomeric Cu 2§ ions, and that the oxygen bridged species (Cu-O-Cu) 2§ can be easily reduced to Cu § species by heating under vacuum at 400~ ; however a substantial quantity of isolated Cu 2+ ions remains. The reducibility of Cu 2+ ions located in the framework of the molecular sieve plays an important role in the catalytic NO reduction activity. The values of the peak maxima for the HE-TPR of our catalysts are shown in Table 2. The HE-TPR curves of the Cu-ETS samples have two peaks : one around 167~ and the other between 180 and 200~ The Cu-ETS-5 sample displayed a shoulder near 210~ where the higher temperature peak existed. The first and second peaks can be attributed to the reduction of CuO to Cu20 and the reduction of Cu20 to Cu ~ respectively. The Cu-ZSM5 samples also presented two peaks. The first peak was

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Table 2 Temperature of peak maxima of H2-TPR and O2-TP0 curves of selected materials Catalysts Temperature of peak maxima Temperature of peak maxima (~ (TPR) . (~ (~.TPO) Cu-ETS-04 160(s) 181 153 220 275 Cu-ETS-1 167(s) 187 167 222 265(s) Cu-ETS-5. ' 168(s) 200 210(s) 191 276 Cu-ZSM5-02 191 275 Cu-ZSM5-5 187 205(s) 212(s) 206 275 Cu-SA 197 209(s) 229 144 268 (s) " shoulder probably caused by the reduction of Cu 2+ to Cu +, while the second peak was smaller and possibly due to the reduction of CuO to Cu ~ The stability of copper in a reduced state was examined by TPO analysis. Oxidation of the Cu-ZSM5 samples began at a higher temperature than for the Cu-ETS samples. The zeolite structure strongly affects the oxidation properties of Cu ~ The Cu-ETS samples present three peaks, which may be assigned to the oxidation sequences of Cu ~ i.e. Cu ~ -~ Cu 2+ or Cu +, Cu ~ -~ Cu20 and Cu20 --~ CuO. Catalytic activity in the NO-C2H4-O2 reaction on the differently supported copper catalysts was studied in order to determine the relationships between the properties of the active copper sites and their activity. Deep differences in reactivity emerged among catalysts, reflecting the dramatic influence of the support in stabilizing Cu centers in geometrical and electronic situations suitable for activity. The two Cu-ZSM5 catalysts, as expected, had very high activity. The TOF values were in the range 55-80 and 10-20 molN2.molcd~-s1 for Cu-ZSM5-02 and Cu-ZSM5-5, respectively, at reaction temperatures in the 300-500~ interval. The decrease of TOF with increasing Cu loading suggests that only the isolated copper ions dispersed in/on the zeolite matrix are active; the over-exchanged copper, present mainly as 100 i

r

.9, ~ O z

'1

.~

20

o" rj

~,,,.~

-~....

O

*" o

c 0 '-

I

~i~ ~

3O z

i

9

~

O

c 0

~D cO

o 0

"'

100

I

200

I

300

I

400

Temperature (~

,,

..'"

8O

60 i

ol

c o_.

10

S

40

t

9

I/,g.7

#~+~,..~'/~,~:

/' ,"

20

I-~-cu'ETs'~

I

l-O-Cu- ETS-03 I

['~176176

1

500

0

100

I

I

I

I

200

300

400

500

Temperature (.~

Fig 3 Dependence of NO conversion to N2 (A) and of C2[]4 conversion to CO2 (B) on the reaction temperature for the Cu catalysts on the ETS-] 0 support

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CuO [6], was not catalytically active. The same behavior could be observed for the copper catalysts on ETS10 support. TOF values were in the range 13-23 for the least Cu-loaded catalyst (Cu-ETS-02) and regularly decreased with increasing Cu content, down to 2-5 mols2mold~-s ~ for the catalyst with high Cu content (Cu-ETS-1). It can be inferred that small copper aggregates are much more active than large ones, and therefore that the reduction of NO by hydrocarbons seems to be a "structure-sensitive" reaction. The over-exchanged catalyst (Cu-ETS-5), which has lost most of its microcrystalline character, was completely inactive toward the N2 formation, only the quantitative combustion of C2I-I4 at high temperature was observed. Moreover, the Cu-SA catalyst, which displayed scarce dispersion of the Cu ions, was ineffective for NO reduction. Presumably, these two samples (Cu-ETS-5 and Cu-SA) contain CuO crystallites which are very active for hydrocarbon combustion, therefore making C2H4 combustion the predominant reaction. The most interesting results in the NO-C~H4-O2 reaction for the catalysts of the ETS series are presented in Fig. 3 in terms of N2 and CO2 productions. The amount of N2 produced was about the same for every catalyst, and the temperature of maximum N2 production was not clearly related to the Cu content of the catalyst. A shift towards higher temperatures was observed for Cu-ETS-02 only. On the other hand, a clear dependence on Cu content was observed for the C2I-L combustion, confirming that large CuO crystallites had better combustion ability. 4. CONCLUSION The characteristics of the support and the preparation method greatly influenced the deposition of copper in the final catalyst. Copper can be stabilized in different forms: Cu § and Cu 2+ ions and crystalline aggregates of Cu20 and CuO oxides. In the NO-C2H4-O2 reaction, highly dispersed copper centers with high reduction ability are much more effective than stabilized copper centers. The study of the adsorption of different reactive probes using volumetry linked to microcalorimetry, with the help of infrared spectroscopy data from the literature, made it possible to quantify the amounts of the various copper-containing species. REFERENCES

1.

M.C. Kung, K.A. Bethke, J. Yan, J.-H. Lee, H.H. Kung, Appl. Surf. Sci., 121/122 (1997) 261. Y. Traa, B. Burger, J. Weitkamp, Micropor. Mesopor. Mater., 30 (1999) 3. 3. N.W. Cant, A. D. Cowan, Catal. Today, 35 (1997) 89. 4. A.T. Bell, Catal. Today, 38 (1997) 151. 5. A. Auroux, A. Gervasini, C. Guimon, J. Phys. Chem., (1999) in press. 6. A. Gervasini, C. Picciau, A. Auroux, Micropor. Mesopor. Mater., (1999) in press. 7. A. Auroux, Topics in Catalysis, 4 (1997) 71. 8. Z. Chajar, M. Primet, H. Praliaud, M. Chevrier, C. Gauthier, F. Mathis, Stud. Surf. Sci. Catal., 96 (1995) 591. 9. K. Hadjiivanov, L. Dimitrov, Micropor. Mesopor. Mater., 27 (1999) 49. 10. G.D. Lei, B.J. Adelman, J. Sarkany, W.M.H. Sachtler, Appl. Catal. B, Environmental, 5 (1995) 245. 11. C.Y. Lee, K.Y. Choi, B.H. Ha, Appl. Catal. B, Environmental, 5 (1994) 7. .

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Reactivity of the NOx surface species formed after co-adsorption of NO + Oz on a WO3/ZrOz catalyst: an FTIR spectroscopic study Stefan Kuba, Konstantin Hadjiivanov* and Helmut Knrzinger Institut for Physikalische Chemie, LMU, Butenandtstral3e 5-10, Haus E, M0nchen 81377, Germany The only stable surface species produced after NO + 02 coadsorption o n WO3/ZrO2 are surface bidentate nitrates. They are thermally stable at 573 K, but they easily oxidize methane at this temperature.

1. INTRODUCTION Presently, the most attractive route toward reduction of NOx-emissions from stationary sources is the selective catalytic reduction (SCR), using ammonia as a reducing agent [1-3]. Many efforts are presently made to develop the SCR of NOx by hydrocarbons (HC-SCR) [48], because the three way catalysts are not efficient for NOx control in lean burn engines, in which the burning of the fuel occurs in excess of oxygen [4]. The first reported catalyst for HC-SCR was Cu-ZSM-5 which operates with C2+ alkenes and C3+ alkanes [5,6]. Many other materials have been tested and shown promising HC-SCR activities [7-9]. A very important result in research of HC-SCR is the discovery of Li and Armor [ 10] that the reduction of nitrogen oxides can be performed with methane on Co-ZSM-5. This provides the principal possibility to replace ammonia as a reducing agent in stationary NOx sources by CH4. The following key results in the studies of CH4-SCR can be outlined: 9 Li and Armor [11] reported that, in addition to Co-ZSM-5, other Co-exchanged zeolites are active in the reaction, while oxide supported cobalt is totally inactive. 9 Adelman et al. [12] reported a comparative mechanistic study on the HC-SCR over Cu-ZSM-5 and Co-ZSM-5. These authors found that surface species characterized by a band at ca. 1526 cm1 (and assigned to nitro groups) are produced during NOx adsorption on Co-ZSM-5, but not on Cu-ZSM-5. These species, in contrast to the nitrates formed on Cu-ZSM-5, reacted easily with methane. 9 Inaba et al. [13] demonstrated that oxide-supported cobalt could also show very good properties in HC-SCR. The authors found a high activity for this reaction of Co/SiO2 prepared via impregnation of silica with cobalt acetate and very poor activity for a sample prepared via impregnation with cobalt nitrates. The authors suggested that highly dispersed

Pemlanent address: Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria.

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cobalt ions are active sites for the reaction. This hypothesis was confirmed later by IR spectroscopy studies [14]. It was found that isolated cobalt ions on silica form the 1526 cm ~ species (assigned here to monodentate nitrates) atter NOx adsorption that are typical for Co-ZSM-5. These species easily oxidized methane. No such species were produced on samples prepared by impregnation with cobalt nitrate. Thus, it appears that a high dispersion of the supported phase favours SCR with hydrocarbons. In addition, IR studies of the reactivity of the surface nitrates correlates well with the activity of the samples. In this work we studied the surface NOx species produced atter NO adsorption and NO + 02 coadsorption on a WO3/ZrO2 sample as a potential candidate as a CI-h-SCR catalyst. Titania-supported vanadia and tungsta are among the commercially applied catalysts for SCR by ammonia. Although tungsta-based catalysts operate at a higher temperature than vanadiacontaining catalysts, we expected that the high surface acidity of WO3/ZrO2 might assist methane activation.

2. EXPERIMENTAL The WO3/ZrO2 sample was prepared by suspending amorphous Zr(OH)4• in an appropriate amount of aqueous solution of ammonium metatungstate. The suspension was refluxed for 16 h at 383 K and then the water was evaporated at 383 K. The solid was dried for 12 h at 383 K and finally calcined at 923 K. The nominal concentration of WO3 in the sample was 17.7 wt. %. This corresponded to a theoretical monolayer of tungsta on zirconia surface. FTIR spectroscopy studies was carried out using a Bruker IFS-66 apparatus at a spectral resolution of 2.0 cm 1 accumulating 128 scans. Self-supporting wafers (ca. 10 mg cm 2) were prepared from the sample powders and heated directly in the IR cell. The latter was connected to a vacuum/sorption apparatus with a residual pressure less than 10-3 Pa. Before the measurements all samples were activated for 1 h in a flow of oxygen at 673 K followed by 1 h evacuation at the same temperature. Raman spectroscopy was performed using a Dilor Omars 89 spectrometer equipped with a Peltier cooled CCD-camera and a Spectra Physics 2020 Ar § excitation source using the 488 nm line at a laser power of 50 mW. The spectral resolution was 5 cm-1. The specific surface area was determined by low-temperature nitrogen adsorption according to the B.E.T. method.

3. RESULTS AND DISCUSSION

3.1. Sample characterization The specific surface area of the sample was 120 m 2 g-l. Analogous zirconia samples, also calcined at 923 K are characterized by a significantly lower specific surface area indicating that the supported tungsta phase inhibits the sintering of the ZrO2. Raman spectroscopy revealed W-O-W linkages which suggested that the WOx surface species consist of small oligomeric clusters grafted onto the zirconia surface. No crystalline

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WO3 was detected. In addition, it was found that the surface WOx-species stabilized the

metastable tetragonal modification of the ZrO2 support (pure ZrO2 is stable in the monoclinic modification). The IR spectrum of the sample activated at 723 K contained, two bands in the v(OH) stretching region with maxima at 3740 and 3620 cm 1, assigned to Zr-OH and W-OH hydroxyls, respectively. In addition, an intense band at 1018 cm ~, due to v(W=O) and the respective overtone at 2025 cm "~ [ 15] are observed. Adsorption of CO at ambient temperature (1.6 kPa equilibrium pressure) on the activated sample results in the appearance of one band at 2201 cm ~ which decreases in intensity with the equilibrium pressure and shifts to 2205 cm -1 before disappearance atter evacuation. This band is typical for CO coordinated to Zr 4§ Lewis acid sites [16] and reveals the existence of bare zirconia surface on the sample. The Bronsted acidity of the sample was studied by low-temperature CO adsorption. In addition to the bands due to Zr4+-CO species, adsorption of CO at 85 K resulted in the appearance of a band at 2155 cm 1 indicating the formation of H bonded CO [17]. Simultaneously, a red shift of the W-OH band from 3620 to 3460 cm -1 was observed, i.e. Av(OH) was -160 cm ~. The value of the shift of the OH groups atter CO adsorption, compared with other oxide systems [17,18], is relatively high thus evidencing a relatively strong Bronsted acidity of the sample.

3.2. Adsorption of NO Exposure of the activated WO3/ZrO2 sample to NO (1 kPa equilibrium pressure) leads to an immediate appearance of two bands at 2286 and 2242 cm 1 in the IR spectrum (Fig. 1, spectrum a). The band at 2242 cm ~ has been observed previously after N20 adsorption on pure zirconia and assigned to the N - N stretching mode of N20 coordinated to Zr 4§ sites [19]. The band at 2286 cm ~ is probably due to N-bonded N20 on tungsta [20]. In addition, a weak band at 2130 cm ~ characterizing NO* species [21] is also visible. Two bands of very weak intensity at 1605 and 1220 cm 1 are assigned to surface bidentate nitrates [22]. Simultaneously with the formation of nitrates the band characterizing W=O bond is red shifted which indicates that at least part of the nitrates are formed with the participation of this oxo-group. Allowing the sample to stay in the NO atmosphere (Fig. 1, spectrum b) results in a strong increase in intensity of the NO + and the nitrate bands. The higher frequency component of the latter is split into two components at 1615 and 1580 cm 1. In addition, a strong band at 1935 cm ~ developed. The latter is assigned to nitrosyl species formed on sites that are affected by nearby nitrates. These results suggest that the following reactions take place on the catalyst surface: 3 NO o N20 + NO2 (1) 2 NO2 o NO + + NO3 (2)

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3.3. Co-adsorption of NO and 02 Addition of increasing amounts of oxygen to the sample placed in a NO atmosphere (Fig. 1, spectra c, d) leads mainly to an increase in intensity of the nitrate bands, i.e. NO is oxidized to NO3. In addition, a weak band at 1748 cm l indicates formation of adsorbed N204 [23]. Evacuation at ambient temperature (Fig. 1, spectrum e) leads to the disappearance of the bands due to NO and N204 and to a slight decrease of the amounts of NO + and nitrates. When the sample is evacuated 423 K (Fig. 1, spectrum f), NO + disappears and the nitrate bands decrease in intensity. After consumption of all NO § however, further evacuation leads to no substantial changes in the spectra. This suggests recombination of NO t with surface nitrates leading to the reverse of reaction (2). The nitrates are still detectable at 673 K.

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wavenumber, cm -1 Fig. 2. FTIR spectra recorded after exposure of the WO3/ZrO2 sample to a NO + 02 mixture, followed by 30 min. evacuation at 473 K (a), after 10 min. interaction with methane (2 kPa) at 473 K (b), subsequent 30 min. evacuation at 573 K (c) and 10 min. interaction with methane (2 kPa) at the same temperature (d). 3.4. Interaction of the surface nitrates with methane It has been reported that the reaction order of HC-SCR is ca. 1 in hydrocarbons and ca. 0 in NO [24]. This suggests that the hydrocarbons react with a catalyst surface modified by NOx. For being intermediates in SCR, the surface NOx species have to be stable at temperatures at which SCR proceeds, but have to react easily with hydrocarbons. The only stable surface NOx species on our sample were the bidentate nitrates, being thus potential candidates for SCR intermediates. For this reason we studied their interaction with CI-h. Surface nitrates were produced by co-adsorption of NO and 02, followed by evacuation at 473 K (Fig. 2, spectrum a). Interaction of this nitrate-containing sample with methane at 473 K leads to a strong decrease in intensity of the nitrate bands and production of small amounts of NO § (Fig. 2, spectrum b). Simultaneously, a strong CO2 band at 2355 cm1 was detected. These results indicate that, even at that low temperature, methane is oxidized by the surface nitrates giving COz as a final product. Unfortunately, we were not able to detect the formation of nitrogen but evidently part of the nitrates was reduced to NO +. The surface nitrates lett on the surface are stable towards evacuation at 573 K (Fig. 2, spectrum c), but all of them react completely with methane at this temperature (Fig. 2, spectrum d).

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4. CONCLUSIONS: 9 The only thermally stable compounds formed upon NO + 02 co-adsorption on a WO3/ZrO2 sample are surface nitrates. 9 These nitrates easily interact with methane even at 473 K giving CO2 as a final oxidation product. The strong Bronsted acidity of the sample is probably involved in the activation of methane.

ACKNOWLEDGEMENTS: This work was financially supported by the Deutsche Forschungsgemeinschaft (SFB 338) and the Fonds der chemischen Industrie. K. H. acknowledges a fellowship of the Alexander von Humboldt-Foundation. REFERENCES 1. F.J. Janssen, in "Handbook of Heterogeneous Catalysis", G. Ertl, H. Kn6zinger and I. Weitkamp (eds.), Willey- VCH, Weinheim, 1997, p. 1633. 2. V.I. Parvulescu, P. Grange and B. Delmon, Catal. Today, 46 (1998) 233. 3. G. Busca, L. Lietti, G. Ramis and F. Berti, Appl. Catal., B: Environ., 18 (1998) 1. 4. E.S.J. Lox and B.H. Engler, in "Handbook of Heterogeneous Catalysis", G. Ertl, H. Kn6zinger and I. Weitkamp (eds.), Willey- VCH, Weinheim, 1997, p. 1559. 5. M. Iwamoto and H. Hamada, Catal. Today, 17 (1991) 94. 6. W. Held, A. K6nig, T. Richter and L. Puppe, SAE Technical Paper 900496, 1990. 7. X. Feng and W. K. Hall, Catal. Lett., 41 (1996) 45. 8. L.J. Lobree, A. W. Aylor, J. A. Reimer and A. T. Bell, J. Catal., 181 (1999) 189. 9. T. Chafic, A. Ouassini and X. Verykios, J. Chim. Phys. Phys. Chim. Biol., 95 (1998) 1666. 10. Y. Li and J. N. Armor, Appl. Catal., B: Environ., 1 (1992) L31. 11. Y. Li and J. N. Armor, in "Natural Gas Convertion II", H.E. Curry-Hyde and R.F. Howe (eds.), Elsevier, Amsterdam, 1994, p. 103. 12. B. Adelman, T. Beutel, G. Lei and W.M.H. Sachtler, J. Catal., 158 (1996) 327. 13. M. Inaba, Y. Kintaichi, M. Haneda and H. Hamada, Catal. Lett., 39 (i 996) 269. 14. B. Djonev, B. Tsyntsarski, D. Klissurski and K. Hadjiivanov, J. Chem. Soc. Faraday Trans., 93 (1997) 4055. 15. G. Ramis, G. Busca, C. Cristiani, L. Lietti, P. Forzatto and F. Bregani, Langmuir, 8 (1992) 1744. 16. V. Bolis, B. Fubini, E. Garrone, C. Morterra and P. Ugliengo, J. Chem. Soc. Faraday Trans. 1, 88 (1992) 391. 17. M. Zaki and H. KnOzinger, Mater. Chem. Phys., 17 (1987) 281. 18. A. Tsyganenko, L. Denisenko, S. Zverev and V. Filimonov, J. Catal., 94 (1985) 10. 19. T. M. Miller and V.H. Grassian, Catal. Lett., 46 (1997) 213. 20. G. Ramis, G. Busca and F. Bregani, Gezeta Chim. Ital., 122 (1992) 79. 21 K. Hadjiivanov, J. Saussey, J.L. Freysz and J.C. Lavalley, Catal. Lett., 52 (1998) 103. 22 D. Pozdnyakov and V. Filimonov, Kinet. Katal., 14 (1973) 760. 23 J. Laane and J. R. Ohlsen, Prog. Inorg. Chem., 28 (1980) 465. 24. K. Yogo and E. Kikuchi, Stud. Surf. Sci. Catal., C84 (1994) 1547.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

The mechanism of the selective NOx adsorption on 12-tungstophosphoric acid hexa-hydrate S.Hodjati, C.Petit*, V.Pitchon and A.Kiennemann LERCSI, UMR 7515 du CNRS - ECPM, 1, rue Blaise Pascal, 67070 Strasbourg, France The paper describes the study of HPW, using conditions simulating as close as possible to a real car exhaust. The amounts of NO• absorbed and desorbed prove very high with NO and NO2 in molar ratio. XRD suggests that the structure is conserved following complete saturation by NOx. The adsorption mechanism proceeds via a substitution of the water molecules by NOx to form an [H+ (NO[, NO+)] complex. 1. INTRODUCTION The elimination of NOx stemmed from Diesel exhaust is a major challenge for automotive industries in the world. Many processes have been developed with two different concepts based upon direct reduction into nitrogen on stream or selective NOx trapping followed by a reduction [1]. For this second concept, Daimler-Benz has produced a very attractive possibility called SNR (Selective NOx Recirculation) whereby NOx are recirculated to the combustion chamber where they are reduced by combustion processes upon selective adsorption [2] The key point in such a device is the efficiency of the adsorbent which should have a high NOx adsorption capacity at low temperature (100-300~ and a good ability to desorb them at temperatures relatively low (maximum 550~ to minimise the input of energy. One of their main features should be a good resistance to SO2 poisoning which so far has not been proposed in the systems existing in the literature. These systems are numerous and are mainly based on inorganic oxides [3], mixed oxides [4,5], well-defined structures such as perovskites [6], cuprates [7] or modified three-way catalysts [8]. The most efficient formulations contain barium, which easily form nitrates [9] which are displaced by SO2 to form irreversible sulphates causing a permanent deactivation [10]. Almost at the same time, appeared in the literature the results of two different teams reporting adsorption of nitrogen oxides on 12-tungstophosphoric acid (HPW), compound of the heteropolyacids family having the formula H3PW~2040, 6H20 [11, 12]. HPW is an ionic solid; its structure is formed of primary units also called Keggin anions made of 12 octahedra WO6 surrounding a central tetrahedron PO4. The Keggin's polyanions are linked together by H+(H20)2 bridges to form the so-called secondary structure. The structure was fully described in 1977 in Acta Crystallography by G.M.Brown et al. [ 13]. The mechanism of adsorption of NOx proposed by the two teams is somewhat different; For Chen and Yang, NO only interacts with the HPW to substitute water molecules at low temperature at 50-230~ and low NO concentrations. Upon rapid heating, NO is decomposed into N2. The mechanism invokes the formation of NO via a (NOH) + complex replacing the H+(H20)2 linkages in the structure based upon NO thermodesorption, XRD results and bond lengths calculations. In their case, B61anger and Moffat have observed that NO2 is adsorbed with a stoichiometry of 3 molecules of NO2 per Keggin anion. Their mechanism involves the formation HNO2+ resulting from the reaction between NO2 and surface or bulk protons. The exposure of the solid to water vapour

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at 150~ led to a partial desorption of NO2 into the form of HNO3. NO is not able to penetrate into the bulk of the structure and therefore is not adsorbed on the structure. Nevertheless, if NO2 is preadsorbed some NO adsorption is observed and such phenomenon is related to the formation of N203 as put in evidence by infrared spectroscopy. One of the main objectives consists of the development of a material able to adsorb selectively NOx issued from both Lean-Bum or Diesel engines. From this point of view, HPW seemed to be very promising but there is still a strong necessity to study its behaviour towards NOx adsorption using conditions as closed as possible from a real car exhaust, containing for example CO2, SO2 or hydrocarbons. The existing results which are reported so far are not taking into account this aspect and also seemed to lead to contradictory conclusions. Our approach was to use other experimental techniques to bring new evidence allowing the elucidation of the mechanism of both NOx adsorption and desorption on HPW. 2. EXPERIMENTAL

The catalytic material was purchased from Strem Chemicals. It was characterised by XRD before and after reaction. In some cases, 1% of platinum was added by impregnation of H2PtCI6 solution. The experimental set-up is detailed elsewhere [9]. Briefly, The experiments were conducted in a flow reactor using a series of mass flow controllers with diluted gases (NO, NO2, CO, C3H6, 02, CO2, SO2, with N2 as the diluting gas). The reactor was a quartz tube of 12 mm in diameter and a sinter, sealed up with Cajon couplings. The uptake and desorption capacity of NO/NO2 were measured according to the temperature using a furnace and a temperature controller with a mass of catalyst of 330 mg and a flow of 300 cm3.min -~. Water was added using a saturator maintained at the required temperature in order to have 5% humidity in the flow. The water was removed from the stream with a permeation tube and the NO and NO2 were specifically examined by infrared analysers (Rosemount, accuracy +/- 10 ppm). Thermogravimetric analyses were using a TG-DTA 92-10 from Setaram. Two gas compositions were used in simulation of a Diesel (NO 1000 ppm, C3H6 50 ppm, CO 300 ppm CO2 5%, O2 10% and H20 5%)or a Lean-Burn (NO 500 ppm, NO: 500ppm, CO2 5%, 02 10% and H20 5%) exhaust. The tests were made according to the following procedure: the catalyst was heated up to 80~ in air and the gas flow was switched according to the type of experiments and maintained at this temperature for 15 min. The temperature was then raised to 550~ or 170~ with a ramp rate of 4~ -~ After 15 min. at this temperature, the gas flow was switched to synthetic air (O2/N2), and the temperature cooled to 80~ The overall experiment was repeated at least twice to measure reproducibility. The adsorption capacities are expressed in mg of NOx per gram of catalyst. 3. RESULTS AND DISCUSSIONS The first test was conducted by raising the temperature up to 550~ under a Lean-Burn mixture after a dwell of 15 minutes at 80~ The operation was repeated twice. For the first run, an adsorption of NOx between 100 and 300~ is observed followed by a desorption between 300 and 550~ as described by figure 1.The amount of NOx adsorbed and then desorbed are equal and of 38 mg/g of HPW. For the two following cycles this amount drops drastically down to 4.5 mg/g. The literature reports a thermal fragility of the heteropolyacids structure [14]. Indeed, after one experience at 550~ we observe a complete loss of cristallinity by XRD and deep modifications of the infrared spectrum. This degradation of structural properties corresponds to the loss of the H+(H20)2 bridges. To understand the process of water loss we have undertaken a TGA analysis (figure 2).

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There is an overall mass loss of 12.4% spread over three zone of temperature. The first one between 25 and 75~ represents 8.6% and corresponds to physisorbed water. The second between 100 and 240~ correspond to the loss of the 6 water molecules of the structure with a percentage of 3.2% and the third one of 0.6% between 375 and 475~ is water issued from

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decomposition. At this level the Keggin units are transformed into simple oxides according to the equation: H3PW12040 ~ (1/2)P205 + 12WO3 + (3/2)H20. Therefore, the degeneracy of structural properties can be correlated with the loss of adsorption capacity after the first run. In order to evaluate the limits of the structural stability of the adsorbent as well as to define an optimal working conditions, several temperature of adsorption were tried. The best results were obtained by using 170~ as a maximum as displayed by figure 3. A very peculiar phenomenon was observed in the broad range of temperature tested. The amount of NOx adsorbed and desorbed were always different. This discrepancy is explained by the fact that on such material desorption is observed during the cooling phase. This type of behaviour is very unusual and has never been reported before.

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Fig. 3 Detailed profile of adsorption-desorption of NO and NO2 Adsorption in a Lean-Burn with a maximum temperature of 170~ and desorption in wet air. Another important point to note from figure 3 is the fact that NO and NO2 are adsorbed and desorbed in molar ratio. In this case, the ratio NO/NO2 was equal to one. It was varied between 0.2 and 5 and the same observations is available, i.e. in all cases the quantity of NO adsorbed or desorbed is equal to the one of NO2. For this reason, no adsorption was observed in the case of a NO2 free mixture, i.e. in Diesel conditions. Nevertheless, the addition of 1% of platinum provokes the oxidation of NO into NO2 between 180 and 275~ In this range of temperature, a consumption of 30 mg/g became possible. A solid containing no Pt was saturated by NO and NO2 at 170~ and then examined by infrared spectroscopy. The HPW is diluted in KBr and the spectrum recorded at 25~ is shown on figure 4b and compared with a fresh sample on figure 4a.On the reference HPW a broad band with two maxima at 1620 and 1710 cm-" are identified and correspond respectively to water and H+(H20) bridges in the secondary unit. Upon submission to NOx new bands at 1295, 1384 and 2261 cm -~ appear. According to B61anger and Moffat [15] the band at 2261 cm -~ is attributed to a pertubated NO2+ species and more particularly to HNO2 +. In the case of NO adsorption on HPW presaturated with NO2, the appearance of a new band at 1304 cm -1 is observed and is attributed to the N203 species. The absorption bands of Nujol used the IR spectra record are located in the zone between 1350 and 1450 cm -1, which possibly hide the band at 1384 cm ]. According to Yang and Chen ]~, the band at 2261 cm ~ is due to the presence of NO + species and more

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particularly to (N-O-H) + . Likewise, the same authors reckon that the bands at 1384 and 1295 cm 1 are due to the interaction of NO with KBr used to press the HPW sample [16]. Our interpretation is somehow very different. According to the fact that NO and NO2 are adsorbed simultaneously and in the same proportions on one hand and based upon literature [17, 18] on the other hand, we attribute the band at 2261 cm -~ to an NO + species in interaction with the proton of the structure, the modification of the electronic environment of NO + could explain the shift in energy observed when compared to free NO + which is normally observed~ at 2330 cm z. IR vibrations for nitro and nitrito species are reported in the 1200-1400cm- region [ 17]. We attribute the bands observed at 1295 and 1384 cm -~ to a (- O-N=O) species stabilised in the acid structure in interaction both with the proton and the terminal oxygen of the Keggin units. _

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Wave Number (em "1) Fig. 4 :IR spectra of HPW: a) fresh b) saturated with NOx. One of the key factor for the NOx adsorption/desorption process is the presence of water. In the absence of water in the phase of adsorption, the temperature of adsorption is shifted of c.a., 20~ towards lower temperature indicating a competition between NO/N2 and water for the adsorption sites. In the absence of water in the phase of desorption, no desorption at all is observed. As soon as water is added to the mixture, immediate desorption of NOx becomes possible. These successive operations of adsorption-desorption with and without water are perfectly reversible and non-destructive since the initial capacity of 38 mg/g is always recovered. All those observations led us to propose a mechanism of adsorption and desorption of NOx on HPW. The adsorption proceeds via a substitution of the water molecules by NOx to form an [H+(NO2", NO+)] complex as described in figure 5. This process is a bulk phenomenon and is not related to surface since the amount of NOx adsorbed corresponds to a complete substitution of water, i.e. stoichiometrically, 6 molecules of water are replaced by 3 (NO +, NO2). Moreover, the temperature at which adsorption arise is perfectly correlated with the TGA experiment (figure 2) which shows that structural water evolves between 100 and 300~ Once the substitution is achieved, the cohesion of the secondary unit is maintained by this complex [H+(NO2-, NO+)]. A complementary evidence of such a fact is that by XRD

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the HPW structure is observed after complete saturation by NOx save for a shift of the lines towards higher 20. This co-adsorption of NO and NO2 implies that molecules such as CO2 cannot interact with the proton.

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~

Figure 5 : Mechanism ofNOx adsorption-desorption on HPW 4. CONCLUSIONS We can therefore summarise the different steps of the adsorption-desorption process in the following manner: In the first step, physisorbed water is evacuated between 25 and 80~ Then because of the built-in polar character of the HPW molecule between the proton and the oxygen and therefore the presence of an electrostatic field, a reaction transfer is operated according to: NO + NO2 ~ NO + + NO2-. Once the temperature reaches 100-120~ the simultaneous adsorption of NO and NO2 occurs by substitution of structural water molecules with formation of a [H+(NO2, NO+)] complex. During the cooling phase and in presence of water, around 100~ reverse substitution occurs.

Acknowledgement The authors would like to thank the European Community for financial support (BE 95-0054). REFERENCES 1 A.Fritz, V.Pitchon, Appl. Catal., B, 13 (1997) 1. 2 Daimler-Benz Patent DE 4319 294 (1993). 3 H. Arastpoor, H. Hariri, Ind. Eng. Chem. Process Des. Dev. 20 (1981) 223. 4 K. Eguchi, M. Watabe, S. Ogata et H. Arai, J. Catal, 158 (1996) 420. 5 M.Machida, H.Arai, Catal. Today, 27 (1996) 297. 6 J. M. D. Tascon, A. M. O. Olivan, L. G Tejuca, A. T. Bell, J. Phys. Chem, 90, (1986) 791. 7 M.Machida, H.Murakami, T.Kijima, Appl. Catal. B, 17 (1998)195. 8 Toyota Patent EP 589 393 A2, and EP 562 516 A1 9 S.Hodjati, P.Bernhardt, C.Petit, V.Pitchon, A.Kiennemann, Appl. Catal. B 19 (1998) 209 10 S.Hodjati, P.Bernhardt, C.Petit, V.Pitchon, A.Kiennemann, Appl. Catal. B 19 (1998) 221 11 R.T.Yang, N.Chen, Ind. Eng. Chem. Res., 33 (1994) 825 12 R.B41anger, J.B.Moffat, J.Catal., 152 (1995) 179 13 G.M. Brown, M.R.Noe-Spirlet, W.R.Busing, H.A.Levy, Acta. Cryst. B 33, (1977) 1038. 14 M. Misono, Catal. Rev. Sci. Eng., 29, (1987) 269. 15 R. B61anger, J. B. Moffat, J. Mol. Catal. A, Chem., 114 (1996)319. 16 N. Chen, R. T. Yang, J. Catal., 157 (1995)76. 17 K. Nakamoto, 4th Edition, Wiley-Interscience publication ISBN 0-471-01066-9, (1986). 18 W. R. Angus, A. H. Leckie, W. Ramsay, Proc. R. Soc. A., 149,(1935) 327

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Activity and DRIFT spectroscopy studies of NO oxidation, and reduction by C3H6 in excess 0 2 over A1203 and Au/AI203 G. R. Bamwenda*, A. Obuchi, S. Kushiyama, and K. Mizuno National Institute for Resources and Environment, AIST, 16-30nogawa, Tsukuba, Ibaraki, 305-8565 Japan The activity and in situ DRIFT spectroscopy studies of NO and NO2 SCR were performed on y -A1203 and Au/? -A1203. The oxidation of NO to NO2 over Au/AI203 increased with temperature passing over a maximum of 25% at --450 ~ before declining to near equilibrium conversions of ~20% at ca. 500 ~ The conversion of NOx over Au/AI203 increased with temperature, reached a maximum of~ 40% at 450 oC. When NO2 was used as a source of NOx, the NOx conversion reached up to 53% at 400 oC. The activity and DRIFT results indicate that the initiation of NO SCR on Au/~, -A1203 involves the oxidation of NO to NO2 followed by coupling of the NO2 or its adspecies, NO'x, with activated C3H6 on active sites on A1203 to form CnHmNxOy species, which are responsible for the propagation step. Its subsequent internal rearrangement and decomposition leads to the formation of N2 and other products. Taken together, the obtained results suggest that the NO2 intermediacy benefits the NO SCR over gold catalysts as a reaction initiator or oxygen transfer agent. 1. INTRODUCTION The catalytic reduction of NOx with hydrocarbons in the presence of oxygen is widely studied because of the possible application of this process in the NOx removal from automobile emissions. Until recently, most studies have focused on the development of catalysts in order to improve the activity for NOx conversion and selectivity to di-nitrogen molecule [1]. The objective of this study was to determine the activities, and to obtain information concerning the species adsorbed on Au/y-AI203 during the oxidation of NO, and during the reduction of NO and NO2 by propylene in the presence of oxygen, in order to identify chemisorbed species which could act as catalytic intermediates.

2. EXPERIMENTAL The ~,-A1203 (208 g/m 2) was obtained from Sumitomo Chemicals and was used after calcining it in air at 600 oC for 5 h. A 3% Au/A1203 catalyst was prepared by impregnating ~, -A1203 with a solution of HAuC14.4H20 at 60 oC. The resulting slurries were freeze dried overnight at 15 Pa, then calcined in flowing 20% O2/He at 500 ~ for 4 h. The obtained samples were washed several times in hot water for removal of CI-, which induces a strong modification of the Bronsted and Lewis acid site distribution on ~,-A1203, and calcined again at 400 oC for 2 h. The surface area of the obtained 3%Au/AI203 was 177 m2/g. The gold particles on Au/A1203 spent catalysts were in the 1-90 nm size range with the Au crystallite

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population being dominated by 30-60 nm particles, as determined from HRTEM. DRIFT spectra (100 scans, 4 cm" 1 resolution) were recorded on a JASCO FTIR-7300 spectrometer equipped with an MCT (HgCdTe) detector and an IR stainless-steel cell (JASCO DR-800/H) designed to allow the flow of reactants and to treat the samples in situ up to 600 oC. The spectra obtained were corrected for background by subtracting the original spectrum acquired in He under the same recording conditions. The NO conversion runs were carried out at atmospheric pressure using a quartz tube with 8 mm i.d. and 0.2 g of the catalyst. The feed gas consisted of 1000 ppm NO or NO2, 1000 ppm C3H6, and 5% 02 with He as the balance gas, ata total flow rate of 200 cm3/min (GHSV= 27,000 h-l). The activity at steady-state condition was determined by continuously measuring the content of NO and NO2 at the reactor inlet and in effluent streams with chemiluminescent analyzers. The N2, N20, CO and CO2 were analyzed by on-line GC. 3. R E S U L T S

AND DISCUSSION d e'l

Z 25

,=O,,=3%Au/AI203 ~ r "AI203 - . . NOEquil.Cony.

--

~

.

f't ~ L

50

3.1 Catalytic activity No~.,mo,oz J "i) Figure 1 shows the activities of y 20 ~1~ AI203 ~k/ I -~ 40 -A1203 and Au/A1203 for the reaction NO+l/202=NO2 and its reverse reaction. The oxidation of NO to NO2 over Au/AI203 10 20 ~ increased progressively with temperature passing over a maximum of 25% at ~450 ~ s before declining to near equilibrium conversions of ~20% at ca. 500 ~ Only a 200 300 400 500 slight oxidation of NO was observed on 1t -A1203. On the other hand, both materials Temperature, ~ were active for the partial decomposition of ure 1. The NO+O2 reaction over A1203 and ;~ Au/AI203: 0.1%NO or NO2, 5 % 0 2 , GHSV:27,000. NO2 into NO and 02 at T> 400 ~ The catalytic effects on the SCR of NO and NO2 over ), -A1203 and Au/A1203 are summarized in Table 1. The conversion of NO over Au/A1203 increased with temperature, reached a maximum of-~40% at 450 oC and then declined with increasing reaction temperature. This result is in accordance with that of Ueda et. al. [2]. Note that the maximum NOx conversions is attained at almost the same temperature as that of maximum NO oxidation to NO2 (see Fig. 1). The fall in activity at T> 450 ~ is due to increased competition of NOx and 02 for C3H6. At T> 450 ~ C3H6 was mostly oxidized to CO2. Appreciable NO SCR was also noted on y-A1203 at T> 450 ~ [4]. Both materials showed a high conversion of NOx to N2 with low formation of N20. When NO was replaced with NO2, the NOx conversion over Au/A1203 increased steadily with temperature reaching conversions up to 53% at 400 oC before gradually falling off with temperature. The NOx conversion over ),-A1203 increased steadily with temperature and remained near 100% at temperatures > 400 ~ The NOx conversion in the NO2+C3H6+O2 system was higher than that of the NO+C3H6+O2 reaction because NO2 easily activates C3H6 either by oxidation or nitration. When NO2 is used as feed, the low activity of Au/AI203 in comparison to that of y -A1203 stems from the fact that the fast oxidation of C3H6 on Au/AI203 prevents adequate

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secondary interactions. In addition, in this case Au masks some of the active sites on A1203 and makes them inactive for propagation reactions.

3.2. DRIFT studies 3.2.1. NO+1/202= NO2 reaction

The results from DRIFT studies for the NO+ 1/202=NO2 and NOx SCR are presented in Figures 2 to 4. Flowing NO+O2 on A1203 and 3%Au/A1203 resulted in the formation of

Table 1. Catalytic activities and major products from the SCR of NO and NO2 with C3H6 over ~,-A1203 and 3%Au/AI203 NO2+C3H6+O2

NO+C3H6+O2 Catalyst

T

SN2

SN20

SN2

SN20

(%)

(%)

(%)

(%)

(%)

(%)

(%)

(%)

200

4

0.2

85

15

7

1

67

33

300

6

2

97

3

25

6

87

13

400

8

3

90

10

92

32

92

8

450

18

15

92

8

96

51

91

9

500

34

68

76

24

99

67

88

12

200

2

0.3

86

14

3

1

70

30

3 00

3

1

99

1

16

5

90

10

(~

XNOx XC3H6

X N O x XC3H6

1t-A1203

3%Au/AI203

400

22

21

89

11

53

68

94

6

450

41

94

88

12

48

98

97

3

500

33

99

72

28

40

99

98

2

Conditions: NO, NO2 =940-970 ppm, C3H6=1000 ppm, 02=5%. GHSV= 27000 h-1. Where XNOx: NOx conversion, SN2: selectivity to N2 and SN20: selectivity to N20. intense bands characteristic of NO adsorption complexes, &g., nitro and nitrate groups in the region 1630-1300 cm -1, and N204 (1749 cm -1) [5,6], as shown in Fig. 2. The nature of these species was partly confirmed by doping A1203 with NaNO2 and NaNO3, which gave strong bands at 1436, 1329, and 1252 cm-1 characteristic of the -NO2- group vibrational modes, and at 1792, 1753, 1557, 1539, 1523 cm -1 corresponding to-NO3- groups. The intensities and stability of NO adspecies depended on exposure time and temperature. The adsorbates reached a steady-state condition after 30-60 minutes. The spectral series obtained from A1203 and 3%Au/A1203 when the NO+O2 flow at room temperature in Figs. 2a and b was replaced with He flow and the temperature raised to 300-400 oC are shown in Fig. 2c-e. Purging the samples in He (with flow rate 200 cm3/min) for 30 min at 25 ~ did not cause any significant changes in the spectrum of Fig. 2 a and b. By

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raising the temperature in the range 60-100 oC, the bands in the 1550-1350 cm -1 region disappeared progressively and disappeared completely at T> 200 oC. But as seen in Fig. 2c-e, the other sorbed NOx species were thermally stable and remained adsorbed, with slight band shifts, up to 450 oC. It seems probable that, in the presence of a reductant, such as propylene, these sorbed complexes may take part in the surface NO SCR.

r ==

0

"

l_ 2400

400o

I 1800

I 1400

1000

W a v e n u m b e r , (cm" 1) Fig. 2. Reaction of 1000 ppm NO+ 5% 02 at 25oc on A1203 (a) and 3% Au/A1203 (b) followed by He purge, 200 cm3/min, at 300-400 ~ A1203" 300 ~ (d)" 3% Au/AI203:300 ~ (c) and 400 o c (e). tscan = 2h.

3.2.2.

N O x + C 3 H 6 + O 2 reaction Figure 3 and 4 show the species observed during the NO+C3H6+O2 and NO2+C3H6+O2 reactions over Au/A1203. At T< 350~ the NO+C3H6+O2 reaction on Au/A1203 produced species such as adsorbed hydrocarbons (3000-2900 cm-1), CO2 (2349 cm-1), a strong cyanide band (2195 cm'l), CO (1952 cm-1), carbonate or carboxylate (1579, 1472-1458 cm-l), and formate species (1591, 1390-1377 cm-1), the latter not shown for brevity [3]. Elevating the temperature led to the decrease in intensity of the band at 2194 cm-1, and the appearance of a weak isocyanate band at 2232 cm -1, and a -CN band at 2128 cm -1. These bands were strongest between 350-450 ~ they disappeared from the surface at T> 500 ~ When 15NO was substituted for 14NO, the 2232, 2194 and 2128 cm" 1 bands appeared at 2216 and 2159 and 2097 cm -1, indicating that these features are due to species containing nitrogen atoms. The intensity of the bands in the 3000-2900 cm- 1 region, corresponding to CH vibrations in alkyl species, increased progressively with temperature, reached their maximum at ca. 300 ~ and then declined with further increase in temperature. Only slow

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NO+C3H6+O2 3%AulAI203

~ o

~

r O

eq it) (",l

~

~lt.

<

.._.r ooo

I

tscan=

3200 ,

30

rain

,

,

i

3\

" .......... "'""" . . . . . . . . . . . . . . . . . . . . . . . . . . . "J

4 0 0~

93 5 0

. eq eq e q ^ /I ~ ~ - _

0

Z .........................................................

0

~

f, .......

--9. . . . . . . . . . . . . . . . . . . . . . . . . .

.0

~ m

eq m

4500

.........................

_~

~ ~

..... i ' -

"\ .

r

.

.

.

.

.

....

.1

,

'

2 100'5

'

'

'

2 0 i0 0

,

18 00

W a v e n u m b e r / cm- 1 Fig. 3. In situ DRIFT spectra of species adsorbed on 3%Au/AI203 during the NO+C3H6+O2 reaction at various temperatures: Reactants: NO =1000 ppm, O2=5%, C3H6=1000 ppm, He balance, GI--ISV" 27,000 h- 1.

w,l t t i Mpee i

NO2+C3H6+O2

9 J"

_,

o , oo

~

o

u

~'~_---~

"~"~'"--

tscan= _

3200

,

,i

.. ," ! I

. . . . . . . . . . 9. . . . . . . . . . . . . . . . . .

,,-,...J"

<

30 I

Ii

.,,: ................. \,

..,.,-.-"

.)

rain ,

In

t,~

~

,'-'-----------~

', \.,.. ~ t

t

400

....., ' f !,i ! i'..".:: , . . . . . . . .

.......................................

~-.........

,--, ~, ,', !:':,i tl 1~ l'

......................

o ,.Q

~

,,':I'~

T

t-q

H~

~,

\...~ /,~

;

:,,. . . . . . . . . . . . . . . . . . . . . .

/ \ ,.,

350

3000

. .9. . . . . . . . . . . . . . . . . . . . . . .

_..

~ ~

-" --

--.--""

.....

ea i

,

i

!

2500

i

i

l

I

I

2000

i.

1800

W a v e n u m b e r / cm- 1 Fig. 4. In situ DRIFT spectra of species adsorbed on 3%Au/AI203 during the flow of NO2+C3H6+O2 at various temperatures. Conditions as in Figure 3.

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temperature-dependent effects were observed for species below 1700 cm- 1 (not shown). Over A1203, only hydrocarbons (3000-2900 cm'l), CO2 (2349 cm'l), CO (1952 cm-1), carbonate or carboxylate (1579, 1472-1458 cm'l), and formate species were noted. With temperature ramp, apart from the disappearance of hydrocarbon species, no significant changes were noted in the spectrum. In the NO2+C3H6+O2 system, stronger interactions between NO2 and activated C3H6 interactions were observed even at low temperatures (see Fig. 4) than those noted in the NO+C3H6+O2 system. On Au/A1203, at T< 250 ~ the -NCO band at 2235 cm-1 was weak compared with the -CN band at 2194 cm -1. Raising the temperature from 250 to 300 oC, the the -NCO band and bands at 3000-2800 grew in intensity. Between T= 300-400~ the -NCO band had the highest intesity. At T> 400 oC the -NCO band gradually diminished whereas -CN band at 2128 cm-1 intensified. These species remained on the surface up to 500 ~ where they disappeared. Although not shown, the exposure of A1203 to a NO2+C3H6+O2 flow at 300400 oC, in addition to the bands due to hydrocarbons, CO2, CO, carbonate or carboxylate, and formate bands, a peak at 2251 cm-1, due -NCO species adsorbed at A13+ sites was noted. Based on the above and our previous results [7] and those of others [8-10], in which the reactions of isocynate and cyanide species with NOx and 02 were examined, the following reaction scheme for the NO SCR with propylene in excess oxygen on A1203 and Au/A1203 is proposed. The initiation of NO SCR on Au/y -A1203 involves the oxidation of NO to NO2 followed by coupling of the NO2 and/or NO2 adsorbed complexes (that are fairly strongly held on Au/A1203 even at elevated temperatures)with activated C3H6, possibly allyl species, on active sites on A1203 to form CnHmNxOy species, which are responsible for the propagation step. Its subsequent internal rearrangement and decomposition leads to the formation of N2 and other products. 4. C O N C L U S I O N From the above results it is concluded that: The oxidation of NO to NO2 is a prerequisite for NO SCR to proceed on Au/A1203. NO2 and its adspecies NOx-, are primary intermediates. Interaction of allyl species with NO2 and/or its adspecies lead to the formation of nitrogen-containing organic species, such as -NCO and -CN which are secondary intermediates. Due to the high conversion of NO2 to N2, A1203 and Au/A1203 can be used as secondary components in multi-bed staged systems. REFERENCES

1. M. Iwamoto and N. Mizuno, Proc. Instn. Mech. Engrs., J. Automobile Eng., 207 (1993) 23.

2. A. Ueda, T. Oshima, and M. Haruta, Appl. Catal. B., 12 (1997) 81. 3. G.R. Bamwenda, A. Ogata, A. Obuchi, J. Oi, K. Mizuno, and J. Skrzypek, Appl. Catal. B, 6 (1995) 311. 4. H. Hamada, Y. Kintaichi, M. Sasaki, T. Ito and M.Tabata, Appl. Catal., 70 (1991) L15. 5. K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, John Wiley, New York, 1997. 6. N.D. Parkyns, Procced. 5 th Inter. Congress Catalysis, Palm Beach, (1972) 255. 7. G.R. Bamwenda, A. Obuchi, A. Ogata and K. Mizuno, Chem. Lett., (1994) 2109. 8. A. Obuchi, C. Wogerbauer, R. Koppel and A. Baiker, Appl. Catal. B: 478 (1998) 1. 9. H. Takeda and M. Iwamoto, Catal. Lett., 38 (1996) 21. 10. A.Y. Aylor, L.J. Lobree, J. A. Reimer, and A.T. Bell, J. Catal., 170 (1997) 390.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Transient study of NO, N20, C3H6 and C3Hs interactions over a silicasupported Pt catalyst P. Denton", Y. Schuurmanb, A. Giroir-Fendler a, H. Praliaud a, M. Primet" and C. Mirodatos b aLaboratoire d'Application de h Chimie ~ rEnvironnement (LACE); UMR 5634 (CNRS-UCB Lyon 1); 43, boulevard du 11 novembre 1918; 69622 Villeurbanne Cedex; France bInstitut de Recherches sur h Catalyse; UPR CNRS 5401; 2, avenue Albert Einstein; 69626 Villeurbanne Cedex; France The generation of N2 and N20 during NO decomposition and reduction on Pt/SiO2 was investigated by means of a TAP reactor. With rich mixtures the hydrocarbon serves to create free surface sites for NO decomposition and/or to induce a direct reaction of NO with carbonaceous residues, especially for C3H6. Furthermore, results of both transient and steadystate experiments indicate that adsorbed N20 is an intermediate leading to N2. I. INTRODUCTION Selective catalytic reduction of NOx by hydrocarbons (HC) in the presence of oxygen can be performed with platinum-based catalysts, but the activity-temperature window is narrow, and the selectivity toward N20 is high, at the expense of N2 [1-3]. The kinetics and mechanism of SCR-HC over Pt have already been studied. With C3H6 the reaction seems to involve dissociation of NO on reduced Pt sites [2-4]. A direct reaction between hydrocarbonderived adsorbates and NO has, however, also been considered [5]. N2 is often considered to be formed from two dissociated NO molecules, and N20 from one N atom and molecularly adsorbed NO, but the possibility that N20 is an intermediate leading to N2 is still under debate. In this study, we used a Temporal Analysis of Products (TAP) reactor to seek further information on the possible pathways. In particular, the formation of N2 and N20 during NO decomposition or reduction on Pt/SiO2 was examined.

2. EXPERIMENTAL The support was a mesoporous silica, supplied by Grace Davison, with an average pore radius of 3 nm and a specific surface area of 596 m2 g~. The Pt/SiO2 catalyst was prepared by wet impregnation of the support by Pt(NI-I3)4(OH)2,followed by calcination under air at 773 K (heating rate 1 K min~, plateau 8 h) and reduction overnight under H2 at 573 K (heating rate 2 K min~). The Pt content was 0.90 wt. % and the dispersion was 34 % (metallic surface area of 83 m 2 g~ and homogeneously dispersed, 3 nm diameter Pt particles). A TAP experiment consists of introducing narrow pulses of gaseous reactants in a continuously-evacuated microreactor. The response of each pulse is detected by a quadrupole mass spectrometer at the reactor exit. The shape of the response reflects diffusion, adsorption,

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desorption and reaction of the reactants and products. The principle of the TAP experiment and its applications are extensively described by Gleaves et al. [6-7]. Pulses of reactant (10 ~5to 5.5"10 is molecules each, for 1.8'1017 or 9.5"10 ~6 Pt,) were introduced until steady-state coverage was reached. The internal standard was usually Ar, but Xe was used for some of the experiments with C3I-I~. Several reactions were considered" - Adsorption and dissociation of NO (or N:O) at various temperatures on a reduced (bare) or oxidized surface. NO or N20 was pulsed and the responses of inert gas, NO, N20 and N2 were measured sequentially. The fragment at m/e - 14 was used to identify Ne. - Adsorption and reactions of NO on hydrocarbon-saturated surfaces. - Adsorption of HC (C3I-I6or C3I-h) on a bare, oxidized or NO-saturated surface. Conversions, selectivities, yields (i.e., products of conversion and selectivity) and mass balances were calculated from the areas under the response signals, with the calibration factor of each species taken into account. Residence times were also considered. In steady-state conditions at 478 K, the effect of space velocity was studied with a flow containing 4000 vpm NO-2000 vpm C3I-I6-5 vol. % O2-He (total flow 4 to 14 L h~). The conversions of NO into N:, N20 and NO2 were measured. Also, N20 reduction was studied between 423 and 773 K using the following feed: 1000 vpm N20-2000 vpm C3H6-7000 vpm O~-He (total flow 10 L hq). 3. R E S U L T S A N D D I S C U S S I O N 3.1. N O a d s o r p t i o n a n d d i s s o c i a t i o n on a b a r e s u r f a c e

0.6 0.5

5

/

0.4

z

/

f

f

Oo

/

~ o.3>-

f

/

0.20.1

0.0

-

0

I

I

I

I

50

100

150

200

pulse number

Figure 1. NO pulses at 573 K on a bare surface. Yields of N2, N20 and NO and oxygen surface coverage as a function of the number of pulses.

Before each experiment, the catalyst was reduced under H2 at 573 K until the end of H2 consumption and H20 formation, then degassed, leaving Pt~ (hence, a bare surface). NO was pulsed over this bare surface at temperatures between 323 and 723 K. Initially, high yields of N2 were observed. As the number of NO pulses increased, the formation of N2 decreased steadily, while N20 was detected aRer a certain number of pulses depending on the temperature (Fig. 1). After further introduction of NO, N20 production reached a maximum and the NO signal increased. 02 was never detected, which suggests that the O adatoms (O*) resulting l~om the

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dissociation of adsorbed NO (NO*) or adsorbed N~O (N20*) remained on the surface. As well, NO2 was never observed. The broad pulse response of NO indicated a slow desorption rate well beyond the time scale of the experiment (Figure 2). The surface coverages in O and NO (0o, 0so) were calculated. The total calculated NO surface coverage was well beyond one 1.0 NO monolayer since the NO desorption was not taken into account. The N2 0.8 formation of N2 and N~O depends on =. t~ surface oxygen concentration. With increasing 0o, N~ formation 0~= 0.6 10 decreased continuously. At a) ._. intermediate oxygen coverages, N20 "i 0.4 formation increased along with the concentration of molecularly 0.2 adsorbed NO (NO*), indicating that the latter species must be present on the surface for N20 to be formed. 0.0 This N20 formation passes through 0.0 0.1 0.2 0.3 0.4 a maximum and then drops off on an time (s) oxidized surface. 0t--

I

1

1

I

}

Figure 2. NO pulses at 573 K on a bare surface. Average responses for Ar, N20, N2 and NO as a function of time.

Surprisingly, the mean residence time of N20 was shorter than that of N2 (Fig. 2), so N2* was more strongly adsorbed than N20*. According to Burch et al. [8], N* may be a relatively stable surface species. By correlating selectivity and ease of desorption, we may consider the following reaction scheme. 2 NO (g)

N20 (g)

2 NO* ~

N20* + O* --> N2* + O*

1',~

1'$

N2 (g)

1'

At this stage, we cannot yet say i f N 2 0 is necessarily an intermediate leading to N2. NO was more strongly adsorbed than N2, contrary to the observations of Lacombe et al. [5] with a polycristalline Pt sponge catalyst. This can be explained by an influence of particle size. With temperature programmed desorption experiments (TPD NO) on several Pt/SiO2 solids, we have found, in agreement with Zafiris et al. [9], that larger Pt crystallites lead to weaker Pt-NO bonds. 3.2. NO adsorption on an oxidized surface

When NO was pulsed over an oxidized catalyst, no adsorption and no dissociation were observed, in agreement with the observations ofBurch et al. [3] and in contrast with those of Eckhoffet al. [4].

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3.3. NzO adsorption and dissociation on a bare surface

Pulsing N20 at 573 or 723 K over the bare catalyst led to decomposition with the formation of N2 and O adatoms (Fig. 3), contrary to the findings of Li and Armor [10]. The following reactions are thus considered: N20 (g) + * ~ - N20* and N20* --> N2 (g) + O*. As the number of pulses increased, 0o increased and N2 yield decreased, until the surface was saturated. N20 is not dissociated on a fully oxidized surface. NO was never observed (except as a fragment of N20), so the NO* ~ N20* transformation in the above scheme is irreversible. Figure 4 shows that N2 is formed more quickly from N20 than from NO.

1.0

1.0 N~O

0.8

0.8 -4

r

o

~

0.6

~

z

x, ~"0.4

0.2

0.0

0

.

6

0.4

/'

0.2

0.0 I

I

20

40

I

I

I

60

80

100

' I

120

pulse nurrber

Figure 3. N2Opulses at 573 K on a bare surface. Yields of N2, NeO and the oxygen surface coverage as a function of the number of pulses.

0.0

0.1

"

I

I

0.2

0.3

0.4

time (s)

Figure 4. N2 response curves at 573 K. A: N2 pulse, B: N2 producthgn from N20, C: N2 production.from NO.

3.4. Discussion. Formation of N2 and N20 N 2 0 c a n be formed either by recombination of N* and NO* or by recombination of 2 NO*. We can exclude the reactions 2 N* + O* --->N20* + 2* and N* + NO (g) ~ N20*. N2 is formed either by recombination of two N adatoms (N*) resulting from the dissociation of NO* or by decomposition of N20* into N2 (g) and O*. We note that, in steady-state conditions, N20 can be reduced by C3I-I6 into N2. In the presence of 7000 vpm 02, the conversion of N20 into N2 reached 10 % at 573 K, 60 % at 673 K and 75 % at 773 K. Furthermore, during NO reduction at 478 K, N2 selectivity depends on contact time, in contrast with the results of Butch and Watling [11 ]. The overall conversion of NO decreased (from 44 to 16 %) as did that of C3H6 (from 41 to 12 %) as the space velocity increased from 6 600 to 23 000 h4. The intrinsic rates of NO reduction into N20 and of oxidation of C3I-I6 by 02 were independent of contact time, but the intrinsic rate of NO reduction into N2 decreased

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as the space velocity increased. Thus, N2 selectivity, defined as N2/(N2 + N20), was equal to 33 % at 6 600 hl and 2 % at 23 000 h l. We can conclude that adsorbed N20 is a precursor for N2 formation according to the reaction N20* ~ N2 (g) + O*. This does not mean that N20* is the only possible precursor for N2. N20*, however, is probably formed by recombination of 2 NO* rather than that of N* and NO*. If the latter were true, the N20 mean residence time would be close to that of N2. 3.5. Hydrocarbon adsorption and reactions At 573 or 723 K, hydrocarbons were adsorbed on a bare surface (0HC = 1 if we consider 1 C3 for 1 Pts) and regenerated the Ptfl sites on a surface oxidized by 02 or NO. CO2 (with its CO fragment) and H20 are thus detected. Mass balance calculations indicated a PtO2 stoichiometry for HC pulses on an O2-saturated surface and for 02 pulses on an HC-saturated surface. During HC pulses on an NO-saturated surface, CO2 and H20 were detected but not N2 or N20; therefore, the surface was saturated with O* but not with NO* or N20*. 3.6. NO pulses on an HC-saturated surface and comparison with NO on a bare surface For NO pulses on an HC-saturated surface, the formation of N2 and CO2 was observed, with less CO2 than expected from the C3H6/NO and C3Hs/NO reaction stoichiometries. This suggests that part of the surface is bare, while the rest is covered in carbonaceous residues, and so two different types of reactions can occur. Reaction (1) describes the behavior of NO on a bare surface, while reactions (2) and (3) describe its behavior on surfaces saturated with C3H6 and C3I-Is, respectively. Reactions (2) and (3) may occur either by direct reaction of N- and Ccontaining intermediates or by direct decomposition of NO following the cleaning-off of adsorbed oxygen by the hydrocarbon.

(1) (2) (3)

2 N O - ~ N 2 + 2 O* 9 NO + C3H6 ~ 4.5 N2 + 3 CO2 + 3 H20 10 NO + C3I-I8 -~ 5 N2 + 3 CO2 + 4 H20

For instance, at 573 K, the percentage of N2 arising from NO decomposition is higher for C3Hs (50 %) than for C3H6 (20 %). In both cases, part of the metallic surface remains free of HC residues, but the coke deposits arising from C3H6 leave fewer l~ee sites for NO decomposition than the deposits arising from C3Hs. As the number of NO molecules was increased between 0.3 and 3.6 times the number ofPt~ (9.5"1016), N2 and N20 production (yields, and duration of N2 formation) was compared for the bare surface and surfaces saturated with C3H6 or C3I-Is. For instance, at 573 K, on a bare surface or on a surface saturated with C3H6, the initial N2 yield is strong (Fig. 5), since the surface presents free Pt~ sites able to dissociate NO or reactive carbon species able to reduce NO. When the Pt~ sites are oxidized and/or CxHy species consumed, the N2 yield decreases and becomes weaker than with C3I-h. When the surface is saturated with C3I-h, carbon deposits are less reactive and N2 formation is observed for a long time with a medium N2 yield (Fig. 5). The duration of N2 formation obeys the sequence C3I-h > C3I-I6 > bare surface. Let us recall that, in the presence of 02 and in steady-state conditions, C3I-h is far less effective than C3H6 for SCR.

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4. CONCLUSION 0.8

0.6

"o (D

"~, 0.4 ol

C~HB

7

0.2

Ot

0

I

I

i

f

0.9

1.8

Z7

3.6

bid irt'c~__red__(rn:xada/em) Figure 5. N2 yields at 573 K as a function of the number of NO molecules (expressed as a function of the monolayer with 1 NO per PG).

With rich mixtures, it appears that two processes occur: NO (or N20) decomposition on reduced Pt, and NO reduction involving hydrocarbon-derived adsorbates (perhaps via the cleaning-off of adsorbed oxygen), the reduction process being more important with C3H6 than with C3I-h. In the absence of hydrocarbon, N2 is formed mainly by the decomposition of adsorbed N20. This N20 originates principally from the recombination of two adsorbed NO.

REFERENCES ~

2. 3. 4. ~

6. 7. ~

9. 10. 11.

M.D. Amiridis, T. Zhang and R.J. Farrauto, Appl. Catal. B, 10 (1996) 203. R. Butch and P.J. Millington, Catal. Today, 26 (1995) 185. R. Burch, P J. Millington and A.P. Walker, Appl. Catal. B, 4 (1994) 65. S. Eckhoff, D. Hesse, J.A.A. van den Tillaart, J. Leyrer and E.S. Lox, Stud. Surf. Sci. Catal., Vol. 116, Eds. N. Kruse, A. Frennet and J.M. Bastin, Elsevier 1998, p. 223. S. Lacombe, J.H.B.J. Hoebink and G.B. Matin, Appl. Catal. B, 12 (1997) 207. J.T. Gleaves, J.R. Ebner and T.C. Kuechler, Catal. Rev. Sci. Eng., 30 (1988) 49. J.T. Gleaves, G.S. Yablonskii, P. Phanawadee and Y. Schuurman, Appl. Catal. A., 160 (1997) 55. R. Burch, A.A. Shestov and J.A. Sullivan, J. Catal., 182 (1999) 497. G.S. Zafiris and R.J. Gorte, Surf. Sci., 276 (1992) 86. Y. Li and J.N. Armor, At~I. Catal. B, (1992) L21. R. Burch and T.C. Watling, Stud. Surf. Sci. Catal., Vol. 116, Eds. N. Knase, A. Frennet and J.M. Bastin, Elsevier 1998, p. 199.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) @ 2000 Elsevier Science B.V. All rights reserved.

Electrochemical Promotion of NO reduction by C3H6 on Rh/YSZ catalystelectrodes and investigation of the origin of the promoting action using TPD and WF measurements C. Raptis, Th. Badas, D. Tsiplakides, C. Pliangos and C.G. Vayenas University of Patras, Dept. of Chemical Engineering, Patras GR 26500, GREECE The effect of Electrochemical Promotion (or Non-faradaic Electrochemical Modification of Catalytic Activity- NEMCA) was used to promote the selective catalytic reduction of NO by propylene in the presence of excess oxygen. The catalyst was in the form of a Rh polycrystalline film, interfaced with Yttria-stabilized-Zirconia (YSZ, an 02- ion conductor). It was found that dramatic changes of the catalytic activity can be obtained under NEMCA conditions, accompanied by significant alterations in the product selectivity. The origin of the electrochemical promotion is discussed in the light of temperature-programmed desorption (TPD) and Work Function (WF) measurements. I. INTRODUCTION The catalytic performance (activity, selectivity) of metals interfaced with solid electrolytes can be affected significantly upon external current or potential application. The electrochemically induced change in catalytic rate can be several orders of magnitude higher than the rate of ion transport through the solid electrolyte. This phenomenon is known as Non-faradaic Electrochemical Modification of Catalytic Activity (NEMCA) [1,2] or Electrochemical Promotion [3]. It has been studied for more than 55 catalytic systems [1,2] using various metal or metal-oxide catalyst-electrode films (Pt, Pd, Rh, Ag, Ni, IrO2, RuO2, etc.) interfaced with O 2-, Na +, H +, F-ionic conductors, mixed electronic-ionic conductors (TiO2, CeO2) [4,5] or aqueous alkaline solutions (LiOH, KOH) [6] for several groups of catalytic reactions (i.e., oxidations, reductions, hydrogenations, decompositions, etc.). Work in this area has been reviewed recently [2,7]. The importance of NEMCA in catalysis has been addressed by several authors [3,8,9] and a considerable number of research groups has made an important contribution in this area [10-14]. The observed promotional phenomena have been shown to be due to the electrochemically controlled reverse spillover of ionic species migrating between the solid electrolyte support and the catalyst surface [1,2,15,16]. Consequently, an "effective" electrochemical double layer [2] is, thus, established on the catalyst surface which changes the catalyst-electrode work function, eO, by A(e~) = eAVwR (1) where VWR, is the catalyst (working) - electrode potential with respect to the reference electrode. Eq. (1) has been confirmed by Kelvin probe [ 15] and UPS [ 17] measurements. The magnitude of Electrochemical Promotion is given by the Faradaic efficiency, A, and the rate enhancement ratio, p, defined from [ 1,2,7]:

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A = Ar / (I/2F) ; p = r/r~ (2) where, r and r ~ are the electrochemically promoted and the open-circuit (unpromoted) catalytic rate values respectively, Ar=r-r~ is the electrochemically induced change in catalytic rate,/, is the applied current and F is the Faraday constant. According to this definition, a catalytic system exhibits the effect of Electrochemical Promotion, when IA]>I [1,2,7]. A and 9 values as high as 3x 105 [16] and 150 [18] have been observed, respectively. In this paper the effect of Electrochemical Promotion was used to promote the NO reduction by C3H6, in the presence of excess oxygen. The origin of Electrochemical Promotion is also discussed, using Temperature Programmed Desorption of oxygen from Pt and work function measurements on Pt films deposited on YSZ during C2H4oxidation. 2. E X P E R I M E N T A L The experimental setup used for the Electrochemical Promotion of the NO reduction by propylene consists of the flow system, the reactor and the gas analysis unit. The reactor corresponds to the "single pellet" design [1] and has been described in detail elsewhere [ 1,2,7], together with the metal film deposition technique. Rh polycrystalline films deposited onto the one side of YSZ pellets serve both as the catalyst and as the working electrode, while two Au films, deposited on the other side of the disc, serve as reference and counter electrodes. All electrodes are exposed to the same gas mixture. In all cases a series of blank experiments was carried out to confirm that the catalytic activity of the Au electrodes was negligible in comparison to that of the Rh catalyst. Reactant and product analysis was performed by means of a Perkin Elmer S-300 gas chromatograph fitted with porapak Q (C3H6, CO2, N20) and molecular sieve (O2, N2, CO) columns. Nitrogen oxides (NO, NO2) concentrations were recorded on line by means of a Teledyne mod.911/912 NO/NO• analyzer. In addition, an Anarad infrared analyzer was used to monitor the CO2 concentration in the effluent stream, while currents or potentials were applied by means of an Amel 533 Galvanostat/Potentiostat. The temperature-programmed desorption experiments were carried out in an ultrahigh vacuum chamber (base pressure 10-1~ Torr after baking) equipped with a quadrupole mass spectrometer (Balzers QMG 420) and a differentially pumped gas inlet system. The polycrystalline Pt catalyst film was deposited in the form of a half-ring on the outer surface of a tubular YSZ specimen (OD 19mm, thickness 3mm, length 30mm). Au counter and reference electrodes were deposited on the inside wall of the tubular YSZ element opposite to the catalyst film. Details on the preparation and characterization of Pt and Au electrodes are given elsewhere [2]. A series of blank experiments showed that the TPD spectra reported here can be attributed to oxygen chemisorption on the catalyst film only. Constant currents between the catalyst film and the Au-counter electrode (galvanostatic operation) or constant potentials VWR between the catalyst film and the Au-reference electrode (potentiostatic operation) were applied using an AMEL 553 galvanostatpotentiostat. The specimen was heated radiatively using an Osram Xenon lamp located inside the tubular YSZ specimen. A type-K thermocouple attached on the catalyst film was used to measure the temperature. The temperature was varied linearly at a heating rate of 1 K/s, using Eurotherm programmable temperature controller. The experimental setup for the WF measurements during C2H4oxidation on Pt was similar to the one used for the electrochemical promotion experiments described above. Reactants were certified standards of ethylene (4.5% C2H4in He) and oxygen (20% O2 in He). Typical

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flow rates were around 200 mL/min STP with a gas composition of 15% 02 and 0.2% C2H4 in He. Reaction temperature was maintained at 400~ A Kelvin probe (Besocke/Delta Phi-Electronik, Probe "S"), with a 2.5 mm diameter gold grid vibrating electrode placed -~500 ~tm from the catalyst surface, was introduced in the reactor for measuring the catalyst work function. In the Kelvin probe'S' operation, the work function signal is drawn from the vibrating gold grid, so that the Kelvin probe lock-in amplifier circuit is entirely independent of the solid electrolyte cell circuit. Details of the electrical circuit have been given previously [ 19]. A three-electrode cell was used in all experiments. The electrolyte was a thin disk (25mm diameter and lmm thick) of YSZ. On the one side of the electrolyte the Pt catalyst film/working electrode and the Au reference electrode were deposited. Gold counter electrode was deposited on the opposite side. All electrodes were exposed to the reaction gas mixture. 3. RESULTS AND DISCUSSION 3.1 Electrochemical Promotion of NO reduction by C3H6 Rh metal catalysts in the form of polycrystalline films interfaced with Yttria- stabilizedZirconia were used in this investigation, in galvanic cells of the type:

C3H6, NO, 02, products, Rh / YSZ / Au, C3H6, NO, 02, products It was found that both the catalytic activity and the selectivity of the Rh catalyst-electrode is affected dramatically upon varying its potential with respect to the Au reference electrode. The rates of CO2 and N2 production are enhanced dramatically both with positive and negative potentials or currents. The induced changes in catalytic rates are typically 3 orders of magnitude higher than the rate I/2F of O 2- supply or removal to or from the Rh catalyst electrode. Thus, the Faradaic efficiency, A, was found to take values as high as 4000 and as low as-6000. The rate enhancement ratios of the three main products (COz, N2, N20) were also found to take very high values. Positive current (or potential) application leads to an up to 150-fold enhancement in the rate of CO2 production and an up to 60-fold enhancement in the rate of N2 production [18]. The Pco2=150 value is the highest reported so far in NEMCA studies utilizing YSZ. A typical galvanostatic NEMCA experiment is shown in Figure 1, which depicts the transient effect of a constant positive current application on the rates of formation of CO2, N2 and N20, on the NO conversion and on the catalyst potential, VWR. The open-circuit (unpromoted) rates correspond to t<0 in the figure. At t=0 the galvanostat is used to apply a constant current I=60~tA between the working and counter electrode. At the new steady state, reached after 80 min, the catalyst-electrode performance has changed dramatically. There is a 1300% increase in NO conversion and a 5200%, 1000% and 300% enhancement in the rates of CO2, N2 and N20 formation (Pco2=52, 9N2=10, PN20=3). The increase, Arco2, in the rate of CO2 formation is 380 times larger than the rate, I/2F, of supply of O 2- to the Rh catalyst. The product selectivity to N2 (vs N20) is also significantly enhanced. It is worth noting in Fig. 1, that after current interruption the effect is not totally reversible, since the catalytic rates do not return to their initial open circuit values (i.e., there is a 6-fold permanent enhancement of the catalytic activity for CO2 production and a 3-fold permanent enhancement in NO conversion). This is a manifestation of the "permanent NEMCA" effect studied by Comninellis et al. [19]. We found that the initial rates can be restored after treating the catalyst with C3H6 for a few minutes.

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[o.c_ 5 "~' i

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1200 ppm C3H6 950 ppm NO

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Q=115 cc/min

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The observed pronounced promotional phenomena can be rationalized by taking into account the effect of changing the potential, and thus the work function [1,2], on the chemisorptive bond strength of oxygen, NO and propylene. In particular, it appears that the pronounced weakening in the Rh=O bond, induced at high potentials [1], enables NO to adsorb and dissociate even in presence of significant amounts of gaseous oxygen. This may be of significant practical importance. Also the significant weakening in the C3H6 chemisorptive bond at negative potentials, may again enhance NO adsorption and dissociation, causing the observed electrophilic behaviour.

...

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3.2

Temperature-Programmed

Desorption and Measurements

Work

Function

The creation of two types of chemisorbed oxygen on Pt surfaces interfaced with YSZ and subject to electrochemical promotion conditions is manifested clearly by temperatureprogrammed desorption (TPD). Figure 2 shows typical oxygen thermal desorption spectra obtained a~er gaseous oxygen adsorption at Po2 =4x10-6 Torr for 1800 s (7.2 kL) at 673 K followed by Fig. 1. Transient effect of a constant applied current on the catalytic rates of CO2, N2 and N20 production, on NO conversion (XNo) and on catalyst potential (VwR).

electrochemical supply of O 2, at a rate I/2F, at 673 K for various current application times, t~, indicated on each curve. Gaseous oxygen supply (dashed line, ti=0) creates a single adsorption state (Tp=743 K), corresponding to chemisorbed atomic oxygen. Additional electrochemical oxygen supply in presence of preadsorbed oxygen creates two oxygen states. Figure 2 provides a clear explanation of the NEMCA effect when using O z- conductors. Electrochemically supplied 02- produces strongly bonded "ionic" or "backspillover" oxygen (Tp=723 to 773 K) with a concomitant pronounced lowering (up to 50 K, i.e. from 743 to 693 K) of the desorption temperature of the more weakly bound atomic oxygen obtained via gas phase adsorption. This pronounced weakening in the Pt=O bond causes the observed dramatic catalytic rate enhancement in NEMCA studies, due to the very fast oxidative action of this weakly bonded oxygen. The strongly bonded oxygen state is significantly less reactive than the atomic oxygen and acts as a sacrificial promoter. Figure 3 shows the effect of Pt catalyst overpotential on catalytic rate and catalyst work function during CaH4 oxidation on Pt deposited on YSZ. The catalytic rate increases both with

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positive and negative overpotentials. However, the rate increase is more pronounced for positive applied potentials, exhibiting an electrophobic behavior [2]. 20

2

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700

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AVwg,V Fig. 3: Effect of catalyst ohmic-drop free overpotential on reaction rate (e) and on catalyst work function changes (I).

The catalyst work function was also monitoring during the catalytic activity measurements. As shown on Figure 3 the imposed changes in catalyst overpotential, AVwR, cause almost quantitatively the same changes in catalyst work function, A(e~). Some deviations from the experimentally observed [ 1,2] and theoretically explained [2] equality (eAVwR=A(eO), dashed line) are observed for high positive overpotential. Two factors could be responsible for these deviations: (a) Under these high rate conditions, the lifetime of backspillover oxygen ions is not long enough (due to their reaction with C2H4) to establish a full electrochemical double layer which would result to an one-to-one relation between catalyst overpotential and work function changes. In more oxidizing atmospheres, or with pure oxygen mixtures, this equality was found to be exactly valid. (b) Some insulating carbonaceous species could be deposited on the Pt surface during preparation or after carbon deposition reaction from gas phase reactants. Such an insulating species could act as a means of storing electric charge on the catalyst surface and thus lead to deviations from eAVwR=A(e~) [2]. 4. CONCLUSIONS The present study shows that Electrochemical Promotion can affect dramatically the reduction of NO by propylene, in the presence of oxygen. The catalytic activity and selectivity of the Rh catalyst-electrode is promoted very significantly upon varying its potential. The rate enhancement ratios, p, of the CO2 and N2 production were found to take values as high as 150 and 60 respectively, while minor catalytic rate enhancements of the N20

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formation were observed. This behavior leads to an enhancement of the product selectivity to N2. Particularly, imposition of positive potentials causes up to 2-fold enhancement of the nitrogen selectivity in the lower temperature range of the present investigation. From the TPD experiments it was found that electrochemical 0 2- pumping to Pt catalysts in presence of preadsorbed oxygen causes backspillover of large amounts of oxygen on the catalyst surface and leads to the formation of two oxygen adsorption states, i.e. a strongly bonded anionic oxygen along with the weakly bound atomic oxygen. The two oxygen species coexist on the surface. The enhancement in catalytic rate is due to the high reactivity of the weakly bonded oxygen while the electrochemically supplied strongly bonded oxygen is A times less reactive than the weakly bonded oxygen and acts as a sacrificial promoter. The WF measurements during C2H4 oxidation have confirmed that the catalyst electrode work function is dictated by the applied potential. This electrochemically induced change in WF affects the chemisorptive bond strength of all reactants and intermediates and is thus causing NEMCA. Acknowledgment: We thank the B RITE III programme for financial support. REFERENCES:

1. C.G. Vayenas, S. Bebelis, I.V. Yentekakis and H.-G. Lintz, Catal. Today, 11 (1992) 303. 2. C.G. Vayenas, M.M. Jaksic, S. Bebelis and S.G. Neophytides, in "Modem Aspects of Electrochemistry" (J.O'M. Bockris, B.E. Conway and R.E. White eds) Vol. 29, pp. 57-202 Plenum Press, NY, (1995). 3. J. Pritchard, Nature (London), 343 (1990) 592. 4. C. Pliangos, I.V. Yentekakis, S. Ladas, and C.G. Vayenas, J. Catal., 159 (1996) 189. 5. P.D. Petrolekas, S. Balomenou and C.G. Vayenas, J. Electrochem. Soc., 145(4) (1998) 1202. 6. S. Neophytides, D. Tsiplakides, P. Stonehart, M.M. Jaksic, and C.G. Vayenas, Nature (London), 370 (1994) 45. 7. C.G. Vayenas, and I.V. Yentekakis, in "Handbook of Catalysis" (G. Ertl, H. Knotzinger, and J. Weitcamp eds.), VCH Publishers, Weinheim, pp. 1310-1338 (1997). 8. J.O'M. Bockris and Z.S. Minevski, Electrochim. Acta, 39 (1994) 1471. 9. B. Grzybowska-Swierkosz and J. Haber, in Annual Reports on the Progress of Chemistry, Vol. 91, p.395, The Royal Society of Chemistry, Cambridge (1994). 10. T.I. Politova, V.A. Sobyanin and V.D. Belyaev, React. Kinet. Catal. Lett., 41 (1990) 321. 11. L. Basini, C.A. Cavalca and G.L. Haller, J. Phys. Chem., 88 (1994) 10853. 12. I. Harkness and R.M. Lambert, J. Catal., 152 (1995) 211. 13. P.C. Chiang, D. Eng and M. Stoukides, Interface, 139 (1993) 683. 14. E. Varkaraki, J. Nicole, E. Platmer and Ch. Comninellis, J. Appl. Electrochem.,25 (1995) 978. 15. C.G. Vayenas, S. Bebelis, and S. Ladas, Nature (London), 343 (1990) 625. 16. S. Bebelis, and C.G. Vayenas, J. Catal., 118 (1989) 125. 17. W. Zipprich, H.-D. Wiemhoefer, U. Voehrer, and W. Goepel, Ber. Bunsenges. Phys. Chem., 99 (1995) 1406. 18. C. Pliangos, C. Raptis, Th. Badas and C.G. Vayenas, Solid State Ionics, submitted (1999). 19. J. Nicole, D. Tsiplakides, S. Wodiuning and Ch. Comninellis, J. Electrochemical Soc., 144(12) (1997) 312.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

The reaction of propane with mixtures of NO, N20 and 02 over platinum, palladium and rhodium catalysts Dean C. Chambers, Dennys E. Angove and Noel W. Cant Department of Chemistry, Macquarie University NSW 2109, Australia The reactions of propane with Oz, N20 and NO over Pt, Pd and Rh catalysts have been investigated under near-stoichiometric conditions. In general, Pt is the most active catalyst for propane oxidation, followed by Pd and Rh. In individual tests, the reactivity of the oxidants varies widely, the order being 0 2 > > N20 > NO for Pt, 02 ~ N20 >> NO for Rh, and N20 >> NO or 02 for Pd, where the reaction involving O2 is dependent on pretreatment and history. Interactions between oxidants alter the behaviour when all three are present. The order of reactivity changes to 02 > NO > N20 for Pt, 02 > N20 > NO for Rh, and 02 > NO > N20 for Pd. The findings imply substantial competition between oxidants, especially with Pt and Pd. 1. INTRODUCTION The selective reduction of nitric oxide by alkanes in the presence of excess oxygen over supported platinum has received considerable attention because of its possible application for pollution control on lean burn engines [1,2]. Partial reduction of NO, resulting in the formation of a large proportion of N20 rather than N2, is a known problem [1,3]. By contrast there is very little information concerning this reaction during conventional three-way catalysis with near-stoichiometric air/fuel ratios. This is somewhat surprising given that alkanes are much less readily removed than either alkenes or carbon monoxide [4,5]. As a result the final stage of pollutant removal under chemically controlled conditions is likely to involve competition for alkanes by the three oxidants O2, NO and N20, the latter having been formed largely through the prior reduction of NO by CO. The aims of the present work were to determine the intrinsic characteristics of Pt, Pd and Rh for the reaction of one alkane (propane) with these oxidants both individually and in a mixture. 2. EXPERIMENTAL The 1.1 wt% Pt/SiO2 (40% dispersion) catalyst was from the batch designated 40-SiO2-PtC1-L [6], and the 0.5 wt% Pd/SiO2 catalyst (79% dispersion) from that designated 79.1-Pd/SiO2-IV [7], in the series prepared and characterised by Burwell et al. The 0.5 wt% Rh/SiO2 catalyst was prepared here in a similar way by incipient wetness impregnation of the same silica (Davison grade 62, 180-210 ~tm, 285 m2/g) with a solution of RhC13, followed by reduction in 10% H2/He at 350~ for 2 hours. TEM analysis showed that the Rh particles approximated 3 nm diameter spheres, which corresponds to a dispersion of ca. 40%. Comparison testing was carried out in a flow system with 75 mg samples of catalyst and a total flow rate of 100 cm3/minute (GHSV = 32000 hr -~) with helium as the carrier. The propane concentration was set at 200 ppm with, in individual tests, a very slight excess of the oxidant (approximately 1000 ppm when using 02 or 2000 ppm with either NO or N20). With

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mixed oxidants the ratio was also near stoichiometric with equal amounts of O atoms available from each oxidant (approximately 333 ppm 02, 666 ppm NO and 666 ppm N20). The temperature of the sample was ramped up and down several times, usually at 2~ per minute, to test reproducibility. Hysteresis, assessed from the temperatures required for 50% conversion was no more than 10~ after the initial run, except for some Pd/SiO2 experiments as noted later. The exit gas stream was sampled by a high-speed gas chromatograph (MTI model M200) for 02, Nz and C3H8analysis. Other species (NO, N20, CO2, CO, NO2 and H20) were analysed by FTIR spectroscopy (Nicolet Magna-IR 550), using a single-pass cell with 16 cm path length, and a least-squares spectral fitting routine [8]. 3. RESULTS Figure 1 shows the conversion of propane as a function of temperature in reactions with 02, NO and N20 separately over the Pt/SiO2 catalyst. Oxidation of propane is fastest with 02 and slowest with NO, N20 being of intermediate reactivity. Reaction of NO forms N20 with a sharp peak in production (460 ppm) at 285~ followed by a steep decline during the latter stages of NO removal. The conversion versus temperature profiles are significantly different when propane is reacted with the mixed oxidant feed (Figure 2). Reaction with 02 is delayed by approximately 50~ and NO reacts ahead of N20. Formation of N20 from NO results in a substantial temperature range over which the conversion of N20 is apparently negative (i.e. more N20 in the product than the feed stream) and N20 is the sole residual oxidant when the conversion of propane is complete. Approximate kinetic orders were determined to see if the shifts in reactivity evident between Figures 1 and 2 were attributable to the threefold lower concentration of each oxidant in the mixed feed. Measurements were made at low conversion holding the concentration of one reactant at the value used in the individual experiments of Figure 1 and varying the

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Fig. 1. Conversions versus temperature using individual oxidants over Pt/SiO2 (ca. 1000 ppm 02 or 2000 ppm with NO or N20).

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Fig. 2. Conversions versus temperature using mixed oxidants over Pt/SiO2 (ca. 333 ppm 02, 666 ppm NO and 666 ppm N20).

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Table 1. Approximate kinetic orders for reactions of 02, NO and N20 with propane Temp./~ Pt/SiO2 Temp./~ Rh]SiO2 Temp./~

02 C3H8 NO C3H8 N20 C3H8

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324

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1 -0.1 -0.9 1.1 0.7 -0.4

341 188

concentration of the other reactant 50% either side of the corresponding value. The temperatures used, and the orders obtained from the slopes of log-log plots of CO2 formation versus concentration, are summarised in Table 1. The reaction of propane with oxygen is strongly negative order in 02, in which case the lower concentration of 02 in the mixed feed would be expected to reduce the temperature needed for reaction contrary to that observed. Thus competition from nitrogen oxides, not concentration differences, is the cause of the slower reaction with O2 in the mixed feed. The reactions of the two nitrogen oxides with propane show different kinetic orders, near zero order in oxidant for NO but strongly positive order in oxidant for N20. Lowering the concentration should then favour the reaction of NO over N20, which may be a partial cause for the delayed reaction of N20 in the mixed feed. However, the steepness of the NO and N20 curves in Figure 2 compared to Figure 1 implies competition between them, and from oxygen as well in the case of NO. Figure 3 shows that the activity of the Rh/SiO2 catalyst for the separate reactions of propane with each of the three oxidants is lower than that of Pt/SiO2 for the corresponding reactions (Figure 1). The order of reactivity for the oxidants, 02 > N20 > NO, is the same but the reactivity of N20 is considerably closer to that of 02. NO is surprisingly unreactive, with conversion incomplete even at 500~ The peak yield of N20 during the reaction with NO

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Fig. 3. Conversions versus temperature using individual oxidants over Rh/SiO2 (ca. 1000 ppm O 2 or 2000 ppm with NO or N20 ).

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Fig. 4. Conversions versus temperature using mixed oxidants over Rh/SiO2 (ca. 333 ppm 02, 666 ppm NO and 666 ppm N20).

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(90 ppm) is much less than with Pt/SiO2, seemingly because the greater reactivity of N20 relative to NO on Rh/SiO2 allows reaction of N20 concurrently with its formation. Figure 4 illustrates the behaviour of Rh/SiO2 for the oxidation of propane in the mixed oxidant situation. There is a small displacement of each curve to higher temperature compared with that required for the individual reactions, the shift being greatest for N20. This must be attributed to competition effects rather than differences in oxidant concentrations, since the kinetic order in the oxidant is near zero for each reaction (Table 1). The apparent conversion of N20 is never negative reflecting its relative ease of consumption in the presence of NO. Some propane remains unconverted at 500~ and the residual oxidant is predominantly NO. The behaviour of the Pd/SiO2 catalyst for the oxidation of propane by 02 alone was strongly dependent on the pretreatment and, to some extent, on the operating history. The two extreme situations corresponding to prereduction (in 1% H2 for 30 minutes at 350~ and preoxidation (in 1% O2 under the same conditions) are shown in Figure 5. The preoxidised state is much more active. In contrast to the reaction with 02, Pd/SiO2 was quite stable for the oxidation of propane by N20 alone, or by NO alone, with behaviour as shown in Figure 5. Unlike Pt and Rh, N20 is the best individual oxidant and the separation between its removal curve and that for the corresponding reaction of NO is also the largest. Even so, reaction with NO does yield significant N20, 270 ppm at the maximum, and N20 rather than NO remains when propane conversion is complete. This indicates that NO reacts in preference to N20 when both are present. The reaction of propane with the mixed oxidants showed hysteresis on the first cycle with higher activity during the initial ascending ramp. However it was stable thereafter with behaviour as shown in Figure 6. Oxygen reacts first with behaviour intermediate between that for the two situations involving 02 in Figure 5. N20 and NO react simultaneously but only after consumption of 02 is complete. The lower reactivity of N~O in the presence of O2 is particularly noteworthy. The temperature required for 50% conversion is raised by 200~ compared with that when N20 reacts alone (Figure 5) indicating very strong inhibition by 02.

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|9

-20 100

/

260'360'460'500

Temperature/~ Fig. 6. Conversions versus temperature using mixed oxidants over Pd/SiO2 (ca. 333 ppm 02, 666 ppm NO and 666 ppm N20).

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4. DISCUSSION

The situation with Pt/SiO2 is reasonably clear cut. The reaction orders for the C3H8/O2 reaction (Table 1) are strongly positive in propane and strongly negative in oxygen. This is the same as that reported by Yu Yao [9], who explained it in terms of a high but incomplete coverage by oxygen with competition between propane and oxygen for the remaining surface. The kinetic orders in oxidant for the other two reactions are different, near zero with NO and positive with N20. This indicates less complete oxygen coverage, probably because the dissociation rates of NO and N20 are lower than for 02, which also accounts for their slower rates of reaction (Figure 1). In the case of NO, the high yield of N20 points to a surface populated by a mixture of NO molecules and N atoms. Propane gains access at vacant sites before reacting with oxygen atoms. The situation with N20 is explainable by extension. Molecular adsorption of N20 is weak, leading to a situation in which the surface becomes dominated by partially dissociated hydrocarbon and competition occurs between C3H8and N20 for vacant sites. The reaction rate should then increase steeply with N20 concentration, and be inhibited by higher concentrations of propane, as observed. The order of reactivity when using the mixture of oxidants (Figure 2) follows similarly. The most readily dissociated one, 02, reacts first, although this seems to be hindered to some extent by competition from NO. Molecularly adsorbed NO may reduce the number of vacant sites available for propane adsorption. N20 starts to react only when all 02 is removed and when the temperature and NO concentrations are such that little molecular NO remains adsorbed. The kinetic orders for the reaction of propane with 02 over Rh/SiO2, fractional in propane and near zero in oxygen (Table 1), are also similar to those observed by Yu Yao [9]. The zero order in oxygen points to a fully oxidised surface. However, unlike Pt, the reactions with NO and with N20 are also zero order in oxidant over Rh suggesting that their dissociation rates are sufficiently fast to maintain high oxygen coverages. The rates of the 02 and NaO reactions are similar (Figure 3), as might be expected if the same steps (e.g. fragmentation of propane on nearly identical oxygen-covered surfaces) are occurring. The reaction with NO is slower, perhaps because the surface layer also contains nitrogen atoms isolated between immobile oxygen adatoms and this hinders attack by propane. The only difference with the mixture of oxidants (Figure 4) is that reaction of N20 is somewhat delayed, presumably because it is less readily dissociated than 02 on gaps created by the removal of reaction products. The situation with Pd is complicated by the possible formation of an oxidised phase which is marginally stable under reaction conditions. According to Yazawa et al. [ 10] the activity of Pd/A1203 for propane oxidation depends on the ratio of PdO to metallic Pd which is influenced both by temperature and the ratio of oxidant to reductant. Similarly, Carstens et al. [11] have shown that the presence of small areas of metallic palladium can enhance the activity of PdO for methane oxidation. The very small metal particle size of the Pd/SiO2 catalyst used here is likely to make it sensitive to transitions between metallic and oxidic forms and this is the probable reason for its variable behaviour. The kinetic orders observed here for the reaction of propane with 02 alone, near first order in oxygen and zero order in propane (Table 1), are almost the reverse of those found by Yu Yao [9], who used a strongly oxidising feed over preoxidised Pd/A1203. The present values suggest that the amount of oxygen on the metal is less than optimal. The kinetic orders for the reaction of propane with NO are quite different, near first order in propane and negative order in NO, indicating

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over-oxidation. Whilst surprising at first sight it is not unreasonable since the conversion of Pd to PdO is thermodynamically more favourable with NO than 02 due to the positive free energies of formation of nitrogen oxides in general. The reaction with N20, which occurs at a much lower temperature (Figure 5), shows different kinetics again, with fractional positive order in N20 and negative order in propane. The probable explanation is that the dissociation rate of N20 is never sufficient to produce an oxidised form of palladium. The metal surface then becomes partially covered by hydrocarbons, with the reaction proceeding when N20 dissociates on the remaining vacant sites. Increases in N20 concentration lead to an increased rate, while higher propane concentrations block more surface sites and hinder the reaction. The results for the mixed oxidants (Figure 6) are consistent with this picture. The higher dissociation rate of 02 transforms the system to a state in which N20 cannot react. The near plateau in the formation of N20 during the C3HdNO reaction indicates that N20 also has difficulty reacting in the presence of NO, as expected if the system is oxidic. In summary, the three reactions studied here show a variety of behaviours depending on the metal and oxidant. Considerable additional work with binary mixtures and Pd in particular will be required to build a complete understanding of them. Nonetheless, it is clear that Pt has by far the best performance when the oxidants are mixed. The final oxidant, N20, is largely removed below 300~ compared to 400~ with Pd and higher still with Rh. This confirms our previous conclusions with simulated exhaust mixtures that Pt is the key metal for minimising NaO emissions [5]. Loss of this activity may be a partial cause for the increases in N20 emissions seen as catalytic converters age on vehicles [12]. ACKNOWLEDGEMENTS This work was supported by a grant from the Australian Research Council. We are grateful to Professor R. Burwell for providing samples of the Pt/SiO2 and Pd/SiO2 catalysts. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

R. Burch and T.C. Watling, J. Catal. 169 (1997) 45. R. Burch and P.J. Millington, Catal. Today 26 (1995) 185. M. Inaba, Y. Kintaichi and H. Hamada, Catal. Lett. 36 (1996) 223. C. Howitt, V. Pitchon and G. Maire, J. Catal. 154 (1995) 47. N.W. Cant, D.E. Angove and D.C. Chambers, Appl. Catal. B 17 (1998) 63. T. Uchijima, J.M. Herrmann, Y. Inoue, R.L. Burwell, J.B. Butt and J.B. Cohen, J. Catal. 50 (1977) 464. B. Pitchai, S.S. Wong, N. Takahashi, J.B. Butt, R.L. Burwell and J.B. Cohen, J. Catal. 94 (1985) 478. D.W.T. Griffith, Appl. Spectrosc. 50 (1996) 59. Y-F. Yu Yao, Ind. Eng. Chem. Prod. Res. Dev. 19 (1980) 293. Y. Yazawa, H. Yoshida, N. Takagi, S. Komai, A. Satsuma and T. Hattori, Appl. Catal. B 19 (1998) 261. J.N. Carstens, S.C. Su and A.T. Bell, J. Catal. 176(1998) 136. V.F. Ballantyne, P. Howes and L. Stephenson, SAE Technical Paper 940304

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Kinetics of NO reduction by CO on R h ( l l l ) : A molecular beam study F. Zaera and C. S. Gopinath Department of Chemistry, University of California, Riverside, Riverside, California 92521, USA A brief overview of results from isothermal kinetic studies on the reduction of NO by CO on rhodium (111) single-crystal surfaces is provided. These studies were performed under well-controlled ultra-high vacuum, but by using high-flux collimated molecular beams that reproduce catalytic conditions.

1.

General considerations

The experimental procedure used here for rate determinations in the NO+CO/Rh(111) system is based on exposing the clean rhodium surface, a specific premixed ratio of NO and CO, and on following the partial pressure of the different gases of interest as a function of time by mass spectrometry [ 1, 2]. As an example of the results obtained with this approach, Figure 1 shows raw kinetic data for the evolution of the partial pressures of N 2, N20 (not produced ever in these experiments), NO, CO, and CO 2 over time in the case of a 1:1 NO:CO ratio and a reaction temperature of 475 K. A series of actions are taken during these experiments, as follows: (1) At time t=l 0 s the NO+CO molecular beam is turned on with the flag in the intercepting position so the crystal is not yet exposed directly to the beam. At this point the reactants (NO and CO) are scattered throughout the vacuum chamber, and their partial pressures increase up to new steady-state values. (2) At approximately t=20 s the flag is removed from the path of the NO+CO beam in order to allow for its direct impingement on the surface. This causes both a decrease in the partial pressures of the reactants and an increase in the signals of the products during the transition from a clean surface to the steady-state. The dips in the CO and NO signals are proportional to their apparent sticking coefficient, and the CO 2 and N 2 pressures increase to reflect the rate of their formation. Notice that while a change in the CO 2 signal is seen immediately after unblocking of the beam, the signal for the formation of N 2 rises only after a delay of about 20 s from that point. (3) The system is let to evolve until a steady state is reached, which in general happens within 50 s after the unblocking of the beam. Neither the adsorption of the reactants nor the desorption of the products change with time in this steady-state condition. (4) During the steady-state regime the molecular beam is blocked and unblocked deliberately by raising and lowering the flag (at times t=70 and 90 s in this example, respectively) to check the reaction rate values. The clear increases in the partial pressure of the reactants and the accompanying drops in the partial pressure of the products are proportional to the steadystate reaction rates. (5) At about t=200 s the molecular beam is turned off. After the partial pressures of reactants return to their background levels, the crystal temperature is lowered below 250 K, and the surface is saturated with CO if titration of the O atoms left on the surface is to be performed. (6) Finally, the crystal temperature is ramped at a constant rate of 10 K/s to record the TPD traces for CO and NO (to measure the amount of any unreacted molecules), CO 2 (from CO+O recombination), N 2, and 0 2. The CO 2 (from the titration experiments) and N 2 TPD traces allow for the calculation of the coverages of O and N atoms that remain on the surface after stopping the steady-state reaction.

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II 15NO+CO/Rh(111) Kinetic Run Isothermal Molecular Beam Kinetics

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Figure 1. Left: Raw data from a typical isothermal kinetic run of the type described in this report. Right: Temperature-programmed desorption (TPD) spectra recorded after the isothermal kinetic run. The heating rate used in the TPD was 10 K/s. Reaction rates and surface coverages can be calculated from these raw data. 2.

Steady-state

rates

Systematic steady-state studies were carried out as a function of surface temperature, N O + C O beam composition, and total beam flux [3]. A summary of the resulting rates is provided in Figure 2. A maximum in reaction rate is observed between 450 and 900 K, the exact temperature depending on the NO:CO beam ratio. A synergistic behavior is seen between increasing CO concentrations in the beam and higher surface temperatures. The desorption of molecular nitrogen was determined to be rate limiting for the overall NO reduction process. In particular, it was found that while the rate of CO 2 formation responds immediately to changes (blocking and unblocking) in beam flux, the nitrogen desorption traces do not (see Figure 1). 15NO + CO/Rh(111) Kinetics Steady-State Rate vs. Beam Composition and Temperature 0.02 %"

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Transient from clean R h ( l l l ) to steady-state

The transient of the NO+CO reaction from the clean Rh(111) surface to the steady state was studied as well [4]. The systematic variations observed in this transient correlate well with the overall steady-state reaction rates. Specifically, there is a time delay in the production of molecular nitrogen because of the need to build up a threshold atomic nitrogen coverage on the surface before the start of the reaction. The coverage of this nitrogen, as calculated by the time delay in the transient state, corresponds to that observed by TPD afterwards (see below). That coverage increases at a given temperature as the beam becomes richer in CO. Coverage differential parameters (aoT'aas), as defined by the difference between the coverages of the surface species (nitrogen and oxygen atoms) deposited during the initial stages of the reaction and those expected if steady-state were to be reached immediately after the start of the reaction, were found to correlate well with the behavior of the steady-state rates as a function of temperature and beam composition. The data for the case of nitrogen are shown in Figure 3.

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Initial sticking coefficients were also determined for both NO and CO in NO+CO mixtures as a function of surface temperature and beam composition [4]. The changes in S~ with temperature in particular are quite interesting, as they differ from those on clean Rh(111) [5]. While on clean rhodium the sticking coefficient of NO is temperature-independent below 800 K, in the presence of CO it increases both with NO:CO ratio and with temperature (Figure 4, left). This indicates that competition with CO alters the kinetics of NO adsorption. In general, the value of S~ in NO+CO mixtures is lower than that measured for pure NO (about 0.7), but becomes comparable for close to stoichiometric (NO:CO=1:3 to 1:7) mixtures and moderate (500-700 K) temperatures. A similar argument can be made for CO, for which the initial sticking coefficient is seen to decrease significantly above about 500 K (Figure 4, right). Ultimately, however, S~ and S~ only change by a factor of 2-3 over the most relevant temperature range, and account for a significant component of the variations of the overall NO+CO conversion rate in the steady-state regime only in the case of CO-rich beams.

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0.8

15NO+CO/Rh(111 ) Kinetics

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Figure 4. Initialsticking coefficients for the adsorption of NO (S~ left) and CO (S~ right) from NO:CO beams on Rh(111)surfaces as a function of surface temperature and beam composition. The total flux was kept constant in all these experiments at Fro,a~=0.50 ML/s. In general, s~ goes through a maximum at the conditions required for optimum surface stoichiometric NO and CO coverages, while S~ often starts to decrease at lower temperatures than S~ . As opposed to the weak dependence that reaction rates display on sticking coefficients, the differences in adsorption energies between NO and CO on Rh(111) are of great significance to the steady-state coverages, and as a consequence, to the steady-state rates. The adsorption of NO is clearly stronger than that of CO, as evidenced by the fact that CO is displaced by NO [4]. Assuming that optimum rates are reached with equimolar coverages of CO and NO on the surface (as inferred from steady-state measurements), the energy of adsorption of NO was estimated to be approximately 11.5+_1.0 kcal/mol higher than that of CO. This is the reason why the rate of NO+CO conversion is optimum with somewhat COrich beams. 4.

Surface coverages

The steady-state NO+CO conversion rates were found to be directly proportional to the coverage of atomic oxygen on the surface [6]. The relation between those rates and nitrogen coverage, however, proved to be much more complex. Two types of kinetically-different nitrogen atoms were identified on the surface (Figure 5). As mentioned above, the deposition of a critical coverage of strongly-bonded nitrogen is required for the start of the nitrogen recombination step to N 2. This threshold coverage is quite large at low temperatures, amounting to over half a monolayer around 400 K, but decreases abruptly above 600 K, and is quite insensitive to the ratio of NO to CO in the reaction mixture. An additional small amount of nitrogen appears to be present on the surface during catalysis but to desorb rapidly after the removal of the gas-phase reactants. The NO reduction rate displays an approximately first-order dependence on the coverage of these labile N atoms.

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0.8 -

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Figure 5. Nitrogen steady-state coverages as a function of reaction temperature and beam composition. Strongly-bonded N coverages (left) were calculated from TPD data (inset), while weakly-bonded N coverages (right) were estimated from data obtained by blocking the beam during the steady-state portion of the kinetic runs (inset). Note that the coverage of the strongly-held nitrogen decreases sharply with reaction temperature. The coverage of the weakly-bonded, reactive nitrogen is usually low, never exceeding 0.2 ML, and peaks around 500-600 K and close to stoichiometric NO+CO mixtures. Isotope switching experiments indicated that the two types of kinetically-different nitrogens are not likely to represent different adsorption sites, but rather similar adsorption states with adsorption energetics modified by their immediate surrounding environment on the surface. These data can be explained by a model in which the nitrogen atoms form surface islands and where the atoms at the perimeter of those islands react preferentially, perhaps via N + N O

recombination and decomposition to N 2 (Figure 6). NO+CO/Rh(111) Kinetics Surface Nitrogen Isotopic Exchange

0.20

Island Model for 14N Removal During Isothermal lSNO+CO Reactions on Rh(111)

Nitrogen TPD After

250 s Exposure to 1:1 14NO+CO Beam

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Figure 6. Left: 14N and ~SN surface coverages during the steady-state conversion of NO as a function of the time a ~4N-saturated surface is exposed to a 'SNO+CO beam (circles). The original ~4N is slowly replaced by new ~SN by following a complex kinetics explained by the islanding model depicted above (lines).

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5.

Correlations

The results from the transient and steady-state measurements can be put together with the data for the surface coverages to come up with a more complete picture of the mechanism of the overall NO+CO/Rh(111) process. Figure 7 displays a selected example of this for a NO:CO beam ratio of 1:7.

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Transient Kinetics

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Figure 7. Correlations between coverages and reaction rates for a 1:? NO:CO beam as a function of

temperature. Left: Direct correlation between weak nitrogen coverages and steady-state rates. Rate maxima for specific beam compositions are observed when AONv....=0. Right: Strong negative correlation between R,s and moo lrans.

Funding for this research was provided by a grant from the National Science Foundation. 6. [1] [2] [3] [4] [5] [6]

References J. Liu, M. Xu, T. Nordmeyer and F. Zaera, J. Phys. Chem. 99 (1995) 6167. H. Ofner and F. Zaera, J. Phys. Chem. 101 (1997) 396. C.S. Gopinath and F. Zaera, J. Catal. 186 (1999) 387. C.S. Gopinath and F. Zaera, J. Phys. Chem. B (2000) in press. M. Aryafar and F. Zaera, J. Catal. 175 (1998) 316. F. Zaera and C. S. Gopinath, J. Chem. Phys. 111 (1999) 8088.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) @ 2000 Elsevier Science B.V. All rights reserved.

The surface migration of NOx adspecies as a factor determining the reactivity of supported Pt catalysts for the treatment of lean burn engine emissions G.E. Arena, I A. Bianchini, 2 G.Centi, 1'2 and F. Vazzana 1 i Dip. di Chimica Industriale, Universit& di Messina, Salita Sperone 31, 98166 Messina, Italy. Phone: +39-90-393134, fax: +39-90-391518, e-mail: [email protected] 2 Dip. di Chimica Industriale e Materiali, University of Bologna, Italy. The dynamic of surface phenomena on a Pt(1%)/A1203 catalyst for the reduction of NO by propene/O2 has been studied by in situ DRIFT experiments and parallel transient reactivity data using the RWF (Rectangular WaveFront) method. The results indicate that NO strongly chemisorbs over Pt oxidized surface and this inhibits Pt reactivity towards hydrocarbon oxidation, but then progressively migrates over alumina surface from which can desorb as NO2. At the temperature of maximum activity in the reduction of NO, the rate of migration is slow and thus the hydrocarbon reacts with the nitrogen oxide adsorbed species over the metal surface, whereas at higher temperature the rate of migration is faster leaving free oxidized metal surface for hydrocarbon combustion.

Introduction Lean burn or diesel engines are characterized by a lower fuel consumption and C 0 2 emissions than current engines operating at a stoichiometric air/fuel ratio, but the presence of O2 in their emissions prevents the use of current "three way" type catalysts. It is thus necessary to develop catalysts able to reduce NOx to N2 in engine emissions containing oxygen. In the last years, the interest has been focused on the possibility of modifing conventional supported noble metal catalysts in order to improve their activity and temperature operative window in the reduction of NO to N2 in the presence of O2 and hydrocarbons [ 1]. This requires a better understanding of the catalytic phenomena and the dynamic of the adsorbed species [2]. The aim of this work is to contribute to the understanding of these questions using a combined approach of in situ DRIFT studies and transient reactivity investigation using rectangular modulations in the feed composition (RWF -Rectangular WaveFront method).

Experimental The Pt(1% wt.)/A1203 catalyst has been prepared using commercial "/-A1203 (RP 535) to which platinum has been added by incipient wet impregnation method using an aqueous solution of H2PtC16. After drying and calcination up to 550~ the catalyst was reduced at 400~ for 12 h with a flow of pure hydrogen and then reoxidized in mild conditions. In situ DRIFT studies have been made using an environmental diffuse reflectance chamber in series to a reactor for varying gas composition and connected to an on-line mass quadrupole detector for real-time analysis. Transient reactivity data using the RWF method have been made using a quartz fixed bed reactor. Inlet and outlet reactoi composition has been

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monitored by a mass quadrupole apparatus. Other details on the apparatus and conditions for the catalytic tests have been reported previously [3]. Results and Discussion , , o, , Steady-state activity. Figure 1 shows the cata400 . . . . . ~ + Conv. of NO lytic behavior of the Pt/AI203 sample used in - O - - Conv. of C3H6 --~ 9Conv. to NO2 this work in the steady-state conversion of NO 80 ~ Conv. to N20 in lean conditions and using propene as reductant (0.1% NO, C3H6]NO = 1, 5% O2, remain- =ing He; space-velocity 60.000 h1). In agree- .o 6o ment with literature data [4], the catalyst shows ,~ g a sharp maximum in the conversion of NO L) 40 (about 60%) centred at about 250~ (selectivity to Nz of about 50%), in correspondence of the 20 sharp increase in the conversion of propene. At higher temperatures the conversion of NO decreases and significatively increases the for0 200 300 400 mation of NO2. Reaction temperature, ~ Data reported in Figure 1 pointed out that Pt/A1203, which can be considered as a model Fig. 1 Catalytic behavior of Pt/AI203 in steadycatalyst for commercial supported noble metal state conversion of NO in the presence of three-way catalysts, is characterized from the NO and propene. presence of a sharp maximum in the conversion of NO to N2 centred at about 250~ Although this behavior can be described in kinetic terms and the model derived can correctly simulates the behavior of the performances of a light-duty diesel vehicle [5], a competition between NO and 02 chemisorption only at high temperatures must be assumed. This hypothesis do not agree with experimental evidences with labelled compounds [2] and thus a different reaction model must be considered. i,

wv

9

vv

Transient reactivity. Transient reactivity data using the rectangular wavefront (RWF) method are summarized in Figure 2. In these experiments, in a flow of 0.1% NO in He a rectangular modulation in the oxygen concentration is made for about 3 min (RWF-O2 experiments), followed by a period of about 10 min using the first feed composition. The cycle is repeated several times, after which the NOx species remaining on the catalyst are analyzed either by thermodesorption in an inert flow or at the same temperature by sending a series of Hz-RWF modulations. Figure 2 (graphs al and a2) shows RWF-O2 experiments on Pt/AI203 catalysts at the temperature of the maximum in the activity of the catalyst in the reduction of NO (see Figure 1) during a sequence of cycles (for a better comparison, the time of the starting of the cycle was normalized to zero in all cycles). Graph al shows the conversion of NO as a function of time in the various cycles and the change of the oxygen concentration during a single RWF modulation. Graph a2 reports the corresponding conversion of NO to NO2 in the various cycles. The effect of the reaction temperature (in the 5th pulse) during these RWF-O2 tests is instead summarized in graphs b 1 and b2 of the Figure 2. No other N-contaning products were detected in these conditions. The catalyst was pretreated at 550~ in He, then at 350~ with H2 for 3 h and then reoxidized with 20% O2 in He at the same temperature to clean the surface. This cleaning procedure of the catalyst is used

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Fig. 2 RWF-O2 experiments on Pt/AI203: change of the conversion of NO, concentration of 02, and conversion of NO to NO2 during a sequence of RWF-O2 modulations at 240~ (graphs a) or during the 5thpulse at different reaction temperatures (240-350~ range; graphs b). prior of each sequence of cyclic RWF tests. In the first RWF modulation at 240~ (Graph al) there is a strong initial chemisorption of NO which does not corresponds to the formation of other N-containing species. NO2 starts to form only later and continues to grow during the 3 min of the RWF modulation. The conversion of NO reaches instead a constant value after about 1 min, although the conversion of NO is higher than the formation of NO2, indicating that a large part of NO still remains chemisorbed. Switching to the anaerobic flow after 3 min, the conversion of NO decreases to zero, apart from an initial negative conversion due to a partial desorption of the chemisorbed NO in the absence of 02 in the feed. In the consecutive RWF modulations, the behavior is quite similar, apart from a slightly lower initial chemisorption of NO and a progressive increase in the formation of NO2. The amount of NO chemisorbed during the first minute in the first modulations corresponds to a degree of Pt surface coverage of about 40% (assuming a 4 A 2 as the space occupied by a single NOx adspecies on Pt and a 100 m2/g dispersion of Pt). It is thus evident that the presence of the initial chemisorption in the consecutive pulses implies that the NOx species migrate from the Pt surface to the alumina from which can desorb as NO2. This explains the delay in the formation of NO2 and its progressive increasing formation with the number of modulations (graph a2). It was also verified that in the absence of Pt, the adsorption of NO on alumina is negligible. Increasing the reaction temperature above the maximum in the conversion of NO in the

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presence of propene/O2 (240~ the conversion of NO reaches quickly a nearly constant value (graph bl in Figure 2), as well as NO2 immediately starts to form (graph b2). Furthermore, the adsorption of NO (difference between the conversion of NO and the formation of NO2) is much lower in comparison with that observed at 240~ This change in the behavior can be reasonably interpreted as a faster process of migration of NOx species from Pt to alumina surface [3]. RWF-O2 experiments to ..-.. 100 analyze catalyst activity in pro- g pene oxidation at 240~ [temperature of the maximum in NO ~ 80 conversion to N2] are summarized in Figure 3. In these tran- " 60 sient reactivity tests, the flow is g changed from a flow on 0.1% w C3H6 in He to a flow of 0.1% ~" 40 C3H6 and 0.1% NO in He and zo back. ,o I 20

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When NO is absent in the feed, the conversion of propene quickly reaches 100%, when the 00 50 100 150 200 flow is switched to that conTime, sec taining oxygen. The trend of propene conversion follows Fig. 3 RWF-O2 experiments on Pt/AI203 at 240~ to analyze the transient reactivity of propene in the absence and presence closely the change in oxygen of NO during O2 RWF pulses. concentration. When NO is also present in the feed, a clearly distinct behavior is noted. In this case, initially the conversion of propene follows that in the absence of NO (up to about 60% of conversion), but then conversion of propene decreases following a trend close to that of the conversion of NO. These results show that catalyst reactivity is deactivated by the cofeeding of NO, due to the formation of strongly bound oxidized nitrogen oxides over platinum surface. The analysis of the thermodesorption of NO (using He as the carrier gas) after the RWF-O2 experiments indicates that different type of nitrogen oxide species remain adsorbed on the catalyst depending on the presence or not of cofeed propene. While in the absence of C3H6, a broad peak centred between 350-400~ is observed, a less intense peak centred between 250-300~ is detected when propene is cofeed together with NO. This pointed out that the nitrogen oxide species desorbing at the higher temperature is probably consumed by the reaction with propene. Temperature programmed desorpton experiments [3] suggest that these species can be related to NOx species on Pt surface. m

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In situ DRIFT studies. In situ DRIFT experiments of the formation of NOx adspecies during experiments similar to those shown in Figure 2 (graphs a) are reported in Figure 4a. In the absence of oxygen in the feed, minor amounts of NOx species form on the catalyst. The NOx species are characterized by bands at 1470 and 1330 cm "], and 1230 cm -], indicating the presence of linearly coordinated and bridging nitrito species (probably on Pt surface), respectively. A band at 1581 cm ] is also observed, assigned to bridged NO on Pt. No bands due to linearly coordinated NO, either partially negatively or positively charged, could be detected in

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cating the presence of nitrate species. For longer times of ',, , ~i . ~ ~, ~ , contact of the catalyst with j,, '', 'm the feed containing NO and oxygen, the spectrum does r L\"', A . , "" ,J=';i'~l not change, but grows in in-" ~ \ \ N , ;, N-~I tensity. ,i

200~

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significative amounts (apart from a very weak and broad band at 1760 cm ~ - P t - N & ) in these in-situ experiments. These bands are detected with minimal differences by adsorption in all the 100250~ temperature range, although at the lower temperatures (100 ~ the presence of up to 1% of oxygen in the feed does not change their feature and intensity. At 250~ instead, the presence of oxygen in the feed leads to I a very fast growth in inten1200 sity (see Figure 4a) with the spectrum being dominated | from two intense bands at b t:" 1560 and 1300 cm 4 with a a

base

,~~

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spectrum of NOx adspecies changes, but the change in . . . . . . . the nature of NOx adspecies 0.00 could be better monitored, 2000 1800 1600 1400 1200 -1 when, after adsorption at low temperature (100~ flow of Fig. 4 In situ DRIFT spectra during NO or NO+O2 interaction with 0.1% NO and 1% 02 in He), Pt/AI203 at 100~ (a) or during thermodesorption in He flow the change of the spectrum after interaction at 100~ with 0.1%NO + 1% 02 for lh. in recorded as a function of the increase of the temperature during in situ temperature programmed DRIFT spectra in an inert flow. (Figure 4b).

Wavelenght,cm

Up to the reaction temperature of the maximum in catalyst activity in NO reduction (Figure 1), no significant change of the spectrum could be detected, but for higher temperature, the following changes can be noted: 9 A shift of the maximum of the most intense peak from 1581 cm ~ to 1547 cm -1 9 A shift of the peak at 1324 cm 1 to 1304 cm l with the growth of a clear intense shoulder at 1258 cm l

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9 A decrease of the intensity of the band at 1229 cm 1

Although an unambigous assignement of the change in the spectrum is not possible due to the presence of overapping bands, the modification is well consistent with a migration of the nitrate species from Pt (mainly) to alumina (mainly) surface. In situ DRIFT experiments in the presence of propene were also made as a function of the reaction temperature. Also in this case a significant change in the spectra at temperature below or above the maximum in activity was observed. At a reaction temperature of 300~ in a flow of 1% NO, 2% 02 and 1% C3H6in He, the IR spectrum in the 1900-1200 cm -1 region is characterized by two intense bands at 1560 and 1300 cm 1 similar to those obtained in contact of NO + 02 only. At a temperature of 250~ the spectrum is instead characterized by an intense band at 1660 cm -I with a shoulder at 1720 cm l (plus minor additional bands at 1320 and 1380 cm -~) indicating the formation of acrolein/acrylic acid adspecies. Bands at 2235 and 2265 cm 1 ascribed to isocianate species on Pt and alumina, respectvely, are also detected which intensity parallel the maximum in catalyst activity in NO reduction. Conclusions Transient reactivity and parallel DRIFT experiments suggest that the maximum in activity of Pt/A1203 catalyst in the reduction of NO by propene/O2 may be associated to a faster process of migration of NOx adspecies from Pt to alumina surface with respect to the rate of their formation. NOx adspecies have an effect of moderating platinum activity towards hydrocarbon total oxidation and DRIFT experiments confirm that at the maximum of catalyst activity the presence of partial oxidized products (acrolein/acrylic acid) could be detected together with a maximum in the formation of isocyanate species. This suggests that the maximum in activity in NO reduction is determined by a situation in which part of the Pt surface is covered from NO, species leaving few active sites able to oxidize only partially the hydrocarbon to acrolein/acrylic acid species which, reacting with NOx species, lead to the selective reduction of NO to N2. At higher reaction temperature, the faster migration of the NOx species from Pt to alumina surface leaves a large fraction of free Pt surface which completely oxidizes the hydrocarbon. Acknowledgements The financial support of MURST (Rome) "Progetti di ricerca di rilevante interesse nazionale-1998" is gratefully acknoledged. References [ 1] M.D. Amiridis, T. Zhang, R.J. Farrauto, Appl. CataL B: Env., 10 (1996) 203. [2] F. Garin, P. Girard, S. Ringler, G. Maire, N. Davias, Appl. Catal. B: Env., 20 (199) 205. [3] G.E. Arena, A. Bianchini, G. Centi, F. Vazzana, P. Vitali, Stud. Surface Science and Catal., 122 (1999) 109. [4] (a) R. Burch, P.J. Millington, Catal. Today, 26 (1995) 185. (b) R. Burch, P.J. Millington, A.P. Walker, Appl. Catal. B, 4 (1994) 65. [5] G.P. Ansell, P.S. Bennet, J.P. Cox, J.M. Evans, J.C. Frost, P.G. Gray, A-M Jones, M. Litorell, R.R. Rajaram, G. Smedler, A.P. Walker, Stud. Surface Science and Catal., 122 (1999) 157. [6] G. Centi, S. Perathoner, D. Biglino, E. Giamello, J. Catal., 152 (1995) 75.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Pd/SnO2 as deNOx CATALYSTS" preparation, characterization and activity D.Amalric-Popescu a, J.-M.Herrmann b and F.Bozon-Verduraz a a Laboratoire de Chimie des Mat6riaux Divis6s et Catalyse, Universit6 Paris 7-Denis Diderot, 2, place Jussieu, 75251 Paris Cedex 05-(France), email:[email protected] b Laboratoire de Photocatalyse, Catalyse et Environnement, IFOS ( U M R - CNRS 5621), Ecole Centrale de Lyon, 69131 Ecully (France) ABSTRACT Palladium-tin oxide catalysts prepared from various precursors (Pd(acac)2 and Pd0NIO3)2) and by different preparation methods (grafting, photodeposition) have been used for the elimination of NOx. They are active at low temperature (180~ in the presence of stoichiometric CO-NO-O2 gas mixtures; this activity is related to the high concentration of oxygen vacancies in tin dioxide, which is evidenced by UV-visible DRS, EPR and electrical conductivity measurements. A mechanism involving palladium and oxygen vacancies is proposed. Keywords: palladium-tin oxide, deNOx, photodeposition, oxygen vacancies, UV-visible DRS, electrical conductivity measurements. 1. INTRODUCTION AND OBJECTIVES Tin dioxide is a n-type semiconductor widely used in sensors [1,2] and in CO2 lasers [3] for low temperature oxidation of carbon monoxide arising from CO2 dissociation. Despite its reducibility, it has called much less attention as a catalyst support than titania (which has been largely used in photocatalysis [4]) or ceria [5] (which has focussed attention in automotive exhaust). The objectives of this work are (i) to prepare high surface area SnO2 samples and to characterize their defects by various techniques ; (ii) to obtain well-dispersed SnO2-supported Pd catalysts by original methods including photodeposition; (iii) to study their activity in deNOx reactions. 2. EXPERIMENTAL 2.1 Methods

X-ray powder diffraction (XRD) patterns were recorded using a Siemens diffractometer with CoKot radiation ; the crystallite size was estimated using the Scherrer equation. Transmission electron microscopy (TEM) analyses were made on a JEOL 100CXII equipment. FTIR transmission spectra were recorded on a Perkin-Elmer 1730 spectrometer; the samples were pressed into self-supporting discs (about 25 mg/cm 2) and placed in a stainless steel cell (In Situ Research Instruments) allowing in situ analysis of samples in the 20-500~ range, including CO adsorption. In situ UV-visible-NIR Diffuse Reflectance Spectra (DRS) were recorded on a Cary 5E spectrometer using a Harricks Praying Mantis accessory. The electronic state of Pd was also studied by X-ray photoelectron spectroscopy (XPS).

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2.2. Preparation of catalysts Sn02 was obtained by reaction of nitric acid on high purity metallic tin; the solid obtained after filtering and washing, was dried at 120~ and calcined at 200~ Pd/Sn02 : two preparation procedures, original on SnO2, were carried out: (i) GC sample: grafting of Pd acetylacetonate on tin dioxide from a solution in toluene at 110~ for 4h; filtering; drying of the grafted support at 80~ for 12h and calcination in flowing oxygen at 400~ for 2h ; (ii) PH sample : photodeposition of Pd from a Pd(NO3)2 solution through irradiation of SnO2 by a mercury lamp with 2-propanol as a hole scavenger, followed by drying at 80~ for 12h. The Pd content, determined by emission spectroscopy (ICP), was 3.0 % for both samples. 3. CHARACTERIZATION 3.1. SnO2 The preparation method leads to rutile-type structure SnO2 nanoparticles (table 1). Table 1. Characterization of tin dioxide Particle size, (nm) Surface area, (m2/g) Pores volume, (cm3/g) Pores diameter, (nm) Energy Band gap, (eV)

(XRD) (TEM) 200~ 500~

4 2-4 170 62 0.11 2.9 3.6

Heating in vacuo and CO adsorption give rise to lattice defects (oxygen vacancies, Sn2+); these defects are identified by UV-visible DRS, EPR and electrical conductivity measurements. UV-visible DRS : after heating in vacuo, the spectra show a marked absorption in the visible range, adjacent to the interband (valence--> conduction) transition (fig.l); this is ascribed to donor levels, such as oxygen vacancies, located within the forbidden band.

~

1,6 1,4 1,2 1,0 0,8 0,6 0,4 200' 3b0' 4b0' 500' 6b0' 7 ~ ' 800 WAVE LENGTH, nm

Fig. 1" UV-visible DR spectra of SnO2 after: 1) outgassing at r.t.; 2) outgassing at 400~ 3) adsorption of CO ( under 100 Torr ) at r.t.; 4) outgassing at r.t. ;

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EPR Aider outgassing at 400~

(fig.2) the EPR spectra shows signals at (i) g= 2.002 assigned to O species, (ii) g= 1.99 and g= 1.90 assigned to singly ionized vacancies [6] (Vo+ ); the latter signals disappear after treatment at 400~ under 400 Torr 02 (which indicates that the Vo+ species have been removed). The new signal detected at g= 2.015 is ascribed to 02- entities [7] (fig.3).

3000 :5 At

eq

,,.d

i~ -30~00

3300 3400 3500 3600 3700 B, Gauss

Fig.2: EPR spectra of SnO2 after outgassing at 400~

-3 B, Gattss

Fig.3: EPR spectra of SnO2 after calcination at 400~ under 400 Torr 02

Electrical conductivi~ measurements: heating in vacuo (fig.4) increases the concentration of

vacancies and leads to the degeneracy of the semiconductor (quasi-metallic behaviour). These defects may act as catalytic centres in redox reactions. After heating in vacuo, the electrical conductivity is 104 greater than after calcination in 02 (fig.4), which shows an increase in oxygen vacancies concentration.

-1 -2

0,02

2200 CO-S. / " N

2

,

4+

2 145

~'

] [

u-3

O•Ol

-4 -5

d

1

,

- ,48

1, 0 1,64

Fig.4" Diagram log cr = f (1000/T) 9 1) SnO2 under 400 Torr 0 2 2) in vacuo

50

i

,

j

2200 2150_1 WAVE NUMBER, cm

2100

Fig.5" FTIR spectra of CO adsorbed on SnO2 9under 1) 6 Torr; 2) 30 Torr; and 3) outgassing at r.t.

IRS o f adsorbed CO 9CO- Sn 4+ (2200 cm -1) and CO- Sn 2+ (2145 cm -1) species were detected

upon CO adsorption (fig.5). To our knowledge, this observation is reported for the first time.

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3. 2. Pd/SnO2 catalysts Because of the lack of contrast, TEM observations give no reliable data concerning particle size but some information is obtained from XRD line broadening. In addition, 117spectra of adsorbed CO (fig.6-7; table 2) allow an estimation of the metal fraction exposed (MFE) from absorbance values. Experimental conditions are chosen to avoid the drastic loss of background transmission ot}en induced by CO adsorption (flee electrons absorption). GC sample : According to UV-visible DRS (not shown), palladium is initially present as Pd 2+ ions or PdO clusters Q~maxnear 465 nm) [8], which are immediately reduced to Pd ~ upon CO chemisorption at rt., giving rise mainly to linear CO entities, a few bridged species and some residual ionic Pd species (fig.6). The MFE is estimated to about 0.5. PH sample (Fig.7)" The mean particle size determined from XRD is around 4.5 nm and the MFE is about 0.25. Before the catalytic test, XPS shows the presence of metallic palladium (335.3 eV) and also of residual Pd 2+, which means that the photoreduction was incomplete.

y8

0,2

O0

~g

0%

0,1

0,0

22OO

2 1 0 0 2OOO l_gp0 1800 WAVE NUMBER, cm

Fig. 6: IR spectra of adsorbed CO on GC catalyst: (1) 5 Torr CO 2) 15 Torr 3) after outgassing at rt.

0,00

2200

'210() '2000 '1900 '1800 WAVE NUMBER, C1~ 1

Fig.7: IR spectra of adsorbed CO on PH catalyst under (1) 5 Torr CO, 2) 15 Torr CO and 3) 35 Torr CO

We report below (table 2) the wavenumbers and assignments of the CO species adsorbed on the GC and PH samples. Table 2 IR bands of CO adsorbed on Pd/SnO2 v/cm~ species 2200 O=C - Sn 4+ 2160-2145 O-=C- Pd 2§ O - C - Sn 2§ 2135-2110 O_=C- Pd + 2100-2025 O - C - Pd ~ 1995-1960 Pd2(CO) compressed or bridged on (100) faces 1950-1925 Pd2(CO) isolated or bridged on (111) faces

References this work [8- 10], this work [8- 10] [8- 11] [8- 10] [11]

[8- lO] [11]

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4. CATALYTIC PERFORMANCES IN deNOx REACTION 4.1. Reaction conditions The reaction conditions are the following: CO (1.50 % vol.), NO (0.2 %), O2 (0.65 % vol.) (stoichiometric gas mixture), He; Space Velocity (SV) =70000-140000hq; gas flow = 12 l/h; catalyst mass = 0.125 g-0.250 g; contact time = 0.022s.-0.045s. 4.2. Results (i) SnO2 shows a noticeable activity at 400~ (95% CO and 65% NO conversion) (fig.8) ; (ii) The Pd/SnO2 catalyst prepared by grafting and calcination (GC) is the most active and NO conversion approaches 100 % at temperatures as low as 180~ for a contact time of 0.045 sec. (fig.9). For PH catalyst, NO conversion was only 75% at 180~

100

_._Pd/SnO2 o~ 80 . . . . SnO2

IO(Y: - . - - % ( 3 9

.~/CO-~ ~ ," NO

80

60

8

. .__

]~]d"~Jm~~om

%1"7

urn 9

~

ol~"

%OOooooO 9

60

40

40

20

20 o

6

' 160 ' 260 ' 360 ' 460 TEMPERATURE, ~

Fig. 8: Catalytic activity in deNOx: Sn02 at 400~ Pd/Sn02 (GC) at 220~ SV=140000h -1

500

0

9i t /

' 1~

/9

' 260 ' 360 ' 460

TEMPERATURE, ~

Fig. 9: Catalytic activity in deNOx: Pd/Sn02 (GC) at 180~ ; SV =75000h -1

4.3. Discussion In spite of his stoichiometric nature the gas mixture have a slight reducing effect, which is due to CO. Before the catalytic test, the XP spectrum of the GC sample contains only one Pd 3d 5/2 peak (336.5 eV) assigned to Pd 2+ ions. After the catalytic test, the spectrum presents two peaks at 335.5 eV and 337 eV; the first one is assigned to metallic palladium which demonstrates the reducing nature of this mixture, and the second one to Pd 2+. These results show that CO-NO-O2 reaction takes place even on SnO2 alone, and stresses the role of defects of the oxide support such as oxygen vacancies. In the case ofPd/SnO2 catalysts, it is proposed that the actives sites are oxygen vacancies of the support associated with palladium atoms at the metalsupport interface. The role of Pd is to adsorb CO and to enhance the formation of oxygen vacancies. The proposed mechanism involves the following steps: CO adsorbs on the metal, then spills over the support and finally reacts with oxygen anions of the support, generating CO2 and oxygen vacancies according to:

CO (~a~)+ O-Sn-O ~ C02 + ~ S n-O

(1)

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These oxygen vacancies can either dissociatively trap gaseous oxygen or dissociate NO (equation (2)).

N O + z~- S n-O ~ No~as.) + O-Sn-O

(2)

N:.,~,O + N:od~) ~ N2 "/'

(3)

9 These promising results, which stress the role of SnO2 defects, are now being completed by different tests involving (i) a new ~ colloidal >>SnO2 sample, prepared in hydrazine, which exhibits a very small particle size (1.5-2 nm), with a narrow distribution and a very good resistance to sintering (surface area = 230m2/g at 500~ and (ii) long term activity measurements. Acknowledgements: Engelhard-CLAL is gratefully acknowledged for palladium supply. REFERENCES

1. J.F.Aller, T.Moseley, J.O.Norris and D.E.Williams, J.Chem.Soc. Faraday Trans.I, 83 (1987) 1323-1346. 2. G.Heiland, Sensors and Actuators, 2 (1982) 343-361. 3. M. Sheintuch, J.Schmidt, Y.Lechtman_and G.Yahav, Applied Catalysis, 49 (1989) 55-65. 4. J-M.Herrmann , J. Disdier, P.Pichat, A.Fernandez, A.Gonzalez-Elipe, G.Munuera and C.Leclercq, J.Catal., 132 (1991) 490-497. 5. a) A.Trovarelli, Catalysis Reviews, 38(4) (1996) 439-508. b) S.Bernal, J.J.Calvino, M.A. Cauqui, J.M.Gatica, C.Larese, J.A.Perez Omil and J.M.Pintado, Catalysis Today, 50 (1999) 175-207. c) J.Kaspar, P.Fornasiero and M.Graziani, Catalysis Today, 50 (1999) 285-299. 6. C.Canevali, N.Chiodini, P.Nola, F.Morazzoni, R.Scotti and C.Bianchi, J.Mater.Chem., 7(6) (1997) 997-1002. 7. M.Che and A.J.Tench, Adv. Catal., 32 (1983) 1-148. 8. D. Tessier, A. Raka'f and F. Bozon-Verduraz, J.Chem.Soc. Faraday Trans., 88(5) (1992) 741749. 9. Y.A.Lokhov and A.Davydov, Kinetik i Kataliz, 21 (1980) 1515 and 1523. 10. C.Naccache, M.Primet, M.Mathieu, Adv.Chem.Ser., 121 (1973) 266. 11. A.Badri, C.Binet, J.C.Lavalley, J.Phys.Chem., 86 (1989) 451.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Decomposition of NO over copper-manganese temperature

oxide catalysts at room

I. Spassova, M. Khristova, N. Nyagolova, D. Mehandjiev Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bontchev Str. bl. 11, 1113 Sofia, Bulgaria, Direct decomposition of NO over Cu-Mn oxide catalysts in both presence and absence of oxygen was studied. The catalysts are active in direct decomposition of NO at room temperature. No N20 is formed during the reaction. When conversion proceeds in the absence of oxygen in the gas phase, part of the oxygen evolved during NO decomposition leads to secondary oxidation to NO2 of the undecomposed NO adsorbed on the catalyst surface. No homogeneous oxidation of NO occurs in the presence of oxygen. 1. INTRODUCTION The efforts aimed at reducing the NO concentration in gas emissions are mainly based on reduction of NO using NH3, CO and hydrocarbons [ 1] or direct decomposition of NO to N2 and 02 which requires no introduction of other compounds in the waste gas and no formation of other products such as CO2. Manganese-containing oxide catalysts are interesting due to the presence of various forms of labile oxygen needed for catalytic reactions [2]. In a previous paper [3] we have shown that copper-manganese oxides containing catalysts are highly effective during catalytic reduction of NO with CO to N2 and CO2 at room temperature. The purpose of the present paper was to investigate the direct decomposition of NO in presence and absence of oxygen and to establish how far this type of catalysts are applicable to direct NO decomposition at room temperature. 2. EXPERIMENTAL

2.1. Samples Catalysts with copper: manganese ratios of 1:1 and 1:2 prepared by coprecipitation of the corresponding hydroxides and subsequent thermal treatment at 300~ [4] were used. The composition with Cu:Mn = l:l exhibited a maximum activity in the reduction of NO with CO at 25~ [3]. The catalyst samples were characterized by AAS (Pye Unicam SP90B), the Bunsen-Rupp method [5], BET specific surface area and magnetic measurements with a Faraday type magnetic balance. The data are presented in Table 1.

2.2 Catalytic tests The catalytic investigations were carried out in a flow apparatus described in ref. [6]. Two different gas flows (mixtures) were used in separate experimental stages: A=(NO+Ar) flow, containing 1400-1600 ppm NO; B=(NO+O2+Ar) flow, containing 1400-1600 ppm NO and

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Table 1. Chemical formulas, specific surface areas and magnetic moments of the samples. Formula

Specific surface area, m2/l~

Magnetic moment, BM

1"1

CuMn o.04Mn 0.94(OH)o.0402.94.2.95H20

140

3.5

1:2

CuMn3+l.s0Mn 0.22(OH)l.s003.22.0.02H20

77

6.5

Sample 3+

4+

4+

700-800 ppm 02. The transient response method studies and the thermally programmed desorption (TPD) experiments were performed in an Ar flow. The volume rate of the gas flow was 136 ml/min (8000 hl). The catalytic reaction was studied at 25-300~ The activity of the catalysts remained the same during the 3 h-long experiments. The concentration of NO was measured continuously by a UNOR 5N NDIR (Maihak, Germany) gas analyzer. The analysis of N20 was performed with a Specord 75 IR spectrometer using a lm folded path gas cell (Specac, UK, England). The analysis of NO2 was carried out on a UNOR 5N.3 NDIR (thermal converter, Maihak, Germany) gas analyser. The N2 and 02 contents were controlled periodically by a Pye Unicam 104 (UK) gas chromatograph. Thermally programmed desorption (TPD) of the samples in an Ar flow was also performed in the flow apparatus with a heating rate of 20~ The infrared spectra were recorded on a Bruker model IFS25 Fourier transform interferometer (resolution <2 cm l ) in an in situ vacuum cell using self-supported wafers (~10 mg/cm2). The adsorption of NO was carried out at room temperature as well as at 80~ as described in [7]. 3. RESULTS AND DISCUSSION Comparing the activity of the catalysts investigated towards direct decomposition of NO in gas flow A (NO+Ar) (Fig. 1) we have found that the catalyst with a Cu:Mn atomic ratio of 1"1

o

a) ----Cu:Mn=1"12000 ....................:---Cu:Mn= 1:2

1600 1400 1200

b)

NO ....... Gauss peak

1500

1000

= 800 o 600 o 400 200 0

1000

500 0

""

|

10

9

|

9

i

|

20 30 40 Time, min

"

|

50

'

"

60

0

'

|

50

"

!

"

|

!

"

|

|

100 150 200 250 300 Temperature, ~

Fig.1. Sequential reaction (a) and TPD (b) stages carried out with the catalysts investigated at 25~ with gas flow A (NO+Ar).

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100 80 O

m

60

o

40

0 Z 20ti 0

0

II

III

IV

1'0 ' 2'0 ' 3'0 ' 4'0 ' 5'0 ' 60 Time, min

Fig. 2. Conversion degree of NO depending on time in gas flow A (NO+Ar) at 25~ is more active than the catalyst with Cu:Mn=1:2. The NO conversion degree achieved in 35 min is 34% with the first and 4% with the second catalyst. Since the separate samples exhibit almost the same behaviour, and the catalyst with a 1:1 ratio between copper and manganese is more active, the decomposition of NO on this catalyst is presented. Fig. 2 shows the NO conversion degree depending on time in gas flow A(NO+Ar). As is evident from Fig. 2, the reaction of NO can be divided into four stages. During the first (step I), complete absorption of NO by the gas phase is observed. The chromatographic analysis of the reaction products evidences no presence of nitrogen and oxygen. Nitrogen dioxide is not registered either. Hence, this stage represents NO adsorption on the catalyst surface. Step II consists in decomposition of NO to nitrogen and oxygen. At this stage NO is completely decomposed to nitrogen and oxygen, as is obvious from the GC. No formation of NO2 is registered. The initial mixture is found to contain NO2. GC has revealed the presence of nitrogen and an about three times smaller amount of oxygen. Therefore, a secondary reaction of NO oxidation to NO2 by the oxygen evolved as a result of NO decomposition, takes place. Fig. 2 shows that 1/3 of the converted NO is consumed in the secondary oxidation reaction. No formation of N20 is established during all three stages. TPD analysis has been used to elucidate whether there are different adsorbed forms of NO. Fig. l b presents the TPD spectrum of NO after step III (60 min). The desorption peak visible can be decomposed to three Gaussian peaks with Tmax at 52~ 74~ and 96~ This indicates nitrogen oxide adsorption on three types of active sites close in energy (desorption is observed within a rather narrow temperature range, from 30-125~ Desorption of NO2 is also established. FTIR in situ studies have evidences NO adsorption on the sample surface, oxidized NO adsorption forms only being observed (Fig. 3). Nitrosyl group lines at 1880-1880 cm -1 (attributed in [8] to N20 formation) have not been detected which is also valid for lines at 1950-1900 cm ~. The spectral region below 1770 cm -~ is complicated because of the existence of a wide variety of species due to NO oxidation (nitrate groups, NO2, N202 -2) bonded linearly monodentately or bidentately (chelate or bridging) to the metal ions from the mixed oxide.

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ej

C

,.Q O raq

,.Q

B

,

2400 2200 2000 1800 1600 1400_,1200, 1000 Wavenumbers, cm Fig.3. FTIR spectra of the pure sample (A) and after NO adsorption at ambient temperature (B) and at 80~ (C). The bands in the region 1540-1580 cm -~ are attributed to stretching modes of bidentate and monodentate nitrate species. A band with a maximum at 1460 cm -l is assigned to linearly bonded nitrite groups or to pseudo-nitrite Me-NO-O-Me species. Adsorption of NO at 80~ leads to disappearance of the bands at 1680-1800 cm -~ These are probably active sites adsorbing NO by a more labile bonds. It may be assumed that they play the role of active complexes on which that part of NO is adsorbed that is subsequently decomposed to N2 and 02. The fact that this adsorption does not take place is probably the reason for a drop in the catalyst activity with rising temperature. Above 100~ the catalysts display a constant low degree (5-8%) of NO conversion. This opinion is confirmed by the TPD spectrum in Fig. lb which clearly shows that above 100~ NO is practically not desorbed. The fact that NO adsorption is a necessary stage of its decomposition to nitrogen and oxygen is confirmed by Fig. 4 where the transient responses for nitrogen oxide at the different reaction temperatures are shown. 300~ 250~ 170~

.4.-a

o

100~

9 Z

~

50~ 25~

0

1

2

3 4 5 Time, min

6

7

Fig.4. Initial NO responses obtained in an A (NO+Ar) gas mixture depending on the reaction temperature.

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Evidently, the strongest adsorption corresponds to room temperature, whereas at 300~ adsorption is almost absent. On the basis of the results obtained, the mechanism of NO conversion in the absence of oxygen can be presemed as follows: NO+ *-->NO*ads 2NO*ads--+N2+O2

NO*ads + 1/2Oz--->NO2 The presence of oxygen in gas flow B (NO+Oz+Ar) does not inhibit the decomposition of NO (Fig.5a). Even in that case the catalysts show higher conversion degrees: 38% for the catalyst with C u : M n - 1:1 and 20% when Cu:Mn - 1:2. At the initial NO adsorption stage no NO2 formation is registered. The reaction products are N2 and NO2. Appearance of N 2 0 has not been established. With rising temperature, the conversion degree of NO increases and passes through a maximum at 200~ The TPD spectrum (Fig. 5b) has the same shape as with the reaction in which no oxygen participates, however the temperature range is broader (up to about 200~ The maxima of the three desorption peaks coincide with those in Fig. lb. The desorbed NO amount is the same in both cases. The relative amount of NO adsorbed on different sites only changes. The process kinetics shows identity with Fig. 2. The reaction mechanism follows steps I and II (Fig. 2). No stage of activation is observed, i.e. in the presence of oxygen the latter should not be accumulated in order to ensure the proceeding of a secondary oxidation to NO2 as this reaction occurs simultaneously with the decomposition of NO. With rising temperature the conversion degree of NO increases at the expense of the oxidation of NO.

1500 .. ~a.

g looo

a)

.

=.

b)

20O0_

NO

-..... Gauss Ga peak

1500. 1000.

"i

500 0

500.

0

1'o 2o 3'o ~ 5'o do Time, min

0

. . . . . 100 150

5o

260

250

360

Temperature, ~

Fig.5. Sequential reaction (a) and TPD (b) stages carried out with the catalysts investigated at 25~ with gas flow B (NO+O2+Ar).

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CONCLUSION The catalysts investigated are active in direct decomposition of NO at room temperature in presence and absence of oxygen. No N20 is formed during the reaction. When conversion proceeds in the absence of oxygen in the gas phase, part of the oxygen evolved during NO decomposition leads to secondary oxidation to NO2 of the undecomposed NO adsorbed on the catalyst surface. No homogeneous oxidation of NO occurs since during the NO adsorption in the presence of oxygen no NO2 is formed. The drop in NO conversion degree almost to zero with the increase of temperature unambiguously shows that the decomposition to nitrogen and oxygen is the main reaction taking place. The secondary reaction of NO oxidation to NO2 amounts to 30% of the total amount of converted NO. The presence of different oxidation states of metal ions on the catalyst surface leads to the formation of different acceptor sites which facilitate adsorption and further decomposition of NO. For that reason, copper- manganese oxides are very promising as catalysts? of the direct decomposition of NO at room temperature.

Acknowledgments The authors thank the National Fund for Scientific Investigations for the financial support Project X-807. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.

M. Iwamoto and N. Mizuno, Shokubai, 32 (1990) 462. F. Stone, Advances in Catalysis, Vol. 13, p. 1, Academic Press, New York, 1962. I. Spassova, M. Khristova, D. Panayotov: D. Mehandjiev, J. Catalysis, 185 (1999) 43. I. Spassova and D. Mehandjiev, Proc. 7th Int. Symp. Heter. Catal, Bourgas, part 2, p. 1000, 1991. D. Mehandjiev, I. Spassova, R. Kvachkov, React.Kinet.Catal.Lett., 44 (1991) 337. D. Panayotov, M. Khristova, D. Mehandjiev, Appl. Catal., 34 (1987) 48. D. Stoilova, K. Cheshkova, R. Nickolov, React. Kinet. Catal. Lett., 65, No2 (1998) 265. G. Centi, S. Peratoner, D. Biglno, E. Giamelo, J. Catalysis, 151 (1995) 75.

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Studies in Surface Science and Catalysis 130 A. Corrna, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Catalytic p e r f o r m a n c e of SrTiO3-based catalysts for N O direct d e c o m p o s i t i o n Y. Yokoia, I. Yasudaa, H. Uchidaa, O. Okada b, Y. Nakamura c, H. lshikawac and H. Kimura d "Fundamental Technology Research Laboratory, Tokyo Gas Co., Ltd., 1-16-25 Shibaura, Minato-ku, Tokyo, 105-0023, Japan bFundamental Research Laboratory, Osaka Gas Co., Ltd., 6-19-9 Torishima, Konohana-ku, Osaka, 554-0051, Japan cFundamental Research Department, Toho Gas Co., Ltd., 507-2 Shinpo-Machi, Tokai City, Aichi, 476-8501, Japan dResearch and Development Institute, Saibu Gas Co., Ltd., 1-10-89 Higashihama, Higashi-ku, Fukuoka, 812-0055, Japan The effect of coexisting gases on catalytic performance of the SrTio 8Feo203 catalyst for NO direct decomposition was studied. The catalyst was more stable against H,O vapor than Cu/ ZSM-5, but it was more poisoned by O, than by H~O vapor. The impregnation of the catalysts supported on MgO was found to be effective to reduce the deactivation caused by the coexisting O~ as well as to increase NO decomposition activity. -

1. INTRODUCTION The direct decomposition of NO to harmless N 2 and 02 is an attractive approach for the control of NO emission because of its chemical simplicity [ 1]. Extensive catalyst searching and screening studies for the past two decades have led to the discovery of copper ion-exchanged ZSM-5 catalysts [2], which are known to exhibit the highest activity for NO direct decomposition among various catalysts tested. However, its rapid deactivation by H20 severely limit its potential for practical use [3]. Development of a durable catalyst for NO direct decomposition remains a major challenge in the area of environmental catalysis. We have reported that Fe-doped S r T i O 3 catalysts showed high NO decomposition activity [4]. In particular, SrTi0.sFeo.203 was the most active; its activity in the absence of 02 was about two times as high as that of La0.sSro.2CoO 3 w h i c h has been reported to be the most active perovskitetype oxide catalyst [5, 6]. In the present paper, we report effects of two coexisting gases on NO decomposition activity. The one is the effect of H20 vapor that is unavoidably produced in internal combustion engines. The damaging effect by H20 vapor on the SrTi0.sFe0.203 catalyst and a copper ion-exchanged ZSM-5 catalyst was compared. The other is the effect of 02 that is present in exhaust gases from lean-burn engines. We have already reported that the Fe-doped SrTiO 3 catalysts were deactivated in the presence of 02 [4]. With the aim of overcoming this problem, we attempted to support the Fe-doped SrTiO 3 on MgO.

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2. EXPERIMENTAL

SrTio.sFeo.203 and Lao.sSro.2CoO 3 w e r e prepared by calcining precursor powder synthesized by spray pyrolysis (Praxair Specialty Ceramics) [7]. The calcination was carried out in air at 650~ for I h and at 850~ for 5 h in air. This preparation method (referred to as SP) is the same as that for the previously reported catalysts [4, 8]. Supported SrTio.sFeo.203 catalysts were prepared by the following procedure. Sr (OC3H5)2' Ti (OC4H9) 4 and Fe (OC3H5) 3 were used as starting materials. These were dissolved in an alcohol with a cation ratio Sr : Ti : Fe of 1 : 0.8 : 0.2. The support used in this study was MgO (JRCMGO-4,100A grade, from Ube Materials). The support was impregnated with the adequate amount of the solution to reach perovskite loading of 10 or 20 wt%. The samples were dried, calcined in air at 500~ for 1 h and finally calcined at 850~ for 5 h in air. To study the effects of the supporting on NO decomposition activity, a catalyst obtained by calcining the MgO support alone in air at 850~ for 5 h as well as an unsupported catalyst obtained by calcining the powder made from the solution of the alkoxides in the alcohol (referred to as AL) were prepared. Copper ion-exchanged ZSM-5 catalyst was prepared by the method previously reported [9]. SiO2/AI203 ratio of the ZSM-5 used in this study was 23. The ion exchange level was confirmed to be 148% by ICP-AES, which was calculated on the assumption that one Cu > is exchanged with two Na § For references, all catalysts tested in this study are listed with their preparation methods and abbreviations in Table 1. The phases present in the catalyst were identified by X-ray powder difraction (XRD) using Cu-Ka radiation. The Specific surface area of the catalysts, which is also listed in Table 1, was determined by the nitrogen adsorption at -196~ after the catalyst was degassed at 300~ for 3 h in a high vacuum. To measure the catalytic activity, The calcined powder was pressed into pellets, then crushed, and sieved to 40 - 80 mesh size. The catalytic activity measurements were carried out using a fixed-bed flow system with a quartz tubular reactor (10 mm i.d.) under atmospheric pressure. The exhaust gas from the reactor was analyzed by gas chromatography using a molecular sieve 5A column for N~ and O~ and a Porapak Q column for N~O. The catalytic activity for NO decomposition was evaluated in terms of the conversion of NO to N~ (2[N~]out / [NO]in). _

Table 1 Catalysts tested in this study. Catalyst

Preparation method a~

Abbreviation

SrTio.8Fe0.203 La0.8Sro.2CoO3 MgO 10 wt% STFO/MGO 20 wt% STFO/MGO SrTio.sFeo.203 Cu/ZSM-5

SP SP Calcination Impregnation Impregnation AL Ion-exchenge

STFO-SP LSCO-SP MGO STFO10/MGO STFO20/MGO STFO-AL CZSM

a)See the text

BET area (m 2 g-~) 7.8 5.7 30.8 18.7 16.1 16.8

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3. RESULTS AND DISCUSSION

3.1 Crystalline phases of the catalysts The X-ray diffraction analyses indicated that STFO-SP and LSCO-SP were single-phase perovskites. Fig. 1 shows the XRD patterns of the MgO support (MGO), the supported catalysts (STFO 10/MGO and STFO20/MGO) and the unsupported catalyst (STFO-AL). The pattern of MGO showed five distinguishable XRD diffraction peaks at 20 = 37.0 ~ 42.9 ~ 62.3 ~ 74.7 ~ and 78.6 ~ This pattern was assigned as that of MgO having the rock salt type structure. Some peaks other than those observed in the XRD pattern of MGO appear in the XRD patterns of the supported catalysts. These peaks were commonly found in the XRD pattern of STFO-AL. The intensities of these peaks were roughly proportional to the perovskite loading. Thus, the formation of particles of the single-phase perovskite-type oxide on the MgO support was confirmed. The crystallite sizes of SrTio.sFeo.203and the MgO support calculated by using the Scherrer equation are listed in Table 2.

j~

(a)

.w

(b) ~ , , _ _ _ & . ~

r

(c)

, ~

(d) I

3O

I

I

I

I

I

40

50

60

70

80

20 (degrees) Fig. 1. XRD patterns (Cu-K(~) of the supported SrTi0.sFe0203 catalysts, the unsupported catalyst and the MgO support. (a) MGO, (b) STFO 10/MGO, (c) STFO20/MGO, (d) STFO-AL.

3.2 Catalytic performance in the presence of coexisting gases To study the effects of two coexisting gases, H20 vapor and 02, on the NO decomposition activity, parallel tests were carried out for the two catalysts, STFO-SP and CZSM, under comparable conditions except for the reaction temperature. The results for the effect of H20 vapor are shown in Fig. 2. The test gas of only 1% NO/He

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Table 2 The crystallite size of the MgO support and the SrTio.sFeo.203 caculated using the Scherrer equation. Catalyst

Crystallite size (nm)

MGO STFO 10/MGO STFO20/MGO STFO-AL

MgO

SrTio.sFeo._~O3

30 36 39 --

-'26 28 38

100 z

STFO-SP

80

CZSM

0

9 z

60

=

0

40

>

20

0 .,.i

0

0 A

B

C

Fig. 2. Effect of O~ on NO decomposition activity in 1% NO/He at 800~ for STFO-SP and 500~ for CZSM; W/F - 0.6 g s cm-3; (A) in the absence of H~O vapor, (B) with l0 vol.% of H~O, (C) in the absence of H~O vapor after exposure to H~O. was introduced for 10 h (A). When 10 vol.% of H~O was added to the test gas, the conversion over CZSM decreased significantly while STFO-SP maintained half of the initial activty over the next 5 h (B). Then, the H,O was removed from the stream. STFO-SP recovered the activity nearly to its initial level while CZSM recovered only half of the initial activity (C). These results suggest that STFO-SP is more stable against H~O vapor than CZSM in spite of the higher reaction temperature. The procedure for testing the effect of O, was the same as the test for the H,O vapor except for the step B. The results are shown in Fig. 3. After the step A, when the test gas of 1% NO/He was swiched to the mixture of NO (1%) and O~ (10%) balanced with He, STFO-SP still showed higher conversion. However, the degree of poisoning by 02 is greater for STFO-SP than the poisoning by H,O vapor. When the test gas was returned to 1% NO/He after 5 h residence at step B, both catalysts completely recovered the activity to their initial levels. It was found that the coexisting O 2 poisoned the NO decomposition activity of STFO-SP more than the coexisting H20 vapor did.

3.3 Catalytic activity of supported catalysts To improve the NO decomposition activity, especially in the presence of excess O 2, we tried to support SrTio 8Fe0203 on MgO and studied the effects of the supporting on the NO decompo-

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1323 100

!

化

~ 80

Z O

o z

60

t

'

STFO-SP CZSM

0

=

40

>

20

~0,,,,q

0

A

B

C

Fig. 3. Effect of O, on NO decomposition activity for STFO-SP at 800~ and for CZSM at 500~ W/F = 0.6 g s cm-3; (A) 1% NO/He, (B) 1% NO + 10% O,/He, (C) 1% NO/He after (B). sition activity. Fig. 4 shows the comparison of NO decomposition activity per unit weight of SrTio.sFeo.203 between the supported and unsupported catalysts. STFO 10/MGO showed almost 100% of conversion of NO under this typical reaction condition, although it must be taken into consideration that the NO conversion of the MgO support (MGO) is 11% under the same reaction condition. The weight-normalized activity of STFO 10/MGO was about two times as high as that of STFOAL. The activity is considered to depend on the crystallite size of SrTio.sFeo203 listed in Table 2, since the activity decreased with the increase in the crystallite size. It is interesting that the NO conversion of STFO-AL was about half of that of STFO-SP in spite of larger BET area of STFOAL than STFO-SP. This suggests that the differnce in preparation method can greatly influence the NO decomposition activity. Fig. 5 shows the NO decomposition activity as a function of contact time for STFO 10/ MGO, STFO-SP, LSCO-SP and MGO. Fig. 6 shows the NO decomposition activity as a function of the coexisting O, concentration. The degree of the deactivation was found to be greatly _

100

m Z

80

0

o z

60

0

40

=

~0,,,,q

20 0

STFO 10 /MGO

STFO20 /MGO

STFO-AL

STFO-SP

MGO

Fig. 4. Effect of MgO-supporting on NO decomposition activity. W / F - 3.0 g s c m -3, 1% NO/He balance, 800~ Closed bars: W is weight of SrTio.8Fe0.~_O3,Open bar: W is weight of MGO.

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100

化

100

STFO 10/MGO zo

80

ZO

60

o

40

~

Z o

STFO-SP

80

9 z 60

L S C ~

o = o

40

>

20

~ ,,,,q

> o r..)

20

o 0 0

1

2

3

W / F ( g s c m -3)

Fig. 5. NO decomposition activity as a function of contact time. W/F = 3.0 g s c m -3, 1% NO/He balance, 800~ Closed symbols: W is weight of perovskite, Open symbols: W is weight of MGO.

5

02 concentration (%) Fig. 6. Effects of O, concentration on NO decomposition activity. W (Perovskite)/F- 0.6 g s c m -3, 1% NO/He balance, 800~

reduced by the supporting. The conversion of the supported catalyst in the presence of 10% O~ is as high as 60%, which is the same level as that of the unsupported catalyst in the absence of O~. In this way, the impregnation of the catalysts supported on MgO was found to be effective to reduce the deactivation caused by the coexisting O~ as well as to increase the NO decomposition activity. _

4. ACKNOWLEDGMENTS This work was performed as a part of the national project "Development of Ceramic Gas Engine" supervised by the Japan Gas Association. Financial support from MITI (Ministry of International Trade and Industry, MITI) is greatly acknowledged. REFERENCES

1. J.W. Hightower and D.A Van Leirsberg, The Catalytic Chemistry of Nitroge Oxides, R.L. Klimish, J.G. Larsonv, Eds., Plenum Press, New York, 1975, p. 63. 2. M. Iwamoto, H. Furukawa, Y. Mine, F. Uemura, S. Mikuriya and S. Kagawa, J. Chem. Soc., Chem. Commun., (1986) 1272. 3. M. Iwamoto, H. Yahiro, K. Tanda, Stud. Surf. Sci. Catal., 37 (1988) 219. 4. Y. Yokoi, I. Yasuda, H. Uchida, O. Okada, Y. Nakamura and S. Kawasaki, 2 "d world Congress on Environmental Catalysis, Miami Beach, 1998. 5. Y. Teraoka, H. Fukuda and S. Kagawa, Chem. Lett., (1990) 1. 6. Y. Teraoka, S. Kagawa and T. Harada, J. Chem. Soc., Faraday. Trans., 94 (1998) 1887. 7. C.B. Martin, R.P. Kurosky, G.D. Maupin, C. Han, J.Javadapour and I.A. Aksay, Ceramic Powder Science ~ , G.L. Messing and S. Hirano Eds., American Ceramic Society, Ohio, 1987, 8. Y. Yokoi and H. Uchida, Catal. Today, 42 (1998) 167. 9. M. Iwamoto, H. Yahiro, Y. Mine and S. Kagawa, Chem. Lett., (1989) 213.

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化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

CO and NO elimination over Pd-Cu catalysts M. Fermindez-Garcia, a'* A. Martinez-Arias a, J.A. Anderson b, J.C. Conesa a, J. Soriaa a Instituto de Cat~lisis, CSIC, Campus Cantoblanco, 28049-Madrid, Spain b Dept. of Chemistry, University of Dundee, Dundee DD 1 4HN, Scotland, UK The behaviour of a series of Palladium and Palladium-Copper catalysts supported on ceria/alumina and ceria-zirconia/alumina for the CO+NO+O2 reaction has been analysed by a combination of Electron Transmission Microscopy, Infrared and Electron Paramagnetic spectroscopies and catalytic test studies. In both systems, the catalytic behaviour is dominated by the properties of the Metal-Support Interface. While Palladium systems show a similar behaviour in both types of supports, Cu only modifies the behaviour of the ceria/aluminasupported one. 1. I N T R O D U C T I O N The strengthening of exhaust emission legislation expected for the XXI century will make necessary the improvement of the actual catalytic converter technology in automobiles. In stoichiometric conditions, low light-off catalysts, which start working earlier by decreasing the initial temperature of operation, constitute one of the most promising ways explored recently to achieve this objective [1,2]. As lead and sulphur levels have become lower in fuels, the use of palladium has increasingly attracted the attention of the catalytic community in order to develop efficient catalysts for hydrocarbon and CO oxidation at low temperatures [1,3]. However, the performance of Pd-based systems to eliminate NO requires usually improvements to become industrially applicable [1,2]. As it is well known, substitution of Rh in three-way catalysts (TWCs) is another desirable objective from an economical point of view and promotion of Pd with a second "active" metal has been revealed as a useful way to achieve this goal. Cu has been shown to enhance significantly the NO elimination activity of Pd in typical ceria/alumina three-way catalysts (TWC) [4]. Here we will extend the analysis of Pd and Pd-Cu loaded systems, comparing the chemical behaviour and catalytic performance of ceria/alumina and ceria-zirconia/alumina supported catalysts in the CO+NO+O2 reaction. 2. EXPERIMENTAL A T-A1203 powder (Condea), here designated A, was used as base support. The CA support was prepared from it by incipient wetness impregnation with aqueous cerium(III) nitrate, in an amount such as to give a final CeO2 content of 10 wt.%. The ceriazirconia/alumina (CZA) was prepared using zirconyl nitrate and cerium(III) nitrate (Ce:Zr ratio 1"1) in an inverse microemulsion (water in a heptane organic solvent, using Triton X-100 as surfactant and hexanol as cosurfactant). The amount of the alumina support required to achieve 10 wt.% of CeZrO4 was added to this microemulsion prior to mixing with a similar emulsion containing tetramethylammonium hydroxide as base. The resulting suspension (with all Ce and

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Zr coprecipitated) was stirred for 24 h, centrifuged, decanted and rinsed with methanol. After drying at 353 K and calcination of the CA and CZA precursors in air at 773 K, all supports displayed BET surface areas similar to the parent alumina (180 m 2 g-l). A microscopy (TEM/diffraction) analysis of the CZA material shows that it contains particles of a 1:1 Ce-Zr mixed oxide phase of size around 20 A, homogeneously dispersed in the alumina support [5]. Fractions of the above support materials were (co)-impregnated with Cu(II) nitrate and/or Pd(II) nitrate and further calcined at 773 K. Metal loadings were 0.5 wt.% of Pd and 1.0 wt.% of Cu. On samples pre-calcined at 773 K, T-programmed catalytic tests at 10 K/min using a 1% CO + 0.1% NO + 0.45 % 02 (N2 balance) mixture at 30.000 h -~ were performed in a glass reactor system, coupled with a Perkin-Elmer 1725X FTIR spectrometer for on-line analysis of the gas phase composition. Characterisation of samples was carried out using a JEOL 2000 FX (0.31nm point resolution) electron microscope equipped with a LINK (AN 10000) probe for EDS analysis. DRIFTS analysis of adsorbed species present on the catalyst under reaction conditions was performed using a Perkin-Elmer 1750 FTIR fitted with an MCT detector. EPR spectra were recorded at 77 K with a Bruker ER 200 D spectrometer operating in the X-band and calibrated with a DPPH standard (g=2.0036). In this last case, samples were handled using a conventional dynamic high-vacuum line. In all cases, they were subjected to calcination pretreatment under 300 Torr of Oz at 773 K. Then they were reduced in 100 Torr of CO at 423 K followed by outgassing at the same temperature. Other sample aliquots were subjected to the same reduction treatment, aRer which they were treated with 10 Torr of NO at 423 K and thoroughly outgassed at room temperature (RT). After both treatments, EPR data were obtained following oxygen adsorption (70 ~tmologl ) plus 30 min outgassing at 77 K.

3. RESULTS 3.1. Catalytic tests The light-off behaviour of Pd and PdCu specimens is depicted in Fig. 1. For CO conversion, copper addition improves Pd performance on both types of supports, CA and CZA, although at high temperatures the CZA-supported materials show only a marginal effect. For NO, bottom of Fig. 1, only CA-containing materials present an enhancement by addition of Cu. In both mono and bimetallic systems, the CA-supported catalysts happen to be more effective than those using CZA (and these more than those on alumina) for both CO and NO elimination, except in the case of NO conversion in the high T range, in which the monometallic CZA-supported catalyst is more active than the CA-based one. Monometallic Cu catalysts gave always conversions (data not shown) much below the Pd-Cu ones.

lO0 80 "~

60

o= ~' ~ ,-,

40 20 0 10 0

~ ~

80 60

~ ~

40 20 0

II

--m--

_~.~

~'

~_..~

i

350

#

.--

,

!

450

PdCu/CA

PdCu/A

--o-- Pd/CA

,

!

550

l I

650

Tem perature (K) Fig. 1. CO and NO conversion profiles for the C O + N O + O 2 reaction over Pd and Pd-Cu catalysts.

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3.2. Drifts data

The Microscopy study (electron diffraction and EDS) was not able to detect any Cu or Pdrelated signal in fresh or used samples, indicating a high initial dispersion (particles are estimated to be smaller than 10A) and the absence of strong sintering processes during reaction 9 The presence of these active metallic phases can be however detected by DRIFTS under reaction conditions. For monometallic Pd samples (Figs. 2A,B), spectra taken at low temperature show mainly (overlapping the two-lobed CO gas band) a band located at 2160 cm -l, attributable to Pd 2+ carbonyls [6]. For Pd/CA, it disappears at 363 K, indicating palladium reduction, being formed again at higher temperatures, coinciding with the onset of NO reduction. The same band is also present in sample Pd/CZA, but only at low temperatures (< 363 K). A band at 1970-1960 cm l evidences the presence of CO bridging two Pd(0) atoms [7] for both CZA-supported samples (Figs. 2B,C); the bimetallic one displaying it only at T > 425 K. For Pd/CZA, an additional contribution was also detected initially around 2090 cm -~, with a downward shit~ of 30 cm q at increasing temperatures; this band is assigned to CO ontop of Pd(0) atoms [7]. A common feature to all spectra is the presence of NCO species, giving bands at 2230 and 2255 cm l [8]. Notice that this species has been postulated as a real intermediate in NO reduction [4]. In agreement with this, Figs. 1 and 2 display a correlation between the NO light-off behaviour and the temperature range of existence and band intensity of NCO species. This species appears from 393 K for Pd-Cu/CA (spectra not shown), together with strong peaks in the 2150-2100 cm -l range which, like the similar ones observed here for Pd-Cu/CZA, are mainly due to Cu+-CO complexes [4,9]. .

,

.

,

.

,

.

~d

I

9

I

'

I

9

I

9

I

0.5 ,

|

,

|

,

i

,

Pd-Cu/A ~ A

_.aa .

i

.

i

.

,

u/CZA

'e

K,..

Pd-Cu/CA

~d

23( 0

Pd/CA

2200

2100

2000

Wavenumber (cm "~)

1900

Fig. 2. IR spectra of A) Pd/CA, B) Pd/CZA, and C) Pd-Cu/CZA in a) O 5 flow at RT, then in a CO+NO+O 2 flow at b) 303, c) 363, d) 423, and e) 453 K.

I

I

I

I

I

2300

2200

2100

2000

1900

Wavenumber (cm i) Fig. 3. IR spectra after CO adsorption at RT on used catalysts.

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After running the CO+NO+O reaction up to 523 K (and 473 K for the ceria/alumina based systems) and purging in N2 at the same temperature, CO adsorption at RT (Fig. 3) gave rise to bands at 2090 and 1970-1960 cm ~ attributable to metallic Pd in the monometallic and Pd-Cu/CZA specimens. This evidences the presence of Pd metallic particles after this treatment; these bands however seem to be absent in the CA- and A-supported bimetallic catalysts, a fact ascribed to the formation of alloyed Pd-Cu particles [4], which has been confirmed from in-situ T-programmed XANES data [12]. All Cu-containing samples yield bands in the 2150-2100 cm -~ range, characteristic, as mentioned before, of CO adsorption on Cu + and Cu 2+ centres [4,9].

3.3. EPR data The EPR results show that oxygen adsorption on ceria-containing catalysts reduced in CO leads to formation of superoxide radicals (Ce4§ -) following a process that can be envisaged as Ce3+-V~ + 02 ---->Ce4+-O2", where V ~ denotes a doubly ionised anion vacancy. Evaluation of the intensity and nature of such species gives information on the degree of reduction achieved by the sample and on the nature of the reduced centres involved in this process [4,10]. For the three samples examined, the spectra are mainly formed by similar superoxide signals in which the Ce3+-V ~ adsorption centres belong to two-dimensional entities dispersed on the alumina surface; these are characterised by presenting gz < 2.028 and gx > 2.017 (with gy = 2.012), in contrast with species formed on ceria and/or zirconia-ceria mixed oxide aggregated phases [10,11]. As shown in Table 1, more cerium ions are reduced by CO in the Pd-Cu/CZA case. However, NO interaction with the anion vacancies created by CO reduction, which leads to destruction (reoxidation) of part of them, is more efficient for Pd-Cu/CA, for which a very low intensity (disallowing quantification) is observed after NO treatment. For the Pd-Cu/CZA sample, actually, reaction with NO produces a degree of decrease in the number of O2-forming centres somewhat smaller than for Pd/CA. Thus, it would seem that the reactivity of these centres towards NO is lower on the CZA-supported specimens. Table 1. Concentration of superoxide species (btmol per gram of sample) detected after oxygen adsorption at 77 K on the samples treated in CO (+ vacuum) or in CO (+ vacuum) and subsequently in NO at 423 K; the ratio between both magnitudes is included as well. Sample

Intensity (101 [tmol.g q) [O2] co [O2] co+so

Pd-Cu/CZA 5.2 2.9 Pd-Cu/CA 1.2 n.e.* Pd/CA 2.2 0.9 n.e. not evaluated due to the small intensity of the signal

[O2"]CO+No/[O2-]CO 0.56 <0.2 0.41

4. DISCUSSION The catalytic activity of noble metal TWCs for CO and NO elimination depends on the properties of the metal-support interface as this governs the chemical (oxidation) state of the noble metal and the number and distribution of anion vacancies present in the Ce-containing material [3,4,9,10,11 ]. For Pd monometallic systems, the CZA support seems to enhance the Pd reducibility

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with respect to CA, i.e., to increase the presence of superficial Pd(0) centres and decrease that of Pd 2§ during reaction (Figs. 2A,B). This reducibility seems to be further exalted by Cu in the CZA material, where pd2§ bands are absent (only the CO gas contribution is observed in Fig. 2C in the corresponding frequency region around 2160 cml). However, addition of Cu, which on the A and CA supports leads to extensive reduction of surface and bulk Pd entities by formation of alloys during reaction [4], on CZA only promotes, as mentioned above, a reduced state of Pd without formation of an alloyed phase, at least at high temperatures. This absence of alloying is evidenced by the presence of a well-developed band at 1970 cm "1 in the in-situ run (Fig. 2C) and particularly in the post-reaction experiment shown in Fig. 3. This suggests that Cu interacts preferentially with the CZ mixed phase and this, in turn, affects the Pd oxidation state. Interesting to mention is that preliminary analysis of XANES results carried out in similar T-programmed reaction conditions shows on both CA and CZA supported materials alloying of Pd-Cu starting at intermediate reaction temperatures. However, for the CZA specimen, this binary phase disappears at higher temperatures, in agreement with the evolution of the 1970 cm -1 peak. This behaviour suggests that a specific interaction of Cu or Pd with the ZrCeO4 phase is energetically favoured with respect to the Pd-Cu one. A tentative explanation can be reached from analysis of the Cu K-edge XANES data, where a larger degree of reduction is observed when zirconium is present in the catalyst. Although the XANES study is preliminary and will be described elsewhere in more detail [ 12], it allows to hypothesise that the larger energy stabilisation reached by reduced Cu states in contact with the ZC phase may be the driving force to extract this metal from the Pd-Cu alloy. In spite of the larger Pd reducibility induced by Cu addition in CZA-supported systems, no significative improvement in CO and/or NO elimination is observed, in contrast to CA-based systems. Considering this last support, results of Fig. 1 show that Cu addition here enhances Pd activity towards CO oxidation and particularly NO reduction. For PdCu/CA, alloying of active components is detected by IR after reaction (low intensity of the 1970 cm -l peak, Fig. 3), this being confirmed by XANES [12]. This phenomenon allows Pd to stay in a reduced state in reaction conditions, modifying also its electronic state in the adequate direction for a stronger NO activation [13]. However, comparison of catalytic performance of Pd-Cu/CA and Pd-Cu/A, both containing Pd in an alloyed phase, shows that the cerium-containing phase plays a key role in governing the catalytic behaviour. As reported previously, cerium oxidic compounds promote Pd reduction by CO even from RT by forming anionic vacancies at the metal-support interface which favour 02 and NO dissociation processes [2,3,4]. This new path allows to obtain catalytic activity from low temperatures; close to RT for CO oxidation and above 373 K for NO elimination [3,4]. As can be seen in Table 1, the metal-promoted CZA support contains the largest number of oxygen vacancies after CO treatment at 423 K. This coincides with the well-known fact that Zr-Ce mixed oxides have higher capacity for oxygen release/uptake than ceria, and thus indicates that the dispersed phase present in these catalysts has a mixed Zr-Ce oxide composition [11,14]. However, results of Table 1 show that the redox activity of these vacancies seems to be lower for Pd-Cu/CZA than for Pd-Cu/CA, presenting a behaviour similar to that shown by Pd/CA. This follows from the fact that NO-induced reoxidation is less pronounced in those systems than for Pd-Cu/CA. Thus, comparing Pd-Cu/CZA and Pd-Cu/CA, we can conclude that for the former the activation/dissociation process of such molecule is less promoted by the presence of Cu, which would be a consequence of the absence of alloying in the PdCu/CZA case and may be also of a lower activity of the anion vacancy sites.

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5. CONCLUSIONS

Ceria-zirconia additive (as present in CZA) seems to promote Pd monometallic systems in a similar way as CA, although the larger metal reducibility induced by it (particularly at high temperature) and the higher number of anion vacancies that can be formed by reduction on it may explain the improvement of the NO reduction at high temperatures observed in Fig. 1. The enhancing effect of Cu in Pd catalytic activity detected in CA-supported systems [4] is much smaller for CZA-containing systems (specifically in the NO reduction reaction); this is attributed mainly to the absence of alloying between these active metals, which might be related to a specific stabilisation of reduced Cu species by the ZrCe04 phase. More work will be necessary to determine the best way (preparation method, change in component proportion, etc.) for making compatible in Pd catalysts the advantages of promotion by Cu and those of using ZrCe04 instead of CeO/. Acknowledgements Support from CAICYT (Project no. MAT 97-0696-CO2-O1) and CAM (Project no. 06M/084/96) is appreciated. M. F.-G. thanks the "Ministerio de Educaci6n y Cultura" of Spain for a Postdoctoral contract. A. M.-A. thanks the "Comunidad de Madrid" for financial help under the "Ayudas para Estancias Breves en el Extranjero" program. We also thank the Royal Society (London) for a European Exchange Fellowship (M. F.-G.) and a University Research Fellowship (J.A.A.).

REFERENCES 1. R.A. Searles, Stud. Surf. Sci. Catal., 116 (1998) 23. 2. A. Trovarelli, Catal. Rev. Sci. Eng., 38 (1996) 439. 3. M. Fem~ndez-Gareia, A. Martinez-Arias, N.L. Salamanca, J.M. Coronado, J.A. Anderson, J.C. Conesa and J. Sofia, J. Catal. 18 (1999) 475. 4. M. FemAndez-Garcia, A. Martinez-Arias, C. Belver, J. A. Anderson, J. C. Conesa and J. Sofia, J. Catal. (accepted). 5. M. Fem~adez-Garcia, unpublished results. 6. B. Skarman, L.R: Wallenberg, P.-O. Larsson, A. Andersson, J.-O. Bovin, S.N. Jacoben and U. Helmersson, J. Catal., 181 (1999) 6. 7. P. Hollins, Surf. Sei. Rep., 16 (1992) 51. 8. J.A. Anderson, C. M~rquez-Alvarez, M.J: L6pez-Mufioz, I. Rodriguez-Ramos and A Guerrero-Ruiz, Appl. Catal. B, 14 (1997) 189. 9. A. Martinez-Arias, M. Fem~dez-Gareia and J. C. Conesa, J. Catal., 182 (1999) 367. 10. A. Martinez-Arias, J.M. Coronado, J.C. Conesa and J. Sofia in Rare Earths. R. Sfiez Puehe and P. Caro (eds.). (Complutense. Madrid, 1997). Page 299. 11. A. Martinez-Arias, M. Femhndez-Garcia, V. Ballesteros, N. L. Salamanca, J. C. Conesa and J. Sofia, Langmuir, 15 (1999) 4796. 12. M. Femhndez-Garcia, A. Martinez-Arias, J. C. Conesa and J. Sofia, in preparation. 13. F. Illas, N. L6pez, A. Clotet, J. M. Ricart, J. C. Conesa and M. Fem~dez-Garcia, J. Phys. Chem. B, 102 (1998) 8017. 14. G. Vlaic, R. Di Monte, P. Fornasiero, E. Fonda, J. Ka~par and M. Graziani, J. Catal., 182 (1999) 378.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V, Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

NO~ storage and reduction over Pt/BaJAI203 James A. Anderson ~, Allan J. Paterson ~ and Marcos Femhndez-Garcia 2 1Department of Chemistry, The University, Dundee, DD1 4HN, Scotland, UK 2Instituto de Catfilisis y Petroleoquimica, CSIC, Campus UAM, 28049, Madrid, Spain Pt/BaO/A1203 catalysts containing stored NOx have been heated from 298-673 K under air or air/propene/N2 flows to determine the stability of the stored NOx under conditions akin to those experienced during warm-up. Under lean conditions the stored NOx is stable but is destabilised if the experiment is performed under stoichiometric conditions. Formation of bulk Ba(NO3)2 is favoured by the presence of Pt. The absence of isocyanate and the detection of CO coadsorbed with oxygen indicated that released NO2 maintained oxygen coverage of the Pt surface thus preventing NO dissociation and thereby eliminating this particular route to N2 formation under stoichiometric conditions.

I. INTRODUCTION Storage and reduction catalysts offer one possibility of controlling NOx emissions from automobile sources while permitting operation under predominantly lean-bum conditions [1 ]. NOx is stored under lean conditions on an alkaline earth oxide component and then rel,, ed during intermittent rich/stoichiometric periods where the stored NOx is released and reduced by CO or HC over the noble metal component. Although certain mechanistic details of the processes involved are now emerging [2,3], much detail is still required. For example, it is uncertain how stable the stored NOx will be under warm-up conditions. In order to address this aspect, combined TPD/FTIR studies have been performed over Pt and Pt-free BaO/A1203 catalyst under both lean and stoichiometric conditions over samples which have been previously exposed to NO2. 2. EXPERIMENTAL BaO/AI203 samples (1 to tO wt% BaO) were prepared by precipitation of BaOH from a nitrate solution onto Degussa Aluminoxid C v-alumina using an ammonia solution. The prepared material was filtered, washed, dried at 363 K (16 h) and calcined in air (100 cm3 rain I) at 773 K (2 h). A portion of each sample was wet impregnated with 1 wt% Pt (H2PtC16). Excess solvent was removed by heating under continuous stirring and the resulting powder dried overnight at 363 K prior to calcination in air (100 cm3 min ~) at 773 K (2 h). XRD of the dried powder samples was carried out using Cu K~ radiation and BET surface areas measurements were performed using Ar as adsorbate on samples outgassed at 573 K. Pt dispersion was measured using CO pulsed chemisorption on samples reduced in H2 at 573 K. Barium dispersion was determined from CO2 adsorption isotherms at 298 K on sa-,oles heated to 1013 K under N2. DR/FT spectra were recorded at 4 cm ~ resolution (100 ,~ans)

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using an MCT detector and Harrick environmental cell. 150 nag of powdered sample was used. A computer-controlled gas blender was attached to the IR cell, with the exit gases passed to a chemiluminescence detector for NO/NOz/total NOx analysis. Samples were calcined m situ in the DRIFT cell at 673 K (1 h) in a flow of dry air (50 cm 3 minq), cooled to 298 K and exposed to NO2. The cell was then flushed in air before a TPD of NOx from the samples was performed under air or air/propene/N2 flows (50 cm 3 min "1) between 298 to 673 K at 3 K min q. NOz (99.5 %) and propene (99.99 %) were used as supplied.

3. RESULTS AND DISCUSSION 3.1 Sample characterisation Table 1 Sample characteristics

1.0 0.9

....Material ............... BET --- D--CO2-i-BaO(m2g"1) (%) Ratio

o.8 0.7

........................................................................................................................................................................

A1203 1% BaO/AI203 2.5 % BaO/Al203 5 % BaO/Al203 7.5 % BaO/A1203 10% BaO/Al203 Pt, 1% BaO/Al203 Pt, 5 % BaO/Al203 Pt, 10 % BaO/A1203

93.5 88.3 91.1 87.1 92.7 101.7 93.8 92.6 90.9

35.2 46.3 51.8

0.45 0.36 0.27 0.16 0.09 0.22 0.23 -

0.6 ~ 0.5 ~ 0.4 ~ 0.3 r

0.2 o.1 0.0

15

,

I

20

,

I

25

,

I

30

,

I

35

i

I

40

,

I

45

~

I

50

~

I

55

2e Fig. 1 X E D patterns of samples (a) 1, (b)

2.5, (c) 5, (d) 7.5 and (e) 10 wt% BaO/A1203 and also (f) Pt-5%BaO/A1203 following exposure to NO2. BET surface areas (Table 1) were reasonably constant with little change following the addition of baria, or subsequent addition of Pt and further calcination. Platinum dispersion (assuming Pt:CO =1) unexpectedly increased as the BaO loading was increased, even though the fraction of surface covered by baria was greater at higher loadings leaving less of the alumina surface free for interaction with the Pt salt. The subsequent addition of Pt appeared to effect the dispersion of baria with the lowest BaO loading suffering the greatest loss. BaO dispersion showed an almost linear decrease as a function of loading. Consistent with baria dispersion measurements, XRD patterns (Fig. 1), although dominated by features for alumina, showed evidence (24 and 42 ~ 20) for crystalline Ba(CO3) for samples exposed to air, with the lwt% sample showing the weakest peaks.

3.2 Exposure to NO2 Exposure to NO2 led to a change in the observed XRD pattern for Pt containing catalyst (Fig. 1, f) with losses in intensity at 24 and 42 ~ 20 and the appearance of peaks at 19 and 36.6 ~ 20, which are consistent with peaks in the diffractogram of bulk barium nitrate. These

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peaks were retained alter heating to 673 K in air but depleted to some degree following heat treatment in the stoichiometric gas mixture. 3.3 Decomposition under air

1400

~0 1200 Q- 1000

"~

400

0

a

o300

350

400

450

500

550

600

650

Temperature/K Fig. 2. TPD of N02 desorbed in air from (a) A1203, (b) ], (c) 5 and (d) 10 wt% Ba0/AI203 and from (e) Pt-5% Ba0/AI203. 22

-

1294

20 18

lo 1624

--

68 4 2 0

[i

1586

(e~.,.~

~ \ \ Xd) /"'/-\" ~ " " ' c )

(c)

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--e) 254

,

, oo

,

,goo

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,goo

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,

I

,.oo

,

,3'oo

,

I

, oo

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Fig. 3. Spectra of 5 wt% BaO/AI203 after (a) calcination in air, (b) exposure to NO2 with subsequent heating in air (50 cm 3 min -1) at (c) 345, (d) 440, (e) 530, (f) 605 and (g) 673 K. Fig.2 shows profiles of NO2 desorbed from samples, which, according to XRD patterns may be expected to contain bulk nitrates. FTIR spectra (Figs. 3,4) also indicate that various forms of surface nitrate and other NO containing species are present. NO desorption in all cases was less significant and in general, followed the NO2 desorption profiles. NO2 was initially desorbed in a single, sharp peak below 340 K, most likely due to NO2 which was weakly held by the surface. FTIR spectra contained bands at 1625 and c a . 1595 cm "1 which correspond with the frequencies of the R and P branches of Vasym(NO2)and a weaker maximum at c a 1750 cm 1 which could correspond to V~ym (N204) [3]. However, their presence in spectra recorded at higher temperatures indicates that gaseous or physisorbed NO2 (N204) are not responsible for the first desorption peak. The 1750 r ~ maximum along with others at 1554, 1469, 1435, 1351 and 1255 cm l are present in the calcined samples (Figs. 3a, 4a, 6a) and due

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to a combination of bulk (Fig 1) and surface barium carbonates. A 2"d TPD maximum, between 325 and 425 K, corresponds with the 1't maximum reported by Fridell et al [2] for similar systems, while the peak at 500 K appears unique to Pt containing samples. Even considering the presence of coadsorbed oxygen, this is not thought to involve NO2 desorption from Pt [4]. 11% spectra recorded over the latter range show significant changes for Pt containing sample (Fig. 4 d,e) which are not apparent for Pt-free samples (Fig. 3 d,e). Decreases in intensity at 1623 and 1321 cm4 occur while intensity is gained at c a . 1595 and 1360 cm 1 and NO2 is released. As heating is continued, two clear maxima shifting to 1413 and 1362 cm 1 appear (Fig. 4f, g). Although the higher frequency components are also observed in for alumina (not-shown) and B a J A I 2 0 3 (Fig. 3) and probably due to differem forms of nitrate on the alumina surface [5], the 1413/1362 cml band pair were only developed in the presence of Pt. These bands are consistent with those observed for Pt/Ba/SiO2 [3] and corresponds with maxima assigned to Ba(NO3)2 [1,3] and would support conclusions that NOx storage as nitrate requires the presence of atomic oxygen which is only produced in the presence of a noble metal component [2]. Bulk nitrate was still detected by XRD after completion of the TPD experiment upto 673 K. The high temperature desorption peak between 550-673 K, present for all samples and the support, is consistent with the maximum at 623 K in TPD profiles for NO2 from A1203 [2] and due to decomposition of surface nitrates. The apparent inconsistency of increased intensity at 1585 and 1294 cm4 (Fig. 3 g) while surface nitrates are lost, is due to reduced intermolecular interactions between adjacent adsorbed species and the remaining adsorbed species gaining homogeneity. 14

-

12

-

9

"

1447

6.: 4

"~

2-

~

(<,>>-.._\

.

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1362 //"~

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k'l ~21 ,.,.(c)

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-

1242 (d .

.

.

.

.

1351 -2

,

r

,

goo

i

i

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i

i

Wavenumber/cm -~ Fig. 4. Spectra of Pt5BaO/AI203 after (a) calcination in air and (b) exposure to NO2 with subsequent heating in air to (c) 363, (d) 398, (e) 523, (f) 623 and (g) 673 K. 3.4 Decomposition under stoichiometric conditions TPD profiles obtained in propene/air/N2 show the same initial sharp desorption peak at c a 325 K but show significantly enhanced desorption between 340-400 K (Fig. 5) when compared with heat treatment in air alone (Fig. 2). The intensity of this peak shows some form of (inverse) correlation with baria dispersion and can therefore be attributed to desorption from the alumina support. The main desorption peak appears between 400-500 K, independent of the presence of Pt and/or baria. As the highest temperature peak that appeared while heating in air (Fig.2) was no longer detected, it can be deduced that this desorbed NO2 is derived from the same source, which is destabilised under stoichiometric conditions. The presence of this

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1335

maximum for alumina alone supports the assignment given under lean conditions to decomposition of surface nitrate species from the support. Infrared spectra (Fig. 6 e,f) show losses in intensity at 1625 and 1583 cm~ b e t w e e n the temperatures corresponding with the main desorption peak. Unlike the spectra for alumina alone or to a lesser extent BaJAl203 (not shown) where the lower region was dominated by a single sharp band at ca. 1300 cml, spectra for Pt containing BaJAI203 showed maxima in the 1450-1350 cm "1 region, indicating the presence of additional NOx species. Unlike the treatment in air alone, a clear doublet due to Ba(NO3)2 [ 1,3] at 1413/1362 cm ~ was not detected for Pt/Ba/Al203 in propene/air at 673 K, and XRD showed less intense characteristic diffraction features due to the phase. An NO desorption peak was observed at 500-600 K only in the case of Pt/Ba/Al203.

i.

1000

,,

-

!,

300

(e)

.

350

l

400

.

I

450

500

550

600

650

Temperature/K Fig. 5. TPD of N02 desorbed from (a) A1203, (b) 1 (c) 5 and (d) 10 wt% Ba0/Al203 and from (e) Pt-5% Ba0/AI203 in a stoichiometric propene/air/N2 flow. 24

-

X C

16

,

12

-

(d)~1438/(e)

1357

--~..

"

( d ) 1 5 .|8 3 / 1 5 6 5

1625

1250

a-

V'

. 800

~ ,

~ oo

/

'Y,J ,6oo

--

"(a),

,

Wavenumber/cm

, ,,oo

,

/ ,3oo

,

, ,

-~

Fig. 6. Spectra of Pt5%BaO/A1203 (a) calcined in air then (b) exposed to NO2 and heated in a stoichiometric propene/air/N2 mix at (c) 333, (d) 400, (e) 455, (f) 525 and (g) 623 K. Samples containing Pt also show a weak band at c a 2130 cm"1 which is somewhat beyond the frequency expected (2120 cm l) for CO adsorbed on oxidised Pt sites. However, coadsorption of NO2 and CO gives rise to such high carbonyl frequencies [6] due to the formers ability to generate high surface concentrations of adsorbed oxygen [4]. The presence of both

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1336

adsorbed CO, (from propene partial oxidation) and NO (from NO2 dissociation under these conditions [4] or direct release from the surface), would be expected to generate isocyanate (NCO) species which are formed from NO/CO mixtures over clean, reduced Pt/A1203 surfaces at temperatures of 473 K and above [7]. Recent results for NOx trap catalysts operating at constant temperature indicate that NCO is absent during lean conditions and abundant during rich operation with the maximum in isocyanate concentration correlating with the temperature where NOx storage is a maximum [2]. The failure to detect the distinctive band pair at 2272/2236 cm -~ due to isocyanate species [7] indicates that N adatoms are not generated and therefore no route to N2 formation is available under stoichiometric conditions during the 298-673 K temperature ramp used here. CO2 production in the 1R cell did not reach significant levels, and direct oxidation of propene by oxygen may have been responsible for this. NOx released from the catalysts under these conditions (but otherwise retained by the surface under lean conditions) is thus released to the atmosphere directly. The inability to dissociate NO (from NO2 or the traces of NO produced) during the temperature ramp can be attributed to the presence of adsorbed oxygen on the Pt surfaces. This is shown by the fact that although CO is generated, bands between 2085 and 2055 cm -~ characteristic of adsorption on Pt~ are never detected. The presence of propene, although in a stoichiometric balance with 02 present in the gas phase is clearly insufficient to maintain the Pt surface oxygen-free. Released NO/shifts the balance to produce a net-lean mixture, and it's ability to dissociate readily on Pt to generate high coverages of O adatoms and NO [4] (the latter of which may then be released to the gas phase) means that the Pt surface is never oxygen-free. Coadsorption with oxygen strengthens the NO bond making it less likely to undergo dissociation. NO2/CO mixtures did not generate NCO over prereduced Pt/A1203 catalyst [6]. 4. CONCLUSIONS NOx stored on BaJA1203 is stable during light-off if heating is performed under lean conditions although minor release of NO2 may accompany the transformation of surface to bulk nitrate. The latter process is catalysed by the presence of Pt. In contrast, when heated under stoichiometric conditions, NO2 is released in a large pulse between 400-500 K, when the noble metal has not yet started to dissociate NO which results in the release of desorbed NOx to the atmosphere. REFERENCES

1. N. Takahashi, H.Shinjoh, T. fijima, T. Suzuki, K. Yamazaki, K. Yokota, H. Sumhki, S. Matsumoto, T. Tanizawa, T. Tanaka, S. Tateishi and K. Kashara, Catal. Today, 27 (1996) 63. 2. E. Fridell, M. Skogludh, B. Westerberg, S. Johansson and G. Smedler, J. Catal., 183 (1999) 196. 3. J.M. Coronado and J.A. Anderson, J. Molec. Catal., 138 (1999) 83. 4. M.E. Bartram, R.G. Windham and B.E. Koel, Surf. Sci., 184 (1987) 57. 5. N.D. Parkyns, Proc. 5~ Inter Cong. Catal., p-255, J.W. Hightower (ed.) North Holland, Amsterdam, 1973. 6. J.A. Anderson and C.H. Rochester, J.C.S. Faraday Trans., 87 (199 l) 1485. 7. M.L. Unland, J.Phys. Chem., 77 (1973) 1952.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Nitrogen emission process in the decomposition of nitrogen oxides on Pd(ll0); an angular and velocity distribution study Ivan KOBAL,~~ KIMURA, b Yuichi OHNO, ~ Hideyuki HORINO, b lzabela RZI:ZNICKA, "~ and Tatsuo MATSUS IM ~ "Catalysis Research Center, Hokkaido University, Sapporo 060-0811 Japan bGraduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0811 Japan CJ. Stefan Institute, Ljubljana, Slovenia ~nstitute of Inorganic and Ezological Chemistry, Technical University o f ~ 90-924 Lodz, Poland The decomposition of N20 and NO was studied on Pd(110) through analysis of the angular and velocity distributions of desorbing products in both thermal desorption and a steady-state NO+CO reaction by using cross-correlation time-of-flight techniques. The product N2 showed a highly inclined desorption and hyper-thermal energy in both decompositions, indicating a common intermediate. It was proposed that this peculiar desorption was due to the decomposition of aligned N20(a). 1. INTRODUCTION NO decomposition on noble metals is one of the key processes of the actual three-way catalysts for automotive exhaust gases. This paper delivers evidence that N2 emission occurs v/a the process of N20(a)---)N:(g)+O(a). N20 is decomposed on metal surfaces below 200 K, releasing only oxygen on them. Although it is easily formed l~om N(a) and NO(a), its surface residence time must be extremely short at steady-state reaction conditions because of the low adsorption heat [1]. These short-lived and highly active characteristics have obscured the observation of its properties on catalysts. The spatial and velocity distributions of desorbing products are useful for examining such chemical species in the course of catalyzed reactions. We successfully applied cross-correlation time-of-flight (TOF) techniques combined with angle-resolved thermal desorption (TD) [2] or steady-state (SS) experiments to N20 and NO decompositions on Pd(110) at surface temperatures (Ts) above 120 K or to a NO+CO reaction above 350 K. 2. EXPERIMENTAL A UHV system with three chambers was used [2]. The reaction chamber was equipped with LEED-AES, an Ar+ gun, and a mass spectrometer for angle-integrated (AI) analysis. The chopper house had a slit on each end and a pseudo-random chopper [3]. The analyzer had another mass spectrometer for angle-resolved (AR) and velocity measurements. A Pd(110) crystal was rotated to change the desorption angle ( 0 ;polar angle) mostly in a plane in the [001] direction (Fig. 1). For TD experiments, it was heated at a constant rate after 1%Oexposure below 135 K. In SS experiments, it was kept in a constant reactant (~lO and CO) flow at fixed Ts values. Time resolutions for TOF

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1338

measurements were 15 U s for ~N2 and 30-45 u s for l~lO and ~SNzO. Hereafter, l~q is simply designated by N. 3. RESULTS

3.1 TD spectra N 2 spectra were sensitive to N20 exposure (Fig. 2). A single N 2 peak ( ~ 0-Nz)appeared around 140 K below 0.5 L (1 Langmuir =133 # Pa s). Above this level, it split into two peaks, /3 ~-N2at 152 K and /~ 2-N~at 134 K. AR-TD spectra were sensitive to the desorption angle. /3 o-N~was more intense at -43 ~ than at the normal direction. /3 ~- N2 showed similar behavior /3 2-N2 was completely suppressed when the surface was pre-c~vered by a 0.04 mono-layer of oxygen (Fig.2, inset). N20 desorption also showed a single peak below 0.5 L which split into two peaks, a ~-N20 (major) at 152 K and a 2-N20 at 140 K. These peaks overlapped considerably. AI-TD spectra involved desorption from ill-defined parts and crystal supports because AR-TD spectra were insensitive to 0 although significant differences were found between AI- and AR-TD. Thus, typical spectra in AR form were shown at 0 = 0~ []]o] &

:

~

[7"

,[001

]

-

i

I

7 ~ PdI ~ NzO/ (I I0

I

(0) N 2 / A I

3.9 A Fig. 1 (up) Pd(110) structure and proposed transient orientation of N~O. The size of physisorbed N2 is also shown.

//

(b) N2 / A R

I

I

(c) N20

,Bz-Nz ,~I-N2

]~2-N2

,

(:Z2_N20

I I

I

N

-43 ~

I

II

a~-N20

0.04 ML O(od) , I -i- 0.96 L N20 I~

I

Ijl~

Fig. 2 TD spectra in AI and AR forms exposures. The pre-adsorbed oxygen effect is shown in the inset. Typical deconvolutions based on Gaussian forms are indicated by dotted lines. The heating rate is shown.

i-'--

0~

of N2 (a,b) and N~O (c) at various Y~O

0.72

oN2

-43 ~

. ,~

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4s L

~00

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!

~so 200

//

I

~00

~

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I

~ Z

~50 200

Temperature / K

oo

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化

(a)

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.... ( 4 ) "3 ~' " ' 13o-N2

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Fig. 3 (left column) Angular distributions of N2 and N20 in a plane in the [001] direction. (a)/3 o-1'42at 0.20 L ofN20. (b)/3 rN2 at 0.96 L ofN20. (c)/3 2-N2 at 0.96 L of N20, and (d) c~rN20 at 0.60 L. Typical deconvolutions are shown by broken lines. The solid and dashed lines indicate the summation. Fig. 4 (right column) Velocity curves of N2 and N20 in N20 decomposition. (a)/3 0-N2at 0.20 L of N20 and 0 = 43 ~ , (b)/3 rN2 at 0.96 L and 0 = -43~ , (c)/3 2-N2 at 0.96 L and 0 = -43 ~ , and (d)c~ ]N20 at 0.60 L and 0 = 0 ~ . Broken and solid lines present components and their summation, respectively. The dashed curves indicate a Maxwell distribution at 'Is.

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(b)

化

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_(a) --t~

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.

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Fig. 5 Steady-state NO+CO reaction at Puo = 26 tz Pa and Pco = 13 tt Pa. (a) Formation rates of N20 and CO2 at 0 = 0~ and of N2 at 0 = 0~ and 41 ~ . (b) Angular distribution of N2 at Ts/K= 500(A), 510(I-1), 520(~), 540(C)), 550(V), 570(A), 600(Y), and 620(O). It is described by cos28( 041)+cos28( 0 +41), and (c) Angular distribution of CO2, as followed in a form of 0.50 cos 0 +cos ~50 and of N20 as cos 0. Ts = 540 K. (d) Velocity curves of N2 at 0 = 41 ~ and Ts = 540 K. The observed signals (/k) and the fast components after subtraction of a Maxwell distribution at Ts (Q)). 3.2 A n g u l a r distribution

/3 0-N2 showed a sharp two-directional desorption collimated at 0 - - 4 3 ~ , accompanied with a broad form. Its signal was described as 1.2cos 0 +cosS~ 0 +43)+cosS~ 0 -43) (the solid and dotted lines in Fig. 3a). B 1-N2 showed only two-directional components as a form of cosS~ 0 +43)+cos5~ 0 -43) in a plane along the [001] direction_ No B :N2 signal was found in the plane perpendicular to it. On the other hand, /3 2-N2 was mostly composed of a cosine form and the inclined one was barely noticeable. The latter was contributed fi'om/3 :N2 by overlapping with TD peaks because in the plane along the trough (in the [110] direction), only the cosine component remained and no inclined component was found. The curve in Fig. 3c was represented by cos 0 +0.40 cosS~ 0 -43)+0.40cos5~ 0 +43). N20 desorption in cr :N20 was isotropic and obeyed a cosine form.

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3.3 Velocity /3 0- and /3 ~-N2 showed hyper-thermal energy. Figure 4 displays the velocity curves of N2 at 0 = +43 ~ . The curves were bi-modal at the chopper gate time of 15 # s and then were deconvoluted into two components, each represented by a modified Maxwell form. The translational temperature defined as the mean kinetic energy (<E>) divided by 2k (k is the Boltzmann constant), i.e., T,~=<E>/2k, is inserted. Each component in /3 0-N: showed a translational temperature of 1300 K and 2700 I~ Experiments of /3 0-N2with the gate time of 40 tz s showed another slow component with 140 K at 0 = 0~ . It is known that a slow component is suppressed in the deconvolution procedure when the gate time of a random chopper is far l~om a suitable value [3]. /3 ~-N: involved two components with 580 K and 1930 K, whereas /3 :-N: showed far fewer signals due to the fast components (Fig. 4c). The slow component of 134 K in /3 2-N: appeared at 0 = 0~ at the gate time of 40 lz s. Desorbing N:O showed a Maxwell distribution at Ts as shown in Fig. 4~ 3.4 NO decomposition N2 desorption peaked around 490 K in TD work of NO. Desorbing N2 collimated at + 41 ~ in the plane along the [001] direction [4]. Its velocity curve also showed two components with 2700 K and 1300 K, similarly to/~ o-N2 [ 1].

3.5 Steady-state reaction In NO+CO reaction, the formation of N:O, CO2 and N2 suddenly increased at around 490 K, reached its maximum at 520 K and then decreased at higher Ts (Fig. 5). The signals due to N20 and C O 2 peaked in the normal direction, whereas the N 2 signal was maximized around 0 = + 41 o in the plane along the [001 ] direction and disappeared at 0 = 0~ . No N2 signal was found in the plane perpendicular to it. The N2 velocity curve again showed two components, a fast component of 2660 K and a slow one of 1300 K. An additional component with 540 K was due to the background in the reaction chamber. CO2 desorption was composed of a sharp component collimated at 0 = 0~ and a weak cosine component, whereas N20 desorption still followed a cosine form. 4. DISCUSSION 4.1 Inclined desorption Highly inclined components and a hyper-thermal energy are characteristic of N2 desorptiort Inclined N2 desorption was once reported on Si(100), where a simple photo-dissociation of N20(a) oriented off the surface-normal direction was proposed [5]. On metal surfaces, however, N20(a) was proposed to be in a tilted form v/a its temfinal N atom [6]. It must move its O atom toward the surface to a lying position or a flu/her inclined one v/a an oxygen-metal bond immediately prior to decomposition (Fig. 1). From this state, inclined desorption becomes possible when nascent N2 receives Pauli repulsion due to its formation in dose proximity to the surface and additional forces operative in parallel to the surface as found in surface-aligned reactions [7]. There must be a mechanism that N20(a) dissociates when it is oriented along the [001] direction. The hyper-thermal energy must largely be due to the heat ofreaction for N20(a)~N2(g)+O(a), AH=-190 kJ mol"~[1].

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4.2 Actual desorption Dissociation of N:O(a) at low coverages can yield /3 o-N2 with a high energy because a highly exothermic process is possible on oxygen-free surfaces. In fact, this exothermic reaction is mostly due to the formation energy of a Pd-O bond [1]. /3 l-N2 is formed after high N20 exposures and the N 2 energy may be reduced because the exothermic heat is suppressed by pre-adsorbed oxygen_ The desorption temperature of cz i=N20 and /31-N2was shifted to slightly higher values because of site modifications in the prec~ing N20 dissociation. The two N2 components with different energies might be assigned to vibrationaUy excited molecules at different levels, since the difference is close to the first vibrational level of N2. /3 2-N2 is desorbed from N2(a) formed in the dissociation as N20(a)~N2(a)+O(a). This desorption is not repulsive, yielding a cosine form. It is observable below 135 K because of the low adsorption heat of N2(a) [8]. The dissociation is easily suppressed by a small amount of oxygen. This explains the significant cosine component in B 0-N2and the lack of contribution to/3 l-N2.

4.3 NO+CO reaction In the steady reaction, CO 2 desorption sharply collimated along the 0 = 0 ~ direction. It is identical to the CO2 distribution in CO oxidation [2], where the surface was not reconstructed. This is reasonable since Pd(110) is reconstructed at high oxygen coverages. The coverages of CO(a), NO(a), N20(a) and N(a) are low above 500 K because of low adsorption heats or high reactivity. Thus, N2 may fairly be desorbed v/a the /3 0-N2 channel. In fact, both angular and velocity distributions at SS are similar to those for B 0-N2. A noticeable difference (about 2~ ) was found in the collimation angle between NO and N20 decompositions, possibly due to a thermal effect. N2 desorption in NO decomposition occurs above 500 K, a much higher temperature than the 152 K required for N20. The collimation angle in the former may be shifted towards 0 = 0~ by enhanced thermal motions. ACKNOWLEDGMENTS Ivan Kobal acknowledges the support fi'om the Ministry of Education, Science Sports and Culture of Japan through the foreign-researcher (COE) invitation program. Izabela Rzeznicka is indebted to the Japan Intemational Cooperation Agency. The authors thank Ms Atsuko I~atsuka for the figures. REFERENCES

1. Y. Ohno, I( Kimura, M. Bi and T Matsushima, J. Chem. Phys. 110(1999) 8221. 2. T Matsushima, Heterogen.Chem. Rev. 2 (1995) 51. 3. G. Comsa, R. David and B.J. Schumacher, Rev. Sci. Instmm. 52 (1981) 789. 4. M. Ikai and K. Tanaka, Surf. Sci. 357-358 (1996) 781. 5. J. Lee, H. Kato, K. Sawabe and Y. Matsumoto, Chem. Phys. Lett. 240(1995) 417. 6. P. Vaterlein, TKrause, M.Bassler, P,.Fink, E. Umbach, J.Taborski, V. Wustenhagen and W. Wurth, Phys. Rev. Lett. 76(1996) 4749. 7. M. Sano, Y. Ohno, T Yamanaka, T Matsushima, E. B. Quinay and K. Jacobi, J. Chem. Phys. 108 (1998) 10231. 8. Y. Kuwahara, M. Jo, H. Tsuda, M. Onchi and M. Nishijima, Surf.. Sci. 180 (1987) 421.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Reduction of NO by propylene over modified Rh/TiO2 catalysts in the presence of oxygen T.I. Halkides, D.I. Kondarides and X.E. Verykios Department of Chemical Engineering, University of Patras, GR 26 500, Greece

Selective catalytic reduction (SCR) of NO by propylene under lean conditions has been examined over modified Rh/TiO2 catalysts in the temperature range~ of 150-550~ Modification of the electronic structure of the supported Rh catalysts has been achieved by doping the TiO2 semiconductive carrier with small amounts of W 6§ cations. Parameters examined include dopant type and concentration, calcination temperature and preparation method. Structural parameters of supported Rh catalysts were investigated by altering the dispersion and average crystallite size of the Rh particles. It has been found that the catalytic performance of Rh is significantly improved when supported on W6§ TiO2 carriers. Over the optimized catalyst, conversion of NO to reduction products reaches 65% with selectivity toward N2 of ca 80%, at 310~ 1. INTRODUCTION Selective catalytic reduction of NO by hydrocarbons has recently received considerable attention due to its potential use in lean-burn and diesel-fueled vehicles. Standard three way catalysts (TWC) exhibit poor performance when operating under lean burn conditions [1], a behavior which is associated with competitive chemisorption of NO and 02 on the catalytic surface and the concomitant oxidation of the reduced active sites. Platinum group metals and zeolites are under investigation, as potential catalysts, due to their high activity toward NO reduction at low temperatures. The former group of catalysts also exhibits good sulfur resistance and tolerance toward steam while the latter group exhibits a wide temperature window of NO reduction activity. However, the low selectivity of Pt-containing catalysts toward N2 and the rapid deactivation of zeolites caused by the presence of inhibitors (SO2 and H20) in the feed are major problems for the present application. On the other hand, Rh containing catalysts exhibit superior behavior as far as selectivity toward N2 is concerned and they have relatively good tolerance toward sulfur and steam while they exhibit relatively low light-off temperatures[2]. Work has been carried out in this laboratory to study the effects of catalyst support modification on catalyst performance. It has been shown that the activity of metal crystallites can be significantly enhanced when dispersed on doped semiconductive carriers [3-5]. In the present study, these ideas have been applied toward the development of an efficient catalyst for the SCR of NO by propylene in the presence of excess oxygen.

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2. EXPERIMENTAL

2.1 Catalyst preparation and characterization Titania carriers doped with small amounts of tungsten (0-0.68 at. %) were prepared by the method of solid state diffusion, using TiO2 P-25 (Degussa) and (NH4)I2WI2041.SH20 (Alfa) as starting materials. Diffusion of the dopant cation into the crystal matrix of TiO2 was achieved by calcination of the samples in the temperature range of 850-1000~ following a procedure which has been described elsewhere [3-5]. Preliminary experiments showed that the optimum calcination temperature lies between 900-950~ and, thus, doped materials employed in the present study were calcined at 900~ Carriers were characterized in terms of their specific surface area employing nitrogen physisorption, and in terms of the crystalline mode of TiO2 employing the XRD technique. The acidity of the carriers was examined by NH3 adsorption at 25~ followed by TPD experiments. Dispersed Rh catalysts, with metal loading varying between 0.5 and 4.0 wt.%, supported on undoped and W+6-doped TiO2 carriers, were prepared by the wet impi'egnation method using Rh(NO3)3 as the metal precursor. After preparation, all samples were reduced in flowing H2 at 300~ for 2 hours. Rh dispersion and mean particle size were determined by chemisorption of H2 at room temperature and extrapolation of the linear part of the chemisorption isotherm to zero pressure. 2.2 Catalytic performance experiments Catalytic experiments were performed in a quartz, fixed-bed reactor (i.d. 6 ram), in the temperature range of 150-550~ with a feed composition of 1000 ppm NO, 3500 ppm C3H6 and 5% 02 in He. The catalyst weight was 60 mg and the feed flow was 200 cc/min (W/F= 0.018 g.s/cc). On-line analysis of reactants and products was accomplished with the use of a gas chromatograph and a NO/NO~ analyzer. Separation of N2, 02 and CO was achieved using a molecular sieve column while CO2, N20 and C3H6 were separated using a porapak-Q column. The concentrations of NO and NO2 at the effluent of the reactor were continuously monitored with the use of the NOx analyzer. High purity reaction gases (1% NO/He, I%C3H6/He and 20% O2/He) were obtained from Messer-Griesheim. Prior to each experiment, the sample was in-situ conditioned under He flow for 1 h at 550~ 3. RESULTS & DISCUSSION

3.1 Catalyst characterisation. Calcination of the undoped carrier results in significant loss of surface area which drops from 50 m2/g for P-25 to 13 m2/g after treatment at 700~ and to less than 6 m2/g after treatment at 900~ However, doping TiO2 with W 6+ cations preserves this loss in a manner which depends on dopant concentration (Table 1). Apart from the effect on surface area, thermal treatment during carrier preparation results in complete transformation of titania to its rutile form. Since the undoped TiO2 carrier calcined at 700~ has comparable characteristics with those of the doped materials, it was used as the reference support. Results concerning Rh dispersion and mean particle size of Rh/TiO2(W 6+) catalysts are also presented in Table 1. It is observed that dispersion of Rh increases with increasing dopant concentration, probably due to the increased surface areas of the doped carriers. Increasing Rh content from 0.5 to 4.0 wt.% results in a significant decrease of dispersion, from 68% to 19%. Similarly, calcination of a series of 0.5% Rh/TiO2(0.45% W 6+) catalysts in air, in the

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temperature range of 600 to 800 ~ sintering of the Rh particles.

results in significant decrease of metal dispersion due to

Table 1 Specific surface of doped TiO2 carriers, dispersion and mean particle size of dispersed Rh/TiO2(W 6*) catalysts. Dopant concentrati on (at.% W 6+)

Rh Ioadi ng (wt. %)

S BET (m2/g)

Di spersi on (%)

Particle size (nm)

0 * 0.23 0.34 0.45 0.56 0.68

0.5 0.5 0.5 0.5 0.5 0.5

13 10 12 14 15 16

71 57 62 68 70 80

1.5 1.9 1.8 1.6 1.6 1.3

0.45 0.45 0.45 0.45

0.75 1.0 1.5 4.0

59 51 20 19

s 1.9 2.2 5,0 5.8

0.45 0.45 0.45 * Carrier calcined

**' l 0.5 58 1.8 **.2 0.5 37 2.9 **' 3 0.5 36 3.0 at 700"C; ** Catalysts calcined after preparation at 600 (~), 700 (2) and 800 (:~)"C for 2 hours.

The acid properties of the carriers were determined employing NH3 adsorption at room temperature followed by temperature programmed desorption. Typical TPD spectra of NH3 obtained from TiO2 and TiO2(0.45%W 6+) are shown in Figure 1. It is observed that NH3

.

.

.

.

.

.

TiO 2

.

2000

9

.

~

+)

1500 E

6

o~~

~E 5 -r~ Z

1000

~ o ~

~

500 O

-

|

100

-

|

200

-

|

300

g

400

Temperature ( ~

-

i

500

-

i

600

)

Figure 1. TPD spectra obtained following NH3 adsorption on TiO2 and TiO2(0.45%W 6§ at 25~ fl=30~

olo

o12

o~4

o~s

Dopant Concentration (at. %)

0.8

Figure 2. Amount of NH3 desorded during TPD experiments as a function of dopant concentration.

desorbs from undoped TiO2 exhibiting three maxima at ca. 120, 230 and 350~ The peak at low temperature is due to weakly adsorbed species while the second and third peaks are related to strongly adsorbed ammonia onto the acid sites of the material [6]. Doping TiO2 with W 6§ cations results in higher desorbed amounts of NH3 and domination of the low temperature peak, indicating that doping results in an increase of the acidic nature of the

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support, particularly of the sites which have weak acidity. Results are summarized in Figure 2, where the amount of NH3 desorbed per square meter of the carrier are plotted as a function of dopant concentration. It can be observed that the amount of NH3 goes through a weak maximum at a dopant content of 0.34 at % W 6§

3.2 Catalytic performance of Rh~iO2 (W 6+) catalysts. 3.2.1 Effect of dopant concentration Typical catalytic activity results obtained over the doped 0.5 % Rh~iO2 (0.45 at % W 6+) are presented in Figure 3, where the yields of N2, N20, NO2, as well as the total conversion of NO and C3H6 are shown as a function of reaction temperature. The conversion of C3H6 is observed to increase slowly with increasing reaction temperature, up to a critical temperature (= 300 ~ above which C3H6 conversion is complete. This behavior is accompanied b)~ a parallel increase of the yield of N2 and N20 which pass through a maximum at ca. 320 "C. Above this temperature, reduction of NO is suppressed, probably due to unfavorable competition with oxygen for the same surface sites. This is paralleled by an increase in the yield of NO2 which goes through a maximum at 420 ~ and decreases again, following thermodynamic equilibrium constrains. 60 100, ............... c .o

~ to 0

i ..................

80.

60

................i...

i7--- " : L ~ i]~'

100 ......

$_

80

i --v--iY~

0.5% wt Rh/TiO 2 (x% at W ~)

-<

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i

/i

60 ~ z

/

40 z

40

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~ 2o.

.

0 z

0. i

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400

500

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Figure 3. Catalytic performance of 0.5 wt % Rh supported on doped TiO2 with 0.45 % at W 6§

o,,..

40 9. . . . . . . . . . . . 9

,20 ~ .0

X

30

0'.0

0'.2 0'.4"0'.6 0.8 Dopantconcentration(at. %)

Figure 4. Effect of dopant concentration on the maximum NO conversion to reduction products over 0.5%RhffiO2(W 6§ catalysts

Figure 4 shows the maximum NO conversion (XNo) tO reduction products (N2 + N20) obtained over the 0.5%RhffiO2(W 6§ catalysts as a function of dopant concentration. It is observed that XNO increases from 38% over the undoped catalyst to 52% over the catalyst doped with 0.45% at. W 6§ at which level conversion goes through a sharp maximum. The variation of reduction activity with dopant content is qualitatively similar to that observed for the effect of dopant concentration on the capacity of the carrier toward NH3 adsorption (Fig. 2), indicating that a correlation may exist between the acidity of the carrier and the catalytic performance of supported Rh. A first explanation can be derived from the fact that defect-site reactions occur when TiO2 is doped with W 6§ cations, resulting in the appearance of vacancies onto the doped TiO2 surface, which may act as acid sites responsible for the larger amounts of

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adsorbed NHa or as oxygen vacancies during the reactions of NO reduction. Higher amounts of dopant cations may accumulate on the surface resulting in reduction of surface acidity and NO reduction activity. It was also found that selectivity toward N2 is approximately the same for all catalysts (75-80%). In addition to alterations of NO reduction activity, doping of the carrier also results in a significant suppression of NO2 formation, which occurs above 350~ Apart from the alterations of surface acidity of the support, the observed beneficial effect in the catalytic performance of Rh induced by doping of TiO2 with W 6* cations, may be also attributed to electronic modifications of the supported Rh crystallites, which result from an electronic interaction which develops at the metal-semiconductor boundary layer [3-5]. It has been shown [7] that these electronic modifications promote the formation of RhNO species, which are responsible for NO dissociation-the key reaction step for NO reduction over Rh supported catalysts- and lowers the N-O bond strength resulting in higher propensity of these species to dissociate and thus to higher dissociation and NO reduction rates. 3.2.2 Effect of Rh crystallite size and Rh loading. The effect of crystallite size has been examined over a series of 0.5% Rh/TiO2(0.45% W 6+) catalysts of variable dispersion. This has been achieved by calcining the sample in air at temperatures in the range of 600 to 800~ (Table 1). As shown in Fig. 5 maximum NO conversion decreases with mean Rh crystallite size while turnover frequency of NO reduction is only slightly affected by crystallite size. These results indicate thet reduction of NO is structure-insensitive, however the competition between NO reduction and NO oxidation or between reduction of the catalytic surface by C3H6 and oxidation by O2 may be affected by the size of the metal crystallites. 60

r

55

._o r 0

0 0 z

O .-! O

i

50

"I

"13

t~.k

45

"'"'"O.Q

40

115

210 215 310 Crystalline size, nm

Figure 5. Effect of Rh particle size on the catalytic performance, and on the specific activity, TOF of 0.5 wt % RhffiO2 (0.45% W 6+) catalysts. Similar phenomena were investigated by variation of the metal loading of the catalysts. Results obtained over Rh/ TIO2(0.45% W6+) catalysts with variable metal loading are presented in Figure 6-A. It is observed that increasing Rh loading from 0.5 to 1.5 wt.% results in an almost linear increase of NO conversion from 52% to almost 65%. Further increase of metal content does not result in significant change of catalytic activity. The plateau observed can be explained from the fact that the support material has a relatively low specific surface area, which results in decreased Rh dispersions (Table 1). This results in reduced maximum

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NO conversion, as illustrated in Fig. 5. The light off temperature, defined as the temperature at which conversion of propylene is 10%, is also affected by increasing metal loading and decreases from about 290 to 260~ under the experimental conditions employed (Figure 6-B). Similar behavior was observed over Pt-containing catalysts [8], where the activity toward NO reduction was increased with increasing Pt loading from 0.5 up to 10 % wt, while the TOF was rather insensitive to crystallite size. Thus we can assume that NO reduction is strongly affected by the population of active sites, while the geometric properties of the active sites seem not to significantly affect specific activity, under the experimental conditions employed. A combination of the results obtained from variation of the crystallite size either by sintering of Rh crystallites or by variable metal loading supports the above conclusion.

70 x% Rh/TiO2 (0.45% W6§

...65-

290

.s,O ...........

~

g 280

0

on,.60"

(a)

o

/"

e.o

300

(A)

2

!

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270

0z 55

I---

i

50

o

i

~

~

~

Metal loading (% wt Rh)

s

260 aS~

~

~

~

,i

Metal Loading (% wt Rh)

Figure 6. Effect of Rh loading on the catalytic performance (A), and the light off temperature (B) of RhffiO2 (0.45% W 6§ catalysts. 4. CONCLUSIONS Results of the present study clearly show that the SCR of NO by C3H6 in the presence of excess oxygen is significantly influenced by modifications of the electronic properties and structural characteristics of TiO2-supported Rh catalysts. In particular, doping TiO2 with small amounts of W 6+ cations results in increased NO conversion to reduction products, which over the optimized 1.5% RhffiO2(0.45% W 6§ catalyst reaches 65% at 310 ~ with selectivity toward N2 of ca 80%. No significant effect of mean Rh crystallite size was observed on specific activity, indicating that NO reduction is rather insensitive to the geometric properties of the Rh crystallites.

REFERENCES 1. 2. 3. 4. 5. 6. 7 8.

J.N. Armor, Catal. Today, 26 (1995) 99. M.D. Amiridis, T. Zhang, R. J. Farrauto Appl. Catal. B 10 (1996) 203. Z. Zhang, A. Kladi and X.E Verykios, J. Phys. Chem. 98 (1994) 6804. T. Ioannides and X.E. Verykios, J. Catal., 145 (1994) 479. T. Ioannides and X. E. Verykios, J. Catal. 161 (1996) 560. J. Fung and I. Wang, Applied Catal. A 166 (1998) 327 T. Chafik, A. M. Efstathiou and X. E. Verykios, J.Phys. Chem. 101 (1997) 7968 R.Burch and D. Ottery, Applied Catal. B 13 (1997) 105

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Effect of Sulfur on the Oxygen Storage/release Capacity of Rh/CeO2 and Rh/CeO2-ZrO2 Model TWCs Marta Boaro, Carla de Leitenburg, Giuliano Dolcetti and Alessandro Trovarelli Dipartimento di Scienze e Tecnologie Chimiche, Universita di Udine, via del Cotonificio 108, 33100 Udine, Italy In the present work we have examined the effect of sulfur in CO oxidation carried out under redox conditions over Rh supported on ceria and ceria-zirconia. Presence of SO2 (30-70 ppm) is shown to suppress the oxygen storage activity at low temperature of both ceria and ceria-zirconia based catalysts due to formation of bulklike sulfate species. However mixed oxides better recover catalytic activity after regeneration. It appeared that the presence of ZrO2 facilitates removal of sulfate species under reductive atmospheres, while no differences are found following regeneration under oxidizing conditions. This carries important implications for use of these materials in TWC formulations.

1. INTRODUCTION Ceria has an important role in the removal of noxious compounds from gaseous stream originating either from stationary or mobile sources and it is present as a component of fluid catalytic cracking catalysts (FCC)(1) and three-way catalysts (TWC)(2). In the latter, the main role of ceria is to provide oxygen buffering capacity during rich/lean oscillation of exhaust gases; however, ceria contributes to a number of other catalytic functions. In particular, it seems that ceria is also involved in the formation and storage of sulfates due to sulfur impurities present in the fuels (3). These impurities are oxidatively adsorbed on ceria with its simultaneous reduction to CeO2.x and subsequently they decompose to SO2 or H2S under reducing conditions (4-7). At present, the textural stability of ceria is not high enough to meet the requirements of several gas-phase catalytic applications. Therefore, much effort has been directed in recent years to find catalyst formulations which can enhance the thermal stability of ceria without diminishing its specials features, such as the redox properties. These characteristics can be obtained by doping ceria with transition and rare-earth metal oxide like Zr, La and Pr (8). In particular, ceria-zirconia solid

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solutions have been used in the formulations of car exhaust catalysts since the early 1990s (9). They show a higher thermal resistance compared with conventional CeO2containg TWC, a higher reduction efficiency of the redox couple Ce4§ 3§ and a good oxygen storage/release capacity (10-13). However, a few fundamental studies exist on the sulfation of these materials (7,14), although this is a key issue in the evaluation of catalyst formulations. In these studies it was shown that formation of both surface and bulklike sulfate species occurs either in the presence or in the absence of the metal, and these species of different thermal stability can be partially removed after vacuum treatment at high temperature. With the present study we want to gain further information on sulfur adsorption in these materials and especially on the impact of sulphur on the oxygen storage capacity.

2. EXPERIMENTAL

CeO2 and CeO2-ZrO2 solid solutions were provided by Rhodia and the characterization of these samples was recently reported in the literature (15). Catalysts containing Rh were prepared by incipient wetness impregnation of the oxides previously calcined at 1173K with an aqueous solution of RhC13-3H20, followed by drying (373K for 15 h) and calcination at 773 K for 2 h. The specific surface area was measured by the BET method (see Table 1) and the effective Rh loading was checked by atomic adsorption spectrometry. Prior to catalytic testing the Rh-supported catalysts were treated in situ under H2 for one hour at 473 K. Table1 : characteristics of samples used in this study. Sample composition SA (m2/g) Rh/CZ100 Rh/CeO2 38 Rh / CZ80 Rh/Ce0.8Zr0.202 49 Rh / CZ50 Rh / Ce0.sZr0.sO2 53 Rh / CZ 15 Rh / Ce0.152Zr0.84802 55 Rh/CZ0 Rh/ZrO2 8

Rh(wt%) 0.50 0.59 0.52 0.41 0.45

The oxygen storage measurements and the deactivation tests were carried out in a microreactor system operating under steady-state and oscillating conditions, and will be described elsewhere (16). Under cycling operating mode the catalyst (0.1g) was treated with an oscillating feedstream (frequency 0.5 Hz, total flow 0.12 N1/min) containing alternately CO (4% in He) and 02 (2% in He). Conversion to CO2 was taken as an estimate of oxygen storage activity. Catalyst deactivation was performed at 473K by adding SO2 to the feedstream (30-70 ppm) and measuring the degree of CO conversion vs. time with a gas chromatograph equipped with a TCD detector.

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The characterization of the spent catalysts were carried out by combined TPR/TPO/TPD. They were performed by heating the spent catalysts (from 298K up to 1373K, 10K/rain) in a stream of H2 (5% in Ar), 02 (1% in He), and He respectively, with a total flow of 35ml/min. The reaction products, such as SO2 and H2S, released during the course of temperature programmed measurements were determined by means of mass spectrometry (Omnistar GDS-300 Balzers). FT-IR measurements were carried out with a FTS-40 instrument (Biorad).

3. RESULTS AND DISCUSSION In the presence of Rh, both ceria and ceria-zirconia solid solutions underwent a rapid and strong deactivation (Fig. la) and the conversion drops to less than 10% of the initial value in one hour of reaction. The deactivation rate is different; for ZrO2rich oxides the activity drops slowly with time, while a faster deactivation is observed with Rh/CZ100 and Rh/CZ80. However, after 1 h of reaction similar activities are found for all samples. The content of sulfur in the spent catalysts is near the theoretical value as expected if all the SO2 were adsorbed. This corresponds to ca. 0.01 gS/gcat and this value does not depend on the content of zirconium in solid solutions, in agreement with results reported by Bazin et al. (13) who did not found difference in the sulfur adsorption capacity of ceria and ceria-zirconia in the presence of oxygen. It seems therefore that the presence of Zr does not interfere with the adsorption properties. lOOI o

N

~

80

=

~

60-

-~

,.

~ 40 ~.

0~9

~9 20! 0

10 20 30 40 50 60 70 80 90 100 0 1'0 2'0 3'0 4'0 5'0 6'0 7'0 8'0 9'0 100 Time (min.) Time (min.) Fig. 1. Oxygen storage activity vs time in the presence of 70 p p m of 902 before (a) and after regeneration under reducing conditions (b): Rh/CZ100 ( , ) , Rh/CZ80 (O),

Rh/CZ50 (A), Rh/CZ15 (U), Rh/CZ(J,).

The nature of the species formed were investigated by FT-IR spectra collected on samples after deactivation. It seems that bulklike sulfate species are the majority either in Rh/CZ100 and Rh/CZ50, with a broad band at ca. 1120 cm 1. Only traces of sulfites and surface sulfate species were observed. It is likely that sulfites, if initially

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present, are transformed to sulfates under our reaction conditions (lh under CO/O2 at 473K), as observed for sulfites in Pt containing catalysts (7). The presence of mobile oxygen species in these materials, which can be easily released, may explain the formation of sulfates also in a slight reducing environment. The absence of surface sulfate species is more puzzling; they are generally observed after sulfation of high-surface area ceria samples both in the presence and in the absence of the metal (6), while they are not formed on low surface area samples. Our surface area is not high if compared to that reported in ref. 7 (38 m2/g vs 13); moreover, the formation of bulklike sulfates from surface sulfates is promoted by increasing the temperature (7,17). Both conditions, which apply to ours, can explain the absence of surface sulfates. The thermal stability of adsorbed sulfur species was studied on deactivated samples by heating under different atmospheres and monitoring the evolution of gaseous products. It is reported that sulfur species on zirconia and ceria-based catalysts are stable up to ca. 673K under vacuum (13). Our temperature programmed experiments reveal that the nature of the support does not influence the stability of the sulfur species adsorbed. In Fig. 2 are reported the temperature programmed traces of SO2 and H2S formed under oxidizing and reductive conditions respectively over Rh/CZ100 and Rh/CZS0.

ei o

."

250

450

,,.~176

650 850 10'50 Temperature (K)

~,. " ~.~

12'50 1450

Fig. 2. Temperature programmed profiles for Rh/CZ50 (a) and Rh/CZ100 (b)" SO2 signal from TPD (---), S O 2 signal from TPO (......), H2S signal from TPR (-). Desorption starts at around 700K regardless of the support used and the traces are qualitatively similar. However the differences are related to the amount of gases desorbed at different temperatures; particularly it can be evidenced that the integrated amount of H2S desorbed from Rh/CZ50 is higher at low temperature than the corresponding amount formed on Rh/CZ100 (Fig. 3). This indicates that under reductive conditions formation of H2S is facilitated on solid solutions and under

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these conditions removal of sulfates is enhanced at lower temperature. Therefore the presence of ZrO2 promotes the reduction of sulfates, in agreement with its known role in the promotion of Ce 4§reduction (9).

o

.o

a r~

/

680

RtdCZ100 880

730 780 830 Temperature(K)

Fig. 3. Formation of H2S during TPR experiments. Removal of sulfates results in recovery of the catalytic activity, and the activity level after regeneration is closely dependent on the conditions used. In accordance with TP data, regeneration under reductive or slightly reductive conditions (either H2 or CO/O2 environment) restores ca. 80% of oxygen storage activity on Rh/CZ50 and less than 50% (70% with CO/O2) on Rh/CZ100 (Fig. 4). 100 o =

Rh/CZ100

80 60

r~

40 oo 9 20

o

2.._1_[Z21

c d e f Fig. 4. Oxygen storage activity of fresh (a) and deactivated catalyst (b). Activity after treatment of the deactivated catalyst at 773 K for lh under H2 (c), CO/O2 (d) He (e) and air (f). a

b

c

d

e

f

a

b

Regeneration under inert atmosphere (He) or under oxidative condition does not lead to substantial reactivation with both catalysts. Similar features are observed with the other ceria-zirconia compositions (Fig. lb). After regeneration under hydrogen the recovery of Rh/CZ catalysts is higher and the deactivation profiles are qualitatively similar to those observed on the fresh catalysts.

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In summary, the formation of similar sulfate species on ceria and ceria-zirconia is observed, and the presence of ZrO2 does not strongly influence their composition and thermal stability. Instead, their regeneration is strongly influenced by the presence of ZrO2. The formation of intermediate cerium oxysulfides may be suggested (18). The higher recovery of solid solution should involve migration of oxygen and sulfur ions from bulk to surface, thus exploiting the intrinsic higher anion mobility of these catalysts. ACKNOWLEDGMENTS: The authors wish to thank Rhodia for providing samples

for this study. We are also grateful to dr. Sandro Recchia (Department of Chemistry, Universita degli Studi dell'Insubria) for determining Rh loading. REFERENCES

1. A. K. Bhattacharyya, G. M. Woltermann, J. S.Yoo, J. A. Karch and W. E. Cormier, Ind. Eng. Chem. Res., 27 (1988) 1356. 2. A. Trovarelli, Catal. Rev. Sci. Eng., 38 (1996) 439. 3. B. Harrison, A. F. Diwell and C. Hallett, Platinum Metal Review, 32 (1988) 73. 4. A. F. Diwell, S. E.Golunski, J. R. Taylor and T. J. Truex, Stud. Surf. Sci. Catal., 71 (1991) 417. 5. S. Lundgren, G. Spiess, O. Hjortsberg, E. Jobson, I. Gottberg and G. Smedler, Stud. Surf. Sci. Catal., 96 (1995) 763. 6. M. Waqif, P. Bazin, J. C. Lavalley and G. Blanchard, O. Touret, Appl. Cat: B: Env. 11 (1997) 193. 7. P. Bazin, O. Sour, J. C. Lavalley, G. Blanchard, V. Visciglio, and O. Touret, Appl. Catal. B: Env. 13 (1997) 265. 8. A. Trovarelli, C. de Leitenburg and G. Dolcetti, CHEMTECH, 27 (1997) 32. 9. J. Kaspar, P. Fornasiero and M. Graziani, Catal. Today, 50 (1999) 285. 10. P. Fornasiero, R. Di Monte, G. Ranga Rao, J. Kaspar, S. Meriani, A. Trovarelli and M. Graziani, J. Catal., 151 (1995) 168. 11. C. E. Hori, H. Permana, Ng, K. Y. Simon, A. Brenner, K. More, K. M. Rahmoeller and D. Belton,, Appl. Catal. B: Env. 16 (1998) 105. 12. A. Trovarelli, F. Zamar, J. Llorca, C. de Leitenburg, G. Dolcetti and J. Kiss, J. Catal. 169 (1997) 490. 13. H.-W. Jen, G.W. Graham, W. Chun, R.W. McCabe, J.-P. Cuif, S.E. Deutsch, and O. Touret, Catal. Today 50 (1999) 309. 14. P. Bazin, O. Saur, J. C. Lavalley, A. M. Le Govic and G. Blanchard, Stud. Surf. Sci. Catal., 116 (1998) 571. 15. G. Colon, M. Pijolat, F. Valdivieso, H. Vidal, J. Kaspar, E. Finocchio, M. Daturi, C. Binet, J. C. Lavalley, R.T. Baker, and S. Bernal, J. Chem. Soc. Faraday Trans., 94 (1998) 3717. 16. M. Boaro, G. Dolcetti and A. Trovarelli, manuscript in preparation. 17. J. Twu, C. J. Chuang, K. I. Chang, C. H. Yang and K. H. Chen, Appl. Catal. B: Env. 12 (1997) 309. 18. Y. L. Suponitskii, G. M. Kuz'micheva and A. A. Eliseev, Russ. Chem. Rev. 57 (1988) 209.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

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Thermal Stability and O x y g e n Storage Capacity of Noble Metal/Ceria-Zirconia Catalysts for the A u t o m o t i v e Converters with the On-Board-Diagnostics (OBD) P.Fornasiero, R.Di Monte, T.Montini, J.Ka~oar, and M.Graziani _

Dipartimento di Scienze Chimiche, Universitb. di Trieste, 34100 Trieste; Italy. Fax +39040-6763903; e-mail [email protected]. Reduction behaviour and oxygen storage capacity (OSC) of Rh-loaded and metal-free Ce0.sZr0.502 mixed oxides is investigated. It is shown that use of different synthetic methods and the homogeneity of the solid solution strongly affect both the reduction behaviour and the totalOSC. Presence of supported noble metal (NM) minimises the differences between the different samples due to the ability of the NM to promote reduction of the Ce0.sZr0.502 by activating hydrogen. 1. INTRODUCTION Automotive emission regulations have become more and more stringent. In the European Union: targets of 2.7, 0.10 and 0.08 g km l were indicated by the EURO phase IV (year 2005) for emission of respectively CO, HC and NOx. Further, a lifetime of 160.000 km will be required. At present three-way catalysts (TWCs) represent the most competitive technology able to meet such targets. The so-called OBD (on-board diagnostics), which monitors the efficiency of the oxygen storage/release capacity (OSC) of the TWCs is being employed, since decline of OSC due to thermal ageing has long been recognised as a major pathway for TWC deactivation. To minimise start-up emissions the converter is close-coupled to the engine, which exposes TWCs to temperatures up to 1273 K. Exceptionally high thermal stability of the OSC is therefore mandatory for a modem TWC application. CeO2-ZrO2 mixed oxides show improved thermal stability and oxygen storage capacity compared to traditional CeO2 based TWCs (1). However, the origin of these properties and the influence of synthesis and ageing conditions on the phase stability have not been addressed yet. We have investigated these systems in detail showing that the high oxygen mobility in the bulk could be a key factor in improving the OSC property (2). Here the effects of the synthesis method and ageing procedure of the OSC of metal-free and Rh-loaded Ce0.sZr0.502 single phase mixed oxide are reported. The comparison with previous data reported on non-homogenous Ce0.sZr0.502 systems reveals that reduction behaviour benefit from use of single-phase product. 2. EXPERIMENTAL CeO2-ZrO2 mixed oxides were synthesised by employing homogenous gel-type synthesis route using alkoxide precursors or the citrate route (3,4). Here in after these samples will be indicated respectively as CZ-A and CZ-C (alkoxide and citrate routes). After calcination at 773 K for 5h, surface areas of 65 and 40 m 2 g-i were obtained respectively for CZ-A and CZ-C. The TPR experiments were carried out as described previously (5). Before the TPR experiments,

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samples were cleaned using a standard cleaning procedure in O2/Ar mixture at 823 K (6). Effects of redox ageing were investigated by subjecting the samples to TPR/MO (mild oxidation, 700 K, pulses of 02) or TPR/SO (severe oxidation, 1273 K, pulses of 02) sequences. IH2 TPR and 2H2 TPR were carried out in situ, on the same sample to ensure the reproducibility of the results. Thermal conductivity detector (TCD) and mass spectrometer (MS) were employed as detectors in the experiments. 3. RESULTS AND DISCUSSION 3.1. Characterisation of the Ce0.sZr0.sO2 samples CZ-A sample was from a previous study (3). This sample consists of a mixture of tetragonal t"-phase Ce0.sZr0.502 solid solution plus a small amount (<10%) of a CeO2-rich phase not incorporated into the solid solution. The t"-phase is characterised by a fluorite type cation sublattice, while the oxygen atoms experience a tetragonal distortion (7). XRD patterns of as prepared and calcined 1273 K, 5h CZ-C samples are reported in Figure 1. Symmetrical peaks typical of a fluorite type cation sub-lattice are observed in both spectra, indicating that a homogeneous solid solution has been obtained by this method. This was confirmed by a profile fitting of the spectrum shown in Figure 1, which gives a cell parameter of 0.52983 nm. This value is consistent with formation of a solid solution. Raman spectra feature bands at 465 cm -I with a shoulder at about 550 cm -I and two bands at 313 (w) and 140 (vw) cm l , indicating that a crystallographically pure t"-phase is present. The presence of a single phase Ce0.sZr0.502 after calcination at 1273 K is a remarkable result since this treatment leads to phase separation as detected by asymmetric peak profiles (8,9). This is also a strong indication that the as prepared CZ-C sample is a homogeneous solid solution. No appreciable structural modification has been detected after loading the supports with NMs.

g

8

A

25

35

fl

45

~~

,--

1273 K, 5h .

55

65

75

85

95

.

.

105

o

.

115

125

135

2 Theta Figure 1. XRD patterns of CZ-C sample: fresh (calcined 773 K) and calcined at 1273 K, 5h, profile-fitted and difference spectra.

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Reduction Behaviour and Oxygen Uptake 3.2. TPR technique and 02 uptake measurements at 700 K were employed to investigate redox behaviour of the fresh CZ-C and Rh/CZ-C samples. At 700 K the re-oxidation of the mixed oxide is complete and the contribution of the so-called reversible CeO2 reduction, i.e. adsorbed hydrogen and/or hydrogen bronzes, is eliminated allowing a direct measure of oxygen vacancies created upon reduction. 3.2.1. Redox behaviour of Ceo.sZro~Oz: Effect of synthesis and H2/D2 mixtures Figure 2 compares TPR profiles of CZ-C sample and the effects of redox ageing using an reoxidation step at 700 K (MO) and, eventually, a further oxidation at 1273 K (SO). For sake of comparison TPR profiles of CZ-A from a previous study is included (3). The single-phase product features a single broad reduction peak centred at 858 K. In contrast, CZ-A features two peaks respectively at 880 and 1010 K. Subjecting the reduced CZ-A sample to an in situ MO results in strong modification of the subsequent TPR with the disappearance of the peak at 1010 K and formation of peaks/shoulders in the range 600-950 K. In contrast, the TPR profile of the CZ-C is almost unaffected by this redox cycle. The overall picture is that the single-phase product gives rise to a stable reduction behaviour in comparison to the non-homogenous one. In view of these results and comparison with the literature data, we infer that presence of multiple reduction peaks can be taken as an indication of non-homogeneity of the solid solution as long as equal/MO pre-treatment conditions are employed (I0). 1

nYe =19

.,-:..

cO .m Q.

E

,

~

,

,

,

,

,

,

,

,

g3 tO

=

0

4

o~

1"

9

300

500

700

900

1100

Temperature (K)

1300

2~

,

4~

,

,

6~

,

,

8~

,

,

,

,

,

1~12001~0

Temperature(K)

Figure 2.TPR-TCD profiles of CZ-C (1-5) and CZ-C (6-7): pre-treatments: fresh (1,6), MO (2,4,7), SO (2,5). Figure 3. TPR-MS of CZ-C using D2/Ar as reductant: Effects of pre-treatments SO (1) and MO (2) on D2 consumption, hydrogen scrambling and water evolution. The application of the SO to the TPR treated CZ-C sample modifies the subsequent reduction profile as most of the reduction process occurs around 660 K, which is now the major reduction peak. Notice that an equal reduction profile has been obtained on a CZ-C sample calcined at 1273 K, subjected to TPR and MO, indicating that the combination SO/TPR/MO is necessary to

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induce the low-temperature reduction TPR pattern. As indicated in Figure 2, the reduction behaviour, e.g. low-temperature vs. high-temperature patterns, can be reversibly induced by the combination of SO/MO treatments. This kind of behaviour was recently reported for a Ce0.68Zr0.3202 sample (11). Our data confirm that the reversible modifications of the reduction behaviour induced by these treatments appear to be e general property of the CeO2-ZrO2 mixed oxides. Accordingly, we found that such kind of reduction behaviour could be induced also in a Ce0.2Zr0.802 sample, i.e. at a ZrO2 rich level. To obtain indications on both the origins of this effect and the reduction mechanism we used D2 as reductant. An isotope independent profile is obtained for all the reduction processes below 700 K, while for the TPR peaks around 900 K an isotopic effect is found, as detected also by the apparent activation energies reported in Table 1. In the D2-TPR hydrogen scrambling is observed at a temperature 150-200 K lower than that of water evolution independently of the pre-treatment, e.g. the temperature of the peak at m/e=3 reversibly shifts with the pre-treatment (Figure 3). This indicates that H2 activation should not be rate limiting the reduction under the TPR conditions. It appears that H20 formation/desorption limits the rate of reduction at 900 K, i.e. after SO, while migration in the bulk, i.e. a reductant insensitive step, might be rate limiting the processes occurring below 700 K. Table 1. Results of TPR/O2 uptake measurements over CZ and NM/CZ (NM= Rh,Pd). Sample/pretreatment" CZ-A/fresh CZ-A/MO CZ-C/fresh CZ-C/MO CZ-C/SO CZ-C/SO CZ-C]MO

Reductant Hz Hz

H2 Hz

H2 Dz

D2

Peak Temperatures (K) 880, 1010 690/5, 845 858 659, 896 914 921 659, 908

02 uptake (mmol mol oxide l ) 78 72 129 115 115 115 114

Activ. Energy b (kJ mol -l)

39,91 90 134 42, 120

'~Except for the fresh samples, sample was recycled from a previous TPR run. b Calculated from TPR runs for each reduction peak, according to ref. (12), standard dev. _+4 kJ mol i.

3.2.2. Redox behaviour of NM/Ceo.sZro.sO2: Effect of synthesis and redox ageing Reduction behaviour of the NM/CZ samples was investigated by TPR-MS technique. Addition of NM (NM= Rh, Pt and Pd) to CZ strongly modifies the reduction profiles of both CZ-A and CZ-C. For sake of conciseness, here we report some representative experiments for Rh samples (Figures 4 and 5). Independently of the nature of the support, e.g. CZ-A or CZ-C, the reduction process shifts to low temperatures. The two-peak pattern is still observed in NM/CZ-A, the peak at 880 K being now shifted to approximately 600 K, depending of the nature of the noble metal. Re-cycling the sample in a further TPR the high temperature peak disappears, due to incorporation, under reducing conditions, of the CeO2-rich phase into a mixed oxide (8,13). In agreement with this interpretation, the TPR-MS profiles of fresh Rh/CZ-C (Figure 5) reveals the presence of single reduction peak. The comparison of the HE and H20 traces shows that H2 adsorption starts significantly earlier than HEO desorption. This is easily seen in the TPR profile of the recycled sample (Figure 4.2, 5.2-3), due to broad water desorption features in the fresh high surface area sample (vide infra). We recall that treatment under TPR conditions up to 1273 K leads to a strong sintering of the support (3,13). This phenomenon seems not to be related to H20 retention on the surface of the catalysts since evolution of water was observed below 500 K in the cleaning pre-treatment.

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Likewise it cannot be attributed to a transfer-line generated delays in H20 detection. In fact, the difference in the temperatures of H2 minima and H20 maxima depends on the sample/pretreatment. It appears that the NM plays a key role in the reduction process by favouring adsorption of H species onto the support before the irreversible reduction occurs, e.g. creation of oxygen vacancies. Presence of active H species at the support makes creation of oxygen vacancies easier compared to the bare support. Consistently, we find that hydrogen activation at the support, as detected by hydrogen scrambling, always precedes the reduction, e.g. water evolution.

(IL

(2) 1 -~,11,,, l , ~ l " r o l l

=2

-g ~m/e = 3 9

9

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,

9

,

9

~:....-. _ ~ - +-y-~:2 . L _ . ~ .

' ~

~ ~.-

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-:_ - 2 .

.--.-.r. i

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i , '

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i

,

i

.

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300 500 700 900 1100 Temperature (K) S

m/e = 18 i

300

500

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i

700

!

i

i

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i

900 1100 1300

Temperature (K) Figure 4. TPR-MS of Rh-CZ-A fresh (1) and recycled, after MO pre-treatment (2). Figure 5. TPR-MS spectra of Rh-CZ-C, fresh (1), and recycled: MO-treated (2) and SO-treated (3).

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An important observation is that the presence of Rh minimises the "negative" response of the systems to the SO treatment, as the shift to high temperature is rather limited. We attribute this effect to the ability of Rh. to provide active hydrogen species to the system, making the overall reduction process easy. 4. CONCLUSIONS The comparison of homogeneous and non-homogenous Ce0.sZr0.502 mixed oxides disclosed that single-phase solid solution features favourable redox properties both in terms of stability and the ability to induce a low-temperature reduction generated by a SO/TPR/MO sequence. When MO is applied during the redox-ageing treatments, no significant modification of the TPR profile/redox behaviour is found. Notice that stability of OSC is a desirable property for TWCs. NM favours the reduction of the mixed oxide and minimises the unfavourable effect of SO, indicating that such systems are suitable for durable TWCs as required by the OBD technology. Acknowledgements. Helpful discussion with Prof. S.Bernal, University of Cadiz (Spain) are acknowledged. University of Trieste, the Ministero dell'Ambiente (Roma), contract n. DG 164/SCOC/97, CNR (Roma) Programmi Finalizzati "Materiali Speciali per Tecnologie Avanzate II, Contract n. 97.00896.34 are gratefully acknowledged for financial support. REFERENCES

1. J.P. Cuif, G. Blanchard, O. Touret, A. Seigneurin, M. Marczi, and E. Qu6mer6, SAE, 970463 (1997) 2. P. Fornasiero, R. Di Monte, G. Ranga Rao, J. Kaspar, S. Meriani, A. Trovarelli, and M. Graziani, J. Catal. 151 (1995) 168. 3. P. Fornasiero, G. Balducci, R. Di Monte, J. Kaspar, V. Sergo, G. Gubitosa, A. Ferrero, and M. Graziani, J. Catal. 164 (1996) 173. 4. P. Vidmar, P. Fornasiero, J. Kaspar, G. Gubitosa, and M. Graziani, J. Catal., 171 (1997) 160. 5. P. Fornasiero, J. Kaspar, and M. Graziani, Appl. Catal. B Environ., 22 (1999) L11. 6. M. Daturi, C. Binet, J.C. Lavalley, H. Vidal, J. Kaspar, M. Graziani, and G. Blanchard, J. Chim. Phys., 95 (1998) 2048. 7. M. Yashima, H. Arashi, M. Kakihana, and M. Yoshimura, J. Amer. Ceram. Soc. 77 (1994) 1067. 8. G. Colon, M. Pijolat, F. Valdivieso, H. Vidal, J. Kaspar, E. Finocchio, M. Daturi, C. Binet, J.C. Lavalley, R.T. Baker, and S. Bernal, J. Chem. Soc., Faraday Trans., 94 (1998) 3717. 9. C.E. Hori, H. Permana, K.Y.S. Ng, A. Brenner, K. More, K.M. Rahmoeller, and D.N. Belton, Appl. Catal. B Environ. 16 (1998) 105. 10.J. Kaspar, P. Fornasiero, and M. Graziani, Catal. Today, 50 (1999) 285. 11.R.T. Baker, S. Bernal, G. Blanco, A.M. Cordon, J.M. Pintado, J.M. Rodriguez-Izquierdo, F. Fally, and V. Perrichon, Chem. Commun., (1999) 149. 12.D.A.M. Monti and A. Baiker, J. Catal., 83 (1983) 323. 13.P. Fornasiero, J. Kaspar, V. Sergo, and M. Graziani, J. Catal., 182 (1999) 56.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

N20 formation over CeOz-ZrO2 mixed oxides supported metal (Pt, Pd, Rh) catalysts for automotive exhaust control J.L. Marc a, J.R. Gonzfilez-Velasco a *, M.A. Guti6rrez-Ortiz a, J.A. Botas a, M.P. Gonzfilez-Marcos a and G. Blanchard b Departamento de Ingenieria Quimica, Universidad del Pais Vasco /EHU, P.O. Box 644, 48080 Bilbao, Spain. Tel.: +34-94-601-26-81 ; Fax: +34-94-464-85-00; *E-mail: [email protected] b Rhodia Recherches, 52 rue de la Haie Coq, 93308 Aubervilliers Cedex, France

a

N20 formation under stoichiometric cycled condition over three monometallic ceriazirconia catalysts has been investigated. A strong dependence between the N20 production and the temperature at which NO reduction occurs was noted, i.e. the lower temperature the higher the amount of NzO produced. Among the three monometallic catalysts, the one with Rh produces lower N20 concentration. The NzO disappearance observed at high temperature is suggested to be due to both its removal and unfavorable conditions for its formation. 1. INTRODUCTION Noble metals supported on ceria-zirconia mixed oxides are being intensively investigated as good candidates for a new generation of three-way catalysts (TWC). The total elimination of NOx in the catalytic converter selectively to nitrogen and not to other compounds such as N20, HCN or NH 3 is an important challenge in the actual research and some improvements have to be made in that direction. When the three (Pt, Pd, Rh) monometallic ceria-zirconia supported catalysts were tested in a close-looped system which resembles the real conditions at the catalytic converter with simulated full composition feedstream (mixture of CO, NO, CO2, 02, C3H6and H20 under cycled stoichiometric conditions), no NH 3 is detected but significant production of NzO is observed. In this work we propose to focus on these N20 production and to understand why N20 is not detected in the outlet gas for the high temperatures. The influence of the loaded metal nature and of the specific activity of the catalyst for the NO elimination on the N20 production will be discussed. 2. EXPERIMENTAL The three monometallic Pd, Pt and Rh ceria-zirconia catalyst supported were supplied by Rhodia. Metal was deposited on high surface Ce0.68Zr0.3202support by incipient wetness impregnation from an aqueous solution of chlorine-free precursors. Then, the catalysts were dried in air at 110~ for 2 hours and calcined in dry air at 450~ for 5 hours. The metal

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content in weight percentage was 0.64 % Pd, 0.56 % Pt and 0.69 % Rh, and their surface area about 100 m2/g as fresh and 25 m2/g after aging. The samples, in the shape of pellets (size: 0.5-1.0 mm), were cleaned by an oxidizing pretreatment under O2/N2 (5:95) for lh at 550~ The cleaned catalyst thus obtained constituted the fresh catalyst. Before aging and submission to the catalytic tests, catalysts were heated at 100~ in N2 flow during 12 hours. Aging treatment consisted of heating the catalyst at 8~ from 100~ up to 900~ in an oxidizing-reducing cycled feedstream (cycling frequency of 0.0166 Hz), keeping the temperature for 5 hours at 900~ and then slow cooling under the same gas environment to 150~ Finally, the catalyst was slowly cooled to room temperature in N2 flow. The volume percentage of each component in the gas mixture were: Reducing flow: CO, 1.6%; C3H6, 900 ppm; NO, 900 ppm; 02, 0.465%; CO2, 10%; H20, 10% and N2 balance, A/F = 14.13. Oxidizing flow: CO, 0.4%; C3H6, 900 ppm; NO, 900 ppm; 02, 1.260%; CO2, 10%; H20, 10% and N2 balance, A/F = 15.17. Catalytic tests were performed under the same oxidizing-reducing cycled feedstream (average A/F=14.63) than aging. Temperature was increased from 100 to 600~ at 3~ with a space velocity and a cycling frequency of 125,000h -~ (1.8 g of catalyst diluted in inactive ceramic pellets to obtain 3.5 cm 3) and 1 Hz respectively. The conversion data were continuously analyzed by specific on-line analyzers, i.e. chemiluminescence for NO and NOx, non-dispersive infrared for NH3, N20 , CO and CO2, flame ionization for C3H6 and magnetic susceptibility for 02. From the temperature-programmed conversion curves of CO, C3H6, and NO, the temperature for 50% conversion (Ts0) and the conversion at 500~ (Xs00) were determined for the three pollutants. In some experiments the catalyst was prereduced before reaction by heating in H2/N2 (5/95) flow at 300~ for 1 hour. 3. RESULTS AND DISCUSSION Each monometallic catalyst has been tested fresh, after prereduction, and after aging and then prereduction under the conditions described above. The activity of the catalysts, represented by Tso and Xsoo, is reported in table 1. Table 1: Evolution of Tso and Xso0 for the three components (CO, C3H6, NO) and the three monometallic catalysts as a function of the pretreatment suffered Catalyst Fresh Pt FreshprereducedPt Aged prereduced Pt Fresh Pd Fresh prereduced Pd Aged prereduced Pd Fresh Rh Fresh prereduced Rh Aged prereduced Rh

T~o (~ CO 364 127 240 137 133 306 187 178 282

NO 437 365 279 255 276 320 325 323 287

C~H~ 423 329 264 226 250 308 255 244 317

X~00(%) CO NO 97.3 92 100 88.5 100 97.5 100 80 100 96 100 86 100 90 100 89 100 91.5

C3H~ 93.7 99.9 99.4 100 100 100 100 100 99.3

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The activity of the catalyst depends on the nature of the active sites, those depending strongly on the pretreatment conditions. Thus, prereduction and aging pretreatments modified the specific activity of each monometallic catalyst for the elimination of the three pollutants (CO, C3H6 and NO). Considering particularly Tso data, when 160 100 fresh, the Pd catalyst can be considered as the most active for the consecutive 80 120 E elimination of the three pollutants. Q. .< D. When prereduction was carried out on :; 60 c 8o ~ the fresh catalysts, an improvement of the O 0 o o 40 activity for both Pt and Rh in the o Z elimination of the three pollutants was 4o ~ 20 observed, while for the Pd catalyst only CO elimination was enhanced. 0 0 The main consequences of the aging 200 300 400 500 600 treatments were: deactivation of the Pd 100 160 catalyst, improvement of the activity of the Pt catalyst and enhancement of the NO elimination for the Rh catalyst. 80 120 E Concerning NO elimination, aging leads Q. eo to a Tso(NO) decrease and to a Xsoo(NO) 8 80 8 increase for both Pt and Rh catalysts. ~ 4o The NO conversion curves as well as 20 40 0 the concentration of N/O produced during testing are reported in Figure 1. N20 production begins when the NO conversion 0 0 starts; its formation seems to be more 200 300 400 500 600 favored at lower temperatures. At higher 160 lOO 1 temperatures, N20 concentration decreases, and a curve passing through a maximum is 80 f c 120 E obtained with temperature. The produced O., amount of N20 seems to be dependent on 60 the temperature at which the NO 8o ~ 0 o 40 conversion occurs, i.e. the lower the o temperature the higher the N20 production. 4o ~ 20 We can observe that the N20 concentration maximum and the temperature at which 0 0 this maximum was reached are quasi 200 300 400 500 600 linearly dependent for each metallic Temperature, *C system. In this way, Rh catalyst is the most Figure 1: NO conversion (black symbols) efficient in NO elimination producing the and N20 concentration (opened symbols) lowest quantity of N20. The N20 disappearance is completely for the fresh (circles), fresh prereduced (squares) and aged prereduced (triangles) independent of the catalyst activity, because at 600~ the concentration of N20 with Pd (a), Pt (b) and Rh (c) catalysts was zero or very low. EL

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N20 removal has been interpreted by some mechanisms reported in the literature; schematically the N20 disappearance can be explained by the absence of its formation and the dependence existing between the rate of NO dissociation and the temperature. At lower temperatures the dissociation of the adsorbed NO is low and some NO recombines with N atoms to form N20, whereas at higher temperatures the NO dissociation increases and the N20 formation decreases because the most favored reaction becomes the recombination of two adsorbed N atoms to give N2 [ 1]. Other reported mechanisms explain the N20 elimination by its reaction with other reducing species present in the reacting gas, such as CO or HC [2,3]. While many studies are particularly focused on the NO reduction by CO, they stress the fact that in the presence of oxidizing species, such as NO and 02, N20 consumption is not favored. Considering our experimental conditions, N20 should be formed by NO reaction with CO and C3H6. The participation of H2 as reducing species to the NO reduction is not probable in our case because H2 formation by Water Gas Shift or Steam Reforming can only be effective at temperatures higher than those for which N20 presence was observed. In order to check the influence of the redox character of the feedstream on the N20 production, two tests have been carried out with the fresh prereduced Pt catalyst under reducing (CO+NO+C3H6) or oxidizing (CO+NO+C3H6+O2) steady state conditions. Concentration of the components present in the feedstream remained the same as detailed above (except for O2=1.51%). The absence of some components was compensated with nitrogen, so that the space velocity was unchanged. The results are presented in Figures 2a and 2b. 100

100

100

80

80 Q. E

o 60

60 cu

>- 9 c 0 o

0 o

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100

b

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E Q.

u 60

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411 0

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0

0

d 0

2o

2O

d 0

2o

2O

0

100

200 300 400 500 Temperature, ~

600

100

200 300 400 500 Temperature, ~

600

Figure 2: Fresh prereduced Pt catalyst, CO (Q), C3H6 (~) and NO (A) conversion and N20 (O), NO2 (Ui) and NH 3 (A) production, (a) reducing conditions (A/F=14.02) and (b) oxidizing conditions (A/F= 15.13). These two figures show that N20 can be formed in both reducing and oxidizing conditions. Under reducing conditions N20 is formed by NO reaction with CO. NO reduction by C3H6 produced only a small amount of N20 , while NH3 was produced in large quantity. Under oxidizing conditions, N20 is probably formed by NO reaction with both CO and C3H6. On the other hand, at higher temperatures, in addition to N20 , NO elimination leads also to formation of NO 2.

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The question of N20 disappearance at high temperature is quite complex and both the N20 removal by reaction with CO and C3H6 as well as the absence of N20 formation can be considered. In order to determine if N20 could be easily eliminated during our catalytic testing conditions, 130 ppm of N20 were introduced in the feedstream in addition to the other components. As it can be observed from the light-off curves presented in Figure 3, N20 was easily removed even at low temperatures. Between 240~ and 410~ N20 is produced, while at higher temperatures it is easily consumed. Thus, N20 reaction with CO and C3H6 could explain the absence of N20 observed at higher temperatures, even in the presence of 02, NO and H20. Complementary experiments were carried out in order to confirm the influence of temperature on the N20 formation. They consisted of a reduction of the Pt catalyst at 300~ under Hz/N2 (5:95) for 30 minutes and then an oxidation by introducing NO (conditions: 900 ppm of NO, nitrogen to balance and a space velocity of 125,000h -1) at two different temperatures: 70~ and 550~ NO conversion, N20 production and selectivity to N 2 versus time are reported in Figure 4. As O2 was not detected in the outlet gas, oxygen comming from NO decomposition was consumed to reoxidize the catalyst.

100

.......................... 250

80

210

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511

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Figure 3: Catalytic test carried out with the fresh prereduced Pt catalyst (Feedstream (A/F=14.63) + 130 ppm N20), CO (Q), C3H6 (m), O2 (O) and NO (A) conversion and N20 (O) concentration.

I~

0

o eO

Z

0 0

5

10

15

20

25

Time,rdn. Figure 4: Reoxidation of the fresh

prereduced Pt catalyst with NO at 550~ (opened symbols) and 70~ (black symbols): NO conversion (triangles), N20 concentration (rhombus) and selectivity to N2 (circles).

Reoxidation was found to be quite fast for the first few minutes and then, after 15 minutes became slower. The amount of N20 produced at 550~ is very low, while at 70~ a significant amount of N20 was detected. At the lower temperature, the selectivity to N2 started from about 70% and then decreased with time. A feasible explanation of the N2 selectivity decrease versus time is that the active surface is probably covered with adsorbed oxygen atoms, comming from the dissociative NO adsorption, as well as with adsorbed NO. The dissociative NO adsorption becomes increasingly inhibited by the decrease of unoccupied surface reduced sites. Consequently,

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the N 2 formation decreases due to the lower adsorbed N atoms concentration. In this sense, as NO dissociation is probably more favored at higher temperatures, N20 was practically not formed at 550~ even at long reoxidation times. These results suggest that N20 is formed from the dissociative NO adsorption over the reduced active sites. Both CO and C3H 6 should participate as reducing species to provide active sites for the NO reduction. N20 production depends on both temperature and oxidation state of the active sites. N20 is not detected in the outlet gas at high temperature probably for two reasons: firstly, its formation is less favored and, secondly, N20 product is readsorbed on the active site and eliminated to give N2 under stoichiometric conditions and, maybe, NH 3 and NO2 in addition under reducing and oxidizing conditions, respectively. 4. CONCLUSION All monometallic ceria-zirconia catalysts presented a maximum in volcano shaped curve of N20 production versus temperature. The amplitude and the position of the N20 peak with temperature is related to the corresponding NO conversion curves. For each monometallic catalyst, catalytic activity for NO elimination was specifically changed by the aging or prereduction treatment: the lower the temperature at which NO reduction occurred, the higher the amplitude of the N20 peak. Among the three monometallic catalysts, the one with Rh trends to produce lower amount of N20, probably, as underlined in the literature, due to its ability to dissociate NO more easily than Pt or Pd [4]. The question of the N20 disappearance observed at high temperature is quite complex. The present results suggest that it is a combination of both unfavorable formation and removal at high temperature. ACKNOWLEDGMENTS This work has received financial support from the TMR Program of the European Union (Project CEZIRENCAT, Contract No.: FMRX-CT-96-0060). REFERENCES 1. R. Burch and P. J. Millington, Catal. Today, 26 (1995) 185. 2. B. K. Cho, J. Catal., 148 (1994) 697. 3. N. W. Cant, D. E. Angove, D. C. Chambers, Appl. Catal. B: Environmental, 17 (1998) 63. 4. M. Shelef and G. W. Graham, Catal. Rev.-Sci. Eng., 36 (1994) 433.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Model Studies of Automobile Exhaust Catalysis Using Single Crystals of Rhodium and Ceria/Zirconia C.H.F. Peden *a, G.S. Herman l, S.A. Chambers l, Y. Gao l, Y.-J. Kim l, and D.N. Belton 2 1pacific Northwest National Laboratory*, P.O. Box 999, Richland, WA 99352 USA. 2General Motors Powertrain, 3300 GM Rd, MC 483-302-137, Milford, MI 48380 USA In this brief proceedings paper, we describe a continuing program aimed at defining and understanding the reaction kinetics over well defined metal single crystal catalysts under conditions of temperature and pressure comparable to those encountered in automotive exhaust. By studying such well-defined model catalysts, we are able to isolate the activity of the noble metal component of the catalyst free from complicating factors such as metal particle size and catalyst support effects. Because single crystals have well-defined surface structures, surface areas, and no support effects, they are ideal for activity comparisons between metal surfaces with varying geometric structures. A particular focus of our current studies is on how the catalytic chemistry is effected by the oxygen uptake, storage, and release processes carried out by the oxygen storage material. For these latter studies, we have prepared and characterized a number of model, single crystal thin films of ceria with and without Zr-doping. Here we provide an overview of some of the results from these studies. 1. INTRODUCTION The 1990 Clean Air Act Amendment mandates that automobile manufacturers produce cars with significantly lower hydrocarbon (HC), carbon monoxide (CO), and nitrogen oxide (NOx) emissions to be phased in over the next 10 years [1-3]. In recognition of the serious pressures on the current emission control technology described above, the recent DOEcosponsored workshop, Basic Research Needs for Vehicles of the Future [4], identified a critical need for greatly improved catalyst activity and durability. To effect these dramatic improvements that are so crucial for meeting the very stringent future regulatory requirements, the need for detailed chemical kinetics information on realistic and model catalyst materials, directly coupled with extensive catalyst characterization cannot be overemphasized. Indeed, more specific recommendations from the Vehicles of the Future workshop include the need for fundamental advances in the modeling of transient NOx kinetics, and a determination of the mechanisms of catalyst deactivation in the current generation of catalyst materials. *Pacific Northwest National Laboratory is a multiprogramnational laboratory operated for the U.S. Department of Energy by Battelle Memorial Instituteunder contract# DE-AC06-76RLO1830. *Authorto whom correspondenceshouldbe addressedat: [email protected].

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Dramatic improvements have been made in automobile exhaust converter catalysts by the incorporation of oxygen storage (OS) materials, usually consisting of ceria or modified ceria, that can effectively damp deviation in the exhaust air/fuel (A/F) ratio bringing the gas phase closer to the stoichiometric point. Although there is some limited understanding of the activity and durability of these 1990-vintage catalysts, virtually no fundamental understanding of this type exists for the current generation of catalysts that incorporate ceria-zirconia solid solutions as the OS material. In the work to be described here, our overall objective is to obtain detailed chemical kinetics data on idealized but well-characterized catalyst systems useful for understanding the important elementary catalytic converter reactions, and how they are effected by the oxygen uptake, storage, and release processes of the OS material. 2. EXPERIMENTAL The catalytic activity measurements were performed in a custom built system that combines an UHV analysis chamber connected to a high pressure (< 760 Torr) reactor [5]. The UHV analysis chamber is equipped with several surface analytical techniques. For this study we used x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED). The sample was mounted to a retractable transfer rod that can be moved between the reactor and analysis chamber when both are under UHV conditions. Elsewhere [5] we describe the mounting and preparation of the single crystal Rh samples, and details about the methods used for CO+NO reaction rate measurements over these model catalysts. Epitaxial growth and in-situ characterization of Cel_xZrxO2 films were carried out in a dualchamber MBE system specially designed for growth of oxide thin films and described in detail elsewhere [6]. Polished, pure and 0.15 wt.% Nb-doped SrTiO3 (001) single crystals were used as substrates for epitaxial growth of Cel.xZrxO2 solid solutions as the oxygen sublattices of these two crystals are very well matched. Post-growth compositional and structural measurements included x-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED). The film composition was also determined e x - s i t u by Rutherford backscattering spectrometry (RBS). The film composition determined by XPS and RBS agreed with that measured by a quartz crystal oscillator (QCO) deposition rate monitor during growth to within a few percent. In addition, the Cel.xZrxO2 epitaxial films were characterized ex-situ by x-ray diffraction (XRD), ion channeling, and atomic force microscopy (AFM). The combination of these probes allows us to determine the film structure and composition, the oxidation states of both Ce and Zr, crystal quality, surface morphology, and Zr incorporation in the CeO2 lattice. Studies of the surface chemistry of these films were performed in a separate ultrahigh vacuum (UHV) system that contains capabilities for XPS, Auger electron spectroscopy (AES), temperature programmed desorption (TPD), LEED, and a sputter gun for sample cleaning. This system also has a sample holder that allowed sample temperatures to be varied from 80 K to 1600 K by liquid nitrogen cooling and resistive heating. Angle-resolved massspectroscopy of recoiled ions (AR-MSRI) measurements were performed in a separate, custom UHV chamber described elsewhere [7], where we also discuss sample mounting and temperature measurement. Based on the position of the water multilayer desorption peak, the TPD peak temperatures reported here should be close to the actual sample temperature.

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3. RESULTS AND DISCUSSION

3.1 Characterization and Reactivity of Single Crystal Rh Catalysts A direct comparison of the NO consumption rate over the Rh(100), Rh(111) and Rh(110) surfaces represents the best relative measure of overall activity of the NO-CO reaction since it does not depend on the selectivity of the reaction [5]. While the Rh(111) and Rh(110) data, plotted in Arrhenius fashion, can be described by a single straight line with Ea values of 34.8 and 27.2 kcal/mol, respectively, the Rh(100) data has a LT and HT region with Ea values of 35.3 and 20.5 kcal/mol, respectively. In Figure 1 the temperature sensitivity of the N20 selectivity for NO-CO reaction over the same three single crystal Rh surfaces is plotted. In this case, the N20 selectivity is defined as S(N20) = 100 x [moles N20/(moles N20 + moles N2)]. These results can be understood in terms of the relative surface coverages of adsorbed NO and N-atoms on the three surfaces. Notably, higher steady-state N-atom coverages catalyst surface favor N-atom recombination (N2 formation) more than the NO+N reaction (N20 formation). Figure 2 shows post-reaction XPS of the three different surfaces where it is clear that the three different surfaces have different relative concentrations of N(ad) and NO(ad). The Rh(111) surface consists of only NO(ad), while the Rh(100) and Rh(110) surfaces contain two peaks that can be assigned to NO(ad) and N(ad) with binding energies of 400.3 and 397.6 eV, respectively. The relative intensity of the N(ad) feature increases with decreasing rhodium surface atomic density (surface atomic density: Rh(111) > Rh(100) >

Rh(110)). 100

Nls

PNO= Pco = 8 Torr

P:: = 8 TT~

t

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,m 0

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~~]!_, Rh(lO0)

<

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v

0 Rh(100) ~ ~ [] Rh(110) \ A Rh(111)

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_=

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500

I

600

I

700

I

800

410

900

400

|

|

|

I

|

395

|

|

|

390

Binding Energy (eV)

Temperature (K) Figure 1: N20 selectivities for the NO-CO reaction over Rh(100), Rh(110), and Rh(111) as a function of temperature. Pco = PNO = 8 Torr [5].

405

Figure 2: Post-reaction N(ls) XPS results for the NO-CO reaction at 528 K over Rh(100), Rh(110), and Rh(111). Pco = PNO = 8 Torr

[5].

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3.2 Preparation and Characterization of Model Ceria/Zirconia Thin Films Cerium dioxide (CeO2) is used as an oxygen storage material in an automobile catalytic converter because of its redox properties (switching rapidly between +3 and +4 oxidation states), the high oxidizing power of Ce§ and the high 0 -2 mobility in the ceria lattice [8-10]. The oxygen-storage properties of ceria are generally attributed to facile oxygen exchange over a fairly wide range of oxygen stoichiometry. Recently, several doped cerium oxides have been reported to enhance the oxygen storage properties of pure ceria and to increase its thermal stability [11-20]. For example, doping of Zr into the ceria lattice to form Cel.xZrxO2 solid solutions significantly improved the oxygen storage capacity and the rate of the oxygen release reaction [20-23]. These studies clearly show that the enhancement of the oxygen storage capacity by foreign atoms occurs, and suggests that some kind of surface or even bulk modification effect is operative. Though several possible mechanisms have been proposed to explain some of the effects of the doping of ceria, they are still not well understood. Studies performed to date have been carried out on polycrystalline ceramic samples, presumably due to the lack of commercially available doped ceria single crystals. As a result, it has not been possible to determine at the atomistic level the cause(s) of the enhanced catalytic activity. Important insight into the cause(s) of this enhancement will be gained by careful surface chemistry studies that employ high-quality, well-characterized single crystals or epitaxial films of doped ceria. However, neither doped ceria single crystals are commercially available, nor doped epitaxial thin films have been produced to the best of our knowledge. In this study, we have prepared epitaxial films of Cel.xZrxO2 solid solutions for such surface chemistry studies in order to elucidate the role of Zr in enhancing the oxygen release reaction and the oxygen storage capacity of pure ceria. Such knowledge of the surface reactivity of Cel.xZrxO2 is very important for understanding the role of this material in automobile catalysis. For example, using epitaxial films of Cel_xZrxO2 will allow us to study how the presence of Zr in the ceria lattice affects the kinetics of the CO oxidation, the NO reduction, and the water-gas shift reactions. The results of the surface chemistry studies will be reported elsewhere. In this paper, we describe epitaxial growth of Cel.xZrxO2 films by molecular beam epitaxy (MBE), and structural and chemical characterization of these films. Epitaxial films of mixed Cel.xZr• oxides with x < 0.3 have been grown on SrTiO3(001) by oxygen-plasma-assisted molecular beam epitaxy. The film growth at 600 ~ is predominantly nucleation and growth of 3-D islands. The films become much smoother after rapid thermal annealing at 700 ~ for 30 seconds in the oxygen plasma. High-energy ion channeling reveals that Zr atoms substitutionally incorporate at cation sites in the CeO2 lattice for all doping levels, leading to Cel.xZrxO2 solid solutions. Analysis of Zr(3d) and Ce(3d) core-level binding energies shows that the oxidation state of both Zr and Ce is +4. Lattice distortion induced by incorporation of Zr in the CeO2 lattice becomes prevalent for high doping levels, and surfaces roughen accordingly. Furthermore, changes in the Ce(3d) XPS spectrum at these high doping levels indicates the presence of some partially reduced Ce. Though formation of Ce +3 is still not understood yet, one possible explanation is the formation of oxygen vacancies in the surface region due to oxygen displacements along the c axis from ideal fluorite position caused by incorporation of Zr in the CeO2 lattice [24]. The surface structure and water adsorption properties of the pure CeO2(001) films have been further characterized in our recent work [7,25]. Angle-resolved mass-spectroscopy of recoiled ions (MSRI) measurements were obtained for this surface [7]. Prior theoretical studies [26] have suggested that the (lxl) unreconstructed surface, terminated with bulk equivalent layers of either cerium or oxygen, is unstable due to a non-zero surface dipole. We

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have experimentally determined [7] that the surface is terminated with 0.5 monolayers of oxygen; a model that should give a stable surface with a zero dipole moment. This surface structure is shown below in Figure 3(a). The interaction of D20 with the CeO2(001) surface was further studied with temperature programmed desorption (TPD) and XPS [25]. It was found with TPD that D20 desorption occurs in three states with temperatures of 152, 200 and 275 K which are defined as multilayer D20, weakly bound surface D20, and hydroxyl recombination, respectively. O(ls) XPS measurements (illustrated in Figure 4) for high D20 exposures, where multilayer water desorption was observed in the TPD, resulted in emission from only the substrate and surface hydroxyls. This is likely due to a non-wetting behavior of D20 on this surface with the formation of nanosized clusters. An analysis of the O(ls) XPS data indicates that the surface has a hydroxyl coverage of 0.9 monolayers for large water exposures at 85 K. This is consistent with a model, shown in Figure 3(b), in which the polar CeO2(001) surface can be stabilized by a reduction of the dipole in the top layer by the formation of full monolayer of hydroxyls.

(a) CEO2(001 ) CeO2 (001) o is, AI K(~ T=85 K D20 e x p o s u r e = 2 2 0 s c-

(b) HydroxylatedCe02(001)

I

(/} r (D

I

_=

~o ,

[O~lb [100] Figure 3: Structural models for the non-polar reconstructions of the CeO2(001) surfaces. The cerium, oxygen and hydrogen ions are represented by gray, white and black circles, respectively. The oxygen ions in the third layer are shown as smaller circles with respect to those in the first layer. Model (a) is a vacancy model with a 1/2 ML of oxygen removed from the topmost layer. Model (b) is a fully hydroxylated surface.

I

534

,

I

dH ,

I

532

,

6~ I

,

I

530

,

I

,

I

528

,

Binding Energy (eV)

Figure 4: O ls x-ray photoelectron spectra from the CeO2(001) surface held at 85 K before exposing to D20 (lower), and after exposing to D20 for 220 seconds (upper).

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4. ACKNOWLEDGMENTS

The w o r k described here was funded by the U.S. Department o f Energy (DOE), Office o f B a s i c E n e r g y Sciences, Division o f C h e m i c a l Sciences. M o s t o f the e x p e r i m e n t s w e r e p e r f o r m e d in the Environmental Molecular Sciences Laboratory (EMSL) at Pacific N o r t h w e s t N a t i o n a l L a b o r a t o r y (PNNL), a scientific user facility sponsored by the D O E ' s Office o f Biological and Environmental Research.

REFERENCES 1. 2. 3. 4.

K.C. Taylor, Chemtech 20, 551 (1990). K.C. Taylor, Catal. Rev.-Sci. Eng. 35, 457 (1993). J.G. Calvert, J.B. Heywood, R.E. Sawyer, and J.H. Seinfeld, Science 261, 37 (1993). P.M. Eisenberger (ed.), Basic Research Needs for Vehicles of the Future. Princeton Materials Institute (Princeton, NJ) 1995. 5. G.S. Herman, C.H.F. Peden, S.J. Schmieg and D.N. Belton, Catal. Lett., 62, 131 (1999). 6. S.A. Chambers, T.T. Tran, and T.A. Hileman, J. Mater. Res. 9, 2944 (1994). 7. G.S. Herman, Phys. Rev. B 59, 14899 (1999); (b) G.S. Herman, Surf. Sci. 437, 207 (1999). 8. M. Iwamoto, Catal. Today 22, 1 (1994). 9. A. Trovarelli, Catal. Rev. - Sci. Eng. 38, 439 (1996). 10. Y. Gao, G.S. Herman, S. Thevuthasan, C.H.F. Peden, and S.A. Chambers, J. Vac. Sci. Technol. A 17, 961 (1999). 11. S.H. Oh, J. Catal. 124, 477 (1990). 12. J.G. Nunan, H.J. Robota, M.J. Cohn, and S.A. Bradley, J. Catal. 133, 309 (1992). 13. T. Muroto, T. Hasegawa, and S. Azasa, J. ,4lloys and Compounds 193, 622 (1993). 14. B.K. ChoJ. Catal., 131, 74 (1991). 15. T. Mikki, T. Ogawa, M. Haneda, N. Kakuta, A. Ueno, S. Tateishi, S. Matsuura, and M. Sato, J. Phys. Chem. 94, 6464 (1990). 16. R.K. Usmen, G.W. Graham, W.L.H. Watkins, and R.W. MacCabe, Catal. Lett. 30, 53 (1995). 17. G. Ranga Rao, J. Kaspar, S. Meriani, R. Di Monti, and M. Graziani, Catal. Lett. 24, 107 (1994). 18. C. de Leitenburg, A. Trovarelli, F. Zamar, S. Maschio, and J. Llorca, J. Chem. Soc., Chem. Commun. 2181 (1995). 19. F. Zamar, A. Trovarelli, C. de Leitemburg, and G. Dolcetti, Stud. Surfi Sci. Catal. 101, 1283 (1996). 20. P. Fomasiero, R. Di Monte, G. Ranga Rao, J. Kaspar, S. Meriani, A. Trovarelli, and M. Graziani, d. Catal. 151, 168 (1995). 21. P. Fornasiero, G. Balducci, J. Kaspar, S. Meriani, R. Di Monte, and M. Graziani, Catal. Today 29, 47 (1996). 22. H. Hamada, Catal. Today 22, 21 (1994). 23. M. Ozawa, M. Kimura, and A. Isogali, J. Alloys Comp. 193, 73 (1993). 24. M. Yashima, S. Sasaki, Y. Yamaguchi, M. Kakihana, M. Yoshimura, and T. Mori, Appl. Phys. Lett. 72, 182 (1998). 25. G.S. Herman, Y.-J. Kim, S.A. Chambers, and C.H.F. Peden, Langmuir 15, 3993 (1999). 26. (a) T.X.T. Sayle, S.C. Parker, and C.R.A. Catlow, Surf. Sci. 316, 329 (1994); (b) J.C. Conesa, Surf. Sci. 339, 337 (1995).

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

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Thermally stable, high-surface-area, PrOy-CeOz-based mixed oxides for use in automotive-exhaust catalysts A. N. Shigapov a, H.-W. Jenb, G. W.

G r a h a l T l b,

W. Chunb, and R. W. McCabe b

alnstitute of Chemistry and Chemical Technology, 42 K. Marx St., Krasnoyarsk 660049, Russian Federation bFord Research Laboratory, MD3179/SRL, P.O. Box 2053, Dearborn, MI 48121, USA High-surface-area praseodymia-ceria-based mixed oxides were prepared by an organictemplating method and evaluated as support materials for palladium in model automotiveexhaust catalysts. Results demonstrate that these materials can provide much more oxygen storage capacity than ceria-zirconia at low temperature (___350~ Addition of zirconium, yttrium, or calcium to praseodymia-ceria increased the surface area and its thermal stability but decreased the low-temperature oxygen storage capacity. Overall, these appear to be viable materials for use in practical catalysts. 1. INTRODUCTION The main function of ceria in automotive-exhaust catalysts is to provide oxygen storage capacity under transient operating conditions. Praseodymium-cerium mixed oxides are capable of providing even greater storage capacity than ceria, comparable with that of ceria-zirconia, with the additional advantage of being able to release oxygen at lower temperatures [1], but they need to be developed into high-surface-area, thermally stable form. In this work we prepared such materials, stabilized with low levels of zirconium, yttrium, or calcium, and compared their oxygen storage and release properties in model palladium catalysts with those of similar catalysts made from commercial state-of-the-art ceria-zirconia. 2. EXPERIMENTAL DETAILS Mixed oxides were prepared by an organic-templating method in which a high-purity microcrystalline cellulose material (Whatman 50 filter paper) was fully wet with 0.1M aqueous solution of metal salts (praseodymium nitrate, cerium nitrate, and zirconium citrate, ammonium complex, yttrium nitrate, or calcium nitrate) of the appropriate molar ratio, followed by drying and combustion of the cellulose (performed by heating at 10~ to 650~ and holding for 2h) in air. Resulting (fresh) materials were characterized by a combination of surface area (BET), XRD, and TGA measurements. Catalysts were made from the fresh materials by impregnation to incipient wetness with aqueous palladium nitrate solution followed by drying and calcination at 600~ for 12h. The palladium loadings were between 1 and 3wt%. The oxygen exchange properties of the catalysts were evaluated by a pulsed CO/O2 method in which each catalyst was first heated to 50~ in

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2.5% 02 (in He) [2]. A series of 5 four-pulse sequences (10s of 2.5% 02 followed by 10s of He followed by 10s of 5% CO followed by 10s of He) was then applied to the catalyst, and the gas exiting the catalyst bed was analyzed by mass spectrometry. The temperature was then raised by 50~ in 2.5% 02 and the measurement repeated, until reaching 350~ Oxygen storage capacities deduced from consumption of CO, production of CO2, or subsequent consumption of 02 were completely consistent. 3. RESULTS

The specific surface areas of a few fresh materials made with varying levels of zirconium are listed in Table 1 together with areas measured after an additional 12h of heating at 1050~ in air. Powder XRD measurements showed that each of the oxides consisted of a single phase with the fluorite structure, characterized by lattice constant, a.

Table 1 Specific surface areas and lattice constants of praseodymium-cerium-zirconium mixed oxides Pr : Ce : Zr

BET surface area (m2/g)

a (nm)

fresh

air aged

0.50:0.50:0.00

36

2.7

0.540

0.45 :0.45

:0.10

118

8.1

0.541

0.425 : 0.425 : 0.15

103

11.2

0.540

0.40:0.40:0.20

121

15.1

0.539

The specific surface areas of the zirconium-containing mixed oxides in both fresh and airaged states are comparable with those of commercial ceria-zirconia materials [2], and the positive effect of zirconium on surface area is apparent. On the other hand, the decrease in lattice constant with increasing level of zirconium is much less than for ceria-zirconia, suggesting that the addition of zirconium tends to promote an offsetting transformation of Pr+4 to Pr+3 in praseodymia-ceria-zirconia [3]. This was confirmed by thermal gravimetric analysis which showed that the amount of oxygen reversibly released upon heating from 200 to 800~ in air decreases with increasing level of zirconium (994gmol O per gram of 0.50PrOy-0.50CeO2 to 510gmol O per gram of 0.45PrOy-0.45CeO2-0.10ZrO2 to 378~tmol O per gram of 0.40PrOy0.40CeO2-0.20ZrO2). Results of oxygen storage capacity measurements, starting at 100~ from fresh catalysts made with varying levels of zirconium are summarized in Table 2. Note that results from both the first and fifth pulse sequences are listed.

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Table 2 Oxygen storage capacities of fresh catalysts in first (fifth) pulse sequence (gmol O/g catalyst) Evaluation Temperature (~ Pr : Ce : Zr 0.50:0.50

Pd(wt%) 2.0

100

150

200

250

300

350

297(11) 209(75) 492(295) 851(476) 1048(661) 1026(882)

0.45 : 0.45 : 0.10 1.32

206(13) 232(35) 323(148) 503(283)

635(445)

646(610)

0.40:0.40:0.20

153(18) 213(41) 292(152) 406(248)

492(393)

545(545)

1.54

0.31 : 0.31 : 0.38 1.9

49(31)

74(24)104(61)194(145)

324(271)436(442)

0.62:0.38

66(44)

56(51)149(93)323(281)

598(582)

2.0

772(771)

For temperatures in the range of 100 to 250~ catalysts made from the praseodymiumcontaining mixed oxides with low levels of zirconium (_< 0.20mo1%) have significantly higher oxygen storage capacities in the first pulse sequence than the catalyst made from the commercial ceria-zirconia material (last entry in Table 2). The large difference between first and fifth pulse sequences in this temperature range reflects the relatively slow re-oxidation kinetics of praseodymia. As the level of zirconium increases in the praseodymium-containing mixed oxides, the oxygen storage capacity in the first pulse sequence at low temperature decreases, as does the difference between the first and fifth pulse sequences, apparently because the fraction of Pr +4 decreases [4]. At temperatures higher than about 350~ praseodymia becomes less useful for oxygen storage than ceria because of the lower thermodynamic stability of Pr+4than C e +4. In an attempt to further optimize praseodymia-ceria, a few yttrium-containing compositions were also prepared since yttrium is known to improve the oxygen storage properties of ceriazirconia [5]. The result for a catalyst made from praseodymium-cerium mixed oxide with 20mo1% Y is shown in Table 3 together with that of the catalyst made from the corresponding zirconium-containing mixed oxide, taken from Table 2. Although the yttrium-containing mixed oxide provides somewhat less capacity at low temperatures, it has much more capacity at higher temperatures. Table 3 Oxygen storage capacities of fresh catalysts in first (fitlh) pulse sequence (gmol O/g catalyst) Evaluation Temperature (~ P r : Ce : Z r : Y Pd(wt%) 100 0.40:0.40:0.20 0.40:0.40

1.54 :0.20 2.62

150

153(18) 213(41) 106(14)

200 292(152)

108(26) 230(94)

250

300

350

406(248)

492(393)

545(545)

616(378)

919(632)

957(799)

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Finally, mixed oxides were prepared with an even lower concentration of zirconium (5mo1%) as well as with calcium. Results of the evaluation of catalysts made from them and also the mixed oxide made with a low yttrium concentration (5mo1%) are shown in Table 4. The results for the catalysts made from pure praseodymia-ceria and the commercial ceriazirconia, taken from Table 2, are repeated for ease of comparison. (The Pd loading in all of these catalysts is 2wt%.)

Table 4 Oxygen storage capacities of fresh catalysts in first (filth) pulse sequence (gmol O/g catalyst) Evaluation Temperature (~ Pr : Ce : Zr : Y : Ca

100

150

200

250

300

350

0.500:0.500

297(11) 209(75) 492(295) 851(476) 1048(661) 1026(882)

0.475:0.475:0.05

171(8) 116(39) 363(224) 623(381)

815(569) 824(766)

186(8) 112(28) 363(221) 699(419)

955(614)

941(822)

68(18) 193(6)

108(49) 542(265)

902(474)

927(690)

66(44)

149(93)323(281)

598(582)

772(771)

0.475:0.475

:0.05

0.475:0.475 0.620:0.380

: 0.05

56(51)

As before, the catalyst made from pure praseodymia-ceria has the largest capacity, and all three stabilizers (Zr, Y, and Ca) produce about the same reduction in capacity at the 5mo1% level. In the case of Zr, concentrations of 10 and 20mo1% produce a progressive reduction in capacity mainly at higher temperatures (> 250~ relative to 5mo1%. The effect of high-temperature aging in automotive exhaust on some of the catalysts was simulated by heating them at 1050~ for 12h in a mixture of gas, which was alternatively oxidizing and reducing (redox aging) [2]. The evaluation of oxygen exchange properties was performed as before, using the pulsed CO/O2 method, but due to an overall reduction in oxygen storage capacities, the evaluation was commenced at 200~ rather than 50~ The results are shown in Table 5 together with additional values taken at 500~ Still, the catalyst made from pure praseodymia-ceria has generally the largest capacity, followed by the catalysts made from praseodymia-ceria materials with 5mo1% Y, Zr, and Ca (in that order), then 20mo1% Y, and finally, 10mol% Zr. In the last instance, only at 350~ does the catalyst made from the zirconium-containing praseodymia-ceria mixed oxide have a higher capacity than that made from the commercial ceria-zirconia.

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Table 5 Oxygen storage capacities of redox-aged catalysts in first (filth) pulse sequence (~tmol O/g catalyst) Evaluation Temperature (~ Pr : Ce : Zr : Y : Ca

Pd (wt%)

250

300

350

500

0.500:0.500

2.0

128(49)

279(59)

823(199)

778(669)

0.475:0.475:0.05

2.0

142(15)

166(NA)

546(123)

626(499)

2.0

169(25)

199(65)

695(144)

785(653)

2.0

51(0)

91(43)

471(50)

697(564)

1.32

73(18)

97(43)

343(104)

430(363)

2.62

77(17)

100(33)

382(104)

694(621)

2.0

85(16)

102(17)

103(57)

856(806)

0.475:0.475

:0.05

0.475:0.475

:0.05

0.450:0.450:0.100 0.400:0.400

:0.200

0.620:0.380

The specific surface areas of the various catalysts aider redox aging are shown in Table 6. Again, the positive effect of zirconium on surface area is apparent, but its magnitude at the lowest concentration, 5mo1%, is small, and it would seem to be more than offset by the larger negative effect of zirconium on oxygen storage capacity. Yttrium appears better in this regard while calcium is worse. In no case is the surface area less than 1/4 that of the catalyst made from commercial ceria-zirconia. Table 6 Specific surface areas of redox-aged catalysts Pr : Ce : Zr : Y : Ca

Pd (wt%)

BET surface area (m2/g)

0.500:0.500

2.0

2.0

0.475:0.475:0.05

2.0

2.3

2.0

2.8

2.0

1.8

1.32

2.8

2.62

2.4

0.425 : 0.425 : 0.150

1.8

5.8

0.400:0.400:0.200

1.54

7.4

2.0

7.0

0.475:0.475

:0.05

0.475:0.475

:0.05

0.450:0.450:0.100 0.400:0.400

0.620:0.380

:0.200

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4. CONCLUSIONS

In summary, a method for preparing high-surface-area praseodymia-ceria-based mixed oxides has been demonstrated, and model catalysts made from the materials produced have been shown to exhibit oxygen storage properties at low temperature that are superior to those of catalysts made from commercial state-of-the-art ceria-zirconia. Although these mixed oxides are not quite as stable as ceria-zirconia with respect to maintenance of surface area or relative oxygen storage capacity after severe high-temperature redox aging, they appear to be viable materials for use in automotive-exhaust catalysts. REFERENCES

1. M. Yu. Sinev, G. W. Graham, L. P. Haack, and M. Shelef, J. Mater. Res., 11 (1996) 1960. 2. H.-W. Jen, G. W. Graham, W. Chun, R. W. McCabe, J.-P. Cuif, S. E. Deutsch, and O. Touret, CataL Today, 50 (1999) 309. 3. C. K. Narula, L. P. Haack, W. Chun, H.-W. Jen, and G. W. Graham, J. Phys. Chem. B, 103 (1999) 3634. 4. Note: The variations in Pd loading should not affect these and subsequent comparisons since (a) PdO, which may also contribute to oxygen storage capacity, has already been substantially reduced during the lowest-temperature measurement, and it does not re-form under these conditions, and (b) we have found that oxygen storage capacities of model catalysts made from the commercial ceria-zirconia, for example, are not very sensitive to Pd dispersion for the types of loadings and aging conditions used here. 5. H. Tanaka and M. Yamamoto, SAE Paper 960794 (1996).

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Studies in Surface Science and Catalysis 130

A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors)

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9 2000 Elsevier Science B.V. All fights reserved.

Improvement of three-way catalytic performance by optimizing ceria and promoters in Pd-only catalyst prepared by sol-gel method H.-S. Soa, O-B. yanga, D. H. Kimb and S. I. Woob aSchool of Chemical Engineering and Technology, Chonbuk National University, Chonju, Chonbuk 561-756, Korea, E-mail" [email protected] bDepartment of Chemical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Taejon 305-701, Korea Pd-only three-way catalysts containing bulk ceria and/or stabilized ceria (CeO2-ZrO2) and promoters such as V and Zr were prepared by sol-gel method and characterized by threeway catalytic performances test, X-ray diffraction, BET surface area and pore size distribution, temperature-programmed reduction and oxygen storage capacity measurement. It was found that bulk and stabilized ceria should be physically mixed to improve the three-way catalytic performance. Optimally formulated physical mixture catalyst showed excellent activity and thermal stability even after aging at 1000~ for 50 h, which seemed to be ascribed to the significant suppression of ceria sintering by anchoring effect, and to the separation of easily agglomerating ceria from palladium by physical mixing. 1. INTRODUCTION Ceria is one of major components in current three-way catalyst (TWC). Its major roles in the catalyst are the stabilization of the active metal and the alumina support, the promotion of the water-gas shit~ reaction and the enhancement of oxygen storage capacity (OSC), thus enlarging the air/fuel window [ 1]. However, it drastically reduces the OSC and the catalytic performance by sintering at high temperature [2]. In our previous work, Pd-only three-way catalyst prepared by sol-gel method showed a good thermal stability, high low-temperature activity and strong SO2 resistance [3]. It was also reported that the optimally formulated sol-gel catalyst, Pd-V-Zr-A1203 (PVZA), was not nearly deactivated up to 800~ and kept its low-temperature activity even up to 900~ However, because of the narrow air-to-fuel (A/F) window of the PVZA catalyst, it is required

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to introduce the efficient and thermally stable CeO2-based mixed oxides to enlarge the A/F

window and to improve the three-way catalytic performance. In this study, we investigate the effect of ceria and promoters in Pd-only TWC on the three-way catalytic performance and the thermal stability. Also the catalysts were characterized with X-ray diffraction (XRD), BET surface area and pore size distribution, temperature-programmed reduction (TPR) and OSC measurement 2. EXPERIMENTAL All of the catalysts containing 1 wt% palladium prepared by sol-gel method in pH 10. Aluminum isopropoxide (All, Aldrich) was dissolved in distilled and de-ionized water by H20/All molar ratio of 100. Then NH4OH as a hydrolysis and condensation catalyst was added to the A l l solution. For the preparation of Pd(1 wt%)-V(2 wt%)-Zr(10 wt%)-A1203 catalyst which is abbreviated to PVZA in this paper and ceria incorporated PVZA (Pd(1 wt%)-V(2 wt%)-Zr(10 wt%)-Ce(29 wt%)-A1203), acetylacetonate form precursors of Zr, V and Ce in acetone were sequentially mixed with the solution and then stirred at 50~ for 1 h per promoter. Finally, palladium acetylacetonate (Pd(AcAc)2, Aldrich) as a palladium precursor in acetone was added and stirred at 50~ for 4 h. Fresh catalyst was obtained by drying at 110~ for 24 h and calcining at 500~ for 5 h after removing the solvent. Aging of the catalyst was carried out in the simulated automotive exhaust gas at 1000~ for 2 h or 50 h. Bulk ceria (b-Ce) was prepared by sol-gel method using a precursor of cerium hydroxide

(Ce(OH)4), Aldrich). Similarly, zirconia incorporated ceria compound, CeO2-ZrO2 which was denoted as a stabilized ceria (s-Ce) was prepared by the same sol-gel method except adding the zirconium acetylacetonate solution in ceria hydroxide aqueous solution with a Ce/Zr mole ratio of 2.3. PVZA, s-Ce and b-Ce were mixed at an appropriate ratio and ground in a mortar to prepare the physical mixture catalysts that are represented as x% PVZA + y% s-Ce in case of PVZA mixed with s-Ce in a weight ratio of x/y. Three-way catalytic activity was measured with simulated automotive exhaust gas containing 6000 ppm CO, 1500 ppm NO, 500 ppm C3H6, 3000 ppm H2, 6000 ppm 02, 13% H20 and N2 balance in a quartz reactor at a gas hourly space velocity of 72,000 h ~. TPR was performed at a ramping rate of 10 K/min with 5% H2 in N2. After the catalyst was reduced by Hz at 700 K for 2 h, OSC was measured by injection of 0.1 ml 02 pulses until no more adsorption of 02. XRD experiment was carried out with a scan speed of 4~

in the

target of Cu K~ (~.=1.540598 A) operated at 40 kV and 45 mA. BET specific surface area, pore size distribution and pore volume were measured by nitrogen adsorption isotherm at 77 K in a BET equipment (ASAP 2010C, Micromeritics).

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3. RESULTS AND DISCUSSION The surface areas and pore volumes of the catalysts were summarized in Table 1. Surface area of bulk ceria (b-Ce) and stabilized ceria (s-Ce) is around 30 m2/g, which is less

than those of the others as shown in Table 1. Accordingly, the surface areas of the physically mixed catalysts with ceria were drastically decreased as increasing the ratio of ceria. However, OSC of the catalyst was significantly increased as increasing the ceria contents as shown in Table 2. In the physically mixed catalysts, the amount of OSC increasing by adding of s-Ce was much higher than that by adding of b-Ce. TPR spectra of various catalysts were shown in Fig. 1. Hydrogen uptakes in TPR spectra are ascribed to the reduction of oxidized state of PdO and CeO2 to reduced state of Pd and

Ce203,respectively.

Table 1 BET surface area, pore volume and average pore radius of the catalysts Surface Pore Catalyst area volume (m2/g) (cm2/g) Pd-AI203 315.0 0.48 PVZA (fresh) 324.8 0.54 PVZA (aged, 2 h) 119.2 0.44 Pd-V-Zr-Ce-A1203 259.4 0.46 b-Ce 37.5 0.12 s-Ce 31.1 0.07 90%[70% PVZA+30% s-Ce] + 10% b-Ce (fresh) 203.0 0.36 90%[70% PVZA+30% s-Ce] + 10% b-Ce (aged, 50 h) 56.7 0.27 Table 2 Oxygen storage capacities of various catalysts Catalyst

OSC x 105 (mol-02/g-catal.)

PVZA (fresh) PVZA (aged, 50 h) Pd-V-Zr-Ce-A1203 90% PVZA + 10% s-Ce 80% PVZA + 20% s-Ce 70% PVZA + 30% s-Ce 90%[70% PVZA + 30% 90%[70% PVZA + 30% 70%[70% PVZA + 30% 50%[70% PVZA + 30%

0.11 0.06 18.40 4.22 11.05 16.57 23.82 5.65 35.27 46.44

s-Ce] s-Ce] s-Ce] s-Ce]

+ + + +

10% b-Ce (fresh) 10% b-Ce (aged, 50 h) 30% b-Ce 50% b-Ce

Average pore radius (nm) 5.00 3.34 7.36 3.56 4.62 3.51 9.56

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G

IF E

C

A

..... ,, |

O

200

i 4OO

i 6OO

| 800

T e m p e r a t u r e

i

1OOO

("C)

Fig. 1. TPR profiles of PVZA (A), 90%[70% PVZA + 30% s-Ce] + 10% b-Ce (fresh) (B), 90%[70% PVZA + 30% s-Ce] + 10% b-Ce (aged, 50 h) (C), Pd-V-Zr-Ce-A1203(fresh) (D), Pd-V-Zr-Ce-A1203(aged, 50 h) (E), b-Ce (F) and s-Ce (G). Therefore, the peak intensity of TPR spectrum is directly proportional to the amount of OSC of the catalyst. In TPR spectrum of s-Ce (Fig. 1 (G)), the strong peak around 490~ medium peak around 845~

and

result from the reduction of surface ceria and bulk ceria,

respectively [1 ]. There is most of bulk oxygen and small amount of surface oxygen in b-Ce as shown in Fig. 1 (F). Physically mixed catalyst contains both surface and bulk oxygen as shown in Fig. 1 (B). However, there is not much oxygen stored in ceria in the Pd-V-Zr-CeA1203, which shows the strong reduction peak of PdO to Pd around 140~ A little amount of oxygen was stored in PVZA catalyst. This result suggests that stabilized ceria which is homogeneous solid solution of Ce0.7Zr0.302 is essential for the enhancing the amount of surface oxygen which may be more easily used as an oxygen atom than the bulk oxygen strongly bounded in bulk ceria. Fornasiero et al. reported that the insertion of zirconia into the CeO2 lattice induced formation of defective sites also in the bulk and resulted in the enhancement of OSC [4]. XRD spectra of fresh and aged catalysts are shown in Fig. 2 and Fig. 3, respectively. In the aging of the Pd-A1203 catalyst, highly dispersed palladium and ],-A1203 was significantly sintered to large crystalline PdO and c~-A1203 and 0-A1203, respectively, resulting in the severe decreasing of surface area. However, aged PVZA was not much sintered even after aging, which might be ascribed to the role Zr as a promoter to enhance the thermal stability of Pd. Even after aging at 1000~ for 50 h, relatively high OSC was kept in the physically mixed catalyst in spite of the significant sintering and large decreasing of the

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A1

v

9

I (7-AlaO3), 9(CeO a), 0 (Zr~ a), * (PdO)

e (~,-AI=03),0 (O-AI=O3), Q (~-A1203),@ (ZrO 2), ,k (PdO),A (CeO a)

m

r r

.5 c

A

10

20

30

40

20

50

60

70

80

Fig. 2. XRD spectra of Pd-A1203(fresh) (A), PVZA(fresh) (B), Pd-V-Zr-Ce-A1203 (fresh) (C) and 90%[70% PVZA+30% s-Ce] + 10% b-Ce (fresh) (D).

9

10

2o

30

i

.

,0

i

60

,

i

,

80

2o

Fig. 3. XRD spectra of Pd-A1203 (aged) (A), PVZA (aged) (B), and 90%[70% PVZA + 30% + s-Ce] + 10% b-Ce (aged) (C).

surface area upon aging, which accounted for its highest activity before and after aging as shown in Fig. 4. This excellent activity and thermal stability of the physically mixed catalyst upon aging seems to be attributed to the significant suppression of ceria sintering by anchoring effect which results from sol-gel method by adding the zirconia, and to the separation of easily agglomerating ceria from palladium by physical mixing. Ts0 indicates the temperature at which the conversion of each reactant reaches 50%. The physically mixed catalyst showed higher activity than ceria incorporated PVZA, Pd-V-Zr-Ce-A1203 and conventional Pt-Rh/A1203 three-way catalyst. Especially, Pd-V-Zr-Ce-A1203 catalyst was drastically deactivated upon aging as shown in Fig. 4. Therefore, ceria should be physically mixed to enhance OSC and thermal stability of Pd-only three-way catalyst. Recently, extensive researches are focused on the catalysts show the high activity at low temperature because most pollutants of the automotive emission occurred during the cold-start [5]. This physical mixture of PVZA, bulk ceria and stabilized ceria showed quit high activity at low temperature because of the enough OSC contained in bulk and stabilized ceria. 4. CONCLUSIONS Physical mixing of bulk and stabilized ceria in the Pd-only TWC is essential to improve the three-way catalytic performance and the thermal stability. The physical mixture of PVZA, bulk ceria and stabilized ceria accomplished the excellent three-way catalytic

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化

450

400 350

f

" [1-'~a34

300

~

250 200

INO

I~

359

342 342 300

351 EEE3

CO

jHC

334 331 I

284

~ 25R ~

(A)

~ i

(B)

~255

(C)

265 ~

243 ~

(D) (E) Catalysts

Fig. 4. Light-off temperature (Ts0) of NO, CO and

c 3 n 6 over

(F)

I

(G)

Pd-AI203 (A), PVZA(fresh) (B),

PVZA(aged, 50 h) (C), Pd-V-Zr-Ce-Al203(fresh) (D), Pd-V-Zr-Ce-A1203(aged, 50 h) (E), 90%[70% PVZA + 30% s-Ce] + 10% b-Ce(fresh) (F) and 90%[70% PVZA + 30% s-Ce] + 10% b-Ce(aged, 50 h) (G). performance by the high activity at low temperature and by the improved thermal stability at high temperature, which seemed to be attributed to the significant suppression of ceria sintering by anchoring effect, and to the separation of easily agglomerating ceria from palladium by physical mixing. ACKNOWLEDGEMENT This research was funded by a national project granted from Ministry of Commerce, Industrial and Energy and Ministry of Science and Technology (1995 - 1998). REFERENCES 1. K.C. Taylor, Catal. Rev. Sci. Eng., 35 (1993) 457. 2. S.J. Schmieg and D.N. Belton, Appl. Catal. B, 6 (1995) 127. 3. J. Noh, O-B. Yang, D. H. Kim and S. I. Woo, Catalysis Today (1999) in press. 4. P. Fomasiero, R.D. Monte, G.R. Rao, J.Kaspar, S. Meriani, A. Trovarelli and M. Graziani, J. Catal., 151 (1995) 168. 5. J.C. Summers and W.B. Williamson, in 9Environmental Catalysis, ACS symposium series, Vol. 552, eds. J.N. Armor (American Chemical Society, 1994) p. 94.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Flue gas NOx removal by SCR with NHa on CuO/AC at low temperatures Z. P. Zhu a, Z. Y. Liu a, S. J. Liu a, H. X. Niu a, T. D. Hu b, T. Liu b, and Y. N. Xian b a State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, P.R.China b State Key Laboratory of Synchrotron Radiation, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, 100039, P.R.China ABSTRACT NO reduction with NH3 over activated carbon supported copper oxide (CuO/AC) catalyst was studied at low temperatures (30~250~ The attention was focused on the effects of preparation parameters on catalyst structure and activity. The catalyst shows high activity for NO reduction with NH3 in the presence of 02 at temperatures above 180 ~ Some amount of Cu20 exists in the catalyst and results in a low initial activity, but it is easily oxidized into active CuO by 02 during the NO-NH3-O2 reaction. The catalytic activity is influenced by the calcination temperature and Cu loading, and the catalyst with 5wt% Cu loading and calcined at 250~ shows the highest activity. The lower activities of the catalysts with higher Cu loadings and/or calcined at higher temperatures are mainly due to aggregation of copper species. 1. INTRODUCTION Simultaneous removal of SOx and NOx from stationary sources has received more and more attention because advantages in economy and in removal efficiency in contrast with the combined removal techniques. A suitable location for NOx and SOx removal systems is at the exit of the particulate control device, where the flue gas temperature are generally around 150 ~ CuO/A1203 and activated carbon (AC) have been developed for simultaneous SOx and NOx removal using selective catalytic reduction (SCR) of NO with NH3 and the oxidation of SO2[ 1]. However, CuO/A1203 catalyst shows high activity only at temperatures of above 300 ~ while the AC only at temperatures of less than 100 ~ In contrast, at temperatures of about 150 ~ activated carbon supported copper oxide (CuO/AC) catalyst has high activity for both NO reduction with NH3 [2,3] and oxidation of SO2 [4], which makes it possible to be used in simultaneous SOx and NOx removal at low temperatures (<250~ However, the previous reports [2,3] only dealt with activity measurements of CuO/AC catalyst for the SCR reaction. The present paper focuses on the relationship between the preparation parameters and the activities of CuO/AC catalyst for the NO reduction with NH3.

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2. EXPERIMENTAL

2.1 Catalyst preparation The support, activated carbon (AC), was prepared from a commercial coal-derived semicoke (Datong Coal Gas Co., China) through steam activation at about 900"(2. Before being used, the AC was generally preoxidized with concentrated HNO3 (3ml/gAC) at 60~ for lh, followed by filtration, fully washing with distilled water and drying at 120~ for 5h. The AC supported copper oxide catalysts (CuO/AC) were prepared by pore volume impregnation of the AC with aqueous solution of Cu(NO3)2. 3H20 (>99.5%). After the impregnation, the catalysts were dried overnight at 50 ~ and then at 120 ~ for 5h, followed by calcination in Ar stream for 2h at desired temperatures. 2.2 Activity test Activity tests of the catalysts for the NO+NH3+O2 reaction were carried out in a fixed bed quartz reactor. 0.5%NO in Ar (NO/Ar), 0.5%NH3 in Ar (NH3/Ar) and 15%02 in Ar (O2/Ar) were used as the source of the flue gas components, and Ar was used as a balance gas. Gas flow rates were controlled by mass flow controllers for NO/Ar and NH3/Ar and by rotor flow controllers for O2/Ar and Ar. In all the test runs, the total gas flow rate of 300 ml/min was used. Prior to being fed into the reactor, the reaction gases were mixed in a chamber filled with glass wool. The concentrations of NO, NO2, and 02 at both the inlet and the outlet of the reactor were continuously analyzed by an online Flue Gas Analyser (KM9006 Quintox, Kane International Limited), which is equipped with NO, NO2, CO, and 02 sensors. 2.3

XAFS measurement To determine the chemical forms of the copper species in the CuO/AC catalysts, X-ray absorption fine structure (XAFS) technique was used and measured on wiggler beam line 4W1B at the Beijing synchrotron radiation facility. A double crystal S i ( l l l ) was used to monochromatize X-rays from the 2.2GeV electron storage ring, and the Cu K-edge absorption spectra were recorded in the transmission mode. Fourier transformation was performed on the k3 - weighted EXAFS oscillation in the range of 3 - 14 Al. 3. RESULTS AND DISCUSSION 3.1 EXAFS characterization Fig. 1 illustrates the radial structure functions (RSF) of the catalysts calcined at 180, 250, 350 and 450~ The RSF spectrum of the un-calcined sample is also shown as a comprison. The un-calcined sample and the catalyst calcined at 180~ show a strong peak at 1.56 .~, which is associated with the Cu-O distance in CuO. With increasing calcination temperature this peak becomes weaker and shifts to 1.6 A, and a new peak appears at about 1.4 A and becomes stronger. The latter peak is relevant to the Cu-O distance in Cu20. Additionally, a strong peak apears at about 2.1 A for the 3500C and 4500C calcined samples. This peak is associated with the Cu-Cu distance in metallic copper. These observations indicate that with increasing calcination temperature part of CuO is reduced by the activated carbon into Cu20

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E

~ i

k

E P-/~ / ~ \ A

.o

~,l_/'-Jl

~~ 0

\\~

v~'~

"-------~

,,,-.,. _ . . _

~8o'c+

\. . . . . 2

+

,.,. / / ~ . . . . ' - ~ 4 5 0 *C

0

Drying~ 4

6

Distance (i)

Fig. 1 The Cu K-edge EXAFS RSF spectra of the CuO/AC catalysts with 5wt% Cu loading and calcined at different temperatures.

i

0

2

i

4

6

Distance (A) Fig. 2 The Cu K-edge EXAFS RSF spectra of the CuO/AC catalysts with different Cu loadings and calcined at 250 ~

and subsequently into metallic Cu. Interestingly, for the samples calcined at temperatures of below 2500C, the peaks in high-R region are very weak, which is attributed to the small particle size of copper species in the catalysts. On the other hand, the catalysts calcined at temperatures above 350~ show relatively strong peaks in the high-R region, which is attributed to large particle size of copper species in the catalysts. As a conclusion, the copper species loaded on the AC surface is reduced and aggregated at calcination temperatures above 350~ The RSF spectra of the catalysts with different Cu loadings are shown in Fig.2. All samples were prepared by calcination at 250~ The sample with Cu loading of 5wt% shows double peaks at about 1.4 A and 1.6 A and exihibits very low intensity in the high-R region, which indicates that part of CuO is reduced by carbon into Cu20 during the calcination and that both CuO and Cu20 are well dispersed on the AC surface. Unlike this, the samples with the Cu loading of 10wt% and 20wt% show three peaks at 1.56 A, 2.6 A and 3.0 A, which indicates that CuO species are hardly reduced but significantly aggregated. In addition, the aggregation becomes heavier with increasing copper loading. It may be resulted from multilayer deposition of copper species on the AC surface. 3.2 Effect of temperature Fig. 3 shows the NO conversion versus reaction temperature over the support AC and the CuO/AC catalyst, with 5wt% Cu loading and calcined at 250~ For the AC, NO conversion significantly decreases with increasing temperature. In contrast, the activity of the CuO/AC catalyst initially decreases with increasing temperature within 30-90~ and then increases

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greatly when temperature is over 90~ This suggests that the SCR reaction over the CuO/AC catalyst occurs at carbon sites at temperatures below 900C and mainly occurs at copper sites at temperatures above 90~ In the flue gas temperature range of 120-250 ~ the activity of the CuO/AC catalyst is much higher than that of the AC.

100

100

~

8O

80

.~

60

60

r

0

0

0

0

oo

o t

180"C i

oo

0o o ~176

-

Oo00

A

o l o50

o

i

---O-- AC

40

o 40 e,.)

9 Z 2o I

I

I

I

50

100

150

200

5 x

:

250

i

i

_

!

_

i

_

0

Temperature (~

120~

,

20

)

0

Fig. 3

f(~ O

I

I

I

100

200

I

300

t

I

400

!

500

Ymleon stream(rnm)

NO conversion versus temperature

Fig.4

over the AC and 5wt%CuO/AC catalysts. calcined at 250~ Reaction conditions:

over

Reaction conditions" 500ppm NO; 560ppn

500ppm NO; 560ppm NH3; 3.3 vol% 02;

NH3; 3.3 vol% 02; WHSV, 36000h -~.

Activity versus time and temperatur~ 5wt%CuO/AC

calcined

at

250 ~

WHSV, 10000h -1. The stability of the CuO/AC catalyst is also greatly influenced by the reaction temperature. Fig.4 shows typical results for 5wt%CuO/AC catalyst. At temperatures below 180~ the catalytic activity decreases with reaction time, however, at temperature of 180 ~ NO conversion is high and stable. This is possibly due to the formation of some special surface species of NO and/or NH3, which may be inactive at low temperatures and subsequently cover the active sites. 3.3 Effect of preparation parameters Fig. 4 also shows that the activity of the 5wt%CuO/AC catalyst is initially very low, it increases with reaction time and reaches the highest value in about 25rain. This phenomenon was also observed for other catalysts prepared, especially for those calcined at high temperatures (350~ 450~ The low initial activity of the catalyst may be derived from the existance of some reductive copper species in the prepared catalysts shown by EXAFS (Fig. 1). The reductive copper species are inactive for the SCR reaction. During the reaction they may be oxidized by 02 (and/or NO) into active CuO, which hence results in the gradually

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100

~.

80

~ 6o ~ 4o 20

0

1

1

I

I

2

3

4

0

1

5

6

0

Distance (A) Fig. 5

The Cu K-edge EXAFS RSF spectra

5

10

15

20

Cu loading (wt%) Fig. 6

The effect of the Cu loading on

of the CuO/AC catalysts with 5wt% Cu the activity of the CuO/AC catalyst. loading and calcined at 250~

(a) original, (b Reaction

conditions:

500ppm

NO;

after two hours reaction of NO-NH3-O2 at 180 560ppm NH3; 3.3 vo1%O2; WHSV, ~ (c)after preoxidization by 10vol%O2/Ar 36000h -l" reaction temperature, 180~ 9 at 180~ for 30 min.

catalyst, calcined at 250~

increased activity. This suggestion is supported by EXAFS results as shown in Fig.5. The 5wt%CuO/AC catalyst shows double peaks at 1.4 i and 1.6 i , indicating a coexistance of CuO and Cu20. But, after the reaction, it shows a single peak at 1.56 i , indicating that only CuO exists in it. Additionally, the catalyst preoxidized by 10vol%Oz/Ar a t 180~ for 30min contains only CuO and hence shows a high initial activity. Therefore, to obtain a stable CuO/AC catalyst with high initial activity, a preoxidization is needed. Fig. 6 shows the dependence of catalytic activity on Cu loading. Within the studied Cu loading range (1-20wt%), the sample with 5wt% Cu exchibits the highest catalytic activity. For the catalysts with copper loadings of less than 5wt%, their low activities may be due to the low coverage of copper on the AC surface. For the catalysts with copper loading higher than 5wt%, the aggregation of copper species occurs as suggested by EXAFS results (see Fig. 2) and thus results in the declined activities. The effect of calcination temperature on the catalytic activity was also studied. It was found that the catalyst calcined at 250~ shows the highest activity, with a complete NO conversion under employed reaction conditions, while the catalysts obtained at 180~ 350 ~ and 450~ show relatively low activities, especially the last one. The calcination temperature of 180~ may be too low to allow complete decomposition of the impregnated copper nitrate. The calcination temperatures of 350~ and 450~ result in the formation and aggregation of the reductive copper species, as suggested by EXAFS in Fig. 1, and hence the low activities.

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ACKNOWLEDGEMENTS

The authors gratefully acknowledge the financial support from the Natural Science Fund of China (29633030, 29876046), Shanxi Natural Science Fund and State Key Laboratory of Coal Conversion. REFERENCES

1. H.Bosch and F. Janssen, Catal. Today, 60 (1987) 115. 2.

L. Singoredjo, M. Slagt, J. Van Wees, F. Kapteijn, and J.A. Moulijn, Catal. Today, 7 (1990) 157.

3.

F. Nozaki, K. Yamazaki, and T. Inomata, Chem. Lett. (1977) 521.

4.

S. Liu, Z. Liu, Z. Zhu, H. Niu, 16th. North Am. Catal. Soc. Meet., Boston, Massachusetts, 1999, PI-038.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Low temperature monolithic SCR catalysts for tail gas treatment in nitric acid plants J. Blanco, a P. Avila, a L. Marzo, b S. Suarez a and C. Knapp a a Instituto de Catfilisis y Petroleoquimica, Camino de Valdelatas s/n, 28049 Madrid, Spain, Tel: 34-91 585 4602, Fax: 34-91 585 4789, e-mail" [email protected] b ESPINDESA, Arapiles 13,28015 Madrid, Spain The performance of new monolithic catalysts for low temperature Selective Catalytic Reduction of nitrogen oxides from nitric acid plants has been studied. Low temperature and low pressure drop offer a lower cost solution for NOx emission control. The porous structure of the catalyst has been improved, thus increasing its effectiveness factor by 5%. This new catalyst concept allows to achieve NOx conversions of about 90 %, with very low NH3 emissions at 180~ and GSHV of 5900 h l (NTP) with a pressure drop of only 1 cm.2 o m -I

1. INTRODUCTION Catalysts containing copper oxide stabilised with nickel oxide and supported on ),-alumina have given excellem results in its industrial application for the Selective Catalytic Reduction of nitrogen oxides from nitric acid plants tail gas [1]. These catalysts have been generally used as pellets, defining their sphere-size to minimise the pressure drop without increasing too strongly the internal diffusion limitations. However, a more convenient solution is the use of catalysts in monolithic form, which reduce the pressure drop by about two orders of magnitude, compared to that produced by catalysts with conventional forms such as pellets

[21.

As the SCR reaction rate is high, the internal diffusion resistance in the monolithic pore structure -total pore volume and pore size distribution- has been shown to have a remarkable influence on the catalysts performance at temperatures above 300~ [3]. The aim of this work has been to prepare monolithic CuO-NiO/y-alumina catalysts, in which the reduction of the internal diffusion resistance allows operation at low temperature (180-230~ The influence of various parameters related to the textural properties and operating conditions on their catalytic behaviour were studied. Evaluation of the selected catalyst performance in pilot-scale with real gases was carried out. 2. EXPERIMENTAL

2.1. Monolithic catalyst preparation Monolithic supports were manufactured in an industrial size of 500x87x87 mm 3 by extrusion of doughs prepared by kneading of bohemite powder (as y-alumina precursor), silicates as binders and temporal additives. The monolith shaped dough was subsequently

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dried and heat treated at 500~ for 4 hours in air. Non-permanent additives were used in order to obtain supports with different textural properties. Catalysts with 5.2 wt% CuO and 0.5 wt% NiO were prepared by impregnation with a copper and nickel nitrate solution and subsequently treating at 500~ in air. The monolithic catalysts had a square-cell size of 2.5 mm, wall thickness of 0.9 mm, and geometric area of 950 mEm"3.

2.2. Catalytic activity tests The activity of the catalysts in the selective reduction of nitrogen oxides was studied using a glass reactor and synthetic gas mixtures with a typical nitric acid plant tail gas composition: [NO] = 500 ppm, [NO2] = 500 ppm, [02] = 4 vol.-%, [N2] = balance, adding ammonia as a reductant: [NH3] = 500-1000 ppm. The gas mixture to be treated was at 170~ this temperature was increased by heat-exchange with the outlet gases up to 180-230~ simulating operation of an industrial unit. The NO and NO2 concentrations were determined with a Signal chemiluminescence analyser (model 4000), and the determination of NH3 was carried out with an ADC IR spectroscopy analyser (Lufl type Infra-red Gas Analyser). 2.3. Characterisation techniques The catalysts textural properties were characterised by Nitrogen adsorption/desorption isotherms and by Mercury intrusion porosimetry analyses. The nitrogen isotherms were obtained using a Micromeritics device (model 1320 ASAP), outgassing the samples overnight at 250~ to a vacuum of <10 "4 torr to ensure a dry clean surface. Mercury intrusion measurements were performed with a Fisons Instruments porosimeter (model Pascal 140 and 240), after drying the samples in an oven at 110~ overnight to ensure that the materials were completely dry. 3. RESULTS AND DISCUSSION The influence of the textural characteristics of the monolithic catalysts on their activity at low temperature (180-230~ was studied, observing that the pore structure of the support plays a key role in the process. These results allowed improvement of the catalysts efficiency by decreasing the internal diffusion limitations.

3.1. Estimation of effectiveness factors Catalysts with different pore size contributions were prepared in order to improve their internal effectiveness factors, q, that was estimated as a function of the Thiele modulus, ~b,for a flat porous plate: tanh~b r/ . . . . . .

(1)

where the Thiele modulus, ~b, is:

(k.pp

(2)

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where L is the characteristic length (half wall monolith thickness = 0.45 mm), k is the kinetic constant, pp is the monolith density, and De is the effective diffusion coefficient. As the catalysts were prepared following the same procedure and had the same final composition, the same value of k was taken for both cases. The kinetic constant at 180~ was estimated from the kinetic equation of the catalyst, obtained operating in differential regime

[4]:

log k =-0.0068 T + 4.7823

(3)

where T is the operating temperature 9At T = 453 K (180~ the value of the kinetic constant was k = 50 cm3gls l . The monolith density, pp, was estimated from mercury intrusion porosimetry results. The effective diffusion coefficient, De, was estimated following Smith [5]" De =

O c 9

P

(4)

where Cp is the porosity of the catalyst, r is the tortuosity factor, and Dc is the combined diffusion coefficient, defined as follows, if there are more than one pore size contributions:

Dc : E

(5)

z,./v .........

.4_

DNO-N, (Dx), where D NO_N~is the molecular diffusion coefficient of nitrogen oxide in nitrogen, and

(Dx)i is the Knudsen diffusion coefficient for the pores size contribution i. Vt is the pore volume of contribution i, and V is the total pore volume, both estimated from nitrogen adsorption/desorption and mercury intrusion porosimetry data. The value of the tortuosity factor was considered to be r = 4, as indicated by Smith [5] for this type of porous ceramic material. DNO_N2was estimated by the Chapman-Enskog equation [5]:

DNO/N~ = 1.8583 91 0

-3

T 3/z 9

_-- -I 1_MNo

.Jf.

11~ MN~

9

Pt

9 O ' N2O / N

(6)

2 " ~"~ N O / N 2

where T is the temperature = 453 K (180~ Pt is the total pressure of the gas mixture Pt = 1.18 atm, MNO and M N, are the molecular weights of the nitrogen oxide and nitrogen (30.01 and 28.02, respectively), f2NO/N~ is the collision integral (0.870), which is a function of

k B 9T/eNO/N~ ; CrNO/N2 (3.575 A) and CNO/N~ (1.440"1025 J) are parameters of the potential

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Table 1 Porous structure data of the monolithic catalysts Catalyst

A

Pore size range, nm 8-12 12-200 200-2000 8-12

Mean radius F, nm 10 106 1000 10 106 1000

12-200

200-2000

(D r),, c m 2 s-I

V,, cm3Vg"1

Vp/V

0.377 0.399 3.768 0.377 0.399 3.768

0.131 0.220 0.099 0.102 0.242 0.287

0.291 0.489 0.220 0.162 0.384 0.455

function of Lennard-Jones for the couple NO]N2, and kB is the Boltzman constant = 1.3805.1023 JK l. In these conditions, a value of DNO/N 2 = 0.357 cm 2 s ~ was obtained. The Knudsen diffusion coefficient was estimated by assuming that the pores behave as cylinders with a mean radius F"

(DK)' = 9"70"103 "~"

I

T

(7)

MN ~

where ~ is the pore mean radius of the selected pore size range, determined by mercury intrusion porosimetry, T is the temperature in K, and M is the molecular weight of the diffusing molecule, NO. Taking into account three main contributions of pore sizes (8-12 nm, 12-200 nm and 2002000 nm) observed by mercury intrusion porosimetry, the data that are collated in Table 1 allowed the estimation of the combined diffusion coefficient, Dc, for both catalysts by using Eq. 5. From these data, the effective diffusion coefficient (De, Eq. 3), the Thiele modulus, (~b, Eq. 2), and the effectiveness factor (r/, Eq 1) were calculated. The results of the estimations for both catalysts are presented in Table 2. These results indicated that increasing the macro-pores contribution from 0.099 to 0.287 cm 3 g-i (Table 1), the effectiveness factor increases from 0.44 to 0.49.

Table 2 Parameters and results of the effectiveness factor estimation at 180~ Catalyst A B

ep 0.55 0.62

pp, gcm "3 1.18 0.99

Dc, cm2s-1 0.174 0.167

De, cm2s-1 0.024 0.026

r

,j

2.236 1.967

0.44 0.49

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化

100

[

=o

60

(D

> cO to

200

[

t~

150 40

100

x

0 Z

E Q.

/

20 0

,

160

'

50 I

180

l

I

200

'"

'

1

'"

o r "lZ

i

220

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Temperature, ~ Figure 1. NOx conversion and ammonia slip of two monolithic catalysts with internal effectiveness factor values of (El) 0.44, and (A) 0.49 estimated at 180~ Operating conditions: GHSV (NTP) = 5900 h "l, vl = 0.73 Nm s1, P = 120 kPa; feed: [NOx] = 1000 ppm, [NO/NOx] = 0.5, [NH3]/[NOx] = 1, [02] = 4 vol.-%, [N2] = balance.

3.2. Catalytic activity The activity studies indicated a remarkable benefit of reducing the internal mass-transfer resistance, as shown in Figure 1, where the catalytic behaviours of the two catalysts with estimated internal effectiveness factors of 0.44 and 0.49 (at 180~ are presented. Increasing the effectiveness factor by improving the catalysts macro-porosity gave rise to an increase of the NOx conversion, as well as a significant reduction in the ammonia slip over the studied operating temperatures (180-230~ 100 ~

=o

. m

I,,-

(1)

80

E E1

60

> to o

40

oZ

2o

m

100 50

0,2

0,4

0,6

0,8

o ZIZ Z

1

NHdNOx, molar ratio Figure 2. Influence of the [-NH3]/[NOx] molar ratio on NOx conversion and ammonia slip of a monolithic catalyst with an internal effectiveness factor of 0.49 estimated at 180~ Operating conditions: GHSV (NTP) = 5900 h l , vj = 0.73 Nms -1, P = 120 kPa; feed: [NOx] = 1000 ppm, [NO/NOx] = 0.5, [02] = 4 vol.-%, [N2] = balance.

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Studies at 180~ on the influence of the NOx/NH3 ratio of the selected catalyst (internal effectiveness factor at 180~ = 0.49) indicated a satisfactory behaviour (Figure 2). At a NH3/NOx ratio of 0.9 and GHSV = 5900 h l (NTP) a 90 % NOx conversion was reached, with only 3 ppm ammonia in the reactor outlet. The performance of the selected monolithic catalyst was evaluated in pilot-scale at the nitric acid production plant of SEFANITRO in Bilbao (Spain), achieving a 77% NOx conversion with an ammonia slip of 30 ppm, operating in the following conditions" T = 220~ GHSV(NTP) = 14120 h l, v~ = 1.0 Nm sl, P = 326 kPa; feed: [NOx] = 1000 ppm, [NO/NOx] = 0.7, [NH3]/[NOx] - 0.8, [ 0 2 ] -" 2 vol.-%, I N 2 ] = balance.

4. CONCLUSIONS The effectiveness factor of copper-nickel y-alumina supported monolithic catalysts has been improved by increasing the macro-pores contribution to the catalysts porosity. The optimised catalysts gave rise to a better NOx conversion at low temperature (180230~ as well as a significant reduction in the ammonia slip. Promising results were obtained by testing the new catalysts in a pilot-plant with real gases from a nitric acid production plant.

ACKNOWLEDGEMENTS We are grateful to the CICYT (Projects AMB97-0946 and 2FD-97-0035) and to the Comunidad Aut6noma de Madrid (Project 07M/0067/1998) for supporting this work. We thank also Dr. M. Yates for valuable discussions.

REFERENCES 1. J. Blanco, P. Avila and L. Marzo, Catal. Today, 17 (1993) 325-332. 2. S. Irandoust and B. Andersson, Catal. Rev. Sci. Eng., 30 (1988) 341-392. 3. P. Forzatti and L. Lietti, Heterogen. Chem. Rev., 3 (1996) 33-51. 4. J.M.R. Bias, Desarrollo de Catalizadores Monoliticos Basados en Oxidos de Cobre y Niquel para la Eliminaci6n de Oxidos de Nitr6geno, Doctoral Thesis, Universidad Complutense de Madrid, Madrid, 1997. 5. J.M. Smith, Chemical Engineering Kinetics, 3rd Ed., Mc Graw-Hill, New York, 1981.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Effect of La203 concentration in La2Oa-A1203 supports and P d / L a 2 0 3 - A I 2 0 3 catalysts in reduction of NO by H2 N.E. Bogdanchikovaa, A. Barrerab, S. Fuentes a, G. Diaz c, A. G6mez-Cort6s c, A. Boronin d, M. H. Farias a, M. Viniegra R. b aCentro de Ciencias de la Materia Condensada, UNAM, Apartado Postal 2681, 22800, Ensenada, B.C., M6xico bDepartamento de Quimica, UAM-Iztapalapa, C.P. 09340, M6xico D.F., M6xico ClFUNAM, A.P. 20364, 01000, D.F., M6xico dInstitute of Catalysis, 630090 Novosibirsk, Russia It was obtained that the selectivity of La203-A1203 oxides in the NO reduction by H2 depends on the La203 concentration and reaction temperature. The surface area as well as the structure of the supports is varied with the lanthana concentration. All mixed supports are highly selective to the production of N2 when the catalytic reaction is conducted at 973 K. La 3d5/2 binding energy measured for mixed supports is higher than that reported for La203 and it is comparable with value published for dispersed lanthanum phases in alumina. XRD data also show that lanthanum is highly dispersed in A1203. Lanthana concentration does not influence significantly on NO conversion with temperature neither for Pd catalysts nor for supports with an exception for 50 % LaEO3-A1203 support. Its activity is higher than activity of other supports. Application of coprecipitation method for preparation of La203-A1203 supports results in" l) significant enhance of N2 selectivity (at high temperatures) on La2Oa-A1203 supports and Pd/La203-A1203 catalysts and 2) increase of NO conversion on La203-A1203 supports compared with sol-gel method. 1. INTRODUCTION The addition of lanthanum to Pd/AI203 three-way catalysts improves their activity in the reduction of NO [ 1]. Usually Pd-lanthanum-alumina catalysts are prepared by impregnation of A1203 with lanthanum nitrate solutions and thereafter with Pd to obtain Pd/La203/A1203 catalysts [2,3]. In a previous work, we reported that Pd/La203-AI203 catalysts prepared by

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sol-gel method show high activity in the NO reduction by H2 and are highly selective to the production of NH3 [4,5]. At temperatures higher than 673 K moderate activity with predominant ammonia formation was detected for La203-A1203 support [5]. Comparison of data for reduction of lanthana in Pd/La203-A1203 and catalytic activity of La203-A1203 support suggests that reduced sites of lanthana are able to reduce NO in the presence of hydrogen. High ammonia selectivity of Pd/La203-A1203 catalyst was concluded to be due to high ammonia selectivity of La203-AI203 support. Coprecipitation is another known method for the synthesis of multicomponent homogeneous materials. It is simpler than sol-gel method. In the present work we study the effect of La203 concentration in La203-AI203 supports and Pd/La203-A1203, prepared by coprecipitation on the NO reduction by H2. 2. EXPERIMENTAL

2.1. Preparation Mixed La203-A1203 supports with varying concentration (2, 6, 15, 25 and 50 wt. % of La203) were synthesized by coprecipitation method. Lanthanum acetylacetonate was added to the solution of aluminum sec-butoxide in 1-butanol with following water addition. Dried samples were heated in N2 flow first at 523 K for 4 h and then at 723 K for 12 h. Preparation of another sample set included additional calcination in oven at 923 K for 4 h. A1203 support was prepared by the same method as La203-AI203 supports without Lacomponent addition. La203 support was prepared by calcination of lanthanum oxide carbonate at 923 K. Supports after calcination at 923 K were impregnated by water solution of palladium chloride, dried in N2 flow at 383 K for 12 h and then calcined at 923 K for 3 h. 2.2. Characterization Surface area measurements (SaET) were carried out by low temperature nitrogen adsorption in volumetric equipment Gemini 2600 from Micromeritics. Calculations were performed on the basis of the BET isotherm. Samples were pre-treated in argon at 473 K for 2h. X-ray diffractograms were recorded in a Philips X'pert diffractometer using the CuI~ (2, = 0.154 nm) radiation. Particle size was calculated from the full width at half maximum intensity of peaks by using the Scherrer equation and assuming shape factor value K = 0.9. X-ray photoelectron spectroscopy analyses were performed using an analysis chamber equipped with a Riber-CAMECA Mac-3 system with A1K~ radiation (hv = 1486.6 eV). Calibration of the energy position of peak maximums was made using the binding energy of carbon 1s peak at 284.8 eV. 2.3. Catalytic studies Reaction of NO with H2 was studied in a continuous flow microreactor Multipulse RIG 100, In Situ Research Instrument under atmospheric pressure. Sample weight was 100 mg.

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化

IO0

"1o 8

E u.I an

Or)

0

!

0

,

,

20

,

,

40

.

.

.

60

.

.

80

T

100

La203 concentration, wt. % Fig. 1. Specific surface area (BET) as a function of lanthana concentration prepared at 723 K (1) and at 973 K (2).

T~ature, K Fig. 2. NO conversion as a function of temperature for Pd catalysts with 0, 2, 6, 25, 50 % La203 (1-5) and supports with 0, 2, 6, 15 (710), 50 (6) % La203.

Pretreatment of catalysts before the catalytic experiments consisted in heating in H2 flow at 673 K for 1 h. The concentration of the reaction mixture in helium was 5% with NO/I-I2 molar ratio equal to 0.5. The flow rate of reaction mixture was 120 cm3/min (at room temperature and under atmospheric pressure). The reaction products were detected by gas chromatography using a Tremetrics Gas Chromatograph model 9001. 3. RESULTS AND DISCUSSION 3.1. BET surface a r e a (SBET) Figure 1 shows that SBETof LaEO3-AI203 supports prepared at 723 and 923 K depend on La203 concentration IFig. 1). In the range 5.8 - 25 wt. % of La203 SBETreaches a maximum 390 and 215 - 290 m g for preparation temperatures 723 and 923 K, respectively. Obtained results (Fig. 1) show that La203 is textural promoter in the range ca. 2-25 % and 2-15 % of lanthana for preparation temperatures 723 and 923 K, respectively. In contrast, lanthana is textural inhibitor for higher La203 concentrations.

3.2. X-ray diffraction (XRD) Diffraction patterns show a structure typical of )'-A1203 for supports with lanthana concentration from 0 to 15 wt. %. An average crystallite size for these supports is ca. 3.7 nm.

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XRD patterns of supports with 25 and 50 wt. % of La203 do not contain any well-pronounced peaks. This implies that particles of these samples are amorphous. On the other hand, the results of SBETmeasurement show that SBETof supports with La203 concentration < 25 wt. % are higher than SBET of supports with large La203 concentration (Fig. 1). Hence, according to SBET data, average particle size of supports with low La203 concentration is higher than that one of supports with large La203 concentration. These XRD and SaEx data can be observed when the particles of the supports with 25 and 50 wt. % of La203 are large but amorphous. Amorphous character of supports with La203 concentration > 25 wt. % could be a result of the disruption of y-AI203 spinel structure as suggested by Ledford et al [6]. The lack of peaks corresponding to lanthanum phases in XRD patterns of all lanthana-alumina supports suggests the presence of dispersed lanthanum species within the alumina. The same suggestion was made by Haack et al. [7]. 3.3. XPS data Maximum peak position for La 3d5/2 binding energy for La203 is at 835.1 eV. It was deconvoluted into two peaks at 833.9 and 835.6 eV those are typical for La203 and slightly (0.5 eV) shifted towards higher energy as compared with La203, respectively [8]. For La203-A1203 supports with 2, 6, 25 and 50 wt. % of La203, peak maximums for La 3d5/2 binding energies are in the range 835.8-836.1 eV with average at ca. 835.9 eV. That is 0.8 eV higher than were measured for pure La203. Thus, XPS spectra show that the state of lanthanum in mixed La203-A1203 supports differs from the state of lanthanum in La203. This La 3d5/2 binding energy is very close to value (836.2 eV) for dispersed lanthanum--containing phase in alumina after calcination at 873 K measured by Ledford et al. [6]. 3.4. Catalytic data Activity of Pd catalysts is significantly higher than that one of supports (Fig. 2). It can be seen in Fig. 2 that La203 concentration does not influence significantly on NO conversion with temperature neither for Pd catalysts nor for supports with an exception for 50 % La2OaA1203 support. Its activity is higher than activity of other supports. All studied supports are not selective to N20 (Figs. 3). Ammonia is the main product only for supports with > 2 % of La203 at 773 K. At 873 and 973 K nitrogen is the main product on all supports (Fig. 3). The supports with 2 and 5.8 wt. % of La203 seem to be suitable supports in order to obtain high NO conversion into N2 with minimum production of NH3. In contrast, on La203-AI203 supports prepared by sol-gel method in this temperature range N2 selectivity was low (20 %) [4]. All Pd/La203-A1203 catalysts at 473-573 K behave similar to Pd/AI203 catalyst (Fig.3). (Data at 423 K we do not consider because at this temperature the activity increases and selectivity changes very rapidly with temperature.) Higher 573 K the behavior of Pd/La203A1203 catalysts with 2 % and 6-50 % La203 become different (Fig. 3). The behavior of Pd/2 %-La203-A1203 at higher temperatures is still very similar to Pd/AI203 catalyst. Hence, 2 % of La203 is concentration not enough to change activity and selectivity of Pd/A1203 catalyst,

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B

化

A

60~I~.

=------------I= 973 K 973 K

-----, l,,,,l,,,,,l,,,,l,,,,l,,,,

60

-

~

60 o~ 3

0

~

873 K

873 K

773 K

773 K

0 673 K

6o

~ 30

10 20 30 40 50 60 La203 concentration, wt. %

j.....................,........_L,_,_._J ...

l~

I

I

I

I

60 ~ 573 K 30,L. ..... 0 III1~;, ~,i-~-h~-.~-~,~-Ti-~-~-,---;.... _~ .. ~. 523~K 60-" " " 30 .~ ; 0

60

3

-,-I-,-, , , - l - i

I_ 0

0

..

, , il

Fig. 3. Dependence of the selectivity of Pd catalysts (A) and supports (B) on La203 concentration at different temperatures" circles - NH3; triangles - N20; squares - N2.

....

473 K ~

', I,,,

lla'*,l'','l

IIII~III

I

6O 3O ,,,,,, , 10 20 30 40 50 60 La203 concentration, wt.%

0"~,:~<,,,,,

0

. . . .

. . . .

. . . .

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while it is enough to change activity and selectivity of A1203 support and chemical state of lanthanum in the support. At high temperatures the behavior of Pd/La203-AI203 catalysts with 6-50 % La203 becomes similar to behavior of supports (N2 selectivity reaches 80 % at 973 K). Selectivity results suggest that in Pd/La203-AI203 catalysts with 2 % La203 catalytic reaction occurs on Pd in the wide temperature range (423-973 K). NO conversion on La203-A1203 supports prepared by coprecipitation is considerably higher than on supports prepared by sol-gel method [4]. At 973 K NO conversion on majority of supports prepared by coprecipitation is 100 % (Fig. 2) while on sol-gel support it is only 40 % [4]. NO conversion on Pd/La203-A1203 catalysts prepared by coprecipitation (Fig. 2) is similar to that one on catalysts prepared by sol-gel method [4]. Hence, application of coprecipitation method for preparation of La203-AI203 supports results in: 1) significant enhance of N2 selectivity (at high temperatures) on La203-A1203 supports and Pd/La203-AI203 catalysts and 2) increase of NO conversion on La203-A1203 supports compared with sol-gel method.

Acknowledgements The authors thank M.E. Aparicio and G. Soto for technical assistance in experiments and Dr. V. Petranovskii for fruitful discussions. This work was supported by CONACYT grant No 25381-A.

REFERENCES 1. H. Muraki, H. Shinjoh, H. Sobukawa, K. Yokota, and Y. Fujitani., Ind. Eng. Chem. Prod. Res. Dev., 25 (1986) 202. 2. J. S. Church, N. W. Cant, and D. L. Trimm, Applied Catalysis A: General, 101 (1993) 105. 3. L. P. Haack, J. E. DeVries, K. Otto and M. S. Chattha, Applied Catalysis A: General, 82 (1992) 199. 4. S. Fuentes, N.E. Bogdanchikova, G. Diaz, M. Peraaza and G.C. Sandoval, Catalysis Letters, 47 (1997) 27. 5. N. E. Bogdanchikova, S. Fuentes, M. Avalos-Borja, M. H. Farias, A. Boronin, G. Diaz, Applied Catalysis B: Enviromental, 17 (1998) 221. 6. J. S. Ledford, M. Houalla, A. Proctor, D. M. Hercules and L. Petrakis, J. Phys. Chem. 93 (1989) 6770. 7. L. P. Haack, C. R. Peters, J. E. De Vries and K. Otto, Applied Catalysis A: General, 87 (1992) 103. 8. J.F. Moulder, W.F. Stickle, P.E. Sobol and K.D. Bomben (eds.), Handbook of X-Ray Photoelectron Spectroscopy. A Reference Book of Standard Spectra for Identification and Interpretation of XPS Data, Perkin-Elmer Corporation, Physical Electronics Division, Eden Prairie, 1992.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and 3.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Molecular Basis for the Design of Transition-Metal Oxide Catalysts for Selective Catalytic Reduction of NO by Hydrocarbons K. Shimizu a, H. Maeshimaa, H. Kawabata a, H. Yoshida a, A. Satsuma a and T. Hattori b aDepartment of Applied Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan* bResearch Center for Advanced Waste and Emission Management, Nagoya University, Nagoya 464-8603, Japan

This study demonstrates the intrinsic factors affecting the activity of transition metal oxide catalysts for C~H6-SCR. On the basis of the molecular information from XAFS and in-situ FTIR spectroscopies, the structure - activity relationship is examined in a molecular level to obtain intrinsic factors affecting the catalytic activity. The activity of metal oxide catalysts was determined not only by the kind of metal oxides but also by the dispersion, coordination, and local electronic states of the metal cation, which can be influenced by the surrounding oxide matrix. The results provide a criterion for a rational design of effective HC-SCR catalysts. 1. INTRODUCTION Zeolite-based catalysts such as Cu-ZSM-5 are known to be effective for selective catalytic reduction of NOx by hydrocarbons (HC-SCR) [1]. However, due to the lack of stability zeolite-based catalysts are unlikely to be suitable as an automotive catalyst. Metal oxide catalysts is also known to catalyze this reaction [2-5]. Recently, we have reported that highly dispersed metal oxide catalysts, such as Cu-AleO3 [4] and Ga-AleO3 [5], exhibit high de-NOx activity comparable to the corresponding ZSM-5 based catalysts, indicating that the use of zeolite as a matrix is not essential for this reaction. In addition, we have shown that Cu-A1203 is more promising than Cu-ZSM-5 in terms of stability [4]. With further improvement of the activity, the metal oxide catalysts can be attractive candidates for practical use. However, fundamental factors for controlling the catalytic properties of metal oxide catalysts have not well investigated. Kung et al. suggested, as a first approximation, that the de-NOx properties of metal oxide catalysts are determined by the dispersion of the metal ions and the kind of the metal ions [2]. On the other hand, we have recently reported that the coordination symmetry of gallium atom and surrounding matrix have also a strong effect on the intrinsic activity of gallium oxide catalysts for CH4-SCR [7,8].

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This study shows the fundamental factors affecting the activity of transition metal oxide catalysts for CaH6-SCR. The effect of chemical nature of the metal ions is first studied. Next, for Cu-based catalysts as an example, the effect of the intrinsic factors (dispersion, coordination, and the local electronic states of the metal ion) on the intrinsic activity are examined by changing the preparation condition, Cu content and the kind of surrounding oxide matrix. 2. E X P E R I M E N T A L Transition metal aluminate and other copper containing mixed oxide catalysts were prepared by coprecipitation method [4]. The coprecipitated hydroxides were dried, and calcined at 1073 K for 12h except for Cu-ZrO2, which was calcined at 773 K for 4 h. A1203 supported CuO catalysts (CuO/A12Oa) were prepared by impregnation method and calcined at 673 K. XRD and UV-VIS results showed that CuO/A1203 contains CuO particles as a predominant Cu species, while Cualuminate consists of Cu 2§ cations dispersed in an aluminate phase [4,8]. Copperaluminate with Cu content of 33 mol% corresponds to the CuA1204 (stoichiometric spinel). Cu L-edge XANES spectra were measured on BL-7A at UVSOR [8]. Cu K-edge XANES was taken at BL-10B of Photon Factory in High Energy Accelerator Research Organization. The catalytic test was performed with a flow reactor by passing a mixture consisting of 1000 ppm NO, 2000 ppm C3H6, and 6.7 % 02 in He at a rate of 100 cm 3 min -~ over 0.04-2.0 g catalyst. The reaction rates of NO reduction to N2 were measured under the condition where the conversions were below 30%. In situ IR spectra were recorded on a JASCO FT/IR-300 equipped with the IR cell connected to a conventional flow reaction system [9,10]. All the spectra were measured under the same conditions as the reaction studies. 3. RESULTS AND DISCUSSIONS 3.1 Effect of m e t a l cation species Figure 1 shows the activity of various transition-metal aluminate catalysts. The activity was dependent on the kind of metal cation (factor of 101), which is consistent with the result reported in our previous study [4] and that reported by Kung et al. [2]. Further, the activity pattern, Ag < Rh > Cu > Ga > A1, exhibited the volcanoshape with respect to the heat of formation of the transition metal oxide ( - A HP). Hence, the effect of the specific transition metal ions on the NO reduction activity can be explained to be due to the changes in the chemical nature of the metal cation, such as redox property.

'~

tRh(1)Cu(2)

102 E 9 Z

fAg(l)

N~)

I~Ai(100) 101/ 0

,

I

,

I

,

I

I

,

100 200 300 400 500 600 _/1Hf~

Fig. 1. NO reduction rate at 623 K ( C ) ) on transition-metal aluminates as a functio~n of heat of formation of oxide (-A H~) per tool of oxygen. Metal content (tool%) are designated in parentheses.

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3.2 Effect of Cu dispersion states

Figure 2 shows the effect of Cu content and bulk structure (Cu-aluminate or A1203 supported CuO) on the NO reduction activity. Cu-aluminate showed higher activity than CuO/A1203. For Cu-aluminate, the activity (per surface area of the catalyst) increased as Cu content increased. This trend was consistent with the increase in the concentration of surface Cu 2+ species (Cu]Cu+A1 ratio) determined from XPS analysis. From these results, it is shown that Cu 2§ species atomically dispersed in the aluminate phase is responsible for this reaction. Thus, the primary role of the alumina matrix should be to disperse Cu 2§ cation in an atomic level as established in previous studies [4,11].

3.3 Effect of Cu coordination states

For Cu-aluminate samples, UV-VIS spectra showed that Cu 2§ is present in both tetrahedral and octahedral sites [8]. The fraction of octahedra and tetrahedra has been determined by the deconvolution analysis of Cu L3-edge XANES. As shown in Figure 3, the fraction of octahedra decreased as Cu content increased [8]. This may be due to the strong preference of Cu 2§ for octahedral site relative to A13§ in A1203 matrix [12]. The NO reduction activity per exposed Cu 2§ species (TOF) was estimated by using surface Cu 2§ concentration and was plotted in Figure 3. Clearly, the TOF decreased as Cu content increased, which coincides with the decrease in the fraction of octahedra. Thus, it is shown that the coordination symmetry of Cu 2§ cation, which is affected by A1203 matrix, influences the activity (factor of ~-101); Cu 2§ cation in octahedral site is responsible for the higher activity. • 10-4 "-7

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30

2

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20

10

10o h 9

p

O

50 ~

~D

O Z

10

.~" ( C

|

|

I

10 20 bulk Cu/mol%

~

o

I

30

Fig. 2. Rate of NO reduction at 573 K on Cu-aluminate ( O ) and CuO/AI 203 ( V ) and the surface Cu concentration of Cualuminate (+) and CuO/AI 203 ( • as a function of the bulk Cu concentration.

Z

5

9

flg ,i,,~'1

0

,

i

,

i

10 20 30 Cu content/n~l%

Fig. 3. TOF of NO reduction ( O ) at 573 K and fraction of Td ( A ) or Oh ( A ) as a function of Cu content of Cu-aluminate.

O

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3.4 Effect of Cu e l e c t r o n i c s t a t e s

Another effect of matrix on the activity of Cu 2§ cation was investigated by using various mixed oxide catalysts (1 wt% Cu in Ga203, ZrO2, A1203, Mg~204, and MgO). As shown in Figure 4, the specific rate of NO reduction was greatly dependent on the matrix (factor of "~ 102), and followed the sequence of Ga203 > ZrO2 > A1203 > MgA1204 >> MgO. It should be noted that the activity increased with a decrease of the enthalpy of formation of the oxide matrix (-A H~). Previously, Kung et al. suggested that the de-NOx properties of metal oxide catalysts are determined more by the nature of the metal ions and less by the nature of the support. However, the present result suggests that the nature of the support (oxide matrix) is an another critical factor. XRD pattern of these catalysts showed no lines due to CuO. In UV-VIS spectra of these catalysts, an absorption band due to octahedral Cu 2§ (around 750 nm [8,13]) was observed, but no peaks due to tetrahedral Cu 2§ (at around 1500 nm [8,13]) nor CuO (620 nm [13]) were observed. These results suggest t h a t dispersed Cu 2§ are in the distorted octahedral sites of mixed oxide phase. In order to discuss the effect of matrix on the electronic state of dispersed Cu 2§ Cu K-edge XANES spectra of these catalysts were measured. As shown in Figure 5, the XANES spectra of the samples did not exhibit a peak due to Cu § (8983 eV [14]) nor a peak due to CuO (8986 eV [14]). In addition, each sample exhibited characteristic XANES spectrum, which indicates that electronic state of Cu 2§ species in each sample differs through an interaction with the surrounding matrix. In addition, it appears t h a t the intensity of the peak at about 8990 eV was lower for the catalyst of higher activity. The 8990 eV peak corresponds to the electron transition from Cu ls to the unoccupied antibonding molecular orbital (MO) originating from the Cu 4p and O 2p orbitals [15]. Therefore, it is suggested that the higher electron density in this antibonding MO, or, in other word, the more labile Cu-O bond is required for the higher SCR activity. 9oev/"~ ~,_.,

Ga ~|

"~....~~.GI1203

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.O" A1 40 gal

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300

i

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400

i

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500

- A He~

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600 mol

,

'

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0

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Fig. 4. NO reduction rate at 573 K ( O ) and area of 8990eV peak in Cu K-edge XANES ( ~ ) for Cu-d~spersed catalysts as a function of- A Hf of oxide per mol of oxygen.

f

8980

t

i

I

i

9000 9020 photon energy/eV

Fig. 5. Cu K-edge XANES spectra of Cu-dispersed catalysts.

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NO+C3H6+O2 (for 180 min)._ ..

NO+O2

II

Cu-Mgml204 Cu-Al203 Cu-ZrO 2

Cu_Ga203 ,

,

0

,

i

i

i

l

100 time/min

i

I

I

'

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Fig. 6. Changes in the relative intensity of the acetate band normalized by intensity at the steady-state (t=0) as a function of time in a flow of NO+O 2 at 573 K.

Additional information about the surface reactivity of the Cu-dispersed catalysts was obtained by a transient in-situ IR experiment. IR result showed that the main adspecies during the reaction was the acetate for all the samples except for Cu-MgO. Recently, we have studied the mechanism of C3H6-SCR on Cualuminate catalysts and showed that the acetate adsorbed on the surface Cu-O site is a possible intermediate, which reacts with NO+O2 to produce N2 and CO2 [9]. The reactivity of the acetate on Cu-dispersed catalysts was tested by the transient response of the IR spectra. After obtaining the IR spectrum under steady-state condition, the flowing gas was switched to NO+O2. Figure 6 shows the change in the integrated intensity of the acetate band (around 1452 cm ~) in a flow of NO+O2. The acetate band decreased in NO+O2, indicating that acetate is highly reactive toward NO+O2. The initial rate of the acetate reaction followed the sequence of GaeO~ > ZrO2 > A1203 > MgAleO4, which is consistent with the order of the NO reduction activity. From these results, it is suggested that the nature of the matrix, such as reducibility, influences the activity of the Cu 2§ species; the more reducible the matrix the more labile the surface Cu-O bond, leading to the higher reactivity of the acetate intermediate toward NO+O2. 4. CONCLUSION This study demonstrated the fundamental factors affecting the activity of transition metal oxide catalysts for C3H6-SCR. The activity was determined by the nature of the transition metal element, which could influence the red-ox property of the active center. The local structure was found to be another critical parameter; the activity was dependent on the dispersion, coordination, and local

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electronic states of the metal cation. Considering the higher stability of metal oxides than zeolite matrix, the active and stable SCR catalysts will be designed by using transition metal cation with a moderate red-ox property and by controlling the above intrinsic factors through an interaction of metal cation with surrounding matrix.

ACKNOWLEDGMENT The authors thank Mr. H. Ishikawa of Toho Gas Co., Ltd. for his technical assistance on XPS measurement. K.S. acknowledges support by the Fellowship of JSPD for Japanese Junior Scientists. 5. R E F E R E N C E S

1. M. Iwamoto and H. Yahiro, Catal. Today, 22 (1994) 5. 2. K.A. Bethke, M. C. Kung, B. Yang, M. Shah, D. Alt and H. H. Kung, Catal. Today, 26 (1995) 169. 3. H. Hamada, Catal. Today, 22 (1994) 21. 4. K. Shimizu, H. Maeshima, A. Satsuma, T. Hattori, Appl. Catal. B, 18 (1998) 163. 5. K. Shimizu, A. Satsuma, T. Hattori, Appl. Catal. B, 16 (1998) 319. 6. K. Shimizu, M. Takamatsu, K. Nishi, H. Yoshida, A. Satsuma, T. Hattori, Chem. Commun., (1996) 1827. 7. K. Shimizu, M. Takamatsu, K. Nishi, H. Yoshida, A. Satsuma, T. Tanaka, S. Yoshida, T. Hattori, J. Phys. Chem. B, 103 (1999) 1542. 8. K. Shimizu, H. Maeshima, H. Yoshida, A. Satsuma, T. Hattori, Jpn. J. Appl. Phys., 38 (1999) 44. 9. K. Shimizu, H. Kawabata, H. Maeshima, A. Satsuma, T. Hattori, J. Phys. Chem. B, submitted. 10. K. Shimizu, H. Kawabata, A. Satsuma, T. Hattori, J. Phys. Chem. B, 103 (1999) 5240. 11. Z. Chajar, M. Primet, H. Praliaud, J. Catal., 180 (1998) 279. 12. Navrotsky and O.J. Kleppa, J. Inorg. Nucl. Chem., 29 (1967) 2701. 13. M. C. Marion, E. Garbowski, M. Primet, J. Chem. Soc. Faraday Trans., 86 (1990) 3027. 14. L.-S. Kau, K. O. Hodgson, E. I. Solomon, J. Am. Chem. Soc., 111 (1989) 7103. 15. N. Kosugi, H. Kondoh, H. Tajima, H. Kuroda, Chem. Phys., 135 (1989) 149.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

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Silver as a Promoter for the Catalytic Decomposition of NOx under Oxygenexcess Condition: Evidence for Oxygen Spiilover from Noble Metals to Silver W. X. Huang"

J.W. Teng a'b T.X.

Cai b

and X. H. Bao"*

aState Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, 116023 Dalian, China b School

of Chemical Engineering, Dalian University of Technology, Dalian 116012, China

Metallic silver as an effective additive in noble metal based catalysts enhances the migration of the tightly bound oxygen on noble metal atoms, as evidenced by TDS and photoemission electron microscopy (PEEM) observations. The actual spillover of oxygen and the unique thermal redox properties of silver result in a significant lowering of the temperature for the removal of the surface oxygen, which keeps the noble metal sites available for the adsorption of reactants, particularly in the presence of excess oxygen, and leads to an increase of the activity. For the catalytic decomposition of NO over the A1203-supported 1%Pd catalyst, the presence of 0.1% Ag as a promoter resulted in about 18% increase of the conversion of NO to N 2 at 873K.

1. Introduction The effective removal of NOx from industrial waste gases and automobile exhausts has been a major concern of environmental protection. But until now no suitable catalyst which can effectively convert NO into molecular N 2 and 02 has been found [1'2]. Studies have shown that highly active catalysts which can decompose NO, for example noble metal catalysts, usually also have strong affinities towards atomic oxygen. Adsorbed atomic oxygen does not easily desorb from these surfaces, since these oxygen atoms occupy the active centers for NO adsorption, thus impede the adsorption and decomposition of NO on the catalyst surfaces, resulting in the deactivation of the catalysts [3]. Silver is a metal which exhibits unique thermal redox properties. Gas-phase oxygen adsorbs on the silver surface and dissociates into adsorbed atomic oxygen. Upon heating, except a few percent of adsorbed surface oxygen atoms diffuses into the bulk phase of silver, most of them recombine at the silver surface as dioxygen and desorb from the surface into the gas phase at about 600K. This gives rise to a recreating of free silver surface at this temperature [46]. This implies a possibility that silver is a good catalyst for NO decomposition or is an effective additive to improve the catalytic preference of noble * To whom the correspondence should be addressed. Fax: 0086 411 4694447, Email: [email protected]

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metal based catalysts in the presence of excess oxygen [7-91. In the present presentation, the catalytic chemistry of silver itself as a catalyst and as an additive of Pd-based catalysts for the catalytic decomposition of NOx in the presence of excess oxygen is reported. The variations of the surface structures during the catalytic reactions have been characterized by the application of a variety of surface techniques.

2.Experimental Supported Pd and Pd-Ag catalysts were prepared by incipient-wetness impregnation using aqueous solutions of PdC12 (First Chemicals Company of Shanghai) and AgNO3 (Chemicals Company of Beijing) at RT on 7-A1203 (from DICE B.E.T. surface area 140.2m2/g). Catalytic tests were carried out by using a quartz tubular fixed-bed reactor (6 mm i.d.) containing 0.25g catalyst, with 2g of quartz chips located prior to the catalyst bed to preheat the feed gases. After flushing with He (15ml/min) at 600 K for 30 min, a feed gas mixture of 2.0% NO, with He as the balances, was introduced into the reactor at a space velocity of 600ml/gcat.h through a mass flow controller. Products, such as N2, NO, 02, N20 and NO2 etc., were separated and analyzed on-line by a gas chromatograph (Model Shanghai 102G) equipped with a column of molecule-sieve 5A and a thermo-conductive detector (TCD). The activities were evaluated in terms of conversion to N2=2[N2]oL,t/[NO],,,* 100%. The photoemission electron microscopy (PEEM) studies were performed in a UHV chamber equipped with devices for low-energy electron diffraction, Auger electron spectroscopy, MS, PEEM and an Ag evaporator PEEM forms a magnified image of the surface due to contrast in the intensity of photoelectrons emitted from the surface irradiated by UV light f~01.It reflects the work function in the observed surface area so that areas with low work functions give rise to a high photoelectron current and yield bright regions in the PEEM image. A P t (110) crystal was polished again before being put into the UHV chamber, and the surface was cleaned in UHV by the standard procedure of repeated cycles of sputtering with Ar t, heating in oxygen and annealing. Deposition of Ag was accomplished by evaporating Ag from a resistively heated tungsten shape using a square grid as a mask, and the amount of Ag on Pt (110) was estimated from the intensity ratio of Ag to Pt.

3. Results and Discussion Metallic Pt and Pd have been found to be unique components for catalyzing the direct decomposition of NO. Fig. 1 (curve a) illustrates the variation of NO conversion to Nz with the reaction temperature over a l%Pd/A1203 catalyst. Similar to that described by other investigators N, it was found that the ignition of the NO decomposition reaction over Pd-based catalysts required higher temperatures, and there existed a pronounced dependence of the reaction activity on the reaction temperature. Moreover, the catalytic performance was restricted by the amount of excess oxygen in the system. Metallic silver has been demonstrated to exhibit only minor catalytic activity for the dissociation of nitrogen oxides due to its inertness for the adsorption of NO. However, the participation of metallic silver in

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化

60

50

40 0 r

.o_ C

o o

3()

20

10

e I

800

900 1000 Temperature/K

11O0

600

800 1000 Temperature/14:

Fig.1. Temperature dependence of the catalytic decomposition of NO over (a) 1Pd/A1203; (b) 1Pd-0.1%Ag/A1203 ; (c) 1Pd-0.2%Ag/A1203 ; (d) 1Pd-I%Ag/A1203 and (e) 1Pd-2%Ag/A1203 (2.0% NO in feed; space velocity, 1200 cm3g-lhour 4 ). Fig.2. Oxygen thermal desorption spectra taken after exposed 4.2% OJHe at 773K for 2 h, followed by various cooling procedures (The heating rate is 10.5K/min): (a) 1Pd /AI203 and (b) 1Pd-I%Ag/A1203, cooled down to RT in 4.2% O2/He; (c) 1Pd1%Ag/A1203, before cooled down to RT in He, swept at 773 K in He for 2h; (d) 1Pd-I%Ag/A1203, before cooled down to RT in He, swept at 773 K and cooled down to 443K in He and than held at 443K in He for 2h. the Pd-based catalysts gives rise to a distinct alteration of the catalytic behaviors. As shown in Fig.l, a small amount of Ag, 0.1% and 0.2%, respectively, promotes the catalytic activity through the lowering of the reaction temperature and enhancing the actual conversion of NO at the given temperatures. For a 30% NO conversion, the reaction temperature needed over the 1%Pd-0.1%Ag/A1203 was about 60 K lower than that for the pure Pd catalyst (1%Pd/AlzO3). The increase of the silver content in the catalyst causes an obvious restriction of the reactivity of NO dissociation. As shown in curves d and e in Fig.l, the conversions for the catalysts of Pd-Ag/A1203 with respectively 1% and 2% silver loadings were only about one fifth of the NO conversion at 1078K, which could be attributed to the covering of Ag on the Pd sites, which were the active centers for the decomposition of NO. As demonstrated, the rate-limiting step in the NO decomposition reaction over noble metal based catalysts is the efficient removal of oxygen species which were formed from the dissociation of NO or from the adsorption of excess oxygen in the gas phase [3]. In such cases, successful enhancement of the activity should be attained by the weakening of the interaction of oxygen with the noble metals such as Pd and Pt. Fig.2 presents the oxygen TDS results taken from 1%Pd/A1203 and 1%Pd-1%Ag/Al203 catalysts. Curves a and b were recorded after exposing respectively I%Pd/AI203 and I%Pd-I%Ag/A1203 to 4.2% O2/He mixture at 773K, and then cooling down to RT in the same atmosphere. As can been seen from curve a, on the A1203-supported Pd surface only a single oxygen species which is attributed to the surface

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state (%) existed. Compared with the results obtained from the Pd (110) surface [111,except for the absence of a weakly bound state oxygen (a~) which is designed as subsurface oxygen, the desorption temperature of the surface state oxygen on the A1203-supported Pd surface appeared to shift about 150K towards high temperatures. The addition of Ag did not cause significant change in the desorption behaviors of the surface state oxygen from the Pd-related sites (%), but it gave rise to a new oxygen species desrobed at a lower temperature (670K), as shown in curve b of Fig. 2. By comparison with the results of the OJAg system, the new species could reasonably be assigned to the atomically adsorbed oxygen on Ag-relating sites, and the 70K increase of the desorption temperature with respect to that of the bulk metallic silver can be ascribed to the strong interaction between Ag and the A1203 carrier. The spectral invariability of the oxygen desorbed from Pd-relating sites and the appearance of the Agrelated oxygen species reveal that the participation of Ag in the Pd/A1203 did not lead to the active formation of an Ag-Pd alloy, as it has been demonstrated that, if there is the formation of an alloy, there will be an obvious shift of the oxygen desorption peak towards lower temperatures. Instead, some isolated Ag-related sites were created, which exhibit the characteristics of metallic silver. As shown in curve c of Fig.2, when the sample of l%Pd1%Ag/A1203 was exposed to 02 at 773K and cooled down in He instead of in O2/He mixture to RT, subsequent oxygen desorption gave only one dominant peak representing the oxygen species on Pd-related sites. However, when the oxygen-saturated 1%Pd-1%Ag/A1203, before being cooled down to RT, was swept at 773 K and cooled down to 443K in He and than held at 443K in He for some time, then besides the expected Pd-relating oxygen desorption peak, a clear uptake at about 650K emerged. The appearance of the Ag-related oxygen species reveals an actual migration of the oxygen atoms, i.e., oxygen spillover, from the Pd-related centers to the Ag sites at appropriate temperatures. A direct observation of the analogous phenomenon of oxygen spillover has been carried out by using photoemission electron microscopy (PEEM) on the Ag-deposited Pt(110) surface. The use of the grid permits us to identify easily the pure Pt(110) region and the Ag-modified

140 "--:"~::3130 .__

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Fig.3. The left part shows the PEEM images of Pt(110) surface after exposed to 3.0x10 5 Pa 02 and NO mixture (O2:NO=l:l) at 400K for 600Sec (upper) and then heated to 700K in UHV (lower). The right part presents the corresponding variation of image brightness of Ag area (a) and Pt area (b) with heating time (The heating rate is about 4-6K/sec).

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region. Fig.3 presents the PEEM images taken from the Ag-modified Pt(110) after exposing to the mixture of 50%02 and 50%NO at 3.0x 103mbar of the total pressure at 400K for 10min (upper part) and heated to about 700K in UHV (lower part). In the pictures presented, (a) and (b) denote the Ag area and Pt area on the surface, respectively. Fig. 3 also presents the variations of the image brightness (photo-current) in Ag and Pd areas with the surface temperature (temperature ramp about 4-6K/sec), which were obtained by an additional computer analysis. On the clean surface (not presented), the Pt(110) regions are darker than the Ag region, which is in agreement with the fact that the work function of Pt(110) ( 5.49 eV I~21) is higher than that of Ag, being 4.2-4.7 eV depending on the configuration of the surface atoms. After exposed to the 02 and NO mixture, the Pt areas (lines) became distinctly darker, while the Ag areas (squares) remained more or less unchanged, which reflects a preferential adsorption of 02 and NO on the Pt areas. By heating the sample, the initial lowering of the brightness in both the Pt and the Ag areas reflexes the dissociation of adsorbed NO or NO2 , in which adsorption of the released atomic oxygen led to an increase of the surface work function. When rising to about 550K (22 sec), the brightness of the Pt areas began to enhance, and the Ag areas became darker correspondingly. It has been shown that the interaction of oxygen with Pt surface is very strong, and the desorption of adsorbed oxygen species occurs commonly at temperatures above 850K. On Ag-modified Pt surface, the actual variation of the work function of the oxygen-covered Pt areas and Ag-areas below 600K, as indicated by the enhancement of the PEEM brightness, reveals definitely an active migration (i.e. oxygen spillover) of the adsorbed oxygen from the Pt areas onto the Ag areas.

It has been shown that the activation energy for surface diffusion of adsorbed oxygen species over noble metal surfaces (e.g. Pd and Pt) is low, as compared with their heats of desorption [~31.Therefore, these species migrate along the surface and visit various adsorption sites during their surface residence time. Thus, although the apparent binding of oxygen with noble metal atoms are tighter than that with the silver atoms, the actual mobilization of the adsorbed oxygen species on the surfaces gives rise to the possibility for silver to trap and catch the mobile oxygen atoms, due to its higher affinity for 02 [~4].

In conclusion, on the noble metal-modified catalysts, NO molecules adsorb preferentially on metal active sites and dissociate at elevated temperatures, forming atomically adsorbed

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Fig.4 Visualization of adsorption and decomposition of NO and "-~ the removal of adsorbed oxygen on Pd/Pt(upper) and Pd/Pt-Ag (lower) surfaces.

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oxygen and nitrogen on the surfaces. The resulting oxygen species exhibit a strong interaction with the noble metal atoms and are not easily to be desorbed. Therefore, they occupy the active centers, which are essential for NO adsorption. This will impede and slow down the adsorption and decomposition of NO on the catalyst surface, resulting in catalyst deactivation. As visualized in Fig. 4, the addition of Ag in the Pd/Pt-based catalysts opens a possible channel for the low-temperature removal of tightly bound surface oxygen on noble metal based catalysts. This will keep the noble metal sites available for the adsorption of nitrogen oxides and thus increase the activity of the decomposition reaction, particularly in an atmosphere of excess oxygen.

Acknowledgement The project was supported financially by the National Natural Science Foundation of China and the Ministry of Science and Technology of China through the Climbing Project. Helpful discussions with Professor G. Ertl and Prof. Schl6gl of the FritzHaber Institute (Berlin/Germany) are gratefully acknowledged.

References: [ 1] J.N. Armor, Appl. Catal. B: Environmental, 1 (1992) 221 [2] A. P. Walker, Catalysis Today 26 (1995) 107 [3] M. Iwamoto and H. Yahiro, Catalysis Today 22 (1994) 5 [4] M. A. Barteau, R. J. Madix, in: The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 4, Eds. D. A. King and P. Woodruf (Elsevier, Amsterdam) [5] X. Bao, M. Muhler, R. Schl6gl, G. Ertl, Surf. Sci., in press [6] X. Bao, J. V. Barth, G. Lehmpfuhl, R. Schuster, Y. Uchida, R. Schl6gl, G. Ertl, Surf. Sci. 284 (1993) 14 [7] D.Y. Zemlyanov, A. Hornung, G. Weinberg, U. Wild and R. Schl6gl, Langmuir 14 (1998) 3242 [8] R.J. Wu, T.Y. Chou and C.T. Yeh, Appl. Catal. B: Environmental 6 (1995) 105 [9] H. Hamada, Y. Kintaichi, M. Sasaki, et al., Chem. Lett. 1 (1990) 1069 [ 10] W.Engel, M.E.Kordesch. H.H.Rotermund, S.Kubala, A. Von Oertzen, Ultramicroscopy 36 (1991) 148 [11 ] J.W. He and P.R. Norton, Surf. Sci. 204 (1988) 26 [12] R. Liege J.M. Bull, Soc. 45 (1976) 103 [13] G.A. Somorjai, Annu. Rev. Phys. Chem., 45 (1994) 721 [ 14] C. Rehren, M. Muhler, X. Bao, et al., Z. Phys. Chem. 174 (1991) 11

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Reactivity of surface species in the reduction of NO~ by ethanol on Ag/AI203 Tarik C h ~ l'a'b, Satochi Kameoka a, Yuji Ukisua, and Tatsuo Miyaderaa, "National Institute for Resources and Environment 16-3, Onogawa, Tsukuba, Ibaraki 305, Japan b Department of Chemical Engineering, University Abdelmalek Essadi Facult6 des Sciences et Techniques, PO. Box 416, Tangiers, Morocco In situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy has been used to investigate the surface species involved in NOx reduction by ethanol over Ag/AI203 catalyst. The DRIFT measurements, carded out in dynamic conditions, were combined with gas chromatography (GC) analysis to monitor the N2 formation under CEH5OH/NO/OE/He mixture and after a switch to He followed by heating under He (mixture, 250~ 250~ under He). A parallelism has been observed between the change in isocyanate band area and the N2 formation during step change experiment. The isocyanate species (NCO) were found to be generated from the decomposition of adsorbed organic nitro compounds formed under ethanol/NO/O2/He and ethanolfNO/He. The role of oxygen in NOx reduction was investigated by performing the same set of experiment with a reaction mixture without oxygen. I. INTRODUCTION The desire for improved fuel economy, and for lower emissions of greenhouse gases such as CO2, is projected to increase the demand for diesel engines throughout the world. It is therefore of great importance to develop catalyst technology that will allow NOx reduction for diesel engine emissions. The available treatment technologies based on NH3-SCR and three way catalyst are still not perfect because of NH3 toxicity and the ineffectiveness of the three way converter to treat NOx exhaust produced with high oxygen concentration. Miyadera et al [ 1] recently reported an interesting finding related to the use of oxygen-containing organic compounds (such as ethanol) for reducing effectively NOx over mg/ml203 even in the presence of water vapor and excess of oxygen. At present, this system has been practically used to achieve around 80% of NOx removal from diesel engine emissions [2]. Therefore, further information must still to be provided for elucidating the mechanism of NOx reduction process. In an attempt to address this issue, Ukisu et al. [3] recently reported the formation of adsorbed isocyanate species o n A g / A 1 2 0 3 catalyst on the basis of FTIR experiments of C 2 H 5 O H f N O / O 2 adsorption preformed in static condition. However, the role of these species as reaction intermediates has not been fully established. The present study was undertaken to determine the reactivity the surface species involved in NOx reduction by ethanol on Ag/A1203. This issue has been addressed using an analytical approach based on combined in situ diffuse reflectance IR spectroscopy (DRIFT) and gas chromatography. zCorrespondingauthor. Fax: (212) 9 39 39 53, Email: [email protected] T. C. gratefullyacknowledgethe Science and TechnologyAgencyof Japan for the financial support

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2. EXPERIMENTAL

The catalyst used in this work was an alumina-supported silver catalyst (Ag/A1203) prepared by an ordinary impregnation method according to the previously outlined procedure [2]. The silver content in the catalyst was 2% by weight corresponding to the reported highest activity catalyst composition [2]. The infrared spectrometer used was a Nicolet Magna 550 equipped with a MCT detector and a KBr beamsplitter. The catalyst pretreatment and in-situ DRIFT experiments were conducted in a DRIFT cell permitting measurements in a controlled gas atmosphere and temperature. Prior to the reaction, the catalyst was treated in-situ under 10% O2/Ar flow at 500 ~ for 3 h followed by He purging for 30 rain. A clean catalyst surface spectrum was then recorded at reaction temperature to be used as a background to which experiments spectra were corrected, and the final spectra were presented in Kubelka-Munk form. All the gases used to make a simulated exhaust stream were of ultra high purity. A reaction mixture flow of 500 ppm ethanol/800 ppm NO/10%O2 and He balance was directly introduced into the DRIFT cell at a flowing rate of 60 cm 3 rain1. As for N2 formation analysis, the same mixture was flowed at 200 cm3 rain-t through a quartz continuous flow reactor connected to a gas chromatograph equipped with a molecular sieve 5~ column and TC detector. A linear heating rate of 10 ~ rain-~ was applied to the quartz reactor and to DRIVI" cell for heating under He during the experiments. 3. RESULTS AND DISCUSSION 3.1 Identification of surface species involved in NOx reduction by ethanol In order to identify the surface species involved in NOx reduction by ethanol, in situ DRIFT measurement has been carried out under different reaction mixtures at 250 ~ ethanol/NO/O2/He, ethanol/NO/He and NO/O2/He corresponding to the spectra (a), (b) and (c), respectively (Figure 1). The choice of this temperature is justified by the better detection of surface species resulting from the C2H5OH~O/O2/He reaction [4] and the reasonable activity of NOx reduction by ethanol on Ag/A1203 at 250 ~ [2]. Io.ot

9r

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(a)

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(b) ethanol/NO/He (c) N O / O 2 / H e

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Fig 1: In situ FTIR spectra of Ag/AI203 after 30 rain at 250~ under different reaction mixtures at 250~

Fig.2" FTIR spectra recorded after adsorption and evacuation of isocyanic acid (HNCO) on Ag/AI203 at 250~

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The IR bands identification of spectrum (a) in the 1300 and 1750 cm -~ spectral region, is rather complex. According to the assignment reported in the literature, one attribute the observed bands to several surface species such as formate, formyl, aldehyde and carbonyl resulting from the partial oxidation of ethanol which produce IR bands located between 1300 and 1750 cm ~ [6-10]. Moreover, the high intensity of IR bands at 1580 and 1635 cm -~ (spectrum (a)) may generate an overlapping of other surface species IR bands, such as nitrogen-containing species which also exhibit IR bands in the spectral range between 13001750 crn-~. Tanaka et al. [5] observed IR bands at 1565 and 1405 cm -~ assigned to adsorbed organic nitro compounds on the basis of isotopic shift for ~sNO and comparison with the spectra of standard organic nitro compounds. Similar features were also reported by Hayes et al. [6]. It could be concluded that the IR features observed in the spectra (a) at 1300-1750 cm -~ region are attributed to some CxHyOz and adsorbed nitrogen-containing species (nitro-organic compounds). More on this issue is reported elsewhere [4].

At the 2000-2500 cm -~ spectral range, the intense band at 2235 cm -~ is attributed to isocyanate species. This assignment is evidenced on the basis of FTIR measurement of isocyanic acid vapor adsorption on Ag]ml203 catalyst followed by a prolonged HNCO gas phase evacuation (Fig. 2). The FTIR spectra were recorded in static conditions by performing an in-situ depolymerization of cyanuric acid [(HOCN)3] at 450 ~ according to the procedure outlined by Herzberg and Reid [ 13]. The formation of NCO was clearly indicated by an intense IR band at 2235 cm ~, accompanied with NCO stretching vibration band at 1372 cm-~[13]. The IR bands at 2130, 2165 cm -~ (a) are tentatively assigned to two different CN species, according to the results reported for the NOx reduction by ethanol on Cu-ZxO2 in excess of 02 [ 11, 12]. As for the spectrum (b) obtained under ethanol/NO mixture, although the formed IR bands present lower intensity, the absence of oxygen in the mixture did not generate any change in the position of the IR band at 1300-1750 cm -~ region and therefore the same assignment may be applied. However, it should be noted that the intensity of NCO band, observed in the spectrum (b), is very small and hardly detected at 2216 cm -~. The spectrum (c) recorded under NO/O2/He mixture showed the formation of intense bands at 1580 and 1295 cm -~ assigned according to literature to surface nitrates (NO3_) species resulting from the oxidation of NO by oxygen [14]. The presence of IR bands at the same position in the spectra (a) and (b) suggest also the formation of NO3_ species when the catalyst is exposed to ethanol/NO/O2/He or ethanol/NO/He mixtures. In-situ DRIFT study of ethanol/NO/O2/He adsorption on Ag/A1203 catalyst, revealed the formation of several surface species such as nitro-compounds, nitrate and isocyanate as well as CxHyOz species resulting from the partial oxidation of ethanol. It is of interest to investigate the dynamic behavior and the reactivity of the observed species simultaneously with N2 formation during step change experiments followed by heating under He flow.

3.2 Surface species evolution during step change (C2HsOH/NO/OTJHe--,He) Figure 4 shows the spectra change upon the switching of gas from reactant mixture (after exposure at 250 ~ for 30 min) to He followed by heating under He flow. The isothermal exposure of the catalyst to the He flow leads to a decrease in NCO band intensity, meanwhile the IR bands in the 1300-1750 cm l spectral region seem to be hardly affected. However, the heating under He generates a drastic change in the recorded spectra; the increase of the NCO band is accompanied with a clear decrease in the IR bands at 1405 and 1565 cm -~ corresponding to the organic nitro species. These features are clearly showed by the spectrum

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(e) recorded at 400~ where the NCO band reached i~ maximum. The spectrum (d) recorded at 500~ showed only the presence of the two intense Ill bands at 1580 em ~ and 1470 cm ~ as well as the band at 1330 cm ~, which disappeared later on during the heating process (spectra not shown). These bands may be assigned to some NO~_ and/or carboxylates and formates species considered as spectator species according to their behavior. c "2235

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~000

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~000

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Fig. 3: FTIR spectra change during the step change (CzI-lg3H/NO/Oz/I-Ie (30 min), 250~ He, 250~ heating under He flow), (a) 30min under mixture, (b) 15 min under He at 250~ (c) heating 400~ (d) 500~

9 10

20

30

i 50

40 "l'm~e

i 60

, 70

80

(rain)

Fig.4: Evolution of Nz production and the IR band area at 2235 eral during the step change experiment (CzH~OH/NO/O2/He, 250~ He, 250~ -* Heating under He flow)

From these results, correlation between the adsorbed NCO formation and the decomposition of organic nitro species can be made. Apparently, the regeneration of surface NCO species observed during the heating under He process, is originating from the decomposition of adsorbed organic nitro compounds. This finding is illustrated by IR studies of model organic nitro molecule decomposition (nitro-ethane) on Ag/AI203 (Fig. 5). It is clearly indicated a decrease in the intensities of the IR bands at 1565-1555, 1405-1395 and 1380-1370 cm -~ due to adsorbed C2HsNO2 [15], accompanied by the appearance of NCO band at 2260 cm ~. It should be noted that the NCO formation by thermal decomposition of nitro-ethane on A1203 alone was significantly suppressed (Fig.5 dotted line). Further confirmation of the role of NCO species in NOx reduction is demonstrated in Figure 4 showing the evolution of the IR band area at 2235 crn"~ and N2 production during the step change experiment ( C2HsOH/NO/O2/He, 250 ~ He, 250 ~ heating under He flow). The shape of N2 formation monitoring shown in Fig. 4A is similar to the NCO band area curve (Fig. '~ ;,h " m ...... ,H .... ,H . . . . - , , s 4B). Under reaction mixture flow, the Fig. 5: IR spectra recorded following the C2HsNO2 detected N2 is originated from NOx reduction. exposure at 25~ and heating stepwise up to 350~

Io.1

A~AI20~

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The switch from the reaction mixture to He at 250 ~ led to a decrease in the intensity of NCO band accompanied with a diminution of N2 formation. Heating under He flow led to a detection of N2 and a regeneration of NCO species. The latter is resulting from the decomposition of adsorbed organic nitro compounds occurring in the course of heating process. The N2 formation and NCO peak area evolution curves during the heating process, exhibited the same maximum around 400~ The parallelism between the two curves (Fig. 4A and B), strongly support the participation of NCO species into the NOx reduction to N2. Figure 6, shows the change of NCO IR bands on Ag/A1203 and A1203 during the stream under NO, 02 and NO+O2 flows. In the case of Ag]A1203, NCO band decreased significantly under 02 or NO+O2, while little change was observed under NO. With A1203, the decrease of NCO band was much faster under NO+O2 flow than NO or O2.These results suggest that NCO reacts with NO+O2 (presumably NO2) even in the absence of silver, while the reaction of NCO with 02 proceeds only in the presence of silver. Products of these reactions were investigated by pulse reaction technique [ 16]. N2 was mainly detected, especially during 02 and NO+O2 reaction with NCO on Ag/A1203. (Fig.7). Details on this issue are presented elsewhere [ 16]. 0 ,

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Ag/Al203 and A1203

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, 8

exposure to NO+O2 mixture, NO and 02 at 250~

9 II /

Fig.7 9N2 formation as function of temperature during the reaction of NCO species with NO (1000ppm)+O2 (10%), 02 (10%) and NO (1000ppm). Pulse (lml/pulse; He balance; NO:41 nmol/pulse; 02:4115 nmol/pulse)

3.2 Role of oxygen in the NOx reduction by ethanol on Ag/AI203 In order to investigate the role of oxygen in NOx reduction by ethanol, in-situ step change experiments were performed with a reaction mixture without 02 (Fig. 8). It was previously stated, that the IR bands observed under C2H5OH/NO/He mixture were almost similar to those obtained in the presence of 02 leading to nitro compounds, nitrate, CxHyOz., and isocyanate (2216 cm-l). The switch to He led to an increase of NCO IR band intensity, which might arise from the same decomposition profile of the nitro organic compounds. The NCO IR band intensity after switching to He at 250~ and during heating was smaller as compared with the one formed the presence of 02.

1 o.o12

i

il~

~~=~

9

2500

d

~

j

/

'

(a) 30 min ( reactmixture) (b) 10 min underHe (c) heating400~ (He) (d) heating500~ (He)

'~

/ll

2000

~-~.___._~,.

2000

1500

1000

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Fig.8" FYIR spectra change during the step change (C.~H5OH/NO/Hc,250~ tc,250~ under I Ic)

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In addition, the N2 formation is hardly detected (GC analysis) during step change experiments with ethanol/NO/He under reaction mixture or during the heating process. From the above experiments, it could be concluded that the absence of oxygen in the reaction mixture did not affect the nature of the nitrogen-containing surface species formation as well as their thermal decomposition. However, the presence of oxygen strongly enhanced N2 production during C2HsOH/NO/O2/He reaction on Ag/A1203. The fact that the NCO formation from nitrocompounds decomposition took place under the absence of oxygen in gas phase mixture, strongly justify the implication of adsorbed oxygen on silver, stored during the oxidative pretreatment of the catalyst. In support of this, the recent result of Aoyama et al. [ 17] where the oxidized Ag/A1203 was reported to be highly active for NOx reduction with ethanol and propene rather than the reduced catalyst. This performance was explained in term of specific oxygen/silver interaction [17]. Considering silver oxygen affinity in Ag/A1203 catalyst [ 18], it seems to be clear that the role of oxygen in NOx reduction process is not limited only, (i) to convert NO to NO2 (ii) and/or to clean the catalyst surface from carbonaceous deposit (iii) and/or to keep the metal particles in high oxidation state, but also to participate in the process of NOx reduction by ethanol on Ag/A1203 by enhancing the decomposition of organic nitro compounds to NCO surface species and the N2 formation. 4. CONCLUSION The In-situ DRIFT measurement of surface species involved in NOx reduction by ethanol combined with N2 analysis, revealed that the decomposition of organic nitro compounds, formed with both C2HsOH/NO/He and C2HsOH/O2/NO/He mixtures, led to the generation of adsorbed isocyanate which react with NO+O2 or 02 to form N2. However, it is to be noted that the presence of oxygen in the reaction mixture is necessary to drastically enhance the N2 formation as well as the nitrogen-containing species decomposition to adsorbed NCO species. REFERENCES [1] [2] [3] [4] [5] [6] [7]

T. Miyadera, Appl. Catal. B, 2 (1993) 199. T. Miyadera, A. Abe and K. Yoshida, Advanced Mater. 93V/A: Ecomaterials (1994) 405. Y. Ukisu.,, T. Miyadera, A. Abe and K.Yoshida,. Catal. Lett., 39 (1996) 265. T. Chafik, S. Kameoka, Y. Ukisu and T. Miyadera, J. Mol. Catal., A 136 (1998) 203. T. Tanaka, T. Okuhara, and M. Misono. Appl. Catal. B,4 (1994) L1. N.W Hayes,R. W. Joyner, and E.S. Shpiro, Appl. Catal. B, 8 (1996) 343. A. Kiemann, J.P. Hindermann, in: S. Kaliaguine (Ed.), Keynotes in Energy Related Catalysis, Vol. 35, Elsevier, Amsterdam, 1988, p. 181. [8] K.Nakamoto, Infrared and Raman Spectra of Coordination Compounds,New York, 1986. [9] T. Chafik, D. Bianchi.and S.J. Teichner, Top. Catal.,2, (1995) 103. [10] D. Bianchi, T. Chafik and S.J. Teichner, Appl. Catal., 105 (1993) 223. [11] C. Li, K.A. Bethke, H.H. Kung, M.C. Kung, J. Chem. Soc., Chem. Commun. (1995) 813. [ 12] B.L. Yang, M.C. Kung, H.H. Kung, B.W. Jang, J.J. Spivey, Paper 83e presented in the 1994 AIChE Annual Meeting, San Francisco, November 1994. [13] G. Herzberg and C. Reid, Disc. Faraday Soc., 9 (1950) 92. [14] J.W. London and A.T Bell J. Catal., 31(1973) 32. [ 15] S. Kameoka, T. Chafik, Y. Ukisu and T. Miyadera, Catal. Lett., 51 (1998) 11. [16] S. Kameoka, T. Chafik, Y. Ukisu and T. Miyadera, Catal. Lett., 55(1998) 211. [ 17] N.Aoyama, K Yoshida, A. Abe and T. Miyadera, Catal. Lett., 43 (1997) 249. [18] D.I. Kondarides, and X. E. Verykios, J. Catal., 143 (1993) 481.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Modelling of uncatalysed and barium catalysed NO reduction by activated carbon S. A. Carabineiro a, F. B. Fernandes b, J. S. Vital a, A. M. Ramos a, I. F. Silvaa* aDepartamento de Quimica, C.Q.F.B., bDepartamento de Ci~ncia dos Materiais, CENIMAT, Universidade Nova de Lisboa, F. C. T., 2825 -114 CAPARICA, Portugal The kinetics of the reaction of NO with activated carbon without catalyst and impregnated Barium was investigated. The conversion of NO was studied up to 900~ using a fixed bed reactor whose effluents were analysed using a GC/MS on line. Based on the in situ X R D results, Barium catalysed NO reduction mechanism was proposed and adjust to isothermal kinetic results obtained at 400~ and 700~ Also a mechanism was proposed to the uncatalysed reaction and adjusted to isothermal kinetic results obtained at the same temperatures. 1. INTRODUCTION NO removal from exhaust streams of various combustion sources has become increasingly important in the past few years [1-14]. Recently catalytic reduction of NO with carbons as reducing reagents has been intensively investigated [2-8, 10-12, 14]. The reaction depends significantly on the composition of the catalyst and nature of the reacting carbon [8]. The major factors that influence the rates of reactions with carbon are: i) The concentration of active sites on the carbon surface, ii) The cristallinity and the structure of the carbon, iii) The diffusion of reactive gases to the active sites. In attempt to understand and propose a mechanism for catalytic carbon gasification with oxygen, carbon dioxide, water vapour and hydrogen, McKee studied the effects of alkaline, alkaline-earth and transition metals using TGA/DTA and hot stage microscopy [15, 16]. The author concluded that the catalytic process involves an oxidation-reduction cycle. The oxidation state of the metal determines its performance, and the ability of the precursor to be reduced by carbon to a lower oxidation state is an important factor [ 15, 16]. Also the ability of the catalyst to wet and spread on carbon surface promotes catalyst/carbon contact and reactivity is enhanced [1, 15-21 ]. Illhn-Gomez et al. reported that alkaline, alkaline earth and transition metal catalysts seem to enhance NO chemisorption to different extends and they have an important role in redox cycle, which transfers oxygen from the catalyst surface to the carbon surface to produce CO2 and CO [48]. This mechanism is analogous to that one occurring in other carbon gasification reactions [ 15, 161. According to earlier studies of Suuberg et al, at low temperatures dissociative NO chemisorption results in the formation of a carbon oxygen complex and the decomposition of these complexes (at higher temperatures) leads to production of CO2 and CO [13]. In recent studies Illhn-Gomez et al. reported that the analysis of the reaction products reveals a mechanism for catalytic NO reduction with three stages [4-8]: i) at low temperatures, N2 and or N20 are the only products. Oxygen is retained on the catalyst/carbon surface. *To whom correspondence shouldbe adresscd

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ii) above 3000C, N2 continues to evolve but CO2 evolution occurs and the rate of NO reduction increases. iii) above 500~ N2 evolution becomes constant and CO becomes dominant. The active sites that retain and transfer the oxygen to carbon are different, depending on the nature of the chemical species involved in each case [5-8]. The aim of the present work is to propose a mechanism for the uncatalysed and Barium catalysed NO reduction and adjust it to the experimental kinetic results. The catalytic mechanism proposed for the catalysed reaction is based on data obtained by in situ XRD and isothermal kinetic studies. 2. E X P E R I M E N T A L Charcoal Activated GR MERCK (powder) was used to carry out isothermal kinetic studies of NO. This activated charcoal had a surface area of 1011 m2/g and a pore total volume of 0.54 cm2/g. The charcoal was impregnated with a diluted solution Barium Acetate. Metal salt was diluted in a small amount of distilled water and the obtained solution was added dropwise to the carbon, mixing throughly and the obtained product was evaporated in hot plate. The metal loading was 4 wt %. NO conversion was studied using a fixed bed reactor. The reactor effluents were analysed using a GC/MS (Fisons MD800) apparatus, equipped with a column of GS-Molesieve (30mx0.541mm) type. These essays included heating of sample at 2~ (TPR) to 950~ in a mixture of 0.5%NO in He. The flow rate was 3.3 cm3/s. All samples were pre-treated in He at 500~ for 30 minutes. Isothermal experiments were carried out in a fixed bed reactor at 400~ and 700~ at different contact time (g min/mmol ,W/FNo)), being W the initial carbon weight and FNo the NO molar flow rate. In situ X R D studies were conducted in a Rigaku D/max HI C diffractometer with a Cu (Kct) radiation source (50 kV, 30 mA), equipped with a high temperature special chamber. All the experiments were carried out at exactly the same experimental conditions as in the kinetic studies. 3. RESULTS AND DISCUSSION Figure 1a shows the TPR profiles of samples without catalyst and with Barium. The presence of this metal in the carbon surface seems to increase carbon reactivity at low temperatures. The analysis of the reaction products (Figures l b-c) in TPR using a GC/MS on line show the presence of N20, N2, CO2 and CO together with some unreacted NO. For Barium catalysed reaction only N2 and N20 were detected in the low temperature range (20~176 Above 400~ CO2 is also observed. At higher temperatures, above 500~ or 600~ N20 emission ceases and CO starts to appear. The catalytic effect observed with addition of Barium seems to be related with an enhancement of oxygen carbon complexes and CO2 evolution. Similar results were obtained by other authors [37]. The greater the rate of CO2 evolution the greater the NO conversion. As previously reported by Illhn-Gomez et al [4-8], the analysis of reaction products also reveals the three stage mechanism for catalytic NO reduction already described in the introduction. For the uncatalysed reaction the following mechanism was proposed for both temperatures: T=400~ NO + 2C ~ C(O) + C(N)

T=700~ NO +2C ~- C(O) + C(N)

2C(N) -~--~N2 + 2C

2C(N)

2c(o)--,co2+

c(o)

C(N) + NO

c

-~ C + N20

~ N2 + 2C , co

C(O) + CO

r- CO2 + C

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1423 where C is a free active carbon site, C(O) is a surface carbon oxygen complex and C(N) is a surface carbon nitrogen complex. Based on this mechanism and assuming that the total number of carbon active centres were constant, a kinetic model for a steady state fixed bed reactor was derived and fairy model fittings of the experimental data were achieved (Figures 3a-b). Active phases play also an important role on the catalytic conversion of NO [4-8, 20-21]. Figures 2a-b show in situ X R D patterns obtained in N2 and NO at several temperatures for carbon doped with Barium. The crystal phases detected by in situ X R D after pre-treatment in inert atmosphere at 500~ were: BaCO3/BaO/BaO2/Ba. The peaks show shifts to 20 with increasing temperature, which reflects expansion of the crystal lattices. Pt peaks appear in the spectra resulting from exposure to the sample holder to the X ray beam as carbon burned away. In situ X R D experiments carried

化

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.~

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| 40-4-

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[email protected] 0 2

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-

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400 Te mpe rature

600

800

(~

Figure 1 - NO conversion as a function of temperature (a) and reaction products for sample doped with Barium (b) and carbon parent sample (c) in TPR. C-Carbon 1-Ba 2-BaCO 3 3-BaO/BaO 2 4-Pt

C-Carbon

2

'+ I

a

II 2 3_ _ 4 2'31 21 3 ' '4 [[!I12 32,3 'z.~ ~iz "~ "2 g.A-(~ 4

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2

.

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!

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50(Pe 200~

10

20

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Figure 2 - In situ X R D data obtained in NO (a), N2 (b) on heating sample doped with barium at several temperatures.

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BaO/BaO2

out in nitrogen, the peak attributed to is only clearly visible after sample cooling. These observations suggest that BaO/BaO2 is present as a melted phase. Melting of the catalysts improves wetting of the carbon surface promoting catalyst/carbon interaction [ 1, 15, 16, 19-21]. It has been reported recently that alkali, alkaline earth and transition metals enhance NO chemisorption. The catalytic system is more or less active depending on the ability of the oxide to be reduced by carbon and undergo redox reactions on the carbon surface [4-8]. The same mechanism has been proposed to explain carbon catalysed reactions in other gaseous atmospheres [15-18,20,21]. In situ X R D shows clearly that catalysts are reduced 2,5 E- 04 I to lower oxidation states ~ 2,0E-04 such as ~ 1,5E-04 BaCO3/BaO/BaO2/Ba. The --, catalytic effect observed ~ 1,0E-04 ~ ~, , for the NO conversion can ~ 5,0E-05 be explained by the occurrence of redox processes m 0,0 E + 00 -.~ ~ --which the oxides particles 3,0E-04 1 are reduced by reaction ~ 2,5E-04 i~ b with the carbon at points ~ 2 0E-04 ~i-~ ~-~--~ of contact with the carbon ~ substrate to form lower ~ 1 0E-04 t ! oxides. Figures 3a-b show o , the evolution of the reac- to 5,0E-05 ~0 " tion products versus con0,0E+00 ~ ~ , ,= ~ ~-= tact time at 400~ and 0 5 10 15 20 25 30 700~ respectively. At W/F (g min/mmoi) 400~ the reaction products observed were NO, N20, N2 and CO2. At Figure 3 - Model fitting for the uncatalysed reaction at 400~ (a) a - N2; 9 - CO2. 700~ CO was detected and 700~ (b). 9 NO; 9 - CO; instead of N20. Based on the results obtained for the isothermal kinetic studies and the solid phases present under reaction conditions, two different mechanisms are proposed for Barium catalysed reaction at low (400~ and high (700~ temperature. Using the mechanisms above proposed, nonlinear regression fitting of the kinetic data obtained at 400~ and 700~ was carried out. Assuming that NO and N20 are reversible adsorbed on Barium centres and CO2 on BaO, the following mechanisms are proposed for the reaction at 400~ and 700~ respectively. a

-

-

- - - - - - - - - - ~

1,5E-04~

T=400~ Ba + NO ~ Ba(NO)

T=700~ Ba + NO ~ Ba(NO)

2Ba(NO)

2Ba(NO)

~- 2BaO + N2

BaO + C

~ Ba + C(O)

~ 2BaO + N2

NO + Ba(NO) N20 + Ba ~ - .

~- N20 + BaO Ba(N20)

c(o)

~ co

Ba(N20)

~ BaO + N2

CO + C(O)

BaO + C

~ Ba + C(O)

BaO + CO2 ~

BaO + C(O)

~ Ba + CO2

BaO + CO2 ~ - ~ BaCO3

~ CO2 + C

BaCO3

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4,0E-04

Based on this mechanism and assuming that the total ~ 3,0E-04 number of barium and carbon active centers were ~ 2,0E-04 constant, a kinetic model for a steady state fixed bed 9 1,0E-04 reactor was derived. Fairy 0,0E+00 model firings of the ex3,0E-04 perimental data, were achieved (Figures 4a-b). ~ 2,5E-04 I b The CO~ concentration 9 2,0E-04 m value for low contact times, ~ 1,5E-04 is probably due to CO2 ~ 1,0E-04 O desorbed from carbon sur5,0E-05 face functional groups or ! !due to carbonate decompo0,0E+00 0 5 10 15 20 25 30 sition. The fittings obtained for the reactions only show W/F (g min/mmol) that the proposed models Fig. 4. Model fitting for the Barium catalysed reaction at 400~ are possible, not invalidat(a) and 700~ (b). 9 - NO; * - CO; 9 - N2; 9 - CO2. ing others, however the mechanisms proposed are in agreement with those referred by other authors [2-11, 13, 15, 16, 20-21]. The reduction temperature of the metal oxide seems to be a key factor controlling the activity of the metal as catalyst. Illfi.n-Gomez et al. reported that a relationship exists between the ease of the oxidation-reduction (to metallic or lower oxidation state) and the lattice energy of oxide or the free energy of oxide formation [7]. When TPR experiments were performed after NO chemisorption, N2 and or N20 were not observed at low temperatures [8]. The catalyst became inactive due to its conversion to the oxidised state and negligible NO reduction took place. It was only when CO2 evolution begun that NO reduction occurred on an oxygen saturated surface [8]. In our work it was shown by m sire X R D that after pre-treatment at 500~ in inert atmosphere the catalyst is already reduced by carbon to lower oxidation states. Another important factor is catalyst dispersion. As the catalyst dispersion increases, the process of oxidation and reduction of the catalyst is enhanced because the redox mechanism occurs at the catalyst/carbon interface. Thus in the case of the samples doped with Ba, the catalyst/carbon interaction is promoted because the catalyst seems to be presented as a melted phase.( 20-21) In situ X R D shows the probable species involved in the reaction conditions. It seems that a reduced catalyst surface is then required for NO. The active catalysts seem to act as an oxygen accept0r from NO, transferring it to the carbon surface thus recovering the reduced state. Similar behaviour has been reported by several authors [4-8,10,11,20,21 ]. l

11

.--4k

4. CONCLUSIONS This study has indicated that kinetic measurements combined with in situ X R D are useful tool for interpreting catalyst behaviour, being possible to propose a reaction mechanism model. In the CNO reaction a reduced catalyst surface is required to NO reduction. The ability of the catalyst to chemisorb NO going through redox transference of oxygen to the carbon reactive sites seems to explain catalytic activity. The catalytic effect observed with addition of Ba seems to be related with an enhancement of oxygen carbon complexes and CO/evolution.

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ACKNOWLEDGEMENTS

S6nia Carabineiro is grateful to Funda~o para a Ci8ncia e Tecnologia for Grant BD 1275/95. REFERENCES

1. R.T.K. Baker, C.R.F Lund, J.J Chlundzinski Jr, J. Cat., 87 (1984) 255. 2. T. Okuhara, K. IchiTamaka, J. Chem. Soc. Faraday Trans.,l, 82 (1986) 3657. 3. J. Rodriguez-Mirasol, A. Ooms, J. Pels, F. Kapteijn, J. Moulijn, Combust. Flame, 99 (1994) 499. 4. M.J. IU/m-Gomez, C. Salinas-Martinez de Lecea, A. Linares-Solano, Proc. Carbon'94, Granada, Spain, 3-8 July 1994, 460. 5. M.J. Ill/m-Gomez, A. Linares-Solano, L. Radovic, C. Salinas-Martinez de Lecea, Ener. Fuels, 9 (1995) 97. 6. M.J. Illhn-Gomez, A. Linares-Solano, L. Radovic, C. Salinas-Martinez de Leccea, Ener. Fuels, 9 (1995) 540. 7. M. J. Ill/m-Gomez, A. Linares-Solano, C. Salinas-Martinez de Lecea, Ener. Fuels, 9 (1995) 976. 8. M.J. Ill/m-Gomez, A. Linares-Solano, L. Radovic, C. Salinas-Martinez de Lecea, Ener. Fuels, 10 (1996) 158. 9. J. Rodriguez-Mirasol, J. Pels, F. Kapteijn, J. Moulijn, Proc. 22nd Bi. Conf. Carbon, San Diego, CA, 16-21 July 1995, 620. 10. D. Mehardjeive, M. Khristova, E. Bekyarova, Carbon, 34 (1996) 757. 11. C. Marquez-Alvarez, I. Rodriguez-Ramos, Guerrero-Ruiz, Carbon, 34 (1996) 339. 12. S. Carabineiro, I.F. Silva, F.B. Fernandes, Proc. 9th Int Conf. of Coal Science, Essen, Germany, 7-12 Sept. 1997, Vol. II, 1103. 13. I. Aarna, E. Suuberg; Fuel, 76 (1997) 475. 14. S. Carabineiro, I.F. Silva, F.B. Fernandes, A.M. Ramos, J.S.Vital, Proc. Eurocabon 98, Strasbourg, France, 5-9 July 1998, Vol. I, 411. 15. D.W. McKee, Chemistry and Physics of Carbon, Vol. 16, 1, Marcel Dekker, New York, 1981, p. 1. 16. D.W. McKee, Carbon, 8 (1970) 623. 17. I.F. Silva, L.S. Lobo, J. Cat,126 (1990) 489. 18. I.F. Silva, D. Mckee, L.S. Lobo, J. Cat, 170 (1997) 54. 19. I.F. Silva, C. Palma, M. Klimkiewicz, S. Eser, Carbon, 36 (1998) 861. 20. S.A. Carabineiro, F.B. Femandes, A.M. Ramos, J. Vital, I.F. Silva, Catalysis Today, (199.9) in press. 21. S.A. Carabineiro, F.B. Fernandes, A.M. Ramos, J. Vital, I.F. Silva, Catalysis Today, (1999) in press.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Selective Catalytic Reduction of NOx with C3H6 under lean-burn conditions on activated carbon-supported metals J. M. Garcia-Cort6s, M. J. Ill6.n-G6mez, A. Linares-Solano and C. Salinas-Martinez de Lecea Dpto. Quimica Inorganica. Facultad de Ciencias. Universidad de Alicante. Ap. 99, E-03080. Alicante. Spain The Selective Catalytic Reduction of NOx with C3H6 under excess of oxygen has been investigated using different transition metals supported on a carbonaceous material. For this purpose, 1 wt % Pt, Pd, Fe, Co, Ni and Cu supported on an activated carbon, ROXN, have been tested by Temperature Programme Reaction (TPR). A ~as mixture containing 1000 ppm of NO, 1500 ppm of C3H6 and 5 vol. % of O2 (SV = 3600 h- ) has been used to carry out the experiments. In terms of NOx conversion and Nz selectivity, a diversity of behaviours have been obtained. Pt/ROXN presents the highest NOx conversion, almost 100%, but exhibits a moderate N: selectivity. On the other hand, Fe/ROXN presents a 100% N: selectivity, but exhibits low NOx conversions. The results have been discussed attending to the characteristics of the active phase and to the behaviour of the catalysts for C3H6 combustion. Conventional alumina-supported catalysts have been used for comparative purposes: an identical activity trend has been obtained for the studied metals. Nevertheless, the activated carbon-supported metal catalysts show two advantages; i) higher NOx conversions at lower temperatures and ii) the achievement of their maximum values for N2 selectivities, which are similar to the obtained for the alumina-supported catalysts, at lower temperatures.

1. Introduction. The Selective Catalytic Reduction using hydrocarbons as reductant agents (HCSCR) (see reaction l) is, at the moment, one of the most promising solutions to deplete atmospheric NOx emissions. In the last five years, a lot of new catalytic systems have been developed for this purpose. In this way, the authors have recently demonstrated that Pt supported on activated carbons, using C3H6, are an efficient alternative due to its high NOx conversions at low temperature compared with conventional catalytic systems as Pt/Al203 [ 1]. In that study, it was established that the chemical surface properties of the activated carbon play an important role, allowing the support to participate in the reaction. The highest NOx conversion was obtained for the activated carbon with the highest content in CO:-type oxygen groups on the carbon surface. Also, for Pt/AC catalysts, an enhancement of the C3H6 combustion was observed, with a decreasing in the temperature for the maximum NOx conversion respect to the other conventional catalytic systems. The very stable performance shown by these Pt/AC systems indicate that Pt supported on activated carbon can be Cnrre.~nnndin~ alJthnr Tel" +34-965=9093 50" fax" +34=965-903454

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successfully used in the reaction under study. Unfortunately, these catalysts present the disadvantage of having a low N2 selectivity, being the undesired N20 a reaction product, as for most of the Pt based catalytic systems [2]. This drawback has motivated the search for alternative catalytic systems, including different metals able to directly reduce NOx to N2 without N20 formation. The literature presents some promising catalysts to solve this problem, for example, transition metals as Pd, Fe, Co, Cu, etc. as active phase [3]. However, the NOx conversions obtained for all of them diminish considerably regarding the corresponding platinum catalysts.

This paper, considering the advantages of an activated carbon support, attemps to improve N2 selectivities using the above mentioned transition metals. For comparative purposes, y-Al203 support and Pt catalysts will be also used. NO + C3H 6 + 402 ~ ~ N 2 + 3CO2 + 3H20

2.

(1)

Experimental.

2.1. Catalyst preparation All catalysts have been prepared by the excess-solution impregnation method using an activated carbon, ROXN, as catalyst supports. Details of the support preparation and characteristics are found in the literature [1]. H2PtCI6 has been used as platinum precursor while metal nitrate is the precursor for the other metals, Pd, Fe, Co, Ni and Cu. To obtain a 1 wt % metal loading, 10 ml of an aqueous solution with the appropriate concentration, has been added per 1 g of activated carbon. After that, mixture was first submitted to a N2 flow, for water evaporation, and then dried in an oven at 110~ under vacuum, for a period of 12 h. the nomenclature includes the corresponding metal followed by the support, M/ROXN. In addition, for comparative purpose, four catalysts were prepared, following the same preparation process described above but using 3t-A1203 as catalyst support. The metals selected are Pt, Co, Ni and Cu; their nomenclature is similar to the previous one, M/Ai203.

2.2. Catalyst Characterization The metal content of the carbon-supported catalysts has been obtained using Atomic Absorption Spectroscopy. For this purpose, the samples were converted to ash in a muffle furnace at 900~ for a period of 12 hours and the ashes were subsequently dissolved in aqua regia and analyzed. Approximately 1 wt. % of metal has been obtained for all the samples.

2.3. Catalyticperformance measurements The NOx conversion experiments have been performed at atmospheric pressure in a quartz flow reactor (15 mm, i.d.) connecting outlet gases to a gas chromatograph (HP Model 5890 Series II) and to a Chemiluminiscence NOx analyzer (Thermo Environmental Inc, Model 42H). The chromatograph is equipped with a switched dual columns system and with two serial columns (Poropak Q 80/100, for separation of CO2, N20, 1-120and C3H6, and Molecular Sieve 13X, for 02, N2, and CO) joined by a six-way valve. The feed consists of a gas mixture

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containing 1000 ppm NO, 1500 ppm C3I-I6 and 5% 02 balanced with He. The amount of catalyst and the total flow rate is maintained constant, 300 mg and 60 ml/min, respectively. The space velocity for these experimental conditions corresponds to 3600 hz. To study the NOx reduction by C3H6, Temperature Programmed Reaction (TPR) experiments, with a heating rate of 3~ up to 500~ have been carried out. Before to the reaction the samples were reduced at 350"C in 1-12overnight.

3. Results and conclusions. Figure 1 shows the NOx conversions obtained from the TPR experiments for the carbon-supported samples studied. Two sets of curves, corresponding to two different behaviours, can be clearly distinguished. The catalysts containing noble metals (Pt/ROXN and Pd~OXN) present high NOx conversions at low temperatures. However, the non-noble metal (Fe/ROXN, Co/ROXN, Ni/ROXN and Cu~OXN) present moderate NOx conversions at higher temperatures. Furthermore, the curves shape is also different for the two groups of catalysts. The P t ~ O X N and Pd/ROXN samples show a maximum of NOx conversion at 200~ while the other catalysts do not present any maximum in the range of temperature studied. In fact, at 200~ they do not present any appreciable activity, being the NOx conversion significant at temperatures over 240~

20000 -

100 80 =o

, ,t,,,,l

"~' 15000 -

60 "~10000 40

O

9 Z 20

O

0

-

5000 0 r

100

200 T (~

300

400

100

-"

9 ~

..... ,

200 T (~

300

400

....

-.- Pt/ROXN -~ Co/ROXN

-o-Pd/ROXN -~ Ni/ROXN

Figure 1. TPR curves.

---Fe/ROXN -,-Cu/ROXN

--- Pt/ROXN -,- Co/ROXN

§ Pd//ROXN -~-Ni/ROXN

i

-,- Fe/ROXN I -.- Cu/ROXN

Figure 2. Evolution of CO2 during TPR experiments.

This different catalytic performance can be explained considering the reaction mechanism. For the reaction under study (HC-SCR), a two-step mechanism is generally accepted. In the first step, one molecule of hydrocarbon reduces a fraction of metal surface M-O into metallic phase M (oxygen 'clean-off' step). In a consecutive second stage, the adsorption/dissociation of NO proceeds in these reduced sites. The adsorbed NO can recombine with an N-atom, which explains the formation of N20 or, at appropriate temperature, the NO dissociation is favoured and then N2 is formed. Thus, the determining

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step rate seems to be the oxygen 'clean-off'. This hypothesis is supported by experimental results presented in the literature in which is concluded that the metal needs to be in a reduced state to start the NOx reduction [4]. Information about the reduced state of the metal can be obtained following the CO2 evolution because it is formed as a consequence of the reductant consumption. Thus, the CO2 evolution is a useful tool to indicate the easiness to 'clean' the active phase surface of oxygen. Figure 2 shows the evolution of CO2 during TPR experiments. For the Pt/ROXN and Pd/ROXN catalysts, the propene combustion proceeds at low temperatures and, therefore, oxygen 'clean-off' is expected to take place also at low temperatures. On the other hand, for non-noble metal catalysts, the temperatures at which the metal surface is cleaned is observed at higher temperatures because propene combustion is delayed from 200~ to 300~ These results support the previous hypothesis about the determining step of the reaction: until the metal surface is not 'cleaned' of oxygen by the hydrocarbon (then CO2 comes out), the global reaction do not proceds and, consequently, appreciable NOx conversions are not obtained. The discontinuous line of Figure 2 corresponds to the total combustion of the hydrocarbon, i.e., 1500 ppm of C3I-I6, produces 4500 ppm of CO2. Figure 2 also features that the CO2 level surpass the stoichiometric line at certain temperatures. This fact indicates that the activated carbon support is being consumed by O2 combustion. Thus, the evolution of CO2 during TPR experiments is a useful method to analyze the behaviour of the carbon supports versus combustion and therefore, to select the reaction temperature to avoid any loss of carbon. It is remarkable the low support combustion presented by Pd/ROXN, even at high temperatures. For the alumina-supported metal catalysts (Figures 3 and 4), an identical behaviour respect to the activated carbon-supported metal has been obtained. In terms of NOx conversion also two behaviours can be distinguished, corresponding to a noble and non-noble metals: the catalyst containing noble metal (Pt/AI203) presents high NOx conversions at low temperatures, while the non-noble metal catalysts (C0/A1203, Cu/AI203 and Ni/A1203) present moderate NOx conversions at high temperatures. Furthermore, the same NOx conversion trend is followed for the alumina-supported metals studied than the observed for corresponding activated carbon-supported metals (Pt > Cu > Co > Ni).

100-

-,-Pt/A1203 N1/AI203

80 -

~- Co/Al203 + Cu/Al203

4-Pt/~ +IW~

15(XX)-

-,-CNAI203 -,-(MAI203

.o 60l-q ~0

>

o

40-

0 Z 20

O o - " .

100

.

.

.

.

200

Figure 3. TPR curves.

.

.

.

300

T(oc)

.

.

I

400

500

100

-

"1

w

2~30

-

I

I

I

300

400

500

Figure 4. Evolution of CO2 during TPR experiments.

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In terms of C3I-I6 combustion Figure 4 presents the CO2 evolution curves. The lowest temperature for the hydrocarbon consumption corresponds to the Pt/A1203 catalyst, followed by Cu/A1203 and Co/A1203, and finally, the Ni/A1203 catalyst. As for M/AC catalysts, the same sequence is followed by NOx conversion and by C3H6 combustion, indicating that the observed behaviour depends on the metal properties. Thus, for A1203 supported catalysts, the rate determining step seems to be also the oxygen 'clean-off' step.

The comparison of Figure 1 and 3, reveals that the NOx conversions obtained for the activated carbon-supported catalysts are higher than those obtained for the aluminasupported catalysts. The greatest difference is observed for Pt samples, in which an increment of NOx conversion from 55 % at 240~ for Pt/A1203 catalyst, to 95 % at 200~ for Pt/ROXN sample, is achieved. These results can be explained regarding the easiness for the hydrocarbon combustion; the activated carbon catalysts exhibit the complete C3H6 combustion at approximately 50~ lower than alumina catalysts (Figures 2 and 4). This finding shows the effect of support properties in C3H6 combustion and, consequently, in NOx conversion. An additional important point to recall in the selective catalytic reduction of NOx, is the N2 selectivity, usually defined as: N2selectivity-

%N2~ This (% N2 + % N20) oulet selectivity is related with the intrinsic properties of metal used as catalyst and it seems to be independent of the support used [2]. According to the mechanism explained above, once the surface of the active phase is 'oxygen cleaned', the different capacity of metals for dissociative chemisorption of NO will determine the selectivity toward N2. Table 1 shows the maximum N2 selectivities found during TPR experiments, the temperature at which the maximum is observed and the NOx conversion at this temperature for the activated carbonsupported metal catalysts. Table 1. M / A C catalysts N2 selectivity results N2 Selectivity a T (~ NOx conversion c

Pt/ROXN 50 200 Pd/ROXN 70 225 Fe/ROXN 100 200-400 Co/ROXN 50 300 Ni/ROXN 40 300 Cu/ROXN 90 300 a MaximumN2 Selectivitypresentedin the TPR. bTemperatureat the ~ u m N2 Selectivity. ~NOx conversionat the maximumN2 Selectivity.

95 50 40 (300~ 30 20 45

According to these results, Pt shows the highest NOx conversion at low temperatures and a intermediate N2 selectivity while Pd, Fe and Cu feature a higher N2 selectivity but low NOx conversion at low temperature. It has to be pointed out that all the samples are stable in terms of support consumption, at temperatures around 200 ~ and 300~ which are practical temperatures for NOx conversion with noble and non-noble metal catalysts, respectively. An interesting behaviour is observed for the activated carbon catalysts, the temperature for the maximum of N2 selectivity coincides with the temperature at which the C3H6 is completely consumed (see Figure 2) that is, the temperature for oxygen 'cleanoff', with the exception of sample Ni/ROXN, probably because for this catalyst the

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hydrocarbon combustion is delayed and consequently shows low NOx conversions even at high temperature

The maximum value for N2 selectivities encountered for activated carbonsupported metal catalysts are quite similar compared with the corresponding aluminasupported metal catalysts, confirming that selectivity is an intrinsic property of the metal. However, the temperature at which these N2 selectivities are reached are significantly higher (between 50 and 100~ for alumina than for activated carbon samples. This behaviour, which is also observed for temperature of NOx conversion maximum, has to be related with the temperature for metal reduction. It is well known [5] that activated carbon favours the metal reduction because is also a reductor. Therefore, the easier metal reduction in activated carbon than in alumina catalysts has to explain the great performance of M/ROXN catalysts.

4. Conclusions A series of transition metals supported on an activated carbon has been used for the selective catalytic reduction of NOx with propene under lean burn conditions with TPR technique. The results discussed in this paper reveal two behaviours in terms of NOx conversion and N2 selectivity. The metal properties seem to determine the catalyst performance for, both the NOx conversions and the N2 selectivity. The easiness of hydrocarbon to carry out the oxygen 'clean-off" step is the most important point to determine the global performance of the catalysts. For this reason, noble metal catalysts present the highest NOx conversions at low temperature, while non-noble metal catalysts present low NOx conversion even at high temperatures. Conventional catalysts (M/AI203 catalysts) have also been tested, obtaining similar trends. However, the NOx conversions are appreciably lower and they are achieved at temperatures higher than with activated carbon catalysts. Among the catalysts tested, Pt/ROXN shows the highest NOx conversion at the lowest temperature and a medium N2 selectivity, while Pd~OXN, Fe/ROXN and Cu/ROXN feature the highest N2 selectivity and a medium NOx conversion. From these results, it can be deduced that the N2 selectivity of the Pt catalyst can be improved by using other metals, as Pd, Fe and Cu, although with some decrease in the NOx conversion.

Acknowledgement The authors want to thanks CYCIT (project AMB96-0799) for the financial support.

References [1] J.M. Garcia-Cort6s, M.J. Illan-G6mez, A. Linares-Solano and C. Salinas-Martinez de Lecea, Appl. Catal. B, submitted Appl. Catal. B, (1999). [2] G.R. Bamwenda, A. Obuchi, A. Ogata, J. Oi, S. Kushiyama, K. Mizuno. Journal of Molecular Catalysis A: Chemical 126 (1997) 151-159. [3] V.I. P~.rvulescu, P. Grange and B. Delmon. Catal. Today 46 (1998) 233-316. [4] R. Burch, J.A. Sullivan and T.C. Watling. Catalisis Today, 42 (1998) 13-23. [5] M.C. Rom~in-Martinez, D. Cazorla-Amor6s, A. Linares-Solano, C. Salinas-Marinez de Lecea. Current Topics in Catalysis 1 (1997) 17.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Role of acidic sites in the de-NOx process on supported tin oxides A. Aurouxa, D. Sprinceana~b and A. Gervasinic a Institut de Recherches sur la Catalyse, CNRS, 2 av. Einstein, 69626 Villeurbanne, France. email [email protected] b Institute of Physical Chemistry of the Romanian Academy of Sciences, Spl. Independentei 202, 77208 Bucharest 12, Romania. c Dipartimento di Chimica Fisica ed Elettrochimica, Universita di Milano, Via Golgi 19, 20133 Milano, Italy. The influence of the acidic properties of the support on the catalytic performances of supported SnO2 surfaces has been investigated. The acidic properties of low Sn loading catalysts (ca. 3 wt.%) prepared starting from SnCI4, 5 H20 precursor and supported on T-AI203, MgO, SiO2, TiO2, and ZrO2 were characterized by ammonia adsorption microcalorimetry. The catalysts were tested in the NO reduction with ethylene under high oxygen partial pressure at very high space velocity (50.000 hq). The catalysts prepared on the acidic supports (ZrO2, A1203 and TiO2) were the most active. For Sn-ZrO2 at 425~ 47% conversion of 5000 ppm NO to N2 and 77% conversion of 5000 ppm of C2I-I4 to CO2 were obtained in presence of 9% 02. Based on the N2 yield, the following scale of activity was found: Sn-ZrO2 > Sn-TiO2 ~ Sn-A1203 >> Sn-SiO2. 1. INTRODUCTION The selective catalytic reduction (SCR) of NO with hydrocarbons in the presence of oxygen is a very attractive process as a practical way to reach the future stricter emission standards concerning fuel and lean bum engine exhausts [1]. The NO reduction by hydrocarbons on Cu/ZSM-5 and other zeolite based catalysts has given interesting results, but their hydrothermal durability and rapid deactivation have been reported to be a problem. The alumina-supported metal oxide catalysts are more effective at higher temperatures and higher oxygen partial pressures [2-5]. The activity of oxide based catalysts can be attributed to their acidity. In this regard, the cooperation between the support and the active metal centers has to be investigated. In this work, catalysts with about 3 wt% tin oxide loading based on alumina, silica, titania, zirconia and magnesia have been investigated and compared to the support materials. The effectiveness of these catalysts for NOx reduction may depend on the state and dispersion of the tin oxide on the support, on the effect of Sn loading and on the acidity of the support. The acidity was measured by adsorption calorimetry, which is one of the best methods to feature the acidic properties of solid catalysts, providing useful information on both the concentration and strength of sites [6,7]. The changes which may occur in the support matrix due to the addition of tin have been studied.

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2. EXPERIMENTAL

Five supports with different acid-base features and presenting a surface area around 100 m2.gq were u s e d 7-A1203 oxid C from Degussa, TiO2 (anatase) DT51 from Rh6ne Poulene, SiO2 aerosil 130 from Degussa, ZrO2 137 from Degussa, MgO from Carlo Erba. The impregnation was made using solutions of tin tetrachloride pentahydrate (from Aldrich) in distilled water [8]. After drying, the samples were heated in oxygen flow for 6 h at 423 K and for 8 h at 773 K. The amounts of tin deposited were determined by ICP chemical analysis and presented as Sn weight percentages. The samples used in our experiments are listed in Table 1 together with their BET surface area (measured by nitrogen adsorption) and Sn content. Another series of samples was also prepared with a higher loading (-20 wt% Sn) for comparison [7]. Table 1 Physieoehemieal characteristics of the supports and supported SnO2 materials Samples Sn amount BET surface area V ads (wt%) (m2.gq) (lamo~m2) 7-A1203 0 110 1.71 TiO2 0 103 2.74 SiO2 0 130 0.20 ZrO2 0 90 2.60 MgO 0 120 0.23 Sn-A1203 3.1 108 2.00 Sn-TiO2 3.4 90 2.89 Sn-SiO2 3.2 112 0.54 Sn-ZrO2 3.4 78 2.99 Sn-MgO 3.7 54 1.64

V irr (lamol/m2) 0.96 1.60 0.08 1.33 0.09 1.14 1.77 0.26 1.73 0.50

Microcalorimetric studies of the adsorption of ammonia at 423 K were carried out using a heat flow microcalorimeter C80 from Setaram connected to a gas-handling and volumetric adsorption system. The differential heats of adsorption versus adsorbate coverage were obtained by measuring the heats evolved when doses of gas were admitted sequentially onto the catalyst until the surface was saturated by adsorbed species. Adsorption was emended up to an equilibrium pressure of about 130 Pa. After the adsorption was completed, pumping at the temperature of the experiment and consecutive readsorption of ammonia at the same temperature were performed in order to determine the chemisorbed amount (V~). In Table 1, V ~ and V~ are respectively the total adsorbed amount and the irreversibly adsorbed amount, expressed in lamol.m2 of sample, under an equilibrium pressure of 27 Pa. Prior to each experiment, the samples were outgassed overnight at 673 K in vacuum. Catalytic tests of NO reduction by ethylene in high oxidizing atmosphere (NO-C2I-I4-O2) were performed in a fixed-bed quartz tubular downflow reactor, introducing about 0.1 g of catalyst in powder form. The feed gases 2% NO/He, 1% C 2 ~ e , and 20% O2/He or pure 02 were special mixtures supplied by Sapio (Italy) ; they were fed from independent mass flow controllers (Bronkhorst, Hi-Tee.). For all runs the reactant gas was 5000 ppm NO, 5000 ppm C2I-h, and partial pressures of 02 from 5000 to 90,000 ppm, with balance He, at a total flowrate of 5.5 L.h "1. Space velocity corresponded to 50,000 hq (GHSV). The reaction was studied

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in the 200-500~ temperature interval, realizing ten isothermal plateaux and maintaining each temperature for 3 h. Other tests were performed under anaerobic conditions (NO-C2I-h with balance He) at higher contact times. The reactor outflow was analyzed using a gas chromatograph equipped with a T.C.D. detector (Chrompaek) mounting a 60/80 Carboxen1000 column (Supelchem) for the separation of 02, N2, N20, NO, C2H4, CO, and CO2. 3. RESULTS AND DISCUSSION Only a flight decrease of the surface area of the prepared materials was observed with respect to the support, except for the sample based on MgO. The samples were well dispersed, as the characteristic XRD lines for SnO2 were evidenced only for the highly loaded samples (20 wt% Sn). Solid state NMR spectroscopy did not show any signal specific of the presence of SnO ; only SnO2 was detected. Figs. 1a and lb represent the differential heats of NH3 adsorption versus coverage for the listed samples. A small amount of tin oxide enhances considerably the acidity when SnOz is added on SiO2 or MgO, but an increase of the loading from 3 to 20 wt% Sn has less influence [7]. However the respective numbers of strong acid sites (V~r) remain small (29 and 27 lamol.g only), but due to the large decrease in surface area the Sn-MgO sample appears to be more modified. The series of materials using alumina, titania and zireonia as supports are similar in their behavior. The presence of SnO2 on these supports produces only very small increases of the acidity, more visible at low loading than at high loading. The deposition of SnO2 on A1203 increases the number of acid sites on the support in the domain of medium and weak strength but does not affect the heats of adsorption at low coverage, and an increase of Sn loading from 3 to 20 wt% does not change significantly the acidity [7]. Q (kJ/mol)

O (kJ/mol) .......

. . . .

50

o

5

0

~

0.5

1

1.5

2

2.5

NH3 uptake (IJmol/m2)

3

3.5

Figure 1a : Differential heats of NH3 adsorption versus coverage for Sn-Al203, Sn-ZrO2, Sn-SiO2 and the respective supports

0

0.5

1

1.5

2

2.5

NH3 uptake (l~mol/m2)

8

8.5

Figure lb : Differential heats of NHs adsorption versus coverage for Sn-Ti02, Sn-MgO and the respective supports.

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The effectiveness of Sn-yAI203for NO reduction under high oxygen partial pressures and in the presence of water was recently presented in the literature [2,3], while information on the influence of the acidity of the support is not yet available. The low loading of Sn (ca. 3 wt%) on the A1203, TiO2, SiO2, ZrO2 supports led to high activity in the NO-C2I~-O2 reaction, while the loading on MgO support caused activity neither towards the reduction of NO nor towards the oxidation of C2I-h. The anaerobic activity of the SnO2 surfaces (NO-C2I-h reduction in He atmosphere) was poor, while the NO conversion to N2 increased with 02 partial pressure (NO-C2I-h-O2 reaction). Starting from the stoichiometric proportion of 02 with respect to C2I-h fed (10,000 ppm of 02 for 5000 ppm C2I-h), a remarkable increase of activity was observed. This activity was maintained and slightly improved with 02 partial pressures up to 90,000 ppm 02 content. Increasing the Sn loading up to about 20%, a slight modification of the NO reduction activity was observed and a more significant increase of the C2I-h oxidation was observed. Thus, the selectivity of the catalysts with high Sn loading decreased, indicating that large Sn crystallites are more effective than small ones towards the oxidation of the hydrocarbon [9] Based on these results, we detail here the results obtained on the low loading Sn catalyst series (3 wt. %). The five different prepared Sn-based catalysts (see Table 1) were tested in the NO-C2I-h-O2 reaction as a function of temperature (200-500~ at very high space velocity (50,000 h4) in high oxidizing atmosphere (02 content 9%). Fig. 2 reports all the data collected at each reaction temperature, plotted as the conversion of NO to N2 as a function of the extent of the C2I-h conversion to CO2, considered as an index of the extent of the reaction [ 10]. CO2 is indeed the common reaction product deriving both from the reduction of NO with C2I-h and from the oxidation of C2I-h. In this representation, the most active and selective catalyst would have the N2-CO2 curve with the highest slope, giving high N2 formation and low CO2 production. On the contrary, the least active and selective catalyst would have the N2-CO2 50 I I | ~ 40 I/ Z / ~

~ z

I i w 13 Sn-ZrO2" 0 Sn-TiO2 / . ~ ~ * Sn-AI203 ~ ~-~-x-~ A Sn-Si02 / /,"[3/ 0 Sn-Mg0 / ~ ~ - - 0 "

i

./7"

/

, ,0],L /

. A / -

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_

,"

/

,, 9 L r / / /

0

0

0

O-~ 20

I 40

I 60

, 80

100

Conversion of C2H 4 to CO 2 (%)

Figure 2: Conversion of NO to N2 as a function of CO2 produced during the NO-C2H4-O2 reaction (initial concentrations 5000:5000:90,000 ppm respectively) realized at 50,000 h"l over the SnO2-based catalysts. Each marker corresponds to different reaction temperatures in the range 200-500~

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1437 m N2(%) 1777/] N 2 0 ( % )

I--! co2(~) 17~co(~)BB CH4(~!

40-

0

! , | S n - Z r O 2 Sn-TiO 2 Sn-AI20 3 Sn-SiO 2 S n - M g O

i | | ! S n - Z r O 2 Sn-TiO 2 Sn-AI20 3 Sn-SiO 2 S n - M g O

Figure 3" Comparison of the yield of N2 and N20 formed from NO (A) and of the yield of CO2, CO, and CH4 formed from C2I-I403), for the SnO2 based catalysts (reaction temperature" 450~ ; GHSV" 50,000 hq', feed concentration" 5000 ppm of NO and C2H4 and 90,000 ppm of O2). curve with the lowest slope, reaching high CO2 production with low N2 formation. In any case, the conversion of NO to N2 started around 300-350~ (Fig. 2). Above this temperature, the N2-CO2 curves increase regularly with temperature, up to a more or less well-defined point, and then decrease for high temperatures (i.e., high CO2 formation). The highest N2 yield was observed over Sn-ZrO2 catalyst, for which a maximum of N2 formed (47%) was observed at 425~ (77% of C2I-I4 converted to CO2). For Sn-AI203 too, it was possible to detect a maximum of N2 formed (43%) at 475~ (48% of C2H4 converted to CO2). For Sn-SiO2 and Sn-TiO2, starting from 425~ the observed formation of N2 (29% and 34%, respectively) was maintained also for higher temperatures. Among the N-products of reaction, besides N2, N20 was detected at all reaction temperatures, in similar amounts for all catalysts (Fig. 3A); the observed amounts did not follow a regular increasing or decreasing trend with temperature. The presence of N20 could derive from the homogeneous dismutation of the unreacted NO to NO2 and N20 occurring in the reaction atmosphere. Among the C-products of reaction, CO and CH4 were detected besides CO2. Fig. 3B reports the yield of the various C-products at 450~ for the different catalysts. CH4 was only detected on Sn-ZrO2 at high temperatures (_> 450~ CO was mainly formed on Sn-TiO2 and Sn-AI203, and quite absent on Sn-ZrO2 that gave the "cleanest" reaction of NO reduction in comparison with the other Sn-based catalysts. Table 2 collects the temperatures and values of maximum conversion of NO to N2, from which an order of activity can be obtained: Sn-ZrO2 > Sn-AI203 > Sn-TiO2 > Sn-SiO2. The good activity of the SnO2 surfaces is reflected by the very high values of the integral conversion rate of NO to N2 (TOF, in terms of mole of N2 formed per mole of Sn per minute). The competitive factor (c.f. %) can be used in order to compare the selectivity of the catalysts in the NO-C2H4-O2 reaction [11]; a c.f. of 100% would mean that every molecule of hydrocarbon reacting to form CO2 has been used to reduce NO to N2. Values between 8 and 11% are typical of some of the best catalysts reported in the literature [2]. The values of c.f. determined for the SnO2-based catalysts are reported in Table 2. Sn-A1203, Sn-TiO2 and SnZrO2 catalysts have very high values of c.f., indicating a good selectivity and reflecting their ability to reduce NO in the presence of C2H4 in a high oxidizing atmosphere. As a general

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Table 2 Competitive factor (c.f. %) and integral NO conversion rates to N2 (TOF) for the Sn-based catalysts . . . . c.f(%) 102.TOF(mol Nz/mol Sn)-minq Catalyst Temperature (~ Sn-Al203 475 14.9 28.2 Sn-MgO _ Sn-SiO2 475 6.8 24.0 Sn-TiO2 450 11.0 25.8 Sn-ZrO2 425 10.3 30.5

trend, the values of c.f. decreased with increasing temperature (predominance of the C2I-I4 oxidation with respect to the NO reduction). In catalysis, the surface area, the intimate contact between the two oxide components and the size of the resulting metal oxide clusters strongly influence the rate and selectivity of chemical reactions. The notable features of the A12Oa, Z r O 2 and TiO2 supports are their ability to disperse the active metal phase, giving rise to strong metal-support interactions, especially at low loadings. The number of sites which chemisorb ammonia (V~) is always increased when tin oxide has been added on a support. The superior activity and selectivity of catalysts based on alumina, zirconia and titania, which are the most acidic supports, may be the result of a high dispersion of the active metal and the stabilization of the dispersed phase of the second component, tin oxide. The differential heat curves deduced from ammonia adsorption calorimetry indicate a wide distribution of the strength of the acid centres. However a close examination of the data sequence indicates that the surface density of the very strong acid sites (Q>140 lcJ.mol1) is almost constant, while the population of sites around 100-120 kJ.molq is the most affected. In the case of MgO, there is a possible formation of a solid solution which could be responsible of the important decrease of the surface area of the catalyst. So, it appears that depositing a small amount of tin dioxide on a support increases the number of total acid sites by creating a superficial electron deficit in comparison with the support. Moreover the acidity of the support plays a role in stabilizing the active phase in a high dispersion [3]. REFERENCES

1. T. Maunula, Y. Kintaichi, M. Inaba, M. Haneda, K. Sato, H. Hamada, Appl. Catal. B, Environmental, 15 (1998) 291. 2. M.C. Kung, P.W. Park, D.W. Kim, H.H. Kung, J. Catal., 181 (1999) 1. 3. P.W. Park, H.H. Kung, D.W. Kim, M.C. Kung, J. Catal., 184 (1999) 440. 4. T. Miyadera, K. Yoshida, Chem. Letters, (1993) 1483. 5. Y. Teraoka, T. Harada, T. Iwasaki, T. Ikeda, S. Kagawa, Chem. Letters, (1993) 773. 6. A. Auroux, Topics in Catalysis, 4 (1997) 71. 7. D. Sprinceana, M. Caldararu, N.I. Ionescu, A. Auroux, J. Therm. Anal. Cal., 56 (1999) 109. 8. B. Gergely, A. Auroux, ges. Chem. Intermed., 25 (1999) 13. 9. A. Auroux, D. Sprineeana, A. Gervasini, in preparation 10. A. Gervasini, P. Carniti, V. Ragaini, Appl. Catal. B : Environmental, (1999) in press. 11. K.A. Bethke, M.C. Kung, B. Yang, M. Shah, D. Alt, C. Li, H.H. Kung, Catal. Today, 26 (1995) 79.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Sulphated-ZrO2 prepared by impregnation with ammonium, sodium, or copper sulphate: catalytic activity for NO abatement with propene in the presence ol oxygen Valerio Indovina, Maria Cristina Campa, and Daniela Pietrogiacomi Centro di Studio "SACSO" CNR c/o Dipartimento di Chimica, Universit~ degli Studi di Roma "La Sapienza", Piazzale Aldo Moro 5, 00185 Roma, Italy. Fax: +39-6-490324. E-mail: [email protected] 1.it Sulphated zirconia samples were prepared by impregnation of hydrous zirconia dried at 383 K or calcined at 823 K (monoclinic ZrO2) with aqueous solutions of (NH4)2SO4, Na2SO4, or CuSO4. After calcining at 823 K, samples prepared with hydrous zirconia dried at 383 K were tetragonal. At the same sulphate content, (NH4)2SOa/ZrO2 and CuSOa/ZrO2 contained sulphates of the same type. As the total sulphate content increased, IR indicated an increasing (polynuclear)/(mononuclear) sulphates ratio. (NH4)2SO4/ZrO2 and CuSO4/ZrO2 were both active for the abatement of NO with propene in the presence of oxygen. (NH4)2SOa/ZrO2 had several drawbacks: (i) CO formed in large amounts, (ii) N20 formed, (iii) the catalyst surface coked, and (iv) catalytic activity changed as a function of time on stream. CuSOa/ZrO2 catalysts had practically none of these unfavourable features. For a comparison, some catalytic experiments were also run using propane or methane as reducing agent. 1. INTRODUCTION Copper-based catalysts have been intensely studied because of their activity in the selective catalytic reduction of NOx with various hydrocarbons in the presence of 02. Two reviews have addressed the structural features, redox properties, and catalytic activity of copper exchanged in the zeolite framework or supported on oxide matrices [ 1-2]. In a recent study, using propene or ammonia as the reducing agent in the presence of oxygen, we found that the activity for NO abatement of CuOx/ZrO2 depended on copper dispersion. With propene and with ammonia, the tumover frequency (Ncu/NO molecules s" Cu-atom -') was nearly independent of the Cu-content, up to about 2.5 atoms nm L, namely below the limit evidenced by the characterisation to have good copper dispersion [3]. As a catalyst for the abatement of NO with propane in the presence of oxygen, sulphated-ZrO2 was markedly more active than pure ZrO2 [4]. With decane as the reducing agent, up to 773 K, neither sulphated-ZrO2, nor pure ZrO2 were active. To have active catalysts, copper had to be added to the sulphated-ZrO2 [5]. These facts prompted us to study sulphated-ZrO2 samples as catalysts for the abatement of NO with propene, propane or methane in the presence of oxygen. ZrO2 was sulphated to various extents by impregnation with (NH4)2SO4,Na2SO4, or CuSO4. 2. EXPERIMENTAL

2.1. Catalysts

The starting material used for the preparation of catalysts was hydrous zirconium oxide obtained from hydrolysis of zirconium oxychloride, dried at 383 K, Z(383), or calcined at 823 K, Z(823). Samples were prepared by impregnation of Z(383) or Z(823) with aqueous solutions of (NH4)2SO4, Na2SO4, or CuSO4. After impregnation, samples were dried at 383 K and calcined at 823 K. Samples are designated by A-ZSa(x, y), where A is H, Na, or Cu; Z

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stands for ZrO2 and S for sulphate; a specifies the structure after calcining at 823 K (m, monoclinic or t, tetragonal); x specifies the analytical sulphate content (molecules nm ~, alkaline extraction with NaOH 1M, ionic chromatography, Dionex 2000i), and y the analytical metal content (atoms nm 2, atomic absorption, Varian SpectrAA-30). The surface area (SA/m2g "1) of samples was measured by N2 adsorption at 77 K. Phases (m or t) were identified by means of a Philips PW 1729 diffractometer (Cuk~, Ni filtered radiation). The following samples were prepared and used as catalysts: Catalyst H-ZSm(0.2) H-ZSm(0.6) H-ZSm(1.5) H-ZSm(2.4) H-ZSt(1.5) Na-ZSm(1.9)

SA/m 2g'l 52 52 52 52 99 54

Catalyst Na-ZSt(1.9) Cu-ZSm(0.3, 0.3) Cu-ZSm(0.6, 0.7) Cu-ZSm(1.9, 2.3) Cu-ZSm(3.3, 4.2) Cu-ZSt(0.9, 1.1)

SA/m 2g'l 54 52 52 52 52 116

For a comparison, we also studied a commercial sample of sulphated zirconia (type XZO 682/1), obtained from MEL Chemicals (Manchester, England), and containing 2.3 sulphatemolecules nm -2 after calcining at 823 K (tetragonal, SA = 135 mEg'l). This sample will be hereafter referred to as H-ZStMEL. IR spectra were run at RT on an FTIR spectrometer (Perkin Elmer 2000) operated at a resolution of 4 cm "1. The powdered samples were pelletted (pressure, 15-10 ~ kg cm 2) in selfsupporting disks of ca. 15 mg cm 2, and. put in an IR cell which allowed thermal treatments in vacuum or in a controlled atmosphere. Before infrared experiments were run, samples were heated in O2 at 793 K for 0.5 h, and evacuated at the same temperature for 1 h.

2.2. Catalytic experiments

The catalytic activity was measured in a flow apparatus at atmospheric pressure. The apparatus included a feeding section where a gas stream consisting of He, NO (3% in He), 02 (10% in He), and C3H6 (1% in He), or C3H8 (1% in He), or CH4 (2% in He) was regulated by means of independent mass flow controller-meters (MKS mod. 1259, driven by a four channel unit MKS mod. 247 c) and mixed in a glass ampoule before entering the reactor. Gas mixtures were purchased from Rivoira and were used without further purification. The reactor was made of silica with an internal sintered frit of about 12 mm diameter supporting the powdered catalyst. The reactor was positioned upright in an electrical heater, with a thermocouple touching the external wall of the reactor at the middle of the catalyst bed. Temperature was maintained within + 1 K with a commercial device. A reactor bypass was provided by a fourway valve. Reactants and products were analysed by gas-chromatography (Varian mod. 6000, equipped with an Alltech CTR 1 column at 308 K). A thermal conductivity detector was used for detecting N2, N20, CO, CO2, and a flame ionisation detector for the hydrocarbons. Peak areas were evaluated by electronic integration. A fresh portion of catalyst (0.25 g) was treated in a flow of 2 % O2/He mixture, while heating the reactor from room temperature to 773 K in about 1 h and then isothermally at 773 K for 1 h. After this treatment, the reactor was bypassed and the temperature adjusted to the desired value. Catalysis was run with NO:Call6 (or C3H8):O2=4000 ppm:2000 ppm:20000 ppm, or with NO:CH4:O2=4000 ppm:4000 ppm:20000 ppm, with He as balance. After stabilisation of the reactant composition, the four-way valve was switched and the mixture allowed to flow into the reactor, thereby starting a catalysis run. The reaction temperature was changed at random without intermediate activation treatments between consecutive runs. The total flow rate was maintained at 50 cm 3 STP/min. The rate of NO abatement, RNo/molecules slnm "2, and the NO conversion were calculated l 2 from N2+N20 produced. The rate of the total hydrocarbon consumed Rcxn,/molecules s n m , and the hydrocarbon conversion were calculated from CO2+CO produced~ Percent selectivities

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(SscR=100XRNo/RCxHy; SN2=100xN2/(N2+N20); 8CO2--100xCO2/(CO2+CO)) were calculated asumxing the following overall reactions: C3H6 + 2 NO + (7+x-y)/2 O2 -* (l-x) N2 + x N20 + (3-y) CO2 + y CO + 3 H20 C3Hs + 2 NO + (8+x-y)/2 O2 ~ (l-x) N2 + x N20 + (3-y) CO2 + y CO + 4 H20 CH4 + 2 NO + (2+x-y)/2 O2 -* (l-x) N2 + x N20 + (l-y) CO2 + y CO + 2 H20

3. RESULTS AND DISCUSSION 3.1. Infrared eharaeterisation

After activation, spectra of all H-ZSm samples consisted of several bands in the 1250-900 cm l region, typical of S-O symmetric stretching modes (1200, 1060, 1034, 990, 925, and 900 cml), and a complex absorption in the 1420-1350 cm region, typical of covalent S=O asymmetric stretching modes of organic sulphates. The overall intensity of all these bands increased with the sulphate content. As the sulphate content increased, the complex-absorption maximum in the 1420-1350 cm z region shifted from 1364 cm ~ to 1400 cm", indicating an increasing (polynuclear)/(mononuclear) sulphates ratio, in agreement with previous reports [6], (Fig. 1, spectra 2-5).

~J r ,...,

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Wavenumber / cm I Fig. 1. FTIR spectra of sulphated-zirconia samples: ZrO2 (spectrum 1); H-ZSm(0.2) (spectrum 2); H-ZSm(0.6) (spectrum 3); H-ZSm(1.5) (spectrum 4); H-ZSm(2.4) (spectrum 5); NaZSm(1.9) (spectrum 6); Cu-ZSm(0.3, 0.3) (spectrum 7); Cu-ZSm(0.6, 0.7) (spectrum 8); and Cu-ZSm(1.9, 2.3) (spectrum 9). As the sulphate content increased, the intensity of OH-bands, occurring in pure ZrO2 at 3777 cm 1 (terminal-OH) and at 3673 cm 1 (bridged-OH), progressively decreased, predominantly that of terminal-OH, indicating that sulphates preferentially anchored to this hydroxyl type. The preferential anchorage to terminal rather than to bridged-OH of ZrO2 has been also observed with molybdates [7] and vanadates [8]. With increasing sulphate content, the OH-bands shifted to lower frequency (from 3777 to 3758 cm -1 and from 3673 to 3640 cm'l), indicating an increasing Br6nsted acidity of these sites. An analogous shift of the OHbands has been observed [9, 10]. Except for a far more intense absorption in the 1200-1100 cm l region of the CuZSm spectrum, Cu-ZSm and H-ZSm had close!y similar spectra (Fig. 1, spectra 7-9; spectra 8 and 9 are out of the scale in the 1200-1100 cm" region). Conversely, the

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spectruna of Na-ZSm (spectrum 6) distinctly differed, corresponding to a sulphate endowed with a more ionic character, as indicated by the absorption frequency (1280-1230, 1160-1140, 1030, and 980-945 cm "l) close to those of inorganic bidentate sulphates [ 11].

3.2. Catalytic activity With propene as the reducing agent, in the temperature region from 500 to 800 K, catalysts H-ZSm, H-ZSt, H-ZStMEL, Cu-ZSm and Cu-ZSt were all active for the abatement of NO in the presence of oxygen. The main drawbacks with H-ZSm, H-ZSt, and H-ZStMEL were: (i) CO formed in large amounts, (ii) N20 formed, (iii) the surface of the catalyst coked, (iv) an unidentified by-product formed from propene, and (v) the catalytic activity changed as a function of time on stream. The changes were poorly reproducible and depended on the specific portion of the sample analysed. In particular, on the H-ZSm(1.5) sample, during the first 5 hours of the reaction, RNo initially increased by a factor of two and the catalytic activity then stabilised. Cu-ZSm and Cu-ZSt catalysts had not these unfavourable features. 100

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Fig. 2. NO (o) and C3H6 (V1) conversions as a function of temperature (T/K) on H-ZSm(0.6) (section a) and Cu-ZSm(0.6, 0.7) (section b). On H-ZSm, H-ZSt, and H-ZStMEL, propene conversion was substantially lower than NO conversion, up to a temperature which increased from 700 to 750 K with the sulphate content. This finding indicated that a propene fraction transformed into carbon and other by-products. Above 700-750 K, propene conversion became higher than NO conversion, indicating that propene combustion was prevalent (Fig. 2 a). Conversely, on Cu-ZSm and Cu-ZSt, up to a temperature which decreased from 625 to 525 K with increasing the copper sulphate content, propene conversion equalled NO conversion. Above 525-625 K, propene conversion exceeded NO conversion, indicating that as the temperature increased the SSCR selectivity decreased (Fig. 2 b). In particular, on Cu-ZSm catalysts, up to 525-625 K, SscR was nearly 100 %, and substantially decreased at higher, temperature (Fig. 3 a). A remarkable feature of all Cu-ZSm and Cu-ZSt catalysts was the higher Sco2 (80-100 %) and SN2 values (70-100 %) compared with the relevant values of H-ZSm, H-ZSt, H-ZStMEL, Na-ZSm, Na-ZSt catalysts and pure zirconia (Sco2 = 30-80 %, and SN2 = 60-80 %) (Fig. 3 b). On H-ZSm, as the sulphate content increased, Sco2 decreased. On H-ZSm, Rc3H6was nearly independent of the sulphate content (data not shown), whereas RNO increased with the sulphate content up to 1.5 molecules nm", and decreased or remained unchanged on the most concentrated sample (2.4 molecules nm-'). The changes in activity made it difficult to compare H-ZSm, H-ZSt, and H-ZStMEL catalysts. Nevertheless, when we compared the catalysts at their maximum activity level, (i) all sulphated zirconia samples were more active than pure ZrO2, (ii) H-ZSt(1.5) had nearly the same activity as the commercial sample H-ZStMEL, (iii) on monoclinic H-ZS, both RNO and RC3H6were higher than the

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relevant values on tetragonal samples, and (iv) H-ZSm and H-ZSt were far more active than the relevant Na-ZSm and Na-ZSt (Fig. 4 a). 100 ~ ] ~ ] E ] ~ ,~ \

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Fig. 3. Selectivity SSCR (section a) and selectivity Sco2 (section b) as a function of temperature (T/K). Catalysts: (v) H-ZSt(1.5); ([]) H-ZSm(1.5); (<>) H-ZSm(2.4); (+) CuZSt(0.9, 1.1); (O) Cu-ZSm(0.3, 0.3); (O) Cu-ZSm(0.6, 0.7); (zx) Cu-ZSm(1.9, 2.3); and (x) Cu-ZSm(3.3, 4.2). q~ 2"0I

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T/K T/K Fig. 4. Rate of NO abatement (RNo/molecules slnn1-2) as a function of temperature (T/K). Section a: (e) ZrO2; (~]) Na-ZSt(1.9); (o) Na-ZSm(1.9); (x) H-ZSt(1.5); (v) H-ZSm(0.2); (A) H-ZSm(0.6); (O) H-ZSm(1.5); and (o) H-ZSm(2.4). Section b: (<>) Cu-ZSt(0.9, 1.1); (m)Cu-ZSm(0.3, 0.3); (o) Cu-ZSm(0.6, 0.7); (El) Cu-ZSm(1.9, 2.3); CuOx/ZrO2 (from ref. 3): ( 9 Cu-Zm(0.9); and ( I ) Cu-Zm(2.5). On Cu-ZSm, RCaH6increased with the copper sulphate content (data not shown), and RNo reached a maximum at a higher temperature as the copper content decreased. Monoclinic CuZS samples were more active than the tetragonal one. Monoclinic Cu-ZS was also more active than the previously studied CuOx/ZrO2, containing dispersed copper (Fig. 4 b) [3]. As with propene, also with propane, in the temperature region from 500 to 800 K, catalysts H-ZSm, H-ZSt, Cu-ZSm and Cu-ZSt were active for the abatement of NO in the presence of oxygen. Although the same drawbacks encountered with propene also affected H-ZSm and HZSt (Sco2 - 35-100 %, and SN2 -- 60-95 %), with propane as the reducing agent the catalytic activity was stable as a function of time oh stream. With propane, both RNo and RCaHswere nearly the same on monoclinic and tetragonal samples. Also in this case, the unfavourable

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catalytic features operating on H-ZSm and H-ZSt were nearly absent on Cu-ZSm and Cu-ZSt catalysts. In particular, on both Cu-ZSm and Cu-ZSt catalysts (Cu-ZSm being slightly more active than Cu-ZSt), Sscg was 100 % up to about 625 K, and substantially decreased at higher temperature. On Cu-ZSm and Cu-ZSt catalysts, Sco2 was 100 % in the whole temperature range 500-800 K and SN2 was higher than 90 % above 623 K. With methane as reducing agent, from 673 to 800 K, H-ZSm, H-ZSt, and H-ZStMEL catalysts were equally active in the selective abatement of NO, whereas Cu-ZSm and Cu-ZSt were active only in methane combustion. On H-ZSm, H-ZSt, and H-ZStMEL, methane conversion was equal to NO conversion up to about 720 K. Above this temperature, methane conversion was higher than NO conversion (Sscg decreased from 100 to 40 %). The selectivities w e r e S c o 2 = 40-60 % and SN2 -- 75-90 %. On Cu-ZSm and Cu-ZSt, RCH4was substantially higher than on H-ZSm, H-ZSt, and H-ZStMEL. On all samples, both RNO and RcxHy depended on the reducing agent. Specifically, the activity pattern was propene > propane > methane.

CONCLUSIONS The impregnation with (NH4)2804, or CuSO4 yields catalysts containing covalent sulphate species anchored to the ZrO2 surface, which are active in the abatement of NO. Ionic sulphates, anchored to zirconia by impregnation with sodium-sulphate, yield nearly inactive catalysts. When propene or propane are used as the reducing agent, catalysts prepared with (NH4)2804 show several unfavourable features, whereas catalysts prepared with CuSO4 do not. With propene, CuSOa/ZrO2 samples are considerably more active than the previously investigated CuOx/ZrO2 [3]. With methane, copper in CuSOn/ZrO2 is inactive in the selective abatement of NO, thus confirming the behaviour of copper in Cu-ZSM5 catalysts [ 12]. With propene and propane, monoclinic CuSO4/ZrO2 samples are more active than the relevant tetragonal samples. With propene, monoclinic (NHa)2SOa/ZrO2 are more active than the relevant tetragonal samples, whereas with propane and methane monoclinic and tetragonal samples have nearly the same activity.

Acknowledgements

Financial support was provided by MURST (Programmi di ricerca scientifica di rilevante interesse nazionale). Thanks are due to MEL Chemicals (Manchester, England) for kindly providing a sulphated zirconia sample (type XZO 682/1). REFERENCES 1. M. Shelef, Chem. Rev., 95 (1995) 209. 2. G. Centi, S. Perathoner, Appl. Catal. A, 132 (1995) 179. 3. D. Pietrogiacomi, D. Sannino, S. Tuti, P. Ciambelli, V. Indovina, M. Occhiuzzi and F. Pepe, Appl. Catal. B: Environmental, 21 (1999) 141. 4. H. Hamada, Catal. Today, 22 (1994) 21. 5. G. Delahay, E. Ensuque, B. Coq, and F. Figu6ras, J. Catal., 175 (1998) 7. 6. C. Morterra, G. Cerrato, and V. Bolis, Catal. Today, 17 (1993) 505. 7. F. Prinetto, G. Cerrato, G. Ghiotti, A. Chiorino, M. C. Campa, D. Gazzoli, V. Indovina, J. Phys. Chem., 99 (1995) 5556. 8. F. Prinetto, G. Ghiotti, M. Occhiuzzi, V. Indovina, J. Phys. Chem. B, 102 (1998) 10316. 9. L. M. Kustov, V. B. Kazansky, F. Figu6ras, and D. Tichit, J. Catal., 150 (1994) 143. 10. D. Spielbauer, G. A. H. Mekhemer, M. I. Zaki, and H. Kn6zinger, Catal. Lett., 40 (1996) 71. 11. K. Nakamoto, "Infrared and Raman spectra of inorganic compounds", 3th ed.; J. Wiley & Sons, Inc. Publ.: New York, 1978; Table III-19, p. 241. 12. M. C. Campa, D. Pietrogiacomi, S. Tuti, G. Ferraris and V. Indovina, Appl. Cat. B: Environmental, 18 (1998) 151.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Co-based ex-HTlc for the decomposition of N20: Tailoring catalysts for active and stable operation J. P~rez-Ramirez, G. Mul, X. Xu, F. Kapteijn and J.A. Moulijn Industrial Catalysis, DelftChemTech, Faculty of Applied Sciences, Delft University of Technology, J u l i a n a l a a n 136, 2628 BL Delft (The Netherlands) Catalytic decomposition of N~O has been carried out over calcined Co-based hydrotalcites containing different bivalent and trivalent cations. The presence of Rh, La or Pd in the resulting cobalt spinel-type mixed oxide leads to a large increase in the activity in the low temperature region, whereas Mg acts as a structural promoter and a stabilizing element against time-on-stream deactivation.

1. INTRODUCTION The well-known environmental problem caused by nitrous oxide, N20 , as greenhouse gas and ozone layer depletor [1,2], requires the urgent reduction of its emissions, in particular those from point sources like stationary combustion and chemical production processes. Catalytic decomposition offers an attractive end-of-pipe solution for the abatement of N20 in these situations [3]. Calcined hydrotalcite-like compounds (ex-HTlcs) have been reported as very active catalysts for the N~O decomposition, especially those having cobalt as active component [4-6]. What makes HTlcs a general catalyst precursor is (a) their ability to accommodate a large number of bivalent and trivalent cations, (b) the interdispersion of the cations on an atomic scale, leading to catalysts with structural homogeneity and without chemical segregation, and (c) the formation of thermostable materials with high surface area upon decomposition. The first two properties are a result of the precursor and the last property also appears to be characteristic of the hydrotalcite (HT) structure and is probably linked to the decomposition mechanism [7]. These properties allow the occurrence of synergetic effects between the elements, exerting a decisive influence on the catalytic behaviour [8]. This manuscript describes the development of unusual structural and catalytic properties by introduction of different cations in a Co-based ex-HTlc. These additions produce multicomponent catalysts yielding active and stable operation for the decomposition of nitrous oxide at low temperature.

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2. E X P E R I M E N T A L

Hydrotalcite-like materials were prepared by a s t a n d a r d co-precipitation method at low s u p e r s a t u r a t i o n conditions at constant pH and t e m p e r a t u r e , as described elsewhere [5]. All the precursors precipitated in the HT structure. Calcination of these materials was carried out at 723 K. The catalyst characterization by X-ray diffraction (XRD), N 2 adsorption at 77 K, and X-ray fluorescence (XRF), as well as the equipment and experimental conditions for performing catalytic activity and stability tests have been described extensively in a previous publication [5]. Temperature p r o g r a m m e d reduction (TPR) experiments were performed in a home-made set-up, using a high-purity mixture of 7.7% HJAr (flow rate 30 ml/min; pressure 1.0 bar) and a quartz reactor tube (i.d. 4 mm). The sample mass, before dilution with SiC (1:1 vol. ratio), was approximately 20 mg. The t e m p e r a t u r e of the sample was linearly increased from room t e m p e r a t u r e to 1 2 0 0 K at 10K/min. The hydrogen consumption was monitored as a function of the t e m p e r a t u r e with a t h e r m a l conductivity detector(TCD). Infra-red(IR) spectra were recorded using a Spectratech diffuse reflectance accessory and a Nicolet Magna 550 Fourier transform spectrometer, after diluting the sample with KBr. 3. R E S U L T S AND D I S C U S S I O N 3.1. C h a r a c t e r i z a t i o n The various samples, indicated by ex-HTlcs, are listed in Table 1. The catalysts are indicated by the metals involved and the molar ratio between t h e m (if applicable). Ex-Co-A1-HTlc (Co/AI=3/1) has been considered as the reference catalyst.

T a b l e 1. Data of catalysts used. The order in atomic ratio corresponds to the order of elements in the catalytic formula. After calcination @ 723 K Catalysts Atomic ratio SBET Vp~re d .... tal+ (m2/g) (cm/g) (nm) ex-Co-A1-HTlc 3/1 140 0.60 90 ex-Co,Mg-A1-HTlc 2/1/1 155 0.65 55 ex-Co,Mg-A1-HTlc 1/2/1 220 1.00 35 ex-Mg-A1-HTlc 3/1 270 0.50 30 ex-Co-La,A1-HTlc 3/1/1 100 0.40 85 ex-Co,Pd-La,A1-HTlc 3/0.5/1/1 95 0.50 90 ex-Co-Rh,A1-HTlc 3/0.06/1 120 0.75 100 ex-Co,Mg-Rh,A1-HTlc 3/1/0.06/1 150 0.80 80 * from the broadening of the major reflection (integral width)

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The thermal activation of the hydrotalcites at 723 K results in the formation of high surface area mixed metal oxides, containing the cations randomly distributed in an oxygen anion lattice. The TPR pattern (Fig. 1) of the reference catalyst reveals the formation of spinel-type mixed oxides. The first two peaks (at 590 and 685 K) correspond to the reduction of Co~O, to CoO and from CoO to metallic Co [9]. The third peak (950 K) is attributed to the reduction of cobalt aluminate. CoA1204 reduces at much higher temperatures than the other cobalt oxides, due to the presence of A13§ ions. It is suggested that AF § ions polarize the more or less covalent Co-O bonds in the spinel. This will increase the effective charge of the Co ions [10] and, as a consequence, the lattice energy and the reduction temperature. 600 900 1200 300 The DRIFT spectra (Fig. 2) of T[K] different Co-containing catalysts are typical for Co304 and CoA1204, with the Fig. 1. TPR pattern of the reference two strongest absorption bands at 672 catalyst ex-Co-A1-HTlc (Co/A1=3/1). and 569 cm -1 [11]. A shoulder near 540 cm is indicative for the CoA1204 spinel, as well as the MgA1204 spinel. Absorbance (a.u.) Co-spinel phases (CoA1204 and Co~O~) were also identified by XRD (Fig. 3), but due to the similar reflection angles and intensities, it is not possible to distinguish between them. The calculated cell constant for the reference catalyst was 810.0 pm, which is more close to CoA1204 (810.5 pm) (ASTM 10-458) than Co~O~ (808.5 pm) (ASTM 9-418). The single oxide (2/1/1) CoO was not present, due to its instability at the calcination 800 700 600 500 400 temperature. Wavenumber (cm -1) For the ex-Mg-Al-HTlc (Mg/AI=3/1) only the single MgO phase has been Fig. 2. DRIFT spectra of different Co-based ex-HTlcs. identified by XRD. Upon introduction of Co, a gradual development of a spinel structure is observed. Moreover, the MgO reflection in ex-Mg-A1-HTlc is no longer present in the diffraction patterns of Co-based catalysts (Fig. 3). The presence of Mg in these catalysts gives rise to a broadening of the diffraction

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peaks indicating smaller crystallite sizes, and larger specific surface areas (Fig.4). Sa [m2/g] 300 9

Mg/AI = 3/1 ,,!,,

Co-AI (3/1)

9

Co,Mg-AI (2/1/1)

200

100 Co/AI= 3/1

Mg-N(3/1) |

i

I

20

I

30

|

9

40

|

9

50

|

"

60

|

70

0

i

i

I

i

0

1

2

3

2 0 [deg]

Mg/AI [mol/mol]

Fig. 3. XRD patterns of different exHTlcs containing Co and/or Mg. Phases: (O) spinel, (O) MgO.

Fig. 4. Specific surface areas of the catalysts as a function of the Mg/A1 ratio.

The characterization results show that the reference catalyst consists of a mixture of Co~O~ and CoAI~O~ spinels, whereas addition of Mg in the catalyst formulation lead to the formation of magnesium aluminate instead of cobalt aluminate.

X(N20) 1.0 0.8

B 648 K

""

E1 723 K

0.6 0.4

0.2 0.0

~

0/3/1

1/2/1

F' F 2/1/1

3/0/1

Co/Mg/AI [mol/mol/mol]

Fig. 5. N20 conversion as a function of the Co and Mg content. Conditions: 1 mbar N20 in He; Space time=8.105 g.s/mol.

3.3. Activity and stability The introduction of Mg in the spinel-like structure does not only produce structural changes, but also changes of catalytic activity and stability. The conversion of N20 over the exCo,Mg-A1-HTlc increases as a function of an increasing amount of Co and decreasing amount of Mg in the catalyst (Fig. 5). Mg enters the spinel solid solution and therefore Co will be partially substituted. It is suggested that CoA1204 is responsible for the catalytic activity and, as a consequence, its replacement by inactive MgA1204 diminishes the number of active sites, although

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MgA1204 can be prepared with a larger specific surface area than CoA1204. Co will be present as C0304, which seems to be less active than the respective aluminate. Addition of Rh, La, or a combination of La and Pd to the reference catalyst produces a considerable improvement in the conversion of N20 (Fig. 6). The activity curves show a different temperature dependence of the N20 conversion, demonstrating different apparent activation energies. This behaviour has been previously reported for Rh in Zn- and Co-based ex-HTlcs [4,5,12]. The presence of additional cations in a homogeneous and well-interdispersed mixed oxide may provoke the development of synergetic effects between the elements involved. X(N20) 1.0

X(N20) 1.0 0.9

0.8

~ooo<>%~<>oo<><x:~<>o~

0.8

0.6

Co,Mg-Rh,AI

0.7

0.4

9

0.6

0.2

0.5

0.0 450

0.4

525

600

675

750

T [K]

Fig. 6. N20 conversion as a function of the temperature. Conditions: 1 mbar N~O in He; 8.10~g.s/mol.

Co-Rh,AI

&

Co,Pd-La,AI

0

I

I

I

I

20

40

60

80

100

Time-on-stream [h]

Fig. 7. Evolution of N20 conversion during time on stream. Conditions: 1 mbar N~O in He; 580 K; 8.105 g.s/mol.

The stability during time-on-stream was checked for the most active catalysts (Fig. 7). Ex-Co,Pd-La,A1-HTlc presented a slight decrease in the conversion. ExCo-Rh,A1-HTlc showed a significant initial deactivation during 30 h under similar conditions. After addition of Mg to this catalyst (ex-Co,Mg-Rh,A1-HTlc), the conversion was maintained during the whole period at the same initial value as for ex-Co-Rh,A1-HTlc. This indicates that Mg does not affect the amount of N20 converted, but interacts with the active component in the catalyst, stabilising its properties for the reaction. As discussed previously, the activity of Co-Mg catalysts depends on the ratio between these two metals. The substitution of Mg by Co in the cobalt aluminate spinel negatively affects the conversion. However, in the catalyst containing Rh, the presence of Mg does not decrease the catalytic activity, suggesting that Rh is mainly responsible for the catalytic activity, even though only minor amounts are present relative to Co. With respect to the stability behaviour shown by ex-

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Co,Mg-Rh-A1-HTlc, it is suggested that the active Rh phase is stabilized by the presence of the magnesium aluminate spinel. The extensive interaction between the components in the catalyst structure should also play a decisive role in this behaviour, leading to a more active catalyst compared to that reported by Centi et al. [13], which was prepared by impregnation of Rh on an ex-Mg-A1-HTlc.

4. C O N C L U S I O N S

The presence of different bivalent and trivalent cations in the ex-Co-A1-HTlc has a large impact on the catalytic performance in N20 decomposition. Addition of Rh, La, or combinations of La and Pd to the Co-based spinel-type mixed oxide strongly improves the catalytic activity in the low temperature region, while Mg promotes an increase in the surface area of the catalysts and improves the timeon-stream stability. The results suggest that these additions affect the catalyst behaviour via interaction of the well-interdispersed cations in the structure, leading to the development of unusual structural and catalytic properties by synergetic effects. REFERENCES

[11 P.L. Crutzen, J. Geophys. Res., 76 (1971) 7311. [2] J.C. Kramlich and W.P. Linak, Prog. Energy Combust. Sci., 20 (1974) 149. [31 F. Kapteijn, J. Rodriguez-Mirasol and J.A. Moulijn, Appl. Catal. B, 9 (1996) 25. [4] J.N. Armor, T.A. Braymer, T.S. Farris, Y. Li, F.P. Petrocelli, E.L. Weist, S. Kannan and C.S. Swamy, Appl. Catal. B, 7 (1996) 397. [5] J. P~rez-Ramirez, F. Kapteijn, J. Overeijnder and J.A. Moulijn, in p r e s s Appl. Catal. B, 1999. [61 J. P~rez-Ramirez, F. Kapteijn and J.A. Moulijn, in p r e s s Catal. Lett., 1999. [7] M. Belloto, B. Revours, O. Clause, J. Lynch, D. Bazin and E. Elka/m, J. Phys. Chem., 100 (1998) 8535. [8] F. Cavani, F. Trifirb, and A. Vaccari, Catal. Today, 11 (1999) 173. [9] P. Arnoldy and J.A. Moulijn, J. Catal., 93 (1985) 38. [10] H.H. Kung, J. Catal., 73 (1982) 387. [11] G. Busca, V. Lorenzelli and V. Bolis, Mater. Chem. Phys., 31 (1992) 221.. [12] J. Oi, A. Obuchi, A. Ogata, G.R. Bamwenda, K. Tanaka, T. Hibino and S. Kushiyama, Appl. Catal. B, 13 (1997) 197. [13] G. Centi, A. Galli, B. Montanari, S. Perathoner and A. Vaccari, Catal. Today, 35 (1997) 113.

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Studies in Surface Science and Catalysis 130

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A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Cold Start Vehicle Emission Control Using Trapping and Catalyst Technology N.R. Burke~, D.L. Trimm a and R.Howe b "~School of Chemical Engineering and Industrial Chemistry, University of New South Wales, Sydney, NSW, AUSTRALIA 2052 bSchool of Chemistry, University of New South Wales, Sydney, NSW, AUSTRALIA 2052 Increasingly stringent legislation has focused attention on pollutants emitted when an engine is cold. High silica zeolites have been found to adsorb pollutants at low temperatures and to desorb the gases at temperatures =200~ when an exhaust catalyst is operational. Testing of Beta-zeolites with Si:AI ratios of 10 and 300 showed that adsorption capacity decreased in the presence of water. Smaller molecules were affected more severely than larger components in the exhaust gas. High temperature aging had little effect on the adsorption of large molecules, but reduced the capacity for small molecules as a result of structural changes in the zeolite. Desorption temperatures were lower in the presence of water, but still sufficient to ensure that the emission control catalyst was operational and the desorbed pollutants were oxidised. 1. INTRODUCTION The application of catalytic emission control to vehicle exhausts has led to dramatic reductions in hydrocarbon, carbon monoxide and nitrogen oxides emissions [1]. The catalysts use combinations of precious metals suspended in a washcoat, which is deposited on a monolith. The control of emissions is efficient once the catalyst temperature is greater than 170-220~ depending on catalyst age. Increasingly stringent legislation now requires more efficient emission control. Since up to 70% of the unwanted emissions are produced before the catalyst warms up, it is essential to focus on the cold start period. Some of the methods suggested to solve the problem include electrically heated catalysts, close coupled systems and fuel substitution [1,2]. One other possibility involves the use of adsorbents that can adsorb pollutants at low temperatures and release the trapped material at temperatures at which the catalyst is operational [3,4]. The efficiency of various adsorbents has been examined [4], with zeolitic adsorbents showing most promise. The efficiency of such adsorbents is dramatically affected by the fluctuating conditions found in vehicle exhausts. If the adsorbent is to be included in the washcoat, temperature

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stability to I100~ and stability in the presence of 5-10% water in the exhaust gases is essential [6]. A wide range of pollutants may need to be adsorbed, ranging from nitrogen oxides to aliphalic and aromatic hydrocarbons [5]. Since water is present at higher concentrations, preferential adsorption of steam may rule out otherwise suitable adsorbents

[5,6].

Previous studies have shown that activated carbon and zeolitic materials offered most promise [4]. Zeolites containing high silica to alumina ratios are known to be less affected by the presence of water [4,6] and their thermal and adsorptive characteristics may be altered by ion exchange or doping [7]. As a result attention was focused on Beta zeolites with Si:A1 ratios of 10 and 300. 2. E X P E R I M E N T A L Na-Beta zeolite was supplied by Zeolyst (USA) with a Si:A1 ratio of 10 and a pore volume of ca lml g-I [8]. H-Beta zeolite with a Si:A1 ratio of 100 was obtained from the same source. 5g Na-Beta was mixed with 100 ml lanthanum nitrate solution and heated at 70~ for 15h. The solid was then filtered, dried and calcined (500~ -4h). The procedure was repeated three times to give a La concentration, as measured by ICP of 15mo1%. The material was described as La-Beta. Adsorbents were aged by heating in 5% steam:nitrogen mixtures at 800~ for 200h. Gas adsorption experiments were carried out using individual gases (0.2% propene or 1% toluene) in dry and wet (5% water) atmospheres. Helium was the diluent/carrier gas. Simulated exhaust gases were also used (propane 240ppm, propene 360ppm, water 5%, NO l l00ppm, CO 1.1%, hydrogen 2800 ppm, oxygen 0.9% balance He) [5]. Gas adsorption/desorption was measured using a flow system. Gas flows were controlled using Brooks 5850E mass flow controllers. Liquid vapour was introduced from controlled temperature saturators. Ca 0.1g adsorbent was mounted in an 8ram o.d. silica tube and placed in a furnace that could be isothermally controlled (_+2~ or temperature programmed. Exit gas analysis was carried out using a Baltzers Thermostar mass spectrometer or a thermal conductivity detector (TCD)/flame ionisation detector (FID) setup (see reference 4 for details). Samples were degassed in helium at 500~ for 45min before use. Catalytic removal of desorbed gases was studied using a monolithic catalyst held in a flow system. The catalyst (Johnson Matthey), CSD, contained 0.3wt% Pt, Pd + Rh supported on a ceria-alumina washcoat. Individual gases or a mixture of gases (0.5%H2, 1.6% CO, 0.13%C3 H8, 13.4% CO2, 15.1% H20, balance N2) were fed to the reactor via Brooks 5850E mass flow controllers. Exit gases were analysed by gas chromatography, separating components on a CTR-1 column before either a TCD or a FID detector. Catalytic activity was determined both in terms of conversion and in terms of light off temperature [9].

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3. RESULTS AND DISCUSSION

3.1. Single component adsorption/desorption Toluene and propene adsorption capacities for La-beta, Na-beta and the high silica zeolite, H-beta, are shown in Figure 1. Toluene adsorption on La-beta and Na-beta were the same within experimental error (5%). Propene adsorption on La-beta was 16% higher than on Nabeta. H-beta adsorbed 55% less propene than Na-beta and 60% less than La-beta and 15% less toluene than Na and La-beta. This was expected in view of the higher Si:A1 ratio of Hbeta[ 10]. The results suggest that propene adsorption may be directly related to the surface ion or the aluminium content of the zeolite. Toluene adsorption showed less dependence on aluminium content. The difference in propene adsorption on Na- and La-beta may be explained in terms of the difference in valency of the metal ions used. The valencies may affect the rt-bonding of propene. This is reflected in the results not only in terms of higher adsorption capacity, but also in the higher desorption temperature of propene from the La zeolite.

Fig 1: Dry Atmosphere Adsorption c

o

4.E-03

. . . . . . . . . I~ propene

o ,~, 2.E-03

~'~ "~ a3

00.E+O0

:.El toluene

. . . . . . . . . . . . H-beta Na-beta La-beta Zeolite

Adsorption-desorption cycling produced a small decrease in adsorption capacity of propene (ca 10% on Na-Beta and 5% on La-Beta) resulting from polymerisation/coking at higher desorption temperatures [7]. Small traces of water minimised the decrease by facilitating coke removal [7]. NMR analysis showed oligomerisation on the samples treated with propene. This suggests the zeolite surface ions were catalysing the propene polymerisation reaction. Heating a sample of coked zeolite in a TGA setup resulted in a 3% weight loss and a colour change from black (coked) to white (original colour). Desorption was initiated by increasing the adsorbent temperature at a rate of 30~ min j The temperatures at which maximum desorption was observed are summarised in Table 1. Temperatures greater than 200~ were necessary to desorb propene (Table 1), but toluene desorbed at lower temperatures. This illustrates the use of stronger ~--bonds in the adsorption of propene. Toluene bonding is more physical in nature. The projected diameter of the toluene molecule (ca 0.65nm) is larger than that of propene (ca 0.6 nm). This difference should not prevent toluene entering the zeolite's channels but the orientation of adsorbed toluene may prevent the formation re--bonds.

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The presence of water produced a decrease in propene adsorption capacities of all samples (Figure 2). H-Beta capacity decreased by ca 60%, while Na-Beta and La-Beta reduced capacity by over 90%. Adsorption of toluene was not so badly affected, with the adsorption capacity of Na-Beta being reduced by 79%. 60% and 86% losses of capacity were observed for toluene on H-beta and La-beta respectively. The large loss of propene adsorption capacity seemed to suggest that water successfully competed for the sites onto which propene adsorbs.

Fig 2: Wet Atmosphere Adsorption ~-tO .o E t'3~

o _. t_

"0

1 .E-03

................................................................................................................................... W propene

.e-04

El toluene

~

0

O.E+O0

. . . . . . . . . H-beta

Na-beta

La-beta

Zeolite

No loss of capacity on cycling was observed. Desorption temperatures were of the same order as dry propene, with the exception of H-beta. Toluene's desorption temperatures decreased slightly for H-beta but increased for La- and Na-beta (Table I).

Fig 3" Aged Zeolite Adsorption O}

~-2.E-03 0 .o E c-

o ~ ~-~ O

1.E-03

i .......... i

j i

~1 propene [] toluene

.........X4 .....

0.E+00 H-beta

Na-beta

La-beta

Zeolite

Over 90% of propene's adsorption capacity on all zeolites was lost as a result of aging (Figure 3). This reduction in capacity is a result of structural changes in the zeolite. X-ray diffraction (XRD) measurements of fresh and aged zeolite samples showed a decrease in crystallinity for all the zeolites. La-beta showed the greatest decrease followed by Na-beta and H-beta. The fact that H-beta was the least affected of the zeolites and toluene was not so badly affected by the aging suggests the process of structural change probably involved dealumination [11 ]. A reduction in toluene adsorption of 49% was observed with Na- beta, 14% with H-beta and 31% with La-beta. Aging decreased desorption temperatures by ca. 15~ on all of the zeolites (Table 1).

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3.2. Simulated exhaust gas adsorption/desorption The adsorption capacities of Na-Beta and La-Beta in the absence and presence of water are shown in Figure 4. Although the performance of La- and Na-Beta was similar under dry conditions, the La doped sample adsorbed some 26% more gas in wet conditions. In both cases, propene adsorption was significantly reduced (ca 90%) in the presence of water. Again this is thought to result from water occupying the propene adsorption sites.

It is clear that water was adsorbing in the zeolite pore channels and displacing propene. Given the relative amounts of the two gases in the simulated exhaust (5% water, 0.2% propene) this is not surprising, but it does indicate the susceptibility of 7t-bonding molecules such as propene, to changes in the zeolite structure. For larger aromatic molecules, trapping even in the presence of water was more efficient. Given the combustion efficiency expected for components of gasoline in the engine [5], the cold start trapping of residual emitted gases may just be sufficient to meet new legislation.

Fig 4: Exhaust Gas Adsorption -~ 0 oE -= v C:L~ t--

I,..

0

>"

o~ "o

3.E-03

~1 p r o p - dry

2.E-03

II prop - wet

1.E-03

[ ] tot - dry

(~ O.E+O0 C)

[] tot- wet Na-beta

La-beta Zeolite

The destruction of pollutants desorbed from the molecular sieve was confirmed using a three- way-catalyst system. The oxidation of hydrocarbons was enhanced by the presence of steam, complete removal being effective at ca 250~ rather than at 325~ Carbon monoxide was totally oxidised at 150~ The reduction of desorbed NOx was not studied, but it is known that Pd and Rh will catalyse the production of nitrogen under these conditions [7]. 4. CONCLUSIONS High silica content zeolites are more efficient as cold start hydrocarbon traps than are the lower silica content zeolites. The high silica zeolites exhibit both better adsorption capacity in the presence of steam and better durability. Adsorption capacities for smaller gases are significantly reduced by the presence of water but trapping efficiency remains appreciable for larger organic molecules. The presence of La shows a slight improvement over the Na zeolite in simulated exhaust conditions and after aging. H-beta displays the highest desorption temperatures for all conditions. Destruction of gases desorbed at temperatures above 200~ has been demonstrated.

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ACKNOWLEDGEMENTS Thanks are due to the Australian Research Council for financial support.

Table I

Characteristics of adsorbents and desorption temperatures

Sample

State

S.A. a m2/g

pore size nm

pore vol. b m3/g

5% water

Na-beta

Fresh

715

0.57-0.6

0.69

Aged Fresh

720

0.6-0.8

0.95

La-beta H-beta

Aged Fresh

490

0.5-0.63

0.28

dry wet dry dry wet dry dry wet dry

aged

Propene des. temp (~ 189, 238 193 175 197 200 210 290 268 242

Toluene des. temp. (~ 140, 322 205 125 155 196 140 170 163 152

Determined by BET nitrogen adsorption Determined by nitrogen desorption

REFERENCES

.

3. 4. .

.

.

.

9. 10. 11.

R.M. Heck and R.J. Farrauto : Catalytic Air Pollution Control : Van Nostrand Reinhold (1995) K.C. Taylor, Catal.Rev.Sci.Eng., 35, (1993) 457 T.H. Ballinger, W.A Manning., (1997) Lafyatis DS, SAE 970741 N.R. Burke, D.L. Trimm, R. Howe and N.W. Cant, Proc.Chemeca 1998, Article 87 : Inst.Eng.Australia, Australia (1998) E.W. Kaiser, W.O. Siegel, Y.I. Henig, R.W. Anderson and F.H Trinker. : Environmental Sci and Tech., 25, (1991) 2005 K. Otto, C.N. Montreuil, Todoro, R.W. McCabe, and H.S. Gandhi : Ind.Eng.Chem.Res., 30, (1991) 2333 D.L. Trimm, Handbook of Heterogeneous Catalysis, Vol 3, p 1280 : Wiley VCH, NY (1997) R. Szostak : Handbook of Molecular Sieves, Van Nostrand Reinhold NY (1992) N.P. Siswana & D.L. Trimm, Catal.Lett, 46, (1997) 27 O. Taln, S. Zhang, D.T. Hayhurst :J.Phys.Chem. 97, (1993) 12894 J. K. Lampbert, M. Deeba and R. J. Farrauto: Development and Performance Characteristics of a Hydrocarbon Trap for Gasoline Cold Start Conditions, 2 nd World Congress on Environmental Catalysis, (1998)

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Oscillation of the NOx Concentration in Its Selective C a t a l y t i c Reduction on Platinum Containing Zeolite Catalysts Y. Traa, B. Burger and J. Weitkamp Institute of Chemical Technology I, University of Stuttgart, D-70550 Stuttgart, Germany Oscillations of the NOx concentration were observed in the selective catalytic reduction of NOx with propene on Pt-V/H-zeolites. It is assumed that the oscillations have their origin in a periodic transition of Pt between a metallic and an oxidic phase, which is catalyzed by V. 1. INTRODUCTION The selective catalytic reduction of nitrogen oxides by hydrocarbons is a promising process for the purification of exhaust gases from lean-burn gasoline- and diesel-powered engines. Platinum containing catalysts have proven to be efficient catalysts for this process because of their high hydrothermal stability and low lightoff temperature. In order to tune the activity and selectivity of the platinum containing catalysts we used a second metal as promoter. On Pt-V/H-zeolites we observed oscillations of the NOx concentration at high NOx conversions. 2. EXPERIMENTAL

SECTION

The zeolites were transformed into their ammonium forms and loaded with about 3 wt.-% Pt and 1 wt.-% V. The catalytic experiments were performed in a flow-type apparatus with a fixed-bed reactor at atmospheric pressure. 200 mg of the hydrated zeolite were activated in a flow of synthetic air for 1 h at 500 ~ and overnight at 450 ~ The gaseous feed components were premixed and afterwards saturated with water at 45 ~ Typically, the feed contained ca. 0.02 vol.-% NOx, 0.06 vol.-% C3H6, 0.03 vol.-% CO, 4 vol.-% CO2, 9 vol.-% O2 and 10 vol.-% 1-120 in He at a flow rate of 150 cma/min. The product stream was analyzed for nitrogen oxides with a chemiluminescence detector; all other components were analyzed by capillary gas chromatography. Prior to the temperature-programmed reduction with H2 (10 vol.-%) in Ar, the catalyst samples (400 mg) were oxidized in a gas stream containing O2 (10 vol.-%) in He. 3. RESULTS

AND DISCUSSION

Figure 1 shows the oscillation of the NOx conversion observed in the selective catalytic reduction of NOx with propene on 2.6Pt-l.0V/H-ZSM-35 zeolite. (The numbers before the metals indicate the metal content in wt.-% in the dry catalyst.) The oscillations could only be observed at high NOx conversion and within a temperature region of about 20 K. With

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100

,

,

,

,

,

90 0-.9.

80

70

OO o

oooc,. ,,

a =

Tea =

0

i

97 ~

Tea t

100

200

300

T I M E / min

Fig. 1. NOx conversion on 2.6Pt-l.0V/H-ZSM-35 (nsJn~ = 8) for different catalyst temperatures. increasing temperature, the amplitude of the oscillations decreases, whereas their frequency increases. The oscillations were fully reproducible, i.e., they occurred as well on other freshly prepared Pt-V/H-ZSM-3 5 batches and at slightly changed experimental conditions. Only minor changes occurred in the amplitude and the frequency of the oscillation, even over a time period of 17 h. While oscillations in the NOx reduction with hydrocarbons have so far only been reported in systems using feed gas streams without water [ 1-5], the presence of relatively large amounts of water seems to be crucial in our systems: We observed that the amplitude and the frequency of the oscillations decrease with decreasing water concentration (cf. Fig. 2). 100 t 90

'

i

'

9.9 vol.-% H20

I

'

i

'

I

3.3 vol.-% H20

6.6 vol.-% H20

i --- J x~

!

~-~t 1

7o

~176 f 50

0

,

I 100

,

I 200

,

I 300

,

I 400

500

T I M E / min

Fig. 2. NOx conversion on 2.6Pt-l.0V/H-ZSM-35 (nsi/nAj = 8) for different partial pressures of water.

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NborTainstead ofV ~H2O~ /

~

N%-ZSM-35insteadof NH4-ZSM-35 2[PtCId insteadof [Pt(NHz)4]CI2/ VOSO4instea~o f ~ 3Pt-1bV/H-ZS. l -35

n~nv

mcat"~,

,-------

Pd instead of Pt AI203instead of

/

NH4-ZSM-35

decreasing O'H20 increasing Tcat.

Fig. 3. Effect of different parameters on the oscillations (o -- volume concentration). Figure 3 shows the effect of different parameters on the oscillations. The system where oscillations occurred with maximal amplitude at medium frequency and low catalyst temperatures, viz. 3Pt-1V/H-ZSM-35, is arranged in the center of the figure. With decreasing water concentration (cf. Fig. 2) or use of Nb or Ta instead of V, the frequency and the amplitude of the oscillations decrease. If the water concentration approaches zero, a steady NOx conversion is observed. By contrast, with increasing catalyst temperature, decreasing catalyst weight, decreasing npt/nv ratio, use of VOSO4 instead of VCl3 as V source, use of H2[PtCI6] instead of [Pt(NH3)4]C12 as Pt source and use of Na-ZSM-35 instead of NHn-ZSM-35, the frequency of the oscillation increases, whereas at the same time the amplitude decreases in most cases. If the catalyst temperature is further increased, the oscillations stop. No oscillations can be observed, if the catalyst contains only Pt or only V, Pd instead of Pt or A603 instead of NH4-ZSM-35 as cartier. The oscillations are not limited to zeolite ZSM-35 They could also be observed on zeolites with other pore systems, for example on Pt-V/H-Beta and Pt-V/H-ZK-5. On the contrary, on Pt-V/7-A1203 no oscillations occurred. This can be due to its lack of microporosity or rather due to the lower strength of its acid sites. The latter shall be illustrated with the following example: The amplitude of the oscillations decreases from 2.6Pt-l.0V/H-ZSM-35 (nsi/n~a = 8) over 2.0Pt-l.0V/H-ZSM-35 (nsi/nAl = 9) to 2.7Pt-1.1V/Na-ZSM-35 (nsi/n~a = 8). In this order, the number of acid sites decreases. Switching from Pt-V/Na-ZSM-35 to Pt-V/y-A1203, the oscillations stop. One reason for this could be that Pt-V/Na-ZSM-35 still has a significant number of strong acid sites, which could be verified by 1H-MAS-NMR spectroscopy, whereas Pt-V/y-AI203 only disposes of weak acid sites, which might be too weak to initiate the oscillations. However, at this stage of the investigations, it cannot be decided whether the

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microporosity or the strength of the acid sites is the key parameter for the occurrence of the oscillations.

Another item which should be addressed is the fact that on Pd-V/H-ZSM-35 no oscillations occur (cf. Fig. 3). On Pt-V/H-ZSM-35, oscillations could be observed at high NOx conversion, and on Pd-V/H-ZSM-35, the maximal NOx conversion amounted only to 35 % at 239 ~ One explanation for the absence of oscillations on Pd-V/H-ZSM-35 could therefore be that the NOx conversion is not high enough and is reached at high temperatures only. In other words, a prerequisite for oscillations to be observed is a high catabtic activity at relatively low temperatures. 100

o" >
70

60

,

,

,

1

In further experiments, the Pt and V concentrations were varied. On zeolites containing Pt or V only, no regular oscillations of the NOx concentration were noticed, but irregular fluctuations on Pt/I-I-ZSM-35, which occurred exclusively just under the temperature of the maximum NOx conversion (cf. Fig. 4), i.e., the simultaneous presence of Pt and V is indispensable for the occurrence of regular oscillations with large amplitudes.

In an attempt to elucidate the reasons for this synergism, temperature-programmed reduction with H2 [ ! 50 was applied. This method showed that, on H-ZSM-35 0 50 100 loaded with Pt only or V only, no significant hydrogen T I M E / min consumption occurred at temperatures below 350 ~ Fig. 4. Fluctuation in the NOx con- presumably because the metals are located well version on 2.9Pt/H-ZSM-35 stabilized on cation exchange sites of the zeolite (cf. (nsi/n~a = 8), Teat. = 165 ~ Fig. 5). By contrast, on Pt-V/H-ZSM-35, large amounts of hydrogen were consumed already at 150 to 200 ~ i.e., in the temperature range in which the oscillations occur. (The small peak at 70 ~ is due to desorption of Ar which was previously adsorbed on the zeolite: the resulting smaller 1-12 concentration is recorded as apparent H2 consumption.) This means that Pt catalyzes the reduction of V and vice versa, which can be explained by the formation of bimetallic oxide clusters, in which the metals can be reduced more easily. Another possibility is that H2 dissociates on the first formed Pt and reduces via a spillover mechanism the V compound located elsewhere. In TPR experiments with further samples, it was noticed that on all catalysts on which oscillations with large amplitudes occurred the 1-12consumption was relatively large in the temperature range with high NOx conversions (< 350 ~ By contrast, on samples with small oscillation amplitudes, significantly less 1-12 was consumed in this temperature range compared to 2.6Pt-l.0V/H-ZSM-35, i.e., a certain amount of easily reducible metal has to be present so that oscillations with relatively large amplitudes can be initiated.

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For comparison, in Figure 6 the corresponding TPR results on the alumina system are depicted. On Pt/AI203, the Pt is already reduced at a lower temperature compared to the zeolitic system, because the metal is not stabilized at ion exchange sites. Besides, the artifact at 70 ~ is absent because of the much lower Ar adsorption capacity of alumina compared to the zeolite. However, the essential effect, the synergism of Pt and V, is the same with alumina and ZSM-35, i.e., Pt and V mutually catalyze their reduction, and the HE consumption on Pt-V/AI203 is significantly higher than o n P t / A l 2 0 3 and V/AI203. '

I

'

I

'

'

I

"~

I

'

Z

Z

O

0

a..

I'13..

1.0V/H-ZSM-35///~

O9 Z

0t) z

0 o

0 ro

2.7Pt/H-ZSM-35

z Ill (9 0

3.0Pt/AI203

Z iii

(.9 0n," a

a

>--F

>-r"

,

0

I

,

I

200 400 TEMPERATURE / ~

,

600

0

200 400 TEMPERATURE / ~

600

Fig 5. Results of the temperature-programmed Fig. 6. Results of the temperature-programmed reduction on different metal containing reduction on different metal containing H-ZSM-3 5 zeolites, aluminas. This comparison shows that the presence of a certain amount of easily reducible metal is only a necessary, but not a sufficient condition for the occurrence of the oscillations. 4. CONCLUSIONS The whole body of experimental results suggests that oscillations with large amplitudes will occur only if four conditions are fulfilled, viz., 1)

the partial pressure of water in the feed is high,

2)

the catalyst possesses acid sites with a certain strength and concentration and/or disposes of microporosity,

3) 4)

the catalyst displays a high activity already at relatively low temperatures and a large portion of the metal can be easily reduced.

Probably, requirements 2) and 3) have to be met, because the oscillations can only be observed in the range of high NOx conversions, i.e., on a well working catalyst. By contrast,

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items 1) and 4) allow to draw conclusions on the reaction mechanism: At this stage of the investigations we believe that the observed oscillations have their origin in a periodic transition of Pt between a metallic and an oxidic phase. This phase transition is possible because of the simultaneous presence of propene, carbon monoxide and oxygen in the feed. In the literature, oscillations on Pt catalysts are thought to be due to a periodic formation and decomposition of surface Pt oxides as well, e.g. [6,7], and the formation of these surface oxides was proven under comparatively mild conditions (160 ~ 5 vol.-% 02) [8]. In our system, the phase transition should be even easier because of the simultaneous presence of Pt and V: Both metals can change their oxidation states, i.e., a combined redox equilibrium could exist, e.g., Pt + 4

V O 2+ + 4 H §

<

> PtO2 + 4 V 3+ + 2

H20

Water could play a role in a slow reversible adsorption step as kind of a buffer step so that the oscillations could possibly be explained with a model comparable to Eigenberger's buffer step model [9]. This buffer step serves to block active sites after the ignition of the reaction and makes them available again after the extinction of the reaction. This could explain the necessity of large amounts of water in the feed (water is formed as reaction product, but probably not in sufficient amounts). Further attempts to elucidate the reasons for the occurrence of the oscillations are under way, but due to the complexity and corrosiveness of the feed in situ characterization and modelling of the system is quite demanding. ACKNOWLEDGMENTS The authors gratefully acknowledge financial support by Deutsche Forschungsgemeinschafi, Fonds der Chemischen Industrie and Max Buchner-Forschungsstifiung. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.

B.K. Cho, J.E. Yie and K.M. RahmoeUer, J. Catal. 157 (1995) 14. I. Halasz, A. Brenner, M. Shelefand K.Y.S. Ng, J. Phys. Chem. 99 (1995) 17186. A. Obuchi, M. Nakamura, A. Ogata, K. Mizuno, A. Ohi and H. Ohuchi, J. Chem. Soc., Chem. Commun. (1992) 1150. U.S. Ozkan, M.W. Kumthekar and G. Karakas, J. Catal. 171 (1997) 67. B.K. Cho, J. Catal. 178 (1998) 395. C.G. Vayenas, C. Georgakis, J. Michaels and J. Tormo, J. Catal. 67 (1981) 348. I.V. Yentekakis, S. Neophytides and C.G. Vayenas, J. Catal. 111 (1988) 152. J.J. Ostermaier, J.R. Katzer and W.H. Manogue, J. Catal. 41 (1976) 277. G. Eigenberger, Chem. Eng. Sci. 33 (1978) 1263.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Experimental and theoretical description of transition metal ion structures in zeolites relevant to deNOx catalysis Zden6k Sobalik, Judit E. Sponer and Blanka Wichterlov~ J. Heyrovsk~ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolej~kova 3, 182 23 Prague 8, Czech Republic

ABSTRACT Description of the perturbation of zeolite frameworks with topologies of MFI, FER and BEA by cations coordinated to the framework oxygens and bonding potential of deNOx reaction components, was provided by FTIR measurements. For identification of the nature of such local framework perturbation and the cation environment, a combination of the analysis of the antisymmetric internal vibrations obtained by FTIR and the results of ab initio quantum chemical model calculations has been used. 1. INTRODUCTION Transition metal ions coordinated to zeolite frameworks containing low concentration of aluminium of MFI, FER, and BEA topologies provide for unique activity/selectivity in several redox reactions, namely decomposition of NO and N20, and selective catalytic reduction of NO with hydrocarbons. As such activity is not found with well-dispersed metal oxide species on amorphous inorganic carriers, the unusual coordination of transition metal ions and the strong cation-framework mutual interactions in zeolites [1-3], can be expected to have a decisive impact on the metal ion reactivity. Transition metal ions and their complexes with reactants planted in defined sites of crystalline matrices, bring an opportunity to study on a "molecular" scale their structure, redox properties and catalytic activity by both experimental and theoretical methods. The aim of the presentation has been to describe, using FTIR spectroscopy and quantum chemical calculations, bonding of the transition metal ions in zeolites relevant to deNOx catalysis and identify a structural parameter which can characterise the extent of a zeolite framework perturbation. 2. EXPERIMENTAL

2.1. Metal-exchanged zeolite preparations Metal ion exchanged zeolites containing Co(II), Ni(II), Fe(II), Fe(III), Mg(II), Mn(II), Ni(II), Cu(II), Cu(I) or Ag(I) were prepared by ion exchange of the parent zeolites, i.e. ZSM-5 (MFI; Si/A1 11.4), ferrierite (FER; Si/A1 8.5), and beta (BEA; Si/A1 11.5), faujasite The authors gratefully acknowledgethe financial support of the Grant Agencyof the Czech Republic (project no. 203/96/1089) and Air Products & Chemicals, Inc., Pennsylvania.

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(Y; Si/A1 2.7) structures. Metal acetate or nitrate solutions (Cu, Co, Mn), chloride (Mg, Ni) or oxalate (Fe) were applied. Reduction of Cu(II) to Cu(I) was accomplished by evacuation of the sample at 450 ~ while oxidation of Fe(II) to Fe(III) by heating in 02 at 450 ~

2.1. FTIR spectra FTIR spectra of samples in the form of self-supported thin pellets (ca 5 m g / c m 2) were recorded on a Magna-IR System 550 FTIR Nicolet using a heatable cell connected to a vacuum/gas manifold. Usually 200 scans were recorded at resolution of 2 cm l for a single spectrum. For time-resolved spectra the resolution of 8 cm ~ with time resolution up to 15 spectra per second was applied. Prior to spectra monitoring the zeolites were heated in vacuum up to 480 ~ and/or with subsequent adsorption of gases (NO, CO, or NO2).

2.2. Model calculations Ab initio quantum chemical calculations were performed at DFT/Becke3LYP level of theory as implemented in the Gaussian94 computer code. All atoms but the metal were described by a double ~ quality basis set developed by Dunning. On the metal cations the relativistic pseudopotential of Christiansen's group was imposed. In case of transition metals a high spin state electronic configuration was assumed. Natural charges were derived from the optimised wavefunction.

3. RESULTS AND DISCUSSION

3.1. FTIR measurements A local perturbation of the framework bonds due to formation of a bare cation-zeolite framework complex (i.e. cation ligated exclusively to framework oxygens) was observed for M(II)-zeolites studied as well as Cu(I)- and Fe(III)-zeolites. The decomposition of the metalaquo complexes with intermediate structures representing a low metal-aquo complex (BML

08

b

B

|

1 BML

IBM

.o,4

g

Q2

~ 0,2

O0

0,0

930

~

98

9t)

930

980 ' 960 '

'

' 960 ' 880

wavenumber, cm -1 Figure 1. Infrared spectra of Mg-FER (part A) and Ni-FER (part B) of the ion exchanged samples (a) and after their evacuation at 150 (b), 250 (c), 350 (d), 450 (e) and 480 ~ (f) in the transmission window. The background, representing the parent zeolite, was subtracted.

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band), with formation of a "bare" cation (BM band) manifested in the FTIR spectra, is illustrated for Mg-FER and Ni-FER in Figure 1. The position of the main BM band, assigned to the shifted antisymmetric T-O-T vibration [2,3], and representing the prevailing bare cation-zeolite complex in 15-site, identified as the distorted six-member ring of zeolites [ 1], are summarised in Table 1. Among the monovalent cations studied, the BM band was observed only for Cu(I)-zeolite. For others (i.e. Ag(I), Na(I) or NH4+) no band on the slope of the intensive antisymmetric band of the unperturbed zeolite T-O bond was identified. Thus, it should be assumed that the value of the shift for Ag(I) is actually below the limit of detection under the experimental setup. Deformation shifts observed upon binding of cations obtained for FER indicate the following sequences: Fe (III) >> Ni (II) > Fe(II) > Cu(II) > Co(II) > Mg(II) >Mn(II) >>Cu (I) >>Ag (I). A similar sequence was obtained for the already collected data for M(II)-BEA and M(II)-MFI. Moreover, the actual values of BM bands for FER and BEA are nearly equal, while for MFI are systematically shifted to higher wavenumbers. A linear correlation has been established between the level of the metal ion exchange and integral intensity of the BM band (including the side bands) [ 1,4]. Thus, all the cations, bound at cationic sites, are reflected in the intensity of this band. A general phenomenon of a decrease of the framework perturbation after external ligand interaction with the "bare " cation was manifested by the relaxation shift found for all studied zeolites. The character of the process and the possibility to detect the strict time parallel between a decrease of the framework perturbation and the increase of the concentration of the metal-ligand complex is illustrated on the time-resolved FTIR experiments, giving simultaneously a time sequence of metal-nitrosyl formation and the "relaxation shift" during NO adsorption on Fe(II)- and Co(II)-FER (see Figure 2). Relaxation shift increase

M-NO formation A

I O. ,

I

,

I

l

,

I

,

I

,

o2 ,

2000

I

1900

,

i

1800

|

960 wavenumber, cm "1

,

920

!

880

Figure 2. Adsorption of NO (20 Torr) on Fe(II)-FER (Part A) and Co(II)-FER (Part B) followed by time-resolved FTIR. Time scale between the first and last spectrum is 1.5 sec in both experiments.

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Table 1 Position of the main BM and BML bands, and calculated P-values for metal exchanged zeolites MFI BM a -I cm

Co(II) Ni(II) Cu(II) Cu(I) Mg(II) Mn(II) Fe(II) Fe(III) Na(I)

931 930 960 939

> 980

BEA BM cm -1 915 915

925 926 914 > 980

BM cm -I 918 914 915 966 922 927 914 88O > 980

PER BM-CO b cm -I 930 933 f g 937 939

FER Ac cm -1 12 20

A2

PM__COe A2

0.20 0.17

0.24

15 12

0.62 0.24 0.36

pM d

0.28

0.67

aBM: position of the main band of the antisymmetric T-O-T skeletal vibration perturbed by bare metal cations; low intensity side bands are not included; bBM_cO: position of the main band of the antisymmetric T-O-T skeletal vibration perturbed by M-CO complex; cA: relaxation shift calculated as BM-cO- BM; dpM: calculated P-value for bare metal cations coordinated to the distorted six-member ring of ferrierite; epM__CO: P-value for M-CO complex coordinated to the distorted six-member ring of ferrierite; fNo interaction was indicated between Cu(II) and CO; gFor Cu(I)-CO the BML value was beyond the transmission window region. The values of the "relaxation shift" for several M-CO complexes in FER are summarised in Table 1. The relaxation shifts for various ligands change substantially with a zeolitemetal/ligand combination showing differences in ligand interactions with the cation-zeolite systems. Thus, for the NO2 ligand a value of 32 cm -! for Co-MFI, while 11 cm -! for Co-BEA was found. For the NO ligand, values of 7 cm 1 for Fe(II)-FER and 14 cm -1 for Co-FER and nearly 30 cm -1for Co-MFI were observed. Bonding of CO ligand induces a relaxation shift of 12 cm -1 for Co-FER or Co-BEA, but only of 5 cm -1 for Co-MFI. It could be summarised that the extent of zeolite framework perturbation, as revealed by FTIR experiments, is a cation-specific effect. For FER and BEA structures, the actual BM values are nearly identical, in agreement with the similar structure of the distorted six-member ring, identified as the main cationic site position responsible for the main BM band. A general tendency of relaxing the perturbation upon bonding of an extemal ligand was established.

3.2. Quantum chemical calculation The computed models (see Figure 4) follow the arrangement of T-atoms in the deformed six-member ring of FER, i.e. the structural unit preferentially occupied by metal ions [1 ], and represented by the main BM band. Mg(II), Mn(II), Co(II) and Ni(II) have been selected for computations. With monovalent Na and Cu cations, a simplified version of the model was used with only one A1 and one Si atoms at the T positions. The effect of cation interaction with an extra lattice ligand - manifested in the "relaxation shift"- was exemplified on CO adsorption with Mg(II)- and Ni(II)-FER.

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(B)

化

(A) d1=5.40A d2=3 16 A d3=1".0 A d4=1.49A a1=109.5 de9. a2 =140.0 deg.

H.

~'

~

d3

H, ,H '

d4

~-!, d4 (H.,

M=Na

M=Na

M=Ni

(a)

(b)

(c)

Figure 4. Computed models and molecular electrostatic potentials (MEP) for monovalent and divalent metal cations. Simplified model used for monovalent (A), and the larger model used for divalent cations (B). MEP for the optimized model with Na(I) of the A-type (a), and B-type with 1 A1 (b), and for Ni(II) with the B-type model and 2 A1 (c). In Table 2 are summarized the characteristic electronic and structural parameters computed for models of cations in the deformed ferrierite six-ring. A substantially lower (0.17 atomic units) positive charge on Cu + indicates that contribution of a framework to metal charge transfer plays an important role in stabilization of transition metal cations over the exchange sites of zeolites, while Na + ions are stabilized by electrostatic forces. Operation of the extra stabilizing effect, i.e. the covalent contribution, is reflected by the sum of interatomic distances measured between the metal and the main electron donor species of the skeleton (d(Me-A1)+d(Me-O)), as being 0.41 A lower in the model with Cu(I) (4.51 A) than in that with Na(I) (4.92 A). Charge transfer effects play a significant role in divalent metal containing zeolites as well. The computed charges on the divalent transition metal cations are much lower than on the Mg(II) cation. Based on the computed results, we suggest the following empirical formula as a measure of the perturbation caused by the metal binding to the zeolite framework: P = k . ,5 +( M e ) . E [ d ( M e - O ) -

d(Al-O)]

(1)

where k is a "scaling" factor, introduced to normalize the equation to the neat perturbing effect of an ideal charge of +1 or +2, for monovalent and divalent ions, respectively, which is partially delocalized over the zeolite skeleton to the extent determined by the optimized wavefunction coefficients obtained for a particular cation. For divalent cations k=-0.5 and for monovalent cations k=2; &*(Me) is the computed partial charge on the metal cation; E[d(Me-O) - d(A1-O)] can be obtained by summing up the differences between those A1-O and Me-O bond distances which are joined through a common skeletal oxygen atom. (Definition and physical content of the P-value is discussed in details in Ref. [5].) Thus, the physical meaning of parameter P can be considered as a superposition of an ionic (represented by ~*(Me)) and a covalent contribution (represented by Y.[d(Me-O)- d(A1-O)]). Parameter P has been found to correlate well with the experimentally measured positions of the BM bands.

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Table 2. Computed atomic charges (atomic units) and selected geometrical parameters (distances in A)

Parameter Mg(II) Mn(II) Co(II) Ni(II) Na(I) Cu(I) O+(Me) 1.84 1.58 1.49 1.48 0.96 0.79 d(AI-O l)+d(A1-O2) 3.72 3.66 3.71 3.72 3.68* 3.62* d(Me-Ol)+d(Me-O2) 3.98 4.11 3.98 3.95 4.38** 4.40** * Calculated as 2-d(AI-O) for monovalentcations. **Calculatedas 2-d(Me-O) for monovalentcations.

In general, the P-value increases when adding a CO ligand to the model (cf. Table 1). This is in a sound agreement with the observed relaxation upon adsorption of external ligand, with the BML band at higher wavenumbers than the BM band. As seen from the molecular electrostatic potentials (MEP), presented in Figure 4, the Pvalues are primarily dependent on the symmetry of the cationic site. This is the reason, why the experimentally measured BM band positions are the same for M-FER and M-BEA series with similar six-member ring symmetries, while distinctly different for M-MFI. CONCLUSIONS Metal cations at exchangeable sites induce perturbation of the framework bonds as reflected in the shifted position of the antisymmetric T-O-T vibration. This perturbation is a quantitative measure of a degree of metal ion exchange, moreover distinguishing different cationic sites, strength of the cation-framework oxygen bonds, and interaction with an extraframework ligand. A sound correlation has been found between the computed P-value and the experimentally observed wavenumbers of the BM band, characterising experimentally the framework perturbation upon cation binding, as well as the relaxation effect due to external ligand coordination to the cation. The computations have shown that, besides electrostatic forces, the framework-metal charge transfer effects cannot be disregarded when discussing the binding properties of the transition metal cations at the cationic sites of zeolites. This charge transfer causes a local perturbation of the electronic distribution over the exchange sites of the aluminosilicate zeolite framework. REFERENCES 1. B. Wichterlovfi, J. D6de~ek, and Z. Sobalik, Proc. 12th Int. Zeol. Cone Baltimore 1998, M.M.J. Treacy, B. Marcus, J.B. Higgins,(Eds.), Materials Research Society, Warrendale, Pennsylvania, p.941. 2. W.P.J.H. Jacobs, J.H.M.C. van Wolput, and R.A. van Santen, Zeolites 13 (1993) 170. 3. J. S/trk~iny and W.M.H. Sachtler, Zeolites 14 (1994) 7. 4. Z. Sobalik, A.A. Belhekar, Z. Tvarfl~kov~i, and B. Wichterlov~i, Appl. Catal. A: General 188 (1999) 175. 5. J.E. Sponer, Z. Sobalik, B. Wichterlovfi, Angew. Chem. Int. Ed., submitted.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

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C O O P E R A T I O N O F P t A N D Pd OVER H - M O R D E N I T E L E A N S C R O F N O x BY M E T H A N E .

FOR THE

Consuelo Montes de Correa, Fernando C6rdoba C. and Felipe Bustamante L. Department of Chemical Engineering, Universidad de Antioquia Apartado A6reo 1226, Medellin-Colombia. Email: [email protected] A cooperation effect between low levels of Pt and Pd supported over H-MOR leads to an active and selective catalyst for the lean SCR of NOx by methane. It is suggested that the role of Pt is to favor NO oxidation. Moreover, the support acidity and Pd promote the reduction of NOx. The activity of the bimetallic catalyst is highly increased by pretreatment at 450~ in a stream of NO/O2/He during 3 h. Besides, the bimetallic catalyst is more tolerant to the presence of water and SO2 than the corresponding monometallie Pt- and Pd-H-MOR. However, after several runs the bimetallic catalyst gradually loses its catalytic activity for the reduction of NOx, likely due to dealumination. I. INTRODUCTION It is generally accepted that the reduction of NOx with hydrocarbons in excess oxygen proceeds by a series of steps probably catalyzed by different catalytic species. Therefore, it is expected that adequate combinations of catalytic species may give highly active catalysts [ 1]. Among transition metal zeolites, Pd/H-ZSM-5 and Pd/H-MOR have shown [2,3] high activity for lean CI-I4-SCR. This suggests that palladium activates methane whereas the acidity promotes the formation of N2. However, Pd/H-zeolites suffer severe dealumination after hydrothermal aging [4]. Recently, it has been reported [5-8] that combinations of catalytic species may improve the tolerance of catalysts to water vapor. Ogura et. al. [5] observed that Pd-Co/HZSM-5 had a considerably higher activity in the presence of 10% water than the corresponding Pd/H-ZSM-5 and Co/H-ZSM-5. Similar results were obtained with PdCo/HMOR [6]. More recently, it was found that when both Co and Pt are exchanged into Mordenite, ZSM-5 and Ferrierite the resulting catalysts are more tolerant to wet conditions [7]. Here, we report the performance of Pt-Pd/H-MOR on the lean SCR of NOx by methane in water rich atmospheres and in the presence of SO2. 2. EXPERIMENTAL The parent sodium MOR (Si/AI =7), was synthesized in our lab and converted first to the NH4-form by exhaustive ion exchange with ammonium chloride. Pd-NI-h-MOR was prepared by incipient wetness impregnation using the water soluble Pd(NH3)4CI2, drying at 60~ and calcining at 450~ in flowing air for 3 h. Pt-Pd/HMOR was obtained by impregnating

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Pd/H-MOR and pretreating at 450~ in flowing air for 3 h. Catalytic activity measurements were carried out in a pyrex fixed-bed flow reactor containing 100 mg catalyst. The gaseous feed consisted of 1000 ppm NOx, 2000 ppm CH4, 6% 02, and balance helium. The effect of water and SO2 was examined by adding 10% H20 and 30 ppm SO2 to the feed stream. The composition of the reaction mixture was controlled by electronic mass flow controllers (Brooks 5850E). The total flow rate was 100 ml/min and the temperature range was 150-530~ The feed and effluent gases were sampled and analyzed by an on-line gas chromatograph (Varian Star 3400) in series with a chemiluminiscence NOx analyzer (Ecophysics CLD 700 E1 Ht). Temperature-programmed desorption experiments were carried out in the same reactor system. A 100 mg sample of 0.04 wt-% Pt 0.04 wt% Pd-HMOR calcined 6 h in helium and sieved to 60-80 mesh was pretreated in situ at 450~ in flowing NOx/O2/He for 3 h. AJter cooling in flowing He to 25~ 5000 ppm NOx/He was adsorbed on the catalyst at room temperature. The effluent of the reactor was continuosly monitored by a FTIR multicomponent gas analyzer (Temet, model Gasmet DX 4000), and the leveling off in the NO concentration indicated the saturation of the sample with NO. ARer NO adsorption the sample was flushed with He to remove weakly adsorbed NO species. As the NO concentration monitored in the analyzer returned to near the background level the sample was heated to 530~ at a ramp rate of 5~ into either He, 1% O2/He, 1% CI-I4/He and 1% CH4+l% O2/He flowing at 100 cm3/min. The desorbed species were monitored continuosly by the FTIR multicomponent gas analyzer previously calibrated for NO, NO2, N20, NH3, CH4, CO2, CO, H20.

Pt(NH3)4CI2 o v e r

3. RESULTS AND DISCUSSION Fig. 1A shows that the activity for NOx reduction is enhanced on 0.04 wt-% Pt - 0.04 wt-% Pd-HMOR catalyst (0.04PtPd-HMOR) as compared to that over 0.04 wt-% Pt- lwt-% Pd-HMOR. Besides, the former catalyst appears to be more selective since methane conversion is lower than on the latter catalyst (see Fig. 1B). No significant SCR improvement to that observed in Fig. 1 A was obtained using different catalysts containing Pd and Pt in the range between 0.02-1wt-%. Therefore, 0.04PtPd-HMOR catalyst was chosen for additional tests. :

100

~

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-,- 0.04PtPd-HMOR --- 0.04 % Pt, 1% Pd-HMOR

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> C

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ql"

Z

,-

150

i

300

450

Temperature ~

600

100

-,- 0.04PtPdHMOR

75 50

"!" tO

25

=<

o

/ ,; /,'/;

B

/; //

---0.04% Pt, // ' / ' 1% Pd// HMOR /f/ i

0

150

300

450

600

Temperature ~

Fig.1 Lean SCR of NOx by methane over bimetallic PtPd-HMOR catalysts (A) NOx conversion, (B) methane conversion. Dry Feed: [NOx]=1000 ppm, [CH4]=2000 ppm, [O2]=6%, balance He, GHSV=30.000. Samples pretreated in flowing air at 450~ for 3 h.

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Figs. 2A-B show the effect of SO2 and water on NOx and methane conversions over 0.04PtPd-HMOR. In the presence of 30 ppm SO2 both NOx and methane conversions are enhanced. Platinum has shown [8] to catalyze the oxidation of SO2 to SO3. Since no significant poisoning of Pt by treatment in SO3 alone was previously observed [9], it appears that the resistance of 0.04PtPd-HMOR to SOz is likely due to the presence of Pt. Under wet conditions and in the absence of SO2, NOx reduction is improved but, methane conversion is inhibited. The enhancement of NOx reduction in the presence of water over 0.04 PtPd-HMOR catalyst is consistent with previous results in which the addition of Pt to transition metal zeolites lead to more water resistant catalysts [6-7]. However, water inhibited methane conversion which is favored over the Pd component of the catalyst[10], suggesting that palladium hidroxides known to be less active for methane combustion may have formed in the presence of water. The presence of both 10 % water and 30 ppm SO2 inhibits both NOx and CI-Lt conversions. Since the sample used in this experiment was the same as in the previous two runs, it is likely that the observed inhibition is due to zeolite dealumination. For dealuminated samples protons needed to stabilize the precious metal cations become blocked and the growth of oxide particles outside the zeolite crystallites is favored [ 11 ]. No activity for NOx reduction was observed after aging a 0.04PtPd-HMOR sample in the feed stream containing 14 % water and 30 ppm SO2 for 72 hours at 500~ A 25% decrease in BET surface area, as compared to the fresh catalyst was measured on the aged catalyst. However, the structure of mordenite was maintained highly crystalline. ~00 |

: 100 0 ~ H20=10%, S02-30i~Pm No H20, No S O2 / :" 9 ~.......S02=30__00m/ /' ~"; / 50 ; ~ - H 2 0 = 1 0 % ' ,

- - N o H20, No SO2 --- H20=10%, SO2=30 ppm .........SO2=30 ppm .?".-"-" .... H20=10%

, m

75

c

o 50 r

O

x

O 25

i

! :

,

9 / , oO ,~

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r

Z

0 0

150

300

450

Temperature ~

600

0 ~ ........... 0 150 300

450

600

Temperature ~

Fig.2 Effect of water and SOz on the lean SCR of NOx by methane over 0.04 PtPd-HMOR catalyst (A) NOx conversion, (B) methane conversion. Dry Feed [NOx]=1000 ppm, [CH4]=2000 ppm, [O2]=6%, balance He, GHSV=3 0. 000. Samples were pretreated in flowing air at 450~ for 3 h. While not shown, the corresponding monometallic 0.04Pt-HMOR and 0.04Pd-HMOR catalysts are much more sensitive to wet streams containing SO2 than the bimetallic 0.04PtPdHMOR. Thus, there is an evident cooperative effect of supported Pt and Pd on H-MOR even in such a low concentration of the precious metals. Interestingly, the activity of 0.04PdPtHMOR depends on the exact pretreatment conditions. It can be increased by about three times if the He calcined 0.04PtPd-HMOR sample is exposed during 3 h at 450~ to the NOx/O2/He

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feed before reaction. However, no improvement was observed when the pretreatment was conducted in a reducing atmosphere, i. e. CO or H2 in He. The temperature-programmed desorption (TPD) of NO in He is shown in Fig.3A. NO and NO2 desorb up to 300~ NO2 desorbs in three well defined peaks at 10 I~ 134~ and 162~ A shoulder at 114~ and peaks at 145~ and 158~ are observed on the NO spectrum. Nearly equimolar quantities of NO (0.215) mrnol/g) and NO2 (0.213 retool/g) are desorbed. N20 and NH3 desorption are insignificant as compared to NO and NO2 desorption. Besides, since N2 does not absorb in the IR, it was not monitored. The total amount of nitrogen species (as NO and NO2 ) is 0.43 mmol per g of catalyst. As illustrated in Fig. 3B the NO desorption spectrum in flowing l%O2/He shifted to lower temperatures. The spectrum exhibits three shoulders (71~ 121~ 162~ and a peak at 137~ The desorption ratio of NO2/NO in flowing 1%O2/He is about 1.8 as compared to a ratio of 1.0 which is observed in flowing He. This suggests a transformation of some of the adsorbed NO to NO2 in the gas phase. The total amount of nitrogen-containing species desorbing in 1%O2/He is 0.38 mmol/g catalyst. 400

E

Q.

300

NO2~

500

A

E

400

'~~N02

~. 300

200

/

\

200

100

100 100 200

300

400

Temperature(~

500

0 0

100

200

300

400

Temperature (~

500

Fig. 3 Temperature Programmed Desorption of NO from 0.04PtPd-HMOR: A. in flowing He. B in flowing l%OJHe. Fig. 4 shows the TPD of NO into a stream containing 1% CH4 in He taken after exposing 0.1 g of catalyst to 5000 ppm NO at room temperature for 30 min. NO, NO2 and N20 desorption profiles are illustrated in Fig. 4A. The N20 desorption profile was increased by 10 times to compare with NO and NO2 in the same plot. The NO2 desorption profile exhibits two peaks (98~ 140~ and a shoulder (150~ The amount of NO2 desorbed is 0.33 mmol/g. The spectrum of NO shows two desorption peaks (98~ 180~ and two shoulders at 140~ and 180~ The quantity of NO desorbed is 0.31 mmol/g. Peaks at 80~ and 150~ as well as shoulders at 70~ 100~ and 175~ can be seen in the N20 desorption spectrum. However, the amount of N20 desorbed, 0.015 mmol/g, is very small. The total nitrogen containing species monitored amounts 0.67 mmol/g which is much higher than the values observed in the two previous TPD runs. It appears that methane helps to clean the precious metals removing residual nitrogen species held on the metallic surfaces that are not removed by He or 02. Under our experimental error we did not observed any N2 by gas chromatography. Actually if some nitrogen formed it should be insignificant in comparison with the amounts of NO and NO2 desorbed. Therefore, we assume that the NO adsorption capacity of 0.04PtPd.HMOR is around 0.67 mmol/g.

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化

800

E

600

200 ~NO2

A

150

B

CO2/~

E 100

o.. 400

"

200 100 200 300 400 500

Temperature(*C)

50

~ 0

/ o! C

100 200 300 400 500 Temperature (=C)

Fig. 4 Temperature Programmed Desorption of NO from 0.04PtPd-HMOR in flowing 1% CH4/He. A. nitrogen containing species, B. CO and COz. Fig. 4B shows a small amount of CO formation ( 0.03 retool/g) at temperatures higher than 400~ Besides, CO2 formation (0.16 retool/g) occurs in two steps. A broad peak is observed at temperatures lower than 300~ and at temperatures higher than 300~ CO2 formation starts up again and peaks at about 440~ At low temperatures, the appearance of CO2 suggests that CH4 reduces NO2 via the reaction 4NO2 + CH4~ 4NO + CO2 + 2H20. The CO2 decrease at about 440~ and the CO formation at temperatures higher than 400~ may suggest that the reaction taking place under these conditions is: 2CI-I4 + 6NO ~3N2 + 2CO +

4H20. 500

The TPD of NO into a stream containing 1% CH4 + 1% O2[He presented in Fig.5 is similar 400 N02~,~ to the TPD of NO into 1% CH4/He. However, NO2 is observed in the gas phase E 300 between 60~ and 300~ NO is also seen in 2O0 the gas phase beginning at 40~ and 100 continuing to temperatures of more than 300~ N20 is observed in the gas phase 0 between 80~ and 240~ No CO and CO2 100 200 300 400 production was observed over the Temperature (~ temperature range of 25-380~ Therefore, in the presence of 02 the above reactions of methane with NO2 and NO are inhibited over Fig. 5 Temperature Programmed Desorption 0.04PtPd-HMOR. Here again as in flowing of NO from 0.04PtPd-HMOR in flowing 1% He, equimolar quantities of NO and NO2 are CI-I4+ 1%Oz/He. observed in the gas phase. The amount of nitrogen containing species into the stream 1% CH4 + 1% 02/He (about 0.47 mmol/g) is much lower than that observed into the stream containing 1% CHUHe (0.67 mmol/g). Besides, the range of temperature for NO and NO2 desorption into the 1% CI-I4+ 1% OJI-Ie is broader than that into 1% CHUHe. Therefore, the adsorption strength of NOx appears to be enhanced by

02.

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From the above TPD experiments it is interesting to notice that NO2 formation is significant over 0.04PtPd-HMOR even in the absence of oxygen. Previous TPD studies of NO over Pd-HZSM-5 in flowing He [12] have shown that NO2 desorption is about 14 times lower than NO desorption. Therefore, the observed enhancement of the lean SCR of NOx over bimetallic 0.04PtPd-HMOR appears to be due to a cooperative effect of Pt and Pd supported on H-MOR in which the role of Pt is to favor the oxidation of NO to NO2. Besides, it is well established that the acidity of H-MOR and Pd are needed for the reaction between NOx and methane to form N2, CO2 and H20. It is important to point out that NO2 is not necessarily an intermediate in the reduction of NO over Pd loaded zeolites but, it is more reactive than NO for the lean SCR over palladium loaded on acidic supports [ 12].

4. CONCLUSIONS Cooperation of low levels of Pt and Pd supported over H-MOR leads to a more tolerant catalyst to water and SO2 than monometaUic Pt- or Pd-HMOR. The role of Pt is to favor NO oxidation and both the acidity and Pd promote the reduction of NOx. The catalyst performance is highly enhanced by pretreatment in the NOx/OJHe feed for three hours before reaction. However, stability toward dealumination after repeated exposure of the catalyst to high temperature streams containing water and SO2 needs to be improved.

ACKNOWLEDGEMENTS

The authors are grateful to COLCIENCIAS and UNIVERSIDAD DE ANTIOQUIA for supporting this work through the project 1115-13-133-95.

REFERENCES

1. 2. 3. 4.

H. Hamada, Catal. Surveys from Japan 1 (1997) 53. Y. Nishizaka, M. Misono, Chem. Lett. (1994) 2237. H. Uchida, K.I. Yamaseki, I. Takahashi, Catal. Today 29 (1996) 99. C. Descorme, P. G61in, M. Primet, C. L6cuyer, J. Saint Just, in L. Bonneviot and S Kaliaguine (Eds.), Studies in Surface Science and Catalysis, Vol. 97, Elsevier, Amsterdam, 1995, p. 287. 5. M. Ogura, Y. Sugiura, M. Hayashi and E. Kikuchi, Catal. Lett. 42 (1996) 185. 6. C. Hamon, O. Le Lamer, N. Morio, J. Saint-Just, WO Patent Application 98/15 339, assigned to Gaz de Grance, 16 April 1998. 7. L. Guti~rrez, A. Boix, J. Petunchi, in J. Armor (Ed.) Extended Abstracts of the 2"a World Congress on Environmental Catalysis, Miami, 1998, p. 68. 8. H.S. Gandhi, M. Shelef, Appl. Catal. 77 (1991) 175. 9. D.D. Beck, M.H. Krueger, D.R. Monroe, SAE Techn. Paper No. 910844, 1991 10. R. Butch, P.K. Loader, F.J. Urbano, Catal. Today 27 (1996) 243. 11. B.J. Adelman, W.M.H. Sachtler, Appl. Catal. B 14 (1997) 1. 12. L. J. Lobree, A.W. Aylor, J. A. Reimer and A.T. Bell, J. Catal. 181 (1999) 204.

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Studies in Surface Science and Catalysis 130 A, Corrna, F.V. Melo, S, Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B,V. All fights reserved.

The selective catalytic reduction of N 2 0 by NH3 on a Fe-BEA Catalyst Mathias Mauvezin a, G&ard Delahay a, Bernard Coqa and Stephane Kiegerb, aLaboratoire de Mat6riaux Catalytiques et Catalyse en Chimie Organique, UMR CNRSENSCM 5618, ENSCM, 8 rue d l'Ecole Normale, 34296 Montpellier Cedex 5, France bGrande Paroisse, 12 place de l'Iris, 92062, Paris-La D6fense Cedex, France A Fe-exchanged zeolite beta is shown to be very active in the reduction of N20 to N2 by NH3 in presence of 02. The temperature of 50% N20 conversion is lower by ca 80 K than that of its catalytic decomposition in absence of NH3. Kinetics with 14N20 and 15NH3 has shown that N20 is converted at ca 86% through N-O splitting and 14% N-N splitting. The former reaction involves the redox cycle Fe 3+ ,-~ Fe 2+ where the active species are likely binuclear Fe oxocations. This conclusion was reached from TPR, TPO and TPD experiments. The intimate mechanism and the sites responsible for N-N splitting were not yet identified. 1. INTRODUCTION Nitrous oxide exhibits a high potential contribution to the greenhouse effect and takes part to the destruction of the stratospheric ozone layer. N20 is produced in considerable quantities in fluid-bed coal and biomass combustion processes, and acid adipic plants. Additionally, this nitrous oxide is also present, produced as a by-product, in high temperature Selective Catalytic Reduction units that are used for nitrogen oxides (NOx) reduction in power plants and nitric acid plants. Although N20 sources have not yet been exactly evaluated, emissions of N20 from fossil fuels combustion have been estimated for 1996 in 190-520 kton/year from stationary sources, and 400-850 kton/year for mobile sources. At the Kyoto's Conference in 1997, the EU committed itself to reduce by 8% in 2010 the emissions of greenhouse gases based on the values of 1990. The simplest control technology for N20 release is based on the catalytic decomposition to N2 and 02 [1], and Fe-zeolite catalysts have been proved to be amongst the most efficient materials in O2-rich atmosphere. However, these catalysts do not generally exhibit sufficiently high activity below 673 K. Otherwise, the catalytic conversion of NO to N2 in the presence of O2 is greatly enhanced on Cu-zeolites by the use of NH3 as reductant, and recent studies have moreover demonstrated that N20 conversion to N2 was accelerated on Fe-zeolite thanks to the presence of CO [2-5], C3H6 [6,7], C3H8 [8,9], and NH3 [10,11]. A mechanism proposed for the decomposition of N20 over Fe-exchanged zeolites is based on the redox cycle Fe 3+ ~ Fe 2+ with the desorption of adsorbed oxygen as slow step [1]. The boosting effect of reductant addition on the decomposition of N20 to N2 might be interpretated by making the removal of surface oxygen easier [5]. However, fundamental studies on that point are very scarce [5], and do not exist in the case of reduction of N20 by

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NH3 in presence of 02. On that account, the aim of the present work was to elucidate some aspects of the mechanism of N20 reduction by NH3 in presence of 02 on a Fe-BEA catalyst.

2. EXPERIMENTAL A Fe/zeolite-beta (Fe-BEA)was prepared by ion exchange of H-BEA (PQ corporation, SBET = 680 m 2 g-l, Si/A1 = 12.5) with Fe(NO3)3,9H20 at pH = 4. After filtration, the solid was dried at 393 K then calcined at 823 K in air overnight. The chemical analysis was performed at the Service Central d'Analyse du CNRS (Vemaison, France) by ICP. The amount of Fe (1.9 wt%) corresponds to a theoretical exchange degree of 97% (300Fe/AI). The nature and the amount of various Fe species were determined by TPR and MOssbauer spectroscopy. TPR of the calcined sample at 823 K was carried out with a Micromeritics 2910 apparatus with H2/Ar mixture (25/75, vol/vol) at 10 K min -1 from 293 to 1373 K. Before the catalytic tests, the sample (= 200 mg) was reactivated m situ at 823 K in air in a quartz fixed-bed reactor. The decomposition of N20 (N20/O2/He = 0.2/3/96.8) and the SCR of N20 (N20/NH3/O2/He = 0.2/0.2/3/96.6) were performed in the temperature programmed surface reaction mode (ramp: 10 K min -], WHSV: 35,000 h-]). Some experiments with ]SNH3 were carried out at the steady state (T = 623 K) in the SCR of N20. The detection was carried out with a quadrupole mass spectrometer BALZERS Omnistar and a Bruker Equinox 55 FTIR spectrometer. Calibrations were done on selected masses before and after each catalytic experiments. 3. RESULTS AND DISCUSSION After calcination at 823 K in air, iron in Fe(97)-BEA is composed of Fe ions with traces of Fe203 (< 2%) as shown by MOssbauer spectroscopy (Fig. 1) and TPR experiments (Fig. 2). The MOssbauer spectrum is well simulated by three contributions of Fe 3+ in octahedral coordination, which could be compared to the three families of framework sites with related geometries in zeolite beta [ 12]. In TPR experiments the choice of the reducing gas composition '

I

'

I

'

I

'

I

'

I

'

I

'

I

'

.... 'l .... Ar desorption

o

S

I ....

/

~ .... ~L

1 200

t~

-4

1400

~-

-

I000 800 600

= -rn

-4

I

-3

=

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n

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,

I

,

I

,

I

-2 -1 0 1 2 VelociW (ram s 1)

n

I

3

"= G)

c:z. E

400

,

4

Fig.1. M0ssbauer spectum of Fe(97)-BEA as stored after calcination at 823 K.

50

1 O0

150

200

T i m e (rain)

Fig.2. TPR by H2/Ar (25/75) of Fe(97)BEA after activation in air at 823 K.

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(H2/Ar, 25/75) was done with respect to a balance between reduction of Fe species below 1300 K and sensitivity. In the Fe(97)-BEA sample, the peak at 623 K corresponds to the reduction of isolated Fe 3+ to Fe 2+ and of Fe203 clusters to Fe304. The later species are then reduced to FeO and to Fe 0 clusters at 870 K. Fe 2+ reduces to Fe ~ with collapse of the zeolite network above 1100-1200 K. The SCR of N20 in presence of NH3 and 02 on Fe-BEA starts at ca 570 K and reaches 90% at ca 650 K (Fig. 3). N2 and H20 were only detected as products. The decomposition of N20 is initiated at ca 630 K to yield N2 and 02 (Fig. 3). It should be pointed out that a possible small release of 02 could not be identified in the SCR of N20 by NH3 due to the large excess of 02. However, additional experiments of N20 reduction by NH3 in absence of 02 has not shown any formation of 02 below 740 K, and the stoechiometry of the reaction was N20/NH3 = 3/2. There is thus no interference between the SCR of N20 by NH3 and its catalytic decomposition below 620 K. The boosting effect of NH3 on the N20 reduction to N2 is thus put in evidence with a decrease of the light-off temperature (50% N20 conversion) by ca 80 K compared to N20 decomposition. We have previously reported that the promoting effect of NH3 on the N20 reduction on Fe-zeolite depended on the nature of the zeolite [13]. It is very low for Fe-FER, and the largest for Fe-BEA and Fe-MFI. In order to understand this promotion by NH3 it is necessary to identify the possible reaction paths and mechanisms. For this goal, the reaction of N20 was carried out at 623 K (steady state conditions) with 15NH3 and high N20 conversion. We will see later that 02 does not influence the rate of N20 reduction (vide infra). The reaction was therefore performed in 02free atmosphere for a sake of clarity. Moreover, the experiment was started on a fresh sample to prevent any contamination with adsorbed 14NH3. The distribution of labelled dinitrogen species is shown in Fig. 4 as a function of time. Steady state concentrations of nitrogen species are reached after 30 min under stream. As expected, 14N2 and 15N2 are identified as products, but very surprisingly 13% 14N15N is also formed, which indicates some extent of N-N bond cleavage in 14N20. When switching from ]SNH3 to 14NH3 there is a transient period where the concentration of nitrogen isotopomers fluctuate. We have not yet identified if this was an artefact or a real chemical phenomenon. Otherwise, 14N15N disappears 100

!

!

A

.-. 80 o~

80

o

o

60

-~

40

~ 40

20

~ eo

Z

0 500 550 600

~ ~ =' ,

100~,

,,

_

Temperature (K)

Fig.3. SCR of N20 by NH3 (O) and N20 decomposition (~) as a function of temperature on Fe(97)-BEA.

, __-

-

14N2

60

14NH 3

"

O r

650 700 750 800

_-__

~ ~ ,

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,

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,

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100

120

Fig.4. Distribution of labelled dinitrogen in the SCR of N20 by NH3 on Fe(97)-BEA at 623 K; steady state conditions,

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more slowly; this is likely due to the strong adsorption ofNH3. TPD experiments have indeed shown that NH3 still desorbed above 1000 K [14]. In its simplest form the catalytic decomposition of N20 can be described as an adsorption of N20 at the active site, followed by a decomposition giving formation of N2 and a surface oxygen O*. this surface oxygen can desorb by combination with another O* or by direct reaction with N20. These three steps are indicated by Eqs. 1-3"

N20 + * *-" N2 + O* N20 + O* *-~ N2 + 02 20* *-~ 02

(1) (2) (3)

In the presence of NH3, the surface oxygen can also be removed according to Eqn.4: (4)

2NH3 + 30* *-- N2 + 3H20

The inhibition by 02 of the N20 reduction, when occuring, can be understood by Eqn.3 backward. This inhibition is generally low for isolated Fe species exchanged in a zeolite matrix [1,10]. For the present Fe(97)-BEA catalyst, when varying the 02 content from 0 to 4%, the rate of N20 reduction to N2 by NH3 at 618 K, and low conversion, remained the same (5.8 + 0.2 mol h -1 g-l). In contrast, 02 intervenes in the oxidation of NH3 to N2, which is accelerated and reached a plateau above 2000 ppm 02 (Figure 5). This behavior means that NH3 reacts with two different surface oxygen in Eqn.4, coming either from N20 or from 02. The occurrence of two different surface oxygen species can be correlated by the different profiles of TPD of oxygen from Fe(97)-BEA activated at 773 K in N20/He (5/95) or O2/N2 (20/80) (Figure 6). It clearly appears that a different surface oxygen exist after N20 treatment, 25

'

v ~o

20

'~

15

t,=,,

'

~ I

'

'

'

I

~

'

'

I

'

'

'

I

'~

'

'

I

'

'

'

!

u g

g g~

)

s o 0

0

1

2 3 P(02) (kPa)

4

5

Fig.5. Rate of NH3 oxidation as a function of P(O2) in the course of the reaction between N20, NH3 and 02 at 618 K on Fe(97)-BEA.

: 200

./,.,.,.. 400

600

800

Temperature (K)

1000 1200

Fig.6. TPD of oxygen from Fe(97)-BEA activated at 773 K in N20/He (5/95/(a), or O2/N2 (20/80) (b); ramp: 10 K min -1.

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this species desorbing at low temperature (-~ 425 K). One can postulate that this surface oxygen is analog to that recently proposed to be formed by interaction of N20 with Feexchanged MFI, and called a-oxygen [ 15]. The a-oxygen is highly reactive at low temperature in the oxidation of benzene to phenol and methane to methanol [7]. It was proposed that aoxygen is probably associated with binuclear Fe complexes, rather than with isolated atoms [161. In the catalytic reduction of N20 by NH3 we would have normally expected that at low temperature (< 740 K) the reaction was regulated by Eqs. 1,4, since 02 formation was never detected. On that account, in the reaction of 14N20 with 15NH3, the isotopic distribution oflabelled dinitrogen would have been 25% 15N2 and 75% 14N 2. Surprisingly, about 13% 14N15N was formed which could not be taken into account by Eqns. 1,4. We can thus suggest that the catalytic transformation of N20 to N2 proceeeds according to the following parallel reaction paths: i) a redox cycle of Fe ions involving N20 and NH3 as oxydant and reductant interacting on separated sites with the splitting on N-O bond in N20, and ii) a coupling reaction between N20 and NH3 on a same site with the splitting of N-N bond in N20. TPR and TPO experiments were carried out to provide evidence of the occurrence of the redox mechanism. After treatment with H2/Ar (25/75)at 823 K, Fe 3+ was reduced to Fe 2+ in Fe(97)-BEA. Upon interaction with N20 in TPO conditions (N20/He = 0.2/99.8, ramp = 10 K min-1), there is a first N20 consumption with N2 release only between 373 and 573 K. Above 623 K the catalytic decomposition of N20 with the concurrent release of N2 and 02 then occurs. The N20 taken up in the low temperature range (373-573 K) was N20/Fe = 0.47 mol/mol with the equivalent release of N2, and Fe2+ was transformed into Fe 3+ as shown by M6ssbauer spectroscopy. The following reaction can be proposed for the oxidation of Fe2+:

2 Fe2+(NH3)n + N20 ~ [(NH3)nFeOFe(NH3)n] 4+ + N2

(T < 573 K)

(6)

NH3 ligands are maintained on Fe since total NH3 desorption still maintained at 1000 K [14]. The occurrence of a binuclear Fe complexe was proposed above upon interaction of Fe-MFI with N20 [ 15]. After treatment with 02 at 823 K, Fe(97)-BEA was reduced with NH3 in TPR conditions (NH3/He = 0.2/99.8, ramp = 10 K min-1). There is a first and small N20 release centered at 350 K (N20/Fe ~0.02) followed by a clear release of N2 lying from 373 to 773 K (N2/Fe ~ 0.15-0.2). Assuming that Fe is in the form of the above binuclear complexe (eqn. 1), the following step of Fe 3+ reduction can be tentatively postulated: 3 [(NH3)nFeOFe(NH3)n] 4+ + 2 NH3 ~ 6 Fe2+(NH3)n + N2 + 3 H20

(7)

NH3 and N20 partake to the upkeep of the Fe 2+ ,--, Fe 3+ catalytic cycle, and the SCR overall reaction of this redox process becomes: 3 N20 + 2 N H 3 ~ 4 N2 + 3 H20

(8)

14NlSN formed in the SCR of N20 by 15NH3 comes from the direct interaction of the two reactants on a specific active site, and according to a mechanism which was not identified till now. Works are in progress to elucidate this point.

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From the data presented in Figure 4 we could conclude that ca 86% of N20 is transformed through N-O splitting, and ca 14% through N-N splitting. 4. CONCLUSION

The introduction of NH 3 as reductant speeds up the catalytic conversion of N20 in the presence of 02 on Fe-BEA catalysts. The light-off temperature (50% N20 conversion) is thus lowered by 80 K compared to the catalytic N20 decomposition. Two mechanisms operate based on i) the redox cycle Fe 2+ --, Fe 3+ maintained with the help of N20 and NH3 with N-O splitting in N20, and ii) a coupling mechanism which involved N20 and NH3 with N-N splitting in N20. The occurrence of these two mechanisms was proved by SCR with labelled 15NH3 and by TPR and TPO experiments. REFERENCES AND NOTES. 1. F. Kapteijn, J. Rodriguez-Mirasol and J. A. Moulijn, Appl. Catal. B: Environmental 9 (1996) 25. 2. J. O. Petunchi and W. K. Hall, J. Catal. 78 (1982) 327. 3. H. Randall, R. Doepper and A. Renken, Appl. Catal. B: Environmental 17 (1998) 357. 4. C. Pophal, T. Yogo, K. Yamada and K. Segawa, Appl. Catal. B: Environmental 16 (1998) 177. 5. F. Kapteijn, G. Mul, G. Marban, J. Rodriguez-Mirasol and J. A. Moulijn, in "Proceedings 1lth International Congress on Catalysis" (J. W. Hightower, W. N. Delagass, E. Iglesia and A. T. Bell, Eds.), Elsevier, Amsterdam, 1996, p. 641. 6. C. Pophal, T. Yogo, K. Tanabe and K. Segawa, Catal. Lett. 44 (1997) 271. 7. G. I. Panov, A. K. Uriarte, M. A. Rodkin and V. I. Sobolev, Catal. Today 41 (1998) 365. 8. M. KOgel,, V. H. Sandoval, W. Schwieger, A. Tissler and T. Turek, Catal. Lett. 51 (1998) 23. 9. M. KOgel,, R. MOnnig, W. Schwieger, A. Tissler and T. Turek, J. Catal. 182 (1999) 470. 10. M. Mauvezin, G. Delahay, B. Coq and S. Kieger, Appl. Catal. B: Environmental, in press. 11. Grande Paroisse S.A., French Pat. demand 99,0199. 12. I. Papai, A. Goursot, F. Fajula and J. Weber, J. Phys. Chem. 98 (1994) 4654. 13. M. Mauvezin, G. Delahay, B. Coq and S. Kieger, Catal. Lett., in press. 14. G. Delahay, M. Mauvezin, unpublished data. 15. G. I. Panov, A. Kharitonov and V. I. Sobolev, Appl. Catal. 98 (1993) 1. 16. G. I. Panov, V. I. Sobolev, K. A. Dubkov, V. N. Parmon, N. S. Ovanesyan, A. E. Shilov and A. A. Shteinman, React. Kinet. Catal. Let. 61 (1997) 251. 17. H. Y. Chen, T. Voskoboinikov and W. M. H. Sachtler, J. Catal. 180 (1998) 171.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

A new mechanism for the selective catalytic reduction of NOx by ~ Cu-zeolite catalysts

on

B. Coq a, S. Kiegerb, G. Delahay a, D. Berthomieua and A. Goursot a aLaboratoire de Mat6riaux Catalytiques et Catalyse en Chimie Organique, UMR CNRSENSCM 5618, ENSCM, 8 rue d l'Ecole Normale, 34296 Montpellier Cedex 5, France bGrande Paroisse, 12 place de l'Iris, 92062, Paris-La Ddfense Cedex, France The selectivecatalyticreduction (SCR) by NH3 is the most important technology to controlthe emissions of NOx from stationarysources.A new mechanism is proposed for the SCR of N O on Cu-exchanged F A U catalystswhich markedly differsfrom that occurring on the V2Os-TiO2 catalysts;N O takes partin the reoxidationof Cu + to Cu 2+, but not of V 4+ to V 5+. This conclusionwas achieved by decomposing the catalyticcycle intothe oxidationand reduction steps of Cu species by reactantmixtures composed of NO, 02, and NH3 and the comparison with the global SCR reaction.A quantum chemical (QM) modeling of the catalytic sitehas been untertakcn,leadingto a betterknowledge of the Cu electronicstructure. 1. INTRODUCTION The emission of NOx from anthropogenic activities is a major enviromental issue. The selective catalytic reduction (SCR) of NOx by NH3 is the main control technology for emissions from chemical plants and stationary power sources. To face this problem, a great number of catalysts were developed based on noble metals, V2Os-TiO2 mixed-oxides, and zeolitic materials [ 1]. In particular, catalysts composed of faujasites exchanged with Cu (CuFAU) were attractive for some medium and high temperature applications [2]. A reactant ratio NH3/NO close to one is often used in agreement with the global reaction: 4 NO + 4 NH3 + 02 --" 4 N2 + 6 H20

(1)

claimed for SCR on V2Os-TiO2 catalysts[1,3].However, recent practice in industrialSCR processes on Cu-zeolitc has shown that with NH3/NO molar ratiosbetween 0.75 and 0.9, 95% of NOx removal isneverthelessachieved [4].This would demonstrated that reaction(1) does not apply to Cu-zeolitccatalysts. The aim of this report is to present a new SCR overallreactionand catalyticmechanism which can be applied to Cu-zeolitc catalysts.The informations provided from temperature programmed reduction (TPR), temperature programmed oxidation (TPO) and temperature programmed dcsorptionof probe molecules (TPD), as well as SCR experiments with labelled ISNH3 will show that(i)the NH3/NO stoechiomctryin the overallSCR reactionis of 0.77 or 0.86 depending on the reaction temperature, (ii) the reaction scheme is based on the

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Cu 2+ ~ Cu + cycle, where the oxidation of Cu§ to Cu2+ is faster by the NO + 02 than by 02 alone. A new mechanism of the catalytic cycle will be proposed.

2. EXPERIMENTAL A Cu-faujasite (Cu-FAU) catalysts (5.7 wt%Cu) was prepared by ion-exchange fi'om FAU (Si/AI = 2.55) and Cu~O3)2. ARer filtration, the solid was dried at 393 K, then calcined at 773 K in air overnight. The amount of Cu corresponds to a theoretical exchange degree of 56% (200Cu/AI, mol/mol). The sample was characterized by TPR and TPO. TPR conditions: activation of the sample at 773 K in O2/He (3/97) to yield Cu2+=FAU, then TPR with NH3/NO/He (0.2/0.2/99.6), ramp: 5 K rain-l, VVH = 250,000 h =l. TPO conditions: activation of the sample at 773 K in NH3/I-Ie (10/90) to yield Cu+-FAU, then TPO with NO/He (0.2/99.8), O2/He (3/97), and NO/O2/I-Ie (0.213/96.8), ramp: 5 K rain-l, V I I = 250,000 h =1. Before the SCR the sample was activated at 773 K in O2/He (3/97) and the reaction temperature decreased from 773 to 350 K by step of 25 K for 1 h. Standard SCR conditions: NO/NH3/O2/He (0.2/0.2/3/96.6), VVH = 250,000 h =1. For kinetic studies the concentration of the reactants was in the ranges: 200 < [NO] < 4000 ppm, 500 < [NH3] < 4000 ppm, 0 < [02] < 5%. Experiments with 1 5 N H 3 w e r e carried out at steady state conditions by 14NH3/lSNH3 switches during the SCR experiments. 3. RESULTS AND DISCUSSION Copper is in the form of Cu2+ after activation of Cu-FAU at 773 K in air [5]. In TPR conditions, Cu2+ is reduced to Cu+ (color blue to white) by NH3/NO/He at ca 420 K with the I

I

Cu2+

I

I

,-

Cu

I

A +

100

I

I

-

I -

-

80e-

._o L_

. . . . . . . .

,,.

CD > ~

e o

6040-

v

-r.

(/) r u

/r

;

c-

300

Cu +

l_:

4 0' 0

5 0' 0

Temperature (K)

,,

"-o6o

"0

Cu 2+ i

20-

rl ' ~

700

aoo F

Fig. 1. TPR and TPO profiles of Cu-FAU; Na evolution during the TPR of Cu2+-FAU by NO+NI-I3 (a), equivalent oxygen (O) consumption during the TPO of Cu+-FAU by 02 (b) and by NO+O2.(c).

0 z

0 ~ 300

J'' 400

I

500

Temperature (K)

I

600

I

700

800

~"-

Fig. 2. NO (--) and NH3 (--) conversions in the SCR of NO on Cu-FAU.

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release of I-I20 and N2 (N2/Cu = 0.95, mol/mol) (Figure la). The use of 15NH3 provides evidence of the coupling between NO and NH3 with the formation of 14N15N. In TPO conditions, Cu + is not reoxidized by NO below 773 K, but 02 and the NO+O2 mixture can do it, at 640 and 590 K, respectively (Figure lb,c). There is thus a synergistic effect of NO for oxidizing Cu+ to Cu 2+ by 02. Moreover, a first contribution of Cu + oxidation by NO+O2 (about 20-30% ofCu + species) occurs at lower temperature (= 460 K), assigned to Cu+ species located in the supereages of FAU (Figure l c) [5]. The molar ratio between NO and 02 consumptions in the oxidation of Cu + is about 3 at 460 K and 1 at 590 K. The consumption of NO is correlated with the corresponding release of N2. This key step emphasize the different behavior of Cu-FAU [5] compared to V2Os-TiO2, for which NO did not intervene in the reoxidation step V 4+ ~ V 5+ [3]. The changes of color of the sample from blue-green (Cu2+) to white (Cu+) were in perfect agreement with the above oxidation-reduction steps. The SCR in standard conditions, shows that the conversion of NH3 is lower by ca 20% than the conversion of NO (Figure 2). Another vital point is the occurrence of two waves of NO conversion in the range 400-550 K and above 600 K. This behavior was assigned to NO conversion occurring on two different Cu sites [5]. It is proposed that in the range 400-550 K NO is reduced on specific Cun+ sites located in the supercages, these Cu species being stabilized by NH3 ligands. Above 600 K, undifferentiated Cun+ sites in both socialite cavities and superc~es of FAU are active. It should be pointed out that the selectivity to N2 is close to 100% among the products formed, and only trace mounts of N20 were observed below 600 K. However, N20 formation can reach 200 ppm at 750 K. In the range where the first wave of NO reduction occurs (400-550 K), the TPO of Cu+ to Cu 2+ (Figure le) is initiated with the consumption of 3 NO for 1 02 molecules. We have therefore proposed that the SCR process is the result of the following set of oxidation and reduction of isolated Cu species: 10 Cu+(NH3)2.n + 10 H + + 3 NO + 02 --" 10 Cu2+(NH3)2.n + 5 H20 + 3/2 N2

(2)

10 Cu2+(NH3)2.n + 10 NO ~ 10 [Cu(NH3)2_n(NO)] 2+

(3)

I0 [Cu~3)2.n(NO)] 2+ "~ I0 Cu+(NH3)I.n + I0 H + + I0 N2 + I0 H 2 0

(4)

with n = 0,1. Unreacting NI-I3 ligands are maintained coordinated on Cu species because the last NH3 ligand is not desorbed below 550-600 K. From these sequential reactions the overall SCR reaction comes as: 10 NI-I3 + 13 NO + 02 ~ 15 H20 + 23/2 N2

(5)

The stoichiometric ratio NHa/NO ~ 0.77 lower than unity is due to the self-reduction of NO to N2 in reaction (2). This is further confirmed by a SCR with transient 14NH3/15NH3 switches, in which theproportion of 14N2 and 14N15N is of 16% and 84%, respectively. This explains the lower NI-I3 conversion compared to NO conversion in the SCR (Figure 2), and is in perfect agreement with industrial practice of SCR on Cu-FAU catalysts [4]. Above 550 K, the mechanism is basically the same as represented by reactions (2-4), with the exception that the NO/O2 consumption ratio in the oxidation step Cu + to Cu2+ is equal to one. The oxidation and reduction steps of isolated Cu species become: 6 Cu+ + 6 H + + NO + 02 ~ 6 Cu 2+ + 3 1-120 + 1/2 N2

(6)

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化

10 8

N2, ~r~ ~ - . ~ P x[Cu+(NH3)2 H20~ xH+ x[Cu~(NH 3)2-nNO] ) NO

6

x[Cu+(NH3)2.n](NyOz),xH + &

4

d ,_o 0

x[Cu2+(NH N~

H20

0

2

r

exp.

Fig. 3. Proposed scheme for the SCR of NO

on Cu-FAU

4

6

8

10

(retool h4 g'~)

Fig. 4 C~culated (red,:) and experimental (rexp./mmol g.1 h-l) rotes of the SCR of NO on Cu-FAU at 458 K (E), 483 K (A) and 513 K (S) and various concentrations of NO, NH3 and 02.

6 Cu2+ + 6 NH3 + 6 NO --~ 6 [Cu(NI-I3)(NO)] 2+

6 [Cu(NH3)(NO)] 2+ ~ 6 Cu+ + 6 H + + 3 N2 + 6 H20

(7) (8)

There is no more NH3 ligand permanently coordinated to Cun+, as shown by TPD experiments. The overall SCR reaction is then: 6 NH3 + 7 NO + 02 --- 9 H20 + 13/2 N2

(9)

The ratio NH3/NO=O.86 and the 14NI-I3/15NI-I3 switches during SCR experiments provide evidence of the presence of 8% 14N14N and 92% 14N15N in the products. A simplified catalytic cycle for the SCR of NO by NH3 is presented in Figure 3. It is clearly different from that occurring on V205-TiO2, on which NO was not involved in the oxidation step V 4+ ~ V5+ [3]. Kinetic studies were carried out to evaluate the proposed formal reaction scheme and to provide more details on some of its aspects. The SCR were carried out at 458, 483 and 513 K, and varying the NO, NH3 and 02 concentrations, and the NO conversion was maintained lower than 20%, by tuning the space velocity, in order to prevent significant mass transfer limitations. In the range of reactant concentration and temperature investigated, the rate of NO reduction varies by one order of magnitude. Various models based on the catalytic cycle presented in scheme 1 (Figure 3) were evaluated and fitted on the data from kinetics using a non-linear least square error metho& The coupling between NO and NH3 (step 3) is well established [6,7]. The fit to experimental data was the same using an Eley-Rideal (ER) type model of reaction between adsorbed NH3 and gaseous NO, or a Langmuir-Hinshelwood (LH) type model of reaction between adsorbed NH3 and weakly bonded NO; we chose the latter. In the absence of any more information about the nature of x[Cu(NH3)]+(NyOz) (species A, Figure3), the decomposition oft his complex (step 2) was not modulated. Actually, the main attention was

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paid to step 1, which appears as the key step in the overall scheme. The reoxidation of Cu+ to Cu2+ was thus proposed as slow step on Cu-zeolite [8-10]. Three different proposals can be postulated for the interaction between NO and 02 in this step: (i) a LH type model between two adsorbed species, (ii) an ER type model between adsorbed NO and 02 from the gas phase, and (iii) an ER type model between adsorbed oxygen and NO from the gas phase. The best fit was obtained with the LH type model for the interaction between adsorbed oxygen atom and weakly bonded NO, and this on an active site composed of 3 Cu atoms (x = 3) [ 11]. The requirement of several near next neighbor (NNN) Cu atoms to form the active site in step 1 could explain the increase of the turnover frequency of the SCR reaction (number of NO molecules converted per Cu atom and per unit time/h-l) as the function of Cun+ concentration in Cu exchanged MFI, MOR and FAU zeolites [5,8] that is, with the probability to encounter NNN Cu n+. On the basis of these arguments, step 1 can be decomposed in three reactions: (1) the non-dissociative equilibrium of 02 adsorption; (2) the dissociation of adsorbed 02, in order to form a dimer [CuOCu]2+; (3) the reaction of activated O atoms with weakly bonded NO. These last two reactions would need three NNN Cu atoms to proceed. The interaction betweeen adsorbed O atom and NO on several Cu atoms had been suggested as an important step in the SCR of NOx on Cu-zeolites catalysts [8,12]. A rather complex rate equation was derived from this simplified reaction scheme. It allows a comparison to be made between the calculated and the experimental values of the rate of NO removal (Figure 4). A fairly good fit is observed for the rate of SCR reactions carded out from 458 to 513 K. The and mechanism presented above forms an overall comistent picture of the SCR of NO by NH3 on Cu-FAU. However, several unknown features remain in the catalytic cycle, in particular the structures of the intermediate complexes of the Cu site with NH3, NO and 02. We have therefore undertaken a QM study of this reaction, starting with the analysis of the Cu-FAU site. We intend to give some indication on its electronic structure, in relation with the formal charges +2 and + 1, associated wi~ the Cu centre. According to the X-Ray structure of Cu-FAU zeolite [13] and our own experiments, site HI Cu2+ cations, associated with bridging oxygens of the four-membered (4m) tings are good candidates for being the catalytic sites. The results presented here correspond to neutral 4m-models with formulae (Si204A12)(OH)aCa, (Si204A12)(OH)gNaH, (Si204A12)(OH)aCuH and (Si204AI2)(OH)aCu. The terminal hydrogem have been placed along the O-Si bonds of the solid and have been kept fixed during the geometry optimization, as described in ref. 14. Counterions with a global +2 charge must compensate the presence of the two A1, i.e. Ca2+, Na+H +, Cu+H+ and Cu2+. It is worth to compare the electronic structure of both alcali and transition metal models, on one hand, double-charged and mono-charged cations, on the other hand. Several electronic properties of these systems are presented in Table 1. These results suggest the following remarks: 1) The Ca and Nail systems possess comparable frontier orbitals, with a HOMO delocalized on zeolite oxygens and a LUMO characteristics of the empty s orbital of the cation (4s for Ca2+, 3s for Na+). 2) Cu 3d orbitals participate in the HOMO for the CuI-I and Cu models. As for the alkali cations, the empty 4s Cu orbital has a large participation in the HOMO of the Cull cluster, whereas the very low-lying LUMO of the Cu cluster is delocalized on oxygens of the zeolite and Cu 3d orbitals. 3) The Ca, Nail and Cull 4m-clusters show a charge transfer of about 0.3-0.6 electron from the zeolite to Na +, Cu+, Ca2+ and H+ (covalent bond). The Cu2+ system behaves differently: this cation, in the presence of the zeolite model, recovers a 3d 1~configuration.

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Table 1 Electronic properties of the 4m-clusters with Ca~ NaH~ Cull and C u counteriom 4m-Ca 4m-Nail 4m-Cull 4m-Cu gap a ..... 3.29 ' 4.31 1.69 " 0.09 HOMO b O(p) 0(p) 0(p), Cu(d) 9(P), Cu(d) LUMOb ............ Ca(s) Na(s) ...... Cu(s) . . . . . . . . . O(p), Cu(d) q M (I-I)c 1.35 0.85 (0.45) 0.65 (0.45) 0.75 aHOMO-LUM0 gap in eV; bmajor atomic orbital contributions; CMulliken net charge

Hence, comparing the Ca2+ and Cu2+ models, we can say that the zeolite plays the role of a reservoir keeping more electrons when Ca2+ is the counterion, because of the large energy gap, and releasing them easily when Cu2+ is the compensating cation with a very low-lying orbital. We can anticipate from these results that the oxydation/reduction steps in the catalytic cycle are closely related to the molecular orbital pattern of the Cu-zeolite. 4. CONCLUSIONS This study has demonstrated that the overall SCR reaction of NO by NH3 is different on Cu-zeolite than on V205-TiO2 based catalysts. The stoichiometry ratio NI-I3/NO which is one on the latter materials is lower on the former. This is due to the involvement of NO in the reoxidation of Cu+ to Cu 2+, which does not exist for V 4+ to V 5+. Moreover, the QM results indicate that the zeolite framework plays a significant role in the electron distribution. The study of this role during the reaction steps is under progress. The elucidation of the catalytic cycle in the SCR of NO on Cu-zeolite has been made possible thanks to the decomposition of the cycle into elementary oxidation and reduction steps studied by using model reactants mixtures. This information, not previously available, sustains novel industrial technologies by fundamental chemistry basis. REFERENCES AND NOTES. 1. 2. 3. 4. 5. 6. 7. 8.

Bosch, F. Janssen, Catal. Today 2 (1998) 369. G. Deseat, C. Hamon, US Patent No 5,536,483 (1996). N.-Y Topsoe, Science 265 (1994) 1217. Internal note from nitric plant of the Grande Paroisse Society. S. Kieger, G. Delahay, B. Coq and B. Neveu, J. Catal. 183 (1999) 267. W.B. Williamson, J. H. Lunsford, J. Phys. Chem. 80 (1974) 2664. M. Mizumoto, N. Yamazoe, T. Seiyama, J. Cata[ 55 (1978) 119. T. Komatsu, M. Nunokawa, I. S. Moon, T. Takahara, S. Namba, T. Yashima, J. Catal. 148 (1994) 427. 9. G.M. Brandin, L. A. H. Andersson and C. U. I. Odenbrand, Catal. Today 4 (1989) 187. 10. G. Delahay, B. Coq, S. Kieger and B. Neveu, Catal. Today, in press. 11. S. Kieger, PhD Dissertation, Montpellier (France) 1998. 12. G. Centi and S. Perathoner, Appl. Catal. A: General 132 (1995) 179. 13. I. E. Maxwell and J. J. de Boer, J. Phys. Chem. 79 (1975) 187. 14. G. Valerio and A. Goursot, J. Phys. Chem. 103 (1999)51.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

In situ FT-IR study of the selective catalytic reduction of NO by propane on Cu-ZSM-5: evidence of a reaction pathway by oxygen pulses F. Poignant, J.L. Freysz, M. Daturi, J. Saussey and J.C. LavaUey Laboratoire de Catalyse et Spectrochimie, ISMRA, 6, bd Marechal Juin, 14050 Caen Cedex, France

Abstract SCR of NO by propane has been investigated on Cu-ZSM-5 catalyst, using in sire FT-IR spectroscopy in a reactor cell combined with on-line MS and IR gas analysis. We have operated in transient conditions, adding oxygen pulses in a propane + NO flow to put in evidence the reactivity of surface intermediates. A reaction pathway has been deduced involving the formation and evolution of isocyanide and isocyanate intermediates, as well as adsorbed ammonia, coordinated with Cu +. I. INTRODUCTION The mechanism of NOx reduction by hydrocarbons in presence of oxygen on a solid catalyst is not yet established, in spite of the large interest of this reaction in the automotive industry. Formation of organic nitro, nitroso and nitrite compounds has been proposed to arise from the reaction between NOx and hydrocarbons [1]. Nitrile and isocyanate species have been reported by several authors [2-4]. Poignant et al. [5, 6] evidenced by IR spectroscopy the formation of ammonia during the reduction of nitric oxide by propane in presence of 02 over H-Cu-ZSM-5. Recently, Hadjiivanov et al [7] have shown the formation of NO § on H-ZSM-5 and proposed that NO + might be an important intermediate for NOx reduction. The aim of the present paper is to determine reaction pathways using FT-IR studies in working conditions. Due to the lack of detectable reaction intermediates under stationary flow of C3H6 + NO + 02, transient conditions were performed, pulses of one reactant being introduced in a flow of others. 2. EXPERIMENTAL The H-Cu-ZSM-5 catalyst (Si/A1 = 27) was prepared by ionic exchange with a Cu(NO3)2 solution giving a copper content of 1.47 wt.-%. The design of the IR cell for in situ studies has already been reported [8]. It allows reactivity experiments combined with rapid collection of IR spectra of surface species using a Nicolet Magna 750 FTIR spectrometer. Gas products were analysed by FTIR in a gas micro-cell and by mass spectrometry (Balzers TCP 121). The sample was pressed into self-supporting disc (ca. 15 mg cm2 ), heated progressively to 623 K under 5% 02 in helium and maintained for 3h in these conditions. It was then exposed (flow rate = 25 cm3 min~; GHSV = 50000 h1) to different gas mixtures at 623 K. All the spectra of the adsorbed species were obtained by subtracting the spectrum of the activated wafer from the spectrum obtained after introduction of the reactants.

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3. RESULTS AND DISCUSSION

3.1. Influence of the oxygen content Conversions of NO and C3H8 in stationary conditions versus oxygen content in the reactive flow are reported in Figure 1. This experiment shows that the presence of oxygen in the reaction flow is necessary for NO reduction. NO conversion reaches a maximum value of about 80% corresponding to an 02 amount of 1.5%. For higher oxygen values it decreases until a stable conversion level o f - 60% is gained. This activity lowering should be due 80 to a competition between organic species , 70 oxidation by 02 and their reaction with NO. 60 A similar trend for NO SCR by propane has 50 already been reported by several authors 40 [9,10]. On the other side, propane conversion 30 rises up continuously with oxygen content in 20 the flow. One of the main results in this study 10 is that for oxygen values close to the 0 maximum in NO conversion (from 0.5 to 1 0 ;.oodo.. o.e ovo, i %), one propane molecule can reduce two NO molecules. This fact suggests us that the Fig. 1. Effect of the oxygen content in the key intermediate in the reactivity mechanism flow on NO ( . ) and C3H8 (l) conversions has not any more three-carbon atoms, but during NO SCR on Cu-ZSM-5 at 623 K. probably one. (W/F = 0.072 g.s.cm3; flow: 2000 ppm NO, Infrared spectra recorded in the stationary 2000 ppm C3H8, 5% 02, He) state for oxygen contents between 10 and 2% (Figure 2) show only the usual species due to propane combustion: acrylates (bands at 1622 and 1435 cm l [ 11 ]) and acetates (band at 1544 cm-1). Oxygen contents between 2 and 0% give rise ~ ~ to new features, especially o A ', ~, ,,, in the reoions of eth~,lenic v(CH) vibration (3089 and A o Vow:., 2o/oo A 3021 cm "1) and of multiple bonds (2325, 2292, 2262, 2157 and 2044 cm -1). This result suggests the presence of a great number of different adsorbed species; among them reaction 3,500 3000 2500 2000 1500 intermediates can be Wavenumber (cm -l) present. In order to better characterise them and to determine their r61e in the Fig. 2. Influence of the oxygen content on the adsorbed reduction mechanism, it species in a NO and C3H8 flow (2000 ppm + 2000 ppm) at seemed to be necessary to 623 K on Cu-ZSM-5 split the study into two steps: first the behaviour of primary intermediates with 2 or 3 carbon atoms (published elsewhere [12]), second the study of intermediates containing only one ,.==jl

=

\

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carbon atom (this work).

3.2. Characterisation of monocarbonated intermediates Among all the above mentioned infrared bands, that at 2044 crn~ can be univocally generated submitting the catalyst to a NO (2000 pprn) + C3H8 (2000 pprn) flow in helium, followed by a flow of pure helium during one hour at 623 K. This species is stable in those conditions. Its reactivity has been studied towards NO, N20, H20 and 02 additions. Only oxygen immediately gives rise to a reaction. We observe the total disappearing of the band at 2044 crn1 and the simultaneous formation of 4 bands at 3365, 3290, 3193 and 1611 cm~. These 4 bands are characteristic of ammonia adsorption over Cu + [6]. A study of progressive oxidation of the species corresponding to the band at 2044 cm~ has been performed to characterise it and to understand the formation mechanism of Cu+(NH3) complexes. Af~er the formation of the species at 2044 crn1 (Figure 3a), pulses of 10 ttl of oxygen have been sent in a dry helium flow. On the infrared spectrum (Figure 3b) scanned 1 minute after the first pulse introduction, we observe a decrease of the band at 2044 cm] and the appearing of a new band at 2204 crn], together with the 4 bands at 3365, 3290, 3193 and 1611 crn]. Further introduction of O2 pulses every 3 minutes results in decreasing oxygen consumption accompanied by the simultaneous production of N2 and CO2 (Figure 4). The last O2 pulses are not consumed any more. IR gas spectra (not presented here) show that no gaseous ammonia is produced. Infrared band integration of surface species vs. time shows (Figure 5) that the feature at 2044 crn1 quickly decreases after every oxygen introduction; in parallel the band at 2204 crn"1 quickly rises up. It slowly decreases between two successive pulses and disappears when the 2044 cm 1 band is totally consumed. Band intensity of adsorbed ammonia slowly rises up, then reaches a maximum when the 2044 cm] species vanishes and finally disappears at%er the eleventh pulse.

I

0.1 u.a.

~

'

\3365

1

!

a

t

3290/3193

~

,

i

V ,

O2"pulse - - J

-~'~~st

~ /

2157

,x2.204

Before 1 02 pulse

t

3500

,

i

t

,

i

|

3000

t

,

t

!

2500

,

Wavenumber(cm1)

i

,

,

I

2000

t

t

,

i

I

1500

Fig. 3. IR spectra evolution after introduction of 02 pulses on the species characterised by the band at 2044 cm ] over Cu-ZSM-5 at 623 K.

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The infrared band at 2204 cm"1 is characteristic of an isocyanate species on Cu § (Cu~qCO) [ 13]. This species is then due to the oxidation of an isocyanide species Cu+NC corresponding to the band at 2044 cm "]. This isocyanate species reacts with some traces of water to form a carbamide species (undetected), which instantaneously decomposes into CO2 and NH3.

0 2

e, <

C02 ,

0

5

10

15

20

Time (minutes)

25

30

35

Fig 4. Mass spectra of 02 (32),CO2 (44), N2 (28) during 02 pulses introduction (one pulse of 10~tl every 3 minutes) on Cu-ZSM-5 at 623 K.

2 0 4 4 c m "1

2 2 0 4 c m -~ 3 2 9 0 c m -1

5

10

15

20

25

|

J

Fig. 5. Chemigrams of adsorbed species during 02 pulses introduction (101~l) on Cu-ZSM-5 at 623 K. We notice in the above figures that the maxima of NH3 and N2 production are not coincident. This could be explained by the presence of two reaction on the catalyst: the first,

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fast, taking place during an oxygen pulse passage, the other, slow, between two pulses. Cu~NC into Cu~'NCO oxidation is extremely fast (Figure 5), as shown by the instantaneous decreasing of the band at 2044 cm ~. On the contrary isocyanate transformation (band at 2204 cm~), fast during oxygen pulse passage, is not complete at the end of the pulse. Ammonia formed by isocyanate hydrolysis between two pulses cannot readily oxidise, but it coordinates on Cu § ions, where it is relatively stable. The quantity of ammonia formed during this slow reaction would be minoritary respect to that created and oxidised during oxygen pulse. Its accumulation will take place until the complete disappearing of isocyanate from the surface, then it will decrease. A complementary experiment of ammonia adsorption on this catalyst at 350~ shows that this compound can be adsorbed on the surface in two different ways: coordinated on Cu + or in cationic position as ammonium on SiOHAI acid hydroxides. Sending oxygen pulses on these adsorbed species confirms that ammonia oxidise into N2 and that ammonium species is the most reactive. From these results we could say that sharp peaks of N2 production are due to unobserved ammonia oxidation (because too fast), while the superimposed slow phenomenon corresponds to the oxidation of NH3 observed by infrared. The overall reaction pathway (the case of ammonium excepted) takes place on a Cu § ion because: i) isocyanide is formed in reducing conditions, ii) isocyanate wavenumber is typical of Cuq'qCO and iii) only reduced Cu-ZSM-5 zeolite can keep NH3 adsorbed on copper at 623 K under helium flow. Let us remark that our m situ I g studies showed that under NO + propane + O2, even in a large excess of O2, some Cu § sites are present in the catalyst. 3.3. Mechanism and stoichiometry of the reaction pathway

The steps of the process can thus be written in the following way: 2 Cu+NC + 02 2 Cu+NCO + 3 H20 2 Cu+(NH3) + 3/2 02

--> --> -->

2 Cu+NCO 2 Cu+(NH3) + 2 CO2 + 8902 2 Cu + + N2r + 3 H 2 0

( 1) (2) (3)

that gives the global stoichiometric equation: 2 Cu+NC + 2 02

-->

2 Cu + +N2(g) + 2 CO2r

(4)

This equation shows that the overall reaction does not consume water, hence just a small initial quantity is sufficient to start up isocyanate hydrolysis. Being copper mainly reduced for the beginning, it is possible that during first oxygen pulses some Cu § ions are oxidised into Cu2§ forming water molecules as described by the equilibrium equation: 2Cu ++2El ++ 89

~

2 C u 2++H20

which allows to keep electron neutrality of the zeolite framework. From the balance equation a CO2/N2 ratio equal to 2 is awaited. By integrating the signals of mass spectra, we find a little bigger value (2.5), likely due to the oxidation of Cu+CO (band at 2157 cm~) and eventually to N20 traces not computed here. This balance equation has been established to explain N2 formation from isocyanide species oxidation by 02 pulses. But in SCR conditions, in presence of NO, NH3 species adsorbed on Cu § react preferentially with NO. As a consequence step n ~ (3) becomes: 2 Cu+(NH3) + 2 NO + 8902

-->

2 Cu + + 2 Nzcg) + 3 H20

( 3' )

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and the new balance equation is written as:

2 Cu+NC + 2 NO + Oz

~

2 Cu + + 2 N2cg)+ 2 COzen)

( 4' )

This last equation suggests that in stationary conditions the reaction gives a molecule of N2 per copper isocyanide molecule, then it shows out the importance of this species in NO SCR by propane mechanism. That result allows us to propose as an ultimate sequence in the reaction mechanism of NO SCR: Cu+NC

02 > Cu+NCO

ri20 > Cu+(NH3)(+ CO2)

No, 02 > Cu++ N2 (+ H20)

4. CONCLUSION A mechanism pathway for the NO SCR by propane has been established for an oxygen concentration close to the maximum of the NO conversion. This mechanism puts forward the formation of an isocyanide species Cu+NC, its oxidation into Cu+NCO isocyanate and the evolution of this last one into Cu+NH3. N2 formation takes place by reaction of NO with NH3. It is difficult to determine if such a mechanism indeed occur in stationary conditions since in such a case IR spectroscopy is not able to detect any intermediate adsorbed species. However, Radtke et al. [ 14] reported on Cu-ZSM-5 under flow of NOx, propene and 02 the formation of HNCO in the temperature range 513 - 666 K under dry conditions. In presence of water no HNCO was detected, whereas ammonia formation was identified in nice agreement with the results reported in the present study. Possible mechanisms of copper isocyanide species formation will be reported in another paper, attention being paid to acrylonitrile formation and adsorption as a possible intermediate compound [ 12]. REFERENCES .

2.

.

10. 11. 12. 13. 14.

N.W. Hayes, R.W .Joyner and E. Shapiro, Appl.Catal.B, 8 (1996) 343. W. Hayes, W.GrOner, G.J. Hutchings, R.W. Joyner and E. Shapiro, J. Chem. Soc., Faraday Trans., 4 (1994) 531. Y. Ukisu, S. Sato, G. Muratmasu and K. Yoshida, Catal. Lett., 11 (1991) 177; ibid. 16 (1992) 11. Y. Ukisu, S. Sato, A. Abe and K. Yoshida, Appl. Catal. B, 2 (1993) 147. F. Poignant, J. Saussey, J.C. Lavalley and G. Mabilon, J. Chem. Soc., Chem. Comm., (1995) 89. F. Poignant, J. Saussey, J.C. Lavalley and G. Mabilon, Catal. Today, 29 (1996) 93. K. Hadjiivanov, J. Saussey, J.L. Freysz and J.C. Lavalley, Catal. Lett., 52 (1998) 103. J.F. Joly, N. Zanier-Szydlowski, S. Colin, F. Raatz, J. Saussey and J.C. Lavalley, Catal. Today, 9 (1991) 31. G. Centi, S.Perathoner, L. DaU'Ollio, Appl. Catal. B, 7 (1996) 359. M. Iwamoto, H. Yahiro, Catal. Today, 22 (1994) 5. V.S. Escribano, G. Busca and V. Lorenzelli, J. Phys. Chem., 94 (1990) 8939. F. Poignant, J. Saussey, M. Daturi, J.L. Freysz and J.C. Lavalley, to be published. F. Solymosi and T. B/ms~,gi, J. Catal., 156 (1995) 75. F. Radtke, R.A. Koeppel and A. Baiker, J. Chem. Sot., Chem. Commun., (1995) 427.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Catalytic removal of NO: the effect of Cu loading and type of binder on catalytic properties of Cu/ZSM-5 catalyst V. Toma~iO a, A. Ger[3ina b, Z. Gomzi a and S. ZmE3eviO a aFaculty of Chemical Engineering and Technology, Department of Reaction Engineering and Catalysis, University of Zagreb, Maruli[]ev trg 19, 10000 Zagreb, Croatia bFaculty of Chemical Engineering and Technology, Department of Mechanical and Thermal Process Engineering

Catalytic decomposition of NO over Cu/ZSM-5 catalysts has been carried out in a tubular fixed-bed reactor at atmospheric pressure. The influence of the catalyst composition on its physical-chemical and catalytic properties has been investigated. An optimal content of copper at wich Cu/ZSM-5 catalyst demonstrated maximum activity has been established. It has been found that catalyst activity increased with the increase in Si/A1 ratio or with the decrease in aluminium content over zeolite. The addition of binders, such as MgO, ZnO, A1F3 and Na-silicate changed physical properties of the catalyst and at the same time facilitated shaping of the catalyst during the preparation step.

1. INTRODUCTION Nitrogen oxides, NOx (x=l,2), are undesirable products of high-temperature combustion and belong to the most harmful gaseous air pollutants [ 1,2]. Nitric oxide, NO, is the dominating component of the exhaust gases. Thus, direct decomposition of NO over an appropriate catalyst is the most attractive catalytic method of NOx emission reduction. One of the most promising catalysts which has been found to catalyze this reaction is the copper exchanged ZSM-5 zeolite. Recently, Armor pointed out the issues that still limit practical application of this catalyst [3]. This work presents experimental results on a series of the new copper containing ZSM-5 catalyst. The effect of catalyst composition (e.g. copper content and Si/A1 ratio) on the reaction rate is examined at different space times. The influence ofNa-silicate, A1F3, ZnO and MgO on physical and catalytic properties of Cu/ZSM-5 zeolites are also investigated. The activity of these catalysts in NO decomposition is compared to that of the simple copper exchanged ZSM-5 catalyst. The objective of this study was to provide additional information on the relationship between different variables involved in a catalyst preparation and the performance of Cu/ZSM-5 catalyst in NO decomposition.

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2. EXPERIMENTAL 2.1. Catalyst preparation Starting materials for catalysts preparation were commercial zeolites (i) H-ZSM-5 of various Si/AI ratios obtained by Degussa (KM-902, Si/Al=28; KM-903, Si/AI=40) and (ii) HZSM-5 (Y33980, Si/Al=54) supplied by Leuna-Werke. Before the copper ion exchange step, the parent zeolites had been washed with the diluted NaNO3 solution. The Cu-containing ZSM-5 catalysts were prepared by ion exchange of Na-ZSM-5 using aqueous solutions of copper acetate (0.001-0.08 tool din3). Copper content of the samples was from 0.463 wt % to 4.722 wt %, depending on Si/A1 ratio and concentration of the solution during ion exchange. The second series of catalysts was prepared by mixing and shaping of Cu-exchanged sample (1.48 wt % Cu) with Na-silicate, A1F3, ZnO and MgO as binders (75 wt % Cu/ZSM-5 and 25 wt % binder) to form catalyst pellets. The copper containing zeolite powder was pressed into the tablets at 6.08 9106N m 2, crushed and sieved. For catalytic experiments, particle size was in the range 0.315-0.5 ram. 2.2. Characterization methods Crystalline structure of the zeolites after ion exchange was checked by X-ray diffraction analysis. XRD patterns were obtained with Philips PW 1065 diffractometer using Ni-filtered Cu I ~ radiation. Physical properties of the catalysts were determined from nitrogen adsorption (Micromeritics ASAP 2000) at 77 K after outgassing at 473 K for 24 h. Software program included BET and Langmuir surface area analysis, t-plot analysis (Harkins and Jura equation for determination of statistical thickness of the adsorbed layer) and pore size distribution according to Barett-Joyner-Halende model. An avarage pore diameter, dp, was calculated assuming cylindrical geometry of micropores and mesopores. 2.3. Experimental apparatus and procedures Catalytic tests were carried out in a tubular fxed-bed reactor (6.5 mm I.D and 150 mm length) operating at atmospheric pressure. The reactant gas was a standard gas mixture of 4 % NO in helium (Messer Griesheim). The reaction was performed at the temperatures from 623 to 773 k and at different space times (0.896-5.373 s). The space times were changed by varying the total flow rate of the reactant gas at the constant amount of the catalyst (0.5 g). All measurements were made after the reaction had reached steady-state (after l h on stream). NO conversion to N2, calculated as (2 x N2 concentration in the effluent flow)/(NO concentration in the feed stream), was taken as a measure of the reaction extent. The reaction products were analyzed by combining volumetry (NO2) and GC (02, N2, NO), using molecular sieve 5A and a thermal conductivity detector. A detailed description of experimental apparatus and procedures can be found elsewhere [4,5]. 3. RESULTS AND DISCUSSION The copper exchanged ZSM-5 zeolite was characterized by X-ray diffraction and the diffractogram is given in Figure 1. Similar diffraction patterns were obtained for other copper containing samples. It was observed that the introduction of copper into ZSM-5 did not affect

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the structural changes. In the range of 20 = 34 - 40 degree there was no diffraction peak indicating the presence of CuO phase. 100

80

60 4o 20 0

0

'

10

'

,

20

I

,

I

30 40 20 (degree)

J

I

,

50

I

60

Figure 1. X-ray pattern of the copper containing ZSM-5 zeolite. (Si/AI=40; 1.92 wt % Cu)

0.6 0.5 z~

A

0.4 0.3 0.2 0.1 O

0.0 0.0

0.2

0.4

0.6

0.8

1.0

(~/~max) Figure 2. Comparison between the experimental data (points) and the values predicted by the kinetic model, eq. (1) at different Cu content and space times (T-733 K, Si/AI=40) 9 0.463 % Cu, m 1.616 % Cu, A 1.920 % Cu, 9 4.722 % Cu

Figure 2 shows the influence of Cu content on the molar fraction of N2 in the reaction products at various space times. It can be seen that the activity of Cu/ZSM-5 catalyst with Si/A1 ratio of 40 increases with the Cu content up to cca. 2 wt %. However, at higher Cu

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content the conversion of NO into N2 decreases.The explanation of such a behavior may be in the change of the active sites structure, or the change in porosity of Cu/ZSM-5 catalyst due to the copper blocked pores. Figure 2 also shows the comparison between the experimental data and the values predicted by a mechanistic kinetic model

rs

k. C~o (1 + ~JK o Co: )e

(1)

derived on the assumption of bimolecular surface reaction between the adsorbed molecules of NO as the rate determined step. The kinetic data were analyzed by the modified differential method under concomitant estimation of kinetic parameters using Nelder-Mead method of nonlinear optimization. The correlation criterium was the mean square deviation between the experimental and predicted results. As shown in Figure 2, a satisfactory degree of correlation was achieved. 0.6 Si/AI=40 0.4 z ~

0.2

ir

/

Si/AI=28

I

.0

0.0

0.2

i

I

,

0.4

I

0.6

,

I

0.8

1.0

(~/~=x) Figure 3. The influence of Si/A1 ratio on the molar fraction of N2 (YN_,) in the reaction products at 723 K In order to examine the influence of Si/A1 ratio on catalytic activity, the rate of NO decomposition was expressed per atom of copper. The results are presented in Figure 3. The activity of Cu/ZSM-5 catalyst increases with the increase in Si/A1 ratio or with the decrease in aluminium content in the zeolite. The obtained results are in the agreement with experimental reported by Iwamoto et al [6]. They indicate that the most active sites are the isolated copper species. It can be concluded that the NO decomposition over Cu/ZSM-5 catalyst can be considered as an example of the structure-sensitive reaction. Figure 4 displays the influence of a binder components such as MgO, ZnO, A1F3 and Na-silicate on catalytic properties of the catalyst. Comparing the data obtained with different types of binder it was found that ZnO has not only a binder function but also a promoting effect on the catalyst activity. The addition of other binders did not significantly affect the catalyst activity.

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个

1497 0.4

化

~ ~ i

0.3 9

Z ~,

0.2 ~ 0.1 .48 9

.

. ..,,~...

%

,

Figure 4. The influence of a binder component on the molar fraction of nitrogen in the reaction products at 723 K (Si/A1 = 54, 1.48 wt % Cu) The Cu/ZSM-5 catalyst modified with Na-silicate was examined at wider temperature range (623 -773 K)(Figure 5). The results indicate that Na-silicate as a binder caused a change in the catalyst behavior at different temperatures compared to the simple copper containing ZSM-5 catalyst. Physical properties of the catalyst, prepared by mixing and shaping of the copper containing ZSM-5 zeolite samples (Si/Al=54) with various binder components, are given in Table 1. Negative values of the corresponding BET constants have been obtained indicating that the BET equation was not suitable for the calculation of the specific surface area of these zeolite, probably due to micropore filling in the low-pressure region of a nitrogen adsorption isotherm. For that reason total pore volumes, Vp, and specific surface areas of the zeolite catalysts, SLANG,were estimated on the basis of the Langmuir adsorption model. According to the results given in Table 1, a consequence of the catalyst modification by the addition of binders was a considerable change in the physical properties, i.e. a decrease in the specific surface area, pore volumes and micropore volumes.

Table 1 Physical properties of the catalysts (Si/A1 = 54; 1.48 % Cu) Binder Cu/ZSM-5 A1F3 MgO ZnO Na-silicate

(mZ/g)

Pore volume (cm3/g)

Micropore volume (cm3/g)

Average pore diameter (nm)

355.6208 265.1402 229.9071 224.8678 160.6348

0.1520 0.1214 0.1188 0.1081 0.0596

0.1029 0.0708 0.0636 0.0585 0.0005

1.7095 1.8316 2.0665 1.9237 1.4841

SLANG

Pore size distribution curves are presented for typical samples in Figure 6. This parameter was also affected by the type of the binders. In almost all samples a bi-modal pore distribution was created, generating large fractions of pores in the 2-6 nm and 20-80 nm

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regions. In the case of the catalyst modified by Na-silicate, only macropore fraction was present. Micro- and mesopore fractions were omitted probably because of the pore filling with Na-silicate. It is important to emphasis that the addition of binders substantialy facilitated shaping of the catalyst during preparation step, which is of great practical importance. 0.6

"7

0.4

"r,

0.08

~

0.06

o.o

0.2 > )~ 9

0"000

i

i

600

m i

9 i

,

I

,

/

I

,

700 800 900 1000 T/K Figure 5. Molar fraction of N2 in the reaction products as a function of the reaction temperature (-~ - without binder (Si/AI = 40, 1.92 % Cu); 9 - with Nasilicate as a binder)

'O

t

0.02 eL

o. e .~~

9- n

0.00

)~

9

0.04

1

.......

i'o

9--o "go

\o 9annm-m

9

.....

do/nm

16o

Figure 6. Pore size distribution (o - without binder; 9 - with Na-silicate as a binder)

4. C O N C L U S I O N S

This study attempts to present the relation between the structure and the catalytic behaviour of Cu/ZSM-5 catalyst in NO decomposition, covering both compositional and structural aspects. The results reveal the optimal copper content at which the Cu/ZSM-5 catalyst exhibits maximum activity. Activity of Cu/ZSM-5 catalyst increases with the increase in Si/A1 ratio which leads to the conclusion that the active sites are the isolated copper species. The addition of binders does not change significantly the activity of the catalyst in spite of the changed physical properties. ACKNOWLEDGMENT

We extend our grateful thank to the Croatian Ministry of Science and Technology for their financial support to this work. REFERENCES

1. 2. 3. 4. 5. 6.

J.N. Armor, Appl. Catal. B: Environmental 1 (1992) 221. N. de Nevers, Air Pollution Control Engineering, McGraw-Hill, New York (1995) 394. J.N. Armor, Catal. Today 26 (1995) 99. V. Toma~i@, Z. Gomzi, S. ZmSevi[], Appl. Catal. B: Environmental, 18 (1998) 233. V. Toma~i[], Z. Gomzi, S. Zm[]eviD, React. Kinet. Catal. Lett. 64 (1998) 89. M. Iwamoto, H. Yahiro, K. Tanda, N. Mizuno, Y. Mine, S. Kagawa, J. Phys.Chem. 95 (1991) 3727.

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Studies in Surface Science and Catalysis 130 A. Corrna, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Changes in Cation Coordination during Deactivation of C u - Z S M - 5 deNO~ Catalysts S.A. G6mez 1, A. Campero 2, A. Martinez-Hemfindez ~ and G.A. Fuentes ~ ~Area de Ingenieria Quimica, 2Area de Quimica Inorghnica, Universidad A. MetropolitanaIztapalapa. A.P. 55-534, 09340 M6xico D.F., MEXICO. e-mail: [email protected], mx EPR analyses of Cu-ZSM-5, fresh and reaction-tested in the selective reduction of NO in the presence of 10% H20 at 673 K, evidenced the essentially equimolar presence of Cu 2+ in square planar and square pyramidal environments before permanent catalyst deactivation occurred. Once irreversible deactivation ensued, a third Cu 2+ species appeared, presumably associated with the loss in NO activity. This species, most probably located in the pockets of ZSM-5, is difficult to reduce and hence expected to be less reactive. Deactivation in our samples cannot be ascribed to dealumination of the zeolite or Cu compound formation. I. INTRODUCTION Selective catalytic reduction of NOx (SCR) has received a great deal of attention as a plausible car exhaust treatment technology in both lean-bum gasoline engines and Diesel engines. Transition metal exchanged ZSM-5 zeolites comprise a most promising alternative as catalysts, although various hurdles remain, in particular the strong sensitivity of some of these materials against water and SO2 under realistic operating conditions. Cu-ZSM-5 was the first system reported for lean burn SCR [ 1]. However, despite being the subject of numerous studies, it remains only partially understood. In particular, there are still controversial conclusions in the literature concerning the existence of various Cu n+ species in this system before and a~er deactivation [2-8]. In fact, it is not yet clear what is the relationship between those species and activity, as well as how water affects their structure and reactivity. In our opinion, a primary reason for this situation is the large variety of pretreatments used prior to analyses of Cun+ siting and the large Cu loadings used in some cases. The use of accelerated aging conditions, generally involving a temperature excursion during pretreatment or even during reaction [9-11], besides modifying Cun+ geometry or location frequently activates degradation of the zeolite framework as well as the formation of inactive Cun+ compounds [ 12, 13]. These processes may not be significant under less stringent conditions. In spite of the fact that water is present in large concentrations in the engine exhaust, its effect upon activity and upon the structure of lean burn SCR catalysts is not well understood [11, 14-19]. We have been particularly interested in studying the effect of water upon the activity and structure of Cu-ZSM-5, using it as a model system for this class of catalysts. In fact, our findings [12, 13] suggest that at low temperature (T _<673 K) permanent deactivation by H20 can be ascribed to changes in Cu ~+ siting. In this work we report further on the characterization by Electron Paramagnetic Resonance Spectroscopy (EPR) of the changes in coordination environment of Cu 2+ in ZSM-5 after long-

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term isothermal reaction (up to 110 h on stream) in the presence of 10 % H20. Reaction and pretreatment conditions were chosen in order to avoid dealumination of the framework and formation of extraframework Cu compounds. 2. EXPERIMENTAL

Detailed catalyst preparation and reaction studies were described in [12]. Briefly, we prepared Cu-ZSM-5 by aqueous exchange of Na-ZSM-5 (Chemic Uetikon) with Cu(CH3COO)2. The samples were then washed, dried overnight at 383 K, and kept in a dessicator until needed. The resulting Cu loading was 2.3 wt % and Cu/AI = 0.6. Cu-ZSM-5 was reaction-tested during NOx reduction by C3H8/O2 in the presence of 10% 1-120. GHSV up to 180 000 h "~were used. In order to observe significant irreversible deactivation we had to run the catalyst under wet operation for tens of hours. The samples reported here as deactivated were on stream for at least 110 h. The degree of irreversible deactivation was determined by using reaction at 673 K under dry conditions as a standard. In a typical long-term experiment (TOS = 110 h) the catalyst lost 60 % of its initial NO reduction activity and 80 % of its C3H8 oxidation activity [ 12]. EPR spectra were recorded at 77 K on a Bruker ESP300 spectrometer. Catalyst samples were previously dehydrated under vacuum (P < 10-3 Torr) at 673 K for 10 h. They were then exposed to the flow of high-purity 02 at the same temperature for 4 h and finally cooled to room temperature under vacuum. This pretreatment favored the conversion of Cu § to Cu2+ and allowed us to have a common basis for comparison. We determined the fraction of total Cu detected by EPR with the help of an EPR calibration curve prepared using frozen solutions of CuCI2 and Cu-ZSM-5 samples with different Cu loading [13]. We used WlNEPR (v2.11, Bruker) to correct the spectra and to perform a normalized double integration (DI~). In order to find the number and characteristics of Cu 2§ species present, we deconvoluted the four-fold parallel section of the first derivative spectra using Voigt functions. Once a good fit was accomplished (typically, r 2 > 0.999), glJ and A~I were calculated for each component. We used this information to simulate the complete first derivative spectra and determine the location of the perpendicular components [ 13]. That provided us with complete sets of signals corresponding to each Cu 2§ species. We estimated then the fraction corresponding to each species in the samples by numerically fitting the full absorption spectra with sets of gaussians (r 2 > 0.99998) and integrating each set. These were fixed in their position by the previous analysis and only were adjusted in their intensity by our numerical method. 3. RESULTS AND DISCUSSION At 77 K the EPR signal of hydrated Cu-ZSM-5 samples before any pretreatment was anisotropic and showed only one species with glt = 2.38-2.39 and Ata = 147-151 G, assigned to [Cu(H20)6] 2+ located in the zeolite channel intersections [20]. We estimated that roughly 70 % of the total Cu was detected. This was in agreement with our estimates of the fraction of Cu 2§ present in obtained by H2-TPR of flesh catalysts with different Cu loading [ 12, 13]. Dehydration under vacuum at 673 K of Cu-ZSM-5 samples before reaction resulted in the formation of two Cu 2§ species. They were also present in the catalysts tested under reaction for less than 12 h on stream with up to 10 % H20 in the feed. Those have been assigned to square planar (CUPL) (gll = 2.28, Atl = 166-171 G) and square pyramidal environments (Cupv) (gtt =

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2.32-2.33, A~I = 146-154 G) [13, 21]. In all of these samples, irrespective of the reaction conditions used, essentially all Cu was being detected by EPR. This suggested that we primarily had isolated Cu 2+ species, and that Cu + -~ Cu 2+ conversion was indeed occurring during the dehydration-oxidation pretreatment of the samples. It has been suggested that both CupL and Cupv play an important role in the decomposition and SCR of NO [11, 19]. The Cuev/CUpL ratio in all samples was estimated as the fractional area of their corresponding sets of gaussians used to fit the absorption spectra of the samples. We observed that in fresh samples and in those used for short periods with or without H20 in the feed, i.e. before permanent deactivation ensued, the Cupv/CUpL ratio was essentially 1, indicating no preferred environment for Cu, at least at~er our usual dehydration-oxidation pretreatment. Larsen et al. [2] reported that for unused samples the CUpv/CUpL ratio was a function of the Cu loading. Our estimate of CUpv/CUpL matched the value reported by that group for a Cu-ZSM-5 zeolite with Cu loading and Si/Al ratio close to ours. Although during reaction the structure of Cu n+ sites might vary as function of operating conditions, our results indicate that the pretreatment used is enough to restore the original distribution of Cu 2+ sites, i.e. a facile reconversion is possible before deactivation is detectable. A different picture appeared once irreversible deactivation occurred. Besides Cupv and CUpL, deconvolution of the first derivative EPR spectrum of deactivated catalysts indicated the presence of a third paramagnetic site that we referred to as Cu*, as shown in Figure 1. Its parameters were gn = 2.30 and &l = 150 G. This species has been ascribed to Cu 2+ anchored on sites recessed from the main channel, i.e. it is located in the side pockets (five-membered tings) of ZSM-5 [3, 11, 19, 20]. It probably has a distorted square pyramidal geometry. Our numerical analysis of the absorption spectra of deactivated catalysts, shown in Figure 2, indicated that roughly one-third of the total paramagnetic Cu 2+ (in this case equivalent to the total Cu) had changed its coordination to Cu*. The distribution of species in the different samples is given in Table 1. C%y

!

I

I I

I Cu*,

CUpL

--I

|

2600

2700

2800

2900

I

3000

I

3100

H, Gauss Figure 1. C u 2+ species obtained from the first derivative EPR spectrum of CuZSM-5 deactivated atter 110 h on stream. Reaction in the presence of 10 % H20. The experimental and fitted spectra are undistinguishable.

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1502

i

2600

2800

3000

3200

3400

H, Gauss Figure 2. Deconvolution of the EPR absorption spectrum of deactivated Cu-ZSM-5. The experimental and fitted spectra are undistinguishable. A recessed location of Cu n+ sites in the zeolite, by impairing accessibility to reagents, would alter the redox cycle (Cu z+ ~ Cu +) involved in the catalytic reduction of NOx, hence decreasing the overall activity. That would presumably make Cu* less active than Cupy or Cupt.. A significant increase in the temperature needed to start reduction by 1-12 of deactivated CuZSM-5 catalysts further supports this assertion [12]. There are reports in the literature confirming these ideas [3, 11, 19, 20]. Our structural analyses [12, 13] did not show any changes in the zeolite framework, nor indicated the presence of CuO or other Cu compounds. It is then plausible to state that changes in Cun+ distribution were primarily responsible for the loss in catalytic activity. Although Cu* has been previously observed in the context of SCR [ 11, 19], its detection was done aRer harsh pretreatment of the catalyst, and not necessarily aRer wet reaction. As a result, there was no clear-cut correlation between the presence of Cu* and the loss in activity caused by H20 during reaction. Our results indicate that formation of Cu* is indeed associated with permanent deactivation in the absence of other structural modifications. Figure 3 presents deNOx activity as a function of time, as well as the species detected according to our analyses. The residual activity observed in deactivated catalysts when tested under dry conditions [12] Table 1 Fraction of Cu 2§ species in Cu-ZSM-5 catalysts calculated from the EPR absorption spectra Catalyst

CIIpy

CIIpL

Cu*

Fresh 0% 1-120 10% H20 Deactivated

0.49 0.48 0.48 0.34

0.51 0.52 0.52 0.36

0.00 0.00 0.00 0.30

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1503 50 ,~

I__- a " " - ' " Cu r 40 ~ ~ , , PY

~ Cu PL

..,

..

9~, 30 ~

U

20

9 Q

oz lO 0

0

1000

2000

3000

4000

5000

6000

7000

Time (minutes) Figure 3. DeNOx activity of Cu-ZSM-5 at 673 K with 10% of H20 in the feed and detected by EPR.

C u 2+

species

can be ascribed to the re-equilibration of Cupv and CUPL species in the zeolite. Overall, our joint reaction-structure analyses suggest that formation of Cu* is indeed an important, if not the most important, cause of irreversible deactivation during wet selective reduction of NOx below 700 K. 4. CONCLUSIONS Fresh and reaction-tested Cu-ZSM-5 samples have an essentially equimolar ratio of square planar and square pyramidal Cu 2§ species, as determined by EPR. Appearance of a third paramagnetic species (Cu*), probably a distorted square pyramid, is the only relevant structural modification in permanently deactivated Cu-ZSM-5 catalysts. Presence of Cu* coincides with the onset of a permanent loss in activity of Cu-ZSM-5 during NOx reduction at low temperature in the presence of H20. Cu* corresponds to Cu 2§ sites presumably located in less reactive or fully inactive positions off the main channels of ZSM-5. One-third of the Cu n§ cations was present as Cu* in permanently deactivated catalysts aider 110 h on stream. Despite the appearance of Cu*, square planar and square pyramidal Cu 2§ species were still present in equimolar amounts in deactivated catalysts. That suggests that both species contributed equally to the formation of Cu* moieties. ACKNOWLEDGEMENTS We acknowledge the financial support of Consejo Nacional de Ciencia y Tecnologia-M6xico (CONACYT, Projects 400200-5-3420-A and 400200-5-29272-U), Instituto Mexicano del Petr61eo (IMP, FIES 95F-140-III), JICA (Infrastructure grant), and Universidad Aut6noma Metropolitana (UAM-I). AMH acknowledges a graduate scholarship from CONACYT.

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REFERENCES

M. Iwamoto, H. Yahiro, S. Shundu, Y. Yu-u and N. Mizuno, Shokubai, 33 (1990) 430. 2. S.C. Larsen, A. Aylor, A.T. Bell and J.A. Reimer, J. Phys. Chem., 98 (1994) 11533. 3. M. Anpo, M. Matsuoya, Y. Shioya, H. Yamashita, E. Giammello, C. Morterra, M. Che, H.H. Patterson, S. Webber, S. Ouellette and M. A. Fox, J. Phys. Chem., 98 (1994) 5744. J. Dedecek, Z. Sobalik, Z. Tvaruzkovh, D. Kaucky and B. Wichterlovh, J. Phys. Chem., 99 (1995) 16327. A.V. Kucherov, C.P. Hubbard and M. Shelef, J. Catal., 157 (1995) 603. 6. M.L. Jacono, G. Fierro, R. Dragone, X. Feng, J. d'Itri and W.K. Hall, J. Phys. Chem. B, 101 (1997) 1979. A.V. Kucherov, H.G. Karge, and R. SchlOgl, Microporous Mesopororous Mat., 25 (1998) 7. D-J. Liu and H.J. Robota, J. Phys. Chem. B, 103 (1999) 2755. 9. C.C.C. Kharas, H.J. Robota and D.J. Liu, Appl. Catal., B 2 (1994) 225. 10. T. Tabata, M. Kokitsu, O. Okada, T. Nakayama, T. Yasumatsu and H. Sakane in Catalyst Deactivation 1994, Stud. Surf. Sci. Catal. (B. Delmon and G.F. Froment, Eds.), Vol.88, p.409. Elsevier, Amsterdam, 1994. 11. S. Matsumoto, K. Yokata, H. Doi, M. Kimura, K. Sekizawa, and S. Kasahara, Catal. Today, 22 (1994) 127. 12. A. Martinez, S.A. G6mez, and G.A. Fuentes, in Catalyst Deactivation 1997, Stud. Surf. Sci. Catal., (C.H. Bartholomew and G.A. Fuentes, Eds.) Vol.lll, p.225. Elsevier, Amsterdam, 1997. 13. S.A. G6mez, A. Campero, A. Martinez-Hemhndez, and G.A. Fuentes, Appl. Catal. A, (in Press.) 14. R.A. Grinsted, H.W. Jen, C.N. Montreuil, M.J. Rokosz and M. Shelef, Zeolites, 13 (1993) 602. 15. Y. Li, P.J. Battavio and J.N. Armor, J. Catal., 142 (1993) 561. 16. S. Scire and R. Burch, Appl. Catal. B, 3 (1994) 295. 17. S. A. G6mez, G.A. Fuemes and A. Martinez, Actas XV Simp. Iberoam. Catal., 2 (1996) 709. 18. J.Y. Yan, G.-D. Lei, W.H.M. Sachtler and H.H. Kung, J. Catal., 161 (1996) 43. 19. T. Tanabe, T. Iijima, A. Koiwai, J. Mizuno, K. Yokota and A. Isogai, Appl. Catal. B, 6 (1995) 145. 20. M.W. Anderson and L. Kevan, J. Phys. Chem., 91 (1987) 4174. 21. A.V. Kucherov, A.A. Slinkin, D.A. Kondrat'ev, T.N. Bondarenko, A.M. Rubinshtein and Kh.M. Minachev, Zeolites, 5 (1985) 320. .

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

On the role of oxygen in the reaction of selective NO reduction with methane over Co/ZSM-5 catalyst E.M. Sadovskaya, A.P. Suknev, L.G. Pinaeva, V.B. Goncharov, C.Mirodatos* and B.S. Balzhinimaev Boreskov Institute of Catalysis RAS, pr. Lavrentieva 5, Novosibirsk, 630090, Russia, e-mail: [email protected]. *Institut de Recherches sur la Catalyse, 2, av. A.Einstein, Villeurbanne, France, e-mail" mirodato@catalyse, lyon-univ 1.fr. In this work the dynamics of adsorbed NO species formation and their reactivity with respect to methane was studied by DRIFT and SSITKA technique. It was shown that NO2~§ formed via NO interaction with adsorbed oxygen are the most active species to react with CI-I4. Basing on the obtained results scheme of the reaction mechanism was proposed. 1. INTRODUCTION Selective catalytic NO reduction with methane catalyzed with Co/ZSM-5 is one of the ways of NO removal from the exhaust gases [1-3]. The reaction mechanism has been intensively studied [4-7], however, many of details on the nature and reactivity of adsorbed NO species are not clear. According to infrared data [6-8], NO adsorption proceeds with formation of mono- and dinitrosyls of cobalt. Nitrite-nitrate complexes as well as N O 2 ~i+ species are known to be additionally formed in the presence of 02. These species is thought to react with methane thus initiating the reaction. However, infrared data don't allow to reveal the mechanism of these species formation. Furthermore, there are no reliable data on the oxygen participation in oxidation of reaction intermediates. In this work we attempted to study the nature and dynamics of NO adsorbed species formation by means of Diffuse Reflectance Infrared Fourier Transform (DRIFT) Spectroscopy in situ. Steady-State Isotopic Transient Kinetic Analysis (SSITKA) allowed to determine both the concentrations of these species and the consequence of their transformation on the catalyst surface. Following numerical analysis of SSITKA data revealed the reaction pathways involving labelled molecules. 2. EXPERIMENTAL The catalyst was prepared from Na-ZSM-5 (8iO2/A1203 = 37) by ion exchange with cobalt nitrate solution, followed by zeolite filtering, drying and calcination at 300~ for 2 h and 500~ for 2 h. Cobalt content in the catalyst was 1.8 wt.% or 2"102~ at/g so that Co/AI ratio was about 0.4. All the experiments were performed at 450~ For DRIFT spectroscopy, 20 mg of the powdered sample was placed into a SpectraTech diffuse reflectance infrared cell adapted to a Nicolet 550 spectrometer. This cell allows in

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situ spectra recording during any sample treatments, reaction mixtures flowing through the

catalyst bed like in the microreactor. The catalyst was initially heated up to 450~ under flowing He, and various transient sequences (switch from one gas composition to another one) were performed. Following gas mixtures were used in these studies: l%NO + He, l%NO + 5%02 + He and 0.57%CH4 + l%NO + 5%02 + He (flow rate = 100 ml/min). Changes in DRIFT pattern (absorption band intensity) and gas composition were monitored continuously. SSITKA experiments included stepwise replacement of ~4N160 with ~SN~sO (~SN and lSo tractions were 94.6% and 92.9%, correspondingly) at constant chemical flow composition. Following switches were made in plug-flow reactor loaded with 0.22 g of the catalyst (flow rate = 120 ml/min)" 1) 14N160'~ISN180 switch in NO + He flow (Cso = 0.6 and 1.2%); 2) laNl60$1SNlSO switch in NO + 02 + He flow (CNo = 0.6%, Co2 = 0.15, 0.3, 0.6, 1.5, 3% and CNO = 1.2%, Co2 = 3%); 3) laN~60$1SNlSO switch in NO + CH4 + O, + He flow (CNo = 0.6%, Ccm = 0.75%, Co~. = 0.15, 0.3, 0.6, 1.5, 3%). The effluent gases were continuously monitored by a quadruple mass-spectrometer "VG SENSORLAB 200 D". Prior to each experiment the catalyst was treated at 500~ in 6% O2+He for an hour, cooled to operative temperature in this flow and then treated by unlabelled reaction mixture. 3. M A T H E M A T I C A L BACKGROUND The model of isotope label transfer in plug-flow reactor: 1

(~:k 6gk

~kak

=

r~

D~j

N2 -

o, a,% =

,=,

N2 a,

-

,=,

-%)

t=O:

aj =a~

r = O:

% = %'""

(1)

N2 i=l

where a i - are the relative concentration of isotope label, Ck - concentrations of substances in the gas phase, 0j - concentrations of intermediates on the catalyst surface, wi = k if~(C,0)- rates of reaction steps, Ot- is a quantity of exchangeable oxygen in the catalyst, ,B~ - is a coefficient of isotope exchange, ~- is the dimensionless bed length, r - is a residence time, b - is a ratio between total number of catalyst active sites and that of molecules in a free space of the reactor. 4. RESULTS AND DISCUSSION 4. I. DRIFT spectroscopy data

Fig. 1a presents time dependencies of the intensities of adsorption bands corresponding to NO adsorbed species observed in the transient from NO + He to NO + 02 + He flow. In

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化

(a)

9

1.0

r,,n

1.o 9

V

~

0

~ ,i,,~

9

0_5

05

0

0

0

60

120

t

0

time, s.

60

9

120

time, s.

Fig. 1. Time dependencies of normalized intensities of mononitrosyls at 1930 em"n ( 9 NO2s+ species at 2130 cmt (U), nitrite complexes at 1523 cmt (A) and CO2 (V) in the transients fxom NO+He to NO+O~+He (a) and from He to NO+CH4 +O~+He Co) flows. addition to Co 2. mononitrosyls, Co(NO), observed in the absence of oxygen, two types of NO2 adsorbed species differing in the rate of their accumulation, namely NO2s+ species and cobalt nitrites, Co(ONO), were formed in the presence of oxygen. Accumulation of NO2s+ species correlates well with partial (about 30%) decrease of mononitrosyl concentration as well as with attenuation of the AI-OH hydroxyl groups (not shown). Acumulation of Co(ONO) is markedly delayed from that of N 02 8+. A substantial additional consumption of NO was observed during this transient thus allowing us to assume that part of Co(NO) should be oxidized giving rise to new adsorption sites presumably located at the interface of cobalt oxide and the zeolite [8]. After the switch from He to NO+O2+CI-I4+He (fig.lb) fast formation of Co(NO) and, some delayed, of NO2~+very similar to those observed during He to NO+O2+He transient (not presented here, see [8]) occurs. In parallel with the step of their formation an almost complete consumption in the reaction followed by the production of CO2 proceeds. Nitrite complexes were not observed at all, presumably, due to their very low accumulation rate. 4.2. SSITKA data

4.2.1.14N160 replacement with nSNlsO in NO+He flow ISN label (Fig.2a, curve 1) appears several seconds later than Ar and its concentration increases fast reaching steady-state value for 10 s. The amount of nitrogen atoms replaced is 6.4"10 TM at/g (CNo=0.6%) that is considered to be the concentration of cobalt mononitrosyls. Time delay for 180 appearance is 50 s (Fig.2b, curve 1), so that a quantity of fast replaced oxygen is about order of magnitude higher than concentration of cobalt mononitrosyls, indicating the presence of fast oxygen exchange between mononitrosyls and cobalt oxide. Subsequent slower increase of 180 isotope fraction could be due to participation of oxygen of zeolite bulk in the exchange. Indeed, mathematical simulation of the dynamics of oxygen label transfer shows that only a part (1.4"1020 at/g) of oxygen from cobalt oxide [Osta~] participates in fast oxygen exchange with mononitrosyls (13str.F > 30 s'l). This oxygen is thought to bind directly with surface sites capable to adsorb NO as mononitrosyls. The number of these sites was estimated to be 50+70 % from the total amount of cobalt (depending

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1508

10

91 0

O8

08

co

f,,)

06 Q.

~ 0

--

o4

04

02

oo

~~/ 6

(a)

l b '2'0 '3'0 '40 time,

s

I

Ar

(b)

1

234

02 00

o

sb ',6o'I~o 2 o o time,

2~o

s

Fig.2. Responses of ~5N (a) and ~80 (b) isotope fraction in NO at~er 14N160to ~5N~80replacement in NO + He flow (1) mad in NO + Oz + He flows at 0.15% (2), 0.3% (3), 1.5% (4) and 3% (5) of oxygen. on the stoichiometry CoO or C0203). Thus, a part of cobalt atoms was proposed to locate in the bulk of clusters and so can't serve as adsorption sites. Oxygen label transfer from NO adsorption sites into zeolite bulk [OzEoL] proceeds considerably slower (13ZEOL= 0.02 S'l).

4.2.2. 14N160replacement with lSNl80 in NO+1602+He flow Time delays of ~SN label appearance in NO are substantially higher in the presence of oxygen (Fig.2a, curves 2-5) indicating the increase of concentration of fast replaced adsorbed species. A pronounced "slow" components on the response curves could arise from retardation of replacement of the species that are more strongly bound with the surface. In accordance with DRIFT data, we can assign the increase of concentration of fast replaced species to the tbrmation of NO2~§ species, while slow replaced species can be related to nitrite complexes. Since the time delay for ~SO emergence doesn't depend on oxygen presence in the flow (Fig.2b, curves 2-5), a direct oxygen exchange between NO28§ species and cobalt oxide clusters can be excluded. Some decrease of 180 label concentration in NO in the presence of oxygen is determined partially by label transfer into the gas phase oxygen. Mathematical simulation of the dynamics of oxygen label transfer shows that the main reason is an increase of the rate of label transfer into the catalyst bulk (13ZEOL= 0.023 s'l). NO2~§ species being directly bound with zeolite could be responsible for this process. We estimated the rates of the formation of all species depending on oxygen concentration. It was found that the rate of label transfer into 02 by the order of magnitude lower than that of NO2~ formation, i.e. their decomposition isn't accompanied by simultaneous oxygen desorption into the gas phase. Thus, NO28§ species is thought to result from interaction of preliminary adsorbed oxygen with NO. As oxygen concentration is raised, surface concentration of NO 2~+ species is increased rapidly reaching the limiting value at 1.5% of Oz (Fig.3). Since this value isn't change aRer the rise of NO concentration (from 0.6 to 1.2%), it means that equilibrium [O] + NO r [NO28+] is shitted to the fight, and almost all adsorbed oxygen is included into NO2~+. In this case, limiting coverage of NO2~+ (~2"10 ]9 molec./gc.,t) approximately corresponds to the quantity of surface active sites for oxygen adsorption, and amounts to amounts to 10% of the total quantity of cobalt atoms.

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1509

1.0

1.0

O

8 0.5 o

L.

N9 O.5

~

E

L_

E t_

0 I-"

0.0

0.0

O ~

concentration of 0 2

Fig.3. Dependence of normalized reaction rate (o) and NO2~ concentration ( 9 ) o n

02

content.

4.2.3. 14N160replacement with lSNlsO in NO+1602+CH4+He flow Under the reaction conditions the delay for ~SN appearance in NO is smaller compared with corresponding values observed in NO+O2+He flow (Fig.2a); "slow" components are not revealed. Since this decrease of the quantity of adsorbed species (9"10 TM mol/g.cat at Co2=3%) can not be due to consumption of mononitrosyls only (see above), then a conclusion on NO2~* and nitrite species participation in CI-I4 activation can be done. However, taking into account slow accumulation of nitrite complexes, contribution of their reaction with methane to the overall process seems to be negligible. Unlike to mononitrosyls, concentration of NO2~§ species is increased, as 02 content is raised, and correlates well with the behavior of N2 formation rate (Fig.3). Hence, NO2~§ could be considered as a main species to react with methane, and the reaction scheme may be written as follows:

1) 2 [1 + 02 +----+ 2 [O1 2) [O] + NO +---+ [NO2~+1 3) [NO2~+] + CI-I4 > [NCH20] + H20 As was shown above, concentration of adsorbed oxygen [O] is very low, and surface complexes [NCH20] seem to be oxidized via direct interaction with molecular oxygen 4) [NCH20]

+

02 d- NO -----+ C02 + HE0 + N2 + [0]

As can be seen from Fig.4, isotope fraction in N2 is initially higher than in NO (Fig.4, curves 2 and 1, respectively). Excess of isotope fraction in the product over that in labelled reagent can be observed only in a plug-flow reactor. At high rate of NO adsorption-desorption ]SN fraction in the gas phase NO and [NOx] is strongly decreased along the catalyst bed. Calculations show that in this case at almost complete ]4N replacement at the beginning of the bed (including [NOx] and [NCHzO]) a negligible outlet label content in NO can be observed. Outlet isotope fraction in N2 being equal to a half of a sum of length-averaged label fraction in NO and [NCH20] (in accordance with the reaction scheme) should be higher than ~SN fraction in the outlet NO (Fig.4, curve 1). SSITKA allows to estimate the ratio of the rate of intermediate conversion to its concentration (i.e. turnover frequency) basing on the value of the shiR of the response of isotope fraction in the reaction product relative to that in labelled reagent [9]. However, in the

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:~ o

o o

.fn

o

,

1.0

1.0

0.8

0.8

0.6

0.6

o.,

o.4

0.2

0.2

0.0

o

(]

0.0

5

1'0 tim e,

1'5 s

20

,

,

,

3 .-"

-=

/

O 0

5

10

15

20

tim e, s

Fig.4. Experimental (points) and calculated (lines) outlet responses of 1~1 isotope fraction in NO (o, 1) and N2 ( o, 2) as well as calculated length-averaged isotope fraction in NO (3) in NO + CH4 + 02 + He flow at 0.15% (a) and 3% (b) of oxygen. case of plug-flow reactor for such estimates we should compare isotope response in N2 with length-averaged label fraction in NO (Fig.4, curve 3). As seen from this Figure, the values of these shifts are close at the experiments withquite different oxygen concentration. It means that the rate of [NCH20] conversion into N2 isn't limited by oxygen concentration.This can be realized in the case when first nitrogen is formed and evolved into the gas phase at the reaction of this intermediate with NO, remaining C-containing fragment is oxidized to CO2. 5. CONCLUSIONS The main role of oxygen can be reduced to the formation of relatively stable oxidized sites, participating in NO2~+formation. The last species are active in the reaction with methane that was confirmed by correlation between the rate of N2 formation and NO28§ concentration depending on oxygen concentration. Step of oxygen adsorption, however, isn't included into catalytic cycle, it plays a role of buffer in steady-state conditions. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.

J.N. Armor, Catal. Today, 26 (1995) 147. M.D. Amiridis, T. Zhang and R.J. Farrauto, Appl. Catal. B, 10 (1996) 203. C. Montes de Correa and A. Luz Villa de P., Catal. Lett., 53 (1998) 205. A.Yu. Stakheev, C.W. Lee, S.J. Park and P.J. Chong, Catal. Lett., 38 (1996) 271. A.D. Cowan, N.W. Cant, B.S. Haynes and P.F. Nelson, J. Catal., 176 (1998) 329. Y. Li, T.L. Slager and J.N. Armor, J. Catal., 150 (1994) 388. A.T. Bell, Catal. Today, 38 (1997) 151. L.G. Pinaeva, E.M. Sadovskaya, A.P. Suknev, V.B. Goncharov, V.A. Sadykov and B.S. Balzhinimaev, and T. Decamp and C. Mirodatos, Chem. Eng. Sci., 54 (1999) 4327. 9. E.M. Sadovskaya, D.A. Bulushev and B.S. Bal'zlfinimaev, Kin. and Catal., 40 (1999) 54.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

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Effect of Si/Al ratio of Mordenite and ZSM-5 type Zeolite Catalysts on Hydrothermal Stability for NO Reduction by Hydrocarbons Sung Yeup Chung a, Beom Seok Kim a, Suk Bong Hong b, In-Sik Nam *a and Young Gul Kim a a Research Center for Catalytic Technology, Department of Chemical Engineering, School of Environmental Engineering, Pohang University of Science and Technology (POSTECH) P.O. Box 125, Pohang 790-784, Korea b Department of Chemical Technology, Taejon National University of Technology, Taejon 300-717, Korea 1. INTRODUCTION One of the most urgent and demanding challenges in environmental catalysis is to abate NO produced on a massive scale from lean-burn gasoline and diesel engines.

The

discovery by Iwamoto et al. that copper-ion-exchanged ZSM-5 (CuZSM-5) zeolite is much more active than earlier catalysts for selective catalytic reduction of NO in an oxidizing atmosphere by hydrocarbon has led to a tremendous interest in the use of a wide variety of transition-metal ions exchanged into zeolites with different framework structures and compositions as a catalyst for this reaction system [1 ].

However, there is a serious drawback

in the practical application of transition-metal-ion-exchanged zeolites including CuZSM-5 to lean-burn engines, due to poor stability under hydrothermal conditions [1,2].

Thus,

considerable effort has been made to elucidate the origin of the deactivation of the catalyst by H20 [2,3]. Recently, it has been shown that mordenite (MOR) zeolites in both synthetic (HM and CuHM) and natural (CuNZA) forms are highly active for the reduction of NO by hydrocarbons [4].

It was also observed that in the presence of H20, the CuHM catalyst loses

significantly its deNOx activity by the competitive adsorption of NO and H20 on the catalyst surface. Unlike CuHM, however, the CuNZA catalyst showed a peculiar water tolerance for NO removal reaction [4,5].

This result was rationalized by suggesting that the hydrothermal

stability of CuNZA for NO reduction would be better than CuHM, because of its higher Si/A1 ratio.

In the present study, the hydrothermal stability of CuNZA, CuHM and CuZSM-5 with

* To whom all correspondence should be addressed. Phone: 82-562-279-2264.

Fax: 82-562-279-8299.

E-mail: [email protected].

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different Si/A1 ratios for the NO reduction under a simulated lean NOx condition has been

systematically reported.

In addition, changes in the physicochemical properties of the

catalysts induced by hydrothermal aging are also investigated to understand the catalyst deactivation behavior. 2. E X P E R I M E N T A L

A MOR type natural zeolite (NZ), mined from Youngil, Korea [6], and a synthetic MOR (Zeolon 900Na, PQ Corp.) were employed to obtain CuNZA and CuHM, respectively. Prior to Cu 2+ ion exchange, these two MOR materials were also dealuminated by acid treatment and steaming to increase Si/A1 ratio in the zeolite.

Five CuZSM-5 catalysts with

Si/A1 ratios ranging from 14 to 95 were prepared using the corresponding HZSM-5 purchased from Tosoh Corp.

All Cu2+-exchanged zeolite catalysts were prepared by conventional

liquid-phase and solid-state ion exchange methods that are described elsewhere [4,5]. Chemical analysis for Si, A1, Cu in catalysts was performed by AA.

The degree of

dealumination of the zeolites was confirmed by 29Si and 27A1 MAS NMR.

The

physicochemical properties of zeolite catalysts employed in this study are listed in Table 1. A simulated lean-bum condition containing 1,200 ppm NO, 1,600 ppm C3H6, 3.2% 02, 3,000 ppm CO, 1,000 ppm H 2, 10% CO2, 10%

H20 and He (balance) was employed to examine the

hydrothermal stability of the catalysts.

The reaction products were analyzed by on-line gas

chromatograph (Hewlett Packard 5890 Series II).

The hydrothermally aged catalysts were

characterized by XRD, ESR, XANES and BET measurements to investigate the cause of the catalyst deactivation. Table 1. Physicochemical properties of zeolite catalysts employed in the present study Catalyst CuHM 1 CuHM2 CuHM3 CuHM4 CuHM5 CuNZA 1 CuNZA2 CuNZA3 CuNZA4 CuZSM-5-1 CuZSM-5-2 CuZSM-5-3 CuZSM-5-4 CuZSM-5-5

Cu content (wt.%) 2.02 4.20 2.55 1.03 1.73 4.37 1.84 1.75 1.64 2.49 2.90 1.43 1.20 0.44

Si/A1 6 5 12 12 22 4 10 14 19 14 26 27 34 95

Cu/A1 0.16 0.30 0.34 0.14 0.45 0.25 0.24 0.28 0.31 0.53 0.81 0.50 0.53 0.52

Surface area (m2/g) 368 434 450 179 232 128 350 344 400 330

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3. RESULTS AND DISCUSSION 3.1 Hydrothermal stability of MOR and MFI type zeolite catalysts Figure 1 shows the hydrothermal stability of CuNZA2, CuHM1 and CuZSM-5-2 catalysts for NO reduction under the lean NOx condition.

When the catalysts were aged at

800~ with 10% H20 for 6 h, the CuHM1 catalyst exhibited a deNOx efficiency of less than 5% at 450 ~

while 15 and 40% of NO conversion were still observed for CuNZA2 and

CuZSM-5-2, respectively.

Thus, the CuZSM-5-2 catalyst was found to exhibit the highest

hydrothermal stability for NO reduction among these three types of zeolite catalysts. Another important observation is that the activity maintenance of the catalysts was proportional to the Si/A1 ratio of the catalyst.

This clearly shows that the hydrothermal

stability of zeolite catalysts for NO reduction under the lean NOx condition primarily depends on the Si/A1 ratio of the catalyst, despite the differences in the zeolite structure. To further elucidate the role of Si/A1 ratio of the catalysts for the hydrothermal stability,

MOR type zeolite catalysts with different Si/A1 ratio were examined.

Figure 2

shows that deNO~ activity of CuNZA catalyst was significantly improved by dealumination. Especially, the CuNZA4 catalyst with a Si/A1 ratio of 19 still showed more than 35% of NO conversion at 550~ even after the aging at 800~ with 10% H20 for 24 h. 100 0~"

o

z q.. o to "~ o> c o O

l

100

8O

o

sO

Z

~=

t

c

40

1

604

'

i

20

300

400

500

600

700

Reaction Temperature (oC)

Figure 1. Hydrothermal stability of CuNZA2 (O,O), CuHM1 (1,[:]) and CuZSM-5-2 (A,A) catalysts. Closed and open symbols indicate fresh catalysts and hydrothermally aged ones at 800~ for 6 h with 10% H20, respectively.

O,

t

=

300

.

.

400

.

.

.

500

,

.

1

600

700

Reaction Temperature (~

Figure 2. Hydrothermal stability of the dealuminated CuNZA catalsyts 9 CuNZA2 (O), CuNZA3 ( I ) and CuNZA4 (A). These catalysts were hydrothermally aged at 800~ for 24 h with 10% HzO.

3.2 Effect of Si/Ai ratio of MOR and MFI type zeolite catalysts To understand the origin of the significant enhancement of the deNOx activity over the dealuminated CuHM catalysts, the activities of CuHM catalysts with different Si/A1 and Cu/A1 ratios have been examined and are given in Figure 3.

Note that the Si/A1 ratio of

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loo

100

. ao "~

8

8.0

i

4o

i

=o 0

30G

404)

500

84)0

0

7'00

Reaction Temperature (~

]

i

.

.

.

.

300 400 500 600 Reaction T e m p e r a t u r e (~

]

700

Figure 4. Effect of Si/A1 ratio of the CuZSM-5 catalysts on their NO removal activity : CuZSM-5-1 ( O ) , CuZSM-5-3 (ll), CuZSM-5-4 ( 9 and CuZSM-5-5 ( 9

Figure 3. Effect of Si/A1 and Cu/A1 ratms of the dealuminated CuHM catal.ysts on their NO removal actw]ty : CuHM1 (O), CuHM2 (1), CuHM3 ( 9 and CuHM4 ( 9

CuHM1 is quite similar to that of CuHM2, while these two catalysts contain Cu/A1 ratios of 0.16 and 0.30, respectively.

However, the conversion of NO was found to be not distinctive

for the catalysts, even though the reaction temperature exhibiting the maximum NO removal activity of the catalysts is slightly shifted.

For CuHM1 and CuHM4 catalysts with similar

Cu/A1 ratios, on the contrary, a significant improvement in the deNOx activity was observed after dealumination.

A similar result was also observed from the distinctive deNOx activities

of CuHM2 and CuHM3.

Therefore, the improvement in the NO removal activity of CuHM

catalysts upon dealumination cannot be attributed to the difference in Cu/AI ratio of the catalyst but the increase in Si/A1 ratio.

This speculation can be further supported by the

catalytic results obtained from CuNZA and CuZSM-5 with different Si/A1 ratios. shows NO removal activity of CuZSM-5 with similar Cu/A1 ratios.

Figure 4

There may be an

optimal S1/A1 ratio for CuZSM-5 catalyst to remove NO by hydrocarbons.

Therefore, it is

most likely that the Si/A1 ratio of the zeolite catalyst is still one of the most critical properties determining NO removal activity of the catalyst for this reaction system. 3.3 Characterization of the zeolite catalysts

When CuZSM-5-2 catalyst was hydrothermally aged at 900~

structural collapse of

the catalyst was clearly observed by a comparison of the intensity of the XRD peak of the catalyst with respect to the aging duration shown in Figure 5.

As the aging time and

temperature increase, the structure of the catalyst was progressively destroyed. intensity of the characteristic diffraction peaks of the catalyst aged at 900~ significantly reduced as depicted in Figure 5(0.

Thus, the

for 24 h was

This might well reflect not only the loss of

the deNOx activity of the catalyst but also the reduction of the catalyst surface area by hydrothermal aging.

A remarkable decrease of XRD peak intensity by hydrothermal aging

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was also observed for CuHM1 and CuNZA2 catalysts.

For the dealuminated catalysts, the

degree of structural destruction of the catalysts examined by XRD well agreed with the rate of the activity maintenance of the catalyst.

This again supports the speculation that the Si/AI

ratio of the catalyst plays an important role for the destruction of zeolite structure by hydrothermal aging as expected. (a)

~

~_t"L,.

' 10

.

L

20

L

, 30

~

(b)

r .-.,.---

~

(e) -;0

50

60

7'0

80

20 Figure 5. X-ray powder diffraction pattern of CuZSM-5-2 catalysts : (a) fresh, (b) aged at 700~ without H20 for 2 h, (c) aged at 800~ with 10% H,O for 4 h, (d) aged at 800~ with 10% H20 for 6 h,-(e) aged at 800~ with 10% H20 for 24 h and (f) aged at 900~ with 10% H20 for 24 h. The destruction of the catalyst structure upon the hydrothermal aging may alter the ionic state of Cu 2+ ions on the catalyst surface.

Figure 6. Cu 2§ ESR spectra of CuZSM5-2 catalysts : (a) fresh, (b) aged at 800~ with 10% H20 for 4 h, (c) aged at 800~ with 10% H20 for 24 h and (d) aged at 900~ with H20 for 24 h.

An alteration of the chemical environment of Cu 2+

ions on the hydrothermally aged catalyst surface was examined by ESR as shown in Figure 6. The low field hyperfine structure of CuZSM-5-2 catalyst was notably reduced when aged at 900~ with 10% H20 for 24 h.

This suggests that Cu 2§ ions on the catalyst surface were

probably transferred to copper oxide species by the sintering of the metal ions through the structural collapse of the catalyst.

Such an alteration of Cu 2§ ions was also observed for

CuHM1 and CuNZA2 catalysts after hydrothermal aging.

Therefore, it is clear that the loss

of Cu 2+ ions on the catalyst surface occurred upon hydrothermal aging and thereby caused a significant loss of NO removal activity of the catalysts. Additionally, the alteration of Cu 2+ ions to the copper oxides has been confirmed by X-ray absorption near edge structure (XANES).

As shown in Figures 7 and 8, isolated Cu 2§

ions were transformed to C u 2 0 after hydrothermal aging, as already examined by XRD and ESR studies.

Therefore, hydrothermal aging caused the structural destruction of zeolite and

the decrease of surface area, and then Cu 2+ions transformed to copper oxides that may not be active reaction sites for the present reaction system.

Based upon the peak intensity of Cu20,

the zeolite with a higher Si/A1 ratio results in less formation of Cu20 on the catalyst surface.

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A

5 ni

\

8

o <

i

8960

8980

9000

9020 Energy (eV)

8960

9040

Figure 7. XANES spectra of CuHM catalysts : (a) CuHMl(fresh), (b) CuHM3(fresh), (c) CuHM5(fresh), (d) CuHM1 (aged), (e) CuHM3 (aged), (f) CuHM5(aged) and (g) Cu20. These catalysts were hydrothermally aged at 800~ for 24 h with 10% H,O.

|

|

8980 9000 9020 E n e r g y (eV)

9040

Figure 8. XANES spectra of CuNZA catalysts : (a) CuNZA2(fresh), (b) CuNZA3(fresh), (c) CuNZA5(fresh), (d) CuNZA2(aged), (e) CuNZA3(aged), (f) CuNZA5(aged) ) and (g) Cu20. These catalysts were hydrothermally aged at 800~ for 24 h with 10% H20.

Therefore, it can be concluded that the Si/A1 ratio of the zeolite catalyst is one of the most important characteristics for the hydrothermal stability of zeoilte catalyst. 4. CONCLUSIONS Hydrothermal stability of MOR and MFI type zeolite catalysts for NO reduction by hydrocarbons has been observed to become stronger as the Si/A1 ratio of the catalyst increases. It is evident that the Si/A1 ratio of the zeolite catalyst is the most critical characteristic determining the hydrothermal stability of the catalysts for this reaction system.

The

hydrothermal stability of the catalysts can be improved by the optimization of the Si/AI ratio of the zeolite catalysts.

A significant loss in the deNOx activity is mainly due to the

alteration of Cu 2+ions on the catalyst surface through the collapse of the zeolite structure resulting in a decrease of the catalyst surface area. REFERENCES 1. M. Iwamoto and N. Mizuno, J. Auto. Eng., 207 (1993) 23. 2. Y.Li, P. Battavio and J.N. Armor, J. Catal., 142 (1993) 561. 3. K.C.C. Kharas, H.J. Robota and D.J. Liu, Appl. Catal. B, 2 (1993) 225. 4. M.H. Kim, I.-S. Nam and Y.G. Kim, Appl. Catal. B, 6 (1995) 297. 5. M.H. Kim, I.-S. Nam and Y.G. Kim, Appl. Catal. B, 12 (1997) 125. 6. M.H. Kim, U.-C. Hwang, I.-S. Nam and Y.G. Kim, Catal. Today, 44 (1998) 57.

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Studies in Surface Science and Catalysis 130 A. Corrna, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Precipitation of MnO2 onto the external surface of zeolite microcrystals. Structure of the manganese oxide and its role in the removal of NO~ by NH3 M. Richter, H. Kosslick, R. Fricke Institute of Applied Chemistry Berlin-Adlershof, Richard-Willst~.tter-Str. 12, D- 12489 Berlin, Germany A catalyst formulation for the low-temperature conversion of NOx in wet feed beneath 200~ is proposed, based on manganese(IV) oxide and zeolite Y. It is reported that the MnO2 phase structure and the appropriate arrangement are of paramount importance for the catalytic properties of the composite catalyst. The reaction is promoted when intermediate nitrous acid is drained off from the gas phase equilibria through reaction with structural NH4+ ions of the zeolite component. Efficient NOx reduction to N2 requires close neighbourhood of catalyst components and access to the pore system of the zeolite which is ensured by the particular modification technique applied. 1. INTRODUCTION Spatial separation of active components within composite catalysts is one possibility for fine tuning of catalytic properties [1 ]. Enrichment of active components on the extemal surface (the so called egg-shell variant) of porous materials is advantageously for reaction processes proceeding under diffusion control. Zeolite catalysts often operate under diffusion control due to its microporosity. One example is the selective catalytic reduction (SCR) of NOx by NH3 where high space velocities are applied in practice. The present contribution addresses the question how far nature and spatial arrangement of modifying manganese oxides can improve the catalytic properties of zeolite components for the low-temperature reduction of NOx to N2. This reaction was shown to proceed on zeolites through structural NH4+ ions at temperatures lower than 100 ~ under dry conditions and at relatively low space velocities [2]. The reaction requires an intermediary oxidation of NO to NO2 and obeys the overall equation (Z denotes the anionic zeolite framework): 4 ZNH4 + + 4 NO

+ 02 -+

4 N2 + 6 H20 + 4 Z-H+

(1)

NH4+ ions are stoichiometrically consumed while the zeolite is converted into its protonic form [3]. In more detail, conversion of NO• to N2 is thought to comprise two main stages. The first stage is the establishment of simultaneous, coupled equilibrium reactions (2)-(4) starting from NO, 02 and H20 with formation of nitrous acid. 2 NO

+

02 ~ 2 NO2

(2)

NO + NO2 ~ N203

(3)

N203 + H20 ~ 2 HNO2

(4)

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The second stage consists in the selective conversion of HNO2 into N2 by NH4 + ions of the zeolite: HNO2 + Z'NH4 + --} N2 + 2 H20 + Z'H +

(5)

Equation (5) is another formulation of reaction (1) where NO/NO2 mixtures are equivalent to

N203, the anhydride of HNO2. Undesired formation of nitrous oxide is frequently observed. Thus, a selectivity of N2 formation can be expressed in the form S = [N2]/([NE]+[N20]). The approach was that draining off HNO2 from the set of quilibria (2)-(4) by the irreversible reaction step (5) should be of crucial importance for the performance of a composite catalyst. Preliminary screening revealed that manganese(IV) oxide as modifier and faujasite structure Y as zeolite component are superior to other catalyst formulations. Thus, catalyst preparations includes two routes aimed at increasingly close neighbourhood of MnO2 and NH4-Y (1:9 on mass basis all), (i) milling MnO2 powder of different origin with zeolite NH4-Y and (ii) precipitation of MnO2 onto the zeolite microcrystals. It will be shown that MnO2 deposited on the external surface of zeolite Y by precipitation considerably enhances the NOx conversion by NH3 in the presence of water at 130-180~ where the zeolite component serves for NH3 supply. Regarding the outstanding variety of known "Braunstein" phases [4] it was one further aim of the current work to identify suitable phases for the manufacturing of composite DeNO• catalysts. 2. EXPERIMENTAL

2.1. Sample preparation Precipitation of bulk MnO2 was carried out by intense mixing of Mn(II) acetate solution and potassium permanganate at room temperature. The same precipitation technique was applied for covering the external surface of zeolite powder NHa-Y with MnO2 yielding a composite material. Besides the own precipitated MnO2 products, two commercial variants were included, one crystalline form and a second one announced as 'precipitated' (Alfa, Karsruhe, Germany). Zeolite NH4-Y was purchased from Aldrich (Steinheim, Germany) with Si/A1 = 2.3. Details on catalysts samples are given in Table 1. 2.2. Characterization Pore volumes and surface areas were determined by nitrogen adsorption on the ASAP 2010M facility (Micromeritics). Calculation of pore sizes followed the method of Barrett-Joyner-Halender (BJH) according to implemented software routines. Thermogravimetric analysis was performed on a TG/DTA 92 derivatograph (Setaram). The sample was kept at room temperature for 5 min and then heated up to 800~ at a rate of 10 K/min. Temperature Programmed Reduction (TPR) was performed applying the characterization system AMI-1 (Altamira Instruments) equipped with a thermal conductivity detector. For reduction, a feed of 5 vol.-% H2 in Ar was used at a heating rate of 5 K/min. The overall flow rate amounted to 25 cm3/min. The consumption of H2 was determined from the integrated peak areas by calibration. Catalytic measurements were performed in a flow system equipped with an integral flow reactor (internal diameter 10 mm) and on-line sampling.

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Table 1 Samples and textural parameters Sample Mn-0a Mn-0b Mn-Oc Mn-Od Mn-Oe NH4-Y Mn-1 Mn-2 Mn-3 Mn-4 Mn-5

Compositiona

Preparation

MnOi.65,amorphous dried at 150~ MnOi.64,amorphous calcined at 300~ MnOi.66(tx-Mn02) calcined at 500~ MnOi.74,amorphous commercial MnOl.98,(13-MnO2) commercial (NH4)ss(AIssSi1340384) commercial Mn-0e/NH4-Y (10/90) milled Mn-0d/NH4-Y (10/90) milled Mn-0a~H4-Y (10/90) milled Mn-0c/NHa-Y (10/90) milled MnO2/NHa-Y (10/90) precipitated

S (m2/g)b Vp (cm3/g) e

170 n.d. 27 58 0.3 640 n.d. n.d. n.d. n.d. 608

Micro

Meso d

0.05 n.d. 0.02 0.02 n.d. 0.29 n.d. n.d. n.d. n.d. 0.26

0.14 n.d. 0.17 0.18 n.d. n.d. n.d. n.d. n.d. 0.17

adetermined from XRD and TPR (see text), bsurface areas from N2 adsorption isotherms epore volumes, drange of average pore diameters 6.6 nm (Mn-0a), 44.4 nm (Mn-0c), 13.6 (Mn-0d) and 9.8 nm (Mn-5). Mn-1 to Mn-5: mass ratio of MnO2/NH4-Y in parentheses, n. d. not determined. Overall flow rates of 120 cm3/min were applied for 0.9 g or 0.15 g of catalyst (resulting space velocities 8000 and 48000 cm3/g h, resp.). Feed compositions of 1000 ppm NO, 10 vol.% 02, 7 vol.-% H20, remainder He, were introduced into the reactor. According to the NH4§ ions stored on the zeolite, no reductant was necessary at low space velocities. Stationary measurements at the higher space velocity were conducted with permanent supply of 1000 ppm NH3. Reaction temperatures are in the range of 100-200~ Analysis of the product flow was conducted by gaschromatography. Simultaneously, the exit stream composition was continuously analyzed by a Maihak Multigas sensor. 3. RESULTS AND DISCUSSION

3.1. Nature of MnO2 and of composite catalysts Textural data of Mn-0a reveal a high surface area and mesopores with an average diameter of 6.6 nm. Zeolite NH4-Y has only micropores according to its crystal structure. Sample Mn-5 reveals both textural characteristics of samples NH4-Y and Mn-0a. This ascertains that the zeolite pore system is still accessibledespite precipitation of 10 wt.-% MnO2. TPR curves (cf. Fig. 1) show a significant change of the profile between samples Mn-0a, b,c and commercial MnO2 samples (Mn-0d,e). In principle, reduction of MnO2 by H2 leads to MnO via intermediate Mn203 and Mn304 formation [4,5]. Nevertheless, XRD proved the amorphous nature of Mn-0a but identified crystalline ~MnO2 in sample Mn-0c. The structure of Mn-0d is also amorphous and certain similarities to own samples Mn-0a, b are evident in the TPR profile. Sample Mn-0e consists of crystalline I~MnO2 (pyrolusite). Sample Mn-0a can be thermally treated up to 300~ without modification

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of the properties whereas calcination at 500~ modifies the reducibility considerably. The stoichiometry of the MnO2 'phases (cf. Table 1) were calculated from the consumed during the TPR process. The .o crystalline commerical sample E Mn-0e has the composition = Mn02 within the margin of o error. Own precipitations (samples Mn-0a, b,c) have 0 100 200 300 400 500 600 compositions in the range of Temperature (~ MnOi.64-1.66. This oxygen deficit points to the nonstoichiometric character of the sol- Fig. 1. TPR profiles of samples Mn-0a (a, solid line) Mn-0b, ids. From the H2 balance it is (b, dotted), Mn-0c (c), Mn-0d (d), Mn-0e (e), the latter two evident that the calcination has profiles are vertically shifted, m = 25 mg, 13= 5 K/min. not caused a loss of oxygen. Modification of Mn-0a due to calcination is confirmed by thermoanalysis (Fig. 2). Endothermic weight loss occuring from ambient temperature up to 400~ is ascribed to water desorption and dehydration/dehydroxylation (approximately 10 wt.-%). An exothermic peak is centred around 500~ Crystallization processes are of exothermic nature but should not be accompanied by weight increase as it seems to be the case. It is reasonable to assign this effect to a partial crystallization of a-MnO2 requiring oxygen uptake. Sample Mn-0a contains 7 wt.% K which is known to stabilize the phase structure but, on the other side, may lead to K<2MnsOI6 (kryptomelane) as mixed oxide [6]. XRD of Mn-0a, however, gave no indication of this manganese oxide phase. Thermoanalysis of sample Mn-5 (Fig. 3) does not reveal any crystallization process. Instead, weight loss is significantly higher and the endothermic process comprises obviously two steps. Besides dehydration/dehydroxylation of the manganese oxide, water is desorbed t--

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家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1521

from the zeolite component. These processes are superimposed on the high-temperature egde by an exothermic process. At temperatures higher than 200~ ammonia is desorbed which becomes oxidized on the manganese oxide causing a sharp exothermic peak. Thermal release of NH3 from NH4-Y zeolite is found to proceed in the range 200-500~ by other authors [7]. Further changes of the MnO2 phase are not resolved.

3.2. Catalytic properties Catalytic results shown in Fig. 4 were obtained without addition of gaseous NH3. The SCR of NOx is exclusively accomplished by NH4+ ions of the zeolite. The NH4+ ion capacity of zeolite NH4-Y is high enough (ca. 3.5 mmol/g) for the duration of the experiments even at complete conversion of 1000 ppm NOx. However, the pure zeolite component has only marginal activity within the temperature range 130-210~ Addition of crystalline MnO2 (sample Mn-1) leads to only minor improvements. Precipitated MnO2 variants (samples Mn0a,b,c, Mn-0d) admixed to NH4Y are more effective. This underlines the importance of the loo MnO2 structure rather than the role of spatial arrangement of z~ the components. This aspect o 6o gains influence at higher space = O velocities. Shorter residence ~ 4o times make rapid travelling of = o 20 tJ intermediates more crucial and o the spatial arrangement of z 0 100 120 140 160 180 200 220 catalyst components has a higher Temperature (~ impact on the overall performance. Unlike the measurements at low space Fig. 4. NO conversion to N2 v s . temperature for various velocities where NH4+ ion catalyst formulations. NH4-Y (A), Mn-1 (+), Mn-2 (e), concentrations of the zeolite Mn-3 (x), Mn-4 (V), Mn-5 ( l ) , GHSV = 8000 cm3/g h, component are sufficient for feed: 1000 ppm NO, 10 vol.-% 02, 7. vol.-% H20, rereduction, at high space velocity ductant: NH4+ ions of the zeolite. the additional supply of NH3 is necessary. As followed from Figure 5, a discrimination is obvious. It turned out that Mn-5 has the highest performance removing NOx almost completely at temperatures higher than 160~ Moreover, the selectivity to exclusive N2 formation is also highest on the composite catalyst material, whereas Mn-2 causes increasing N20 by-product formation at higher reaction temperatures. Primarily, both reaction stages, viz. formation of nitrous acid and its decomposition, can proceed on the pure components. But, MnO2 is superior to the zeolite for NO oxidation (specifically in the presence of water) and the zeolite is superior in the conversion of nitrite due to its reactive NH4+ ions. As a consequence of close spatial neighbourhood of oxidative (MnO2) and reducing (NH4-Y) catalyst components, NO is efficiently converted to N2. Besides spatial arrangement, easy reducibility of the manganese oxide phase is seen as another prerequesite for yielding active catalysts. It follows from the reduction profiles (cf. Fig. 1) that Mn-0a,b and Mn-0d are easily reducible, because H2 consumption sets in already at ca. 100~ i

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4. CONCLUSIONS

It turned out that the MnO2 phase structure and the appropriate arrangement are of paramount importance for the catalytic properties of the composite catalyst. The approach that intermediate nitrous acid once formed is drained off from its equilibrium with NO/NO2 and H20 by the zeolite component, ! explains why the close spatial lOO neighbourhood of the redox .-. 100 [] A .._..~ component and the zeolite is 80 98 beneficial for NOx conversion to g O N2. Crystallinity of the MnO2 '.o_ 60 96 ~ phase is detrimentral to good > "6 activity. This might be as" 40 94 ~ O O sociated with the reducibility o and the low surface area of the z 20 92 z~ crystalline material. Redox 90 properties are one fundamental 0 130 140 150 160 170 prerequesite for the catalytic Temperature (~ effectiveness of the manganese oxide. At higher space velocities Fig. 5. NO conversion to N2 (full symbols, left) and N2 and permanent NH3 supply, the selectivity (open symbols, right) vs. reaction temperature. adsorption/activation of NH3 NH4-Y (triangles), Mn-2 (circles), Mn-5 (squares). from the gas phase is one GHSV = 48000 cma/g h, feed: 1000 ppm NO, l0 vol.-% additional reaction step. How far 02, 7 vol.-% H20, permanent supply of 1000 ppm NH3. structural NH4+ ions of the zeolite are still involved has yet to be clarified. Nevertheless, a breakthrough to lowtemperature conversion of NOx in wet feed beneath 200~ is achieved with the proposed composite catalysts. 9

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This work is supported by the Senate of Berlin and the Ministry of Education and Research of the FRG (Project 4086 z O w / c / o 40122/1). Ing. R. Eckelt, Dr. H.-L. Zubowa and Ing. R. Dambowsky is thanked for assistance. M. R. and R. F. are indebted to the Verband der Chemischen Industrie (VCI) for financial support. REFERENCES 1. 2. 3. 4.

F. Pinna, Catal. Today 41 (1998) 129. M. Richter, R. Eckelt, B. Parlitz, R. Fricke, Appl. Catal. B: Environmental 15 (1998) 129. M. Richter, B. Parlitz and R. Fricke, J. Chem. Soc., Chem. Commun., 1997, 383. Gmelin Handbook of Inorganic and Organometallic Chemistry, 8th edition, Springer Verlag 1993, Manganese C 1 (1973). 5. E. R. Stobbe, B. A. de Boer, J. W. Geus, Catal. Today, 47 (1999) 161. 6. A. K. H. Nohmann, M. I. Zaki, S. a. A. Mansour, R. B. Fahim and C. Kappenstein, Thermochimica Acta, 210 (1992) 103. 7. U. Lohse, I. Pitsch, E. Schreier, B.Parlitz, K.-H. Schnabel, Appl. Catal. A: General, 129 (1995) 189.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Catalytic properties of mesoporous molecular sieves for the reduction of NO~ A. Jentys*, W. SchieBer and H. Vinek Vienna University of Technology, Institute for Physical Chemistry Getreidemarkt 9/156, A-1060 Wien, AUSTRIA. http://www.physchem.tuwien.ac.at/catalysis/ The catalytic properties of Pt, Rh and Co supported on mesoporous molecular sieves with MCM-41 type structure for the reduction NOx with propene were investigated. Strongly acidic sites were incorporated by co-impregnation of the catalysts with H3PW12040. Pt/MCM-41 was found to be the most active catalyst, however, the selectivity to N2 formation was rather low. Rh/MCM-41 was less active, but had a significantly higher selectivity to N2 formation compared to Pt/MCM-41. Co/MCM-41 had the lowest activity among the series of catalysts studied. In the presence of water vapor a significant increase in the activity was observed for the Pt and H3PW1204o containing catalysts, while for all other catalysts studied the typical decrease in the activity was found. 1. I N T R O D U C T I O N Nitrogen oxides, formed during combustion processes in power plants, waste incinerators and diesel engines, are among the major air pollutants taking part in the formation of photochemical smog and acid rain [1]. Consequently, the catalytic reduction of NOx plays an important role in the transformation of combustion processes into an environmentally friendly technology [2]. At present, catalysts based on titanium- and vanadium-oxides, together with ammonia as reducing agent, are typically applied to remove nitrogen oxides from flue-gas streams of stationary sources [3]. For mobile sources extensive research is currently carried out to find alternative catalytic systems in order to replace NH3 with other reducing agents such as hydrocarbons [4]. Catalysts based on transition metals containing zeolites, e.g., Cu/ZSM5 [5] and Co/ZSM5 [6], were investigated in the first place, but the strong decrease in their activity when H20 and/or SO2 are present in the reactant [7], led to the development of alternative catalysts such as Fe/ZSM5 [8, 9], Pt/ZSM5 [10] and Pt group metals supported on

Si02 and A1203[I 1, 12]. * Present Address: Technische Universit~it Miinchen, Institut fiir Technische Chemie II Lichtenbergstr.4, 85747 Garching, Germany

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To combine the advantages of zeolites and oxide based systems, we studied the catalytic properties of noble and transition metals supported on mesoporous molecular sieves with MCM-41 type structure. As the acid sites of MCM-41 type materials were found to be only weakly acidic [13], the catalysts were additionally impregnated with tungstophosphoric acid, a procedure, which is known to generate strong (Bronsted) acidic sites on this type of material [14].

2. EXPERIMENTAL 2.1. M a t e r i a l s and c h a r a c t e r i z a t i o n The synthesis of siliceous MCM-41 was carried out according to ref. [15] using fumed silica, hexadecyltrimethylammoniumbromide and tetramethylammoniumhydroxide pentahydrate. The support was impregnated with aqueous solutions of PtC14, Rh(NO3)3, Co(NO3)e and H3PWleO4o. The metal loading of the catalysts used was 1.6 wt% for Pt, 1.0 wt% for Co and 1.7 wt% for Rh. The H3PW1204o loading was varied between 0 and 60wt%.(for details see refs. [16, 17]). The structure of the MCM-41 type material was verified by XRD before and after calcination and by Ne sorption, where the usual features of MCM-41 type materials, e.g., four Bragg reflexes in the XRD and a sharp step in the Neisotherm at p/p0-0.4 were observed. The d-spacing of the hexagonal unit cell, determined from the (100) reflex in the XRD, was 39 A, the BET surface area was 1006 meg -1.

2.2. Catalytic r e a c t i o n s The catalytic activity was studied in a quartz reactor (i.d. 8 mm) containing 100 mg of the catalyst. The temperature was measured with a thermocouple placed in direct contact with the catalyst bed. The composition of the reactant gas was: 1010 ppm NO (about 91 ppm NO were directly oxidized in the reaction system to NOD, 1012 ppm propene and 4.9vo1% 02 (balance helium). Up to 8 vol% water vapor could be added into the carrier gas stream using a syringe pump. The total flow of the reactants was 100 cm3/min and resulted in a space velocity of W/F = 6"10 .2 g.s.cm -3. Reactants and products were analyzed with a chemiluminescence NOx analyzer and a gas chromatograph equipped with a PoraPOLT-Q and a Molsieve 5/~ column using a TCD and a FID detector. The conversion was measured in a temperature range between 180 and 500 ~ 3. R E S U L T S AND D I S C U S S I O N The NOx and C3H6 conversions for the MCM-41 supported catalysts and for the H3PW1204o containing catalysts are shown in Figs. 1 and 2, respectively. Independently of the type of metal and the presence of H3PW12040 the conversion of NOx and propene started at the same temperature and increased

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with increasing temperature until the total conversion of propene was reached. At this temperature the maximum conversion of NO,, was obtained. A further increase of temperature led to a decrease in the NO~ conversion, while the hydrocarbon conversion level remained at 100%. 70

~. lOO

--<>- Pt/MCM-41 ..-v--. Rh/MCM-41

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Temperature [~

Temperature [~

Figure 1: NOx and C3H6 conversion for Pt/MCM-41, Rh/MCM-41 and Co/MCM-41 70

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400

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Temperature [~

Figure 2: NOx and C3H6 conversion for Pt/HPW/MCM-41, Rh/HPW/MCM-41 and Co/HPW/MCM-41 (H3PWleO40 loading 30 wt%) The highest NOx conversion was observed for Pt/MCM-41 and it decreased with increasing H3PW~204o loading, while the selectivity to the nitrogen formation increased from 35% (Pt/MCM-41) to 45 % (Pt/HPW(60)/MCM-41). Also the temperature where the maximum NOx conversion was reached increased with the H3PW1204o loading of the catalysts. The activities and selectivities of all catalysts investigated are summarized in figure 3. Rh and Co supported on MCM-41 had a lower activity compared to the series of Pt containing catalysts, while for Rh/MCM-41 the highest selectivity to

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the formation of N2 (67%) was observed. The additional H3PW12040 loading of the Co and Rh containing catalysts led to a lower activity and in contrast to Pt/MCM-41 catalysts also to a lower selectivity to N2 formation. For Co containing catalysts, in contrary to the noble metal containing catalysts studied, the temperature at the maximum NOx conversion decreased after the H3PW12040 loading. 100

90

O O

Pt/MCM41 Pt/HPW(15)/MCM-41 Pt/HPW(30)/MCM-41 Pt/HPW(60)/MCM-41

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N2 selectivity for MCM-41

supported

and

The activity of the catalysts at 300 ~ as a function of the water vapor concentration is shown in figure 4. At this temperature, the activity and selectivity of all Pt containing MCM-41 supported catalysts in water free reaction conditions was similar. As already reported [18], a continuous decrease in the activity with increasing water vapor concentration was observed for Pt/MCM-41, leading to a 15 rel% lower conversion at a water vapor concentration of 8.1 vol%. On the contrary, H3PW1204o containing Pt/MCM-41 catalysts showed an improved activity in the presence of up to 8 vol% water vapor. Note that compared to the reaction in water free atmosphere, the NOx conversion over the Pt/HPW(60)/MCM-41 catalysts increased more than 25 rel% at the presence of 2 vol% water vapor. At higher water vapor concentrations the activity was slightly decreased, however, up to a concentration of 8 vol% H20 all Pt/HPW/MCM-41 catalysts were more active compared to the water free reaction conditions.

家 50

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Rh/MCM-41 Rh/HPW(30)/MCM-41 Co/MCM-41 Co/HPW(30)/MCM-41

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Figure 4: Activity of the catalysts as a function of the water vapor concentration 4. C O N C L U S I O N S Pt supported on siliceous MCM-41 showed a high activity for the reduction of NOx with propene at a relatively low reaction temperature. The co-impregnation with tungstophosphoric acid led to a decrease in the activity, which resulted from a partial coverage of the Pt clusters by tungstophosphoric acid. Contrary to the results published in the literature, the Pt/HPW/MCM-41 catalysts showed a pronounced increase in the catalytic activity during the reduction of NOx with hydrocarbons in the presence of H20 vapor. The Rh/MCM-41 catalyst was less active compared to Pt/MCM-41, but the selectivity to N2 formation was significantly better. However, the decrease in activity in the presence of water vapor was more pronounced compared to Pt/MCM-41. However, Rh/MCM-41 seems to be an interesting option as the selectivity to N2 formation is significantly higher when a MCM-41 type material is used as support compared to ?-A1203. Co/MCM-41 catalysts showed a lower activity compared with both noble metal containing MCM-41 catalysts studied, which might result from an oxidation of Co during the reaction. Additionally, the activity strongly decreased when water was added into the reaction gas mixture. This clearly reflects that noble metals supported on MCM-41 type materials have a higher potential to be applied as catalysts for the reduction of NOx than non noble metals.

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Loading of the catalysts with tungstophosphoric acid led to a decreased activity, while the selectivity to N2 formation reached a similar level on all of these catalysts. Note, that this resulted in an improved selectivity for Pt/HPW/MCM-41, while for Rh/HPW/MCM-41 and Co/HPW/MCM-41 the selectivity was lower compared to the corresponding Me/MCM-41 catalysts.

ACKNOWLEDGEMENT

The work was supported by the project 7119 of the "Hochschuljubil~iumsfonds der 5sterreichischen Nationalbank ". REFERENCES

8 9 10 11 12 13 14 15 16 17 18

C.T. Bowmann, 24 th Symposium on Combustion, (1992) 859. J. N. Armor, Catal. Today, 38 (1977) 163. H. Bosch and F. Janssen, Catal. Today, 2 (1988) 369. J. N. Armor, Catal. Today, 26 (1995) 99. M. Iwamoto, H. Furukawa, Y. Mine, F. Uemura, S. Mikuriya and S. Kagawa, J. Chem. Soc., Chem. Commun., (1986) 1271. Y. Li and J. Armor, Appl. Catal. B, 1 (1992) L31. M. Iwamoto, N. Mizuno and H. Yahiro, Proc. 10th Int. Congr. on Catalysis, Budapest (1992) p. 1285. X. Feng and W. K. Hall, J. Catal., 166 (1997) 368. H.Y Chen and W. M. H. Sachtler, Catal. Lett., 50 (1998) 125. M. Iwamoto, H. Yahiro, H. K. Shin, M. Watanabe, J. Guo, M. Konno, T. Chikahisa and T. Murayama, Appl. Catal. B, 5 (1994) L1. H. Hamada, Y. Kintaichi, M. Sasaki and Y. Ito, Appl. Catal., 75 (1991) L1. R. Burch, P. J. Millington, Catal. Today, 26 (1995) 185. A. Jentys, N. H. Pham and H. Vinek, J. Chem. Soc., Faraday Trans., 92 (1996) 3287. I. V. Kozhevnikov, A. Sinnema, R. J. J. Jansen K. Pamin and H. van Bekkum, Catal. Lett., 30 (1995) 241. C. F. Cheng, D. H. Park and J. Klinowski, J. Chem. Soc., Faraday Trans. 93 (1997) 193. A. Jentys, W. Schiel3er and H. Vinek, Catal. Lett., 47 (1997) 193. A. Jentys, W. Schiel3er and H. Vinek, J. Chem. Soc., Chem. Commun., (1999) 335. W. Schiel3er, H. Vinek and A. Jentys, Catal. Lett., 56 (1998) 189.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

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Catalytic Activity of High Surface Area Mesoporous Mn-based Mixed Oxides for the Deep Oxidation of Methane and Lean-NO~ Reduction V.N. Stathopoulos a, V.C. Belessi a, C.N. Costab, S. Neophytldes, P. Falaras d, A.M. Efstathioub'*, and P.J. Pomonis a'* 9

C

aDepartment of Chemistry, University of Ioannina, Ioannina 45110, Greece 9 bDepartment of Chemistry, University of Cyprus, Nicosia 1678, Cyprus. eFORTH-ICE/HT, P.O. Box 1414, University Campus, 26500 Patras, Greece. dlnstitute of Physical Chemistry-NCSR "Demokritos", 15310 Aghia Paraskevi Attikis, Greece. 1. INTRODUCTION Catalysis has played a major role in environmental protection over the past generation. As the methodology to accomplish environmental protection has developed to include pollution prevention and green chemistry, catalysis has continued to supply the techniques, tools and methodologies which have made some of the most innovative green chemistry technologies possible (e.g., "three-way" catalytic converter). As fuel efficiency coupled with cleaner technologies are required, it is expected that catalysts will play a key role not only in the abatement of emissions but also in the front end of the process to minimize pollution at the source. The most elusive new potential application has been the NOx reduction using hydrocarbons for "lean-bum" engine technology [1, 2]. In addition, the selective reduction of NOx by hydrocarbons (HC-SCR) is becoming a strong alternative for the replacement of NH3SCR industrial process [3]. These challenges will continue well into the 21 st century. The use of catalysts for primary power generation, e.g., catalytic combustion, with virtually no generation of pollutants, is also on the front-edge as new high temperature materials [4]. The present work brings together the concept of mesoporous materials based on manganese and enriches them with other heterocations such as: A1, La, Ce, at will. The materials thus formed have been tested for important environmentally catalytic reactions: CH4/NO/O2 (lean de-NOx) and CH4/O2 (combustion). Their corresponding catalytic performances were found superior to those reported in the open literature for a large number of catalysts tested under similar experimental conditions. A large number of techniques were employed for surface and bulk characterization of the present materials. These include: X-ray photoelectron spectroscopy (XPS) for surface composition, X-ray diffraction (XRD) for crystal phases, surface area measurements (BET) and AFM/Tapping mode experiments for surface texture, and transient methods for in situ determination of adsorption capacities and interaction characteristics of the reaction molecular species with the catalyst surface. The results obtained from these techniques were correlated with the catalytic performance results obtained. Corresponding author.

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2. E X P E R I M E N T A L

The general route for obtaining large surface area, mesoporous and/or microporous manganese based oxidic systems was based on the hydrolysis of trinuclear manganese complex [Mn30(CH3COO)6(pyr)3]C104 and it is a follow-up of work published previously [5, 6]. The trinuclear manganese complex was prepared according to the original report of its synthesis [7]. The main synthesis steps are described in Fig. 1(a)-1 (c). An aquatic drop containing metal cations A n§ and NO3 is added to the acetonic solution of the trinuclear manganese complex. As the solvents mix the chemical species interact, resulting in the hydrolytic decomposition of the complex (Fig.la). The hydrolysis products are OMn4 or MnO4 tetrahedra. Cations A§ adsorb in the vicinity of Mn hydrolysis products and bond to Mn via Mn-O-A bonds (Fig.lb). Finally, as the concentration of Mn-O-A groups increases, they form larger species via Diffusion Limited Aggregation (DLA). The gel-type species upon drying ~ ~,~-~.~-form high surface area solids (Fig. 1c). The specific surface area of the solids (m2g-1) obtained after hydrolysis and drying at various temperatures, and Mn ~)H qA [ their pore size distributions were measured by multi-point Mn~"Mn Ho"Mn-oH A..o'M~. O_A ] BET method at 77K by N2 adsorption. The surface ~v,. +o.: 0%. +,+ A-Oo.~ l /tg,,M,,n - Mn Mn composition of the Mn-based samples was determined by ~Vln -" H +, ,I /O"Mn Mn/?'.lVln Mn-'~Mn..o AI means of X-ray Photoelectron Spectroscopy (SPECS LH- M~'O~ Mnx,,Mn HO Mn OH A-O ,Mn - I OH 6-A l 10). The powders were pressed firmly into a carved (b) ~. stainless steel holder so that they could be introduced into the Ultra High Vacuum chamber (8x10 -l~ mbar). Calculations of the various components composition of the A.o/Mn-o.A A-Oo_ A . solids were based on the spectra of the Mn(2p), Al(2p), La(4d), Ce(3d) and O(ls) photoelectrons. Sensitivity Mn"~'Mn,,c~ a (~'~ factors were reproduced from Wagner et al. [8]. The texture A-O" l~ln ' - ' ' " (c) o-A and morphology of the Mn-based oxides were also investigated by atomic force microscopy (AFM Nanoscope Fig. 1. Schematic representation III, Digital Instruments) in the tapping mode. Observations of the process synthesis of Mnwere performed on thin films deposited on microscope based mesoporous oxides. glass slides. X-ray diffraction patterns of the Mn-based oxides after heating at 500~ were obtained using CuKa radiation at a 2~ scanning rate in a Siemens 500 system. Details on these characterization techniques can be found elsewhere [6, 9]. Table 1 reports the main results of the physicochemical characterization of Mn-based materials based on the above mentioned techniques. In Table 1, the fractal dimension reflects the geometric complexity of the catalyst surface and it was evaluated based on a detailed fractal analysis [ 10]. Catalyst testing of the samples fired at 500~ was conducted in a conventional flow quartz microreactor. Transient experiments were performed in a flow-system which has been described elsewhere [ 11]. Continuous monitoring of the transient responses from the reactor was performed by on line mass spectrometer (MS) (Omnistar, Balzers) equipped with a fast response inlet capillary/leak valve system. The Temperature-Programmed Desorption (TPD) experiments were performed with a 0.3g sample (powder form), 30scc/min He flow and a heating rate of 30~ The analysis of the effluent of reactor in the case of CH4/O2 reaction was done by the use of a gas chromatograph (GC), while that of NO/CH4/O2 reaction by the use of a GC/MS system [11].

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Mass numbers (m/z) used were: 15, 30, 32 and 46 for CH4, NO, 02 and NO2, respectively. Catalytic activity measurements for the deep oxidation of CH4 reaction were conducted in the range 235-500~ at a flow rate of 115 scc/min (GHSV,~70000 hl), and using CH4=4.34%, 02=8.68% and He as balance. For the lean de-NOx reaction, catalytic tests were conducted in the range 200-500~ at a GHSV~12000h ~, and using CHa=6700ppm, NO=2000 ppm, 02=5% and He as balance. 3. RESULTS and DISCUSSION

3.1 Catalyst Characterization As shown in Table 1, the specific surface area and the mean pore diameter (Dp) of the Mnbased solids strongly depend on the A-cation used (A=AI, La, La+Ce). The reasons for these differences is believed to be related to the hydrolytic paths of the cations A n+, originally in the aquatic drops, as soon as they find themselves in the buffered microenvironment created by the acetic and pyridine groups. In a previous study [6] it was shown that the gradual addition of Mn3complex into aquatic solution containing a variety of cations, always leads to a final solution of pH=4.75, which corresponds to the pKa of acetic acid. The nitrate solutions of the A-cations used (A=A1, La and La+Ce) fed dropwise into Mn3/acetone solution hydrolyzing the complex. At the same time coprecipitation of the cations occurs [ 12] at a different degree leading to final products of various surface morphology and composition. For example, the A13§ ions can form oligomeric species at this pH region, among them the well-known Keggin ions [AlI3Oa(OH)24(H20)12]7+. Such species can be adsorbed on the MnOx(OH)y nano-entities resulting in an increased product surface area with resistance to sintering. More discussion on the mechanism of preparation of the present materials can be found elsewere [9]. Table 1 Ph)zsicochemical characteristics of Mn-based mixed oxides materials Property Calcination T(~ Sample Mn-A1-O Mn-La-O BET (m2/g) 711 247 300 310 166 500 3.6 4.8 Dp (rim) 300 3.8 5.1 500 Amorphous Amorphous Crystal structure/XRD 500 1.70:1 0.85:1 Mn:A in hydrolysis 0.28:0.11:0.61 0.33:0.05:0.62 Surface composition/XPS 500 9.76 13.53 Roughness/AFM (nm) 300 20 10-15 Particle size/AFM (nm) 300 2.22 2.29 Fractal dimension/AFM 300

Mn-La-Ce-O 170 122 2.6 3.5 Amorphous 0.85:0.5:0.5 0.18:0.11:0.28:0.53 -

Figure 2 presents XPS results for the Mn-La-Ce-O solid, while Table 1 shows the calculated atomic surface composition of various Mn-based mixed oxides based on the XPS results obtained. In all four samples, Mn(2p3/a) is detected at 641.6 + 0.2 eV (Fig.2). According to the literature [13], Mn(2p3/a) is observed at 642 eV for MnO2, 641 eV for Mn304, 641.4 eV for Mn203 and 641.2 eV for MnO. It is, therefore, rather dificult to conclude on the valence of manganese. However, from the surface composition it seems rather probable that the valence is

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between Mn § and Mn § The atomic surface composition calculations were carried out considering that A1 is as A1203, and La as La203. In the case of Ce, the XP spectrum (Fig.2) clearly shows the coexistence of CeO2 and Ce203 phases. The satellite peak at 916.6 eV is characteristic of CeO2, while the poor resolution of the rest spectrum indicates the coexistence of Ce203 [14]. The O(ls) spectra (not shown) exhibited one peak at 529 + 0.2 eV corresponding to the oxide peaks of all metals except A1, which is detected at 531.6 eV. 660

655

650

645

...........

640

635

Mn_(2p3/2i ' t

II N 2 Selectivity

Mn-La-Ce-O

I--'1

"LT,,'~

t~

60

d

|

920

9

,

910

9

|

900

sn)

9

( % ) 40

|

890

9

i

880

,

|

870

Binding Energy (eV) Fig. 2. X-ray photoelectron spectra for the Mn-La-Ce-O catalyst

0

200

250

300

Temperature ( o C)

350

Fig. 3. NO conversion and selectivity towards N2 formation for the CHa/NO/O2 reaction on Mn-La- O and Mn-La-Ce- O solids.

3.2 Catalyst Performance Figure 3 presents results of the CH4/NO/O2/He reaction in terms of NO conversion and N2 selectivity in the low-temperature range of 200-350~ over the Mn-La-O and Mn-La-Ce-O materials. At 250~ the NO conversion is 25%, while the N2 selectivity is 80% in the case of Mn-La-Ce-O solid. The CH4 conversion varied between 3.5 and 10% in the range 200-350 ~ on both catalysts. Addition of 4% H20 in the feed stream resulted in the remarkable 98% N2 selectivity value, while the maximum NO conversion remained at the same level, but shifted to slightly higher temperatures. This behavior remained constant even after 24h of continuous run. It is noted that a Mn-ZSM5 catalyst was reported to be active only at T>350~ for the same reaction [ 15]. To appreciate the low-temperature "lean de-NOx" catalytic performance of the MnLa-Ce-O material reported in Fig. 3, a comparison is made with one of the best metal supported catalyst (Pt/A1203) reported with potential applications in diesel and "lean-bum" engine [1]. Under C3H6/NO/O2 reaction (C3H6=2350 ppm, NO=500 ppm, O2=5%, GHSV=60000h -1) at 250~ the NE selectivity was only 38% [ 16] to be compared with the value of 80-98% observed over the present material. It is appropriate to state that CH4 activation is expected to be more difficult than that of higher hydrocarbons over metal oxide surfaces. Therefore, a higher de-NOx activity is expected by using other hydrocarbons than CH4 as reductant species [ 17]. Figure 4 presents results of the catalytic performance of the present Mn-based porous oxidic materials (MANPO) in terms of the temperature necessary for 50% conversion of CH4 (T50%). To appreciate the performance of these materials, a comparison is made with few of the best catalysts reported in the literature [18, 19] for the same reaction. It is apparent that MANPO materials are by far the best catalysts for CH4 combustion under the investigated conditions. Indeed detailed calculations show that the ratio (W/F)sarraco over the ratio (W/F)MANPOequals 20. Thus revealing more clearly the better performance of the MANPO solids.

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600

T50%

(~

" Tejuca et al. [18]

Saracco et al. [19]

MANPO

-

500 400

300 200 o

100

La

ao.6Sro.4MnO 3 Pt/AI203 ).8Sro.2MnO3

Ai-Mn-O LaMn0.sM g 0.503 LaMn0.6Mg0.403 La-Ce-Mn-O La-Mn-O LaMn0.8Mg0.203

Fig.4. Catalytic performance of Mn-based porous oxidic materials (MANPO) for CH4 combustion. For comparison, the temperature necessary for 50% conversion (T50%) of CH4 over MANPO and other materials reported [ 18, 19] is indicated. Figure 5 presents temperature programmed desorption (TPD) profiles of NO and 0 2 o v e r MnLa-O and Mn-La-Ce-O solids. After adsorption of NO at T=500~ for 15 min from a 0.5%NO/He mixture, the catalyst was cooled in NO/He to 25~ and left for 15 min. The feed was then changed to He (5 min at 25~ and the catalyst temperature was then increased to 750~ to carry out a TPD run. In the case of 02 TPD, the adsorption conditions were: T=500~ for 15 min followed by cooling of the reactor to 25~ in 5%O2/He flow. The TPD profiles of Fig. 5 show significant differences among the two samples. Table 2 Amounts (~tmol/g) of 02 and NO adsorption measured durin8 various transient experiments Experiment Catalyst Sample Mn-La-O Mn-La-Ce-O 1.TPD of 02 128.8 40.3 2.TPD of NO

27.3

10.8

3.Reaction in CH4/NO/02 at 400~

NO = 77.9 0 2 = 206.0

NO = 9.1 0 2 = 37.8

~, 6000 ,~ 500( Q 4000 3000

O2 . . . . . NO

Mn-La-O

2000 o

Ce-O

1000 0 25

125 225 325 425

525 625 725

Temperature (~ Fig.5. TPD spectra of 02 and NO over Mn-La-O and Mn-La-Ce-O catalysts.

With respect to the amounts of adsorption, the Mn-La-Ce-O sample exhibits much lower uptakes (~tmol/g) in both NO and 02 than the Mn-La-O sample. These quantities are given in Table 2. With respect to the strength of adsorption of NO and 02 with the catalyst surface, Fig. 5 clearly indicates that the binding energy of NO is much lower in the case of Mn-La-O than MnLa-Ce-O sample (TM=125~ vs. 425~ while the opposite is true for the oxygen binding energy. In both samples, desorption of oxygen starts at about 225~ where the Mn-La-O surface exhibits two distinct 02 TPD peaks (TM=405 and 610 ~ while the Mn-La-Ce-O sample exhibits a single 02 TPD peak (TM=375~ with a shoulder at the high temperature falling part of it. The amounts of chemisorbed NO and oxygen species during CH4/NO/O2 reaction at 400~ were also determined. Following reaction for 20 min, the catalyst was quickly cooled to 200~ under the reaction mixture. The feed was then switched to pure He, while the NO and 02

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desorption was measured isothermally at 200~ Subsequently, a TPD run in He flow was performed. The total amounts of NO and O2 calculated under the isothermal and TPD runs are given in Table 2. Astonishing results from this experiment is that, in the case of Mn-La-O sample, the amount of NO and 02 adsorption was significantly enhanced under reaction conditions a result opposite for the Mn-La-Ce-O sample. The transient results shown in Fig.5 and Table 2 can contribute in explaining the differences in the lean-NOx activity and selectivity of the two samples. The Mn-La-Ce-O sample is by far a better "lean-NOx" catalyst in the range 200300~ than the Mn-La-O one. At 250 ~ the Mn-La-Ce-O exhibits about twice NO conversion and N2 selectivity than the Mn-La-O. The much higher chemisorption of oxygen and NO on MnLa-O could suggest that formation of NO2 proceeds with a lower rate than in the case of Mn-LaCe-O solid, if oxidation of NO to NO/ is considered as an important step for reaction with adsorbed hydrocarbon CHx species to form N2. The XPS results may also suggest that a lower concentration of Mn with a mixed oxidation states, and the presence of Ce, are likely reasons for an enhanced activity. Worth of noting is also the TPD results of Fig.5 where in the case of MnLa-Ce-O the binding energies of NO and 02 are similar, a result opposite for the Mn-La-O sample. This could also be an intrinsic reason that largely influences the activity of the present catalysts. REFERENCES

1. A. Fritz, V. Pitchon, Appl. Catal. B: Envir. 13 (1997) 1. 2. E.S.J. Lox and B.H.Engler, "Handbook of Heterogeneous Catalysis", G. Ertl, H. Knozinger and J.Weitkamp, Eds., VCH, Weinheim, v.4, 1997. 3. V.I. P~rvulescu, P. Grange, B. Delmon, Catal. Today 46 (1998) 233. 4. T. Seiyama, Cat.Rev.-Sci.Eng., 34 (1992) 281. 5. C.S. Skordilis, P.J. Pomonis, J. Colloid Interface Sci. 166 (1994) 61. 6. A.D. Zarlaha, P.G. Koutsoukos, C.S. Skordilis, P.J. Pomonis, J. Colloid Interface Sci. 202 (1998) 301. 7. J.B. Vincent, H.R. Chang, K. Folting, J.C. Huffman, G. Christou and D.N. Hedrikson, J. Amer. Chem. Soc. 109 (1987) 5703. 8. C.D. Wagner, L.E. Davis, M.V. Zeller, J.A. Taylor, R.H. Raymond and L.H. Gale, Surf. Interf. Anal. 3 (1981) 211. 9. V.N. Stathopoulos, D.E. Petrakis, M. Hudson, P. Falaras, S.G. Neophytides and P.J. Pomonis, "Characterization of Porous Solids-V', Elsevier 1999, accepted for publication. 10. A. Provata, P. Falaras, A. Xagas, Chemical Physics Letters 297 (1998) 484. 11. C.N. Costa and A.M. Efstathiou, J. of Catalysis, submitted for publication. 12. J. Kragten, in "Atlas of Metal-Ligand Equilibria in Aqueous Solutions", Ellis and Horwood, J. Willey, New York, 1978. 13. C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, "Handbook of X-ray Photoelectron Spectroscopy", Ed. G.E. Mullenberg, Minnesota (1978). 14. M. Alexandrou, R.M. Nix, Surf. Science, 321 (1994) 47. 15. A. Aylor, L. Lobree, J. Reimer, and A.T. Bell, J. Catal. 170 (1997) 390. 16. R. Burch, D. Ottery, Appl. Catal. B: Envir. 9 (1996) L 19. 17. R. Burch, P. Fomasiero, T.C. Watling, J. Catal. 176 (1998) 204. 18. L.G. Tejuca, and J.L.G. Fierro, in "Properties and Applications of Perovskite-Type Oxides", Marcel Dekker, New York, 1993. 19. G. Saracco, F. Geobaldo, G. Baldi, Appl. Catal. B: Envir. 20 (1999) 277.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Characterization of the role of Pt on CoPt and InPt Ferrierite Activity and Stability upon the SCR of NO with CH4 Laura Gutierrez, Laura Cornaglia, E. Mir6 and J. Petunchi # Instituto de Investigaciones en Cathlisis y Petroquimica - INCAPE (FIQ, UNL-CONICET) Santiago del Estero 2 8 2 9 - 3000 - Santa Fe - Argentina. Phone/Fax: 54-342-4536861. e-mail: [email protected] The selective catalytic reduction (SCR) of NO• with CH4 in excess of oxygen was studied over a series of ferrierite-supported monometallic (Co, In or Pt) and bimetallic (Pt-Co or Pt-In) catalysts. Pt promoted the NOx to N2 conversion of both Co and In ferrierite aider the samples were reduced with 1-12at 350~ either under dry or wet reaction stream. The addition of 2% of water to the feed stream also increased the activity of the calcined PtlnFerrierite for the said reaction. Temperature Programmed Reduction and XPS results show the presence of Co ~ Pt ~ Co +2, Pt +2 and In at exchange position in the samples with the highest activity and selectivity for the SCR of NO• I. INTRODUCTION The cooperation effect of catalytic species has recently been studied for the selective catalytic reduction (SCR) of NO• in order to obtain a suitable catalyst under real reaction stream (1). In this vein Kikuchi and coworkers (2-5) performed several studies on the effect of the addition of precious metals (Pt, Rh and Ir) to In/HZSM5. They reported that such solids are highly selective for the reduction of NO with CH4 in feed streams containing up to 10% of water vapor. The role of the precious metals would be to accelerate the NO to NO2 oxidation path even in the presence of water vapor. These authors also suggested that the bifunctional catalysis of such solids is remarkably facilitated by the co-existence of the active sites in the pore of the zeolite, what they call "interpore catalysis". We also found that Pt added to CoZeolite promote the NO to N2 conversion with CH4 and increases the water resistance of such solids (6, 7). In such studies the best performance was obtained with PtCoMordenite (Co/Pt =15) after being reduced with H2 at 350~ This contribution investigates the role of the Pt-Co and Pt-In interaction on the activity and water resistance of bimetallic ferrierite and the nature of the species present, using TPR, XPS, XRD and the catalytic test.

# Our recognition and gratitude to Prof. Juan O. Petunchi for his lifelong lasting contribution to Catalysis. The authors wish to acknowledge the financial support received from CONICET, UNL (CAI+D "96 Program) and ANPCyT. They are also grateful to Claudio Maitre for his technical support and to Prof. Elsa Grimaldi for the edition of the English manuscript.

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2. E X P E R I M E N T A L

Catalyst preparation. The starting materials were K-Ferrierite (Si/AI = 9) and ml203. The Ferrierite-based samples were prepared by ion exchange and the alumina-based ones were obtained by the wet impregnation method. Further details are supplied elsewhere (6). The solids obtained were: CoFer, PtFer, PtCoFer, InFer, PtlnFer, Pt/AI203, Co/A1203, Pt/Co/AI203, In/A1203, Pt/In/Al203. Table 1 shows the prepared catalysts. Table 1 Prepared solids Catalysts

Metal content (wt %) Pt

PtFer a

0.44 0.5 0.52

CoFer a

InFer a PtCoFer ~ PtlnFer Pt/AI203

b

C0/A1203 In/A1203

Pt/In/Al203

b

1.38 1.8 -

0.52 0.49

-

-

-

2.0

2.0

b b

In -

0.5 b

Pt/Co/ml203

Co -

-

-

0.5

2.0

-

0.5

-

2.0

a Prepared by cation exchange. b Prepared by wetness impregnation.

Catalysts characterization. TPR experiments were performed with 0.100 g of pretreatment catalyst in an Okhura TP-2000S instrument with a heating rate of 10~ in a 5% flow of H2 in Ar. XPS data were obtained in a Shimadzu ESCA-750 spectrometer equipped with a Mg anode (MgK = 1253.6 eV). XRD patterns were obtained in an XD-D1 Shimadzu instrument with monochromator employing CuKa radiation and scanning rate of 1degree, min ~ . Catalytic measurements. The reaction was carried out using a fixed-bed reactor. The typical mixture was NO = C H 4 = 1000 ppm, 02 = 2%, H20 = 2% balanced to 1 atm with He. The catalytic activity and the composition of the reacting gases were analyzed with a SRI chromatograph. 3. RESULTS AND DISCUSSION The calcined Co and InFer were active for the SCR of NO• with CH4. Both samples converted c.a. 60% of NO to N2 under dry reaction stream. InFer reached such conversion at 400~ whereas CoFer did at 450~ The monometallic Co and In samples impregnated in A1203 were almost inactive up to 450~ (Fig. 1.A). The incorporation of 0.5% of Pt did not modify the activity of the oxidized monometallic Ferrierites in the 350-500~ temperature range (Fig. 1.B and Table 2) (Note the lower In loading of PtlnFer). ARer the addition of water to the reaction stream, the activity of both bimetallic solids

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was significantly affected at low temperatures (Fig. 1.C), but a dissimilar behavior of Pt-Co and Ptln samples was observed at temperatures higher than 400~ In fact, the NO to N2 conversion leveled off about 30% at 500~ on PtCoFer, whereas in the Pt-In sample, as the temperature increased, the solid recovered the activity and at 500~ it reached 85% of NO conversion. Interestingly, when water was removed, the PtlnFer presented higher activity than the fresh sample (Table 2). When the samples were reduced under flow H2 for l h at 350~ the results obtained were quite different as shown in Fig. 1.B and C. The PtlnFer presented 70% of NO conversion at 500~ under both dry and wet reaction stream, while in the PtCoFer samples, the NO conversion reached almost 100% under dry stream at 450~ However, when 2% of water was added, the maximum conversion occurred at 550~ 80% against 35% in the calcined sample and 20% in the CoFer. 00 90 A . . . . . . . . . . . 80

..............

70

.

-=o=~> 4050.ii:./.~:ii ='~~tiil _~. ~30 i 60

10

.

.

.

.

........,~IC'"U_:._::2.g- -

.** .

. . . . . . . .

......

0~

........

o

3T04~0 4T0 5"~0 5T0 3: 0 400 450 500 550 350 400 450 500 550 Temperature (~ Temperature (~ Temperature (~ Figure 1 : Catalytic behavior of solids. A- Calcined monometallic solids under dry reaction conditions: 0 Pt/ml203, A C0/A1203, V In/A1203, [ ] InFer, | CoFer, 9 PtFer. B- Bimetallic solids under dry reaction conditions: A Pt/Co/A1203, V Pt/In/A1203, F'l, l PtCoFer, ~ ,O PtlnFer,. Open symbols: calcined samples, Close symbols: reduced samples. C- Mono and Bimetallic solids under wet reaction conditions: 12], l PtCoFer, ~ O PtlnFer, ~ InFer, | CoFer, 0 Alumina-based solids. Open symbols: calcined samples, Close symbols: reduced samples.

The TON of calcined InFer and PtlnFer were comparable (Table 2), while an increase

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o f the In specific activity (TON) was observed in the reduced bimetallic samples (Table 2). The same effect appeared after the sample had been under reaction stream with 2% o f H20. These results would suggest that atter such treatments some interaction occurred between Pt and In. Table 2 Turnover Frequencies (TON) of PtCo and PtlnZeolites for NO Reduction a

Catalyst

Pretreatment

TON x 104 (s-l,) b 400 ~ 450 ~

350 ~

PtlnFer calcined 10.40 21.25 24 PtlnFer calcined c 8.03 29.75 35 PtlnFer reduced d 7.08 28.30 35 PtCoFer calcined 0.13 1.52 2.44 PtCoFer reduced d 0.89 3.04 6.30 CoFer calcined 0.86 3.53 5.0 InFer calcined 1151 26.70 26 InHZSM5 e calcined 2 86 4.62 4.62 a Reaction conditions: See Figure 1. b TON: number of NO molecules converted per second per Co or In respectively, c After water removal from the reaction stream, d Reduction at 350~ with H2, one hour. e Ref. 4. The XPS results reveal that in CoFer and PtCoFer, in both calcined or reduced state, the only species detected was Co +2 located at exchange position within the zeolite structure. In fact the B.E. o f Co2p3/2 was 783.7 eV against 781.7 in Pt/CoA1203 where the Co would form cobalt oxide species (Table 3, Figure 2.B). In calcined In/A1203 only a well defined signal of In3ds/2 of 445.1 eV was detected, while a shift to 445.4 eV was observed in Pt/In/Al203 (Table 3). These B.E. were different from the one reported by M6riaudeau et al (8) for In § in bulk In203 (B.E. 444.1 eV). In InFer a broad signal was observed, with a B.E. centered at 446.7 eV. However, this signal could include supported In species like in In/ml203 (B.E. = 445 eV) and In exchanged species with a B.E. = 446.5. These values are in agreement with the one reported for InNaY by Meriaudeau et al (8). The addition of 0.5% of Pt to InFer is likely to increase the amount of In at exchanged location. In fact, in this sample a well defined signal o f In3ds/2 with B.E. 446.5 eV was observed (Table 3). Table 3. XPS atomic ratios and binding energies of calcined catalysts

Catalysts l~t/Co/A1203 PtCoFer In/A1203 Pt/In/m1203 InFer PtlnFer

Co 2p3/2 (eV)

Co/A!

In 3d~/2 (eV)

In/A!

781.7 783.7 -

0.08 0.23 -

445.1 445.4 446.7 446.5

0.014 0.017 0.04 0.024

-

-

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The Pt4f signal partially overlaps with that of Al2p, consequently, it was not possible to accurately determine the signal corresponding to Pt in the monometallic samples with 0.5 wt% (6). The TPR profiles of InFer also suggest the presence of two In species (Fig. 2A). One of them with a maximum at 250~ is also present in In/Al203 (Fig. 2.B). This species show a low activity for the SCR of NOx (Fig. 1.A). The broad reduction peak with a maximum at 550~ could be due to In at exchange location in Ferrierite, this species being active for the said reaction in agreement with the findings reported by Kikuchi and coworkers (4) in other In zeolite systems. When 0.5% of Pt were added to the InFer, the TPR profile of the bimetallic samples showed that some reorganization of Pt and In species occurred. The lower temperature peak shifted to 150~ which would suggested that Pt promoted the reducibility the In species. The reduction profiles of Co exchanged ferrierite shows two peaks. One with a maximum centered at 300~ and the other with a maximum at 700~ The 300~ peak could be assigned to a fraction of the cobalt supported as oxide on the external surface of ferrierite. This species is also present in Co/Al203 (Fig. 2 A and B). They appear not to participate as active sites for the SCR of NOx (Fig. 1 A). The maximum at 700~ may be due to the Co +2 reduction at exchange position. The addition of 0.5% of Pt to the CoFer yields a TPR profile which does not correspond to the sum of its respective monometallic CoFer and PtFer, with maximum at 250, 400 and 520~ which suggests the Pt and Co species of different reducibility are present in ferrierite structure. Most of them could be assigned to Co § in exchanged location (Table 3). Pt promoted the reducibility of such Co species.

IB

J 9

/ ~

i

i

i

i

i

:

.

100 200 300 400 500 600 700 800 Temperature (~

rt/Xi 0

.

2

~

~

o

'.~-.....__~ i

9

:

"

. i.

i

9 :

"

~

-

:

_

9

!3

,

2.

3

-:

: 2 : 3

.... ,

100 200 300 400 500 600 700 800

Temperature (~ Figure 2: TPR Profiles. A- Ferrierite - based catalysts. B- Alumina- based catalysts.

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1540

The TPR results (Fig. 2) of PtCoFer and PtlnFer also suggest the presence of Pt ~ Co ~ Pt +2, Co § and reduced In species (probably at exchange positions) in the samples after they have been reduced at 350~ for one hour. In no case did XRD patterns show metal or oxides particles, which suggests that the above species are highly dispersed in the zeolitic matrix. As was point out before and in agreement with our previous findings for the PtCoMordenite (7) neither Co O nor Co203 contributed to the enhancement of the bimetallic PtCoFerrierite. Co § and at exchanged position appear to play a central role as active site for the NO2 reaction with methane. The same role could be attributed to In species at exchanged position like the one species reported by Kikuchi and coworkers and Mir6 et al (9). The main role of Pt in both PtCoFer and PtlnFer would be to catalyzed the oxidation of NO to NO2.

4. CONCLUSIONS 9 Pt promotes the NO to N2 conversion of Co and InFer on the SCR with CH4 under wet reaction stream, the activity order being PtlnFer>PtCoFer>InFer>CoFer>>Pt/Co/Al203=Pt/In/A1203. 9 Pt ~ Pt 2+, Co 2+ and In in exchange positions play a role as active sites in the bimetallic Ferrierite. 9 The presence of Pt during treatment with H2 or wet stream facilitates the exchange of In in Ferrierite, probably following the mechanism proposed by Kikuchi and co-workers (4).

REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9.

Hamada, H., Catal.Surveys from Japan 1(1997)53. M. Ogura, M. Hayashi and E. Kikuchi, Catal. Today, in press. M. Ogura, S. Hiramoto and E. Kikuchi, Chem. Lett. (1995) 1135. E. Kikuchi, M. Ogura, N. Aratmi, Y. Sugiura, S. Hiramoto and K. Yogo, Catal. Today 27 (1996) 35. M. Ogura, M. Hayashi and E. Kikuchi, Catal. Today 42 (1998) 159. Gutierrez ,L., Boix, A. and Petunchi, J., J.Catal. 179(1998)179. Gutierrez ,L., Boix, A. and Petunchi, Cat. Today, in press. P. M6riaudeau, C. Naccache, A. Thangaraj, C. Bianchi, R. Carliand S. Narayanan, J. Catal. 152(1995)313. E. Miro, L. Gutierrez, L. M. Ramallo L6pez and F. G. Requejo, Subbmited to J. of Catal. (1999)

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

化

1541

Cu-Cr O X I D E C A T A L Y S T S F O R C O M P L E T E AROMATIC HYDROCARBONS

OXIDATION

OF

V. G e o r g e s c u Institute of Physical Chemistry, Romanian Academy, Spl. Independentei 202, 77208 Bucharest, Romania

Supported mixed metal oxide systems, Cu-Cr on y - AlzO3 and ~/- Al203 + SiO2 were prepared and studied. They exhibited catalytic activity in the complete oxidation of aromatic hydrocarbons.

I. Introduction Complete oxidation of hydrocarbons or of different waste gases containing hydrocarbons on platinum catalysts is a well developed procedure [1-4]. However, for evident reasons searching for conventional efficient oxide catalysts is at present the scope of many investigations. The aim of the present paper is to check the catalytic activity and stability of a set of supported mixed oxides prepared via a complex of Cu-Cr with tartaric acid.

2. Characterization of catalysts 2.1. Samples preparation The precursor complex was obtained by precipitation at pH 7 of a mixture of Cu-Cr nitrates in aqueous solution mixed with tartaric acid, in a solution of ethanol and ammonium hydroxide 10% in volumetric ratio 1:1. After precipitation, the resulted compound was dried in vacuum at 90~ It was subsequently submitted to chemical analysis, IR spectrometry, magnetic measurements and thermal analysis. All the above methods resulted in the following formula for the precursor: [ Cr Cu4 Ta6 ]. 5H20 Ta = tartaric acid. In order to prepare the supported catalysts two procedures have been used : 9The first one consists of the binding of the precursor on the support by sueesesive impregnation of the solution of tartaric acid and of the nitrate mixtures. 9The second consits in the synthesis of the precursor, its solubilization and deposition on the support by impregnation. Two types of supports were used for impregnation, Al203 tablets (+ = 6 ram) and a mixture of AlzO3 + SiO/grains (+ =3-5 mm).The obtained metalic content of the samples is 8,5-10 %.

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The prepared catalysts were denominate: C1 and Cs for Cu-Cr / ~, - A1203, C2 and C6 for Cu-Cr/y-AI203 + SiOz. Cj ,C2 were prepared by the first procedure and C5 ,C6 by second. The supported catalysts prepared by the two above mentioned procedures, have been dried for 12 h at 90~ and calcined at 650~ for 6 h. Table 1 prezents the surface area of the samples.

Table 1. The surface area of the sample Surface area ( m 2/g ) Sample Uncalcined samples Calcined samples

"y-A1203

7 - A1203 + SiO2

C1

C2

C5

C6

161

153

160 167

124 155

140 152

123 143

2.2 Chemical and physical properties IR and electronic spectra as well as magnetic measurements have indicated that both C O O and partially HO- are coordinated at metalic ions m the precursors. An IR investigation was carried out for all the samples m the spectral range 4000 - 400 CI][1-1. The mare characteristic IR absorption bands are presented in Table 2. Table 2 The characteristics of the IR spectra Sample

v ( OH ) , v ( OH ), v tmsr~nm, V symm, r ( O H ) , y ( O H ) , v ( C-C ), v ( M e - O ) R-COOH

H20

R-OH

(C-O) (C-O)

v(C-O)

R-COOH

R-COOH

~5(OH)

R-OH

R,COOH v( CH3 )

Cu-Cr-Ta (unsupp.complex) Cu-Cr-Ta/y-AI203

3475 3100 3460 3200 Cu-Cr-Ta/y-Al203+SiO2 3480 3340

2940 2850 2960 2850 -

1628 1620 1640 1620

1375 375 375 -

1120 1062 1088 1040 1075 1045

700 680 840 710 810 720

v,m~-m Vs~m (Me-O) 590 460 610 480 630 510

A large and intense absorption band at 3400 - 3000 em 1 is typical for stretching vJ~a-ations of associated H O groups (VoH) and could be found m all the samples. The bands at 1620 - 1580 cm a as well as the sharp band at 1375 cm -1 are usualy attributed

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to asymmetric stretching vibration and symmetric vibration of the C-O bonds m carboxylic ion, respectively. These are also common to all samples and could be assigned to tartaric acid salts ( Cu- Cr ). Sharp and weak bands are found in the region 590 - 420 em-~ which are attributed to Me-O bonds. These are sometimes screened by intensive absorption bands of the supports. Finally, we think that there are credible arguments for both COO" and partially H O coordination even on alumina and alumosilica. The UV-VIS reflection spectra indicated the octahedral coordination of tartaric acid to both metalic ions [5, 6]. The 410 nm band for the Cr Cu4Ta6 5H~O complex, ascribed to the 4A2g ~ 4T lg ( F ) transition of the Crm ( d3 ) in oetahedral configuration, whereas the 590 nm peak is c,lmracteristie to an oetahedral configuration of the Cun (d9 ) ion. The transfommtion of the complex into oxides were checked by TGA analysis. Although the mass loss process stops at 550~ for all samples, the tested catalyst were calcined at 650~ m order to stabilize the oxide phase. XRD spectra of the calcined samples revealed CuO, CuCr204 and Cu2Cr204 on the copperchromium supported catalysts.

3. Experimental TPR and TPD measurements were carried out within a gas-chromatographic setup, with a thermal conductivity cell [7,8], by measuring the hydrogen consumption from a gaseous mixture of argon with 10% hydrogen and recording the hydrogen evolved in argon ( by two measuring cycles; HCR1 and HCR2 ), which was used as the carrier gas, respectively. A linear heating program of increasing the temperature in the range 20-500 ~ with a rate of 10 K min-1 was applied. The catalytic activity of the samples was measured in a flow reactor coupled with a gas-mixing system, a thermostat regulator for hydrocarbon vapour control and needle-valve controlled air flow. The analysis was carried out with an m line Hewlet Packard Gas Chromatograph model 5840A and using a Chromosorb 102 and Porapak Q packed column. The catalysts were tested in the complete oxidation of benzene, toluene, ethylbenzene and isopropylbenzene at 500, 700, 1000 p.p.m concentration of hydrocarbon in air and at 5000, 10000, 20000 h -~ space velocities.

4. Results and discussion Figs. 1 a,b,c and d shows the HCR1 and HCR2 for C1, C2, C5 and C6 supported samples respectively. For C1 and C2 samples, the HCR1 and HCR2 curves are given in Figs.1 a and b which show that the highest hydrogen consumption at lower temperatures is due to sample C1. From the same figures one can see that sample C2 exhibits the low temperature peak only in the HCR2 curve. The sample C5 ( Fig. 1c) exhibits a significant peak only at low temperatures, on the HCR1 as well as the HCR2 curves. The slight increase in the hydrogen consumption at temperatures higher than 300 ~ C can be assigned to adsorption on the support. The HCR1 curve for sample

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Fig.1 HCR1 (e) and HCR2 (=) for the samples: a- C~, b- C2, c- C5, d- C6

C6 (Fig. 1d ) does not exhibit the low temperature peak which appears only in HCR2 curve. The other samples give similar TPD curves characterized by continuous increase in evolved hydrogen at temperatures above 300 ~ C. According to the literature data [9,10], the TPR curves for the CuO/alumina catalyst exhibit a single peak located in the temperature range 280 - 367~ The temperature of the peak depends on the procedure used to prepare the sample and the experimental conditions (hydrogen concentration, flow rate and heating rate ). For the CuO/alumosilica catalysts, the TPR records exhibit either three peaks [11 ] or a peak and a shoulder [12] depending of the procedure of preparation as well as on the character of the interaction between the support and the Cu 2§ ions. The peaks are located in the temperature range 300 - 800 ~ C and have been assigned to the ~ e s

:

Cu 2§ - ~

Cu §

Cu

For the chromium oxide supported on alumina or alumosilica [13] , the TPR records exhibits two or more peaks at temperatures higher than 360 ~ C. Analysis of X-ray diffractograms of the samples showed that they contain CuO as well as CuCr204 and Cu2Cr204. The TPR curves of the supports [7] show a low hydrogen consumption ordy at high temperatures, which can be assigned to the adsorption on alumina taking

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Fig.2 Variation of concentration with temperature at 1000 p.p.m concentration of hydrocarbons in air and 20000 h4 space velocity for: a- C1, b- C2, c- Cs, d- C6, (1- benzene, 2- toluene, 3- ethylbenzene, 4- isopropylbenzene).

into account that alumosilica exhibits similar TPR curves. The low temperature peak on the TPR curve for the unsupported samples [ 8 ] can be assigned to the reduction of CuO. The high temperature peak on the HCR1 curve is probably due to the transformations : CuCr204 --, Cu2Cr204 As shown on the HCR2 curve, the sample has been restructured, not completely reduced, and, consequently, one still records two forms of hydrogen consumptiort The low-temperature form can be assigned either to hydrogen adsorption on metallic copper after the first reduction, or to the reduction of the residual forms of CuO. For the samples supported on alumina ( C1 and C5 ), their TPR curves exhibit only one lowtemperature peak which can be assigned to the reduction of CuO. The TPR curves of samples C2 and C6 supported on alumosilica show quite an unusual behaviour, ilustrated by the fact that the high temperature peak appears only after a previous

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reduction. This shows once more that the samples undergo reconstruction. These curves show only hydrogen forms which desorb at high temperatures, i.e. strongly bonded. The weakly bonded forms can be desorbed in the gas flow at room temperature. The copper-chromium catalysts have a higher activity in the complete oxidation of aromatic hydrocarbons even at temperatures below 300 ~ C, (Figs. 1. a ,b,c and d). The catalytic activity is m according to the reduction capacity at low temperature of the samples. The catalysts supported o n y-AI203 are more active than the catalysts supported on y-AI203+SiO2. With respect to the method of preparation the catalysts obtained by precursor-complex formation directly on the support are more active ( C~ and C2 ). With respect to the organic substrate, the activity of the catalysts increases in order : benzene < toluene < ethylbenzene < isopropylbenzene.

5. Conclusion

Egg-shell type catalysts were obtained by impregnating the supports with precursor coplexes. The presented data concerning the TPR and TPD of hydrogen of copper and chromium oxides unsupported and supported on alumina and alumosilica showed that the reduction is generally accompanied by surface reconstruction. In comparison with older results concerning simple oxides supported through nitrates impregnation [ 14 ] the mixed Cu-Cr catalysts obtained via precursor complexes exibited better activity and stability. References [1] J.A. Bamard and D.S. Mitchell, J. Catal., 12 ( 1968 ) 386. [2]/L Schwartz, L Holbrook and H. Wise, J. Catal., 21 ( 1971 ) 199. [3] C.I. CuUis and B.M Willott, J. Catal., 86 ( 1984 ) 187. [4] A. Inglot, React. Catal. Lett., 2 ( 1989 ) 241. [5] V. Pocol, L. Patron and P. Spacu, Rev. Roumame de Chim, 39 ( 1994 ) 1113. [6] L. Patron, V. Pocol, N. Stanica and D. Crisan, Rev. Roumaine de Chim, in press. [7] M Teodorescu, I. Sitaru, A. Banciu and E Segal, The~mochim Acta, 233 ( 1994 ) 233. [8] M Teodorescu, I. Sitaru, V. Georgescu, MI. Vass, E. Segal, Thermochim Acta, 282/ 283 ( 1996 ) 61-68. [9] S.J. Gentry, N.W. Hurst and/L Jones, J. Chem. Soc. Faraday Trans., 1 ( 1981 ) 77. [10] S. D. Robertson, J.H.de Bass, S.C.Kloet and J.W. Jenkins, J.Catal., 37(1975) 424. [ 11 ] M Shimokawabe and H. Kobayeski, Bull. Chem Soc. Jpn., 56 (1983) 133. [12] F.S.Ddk and/LVavere, J.Catal., 85 ( 1984 )380. [13] B.Parlitz, W.Hanke, R. Frike, M.Richter, W.Roost and G.Oghnann, J.Catal.,94 (1985) 24. [14] V.Georgescu and MI.Vass, Rev. Chim. 26 (1988), 341.

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化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Ag/Lao.6Sro.4MnO3/y-Al203 catalysts for complete oxidation of methanol at low concentration H.-B. Zhang, W. Wang, Z.-T. Xiong and G.-D. Lin Department of Chemistry & State Key Lab of Physical Chemistry for the Solid Surfaces, Xiamen University, Xiamen 361005, CHINA Ag-modified Lao.6Sro.4MnO3-based catalysts with the perovskite structure were prepared, and their activity for methanol complete oxidation was evaluated. The results showed that over a 6%Ag-modified 20%Lao.6Sro.4MnO3/~,-Al203 catalyst, the "1"5oand T95 were as low as 395 and 413 K, respectively, and even at so low reaction temperature, the contents of the reaction intermediates HCHO and CO in the exit-gas were both under the detection limits. The origin of promoter action by Ag was discussed together with the results of spectroscopic characterization of the catalysts. I. INTRODUCTION Methanol or its mixture with gasoline has been suggested as alternate fuel for car operation to reduce gasoline consumption under certain circumstance [1]. However, the exhaust gas of methanol (or methanol-gasoline mixture)-fueled cars contains unburned methanol and formaldehyde [2]. These compounds may have an undesirable impact on air quality. Most of the existing catalysts for complete oxidation of methanol are precious metal (Pt, Pd, Rh etc.) and Ag catalysts supported by ~-AI203 [2,3]. In the present work, a highly active Ag/Lao.6Sro.4MnO3/~,-A1203 catalyst for complete oxidation of methanol at low concentration is reported. The results have potential significance for the catalytic removal of methanol and formaldehyde contaminants from the exhaust of methanol-fueled cars. 2. EXPERIMENTAL A perovskite-type Lao.6Sro.4MnO3 host catalyst was prepared by using a sol-gel method. Another supported host catalyst, y%(mass percentage) Lao.6Sr0.4MnO3/~-ml203, was prepared by conventional impregnating ~/-A1203 support (with a N2-BET-surf. area of 129m2g-1) with the calculated amounts of La(NO3)3, Sr(NO3)2, and Mn(NO3)3 in aqueous solution, followed by drying at 393K for 4h, and then calcining at 1073K for 4h in air. The Ag-modified Lao.6Sro.4MnO3 and y%Lao.6Sro.4MnO3/),-A1203 catalysts (marked as x%Ag/Lao.6Sro.4MnO3 and x%Ag/y%Lao.6Sro.4MnO3/),-A1203) were prepared by impregnating the corresponding host catalyst with the calculated amount of AgNO3 in aqueous solution, followed by drying at

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393K for 4h and then calcining at 773K for 4h. The ),-A1203-supported catalysts, Ag/~'-AI203, Pt/y-A1203, and Pd/y-ml203 were prepared following the procedure described above and

according to the literature [2], respectively. All these catalyst samples were pressed, crushed, and sieved to size of 70-100 mesh. The catalyst activity was evaluated in a fixed-bed continuous flow reactor-GC combination system packed with a catalyst amount of 300mg. The complete oxidation of methanol at low concentration over the catalysts was conducted at a stationary state under reaction conditions of 0.1MPa, feed-gas CH3OH/O2/N2= 0.2/1.0/98.8 (molar ratio), GHSV= 58000 h 1, and at a series of reaction temperature. The reactant and product were analyzed by an on-line GC (Model 102GD) equipped with a hydrogen flame ionization detector and 3m long GDX-103 column. The catalyst activity is expressed by the temperatures, "I'50and T95 (K), at which methanol conversion reached 50% and 95%, respectively. X-ray diffraction measurements were carried out by using a Rigaku D/Max-C X-ray Diffractometer with Cu-Ket radiation at a scanning rate of 8~ XPS spectra were taken by a VG ESCA LAB MK-2 machine with Mg-Kct radiation (10kV, 20mA, hv=1253.6 eV) and UHV (lxl07pa), calibrated internally by the carbon deposit C(ls) (B.E.) at 284.6 eV. BET-surface area of catalyst was measured by N2 adsorption using an SORPTOMATIC-1900 (CARLO ERBA) machine. Hydrogen temperature-programmed reduction (H2-TPR) was conducted on a fixed-bed continuous flow reactor-GC combination system packed with 20mg of catalyst sample at 10 K/min rate of elevating temperature; a N2-carried 5v%H2 gaseous mixture was used as reducing gas, with an on-line GC (Model 102GD) equipped with a thermal conductivity detector monitoring change of hydrogen-signal. 3. RESULTS AND DISCUSSION

3.1. Assay of catalyst activity The results of evaluation of the catalyst activity for methanol complete oxidation are shown in Table 1. It can be seen that the La0.6Sr0.4MnO3 catalyst with perovskite-phase structure displayed the activity rather higher than that of the supported noble metal catalysts, 0.1%Pd/y-ml203 and 0.1%Pt/y-A1203. 6%Ag-modification to the La0.6Sr0.aMnO3 host catalyst improved pronouncedly its catalytic performance for methanol complete oxidation, making the reaction temperatures for conversion of 50% and 95% methanol, "1"50and T95, dropped by 29 and 44 K, respectively. Over the 6%Ag/20%La0.6Sr0.4MnO3/~,-A1203 catalyst, the Ts0 and T95 lowered further down to 395 and 413 K; and even at so low reaction temperature, the contents of the reaction intermediates, HCHO and CO, in the exit gas were both under the detection limits. While on the 0.1%Pd/y-A1203 and 0.1%Pt/y-ml203 catalysts under the same reaction conditions, the content of HCHO in the exit gas attained 250 ppm and 630 ppm at their T95 temperatures, respectively. In order to directly gain information about the catalytic oxidation of CO (an intermediate of methanol complete oxidation), the oxidation activity of CO at low concentration over those catalysts was tested under reaction conditions of 0.1MPa, feed-gas CO/O2/N2=2.3/20.5/77.2 (v/v), and GHSV=23000h -1. The results are listed in Table 2 and show that the prepared

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Table 1 Results of the activity assay of the catalysts for complete oxidation of methanol in low concentration* Catalyst I"5o (K) %5 (K) Lao.6Sro.4MnO3 10% Lao.6Sro.4MnOrA/-Al203 20% Lao.6Sro.4MnOJ~-A1203 27%Lao.6Sro.4MnO3/~,-A1203 1%Ag~ao.6Sro.4MnO3 4%Ag/Lao.6Sro.4MnO3 6%Ag/Lao.6Sro.4MnO3 10%Ag/Lao.6Sro.4MnO3 6%Ag/),-AI203 6%Ag/10%Lao.6Sro.4MnO3/),-A1203 6%Ag/20%Lao.6Sro.4MnOa/),-A1203 6%Ag/27%Lao.6Sro.4MnOa/),-A1203 0.1%Pd/~,-A1203 0.1%Pt/),-AI20 ~ *Reaction condition: 0.1MPa, feed-gas 58000h -1.

437 483 478 450 428 411 408 413 408 398 395 399 460 442 CH3OH/O2/N2 =

465 531 520 507 445 432 421 421 448 415 413 423 512 468 0.2/1.0/98.8 (molar ratio), GHSV=

Table 2 Activity of complete oxidation of CO at low concentration over the x%mg/Lao.6Sro.4MnO3 catalysts and related systems** Catalyst %0 (K) T95 (K) Lao.6Sro.4MnO3 2%Ag/Lao.6Sro.4MnO3

415 368

420 388

4%Ag/Lao.6Sro.4MnO3 358 383 6%Ag/Lao.6Sro.4MnO3 353 370 10%Ag/Lao.6Sro.4MnO3 398 413 6%Ag/~/-A1203 438 470 0.1%Pd/),-AI203 476 482 0.1%Pt/),-A1203 523 531 **Reaction condition: 0.1MPa, feed-gas CO/OJN2= 2.3/20.5/77.2 (v/v), GHSV= 23000h -1.

Lao.6Sro.4MnO3 catalyst displayed higher activity for catalytic CO oxidation than that of 6%Ag/~/-AI203, 0.1%Pd/),-A1203 or 0.1%Pt/),-A1203. The modification of Ag resulted in a pronounced enhancement of oxidation activity of CO: over the 6%Ag/Lao.6Sro.4MnO3, the "1"5o and T95 came down to 353 and 370 K, respectively.

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3.2. Spectroscopic characterization of catalyst Figure 1 shows the results of XRD measurements of the x%Ag/Lao.6Sro.4MnO 3 (x=4 or 6) and related systems. Only the diffraction peaks ascribed to ABO3-type perovskite-phase were observed for the LaMnO3 and Lao.6Sro.4MnO3 systems (see Figures le and la); for the Lao.4Sro.6MnO3 system, in addition to the features of perovskite phase, quite weak peaks ascribable to the discrete SrO phase were also detected (see Figure ld). According the wellknown Scherrer's equation, it can be estimated that the particle size of the prepared Lao.6Sro.4MnO3 sample was -35 nm. Figures lb and lc were the XRD patterns of the two Agmodified Lao.6Sro.4MnO3 catalysts. Besides the features ascribed to the Lao.6Sro.4MnO3 host phase, two weak peaks were present at 20 = 38.14 ~ and 44.34 ~ which got somewhat more evident as the Ag-doping amount increased from 4% to 6% and were in agreement with the known 20 values for metallic silver, thus attributable to crystallite phase of metallic silver. On the other hand, the signals at 20 = 32.8" and 38.0 ~ which should have been the XRD features of Ag20 phase, were extremely weak and too weak and ambiguous to identify. Thus, from the XRD results, no conclusive evidence was obtained for existence of Ag20 in crystallite phase on the 4%(or 6%)Ag- modified Lao.6Sro.4MnO3 systems.

1023 ,r~

596

/ \

d 1023 ~t~ll~

S

~

~

~ ~

c

a -

23

30

40

50

20, degree

60

70

Figure 1 XRD patterns of: a) Lao.6Sro.4MnO3,b) 4%Ag /Lao.6Sro.4MnO3, c) 6%Ag/Lao.6Sro.4MnO3,

d) Lao.4Sro.6MnO3, e) LaMnO3. (* peaks due to the perovskite phase).

323

!

523

i

723

989

ii23

TEMPERATURE, T Figure 2 H2-TPR spectra of: a) Lao.6Sro.4MnO3; b) 2%Ag/Lao.6Sro.4MnO3.

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Table 3 The results of the least-square fitting of the observed O(ls)-XPS spectra Sample

02(lattice)

O22/O(a)

OH/CO32-(a)

B.E.(eV)

Content

B.E.(eV)

Content

B.E.(eV)

Content

of O(ls)

(Mol.%)

of O(ls)

(Mol.%)

of O(ls)

(Mol.%)

528.9

65.5

530.5

17.6

532.1

16.8

6%Ag/Lao.6Sro.4MnO3 528.6

51.5

530.1

33.1

532.1

15.4

Lao.6Sro.4MnO3

The XPS measurement of the Lao.6Sro.4MnO3 host catalyst showed that the surface Mnspecies on the Lao.6Sro.4MnO3 existed mainly in +3 valence-state with the B. E. of Mn3+(2p3/2) at 641.7 eV. For the 6%Ag-modified Lao.6Sr0.4MnO3 system, the XPS spectrum of Mn(2p3/2) displayed duplex peaks with the B.E. at 641.4 and 642.1 eV, indicating that there co-existed Mn 3§ and Mn 4§ at the surface of the 6%Ag/Lao.6Sro.aMnO3 [4]. On the other hand, the Ag(3d5/2)-XPS spectra of the 6%Ag-modified system displayed a main peak at 367.5 eV (B.E.) and a shoulder at 367.9 eV (B.E.), and the latter was somewhat strengthened with increasing Ag-doping amount. According to the literature [4] and the above XRD results, the observed shoulder peak at 367.9 eV was ascribable to Ag~ of metallic Ag-species at quite a low surface concentration, and the main peak at 367.5 eV was due to Ag§ This result demonstrated that most of the supported Ag20 component did not deoxidize to metallic Ag o after the calcination at 773K in the preparation procedure, and still maintained in Ag § valence-state. In addition, the XPS measurements also provided evidence of existence of oxygen-species with mixed valence-states at the surface of these systems (see Table 3): lattice O2(ls) at 528.6-528.9 eV (B.E.), adsorbed 022-/0 . (is) at 530.1-530.5eV(B.E.), and the O(ls) of oxygen-containing contaminants OH/CO3 2- at -532.1 eV (B.E.). Analysis and fitting of these O(ls)-XPS spectra revealed that the relative content (molar fraction) of the adsorbed O22-/O species in the total surface oxygen on the 6%Ag/Lao.6Sr0.4MnO 3 was almost twice as much as that on the Lao.6Sro.4MnO3. H2-temperature-programmed reduction (TPR) of the catalyst provided useful information about reducibility of the Mn and Ag components in these catalysts. On the Lao.6Sro.4MnO3 host catalyst, three peaks (623, 723, 1023 K) were observed (see Figure 2a). In view of the nonreducibility of the La 3§ and Sr2§ under the condition of H2-TPR, the observed three H2-TPR peaks should be due to the reduction of Mn 3§ species. The former two peaks may be attributed to the reduction of part of the Mn 3§ to Mn 2§ in company with the formation of a corresponding number of anionic vacancies, i.e., Lao.6Sro.4MnO3 + x H2 ---" La0.6Sr0.4MnO3_x+ x H20; and the partially reduced Lao.6Sr0.4MnO3_x intermediate with a certain number of anionic vacancies still maintained the perovskite phase structure as a whole. The 1023K peak was corresponding to the deeper reduction, which led finally to disintegrating the perovskite phase to the discrete oxide phases, La203, SrO, and MnO. On the 2%Ag/Lao.6Sro.4MnO3 (See Figure 2b), the high temperature (1023K) TPR-peak leading to the disintegration of the perovskite ohase was unchanged in the location and shane in comnarison with that of A~,-undnned

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and area were quite apparent. It is known from the above XPS results that the supported Ag20 component at the surface of the Ag/La0.6Sr0.4MnO3 after the calcination at 773K was a kind of Ag20 species unable to pyrolyze into metallic Ag in air, and that the doping of Ag resulted in formation of a certain quantity of Mn 4§ Thus, the 403K little peak may be assigned to the reduction of the kind of Ag20 species: Ag20 + H2 ---" 2 Ag + H20, and the 523K peak was due to the reduction of the Mn 4§ to Mn 3§ and the 596K peak may originate from the reduction of part of the Mn 3+ species to Mn 2§ Therefore, it may be believed that the doping of Ag brought about an increase in the quantity of reducible Mn n§ species, simultaneously with descending obviously in their reduction temperature.

3.3. Origin of promoter action of Ag The results shown in Table 1 indicated that, on all the 7-AlzO3-supported La0.6Sr0.4MnO3 systems, conversion activity of methanol was lower than that on the unsupported single La0.6Sr0.aMnO3 system, probably due to the formation of inactive aluminates, and tended toward the latter as the loading of La0.6Sr0.4MnO3 increased and the perovskite-phase formed. This implied that not only the surface, but also the bulk perovskite-phase, both played important roles in the catalytic methanol oxidation. The use of 7-Al203 as the bottom carrier would be conducive to generating a relatively large operating surface for y-A1203-supported Ag/La0.6Sr0.4MnO3 systems, which would be in favor of improving dispersion of the Ag component on the surface. The Ag + in a proper amount doped onto the surface of La0.6Sr0.4MnO3 could partially take the place of La 3+ and Sr 2§ and occupy the A-sites in the ABO3-type lattice due to its ionic radius (0.126nm) quite close to that of La3§ and Sr2§ thus being better stabilized and favorable to inhibiting aggregation of the Agspecies at the surface. On the other hand, modification of the Ag § to the surface of La0.6Sr0.4MnO3 would bring about an increase in relative content of the surface 022-/0 . species highly reactive toward CH3OH, HCHO and CO. Besides, solution of a small amount of lowvalence metal oxides AgOx and SrO in the lattice of the trivalence-metal composite oxide LaMnO3 will also result in the formation of anionic vacancies and an increase in content of the reducible Mnn§ under the reaction temperature, which would be conducive to the adsorption-activation of oxygen on the surface of the functioning catalyst and the transport of the lattice and surface oxygen species. All these factors would be in favor of enhancing the catalyst activity for the complete oxidation of methanol. REFERENCES [1] [2] [3] [4]

L.V. MacDougall. Catal. Today, 8(1991)337 R.W. McCabe, P.J. Mitchell. Appl. Catal., 27(1986)83 H.K. Plummer, Jr.,W.L.H. Watkins, H.S. Gandhi. Appl. Catal., 29(1987)261. Handbook of X-ray photoelectron spectroscopy. Eds: C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, and G.E. Mullenberg. Published by Perkin-Elmer Co., Physical Electronics Division, Minnesota, p.74 and p.l12.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

T r e a t m e n t o f a q u e o u s s o l u t i o n s of o r g a n i c p o l l u t a n t s by h e t e r o g e n e o u s c a t a l y t i c w e t air o x i d a t i o n (CWAO). Mich~le B E S S O N ~, J e a n - C h r i s t o p h e BEZIAT ~, B e r n a r d BLANC ~, Sylvain DURECU b and Pierre GALLEZOT ~ a Institut de Recherches sur la Catalyse-CNRS, 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex, France b TREDI, D~partement Recherche, 54505 Vandoeuvre-l~s-Nancy, France Wet air oxidation of model and industrial organic pollutants in aqueous media was carried out in batch and continuous reactors, using supported Pt and Ru catalysts. The reactivity of pollutants was studied and the effect of the temperature on the stability of these heterogeneous catalysts was investigated.

1. INTRODUCTION As a result of stricter environmental regulations, w a s t e w a t e r t r e a t m e n t has become a major issue. Wet Air Oxidation (WAO) is a liquid phase oxidation process by molecular oxygen which is efficient for the t r e a t m e n t of aqueous effluents containing high concentrations of toxic or poorly biodegradable organic materials, but the stringent conditions of temperature (200-450~ and pressure (7-25 MPa) severely affect the economy of this technology. The use of catalysts may enhance reaction rates and enable one to carry out the process under milder conditions. Soluble copper salts are generally effective for t h a t purpose [1], but h e t e r o g e n e o u s catalysts are preferred because no catalyst recovery step is required. Copper containing mixed oxides exhibit good activity, but some leaching of transition metal ions from the catalyst matrix may occur [2]. The purpose of this study was to investigate the CWAO on supported platinum and r u t h e n i u m catalysts, of model pollutants and of industrial effluents in batch and in trickle-bed reactors, with particular emphasis on the stability of these heterogeneous catalysts. Small carboxylic or dicarboxylic acids (such as formic, maleic, acetic and succinic acids) which are intermediate or stable end products of oxidation from n u m e r o u s waste streams were chosen as test r e a c t a n t s to evaluate the catalysts.

2. EXPERIMENTAL The oxidation experiments were carried out in a 250 ml Hastelloy C22 autoclave equipped with a magnetically driven impeller [3]. They were also performed in a trickle-bed reactor with cocurrent downflow circulation of air and effluent, described elsewhere [4]. The overall degradation yield was calculated according to the total organic carbon (TOC) which was determined using an automatic TOC 5050 Shimadzu analyzer. High performance liquid chromatography (HPLC) was used for analysis of organic products, using an ion-exchange column (Sarasep

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Car-H) with dilute H2SO4 solutions as eluent, and with UV and RI detectors in

series.

P l a t i n u m catalysts were p r e p a r e d by ion-exchange of the CECA 50S active charcoal (1400 m2g~) with Pt(NHs)4C12 salt in ammoniacal solution. R u t h e n i u m catalysts were provided by Engelhard (2.8%Ru~iO2, Q500-069) or p r e p a r e d by incipient-wetness impregnation of different t i t a n i a and zirconia supports by aqueous solutions of r u t h e n i u m chloride. After washing and drying, they were reduced under a flow of hydrogen (250 ml min "1) at 573K, cooled to 300K u n d e r hydrogen and finally brought into contact with air. 3. R E S U L T S AND D I S C U S S I O N

3.1. Carbon-supported platinum catalysts

Platinum catalysts supported on active charcoal can be used under very moderate conditions (air at atmospheric pressure at 326K or lower temperatures) to oxidize formic and oxalic acid into carbon dioxide and water. The catalytic oxidation of maleic acid was more difficult to achieve and an air pressure of 1.5 MPa was necessary to oxidize aqueous solutions at 403K; during this oxidation, no partial oxidation of maleic acid was detected. Acetic acid, which is known to be the most refractory product in WAO, was not oxidized in these conditions [5]. Table 1 summarizes the initial reaction rates of oxidation of the easily oxidized acids. Table 1 Oxidation rates of aqueous solutions of carboxylic acids (5g L 1) in the presence of 5%Pt/C catalyst Substrate formic acid oxalic acid maleic acid 323 503 Temperature (K) 323 0.9 0.28 Initial rate 36 (tool h 1 g~-l)

3.2. Carbon-supported ruthenium catalysts

To oxidize acetic acid [6] and C4-C 6 carboxylic diacids [7], r u t h e n i u m catalysts supported on active carbon or graphites operating at 453-473K and 5 MPa air pressure were employed. Figure 1 gives the proposed simplified p a t h w a y for wet oxidation of succinic acid and the distribution of products as a function of time. The oxidation of succinic acid occurs via three routes: the direct oxidation to CO2 was the p r i m a r y route, acrylic and acetic acids, which in t u r n were oxidized to carbon dioxide and water, were formed as reaction intermediates. The oxidation of acetic acid was slow, so t h a t all acetic acid formed from succinic acid accumulated in the solution. But it was consecutively oxidized, and at the end of the reaction, only traces could be detected and a TOC removal efficiency of more t h a n 99% was observed within 4 h. No leaching of r u t h e n i u m was observed, but a substantial consumption of the carbon support was noticed, wich makes unable the use of this type of catalyst at the operating conditions required for the refractive organic molecules.

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化

IO0 " 0

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Fig. 1. Oxidation of an aqueous solution of succinic acid (43 mmol 1~) in presence of 5% Ru/C, composition vs. time. Reaction conditions: 463K, 5 MPa total pressure. 9 succinic acid (SUC), m acetic acid (ACE), h acrylic acid (ACR), 9 TOC abatement 3.3. T i t a n i a or z i r c o n i a - s u p p o r t e d r u t h e n i u m c a t a l y s t s Titanium dioxide and zirconium dioxide were then selected as the support for ruthenium in view of their stability in acidic and oxidizing medium. A-Succinic

a c i d a n d a c e t i c a c i d d e r i v a t i v e s in b a t c h r e a c t o r

Catalytic studies were then carried out on aqueous solutions of succinic acid and other carboxylic acids with a Ru/TiO2 catalyst loaded with 2.8 wt% metal obtained from Engelhard. The results in figure 2, giving the TOC abatement vs. time, demonstrate the possibility of a complete mineralization of different carboxylic acids into CO2 and H20. The presence of a substituting group (NH2, C1 or OH) on the a-carbon of acetic acid resulted in higher oxidation rates compared to acetic acid (13 molc h 1 mol zuX). Succinic acid had an intermediate activity (43 molc h ~ mol zuX). The catalyst was recycled in five successive batch oxidations of succinic acid, without loss of activity. The absence of metal ions (Ru, Ti) within the detection level of ICP-AES (0.5 ppm and 0.1 ppm, respectively) in the effluents after reaction and the absence of particle sintering by Transmission Electron Microscopy indicate also a high stability of the catalyst under the conditions employed.

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化

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Fig. 2. TOC a b a t e m e n t as a function of time. Reaction conditions: batch reactor, 150 ml of 5g 1x acid aqueous solutions, 5 MPa air, T = 190~ lg 2.8%Ru]TiO2. + acetic acid, a succinic acid,, glycine, A chloroacetic acid, x glycolic acid. B-Succinic

a c i d in t r i c k l e - b e d and support effect

reactor: stability of the catalyst

The e x p e r i m e n t s were then conducted in the micropilot trickle-bed reactor, operating under integral conditions or differentially. A set of tests of succinic acid was carried out, using the former catalyst 2.8%Ru/TiO2 compressed in grain form (0.8-2 mm), or catalysts prepared by wet impregnation of formed supports (0.9% Rufl~iO2, Degussa P25, a n a t a s e + rutile, l m m extrudates, and 1.4% Ru/ZrO2, Engelhard, 0.8-2 mm grains). Transmission Electron microscopy studies showed t h a t the particle size of these catalysts was comparable (2-5 nm). Figure 3 shows the concentration profiles of reaction products and TOC concentration as a function of the residence time. Increasing residence time were obtained by repeated recycling of the treated solution, while maintaining the liquid (60 ml h -x) and gas (51 h x) rates. 2500

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1.4% Ru / Zr02 Engelhard Fig. 3. Oxidation of succinic acid at 463K and 5 MPa air pressure in the tricklebed reactor: effect of the nature of the support succinic acid, m acetic acid, A acrylic acid. By comparing the experimental results of oxidation obtained, one can notice t h a t the p r e p a r a t i o n of the catalysts was crucial on the activity and the relative importance of the three routes of oxidation of succinic acid. The catalyst prepared on zirconia favored the direct oxidation of succinic acid to carbon dioxide, thus m i n i m i z i n g the formation of refractory acetic acid. As a consequence, the decrease in TOC concentration is higher (less than 7.3 mg 11 at 1.43 h gRu-111). 3.4. O x i d a t i o n

of an effluent from a bleach plant

Among the various industrial w a s t e w a t e r streams, those from the bleaching operation of kraft wood pulp deserve some attention. Wet oxidation of different effluents, generated after the chlorine dioxide stage (acidic pH) and from alkaline extraction (pH 9), was carried out at 463K and 5 MPa air in the slurry reactor. The reactivity of the wastewater was found to depend greatly on the pH of the effluent, and decreased with increasing pH. Figure 4 gives, for instance, the TOC removal versus time profiles for a concentrated basic effluent (initial TOC 8690 mg 11).

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A rapid initial decrease in COT was observed ; as the organic compounds present initially were converted to more refractory ones, the rate at which the TOC decreases declined t h e n significantly. However, the overall TOC content could be drastically reduced by over 90%, the color removal was significant (it changed from dark brown to pale yellow, i n d i c a t i n g a successful destruction of m a j o r organic compounds responsible for coloration), a breakdown of the large molecules was observed (as shown by GPC), and the reduction of AOX (adsorbable organic halides) was effective (32.2 to 0.1 mg 11).

10000

.._, E

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6000 ~

0 4000 0 0 0

2000

0

.

0

.

.

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Fig. 4. TOC concentration vs. time, during oxidation of a concentrated basic effluent from pulp mill industry. Reaction conditions: batch reactor, 190~ 5MPa air, lg 2.8% Ru~iO2.

CONCLUSION The complete degradation of organic compounds in wastewaters to inorganic compounds can be achieved by heterogeneous catalytic wet air oxidation The nature of the pollutant is critical in the selection of a catalyst: Pt/C can be used in moderate conditions to oxidize easily oxidized acids, while RufriO2 or Ru/ZrO2 oxidize more refractive compounds at higher temperatures. These catalysts showed high activity. They are physically and chemically stable in hot acidic oxidizing solutions for a prolonged use. The performances of the catalysts are dependent on the support (TiO2 or ZrO2). REFERENCES

1. V.S. Mishra, V.V. Mahajani and J.B. Joshi J, Ind. Eng. Chem. Res. 34 (1995) 2. 2. S. Imamura, H. Nishimura and S. Ishida, Sekyu Gakkaishi, 30 (1987) 199. 3. J.C. B~ziat, M. Besson, P. Gallezot and S. Dur~cu, J. Catal., 182 (1999) 129. 4. J.C. B~ziat, M. Besson, P. Gallezot and S. Dur~cu, Ind. Eng. Chem. Res., 38 (1999) 1310. 5. P. Gallezot, N. Laurain and P. Isnard, Appl Catal. B, 9 (1996) L l l . 6. P. Gallezot, S. Chaumet, A. Perrard and P. Isnard, J. Catal., 168 (1997) 104. 7. J.C. Bdziat, M. Besson, P. Gallezot, S. Juif and S. Durecu, in Studies in Surf. Sci. and Catal. 110 (1997) 615

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Effect of the Support for Pt Catalysts on Soot Oxidation Akira Obuchi, Junko Oi-Uchisawa, Ryuji Enomoto, Shetian Liu, Tetsuya Nanba and Satoshi Kushiyama Atmospheric Environmental Protection Department, National Institute for Resources and Environment, 16-30nogawa, Tsukuba 305-8569, Japan

Abstract The effect of the support for Pt catalysts on the oxidation of carbon black, a model diesel-exhaust soot were examined. Among eight kinds of Pt-supported metal oxides, Pt/Ta205 showed the highest activity towards the oxidation of carbon black in a model diesel exhaust, containing 02, 1-120, NO, and SO2 in N2. Pt catalysts supported on other non-basic metal oxides such as Nb2Os, WO3, SnO2, and SiO2 showed similar high activities. The cause of the high activity for these catalysts was investigated in relation to the roles of NO2 and SO3 (or H2SO4) in this catalytic process. 1. INTRODUCTION The emission of soot from diesel engines can be reduced by placing a filter device made of heat-resistive materials in the exhaust stream. To achieve this reduction, however, the soot collected in the filter must be continuously or periodically removed from the filter by combustion. Normally, solid carbon in the soot burns only above 600 ~ which can not be realized in the exhaust stream under typical engine operating conditions. Oxidation catalysts can lower this combustion temperature. Various kinds of metal oxides [1], mixed oxides (especially those containing vanadium, molybdenum and/or copper [2], and perovskites [3]), chlorides [4], and sulfates [5] have been reported to be active catalysts for carbon oxidation. Pt catalysts exhibit the highest activity at low temperatures [6,7]. Platinum is believed to promote soot oxidation indirectly, i.e., by oxidizing the NO, normally coexisting in the exhaust gas, to NO2, which subsequently oxidizes soot to CO and CO2. Furthermore, we have recently found that SO2 and 1-120 present in the reactant in addition to NO, substantially promote carbon oxidation in the presence of Pt/SiO2 [8]. SO3 or H2SO4 produced from SO2 oxidation on Pt surface is believed to catalyze the oxidation reaction of carbon by NO2. Water is believed to promote this reaction by decomposing intermediate species through hydrolysis [9]. Here we report the effect of the Pt-catalyst support and discuss the results, based on the above reaction mechanism. 2. EXPERIMENTAL Pt (0.3 wt%) was supported on the granular metal oxides, Ta205, Nb205, WO3, SnO2, SiO2, TiO2, A1203, and ZrO2, having sizes ranging from 0.15 to 0.25 mm. In the case of WO3 and Nb2Os, the oxide granules were prepared from a commercially available oxide in the

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form of powder, by kneading with aqueous ammonia, followed by drying, calcination at 600 ~ in air for 2 h, crushing the aggregates, and sieving. Pt was supported on these oxides by the incipient-wetness method using a solution of Pt(NH3)4(OH)2, followed by drying and calcination in air at 600 ~ The prepared catalyst samples were characterized by BET surface area and Pt dispersion, i.e., (Pt exposed) / (Pt total) expressed as a percentage, determined by CO pulse adsorption at 50 ~ or Pt particle-size observation by TEM. Furthermore, temperature-programmed desorption (TPD) of CO2 was carried out to evaluate the basicity of the support materials. After calcination in air at 600 ~ the sample was exposed to 100% CO~ for 10 min at 100 ~ purged with N2 and heated at a rate of 10 ~ under a N2 flow, during which the CO2 desorption rate was continuously measured. Temperature-programmed reactions (TPR) were carried out to evaluate the catalytic performance for carbon oxidation. Commercially available carbon black (CB; Nippon Tokai Carbon 7350F, primary particle size = 28 nm, BET surface area = 80 m2/g, elemental analysis C = 97.99 wt%, H =1.12 wt%, N = 0.06 wt%) was used as a model soot. The Pt catalyst (0.5 g) and CB powder (0.005 g) were mixed together with a spatula to attain "loose" contacts between the catalyst and carbon [10]. Reactant gases containing 1000-ppm NO, 100-ppm SO2, 7% 1-120, and 10% 02 in N2, were passed through the mixture of catalyst and CB at a flow rate of 0.5 dm3/min. The reactor temperature was raised by 10 ~ from 80 to 750 ~ and the concentrations of CO2, CO, and NO2 in the product gas were continuously measured using non-dispersive IR gas analyzers and a chemiluminescence-type NOx analyzer. In addition, isothermal reaction tests were carried out at 350 ~ under the same gaseous conditions to determine if the combustion of the CB is complete at this temperature. Similarly, transient response reactions were carried out at the same temperature to investigate the effect of SO2 on the catalytic NO oxidation, under the same gaseous conditions as TPR except that no CB was mixed with the catalyst sample, and the SO2 concentration was switched between 0 and 100 ppm. 3. RESULTS AND DISCUSSION The BET specific surface area and Pt dispersion of the catalyst samples are summarized in Table 1, as well as the source of the metal oxides and the pretreatment conditions for supporting Pt. The Pt dispersions on SiO2, ZrO2, A1203, and TiO2 exceeded

Table 1 Preparationmethod and physical properties of the Pt supported catalysts Support

Manufacturer/ Code

Pt preparation BET surface Pt dispersion condition ") area / m2.g1 / % SiO2 Wako chemicals / Wako-gel 100 H/A 434 13 ZIO 2 Daiichi-Kigenso H/A 44 129 A1203 SumitomoChemicals / KHS-24 H/A 152 14 TiO2 IshiharaSangyo/ ST-B11 H/A 29 21 SnO2 Rare Metallic A 3.0 3.8 WO3 Kanto Chemicals A 4.7 --- b) Nb205 WakoChemicals A 2.7 3.3 Ta205 WakoChemicals A 2.5 3.4 ")H" reduction in 3% H, in N~ at 400 ~ for 4h. A: calcination in air at 600 ~ for lh.

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10% even after a pretreatment of calcination in air at 600 ~ for 1 h. It is not clear why the Pt dispersion exceeded 100% in the case of ZrO2. This result suggests that more than one CO molecule were adsorbed on a Pt atom, such as a 'twin CO.' No Pt particles were visible on ZrO2 using TEM at a magnification as high as 3,000,000, which implies a very high Pt dispersion. On the other hand, the Pt dispersions for SnO2, Nb2Os, and Ta205 estimated by TEM were below 4%. The low surface area of these supports may cause the relatively low Pt dispersions. Figure 1(a) shows CO2-emission curves from the TPR experiments for the Pt catalysts supported on the various oxides examined. The CO emission was negligible in all the cases. For Pt/SiO2, the initial and peak temperatures were 280 and 500 ~ respectively, the former being defined as the temperature for which the CO2 concentration exceeds 100 ppm, with a shoulder located at 350 ~ The initial temperatures for Pt/ZrO2, Pt/Al203, and Pt/TiO2 were 100 to 130 ~ higher than that for Pt/SiO2. On the other hand, Pt/Ta2Os, Pt/Nb2Os, N/WO3, and Pt/SnO2 showed higher activities than that for Pt/SiO2 near 350 ~ although the initial temperatures were almost the same. Pt/Ta205 showed a CO2-emission rate nearly twice that for Pt/SiO2 at this temperature. Figure l(b) shows NO2-emission curves obtained during the same TPR experiments. NO2 production started at temperatures as low as 200 ~ for all the catalyst samples except Pt/ZrO2 and Pt/AI203, which indicates that the catalytic activity for NO oxidation to NO2 is very high for these materials. As the temperature rose, the NO2 concentration temporarily decreased near 280 ~ which almost coincides with the initial temperature for CB oxidation. This implies that NO2 oxidizes CB while simultaneously being reduced to NO under the TPR conditions. Above 450 ~ the NO2-emission concentration reached values predicted by the 2000

' li

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,

,

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Fig. 1 CO2 emission (a) and NO2 emission (b) curves from the TPR experiments evaluating the carbonblack oxidation activity of Pt catalysts supported on various oxides. For the experimental conditions, see the exoerimental section in the text.

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thermodynamic equilibrium among NO, NO2, and 02, thus decreasing with a rise in temperature. By contrast, in the case of Pt/ZrO2, NO2 production occurred only above 300 ~ and the concentration was also substantially lower than the equilibrium value predicted for higher temperatures. It is believed that Pt highly dispersed on ZrO2 decreases its catalytic oxidation activity, most likely by strongly interacting with the support. The behavior of Pt/TiO2 appears strange at first sight; while NO2 began to be produced at 230 ~ it was not consumed until 390 ~ Similar behavior was also observed in the case of Pt/AI203. These results suggest that factors other than NO2 production are critical in the catalytic oxidation of CB, which will be discussed later. Figure 2 shows time courses of the carbon oxidation rate at a constant temperature, 350 ~ (from 25 to 110 min), for the same Pt-supported oxides tested in the above TPR experiments. In the case of Ptffa205 and PtfNb2Os, which showed the highest performances in the TPR experiments, the CB was oxidized almost completely in 40 and 60 min at 3 50 ~ respectively. It took approximately 100 min for the CB to be removed in the Pt/SiO2 experiment. On the other hand, for Pt/Zr02 and Ptfrio2 the carbon oxidation rates were much lower, and it was necessary to raise the temperature to oxidize the carbon completely. Figure 3 shows results of transient responses of NO2 emission over different Pt catalysts at 350 ~ after the addition and subsequent cut-off of SO2. Carbon black was not mixed with the catalysts in this case. Except for Pt/ZrO2, the NO2 concentrations were 500 to 600 ppm before adding SO2. One hour after adding SO2, the NO2 concentrations produced by Pt supported on ZrO~, TiO2, A1203, SiO2, SnO2, WO3, Nb2Os, and TarO5 were higher in the stated order, which coincides with the order of the carbon oxidation rates, except for an exchange in the position of A1203 and TiO2. Following the addition of SO2, Pt/Ta~O5 showed a sudden decrease in NO2 of approximately 100 ppm, but with the cut-off of SO2, the concentration quickly returned to the original level. Similar results were obtained for Pt/Nb2Os, Pt/WO3, and Pt/SnO2. With Pt/SiO2, the response to adding SO2 was slower but the decrease became eventually more prominent; aider the SO2 cut-off, the NO2 concentration approached the original level with a slower recovery rate. Furthermore, in the case of Pt/TiO2, once SO2 was added, the NO2 concentration continuously decreased with a negligible 10001

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i

~o 600

------or---Pt/SiO = Pt/TiO2

!

J

I

t.,

I

/ /

I

/

"~. 400 o

/

i 1000

/ 800 ., -"1

"

""

600 400

200 0

I

~ %

200 0

20

40

60

80 Time / min

100

120

0 140

Fig. 2 Timecourses of carbon-black oxidation at 350 ~ with Pt catalysts supported on Ta:Os, Nb2Os, SiO2, TiO2, and ZrO2. The reactant-gas compositionwas the same as that shown in Fig. 1.

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个

1563 700

化

SO2 add

I

/

~

SO2 cut

I

t

600 500

0 .~,,t rd~

Support:

-

400 300

Ta O

SiO -

eq

200

-

---,..._~---

100 I 0

~ u,.-..,u~x_,-~.i3t~(3

~ .

ZIO I 50

100

150

I 200

I 250

Time / min Fig. 3 Transient responses of the rate of NO oxidation to NO2 over different Pt catalysts at 350 ~

after the addition and subsequentcut-offof SO2 in the reactant gas. The reactant-gascompositionwas the sameas that shown in Fig. 1, exceptfor SO2. recovery following the SO2 cut-off. Similar tendency was more remarkable for Pt/ZrO2. The total sulfur amount, which passed through the catalyst samples during this experiment, corresponded to 1.43 wt% of the catalyst sample. The amounts of sulfur present in Pt/Ta2Os, Pt/Nb2Os, Pt/WO3, Pt/SnO2, Pt/SiO2, Pt/TiO2, Pt/AI203, and Pt/ZrO2 measured after the experiment were 0.03, 0.05, 0.02, 0.05, 0.02, 0.99, 1.35, and 0.45 wt%, respectively. Only the last three samples strongly adsorbed sulfur. These results indicate that Pt/Ta2Os, Pt/Nb20~, Pt/WO3, and Pt/SnO2 have a strong resistance (with Pt/SiO2 having a more limited resistance) to poisoning by sulfur (SO3 or H2SO4 derived from SO2) for NO2 production, whereas Pt/TiO2, Pt/AI203, and Pt/ZrO2 lose their catalytic activity by poisoning at this temperature. Furthermore, for the latter three catalysts, SO3 (or H2SO4), which is a catalyst to promote the oxidation of carbon by NO2, is trapped by the supports and does not reach the carbon to be oxidized. Figure 4 shows the results of CO2 TPD experiments to evaluate the basicity of the supports. The CO2 emission was observed only for ZrO2, Al~O3 and TiO2, all of which demonstrated low activity as the support for Pt catalysts. Among these, ZrO2 showed the highest total amount of desorption, reaching 0.024-m01 CO2 per mol ZrO2. By contrast, the other five oxides did not adsorb CO2 at all. A high negative correlation between the basicity and activity of the support was found. SiO2 and the metal oxides with a lower basicity have a reduced affinity toward S03 (or H2S04), and the supported Pt is poisoned to a lesser degree. As a result, the SOa (or H2S04) produced can reach the carbon surfaces more easily, thereby promoting carbon oxidation. 4. CONCLUSION Eight kinds of Pt-supported metal oxides were tested and compared for their catalytic performance in the oxidation of carbon black, mechanically mixed with the catalyst sample; the tests used a model diesel exhaust gas containing 02, 1-120,NO, and SO2. Pt/Ta205 was the

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化

2500

~

I -::1 I =

2000 ~'

I I I

Ti~ ":'"1 --'C}~ SiO2 I

1500

1000 r~

i~

!

500

100

200

300 400 Temperature

500

600

Fig. 4 CO2TPD fromPt/ZrO2,Pt/Al203, Ptfrio2, and Pt/SiO2. most active catalyst. Pt catalysts supported on other non-basic oxides, such as Nb2Os, WO3, SnO2, and SiO2, showed similar high activities. We attribute the high activity for these catalysts to their non-basicity and negligible affinity for SO3 (or H2SO4), which results in less poisoning of the supported Pt and in a smooth supply of SO3 (or H2SO4) to the carbon surface, the oxidation of which by NO2 is catalyzed by SO3 (H2SOa).

REFERENCES

1. J.EA.Neel~ M.Makkee, H.A.Moulijn, Appl. Catal. B, 8 (1996) 57. 2. A.Bellaloui, J.Varloud, P.Meriaudiau, V. Perrichon, E.Lox, M.Chevrier, C.Gauthier, EMathis, Catal. Today 29 (1996) 421. 3. Y.Teraoka, K. Nakano, S.Kagawa, W.F.Shangguan, Appl. Catal. B, 5 (1995) L181. 4. G.Mul, F.Kapteijn, J.A.Moulijn, Appl. Catal. B 12 (1997) 33. 5. Z. Zhao, A.Obuchi, J.Uchisawa, S.Kushiyama, Stud. Surf. Sci. Catal. 121 (1999) 387. 6. P. Hawker, N. Myers, G. Huthwohl, H. T. Vogel, B. Bates, L. Magnusson and P.Bronnenberg, SAE Technical Paper Series, 970182, 1997. 7. B. J. Cooper and J. E. Thoss, SAE Paper Series No. 890404 (1989). 8. J.Oi-Uchisawa, A.Obuchi, Z.Zhao and S.Kushiyama, Appl. Catal. B 18 (1998) L183. 9. J.Oi-Uchisawa, A.Obuchi, A.Ogata, R.Enomoto, S.Kushiyama, Appl. Catal. B 21 (1999) 9. 10. J. P. A. Neefl, O. P. Pruissen, M. Makkee and J. A. Moulijn, Appl. Catal. B, 12 (1997) 21.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Dimethyl carbonate synthesis from carbon dioxide and methanol over Ni-Cu/MoSiO(VSiO) catalysts S.H. Zhong, J.W. Wang, X.F. Xiao and H.S. Li College of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China The surface structures, chemisorption properties and catalytic behaviors of Ni-Cu/MoSiO and Ni-CuNSiO catalysts have been investigated by using the techniques of XRD, TPR, IR, TPD and micro-reactor. Three types of active sites, metallic site M (Ni-Cu alloy), Lewis acid site M n§ (Mo 6§ or V 5§ and Lewis base site M=O (Mo=O or V-O), have been found on the surface of the reduced catalysts. With the cooperation effects of these active sites, CO2 can be chemisorbed on the metallic site M and Lewis acid site M n§ to form a new type of CO2 horizontal adsorption state, M--(CO)--O->M n§ and CHaOH can be dissociated on the Lewis base site M=O and Lewis acid site M n§ to form a dissociative adsorption state, Mn§ and HO--M. The selectivity of dimethyl carbonate synthesized from these adsorption species is over 85% and a surface reaction mechanism of this catalytic reaction is proposed based on the experimental results. 1. INTRODUCTION Carbon dioxide, the major man-made greenhouse gas, is mainly emitted from combustion of fossil fuels, and dimethyl carbonate (DMC) is a kind of important intermediate in organic synthesis. Using carbon dioxide and methanol as the raw materials to synthesize DMC is an important reaction in the field of synthetic chemistry, utilization of carbon resource and environment protection [ 1]. The activation of CO 2 molecule is a key problem in the course of DMC synthesis from carbon dioxide and methanol. Some exploratory researches have been done in aspect of CO2 activation on the surface sites of the transition metals, transition metal oxides or the both cooperation effects of them [2-~5]. Using ethylene oxide, carbon dioxide and methanol as the reactant materials to synthesize DMC with the liquid phase method has been reported in the literature [6]. In the present work, we have focused on the molecular design of catalysts in order to synthesize DMC directly from carbon dioxide and gaseous methanol. Our results are briefly summarized and discussed here. 2. EXPERIMENTAL

2.1. Catalyst preparation The surface complex oxide supports of MoO3-SiO 2 (MoSiO) and V205-SiO 2 (VSiO) with 50% covering degree of MoO3 or V205 onto the surface of silica carrier were prepared by the method of modifying the silica surface character in reaction with hydrochloric acid solution of MoOC14 and VOC13 [7,8]. The catalyst samples, Ni-Cu/MoSiO and Ni-CuNSiO, used in this study were prepared by incipient wetness impregnation of the MoSiO or VSiO with a mixed aqueous solution of Ni(NO3)2 and Cu(NO3)2 . After impregnation, the samples were dried overnight at 393 K, calcined at 723 K in flowing air for 5h, and reduced in flowing HE/N2 mixed gases at a suitable temperature for 2h then stored in bottles. The metal loadings of the

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catalyst samples is about 3 wt% with the Ni/Cu atomic ratio 2.

2.2. Catalyst characterization Temperature-programmed reduction (TPR) experiments of calcined NiO-CuO/MoSiO and NiO-CuONSiO samples were carded out by heating about 100 rng of the samples in a flow of 20 ml/min of a H2/N2 = 1/10 mixture and measuring the hydrogen consumption using a TCD detector. During the experiment, the temperature was raised from 300 K to 1073 K. XRD measurements of reduced Ni-Cu/MoSiO and Ni-Cu/VSiO catalysts were performed using Rigaku 2038 model diffractometer and Ni-filtered Cu Ko radiation. Lattice constants were calculated from centroid positions and particle sizes from the integral half width of the line profiles. Infrared spectra were recorded using a stainless-steel cell equipped with KBr windows held in place by threaded caps. The cell is connected to a gas handling and vacuum system. Ultimate pressures of 5 • 10.6 tort were obtainable under dynamic conditions. A Hitachi 27030 spectrometer was used to measure the infrared spectra of self-supporting wafers after their drying, reduction and exposure to CO2 or CH3OH. The procedures were described in more detail elsewhere [9]. 2.3. Reaction evaluation The temperature-programmed desorption (TPD) system was similar to that described previously [10]. A 50 mg catalyst sample rested on a quartz frit in a tubular, downflow reactor (6 mm OD). The gaseous product distribution of TPD was measured by a LZL-203 quadrupole mass spectrometer located in a diffusion-pumped vacuum chamber. Before the measurements were taken, the catalyst sample was rereduced in flowing H2 at 473 K for 2 h and evacuated for 3 h at the same temperature, following cooling to 300 K. Absorbate gas (50 torr), CO2, CH3OH or CH3OH/CO2=2, flowed over the catalyst until saturation was obtained at 300 K, then evacuated the system to 104 torr. The catalyst temperature was increased linearly at a heating rate of 8 K/min to a final temperature of 673 K in a He flow rate of 20 ml/min. The catalytic reactions were performed in a stainless steel tubular fixed-bed reactor. A MRS-901 micro-reactor device controlled by a computer was employed in this work. 3. RESULTS AND DISCUSSION 3.1. Surface structures of catalysts The surface structures of the complex oxide supports MoSiO and VSiO characterized by IR technique in our previous studies [7,8] showed that they are the valence bonding type of M o O 3 and V205 combined with the SiO2 surface through oxygen bridge. The reduction character of the MoSiO and VSiO supported NiO-CuO samples measured from the TPR experiments showed that the metal oxides NiO.yCuO on the surface of MoSiO and VSiO can be all reduced at the temperature of 353 and 330 ~ respectively, and that the Mo=O and V=O in the surface of MoSiO and VSiO support can be partly reduced at the temperature of 580 and 515 ~ respectively. The metallic-phase composition of the reduced Ni-Cu/MoSiO and Ni-CuNSiO examined from the XRD measurements is a homogenous-phase Ni-Cu alloy cluster with the average particle size of 38 A and 42 A for the Ni-Cu/MoSiO and Ni-CuNSiO catalyst respectively. By combining the results mentioned above in one model, the surface constructions of the Ni-Cu/MoSiO and Ni-CuNSiO catalysts were characterized as it is shown in the Figure 1 a and b. It is clear that three types of the active sites, metallic site M (in the form of Ni-Cu

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alloy), Lewis acid site M"+(Mo 6+ or V 5+) and Lewis base site M=O (Mo=O or V=O) appear respectively on the surface of Ni-Cu/MoSiO and Ni-Cu/VSiO catalysts. The d-electron density of the Ni-Cu alloy may be changed by Cu atoms alloying with Ni atoms and the SiO2 support has an influence on the intensity of Lewis acid site M~+(Mo6+ or V 5+) and Lewis base site M=O (Mo=O or V=O). O _ O _ IMIo ~

"o"

O --O--~

o/\o\

o"

n+O / 0 / I 0, ~..A 0I x O\

,o, !"o, i"o,

a, Ni-Cu/MoSiO b, Ni-Cu~SiO Fig. 1. Surface constructions of catalysts 3.2. COs adsorption on catalysts IR spectra of CO2 adsorbed on Ni-Cu/MoSiO and Ni-Cu~SiO are shown in the Figure 2. By comparison with the IR band characters of different CO2 adsorption states occured on the transition metals or transition metal oxides [5], the IR results of CO2 adsorption on the catalysts may be identified and synthesized in the Table 1. TPD profiles of CO2 adsorbed on Ni-Cu/MoSiO and Ni-Cu/VSiO are shown in the Figure 3. Based on the analysis of CO2 adsorbing strength measured from the IR spectra, the TPD results of CO2 adsorbed on the catalysts were also synthesized in the Table 1. --'-dL-.~-.

-

~,~ J. A

~

d ~

9 ~,,,I

C

0

r

I

2500

2000

1600

Wavenum

1400 1200

1000

b e r ( c m "l)

Fig.2. IR spectra of CO2 adsorption. a Ni-Cu/MoSiO, b Ni-Cu/MoSiO - - CO2(ads), c Ni-Cu/VSiO, d Ni-Cu/VSiO -----CO2(ads)

800

40

|

I

|

I

,

I

80 120 160 Temperature (*C)

,

200

Fig.3. TPD profiles of C O 2 adsorption. a CO2 ads.--CO 2 des. on Ni-Cu/MoSiO, b CO2 ads.---CO des. on Ni-Cu/MoSiO, c CO2 ads.--CO2 des. on Ni-Cu~SiO, d CO2 ads.---CO des.on Ni-Cu/VSiO

It is clear that CO2 can be adsorbed at Lewis base site M=O (Mo=O or V=O) to form bidentate carbonate and at metallic site M (Cu or Ni in Ni-Cu alloy) to form linear and sheafing type of adsorbing species on the surface of catalysts. These common adsorption species are a reversible adsorption state and will be desorbed in the form of CO2 at the temperature range of 70-~95~ It is noteworthy that there is a new type of CO2 adsorption state, horizontal M -- (CO) -- O---~Mn§ appeared on the metallic site M(Cu or Ni in Ni-Cu alloy)

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and Lewis acid site M n+ (Mo6+and V 5+) of the catalysts. This is an irreversible adsorption state and will be desorbed in the form of CO at the temperature range of 130-140 ~ Table 1 The analysis results of IR spectra and TPD profiles of CO2 chemisorbed on catalysts Wavenumber (cml) Ni-Cu/ MoSiO

Adsorbing species

1 6 3 2 1992 1 9 7 0 1303

1594 1369

1568 1328

~

%.~o

y ~ _

Ni-Cu/ VSiO

Adsorbing species

~ ~

~i

1630 1985 1964 1300 o o 1o ~ ~o \V/

Desorption temp. ( ~

~r§

du

72(C02)

Desorption temp. (~ Wavenumber (cmt)

~ ~

1080 960

l~i

1055 960 ~

~8 +

90(C02) 1588 1560 1355 1316

135(C0) 1093 1064 945 945

o, o

%-0, ~ w

oyo

Cu

Cu

75(CO2)

94(CO2)

138(CO)

3.3. CH3OH adsorption on the catalysts IR spectra of CH3OH adsorbed on the Ni-Cu/MoSiO and Ni-CuNSiO are shown in the Figure 4. TPD profiles of CH3OH adsorbed on Ni-Cu/MoSiO and Ni-CuNSiO are shown in the Figure 5.

I I/3 r

t"q

9

I

3500

,

i

9

3000

t/W

1200

,

I

1100

1000

i

900

Wavenum ber (era "l) Fig.4. IR spectra of CH3OH adsorption. a Ni-Cu/MoSiO, b Ni-Cu/MoSiO--CH3OH (ads), c Ni-Cu/VSiO, d Ni-Cu/VSiO--CH3OH (ads)

I

200

i

I

,

250

I

,

300

Temperature (~ Fig.5. TPD profiles of CHsOH adsorption. a CH3OH ads.-CH3OH des. on Ni-Cu/MoSiO, b CH3OH ads.-CH20 des. on Ni-Cu/MoSiO, c CHaOH ads.-CH3OH des. on Ni-Cu/VSiO, d CH3OH ads.-CH20 des. on Ni-Cu~SiO

There are four absorbing bands at 3652, 2976, 2864 and 1026 cm1 and at 3658, 2976, 2864 and 1031 cm ~ appeared respectively on Ni-Cu/MoSiO and Ni-CuNSiO. By contrast with the IR spectra obtained from the gaseous CH3OH, the band at 2976 and 2864 cm1 was assigned to the stretching vibration modes of C--H bond and at 1026 and 1031 cm-~ was assigned to the stretching vibration modes of C--O bond in the oxymethyl occurred on the Lewis acid

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site Mo 6+and V 5+respectively. The band at 3652 and 3658 cm~ was assigned to the stretching vibration modes of O--H bond in the hydroxyl occurred on the Lewis base site Mo=O and V=O respectively. It turns out that the O--H bond in CH3OH interacts with the Lewis acid site M"+ and Lewis base site M=O on the surface of Ni-CtgMoSiO and Ni-Cu/VSiO to form dissociative type of adsorbing species Mn+--OCH3 and H - - O - - M . This is a kind of irreversible adsorption state and will be desorbed in the form of CH20 at the temperature range of 240---260~

3.4. Reaction performances of catalysts The TPSR results of CO2 and CH3OH chemisorbed on Ni-Cu/MoSiO and Ni-Cu~SiO are recognized as it is shown in the Table 2. It must be pointed out that the product DMC did not be detected since the limit of the LZL-203 quadrupole mass spectrometer employed in our experiments. By comparison with the TPD results of CO2 or CH3OH alone adsorbed on the catalysts, it is enough to prove that the CO2 adsorbing types of bidentate carbonate, linear and shearing are not the reaction species, that the horizontal type of CO2 adsorbing species may be able to react with the dissociative species of CH3OH to form DMC and H20 at the temperature around 126~ and that the dissociative type of CH3OH adsorbing species will be transformed into CH20 when the reaction temperature over 270~ Table 2 TPSR results of CO~+CH3OH chemisorbed on catalysts Ni-Cu/MoSiO

Temperature (~ Products

73, 90 CO2

125 H20

137 CO

278 CHzO

CH3OH

Ni-CuNSiO

Temperature (~ Products

78, 94 CO2

128 H20

140 CO

273 CH20

--CH3OH

The catalytic performance of Ni-Cu/MoSiO and Ni-CuNSiO determined from the microreactor technique showed that they are excellent catalysts for DMC synthesis from CO2 and gaseous CH3OH and the reaction products are mainly DMC, CH20, CO and H20. It has been found that the conversion rate of CH3OH and the selectivity of DMC in the reaction products depended upon the reaction temperature, reaction pressure, space velocity and feed composition of CH3OH/CO 2. A typical reaction result is shown in the table 3. Table 3 A typical reaction results for Ni-Cu/MoSiO and Ni-Cu~SiO catalysts Catalysts

P (MPa)

Temp. (~

SV (hl)

CH3OH/ CO2 (mol.)

Conv. of CH3OH(%)

Ni-CufMoSiO Ni-CuNSiO

0.1 0.1

140 140

2000 2000

2:1 2:1

16.37 14.54

Selectivity (%) DMC CH20 CO 86.52 87.84

6.45 5.73

7.03 6.43

Based on the experimental results above, a surface reaction mechanism of DMC synthesizing from CO2 and CH3OH has been set up as it is shown in the Figure 6. It will be seen from this model that the surface reactions of Mn+--OCH3 with M--CO from to DMC and M--OH with M--OH form to H20 and M=O, and that the by-products of CH20 comes from the reaction of M"+--OCH3 with M=O and the M--CO may be desorbed directly to CO. The crystal-frame oxygen M=O takes part in the courses of the surface reactions.

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o

O

Mo+

1~~n1+~ -

h M

[co +2c. o. 1

L...

l0~ l ~ ~ n + I

O I\ C O ,1 Mn+

"- ~,,ln+

" , 3

Sii~ii'............~

OH

I

(a) O O,,..oCH3 1~1 / C Mn+O

OH CH3 j. v M

(b)

IH,Ol

../

o

~

o ~

CH3 ~H3 II~n+O

" ~ ...............'S'i'i~ii'............

(d) (c) Fig.6. A reaction mechanism of DMC synthesizing from CO2 and CH3OH 4. CONCLUSIONS Ni-Cu/MoSiO and Ni-Cu~SiO are the effective catalysts for DMC synthesizing from carbon dioxide and methanol. Under a mild reaction conditions of atmospheric pressure and 140~ the conversion rate of CH3OH is about 15% and the product selectivity of DMC is over 85%. The excellent reaction behaviors of the Ni-Cu/MoSiO and Ni-Cu/VSiO catalysts come from the proper surface constructions. The interaction of the metal Ni-Cu alloy cluster with the valence bonding type of MoSiO and VSiO complex oxides can not only bring about the three types of active sites, including metallic site M (Ni-Cu alloy), Lewis acid site M "+ (Mo 6r or V 5r) and Lewis base site M=O (Mo=O or V=O) on the surface of the catalysts but also change the d-electron density of Ni-Cu alloy and the Lewis acid or Lewis base intensity of the M "r or M=O to suit the activation of CO2 and CH3OH molecules. With the cooperation effects of these active sites, CO2 can be chemisorbed on the metallic site M and Lewis acid site M ar to form a horizontal adsorption state M--(CO)--O---~M nr (Mo 6+ or Vs§ and CH3OH can be chemisorbed on the Lewis acid site M ar and Lewis base site M=O to form a dissociative adsorption state CH30--M nr and HO--M. The M=O of crystal-frame oxygen takes part in the surface reaction and the horizontal adsorbing type of CO2 and dissociative adsorbing type of CH3OH play an important role in the course of DMC synthesis. REFERENCES

1. 2. 3.

J.N. Armor. Catalysis Today, 38 (1997) 163. F. Solymosi. J. Molecular Catalysis, 65(3) (1991) 337. D. Bianchi, T. Chafik, M. Khalfallah and S.J. Teichner. Appl. Catal. A: General, 112 (1994)219. 4. F.M. Hoffmann, M.D. Weisel and J. Paul. J. Surf. Sci., 316 (1994) 277. 5. J.W. Wang and S.H. Zhong. Chinese Chemistry Progress, 10(4) (1998) 374. 6. J. Haggin. C & EN. 70 (1992) 25. 7. Y. Shao and S.H. Zhong. Chinese J. Mol. Catal., 11(5) (1997) 343. 8. Y. Shao and S.H. Zhong. Chinese J. Fuel Chem., 26(1) (1998) 71. 9. S.H. Zhong. J. Catal., 100 (1986) 270. 10. S.H. Zhong and J. Wang. Chinese J. Catal., 16(4) (1995) 263.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Meio, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

P r o m o t i o n o f Support in S y n t h e s i s of Propylene Carbonate from CO 2 and Propylene Oxide Tiansheng Zhao a, Yizhuo Han b, and Yuhan Sun b aKey Laboratory of Enert~r Sources & Chemical Engineering, Mailbox 114, Ningxia University, China 750021 bState Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan, China 030001 The catalytic activity of potassium iodide towards the cycloaddition of carbon dioxide with propylene oxide could be remarkably promoted by its dispersion on the surface of metal oxide, particularly using amphoteric ZnO as the support. IR and XPS-AES characterization indicated that the high activity of ZnO supported potassium iodide could be closely correlated to the acidbase character of ZnO. An intermediate with Lewis acid-base pair sites was proposed for the cycloadditions.

I. INTRODUCTION The cycloaddition between CO2 and epoxides is important for CO2 activation transformation 1-2,which could be catalyzed by binary systems such as organometallic compounds and organic bases a-s. But neither organometallic halide nor organic promoter individually showed good catalytic activity 4. These also gave rise to the difficulty in both preparation and separation of the catalysts. A novel catalyst, metal oxide supported potassium iodide, was therefore developed for the cycloaddition between propylene oxide and carbon dioxide at the present work, which could lead to a heterogeneous process, and showed its high catalytic activity towards the cycloaddition. The catalytic mechanism was then proposed based on the characterization.

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2. EXPERIMENTAL 2. I. Catalyst preparation and characterization

C a t a l y s t s were p r e p a r e d by i m p r e g n a t i n g zinc oxide with a q u e o u s KI s o l u t i o n a n d t h e n dried at 60~ for 10h. Infrared s p e c t r a were r e c o r d e d at r o o m t e m p e r a t u r e on D S - 4 0 2 G s p e c t r o m e t e r . The c a t a l y s t s a m p l e s were p r e s s e d into a s e l f - s u p p o r t e d wafer with a weight of ca. 2 0 m g for the m e a s u r e m e n t . X-ray p h o t o e l e c t r o n s p e c t r o s c o p y (XPS) a n d Auger electron s p e c t r o s c o p y (AES) were d e t e c t e d on VG ESCA LAB5-electron s p e c t r o s c o p y i n s t r u m e n t with a b a s e p r e s s u r e of 10 .9 Torr. The c a t a l y s t s a m p l e s were g r o u n d a n d m o u n t e d on the s t a n d a r d h o l d e r by m e a n s of d o u b l e - s i d e d adhesive tape. A1 Ks 1, 2 (hv = 1486.6 eV) r a d i a t i o n (9 KV a n d 18.5 mA) w a s u s e d to record the spectra. The a n a l y z e r e n e r g y w a s 100eV a n d the step 0.1 eV. The c o n t a m i n a t i o n C w a s u s e d a s the i n t e r n a l reference a n d the b i n d i n g energy of C I s w a s 284.6eV.

2.2. Activity t e s t s Activity t e s t s were carried o u t in a s t a i n l e s s steel autoclave reactor. A typical p r o c e d u r e w a s as follows: 0.1 mol of propylene oxide a n d 0.5g of c a t a l y s t were p u t into the autoclave, a n d CO2 w a s i n t r o d u c e d into the a u t o c l a v e with the initial p r e s s u r e of 5.0MPa at r o o m t e m p e r a t u r e . The autoclave w a s h e a t e d u p to the p r e s c r i b e d t e m p e r a t u r e u n d e r m a g n e t i c a l l y stirring for 2 h o u r s reaction. The p r o d u c t s were d e t e c t e d by m e a n s of GC with t h e r m a l c o n d u c t i v i t y detector.

3. RESULTS 3.1. Catalytic performance The catalytic p e r f o r m a n c e w a s d e p e n d e n t on KI c o n t e n t as s h o w n in Table 1, s u g g e s t i n g t h a t KI w a s the m a i n active c o m p o n e n t . As a c o m p a r i s o n , n e i t h e r KI n o r ZnO w a s individually active t o w a r d s the cycloaddition. The c o n v e r s i o n of PO a n d the yield of PC were lower t h a n 3.50 a n d 2.63%, respectively, implying a synergistic effect b e t w e e n KI a n d ZnO t o w a r d s the reaction. W h e n KI c o n t e n t w a s above 1.4 m m o l / g , K I / Z n O s h o w e d a high a n d stable activity (see Fig. 1). B o t h c o n v e r s i o n a n d yield of PC i n c r e a s e d with a rise in the r e a c t i o n t e m p e r a t u r e . The o p t i m a l t e m p e r a t u r e for the cycloaddition w a s 150 ~ for K I / Z n O ( see Fig. 2 ). In addition, the reaction p r e s s u r e a n d t e m p e r a t u r e were o b s e r v e d as f u n c t i o n of reaction time (see Fig. 3). After a n i n d u c i n g period of c a . . 10 min, the reaction p r e s s u r e d r a m a t i c a l l y d e c r e a s e d from 9.2 to 7.2 MPa

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in 20 min. The reaction t e m p e r a t u r e s i m u l t a n e o u s l y rose p r o m p t l y to 178 ~ After 22 min, it w e n t down to the original 150 ~ This d e m o n s t r a t e d t h a t a large a m o u n t of reaction h e a t released a n d the cycloaddtion of PO with CO2 took place very fast in the p r e s e n c e of KI/ZnO. Table

1

Catalytic p e r f o r m a n c e of ZnO s u p p o r t e d KI for cycloaddition a KI c o n t e n t 0.06 m m o l / g KI c o n t e n t 3 m m o l / g Catalysts PO conv.(%) PC yield (%) PO conv.(%) PC yield (%) KI/ZnO 47.3 41.4 95.3 94.3 ZnO b 3.50 2.63 KI c 0.70 0.53 a Reaction conditions" 150~ b No KI. c U n s u p p o r t e d , 0.166g.

1 O0

~90

. _ _ _ ~ _ _ _"---------* _ _ _ _ ~ f 100

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o

'm 70 [-70 60 ~60-C~ o~ 50 t 50~ 9 [-40 a., 40 . . . . . . . . . . . . . . . . . . 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.~ 4.0 4.5 KI content/mmol g Fig. 1. Influence of KI c o n t e n t on the activity of KI / ZnO.

r~

.

~

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.

.

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.

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145 40 60 80 100 120 Time / min. Fig.3. Reaction p r e s s u r e a n d t e m p e r a t u r as function of reaction time with K I / Z n O (3 m m o l / g ) catalyst. 0

20

1 O0

180

Fig.2. Influence of reaction t e m p e r a t u r e on the activity of KI/ZnO (3 m m o l / g ) .

9

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80 ~9 60 A ~ ~ 40 o o 20 O 0 80 100 120 140 Temperature/~

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3.2 Surface structure o f the c a t a l y s t The IR s p e c t r u m of ZnO showed a b s o r b a n c e s at 530, 490 a n d 445 cm -1, respectively, which were attributed to Zn-O stretching as s h o w n in Fig.4. C o m p a r e d with t h a t of ZnO, a new adsorption peak appeared at 415 cm -~ for KI/ZnO, a n d the b a n d s at 530, 490 a n d 445 cm 1 c h a n g e d slightly. Considering t h a t KI hardly showed the IR adsorption in the 7 0 0 - 2 0 0 c m - 1 region, the new peak should be associated with the effect of KI on the Zn-O bond, a n d s u c h interaction between KI and ZnO was w e a k at room temperature.

II

o m m =N

J

700 660 560 460 3C)0' '24.0 '200 Wavenumbers/cm-1

Fig. 4. IR spectra of (a) ZnO and (b) KI/ZnO (3 m m o l / g ) at room t e m p e r a t u r e . XPS spectra and AES spectra for both KI a n d KI/ZnO are shown in Fig. 5. The strongest XP binding energy Eb(xP) of I(3ds/2) for KI and KI/ZnO was 628.5 and 633.2eV(see Fig.5a and 5b), respectively. Considering the effect of the ~9o5 9-Io 9-15 9~o 985 g;o 9;5 1o'oo'Idol charging potential on the m e a s u r e m e n t of binding energy, a modified Auger p a r a m e t e r was u s e d to characterize the chemical state of I [6]. S u c h p a r a m e t e r was defined as 0~ = Ek(AE ) + Eb(xp)=1486.6- Eb(nE) + Eb(xP). The strongest AE binding o20 o25 o30 o~5 o~.o o~,5 650 ' B i nding e n e r g y / e V energy Eb(nE)Of I (corresponding to atomic orbital layer M4NsNs, 3d3/24ds/24ds/2) for KI and KI/ZnO Fig. 5. X-ray photoelectron spectra of (a) KI and (b) KI/ZnO (3 mmol/g) and Auger electron was 989.2 a n d 991.5eV(see Fig.5c spectra for (c) KI and (d) KI/ZnO (3 mmol/g). and 5d), respectively. But the modified Auger p a r a m e t e r of I.for KI o_ c-

I.-

i

I

,

I

|

,

I

|

,

'

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'

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I

'

'

'

'

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(al) a n d for KI/ZnO (a2) was 1125.9 a n d 1128.3eV, respectively, leading to a shift of 2.4 eV. As the Auger p a r a m e t e r was very sensitive to the chemical state, this shift indicated a change of the chemical state of I, which should result from the interaction between KI and ZnO. 4. D I S C U S S I O N

Supported on the surfaces of ZnO, KI was highly active towards the cycloaddition compared with individual KI or ZnO as the catalyst (see Table 1). The comparative study (using 7-A1203, ZrO2, SiO2 and NaX as supports) showed t h a t no direct correlation could be observed between the activity and BET surface a r e a s of the supports. Thus, the e n h a n c e d activity should be related to the surface properties of the supports. It should be noted t h a t Z n O 7-9 was considered to show amphoteric character and then have the acid-base sites on the surfaces. The behavior of KI changed when supported. A weak interaction was observed for KI on the support with amphoteric character as IR a n d XPS-AES characterization illustrated. In addition, TGA of the catalysts also indicated that the interaction became stronger as the t e m p e r a t u r e increased. The dependence of the activity of KI/ZnO on the reaction t e m p e r a t u r e (see Fig. 2) revealed t h a t the importance of the interaction between KI a n d ZnO. Generally, the nucleophilicity of the anion in metal salts can be reduced because of the interaction with the cation. Under the reaction t e m p e r a t u r e the Lewis acid-base pairs of Ia-Zna+and Ka+O~- might be formed when KI was supported on ZnO. This obviously reduced the interaction between K§ a n d Iand then favored their activity towards epoxides. As a result, the interaction between KI a n d ZnO might promote the reactivity of KI. On the other h a n d , the combination of s u c h acid-base pairs would generate the effective bifunctional active sites for the cycloaddition and simultaneously led to the activation of CO9 a n d PO. Thus, the m e c h a n i s m was proposed as that in Scheme 1. ~Zn~O~Zn~

'-I

l-Zn +

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co

......

I -Zn +

i

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2

3

CH3

Scheme. 1. Proposed diagram of reaction mechanism.

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5. CONCLUSION

A supported KI catalyst has been successfully developed for the cycloaddition of CO2 and PO. The cycloaddition smoothly proceeded in the presence of supported KI within a short reaction time. The activity of KI could be strongly enhanced by using ZnO as both support and promoter. The suitable loading a m o u n t of KI on ZnO and the reaction t e m p e r a t u r e were needed for high catalytic activity. The amphoteric character of the supports played an important role in the reaction. The synergetic bifunctional Lewis acid-base pairs of IS-Zn~+and KS§ s- might be the active sites for activation of CO2 and PO. The a u t h o r s t h a n k the financial supports from the NSF of China and the Natural Science foundation of Shanxi province. REFERENCES

1 B. Denise, R. P. A. Sneeden, CHEMTECH, (1982) 108. 2 D. J. Darensbourg, M. W. oltcamp, Coord.Chem.Rev., 153 (1996)155. 3 R. Nomura, M. Kimura, S. Twshima, A. Ninagaa, H. Matsuda, Bull. Chem. Soc. Jpn., 55 (1982)3200. 4 A. Baba, T. Nozaki, H. Matsuda, Bull.Chem.Soc.Jpn., 60(1987)1552. 6 V. I. Bukhtiyarov, I. P. Prosvirin, R. I. Kvon, S. N. Goncharova and B. S. Bal' zhinimaev, J.Chem. Soc.,Faraday Trans., 93 (1997)2323. 7 R. P. Eischens, W. A. Pliskin, M. J. D. Low, J. Catal., 1 (1962)180. 8 A.L. Dent and R. J. Kokes, J. Phys, Chem, 74 (1970) 3653. 9 H. Nakabayashi, Chem.Lett., (1996) 945.

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化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

A novel process for the removal of nitrates from drinking water Albin Pintar', Jurka Batista and Gorazd Ber6i6 Laboratory for Catalysis and Chemical Reaction Engineering, National Institute of Chemistry, Hajdrihova 19, P.O. Box 3430, SI-1001 Ljubljana, Slovenia A nitrate removal process that drastically reduces salt consumption and waste discharge has been developed on a bench scale. Nitrate is removed by chloride ion exchange, and the strongbase anion resin is completely regenerated at mild reaction conditions (i.e., ambient temperature, atmospheric pressure) in a closed circuit containing a single-flow fixed-bed reactor packed with a Pd-Cu/7-A1203 catalyst. The combined treatment system avoids direct contact between the denitrification reactor and the water to be treated, and minimizes operational problems associated with each separate technique. 1. INTRODUCTION Nitrate concentrations in surface water and especially in groundwater have increased in many locations in the world. As a result, there is renewed interest in the removal of nitrates from raw water. State-of-the-art of treatment methods for the removal of excessive quantities of nitrates from drinking water is discussed in [1 ], and various treatment options are compared in terms of their effectiveness, ease of operation, and cost. Physicochemical methods allow effective removal of nitrate ions from contaminated groundwater, however, by concentrating them in a secondary waste stream. Among them, the capital and operating costs are the lowest for the ion exchange process; nevertheless, it is very difficult and costly to dispose of large quantities of spent regenerant brine in noncoastal locations where natural evaporation is impossible. The most promising techniques for nitrate removal, without any occurrence of wastewater, are biological digestion and catalytic denitrification by using noble metal catalysts [2, 3]. The main reasons for the slow transfer of biological denitrification to drinking water purification are concerns over possible bacterial contamination of treated water, the presence of residual organics in treated water, and the possible increase in chlorine demand of purified water. The reduction of aqueous nitrate solutions by using hydrogen over a solid Pd-Cu bimetallic catalyst offers an alternative process to biological treatment as a means of purifying drinking water streams. The reaction is carried out in a two- or three-phase reactor operating under mild reaction conditions (e.g., T = 278-298 K, p(H2) up to 7 bar), and obeys a consecutive reaction scheme in which nitrite appears as an intermediate, while nitrogen and ammonia are the final products. To maintain electroneutrality of the aqueous phase, consumed nitrates are replaced by hydroxide ions. Supported Pd-Cu and Pd-Sn bimetallic catalysts exhibit the highest activity for nitrate reduction and chemical resistance, but still inadequate selectivity *Corresponding author. Phone: (+386-61) 17 60 282; Fax: (+386-61) 12 59 244; E-mail: [email protected].

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TREATED WATER

~, "

/X

7

RAW WATER

PUMP

PUMP

Fig. 1. Schematic layout of the combined ion exchange/catalytic denitrification process for nitrate removal. 1, column with the packed bed of ion exchange resin; 2, mixing point of the treated and untreated water; 3, saturator; 3a, mixing nozzle; 4, 5, pump; 6, separator. towards nitrogen production. The nitrate disappearance rate as well as nitrogen production yield decrease appreciably in the presence of hydrogencarbonates in tap water [4]. Recent results of nitrate reduction carried out in continuous-flow reactors (i.e., bubble-column fixedbed, trickle-bed) allude that by using up-to-date Pd-Cu bimetallic catalysts direct treatment of drinking water with higher amounts of hydrogencarbonates seems to be unfeasible [5]. To overcome operational problems associated with the regeneration of exhausted ion exchange resins and direct purification of drinking water by means of catalytic hydrogenation, an integrated process was invented, which efficiently combines a conventional single-bed ion exchange unit with a catalytic denitrification reactor in such a way that drawbacks of each separate technique are effectively eliminated [6]. A schematic layout of the combined process is illustrated in Figure 1. The nitrate-free effluent can be blended with a predetermined fraction of bypass raw water to produce a water stream of acceptable nitrate concentration. 2. EXPERIMENTAL The ion exchange step made use of a packed-bed of anion resin in the chloride form. In this work, a macroporous strongly basic anion exchange resin IMAC HP-555 (Rohm and Haas Co.), which is nitrate selective, was used to remove nitrate ions from groundwater. Total exchange capacity of the employed resin determined by means of both breakthrough curves (not shown here) and potentiometric titration with a A g N O 3 solution was found to be equal to 0.00286 mol/g (i.e., 0.92 eq/L). In a typical run, removal of nitrates by ion exchange was conducted in the upflow mode at T=298 K, Ptot.=l bar and OvoI.,L'-8.0 mL/min. When the resin was saturated with nitrate ions, column exhaustion was terminated. Meanwhile, H2 was introduced into the saturator (3), which operated at atmospheric pressure and was filled with 700 mL of aqueous solution of NaC1 with the initial concentration of 5.0 g/L. A centrifugal

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pump (4) recirculated the solution of chloride ions through the saturator and upflow catalytic reactor at a flowrate of 5.3 L/min. The saturator was equipped also with a heating-cooling coil (T=298 K) and mixing nozzle (3a), which provided good contacting between the gas and liquid phase. When the latter was saturated with H2, a peristaltic pump (5) started to recirculate the regenerant through the bed of ion exchange resin and saturator at a flowrate of 10.0 mL/min. The chloride ions from the regenerant substituted the nitrate ions on the ion exchange resin. Due to the continuous recirculation of aqueous solution of chloride ions through the two-phase catalytic reactor, packed with an egg-shell type Pd(1.0 wt. %)-Cu(0.3 wt. %) bimetallic catalyst, nitrate ions from the regenerant solution reacted with H2 chemisorbed on the catalyst surface and converted into nitrogen (and ammonia). The neutralization agent, i.e., the aqueous solution of HC1 (0.25 M), was applied by means of an autotitrator (Metrohm, model 751 GPD Titrino) in order to keep the pH value of regenerant constant at 5.5. The use of HCI had the advantage that no further makeup of the regenerant with NaC1 was required, since the stoichiometric required amount of chloride ions for a subsequent regeneration cycle was simultaneously introduced to the process via the neutralization of hydroxide ions produced during the liquid-phase nitrate reduction. The regenerant continued to be recirculated through the ion exchange column and the denitrification reactor, until both nitrate and nitrite ions were completely consumed. The regenerant free of these species was used in subsequent regeneration cycles. In the catalytic reactor, the catalyst layer was formed from 7-A1203 spherical particles (do=l.7 mm) on which metallic Pd and Cu phases were deposited in such a sequence that the catalyst surface was enriched with Pd clusters. Detailed preparation of the catalyst used in this process is described elsewhere [7].

3. RESULTS AND DISCUSSION It was found out in this study that regeneration of nitrate loaded IMAC HP-555 resin is possible with a diluted solution containing 2.5-10.0 g/L NaC1. Compared to the conventional regeneration procedure with 50-100 g/L NaC1, a flowrate of 2-4 BV/h and a period of approximately 30-50 min, more time and a higher flowrate are needed. However, with 5 g/L NaC1 and a flowrate of 3.0 mL/min (i.e., 25 BV/h) complete regeneration of the resin is possible in 240 min; one should note that prolonged time of regeneration does not represent any drawback for an integrated process. Although small amounts of NaC1 were consumed in performed regeneration runs, the regeneration efficiency, defined as the ratio of equivalents of nitrate removed from the resin during regeneration to the equivalents of regenerant used, is rather low and found in the range of 0.07-0.15 eq nitrate/eq chloride. Low regeneration efficiency is not a serious problem in a closed regeneration system, because the excess of NaC1 will stay in the system and is not lost in the disposed brine. In other words, the value of regeneration efficiency can be easily increased by reusing the regeneration solution. This is particularly true when it has been made free of nitrates, which can be achieved, e.g., by means of the combined process shown in Fig. 1. Of course, all advantages of the process described above come at the price of increased capital cost and process complexity. The breakthrough curves (or effluent histories) for nitrate from the IMAC HP-555 resin bed with regenerant reuse are shown in Figure 2. The breakthroughs are nearly identical and do not suggest any trend to shorter or longer nitrate removal runs. It can be thus concluded that reusing the denitrified regenerant containing 5.0 g/L NaC1 did not negatively influence the

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column performance. A major concem when a spent regenerant is reused is the accumulation of ions stripped from the resin during regeneration. These include nitrate, sulfate and hydrogencarbonate. Nitrate (and nitrite) should be eliminated by denitrification, hydrogencarbonate by neutralization with HC1 in the saturator, but sulfate can accumulate during reuse when there is a net removal of these ions during exhaustion. It was found in this study that only small amount of sulfate ions is accumulated in the regenerant solution (about 8 mg/L sulfate per regeneration cycle), which suggests that more than one-half of sulfate has been dumped from the resin during exhaustion. The results depicted in Fig. 2 demonstrate that in each run the nitrate capacity in the exhaustion mode was found to be unaffected by the amount of sulfate in the regenerant 100 ~0~ ~ 1 7 6 beforehand. After the fourth Exhaustion run #. ..I exhaustion-regeneration cycle of g resin with regenerant recycling, it "~ 80 ~_ o 1 (fresh) 9 2 was found out by means of ,a 3 ~] argentometric titration that the 9 4 IMAC HP-555 resin maintains 99 % T: 298K of its initial capacity for nitrate Ptot: 1.0 bar loading. mresin: 0.75 g IMAC HP-555 13 Experimental results of regen_ [] c(NC~)feed: 100.0 mg/L eration of ion exchange resin 1"1 (t)va.,L: 8.0 mL/min rl n saturated with nitrate and the simultaneous removal of the nitrate Z O~ ion from the regeneration solution lime,rain

J

Fig. 2. Nitrate breakthrough curves for different using catalytic hydrogenation in a two-phase fixed-bed reactor are exhaustion runs during regenerant reuse, presented in Figure 3. Temporal

course of the destructive nitrate reduction obtained in the presence of Pd(1.0 wt. %)-Cu(0.3 wt. %)h{-A1203 was followed by measuring the instantaneous concentrations of nitrate, nitrite and ammonium ions in the regenerant solution by means of flow-injection analysis (Perkin-Elmer). At the given reaction conditions, contacting time of the liquid-phase in the single-flow reactor was equal to 30 msec, and the latter operated in the kinetic regime. Although the volumetric flowrate of regenerant solution through the ion exchange column was rather low (10.0 mL/min), nitrate disappearance rate was not affected by the rate of nitrate desorption from the ion exchange resin. It is evident from Fig. 3a that by employing this process total removal of stripped nitrate ions from the regenerant is attained, which means that the latter can be used in subsequent regeneration cycles. Furthermore, one can see that the activity of the catalyst for nitrate removal decreases with a number of exhaustion-regeneration cycles. This cannot be attributed to the dissolution of Pd and Cu metallic phases, since no metal ions were detected in the regenerant solution by means of ICP-AES examination. The observed decline of the catalyst activity is due to the following reasons: (i) concentration of chloride ions in the regenerant solution increases with a number of regeneration cycles (see below); (ii) isoelectric point of the catalyst decreases during the liquid-phase nitrate reduction, which is ascribed to the consumption of weakly bound protons on the catalyst surface with hydroxide ions formed [8].

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nitrite ions were accumulated in aqueous phase during the regeneration (Fig. 3b). This is ascribed to the low contacting time of liquid-phase in the reactor and much higher affinity of nitrate ion towards the Pd-Cu active sites on the catalyst surface in comparison to nitrite. The concentration of nitrite ion in the regenerant, after the regeneration process of saturated ion exchange resin and simultaneous destructive hydrogenation had been finished, was lower than that prescribed by the regulation of the law (i.e., 0.02 mg/L). However, it is obvious from Fig. 3b that longer reaction time is needed to achieve this goal in subsequent regeneration cycles. This is again attributed to the fact that in each reduction run the catalyst surface becomes more negatively charged, which consequently increases the repulsion between intermediate nitrites and active sites on the catalyst surface. A detailed mechanism of catalyst deactivation observed in the process of liquid-phase nitrite reduction is described in detail by Pintar et aL [8]. 101)

la

T: 298 K ..i 80 o ~ P(H2 ): 1.0 bar; F~ot.: 1.0 bar .:-. mcat : 3.0g; dp" 1.7 mm "~ o.-~T, mresin : 0.75 g IMAC HP-555 -.!. . Vsat : 700 mL; c NaCI = 5.0 g/L 60 o!.- . . -. . pH: 5.5 (const.) 0 -.~.

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In Fig. 3c the concentration of ammonium ions produced is shown as a function of time for various regeneration cycles. Based on these data, about 70 mol % of nitrate stripped from the ion exchange resin is transformed to ammonia. However, this value can be easily miniI ~~ mized to, e.g., 15 mol % by carrying out the 1:: 60 mmmmmm~:x .=l / 9 Regeneration step #: catalytic nitrate reduction at lower hydrogen partial pressures. As the denitrification process i m o9 2 does not take place in direct contact with the ~t oJ A 3 groundwater, there is no risk that ammonia production will affect the water quality. Data Ode , ' 0 1000 2000 300 shown in Fig. 3c confirm that no ammonia was Time, rain transferred into the gas-phase at the given Fig. 3. Temporal course of nitrate (a), nitrite reaction conditions. (b) and ammonium (c) ions for consecutive Figure 4 shows the concentration-time proregenerations of IMAC HP-555 resin by file of chloride ions in the regenerant for differmeans of catalytic denitrification. ent regeneration cycles. It is seen that the concentration of chlorides measured in the saturator increases with time, which arises from the: (i) .J / "~ 120 I. | / 90 I-

P(H2): 1.0 bar; Ptot. : 1.0 bar meat : 3.0 g; dp: 1.7 mm **" mresin: 0.75 g IMAC HP-555 TT Vsat :700mL; CNaCi =5.0g/LT* pH: 5.5 (const.) zxzxa~

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favorized formation of ammonium ions, which accordingly to the reaction stoichiometry increases the consumption of HCI by a factor of two; (ii) neutralization of stripped hydrogencarbonate ions from the ion exchange resin by means of HC1; (iii) reference electrolyte (3 M KC1) I mresin:0.75g IIW~!-IP-555 I Vs~: 700 rrL outflow from the pH electrode. Additional experiments of liquid4.50[ 1(~) pi--t5.5(,c~.) 2000 300 phase nitrate reduction carried out in Tirn~ rnin a batch-recycle reactor in the Fig.4. Concentration of chloride ions (expressed presence of the same catalyst as used as NaCl) as a function of time in the reused regenein this study, show that the nitrate rant solution. (and nitrite) disappearance rate decreases by increasing the concentration of sodium chloride in the aqueous solution. Also, the inhibitive impact of produced hydroxide ions on the catalyst activity is more pronounced in the presence of chlorides. Due to the synergistic effect of these species on the reaction behavior, no kinetic analysis of data shown in Fig. 3a is possible. However, the results depicted in Fig. 4 demonstrate that no make-up of the regenerant solution is required between exhaustionregeneration cycles. In this work, no organic fouling of both the ion exchange resin and catalyst, which can be caused by humic and fulvic acids accumulated in a closed regeneration circuit, was observed. This implies that after the exhaustion-regeneration cycle has been completed, no disinfection of the unit with a solution of, e.g., peracetic acid is needed. In this respect, the denitrification process shown in Fig. 1 is advantageous over the combined ion exchange/biological denitrification techniques [9]. III

9

IV

o

$

4. CONCLUSIONS The described catalytic/physical chemical process is a very attractive technique for nitrate removal from groundwater. Compared to ion exchange brine production is very low and regeneration salt requirement is minimal. As the denitrification process does not take place in direct contact with the groundwater, there is no risk that nitrite and ammonia production will affect the water quality. Also groundwater with a high sulfate content can be treated with this technique, when a nitrate selective resin, for example IMAC HP-555, is used. REFERENCES 1. 2. 3. 4. 5. 6. 7.

A. Kapoor and T. Viraraghavan, J. Environ. Eng., 123 (1997) 371. K.D. Vorlop, T. Tacke, M. Sell and G. Strauss, German Patent No. 3830850 A1 (1988). U. Prttsse, S. Hrrold and K.D. Vorlop, Chem. Ing. Tech., 69 (1997) 93. A. Pintar, M. Setinc and J. Levec, J. Catal., 174 (1998) 72. A. Pintar and J. Batista, Catal. Today, 53 (1999) 35. A. Pintar, J. Batista, G. Berri~ and J. Levec, PCT Application No. SI99/00018 (1999). A. Pintar and J. Levec, SI Patent No. 9500357 (1998). 9

T~"

,

~

n

____v. y

--_

'11 t

T

A T/'~l_'lf~.

9

A A

/ 1 N N O \

"~"~10/"~

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

化

1583

Greenhouse gases and emissions control by new catalysts free of precious metals V. Gryaznov and Yu. Serov Russian University of Peoples' Friendship, 6 Miklukho-Maklay St., 117198 Moscow, Russia There were fbund the effective catalysts without precious metals for dry reforming of methane, carbon dioxide hydrogenation into ethylene and propylene and detoxication of the gas emissions of cars and industrial enterprises.

Carbon dioxide usage as chemical raw material is very important for the elimination of technogenical damages of the atmosphere. The transformation of two main greenhouse gases - CO2 and CH4 - by dry retbrming CH4 + CO2 = 2CO + 2H 2

(1)

has a limited application because of swift cooking of many proposed catalysts. The systematic study of catalytic properties of ultradispersed powders (UDP) of iron group metals performed at Russian University of Peoples' Friendship (RUPF) reveal its activity in dry reforming of methane and in carbon oxides hydrogenation with olefins C2-C 3 formation. The UDP were prepared in collaboration with Moscow Institute of Steel and Alloys (MISA) and industrial enterprise Red Star (RS) by decomposition of metal carbonyls in hydrogen plasma, electric explosion of wires, evaporation-condensation and criochemically by condensation of metal vapour in hydrocarbon matrix cooled by liquid nitrogen. The preparation method and the size distribution of UDP influence on the catalytic activity and selectivity as well as the nature of refractory oxides powders which were used for making of matrixes with metal UDP's. For example the proper choice of matrix material permits minimization of cook fbrlnation during dry reforming of methane. The most active and stable in reaction (1) without diluents at atmospheric pressure, temperature of 1013 K and space velocity 4500 h z is a system of 10 wt. per cent UDP of Fe in the refractory oxide powder. The conversion of initial substances is 94 per cent. The colour of catalyst did not change during 70 h of operation. It does mean that the cooking of catalyst is very low. Hydrogenation of CO 2 by the known catalysts gives paraffins only and the trace amount of olefins. The other result was achieved by usage of mixture of electrolitically produced the thread-shaped crystals of cobalt and iron l-10 micron length and 20-30 nm in diameter with a powder of very pure quartz. This catalyst was tested in flow reactor at atmospheric pressure and CO2/H2ratios fiom 2"1 to 1:4. The content of ethylene and propylene in products was 10

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per cent at temperature of 573 K and CO2/H2 ratio 1:4 on the catalyst with 2.5 per cent Co and 7.5 per cent Fe. The next preparation of this catalyst showed that at CO2/H2 equals 2 and 613 K the content of olefins C2-C3 increased to 14 per cent. Similar data were not found in the literature. The catalytic activity of mixtures of UDP of iron group metals in the oxidation of carbon monoxide, light olefins as well as in the reduction of nitrogen oxides was determined at 373-1023 K in a flow reactor at atmospheric pressure, with space velocities from 3000 to 30000 h -~. The catalyst sample was placed into a quartz reactor equipped with a soldered quartz filter to prevent catalysts loss by being carried away. The effluent reaction products were analyzed by a gas chromatograph equipped with both thermal conductivity and flameionization detectors: Riken Keiki analyzers of CO, CH, NO,, with sensitivities of 50 (CO), 15 (CH), and 1 ppm (NO,) were also ernployed. One series of the catalysts was prepared by mixing UDP of iron, cobalt, nickel, and manganese oxides with AI_~O3and SiO2 powders. According to X-ray diffraction analysis and transmission electron microscopy, the average size of the particles is 10-12 nm. The specific surface area, as measured by the BET method is equal to 40-75 m2/g for the metallic samples under investigation and 44 m-'/g for manganese oxide. The catalysts of the second type were obtained by impregnation of A1203 supports with aqueous solutions of cobalt and manganese nitrates followed by exposure of the samples to an argon-oxygen flow at 623 K: ultrafine iron powders were produced in these systems by hydrogen-plasma treatment of the iron pentacarbonyl. The BET specific surface area of these samples ranges from 17 to 51 m-~/g. The maximum content of the metallic phase in the catalysts studied did not exceed 10 wt.%. Table 1 characterizes some of the employed catalysts. x, % CO

x, % NO~

1 O0

ioo

_

I

A 2 3

50

5O I f

/~

~/

o.I Ax III

I

473

573

673

,,

I

"i'~K 773

Fig. 1. The degree of CO oxidation on catalysts Nos. 1-3 (curves 1-3, respectively), at the space velocities in h-~: (I) 4.4x103, (II) 8x 103, (III) 14.4x 10 ~. (IV) 24x 103.

I

673

,

1

773

T,K

Fig. 2. Reduction of nitrogen oxides by carbon monoxide on catalysts Nos. 1-3 (curves 1-3, respectively).

Oxidation of CO by air on iron UDP becomes noticeable at 480 K (Fig. 1, curve 1), and conversion rises to 90% at 690 K. Nickel powder (Fig. 1, curve 2) converts CO to 96% at

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720 K. The oxidation 99.9% of CO over the cobalt UDP occurs at 503 K (curve 3 in Fig. 1). Variation in the velocity of the initial mixture supplied to the reactor from 4.4x103 to 24x103h ~ only slightly influences both CO conversion and the temperature of complete oxidation. Fig. 2 depicts that the sequence of Fe, Ni and Co activities in reduction of nitrogen oxides by carbon monoxide was reverse. Table 1 Characterization of the catalysts used (s sp is the specific surface area; d av is the average diameter of the particles) No. 1 2 3 4 5 6 7 8

Catalyst First series 3% Fe / AI203 3%Ni / AI203 3%Co / AI203 3 % F e - 3%Co / AI_~O~ 3 % F e - 3 % C o - 3%MNO2 / AI203 3 % F e - 3%Co / SiO, 3%Fe - 3%Co - 3%MnO, / SiO, 3 % F e - 3 % C o - 3%Ni / SiO~ Second series 2 . 3 % F e - 4.5%COO / A1203 5 % F e - 5%COO / AI20~ 7.6%Fe - 5%MNO2/AI20 ~ _

_

_

s sp, m2/g

d

av,

nm

66 33 67 68 44 18 25 18

10-12 20 10-12 10-12 10-12 10-12 10-12 10-20

26 17 51

10-12 10-12 10-12

.

9 10 11

The high catalytic activity of the polymetallic systems containing manganese oxide in nitrogen oxides reduction can be explained by the fact that N20 is formed more intensely from NO in the presence of manganese oxide than in its absence (Fig. 3, curves 1 and 3), the mechanism as follows" 2NO + CO = N20 + CO2

(2)

N20 + CO = N 2 + CO 2

(3)

Over the catalysts containing no manganese oxide, the predominant reaction path leads to the formation of molecular nitrogen: 2NO + 2CO = N 2 + 2CO 2

(4)

It is well known [3-5] that at low temperatures the occurrence of this process via the stage of N20 formation is thermodynamically more favorable than direct reduction of NOx; hence, the rate for reaction (3) is considerably higher than that of reaction (4). Thus, iron-manganese catalyst No. 11 provides a higher conversion of NOx in the temperature range 550-620 K than do iron-cobalt catalyst No. 9. As for catalysts Nos. 9 and 1 l, which are the most active in

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reduction of nitrogen oxides with carbon monoxide, it is worth noting that they performed reduction by propane at considerably higher temperature 973 K. x,% 100 -

50-

ooi AAH

J-

,

~

573 Fig. 3. Temperature dependence of the and No. 11 (II).

L

673

N20 (1,3)

T,K

773

and N: (2,4) yields over catalysts No. 9 (I)

The experiments for destroying the toxic components of engine emissions containing oxygen and water vapors were carried out with the catalysts Nos. 9 and 11 (see Table 2). Table 2 Conversion degree (z in the reactions of CO oxidation and NO, reduction Reaction

Catalyst

2CO + O~ = 2CO~ _

2NO + 2 C O - N~ + 2CO~ _

10NO+C3Hs=5N2+3CO2+4H20

9 11 9 11 9 11

(z=25% 473 463 573 483 858 793

Temperature, K cz=50% cx=75% 490 503 483 500 603 623 523 573 893 913 858 893

c~=99% 563 553 673 623 963 973

A 95% reduction of NO with CO at 623 K is obtained (Fig. 4, curve 1) in a mixture containing 0.1% NO~ and 0.6% CO. After the addition of 2.5% O~ this result still holds (curve 2), however, 16% water vapour addition decreased the extent of NOx reduction (curve 3). The comparison of curves 1-4 of Fig. 5 showes that oxygen presence in the gaseous phase increases the temperature of complete reduction of nitrogen oxides by propane approximately on 100 K. This can be attributed to blocking of the active sites of the catalyst with water formed 99% conversion of NO, is achieved over an iron-cobalt catalyst at 973 K (Fig. 6, curve 1) in a mixture containing no water vapour; this temperature increases to 1023 K _

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(curve 2) in the presence of water. At the same time, 99% conversion of NOx over an ironmanganese catalyst is observed in the presence of water vapors only above 1083 K (Fig. 6, curve 4). x %NO x 100 -

..~.

A

A~2

\. 50

1

I

573

673 T,K

Fig. 4. Reduction of nitrogen oxides by carbon monoxide on an iron-manganese catalyst: in the absence of oxygen ( 1), with 2.5% 02 added (2), with 16% water vapour added (3). A 50% conversion of NO, was found on the iron-manganese catalyst even at 573 K and reached 70% at 593-623 K (Fig. 7, curve 1). The extent of propane oxidation (curve 3) rises gradually with increasing temperature and reaches 99% at 973 K. The conversion of carbon monoxide approaches 99% at 593 K and remains unchanged at higher temperatures (curve 2). As carbon monoxide is consumed, the conversion of nitrogen oxides reduces to zero at 723 K, while the interaction of NOx with propane commences above 823 K (Fig. 7, curve 1). The addition of water vapor to this reaction mixture does not affect the oxidation of either CO or propane but decreases the extent of reduction of nitrogen oxides with carbon monoxide and propane (curve 4). x % NOx 100 -

o--~x~A

.,

/A" J t~

773

A/

o ~

A7

07 1

873

/.i~ i

I"

I

973

o/

i

1

a/

9

/era /

A~.5--" ..~A-i O ~

o/

/e ~ -

,/I" ~,"

o

9

"-%~x d yo A

,o

7 y,/+ A' o i

x, % NOx 100 -

773

I1'"-

873

1

973

I

1073,rv

1073T,K

Fig. 5. Reduction of NOx with propane on iron-cobalt (1,2) and iron-manganese (3,4). Catalysts in mixtures containing no oxygen (1,3) and in the presence of 0.6% 02 (2,4).

Fig. 6. Reduction of NOx with propane on iron-cobalt (1,2) and iron-manganese (3,4) catalysts with no water (1,3) and 16% water vapour (2,4).

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Since water vapour also hampers neutralization reactions of toxic components over the catalysts prepared with noble metals [6,7], the findings of this work suggest that more detailed tests of iron-manganese catalysts for purification of automobile exhausts and industrial emissions is desirable. x% 100 I 0 ~

2

O-- @-- 0------0---- @ - -

O--

o

@------ 0 A n DA

-7

50

673

773

873

973 T,K

Fig. 7. Neutralization of a mixture containing 0.13% NO, (1), 0.15% CO (2), or 0.18% C3H s (3) and 0.6% 02 on an iron-manganese catalyst. Curve 4 refers to the mixture of 0.13% NO,, 0.15% CO, 0.18% C3Hs, 0.6% 02, and 10% water vapour.

REFERENCES 1. I. Hoshiyama, J.M. Song and C.S. Kim, Trans. Jpn. Mech. Eng., 1983, v. 49B, No. 6, p. 2465. 2. I. Hoshiyama and H. Yonejima, J. Chem. Soc. Jpn., Chem. Ind. Chem., 1984, No. 6, p. 1035. 3. W.C. Hecker and A.T. Bell, J. Catal., 1984, v. 84, No. 1, p. 200. 4. M.A. Ismailov, R.B. Akhverdiev, V.S. Gadzi-Kasumov, R.G. Sarmurzina, V.I. Parchishnyi and V.A. Matyshak, Kinet. Katal., 1993, v. 34, No. 1, p. 117 [Kinet. Catal. (Engl.Transl.)]. 5. N. Mizuno, M. Tanaka and M. Misono, J. Chem. Soc., Faraday Trans., 1992, v. 88, No. 1, p. 91. 6. A. Kudo, M. Steinberg, A.J. Bard, A. Campion, M.A. Fox, T.E. Mallouk, S.E. Webber and J.M. White, J. Catal., 1990, v. 125, No. 2, p. 565. 7. R.A. Gazarov and V.P. Moiseev, Kinet. Katal., 1980, v. 7, No. 1, p. 113.

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化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Study on the initial steps of the polyethylene cracking over different acid catalysts D.P. Serrano*; R. Van Grieken; J. Aguado; R.A. Garcia; C. Rojoa; F. Temprano a Department of Chemical Engineering. Faculty of Chemistry, Complutense University, 28040 Madrid, Spain. aRepsol S.A., Technology Center, C/Embajadores 183, 28045 Madrid, Spain The initial steps of the thermal and catalytic cracking of low-density polyethylene (LDPE) at different temperatures and reaction times have been studied in batch operation. The catalysts employed were silica-alumina, USY zeolite and MCM-41. The thermal degradation follows a random scission mechanism with little contribution of isomerization and aromatization reactions; cracking over SIO2-A1203 shows both products coming from random scission and isomerization processes. On the contrary, USY zeolite favours an end-chain cracking mechanism with pronounced isomerization and aromatization. LDPE degradation over MCM-41 proceeds through a random scission mechanism due to their large pores, while hydrogen transfer reactions involving high molecular weight molecules are also favoured by this catalyst. 1. INTRODUCTION Large amounts of plastic wastes are generated annually which end up in landfills or incineration plants. Plastics materials account for about 8 wt% of the municipal solid wastes (MSW), whereas this proportion increases over 20% in volume due to their low density [ 1]. Recently, feedstock recycling has shown to be an interesting alternative to transform back the plastic wastes into chemicals and fuels. Recent works on the conversion of polyolefinic plastics into fuels have revealed the advantages of catalytic processes to improve the quality of the degradation products. Mordi et al. [2] have examined the product distribution from the catalytic degradation of polypropylene over various types of zeolite catalysts under batch conditions. Uddin et al. [3] have studied the influence of silica-alumina catalysts on the polypropylene degradation. Uemichi et al. [4] have used metal-supported activated carbon for the conversion of polyethylene into aromatic hydrocarbons. Aguado et al. [5, 6] have studied the degradation of different polyolefins over a mesoporous catalyst, MCM41, showing that the use of this material leads to the formation of both gasoline and middle distillate fractions as main cracking products. Correspondence concerning this paper should be addressed to Dr. D.P. Serrano, Departamento de Ingenieria Quirnica, Facultadde Ciencias Quimicas,UniversidadComplutensede Madrid, 28040 Madrid, Espafia. e-mail: [email protected] phone number: 34-91-394 41 85 fax number: 34-91-39441 14

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While most of the previous works on the catalytic cracking of polyolefins deal mainly with the analysis of the volatile products coming out from the reactor, in the present research work the solid residue has been deeply characterised in order to get information on the initial stages of the degradation. In this way, a different cracking pathway has been observed when using MCM-41 as catalyst, compared with conventional acid solids such as zeolites and amorphous silica-alumina. 2. E X P E R I M E N T A L

SECTION

2.1. M a t e r i a l s

Low-density polyethylene (LDPE), supplied by Repsol S.A., was used as raw material. Commercially available catalysts, amorphous silica-alumina (Stidchemie, KA-3) and zeolite USY (Grace Davison), were employed without modification. MCM-41 with Si/Al=76 was synthesised in our laboratory according to a room temperature method [7]. 2.2. R e a c t i o n s e t u p

The catalytic degradation of polyethylene was carried out in a stirred batch reactor provided with a helicoidal stirrer (120 r.p.m.), as it is shown in figure 1. The experiments were carried out at three temperatures (380, 400 and 420~ and different reaction times (0380 minutes) under nitrogen flow, the volatile products being collected and separated into gases and liquids at 0~ Both phases were analysed by GC (Varian 3800). The liquid products and the solid residue remaining in the reactor were characterised through 1H NMR measurements (Varian 300 MHz).

I" _.,_.q

~l;~-:;~

-

CONTROLPANEL !~..,

~176 "

' ~---'-'-'-'-'-~"~-------~/ ~ - ~

/

"

' '

\ \ /

N2

\\

Fig. 1. Schematic diagram of the experimental reaction system.

/

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1591

3. RESULTS AND DISCUSSION The products obtained from the thermal and catalytic cracking of low-density polyethylene were grouped as gases (C1-C5), liquids (C6 and higher hydrocarbons retained in the cold trap) and solid (the residue inside the reactor). Table 1 shows the results obtained in both thermal and catalytic experiments. Thermal cracking at 400~ shows little activity, whereas at 4200C a decrease in the solid yield with the reaction time is observed. On the other hand, an improvement of the LDPE cracking takes place when a solid acid, as ultrastable Y zeolite or MCM-41 are used as catalysts. The latter shows higher activity than USY zeolite and amorphous SIO2-A1203. Significant differences in the product distribution are also observed, since MCM-41 and in less extension amorphous SIO2-A1203 lead mainly to liquid hydrocarbons, whereas a high proportion of gases are obtained over USY. Table 1 Results obtained in thermal and catalytic cracking. Catalyst

T (~

t

(min)

Solid Yield (%)

Liquid Yield (%)

Gas Yield ( % )

97.5

0.1

2.4

--

400

20

--

400

120

92.1

3.5

4.4

--

420

0

97.7

0.6

1.6

--

420

20

90.0

7.2

2.8

--

420

90

61.0

35.8

3.2

SIO2-A1203

380

30

98.6

--

1.4

SIO2-A1203

400

30

96.1

--

3.9

SIO2-A1203

420

30

73.0

22.1

5.0

USY

380

30

94.4

3.7

4.9

USY

400

30

87.9

6.6

5.5

USY

420

30

72.3

19.5

8.2

MCM-41

380

30

83.7

12.5

3.8

MCM-41

400

30

77.2

20.8

2.0

MCM-41

420

30

42.3

45.9

11.8

Figure 2.a shows the GC analysis of the liquid product obtained in the LDPE thermal degradation (T=420~ t=90 min), in which pairs of a-olefins/n-paraffins are clearly shown, coming directly from the cracking of C-C bonds in the polyolefinic chains. On the other hand, when MCM-41 is used as catalyst, the liquid fractions are rich in hydrocarbons within the range C6 to C~2 (Figure 2.b), with a much broader product distribution related to the extension of aromatization and isomerization reactions over this catalyst. ~H NMR of the liquid product indicates a higher degree of aromatization when MCM-41 is used as catalyst in comparison to thermal and catalytic cracking over amorphous SIO2-A1203. Likewise, the liquid fraction obtained in the LDPE degradation over USY zeolite presents a high content of aromatics and branched isomers, indicating that aromatization and isomerization reactions are also favoured by the use of this catalyst.

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1592

n-

)araffin

-o tei in /, \ t 1 I

b)

i

C6C8 ClO Cl2 C14C16 C18C20 J

Cl0Cl2 C14C16C18C20 l

Jl

1

I

"~""/"~

I

2 4 6 8 10 12 4 i 6 i ' 8 20 2 4 6 8 10 12 i4 16 18 20 22 Fig. 2. GC analysis of liquid product obtained in: a) LDPE thermal degradation (T=420~ t=90 min); b) LDPE catalytic cracking over MCM-41 (T=400~ t=30 min). Figures 3 and 4 show the IH NMR spectra corresponding to the solid residue of the thermal degradation and catalytic cracking over MCM-41, respectively. They exhibit two main peaks at 0.9 and 1.3 ppm, which correspond to protons on methyl and methylene groups. The spectra also show significant differences in the area of the olefinic protons (4-6 ppm), not only in regards to the decrease in the olefinic character when MCM-41 is used as catalyst but in the different type of olefins present in the solid. Likewise, an increase in the region of aromatic protons at around 7 ppm is observed in the spectrum of the catalytically degraded solid compared to that obtained trough thermal cracking. A correlation between the olefinic protons and the cracking extension must exist, since the cleavage of the C-C bonds leads to the formation of o~-olefins and n-paraffins as primary products.

Solvent (C12CDCDC12)

B.0

7.5

7.0

6.5

6.0

/

5.5

5.0

4.5_

~'[~''''~'~'`~'~'~~'~''~'~''~'''~'~'~''~'~'~'~''~'

8

7

6

5

4

ppm

",',,~,,

3

2

1

0

Fig. 3.1H NMR spectra of the solid residue obtained by LDPE thermal cracking (T=400~ t=120 min).

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

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Solvent (C12CDCDC12)

. . . . . . . . .

8.0

8

......

,

..........

,

7.5

'"'1

7

. . . . . . . . . .

7.0

..........

I . . . . . . . . . .

6.5

I .....

' ....

6

, . . . . . . . . . .

6.0

I " "

.....

5

, . . . . . . . . . .

5.5

, . . . . . . . . . .

5.0

'1 ..........

, . . . . . . . . . .

4.5

I ..........

4

I

4.0 j

I ..........

3

2

I ........

1

''

0

ppm

Fig. 4. 1H NMR spectra of the solid residue obtained by LDPE catalytic cracking over MCM-41 (T=400~ t=30 min). Assuming that the relative number of protons from methyl groups can be considered a measurement of the cracking extension, figure 5 shows the results from the ~H NMR analysis of the solids obtained in different cracking conditions. The residues from thermal treatment show an asymptotic trend, with a pronounced increase in the olefinic character of the solid during the initial stages of the cracking process. Similar results to those shown by thermal degradation are obtained when SIO2-A1203 or USY are used as catalysts. 0.9

9Thermal (90 and 120 min ) tx Zeolite USY (T=380,400,420"C at 30 min) 9MCM-41 (T=380,400,420"C at 30 min) o Silica -alumina (T=380,400,420"C at 30 min oThermal (0 and 5 min)

0.8 0.7

o 0.6

~-~"

o =r 0.5

/

.-= 0.4

.

_e

/

0.2

a;

0.1

1"3

,,I

/

'/~ I

1

,~'~"

~"

0/."

i

lI

o 0.3

~s

/

I ~I

I

o I |

0

,

,

2

4

,

6

Methyl protons

,

8

,

10

12

Fig. 5. Olefinic protons vs. methyl protons in the solid residue into the reactor. The only solid products showing a different trend are those coming from the catalytic cracking over MCM-41. In this case, these products present a clear decrease in olefinic character in regards to the ones coming from the thermal cracking or those obtained using the other types of acid catalysts in adequate combination of acid strength, high surface area and uniform pore size allows MCM-41 to promote hydrogen transfer reactions among large hydrocarbon molecules. The hydrogen generated in the aromatization and dehydrocyclization reactions of olefins is consumed by bulky radicals or carbocation intermediates, leading to a more paraffinic residue.

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Table 2 summarises the results obtained in a serie of experiments carried out over MCM-41 at 380~ at different reaction times. As the solid yield decreases, a certain increase in the aromatic character is observed, whereas the contents in olefinic protons does not seem to change significantly in the solid product. Regarding the nature of the liquid product the higher content of olefins confirms that hydrogen transfer reaction affect mainly to bulky radical and carbocations species. These results suggest that the aromatization activity of MCM-41 does not show a decline at least for the reaction times tested in this work. Table 2 Results obtained in catalytic cracking over MCM-41 at 380~ Solid Liquid Yield ~HNMR Yield ~HNMR Catalyst T (~ t (min) (%) % olef'mic % aromatic (%) % olef'lnic % aromatic MCM-41 380 30 83.7 0.106 0.012 12.5 4.25 0.16

Gas Yield (%) 3.8

MCM-41

380

60

82.1

0.094

0.033

16.6

5.04

0.16

1.2

MCM-41

380

120

67.6

0.087

0.042

24.3

4.92

0.24

8.2

4. CONCLUSIONS Thermal degradation follows mainly a random scission mechanism with little contribution of isomerization and aromatization reactions. Catalytic cracking over SIO2-A1203 shows also products coming from random scission but in this case isomerization reactions are present. On the contrary, USY zeolite promotes an end-chain cracking mechanism with pronounced isomerization and aromatization. The high pore size of MCM-41 catalyst leads to a random cracking mechanism, promoting hydrogen transfer reactions between high molecular weight molecules. The results show a relationship between the reaction mechanism and the catalyst properties. Thus, USY zeolite is a microporous catalyst which promotes endchain scission reactions, whereas LDPE cracking over mesoporous materials, MCM-41 and SiO2-AlzO3 , proceeds mainly trough a random cleavage of C-C bonds. However, the high surface area of MCM-41 and its uniformity of pore sizes favour the extension of hydrogen transfer reactions between bulky molecules. ACKNOWLEDGEMENT Financial support from the Comunidad Autrnoma de Madrid (Project n~ 07M/0420/1997) is gratefully acknowledged.

REFERENCES 1. 2. 3. 4. 5. 6.

R.J. Rowalt. Chemtech., 23, (1993), 56-60. R.C. Mordi; R. Fields; J. Dwyer. J. Chem. Soc., Chem. Commun., (1992) 374. Md A. Uddin; K. Koizumi; K. Murata; Y. Sakata. Polym Deg. Stab., 56 (1997) 37. Y. Uemichi; Y. Makino; T. Kanakuza. J. Anal. Appl. Pyrol., 14 (1989) 331. J. Aguado; D.P. Serrano; M.D. Romero; J.M. Escola. Chem Commun., (1996) 725. J. Aguado; J.L. Sotelo; D.P. Serrano; J.A. Calles; J.M. Escola. Energy Fuels, 11 (1997) 1225. 7. J. Aguado; D.P. Serrano; J.M. Escola. Microporous Mesoporous Mater., 34 (1) (2000) 43.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Catalytic conversion of tars in biomass gasification fuel gases with nickelactivated ceramic filters D.J. Draelants, H.-B. Zhao and G.V. Baron Department of Chemical Engineering, Vrije Universiteit Brussel Pleinlaan 2, B-1050 Brussel, Belgium, Phone: +32-2-6293262, Fax: +32-2-6293248 E-mail: [email protected], [email protected] The problem of efficient, reliable and environmentally sound tar and particle removal is common to most applications of biomass gasification technology. In this work, a novel catalytic reactor for simultaneous tar and particle removal is introduced. It consists of a ceramic candle filter, which contains a nickel-based tar cracking catalyst in the support body. This concept simplifies the entire gas cleaning process with a potential reduction in investment costs. The catalytic feasibility of the concept was screened on a laboratory-scale with a sulphur- and dust-free synthetic biomass gas and with benzene and naphthalene as tar model compounds. The catalytic filter was represented by a disc-shaped ix-alumina cartridge, activated with nickel through a deposition-precipitation technique with urea. This paper gives an overview of the screening experiments that included variation of reaction temperature, flow rate and nickel loading. Above 850~ a high performance for converting benzene and naphthalene was found using gas velocities typically encountered in candle filtration. 1. INTRODUCTION Tars (benzene, naphthalene and heavy aromatics) are one of the undesirable co-products in production of fuel gas for power production by biomass gasification, because they can cause fouling of equipment and are an environmental hazard, if released [1]. Hence, in many applications the tars must be removed before the fuel gas can be utilised. Preferably, this gas cleaning is performed at temperatures close to the ones at the gasifier exit (700-900~ since this may lead to a higher energy efficiency but more importantly to simplified processes and lower cost, avoiding several high temperature heat exchangers. In addition, this cleaning needs to be performed with the smallest possible extra investment so as to make its use economically viable, especially for small-scale units. The previously proposed or existing solutions for gas cleaning, like wet scrubbing methods, do not fulfil these conditions. The use of catalysts to eliminate tars in biomass fuel gas is a good alternative to wet scrubbing because the tars can be directly converted to useful components of the fuel gas (H2 and CO), avoiding a loss of the thermal value of biomass fuel gas. Two types of catalysts (naturally available dolomites and steam reforming nickel-based catalysts) have been used, usually in a packed bed reactor at 800-900~ with the commercial nickel-based catalysts more active than calcined dolomites [2]. Nickel-based catalysts can however be deactivated by coking when the amount of the tars is high and by sulphur compounds in the fuel gas like H2S [3]. It is recognised that the catalytic bed can work under severe internal diffusion

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limitations, which prevents the efficient use of the catalyst [4]. In general, the reaction schemes of catalytic tar conversion are based on gasification of adsorbed hydrocarbon species on the catalyst surface by H20 and/or CO2 [2]. Another important fuel gas cleaning step involves the removal of the particles, causing plugging and abrasion of downstream equipment. At present, this is performed with commercially available ceramic candle filters for hot gas cleaning, which are usually made of silicon carbide or aluminium oxide. Mostly, those filters are formed of two layers, i.e., a thin filtration membrane supported on a mechanically stable large-pore support body [ 1]. In this work, a novel catalytic reactor for tar removal is studied. It consists of a classic ceramic candle filter, but which contains a nickel-based tar cracking catalyst in the support body. This concept simplifies the entire gas cleaning process by integration of the removal of particles and tars in one unit, with a potential reduction in investment costs [5]. In addition, due to the intrinsic pore structure of the filter, the gas flows now through the catalytic active pores and internal diffusion is no longer a limiting factor for the tar conversion as in the conventional packed bed reactors. Figure 1 gives a schematic representation of such a catalytic candle filter. Tar components in gaseous form flow through the catalytic filter (1 m long, 1 cm wall thickness), and are converted in the interior of the filter support body. The particles are trapped on the outside thin filtering membrane where a dust cake is formed, which should not affect the catalytic activity, because the catalytic material is situated only in the interior of the porous support structure [6]. This project focuses on the catalytic performance of the catalytic filter, since this is the intrinsic new addition to the candle filter. It involves preparation chemistry for incorporation of nickel into lab-scale filter cartridges similar to the porous support body of a candle filter, determination of their tar cracking ability and modelling of the mass transfer. At present, a preparation route to incorporate pure nickel into the preformed filter substrates has been developed and some activated filter substrates were screened with major tar model components like benzene and naphthalene in a sulphur- and dust-free biomass fuel gas. This paper gives an overview of the results of this first batch of catalyst screening experiments. Dust and tar removed fuel gas

Raw fuel gas

m

m

Fig. 1. Schematic representation of a catalytic candle filter

Pore-wall catalytically modified pore

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2. EXPERIMENTAL

2.1. Catalyst preparation The porous alumina filter substrates used (Schumacher, Germany) were disc-shaped (1 cm thickness, 3 cm diameter) and consisted of non-porous ~-A1203 grains (100-350 ~tm). They had a mean pore radius of 26 ~tm, a pore volume of 0.1 ml/g and a BET specific surface area of 0.33 m2/g and were similar to the support structure of alumina candle filters. These substrates were catalytically activated with nickel using the precipitation-deposition method with urea. The substrates are vacuum impregnated with a solution containing appropriate amounts of urea and nickel nitrate. After the excess solution is drained off, the substrates are put in a closed glass vessel and placed in an oven at 90~ during a certain period for reaction, resulting in precipitation of the nickel precursor by the slow decomposition of urea in the pores of the disc [7]. After reaction, the disc was dried at 110~ and subsequently calcined at 450~ for 4 h in an air atmosphere to decompose the precipitated nickel precursor to nickel oxide. This technique allows us to deposit up to 2 wt% of nickel by one impregnation cycle. We have demonstrated that a fairly uniform distribution of the nickel precursor throughout the substrates can be obtained and that one impregnation cycle hardly changes the porosity of the filter cartridges. More details about the catalyst preparation can be found in another publication [8]. Before being used for reaction tests, the activated substrates were reduced in 10 to 50 v% H2 in N2 overnight at 900~

2.2. Reaction set-up A laboratory-scale reaction set-up has been constructed to perform catalyst screening tests, long-term tests, deactivation studies and reaction kinetic studies. The gas mixing zone allows to feed a representative dust-free synthetic biomass fuel gas (N2, HE, CO, CO2, H20 and CH4) to the reactor, with or without addition of impurities like NH3, HES and tar (benzene and naphthalene). The inlet gas is preheated till 150~ before and after introduction of water, benzene and naphthalene to prevent condensation. The total gas flow rate can be set as such that the superficial gas velocity in the reactor is comparable to the face velocity used in candle filtration (1 - 4 cm/s). The reactor consists of a horizontal, dense alumina tube (i.d. 30 mm, length 500 mm), to minimise catalytic wall effects. The catalytic filter substrate is fixed in the middle of the tube by means of an alumina powder cement. The reactor is heated in a tube oven. A differential pressure sensor measures the pressure drop across the reactor because an increase in pressure drop can be an indication for e.g. carbon deposition. The gas leaving the reactor remains heated till 150~ to prevent condensation and finally led either into a tar sampling device or into a by-pass line. In the latter, tar and water are removed from the gas by a cold trap (0~ and sulphuric acid before the gas is led to an online gas chromatograph (Varian 3400 GC with TCD) to determine the content of the main gas components (N2, HE, CO, CO2 and CH4). The tar sampling train is composed of two washing bottles in a bath of cooling liquid at -20~ The first bottle is empty to trap most of the water and naphthalene, while the second bottle contains dichloromethane as a solvent to absorb the benzene and other tars. After sampling, the bottles are rinsed with dichloromethane and the content of the tar compounds in the solvent is off-line analysed by gas chromatography (HP 6890 GC with FID). To monitor an immediate change in behaviour of the catalytic filter substrates during the tests, the outlet gas composition was qualitatively followed by an on-line mass spectrometer (Balzers QMG 420) for N2, HE, CO, CO2, H20, CH4 and benzene.

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2.3. Reaction experiments In this study, a typical sulphur- and dust-flee biomass feed gas contained 51 v% N2, 12 v% CO, 10 v% HE, 5 v% CH4, 11 v% CO2 and 11 v% H20, 10-30 g/Nm 3 benzene and 4.5 g ~ m 3 naphthalene. The screening tests involved variation of temperature, inlet flow rate and nickel loading. The temperature was varied from 900~ till 750~ in decreasing steps of 50~ Table 1 gives an overview of the inlet flow rates used, together with their respective space velocity (at normal conditions and based on the reaction volume) and superficial gas velocity (in the reactor tube at 900~ for the two nickel loadings studied. The gas velocity is similar to the face velocity typically used in candle filtration (1-4 cm/s). A gas velocity of 6 cm/s is maybe not realistic for filtration, but was selected to test if it was still possible to get high conversions with such a low contact time. The reaction data were obtained at steady state of the reactor, based on the MS analysis.

Table 1 Flows used during the screening experiments Nickel loading of substrate

inlet flow (Nml/min)

space velocity (h -1)

Superficial velocity (cm/s)

0.5 wt% Ni

195

1655

2

585

4965

6

245

2080

2.5

395

3350

4

590

5008

6

1 wt%Ni

3. RESULTS AND DISCUSSION Figure 2 gives an overview of the conversion of the tar model compounds benzene and naphthalene for the different flow rates, temperatures and nickel loadings studied. 3.1. Effect of gas flow rate To limit the pressure drop across a candle filter, face velocities between 1 and 4 cm/s are normally used. Consequently, the contact time with the catalyst in a catalytic candle filter is imposed by the filtration step. It was not known if this limitation was compatible with the catalytic reactions that have to take place on the low surface catalyst. Our experiments show that, on the condition that the temperature is high enough (850~ the range of tested filtration gas velocities (2-4 cm/s) is adequately to obtain very high conversions of benzene and naphthalene. This indicates a high intrinsic catalytic activity for tar elimination. Mass balance calculations with the inlet and outlet gas compositions, confirmed that benzene and naphthalene were converted to gaseous components like H2 and CO. Below a certain temperature, the conversions decrease as the flow increases, which means that the reaction is then limited by contact time. Nevertheless, this decrease in conversion remains limited in comparison with the variation of the flow rate itself. As already mentioned, a velocity of 6 cm/s (590 Nml/min) is not so realistic for candle filters, but the naphthalene conversion remains very high above 850~ with this high flow rate. However, this velocity is too high to obtain full conversion of benzene (more stable molecule than naphthalene), even at 900 ~

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0.5 wt% nickel

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750

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Reaction temperature (~

Fig. 2. Conversions of the tar model compounds benzene and naphthalene 3.2. Effect of temperature The benzene conversion is strongly affected by the temperature, and this for all tested flow rates. The experiments show that at least a temperature around 850~ is necessary to obtain a high activity for benzene conversion with typical filtration velocities. Further improving the catalyst can of course decrease this temperature. The working temperature of the catalytic filter is a very important parameter since a lower working temperature certainly improves the life time of the candle filter itself and is beneficial to oppose sintering of the nickel catalyst. It also allows operation of the gasifier at lower temperature such as with lower grade biomass. The naphthalene conversion is less affected by temperature and it seems that 800~ is sufficient for the typical filtration velocities used. 3.3. Effect of nickel loading There seems to be no important difference in activity between the two tested substrates. The conversions with the 1 wt% Ni substrate are only slightly higher compared with the 0.5 wt% Ni substrate, despite of the fact that the nickel loading was doubled. Naturally, the total nickel loading doesn't give any information about the nickel dispersion, which indicates how much of the deposited nickel atoms are really available at the surface for the catalytic reactions. In our case, increasing the loading did not seem to really improve the dispersion and hence the activity. This indicates that the original available surface for nickel deposition

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(i.e. the outer surface of the non-porous a-alumina grains) is nearly completely covered with nickel for a loading of only 0.5 wt%. The fact that a low loading of nickel is sufficient simplifies the catalytic filter preparation since multiple impregnation steps with the urea method to increase the loading do not need to be considered. The commercial nickel on (xalumina catalysts, tested for tar conversion in packed beds, had a nickel loading of about 10 20 wt% [2, 4]. In practice, they consist of porous mm-sized particles. Consequently, almost all the nickel is situated inside these particles but can only be reached by the gas by diffusion into the small pores of the particles. In practice, this leads to a low efficient use of the catalyst due to internal diffusion limitations. In a catalytic filter, all the nickel is situated on the outer surface of a-alumina grains of a few 100 ~tm diameter and the gas can instantly react with the nickel when it flows through the filter. 4. CONCLUSIONS AND FUTURE WORK In view of the application background of this project, the first catalyst screening tests have positively demonstrated the principle of activating candle filters with nickel to eliminate tars from a sulphur-free biomass fuel gas. Above 850~ a high performance for converting benzene and naphthalene was found with gas velocities typically encountered in candle filtration. These results are encouraging to further develop this system and continue more screening. However, biomass gasification gas may contain 50-200 ppmv HaS, which is a known poison for nickel catalysts. Additives like Ca and Mg may increase the sulphur resistance of the catalyst and its long-term stability. This implies an adjustment of the catalyst preparation procedure and this will be implemented in the near future in collaboration with a catalyst manufacturer. The operation time was too short to extrapolate to long term activity, but some experiments with a larger time-on-stream will be performed. In addition, future experiments may include the study of the conversion of the NOx-precursor ammonia in the biomass fuel gas, which is present in concentrations of a few thousand ppmv and can also be decomposed with nickel. REFERENCES 1. E. Kurkela, "Formation and removal of biomass-derived contaminams in fluidized-bed gasification processes", VTT Publications, Espoo, 1996. 2. P. Simell, "Catalytic hot gas cleaning of gasification gas", VTT Publications, Espoo, 1997. 3. J. Hepola and P. Simell, Applied Catalysis B: Environmental, 14 (1997) 287. 4. I. Narv~ez, J. Corella and A. Orio, Ind. Eng. Chem. Res., 36 (1997) 317. 5. G. Saracco, S. Specchia and V. Specchia, Chem. Eng. Sci., 51 (1996) 5289. 6. K. Hiibner, A. Pape and E.A. Weber, "High temperature gas cleaning", E. Schmidt et al. (Eds.), Institut Ftir Mechanische Verfahrenstechnik und Mechanik, Karlsruhe, (1996) 267. 7. L.M. Knijff, P.H. Bolt, R. van Yperen, A.J. van Dillen and J.W. Geus, Stud. Sur. Sci. Catal., 63 (1991) 165. 8. H. Zhao, D.J. Draelants, and G.V. Baron, Catalysis Today, in press.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

An e n v i r o n m e n t a l friendly c a t a l y s t for t h e high t e m p e r a t u r e shift r e a c t i o n G. C. Araujo and M. C. Rangel Instituto de Quimica, Universidade Federal da Bahia, C a m p u s UniversitY_rio de Ondina, Federa~gto. 40170-280 Salvador, Bahia, Brazil The need for environmental protection has demanded the searching nontoxic catalysts easily handled and discarded. This work deals with the use of a l u m i n u m as a substitute of chromium, in iron- and copper-based catalysts for the high temperature shift reaction. Catalysts were prepared in the commercial form (hematite) and was evaluated in more severe operational conditions t h a n the u s u a l industrial one (e.g. steam to carbon ratios, R, lower than 0.6). It was found that a l u m i n u m can replace c h r o m i u m in these catalysts leading to better catalytic properties as compared to chromium. The catalyst h a s the advantage of being non toxic and can work in low a m o u n t s of steam.

I. INTRODUCTION The production of hydrogen from the natural gas or n a p h t h a feedstock is usually increased by m e a n s of the water gas shift reaction (WGSR) [1]: CO(g) + H20(g} ~ CO2 + H20(~) AH= - 4 1 . 1 kJ.mo1-1 which is also an important stage in the commercial production of hydrogen. This reaction is a reversible and exothermic one and t h u s is favored by low temperatures and excess of steam. However, it requires high t e m p e r a t u r e s to achieve rates high enough for industrial applications. Therefore, the WGSR is often carried out in two steps, the first being performed in the range of 320-450oC (named high temperature shift, HTS). In the second stage (known as low temperature shift, LTS), carbon monoxide is removed from the feed stream in thermodynamically favorable conditions in the range of 200-250~ [ll. The HTS stage is often carried out over catalysts of chromium-doped iron oxides, available as hematite. The catalyst is reduced in situ to produce magnetite which is found to be the active phase. In a m m o n i a plants, the reduction is performed with a gaseous mixture of carbon monoxide and dioxide, hydrogen and nitrogen. This reaction is highly exothermic and should be carefully controlled to prevent damage to the reactor or to the catalyst. The production of metallic iron should be particularly avoided since it may catalyze the hydrocarbon formation [1,2]. In order to assure the magnetite stability in industrial processes, large a m o u n t s of steam are used. However, this procedure increases the operational costs and t h u s new

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systems that do not generate metallic iron, even at low contents of steam, are d e m a n d e d [3]. In spite of the chromium-doped hematite catalysts have shown high stability in performance, they have been recently modified by the addition of small a m o u n t s of copper, resulting in even more active a n d selective catalysts. However, because of environmental restrictions concerning the discarding of c h r o m i u m compounds, the search for non-toxic catalysts which could be easily handled and discarded is m u c h needed. With this goal in mind, this work deals with the replacement of c h r o m i u m by a l u m i n u m in HTS catalysts. In order to save the energy related to the steam consumption, the catalysts were also evaluated in more severe operational conditions t h a n the industrial one, e.g. lower s t e a m to carbon ratios. 2. EXPERIMENTAL

The reagents used were analytical grade. The catalysts were prepared by copreciptation techniques at room temperature, followed by heating at 500~ for 2h u n d e r nitrogen flow (100 ml. min-1). Four samples were prepared: (i) with a l u m i n u m and copper (HAC sample); (ii)with only a l u m i n u m (HA sample); (iii) with only copper (HC sample) and (iv)without any dopant (H sample). For all samples an iron to the dopant molar ratio of 10 was used. The a l u m i n u m and copper-based catalyst was prepared by adding aqueous solutions of Fe(NO3)3.9H20(1N) and AI(NO3)a.9H20(0.1N) and a concentrated (25% w/w) a q u e o u s solution of a m m o n i u m hydroxide to a beaker with water, u n d e r stirring. The final pH was adjusted to 11 and the system was kept u n d e r stirring for additional 30 min. The sol produced was centrifuged {2000 rpm, 5 min) followed by a water rinsing to remove the nitrate ions from the starting material. After a second centrifugation step, the gel was impregnated with an a q u e o u s solution of Cu(NO3)2.3H20 (0.06N) for 24h u n d e r stirring, centrifuged again and dried in an oven at 120~ The same procedure were used to prepare the other samples. For the catalysts without copper, the gel was kept in pure water for 24h, u n d e r stirring, in order to simulate the same experimental conditions used to prepare the other samples. The qualitative analysis of nitrate was performed by adding about 1 ml of concentrated sulfuric acid to 10 ml of the s o b r e n a d a n t after centrifugation. The formation of [Fe(NO)I 2§ was detected by a brown ring [4]. The absence of nitrate in the solid was confirmed by infrared spectroscopy in the range of 4000-650 cm -1 using a Shimadzu model IR-430 spectrometer and KI discs. The metal contents were determined by inductively coupled p l a s m a atomic emission by using an model Arl 3410 equipment. X-ray diffractograms were recorded at room temperature with a Shimadzu model XD3A i n s t r u m e n t using Cu Kcz radiation generated at 30 kV and 20 iliA. The surface area (BET method) was m e a s u r e d in a Micromeritics model TPD/TPO 2900 equipment on samples previously heated u n d e r nitrogen (150~ 2h). The t e m p e r a t u r e

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p r o g r a m m e d reduction (TPR)was performed in the s a m e equipment, using a 5% H2/N2 mixture. The copper area was m e a s u r e d in a CG-2001 e q u i p m e n t using the N20 pulse technique [5,6]. X-rays microanalysis were performed in a Noran microprobe coupled to a model JSM-T300 microscope operating at 20-30KV. The catalyst performance was evaluated using 0.2 cm 3 of powder within 50 and + 325 m e s h size and a fixed bed microreactor consisting of a stainless tube, providing there is no diffusion effect. All experiments were carried out u n d e r isothermal condition (370~ and at atmospheric pressure, employing a gas mixture with composition a r o u n d 10% CO, 10 % CO2, 60% H2 and 20% N2. It was used several steam to gas molar ratios: 0.2, 0.4 and 0.6. The g a s e o u s effluent was analyzed by on line gas chromatography, using a CG-35 i n s t r u m e n t . A commercial catalyst, based on chromium, copper and iron oxides, was used to compare the performance of the samples prepared in this work. In order to determine the role of copper in these catalysts, a m e c h a n i c a l mixture of the sample with a l u m i n u m (HA sample) and copper oxide was also evaluated. In this case, the catalyst tests were r u n at 200~ because of the high tendency of copper of sintering at 370oC [1]. After each experiment, the Fe(II) content in the catalysts was determined to follow the iron reduction u n d e r reaction atmosphere. Samples were dissolved in concentrated chloridric acid, u n d e r carbon dioxide a t m o s p h e r e and then titrated with p o t a s s i u m dichromate [7].

3. R E S U L T S AND D I S C U S S I O N

As shown in Figure 1, hematite was detected in the flesh catalysts while magnetite was found in the used catalysts; a l u m i n u m t e n d s to impair crystallization w h e r e a s copper tends to favor it. No other p h a s e besides hematite and magnetite was detected. The presence of either a l u m i n u m or the two d o p a n t s increased the surface a r e a of the solids (Table 1), which was not affected by doping the solid with copper alone. For the used catalysts, a l u m i n u m increased the surface area while copper decreased it. The characteristics of the spent catalysts did not show any significant dependence on the s t e a m to gas molar ratio (R) u s e d in the test reaction. The catalyst reduction strongly decreased the surface area, showing that the phase change is followed by a coalescence of particles a n d / o r porous. A l u m i n u m increased the copper areas showing t h a t this metal prevents copper sintering. The a l u m i n u m presence on the surface was confirmed by X-ray microanalysis. The TPR curves of hematite (Figure 2) showed one peak at 510oC, due to magnetite formation and another a r o u n d 770oC, attributed to the formation of metallic iron [8]. In the a l u m i n u m - d o p e d sample, the first peak was shifted to lower t e m p e r a t u r e s (400~ whereas the higher t e m p e r a t u r e peak was not affected; this behavior shows that this metal eases the formation of the active p h a s e but does not affect its stability. The copper-doped catalyst showed a TPR curve with a peak a r o u n d 210 ~ assigned to metallic

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copper [9] and other peaks a r o u n d 230 and 600oC; these TPR peaks show that copper favors the active phase formation and destruction. The a l u m i n u m and copper-doped solid showed a curve with the characteristic peak due to magnetite formation displaced to 300oC and the peak related to metallic iron at 770 oC. It m e a n s that the synergetic effect of the d o p a n t s is HACR

HAC

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80

(b)

Figure 1. X-ray diffractograms of (a) fresh and (b) used catalysts. H= hematite; A= a l u m i n u m and C = copper Table 1 Surface area {Sg) and copper area of the samples Sg (m2.g-1) Sg (m2.g -I) Sample (after reaction) (before reaction) R=0.6 R=0.4 R=0.2 8 9 9 27 H 24 25 28 59 HA 6 4 6 26 HC 17 22 24 86 HAC R = s t e a m / p r o c e s s gas (molar)

Copper area (fresh catalysts) - - - -

_

_

39 44

to ease magnetite formation but not to affect the production of metallic iron. The commercial catalyst showed a TPR curve with peaks a r o u n d 270, 340 and 750~ showing that c h r o m i u m is less effective in prevent the active phase formation and destruction.

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All catalysts were active to HTS reaction, as s h o w n in Table 2. A l u m i n u m leads to a slight increase in activity, which is due to a textural effect. On the other h a n d , copper increased both the activity a n d the activity per area, showing t h a t it acts as a s t r u c t u r a l promoter. The copper a r e a values, as well as the TPR results, suggest t h a t the higher activity of the copper promoted catalyst is due to an increase of the n u m b e r of active sites; it is well k n o w n t h a t copper is also active in the HTS reaction [1,2]. However, the m e c h a n i c a l mixture of the sample with a l u m i n u m {HA) a n d copper oxide (3x10 -4 mol.g-lh -~) showed a lower activity t h a n the HAC sample {7x10 -4 mol. g-~.h -I) at 200oC, suggesting t h a t there is probably an electronic effect of copper. Commercial

~

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Figure 2. TPR catalysts. H = hematite; A = a l u m i n u m a n d C = copper The e x p e r i m e n t s carried out at lower s t e a m to gas ratio showed that the catalysts can work at R=0.4 without any d a m a g e to both activity or selectivity to c a r b o n dioxide. At R=0.2, however, the activity a n d selectivity strongly d e c r e a s e d with the a m o u n t the steam. The analysis of the used catalysts showed t h a t this behavior was likely due to the lower Fe(II)/Fe(III) ratio, which m e a n s a lower a m o u n t of the active sites. Therefore, one can a s s u m e t h a t u n d e r lower R larger a m o u n t s of metallic iron w a s produced a n d h y d r o c a r b o n formation was catalyzed. The catalyst with both d o p a n t s showed higher activity t h a n a c h r o m i u m a n d copper-doped commercial catalyst (25x10 -4 mol.g-l.h-1). This sample p r o d u c e s the active p h a s e more easily t h a n the other catalysts, shows resistance to a f u r t h e r magnetite reduction a n d can work at severe conditions, t h a t is, low s t e a m to process gas ratio.

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Table 2 Catalytic activity (a) and selectivity selectivil (S) of the samples a/Sgxl0 s Sample axl04 Selectivity (%) (mol.g-l.h -1) (mol.m-2. h-~) R=0.6 R=0.4 R=0.2 R=0.6 R=0.4 R=0.2 R=0.6 R=0.4 R=0.2 H 7 6 5 9 7 6 50 50 20 HA 9 8 6 4 3 2 60 20 10 HC 24 23 16 40 58 27 80 60 60 HAC 34 34 17 20 15 7 70 70 80 R = s t e a m to gas molar ratio 4. CONCLUSIONS

The presence of both a l u m i n u m and copper in h e m a t i t e - b a s e d catalysts to HTS reaction favors the formation and stability of the active phase. A l u m i n u m acts as textural promoter and copper acts as a s t r u c t u r a l one. The a l u m i n u m , copper-doped hematite catalyst is more active t h a n a commercial catalyst. Therefore, a l u m i n u m is a promising dopant to replace c h r o m i u m in HTS catalysts since it shows two important features: first, it is non toxic, in comparison to the high c h r o m i u m toxicity, and second it leads to better HTS catalyts as compared to the c h r o m i u m - d o p e d ones. ACKOWLEDGEMENT

The a u t h o r s

CNPq/PADCT.

thank

the

financial

support

from CNPq,

FINEP and

REFERENCES

1. H. Bohlbro, An Investigation on the Kinetics of the Conversion of Carbon Monoxide with Water Vapor over Iron Oxide based Catalysts, The Haldor Topsoe Laboratory, Vedback, 1960. 2. Newsome, D. S., Catal. Rev.- Sci. Eng., 21 (1980) 275. 3. M. V. Twigg, Catalyst Handbook, Wolfe Scientific Books, London, 1970. 4. F. Fergl, V. Anger and R. E. Oesper, Inorganic Analysis, Elsevier, Amsterdam, 1972. 5. C. R. F. Lund, J. J. Schorfheide and J. A. Dumesic, J. Catal., 57 (1979) 103. 6. M. J. Luys, P. H. van Oeffelt, P. Pieters and R. Terven, Catalysis Today, 10 ( 1991) 283. 7. A. I. Vogel, Quantitative Inorganic Analysis, Longman, London, 1961. 8. J. C. Gonzalez, M. G. GonzNez, M. A. Laborde and N. Moreno, Appl. Catal., 2 0 ( 1 9 8 6 ) 3 . 9. S. Pinna, T. Fantinel, G. Strukul, A. Benedetti, and N. Pernicone, Appl. Catal., 49(1997)341.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Direct catalytic conversion of chloromethane to higher hydrocarbons over various protonic and cationic zeolite catalysts as studied by in-situ FTIR and catalytic testing Denis JAUMAIN and Bao-Lian SU* Laboratoire de Chimie des Mat6riaux Inorganiques, ISIS, University of Namur, 61, rue de Bruxelles, B-5000 Namur, Belgium The direct catalytic conversion of chloromethane to higher hydrocarbons over a series of zeolites such as Y, Beta and ZSM-5 in protonic and cationic form has been studied by means of in-situ FTIR and catalytic testing. This study shows that the acido-basic and structural properties of the zeolites influence the conversion rate and the selectivity of the products. Both protonic and cationic zeolites are very active for the conversion of CH3C1. Hydrocarbons such as aromatics, alkanes and alkenes are formed. The IR observations indicate a production of HC1 by elimination from CH3CI during the first step of the conversion. In protonic zeolites, the Bronsted acid sites are active sites, whereas a cooperative role of counter-ions and the framework oxygen atoms is responsible for the conversion of CH3CI in cationic zeolites. 1. INTRODUCTION In 1985, Olah and his co-workers reported an interesting three-steps route for converting natural gas to higher olefins Ill. In this new process, methane is first converted to methyl halide by a selective monohalogenation over supported acid or platinum metal catalysts. In the second step, methyl halide is transformed to methanol by a catalytic hydrolysis reaction and finally to hydrocarbons by MTG process on HZSM-5 catalyst. However, our recent studies have shown that this process can be improved by an efficient direct conversion of methyl halide into a mixture of hydrocarbons using zeolites as catalysts t2'31. This process can be finally reduced to two steps. Furthermore, this direct conversion should present an interesting alternative for the treatment of the chlorinated organic compounds, which are often the industrial hazardous solvent wastes. Their catalytic destruction or conversion to useful hydrocarbons will be an interesting matter for environment protection. It has been shown that this direct conversion can be carried out both on acid zeolites like HZSM-5 and HBeta t2-41 and on cationic zeolites such as faujasites X and Y and ZSM-5 exchanged with mono- or divalent cations [4'51. However, the initial fragment of CH3C1 molecules on zeolite catalysts is not well known. The real roles of the alkali cations and zeolite framework oxygen atoms, as basic sites, in the conversion are not clear and the reaction mechanism remains still preliminary. The aim of this work is to elucidate the mechanism of this reaction from the initial fragments of CH3CI to the formation of the C-C bonds and to study the effect of acido-basicity and structure of zeolites on the activity of catalysts and on the formation of products. A series of zeolites with different structures and Si/AI ratios such as faujasite Y (Si/A1 = 2.3), Beta (Si/A1 = 17.5) in protonic and Na form and ZSM-5 (Si/AI = 21.0) in protonic, cationic (Na, K and Cs) and as-synthesized (Na,H) form were used.

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2. EXPERIMENTAL

In-Situ FTIR. Self-supported zeolite wafers (15 mg/cm 2) were first calcined in a flow of dry oxygen at 450~ for 6 h and then in vacuum for 4 h. The spectrum of zeolite phase alone was recorded as a reference using a Perkin-Elmer Fourier Transform Spectrometer Spectrum 2000. After cooling to room temperature, adsorption of known amounts of CH3CI was conducted on zeolite wafers. The conversion of chloromethane was then performed msitu at different reaction temperatures in a range of 100-400~ during 15 min. After reaction, the samples were cooled to stop the reaction and the IR spectra were then recorded. Catalytic testing. The catalytic activity and selectivity of zeolite catalysts were evaluated by using a conventional catalytic microreactor in a flow condition under atmospheric pressure. The samples were first calcined in a flow of dry oxygen at 450 ~ for 10h and then the oxygen atmosphere is replaced by He and the temperature of reactor is adjusted to the desired temperature. The CH3CI was diluted by He. CH3CI and He flows were fixed by mass flow controllers and mixed in a mixer. The products of the conversion were analysed using a gas phase chromatograph with a FID detector and a HP PLOT Q column. 3. RESULTS AND DISCUSSION 3.1. FTIR study On both protonic and cationic zeolites, at room temperature, no conversion of chloromethane is observed. However, with increasing reaction temperature, peaks around 1387, 1465 and 2930 cm 1, assigned to -CH2- vibrations and at 1647 cm -~, corresponding to the vibrations o f - C = C - are observed from 100~ The observation of these vibrations indicates clearly the formation of C-C bond and higher hydrocarbons. It is interesting to observe a general trend on cationic zeolites, that a broad feature in the range of 3100-3600 cm -l appears due to the interaction of hydroxyls with the olefins or aromatics formed from the conversion of chloromethane. The hydroxyls on these cationic zeolites are probably generated by the interaction of framework oxygen atoms with HCI as a product of the conversion. Attributions of all the vibrations observed are given in Table 1. Adsorption and conversion of CH3C1 on HY and NaY. Figures 1Aa and 1Ba report the IR absorbance spectra of the activated zeolites HY and NaY, respectively. On these two zeolites, a small peak at 3740 cm -1 is observable and corresponds to external silanols. On HY, two other peaks at 3638 and 3535 cm ~ are detected and attributed to the bridging Si-OH-A1 present in the large cages and small cages of the framework, respectively. The introduction of chloromethane at room temperature brings the appearance of four new peaks at 2965, 2865, 1445 and 1350 cm 1 corresponding respectively to -CH3 stretchings and bendings of CH3Cl (Figures lAb and 1Bb). The broad band centered at 3314 cm -1 on the Table 1 9Wavenumbers and assignments of lR Wavenumbers (cm -1) Assignments 3000 o(C-H) of =CH2 2975 o(C-H) of =CH2 2960 o(C-H)as of-CH3 2933 o(C-H)as of-CH22872 u(C-H)s of-CH3 1627 o(C=C) ofalkenes 1610 o(C=C) of alkenes

peaks of the products formed on the zeolites Wavenumbers(cm -1) Assignments 1535 (C=C) of coke 1504 (C-C) of aromatics 1465 /5(C-H) of-CH21450 ~i(C-H)as of-CH3 1387 (CH2) deformation 1375 (CH2) deformation 1348 /5(C-H)s of-CH3

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IR spectrum of I05 , 1105 B HY is due to a ,o~ I l "o .~ shitt of the framework -~, ~_ o I ~ *~ hydroxyl of the ",/ \ o~ o~ -"3-1 .~_' lF ~ ,~ t large cage (3638 ,:Z,,~ o,~,~'~-~ t . . & , ~ II ~ ?,,il -"-'--.-~ / - ~-I cm ~) because of ~ r-5,' ~ ~ o ~ ,,,:,_~\ I h - . . . ~ ~ Io ~ .,. ,I an interaction with the adsorbed CH3CI molecules. Atter heating HY for 15 minutes at 400~ t ~ peaks in the range of 2800-3000 cm 1 (Figure 1Ac) can be observed and attributed to-CH24000 3500 3000 2500 2000 4000 3500 3000 2500 20'00 15'00 asymmetric and Wavenumbers (cm1) Wavenumbers (cm"l) symmetric Fig. 1. Adsorption and conversion of CH3CI over: A: HY, B: NaY, stretching a: Activated, b: CH3CI at RT, c: Atier heating 15 min. at 400~ d: vibrations, which After 30 min. of desorption at RT. is confirmed by the presence of the corresponding bending vibration at 1466 cm -1, and =CH2 stretching. Furthermore, the presence of several broad bands in the range of 3100-3600 cm 1 indicates that olefins seem to interact with OH sites in the zeolites. In comparison with its proton form, NaY shows almost the same behaviour (Figure 1Bc). The peaks in the range 2800-3000 cm ~ suggest the existence of important quantity of unreacted CH3CI. However, the presence of a broad band centered at 3580 cm1 and the appearence of a new peak at 3650 cm -~, corresponding to OH groups, after desorption (Figure 1Bd), imply strongly the production of HCI molecules by elimination from CH3C1 during the conversion. These HCI molecules interacting with oxygen atoms of the framework should create new OH groups, which are normally not present on a fully cation-exchanged zeolite and whose interaction with the products of the conversion gives a broad band centered at 3580 cm -1. These observations bring important informations on the first step of the conversion. The desorption of species adsorbed on HY at room temperature (Figure 1Ad) results in an important decrease of the intensity of hydrocarbons peaks, while the OH peak at 3638 cm -~ is simultaneously restored. This indicates a quite weak adsorption strength. Adsorption and conversion of CH3C1 on HBeta and NaBeta. After pretreatment, only a sharp peak at 3745 cm ~ and a small band at 3674 cm~, corresponding to external silanol and extra-framework A1-OH respectively, are present in NaBeta (Figure 2Ba). For HBeta a supplementary peak at 3612, corresponding to the bridging framework OH groups is observed. (Figure 2Aa). With introduction of CH3CI at room temperature, -CH3 stretchings and bendings peaks appear (Figure 2Ab and 2Bb). As the silanol band of NaBeta decreases in intensity upon adsorption of CH3C1, a broad band centered at 3621 cm-1 is observed and belongs to the interaction of CH3CI with silanols. For HBeta, as in the case of NaBeta, a broad band at 3602

rs- ,l

t~.~N[~

TM

-, ,

~

~,..~,..~

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cm "~, due to the ,.-* tt3 e~ O O '~I"tt" ~t3 ee~ ~t~ 05 ~ 0 O en 9 ~ o O~ ~ - O~ A _ . 5 , ", ~' , o t~. . o ~ oe~n B ~,~ interaction between CH3C1 and silanols, can I Iv -~ 1~ A / ~-I~nP'-'-d /~-~ /x/v be observed. The interaction between CH3CI and the bridging framework hydroxyls gives another band at 3223 cm 4. After heating NaBeta at 400~ for 15 min., (Figure _ Iv,. [ 2Be) the peaks assigned to -CH3 4000 3500 3000 2500 2000 4000 3500 3000 2500 2000 1500 vibrations of Wavenumbers (cm-1) Wavenumbers (cm-1) chloromethane decrease and new Fig. 2. Adsorption and conversion of CH3CI over: A: HBeta, B: vibrations appear, NaBeta, a: Activated, b: CH3C1 at RT, c: Aider heating 15 min. at which are 400~ d: After 30 min. of desorption at RT. caracteristic of hydrocarbons such as aromatics (C-C 1504 cm-~), alkanes and alkenes (C=C 1627 cm -1, -CH3 2960 cm 1, -CH2- 2933, 1465, 1387 and 1375 cm-1). The OH region also presents some new bands at 3651, 3558, 3479 and 3219 cm 1. On the basis of previously reported results, they arise from the interaction of OH groups with 7t-bonds of alkenes or aromatics or with alkanes. This confirms the formation of higher hydrocarbons. The OH groups are normally not present in NaBeta, they should be generated by the presence of HCI which has been produced by dehydrohalogenation from CH3C1 during the conversion. HBeta zeolite shows an important reactivity. The peaks of the reactant disappear after heating this zeolite at 400~ for 15 min. (Figure 2Ac), and are replaced by peaks corresponding to alkanes, alkenes and aromatics. As in the case of NaBeta, broad bands at 3652, 3497 and 3203 cm -1 appear, due to the interaction of products with new OH groups formed by the presence of HC1 produced from CH3C1. Desorption of species adsorbed on these two zeolites at room temperature (Figures 2Ad and 2Bd) does not result in a significant modification, implying that the formed products adsorb strongly on HBeta and NaBeta compared to CH3C1, since a same desorption can remove all the CH3C1 from this zeolite. Adsorption on HBeta and NaBeta is also stronger than that on HY and NaY. Adsorption and conversion of CH3CI on HZSM-5 and NaZSM-5. The IR absorbance spectra of the activated HZSM-5 and NaZSM-5 are shown in figures 3Aa and 3Ba respectively. On both zeolites, a small peak at 3747 cm -1 is observable and corresponds to external silanols. On HZSM-5, a peak at 3612 cm ~ is detected and is attributed to Si-OH-AI. The small peak at 3724 cm~ and the broad band in the range of 3745-2900 cm 4 are caused by isolated internal silanols and the hydrogen bonds formed by the internal silanols respectively. The introduction of CH3CI at room temperature brings the same-CH3 stretching and bending vibration bands (Figures 3Ab and 3Bb) as in Y and Beta zeolites. The broad band at 3203 cm 1 in the 1R spectrum of HZSM-5 is the framework hydroxyl (3612 cm 1) shifted ,

1

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,

~

,

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,

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,

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,

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~

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,

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l

toward lower B r0.5 A 0.5 ~,~ wavenumber e,,l ~ .-. ~,I because of its t~ O ' ~ " ' t',,I e~ ~..~ ~!" interaction with , . ,o ~ ~ I1__~. ,,, . ~ J/i CH3CI molecules. On NaZSM-5, the same kind of _ I1 ~ --~ interaction t'.. ~.O e ~ ,"X~ e ~ I I [--t".. , ,., ,...4 i between CH3CI and external silanols induces a shit~ of the silanols to 3653 , ~ _lkAY cm -1 After heating for 15 minutes at 400~ (Figures 3Ac and 4000 3500 3000 2500 2000 4000 3500 3000 2500 2000 1500 3Bc), vibrations Wavenumbers (cm -~) Wavenumbers (cm-]) caracteristic of Fig. 3. Adsorption and conversion of CH3CI o v e r A: HZSM-5, B hydrocarbons (NaZSM-5, a: Activated, b: CH3C1 at RT, c: After heating 15 min. at CH2- strecthings 400~ d: After 30 min. of desorption at RT. and bendings, C=C stretchings and deformation) are observed on the two zeolites. For HZSM-5, the broad band centered at 3205 cm ~ is so important that it hides other interaction bands. This indicates that a big quantity of CH3Cl remains unreacted. For NaZSM-5, several broad bands in the range of 3200-3700 cm -1 indicates that olefins seem to interact with OH groups in the zeolites. This shows that, as for NaY and NaBeta, a dehydro-halogenation occurs on NaZSM-5 during the conversion and that new OH groups are created. The desorption of species adsorbed on HZSM-5 at room temperature (Figures 3Ad and 3Bd) results in an important decrease of the broad band, indicating the removal of unreacted CH3C1. While on both zeolites, hydrocarbons peaks are still observable, indicating that some products are strongly adsorbed on the framework, as in the case of the Beta structure.

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3.2. Catalytic testing Conversion rate and selectivity results are given in Table 2. The effect of reaction temperature on the catalytic conversion was first studied. It is found that both on protonic and cationic zeolites, the higher reaction temperature favors the conversion. At each temperature, for a given zeolite structure, protonic and cationic forms give the similar conversion rate of chloromethane. It is observed that in the studied temperature range, C2 to C6 hydrocarbons such as ethylene, propane and butane isomers are the major products both on protonic and cationic zeolites. Catalyst aging tests show that cationic zeolites and the large pore zeolites are deactivated more quickly than their protonic forms and the zeolites containing only medium pores. However, the selectivity of the products seems not to be affected at least not significantly by aging, zeolite form and reaction temperature. Y zeolite, whatever it is in protonic or Na form, gives lower conversion rate and higher deactivation. Furthermore, an important quantity of undesired methane is produced. The large pore Beta zeolite favors C4 products, principally isobutane. While the medium pore

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ZSM-5 seems to favor lighter products such as ethylene and propane. Both protonic and Na forms show good conversion rate, but the K and Cs forms (which are more basic since the framework is more negatively charged) give lower activity. This is probably due to the size and Lewis acidity of the counter ion, which interact more weakly with the chlorine atom during the conversion. This interaction should be more efficient with proton and sodium. For all the zeolites, ethyl chloride is found, this is because of a reaction between ethylene, one of the main products, and hydrogen chloride, produced by elimination from CH3CI as observed by m-situ FTIR, or between a CH3CI and methyl group as intermediate of reaction. Table 2 : Products distribution from chloromethane conversion over Y, Beta and ZSM-5 under protonic and cationic form (400~ WHSV = 0.76 h-l). Zeolite Total Products distribution (mol. %) conversion (mol. %) CH4 C2H4 C 3 H 8 C 4 H 8 iC4H10 nC4H10 C2HsCI HY 34.7 19.7 40.8 24.8 2.1 10.2 0.8 1.6 NaY 42.0 28.6 31.8 24.6 3.0 9.2 0.5 2.3 HBeta 45.4 15.9 31.6 16.5 1.7 29.1 3.7 1.5 NaBeta 54.4 4.3 24.1 26.5 2.8 36.4 3.6 2.3 HZSM-5 49.3 5.1 39.0 37.3 4.6 6.1 3.7 4.2 Na, HZSM-5 56.2 4.7 35.2 47.0 6.0 1.4 0.9 4.8 NaZSM-5 55.8 3.2 38.2 40.7 5.0 4.6 3.6 4.7 KZSM-5 26.6 6.7 13.2 61.7 13.4 0 0 5.0 CsZSM-5 20.6 2.3 5.7 3.4 88.6 0 0 0 4. CONCLUSION The present work shows that both protonic and cationic zeolites could be the efficient catalysts for the direct conversion of chloromethane to higher hydrocarbons such as ethylene, propane, butane isomers and aromatics. However, zeolites exchanged with Na cations are more active than the corresponding protonic form. The acido-basic and structural properties of zeolites infuence strongly the conversion and aging of catalysts. On the basis of the above results, a reaction mechanism implying an initial fragment of chloromethane by a dehydrohalogenation both on protonic and cationic zeolites is proposed. In protonic zeolites, the Bronsted acid sites are active sites, whereas a cooperative role of counter-ions and the framework oxygen atoms is responsible for the conversion of chloromethane in cationic zeolites. REFERENCES [ 1] G. A. Olah, B. Gupta, M. Farina, J. D. Felberg, J. Am. Chem. Soc., 107 (1985), 7097 [2] B. L. Su, D. Jaumain, K. Ngalula, M. Briend, Proceedings of 12th IZC, MRS (1999) 2681 [3] X. R. Xia, Y. L. Bi, T. H. Wu, Catalysis Lett., 33 (1995), 75 [4] B. L. Su and D. Jaumain, Book of extended abstracts, International Symposium on Zeolites and Microporous Crystals, ZMPC'97, p 198 [5] D. K. Murray, J. W. Chang, J. F. Haw, J. Am. Chem. Soc., 115 (1993) 4732

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Catalytic conversion of 2-chloropropane in oxidizing conditions: a FT-IR and flow reactor study. C. Pistarino a, F. Brichese a, E. Finocchio a, G. Romezzano a, R. Di Felice a, M. Baldi b and G. Busca a a Dipartimento di Ingegneria Chimica e di Processo, Universit/l di Genova, P.le J.F. Kennedy, I-16129 Genova, Italy. Fax: +39-010-3536028, e-mail: [email protected] b Dipartimento di Ingegneria Idraulica e Ambientale, Universit~ di Pavia, via Ferrata 1, 1-27100 Pavia, Italy. Fax +39-382-505589. The conversion of 2-chloropropane (2CP) in the presence of oxygen has been investigated over a number of oxide catalysts. Mn oxide and Mn containing oxides are deactivated as combustion catalysts by chlorine, but are active in converting 2CP into propene and HC1. VWTi SCR catalysts are also very active in converting 2CP into propene, and, if vanadium content is relatively high, they also catalyse the burning of the resulting propene into CO and CO2. Acid catalysts such as alumina, silica-alumina and ZSM5 zeolite also catalyse the dehydrochlorination of CP to propene. However, HZSM5 is rapidly deactivated by coking. On silica alumina, dehydrochlorination occurs selectively at low temperature. FT-IR data and preliminary kinetic studies allowed to propose a reaction mechanism where an irreversible and fast conversion of 2CP into 2-propoxides is a key reaction step. 1. INTRODUCTION Chlorinated hydrocarbons have a large number of industrial applications as chemical intermediates, solvents, monomers, pest control agents. Problems arise from their disposal and their abatement from waste gases: catalytic oxidation can be a good solution for their treatment [ 1]. Noble metal based catalysts are known to be very active in the catalytic incineration of chlorinated volatile organic compounds [2], but they can also be easily deactivated by chlorine species. Alumina-chromia based catalysts, very active in catalytic incineration [3], represent themselves not environmentally friendly materials due to their chromium content. In the present paper we summarize some results of a study undertaken to test the activity of pure and supported metal oxide based catalysts in the conversion of chlorinated hydrocarbons in oxidizing atmospheres. 2-Chloropropane (2CP) has been chosen as a simple partially chlorinated model reactant. FT-IR surface studies, GC-MS analysis of reaction by-products and preliminary kinetic experiments were used to determine the reaction path.

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2. EXPERIMENTAL

The catalysts studied are reported in table I. Manganese, vanadium and tungsten based catalysts were prepared by conventional coprecipitation-impregnation procedures. SCR and acid catalysts are commercial samples. Table I. Characteristic of catalyst. Catalysts ~r1304

Composition -

Surface (m2/g)

area

24

Hausmannite

Mn-W-A1203

Mn304-WO3/A1203 =

144

MnWO4 on y-A1203

WO3-A1203 CuC12-A1203 A1203 Mn-A1203 H-ZSM-5

10.5: 30.5:59 (wt/wt) 26.5% WO3, 73.5% A1203 9% Cu/A1203 (wt/wt) Atomic ratio Mn:A1=1:1. Si:AI=30:1

147 117 225 106 400

amorphous 7-A1203 7-A1203 (from STREM) ct-Mn3Oa/~-Mn203/7-A1203 ZSM-5 zeolite (from Zeolyst)

SIO2-A1203

87% SIO2, 13% A1203

330

SCR1

VEOflWO3/TiO2

70

Amorphous silica-allumina (from STREM) Commercial SCR catalyst

SCR2

0.51/9.93/89.56 V2Os/W'O3/TiO2 3.7/9.3/87

47

Commercial SCR catalyst

2CP / oxygen / helium gaseous mixtures were taken from commercial cylinders from SIAD; liquid 2-chloropropane was from Aldrich. Catalytic tests were carried out at atmospheric pressure in a continuous flow fixed bed tubular glass reactor. 0.1 to 0.5 g of catalyst were loaded in form of fine powder (particle size 60 + 70 mesh) mechanically mixed with 0.4 g of inert, low surface area material (quartz). The total gas flow varied from 300 to 370 ml / min and the feed composition presented always a large excess of oxygen and a 2CP content from 0.05 % to 3.5 % in He. The reactants and the reaction products were analysed using two on-line gas chromatographs (HP 5890). Permanent gases (02, CO and CO2) were separated using a Carbosieve S-II (Supelco) packed column connected to a TCD detector. Other reaction products, as well as the organic reactant and by-products, were analysed employing a wide-bore VOCOL column (Supelco), connected to a FID detector and a GC-MS instrument equipped with a HP-VOC capillary column. CI ions together with hypochlorite ions have been quantified by mean of ionic chromatography, after adsorption of the effluent from the reactor in NaOH solution. This analysis showed that C1/was never produced in detectable amounts. The IR spectra were recorded by a Nicolet Magna 750 FT-IR instrument. The adsorption and oxidation experiments were performed using pressed disks of the pure powders activated by outgassing at 300-1070 K.

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3. RESULTS

3.1. Transition metal oxide oxidation catalysts Catalysts very active in the total oxidation of hydrocarbons and oxygenates [4], such as pure and alumina supported Mn oxide, do not give rise to oxidative conversion of 2CP to COx, due to the almost irreversible poisoning of the catalyst surface by HC1 and / or by the chlorinated hydrocarbon itself. However, the conversion of 2CP can be complete, producing

2CP with s t e a m

over

2CP with s t e a m

Mn304

100

/

80 60 40 20 0 300

over

0,5 % V205 --r O2(C) t -m-- MCP(C) i -o--C3H6(S) - e - CO2(S) .-,a- CO(S)

100

400

N

8(1

-I-- C3H6IS) ~ N co2 (S) 40

I

/

I

f [

0

500

600

700

T (K)

Fig. 1.2CP conversion and product selectivities over Mn304 catalyst

800

300

400

500

600

700

800

T (K)

Fig 2. 2CP conversion and product selectivities over 0.5 % V 2 0 5 SCR catalyst

exclusively propene. In presence of steam (Fig. 1), propene selectivity is a little lower (95 %), with additional production of COE. The supported Mn oxide and the MnWAI catalyst show the same dehydrochlorination activity, the latter at even lower temperature (500 K). For these oxidation catalysts the GC-MS analysis shows the presence of several different chlorinated organic by-products in traces in the effluent from the reactor. The alumina-supported copper chloride catalyst totally converts 2CP near 550 K with non negligible selectivity to CO2 (near 20%). Very high selectivity to propene is observed at low temperatures but it decreases at higher temperatures, due to the formation of highly chlorinated hydrocarbons. The GC-MS analysis shows the presence of several chlorided hydrocarbons (monochloropropenes, dichloropropanes, dichloropropenes and halogenated C 1).

3.2. SCR-type catalysts The conversion of 2CP has been studied over two V205-WO3-TiO2commercial SCR catalysts [5]. These materials differ for the amount of vanadium oxide component and for their application. The sample with 0.5 % V205 is applied to the treatment of waste gases of power plants through the SCR process, while the sample with 3.7 % V205 is applied to the treatment of waste gases from urban solid waste incinerators. In Fig. 2 is shown the behavior of the 0.5 % V2Os-WO3-TiO2 in presence of steam. At very low temperature, namely 430 K, 2CP is already totally converted, propene being the only product. However, by increasing temperature, propene bums to a mixture of CO and CO2 and oxygen is consumed in parallel. A similar behavior is observed even without steam and for the sample with the higher V content, but oxidation is even faster in this case. This shows that the V2Os-WO3-TiO2 SCR catalysts are very active in the dehydrochlorination of 2CP to propene + HC1, but they still

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display oxidation catalytic activity, allowing the burning of propene. They are consequently not deactivated (at a laboratory measurement scale) by chlorine.

3.3. Acid catalysts The tested acid catalysts (SIO2/A1203, WO3/A1203, 7-A1203 and ZSM5) show high catalytic activity for 2CP dehydrochlorination to propene. Pure Al203 and silica-alumina (Fig. 3) convert chloropropane to propene with selectivity near 100% at temperatures near 400 K and almost no chlorinated by-products. Over WO3/A1203 catalyst (Fig. 4) higher hydrocarbons are mostly formed as byproducts. 2CP overSiO2-A1203

2CP over W-Al2Oa

100

8 ~9

613

m

40

.~ 20 0 3oo

400

100

/

1

5OO

- - - " -

- '-;- - - ~

..--n

8o

l+2cPcc) l ..... c 3 ~ s )

60

700

800

T (K)

Fig. 3.2CP conversion and product selectivities over SIO2/A1203 catalyst.

I+ 2CP (C) I - m - C3H6(S)

4O

0 600

..................F ..................... ' - - - .

I

r

:300

400

500

600

700

800

T (K)

Fig. 4. 2CP conversion and product selectivities over WO3/A1203 catalyst.

Over ZSM5 zeolite 2CP is almost totally converted above 650 K. Propene is the predominant product (selectivity near 60%) but a number of heavy byproducts (mainly nonhalided hydrocarbons) are formed. An analysis of the catalyst IR spectrum shows the deep coking of the sample, with the formation of strong bands in the region 1600-1300 cm -1. 3.4. 1-chloropropane versus 2-chloropropane conversion. Over several of the cited catalysts, the conversion of the isomer 1-chloropopane has also been investigated. In all cases the reaction products from the two isomers are the same and the selectivities are very similar. On the other hand, the conversion of 1-chloropropane is always slower than that of 2CP. This can be interpreted below on the basis of the kinetic and mechanistic studies. 3.5. Kinetic consideration and FT-IR mechanistic studies. In spite of the oxidizing atmosphere we used for our experiments, the dehydrochlorination elimination reaction appears to be the predominant one in all cases, at least at low temperature. The analysis of the thermodynamics of the dehydrochlorination reaction indicates that, in our experimental conditions, the equilibrium is almost totally shifted towards propene + HC1. Preliminary kinetic evaluations have been performed on the silica-alumina catalyst, that appeared to be the most active and the most selective to propene in the dehydrochlorination step. Experiments performed by varying the 2CP concentration in the range 0.1% - 3.5 % at constant contact time (x = 1,07 + 0.02 goat sec / mol2cp) do not evidence significant changes in the reaction rate at a given temperature. This supports that the 2CP reaction order is zero in our conditions. Activation energy has also been evaluated by the help of the Arrhenius plots of different experiments and its average value was found to be 14.5 kcal/mol (60.6 kJ/mol).

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FT-IR studies have been performed on Mn oxide, on alumina and on silica-alumina. In all cases it has been shown that molecular adsorption of the reactant occurs reversibly. However, a nucleophilic substitution by oxide ions over the chlorine atom is evident giving rise to 2propoxy groups. These species are strongly adsorbed, but evolve giving rise to gas-phase propene. Independently, HC1 desorption is also found, thus closing a catalytic cycle. 3, 0] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2,52 2,0-:

1.5-

i8 :~

1,0! o, 5: o,o"

~is _o.~!

i

C

i

"

'

i

b

2, 2, 1500

1400

1300

1200

..................................................................................... W.a.venum...b.er.s...!..cm.:.l.!........................................................................ Fig. 5. FT-IR spectra of the surface species arising from: 2CP adsorption over A1203 in the presence of the gas (a), after outgassing at room temperature (b), IPA adsorption over the same surface (c). In Fig. 5 spectra of the adsorbed species over alumina surface are reported: bands due to isopropoxy species are detectable in the region 1500-1350 and 1100-1200 cm -1, due to CH bending modes and CC/CO stretching modes respectively. Isopropoxy groups arise both from 2CP and isopropanol (IPA) adsorption already at room temperature. The kinetics of the reaction can be rationalized in terms of the following mechanism:

H3C\

H/C\c1

H

H3% FH3 D

0

HoC\~I

,C--CH2 HC1

H3% / C H 3 H/C\o -

[ el

_

HO cr

J

o

The irreversible fast conversion of adsorbed 2CP into 2-propoxides results in the saturation of all active sites constituted by a Lewis acidic center (allowing the coordination of the chlorine atom of 2CP) and a nucleophilic oxygen atom (responsible for the nucleophilic substitution). The slow step consists in the evolution of propene from 2-propoxy-groups, or, alternatively, in the desorption of HC1. The saturation of the active sites justifies the zero order measured for the reaction.

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4. CONCLUSION

Among the studied acidic catalysts, silica-alumina shows the best dehydrochlorination activity around 400 K, and the best selectivity, propene being the only relevant reaction product. In our condition the reaction is zero order with respect to 2CP. On the other hand, the conversion of 2CP is faster than that of its isomer 1-chloropropane. FT.-IR studies show that the molecular adsorption of 2CP (and in general of chlorocarbons) is relatively weak and reversible. However, a nucleophilic substitution occurs giving rise to strongly adsorbed alkoxide species (2-propoxides from 2CP). This species is, on silica alumina, stable up to 400 K where it disappears giving rise to gaseous propene. Thus, the zero order kinetics and the faster conversion of 2CP with respect to 1CP agree assuming that the rate determining step is the decomposition of the strongly adsorbed alkoxides with an E1 mechanism, i.e. through a carbenium ion species. The average activation energy of 2-CP measured over silica-alumina, 14.5 kcal/mol (60.6 kJ/mol), appears to be consistent with this picture and confirms that our experiments have been performed in a chemical kinetic regime. Pure and alumina supported Mn and Mn/W oxides are very likely poisoned by HC1 and still display good dehydrochlorination activity in spite of a loss in their oxidative activity. SCR type catalysts are also active in 2CP dehydrochlorination but they still show oxidation activity (depending on the V205 content) giving rise to COx at high reaction temperatures. REFERENCES

[1] J. N. Armor (ed.), Environmental Catalysis, ACS Symposium Series, Washington, DC 1994 [2] Y. Wang, H. Shaw and R. J. Farrauto in Catalytic Control of Air Pollution Mobile and Stationary Sources, R. G. Silver et al. (eds.), ACS Symposium Series 495, Washington, DC 1992, p.125 [3] S. K. Agarwal, J. J. Spivey, G. B. Howe, J. B. Butt and E. Marchand, Catalyst Deactivation 199 i, C. H. Bartholomew and J.B.Butt (eds.), Elsevier, Amsterdam, 1991 [4] M. Baldi, E. Finocchio, F. Milella and G. Busca, Appl. Catal. B: Environmental, 16 (1998) 43-51 [5] E. Finocchio, M. Baldi, G. Busca, C. Pistarino, G. Romezzano, F. Bregani and G.P. Toledo, Catal. Today, in press

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Catalytic Degradation of Polychlorinated Biphenyls at Low Temperature Dong-Keun Lee, Eun-Suk Byun, In-Cheol Cho t and Sung-Woo Kim'* Department of Chemical Engineering/Environmental Protection, Research Institute of Environmental Protection, Gyeongsang National University, Kajwa-dong 900, Chinju, Kyongnam 660-701, Korea The chemical degradation of PCBs in water was carried out at 90~ in the presence of methanol, 4,4'-Dipyridyl, Fe 3§ and H202. The PCB mixtures could successfully be transformed into CO2 and CI, and the degradation reaction was suggested to occur with hydroxyl radical(" OH) . The hydroxyl radical was believed to be formed from the reaction of H202 with Fe 2§ which had been reduced from Fe 3§ by the charge transfer between methanol and 4,4'-Dipyridyl. The hydroxyl radical-induced PCBs degradation was composed of two sequential steps, that is, the dechlorination and the oxidation of the dechlorinated intermediates into carbon dioxide. The produced dechlorinated intermediates were proposed to be oxidized completely and very quickly. 1. INTRODUCTION Polychlorinated biphenyls(PCBs) are a family of compounds produced commercially by the direct chlorination of biphenyl, with 209 different PCB congeners possible(I). Because of their excellent flame resistance, electrical properties, and chemical stability, PCBs have been used worldwide as heat-transfer fluids, hydraulic fluids, solvent extenders, plasticizers, flame retardants, organic diluents, and dielectric fluids(l). Such an extensive application of these chemically and thermally stable compounds has resulted in widespread contamination(2,3). Due to potential adverse human health and environmental effects, the use of PCBs has ceased, and remediation methodology for existing PCB contamination has become of substantial interest(4,5). PCBs are principally being destroyed by incineration(6), which has become the most widely used technique for their removal(7). Incineration, however, often produces more toxic compounds if it is not carefully controlled. Erickson et al.(8), for example, indicated that

*Present address : Kyong Sang Nam-Do Provincial Government Institute of Health & Environment, Kyongnam, Korea. "Present address : Sam Hyeob Resource Development Co~, Ltd., Kyongnam, Korea. * This project was supported by Clean Technology Center at Seoul National University.

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polychlorinated dibenzofurans(PCDFs) and polychlorinated dibenzodioxins(PCDDs) were both observed in the combustion of PCBs. About 1% of the PCBs in the system was converted into PCDFs. Especially when PCBs are present at low concentration in aqueous media, incineration becomes inadequate for the treatment of PCBs. Other methods for the destruction of PCBs that have been proposed include wet air oxidation(9), biodegradation( 10-12), reaction with superoxide(13), photodechlorination(14), sodium metal-promoted dehalogenation(15), electrolytic reduction(16, 17), and dechlorination with zero-valent iron(18-20). In this study a chemical degradation of PCBs in water was carried out at low temperature(90 ~ in the presence of methanol, 4,4'-Dipyridyl, Fe 3+ and H202. Charge transfer between methanol and 4,4'-Dipyridyl was supposed to transform 02 into 02-which in turn would reduce Fe 3+ to Fe 2+. Hydrogen peroxide was then going to react with Fe 2§to produce hydroxyl radical(" OH) . The produced hydroxyl radical will participate in the oxidation of PCBs into CO2 and CI. 2. EXPERIMENTAL

The PCB mixtures(Aroclor 1242 and 1248) were purchased from Supelco. 4,4'Dipyridyl(98%), hydrogen peroxide(30wt.%), Fe2(SO4)3"5H20(97%), methanol(THM grade) and hexane(+99%) were supplied from Aldrich. The degradation of PCB mixtures was performed in a batch reactor. Twenty milligrams of PCBs, dissolved in 2mL methanol, were added to the reactor containing lmL 10mM 4,4'Dipyridyl in methanol solution. 950mL distilled deionized water was then introduced into the reactor. The mixture solution was stirred mechanically with a glassed blade and warmed to 90~ Additional 47mL distilled deionized water solution containing lmL H20 z and lmL 100mM Fez(SO4)3"5H20 was successively added at once to the mixture while being stirred. 10mL aliquots for different reaction times were withdrawn from the reactor. The samples were twice extracted with 10mL hexane. One ~ of the hexane layer was injected into GC(HP 5890) equipped with ECD detector and into GC/MS(HP 5972). The aqua layer was analyzed for chloride ion concentration with ion chromatography(Dionex 300 series) after diluting with deionized water. Carbon dioxide gas produced in the degradation reaction was introduced into 0.1N Ba(OH)2 solution with 50mL/min suction speed and its concentration was determined by an aqueous pH titration with 0.1N HC1 GC conditions for PCBs and other hexane extracts were as follows: column, HP-5 capillary column(30m • 0.20mm i.d.) with 0.32 llm film thickness; column temperature, 150~ for 1min isothermal, 3 ~ to 220 ~ 2 ~ 250 ~ 250 ~ isothermal for 10 min; carrier gas, nitrogen, 0.9mL/min. GC/MS conditions were almost the same as those of GC except the column and carrier gas. Instead of HP-5 and nitrogen, HP-1 and helium were used as the column and carrier gas. Ion chromatographic conditions for chloride ion were as follows: column, Dionex ionpec AS 14(13 tan, 25cm • 0.40cm i.d.) equipped with a 4cm • 0.40cm i.d. precolumn having AG 14(13/an); eluent, 3.5mM Na2CO3-1.0mM NaHCO3, flow-rate 1.3mL/min.

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3. RESULTS AND DISCUSSION

Figure 1 and 2 show the gas chromatograms of Aroclor 1242 and 1248 solutions before and after reaction at 90 ~ Aroclor 1242 and 1248 PCB mixtures were composed mainly of di-, tri-, tetra- and penta chlorinated congeners, and essemially all the PCB congeners disappeared after 1 h reaction. The result in Figure 1 is stmamarized in Figure 3 which shows the reaction occurring with Aroclor 1242 at different reaction times. The relative peak area recorded on the Y-axis is the ratio of the GC peak area of Aroclor 1242 before reaction and the peak area during reaction. Most of the dechlorination reactions took place within 20min. The total concentration of chloride ion produced after reaction for l h was 8.3 mg/L which is nearly the same as the theoretical value of 8.4 mg/L for the complete dechlorination of Aroclor 1242. When Aroclor 1248 was used, similar results were obtained. Since Aroclor 1242 and 1248 are the mixtures of congeners having differem number of chlorine atoms, the dechlorination rates of the congeners might be different to each other. The differem dechlorination rates will inevitably result in the differem composition of congeners. In Table 1 are listed the composition of congeners in Aroclor 1242 and 1248 solutions before and during reaction at 90 ~ The fraction of higher chlorine-comaining congeners increased with reaction time, which indicates that the lower chlorine-containing PCBs were more

I

a

Figure 1. Gas chromatograms of Aroclor 1242 solution. (a) before reaction. (b) reacted for 5min at 90 ~

Figure 2. Gas chromatograms of Aroclor 1248 solution. (a) before reaction (b) reacted for 5min at 90 ~

(c) reacted for 20min at 90 ~

(c) reacted for 20min at 90 ~

(d) reacted for l h at 90 ~

(d) reacted for l h at 90 ~

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efficiently dechlorinated. This result is consistent with that of Matsunaga et al.(13) who carried out superoxide radical-induced degradation of PCBs. Since an electron capture detector (ECD) of GC is not so effective for the detection of other products, for example hydrocarbons, except halogenated compounds, the solutions of Aroclor 1242 and 1248 were analyzed again with GC/MS. As shown in Figure 4 and Figure 5 for Aroclor 1242 and 1248 solution, no detectable compounds other than the PCB congeners were observable. The production of carbon dioxide gas a~er the reaction of Aroclor 1242 and 1248 solution for 1 h was recognized by the formation of the BaCO3 precipitate, and the amounts of the produced CO2 was about 46% of the theoretically produced CO: for the complete oxidation of PCBs. Although the reason why the concentration of carbon dioxide had not been coincided with the complete oxidation of PCBs is not clear, part of the produced CO2 might be dissolved in the aqueous solution of the reactor. The absence of dechlorinated products such as biphenyl and aliphatic compounds, and the production of chloride ion and carbon dioxide suggest tha~ the produced dechlorinated intermediates had been oxidized to carbon dioxide completely and very quickly. The dechlorination of PCBs and the oxidation of dechlorinated intermediates are believed to proceed under the action of hydroxyl radicals(" OH) which were produced via the steps described as follows : Methanol(MeOH) ~-

+

4,4'-Dipyridyl(DIP)

DIP"

+

02

DIP

- [ - O 2"

02

+

Fe 3+ ~

02

+

Fe 2+

+

H20 2 *~- Fe 3+ +

(MeOH) + +

DIP.

Fe2+ OH-

+

"OH

1.0

10

m 0.8

8~

L_ W

0.6

I ~ Chlorideion Aroc10r1242

ID

ne O.2

E. 0 0 c

Q.

~9 0.4

~,,

4.~ .e0

2"~ 0

0.0

v

0

20

time(rain)

40

60

Figure 3. Changes in Aroclor 1242 and chloride concentration ion with reaction time.

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Table 1. Product distribution of congeners in Aroclor 1242 and 1248 solution before and after reaction at 90"C PCBs

Reaction time(min) 0

1 Aroclor 1242

Aroclor 1248

3 5 10 20

C12 16 13 10

8 6 6 0 0 0 0 0

0 3 5 10 20

C12 = dichlorinated biphenyls, C14 = tetrachlorinated biphenyls,

Product distribution (wt%) C13 C14 41 41 40 41 38 48 35 53 33 57 31 58 37 55 33 58 30 61 29 61 27 62

C15 3 3 4

4 5 5 8 9 9 10 11

C13 = trichlorinated biphenyls C15 = pentachlorinated biphenyls

I

,

9

.t.l

.

1 .

.

.

t

. . . . .

a

_ . . . . . .

.,,,,4

t_

"o

time(min)

Ll, Llt..... time (min)

Figure 4. GC/MS Chromatograms of Aroclor 1242 solution. (a) before reaction. (b) reacted for 5min at 90 ~

Figure 5. GC/MS Chromatograms of Aroclor 1248 solution. (a) before reaction. (b) reacted for 5min at 90 ~

(c) reacted for 20min at 90 ~

(c) reacted for 20min at 90 ~

(d) reacted for lh at 90 ~

(d) reacted for l h at 90 ~

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4. CONCLUSION

The hydroxyl radical-induced degradation of the PCB mixtures could successfully be carried out at 90*(2. Most of the PCBs were dechlorinated and oxidized into CO2 and CI within 20 min. The dechlorinated intermediates were completely oxidized and their oxidation rates were very high. Hydroxyl radical was proposed to be formed from the reaction of H202 with Fe 2+ which had been reduced from Fe 3+ by the charge transfer between methanol and 4,4'-Dipyridyl. REFERENCES

1. Hutzinger, O., Safe, S. and Zitko, U, The Chemistry of PCBs, CRC Press, Cleveland, OH, 1974. 2. Jensen, S., New Sci., 32(1996)612. 3. Buckley, E.H., Science, 216(1982)520. 4. Waid, J.S., PCBs and the Environment, CRC Press, Boca Raton, FL, 1986 5. D'ltri, F.M. and Kamrin, M.A., PCBs : Human and Environmental hazards, Butterworth Publishers, Boston, 1983. 6. Wentz, C.A., Hazardous Waste Management; Clark, B.J., Morriss, J.M., Eds.l; McGrawHill Chemical Engineering Series; McGraw-Hill, New York, 1989. 7. Hawarl, J., Demeter, A. and Samson, R., Environ. Sci. Technol., 26(1992) 2002. 8. Erickson, M.D., Swanson, S.E., Flora Jr., J.D., and Hinshaw, G.D., Environ. Sci. Technol., 23(1989)462. 9. Randall, T.L. and Knopp, P.V., J. Water Pollut. Control Fed., 52(1980)2117. 10. Thomas, D.R., Carswell, K.S. and Georgiou, G., Biotechnol. Bioeng., 40(1992)1395. 11. Williams, W.A. and May, R.J., Environ. Sci. Technol., 31(1997) 3491. 12. Commandeur, L.C.M., van Eyseren, H.E., Opmeer, M.R., Govers, H.A.J. and Parsons, J.R., Environ. Sci. Technol., 29(1995) 3038. 13. Matsunaga, K., Imanaka, M., Kenmotsu, K., Oda, J., Hino, S., Kadota, M., Fujiwara, H. and Mori, T., Bull. Environ. Contam. Toxicol., 46(1991) 292. 14. Yao, Y., Kakimoto, K., Ogawa, H.I., Kato, Y., Hanada, Y., Shinohara, R. and Yoshino, E., Bull. Environ. Contam. Toxicol., 59(1997) 238. 15. Weitzman, L., Treatment and Destruction of Polychlorinated Biphenyls and Polychlorinated Biphenyl-Contaminated Materials: in Exner, J.H.(ed.), Detoxification of Hazardous Waste, Ann Arbor Science, Ann Arbor, 1982. 16. Zhang, S. and Rusling, J.F., Environ. Sci. Technol., 27(1993)1375. 17. Sugimoto, H., Matsumoto, S. and Sawyer, D.T., Environ. Sci. Technol., 22(1988) 1182. 18. Liu, Y., Schwartz, J. and Cavallaro, C.L., Environ. Sci. Technol., 29(1995) 836. 19. Wang, C-. B. and Zhang, W-. X., Environ. Sci. Technol., 31 (1997) 2154. 20. Grittini, C., Malcomson, M., Fernando, Q. and Korte, N., Environ. Sci. Technol., 29(1995) 2898.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Microwave Effects in Exhaust Catalysis Michael Turner a, Robert L. Laurence a, K. Sigfrid Yngvesson b and W. Curtis Conner a aDept. Chemical and bElectrical Engineering University of Massachusetts Amherst, Massachusetts 01003 USA* Many attempts have been made to employ electromagnetic energy as a selective coreagent in catalytic reactions. Photocatalysis employs visible light for a variety of oxidation reactions on a very limited series of catalysts, those employing TiO2. In contrast, attempts to excite specific atom-atom bonds of adsorbed species in the infrared by vibrational resonance have not proved to break the specific bonds selectively. The differences are significant. Many recent studies have suggested that microwave energy can be employed in catalysis and that the results differ from "conventional" heating of the systems studied. Although "photocatalysis" is well accepted, "microwave-catalysis" has not here-to-fore been accepted as an approach to change the selectivity or efficiency of catalysis in the presence of microwave energy. We have studied the influence of microwave energy on sorption and catalysis, particularly on automotive exhaust catalysis. The development of a new generation of automotive exhaust catalysts faces several significant challenges, which might be overcome by the use of microwave energy. The first challenge is to shorten the time required for catalyst "light-off," since a disproportionate fraction of the pollutants are produced as the catalyst is heated up to operating temperature. The second challenge is the influences of sulfur contaminants that impede the catalytic activity, particularly during light,off. Based on preliminary experiments, we propose that indeed microwave energy can induce catalyst light-off more efficiently than conventional heating and can reverse the poisoning by SO2 for a commercial three-way catalyst. 1. BACKGROUND Numerous studies have demonstrated that the sulfur present in gasoline decreases the effectiveness of catalyst for the control of contaminants (hydrocarbons, CO and NOX) in auto exhaust (1-4). The current "state-of-the-art" catalysts used in the control of vehicle emission will need to be modified or replaced to meet the future requirements. It is generally believed that oxides of sulfur are the dominant forms of sulfur present in auto exhaust. These species will adsorb on the catalyst surface to form sulfates and sulfides by reaction with surface species. Sulfur is perceived to react with the noble metals to form sulfides and with the ceria to form sulfates. Ideally, microwave energy alone might decompose the *This research was sponsoredby the CoordinatingResearch Council, CRC, and the National Science Foundation division of CRP, engineering.

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sulfides and sulfates on the surface. These are the most polarizable bonds in the entire system and would be most receptive to absorb microwave energy.

1.1 Sulfur Poisoning of Exhaust Catalysts Although there are several mechanisms by which sulfur (and other "poisons") may inhibit catalytic activity(5), the effect of sulfur containing contaminants on auto exhaust catalysts is reversible (1). The reversibility also suggests that the influence of sulfur involves reaction between the sulfur containing compounds present in automotive exhaust and the catalyst surface.., possibly involving other species present in the exhaust. All proposed influences of sulfur on automotive exhaust catalyst involve surface reactions between sulfur oxides (primarily $02) and the active components. Sulfur is perceived to react with noble metals to form sulfides. These metal sulfides are not (equally) catalytically active for the required reaction of the CO, NOx and unburned hydrocarbons present in auto exhaust. The mechanism by which metal sulfides are formed from SO2 and a surface metal (Pt is employed as an example) in the presence of a reductant (atomic hydrogen, as Hs, is employed for illustration) is shown below: Pt + SO2 + 4Hs-> PtS + 2 H20 (1) Similar mechanisms can be written for the formation of PdS or RhS and/or involving CO or other reducing species. The point is that the active metals can form sulfides which are less active (or are probably totally inactive). Ceria is a crucial component in the most active, state-of-the-art, automotive catalysts. Ceria (as CeO2 or Ce203 ... or mildly defected forms) is believed to be required to store and release oxygen during catalytic cycles. SO2 (or SO3 formed by in situ oxidation of SO/) is commonly understood to react with ceria to form cerium sulfate or oxysulfate on the surface. This is illustrated below for the formation of cerium sulfate from SO2 and ceria, as CeO2 ; although, similar mechanisms exist for the formation of sulfates and sulfites from Ce203 or defected ceria, viz.: (n+m)CeO2 + (2n+m)SO2 + (2n/x+m/x)O x -> nCe(SO4)2 + mCe0SO 4

(2)

The formation of cerium sulfates (or sulfites) impedes their ability to promote the catalytic activity. Indeed, the formation of cerium sulfates and sulfites are commonly believed to be the primary mechanism by which overall catalytic activity is reduced due to exposure to sulfur (as SOx). Several investigators(I,2) have also suggested that SO2 (or SO3 formed by in situ oxidation of SO2) can react with the alumina support to form aluminum sulfate or oxysulfates, e.g.: A1203 + x SO 2 -I-Ox-> A1203_x (SO4)x

(3)

These mechanisms [1-3] may not be precisely those which exist on the surface of active exhaust catalyst to result in loss in activity; however, it is certain that the oxides of sulfur present in exhaust will react with the surface of the catalyst forming sulfides, sulfates and sulfites.

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1.2 Microwave Radiation in Sulfur Catalysis Most recently, Deng and Lin have showed that microwave radiation can be employed to prepare Cu dispersed on dealuminated Y zeolite (6) for SO2 removal from a process effluent and its eventual decomposition. U. S. Patent 4,322,394 (1982) describes a process of rapid regeneration of non-carbon adsorbents by the use of microwave heating. Their examples involve the separation of CO2 and H2S (from >20% to <1% total ) from natural gas over a 4A molecular sieve using a microwave oven (2.45 GHz) to desorb the contaminants. The effluent during the regeneration contained more than 95% contaminant (at times) and they claim the process is more rapid, efficient and safe than conventional regeneration procedures; further, the adsorbent does not change its performance during many regeneration cycles. Promising results have also been found for the decomposition of solids containing heteroatoms such as the desulfurization of hydrocarbon pitch (Wan and Kritz, U.S. Patent 4,545,879, 1985 and (7)). Cha (8) showed that SO2 and NOx are decomposed effectively (>98%) when they are first adsorbed onto activated (pyrolitic) carbon and exposed to microwave radiation (2.45 GHz). The products are elemental sulfur, N2 and 02.

1.3 Regeneration of Catalytic Activity by Microwave Energy Regeneration of catalytic activity due to the "poisoning" would involve reactions by which the sulfur would be removed from the noble metals (decomposing the sulfides) and/or from the ceria (or alumina) decomposing the sulfates or sulfites. Obviously these surface species are stable under normal (periodic) operating conditions; otherwise, there would be no long term loss of activity. Conventional surface chemical regeneration involves higher temperatures and specific treatments with oxidizing/reducing gases. The higher temperatures are required to provide the conditions for favorable reaction kinetics and equilibria for the decomposition of the surface species which have poisoned the reaction. The whole system [bulk as well as surface] must be heated to the temperature required for the decomposition of the compounds responsible for poisoning (loss of catalytic activity). The catalyst must then be exposed to highly reducing or oxidizing environments; often in a specific sequence. We have shown that microwave energy can be absorbed selectively on the surface of oxide supported catalysts (9, 10). The result is a rapid, efficient, selective heating of the surface. As a consequence, desorption can be induced without the necessity of heating the whole system to the same temperature. Potentially, catalytic reactions at the surface can be induced by less energy while the bulk is still at a lower temperature, such as during light-off. 2. EXPERIMENTAL We have studied the influence of microwave energy on a commercial, state-of-theart, three-way catalyst (Englehard Corp.). Continuous microwave energy at different power levels and 2.45 GHz was employed in a flow system with a mass spectrometer to monitor the effluent (10). A fiber optic probe was employed to measure the temperature in the monolith and in the gas phase before and after the bed. The feed contained 650ppm C3H8

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and 6000ppm CO with oxygen above and below stoichiometric. 802 was introduced at -~200ppm. The space velocity of these studies was 20,000. A tube furnace upstream from the catalyst allowed us to raise the temperature of the gas entering the 5cm long x 18mm diameter monolith. The monolith was enclosed in a glass tube, which passed through a microwave waveguide perpendicular to the short axis. Metal chokes (short grounded metal tubes) enclosed the tubular glass reactor and prevented the leaking of microwave radiation outside the waveguide. The electric field inside the waveguide is in the TEl0 mode at 2.45GHz. This system allowed us to Study the light-off of the catalyst as the temperature was increased with and without the presence of microwave energy exposed to the catalyst. Further, the influence of SO2 on the catalytic activity can be measured with and without exposure to microwave energy. 3. RESULTS AND DISCUSSION Two types of experiments were performed. In the first, the light-off of commercial auto-exhaust catalysts was studied by conventional heating compared to heating of the catalyst augmented by microwave radiation. In the second set of experiments, conventional heating was employed to initiate activity. Subsequently, sulfur dioxide was introduced to suppress the catalytic activity. Finally, microwave radiation was employed to determine its influence on the dynamic poisoning.

3.1 Light-off Augmented by Microwave Energy In all cases studied, the time required to initiate catalytic activity was significantly decreased when microwave energy was employed during the initial light-off of the catalyst. Further, the reaction appeared to light-off at a lower temperature (measured in the effluent gas) than measured for conventional increased heating of the inlet reactant stream. We do not believe that this means that microwave radiation is specifically promoting the catalytic reaction(s), but that the surface (the active phase) is able to more effectively absorb microwave energy than the bulk of the catalyst. There was no way to monitor the "effective" surface temperature which, we propose, was higher than for conventional heating alone. We propose that the selective microwave heating of the surface results in a non-isothermal system. The catalytic activity reflects the surface temperature (not measured) while the transfer of heat to the support and gas phase was too slow to achieve thermal equilibrium. Thus, the exposure of microwave energy to a system that comprises a strongly absorbing (surface) on a weakly absorbing (A1203) support can result is a significant temperature difference. Thus, the catalytic activity will not be characteristic of the "bulk" or gas-phase temperatures. Further, less energy (and time) will be required to initiate the catalytic activity since it is not necessary to heat the whole system to the effective reaction temperature.

3.2 Reversal of Sulfur Poisoning by Microwave Energy As the temperature of inlet gas was increased by conventional means, the conversion of CO (m/e = 28) and the increase in CO2(m/e = 44) was evident as the outlet temperature increased above 135~ When SO2 was introduced, the conversion decreased significantly as the CO increased in the effluent while the CO2 decreased. The SO2 poisoned the CO

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oxidation at light-off and the influence increased with time of exposure to CO. When 300W of microwave energy was then turned on, the CO conversion increased to 80+% of that without SO2 present and the conversion remained stable. The microwave energy was able to reverse the influence of the SO2 to poison the catalysis. However, only a fraction of the microwave energy was adsorbed in this experiment, most of the energy passed through the wave guide in this configuration. Again, however, there is a problem in measuring and understanding the influence of surface temperatures in these experiments. The initial experiments (above) suggest that we cannot measure the "effective" temperature directly. 4. CONCLUSIONS

We conclude (propose) that microwave energy is selectively absorbed by the surface/adsorbed-layer of the three-way catalyst. The catalyst lights off at lower temperatures than if the whole catalyst were heated by conventional means. The "effective" temperature is very local and the rate of heat transfer is not as rapid as desorption/reaction. Thus, the "effective" surface temperature is higher than the support or gas-phase temperatures. We cannot measure the local "effective" temperature. These studies give considerable insight into the potential influence of microwave energy on heterogeneous catalytic reactions, a "microwave effect". Specifically, the influence will be greatest if the energy can be directed to the surface, i.e., the bulk is transparent to microwave energy. The concentration of energy due to microwave energy on the surface will depend on the nature of the surface (hydroxyls, etc.), active sites (metals), and the specific adsorbing (reacting or desorbing) species. It is also important that the reaction, or desorption, can occur prior to thermal equilibration; otherwise, there will be little difference for microwave induced sorption/reaction compared to conventional heating. These preliminary experiments suggest that microwave energy can be employed in auto exhaust to decrease the time required for light-off and, thus, can significantly decrease the overall emissions. Further, microwave radiation may be effective in reversing the poisoning of catalytic activity due to SO2 in the exhaust. More studies are needed to document the effects and to optimize the use of microwave energy. Microwave energy can be controlled more precisely than conventional methods of heating. The heating can be instantaneously increased, decreased or pulsed. This control permits unique possibilities to optimize its use for auto exhaust control. Further, we are faced with an interesting challenge! How can we separate the influence of temperature on the catalytic activity when we conclude that the system is not isothermal, i.e., we infer that exposure of a catalytic system to microwave radiation can result in significant temperature differences within the system. REFERENCES

1. CRC Auto/Oil Symposium, 11-12 September 1997, Dearbom, MI. 2. R. Heck and R. J. Ferrauto, Catalytic Air Pollution Control, Van Rostram Reinhold, pbs., New York, 1995 3. Gandhi, H. and Shelef, M., "Effects of Sulfur on Noble Metal Automotive Catalysts", Applied Catal. 77, 175-86 (1991)

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4. Ferrauto, R. and Wedding, B. "Poisoning by Sulfur Oxides of Some Base Metal Auto Exhaust Catalysts", J. Catal. 33,249-55 (1974) 5. W. Conner, J.R. Kittrell and J.W. Eldridge, "Deactivation of Stationary Source Air Emission Control Catalysts" A Chapter in Catalysis volume 9, J. J. Spivey, editor, The Royal Society pbs., Ch. 3, p. 126-182 (1992) 6. Deng, S., Lin, Y., Chem. Eng. Sci., 52 1563 (1997) 7. Wan, J. K. S., and Kock, T. A., in Microwave-Induced Reactions Workshop Pacific Grove, California, 1993), p. A-3. 8. Cha, C. Y., Res. Chem. Intermed, 20(1), 13, (1994). 9. W. Conner, M. Turner, K. S. Yngvesson and R. L. Laurence "The Influence of Microwave Radiation on Sorption in Zeolites" International Congress on Zeolites, Baltimore, M.R.S.. Pbs., M. Treacy, ed. V 1 p 97-102 (1999) 10. M. Turner, R. L. Laurence, K. Sigfrid Yngvesson and Wm. Curtis Conner, "The Influence of Microwave Energy on Zeolite Sorption: Desorption and Competitive Sorption" in press AIChE Journal 1999

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

A new way to Ti-containing zeolite beta catalysts for selective oxidation Francesco Di Renzo l, Sylvie Gomez l, R~mi Teissier 2 and Francois Fajula l* ILaboratoire de Mat6riaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 ENSCM-CNRS, 8, rue de l'Ecole Normale, 34296 Montpellier Cedex 5. France 2Elf-Atochem, Centre de Recherche Rh6ne-Alpes, BP 20, 69310 Pierre-B6nite. France Ti-containing zeolite beta has been prepared by a simple and rapid post-synthesis treatment consisting of reacting the crystals in a concentrated solution of perchloric or nitric acid in the presence of a soluble source of titanium. Simultaneous dealumination and Ti TM incorporation into the framework leads to catalysts that demonstrate good activity and selectivity for the epoxidation of olefins and oxidation of thioethers into sulfones using hydrogen peroxide as the oxidant. 1. INTRODUCTION

Ti-containing zeolite beta (Ti-BEA) catalysts have demonstrated interesting properties for the selective oxidation by hydrogen peroxide of substrates that cannot enter the restricted pores of the MFI structure of TS-1. Several synthesis procedures and post-synthesis treatments (1- 5) have been proposed, with mixed success, to incoporate isolated tetrahedrally coordinated Ti in the framework of purely silicic zeolite beta. Most methods are affected by the presence of small amounts of trivalent elements in the lattice or by the formation of dense Ti-containing phases which negatively affect selectivity in oxidation reactions. We report here on an effective and very simple post-synthesis method of Ti incorporation in zeolite beta based on the attack of a parent zeolite by concentrated acid solution containing a dissolved Ti source. 2. EXPERIMENTAL

Ti-containing zeolites beta (BEA) have been synthesized either by direct synthesis, by incorporating Ti(C4H90)4 as a titanium source to a crystallization medium containing small amounts of aluminium or boron (1,2) or by an original post-synthesis procedure allowing simultaneous dealumination of the framework and insertion of Ti atoms in the lattice (6). The latter procedure consisted in refluxing the zeolite (as-made or in a decationized form) in a solution of concentrated acid (4 - 16 mol/L) in the presence of a soluble source of Ti. Several mineral acids (orthophosphoric, sulphuric, hydrochloric, nitric and perchloric) and Ti sources (Ti(CaH90)4, TiF4, TiO2) were investigated with the results described below. The final materials were recovered by filtration, oven dried at 80~ and characterized by XRD, UV-Vis, FTIR, HR NMR spectroscopies, elemental analysis and nitrogen adsorption.

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Catalytic performances of selected representative samples were evaluated at 80~ for the epoxidation of olefins (1-octene, cyclohexene), and at 30~ for the oxidation of sulfides into the corresponding sulfones. In this latter case the conditions were set in order to obtain quantitative efficiency of the oxidant and the results were compared to those obtained on a TS-1 catalyst (Ti/Si = 0.01). For the epoxidation reaction, results were expressed on the basis of the efficiency of hydrogen peroxide towards decomposition and oxygen incorporation into primary (epoxide) and secondary ( diol, other heavies) products. The epoxidation reactions were carried out in bis(2-methoxyethyl)ether for comparison with the test conditions of industrial catalysts non described in this study. 3. RESULTS AND DISCUSSION 3.1. Incorporation of Ti into the framework of zeofite beta The nature of both the Ti source and the acid appeared critical for Ti incorporation. Regarding the former, TiO2 (the less soluble source of titanium in all the media investigated) led to materials exhibiting two signals in the UV-Vis spectra, at 48 000 and 32 000 cm ~, characteristic of framework Ti TM and of titanium in TiO2, respectively (7). Ti(CaH90)4 and TiF4 proved very efficient for Ti incorporation but the final amount of Ti recovered in the solid, as well as the state of it, depended on the type of acid used. Figure 1 shows as an example the UV-Vis spectra of samples of zeolite treated in the presence of Ti(CaH90)4 (Ti/Si = 0.011) by 14 M solutions of sulphuric, hydrochloric and nitric acids. Although Ti in all samples was incorporated mainly as framework Ti TM, it is clear that sulphuric acid and hydrochloric acids led to the incorporation of various amounts of Ti w (band at 36000 cm~). By contrast, the concentration of the acid did not appear as a critical parameter for Ti incorporation, but for molarities lower than 10 M, the extraction of aluminium was less effective (8). When considering the whole data, a clear relationship could be set between the yield of titanium incorporation (expressed as the ratio between the amount of Ti in the solid and the amount of Ti in the synthesis batch) and the average electronegativity of the acid anion, as illustrated in Figure 2.

.5

9

2.0 !

9

9

9

Figure 1. UV-Vis diffuse reflectance spectra of zeolite beta after treatment by Ti(CaH90)4 in (dotted line) sulphuric acid, (dashed line) hydrochloric acid, and (solid line) nitric acid.

!

\

1.5 9

p

\

1.0 0.5 ....

500010 o4

\\ \ \\ \ , ,~'~,__, 4.0x 10 04 3.0x 10.04 cm-I

"2.0x 10 04

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1.00

A

化

Figure 2: Yield of titanium incorporation v s the average electronegativity of the acid anion. (Solid line for visual aid)

0.75 o,~

E ._= 0.50 t~

[..,

o,..d

0.25 -

0"0~. 0

~2 2.4 2.6 2.8 Anion electronegativity

In other words, in the presence of less electron-donor acid anions (perchlorate and nitrate) with high standard reduction potential, the zeolite lattice competes successfully for the complexation of the transition-metal cation and its incorporation as Ti TM in framework position. Further details on the incorporation of Ti in zeolite beta, mordenite and faujasite are reported elsewhere (9). 3.2. Olef'm epoxidation Cyclohexene and 1-octene epoxidation has been investigated over the series of catalysts prepared according to the procedures listed in Table 1 and the results are summarized in Table 2. Table 1 Preparation procedures and properties of the epoxidation catalysts Catalyst BEA 1

Preparation procedures

Si/A1

Si/B

Ti/Si

AI-BEA synthesized in the presence of tetraethylammonium (TEA) and Na cations

19

Ti-BEA 2

Synthesized according to ref 2

170

Ti-B-BEA 3

Synthesized according to ref 1

170

11.5

0.017

Ti- BEA 4

Ti-B-BEA 3 treated with 12 M HNO3

> 1000

>100

0.01

Ti-BEA-5

As made BEA-1 (5 g) treated with a 10 M solution of HCIO4 (500 mL) containing 0.175 mL of Ti(OBu)4)

Ti-BEA 6 Ti-BEA 7

As made BEA-1 (5 g) treated with a 16 M solution of HNO3 (500 mL) containing 0.25 mL of Ti(OBu)4) Sample Ti-BEA 5 calcined in flowing dry air at 500~

0.007

> 1000

0.01

> 1000

0.01

> 1000

0.01

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The results of the epoxidation tests are in line with what is known on the dramatic influence of acidity (resulting from the presence of trivalent elements or silanol groups) on the selectivity of the reaction. Actually, consecutive reactions leading to diol and heavy products and/or H202 decomposition could not be totally suppressed under the conditions of this study, particularly when using 1-octene as olefin. For instance, the extensive removal of aluminium and boron from sample Ti-B-BEA 3 allowed to reduce the formation of oligomers and to increase the epoxide yield but resulted in a significant increase of the rate of oxidant decomposition (Ti-BEA 4). Similarly, partial healing of the silanol groups (8) by calcination at 550~ (Ti-BEA 5 vs Ti-BEA 7) hardly affected the product distribution. Good results (High epoxide yield, no heavies and marginal H202 decomposition) were nevertheless obtained for the epoxidation of cyclohexene using Ti-BEA 6. Table 2 Oxidation of cyclohexene and 1-octene Catalyst

H202 Conversion

(%)

H202 efficiency (%) towards Decomposition Epoxide Diol

Others

Ti-BEA 2*

80

10

33

34

23

Ti-B-BEA 3*

92

13

5

22

60

Ti-BEA 4*

92

43

43

9

4

Ti-BEA 5*

100

0

48

20

32

Ti-BEA 6*

84

14

74

14

0

Ti-BEA 7*

98

0

40

28

30

Ti-BEA 5**

90

36

13

7

44

Reaction conditions: Stainless steel stirred autoclave, T = 80~ 2.6 g catalyst, olefin (*cyclohexene, **l-octene) = 0.5 mol., H202 = 0.026 mol., solvent - Bis(2methoxyethyl)ether (26 g). Data after 3 h reaction. 3.3. Oxidation of sulfides to sulfones The results of the low temperature oxidation of thioethers into sulfones using a variety of substrates on Ti-beta (Ti-BEA 6) and TS-1 are reported in Table 3. In this reaction, in the experimental conditions used, the H202 efficiency and selectivity to sulfone are higher than 97%, therefore catalyst performances can be directly compared from the conversion level achieved under standard conditions. Ti-beta and TS-1 feature similar activity for the oxidation of small substrates such as diethylsulfide using methanol as the solvent, suggesting that the efficiency of the active sites is the same for the two types of catalysts. By contrast, in the presence of t-BuOH as the solvent or when bulkier thioethers are used, the conversion levels achieved with Ti-BEA catalysts are definitely higher than with TS-1. Actually, TS-1 proved inactive for the conversion of diphenylsulfide due to restricted transition state shape selectivity and diffusion limitations in the narrow pores of the structure. Moreover, the reactivity order found over

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zeolite beta agrees with the nucleophilicity of the S atom, alkyl sulfides being more easily oxidized than allyl or aryl sulfides by an electrophilic oxidant like H202 (10, 11). Table 3" Oxidation of thioethers. Influence of the substrate Substrate

Solvent

Conversion (%) Ti-BEA TS-1

Et2S

MeOH

95

87

PraS

t-BuOH

92

-

AllyI2S

t-BuOH

80

-

Bu2S

MeOH

86

67

PhMeS

t-BuOH

70

-

Ph2S

MeOH

38

0

t-BuOH

18

5

Reaction conditions: T=30~ R2S = 2 mmol., catalyst.Data after 2 h reaction

H202 =

2 mmol., 10 mL solvent, 40 mg

The influence of the solvent on catalytic activity was investigated in more detail for the oxidation of diethylsulfide. Figure 3 compares the results obtained over TS-1 and Ti-BEA and in the absence of catalyst when using six different solvents. The results show that the order of reactivity of TS-1 parallels the one measured in the absence of any catalyst, with methanol as the best solvent. The conversions achieved with Ti-BEA where in all cases higher than with TS-1, with the three protic solvents and acetonitrile leading to very similar activities, larger than those reached with THF and acetone. The relative reactivity order in the various solvents - which follows the solvent polarity - has been explained (12, 13) by assuming a heterolytic mechanism for the cleavage of the peroxide bond leading to transfer of oxygen to the organic substrate, where the partially or totally ionic intermediates are stabilized by the solvent. 100 O

1= C

o

90

60

8

40

II

E =

BTS-1

70

m=

E |

IlWithout

80

Figure 3 9Oxydation of Et2 S at 30 ~ C. Influence of the solvent

I I T i -BEA

so 3o 20 10 0

THF

Me2CO

tBuOH

MeCN

EtOH

MeOH

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4. CONCLUSIONS

Ti-containing zeolite beta with promising catalytic properties in selective oxidation reactions using hydrogen peroxide can be prepared in an efficient way by contacting parent crystals with a concentrated acid solution containing a dissolved source of Ti. The insertion of tetrahedral Ti in the lattice of the zeolite is governed by the redox and complexation properties of the acid anion. The various acids investigated can be ranked in the following sequence of increasing effectiveness for titanium incorporation: orthophosphoric acid < sulphuric acid < hydrochloric acid < nitric acid < perchloric acid. This very simple strategy of preparation of oxidation catalysts has been also successfully applied to mordenite and faujasite zeolites (6, 9). REFERENCES .

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

M. Derewinski et al., Stud. Surf. Sci. Catal., 69 (1991) 127. M.A. Camblor et al., J. Chem. Soc., Chem. Commun., 589 (1992) J.C. van der Waal., Stud. Surf. Sci. Catal., 105 (1997) 1093. B. Kraushaar et al., Catal. Let., 1 (1988) 81. J. Sudhakar et al., Stud. Surf. Sci. Catal., 94 (1995) 309. F. Di Renzo et al., French Pat. Appl. 95 09436 (1995) F. Geobaldo et al, Catal. Let., 16 (1992) 109. E. Bourgeat-Lami et al., Microporous Mater., 1 (1993) 237. F. Di Renzo et al., submitted to Microporous and Mesoporous Mater. V. Hulea et al., Stud. Surf. Sci. Catal., 108 (1997) 361. C. Walling, Acc. Chem. Res., 8 (1975) 125. V. Hulea et al., J. Mol. Catal., A: Chemical, 113 (1996) 499. P. Moreau et al., Appl. Catal., A: General, 155 (1977) 253.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 92000 Elsevier Science B.V. All rights reserved.

Synthesis of Organically Modified Titania-Silica Aerogels: Application for Epoxidation of Cyclohexenol A. Gisler, C.A. M(~ller, M. Schneider, T. Mallat and A. Balker Laboratory for Technical Chemistry, Swiss Federal Institute of Technology, ETH Zentrum, CH-8092 Z0rich Fax: +41 1 632 11 63; e-mail: [email protected] Mesoporous titania-silica mixed oxides with covalently bound amino and acetoxy groups were prepared from trimethoxysilane which carries the functional group, tetramethoxysilane and titanbisacetylacetonatdiisopropoxide using a sol-gel process and ensuing low temperature supercritical extraction with CO 2. The aerogels containing 10 wt% TiO 2 were characterized by thermal analysis, N2 and NH 3 adsorption, infrared spectroscopy, NMR, X-ray photoelectron spectroscopy and transmission electron microscopy. The amorphous mesoporous structure of the aerogels depended markedly on the nature of the modifier. Titanium dispersion within the silica matrix was slightly lower for the modified materials, as compared to unmodified titania-silica. 29Si CP-MAS NMR measurements indicated a more cross-linked network for the unmodified catalyst. Modifying groups located at the surface replaced part of the silanol groups present in the unmodified aerogel, resulting in a decreased adsorption of ammonia. The materials were tested in the epoxidation of 2-cyclohexen-l-ol. Organic modification of the titania-silica aerogel led to enhanced epoxide yield. 1. INTRODUCTION

Recently, titania-silica aerogels [1] were shown to be highly active and selective for the epoxidation of a variety of bulky cyclic alkenes, alkenones and alkenols [2, 3]. These sol-gel derived materials require alkylhydroperoxides as oxidant; the use of aqueous H20 2 deactivates the hydrophilic materials. Klein and Maier [4], and Kochkar and Figueras [5] successfully reduced the hydrophilicity of sol-gel titania-silica by organic modification of the silicon matrix (surface methyl and phenyl groups). However, the catalytic activities and selectivities observed with aqueous H20 2 as oxidant were low compared to the reaction with t-butylhydroperoxide (TBHP) in organic medium. We applied a different strategy, various polar organic groups were built into the titania-silica matrix with the aim of improving the activity and selectivity of mixed oxides in demanding epoxidation reactions, using TBHP as oxidant. Several trialkoxysilane precursors RSi(OMe)3 were synthesized by varying the modifying group R in order to study its influence on the aerogel structure and the catalytic behavior in liquid phase oxidation reactions. Esters and amines were used as functional groups because of their ability to coordinate to Ti. The precursors and aerogels derived from them are listed in Table 1.

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Table 1: Aerogels and corresponding modifiers Aerogel a

Modifier "

EDAP-A

---si~ ~ ," v

PDAP-A

- "'" --.SigN ,,

H

v

v

~

H ~......

-NH2

"'~:Si~N~NH2 "'-.

AcOB-A

/N~

Modifier content in aerogel [%]

*" "~ ~,

H

.

- ~ s , ~

NH2

5

5

10 OAc

DAcOB-A

.... si

10 ~"'"'"~OAc

OAc a

acronyms are composed of abbreviation of modifiers and -A, which stands for aerogel.

2. EXPERIMENTAL 2.1. Catalyst Synthesis The syntheses of all precursors were carried out under an argon atmosphere using Schlenk-tube techniques. All compounds were used as received unless otherwise stated. 2.1.1. Amine precursors 30.5 ml (450 mrnol) ethylenediamine (Fluka, >99.5%) and 16.5 rnl (90 rnmol) (3-chloropropyl)trimethoxysilane (Aldrich 97%) were heated in a 100 ml round bottom flask and refluxed at 100~ The mixture was stirred at this temperature for 2 h and then cooled to room temperature. Two layers formed, the lower layer containing the ammonium hydrochloric salt and propylenediamine was removed and the upper layer with the desired product was distilled under reduced pressure. Distillation (81~ 0.5Torr) yielded 14.8 g (66.5 retool) (ethylenediaminopropyl)trimethoxysilane (EDAP). (Propylenediaminopropyl)trimethoxysilane (PDAP) was synthesized as described above. Distillation (90~ Torrlyielded 14.2 g (60.4 mrnol) of the desired product. According to the I H-, 13C-and 29cSi-NMR spectra, two isomers of PDAP could be detected (Table 1). The ratio of these two isomers was 1 2.7, 9 according to the 1HNMR spectra.

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2.1.2. Acetoxy precursors

Previous experiments showed that hydrosilylation does not take place, if an unprotected alcohol group is present, but a Si-O bond is formed instead. Therefore the alcohol modifiers had to be protected by acylation. Acy/ation of 3-buten-2-ol: 25.24 g (30.2 ml; 0.35 mol) 3-buten-2-ol (Fluka, >97%) were dissolved in 50 ml CH2CI 2 (dist. over Call2) and degassed. Then 890 mg (7.28 mmol) DMAP (Fluka, >98%) and 42.5g (0.42 mol) Et3N (Fluka, >99.5%) were added. The reaction mixture was cooled to 0~ and subsequently 43 g (0.42 mol) acetic anhydride (pract., distilled) was added dropwise. The solution was warmed to room temperature and the reaction was monitored by GC. After 16 h no reactant could be detected anymore and the solvent was removed. The residue was worked up by distillation and yielded 36.52 g (0.32 tool) of 3-buten-2-yl acetate. Hydrosilylation of 3-buten-2-yl acetate: In a 50 ml Schlenk-tube 205 mg (5.10 .4 mol) H2PtCI 6 (Fluka, -38% Pt) were degassed and the catalyst was covered with 11.4 g (0.1 mol) 3-buten-2-yl acetat. The reaction tube was protected against light by an aluminum foil. The reaction mixture was degassed and cooled to -78~ in a dry ice/ i-PrOH bath. 14.6 g (0.12 mol) HSi(OMe)3 (Fluka, pract -95%) was added dropwise with a syringe. The mixture was slowly allowed to reach room temperature. The reaction was controlled by IR-spectroscopy. As soon as the H-Si bond could not be detected anymore, the reacion mixture was distilled under reduced pressure. Distillation (60~ Torr) yielded 14.9 g (63 retool) of (acetoxybutyl)trimethoxysilane (AcOB). 3-Buten-1,2-diol (Fluka, >99%) was acylated according to the procedure described above. Hydrosilylation of the acylated precursor was performed as described above, yielding (diacetoxybutyl)trimethoxysilane (DAcOB).

2.1.3. Aerogel synthesis and characterization

The aerogels were prepared according to procedures previously published [1, 6]. Sol-gel processes were carried out in a glass reactor at room temperature under an Ar atmosphere. The total volume of the liquid was ca. 170 ml and the corresponding molar ratios water : silicon alkoxide : acid :THA were 5 : 1 : 0.1 : 0.15. Prehydrolysi$ of the precursors in i-PrOH with aqueous HNO 3 hydrolysant under vigorous stirring (1000 rpm) lasted 6 h. The prehydrolysis was necessary to compensate for the different sol-gel reactivity of the precursors [6, 7]. Subsequently, tetramethoxysilane (TMOS; Fluka, puriss.) and titanbisacetylacetonatdiisopropoxide (TIBADIP, 75% in i-PrOH; Aldrich puriss.) in i-PrOH were added. The titania content of all aerogels was 10 wt% TiO 2 for a theoretical catalyst TiO2-SiO2. After 24 h, trihexylamine (THA, Fluka >97%) in i-PrOH was added and the stirring speed reduced (500 rpm). Gelation to an opaque monolithic body occurred within 1 h. Different preparation conditions for the amine-modified aerogels had to be chosen because the amine precursors themselves act as base catalyst. A solution of TMOS and TIBADIP in i-PrOH was mixed with the acidic hydrolysant. After 6 h, THA and the amine modifier in i-PrOH were added and gelation occurred immediately. All gels were aged for 7 days. Semicontinuous extraction with supercritical CO 2 was carried out at 40~ and 230 bar. A glass liner was used to prevent contamination originating from the steel autoclave. The as-prepared aerogel clumps were ground in a mortar and calcined in a tubular reactor with upward flow at 100~ All samples were heated at a rate of 10~ min -1 in an air flow of 5 I min 1 and kept at the final temperature for 1 h. The calcination temperatures were chosen on the basis of thermal analytical investigations. The

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composition of the samples with regard to Si, Ti and Fe was determined by inductively coupled plasma atomic emission spectroscopy (ICPAES). The Si to Ti ratio was nominal and the Fe content was below 0.01% (detection limit). The aerogels were characterized by thermal analysis, N2 and NH 3 adsorption, IR, 29Si- and 13C-NMR, XPS, and HRTEM as described elsewhere [6].

2.2. Catalytic Tests 2-Cyclohexen-l-ol (Fluka, ca 97%) and tert-butylhydroperoxide (TBHP, Fluka, ca. 5.5 M solution in nonane, stored over molecular sieve 4 A) were used as received. Toluene (Riedel-de Hahn, >99.7%) was distilled over sodium and stored over molecular sieve 4 A (Chemie Uetikon). In a 25 ml round bottom flask with reflux condensor and thermometer, 70 mg of the catalyst was heated to 100~ under a He stream for 2 h. After cooling to room temperature, a solution of 2 ml toluene, 20 mmol olefin, 0.4 g internal standard and 5 mmol TBHP was added and the mixture was stirred. The reaction was held under a He atmosphere to prevent contamination with oxygen or moisture. Samples were taken via syringe, filtered (Machery-Nagel 0.2 ram) and analyzed using a HP 6890 gas chromatograph (cool on-column injection, HP-FFAP column). Yield, olefin conversion and olefin and peroxide selectivity are defined as follows: Yield = [Epoxide] [TBHP] 0

Olefin conv. 9XOlefi n =

[Epoxide] Peroxide sel." Speroxide = [TBHP] 0 - [TBHP] [ Olefin ]0 - [ Olefin ] [TBHP] 0

[Epoxide ] Olefin sel.: SOlefin = [Olefin]0 [Olefin]

3. RESULTS 3.1. Structural properties For each modifier, the sol-gel process had to be adjusted to ensure the desired properties: i) well dispersed Ti in the silica matrix, ii) mesoporous structure providing access for bulky reactants to the active sites, and iii) covalently anchored organic functional groups stable under the conditions of catalyst preparation and epoxidation reaction. Table 2 compares some structural properties of the synthesized aerogels. Table 2" Structural properties of unmodified and organically modified aerogels Aerogel

SBET [m2g-1]

Vpa [cm3g -1]

dmaxb [nm]

Q4/Q3 c

unmodified

813

2.3

62

3.0

EDAP-A

372

1.58

80

1.67

PDAP-A

436

1.38

32

1.41

AcOB-A

172

0.52

2.4, 67

0.79

DAcOB-A

251

2.02

62

1.83

_

..,

a Total volume of the pores with a diameter between 1.7 and 300 nm b graphically determined maximum of the pore size distribution c Qn stands for Si(OSi)n(OX)4_ n determined from 29Si CP-MAS NMR (X stands for H or Ti)

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All modified aerogels showed significantly lower surface areas and pore volumes than the unmodified titania-silica aerogel. AcOB-A had the lowest BET-surface area and pore volume and exhibited a bimodal pore size distribution. The pore size distribution of all other aerogels were monomodal and indicated mesoporous structure. All modified aerogels showed considerably lower degree of crosslinking, expressed as Q4/Q3, than the original unmodified aerogel. Generally, the textural properties (SBET, Vp and dmax) depended significantliy on the modified silicon precursor used. The thermal stability of the catalysts was limited by the decomposition behavior of the organic groups. Despite this restriction, the precursor structures of the organic modifications were preserved in the aerogel. Crosslinking in the bulk structure was elucidated using 29Si-NMR, confirming the expected aerogel-type structure. Preservation of the structure of the covalently bound modifiying groups after synthesis was confirmed by 130NMR, infrared spectroscopy and thermal analysis combined with mass spectrometry. Transmission electron microscopy corroborated the amorphous structure of the aerogels.

3.2. Catalytic properties

The aerogels were tested in the epoxidation of cyclohexenol with TBHP. Epoxidation activity and selectivity varied depending on the structure of the modifying group. Table 3 illustrates the performance of aerogels in the epoxidation of cyclohexenol. All catalysts showed preferential formation of the cis-epoxide. The fraction of cis-epoxide was 72 % for the unmodified aerogel, 74 % for the amine modified aerogels and reached 79 % for DAcOB-A.

Table 3: Epoxidation of 2-cyclohexene-l-ol (90-~ toluene, TBHP, 2h) Aerogel

Initial activity

Yield120 min [mmolglmin-1] [%]

XOlefin [%]

Solefin [%]

Speroxide [%]

unmodified

4.9

63

88

72

74

EDAP-A

4.3

69

89

77

74

PDAP-A

5.8

77

93

83

81

AcOB-A

5.0

72

=100

72

73

DAcOB-A

8.7

75

=100

76

72

Comparison of the two aerogels modified by bidentate amino groups, EDAP and PDAP with the unmodified aerogel shows an increase in olefin selectivity and yield. The best catalytic performance was achieved with PDAP-A, which afforded highest yield, olefin and peroxide selecitvities. Acetoxy modified aerogels (AcOB-A and DAcOB-A) also showed improved yield. Olefin selectivity was slightly improved with DAcOB-A, whereas peroxide selectivity remained almost unchanged. Highest initial activity was observed with DAcOB-A. Generally catalytic tests were carried out with an excess of olefin corresponding to an olefin : peroxide ratio of 4: 1. In order to elucidate the influence of the olefin : peroxide ratio on olefin selectivity, reactions were also investigated with a ratio of 1:1 with unmodified aerogel and PDAP-A. Application of a ratio of 1:1 resulted in a selectivity decrease of SOlefin from 72 to 57 % with the unmodified catalyst, and 83 to 78 % for

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PDAP-A. In contrast, the peroxide selectivity Speroxide decreased more strongly for PDAP-A (81 to 73 %) than for the unmodified aerogel (74 to 70 %).

4. DISCUSSION

The modified aerogels showed considerably improved olefin selectivity. An exception is AcOB-A for which no significant improve in SOlefin could be observed. A possible explanation for the better performance of organically modified titania-silica aerogels is the interaction of the amino- and acetoxy-alkyl groups with the Lewis-acidic Ti active sites and the Br6nsted acid Si-OH functions. In the former case the amino and acetoxy groups act as electron donor ligand of Ti, which can modify the acidity and thus the activity of the Ti-peroxo complex. On the other hand, the H-bonding interaction with the surface silanol groups can block these acidic sites and prevent undesired acid-catalyzed side reactions of the reactant and/or product epoxide [3]. The lack of influence on the peroxide selectivity observed for most modifiers suggests that the modifying group does not influence peroxide decomposition or peroxide consumption by undesired oxidation reactions. Interestingly acetoxy groups did not hydrolize during sol-gel process as could be expected based on the acidic (basic) reaction conditions. 130 NMR and DRIFT measurements of the aerogels showed preservation of the acetoxy structure. Concerning the influence of the textural properties on the catalytic performance, there is no clear effect discernible. It seems that the catalytic properties of the mesoporous aerogels were not significantly affected by the textural properties. This is reflected by the fact that activity changed only moderately for aerogels with significantly different textural properties. 5. CONCLUSIONS

Amino and acetoxy groups were incorporated into the matrix of titania-silica aerogels using correspondig modified trimethoxysilane precursors, tetramethoxysilane and titanbisacetylacetonatdiisopropoxide in a sol-gel process. Supercritical extraction of the sol-gel products afforded amorphous mesoporous aerogels which were tested in the epoxidation of cyclohexenol with TBHP. All modified aerogels showed improved epoxide yields compared to the unmodified aerogel. The improved yield is mainly due to higher olefin selecitivity, whereas peroxide selectivity was only enhanced for the aerogel derived from (propylenediaminopropyl)trimethoxysilan. REFERENCES

1. Dutoit, D. C. M., Schneider, M., and Baiker, A., J. CataL 153, 165 (1995). 2. Hutter, R., Mallat, T., and Baiker, A., J. CataL 157, 665 (1995). 3. Dusi, M., Mallat, T., and Baiker, A., J. Mol. Catal. A: Chem., 138, 15 (1999). 4. Klein, S., and Maier, W. F., Angew. Chem. 108, 2376 (1996). 5. Kochkar, H., and Figueras, F., J. CataL 171,420 (1997). 6. M(~ller, C.A., Maciejewski, M., Mallat T., and Baiker A., J. CataL 184, 280, (1999) 7. Brinker, C. J., and Scherer, G.W., "Sol-Gel Science,". Academic Press, Boston, 1990. 8. Notari, B., Adv. CataL 41, 107 (1996).

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化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Characterization and Catalytic Properties of Titanium Pillared Clays in the Epoxidation of Allylic Alcohols L. Khalfallah-Boudali, A. Ghorbel, F. Figueras* and C. Pinel* Laboratoire de Chimie des Mat6riaux et Catalyse, D6partemem de Chimie, Facult6 des sciences, Tunis, 1060 Tunisia and *Institut de Recherches sur la Catalyse. 2, Avenue Albert Einstein. 69626 Villeurbanne C6dex, France Abstract: The conditions of preparation of montmorillonite pillared by titanium polycations have been obtained. This Ti-PILC shows a surface area of 350 m2/g, micropore volume of 0.16 mL/g, basal spacing of 2.4 nm, is stable up to 773 K and then the characteristics of a true PILC. It has been used in the gas phase for the conversion of isopropanol at 373 K and exhibits essentially an acid behaviour with a significant selectivity to ether, attributed to a high Lewis acidity. In the liquid phase, it is a good catalyst for the epoxidation of allylic alcohols with t-butyl hydroperoxide in presence of (+) diethyl tartrate. The yields to the corresponding epoxides are comparable to those reported in the literature. The determination of the enantiomeric excess in the product show some participation of asymetric catalysis.

1. INTRODUCTION Redox clays, i.e. clays pillared by transition metal cations (PILC) have recently been pointed out as potential catalyst for liquid phase oxidation (1). Cu-A1 pillared clays can be used for the complete oxidation of phenols (2) and several reports claim the asymmetric epoxidation of allylic alcohols on clays pillared by V (3), Cr (4) or Ti (5). This reaction has been reported to be catalysed by Ti-alkoxydes in the homogeneous phase (6) and the Ti clay had been prepared by deposition of this compound on K10 (5). In the usual preparation of PILC a calcination is required to anchor the pillars to the clay sheet (7), but calcination of Ti-K10 destroyed the catalytic properties, therefore the experiments were performed on a non calcined clay. In non calcined clays the bond between the pillars and the surface is loose and the possibility exists of partial dissolution of the Ti-alkoxyde and therefore of an homogeneous catalysis. These previous results have not been confirmed or contradicted, probably because of the difficulty of preparation of these Ti-PILC. Indeed the difficulty resides in the necessity to introduce a bulky cation, about 1.6 nm in size into the interlayer space of the clay. Sterte (8) used a pillaring solution comisfing of TiCI4/HCI and reached a surface area in the range 200-350 n~/g with an average size of the pores 2V/S = 1.9 nm. Using Ti-isopropoxide as precursor, Yamanaka et al (9) obtained interlayer spacings of 1,35-1,7 nm with surface areas 300 m2/g. Bemier et al (10) investigated the effect of the temperature of intercalation and observed the larger pore size working

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at 298 K. In that case the pore size was 1,4 nm and the surface area stable up to 673 K was 349

m2/g.

We described in an earlier report the preparation and some characterizations of Ti-PILC obtained from pillaring solutions consisting of TiCI4 hydrolysed in aqueous solutions of hydrochloric or sulfuric acids (11). The nature of the acid sites of the solid changed significantly with the type of preparation: the sulfated solid showed relatively strong Bronsted acidity, whereas the Ti-PILC prepared from HC1 solutions was essentially a Lewis acid. Solids can be valuable epoxidation catalysts, and we present here an exploratory study of the epoxidation of aUylic alcohols on well characterised Ti-PILC. 2. EXPERIMENTAL METHODS

2.1. Catalyst preparation The preparation of the pillared clay consists in an ion exchange between a pillaring solution containing TiCI4 and the clay. The original material is a Volclay montmorillonite from CECA (Honfleur, France) of chemical composition SiO2 : 58.5%, TiO2 : 0.3%, A1203 : 20.4%, Fe203 : 3.4%, MgO 1.9%, K20 : 0.2%, Na20 3.1% and H20 : 12.1%. The pillaring solution is obtained by a slow dissolution of TiCI~ into a 6 M HCI solution under vigourous stirring. Final concentrations of 0.82 M in titanium and 0.2, 0.5 or 1 M in HC1 were reached by adding water. IT/Ti values of 0.12, 0.24, 0.6 and 1.2 were thus obtained. The fresh pillaring solution was then added dropwise to 500 cm3 of a suspension containing 2 g of clay, in such quantities that a final Ti/clay ratio of 6, 10, 20 and 40 mmol/g were obtained. The kinetics of intercalation was followed by sampling the solid and analysing by X Ray diffraction. After intercalation the solid was separated by filtration and washed extensively with distilled water, dried at room temperature, then calcined in a flow of air with temperature prograrnmation (1 K/min) to avoid self steaming. The samples are referenced A-C1-B in which A is the IY/Ti ratio and B the Ti/clay ratio. 2.2. Characterizations. The chemical composition was determined by UV spectroscopy analysis after dissolution of the sample in 6N sulfuric acid at 353 K, and addition of 5 mL 1-[O2 per mg of Ti (12). The basal spacings were measured by X Ray diffraction using a CGR diffractometer and CuK~ radiation: oriented samples were used before drying and powder samples for calcined solids. BET surface areas and porosities were obtained from the isotherms of N2 adsorption determined on a Micromeritics ASAP 2000 instrument. According to Plee et al. (13) the interlamellar porosity can be approximated by the micropore volume ~fn obtained by application of the Dubinin equation. Since the original clay shows no microporosity, the micropore volumes measure the extent of pilla-

r~ag.

The catalytic properties were determined for the conversion of isopropanol in the gas phase and for the liquid phase epoxidation of (E)-2-hexen-l-ol and of (E)-3-phenyl-2-propenol by tertbutyl hydroperoxide (TBHP), in presence of (+) diethyltartrate (DET). In that case a 250 mL three neck glass flask equipped with a condensor, mechanical stirring and septum for sampling was used as batch reactor. The reactor was maintained under a flow of nitrogen (containing ppm amounts of water) or helium (perfectly dry). The solvent was anhydrous CI-I2CI2, and the temperature was varied from 253 K to 298 K. The traces of water contained in TBHP were

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eliminated by azeotropic distillation in toluene. The reactants (Fluka) were also distilled before use. For the reactions at 253 K, the reaction medium composed ofDET, dichloromethane and 60 mg of the Ti-PILC (0.4 mmol Ti) was cooled to the reaction temperature. When the temperature was stabilised, TBHP in toluene was added dropwise, then the allylic alcohol in dichloromethane. The reaction temperature was controlled between 253 and 258 K. At the end of the reaction the catalyst was filtered, and the solvents were evaporated. The catalyst used in these experiments was first calcined in air at 673 K , reached with a ramp of 1 K/min and temperature at 673K maintained for 3h. The products were separated by column liquid chromatography using a mixtm'e ethylacetate/hexane (2:8) from Prolabo as eluent. They were analysed and quantified by IH NMR. The enantiomeric excess was measured on a Perkin Elmer 141 apparatus. It has to be pointed out that this procedure permits to measure only the desired products of the reaction. 3. RESULTS and DISCUSSION

3.1. Preparation of the PILC The amount of TiO2 retained by the clay as a function of the experimental conditions is reported in Table 1. Changing the acidity of the pillaring solution (H+/Ti) has a strong impact on the amount of Ti fixed by the clay : a decrease is noticed when the acidity increases, at a constant ratio Ti/clay = 10. This suggests that the charge on the polycations decreases by hydrolysis at low acidity, therefore a larger amount of Ti is required to neutralise the charge of the lattice. The increase of the Ti concentration of the pillaring solution (ratio Ti/clay) does not increase the amount of Ti on the PILC as expected. Indeed in several cases a decrease is observed. Cation exchange is fast and therefore controlled by diffusion, and not by equilibrium. Table 1. Amount of TiO 2 retained by the clay using different conditions of intercalation. H+/Ti Ti/clay wt% TiO2 BET surface area Micropore m2/g volume (mIJg) 0.12 10 54.2 268 0.038 0.12 20 37.4 328 0.125 0.12 40 34.4 362 0.13 0.24 10 39.3 316 0.166 0.6 10 20.5 240 0.045 1.2 10 20.6 192 0.023 1.2 20 14.5 135 0.02 1.2 40 13.6 130 0.01 A further evidence can be found in the changes of the X R spectrum of the pillared clay reported earlier (11) as a function of the number of washings. The XRD pattem contains two lines, one at 1.48 nm attributed to the fraction of the clay non intercalated and one at 2.62 nm corresponding to a pillared material. Extensive washing of the material leads to the development of the line at low angle, with a concomittent decrease of the line at higher angle. This behaviour evidences the diffusion of the Ti species within the particles, and the better quality of the resulting PILC. Since the line at 1.48 nm can contain the 001 reflexion of the non pillared clay and the 002 reflexion of the Ti-pillared clay, it is difficult from XRD alone to conclude on the quality of pillaring.

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The micropore volume can give a good idea of the quality of the product since microporosity is due to pillaring. It can be concluded from the results of Table 1 that surface area and micropore volume go through a maximum at about 40 % TiO2. At this maximum the micropore volume is close to that reported by Plee et al. (13) for a well intercalated beidellite, and suggests then a good quality for the Ti-PILC. A flaker evidence can be found in the results of the study of the thermal stability reported in Table 2. Thermal treatment induces first a shift of the basal spacing from 2.63 to 2.37 ran corresponding to dehydroxylation of the Ti polycation, but after dehydroxylation the solid is stable since the lattice spacing, surface and micropore volume show little change. Table 2. Stability of the Ti-clay (0.24-C1-10) to calcination evaluated from different criteria Calcination temperature D001 Surface area Micropore vol (K) (rim) (m2/g) (cm3/g) 373 2.63 1.59 316 0.166 473 2.36 305 573 2.37 1.26 295 0.142 773 2.30 278 0.12 In the case of a poor intercalation, only the outer fraction of the particles are exchanged and the chore is exchanged by the more mobile protons, present in the acidic pillaring solution. Upon calcination, dehydroxylation of the pillars occurs, and the H-form of the clay hydrolysed by the water produced by this process becomes amorphous. This sintering modifies the surface area of the PILC, and the changes of surface area and porosity are then related to the quality of pillaring. The good stability of the sample referenced 0.24-C1-10 is then consistent with its high surface area and micropore volume. In conclusion of this preparation, the 0.24-C1-10 Ti-PILC shows all characteristics of a homogenously distributed Ti pillars, and was choosen as standard catalyst for the catalytic reactions.

3.2. Catalysis of organic reactions. 3.2.1. Conversion of isopropanol. This reaction was used to estimate the acidity of the solid. The reaction was investigated at low conversion between 338 and 673 K, in a flow reactor using He (70 mL/min) saturated with the partial pressure of isopropanol at 298 K (about 6000 Pa) and 0.1 g of catalyst. The products are propene, di-isopropyl ether and acetone. The catalytic properties measured at 373 K are reported in Table 3. Table 3. Conversion of isopropanol on Ti-PILC at 373 K Sample %TIO2 Activity 10.8 mol.g~.s ~ propene 0.24-C1-10 39.3 3.1 55 0.6-C1-6 18.7 36 85 0.12-C1-20 14.5 5.9 33

Selectivity to acetone 8 1.5 46

ether 37 13.5 20

The standard catalyst showed the lower acidity as evaluated from the rate and a high selectivity for di-isopropyl ether. This high selectivity for the ether suggests a predominance of Lewis acidity (14),

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which is in good agreement with the previous observation of this acidity by FTIR study of pyridine adsorption. A high Lewis acidity is considered to be a good situation for epoxidation (15). 3.2.2 Epoxidation of allylic alcohols. Two reactants were tried : hexene-ol and 6mmmic alcohol (scheme 1). The first series of experiments was carried out with (E)-2-hexen-l-ol, using nitrogen as flush gas, the results are summarised on Table 4. The non calcined clay was inactive for epoxidation at 298 K, or produced at 253 K the hexenoic acid corresponding to the substrate. This is a major difference with earlier results of Choudary et al. (5). Moreover the reaction of Sharpless (6) has been described with Ti-alkoxydes and not chlorides, and is not likely to proceed here. At low temperature the substrate is therefore adsorbed by the alcohol function, and this favours the parallel oxidation of the alcohol to the acid. The shift to the epoxide at higher temperature may simply be due to a higher activation energy for epoxidation. O

R/~''x'j'"

OH 3?~2"/I/3~

~6~

R~

~

OH

O

H

R = C3H7or Ph Scheme ]. Reactions of aHylicalcohols onthe Ti-PILC. Table 4. Results of the reaction of (E)-hex-2-en-1-ol with TBHP in presence of (+) diethyl tartrate on a Ti-PILC. Reaction time (h) Reaction temp (K) Catalyst Product Yield (%) pretreatment 4 253-258 non calcined acid 30 4 253-258 calcined 673 K acid 45 4 273 calcined 673 K acid 26 6 298 calcined 673 K epoxyde 48 23 298 calcined 673 K epoxyde 37 18 298 non calcined none 0 Reaction conditions : Tgtartrate =0.52, alcohol/tartrate=l 6.66 and alcohol/TBHP =- 0.118. The epoxidation of cinnamyl alcohol was investigated in different experimental conditions at 253 K in a nitrogen atmosphere. In the conditions of Gao et al. (6a). or those of Choudary (5) the Ti-PILC was inactive, but a set of conditions could be found in which the uncalcined clay and the PILC were active. However as illustrated in Table 5 the product is then not the epoxide but the aldehyde (E)-3-phenyl-2-propenol or (E) -3 phenyl-2-propenoic acid obtained by oxidation of the reactant. In the same conditions but in helium atmosphere the selectivity to the epoxide becomes reasonably good, and reaches values comparable to those reported in homogeneous phase. It must be pointed out that no Ti trace could be analysed in the solution after filtration of the solid, therefore homogeneous catalysis is discarded.

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An attempt was made to measure the enantioselectivity of the epoxide by measuring the optical rotation as reported by Gao et al. (6). The enantiomeric excess measured by this method is about 15%. However the product was not perfectly pure and probably contained some DET, with an opposite polarisation, therefore the measured enantiomeric excess is probably underestimated. In conclusion, the epoxidation of aUylic alcohols can be obtained on Ti-PILC with reasonable selectivities. The success depends on the preparation of the sample but also on the reaction conditions and particularly on the amount of water present in the medium. Table 5. Reaction of (E) 3-phenylprop-2-en-1-ol with TBHP in presence of DET Reaction time (h) Temperature (K) Solid Product Yield (%) Operation with nitrogen flush 4 253-258 uncalcined aldehyde 26 4 253-258 calcined acid 40 4 298 calcined epoxide 15 20 298 uncalcined none 0 20 298 calcined epoxide 29 dry helium as flush 20 298 calcined epoxide 72 23 298 calcined epoxide 75

REFERENCES

1) R.A. Sheldon and J. Dakka, Catal Today, 19 (1994) 215-246. 2) J. Barrault, C. Bouchoule, K. Echachoui, N. Frini-Srasra, M. Trabelsi, F. Bergaya, Applied Catal B 15 (1998) 269-274 3) B.M. Choudary, V. L. K. Valli and A. Durga Prasad, J. Chem. Soc. Chem. Commun (1990) 721. 4) B.M. Choudary, V.L.K. Valli and A. Durga Prasad, Tetrahedron Lett. 31 (1990) 5785. 5) B.M. Choudary, V.L.K. Valli and A. Durga Prasad, J. Chem. Soc. Chem. Commun (1990) 1186. 6) a) Y. Gao, R. M Hanson, H. Masamune and K. B. Sharpless, J. Amer Chem Soc 109 (1987) 1186. b) Y. Gao, R.M. Hanson, J.M. Klunder, S.Y. Ko, H. Masamune and B.K. Sharpless, J. Amer. Chem. Soc. 109 (1987) 5675. 7) F. Figueras. Catal.Rev.Sci-Eng., 3__0,457-499 (1988) 8) J. Sterte, Clays & Clay Minerals, 34 (1986) 658. 9) S. Yamanaka, T. Nishimara and M. Hattori, Materials Chmistry and Physics 17 (1987) 87. 10) A. Bemier, L.F. Admiai and P. Grange, Applied Cata. 77 (1991) 268. 11) L. Khalfallah Boudali, A. Ghorbel, D. Tichit, B. Chiche, R. Dutartre and F. Figueras, Microporous Materials 2 (1994) 525. 12) G. Chariot <>Vol 2, Masson Ed, Paris (1974) 13) D. Plee, L. Gatineau, and J.J. Fripiat, Clays Clay Miner. 35, 81 (1987). 14) W. H. Saunders, Jr. and A. F. Cockerill, Mechanisms of Elimination Reactions, J. Wiley, New York, 1976, p. 256. 15) Sheldon R.A. and van Doom J. A.,J. Catal. 31,427 (1973).

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Epoxidation of allylic alcohol by v a n a d i u m - m o n t m o r i l i o n i t e

catalyst

I. Khedher and A. Ghorbel Laboratoire de chimie des mat6riaux et catalyse. D6partement de Chimie. Facult6 des Sciences de Tunis 1060 TUNIS. TUNISIE This study deals with an effecient procedure for epoxidation of allylic alcohol using TBHP as oxidant and clay containing vanadium prepared by interaction of VOCI3 and H-montmorillonite as catalyst. The IR and ESR spectroscopies show the presence of VO2§ species in the interlayer of the clay which are likely the active sites responsible for the epoxidation reaction. Furthermore, XRD and UV-visible studies reveal the crytallization of vanadia species inserted into the clay to a new phase V205. 1 .INTRODUCTION The use of solid clay containing active elements in liquid phase for selective organic transformation and catalysis for fine chemicals has been relatively few explored. Pillared clays obtained by the intercalation of metal species in smectite clays, have been recently subjected to liquid phase catalysis for selective organic transformations such as oxidation and epoxidation [ 1-3]. The advantages envisaged in the use of pillared clay catalysts over of conventional heterogenous catalysts include the non-migration of the species on the surface of support during calcination in the pillared clays, which have high thermal stability and more approchable active sites. In this study, we will describe the synthesis and characterization of vanadium-montmorillonite catalysts (V-PILC) and its application in epoxidation reaction of allylic alcohol.

2 .EXPERIMENTAL Na-montmorillonite, was prepared from natural clay, by the procedure of Posner and Quirk [4], and then treated with 0.1N HC1 solution to afford H-montmorillonite [5]. The original material is a clay montmorillonite from CECA (France) of chemical composition: SIO2" 58.5%, A1203: 20.4%, Fe203: 3.4%, MgO: 1.9%, Na20: 3.1%, K20: 0.2%,TIO2: 0.3% and H-,O: 12.1%. V-PILC was synthesised by refluxing VOCI3 in dry toluene with H-montmorillonite under inert atmosphere until the clay suspension turned to green colour (within approximately 5h). The solid was then filtred and washed with dry toluene to remove the excess of VOC13. It was oven dried at 110~ and calcined at different temperatures. H-montmorillonite and clays containing vanadium calcined at different temperatures were characterized by X-ray diffraction and BET surface area and porosity measurments in order to study the intercalation of vanadium species into the clay. To get further insight about the location and the nature of vanadium species present in these catalysts, ESR, UV-vis and IR investigations were performed on all samples.

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Chemical analysis was carried out by atomic absorption spectrometry on a Perkin-Elmer 3100 apparatus, after sample dissolution through acid attack. X-Ray Diffractograms were recorded on a Philips PW 1130/00/60 diffractometer using Ni-filtered CuKct radiation and a Siemens goniometer. BET surface areas and pore volumes of the samples were measured on a Micrometrics ASAP 2000 apparatus. Infrared study was realised with Perkin-Elmer FFIR paragon 1000 PC. ESR spectra were recorded with a spectrometer Bruker ER 200tt at 77K. UV-visible diffuse reflectance spectra were measured at room temperature with Perkin-Elmer lamda 9 with the use of BaSO4 as reference.

The epoxidation of allylic alcohol was performed by stirring 50 mg of V-PILC in the presence of azeotropically dried tert-butylhydroperoxide (C= 5.8M in DCM, 4 mmol) in dry toluene under an inert atmosphere for 20mn, followed by the addition of the allylic alcohol (7 mmol). The reaction mixture was stirred at 80~ for 5h. The catalyst was flitted after the completion of the reaction which has been periodically monitored by CPG analysis. The filtrate was concentrated and purified by column chromatography or distillation. The product thus synthesised was analysed by IHRMN and IR spectroscopies. 3 . R E S U L T S AND DISCUSSION 3 . 1 . C h a r ac t er i z a t i o n

Table-I- Evolution of vanadium quantity retained in the clay as a function of the concentration of vanadium in the solution and calcination temperature. Catalysts V-PILC(I) a V-PILC(I) b V-PILC(I) c V-PILC(II) a V-PILC(II) b V-PILC(II) c

V in solution/clay (mmol/g)

Calcination temperture (~

V retained/clay (mmol/g)

5

110 200 300

0.90 0.88 0.87

10

110 200 300

1.73 1.57 1.45

Table-I- shows that the Vanadium quantity retained in the clay depends on the vanadium concentration in the solution used but in all cases it is higher than the calculated value on the assumption of simple exchange with the clay (0.4 mmol/g). FF-IR spectra for all samples whatever their calcination temperature show bands at 1053 and 830 - 820 cm-l due to V=O and V-O-V stretching vibrations respectively. Bands in the range 600 - 400 cm-J can be assigned to the V-O-V rocking vibrations and a band at 948 cm-I characteristic of V-O-Si fragment, shows that the vanadia species in the interlayer space are linked with the silica sheet of the clay [6]. XRD patterns of H-montmoriUonite and V-PILC(I) calcined at different temperature are shown in Fig.l. The most intensive XRD peak of these samples is (001). The d0olbasal spacing in V-PILC(1) samples give an expansion of about 5/~ in the interlayer space of the anhydrous Na-montmorillonite. This result is in agreement with the relatively low BET surface area of 25m2/g obtained with this sample. The evolution of XRD patterns of V-PILC(I) as a function of calcination temperature shows a slight displacement of (001) peak to a higher 0 values, which suggest a partial loss of vandia species presented in the interlayer space when the temperature

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increases. Furthemore, the peaks at 4.49, 4.25 and 4.06/~ in the V-PILC(I) samples indicate the transformation of vanadia species on heat treatement to a crystalline new phase probably V2Os [71. 1) A(O0 4 49 "14.25

I 1~.o6

I

~~.~~~_~a) f"

lb

-1

15 eb :s 20 F i g . I.XRD patterns of H-montmofillonite (a) and V-PILC(I) calcined at 110~ (b), 200~ and 300~ (d).

The ESR spectra of V-PILC(I) (Fig.2) show an hyperfine splitting due to the interaction of the single unpaired 3d electron with 5 IV nucleus which has a nuclear spin (I) of 7/2. ESR signal is typical of paramagnetic species in axial symmetry surrounding with gH = 1.927 and g_t =1.982 assigned to the V4+ species in vanadyl group VO2+ [8]. However, the ESR spectra show a poor resolution of the hyperfine structure in all samples in particular for V-PILC(I) calcined at 300~ which is likely due to the presence of immobile vanadyl group anchored into the clay. Furthemore, the signal intensity of V-PILC(I) decreases with increasing calcination temperature. This result could be explained by a partial oxidation of V4+ to Vs+ in good agreement with XRD results. 357 I

43~

g_L= 1 . 9 8 2 gll = 1 . 9 2 7

(a)

"

i

635 300"C

\,,~7-

/

f

,u,

(c)

{ H-mont

25(x)

3'50() Gauss

4.500

F i g . 2 . Evolution of ESR spectra of V-PILC(I) as a function of calcination temperature (a) at 110~ (b) at 200~ and (c) at 300~

2o0

~

"

~"

Wavelength (nm)

Fig.3.UV-visible RD spectra of H-mont andV-PILC(1) calcined at different temperatures.

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For materials containing V v (electronic configuration 3d0), very strong absorption due to charge transfert (CT) transitions between oxygen ligands and the central V atoms are generally observed in the UV-vis region. The values of these electronic (CT) energies are strongly influenced by the local structure of V sites.

The diffuse reflectance spectra of V-PILC(I) (fig.3) show two bands at 635 and 600 nm attributed to V(IV) d-d transitions. The intensities of these bands decrease with increasing calcination temperature, which may be explained by a partial oxidation of V(IV) to V(V) as it was yet shown by ESR study. Furthemore, spectra of V-PILC(I) calcined at 110~ and 200~ present three bands situated at 273,420 and 468nm assigned to charge transfert O--->V. When the calcination temperature increase, we notice the displacement of bands at 420 and 468nm to 375 and 43 l nm with intensification due probably to the modification of dispersion state of vanadium. However, on the basis of the litterature data [9] the three main absorptions at 273. 375 and 43 lnm illustrate the crystallization of V205 as it was yet shown by XRD study.

3.2.Catalytic properities Catalytic properties of V-PILC are tested in the epoxidation of Trans-2-hexen-l-ol to 2,3-epoxihexan-1-ol using Tert-butylhydroperoxide (TBHP) as oxidant and toluene as solvent. T a b l e - l l Evolution of epoxide yield as a function of calcination temperture. Catalysts

Epoxide yield (%)

V-PILC(I) a V-PILC(I) b V-PILC(I) c V-PILC(II) a V-PILC(II) b V-PILC(II) c

30.7 33.3 83.4 30.5 31.0 89.5

The results summerized in table-ll show that the epoxide yield depend essentially on the calcination temperature. In fact the epoxidation reaction lead to epoxide with a high yield in the presence of both catalysts V-PILC(I) and V-PILC(II) calcined at 300~ This result indicate the perfectly anhydrous conditions needed to prevent the presence of water in the reaction medium since it exerts an inhibition effect in this type of reaction. Other parameters influence the rate of epoxidation reaction and thus catalytic activity, in particular TBHP/alcohol ratio and reaction temperature. 1o0 80

~'60 r~40 (-20 0

280

'

I

'

I

300 320

'

I

340

'

360

Rcaction temperature(K)

F i g . 4 Evolution of epoxide yield as a function of reaction temperature.

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The effect of reaction temperature is studied between 20~ and 80~ with V-PILC (II) calcined at 300~ (Fig.4).The increase of reaction temperature enhances the yield of epoxide. Activation energy deduced from Arrhenius linear transform is 36,56 KJ/mol.

Table -III- Influence of TBHP/alcohol ratio on the yield.

~

'~~.~HP/alcohol Catalysts

V-PILC(1) a (50mg) V-PILC(II)a (50mg) V-PILC(1) c (25mg)

2/7

4/7

26.6 12.4 23.0

8/7

30.7 30.4 45.7

92.3 94.3 80.0

Table-III shows that the TBHP/Alcohol ratio exerts a great effect on the yield of reaction. In all cases the yield is improved when this ratio increases in presence of the two catalysts used whatever their calcination temperature, but this effect is more pronounced with V-PILC calcined at 300~ which is due probably to the absence of water in this catalyst. In order to compare the yield in homogenous system with the catalytic activity of V-PILC and to better understand the nature of active species involved in the epoxidation reaction, the catalysis of epoxidation of allylic alcohol whith VO(acac)2 which have been widely used in homogenous medium [ 10], is performed in the same reaction conditions. Table-IV Results of epoxidation reaction using V-PILC(I)c and VO(acac)2 Catalysts Vanadium quantity (mmol) Epoxide yield(%)

VO(acac)2

V-PILC(I)C

0.05

0.05

63.2

83.4

The results obtained show clairly the higher catalytic activity of V-PILC (I) c in epoxidation reaction compared to VO(acac)2 one. In fact, this high activity exibited by the vanadium clay, may be attributed to the ease of complexation of TBHP with vanadyl species and alcohol and the facile delivery of oxygen to form epoxide. The high selectivities and the effect of olefin structure on the rate of epoxidation are consistent only with heterolytic mechanism involving electrophilic attack on the olefin, suggeted earlier by Sharpless and Coll. [ 11 ]. Activity of V-PILC in this reaction is assumed to result from the site isolation of monomeric vanadia center, which may be vanadyl VO2+ species. The complex formation between the metal catalyst and the tea-butyl hydroperoxide renders the peroxidic oxygen more electrophilic and, hence, more liable to attack by an olefinic double band. The reaction of the catalyst-hydroperoxide complex with the olefin involves a cyclic transition state where a vanadyl group (V=O) in the catalyst, functions in a similar manner than the carbonyl group in organic peroxyacids (Eq. 1).

+ R

N/ON/ / \

Eq.1

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In heterogenous catalysis and fine chemistry and in particular in the field of investigation of clean ways for chemical synthesis, it is interesting to test the recoverability and the recycling of the catalyst, in order to re-use it in the same reaction. In this aim, two modes of regeneration are tested, the first consists in a new use of the catalyst simply oven dried at 110~ the second proceeds by its recaicination at 300~ T a b l e - V Tests of the recoverability and the recycling of catalysts

Catalyst Epoxide yield(%)

fresh catalyst calcined at 300~ 45.7

used catalyst oven dried at 110~

used catalyst recalcined at 300~

60.0

41.6

Conditions: All these reactions were performed using 25 mg of catalyst. The results obtained (table V) clearly show the recovery of catalytic properties and the possibility of recycling the catalyst for a new use in the same reaction by simple filtration and drying at 110~ 4 .CONCLUSION The catalytic system described here is efficient for the epoxidation of aUylic alcohol with the advantage of high activity and selectivity in particular with V-PILC calcined at 300~ which is appropriate for the perfectly anhydrous conditions needed in this type of reaction. The IR and ESR investigation indicate the presence of vanadyl group VO2+ in the interlayer space of the clay which are probably the active sites in epoxidation reaction. Furthemore, the use of V-PILC provides easy separation by simple filtration and recycling of the catalyst is possible for a new use in the same reaction. REFERENCES

1. H. Yoneyama, S. Haga and S. Yamanaka, J. Phys. Chem., 93 (1989)4833. 2. B. M . Choudary, V. L. K. Valli and A. Durga Prasad, J. Chem. Soc. Chem. Commun., (1990) 721. 3. B. M . Choudary and V. L. K. Valli, Chem. Soc. Chem. Commun., (1990) 1115. 4. A. M. Posner and J. P. Quirk, Proc. R. Soc., London, Ser. A., 35 (1964) 278. 5. W. F. Spencer and J. E. Gieseking, J. Phys. Chem., 56 (1952) 751. 6. J. Cata., 76(1982)144 ; G. Busca, G. Rmis, and V. Lorenzelli, J. Mol. Cata., 50 (1989) 231. 7. F. Theobald, Th/~se, Universit6 de Besancon (1975). 8. O. Bradelli, R. C. Barklie and D. H. Doff, Clay Minerals (1990), 15-25. 9. M. Morey, A. Davidson, H. Eckert, and G. Stucky, Chem. mater., 8 (1996) 486-492. 10. R. A. Sheldon and J. A. Van Doom, Journal of Catalysis, 31 (1973) 427-437. 11. K. B. Sharpless and R. C. Michaelson, J. Am. Chem. Soc., 95 (1973) 6536; S. Tanaka, H. Yamamoto, H. Nozaki; K. B. Sharpless, R. C. Michaelson, and J. D. Cutting, ibid.,96 (1974) 5254.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Activity and stability of thermally stable polyimide-supported Mo(VI) complexes for epoxidation of olefins J.H. Ahna, C.G. Oh b, S.K. Ihm b and D.C. Sherrington c aDepartment of Chemical Engineering and ReCAPT, Gyeongsang National University, 900, Kajwa-dong, Chinju 660-701, Korea* bDepartment of Chemical Engineering, Korea Advanced Institute Technology, 373-1, Kusung-dong, Taejeon 305-701, Korea*

of Science and

CDepartment of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow G1 1XL, United Kingdom

Functional polyimides have been prepared in a bead form by non-aqueous suspension polycondensation. Molybdenum(VI) complex has been supported on a functional polyimide bead containing a triazole group and used as a catalyst in the liquid-phase epoxidation of olefins with tert-butylhydroperoxide (TBHP), as oxygen source. The polyimide-supported Mo catalyst was highly active and selective, and has been recycled 10 times with no detectable loss of Mo from the support.

1. INTRODUCTION In recent years the design and synthesis of catalytically active polymer-supported metal complexes has received considerable interest because these heterogenized catalysts offer several practical advantages over their homogeneous soluble counterparts [1]. Functionalized polystyrene copolymers have been used widely as support materials. These polymers, however, have a drawback in their limited thermo-oxidative stability [2,3]. The scope for application is therefore restricted, particularly in polymer-supported transition metal complex oxidation catalysts [4]. Consequently there is a need for the development of polymer supports with a much higher intrinsic thermo-oxidative stability. Polybenzimidazoles and polyimides are likely candidates in this respect. Polybenzimidazole resin in a porous bead has been applied as a polymer support for homogeneous metal complexes [5,6]. The polybenzimidazole-supported Mo(VI) species

*This work was carried out by G-7 Environmental Technology Development Program, and the financial support is gratefully acknowledged.

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showed retention of high activity in the epoxidation of propylene, but progressive loss of activity on recycling in the epoxidation of cyclohexene. Unlike polybenzimidazole-based thermo-oxidatively stable supports, polyimides can be prepared under relatively mild conditions from starting materials of only low or modest cost [7,8]. Polyimide particulates were prepared in a bead form without a functional group[9,10] or with functional groups [11,12]. The presence of the functional group in the polyimides allows further chemical exploitation, particularly as a catalyst support capable of operating under rather severe oxidative conditions. In this work, functional polyimide particulates carrying a triazole group were prepared, and Mo(VI) complexes were then supported on the polyimide supports and employed as heterogeneous catalysts in the liquid-phase epoxidation of alkenes using t-butylhydroperoxide (TBHP), as the oxidant.

2. EXPERIMENTAL 2-1. Materials N,N'-Dimethylacetamide (DMAc) (Aldrich, HPLC grade) was used without further purification. Acetic anhydride (BDH) was pre-dried over anhydrous sodium acetate. Pyridine (Aldrich, anhydrous) was distilled from KOH prior to use. Poly(maleic anhydride-co-octadec-l-ene)(l:l) (Polysciences), as a polymeric stabilizer, was used as supplied. Paraffin oil (A.J. Beveridge Ltd., liquid paraffin 5LT) was used as a suspending medium. Pyromellitic dianhydride (PMDA) (Aldrich) was recrystallized from butan-2-one before use. 3,5-Diamino-l,2,4-triazole (Aldrich) and tris(2-aminoethyl)amine (Tokyo Kasei) were used without further purification. 2.2. Preparation of Functional Polyimide Particulates A procedure similar to that which we have already reported was employed [9-12]. This involves the preparation of a pre-polymer poly(amic acid) (PAA) solution in DMAc, followed by imidization in suspension in paraffin oil. A typical procedure for the preparation of linear functionalized spherical polyimide particulates (PI-DAT) was as follows. A round-bottomed 3-necked flask was flushed with N2 and charged with 3,5-diamino-l,2,4-triazole in DMAc. The diamine was completely dissolved in DMAc. While solution was mechanically stirred, finely ground pyromellitic dianhydride was added to the mixture on an ice bath in small portions, and then stirring continued overnight at room temperature. Paraffin oil with poly(maleic anhydride-cooctadec-l-ene)(l:l) (0.5wt% in oil) as a suspension stabilizer was added to the flask. The PAA solution was suspended for 2hr at 60~ at the speed of 400rpm. After that, imidization was initiated by dropwise addition of a mixture of acetic anhydride (4.0 molar excess of PMDA used) and pyridine (3.5 molar excess of PMDA used). After 24hr, the polyimide particulates were filtered, washed and extracted with dichloromethane, and then dried at 80~ in a vacuum oven. To obtain crosslinked polyimide particulates (CPI-DAT) tris(2-aminoethyl)amine as a crosslinking agent was added to the PAA solution suspended in paraffin oil. After 24hr, a mixture of acetic anhydride and pyridine was added to give crosslinked polyimide.

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2.3. Preparation of polyimide-supported Mo complex The functional polyimide bead bearing a triazole residue was refluxed with molybdenyl acetylacetonate, MoOa(acac)2, in ethanol for 3days. Upon completion, a polyimide-Mo complex catalyst was filtered and extracted with ethanol in a Soxhlet apparatus for 3 days. The supported complex was dried thoroughly under vacuum. The molybdenum content was measured by inductively coupled plasma (ICP) to be 1.08mmolg l for PI-DAT.Mo and 1.10mmolg -~ for CPI-DAT.Mo.

2.4. Catalytic Epoxidation Catalyst (0.08g), cyclohexene (7.5ml), and bromobenzene (0.5ml) were placed in a three-necked thermostated reaction vessel equipped with condenser, septum cap, and stirrer, and left to thermally equilibrate at 60~ for 20min. Anhydrous TBHP solution (2ml, 5mmol TBHP) was added. Samples were withdrawn by syringe, and the concentration of cyclohexene oxide was monitored by gas chromatography (HP5890 Series II plus) with a capillary column (Ultra 2). 2.5. Analytic Methods Particle size distribution of functional polyimide particulates was determined by sieving: mesh 38/1 m, 75/1 m, 106/2 m, 212/2 m and 425/2 m. Each size fraction was represented by wt%. FTIR spectra were recorded on a Nicolet SX20B instrument with KBr discs.

3. RESULTS AND DISCUSSION The triazole-containing polyimide bead (PI-DAT) was prepared using the suspension polycondensation methodology [9-12]. The PAA solution in N,N'-dimethylacetamide was prepared from PMDA and 3,5-diamino-l,2,4-triazole. The PAA solution was then dispersed as droplets in paraffin oil containing poly(maleic anhydride-cooctadec-l-ene)(l:l) as a suspension stabilizer, and imidization induced at 60~ by addition of a mixture of acetic anhydride and pyridine. A crosslinked analogue of this (CPI-DAT) was also produced by inclusion of tris(2-aminoethyl)amine. Typically 90---100% of mainly spherical polyimide particulates (---20g) were obtained after washing and drying (Figure 1). Figure 2 shows the FTIR spectra of the triazole-containing polyimide bead (PI-DAT). The characteristic absorption bands were obtained at 1780, 1720(heterocyclic carbonyl vibration), 1348(C-N stretch vibration) and 720cm~(imide ring deformation). The thermogravimetric analysis for the polyimide beads showed the progressive loss below ---300~ and it is considered that this almost certainly corresponds to the physically trapped components. Serious degradation of the functional polyimide particulates in air does not happen until ---400~ All the polyimides therefore show good prospects for high temperature application as supports, certainly in reaction up to 300~ The homogeneous Mo complex, MoO2(acac)2, was supported on the PI-DAT. The FTIR spectra of PI-DAT.Mo (Figure 2) showed a band attributable to Mo=O stretching mode at 960cm~, indicating the presence of oxomolybdenum centers. There was no evidence in the IR spectrum of PI-DAT.Mo for an Mo-O-Mo bridge and so the

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Fig. 1. Optical Photograph of PI-DAT beads.

structure of the Mo center in this polymer catalyst remains unclear. However, it is considered that all Mo derivatives on the supported catalysts probably contain oxomolybdenum centers from the initial rate data with no induction

PI-DAT.Mo

PI-DAT

i

2000

i

1600

i

1200

i

800

i

400

Wave number(cm -1)

Fig. 2. FTIR spectra of PI-DAT and PI-DAT.Mo.

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100

9

~.~

_..A

////y..//~ ~x

60-

/I,, ." /,~r

o ._~ >"

I "

/I}1 // //

[

/ ./'

20 -

-#/u

9

~

60

9

9

l

,~,,~

0

_ _ Toluene Chlorobenzene t-guOH Ethanol

. . . . . .

120

180

240

300

Time(rain)

Fig. 3. Solvent effect in the epoxidation of cyclohexene using PI-DAT.Mo complex at 60~

100:

80

!, \\

60

~

\

oxrl Q~ ir o 40 _

\

\ \

._m >-

~ - ~ - -A,, 9

20

cis-cyclooctene

9 cyclohexene

-

9 1-octene 0

1

/

/

\

\ \

\,g/

~

L

I

I

I

t

I

t

t

2

3

4

5

6

7

8

9

0

Run

Fig. 4. Recycling of PI-DAT.Mo catalyst in the epoxidation of various alkenes at 80~

period. One of the authors has already reported on the possible structures of polymer-supported Mo complexes [6,13]. Figure 3 shows the effect of solvent on the epoxidation reaction. Since the level of imbibition into the catalyst for all the liquids involved was very similar (TBHP, cyclohexene, toluene, chlorobenzene, 1,2-dichloroethane ), the observed effects are unlikely to be associated with differences in the swelling characteristics of the polyimide. Rather they are more likely to be associated with the polarity of the solvent, but the rate was reduced in donor polar solvents, t-BuOH and ethanol. The donor solvents are very strongly associated with the catalytic species. The results are consistent with homogeneous and polymer-supported complex catalysts reported before. High activity and selectivity in the epoxidation of various olefins using the supported Mo complex catalysts were observed under favorable conditions (Figure 4). The prolonged activity of polymer-supported heterogenized catalysts is probably the most important factor in their performance. The deactivation of the catalyst by either degradation of the polymer support itself or by leaching of active species from the supported catalyst is unfavorable. Figure 5 shows the yield of cyclohexene oxide after 120min. The polyimide-supported Mo catalysts were recovered at the end of each run and used repeatedly under identical

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conditions. The catalyst shows substantial retention of activity over 10 recycles unlike an earlier polybenzimidazolesupported Mo complex, where the latter displayed rapid deactivation on recycling. The presently reported retention of activity is most encouraging, and suggests that catalysts based on functional polyimide particulates might form the basis of a range of stable polymer-supported metal complex catalysts, where the support is readily synthesized and is highly cost-effective. Application on the both a laboratory and a technical scale also seems feasible.

,IL

100

A

A

~ 80 .-rx o ~, t-x

A

A

~

\\

60

\I(

\

J

\ / A"

~-6 >" 40 t.) ,.. o _~ 7- 2o o

9

C P I - D A T . M o 70~

9

C P I - D A T . M o 60~

9

PI-DAT.Mo

60~

0 I

I

I

I

I

I

I

I

]

1

2

3

4

5

6

7

8

9

10

Recycle number

Fig. 5. Recycling of polyimide-supported Mo catalysts in the epoxidation of cyclohexene by TBHP at 120min. REFERENCES

1 F.R. Hartley, Supported Metal Complexes, Reidel, Dordrecht, 1986. 2. P. Hodge and D.C. Sherrington (eds.), Polymer-supported Reactions in Organic Synthesis, Wiley-Interscience, Chichester, 1980. 3. Y.I. Yermakov, B.N. Kuznetsov and V.A. Zakharov, Catalysis by Supported Complex, Elsevier, Amsterdam, 1981. 4. D.C. Sherrington, Pure Appl. Chem., 60 (1988) 401. 5. M.M. Miller, D.C. Sherrington and S. Simpson, J. Chem. Soc. Perkin Trans. 2 (1994) 2091. 6. M.M. Miller and D.C. Sherrington, J. Catal. 152 (1995) 368, 377. 7. D. Wilson, H.D. Stenzenberger and P.M. Hergenrother (eds.), Polyimides, Chapman and Hall, New York, 1990. 8. L.H. Lee (ed.), Adhesives, Sealants and Coatings for Space and Harsh Environments, Polymer Science and Techology, Plenum, New York, 1988. 9. T. Brock and D.C. Sherrington, J. Mater. Chem., 1 (1991) 151. 10. T. Brock, D.C. Sherrington, and J. Swindell, J. Mater. Chem., 4 (1994) 229. 11. J.H. Ahn and D.C. Sherrington, J. Chem. Soc. Chem. Commun., (1996) 643. 12. J.H. Ahn and D.C. Sherrington, Macromol., 29 (1996) 4164. 13. S. Leinonen, D.C. Sherrington, A. Sneddon, D. McLoughlin, J. Corker, C. Canevali, F. Morazzoni, J. Reedijk, and S.B.D. Spratt, J. Catal. 183 (1999) 251.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

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Epoxidation of Soybean Oil Catalysed by CH3ReOafI-1202 H. Sales, a'b R. Cesquini, a S. Sato, b D. Mandelli c and U. Schuchardt a a Instituto de Quimica, Universidade Estadual de Campinas, P.O. Box 6154, 13083-970, Campinas, SP - Brazil. b Henkel S/A Indfstrias Qufmicas, P.O. Box 050, 12300-970, Jacaref, S P - Brazil c Instituto de Ci~mcias Biol6gicas e Qufmica, Pontiffcia Universidade Cat61ica de Campinas, P.O. Box 111 l, 13020-904, Campinas, SP - Brazil.

The epoxidation of soybean oil with hydrogen peroxide catalysed by CH3ReO3 was investigated in order to evaluate the effect of nitrogen-containing bases (pyridine, 2-picoline, 3-picoline, 4-picoline, pyrazole and bipyridine N,N'-dioxide) on the activity and selectivity of the catalytic system. 1. INTRODUCTION The oxidation of hydrocarbons with hydrogen peroxide or dioxygen catalysed by transition metal complexes is an important industrial process which produces compounds with a higher added value. Epoxidised oils, especially soybean oil, are produced On an industrial scale (--104 ton/year) and are used as stabilisers and plasticisers in the production of polyvinyl chloride [1]. These componds can be prepared in the presence of CH3ReO3 as a catalyst. This alkylrhenium complex reacts with H202 forming the bis-peroxo species CH3ReO(O2)2H20, which is active in the epoxidation of olefins [2]. The current industrial process for the production of epoxidised olefins is via percarboxylic acid, which is a slow reaction, producing acids as side products. The epoxidation catalysed by CH3ReO3 (A) shows a much higher reaction rate and can be carried out at room temperature, reducing the amount of side products. (A) reacts with two H202 molecules to form (C), which is the catalytically active intermediate for the epoxidation of alkenes, as shown in Figure 1 [2]. (C) transfers one oxygen atom to the alkene, forming the epoxide and regenerating (B).

~H3 Re

+H202

-H O

-~

O~

?

+H202

-H O OH 2

=-

[ / Re OH2

(A) (B) (C) Fig. 1. Formation of peroxo and bis-peroxo species from CH3ReO3 and H202 [2].

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A serious problem in these processes is the acid catalysed ring-opening reaction of the formed epoxide [3]. Recently, Sharpless and co-workers have reported that the addition of pyridine derivatives in a biphasic system (solvent/water) suppresses the ring-opening reaction, with no effect on the reaction rate [4]. Pyridine stabilizes the CH3ReO3/H202 system and increases the generation of the peroxorhenium species. The base coordinated to the bisperoxorhenium complex seems to be also responsible for the acceleration of the epoxidation reaction. The basicity of pyridine also lowers the activity of hydronium ions in the reaction medium, reducing the rate of epoxide ring opening [5,6]. A large excess of pyridine is required to maintain the catalytic activity due to the rapid oxidation of pyridine to pyridine Noxide, in the presence of CH3ReO3. Pyridine N-oxide also forms a highly selective, but less active co-catalyst with CH3ReO3/H202. The presence of the aqueous phase ensures high epoxidation activities, due to the rapid and efficient removal of the pyridine N-oxide [6] from the organic media, avoiding its contact with the catalyst. 2. EXPERIMENTAL 2.1. Materials Aqueous hydrogen peroxide (30 wt.%), dichloromethane, and the nitrogen-containing bases (pyridine, 2-picoline, 3-picoline, 4-picoline, and pyrazole) were used without further purification. The bipyridine N,N'-dioxide was prepared by a procedure described in the literature [7]. Soybean oil was obtained from Henkel S.A Ind~strias Qufmicas. CH3ReO3 was prepared as described in the literature [3]. 2.2. Epoxidation reactions In the epoxidation reactions, soybean oil (5.0 g, 24,4 mmol of double bonds), CH3ReO3 (25 mg, 0.1 mmol), nitrogen-containing base (2.4 mmol), H202 30% (3.3 g, 29.1 mmol) and hexadecane (0.65 g, 2.9 mmol, internal standard) were added to 10 mL of CH2C12. The mixture was kept at 25~ under magnetic stirring for 7 h. The reaction was quenched by adding a catalytic amount of MnO2 to decompose any remaining H202 and a stoichiometric amount of anhydrous MgSO4 to remove the formed water. The suspension was then filtered and submitted to transesterification with methanol, following a procedure described in the literature [8]. The methyl esters obtained were quantified by gas chromatography. 2.3. Product characterisation The yields of the epoxidation products were determined by GC, using a Hewlett-Packard gas chromatograph (Hewlett-Packard 5890 II) equipped with a split/splitless injector, flame ionization detector and fitted with a Ultra II HP-5 column (Hewlett-Packard). External standard calculations were applied to obtain the response factor of the epoxidized soybean oil methyl ester products. The structures of the products were determined by GC/mass spectrometry (HewlettPackard 5970) and by comparison of their retention times with those of authentic samples, which were obtained in a large scale epoxidation experiment via peracids, and isolated using column chromatography. These authentic samples were analysed by ~H NMR and ~3C NMR, IR and GC-MS. The ~3C and ~H NMR spectra were recorded on a Bruker AM 300 spectrometer at 300 MHz, using CDC13 as the solvent. Infrared spectra were obtained on a Perkin-Elmer 1600 FTIR M-80 Specord spectrometer.

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3. RESULTSANDDISCUSSION The typical fatty acid composition of soybean oil is shown in Figure 2.

~

O

H

PalmiticAcid 11%

~

O

H

StearicAcid

4%

~

O

H

OleicAcid 25%

~

O

H

LinoleicAcid 51%

~

O

H

Linolenic Acid 9%

Fig. 2: Typical fatty acid components of soybean oil The principal products obtained in the epoxidation of soybean oil using CH3ReO3/H202 (after transesterification) are shown in Figure 3.

(l)

~~~~~~~~o"

~

o

"

(2)

(3) ~ ~ ~ . v ~ V ~ ~ o . .

~ ~ v ~ ~ ~ ~ o . .

(4)

(5) ~

~

(6)

O

"

O

OH

" .

(9) ~

O

"

~

O

OH

"

(10)

OH Fig. 3: Principal products obtained in the epoxidation of soybean oil. Oleic, linoleic and linolenic esters present in the soybean oil are thus converted to mono-, di-, tri-epoxides and their respective diols. A typical chromatogram obtained for the epoxidised soybean oil after transesterification with methanol is shown in Figure 4. The epoxidation of olefins with CH3ReO3 is a stereospecific (cis) addition. Since the starting olefin (unsaturated vegetable oil) is a cis isomer, the resulting epoxide will be cis as well. The bis-peroxo complex can be coordinated on both sides of the double bond. Thus, oleic esters give two enantiomeric epoxystearates, which appear as a single peak, correponding to the racemic mixture (5). Linoleate gives two (9) diasteroisomeric racemates.

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The diasteroisomeric racemates are not necessarily formed in equal amounts [9]. The racemic mixtures from the linolenate epoxide (12), were not detected by gas chromatography. These epoxides react, giving oligomers detected by GPC (gel permeation chromatography).

solvent standard

(1)

(3,4)

(7)jJ5)

(2)

10

20

30

40 time (min)

Fig. 4. Chromatogram of methyl esters of epoxidised soybean oil.

3.1 Epoxidation of soybean oil in the presence of pyridine. The results for the epoxidation reaction of soybean oil with CH3ReO3/H202 in the absence and presence of pyridine are shown in figure 5 and 6.

&,

100

0

~

0

9

.0

X--

X

80

~

60

~e~

D - - Select 7,8 ~a~

40

Conv Select 5

x ~ Select 9

20

~o~ 0

1

2

3

4

5 6 time (h)

Select 6

7

Fig. 5: Epoxidation of soybean oil without pyridine. CH3ReO3/H202/double bonds/ hexadecane = 1/291/244/46 The addition of pyridine causes an increase in the reaction rate. The reaction is practically finished after 60min, while it takes 300min in the absence of pyridine. The monoepoxide intermediates (7) and (8) react faster, forming the diepoxides (9) in a shorter reaction time. In the absence of base, 0.3 mol of the diol (6) was produced, and no diol (6) was produced in the

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1665

presence of pyridine. This Lewis base accelerates the generation of the bis-peroxorhenium species, which coordinates to the olefin. The acceleration of the epoxidation reaction is explanied by its higher concentration with the presence of pyridine [5].

/

100"

"~ 80

o---o

o-

-o

9

- - o - - Conv x - - o - - Select 7,8

60.~ , / x - - - - - - - - - x ~ X 40.x

~

j z x ~l z x \_ _ ~~_ z x l_ _ _~_ _ _x_ _ _ _ z

zx~ Select 5 ~ x - - Select 9

20.

0, 0

~o~ 1

2

3

4

5 6 time (h)

Fig. 6: Epoxidation of soybean oil bonds/hexadecane/pyridine = 1/291/244/46/24

with

Select 6

7 pyridine.

CH3ReO3/H202/double

3.2 Epoxidation of soybean oil in the presence of other bases.

We have compared the effect of pyridine, 4-, 3- and 2-picoline, pyrazol and bipyridine N,N'-dioxide in the epoxidation of soybean oil with CH3ReO3/H202. The results are shown in Figure 7. 100 v

80 60 40

20

, ~ 5 r162

II Select. mono-epox (%) 17 Select. di-epox (%)

Fig.7. Effect of the addition of bases in the epoxidation of soybean oil after 60 min.

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The addition of bases normally increased both conversion and selectivity for the di-epoxide (9). This effect was more pronounced for pyridine, showing that the intrinsic basicity of the base was not proportional to the yield of the products, since 3- and 4-picoline have a higher pKb. The bases also prevented the formation of the diol (6). The addition of 2-picoline reduced the activity of the catalyst, giving a 5% yield to the mono-epoxide after 60 min, which is probably due to the steric hindrance of the active site caused by the methyl group at the ortho position. A similar result was described by Sharpless in the epoxidation of cyclooctene [4]. The reactions carried out with bipyridine dioxide and pyrazole showed low conversions (approx. 40 and 18%, respectively). According to the literature [7,10], these bases increase the activity of the catalyst and the selectivity for the epoxide in the epoxidation of t~-olefins and styrene, respectively, but were rather inactive when soybean oil was used as the substrate. 4. CONCLUSIONS We have found that CH3ReO3/H202 is an active catalytic system for the epoxidation of soybean oil. The activity of this system can be increased by the addition of nitrogencontaining bases, which also prevent the formation of diols, thus increasing the yield of the desired epoxides. Pyridine proved to be the most efficient co-catalyst, whereas typically efficient bases such as pyrazol and bipyridine N,N'-dioxide are not active. 2-picoline poisons the catalyst due to steric hindrance of the active site. ACKNOWLEDGEMENTS This research was supported by Henkel S.A. Indtistrias Qufmicas, FAPESP and CNPq. The authors are grateful to Dr. Ricardo Sercheli and Ir. Michiel van Vliet for helpful discussions. REFERENCES:

1. F.D. Gustone, J. Am. Oil Chem. Soc., 70 (1993) 1139. 2. A.M. al-Ajlouni and J.H. Espenson, J. Org. Chem., 61 (1996) 3969. 3. W.A. Herrmann, R.W. Fischer, M.U. Rauch and W. Scherer, J. Mol. Catal., 86 (1994) 243. 4. J. Rudolp, K.R. Reddy, J.P. Chiang and K.B. Sharpless, J. Am. Chem. Soc., 119 (1997) 6189. 5. W. Wang and J.H. Espenson, J. Am. Chem. Soc., 120 (1998) 11335. 6. W.A. Herrmann, H. Ding, R.M. Kratzer, F.E. K0hn, J.J. Haider and R.W. Fischer, J. Organomet. Chem. 549 (1997) 319. 7. M. Nakajima, Y. Sasaki, H. Iwamoto and S. Hashimoto, Tetrahedron Lett., 39 (1998) 87. 8. U. Schuchardt and O.C. Lopes, J. Am. Oil Chem. Soc., 65 (1988) 1940. 9. L.T. Han, J. Am. Oil Chem. Soc., 71 (1994) 669. 10. W.A. Herrmann, R.M. Jratzer, H. Ding, W.R. Thiel and H. Glas, J. Org. Chem., 555 (1998) 293.

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化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Epoxidation of limonene with layered-double hydroxides as catalysts M. A. Aramendia, V. Borau, C. Jim6nez,* J. M. Luque, J. M. Marinas, F. J. Romero, J. R. Ruiz and F. J. Urbano

Departamento de Quimica Orgdnica, Facultad de Ciencias, Universidad de C6rdoba, Avda San Alberto Magno s/n, E-14004 C6rdoba, Spain This paper reports and discusses the results obtained in the epoxidation of limonene to limonene oxide by using H202 over different layered double hydroxides (LDHs). The solid obtained by calcining Mg/AI LDH at 773 K was found to be the most active among the Mg/M(III) LDHs tested (M = AI, Ga, In). The results are compared with those obtained by using conventional MgO and A1203 catalysts. I. INTRODUCTION Hydrotalcite, [Mg6A12(OH)i6]CO3-4H20, is an anionic clay whose name is used to designate a large number of isomorphic compounds and polytypes known collectively as Alayered double hydroxides (LDHs). The structure of an LDH is similar to that of brucite, Mg(OH)2, with Mg 2+ cations occupying the centres of octahedra joined by their edges to form infinitely superimposed layers linked by hydrogen bonds between hydroxyl groups at octahedral vertices [1]. In hydrotalcite and LDHs, some Mg2+ ions are substituted by tervalent ions; the latter introduce a charge deficiency in the layers that is neutralized by anions in the interlayer region, which also contains crystallization water. Alkenes were initially epoxidized with percarboxylic acids [2] since the use of metals in the presence of hydrogen peroxide led to slow reactions subject to the competition of other processes. Improved results have been obtained in recent years by using molecular sieves containing titanium

[3-5].

2. EXPERIMENTAL 2.1. Preparation of catalysts The three LDHs tested were synthesized following the traditional precipitation procedure at a constant pH, which involves mixing two solutions of the metal nitrates and pouring the resulting solution over one of Na2CO3 at pH 10 and 333 IC The pH was kept constant during precipitation by adding appropriate volumes of 1 M NaOH. Once synthesized, the LDHs were exchanged with carbonate in order to remove nitrate ions from the interlayers. The resulting solids were named HT-Mg/AI, HT-Mg/Ga and HT-Mg/In. All were calcined in a nitrogen atmosphere to obtain the catalysts called HT-Mg/AI-773, HT-Mg/Ga-773 and HT-Mg/In-773, respectively. An MgO-based solid obtained by calcination of commercially available Mg(OH)2 at 873 K and an also commercially available, A1203 solid, were also used for comparison.

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2.2. Characterization of catalysts The elemental composition of the samples was determined on a Perkin-Elmer 1000 ICP spectrophotometer under standard conditions. X-ray diffraction patterns were recorded on a Siemens D-500 ditfractometer using CuK~ radiation. The specific surface areas of the solids were determined by the BET method, which was implemented on a Micromefitics ASAP 2000 analyser. 27A1and 7~Ga MAS NMR spectra were recorded at 104.26 and 121.98 MHz, respectively, on a Bruker ACP-400 spectrometer. Finally, solid basicity was measured by CO2 chemisorption.

2.3. Oxidation reactions Oxidation reactions were conducted in a 250 mL flask containing the catalyst (0.450 g), limonene, the nitrile and hydrogen peroxide. Methanol (70 mL) was used as solvem. The reaction was conducted at 333 K and monitored by gas chromatography, using a 30 m-0.25 mm i.d. DB-1 column. Reaction products were identified by mass spectrometry.

3. RESULTS AND DISCUSSION 3.1. Characterization of catalysts The elemental analysis of the carbonate-exchanged LDHs provided Mg/M(III) ratios consistent with the theoretical predictions. Table 1 gives the formulae of the three exchanged LDHs. The XRD analysis of the solids revealed a high crystallinity in all. As can be seen from the XRD patterns of Fig. l, the solids had typical structures, with stacked layers similar to those previously found by Reichle et al. [6]. Calcination at 773 K of the three LDHs yielded a mixture of magnesium oxides and the corresponding tervalent metal where MgO exhibited a periclase-like structure and the M203 oxides were amorphous (Fig. 1). Table 1 also summarizes the textural properties and basicity of the three LDHs and their calcination products, together with those of the pure magnesium and aluminium oxides studied.

1 g,,,~.-_-

~

,

'

HT-Mg/A1 E ~

'..,..

'~.,j

-w,.~-~,w....,. ~ ]

: r,o

.

.,~,4

HT-Mg/Ga

.,

i'

10

~, ~ x . . . . . , ~

!~

I!

20

k HT-Mg)Ga-773

~ - - - ~ ~

HT-Mg/In 1

,~

30

,

"I

40

:

i

50

60

2 Theta (~

!HT-Mg/In-773 70

80

10

20

30

40

50

60

70

80

2 Theta (o)

Figure 1. XRD patterns for the LDHs and their calcination products. The 27A1MAS NMR and 71Ga MAS NMR spectra for solids HT-Mg/AI and HT-Mg/Ga, and those of their calcination products at 773 K, reveal that the tervalent metal adopts octahedral

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1669

coordination in the LDH, whereas both metals can be in octahedral and tetrahedral coordination upon calcination at 773 K. These results are consistent with previous findings [7,8].

Table 1. Chemical formulae and surface properties of the catalysts. Catalyst Formula SBET(m2"g-~) HT-Mg/AI Mg075A10.25(OH)2(CO3)0.125"0.70H20 56.2 HT-Mg/Ga Mg078Ga0.23(OH)2(CO3)0120-0.62H20 51.7 HT-Mg/In Mg075In025(OH)2(CO3)0125"0.53H20 98.5 HT-Mg/AI-773 143.5 HT-Mg/Ga-773 116.9 HT-Mgfln-773 120.1 MgO 116.0 A1203 59.3

Ftb (ILtmolCO2"g-')

330 182 193 257 43

3.2. Catalytic activity The epoxidation oflimonene under the reaction conditions used is a complex process as regards both the reactants, the catalysts and the resulting products. It is thus crucial to determine the role played by each reactant as a function of its nature and concentration. This entails conducting a series of kinetic tests ensuring a constant behaviour in the reactants with time. The overall process can be fitted to a general kinetic equation such as v = k[limonene][H202][nitrile] which contains a concentration term for each reactant. This involves the unsurmountable difficulty of mixing arising from the low aqueous solubility of the organic products over some concentrations ranges. We those chose to conduct our experiments under reaction conditions where the orders with respect to H202 and the nitrile used would be zero, so the overall kinetic equation would simplify to v-- k'[limonene] and the results obtained would only be a consequence of changes in the limonene concentration. Consequently. the experiments described below were carried out in excess H202 and nitrile relative to the stoichiometric amounts. Figures 2 and 3 show the results of two experiments where the conversion obtained by adding successive portions of H202 or benzonitfile, respectively, was monitored at regular intervals. From the graphs it follows that, when these reactants are at low concentrations, the conversion responds to different orders in such concentrations: however, a portion exists in both curves where the apparent reaction rate, as determined from the slope, does not change upon addition of either reactant, which suggests that both possess a zeroth kinetic order. Accordingly, in subsequent experiments we ensured that the H20 and nitrile concentrations would never fall below such levels. Double bonds can be epoxidized in the absence of acid-base catalysis. In order to check the effect of the blank reaction on our results, we conducted a series of experiments where one of the reactants was excluded from the start. As can be seen from Fig. 4, the reaction can develop in the absence of the catalyst and nitrile; however, both are required in order to achieve the conversion and selectivity levels reported in this paper. The limonene/benzonitrile/H202 mole proportion was 1:5.5" 19.6. Hydrogen peroxide was used in a large excess and was only partly employed in the epoxide formation-it also disappeared through formation of the diepoxide and the direct hydroxylation oflimonene. Figure 5 shows the variation of the limonene and H202 concentrations.

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as determined by iodometry, with time. Alter 5 h of reaction, 85% (in mol) of the initial amount oflimonene was converted and so was 22% of the initial H202 used. However, the H:O2 continued to deeongmse up to 50% (of the initial amount) at 2 h. The yield ofthe process, measured as mol 1-1202 used per mol oflimonene converted, was greater at short reaction times. As the epoxidation rate started to decrease, the presence of byproducts (diepoxides, diglyeols) became significant. In order to boost the selectivity towards the epoxide one must therefore have the reaction complete within as short a time as possible by favouring any factor contributing to a faster reaction. Tests involving limonene oxide as the starting substrate conducted under the previous reaction conditions revealed that the 1,2-glycol was not formed through opening of the oxirane ring but via a direct transformation of the double bond. /// // 2.5~

I

O

I

2.0-

~9

;/

1.O

o.o

~

'-

II I I I

E

I II

illll 14.7 mmol 24.5 mmol 34.3 mmol

2.8 mmol 5.6 mmol 11.3 mmol 22.6 mmol 45.3 mmol 90.6 mmol 121.3 mmol

I I I

i

,,ii !li

I 0

3

6

9

12 15 18 21 24 27

t (h) Fig. 2. Variation of limonene conversion with the addition of H202.

0

2

4

6

8

21

24

27

30

t (h) Fig. 3. Variation of limonene conversion With the addition ofbenzonitrile

Other nitriles tested provided poor yields. As can be seen from Table 2, if the nitrile used contained acid ot protons (e.g. in acetonitrile and butyronitrile), the reaction hardly developed. These substrates reportedly form carbanions very easily and such species may be retained on the hydrotalcite surface, thereby hindering the reaction. When the nitrile contains no acid protons, the reaction is efficiently accelerated by the catalyst. It was outside the scope of this work to elucidate the role of the nitrile in the process, which is currently being investigated. However, we have found that it becomes benzamide during the process. Table 2. Influence of the particular nitrile used on limonene conversion. Nitrile Reaction time (h) Conversion (%) Acetonitrile 8 30.9 Butyronitfile 8 22.2 Benzonitrile 8 86.3 The next step involved examining the influence of the catalyst. For this purpose, the different LDHs and their calcination products were tested under the previously established reaction conditions. As can be seen from Fig. 6, the LDHs exhibited only slight differences in activity. By contrast, the activity of the calcined LDHs decreased with increasing ionic radius of the tervalent

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metal. Table 3 gives the activity and selectivity data extracted from the graphs. The selectivity for the epoxide also appears to be related to the nature of the tervalent metal in the LDH. Like the conversion, it decreases ~om A13+to In 3§ In all respects, solid HT-Mg/AI is the best catalyst for the process, even in relation to the conventional catalysts tested (Fig. 6). 25 20 =

o 9~

15

A

,

~, 10

I

o

o

[]

Blank without peroxide Blank without catalyst Blank without nitrile

5 0 0

2

4 6 8 t(h) Figure 4. Variation oflimonene conversion with time in various blank tests. 60 i

50

_

j]

40 I,

9

H202

l

o

Limonene I

_

I, / !oi/

.

.

.

.

.

.

.

.

.

.

.

6.2 (, 00% 1 ~ . ~._~_~.-.~:_~.~c3:--0--~_; . . . .

0

5

10

....

.

15

~.,

[

20

25

t (h) Figure 5. Variation of limonene conversion and the H202 concentration with time. Table 3. Activit), and selectivity achieved in the epoxidation of limonene with various catalysts. Catalyst Conversion (%) Selectivity 1,2-epoxy + 1,2-glycol Others HT-Mg/AI 86.3 88.1 11.9 HT-Mg/Ga 86.0 88.8 11.2 HT-Mg/In 80.0 82.7 17.3 HT-Mg/AI-773 92.1 89.6 10.4 HT-Mg/Ga-773 78.0 77.3 22.7 HT-Mg/In-773 50.0 57.3 42.7 MgO 85.3 79.2 20.8 AbO3 39.3 96.0 4.0

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100

~

80-

=o

60 4020 0

v

0

2

4

6

80

t (h) 9 A 9 9

HT-Mg/AI HT-Mg/Ga HT-Mg/In AI203 MgO

I

1

2

4 t (h)

9 A 9 o

HT-Mg/AI-773 HT-Mg/Ga-773 HT-Mg/In-773 AI203 MgO

Figure 6. Influence of the catalyst on limonene conversion 4. A C K N O W L E D G E M E N T S

The authors wish to express their gratitude to Spain's DGESIC (Project PB97-0446) and the Consejeria de Educaci6n y Ciencia of the Junta de Andalucia for funding this work. The Nuclear Magnetic Resonance and Mass Spectrometry Services of the University of C6rdoba are also gratefully acknowledged. REFERENCES ~

2. 3. .

5. .

7. 8.

S. Miyata, Clays Clay Miner., 23 (1975) 369. D. Swern, in R. Adams (ed.), Organic Reactions, Wiley, Mew York, 1953, p. 378. A. Corma, J. L. Jordd, M. T. Navarro and F. Rey, J. Chem. Soc., Chem, Commun., (1998) 175. E. Jorda, A. Tuel, R. Teissier and J. Kervennal, J. Catal., 175 (1998) 93. V. Hulea, E. Dumitrin, F. Patcas, R. Ropot, P. Graffm and P. Moreau, Appl. Catal. A: General, 170 (1998) 169. W. T. Reichle, S. Y. Kang and D. S. Everhardt, J. Catal., 101 (1986) 352. A. Corma, V. Fom6s and F. Rey, J. Catal., 148 (1994) 205. M. A. Aramendia, Y. Avil6s, J. A. Benitez, V. Borau, C. Jim~nez, J. M. Marinas, J. R. Ruiz and F. J. Urbano, Microp. Mesop. Mater., 29 (1999) 319.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Epoxidation of Electron-Deficient Alkenes using Heterogeneous Basic Catalysts J. M. Fraile, a* J. I. Garcfa, a D. Marco, a J. A. Mayoral, a E. Sfinchez, a A. Monz6n, b E. Romeo b a Dep. Qufmica Orgfinica, Fac. Ciencias, Inst. Ciencia de Materiales de Arag6n, Univ. Zaragoza-C.S.I.C., E-50009 Zaragoza, Spain. b Dep. Ingenierfa Qufmica y Tecnologfa del Medio Ambiente, Fac. Ciencias, Univ. Zaragoza, E-50009 Zaragoza, Spain. 1. INTRODUCTION The development of heterogeneous catalysts to promote selective organic reactions is a field of growing interest. Whereas heterogeneous acid catalysis has been applied to a good number of processes, basic catalysis is far less developed. Oxidation reactions are among the most interesting processes from the industrial point of view, and epoxidation is one of the most important oxidation reactions, given the usefulness of epoxides as synthetic intermediates. The epoxidation of electron-deficient alkenes can be carried out with hydroperoxides in the presence of a base. Our group described for the first time the use of hydrotalcites as basic solids in the epoxidation of electron-deficient alkenes with hydrogen peroxide [ 1]. This system has the advantage of using one of the oxidants of choice in industry, but it suffers from the drawback of a number of side reactions. Another heterogeneous system uses KF/alumina as the base and tert-butyl hydroperoxide (TBHP) as the oxidant [2]. However, this method suffered from problems associated with solubility, but these were overcome by the use of toluene [3]. The present communication describes our most recent results in this type of epoxidation of alkenes using both systems. Two relevant aspects are studied; firstly the influence of the hydrotalcite composition on the catalytic activity and the product selectivity and, secondly, the use of this methodology in the diastereoselective epoxidation of chiral alkenes. 2. EXPERIMENTAL

2.1. Preparation of the hydrotalcite catalysts The catalysts were prepared by coprecipitation of a 1 M aqueous solution of a mixture of the metallic nitrates (Mg and A1) with a 2.4 M aqueous solution of KOH and K2CO3 (KOH/K2CO3 ratio = 16.42). The pH and temperature of the slurry were kept at 10.2_+0.2 and 60 ~ respectively, during coprecipitation. The hydrated precursors were separated by filtration, washed with hot bidistilled water and dried overnight at 70 ~ The corresponding * Corresponding author. FAX: +34 976 762077. E-mail: [email protected]

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mixed oxides were obtained by calcination of the dried precursors in a flow of N2 at 500 ~ for 14h.

2.2. Characterization of the catalysts The specific surface area was measured by adsorption of NE at 77 K with a Micromeritics Pulse Chemisorb 2700. The XRD patterns were recorded within the range of 5 to 80 ~ (20) using a Rygaku/Max system diffractometer. All the precursors showed the expected structure of hydrotalcite. The number of basic sites was measured by adsorption of phenol. The hydrotalcite (10 mg) was added to a 0.016 M solution of phenol (3 mL) in cyclohexene, containing decane as an internal standard. The mixture was stirred at room temperature for 24 h and the solution analysed by gas chromatography. The amount of phenol adsorbed was calculated by difference. 2.3. Epoxidation with hydrogen peroxide and hydrotalcite To a solution of the alkene (1 mmol) in methanol (1 mL) was added hydrotalcite (150 mg) and 30% hydrogen peroxide (0.35 mL). The reaction mixture was stirred at room temperature for the appropriate time (1 or 24 h), filtered and the catalyst washed with methanol (5 mL). 100 mg of ethylene glycol dimethyl ether as an internal standard was added and the solution was analysed by gas chromatography. The ratio epoxide/dioxolane was determined by 1HNMR spectroscopy. 2.4. Epoxidation of chiral alkenes with hydrogen peroxide/hydrotalcite The hydrotalcite (285 rag) was added to a solution of the alkene (1.94 retool) in methanol (25 mL). 30% hydrogen peroxide (0.68 mL, 6.6 mmol) was added and the mixture was stirred at room temperature for 24 h. The catalyst was filtered off and washed with methanol. The solvent was evaporated under reduced pressure and the crude product was analysed by ~HM R spectroscopy. 2.5. Epoxidation of chiral alkenes with TBHP and KF/alumina To a solution of alkene (0.569 mmol) in anhydrous toluene (10 mL) was added KF/alumina (95 rag, Fluka, -5.5 mmol F/g) and TBHP (0.57 mL, 3 M in isooctane, 1.71 mmol). The mixture was stirred at room temperature under an Ar atmosphere for 24 h. The catalyst was filtered off and washed with toluene. The solvent was evaporated under reduced pressure and the crude product analysed by 1H-NMR spectroscopy. 3. RESULTS AND DISCUSSION 3.1. Effect of Mg/AI ratio Our preliminary results with acyclic unsaturated ketones, such as mesityl oxide (1) (Scheme 1), showed that the system hydrotalcite/H202 led not only to the epoxide (2) but also to the corresponding 3-hydroxy-l,2-dioxolane (3).

0 1

2

Scheme I

0--0 3

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Recently it has been described that an increase in basicity, by anion exchange with the t e r t butoxy anion, increases the activity of the hydrotalcite [4], although no mention of selectivity problems was made. Thus, we decided to study the effect of the basicity of the hydrotalcite on this reaction by changing the Mg/A1 ratio, both on the catalytic activity and the selectivity epoxide/dioxolane. To ensure the reproducibility of the results a precise control of the pH in the synthesis of the hydrotalcite is necessary, so an automatic pH controller system was used. The effect on the epoxidation of another ketone that does not have problems of selectivity, such as isophorone (4) (Scheme 2), was also studied for comparison. In both cases, conversion after 1 h was taken as indicative of the catalytic activity and conversion after 24 h was also measured. O

~

O

Scheme 2 Most of the basicity measurements on calcined hydrotalcites have previously been carried out by microcalorimetry with CO2 [5,6]. However, we decided to try a new method that is closer to the experimental conditions of the reactions. This new method involves adsorption in the liquid phase at room temperature. The probe molecule chosen was phenol and it was analysed by gas chromatography in the liquid phase after adsorption. The amount of adsorbed phenol was then calculated by difference. The values obtained in this way lead to the conclusion that the solid with Mg/A1 = 3 shows the highest number of basic sites (Figure 1), which contrasts with results previously described [5,6], in which magnesium oxide is the solid with the greater number of basic sites. This may be due to the different acidity of the probe molecule used (CO2 vs. phenol). phenol (mmol/g)"

S (m2/g)

2.5

8O

240 220

2.0

60

200 180

1.5

160

.,40

w_. o

20

140

1.0 0.0

0.1

AI/Mg+AI

0.2

0.3

120

Figure 1. Surface area (filled symbols) and amount of phenol adsorbed (open symbols) on hydrotalcites of different Mg/AI ratio.

o.o

AI/Mg+AI

0.3

o.,

Fig 2. Conversion after 1 h of isophorone (open symbols) and mesityl oxide (filled symbols) with H202 and hydrotalcites of different Mg/AI ratio.

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The most relevant results of reactivity are gathered in Figure 2, which shows the conversion of alkene after 1 h plotted against the A1/Mg+A1 ratio of the catalyst. As can be seen, there is a maximum for the catalytic activity and with both ketones, isophorone and mesityl oxide, the hydrotalcite showing the highest catalytic activity has a ratio Mg/AI = 3. This fact seems to indicate that the number of basic sites able to abstract a proton from hydrogen peroxide should be higher in the hydrotalcite with Mg/A1 = 3. This result is in agreement with the previous measurements of the catalyst basicity, which are shown in Figure 1. On the other hand, the values of surface area (Figure 1) do not show any relationship with the behaviour in catalytic activity. The formation of dioxolane (Table 1) is also influenced by the basicity of the solid. The higher basicity of the catalyst, the higher amount of the dioxolane obtained at short reaction times. According to the reaction mechanism (Scheme 3), the formation of dioxolane should be a reversible reaction, whereas the formation of the epoxide should be irreversible. So, at long reaction times the preferent obtention of epoxide is always observed. Consequently, the use of solids of high basicity and short reaction times are necessary for the efficient production of dioxolane. Table 1 Epoxide/dioxolane ratio in the reaction of mesityl oxide with H202/hydrotalcite. A1/Mg+A1 epoxide/dioxolane (1 h) epoxide/dioxolane (24 h) 0.00 51/49 76/24 0.14 28/72 95/5 0.25 8/92 95/5 0.33 32/68 64/36

'

O

i

O

O--O

ASo. O--O

Scheme 3

Finally, we considered it interesting to test the reusability of these catalysts. The solid with Mg/A1 = 3, filtered off and washed after 1 h reaction with isophorone, was dried and reused without further activation. The catalyst maintained its activity during at least two recycling experiments.

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3.2. Epoxidation of chiral alkenes Another objective of this work was the application of heterogeneous systems to the epoxidation of chiral alkenes. With this aim in mind, two substrates with interest from a synthetic point of view were selected, a ketone (6) and a cyanoester (7) (Scheme 4), which are precursors of sugars and polyhydroxyamino acids, respectively. In fact, only two related examples have been described previously in the literature and these involve homogeneous systems and an unsaturated sulfone [7] or a ketone [8] as substrates. With alkenes 6 and 7 we decided to compare the system HEO2/hydrotalcite (Mg/A1 = 3) with KF-alumina/TBHP [2,3].

••'0

Y

0~~~~/x

oxidant ~ ' ~ 0 base" O ~

Y x S

X•O

y

R ....

6: X = COCH3,Y = H 7: X = COOCH3,Y = CN Scheme 4 As can be seen in Table 2, both systems are efficient in the epoxidation reaction. However, the use of hydrogen peroxide leads to some by-products, such as the Michael addition of methoxide to the ketone (6) or the partial hydrolysis of the nitrile group to the amide in the cyanoester (7) (Scheme 5). With both substrates, which bear the same chiral auxiliary, the diastereoselectivity is nearly the same, with up to 50% de obtained with the system KFalumina/TBHP. The use of hydrogen peroxide leads to lower diastereoselectivities, due probably to the smaller size of the peroxide anion, which attacks the double bond. However, solvation effects cannot be ruled out given that in the two heterogeneous systems different solvents are used.

Table 2 Epoxidation of chiral alkenes with heterogeneous systems. % yielda % d.e. a alkene oxidant base % conv. a H202 hydrotalcite 70 60 b 10 6 6 TBHP KF/alumina 100 100 40 7 H202 hydrotalcite 100 70 c 36 7 TBHP KF/alumina 100 100 48 a Determined by 1H-NMR spectroscopy, b 10% yield of several by-products, c 30% yield of nitrile hydrolysis.

The hydrolysis reaction can be used in a preparative way with total selectivity, given that the H202/hydrotalcite system led to a quantitative conversion of the nitrile group in the molecule bearing ketal, epoxide and ester groups (Scheme 5) without problems of hydrolysis of any of them.

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H202

COOCH3

hydrotalcite

~ " ~ .O

CONH 2

O~COOCH

3

Scheme5 4. CONCLUSIONS The catalytic activity of the hydrotalcites in the epoxidation of electron-deficient alkenes depends on the basicity of the solid, which can be controlled by the Mg/A1 ratio. The highest catalytic activity corresponds to a hydrotalcite with a ratio Mg/A1 = 3. The solid with this Mg/AI ratio is also the catalyst with the highest number of basic sites, as measured by phenol adsorption in liquid phase. The epoxide/dioxolane selectivity also increases with the basicity of the solid and the reaction time, showing the reversibility of the reaction of dioxolane formation. The hydrotalcite is recoverable and reusable at least twice without loss of catalytic activity. The heterogeneous systems hydrotalcite/H202 and KF-alumina/TBHP are also active in the epoxidation of chiral alkenes derived from D-glyceraldehyde, which are precursors of interesting products from the point of view of biological activity. Up to 50% de is obtained with the system KF-alumina/TBHP, and this is the most efficient in terms of yield, selectivity to epoxidation and diastereoselectivity. The size of the oxidant (TBHP>H202) and solvation effects (toluene v s . aqueous methanol) may account for this higher efficiency.

Acknowledgement:This

work was made possible by the generous financial support of the

C.I.C.Y.T. (Project MAT96-1053).

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.

C. Cativiela, F. Figueras, J. M. Fraile, J. I. Garcfa, J. A. Mayoral, Tetrahedron Lett. 36 (1995) 4125. V.K. Yadav, K. K. Kapoor, Tetrahedron Lett. 35 (1994) 9481. J.M. Fraile, J. I. Garcfa, J. A. Mayoral, F. Figueras, Tetrahedron Lett. 37 (1996) 5995. B.M. Choudary, M. L. Kantam, B. Bharathi, C. V. Reddy, Synlett (1998) 1203. J. Shen, J. M. Kobe, Y. Chen, J. A. Dumesic, Langmuir 10 (1994) 3902. J.I. Di Cosimo, V. K. Dfez, M. Xu, E. Iglesia, C. R. Apestegui'a, J. Catal. 178 (1998) 499. A.D. Briggs, R. F. W. Jackson, P. A. Brown, J. Chem. Soc., Perkin Trans. 1 (1998) 4097. P.C. Ray, S. M. Roberts, Tetrahedron Lett. 40 (1999) 1779.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

H y d r o x y l a t i o n o f benzene to phenol with nitrous oxide on Fe-silicalites R. Monaci a, E. Rombi "~,M.G. Cutrufelio a, V. Solinas a and G. Berlier l', G. Spotob "Dipartimento di Scienze Chimiche, Universi~ di Cagliari, via Ospedale 72, 09124 Cagliari, Italy bDipartimento di Chimica IFM, Universit~i di Torino, via P. Giuria 7, 10125 Torino, Italy The title reaction was studied, at 623 K and atmospheric pressure, over Fe-si|icalites with different Fe203 content. The best results were obtained for the sample with a Fe203 content of 0.82 wt %. FTIR spectroscopy was used to investigate the nature of the extraframework iron species in Fe-silicalites using NO as probe molecule. 1. INTRODUCTION Phenol is an important intermediate for the production of petrochemicals, agrochemicals and plastic products. More than 90 % of the phenol produced world-wide is obtained by the multi-step process of cumene oxidation. An economical single-step process by means of the direct hydroxylation of benzene to phenol has been studied for some time. Conventional partial oxidation methods with molecular oxygen as oxidant have not given satisfying results, as the reaction mainly leads to the destruction of the aromatic ring [1]. Better results have been obtained in the hydroxylation of benzene with an oxygen/hydrogen mixture, using silicasupported precious metals as catalysts [2]. More promising results seem to derive from the use of alternative oxidants as nitrous oxide. In the first studies on the benzene oxidation by N20, silica-supported vanadium, molybdenum and tungsten oxides had been used as catalysts [3]. More recently, zeolite catalysts as Fe,Na-ZMS-5 and Fe-silicalites structurally similar to ZSM-5 zeolites have been used, providing very' interesting results [4,5]. The iron species located in the so called a-sites, which are particularly suitable for the N20 decomposition, seem to play a key-role on the catalytic performance of these materials, as shown by Panov and co-workers [6,7]. According to them, the N20 decomposition occurs on extraframework iron microclusters and leads to surface reactive oxygen species (a-oxygen), which are responsible for the oxidation of benzene to phenol [8]. The aim of this work is to investigate the influence of the iron content on the catalytic activity of Fe-silicalites and give useful additional information on the surface properties of these materials, by studying the adsorption of probe molecules by FT1R spectroscopy.

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2. EXPERIMENTAL

2.1. Materials Benzene was a > 99 % pure Aldrich reactant; nitrous oxide was a 99.998 % pure SIAD product. 2.2. Catalysts Four different samples of Fe-silicalite, synthesised as reported elsewhere [9], were used. The iron content of the catalysts, expressed as percent by weight of Fe203, is reported in Table 1. 2.3. Apparatus and procedure The experimental runs were performed at atmospheric pressure in a continuous fixed-bed quartz-glass microreactor, with the following operative conditions: temperature, 623 K; space velocity. (WHSV), 4 h-l; R (N20 moles / benzene moles), 5. Hourly collected products samples were dissolved in acetone and analysed by a Carlo Erba 4160 gas chromatograph fitted with a fused silica capillary column (50 m Petrocol DH 50.2, 0.25 mm I.D., 0.25 lure film thickness; Supelco) and a flame ionisation detector. Products were identified by gas chromatography / mass spectroscopy (GC,qVIS) analysis. Phenol was the main product; only at higher temperatures (> 673 K) small amounts of byproducts, formed by further oxidation of phenol, were detected (mainly benzoquinone and benzofuran). The complete oxidation of benzene to carbon dioxide was the main undesired reaction. The results have been expressed in terms of benzene conversion (moles of benzene reacted / moles of benzene feeded) and of selectivity to phenol (moles of phenol formed / moles of benzene reacted). 2.4. IR measurements The IR experiments were carried out on a Bruker IFS 66 FTIR instrument equipped with a cryogenic MCT detector and running at 2 cm -1 resolution. The Fe-silicalite samples were in form of self-supporting pellets suitable for measurements in transmission mode. 3. RESULTS AND DISCUSSION Activity comparison runs were performed on fresh catalyst samples at the previously mentioned standard conditions. To point out the influence of the iron content of the catalysts on their activity, Figure 1 shows the selectivity towards phenol for three Fe-silicalites at conversion values of 10 mol % (3 h on-stream) and 4 mol % (7 h on-stream), for different weight percentages of Fe203. The Fe-Si-1 sample, containing 0.06 % of Fe203, turned out to be inactive. As it can be seen from the figure, selectivity increases with the Fe203 percentage, reaching the highest value for an Fe203 content of 0.82 % (Fe-Si-3). Higher Fe203 contents mainly favour the complete oxidation at the expense of the phenol fbrmation. Figure 2 shows the change in conversion and selectivity to phenol with time-on-stream for the Fe-Si-3 sample. Conversion decreases during the run, whereas selectivity increases. The same trend was shown by all the samples.

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Table 1. Iron content of Fe-silicalite catalysts. Catalyst

Fe-Si- 1

Fe-Si-2

Fe-Si-3

Fe-Si-4

FezO3 (wt %)

0.06

0.36

0.82

1.70

100-

o

L%_, /

80-

E 60-

~

I

J /

/l .-ill ~Jm

~

40r~

I

20O0.36

/Am IWT

0.82

1.70

F e 2 0 3 ( w t o4)

Figure 1. Selectivity to phenol at 10 % (m) and 4 % (i--1) conversion.

100 80

Z

60

o

E

40 20

~.......................i't!!!i...... ,::. I

1

t

|

2

I

i

3

I

|

4

I

|

5

I

!

6

l

I

7

'";~;':'

8

t-o-s (h) Figure 2. Conversion (ii!ii~)and selectivity (A) vs. time-on-stream; Fe-Si-3 catalyst, T - 623 K, WHSV = 4 h -1, RN20/C6H6 = 5.

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As reported by other authors [ 10], the benzene oxidation by N20 is accompanied by coke fbrmation and subsequent catalyst deactivation. Removing coke from the catalysts, by burning it out in air flow, resulted in the recovery of their catalytic activity. The decrease in the conversion value during the run and the complete deactivation of the catalyst within 10 hours on-stream can therefore be ascribed to the production of phenol and polyphenols, which are coke precursors. A possible interpretation of the selectivity increase with time-on-stream could be the presence, on the surface, of catalytic sites which can activate N20 in different ways and then produce phenol or favour the total oxidation. The nature of the extraframework iron species was studied by FTIR spectroscopy, using NO as probe molecule, on the most selective catalyst (Fe-Si-3). The IR spectrum of the sample (previously outgassed at 773 K), in equilibrium with 10 torr of NO at room temperature, is shown in Figure 3 (dotted line). In the same figure, the effect of the gradual decrease in the equilibrium pressure of NO is also reported (full and dashed lines). Examining the figure, it can be said that: i) at the maximum NO coverage, the spectrum is dominated by a strong absorption at 1808 cm ~ superimposed on a second band centred at 1839 cm -~. Weaker bands appear at 1914 and 1765 cm-1; ii) reducing the NO pressure, the bands at 1914 and 1808 cm- are progressively erosed and in the end completely disappear. This phenomenon is 1.5

1.5-

~ ,

t2

~ ~

! !

1.2-

0.9

D.9-

=o

r-

tO

<

~o 0.6

0.6.-I

0.3

0.3"

0.0-_

1950

.

,-

-i

'"

1900

"

"'

i ~

1850

=,

-t .....

1800

,

1

1750

'

Wavenumber(cm"1)

Figure 3. FTIR spectra of NO adsorbed at room temperature on Fe-Si-3. Dotted line: in presence of 10 torr of NO; dashed line: after outgassing for 15 minutes; full lines: intermediate NO coverage.

-

1 '00

0.C 19~o

["

/

/,, i

I

19oo

i

~o

i

~6o

~

1~o

i

1700

Wavenumber(cm")

Figure 4. FTIR spectra of NO adsorbed on Fe-Si-3 (dashed line) and on Fe-Si-3 containing preadsorbed H20.

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accompanied by the simultaneous (and parallel) intensification of the signals at 1839 and 1765 cm ~ (which finally dominate the spectrum: dashed line in Figure 3). According to literature data [11,12], the bands at 1914, 1839, 1808 and 1765 cm -~ can be assigned to nitrosylic complexes formed on extraframework (or partially extraframework) ferrous species (FeA sites). As previously seen (Figure 2), during the first hours on-stream the complete oxidation of benzene prevails; this reaction leads to the formation of three moles of H20 for every mole of benzene reacted. Therefore, to test the influence of water on the surface of Fe-silicalites, FTIR measurements on the Fe-Si-3 sample containing preadsorbed H20 were carried out. Figure 4 shows the spectra collected with the maximum NO coverage on a sample simply outgassed at 773 K (dashed line) and on a sample previously contacted with 5 torr of H20 at room temperature, evacuated under high vacuum for 10 minutes at the IR beam temperature and finally contacted with NO (full line). It can be noted that by pretreating with water: i) the bands at 1914, 1808 and 1765 cm -~ become stronger; ii) the increase in their intensity is accompanied by a decrease in the abso~tion at 1839 cm -1. The erosion of the band at 1839 cm-, due to the water adsorption, can also be seen at low coverage [ 13]. These findings lead to the conclusion that the absorption at 1839 Cln-~ originates from the presence of at least two different components, one of which is selectively depleted by the absorption of water. To explain this thct it can be thought that on the Fe-silicalite surface, together with the FeA sites, another family of ferrous sites (FEB) is present. On the samples without water, these centres form, by NO adsorption, mononitrosylic complexes, which absorb at 1839 cm ~. After H20 adsorption, the sites become able to further interact with NO, forming polynitrosylic complexes, which are spectroscopically indistinguishable from those formed on the FeA sites. Thus, water translbrms the Fen sites into a new species with NO absorption properties similar to those of the FeA centres. As regards the nature of these t~rrous centres, their different tendency to adsorb NO (at least a trinitrosyl can be tbnned by FeA and only a mononitrosyl by Few), is a clear clue to different coordinative states, higher for FeB than for FeA. Spectroscopic results suggest that the FeA sites are isolated, low coordinated iron centres (probably mononuclear) ~afted to the framework, while FeB species belong to (FeO)n extraframework (or partially extraframework) clusters which can be broken up by H20, producing more reactive Fea-type species. It is therefore probable that the catalytic surface initially consists of a certain number of Fea and FeR sites, the former being more selective towards phenol and the latter being responsible for the reaction of complete oxidation. During the first hours on-stream, the complete oxidation prevails, as shown by the high quantity of CO2 initially produced. That also implies a remarkable production of Ha0, whose effect on the sites already present on the surface is to transform the FeR centres into FeA-type, as shown by the IR spectra. The increase in the number of FeA-type sites can be correlated with the higher selectivity towards phenol (Figure 2). Thus, as the Fe~ sites turn into FeA-type, the production of CO2 decreases in favour of phenol. The variation in selectivity, due to the presence of the different ferrous sites, FeA and FeB, can be explained assuming that they adsorb N20 producing oxygen species of different nature. In our opinion, the prevailing formation of phenol could be ascribed to a nucleophilic species like 02-, responsible tbr the partial oxidation; on the other hand, species with an

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electrophilic character, like O2- and O, would attack the aromatic ring in the region of the highest electronic density, leading to the rupture of the benzene ring and the prevailing formation of CO2. 4. CONCLUSIONS Within the tested series of Fe-silicalite catalysts, the best results were obtained with the sample containing an Fe203 percentage around 1%. All the catalysts deactivated during the run and the selectivity towards phenol seemed to be correlated with extraframework ferrous sites, identified by FTIR spectroscopy, which would be able to provide nucleophilic oxygen species responsible for the partial oxidation. AKNOWLEDGEMENTS The financial support ofMURST (Cofin 98) is gratefully acknowledged. REFERENCES

1. T. Fukushi, W. Ueda, Y. Morikawa, Y. Moro-oka, T. Ikawa, Ind. Eng. Chem. Res., 28 (1989) 1587. 2. T. Miyake, M. Hamada, Y. Sasaki, M. Oguri, Appl. Catal., 131 (1995) 33. 3. M. [wamoto, J.I. Hirata, K. Matsukami, S. Kagawa, J. Phys. Chem., 87 (1983) 903. 4. G.I. Panov, G.A. Sheveleva, A.S. Kharitonov, V.N. Romannikov, L.A. Vostrikova. Appl. Catal. A, 82 (1992) 31. 5. A.S. Kharitonov, G.A. Sheveleva, G.I. Panov, V.I. Sobolev, Ye.A. Paukshtis, V. Romannikov, Appl. Catal. A, 98 (1993) 33. 6. G.I. Panov, V.I. Sobolev, A.S. Kharitonov, J. Mol. Catal., 61 (1990) 85. 7. V.I. Sobolev, G.I. Panov, A.S. Kharitonov, V.N. Romannikov, A.M. Volodin, Kinet. Katal., 34 (1993) 797. 8. V.I. Sobolev, G.I. Panov, A.S. Kharitonov, V.N. Romannikov, A.M. Volodin, K.G. Ione, J. Catal., 139 ( 1993) 435. 9. S. Bordiga, R. Buzzoni, F. Geobaldo, C. Lamberti, E. Giamello, A. Zecchina, G. Leothnti, G. Petrini, G. Tozzola, G. Vlaic, J. Catal., 158 (1996) 486. 10. M. Hfifele, A. Reitzmann, D. Roppelt, G. Emig, Appl. Catal. A, 150 (1997) 153. 11. S. Yuen, Y. Chen, J.A. Dumesic, N. Topsoe, H. Topsoe, J. Phys. Chem, 86 (1982) 3022. 12. L.M Aparicio, W.K. Hall, S. Fang, M.A. Ulla, W.S. Millman, J.A. Dumesic, J. Catal., 108 (1987) 233. 13. G. Spoto, A. Zecchina, G. Berlier, S. Bordiga, M.G. Clerici, L. Basini, J. Mol. Catal., (1999), submitted.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

化

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A m m o x i d a t i o n o f p r o p a n e to a c r y l o n i t r i l e o v e r V - S b - A l - o x i d e c a t a l y s t s - I n f l u e n c e o f c a t a l y s t p r e p a r a t i o n on r e a c t i o n k i n e t i c s O. Ratajczak

a,

H. W. Zanthoff b and S. Geisler

a

a Lehrstuhl ftir Technische Chemie, Ruhr-Universit~it Bochum, D-44780 Bochum, Germany b Institut de recherches sur la catalyse, CNRS, 2. Av. Albert Einstein, 69626 Villeurbanne Cedex, France The influence of the preparation method and the aluminium precursor for V-Sb-Me-AI-O catalysts on their reaction kinetics and catalytic performance in ammoxidation of propane were studied under atmospheric conditions in a micro catalytic fixed bed reactor. Catalyst prepared from A1203 were more active than one prepared from AI(OH)3. The obtainable selectivity to acrylonitrile depends on the acidity of the alumina precursor. Residual amounts of alkali on the catalyst surface were found to be detrimental for obtaining high yields to acrylonitrile. 1. I N T R O D U C T I O N The direct ammoxidation of propane is an alternative route of technical interest for the formation of acrylonitrile which is presently produced by ammoxidation of propene using the SOHIO/BP process. Modified multicomponent oxides based on vanadium and antimony, e.g. VSbxMezAlaOy with Me = W, Mo, Te, Sn, P etc., exhibit a high activity and selectivity for this reaction [1-4]. Yields to acrylonitrile of up to 58.9 % (Xc3H8 = 85.2 %) have been reported so far [4]. However, knowledge of the relation between the physico-chemical properties and catalytic performance might lead to the development of even better catalysts being viable for technical application. Very little is known about the role of the preparation method and the precursors of the V-Sb-Me-AI-O catalysts for their catalytic performance. The present study aimed at the investigation of the influence of the preparation method of a VSbsWOx(30 wt%)/A1203 catalyst on the catalytic performance in the ammoxidation of propane with special emphasis on the role of the alumina precursor on the kinetics of this reaction. 2. E X P E R I M E N T A L 2.1. Catalyst p r e p a r a t i o n

VSbsWOx(30 wt-%)/A1203 catalysts were prepared in an aqueous redox reaction as described earlier [3] using NH4VO3, Sb203 and H26N6040W12 as starting materials. Alumina was added as AI(OH)3 (in the following named Cat-a) and T-A1203 of different acidity: pH = 4.1 (Cat-b), pH -- 6.2 (Cat-c) and pH = 9.5 (Cat-d). The resulting solids were calcined in air for 24 h at 623 K, 3 h at 773 K and 3 h at 900 K. After subsequent crushing and sieving the fraction with a particle size of dp = 250 to 355 lam was used for the experiments.

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2.2 Catalyst characterisation

Bulk and surface physico-chemical properties of the samples were investigated using FTIR, XRD, 27AI-MAS-NMR, BET, and XPS. FTIR spectra (400 to 4000 cm -1) were taken from samples mixed with KBr (1:10) using a Nicolet FTIR-Spectrometer Proteg6 460. XRD spectra were recorded in the range of 20 = 10 ~ to 80 ~ using a Siemens powder diffractometer D 500 with Cu-Ka-radiation. BET surface areas were determined applying N2-adsorption at 77 K. XPS data were obtained in a Leybold LHS-10 MCD equipment using A1Ka radiaton.

2.3. Catalytic performance and kinetic measurements The catalytic performance of the different solids was investigated in a micro catalytic fixed bed reactor (i.d. = 14 mm) made of quartz under steady state operation conditions. In order to avoid temperature hot spots the catalysts were diluted with quartz at a ratio of 1:2 of the same particle size. Inlet gas mixtures with C3H8/O2/NH3/Ne = 1:1.75:1.25:1 and 1:2:2:2.5 were used (Ptot = 0.1 MPa; TR = 773 K; mc,,t./V = 0.635 to 2.541 g.s-ml-l). The gas compositions were analysed using a SATOChrom gaschromatograph. The following products were detected: C3H6, C2H4, acrylonitrile (ACN), acetonitrile (AcCN), acroleine (ACR), HCN, CO, CO2 and N2. HCN which only occurred at high conversion levels was not quantified. Carbon mass balances were between 95 and 100 %. Kinetic measurements were performed under differential conditions (all Xi < 15 %) in the micro catalytic fixed bed reactor described above (V = 50 to 200 ml.min -~, meat= 0.1 tO 3.0 g, TR = 673 to 823 K) using the catalysts Cat-a and Cat-b. The influence of feed partial pressure of C3H8, 02 and NH3 (Pc3n8= 0 to 34 kPa; Po2 = 5 to 45 kPa; PNH3= 0 to 50 kPa) on conversion and product formation rates was investigated. The values of the calculated Dart- and Weisz numbers showed that neither external nor internal mass transport limitation occurred under the reaction conditions applied.

2.4. Determination of kinetic parameters (ko, Ea, Kads) Using the results of our earlier mechanistic studies [5,6,7,8] and the results of the present kinetic measurements the ammoxidation of propane over V-Sb-Oxides occurs in a complex reaction network as proposed in Figure 1. L.~.w..~J.=.=, ( ~

I .......

c,..- G '

!"

co...;-;.=

: i

...'".:\

C=H4" (~ ~AcCN .~

co, _,

,,=f=~=~l..===

ACR

!(~

NH3- (~ ~

(NOx)- (~ --~ N2

}

|

,,,CN4.i i

Fig. 1. Proposed reaction network of the ammoxidation of propane on V-Sb-oxide based catalysts (dotted arrows: consecutive reactions)

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C3H6, C2H4, and CO were found to be the only carbon containing primary products. CO2 was clearly identified as a consecutive product since CO2 selectivity approaches zero if extrapolating to zero conversion. CO cannot be oxidised in significant amounts to CO2 under the conditions applied as was found in separate experiments. The kinetic parameters of the four primary reactions (no. 1-4) were determined using a two site redox- model applying Mars-van-Krevelen rate equations for all hydrocarbon reactions (eq. 1-3), a Langmuir-Hinshelwood rate equation for N2 formation (eq. 4) and an isothermal, homogeneous plug-flow reactor-model for the reactor hydrodynamics.

r

k o, "P(OE)'kc, H" p ( C 3 H s ) C.~H~"

k o, . P ( O E ) + • 2 c,H~ 9P ( C 3 H 8 ) + 2• c,H, 9P(C3Hs)

(1)

ko, . P(O2)'kc:H, " p ( C 3 H s ) rc., Ha

ko, " p ( O 2 ) + • 2 c,H~ 9p ( C 3 n s ) + 8 9

k~ " P(O2)" rCO --

(,I

kN~

7

+ -2 kco" P(C3Hs)

K o " P(02)" KC3H 8 9P ( C 3 H 8 ) A PNH3

(2)

1 + p ( 1N H 3 ) -I" kc~ " P(C3Hs)

k~ 9P(02)" I + p ( N H 3 )

rN2

p(C3H8)

(1 + K;2 . P(02) + KC3H 8 9P ( C 3 H 8 ) ) 2

(3) (4)

The kinetic parameters were estimated using a database of 45 experiments for each catalyst applying a genetic algorithm followed by a simplex optimisation. The consecutive products AcCN, ACR, ACN and CO2 were also observed in minor amounts under differential conditions. Their amount was lumped together with C3H6 to a pseudo component C3H6+. 3. RESULTS AND DISCUSSION

3.1 Physico-chemical properties of the catalysts The catalysts prepared exhibited similar bulk compositions. All catalysts are nearly x-ray amorphous; only minor signals of crystalline Sb204 are detectable. =VSbO4 and A1203 were observed as microcrystalline phases [3]. FTIR and 27AI-MAS-NMR investigations showed that alumina is present in the catalysts exclusively as T-A1203. No indication for significant formation of Al-mixed phases (e.g. A1SbO4 or A1VO4) was found. BET surface areas of the fresh catalysts amounted to 100 mZ.g-I (Cat-a) and 66 to 77 mZ.g-I (Cat-b, Cat-c and Cat-d). After >10 h ammoxidation reaction all catalysts exhibited surface areas between 51 and 77 mZ.g-~. XPS analysis was performed for Cat-a and Cat-b only. 3 % Na was observed on the surface of Cat-a whereas the amount of Na on the Cat-b was below the detection limit. As proven by ICP technique the Na originated from the catalyst starting materials.

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3.2. Catalytic performance The prepared catalysts exhibited different activities and selectivities in the ammoxidation of propane to ACN (cf. Figure 2). The activity was found to decrease in the order Cat-d > Catc = Cat-b > Cat-a. The catalyst Cat-a which was prepared using AI(OH)3 exhibited lowest conversion and ACN selectivities. Among those catalysts prepared using the different A1203 Cat-d exhibited lowest ACN selectivities whereas the highest ACN selectivities were obtained for Cat-c at a given degree of conversion. On the contrary, the highest propene selectivities were obtained for Cat-d. A maximum propene yield of 10.3 % was obtained for the Cat-b (Xc3rm = 27 %). Using an optimised C3H8/O2/NH3/He-ratio of 1:2:2:2.5 a maximum ACN selectivity of 51,3 % was obtained for Cat-b (Xc3H8 = 44.4 %, 5C3H6- 2 0 . 1 % ) , which is higher than values reported in the literature so far for similar reaction conditions (SAcN= 45 %, XC3H8 -- 45 %, 5C3H6 = 15 %, C3H8/O2/NH3/H20/He = 1:2:1:0.5:2.5; TR = 753 K) [2].

100 50 ..,...

~

80

-

4o

z 0< 30 o9 20

zx, <>

co

-1- 60 0 40

,

9 Cat-d 9 Cat-c 9 Cat-b 9 Cat-a

co

jJ

t,'Y

10 I , ' r

[,~,

O0

"9

Cat-c A , 9 Cat-b 0 , 9 Cat-a

, lo

2'o

' a'o

X(C3Hs) / %

' 4o

20 O0

l

10

,

,

2'0 3'0 X(C3Hs) / %

|

I

40

Fig. 2. The dependence of ACN and C3H6 selectivity on propane conversion during ammoxidation of propane. (closed symbols: C3H8/O2/NH3/Ne = 1"1.75"1.25"1" open symbols: C3H8/O2/NH3/Ne = 1:2:2:2.5" Ptot = 0.1 MPa; TR = 773 K; dashed line: Nilsson et al. 1997 [2]) 3.3. Kinetics of the ammoxidation of propane The kinetics of the primary reaction steps of the low active catalyst Cat-a (prepared from AI(OH)3) and the highly active catalyst Cat-b (prepared from ?-A1203) were determined in the temperature range from 673 K to 823 K. Figure 3 shows as an example the dependence of the C3H6+, C2H4 and N2 formation rates on the 02 partial pressure for both catalysts. Cat-b exhibits higher reaction rates towards C3H6 but unfortunately also towards the undesired side product N2. Under the differential reaction conditions applied NH3 is converted to N2 with selectivities > 90 %. The reaction network can be modelled using Mars / van-Krevelen rate equations for the formation of propene, ethylene and CO with two different reactive sites and using a Langmuir-Hinshelwood approach for the N2-formation. From parity plots (not shown as figures) it can be deduced that the product partial pressures (C3H6+, C2H4, and N2) are fitted with an error lower than + 20 %, whereas that of the feed gases (C3H8, 02 and NH3) could be fitted with an error lower than + 10 %. The partial pressures of CO exhibited a higher error

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because the formation of CO under differential conditions is low, close to the detection limit. The application of models using only one active site did not result in acceptable agreement between experimental and fitted data. Furthermore, the rate equations proposed by Catani et al. [1] for the same type of catalyst did not result in an acceptable agreement, especially because their model cannot describe the reaction in the absence of NH3 and the N~ formation rates. The kinetic parameters obtained are listed in Table 1 for both catalysts. The most characteristic difference between the parameters for the Cat-a and Cat-b is that the activation energies for the reoxidation of the active sites are much lower for the more reactive catalyst Cat-b. "7, {:}}

..~

"7,

o

~

Cat-a

3,0

4ol \

9

2,5

E 2,0 ~'o,_ 1,5

/

9

"~r

773 K

301

Cat-a

9 9~

,

1,4

"

773K

Cat-a

~'=

":'~

9 9 723 K

-~.

823 K

T z

o ~ 05

" ~176 i'o2'o

}

o,%

3'04'0

~

~

"

3,0

Cat-b * 673K 9 723K

2,5

20

9 77aK

- ls ff ~o

L)

0,5 ~

IT"

A

0'% ~ : - 1 0

9

9

~

0_..0,2 ~ r z - - - - z - - :

~ 4,0 -r, "~ 3,0

9

%

2o

z" lo

~

v

Cat-b *

~

673K 9 723K

r 9 0,3 "t

~ o,2

*

Cat-b 673 K 9 723 K 9 773 K

~~ o,1 "q,

:.

==

9

9 9

9

20

30

p(O2) / kPa

4'0

0,%

.

p(O2) / kPa

9 77aK

.

._

~'o 3'o '.'o "

p(O2) / kPa

9

~

o

~'o 2'o 3'o~.'o=~ = ~176

P(O2) / kPa urJ

_

10

20

30

P(O2))

40

0,00-;

10, ; .

2'0" . a0. : . 4*0 * P(O2) / kPa

Fig. 3. Dependence of the C3H6 +, N2 and C2H4 formation rates on oxygen partial pressure (symbols" experiment; lines: model; TR= 673 K - 823 K, C3H8 9NH3 = 2:2.5, XC3H8 < 10 %) The XPS characterisation results presented above revealed that Cat-a exhibits a significant amount of sodium ions on the surface (3 %) whereas for Cat-b the surface coverage of Na + was below the detection limit. No other significant differences were observed for these two catalysts. It might be therefore assumed that the Na ions hinder the reoxidation of the catalytically active sites in Cat-a (prepared from AI(OH)3). In fact in further preliminary experiments we observed that the activity of the catalyst is reduced if the surface is poisoned with additional amounts of sodium ions after catalyst preparation. A decrease of activity and selectivity due to poisoning with alkali has been observed earlier by Centi et al. [8] for the butane oxidation over a K modified (VO)2P207 catalyst. The site blocking effect there, however, was attributed to the decrease in the surface acidity whereas we have shown earlier that surface acidity on modified V-Sb-Al-oxide catalysts mainly affects ACN selectivity rather than catalyst activity [3].

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Table 1. Parameter estimates for reactions 1 - 4 for the catalysts Cat-a and Cat-b Cat-a

Cat-b

k773K mol.kg-l.s'l.kPa -I

EAkt. kJ.mol -I

k773K mol.kg-l.s-l.kPa -I

EAkt. kJ.mol -I

k (O l)

2.82.10 -~

100.9

5.03.10 ~

40.3

k (0 2)

3.78.10 -~

125.8

2.62.10 .03

29.4

k(C3H6 +)

1.38.10 -~

103.6

2.34.10 ~

95.9

k(CO)

3.60-10 .05

89.9

9.51.10 .06

109.8

k(C2H4)

3.40.10 .06

166.2

2.86.10 .06

135.0

k(N2)

1.86.10 .03

209.5

4.85.10 .03

76.3

K(O2)

3.83.10 -~

1.0

2.17.10 -~

3.5

K(C3H8)

1.04.10 -~

243.6

5.90.10 -~

1.5

4. CONCLUSIONS The influence of the catalyst preparation method on the catalytic performance was investigated by changing the type of alumina precursor. It was found that catalysts prepared from ]t-AI203 are much more active in the ammoxidation of propane than those prepared from a AI(OH)3 compound. The ammoxidation reaction under differential conditions can be well described (+ 20 %) using a two-site-redox-model applying Mars/van-Krevelen rate equations for all primary hydrocarbon reactions and a Langmuir-Hinshelwood rate equation for N2-formation. The analyses of the kinetic parameters for the primary reaction steps on a catalyst prepared with AI(OH)3 and one prepared with A1203, which exhibited largest differences in catalytic activity, showed that the estimated kinetic parameters disclose much lower activation energies for the reoxidation rate constant of the active sites of the catalyst prepared with A1203. These explain its higher activity compared to the catalyst prepared with AI(OH)3. REFERENCES 1. R. Catani, G. Centi, F. Trifiro, R.K. Grasselli, Ind. Eng. Chem. Res. 31 (1992), 107. 2. J. Nilsson, A.R. Landa-C~novas, S. Hansen, A. Andersson, Stud. Surf. Sci. Catal. 110 (1997), 413. 3. H.W. Zanthoff, S. Schaefer, G.U. Wolf, Appl. Catal. A: General 164(1-2) (1997), 105. 4. X. Wu, Q. Zhang, D. Lu, Stud. Surf. Sci. Catal. 110 (1997), 1107. 5. S.A. Buchholz, H.W. Zanthoff, ACS Symp. Series 638 (1996), 259. 6. H.W. Zanthoff, S.A. Buchholz, O.V. Ovsitser, Catal. Today 32(1-4) (1996), 291. 7. S.A. Buchholz, H.W. Zanthoff; DGMK-Berichte 9803 (1998), 37. 8. G. Centi; G. Golinelli; F. Trifiro; Appl. Catal. 48 (1989), 13.

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化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Ammoxidation of propylene to acrylonitrile catalyzed by multimetal molybdate- and iron antimoniate-based active compounds dispersed in oxidic matrixes: the effect of the dispersing oxide on the catalytic performance R. Catani b, F. Cavani a, U. Cornaro b, A. Del Bianco a, E. Frontani a, D. Ghisletti b, A. Tasso b, F. Trifir6 a aDipartimento di Chimica Industriale e dei Materiali, Viale Risorgimento 4, 40136 Bologna, Italy bSnamprogetti, Via Maritano 26, 20097 S. Donato Milanese, MI, Italy The effect of three different dispersing agents, silica, y-alumina and tin oxide, on the catalytic performance of multimetal molybdate- and antimoniate-based catalysts was evaluated in the reaction of propylene ammoxidation to acrylonitrile. Silica provided the better dispersing effect on the active components, due to both a negligible direct contribution of silica to the reaction, and a weak chemical interaction with the active components. 1. INTRODUCTION The ammoxidation of propylene to acrylonitrile is carried out in fluidized-bed reactors, catalyzed by heterogeneous multicomponent systems which consist of either multimetal molybdates, or antimoniates of U or Fe, or mixed molybdates/antimoniates [1]. In all cases the active phase is dispersed inside a silica matrix, to achieve a resistant and fluidizable catalytic material. Silica is considered the most suitable dispersing material, since it is the least reactive towards organic substrates, and gives the final catalyst the best mechanical and chemical-physical properties. In this work we compare the effect of three dispersing oxides: i) silica, ii) ~/alumina and iii) cassiterite SnO 2, on the catalytic performance of multimetal molybdates and of iron antimoniate in the ammoxidation of propylene to acrylonitrile. This comparison makes it possible to evaluate the use of oxidic materials with different surface properties as dispersing agents for mixed oxides in complex catalytic systems used for several applications in the field of selective oxidation of organic substrates.

2. EXPERIMENTAL Three different dispersing oxides were prepared:

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1) silica, from a SiO2 sol Ludox AS 40 Du Pont; after spray-drying and calcination at 500~ the surface area was 125 m2/g; 2) t-alumina, from a A1OOH sol AL-20 Nyacol; surface area of calcined material was 188 m2/g; 3) cassiterite, from a SnO2 sol SN15CG Nyacol; surface area 50 m2/g. Two different mixed oxides were prepared as active compounds: 1) Fe/Sb 1.0/1.5 (atomic ratio); a slurry was prepared which contained fused Fe(NO3)3.9H20, Sb203 and HNO3 in the right proportions (slurry-redox method). Drying of the slurry and calcination o-T~e solid at 500~ led to the development of the rutile-type FeSbO4 compound, as shown by means of DRIFT spectroscopy and XRD. Other compounds identified were Sb204 cervantite (27 wt.%), Sb203 senarmontite (9 wt. %) and Fe203 (9 wt. %). The same compounds were also identified in dispersed, spray-dried systems. Only in the case of tin oxide-dispersed catalyst was identification of crystalline compounds present in the active phase not possible, since SnO2 and FeSbO4 are isostructural. 2) Bi/Fe/Ni/Co/Mo 1.0/1.9/4.6/9.7/18.6 (atomic ratios); aqueous solutions of (NH4)6Mo7024.4H20 with NH4OH and Bi(NO3)3.5H20 with HNO3 were mixed in HNO3 under stirring (pH = 2, maintained through addition of NH4OH). A precipitate was obtained, which was filtered and washed, dried and calcined at 500~ Separately, aqueous solutions of (NH4)6Mo7024.4H20 with NH4OH and Fe(NO3)3.9H20 were mixed under reflux temperature while stirring. The resulting precipitate was filtered and washed, dried, and calcined at 500~ A third precipitate was obtained by mixing an aqueous solution of (NH4)6Mo7024.4H20 and NH4OH with a second solution containing Co(NO3)2.6H20 and Ni(NOg)2.6H20, under reflux while stirring. The precipitate was filtered and washed, dried, and calcined. The three precipitates were then mixed through redispersion in water, in the relative amounts necessary to obtain the desired composition; the resulting slurry was filtered and the solid was washed and dried. Calcination of this solid at 500~ led to the development of the following crystalline compounds: Bi2Mo3012 (34 wt.%), Fea(MoO4)3 (42 wt.%), [3-NiMoO4 + ~-CoMoO4 (24 wt.%). When dispersed in ~/-alumina, only crystalline Fe2(MoO4)3 and [3-(Ni,Co)MoO4 could be unambiguously identified (the presence of Bi3FeMo2012 however could not be excluded). When dispersed in silica, the following crystalline compounds were identified: ~-(Ni, Co)MoO4, BiaMo209, Fe2(MoO4)3, Bi3FeMo2012. Finally, dispersion in tin oxide led to a catalyst where it was not possible to identify any crystalline compound relative to the active phase. In all cases, the active phases were dispersed in the oxidic gels (60 wt.% of the dispersing oxide), and then the slurry was homogeneized and spray-dried at 300~ using a laboratory spray-drier ((Lab-Plant SD-04). The solid was finally calcined at 500~ The surface area of the dispersed active phase B i / F e / N i / C o / M o / O was 128 m2/g for the alumina-dispersed system, 89 m2/g for the silica-dispersed system and 81 m2/g for the tin oxide-dispersed system. Catalytic tests were carried out in a fixed-bed glass laboratory reactor, loading I g of catalyst, and operating at atmospheric pressure. The typical feedstock composition was the following: 7.5 vol.% propylene, 10.4 vol.% ammonia, 17.3 vol % oxygen,

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remainder nitrogen. The residence time (W/F) was equal to 2 g m1-1 s. Analysis of products was carried out through on-line sampling of a defined volume of the effluents to gas-chromatographs equipped with TCD. 02, N2 and CO were analyzed using a MS 5A packed column, while all the other unconverted reactants and products formed were analyzed using a Hayesep-T packed column. /" 3. RESULTS AND DISCUSSION 3.1 The reactivity of the dispersing oxides First the reactivity of the dispersing oxides alone, calcined at 500~ was studied. Figure 1 reports the conversion of propylene, the conversion of ammonia and the selectivity to N2, which was formed by combustion of ammonia, as functions of the reaction temperature for the three oxides. Figure 2 reports the distribution of the Ccontaining products at 465~ 70

100

60

80

50

7

.o 40 u')

or)

60

>

40

o

20

E

>

30

c- 20 o o 10

cO

-

-

w--~

0

400

420

440

temperature,~

460

480

400

r

r

=

I

I

I

420

440

460

480

temperature,~

Fig. 1. Conversion of propylene (left), and ammonia (right, full symbols) and selectivity to N2 (right, open symbols) as functions of temperature for the three dispersing agents: ?-alumina (e), silica (11), and tin oxide (&). The three dispersing oxides showed considerable differences in reactivity towards propylene. On the contrary, the interaction with ammonia and its transformation either to C,N-containing products (acrylonitrile, acetonitrile, cyanidric acid) or to the combustion product N2 (selectivity to N2 in Figure 1) was similar for all the materials. Silica and alumina showed quite different product distributions. In the case of ?alumina the prevailing products were CO and acetonitrile (no formation of C3 products was observed), while with silica the main product was HCN. Tin oxide yielded almost exclusively CO2. In the case of alumina, surface acidity can be responsible for the formation of an ionic intermediate, which is then cracked to yield C1 and Ca fragments, precursors of CO2 and acetonitrile formation, through unselective oxidative attack and ammonia insertion, respectively. It is worth mentioning that the molar productivity of CO2 and acetonitrile was the same, thus suggesting a common intermediate.

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100 80

>~

-

Elacrylonitrile

-

BHCN

Ilacetonitrile 60

~>

o

(D

40

Ilacrolein

-

E]CO ielCO2

20 0

-------alumina

silica

tin oxide

Fig. 2. Selectivity to the different products obtained in propylene ammoxidation with the three dispersing oxides (calcined at 500~ Temperature 465~ However, the product which was formed to the greatest extent was CO, the formation of which probably arose from another type of reaction intermediate. It is reported [2] that the interaction between Lewis-type acid centers on coordinatively unsaturated A13+ sites and the double bond of propylene, generates an allylic radicalic intermediate via H. abstraction by the O atom bridging two A13+ions. In the case of silica, the quite different distribution of products suggests a reaction mechanism different than with alumina. It is known that silica is able to activate paraffins and oxidize them, even though in many cases this role has been attributed to the impurities of metals present in silica [3]. However, with extremely pure silica the formation of 02.- species occurring on defect surface sites has been demonstrated, which might be responsible for the transformation of propylene to C1 fragments containing N and O (HCN and COx) [4]. Finally, tin oxide was extremely active, and catalyzed the combustion of propylene to CO2. Tin oxide is a n-type, wide-gap semiconducting oxide, and it is known that it can catalyze redox-type reactions in the presence of 02 and of a reductant (an hydrocarbon), with involvment of the couple Sn4+/Sn 2§ However, it is not able to promote selective O2--insertion onto activated hydrocarbons, but rather it catalyzes oxidehydrogenation reactions, such as the formation of ethylene from ethane or of formaldehyde from methanol [5]. In our case, in the absence of NH 2-- or of O2--insertion sites the intermediate formed undergoes unselective oxidative attack with formation of CO2. Notwithstanding the differences observed in propylene ammoxidation, all three oxides behaved similarly in the activation of ammonia, in terms of both conversion and selectivity; in all cases the main N-containing product was N2. This suggests that the surface sites responsible for propylene and ammonia activation are likely not the same. Alternatively, it is possible that in these cases the mechanism of ammonia activation and combustion is not heterogeneous, but rather mainly homogeneous.

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3.2 The reactivity of the active compounds dispersed in oxidic matrixes Figure 3 (left) shows the propylene conversion as a function of temperature for Fe/Sb/O, dispersed in alumina, silica and tin oxide. Figure 3 (right) reports the distribution of the products at 439~ for the same catalysts. Figure 4 shows the propylene conversion and the distribution of the products for the multimetal molybdate, dispersed in the three oxides.

~" C) ~)

o

80

100

60

8O

40

eo .4..J

cD CD {/)

2o

410

I

I

I

I

I

420

430

440

450

460

temperature, ~

40 20

470

alumina

silica

tin oxide

Fig. 3. Conversion of propylene (full symbols) and ammonia (open symbols) (left) and selectivity to the different products at 439~ (right) for the active phase Fe/Sb/O dispersed in ~,-alumina (o), silica (m), and tin oxide (A). Captions for Fig. 3 (right) as in Figure 2. Figure 3 shows that in all cases the conversion was independent of the reaction temperature. This was because oxygen was the limiting reactant (in all cases conversion was already practically complete at the lowest temperature), while the conversion of ammonia was the highest for the silica-dispersed system. The distribution of the products was similar for all catalysts, and this indicates that in all cases the reactivity of the dispersing matrix was negligible with respect to the activity of the antimoniate (except maybe in the case of the tin oxide-dispersed system, for which the highest selectivity to CO2 was obtained). The very low selectivity to acrylonitrile for all catalysts was likely due to the presence of Fe203, as evidenced by XRD. Calcination of the Fe/Sb/O system at 700~ is likely necessary to obtain complete development of the rutile structure. In the case of the active phase B i / F e / N i / C o / M o / O , considerable differences in reactivity between the three systems could be observed, both in propylene and in ammonia activation (Figure 4). The conversion of the silica-dispersed system increased with increasing temperature; only in this case was total conversion of oxygen reached at the highest temperature. In the case of the alumina- and tin oxidedispersed systems the conversion of propylene was instead limited by oxygen. In the tin oxide-dispersed catalyst the support was clearly responsible for the higher formation of CO2 (with a consequent lower maximum conversion of propylene). Differences between alumina- and silica-dispersed systems (which had comparable

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behavior in the case of the Fe/Sb/O active phase, and which themselves were both characterized by low activity in propylene ammoxidation) can be attributed to a role of alumina either i) on propylene transformation (this however can be neglected, also taking into consideration the distribution of products of the support alone, Figure 2), or ii) in favouring the combustion of the acrylonitrile formed, or iii) in affecting the nature of the active components in the catalyst. The latter effect likely is the most important, since i) the non-dispersed B i / F e / N i / C o / M o / O also behaves similarly to the silica-dispersed catalyst, and ii) there are considerable differences in the characteristics of the silica- and alumina-dispersed catalysts: indeed in the aluminabased system it was not possible to detect the presence of any crystalline Bi/(Fe)/Mo/O compound. 100

80

80

~

eo

"~ 40

o

"~

20

40 410

430

450

470

temperature, ~

490

0 alumina

silica

tin oxide

Fig. 4. Conversion of propylene (full symbols) and ammonia (open symbols) (left) and selectivity to the different products at 439~ (right) for the active phase B i / F e / N i / C o / M o / O dispersed in ~,-alumina (e), silica (11), and tin oxide (&). Captions for Fig. 4 (right) as in Figure 2. 4. ACKNOWLEDGEMENTS

Snamprogetti is gratefully acknowledged for financial support. REFERENCES

1. R.K. Grasselli, in "Handbook of Heterogeneous Catalysis", G. Ertl, H. Knozinger and J. Weitkamp (Eds.), 1997, p. 2302 2. T.A. Gordymora and A.A. Davidov, Kinet. Catal., 20 (1979) 604 3. A. Parmaliana, V. Sokolovskii, D. Miceli, F. Arena, N. Giordano, J. Catal., 148 (1994) 514 4. Y. Barbaux, D. Bouqueniaux, G. Fornasari, F. TrifirG Appl. Catal., A: General, 125 (1995) 303 5. R. Burch and E.M. Crabb, Appl. Catal., A: General, 97 (1993) 49

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Mastering the chemical state of Mo at the surface of oxide catalysts in the selective oxidation of hydrocarbons: towards a fine optimization of the catalytic performances E.M. Gaigneaux*, M.-L. Naeye, E. Godard, H.M. Abdel Dayem, P. Ruiz and B. Delmon Unite de catalyse et chimie des matdriaux divisds, Universitd catholique de Louvain, Place Croix du Sud 2/17, B-1348 Louvain-la-Neuve, Belgium. The objective of this work is to show the crucial role of oxide surfaces stabilized in a slightly reduced state in selective oxidation reactions. The results concern four catalytic oxidation systems: the selective oxidations of propene and isobutene to acrolein and methacrolein, and the O2-assisted dehydration of 2-butanol to butene, on MoO 3 and Bi molybdates catalysts. Although conditions are very different, identical correlations between the performances of Mo-containing oxides and the state of their surface during the oxidation process are found. The highest performances are obtained on oxides with their surface stabilized in a Mo suboxidic state, this being tuned by jointly adjusting the catalyst formulation and the reaction conditions. This joint adjustment thus constitutes a powerful tool to maximize the number of selective sites on oxide surfaces and their reactivity in selective oxidation reactions. I. INTRODUCTION AND OBJECTIVES The reactivity of oxides, in particular .MoO3 and Bi molybdates, in hydrocarbon selective oxidation reactions is dictated by the Mars-van Krevelen mechanism [1]. The oxygen atoms inserted into the oxidation products are pumped out from the catalyst surface, leaving behind deactivated reduced sites. These sites are regenerated afterwards by getting reoxidized by gasphase oxygen. For the catalyst to be "ideal", the reduction and the re,oxidation of the oxide surface must occur at equal rates. If not, "accidental" deeply reduced sites, which are difficult to be reoxidized, might form, and the catalytic surface might rearrange to progressively more Odeficient states with Mo suboxidic stoichiometries. The optimal balance of the Mars-van Krevelen mechanism obviously demands that the extent of reduction of the catalysts at the steady state be limited, because these must keep a large number of O atoms available for hydrocarbon oxidation. But, in addition, the reactivity and the stability of the (sub)oxides are also dictated by the energy they need to transfer oxygen to the hydrocarbon and by their easiness to refill subsequently their reduced catalytic sites using O 2. However, except some studies on VPO catalysts in the selective oxidation of n-butane [2], there is presently a lack of knowledge of the chemical state of catalytic atoms at the surface of oxides during the catalytic processes, and of the way this state affects the catalytic performances. This work aims at identifying the Mo reduction state at the surface of Mo-containing oxides that corresponds to the highest selective oxidation performances. Additionally, we wish to point to some approaches to stabilize this most performant state during the catalytic reaction. This contribution includes results from four catalytic oxidation systems" the selective oxidations of propene or isobutene to acrolein or methacrolein, respectively, and the oxygenassisted dehydration of 2-butanol to butene, on MOO3, MOO3_x and Bi molybdates (which are closely related to MOO3). The latter reaction was chosen on the basis of the strong parallelism of oxygen-aided processes on oxide catalysts to oxidative dehydrogenations and "true" selective oxidations of hydrocarbons [3]. The influence of the catalyst formulation is explored by comparing the performances of MOO3, MosO23 and Bi molybdates with those of their physical * E.M. Gaigneaux is "Charg6 de recherches" researchfellow for the National Foundation for Scientific Research (FNRS) of Belgium. Fax: +32 (0) 10/47 36 49, e-mail: [email protected]

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mixtures with ~-Sb204. The influence of the reaction conditions was studied by modifying the hydrocarbon and O2 partial pressures in the gas feed. The catalysts were characterized before and after reaction mainly by XRD, XPS, Raman spectroscopy and SEM.

2. EXPERIMENTAL 2 . 1 . Preparation of the catalysts MoO3 was prepared by recrystallization of a commercial Mo trioxide (BDH, 99.5%+) under pure 02 at 873 K during 12 h. MoO 2 was used as purchased (Aldrich, 99%). MoaO23 was synthesized by calcination of a mixture of commercial MoO 3 and MoO2 with a 7/1 molar ratio in a sealed evacuated tube at 1023 K during 96 h. Bi molybdates (c~-Bi2Mo3Ox2, 13-Bi2Mo209 and y-Bi2MoO~) were synthesized by decomposition at 573 K during 16 h, and subsequent calcination at 743 K during 16 h, of amorphous precursors obtained by drying under vacuum at 353 K during 16 h viscous solutions of Bi(NO3)3.5H20 and (NHa)rMo7024.7H20 (with Bi/Mo atomic ratios of 2/3, 1/1 and 2/1, respectively) complexed with an equivalent quantity of citric acid and concentrated by evaporation under vacuum at 303 K. c~-Sb204 was obtained by calcination of Sb203 (Aldrich, 99%) in air at 773 K during 20 h. Physical mixtures were prepared by interdispersing in n-pentane adequate quantities of the pure phases obtained as described above, followed by evaporation of the liquid under vacuum at 303 K and drying of the resulting powder in air at 353 K during 16 h. Identical "mixing" procedure was applied to the pure phases before the catalytic activity measurements.

2 . 2 . Catalytic activity measurements

The three catalytic reactions were performed at atmospheric pressure in a fixed bed flowmicroreactor. For the O2-assisted dehydration reaction, the feed was 90 ml min-1 of dry air with a partial pressure of 23.46 kPa of 2-butanol. The reaction temperature was 493 K or 523 K depending on the catalyst. The oxidation of isobutene was performed at 693 K with 30 ml minof a gas mixture containing 10.13 kPa of isobutene and 20.26 kPa of 02 in He. The oxidation of propene was carried out at 673 K with a flow of 30 ml min ~. The partial pressures of propene and 02 in He were 5.07 kPa, 10.13 kPa and 20.26 kPa depending on the experiment.

3.

OXYGEN-ASSISTED

DEHYDRATION

OF 2 - B U T A N O L

ON

MOLYBDATES AND THEIR PHYSICAL MIXTURES WITH a-Sb204

MOO3,

Bi

The performances of the mixture of MoO 3 and c~-Sb204 indicated the occurrence of a cooperation between them: while leading to a selectivity to butene similar to that of pure MoO3, the mixture led to 2-butanol conversion and yield to butene conspicuously higher than the corresponding calculated values expected if the phases had behaved independently (Table 1). Table 1 Conversion of 2-butanol (%C), yield (%Y) and selectivity (%S) to butene on MOO3, (3~-5b20 4 and their physical mixture at 493 K. The values in 0 are those calculated from the performances of pure oxides assuming the absence of interaction between M o O 3 and ~-Sb204. Catalysts MoO3 Mixture a- Sb704

Mass and composition of catalyst 1250 mg 1250 mg MoO 3 + 480 mg of a-Sb204 480 mg

%C 34 57 (34) 0

%Y 12 18 (12) 0

%S 35 32 (35) 0

When used alone, MoO3 underwent a deep reduction to MoO 2. The phenomenon was evidenced by the presence of tungarinovite peaks on the XRD pattern of pure MoO3 after 12 h of catalytic reaction. The presence of a Mo 4+ contribution in the XPS MO3dband of the sample was also consistent with the deep reduction of pure MoO 3. In addition, the relatively small Mo s+ contribution suggested that the destruction of the lattice structure of MoO3 to MoO 2 happened without intermediate (Fig. 1). This hypothesis was supported by SEM micrographs showing that the formation of MoO2 particles was associated with a violent fragmentation of the MoO 3 crystals (Fig. 2). Most likely then, the surface of MoO 3 crystals reacted alone was not covered

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Pure MoO3

by any continuous layer of Mo suboxides during the reaction, v~[Mo5+ = 35.4% t ~ AI~r but rather by MoO 2, just before a '1 Mo4+ = 5 5%'~I MoO 2 particle detached, and MoO 3, just after. When ~-Sb204 and , f MoO 3 were mixed together, they kept their crystallographic 240 238 236 234 232 230 228 240" 238 ' 236 ' 234' 232 ' 230 ' 228 structures unchanged. No Binding energy (eV) Binding energy (eV) ternary phase was formed and MoO 3 did not reduce to MOO,'. "~[ Mo6+ = 100% A Before l neither XRD nor SEM indicated the presence of other phases than MoO 3 and cx-Sb204 in the mixture after 12 h of reaction. But, the presence of an important Mo s§ contribution in the XPS MO3d band indicated 240 238 236 234 232 230 228 240 238 236 234 232 230 228 Binding energy (eV) Binding energy (eV) that MoO 3 surface was stabilized Figure 1. XPS MO3d band of MoO 3 before and after a 12 h in a suboxidic state (Fig. 1). use in the O2-assisted dehydration of 2-butanol at 493 K This conclusion was supported by SEM which showed that alone and in the physical mixture with o t - S b 2 0 4. MoO 3 crystals acquired a morphology with facetted edges between the (010) and (100) faces (which is typical of compounds like MoxsO52 [4]) during the reaction in the presence of ot-Sb204 (Fig. 2). Under the same conditions of reaction, MoO 2 had a selectivity to butene (36%) similar to that of MoO 3 (35%), but an intrinsic 2-butanol conversion rate (2.66x10 -5 mole s -1 m -2) lower than that of MoO 3 (4.15x10 5 mole s 1 m2). The reduction of MoO 3 to MoO 2 must thus be interpreted as a deactivation. The synergetic improvement of the performances of MoO3 in the mixture with ot-Sb204 is due to the fact that the latter inhibits the deep reduction of MoO 3 or, in other words, that ot-Sb204 stabilizes the surface of MoO 3 to just a slightly reduced suboxidic state with relatively high performances [5]. Table 2 shows the performances obtained in the O2-assisted dehydration of 2-butanol on the three Bi molybdates and their physical mixtures with ct-Sb204. All Bi molybdates suffered a dramatic loss of performances when reacted in the presence of ct-Sb204: their 2-butanol conversions and, for the 13 and y phases, selectivities to butene were lower than the values expected if the phases had behaved independently in the reactor. As the phases in the mixtures kept their lattice unchanged after reaction (as ascertained by XRD and Raman spectroscopy), the inhibition must be interpreted on the basis of two phases remaining completely separate and mutually non-contaminated during the catalytic process.

~'i Mo'6~'-"h~.8~ ' "~ . . . . . . . . . .

~

"~iuo6~'-54.7~" "~ . . . . . . '

1 . . . 1 . . . | . . . 1 , . . 1 . . . , . . . ,

, . . .

, . . . , . . .

,

. . . ! . . . , . . .

9 9 9,

9 9 9,

9,

9,

9 .-;'T.

9 9 , --%r~-

,

Figure 2. SEM micrographs of pure MoO3 (,---, bar= 10 ~tm) fresh, and (1', bar=2 ~tm) after 12 h of use in the O:assisted dehydration of 2-butanol (white particles are MOO2), and (---,, bar= 1 ~tm) of the MoO 3 crystals having acquired a morphology with facetted edges between the (010) and (100) faces during the reaction in the presence of ot-Sb204.

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Table 2 Conversion of 2-butanol (%C), yield (%Y) and selectivity (%S) to butene on the Bi molybdates, ct-Sb204 and their physical mixtures (50/50 %wt.) at 523 K (mass of catalyst: lg). The values in 0 are those, calculated from the performances of pure oxides, assuming the absence of interaction between the Bi molybdates and ~-Sb204. Catalysts ct-Bi2Mo3012 13-Bi2Mo209 )'-Bi2MoO6

~-SbTO4

Pure phase %C %Y 94.0 34.2 50.0 16.8 45.5 15.6 1.8 0

%S 36.4 33.7 34.4 .

.

Mixture with t~-Sb204 %C %Y 19.5 ( 4 7 . 9 ) 6 . 7 3.2 (25.9) 0.5 2.9 (23.6) 0.2 . . . .

(17.1) (8.4) (7.8) .

%S 34.3 16.7 5.7

(35.7) (32.5) (33.0)

The XPS MOad band of fresh Bi molybdates, alone and in the mixtures, exhibited a significant Mo s+ contribution, suggesting that their surface was in a suboxidic state. The Mo 5+ signal was unchanged after the Bi molybdates had reacted alone. But it completely disappeared and only a fully oxidized Mo 6+ contribution was found after reaction in the presence of ~-Sb204. The effect on the Bi molybdates is consistent with that observed on MoO 3 in the same reaction: ~-Sb204 maintains the surface of Mo-containing catalysts in a more oxidized state than when the catalysts are reacted alone under identical conditions. When mixed together, and when mixed with MoO3, Bi molybdates did not suffer any activity drop, but rather exhibited synergetic enhancement of performances. In all cases, the catalysts always remained in a state possessing Mo 5+ species at their surface [6]. These results, together with those reported above, thus unambiguously indicate that the chemical state of the Mo atoms at the surface of the Bi molybdates dictates the reactivity of the catalysts in the OEdehydration of 2-butanol: high performances are obtained on Bi molybdates with their surface in a suboxidic state, but fully oxidized catalysts are only weakly active and selective. The results from the two series of experiments are consistent. They show (i) that the Mo state with the highest performances for the O2-assisted dehydration of 2-butanol is a slightly reduced suboxidic state. Fully oxidized Mo 6+, as well as too reduced Mo species, are less active and/or selective. (ii) tx-Sb204 can advantageously limit the reduction of an excessively reducible oxide, like MoO3, but it correlatively diminishes the beneficial slight reduction that is stabilized in oxides with low reducibility, like Bi molybdates, when these are reacted alone.

4. SELECTIVE OXIDATION OF ISOBUTENE ON MOO3, MosO23 AND THEIR PHYSICAL MIXTURES WITH a-Sb204 When reacted in the presence of r in the selective oxidation of isobutene, MoO 3 crystals had their surface stabilized in a suboxidic state with a stoichiometry reflecting that of MolsO52. But, when reacted alone, the surface of MoO3 further reduced, likely to Mo4Oll. Delmon argued that MolsO52 possesses a higher reactivity than M0401~ in oxidation reactions. Summarizing his opinion, the phenomenon is due to the higher ability of MolaO52 to exchange oxygen thanks to its special crystallographic structure and morphology: namely, it is able to smoothly accomodate additional oxygen vacancies (while Mo40~ must undergo an energetically expensive transformation of its lattice to that of MoO 2 when losing an additional O) and to easily refill these vacancies using O2 (while this ability is not proven for MO4Oll) [7]. The difference of stoichiometry stabilized at the surface of its crystals during the reaction thus perfectly accounted for the synergetically improved performances reached when MoO 3 is reacted in the presence of ct-Sb204, while its performances remained low when it is used alone [8]. Under identical conditions of reaction, pure MosO23 underwent a progressive deactivation with time-on-stream (Fig. 3). XPS indicated that its surface further reduced during the reaction. The finding is consistent with the behavior of pure MoO 3. But, XRD showed that, simultaneously, MoaO23 underwent an unexpected reoxidation of its bulk to MoO3, with correspondingly the disappearance of its shear-structures. The phenomenon is due to the dissociation of 02 at the places where the shear-structures of MOsOE3 appear on the surface [9]

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and its subsequent migration along them towards the bulk. Isobutene molecules are too large to migrate along the shear-structures, so making the diffusion flow from the surface to the bulk only composed of oxygen. This inevitably leads to the observed oxidation of MosO23 to MoO 3. According to opinions of Haber et al [10] and Delmon [7], respectively, the combined loss of shear-structures and deep reduction of the surface thus account for the deactivation of MosO23. 25

When mixed with ct-Sb204, the

Mo8023 ---m---MogO23/Sb204

bulk of MosO23 reoxidized to MoO 3

identically to MoaO23 reacted alone. But, contrary to pure MosO23, the surface of MosO23 mixed with r progressively reoxidized during the reaction and stabilized in a state possessing Mo 5§ species. The latter likely reflected a stoichiometry close to that of - :m m 9 Ii ii "~10 Mo~sOs,_. In parallel with the Mo S "of re,oxidation, the selectivity to methacrolein "l I--,4 0 on the mixture of Mos023 and ~x-Sb,04 5 increased with time-on-stream (Fig. 3). 0 20 40 60 80 100 120 140 160 The results show (i) that the shearTime-on-stream / rain structures in the bulk of Mo suboxides are Figure 3. Evolution with time-on-stream of the selectivity to methacrolein on Mo8023 and its not necessarily the origin of the high physical mixture (50/50 wt.) with a-Sb204 (mass of catalytic performances on Mo-containing oxide catalysts. Consistently with our catalysts: 500 mg). previous findings, the results also confirm (ii) that the highest performances for the selective oxidation of isobutene are reached when Mo oxide is maintained in a slightly reduced chemical state, most likely Mo~sO52, and (iii) that this state can be stabilized when the catalyst is used in the presence of cx-Sb204. It is remarkable that the final stable chemical state of Mo oxide in the presence of ~x-Sb204 is identical (Mo~sO52) when starting from either a more oxidized phase (MOO3) or a more reduced one (MosO23). This fact suggests (iv) that the stabilization of Mo~sO52 corresponds to the occurrence of a delicate dynamic equilibrium at the surface of the catalyst in which the reduction and the reoxidation steps of the catalytic redox cycle are perfectly balanced under the reaction conditions used here. In the results described before, both for the O2-assisted dehydration of 2-butanol and the selective oxidation of isobutene, the effect of cx-Sb204 on the Mo chemical state is interpreted according to the remote control mechanism theory. Spillover oxygen species (Oso) produced by ct-Sb204 react with the surface of the Mo-containing phase and assist its reoxidation, so leading to stabilization of more oxidized state than in the absence of Oso [11,12].

_~ 20

....

....

5. S E L E C T I V E O X I D A T I O N OF P R O P E N E ON MoO 3 A N D ITS P H Y S I C A L M I X T U R E S W I T H ~-Sb204" F I N E T U N I N G OF T H E S U R F A C E S T A T E After concluding that the stabilization of a suboxide at the surface of MoO3 corresponds to a perfect balance between the rates of its reduction by the hydrocarbon and reoxidation by 02, one can now predict that a modification of the partial pressures of 02 or hydrocarbon in the feed gas should affect the equilibrium ruling the Mo state at the surface of the catalysts, and correspondingly their selective oxidation performances. One can also expect that the quantity of Oso in the catalytic system would similarly dictate the reactivity of the catalyst. The results shown in Tables 3 and 4 confirm these predictions. For pure MOO3, higher partial pressures of 02 led to sligthly higher performances, likely corresponding to slightly more oxidized Mo states at its surface (Table 3). Correspondingly, one might have expected that higher partial pressure of propene would lead to more reduced Mo states and lower performances. Such effect is not observed for pure MOO3, likely because its surface was very reduced under all conditions, so that more propene in the feed cannot induce further reduction. But, the expected dependence on

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Table 3 Effect of the partial pressures of 02 (pO2) and propene (pProp) on the selectivity to acrolein (%S) at 673 K on pure MoO3 and its mixture with a-Sb204 (25 %wL of MOO3) 00~ (kPa) 20.26 10.13 5.07 10.13 10.13

pProp (kPa) 10.13 10.13 10.13 20.26 5.07

%S on pure MoO.~ 14.9 14.3 13.2 13.2 11.7

%S on the mixture 50.2 49.0 49.2 43.9 48.8

Table 4 Effect of the composition of mechanical mixtures of M o O 3 and ct-Sb204 on the selectivity to acrolein (%S) at 673 K (pO2=pProp=10.13 kPa) (MF: mass fraction)

hydrocarbon partial pressure was observed for a mixture of MoO 3 and ct-Sb204 which exhibited its lowest selectivity to acrolein at the highest propene pressure (Table 3). MF of MoO~ MF of ct-SbzO4 %S Globally, except (to some 100 0 14.3 extent only) the point at high propene 75 25 33.8 pressure, the performances of MoO 3 in 50 50 37.6 the mixture were not deeply affected 25 75 49.0 by the gas composition and always remained high (Table 3). Table 4 points to the origin of the phenomenon. Even when produced in little quantity (like in the mixture with the smallest mass fraction of ct-Sb204), the presence of Oso in the system led to huge improvement of the selectivity of MoO 3. This confirms the high efficiency of Oso to reoxidize the catalyst and maintain its surface in a state with higher performances. This also accounts for the sensitivity of the mixture to increase of propene pressure (Table 3), because its surface can reduce with respect to the state stabilized at lower propene pressure. The results also show that with more Oso in the system, ie. with more a-SbzO 4 in the mixture, progressively higher selectivity is obtained (Table 4). This fits the prediction of the dependence of MoO 3 performances on the quantity of Oso in the system. 6. C O N C L U S I O N S The highest selective oxidation performances are obtained by stabilizing the surface of the oxide catalysts to a Mo chemical state with a slightly reduced suboxide stoichiometry. The stabilization of such chemical state at the surface corresponds to a dynamic equilibrium in which reduction and reoxidation of the oxide are perfectly balanced in the catalytic cycle. To some extent, the stoichiometry of the surface can be tuned by adjusting the 02 and hydrocarbon partial pressures ratio in the gas feed. But an additional more efficient tuning is achieved by adjusting the quantity of spillover oxygen (Oso) in the system. The flow of Oso is mastered by adjusting the fraction of Oso donor, e.g. ct-SbzO4 [ 12], mixed with the Mo-containing oxide catalyst.

REFERENCES 1. 2.

P. Mars and D.W. van Krevelen, Chem. Eng. Sci., 3 (1954) 41. K. AitLachgar, A. Tuel, M. Brun, J.M. Hermann, J.M. Krafft, J.R. Martin, J.C. Volta and M. Abon, J. Catal., 177 (1998) 224. 3. Y. Moro-Oka, Appl. Catal. A, 181 (1999) 323. 4. J.C. Volta, O. Bertrand and N. Floquet, J. Chem. Soc., Chem. Commun., (1985) 1283. 5. E.M. Gaigneaux, M.-L. Naeye, P. Ruiz and B. Delmon, Shokubai, 41 (1999) 82. 6. E. Godard, E.M. Gaigneaux, P. Ruiz and B. Delmon, submitted in Catal. Today. 7. B. Delmon, Stud. Surf. Sci. Catal., 111 (1997) 39. 9. P.L. Gai-Boyes, Catal. Rev., 34 (1992) 1. 8. E.M. Gaigneaux, P. Ruiz and B. Delmon, Catal. Today, 32 (1996) 37. 10. J. Haber and E. Lalik, Catal. Today, 33 (1997) 119. 11. E.M. Gaigneaux, P. Ruiz and B. Delmon, submitted in J. Phys. Chem. B. 12. L.T Weng and B. Delmon, Appl. Catal. A, 81 (1992) 141.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

O l i g o m e r i z a t i o n o f higher olefins and oxidation in a series o f i s o b u t a n o l isobutyric aldehydeisobutyric acid in the presence o f some heteropoly compounds S.M. Zulfugarova, M.K. Munshieva, D.B. Tagiev, P.S. Mamedova Institute of inorganic and physical chemistry Azerbaijan Academy of sciences, Baku, 370143, H.Javid avenue, 29, Azerbaijan Republic

The results of oligomerization of C6-olefins and oxidation in a series: isobutanol isobutyric aldehyde - isobutyric acid. in the presence of some 12-Heteropoly acids (HPA) and their compounds are given. It is shown that the reaction may takes place on the surface of these catalysts as well as in their bulk. By modifying the studied heteropoly acids and their compounds or supporting them on the surface of some substrates can increase its efficiency. 1. INTRODUCTION Heteropoly compounds have recently attracted considerable attention as catalysts for various industrial processes. The wide diapason of catalytic possibilities allows their use as homogeneous and heterogeneous catalysts for acidic and oxidation reactions [ 1]. In the present paper we wish to report the results of oligomerization of C6-olefins and oxidation in a series: isobutanol - isobutyric aldehyde - isobutyric acid. in the presence of some 12-Heteropoly acids and their compounds. 2. EXPERIMENTAL HPA and their compounds were synthesized, according to [1,2]. For identification of obtained compounds the method of IR-spectroscopy was used: the bands of absorption at 1060, 970, 870 and 790 cm l, belonging to the Keggin's cell were observed. HPA was investigated in free form and supported on the various substrates, such as CaCO3, silica with the specific surface 120-300 m2/g and zeolite HZSM with SIO/A1203=33. The oligomerization process was taking place at the boiling point of C6-olefins (6572~ and atmospheric pressure. The reaction of the oxidative conversion in a series: isobutanol - isobutyric aldehyde isobutyric acid, was studied in the flow reactor at the temperature 250-460~ molar correlation - raw material: oxygen between 1:1-1:4 and at the volume velocity 2-3 hours 1. The products of these reactions after separating from the catalyst were analyzed by method of GLC.

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3. RESULTS AND DISCUSSIONS

Free and supported HPA were researched in oligomerization process. For correct comparison of activity of these catalysts, the amount of active mass in both cases was equal. (Table 1). Table 1 Conversion of C6-01efins in the presence of some heteropoly acids The Conversion The yield of products, % mass Catalyst amount of C6-01edimers tri- and aromatics of fins, % oligomers HPA, mass g 0.8 55.0 100 H3PWI204o 1.0 100 61.2 56.0 0.5 68.6 5.7 25.7 25% H3PW1204o/SiO2 1.0 630 72.2 5.6 22.2 30% H3PWI204o/SiO2 24.0 0.15 83.3 16.7 0.50 470 87.0 13.0 HsPWloV204o 650 1.0 88.5 11.5 24.0 0.15 83.3 16.7 8% HsPWloV2040/SiO2 0.46 86.8 13.2 47.0 30% HsPWloV204o/SiO2 88.5 11.5 650 0.92 05 93.0 6.1 0.9 6O 5 H4SiWl204o 94.0 4.5 1.5 69 0 10 6O 5 0.5 93.0 6.2 0.8 25% H4SiWl204o/SiO2 94.2 4.3 1.5 69.0 10 30% H4SiWl204o/SiO2 100 26 0 O8 H3PMo1204o 100 29.0 10 45 55 8.0 O7 H4PMOllVO4o 50 50 14.0 10 23 77 4.0 1.0 HsPMoloV204o 95.7 4.3 0.5 57.5 25% H3PW1204o/SiO2 95.4 4.6 77 1.0 30% H3PW1204o/SiO2 95.0 5.0 62 0.5 25% H4SiWl204o/SiO2 94.9 4.4 0.7 68.5 1.0 30% H4SiWl2040/SiO2 100 10 0.5 25% HsPWloV2Ono/CaCO3 90 10 41 30% H3PW12Oao/HZSM 1.0 90 10 3 1.0 30% H3PWI204o/HZSM 100 53 2.0 NH4HEPW1204o The samples 2,4,6 were prepared by use of silica with the specific surface - 120 m2/g as a support. In the samples 10.11 as a support was used the silica with the specific surface 300 m2/g. It is shown that the activity and selectivity of HPA are dependent on their nature. So PW and SiW HPA allow to conduct the dimerization process of C6-olefins with 100% selectivity [3]. The olefin conversion is within the limits 60-70%. As opposed to the

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aforementioned catalysts PMo I-IPA has lower activity (approximately 30%) and there the aromatizasion process is taking place. The substitution of Mo- and W-ions for V-ion influences on the nature of HPA's acidic centres. It is established that when the dimerization process is going on in the presence of PW HPA then both dimerization and oligomerization are taking place on the PWV HPA. But the replacement of Mo-ions by V-ones decreases the catalytic activity of HPA in oligomerization of olefins The free HPA are not thermal stable and their specific surface is 1-5 m2/g. Therefore we supported these catalysts on the different substrates having surface hydroxyl groups and minimum surface area of about 80 m2/g. Our investigations showed that the strong interaction of HPA with CaCO3 having the basic properties decreases their catalytic activity in oligomerization process. HPA supported on the HZSM interact with the surface hydrogen atoms of zeolite. Therefore it is supposed the acidic properties ofHPA's surface become stronger. However, these catalysts exhibit a lower activity in the studied reaction. Optimal support among the researched ones was silica-SiO2, the surface of which has undergone the physical adsorption of HPA in distinction from chemisorption in the presence of other supports. The substantial distinction of supported catalysts from free ones in oligomerization of olefins is the decrease of activity of free I-IPA after 4-6 experiments and maintaining of high activity of suppoIled HPA even after numerous (20-25) experiments. Thus, the support of HPA on the surface of silica stabilizes the activity of HPA [4,5]. According to the experimental data, the conversion of olefins on the free and silica supported HPA is almost identical. Therefore, it is assumed that the process of oligomerization takes place on the outer surface of both catalysts. The hydrated protons included in the structure of the surface acidic centres, probably, have located on the oxygen atoms of angle of bridges O which have linked the metal atoms in triads M3013. M/\M Monosubstituted NH4 salts of PW HPA (NH4H2PW12040) also exhibit high catalytic activity in oligomerization of C6-olefins. The conversion of olefins reaches 55%, after 10-12 experiments it decreases to 12-14%. The investigation of the structure and adsorptive capabilities of salts of monovalent cations of HPA, recorded by method BET shows the dependence of surface on size of cation has maximum for NH4 and equals 100-200 mZ/g, according to [6]. Therefore, ifHPA is characterized by virtual absence of evident porosity the reaction mainly takes place on their surface or on boundary layers. The substitution of H + for other cations leads to expansion of pores and bulk acidic centres become more accessible to reacting molecules, which explains the high activity NH4H2PW12040 in oligomerization process. The study of HPA acidic properties was carried out by IR-spectroscopy of adsorbed pyridine. The samples of silica supported H~PW10V2040 before and after oligomerization of C6-olefins in their presence were chosen for this purpose. It is noted that these samples hadn't been exposed to high temperature treatment. Before the pyridine adsorption the catalysts were only pumped out at the room temperature. In this case the sharp decrease of the band at 3500 cm 1, which belongs to the OH-groups, and the appearance of the strongest, band at 1540 cm x, characteristic of the bond of pyridinium-ion with HPA surface and the strength of the Bronsted acidic centres are observed.

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Atter the preliminary thermal treatment of catalysts at 140~ and subsequent adsorption of pyridine the band at 1450 cm 1 , belonging to the Lewis acidic centres appears in IR spectra. This band is more intensive than the aforementioned one at 1540 cm -1. After pyridine pumping out the intensity of band at 1540 cm ~ decreases, but the band at 1450 cm -~ doesn't change. This fact characterizes the firmness of the link of pyridine with the Bronsted acidic centres. The essential role of these centres of HPA in oligomerization of olefins is confirmed by the experiments in which the catalyst samples heated at 150-250~ appeared to be less active than those which hadn't been exposed to any thermal treatment. The investigation of PMo, PW, PMoV and PWV acids and some of their salts in oxidation in a series of isobutanol- isobutyric aldehyde- isobutyric acid allowed to establish undermentioned regularities: PMo acid does not practically show any activity in oxidation of isobutanol in a isobutyric aldehyde. The deep oxidation process and the formation of acetone, methylketone and acetate acid take place, in particular, in the presence of this catalyst. The substitution of atoms of Mo for V-ones in the initial PMo acid leads to decrease of the total conversion of isobutanol and the insignificant increase of the yield of isobutyric aldehyde. (Figure 1). 6f

80

4 6o ~

40

......

3L

o

r..)

20 /J V Mo

I-

I

!

1 11

2 10

3 9

[-

4 8

~r Mo

1 11

2 10

3 9

4 8

Figure 1. The oxidation of isobutanol to isobutyric aldehyde over H3PMo12-nVnO40/SiO2 (n--0, 1, 2, 3, 4). The content of H P A - 30% wt. PW acid exhibits a higher activity in oxidation of isobutyric aldehyde into a isobutyric acid in the comparison with PMo-one; the yield of isobutyric acid reaches 14% mass in the presence of H3PW12040/SiO2. The substitution of two W-atoms for V-ones leads to the increase of the yield of isobutyric acid to about 38% mass, but in the case of replacement of W-atoms by Mo- or Si-ones the yield of isobutyric acid decreases to about 2% mass (Table 2). In oxidation of isobutyric acid PMo-, PMoV-, PWV HPA have not exhibited the notable activity. The investigation of Cs salts of these HPA showed their higher activity in oxidation of isobutyric acid to methacrilic one. This is a result of expansion of the channels between the different heteropolyanions and the formation of stable structures while modifying

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Table 2 Oxidative conversion of isobutyric aldehyde Catalyst: HPA/SiO2 The amount of HPA, %wt H3PW12040 20 30 H3PWloV204 10

H3Px,V6Mo604o HaSiMol204o

20 30 30 30

The yield of isobutyric acid, % mass 10 14 25 30 38 5 2

I-[PA with cations with a large radius. Therefore, in the presence of Cs2H2PMol1VO40 the yield of the methacrylic acid totals 60% with a selectivity of about 80% (Table 3). Table 3 The catalytic activity of some heteropoly compounds in the oxidative conversion of isobutyric acid to methacrylic one Catalyst Temperature, Conversion of The yield of meSelectivity T~ isobutyric acid, thacrylic acid, % % mass % mass CsH2PW12040 460 45 22 49 CsH4PW10V2040 460 36 11 30 Cs3PWI2040 460 40 CsH2PMol2040 400 88 C szH 2PMo I 1VO40 400 75 60 80 CszH3PMoloV204o 400 80 55 68 380 32 10 31 Mol0VPCu0.2 440 32 16 50 460 46 22 48 CuH2PMo I 1VO40 400 73 27 38 460 69 31 41 The investigation of the complex substance catalyst prepared by including the different elements in its composition in one stage showed the small activity (23%) in formation of the methacrylic acid. The modification of this catalyst with CsNO3 almost tripled its initial activity. Perhaps, the inclusion of Cs + ions in the composition of catalyst changes not only its main acidic capabilities but also the structure. As a result of this, the active V centres O=Mo 6+ become accessible for the reagents. 2 M o 6+ + O2+ 2 V 5+ ---~2Mo5+ +2V 5+ / > 2Mo6++2V4+ isobutyric acid ~ methacrylic acid In the reactions of selective oxidation the important role belongs to the absorbed oxygen stability of its linkage with catalyst and the amount of reaction capable oxygen. To

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make clear this problem the thermodesorption of oxygen from the surface of various HPA was investigated. For this purpose, the adsorption of oxygen was carried out at 300-450~ for 20-30 min. Aiter this, the system was cooled until the room temperature and then it was heated with the linear velocity of 15~ until 500~ On the spectra of thermodesorption of oxygen from the catalysts HsPWIoV2040/SiO2 three peaks with the maximum of temperature 120-160, 200-290 and 440-450~ are observed (Table 4). The displacement of W-atoms by V-ones leads to a slight removal of the maximum of temperature to the side of low temperature, which can testify the weak linkage of oxygen with lhe catalyst. The amounts of adsorbed oxygen corresponding to these forms also differ. According to the data presented in the Table 4, it follows that under the substitution of W-atoms for V-ones the amount of low temperature oxygen decreases but, on contrary, the volume of high temperature oxygen increases. Table 4 Thermodesorption of oxygen from HPA Sample

H3PWlzO40/SiO2 HsPW10V2040/SiO2

The temperature of maximum, T~ 160 290 440 120 200 440

The amount of desorbed oxygen, mmol/g 1.04 0.630 0.554 0.545 0.668 0.682

Therefore, the catalytic activity of HPA in selective oxidation of isobutyric aldehyde into a isobutyric acid also increases. REFERENCES

1. J.V.Kojevnikov, Russian Chemical Review, No. 9 (1987) 1417. 2. E.A.Nikytina, Heteropoly Compounds, Mir, Moscow, 1961. 3. S.M.Zulfugarova, M.K.Munshieva, D.B.Tagiev, P.S.Mamedova, The Method of the Obtaining of C6-, C8-olefins Dimers, Author Certificate (USSR) No. 1723792 (1991). 4. D.B.Tagiev, S.M.Zulfugarova, M.K.Munshieva, P.S.Mamedova, The Catalyst for Oligomerization of C6-olefins, Author Certificate (USSR) No. 1743035 (1992). 5. S.M.Zulfugarcwa, D.B.Tagiev, M.K.Munshieva, P.S.Mamedova, The Method of Oligomerization of C6-olefins, Author Certificate (USSR) No. 1823415 (1992). 6. J.B.Mollat, G.B.Garvey, NATO Adv. Res. Workshop, Chattily, London, (1990) 193.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G, Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Modifications of vanadium phosphorus oxides by aluminium phosphate for n-butane oxidation to maleic anhydride Steve Holmes a'b, Louisa Sartoni a'c, Andy Burrows d, Vincent Martin a Graham J. Hutchings c, Chris Kielyb'd, Jean-Claude Volta a a. Institut de Recherches sur la Catalyse, 2 Avenue A. Einstein, 69626, Villeurbanne C6dex, France. b. Leverhulme Centre for Innovative Catalysis, Department of Chemistry, University of Liverpool, Liverpool, Merseyside L693BX, United Kingdom. c. Department of Chemistry, Cardiff University, Cardiff, PO Box 912, Cardiff CF10 3TB,United Kingdom. d. Department of Engineering, University of Liverpool, Liverpool, Merseyside L693BX, United Kingdom Aluminium phosphate associated with vanadium phosphorus oxide is demonstrated in improving both the catalytic performance for the production of maleic anhydride from n-butane and the time of activation of the catalyst as compared to conventional v a n a d i u m p h o s p h o r u s oxide catalysts. A l u m i n i u m phosphate is prepared at a fixed pH and then added during the preparation of the VOHPO 4, 0.5 H20 precursor. The morphology of the VPO precursor is influenced by the nature of the A1PO material. The VPO-A1PO catalyst is then activated in situ under an-butane/air atmosphere. During the activation, ( V O ) 2 P 2 0 7 is formed and the original amorphous A1PO material crystallizes. No new ternary VA1PO phase is observed. The improvement in the catalytic properties of the mixed VPO-A1PO oxide system cannot solely be explained in terms of a doped V(A1)PO catalyst (A1/V= 1%) prepared independently, for which the catalytic performances are lower. 1. INTRODUCTION Few examples exist in the open literature on the possibility of supporting vanadium-phosphorus oxides (VPO) catalysts [1-7]. If some of these supports, such as alumina, silica or titania are considered favourably for mild oxidation of olefins, the catalytic performances for n-butane oxidation to maleic anhydride are usually poor. Aluminium phosphate has been reported as a support material for VPO catalysts for selective oxidation of 1-butene to maleic anhydride [4]. In this communication, we show that there is a significant benefit in using a l u m i n i u m phosphate associated with VPO to improve both the catalytic performances for

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the production of maleic anhydride from n-butane and the time of activation of the catalyst. The interaction between VPO and A1PO is studied. 2. EXPERIMENTAL

2.1. Catalysts preparation A1PO materials were first prepared in aqueous media by precipitation at constant pH (ranging from 4 to 9) of aluminium nitrate by ammonium hydroxide in a phosphoric acid solution (P/A1 = 1.6). The materials were then calcined under air at 500~ Two A1PO materials precipitated at pH 6 and pH 9 (500~ calcined) and called A1PO-6 and A1PO-9 were respectively chosen for a series of further VPO-A1PO preparations. For the synthesis of the VPO-A1PO precursors, the V/A1 ratio was chosen on the basis of a theoretical monolayer of the VOHPO 4. 0.5 H20 precursor on the A1PO material, taking into account the BET area of the support. The incorporation of the VOHPO 4. 0.5 H20 precursor on the A1PO's was effected during the preparation of this precursor. Thus, V20 s (11.Sg) was first refluxed with isobutanol (250 ml) for 16 hours. The mixture was then cooled at room temperature and A1PO was then added with phosphoric acid (16.49 g, 85%) at a suitable V/A1 ratio and the mixture was refluxed for a further 16 hours. The light blue suspension was then separated from the organic solution by filtration and washed with isobutanol (200 ml) and ethanol (150 ml, 100%). The resulting solid was refluxed in water ( 9 ml H20/ g solid), filtered hot, and dried in air (110~ 16 hours). 2.2. Characterisation techniques The atomic composition of the materials was determined by atomic absorption spectroscopy. Powder X-ray diffraction analyses were performed using a Siemens goniometer equipped with a quartz front monochromator (Cu K~ radiation). UV-visible spectra were recorded on a Perkin-Elmer Lambda 9 spectrometer equipped with an integration sphere . An Hitachi $800 scanning electron microscope was employed to obtain topographical information from the precursors and catalysts. TEM observations were made in a JEOL 2000 FX electron microscope equipped with a LINK system EDX detector. The 31p and 27A1 measurements were performed on a Bruker MSL 300 NMR spectrometer. Conventional spectra were obtained at 121.5 MHz using a 90~ sequence. Raman spectra of the precursors and catalysts were recorded on a Dilor Omars 89 spectrophotometer equipped with an intensified photodiode array detector. The 514.5 nm emission line from an Ar § ion laser was used for excitation. 2.3. Catalytic measurements The oxidation of n-butane was performed at 400~ at atmospheric pressure under oxidizing conditions ((nCa = 2%, 02 = 18%, He = 80%) - GSHV = 2000h 1. Analysis of reactants and the products of reaction was performed by on-line gas chromatography.

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3. RESULTS A N D DISCUSSION

After calcination at 500~ the A1PO solids exhibited a BET area ranging from 65 m2/g (pH = 6) to 91 m2/g (pH = 9). They are amorphous and mesoporous (pore size 6.7-15 nm) with a type IV isotherm. Calcination at 1000~ gave a well crystallized A1PO4 tridymite structure. Figure 1 shows the XRD spectra of the VPO-A1PO precursors prepared from A1PO-6 and A1PO-9 materials. The peaks present are both characteristic of only the VOHPO4. 0.5 H20 hemihydrate and of the A1PO4 tridymite phases. It is interesting to note that crystallized A1PO4 (trid.) is observed when the initially A1PO amorphous materials have been added during the synthesis of the VPO precursor. The VOHPO 4. 0.5 H20 diffraction spectrum is typical of the so called ~ V P D - morphology (broad (001) and slight intense (220) line) which is known to be obtained by reduction of VOPO4,2 H20 by isobutanol [8]. Intensity

(a.u.)

~ ~"

,.q "-'al

10

15

2~)

A

~

i

i ~

iI1~

30

o VOHPO4. 0.5 H20 4, ALP04(trid.)

VPO-AIPO-9 ,'~ ,~,

3~5

4~) 20( ~)

Fig. 1 : XRD spectra of the VPO-A1PO-6 and VPO-A1PO-9 precursors The SEM examination of both VPO-A1PO precursors reveals the existence of two morphologies : the first is characteristic of <~sand-roses - agglomerates of platelets typical of the VPO structure, while the second is an agglomeration of small crystallites into spheres (Figure 2). A STEM-EDX study of these two morphologies evidenced, firstly the presence of about 1 at.% A1 in the VPO structure and, secondly, of around 1 at.% V in the spherical structures which were identified as characteristic of the A1PO structure. Differences were found to exist between the VPO-A1PO-6 and VPO-A1PO-9 precursors : for VPO-A1PO-6, the VPO-platelets are comprised of thin mixed sheets (thickness 0.05 ~tm) while for VPO-A1PO-9, the VPO-platelets (thickness 0.03-0.05 ~tm) are gathered into <~sand-roses - type structures. The A1PO morphology is preserved in both precursors. It may be deduced from these observations that the conditions of synthesis of the VPOA1PO precursors in acidic (H3PO4)media have favoured the recrystallization of the initially amorphous A1PO material prepared under different conditions and modified the VPO counterpart, depending on the pH of preparation of the starting amorphous A1PO, thus favouring the ~ V P D - type morphology. For VPO/A1PO precursors, only octahedral V4§ signal at 620 and 820 nm, typical of VOHPO 4. 0.5 H20 were identified by diffuse-reflectance UV. The presence of the

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only VOHPO a. 0.5 H20 precursor with the A1PO material was also confirmed by spin echo mapping 3~p NMR (signal at 1625 ppm for the VPO precursor)[9] and by Raman spectroscopy (signals at 979, 1105 and 1149 cm~)[10]. The 27A1NMR spectra indicated mainly the presence of Alrd with some A1ee~tand Aloh for A1PO-9. The two last coordinations were associated with the presence of water connected to the A1PO support.

r

I

,

:

,. :

..~

~.

" !~;%:d,~,

,

,

....~2~,~Y.~'

'

"!:?:}%:,'. ...~.

VPO-AIPO-9

VPO-AIPO-6

,

Ip.m

_ .

1Wrl

Ilxm

1lain lk,.,I

Fig. 2 9SEM examination of the VPO-A1PO precursors. As compared to other VPO-supported catalysts, the VPO-A1PO catalysts are very active and selective for MA production (Figure 3). For example for VPO/A1PO-9" Conv. C 4 = 90% for Select. MA = 45% which compares favourably with bulk VPO 9 C 4 = 64% for Select. MA = 62%). A1PO 4 was verified to be inactive in the corresponding reaction. A comparison with previous studies of VPO on other supports shows the benefit in associating VPO with A1PO (on VPO/TiO 2 (2.9% V loading) only 15% Conv. C 4 for 40% Select. MA [6], on VPO/SiO 2 (8.7 % V loading) only 40% Conv. C4 for 40% Select. MA [7], under very similar catalytic conditions). Another interesting observation is the fact that the activation of the VPO/A1PO catalyst is faster than for bulk VPO (20 hours to reach the stationary state for VPO/A1PO-9 as compared to 100 hours for bulk VPO). The characterisation of the activated VPO/A1PO catalysts by XRD and TEM shows that the ( V O ) 2 P 2 0 7 phase is the only significant phase present in conjonction with the A1PO a tridymitecrystalline structure. An enlargement of the (200) ( V O ) 2 P 2 0 7 line as compared to the other lines of this phase is indicative of thin (VO)2P207

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化

Cht i

PO-AIPO-9

20

.."....:=~.'=.".2.......... .~.--"-

0

Scoscoz . i ._ 30 Time(hours)

15 VPO-AIPO.6

cau, 9 " .~.~.~, ......................... 40[ ~ ' - - ' - " ' ~ ' ............. 20 ..-."''" ................................................................................. 0

15

30

45

S~tx Sco Sco2

60

Time

(hours)

Fig. 3 9Activation of VPO-A1PO-6 and VPO-A1PO-9 at 400~ (C4 = 2%, 02 = 18%, He = 80%, GSHV = 2000 h "1) crystals largely developing the (100) face. Both 31p NMR (MAS and spin echo mapping) revealed a unique environment of the phosphorus nuclei, typical of (VO)2P20 7. Characteristic V4§ bands at 620 and 820 nm with slight V 5§ bands in the 450-550 nm range, typical of a working VPO catalyst, were observed by diffusereflectance UV (Figure 4). Examination of the VPO/A1PO catalysts and the comparison with the A1PO support by Raman spectroscopy revealed only the spectrum of (VO)2P207, very well resolved, with characteric bands at 921, 1124 and 1182 cm 1.

~ V

S"oct

V4*ocl

i v,.o~' [ (a.u.)

x6 500

1000

1500

k/nm

Fig. 4" Diffuse-reflectance UV spectra of VPO-A1PO-6 and VPO-A1PO-9 catalysts as compared to (VO)2P20 7 In order to try to explain the interesting catalytic properties of the VPOA1PO-9 by a doping effect, as observed by EDX, we prepared separately an A1 doped

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VOHPO a. 0.5 H20 precursor and a V-doped A1PO-9 in order to test t h e m individually. Whereas the doping of VPO by A1 is quite possible by solubilizing A1 as AI(OBu) 3 in isobutanol during the preparation of the corresponding precursor ( A1/V = 1%), the doping of A1PO by V is found to be almost impossible (V/A1 = 0.03%). The activation of the V(A1)PO precursor follows the same evolution as for the VPO/A1PO-9 catalyst but with poorer catalytic performance (nC4 conversion : 40% and MA selectivity : 50%) and a longer time of activation (80 hours). Under the same catalytic conditions, the AI(V)PO catalyst is almost inactive (3% nC a conversion) giving maleic anhydride : 8% and mainly 2-butene : 24% and essentially CO (60%). It was thus concluded that the synergistic effect induced by incorporation of A1PO-9 with VPO was essentially due to doping of VPO by A1. However other possible interactions between adjacent VPO and A1PO phases should not be rejected. 4. CONCLUSIONS The improved catalytic behaviour of the VPO-A1PO catalytic system is interesting since the VPO-A1PO-9 catalyst may be explained by a doping of VPO by A1. However, we do not exclude the possibility of cooperative effect occuring between the V(A1)PO and AI(V)PO structures. This has to be studied in more depth. A second inactive oxide (for n-butane mild oxidation - this was the case for A1PO), on interacting with (VO)2P207may change the catalytic properties of the VPO catalyst and modify the Vs§ 4§ balance on the surface of ( V O ) a P 2 0 7 9 This has yet to be verified by XPS. This work opens up new routes for metallic phosphates to be associated with the well known VPO catalytic system for the mild oxidation of light alkanes. 5. ACKNOWLEDGEMENTS This work was carried out under the framework of the European Associated Laboratory ~ High Specificity Catalysis ~. The authors thank CNRS (France) and EPSRC (UK) for the financial support. REFERENCES

[1] M. Nakamura, K. Kawai and Y. Fujiwara, J. Catal., 34 (1074) 345. [2] A. Ramstetter and M. Baerns, J. Catal., 109 (1988) 303. [3] R.L. Varma andD.N. Saraf, Ind. Chem. Eng., 20 (1978) 42. [4] P.S. Kuo and B.L. Yang, J. Catal., 117 (1989) 301. [5] M. Martinez-Lara, L. Moreno-Real, R. Pozas-Tormo, A. Jimenez-Lopez, S. Bruque, P. Ruiz and G. Poncelet, Can. J. Chem., 70 (1992) 5. [6] R.A. Overbeek, P.A. Warringa, M.J.D. Crombag, L.M. Visser, A.J. van Dillen, J.W. Geus, App1. Catal. A, General, 135 (1996) 209. [7] R. A. Overbeek, A.R.C.J. Pekelharing, A.J. van Dillen, J.W. Geus, Appl. Catal. A, General, 135 (1996) 231. [8] C. Kiely, A. Burrows, S. Sajip, G.J. Hutchings, M.T. Sanan6s, A. Tuel and J.C. Volta, J. Catal., 162 (1996) 31. [9] M.T. Sanan6s, A. Tuel, G.J. Hutchings and J.C. Volta, J. Catal., 148 (1994) 395. [10] G.J. Hutchings, A. Desmartin-Chomel, R. Olier and J.C. Volta, Nature, 368 (1994) 41.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

S e l e c t i v e o x i d a t i o n of n - b u t a n e o v e r n o v e l V-P-O/silicac o m p o s i t e s prepared t h r o u g h intercalation and exfoliation of l a y e r e d precursor N. Hiyoshi, N. Yamamoto, N. Terao, T. Nakato a and T. Okuhara* Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, J a p a n apresent address: Department of Environmental Natural Resource Science, Faculty of Agriculture, Tokyo University of Agriculture and Technology, Fuchu 183-8509, J a p a n Vanadium-phosphorus oxides-silica composites were prepared through intercalation-exfoliation of layered VOPO42H20. Intercalated compounds of VOPO42H20 with 4-butylaniline, acrylamide and 2-octanol were formed retaining its morphology and lattice structure. These compounds were exfoliated in polar solvents into delaminated sheets. The composites of silica were obtained by impregnating silica with the solution containing the delaminated layers. It was found that the SiO2-composite obtained by exfoliation and reduction only with 2octanol was highly selective (54%-selectivity), while the SiO2-intercalated materials with the N-containing compounds were less selective. 1. I N T R O D U C T I O N

Selective oxidation of n-butane to maleic anhydride (abbreviated as MA) is only commercialized application in the gas-solid reaction of alkane. Vanadyl pyrophosphate, (VO)2P207, is selective phase for this reaction to MA [1-3] and is a main component of the commercial catalyst. However, the yield of MA is still about 60%, it is desirable to improve the catalytic performance greatly. The catalytic activity and selectivity of (VO)2P207 have been claimed to largely depend on its crystal plane [4,5]. Okuhara et al. [4] showed that the basal plane, on which the characteristic pair sites, V4+(=O)-O-V4§ are located, is selective for nbutane oxidation, but the side planes are non-selective. In addition, it was revealed that the reaction proceeds via a redox mechanism on the surface layers [6,7]. Considering the above findings, catalysts consisting of thin layered of (VO)2P207 would exhibit high catalytic performance. Intercalation is a technique useful for nanostructural modification of layered materials. Recently, "exfoliation" technique has been developed from intercalation for some layered materials which are metal chalcogenides [8], clay [9], zirconium phosphate [10], niobates [11], and titanates [12]. Exfoliation is a method of delaminating stacked inorganic sheets in a solvent by infmite swelling of

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their interlayer spaces where appropriate guest species are intercalated. In the present study, we a t t e m p t e d to prepare a new type of catalyst through the intercalation, exfoliation, and impregnation by using the layered VOPO42H20 as a starting material. The preparation processes are illustrated in Fig. 1.

Intercalationf ~i

Exfoliation ~ ~ . , . , . R e c o n s t r u c t i o n ~ i02

,,"

!

VOPO4"2H20

Intercalation compound

Exfoliated layers

SiO2 composite

VPO -

Fig. 1. Preparation processes ofVP oxides-SiO 2 composite catalysts

2. Experimental VOPO42H20 was prepared according to the literature [13] by refluxing a mixture of V205, 85%H3PO 4 and H20 a t 403 K, followed by washing with acetone. (VO)2P20 v was prepared from VOHPO40.5H20 obtained by reduction of VOPO4 2H20 with 2-octanol and 2-butanol according to the literature [14]. These catalysts are designated as C-4(Oc) and C-4(Bu), respectively. The intercalation compound with 4-butylaniline was performed by the direct reaction of VOPO42H20 (3 g) with 4-butylaniline (30 cm 3, 0.33 mmol) at room t e m p e r a t u r e for 15 min [15]. This compound is denoted to VP-BA. VOPO4-acrylamide intercalation compound was prepared from VOPO42H20 (1 g) with 20% ethanolic solution (100 cm 3) of acrylamide at 303 K for 72 h. This compound is denoted to VP-AA. These compounds were washed with acetone at room temperature. VP-BA (10 g) was reduced with 2-octanol under reflux (408 K) for 18 h. Then the reduced compound was added to THF, and was exfoliated under the reflux conditions for 24 h. The dark-brown s u p e r n a t a n t of the obtained suspension was used for the impregnation of silica (Aerosil 3 0 0 , 3 1 3 m2gl). The obtained SiO 2composites will be designated as VP-BA/SiO 2. VP-AA (0.5 g) was added into 2octanol (100 cm 3) at 373 K to form homogeneous solution. As will be described below, VP-AA would be exfoliated at this step. This solution was stirred at 408 K for 10 h to become light green. To the solution, SiO 2 (2 g) was added. Again, the suspension was stirred at 408 K for 20 h. By removing the solution by evaporation, the silica composites were obtained (designated as, e.g., VP-AA/SiO 2. VOPO42H20 (0.5 g) was directly added to 2-octanol (100 cm 3) at 373 K or 2butanol (100 cm 3) at 353 K and the suspension was stirred until the suspension became light blue homogeneous solution (at this step, the exfoliation took place probably). SiO 2 (2 g) was then added to the solution, and the suspension was stirred at 408 K (for 2-octanol) or 368 K (for 2-butanol) for several hours. By removal of the solution, VP-Oc/SiO 2 or VP-Bu/SiO 2 was obtained. VP-Oc/SiO2 washed with acetone will be denoted to VP-Oc/SiO2(w). The loading amount will be expressed by wt% of V-P oxide. The obtained silica-composites were characterized with powder-XRD, IR (KBr), elementary analysis, and SEM.

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Catalytic oxidation of n-butane was performed in a flow reactor (Pyrex tube, 12 m m of inside diameter) under an atmospheric pressure at 703 K. The reactant mixture consists of n-butane 1.5vo1%, 02 17vo1%, and He (balance). The catalyst temperature was elevated in the reactant mixture (50 cm3min 1) from room temperature to the reaction temperature, 703 K, at a rate of 5 K min ~. Time course was followed until the stationary state was obtained (about 15 h). The products were analyzed with gas chromatographs (a high speed C~ (TCD), Aera J a p a n M-200) and a Shimadzu 8A (FID) equipped with columns of Molecular Sieve 5A and Porapak QS. 3. R E S U L T S AND D I S C U S S I O N 3.1. I n t e r c a l a t i o n a n d e x f o l i a t i o n

Elementary analysis gave the compositions of VP-BA and VP-AA to be (C4H9CsH4NH2)I.2VOPO 4 and (C2H3CONH2)0.17(C2HsCO)0.24VOPO4, respectively. Figure 2 shows the XRD patterns and IR spectra ofVOPO42H20 , VP-BA and VPAA. The XRD pattern showed that the basal spacings of VP-BA and VP-AA (2.36 nm and 0.90 nm) were larger than that of VOPO42H20 (d001, 0.74 nm) and anhydrous VOPO4(d001, 0.41 nm) [16]. This indicates that the layered VOPO 4 was intercalated with the organic molecules. IR spectra of the intercalated compounds exhibited absorption bands characteristics to 4-butylaniline (vC-H (2969-2850 cml), 5N-H (1515 and 1460 cm 1) and vC-N (1280 and 1265 cm -I) in Fig. 2b) and to acrylamide (vC=O (1686 cml), vN-H (-1600 cm-1), and vCN (1277 cm 1) and acetone in Fig. 2c). Broad bands due to lattice vibration of VOPO~ layers were observed at around 900-1100 cm -1, and the profile of this band was quite similar to those previously reported [17]. As shown in Fig. 3, SEM images demonstrate that both VOPO42H20 and the intercalation compound consists of square platelets with the lateral dimension of around 10 ~m and the thickness of less t h a n 1 ~m. Namely, plate-like morphology of initial VOPO42H20 was retained after intercalation, reflecting the layered nature of the samples, although the warped shape of the intercalated platelets would be based on mechanical stress by the interlayer expansion. The powder of the intercalation compound (0.3 g) was then added to THF (40 cm 3) followed by stirring at room temperature for 24 h, thereby a nearly homogeneous suspension was obtained. A brown transparent solution was obtained by removal of a small amount of remaining solid. We deduce that VPBA was exfoliated in THF, and VOPO 4 layers thus were highly delaminated. To confirm the exfoliation, a small amount of the solution was dropped and dried on fine particles of a-alumina (Sumitomo Chemical, AA-10)and SEM was attempted. As shown in Fig. 3c, twisted sheets like thin silks dressed by the a-alumina particles. The sheets were much thinner than the platelets of the intercalation compound before the treatment with THF. This is evidence for the exfoliation of the VOPO 4 layers [15]. In the case of VP-AA, we obtained 1-butanol homogeneous solution (50 cm 3) at room temperature containing 0.27 g of PV-AA. Furthermore, similar homogeneous solutions were obtained from 2-butanol and 2octanol at 353 and 373 K, respectively, suggesting that the exfoliation of layered compounds took place.

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=

,,

,,

,

,

,,,

化

...

.90

nm

O e"

| Jl

(b)

I(b) ~

L. O

0.74 nm (a) I 10

l,,, I 15 20 2 0 / deg.

l 25

i

I ....

I I 3000 2000 Wavenumber/cm

1000 -1

Fig. 2. XRD patterns (left) and IR spectra (right) of (a) VOPO42H20, (b) VOPO42H20-butylaniline intercalated compound, and (c) VOPO4-acrylamide intercalated compound. Asterisks indicate the absorption bands due to the intercalated organic molecules.

@ ~ , i~::: :~ ~,

~

.......~'~i i !i i i!i il

20

L~ I'n

|

L

Fig. 3. SEM images of (a) VOPO42H20, (b) VOPO4-4-butylaniline intercalated compound, (c) the sample obtained from the suspension of the intercalation compound dropped onto a-alumina and (d) bare a-alumina. 3.2. Catalytic oxidation of n-butane At the initial stage of the reaction (0-5 h), the conversion decreased slightly

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1719

with time over C-4 (Bu) and C-4 (Oc) and the selectivity increased gradually. On the other hand, the conversion and the selectivity tended to increase with time over the VP-SiOz-composites. After 15 h, the conversion and selectivity became nearly constant. Thus the reaction rate and selectivity were calculated from the data after about 15 h. The carbon balance was more t h a n 95% on the basis of catalyst weight. Figure 4A shows the dependence of the conversion on W/F, where W is the catalyst weight and F is the flow rate of n-butane. While the conversion increased as W/F increased, the slope of the curve became smaller at higher values of W/F. The catalytic activities were evaluated from the slopes of the curves at the low range of W/F. Both C-4's gave similar activities. It is clear from Fig. 4 that the most VP-SiO 2 composites were more active t h a n C-4's The MA selectivities are plotted against the percent conversion of n-butane in Fig. 4B. The selectivities of C-4's were about 60%, which is consistent with the previous report [14]. It is noted t h a t the selectivities of 13wt%VP-Oc/SiO2(w) and 17wt%VP-Oc/SiO 2 were 54 and 42%, respectively. On the other hand, the other SiO2-composites were much less selective (15-30 %). It should be emphasized t h a t the specific activities (per unit weight of VP oxides) of these SiO2 composites were higher t h a n those of C-4. As a result, the productivity of VP-Oc/SiO2(w) for MA was about 5 times higher than those of C-4. 80

90 A

B

!1

u

-

6O

-

40 .__.

I

60

~

O k,. >, O

(13

~ 30

20 ""--c I

I

300 600 W.F-1 / g.h.mol-I

I

900 0

I

20

I

I

40 60 Conversion / %

0

80

Fig. 4. Dependence of the conversion on W/F (A) and selectivity to MA as a function of the conversion (B). (m): C-4 (Oc), (n); C-4 (Bu), (0); 17wt%VP-Oc/SiO 2, (V); 13wt%VP-Oc/SiO2(w), (O); 17wt%VP-Bu/SiO 2, (ix); 15wt%VP-AA/SiO2, (A); 29wt%VP-AA/SiO 2. XRD of 17wt%VP-Oc/SiO 2 and 13wt%VP-Oc/SiO2(w) showed t h a t weak peaks due to VOHPO40.5H20 were detected before the reaction. After the reaction, broad and weak peaks due to (VO)2P207 appeared for these VP-Oc/SiO2, while these peaks were not clear for other VP-SiO 2 composites. It is likely t h a t the thin layered (VO)2P207 is the active surface phase of the SiO2-composites for the MA

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formation. The lower selectivities of these composites than those of C-4 are probably due to the presence of V 5§ species and low crystallinity of VP species. Benziger et al. inferred that VP oxides prepared from the compounds of VOHPO40.5H20 with amines showed an activity with the selectivity for MA (19 50%), while the activity was low [18]. Kung et al. [19] reported t h a t VPoxide/SiO 2 (P/V = 1) obtained from NH4VO 3 and NH4H2PO 4 gave 20 - 30% selectivity to MA. Furthermore, Herron et al. claimed [20] that VP oxides/SiO 2 prepared from a complex like (VO)3(C2H50)2PO2)6CH3CN , was highly active, while the selectivity was 30%. In conclusion, our novel method utilizing intercalationexfoliation is useful for the catalyst preparation of selective oxidation of n-butane and will be applicable to other types of catalyst. This work was partly supported by NEDO. REFERENCES

1. T. Shimoda, T. Okuhara, and M. Misono, Bull. Chem. Soc. Jpn., 58 (1985) 2163. 2. T.P. Moser and G. L. Schrader,. Catal., 92 (1985) 216. 3. G. Busca, F. Cavani, G. Centi, F. Trifiro, J. Catal., 99 (1986) 400. 4. T. Okuhara, I~ Inumaru, and M. Misono, ACS Symposium Series 523 (1993) 156; tL Inumaru, T. Okuhara, and M. Misono, Chem. Lett., (1992) 1955. 5. E. Bordes, Catal. Today, 16 (1993) 27. 6. M. Pepera, J. L. Callahan, M. J. Desmond, E. C. Milberger, P. R. Blum, N. J. Bremer, J. Am. Chem. Soc., 107 (1985) 4883 7. G. Koyano, T. Okuhara, and M. Misono, J. Am. Chem. Soc., 120 (1998) 767. 8. P. Joensen, R. F. Frindt, and S. R. Morris, Mater. Res. Bull., 21 (1876) 457. 9. E. R. Kleinfeld and G. S. Furgson, Science, 265 (1994) 370. 10. S. W. Kelle, H.-N. Kim, and T. E. Mallouk, J. Am. Chem. Soc., 116 (1994) 8817. 11. R. Abe, tL Shinohara, A. Tanaka, M. Hara, J. N. Kondo, and tL Domen, Chem. Mater., 9 (1997) 2179. 12. T. Sasaki, S. Nakato, S. Yamauchi, and M. Watanabe, Chem. Mater., 9 (1997) 602. 13. G. Ladwig, Z. Anorg. Allg. Chem., 338 (1965) 266. 14. H. Igarashi, tL Tsuji, T. Okuhara, and M. Misono, J. Phys. Chem., 97 (1993) 7065. 15. T. Nakato, Y. Furumi, and T. Okuhara, Chem. Lett., (1998) 611. 16. E. Bordes, P. Courtine, and G. Pannetier, Ann. Chim., 8 (1973) 105. 17. H. Nakajima and G. Matsubayashi, J. Mater. Chem., 5 (1995) 105. 18. J. B. Benziger, V. Guliants, and S. Sunderesan, Catal Today, 33 (1997) 49. 19. tL E. Birkeland, S. M. Babitz, G. tL Bethke, H. Kung, and G. W. Coulson, S. R. Bare, J. Phys. Chem. B, 101 (1997) 6895. 20. N. Herron, D. L. Thorn, R. L. Harlow, and G. W. Coulston, J. Am. Chem. Soc., 119 (1997) 7149.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Molecular Structure-Reactivity Relationships in n-Butane Oxidation over Bulk VPO and Supported Vanadia Catalysts: Lessons for Molecular Engineering of New Selective Catalysts for Alkane Oxidation V.V. Guliants a*, J. B. Benzigerb, S. Sundaresanb, and I. E. Wachs c

'Department of Chemical Engineering, University of Cincinnati, Cincinnati, OH 45221-0171 bDepartment of Chemical Engineering, Princeton University, Princeton, NJ 08544 CDepartment of Chemical Engineering, Zettlemoyer Center for Surface Studies, Lehigh University, Bethlehem, PA 18015 The present study addresses the nature of the promoter effect in the bulk VPO and supported vanadia catalysts. No correlation was found between the electronegativity of the promoter or oxide support cation and the catalytic properties of these two catalytic systems. The enhancement of surface acidiw had a beneficial effect on both the rate of n-butane oxidation and selectivity to maleic anhydride over the bulk VPO and supported vanadia catalysts. These findings suggested that the activation of n-butane on both the bulk and supported vanadia catalysts may require both a redox and an acid site. The results of the present study further demonstrate that the supported vanadia catalysts represent a suitable model for bulk VPO catalysts. 1.

INTRODUCTION

1,

The partial oxidation of n-butane to maleic anhydride over the bulk VPO catalysts is the only known commercial process for an alkane oxidation. Vanadyl(IV) pyrophosphate, (VO)2P207, displaying preferential exposure of the (100) crystal planes is critical for active and selective VPO catalysts [1, 2]. The catalytic activity of the bulk VPO catalysts is confined to a very thin surface region of the (100) planes of (VO)2P207 [3, 4], suggesting similarities between the bulk VPO and supported catalysts. The vanadyl dimers present in the hypothetical surface (100) planes of (VO)2P207 have been suggested as the active sites for n-butane oxidation to maleic anhydride [ 1, 5-7]. Supported vanadium(V) oxide catalysts have recently attracted attention as promising model catalysts for selective oxidation of n-butane [8]. Unlike the bulk VPO catalysts, the model supported vanadia catalysts possessed the surface molecular structures that could be reliably established by a variety of spectroscopic techniques [9]. The recent study [10] suggested the critical involvement of the bridging V-O-support bond, and particularly V-O-P bond in n-butane oxidation. Moreover, this oxidation reaction was more efficient when multiple surface vanadia sites were present at high coverages, which is similar to the proposed models of n-butane oxidation over the bulk VPO catalysts [ 1, 5-7]. These findings indicate that as a model system, the supported vanadia catalysts can provide insights into the mechanism of n-butane oxidation over bulk VPO catalysts.

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The purpose of this paper is to study the promoter effect and further elucidate the structure-reactivity relationships for n-butane oxidation to maleic anhydride on well-defined promoted VPO and supported vanadia catalysts. 2. 2.1.

EXPERIMENTAL Synthesis The model bulk organic VPO precursor, VOHPO4x0.5H20, was prepared by a twostep method [11] and transformed thermally into (VO)2P207 in 1.2 vol. % n-butane in air at 673K. Prior to this transformation, ca. 0.25 wt.% of promoter elements (Si, Ti, Zr, and V alkoxides, Aldrich, Inc.) dissolved in anhydrous ethanol were introduced via incipient wetness impregnation of VOHPO4x0.5H20. The details of this preparation method may be found elsewhere [ 10]. A reference unpromoted VOHPO4x0.5H20 precursor was prepared by a similar impregnation method using anhydrous ethanol. The supported vanadia catalysts were prepared by the incipient-wetness impregnation of the metal oxide supports (SiO2, TiO2, ZrO2, Nb2Os, and A1203) with a vanadium isopropoxide solution in methanol (Alfa, 95-99% purity) [10]. 2.2.

Characterization The powder X-ray diffraction, Raman and BET procedures have been previously described [2]. The XPS analysis was performed using a Model DS800 XPS surface analysis system (Kratos Analytical Plc.). During kinetic tests, ca. 1 g of promoted VOHPO4x0.5H20 or supported vanadia catalyst was placed into a U-tube Pyrex glass reactor inside an aluminum split block. The kinetic study was conducted in 1.2% n-butane in air at 653K and 494K for the promoted VPO and supported vanadia catalysts, respectively. The details of the kinetic tests and product analysis may be found elsewhere [2]. 3.

RESULTS The XRD patterns and Raman spectra of the bulk model organic VPO precursors as well as the flesh and equilibrated catalysts showed the presence of only VOHPO4x0.5H20 and (VO)2P207, respectively, and are not shown here. The pyrophosphate Raman band was observed at 924 cm ~, suggesting the presence of some VOPO4 phase [2]. The intensity ratio of the "interlayer" to in-plane reflection of (VO)2P2Ov, I200/I042,which is frequently used as an indicator of the crystal morphology and disorder [2] showed little variation among the promoted catalysts. The (VO)2P207 phase in these catalysts possessed a thin platelet morphology that can be observed in the SEM pictures (Figure 2 in [5]). These platelets preferentially exposed the (100) planes, which have been proposed to contain the active and selective sites for n-butane oxidation according to several recent models [ 1, 5, 12]. Based on the SEM observations, the surface (100) planes accounted for nearly 90% of the total surface area of these catalysts. The surface areas of the VPO catalysts were low (ca. 4.5 m2/g), reflecting the large size of the platelet crystals. The promoter surface coverage was calculated based on several assumptions. It was assumed that the promoters were completely localized at the surface and formed a square close-packed lattice of the surface metal oxide phase. Significant surface enrichment in the promoter elements was indeed confirmed by the XPS surface measurements of the Si- and Nb-promoted VPO catalysts (Table 1). The Sipromoted catalyst showed higher surface enrichment in the promoter element than the Nbpromoted catalyst despite the lower silica content. The promoter surface coverage was estimated from the knowledge of the quantity of the promoter applied, the surface area of the promoted VPO catalysts and the promoter metal-oxygen bond distance. The average metaloxygen bond distances were taken from the published crystal structure data [13] for the corresponding metal oxides. The estimated promoter surface coverages (Table 1) are very

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close to 0.25 monolayer with the exception of the Si-promoted catalyst which had a slightly lower coverage (19=0.14).

Table 1 The effect of promoters on n-butane oxidation to maleic anhydride (MA) on bulk VPO catalvsts at 653K in 1.2 vol.% n-butane in air. Promoter (M)

Wt. (g)

Flow (cc/min)

Conv(C4) (mol.%)

s(MA) (mol.%)

C'4 TOF (10"5/s)

MATOF (105/s)

19

R

none EtOH Si Ti Zr

0.29 0.34 0.49 0.36 0.36

15.5 14.4 15.7 16.1 18.2

20 21 23 13 20

35 37 47 16 11

53 40 39 36 62

19 15 18 6 7

0 0 0.14 0.26 0.25

29 nc nc

V Nb

0.35 0.35

12.3 25.0

19 17

36 53

43 80

16 43

0.26 0.26

nc 19

Note: Wt. is the catalyst weight; 19, the promoter surface coverage, R, the ratio of the promoter concentration in the 2-4 nm surface region (XPS) to its total concentration; nc, not collected. The results of n-butane oxidation over a number of promoted VPO catalysts are summarized in Table 1. The model organic VPO catalysts displayed lower selectivity to maleic anhydride as compared to the conventional organic VPO catalysts [2]. The presence of some VOPO 4 phase suggested by the Raman band shift of pyrophosphate [2] may be responsible for the inferior catalytic performance of the model organic system. The time required to reach the steady state did not appreciably vary among the promoted VPO catalysts of this study and was ca. 240h under catalytic reaction conditions. 70

9 none A D

2 50

a Si

D DD

9

DD

ITi

9 D

+Nb

30

oZr

~'~ 20

DV

e ~ ell

~

10

OO

r.~ 0

i

0

5

1

10

i

15

i

9

20

25

n-Butane Conversion (rml.%) Figure 1. Catalytic performance of the bulk VPO catalysts in oxidation of n-butane to maleic anhydride in 1.2 vol.% n-butane in air at 653K.

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The reaction rates were calculated assuming a pseudo-first order reaction [ 1]. Maleic anhydride and carbon oxides were the main oxidation products detected. The thin platelet morphology of the bulk VPO system was particularly suitable for studying fundamental structure-catalytic property relationships of this system. In the present study, the relationship between the catalytic activity and the number of the surface vanadium ions present in the crystallographic (100) planes of vanadyl pyrophosphate (5.034x10tS/m 2 [14]) corrected for the promoter surface coverage was investigated. Both the catalytic activity and selectivity to maleic anhydride were significantly affected by the presence of the promoters (Table 1 and Figure 1). Only the Zr- and Nb-promoted bulk VPO catalysts possessed n-butane oxidation activity superior to that of the unpromoted catalysts, and only the Si- and Nb-promoted catalysts were more selective to maleic anhydride. 4.

DISCUSSION The promoter elements were introduced in the present bulk VPO catalysts by the impregnation of the VOHPO4x0.5H20 precursor at a level that was too low for the promoters to have a structural effect. Therefore, according to a classification proposed by Hutchings [15], these promoted bulk VPO catalysts belong to the Type 2 systems. Recent studies suggested that the bulk VPO promoters present at a low level may function as selective poisons which block unselective surface sites present in the surface (001) planes of (VO)2P207 [16, 17]. Other promoter effects have been also suggested: the enhancement of oxidation of V~Vpo phases into VvoPo4 in fresh catalysts, which accelerates the attainment of the steady state and optimizes the surface Vs+/V4§ distribution [18], formation of ((VO)xMI-x)2P207 solid solutions, which display improved catalytic properties [15], and intercalation and cleavage of the hydrogen phosphate layers in the catalyst precursor structure, VOHPO4x0.5H20 which leads to preferential exposure of the (100) planes of (VO)2P207 in equilibrated catalysts [19]. Promotion by poisoning unselective surface sites may be discerned by observing a decrease in the rate of n-butane oxidation upon addition of an otherwise catalytically inactive promoter as the selectivity to maleic anhydride is improved at fixed catalytic reaction conditions. Examination of the kinetic data in Table 1 suggests that the promoter effect by poisoning the unselective sites may only play a role in the case of the bulk VPO catalyst promoted with silica, which by itself is inert in this hydrocarbon oxidation. The catalytic performance data for the other promoted bulk VPO catalysts shown in Table 1 do not support this mechanism of promoter action. Formation of oxidized phases in the fresh or equilibrated bulk VPO catalysts was detected by neither XRD nor Raman. Moreover, the attainment of the steady was not affected by the presence of promoters in the bulk VPO catalysts. Therefore, this mechanism of promoter action does not appear to be important for the promoted bulk VPO catalysts of this study. The intercalation of the promoters into the layered structure of the VOHPOax0.5H20 precursor and the cleavage of its (010) planes should result in an increase of the surface area and preferential exposure of the (100) planes of (VO)2P207[19]. However, the surface area and relative exposure of the (100) planes of (VO)2P207in the catalysts of the present study remained essentially unchanged. Furthermore, the promoter elements in the most active and selective Nb- and Si-promoted catalysts were concentrated in the surface region. The Si-promoted catalyst was characterized by a much higher RM ratio than the Nb-promoted catalyst despite the lower promoter content, which probably indicates higher solubility of the Nb promoter in the VPO matrix. Therefore, it appears that the promoter elements may partially form a solid solution. However, this process is primarily limited to the surface region and affects the catalytic properties of the bulk VPO catalysts.

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Lessons for Molecular Design of New Oxidation Catalysts Previously, it was concluded that the bridging V-O-support bonds were the kinetically critical functionalities in the selective oxidation of butane to maleic anhydride. The catalytic activity of the supported vanadia catalysts was found to be a very strong function of the specific metal oxide support (Table 1 in [10]). However, the catalytic activity of the supported vanadia catalysts did not correlate with the Sanderson or Pauling electronegativity of the metal cation of the oxide support [10]. Similar to the supported vanadia system, the catalytic activity of the promoted bulk VPO system did not correlate with the abovementioned electronegativity scales. The bulk VPO catalyst treated with the acidic Nb promoter was the most active, which suggests that the surface acidity plays an important role in n-butane oxidation (see below). The treatment of the unpromoted bulk VPO precursor with anhydrous alcohol had a somewhat negative effect on the catalytic activity of the unpromoted bulk VPO catalyst (Table 1) and no effect on its surface area. The surface areas of all equilibrated catalysts of the present study remained relatively constant at ca. 4.5 m2/g within the accuracy of the BET method. Similar alcohol treatment in an earlier study [19] resulted in a significant improvement in the surthce area and catalytic properties of the unpromoted bulk VPO system. It is possible that such treatment in the earlier study [19] facilitated removal of some inactive surface components, such a s VO(H2PO4) 2 or excess orthophosphoric acid. The maleic anhydride selectivity trends revealed that the electronegativity properties [20] of the promoter or bridging V-O-support bond were not related to selectivity, since the selectivity trends were Nb>Si=unpromoted>V>Ti>Zr and AI>Nb>Ti>Zr [10] for the promoted bulk VPO and supported vanadia catalysts, respectively. However, these selectivity trends parallel the strength of the Lewis acidity of the oxide supports and promoter cations, since alumina possesses the strongest Lewis acid sites followed by niobia [21 ]. The other supports and promoter cations of this study possessed only weak Lewis acidity. The silica overlayers were inert in n-butane oxidation, and the improvement of selectivity to maleic anhydride observed in this case was possibly due to selective blockage of surface sites responsible for the total oxidation of n-butane. The V, Ti, and Zr-promoted bulk VPO catalysts possessed the catalytic activity similar to the unpromoted bulk VPO system. However, these promoter cations were less selective in n-butane oxidation to maleic anhydride. These observations indicated the importance of surface acidity for high activity and selectivity of the bulk VPO and supported vanadia catalysts for selective oxidation of nbutane. Similarly, Zazhigalov et al. [22] observed a correlation between the selectivity to maleic anhydride and the surface acidity of the promoted bulk VPO catalysts. According to Zazhigalov et aL [22], moderate surface acidity facilitates desorption of maleic anhydride and prevents its complete oxidation to carbon oxides. In fact, the acidic promoters, such as Nb, employed in this study had a beneficial effect on the selectivity to maleic anhydride over bulk VPO and supported vanadia catalysts, suggesting that these promoters play a crucial role in controlling further kinetic steps of n-butane oxidation. According to several recent models of the active surface sites [ 1, 5-7], pairs of active surface vanadium sites present in the (100) plane of vanadyl pyrophosphate were required for selective oxidation of n-butane on bulk VPO catalysts. The multiple surface vanadia sites present at ca. 0.75 monolayer coverage of vanadia on TiO, were indeed more efficient at oxidizing n-butane to maleic anhydride than the isolated surface vanadia sites present at lower surface coverages (Figure 11 in [10]). Further enhancement in catalytic activity and selectivity to maleic anhydride in the model vanadia/TiO2 system was observed when an acidic metal oxide, Nb2Os, was present at the surface of the V2Os/TiO 2 catalyst, (| Or=0.17 in Table 3 [10]). These findings suggest that the efficiency for maleic anhydride formation on supported vanadia catalysts might be related to the presence of two adjacent surface vanadia sites or a combination of a surface vanadium oxide redox and surface acid

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sites. Similar to the Nb-promoted supported vanadia system, the Nb-promoted bulk VPO catalyst (| displayed the highest activity and selectivity among the bulk VPO catalysts (Table 1). The observed similarities in the catalytic behavior between the supported vanadia and promoted bulk VPO catalysts suggest that selective oxidation of n-butane on the bulk VPO catalysts may also require a Lewis or Bronsted acid site in combination with a surface vanadium redox site [ 10, 23]. 5.

ACKNOWLEDGEMENT This work was supported by the AMOCO Chemical Corporation (Princeton), National Science Foundation Grant CTS-9100130 (Princeton), and the Division of Basic Energy Sciences, Department of Energy under Grant DEFG02-93ER14350 (Lehigh). REFERENCES [1] G. Centi, F. Trifirb, J. R. Ebner, and V. M. Franchetti, Chem. Rev. 88 (1988) 55. [2] V.V. Guliants, J. B. Benziger, S. Sundaresan, I. E. Wachs, J. M. Jehng, and J. E. Roberts, Catal. Today 28 (1996) 275. [3] M.A. Pepera, J. L. Callahan, M. J. Desmond, E. C. Milberger, P. R. Blum, and N. J. Bremer, J. Am. Chem. Soc. 107 (1985) 4883. [4] M. Abon, K. E. Brrr, and P. Delich~re, Catal. Today 33 (1997) 15. [5] V.V. Guliants, J. B. Benziger, S. Sundaresan, N. Yao, and I. E. Wachs, Catal Lett. 32 (1995) 379. [6] B. Schiott, K. A. J~rgensen, and R. Hoffmann, J. Phys. Chem. 95 (1991) 2297. [7] P.A. Agaskar, L. DeCaul, and R. K. Grasselli, Catal. Lett. 23 (1994) 339. [8] V.V. Guliants, Catal. Today 51 (1999) 255. [9] I.E. Wachs and K. Segawa, Characterization of Catalytic Materials, I.E. Wachs (Ed.), Butterworths, Stoneham, MA, 1992, p. 69. [ 10] I.E. Wachs, J.M. Jehng, G. Deo, B. M. Weckhuysen, V. V. Guliants, J. B. Benziger, and S. Sundaresan, J. Catal. 170 (1997) 75. [11] H. Igarashi, K. Tsuji, T. Okuhara, and M. Misono, J. Phys. Chem. 97 (1993) 7065. [ 12] G.J. Hutchings, C. J. Kiely, M. T. Sananes-Schulz, A. Burrows, and J. C. Volta, Catal. Today 40 (1998) 273. [ 13] Metal oxide structure files, Cerius 2 version 3.8 molecular modeling package (1998), Molecular Simulations Inc., 9685 Scranton Road, San Diego, CA, 92121. [ 14] Yu. E. Gorbunova and S: A. Linde, Dokl. Akad. Nauk SSSR 245 (1979) 584. [ 15] G.J. Hutchings, Appl. Catal. 72 (1991) 1, and references therein. [ 16] I. Matsuura and M. Yamazaki, in: New Developments in Selective Oxidation, eds. G. Centi and F. Trifirb (Elsevier, Amsterdam, 1990), 563. [ 17] T. Okuhara, K. Inumaru, and M. Misono, in: Catalytic Selective Oxidation, eds. J. Hightower and S. T. Oyama (ACS, Washington, DC, 1993), 156. [18] J.C. Volta, Catal. Today 32 (1996) 29. [19] D. Ye, A. Satsuma, A. Hattori, T. Hattori, and Y. Murakami, Catal. Today 16 (1993) 113. [20] R.T. Sanderson, Polar Covalence (Academic Press, New York, 1983); L. Pauling, The Chemical Bond (Cornell Univ. Press, Ithaca, NY, 1967); R. G. Pearson, Inorg. Chem. 27 (1988) 734. [21] J. Datka, A. M. Turek, J. M. Jehng, and I. E. Wachs, J. Catal 135 (1992) 186. [22] V.A. Zazhigalov, J. Haber, J. Stoch, I. V. Bacherikova, G. A. Komashko, A. I. Pyatnitskaya, Appl. Catal. A 134 (1996) 225. [23] F. Cavani and F. Trifirb, in: 3rd World Congress on Oxidation Catalysis, eds. R. K. Grasselli, S. T. Oyama, A. M. Gaffney, and J. E. Lyons (Elsevier Science B.V., Amsterdam, 1997), 19.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

The n a t u r e of the Cobalt Salt affects the Catalytic P r o p e r t i e s of P r o m o t e d VPO L a u r a Cornaglia, Carlos Carrara, J u a n Petunchi and E d u a r d o Lombardo Instituto de Investigaciones en Cathlisis y P e t r o q u i m i c a - INCAPE (FIQ,UNLCONICET), Santiago del Estero 2829 - 3000 S a n t a Fe - Argentina. FAX # 54-342-4536861. E-marl : [email protected] Cobalt-impregnated VPO catalysts containing 2 and 4% of the metal by weight were p r e p a r e d using two different cobalt salts. The catalytic tests showed t h a t cobalt impregnation significantly increased the overall activity. The use of cobalt acetylacetonate led to a more selective high loading catalyst. To investigate the origin of the cobalt effect, the solids were characterized using XRD, R a m a n Spectroscopy, FTIR and XPS. No structural effects were detected through XRD. After several h u n d r e d hours on stream, the only phase detected in all cases was V(IV) vanadyl pyrophosphate. The surface oxidation state of v a n a d i u m was V(IV). The Co 2p XPspectrum has an intense shoulder at 788 eV, indicating t h a t Co(II) species are present. 1.INTRODUCTION The VPO system exhibits an unique ability to activate and selectively oxidize alkanes. Vanadyl pyrophosphate is used commercially to catalyze the selective oxidation of n - b u t a n e to maleic anhydride. It is generally agreed upon t h a t the best catalyst precursor is VOHPO4.0.5H20 which is converted to (VO)2P207 [1] during activation. Among them, several VOPO4 phases have been claimed as necessary for the catalytic act [2]. However, these phases have never been detected in the so-called equihbrated catalysts t h a t have been on stream for over several h u n d r e d hours [3]. A variety of cations have been added to v a n a d i u m p h o s p h a t e catalysts to improve activity and selectivity. It has been claimed t h a t the addition of the first row transition metal produces an improvement in maleic anhydride yield [4,5 ]. Several papers [4-9] refer to the effect of cobalt used as a promoter. The aim of our work is to study the effect of the Co salt impregnation on precursors prepared by the conventional organic method. Our recognition and gratitude to Prof. Juan Petunchi for his lifelong lasting contribution to Catalysis. The financial support was received from ANPCyT(PICT N~ 14-00000-00719) and UNL (CAI+D 96 Program). The authors are grateful to the Japan International Cooperation Agency (JICA) for the donation of the major instruments used in this study. Thanks are given to Prof. Elsa Grimaldi for the edition of the English manuscript

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2. E X P E R I M E N T A L

Catalysts were p r e p a r e d by refluxing v a n a d i u m pentoxide in a mixture of isobutanol and benzyl alcohol during three hours. Orthophosphoric acid (100%) was then added and refluxed for two hours to obtain the desired P/V ratio in the precursor. The suspension was filtered and the solid dried at 390 K for 24 hs. The promoter was i m p r e g n a t e d on the precursor using isobutanol solutions of two different cobalt salts: Co acetate (VPCox-I1) and acetylacetonate (VPCox-I2), where x indicates the Co w%, I1 or I2 the i m p r e g n a t i o n salt. The catalysts were activated in two steps: 1) The precursor was h e a t e d up to 603K while a flowing mixture of 0.75% n - b u t a n e in air, with a gas hourly space velocity (GHSV) of 900 h 1. 2) The concentration of n-butane was increased to 1.5%. The t e m p e r a t u r e was increased with the limitation t h a t conversion never exceeded 80% and then the GHSV was raised to 2500 h -1. These conditions were m a i n t a i n e d for at least 500 h until stable conversion and selectivity values were obtained (equilibrated catalysts). The catalysts were characterized t h r o u g h Xray Diffraction and Raman, FT Infrared and X-ray Photoelectron Spectroscopies. 3.RESULTS AND DISCUSSION Figures 1.a and u n p r o m o t e d solids.

1.b show the

catalytic results of

promoted and

50 a

70

40

60

30

r..) m 50

20

40

10

30 640 TEMPERATURE/K

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.

.

.

.

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.

.

.

.

.

.

.

.

.

--A--VPCo4-I 1 --O--VPCo4-I2

0640

750 TEMPERATURE/K

Figure 1. Catalytic behavior of equilibrated catalysts. Reaction c o n d i t i o n s 1.5% n-butane/air, G H S V - 2500 h 1.

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50

_j -

Z 40

'-.,

VPCo4-I2 _.__.i"

30

f,--f" -'/~J/"

. . . . -~

......

~.

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1800

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17'00

'

16'00

1500

1400

W A V E N U M B E R / c m -1 Figure 2. FTIR spectra of VPO, VPCo4-I2 and VPCo4-I 1 precursors The Co-promoted catalysts present a higher yield of maleic anhydride. The impregnated solids present a significantly higher conversion in the whole temperature range, with the exception of the VPCol.5-I. This solid shows the lowest conversion of all the catalysts. The VPCo4I-2 presents a better selectivity at temperatures higher than 663 K. At these temperatures the yield is comparable to industrial catalysts. To investigate how cobalt affects the structure and surface of the VPO catalysts several techniques were used. The FTIR spectra of the precursors are shown in Figure 2. All of them show the characteristic vibrations of the VOHPO4.0.5H20. The C=C vibrations in the plane of the aromatic ring are also observed. These bands are due to benzyl alcohol and benzaldehyde retained in the solid after drying. In the VPCo4-I1 spectrum, bands at 1564 cm -1 and 1430 cm-1 are observed, while in the VPCo4-I2 spectrum, bands at 1523 cm 1 and 1430 cm-1 are detected. They are coincident with the most intense ones of cobalt acetate and cobalt acetylacetonate, respectively. This indicates that even after drying, the cobalt salts are present in the precursor. However, after activating the solids, these bands are not observed and the presence of cobalt oxides is not detected, either. Zazhigalov et al. [8] observed the appearance of Co2P207 for the catalysts with C o N > 0.15. In our case (CoN = 0.13), neither XRD nor FTIR indicate the presence of Co-containing phases. After several h u n d r e d hours on stream, vanadyl pyrophosphate was the only crystalline phase detected. No structural modifications (Table 1) were observed on impregnated solids through XRD. The Raman spectra show that none of the equilibrated catalysts contain VOPO4 p h a s e s . However, Volta and coworkers [7]

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sustain that Co and Fe dopants can modify the VOPO4/(VO)gPgO7 dispersion. They concluded that the activity for maleic anhydride formation is a function of the concentration of the V(V) phases present. In our catalysts, cobalt impregnation produces an increase of the precursor surface P/V ratio. The P/V surface ratio increases upon equilibration in the unpromoted VPO, whereas this ratio decreases in the Co containing catalysts.

Table 1 XRD and XPS data of precursors and equilibrated catalysts. Solid

VPO VPCo 1.5I 1 VPCo4-I 1 VPCo2-I2 VPCo4-I2 a) b) c) d)

FWH1Vro (001) 1.0 1.0 1.14 1.2 1.2

Precursor . P/V)~,: CoN)~ 2.15 3.80 3.70 3.40 3.40

Equilibrated catalyst FWHM d PN)~ c CoN)~ c

0.36 0.60 0.38 0.70

(200) 1.0 1.0 1.1 1.0 1.0

2.8 2.6 2.6 2.7 2.5

0.31 0.26 0.25 0.32

Bulk P/V ratio = 1.26 VOHPO4.1/2H~O (20=15.5~ FWHM in 20 Surface atomic ratios measured by XPS (VO)~P207 (20= 23.2o). FWHM in 20

The surface oxidation states of vanadium were studied by XPS through the analysis of the separation of the signals corresponding to O ls and V 2p.~/2 [10]. This method avoids the use of a reference as we concluded in a previous study [10]. The results are shown in Table 2. This value is equal to 14.2 + 0.2 eV for promoted and unpromoted catalysts. Coulston et al. [12] have reported a correlationship between the A value and the average vanadium oxidation state. These calculated values are around 4. In all the cases studied, the curve fitting leads to a single, well-defined binding energy for V 2p3/2 (517.7+0.1 eV) assigned to V(I\/) in agreement with the values reported by Lbpez Granados et al. [3] for unpromoted catalysts. Both methods support the overwhelming presence of V(IV) on the surface of the VPO equilibrated catalysts. The addition of cobalt does not produce modifications on the vanadium surface oxidation states. Binding energies for cobalt were also measured by XPS (Table 2) for the high loading catalysts. For the VPCo4-I2 and VPCo4-I1 catalysts, the Co 2p3/2 binding energies were different. The value of 783.6 eV indicates the presence of CoO on the surface is unlike. The Co 2p3/2 B.E. is 780.1 eV for CoO. For Co promoted vanadyl pyrophosphate the binding energy is much higher. The Co 2p spectrum has an intense shoulder at 788 eV. In addition, the 2p1/~-2p~2 spinorbit splitting of 15.7 eV for this catalyst indicates that Co(II) species are

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present. This shift might be related to strong interactions between the metal and other atoms of the pyrophosphate matrix and/or to a highly dispersed cobalt. The different B.E. between the two impregnated catalysts could be related to a different surface Co(II) species. Binding energies similar to those were reported [11] for the ion exchanged form of cobalt in zeolites. In these solids, the higher than normal binding energy is suggestive of cobalt in a highly oxidize environment. Zazhigalov et al. [8] reported a similarly high binding energy of 783.0 eV for Co-promoted VPO catalysts. They compared their results with the value of 782.8 found in cobalt molybdate or 782.2 and 783.0 for COC12 and CoF2 and concluded t h a t the metal ligand bonds are polarized in these reference compounds and a similar structure may be expected for cobalt phosphates. They sustained that the high binding energy values of 782.7-783 eV observed in their catalysts can be considered as an additional indication of the formation of cobalt phosphate (detected by XRD). On the other hand, they suggested the existence of strong interactions between cobalt atoms and phosphate groups with a shift of bonding electrons towards the phosphate anions. In our catalysts the C o N nominal ratio is lower than 0.13, and no cobalt phosphates were detected by XRD, FTIR and Raman. In agreement with our results, Hutchings and Higgins [5] have recently reported that the XRD patterns do not even hint the formation of solid solutions at low promoter ratios, M/V < 0.12. Hutchings [4] propose that the promoters at high promoter/vanadium atomic ratios, act as phosphorus scavengers by either formation of metal phosphates or by formation of sohd solutions. Table 2. The surface oxidation states of the equilibrated catalysts Solids

V 2p:~/2"~." A[O Is -V 2p3/2] Vox b Co 2p3/2 c B.E.(eV) FWHM (eV) B.E.(eV) VPO 517.9 2.3 14.3 4.09 -VPCo4-I1 517.8 2.2 14.4 4.03 782.6 VPCo4-I2 517.8 2.3 14.3 4.09 783.4 a Binding energies determined by curve fitting b Using the following correlation : Vox = 13.82 -0.68(A[O ls -V 2p3/2]), from Coulston et a1.[12] c O ls Binding energy equal to 532.2 eV was used as reference. The Co addition significantly increases the overall activity. This seems to correlate with the presence of cobalt on the equilibrated catalyst surface. Using cobalt acetylacetonate for the precursor impregnation produces a solid with higher selectivities and performances comparable to commercial catalysts. In this complex structure the promoter might be incorporated in the crystal lattice of (VO)2P2OT, as reported by Takita et al.[6]. They studied the incorporation of

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promoter elements employing metal acetylacetonates added during precursor synthesis. Incorporation of the promoter elements into the crystal lattice of (VO)2P207 brings about the shift in the IR absorption bands of the V=O stretching mode. In the case of the catalyst prepared in an aqueous medium, in which Zn is located at the surface of vanadyl pyrophosphate, no shifts in wavenumber were observed. In our catalysts the V=O vibration was not modified by the presence of cobalt. 4.CONCLUSIONS The addition of Co significantly increases the overall activity in all the temperature range, while slightly decreasing the maleic anhydride selectivity at low reaction temperatures. The impregnated VPCo4-I2 catalyst has shown the highest selectivity and yield. This seems to be due to a different surface COOI) species present in this equilibrated catalyst. REFERENCES 1. G.Centi, F. Trifir5, J. Ebner and V. Franchetti, Chem. Rev. 88 (1988) 55. 2. F. Ben Abdelouahab, Olier, R. , Ziyad, M. and J. C. Volta, J. Catal. 134 (1992) 151. 3. M. L6pez Granados, J.L.Garcia Fierro, F. Cavani, A. Colombo, F. Giuntoli and F. Trifir5, Catal. Today, 40 (1998) 251. 4. G.Hutchings, Appl.Catal., 72 (1991) 1. 5. G. Hutchings and R. Higgins, J. Catal., 162 (1996) 153. 6. Y. Takita, K. Tanaka, S. Ichimaru, Y. Mizihara, Y.Abe and Y. Ishihara, Appl. Catal. A, 103 (1993) 281. 7. G.Hutchings, C. Kelly, M.T. Sanan6s-Schulz, A. Burrows and J.C. Volta, Catal. Today, 40 (1998) 273. 8. V.A. Zazhigalov, J. Haber, J. Stoch, A. Pyatnitzkaya, G.A. Komashko and V.M. Belousov, Appl. Catal. A, 96 (1993) 135. 9. L. Cornaglia, C. Carrara, J. Petunchi and E. Lombardo, Appl. Catal.A, 183 (1999) 177. 10.L. Cornaglia and E.A. Lombardo, Appl. Catal. A, 127 (1995) 125. 11.J. Stencel, V. Rao, J. Diehl, K. Rhee, A. Dhere and R. DeAngelis, J. Catal, 84 (1983) 109. 12.G.W. Coulston, E.A. Thompson and N. Herron, J. Catal., 163 (1996) 122.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Oxidation of toluene to benzaldehyde over VSbl.xTixO4 catalyst. Kinetics studies. A. Barbaro, S. Larrondo and N. Amadeo a a Labomtorio de Procesos Cataliticos. Depto.de Ing. Quimica, Facultad de Ingenieria, UBA. Pabell6n de Industrias, Ciudad Universitaria, 1428 Buenos Aires, Argentina. In this work a kinetic study of the vapor phase oxidation of toluene by air using titanium doped vanadium antimonate as catalytic system, is reported. The catalytic tests were performed in an integral fixed-bed reactor operating at atmospheric pressure. From the results obtained in the introductory kinetic tests, a kinetic scheme that considers the partial oxidation of toluene to benzaldehyde, a consecutive oxidation of benzaldehyde to carbon oxides and a parallel oxidation of toluene to carbon oxides, is proposed. The corresponding kinetic parameters are estimated using non-linear regression. There is a good fitting between the regression curves and the experimental data. The oxidation of toluene takes place on a partially reduced catalyst surface. The overall rate depends on the partial pressures of the reactants. The three steps of the kinetic mechanism have similar activation energies indicating that the same intermediate is involved. In the range of the operating conditions, there is no rate-controlling step. 1. INTRODUCTION The processes of catalytic partial oxidation of aromatic hydrocarbons in vapor phase are of great importance in chemical technology[1 ]. The partial oxidation of hydrocarbon has been subject of many research works. This is because oxidation catalysis is complex and involves many parallel and consecutive reactions. There have been few comprehensive studies of the kinetics of selective oxidation reactions. In particular, for toluene oxidation, kinetic equations are usually of the power rate law type and Mars-van Krevelen redox model type [2-6]. In many of these works [4-6], low hydrocarbon pressures and high oxygen pressures were used. In consequence, the rate expressions are nearly first order in the hydrocarbon pressure and zero order in oxygen pressure. The data available in the literature about the vapor phase oxidation of toluene show that it is common to obtain selectivities to carbon oxides of 40% and higher, even at low toluene conversion levels [7-8]. This would indicate the presence of a route for direct oxidation of toluene to carbon oxide. However, there are few works [2] considering this direct oxidation of toluene by a parallel route, in the kinetic studies. In a previous work it was found that the vanadium antimonate doped with titanium, with nominal composition VSb0.8Ti0.204 [9], presents good catalytic performance in the toluene oxidation reaction. The products obtained with this catalytic system were benzaldehyde and carbon oxides. In the present work a kinetic study of vapor phase oxidation of toluene by air, using a titanium doped vanadium antimonate as catalytic system, is reported. The aim is to

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propose a kinetic scheme suitable to identify the main reaction steps as well as to determine a kinetic expression that provides a good fitting with the experimental data. Further studies will optimize the operation conditions in order to obtain better selectivites to benzaldehyde. 2. EXPERIMENTAL

2.1. Catalyst Preparation The catalysts were prepared by solid state reaction of mechanical mixtures of pure vanadium (V) oxide, antimony 0ID oxide and titanium (IV) oxide. The amount of each oxide in the mixtures, was the necessary for obtaining a solid catalyst with a nominal composition of VSb0.sTi0.204. The samples were put in quartz crucibles and heated in air by a conventional furnace with a temperature calcination programme, according to the method published elsewhere [9]. X-Ray powder diffraction (XRD) technique was used to check that the structure of the phase obtained was similar to that previously reported as vanadium antimonate with 20% of titanium doping [ 10].

2.2. Catalytic tests The catalytic oxidation of toluene was studied in an integral fixed-bed reactor operated at atmospheric pressure. The reactor was made of a Pyrex glass tube of 13-mm inner diameter. Toluene was fed by means of a carrier air stream flowing through a saturator. The feed toluene/air molar ratio was controlled by adjusting both the saturator temperature and the input air flow rate. The reaction temperature was measured with a sliding thermocouple placed inside the bed. Since the oxidation reactions are highly exothermal, the catalyst bed was diluted (1:10) with glass particles, of the same diameter range, in order to avoid adverse thermal effects.The composition of the feed gas and the effluent were analysed by on-line gas chromatography. The reactor was operated in steady-state conditions. The catalytic tests were performed under the following conditions: catalyst mass: 100-400 mg; total feed rate: 200-600 ml.min 1, toluene molar fraction: 0.001 to 0.01; oxygen molar fraction: 0.04 to 0.2 ; temperature range: 673-723K; particle diameter <120 ~tm. The experiments were carried out at fifteen space times, five temperatures, ten toluene partial pressures and six oxygen partial pressures.

2.3. Introductory kinetic tests. In order to insure that the kinetic experiments give meaningful results, preliminary catalytic tests were carried out at different residence times, particle diameter, total gas flow and temperatures. Some of them were performed without catalysts in order to study the contribution of the thermal oxidation. These tests showed a negligible contribution of homogeneous oxidation to total conversion and the absence of internal and external diffusion limitations, for a particle diameter below 120 l,tm and total gas flow equal or greater than 100 ml.min -1, respectively. The catalyst showed activity without the necessity of a pretreating and was stable during a typical run period of 12 hours. The main oxidation products formed were benzaldehyde, carbon monoxide and carbon dioxide. It was also detected benzoic acid in very small quantities. In Figure 1 selectivities are plotted as a function of toluene conversion at 713K. The shape of the curve representing the selectivity to benzaldehyde shows that the rate of benzaldehyde oxidation is greater than the rate of formation. The extrapolation of both curves to zero

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conversion of toluene lead us to the conclusion that nearly 30 to 40% of toluene would be oxidized by parallel reaction to carbon oxides. Therefore, the reaction kinetic scheme proposed is the following: 100% Y~ 0.55 mol-% T = 713 K Toluene 9 Benzaldehyde t~'''''80*,6 y~ =19.9 m o l - ~ ~

"--...

9~ 60%

OSco~ ASBA

~ 40.,6 20*,6 0%

0.0

0.1 0.2 0.3 0.4 Toluene Conversion

0.5

/

CO~ Benzoic acid was detected as product only in few experiments and in a very low concentration. Then, only the total oxidation of benzaldehyde was considered. 3. KINETIC MODEL

The classical redox mechanism proposed by Mars and Van Krevelen[3] was adopted in this work. The basic concepts of this mechanism may be interpreted by assuming two sucessive steps:

Figure 1. Selectivities vs. Toluene conversion

1. Hydrocarbon + Oxidized Catalyst 2. Reduced Catalyst + 02 (g)

Oxidized Products + Reduced Catalyst Oxidized Catalyst

The rate expressions derived from this mechanism are presented in Table 1. First order in hydrocarbon and order "n" in oxygen partial pressures were assumed. Tablel. Rate expressions for each step of the kinetic mechanism proposed. Reaction step 1. Tol + Oox ~

BA + H20

Rate expression + (~r

rl = k l

9PTol

.0

2. Tol + Oox ~

7 COx + 4 H20 + Or

r 2 = k 2 "PTol

"0

3.BA + Oox ~

7 COx + 3 H20 + (~r

r3 = k3 " PBA

.0

4. Oox + O2

=

r4=k 4.Pn .0_0 ) 02

(2Jr

Under steady state conditions, the rate of catalyst reduction equals the rate of catalyst reoxidation. Then, the surface degree of coverage by oxygen, 0, is n

O =

k4P02 k 4 P 0n2

+ (kl + fl.k2)PTol + 2.k3 PBA

(I)

where fl and 2 are stoichiometric constants that represent the number of moles of oxygen consumed by total oxidation of one mole of toluene and one mole of benzaldehyde, re~bectively. The values of fllese constants are fl = 8.11 and 2 = 7.11. When the surface is

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completely oxidized (O = 1) carbon oxides are mainly formed. The progressive reduction of the surface increases the formation of partial oxidized products (O <1). Theoretical calculations predicted that at O values in the 0.3-0.4 range, maximum selectivities would be achieved [11 ]. The differential equations for an ideal plug-flow reactor, with the corresponding inlet conditions are summarized in Table 2. Table2. Differential equations and inlet conditions Differential equations

dy~o,

Inlet Conditions

(k, + k 2 ) . k 4 "Yro, "Yo,

dm~,,t = - F . P .

k4 "Yo, + ( k , + f l . k 2 ) . y r o I + ./]..k 3 "YBA

dYo2 = _ F . p . ( ( k , dm~t

+ fl'k2)'Yro,

+k3"2"YsA)'k4"Y02

k4 " Yo= + (k, + fl . k~ ). Yro' + 2 - k 3 "YsA

dYBA =

F'P"

= 7.F.P.

dmcat

YO 2 = Y O 2 exp. 0

k4 "Yo, +(k~ + f l . k 2 ) ' Y r o , + 2 - k 3 "YBA

dm~a, a"c~

(kl " Yrol - k3 "YeA )" k4 " Yo,

o 0 Y]'ol = YTol exp.

(k~ "Y~ol + lc~" YB,)" k4 "Yo, k4 " Yo2 + (kl + fl " k: )" Yrot +/]"k3 " YBA

=

Y BA

0

YgO x - 0

4. RESULTS The reaction parameters of the former rate expressions were determined by minimizing the sum of the squares of the deviations between measured and calculated molar fractions of reactants and products. A specific optimization routine for multivariable non-linear regression, developed by Quiroga and Gottifredi [12] coupled with a standard routine of numerical integration of the differential equations presented in Table 2 was used. In a first step, the parameter estimation was done using the experimental data obtained at 713K. The goodness of the fitting of the regression curve and experimental data assuming n-1 in the oxygen partial pressure is shown in Figures 2,3 and 4.

'0~y~ 055mo1~ T713~ L ,00~ ~yoo~ 199 mo,~ ~ ~ - - ~~- -s-~~ / ~0~1Y~~ ~ 0T~4mo, 7 1 ~ 40~~ ' ~

O

r~

oXTo~ / ~40~1 :

0.0 0.2 0.4 0.6 0.8 1.0 1.2 Space-time W/F ( gr cat. min/ml ) Figure 2. Effect of space-time

2%

~"~

6% 10% 14% 18% Oxygen Molar Fraction

22%

Figure 3. Effect of oxygen molar fraction (W/F = 0.6 gr cat.min/ml)

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A significant decrease in the fitting quality for n: 0, 0.5 and 2 was observed. The surface degree of coverage by oxygen, 8, was calculated with the parameters estimated at 713K. Values within the 0.2 to 100% yO 0.8 range were found. Therefore, the oxidation 9~ 02 = n ,tool-% , 1 9 T " =9713 K takes place over a partially reduced surface 80% with the overall rate depending on oxygen

~60%

n S COx a S BA o X To l

and toluene partial pressures (Figures 3 and 4). Likewise, it was observed that 40% temperature has no significative influence on "~ -o~.._ ... selectivities to partial and total oxidized products (Figure 5). This fact suggests that all 20% o the steps of the kinetic scheme have the same 0% activation energy. The preexponential factors 0.0% 0.2% 0.4% 0.6% 0.8% 1.0% and the activation energy (Arrhenius law, Toluene MohrFraction k~= kfl * exp(-Ea/RT)), were determined considering all the experiments performed. Figure 4. Effect of toluene molar fraction The results are presented in Table 3. It is (W/F = 0.6 grcat.min/ml) important to remark that estimated activation energy for the reoxidation reaction was close to those of the oxidation reactions. Table 3. Estimated parameters Parameter

Estimated Values ...............

k ~ mol.(min.atrn.grcat) "l

5.22E+06

k ~ mol.(min.atm.grcat) "1

3.28E+06

k ~ mol.(min.atm.grcat) "l

7.24E+07 2.02E+06

k4~ mol.(min.atm.grcat) "1 Ea kcal.mol 1

27.6

The kinetic scheme has a good fitting of the data, as can be seen in Figures 2 to 5 and in the good correlation between the experimental and the calculated toluene molar fraction showed in Figure 6. lO0~ Y~ = 0.55 mol-% y~ 2 =19.9 mol-% 80%

~ * ~ ~ ~ A 723 K SCOx n713K o 703 K -x~ o 693 K 73 K

.~ 60~ ,, ~

~

40% "

20%

0% 0%

~176 . r "

~" 0.0% . . . . . 10% 20% 30% 40% 50% m 0.0% 0.2% 0.4% 0.6% 0.8% 1.0% Toluene Conversion Calculated Molar Fraction Figure 5. Selectivities vs. Toluene conversion Figure 6. Parity plot for toluene molar fraction

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5. CONCLUSIONS

The reaction network was modelled assuming the redox mechanism proposed by Mars and Van Krevelen. The approach is oversimplified with respect to the true reaction mechanism, but provides a better description of the experimental data than the empirical approaches (power law equations). The kinetic-scheme proposes three steps: the partial oxidation of toluene to benzaldehyde, a consecutive oxidation of benzaldehyde to carbon oxides and a parallel oxidation of toluene to carbon oxides. The good fitting of the experimental results for the model proposed does not imply that this model represents the true reaction mechanism. Detailed mechanistic theories are out of the scope of the present work. The similar values found of apparent activation energies for each step, suggest that benzaldehyde and carbon oxides are produced from the same intermediate. Similar behaviour was obtained by Van der Wiele and Van den Berg [2] in the oxidation of toluene over bismuth molybdate catalysts. The activation energy value obtained in this work is similar to that reported by Trimm and Irshad in the oxidation of toluene over molybdenum trioxide catalyst (27.4 Kcal/mol) [5]. The overall rate was found dependent of the partial pressures of the reactants. The oxidation takes place over a partially reduced surface. The selectivity to benzaldehyde was in the 15% to 50% range and increases as the toluene molar fraction increases and oxygen molar fraction decreases. In the range of the operating conditions, there is no rate-controlling step. Further studies will be carried out in order to find the optimal conditions for benzaldehyde production. NOMENCLATURE BA : Benzaldehyde COx: Carbon oxides. Ea :activation energy. exp.: experimental F : molar feed rate. F~o : volumetric feed rate. ki: specific rate constant. ki~ Arrhenius law preexponential factor. m: catalyst mass Pi : partial pressure of component i.

P :total pressure :reaction rate of step i. T :temperature x :toluene conversion yi : molar fraction of component i. TOL : Toluene. ~ox : oxidized catalyst sites. Qr : reduced catalyst sites 0 : surface degree of coverage by oxygen W/F: space-time ri

REFERENCES 1. A.Bielanski and J. Haber, Oxygen in Catalysis, Marcel Dekker Inc., New York, 1991. 2. K. Van der Wiele and P.J. Van den Berg, J.ofCatalysis, 39 (1975) 437. 3. P. Mars and D. W. Van Krevelen, Special Supplement to Chem. Eng.Sci., 3 (1954) 41. 4.G. Gunduz and O. Akpolat, Ind. En. Chem. Res., 29 (1990) 45. 5. D.L. Trimm and M. Irshad, J.of Catalysis, 18 (1970) 142. 6. K. Papadatos and K. Shelstad, J.of Catalysis, 28 (1973) 116. 7. M. Ponzi, C. Duschatzky, A. Carrascull, E.Ponzi, Applied Catal. A: Gen., 169 (1998) 373. 8. Andersson S.L.T., J.ofCatalysis, 98 (1986) 138. 9. A. Barbaro, S. Larrondo, S. Duhalde, N. Amadeo, Applied Catal. A: Gen. (in press). 10. Berry F., Smart L., S. Duhalde, Polyhedron, 15 (1996) 651. 11. S.T. Oyama, A.N. Desikan, J.W. Hightower, ACS Symposium series, 523 (1992). 12. O.D. Quiroga and J.C. Gottifredi, Avances en la determinaci6n de parhmetros cin6ticos en sistemas de reacciones quimicas complejas, CONICET, Buenos Aires, (1982).

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化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Interaction of Surface- and Bulk-Oxygen in Mo/V-mixed Oxides during the Partial Oxidation of an unsaturated Aldehyde - A Concentration-Programmed-Reduction StudyR. B6hling, A. Drochner, M. Fehlings, K. Kratff5, H. Vogel Darmstadt University of Technology, Department of Chemistry, Institute of Chemical Technology, Petersenstr. 20, D-64287 Darmstadt, Germany Mixed oxides based on molybdenum and vanadium are used as catalysts in partial oxidation reactions. Transient examinations, in particular the concentration programmed reduction method with acrolein, proofs the presence of at least two different oxygen species participating in the reaction. A mechanistic view involving surface- and bulk-0xygen as well as a simulation are given.

I. INTRODUCTION The heterogeneously catalysed partial oxidation of unsaturated aldehydes is an important reaction class for the synthesis of unsaturated carboxylic acids. Besides metallic systems, the main catalysts used are solid-state oxide systems. As a rule these can store oxygen reversibly. Selective oxidation of the organic educt takes place as a result of the oxygen stored in the catalyst, various oxygen species (e.g. surface- or bulk-oxygen) being capable of oxidative activity. The catalyst is re-oxidised by molecular gas-phase oxygen (Mars- van Krevelenmechanism [1]). The catalysts investigated were Mo/V-mixed oxides, which are the basic material for many industrial catalyst systems. Acrolein served as an example of an unsaturated aldehyde. The purpose of the present work was to employ transient methods to obtain more insight into the interaction of such catalytic systems. Isothermal concentration-programmed reductions (CPR) were used, in which the interference factor was the educt concentration. By means of this transient investigation method, the individual reactions on the catalyst system could be separated and characterised with regard to the interaction of surface- and bulk-oxygen.

2. EXPERIMENTAL The experimental procedure for the CP measurements was divided into an in situ pretreatment of the sample and the actual measurement. The sample was pre-treated at a specific temperature (300 ~ and gas-phase composition (10 vol% O2 in N2) to bring it into a specific, reproducible initial state with regard to its degree of oxidation and surface substances. The gas composition was analysed online by QMS. In the concentration-programmed experiments the

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concentration jumps were produced by 6-way pneumatic valves. After the usual sample pretreatment, an acrolein concentration jump was applied under isothermal conditions.

Apparatus The measurements were carried out on a self-constructed micro-apparatus (Fig. 1), which allows fully automatic changes for temperature- and concentration programmed investigations.

o; from gas-supply and saturators

I

~ 1

i ~ 02, H2

"

i

I i I _

exhaust

1 ~

gas

A1 [~

comp sse inert / reference carriergas

N~ ii ii

Fig. 1

|!!

! i

spectraSSeter

eleelr.

i

'm;

Part of the flow sheet showing the measuring apparatus (B 1 represents a gas mixing device, A 1 represents an absorption unit).

The reactor consists of a quartz glass U-tube (inner diameter 4 mm). The catalyst is retained by quartz glass wool. The reactor is placed in an air-stirred electric furnace, which has computer aided temperature/time controls. The temperature measurements are carried out in a parallel arranged U-tube. The thermocouple is secured in a fixed bed of inert material (alumina). Educts (acrolein or oxygen) and inert gas (nitrogen/helium) are fed into the reactor by mass flow controllers (Bronkhorst). Two identical saturators arranged in series enable a reproducible load of the inert gas flow with acrolein to take place (Fig. 2). The desired concentration (here 5 vol%) is adjustable with the aid of the vapour pressure curve of acrolein [2] by varying the lower temperature in the second vessel [3]. In this work, reaction-gas mixtures were analysed with online mass spectroscopy (Balzers, QMG 511). The time-resolution, depending on scanned masses, ranges between 4 and 6 Hz. The intensities of mass spectrometric signals are transformed online into volume

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fractions with the aid of the correction matrix [4]. A mixture of 5 vol% helium in nitrogen is used as inert carrier gas. The noble gas represents the internal standard. This reference ensures that volume changes caused by reaction, addition of gases and pressure changes at the outlet of the reactor can be corrected.

dry inertcarriergas

coolant

T1

Fig. 2

to reactor

@

~

lant

T2

Two-stage saturator for acrolein. The volume fraction of acrolein is adjustable by temperature in the second stage (Tl > T2).

Six-way-valves that are common in chromatography are available to perform CP-experiments. In order to perform almost ideal concentration jumps, the dead volume and the use of 1/8"-supply tubes and fittings had to be minimised wherever possible. From inside the reactor a partial flow of the reaction gas is drawn off employing a 1/16"-tube, conveying the flow directly into the recipient of the mass spectrometer. All supply and exhaustion tubes are heated electrically (>140 ~ in order to avoid undesired condensation effects. Tailing and memory effects are suppressed to the greatest possible extent in this way. Chemicals 9 acrolein, 95 %, Merck AG 9 acrylic acid, 99 %, Fluka Chemie AG 9 acetic acid 100 %, Riedel-de Hahn AG 9 nitrogen (4.6), helium (5.0), oxygen (4.5), carbon dioxide (4.5), Linde Gase AG 9 5 vol% helium in nitrogen, Linde Gase AG 9 Mo/V-mixed oxide, BASF AG [5]

3. RESULTS AND DISCUSSION The acrolein-jump emitted on entry into the reactor, yields an acrylic acid answer signal (volume fraction of AS as a function of time cpAs(t)) practically without any time lag. This runs through a maximum on the grounds of the limited oxygen capacity of the mixed oxide. The above experiment is reproducible numerous times ever after a re-oxidation of the mixed oxide with gas phase oxygen. This illustrates the load-bearing capacity and stability of catalyst materials of that type.

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The AS-answer signals thus obtained, surprisingly show a second maximum in the temperature range of 200 - 250 ~ i.e. the reduction of the mixed oxide is accelerated once again. Analogous thermogravimetric investigations carded out, confirm this observation; a shoulder can be clearly recognised in the DTA-signal (Fig. 3). 0,6 0, 4 8 4

-12

55

-13

45 35

-14

25 -15

0,2

15

-16

0,0

-17 0

2

4

6

t / min Fig. 3

5

8

126

129

132

-5 35

t / min

Acrylic acid answer signal (A) and thermogravimetric measurement (B) during a concentration jump with acrolein (5 vol%) on a Mo/V-mixed oxide (220 ~ A: 50 mg, 20 ml minl; B: 22 mg, 100 ml minl).

The occurrence of the second maximum can be explained e.g. by at least two different species of oxygen being present in the mixed oxide. If the oxygen reservoir of the mixed oxide were to consist of only one species of oxygen, then the AS-answer signal would not rise any more after the breakthrough of acrolein. The observed phenomenon can be described in accordance with the work of Leary [6] by the model described in what follows, which depends on a simple distribution of the species of oxygen on the surface- and bulk of the mixed oxide.

4. MODEL AND SIMULATION First of all the gaseous reducing agent (G, here acrolein) reacts with the active oxygen on the surface of the mixed oxide (Os) so that this becomes partially reduced. The oxygen vacancies ( | thus resulting, can be filled again subsequently with a diffusing species of oxygen from the bulk (OR). kR G +Os---~GO+| s ko OB + | S ---~Os + | B

(1) (2)

With an increasing degree of reduction of the surface (a), the degree of reduction of the bulk (y) also rises, along with the number of oxygen vacancies of the bulk phase (| B )" The initially accelerated diffusion of the bulk oxygen thereby leads to a rise of the reaction rate on the surface, and becomes compensated by the limited oxygen capacity of the sample.

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a = 1

n~

... y = 1 - . nob n o s ,0

n o s ,0

(3) and (4)

The sizes nos and non are the quantities of surface- and bulk oxygen at the point in time t of the reduction. The sum of the initial quantities of both oxygen species, of the highly oxidised Mo/V-mixed oxide (nos, o + non, o) supplies the integration of the acrylic acid answer signal (tpAs(t)). As already mentioned above, practically only acrylic acid is formed during the CPR (selectivity ~ 1), it corresponds to the quantity of oxygen supplied by the mixed oxide MoxVyOz and therefore to the quantity of acrylic acid formed. This also correlates with the thermogravimetric measurements.

~

.,,,,.~0

+

M~

Oz

~

y

O

+

M~

Oz-1

(5)

OH acrolein (ACR)

acrylic acid (AS)

n ~ 1 7 6= n ~ 1 7 6+ n~176 - R i } "

tPAs(t)dt

(6)

0

The conversions were very low during the CPR's (max. 0.1), so that in the first approximation acrolein was present in excess, and whose concentration could be considered as constant (kR.CAc R = k 'R). Under the named simplification, the following differential equation system arises for the CPR-experiments with regard to the mixed oxide. d~= i" = -

HOs n o s ,0

: k,~ .(1- a ) - k D .(1- y). a . noB,o

nOB_. n o B ,0

kD

"(1-r)'a'nOs,O

(7)

(8)

The ratio of nos, o to noB, o and the rate constants kR' and kD give the simulation parameters. A numeric solution of the differential equation system (eq. 7 and 8) is represented in Fig. 4. The acrylic acid answer signals measured, correlate to the first derivative of the total degree of reduction according to time (o-). =

n o S ,0

n o s ,o + n o , ,o

96 +

nO B ,0

n o s ,o + n o B ,o

.~;

(9)

The simulation reconciles not only an insight into the time scale of the proceedings, but it additionally inaugurates the separate consideration of the surface- and bulk species of oxygen. Through the diffusion which occurs with bulk oxygen, with increasing concentrations of oxygen vacancies, the surface becomes partially re-oxidised. This has the consequence of the

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reduction rate of the surface (&) having a local minimum, and indeed rather at the point in time where the reduction rate of the bulk (~;) is a maximum (Fig. 4). Within a specified period of time, the re-oxidation rate of the surface is in the same range as its reduction rate. 2,0 9-

1,5

~

1,0

0,6

"•

0,4 -

~

"~ 0,5 0,0

I

0

2

t / min Fig. 4

0,0

I

4

6

8

t

0

2

4

6

8

t / min

Simulated curves of the modelled reaction scheme given in eq. 1 and 2; the first derivative of the total degree of reduction shows a shoulder according to an accelerated reaction rate.

5. CONCLUSION CP-experiments augment the arsenal of methods for explaining the oxygen dynamics of Mo/V-mixed oxide catalysts. The possibility thereby given of obtaining kinetic information, with the simultaneous presence of different species of oxygen, yields an enormous potential for the understanding and further development of catalysts of that type. Formulated questions still open, will be pursued in further work, as e.g. what significance and implications the participation, as well as the dynamics of the bulk oxygen has on the catalytic process, and how and where the oxygen transport occurs in the bulk, if gas phase oxygen is involved.

REFERENCES

1. P. Mars, D.W. van Krevelen, Special Supplement to Chemical Engineering Science, 3 (1954) 41-59. 2. D.R. Stull, Industrial and Engineering Chemistry, 39 (1947) 517. 3. R. B6hling, A. Drochner, M. Fehlings, D. K6nig, H. Vogel, Chemie Ingenieur Technik, 71 (1999) 226-230. 4. E. Mtiller-Erlwein, H. Hofmann, Chemiker Zeitung, 11 (1987) 325. 5. R. Krabetz, W. Ferrmann, H. Engelbach, P. Palm, (BASF AG), Eur. Pat. Appl. 0017000 A 1 (1980). 6. K.J. Leary, J.N. Michaels, A.M. Stacy, AIChE Journal, 34 (1988) No. 2, 263.

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Studies in Surface Science and Catalysis 130 A. Corma,F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rightsreserved.

化

1745

Steady State and Transient Kinetic Experiments For Rational Catalyst Design" Differences of Acrolein and Methacrolein Oxidation on Mixed Oxide Catalysts H. B6hnke, J.C. Petzoldt, B. Stein, J.W. Gaube Technische Universit~it Darmstadt, Institut f'tir Chemische Technologie, 64287 Darmstadt

Acrolein (ACR) is oxidised with exceptionally high selectivity and activity on Mo-Voxides to give acrylic acid (AA). On the other hand heteropoly acids are the best catalysts for the partial oxidation of methacrolein (MAC). Our experiments have shown, that the oxidation of both starting materials is independent of the further oxidation of the corresponding acids and vice versa [1, 2]. This implies that the selective oxidation and the further oxidation proceed on different domains of the catalyst. Crosswise exchange of catalysts and reaction leads to considerably lower selectivities of both reactions. The variation of the Mo/V-ratio of heteropoly acid catalysts (HPA) has shown that catalysts of high selectivity generally exhibit low activity [3]. Due to this, variation of redox properties of the catalyst was restricted to mixed oxides. Alteration of the Mo/V-ratio of the mixed oxide has revealed that the different catalyst domains can be separately influenced, thus leading to an optimized selectivity. EXPERIMENTAL Steady state kinetic experiments were carried out in a differential recycle reactor which behaves like a CSTR. The composition of the gas phase is analysed by GC. The reaction rates can be directly determined by analysing the gas flow at the inlet and the outlet of the reactor [21. acrolein

jet loop reactor condensation

N2

O2 acrylic acid N2

vaporation i

~

H20

GC-

co co l

]saturation

continuous analysis and data registration Figure 1" Experimental unit for steady state kinetic measurements Transient experiments and sorption studies were carried out in a static adsorption apparatus under reaction conditions comparable to those of the industrial process in temperature and

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partial pressures. Furthermore, the reduction degree of the catalyst can be adjusted and evaluated (by reduction and back-titration with oxygen, respectively) and the BET-surface can be determined.

~ lC

Dosing of O, H, H, O ,acrolein, acrylic-acid and Ar

PIR

ition

Gas[Chrimatograph

~

sample- ~ loop

II II ass-

I I ~ I spectr~ online MS

Ho

. ~ . t ~ adsorption cell

,

(~vacuum

Figure 2: Experimental setup for adsorption and transient kinetic measurements RESULTS In stationary kinetic experiments on a mixed oxide catalyst used in the ACR oxidation, the activity in the MAC conversion turned out to be less than 10 % when compared to ACR. Furthermore it is evident that the reaction becomes independent of the MAC concentration at a very low partial pressure as presented in figure 3. - - " - - Methacrolein i .... 9-- Acrolein

9I Mo-V-oxide .i-..-~7 o6

/ / /

O3

v

0,70,6

t- 5 o 4 E u

,__._.,

/

/

/

/

/

/

~ /

/ J'

T = 280 ~ 10%O 2 10 % H20

I Cs~H2[PMollVO4?] i

0,5

T = 300 ~ 10 % O.. .. ......~ 6 % H2~) ................ 9 ..o/

0,4 0,3 0,2

/

p ~ m

0,1 - /

m~.m__

m

/O

,,,,11.-----I1~11~11--------...----i1..-----------~11 '

I

'

I

'

I

,

,0,0

,-

2000 4000 6000 8000 0 Pi [Pa]

.

,

2000

,

,

4000

Figure 3" Rate of oxidation of ACR and MAC on Mo-V-oxide and HPA

,

,

6000

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In contrast to this finding, the conversion of both starting materials on heteropoly acids hardly shows any difference. But for both catalysts it is observed that for the conversion of acrolein the oxidation of the aldehyde is the rate determining step even at rather high aldehyde concentration whereas for the methacrolein oxidation the reoxidation of the catalyst determines the reaction rate. This can be interpreted by an easier oxygen transfer to the former. Transient experiments have shown that only ACR is able to use substantial amounts of the catalyst oxygen, whereas with MAC the catalyst remains in a high degree of oxidation as shown in figure 4. These results support the interpretation mentioned above, that the oxygen can be transferred easier to acrolein than to methacrolein. 1. A C R - P u i s e , R = 0.5 %, T = 473 K 40

1. M A C - P u l s e , R = 0.5 %, T = 473 K

40

A

ACR

e,..~

"'O"

A A

~'~ 30 0

--e- C3 adsorbed - - Monolayer

C

59 ?

d

,~

.

'

~ 10

'.A.

0

....

i 60

I

-A . . . . .

A-

--

20

- : ~ M0n0!ayer

zx

~

..... *................

.~I;

0 ~**-

............

I

-zx- M A C --*-. M A A - e - C3 adsorbed

-

A

,,,,

/k ......

i

120 180 t [s]

--] ........ ,~ 240

300

;~.~ 0

,. 60

. 9. . . . . . . . 120

t [s]

180

~,......... -. 240

300

Figure 4: Transient kinetics of ACR and MAC on Mo-V-oxide in absence of oxygen Sorption studies have revealed that this difference in substrate activity is not due to different amounts of the aldehydes adsorbed on the catalyst. Therefore the independence of the MAC conversion of the MAC partial pressure is not the result of different amounts adsorbed, but is the consequence of the reoxidation being the rate determining step. Thus, ACR is able to strongly reduce the catalyst, thereby leading to a high rate of reoxidation. In contrast, the very low rate of reoxidation in MAC conversion is the consequence of the high degree of oxidation in the steady state. Reoxidation studies have revealed that the rate of reoxidation decreases strongly with increasing degree of oxidation. Comparing the reaction rates of the acrolein oxidation on the mixed oxide and the HPA catalyst, it is obvious that the HPA shows an intrinsically much smaller oxygen activity. This conclusion has been further proven by measurements of the degree of reduction of used catalyst samples. As we aimed for high catalyst activity, variation of catalyst stoichiometry only was carried out for the mixed oxide. As the Mo-V-oxide catalyst used in this study is not well suited for the oxidation of MAC, the molar Mo/V-ratio, regarded as crucial for the redox properties, was varied. In reoxidation experiments, it turned out that the Mo/V-ratio indeed strongly effects the reoxidation rate: with decreasing Mo/V-ratio the reoxidation rate increases (Table 1). Consequently, the rate of the MAC steady state conversion was found to increase in the same manner (Figure 5).

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- - o - - MoN = 9

,..., 7

- - l i - - M o N = 3.47

~I'

Table 1: Reoxidation kinetics, transient ex ~eriments r(02)

//.-~,../r ,v/'~~_~~e~ ----'~

9 3.47

2 H20=12%

1 ~,p

'

0

I

10

'

[mol/(hkgaaive)] 18 26 39 48

I

20 Po2 [kPa]

Figure 5" Oxidation of MAC--rMAC =

O/o

T=573K' '

I

'

30

f(p(O2))

1.8

I

40

for different Mo-V-ratio

On the other hand, it was found that the acid conversion is comparably sensitive with regard to the redox properties as well, leading to an optimum in selectivity for Mo/V - 3. Figure 6 shows the shares of the parallel and consecutive oxidation of MAC lowering the selectivity toward MAA.

~

lOO 90 80 70 60 5o 40 30 20 10 0

n

I

1,80

I

3,00

IIII

~

I

3,47

9,00

Mo/V

I nS(MAA) nconsecutive reaction Ilparallel reaction I Figure 6: Reduction of selectivity to MAA by the different reaction paths of by-product formation, parallel oxidation of MAC and consecutive oxidation of MAA. PMAA= 200 Pa, p(O2) = 14.5 kPa, pMAC= 2.5 kPa, T = 573 K

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Comparing the rate of further oxidation of AA and MAA, the latter turned out to be oxidized faster, but we do not observe such extreme differences as in the case of the aldehydes. The shape of the curves show that the MAA oxidation reaches saturation at very low partial pressures, whereas AA shows a constant increase with partial pressure. This suggests that the adsorption of MAA approaches saturation already at a very low partial pressure in contrast to AA. Mo-V-Oxide

0,30 ,-~---,_0,25

A

tA

~

A--~---______ A

,- 0,20 O

E0,15 ~,.~-

0,10

T = 280 ~ 10%0:, 10 % H20

0,05 0,00

0

I

I

1000

- - a - - MAA 9 AA I

2000 3000 Pi [Pa]

'

I

I

4000

5000

Figure 7: Rate of oxidation of AA and MAA on Mo-V-oxide In adsorption experiments we could show that the maximum amounts of acid adsorbed are quite similar but that saturation is reached at a lower partial pressure for MAA which is therefore stronger adsorbed than AA (Figure 8). MAA-Adsorption, partially reduced Catalyst, T = 443 K 100

g

10

8O

100 "~ O

8

6O

0 ,

I

0

2

4

% Saturation~ n adsorbed 4 6 8 p(MAA) [kPa]

2

cO

.m

6

!

40

AA-Adsorption, partially reduced Catalyst, T = 443 K ,

80

84

60

84

,

4. ut ' ~

,

.'7".'.'.'.'.'.'.'.'~.

o)

t o|

o ~-

"E"

o

40 . . . . . . . . . . . . . . . . . . . . . .

---4."-. 2 ~"

20 1. . . . . . . . . . . . . . . . . . . . . . . . . . IN

I

0

2

Figure 8" Adsorption isotherms of MAA and AA on Mo-V-oxide

I

t

4 6 p(AA) [kPa]

0

8

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CONCLUSIONS

As our work shows, the combination of steady state and transient experiments as well as sorption studies is a very powerful tool. It renders possible to determine the steps of the Marsvan Krevelen mechanism, that are decisive in the oxidation of MAC and ACR, thereby leading to an interpretation of the large difference in the conversion of these aldehydes. As MAC or its corresponding intermediate is hardly able to mobilize the catalyst oxygen, altering the reoxidation rate by changing the redox properties did not lead to a MAC conversion rate similar to that of ACR. As we have described earlier the selective oxidation of the aldehyde and the further oxidation of the corresponding acid proceed on two different domains of the oxide surface. Therefore two strategies to develop an improved catalyst are imaginable. First the activity of the aldehyde domain must be enhanced without increasing the rate of further oxidation thus leading to a higher selectivity. On the other hand a separately achieved lowering of the oxidation activity on the acid domain would promote the selectivity as well. Despite the differences between acrolein and methacrolein conversion on Mo-V-oxides the optimum selectivity was found for a Mo/V-ratio of three. This is in accordance with the findings of Andrushkevich concerning the acrolein conversion [4]. It turned out that the rate of MAC-oxidation could not be increased for the aldehyde domain separately, but was accompanied by a higher rate of further oxidation as well. Therefore the chances to improve the catalytic performance of the aldehyde domain independently by variation of the Mo/V ratio seem to be exhausted for the selective oxidation of MAC. Further work therefore focusses on the variation in catalyst structure such that more active oxygen is supplied exclusively to MAC. At the same time, certainly the MAA conversion has to stay unchanged to reach an improvement in selectivity. At the moment attempts are made to achieve a selective hindrance of the oxidation of the acid. Such modification of the acid domain must not lead to a lowered BET-surface or a decreased activity for the aldehyde conversion. First results have shown that lowering of solely the acid conversion is possible by refined catalyst preparation [5]. REFERENCES 1 L.M. Deuger, J.C. Petzoldt, J.W. Gaube, H. Hibst, Ind. Eng. Chem. Res., 37 (1998) 3230. 2 B. Stein, C. Weimer, J. Gaube, Studies in Surface Science and Catalysis 110 (1997) 393. 3 L. Deuger, J.W. Gaube, F.-G. Martin, H. Hibst, Studies in Surface Science and Catalysis 101 (1996) 981. 4 A. Davydov, T.V. Andrushkevich, Catal. Rev.-Sci. Eng., 35, (1993), 213. 5 J.C. Petzoldt, Dissertation, TU Darmstadt, 1999.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

The effects of particle size on the performance of Er203 and 30 mol% BaCI2/Er203 catalysts in the selective oxidation of ethane to ethylene W. Zhong, a' b H. X. Dai, a C. F. N g 1 and C. T. Au " * ' Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong, P.R. China. E-mail: [email protected]. bNational Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P.R. China. A comparison of the nanoscale Er203 and BaCI2/Er203 catalysts prepared by sol-gel method with their large-size counterparts has been made for the oxidative dehydrogenation of ethane (ODE) to ethylene. The nanoscale catalysts showed better catalytic performance than the corresponding large-size ones. We detected that the extent of cubic Er203 lattice enlargement in nanoscale BaCI2/Er203 was larger than that in the large-size one and more defects were generated in the nanoscale catalyst. TPR results indicated that compared to the large-size catalysts, there were more reducible oxygen adspecies present on the nanoscale ones. ht situ Raman studies revealed that the reduction in particle size was beneficial for the activation of oxygen molecules. Based on the activity and Raman results, we suggest that dioxygen species are responsible for the selective oxidation of C2H6 to C2H4. 1. INTRODUCTION In contrast to conventional catalysts, nanometer catalysts have an appreciable fraction of atoms residing in defect environments. It is usually envisioned that by reducing the scale of a catalyst to nanometer range, one could induce unique and fascinating properties that could be related to the interfaces. Although nanoscale materials have been investigated as catalysts for numerous heterogeneous reactions, work done on the ODE reaction is scarce. Most of the ODE catalysts, as widely reported in the literature [1-3], were large-size traditional materials. In the present work, we have (i) prepared the nanoscale polycrystalline ErzO3 and BaClzmodified Er203 catalysts; (ii) studied the catalytic performance of nanoscale catalysts for the ODE reaction; (iii) investigated the role of BaCI2 in improving the catalytic properties of Er203; and (iv) made comparisons with their large-size counterparts. 2. EXPERIMENTAL Nanoscale Er203 was synthesized by a modified sol-gel method. Commercial Er203 (Aldrich, 99.99%) was dissolved in dilute HNO3 solution at 70~ suitable amounts of citric acid and ethylene glycol as coordinate agents were added and a complete homogeneous transparent solution was achieved. This solution was subject to slow evaporation at 70~ until a highly viscous residual was formed and a gel was developed during heating at 170~ The gel

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was thermally treated at 400~ for 5 h, and then ground. We designate this material as Sample A. After be!ng calcined in air at 700~ for 5 h, Sample A was adopted as the nanoscale Er203 catalyst. Nanoscale BaCl2-modified Er/O3 catalysts were prepared by impregnation method. Sample A was added to deionized water and the resulted slurry was kept at 70~ With constant stirring, a solution of BaCI2 was slowly added. After evaporation at 70~ the material was dried in air at ca 120~ and the resulting solid was ground and calcined in air, respectively, at 500 and 700~ for 5 h. The ODE reactions were carried out with 0.5 g of the catalyst secured in a fix-bed quartz flow micro-reactor (i.d. = 4 mm). Reaction mixtures exiting the reactor were analyzed using an on-line gas chromatograph (Shimadzu 8A) equipped with Porapak Q and 5 A molecular sieve columns. All studies were carried out at atmospheric pressure with a C2H6/O2/N2 molar ratio of 2/1/3.7. Phase identification and structural analysis of the catalyst were performed on an X-ray powder diffractometer (D/Max-RA, Rigaku). High-purity silicon powder was used as an internal standard for lattice parameter determination. The chlorine contents of fresh and used catalysts were determined gravimetrically using AgNO3. The morphology of nanoscale catalyst was examined by direct observation via transmission electron microscopy (TEM). The average crystalline size of the catalysts was estimated by means of X-ray diffraction line broadening analysis. The specific surface area was measured in a N2 physisorption flow apparatus (Nova 1200). The magnetization of the catalysts was measured at low temperature (-173~ by using a vibrating sample magnetometer (LDJ 9600) in a magnetic field of 5 kOe. ht situ laser Raman spectroscopy experiments were performed over a Nicolet 560 FT Raman spectrometer with 64 scans at 4 cm -1 resolution. The Raman spectra reported in this paper were recorded at room temperature. Temperature-programmed reduction (TPR) was carried out by using a thermal conductivity detector with the sample (0.2 g) being heated at a constant rate of 10~ min -1 to 820~ in a N2 + H2 (7% v/v) mixture. 3. RESULTS AND DISCUSSION

3.1 Catalytic performance In a blank experiment, quartz sand gave 4.9% C2H6 conversion and 95.0% C2H4 selectivity at 660~ It indicates that below 660~ homogeneous reaction of C2H6 with O2 is insignificant. One can observe from Figure 1 that C2H6 and 02 conversions increased significantly and C2H4 selectivity stayed at a high value when the particle size was reduced to nanometer scale. The effects of particle size on C2H6 conversion were more obvious, especially at lower temperatures. When Er203 was modified by 30 mol% BaCI2, C2H4 selectivity increased markedly. Compared to the large-size counterpart, throughout the temperature range, the nanoscale BaCIJEr203 catalyst showed higher C2H6 conversion but similar C2H4 selectivity. As shown in Table 1, the surface areas of the nanoscale catalysts were an order of magnitude higher than their large-size counterparts, respectively. The mass specific activities of the former catalysts were superior to those of the latter, whereas the area specific activities were the other way round. It is apparent that the enhanced surface areas of the nanoscale catalysts contributed to the better performances. Figure 2 demonstrates the effect of space velocity on the catalytic performance of largesize and nanoscale 30 mol% BaCI2/Er203 at 660~ The flow rate was kept constant and the

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amount of catalyst was varied. Over the nanoscale catalyst, the conversion of ethane was much higher than that of the large-size counterpart, especially at very large space velocities. 100

~

100

8o

80

6O

"~

o~

4o

"~ c

2O

~

0

C H

_

2

(a)

~

conversion 6

C2H 4 selectivity

60

4O 20 .

100 ao

,-

o~

.

.

.

.

.

~

|

.

,

.

,

.

BaCl2/ErzO ~

~

3

80

Ib

6O

0

0

60

40

40

20

20

0

4;o4;osc;os;o

600

Temperature

eso

6000 3 0 0 0 0

75000

120000

S p a c e v e l o c i t y ( m L h - l g -1) Fig.2. Effects of space velocity on the catalytic performance of (a) large-size and (b) nanoscale BaCI2/Er203 at 660~ Data were taken after ca 50 h on-stream reaction.

(oC)

Fig. 1. Catalytic performance of nanoscale and large-size catalysts. Square: C2H6 conversion; triangle:C2I-I4 selectivity; circle: 02 conversion (solid: nanoscale; hollow: large-size).

100 n.__.__na 100

~

9o

v

"o

8O

m

70

>'

60

"o c

50

1

V.

40

m

~

C

2

H

C

30

"

20

c:

10

0 0

2

H

~ C O

"v

0

4

9

0

i

,

10

R ea

|

,

20

ctio

,

3'0

'

40

n

tim

' ,

Y ie Id

4

2

n

sel.

sel.

.w,v.v ,

v,

~

.,,,,

,v ,

|

50

60

e

(h)

,

70

Fig.3. The catalytic performance of nanoscale Fig.4. Transmission electron micrograph BaC12/Er203 as a function of reaction time. of nanoscale BaCI2/Er203.

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The catalytic performance over nanoscale 30 mol% BaCI2/Er203 at 660~ as a function of reaction time is depicted in Fig.3. One can see that C2H6 conversion increased from 47.3 to 68.3 % over a period of 50 h before stabilizing. Meanwhile, C2H4 selectivity decreased from 78.0 to 73.0% and then slowly increased to 83.0%. A C2H4 yield in excess of 55.0% was obtained after 35 h. After 60 h of operation, the loss of chlorine was about 15% of the initial content, considerably smaller than that observed over Li+-MgO-CI -, where the loss of chlorine was 25% atter 50 h of use [4]. 3.2. Defective structure

One can see from Fig. 4 that nanoscale BaCI2/Er203 particles are ultrafine and uniform. The average particle sizes are estimated to be ca 40 and 50 nm, respectively, for Er203 and BaClz/Er203, larger than the average crystalline sizes calculated from the Scherrer formula (Table 1). This is because the observation from SEM or TEM micrograph reflects the size of the secondary particles whereas X-ray line broadening analysis determines the size of primary nanoparticles. Surface areas of all the catalysts measured before and after 30 h reaction are rather similar, indicating the non-occurrence of sintering. One can observe that the ao value of cubic Er203 lattice in nanoscale BaCIz/Er203 was larger than that in its large-size counterpart, indicating that there are more defects generated in the nanometer material. The formation of defects such as trapped electrons and O- centers is a result of ionic exchange between Ba z+ and Er 3+ and/or CI- and 02- [5]. The existence of such ionic exchanges is evidenced by the magnetic measurement results. BaCI2 is diamagnetic; if there is no interaction between BaCI2 and Er203, the magnetization ~5 value should reduce rather than increase. The rise in ~5 value for nanoscale BaCI2/Er203 clearly indicated the presence of strong interaction between the BaCI2 and Er203 phases. In other words, the extent of ionic exchanges in nanoscale BaCI2/Er203 is larger than that in large-size BaCI2/Er203. The absence of BaCi2 phase in nanoscale BaCI2/Er203 as indicated by XRD also confirmed this point. Table 1 Physical properties of catalysts

Sample

Average size (nm) TEM XRD > 5000 ~

Surface area (m 2 g-l) Fresh Used a 1.3 1.5

Magnetization cy5 (emu g-l) 4.48

Crystal phase

Large-size Er203 Er203 Nanoscale 40 15 20.5 19.8 5.63 Er203 Er203 Large-size > 5000 0.9 1.0 4.27 Er203 BaCl2/Er203 BaCI2 Nanoscale 50 18 12.1 11.8 6.04 Er203 BaCI2/Er203 a Sample used in ODE reaction for 30 h at 660~ b Lattice parameter of cubic Er203 phase in catalyst measured at room temperature.

b ao (nm) 1.0548 1.0526 1.0604 1.0618

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3.3. In situ Raman and TPR studies Figure 5' shows the Raman spectra of Er203 and BaCI2/Er203 treated in 02 at 800~ for 15 min. Only a broad and weak band centred at ca 1168 cm -l (02-) [6] was observed over the large-size Er203 sample (Fig.5a). For nanoscale Er203, at least three kinds of dioxygen species: 02 n- (l
,

,

~.

.

.

.

.

20"

Large-size BaCI2/Er203 4-1

"~ ~ ~

10

b'

1168

j b

Nanoscale Er203 979 1060 ,~~029:

1149

Large-size Er203 a

0

~

~

"

800

x2

:

-:-

1000

--

I

!

1200

I

Raman shift(cm "1)

.i ,,

1400

Fig.5. In s'itu Raman spectra of (a, a') E r 2 0 3 and (b, b') BaCIjEr203 treated in 02 at 800~ for 15 rain. a' and b' denote the nanoscale catalysts.

2oo

~

;

x2 ~

.......

ado -46o

sdo '6do '~do 'ado Temperature (~ Fig.6. TPR profiles of catalysts.

The TPR profiles of the four catalysts are shown in Fig.6. Compared to the large-size counterpart, the nanoscale Er203 catalyst displayed reduction bands of lower temperatures and larger amounts of reducible oxygen; the nanoscale BaCldEr203 sample showed a much bigger band centered at ca 702~ These results indicated that there are more reducible oxygen species present on the surfaces of nanoscale catalysts than those on the large-size ones. That is to say, the reduction in particle size is beneficial for the activation of oxygen molecules.

3.4. Active oxygen species Generally speaking, the chemisorption of oxygen molecules follows the sequence: Oz (g) r 02 (ads) r 02- (ads) ~ O22- (ads) r 20- (ads) r 202- (ads) r 202- (lattice) The nature and concentration of oxygen species are dependent on the surface property of the catalyst. Nanoparticles have high specific surface areas and exert influence on surface and bulk

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properties by introducing defects, strain fields, short-range correlated static or dynamic displacements of atoms from their ideal crystal lattice positions [8]. As shown in Table 1, compared to the large-size catalysts, the larger enlargement of cubic ErzO3 lattice and the greater increase of magnetization in nanoscale BaClz/Er203 demonstrate the existence of more lattice defects. These defects include trapped electrons and O- centers [5]. Oz molecules can pick up the trapped electrons to form Oz-, Ozz-, O- or Oz- adspecies according to the above sequence. The reactivity of various oxygen species over the Li/MgO system with light hydrocarbons has been reported to follow the order of O- >> Oz2- >> Oz- > O z- [9]. Oabstracted a hydrogen from CzH6 to form an ethyl radical, the ethyl radical combined with surface lattice O z- to give an ethoxide which then decomposed to generate C2I-I4 [9]. Since Ois highly active, an excessive amount of it inevitably leads to C2H6 and C2H4 deep oxidation. However, dioxygen species such as Oz-, O2~-, Oz"-, and O2z- showed moderate reactivity towards C2H6. As illustrated in the TPR studies (Fig.6), there were more reducible oxygen species present on the nanoscale catalysts than on the large-size ones. From the Raman results (Fig.5), one can realize that there were more dioxygen species on the nanoscale Er203 and BaCI2/Er203 catalysts than on their large-size counterparts and that the amounts of 02- species over BaCIJEr203 were significantly more than those over ErzO3. The results affirm that superoxide species favoured C2H6 selective oxidation because compared to the undoped Er203 catalysts, the BaClz-doped ones exhibited significantly higher C2H4 selectivities. The higher C2H6 conversion and CzI-h selectivity obtained over the nanoscale materials can be closely associated with the nature and concentration of oxygen adspecies; the activation of 02 is governed by the lattice defects induced in ionic substitution as well as in the reduction of particle size. We conclude that the Er203 and BaClffEr203 catalysts with nanometer dimensions possess properties different from their large-size counterparts in lattice defects, oxygen chemisorption, and catalytic performance. The superoxide species are likely to be responsible for the selective oxidation of C2H6 to C2H4. ACKNOWLEDGMENTS The work described above was fully supported by a grant from the Hong Kong Baptist University (FRG/97-98/II-04). HXD thanks the HKBU for a Ph.D. studentship. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.

S.J. Conway and J.H. Lunsford, J. Catal., 131 (1991) 513. C. Shi, M.P. Rosynek and J.H. Lunsford, J. Phys Chem., 98 (1994) 8371. C.T. Au, K.D. Chen, H.X. Dai, Y.W. Liu, J.Z. Luo and C.F. Ng, J. Catal., 179 (1998) 300. D.J. Wang, M.P. Rosynek and J.H. Lunsford, J. Catal., 151 (1995) 155. C.T. Au, X.P. Zhou and H.L. Wan, Catal. Lett., 40 (1996) 101. M. Kozuka and K. Nakamoto, J. Am. Chem. Soc., 103 (1981) 2162. A.B.P Lever, G.A. Ozin and H.B. Gray, Inorg. Chem., 19 (1980) 1823. R.W. Siegel, Annu. Rev. Mater. Sci., 21 (1991) 149. (a) K. Aika and J.H. Lunsford, J. Phys. Chem., 81 (1977) 1393; (b) K. Aika and J.H. Lunsford, J. Phys. Chem., 82 (1978) 1794; (c) Y. Takita and J.H. Lunsford, J. Phys. Chem., 83 (1979) 683; (d) M. Iwamoto and J.H. Lunsford, J. Phys. Chem., 84 (1980) 1710; (e) Y. Takita, M. Iwamoto and J.H. Lunsford, J. Phys. Chem., 84 (1980) 3079.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Perovskite-type chloro-oxide SrCoO3_~Clo: a novel and durable catalyst for the selective oxidation of ethane to ethene H.X. Dai, C.F. Ng, and C.T. Au* Department of Chemistry and Center for Surface Analysis and Research, Hong Kong Baptist University, Kowloon Tong, Hong Kong, P.R. China. E-mail: [email protected]. The catalytic performances and characterization of SrCoO3_0.401 and 8rCoO3_0.450C10.126 catalysts have been investigated for the oxidative dehydrogenation of ethane (ODE) to ethene. XRD results indicated that both catalysts were of cubic perovskite-type structures. The incorporation of chloride ions in the SrCoO3_~ lattice can significantly enhance C2H6 conversion and C2H4 selectivity. The catalyst showed 89.6% C2H6 conversion, 67.1% C2H4 selectivity, and 60.1% C2H4 yield under the reaction conditions of C2H6/O2/N2 = 2/1/3.7, temperature = 660~ and space velocity = 6000 mL h-~ g-1. Over SrCoO3_0.450C10.126, with the rise in space velocity, C2H6 conversion decreased, whereas C2I-I4 selectivity increased. Life studies showed that SrCoO3_0.450C10.126 was durable. The results of XPS and pulsing studies indicated that the inclusion of chloride ions in the SrCoO3_a lattice could promote lattice oxygen mobility. Based on the results of O2-TPD and TPR studies, we suggest that the oxygen species desorbed at ca 695~ were active for the selective oxidation of ethane. By regulating the oxygen vacancy density and Co4+/Co ratio in the perovskite-type chloro-oxide catalyst, one can generate a durable catalyst with excellent performance for the ODE reaction. I. INTRODUCTION In the past decades, many efforts have been made to develop good catalysts for the oxidative dehydrogenation of ethane to ethene. Among the numerous catalysts studied, Li+MgO-C1- seems to be highly effective (ca 58% C2H4 yield at 620~ [1]. Although KSr2Bi304C16 [2], a layered complex compound, gave a C2H4 yield of ca 70% at 640~ the catalyst deteriorated due to C1 leaching. In recent years, we have reported a number of alkaline earth halide-modified rare earth oxide catalysts for the OCM (oxidative coupling of methane) [3a] as well as ODE [3b, c] reactions but halogen leaching appeared to be a general problem. Perovskite materials (ABO3) are known to be thermally stable and active for the complete oxidation of hydrocarbons [4]. These behaviours are associated with defective structures such as oxygen vacancies and multivalent B-site cations. The increase in oxygen vacancy density is favourable for the deep oxidation of hydrocarbons whereas the rise in hypervalent B-site

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cations is beneficial to the enhancement of redox ability of perovskite catalysts [4]. In the ODE reaction, selective oxidation rather than deep oxidation is desired. If one could regulate the oxygen vacancy density and the oxidation states of B-site cations, one might achieve optimum catalytic performance. Compared to the size of O z- (radius, 1.40 A~), CI- (radius, 1.81 A) is larger. Incorporating the halide ions in the positions of oxygen vacancies can undermine the deep oxidation of hydrocarbons, in the meanwhile the oxidation states of the B-site cations are increased, thus improving the redox ability of the catalyst. Based on such an idea as well as for the sake of stabilizing the halide ions, we made an attempt to incorporate halide ions into the perovskite-type oxides. By so doing, we have generated a class of selective catalysts stable for the ODE reaction. 2. EXPERIMENTAL The catalysts were prepared by adopting the method of citric acid complexing [5]. The C1ions required were added in the form of SrCI2. The catalysts were calcined at 950~ for 12 h. The as-generated powders were in turn ground, pressed, crushed, and sieved to a size range of 40-80 mesh. Catalyst evaluation was carried out at atmospheric pressure in a fixed-bed quartz micro-reactor (i.d. = 4 mm) under the conditions of C2H6/O2/N2 = 2/1/3.7, temperature = 500680~ and space velocity = 6,000 mL h-1 g-1. The products (C2H6, C2H4, CO, CO2, and CH4) were determined on line by gas chromatography (Shimadzu 8A TCD) with Porapak Q and 5A Molecular Sieve columns. For the variation of space velocity, the reactant flow rate was varied at a fixed catalyst mass of 0.5 g. X-ray diffractometer (XRD, D-MAX, Rigaku) was used to identify the crystal phases. Xray photoelectron spectroscopy (XPS, Leybold Heraeus SKL-12) was used to determine the Ols binding energy of surface oxygen species. Before XPS measurements, the samples were treated in He (20 mL min-1) at 600~ for 1 h and then cooled in He to room temperature. The samples were then outgassed in a vacuum of 10-9 torr for 0.5 h. The Cls line at 284.6 eV was taken as a reference for binding energy calibration. The specific surface areas of the catalysts were measured using a NOVA 1200 apparatus. We performed pulse experiments to investigate the reactivity of surface oxygen species. A catalyst sample (0.2 g) was thermally treated in a micro-reactor at a desired temperature for 0.5 h before the pulsing of C2H6 or C2H6/O2 (2/1 molar ratio), then the effluent was analyzed on line by a mass spectrometer (HP G1800A). The pulse size was 65.7 ~tL (at 25~ 1 atm) and He (purity > 99.995%) was the carrier gas. The O2-TPD (temperature-programmed desorption) investigation was conducted on a mass spectrometer (HP G1800A) and the TPR (temperature-programmed reduction) experiments were performed by using a thermal conductivity detector with a mixture of 7% H2-93% Nz (v/v) [3b,c].

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The chloride content was analyzed by titrating the digested (in 2 M HNO3) catalyst against standardized 0.1 M silver nitrate solution with 0.005 M potassium chromate as indicator. The Co 3. content was determined by employing the method of iodometry [6]. 3. RESULTS AND DISCUSSION

Table 1 shows the crystal phases, compositions, and surface areas of SrCoO3_8 and SrCoO3_sCl~. Based on the CI content and Co4+/C0 ratios as well as the assumption of electroneutrality, the value of 8 was estimated to be 0.401 for SrCoO3_s, whereas 8 and a values were 0.450 and 0.126 for SrCoO3_sCI,,, respectively. By comparing the XRD results with the JCPDS data (No. 38-1148), we deduced that the flesh and used (40 h of on-stream ODE reaction) SrCoO3_0.4ol and SrCoO3_0.450C10.126 catalysts adopted a cubic perovskite structure. The single-phase perovskite structure of the Cl-doped material confirmed that the CI- ions were incorporated into the SrCoO3_~ lattice. From Table 1, one can realize that the inclusion of chloride ions in the SRC003_0.401 lattice has caused the C04+/C0 ratio to rise and the oxygen vacancy density to decrease. According to the BET method, the surface areas of SrCoO3_0.401 and SrCoO3_0.450C10.126were estimated to be 1.32 and 1.29 m 2 g-~, respectively. The addition of C1- ions to SrCoO3_0.401 did not induce any significant change in surface area. Table 1 The crystal phases, compositions, and surface areas of SrCoO3_~ and SrCoO3_sClo catalysts. Catalyst

Crystal phase

CI Content (wt%)

SrCoO3_~ Cubic SrCoO3_~CI,~ Cubic 2.33 (2.32) a Note: a After 40 h of on-stream ODE reaction.

Co4+/Co (mol%) 19.8 22.7

~

(3"

0.401 0.450

-0.126

Surf. area (m 2 g-~) 1.32 1.29

In a blank experiment, 5.0 g of quartz sand gave a C2H6 conversion of 7.6%, a C2H4 selectivity of 89.2%, and a C2H4 yield of 6.8% at 680~ It indicates that quartz sand is poor in catalytic activity. Figure 1 shows the catalytic performances of 5rCoO3_0.401 and SrCoO3_ 0.450C10.126 at temperatures ranging from 500 to 680~ With the increase of reaction temperature, C2H6 conversion and CI-h selectivity increased whereas C2H4 selectivity and C2H4 yield first increased and then decreased over 5rC003_0.401. The 02 conversion within the whole temperature range was 100%. The highest C2H4 selectivity (60.5%) was observed at 620~ with a C2H6 conversion of 53.5%, the C2H4 yield was 32.4%. Above 640~ C2H6 conversion increased. However, due to the rise in COx (CO + CO2) and CH4 selectivities, C2H4 yield decreased. Over the SrCoOa_0.450C10.126catalyst, with the increase of temperature from 500 to 680~ C2H6 conversion, CH4 selectivity, and O2 conversion increased. At 600~ C2H4 selectivity reached a maximum value of 80.4% and the corresponding C2H6 conversion and

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COx selectivity were 37.7% and 17.6%, respectively. The highest C2H4 yield (60.1%) was observed at 660~ where the C2H6 conversion and C2H4 selectivity were 89.6% and 67.1%, respectively; the 02 conversion was 88.1%. Based on these results, one can conclude that the SrCoO3_o.45oC1o.126 catalyst is much superior to the SrCoO3_0.4ol catalyst in catalyzing the ODE reaction. With the rise in space velocity from 4,000 to 10,000 mL h-I g-l, C2H6 conversion decreased whereas C2I-I4 selectivity increased, giving a maximum C2H4 yield of 60.1% at 6,000 mL h-~ g-l. Similar results were obtained when this catalyst was well dispersed in quartz sand. This indicates that the problem of hot spots was not significant. The 8rCoO3_o.45oC1o.126catalyst showed stable performance within a period of 40 h. As shown in Table 1, the CI contents in the fresh and used Cl-doped catalyst were rather similar, confirming that the 8rCoO3_o.45oC1o.126 catalyst is durable. In the oxidation of C2H4 under similar reaction conditions at 660~ the C2H4 conversion was 16.4% over the Cl-doped catalyst whereas over 8rC003_0.401, it was 40.2%. In other words, the introduction of CI- ions to SrCoO3_s resulted in the reduction of C2H4 deep oxidation. 100 80" '~

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Figure 2 shows the O2-TPD and TPR profiles of 5rCoO3_o.4oi and SrCoO3_o.45oC1o.126. There were three desorption peaks in the SrCoO3_0.401 profile. According to the nature of desorbed oxygen, the desorption peaks at ca 420 and 510~ could be assigned to ~ oxygen, whereas the third one at ca 801~ to 13 oxygen. In the 8rCoO3.o.45oC10.126 profile, however,

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there were only two desorption peaks at ca 440 and 695~ the former could be assigned to ct and the latter to 13 oxygen. Compared to those of SrCoO3_0.401, the ot oxygen of SrCoO3_ 0.450C10.126was lower whereas the 13 oxygen was higher in intensity; furthermore, the desorption temperature of the 13 oxygen was obviously lower. In the TPR profiles, there were two reduction bands at ca 482 and 800~ for SrCoO3_0.401, and at ca 436 and 682~ for SrCoO3_ 0.450C10.126. The two reduction bands are rather similar to the O/-TPD peaks in intensity and temperature. Based on the results of O2-TPD and TPR studies, we propose that with the addition of C1- ions to SrCoO3_~, the content of ot oxygen would decrease whereas that of 13 oxygen increase. It is well known that ot and 13 oxygen desorptions in O2-TPD profile are characteristics of most perovskites, ot oxygen is accommodated in oxygen vacancies and is responsible for the complete oxidation of hydrocarbons; the desorption of 13 oxygen (i.e. lattice oxygen) is attributed to the partial reduction of B-site cation by lattice oxygen and is responsible for the selective oxidation of hydrocarbons [4]. The inclusion of CI- ions in the SrCoO3_~ lattice would give rise to two effects: (i) the decrease in the amount of oxygen vacancy, i.e. the decrease of ot oxygen; (ii) the rise in Co4+/Co ratio, i.e. the increase of 13 oxygen (ascribable to the partial reduction of cobalt from 4+ to 3+ or even 2+) desorption. In SrCoO3_0.401, the amount of nonstoichiometric oxygen is 0.401. In SrCoO3_0.450C10.126, the CI- ions could enter into oxygen vacancies and/or positions originally occupied by 0 2- ions. If an 0 2- ion is replaced by a C1ion, in order to maintain electroneutrality, one CI- ion has to enter into an adjacent oxygen vacancy or the oxidation state of a cobalt cation has to decrease. As shown in Table 1, the introduction of CI- ions into SrCoO3_~ led to the increase rather than the decrease of Co4+/Co ratio. This indicates that the C1- ions have occupied a certain amount of oxygen vacancies. Therefore, we suggest that the incorporation of CI- ions into the SrCoO3_~ lattice has caused the bulk oxygen vacancy density to decrease, and so the complete oxidation reactions were reduced as a result.

Table 2 The catalytic performances of SrCoO3_0.401 and SrCoO3_0.450C10.126 in a C2H6 or C2H6/O2 pulse after thermally treated in He at various temperatures for 0.5 h. Catalyst

500~ a

600~ ~

690oc

C2H6 C2H4 C2H6 C2H4 C2H6 C2I--I4 Cony. (%) Sel. (%) Conv. (%) Sel. (%) Conv. (%) Sel. (%) SrCo03_0.401 18.9 b 23.6 33.1 54.4 57.4 56.7 (20.1) c (19.2) (37.2) (52.1) (76.1) (33.6) SrCoO3_0.450C10.126 1.9 64.5 32.3 82.6 92.2 86.7 (2.2) (62.8) (36.2) (81.3) (90.8) (60.8) Note: a Temperature for thermal treatment and reactant pulsing; b in a pulse of C2H6; c the values in brackets were obtained in a pulse of C2H6/O2 (molar ratio - 2/1).

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With the rise of temperature, C2H6 conversion and C2H4 selectivity increased in C2I-I6pulsing, whereas in C2H6/O2-pulsing, C2H6 conversion increased while C2H4 selectivity reached a maximum value at 600~ over the two catalysts (Table 2). According to the Ols XPS spectra, there is only one 01s peak at 529.9 eV (lattice oxygen) binding energy for a SrCoO3_ 0.450C10.126 sample treated in He at 600~ As for a 600~ SrCoO3-0.401 sample, there are two 01s peaks at 529.2 eV (lattice oxygen) and 531.3 eV (adsorbed oxygen). The incorporation of C1- ions in SrCoO3_~ caused the 01s binding energy of lattice oxygen to increase from 529.2 to 529.9 eV. Since CI- ions are slightly larger than 02- ions in size, accommodating CI- ions in SrCoO3_~ lattice would cause the crystal lattice to enlarge. Based on the XRD results, the lattice parameter, a0, was estimated to be 3.842 A for SrCoO3_0.401, and 3.888 A for 8rCoO3_0.450C10.126.In other words, the introduction of CI- ions would weaken the coulombic force between the cations and 02-. As a result, the lattice 02- became more mobile. The increase in C2H4 selectivity over the Cl-doped catalyst (Fig. 1 and Table 2) and the lowering of 13 oxygen desorption temperature (Fig.2) are supporting evidences for this point. With the results of pulsing, XPS, O2-TPD, and TPR studies, it demonstrated that the ct oxygen fomented total oxidation whereas the 13oxygen favoured selective oxidation of C2H6 to C2H4. Based on the above results and discussion, we conclude that (i) the inclusion of halide ions in the SrCoO3_~ lattice could promote lattice oxygen mobility; (ii) the 13 oxygen species that desorbed within the 680-710~ range are active for the selective oxidation of C2H6; (iii) the excellent and sustainable catalytic behaviour of the Cl-doped perovskite could be related to the decrease in oxygen vacancy density and the rise in C04+/Co ratio. ACKNOWLEDGMENTS The work described above was fully supported by a grant from the Research Grants Council of the Hong Kong Administration Region, China (Project No. HKBU 2050/97P). HXD thanks the HKBU for a Ph.D. studentship. REFERENCES 1. D. Wang, M.P. Rosynek and J.H. Lunsford, J. Catal., 151 (1995) 155. 2. W. Ueda, S.W. Lin and I. Tohmoto, Catal. Lett., 44 (1996) 241. 3. (a) C.T. Au, Y.W. Liu and C.F. Ng, J. Catal., 176 (1998) 365; (b) C.T. Au, K.D. Chen, H.X. Dai, Y.W. Liu and C.F. Ng, Appl. Catal. A, 177 (1999) 185; (c) C.T. Au, K.D. Chen, H.X. Dai, Y.W. Liu, J.Z. Luo and C.F. Ng, J. Catal., 179 (1998) 300. 4. T. Seiyama, Properties and Applications of Perovskite-Type Oxides (L.G. Tejuca, and J.L.G. Fierro, eds.), Marcel Dekker, New York, 1993, p.215. 5. M.L. Rojas, J.L.G. Fierro, L.G. Tejuca and A.T. Bell, J. Catal., 124 (1990) 41. 6. Y. Wu, T. Yu, B.S. Dou, C.X. Wang, X.F. Xie, Z.L. Yu, S.R. Fan, Z.R. Fan and L.C. Wang, J. Catal., 120 (1989) 88.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Preparation and activity of Copper, Nickel and Copper-Nickel-A! Mixed Oxides Via Hydrotalcite-like Precursors for the Oxidation of Phenol Aqueous Solutions A. Alejandre*, F. Medina* 1,X. Rodriguez*, P. Salagre**, and J.E. Sueiras *

* Departament d'Enginyeria Quimica, ETSEQ. **and Facultat de Quimica, Universitat Rovira i Virgili, Pl. Imperial Tarraco 1, 43005 Tarragona, Spain; Various solids with a hydrotalcite-like structure and containing various amounts of Cu2+/Ni2+/AL 3+ cations were prepared. Several catalysts were obtained by calcining these hydrotalcite-type precursors at different temperatures between 373 K and 973 K We performed thermogravimetric analysis, X-ray diffraction, BET areas and FT-IR spectroscopy to characterize the hydrotalcite-like compounds synthesized and the chemical changes which take place during their calcination process. We also tested their catalytic behaviour for the oxidation of phenol aqueous solutions using both a continuous triphasic tubular reactor and a semi-batch reactor for comparison, and air with an oxygen partial pressure of 0.9 MPa at a reaction temperature of 413 K. The hydrotalcite samples calcined at 373 K were practically inactive. Samples calcined at temperatures around 623 K showed the highest initial activities (>95% of phenol conversion for copper catalysts and >80% for nickel ones). However, nickel catalysts showed lower amounts of intermediate oxidation products such as quinones and carboxylic acids. Phenol conversion decreased continously over time for these samples mainly because of the continuous loss of the CuO and NiO phases by elution during the reaction. On the other hand, the spinel phases (obtained for the samples calcined at 973K) showed higher conversions (between 40-75%) and were stable at these reaction conditions. They did not show any loss in activity after a continuous working run of 15 days using a trickle-bed reactor. However, when the experiments were performed in an autoclave a loss of activity was observed for the copper spinel phase (probably due to polymer formation) but not for the CuNi-spinel catalyst.

1. Introduction Hydrotalcite-like materials (HT) belonging to the class of anionic clays have a brucite-like Mg(OH)2 network in which Mg 2+ is isomorphously substituted by a trivalent element M 3+. The structure of the hydrotalcite is very similar to that of brucite. In brucite, each magnesium cation is octahedrally surrounded by hydroxyls [1,2]. The resulting octahedron shares edges to form infinite sheets with no net charge. When Mg 2+ ions are replaced by a trivalent ion (the radius of which, like A13+, is not too different), a positive charge is generated in the brucite sheet. The positively charged Mg-A1 double hydroxide sheets (or layers) are charge-balanced by the carbonate anions residing in the interlayer sections of the clay structure. In the free space of this interlayer, the crystallization water also finds a place [ 1-3]. Anionic clays based on hydrotalcite-like compounds (HT) can be used as catalysts, catalyst supports, ion exchangers, stabilisers, adsorbents etc., mainly because of their variable chemical

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compositions. Recently, they have been among the most widely investigated catalyst precursors [1-3]. Furthermore, all the divalent metals from Mg 2+ to Mn 2+ form hydrotalcitelike compounds, except Cu 2+, which forms HT only when other bivalent cations such as Zn, Cr, Co, Mg, Mn, etc. are present [ 1]. Reichle [4] reports the formation of CuAICO3- HT when the gel obtained by using aqueous bicarbonate solutions as a precipitant is crystallized at relatively high temperatures. However, CuA1-HT are always mixed with other phases, such as malachite or gerhardtite, due to the Jahn-Teller effect of the Cu 2+ ion [ 1]. Supported metal oxide catalysts are also widely used in oxidation processes. The oxidation of dilute aqueous solutions for organic pollutants using air or oxygen over a solid catalyst is a useful and inexpensive process in which the organic compounds are oxidised to carbon dioxide and water. Batch and semi-batch approaches have previously been studied, and recently continuous processes in a trickle-bed reactor have been reported [5-7]. However, the copper and nickel catalysts that have been used up to now for long run-times (e.g. more than 10 days) undergo a considerable loss in activity due to the strong oxidation conditions of the processes, and the solubilisation of the active phases and/or the formation of polymers [5]. Mixed oxides crystallizing in the spinel form are potential catalysts for the oxidation of phenol in aqueous solutions because they are stable in these reaction conditions and their activities can be related to the BET area of the spinel form [5]. This paper studies the catalytic behaviour of several copper-nickel-aluminium mixed-oxide samples with high BET areas, using hydrotalcite-like phases as precursors for the oxidation of phenol aqueous solutions in a trickle-bed and in a semi-batch reactor for comparison. We also study the nature and characteristics of these copper, nickel and copper-nickel- hydrotalcite-like compounds and the chemical changes which take place before the spinel phases are formed.

2. Experimental section 2.1 Sample preparation Seven samples of copper-nickel-aluminium hydrotalcite-like compounds were synthesized with different Cu/Ni/A1 ratios (see Table 1). They were obtained by coprecipitation from two aqueous solutions at a constant pH between 8 and 8.5 __ 0.2, one of which contained appropriate amounts of Cu(NO3)2 x 6H20, Ni(NO3)2 x 6H20 and Al(NO3)3 x 9H20 while the other contained an aqueous solution with triethylamine (1M). The addition was completed in 3h under vigorous stirring. During the coprecipitation process, a constant flow of CO2 was bubbled through the glass reaction vessel. This led to carbonate anions forming in the interlayer region of the solids. The precipitated gel was filtered, washed several times with warm distilled water and then dried in a vacuum at room temperature for 48 h. The calcination process of the samples was performed with a heating rate of 1K/min in air and the final calcination temperature was maintained for 16 h. The copper-nickel-aluminium samples obtained in this way are written as HT[n:I:m](K) where [n:l:m] are the Cu~i/A1 atomic ratios and K the calcination temperature in degrees K. The Cu/A1 contents in the coprecipitates were determined using a JEOL 2000FXII equipped with a LINK probe for EDS analysis and atomic absorption spectroscopy (Hitachi Z-8200). The results obtained from these techniques are very similar and give the following Cu/Ni/A1 atomic ratios: 0.9/0/2 for HT[1:0:2], 5.8/0/2 for HT[6:0:2], 0/0.97/2 for HT[0:1:2], 0/5.9/2 for HT[0:6:2], 0.24/0.77/2 for HT[0.25:0.75:2], 0.49/0.5/2 for HT[0.5:0.5/2] and 0.71/0.24/2 for HT[0.75:0.25:2].These values were similar to the expected values.

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2.2 Sample characterization BET surface areas were calculated from the nitrogen adsorption isotherms at 77 K with a Micromeritics ASAP 2000 surface analyser. Powder X-ray diffraction (XRD) patterns of the catalysts were obtained with a Siemens D5000 diffractometer. The structural evolution during thermal treatment in air was monitored in situ with a high-temperature chamber (HTK Anton Paar) attached to the XRD. The FT-IR spectra were recorded on a Nicolet 5ZDX Spectrometer in the 4000-400 cm -1 wave number range using pressed KBr pellets. Thermogravimetric analyses (TG) were carried out on a Perkin-Elmer TGA 7 microbalance with an accuracy of 1 mg and equipped with a 272-1273 K programmable temperature furnace. The nature of the gases evolved during the thermal decomposition process was monitored with a QTMD FISONS mass spectrometer. 2.3 Catalytic measurements The catalytic behaviour of the samples for the oxidation of an aqueous solution of 5g/L in phenol using air as oxidant was tested using a trickle-bed and a semi-batch reactor for comparison. The reaction conditions were the following: the temperature was 413 K, the air flow-rate 8.3 NL/h and the partial oxygen pressure was maintained at 0.9 MPa. For the continuous reactor, the liquid flow-rate was 35.4 ml/h working at 0.41 h of inverse WHSV (weight hourly space velocity), the particle diameter of the catalysts was 25-50 mesh and 14 g of the catalyst was loaded into the reactor; for the semi-batch reactor (a 1000 ml autoclave), 125 ml of the aqueous solution of phenol and 2 g of catalyst were loaded into the autoclave. Phenol conversion and product distribution were analysed by HPLC (Beckman System Gold) and TOC (Shimadzu 5050). 3. Results and discussion Table 1 shows the results of the BET areas and XRD phases detected for the catalysts. The composition of the samples and the calcination temperatures have a considerable influence on the surface areas. When both the copper or nickel concentration of the sample and the calcination temperature increase, the BET area values decrease. Samples HT[6:0:2] and HT[0:6:2], which have the highest amounts of copper or nickel respectively, have the lowest BET area values. All the samples show a maximum BET area at temperatures around 623 K (which corresponds to the disappearance of the HT-XRD pattern). When the calcination temperature increases to 973 K, the BET area decreases. The BET area values detected for the samples which have a divalent/trivalent ratio of 0.5 were higher because of the formation of more amorphous phases such as hydroxides-carbonates and alumina hydrate (gibbsite) instead of the hydrotalcite phase during the coprecipitation process. The pure and mainly pure hydrotalcite-like phase is only detected for the HT[0:6:2] and HT[6:0:2] samples, respectively, at 373 K. This HT phase is also detected for the other samples at 373 K but is poorly crystallized and is always accompanied with other side phases such as malachite and gibbsite (probably also amorphous hydrotalcite-like phases). The yields and crystallinity of the hydrotalcite-like phase are therefore higher when the copper or nickel content of the sample increases.This is because a divalent/trivalent molar ratio equal to or larger than 2 is needed to prepare crystallographically pure hydrotalcites [6]. However the HT[6:0:2] sample also shows traces of malachite phase. This can be attributed to the lower stability of the copper hydrotalcite phase due to the Jahn Teller effect in the Cu 2+ ion which improves the formation of the more stable malachite phase. A pure hydrotalcite-like phase is only

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obtained for the sample HT[0:6:2]. The samples HT[6:0:2] and HT[0:6:2] calcined at 623 K yield incipient CuO and NiO phases, respectively, with crystallite sizes lower than 9 nm. The calcination of the other samples at this temperature (except for HT[1:0:2]) yields incipient NiO and spinel phases with particle sizes lower than 4 nm. Practically pure spinel phases are detected, at calcination temperatures around 973 K, for these samples (traces of NiO phase are also detected) with particle sizes lower than 7 nm. The copper spinel phase is detected for the sample HT[I:0:2] at temperatures around 973 K. On the other hand, nickel and/or coppernickel spinel begins to form at lower calcination temperatures (623 K). Table 1 Some Characteristics of the Cu-Ni-Based Catalysts BET HT samples Calcination XRD Area (m2/g) Cu/Ni/Al Temperatures (K) Phases (b) [ 1:0:2] [6:0:2] [0:1:2] [0:6:2] [0.25:0.75:2] [0.5:0.5:2] [0.75:0.25:2]

373 623 973 373 623 973 373 623 973 373 623 973 373 623 973 373 623 973 373 623 973

HT,Ma,Gi CuSp HT,Ma T CuSp, T HT, Gi Bu, NiSp NiSp, Bu HT Bu Bu, NiSp HT, Gi Bu, CuNiSp CuNiSp HT, Gi T, CuNiSp CuNiSp HT, Gi CuNiSp CuNiSp

180 220 145 45 55 50 90 190 80 19 115 85 40 167 102 42 155 87 37 163 98

XRD Phases (d)

T,M, B CuSp T,M,B,HT T,M,CuSp Bu,NiSp,B Bu,NiSp Bu,HT,B Bu,NiSp Bu,T,CuNiSp,B CuNiSP Bu,T,CuNiSp, CuNiSp T,CuNiSp,B CuNiSp

XRD Phases (a) B CuSp B CuSp NiSp,B NiSp HT,B NiSp CuNiSp,B CuNiSp CuNiSp,B CuNiSp CuNiSp,B CuNiSp

HT = Hydrotalcite-like phase, Ma=Malachite, Gi=Gibbsite, CuSp=CuA1204, NiSp = N i A I 2 0 4 , CuNiSp = Copper-Nickel Spinel phase, B=bohemite, Bu = Bunsenite (NiO), T = Yenorite (CuO), (b) (d) and (a) = Before, during and after reaction. The FT-IR and TG equipped with a mass detector show carbonate (mainly) and nitrate (traces) as counteranions interacting in the interlayer region. Loosely bound carbonate and nitrate anions and one strongly bound type of carbonate (mainly for copper samples) were found. The hydrotalcite-like samples calcined at 373 K show practically no conversion for the catalytic oxidation of phenol in aqueous solutions under the conditions given in the experimental section. Figure 1(A,B) shows the conversion during a 30-day run for the samples calcined at 623 K.

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o

+ o

9.$ ..e - 9 9 o % "o*o

x

60

'% '% '% '% '% x+ ,% x+ ~

9

+

9

o

[]

40

30 15 0

9

40

O

e

o

@

[]B

"i 40

0

9

o

60

+e+ 9

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@

I

0

5

,

I

10 Time (days)

x

20

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o

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"o

m

==

15

0

.

I

1

.

I

2

.

I

3

.

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.

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Time (hours)

Figure 1 (A,B,C,D) Evolution of the phenol conversion for several samples calcined at 623 K (A,B) and at 973 K (C,D) using trickle bed (A,B,C) and semi-batch (D) reactors. Symbols for Figure 1 A,B,C : ~' HT[6:0:2], $ HT[I:0:2], o HT[0,75:0,25:2], 9 HT[0,5:0,5:2], o HT[0,25:0,75:2], x HT[0:6:2], + HT[0:I:2] . Symbols for Figure 1 D : + HT[I:0:2] (run 1), 9 HT[I:0:2] (run 2), o HT[I:0:2] (run 3), [] HT[0,5:0,5:2] (run 1), 9 HT[0,5:0,5:2] (run 2), x HT[0,5:0,5:2] (run 3).

As can be seen, all the catalysts show a more or less constant decrease in phenol conversion. Therefore, the reaction products for all the catalysts are CO2 (>70%), diphenols, quinones and organic acids (oxalic, formic, acetic, succinic, etc.). However, it should be pointed out, that pure nickel catalysts (HT[0:6:2] and HT[0:1:2]) show lower amounts of intermediate products than pure copper catalysts (HT[6:0:2] and HT[I:0:2]). These nickel catalysts mainly convert phenol into CO2 (>90% instead of >70% for copper ones) but their initial conversions are always lower (see Fig. 1 A). There are several reasons for the activity loss observed for these nickel and copper catalysts (see Fig.1 A,B). For example the solubilisation of the active phase (i.e. mainly CuO and/or NiO), formation of new crystalline phases such as copper oxalate and boehmite, and the regeneration of the hydrotalcite phase (mainly for the HT[0:6:2] sample)

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because of the reaction conditions (water, higher temperature reactions and formation of CO2) (see Table 1). It should also be pointed out, that the spinel phases (see Fig. 1 C) (obtained for the samples calcined at higher temperatures and subjected to HC1 treatment until no elution of copper or nickel oxide was detected) showed higher conversions (>40%) and were stable in these reaction conditions. They showed no loss in activity after a continuous working run of 15 days using the trickle-bed reactor. The pure copper spinel phase is more active than the pure nickel spinel phase (65% and 40% conversion respectively). However, the activity of the copper-nickel spinel phase of the sample HT[0.5:0.5:2](973K) is the highest (around 75% conversion). It seems that there is a synergic effect between copper and nickel. Copper shows higher conversion and nickel shows lower intermediate oxidation products. It seems that nickel is less efficient at cleaving the phenol ring (which is the limiting step of the oxidation process of phenol), but surprisingly is more active at transforming the quinones and carboxylic acids to CO2 than the copper catalysts. The conjunction of these two effects may explain the higher activity and selectivity towards CO2 (>90%) detected for the HT[0.5:0.5:2](973K) catalyst. It should also be pointed out that polymerisation products do not form when this trickle-bed reactor is used. However, when the experiments were performed in the autoclave there was a loss of activity for the copper spinel phase for runs 1-3 (due to polymer formation) but not for the Cu-Ni-spinel phase (see Fig.lD). The polymers are formed by two reactions in the liquid phase: the addition of glyoxal to phenol or the polymerisation of glyoxal [5]. So, the Cu-Nispinel phase also prevents polymers from forming during phenol oxidation, even in an autoclave reactor, because the conversion to intermediate products is lower. This means that the catalyst potentially has a longer life. REFERENCES

1. F. Cavani, F. Trifiro, and A. Vaccari, Catal. Today, 11 (1991) 173. 2. F.Trifiro and A Vaccari, Comprehensive Supram.Chem.,7 (1996) 25. 3. M. J. Climent, A. Corma, S. Iborra and J. Prima, J. Catal., 151 (1995) 60. 4. W.T. Reichle, J. Catal. 94 (1985) 547. 5. A. Alejandre, F. Medina, P. Salagre, A. Fabregat, and J. E. Sueiras, Appl. Catal. B: Environ. 18 (1998) 307-315. 6. A. Pintar, and J. Levec, J. Catal. 135 (1992) 345. 7. J.Beziat, M. Besson, P. Gallezot and S. Durecu, Ind. Eng. Chem. Res. 38 (1999) 1310.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Influence of temperature and catalyst loading on the aqueous-phase catalytic oxidation of phenol A. Santos, P. Yustos, B. Durbfin and F. Garcia-Ochoa Departamento Ingenieria Quimica. Facultad. CC. Quimicas. Universidad Complutense. 28040- Madrid. Spain. ABSTRACT The oxidation of phenol over a copper oxide-copper chromite catalyst has been studied in a basket stirred tank reactor in the temperature range 127-180~ with a catalyst loading range of 0-120 g/L and using an initial pH of 3.5 and 10. The phenol conversion and its mineralization to carbon dioxide have been measured. A higher oxidation rate of phenol was found when the initial pH was acid. At the lower temperatures studied (127 and 140~ an increase of both the phenol oxidation rate and the CO2/phenol conversion ratio was observed when the catalyst loading was increased. At the highest temperatures used (160 and 180~ the homogeneous mechanism is the main contribution to the phenol oxidation rate. 1. INTRODUCTION The problem of the pollution reduction of the aqueous streams containing phenolic compounds is of great importance in wastewaters from many industries (agroalimentary processes, pharmaceutical, fine chemical, petrochemical, etc). These effluents can not be treated through conventional processes of biological oxidation, because of their little biodegradability. The process currently used is the wet oxidation or supercritical oxidation, that requires high pressures and temperatures, and consequently high cost (1). An interesting alternative is the heterogeneous catalytic oxidation, because the catalyst permits a significant reduction of the temperature and pressure improving the economy of the process (2-4). Studies carried out about this topic show that the reaction involves a free radical mechanism, with an induction period before the steady-state regime is achieved, considering both homogeneous and heterogeneous reaction contributions. The influence of variables such as pH, catalyst loading and temperature on the induction period and steady-state rate are not yet clarified and results obtained from different authors often disagree (5-7). Most of the studies have been carried out in slurry reactors, with a maximum catalyst loading of 4-5 g/L. The few papers where the reaction is carried out in fixed bed (usually as trickle bed reactors) did not study the effect of different catalyst weights in the reactor. Hence the catalyst loading influence on the phenol conversion and its mineralization to carbon dioxide requires further studies. Furthermore, most of the papers do not consider the conversion to CO2 but only the conversion of phenol. However the phenol oxidation does not produce directly carbon dioxide but many intermediates have been detected in the oxidation route, being the main

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oxidation intermediates the dihidroxyphenols (cathecol and hydroquinone), the benzoquinones, the C-4 and C-2 dicarboxilic acids, the acetic acid, etc. The best catalyst should be as less selective as possible and promote the oxidation of these intermediates to carbon dioxide.

2. EXPERIMENTAL 2.1. Catalyst A commercial catalyst from Engelhard (Cu-0203) with copper as active component has been employed. The catalyst properties are summarized in Table 1. Leaches of metallic ions Cu and Cr have been measured after the catalyst has been in contact with an acid aqueous phenolic solution at 140~ and 16 atm of pressure during 10 hours. An almost negligible amount of those ions in solution was found, as can be seen in Table 1. Table 1. Properties of the Catalyst Engelhard Cu-0203

Chemical Composition ~

Compounds Copper oxide 67-77 Copper chromite 20-30 Synthetic graphite 1-3

~

Metals 60% Cu 10% Cr

Physical Properties

Metal leaches after 10 h (mg ion/g cat) T=140~ P=I 6 atm.

Sg m2/g

V cm3P/g

dp mm

Cu

Cr

10

0.10

3.175 (1/8")

0.241 0.0393 % Cuo

9.3.10 -4 9.10 -4 % Cro

2.2. Experimental Set-Up and Procedure A basket stirred tank reactor (BSTR) from Autoclave Engineers (500 ml Spectrum Reactor) was employed to carry out the experimental runs. A volume of 250 ml of the aqueous solution was introduced in the reactor after setting the pH to an initial value (pHo). The catalyst was boiled twice in distilled water before its use and placed in the basket of the reactor, and the rotating speed of the stirrer was fixed at 700 rpm. The reactor was pressurized at 16 atm. Steady state conditions for temperature and concentration of dissolved oxygen were achieved by means of a continuous flow of oxygen (0.2 L lnin -R at NTP conditions). Then, the volume of a loop containing a concentrated solution of phenol was injected being the initial concentration of phenol in the reactor liquid phase of 1200 ppm. Experimental runs were carried out at different temperatures (127 to 180~ catalyst loadings (0 to 120 g/L) and initial pH (3.5 and 10). Gas and liquid samples were taken periodically from the reactor and analyzed. Residual phenol concentration and some intermediates in the samples from the reactor were determined by HPLC with a Diode Array detector (HP G1315A). Other reaction intermediates were identified and quantified by means of a GC/MS technique (Hewlett Packard model 6890). Main intermediates identified at acid pH were 1,2 benzenediol, 1,4 benzenediol and 2,5-cyclohexadiene-l-4dione. Also maleic and oxalic acids were detected. Total organic carbon in the liquid phase values (TOC) have been

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obtained in a SGE analyzer. Copper and chromium leaches have been determined by means of a fotometer (Aqualytic AL282). The pH values of the liquid samples were also measured with time.

3. RESULTS and DISCUSSION 3.1. Influence of the pH on phenol and TOC conversion

The results obtained at 140~ and using a catalyst loading of 0 and 60 g/L for both initial pH 3.5 and 10 are shown in Figures 1a and lb. As can be seen, the phenol conversion is much slower at basic pH, probably due to the free radical mechanism proposed for this reaction (5). Regarding to the mineralization of the phenol to CO2 similar curves with and without catalyst are obtained at each pH. However at basic pH the mineralization achieved for a given phenol conversion is slightly higher than the obtained at acid pH.

X phenol

X

1.0

)

1.0

0.8

0.8

0.6

0.6

0.4 0.2 0.0

,~ ()

- - l - - pH,=3.5 Ccat=0 g L-'

0.4

--A-- pH,,=3.5 Ccat=60 g Lj

0.2

--o-- pH,,=l 0 Ccat=0 g L~ 5'0

160

--A-- pH,,=10 Ccat=60 g L~

time (min)

Figure 1. pH influence. T=140~

TOC

''

:

Pp:]]~33155 ((]::tt~060gL-L.'

"

E] PH,,= 10 Ccat=0 g L-~

'

" b)/~

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/

I

0.0

0'.0

o'.a

0'.4

•

o'.6

0'.8

1'.0

phenol

P=16 atm. a) Phenol conversion b) Phenol mineralization.

3.2. Influence of temperature

Figures 2a,b and 3a,b show the results obtained for the phenol and the TOC conversion with time at the two extremes catalyst loading employed. As can be seen, for both catalyst loadings in the reactor, 0 and 120 g/L, the phenol conversion is highly affected by the temperature until 160~ then the increase of this variable does not produce an increase of the phenol conversion rate. On the other hand an increase of the temperature increases the mineralization grade if no catalyst is added. When the reaction is carried out at 120 g/L of catalyst loading no effect of the temperature on the mineralization achieved was noticed.

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X phenol

Xphenol i.0

9

,

,

9

,

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,

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T=,80oc ~

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, &

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9

9

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0.0

2'0 ' 4'0 " 6'0 ' 8'0 " 100 1 89 140" 160

6

2'0

time (min)

T=!

60~

9 T=I80 C

4'0

6'0

80

time (min)

Figure 2. Effect of the temperature on the phenol conversion, pHo =3,5, P-16 atm. a) Ccat=0 g/L b) Cc.~t=120 g/L XTOC

1.0

'

0.8

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9

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9

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9

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|.

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1.0

Figure 3. Effect of the temperature on the mineralization of phenol, pHo=3,5, P=16 atm. a) Ccat-0 g/L b) Ccat=120 g/L

3.3. Effect of the catalyst loading The effect of the catalyst concentration on the phenol conversion and its mineralization to CO2 at the four temperatures studied is shown in Figures 4a,b,c and 5a,b,c. As can be seen in Figure 4c at temperatures higher than 160~ the addition of the catalyst does not improve the phenol conversion. Thus, the main contribution to the phenol oxidation rate is due to the homogeneous reaction. At 180~ a similar behavior for the mineralization was found with and without catalyst. On the contrary, at 160~ the addition of the catalyst yields a higher mineralization of the intermediates produced from the oxidation of phenol. At the lower temperatures used, 127 and 140~ a significant positive influence of the addition of catalyst on both phenol and TOC conversion was found, as can be seen in Figures 4a,b and 5a,b. At these temperatures no differences in the grade of mineralization of phenol were found for a catalyst loading over 30 g/L.

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X

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--A--T=I80"C Ccat=120g L4 .

/,a~,

9 Ccat=OgL-

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Ccat =

0.6

,//~?

9 Ccat=60g L-~

0.0 ~

140

.

time (min)

• 0.81.0 ~

'

9 Ccat=4g L-~

~o

--.-- Ccat=4 g L-'

/m/

0.0

0.0

.

0.6

f/p/

/e,

0.0

0.8

0.6

0.2

9

/

0.2

!.0

b)

9

0.4

.

..-

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,.o

.

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3henol I

.

a ) ...--"

9 C cat=60 g L4

0.4

time (min) X

.

. - / / / ~ I/

v. V

//j/

[]

[]

0.0 50

time (rain)

0.0

0.2

0.4

0.6

0.8

1.0

Xphenol

Figure 4. Effect of the catalyst loading on the phenol conversion, pHo=3,5, P=16atm.

Figure 5. Effect of the catalyst loading on the mineralization, pHo=3,5, P=I 6 atm

a) 127~ b) 140~ c) 160, 180~

a) 127~ b) 140~ c) 160, 180~

In Figures 6a,b,c the pH change with time is shown at the two extremes catalyst loading used (0 and 120 g/L) for the temperatures studied. It can be seen that the catalyst loading has a significant influence on the pH change. When the catalyst was added the pH decreased initially with the increase of the phenol conversion, achieving a minimum, and then increased. This can be explained considering that the acid compounds (mainly C-2 and C-4 dicarboxilic acids) are the last intermediates of the oxidation route before the mineralization is achieved. The catalyst produces a higher oxidation rate of these acid compounds. The higher

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oxidation of the acid compounds achieved with the catalyst can explain the differences in the mineralization grade observed in Figures 5a,b,c. When the temperature increases the results obtained with and without catalyst for the oxidation of those acid compounds are closer. For example, negligible differences in pH change with time were found with and without catalyst at 180~ and consequently the same TOC abatement vs. phenol conversion was also observed. )H

pH ,

T=127"C

5.0

a)

,

5.54 . 5.0 ]~

9 Ccat=-4gL~

4.5 i

.

.

.

. . . T=140"C

4.5

--T-- Ccat=120 g Lt

4.0

4.0 3.5

|

b)

9 Ccat=0g L" v_ Ccat=___~120g L 2

" ~

3.032.5.5 I

;

-

w

-

-

'

x

~

3.0

() " 2'0 " 4'0 " 6'0 " 8'0 1001 89

. . . . l zi0 160

2.0

d

2'0 4'0

time ( 9

6'0

8'0 ~bo 1Ao l~o

time (min)

pH

-- 9 T=160"C Ccat= 0 g L-~

5.5 5.0 4.5

'

--A-- T=160"C Ccat= 120 g L-~

z~

--0-- T=180"C Ccat=0 g L"

' C)

4.0 3.5 3.0 2.5 2.0

0

20

40

60

80

100

time ( 9 Figure 6. Effect of the catalyst loading on the pH change, a) 127~

b) 140~ c)160, 180~

ACKNOWLEDGEMENTS This work has been supported by CAM under contract no. 07M/0035/1998. The authors wish to thank Engelhardt who kindly supplied the catalyst. REFERENCES

1. T.D. Thornton and P.E. Savage, J. Supercritic. Fluids 3(1990) 240. 2. A. Pintar and J. Levec, Catal. Today. 24 (1995) 51. 3. Z. Ding, S.N. Aki and M.A. Abraham, Env. Sci. Tech. 29 (1995) 2748. 4. S.H. Lin and S.J. Ho, Appl. Catal. B: Env. 9 (1996) 133. 5. A. Sadana and J.R. Katzer, J. Catal. 35 (1974) 140. 6. H. Otha, S. Goto and H. Teshima, Ind. Eng. Chem. Fundam. 19 (1980) 180. 7. A. Pintar and J. Levec,, Ind. Eng. Chem. Res. 33 (1994) 3070

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Giyoxal Synthesis by Vapour- Phase Ethylene Glycol Oxidation on a Silver and Copper Catalysts O.V. Vodyankina a, L.N. Kurina a, A.I. Boronin b and A.N. Salanov b aTomsk State University, Tomsk, 634050, Lenin str. 36, Russia bBoreskov Institute of Catalysis SB RAS, Novosibirsk, 630090, Lavrent'ev pr. 5, Russia The process of the ethylene glycol oxidation with using a number of different catalytic systems has been studied. Efficiency of supported Ag catalysts is associated with the stabilization of ion Ag +~ state under the influence of oxygen of the support lattice. It is shown that the application of the double- layer Cu - Ag catalytic system increases the selectivity of the ethylene glycol oxidation into glyoxal. 1. INTRODUCTION Glyoxal widely using in industry is prepared by both nitric acid oxidation of acetaldehyde and catalytic oxidation of ethylene glycol. The latter process is more perspective method because this process is realized in gas phase, and it is characterized by higher productivity. Ethylene glycol oxidation is realized at 773-923 K on Ag and Cu catalysts generally. In Ref. [1,2] a SiC supported silver catalysts were prepared. Without phosphorous promoters the efficiency of supported Ag catalysts is essentially lower in comparison with double - layers C u - Ag catalysts [3]. The reasons are the subject of investigation in the present work. 2. EXPERIMENTAL The samples of metal catalyst were prepared according to Ref. [4,5]. Supported silver catalysts were.prepared by chemical reduction method. Catalytic activity has been examined in quartz fixed bed reactor (0.82 sm2). The catalyst mass was 1.9 g. The analysis of the liquid products has been performed according to Ref. [6]. The interaction of ethylene glycol on the clean and oxidized surface of the catalysts has been studied by thermal programmed reaction method (TPR). Adsorption of ethylene glycol took place at T = 473 K and 573 K. Adsorption of O2 was carried out at T=473 K. The coke formation has been studied in a flowing reactor, which makes it possible to observe in the regime in situ the growth of the coke deposits (CD). The X-ray photoelectron spectra (XPS) were recorded with a VG ESCALAB electron spectrometer using AII~ radiation. Experimental resolution characterized by full width at half maximum (FWHM) of Ag3ds/2 line is 1.3 eV if pass energy of analyzer equals to 20 eV. Spectra were calibrated against Eb(Ag3d5/2) = 368.1 eV and Eb(Cu2p3/2) =932.7 eV [7]. Investigations by Scanning Electron Microscopy (SEM) have been performed with aid of microscope BS-350 (TESLA) equipped with electron gun of 16 kV. 3. RESULTS AND DISCUSSION Table 1 gives the experimental data on different catalyst systems. It is known that an active component in the supported Ag catalysts does not wholly cover support surface [8], but it is situated as crystallites with dimensions of 50 - 90 nm. Thus, a part of the support surface is opened.

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Table 1 Catalytic activity of different system in the ethylene glycol oxidation process Simple Composition T, K Molar Conver- Glyoxal Concentration of acid of support ratio of sion, % yield, % centers, (mol/m 2) x 103 SiO~/AbO~ O~/EG Before After Ag el. crystals 823 1.0 80.8 56.0 28.28 19.04 Double Cu-Ag 853 1.0 97.2 78.4 10% A g / 1 ~ - A1203 853 0.9 87.5 35.3 1.52 0.32 10% Ag / 2 80/20 853 0.9 94.3 38.1 10% A g / 3 40/60 853 0.9 96.1 55.0 4.90 5.98 40% Ag / 4 70/30 853 0.9 90.7 42.5 3.46 8.26 10% A g / 5 c~- A1203 853 1.1 93.1 28.2 1.57 0.20 10% Ag / 6 SiO2 853 1.1 81.3 23.6 -

In the previous work [3] it was shown that on clean surface of carriers the unselective oxidation of ethylene glycol (EG) occurred. Besides during the time catalytic systems prepared on the base of ~ - A1203 (sample 1,5) and SiO2 (sample 6) decrease their catalytic activity. The reason of catalyst activity decrease may be associated with agglomeration of Ag on the support surface under the action of high temperature reaction media. One can see from the data of Table 1 that both activity and selectivity of supported Ag systems depend strongly on SiO2 containing in supports. It may be associated with stabilization of Ag in high - dispersity state on surface of alumina - silicate supports as it was shown in Ref [9]. H i g h - dispersity Ag state may appear as the result of both the formation of chemical surface compound of Ag with support and the Ag +5 state formation under the influence of oxygen of support lattice. Taking into account the silver ions are stabilized by oxygen atoms of the support lattice, the electron density of such oxygen ions effects on the polarization Ag 6+-- 05.- Me bond. On the surface of supports prepared on the base of individual oxides (1,5,6 samples) such stabilization is practically absent and Ag is able to agglomerate leading to the decrease of catalytic activity. To estimate the degree of the ligand influence of oxygen of the support lattice on the Ag +5 state, we used method of potentiometric titration of the supported Ag catalysts by potassium ethylate (CzHsOK) in the media of non - proton organic solvent [10]. These data are presented in Table 1. Four types of acid centers, which are different in strength, have been found for the catalyst surface in agreement with the experimental potentiometric curve. At first the strongest acidic centers are titrated by CzHsOK then weaker centers interact with C2HsOK [10]. One can see that the surface of massive Ag crystals is characterized by the highest acid properties. Thus acid properties of supported Ag systems depend on the distribution of active component along support surface. Treatment of catalysts by the reaction mixture leads to both the increase of acid center concentration on surface of alumina- silicate Ag supports and simultaneously the acid center concentration decrease on the Ag catalysts prepared on the base of individual ~ - A1203. This may be associated with the formation of high - despersity Ag particles such as isolated Ag ions and two - dimensional clusters characterizing by highest effective charge on the surface of alumina- silica supports. Under action of the reaction mixture amount of high - dispersity Ag state increase in the result of partial decomposition of more large clusters. The similar correlation is observed at the investigation of catalytic activity of these systems in the process of ethylene glycol oxidation.

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To elucidate details of the ethylene glycol oxidation process into glyoxal on 493 ~ 683 the catalyst surface, we used methods of TPR. Ethylene glycol is reversibly adsorbed on the clean Ag (Fig.la). According to calculated kinetic parameters for this TPD spectrum (activation energy = (CHO)A2~r H2 b ~ / ~ ('(HO),) 59.6 + 0.8 kJ/mol and desorption order = 0.9) the multilayer of ethylene glycol was H2O453 I-t20~ / ~ ~ :02 formed at such conditions. Similar conclusion was made at the investigation of the ethylene glycol interaction on the clean Ag (110) surface in Ref. [11]. The 4m -9" ~ CO.> interaction of ethylene glycol with oxidized surface of the silver catalyst depends on both O-Ag bond strength and the ~,,~ ) 373 573 773 T,K 473 673 883 adsorption species of alcohol. The reaction of ethylene glycol dehydrogenation to Fig. 1. Tile etlvlene glycol reaction on file clean (a) glyoxal occurs on the oxygen - containing ,'uld o~gen - doped catalyst smthce (b,c) under folcenters of Ag surface at Tads = 473 K lowilN conditions: (a) T~.~1~EG=473 K, (b) Tads 02, Tads EG=zI73 K, (c) Tads 09=573 K, Tads EG=d73K (Fig. lb). While the temperature of oxygen adsorption increases until 573 - 673K the glyoxal formation by the oxidation way takes place (Fig 1c). It is reliably evidenced by the absence of hydrogen in the desorption products. Another part of ethylene glycol is adsorbed on the oxidized Ag surface where the process of the EG decomposition leads to the formation of secondary products: formaldehyde and CO2 (Fig. 1b,c). In contrast to the silver catalyst the chemisorption of EG on the clean Cu surface is characterized by the process of ethylene glycol dehydrogenation (Fig. 1a). At low coverage of oxygen the formation of glyoxal occurs along the oxidative route predominantly (Fig.lb). While the adsorption temperature increases up to 573 - 6 7 3 K the reaction transition from the copper catalyst surface to the reactor volume is observed. Thus, the application of the double layer Cu - Ag catalytic system increases the selectivity of the ethylene glycol oxidation to glyoxal although the basic process is realized inside the silver catalyst layer. The efficiency of Cu layer grows up at the decrease of oxygen concentration and temperature. While using of double - layer Cu - Ag system as a catalyst in the selective ethylene glycol oxidation process the intense coke formation has been observed in the Cu layer [3]. To study the catalyst morphology, nature and states of carbon in the coke deposits, we used a number of physical methods such as SEM, TGA and XPS together with catalytic methods. In our previous work [12,13] it is shown that the quantity of the coke deposits on Cu is significantly higher then on Ag. This fact may be associated with the difference of the coke formation mechanisms on the surface of Ag and Cu catalysts. According to TPR data glyoxal formed on the unoxidized Cu surface unlike Ag where this process is realized only on o x y g e n containing active centers. Therefore the process of carbon deposit (CD) formation at the oxygen absence conditions goes on the clean Cu surface because glyoxal is the basic coke formation agent. TGA analysis of CD has shown that on Cu surface the formation of two types of carbon deposits burning out at 723 and 653 K is possible in dependence on the composition of the reaction mixture. TG analysis made for Ag samples shows burning of CD at temperature of 663 K only. Thus, in contrast to Ag on the Cu catalyst surface two types of CD are formed.

Ag

-t

I

Cu

!

t~,

..... i

I

I

i .............

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Cls

In Fig.2 C ls spectra of the silver and copper samples investigated after the action of the reaction mixture are presented. The treatment of Cu catalyst with pure EG produces C 1s spectrum (curve a) where the main peak with Eb(C l s) = 284.2 eV together with [ ' k 286.8 the satellite line (Eb = 290.5) can be reliably associated with a graphite - like structures. The variation of the reaction mixture composition leads to the essential changes in C ls spectra where the 285.1 chemical shift of main component 0.5+0.9 eV is observed (Fig.2, b,c). We propose that the basic components of Cls spectra with Eb = 285.1+284.7 eV correspond to oxygen- carbon structures such as C O - C which is formed by the cyclization of ethylene glycol or glyoxal on the oxidized catalyst surface. c) te) Thus, XPS spectra demonstrate that carbon deposits contain not only carbon atoms, but they consist of oxycarbon structures on the surface. In the result of the catalysis action the surface morphology of the Ag, Cu samples changes ......... " ........ dramatically. Initially the Ag, Cu samples represent 280 285 290 295 the entire openwork object plaited from the threads with the diameter of 20 - 30 ~tm for Ag and 2 0 0 - 300 ~tm for Cu. At first, the formation of holes with a Fig.2. C ls spectra obtained after the mean size around 0.1 ~tm is observed on the Ag action of reaction mixture on copper surface in the result of the reaction mixture action at (curves a,b) and silver (curves c) 823 K for l h (Fig.3 a,b). The similar phenomenon is revealed in Ref [ 14] catalysts: (a) O2/EG = 0.0, (b) O2/EG in the course of methanol oxidation on Ag foil. = 0.3, Y = 873 K, EG/H20 = 0.6; (c) Authors concluded that the formation of holes O2/EG - 1.0, EG/H20 = 0.4, T resulted from the reaction of b u l k - dissolved 773 K. hydrogen and oxygen. However, it is noteworthy that the treatments of Ag samples by gaseous mixtures (H2 + 02 +N2, H2 + N2 etc.) at such conditions did not lead to the morphological changing of surface (not shown here). Comparison with XPS data allows us to suppose that holes serve as storage of carbon containing depositions. According to SEM data the holes penetrate to Ag crystal from beginning to end (Fig.3c). Besides the process of the hole formation is only realized when the reaction conditions are favor for selective oxidation of ethylene glycol into glyoxal. This is allowed to suppose that the formation of holes is realized with the participation of glyoxal. Probably the process of coke formation is realized in the bulk of Ag crystal in the direction from volume to surface. At the elevated temperature the increase of mobility of Ag surface layers leads to the appearance of the coke on surface, where it weakly bum out forming channels in the presence of oxygen of gas phase. Taking into account the morphology of observed channels such coke deposits must have filament structure. In contrast to Ag the Cu surface is covered the shallow cracks (Fig.3d). According to XPS data this effect is associated with the formation of the surface layer of copper oxide (I). The increase of the treatment time of catalyst surface by the reaction mixture leads to the formation of thick films of the carbon-containing deposits. The thickness of the film on Ag catalyst is 0.5 ~tm while it is about 3 ~tm on the Cu. On the Ag surface the deposited carbon

a)

/~84.8

b)

]~286.9

_._J

~

e

Binding Energy,eV

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Fig.3. SEM images of the surface of silver (a,b,c,e) and copper (d,f) catalysts after the action of reaction mixture: (a) T = 823 K, (b) T = 973 K, (c) T = 873 K, (e) T = 898 K, O2/EG = 1.0, EG/H20 = 0.4; (d) T = 773 K, (f) T = 873 K, O2/EG = 0.5, EG/H20 = 0.6.

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film looks as a compact crust (fig.3 e). Close inspection of this crust shows that it consists of carbon filaments tightly plaited. The morphology of carbon deposits on Cu is different one. The deposited film on the Cu catalysts consists of carbon filaments tight-fitting to each other, which are perpendicular to the surface (Fig. 3 f). Thus, the growth of the oxycarbon structures occurs as filaments on the oxidized catalyst surface. According to catalytic data presented earlier in [12,13] the oxygen - content increase up to 0.6 - 0.9 in the reaction mixture is accompanied by the glyoxal yield growth on both Ag and Cu catalysts. This phenomenon could not take place because of self- regeneration process only, but one can assume that the oxycarbon layers are likely a reservoir of an additional condensed glyoxal. 4. CONCLUSION The data presented in this work show that efficiency of supported Ag catalysts is associated with the stabilization of active component in the h i g h - dispersity Ag +~ state under the influence of oxygen of the support lattice. However, uncovered by Ag lots of catalyst surface is answerable for unselective ethylene glycol oxidation. The application of the doublelayer Cu-Ag catalytic system increases the selectivity of the ethylene glycol oxidation into glyoxal. One of the main reasons is the growth of efficiency of Cu layer located in the field of reactor where more low oxygen concentration and temperature in comparison with Ag layer take place. One can assume that the coke deposition on the oxidized metal catalyst surface exert favorable influence on selectivity of the major process. The coke deposits are the oxycarbon structures which grow as filaments. It is proposed the destruction of oxycarbon layer under the oxygen action leads to the glyoxal appearance in the reaction products. REFERENCES 1. P. Gallezot, S. Tretjak, Y.Christidis et al, J. Catal. 142 (1993) 729. 2. J. Deng, J. Wong and X.Xu, Catal. Lett. 36 (1996) 207. 3. O.V. Vodyankina, L.N. Kurina, L.A. Petrov et al., Chem. Ind. 12 (1997) 802. 4. L.A. Petrov, I.G. Rozanov, A.P. Ckhramov et al., EMRS Fall Meeting, 4 European East West Conference, St.-Petersburg (1993), A4. 5. Antsiferov V.N., Ovchinnikova V.I., Makarov A.M. Open cell foam structures, catalysts supported there by and method of producing the same, US Patent WO 95/11752 (1995). 6. O.V. Vodyankina and S.I. Galanov, Ind. Lab. 8 (1995) 13. 7. D. Briggs, M.P. Seah (Eds.), Practical surface analysis by Auger and X- Ray photoelectron spectr., John Wiley&Sons Ltd., Chichester-New-York-Brisbane-TorontoSingapore, 1983. 8. S.M. Brailovskii, O.N. Temkin and I.V. Trophimova, Probl. Kinet.Catal. 19 (1985) 146. 9. A.N. Pestryakov, A.A. Davidov and L.N. Kurina, J. Phys. Chem. 62 (1988) 1813. 10. V.N. Belousova, O.G. Kuznetsova and L.N. Kurina, J. Appl. Chem. (1984) 921. 11. A.J. Capote, R.J. Madix, J. Amer. Chem. Soc., 3 (1989) 3570. 12. L.N. Kurina, L.A. Azarenko and O.V. Vodyankina, React. Kinet. Catal. Lett. 63 (1998) 355. 13. L.A. Arkatova, L.N. Kurina and O.V. Vodyankina, J. Appl. Chem. 72 (1999) 534 14. A. Nagy, G. Mestl, T. Ruhle et al, J. Catal. 179 (1998) 548.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

P r o d u c t i o n of A l k e n e s t h r o u g h O x i d a t i v e C r a c k i n g of n - B u t a n e o v e r OCM C a t a l y s t s M. N akamura, S. Takenaka, I. Yamanaka and K. Otsuka* Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan Oxidation of n-butane was carried out over rare-earth metal oxide catalysts. Oxidative pyrolysis of n-butane proceeded over these catalysts to form ethene and propene. Modification of the catalysts by Li + brought about the suppression of total oxidation and the improvement of selectivity to alkenes. Alkene yield was the highest over Li+-added-Y203 catalyst, and its catalytic activity was very stable. XRD patterns and O K-edge XANES spectra of the catalysts suggested that solid solutions were not formed between Y203 and Li*, but Y20~ was covered with a thin layer of Li2CO3.

1. INTRODUCTION The utilization of light alkanes to produce more valuable chemicals is always attractive. One of the promising routes is the conversion to unsaturated hydrocarbons. The production of alkenes from alkanes by catalytic oxidation is preferable because the process can overcome the thermodynamic limitation in pure dehydrogenation processes as well as the deactivation of catalysts due to coking. Many efforts have been made toward the development of active catalysts for the oxidative dehydrogenation of light alkanes. Most of the catalysts reported so far contain vanadium or molybdenum oxides as active species. In oxidation of hydrocarbons over these types of catalysts, the lattice oxygen species in the catalysts activate the light alkanes to form products. For example, vanadium-magnesium mixed oxide catalyst (VMgO) was reported to be active for the oxidative dehydrogenation of n-butane to butenes and butadienes [1]. However, this catalyst enhanced the deep oxidation of n-butane considerably because of strong oxidizing activity of vanadium oxide. Oxidative coupling of methane (OCM) is believed to proceed in the gas-phase via methyl radicals which have been generated on the catalyst surface [2]. We have already reported that basic metal oxides, such as rare-earth metal oxides, are effective catalysts for OCM, that is, the successive oxidations of the formed methyl radicals and ethene are suppressed due to a basic property of the catalysts [3]. Therefore, we expected that the basic metal oxide catalysts efficiently generate alkyl radicals from light alkanes, and the catalysts do not oxidize the formed alkyl radical species and alkenes deeply into COx. In addition, in the oxidative coupling of methane over basic metal oxide catalysts, oxygen species adsorbed on the catalysts are believed to activate methane, generating methyl radicals [2,4]. The adsorbed oxygen should exhibit catalytic performance

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different from the lattice oxygen in vanadium or molybdenum oxide catalysts mentioned above. In this study, we carried out oxidation of n-butane over rare-earth metal oxide catalysts. In addition, we investigated the effect of the addition of Li § to the catalysts on the catalytic performance. 2. E X P E R I M E N T A L

Catalysts Li*-added metal oxide catalysts were prepared by impregnating the metal oxides with aqueous solutions of Li2CO3, and drying up the impregnated samples at 353 K. The samples were calcined at 1023 K for 5 h under an air stream. The amounts of added Li § were adjusted to be a molar ratio Li*/X = 0.4 (X; metal ions in metal oxides). V205(20 wt%)/MgO and MOO3(36 wt%)/MgO were prepared by impregnation of aqueous solutions of NH4VO3 and (NH4)6Mo7024"4H20 into MgO, respectively. The samples were dried at 363 K for 12 h and calcined at 1023 K for 5 h in an air stream. The catalysts were pressed into pellets. The pellets were crushed and sieved to 20/45 mesh size. Reactions

The oxidation of n-butane was performed in a conventional gas flowing system with a fixed-bed reactor at an atmospheric pressure. The cylindrical reactor made from alumina had an i.d. of 5.5 mm and a length of 360 mm. In order to minimize the contribution from any gas-phase reaction, quartz sands filled the space above and below the catalyst bed in the reactor. Catalyst (0.2g) was introduced in the reactor and was heated to 1023 K in a flow of oxygen, prior to the reaction. The temperature profile was measured using a ChromelAlumel thermocouple, which was placed in an axial thermowell and centered in the catalyst bed. n-Butane and oxygen were fed with a helium carrier over the catalyst, and gaseous products were analyzed on-line by gas chromatographs.

Characterization of Catalysts X-ray absorption experiments were carried out on beamline BL-8B1 at UVSOR, Institute for Molecular Science, Okazaki, Japan. O K-edge XANES spectra of catalysts were recorded at room t e m p e r a t u r e in the total electron yield mode. The sample was put on Cu-Be dinode which is attached to the first position of the electron multiplier. X-ray diffraction patterns (XRD) were collected using a Rigaku RINT 2500V diffractometer. 3. R E S U L T S A N D D I S C U S S I O N

The oxidation of n-butane was performed over various rare-earth metal oxide catalysts to investigate the catalytic performance of basic metal oxides. In addition, non-catalytic experiments were carried out in order to examine the contribution of the gas-phase reaction. In this case, quartz sands were packed in the catalyst bed. The results were shown in Table 1. For the reactions over all rare-earth metal oxide catalysts, the conversions of n-butane and oxygen were much higher t h a n those over quartz sands (in the absence of catalysts), suggesting t h a t rare-earth metal oxides catalyze the oxidation of n-butane. For the reaction over these catalysts, ethene, propene, CO, and CO2 were formed mainly, and little amounts of methane, ethane, and propane were observed. The results indicated that the oxidative pyrolysis of n-butane took place selectively

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Table 1

1783 Oxidation of n-butane over rare-earth metal oxide catalysts a)

catalyst

temp. b) conv. / % selectivity / % /K C4Hlo 02 C2H4 C3H6 C4Hs C~H~ Y203 973 65.9 83.4 36 26 4 tr 1023 80.6 91.7 33 21 3 tr Sm203 973 66.5 92.8 35 23 4 tr 1023 74.1 97.4 36 23 4 tr Gd~O3 973 66.3 85.1 34 24 4 tr 1023 77.7 92.7 34 22 4 tr La203 973 66.1 91.5 34 20 3 tr 1023 73.2 96.6 34 19 4 tr CeO2 973 38.8 98.2 29 12 2 tr 1023 54.3 98.4 34 15 1 tr quartz 973 10.1 3.0 34 45 6 tr 1023 42.3 18.6 37 41 5 tr a) P(n-CtHlo)=P(O2)=8.4 kPa, P(He)=84.5 kPa, flow rate=180 ml-min. b) reaction temperature, c) total yield of alkenes.

CO 14 12 12 12 12 13 10 10 5 6 1 3

CO2 11 10 14 13 12 12 15 14 38 25 tr tr

O.y.c~/% 42.9 46.3 41.3 45.9 41.3 46.0 37.9 41.7 16.7 27.5 8.6 35.0

over rare-earth metal oxide catalysts. The yield of alkenes was the highest over Y203 catalysts among all the rare-earth metal oxide catalysts. Over CeO2 catalysts, the conversion of oxygen was high at relatively low temperatures (N923 K), and total oxidation to CO2 was appreciable. In order to improve selectivities to ethene and ethane in oxidative coupling of methane, alkali ions were often added to metal oxide catalysts, for examples Li § added MgO [5] and Sm203 [6]. Therefore, we tried to apply this modification of the catalysts to n-butane oxidation. The results of the oxidation over Li+-added catalysts are shown in Table 2. The addition of Li § into any of the rare-earth metal oxide catalysts resulted in the decreases in conversions of n-butane and oxygen. Over CeO2 catalyst, the conversion of oxygen was significantly suppressed by addition of Li § On the other hand, selectivity to CO2 became low, and those to alkenes, especially propene, became high by Li*-addition, t h a t is, oxidative pyrolysis occurred selectively. Although alkene yields decreased slightly by addition of Li § for most of the catalysts due to the effects of Li § mentioned above, the alkene yield over Y203 catalyst increased by Li+-addition, exceeding 50 %. The highest alkene yield of 58 % was obtained with this catalyst when the gas-catalyst contact time was increased by decreasing the gas-flow rate to 60 ml.min 1. There have been few reports on the oxidation of light alkanes to alkenes over basic metal oxide catalysts. As described above, Li+-modified rare earth matal oxides, in particular Li§ Y208, are concluded to be effective catalysts for the oxidative pyrolysis of n-butane. For comparison, the results of the oxidation over V2Os/MgO and MoO3/MgO catalysts are shown in Table 2. Selectivities to butenes and butadiene were significantly high for both catalysts, which is in striking contrast to the results of the rare-earth matal oxide catalysts both with and without Li § It is accepted t h a t in oxidation of hydrocarbons over the catalysts containing vanadium or molybdenum oxides as active centers, the lattice oxygen species in the catalysts activate hydrocarbons and the removed lattice oxygen species are reproduced by gaseous oxygen. On the other hand, it is reported t h a t in the oxidation over basic metal oxide catalysts, such as Sm203, molecular oxygen species adsorbed on the

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Table 2

Oxidation of n - b u t a n e over r a r e - e a r t h m e t a l oxide c a t a l y s t s a)

catalyst

temp. b) conv./ % selectivity/% C2H4 C3H6 C4Ha C4H6 /K C4Hlo 02 Li§ 973 51.2 38.3 39 33 5 tr 1023 70.7 78.5 40 26 6 tr Li+-Sm~O3 973 26.1 18.4 37 36 6 tr 1023 57.1 41.3 38 31 5 tr Li+-Gd903 973 29.9 18.8 37 36 3 tr 1023 57.4 42.7 40 33 3 tr Li+-La~Oa 973 24.1 13.7 36 37 6 tr 1023 53.0 30.8 38 33 5 tr Li+-CeO~ 973 16.4 10.0 37 40 5 tr 1023 43.8 23.2 39 36 5 tr V~.Or/MgO 753 14.4 45.4 tr tr 38 21 773 32.8 97.5 1 tr 24 34 MoOJMgO 973 30.6 43.8 15 8 27 37 1023 51.6 60.7 30 20 12 21 a) P(n-C4Hlo)=P(O~)=8.4 kPa, P(He)=84.5 kPa, flow rate=180 ml-min. b) reaction temperature, c) total yield of alkenes.

O.Y.C~/% CO 2 2 2 2 1 2 2 3 1 2 11 13 6 9

CO2 9 13 6 6 6 6 4 4 3 3 29 23 4 4

39.2 51.5 20.7 42.3 24.0 44.6 19.1 40.4 13.5 35.0 8.5 19.4 26.6 42.8

c a t a l y s t s a c t i v a t e hydrocarbons [4]. The selectivities to a l k e n e s described above m i g h t be d e t e r m i n e d by t h e k i n d s of active oxygen species on t h e catalysts. It w a s r e p o r t e d t h a t in oxidative ~ 100 coupling of m e t h a n e over Li+-added MgO ~ 80 conv. catalyst, t h e conversion of m e t h a n e and t h e -~-yield of e t h e n e decreased w i t h the t i m e on ~ ~ s t r e a m [7]. The cause of this deactivation ~ .~ 60 ~ 40 ~---'-------'............................... -alkeneyield .... w a s a t t r i b u t e d to t h e loss of Li § from t h e 8~ .~ c a t a l y s t s d u r i n g t h e oxidation. The loss of ~-~ 20 ............................................................................... Li + m a y occur in our case. Therefore, we aa i n v e s t i g a t e d t h e stability of the Li§ ~ o , ,,, , , , Y2Oa catalyst. F i g u r e 1 shows t h e r e s u l t s ~ 3 .....0....._.........~ ............ 4 0 C2H4_._ of t i m e - c o u r s e of t h e n - b u t a n e oxidation over Li+-added Y 2 O a catalyst. The conversion of n - b u t a n e , yield of alkenes a n d selectivities to all products were C3H6 ] i~ 20 I -.............................................................................. a l m o s t c o n s t a n t d u r i n g t h e t e s t e d period, C0 2 l i n d i c a t i n g t h a t Li§ Y2Oa c a t a l y s t w a s 10 "-AA---~t---~,--at----,t- ~t---A--at- i---~:---A ^ C v e r y stable. In oxidative coupling of m e t h a n e over Li+-added MgO catalyst, it 0 2 4 6 8 10 12 w a s i n d i c a t e d t h a t t h e introduction of CO2 time on stream / h into a m i x t u r e of feed gases p r e v e n t e d t h e Figure 1. Time course of the oxidation d e a c t i v a t i o n of t h e c a t a l y s t s [7]. The of n-butane over Li+-added Y203 catalyst. cofeed of CO2 b r o u g h t about t h e formation Reaction conditions: catalyst 0.2 g, of Li2COa on t h e catalysts. The Li2COa, flow rate 180 ml.min-1 t h e source of t h e catalytic active sites for P(n-C4H10) = P(O2) = 8.4 kPa, t h e oxidative coupling of m e t h a n e , would P(He) = 84.5 kPa, temperature 1023K be stabilized in t h e presence of a high

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partial pressure of CO2. In oxidation 35 of n-butane over Li+-added Y~O8 30 catalyst in this work, a relatively ...................1 o I.....................t high partial pressure of CO2 was ~" 25 present due to a high conversion of - ............................i g .~ ..................... 1 n-butane, compared to the i~ 20 oxidation of methane. The CO~ .....................1 15 : ............................1 " formed during the oxidation of n- 0 10 butane may stabilize the catalytic activity of the Li§ Y208 catalyst. In an attempt to obtain 0 1 2 3 4 5 6 7 evidence for the reaction pathway W/F / 10-3g rain ml-1 in the oxidation over Li§ Y203 catalyst, the effect of varying Figure 2. Effect of W/F on the oxidation the residence time (W/F; W: of n-C4Hlo over Li+-added Y203 catalyst. catalyst weight, F: flow rate of Reaction conditions: catalyst 0.2 g, e m p e r a t u r e 893 K, flow rate 25 to 240 ml.min-1, feedstock) on product distributions tP(n-C4Hlo) = P(O2) = 8.4 kPa, P(He) = 84.5 kPa was studied. Figure 2 shows the change in selectivities with W/F. In this reaction, flow rates of feedstock were changed from 25 to 240 ml.min 1, with a constant weight (0.2 g) of the catalyst. In spite of change of W/F, the selectivites to propene and ethene were kept at constant values, although the conversion of n-butane increased from 7 to 28 %. The selectivity to CO decreased and that to CO2 increased slightly with the W/F, but the sum of these carbon oxides was constant. The results indicate that the successive oxidation of the alkenes does not occur in the oxidation over Li§ Y208 catalyst. We consider that the oxidation of alkenes was suppressed due to a basic property of the Li§ Y208 catalyst. Oxidation to alkenes and that to COx may proceed on different active sites and/or by different active oxygen species on the catalyst. The results of the n-butane oxidation indicated the improvement of catalytic performance of Y203 by addition of Li § The addition of Li § to Y208 is expected to form mixed compounds or to cause the structural change of Y203. Therefore, we measured XRD patterns of Y20~ and Li § added Y203 samples. Figure 3 shows the Li § added Y203 XRD patterns. By addition of Li § into Y203, the peak positions of Y208 did not change, although small peaks due to LiYO2 were observed at 20 = 21.8, 41.0, 54.4 ~ in o the XRD patterns of Li+-added Y208. The formation of LiYO2 may cause the improvement of catalytic performance by Li § addition. To clarify the assumption, I I I I I I we carried out oxidation of n-butane over 20 25 30 35 40 45 50 LiYO2 catalysts. However, the catalytic 2e activity was much lower than that over Figure 3. ~]~D patterns of Y203 Li+-added Y 2 O s catalyst, although and Li § added Y203 . selectivities to alkenes were almost the

!!

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same levels as those over Li*-added Y208 catalyst. Therefore, we consider t h a t LiYO2 was not the active species for the oxidation. The XRD patterns provide the physical information about bulk structures of the ~ e r samples, while O K-edge XANES spectra measured in total electron yield mode give the information of the surface of the samples. o Figure 4 shows O K-edge XANES spectra of +-added Y203 Y203 and Li§ Y203 catalysts, as well as Li2CO 3 and the physical mixture of Li2CO3 and Y203 (Lily = 0.4). In the XANES spectrum of Y203, two peaks were observed at ca. 534 and 538 eV. The first peak is I I I I assigned to the transition from O ls to the 530 540 550 560 5 2 0 hybridized orbital of the oxygen 2p and photon energy/eV yttrium 4d (t2g), and the other to eg [8]. By the addition of Li + to Y203, the feature of the Figure 4. O K-edge XANES spectra XANES spectra changed. The XANES of the catalysts and reference samples. spectrum of Li+-added Y203 was almost compatible with t h a t of Li2CO 3. The XANES spectrum of the physical mixture of Y203 and Li2CO3 at a molar ratio Li/Y = 0.4 was different from t h a t of Li§ Y203, and was similar to that of Y203. These observations suggest t h a t the surface of Li§ Y203 was covered with Li2CO3. For the XRD profile of the Li§ Y203 catalyst, no peak due to Li2CO3 was found. Hence, Li2CO3 would be present on Y203 surface as a thin layer. The attainment of the higher yield of alkenes m a y be ascribed to the formation of Li2CO3 layer which enhances the formation of active oxygen species but reduces the deep oxidation of alkenes. 4. C O N C L U S I O N Rare-earth metal oxides, especially Y203, are effective catalysts for oxidative pyrolysis of n-butane to form ethene and propene. Modification of the metal oxide catalysts by Li § brought about the increase in selectivities to alkenes. The alkene yield attained to 58 % in the oxidation over Li*-added Y203 catalyst, and the activity of the catalyst was very stable. For the Li§ Y208 catalyst, the surface of Y203 was covered with a thin layer of Li2CO3. REFERENCES

1. H. H. Kung, Adv. Catal., 40 (1994) 1. 2. J. H. Lunsford, Stud. Surf. Sci. Catal., 81 (1994) 1. 3. K. Otsuka, K. Jinno, and A. Morikawa, J. Catal., 100 (1986) 353. 4. K. Otsuka, and K. Jinno, Inorg. Chim. Acta, 121 (1986) 237. 5. T. Ito, J. X. Wang, C. H. Lin, and J. H. Lunsford, J. Am. Chem. Soc., 107 (1985) 5062. 6. K. Otsuka, Q. Liu, and A. Morikawa, J. Chem. Soc., Chem. Commun., 586 (1986). 7. E. E. Wolf (eds.), Methane Conversion by Oxidative Processes, Van Nostrand Reinhold, New York, 1992. 8. J. G. Chen, NEXAFS Investigations of Transition Metal Oxides, Nitrides, Carbides, Sulfides and Other Interstitial Compounds, Elsevier, 1997.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Selective oxidation of monosaeeharides using Pt- containing catalysts E. Sulman a, N. Lakina a, M. Sulman a, T. Ankudinova a, V. Matveeva a, A. Sidorov a, S. Sidorov b aBiotechnology and Chemistry Department, Tver Technical University, A. Nikitina str., 22, Tver, 170026, Russia bA.N. Nesmeyanov Institute of Organoelement Compounds RAS, Vavilova str., 28, Moscow, 117813 Russia L-sorbose oxidation selectively to 2-keto-L-gulonic acid has been studied using Ptpolymer catalyst derived from hypercrosslinked polystyrene containing Pt nanoparticles (HPS). Varying the reaction conditions, such as sorbose and soda concentration, catalyst amount, temperature, oxygen feed rate, and stirring rate, the optimal conditions for achieving the selectivity of 98% have been found. Studying the reaction kinetics allowed to choose the mathematical model which describes well the experimental data. Kinetic experiments at various reaction temperatures permitted to obtain apparent activation energy value of 36 kJ/mol.

1. INTRODUCTION In recent years the problem of selectivity has become one of the most important problems of catalysis. With decreasing selectivity, side products are formed, thus the quality of the whole product deteriorates. Nowadays the reactions of selective oxidation of monosaccharides underlie the methods for the production of biologically active compounds. Thus, D-glucose oxidation to D-gluconic acid is used in the production of calcium gluconate which is both a medicine and an intermediate in riboflavin (vitamin B2) synthesis. The main intermediate in the production of ascorbic acid (vitamin C) is 2-keto-L-gulonic acid obtained by L-sorbose oxidation [ 1, 2]. Literature analysis on direct catalytic oxidation of L-sorbose to 2-keto-L-gulonic acid [3-6] shows that oxidation of monosaccharides with oxygen is recommended to be conducted on Pt, Pd, Os catalysts, better applied on activated charcoal at normal or increased temperatures (50-70~ and pressure, at neutral or alkaline pH. In acid solutions the process doesn't take place. Strongly alkaline medium causes by-products formation. Optimal pH values are within the limits of 8-10. As alkalizing reactants, alkaline salts of weak organic or mineral acids were suggested. Earlier we studied selective heterogeneous catalytic oxidation of D-glucose to Dgluconic acid (Scheme 1) [7]. There have been used platinum- and palladium-containing catalysts (including modified) supported on A1203, SiO2, carbon material "Sibunit". The regularities of D-glucose oxidation have been found. The reaction mechanisms have been detailed. The kinetic models with adequate description of the experiments have been also developed.

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H~C--O H~C HO~C

I I

H--C

f H--C I H2C

NaO~C

~O

r I [o], Na~_ H O - - C ~ H

OH

H~C

H

Cat

OH

-

I H~C

OH

H~C

OH

I I H2C

OH

OH

OH

D-glucose

OH

sodium D-gluconate Scheme 1

As for solving the selectivity problem in direct L-sorbose oxidation (Scheme 2), most of authors search for selective modified catalytic systems on the basis of Pt and Pd [8-11]. Under optimal conditions when Pt/C (5% Pt) catalyst was used, 28-37% yield of 2-keto-Lgulonic acid was achieved at 100% L-sorbose conversion [10]. So, such a catalyst did not provide high selectivity in L-sorbose oxidation. Lately attention was focused on synthesis and studying of catalytic properties of colloidal metal particles of nanometer size (nanoparticles), incorporated into polymer matrix. It provides a stabilization of small particles with huge surface areas and high reactivity [12]. In the present paper we report on catalytic properties of new Pt catalyst derived from hypercrosslinked polystyrene (HPS) [13] containing Pt nanoparticles. The kinetics of Lsorbose selective oxidation is discussed. CH2OH

I H0--C I HO--C--H I H~C--OH I HO~C~H I H2C

H

COOH

C=O

I HO--C ~ I HO--C~H O

L-sorbose ( B 1 )

O2~ I Cat~ H ~ C - - - O H

I I H 2 C ~

COOH

I HO~C I HO--C~H O

HO~C~H

L-gulosone

o2

]

Cat ~ H ~ C - - O H

I I H2C

HO~C~H

I I C

C--O HO O

H

O2~ I Cat~ H - - C - - O H

I I H2C - -

HO--C~H

OH

2-keto-L-gulonic acid (B2) Scheme 2

2. EXPERIMENTAL TECHNIQUE 2.1. Catalyst preparation

Catalyst was prepared at the A.N. Nesmeyanov Institute of Organoelement Compounds by impregnation of HPS with a solution of H2PtC16 in THF [13]. After drying procedure (3 days in vacuum of 1.5 Mbar), HPS-Pt contained 3-6 wt.% Pt.

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2.2. L-sorbose oxidation technique The experiments have been conducted in a specially constructed apparatus [ 14] which allows to carry out a batch process varying such parameters as sorbose and soda (NaHCO3) concentration, catalyst amount, temperature, oxygen feed rate, and stirring rate. The catalyst and L-sorbose solution at predetermined concentration were placed in thermostated set-up supplied with magnetic stir bar and reflux condenser. Oxygen-containing gas feed was controlled by rotameter. Equimolar quantity of alkalizing agent (sodium hydrogencarbonate, NaHCO3) was fed to the set-up once or gradually in certain periods to maintain constant pH. High rate stirring allows to carry out the process without diffusion limitations. During the reaction, probes of the reaction mixture were taken for analysis at certain periods of time. After the end of the experiment the catalyst was separated by filtration. The filtrate was analyzed for the presence of unreacted L-sorbose and 2-keto-L-gulonic acid formed. The determination of residual monosaccharide was carried out by gas chromatography (GC) using "Chrom-5" chromatograph. It is supplied with flame-ionoization detector and glass column filled with 5% SE-30 on Chromaton N-AW at a constant temperature. The quantity of the product, 2-keto-L-gulonic acid, was determined by Heinz classical iodometric method [ 15]. 3. RESULTS AND DISCUSSION The kinetics of the selective L-sorbose oxidation on the HPS-Pt catalytic system has been studied. The experiments have been conducted while varying the ratio between the initial L-sorbose concentration (Co) and catalyst (HPS-Pt) concentration (CD, q - C0/Co, from 0.77 g/g to 2.05 g/g. The experimental conditions have been the following: T was 70 ~ oxygen-containing gas flow rate was 20.10 -6 m3/s; stirring rate was 1000 rpm. From the dependencies obtained in the plot of "sorbose conversion (X1) vs. reaction time" (Fig. 1.). It follows that the higher the q value, the longer the reaction time.

1.0{ 0.9O.80.7-

X 0.6

-

0.5

-

0.4

-

0 , 0

9 o v x7

q=0.77 q=0.88 q=l.02 q=2.05 I

I

I

I

I

3000

6000

9000

12000

15000

T, S

Fig. 1. Sorbose conversion (X1) vs. reaction time

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The primary analysis of the experimental data has allowed to reveal the dependence of the time of 30% L-sorbose conversion on q. This dependence is shown by the expression: (n=0.5)

1:0.3 "~ qn

According to GC analysis on catalysate, the process was established to occur selectively and only a few by-products were formed. So the following process scheme has been suggested: B1 k, > 9 2 , where B1 is L-sorbose and B2 is 2-keto-L-gulonic acid. For the reduction of the experimental data belonging to different Co and Cr values to a family of curves, an independent variable has been introduced: O = r / q " , where r is the time and n - 0.5. The X : - 0 dependence obtained, where X2 is the current concentration of B2, is presented in Fig. 2.

0.6

0.5-

o/r

0.4r

,/

0.3

0.2-

I I

4000

q = 0.77 q = 0.88

v

q = 1.02

Calculation

I

0.1-

0

9

9 o

r

v

v

T

o I

6000

i

i

i

8000

10000

12000

|

~/q0.5

14000

16000

Fig. 2.2-keto-L-gulonic acid yield vs. |

The application of the explicit integral method has permitted to develop the mathematical model for L-sorbose oxidation on the HPS-Pt catalyst describing the experiments: W -- - k . X 1 - ~

where W is the reaction rate, k is the rate constant.

(1)

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The comparison of experimental and computed data (Fig. 2.) shows that this model agrees well with the experimental values. From Fig. 2 one can see an induction period. The existence of the induction period is apparently connected with unsteady state of the catalyst surface in the oxygen flow and with a formation of the catalyst active sites. Such a phenomenon takes place in many catalytic reactions [ 16]. To obtain supplementary information about the regularities of L-sorbose oxidation on the HPS-Pt catalytic system, the influence of the temperature on this reaction has been studied. L-sorbose aqueous solution with 0.119 mol/L concentration and q equal to 0.88 g/g has been oxidized. Temperature has been varied from 60 to 80 ~ The dependences obtained while analyzing experimental data reveal that temperature increase from 60 to 80 ~ results in increase of L-sorbose oxidation rate. At the same time, selectivity decreases at a temperature higher than 70 ~ It can be caused either by 2-keto-Lgulonic acid overoxidation products (because of reaction rate increase) or by the decomposition of L-sorbose which is a thermolabile compound. On the basis of the experimental results at various temperatures and using model (1), k parameter has been computed. Arrhenius dependence (Ink vs. l/T) for the apparent activation energy determination has been plotted. The Eapp. value equal to 36 kJ/mol was found to be higher for HPS-Pt than for Pt/SiO2 catalyst (Eapp. = 28 kJ/mol) described in [ 14]. On the other hand, the relative oxidation rate for HPS-Pt was 1.2-10-3 mol B2/gPt.mol B l-S which is twice as high compared to the relative rate for Pt/SiO2 (0.6-10 -3) [14]. In the case of polymer catalyst, the number of active sites taking part in the reaction is supposed to increase. To determine oxidation optimal conditions, the following parameters have been varied: catalyst amount (Cc) from 20 to 75 g/L, sorbose concentration (Co) from 0.22 to 0.44 mol/L, NaHCO3 (alkalizing reactant) concentration (Calk.react) from 0.22 to 0.44 mol//L; reaction temperature (T) from 60 to 80 ~ oxygen feed rate (V0) from 6-10-6 to 14.10 -6 m3/s; stirring rate from 200 to 1000 rpm. Maximum selectivity (98%) has been achieved under the following conditions" Cc = 63 g/L, Co = 0.36 mol/L, Calk.react. = 0.36 mol/L, T = 70 ~ V0 = 14.10 -6 m3/s, stirring rate equal to 1000 rpm. Reaction was carried out as a batch process with continuous alkaline agent feed. 4. CONCLUSION

The catalyst derived from Pt-containing hypercrosslinked polystyrene (HPS-Pt) has showed high catalytic activity and selectivity in the reaction of L-sorbose direct oxidation. Varying of the parameters of L-sorbose oxidation has allowed to find optimal conditions of Lsorbose selective oxidation with selectivity of 98%. The kinetics of L-sorbose oxidation was studied. The kinetic data obtained have allowed to chose the mathematical model which describes well the experimental results. In the future this model will be used for the computation of the industrial reactor for 2-keto-L-gulonic acid synthesis. ACKNOWLEDGMENTS We thank Russian Foundation for Basic Research (Grant No. 98-03-33372) and Federal Program "Integration" (Educational Research Center on Chemistry and Physics of Polymers) for financial support of this research.

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REFERENCES

1. L. Shnaidman, Vitamins productions, Russia, Moscow, 1973. 2. V. Berezovsky, Chemistry of vitamins, Russia, Moscow, 1973. 3. E. Merck, Verfahren zur Herstellung von 2-keto-L-gulonsaure, Germany Patent No. 692 897 (1965). 4. O. Dalmler and K. Heyns, Process for the Production of Ascorbic Acid from Sorbose, US Patent No. 2 189 778 (1956). 5. O. Dalmler and K. Heyns, Process for the ProduDtion of Keto Culonic Acid from Sorbose, US Patent No. 2 190 377 (1954). 6. E.Merck, Procede pour la preparation de 1 acide 2-ceto-l-gulonigue, France Patent No. 829 236 (1968). 7. E.M. Sulman, O.B. Sannikov, A.I. Sidorov, M.V. Avtushenko, A.T. Kirsanov, Method for calcium gluconate synthesis, Russian Patent No. 2 118 955 (1996). 8. C. Bronnimann, T. Mallat and A. Baiker, J. Chem. S o c . - Chem. Comm., 13 (1995) 1377-1378. 9. C. Bronnimann, and Z. Bodnar, and R. Aeschimann et al, Joumal of Catal., 161, 2 (1996) 720-729. 10. T. Mallat, and C. Bronniman, and A. Baiker, Applied Catal. A, 149, 1 (1997) 103-112. 11. T. Mallat, C. Bronniman, A. Baiker, J.Molec. Catal. -A, 117, 1-3 (1997) 425-438. 12. H. Hirae, Macromol. Chem. Suppl., 14 (1985) 55. 13. L. Bronstein, S. Sidorov, A. Gourkova, P. Valetsky, J. Hartmann, M. Breulmann, H. C61fen, M. Antonietti, Inorg. Chim. Acta, 280, 1-2 (1998) 348-354. 14. M.V. Avtushenko, Catalytic oxidation of monosaccharides - Thesis, Tver, Russia, 1996. 15. K. Heyns, Analyt. Chem., 558 (1947) 171-192. 16. E.M. Sulman, Russian Chemical Reviews, 63, 11 (1994) 923-936.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Synthesis of New Neo-carboxylic Acids via Rearrangement and Oxidation of 1,3-Dioxanes + Jutta Fischer and Wolfgang F. H61derich* Department of Chemical Technology and Heterogeneous Catalysis University of Technology RWTH Aachen, Worringerweg 1, 52074 Aachen, Germany I. ABSTRACT Several methods for the preparation of new interesting and valuable compounds 3-alkyloxypivalic acids starting from substituted 1,3-dioxanes were examined. A two-step environmentally benign method could be established using O2 as oxidant in the presence of heterogeneous catalysts. The scope of the method was tested by use of several 1,3-dioxanes and the isolated new pivalic acid derivatives were characterised. 2. INTRODUCTION The isomerisation of 1,3-dioxanes to 3-alkoxypropanals in the presence of various heterogeneous catalysts has been discussed in detail [ 1,2]. Catalysts include zeolites [3], metal oxide impregnated silica [4], acid phophates having a zeolite structure [5] and acid treated metal oxides [6]. Highest activity and selectivity, however, are displayed by pentasil zeolites [7,8]. By employing 5,5-dimethyl substituted 1,3-dioxanes in the isomerisation reaction, pivalic aldehyde derivatives are formed. The corresponding 3-alkoxypropanols or 3-alkoxypropyl amines can be obtained by introducing hydrogen or a combination of H2 and ammonia into the reaction system using metal modified catalysts [9,10]. These neo-compounds can find use in organic synthesis for the preparation of biologically active chemicals. Also, the phthalates of the alcohols can be used as effective plasticisers for PVC. Therefore, it is surprising that the synthesis of 3-alkyloxypivalic acids which could be valuable building units for organic synthesis or lubricants has not been studied so far. Only 3-benzyloxypivalic acid has been mentioned in literature, synthesised from pivalolactone or 2-1ithio-2-methyl-propionic acid as starting materials [11,12] which are expensive and thus unsuitable for synthesis in commercial scale. Also, oxidation of 3-benzyloxypivaldehyde could be achieved with potassium permanganate or with oxygen catalysed by cobalt or manganese salts [ 13]. These oxidations can be environmentally hazardous because of the high amounts of inorganic salts formed and the difficult separation of homogeneous catalysts from the reaction mixture. Thus, it was our aim to find an easy, environmentally benign and generally applicable synthesis route for various 3-alkoxypivalic acids. + This work carried out within the Sonderforschungsbereich SFB 442 was sponsored by the Deutsche Forschungsgemeinschaft to whom we extend our thanks. * to whom correspondence should be addressed

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3. EXPERIMENTAL

3.1 Reaction conditions Batch experiments were carried out in a 70ml steel autoclave tested for pressures up to 100 bar equipped with a glass coated magnetic stirrer and two high pressure valves. Starting materials (0,1g catalyst, lg 1,3-dioxane, 2ml benzene) were given into the autoclave; after sealing the autoclave, oxygen was added while stirring. The autoclave was heated using a heating band and a Eurotherm temperature controller. In case of two-step reactions, oxygen was added after cooling following the isomerisation step. Samples were prepared for GC by adding CH2N2 to form the ester of the acids. Continuous gas phase experiments were carried out in a tube reactor under atmospheric pressure. 2g of catalyst pellets were placed in a fixed bed. A solution of lg 1,3-dioxane in 5g of toluene was introduced into the evaporating zone where the carrier gas entered the reactor. The reactor was placed in a furnace where isothermic reaction conditions were maintained. The reaction mixture was collected in a double-walled glass flask equipped with a reflux condenser. In the two-step reaction, oxygen was introduced via a tube led through the condenser into the flask which was heated using its double wall. Samples were modified by CH2N2 for GC-Analysis. 3.2 Starting materials and catalysts 1,3-dioxanes were prepared by reaction of neopentylglycol with the appropriate aldehydes according to known procedure [14] catalysed by Amberlyst 15~ and purified by destillation under reduced pressure. Starting materials as well as solvents were obtained from Fluka and used without further purification. BMFI-catalysts were kindly provided by BASF company and A1MFI-catalysts from Degussa company. Silica-alumina was bought from Aldrich company. Synthesis of V-MCM41 [ 15] as well as impregnation and other treatments were carried out in our laboratories. For metal oxide impregnation of the catalysts, 10g of zeolite powder were stirred in 50ml of water and the appropriate amount of metal salts (0,159g silver nitrate; 0,263g ammoniumtetravanadate, 1,923g copper nitrate trihydrate) were added. The mixtures were stirred for 30min at room temperature and dried at 60~ under reduced pressure. The materials were calcined at 550~ to form the oxides. 3.3 Reaction products Yields were determined by GC-analysis. Reaction products were characterised by and after isolation by IR- and NMR-spectroscopy. Isolation of the products was achieved by selective extraction. Washing of the mixture with sodium bicarbonate solution isolated acidic components. The aqueous was washed with diethylether and acidified with HC1. The separating solid or extracted with diethylether, which was dried with Na2SO4 and evaporated.

GC-MS reaction solution oil was

4. RESULTS AND DISCUSSION

4.1 Isomerisation of 1,3-dioxanes in oxygen atmosphere In analogy to the "one-pot"-synthesis of neoalcohols and neoamines, our first strategy was introducing oxygen into the reaction system to allow oxidation of the formed aldehyde to the corresponding acidsimultaneously with the rearrangement. This was tried both in batch reactors and continuous gas phase reaction. As a test molecule 2-phenyl-5,5-dimethyl-l,3dioxane was used.

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Conversion according to (1) gives the known 3-benzyloxypivalic acid as a product. O .~

0

1

OH

(l)

2

Problems were anticipated since the isomerisation runs effectively only at temperatures higher than 250~ a temperature favouring decarbonylation of radical species formed during the oxidation of the aldehyde. Furthermore, an ether function in the isomerisation product offers an point of attack for both oxygen and the acid catalyst. In batch reactions, to avoid decarbonylation the reaction was performed under pressure; in gas phase reactions, short contact times at the catalyst were used to suppress consecutive reactions. However, both ways were unsuccessful for the preparation of component 2. In both reaction types, benzoic acid 3 was the main product (selectivity up to 60%) due to cleavage of dioxane I and products to benzaldehyde 4 and subsequent oxidation. Only trace amounts of 2 and no aldehyde intermediate (component 5) could be detected. As by-products, mainly benzoates are formed from the fragments of product degradation. Consecutive reactions are obviously dominating under batch conditions Therefore, the reaction was transferred to the gas phase. A boron containing pentasil zeolite (catalyst A) known as the best catalyst for the rearrangement of the 1,3-dioxanes to the neo-aldehydes under these conditions was impregnated with metal oxides to promote oxidation, such as V205 (B), CuO (C) and Ag20 (D). Additionally, a vanadium containing MCM-41-material was used (E)to reach a more selective oxidation by isolated V-species and to reduce diffusion constraints through its larger pore size, avoiding consecutive reactions. Table 1 Comparison of different catalysts in the isomerisation of 2-phenyl-5,5-dimethyl-1,3-dioxane under oxidising atmosphere; 290~ 15 1/h 02, WHSV = lh -1, TOS = 6h catalyst

conversion 1/ %

selectivity 5/ %

A B D E

69 54 51 100

23 46 65 6

selectivity 2/ % 0 1 2 0

selectivity 4/ % 6 17 10 69

selectivity 3/ % 4 4 6 17

Table 1 shows a low selectivity to component 5 obtained by catalyst A, not to be expected from published results. Presence of oxygen has to be a limiting factor. The metal oxide impregnated catalysts B and D give higher selectivities, but only very low amounts of the corresponding acid 2 are formed. These catalysts are less active due to blocking of some acidic centres by metal oxide. Vanadium seems to favour the formation of benzaldehyde, either because its oxophilic character impedes resorption, allowing consecutive reactions, or by facilitating an electron shift in the starting material by its ease in changing oxidation states. The results obtained with Cu-BMFI (C) are not shown because of complete blocking of the reactor after one hour time on stream (TOS) due to coke formation.

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4.2 Two-step reaction for the synthesis of 3-benzyloxypivalic acid For the aforementioned reasons, 2 had to be synthesised in two steps according to equation (2), separating the high temperature isomerisation from the low temperature oxidation. 0 cat,

1

0

0~, > ~...0~.~

5

OH

(2)

2

In batch isomerisations, catalyst A was studied by variation of several reaction parameters. It was determined that selectivity to the aldehyde drops slowly after 16 hours, while formation of benzaldehyde increases. Furthermore, the combined influence of oxidation and isomerisation catalyst on the oxidation was examined. While the aldehyde is nearly completely oxidised without a catalyst, Ag20 slightly increases conversion. The presence of catalyst A does not impede oxidation. Combining these results, the yield of 2 was optimised up to a selectivity of 48% (table 2). An aluminum containing catalyst (F) led to the formation of higher amounts of benzaldehyde since its stronger acid centres slow down desorption of the formed aldehyde. Catalyst C is less active in the isomerisation because active centres are blocked by CuO. Table 2 Comparison of different MFI-based catalysts in the two-step batch reaction synthesis of 3-benzyloxypivalic acid; isomerisation: 300~ 16h; oxidation: 50~ 20 bar 02, 6h; m (dioxane)/m (catalyst) - 10 for both reactions catalyst F A C

conversion 1/ % 97 70 61

selectivity 5/ % 2 0 0

selectivity 2/ % 27 48 26

selectivity 4/ % 18 8 10

selectivity 3/ % 4 3 5

By selective extraction (section 3.3) all acidic components are isolated. This leads generally to contamination of isolated 2 with benzoic acid formed from benzaldehyde present from the isomerisation step. To avoid this, a combination of gas phase isomerisation and liquid phase oxidation in a semi-continuous reaction mode was developed (section 3.1). As in the case of the batch reactions, reaction parameters in both reaction steps were varied. Optimal isomerisation conditions using catalyst A were found to be about 300~ and a WHSV of lh -1 which is in accordance with data given in the literature[7]. Yields of 5 are about 65%. For the oxidation step, the optimum of temperature and the necessity of a catalyst were the main topics of interest. It was found that a temperature of 80~ offers a compromise between high conversion of the aldehyde and acceptable selectivities to component 2. Several silver and silver oxide containing materials were found to slightly increase conversion (table 3). More important, however, is the enhancement of selectivity using highly dispersed silver on alumina or charcoal, giving about 90% yield of 2 in the oxidation step. Thus, a combination of optimised conditions for both steps yields more than 50% of 2.

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Table 3 Comparison of oxidation catalysts in the semi-continuous two-step synthesis of component (2); 80~ 02 20 l/h catalyst without Ag20 5% mg on A1203 50% Ag20 on A1203 15% Ag on charcoal - _ -. . . . . . . . . . . . . . . . . . : _ - . : . . . . . . . . . . : _ - _ - _ - _ - : _ - _ - . : . : . : : . : . : . : . : _

loading (g 1/g metal _c__0mp0nent)

.:_ .:.:_ _ .: . . . . . . . . . . . . . . . .:.:.:.:.:_-. -_ . . . .

conversion of 5 / %

selectivity to 2/%

90 95 91 96 91

76 65 89 80 92

1200 3000 1200 3000 .:.:.:.:.:_-_-.:.:.:.:.:.:.:.:.:.:.:.:.:_-.:_

_-_ _ .:_. -_ . . . . . . . . . . . . . . .

-_ . . . . . . . .

_-_ _ _ .:.:_-.:.:.:.:.:_

_ .:.:.:.:_-.:.:.:.:_-.:.:.:.:_-

. . . . . . . . . . . . . . . . . . . . . . . . .

_ -. . . . .

_-.:.:.:.:.:.:.:_-.:.:_-_-

. . . . . . . . . . . . . . . . . . . . . . . . . . .

In transferring the reaction to a pilot plant, also the duration of the activity of catalyst A was tested. During 50 hours time on stream (TOS) catalyst activity decreased. Deactivation set in rapidly but proceeded more slowly after a TOS of 20 hours, being stable at 45% during the last 25 hours. Selectivity is very high at 90% and does not change significantly. In the oxidation, a scale up leads to lower conversion of the aldehyde. Reason for this is the relatively lower amount of oxygen brought into the reaction mixture since the reflux condenser could not accommodate a higher gas stream. On the other hand, a higher selectivity to 2 is observed, this way keeping the over all yield (see table 4). Table 4 Scale up of the oxidation; 80~

02 20 l/h, m(dioxane) 9m(catalyst) = 1200, catalyst Ag20

scale

conversion of 5/%

selectivity to 2/%

laboratory (70ml reaction volume)

95

65

scale up (11 reaction volume)

86

84

4.3 Synthesis of new 3-alkoxypivalic acids To examine the scope of the developed reaction route, several 1,3-dioxanes were converted to acids under conditions optimised for I (figure 1):

0 H3C

0

CH3

3-propyloxypivalic a c i d 6

H3C~O.'~~. H3C/ H3C

H3C

0

CH3

~ H3C CH3

CH3

3-butyloxypivalic acid 7

3-isobutyloxypivalic acid 8

OH CH3

O.'~~_..OH H3C CH3

0

3-(2-ethylbutyloxy)-pivalic acid 9

CH3

[ ~

0

3-(2-methylphenylmethoxy)-pivalic acid 10

Fig. 1 New 3-alkoxy pivalic acids prepared by two-step synthesis from 1,3-dioxanes

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Table 5 Conversion of different 1,3-dioxanes to the corresponding 3-alkoxypivalic acids; 300~ carrier gas N2 12 l/h, WHSV = lh "1, TOS = 6h, atmospheric pressure

acid 6 7 8

9 10

e-onve-rsion....... -seieciivity to 6 -- i (JJ/-eonv~e:rsi-0-n~ ili-e" 1,3-dioxane/% % oxidation/% 64 55 82 66 53 88 45 62 86 27 48 85 50 42 61

sdeetMiy t0 6 :-~i0~:~tile .... oxidation/% 89 67 62 66 87

All corresponding acids 6 to 10 could be isolated by the process described in section 3.3 and were characterised by MS, IR and 5. CONCLUSIONS An easy and generally applicable method for the synthesis of 3-alkoxypivalic acids has been developed and five new substances of this class were isolated and characterised. A discontinuous liquid phase oxidation reaction was the best solution for the production of the acids. Good yields of 3-benzyloxypivalic acid can be achieved. Yields up to 40% over both steps could be obtained and the acid could be isolated in 98% purity. In a scale up of the reaction both the isomerisation and the oxidation step gave satisfying results. Employing several 1,3-dioxanes as starting materials, the broad scope of the developed method could be illustrated. This way, a wide variety of pivalic acid derivatives containing different ether groups could be obtained. REFERENCES

[11 [21 [31 [4] [5] [6] [7] [8]

[9] [10] [11] [12] [13] [14] [15]

C.S. Rondestvedt, G.J. Mantell, d. Am. Chem. Soc., 1960, 82, 6419. C.S. Rondestvedt, J. Am. Chem. Soc., 1962, 84, 3319. W.F. H61derich, F. Merger, R. Fischer, EP 0 199 210 B1, (16. 10. 1991), BASF AG, Ludwigshafen. H.-J. Arpe, DE 29 22 698, (11. 12. 1980), Hoechst AG, Frankfurt. W.F. H61derich, F. Merger, EP 0 291 807 B1, (15. 01. 1992), BASF AG. W.F. H61derich, F. Merger, H. Lermer, EP 0 291 806 B1, (26. 08. 1992), BASF AG. M.E. Paczkowski, W.F. H61derich, Stud. Surf. Sci. Catal., 1994, 83, 399. W.F. H61derich, F. Merger, R. Fischer, EP 0 199 210, (29. 10. 1986), BASF AG, Ludwigshafen. W.F. H61derich, M.E. Paczkowski, D. Heinz, DE 196 21704, (30.05. 1996), Hoechst AGRuhrchemie. W.F. H61derich, M.E. Paczkowski, D. Heinz, Th. Kaiser, DE 196 21703, (30.05. 1996), Hoechst AG-Ruhrchemie. U. Luhmann, W. Liittke, Chem. Ber., 1972, 105, 1350. Y-I. Matsushita, Chem. Lett., 1983, 1397. A. Abiko, J.C. Roberts, T. Takemasa, S. Masamune, Tetrahedron Lett., 1986, 27(38), 4537. F.A.J. Meskens, Synthesis, 1981, 501. A.B.J. Amold, J.P.M. Niederer, T.E.W. Niegen, W.F. H61derich, Microporous Mesopourous Mater, 1999, 28, 353.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Low Temperature Radical Oxidation of Butane over in-situ Prepared Silica Species A. Satsuma ~*, N. Sugiyama l, Y. Kamiya 1, T. Kamatani I and T. Hattori 2 ~Department of Applied Chemistry, Nagoya University, Nagoya 464-8603, Japan. 2Research Center for Advanced Waste and Emission Management, Nagoya University, Nagoya 464-8603, Japan.

Silica species prepared by in-situ method using silicon alkoxide as a precursor were found to catalyze the radical oxidation of n-butane at low temperatures around 650 K. The silica species play a role of an initiator, and the reaction propagates in gas-phase. The in-situ hydrolysis of silicon alkoxide also resulted in the formation of active silica species. It was indicated that the active sites were generated from surface silanols on silica surface. 1. INTRODUCTION Silica itself is generally thought to be inactive for catalytic reactions. Although there are some reports about the catalysis of silica [1-5], the catalytic activity of silica is very low and the reaction conditions are much limited in these cases. For example, partial oxidation of methane over silica is achieved only at high temperatures around 8 0 0 - 1000 K [1,2]. It is also reported that the evacuation of silica powder at high temperatures generates catalytic activities for dehydrogenation [4] and photocatalytic oxidation [5]. The requirement of such high temperatures may suggest the difficulty in the activation of silica surface. Previously, we have found that silica species prepared by in-situ method are significantly active for partial oxidation and oxidative dehydrogenation of butane at lower temperatures around 650 K [6]. The introduction of tetraethylorthosilicate (TEOS) vapor into nbutane/Oz/N2 mixture gas (in-situ preparation) resulted in the formation of active silica species and the conversion of butane. The activity of the in-situ prepared silica was comparable to that of (VO)2P207 which is known to be efficient catalyst for the industrial production of maleic anhydride [7]. Although the radical oxidation was suggested, the reaction mechanism of silica species is still unclear. In the present work, roles and active structure of silica species are investigated, and the reaction mechanism over in-situ prepared silica species is discussed. 2. E X P E R I M E N T A L To identify the active silica species, the preparation of silica species and the butane oxidation were performed separately. The continuous flow reaction was carried out in a

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conventional flow apparatus using an empty Pyrex-tube reactor at atmospheric pressure. A mixture of n-butane/O2/N2 (2%/18%/balance) was used as a feed gas at a flow rate of 64.2 cm 3 min ~. The pulse reaction was carried out in the same flow apparatus equipped with a sampling column for the injection of n-butane/O2/N2 mixture (2%/18%/balance, 0.80 cm 3) into flowing He as a carrier gas. The Pyrex-glass reactor was mainly made of capillary glass tube (I.D. 1 mm, O.D. 6 mm) and partly large space for reaction (I.D. 9.6 mm, O.D. 12 mm, and length 100 mm). Silica species were prepared by the following two methods: in-situ preparation and in-situ hydration. For in-situ preparation, tetraethylorthosilicate (TEOS, 0.037%)was introduced in the stream of n-butane/O2/N2 mixture at 643 K for 1 h. For in-situ hydration, 0.25 cm 3 of TEOS was injected in flowing 10%H20/N2 at 643 K. SiO2 (JRC-SIO-8, 303 m2g-1) was supplied from the Committee on Reference Catalyst of the Catalyst Society of Japan. 3. RESULTS AND DISCUSSSION 3.1 B u t a n e O x i d a t i o n o v e r in-situ P r e p a r e d Silica Species Fig.1 shows the conversion of butane and oxygen in the empty reactor in the absence and presence of TEOS. Although the butane conversion was negligibly small in the absence of TEOS, the conversion steeply increased after introduction of TEOS. When TEOS vapor was removed from the feed stream, the butane conversion remained unchanged for longer than 1000 min. It should be mentioned that color of the reactor turned to white after the reaction, suggesting the deposition of silica on wall of the reactor. Based on the assumption that all the loaded TEOS molecules contributed to the butane conversion, turnover number was above 80. Therefore, the oxidation is not a homogeneous reaction in gas-phase but a heterogeneous catalytic reaction over the solid silica species derived from TEOS. 100

,

100

I

',with', ',TEOS',

~. 8O

without TEOS

~ 80 o

I I I

o 60

~ 60

9 ~-,,i

Butane

t:x:rT-o----

o

~ 40

=o 40 9

20 t

m 20

Oxygen

,, I

0

|

500 1000 Time / min

1500

Fig. 1 Conversion of butane ( O ) and oxygen (O) in an empty reactor at 643 K in the absence and presence of TEOS.

600

650

700

Temperature / K Fig. 2 Conversion of butane in an empty reactor ( A ) , over in-situ prepared silica species ( O , O ) , and over SiO2 (E-l).

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Fig.2 shows the temperature dependence of butane conversion. The silica species were prepared in flowing TEOS/butane/O2/N2 at 643 K for 1 h in the empty reactor. Then, the butane oxidation was carried out in the absence of TEOS. The temperature dependence of butane conversion over the in-situ prepared silica was quite unique and entirely different from conventional catalytic reactions. The butane conversion increased steeply from 603 K, reached to the maximum at 643 K, and gradually decreased at higher temperatures. This profile was reproducible on the increase (open symbols) and decrease (closed symbols) in the reaction temperature. The steep increase in the conversion between 603 K and 623 K corresponds to the activation energy of 550-700 kJ mo1-1 which is extremely higher than those in usual catalytic oxidation reactions. Above 643 K, negative temperature coefficient of reaction rate was observed. The butane conversion over SiO2 gradually increased with reaction temperature, but the activity was negligibly smaller than that of the in-situ prepared silica. Table 1 shows the product distribution on the in-situ prepared silica. Butene and acetaldehyde were the main products in the organic molecules. The selectivity to butene increased with the increase in the reaction temperature, while the selectivity to acetaldehyde decreased from 30.0 to 18.4 %. As for the oxygenated products, alcohol, aldehyde, ketone and furan were observed. Maleic anhydride was not produced at all, though this is the main product of butane oxidation over (VO)2P207 [7]. The product distribution was also different from that over SiO2 at 793 K, at which the conversion was comparable to that over the in-situ prepared silica species. Table 1 also shows the product distribution in gas-phase radical oxidation reported by Euker and Leinroth [8]. The product distribution in our experiment was quite similar to that of gas-phase radical oxidation, indicating the butane oxidation over the in-situ prepared silica species proceeds as a radical oxidation in gas-phase. Fig. 3 shows the effect of space volume in the reactor. In this series of experiments, the thicker reactor (I.D. 13.6 mm, O.D. 16 mm, and length 100 mm) was used, controlling the space volume by insertion of glass rod (O.D. 3.0 - 8.0 mm). The butane conversion significantly decreased with the reduction of the space volume. The strong dependence on the

Table 1 Conversion and selectivity in butane oxidation. Catalyst/Condition In-situ prepared silica

a)

SiO2 a) (JRC-SIO-8) 683 793

gas-phase t,j oxidation 598

Temperature / K 623 643 Conversion/% butane 41.7 57.8 25.5 26.5 16.9 Selectivityc / % butene ) 15.9 14.0 36.9 13.1 26.7 C2-C3 olefins 5.4 6.5 8.4 13.3 6.6 acetaldehyde 30.0 20.6 18.4 3.2 14.9 C3 oxygenated d) 6.0 3.9 4.4 0.1 2.1 C4 oxygenated e) 2.1 1.3 2.0 0.9 1.2 furan + dihydrofuran 4.0 3.3 3.1 8.2 7.5 C1-C3 paraffins 0.1 0.2 0.1 0.2 0.9 others 7.2 7.7 8.6 0.4 16.5 CO + CO2 29.3 42.7 18.1 60.5 23.7 a) This work, n-butane/O2 = 2%/18% in Pyrex glass tube, b) reported by Euker et al., nbutane/O2 = 7% /14% in stainless tube, c) 1-butene, cis-2-butene, and trans-2-butene, d) propionaldehyde, acetone, and propanol, e) methyl ethyl ketone, butanol, and crotonaldehyde.

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space volume suggests that the oxidation involves chain propagating steps in gas-phase and the butane oxidation terminates on the l~176 / wall of the reactor. 14.5 cm 3 The negative temperature coefficients of ~-~ 8o reaction rate (Figs. 2 and 3) can be observed in .2 13.8 cm 3 free radical gas-phase oxidation of ~ 6o hydrocarbons at low temperatures, which is o= called "cool flame" [9,10]. The reaction starts ,~ 4 0 from the abstraction of hydrogen from hydrocarbons followed by formation of m 20 peroxide radical [11]. At lower temperatures, isomerization of peroxide radical to a chain 0 600 650 700 branching step containing the formation of Temperature / K aldehydes and ketones is predominant, while olefins are formed through decomposition of Fig. 3 Effect of space volume on peroxide radical as a non-chain branching step at higher temperatures. Thus, these different butane oxidation in silica coated empty reaction routes result in the negative reactor at 643 K in the absence of TEOS. temperature coefficient of reaction rate [9,10]. Based on this reaction mechanism, acetaldehyde is the main product of the chain branching reaction. As shown in Table 1, the selectivity to acetaldehyde decreased with the increase in the temperature, which is in accordance with the reaction mechanism of cool flame phenomena. From the profiles of the catalytic activity and the selectivity of the in-situ prepared silica species, it can be concluded that in-situ prepared silica species act as the initiator for the gasphase radical oxidation, and they act as the sold catalysts on the wall of the reactor.

//

3.2 Structure of Active Silica Species Table 2 shows the effect of the feed gas in the preparation of silica species. The active silica species was generated only in the presence of butane and oxygen. As for the preparation temperature, the active silica was only generated at 603 - 643 K, at which butane converted in the presence of TEOS. These results suggest that the reaction of butane and oxygen is essential for the generation of the active silica species, i.e., any products may contribute to the generation of the active silica species. Furthermore, the active silica species were also generated by the in-situ hydrolysis of TEOS in flowing 10%HaO/N2. The activity and selectivity were the same as that of the in-situ prepared silica species. Fig. 4 shows the initial activity of the silica species generated by the in-situ hydrolysis of TEOS, as investigated by pulse reaction with butane/O2 mixture. Since no induction period was observed in the initial stage of the reaction, there is no contribution of any carbonaceous species to the active species. Thus, the active species should be directly generated from highly hydrated silica surface. The presence of radicals on the silica species was confirmed by introduction of NO as a radical scavenger. The addition of 8.7 x 10.5 mol of NO to the in-situ hydrated silica species suppressed the butane conversion from 30.7 % to 10.4 %. Figs. 5 and 6 show the effect of drying and water vapor at 643 K on the in-situ hydrated

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Table 2 Effect of preparation conditions and catalytic activity of silica species for butane oxidation. Preparation Reaction a) Atmosphere Temperature Conversion /K /% TEOS 643 0.0 butane, TEOS 643 0.0 0 2 , TEOS 643 0.4 b u t a n e , 0 2 , TEOS m 573 0.0 butane, 02, TEOS m 603 9.7 butane, O2, TEOS m 643 57.8

.............

N;6;-

i

iS-g

............

.....................

"i~ 40 O

O

O O o

20

,l,..a

10 0

........

a) Flow reaction at 643 K, n-butane/O2/N2 = 2% /18%/balance, b) TEOS/n-butane/O2/N2= 0.046% /2%/18%/balance, c) TEOS (0.25 cm 3) was fed as a pulse in the stream of 10%H20/N2.

,

I

1

,

I

2

,

3

Amount of feed gas/cm Fig.

4

Initial

activity

of in-situ

hydrated silica species as a function of pulse amount of the feed gas.

silica species. As shown in Fig. 5, both the treatments deactivated the silica species. After the water vapor treatment, the butene conversion immediately recovered in the stream of butane/O2~a mixture at 643 K (Fig. 6). On the contrary to this, the activity slowly recovered after the drying in He. These series of experiments indicate that the highly hydrated surface is required for the butane oxidation, i.e., the active silica species should be generated from silanol species on the silica surface. 50 -~ 40 O

O

..,,~

.,.-i r~

30( ' O o

O o

20

;::I

mlO

m lOO 200 300 Time / min

400

Fig. 5 Butane conversion in pulse reaction over in-situ hydrated silica species as a function of the treatment time in flowing He (O) and 10% H20/N2 (O) at 643 K.

0

100 200 300 400 500 600 Time / min

Fig. 6 Butane conversion in flow reaction over in-situ hydrated silica species after the treatment in drying for 2 h (O) and in water vapor for 12 h (O) as a function of time.

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The activation of ordinary silica is well investigated by Low et al. [ 12]. They reported that ordinary silica can be activated by a procedure consisting of methylation, pyrolysis of the methyl layer, and high temperature degassing. The reactive silica can adsorb oxygen to form radical site, which is active for ring disruption of cyclopropane, benzene and toluene. Although the preparation conditions and the reactivity of the in-situ prepared silica specis are different, it can be expected that the active silica species were generated in the similar way: The elimination of OH or H from surface silanols may produce radical sites for the initiation of the catalytic oxidation and dehydrogenation. 4. CONCLUSION Based on the above results, the catalysis of in-situ prepared silica species can be proposed as shown in Fig. 7. The butane oxidation proceeds over the silica species as follows: (1) The active sites are directly generated from surface silanol species, (2) the reaction is initiated by radical sites derived from silanol species, (3) high butane conversion is achieved by the role of radical species in gas phase.

products

Si(O02H5)4 04Hlo+O2 { ,.,o~ ~, ~k, H20 H ~ ' ' ~ ~ hase~*~ 04H1~

_.

Fig. 7

tiat, on

9termination

wallof reactor

Schematic model of butane oxidation over in-situ prepared silica species.

REFERENCES 1. 2. 3. 4. 5.

S. Kasztelan and J. B. Moffat, J. Chem. Soc., Chem. Commun., (1987) 1663. G.N. Kastanas, G. A. Tsigdios, and J. Schwank, Appl. Catal., 44 (1988) 33. J.N. Armor and P. M. Zambri, J. Catal., 73 (1982) 57. Y. Matsumura, K. Hashimoto, and S. Yoshida, J. Catal., 117 (1989) 135. H. Yoshida, T. Tanaka, M. Yamamoto, T. Funabiki, and S. Yoshida, Chem. Commun., (1996) 2125. 6. A. Satsuma, N. Sugiyama, Y. Kamiya, and T. Hattori, Chem. Lett., (1997)1051. 7. F. Cavani and F. Trifiro in Catalysis vol. 11, Royal Society of Chemistry, Cambridge, (1994), p.246, and references there in. 8. C.A. Euker, Jr. and J. P. Leinroth, Combust. Flame, 15 (1970) 275. 9. N.N. Semenov, Some Problems in Chemical Kinetics and Reactivity, Chap.7, Princeton Univ. Press, Princeton, New Jersey, 1958. 10. I. Glassman, Combustion (Third Edition), Chap.2, Academic Press, San Diego (1996). 11. M. Carlier and L-R. Sochet, Combust. Flame, 25 (1975) 309. 12. M. J. D. Low, E. McNelis, and H. Mark, J. Catal., 100 (1986) 328.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

M e c h a n o c h e m i c a l preparation of V-Ti-O catalysts for o-xylene low temperature oxidation V.A. Zazhigalov l*, J. Haber l#, J. Stoch l#, A.I. Kharlamov 2, A. Marino 3 L. Depero 3 and I.V. Bacherikova I*, 1) Ukrainian-Polish Laboratory of Catalysis: *Institute of Sorption and Problems of Endoecology, National Academy of Sciences of Ukraine, Gen.Naumova 13, Kyiv, 252680, Ukraine #Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul. Niezapominajek l, Krakow 30-239, Poland 2) Institute for Materials Science Problems, National Academy of Sciences of Ukraine,, ul.Kzhyzhanovskogo 3, Kyiv, 250680, Ukraine 3) Structural Chemistry Laboratory, Dip. di Chimica e Fisica per l'Ingegneria e per i Materiali, University of Brescia, 9 Via Valotti, 25123 Brescia, Italy

Abstract Catalysts were prepared by mechanical treatment of the powder mixture of V205 and TiO2 in air, water and ethanol as dispersing medium. When prepared in alcohol they showed already at 573 K 89% conversion of o-xytene with 87% selectivity to phthalic anhydride. This may be related to the increased exposure of (010) plane of V205 and its decoration with Ti ions, as revealed by XRD, XPS and Raman spectroscopic analysis. 1. INTRODUCTION The V-Ti-O oxide system is a catalyst of many processes, including industrial o-xylene oxidation and NOx reduction [ 1,2]. They are usually prepared by one of following methods: i) impregnation of TiO2 with different vanadium compounds, ii) vanadium chemical bonding with functional groups on TiO2, and iii) V and Ti co-precipitation from compounds of Me(OR)n type [2,3], iv) from the powder mixture of vanadia and titania oxides by grinding them in an agate mortar [4]. In this paper a possibility of the catalyst synthesis by a mechanochemical treatment of the V205 and TiO2 powder mixture is considered. 2. EXPERIMENTAL Initial reagents were TiO2 powder (Aldrich, p.a.), V 2 0 5 - p (powder, REACHIM, p.a.) and V205 - m melted in air and then crushed. Powders were thoroughly mixed in an agate

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mortar and then ground for different periods in a planetary mill (3000-rpm). Ethanol or water were used as dispersing medium. Mass of mixed oxides was 25 g, mass of balls 450 g. After the treatment was completed, catalysts were filtered and dried at 393 K. Specific surface area was determined by thermal desorption of argon (GASCHROM-1). A phase composition was found with a Philips MPD 1880 diffractometer (CuKcx excitation, graphite monochromator). Relative line intensity and width (FWHM) were determined with Automated Powder Diffraction Program (version 3.5, Philips). Micro Raman spectra were obtained with Dilor Labram spectrograph (HeNe l a s e r - 632.8 nm). The spectrograph was linked to a microscope for selection of proper analysis spot (resolution better than 1 ~tm). Surface composition was determined with XPS using VG ESCA-3 spectrometer. The spectra were calibrated using C ls line at 284.8 eV; the spectra handling were described elsewhere [5]. Catalytic properties of the samples (fraction 0.25 - 0 . 5 0 mm) were studied in a flow system with steel microreactor loaded with 0.5 cm 3 of catalyst. The contact time was modified by controlling flow rate, of the feed mixture in the range 12-65 cm3/min. Reactions of oxylene oxidation (0.9 vol. % in air) and n-pentane oxidation (1.6 vol. % in air) were selected as test reactions. Outlet gases were analyzed in the on-line system of two chromatographs (Chrom-5) equipped with: a) 2.5 m column filled with 4.3% F-50 on Chromosorb G (temp. program 373-598 K) for analysis (DTP) of products of partial oxidation of o-xylene, b) 2 m column filled with silicagel KSK-2.5 (temp. program 323-393 K) for analysis of CO2 and hydrocarbons, and c) 2 m column filled with molecular sieves CaA (temp. 273 K) for analysis of 02 and CO. Chromatograms were recorded and then analyzed on a PC 486 DX 100 computer.

3. RESULTS AND DISCUSSION The XRD spectra of V205 - p and V 2 0 5 - m revealed the most intense reflection at 2 0 = 20.30 corresponding to the (010) plane. However, while in the first sample the reflection intensity ratio (010)/(110) was near 3/1, in the second one it was about 10 times higher. This indicates that V205 - m was in the form of platelet-like crystals developed along the (010) plane. The XRD spectrum of TiO2 was typical for anatase with strongest reflection at 2| = 25.30 corresponding to the (101) plane. In the case of V2Os/TiO2 mixtures and after their mechanochemical treatment the XRD spectra were a superposition of diffractograms of simple oxides. Simultaneously this treatment changed the intensities of all reflections, initially typical for simple oxide, leaving their FWHM unchanged. For V2Os-m/TiO2 an increase in time of milling did not change the reflection intensity ratio (010)/(101) though it was remarkable larger comparing to V2Os-p/TiO2. This is connected with differences in intensity of the (010) reflections in initial forms of vanadium oxide. In the mixture of V2Os-p/TiO2, after 10 min of the treatment in alcohol, the intensity ratio of (010)/(101) reflections (informing about Ti/V ratio) decreased from about 12 to 6. However, when time of the treatment increased from 10 to 30 min, the ratio rose from 5.9 to 8.3. In water this ratio already after 10 min of the treatment decreased to 10 and remained almost constant after further exposition. Somewhat lower intensity of the (010) reflection after the treatment in water can be caused by partia dissolution of V205 observed already earlier during its mechanochemical treatment [6].

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14001200-

A

,i

v

1000"

1oooo

v

800.rl

600

5000

400

I-4

200

,,9

AI0

~"-"'--'-------'--W T

I

500

1000

15000-

9 ----"-'----~

1000

Wavenumber (cm-l)

A30

W a v e n u m b e r (cm-l)

'

50tl

800A

A

600

1oooo-r,I II1

4-1

5000-

400

F-t

200

o-

r

500

B10

kJ

Wavenumber (cm-l)

500

1000

B30

1ti00

Wavenumber (cm-l) t

Figure 1. Micro-Raman spectra of V205-TiO2 mixture after 10 and 30 min of the mechanochemical treatment in water for different spots A and B. The Raman spectrum of TiO2 presented typical set of bands at 145 (very strong- v.s.), 400, 520 and 645 (v.s.) cm "1. Bands at 155 (v.s.), 205, 295 (v.s.), 307, 405,487, 540, 705 and 1000 (v.s.) cm -l were characteristics of both V205 samples. The spectra of the V2Os/TiO2 mixture before and after mechanochemical treatment were the result of the superposition of those of initial oxides. However, micro-Raman spectra taken for two different points (A and B in Fig.l) of the near-surface region showed that, in the case of the VEOs-p/TiO2 after 10 (Fig.l, A10, B10) and 20 min of the treatment in water, differences in intensity of absorption bands 145/155 and 645/1000 cm 1 in points A and B were reflecting heterogeneity of the sample. After 30 min (Fig. 1, A30, B30) almost equal ratios of the intensity band in these points were observed, indicating homogeneity of the V2Os-p/TiO2 system. On the treatment in

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alcohol, already after 20 min the system was nearly homogeneous, while after 30 min it was fully homogeneous. The treatment of the V2Os-rn/TiO2 system in ethanol after 20 min also gave a uniform component distribution. As follows from XPS data (Table 1), binding energies of electrons in the V2Os/TiO2 system was not much influenced by the mechanochemical treatment being close to that already published [7]. This shows that the oxidation state of elements did not change under the treatment. However, the atomic ratio of surface elements was substantially changed. Treatment in ethanol is increasing the Ti/V ratio, more significant in V2Os-m, while in water this ratio decreased (Table 1). Table 1 Surface properties of the V-Ti-O composition after the mechanochemical treatment Treatment XPS binding energy, eV Composition Medium Time. Ti 2p V 2p O 1s V/Ti V/Ti (XRD) V205 (XRD) min (XPS) (010)/(101) (010)/(11 O) 2

3

4

5

6

7

8

Ethanol Ethanol Ethanol

10 20 30

459.2 459.1 459.1 459.3

517.7 517.6 517.6 517.6

530.8 531.3 531.0 530.9

0.77 0.50 0.56 0.48

0.08 0.17 0.16 0.12

3.1 5.3 4.7 4.5

Water Water Water #

10 20 30

459.2 459.4 459.3

517.6 517.9 517.8

531.0 531.4 531.1

1.10 1.10 0.91

517.4 517.6

531.0 530.9

0.38 0.43

0.10 0.08 0.09 4.76 0.30 0.29

3.0 3.0 3.2 45.5 9.0 25.0

l

#

Ethanol ^ Ethanol ^

10 459.1 20 459.2 ^V205-m. # untreated mixture

The data obtained, including those concerning mechanochemical modification of V205 [6,8] allowed us to assume the following picture of changes accompanying the mechanochemical treatment of the V2Os/TiO2 mixture. As already established the mechanochemical treatment of V205 in ethanol produces anisotropic crystal deformation [6,8] resulting in an increase of the (010) plane exposure. Such change in the contribution of (010) crystal plane was observed in the sample V205-p after 10 min of the treatment (Table 1, Column 8) since the (010)/(110) ratio increased from 3.1 to 5.3. But further exposition unexpectedly brought small reversion of the process, i.e. slight decrease in the vanadium XRD (010)/(110) ratio (Table 1, column 8). Consequently, the XRD (010)/(101) ratio initially increased by the factor 2, mostly as the result of strong development of the (010) vanadium plane. On further treatment this plane intensity ratio was consecutively decreasing from 0.17 to 0.12, partly reflecting increase of the relative vanadium (010) plane content and partly by loss of the vanadium visibility by XRD in highly dispersed oxide. Changes in XRD data (table 1, col. 7 and 8) suggest that comparing TiO2 and V205 oxides, the last one is more susceptible to destruction and is easily dispersed. In conclusion, our results show that prolonged mechanochemical treatment of the V2Os-p/TiO2 composition in ethanol produces a mixture of larger TiO2 crystals and much

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smaller V205 particles. Since the XPS V/Ti ratio remained small, there is no reason to suppose coating of TiO2 with V205, but rather agglomeration of dispersed vanadia should be assumed. Different picture is observed in V2Os-m. Very high XRD (010)/(110) ratio in the starting powder (Table 1, col.8) points to strong preference for the (010) plane. During the treatment these large, initially fiat crystals are crushed vertically to this plane, which reduces its relative exposure, but after some time, gliding of layers along the (010) plane begins to be dominant. Considering the treatment of the V2Os-p/TiO2 mixture in alcohol, the most striking feature is the interruption of evolution of vanadium (010) plane exposure with time of treatment after its initial growth, as opposed to pure V205. With exactly the same treatment, the only reason for the effect was the presence of titanium atoms. On the other hand each inorganic compound dissolves to some extent in every solvent. These trace amounts of both substances are available during the treatment. A hypothesis may be advanced that single titanium atoms or oxygen-titanium polyhedra become incorporated to the V2Os-p lattice, decorating edges or staking-faults and in consequence hindering the gliding of vanadia (010) planes. The surface enrichment in titanium could be detected by XPS which shows the decrease of the surface concentration V/Ti ratio could not be detected by any XRD data because of XRD insensitivity in such systems. When V2Os-m was used, the anisotropic deformation was less extended because of its higher hardness. This is evidenced by the absence of changes in the XPS and XRD composition during the treatment (Table 1, col. 6 and 7). In the presence of water a chaotic destruction of crystals [8] and partial dissolution [6] proceed in V205. As the result, the lowest values of the (010)/(101) reflection intensity ratio and the V/Ti ratio at the surface have been observed.

Table 2. Catalytic properties of the V-Ti-O composition after the mechanochemical treatment Treatment n-Pentane oxidation. T = 573 K o-Xylene oxidation. T = 533 K Time Conversion% Conversion SMA.* SPH. * SMA. SPH. % Medium mol. % mol. % mol. % mol. % # 18 12 3 38 12 48 Ethanol 10 21 34 3 73 61 Ethanol 20 72 8 70 Ethanol 30 23 37 2 83 9 79 Water 10 35 36 12 59 21 48 Water 20 67 16 52 Water 30 54 45 10 69 14 56 Ethanol ^ 10 86 6 85 Ethanol ^ 20 89 3 87 * SMAand SPH- selectivity in maleic and phthalic anhydride formation, respectively, ^V2Osm. # untreated mixture Mechanochemical modification of V-Ti-O samples leads to the change of their catalytic properties (Table 2). The treatment in ethanol did not influence much n-pentane conversion, while the conversion of o-xylene significantly increased. After the treatment in water

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conversion of both hydrocarbons also increased, although the activation energy remained constant and amounted to 126 kJ (30 kcal)/mole for o-xylene and 142 kJ (34 kcal)/mole for npentane. In n-pentane oxidation selectivity towards maleic anhydride increased independently of the environment of treatment, while that towards phthalic anhydride increased only when water was used. On the contrary, in o-xylene oxidation an increase in both activity and selectivity to phthalic anhydride was very pronounced, independently of the nature of dispersing medium. Moreover, high yield of phthalic anhydride was observed already at relatively low temperature of 533 K. The treatment did not change significantly the selectivity to maleic anhydride. This study showed that catalytic properties could be significantly influenced by modification of the catalyst accompanying the mechanochemical treatment. It seems that the mechanism is not directly related to the chemical action, but the chemical action controls the physical process, which produces the more effective product. Concluding, the mechanochemical modification of V-Ti-O compositions enhances their catalytic ability to selective oxidation of n-pentane and o-xylene.

Acknowledgment This work was supported by the Polish State Committee for Scientific Research, Grant 3T09A016 16.

4. REFERENCES V.Nikolov, D.Klissurski, A.Anastasov. Catal. Rev.-Sci.Eng., 33 (1991) 319. C.R.Dias, M.F.Portela. Catal. Rev.-Sci.Eng., 39 (1997) 169. B.Grzybowska-Swierkosz. Appl. Catal., A. 157 (1997) 263. G.Centi, E.Giamello, D.Pinelli, F.Trifiro. J. Catal., 130 (1991) 220. V.A.Zazhigalov, J.Haber, J.Stoch, A.I.Pyatnitskaya, G.A.Komashko, V.M.Belousov, AppL CataL, A, 96 (1993) 135. 6. V.A.Zazhigalov, J.Haber, J.Stoch, A.I.Kharlamov, L.V.Bogutskaya, I.V.Bacherikova, A.Kowal, SolidState Ionics., 101/103 (1997) 1257. 7. B.M.Reddy, B.Chowdhury, I.Ganesh, E.P.Reddy, T.C.Rojas, A.Fernandez. J.Phys. Chem., B., 102 (1998) 10176. 8. V.A.Zazhigalov, A.I.Kharlamov, I.V.Bacherikova, G.A.Komashko, S.V.Khalameyda, L.V.Bogutzka, O.G.Byl, J.Stoch, A.Kowal, Teoret. Experim. Khim., 34 (1998) 180.

1. 2. 3. 4. 5.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

A Common Concept Accounting for Selectivity in Mild and Total Oxidation Reactions: the Optical Basicity of Catalysts. P. Moriceau, B. Taouk, E. Bordes and P. Courtine D6partement de G6nie Chimique, ESA 8067, Universit6 de Technologie, B.P. 20529, 60205 Compi6gne, France. Correlations are settled between the "optical basicity", A, which is defined as the electron donating (accepting) power of the lattice oxygen (cation) of the solid, and its equivalent for the gaseous phase: the ionization energy of the molecules. The optical basicity of any catalytic oxide is easily calculated and depends on the cation(s) valence and coordination. The difference between the ionization energies of reactant and of product, AI, accounts for the selectivity which is driven by the catalyst. Linear relationships AI = aA + b are drawn by gathering reaction-to-product/selective catalyst couples. The a and b constants depend on the nature of the reactant (paraffin, olefin, alcohol) and on the type of selective reaction (mild oxidation/oxidative dehydrogenation, ammoxidation, total oxidation). Some theoretical considerations are proposed to justify the linearity of the correlations, which permit to select a priori a selective catalyst for a given reaction.

1. INTRODUCTION Several properties of oxide catalysts able to selectively oxidize organic compounds have been used in the past to set up correlations with catalytic properties. In our research group, we are mostly working on the role of structural properties and solid state reactivity in selective oxidation, because the way the oxygens of the solid participate to the reaction is strongly related to them [1, 2]. At least two steps depend on the characteristics (strength, energy, spatial display) of the metal-oxygen bonds: the activation of the C-H bond of the reactant by surface nucleophilic 02- specie and the insertion of oxygen into the intermediate [3-6]. The selectivity of the process depends mostly on this second step, the catalyst oxygen being found in the oxygenated product (case of mild oxidation, MOx), and/or in water (case of oxidative dehydrogenation, ODh). At least two types of nucleophilic oxygens (linked to the oxidizing cations) are therefore needed in mild oxidation reactions [3]. We have searched for a parameter which could account for the specific action of each solid catalyst in a given reaction. The aim of this paper is to show that all reaction/catalyst couples can be related to a common parameter ruling selectivity, the 'optical basicity' of the solid. The optical basicity, A, of an oxide represents the electron donating power of the lattice oxygen (or the electron acceptor power of the associated cation). It is widely used

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in inorganic chemistry and allows to foresee the progress of reactions [7], including acidbase and redox systems. The optical basicity can be calculated for any formula and varies with the valence, coordination and spin state of the cation [7-9]. We have shown recently [10] that the optical basicity of an oxide which is selective in a given reaction is related to the difference between the ionization energies of the organic molecules of the reactant and of the product. This difference, AI, is assumed to represent selectivity, its absolute value accounting for the electron exchange during the considered reaction which is ensured by the selective catalyst. The former study in which linear relationships were drawn for mild oxidation of saturated and unsaturated hydrocarbons [10] is completed in the present paper by other types of reactions involving oxygen.

2. RESULTS AND DISCUSSION 2.1. Optical basicity Experimental values of the optical basicity of ionic oxides were first obtained by Duffy [7] from the energy of ultraviolet absorption bands. The adaptation of this concept to more covalent oxides was done by Lebouteiller and Courtine who provided data of optical basicity A of transition metal cations and oxides [9, 10]. The calculation of A for a mixte oxide Axa+Byb+On2- is simply made by using the equation A = 1/2n (aXAA + byAB), where a and b, AA and AB are the valence and optical basicity of A and B, respectively. The optical basicity ranges from 0.30 (CO2, acidic) to 1.7 (Cs20, basic). Going from acidic to basic catalysts, the trend is as follows (A in parentheses): H4PMollVO40 (0.48) < (VO)2P207 (0.49) < WO3 (0.51) < MoO3 (0.52) = Fe2P207 < V205 (0.63) < ~-CoMoO4 (0.64) < V6013 (0.68) < Mg2V207 (0.72) < Fe203 (0.77) < Bi2MoO6 (0.86) < NiO (0.91) < Cu20 (0.98) < Ag20 (1.25), and for some common supports: SiO2 (0.48) < A1203 (0.60) < ZrO2 (0.71) < TiO2 (0.75) < MgO (0.78) < SnO2 (0.87). An acidity/basicity scale of oxidic compounds is therefore obtained, in which any combination of cations can find its place. For example, the optical basicity of PMol2Valr0.02Cu0.1Co0.aK0.3On which is claimed in a patent [11] to be selective in the oxidation of isobutane to methacrylic acid is A = 0.55. This example gives the opportunity to point out a drawback of optical basicity which is a theoretical parameter [7, 9]. To calculate the oxygen stoichiometry (n = 44) and A, the valence and coordination of cations have been supposed to be the highest. This is only an assumption because their actual values in operating conditions are not known. In cases where A varies strongly with valence and/or coordination, like for 6-coordinated molybdenum (Mo6+: A = 0.52; MoS+: A = 1.17), the A value may be far from reality [10].

2.2. Correlations between A (catalyst) and AI (gaseous or fiquid molecules). Each selective catalyst is responsible for the fact that a given reactant molecule is transformed into a definite product because of its electron donating-accepting capacity. For example, (VO)2P207 is used to oxidize n-butane to maleic anhydride [12] (AI = I IRIpI - 0.27 eV) but not to butadiene (AI = 1.46 eV). It is not selective in the oxidation of,

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e.g., propane to acrolein (AI = 0.85 eV). When supported on TiO2, V205 is selective in the oxidation of o-xylene to phthalic anhydride (AI = 1.44) [13], not in the oxidation of toluene to benzaldehyde (AI = 0.67 eV) [14]. Various reactions of MOx and ODh have been considered and the corresponding reactant-product/catalyst couples characterized by their {AI,A} values [10]. By plotting AI against A, two straight lines were obtained which gathered saturated (mild oxidation of 'paraffin', line MOP) on the one side and unsaturated (mild 'olefin' oxidation, line MOO) hydrocarbons on the other (Fig.). The less demanding the reaction, the more numerous the catalysts, so that for a same AI several values of A may appear. For example, in the ODh of but-l-ene to butadiene (AI = 0.51 eV), A of Bi2MoO6, FeSbO4, USb3Ol0 [15] and Sb2Oa/SnO2 [16] have been considered. This is true also for ODh of C2-C4 alkanes. When only the alkyl group is modified (e.g., toluene-benzaldehyde, o-xylene-phthalic anhydride, etc.), the {AI,A} of unsaturated hydrocarbons fit the line MOP obtained for paraffin activation. The thermodynamic method we use, which consists in looking at the initial and final states and not in considering the mechanism, is the explanation. For the latter reason, ethylbenzene-styrene or methane-ethylene fits MOP line, although a carbon layer or radicals are supposed to be involved in the reaction, respectively. 5

4

3

2

MOO

~

llr/ll

MOA

AP "%4IN, hiP r

0.4

I

I

I

I

0.6

0.8

1

1.2

A

Figure. Correlations between [A(I)[ and A for combustion of paraffinic bonds (full squares, line CP) and olefins (open squares, line CO), mild oxidation of alcohols (open lozenges, line MOA), of paraffins (full triangles, line MOP) and of olefins (open triangles, line MOO), and ammoxidation of paraffins (full circles, line AP).

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Following the same methodology, we have examined other types of reaction, like ammoxidation of hydrocarbons, mild oxidation of alcohols and total oxidation. In the latter case, the correction of AI by the np/n R ratio has been systematically applied (n R and

np are carbon number in reactant and product, respectively). Linear relationships AI = aA + b have again been obtained (Fig.). The range of AI and A, the a and b coefficients and reliability factors R 2 are gathered in the following Table. Table. Characteristics of linear relationships between AI and A obtained for various reactions Reactions AI range A range AI = aA + b Reliability (eV) a b factor R 2 'Alkane' Oxidation 0.18-2.00 0.48-0.91 4.09 - 1.73 0.94 'Olefin' Oxidation 0.06-1.22 0.48-1.25 - 1.51 + 1.90 0.93 Alkane Ammoxidation 0.11-0.34 0.49-0.93 0.58 - 0.15 0.81 Alcohol Oxidation 0.02-0.60 0.53-1.03 1.11 - 0.56 0.95 Alkane Combustion 1.26-4.90 0.87-1.07 19.39 -15.72 0.96 Olefin Combustion 3.20-4.54 0.85-0.93 18.94 - 12.95 0.89 During the ammoxidation of hydrocarbons in which NH3 is added to 02, the surface lattice oxygens are known to be responsible for selectivity [17]. The ammoxidation of propane to acrylonitrile and toluene to benzonitrile with various multicomponent catalysts based on molybdenum or vanadium (Fig., line/kiP) has been mainly considered here. The mild oxidation of Cl to C6 alcohols to aldehydes or ketones is presented (Fig., line MOA). For example, Mn304 is selective in MOx of propanol to acetone (AI = 0.91 eV) as well as of allylic alcohol to acrolein [ 18], which has the same value of AI. Finally, selective combustion reactions have been considered. Because of the large contribution of CO2 in the calculation of AI, the slopes of the combustion of paraffins C1 to Ca (line CP) and of olefins C2 to C4 (line CO) are very steep. Real selective catalysts for combustion are not so numerous and the range of A is small (Table). Perovskites like LaCoO3 (A = 0.88) are known as good catalysts for the total oxidation of isobutene. Another very good catalyst is MnO2 which has the same value, A = 0.88.

2.3. Attempts to justify linear correlations between AI and A. In a study of the ionization of aromatics condensed molecules owing to the action of various cation-exchanged faujasites, Richardson [19] used the charge transfer theory to express the number of cation radicals Ni + obtained by ionization of N molecules i as a function of their ionization energy Ii and of the electron affinity Ac of the catalyst cation: l n ~ , 9-A~+W-IikT

(k is Boltzman constant and T is temperature)

(1)

(W is the dissociation energy of the excited state of the charge transfer complex). In selective oxidation the redox mechanism is widely used. During the first step, the

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reactant R adsorbed on the oxidized form KO of the catalyst is transformed into the product P, which desorbs from the reduced form K of the catalyst. Eq. (1) becomes: lnN~ AKo+WR--IR and NR kT -

I-N~-AK+Wp-IP .U~aakT

(2)

These equations may be rewritten as a function of I to make AI appearing: AI =IR-IP =--AWR-P -AAKo-K + k

NR -In NP ln~-~-R

=--(~KO-K +AWR-0 +kl~ lnN--~++R +InNP / L NP NR J

(3)

where AAK-KO = AKo-AK and AWR_p = WR-Wp respectively. If the ratio Np/NR is considered to be constant at the steady state, eq. (3) is reduced to: AI =IR--IP =C lnN--~++d NP

(4)

where d =-AAKo-K-AWR-P +kT lnNP is constant for the considered reaction/catalyst NR couple and c = kT is little varying (T = 623-923 K). This equation already shows that a relationship exists between AI and a characteristic of the catalyst, which is the electron affinity of the free ion in the present case. On the other hand, according to Duffy [7], there is a linear relationship between A and the ratio of concentrations of the oxidized and reduced cations in the case of (ideal) MZ§ redox systems, which can be applied to K/KO: In-[Mz+[M(Z")--kpA+q

becomes ln[~K~]=~A+I3

(5)

This type of relationship is valid provided that the reduced and oxidized forms belong to the same phase, which can be assumed to be the case on the surface of a catalyst. It is not yet possible to find an equation by combining eqs. (4) and (5) but we think that, by this approach, the empirical relationships AI = aA + b should be validated in the near future. 3. CONCLUSION

The above results have been obtained by using a thermodynamical method in which only the initial and final states of the reaction are considered, in something similar to "standard" conditions since temperature is not taken into account. As a consequence, nothing is said about the mechanism (radicals, carbanions, etc.), the activity or the number and display of active sites on the surface. However, the method is powerful and allows to correlate the Lewis-type basicity/acidity of the reaction (by means of AI) with acidity/basicity (by means of A) of the selective catalyst. Linear relationships AI = aA + b can be drawn by plotting AI against A, each one characterizing a type of reactant (paraffin-type, unsaturated hydrocarbons, alcohols) involved in a type of reaction (mild

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oxidation and oxidative dehydrogenation, ammoxidation, total oxidation). The slope a may be positive or negative, and steep or not, depending on the relative Lewis acidity of the reactant and of the product, and on the extent of oxidation: total combustion is steeper than mild oxidation. The physical meaning of the {AI,A} correlations by use of the charge transfer theory is in progress. As a consequence, it becomes possible to use these correlation lines to search for new catalytic formula that would be selective in a chosen reaction. The main drawback of the method is that assumptions have to be made on the coordination, valence and spin state of the cation(s) in real operating conditions.

Acknowledgement C. Pham and Elf-Atochem are thanked for helpful discussions and funding.

REFERENCES P. Courtine, in: Solid State Chemistry in Catalysis, Eds. R.K. Grasselli and J.F. Brazdil, ACS Symp. Series, 279 (1985) 37. E. Bordes, in: Elementary Reaction Steps in Heterogeneous Catalysis, ed. R.W. Joyner and R.A. van Santen, Kluwer Academic Publ., 1993, 137. V. D. Sokolovskii, Catal. Rev.-Sci. Eng., 32 (1990) 1. J. Haber in: Solid State Chemistry in Catalysis, Eds. R.K. Grasselli and J.F. 4. Brazdil, ACS Symp. Series, 279 (1985) 3. M. Che and A.J. Tench, Adv. Catal., 32 (1983) 1. J. Haber and M. Witko, J. Molec. Catal., 9 (1980) 399. J. A. Duffy, Geochim. Cosmochim. Acta, 57 (1993) 3961. J. A. Duffy, J. Non Cryst. Solids, 86 (1886) 149. A. Lebouteiller and P. Courtine, J. Solid State Chem., 137 (1998) 94; A. Lebouteiller, Ph.D. thesis, Compi6gne 1997. 10. P. Moriceau, A. Lebouteiller, E. Bordes and P. Courtine, Phys. Chem. Chem. Phys., 1 (1999) 5735. 11. Mitsubishi Rayon Co. Ltd, T. Kuroda, O. Motomu, JP 04,128,247, 1992. 12. F. Cavani, F. Trifir6, Appl. Catal. A General, 88 (1992) 115. 13. M. Gasior, I. Gasior and B. Grzybowska, Appl. Catal., 10 (1984) 87; B. Grzybowska, Catal. Today, 1 (1987) 341. 14. J.E. Germain, R. Laugier, Bull. Soc. Chim. Fr., (1972) 2541. 15. B.C. Gates, J.R. Katzer and G.C.A. Schuit, in: Chemistry in Catalytic Processes, McGraw-Hill, 1979, 366. 16. F. Sala and F. Trifir6, J. Catal., 41 (1976) 1. 17. R. K. Grasselli and J.D. Burrington, Adv. Catal., 30 (1981) 133. 18. B. Grzybowska, J. Haber and J. Janas, J. Catal., 49 (1977) 150. 19. J. T. Richardson, J. Catal., 9 (1967) 172. .

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Copper species in CuCIz/3c-AI203 catalyst for ethylene oxychlorination M. Garilli ~, D. Carmello a, B. Cremaschi a, G. Leofanti at, M. Padovan a*, A. Zecchina b, G. Spoto b, S. Bordiga b, and C. Lamberti b ~EVC Italia, Inovyl Technology Centre, Via della Chimica 5, 1-30175 Porto Marghera, Italy bDipartimento di Chimica IFM, Via P. Giuria 7, 1-10125 Torino, Italy CuCI2 on 7-alumina, the base catalyst for oxychlorination reactions, has been investigated by several techniques as a function of Cu loading, sample aging and heating. The results point out the formation of a surface Cu aluminate species and a highly dispersed CuCI2. The latter slowly hydrolyzes under aging, but CuCI2 is restored under heating and is the active species in oxychlorination as demonstrated by activity tests. 1. INTRODUCTION Nowadays almost all the total world production of vinyl chloride is based on cracking of 1,2-dichloroethane, in its turn produced by catalytic oxychlorination of ethylene with hydrogen chloride and oxygen: C2I-I4 + 2HC1 + 89O2 ---) C2H4C12 + H20, [ 1]. The reaction is performed at 490-530 K and 4-6 atm, using both air and oxygen in fluid or fixed bed reactors. Commercial catalysts are produced by impregnation of alumina with CuCI2 (4-8 wt% Cu) and, preferably, also with other chlorides (mainly alkaline or alkaline earth ones) in a variable concentration. The other chlorides are added in order to improve the catalytic performances making the catalyst more suitable for the use in industrial reactors. In spite of several papers published since 1973 the problem to identify the species present on the catalyst is not fully resolved, even in the case of the basic catalyst [2-7], i.e. alumina supported CuCI2 without additives. The aim of our contribution is threefold: i) the identification of the species present at different Cu content, ii) the determination of their concentration and iii) the investigation about the possibility of their transformation during catalyst life.

2. EXPERIMENTAL This study has been performed on a set of seven CuClz/alumina samples (having a Cu content of 0.035, 0.25, 0.5, 1.4, 2.3, 4.6 and 9.0 wt% respectively) prepared by impregnation of 7-allumina (Condea Puralox SCCa 30/170, surface area: 168 mZg-1, pore volume: {).50 cm3g -~) with an aqueous solution of CuClz'H20 following the incipient wetness method. Samples were examined in different conditions: i) immediately after impregnation of alumina, ii) after 1 day and 6 months aging at RT, iii) under heating up to typical oxychlorination reaction temperature (500-550 K. To fulfil this task, different complementary techniques such as XRD (Siemens D500), UV-Vis-NIR DRS (Perkin-Elmer Lambda 9), EPR (Varian E 109), EXAFS (EXAFS1, beamline at LURE DCI with a Si(331) monochromator), solubility test in ethyl alcohol and activity test in ethylene oxychlorination (pulse method) have been adopted. Present address: Via Firenze 43, 20010 Canegrate (Milano), Italy * Present address: Villa Mirabello 1, 20000, Milano, Italy

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3. RESULT AND DISCUSSION

3.1. Fresh catalyst: low Cu loaded samples As far as the fresh catalyst is concerned, the main results can be summarized as follows. At low Cu content the formation of an insoluble surface aluminate species takes place, where isolated Cu n ions occupy octahedral vacancies of alumina surface. The chlorine released by CuC12 during its interaction with alumina reacts with the support giving rise to >AI-C1 species as documented by the constant CI/Cu ratio = 2 as found by elemental analysis. The formation of surface aluminate has been evidenced by UV-Vis, EXAFS, EPR and solubility test, where the experimental features are very similar to those observed on chlorine free samples prepared from nitrates, for which the formation of the surface aluminate is well documented (see [8-11 ] and refs. therein). In particular, UV-Vis spectra are characterized by a low intensity d-d 2Eg --> 2T2gtransition at about 13000 cm l, typical of an octahedral Cu n species containing chemically equivalent ligand, and by a single CT band in the 40000-43000 cm -~ range (Fig. 1). These features are not affected by drastic thermal treatments (up to 923 K), able to remove all the chlorine from the sample. EXAFS analysis (performed on the 1.4, wt% sample) indicates that Cu n ions are surrounded by five oxygen ligands at about 1.92 A and discards the presence of C1 in its first coordination shell (see section 3.2). No evidence of a significant Cu-Cu second shell is observed, indicating that we are dealing with a dispersed phase. EPR spectra exhibits an axial (gxx = gyy ~- g• and gzz = g//) symmetry (Fig. 2). The spectrum of the 0.035 wt% loaded sample is typical of isolated Cu H species, where the hyperfine splitting into quartets, due to the interaction between the unpaired electron and the copper nucleus (3/2 nuclear spin), is clearly visible. By moving to the 1.4 wt% sample, a progressive loss of the resolution occurs, particularly relevant in the perpendicular component. This is the clear manifestation of spin-spin relaxation phenomena. The data extracted from the solubility test (vide infra Fig. 4) indicate that the maximum achievable concentration of surface aluminate is 1.6 wt% Cu, corresponding to 0.95 wt% Cu/100 m 2 and to 1.06 wt% CI/100 m 2. These values represent the maximum capacity of the alumina surface to accommodate Cu and C1 ions deriving from copper chloride complexes or, in other words, its saturation point. 3.2. Fresh catalyst: high Cu loaded samples At higher Cu concentrations excess copper chloride precipitates directly from solution during the drying in an amorphous phase, overlapping progressively the surface aluminate as pointed out by solubility test, UV-Vis and EPR. The first argument in favor of this hypothesis is the presence, at high loading, of a consistent fraction of soluble Cu. In fact, copper chloride is the only Cu compounds, among chlorides, oxychlorides, hydroxychlorides and oxides, having an appreciable solubility. Moreover, the soluble fraction increases linearly with Cu concentration (vide infra Fig. 4, A data): this is the expected behavior if, after completion of surface aluminate, the excess CuCI2 precipitates from solution inside the pores during the drying process, without interaction with the support surface. As far as UV-Vis is concerned, samples with Cu concentration higher than 2.3 wt% show the development of a second CT band in the 28000-31000 cm -1 range, progressively overlapping the band in the 40000-43000 cm -1 range present in the low copper concentration samples (Fig. 1). The consistent difference (about 9000-11000 cm -~) between the CT of the two species (surface aluminate and supported CuC12.H20) clearly indicates that a partial substitution of oxygen ligands occurs by passing from low to high concentration species. This model agrees with the Jorgensen empirical equation [12] (E(CT) = 30000 [Zopt(ligand ) - Z opt(metal)I, in cm -1) since the difference in optical electronegativity between chlorine (Zopt = 3.0) and alternative ligands (O, Zopt3.2-3.3"

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OH-, ~opt 3.5; HzO, ~opt3.5) gives a difference between the corresponding CT band maxima of 6000-15000 cm -1. A further proof of the presence of CuClz'H20 is given by the abrupt change in intensity of the d-d transition band observed at copper loading _=_ 2.3 wt% and by the comparison with the spectrum of unsupported CuCIz'H20 (Fig. 1): note that the high intensity of the d-d band of CuC12"HzO is due to the presence of two chemically different ligands (C1 and O) in the coordination sphere of Cu n. In fact, the spectra of 9.0 wt% loaded sample and of hydrated CuCI/ are very similar in the whole range, although the former is less resolved, probably because of the disorder induced by the support.

~~'l

I

'

.

9"%~

10

.. j

.

ee

: :

Z'~176 I

o t

~

......

'

Cu9.0

~

,

9

...

Cu4.6

...

9

Cu2.3 ~

Cu1.4 CuO.5

Cu0.25 Cu0.035

0

-

20000

Wavenumber

40000

9

( c m -~)

Fig.1. Full lines: UV-Vis DRS spectra of 1 hour aged samples. From top to bottom: 9.0, 4.6, 2.3, 1.4, 0.5 and 0.25 Cu wt% loaded samples. Dotted line correspond to the spectrum of CuC12.H20

!

2500

9

3000

,

!

3500

B (Gauss)

9

40(30

,

Fig. 2. 77 K EPR spectra of all samples. The amplitude of the EPR signal of all samples has been multiplied by different factors for graphical reasons. Before cooling, samples were evacuated at RT up to 10-3 Torr.

The presence of a second C u II species is also supported by EPR data, as shown in Fig. 2 where, by moving to high loaded samples, any vestige of the hyperfine structure is progressively lost, being totally absent at 9.0 wt%. This sample shows a broad EPR spectrum, similar to that obtained on copper chloride treated under similar conditions (not reported), and typical of an isotropic g tensor (gxx = gyy gzz- giso) [3-5]. It is worth recalling that XRD gives no useful information for the identification of the structure of such species since the XRD pattems of all samples are identical to that of bare alumina. This implies that the species present at high concentration must be in an amorphous state, or in form of nanoclusters having a size falling in the undetectable region of the diffraction technique (less than about 20-30/k). =

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3.3. Aged catalyst

The behavior of the sample upon aging depends on Cu concentration. At copper content lower than 0.95 wt% Cu / 100 m2 support, the Cu surface aluminate, is stable: no changes have been observed in UV-Vis, EPR spectra and in results of solubility tests. Over the alumina adsorption capacity, Cu chloride slowly hydrolyses under aging, forming various insoluble copper hydroxochlorides, depending mainly on drying conditions. In any case the final product of hydrolysis is crystalline paratacamite, as evidenced by UV-Vis, XRD, EPR, EXAFS and solubility test. The most relevant modifications in UV-Vis spectra induced by aging can be summarized as follows: i) the intensity of the d - d transition band undergoes a strong decrease; ii) the component at 40000-43000 cm -~ in the CT region, clearly visible in the as prepared sample, disappears upon aging. The resulting spectrum is very similar to that of paratacamite (spectra not reported for brevity). This evolution is not associated with loss of chlorine (as pointed out by the elemental analysis). Also XRD shows the progressive development of paratacamite diffraction lines. An evolution with time has also been observed by EPR: a progressive gain in resolution is observed upon aging. This reflects the evolution from amorphous, highly dispersed, CuClz'2H20 to crystalline paratacamite, i.e. the evolution from a strong heterogeneous Cu lI environment to a much more homogeneous one (spectra not reported). The upper curve reported in Fig. 3 is the radial distribution obtained from the EXAFS data of CuClz'2HzO model compound. This spectrum is dominated by two peaks (at 1.49 and 1.94 A without phase correction) due to two oxygens at 1.95 /~ and two chlorines at 2.29 /~ and will be used to evaluate, in a qualitatively way, the average fraction of oxygen and chlorine atoms in the first coordination shells of copper in the different samples (middle spectra in Fig. 3). For the 1.4 wt% Cu loaded sample (dotted line), quantitative EXAFS data analysis gives rise to N=4.8(_+0.5) and r(CuO)=1.92(_+0.02)/k, and is interpreted on the basis of a dispersed surface aluminate phase. On the contrary, 9.0 wt% sample (dashed line) exhibits a o " much broader first shell peak, clearly shifted at 9.0 wt% A k o higher R values (maximum at 1.63 A and shoulder at 1.85 A) indicating a coexistence of both Cu-O and Cu-C1 contributions from different scattering atoms located at different distances in three different phases (surface aluminate, CuC12"H20 and paratacamite). For this reason, quantitative EXAFS ' ) RiA) results can not be safely extracted. The 2.4 wt% sample (full line) is in an intermediate situation, Fig.3 k3-weighted, phase uncorrected being more close to the 1.4 wt% one. The above FT of the EXAFS functions for 6-m. picture is further supported by the results obtained aged samples and CuC122H20 from solubility test, which allows to measure quantitatively the copper chloride content of each sample. Fig. 4 gives an overall view of the behavior of the different samples under aging. In the graph the copper solubility is reported vs. the total Cu concentration. Full line represents the reference, i.e. the value corresponding to a hypothetical total solubility of the copper as CuCI2. Dashed, dotted and dot-dashed lines represent the solubility data of 1 hour, 1 day and 6 months aged samples respectively. At each abscissa value, the difference between

CuCI2"2~20

;

I

O

9

heated / / ........1.4wtVo ~_ / \ ___2.3wt~176

--aT

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the measured line and the reference line represents the value corresponding to the insoluble fraction. In freshly prepared samples the insoluble Cu (0.95 wt% Cu/100 m1) is due to the surface aluminate species, hosted in octahedral sites of y-alumina surface. At each abscissa value, the difference between the solubility measured at two aging times (1 h-1 day; 1 day-6 months) gives the amount of paratacamite formed during the corresponding period. After 1 day aging, copper chloride is almost absent in the 2.3 wt% sample, but it is still present on

8

,~-_ 8-

A 1 hour aging /

:

'6d~Yonging

...'~

/

~6

_._= 4 oor)

O. 0

0

2

4

6

8

Cu concentration (%)

Fig.4. Dependence of copper solubility as a function of both Cu content and aging. To verify the linearity of the reported trend for 6-months aged samples, a 6.8 wt% Cu loaded sample has been added.

9

m

0 2-

m

9

9

m

m

_-,ooO _oooOO'" ooooO -

[]

] mo~ ~176176 OUU'v" 0

m mm

m m

.

-

2

mmm mmm

9

-o 6-~ ~ > r-o4-

v

0

o

10

Pulses

20

Fig.5. Ethylene conversion to 1,2dichloroethane versus the number of pulses on 9.0, 4.6 and 1.4 wt% samples: m, 9 and A symbols respectively. Symbols and O refers to 9.0 and 4.6 wt% loaded samples normalized by "active" Cu concentration, see text.

samples with higher Cu content. In the tbllowing months, the formation of paratacamite continues: however, even after 6 months, the copper chloride is not completely transformed into the hydroxochloride. This suggests that, once the basic sites of alumina are consumed by HC1 (formed in the chloride hydrolysis), the reaction stops or, at least, becomes much slower. 3.4. Heated catalyst

Samples with low Cu content i.e. samples containing only surface Cu aluminate, are not affected by thermal treatments, even at temperatures as high as 923 K. On the contrary, in high loaded samples the alumina releases partially the fixed HC1 under heating and the reverse transformation of paratacamite into copper chloride can take place as evidenced by XRD, UV-Vis, EXAFS and solubility test. Temperature dependent XRD measurements indicate that paratacamite starts to disappear at a temperature comprised between 383 K and 438 K and that it approaches the completion at 493 K. UV-Vis DRS spectra (not reported), collected on high Cu loaded samples heated at 523 K and re-exposed to air for 2 hours to rehydrate the chloride are fully comparable with those of the fresh samples (see Fig. 1). Thermal activation implies a loss of the resolution of the hyperfine signal of EPR spectra, progressively appeared upon aging up to 6-mounths. This experimental evidence reflects the evolution from a Cu n environment with long range ordering (crystalline paratacamite) to an highly heterogeneous one (amorphous, highly dispersed, anhydrous CuClz). EXAFS data of

I

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the 9.0 wt% sample aged and heated (Fig.3 bottom curve) indicates that heating removes nearly all the oxygen from the first coordination shell of Cu (only the small fraction of surface aluminate being not affected) and gives rise to a quantitative data analysis fully compatible with the local structure of anhydrous CuC12:4 chlorine atoms at 2.26 A.

3.5. Working catalyst From what learnt in the previous sections, in the oxychlorination reaction environment (ethylene, 02 and HCI) paratacamite can not be present and only surface aluminate and copper chloride or products arising from the interaction of these compounds with reactants or reaction products are present on the catalyst. Activity tests have evidenced that only CuCIz is the active phase and that the reaction takes place by C1 donation from CuClz to ethylene with formation of CuCI, and dichloroethane. In order to explore the reaction mechanism, we have performed depletion tests, where ethylene is sent on the sample in absence of HC1 reactant. In such a way, the only chlorine source available for the C2H4 conversion into CzH4Clz, is on the catalyst itself. The total ethylene conversion obtained on samples with 9.0, 4.6, 1.4 wt% Cu are reported in Fig. 5 (solid symbols) as a function of the number of pulses. We observe that only catalysts containing CuCI2 (9.0 and 4.6 wt% Cu) are active. The increment of conversion measured by moving from 4.6 to 9.0 wt% Cu catalyst is more than directly proportional to the increment of total copper. This fact is not surprising since we have demonstrated that the fraction of Cu forming the surface aluminate (1.6 wt% Cu for our support having 168 m2 g-L) is inactive: this means that the fraction of active copper species in samples 4.6 to 9.0 wt% Cu loaded is only 3.1 and 7.5 wt% Cu respectively. By re-plotting the activity curves of these samples renormalized by factors 1/3.1 and 1/7.5, we see that they overlaps rather well, see Fig. 5 open symbols, giving a further proofs of the validity of our model. We are strongly indebted to G.L. Marra (EniChem, Istituto G. Donegani) for XRPD measurements, to G. Vlaic (Sincrotrone Trieste and Dipartimento di Chimica Universit?~ di Trieste) and F. Villain (LURE) for their relevant and friendly support during EXAFS measurements, to G. Turnes Palomino and E. Giamello (Dipartimento di Chimica IFM Universit~t di Torino) for EPR measurements and related enlightening discussion and to B. Cremaschi (EVC, Porto Marghera) for her analytical support.

REFERENCES 1. M. Garilli, T.L. Fatutto, F. Piga La Chimica e l'Industria, 80 (1998) 333. 2. A. Arcoya, A. Cortes and X.L. Seoane, Can. J. Chem. Eng., 60 (1982) 55, 3. P. Avila,, J. Blanco, J.L. Garcia-Fierro, S. Mendioroz, and J. Soria, Stud. Surf. Sci. Catal., 7B, 1031 (1981). 4. A. Baiker, D.Monti, and A. Wokaun, Appl. Catal., 23 (1986) 425. 5. L. Garcia, D. E. Resasco, Appl. Catal., 46 (1989) 251. 6. G. Zipelli, J.C. Bart, G. Petrini, S. Galvagno, and C. Cimino, Z. Anorg. Allg. Chem., 502 (1983) 199. 7. J. Rouco; Appl. Catal. A 117 (1994) 139. 8. P.A. Berger, and J.F. Rooth, J. Phys. Chem., 71, (1967) 4307. 9. B.R. Strohmeier, D.E. Leyden, R.S. Scott Field and D.M. Hercules, J. Catal., 94(1985)514. 10. M.C. Marion, E. Garbowski, and M. Primet, J. Chem. Soc. Farad. Trans., 86, (1990) 3027. 11. R.M. Friedman, G. J. Freeman, and F.W. Lytle, J. Catal., 55, (1978) 10. 12. C.K. Jorgensen, Progr. Inorg. Chem., 12 (1979)101.

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Studies in Surface Science and Catalysis 130 A. Corrna, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

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A study of the main path and of side-reactions oxychlorination over CuCl2=Al203 based catalysts.

upon

ethylene

A. Marsella l, D. Carmello l, E. Finocchio 2, B. Cremaschi l, G. Leofanti 3, M. Padovan 4 and G.Busca2 EVC Italia, Inovyl Technology Centre, Via della Chimica 5, 30175 Porto Marghera, Venezia (Italy) Fax -39-041-2912685; e-mail: [email protected] Dipartimento di Ingegneria Chimica e di Processo, Universit/t di Genova, P.le J.F. Kennedy, I-16129 Genova, Italy. Fax: 39-010-3536028, e-mail: [email protected] 3 present address: Via Firenze 43- 20010 Canegrate (MI) (Italy) 4

present address: Via Villa Mirabello 1-20125 Milano (Italy)

CuC12/A1203 based ethylene oxychlorination catalysts have been characterized by using IR spectroscopy of the surface hydroxy-groups and of adsorbed pyridine and CO2. The ethylene oxychlorination reaction to 1,2-dichloroethane and the dehydrochlorination of ethylchloride (chosen as a probe molecule) have been investigated by using both pulse reactor measurements and in-situ IR experiments in static conditions. Experiments performed over doped alumina and doped CuC12/A1203 confirmed that the exposed support surface can be responsible of the dehydrochlorination of 1,2-dichloroethane (EDC) to vinyl chloride monomer (VCM), with a consequent loss in selectivity in the oxychlorination reactor. Doping alumina with MgC12 and in particular with KC1 limits the activity of the bare support in this side-reaction. 1. INTRODUCTION. The heterogeneously catalyzed gas-phase oxychlorination of ethylene to 1,2-dichloroethane (ethylene dichloride, EDC): H2C=CH2 + 2 HC1 + 8902 ---> C1-CH2-CH2-C1+ H20 followed by the dehydrochlorination of EDC represents the main way for the production of vinyl chloride monomer (VCM) and of its polymer polyvinyl chloride (PVC). The catalysts currently used are based on CuC12 supported on alumina and doped with other metal chlorides. Several studies have been devoted to the characterization of the industrial and model catalysts, mostly regarding the characterization of the Cu centers. This study summarizes the results of i) the characterization of the overall surface of both undoped and doped CuC12/alumina catalysts by IR spectroscopy of adsorbed probe molecules; and ii) reactivity in oxychlorination and dehydrochlorination (the most important side reaction) by in situ IR spectroscopy and pulse microreactor experiments.

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2. E X P E R I M E N T A L

The samples under study are summarized in Table I. Table I. Samples studied. Sample Composition Impregnating salt notation A 7-A1203 " -1CuC1A 1% Cu / y-A1203 CuC12.2 H20 5CuC1A 5 % Cu / y-A1203 " 9CuC1A 9 % Cu / y-AI/O3 " 5CuKC1A 5 % Cu - 2.8 % K / y- CUC12.2H20 + KC1 A1203 5CuMgC1A 5 % Cu - 1.7 % Mg / CuC12.2H20+ y-A1203 MgC12.6H20 KC1A 2,8 % K/y-A1203 KC1 MgC1A 1,7 % Mg / y-A1203 MgC12.6H20 9 at 503 K

Surf. area Ethylene EtC1 mZ/g Conv. Conv. * 174 0.09 20.8 188 9.19 8.2 154 41.24 7.6 117 51.56 5.1 130 1 165

5.1

140 160

4.6 5.6

The preparation procedure has been described in Ref. 1,2. FT-IR spectra have been recorded by a Nicolet Magna 750 instrument, using conventional IR cells with NaC1 windows, connected to an evacuation / gas manipulation apparatus. The sample powders were pressed into self-supporting disks and outgassed before the interaction experiments. Catalytic reactivity tests on ethylene conversion and ethylchloride have been carried out on a pulse microreactor system as reported in Ref. 1,2.. In the case of dehydrochlorination reaction, the reactants were injected in the microreactor with the following sequence: ethylchloride, HC1 and oxygen at 483 and 503 K. The injection of HC1 and oxygen is required in order to avoid catalyst deactivation (reduction) by the oxychlorination of ethylene produced upon ethylchloride dehydrochlorination. 3.RESULTS 3.1. Ch ar acte riz ation.

The surface of the catalysts has been characterized by pyridine and COz [1] adsorption experiments and by t h e i R analysis of the OH groups (fig. 1) in order to obtain complete informations on acidic and nucleophilic sites and on the coverage degree of the support surface. The analysis of the OH stretching region shows for the catalysts at quite high CuC12 loading (5% and 9% Cu content) the presence of paratacamite-like particles, characterized by two IR main bands at 3455 and 3360 cm l. These bands disappear in the range 523-623 K, temperature at which paratacamite particles decompose, leaving a disordered bulk-like copper chloride species. Impregnation with CuC12 even at 9% (amount sufficient to complete the

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monolayer) still leaves traces of free alumina OHs. On 5CuKC1A and 5CuMgC1A doped catalysts small but detectable amount of paratacamite particles are still detectable at 523 K and a band due to free OHs of uncovered alumina (3730 cm 1) persists, together with a very strong, broad band centerd at 3500 cm l and due to H-bonded OHs. Doping seems to increase the thermal stability of paratacamite-like structure. The obtained results show that the support surface is still exposed in part on the active oxychlorination catalyst, also in the case of the 9CuC1A sample, nominally containing a "full monolayer loading" of CuC12. Pyridine adsorption on the three CuC1A catalysts surface (spectra not reported for sake of brevity) shows that Cu chloride species have medium-strong Lewis acidity. On the sample at highest CuC12 loading, a band centered at 1608 cm 1 has been identified, due to pyridine strongly interacting with Cu centers. C02 adsorption also points out the presence of some "free" alumina surface nucleophilic centers even in the 9CuC1A sample. A study of the effect of pretreatment at high temperature with HC1 shows that HC1 itself is nearly totally dissociated on the catalyst surfaces, and does not give rise to significant amounts of Bronsted acid sites. We can also note that the nature of Lewis centers exposed (A13§ and Cu 2§ coordinatively unsaturated) is not substantially changed. Impregnation with CuC12 and doping with MgC12 and KC1 kills most of alumina nucleophilic sites (exposed oxide anions).

1.8 Ab so rb an ce

1.6 1.4 1.2 1.0

_b

4000

3500 3000 Wavenumbers (cm-1)

2500

Fig. 1. FT-IR spectra of OHs groups: bulk paratacamite (a), 5CuC1A outgassed at 473 K (b) and 523 K (c), 5CuMgC1A out. at 523 K (d), MgC1A out. at 523 K (e), A1203 out. at 523 K(f).

3.2. Study of the oxychlorination activity. The oxychlorination reaction was studied by "in-situ" IR in static conditions [ 1]. The disapperance of the reactants ethylene and HC1 and the appearance of EDC was monitored in the gas phase while, simultaneously, the surface species were also detected. On the pure alumina support there is no evidence of ethylene adsorbed species at room temperature while on copper containing catalysts the formation of IR bands at 1544, 1424 and 1275 cm 1 assigned to copper-ethylene complexes is observed. Acetate and formate species coordinated on Cu centers are observed at 473-523 K, likely involved in total combustion of ethylene.

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We could also observe the oxychlorination process in the IR cells starting from the 9CuC1A catalyst pretreated with HC1 and ethylene. At 573 K gaseous 1,2 dichloroethane, characterized by IR bands at 1235 and 725 cm l , is observed in the gas phase. This study allowed us to propose a mechanism for the activation and oxychlorination of ethylene over the CuC1JA1203 samples, that implies a role of a Cu-ethylene complex as an intermediate in the selective way and of formate species as intermediates in the competitive combustion of ethylene. Pulse oxychlorination catalytic studies show that alumina is essentially inert towards ethylene conversion and that the sample with very low copper loading 1CuC1A is already significantly active although not very selective to EDC because of the formation of byreaction products such as VCM, 1.1.2 trichloroethane, trichloroacetaldehyde, trichloromethane. By increasing Cu loading, the ethylene conversion increases almost proportionally, while the selectivity increases more.

3.3. Study of the main side-reaction: dehydrochlorination. The dehydrochlorination activity of the catalysts and of the bare support has been investigated by both FT-IR and pulse reactor experiments using EtC1 as probe molecule (table I) [21. At room temperature IR studies point out the presence of adsorbed EtC1 over pure alumina surface (fig. 2). The IR spectrum shows bands at 1451, 1382, 1395 cm -l (CH bending modes), 1303, 1286 cm -I (CH2 wagging mode), 1250 cm -I (CH2 twisting mode)and 1075 (CH3 rocking mode) with the corresponding CH stretching bands in the high frequency region. After heating, a complex absorption in the region 1200-1000 cm -~ grows progressively up to 523 K. These bands, associated to the CH3 deformation modes at 1447 and 1388 cm t, can be attributed to the C-C and C-O stretchings of alkoxy groups, they disappear at 573 K, just when we start to detect ethylene in the gas phase. In the same temperature range the bands of adsorbed EtC1 have also fully disappeared. The spectra recorded after treating at temperatures higher than 573 K show bands due to a new strongly adsorbed species: a couple of weak bands centered at 1570 and 1472 cm -~ is formed, due to a strongly adsorbed species. They are assigned, in agreement with results of acetic acid adsorption experiments, to the asymmetric and symmetric -COO stretching of acetate species. In the gas phase features due to ethylene and HC1 are observed starting from 573 K. Diethylether is also detected at lower temperatures as by-product (band at 1146 cm -~ in the gas phase), while CO and CO2 start to be formed in traces at the highest temperatures. These data show that alumina is highly active in the dehydrochlorination of ethyIchloride to ethylene + HC1. IR studies show that two main pathways for the production of ethylene over Cu-free alumina are possible, depending on the chlorination of the catalyst surface. The first reaction path (scheme I) requires the formation of an alkoxy species at the surface involving a coordinatively unsaturated A1 cation coupled with two exposed oxide anions. A nucleophilic substitution occurs on ethylchloride to give ethoxy species. The desorption of ethylene and HC1 restores the active site. A by reaction occurring during this process is the nucleophylic substitution of ethoxy groups on ethylchloride giving rise to diethylether.

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H3C

H2C-CH 2

H--CH 2

\

H2C\ O=

Cl

/ I

~

V AI3+

O=

O-

H2C l

250 ~ O

Q T 3+ AI

CI"

"OH

J

CI" AI3+

Scheme I. Dehydrochlorination activity on pure alumina. It seems evident that HC1 inhibits the mechanism because it interacts with the Lewis acid sites of alumina. On the other hand, this inhibition by HC1 is only partial and reversible. On chlorinated alumina (alumina pre-treated with HC1) another "concerted" elimination mechanism is proposed without the intermediacy of the alkoxide. The interaction of ethyl chloride with 9CuC1A sample shows that up to 373 K the ethyl chloride adsorption is mostly non reactive, while alkoxides become predominant at 473 K; starting from this temperatures we observe the growth of a couple of intense bands near 1590 and 1440 cm -l, assigned to acetate species. In the presence of CuC12 alkoxy ions are formed more slowly than on the pure alumina, and can also be oxidized extensively to acetates by the copper ions, that are consequently reduced. The behavior of the MgC12- (fig. 3) and KCl-doped CuC12-A1203 catalysts is, qualitatively, very similar to the one observed on the corresponding supports, MgC12- and KCl-doped alumina. The only evident difference is that IR bands of acetate species are stronger than bands found in the spectra of all CuC12-A1203 catalysts. This provides evidence that the main dehydrochlorination activity occurs on the support, but oxidizing reactivity typicall occurs on cupric centers.

,, ;:'~

,

0.2

0,4

"t/tl _

18oo

~

nc~,l ~---,-_0-"'~"~,,,..._

~~.___S<~':

o,o

,I--

16oo

14oo

12oo

Wavenumbers Fig. 2. A1203 sample. FT-IR subtraction spectra of the adsorbed species arising from ethylchloride adsorption at room temperature (a), 423 K (b), 473 K (c), 523 K (d), 573 K (e), 623 K (f) and after heating and outgassing at 623 K (g).

! Fig. 3. 5 ~ sample. FT-IR subtraction spectra of the surface species arising from ethylchloride adsortpion at room temperature (a), at 373 K (b), 423 K (c), 473 K (d), 523 K (e), 573 K(f), 623 K (g)

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The comparison of the results obtained with CuC1A, 5CuKC1A and 5CuMgC1A samples shows that MgC12 slightly increases the stability of ethoxides while KC1 strongly increases their stability. Our data indicate that the treatment of alumina with HC1, or the presence of Cu, K and Mg chloride species at the alumina surface, more or less hinder the dehydrochlorination reaction activity of the support. This occurs by reducing either the number and / or the reactivity of the surface oxide species involved in the conversion of EtC1 into ethoxy groups (this seems to occur by treatment with HC1 and by the presence of CuCl2), or by stabilizing these ethoxy groups as less reactive forms (this seems to occur mainly for catalysts doped with KC1 or MgC12). Following these data the site reactivity scale for EtC1 to ethylene is:

bare A1203 > MgC12 on A1203 >> CuC12 on A1203 >> KC1 on A1203 the last species being still inactive at 623 K. As for the Cu-containing catalysts, by combining the above site reactivity scale the dehydrochlorination activity apparently follows the trend: CuC12 on A1203 > MgC12 -CuC12 on A1203 >> KC1-CuC12 on A1203 So, it seems likely that on CuC12-A1203 and on MgC12-doped CuC12-A1203 the support is acting as the catalyst for EtC1 dehydrochlorination, while, on KCl-doped CuClz-AI203, CuCl2 sites are likely acting as the catalyst for EtC1 dehydrochlorination. The pulse reactor studies suggest that the most active alumina sites are actually destroyed already at the lowest CuClz coverages of 1CuA. CO2 adsorption experiments actually showed that, in spite of the very low theoretical monolayer coverage of this sample (9 %), 3 1 % of the nucleophilic CO2 adsorbing sites (the strongest ones) are already destroyed [ 1]. The activity of CuClz-type sites too in dehydrochlorination justifies the almost identical conversion of EtC1, measured on surface area basis, on the three undoped CuA samples. In fact, the activity of the increasing CuClz-type sites can balance the loss of the weakest alumina sites by increasing Cu loading. 4. CONCLUSIONS. The present data show that the active sites for ethylene oxychlorination on CuClz/AI203 based catalysts are cupric chloride centers. The selectivity to EDC depends on the CuC12 coverage mostly because the uncovered support surface can act as a dehydrochlorination catalyst, converting EDC to VCM, which is an undesired product. In fact, VCM can be oxychlorinated to 1,1,2 trichloroethane in the oxychlorination reactor and further transformed to others by-productsOur data suggest that doping CuClz-AlzO3 oxychlorination catalysts with MgCI2 and, mainly, KC1 has the beneficial effect of reducing the dehydrochlorination activity of the support, so increasing the selectivity to EDC and decreasing the production of VCM and of its following oxychlorination products. So, part of the beneficial effect of such dopants is, more than influencing the CuCI2 active sites, in poisoning the unwanted activity of the alumina support. 5. REFERENCES. 1. E. Finocchio, N. Rossi, G. Busca, M. Padovan, G. Leofanti, B. Cremaschi, A. Marsella and D. Carmello, J. Catal. 179 (1998) 606 2. D. Carmello, E. Finocchio, A. Marsella, B. Cremaschi, G. Leofanti, M. Padovan, and G. Busca, J. Catal. in press

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Oxidative Dehydrogenation of Ethane on Lithium Promoted Oxide Catalysts Shaobin Wang, K. Murata, T. Hayakawa, S. Hamakawa and K. Suzuki Department of Surface Chemistry, National Institute of Materials and Chemical Research, Tsukuba, Ibaraki 305, Japan Several lithium chloride catalysts supported on metal oxides such as alumina, silica, titania, zirconia, and sulfated zirconia (SZ) were prepared and tested for the selective oxidative dehydrogenation of ethane into ethylene. It is found that LiCl-promoted oxide catalysts show an excellent performance in the selective oxidation of ethane. The catalytic activity depends on the nature of the support, lithium content, and reaction temperature. LiC1/SiO2, LiC1/TiO2 and LiCI/SZ catalysts exhibit high ethane conversion and ethylene yield. However, LiC1/SiO2 and LiC1/TiO2 show rapid deactivation while LiC1/SZ can maintain a relative long-term stability. 1. INTRODUCTION The oxidative dehydrogenation (ODH) of ethane into ethylene is a potentially important process to produce ethylene instead of the thermal cracking of ethane. So far, a great deal of efforts have been made to seek more effective catalysts, especially those exhibiting both high selectivity to ethylene and good stability in this reaction. Alkali metal and alkaline-earth metal oxides, transitional metal oxides, rare-earth metal oxides and other catalytic materials have been investigated for this reaction in the past decade [1-3]. Several researchers have found that LiC1 supported on some metal oxides such as MgO [4], ZnO [5], and NiO [6] exhibited high activity for the oxidative dehydrogenation of ethane and methane coupling to C2 compounds. Wang et al. [4] reported that they obtained 75% ethane conversion and 77% ethylene selectivity over a Li +MgO-C1- catalyst at 620 ~ after 50 h on stream. We have also reported that LiC1 supported on sulfated zirconia catalysts showed a much better activity and selectivity to ethylene in the oxidative dehydrogenation of ethane [7,8]. In this work, we prepared some LiC1 catalysts supported on other metal oxides such as A1203, SiO2, TiO2 and ZrO2 and investigated their catalytic performance in this reaction. 2. EXPERIMETAL Three commercial catalyst supports, A1203, SiO2, and TiO2, were obtained from Wako Chemicals. ZrO2 was prepared by precipitation of ZrO(NO3)2 using NH3.H20 at pH=10. The deposits were firstly dried at 105 ~ overnight and then calcinated at 300 ~ for 3 h. The sulfated zirconia (SZ) samples were prepared by wetness impregnation on the fresh ZrO2 using a (NH4)2SO4 solution at an appropriate concentration to reach 6 wt% sulfate, dry at 105 ~ and calcination at 700 ~ for 3 h. LiCl-doped catalysts were prepared by impregnation of lithium chloride on the above supports at various lithium loadings and calcination at 700 ~ for 3 h. The BET surface areas of the catalysts prepared were determined by N2 adsorption at -196 ~

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on a volumetric adsorption equipment provided by Shimadzu. XRD measurements were conducted on a Philips PW 180 X-ray diffractometer at 40 kV and 40 mA. The temperatureprogrammed reduction (TPR) experiments were carried out in a fixed-bed reactor. 0.5 g samples were loaded in the reactor and underwent a heat-treatment in a Ar gas flow of 30 ml/min to 700 ~ at a heating rate of 5 ~ and further treated at the same temperature for 30 min. After that, the temperature was reduced to the ambient temperature under the same gas flow. And then a 10%H2/Ar flow with a flow rate of 30 ml/min was fed into the reactor and a heating program was started to increase the temperature to 700 ~ at a heating rate of 3 ~ The concentration of H2 in the flow was determined by a GC-8A equipped with a TCD. The selective oxidative dehydrogenation of ethane was performed at atmospheric pressure in a fixed-bed vertical-flow reactor constructed from a high-purity alumina tube (i.d.= 6 mm) packed with 1 g catalysts and 2 g quartz sand, mounted inside a tube furnace. The reactant stream consisting of 10% ethane, 10% oxygen and 80% nitrogen was introduced into the reactor at a flow rate of 60 ml/min. The reaction temperature varied between 500-650 ~ The products were analyzed by two gas chromatographs (Shimadzu GC-8A) equipped with a Porapak Q column using a FID for analyses of hydrocarbons and a 5A molecular sieve column for CO, CO2, CH4, 02, N2, and H2 using a TCD. Before catalyst testing, the empty tube loaded only with quartz sand was evaluated for the oxidative dehydrogenation of ethane under various conditions. It was found that the homogeneous contribution was negligible because ethane conversion was less than 2 % at 650 ~ 3. RESULTS AND DISCUSSION Table 1 presents the surface areas, catalytic conversions and product distributions over the catalysts in the oxidative dehydrogenation of ethane. It is seen that metal oxides are active catalysts for this reaction; however, they show varying activities. A1203, ZrO2 and SZ exhibit high ethane conversions over 40% but low ethylene selectivities around 10%. The main products over these oxides are carbon oxides (CO and CO2) with selectivities over 80%. Although ethane conversions over SiO2 and TiO2 are lower (20%), the selectivities towards ethylene are both higher than 50% with the ethylene yield at 14.3% and 15.2%, respectively. In terms of ethylene yield, catalytic activity of the tested oxides follows an order of TiO2 ~SiO2 > SZ > ZrO2 ~ A1203. Table 1 The performances of LiCl-doped oxide catalysts in ODH of ethane at 600 ~ SBET Conv(%) Selectivity(%) Catalyst (m2/g) C2H6 02 C2H4 CH4 CO CO2 A1203 181 49.7 100 8.6 4.6 42.1 44.4 SiO2 348 20.0 24.6 71.6 1.8 19.1 7.3 TiO2 9.0 28.6 100 53.0 0.3 0 46.7 ZrO2 21 51.6 100 10.8 0.3 24.5 64.4 SZ 72.5 42.3 69.7 16.6 2.2 38.6 42.6 3.5wt%LiC1/A1203 50.5 64.4 100 41.4 2.5 31.0 16.3 3.5wt% LiC1/SiO2 0.1 99.1 86.4 79.4 1.7 16.0 2.6 3.5wt% LiC1/TiO2 0.7 76.6 66.3 84.6 1.3 7.8 3.8 3.5wt% LiC1/ZrO2 4.6 48.8 69.9 54.6 0.6 7.3 33.7 3.5wt% LiC1/SZ 2.0 62.4 50.2 82.5 0.5 5.4 8.5

C2HsOH 0.09 0.2 0 0 0 7.9 0.1 1.7 3.1 2.4

Yield(%) C2H4 4.2 14.3 15.2 5.6 7.0 26.7 78.6 64.8 26.6 51.5

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After doped with lithium chloride on these oxides, catalytic conversion of ethane and ethylene selectivity on the LiC1/oxide catalysts are generally improved. LiCI/SiO2 shows the highest ethane conversion of 99% with a 80% ethylene selectivity. LiCI/TiO2 exhibits a medium value of ethane conversion and the highest ethylene selectivity ca. 84%. For LiC1/A1203 and LiCI/SZ, ethane conversions and ethylene selectivities are all enhanced. Unlike the behavior of other LiCl/oxide catalysts, LiC1/ZrO2 shows an unchanged ethane conversion as compared with the value of the support; however, ethylene selectivity is enhanced from 10% to 55%. From the viewpoint of ethylene production, one can see that the order of the LiC1/oxide catalysts shows the same variation as that of the oxides. Among these LiCl/oxide catalysts, LiCI/SiO2, LiC1/TiO2 and LiCI/SZ are the better ones, which give ethylene yield higher than those over the Li+-MgO CI [4] and Li/La/CaO [9] ever reported at the similar conditions. The catalytic performance of the three active catalysts, LiC1/SiO2, LiC1/TiO2 and LiC1/SZ at 3.5 wt% Li loading, as a function of temperature is shown in Figure 1. It is seen that ethane conversion and ethylene yield over all three catalysts increase as the temperature increases. However, ethylene selectivity exhibits a different behavior. Ethylene selectivity firstly increases with the increasing temperature and then decreases due to the over-oxidation of ethylene at high temperatures. The temperature at which ethylene selectivity exhibits a decreasing trend differs among the three catalysts. This behavior is a typical characteristic of the halides-containing catalysts in the oxidative dehydrogenation of ethane, due to the fact that the formed ethyl radicals at high temperatures desorbe and form ethylene in the gas phase via reaction with 02 [2]. 100

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Li content

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Fig.2 Effect of Li loading on catalytic activity. (a) L i C l / S i O z, (b) L i C l / S Z .

The effect of LiC1 content on catalytic activity was also investigated. Figure 2 presents the catalytic performance of LiCI/SiO2 and LiCI/SZ catalysts with varying Li loading at 600 ~ As shown, addition of LiC1 on SiO2 greatly increases the ethane conversion and ethylene selectivity. The maximum of ethane conversion and ethylene yield can be achieved at 3.5 wt% Li on

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LiCI/SiO2 catalysts. Although ethane conversion is reduced when LiC1 is introduced into SZ at 2 wt% Li, ethylene selectivity is remarkably increased, resulting in the enhancement of ethylene yield. A LiC1/SZ catalyst at 3.5 wt% Li can produce the maximum of ethane conversion and ethylene yield. High loading of Li increases the surface basicity and thus suppresses the active sites, resulting in a lower activity. Therefore, the optimal LiCl loading on LiC1 modified oxide catalysts is 3.5 wt% Li in terms of ethane conversion and ethylene yield. Figure 3 demonstrates the catalytic performance of the three most effective catalysts at 3.5 wt% Li content as a function of time at 600 or 650 ~ One can see that LiC1/TiO2 and LiC1/SZ catalysts exhibit the same ethane conversion at 650 ~ as that of LiC1/SiO2 at 600 ~ however, LiC1/SiO2, LiC1/TiO2 and LiC1/SZ display different patterns of deactivation. LiC1/SiOe and LiC1/TiO2 deactivate rapidly in the initial 6 h, ethane conversion decreasing from 99% to 20%. Thereafter, LiC1/TiO2 maintains its activity without a significant decrease in conversion and yield whereas LiC1/SiO2 still shows deactivation but at a relatively lower rate. For LiC1/SZ, ethane conversion and ethylene yield decrease gradually within the initial 15 h and then keep the values nearly unchanged thereafter. Ethane conversion and ethylene yield are reduced from 98% and 68% to 70% and 46%, respectively, while ethylene selectivity keeps at the same level during 24 h testing. 100

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Fig.4 X R D p a t t e r n s of various I.i/oxide catalysts before and and after reactio~ (a) fresh catalysts, (b)used cata_lysts.

The XRD pattems of the fresh and reacted LiC1/oxide catalysts prepared in this investigation are presented in Figure 4. Monoclinic and tetragonal ZrO2 are the major phases in LiC1/ZrO2 and LiC1/SZ catalysts. A small amount of Li2ZrO3 was also found in two catalysts; however, the

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intensities of Li2ZrO3 peaks are weaker on LiC1/ZrO2. For LiC1/AI203, Li20 is present apart from the major crystallites 7-A1203. Surprisingly, the peaks for LiCl were not identified in the above LiCl-doped oxide catalysts. This is probably due to the lower amount in amorphous phase or well dispersion of LiCl in catalysts. TiO2 and LiCl coexist in LiC1/TiO2, while four phases, quartz, LiESiO3, LiESiO5 and LiCl, are present in LiC1/SiO2. The XRD patterns of the used catalysts, LiC1/TiO2 and LiC1/SiO2, display different profiles from those of the fresh catalysts, indicating that a phase transformation occurred on these catalysts during the reaction. However, there is no significant phase change on the LiC1/SZ. From Fig.4, it is seen that the intensities of the XRD diffraction peaks of the used LiC1/SiO2 are weaker, suggesting the occurrence of decrystallization. No LiC1 peaks can be found, indicating the loss of LiC1. For LiC1/TiO2, a larger amount of Li2TiO3 is formed after reaction and no LiC1 was detected. It has been found that Li2TiO3 exhibits a much lower activity in the oxidative dehydrogenation of ethane. Therefore, the phase transformation and leaching of LiC1 will account for the deactivation of the LiCl-based catalysts. Figure 5 shows the TPR profiles of the LiCl/oxide catalysts. It is seen that these . . . . . . . uc~s~o~ catalysts demonstrate a different reduction behavior. There is a weak reduction peak LiCl/TiO2 II LiCI/AI203 I between 400-500 ~ in the TPR profile of ---LiCllZrO2 I LiCI/A1203. One weak reduction peak = uc~/sz t~ I appears at 500-630 ~ in the TPR profile of z" .o / LiCI/ZrO2. The reduction of LiC1/TiO2 E J happens at high temperatures around 500 ~ ~, t- = / / while a strong reduction occurs after 600 ~ 8 _ ~ _ .....on LiC1/SZ. For LiCI/SiO2, there are two ~ . . . . . ~ _- -. -~ . ~ ~ ---- ~ ~ weak peaks at lower temperatures and a ~ . . . . . . . . . . . _. . . . . . . strong peak at high temperatures in the TPR .... profile centered at 250, 450 and 660 ~ . . . . . . . . respectively. These results indicate that ~oo zoo 300 400 soo soo 700 LiC1/SiO2 and LiC1/SZ can produce m o r e Temperature(~ oxygen species, which will promote the Fig.5 TPR patterns of various Liloxide catalysts. activation of ethane. It is believed that catalytic activity in the oxidative dehydrogenation of ethane has a close relationship with the acid-base and redox properties of the catalysts. If the surface basicity of a catalyst was very large, strong CO; adsorption would occur on the catalyst surface, resulting in low catalytic activity. For LiCl-promoted catalysts, the acidity/basicity will depend on the support. It is know that SZ and SiO2 are acidic supports, TiO2 and ZrO2 show weak acidity while A1203 is a basic oxide [1]. Therefore, it may be deduced that lower acidity of LiC1/A1203 is attributed to its high basicity while high activity of SiO2 and SZ supported catalysts promotes the contact of ethane and the catalyst surface. The TPR results also demonstrate that LiC1/SiO2 and LiC1/SZ show high reducibility, which may also account for their higher activity. In addition, CI ions in these catalysts also play an important role. CI ions can prevent the active sites from CO2 poisoning and lower the reactivity of oxygen species to ethylene oxidation [10,11]. Chlorine radicals are also thought to favor the homogeneous decomposition of ethyl radicals to ethylene. All these will contribute to the increase of ethylene selectivity over the LiC1/oxide catalysts. Due to the strong interaction of C l ions and basic metal oxides, few chlorine radicals will be produced at high temperatures, resulting in lower activity and ethylene selectivity on the

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LiCI/AI203 and LiCI/ZrO2 catalysts.

4.

CONCLUSIONS

Metal oxides such as A1203, SiO2, TiO2, ZrO2 and sulfated ZrO2 exhibit a moderate activity in the oxidative dehydrogenation of ethane. Addition of LiCl in those oxides promotes the ethane conversion and ethylene selectivity. However, LiCl-based catalysts show a varying catalytic behavior depending on their bulk and surface properties. Among the catalysts prepared, LiCI/SZ was found to exhibit high conversion and a longer period time of stability. The deactivation of catalysts can be attributed to the phase transformation and loss of LiC1. REFERENCES

1. 2. 3. 4. 5. 6. 7.

T. Blasco and J.M. Lopez-Nieto, Appl. Catal., 157 (1997), 117. F. Cavani and F. Trifiro, Catal. Today, 24 (1995), 307. E.A. Mamedov and V. Cortes-Corberan, Appl. Catal., 127 (1995), 1. D. Wang, M.P. Rosynek, and J.H. Lunsford, J. Catal., 151 (1995), 155. D. Wang, M.P. Rosynek, and J.H. Lunsford, Chem. Eng. Technol., 18 (1995), 118. K. Otsuka, M. Hatano, and T. Komotsu, Catal. Today, 4 (1989), 409. S. Wang, K. Murata, T. Hayakawa, S. Hamakawa and K. Suzuki, Chem. Commun., 1999, 103. 8. S. Wang, K. Murata, T. Hayakawa, S. Hamakawa and K. Suzuki, Catal. Lett., 59 (1999), 173. 9. L. Ji, J. liu, X. Chen and M. Li, Catal. Lett., 39 (1996), 247. 10. S.J. Conway and J.H. Lunsford, J. Catal., 131 (1991), 513. 11. R. Burch, E.M. Crabb, G.D. Squire and S.C. Tsang, Catal. Lett., 2 (1989), 249.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Oxidative Dehydrogenation of Ethane to Ethylene over NiO/AI203 Catalyst Xinjie Zhang, Gang Yu, Yunqing Gong, De'en Jiang and Youchang Xie* Institute of Physical Chemistry, Peking University, Beijing 100871, P.R.China Oxidative dehydrogenation of ethane to ethylene was studied over NiO/AI203 catalysts. The effects of various A1203 supports, NiO loading, calcination temperature of the supports and of the catalysts, as well as some dopants were investigated. It was found that P205 is the best dopant. An optimum NiO-P2Os/AI203 catalyst can give ethane conversion of 58.3% and selectivity to ethylene of 72.0% with yield to ethylene of 42.0% at reaction temperature of 450~ 1. INTRODUCTION The thermal pyrolysis of ethane to ethylene has been one of the most important industrial processes for a long time, but it is costly in energy consumption and investment, because it is an endothermic process carried out at high temperature and special alloy reactor is used. The catalytic oxidative dehydrogenation of ethane (ODE) to ethylene has received more and more attention since 70's, because it is an exothermic reaction and can be carried out at low temperature with high conversion and selectivity if a good catalyst is found. For the last two decades, a variety of catalysts such as MoVNbSbCaOx[1-5], Li/MgO[6-8], La/CaOx[9,10], have been examined for ODE. Among them, MoVNbSbCaOx catalyst developed by Union Carbide Corporation is the only type of catalyst that can be operated at low temperature with good conversion and selectivity. Recently, Martin et al.[ 11 ] reported another new type of low temperature catalyst, NiO/SiO2, for ODE. Li et al.[12] improved this catalyst by using A1203 as support, but the highest yield to ethylene over their catalysts is only about 25.3%, much lower than the yield of 50% given by Union Carbide catalyst. The present study is to look for a better NiO/A1203 catalyst by screening various A1203 supports and studying the effects of NiO loading, calcination temperature of the supports and of the catalysts, as well as some additives on the catalytic properties of the catalysts. It was found that a good catalyst can be obtained if suitable amount of NiO is supported on a suitable A1203, and the catalyst is calcined at a suitable temperature with P205 as a promoter. 2. EXPERIMENTAL

2.1. Catalyst preparation In general, the catalysts were prepared by the impregnation of A1203 powders in aqueous solution of Ni(NO3)E'6H20, followed by evaporation to dryness at 140~ ovemight. The resulting solids were then calcined at 450~ in air for 5 hours unless stated otherwise. If an additive was used, it was dissolved in the Ni(NOa)2"6H20 aqueous solution for impregnation. * Corresponding author. E-mail" [email protected]

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The catalysts were crushed and sieved to 40-80 mesh for the evaluation of catalytic property.

2.2. Catalytic property measurement The catalytic tests were carried out with a fixed-bed flow glass microreactor, using 0.5g catalyst and operating under atmospheric pressure. The reaction temperature ranged from 350~ to 450~ The feed consisted of 10(vol)% ethane, 10(vol)% 02 and 80(vol)% N2 with a space velocity of 338 ml C2H6/g cat. h (GHSV at STP). The products were analyzed on an on-line gas chromatography with a 4m Porapak Q column and a hydrogen flame ionization detector. A methanator fitted with Ni catalyst was equipped to the GC for the analysis of carbon monoxide and carbon dioxide. 2.3. Catalyst characterization X-ray diffraction analysis was performed with a BD-86 X-ray diffractometer using Cu Kct radiation at 40 kV and 20 mA. The BET surface areas of the samples were measured by using nitrogen adsorption at 77K. H2-TPR was carried out by using 5 vol% H2 in Ar as reducing gas with a flow rate of 20 ml/min and a heating rate of 10~ 3. RESULTS AND DISCUSSION 3.1. Effect of various Ai203 supports Several A1203 supports made from different precursor aluminum salts were used to prepare NiO/AI203 catalysts. Since these supports have different surface areas, we prepared these catalysts with NiO loading on the base of the surface area of the supports. The catalysts, C1 to C4, have NiO loading of 0.24g/100m 2 surface of A1203. Their catalytic properties are reported in Table 1. Table 1 Catal ic ro erties of the catal sts usin different A1203 su _ys

Cat.*

.......................... .3__5..0.............................................

S% C% .............................. Y%

C2H4 CO2

.40.0

orts

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.................................................

S% C% .............................. Y%

C2H4 CO2

.4_50 .........................

S% C% .............................. Y%

C2H4 CO2

C1 12.0 79.9 20.1 9.6 31.8 75.2 24.8 23.9 57.2 67.1 32.9 38.4 C2 9.2 75.2 24.8 6.9 26.6 71.6 28.4 19.0 52.2 67.6 32.4 35.3 C3 14.7 53.4 46.6 7.8 36.8 52.0 48.0 1 9 . 1 52.7 53.4 46.6 24.1 C4 10.4 76.9 23.1 8.0 29.2 69.6 30.4 20.3 55.6 65.3 34.7 36.3 C%: ethane conversion, S%: selectivity, Y%: yield to ethylene * The A1203 supports used in C1, C2, C3 and C4 were prepared by hydrolysis of aluminum alkoxide, precipitation of aluminum sulphate by ammonia solution, aluminum nitrate by ammonia solution and sodium aluminate by aluminum sulphate, respectively. .............................................

,

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Table 1 shows that the catalyst C 1, which was made with an A1203 support prepared by hydrolysis of aluminum alkoxide, is the best one. It gives good ethane conversion and good selectivity to ethylene for the whole reaction temperature, and the highest yield is obtained at 450~ Therefore, this A1203 support was used to prepare the catalysts in the following experiments.

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3.2. Effect of NiO loading The effect of NiO loading on the catalytic with the A1203 support calcined at 900~ for 5 Fig. 1. In Fig. 1, the NiO loading is counted on the base of 100m 2 surface of the A1203 supports. As the NiO loading increases from 0.03 to 0.21g NiO/100m 2 surface of A1203, the ethane conversion increases, and then the ethane conversion decreases slightly when the NiO loading exceeds 0.2 lg NiO/100m 2 surface of A1203. In Fig. 1, the selectivity to ethylene decreases gradually as the NiO loading increases. The highest yield to ethylene is obtained over the catalysts with NiO loading of 0.21-0.27g NiO/100m 2 surface of A1203.

property was tested over a series of catalysts hours. Their catalytic properties are shown in 0

E 80

O~o~,..

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Fig. 1. Ethane conversion(o), selectivity (o) and yield to ethylene(,,) as a function of NiO loading. Reaction temperature: 450~

3.3. Effect of calcination temperature of the supports A1203 was calcined at 550~ 800~ 900~ 1000~ 1100~ and 1200~ for 5 hours to obtain supports with different surface areas and surface properties. The surface areas of the supports decrease as the calcination temperature increases, as is shown in Table 2. Table 2 Surface areas of A1203 calcined at different temperatures ........................ .Sup.p_o_a_.s_ ............................. . % . 0 . ........... ._s__8._0.0........... _ _s..9__0_9_......... _.S..l__0._0_0_ .......... __S_l_l_0__0_ .......... . % 0 0 _ .....

Calcination temp_.erature.(~

550

According to the results of 3.2, we chose 0.24g NiO/100m 2 surface of A1203 as a suitable NiO loading to prepare the catalysts with the A1203 supports obtained at different calcination temperatures. The ethane conversion, selectivity and yield to ethylene over the catalysts at the reaction temperature of 450~ are shown in Fig. 2. Fig. 2 shows that the calcination temperature of the supports has significant effect on the catalytic properties of the catalysts. Though the amount of NiO on the catalysts decreases as the calcination temperature of their A1203 supports raises, the ethane conversion and yield to ethylene

800

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900

80

1100

1200

~/0 ~0-.-.-----0~ " / 0 - - - - - - - _ 0

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20

o

1000

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500 600 700 800 900 1000"1100 1200 1300 Calcination temperature of the support(~

Fig. 2. Ethane conversion (o), selectivity (o) and yield (m) to ethylene as a function of calcination temperature of supports. Reaction temperature" 450~

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increase slightly rather than decrease over these catalysts as the calcination temperature of supports increases from 550~ to 900~ Fig. 2 also shows that as the calcination temperature of the supports increases from 550~ to 1100~ the selectivity to ethylene over the respective catalysts increases. When the calcination temperature raises to 1200~ the support converts to t~-A1203, and the selectivity to ethylene decreases abruptly. The data in Table 1 and Fig. 2 reveal that the best catalyst can be obtained by using support prepared by hydrolysis of aluminum alkoxide and calcined at 900~ 3.4. Effect of calcination temperature of the catalysts

A series of catalysts with various NiO loading were prepared with the A1203 supports calcined at 900~ for 5 hours, and the catalysts were calcined at different temperatures. The catalysts are designated as NiOxTy, where x represents NiO loading (g/100m 2 surface of A1203), and y represents the calcination temperature (~ of the catalysts. The catalytic properties of the NiO0.24Ty catalysts are shown in Table 3. Table 3 Effe ct 0fca!cinatir0 n temperature 0fthecatalysts on the catal~ic ~pr0Pe~ies

...........

.......... _T(!.c)_ ............................ 3_5.0 ......................................... _40.0......................................... _45_0.....................

S% S% C% .......................... Y% C% .......................... Y% C% C2H4 CO2 C2H4 CO2 NiO0.24T400 23.4 66.9 33.1 15.7 44.7 66.1 33.9 29.5 . NiO0.24T500 15.2 73.8 26.2 11.2 39.7 69.7 30.3 27.7 59.5 NiO0.24T600 4.8 81.0 19.0 3.9 14.9 78.0 22.0 11.6 33.4 NiO0.24T700 . . . . 4.0 83.0 17.0 3.3 11.0 C%: ethane conversion, S%: selectivity, Y%: yield to ethylene Cat.

S% ......................... Y%

C2H4 CO2

.

. . 64.5 35.5 38.4 70.7 29.3 23.6 81.3 18.7 8.9

In Table 3, as the calcination temperature of the catalysts increases, the ethane conversion decreases for all the reaction temperature, while the selectivity to ethylene increases. The other catalysts demonstrate similar trend as the calcination temperature of the catalysts increases. The catalysts were characterized by XRD and H2-TPR to investigate the change of the catalytic property. XRD patterns of NiO0.12Ty and NiO0.30Ty are shown in Figs. 3 and 4.

9NiO

10

20

30

40

50

60

70

2 Theta/~ Fig. 3. XRD patterns of NiO0.12Ty.

10

20

9

30

9

40 50 2 Theta/~

60

Fig. 4. XRD patterns of NiO0.30Ty.

70

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The XRD pattems of NiO0.12Ty catalysts in Fig. 3 have no obvious difference with that of the A1203 support, and there is no evident crystalline NiO on these catalysts. The result indicates that NiO is highly dispersed on the NiO0.12Ty catalysts. In Fig. 4, for the catalysts (NiO0.30Ty) with higher NiO loading, it is observed that there is crystalline NiO on the catalysts. Fig. 4 also shows that almost all the crystalline NiO on the NiO0.30Ty catalysts does not change greatly as the calcination temperatures of the catalysts increase. According to the results of Xie et al. tl31, not all NiO is crystalline NiO on the catalysts if the NiO loading exceeds 0.12g NiO/100m 2 surface of A1203, so there exists both highly dispersed NiO and crystalline NiO on the NiO0.24Ty catalysts. Fig. 5 shows the H2-TPR curves of the NiO0.30Ty catalysts. There are two kinds of reduction peaks. The peaks at low temperature ( 300~176 ) are due to the reduction of crystalline NiO, and the peaks at high 571 temperature ( 400~176 ) are due to the ~NiO0.30T70( reduction of highly dispersed NiO[13]. Though 547 both the low temperature peaks and the high .~~NiO0.30T60( temperature peaks shift to higher temperature as the calcination temperature of the catalysts ----~---~NiO0.30T50( increases, the high temperature peaks shift more than the low temperature peaks. The TPR peaks ~ N i O 0 .i3 0 T!4 0 ( shift to higher temperature maybe ascribes to that 100 200 300 400 500 600 700 800 900 some highly dispersed NiO has formed spinel NiA1204 with A1203 owing to the calcination at Temperature (~ higher temperature. This is consistent with the Fig. 5. H2-TPR curves of NiO0.30Ty. activity decline of the catalysts calcined at higher temperature. *

3.5. Effect of doping oxides Several oxides such as WO3, MOO3, B203, P205, were used to dope NiO/A1203 catalysts, and their effects on the catalytic properties were tested. The data show that P205 is the best dopant. The catalytic property of a NiO-P2Os/A1203 catalyst (0.24g NiO/100m 2 surface of A1203 and Ni:P=I 0:1 (mol)) is shown in Fig. 6, which shows that the dopant P205 can make the selectivity to ethylene increase about 5%, while the ethane conversion changes a little. The P205 doped catalyst gives ethane conversion of 58.3% and selectivity to ethylene of 72.0% with yield to ethylene of 42.0% at 450~

9

,

9

*

9

*

9

,

9

,

9

,

,

,

,

~ 80 _o_

coov Oov** 9

3

~ 60 --o-- Sel. overNiO/AI203 --ZX--Conv. overNiO-P~Os/Ai20 ~ ~ / . ~ _ o --V-- Sel. overNiO-P2Os/Al20.a/~~" 40 =~ 20 O w 340

,

i

,

360

!

380

,

i

400

,

I

420

,

i

440

,

460

Reaction temperature (~ Fig. 6. Ethane conversion and selectivity to ethylene as a function of reaction temperature over the NiO/A1203 catalysts with or without P205 dopant.

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4. CONCLUSION This work investigated oxidative dehydrogenation of ethane to ethylene over NiO/AI203 catalysts. The effects of various A1203 supports, NiO loading, calcination temperature of the supports and of the catalysts, as well as some dopants were studied. The A1203 prepared by the hydrolysis of aluminum alkoxide and calcined at 900~ is an appropriate support. The appropriate NiO loading on the support is 0.21-~0.27g NiO/100m 2 surface of A1203. The appropriate calcination temperature of the catalyst is 450-500~ P205 is a good promoter for the NiO/A1203 catalyst. It can increase selectivity to ethylene, while ethane conversion does not decrease. For an optimum NiO-P2Os/AI203 catalyst, ethane conversion is 58.3% with a selectivity to ethylene of 72.0%, i.e., yield to ethylene of 42.0% is obtained at the reaction temperature of 450~ The NiO-P2Os/A1203 catalyst is a better low temperature catalyst for ODE. ACKNOWLEDGEMENT The authors acknowledge the National Science Foundation of China for financial support (Project No: 29733080 ). REFERENCES

1. E.M. Thorsteinson, T. P. Wilson, F. G. Young and P. H. Kasai, J. Catal., 52(1978)116. 2. F.G. Young and E. M. Thorsteinson, Low temperature oxydehydrogenation of ethane to ethylene, US Patent No. 4250346(1981). 3. J.H. McCain, Process for oxydehydrogenation of ethane to ethylene, US Patent No. 4524236 (1985). 4. J.H. McCain, Process for oxydehydrogenation of ethane to ethylene, US Patent No. 4568790(1986). 5. R.M. Manyik, J. L. Brockwell and J. E. Kendall, Process for oxydehydrogenation of ethane to ethylene, US Patent No. 4899003(1990). 6. E. Morales and J. H. Lunsford, J. Catal., 118(1989)255. 7. S.J. Conway and J. H. Lunsford, J.Catal, 131 (1991)513. 8. S.J. Conway, D. J. Wang and J. H. Lunsford, Appl. Catal. A, 79(1991)L 1. 9. X. Zhou, Z. Chao, J. Luo, H. Wan and K. Ksai, Appl. Catal., A, 133(1995)263. 10. L. Ji, J. Liu, X. Chen and M. Li, Catal. Lett., 39(1996)247. 11. V. Ducarme and G. A. Martin, Catal. Lett., 23(1994)97. 12. T. Chen, W. Li, C. Yu, in: The Development of Catalysis--Theses of the Eighth Catalysis in China (H. Zhang, X. Cai and D. Liao, eds., Xiamen University plb.)(1996) p.273. 13. Y. Xie, Y. Tang, Adv. Catal., 37(1990)1.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendiorozand J.L.G. Fierro (Editors) 9 2000 ElsevierScience B.V. All rights reserved.

化

1841

Oxidative dehydrogenation heteropolycompounds

of ethane

over

catalysts

prepared

via

N. Haddad a, C. Rabia a, M.M. Bettahar b. A. Barama a aLaboratoire de Chimie du Gaz Naturel, Institut de Chimie, USTHB, BP32 E1 Alia Bab Ezzouar, Alger, Algerie bLaboratoire de catalyse h6t6rog6ne, Universit6 de Nancy 1-Henri Poincarr6 54506 Nancyles-Vandoeuvres-Cedex. Catalytic performance of supported catalysts prepared via polyoxometalate compounds H3PMol2040 (PMoI2), n3PW12040 (PWI2), n4PMollVO40 (PMollV) and (NH4)6PMotlNiO40 (PMollNi) were studied in the selective oxidation of ethane at atmospheric pressure, T - 400-650~ and C2H6/O2 -2-10 ratio. The catalysts have been characterized by their BET area and XRD and IR spectras. The main result is the obtention of high selectivity (75%-100%) of ethene at substancial conversion (up to 60%). The activity depended on the reaction conditions, the nature of the metal atom and support. The support played an important role in the stability of the catalysts. 1. INTRODUCTION The production of olefins is gaining more and more interest, especially because of their use as an original material for manufacture of various valuable products. Ethane is an available natural resource, also its oxidative dehydrogenation would be a promising route for ethene production and, further, that of valuable chemicals such as styrene, ethyl benzene, ethanol, acetaldehyde, vinyl acetate, etc. In this way many catalytic formulations have been tested in the selective oxidation of ethane into ethene [ 1-4]. In the present work, we report the results obtained during this reaction at atmospheric pressure on catalysts prepared from heteropolyacids (HPA): H 3 P M o 1 2 0 4 0 ( P M o I 2 ) , H 3 P W I 2 0 4 0 ( P W l 2 ) , H 4 P M o l I V O 4 0 ( P M o l l V ) and (NH4)6PMollNiO40 (PMollNi). The catalysts have been characterized by their BET area, IR spectras and XRD. 2. EXPERIMENTAL The catalysts were prepared by conventional impregnation of the active phase on A1203, Sm203, SiO2, Dy203 or La203 with an aqueous solution of HPA. They were heated in air at 400-650~ for 16h. The specific areas, XRD and IR spectra were performed on a BET Coultronics 2100D, Phillips 1710 diffractometer and Phillips 9800 FTIR respectively.

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The catalytic performances were carded out in a fixed bed quartz tubular reactor under the following experimental conditions: P=I bar, T=450-650~ C2HdO2 = 2-10 and Wcat-0.5g. The reactants and products were analyzed on line using Delsi 121ML FID and TCD GC equipped with porapak (C1-C2 analysis) and carbosieve SII (C1 , C O , CO2 , air analysis) columns respectively. The activity of catalysts has been evaluated at steady state.

3. RESULTS AND DISCUSSION 3.1. Characteristics of the catalysts

The specific surface areas depended on the nature of the support that of the active phase (Table 1). High BET area was obtained with the SiO2 support (237 m2/g), the BET area decreased after reaction as probably an effect of the sintering of the active phase. It is woth to note that, in the used catalyst, the specific area was divided by 2 for the unreducible supports (A1203 and SiO2) but to a lesser extent for the reducible supports ( Sm203 La203 and Dy203). In the case of the PMol2/A1203 catalyst, the BET area (3.7 m2/g) increased by substituting one Mo atom by one V atom (PMol iV/A1203 ) (21.3 m2/g). Table 1 Variation of specific areas of catalysts with PMollV loading , nature of support and precursors formula: Tc = 600~ Tr = 650~ Catalyst 15% PMol IV/support 15%HPA/(x-AI203 (x-AI203 SiO2 Sm203 La203 Dy203 PMoI2 PWI2 PMollVPMollNi S (m2/g) fresh 21.2 237 9.7 11.3 6.3 3.7 4.4 21.3 6.5 S (m2/g) used ll.3 113 7.5 9.4 5.9 . . . . The XRD spectra of the PMollV based catalysts showed sharp bands of well crystallized solids. As expected, the HPA phase decomposed during the heating step [5]. Indeed, besides the support phase spectrum, the MOO3, V204, V205 and P205 crystalline phases were evidenced. For the fresh catalysts, the V205 oxide was observed on the unsupported PMollV catalyst and in the presence of all the supports except La203. Whereas the V/O4 oxide appeared only in the presence of A1203 and La203 supports. The V204 oxide was also observed on the used PMoIIV/AI/O3 catalyst (figure 1 and 2) J~

[counts] 1200

lV~,

i

lO00 J

@ V2Os ]

800

4OO 20O -

: +

2. 0. 4. 0. .

.'1

Fig. 1 9XRD spectra of the fresh 15% PMol!V/AI203

60

J" [)~AI20, [

/|

goo !

+

600

0

1200

AI20.~ | p2os ]

~

1000

Ii

~ I

4-

600' 400 ++

200 ....

+-_

[20]

80

t I +_

_

'

A.+I

,

20

,

40

60

Fig.2 9XRD spectra of the used 15% PMol IV/AI203

[201

80

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The FTIR spectras of PMol2 and PMoltM (M = V, Ni) supported on alumina showed the disappearance of the bands attributed to the Keggin structure (in the 1200-300 cm "l) after heating at 600 ~ thus confirming the decomposition of the HPA phase (Figures 3,4). 0'

80

j _jA

8 70 -~ 60

50"

379

3O 40 96;1 1100

782~

900 .... 700_ 500_ Wavenumber [ em -l]

300

16 4000

3000

2000 Wavenumber[ em"l]

1000 600

Fig.3: FTIR Spectra of PMol~V before heating Fig. 4: FTIR Spectra of PMollV ,Tc=600~ 3.2. Catalytic activity

All catalytic tests were carded out at 650~ over catalysts heated at 600~ These conditions drive to the better performances. The products of the reaction were: C2I-I4, CH4, CO, CO2 and traces of C2H5OH. For the supported catalyst the steady state was reached after about 2 h and the catalytic activity remained constant over 3 days. For the unsupported catalyst a slight and progressive decrease in activity was observed and no well defined steadystate was observed within 6h-48h. Ethane conversion and products selectivities depended on the nature of the metal (Mo, W, V, Ni) atom and the nature of the support. The main results of steady-state activity are reported in Tables (2-4) and Figure (5). 3.2.1. Effect of the reaction conditions

The reaction temperature increased the conversion of C2H6 and 02 with relatively high selectivity to ethene as illustrated in Figure 5 for the 15% PMollV/Sm203 catalyst. For example, when the reaction temperature was increased from 450~ to 650~ conversion of both reactants increased from 9.1 to 60.6% and 2.3 to 100% respectively, whereas the selectivity to ethene decreased from 100% to only 75.9%. Cracking (methane) and combustion (carbon oxides) products were favored at high reaction temperature but their selectivity remained low (5.1-13.8 % and 13.4-10.2% respectively) in the studied range of temperature (Figure 5).

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化

* ConvC2H6

mConv 02

9

9 SCO + C02 mSCH4 Selectivity (%) Conversion (%) 1201 ........................................................................................................................................................................................................................... 120 i

100!

A

9

-

-

_-

100

80

80

60!

60

40

40

~

20

20 ~

0

400

450

.

.

.

.

500 550 Temoerature *r

600

650

Fig.5 9Variation of the conversion and selectivities as a function of the reaction temperature for 15% PMOllV/Sm203 catalyst. C2H6/O2 = 5; Tc = 600~

The effect of C2HffO2 ratio on the performances of 15 % PMol IV/AI203 catalyst was investigated at 650~ The results obtained (Table 2) showed that, when the C2H6/O2 ratio increases from 2 to 10, the conversion of C2H6 passed through a minimum (47.8%) for C2H6/O2 = 5 whereas O2 was totally consumed. In parallel, carbon deposition also passed through a minimum (5.0%). This minimum in activity should be attributed to changes in the surface redox potential with change in the chemical composition of the reactive atmosphere. As to the selectivities, that of C2H4 increased from 59.4 to 82.2 % at the expense, notably, of that of the combustion by-products which decreased from 23.8% to 5.4% [6]. The selectivity to methane was relatively not changed (12.3%-16.7%). Table 2 Effect of the C2H6/O2 ratio on the ODE over 15% PMollV&t-A1203 : Tcal = 600~ Tr = 650~ Rapport Conversion (%) Selectivity (%) % Coke C2H6 02 C2H4 CH4 CO + CO2 2 77.8 100 59.4 16.7 23.8 5.7 5 47.8 100 78.9 13.7 7.4 5.0 10 59.4 100 82.2 12.3 5.4 17.2 3.2.2. Effect of the nature of the support

The catalytic activity results obtained on PMolIV unsupported and supported catalysts at 650~ with a C2H6/O2 ratio of 5 are reported in Table 3. No important changes were observed in conversions nor in selectivities by supporting the active phase nor varying the nature of the support. Thus the conversion of ethane and selectivity of ethene remained

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around 50% and 80% respectively. However, coke deposition was the highest (15.0%) for the unsupported catalyst and high carbon formation (12.8%) was also observed with the lanthanum oxide Support. Then, in the present conditions, the redox properties of a multiphasic mixture of metal oxides seemed to be the driving force of the reaction course. Further investigations are needed for a better understanding of the exact nature of this driving force. The main effect of the support, as reported above, was to stabilize the active phase by a better dispersion. Table 3 Effect of the nature of the support on the ODE over 15% PMollV/support: Teal = 600~ Tr = 650~ C2H6/O2=5 (Feed composition: 50% of C2tt6,10% of 02 and 40% of N2) catalyst Conversion (%) Selectivity (%) (yield %) %Coke C2H6 02 C2H4 CH4 CO + CO2 PMollV 50.0 100 78.3 (27.4) 13.8 (4.8) 7.8 (2.7) 15.0 PMoIIV/A1203 47.5 100 78.9 (33.7) 13.7 (5.8) 7.4 (3.1) 5.0 PMoI~V/SiO2 46.9 100 78.6 (29.5) 11.7 (4.4) 9.6 (3.6) 9.3 PMollV/Sm203 60.6 100 75.9 (39.7) 13.8 (7.2) 10.2 (5.3) 8.2 PMo~V/La203 51.0 100 82.3 (31.6) 10.1(3.8) 7.5 (2.8) 12.8 PMoIIV/Dy203 53.9 100 79.9 (35.3) 10.9 (4.8) 9.1 (4.0) 9.3

3.2.3. Influence of the nature and composition of the metal phase

The influence of the nature of the metal on the C2H6 conversion and products distribution in C2H6 + 02 reaction was studied in the same conditions as above and the obtained results are reported in Table 4. It can be seen that the catalysts prepared via the heteropolymolybdic acid were more active than the catalyst prepared via the heteropolytungstic acid (47.8-61.2% of conversion against 33.5%). However, they yielded more coke (up to 11.2% against 3.7%). The observed difference can be attributed to the Mo redox and acid-base properties in agreement with the literature data [7]. Although the vanadium was more oxidative element than molybdenum, the replacement of one Mo atom by one V atom resulted by notable decrease in C2H6 conversion (57.5 to 47.8%) in good accordance with literature [8]. However, no change in ethene selectivity has been observed (about 80%), although improvement was expected [8]. In contrast, when a molybdenum atom was substituted by a nickel atom, both the C2H6 conversion and the C2H4 yield were improved (respectively 61.2% and 39% against 57.5% and 36%) (Table 4). A synergetic effect between MoO3 and NiMoO4 phases (both present in PMol iNi catalyst) should be the starting point of this improvement[9]. Table 4 Effect of the metal composition on the ODE over 15% PMI2-xM'x/AI203: (M: Mo, W and M':V; Ni, x=0; 1): Teal = 600~ Tr = 650~ C2H6/O2 = 5 Catalyseur % Conversion Selectivity (%) (yield %) % Coke C2H4 CH4 CO + CO2 C2H6 02 PMOl2 57.5 100 77.5 (36.0) 13.2 (6.1) 9.2 (4.2) 11.2 PWI2 33.5 100 72.5 (21.5) 13.5 (4.0) 13.6 (4.1) 3.6 PMollV 47.8 100 78.9 (33.7) 13.7 (5.8) 7.4 (3.1) 5.0 PMollNi 61.2 100 75.6 (38.9) 14.8 (7.6) 9.5 (4.8) 9.7

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4. CONCLUSIONS

This study shows that the catalysts prepared via heteropolycompounds are suitable for ODE. The Selectivity towards C2H4, exceeding 82%, can be obtained at high conversion of ethane. The stability of the active phase is highly sensitive to the nature of the support. The coke deposition decreased in the presence of support, as probably, a consequence of a decrease of acidic surface properties. The catalytic behavior of these solids depends on the reaction temperature conditions, the nature of support and of the metal atom. The ODE was favored with molybdenum based catalyst. The presence of vanadium in the catalytic formula decrease the conversion whereas that of nickel enhanced it. REFERENCES

1- K. Ruth, R. Burch and R. Kiffer J. Catal. 175 (1998) 27. 2- J. Bandira, Y. Ben Tgmrit, Appl. Catal. A. : General 152 (1997) 5. 3- J.Z. Luo, X.P. Zhou, Z.S. Chao, H.L. Wan Appl. Catal. A: General 159 (1997) 9. 4- K. Ruth, R. Kiffer, and R. Burch, J. Catal. 175 (1998) 16. 5- C. Rocchiccioli-Deltcheff, A. Aouissi, S. Launay, M. Foumier, Journal of molecular catalysis A: Chemical 114 (1996) 331-334 6- D. W. Flick and M. C. Huff, Catalysis Letters 47 (1997) 91-97 7- T. Okuhara, N. Mizuno and M. Misono, Advance in catalysis, 41 (1996) 113. 8- B. Grzybowska-Swierkosz, F. Triffiro and J.C. Vedrine eds., Appl.Catal. A General, 157 (1997)1. 9- O.Lezla, E.Bordes, P.Courtine and G.Hecquett, J of catalysis, 170, (1997) 346-356.

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化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

The Importance of Nonstoichiometric Oxygen in NiO for the Catalytic Oxidative Dehydrogenation of Ethane Tong Chen v, Wenzhao Li*, Chunying Yu, Rongchao Jin and Henyong Xu Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110, Dalian, 116023, China, Fax: +86-411-4691570, E-mail: [email protected]

Abstract The catalytic behavior ofNiO, pretreated in oxygen at 773 K and 973 K, respectively, for the oxidative dehydrogenation of ethane (ODHE) was investigated. Nonstoichiometric oxygen in NiO and its role in the selective ODHE reaction was quantitatively and qualitatively characterized by O2-TPD-MS, TGA, in situ magnetic measurements and pulse experiments. The catalytic behavior depends markedly on the amount of nonstoichiometric oxygen in NiO, which can be controlled by the temperature of its pretreatment in an oxygen atmosphere. With decreasing amounts of the nonstoichiometric oxygen species in NiO, the ODHE activity is lower. It is suggested that only the p-type semiconductor NiO containing nonstoichiometric oxygen rather than the intrinsic NiO is the active phase for ODHE to ethylene. The ODHE selectivity seems to be related to the ease of the Ni ~2+5)+-- Ni 2+ (0<8<1) transition. 1. INTRODUCTION Selective catalytic oxidation is one of the most interesting reaction in heterogeneous catalysis. For an understanding of it, it is very important to know the active oxygen species in the oxides. Oxygen species located on the surface or in the bulk phase of the catalyst have been extensively studied in the selective oxidation of hydrocarbons [1-+6]. Lattice oxygen has been widely accepted as the active species for the selective oxidation of propylene, whereas 02 or O-species are suggested to be responsible for the selectivity from light alkanes (methane, ethane) to olefins [7,8]. Previous studies indicated that nickel oxide is an attractive candidate for the ODHE reaction [9-~13]. It is well known that NiO is a typical p-type semiconductor which usually contains nonstoichiometric oxygen in an oxidative atmosphere due to its easier oxygen exchange with gas phase oxygen at moderate temperatures. In this paper, nonstoichiometric oxygen in NiO and its role in the selective ODHE reaction were investigated. 2. EXPERIMENTAL NiO used was prepared by calcining Ni(NO3)2.6H20 at 773 K in air for 7 hr. Continuous ODHE was run in a fixed bed reactor with 0.20 g catalyst in the temperature range of 523 K-773 K. The inlet gas mixture contains 5 vol% ethane, 2.5 vol% oxygen, and the rest Present address: Department of Chemistry, Xiamen University, Xiamen, 361005, P.R. China; E-mail" chentonzca3,iin~xian.xmu.edu.cn * Corresponding author

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helium with a total flow of 60 ml/min. TGA, O2-TPD-MS measurements were carried out on a DT-20B thermal analysis appratus and a HP 5988A GC/MS respectively. Magnetic measurements to detect ferromagnetic nickel were carried out at room temperature in an electromagnet equipped with an extraction system yielding fields up to 21 kOe (2.1Tesla). The saturation magnetization was measured by extrapolating the magnetization variation at I/H = 0, from which the amount of ferromagnetic nickel present in the sample was calculated [ 14].

3. RESULTS AND DISCUSSION 3.1. Relationship between catalytic behavior and the pretreatment temperature of NiO The catalytic ODHE behavior ofNiO, pretreated for 30 rain. at 773 K and 973 K, respectively, is shown in Table 1. On the sample pretreated at 773 K, the reaction starts at 523 K. In the temperature range of 523-~673 K, ethane conversion increased with temperature increment, while ethylene selectivity gradually decreased from 70% to 50%. 19.3% yield of ethylene could be obtained at 673 K. If the reaction temperature exceeded 673 K, the selectivity dropped sharply to zero and the main products were methane, CO and CO2. The pretreatment at 973 K brought about a much lower ODHE activity. The yield of ethylene was only 6.53% at 673 K. It could be concluded that the catalytic behavior of NiO for ODHE depends strongly on the pretreatment temperature.

Table 1 The ODHE catalytic behavior ofNiO pretreated at different temperatures Pretreatment Reaction Conv. (%) Sel. (%) Temp.(K ) Temp.(K) C2H6 C2H4 CH4 CO2 773 523 2.04 67.2 32.8 548 5.05 69.1 30.9 573 7.17 66.5 33.5 623 22.8 55.9 44.1 673 37.5 51.4 48.6 698 100 0 47.1 39.0 973

673 723 773

9.82 19.5 52.0

66.3 53.6 0

19.8

33.7 46.4 42.3

CO 13.9

Yield (%) C2H4 1.37 3.50 4.77 12.8 19.3 0

37.9

6.53 10.4 0

3.2. The role of the nonstoichiometric oxygen in NiO The O2-TPD curves for NiO pretreated at 773 K and 973 K are shown in Fig. 1. Two desorption peaks appeared at 573-723 K (at oxygen species) and 723-~893 K (13 oxygen species) on the former, which seemed most probably to be the nonstoichiometric oxygen (or mobile oxygen) in p-type semiconductor NiO, because the release of the lattice oxygen of NiO is usually at a temperature higher than 973 K [ 15]. Only a small at oxygen peak was detected over the sample treated at 973 K. Obviously, the amount of nonstoichiometric oxygen species

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is closely related to the pretreatment temperature, i.e. the higher the pretreatment temperature, the smaller the amount of nonstoichiometric oxygen species. A similar result to that of 02TPD-MS was found by TGA (Fig. 2). Three weight loss stages in the temperature ranges 623--693 K, 693-~893 K and >1073 K were observed over the sample pretreated at 773 K, which corresponds quite well with the desorption temperature rangev of ct, 13 and lattice oxygen, respectively. According to the oxygen weight loss of the first two stages (before

773K

E1

10.5 %

v

73K 1-"

< O

a

I

o I

4.73

673 873 1073 Temprature / K Fig. 1 The curves of O2-TPD-MS for NiO pretreated at 773K, 973K for 30 rain

i

I

,

I

,

I

a

I

,

i

,

273 473 673 873 1073 1273 Temperature / K Fig.2 The curves of TGA for NiO pretreated at 773K, 973K for 2h in He

973K), it is calculated that there exists 6 mol% nonstoichiometric oxygen species in NiO after a pretreatment at 773 K. Comparing the results of O2-TPD and TGA with that of catalytic performance, we can conclusively suggest that only those NiO containing nonstoichiometric oxygen, i.e. possesing p-type semiconductivity rather than the intrinsic NiO are the active phase for the selective ODHE reaction. The following results of pulse experiments and in-situ magnetic measurements further supported the above postulate. 3.3. Exchange between nonstoichiometric oxygen and gas phase oxygen For investigating the exchange between the nonstoichiometric oxygen in NiO and gas phase oxygen, the O2-TPD for NiO pretreated under various conditions were measured, the NiO sample (0.5g) was first heated in oxygen (ca. 1 atm) at 773 K for 1 h, then cooled down in the presence of oxygen to room temperature and purged by helium at the same temperature for 20 minutes. The first TPD spectrum was obtained (Fig. 3, curve a). Both ct and 13 oxygen were observed. Curve b was taken by a TPD procedure from RT to 673 K over a fresh sample. Only a m-oxygen peak was obtained. The sample was then cooled naturally in oxygen to RT, and a TPD procedure was carried out to 673 K (curve c). A desorption peak of ~ oxygen similar to that in curve b was found again. It indicated that the exchange between ot oxygen

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and gas phase oxygen was quickly and reversible even at room temperature. If the sample was cooled again in helium to RT, the TPD spectrum d was obtained, which only gave a 13-oxygen peak but no ct oxygen. Curve e was obtained atter a rapid cooling in oxygen from 1023K to room temperature, no any desorption peak of oxygen appeared at all. When the sample was

100

50

,-7,.

a

40

9

r~

80 ~.

,0

60

30

.=_

o'

O O

d

t O 250~

O

w 20

i

O-. [-I c!

l0 84

0

l

473

,

i

673

,

l

873

r~

~=:_-.___

p

20 =

=

=

= 0

,

1 )73

Temprature / K Fig.3 The curves of O2-TPD for NiO treated under various conditions

Pulse Number Fig.4 Conversion of ethane(a) and Selectivity for ethylene(b) as function of the pulse member under various temperature

gradually cooled in oxygen from 1023K to RT, a peak of ~ oxygen appeared again, while the ~3 oxygen was detected with only a negligible amount (curve f). This indicated that the exchange between gas phase oxygen and c~ oxygen is easier than that of 13 oxygen. Finally, the sample was kept in oxygen at 773 K for 1 h and then cooled naturally in oxygen to RT to give curve g which clearly showed a great resemblance with curve a. This demonstrated that the exchange between nonstoichiometric oxygen and gas phase oxygen is reversible. The nonstoichiometric oxygen structure of NiO can be recovered in oxygen.

3.4. Study of NiO by pulse experiments For further investigating the role of the m, 13 oxygen species in NiO, pulse experiments were carried out. 0.2g of fresh catalyst was degassed by purging in He at 523 K for 30 rain. Then pure ethane pulses (0. l ml) were introduced into the reactor at the desired temperature and the products were detected by on-line gas chromatography. The results are shown in Fig. 4. We can see that ethane can be oxidized even in the absence of a gaseous oxygen supply. As the pulse number increases, the conversion of ethane decreases monotonously, and then it becomes very low while nearly 100% selectivity for ethylene was obtained aider 4 pulses of ethane. These results suggested that the active oxygen species for ODHE is not gas phase oxygen. Other active oxygen species (~, 13 oxygen) exist on the catalyst surface or in the catalyst bulk which can interact first with ethane. When o~, 13 oxygen were gradually used out, catalytic reactivity decreased. Pure oxygen pulses were then introduced into the reactor after 8

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pulses of ethane, and no products including oxygen itself were detected for the first and second oxygen pulses. After the catalyst was fully re-oxidized (more than 3 pulses of oxygen), ethane pulses were introduced again and the same picture was given as above. It is interesting to notice during the pulse experiments at 723 K, starting from the third pulse, the conversion of ethane sharply increased and the selectivity for ethylene abruptly dropped to zero, and metal nickel, Ni~ was found on the catalyst immediately. Ethane can interact with lattice oxygen and reduce Ni 2+ to Ni ~ after ct, 13 oxygen was used up under a given reaction temperature. It is suggested that the selective oxygen species for ethylene is most probably the nonstoichiometfic oxygen species rather than the lattice oxygen. 3.5. In-situ magnetic measurement of NiO Magnetic measurements can detect minute amounts of ferromagnetic materials. They give rise to signals several orders of magnitude higher than dipara or antiferromagnetic compounds, and are characterized by a saturation which is reached at moderate field strengths. Thus, this technique provides significant information on the characterization of NiO catalysts with less than 0.1% Ni~ In order to relate the catalytic behavior 10 10 of different samples with the different species on the catalysts, the samples were 8 8 characterized by magnetic measurements atter %6 % in situ treatments under reaction conditions. .~4 6 Fig. 5 shows the variations of magnetization of the NiO catalyst with field strength, measured v-~2 ~ f a r 4 "-" ~ after reaction at the given temperature. As can lhat423K be seen, the catalyst contacted with reagents 0 r _- tyaxl~ (~IEa548K 2 :~ -2 • UsedforODHEat623K below 673 K leads selectively to ethylene and v Usedfa-OgHEat698K did not show the formation of a ferromagnetic A

-4 6 5 fo 15 2'0~ phase. From this observation it can be assessed H(kOe) that no metal nickel particles are formed under Fig 5 In dtu ~ ~ ~ as a these conditions. This shows that the active of H utxter ~ lreatrnerl ccrdifiom phase which leads to ethylene do not consist of ~ overN~) catalyst(0.1gCat; at 300K~ ) Ni particles. When the reaction temperature was increased to 698 K, magnetic measurements (MM result increasing about 100 fold) revealed the presence of a ferromagnetic phase, which is assigned to metallic nickel, the most plausible candidate for ferromagnetism. Catalytic measurements have shown that the selectivity of ethylene dropped sharply to zero at 698 K. These observations strongly suggest that the active phase for ethylene formation is not the ferromagnetic phase detected by magnetic measurements and the catalytic activity is related to the ease of the Ni (2+8)+--- Ni 2+ (0<8<1) transition.

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4. CONCLUSIONS NiO containing nonstoichiometric oxygen rather than the intrinsic NiO is the active

phase for the ODHE reaction. The ODHE activity is related to the ease of the Ni ~2+5)+----Ni 2§ (t)<~i
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Oxidative Dehydrogenation of Ethane on Vanadium-Phosphorus Oxide Catalysts J.M. L6pez Nieto ~, V.A. Zazhigalov 2, B. Solsona ~, I.V. Bacherikova 2 1) Instituto de Tecnologia Quimica, Universidad Politecnica de Valencia, Avda los Naranjos s/n, 46022 Valencia, Spain; [email protected] 2) Ukrainian-Polish Laboratory of Catalysis, Institute of Physical Chemistry, NASU, pr. Nauki 31, Kyiv-22, 252022, Ukraine* * Present address: Ukrainian-Polish Laboratory of Catalysis, Institute of Sorption and Problems of Endoecology, NASU, ul. Gen. Naumova 13, Kyiv-164, 252680 Ukraine Bi-doped VPO catalysts have been prepared, characterized and studied in the oxidation of ethane. The catalysts activation strongly influence both the characteristics of catalysts and their catalytic behavior. (VO)2P207 and BiPO4 were mainly observed in catalysts calcined in ethane/oxygen mixtures. These catalysts are highly selective in the oxydehydrogenation of ethane. In addtion, the incorporation of a big ionic radius cation as Bi, favors the increase of yield of ethene as was previously observed in the oxidation of n-butane. On the other hand, [~VOPO4 and BiPO4 were observed in catalysts calcined in air, which present a poor selectivity to selective oxidation products. The presence of Bi, could stabilize both overstoichiometric phosphorus at the surface and the presence of V 4§ on the surface of catalysts. 1. INTRODUCTION The broad use of lower olefins in different technological processes determine the interest to research of their means obtaining [1, 2]. The oxidative dehydrogenation of short chain alkanes offers an attractive alternative route to form olefins from alkanes in simple and low cost reactors. One of these reactions investigated in the last years, is the OXDH of ethane to ethene. Rare-earth metal oxides are effective catalysts for this process but the better results are attained at high reaction temperatures (750-800~ [3]. However, the reaction can selectively be carried out at lower temperatures (450-500~ on V, Mo, Nb mixed oxides [4] and supported vanadium oxide catalysts [2]. VPO catalysts have been also employed in this reaction, although the yield of ethene was not higher than 6 % [3, 5]. Our ideas about mechanism of oxidation of lower paraffins [2, 6, 7], and more specifically on VPO catalysts [6, 7], suppose the ability of attain higher productivities for the oxidative dehydrogenation of ethane. In this way, the incorporation of some additives with big ionic radius can distort the structure of vanadyl pyrophosphate [6, 7]. For activity and selectivity rise in oxidehydrogenation of ethane is also recommended using catalysts whose plane (001) of VOHPOa 0.5H20 or plane (100) of (VO)2P207 which contain of paired vanadyl groups, are not basic. In accordance with those suppositions, the incluence of the Bi-incorporation on VPO catalysts were investigated. The catalysts were synthesized in

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conditions that had allowed to obtaine samples with most intense reflexes of plane, which perpendicular to vanadyl one.

2. E X P E R I M E N T A L 2.1. Catalyst Preparation The VPBiO catalysts were synthesized in a butanol medium with simultaneous addition of vanadium oxide, bismuth nitrate and o-phosphoric acid in reaction mixture [7]. The atomic ratio of the components in catalysts was the same P/V = 1.15, and Bi/V atomic ratio from 0.025 to 0.300. An active mass of the precursor was separated by evaporation of the solvent and sample was dried in vacuum (0.010-0.012 MPa) with increasing of temperature step by step to 543 K. Some characteristics are shown in Table 1.

Table 1. Properties of VPBiO catalysts. Sample

Bi/V

SSA a)

(P/V)xPs

Adsorption and

OXDH of

atomic

(m2g-~)

atomic

TPD of NH~- b)

ethane c)

Ratio

Ratio

A

I)

X, (%)

Sr (%)

8.0

76.9

VPO

0.0

12.2

1.45

0.10

0.43

VPBi025

0.025

13.4

1.48

0.11

0.44

VPBi050

0.050

14.2

1.56

0.12

0.44

8.8

77.9

VPBi 100

0.100

16.1

1.62

0.14

0.47

9.5

77.1

VPBi200

0.200

16.6

1.72

0.17

0.51

15.8

68.2

VPBi300

0.300

17.0

1.83

0.20

0.53

18.7

63.8

BiPO4

0.0

9.4

-

-

-

1.5

11.4

a) Specific surface area of catalysts after the catalytic tests; b) A = amount of adsorbed NH 3 in ml m2; D= amount ofNH 3 desorbed at T > 523 K in ml m-2; c) Conversion of ethane (Xt) and selectivity to ethene (Sc2=) during the OXDH of ethane at 500~ and a W/F of 36 gcat h (moI-C2)- 1

2.2. Catalyst Characterisation The catalysts were characterised by several techniques: XRD (Phillips X'Pert-MPD difffactometer provided with a graphite monochromator, operating at 40 kV and 20 A and employing nickel-filtered CuKa radiation), XPS (VG ESCA-3, A1 Ka-radiation, s t a n d a r d binding energy of C Is-electrons- 284.8 eV; the method of spectra analysis were published previously [7]). The specific surface areas have been obtained by termal desorption of argon (Gasochrom-1). The acid character of catalysts has been studied from the TPD-NH 3 (chromatographic version) and the decomposition of 2-methyl-3-butyn-2-ol (MBOH) as indicated previously [8].

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2.3. Catalytic tests The catalytic tests for the oxidative dehydrogenation (OXDH) of ethane were carried out in a fixed-bed quartz tubular reactor (16 mm i.d., 500 mm length). Catalyst samples 0.51.5 g (particle sizes 0.25-0.42 mm) were mixed with variable amounts of SiC (0.59 mm) to keep a constaint volume catalyst (3 cm3). The reaction was studied at atmospheric pressure in the 400-500~ temperature interval using an ethane/oxygen/helium reaction mixture with a molar ratio of 10/5/85. The total flow was varied from 100 to 200 ml min-' (the time of contact W/F = 15-70 gcath/(mol-C2)"). Analysis of reactants and products was carried out by GC. The catalysts were previously activated "in situ" at 500~ for 4 h with a mixture of ethane/oxygen/helium with a molar ratio of 10/5/85. The oxidation of n-butane and n-pentane (1.7 and 1.6 vol. % in air, respectively) was investigated in a fixed-bed quartz reactor (8 mm i.d., 400 mm length) [7]. The catalysts were used after activation "in situ" with a n-butane/air mixture.

3.

RESULTS AND DISCUSSION

3.1. Catalyst characterization VOHPO4.0.5H20, with a intense x-ray line at d = 0.293 nm (plane 220), and (VO)2P207 phase, with an intense x-ray line at d = 0.313 nm (plane 024), are mainly observed in the VPBi0 sample before and after the catalytic tests, respectively. In the case of Bi-doped catalysts, BiPO4 (with peaks at 0.442, 0.352, 0.303, 0.286 and 0.237 nm) and (VO)2P207 crytalline phases were only observed (Fig. 1). u

d

I I

0

0

0

~ ~

~ --el

20

0

o

~r ~

e,~ ~

30

0

~0 ~

Figure 1. XRD patterns of undoped (a) and Bi-doped VPO catalysts with Bi/V atomic ratio of 0.10 (b), 0.20 (c) and 0.30 (d). Samples activated in reaction conditions. Symbols: (VO)2P207 (I); BiPO 4 (O); and BiPO 4 (A) ~1" ~

40

20 Table 1 shows the characteristics of catalysts determined after the catalytic tests. It can be seen that the addition of bismuth in VPO samples increases their specific surface area. The introduction of Bi in different concentrations did not cause any changes of the binding energy of V2p3 n- and P2p-electrons (517.5 and 133.7, respectively), although the binding energy of O 1-electrons monotonously decreases when increasing the bismuth-content of catalysts. XPS data also showed that the surface P/V atomic ratio raises with the Bicontent. It should be noted that this ratio is higher than its in the case of the sample promoted by bismuth but having preference orientation by (001) plane [9]. It can be explained from the fact that phosphorus (or its oxide) lightly migrates on the structure cavities, which are parallel

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(001) plane [ 10]. In the case of catalysts calcined in air, an increase of the binding energy of V2p3/:electrons (0.9-1.2 eV) has been observed. This fact can be explained by the formation of Vs+OPO4 phase. From the TPD results of Table 1 it can also been concluded that the number of acid sites increases with the Bi-content of catalyst. However, the ratio of Lewis/BrOnsted acid also changes. In fact, it has been observed that the growth of the bismuth content in catalyst lead to increase the acid properties of the catalyst. We must notice that the selectivity to 3-methyl-3buten-l-yne in the MBOH decomposition (which is characteristic of Lewis acid sites) decreases when increasing the bismuth content [8].

3.2. Oxidative dehydrogenation of ethane Table 1 shows comparatively the catalytic results obtained during the OXDH of ethane. Ethene, CO and CO2 were the main reaction products. Oxygen containing products other than carbon oxides were not observed. From the results of Table 1 it can be concluded that ethene can be selectively formed on both undoped and Bi-doped catalysts, although the conversion of ethane increases with the Biloading. Figure 2 shows the variation of the yield of ethene with the Bi-content in the 450500~ temperature interval. It can be seen that the yield of ethene increases with both the reaction temperature and the bismuth content of catalyst. In addition, it can be noted that, at isoconversion conditions, the selectivity to ethene is independent of the Bi-content.

15 - e - 450oC - o - 475oc

A

~0

-A-

Figure 2. Variation of the yield of ethene with the Bi-content in Bi-containing VPO catalysts during the OXDH of ethane at 450, 475 and 500~ and a W/F= 36 gca, h (molc2)-I.

500oc

,,$ "0

5

0 0

I

I

0.10

I

I

0.20

I

I

0.30

Bi-content

BiPO4 shows a very low activity and selectivity in the OXDH of ethane. Thus, it appears that the catalytic activity is related to the presence of V-species. However, the presence of BiPO4, formed in Bi-containing catalysts, could favor the increase of the number of effective active sites. It can be noted that a yield of ethene equal to 18 % can be obtained on the VPBiO (Bi/V = 0.3) catalyst at high contact times. So, the incorporation of bismuth favors a higher production of ethene (three times higher than undoped catalyst) and an ethene productivity of 0.165 kg h -~ kg-cat" for the sample with a Bi/V atomic ratio of 0.30 has been obtained.

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Figure 3 shows the variation of the selectivity to the main reaction products with the conversion of ethane on VPBi300 catalyst. The selectivity to ethylene decreases and the selectivity to carbon oxides increases with the conversion of ethane, although the selectivity to ethene was more than 80 % at paraffin conversion less than 5 %.

100 ..

>,

80

60

(,)

475oc 500oc

v

,,..=,

o

40

~

CO

20 .

.

0

5

.

10

9

. . . . .

15

20

Figure 3. Variation of the selectivity to ethene, CO and CO 2 with the conversion of ethane during the OXDH of ethane on VPBi300 catalysts.

C02

25

30

Ethane c o n v e r s i o n ( % ) Similar trends were observed in all the catalysts studied but differences in selectivity to ethylene between the undoped sample and the Bi-containing catalysts are not observed. On the basis of these results, a mechanism of parallel oxidation of ethane to ethene and CO2, and consecutive formation of CO from ethene can be proposed. It has been established that the conversion of ethane increases with the reaction temperature and the bismuth content in the VPO catalyst. In addition, the specific rate of OXDH of ethane also raises with increasing of bismuth concentration in catalyst and decreasing of O Is-electrons binding energy (Fig. 4). Analogous dependence was observed in the selective oxidation of n-butane and n-pentane on these catalysts (Fig. 4).

2O

1

15

5 I

9

0 531.4

I

531.6

9

'

|

531.8

'

9

532.0

Figure 4. Variation of the specific rate of OXDH of ethane (1) (in 105 moI-C2 h-I m-2) with the binding energy of Ols-electrons during the OXDH of ethane on VPBiO catalysts. For comparison, the dependence obtained for the oxidation of n-butane (2) and n-pentane (3) on the same catalysts is also included.

Binding E n e r g y 0 1 s - e l e c t r o n s , eV

The change in the catalytic properties with the Bi-loading can be explained on the basis of the mechanism of paraffin activation proposed in refs.[2, 4-6]. The first step in the

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alkane activation is the rupture of a proton from the paraffin by unsaturated bridged oxygen ((024) plane) which is more active in O b H - g r o u p formation [11]. So, the increase of electronic density on the oxygen atom of the surface of catalyst (decrease of binding energy of O ls-electrons) with the bismuth incorporation leads to the growth of the proton abstraction probability and, consequently, increases the rate of the ethane oxidation (Fig. 4). This step is common for all paraffins oxidation reaction as confirmed by data represented on Figure 4 for the case of n-butane and n-pentane. Carbanion formed after proton abstraction could connect with vanadium ions (Lewis acid sites) in spite of it take place the abstraction of hydrogen from second carbon atom and desorption of water ObH2 which occured rather easily [ 11 ]. It could be proposed that in the case of oxidized sample, which have strong Lewis centers (VS+), the bond V-C is very firm and the desorption of hydrocarbon fragments is difficult. Decomposition of this fragment leads to coke formation and deactivation of surface catalyst. The second possibility is its deep oxidation to carbon oxides. In this way, the incorporation of Bi decreases the acidity of Lewis acid sites but increases the selectivity to ethene [6, 7].

CONCLUSIONS In this paper we present the possibility of obtain effective catalysts of oxidative dehydrogenation of ethane by the modification of catalyst structure of VPO sample and the incorporation of additives. In this case, the incorporation of a big ionic radius cation as Bi, favors the increase of yield of ethene. The presence of Bi, could stabilize both overstoichiometric phosphorus and the presence of V 4§ at the catalyst surface as suggested previously in the oxidation of n-butane [7]. J.M.L.N. and B. S thank the financial support of CYCIT (Project MAT-97-0561) REFERENCES

1. 2. 3. 4. 5.

6. 7. 8. 9. 10. 11.

F. Cavani and F. Trifir6, Catal. Today 51 (1999) 561. T. Blasco and J.M. L6pez Nieto, Appl. Catal. A:General 157 (1997) 117. M. Baems and O. Buyevskaya, Catal. Today 45 (1998)13. K. Ruth, R. Kieffer and R.Burch, J. Catal. 175 (1998) 16. a) G. Centi and F. Trifiro, Catal. Today. 3 (1988) 151; b) P.M. Michalakos, M.C. Kung, I. Jahan and H.H. Kung, J. Catal. 140 (1993) 226; c) M. Loukah, G. Coudurier, J.C. Vedrine and M. Ziyad, Microp. Mater. 4 (1995) 345. V.A. Zazhigalov, in: Selective Oxidations in Petrochemistry: DGMK, Hamburg, p.249 (1998). V.A. Zazhigalov, J. Haber, J. Stoch, L.V. Bogutskaya and I.V. Bacherikova, Appl. Catal., A 135 (1996) 155. H. Lauron-Pernot, F. Luck and J.M. Popa, Appl. Catal. 78 (1991) 213. J. Haber, V.A. Zazhigalov, J. Stoch, L.V. Bogutskaya and I.V. Bacherikova, Catal. Today 33 (1997) 39. C.C. Torardi, Z.G., Li, H.S. Horowitz, W. Liang and M.H. Whangbo, J. Solid State Chem. 119 (1995) 349. M. Witko, Catal. Today. 32 (1996) 89.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

H i g h - T h r o u g h p u t Synthesis and Screening o f Nb and Ta Containing Mixed Metal Oxide Libraries for Ethane Oxidative D e h y d r o g e n a t i o n Yumin Liu*, Peijun Cong*, Robert D. Doolen, Howard W. Turner and W. Henry Weinberg Symyx Technologies, Inc. 3100 Central Expressway, Santa Clara, CA 95051, USA ABSTRACT Niobium and tantalum containing mixed metal oxide libraries (16x16x16 ternary libraries) of V-A1-Nb, Cr-A1-Nb and Cr-A1-Ta are prepared and screened for ethane oxidative dehydrogenation. An apparatus for screening catalytic activity and selectivity of a 144-member catalyst library prepared on a Y' quartz wafer was constructed. It consists of a reaction chamber, remote and optical and mass selective detectors. In the reaction chamber, each library member is heated individually by a CO2 laser and reactant gases are delivered locally to each member. The reaction products, ethylene and CO2, are detected by photothermal deflection spectroscopy and by mass spectrometry respectively. The catalytic activities obtained showed that V-A1-Nb oxide catalysts are high temperature catalysts and Nb does not affect the catalytic activity of the V-A1 oxides, in contrast to the effect of Nb found in Mo-V-Nb oxides. However, for the Cr-A1-Nb oxide library, the most active catalyst contains about 4% Nb. Similarly, the most active catalyst in the Cr-A1-Ta oxide library contains about 9% Ta. These results suggest that a fine composition mapping is necessary for discovery of new heterogeneous catalysts in the ternary systems. INTRODUCTION Heterogeneous catalysts have played important roles in the chemical and petrochemical industries; however, the discovery of new heterogeneous catalysts is often achieved by trial-and-error approaches, which are labor-intensive processes. The heterogeneous catalysts often contain multiple components and their compositions are critical to their catalytic performance. For example, one of the best known catalysts in ethane oxidative dehydrogenation that is active at low temperature (below 300 ~ contains a mixture of molybdenum, vanadium and niobium oxides with an optimal composition of Mo0.61V0.31Nb0.0sOx[1-3]. The complexity of ternary systems suggests the use of highthroughput synthesis and screening technologies for developing new catalysts. Rapidly synthesizing a large number of chemically distinct materials and screening for desired properties in a chemical reaction can accelerate the discovery of new catalysts significantly. Here we report high-throughput synthesis and screening of mixed metal oxide libraries for ethane oxidative dehydrogenation, illustrated with V-A1-Nb, Cr-A1-Nb and Cr-A1-Ta mixed metal oxide libraries.

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E X P E R I M E N T A L DETAILS

High-throughput synthesis of mixed metal oxide libraries was conducted on a 3" diameter quartz wafer[4, 5]. A mixed metal oxide library was prepared by sol-gel methods with metal alkoxides in 2-methoxyethanol as precursors. A precursor library was prepared with automatic dispensing robotics to deliver aliquots of the metal solutions into wells of a microtiter plate, resulting in a 16 x 16 x 16 triangular array of solutions. Next, 3.0 lal of each of the solutions in the microtiter plate was transferred to discrete locations along a surface of a 3" diameter quartz wafer with a 12 x 12 square array. To better utilize available wafer space, the 16 x 16 x 16 triangular array of solutions was mapped into part of the 12 x 12 square array. Array members consisted of approximately 3 mm diameter droplets with adjacent droplets separated by about 1 mm. The array members were exposed to laboratory moisture at 25 ~ for gelation, then heated to 60 ~ to remove solvent, and calcined to 400 ~ in air. Since the stock solutions of metal precursors are 0.50M, each member contains 1.5 lamol metal oxide. The quartz wafer containing the catalyst library is loaded onto a two-dimensional screening stage underneath a probe with concentric tubing for gas delivery/removal and sampling[4-6]. Only the catalyst that is being tested is exposed to the reactant gases, and the excess reactants are removed from the system by a vacuum line. A catalyst is heated by a CO2 laser to the desired temperature before the measurement commences. No other catalyst experiences heat or the reactant gases since the heating is localized and reactant gases are delivered locally. The reactant gases contain a mixture of C2H6, 02 and Ar with a ratio of 4:1:5 in screening catalysts for ethane oxidative dehydrogenation. Two main products from ethane oxidative dehydrogenation--ethylene and CO2--are detected by a photothermal deflection and mass selective detection systems simultaneously. The measurement of product as well as reactant concentrations is achieved by sampling the gas mixture directly above the catalyst and transporting it through a capillary transfer line to the detectors. Each measurement takes about one minute to complete, and slightly more than two hours are needed to complete a 144-member library. RESULTS AND DISCUSSION We chose to prepare V-A1-Nb oxide libraries and screen their catalytic activity and selectivity for ethane oxidative dehydrogenation because V205/A1203 distinguishes itself from the other literature catalysts with high ethylene productivity[7]. Although the reaction temperature for V205/A1203 is high, by incorporating Nb oxide into V205/A1203 we may obtain more active and selective catalysts since niobium oxide has been reported in numerous catalytic systems to enhance metal oxide catalytic activity and selectivity in producing ethylene[ 1, 8, 9]. A library of V-A1-Nb mixed metal oxide was prepared by sol-gel methods with vanadium triisopropoxide oxide, aluminum triisopropoxide, and niobium pentaethoxide as metal oxide precursors. The V-AI-Nb oxide catalyst library was then screened for catalytic activity and selectivity at 400 ~ Relative ethylene and CO2 productions from new catalysts are compared with those produced by the known Mo-V-Nb oxide catalysts[ 1] prepared on a flat

家

150

化

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1861 150 [

O

C2I'I4 (ppm)

120

8o

120 I 90

oO~

90 C2H4 (ppm) 60

O O

60 30 0.2

0.4

0.6

0.8

1

Fig. 1. Production of ethylene (at 550 ~ versus V content in a V-A1-Nb oxide catalyst library. The most active catalyst contains 86% V and 14% A1.

oo

o

60 Oo c2n4 oO (ppm) 40

0.4

0.5

i

o

i

o 0

i i i

~

20

Fig. 3. Ethylene produced by a Cr-A1-Ta oxide library at 400 ~ Member (row 1, column 1) contains CrO3, member (16,1), A1203 and member (16,16), Ta2Os.

0.2 0.3 Nb

I

................................................................................................................................... i

80

80

0.1

i

Fig. 2. Production of ethylene (At 550 ~ versus Nb content in a V-A1-Nb oxide catalyst library. The most active catalyst contains no Nb.

00 100

I0

1

0

V

60

O

O

i

0

C2H4

O

30

o

(ppm)

[

--

-

oo

+

0.2

0.4 0.6 0.8 1 Ta Fig. 4. Production of ethylene versus the Ta content of the Cr-A1-Ta oxide library. The most active catalyst contains about 9% Ta.

surface[5]. Although the catalyst activity is measured at low conversion, such a measurement correlates well with those measured at high conversion demonstrated in the Mo-V-Nb oxide catalyst library[5]. Ethylene produced at 400 ~ by the V-A1-Nb oxide catalyst library is only about 20 parts per million (ppm) above the background; however, the catalytic activity is sensitive to the catalyst's composition with catalyst containing about 86% V producing the maximum amount of ethylene. This catalyst is less active than the Mo-VNb oxide library prepared similarly (44 ppm ethylene produced at 400 ~ Subsequently, the same catalyst library was screened at 550 ~ as shown in Fig. 1, where the signals are substantially higher than the background and the corresponding member screened at 400 ~

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V0.86A10.14Oxis the most active member, which produced 134 ppm ethylene. However, Nb showed (Fig. 2) no obvious effect in enhancing the catalytic activity for V-A1 oxide suggesting that the synergistic effect is absent in this case[10]. Similarly, we prepared a Cr-AI-Nb oxide library and studied the effect of Nb on the catalytic activity of Cr-A1 oxides[ 11 ], with chromium (III) 2-ethylhexanoate (0.50M) as the Cr precursor. The catalysts with optimum catalytic activity are in the region close to Cro.35A10.55Nb0JoOx, with an ethylene yield of about 178 ppm. This catalyst is much more active than the Mo-V-Nb oxide library (44 ppm ethylene produced)[5]. Due to the similarity of niobium and tantalum, we then prepared a Cr-A1-Ta oxide library by similar method, and screened their catalytic activity and selectivity at 400 ~ for ethane oxidative dehydrogenation. Fig. 3 showed the ethylene produced with maximum ethylene of 86 ppm produced by the most active member, Cro.38A10.52Tao.o9Ox. This library is more active than MoVNb but less active than Cr-A1-Nb oxide library. The most active catalysts contain about 9% Ta. (see Fig. 4). A focused library of Cr-A1-Nb oxide was then prepared with member (1,1) containing

Cr0.14A10.86Ox, member (16,1) containing CrO3, and member (16,16) containing

Cr0.45Nb0.55Ox. The most active member in this library is Cr0.28A10.68Nb0.04Ox, which produces 428 ppm ethylene (see Fig. 5). Clearly, Cr0.28A10.68Nb0.04Oxis a much more active catalyst than either V0.86A10.14Ox or any Mo-V-Nb oxide catalyst for the oxidative dehydrogenation of ethane. Fig. 6 shows the CO2 produced by the focused Cr-A1-Nb oxide library. Although the Cr-AI-Nb oxide library is more active than the MoVNb oxide library in producing ethylene, the ethylene selectivity of the Cr-AI-Nb oxide library (30% to 50%) is lower than that of the MoVNb oxide library (about 90%)[5]. More importantly, the niobium content enhances the catalyst's activity, reaching a maximum at the Nb content of about 4% (see Fig. 7).

400 C2H4

300

ppm

~00

O0

Column

Fig. 5. Ethylene produced by a focused Cr-A1-Nb oxide library at 400 ~ Member (1,1) contains Cr0.14A10.s6Ox,member (16,1 ), CrO3 and member (16,16), Cro.45~Oo.550x.

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1600 1200

C02 ppm

|00

O0

~olumn

Fig. 6. CO2 in ppm produced by the focused Cr-A1-Nb oxide library at 400 ~ Once an interesting catalyst member is identified, its catalytic activity and selectivity can be screened at different temperatures. In the focused Cr-A1-Nb oxide library, very active members (6,4) and (8,7) were screened at temperatures from 350 ~ to 450 ~ Arrhenius plots for production of ethylene and CO2 were obtained for member (6,4) (see Fig. 8) and member (8,7). For production of ethylene, the apparent activation energies for member (6,4) (Cro.28Alo.68Nbo.o4Ox) and member (8,7) (Cro.27Alo.63Nbo.loOx)are 20 + 1 and 21 + 1 kcal/mol, respectively. For the combustion by-product CO2, the apparent activation energies for member (6,4) (Cro.28Alo.68Nbo.o4Ox)and member (8,7) (Cro.27Alo.63Nbo.loOx) are 18 + 1 and 16 + 1 kcal/mol, respectively. The activation energies obtained on the catalyst library on a flat surface are consistent with the activation energies of Mo-V-Nb oxide catalysts obtained on large scales[9]. 45o

400 350 300 I~O0 C2[][4 250 ~ _ _ 200

ppm

0 0 0 ~0 v.

150

50 0

_. 0

% T

,

i

0

0.1

0.2

"l

0.3

!

-o -

"J

0.4

.....

o T

0.5

...........

0.6

Nb

Fig. 7. Production of ethylene versus the Nb content of the focused Cr-A1-Nb oxide library. The most active catalyst contains about 4% Nb.

0.00140

0.00145

0.00150

0.00155

0.00160

1/'1"

Fig. 8. Arrhenius plot of ethylene and CO2 yield for member (6,4) of the focused Cr-AINb oxide library.

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CONCLUSION

V-A1-Nb, Cr-A1-Nb and Cr-A1-Ta mixed metal oxide libraries were prepared on 3" quartz wafers and screened for catalytic activity and selectivity of ethane oxidative dehydrogenation. Despite the effect of Nb in enhancing catalytic activity in the Mo-V-Nb system, no obvious effect of Nb has been observed in the V-A1-Nb oxide library. The catalytic activity reached a maximum at a content of 4% Nb in the Cr-A1-Nb library. Both niobium and tantalum have similar effect in enhancing activity of Cr-A1 oxides. In all cases, the most active members are limited in a narrow range of composition of the mixed metal oxide library. Therefore, high-throughput synthesis and screening of solid materials for heterogeneous catalyst are useful in discovery of new catalysts for those ternary systems. To date, a large number of solid materials have been screened for ethane oxidative dehydrogenation with many lead catalysts discovered. The lead catalysts discovered on a 3" quartz wafer have been successfully scaled up to the 20g scale. A detailed account of these newly discovered catalysts will be published in the future. REFERENCES

.

3. 4.

.

o

8. 9. 10. 11.

E. M. Thorsteinson, T. P. Wilson, F. G. Young and P. H. Kasai, J. Catal., 52, 116-32 (1978). F. G. Young and E. M. Thorsteinson, US Patent: 4,250,346, 1981. J. H. McCain, US Patent: 4,524,236, 1985. P. Cong, R. D. Doolen, Q. Fan, D. M. Giaquinta, S. Guan, E. W. McFarland, D. M. Poojary, K. Self, H. W. Turner and W. H. Weinberg, Angew. Chem. Int. Ed., 38, 484488 (1999). P. Cong, A. Dehestani, R. D. Doolen, D. M. Giaquinta, S. Guan, V. Markov, D. Poojary, K. Self, H. Turner and W. H. Weinberg, Proceedings of National Academy of Sciences, 96, 11077-11080 (1999). W. H. Weinberg, E. W. McFarland, P. Cong and S. Guan, , Symyx Technologies, Inc., US Patent: 5,959,297, 1999. F. Cavani and F. Trifiro, Catal. Today, 24, 307-13 (1995). R. Burch and R. Swamakar, Appl. Catal., 70, 129-148 (1991). O. Desponds, R. L. Keiski and G. A. Somorjai, Catalysis Letter, 19, 17-32 (1993). P. Courtine and E. Bordes, Studies in Surface Science and Catalysis, 110, 177-184 (1997). J. R. H. Ross, R. H. H. Smits and K. Seshan, Catal. Today, 16, 503-11 (1993).

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Chromium-impregnated mesoporous silica as catalysts for the oxidative dehydrogenation of propane. J. M6rida-Robles, M. Alcfintara-Rodrfguez, E. Rodriguez-Castell6n, J. Santamar/a-Gonzfilez, P. Maireles-Torres and A. Jim6nez-L6pez Departamento de Qufmica Inorg~inica, Cristalograffa y Mineralogfa, Facultad de Ciencias, Universidad de M~ilaga, Campus de Teatinos, 29071 M~ilaga (Spain) The activity of chromium-supported Silica catalysts in the oxidative dehydrogenation of propane at 300-550~ has been investigated varying the Cr content between 0.5 and 6.0 wt%. XPS and diffuse reflectance spectroscopy indicate the existence in all materials of Cr species with two different oxidation states: Cr(III) and Cr(VI). After impregnation, textural parameters of the mesoporous MCM-41 silica are barely affected. These catalysts are active in the oxidative dehydrogenation of propane at temperatures as low as 300~ Activity and selectivity are correlated to the presence of highly dispersed Cr(III) species and increase with the reaction temperature. Thus, the catalyst which presents the highest loading of Cr, without evidence of the presence of crystallites of Cr203 (1.5Cr-MCM), is the most active for this catalytic reaction. 1. INTRODUCTION One of the catalytic reactions of increasing interest is the dehydrogenation of light alkanes, which represents an alternative for obtaining alkenes, useful for the synthesis of polymers and chemicals [1]. Thus, for instance, the oxidative dehydrogenation of propane can lead to propylene, a more reactive, and therefore valuable, intermediate. Supported vanadium oxides have been largely reported to be selective catalysts in this reaction [2]. However, chromium(III) supported catalysts have been also used for dehydrogenation of light alkanes [3]. On the other hand, the synthesis of a new family of high surface area mesoporous silica, MCM41-type, has open an unique opportunity to use them as support for the deposition of catalytically active metal species. The advantages of these solids lie in their high specific surface areas, uniform size channels which diameters can be tuned between 15 and 100 ,~ and thermal stability [4]. In a previous paper we reported the synthesis, characterisation and catalytic behaviour of structurally chromium-doped MCM-41 silica in the dehydrogenation of propane, in oxidative and non oxidative conditions [5]. These results indicated that the ODH activity values were tightly related to the Cr content, and always higher than those found in non-oxidative atmosphere. The present work reports the effect of the incorporation of chromium species by impregnation, using a mesoporous silica as support, on their catalytic activity in the oxidative dehydrogenation of propane.

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2.- EXPERIMENTAL

2.1. Preparation of catalysts. The mesoporous silica support was prepared by putting in contact a cetyltrimethylammonium chloride aqueous solution (25 wt.%) and an ethanolic solution of tetraethoxysilane (TEOS). The pH was adjusted to 11 by addition of a tetramethylammonium hydroxide aqueous solution. The resulting gel was stirred at room temperature during three days. Then, the white solid was recovered by centrifugation, washed with ethanol and water and air-dried at 60~ The mesoporous MCM-41 silica support was obtained after calcination at 550~ in air for 6 hours. Chromium species have been deposited on the surface of a mesoporous MCM41-type silica by impregnation, using the incipient wetness method and chromium(Ill) nitrate aqueous solutions containing amounts corresponding to Cr wt.% varying from 0.5 to 6.0. The catalysts, obtained by calcination at 550~ in air, will be designated hereinafter as xCr-MCM, where x denotes the chromium content of the sample, expressed as the weight percent of Cr.

2.2. Catalysts characterisation Powder XRD patterns were recorded on a Siemens D501 diffractometer (graphite monochromator, Cu-KGt radiation). Diffuse reflectance electronic spectra were registered on a Shimadzu MPC 3100 spectrophotometer (BaSO4 reference) and IR spectra on a Perkin-Elmer 883 spectrometer. X-ray photoelectron spectra (XPS) were recorded with a Physical Electronics spectrometer equipped with a Mg KGt X-ray excitation source and multichannel hemispherical electron analyser. N2 adsorption-desorption isotherms were obtained using a conventional volumetric apparatus (77 K, degassing at 200~ and 10 4 mbar overnight). Pore size distributions were calculated with Cranston & Inkley method for cylindrical pores [6]. Thermal-programmed desorption of ammonia (NH3-TPD) was used to determine the total acidity of the samples. Before the adsorption of ammonia at 373 K the catalysts were heated at 500~ in a He flow (35 mL min -~) for 1 h. The NH3-TPD was performed between 100 and 500~ with a heating rate of 10~ min -~. The evolved ammonia was analysed by online gas chromatograph (Shimadzu GC-14A) provided with a thermal conductivity detector.

2.3. Catalytic reaction The catalysts have been tested in the oxidative dehydrogenation of propane, using an automatic microcatalytic flow reactor working at atmospheric pressure. A fixed-bed quartz Utube reactor with a catalyst charge of 0.120 g without dilution was used in all cases. Samples were previously treated at 400~ under a helium flow (30 mL min -~) for 1 hour. The gas reaction mixture was constituted of 7.06 mol% propane in 0 2 : N 2 (19.5 : 73.4, mol%) and a total flow rate equal to 28.32 mL min l and a spatial velocity of 25.7 gcat h/mol propane were used. The analysis of reactants and products was performed on line with a gas chromatograph Shimadzu GC-14B provided with a column (7 m in length and 5 mm internal diameter) filled with sebaconitrile, and using a flame ionization detector. The analysis of CO2 was carried out on line with a packed column Poropack A (2.5 m in length, 1/8" OD) and using a TCD detector. 3. RESULTS AND DISCUSSION Powder XRD patterns of the mesoporous silica support exhibit an intense low angle peak at 35.4 A, which is assigned to the d~00 reflection assuming a hexagonal arrangement. As

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expected, this diffraction signal is barely affected by the incorporation of chromium species by impregnation, confirming the structural stability of the mesoporous host (Fig. 1). However, the catalysts with percentages of chromium of 3.0% or higher exhibit XRD patterns at high diffraction angles which clearly show the typical diffraction lines of crystalline Cr203 at 3.63, 2.67, 2.48, 2.17 and 1.82 .A,. All these results point to that chromium is not very well dispersed as a monolayer but rather the formation of crystallites of Cr203 which appears quite before the formation of a single monolayer of Cr203 on the surface of mesoporous silica.

3.63

2.67

2.48 2.17

"2'.

e

q

d

c b .........

1 2 3 4 5 20/o

1 .........

15

1 .........

25

t .........

35

I .........

45

55

20/~

Fig. 1. X-ray diffraction patterns of (a) Si-MCM-41, (b) 0.5Cr-MCM, (c) 1.5Cr-MCM, (d) 3.0Cr-MCM and (e) 6.0Cr-MCM catalysts. X-ray photoelectron spectroscopy (XPS) has been used to get insights into the chemical nature of the surface of chromium-based catalysts. Figure 2 shows the evolution of the Cr2p core level spectra as a function of the chromium content. It can be observed that the peak with a maximum at 576.5 eV, assigned to Cr(III), can be deconvoluted in two components, one main at low energy (576.7 eV) due to Cr(III) and another, of lower intensity, at higher energy (578.9 eV) attributable to the existence of Cr(VI). The contribution of Cr(VI) decreases with the Cr content. This behavior has been also found in other catalytic systems such as chromiaalumina [7], and indicates that a higher dispersion of chromium favors its oxidation to Cr(VI). Thus, the catalyst with the lowest chromium content attained a percentage of Cr(VI) of 21%. The number of CrO x species per nm 2, calculated from the amount of Cr used in the impregnation process, has also been included in Table 1 and allows us to know the degree of surface coverage, taking into account that the formation of a monolayer of CrOx species is reached with 10 CrOx species per nm 2 [8]. However, the surface Cr/Si atomic ratios, obtained by XPS, remain almost constant along the group of catalysts, being these values lesser than the bulk Cr/Si atomic ratios. Even the value corresponding to a catalyst prepared with a 15 wt% of chromium is lower than the corresponding bulk Cr/Si. Because the XPS signal rises both from support and deposited phase, the atomic ratios Cr/Si could exhibit a slight increment with the coverage degree of the surface with chromium, and only is expectable a important increment of Cr/Si atomic ratios beyond the formation of multilayers on the support when the chromium species quenched the signal of support. All these data point to a poor dispersion on the internal surface of the support with formation of crystallites of Cr203.

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The existence of Cr(VI) on the surface of catalysts has been also confirmed by UV-vis spectroscopy. Thus, a band centered at 269 nm, corresponding to charge transfer transition of Cr(VI) species, are distinguished in the diffuse reflectance spectra. This signal decreases in intensity as the chromium loading increases, while concomitantly a group of three bands at 354-360, 400 and 600 nm, typical of octahedral Cr(III), become more intense.

:i i

r~

mm

595

590

585 580 575 Binding Energy (eV)

570

Fig. 2. X-ray photoelectron spectra in the Cr2p core level region of (a) 0.5Cr-MCM, (b) 1.5Cr-MCM, (c) 3.0Cr-MCM and (d) 6.0Cr-MCM catalysts. Textural parameters of chromium containing catalysts have been determined from N2 adsorption-desorption isotherms at 77 K, and are shown in Table 1. Regarding the shape of the isotherms, they are of Type IV in the IUPAC classification, and exhibit the typical adsorption step of mesoporous solids with uniform porous network. The most important modification corresponds to sample 6.0Cr-MCM with a reduction of 11% in the BET specific surface area with respect to that of the support. The total pore volumes are significative with values ranged between 0.57 and 0.64 cm 3 g-l, being similar to that of the support. The pore size distribution curves exhibit an unique and narrow peak, indicating the existence of a single maximum pore diameter close to 23 *. These data point to a degree of pore blocking very small after the impregnation of the support with chromium species.

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Tablel Chemical and textural characteristics of Cr-impregnated silica catalysts V * dp (av) * Sample CrOx/nm 2 SBET m 2 g -i cm3Pgl Ao Si-MCM-41 938 0.643 0.5Cr-MCM 0.064 895 0.601 1.5Cr-MCM 0.195 889 0.646 3.0Cr-MCM 0.400 869 0.592 6.0Cr-MCM 0.834 833 0.570 9Accumulated pore volume and average pore diameter from

22.8 22.5 24.7 22.8 23.0 the Cranston & Inkley method

As shown in Figure 3, the TPD-NH3 data indicate that, in contrast to the pristine Si MCM41 which barely adsorbs ammonia, chromia supported catalysts are acid solids with a total amount of desorbed ammonia, between 100 and 500~ comprised between 639 and 178 ~tmol g-~. It is noteworthy that total acidity decreases with the chromium loading, which matches very well with the evolution found for the dispersion degree of the active phase found by XPS. Thus, sample containing 6 wt% Cr exhibits a low acidity as a consequence of the formation of microcrystallites of Cr203 on the support surface. In relation to the nature of acid sites, adsorption of pyridine coupled to IR spectroscopy reveals the presence, in all catalysts, of acid sites of Lewis-type. However, pyridine adsorbed on these acid sites is easily removed by thermal treatment, because at 100~ practically all pyridine molecules are removed from the catalyst surface. These Lewis acid sites must be associated to Cr species deficiently coordinated. 800 cD

" 0

r~

600 D 100-200~

..~ 400 200~ N

::t.

N1200-300~ D 300-400~ D400_500oc I total

0

Si

0.5Cr

1.5Cr

3.0Cr

Fig. 3. Total acidity of chromium-impregnated thermoprogrammed desorption of ammonia.

6.0Cr

silica catalysts

as determined

by

These materials have been tested as catalysts in the oxidative dehydrogenation of propane. The reaction temperature was varied between 300 and 550~ increasing the catalytic activity towards propene with the temperature, except for the sample with the lowest Cr content (0.5Cr-MCM) which exhibits a catalytic activity practically independent of the temperature (Figure 4). In all cases, propene and carbon oxides (CO and CO2) were the main reaction products, and methane and ethane as traces. It must be pointed out that these Cr-based catalysts catalyse the oxidative dehydrogenation of propane at temperatures as low as 300~ giving, in the cases of the catalysts with the lower Cr content, activities higher than 0.7 ktmol propene g-~ s-~ (Figure 4). The selectivities towards propene reach in all cases values lower than 15%. Concerning conversion, the two

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catalysts with the lowest Cr content show values, in all the temperature range, comprised between 45 and 65%. The other catalysts exhibit conversions at 300~ lower than 15%, increasing with the temperature until 45% (550~ in the case of the 3.0Cr-MCM sample. A maximum yield at 450~ of 7.2% is obtained for the 0.5Cr-MCM catalyst. This value is higher than those found for Cr-impregnated zirconia, silica or zirconium phosphate at 550~ [9], and similar to that of mixed Cr/Ga oxide pillared zirconium phosphate at 400~ [3b].

1.2

ir'1.a A/

=~ 0.9 0.6

Xj X ~~X

::t. .~

~ 0.3

SX I

0.5Cr - - I - 1.5Cr 3.0Cr I--X-- 6.0Cr

.~

.<

0 250

I

I

350

450

!

650 550 T (~

Fig. 4. Catalytic activity in the ODH of propane as a function of the reaction temperature. These catalytic results can be explained by supposing mononuclear Cr(III) species as the active centres in the oxidative dehydrogenation of propane, since the maximum values of catalytic activity has been found for the catalyst with the highest Cr loading but which does not show evidence for the presence of crystallites of Cr203, that is, the 1.5Cr-MCM catalyst. We thank the CICYT (Spain), project MAT97-906 for finantial support.

REFERENCES 1. H.H. Kung, Adv. Catal., 40 (1994) 1. 2. T. Blasco, J.M. L6pez-Nieto, Appl. Catal. A: General, 157 (1997) 117. 3. (a) S. De Rossi, G. Ferraris, S. Fremiotti, A. Cimino and V. Indovina, Appl. Catal., 81 (1992) 113. (b) M. Alcfintara-Rodrfguez, E. Rodrfguez-Castell6n and A. Jim6nez-L6pez, Langmuir, 15 (1999) 1115. 4. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L.Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. 5. P. Maireles-Torres, M. Alcfintara-Rodrfguez, F. P6rez-Reina, E. Rodrfguez-Castell6n, P. Olivera-Pastor and A. Jim6nez-L6pez, Stud. Surf. Sci. Catal., 118 (1998) 903. 6. R.W. Cranston and F.A. Inkley, Adv. Cat., 9 (1957) 143. 7. O.F. Gorriz, V. Cort6s-Corberfin and J.L.G. Fierro, I&EC Research, 31 (1992) 2670. 8. G.C. Bond, J. P6rez-Zurita, S. Flamerz, P.J. Gellings, H. Bosch, J.G. van Ommen and B.J. Kip, Appl. Catal., 22 (1986) 361. 9. M. Loukah, G. Coudurier, J.C. Vedrine and M. Ziyad, Microporous Mater., 4 (1995) 345.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

T r a n s i e n t k i n e t i c s t u d i e s of the p r o p a n e o x i d e h y d r o g e n a t i o n on V~OsfriO2 c a t a l y s t s S. Pietrzyk and F. Genser Laboratoire de Catalyse de Lille UPRESA 8010/Laboratoire de G6nie Chimique et d'Automatique (LGCA), Ecole Nationale Sup6rieure de Chimie de Lille - Ecole Centrale de Lille, B.P. 48, 59651 Villeneuve d'Ascq CEDEX, FRANCE A transient kinetic study of the title reaction was u n d e r t a k e n to check for its feasibility in a circulating bed reactor, with separated zones of oxydation of propane and catalyst regeneration Credox mode") on VzOdTiOz catalyst, as such and doped with K, between 440 and 720 K (non-doped) or 610 - 670 K (K-doped). The catalysts have been found to resist to reduction by propane, and the amount of transferable oxygene corresponding to about the first layer of lattice oxygen ions could be exceeded at higher temperatures, indicating some oxygen mobility between layers. The partial pressures of products have been compared with those computed on the basis of some plausible reaction mechanisms. The results obtained with two such mechanisms based on that proposed by Oyama are given. 1. INTRODUCTION Kinetic study of a given reaction may be performed by measuring the product stream compositions obtained with various initial reactant mixtures in a steady-state operation (constant feed rate flow and composition, temperature and pressure; enough time on stream to obtain constant products partial pressures), or by measuring the variations of these partial pressures induced by some changes of the above parameters. The most productive case corresponds to variable feed composition, usually step changes of concentrations or very short injections of some reactants (respectively, step and pulse transient experiments). The best known exemple of reactors working in transient mode are TAP (Temporal Analysis of Products) reactors [ 1]. The analysis of results of such experiments is more complex than that of the more usual steady-state ones, and the results may be flawed by phenomens absent or of little importance under steady state conditions. On the other hand, if the care is taken of these problems in the experimental set-up and in the treatment of data, transient methods become very powerful and productive methods to study reaction kinetics. In particular, if the objective of a study is to model and optimize an industrial reactor operated in the forced unsteady state mode ("FUSO" reactor), only the corresponding transient kinetic tests are able to furnish the necessary data [2]. In the special case of a circulating bed reactor (CBR, see e.g. [3]), a solid - selective oxidation catalyst circulates between the reaction

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and the regeneration zones and is, sucessively, reduced and oxidated, being exposed in these zones, respectively, to a mixture containing e.g. an hydrocarbon and to air or oxygen. The activity and selectivity under anaerobic conditions, the quantity of oxygen that can be transferred by a unit mass of the catalyst, the amount and nature of the organic matter adsorbed on the catalyst when it leaves the reaction zone, etc. can be determined most efficiently in specifically designed transient experiments. In this paper we present the results of such a study of propane oxidehydrogenation on V2Os/TiO2 catalysts, in transient mode. 2. E X P E R I M E N T A L

2.1 Catalysts V2OdTiO2 catalysts were prepared in the Institute of Catalysis and Surface Chemistry in Krakow, Poland, by impregnation of an anatase support from Tioxide, France with aqueous solutions of ammonium methavanadate and potassium nitrate for the K-doped catalyst, evaporation of water and drying at 120~ for 18 hrs followed by calcination at 450~ for 5 hrs [4]. The amounts of sels correspond to 5 theoretical monolayers of V205, on the basis of the support BET area of 48 m2/g and 10 V atoms/nm 2, and to a K/V ratio of 0.1 for the K-doped catalyst. Theoretical composition is thus 26.7% V205, or 26.3% V205 and 1.36% K20 for the non-doped and doped catalysts, respectively. BET (N2) surface areas of the fresh catalysts are 37.4 m2/g for the non-doped and 32.9 m2/g for the K-doped, slightly lower after use. The other characteristics of these solids, and the results of some steady-state catalytic tests are given in earlier papers [4, 5, 6]. The K - doped catalyst is less active but more selective than the non doped one.

2.2. Experimental set-up Transient kinetic experiments were performed in a set-up similar to that described by MOiler and Hofmann [7]. Gases from Air Liquide, France (propane N35, oxygen A, helium U) were combined by means of mass controllers into 3 flows: C3Hs + He, 02 + He, and He, and distributed by means of 2 pairs of three-way electrovalves among the reactor and two ballasts, to avoid pressure transients on switching. The total flows of the three gas mixtures were identical, in general 50 Ncm3/min (3.72 10-5 mol/s). The reactor and the two dummies were made of stainless steel, 5 mm i.d.; the load of the catalyst was about 300 mg. The reactor and the dummies were placed in the same oven; the temperature of the reactor was measured by means of a thermocouple touching its external wall. The product gas was analyzed by means of an analytical system QMA 420 (Balzers), composed of a Quadrupole MS (QMG 420) and a capillary - valve assembly permitting to reduce the pressure of the analyzed gas mixture from atmospheric to about 10-8 mbar. To minimize interactions between components, only mixtures containing propane (and corresponding products), or only those containing oxygen (and products) have been analyzed in a given run. This was obtained using a third couple of electrovalves, switching the gas flow in the vicinity of the tip of the capillary of the analytical system, between the reactor effluent and pure helium. The three couples of electrovalves were accentuated in programmed sequences, so that the catalyst bed was fed with O2+He mixture, pure He, C3Hs+He, He, O2+He, etc., the products from selected time intervals of the cycle being analyzed with a frequency of about 1 s~. The lengths of the reaction/regeneration steps were typically of the

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order of 100 s, the He steps about 50 s. Other details of the experimental set-up, analyses and data treatment are given elsewhere [8, 9].

2.3 Experimental procedures A new sample of a catalyst was heated under a flow (50 Ncm3/min) of 20% 02 in He (ramp 5 K/min, maximum temperature 450~ hold lh), and then cooled down under He to the temperature of the first experiment. One or more series of alternating concentration steps were applied at this temperature, each series composed of 6 cycles containing an oxygen step, typically 20% 02 in He, 120 sec, followed by a He step and a propane step, e.g. 20% C3Hs in He, 120 sec, as described in 2.2.; the collected analytical MS results were stocked for later treatment. After that, the temperature was increased to that chosen for the next experiment, the sample being exposed to a He flow. In some experiments the propane mole fraction was varied, between 10 and 40%, or the lengths of concentration steps. For the non-doped catalyst, the temperature was varied between 166 and 445~ and for the K-doped one, between 337 and 397~ : at that last temperature, some transformation of this catalyst occured, which increased its activity, but decreased the selectivity to propene. This changes persisted even if lower temperatures were subsequently applied. 3. RESULTS AND DISCUSSION The products of the reaction obtained under transient conditions applied in this work are propene, mono- and dioxide of carbon, and water, like in steady-state tests [4,5]; variations of partial pressures of this products observed for the consecutive propane steps were practically identical, at given conditions, with exception of the first one, for which some differences were generally observed. Only for the K - doped catalyst, at about 400~ where the abovementionned irreversible transformation was observed, the successive propane steps gave rise to widely different product partial pressures, until the end of the transformation (3 - 4 steps). A catalyst sample usually could be used in many experiments, without noticeable modification of its properties. 3.1. Propane concentration steps, non - doped catalyst. A typical evolution of mole fraction of products during a propane step and shortly after, on a non doped catalyst sample at 374~ is shown in Fig. 1. It can be seen that the mole fraction of propene has a sharp maximum at the beginning of the step, and then decreases rapidly to a fiat plateau. The curves of CO and CO2 are similar, the formation of CO being more important. The curve of water is less sharp, which can be due in part to the adsorption/desorption phenomena in the set-up, and particularly in the analytical system. The selectivity to propene increases, from initial values of about 0.2, to about 0.9 at the end of the propane step, giving a mean value for the whole step of about 0.6, with the mean propane conversion of about 1.5%. Mean values for a propane step of 120 s decrease from 70% to 50% in the temperature interval 220 - 450~ The amount of oxygen given by the solid in the experiment at 374~ shown here could be estimated at about 20% of the monolayer (BET surface area of 36 mZ/g, 10 OZ/nm2); at the highest temperatures studied (about 450~ this amount attained 120% of the monolayer, suggesting strongly some mobility of lattice oxygen at these temperatures.

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After the end of the propane step, the propene production ceases almost immediately, while that of CO and 1-120 continues (Fig. 1). Concerning CO2, the situation is less clear. This result shows that aIter the propane step there remains on the surface some adsorbed residue which can not desorb as propene, but can lead to the formation of carbon oxides, particularly CO. On the other hand, the continuing production of water should be, at least in part, attributed to the formation on the surface of the catalyst of groups OH, which desorb as water. The presence of a carbonaceous residue has been verified by the analysis of the gases formed in the regeneration (reoxidation) step, where the formation of carbon oxides and of water has been confirmed [6, 9]. The amounts of these products correspond to a stoichiometry H/C ~ 2, and the amounts of the carbon removed increase from 3 ~tmol/g at 192~ to 53 ~tmol/g at 375~ (surface coverages between 0.2 and 3.1%, as C3Hx, on the basis of BET surface area 36m2/g and 10 sites/nm2).

3.2 Propane concentration steps, K - doped catalyst. An exemple of the evolution of mole fractionss during a propane concentration step at 377~ is given in Fig. 2. As expected, the selectivity to propene is higher than on the non doped catalyst, 0.6 at the beginning and more than 0.9 atthe end of the concentration step, resulting in a mean value of about 0.8 for a mean prpane conversion of 1.2%. In the temperature interval 337 - 377~ the mean selectivity to propene obtained for steps of 120 sec was almost constant, between 0.8 and 0.9, while the mean conversion of propane increased from 0.2 to 1.2% It shoul be stressed that the conversions have been kept low deliberately, to make possible the use of the model of a concentration-step differential reactor (see 3.3.).

mole fraction 0,014

........2 ................................................

0,012 ................. ~.4................4 ....................................... 0,01 ..................................................... 0,008 0,006

'~'end of step ........i ............................~ [ .........

0,004 ii.i__.!i.~!_i.I _ ~ 0,002 0 ~ 0

mole fraction 0,008 0,007 0,006 0,005 0,004 0,003 0,002

end of step

0,001

J

~

k _ 100 time, sec

Fig. 1. Propane concentration step, nondoped catalyst, 374~ 20% C3Hs + He; 1 C3I-I6, 2 - CO, 3 - CO2, 4 - H20.

0

0

100 time, sec

Fig. 2. Propane concentration step, K doped catalyst, 377~ ; curves as in Fig. 1

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3.3 Modeling of the product formation. To use the results of such a study to optimize an unsteady-state operated reactor, they should be preferably expressed as equations giving the partial pressures of products, selectivities, yields, etc. as functions of reaction conditions, including those representing the unsteady-state character of the operation. This can be done, as for the steady-state operation, on the basis of a convenient reaction mechanism. The suitable mathematical treatment of varying complexity have been described in the litterature [e.g. 6, 10 - 13]. In this case, because of low conversions, and other characteristics of our reactor set-up, we have used a simplified version of the model of differential reactor oerated in concentration step mode [6, 10]. Wa have tried many plausible mechanisms proposed for the HDO reactions on oxide catalysts; the best results have been obtained with two mechanisms based on suggestions of Oyama [ 14, 15] : interaction of a hydrocarbon molecule with surface oxygens (O z') produces loosely adsorbed intermediates which desorb as corresponding dehydrogenated products, while the intermediates adsorbed on transient metal (V) ions are strongly held and give way ultimately to total oxidation products (COx, H20 ). It should be stressed that the presence of strongly held hydrocarbon species, and their transformation to CO x could be inferred from the products observed in propane and regeneration steps (3.1.). Two such mechanisms have been compared, Mechanism I (2 intermediates: C3H7s 1 and C3H7s 2, O s = surface 02- ion, V = vacancy, considered as an exposed V ion), C3H8(g ) + 2 0 s --~ C3H7s 1 + OH s C3H7s 1 + Os ~ C3H6(g) + OHs + Os 2 OH s ~ H20(g ) + O s + V C3H8(g ) + O s + V --~ C3H7s 2 + OH s C3H7s 2 + 2 0 s --~ 3 CO x + 7 OH s - 9 0 s + 5 V Ob+V~O s

(1) (2) (3) (4) (5) (6)

and Mechanism II (only one intermediate, C3H7s2, propane molecules from the gas phase reacting directly with surface oxygens): C3Hs(g ) + 2 0 s --~ C3H6(g ) + 2 OH s

(1')

and the reaction of propene with surface oxygens considered as another total oxidation route: C3H6(g ) + 2 0 s --~ 3 CO x + 6 OH s - 8 0 s + 4 V

(7)

In eqns. 5 and 7, the two carbon oxides have been combined in a "COx", with x - 4/3 corresponding to the average ratio CO/CO 2 ~ 2; left-hand sides of these two equations correspond to the assumed kinetics, while the coefficients on the fight-hand side provide balanced stoichiometries. It was necessary to include the mobility of 0 2- between the bulk and the surface, eqn. 6, particularly ar T > 350~ For the non-doped catalyst, both mechanisms reproduced satisfactorily the shapes of product curves obtained at different temperatures, for a constant partial pressure of propane; the second mechanism was found much better when the variations of the propane partial pressure were included. The rate constants are given in Table 1

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Table 1 Rate constants of reactions (1) through (7), units : seconds, bars, at about 375~ ; activation energies (in the parentheses) in kJ/mol Mechanism kl (k'l) k2 k3 k4 k5 k6 k7 I, non-doped 1.5 (29) 0.025 (25) 0.011 (59) 8 (11) 0.15 (5) 0.1 (19) II, non-doped 0.5 0.009 0.04 0 0 19 I, K-modified 0.002 0.1 10 0.01 0.2 0.02 The behavior of the K-doped catalyst could not be reproduced by either mechanism, with one set of constants. We tried to treat this catalyst by assuming that its surface is composed of

patches modified by potassium, and of the non-modified fragments identical with that of the non-doped catalyst. The results obtained with mechanism I are given in Table 1; about 70% of the catalyst surface would be modified by potassium. CONCLUSION V205/TiO 2 could be interesting as catalysts for the propane ODH in a circulating bed reactor: they can give away all surface oxygen ions (actually more than that, at higher temperatures) and still retain the capacity of rapid reoxidation. The reaction mechanism proposed by Oyama seems to be sufficiently close to the reality, to allow a satisfactory modeling of the results; the influence of potassium can be interpreted within this mechanism as a reduction of rates of interaction of propane with the surface species. Acknowledgements to Prof. B. Grzybowska for catalyst samples and helpful discussion.

REFERENCES 1. J. T. Gleaves, J. R. Ebner, and T. C. Kuechler, Catal. Rev. - Sci. Eng., 30 (1988) 49. 2. Y. Sh. Matros, Can. J. Chem. Eng., 74 (1996) 566. 3. R. M. Contractor, H. E. Bergma, and H. S. Horowitz, Catal. Today 1 (1987) 40. 4. R. Grabowski, B. Grzybowska, K. Samson, J. Sloczynski, J. Stoch, and K. Wcislo, Applied Catal. A, 125 (1995) 129. 5. R. Grabowski, B. Grzybowska, A. Kozlowska, J. Sloczynski, K. Wcislo, and Y. Barbaux, Topics in Catal., 3 (1996) 277. 6. F. Genser and S. Pietrzyk, Chem. Eng. Sci., accepted. 7. E. M~iller and H. Hofmann, Chem. Eng. Sci., 42 (1987) 1705. 8. M. L. Ould Mohamed Mahmoud, Ph. D. Thesis, UTC, Compirgne, 1996. 9. F. Genser, Ph. D. Thesis, UTC, Compirgne, 1998. 10. M. Kobayashi and H. Kobayashi, Catal. Rev., 10 (1974) 139. 11. M. Kobayashi, Chem. Eng. Sci., 37 (1982) 393. 12. T. Salmi, Chem. Eng. Sci., 43 (1988) 503. 13. S. Pietrzyk, M. L. Ould Mohamed Mahmoud, T. Rembeczky, R. Bechara, M. Czernicki, and N. Fatah, Stud. Surf. Sci. Catal., 109 (1997) 263. 14. S. T. Oyama, J. Catal., 128 (1991) 210. 15. S. T. Oyama, A. N. Desikan, and W. Zhan, ACS Syrup. Series, 523 (1993) 16.

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化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Vanadium-Doped Titania-Pillared Montmorillonites as Catalysts for the Oxidative Dehydrogenation of Propane K. Bahranowski a, R. Dttla b, R. Grabowsld b, E.M. Serwicka b, K. Wcis|o b

aFaculty of Geology, Geophysics and Environmental Protection, Academy of Mining and Metallurgy, 30-059 Krak6w, al. Mickiewicza 30, Poland, blnstitute of Catalysis and Surface Chemistry, Poli,~'hAcademy of Sciences, 30-239 Krakdw, ul. Niezapominajek 1, Poland, V-containing titania pillared montmorillonites have been prepared and characterised with XRD, ESlL BET and chemical analysis. Their performance in the oxidative dehydrogenation of propane depends on the manner of doping. Co-pillaring gives catalysts of little V content which are less active but more selective than those containing more vanadium and prepared by means of cation exchange. The latter ensure highest yields of propene. The increased selectivity of the co-pillared sample is tentatively assigned to the presence of V centres of higher rccovalency within bonds between vanadium and bridging oxygens.

1. INTRODUCTION Vanadia, the most popular component of catalysts used in redox processes, including oxidative dehydrogenation (ODH) of alkanes [ 1,2], is usually deposited on oxidic supports such as alumina, silica, titania or zirconia. Over 20 years ago it has been demonstrated that small clusters of these oxides, of nanometer size, can be incorporated into the structure of layered clay minerals [3-5] resulting in the highly porous materials known as pillared interlayered clays (PILCs). The aim of this work was to prepare clay catalysts in which titania pillars would act as support for vanadium phase and to test their performance in the ODH of propane. 2. E X P E R I M E N T A L The montmorillonite used in this study was the sodium-exchanged less than 2 ~tm particle size fraction of Milowice (Poland) bentonite, referred to as Na-mt. Titania-pillared sample was prepared following the procedure described by Sterte [6]. Vanadium was introduced into the samples either by cationic exchange of calcined Ti-pillared clay with VO 2+ ions, or by direct addition of V O 2+ ions to the pillaring solution, in the process referred to as "co-pillaring". The doping procedure was described in detail elsewhere [7]. The samples were denoted as follows: Ti-pillared clay calcined at 450~ - Ti-PILC; Ti-PILC exchanged with VO 2+ (x number of exchange treatments) and calcined at 450~ - Vx-(Ti-PILC); co-pillared sample, 10 at.% V in the pillaring solution, calcined at 450~ - (V-Ti)-PILC. A V/TiO2 sample prepared in a conventional manner by impregnation of a commercial anatase support (Tioxide, 48 mZ/g) with ammonium metavanadate solution, followed by calcination at 723 K and washing with ammonia solution served as a reference in catalytic tests. Treatment with NH3aq. was aimed at

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removal of bulk vanadia from the supported catalyst to leave only V species strongly adhering to the surface. The catalysts were characterised with PXRD (oriented samples on glass slides, DRON-3 diffractometer, Ni filtered Cu Ka radiation), ESR (samples reduced mildly by 5 rain. exposure to isopropanol vapour at 100~ recordings at room temperature and -196~ X-band SE/X Technical University Wroclaw spectrometer, spectra parameters determined from the best-fit computer simulations using the programs SIMAX and COMBI, kindly made available by Dr. Jos6 C. Conesa from Instituto de Catalisis y Petroleoquimica, CSlC, Madrid, Spain), BET (from nitrogen adsorption after outgassing at 200~ using Micromeritics ASAP 2000V apparatus), and chemical analysis (with an ICP-AES Plasma 40 Perkin-Elmer spectrometer), after dissolution of the solids in mineral acids. The oxidative dehydrogenation of propane was carried out in the temperature range 200 520~ in a fixed bed flow apparatus with on-line GC product analysis. The catalyst volume was 0.5 c m 3, particle size 0.2-0.5 mm, the contact time 1 s. The reaction mixture contained 10 vol.% of propane in air. 3. RESULTS AND DISCUSSION The physico-chemical characterisation of the samples is presented in Table 1. The basal spacings and BET specific surface areas of all pillared clays increase with respect to those of the parent Na-mt indicating that the pillaring procedure was successful and the interlamellar spaces became accessible. Table 1 Basal acin S d001, BET specific surface areas and V and Ti content in p 2 i l l a r e d ~ ~sam c l a les Sample d001 SBET V20~ TiO2 [X] [m2g"l] [wt. ~ [wt. %1 Na-mt Ti-PILC Vl-(Ti-PILC) VE-(Ti-PILC) (V-Ti)-PILC

12.5 -25.0 diffuse -25.0 diffuse n.d. -25.0 diffuse

22 323 320 309 286

3.3 5.1 0.1

44.2 42.9 41.4 39.8

Figs. 1 and 2 show the dependence of the propane conversion and of the selectivity to propene on the reaction temperature for all investigated catalysts. Already undoped Ti-PILC matrix shows certain activity but addition of vanadium by means of cation exchange to give Vx-(Ti-PILC) catalysts clearly improves the activity and changes the selectivity profile. The (V-Ti)-PILC sample doped by co-pillaring is the least active of all V-containing catalysts, which is clearly due to the very small content of vanadium that entered the clay structure. Moreover, it is also less active than the undoped Ti-PILC matrix, which shows that formation of centres responsible for the activity of Ti-PILC is suppressed in the process of co-pillaring. The selectivity to propene vs. temperature profiles differ strongly from sample to sample. The curve for undoped pillared clay shows an unusual increase in selectivity with increasing temperature, followed by a decrease at highest investigated temperature. While looking for a possible explanation of the initial increase one can recall that Ti-pillared clays possess Lewis

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- - e - TI-PILC --~-- Vl-(TI-PILC) - - i - V2-(Ti-PILC)

7060II--I

...m

---<>-- (V-TI)-PILC

-.o... 9 V/TiO2(NH3aq)

.

D

50

~ E

o 40

~

/

r-a)o > 30

I

o 20 -

////..

200

~/

.7 /

/

i,"

[]

//.

/

xllu' "

IV'

// /

/"

/

240 280 320 360 400

440

480

520

T e m p e r a t u r e [~

Fig. 1 Conversion of propane as a function of the reaction temperature. and Bronsted acidities which decrease with increasing temperature [8,9]. One of the hypotheses correlating the selectivity in the oxidation of hydrocarbons with the properties of the catalyst surface points to the crucial role of acido-base properties of the solid [ 10,11]. According to this --e--&----~--o-D-. 9

40

Ti-PILC V1-(Ti-PILC) V2-(Ti-PILC) (V-Ti)-PILC V/TiO2(NH3aq)

o-~'-~30 ~!~\\! -

.,~

.

0

__m 2 0 -

~

.

1

~\ \

01

".n...

9

"x, ~%.~...----..a~'~.

10 =

i

-----o..

..,~'- ~-~

/.," -- ~,,~~ "<

.

200 240 280

.

.

320

.

360

"-~_.~

_.~..~.~ ~ ,,,.

.

.

"-o ........m-\__

.

400 440 480

Temperature

[~

520

Fig. 2 Selectivity to propene as a ftmction of the reaction temperature.

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approach, the propene molecules, which can be considered basic, desorb more easily from the less acidic surfaces preventing in such a way the subsequent oxidation to carbon oxides. The co-pillared sample is the most selective one in the whole range of studied temperatures. Of the two Vx-(Ti-PILC) catalysts, the VI-(Ti-PILC) one is more selective up to ca. 400~ where the situation becomes reverse. Above this temperature the selectivities to propene show an increasing trend, the effect being more pronounced for the V2-(Ti-PILC) catalyst. Bearing in mind that the catalysts have been calcined at temperature by 70~ lower than the maximum temperature of the catalytic reaction (520~ a possible modification of the catalyst has to be considered as a potential reason for the phenomena observed above 450~ However, the upward swing in the high temperature part of the selectivity vs. temperature profiles appears also when the catalytic run is repeated over the same catalyst. Thus, it seems unlikely that the effect is due to the changes in the catalyst structure. The more probable explanation is that at high conversions the reaction feed becomes depleted of oxygen and the consecutive oxidation of propene becomes more difficult. The shape of the relevant conversion vs. temperature curves, which become almost horizontal in the high temperature range, also points to the relative exhaustion of oxygen in the reaction mixture. The similar activation energies of the propene formation and of the subsequent total oxidation reported for the vanadia/titania catalysts [ 12] justify the comparison of the selctivities at isoconversions obtained at different temperatures. The higher selectivity at 10% conversion observed for the co-pillared (V-Ti)-PILC sample (18%) with respect to the values obtained for Vx-(Ti-PILC) (13% and 14% for VI-(Ti-PILC) and V2-(Ti-PILC) respectively) may be partly due to the slightly lower surfae area of the co-pillared sample, but the difference in the electronic structure of V centres produced by two methods of doping should not be overlooked. ESR spectra of investigated samples are presented in Figure 3. The signals have been generated by a mild reduction of the samples in isopropanol vapour. All show well resolved hyperfine structure associated with the interaction of the unpaired electron with the V 51 nucleus (1=7/2). Their ESR parameters are gathered in Table 2. From our detailed ESR Table 2 Parameters of the ESR spectra of V-doped titania pillared clays and of the reference c a t , s t reduced at 100~ with i s o p . ~ ~ o u r . ...........

S . a m P . . ! . e

............................

(V-Ti)-PILC VI-(Ti-PILC) V/TiO2NH3a~

...................................

1.935 1.932

..........................

1.976 1.978 1.977

..................

184.0 192.4 187.4

...........................

63.4 65.0 62.8

.................

0.79 0.84 0.82

investigations of these materials presented elsewhere [7,13], it is known that, irrespective of the method of preparation, V species present in titania-pillared clays are bound exclusively to the titania component. Additionally, it has been found that in the co-pillared clay the vanadyl centres are characterised with the lowest value of the ~2 coefficient which informs about the degree of localisation of the unpaired electron on the vanadium dxy orbital. This means that these species possess higher degree of rc-covalency within the V-O bonds of bridging nature than species anchored at the surface of titania pillars by means of cation exchange. Similar effect is observed in signals of pillared clay samples reduced with isopropanol vapour examined in the present work (Table 2). In any reaction mechanism proposed for ODH of propane, irrespective whether the activation step consists in the homolytic or the heterolytic dissociation of C3H8, the created propyl radical or anion radical has to loose H or H in order to transform into a propene molecule [ 1]. This is equivalent to a transfer of a proton and one or

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20013

Fig. 3 ESR spectra of a) (V-Ti)-PILC, b) VI-(Ti-PILC) and c) V/TiO2(NH3aq.) at -196~ two electrons to the lattice. While the recipient of the former may be a surface oxide anion the ultimate sink for electrons in the case of V-based catalysts are vanadium centres. If the bridging oxygens were involved in the step transforming the surface propyl radical into propene molecule, the higher selectivity of the co-pillared samples could be explained in terms of a more facile electron transfer via oxygen-vanadium re-electron system. Indeed, the particular activity of bridging oxygens in selective oxidation processes has been shown both in experimemal studies [ 14] and theoretical calculations [ 15]. Selectivities to CO and CO2 change in a similar manner for all V-containing PILC samples. Selectivity to CO is coupled with that of C3H6, i.e. when one increases the other decreases. Similar behaviour was observed frequently in ODH of propane and indicates that the main source of CO is the consecutive oxidation of propene [ 11]. Selectivity to CO2 generally shows a decreasing trend. The highest yields of propene are obtained on V• samples, prepared by cation exchange method. They are observed for maximum temperatures where the effect of the increase of selectivity appears. It is interesting to compare the performance of V-doped pillared clays with that of a V/TiO2 catalyst which prior to the catalytic test had been subjected to washing with ammonia. The latter treatment is supposed to leave at the surface only firmly bound V species, and it is expected that such material should show similarity to V-doped pillared clays. Indeed, the course of the selectivity/conversion curve for this material, although slightly shined upwards,

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lies parallel to those of the Vx-(Ti-PILC) catalysts (Fig. 3). The ESR spectrum of this sample is similar to those of V-doped clays which confirms that isolated V species dominate at the surface of this catalyst. The g-factors of this signal are similar to those of signals observed for pillared clays, but the difference in the hyperfine splitting constants and, consequently, 132 coefficients, shows that there are also some differences in the nearest surrounding of the V species concerned. The 132value lies between those of (V-Ti)-PILC and VI-(Ti-PILC) catalysts and so does the selctivity/conversion curve which again may point to the importance of the de/o'ee of ~-covalency within the V-O bonds perpendicular to the vanadyl axis. 4. CONCLUSIONS Catalytic performance of the V-containing titani~,-pillared samples in the ODH of propane depends on the method of vanadium incorporation. Co-pillaring gives catalysts of little V content which are less active but more selective than those containing more vanadium and prepared by means of cation exchange. The latter ensure highest yields of propene. The increased selectivity of the co-pillared sample is tentatively assigned to the presence of V centres of higher rc-covalency within bonds between vanadium and bridging oxygens. Beating in mind that in the clay samples V species are associated exclusively with the titania pillars, the materials may be regarded as model catalytic systems for vanadium centres dispersed on the high surface area titania.

Acknowledgements: This work was financially supported by the Polish Committee for Scientific Research within the research project 3 T09A 079 14. The authors wish to thank Professor Barbara Grzybowska for critical reading of the manuscript and helpful comments.

REFERENCES

1. E.A. Mamedov and V. Cortes Corberfin, Appl. Catal. A, 127 (1995) 1 and references therein. 2. F. Cavani, F. Trifir6, Catal. Today 24 (1995) 307-313. 3. G.W. Brindley and R.E, Sempels, Clay Miner., 12 (1977) 229. 4. N. Lahav, U. Shani and J. Shabtai, Clays Clay Miner., 26 (1978) 107. 5. S. Yamanaka and G.W. Brindley, Clays Clay Miner., 27 (1979) 119. 6. J. Sterte, Clays & Clay Miner., 34 (1986) 658. 7. K. Bahranowski, M. Labanowska, E.M. Serwicka, Appl. Magn. Reson., 10 (1996) 477. 8. S.A. Bagshaw and R.P. Cooney, Chem. Mater., 5 (1993) 1101. 9. H.L. del Castillo, A.Gil and P. Grange, Catal. Letters, 36 (1976) 237. 10. M. Ai, Bull. Chem. Sos. Jpn., 49 (1976) 1328. 11. R. Grabowski, B. Grzybowska, K. Samson, J. Stoczyfiski, J. Stoch and K. Wcisto, Appl. Catal. A, 125 (1995) 129. 12. J. Stoczyfiski, R. Grabowski, K. Wcisto and B. Grzybowska-Swierkosz, Polish J. Chem., 71 (1997) 1585. 13. K. Bahranowski and E.M. Serwicka, Colloids Surfaces, 72 (1993) 153. 14. G. Deo, I. Wachs and J. Haber, Crit. Rev. Surf. Chem., 4 (1994), 141. 15. M. Witko, R. Tokarz and J. Haber, Appl. Catal. A, 157 (1997) 23.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Oxidative Dehydrogenation of Propane Over Alkali-Mo Catalysts Supported on Sol-Gel Silica-Titania Mixed Oxides Rick B. Watson and Umit S. Ozkan* Department of Chemical Engineering, The Ohio State University, Columbus, OH 43210, USA A group of novel molybdena catalysts for propane oxidative dehydrogenation (ODH) is presented. These catalysts, which were prepared using a sol-gel/co-precipitation technique, were supported on silica-titania mixed oxides and promoted by alkali metals (Li, Na, K, Cs). While reaction results show no specific trend with the type of alkali promoter, comparisons indicate that promotion with potassium gives a superior catalyst. Propylene yields around 30% were obtained with K/Mo catalysts at 550~ in dilute feed stream experiments. Differences in reaction performance are found to be closely related to surface acidity, oxidation- reduction behavior, and dispersion of the active components. 1.

INTRODUCTION

Olefin feedstocks, such as propylene, find wide use in industrial processes. The strong industry demand for propylene has led to numerous research efforts on the use of a catalytic oxidative dehydrogenation (ODH) process to produce propylene from propane. This process offers several advantages compared to conventional cracking and dehydrogenation processes because ODH is not thermodynamically limited at lower temperatures and usually does not lead to the formation of coke and smaller hydrocarbons. Research efforts on ODH primarily focus on the development of catalysts active at low temperatures (<550~ that show low selectivities to carbon oxides. The most active and selective catalysts studied in recent literature include vanadium-magnesium, vanadia supported on niobium, and nickel molybdates. In particular, promising results have been obtained when molybdate-based catalysts are promoted or supported. For example, Ni-CoMo [1] V-Nb-Mo/TiO2[2], K-MnMoO4[3], and K2MoO414] have shown promise in ODH and other partial oxidation reactions. Furthermore, the increase in activity and/or selectivity of alkali (Li, Na, K, Rb, and Cs) doped metal oxide catalysts has been widely studied [5-7]. These positive effects arise from the alkali's ability to alter oxidation/reduction behavior, affect surface acidity, and/or cause a synergism between alkali and transition metal oxide phases. *To whom correspondence should be addressed Phone: (614)292-6623 Fax: (614)292-3769 E-mail: [email protected]

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Our initial work focused on bulk MoO3 and K2MoO4 catalysts. Bulk, precipitated potassium molybdate catalysts were up to 60% selective to propylene formation but at a conversion less than 6%. Furthermore, these samples showed considerable selectivity to cracking products, i.e. methane, ethane, and ethylene. Un-supported potassium molybdate catalysts had low surface areas because of the precipitation and calcination procedures. In an attempt to form highly dispersed molybdate catalysts and possibly create metal-support interactions, molybdate catalysts supported over the mixed oxides of silica and titania were developed. Study of silica-titania mixed oxides has gained much attention because of their high activity for epoxidation reactions of olefins with hydroperoxides [8]. Silica-titania mixed oxides have been studied extensively [9-14] for attributes such as acidity, porosity, TiO-Si bond connectivity, and phase separations. However, few studies focus on their use for transition metal oxide supports. In this work, alkali-promoted molybdenum catalysts supported over silica-tiania mixed oxides have been studied in regard to their activity for the oxidative dehydrogenation (ODH) of propane. The effect of alkali doping on the catalyst surface characteristics and, in turn, on the catalytic performance has been examined. The catalysts used in this study have been synthesized by a "one-pot" sol gel/co-precipitation technique. The main focus of this work was to investigate the use of different alkali dopants and the effect of alkali/Mo molar ratio. 2.

EXPERIMENTAL

Catalyst preparation Catalysts were prepared using a modified sol-gel/co-precipitation technique. Sol-gel chemistry is extensive [15] and cannot be covered here completely. The method used here involves the reactions of metal alkoxide precursors in an alcohol solvent when contacted with water. Ammonium heptamolybdate (AHM) and alkali (Li, Na, K, Cs) hydroxides were used as molybdenum and alkali precursors, respectively. For silica-titania mixed oxides, tetraethylorthosilicate (TEOS) and titanium(IV)isopropoxide (TIPO) were used. The solvent was isopropyl alcohol. This method is referred to as sol-gel/co-precipitation because as the silica and titania precursors are hydrolyzed and precipitate out of solution, active metal species present in the aqueous solution, that are insoluble in alcohol, also precipitate. A more detailed description of the preparation method has been given elsewhere [ 16]. The catalysts reported in this paper are listed in Table 1. Our previous results have shown that a silica-titania molar ratio of 1:1 support performed the best in the ODH reaction. Therefore, all catalysts were supported over Si:Ti 1:1. These include a series of alkali-doped molybdate catalysts with alkali/Mo molar ratio of 0.1 at constant 10% weight loading of Mo. Potassium-promoted catalysts with different alkali/Mo ratios were also synthesized at a constant Mo loading of 10% and examined in propane ODH. 2.1

2.2

Oxidative dehydrogenation of propane Steady-state reaction experiments were carried out in a fixed-bed, quartz reactor, operated at ambient pressure. Catalyst samples, ranging from 0.1 g to 1.5g, were held in place by a quartz frit. To minimize effects from any homogeneous reaction or surface-initiated gas phase reaction and to provide a short residence time for propylene formed, the dead volume of the quartz microreactor was filled with quartz wool and/or ceramic beads.

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Reaction temperatures ranged from 450 to 550~ The feed consisted of propane, oxygen, and nitrogen usually at a flowrate of 25 cm3/min. Two different feed conditions were evaluated, dilute (5% propane) and concentrated (26% propane). However, the propane/oxygen molar ratio was held constant at 2. The product distributions maintained a carbon balance of 100% (+/- 5%).

Table 1 Catalyst Compositions Composition

BET Surface Area (m2/g)

Si:Ti 1:1 10%Mo/Si:Ti 1:1 10% Li/Mo=0.1)/Si:Ti 1:1 10%, Na/Mo=0.1)/Si:Ti 1:1 10% K/Mo=0.07)/Si:Ti 1:1 10%, K/Mo=0.1)/Si:Ti 1:1 10% K/Mo=0.3)/Si:Ti 1:1 10%, Cs/Mo=0.1)/Si:Ti 1:1

320 229 170 323 136 191 166 188

3.

RESULTS AND DISCUSSION

3.1

Effect of preparation method on physical properties

Alkali-doped molybdate catalysts show no specific trend in BET surface area with differing alkalis or amounts of potassium added. All catalysts containing alkali exhibited lower surface area than the "molybdenum only" catalyst with the exception of the sodiumdoped catalyst. The nitrogen adsorption-desorption isotherm of the Si:Ti 1:1 support indicated a micro to meso-porous structure. The pore size distribution was calculated using the desorption isotherm. This yielded an average pore diameter of 2.1nm and a pore volume of 0.34cma/g. X-ray diffraction of the Si:Ti 1"1 support yielded a pattem typical of a silicatitania sample with one broad peak located at a d spacing of 3.59 A, which is the most intense diffraction line from the anatase structure. Molybdenum species were not detected in the 10%(alkali/Mo) loaded Si:Ti 1:1 catalysts. This suggests that molybdena species on the mixed oxide supports are more finely dispersed than on a silica or titania support alone. Raman spectra of the Si:Ti 1:1 support showed that the bands associated with the anatase structure were shifted to lower wavenumbers than those of pure anatase [8]. Evidence for SiO-Ti connectivity also was observed in the Raman spectra. Bands associated with terminal Mo=O stretches around 950cm -~ were observed on all catalysts studied except for the 10%(Na/Mo=0.1)/Si:Ti 1:1 catalyst. Considering the higher surface area of this catalyst, surface molybdate species may be undetectable. Broad bands arising from Mo-O-Mo vibrations were observed around 850cm -1. From the Raman spectra, there is no evidence of crystalline MoO3. Molybdenum in these catalysts is in the state of surface coordinated molybdena species. Details of further characterization studies are presented elsewhere [ 16].

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3.2

Equal surface area reaction experiments Alkali/Mo catalysts were tested in the ODH reaction using equal surface area loading (65m 2) in the reactor and at temperatures of 450~ and 550~ The feed percentages for these experiments were Nz/C3/Oz: 61/26/13. The results are presented in Table 2.

Table 2 Reaction Results for Alkali/Mo Catalysts 10% (Mo)/Si:Ti 1:1 T (~ T (~ 10%(Li/Mo =0.1)/Si:Ti 1:1 T (~ T (~ 10%(Na/Mo=0.1)/Si:Ti 1:1 T (~ T (~ 10% (K/Mo=0.1)/Si:Ti 1:1 T (~ T (~ lO% (Cs/Mo=O.l)/Si:Ti 1:1 T (~ T (~

C3H 6 11.4 20.0

Yield(%) CO 2 CO 2.3 4.9 3.1 6.7

C2H 4 0.0 0.4

CH 4 0.1 0.8

11.2 18.3

3.5 4.5

6.3 7.5

0.0 0.2

0.0 0.4

15.2 21.0

1.7 1.5

2.2 6.6

0.1 0.5

0.0 0.3

17.2 22.3

2.9 4.9

5.2 6.7

0.0 0.4

0.0 0.8

9.1 21.4

0.7 4.7

0.5 5.6

0.0 0.2

0.0 0.3

Conditions: equal surface area (65m2), %N2/C3/O2:61%/26%/13%, 25cc/min. Results indicate that the alkali promoted catalysts behave rather similarly under these conditions. However, the Cs-promoted catalyst shows a considerably lower activity at 450~ whereas the potassium-promoted catalyst exhibits the highest yield of propylene. The propylene yield of these catalysts increases in the order Cs, Li, Na, K at 450~ and Li, Na, Cs, K at 550~ There appears to be no correlation between catalyst activity and atomic weight of the alkali promoter. However, the differences in performance may be due to not only the size of the alkali but other parameters such as dispersion and interaction of the support with alkali and molybdena species. While the differences were small, the potassiumpromoted catalyst gave a better ODH performance, warranting its further investigation. 3.3

Selectivity comparisons at equal conversions

To compare catalysts with different K/Mo ratios, a series of equal conversion experiments were run at 450~ Equal conversions were obtained by changing the mass of catalyst loaded inside the reactor. The feed concentrations were %N2/C3/O2: 61/26/13. The results are presented in Table 3. It is seen that alkali-promoted catalysts give rise to significantly higher selectivities to propylene and lower selectivities to carbon oxides.

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The selectivities to cracking products do not seem to vary much with the incorporation of the alkali promoter. Another important feature of these comparison experiments is that the product distribution obtained over the bare support heavily favors carbon oxides. Ethylene production over the support is also substantial.

Table 3 Equal Conversion Comparison for K/Mo Catalysts Selectivity (%) C3H6 CO2 CO C2H4 CH4

"~5% Calls Conversion Si:Ti 1"1 Support

32.9 86.0 92.3 96.1

10%(Mo)/Si:Ti 1:1 10%(K/Mo =0.07)/Si:Ti 1:1 10%(K/Mo=0.3)/Si:Ti 1:1

40.8 5.6 5.5 3.2

22.5 8.4 2.2 0.5

2.9 0.1 0.1 0.1

0.6 0.0 0.0 0.1

C2H6 0.3 0.0 0.0 0.0

Selectivity (%) -10% C3Hs Conversion Si:Ti 1"1 Support 10%(Mo)/Si:Ti 1:1 10%(K/Mo =0.07)/Si:Ti 1:1 10%(K/Mo=0.3)/Si:Ti 1:1 Conditions: 450~ ~

3.4

C3H6 CO2

CO

23.7 79.3 88.5 91.7

44.1 13.6 6.4 3.3

27.2 7.0 5.0 4.9

C2H4 CH4 4.1 0.1 0.1 0.1

0.9 0.0 0.0 0.0

C2H6 0.0 0.0 0.0 0.0

25cc/min.

Dilute feed stream experiments

A set of experiments was performed with more dilute propane concentrations, NJCaH8/O2: 92.5/5/2.5, which exhibited a higher molar yield of propylene. The results are presented in Table 4. Under these conditions, the catalyst with a K/Mo ratio of 0.07 gave the highest yield of propylene yield (around 30 %). It was also seen that the propylene yield starts decreasing when alkali loading is further increased. The bare support showed a very small propylene yield for this set of experiments. The potassium-containing catalysts showed Table 4

Reaction Results for Dilute Feed Stream Experiments Si:Ti 1:1 Support

C3H8 Conversion (%) C3H6 CO2 11.2

10%Mo/Si:Ti 1:1

1.4

6.0

C3H6 CO2 39.7

10%(K/Mo=0.07)/Si:Ti 1:1 48.4 10%(K/Mo=0.3)/Si:Ti 1:1 35.0 Conditions: 5s feed residence time, 550~

19.2

Yield (%) CO C2H4 CH4 C2H6 3.4

CO

10.3

5.0

C3H6 CO2

CO

30.1

4.3

9.2

C3H6 CO2

CO

23.6

2.5

8.1

%N2/C3/O2 92.5/5/2.5

0.2

0.1

0.0

C2H4 CH4 C2H6 2.1

2.0

1.1

C2H4 CH4 C2H6 2.0

1.8

0.9

C2H4 CH4 C2H6 0.4

0.4

0.0

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improved yields to propylene and lower yields of CO, CO2, and cracking products when compared to the un-promoted catalyst. It appears that molybdena catalysts supported over Si:Ti mixed oxides has ODH activity, but the incorporation of small quantities of an alkali promoter significantly improves the product distribution in favor of propylene. Our characterization results [ 16], as well as some previous reports in the literature [17,18], suggest that this effect is achieved by suppression of the acidity and reducibility of the molybdenum surface sites. These changes in the surface characteristics, in turn, affect the alkane activation performance of the catalyst and the adsorption/desorption characteristics for the olefins. It appears that there is an optimum alkali/Mo ratio, which provides the best balance between the alkane activation and the olefin combustion rates to give the highest propylene yields. It should also be kept in mind that the differences observed in the ODH performance of these catalysts cannot be explained solely in terms of the direct modification of the surface by the alkali promoters. Other structural and surface characteristics, which may be affected by the sol-gel parameters, can also change the catalytic behavior. Studies are underway to examine the oxygen mobility, dispersion, support structure and the adsorption/desorption behavior of these catalysts. These studies will help correlate the surface and structural characteristics of this group of novel catalysts with their catalytic properties for propane oxidative dehydrogenation. 4. R E F E R E N C E S 1.

2. 3. 4. 5. 6. 7. 8.

9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

Stem, D. L., Grasselli, R., J. Catal., 167 (1997) 550. Meunier, F. C., Yasmeen, A., Ross, J.R.H., Catal. Today 37 (1997) 33. Driscoll, S. A., Gardner, D. K., Ozkan, U. S., J. Catal., 147 (1994) 379. Erdohelyi, A., Mate, F., Solymosi, F., J. Catal., 135 (1992) 563. Mross, W.D., Catal. Rev.-Sci. Eng., 25(4) (1983) 591. Abello, M.C., Gomez, M.F., Cadus, L.E., Catal. Letters, 53 (1998) 53. O'Young, C.L., J. Phys. Chem., 93 (1989) 2016. Notari, B., Adv. Catal., 41 (1996) 253. Gao, X., Wachs, I. E., Catal. Today, 51 (1999) 233. Stakheev, A.Y., Shipiro, E.S., Apijok, J., J. Phys. Chem., 97 (1993) 5668. Lassaletta, G., Femandez,A., Espinos, J.P., Gonzalez-Elipe, A.R., J. Phys. Chem., 99 (1995) 1484. Waiters, J.K., Rigden, J.S., Dirken, P.J., Smith, M.E., Howells, W.S., Newport, R.J., Chem. Phys. Letters, 264 (1997) 539. Kumar, S.R., Suresh, C.,. Vasudevan, A.K, Suja, N.R., Mukundan, P., Warrier, K.G.K., Mat. Letters, 38 (1999) 161. Klein, S., Thorimbert, S., Maier, W.F., J. Catal., 163 (1996) 476. Brinker, C.J, Scherer, D.W., "Sol-Gel Science", Academic Press, New York, 1990. Watson, R.B., Ozkan, U.S., J. Catal, Accepted for publication. Feng, Z., Liu, L., Anthony, R.G., J. Catal., 136 (1993) 423. Grabowski, R., Grzybowska, B., Samson, K., Sloczynski, J., Stoch, J., Wcislo, K, Appl. Catal., 125 (1995) 129.

ACKNOWLEDGEMENTS

Financial support provided by the National Science Foundation (Grant# CTS-9412544) is gratefully acknowledged. The authors would also like to thank Dr. Gurkan Karakas for his technical assistance at the early stages of the project.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

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Vanadium oxide supported on gallium and indium physicochemical and catalytic properties

oxides:

synthesis,

B. Sulikowski, A. Kubacka, E. Wloch, Z. Schay, a V. Cortes Corberdn b and R.X. Valenzuela b Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 1, PL 30-239 Krak6w, Poland. E-mail: [email protected] alnstitute for Isotope and Surface Chemistry, Chemical Research Center, Hungarian Academy of Sciences, Konkoly Thege M. fit. 29/33, H-1525 Budapest, Hungary bInstituto de Cat~ilisis y Petroleoquimica, CSIC, Campus U.A.M. Cantoblanco, 28049 Madrid, Spain Vanadium oxide was supported on gallium(III) and indium(III) oxides, with the vanadium superficial coverage from 0.6 to 1.8 of the theoretical monolayer. A stronger vanadia/support interaction was observed by Raman and XPS spectroscopies for the V-Ga samples. While combustion of alkanes and low selectivity to olefins were observed predominantly for the V-In samples, the V-Ga system exhibited enhanced selectivity to olefins in the oxidative dehydrogenation (OXD) of ethane and propane. 1. INTRODUCTION Unique catalysts can be obtained by depositing of one oxide phase onto a support, which usually is another oxide or mixed oxide. Vanadium oxide is widely used as a major component of numerous mixed oxide catalysts for the partial oxidation reactions of hydrocarbons to oxygenated products [1]. Catalytic properties of supported V205 oxide depend largely on the amount deposited on the support, type of support and presence of other promoters. They do not depend, on the contrary, on a method used for depositing vanadia. The most significant changes in such catalysts are observed at low vanadium loadings, in other words when the surface coverage by vanadia is below the monolayer. One of the most investigated system was V205 on TiO2, which constitutes an important catalyst for oxidation of o-xylene to phthalic anhydride, ammoxidation of alkyloaromatic hydrocarbons and selective oxidation of olefins. Vanadium pentoxide was also deposited onto A1203, SiO2, CeO3, Cr203, ZrO2 and their combinations, just to mention a few. Several systems containing V-Mg-O [2] and V-Nb-Ti-O are also known [3]. A large number of catalysts has been studied in the OXD of alkanes: mixed oxides containing vanadium [1], various molybdates and zeolite catalysts [4, 5]. In particular, Gaand Zn-containing zeolite catalysts were investigated in dehydrogenation of hydrocarbons [6].

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In this paper we wish to report, for the first time, on synthesis and characterization of vanadium oxide supported on gallium(III) and indium(Ill) oxides, respectively. We will also outline catalytic properties of such systems in the OXD of ethane and propane.

2. EXPERIMENTAL

Vanadium(V) oxide (p.a.) was dissolved in the solution of oxalic acid. Then the calculated amount of the vanadium oxalate solution was carefully mixed with gallium(Ill) oxide (Aldrich, surface area BETAr = 27.5 m2/g) or indium(Ill) oxide (BETAr = 5.0 m2/g). After drying at ambient temperature the samples were calcined in He (VIn series) or Ar (VGa series) at 500 ~ for 2 h (heating rate from room temperature to 500 ~ was 15 deg/min). The samples were then cooled down, crushed and sieved to give 0.30 - 0.42 mm fraction used for catalytic tests. A list of the samples is given in Table 1. Table 1

No. 1. 2. 3. 4. 5. ........ ,

a _

Composition of vanadium-containing samples supported on Ga(III) and In(III) oxides

Sample Label 0.6VIn 1.2VIn 1.8VIn 0.6VGa 1.5VGa

BET (Ar) (m2/g) 6.7 7.0 6.3 15.9 18.6

Composition V205 (wt~ 0.58 1.16 1.76 0.59 1.50 .

.

.

.

.

.

.

.

0a

V (atoms/nm 2)

0.58 1.16 1.76 0.59 1.50

5.8 11.6 17.5 5.9 14.9

.

vanadium superficial coverage, calculated assuming the theoretical monolayer is equal to 9.96 V/nm 2 [7].

Laser Raman spectra were collected at ambient temperature with a FT Bruker RFS 100 spectrometer equipped with a liquid-nitrogen-cooled germanium detector. A Nd:YAG laser was used as the excitation source (L = 1046 nm), and the beam power on the sample was adjusted as necessary (50-200 mW). Typically 128 or 256 scans were taken. For the samples 0.6 VGa and 1.5 VGa, exhibiting wery weak Raman signal, 2048 scans were accumulated. Powder XRD patterns were acquired on a Siemens powder diffractometer using Ni-filtered Cu Kcx radiation. XPS spectra were recorded with a KRATOS XSAM-800 machine in FAT mode using 40 eV pass energy. The X-ray source was Mg Ka. The samples for these measurements were prepared from ethanol suspensions. X-ray satellite subtraction of the O l s peak was used to evaluate the V 2p range. The OXD of ethane and propane was performed in a continuous flow quartz reactor (internal diameter 9 ram) under steady-state conditions. Typically, 0.5 g of catalytic particles (0.25-0.42 mm) were mixed with double amount (by volume) of SiC chips (0.50-0.84 mm) and loaded into a tubular, down-flow quartz reactor. Special precautions have been undertaken in order to suppress the homogeneous reaction of hydrocarbons. To this point special design of the reactor geometry was applied. Also, remaining empty space of the reactor was filled with additional amounts of SiC. Decomposition of ethane and propane in the reactor filled only with SiC was also studied. The propane (99.5%) or ethane (99.5%) -

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oxygen (99.995%) mixture diluted with He (99.999%) was fed into the reactor to give a total flow of 100 cm3/min. The He : (propane or ethane) : oxygen ratio was adjusted as desired to 92:4:4 or 88:4:8 to give W/F - 54.2 g catal.* h/gmol Ca or C2). An electronic flow control unit was connected to Brooks 5850 mass flowmeters, to give control of gases flow better than + 2%. Catalytic tests were performed at temperatures 400 - 600 ~ under the atmospheric pressure. The conversion of alkanes was studied under steady-state conditions, which were established after 10 to 30 minutes on stream at given temperature. After the test performed at the highest temperature, the sample was cooled down to the initial temperature, to assess the change of conversion and selectivity, if any. Reactants and products were analyzed on-line with a Varian 3400 CX gas chromatograph equipped with a TCD detector, and two columns filled with Porapak Q and molecular sieves 13X. Most mass and carbon balances were found to be within 100a:2% (occasionally, maximum error of 100-]=5% was allowed).

3. RESULTS AND DISCUSSION The supports and samples containing vanadia were studied by XRD. The corresponding XRD patterns of pure In203, Ga203 and vanadium deposited onto the two supports are visualized in Figures 1 and 2. As seen, upon loading of In203 with vanadium species no changes in the corresponding XRD patterns, which might be ascribed to formation of V-In-O phases, can be discerned. We conclude therefore that, under the conditions used for catalysts preparation, no new phases were formed for the vanadia supported on indium(III) oxide. Indium(III) oxide gives a very good quality Raman spectrum with a high signal-to-noise ratio, in which the most characteristic bands are seen at 132, 306, 365 and 628 cm -l (Fig. 3a). When indium oxide is loaded with vanadia species, a new signal arises at 914 cm -1. This signal increases with the V loading, albeit not proportionally to the amount of V species in the samples (Fig. 3b-d). The signal at 914 cm -l is accompanied by the two other, which can be seen as small humps at 390 and 343 cm -l for the 0.6 VIn, and as the two well separated bands for the other samples. All three signals increase significantly in intensity with V content in the

L

~

,L

) . . . .

_..__,___...~ ~

2'o

3'o

t

. . . .

)

4'o

20

50

6'0

Fig. 1 XRD diffraction pattems of pure indium(Ill) oxide (a); and samples 0.6 VIn (b); 1.2 VIn (c) and 1.8 VIn (d), calcined in a He flow at 500 ~ for 2 h.

2'o

3'o

4'o

5'0

2| Fig. 2 XRD diffraction pattems of pure gallium(Ill) oxide (a); and samples 0.6 VGa (b) and 1.5 VGa (c), calcined in an Ar flow at 500 ~ for 2 h.

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化

r O

.

~

~I" ~-

0)

(D 0)

0 0) e~I

c)

c)

k.

"A b)

b)

,

-~_

~2'00

10'00

~"

800

~L,~k~~ 600

460

a) 260

Raman shift, cm -~ Fig. 3 FT laser Raman spectra of pure indium(Ill) oxide (a); and samples 0.6 VIn (b), 1.2 VIn (c) and 1.8 VIn (d).

b

12'00 1600 ado

abo

4t50

200

0

Raman shift, cm" Fig. 4 Comparison of FT laser Raman spectra of indium oxide (a), vanadium oxide (b), and vanadia supported on In203, sample 1.8 VIn (c).

samples 1.2 and 1.8 VIn (Fig. 3c,d). Upon close inspection of spectra, the fourth weak signal at 218 cm -I can also be found. Vanadium oxides deposited on alumina and titania were extensively studied by, inter alia, Raman spectroscopy. A signal at 930 cm ~ was assigned to the V=O symmetric stretch for 2% V2Os/A1203 [8]. Upon dehydration, the signal in this region shifts to higher frequency at 1034 cm -~. It is also known that the position of the signal around 900 cm -l changes with the support. Accordingly, the signal at 914 cm -~ found for the V-In samples can be assigned to V=O units. Appearance of microcrystals of V205 can be conveniently monitored by Raman spectroscopy, since the Raman cross sections of V oxides are much higher than for the supports [9]. As seen, the major Raman bands for crystalline V205 are present at 994, 701, 284 and 145 cm -~ (Fig. 4b). In all spectra of the VIn series we did not find even the traces of crystalline V205, in a good agreement with XRD results. The signals around 390 and 343 cm-l, clearly seen in the samples 1.2 VIn and 1.8 VIn, may be therefore assigned to v (O-V-O)and 8 (V-O-V) vibrations, respectively [3]. These bands are of course more pronounced for the samples containing more vanadia (Fig. 3). Finally, a very small signal at 218 cm -1 is due to 8 (V-O-V) linkages [10]. Raman spectrum of pure [3-Ga203 shows lines at 112, 144, 168, 197, 345, 416, 475, 653 and 765 cm -1 (Fig. 5a). The bands in the range 300-600 cm -1 correspond to bending vibrations, while the signal at 770 cm l is due to the Ga-O4 tetrahedral stretching modes [ 11 ]. For the VGa samples a very weak Raman signals and high fluorescence were observed. Consequently, spectra with low signal-to-noise ratio could only be obtained, despite the conditions of the measurement. Two spectra shown in Fig. 5b,c differ substantially from these recorded for the VIn series. First, the signals are very broad. Second, the lines corresponding to Ga203 are either completely suppressed or are very weak, even the most intense line at 197 cm l. Third, two broad features around 732 and 904 cm -1 are visible for VGa samples. Earlier studies have revealed that in a V/7-A1203 system the lines around 970 cm 1 were due to polymeric form of vanadia (VO6), while the signal at 830 cm l was ascribed

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to isolated VO43" units. Consequently, the largest signal around 732 cm l for sample 0.6 VGa may be assigned to the isolated tetrahedral V-O species. One would expect decreasing the amount of these species upon increasing vanadium content, and it is indeed to observed (Fig. 5c). In this sample some to ~ 732 r ~ other weak features are also seen, at 145,284, 992 and 1115 cm "1. These are assigned to crystalline V205 (cf. Figi ~, b) 4b). A broad band around 904 cm I corresponds to polymeric form of to r to J vanadia. Finally, and most remarkably, no sharp band characteristic for V=O vibrations can be seen in both VGa samples. 12'00 10'00 860 660 460 260 t) In the V-In samples XPS studies R a m a n shift, c m -~ revealed binding energy (BE) of V 2p equal to 517, 517.3 and 517.5 eV, Fig. 5 FT laser Raman spectra of pure characteristic for V 5+. Contrary to this, gallium(III) oxide (a); and samples 0.6 VGa a broad signal was found for VGa series, (b) and 1.5 VGa (c). and the BE was lower (516.7 eV for 1.5 VGa) indicating oxidation state close to V4+. All these data suggests that a stronger interaction exists between vanadia species supported on gallium oxide. No dehydrogenation of propane was observed on In203 in the temperature range of 400500 ~ Combustion to COx was the only reaction observed, with the conversion not exceeding 11%. At 475-500 ~ cracking of C3H8 to methane and ethene was accompanying combustion. Similar observations were made for transformation of ethane. Small amounts of ethene were formed under all conditions studied (selectivity lower than 2.5 %). Upon supporting vanadia species on In203 the conversion was increased. The sample 0.6 VIn containing the smallest amount of V was the most active (Table 2). However, the selectivity towards ethene at the 10 % conversion level, while higher than for pure In203, was still very low (8 %). When the amount of vanadia was increased, the selectivity was increased up to 27.5 %.

Table 2

Sample 0.5 VGa 1.5 VGa In203 0.6 VIn 1.8 VIn

Selectivity to olefin and temperature required for 10 % conversion level of a hydrocarbon in the OXD of ethane and propane. Helium : CxHy : O2 = 88 : 4 : 8 Ethane Selectivity (%) Temp. (~ 47 405 42 385 1.3 490 8 478 27.5 542

Propane Selectivity (%) Temp. (~ 44 340 42 400

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化

e0

(c) v

(a)

2O

.qo :_.~ 20.

m l raue Fig. 6 Conversion of propane vs. temperature on: (a) Ga203; (b) 0.5 VGa and (c) 1.5 VGa. He:C3Hs:O2 = 88 : 4 : 8.

(c) 6

lb

2'0

~0

go

50

Conversion (tool%)

Fig. 7 The selectivity of propene formation as a function of conversion on: (a) Ga203; (b) 0.5 VGa and (c) 1.5 VGa.

In Figs. 6 and 7 catalytic results obtained for Ga203 and VGa samples are depicted. The sample 0.5 VGa was the most active, while a meaningful conversion of propane on pure gallium oxide was observed only for temperatures exceeding 500 ~ (Fig. 6a). As seen in Fig. 7, the selectivity towards olefin formation was higher up to about 10 % conversion level on 0.5 VGa. More stable selectivity in the whole conversion range was obtained for higher V loading (sample 1.8 VGa, Fig. 7c). In Table 2 the results of catalytic tests on In203, VGa and VIn samples are summarized. We note that low selectivity towards olefin formation was observed for the VIn samples, for which a discrete signal of V=O groups was found (914 cm-1), in contrast to the VGa catalysts. The project was supported by the Polish Academy of Sciences/C.S.I.C. Interchange Agreement and European Commission (INCO-Copemicus N ~ ERBIC15-CT96-0725). REFERENCES

1. 2. 3. 4. 5.

E.A. Mamedov and V. Cort6s Corberfin, Appl. Catal. A, 127 (1995) 1. H.H. Kung and M. C. Kung, Appl. Catal. A, 157 (1997) 105. I.E. Wachs, J. M. Jehng and F. D. Hardcatstle, Solid State Ionics, 32/33 (1989) 904. B. Sulikowski, Z. Olejniczak and V. Cort6s Corber~n, J. Phys. Chem., 100 (1996) 10323. V. Cort6s Corber~n, R. X. Valenzuela, B. Sulikowski, M. Derewinski, Z. Olejniczak and J. Krysciak, Catal. Today, 32 (1996) 193. 6. B. Sulikowski, Heterogeneous Chem. Rev., 3 (1996) 203. 7. J.M. L6pez Nieto, G. Kremenic and J. L. G. Fierro, Appl. Catal. A, 61 (1990) 235. 8. S.S. Chan, I. E. Wachs, L. L. Murel, L. Wang and W. K. Hall, J. Phys. Chem., 88 (1984) 5831. 9. S.S. Chan, I. E. Wachs and L. L. Murel, J. Catal., 90 (1984) 150. 10. M. A. Vuurman and I. E. Wachs, J. Phys. Chem., 96 (1992) 5008. 11. D. Dohy, G. Lucazeau and A. Revcolevschi, J. Solid State Chem., 45 (1982) 180. 12. A. Khodakov, B. Olthof, A.T. Bell and E. Iglesia, J. Catal., 181 (1999) 205.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

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Oxidative dehydrogenation of cyclohexane over heteropolymolybdates S. Hocine l, C. Rabia l, M.M. Bettahar2, M. Foumier2 1- Laboratoire de Chimie du Gaz Naturel, Institut de Chimie, BP32, El-Alia, 16111 BabEzzouar, Alger, Alg6rie 2- Laboratoire de catalyse H6t6rog6ne et Homog~ne, URA CNRS N~ 59655Villeneuve D'ASCQ Cedex, France.

B~timent C3,

The oxidation of cyclohexane in presence of molecular oxygen has been studied at 350 and 400~ over heteromolybdates catalysts pretreated at respectively 350 and 400~ under O2 stream. This study examined the relationship between the acid-base and redox properties of Keggin-type heteropolycompounds (HPA) and their catalytic behavior in the oxidation of various C6 hydrocarbons. The obtained results showed that HPA were very selective for the cyclohexane oxidative dehydrogenation to benzene and have pronounced catalytic activity which depends on pretreatment and reaction temperatures, nature of oxometal (Mo,V,Ni) in the primary structure and counter-ion (H30+,Cs+,Fe2+). 1. INTRODUCTION Particular attention is given to most recent developments in the use of Keggin type heteropolycompounds (HPA) as catalysts for the selective oxidation of hydrocarbons [1-4]. This reaction appear to be an alternative way to functionalize low molecular weight paraffins in substitution of the more expensive olefins to obtain oxygenated compounds. One key factor is the surface acidity of both Br6nsted and Lewis type and redox character useful to activate the C-H bond in the secondary carbon atom. The oxidative dehydrogenation of cyclohexane has been studied over various solids. On modified Ni /A1203 catalysts, it was demonstrated that the added metal (Sb, Pb or Cu ) displays an improving effect on the activity, stability and selectivity [5]. It was also shown that the cyclohexane dehydrogenation proceeded on the paired acid-base site over bifunctional catalyst TiO2-ZrO2 [6] and TiO2- ZrO2- V 2 0 5 [7] . On the other hand, Russell and coll suggested that cyclohexene and/or cyclohexadiene are probably intermediates in the dehydrogenation reaction [8]. Only dehydrogenation products (cyclohexene and/or benzene) were observed over vanadate [9] and molybdate [ 1 0 ] based catalyst. The heteropolycompounds (HPA), having well defined structure and the resulting acidic and redox properties, exhibited high oxidation abilities in the selective oxidation of alkanes in according with literature data [1-4] .Also, we have undertaken the study of the oxidative dehydrogenation of cyclohexane to benzene over HPAs. The present paper's results deals with the effect of the nature of central coordination atoms and that of the counter-ion on the catalytic activity of heterophosphomolybdate anions in this reaction.

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2. EXPERIMENTAL

The HPA compounds have been prepared according to the literature data [ 11-13] and characterized by XRD and FTIR spectroscopy before and after reaction. Prior to the reaction, 0,2 g of each catalyst was treated in an oxygen stream (21/h) for 1 hour at reaction temperature. The reactions were performed in a continuous flow reactor at atmospheric pressure and in the range of temperature 250-400~ The reaction mixture was obtained by passing a carder gas (N2+ 02) through a saturator containing the cyclohexane and maintained at 280 K (partial pressure of cyclohexane: 40 mmHg ) with a total flow rate of 21/h and N2/O2 =1 ratio. Reactants (cyclohexane and 02) and reaction products (benzene, cyclohexene, cyclohexadiene, n-hexane, methane, CO2) were analyzed online by GC equiped with a column WCOT (glass 20m X 0.3mm coating Carbowax 20M). 3. RESULTS AND DISCUSSION

3.1 Catalysts Characterization The initial surface areas of the HPA catalysts are tabulated in table 1. BET surface areas of salts were l~reater than those of the corresponding acids. They ranged from 50m2/g for salts to around 3m'/g for acids. The XRD and FTIR studies showed that the whole catalysts have Keggin structure [14-15]. This structure was stable up to 350~ under reaction steam but decomposed at 400~ into the parent MOO3, P205, V205 simple oxides for the heteropolyacids as illustrated in Figure 1 for PMol2 anions [16]. In contrast, the corresponding salts were much more stable in the same operating conditions as it can be seen in Fig. 2 for CsFePMol2 solid. Table 1 BET surface areas of HPA Catalysts H4PMOI iVO40

H3PMoI204o (NH4)6HPMollNi04o

Cs2,sFeo,osHl,26PMollV040 Cs2,5Feo,osHo,26PMo1204o

"2'0 ...... 3'0....... 4'0....... 50 .... 60'

70 '(20)

Fig. 1" XRD patterms of H3PMol2040 (under air): a) 25 ~ b) 350~ e)400~

Surface areas (m2/g) 5 2,8

2,78 51 51

~l~-zu~ 304050

-~;~0::-----2060 ( )

Fig.2 9XRD patterms of CsFePMol204o (under air): a) 25~ b) 400~

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3.2 Catalytic activity 3.2.1Cyclohexane dehydrogenation The influence of the reaction temperature. The obtained results show that benzene was the major product at low conversion levels, n-hexane, cyclohexene, cyclohexadiene, methane, carbon dioxide and heavy product corresponding likely to a carbon chain rupture and polycondensation of several cyclohexane molecules were also observed. The main testing experiments are reported in Figures 3, 4 and 5 and Table 2. At steady-state, the obtained results showed that the catalytic activity of the solids was very sensitive to the reaction temperature and to the nature of the coordination ion (MoVl,vV, Ni u ) and to the counter-ion " (H30 §,Cs + , Fe 1I, N1.I1 ) (Table 2). A general trend was the increase of the conversion with the reaction temperature and the decrease in the selectivity of benzene. Thus, the conversion and benzene selectivity passed from 8.2 to 55.5% and from 90.2 to 34.3% respectively when the reaction temperature was increased from 250 to 350~ for PMol2. The substitution of a Mo ion by V ion did not influence the conversion (about 50% at 350~ but increased slightly the benzene selectivity (from 34 to 46%) (Table 2). This shows that the protons in anhydrous HPA are accessible to the reactant molecules, thus the high catalytic activity of HPA may be related to high acid strength and high mobility of protons which are responsible to the dehydrogenation of cyclohexane. This agrees with Chang and all. [7] that the dehydrogenation of cyclohexane was due to Br6nsted acid sites on the surface. Also, the substitution of Mo ion by Ni ion lead to a substancial decrease of conversion (from 8,2 to 0,3% ) and benzene selectivity (from 90,2 to 50%) at low temperature (250~ inversely at high reaction temperature (350~ the conversion and benzene selectivity increased from 55,5 to 72,0% and decreased 34,2 to 30,5% respectively (Table 2). In presence of PMo~Ni, heavy products of polycondensation are detected, but not identified, with a selectivity of around 40% at high reaction temperature. It can be result of the different degrees of oxidation of the catalysts. Thus at steady state, the surface of PMollNi is more reduced than PMolIV and PMol2. PMollNi will have a higher tendency to oxidize cyclohexane beyon benzene. PMol2, PMollVand PMollNi catalysts show difference, which could be related to their rates of reduction and reoxidation. More important effects were observed with the HPA salts. Thus,the salts showed higher activity at higher reaction temperature compared with the parent acids. For example, in the case of CsFePMol2, the conversion passes from 2,7 to 80,1% and the selectivity of benzene from 90,1 to 73,1% when the temperature rises from 300 to 400~ It demonstrate that the amount of Lewis acid sites are much higher than the amount of to Br6nsted acid sites on the HPA salts. It suggest that the Lewis acid sites are the active sites on acid-base bifunctional catalysts in the dehydrogenation of cyclohexane reaction as reported in a earlier paper [6]. We conclude that the acidic sites of catalysts play the most important role in the dehydrogenation of cyclohexane. Cyclohexene selectivity was more strongly decreased for the HPA salt than for the parent acid: it was about less than 5% against 22% at 350~ (Table 2). For all solids, in addition to benzene and cyclohexene, methane is also formed and the trend of the methane selectivity against conversion parallels the benzene selectivity (for T> 350~ the methane formation is attributed to the cracking of cyclohexane at high temperature. Besides to these

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products, n-hexane is detected with a selectivity of around 5% or less and cyclohexadiene is observed specially in the presence of acids at 350~ its selectivity is inferior than 8%. Carbon dioxide was the alone combustion product detected in presence of CsFePMoIIV and PMol iNi, with a selectivity less than 10%. The presence of cyclohexene and cyclohexadiene in small quantities among the reaction products means that they are produced as an intermediates in the dehydrogenation of cyclohexane to benzene, as indicated by other authors [8-9-10]. It appears that, selectivity for alkenes decreases with increasing conversion. This behavior agrees with the fact that alkenes are the primary products so their selectivity declines at higher conversion on account of the secondary reactions. Table 2 Oxidation of cyclohexane in ~resence of molecular oxygen over HPA. catalyst T(~ time (h) conv selectivity of product (%)

(%)

H3PMoI204o

H4PMo~IVO4o Cs2.5Feo.ogPMol2

CsFe0.0sPMollV (NH4)6HPMol tNi

250 350 250 350 300 350 400 350 400 250 350 400

8 8 8 8 8 8 8 8 8 8 8 8

8.2 55.4 2.5 56.1 2.7 10.5 80.0 4.9 56.9 0.3 72.0 90.0

Ic .,o Icon. 90.2 34.2 86.2 46.0 90.1 89.7 73.1 86 59.5 50 30.5 42.9

22.2 21.2 0.5 3.4 8.6 43.4 4.0 5.5

7.8 5.2 -

[cr lco I o* 4.4 2.0 0.5 -

-

14.1 -

7.8 -

14.2 -

22.8 -

17.5 15.7

9.8 17.3 13.8 17.8 9.9 9.8 9.8 2.9 10.1 9.1 6.6 4.3 43.7 12.4 23.5 -

-

* Pc "polycondensation product no identified T h e

i n f l u e n c e

of

the

time

on

stream.

The variation of conversion and benzene selectivity were studied as a function of the reaction time (Fig 3, 4, 5). Steady-state activities were obtained within a period of few minutes or hours depending on the nature of catalyst. In presence of heteropolyacid, the steady-state activity of cyclohexane and benzene selectivity is reached after 4h. We observed a trend of increasing activity with decreasing benzene selectivity in initial period and then remains nearly constant. In the case ofheteropolysalts, at 350~ the conversion is lower (<11%) and seemed to be more stable during reaction time, likewise the benzene selectivity also stable, it is higher (>86%). However, pretreated and tested at 400~ (fig. 5), the heteropolysalts showed a different behavior. The conversion and benzene selectivity are higher and decreased slightly versus the time. These results revealed that the presence of Cs-Fe as counter-ions display an improving effect on the activity, stability and the benzene selectivity of HPA in the cyclohexane dehydrogenation.

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120

70q

__

100 80

40 30

I r

i r

H3PMo12 H4PMo11V

40

9

i

--

2O 0

2

3

4

5

6

7

~

~

.1;

.ar..

--

:.

.-

8

time (h)

=

- _

~ Ilm "

:3

-I

1 0

0 ' 1

~

~-,...~lk

60

-

10 m

~_ ~--.

,

'

,

'

2

4

6

8

r

conversionlCsFePMo12

"

conversionlCsFePMol

---IB-- selectivity/C 1V

"

tim e(h)

10

s F 9P M o 12

selectivitylCsFePMo11~/

Fig.4: Selectivity of benzene as a function of time reaction over HPA catalysts. Reaction conditions: T=350~ N2/O2=1.

Fig.3" Cyclohexane conversion as a function of time reaction over HPA catalysts. Reaction conditions: T=350~

3.3 Cyclohexene dehydrogenation The dehydrogenation of cyclohexene on CsFePMol2 at 350~ showed very high conversion and benzene selectivity values (conv. =100%, Sben=90%) (fig. 6). The first step in the dominant pathway for further reaction of alkenes is the loss of an allylic hydrogen to from a surface allylic species. In the case of cyclohexene loss of another hydrogen from the surface allyl to form benzene occurs rapidely [9]. This result suggests that cyclohexene is the intermediate product for benzene formation. This observation correlates with those made with vanadate and molybdate alumina-supported catalysts [ 1-5-6].

120

120

=~ E

0~ 100

lOO 1

~

I

--

-~ ~ 40 ~ 0

>~ 2oi r O

o

r

r

r

#

C

80

= .= 40 t r

0 i

.........

0

2

4

, r

c~nvemionlCsFePMo12

I

conversion/CsFePMo11V

"

=

r

~ .....

+

6

8 time(h;O

I

--I--selectiv~lCsFePMo12~

C

selectivity/CsFePMo11~

Fig.5: Cyclohexane conversion and selectivity of benzene as a function of time reaction over Cs2.sFeo.08PMol2 and Cs2.sFeo.08PMol~V catalysts. Reaction conditions: T=400~ N2/O2=1.

,

20"

o

i i

L

,

r A

---

i

1O0 200 conversion -~ benzene selectivity !

r

300 time(mn)

Fig.6: Cyclohexene conversion and selectivity of benzene as a function of time reaction over Cs2.sFeo.08PMol2 catalyst. Reaction conditions: T=400~ N2/O2=1.

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1900

4. CONCLUSION

In conclusion, the HPA exhibit high catalytic activities for dehydrogenation of cyclohexane. On these catalysts, the product distribution depends on operating conditions and the composition of the catalyst, which suggest that the nature of counter-ion and oxometal affects the properties of the catalyst. Heteropolyacid and heteropoysalts catalysts show interesting differences, which could be related to the different redox and acid-basic properties. The higher benzene selectivities on HPA can be the result of the simultanous presence of several metals (FeMo, FeMoV or MoNi ..... ) in the catalyst at different oxidation degrees. REFERENCES

1. J. B. Moffat, Appl. Catal. A : General 146 (1996) 65-86. 2. N. Mizuno, M. Tateiski, M. Iwamoto, App. Catal.; A. General 128 (1995) L165-L170. 3.W. Li and W. Ueda., 3rd World. Cong. on Catal., Elsevier (1997) 433. 4. W. Ueda, Y. Suzuki, W. Lee and S. Imaoka; 11th. Cong. on Catal., Stud. Surf. Sci.Catal., Elsevier, Vol. 101 (1996), 1065. 5. H. D. Lanh, NG. Khoai, H.S. Thoang and J. Volter, J. Catal. 129, 58-66 (1991). 6. J. Fung and I. Wang, J. Catal. 164, 166-172 (1996). 7. R. C. Chang and I. Wang, J. Catal. 107, 195-200 (1996). 8. W. Russel,W. Maatman, P. Mahassy, P. Hoekstra and C.Addink, J. Catal., 23, 105-117 (1971). 9. M.C. Kung and H.H. Kung, J. Catal. 128, 287-291 (1991) 10. R. Maggior, N. Giordano, C. Crisafulli, F. Caastelli, L. Solarino and J.C.J.Bart, J.Catal. 60, 193-203 (1979) 11. G.A. Tsigdinos and C. Hallada, Inorg. Chem., 7, (1968) 437 12. C. Rabia, M.M. Bettahar, S. Launay, G. Herve, M. Foumier, J. Chem. Phys., 92, 14421456 (1995) 13. S. Hocine, C. Rabia, M.M. Bettahar, M. Foumier, submitted for publication. 14. C. Rocchiccioli-Deltchef, R. Thouvenot, R. Franck, Spectrochim. Acta., 32A, 587, (1976). 15. C. Rocchiccioli-Deltchef, R. Thouvenot, J. Chem. Reasearch, 546, (1977). 16. M. Foumier, C. Feumi-Jantou, C. Rabia, G. Herv6, S. Launy, J. Mater. Chem., 2, 971, (1992).

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1901

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

T r a n s i e n t r e s p o n s e s t u d i e s of i s o b u t a n e o x i d a t i v e d e h y d r o g e n a t i o n over m o l y b d e n u m c a t a l y s t s N. V. Nekrasova), N. A. Gaidai a), Yu. A. Agafonova), S. L. Kipermana), V. Cortes Corberfin h) and M. F. Portela c) a) N. D. Zelinsky Institute of Organic Chemistry, R.A.S., 47 Leninskii Prospekt, 117913 Moscow, Russia b) Instituto. de Catfilisis y Petroleoquimica, CSIC, Campus UAM Cantoblanco, 28049 Madrid, Spain c) GRECAT, Instituto Superior Tecnico, Av, Rovisco Pais, 1096 Lisbon, Portugal Kinetics and mechanism of isobutane oxidative dehydrogenation were studied over cobalt and nickel molybdate catalysts. The data obtained in nonstationary and stationary regimes showed that kinetics and mechanism are the same over both catalysts. Isobutene and carbon oxides are primary reaction products. Lattice oxygen takes part in dehydrogenation reactions. Carbon oxides formation proceed by the interaction with adsorbed oxygen. Cobalt molybdate is more active and selective catalyst for isobutane oxidative dehydrogenation. It was shown t h a t nickel molybdate catalyst is stable only at high oxygen concentration while cobalt molybdate catalyst can work at lower oxygen concentrations. I. I N T R O D U C T I O N Many studies are carried out in the last years on the light alkane oxidative dehydrogenation (ODH) as ODH is an attractive way for olefin production owing to the absence of thermodynamic limitations. Most of the studies are devoted to developing new catalysts and much less attention was paid to the kinetics and, especially, the dynamics of the process. It is known that the study of the dynamic response can get valuable information about reaction mechanisms [1-3]. The works devoted to studies of isobutane ODH mechanism are practically absent. TAP reactor was used for investigation of propane ODH over V-Mg-O catalyst [4]. It was shown t h a t partial and total oxidation of propane occur on the same surface site but involve different forms of reactive oxygen: nucleophilic lattice oxygen takes part in the propane partial oxidation to propene, while adsorbed electrophilic oxygen is involved in the total oxidation process of propane. The transient response method was used in the study of the ammoxidation of propene and propane over an Sb-V-oxide [5]. It was shown there t h a t ammoxidation of

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1902

propane to acrylonitrile occurs t h r o u g h the O D H of p r o p a n e a n d the reaction scheme w a s proposed for this process. However, the t r a n s i e n t r e s p o n s e s were obtained only for the r e p l a c e m e n t of inert gas by reaction m i x t u r e s . In this s t u d y we have i n v e s t i g a t e d t r a n s i e n t processes d u r i n g the a t t a i n m e n t of a s t e a d y s t a t e u n d e r different conditions to obtain the detailed i n f o r m a t i o n for m e c h a n i s m of isobutane ODH over m o l y b d a t e c a t a l y s t s (C00.95M004 a n d aNiMoO~) in a wide i n t e r v a l of process p a r a m e t e r s . Besides, the reaction kinetics of this process was studied u n d e r s t a t i o n a r y conditions over these c a t a l y s t s . 2. E X P E R I M E N T A L

The e x p e r i m e n t s u n d e r n o n s t a t i o n a r y conditions were carried out in special g r a d i e n t l e s s reactors of small volume, connected to a time-of-flight massspectrometer. T r a n s i e n t response m e t h o d [1-3] w a s applied in this case. The relaxation curves describing a t r a n s i t i o n of the s y s t e m to a new s t e a d y s t a t e were obtained after an j u m p w i s e change in the corresponding r e a c t a n t concentrations. A vertical line m e a n s a change of reaction conditions; the r e l a x a t i o n curve is r e l a t e d to the last change of these ones. Since the composition of the reaction m i x t u r e s leaving the reactor was a n a l y z e d every 1 s, the response curves for the time scale indicated in the figures i l l u s t r a t e n e a r l y continuous processes. Initial m i x t u r e s of specified composition such as (O~+C4Hlo+N2), (N2+C4H10), (O~+Ne) were p r e p a r e d by mixing the flows of the corresponding gases, purified and dried prior to the experiments. The residence time, defined as the ratio of the volume of the r e a c t i n g s y s t e m to the flow rate, did not exceed 2 s. It w a s considered at the t r e a t m e n t of the e x p e r i m e n t a l data. For all e x p e r i m e n t s the r e l a x a t i o n time w a s lower t h a n the t u r n o v e r time. This allows to characterize the observed r e l a x a t i o n s as intrinsic ones (i.e., those d e t e r m i n e d only by the reaction m e c h a n i s m ) [6, 7]. The m/e ratios employed were as follows: 15 (methane), 18 (water), 32 (oxygen), 43 (isobutane), 44 (carbon dioxide), 56 (isobutene). Kinetic e x p e r i m e n t s u n d e r s t e a d y - s t a t e conditions were carried out in g r a d i e n t l e s s units at a t m o s p h e r i c pressure. B l a n k r u n s (with a n d w i t h o u t quartz) showed t h a t an exit of the reaction into the volume as well as the homogeneous reaction can be neglected u n d e r e x p e r i m e n t a l conditions. The m a i n reaction products were isobutene, carbon oxides, low hydrocarbons a n d w a t e r . The i n t e r v a l of process p a r a m e t e r s was c h a n g e d in the following ranges: t e m p e r a t u r e 470 - 560 ~ P,:so~,,t,,,: = 0.08 - 0.50, Po.~ = 0.032 - 0.18 a t m . Total conversion (x) and isobutene selectivity varied in the i n t e r v a l s 0.02 - 0.26 a n d 0.70 - 0.95 on CoMoO4. Analogous p a r a m e t e r s on NiMoO4 were 0.05 - 0.20 and 0.50 - 0.80, respectively. 3. R E S U L T S A N D D I S C U S S I O N F i g . l a i l l u s t r a t e s the response w h e n the reaction m i x t u r e w a s i n t r o d u c e d onto CoMoOl a n d NiMoO~l c a t a l y s t s p r e v i o u s l y t r e a t e d by air. The r e l a x a t i o n

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个

1903 0,3

化

0,5

a

0,4

-~

0,3

:~ g

0,2

~

0,2

0,1

0,1 0,0

JF

l

T

,

0

10

20

30

'

40 0

5

1

0

0

10

I

20

S

0

10

20

II

Fig.1. The change of isobutene concentration with the time (s) in the responses" (O2+N2)/(O2+CnHlo) (a) and (O2+C4Hlo)/(O2+N2) (10) over: I) CoMoO_l, 490 ~

Poe / Pb aH,o

0.2; II) NiMoO4, 520 oC

No2 " / Pb 4H,0

= 1.0.

curve has a delay, which can be attributed to some time necessary for isobutane adsorption, and a maximum. This m a x i m u m in the concentration of isobutene at the initial stage of the process is due to the participation of the highest amounts of lattice oxygen at the beginning of the process. Thus, this type of oxygen takes part in isobutene formation. The form of relaxation curves in this case is the same on both catalysts, but for the reverse response (O2+C..,Hlo)/(O2+N2) (see Ib, IIb) the relaxation curve has a m a x i m u m over NiMoOl at a variance of the response over CoMoO4. This can be connected with a higher heterogeneity of Ni-catalysts as compared with Cocatalysts. This heterogeneity of NiMoO4 is further d e m o n s t r a t e d in the response (N2+C4H10)/(O2+C4Hlo) (curves not shown). The change of isobutene concentration is monotonous in the response (N2+C~,H10)/(O2+C4Hlo) over CoMoO4 while it shows a m a x i m u m over NiMoO4. These two relaxation curves have no delay in these responses as in this case no time is needed for the isobutane adsorption. The relaxation curve of isobutene formation observed in the response O2/N2/(O2+C4Hlo) over both catalysts is monotonous after the intermediate blowing off by nitrogen during 1 minute. It means t h a t concentration of reactive lattice oxygen is decreased during the nitrogen t r e a t m e n t owing to a decrease of adsorbed oxygen concentration. The form of relaxation curves in the tests where the introduction of one reactant is shifted from one to the other (e.g. in the response (O2+N2)/(N2+C4Hlo) or the reverse one) confirms the participation of lattice and adsorbed oxygen in the dehydrogenation reaction and CO2 formation, respectively. The relaxation curves of CO2 formation in the responses (O2+N2)/(O2+C4Hlo)

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个

1904

2

0,8

化

a

1,5

0,6

a

b

5 a/ 1

C~ o 0,4

r

0,2

0,5

Y

0

35

s I

II

Fig.2. The change of carbon dioxide concentration with time (s) in the responses: (O2+N2)/(O2+C~H10) (a) and (O2+C4Hlo)/(O2+N2) (b) over: "("H,o - 0.2; II) NiMoO4, 520 oC, " " -1.0. I) CoMoO,, 512 oC, P ~

po/PcH,o

over CoMoO.~ and NiMoO ~are p r e s e n t e d in Fig.2. The characteristic curves of the CO2 concentration in the responses (O2+N2)/(O2+C4Hlo) had a m a x i m u m over CoMoO~ and reached the s t a t i o n a r y level monotonously over NiMoO4. The absence of a m a x i m u m over Ni-catalyst is connected with the excess of oxygen in the reaction mixture (a fivefold higher concentration t h a n in the case of Cocatalyst). In the reverse responses (O2+C4Hlo)/(O2+N2) both curves have a m a x i m u m which is much higher in the case of NiMoO~. It can be explained by hydrocarbon reduction and coke formation on catalyst surface, m u c h higher on Ni-catalysts. The oxygen introduction results in coke burning. This m a x i m u m became much lower with a higher oxygen concentration in the reaction m i x t u r e (P"02/ P"isobutane : 2 . 0 ) . Therefore, an excess of oxygen is n e c e s s a r y for keeping stable the performance of Ni-catalysts in ODH of isobutane. Similar results were obtained in [8] for ODH of propane over NiMoO4 The concentration of CO2 in the response (O2+C.IH10)/(O2+N2) (b) is higher t h a n the one in s t a t i o n a r y state (a). It m e a n s t h a t the oxygen concentration in the reaction m i x t u r e (O2+C4H10) is mostly consumed in filling up the oxygen vacancies or in w a t e r formation but not in coke burning. Therefore the filling up the oxygen vacancies proceeds more quickly t h a n coke burning. A complex s t r a t e g y [9-12] to obtain justified models u n d e r s t a t i o n a r y conditions and to prove their adequacy was used in this investigation. We applied the methods of previous analysis to t r e a t the kinetic d a t a obtained: the dependence of reaction rates on dilution [13], a change of process selectivity in dependence on operation conditions [14]. It was shown t h a t overall order in the n u m e r a t o r in kinetic equation for carbon dioxide formation is higher t h a n the power of the

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

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denominator and the order of the n u m e r a t o r in the kinetic equation for light hydrocarbon formation is equal to the power of the denominator. The following kinetic equations describing different routes of the process over both the catalysts under investigation were found: 1) isobutane formation:

Pc4H,oPo2 r, - k, 1-"o.~ + k, t"-'C4H,o

(1)

2) carbon dioxide formation: 0.5

(k,, PC4H,o +k',, PC4H~+k'i, P c o ) Po~ 1" tl =

NO.5

02 + k~

PC4H8

(2)

3) carbon monoxide formation:

r., - k,.

PC4H~P"o~

o~ Po2 +k2 PC4H~

(3)

4) cracking products formation:

r , . - k , . D..~

PC4H,o

~0~ +k~ PC4H~

(4)

The kinetic data are in accordance with the reaction mechanism proposed in the nonstationary investigation: isobutene formation is characterized by redox mechanism (isobutane reduces the catalyst surface which can be oxidized by oxygen), COz is formed from isobutane, isobutene and CO, formation of CO proceeds mainly from isobutene and cracking products are formed from isobutane. Adsorbed oxygen takes part in carbon oxides formation. The observed rate of isobutene formation (rj) taking into account its transformation into carbon oxides can be described by the following equation: F/- r / -

[(k'.+k,.)Pc4HJPo~ _o

5

Po~ +k2 PC~H~

(5)

The equations (1)-(5) described all the transformations observed in this system. The comparison of isobutene formation rates over the two catalysts shows that it is 1.5 - 2.0 times higher over CoMoO4. This catalyst is more selective for isobutene formation and it needs lower oxygen concentration to keep up a stable performance t h a n NiMoO4, which needs a large excess of air to be stable.

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1906

4. C O N C L U S I O N

The kinetic data of processes taking place under stationary and unstationary conditions over CoMoO4 and NiMoO4 show that the kinetics and the mechanism for ODH of isobutane are the same over both catalysts. The following mechanism was proposed: isobutene formation proceeded according to the redox mechanism, CO2 is formed from isobutane, isobutene and CO; formation of CO proceeds mainly from isobutene; cracking products are formed from isobutane. Adsorbed oxygen takes part in carbon oxides formation. However, both catalysts differ in their catalytic performance and stability. CoMoO~ is a more active and selective catalyst for ODH of isobutane than NiMoO4. Furthermore, NiMoO.l needs a large excess of oxygen to keep its catalytic activity stable, while CoMoO4 can operate at much lower oxygen-tohydrocarbon ratios (air/isobutane ratio = 2) with no change in stability. As a consequence, productivity of CoMoO~ catalyst can be larger than that of NiMoO4 at equal total space velocity. ACKNOWLEDGEMENTS The authors acknowledge INTAS (N 96-1117) for financial support. REFERENCES

1. C. O. Bennet, Cat. Rev.- Sci. Eng., 13 (1976) 121. 2. H. Kobayashi and M. Kobayashi, Cat. Rev. - Sci. Eng., 10 (1974) 139. 3. M. Kobayashi, Chem. Eng. Sci., 37 (1982) 393. 4. A. Pantazidis, S. A. Bucholz, H. W. Zanthoff, Y. Schuurman and C.Mirodatos, Catal. Today. 40 (1997) 207. 5. R. Nilsson and A. Anderson, Ind. Eng. Chem. Res., 36 (1997) 5209. 6. M. I. Temkin, Kinet. Katal., 17 (1976) 1095. 7. F. S. Shub, A. G. Ziskin, M. G. Slin'ko and, M. I.Temkin, Kinet. Katal., 20 (1979) 334. 8. R. Rosso, A. Kaddouri, R. Anouchinskym, C. Mazzocchia, P. Gronch and P. Centola, J. Mol. Catal., 135 (1998)181. 9. S.L. Kiperman, Uspekhi Khim. (russ.), 47 (1978) 3. 10. S. L. Kiperman, D. M. Shopov, A. Andreev, N. E. Zlotina and B. S. Gudkov, Izv. Khim. Bull. of Chemistry, Bulg. Akad. Sci, 4 (1971) 237. 11. S. L. Kiperman, Chem. Eng .Commun., 100 (1991) 3. 12. S. L. Kiperman, Kinet. Catal. (English Edn.), 36 (1995) 7. 13. N. I. Koltsov and S. L. Kiperman, I. Res. Inst. Catalysis, Hokkaido Univ., 26 (1978) 85. 14. N. I. Koltsov and S. L. Kiperman, Teor. Exp. Khim. (russ.), 13 (1977) 630.

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1907

化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Olefins formation by oxidative dehydrogenation of propane over monoliths at short contact times V. A. Sadykov, S. N. Pavlova, N. F. Saputina, I. A. Zolotarskii, N. A. Pakhomov, E. M. Moroz, V. A. Kuzmin, A. V. Kalinkin, A. N. Salanov, I. G. Danilova, E. A. Paukshtis The Boreskov Institute of Catalysis, Siberian Branch of the RAS, Lavrentieva, 5, Novosibirsk, RUSSIA; 630090, Fax: 007(3832343766), E-mail: [email protected]

A specially designed tubular microreactor allowing to rapidly quench reaction products was used to test performance of Pt supported onto corundum micromonoliths in the reaction of propane oxidative dehydrogenation at short contact times. Promoters known as dehydrogenation catalysts (tin, zinc aluminate spinel, transition metal pyrophosphates) were used and reaction mixture composition was tuned to increase propylene yield. FTIRS data on propane interaction with a model Pt/SiO2 system helped to analyze the possible routes and limits of propylene yield improvement. 1. INTRODUCTION Last years, autothermal oxidative dehydrogenation of paraffins over Pt-containing foam monoliths at temperatures in the range of 900 -1000 ~ was shown to be quite efficient in olefins production at short contact times [1, 2]. The most important side reactions leading to decrease of higher olefins yield are cracking and complete combustion. In this work, to change catalytic properties of platinum in propane oxidative dehydrogenation, its combination with non-metallic dehydrogenating promoters (transition metal pyrophosphates, SnO2 and zinc aluminate) as well as lower operating temperatures were used. To reduce probability of propylene secondary reactions (oxidation, cracking, steam reforming), straight channel thin wall corundum and zinc aluminate monolithic supports were used. A special reactor design was elaborated to ensure a rapid quenching of the reaction products thus minimizing secondary gas-phase reactions. For tuning the feed mixture composition, propane/oxygen ratio was varied and water and hydrogen were added. To elucidate the roles of the active component and support in propane transformation, FTIRS studies of propane adsorption on the model Pt/SiO2 system have been carried out. 2. EXPERIMENTAL As supports, proprietary corundum monoliths (surface area - of 5-10 m2/g) annealed at 1300 ~ and bulk zinc aluminate monolithic supports (surface area 15-50 m2/g ) o f - 16 mm diameter with wall thickness 0.25 mm and channel sizes ca 1 mm were used. To suppress the inherent acidity of alumina and decrease microporosity, a mullite surface layer was formed using impregnation of support with silica sol followed by drying and calcination at 1300 C.

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1908

Pyrophosphates sols were prepared by mixing Co or Mn nitrate and acid ammonium phosphate solutions. Corundum micromonoliths were impregnated by sols, purged by air to remove their excess and calcined at 700 ~ thus ensuring pyrophosphate content-~ 5 wt. %. Pt was supported onto monolithic supports via incipient wetness impregnation with aqueous solutions of H2PtC16. Sn was added either to a-A1203 by impregnation with solution of [Pt(SnCI)3C12]2 complex in 2M HC1 [3] or to Pt-containing catalysts calcined at 700 ~ by impregnation with SnC12 solutions. After impregnation, all samples were dried and calcined at 700 ~ Pt and SnC12 concentration in the solution were chosen in such a way as to ensure 0.52.5 wt. % Pt loading and a required Pt: Sn ratio. Usually, samples of micromonoliths with lengths in the range of 2-20 mm were used. Model sample 2 wt. % Pt/SiO2 was prepared using incipient wetness impregnation of high purity silica support (specific surface 300 m2/g) by H2PtC16 solution followed by drying and air calcination at 500 C for 4 h.. The phase composition of catalysts was studied by XRD (URD -63 diffractometer, Cu I ~ radiation). Pt particles sizes were estimated from the diffraction lines broadening using the Scherrer equation. The surface composition of catalysts was characterized by XPS. Photoelectron spectra were recorded for samples dusted onto an adhesive tape without any pretreatment using a VG ESCA-3 spectrometer and A1 Kot radiation. The surface texture of catalysts was studied by SEM (a BS-350 "Tesla" microscope) A specially designed quartz tubular reactor allowing to independently preheat a feeding mixture flow up to desired temperature (usually ca 300 ~ and control the monolith temperature was used for catalysts testing. Pieces of monolithic catalysts with diameters ca 16 mm wrapped in silica-alumina fiber cloth for insulation and preventing reactant bypass were placed into the reactor. A piece of corundum monolith was usually situated up-stream of the catalyst. Catalysts were pretreated in H2 at 700~ for 2 h. To take probes of the gas phase immediately after catalyst, a tube sampler kept at constant temperature to prevent water condensation and stop any homogeneous reactions was used. The catalyst temperature was measured by a thermocouple inserted into the plugged up monolith channel. Inlet and outlet compositions were analyzed by GC, conversion and selectivities were calculated on the carbon atoms basis. Within the usual limits of GC uncertainty (+ 10%), the carbon balance was closed. In all experiments, 02 conversion was shown to be complete. Propane adsorption on Pt/SiO2 was studied using wafers with the densities-10 mg/cm 2 and an IR cell allowing thermal treatments in the controlled atmosphere. The samples were pretreated in 02 (100 Torr) for l h at 773 K, then cooled 298 K and evacuated up to 10-3 Torr. Propane (20 Torr) was added at 298 K followed by sample heating up to 400 ~ IR spectra were recorded using an IFS-113v <>spectrometer. 3. RESULTS AND DISCUSSION

3.1. Samples texture, phase composition and Pt dispersion Some data on samples composition and properties are presented in Table 1. In some samples, metallic Pt phase was not registered by XRD either due to its low content or small particle sizes. XPS is not able to detect Pt due to superimposition of its characteristic 3d5/2 line with the support A1 2p line. For P-3 sample prepared using Pt-tin chloride complex, too low Sn content as judged by XPS can be explained by a partial sublimation of SnC14 during air calcination [3]. As a result, after reaction only Pt phase represented by rather big particles was

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Table 1. Samples composition Sample

Support

Pt, wt.%

P-1 P-2

ot-A1203 oc-A1203 a

3 2

P-3 P-4 P-5

Phase composition c

d

Pt, S n O 2

Pt3Sn+ PtSn;SnO2 Pt Pt3Sn

Pt particle size, A

S~A1 ratio, c/d

200

0.1/0.29

c~-A1203b 2 Pt 360 0.043/0.017 ZnA1204 b 0.5 150 0.56/9 Co2P207/ 0.5 ot-A1203 P-6 MnzP207/ 0.5 ot-A1203 a-Atomic ratio Pt/Sn-1:7, tin was added after Pt supporting; b- Atomic ratio Pt/Sn-1:3, Pt-Sn complex was used; c-before reactions; d- after reaction. observed, and aggregation of tin oxide particles was suggested by a decline of the Sn/A1 ratio. Such a sublimation appears to be suppressed when more basic support is used (sample P-4) as revealed by higher Sn/A1 ratio and formation of Pt-Sn alloy after reaction. Fig. 1 shows typical SEM images for P-2 sample in the initial (oxidized) state and after prolonged working in propane-rich feeds.

,~~ ~ a

b

c

d

i

Fig. 1. SEM images of P-2 sample in the initial (oxidized) state (a, b) and after prolonged working in rich feeds (c, d). a, c-general view of the monolith wall texture; b, d- active component fixed on corundum particle (b) and graphite-like platelet (d).

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1910

Two features are to be stressed: (i) after reaction, the active component particles appear not to be blocked by carbon; in fact, they are now supported on carbon (presumably, graphite) platelets of--- 10 mkm thickness not contacting with the initial support; (ii) in reaction media, recrystallization of the active component proceeds forming rather bulky polycrystalline dendrites of Pt-Sn alloy (see XRD data). Some increase of Sn/AI ratio after reaction (Table 1) can be explained by a screening effect of the carbon layer on AI signal. Hence, carbon build-up certainly proceeds on support, while particles of the active component appear to be clean, presumably, due to their efficiency in carbon gasification by CO2 and H20 even in the absence of oxygen in the gas phase.

3.2. Effect of platinum modification by tin dioxide and support As follows from Table 2, at 1"1 reaction mixture composition, SnO2 has a promoting effect as regards propylene selectivity. This result agrees with the data of Schmidt et al for the Table 2. Effect of tin on performance of Pt supported catalysts Sample

Tcat~

Contact time, sec

Selectivity, %

C3H8conversion, %

P-1 835 0.03 P-2 850 0.1 P-3 840 0.1 P-4* 880 0.1 *SupPort specific surface 50 m2/g

63 47 62 33

CH4 18 11.7 14.5 5.2

C2H4 34 23.5 29.5 12.4

C3H6 17.5 26.4 20.6 25.3

CO 21 2.5 15.2 10

CO2 2.7 28.5 11.5 45

reaction of ethane oxidative dehydrogenation [4]. As judged by decreased methane selectivity, tin promotion helps to decrease propylene cracking. However, promotion by tin has a drawback in decreasing ethylene yield mainly due to a higher carbon dioxide selectivity, especially for P-2 catalyst with a higher tin content and P4 catalyst supported onto ZnA1204 (Table 1, 2). Hence, SnOx addition increases surface oxidation potential. In the case of P-4 sample, very low methane selectivity suggests basic support helps to suppress propylene cracking catalyzed by the surface acidic centers [5]. The highest yield of CO2 for this sample can be explained by too high specific area of this support. As the result, some intra-pore oxidation of olefins can occur.

3.3. Effect of feed composition A substantial gain in propylene selectivity was achieved by tuning propane/oxygen ratio (Fig. 2). For 3"1 mixture, CO2 formation was suppressed, while no channels plugging or performance deterioration with time were observed. Simultaneous increase of both reagents concentration provided their ratio being kept constant was found to increase further propane conversion and ethylene selectivity, while propylene selectivity improved only slightly. At a constant feed mixture preheat, water addition up to 40% was found to decrease the monolith temperature by about 100 ~ without changing propane conversion.

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1911

.. 7O I o~. 60~9 501 40 ~ 30 o 20 ~ 10-

o 0

0

"=4

.7

110

1"5 210 215 310 propane/oxygen ratio

Thus, for P-2 catalyst, at 21,000 GHSV and propane/O2 1"1 mixture, H20 increased propylene selectivity from 20 to 30%, while ethylene selectivity remained unchanged. Concomitant decline of methane and carbon dioxide selectivities (from 6.8 to 4.6% and from 53 to 45%, resp) was observed evidencing suppression of cracking and deep oxidation reactions as the reason for improving propylene yield. Hydrogen addition was found to be also beneficial for olefins production. Thus, on P-2 catalyst at 850 ~ addition up to 10% H2 to feed containing 21% C3H8 and 18 % 02

Fig. 2. C3H8 conversion (1), C3H6 (2), C2H4(3) in N2, increased propane conversion from 45 and CO2 (4) selectivities vs. C3H8/O2 ratio, to 60%, integral olefin selectivity was raised 700 ~ 22 vol. % C3H8. P-2 sample, up to 60% at the expense of carbon oxides selectivity decline from 30 to 20%.. 3.4. Feed tuning to maximize propylene selectivity

For the most promising systems, to maximize propylene selectivity, water and hydrogen were added simultaneously to propane-rich feed. Table 3 shows the results of these experiments. Table 3. Performance of catalysts in propane-rich mixtures (20 vol. %) and H20 Catalyst

H20, vol. %

GHSV, 103h-1

Tcat ~

(C3H8/O2-3)in the presence of H2

Propane cony.,%

P-2 34 660 32 P-4* 38 26 598 20 P-5 40 14 560 18 P-6 38 14 560 13 * Specific surface of zinc aluminate support 16 m2/g

Selectivities, %

CH4 5.4 5.6 2 3

C2H4 18 20 18 19

C3H6 53 72 69 71

CO/CO2 1.7/22 2.5/0 11/0 7/0

As follows from these results, very high integral olefins selectivity (ca 90%) was obtained. In the presence of water, CO2 was absent in the products thus suggesting secondary reactions of olefins oxidation can be efficiently suppressed. Moreover, low methane selectivity indicates suppression of propylene cracking. Nevertheless, ethylene selectivity remains appreciable suggesting its formation in part via the primary reaction of propane activation on Pt. Meanwhile, CO selectivity can be assigned to gasification of the surface carbon by water.

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3.5. IR studies of propane transformation on model Pt/SiO2 catalyst

Propane adsorption at 298 K on oxidized sample was accompanied by appearance of bands at 2960, 2925, 2850, and 2800 cm l with simultaneous decrease of band at 3735 cm 1 corresponding to terminal Si-OH groups. It suggests appearance of Si-O-CnHn+I alkoxy groups due to interaction between terminal hydroxyls and propyl radical formed via homolyric activation of propane on oxidized Pt. Bands assigned to surface formates (1580 cm ~ COO asymmetric stretching; 1370 cm 1, COO symmetric stretching) [6] were also observed. After sample heating to 200-250 ~ C2H4 (v (-C-H) bands at 3025 and 3100 cm !) and C3H6 (v (=C-H) bands at 3010 and 3085 cm l ) [7] were detected in the gas phase. Intensity of bands at 1508, 1540, 1615 cm l (8(HOH) or v(C-C)), 1730 cm ~ (v(C-O) in adsorbed aldehyde or acetone) was increased. At 400 ~ acetone or acetaldehyde (band 1710 cm -1, v(C-O) ) also appears in the gasphase along with propylene and ethylene, while surface coverage by these species increases. These results suggest that both propylene, ethylene and formate species are the primary products of propane transformation on oxidized platinum. Hence, in all conditions, some yield of ethylene is to be observed determined by probability of [3 C-C scission in propyl radical [1, 2]. Carbon oxides seem to be secondary products of formate transformation in the presence of gas-phase oxygen, and their formation can be suppressed by retarding activation of molecular oxygen. Without oxygen, oxygenates such as acetone and aldehydes are formed, and, along with alkyl radicals, they may be important for catalyst-supported homogeneous process. If formation of alkoxy-groups effects the yields of products including coke, the acid-base properties of support can be of a great importance in propane oxidative dehydrogenation. 4. CONCLUSIONS For the reactions of propane oxidative dehydrogenation at short contact times on Pt supported monolithic catalysts, propylene yield was shown to be improved due to quenching of the reaction products, Pt modification by typical dehydrogenation compounds and feed tuning. FTIRS study of propane activation on supported Pt helps to understand these data. Acknowledgments. The financial support of the Engelhard Corporation is gratefully acknowledged. Authors are indebted to R. J. Farrauto for valuable discussion.

REFERENCES 1. 2. 3. 4. 5.

M. Huff. and L.D. Schmidt, J. Catal., 149 (1994) 127. L.D. Schmidt and C. T. Goralski. Studies Surf. Sci. Catal., 110 (1997) 491. N.A. Pakhomov and R. A.Buyanov, Kinetika i kataliz, 22 (1981) 488. C. Yokoyama, S. S. Bharadway, and L. D. Schmidt. Catal. Lett., 38 (1996) 181 B.C. Gates, J. R. Katzer, and G. C. A. Schuit, Chemistry of Catalytic Processes, McGrawHill Book Company, 1979. 6. A.A. Davydov. Infrared Spectroscopy of Adsorbed Species on Surface of Transition Metal Oxides, Wiley, Chichester, 1990. 7. L.M. Sverdlov, M. A. Kovner, E. P. Kraynov, Vibrational Spectra of Multiatomic Molecules, Nauka, Moscow, 1970.

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1913

化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Experimental and Theoretical Investigation on the Roles of Heterogeneous and Homogeneous Phases in the Oxidative Dehydrogenation of Light Paraffins in Novel Short Contact Time Reactors A. Beretta, E. Ranzi, P. Forzatti Dipartimento di Chimica Industriale e Ingegneria Chimica, Politecnico di Milano, piazza Leonardo da Vinci 32, 20133 Milano - Italy -*e-mail: [email protected]

The role of a Pt/AI203 catalyst in the oxidative dehydrogenation of light paraffins was investigated by means of a novel annular reactor which allows to realize very high space velocities and controlled temperature conditions. While a number of previous papers claimed that Pt is extremely active in the selective oxidation to olefins, the experiments in the annular reactor showed that the present noble-metal catalyst is highly active in the non selective oxidation to COx, HE and H20 only. Gas-phase experiments and simulations of a homogeneous reactor showed that the formation of olefins at high temperature and short contact times can be largely explained by the gas-phase oxidative pyrolysis. However, autothermal experiments demonstrated that the catalytic phase can be exploited in an adiabatic reactor for igniting and thermally supporting the pyrolytic process; the combined catalytic-homogeneous reactor performed as a homogeneous reactor, but the presence of the catalyst allowed to produce high yields to oleflns at much lower reactor volume and lower pre-heat temperatures. 1. INTRODUCTION Oxidative dehydrogenation of light paraffins ranks among the novel routes for the chemical conversion of natural gas to valuable chemicals, as it represents a potential altemative to the traditional endothermic processes for the production of short-chain olefins. Very high selectivities to olefms were reported by Schmidt and co-workers [1, 2] in the selective oxidation of light alkanes over Pt-coated foam monoliths, at extremely short contact times (1-10 ms). In spite of the high reaction temperatures related to the adiabatic operation of the foam monolith (700-1000~ possible contributions from homogeneous reactions were ruled out and a thoroughly heterogeneous mechanism was proposed for the synthesis of propylene and ethylene. In this work, a study of ethane oxidative dehydrogenation over a structured Pt/y-Al203 catalyst is addressed and a comparison with previous results of propane oxidative dehydrogenation [3, 4] is made. Catalytic tests were performed at short contact time under both isothermal and adiabatic conditions. Gas-phase experiments were performed as well. The results were compared with the predictions of a detailed kinetic scheme of hydrocarbon oxidative pyrolysis [5] about the expected performance of a purely homogeneous reactor. New

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1914

pieces of evidence were thus obtained on the roles that catalytic phase and gas phase play in the formation of olefins at high temperatures and short contact times.

2. EXPERIMENTAL AND MODELING Catalyst - The catalyst used in the present study was a commercial 5% Pt/y-A1203 system (Engelhard, ESCAT 24). In the annular reactor, the catalyst was tested in the form short (1-5 cm) and thin (50-100 ~tm) layers, deposited onto a mullite tube. Annular reactor - The annular reactor consisted in a 60 cm long quartz tube (Figure 1) wherein the catalyst-coated ceramic tube was inserted. The mullite tubes had a diameter of 4.75 nun, while a 9 mm ID quartz tube and a 7 mm ID quartz tube were used in the catalytic and homogeneous experiments, respectively. Special Cajon fittings were used to keep inner (ceramic) and outer (quartz) tube in coaxial position and to connect the resulting annular chamber to the inlet and Quartz tube outlet lines. The axial Gas Flow ~, temperature profile of the ] Cat. [ Mullite tube reactor was measured from inside the same mullite tube, which was exploited as themocouple-well. Figure 1 - Schematic picture of the annular reactor Millisecond-contact time adiabatic reactor - The adiabatic tests of oxidative dehydrogenation of propane and ethane were instead realized by depositing the catalyst onto Fecralloy fibers and by packing the coated metallic elements in a 7 ID mm quartz tube. Guardbeds of ceramic particles were placed upstream and downstream from the catalytic portion in order to guarantee the insulation along the axial direction. Additional inert material was wrapped around the quartz tube to prevent heat dispersion in the radial direction. The axial temperature profile was measured by sliding a thin thermocouple along a stainless-steel well, placed inside the reactor. Homogeneous reaction scheme - The detailed reaction scheme herein used for the simulation of ethane (and propane) partial oxidation has been deeply described and discussed elsewhere [5]. The model involves more than 150 species in about 3000 elementary and lumped reactions and it has been widely validated for partial oxidation and combustion processes. It consists in a hierarchical and modular structure, which allows easy extension to analyze the effect of new species and new conditions.

3. ACTIVITY TESTS IN THE ANNULAR REACTOR In a previous work, some of the authors have investigated the role of Pt/A1203 in the oxidation of propane [3]. By varying the fiu~ace temperature from 200~ to 700-800~ and operating at short contact time with respect to the catalyst phase, it was found that from low to medium temperature (200-500~ only products of deep oxidation were formed. At higher temperatures, olefins were formed with selectivities as high as 60%. However, comparison with blank experiments and with the predictions of the detailed kinetic model showed that the production of olefins observed in the presence of the catalyst could be well explained by the

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1915

single contribution of radical reactions in the empty volume of the reactor. Additional experiments were then performed under operating conditions which minimised the extent of homogeneous reactions; upon variation of the catalyst load, the production of COx, H20 and H2 increased accordingly, while the small residual formation of olefins due to gas-phase reactions decreased upon addition of increasing amounts of catalysts. The whole bulk of data seemed to suggest that the Pt catalyst was uniquely active in the total oxidation of propane to CO2 and H20, and (above 450~ in reforming routes responsible for the formation of CO and H2; on the opposite, no evidence of any activity of Pt/AI203 in the selective oxidation to propylene could be found. An analogous investigation has been herein addressed with regard to the oxidative dehydrogenation of ethane. Schmidt and co-workers reported in fact very high selectivities to ethylene by using Pt-coated A1203 foam monoliths under adiabatic conditions at 5 ms contact time [1 ]. The authors claimed that the formation of olefins had a purely heterogeneous genesis. Holmen et al. [6] on the opposite found that the oxidative dehydrogenation of ethane in the presence of Pt/Rh gauzes mainly proceeded in the gas-phase while the gauze was effective in igniting the homogeneous reactions. The Pt/A1203 catalyst was tested in the annular reactor under reference operating conditions with a high value of gas hourly space velocity GHSV = lxl06 L(NTP)/kgcat/h, and molar feed composition ethane/oxygen/nitrogen = l/l/4. The effect of temperature was investigated by spanning the oven T from 200~ to over 750~ Already at 200~ conversion of both reactants was observed with prevailing consumption of oxygen (Figure 2). The values of conversion kept almost constant up to 500~ above this T, a sharp increase of ethane conversion was observed; conversion of both reactants was complete at the highest temperature investigated. 100 e--(-

2

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' 600

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760

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Figure 2 - ODH of ethane in annular reactor. GHSV =106 NL/kgcat/h. C2/O2/N2=1/1/4. At low oven temperature, CO2 and water were the only reaction products observed in the outlet stream. As shown in Figure 3, the selectivity of CO2 decreased at increasing temperature in favor of the formation of CO. Production of water decreased as well while the production of H2 progressively enhanced. At 550-600~ the temperature at which ethane conversion had the

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1916

100

....

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sharp increase, ethylene was formed. Its selectivity increased with T up to a maximum value at 650~ Methane was also formed above 600~

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Interesting pieces of evidence were provided by the temperature profiles of the annular reactor, 0 which changed significantly at 1 O0 200 300 400 5C)0 600 700 800 increasing oven T. Figure 4 Oven Temperature, *C compares the axial reactor profiles Figure 3 - Product distribution. Conditions as Fig. 2 with the longitudinal oven Tprofiles in the extreme cases of low and high heating T, in correspondence with the unique formation of combustion products, and the formation of ethylene, respectively. It is evident that while the low T formation of COx was characterised by the presence of an important hotspot in correspondence with the catalyst layer (located at the length 40-41.3 cm of the muUite tube), the high-T formation of olefins was characterised by the onset of a hot-spot directly at the entry region of the reactor well upstream from the catalyst bed, while the catalyst temperature did not provide evidence of any exothermic reaction. 9

20

methane 9 ethylene

-.

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450 400

9 Treactor I 700" 9 Toven

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Figure 4 - Axial temperature profiles (T in ~ of the annular reactor and of the heating wall. Axial coordinate = length of the mullite tube. Catalyst layer is located between 40 and 41.3 cm. This was interpreted as an indication of the possible onset of gas-phase reactions at the high temperatures. In order to better quantify the role of the homogeneous oxidative pyrolysis of ethane, experiments were performed in the empty reactor, in the absence of catalyst. 4. R O L E OF T H E H O M O G E N E O U S

P H A S E : TESTS IN THE EMPTY REACTOR

AND MODEL SIMULATIONS The single effects of reaction temperature, contact time, feed composition were investigated. The experimental results were compared with the simulations of a purely homogeneous isothermal plug flow reactor, operating within the same conditions as those investigated experimentally.

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

I,

0.9

.

1917 T=627~ C T=667"C

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Ethylene

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9

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10

20

30

40

50

60

70

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T = 670 *C 9 T = 725 =C 0 T = 650-725"C

t

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90

Figure 5 - Calculated (left) and experimental (right) dependence of ethylene selectivity vs. % ethane conversion. Feed composition: ethane and air with C2/O2 = 1/1. Experiments and theory showed that gas-phase reactions were active above 600~ and were very selective towards olefins (with over 60% C-selectivity to ethylene at more than 70% ethane conversion), which are in fact primary products of alkane oxidation processes. In general, it was found that for a given feed composition, the product distribution of the gas-phase reactions mainly depends on the degree of alkane decomposition (either ethane or propane), despite of the operating conditions at which the reaction takes place. This is shown for instance in Figure 5 which illustrates, respectively, the calculated and experimental dependence of ethylene selectivity on ethane conversion in ethane oxidative dehydrogenation; experiments and calculations refer to different combinations of temperature and contact time. This unique dependence reminds of the characteristic behaviour of an intermediate species which is involved in consecutive reactions, wherein 0.9 the steps of formation and consumption have 0.8 comparable activation energies; ethylene is indeed 0.7 an intermediate species of the gas-phase oxidation 0.6 0.5 of ethane and the metathesis reactions through 0.4 \ I which the pyrolysis process proceeds have typically 0.3 small activation energies. 0.2 As a consequence, high selectivities to ethylene 0.1 ~ T=1027~ "[ = 0-10 ms J can be guaranteed up to 50-60% conversion of 00 10 20 30 40 50 60 70 80 90 100 ethane operating both at 600~ and contact times Figure 6 - Ethylene selectivity vs. ethane of 1 second, and 1000~ and contact times in the conversion. C2/O2/N2= 1/1/4 order of few milliseconds, as shown in Figure 6. The results of the model simulations and of the gas-phase tests were compared with those of the catalytic tests; additional experiments were then performed in order to study the sensitivity of the product distribution upon variation of the catalyst load under operating conditions which guaranteed a negligible contribution from gas-phase reactions. It was concluded that likewise the results of the previous work with propane/air mixtures [3, 4], the Pt/A1203 catalyst is much active in the total oxidation of ethane already at low temperature; the combustion reaction rate is so high that the process underwent mass transfer limitations in the structured annular reactor. Over 400-450~ the catalyst was also active in the production of syn-gas, likely due to steam or dry reforming reactions of the alkane.

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1918

5. OXIDATIVE DEHYDROGENATION OF LIGHT ALKANES IN ADIABATIC REACTOR: A SYNERGYSM BETWEEN CATALYST PHASE AND GAS PHASE

It was thought that the Pt-catalyst, given its high combustion activity, could be exploited in an adiabatic reactor for igniting the desired gas-phase reactions, highly selective towards olefins. The expected performance of a purely homogeneous reactor was first theoretically analysed. Model predictions showed that autothermal operation is expected to guarantee the realisation of C-selectivities to olefins up to 55-60%, in correspondence with high alkane conversions (>60%). Autothermal experiments were then realised at millisecond contact times by using the Pt-coated Fecralloy fibrous support. The effects of alkane/O2 feed ratio, N2 content, flow rate, reactor design were investigated. Indeed, in the case of fuel-rich propane/air mixtures (C3/O2 > 2) nearly 50% total yield to ethylene+propylene was achieved [4]. In the case of ethane/air mixtures (C2/O2 = 2), over 63% selectivity to ethylene was observed at 71% ethane conversion. The lab-scale adiabatic reactor operated at 5 ms contact time and was preheated at 250~ Simulations showed that the same performance could be obtained from a purely homogeneous reactor at longer contact time (20 ms) or higher inlet temperature (about 600~ The presence of the Pt-catalyst thus accelerated ignition. It is finally observed that the catalyst might have an important role in minimizing the formation of coke; homogeneous tests indicated that this is highly favored under the present high temperatures and low oxygen contents, however stable activity of the Pt-containing autothermal reactor was observed along several days of operation. Preliminary results showed that ignition of the homogeneous reactions could be realized by using combustion catalysts other than the Pt/A1203. When using a BaMnAlllOl9 catalyst, however, coke formation occurred and the catalyst needed a daily reactivation. Further work is presently on-going in order to verify the possible relationship between the absence of coke and the activity of the Ptcatalyst in steam reforming reactions, which are not active over the BaMnAl~ ~O!9 catalyst. The results herein presented support the hypothesis according to which at high T and short contact times the oxidative dehydrogenation of light alkanes to olefins is mainly governed by gas phase reactions. In an adiabatic reactor, a highly active combustion catalyst such as the present Pt/A1203 system can be exploited for igniting the homogeneous process in the surrounding gas volume. Theoretical calculations and experimental demonstration of this concept showed that yields as high as 50% of short chain olefins can be realised without external heat input by exploiting the synergism between heterogeneous and homogeneous phases. This work was finanr supported by ENI and SNAMPROGETTI [ 1] M. Huff, L.D. Schmidt, J. Phys. Chem., 97 (1993) 11815. [2] M. Huff, L.D. Schmidt, J. Catal., 149 (1994) 127. [3] A Beretta, L. Piovesan, P. Forzatti, J. Catalysis 184 (1999) 455. [4] A. Beretta, P. Forzatti, E. Ranzi, J. Catalysis 184 (1999) 469. [5] E. Ranzi, P. Gaffuri, T. Faravelli, A. D'Anna, A. Ciajolo, Combust. Flame 108 (1997) 24. [6] R. Lodeng, O. A. Lindvag, S. Kvisle, H. Reiner-Nielsen, A. Holmen, Appl. Catal. A:General, 187 (1999) 25-31.

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1919

化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Composite nickel-oxide surfaces, modified by palladiun~ R.G.Baisheva, Z.K.Kanaeva, K.A. Zhubanov, Zh.K.Kairbekov Chemical Department AI-Farabi Kazakh State National University 95 Vinogradova St., Almaty, Kazakhstan 480012 The following is the study of influence of composite nickel-oxide (SiO2, TiO2, MgO, V205, and Fe203) electrodes' surface modification by palladium on their electrocatalytic activity and area of real nickel surface in the process of nitrobenzene electroreduction. Electrocatalytic activity of studied electrodes was estimated by the scale of nitrobenzene electroreduction stationary currents in the area of hydrogen evolution under-0.2B potential (reversible hydrogen electrode- r.h.e.). Stationary currents of nitrobenzene electroreduction on composite electrodes modified by palladium have grown 4-5 times. 1. INTRODUCTION Since recently, composite electrodes became widely used for conducting electrochemical reactions. Depending on nature of combining components, it is possible to obtain cheap composite electrocatalysts with sufficiently high electrochemical characteristics. Such variously composed electrodes have high and low overtension of oxygen and hydrogen evolution, which allows for conducting electrocatalytic synthesis of organic compounds with high speed and selectability. Presently such works are not many, which is why the study of electrocatalytic proiperties of composite electrodes under reduction processes of aromatic nitrocompounds is remarkably important for solving a number of electrocatalysis" fundamental problems. Application of metallic and non-metallic sub-mono-layers to the electrodes" surfaces influences the processes of sorbtion and ionization of adsorbed hydrogen [1], the catalytic properties of electrodes [2], whereas under different conditions the efficiency of ad-atoms activity appears to be different [3]. Data represented in Article [3] show, in one example, the role of both macro- (form and character of crystals) and micro-structure (surface energy spectrum) of surface in electrocatalytic processes. 2. EXPERIMENTAL Composite electrodes were prepared the following way. Nickel from chloridesulphate solution was precipitated on steel net with area of 4.5 cm 3. Then, fine-

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1920

disperse powders of oxides (SiO2, TiO2, MgO, V205I and Fe203) were introduced into electrolyte, and repeated electrolytic coating was conducted under intensive mixing at 50~ For electrochemical research were used potentiostat P-5848 and 3-section electrochemical cell with separated cathode and anode spaces. Oxide-mercury electrode served as a reference electrode. Measurement of nickel surface was conducted by galvanostatic charge curve in the area of 0.03-0.18B potentials using method [4] in 1N KON. Nitrobenzene electroreduction stationary currents were measured in ethanol-alkaline solutions. Potentials are showed relatively to reversible hydrogen electrode in the same solution. Contents and structure of composite electrodes' surfaces were studied by X-ray Spectroscopy Microanalysis (RSMA) method using Jeol electronic-probe micro-analyser (Japan). Previously developed method of estimating nickel surfaces for pressed nickel powder was used for estimating real nickel surface of composite nickel electrodes [3,4]. As real surface, accordingly to its definition, is understood the surface on which occur reversible processes of adsorbed hydrogen oxidation and adsorptions of weakly-bound oxygen. [4] shows that on nickel electrode in alkaline solution filling by adsorbed oxygen is quite insignificant up to 0.17B potential. Correspondingly, whole amount of electricity, spent while taking measurements galvanostatic curve from 0.0 to 0.2B, goes to oxidation of adsorbed hydrogen: H,,as + O H - = H 2 0 + e ;

active surface of nickel electrode is calculated by formula: Sa = C/Cs, where C = d Q / d t , under Er=0.1B Cs=1120 m k f / c m 2 [7]. For standardizing nickel electrode surface, special pre-treatment was employed: holding electrode under E=-0.2B for restoring electrode surface, and under E = 0.1-0.2B for removing hydrogen dissolved in the metallic body. Modification of composite electrodes by palladium was conducted by short-time sinking of the electrodes into 1% solution of PdC12, after which they were washed in distilled water and placed in electrochemical research cell. Before each experiment, the electrodes were activated by cathode polarization during 15 min by 50 mA current in 0.1N KONsolution. 3. RESULTS AND DISCUSSION Analysis of surface sectors' contents was conducted by RSMA. In all cases, galvanic nickel layers with 92-97% concentration of nickel and 0.1-3% of correspondent oxide's disperse phase in some areas were observable. Modification of surface by palladium leads to appearance of palladium centers of variable concentrations from 0.25% to 35%. Size of surface agglomerates for all systems changes within the limits of (0.4-1.5mkm), when modified by palladium, grew 1.5-3.0 times. Process of nitrobenzene electroreduction led to formation of surface layer with smaller particles (0.15-0.70 mkm) and to its enrichment in palladium (23-44% Pd). Modification of composite electrodes' surfaces

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1921

by palladium leads to decline in nickel content, in case of NiTiO2 - up to 3% of nickel in some areas. Table I shows data for estimations of real nickel surfaces on freshly prepared composite nickel-oxide electrodes and its change during subsequent reduction of increasing weighted portions of nitrobenzene. Presence of oxide's disperse phase promotes formation of galvanic layer with sufficiently large nickel surface, while its area depends on the nature of oxide: as appears from Table 2, it varies between 0,79 m 2 (Ni+MgO) and 0.03 m 2 (Ni+Fe203). Besides, subsequent reduction of nitrobenzene portions on the same electrode in all cases leads to decrease in nickel surface after the first weighted portion, and its stabilization in the subsequent cycles of reduction, i.e. non-reversible poisoning of surface by nitrobenzene and products of its reduction does not occur. Table 1. Changes in real nickel surface of composite nickel-oxide electrodes in the process of electroreduction of subsequent weighted portions of nitrobenzene

electrode Ni+MgO (Ni+MgO) Pd Ni+V205 (Ni+V2Os) Pd Ni+TiO2 (Ni+TiO2) Pd Ni+SiO2 large cells (Ni+SiO2) Pd Ni+SiO2 small cells (Ni+SiO2) Pd Ni+Fe203 (Ni+Fe203) Pd

Sreal,M 2 concentration of nitrobenzene, C'10 -2 mol/l 1.0 2.5 5.0 7.5 10 0.79 0.74 0.76 0.68 0.55 0.52 0.90 0.74 0.60 0.60 0.58 0.51 0.23 0.09 0.08 0.08 0.08 0.07 0.10 0.09 0.06 0.06 0.05 0.05 0.05 0.03 0.03 0.03 0.03 0.03 0.05 0.03 0.03 0.03 0.03 0.02 0.42 0.29 0.32 0.27 0.27 0.29 0.42 0.25 0.26 0.18 0.18 0.22 0.20 0.21 0.20 0.19 0.18 0.16 0.25 0.23 0.22 0.20 0.20 0.18 0.03 0.02 0.01 0.01 0.01 0.01 0.02 0.02 0.01 0.01 0.01 0.01

Modification of composite electrodes by palladium did not affect the form of charge curve and the area of modified electrodes' nickel surface practically matches that of non-modified electrodes. For systems (Ni+MgO)Pd and (Ni+V2Os) Pd there appear variation in surface areas of freshly prepared samples, compared to non-modified ones, but after the first cycle of electroreduction it disappears. Electroprecipitation of composite coating Ni+SiO2 on steel net with large and small cells leads to formation of different nickel surfaces: 0.42 and 0.20-0.25 m 2. Therefore, under modification the nickel coating with ingrained particles of disperse phase of oxide persists and individual palladium centres appear. Real nickel surface of nickel-oxide composite electrodes remarkably decreases after reduction of the first weighted portion of nitrobenzene and subsequently stabilizes.

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1922

After determining real nickel surface area, the potential equal to 0B was applied to composite electrode, chain was disunited and corresponding weighted portion of nitrobenzene was introduced, after which change in potential was registered over time until establishing stationary value. Table 2. Influence of modification by palladium on values of anode shifts of nickeloxide composite electrodes' potentials under nitrobenzene adsorption from solutions of it's different concentrations.

Composite electrode Ni+SiO2 large cells (Ni+SiO2) Pd Ni+SiO2 small cells (Ni+SiO2) Pd Ni+TiO2 (Ni+TiO2)Pd Ni+V2Os (Ni+V2Os) Pd Ni+MgO (Ni+MgO) Pd Ni+Fe203 (Ni+Fe203) Pd

1.0 0.55 0.49 0.34 0.33 0.30 0.41 0.25 0.62 0.29 0.37 0.25 0.52

2.5 0.55 0.55 0.35 0.37 0.52 0.41 0.30 0.67 0.39 0.43 0.28 0.57

E,B Cmtro~e*lO -2 mol/1 5.0 7.5 0.58 0.60 0.57 0.62 0.37 0.37 0.41 0.41 0.52 0.53 0.43 0.44 0.30 0.31 0.69 0.72 0.43 0.45 0.49 0.51 0.32 0.33 0.61 0.63

10 0.64 0.65 0.39 0.44 0.55 0.46 0.37 0.72 0.46 0.67 0.40 0.65

The value of anode shift increases with rising concentration of nitrobenzene and depends on nature of electrode (Table 2). Surface palladium modification of composite electrodes Ni+MgO, Ni+V2Os and Ni+Fe203 has increased value of electrode potential's shift, whereas with Ni+TiO2 it insignificantly declined. Stationary value of anode potential's shift of composite palladium-modified Ni+SiO2 electrode on large-cell net remained unchanged and increased on small-cell net. After determining stationary values of potential's anode shift under adsorption of nitrobenzene, currents of its reduction were measured, by shifting step by step electrode's potential from initially established at 0.05B toward negative direction. Electroreduction of nitrobenzene is observable already at positive potentials and significantly increases in the area of hydrogen evolution. The Table 3 shows values of specific stationary nitrobenzene electroreduction currents of palladium-modified composite nickel-oxide electrodes under E=-0.2B in comparison to correspondent values for non-modified electrodes. As demonstrated by the Table3, palladium modification of surface leads to increase in electroreduction stationary currents on studied composites for all concentrations of nitrobenzene. Remarkably, specific currents increased 3-5 times, which is observable under 0.1 mol/1 nitrobenzene concentration, compared to nonmodified electrodes. Size of steel nets' cells (large and small) influences values of

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1923

nitrobenzene electroreduction currents on composite Ni+SiO2 (for instance, 95.1 and 35.4 m A / m 2, and for modified - 270.2 and 143.1 mA/m2). Table 3. Specific stationary electroreduction currents under various nitrobenzene concentration on nickel-oxide composite electrodes modified by palladium.

J, mA/m2 Composite electrodes Ni+SiO2 large-cell net (Ni+SiO2)Pd Ni+SiO2 small-cell net

(Ni+SiO2)Pd

Ni+TiO2 (Ni+TiO2)Pd Ni+V205 (Ni+V2Os)Pd Ni+MgO (Ni+MgO)Pd Ni+Fe203 (Ni+Fe203)Pd

C nbl0-2mol/1 1.0 34,77

2.5 37,3

5.0 61,3

7.5 84,3

10 95,1

89,14 0,7

119,04 14,2

200,7 17,7

236,2 26,4

270,2 35,4

18,0 9,3 75,0 20,2 32,2 22,2 24,3 17,4 109,68

34,3 47,7 540,0 24,8 168,5 23,1 26,0 54,64 135,0

73,0 73,1 620,0 28,8 212,7 25,1 36,8 67,5 165,0

102,0 123,1 1006,7 38,7 225,7 31,6 54,9 84,1 247,5

143,1 349,6 1940,0 57,9 300,7 35,9 135,0 135,0 337,5

Reduction of nitroaromatic compounds with purpose of obtaining amines is important process in chemical industry. Due to their high reaction capacity amines are used for synthesis of almost all synthetic paints, as well as in production of medicines, photochemicals, antiseptics, etc. Preparatory electrolysis of o-nitrophenol nitrobenzene was conducted on freshly prepared composite nickel-oxide and palladium-modified electrodes in galvanostatic regime inside stationary cell. As demonstrated by the Table 4, output of correspondent amines depends on the nature of oxide, used in composite coating. Maximum amines output is observed on Ni+MgO and Ni+V205, and minimum- on Ni+Fe203 and Ni+SiO2 composite electrodes. In the same order decreases basic capacity of oxides, which apparently influences the degree of electronic interaction between metals and oxides.

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Table 4.Electrosynthesis of amines on composite nickel-oxide electrodes. (I=0.2 A).

Electrodes

Nitrocompound

Ni+SiO2

Nitrobenzene o-nitrophenol Nitrobenzene o-nitrophenol Nitrobenzene o-nitrophenol Nitrobenzene o-nitrophenol Nitrobenzene o-nitrophenol Nitrobenzene o-nitrophenol Nitrobenzene o-nitrophenol Nitrobenzene o-nitrophenol Nitrobenzene o-nitrophenol

(Ni+SiO2) Pd Ni+TiO2 (Ni+TiO2) Pd Ni+V205 (Ni+V2Os) Pd Ni+MgO (Ni+MgO) Pd (Ni+Fe203) Pd

Amines output by matter, % 81.1 74.0 97.5 85.0 83.5 69.1 97.2 83.7 85.0 82.0 98.0 95.6 87.0 78.3 98.5 94.5 85.8 82.7

Amines output by current, % 79.4 72.0 95.6 83.0 81.9 67.2 95.3 80.2 83.3 79.3 96.0 90.0 85.3 75.5 97.2 90.3 84.1 79.5

4.CONCLUSION Therefore, it follows from the present paper's results that presence of oxide's disperse phase promotes formation of electrodes with remarkably large active surface, size and electrocatalytic activity of which is strongly influenced by the nature of oxide. By the order of increasing activity, under nitrobenzene electroreduction, the nickel-oxide composite electrodes produce the following sequence: Ni-MgO < Ni-V205 < N i - S i 0 2 < Ni-Fe203 < N i - T i 0 2 . Modification of nickel-oxide composite electrodes' surfaces by palladium leads to formation of surface agglomerates of various size, depending on the nature of oxide, to appearance of palladium centres, which significantly increases electrocatalytic activity of modified composites in nitrobenzene electroreduction reaction. Electrochemical reduction of aromatic nitrocompounds has shown, that developed composite electrodes can be recommended for synthesis of amines. REFERENCES

1. M.I.Zhirnova, O.A.Petrii. Elektrokhimiya, 14 (1978) 975. 2. R.R.Adzic, M.D.Spasojevic, A.R.Despic. J.Electroanalyt. Chem., 92 (1978) 31. 3. O.A.Petrii, C.Ya.Vasina, M.I.Zhirnova. Elektrokhimiya, 16 (1980) 348. 4. O.S.Abramzon, S.D.Chernyshev, A.G.Pshenichnikov, Elektroknimiya, 12 (1976) 1667.

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化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Photocatalyst-Coated Acrylic Waveguides for Oxidation of Organic Compounds Lawrence W. Miller, M. Isabel Tejedor-Tejedor, Montserrat Perez Moya, Ramona Johnson, Marc A. Anderson Water Chemistry Program, University of Wisconsin, 660 N. Park Street, Madison, WI 53706, USA Acrylic (polymethyl methacrylate) sheets were coated with a thin film of porous, titanium dioxide (TiO2) semiconducting photocatalyst. The sheets were first passivated with a thin film of silica to prevent them from being oxidized by the TiO2 semiconductor photocatalyst. The coated acrylic structures act as waveguides that propagate ultraviolet (UV) light in an Attenuated Total Reflection (ATR) mode. A recirculating reactor was used to evaluate the acrylic waveguides for their ability to photocatalytically oxidize formic acid in water. The TiO2-coated waveguides utilize activating ultraviolet light four times more efficiently for the photocatal~ic oxidation of formic acid then TiO2 films illuminated directly with diffuse UV light.

I. Introduction Photocatalytic oxidation of organic contaminants on the surface of titanium dioxide (TiO2) is an attractive process for remediating liquid or gas-phase waste streams. Organic compounds can be oxidized to carbon dioxide and water in the presence of TiO2, ultraviolet (UV) light (wavelength of ca. 380 nm or less), and an electron acceptor such as oxygen. This process occurs at room temperature. While the potential of TiO2 photocatalysis for environmental remediation has generated considerable research interest (1), development of commercially viable systems has been hindered in large part by the expense of UV light generation and capture. Conventional heterogeneous photocatalytic reactors are difficult to scale to the dimensions necessary for commercial applications because light is severely attenuated within the reactor through absorption or reflection by the catalyst, the reactant, reactor walls, and catalyst supports. Scaling-up of conventional photoreactors is also difficult and expensive because light intensity diminishes with the distance from the diffuse source. Since the rate of a heterogeneous photocatalytic reaction depends on the light intensity, a scaled-up photoreactor practically requires many light sources closely spaced throughout the reactor volume. The use of waveguides to support and illuminate photocatalysts was initially proposed more than twenty years ago as a way to overcome the light distribution limitations inherent in conventional photoreactor designs (2). In a waveguide photoreactor, light enters the catalyst-coated waveguides (e.g. transparent optical fibers or planar structures) near the source where it is most intense, and it is distributed to the catalyst via successive internal

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1926

reflections at the waveguide surface. Waveguide photoreactors would have several advantages in comparison to conventional photoreactor designs (e.g. packed bed or slurry reactors) in which the catalyst is directly illuminated by sunlight or artificial light. By capturing light from a single source and propagating it internally to the photocatalyst, waveguide reactors should increase the amount of fixed, illuminated catalyst per reactor volume. Furthermore, a waveguide photoreactor would more evenly distribute light from a diffuse source throughout the reactor volume. Some researchers have coated silica optical fibers with TiO2 in an effort to realize the advantages of waveguide supported photocatalysis for environmental remediation (3,4). While these efforts yielded photoreactors that successfully oxidized organic compounds, the TiO2-coated fibers that were developed did not propagate light effectively. Light was refracted out of the fibers at the TiO2-coated surface. At the Water Chemistry Program of the University of Wisconsin, we developed planar silica waveguides coated with TiO2 that propagate UV light in an attenuated total reflection (ATR) mode (5). In an ATR mode, light propagates internally via successive total reflections at the waveguide boundaries. The TiO2 coating absorbs a small portion of the light at each reflection. These systems do not lose light through refraction at the waveguide surface. We used these TiO2-coated waveguides to photocatalytically oxidize formic acid in water. A recent paper by Miller, et al. explains the optics of a low-refractive index substrate coated with a higher refractive index TiO2 film (6). While fused silica is an attractive waveguide medium because of its transparency in the UV spectrum, there are several practical limitations to using silica in commercial reactors. It is expensive, heavy (ca. 4 g/cm3), fragile, and difficult to machine. For this reason, we developed planar waveguides made from acrylic (polymethyl methacrylate) sheet. Acrylic is an attractive waveguide substrate because it is lighter than glass, easily machined, thermoformable and moldable. It is can be formulated to be reasonably transparent in the near UV spectrum (transmittance = ca. 70% cm l at wavelength - 360 nm), and an industry exists for the manufacture of UV-stable and transparent cell-cast acrylic sheet. Here we describe a process for coating acrylic sheet with a mesoporous TiOz film. The acrylic waveguides are passivated with a microporous silica film, and the TiO2 is deposited on top of the silica. The silica film protects the acrylic from photooxidation by the TiO2 semiconductor photocatalyst. We evaluate the waveguides' photocatalytic performance, and propose a reactor design that demonstrates the advantages of TiO2-coated acrylic waveguides for the scale-up of TiO2-based photocatalytic processes.

2. Experimental Section Acrylic sheet (thickness = ca. 0.32 cm) was obtained from Cyro Industries (cat. no. AE-OP4). All chemicals were obtained from Aldrich Chemical Co. (Milwaukee, WI). Laboratory grade water (18 M.Q cm -1) was generated with a Barnstead system. The sol-gel synthetic method was used to prepare a colloidal silica sol. Tetra-ethyl orthosilicate was hydrolyzed in water and KOH (7). The resulting sol is composed of silica particles of ca. 3 nm in diameter and has a pH of ca. 9.5. The particles have a zeta potential of ca. 4.0. The pH of the silica sol was adjusted to ca. 3.5 by addition of 1 wt. % nitric acid in water. Acrylic waveguides were prepared by cutting acrylic sheet to the desired dimensions. The waveguide edges were sanded with successively finer grained sandpaper (finished with

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1927

1000 grit paper). The edges were then polished with 45 ~tm diamond slurry on a polishing wheel. To prepare the acrylic waveguides for coating with silica and then TiO2, they were first saponified in 5M NaOH for 1 hr., rinsed in water and air-dried. This step was necessary to make the acrylic surface hydrophilic. A microporous silica film was deposited onto the saponified acrylic waveguides by dipping them into the silica sol and withdrawing at a controlled rate of 2.5 cm/min followed by drying in air. The thickness of the silica film was adjusted by repeating the coating process. For the waveguides used in this study, three layers of silica were applied. A mesoporous TiO2 film was then deposited onto the silica-passivated acrylic by dipping the waveguides into a colloidal TiO2 sol of pH = ca. 3.5 (8). The withdrawal rate was ca. 8 cm/min. TiO2 films prepared from this method have a high surface area (> 150 m 2 g-~) and a pore radius of ca. 1.5 nm to 4 nm (8). These films have been shown to be effective photocatalysts (9). Six layers of TiO2 were coated onto the waveguides. After coating, the edges were repolished to remove any TiO2 or silica. Profilometry measurements (Tencoro model 2000) made on equivalently coated glass plates were used to estimate the thickness of the silica and TiO2 films. The films were inspected visually and with an optical microscope to determine their uniformity. SEM micrographs were obtained with a LEO model 952 scanning electron microscope. An internal reflection photoreactor described in a recent paper was used to evaluate the TiO2-coated acrylic waveguides for degradation of formic acid in water (5). A schematic of the reactor is shown in Figure 1. With this reactor, a rectangular acrylic waveguide measuring 5 cm x 2 cm x 0.3 cm was held in a flow-through liquid cell. A solution of 0.001 M formic acid in water was recirculated from an oxygenated reservoir across the TiO2-coated face of the waveguide at a rate of 4 mL/min. The geometric surface area of the TiO2 film exposed to the reactant was ca. 7.1 cm. Solution volume measured 10 mL. A fluorescent bulb (Sylvania, model F8T5350blb) positioned ca. 0.5 cm from the polished waveguide edge provided UV illumination. UV light intensity (all wavelenghs < 400 nm) incident on the polished waveguide edge was measured with a radiometer (International Light Corp.). Formic acid concentration was measured using a Total Organic Carbon (TOC) analyzer (Shimadzu, model TOC5000). Measurements were made at the beginning of each experiment and after 4 hours of reactor operation.

ReactantS" 7

Jo Ool

/~ ~

['~flow-thru~

i lcel!

Acrylic Waveguide

i.

I:::t I1 I

i

iI

'] .~~

Fluorescent 0 2 UV source Figure 1: Schematic of recirculating photoreactor

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1928

The reactor was modified to allow light to shine on the directly onto the surface of the TiO2-coated acrylic waveguides. This modification allowed comparisons between reaction rates for TiO2 films illuminated by internally reflected light and for films illuminated directly. Light entered this modified flow-through reactor through the acrylic support and illuminated the TiO2 film that contacted the formic acid solution. Lamp position was adjusted to make the intensity of the light striking the TiO2 film roughly equivalent to that striking the waveguide edge in the above-described series of experiments. TiO2 film thickness was the same in both sets of experiments and the same geometric surface area of TiO2 was allowed to contact the reactant. The reactant solution was 15 mL of 0.001 M formic acid in water. Experiment duration was 25 minutes. In both sets of experiments, the reactor was operated without illumination, and no degradation was observed. Flow rates were varied with no apparent change in reaction rates. For both sets of experiments (internally propagated light and direct illumination of the TiO2), three samples were evaluated in the reactor, and the experiment was repeated three times for each sample. The change in concentration with time was used to calculate an absolute rate of reaction adjusted for geometric TiO2 surface area for each sample (reported in units of mol s-~ cm-2).

3. Results and Discussion

Profilometry performed on equivalently coated glass plates showed a thickness of 50-100 nm for the silica films and 350-400 nm for the TiO2 films. SEM analysis of the coated acrylic waveguides showed similar film thicknesses. A representative SEM micrograph is shown in Figure 2.

Figure 2" SEM Micrograph of TiO2/silica film on acrylic

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1929

Uponvisual examination, the TiO2 and silica films have few observable defects. The coated waveguides exhibit birefringence to visible light, indicating that the films are transparent with boundaries parallel to the acrylic surface. Gentle abrasion of the waveguides when they were immersed in water did not remove the films, indicating reasonable adhesion. However, exposure of the waveguides to methanol, acetone or other solvents that attack acrylic removed the films. No film delamination was observed during operation of the test reactor. The highest quality silica films were obtained when the colloidal silica sol used to coat the waveguides was adjusted to pH = 3.5. At this pH, the surface charge of the silica particles is neutral or slightly negative (7). If one assumes that the saponified surface of the acrylic has a pKa value similar to that of acrylic acid (pKa = ca. 4.2), then the surface of the acrylic is protonated at this pH. We suspect that these conditions allow for favorable film deposition and adhesion. When the recirculating photoreactor was operated in internal reflection mode, the concentration of the 0.001 M formic acid solution was reduced by 36 + 4% in 4 hours of reaction time. Light intensity striking the waveguide edge measured 12 + 1 mW/cm 2. The reaction rate normalized to the geometric surface area of TiO: film exposed to the reactant was calculated using the following relation: r =

CoxXxV txA

(1)

where r is the area-adjusted reaction rate (mol s~ cm-2), Co is the initial concentration of the reactant solution (mol/L), X is the fractional conversion of the reactant, V is the reactant solution volume (L), t is the reaction time (seconds), and A is the geometric surface area of TiO2 film exposed to the reactant (cm2). The calculated rate of formic acid oxidation for the TiO2/silica-coated acrylic waveguides was 3.5 + 0.4 x 10-~1 mol s~ cm 2. For the TiO2/silica-coated acrylic illuminated directly with the same UV light intensity, the overall formic acid degradation measured 43 + 2% in 25 minutes of reaction time. This was equivalent to a rote of 6.0 + 0.3 x 10-~~mol s ~ cm 2. Under the conditions of the experiment, it appears that the TiO2 coating is ca. 17 times more effective when illuminated directly rather than by light propagated internally through the waveguide support. However, this apparent advantage is negated when one considers that a much greater geometric surface area of TiO2 could potentially be illuminated in a reactor consisting of an array of TiO2-coated acrylic waveguides. This difference is illustrated in Figure 3. An annular reactor design, as specified in Figure 3 Design A, wherein the TiO2 film is illuminated directly by a cylindrical UV source (e.g. a fluorescent UV bulb) would have ca. 63 cm 2 of illuminated catalyst. However, an array of coated waveguides as configured in Design B would have an illuminated, geometric catalyst surface area of ca. 4400 cm ~. Thus a far greater amount of TiO2 catalyst can be illuminated in the waveguide reactor design than in the annular design. In fact, the waveguide design should be ca. 4 times more effective at degrading formic acid under identical conditions of illumination.

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

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E O

<. Design Figure

A

Design

B

3: S c h e m a t i c s of two equivalently i l i u m i n a t e d T iO 2 p h o t o r e a c t o r s

We have shown that a mesoporous, photocatalytic TiOz film can be securely attached to acrylic sheet. Furthermore these coated acrylic structures will act as waveguides and can be used to photocatalytically oxidize organic compounds. These waveguides can be incorporated into photoreactor designs to dramatically increase the amount of catalyst that can be illuminated with a single light source. This should aid the scale-up of photocatalytic systems to the dimensions required for commercial applications. Additionally, acrylic waveguides can be easily machined into a variety of geometries, thereby reducing manufacturing costs and allowing greater freedom in the design of heterogeneous photocatalytic reactors. References

1) 2) 3) 4)

5) 6) 7)

8) 9)

Blake, D.M. Bibliography of Work on the Photcatalytic Removal of Hazardous Compounds from Water and Air. National Renewable Energy Laboratory, (1994). R.E. Marinangeli and D.F. Ollis, AIChE J. 23 (4) (1977) 1000. R. Bauer and K. Hofstadler, Environ. Sci. Technol. 28, (1994), 670. N.J. Peill, and M.R. Hoffmann, Environ. Sci. Technol. 29, (1995), 2974. L.W. Miller, M.I. Tejedor-Tejedor, M.A. Anderson, Environ. Sci. Technol. 33 (12), (1999), 2070. L.W. Miller, M.I. Tejedor-Tejedor, B.P. Nelson, M.A. Anderson, J. Phys. Chem. B 103 (40) (1999) 8490. L. Chu, M.I. Tejedor-Tejedor, M.A. Anderson, Mater.Res. Soc. Symp. Proc. (1994), Materials Research Society, 346. Q. Xu, M.A. Anderson, J. Mater. Res. 6 (5), (1991), 1073. M.A. Aguado, M.A. Anderson, Solar Energy Materials and Solar Cells 28, (1993), 345.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

P r e p a r a t i o n of efficient t i t a n i u m oxide p h o t o c a t a l y s t s by an i o n i z e d cluster b e a m m e t h o d and their application for the d e g r a d a t i o n of propanol diluted in w a t e r Hiromi Yamashita, Masaru Harada, Akihiro Tanii, Miwa Honda, Masato Takeuchi, Yuichi Ichihashi, and Masakazu Anpo Department of Applied Chemistry, Osaka Prefecture University Gakuen-cho, Sakai, Osaka, 599-8531, Japan Titanium oxide catalysts were prepared on porous Vycor glass (PVG) and activated carbon fibers (ACF) using an ionized cluster beam (ICB) method and their photocatalytic reactivity for the degradation of 2-propanol diluted in water was investigated. Characterizations of these catalysts by means Of SEM, XAFS and XRD techniques showed that titanium oxide thin films and titanium oxide clusters could be formed on PVG and ACF, respectively, mainly in an anatase structure. UV i r r a d i a t i o n of these catalysts in a d i l u t e d aqueous solution of 2-propanol led to the formation of acetone and CO2, demonstrating t h a t the t i t a n i u m oxide species prepared on these porous materials exhibited efficient photocatalytic reactivity. The present results have clearly shown the ICB method to be useful in the development of titanium oxide photocatalysts combined with porous supports such as PVG and ACF without any calcination pretreatment at high temperatures. 1. I N T R O D U C T I O N The photocatalytic degradation of various toxic compounds diluted in aqueous solutions using UV irradiated TiO2 powder photocatalysts has a t t r a c t e d a g r e a t deal of a t t e n t i o n for t h e i r potential in a d d r e s s i n g environmental concerns [1-7]. The design of t i t a n i u m oxide photocatalysts anchored or embedded onto support materials with large surface areas in order to condense dilute polluted substances would be of great significance,

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1932

not only to avoid the f i l t r a t i o n

and

suspension

of small

photocatalyst

particles, but in order to obtain higher efficiency. Although the sol-gel method is usually applied to prepare TiO2 thin film photocatalysts [8], this is a wet process and requires the use of solvents and calcination t r e a t m e n t s at high t e m p e r a t u r e s .

On the other hand, the Ionized Cluster Beam (ICB)

method we have applied is one of the most effective and applicable techniques in p r e p a r i n g such active t i t a n i u m oxides on support materials u n d e r mild and dry conditions [3-5]. The advantages of using the ICB method are: (1) c o n t a m i n a t i o n with various impurities can be easily excluded because the processes are performed in a high v a c u u m chamber; (2) highly crystalline TiO2 films can be prepared without any calcination at high temperature; (3) the various properties of the thin films such as thickness can be controlled while various substrates can also be applied. In the present study, we deal with the preparation and characterization of titanium oxide thin films prepared on t r a n s p a r e n t porous silica glass (PVG) and titanium oxide clusters highly dispersed on activated carbon fiber (ACF) u s i n g the ICB m e t h o d as well as the successful

utilization

of t h e s e

photocatalysts for the degradation of 2-propanol in water. 2. E X P E R I M E N T A L

Plate (10x8xl mm, -200 mg) of PVG (Corning Co., Code-7930, surface area 250 m2/g) calcined at 723 K and felt (20x30x5 mm, -70 mg) of ACF

Substrates (PVG, ACF)

I,,,

I

\

TiO2

Electric field " - - ~ - - . ~ - + - - - ~ - - ~ ) - - ~ - - Ionized for __._L_.~,.__ . . . . . _z._._ Ti-clusters \ (in 02) / accelating ~. . . . . + " i ~ ~ -

(Toho Rayon Co., FX-300, 900 m2/g) dried

at

373

K

were

used

as

s u b s t r a t e s . T i t a n i u m oxide c a t a l y s t s loaded on PVG and ACF were prepared

_

Electrons for impact ionization

by the ICB method using a Ti metal as the source material under a dilute 0 2 atmosphere.

k~ \

"

1

_

/ \

e'-..-~l~'i~+ / e'.ooo.~ ~ /

\

|

|

I

/

Neutral clusters

\/ Crucible ~

A systematic diagram of

the ICB method is shown in Fig. 1.

Source material (Ti metal)

The c h a m b e r can be e v a c u a t e d by a

Fig. 1. Systematic diagram of the

cryo pump to a pressure of 1 x 10 -7

ionized

Torr.

method.

High purity Ti metal (99.9 %)

cluster

beam

(ICB)

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1933

was used as the source of the titanium oxide. The Ti cluster beam was produced by ejecting of the Ti vapor ionized by the electrons emitted from the ionization filament, and then the produced ionized clusters were accelerated by an accelerating electrode. Active titanium oxide thin films were produced by impingement of the ionized clusters and oxygen gas onto the external surface of support substrate. In these experiments, the acceleration voltage, the substrate temperature, and the oxygen gas pressure were 0.25 kV, 523 K, and 2.0 x 10 -4 Torr, respectively. The loading amount of titanium oxide was controlled by changing the reaction time.

XAFS (XANES and FT-EXAFS) spectra were obtained at the BL-7C facility of the Photon Factory at the National Laboratory for High Energy Physics, Tsukuba. X-ray photoelectron spectroscopy were measured at 295 K with a Shimadzu ESCA-7500 spectrometer using Mg ka radiation. These photocatalysts were transferred into a quartz cell containing an aqueous solution of 2-propanol (2.6x10 -3 mol/1, 25 ml) and irradiated with a TiO2 deposited face of substrates (PVG: 10x8 mm, ACF: 20x30 mm) at 295 K using UV light (~> 280 nm) from a high-pressure Hg lamp under 0 2 atmosphere.

The products were analyzed by gas chromatography.

A

quantum yield for the photocataiytic degradation of 2-propanol was defined as the disappeared number of 2-propanol molecules relative to the number of adsorbed photons by standard potassium ferrioxalate chemical actinometry [5]. 3. R E S U L T S A N D D I S C U S S I O N 3.1 T i O 2 t h i n f i l m s e m b e d d e d

on PVG

T r a n s p a r e n t t i t a n i u m oxide thin films of various thicknesses were prepared on porous silica glass (PVG) by the ICB method. XRD and XAFS measurements of the catalysts showed that the titanium oxide thin films exist mainly in anatase structure. The UV-VIS absorption spectra of the TiO2 thin films are shown in Fig. 2. In these spectra, TiO2 thin films having a thickness larger than 100 nm clearly show specific interference fringes, indicating that t r a n s p a r e n t and uniform TiO2 thin films are formed in these catalysts. As the film thickness becomes smaller, the absorption spectra of the TiO2 thin films were found to shift toward shorter wavelength regions.

This can be

attributed to the quantum size effect of the thin films caused by the presence

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1934

of the small TiO2 particles (< 2 nm) which compose of the transparent TiO2 thin films. UV irradiation of the catalysts suspended in an aqueous solution of 2propanol at 295 K under an 02 atmosphere led to the formation of acetone and CO2 without the induction period. Acetone was further decomposed into CO2 under prolonged irradiation. The quantum yields of the p h o t o c a t a l y t i c degradation of 2-propanol using these titanium oxide thin films were higher than the yields of the standard TiO2 photocatalysts (P-25). Figure 3 shows the effect of the TiO2 film thickness on the photocatalytic reactivity of the thin film photocatalysts. The photocatalytic reactivity of these TiO2 thin films are strongly dependent on the film thickness, and the thinner the films, the higher the reactivity. These results clearly indicate that the ICB method can be applied to prepare highly .reactive t r a n s p a r e n t TiO2 thin film photocatalysts.

(Reaction Scheme) 2-PrOH

I25 %

~ - - -

L

I-~0 ~ 02

Acetone

I-~O ~ 02

C02 + H20

]r4~o / 09-

100

5

8O

(D 0 r-

"o (D -9 >..

E co rt~

E --I

t_

200

500

800

Wavelength / nm

60

-.c-

40

O

20

Powdered Ti02

2

20

1 O0

Film Thickness /nm

Fig. 2. UV-Vis absorption spectra of TiO2/PVG photocatalysts having

Fig. 3. The photocatalytic reactivity of TiO2/PVG photo-catalysts having

various film thicknesses prepared by the ICB method. The film thickness; (a) 50, (b) 100, (c) 300 (nm).

various film thickness and powdered TiO2 for the photocatalytic degradation of 2-propanol diluted in water.

3.2 C l u s t e r e d TiO2 e m b e d d e d o n A C F

Characterizations of TiO2/ACF photocatalysts prepared by the I CB method by the SEM technique showed the titanium oxides to be deposited on

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个

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化

the ACF as small clusters. The migration of titanium ion clusters was not easy on ACF with the numerous micropores, resulting in the formation of titanium oxide clusters and not thin films. XRD and XAFS measurements of the TiO2/ACF photocatalysts suggest crystalline anatase as the main structure present. Figure 4 shows the reaction time profiles of the liquid-phase photocatalytic reaction on the TiO2/ACF photocatalysts. In the initial stage of the reaction under dark conditions, the adsorption of 2-propanol onto the photocatalysts can be observed. When the UV light is turned on, the 2propanol decomposes into acetone and CO2. These TiO2/ACF photocatalysts exhibited a higher and more effective photocatalytic reactivity than the catalysts prepared on ACF by the impregnation method using an aqueous solution of ( N H 4 ) 2 T i O ( C 2 0 4 ) 2 . These results indicate that with the ICB method it is possible to prepare highly crystalline anatase titanium oxide photocatalysts at low calcination temperatures without damaging the microporous structure and adsorption properties of the ACF supports. The adsorption ability of substrates kept the original levels while the amount of deposition TiO2 was less than 2 wt%. With increase in the amount of deposited TiO2 over 2 wt%, the adsorption ability of substrates decreased gradually.

o~

o E Q. o Q_

& o t-"

100

'

I

I~

8o o~ ~

60

6O

40

_

Q

1_.

60

80

/

.m

~ > t.-Q O

~oo o~

"

light on

ce,one -

20 --

Ac ,

-

0 0

60

120

4o

~

~

I adsorption ~1 photocatalysis

1200

~"

40

800

20

400

. _

<

20@

=

0 ~-

Irradiation Time / h

Fig. 4. The reaction time profiles of the photocatalytic degradation of 2-propanol diluted in water on the TiO2/ACF photocatalyst.

'

0 FX-200 (700 m2/g)

FX-300 (900 m2/g)

their

adsorption

ability

L) O

~

I-

G.

FX-400 (1100 m2/g)

Fig. 5. Effects of the surface area of the TiO2/ACF photocatalytst on photocatalytic

,~ ._to >,

and

reactivity for the

degradation of 2-propanol diluted in water.

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Figure 5 shows the adsorption and photocatalytic properties of several TiO2/ACF photocatalysts prepared with various ACF of different surface areas. The photocatalysts prepared with ACF having a larger surface area exhibits a higher adsorption ability and photocatalytic reactivity. These results indicate that combining the superior adsorption properties of ACF and the efficient photocatalytic reactivity of TiO2 clusters allows the efficient degradation of 2-propanol diluted in water. 4. CONCLUSIONS Unique TiO2 photocatalysts loaded on PVG or on ACF by means of a combination of the ICB method and calcination at a relatively low temperature were developed for various significant photocatalytic reactions. These TiO2 photocatalysts had an anatase structure as a main phase and were found to exhibit highly efficient photocatalytic reactivity. The crystalline structure and electronic state of the TiO2 thin films supported on PVG varied with the photocatalytic reactivity of the TiO2 thin film photocatalyst and the thinner TiO2 the films the higher the reactivity. In the case of TiO2/ACF, the ACF remained intact even with the preparation of the TiO2 photocatalysts on carbon fibers at low calcination t e m p e r a t u r e s , showing a high condensation effect and high photocatalytic reactivity. REFERENCES

1. A. Heller, Acc. Chem. Res., 28 (1995) 503. 2. N. Takeda, M. Ohtani, T. Torimoto, S. Kuwabata, H. Yoneyama, J. Phys. Chem. B, 101 (1997) 2644. 3. M. Anpo, Catal. Survey Jpn., 1 (1997) 169. 4. M. Anpo, Y. Ichihashi, M. Takeuchi, H. Yamashita, Res. Chem. Intermed., 24 (1998) 151. 5. H. Yamashita, M. Honda, M. Harada, Y. Ichihashi, M. Anpo, T. Hirano, N. Itoh, N. Iwamoto, J. Phys. Chem. B, 102 (1998) 10707. 6. H. Yamashita, S. Kawasaki, Y. Ichihashi, M. Harada, M. Takeuchi, M. Anpo, M. A. Fox, C. Louis, M. Che, J. Phys. Chem. B, 102 (1998) 5870. 7. H. Yamashita, Y. Ichihashi, M. Harada, M. A. Fox, M. Anpo, J. Catal., 158 (1996) 97. 8. N. Negishi, T. Iyoda, K. Hashimoto, A. Fujishima, Chem. Lett., (1995), 841

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化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Thermal Treatment of Titanium Aikoxides in Organic Media: Novel Synthesis Methods for Titanium(IV) Oxide Photocatalyst of Ultra-high Activity Hiroshi Kominami 1, Jun-ichi Kato 1, Shin-ya Murakami 1, Masaaki Kohno 1, Yoshiya Kera 1, Sei-ichi Nishimoto 2, Masashi Inoue 2, Tomoyuki Inui 2, and Bunsho Ohtani 3 1Department of Applied Chemistry, Kinki University, Higashiosaka, Osaka 577-8502, Japan, 2Department of Energy and Hydrocarbon Chemistry, Kyoto University, Kyoto 606-8501, Japan, 3Catalysis Research Center, Hokkaido University, Sapporo 060-0811, Japan Thermal treatment of titanium(IV) alkoxide in organic solvent was examined. Titanium tert-butoxide was decomposed in inert organic solvents at 573 K to give microcrystalline anatase titanium(IV) oxide (TiO2) powders with crystallite size of ca. 9 nm and a surface area of >100 m 2 g-I. Primary and secondary alkoxides of titanium(IV), however, were not decomposed under the similar conditions, indicating that the thermal stability of C-O bonds in the alkoxides was a decisive factor for their decomposition. Reaction of titanium n-butoxide in alcohols yielded microcrystalline TiO2, suggesting that water generated from alcohols hydrolyzed the alkoxide. The as-prepared TiO 2 powders exhibited much higher rate of carbon dioxide formation than any other active photocatalysts such as Degussa P-25 and Ishihara ST-01 in the photocatalytic mineralization of acetic acid in aerated aqueous solutions. The higher activity of the present TiO 2 photocatalysts is attributed to their high crystallinity and large surface area. 1. INTRODUCTION Titanium(IV) oxide (TiO2) has attracted much attention mainly in expectation of being applied to environmental photocatalytic processes such as deodorization, prevention of stains, sterilization [1], and removal of pollutants from air and water [2-5]. For the realization of their practical application, development of highly active TiO 2 photocatalyst is keenly desired. Based on the kinetic investigation of photocatalytic reactions, parts of authors (S.N. and B.O.) have pointed out that TiO 2 particles having both large surface area and high crystallinity must exhibit higher photocatalytic activity [6]. The former property should increase the amount of surface-adsorbed substrate(s) to enhance the capture of photogenerated electron (e-) and positive hole (h+), and the latter, i.e., less defects acting as the recombination center, should suppress mutual e--h + recombination. For preparation of catalysts, catalyst supports, and photocatalysts, metal alkoxides are widely used as the starting material to avoid the contamination of the inorganic counter anion from the corresponding metal salts [7] which affects the activity and selectivity of the catalyst [8]. In most of the synthetic processes, the alkoxides are hydrolyzed in an alcoholic solution yielding amorphous (or hydrated) metal oxide with large surface area; however, their surface area is drastically decreased on calcination at the temperature where corresponding oxides begin to crystallize [7]. This predicts that the conventional synthetic processes including the calcination of amorphous TiO 2 seldom satisfy the requisites for higher photocatalytic activity, large surface area and high crystallinity. Recently, we have found that thermal decomposition of aluminum(III) and zirconium(IV) alkoxides in inert organic solvents, such as toluene, at 573 K yielded microcrystalline (average diameters of ca. 10 nm) ;~-alumina [9] and tetragonal zirconia [10], respectively, and that they possessed large surface area and exhibited excellent thermal

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stability. The latter fact suggests that these products had high crystallinity. Therefore, it was anticipated that if TiO 2 was prepared by this method, the two requisites for high photocatalytic activity would be satisfied. The present authors applied this novel method to the synthesis of TiO 2 and found that the product had extremely high photocatalytic activities as expected. Moreover, another novel method for TiO 2 synthesis, i.e., hightemperature hydrolysis of alkoxide with water generated from solvent alcohol is also reported. 2. EXPERIMENTAL Titanium alkoxide was dissolved in a 70 cm 3 portion of an organic solvent in a test tube, which was then set in a 200 cm 3 autoclave. An additional 30 cm ~ of the solvent was placed in the gap between the test tube and the autoclave wall. The autoclave was thoroughly purged with nitrogen, heated to desired temperature (453 - 573 K) at a rate of 2.7 deg min -j, and kept at that temperature for 2 h. Autogenous pressure gradually increased as the temperature was raised and usually reached ca. 6 MPa at 573 K. After the autoclave treatment, the resulting powders were washed repeatedly with acetone and dried in air. In the present paper, the TiO 2 samples prepared by thermal decomposition in inert organic solvents will be called as TD-TiO 2 (thermal decomposition) whereas the samples in alcohols as THyCA-TiO 2 (transfer tL~drolytic crystallization in alcohols). A part of the TD- and THyCA-TiO 2 powders was calcined in a box fumace for 1 h in air. The photocatalytic activity of TiO 2 (50 mag) for mineralization of acetic acid (19 or 175 /tmol) in aerated aqueous suspension (5 cm ~) was examined [11-13]. The details of pretreatment, photoirradiation, and product analyses were described in the previous papers [12, 13]. Photocatalytic dehydrogenation of 2-propanol (500/~mol) in aqueous solution (5 cm ~) was also examined as another model system under deaerated conditions; the TiO 2 powders were calcined at 823 K, platinized (0.5 wt%) followed by hydrogen (H2) reduction at 773 K, and then used for the photocatalytic reaction. The details of platinization, photoirradiation, and product analyses were reported elsewhere [14,15]. 3. RESULTS AND DISCUSSIONS 3.1. General features ofTD-TiO 2 Fig. l(a) shows an XRD pattern of TD-TiO 2 prepared from titanium tert-butoxide (TI'B) in p-xylene at 573 K, indicating that anatase was formed without contamination of any other phases such as ruffle or brookite. Addition of water to the supernatant after the autoclaving gave no precipitates, which shows that TTB was completely decomposed according to this equation (Ti(OCH(CH3)2) 4 = TiO 2 + 4CH2CHCH 3 + 2H20). The crystallite size of the as-prepared sample was calculated to be 8.8 nm from the Scherrer equation. TEM observation of the sample revealed that the average particle size was 13 nm (Fig. 2). This value was larger than the XRD crystallite size but is in good agreement with the particle size (14 nm) calculated from the surface area (111 m2g -1) by assuming the density of anatase to be 3.84 g c m -3 [16]. Anyway, it should be noted that TD-TiO 2 consists of TiO 2 nano-crystals. Kondo et al. [17] examined hydrothermal crystallization of alkoxide-derived amorphous TiO 2 and reported that the thus-formed anatase had a relatively large crystallite size (25 nm by the treatment at 523 K). The smaller crystallite size of TD-TiO 2 can be interpreted by the inhibition of crystal growth due to the smaller solubility of TiO 2 microcrystals in the organic solvent under the present autoclaving conditions than in water during the hydrothermal treatment. 3.2. Effect of synthesis conditions and calcination on the physical properties of TD-TiO 2 Several physical properties of the TD-TiO 2 synthesized under the various conditions are summarized in Table 1. TTB was decomposed in toluene even at 473 K to give anatase having relatively large surface area. Since BET surface area of a sample synthesized at 523 K (339 m2g -1) was a little larger than that (295 m2g -1) calculated from its crystallite size, it is strongly suggested that the products synthesized at lower temperature were contaminated with the amorphous-like hydrated phase. This may be attributed to insufficient thermal

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,:.

..

9..,

..

..

9 anatase

l

I I : rutile

_

__

.....

(a)

t~

t

i

I

i

20

30

40

50

60

20

!

degree

i

70

5Onto

Fig. 1 XRD patterns of: ( a ) TD-TiO 2 prepared in p-xylene at 573 K for 2 h; (b) and(c), the samples obtained by calcination of the TiO 2 product at 823 and 973 K, respectively.

Fig. 2 TEM photograph of TD-TiO 2 XRD pattern of which is shown in Fig. l(a).

Table 1 Some physical properties of TiO 2 powders prepared under various conditions and their photocatalytic activities for mineralization of acetic acid in aqueous solutions

Titanium Organic alkoxide a solvent

Temp. Method /K

TTB Toluene TTB Toluene TTB Toluene TTB Benzene TTB p-Xylene TTB Cyclohexane TNB 2-Propanol TNB 1-Butanol TNB 2-Butanol TNB t-Butyl alcohol (P-25)g (ST-O1)g (Amorphous) i

573 523 473 573 573 573 573 573 573 573

Product

After calcination at 823 K

dl01 b SBETc co2d / nm /m2g -1 /~tmol h -1

dl01 b SBETc co2d /nm /m2g -1/~tmol h -1

9.0 TD 5.3 TD <5 TD TD 7.0 TD 8.8 (8.1) f TD THyCA 25 12 THyCA 19 THyCA 14 THyCA 24 7h -

117 339 420 172 111 ND 52 107 63 81 50 h 320 h 452

22.3 6.0 4.6 10.0 17.5 7.5 29.5 17.4 21.3 22.0 8.5 11.6 <1

10 12 12 12 11 16 ND ND 20 ND ND ND 18

100 13.8 108 13.4 85 12.3 95 ND 97 (43) e ND 73 ND ND ND ND ND 56 13.8 ND ND ND ND ND 8.3 29 2.6

aTTB; tert-butoxide, TNB; n-butoxide, bCrystallite size calculated from the 101 diffraction peak of the anatase phase. CBETsurface area. dRate of photocatalytic CO 2 production from acetic acid (175 ~tmol) solution evaluated from the time course of 6 h irradiation. eAfter calcination at 973 K. fThis value included large error due to the less intense XRD peak. gP-25; Degussa P-25, ST-01; Ishihara ST-01. hData reported by the suppliers. ~Alkoxide was hydrolyzed in ethanolic solution at room temperature under atmospheric pressure.

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energy for dehydration and crystallization of TiO 2. Upon raising the synthesis temperature to 573 K, the crystallite size was increased and the amorphous fraction, as indicated by a difference in surface area by XRD and BET methods, seemed to become negligible. The effect of the structure of the alkyl group in the starting alkoxide was also examined. Titanium(IV) alkoxides from primary and secondary alcohols were not decomposed even at 573 K giving no appreciable amounts of solid products. This suggests that the process requires direct cleavage of the C-O bonds in alkoxides, and therefore the thermal stability of the C-O bond may be a decisive factor for the formation of TiO 2. A similar effect of the alkyl group in alkoxides has been observed in thermal decomposition of aluminum and zirconium alkoxides in inert organic solvents [9,10]. As shown in Table 1, the crystallite size of TD-TiO 2 depended markedly on the kind of organic solvent. Komiyama et al. [18] synthesized TiO 2 powder by chemical vapor deposition (CVD) of titanium alkoxide and found that porous amorphous TiO 2 was formed at temperatures <593 K. Although the thermal decomposition of titanium alkoxide similar to the present method took place in the CVD process, the presence of organic solvent in the autoclave might facilitate the crystallization of anatase. As shown in Fig. l(b), an XRD pattern of TiO 2 calcined at 823 K was almost identical to that before calcination except for the fact that the peaks were slightly sharpened. As expected from this pattern, the TD-TiO 2 calcined at that temperature still possessed small crystallite size (10-16 nm) and large specific surface area (ca. 100 m 2 g-l) (Table 1). Even after calcination at 973 K, the TD-TiO 2 (prepared in p-xylene at 573 K) was predominantly composed of the anatase phase and maintained its large surface area (43 m 2 g-l) though partial transformation into rutile took place (Fig. l(c)). 3.3. Photocatalytic activity of TD-TiO 2 for mineralization of acetic acid in aqueous solution under aerated conditions In the previous papers, we reported that microcrystalline TiO 2 synthesized by the similar but distinguished method, hydrothermal crystallization in organic media (HyCOM) [19], had remarkably high photocatalytic activity for dehydrogenation of 2-propanol [15], silver metal deposition [15, 20], and N-cyclization of (S)-lysine [21]. In addition to these reactions under deaerated conditions, mineralization of acetic acid in aerated aqueous solutions was also investigated and the HyCOM-TiO 2 exhibited much high rate for photocatalytic CO 2 formation from acetic acid [12]. This reaction was found to be less sensitive to the conditions, e.g., concentration of acetic acid, and stoichiometric decomposition proceeded by TiO 2 photocatalysts with fairly good reproducibility. Time dependencies of CO 2 yield from acetic acid (19 or 175/tmol in 5.0 cm 3) in an aqueous suspension of TD-TiO 2, synthesized in toluene at 573 K without further thermal treatment in air, are shown in Figs. 3 and 4, respectively. From the dilute solution of acetic acid, stoichiometric amount (38/tmol) of CO 2 was formed by ca. 2 h photoirradiation, indicating that acetic acid was completely decomposed according to the following equation (CH3COOH + 202. = 2CO 2 + 2H20 ). At the initial stage of the reactions, the molar amounts of CO 2 increased linearly in both solutions with photoirradiation time and, irrespective of the concentration of acetic acid, almost the same rate (21.9 and 22.3/tmol h -1 for 19 and 175/~mol, respectively) was obtained, indicating a pseudo-zero-order kinetics independent of the acetic acid concentration [12, 13]. The rate for photocatalytic CO 2 formation from the acetic acid (175/~m.ol) solution by various uncalcined TD-TiO 2 samples is also shown in Table 1. For comparison, results for the amorphous and some commercial TiO 2 samples known to have high photocatalytic activities are also shown. As clearly seen in Table 1, the TD-TiO 2 samples with larger crystallite sizes, which were prepared in toluene and p-xylene at 573 K, exhibited much higher rate of CO 2 production than the commercial TiO 2 samples. Since these TD-TiO 2 samples satisfied the basic requirements for active TiO 2 photocatalyst, i.e., both large surface area and sufficiently high crystallinity [6], the present results can be reasonably accepted. The negligible activity of the amorphous TiO 2 powder, maybe due to defective part (like a crystal defects for crystalline materials) leading to the e'-h + recombination, was also consistent with the hypothesis. Photocatalytic activity of TD-TiO 2 strongly depended on the synthesis conditions,

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especially synthesis temperature. Poorly crystallized TD-TiO 2 samples, e.g., those synthesized at lower temperatures, showed relatively lower activity although they have quite large surface area. As mentioned above, these samples possibly contained the amorphouslike phase with many crystal defects, which act as sites for e'-h + recombination. The effect of heat-treatment (calcination) on the photocatalytic activity for TD-TiO 2 samples synthesized in toluene at various temperatures is also shown in Table 1. Activity of the TD-TiO 2 sample synthesized at 573 K was decreased on calcination at 823 K. In contrast to the well-crystallized TiO 2 samples, activities of the TiO 2 samples synthesized at lower temperatures were greatly improved on the calcination. Such behavior has been commonly observed for amorphous TiO 2 powders [22] and is attributed to the decrease in number of the recombination sites. However, as clearly seen in Table 1, the annealed TDTiO 2 had much higher activities than the sample obtained by calcination of amorphous TiO 2. Sufficient surface areas more than 80 m 2 g-1 of the former samples and, on the other hand, small surface area less than 30 m 2 g-1 of the latter sample could reasonably account for the higher photocatalytic activities of the TD-TiO 2.

60

150

40

E :::L100

o

0,1

0 o

J

4---

o

o

20

50

. ~

0

,

0

2

4 Time / h

6

Fig. 3 Time course of CO 2 yield from an acetic acid (19/zmol) solution by suspended TD-TiO 2. The CO 2 yield corresponding to the complete decomposition or acetic acid is shown by a dashedline.

0

,

2

I

4 Time / h

,

I

6

Fig. 4 Time course of CO 2 yield from an acetic acid (175 /~mol)- solution by suspended TD-TiO2, which was same as that used in the experiment for Fig. 3.

3.4. Photocatalytic activity of TD-TiO 2 for dehydrogenation of 2-propanol in aqueous solution under deaerated conditions Photocatalytic activity of platinized TD-TiO 2 under deaerated conditions (dehydrogenation of 2-propanol: CH3CH(OH)CH 3 = H 2 + CH3COCH3) were also examined. Two TD-TiO 2 samples synthesized in toluene and p-xylene at 573 K showed H 2 production rate of 239 and 259/~mol h -1, respectively, with almost same rate of acetone formation, which were much larger than that for P-25 (125/~mol h-l). It is obvious that, in addition to the aerated condition, TD-TiO 2 has a potential for photocatalytic reaction under the deaerated conditions.

3.5. Synthesis and photocatalytic activity of THyCA-TiO 2 Thermal treatment of titanium n-butoxide (TNB) in alcohols produced anatase TiO 2 in contrast to the treatment in inert organic solvents, indicating that TNB was hydrolyzed with water generated from the solvent alcohols and subsequent crystallization occurred during the

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treatment. Some physical properties of THyCA-TiO 2 samples are summarized in Table 1. Nano-sized anatase TiO 2 particles were obtained by this method and, as expected, THyCATiO 2 samples showed excellent photocatalytic activity for mineralization of acetic acid as well as TD-TiO 2. In both HyCOM and THyCA methods, alkoxides are hydrolyzed with water. In HyCOM method, water, which was separately fed in the autoclave, dissolves in an organic solvent during the thermal treatment and then hydrolyzed alkoxide. In THyCA method, water is homogeneously generated from solvent alcohols during the treatment. Although difference in their physical properties and photocatalytic activities is unclear at this point, the fairly different conditions of hydrolysis of alkoxide allow us to expect. 4. CONCLUSION Nano-sized TiO 2 could be synthesized by two novel methods that use organic solvents, TD and THyCA, and the thus-obtained TiO 2 exhibited superior potential for photocatalytic reactions under both aerated and deaerated conditions. ACKNOWLEDGMENT This work was partly supported by a Grant-in-Aid from the Ministry of Education, Science, Sports, and Culture of Japan (09750861, 09218202, and 09044114). Mr. Hideki Yamagiwa (Kinki University) and Dr. Keiji Hashimoto (Osaka Municipal Technical Research Institute) are acknowledged for help for TEM analysis and useful suggestions, respectively. REFERENCES

1 T. Wakanabe, A. Kitamura, E. Kojima, C. Nakayama, K. Hashimoto and A. Fujishima, p. 747, in: Photocatalytic Purification and Treatment of Water and Air, D. E. Olis and H. A1Ekabi Eds., Elsevier, 1993. 2 M.A. Fox and M. T. Dulay, Chem. Rev., 93 (1993) 341. 3 M.R. Hoffmann, S. T. Martin, W. Choi and D. W. Bahnemann, Chem. Rev., 95 (1995) 69. 4 T. Ibusuki and K. Takeuchi, J. Mol. Catal., 88 (1994) 93. 5 N. Serpone and E. Pelizzetti, Eds., Photocatalysis" Fundamentals and Applications, Wiley, New York, 1989. 6 B. Ohtani and S.-i. Nishimoto, J. Phys. Chem., 97 (1993) 920. 7 I.A. Montoya, T. Viveros, J. M. Dominguez, L. A. Canales and I. Shifter, Catal. Lett., 15 (1992) 207. 8 A. Kurosaki and S. Okazaki, Nippon Kagaku Kaishi, (1976) 1816. 9 M. Inoue, H. Kominami and T. Inui, J. Am. Ceram. Soc., 75 (1992) 2597. 10 M. Inoue, H. Kominami and T. Inui, Appl. Catal. A 97 (1993) L25. 11 K. Kato, A. Tsuzuki, Y. Torii, H. Taoda, T. Kato and Y. Butsugan, J. Mater. Sci., 29 (1994) 5911. 12 H. Kominami, J.-i. Kato, M. Kohno, Y. Kera and B. Ohtani, Chem. Lett., 1051 (1996). 13 H. Kominami, J.-i. Kato, S. Murakami, Y. Kera, M. Inoue, T. Inui and B. Ohtani, J. Mol. Catal. A, 144 (1999) 165. 14 S.-i. Nishimoto, B. Ohtani and T. Kagiya, J. Chem. Soc., Faraday Trans. 1, 81 (1985) 61. 15 H. Kominami, T. Matsuura, K. Iwai, B. Ohtani, S.-i. Nishimoto and Y. Kera, Chem. Lett., 1995, 693. 16 D.R. Lide Eds., Handbook of Chemistry and Physics, 74th Ed., CRC Press, Boca Raton, 1993. 17 M. Kondo, K. Shiozaki, R. Ooki and N. Mizutani, J. Ceram. Soc. Jpn., 102 (1994) 742. 18 H. Komiyama, T. Kanai and H. Inoue, Chem. Lett., (1984) 1283. 19 H. Kominami, Y. Takada, H. Yamagiwa, Y. Kera, M. Inoue and T. Inui, J. Mater. Sci. Lett., 15 (1996) 197. 20 H. Kominami, S. Murakami, Y. Kera and B. Ohtani, Catal. Lett., 56 (1998) 125. 21 B. Ohtani, K. Iwai, H. Kominami, T. Matsuura, Y. Kera and S.-i. Nishimoto, Chem. Phys. Lett., 242 (1995) 315. 22 B. Ohtani, Y. Ogawa and S.-i. Nishimoto, J. Phys. Chem. B, 101 (1997) 3746.

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Photocatalytic water decomposition by layered perovskites Tsuyoshi Takataa, Akira Tanakab, Michikazu Haraa, Junko N. Kondoa and Kazunari Domena,c. aResearch Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan. bNickon Corp., 1-10-1 Asamizodai, Sagamihara 228-0828, Japan. cCore Research for Evolutional Science and Technology (CREST), Japan Science and Technology (JST). Photocatalytic decomposition of water was studied on some layered perovskite type materials. An efficient overall water splitting was attained on a nickel-modified perovskite, K2La2Ti3010, with hydrated layered structure. The incorporation of lead to layer composition was effective for the photoresponse under visible light irradiation. A proper modification of the catalyst and the efficient use of interlayer surface as reaction sites was found to be effective for an efficient water splitting. 1. I N T O R O D U C T I O N

Photocatalytic water splitting has been studied in terms of photon energy storage technique. One of the promising ways to develop this system is an efficient application of semiconductor oxide as photocatalyst. A sort of ion-exchangeable layered materials was found to be advantageous photocatalytic materials in that interlayer space is available as reaction sites [1]. So-called layered perovskite type materials were found to be attractive photocatalytic materials. Photocatalytic behavior of some layered perovskite type titanates and niobates has been investigated on account of their suitable energy levels of conduction bands and valence bands. K2La2TisOlo and KSr2Nb3Olo are the typical examples of layered

1943

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perovskite type niobate and titanate [2, 3], whose schematic crystal structures are depicted in figure 1. The crystal structures of the two materials are quite similar. Two-dimensional oxide sheets composed of mixed oxide based on the ABO8 perovskite structure, form layered structure stacking along the c-axis. Alkaline metal cations are located in between the individual sheets as interlayer cations in order to compensate for the negative charge of the layer. The most distinctive difference in the crystal structure between K2La2Ti8010 and KSr2NbsOlo is the mlmber of interlayer alkaline metal cations due to the difference of their layer charge density. K2La2Ti8010 belongs to Rudlesden-Popper phase, while KSr2Nb3Olo belongs to Dion-Jacobson phase. The difference in the interlayer circumstance of the two phases should result in different ion-exchange and intercalation behavior as well as the photocatalytic behavior. Perovskite structure is one of the most basic crystal structures, and this group has many varieties in composition as well as functions as inorganic solids. We present here the use of some ion-exchangeable layered perovskites as photocatalyst [4].

~

K2La2Ti3Olo

O

K

O

La or Sr TiO6 or NbO6

KSr2Nb3Olo

Figure 1. Schematic structures of layered perovskites. 2. E X P E R I M E N T A L

Layered perovskite type materials were prepared by a usual ceramic technique. Starting materials were carbonates for alkaline metal and alkaline earth metal components, and the origin of the other materials were oxides. The powders of each component were mixed in desired composition

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and were sufficiently ground. The products were obtained by heating the mixtures in air at 1073~1273 K for 2 days with an intermediate grinding. Nickel was loaded by impregnation method from aqueous Ni(NOs)2 solution, and then the catalyst was activated by H2 reduction at 773 K for 2 h followed by the O2 reoxidation at 473 K for 1 h (referred to as R773-O473) in a closed gas circulation system. Pt was loaded by in-situ photodeposition method from aqueous H2PtC16 solution. Photocatalytic reactions were carried out in a closed gas circulation system. The catalyst (1.0 g) was suspended in distilled water (330 ml) containing certain amount of alkaline metal hydroxide. Light irradiation was carried out from the inside of reaction vessel using an inner irradiation type cell made of quartz or Pyrexglass in which a high pressure mercury lamp (450 W) was placed. Wavelengths used for reaction were > 190 n m , > 300 nm and > 400 nm corresponding to the reactions systems using i) quartz cell, ii) Pyrex-glass cell and iii) a cell with aqueous NaNO2 solution filter for the incident light, respectively. Produced gases were analyzed by gas chromatography (TCD, MS-5A column, Ar carrier). 3. R E S U L T S a n d D I S C U S S I O N S Figure 2 shows the time course of H2 and 02 evolutions over

K2La2Ti3Olo and nickel-modified K2La2Ti3Olo under the irradiation with quartz cell. The amount of evolved I-I2 and 02 over bare and

~ 2 ~:~

nickel-modified K2La2Ti3Olo are ~ 1 plotted by open (H2) and filled (02) ~.~> triangles and circles, respectively. K2La2Ti3Olo evolved both H2 and 02, o although the amount of evolved 02 was smaller than the stoichiometry. However, the activity of photocatalytic 0 water splitting was remarkably

1

2

3

4

5

6

reaction time / h

enhanced by the nickel-modification. The amount of optimized nickel-loading Figure 2. Photocatalytic water was 3.0 wt %, and the catalyst was decomposition over K2La2Ti3010.

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1946

treated by R773-O473.

Next, photocatalytic behaviors of some layered perovskites with a general formula of Rb2-xLa2Tis.xNbxOlo (x=0, 0.5, 1.0) were investigated. Rb2.xLa2Tis.xNbxOlO (x=0, 0.5, 1.0) are the relatives of K2La2Ti3Olo, and these with x = 0, 1.0, 0.5 belong to Ruddlesden-Popper phase, Dion-Jacobson phase and the hybrid type of the former two phase, respectively [5]. A partial substitution of Ti4+ by NbS+ reduce the negative charge of layer, and accordingly, the number of interlayer Rb+ ions decrease in order to keep the charge balance. When one-third of Ti4+ in Rb2La2TiaOlo were replaced by NbS+, structural transformation from Ruddlesden-Popper phase to Dion Jacobson phase should take place. Therefore, a proper selection of layer composition enables to control the phase of layered perovskite. Photocatalytic activities of Rb2.xLa2Tia.xNbxOlo under the irradiation with quratz cell as well as their optimum conditions are summarized in table 1. The activities of Rb2La2Ti~Olo and Rbl.sLa2Ti2.sNbo.5Olo did not differ so much, while the activity of RbLa2Ti2NbOlo was lower by about one order of magnitude. The partial substitution by Nb did not lead to so many changes in the character of catalysts such as band gap value or particle size which should affect on the photocatalytic activity. Only conceivable reason for the difference in activity was the availability of interlayer space. RbLa2Ti2NbOlo is anhydrous, while the other are hydrous. It seems that layered perovskites which hold larger amount of alkaline metal cations at the interlayer space facilitate the hydration. As seen from table 1, the Rb2La2TiaOlo and Rbl.sLa2Nb2.sTio.5Olo required larger amounts of nickel loading (4.0~5.0 wt%) for the optimum activities than that (0.3 wt%) of RbLa2Ti2NbOlo. The difference in the optimum amount of nickel loading is probably due to the difference in the available reaction surface area. The hydration of interlayer space seems to be important to attain an efficient water splitting. Table 1. Photocatalytic activity ofRb2.xLa2Ti3.xNbxOlo (x=0, 0.5, 1.0) rate of gas evolution//zmol.h-i 0 0.5 1.0

optimum condition

H2

02

Ni-loading/wt%

pH

869 725 79

430 358 30

4.0 5.0 0.3

12.8 12.6 12.8

1,,

,,,

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1947

KSr2NbsOlo, a layered perovskite type niobates which belongs to Dion-Jacobson phase was studied. Although the same procedures as those of K2La2TisOlo were employed for the nickel-modification, 02 evolution was not observed for KSr2Nb8Olo. KSr2NbsOlo did not show hydration of interlayer space even in an aqueous solution. However, this compound showed the hydration of interlayer space by the substitution of protons for

interlayer K+ ions. Proton-exchanged form of KSr2NbsOlo was prepared by the suspension of the powder in an aqueous HNOs (5 tool.l-l) solution for 3 days. The use of proton-exchanged form of KSr2NbaOlo did not result in an overall water splitting. However, a remarkable enhancement of the activity for H2 evolution was observed for Pt(0.1 wt%)-loaded KSr2Nb3Olo by the proton-exchange, when the reaction was carried out in an irreversible photocatalytic process containing aqueous methanol solution under the irradiation with Pyrex-glass cell as shown in figure 3. Therefore, it is expected that proper modification and efficient use of interlayer space results in an efficient overall water splitting. 3

15 o

o

--___ @4

O4

lO

0

o

o

o

2

o

l O

-

0

.

20

40

,

60

i

80

.

,

|

100 120

reaction time / h Figure 3. Time course of H 2 evolution over KSr2NbsOlo and proton-exchanged KSr2NbsOlo. A, KSr2Nb3010; O, proton-exchanged KSr2NbsOlo.

O

_

6

_

,

1

i

2

,

i

3

,

I

.

4

i

5

reaction time / h Figure 4. Time course of H2 evolution over proton-exchanged RbPb2Nb3010. A, > 300 nm; O, > 400 nm.

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1948

The layered perovskites presented above do not absorb light in the visible region. Those layered perovskites which are composed of metal cations with d o electronic configuration are wide gap semiconductors. Incorporation of lead to those compounds seems to be effective to extend the absorption edge into visible region. The edge of valence band shifts to negative side by the incorporation of lead probably due to the donation of Pb6s electrons to valence band that is ordinary formed by O2p electrons [6]. RbPb2NbsOlo is analogous to KSr2NbaOlo and shows the hydration of interlayer space on substitution of interlayer Rb § ions by protons. Proton exchange was carried out in HNOa (3 mol-l-Dfor i day. Higher concentration of HNOa and/or prolonged period of proton exchange remarkably reduce the photocatalytic activity because of the partial elution of lead component. Figure 4 shows the time courses of H2 evolution over proton-exchanged RbPb2NbaO10. This catalyst evolved H2 from aqueous methanol solution even under the irradiation of light with wavelength longer than 400 nm. 4. SUMMARY Overall water splitting was attained on some layered perovskites by a proper modification. The use of interlayer space as reaction sites resulted in an enhancement of photocatalytic activity. Incorporation of lead to the layer composition enabled the H2 evolution from aqueous methanol solution under visible light irradiation. Perovskite type materials have many combinations in composition, and it is inferred that some of lead-containing novel layered perovskites can be prepared, and work under visible light irradiation. References

1. A. Kudo, A. Tanaka, K. Domen, K. Maruya, K. Aika and T. Onishi, J. Catal., 111 (1987)67. 2. J. Gopalakrishnan, V. Bhat, Inorg Chem., 26 (1987) 4429. 3. M. Dion, M. Ganne and M. Tournoux, Mater, Res. Bull., 16 (1981) 1429. 4. T. Takata, Y. Furumi, K. Shinohara, A. Tanaka, M. Hara, J. N. Kondo and K. Domen, Chem. Mater., 9 (1997) 1063. 5. J. Goparakrishnan, Chem. Mater., 7 (1995) 1265. 6. G. Trinquier and R. Hoffman, J. Phys. Chem., 88 (1984) 6696.

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1949

化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Catalyst Preparation and Reactor Design for Gas-Phase Photocatalytic Oxidation of Trichloroethylene (TCE) Pollutant A. J. Maira, K. L. yeung*, C. K. Chan, J. F. Porter, and P. L Yue Department of Chemical Engineering, the Hong Kong University of Science and Technology Clear Water Bay, Kowloon, Hong Kong 1. INTRODUCTION Heterogeneous photocatalytic oxidation (PCO) is a promising technique for the degradation of volatile organic compounds (VOCs) in process air stream 1. This technique allows the oxidation of gas-phase VOCs to CO2 and H20 at room temperature in the presence of semiconductor catalyst and UV or near-UV light source. PCO can effectively mineralize simple organic compounds, but most pollutants are complex molecules that are difficult to degrade. This demands for better catalyst formulation and careful design of photocatalytic reactor in order to achieve an optimal performance. Titanium dioxide (TiO2) is the most commonly used photocatalyst for PCO process because of its excellent chemical stability, good catalytic activity and easy availability. In this study, nanostructured TiO2 catalysts were synthesized at room temperature using sol-gel method 2-3. By carefully controlling the hydrolysis and condensation of titanium alkoxides, TiO2 catalysts of controlled particle size and morphology were obtained 4-5. This paper will discuss the effects of secondary particle or aggregate size on the catalyst reactivity for TCE degradation. A good photoreactor design must maintain both high catalyst effectiveness and high photon utilization. However, the geometry of the photoreactor is usually constrained by the light source. In this paper the performance of two reactor configurations was compared for gas-phase TCE photo-oxidation. 2. EXPERIMENTAL In this paper, TiO 2 catalysts were prepared using a modified sol-gel process and were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and N2 physi-adsorption methods. During the synthesis, the effects of different parameters on the size and morphology of" the TiO2 particles were investigated. Methods for obtaining TiO 2 with different secondary particle (i.e., aggregate) size were developed. These catalysts were tested for the gas-phase photo-oxidation of TCE in two different reactor configurations.

2.1. Synthesis and characterization of TiO2 nanoparticles Titanium isopropoxide (TIP) was used as the precursor for the sol-gel synthesis of nanostructured TiO2 catalysts. The synthesis was conducted in a dry, nitrogen hood where 1 *Author to whom correspondence shouldbe adressed Tel: 852-2358-7123;Fax: 852-2358-0054;E-mail: [email protected]

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1950

ml of TIP was added, drop-by-drop, into a well-mixed isopropanol-water solution. The addition rate of TIP was precisely controlled using a specially designed glass apparatus. Aider the addition, the mixture was aged for 1 h at room temperature. The effects of water concentration (0.5 _< [H20] < 4.7 M), [H20]/[TIP] ratio (4 < [H20]/[TIP] < 400), TIP addition rate (0.02 < R < 2 ml/min) and mixing rate on the particle size were investigated. The amorphous as-precipitated gel spheres were coated onto a porous stainless steel plate (Mort Metallurgical) with a nominal pore size of 0.2 btm, a thickness of 1 mm and an area of 6.25 cm 2. Following the thermal treatment in air at 450~ and 3 h, the deposits were transformed into active anatase phase. The primary particle size was determined from the line broadening of the (101) XRD (Model PW1830, Philips) diffraction peak for the anatase TiO2, whereas the secondary particle size was measured from the TEM (Philips CM20) micrographs. The catalyst surface area was also analyzed using N2 physi-adsorption (Micromeritics ASAP 2010). 2.2. Photocatalytic Oxidation of TCE A novel reactor was designed for gas-phase photocatalytic oxidation of volatile organic compounds. The reactor can be operated as a conventional fiat-plate reactor with the reactant flowing parallel to the catalyst surface, or as a through-catalyst reactor with the reactant flowing though the catalyst layer. The TiO2 catalysts supported on stainless steel plate and the reactor's Pyrex glass window form a narrow rectangular channel (25mm x 2mm) for the gas flow. The liquid TCE feed was delivered to a constant temperature heat exchanger using a syringe pump (kdScientific 1000). The vaporized TCE was mixed with flowing oxygen (HP, HKO) before entering the photoreactor. The catalyst plate was illuminated by five fluorescent black tubes (6w, BLB Sankio Denki) located 7 mm above the reactor window. The outlet gases were separated using a GS-GASPRO capillary column (0.32 mm x 30 m) and analyzed using a gas chromatograph (HP 6890) equipped with thermal conductivity and flame ionization detectors. In a typical experiment, the sample catalyst plate was placed in the reactor and the desired gas flowrate and TCE concentration were fed to the reactor. Aider reaching a steady state, the outlet gas composition was analyzed to give the initial TCE concentration. Aider switching on the UV-light, the reactor effluent was monitored until a constant composition was obtained. The steady state TCE consumption rate can then be calculated from the initial and final TCE concentrations. 3. RESULTS AND DISCUSSION

3.1. Preparation TiO2 with controlled particle size The modified sol-gel method allows the preparation of amorphous TiO 2 gel spheres of well-defined morphology and particle size. The experimental study showed that the [H20]/[TIP] ratio, TIP addition rate and mixing rate have no effect on the amorphous particle size. This amorphous particle size could be controlled by the water concentration during the sol-gel process. The amorphous titania was transformed into crystalline TiO2 particles using thermal and hydrothermal processes. The crystalline phase was nucleated and formed from the amorphous titania material so that the shape and size of the aggregate or secondary particle were identical to the original amorphous titania. The primary particle size was controlled

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1951

during the thermal or hydrothermal treatment. Figure 1 displays the TEM micrograph of TiO2 catalyst with primary and secondary particle sizes of 7 nm and 900 nm, respectively. Although the primary and secondary particles can be easily distinguished from these micrographs, the TEM was used mainly to measure the secondary particle size while the primary particle size was independently determined from the XRD analysis in order to avoid confusion. In this study, the primary particle size was fixed at 11 nm and the secondary particles have a spheroidal shape.

Fig. 1. Primary (7 nm) and secondary particles (900 nm) of a typical TiO2 catalyst.

1000

50

A

A

800 -

E

r 13.

~

40-

.o

30

C

600-

!1..._

400 200-

0

r

v

0 0

i

i

i

i

0.5

1

1.5

2

-

2010I

2.5

M [H=O]

Fig. 2. TiO 2 secondary particle size (SPS) versus water concentration during sol-gel preparation process.

250

t

t

500

750

1000

SPS (nm)

Fig. 3. TCE conversion versus TiO2 secondary particle size (SPS).

The amorphous particle size and, therefore, the secondary particle size of TiO 2 anatase was controlled by water concentration during sol-gel process. Figure 2 plots the secondary particle size of TiO2 catalyst as a function of water concentration at fixed TIP addition rate of 0.2 ml/min and [H20]/[TIP] ratio of 4. The figure shows that the particle size decreases rapidly

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1952

with water addition reaching a constant value at high water concentrations. This behavior can be explained if we consider that the increase in water content of the synthesis mixture results in higher homogeneous nucleation rate 6 and smaller gel spheres. At a constant [H20]/[TIP] ratio, increasing the water concentration means smaller reaction volume and higher [TIP] concentration. At [H20] > 0.5 M, a critical value was reached and secondary particle size smaller than 80 nm could not be obtained. The TiO2 catalysts shown in Fig. 2 have identical primary particle size of 11 nm and exhibit similar surface area independent of the secondary particle size (cf. Table 1). Table 1. Particle size and surface area of TiO2 catalysts Catalyst P11S100

Primary Particle Size (nm)** 10-12

Secondary Particle Size (nm) * 95-120

Surface Area (m2/g) 93

P11S300

10-12

275-315

92

P11S400

10-12

370-430

93

P11S600

10-12

580-640

94

** primary particle size measured from XRD line broadening * secondary particle size determinedby TEM analysis

3.2. Effects of TiO2 secondary particle size on TCE photo-oxidation The TCE conversions for different secondary particle sizes are shown in Fig. 3. The conversion plot shows that for aggregates smaller than 600 nm, the TCE conversion exhibits rapid decline with increasing particle size. The reaction results indicate that TiO2 photoreactivity is strongly influenced by the secondary particle size although these catalysts have identical primary particle size of 11 nm and a constant surface area of 94 mZ/g. Two plausible explanations can decribe the photocatalytic behavior of these TiO2 catalysts. The lower reactivity of the TiO2 catalysts with larger secondary particle size can be due to larger interparticle diffusional resistance across the aggregate. It is also important to account for the photon penetration through these ensemblies. In larger aggregate, the number of photons reaching the core can be drastically reduced by light scattering resulting in lower catalyst effectiveness. Studies conducted on TiO2 aggregates of different primary particle size tend to support the later explanation.

3.3. Performance of different reactor configurations TiO2 catalyst of 300 nm secondary particle size and 11 nm primary particle size was used to evaluate the performance of the fiat-plate and through-catalyst reactor configurations for TCE photo-oxidation.

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1953

Figure 4 shows the plots of TCE reaction rate as a function of gas flow for the two reactor configurations. The reaction rate increases with flow rate reaching a constant value at flow rates higher than 500 seem. This behavior indicates that TCE photo-oxidation is limited by the mass transfer rate of the TCE to the irradiated surface of the TiO 2 catalyst. 0.8 ,~

Differential

0.6 oi t._

0.4 0.2

0

!

i

i

250

500

750

Flow rate (mllmin)

Figure 4.- Experimental values of the TCE reaction rate, r, versus the gas flow rate for the flat-plate and through-catalyst configurations, using 0.2 g of TiO2 (SPS = 300 nm) and a TCE concentrations of 0.1 mM.

At low flow rate, it was expected that the flat-plate reactor should exhibit low reactivity instead the reactor displays a slight maximum in TCE reaction rate at this condition. This was attributed to the formation of reaction intermediates which at sufficient residence time can further react with the TCE resulting in the higher conversion rate observed in the study. Indeed, the selectivity of the flat-plate reactor for total combustion was significantly lower than the through-catalyst reactor as shown in Table 2.

Table 2. TCE photo-oxidation selectivity toward CO2 for flatplate and through-catalyst reactors. Flow rate (ml/min)

Selectivity (%) Flat flow

through-catalyst

50

39

53

80

16

30

300

15

15

Only carbon dioxide, chlorine and phosgene (COC12) were detected in the photoreactor effluent for both reactor configurations. Carbon and chlorine balance were not obtained because phosgene could not be analyzed quantitatively. Therefore, the selectivity was measure in term of total mineralization of TCE to COz. Dichloroacetyl chloride (DCAC) has also been reported as one of the main products of TCE photo-oxidation. However, it was not detected in this work due to its sort on-stream life of less than 240 ms 7, since longer time were spent by the reactants to reach the detector.

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1954

4. CONCLUSIONS

TiO2 catalysts of controlled primary and secondary particle sizes were successfully prepared using a modified sol-gel method. These catalysts have identical particle morphology. Although the surface area of the TiO2 catalyst is unaffected by the secondary particle size, the activity of the catalyst for TCE photo-oxidation exhibited strong dependency on the aggregate size. Comparison of the two reactor configurations, fiat-plate and differential indicates that the differential reactor provides better mineralization of the TCE pollutant. 5. ACKNOWLEDGEMENT We gratefully acknowledge the financial support from the Hong Kong Research Grant Council (grant RGC-HKUST 6118/97P). We also thank Dr. K. M. Moulding of the Materials Characterization and Preparation Facility (MCPF) of the Hong Kong University of Science and Technology (HKUST) for conducting the TEM analysis of the TiO2 catalysts. 5. R E F E R E N C E

1. M.R. Hoffmann, S.T. Martin, W. Choi and D.W. Bahnemmann, Chem. Rev., 95 (1995) 69. 2. V.J. Nagpal, R.M. Davis, and S.B. Desu, J. Mater. Res., 10 (1995) 3068. 3. A.G. Gaynor, R. J. Gonzalez, R.M. Davis and R. Zallen, J. Mater. Res., 12 (1997) 1755. 4. Z. Zhang, C.-C. Wang, R. Zakaria and Y. Ying, J. Phys. Chem. B., 102 (1998) 10871. 5. N. Xu, Z. Shi, Y Fan, J. Dong, J. Shi and M. Z.-C. Hu, Ind. Eng. Chem. Res. 38, (1999) 373. 6. J.H. Jean and T. A. Ring, Colloids and Surfaces 29, (1988) 273. 7. W.A. Jacoby, M. R. Nimlos and D. M. Blake, Environ. Sci. Technol., 28 (1994) 1661.

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1955

化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

Formation of E t h y l e n e O x i d e b y P h o t o o x i d a t i o n of E t h y l e n e over Silica Modified with Copper Yuichi Ichihashi a and Yasuyuki Matsumura b aOsaka National Research Institute, AIST, Ikeda, Osaka 563- 8577, Japan bResearch Institute of Innovative Technology for the Earth, Soraku-gun, Kyoto 619-0292, Japan Silica and silica modified with copper can catalyze the photooxidation of ethylene with molecular oxygen. Without the modification silica mostly produces carbon dioxide, however, Cu-modified silica yields various oxidative products such as acetaldehyde, ethylene oxide and ethanol. In particular silica containmg only 0.1wt% of copper gives a maximum yield of ethylene oxide, implying that copper is not a major photooxidation site for ethylene oxide. No reaction takes place with the irradiation of light whose wavelength is longer than 280 nm. The UV-visible diffuse reflectance spectra of silica with and without modification of copper suggest that the UV absorption of silica promote the photooxidation of ethylene. Formation of ethylene oxide can proceed even after exhaustion of oxygen in gas phase, indicating that surface oxygen species take important roles in the photooxidation of ethylene to ethylene oxide. 1. INTRODUCTION Silica has been believed as an inert or poor catalyst for most of reactions because its surface is neutral or perhaps slightly acidic. However, some chemical reactions take place on the silica surface, e.g., ethanol can be dehydrogenated to acetaldehyde over highly dehydrated silica [1-4]. Ogata e t al. reported that photocatalytically active sites are present on silica and photooxidation of carbon monoxide proceeds on the surface [5]. Partial oxidation of propylene to the epoxide takes place over some silica-based photocatalysts containing metal oxides, and coordinately unsaturated sites such as Si(OSi)3 were proposed [6,7]. We have also studied these peculiar active sites of silica for photocatalytic reactions,

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1956

because reactants are often easily activated over the photocatalysts u n d e r mild

conditions in comparison with t h e r m a l reactions [8-11]. In the present work we will show t h a t the photooxidation of ethylene to ethylene oxide can take place over silica, and it is significantly promoted by modification with a slight amount of copper while it is suggested t h a t the sites on silica are r a t h e r important to the epoxidation. 2. EXPERIMENTAL Silica was supplied by Fuji Silicia Chemical (Cariact G-10).

Silica whose BET

surface area was 252 m 2 g" was modified with copper (Cu/SiO2; Cu content, 0.05-3 wt%) by evaporation of an aqueous solution of Cu(NO3)2 (Wako Pure Chemical) on silica at ca. 330 K. at 673 K in air.

The wet mixture was freeze-dried, and then, calcined for 5 h

Prior to the reaction, the sample (150 mg) was usually heated in oxygen at 673 K for 1 h, then, evacuated at 973 K for 1 h to 10 .4 Pa.

The powder was spread on

the flat bottom (12 cm 2) of a quartz reaction cell (15 cm 3) connected to a conventional vacuum s y s t e m .

After introduction of ethylene (0.5 kPa, 180 ,amol)

and oxygen (0.5 kPa, 180 ,amol) the catalyst was irradiated with a high-pressure Hg lamp (100 W) through a water filter at 273 K.

The light went through the

thin layer of the catalyst. The products were collected with a liquid nitrogen trap after irradiation and analyzed with a gas chromatograph (Shimadzu GC-8A). Electron spin resonance (ESR) of the samples was m e a s u r e d with a Bruker ESP-300E X band-spectrometer.

Adsorption of oxygen was carried out in a

vacuum system equipped with a Baratron vacuum gauge.

The magnetic field

was corrected by the reference of DPPH (diphenylpicrylhydrazyl).

UV-visible

diffuse reflectance spectra were recorded at room t e m p e r a t u r e with a JASCO V560 spectrometer.

3. RESULTS AND DISCUSSION E t h y l e n e was mainly photooxidized to carbon dioxide over silica evacuated at 973 K (Table 1).

Although a small a m o u n t of ethylene oxide whose selectivity

was 2 % with the ethylene conversion of 26 % was detected after irradiation for 45 min, no e t h y l e n e oxide was d e t e c t e d a f t e r the r e a c t i o n for 3 h.

The perfect

o x i d a t i o n to c a r b o n dioxide a n d water, which is the m a j o r r e a c t i o n , does not

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1957

Table 1.

Product distribution for the photo-oxidation of C~H4 over SiO~ and

Cu/SiO2 catalysts evacuated at 973 K after the reaction for 5 h. ,

,

,,

,

Reduction Rate of Pressure a) . / Torr. min ~ CO2

Cony. /%

Catalysts

i,

,,

,

Select./% CH3CHO

C2H40

C4

C2HsOH

,

,,

SiO 2

33.0

0.028

99.8

0

0

0.2

0

0.05wt% Cu/SiO2

33.6

0.014

98.7

0.5

0.1

0.6

0.1

0.1wt% Cu/SiO2

34.9

0.012

91.3

1.0

6.8

0.7

0.2

0.5wt% Cu/SiO z

33.5

0.012

90.9

1.1

5.9

1.7

0.4

21.1

0.011

92.5

0.7

3.8

2.6

0.4

1.0wt%

Cu/SiO2 ,

L

,

a) Measured in the period of 30 - 40 min of reaction time accompany change in the total m a s s of the reaction mixture, however, the total pressure

8

of t h e r e a c t o r d e c r e a s e d

g r a d u a l l y d u r i n g the reaction b e c a u s e silica a d s o r b e d w a t e r produced. change

in pressure

The

corresponded

m o s t l y to t h e y i e l d of c a r b o n dioxide d e t e r m i n e d after the reaction.

%. ~

~ 7

*--., (b)

Hence,

we c a n follow t h e r e a c t i o n w i t h t h e p r e s s u r e change and Figure 1 shows

t O

9 f)_ .l.

the relation between the photooxidation activity

and

the

dehydration

t e m p e r a t u r e of silica.

The r e d u c t i o n

r a t e of the reaction p r e s s u r e i n c r e a s e d w i t h an i n c r e a s e in t h e e v a c u a t i o n t e m p e r a t u r e up to 973 K and decreased at 1 0 7 3 K.

It was reported

that

maximum

intensity

the

photoluminescence

of

of s i l i c a a r o u n d

o

20

40

60

Time / min Fig. 1. Time profiles of the pressure change in the photooxidation of C2H 4 o n S i O 2 catalysts evacuated at (a) 473 K, (b) 773 K, (c) 973 K, and (d) 1073 K.

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1958

450 nm (excitation energy, ca 250 nm) can be observed by evacuation at ca. 970 K [11,12].

Thus, the photocatalytic activity is probably related to the phenomenon

while the photoluminescence may be attributed to silanone (Si=O) groups as discussed by Goal and Dixon [13]. In the ESR of the unmodified SiO2 UV-irradiated in presence oxygen, a signal of which g, and gi were 2.0089 and 2.0043, respectively, was observed (Figure 2), and it can be attributed to T-shape Si-O3 species [5,16]. Hence, it is suggested that Si-O3 species are formed by the reaction between Si=O and 02 under UVirradiation. Modification of silica with a small quantity of copper produced a significant change in the activity. The products of partial oxidation such as acetaldehyde and ethylene oxide were detected even with silica containing only 0.05 wt% of copper, and the initial rate of the reaction was drastically decreased in comparison with unmodified silica (see Table 1).

280nm a)

1.0

(a)

.,~1.0

b ( )

,L_~

~-~~_~_

rY

0 wt%

i I

0

EL 1.0

gm g•

0.1 wt%

L_~

0 1.0 0

gs gl Fig. 2. ESR spectra of (a) SiO2 and (b) 0.1wt% Cu/SiO2 after UV irradiation in presence of 02 at 77 K.

d

250

'

300

350

400

450

500

Wavelength / n m

Fig. 3. UV-visible diffuse reflectance spectra of Cu/SiO2 catalysts whose Cu contents are (b) 0.05 wt%, (c) 0.1 wt%, and (d) 0.5 wt%, and of (a) SiO2 catalyst. The catalysts were evacuated at 973 K.

There was a peak at 450 nm attributed to Cu § in the UV-visible spectra of the samples containing copper (Figure 3), appearing that the samples were partially

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1959

reduced during the evacuation at 973 K [14,15]. The similar spectra for 0.05 wt% and 0.01 wt% Cu]SiO~ suggest that the electronic states of copper on these

samples are close.

The spectrum for 0.5 wt% Cu/SiO2 was different from the

other samples containing copper probably because the sample is not perfectly reduced to Cu § [14,15].

It would be possible that the photoactive sites on silica

are directly modified with copper. However, the ESR signal for 0.1 wt% Cu/SiO~. UV-irradiated in presence of oxygen was very similar to that for unmodified silica (see Figure 2), suggesting no signffi"cant change in the activation sites of oxygen after the modification of copper. Since the intensity of the ESR signal attributed to T-shape Si-O3 species on 0.1 wt% Cu/SiO2 was considerably smaller, it is rather supposed that copper deactivates the surface oxygen species. It is noteworthy t h a t formation of e t h y l e n e oxide w a s s i g n i f i c a n t l y enhanced with the samples modified with 0.1 and 0.5 wt% of copper. The photo-oxidation did not proceed with i r r a d i a t i o n of U V - f i l t e r e d l i g h t of which w a v e l e n g t h was longer t h a n 280 rim. As shown in Figure 4, the yields of carbon dioxide and ethylene oxide increased

under

~ 30

-

~20 O 4~ --~ 10

Light ; "Light ON I OFF \~

~l

!

\i

; I

~

; I i

I/

" J,

/

,-I,

/4'

2v

% r

-o

the UV

irradiation on 0.1 wt% Cu/SiO2, but the formation of carbon dioxide was

o -2

monitored by the color of the catalyst.

The color of the catalyst, which was

0

2

4

6

0

Time/h depressed after the irradiation for 3 h when oxygen in the gas phase was Fig.4. Time profiles of the photooxidation of almost exhausted. The a m o u n t of C2H4 into CO2 (0) and ethylene oxide ( I ) e t h y l e n e o x i d e f o r m e d by t h e over 0.1wt% Cu/SiO2 catalyst evacuated at following irradiation for 3 h was 1.3 973 K. ~mol a n d the a m o u n t of copper in the sample was 2.4 ~mol. The oxidation state of copper on silica can be yellowish after the evacuation at 973 K, was not changed obviously during the reaction, appearing that the oxygen in ethylene oxide is not mainly from copper oxide on the surface.

Since formation of carbon dioxide can relate to the 03

species which is directly generated from gas-phase oxygen by UV-irradiation and formation of ethylene oxide was not parallel to that of carbon dioxide, smface oxygen species other than 03 are rather responsible for the formation of ethylene

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

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oxide. Ogata et al. showed the ESR data suggesting decomposition of Si-O3 to Si-O2 in presence of carbon monoxide and formation of C2H40 radical species from Si-O2 [5]. We also observed formation of ethylene oxide over unmodified silica; thus, copper does not directly contribute to the activation of oxygen. Presence of copper may cause transformation of the Si-O3 species to the new oxygen species which leads formation of ethylene oxide.

REFERENCES

1. Y. Matsumura, K. Hashimoto, S. Yoshida, J. Catal., 117 (1989) 135. 2. Y. Matsumura, K. Hashimoto, J. B. Moffat, J. Phys. Chem., 96 (1992) 10448. 3. Y. Matsumura, K. Hashimoto, J. B. Moffat, Catal. Lett., 13 (1992) 283. 4. B. A. Morrow, I. A. Cody, J. Phys. Chem., 80 (1976) 1995. 5. A. Ogata, A. Kaxusaka, M. Enyo, J. Phys. Chem., 90 (1986) 5201. 6. H. Yoshida, T. Tanaka, M. Yamamoto, T. Funabiki, S. Yoshida, J. Chem. Soc., Chem. Commun., (1996) 2125. 7. H. Yoshida, T. Tanaka, M. Yamamoto, T. Yoshida, T. Funabiki, S. Yoshida, J. Catal., 171 (1997) 351. 8. M. A. Fox, M. T. Dulay, Chem Rev., 93 (1993) 341. 9. Y. Ichihashi, H. Yamashita, M. Anpo, Stud. Su1r Sci. Catal., 105 (1997) 1609. 10. M. Anpo, H. Yamashita, In Surface Photochemistry, M. Anpo, Ed.; John Wiley & Sons, Chichester, (1996) pp 118-164. 11. Y. Ichihashi, H. Yamashita, M. Anpo, Y. Souma, Y. Matsumura, Catal. Lett., 53 (1998) 107. 12. Y. Matsumura, K. Hashimoto, S. Yoshida, J. Catal.,ll7 (1989) 135 13. J. L. Gole, D. A. Dixon, J. Phys. Chem., 102 (1998) 33. 14. P. W. Baumeister, Phys. Rev., 121 (1961) 359. 15. H. Praliaud, S. Mikhailenko, Z. Chajar, M. Primet, Appl. Catal. B, 16 (1998) 359. 16. M. Che and A. J. Tench, in:Advances in Ctalysis,eds. D. D. Eley, H. Pines and P. B. Weisz, 32 (1983) 1-148.

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化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

P h o t o c a t a l y t i c o x i d a t i o n o f n - b u t a n e at a s t e a d y r a t e o v e r Rb-modified vanadium oxide supported on silica Tsunehiro Tanaka, Tomokazu Ito, Takuzo Funabiki and Satohiro Yoshida Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan. Photocatalytic oxidation of n-butane over various catalysts was examined with a flow reactor. Of the catalysts examined, Rb-ion-modified silica-supported vanadium oxide catalyst (Rb-VS) exhibit a stable activity for photocatalytic oxidation of n-butane to form methylethylketone at 313 I~ Silica-supported vanadium oxide (VS) and titania also exhibit fairly high activity. But quick deactivation took place for VS and deep oxidation is the main path over titania. The Rb-VS responses also to visible light irradiation. The photocatalytic reaction selectivity over Rb-VS was quite different from that over thermally activated Rb-VS. The characteristics of Rb-VS is that secondary carbon is preferentially oxidized rather than an olefinic bond. 1. I N T R O D U C T I O N Silica-supported vanadium oxide is widely known as a photocatalyst for oxidation [1,2] since the publication of photoluminescence work. [3] Previously we have reported that modification of silica-supported vanadium oxide by addition of alkali ions results in improvement of selectivity for photocatalytic oxidation of alkanes as well as enhancement of activity [4]. In the presence of water molecules V2OJSiO 2 changes its structure of an active site by adsorption of water molecules[5] while alkali-ion-modified catalyst does not [6]. These properties let us expect that alkali-ion-modified vanadium oxide supported on silica can be utilized as a catalyst in a practical use. Furthermore, the modified catalyst has another remarkable property that the catalyst can be operated under visible light irradiation [4]. The present paper reports that the catalyst system with Rb-modified vanadium oxide supported on silica [7] can be operated without deactivation at a high rate of production of partial oxidation product.

2. EXPERIMENTAL 2.1. Catalyst preparation Silica was prepared by hydrolysis of distilled tetraethyl orthosilicate as described elsewhere [8]. Alumina (JRC-AL04)was supplied from Catalysis

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1962

Society of J a p a n and titania was commercial one (Degussa P-25). V2OJSiO 2 (VS) and V2OJA120 s (VA) were prepared by impregnation of support with an aqueous solution of NH4VO s, followed by calcination in a dry air at 773 K for 5h. The loading amount of V in each catalyst was adjusted to be 2.5 wt% as V205 unless otherwise noted. A Rb-modifed catalyst was prepared by impregnation of VS or VA with an aqueous solution of RbOH, followed by calcination in a dry air stream at 773 IC Rb-modified VS and VA are denoted as Rb-VS and Rb-VA, respectively. The loading amount of Rb was R b / V =1.5. 2.2. R e a c t i o n s The catalyst 300 mg was diluted with 4.5 g of quartz powder and was mounted on a catalyst bed (3.0 mL) made of Pyrex glass [9]. The pretreatment of the catalyst was carried out by heating at 673 K under an atmospheric stream of He-O 2 (80:20) mixture at a 100 ml/min flow. A reaction was carried out under a total flow rate of 100 cc/min of a gas mixture of n-butane, oxygen and h e l i u m (20:10:70) at an atmospheric pressure. 300 W high pressure Xe lamp was used as a light source with a chilled water filter. The reactor was irradiated through glass filters, UV31 and UV39 which can permit UV/VIS lights with ~. >300 n m and ~. > 380 nm, respectively. The products were analyzed by gas chromatographs connected directly to a flow reactor. During the irradiation, the temperature of the catalyst bed was 313 K measured by an infrared radiation thermometer.

3. R E S U L T S AND D I S C U S S I O N 3.1. P h o t o o x i d a t i o n of n-butane over some photocatalysts. Table I shows the result of the reactions at 8 h time on stream over various photocatalysts. The products were butanone (methylethylketone, MEK), butanal (butyraldehyde, BA), propanal (propionaldehyde, PA) , ethanal (acetaldehyde, AA) and carbon oxides (COx). In case of the irradiation involving UV-ray (UV31), all .__ 0.7 the catalysts exhibited the "~ 0.6 6 activity and the activity for 0.5 -Rb-VS was the highest. As ..= 0.4 .F,,~ ~o shown in Fig. 1, the activity for 4 0.3 Rb-VS was stable and the ~ 0.2 ~ reaction attained a steady state ~2 O by 2 h after irradiation started. 0.1 The color of the catalyst kept 0.0 o0 white during the reaction. The 0 200 400 600 800 Time on Stream / rain selectivity to MEK is appreciably high and the space Fig. 1. Photooxidation of n-butane over time yield of MEK was 3.03 Rb-VS. A UV-31 filter was used. gL-~hx. Fig. 2 shows the result • O" MEK,&: A A , / k : PA, over VS. The main product is V" BA, m" CO 2, and [::]" CO.

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1963

Table 1 Photooxidation of n - b u t a n e over various catalysts, a) Catalystb)

filterr

Conversion 9 a) /%

MEK

VS

31 39

0.04 tr

17 .

31

0.56

44~'

7

39

0.17

61

11

31 39

0.16 tr

28 .

31

0.06

36

39

tr

.

31

0.17

54

20

39

0.07

92

7

V V -

Rb-VS

TiO2 VA Rb-VA

BA

selectivity / % PA AA

34 .

.

1 .

. 36

.

5 .

44

0

8

20

23

5

10

12

tr

70

17

6

4

11

10

-

-

tr

tr .

.

.

5 .

COx

.

.

a) The d a t a are collected at 8 h time on stream. GHSV = 2000 h ~. b) VS, Rb-VS, VA and Rb-VA, see text. c) See text. d ) Based on n-butane, e) STY(MEK) = 3.03 gL-lh -1. e t h a n a l and the activity is directly related to the formation rate of ethanal. Therefore, the activity of VS was reduced along the time. The color of VS changed from the white to d a r k yellow, suggesting that the deactivation of the catalyst was due to the reduction of v a n a d i u m cations as well as hydration. This was confirmed by UV/VIS spectra of the catalyst , i.e., red shift of absorption edge due to w a t e r adsorption and the growth of broad band at a r o u n d 500 n m due to the formation ofV ~ ions after a reaction. On the other hand, TiO 2 showed the stable activity, but the deep oxidation was the m a i n path as shown in Table 1. The partial | oxidation products are m i n o r .~ 0.7 0.10 ones. Although it has ~ 0.6 considerable photoactivity, o 0.08 capability of oxidation is too ~ o.5 strong to be applied to a _~ 0.4 0.06 ' ' 0.3 catalyst for p a r t i a l oxidation. VA and Rb-VA exhibit fair ~ 0.2 0.04 * r activity. The m a i n products = o.1 are M E K and BA. ~ 0.0 0.02 In case of the i r r a d i a t i o n 0 100 200 300 400 500 of visible light (UV39), Rb-VS Time on Stream / min and Rb-VA exhibited activity. Fig. 2. Photooxidation of n - b u t a n e over VS. Of these, the activity of Rb-VS A UV-31 filter was used. Symbols, see was higher. Fig.3 shows the captions to Fig. 1.

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reaction time on stream when ~. irradiated through the UV39 ~ 0.20 filter. Because the total amount ~ 0 0.15 ~ of photon flux was restricted, the activity was reduced to 30 %. 0.10 "'00~ But the total oxidation was o "9 0.5 suppressed, resulting in the 0.05 o high selectivity to MEK. We A I x x have already reported that 0.00 00.0 , P F" , I , Rb-VS m a k e s a response to 0., 0 ~ 400 600 800 visible light [ 4 ] . During the Time on Stream / rain reaction, Rb-VS did not change Fig. 3. Photooxidation of n-butane over the color, indicating that the Rb-VS. A UV-39 filter was used. Symbols, see chemical and physical change of captions to Fig. 1. surface species hardly takes place. This is probably because v a n a d i u m species on Rb-VS does not interact with water molecules directly but v a n a d i u m species on VS is affected directly by water adsorption [5]. Rb-VS is promising as a catalyst for photochemical conversion of alkenes. A responce to visible light by Rb-VA was firstly confirmed and it is now under investigation. 3.2. T h e e f f e c t o f t h e h e a t o n c a t a l y s i s Although the light was irradiated through a chilled water filter, infrared light is still involved and actually the t e m p e r a t u r e of the surface of the reactor was by 15 K higher t h a n the room temperature. Therefore, we may not be able to neglect the effect of the heat. Fig. 4 shows the result of the reaction in the dark at several t e m p e r a t u r e s with the same reactor. At the t e m p e r a t u r e s less t h a n 623 K, oxidation did not take place at all. With increasing temperature, the yield of carbon dioxide, carbon monoxide, butenes, MEK and methylvinylketone increases. Evolution of carbon dioxide and carbon monoxide is remarkable.

35 O

~g5 "~20 9~

15

~10 O

~ 0 600

650 700 Temperature / K

750

Fig. 4. Dark reactions over Rb-VS at several temperatures. +" butenes and v : methylvinylketone. Symbols other t h a n + and V, see captions to Fig. 1.

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1965

Table 2 Photooxidation of 1-butene over VS and Rb-VS. UV Yield b~ ProductcompositionC'/mol% Catal. filter /~tmol AA PA AL BA MEK MVK CA VS Rb-VS

14

31

18.9

12

9

tr

2

tr

72

4

1

tr

39

21.2

8

6

tr

2

tr

75

8

1

tr

.

5 .

4

3

33 .

.

3 .

16

21.5 0.96

.

10

t-B

31 39

.

9

BD

.

.

a) Reaction time; 60 min. 02, 140 ~t mol, 1-butene 70 ~tmol. b) Total amount of detected partial oxidation products, c) AA: ethanal, PA: propanal, AL: acraldehyde, BA: butyraldehyde, MEK: methylethylketone, MVK: methylvinylketone, CA: crotonaldehyde, BD: 1,3-butadiene, t-B: trans-2-butene. The appreciable amount of butenes are observed at high temperature. The product distribution is completely different from the reaction under photoirradiation. Therefore, the reactions taking place under heating the catalyst can be neglected in the present system. 3.3. T h e r e a c t i o n o f 1-butene In our previous report [4], we have report that both 2-methylpropane and 2-methylpropene are photo-oxidized to propanone over Rb-VS. To characterize the present reaction system, a trial of photooxidation of 1-butene is interesting whether a C=C double bond or a secondary carbon is oxidized. Over VS, the predominant path of the photooxidation of 1-butene has been reported to be the cleavage of a C=C double bond to form propanal (PA) [10] and other paths are not neglected resulting in non-selective oxygenation [11]. Table 2 shows the result of photooxidation of 1-butene over VS and Rb-VS in the closed circulating system [10-12]. The composition of the partial oxidation products is indicated. Actually, the reaction over VS produces variety of products in the case using UV31 filter. The predominant product was PA, ethanal (AA) and butadiene (BD). This is well consistent with our earlier data [9]. When UV39 filter was used, VS was inactive as expected. On the other hand, Rb-VS showed the fair activity both in the cases where UV31 and UV39 filters were used. It should be noticed that the main product is methylvinylketone (MVK) and other products are only a few. This result suggests that over photo-irradiated Rb-VS, the secondary carbon atom is preferentially attacked [13] rather than a C=C double bond.

4. C O N C L U S I O N Rb-VS catalyst showed the highest and the most stable activity for photooxidation of n-butane among the photocatalysts examined. Not only high

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activity but also the good selectivity to methylethylketone was exhibited. The space time yield of methylethylketone was 3.03 gLlh 1. Over the catalyst, saturated secondary carbon atom is oxidized preferentially rather than an olefinic bond. 5. A C K N O W L E D G E M E N T

The present work is partially supported by a Grant-in-Aid on Priority-Area-Research "Photoreaction Dynamics" from the Ministry of Education, Science, Sports and Culture of Japan (No. 06239237). REFFERENCES

1. M. Anpo, I. Tanahashi and Y. Kubokawa, J. Phys. Chem., 84 (1980)3440; S, Yoshida, Y. Magatani, S. Noda and T. Funabiki, J. Chem. Soc., Chem. Commun., (1982) 601. 2. S. Kalliaguin, B. N. Shelimov and V. B. Kazansky, J. Catal., 55 (1978) 384. 3. A. M. Gritscov, V. A. Shvets and V. B. Kazansky, Chem. Phys. Lett., 35 (1975) 511. 4. T. Tanaka, S. Takenaka, T. Funabiki and S. Yoshida, Chem. Lett., (1994) 1585; S. Takenaka, T. Kuriyama, T. Tanaka, T. Funabiki and S. Yoshida, J. Catal, 155 (1995) 196; S. Takenaka, T. Tanaka T. Funabiki and S. Yoshida, Catal. Lett., 1997, 44, 67. 5. S. Yoshida, T. Tanaka, Y. Nishimura, H. Mizutani and T. Funabiki, in 'Catalysis, Theory to Practice', M. J. Phillips and M. Ternan (eds.), Proc. 9th Intern. Congr. Catal.,1988, Calgary, vol. 3, p. 1473, 1988, The Chemical Institute of Canada, Ontario; X. Gao, S. R. Bare, B. M. Weckhuysen and I. E. Wachs, J. Phys. Chem. B, 102 (1998) 10842. 6. S. Takenaka, T. Tanaka, T. Yamazaki, T. Funabiki and S. Yoshida, J. Phys. Chem. B, 101 (1997) 9035. 7. T. Tanaka, S. Takenaka, T. Funabiki, and S. Yoshida, J. Chem. Soc., Faraday Trans., 92 (1996) 1975. 8. S. Yoshida, T. Matsuzaki, T. Kashiwazaki, I~ Mori and I~ Tarama, Bull. Chem. Soc. Jpn., 47 (1974) 1564. 9. T. Tanaka, T. Ito, S. Takenaka, T. Funabiki and S. Yoshida, Catal. Today, in press. 10. S. Yoshida, T. Tanaka, M. Okada and T. Funabiki, J. Chem. Soc., Faraday Trans. 1, 80(1984) 119. 11. T. Tanaka, M. Ooe, T. Funabiki and S. Yoshida, J. Chem. Soc., Faraday Trans. 1, 82 (1986) 35. 12. S. Yoshida, Y. Matsumura, S. Noda and T. Funabiki, J. Chem. Soc., Faraday Trans. 1, 1981, 77, 2237. 13. S. Takenaka, T. Tanaka, T. Funabiki and S. Yoshida, J. Chem. Soc., Faraday Trans., 93 (1997) 4151.

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个

1967

化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Photocatalytic Oxidation of Toluene on Platinum/Titania Catalysts M. Catherine Blount and John L. Falconer Department of Chemical Engineering, University of Colorado, Boulder, CO 80309 During heterogeneous photocatalytic oxidation (PCO) at room temperature, aromatics are not as readily oxidized as other organics, and intermediates accumulate that deactivate the catalyst. Toluene was used as a model aromatic on TiO2 and Pt/TiO2 catalysts. Transient isothermal reaction and reaction of a continuous flow of toluene were used to study reaction steps during PCO. Oxygen uptake was rapid for transient PCO of adsorbed toluene, but the rate of CO2 formation was much slower. The Pt/TiO2 catalyst deactivated within 7 h during PCO of a continuous flow of unhumidified toluene. With the addition of water, the rate of CO2 production increased, and the catalyst was still active atler 42 h. Less-reactive intermediates accumulated on the surface during PCO, and they were more strongly bound than toluene, which desorbed at relatively low temperatures. These intermediates were characterized by temperature-programmed hydrogenation (TPH). The strongly bound intermediates either thermally decomposed or oxidized during temperature-programmed desorption (TPD) or oxidation (TPO). However, these intermediates were hydrogenated to more weakly bound species that desorbed during TPH. The TPH spectra indicated that the aromatic ring remains intact during the first steps in toluene PCO. The transient photocatalytic oxidation ofbenzaldehyde was also investigated, but benzaldehyde does not appear to be the main reaction intermediate formed during toluene PCO. 1.

INTRODUCTION Heterogeneous photocatalytic oxidation (PCO) is a promising technique for the removal of organic pollutants present in low concentrations in waste gas streams. During PCO, the catalyst absorbs UV light, which excites electrons from the valence band into the conduction band. The resulting electron/hole pairs can then migrate to the surface and initiate redox reactions with adsorbed organics. Less reactive, strongly bound compounds form during PCO of BETX (benzene, ethylbenzene, toluene, xylenes) compounds (1-6). Accumulation of these partial oxidation products can poison the catalyst surface by blocking reaction sites. Toluene was used in this study as a model aromatic to characterize the surface intermediates that form during PCO. Both transient reaction of a monolayer and reaction of a continuous flow of toluene were studied. The intermediates that form on the surface were characterized by temperatureprogrammed hydrogenation (TPH) because hydrogenation is expected to convert the strongly bound intermediates that form during toluene PCO to more weakly held species that desorb from the surface. These intermediates thermally decompose or oxidize during temperatureprogrammed desorption (TPD) or oxidation (TPO). Platinum was added to the TiO2 since it has been reported to increase the rate of PCO (6-8) and also to take advantage of spillover of H atoms onto the TiO2 during TPH to more effectively remove the PCO intermediates from

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1968

the TiO2 surface. For Pt loading of 0.2%, less than 0.4% of the TiO2 surface is covered with Pt, and thus the photocatalytic processes are expected to by similar to those on TiO2 alone. Transient reaction techniques are useful for studying PCO because adsorption, surface reaction, and desorption steps can be separated in time. For the transient reaction in the current study, an adsorbed layer of toluene was oxidized in the absence of gas-phase toluene, and less than a monolayer was reacted so intermediates are more likely to be the result of toluene PCO and not side reactions. Toluene is weakly adsorbed on TiO2 and desorbs intact at relatively low temperatures (1), and thus unreacted toluene can be readily separated from intermediates that are more strongly bound to the surface. Though transient PCO is ideal for identification of intermediates and reaction pathways, the behavior of these pathways are difficult to predict. Therefore, continuous-flow PCO of both unhumidified and humidified toluene was studied. The PCO ofbenzaldehyde, a suspected intermediate during toluene PCO, was also investigated. 2.

EXPERIMENTAL METHODS The apparatus used for PCO and temperature-programmed experiments is described in detail elsewhere (1). A thin-film, annular reactor with a 1-mm annular spacing was used to ensure high gas flow rates over the catalyst. The catalysts used were Degussa P25 TiO2 (75% anatase with a BET surface area of 50 m2/g) and Degussa P25 with a 0.2% (wt) loading of Pt. Approximately 30 mg of catalyst coated the inside of the Pyrex photoreactor, which corresponds to a catalyst layer of 0.4 ~tm average thickness. Transient experiments used six near-UV lamps (4 W), while the continuous flow experiments used twelve near-UV lamps (8 W). A quadrupole mass spectrometer monitored the reactor effluent immediately downstream of the reactor. Computer-controlled data acquisition simultaneously monitored and recorded selected mass signals, temperature, and elapsed time. The mass spectrometer signals were calibrated by injecting known quantities of gases or liquids into the appropriate gas steam composition, and mass signals were corrected for cracking in the mass spectrometer. The catalyst was heated to 723 K in a flowing O2/He mixture before each experiment to obtain a reproducible surface. For transient PCO, organics were injected upstream of the reactor and allowed to evaporate into the flowing gas and adsorb onto the cooled catalyst. Because toluene is weakly adsorbed on TiO2 and slowly desorbs at room temperature, the reactor was cooled to 273 K and 256 K during toluene adsorption on TiO2 and Pt/TiO2, respectively, to ensure that gas-phase toluene was not present during PCO and that accurate measurements of adsorbed and reacted toluene could be made. Toluene PCO was performed at the temperature maintained during adsorption. Benzaldehyde adsorption and PCO were performed at room temperature. The transient PCO was performed in 0.2% 02 in He (100 standard cm3/min) so that the rate of 02 consumption could be measured. For the continuous flow toluene PCO experiments, a tank containing a known mixture of toluene/He was mixed with He and 02 to produce a 100 ppm toluene/10% O2/He flow (200 standard cm3/min). To create a humidified toluene gas flow, another He stream was bubbled through distilled water and mixed into the flow gas for a H20 concentration of 1000 ppm. After the feed flow reached steady-state, the catalyst was illuminated with near-UV light, and the gas-phase species were detected with mass spectrometer and GC, which sampled the effluent every 3 min. Continuous flow PCO was performed at room temperature. The PCO was stopped by turning off the UV lights, immediately cooling the catalyst surface, and switching to He to flush any gas-phase toluene from the reactor. Species adsorbed on the catalyst surface were analyzed by TPO, TPD, and TPH.

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

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During TPO, a 20% o2/ne mixture (100 standard cm3/min) flowed over the catalyst surface as the temperature was increase at a constant rate of 1 K/s. During TPD and TPH, pure He and H2, respectively, flowed over the catalyst at the same rate. An interrupted TPD to 348 K was performed following toluene PCO, but before TPH, to remove unreacted toluene so that the amount of reaction during PCO could be determined, and any toluene formed during TPH could be distinguished from unreacted toluene. 3.

RESULTS AND DISCUSSION

3.1

Transient toluene PCO

Larson et al (1) observed that when a toluene-covered TiO2 surface was illuminated at 273 K, 02 was immediately consumed; the rate of 02 consumption (Fig. 1) quickly reached a maximum and then decayed. Carbon dioxide was the only gas-phase species detected during PCO, and its rate of formation was much slower than the rate of 02 uptake. Since CO2 adsorption coverage is low on TiO2 (9) and little CO2 was produced during PCO, oxygen was incorporated into a strongly bound surface intermediate. The PCO of toluene on Pt/TiO2 was similar to the PCO on unplatinized TiO2, but Pt increased the PCO rate (Fig. 2). From mass balances, the amount of toluene adsorbed on the TiO2 and Pt/TiO2 was 80 and 100 ~tmol toluene/g catalyst, respectively. The TPD spectra following 20 min toluene PCO on TiO2 is shown in Fig. 3. Carbon dioxide and unreacted toluene desorbed at lower temperatures, and small amounts of CO2, CO, and benzene desorbed at higher temperatures. Figure 4 shows the TPH spectra obtained following a 20-min PCO of toluene and interrupted TPD on TiO2. Interrupted TPD removed all of the unreacted toluene, and only toluene desorbed during the interrupted TPD. Note that though all the unreacted toluene was removed by interrupted TPD, the largest peak during TPH is toluene, and it formed at much higher temperatures than adsorbed toluene desorbs from TiO2. In addition to toluene, benzene, a small amount of methylcyclohexane ( M C H not shown for clarity), CO2 and CO desorbed during TPH. The TPH after 20-min PCO and interrupted TPD on Pt/TiO2 is shown in Fig. 5. Platinum is a more effected hydrogenation catalyst, so the formation of a large amount of MCH was expected. Benzene and toluene are weakly adsorbed on TiO2 (1), so their formation during TPH was reaction limited. As expected for a fully hydrogenated species, MCH does not adsorb to a measureable coverage 0.4 or) or) 0.15 or) >, >" 0.3 co

~

02

o.1

0

E n,'

02 O

0.05

CO2 (x 5)

0 0

A"

0.2

E ~0.1 0

5

10 15 20 2 Time (min) Fig. 1" 02 uptake and CO2 formation during 20 min toluene PCO on TiO2

J.tl

z

9

'

5

.

.

.

.

.

10 15 Time (min)

'

20

A

25

Fig. 2 : 0 2 uptake and CO2 formation during 20 min toluene PCO on Pt/TiO2

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1970

on TiO2 or Pt/TiO2 at room temperature; it is more weakly adsorbed than toluene, and its appearance during TPH was also reaction limited. The formation of MCH and toluene during TPH suggests that at least one intermediate has a methyl group (or oxidized methyl group) attached to an aromatic ring.

3.2

PCO of benzaldehyde The PCO ofbenzaldehyde on Pt/TiO2 was similar to the PCO on TiO2 (1). Unlike toluene PCO, CO2 formation was immediate and then quickly decayed during benzaldehyde PCO (Fig. 6). In addition to CO2 formation during benzaldehyde PCO on Pt/TiO2, a small amount of benzene was detected during the initial stages of PCO. The benzene production was negligible after-~ 8 min. No benzene was detected during toluene PCO ofbenzaldehyde PCO on TiO2 (1). The amount ofbenzaldehyde adsorbed on Pt/TiO2 before PCO was 85 pmol benzaldehyde/g catalyst. The differences in the PCO of toluene and benzaldehyde suggest that benzaldehyde is not the strongly bound intermediate formed during toluene PCO. 3.2

Continuous flow PCO of toluene The PCO of a continuous flow of toluene on Pt/TiO2 is shown in Fig. 7. Upon illumination, toluene was immediately consumed, and its consumption almost immediately reached a maximum. Toluene uptake then quickly decayed, and after 30 min the rate dropped by an order of magnitude. Approximately 10 monlayer equivalents (ME) of toluene or 2.3 ME ofbenzaldehyde were consumed during PCO indicating that the reaction was catalytic. Carbon dioxide formation also reached a maximum almost immediately and then quickly decayed. The CO2 production rate was 1/7th the toluene uptake rates; therefore, as with the transient reaction, most of the toluene was converted to strongly bound species that remain on the catalyst surface. A small amount ofbenzaldehyde and benzene also desorbed during the PCO. Because of the inherent drift in mass spectrometer measurements, the lights were turned off for 7 min throughout the PCO to determine the baseline. When the lights were off, the reaction stopped, and the toluene signal returned to its baseline value. For the continuous flow reaction of unhumidified toluene, the catalyst deactivated after only---7 h of PCO. The maximum toluene uptake rate during PCO of a continuous flow of a humidified toluene stream on Pt/TiO2 (Fig. 8) was roughly the same as the maximum uptake rate for unhumidified toluene. The CO2 formation rate for the humidified stream, however, was -1.7 o~

0.25

A

0.15

>, 0.2 8 o.15 0

E

m

f ~ l U \l Beennezene(x 20)

o 0.1

a~ 0.05 rY

n~ ,

350

,

Tolu

O

E

0.1

0 250

t~

,

450 550 650 Temperature (K)

Fig. 3" TPD after 20 min toluene PCO on TiO2

Benzene

0.05 0 .,. ~ ~ ~ ~ ~ C 300

~ ~ ~ ,

400 500 600 Temperature (K)

700

Fig. 4: TPH after 20 min toluene PCO on TiO2 and interrupted TPD

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1971 0.15

0.2

A

CO

0.15 0 0'}

0.1

CO2

tO

~O E

0.1

9

0.05

n~

N O

CO2

E 0.05

v

O

n,"

0 300

9

,

.

0

,

400 500 600 700 Temperature (K) Fig. 5: TPH after 20 min toluene PCO on Pt/TiOz and interrupted TPD

0

A

9

5

10 15 20 25 Time (min) Fig. 6" CO2 and benzene formation during 20 min benzaldehyde PCO on Pt/TiO2

times higher than the formation rate for the unhumidified toluene flow. A small amount of benzaldehyde and benzene also formed during the PCO. Atter 19 h of PCO, toluene uptake and CO2 formation were still measureable (Fig. 8). Approximately 16 ME of toluene or 3.8 ME ofbenzaldehyde were consumed during 19 h of PCO. After 42 h or PCO, toluene uptake was still barely measureable, but CO2 production was negligible indicating that the catalyst does eventually deactivate even with water present in the flow gas. The TPH spectra obtained following PCO of a continuous flow of unhumidified toluene and interrupted TPD to 348 K is shown in Fig. 9. While 25 l.tmol toluene/g catalyst desorbed during interrupted TPD following transient PCO on Pt/TiO2, only ---3 ~tmol unreacted toluene/g catalyst desorbed following continuous flow PCO. Thus, the catalyst was covered with intermediates during continuous flow PCO, and little unreacted toluene was on the surface. The TPH spectra following PCO of both continuous flow humidified and unhumidified toluene were similar with only the amounts of the desorbing species differing. Unlike the TPH spectra following transient PCO of toluene on Pt/TiO2, little MCH desorbed (<1 I.tmol MCH/g catalyst). Toluene and benzene desorbed at high temperature as well as ethylene, methane, and a small amount ofbenzaldehyde (not shown for clarity). Carbon 0.4 ~" 0.6 ::;::= 2, 0.2 t~ 002 N o.4 B

E O

n,"

o o'} 0.2

I Q~-~-~

0

-0.2

m

,

0

l

2

0

~ 9-0.2

Toluene

-0.4

o

E .-1

,

,

,

,

J

4 6 8 10 Time (hr) Fig. 7: Toluene uptake and CO2 formation during continuous flow PCO of toluene on Pt/TiO2

-0.4

T~luene 9

,

2

,

=

4

.

,

6

.

.

.

.

.

.

.

.

.

.

.

.

.

.

8 10 12 14 16 18 Time (hrs) Fig. 8: Toluene uptake and CO2 formation during continuous flow PCO of humidified toluene on Pt/TiO2

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个

1972

化

0.6

>, m

o 0.4

CO2

o

E ID t~ n,

0.2 0

300

400

500 600 700 Temperature (K) Fig. 9: TPH after continuous flow PCO of

toluene on Pt/TiO2 and interrupted TPD

monoxide and CO2 desorbed in large broad peaks over a wide temperature range. Experiments were performed with 13CO2to determine if the CO2 observed during TPH was adsorbed on the catalyst surface. The 13CO2 flowed over the catalyst after continuous flow PCO of toluene and before TPH. No 13CO2 desorbed during the subsequent TPH indicating that no 13CO2/12CO2 exchange occurred. Therefore, the CO2 observed during TPH was formed from the decomposition or hydrogenation of the strongly bound surface species. 4.

CONCLUSIONS

Temperature-programmed hydrogenation was effective at hydrogenating strongly bound intermediates to more weakly bound species that could desorb during TPH. The intermediate formed during toluene PCO has a stable ring structure with a methyl (or oxidized methyl) group attached. Platinization of TiO/ increased the transient toluene PCO rate. Dramatic differences in the PCO behavior ofbenzaldehyde and toluene indicate that toluene does not react through benzaldehyde to a significant extent. The addition of water to the flow gas during the continuous flow experiments increased the rate of PCO and slowed the rate of deactivation. Experiments with 13CO2 show that CO2 desorbed during TPH was due to the decomposition or hydrogenation of the strongly bound surface species. REFERENCES

1. Larson, S.A., and Falconer, J.L., Catal. Letters 44, 57 (1997) 2. Jacoby, W.A., Blake, D.M., Fennell, J.A., Boulter, J.E., Vargo, L.M., George, M.C., and Dolberg, S.K., J. Air Waste Manag. Assoc. 46, 891 (1996) 3. Sitkiewitz, S., and Heller, A., New J. Chem. 20, 233 (1996) 4. Mendez-Roman, R., and Cardona-Martinez, N., Catal. Today 40, 353 (1998) 5. Augugliaro, V., Coluccia, S., Loddo, V., Marchese, L., Martra, G., Palmisano, L., and Shiavello, M., Appl. Catal. 20, 15 (1999) 6. Fu, S., Zeltner, W.A., and Anderson, M.A., Appl. Catal. B: Environ. 6, 209 (1995) 7. Falconer, J.L., and Magrini-Bair, K.,J. Catal. 179, 171 (1998) 8. St. John, M.R., Furgala, A.J., and Sammells, A.F., J. Phys. Chem. 87, 801 (1983) 9. Luo, S., and Falconer, J.L., Catal. Letters 57, 89 (1999)

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个

1973

化

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

Photocatalytic Degradation of Toluene in Aqueous Suspensions of of the Surfactant Polycrystalline TiO2 in the Presence Tetradecyldimethylamino-oxide V. Augugliaroa, V. Loddoa, G. Marci a, L. Palmisanoa, C. Sbriziolob, M. Schiavelloa and M. L. Turco Liverib aDipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universit~ di Palermo, Viale delle Scienze 90128 Palermo, Italy bDipartimento di Chimica Fisica, Universit~ di Palermo, Viale delle Scienze 90128 Palermo, Italy The heterogeneous photocatalytic method has been used for carrying out the toluene oxidation in synthetic aqueous solutions containing also a zwitterionic surfactant, i.e. tetradecyldimethylamino-oxide (CI4DMAO). A batch photoreactor with immersed lamp and two kinds of polycrystalline TiO2 catalysts were used. The results indicate a substantial increase of the toluene degradation rate in the presence of the surfactant; the complete photooxidation of toluene was achieved in a few hours of irradiation at the used experimental conditions; at higher irradiation times also C14DMAO was completely degraded. The main intermediates of toluene degradation were p-cresol (4-methylphenol) and benzaldehyde. Depending on the used catalyst, benzyl alcohol, benzoic acid, hydroquinone, trans, trans muconic acid and pyrogallol (1,2,3 trihydroxybenzene) were also detected. 1. INTRODUCTION Toluene is a very noxious organic compound and many strategies have been identified to reduce its presence in the environment. Toluene is used as starting reagent for the preparation of many compounds, as for instance benzaldehyde, benzyl alcohol, benzoic acid and chloro derivatives and consequently it can be found in many industrial waste effluents. The photocatalytic method has been largely studied and the potential applications of photo-oxidation reactions for the degradation of toxic compounds, present in the environment both as liquid and gaseous phases, have been deeply investigated [1-5]. In particular, photocatalytic oxidation offers various advantages compared with traditional treatment methods because the photo-reactions occur at room temperature and atmospheric pressure, radiation of the near-UV region is needed, and pollutants can be abated to very low concentration levels at measurable rates. The heterogeneous photocatalytic method has been tested for toluene abatement and previous papers report the partial or complete photocatalytic oxidation of toluene both in liquid-solid [6-8] and in gas-solid systems [9,10]. As far as the liquid-solid regime is concerned, Fujihira et al. [6,7] studied the photooxidation of toluene in aqueous aerated suspensions containing various powdered semiconductors and reported the formation of cresols, benzaldehyde and benzyl alcohol,

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1974

depending on the pH of the solution and on the semiconductor used. The formation of benzaldehyde was confirmed also by Navio et al. [8] which used acetonitrile suspensions and also investigated the influence of the presence of water on the distribution of the products. As far as the gas-solid regime is concerned, the study by Ibusuki and Takeuchi [9] claims the complete photo-oxidation of toluene in the presence of water vapour and polycrystalline TiO2 at room temperature by using four photoreactors in series. Augugliaro et al. [ 10] report, instead, the partial photo-oxidation of toluene mainly to benzaldehyde in the presence of water vapour and polycrystalline TiO2 by clarifying some mechanistic aspects of the reaction. In this work the results of the photocatalytic degradation of toluene in aqueous oxygenated solutions containing polycrystalline TiO2 and a zwitterionic surfactant, i.e. C14DMAO, are reported. The investigation of the behaviour of toluene-surfactant mixtures appears to be useful not only by a scientific point of view but also because the synthetic aqueous systems consisting of both these compounds are similar to some actual civil or industrial wastewaters. The reactivity results indicate that the presence of C14DMAO enhances the toluene degradation rate; no investigation has been carried out in order to identify the aggregated system present in the homogeneous media even if it is possible to state that toluene-surfactant interaction is hydrophobic in nature. The toluene intermediate products, detected by HPLC analysis, were benzaldehyde and p-cresol; also traces of oxygenated aromatic compounds were detected. 2. EXPERIMENTAL The photoreactivity experiments were performed at ca. 300 K in a Pyrex batch photoreactor of cylindrical shape containing 1.5 I of aqueous suspension. Polycrystalline TiO2 Degussa P25 (~80% anatase, ~20% rutile; BET specific surface area = 50 m2/g) or TiO2 Merck (~ 100% anatase; BET specific surface area = 10 m2/g) were used. Initial toluene concentrations ranged from 0.1 to 1.4 mM (well below the solubility limit of toluene in water) while the surfactant concentration was 0.05 mM for the majority of the runs. The molar ratios between toluene and C14DMAO varied from 2 to 28. Some experiments were carried out by varying the surfactant concentration in the 0.01-0.075 mM range with an initial toluene concentration of 0.37 mM. Preliminary experiments were carried out by irradiating mixtures in which photocatalyst or toluene or surfactant were absent. For all the runs the catalyst amount was 0.4 g/l and the initial pH of the suspension was equal to 6.0. A 500 W medium pressure Hg lamp (Helios Italquartz) was immersed within the photoreactor; the lamp was cooled by water circulating through a Pyrex jacket surrounding it. The photon flow impinging on the reacting suspension was measured by using a radiometer UVX Digital and its average value was 19.3 mW.cm -2. The Pyrex thimble together with the water circulation allowed to avoid mixture heating and to cut off any radiation with wavelength below 300 nm. The photoreactor was provided with ports in its upper section for the inlet and outlet of gases, for sampling and for pH and temperature measurements. A magnetic stirrer guaranteed a satisfactory suspension of the photocatalyst and the uniformity of the reacting mixture. The reacting mixture was saturated by bubbling pure oxygen at atmospheric pressure for 0.5 h before switching on the lamp; the gas was continuously bubbled also during the runs that lasted a time ranging from 1.5 to 10 h. For a run air was bubbled; the obtained reactivity results did not show appreciable differences with respect to the corresponding ones obtained with pure oxygen. Toluene and its intermediate oxidation products were analysed by HPLC by using a Varian 9010 Solvent Delivery System coupled

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1975

with a Varian 9050 Variable Wavelength UV-Vis Detector. The analyses were performed after separation of the catalyst by filtration through 0.45 ~tm cellulose acetate (HA, Millipore). Two different Alltech columns were used: an Econosphere C18 3~tm (150 mm long • 4.6 mm i.d., eluant acetonitrile/water 35/65 v/v) to analyse toluene and an Econosil C18 10~tm (250 mm long • 4.6 mm i.d., eluant acetonitrile/water 65/35 v/v) to analyse the other products. Total organic carbon (TOC) analyses were carried out for all the runs by using a Carlo Erba TCM 480 instrument in order to check the complete mineralization both of toluene and surfactant. It was not possible to confidently determine the C]4DMAO concentration; the fate of the surfactant was therefore followed by TOC analyses carried out both in the presence and in the absence of toluene. 3. RESULTS AND DISCUSSION Preliminary runs indicated that both radiation and oxygen are needed for the onset of the reactivity. In the absence of catalyst only a slight decrease (about 19 %) of toluene occurred even after 2 h of irradiation for runs carried out with toluene:Cl4DMAO molar ratio equal to 6:1; for these runs the TOC content was found unchanged with time. Figure 1 reports experimental data of toluene concentration versus irradiation time for two runs carried out in the presence of catalyst with and without the addition of surfactant. It may be noted that the presence of CI4DMAO appreciably enhances the toluene degradation rate. For the previous runs the toluene and TOC concentration values were measured in the time interval (about 30 minutes) preceding the starting of the irradiation. These concentration values were found constant with time thus suggesting that in the dark the toluene and surfactant adsorption on the catalyst is negligible. For a typical run Figure 2 reports the concentration values of toluene and of the main intermediate products detected in the course of the degradation, i.e. p-cresol and benzaldehyde, as a function of the irradiation time; these results were obtained in a run carried out in the presence of TiO2 Degussa P25 by using the highest toluene:surfactant ratio.

8 !m o

m

6

<

dark

I L,~

~" I I I I

I "i I I I I I

0

lamp on e m

e m

r

,

e

m

0.5

e

m

1

e

q~

m

1.5

TIME, h Fig. 1. Toluene concentration versus irradiation time for runs carried out without ( 0 ) and with (m) C]4DMAO. Toluene" C]4DMAO 14 "1 molar ratio. Photocatalyst: TiO: Degussa P25 (0.4 g-l]).

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1976

'~ lamp on

15 "7

dark

"7

10 0

9

0

0.5

1

1.5

2

2.5

TIME, h Fig. 2. Reactivity results of a run carried out with a toluene:C14DMAO molar ratio equal to 28:1. Photocatalyst: TiP2 Degussa P25 (0.4 g.l-~). Symbols: ( 0 ) toluene; (11) p-cresol; and (e) benzaldehyde. From Fig. 2 it must be noted that the pattern of the concentration values of benzaldehyde and p-cresol suggests the occurrence of parallel reactions involving toluene molecules adsorbed on the catalyst surface. As a kinetic modelling of the reactivity results is out of the aims of the present work, the comparison of the different systems has been carried out by using the initial reaction rate parameter. In Table 1 the values of the initial reaction rate referred to the unit volume, r0, or also to the surface area corresponding to the amount of used catalyst, r'0, are reported for the experiments carried out at the various toluene:C~aDMAO molar ratios and by using both catalysts. The r0 and r'0 values increase by increasing the toluene concentration thus indicating the dependence of the toluene oxidation rate on its concentration. On the contrary the runs not shown for the sake of brevity carried out by varying the surfactant concentration and by maintaining constant the initial toluene concentration showed the independence of the toluene initial oxidation rate from the surfactant concentration. On the basis of the macroscopic information obtained in this work it is not possible to undoubtedly establish the role of the surfactant in the toluene photo-oxidation process. All the insights, however, suggest that the enhancement of the oxidation rate is related to the modification of the adsorption properties of the irradiated TiP2 surface in the presence of the surfactant. It is likely that the surfactant molecules can be adsorbed under irradiation on the TiP2 surface through their hydrophilic group. The interaction of the dissolved toluene molecules with the hydrophobic part of the adsorbed surfactant would allow to enhance the toluene adsorption on the catalyst surface. The comparison between the runs carried out by using TiP2 Degussa P25 and TiP2 Merck samples under the same experimental conditions (toluene: ClaDMAO molar ratio

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1977

equal to 28:1) suggests an interesting consideration. Indeed, the r0 values indicate that the photoactivity of TiO2 Degussa P25 is higher than that of TiO2 Merck, whereas the r'o values indicate an opposite behaviour. By considering that in heterogeneous catalysis and photocatalysis the reactions occur on the catalyst surface, our opinion is that the r'o parameter is the correct one for comparing the reactivity results obtained with TiO2 Degussa P25 and TiO2 Merck. In conclusion, when a comparison between different photocatalysts is done, the surface areas must be taken into account in a similar way as for catalytic studies. Table 1 Initial reaction rates, ro and r'o, for photoreactivity runs carried out at various toluene: C~4DMAO molar ratios. Initial C~4DMAO concentration: 0.05 mM Catalyst

Toluene:C 14DMAO molar ratio 14:0" 2:1 3:1 4:1 6:1 14:1 22:1 28:1 10:1 28:1

ro"104 r'o"106 (mol-ll-h "l) (mol-ll.hl-m "2) Deg. P25 3.9 13 " 1.2 4.0 " 1.9 6.3 " 2.8 9.3 " 4.7 16 " 8.1 27 " 9.2 31 " 14 46 Merck 3.6 61 " 9.1 150 "(air) 6:1 1.6 26 run carried out in the absence of C14DMAO,the toluene concentration is equal to that of the 14:1 run. The photo-oxidation runs indicated that toluene degradation occurred via the formation of various intermediates before its complete mineralization. Contemporarily to toluene, also the surfactant was photo-oxidised until its complete mineralization, as indicated by TOC analyses. The complete mineralization of toluene and C~4DMAO occurred after about 4 h for the run carried out by using a 6:1 molar ratio between toluene and C~4DMAO in the presence of TiO2 Degussa P25. As far as TiO2 Merck is concerned, the complete mineralization in similar experimental conditions occurred after about 10 h. The main intermediates found for both the used catalysts were p-cresol and benzaldehyde; it is worth to remind that these compounds were produced by parallel reactions. Benzoic acid, hydroquinone and trans, trans muconic acid were detected only when TiO2 Merck was used, whereas benzyl alcohol was found only in the presence of TiO2 Degussa P25. The explanation of these insights suggesting the occurrence of different photodegradation pathways with the two kinds of photocatalysts is object of work in progress. The formation in liquid-solid regime of benzaldehyde and p-cresol and the subsequent mineralization can be described very roughly as follows: TiO2 + hv

---> TiO2 (e'ob, h+vb)

(1)

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1978

02(ads) +

e"

02(.d~)

2H20

+

O'2(ads)

-+ +

e

~

(2) H202

+

2OH"

(3)

H202 -k e" ~

OH + OH-

(4)

OH'(~u~0 + h+r

--+

(5)

••--

+'OH

OH ~---

~2HO

+ other products ---~ CO2 + H20

(6)

CH3

(ads)

HO-~CH3 4.

+ other products ---~ CO2 + H20

(7)

CONCLUSIONS

The results indicate that the photocatalytic degradation rate of toluene in aqueous suspensions of polycrystalline TiO2 was enhanced by the presence of a surfactant (CI4DMAO) added to the suspension. The role of surfactant is not linked to a direct interaction between the toluene and surfactant molecules; it seems likely that the surfactant positively affects the adsorption properties of the TiO2 surface. Differently from the gas-solid systems for which benzaldehyde was the main intermediate product, in the studied liquid-solid system p-cresol was found to be the main intermediate product. Work is in progress in order to confirm and explain the findings of this investigation. REFERENCES 1. M. Schiavello (ed.), Photocatalysis and Environment. Trends and Applications, Kluwer, Dordrecht, 1988. 2. N. Serpone and E. Pelizzetti (eds.), Photocatalysis. Fundamentals and Applications, Wiley, New York, 1989. 3. D.F. Ollis in E. Pelizzetti and N. Serpone (eds.), Homogeneous and Heterogeneous Photocatalysis, Kluwer, Dordrecht, 1986, p. 651. 4. V. Augugliaro, L. Palmisano, A. Sclafani, C. Minero and E. Pelizzetti, Toxicol. Environ. Chem. 16 (1988) 89. 5. V. Augugliaro, A. Di Paola, V. Loddo, G. Marci, L. Palmisano and M. Schiavello, Catalysts and Catalysis 41 (1999) 73. 6. M. Fujihira, Y. Satoh and T. Osa, Nature 293 (1981) 206. 7. M. Fujihira, Y. Satoh and T. Osa, J. Electroanal. Chem. 126 (1981) 1053. 8. A. Navio, M. Garcia G6mez, M. A. Pradera Adrian and J. Fuentes Mota, J. Mol. Catal. 104 (1996) 329. 9. T. Ibusuki and K. Takeuchi, Atmos. Environ. 20 (1986) 1711. 10. V. Augugliaro, S. Coluccia, V. Loddo, L. Marchese, G. Martra, L. Palmisano and M. Schiavello, Appl. Cat. B: Environ. 20 (1999) 15, and references therein.

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1979

Photooxidation of Benzene to Phenol by Ru Complex Occluded in Mesoporous FSM-16

Keiko Fu]ishima, Atsushi Fukuoka, and Masaru Ichikawa Catalysis Research Center, Hokkaido University, Sapporo 060-0811, Japan

[Ru(bpy)3]Cl2 occluded

in mesoporous FSM-16

shows a high activity in the

photooxidation of benzene to phenol using hydrogen peroxide as an oxidant. based on Ru and the selectivity to phenol was 98%.

The TON was 430

The catalytic oxidation of benzene may

proceed by a photo-excited state of the Ru complex.

1. Introduction Selective oxidation of benzene to phenol is currently one of the challenging goals in mimicking enzymes in heterogeneous and homogeneous catalyses [1 ].

In the case of the Fenton's

reagent method, which is widely studied in the direct oxidation of benzene, the reaction scheme is generally proposed as follows; hydroxyl radical produced from H202by Fe2§ reacts with benzene to form an intermediate hydroxycyclohexadienyl radical.

The hydroxycyclohexadienyl radical reacts

with Fe 3§ to form phenol and regenerated Fe 2+. B iphenyl is usually produced as a byproduct according

to

the

dimerization

0f

two

~--N

N--~

hydroxycyclohexadienyl radical [2]. We have reported the preparation of hybrid

composites

of

metalloporphyrins

and

fullerenes in mesoporous FSM-16 and their activity in photooxidation of propylene [3].

It was also

OR N

Fig. 1. Structure of [Ru(bpy)3]~+.

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1980

found that the fullerenes occluded in FSM-16 catalyzed the photooxidation of cyclohexene and

benzene with 02 [4]. [Ru(bpy)3]Cl2 (Fig. 1) has three excited states: metal-ligand charge transfer (MLCT), ligand charge (LC) transfer and metal charge transfer (MC). luminescence and bimolecular quenching. release ofbpy ligand.

MLCT and LC excited states exhibit

On the other hand, MC excited state results in the

For most Ru( II ) polypyridine complexes, the energy position of the MLCT

is lower than that of the MC, and [Ru(bpy)3]Cl 2 shows strong luminescence and quenching [5]. Here we report that

[Ru(bpy)3]Cl2 (bpy = 2,2'-bipyridyne),

which has unique

photochemical and photophysical properties of excited state, occluded in the mesopores of FSM-16 shows a high catalytic activity in liquid phase photooxidation of benzene to phenol.

2. Experimental FSM-16 (channel diameter 2.7 nm) was prepared according to the literature method [6], and evacuated at 623 K for 2 h.

[Ru(bpy)3]Cl2 (Ardrich Chem. Co.) was occluded in FSM-I 6 by

the impregnation from a methanol/water solution (Ru complex 1 wt%) (Fig. 2). grade)

was

occluded

in FSM-16

in a sealed

glass tube under

C6o (Hoechst gold

vacuum

at

773

K.

[Ru(bpy)a]CI:4FSM-16 and C6o/FSM-16 were washed with appropriate solvents to remove weakly adsorbed [Ru(bpy)3]C12 or C60, and the amount of occluded species were determined by UV-vis. spectroscopy.

Photooxidation of benzene was performed in a quartz vessel for photochemistry,

and a high-pressure mercury lamp (USHIO UM-102 100 W, 300-600 nm) was used as a light source.

!Evacuation,623 K, 2 h ,:

[Ru(bpy)3]Cl2(aq.)= :~ ~

........ St!.m'ng.un.dcrN2a..trn0sphc~, 4 h ....... Vacuumed300 K, overnight ,"-

[Ru(bpy)3]Cl~/FSM-16

.~..

Wash ~m br [Ru(bpyh]CI2/C6o/FSM- 16

Fig. 2. Preparation of [RuCopy)a]Cl2/Ceo/FSM-16.

家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化

1981

A mixture of benzene (Kanto Chem. Ltd.) and aq. H202 (Mitsubishi Gas Chemical, 20 - 30 %) was

stirred with a catalyst under irradiation.

Products were analyzed by GC, GC-MS (Shimazu, DB-

WAX 25 m), and HPLC (Nihon Millipore Ltd., 5~t C~8 100 A).

Glass filter (Toshiba UV-39D and

UV-D33S) were used in experiments for wavelength dependence.

3. Results and Discussion 3.1. Catalytic activities in photooxidation of benzene [Ru(bpy)3]Cl2 was supported in the mesopores of FSM-16 by the impregnation and C6o by the CVD method.

After washing out the weakly adsorbed [Ru(bpy)3]Cl 2 and/or C6o, the loading

determined by UV-vis. were as follows: [Ru(bpy)3]CIJFS-16, 0.1% for Ru metal; C6o/FSM-16, 316%; [Ru(bpy)3]CI2/C6o/FSM-16, 0.1% for Ru and 1.2 % for C6o. Table 1 summarizes the catalytic results of photooxidation of benzene by Ru complexes using H202 as an oxidant (Eq. 1). +

H202

OH + 0

0 +

(1)

In a mixture of benzene and aq. H202, [Ru(bpy)3]Cl2 itself shows a turnover number of 170 for the formation of phenol in 24 h under irradiation (Table 1, Run 1), which was larger than that by that of 83 for RuCI 3. The oxidation of benzene by [Ru(bpy)3]Cl 2 resulted in a high selectivity of phenol; quinone and biphenyl were formed as minor products and their TONs were much lower and catecol, hydroquinone and resorcinol were not detected.

It is interesting to note

that the occlusion of [Ru(bpy)3]Cl2 in FSM-16 gave a higher TON (430) for phenol production (Run 2).

No reaction was observed in flowing 02 (1 atm) as oxidant for [Ru(bpy)3]Cl 2 or Table 1. Catalytic activities for benzene oxidation, a) 9 CatalFsts [Ru(bpy)3]Cl2 [Ru(bpy)3]CI2/FSM-I 6 RuCI3 [Ru(bpy)3]Cl2/C60 [Ru(bpy)3]CI2/C60/FSM-16

phenol 170 430 83 53 86

TON(/Ru) quinone biphenyl 3 0.8 6 0.8 8 0 2 0.1 10 0.1

a) Conditions; Benzene 840 mmol, H202 710 retool, Ru 5.7x10 ~ retool, Coo 1.0xl0 2 mmol, irradiated 300-600 nm, 24 h, room temperature.

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350

[Ru(bpy)3]Cl2/FSM- 16 catalysts. Although

the

oxidation

larklightdark li~lt

.~/,//

by

[Ru(bpy)3]CI~FSM-16 slightly proceeded in dark (TON of phenol: 35 in 24 h) (Fig. 3, line A), the

25O 2OO

irradiation of light greatly enhanced the reaction (Fig. 3, line B).

It suggests that this reaction is

not an autooxidation reaction.

The original

orange color of [Ru(bpy)3]Cl2 solution gradually turned to pale yellow during the irradiation, and the

visible

t

spectroscopy

showed

the

A:Un~ 0

0

5

tO

15 20 Tmle (h)

25

30

Fig. 3. Dependence of benzene oxidation to phenol on irradiation by lRu(bpy)a]CI~/FSM-16. ~

photosubstitution of the bipyridyl ligand from Ru

O.

center (Fig. 4) [7]. The

-"

/.-.

0.6~

dependence

wavelength was studied.

of

TONs

on

35

"

=

;

/

/

Oh

-

3h

'~

;-m\

6h

In the case of

irradiation at 300-400 nm in the MC excited range [5], the TON of phenol was almost the same as whole wavelength irradiation (300-600 0

nm), and the TONs of quinone and biphenyl were decreased.

On the other hand, at 400-600

nm of the MLCT excited range [5], the TONs were decreased.

In all cases except under the

dark, the color of catalyst changed and it suggests the degradation of

[Ru(bpy)3]Cl2.

"-. . . . . . .

300

450

500

Wavel e n g t h (rim)

550

600

Table 2. Wavelength dependence for the benzene oxidation by [Ru(bpy)a]C12.

These data indicate that [Ru(bpy)n] 2+ (n--l-3) species formed from the MC excited species are

300-400 a)

involved in the catalytic photooxidation of

400-600 a) dark . .

Furthermore, the

,

400

Fig. 4. Spectral change of the visible spectra of [Ru(bpy)a]Clg. in H,O~ during the irradiation.

W. L. ( n m ) 300-600

benzene by [Ru(bpy)3]Cl 2.

x...........

350

TON (/Ru) quinone biphenyl 4.0 0.4 100 2.3 ND 41 2.0 0.1 15 0.4 O.1 .

phenol 96

.

initial activity under irradiation was higher than that at 30 h in Fig. 3, which suggests that [Ru(bpy)3] 2+ species is more active than the degradated species.

When a mixture of [Ru(bpy)3]Cl2 and C60 was used as catalyst in the reaction with

H202,

the TON for phenol was lower than that by [Ru(bpy)3]Cl 2, but the decomposition of [Ru(bpy)3]Cl2

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1983

was effectively inhibited. catalyzed

the

C60 also

photooxidation

[Ru(bpy)3]2+* '.

~[Ru(bpy)n]2**/

H202

of

benzene with aq. H202 to produce phenol with a TON of 3.

J

The

/

occlusion of both [Ru(bpy)3] C12 and C60 in FSM-16 increased the TON for

[Ru(bpY)nl2+

',t.

[Ru(bpy)n] 3*

/

0

phenol compared to the non-occluded [Ru(bpy)3]Cl2

and

C60, and

the

formation of quinone was slightly improved.

Furthermore,

in

the

Fig. 5. Proposed reaction cycle for the photooxidation of benzene by [Ru(bpy)3]Cl~.

reaction using 02 as oxidant, [Ru(bpy)3]CI2/C6o/FSM-16 catalyzed the phenol formation with a TON of 2-3.

3.2 Discussion of the reaction mechanism.

Although the mechanism of the photooxidation of benzene is not yet clear at this moment, the excited state of [Ru(bpy)3]Cl2 may promote both the formation of OH radical from H202 and the attack of the OH radical to benzene as proposed for the Fenton reagent [2].

From the experiments

of the wavelength dependence, we propose that the MC excited state of Ru complex and its derivative may be an active species to produce hydroxyl radical from the hydrogen peroxide and hydroxycyclohexadienyl radical in the catalytic oxidation of benzene to phenol. hydroxycyclohexadienyl radical by R u 3+ results in the formation of phenol.

The oxidation of

The dimerization of

the radical gives biphenyl, but in our case the dimerization path is minor because only a trace amount of biphenyl was formed as shown in Table 1.

The Ru(lll) is a strong oxidant, and

reduction to phenol is faster than dimerization to biphenyl. In the reaction by [Ru(bpy)3]Cl2/C60, C60 seems to work as a quencher of the excited [Ru(bpy)3]Cl2 and prevents the excited Ru complex from oxidizing benzene and degradating.

4. Conclusion

[Ru(bpy)3]Cl 2

catalytically oxidizes benzene to phenol by H202, and the TON and

selectivity of phenol are high.

The excited state Ru complex drastically acceralates the reaction.

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C6o may prevent [Ru(bpy)3]Cl 2 from the photo-oxidation by quenching from the excited Ru states.

References 1. Catalytic Oxidation -Principles and Applications, R. A. Sheldon and R. A. van Santen (eds.), World Scientific Publishing Co., Singapore, 1995; Catal. Today, 41, No. 4 (1998). 2. J. R. L. Smith et al., J. Chem. Soc., 2897(1963), A. Kunai et al., J. Am. Chem. Soc., 108 (1986) 6012. 3. J. Tachibana, M. Chiba, M. Ichikawa, T. Imamura, and Y. Sasaki, Supramol. Sci., 5 (1998) 281. 4. A. Fukuoka, K. Fujishima, M. Chiba, J. Tachibana, and M. Ichikawa, Shokubai (Catalyst), 40 (1998) 490, and to be published. 5. A. Juris and V. Balzani, Coord. Chem. Rev., 84, (1988) 85. 6. S. Inagaki, Y. Fukushima, and K. Kuroda, J. Chem. Soc., Chem., Commun., (1993) 680. 7. B. Durham, J. V. Caspar, J. K. Nagle and T. J. Meyer, J. Am. Chem. Soc., 104, (1982) 4803.